Research Highlights
Vapochromism Induced byIntermolecular Electron Transfer Coupledwith Hydrogen-Bond Formationin Zinc Dithiolene Complex
Mori and Ozaki Groups
Reversible control of electronic functionalities ofmolecular materials using external stimuli and perturbations
is important not only for applications to next-generation
switching devices and sensors but also from the viewpointof basic science to elucidate the mechanism and to establish
the material design criteria. Recently we have demonstrated
a proton-electron coupled phenomenon in H (hydrogen)bonded π-conjugated molecular crystals [1-3], in whichmagnetism and electrical conductivity were switched byproton (deuteron) dynamics in an intermolecular H-bond.
If control of the functionalities, such as optical properties, other than magnetism and electrical conductivity bymodulating the electronic states through H-bonding is wellestablished, it will impact various research fields and greatly
expand the design criteria of functional materials. As one of
the functional molecular materials with optical properties
responsive to external stimuli, vapochromic materials, which
respond with color changes when exposed to solvent vapors,have been actively studied as good candidates for their use as
chemical sensors because they can directly visualize external
environmental changes. In this study, we focused on a novel
Zn complex, (Ph4P)2[Zn(4-mxbdt)2] (1 in Fig. 1; 4-mxbdt= 4-methoxybenzene-1,2-dithiolate) to realize the controlof vapochromic behavior by direct modulation of electronic
states through H-bonding. Based on the detailed singlecrystal structure analyses and first-principles calculations
considering intermolecular interactions using the periodic
structure model of the crystal, we disclosed a new mecha-

Fig. 1. A novel vapochromic mechanism by intermolecular electrontransfer coupled with hydrogen-bond formation was realized in azinc dithiolene complex crystals, (Ph4P)2[Zn(4-mxbdt)2] (1, left) and
1·2CH3OH (right).

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nism of vapochromism in 1, that is, not molecular structure
and arrangement changes but electron transfer via H-bondformation for the first time [2].
The Zn complex 1 was synthesized by complexation of zinc(II) chloride and the ligand precursor inmethanol. Yellow block-like crystals were obtained by
the slow evaporation of the crude mixture. From the
X-ray structural analysis, the yellow crystal was determined as (Ph4P)2[Zn(4-mxbdt)2]·2CH3OH (= 1·2MeOH)(Fig. 1). In this crystal, the [Zn(4-mxbdt)2] complex
showed tetrahedral coordination without a π-stacking structure because of the presence of bulky Ph4P+ cations, andthus there were no significant intermolecular interactions
between the complexes. The spaces in the loosely packed
structure were filled with two methanol molecules per
formula. These methanol molecules formed [S···H–O]
H-bonds with S atoms on the [Zn(4-mxbdt)2]2-. Interestingly,
drying the crystals overnight under vacuum induced the color
change from yellow to orange, maintaining its high crystallinity (Fig. 1).
We quantitatively evaluated the color change of eachcrystal by UV-vis absorption spectra. Visible-light absorption
edges of the orange crystal 1 and yellow crystals, 1·H2O and
1·2MeOH, were estimated to be 2.29 eV (541 nm), 2.41 eV(514 nm), and 2.45 eV (506 nm), respectively. A blue shift of
the absorption edge (0.12–0.16 eV) upon methanol or water
vapor absorption was observed, which is consistent withthe visually observed color changes. We also evaluated thephotoluminescence properties of these crystals. Significantly,
a blue shift was observed in the photoluminescence spectra(0.10–0.14 eV) for both 1·H2O and 1·2MeOH, similar to theFig. 2. (a) Crystal orbital energies of 1 and 1·2MeOH (major) and (b)corresponding crystal orbitals in 1·2MeOH (major). Crystal orbitals
from -3.2 eV to -1.3 eV (left; gray lines) are mostly on Ph4P+ cation
molecules.

UV-vis absorption spectra. The apparent color changed from
red to yellowish-orange. These experimental data demonstrate that crystal 1 is a novel vapochromic material having
both visible-light absorption and photoluminescence changes
when exposed to methanol or water sorption with reversible SCSC (single crystal-single crystal) transformations.
Remarkably, this is the first report of metal-dithiolene-based
vapochromic materials with SCSC transformations.
Then, we focused on the mechanism of vapochromismin this crystal, 1·2MeOH. DFT calculations based on theexperimentally observed crystal structures using OpenMX
software revealed the mechanism of the Fig. 2(b) H-bonding
structures in 1·2MeOH. In 1·2MeOH (major), energy gaps
for both α to γ and β to δ transitions increased (α to γ:
2.85 eV to 3.02 eV, β to δ: 2.97 eV to 2.99 eV), where the αand β orbitals were more stabilized than γ and δ (Fig. 2 (a)).
This is because of the fact that the occupied orbital groups(α and β) have a large weight on the H-bonding S atoms,
on which the largest changes in the number of electrons
were estimated by Mulliken population analysis, comparedwith the unoccupied orbital groups (γ and δ). Considering the orbital shape around the H-bond, these crystal
orbitals are derived from the hybridization of the frontier
orbitals (HOMO-1, HOMO, LUMO, and LUMO+1) of the[Zn(4-mxbdt)2]2- complex with the LUMO of the absorbed
vapor molecules at higher energies. This hybridization leads
to the above-mentioned electron transfer to stabilize both theα/β and γ/δ orbitals.
In conclusion, a novel mechanism of vapochromism
based on the intermolecular electron transfer coupled with
H-bond formation was realized in the newly obtainedzinc dithiolene complex crystal, (Ph4P)2[Zn(4-mxbdt)2]
2CH3OH. According to the mechanism proposed in thisstudy, molecules in which molecular orbitals for visible-lightabsorption are significantly distributed to H-bonding parts
can be promising candidates for a new class of vapochromic
materials. The findings here provide important insights to
expand the scope of molecular designs for vapochromic aswell as other stimuli-responsive materials.
References[1] S. Yokomori, A. Ueda, T. Higashino, R. Kumai, Y. Murakami, and
H. Mori, Cryst Eng Commun. 21, 2940 (2019).
[2] S. Yokomori, S. Dekura, T. Fujino, M. Kawamura, T. Ozaki, and
H. Mori, J. Mater. Chem. C. 8, 14939 (2020).
[3] S. Yokomori, S. Dekura, A. Ueda, R. Kumai, Y. Murakami, and
H. Mori, J. Mater. Chem. (to be published).

as a nonlinear transport phenomenon in the non-equilibrium
state carrying finite electric current.
In this study, we predicted the nonlinear AHE in alayered organic conductor α-(BEDT-TTF)2I3, and confirmed
it experimentally. This is the first example of topologicaltransport phenomena in organic conductors. We focusedon the "weak" charge ordering (CO) state in α-(BEDTTTF)2I3, where the inversion symmetry is broken. This
compound exhibits a phase transition to a CO insulator at
135 K at ambient pressure. This CO transition is suppressedby pressure, and a two-dimensional (2D) massless Diracfermion (DF) state appears above the critical pressure Pc ~1.3 GPa. The CO state just below Pc is called the weak CO
state.
First, we experimentally established the fact that the
weak CO state is a 2D massive DF state with a pair of tilted
gapped Dirac cones [1]. We used the temperature-dependent
peak structure of the interlayer magnetoresistance, which ispeculiar to the layered DF system. The magnetic field dependence of the peak temperature observed in the weak CO stateis explained only by the massive DF with a finite gap. Therefore, a finite Berry curvature appears in the weak CO state.
Next, we investigated the nonlinear AHE in the currentcarrying state in the 2D massive DF system [2]. Under thetime reversal symmetry, the sum of Berry curvature over
occupied states is canceled out between the Kramers pair of
Dirac cones, resulting in no AHE in the equilibrium state.
However, if the Dirac cones are tilting, this Berry curvature balance is broken between the Dirac cone pair in thecurrent-carrying state with non-equilibrium distribution. It
revives finite AHE as the current-induced nonlinear AHE.
The efficiency for the appearance of this current-induced
phenomenon is represented by the Berry curvature dipole(BCD) of the system. We demonstrated the anisotropy andrectification characteristics (non-reciprocity) of the nonlinear
AHE in the 2D tilted massive DF system. The generated Hall
current exhibits unidirectionality even under an external AC
field. Furthermore, we quantitatively estimated the nonlinear
AHE expected in the weak CO state in α-(BEDT-TTF)2I3,
and concluded that it is within the measurable range.
In real α-(BEDT-TTF)2I3, there may exist two types of

Authors
S. Yokomori, S. Dekura, T. Fujino, M. Kawamura, T. Ozaki, and H.
MoriNonlinear Anomalous Hall Effect inOrganic Massive Dirac Fermion System
Osada Group
When an electron system has a conduction band with afinite Berry curvature and no time reversal symmetry, it may
exhibit the intrinsic anomalous Hall effect (AHE) even atzero magnetic field. This topologically nontrivial transportphenomena disappears in equilibrium state of the systemwith time reversal symmetry. However, the AHE may revive

Fig. 1. Current-carrying state in the 2D massive DF system with tilted
Dirac cones around (a) k0 and (b) -k0. Lower panels depict the band
dispersion and upper panels illustrate the Berry curvature (contours)
and the Fermi surface of the conduction band. The hatched region
indicates occupied states in the current-carrying state. (c) Schematicof the nonlinear AHE. The balance of average anomalous velocity ofelectrons around k0 and -k0 is broken (dashed arrows) in the currentcarrying state.

ISSP Activity Report 20203

Fig. 2. (a) Dependence of anomalous Hall current j(2) on the electricfield direction θ in the current-carrying state. (b) Nonlinear AHE
observed in the weak CO state in α-(BEDT-TTF)2I3. It shows the rectifying characteristics, that is, the same Hall voltage against reversed
current.

CO domains, which cancel the nonlinear AHE under current.
So, we proposed the "current-field-cooling" technique, formacroscopic emergence of nonlinear AHE [2]. It utilizes
the current-induced orbital magnetization (orbital Edelsteineffect) to enhance the formation of single type of CO
domains.
Finally, we attempted an experimental confirmationof the nonlinear AHE in the weak CO state in α-(BEDTTTF)2I3. We performed transport measurements in the weak
CO state at 1.25 GPa and 4.2 K, and extracted the nonlinear
signal from current-reversed data. We successfully observed
nonlinear AHE with estimated order and rectifying characteristics only below the CO transition temperature [3].

kagomé structure suppresses the magnetic order down to
TN = 4 K, realizing a spin liquid state in a wide temperature
range TN < T < J ⁄ kB.
We find clear thermal Hall signals in the spin liquid
phase in all samples. The thermal Hall conductivity (κxy)
is larger in a higher-quality sample with a larger longitudinal thermal conductivity (κxx), whereas the temperature dependence of κxy is similar (Fig. 1(a)). At 15 T,κxy and κxx are found to show a peak at almost the same
temperature, a telltale sign of a phonon contribution κ phxy
in κxy at high fields. In addition, we find that the fielddependence of κxy turns to be nonlinear at low temperatures and at low fields (Fig. 1(b)), concomitantly withthe appearance of the field suppression of κxx, indicatingsp
the presence of a spin contribution κ xyin κ xy at low
spfields. This is the first observation of both κ phxy and κ xy in thesame magnetic insulator.
sp
Remarkably, by assembling the κxx dependence of κ xy
data of other kagome antiferromagnets, we find that, whereasspκ xy
stays a constant in the low-κxx region (the “intrinsic” linespin Fig. 2), κ xy
starts to increase as κxx does in the high-κxx
region (the “extrinsic” line in Fig. 2). This κxx dependence
bears similarity to that of the anomalous Hall effect inferromagnetic metals; the intrinsic mechanism by the Berrycurvature is dominant in a moderate dirty metal, whereas theextrinsic mechanism by skew scatterings is dominant for a
superclean metal. This good analogy indicates the presence

References[1] K. Yoshimura, M. Sato, and T. Osada, J. Phys. Soc. Jpn. 90, 033701(2021).
[2] T. Osada and A. Kiswandhi, J. Phys. Soc. Jpn. 89, 103701 (2020);
ibid. 128002 (2020).
[3] A. Kiswandhi and T. Osada, arXiv: 2103.00300.
Authors
T. Osada, A. Kiswandhi, K. Yoshimura, M. Sato, K. Uchida, andT. Taen

Thermal Hall Effects of Spins andPhonons in Kagome Antiferromagnet
Cd-Kapellasite
Yamashita, Hiroi, Kawashima,and Sakakibara Groups
The trajectory of an electron bends as it moves througha magnetic field. This Hall effect has been thought to notappear in insulators because of the apparent absence ofmobile electrons. But recent reports of a thermal version ofthis effect, known as the thermal Hall effect, in numerous
insulators has led to broad attention from researchers tounderstand its origin as well as potential applications forthermal current control.
Thermal current in an insulator is carried by spins
and phonons. This leads us to ask how these charge
neutral carriers can be bent by magnetic fields and how
one can separate these two effects. To clarify the issue,
we investigate the magnetic insulator Cd-kapellasite
CdCu 3(OH) 6(NO 3) 2, a kagomé antiferromagnet [1]. In
Cd-kapellasite, Cu2+ ions form a kagomé structure with adominant nearest-neighbor antiferromagnetic interaction
J ⁄ kB = 45 K. The geometrical frustration effect of the

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Fig. 1. (a) The temperature dependence of the thermal Hall conductivityof Cd-kapellasite of different samples at 15 T [1]. All the samples show
a peak at the same temperature, but with different magnitudes. (b) The
field dependence of κxy at different temperatures. The non-linear part(colored region) shows the spin contribution.

Fig. 2. The dependence of the spin thermal Hall conductivity per the 2D
sp,2D
kagomé layer (κ xy) on the longitudinal thermal conductivity (κxx) of
Cd-kapellasite [1], Volborthite [2], and Ca-kapellasite [3]. The blue andpink dashed lines are guides to the eye for the intrinsic and extrinsic
contributions, respectively.

of a similar duality of intrinsic-extrinsic mechanisms for the
spin thermal Hall effect of an insulator.
sp
Furthermore, we find that both κ phxy and κ xy disappear in
the antiferromagnetic ordered phase at low fields, showingthat phonons alone do not exhibit the thermal Hall effect. A
high field above approximately 7 T induces κ phxy, concomitantly with a field-induced increase of κxx and the specificheat, suggesting a coupling of the phonons to the fieldinduced spin excitations as the origin of κ phxy.

point fluctuation not to allow the spins to order. While thepresence of a finite linear residual thermal conductivity,
κ0 ∕ T ≡ κ ∕ T (T → 0) has been shown [2], recent experiments [3, 4, 5] have reported the absence of κ0 ∕ T in somesamples. (Fig. 1). However, in the experiments reporting theabsence of κ0 ∕ T, the phonon thermal conductivity is strongly
suppressed [3]. In sharp contrast, a large phonon thermal
conductivity with a phonon mean free path reaching almost
the boundary limit is observed in the experiments showing
the presence of the κ0 ∕ T. Therefore, it is not clear if theabsence of κ0 ∕ T shows absence of the gapless spin excitations or if the gapless spin excitations are simply suppressed
when the phonons are strongly scattered.
Here we show that both the magnitude of the phonon
thermal conductivity and the presence/absence of κ0 ∕ Tstrongly depend on the cooling process of the sample. Whencooling down very slowly, a sizable κ0 ∕ T is observed. In
contrast, when cooling down rapidly, κ0 ∕ T vanishes and,in addition, the phonon thermal conductivity is strongly
suppressed. These results suggest that possible random
scatterers introduced during the cooling process are responsible for the apparent discrepancy of the thermal conductivity data in this organic system. The present results provide
evidence that the true ground state of EtMe3Sb[Pd(dmit)2]2
is likely to be a quantum spin liquid with itinerant gaplessexcitations.

References[1] M. Akazawa et al., Phys. Rev. X 10, 041059 (2020).
[2] D. Watanabe et al., Proc. Natl. Acad. Sci. USA 113, 8653 (2016).
[3] H. Doki, M. Akazawa, H.-Y. Lee et al., Phys. Rev. Lett. 121,
097203 (2018).
Authors
M. Akazawa, M. Shimozawaa, S. Kittakab, T. Sakakibara, R. Okuma,
Z. Hiroi, H.-Y. Leec, N. Kawashima, J. H. Hand, M. Yamashita
aOsaka University
bChuo University
cKorea University
dSungkyunkwan University

Presence and Absence of Itinerant Gapless
Excitations in the Quantum Spin Liquid
Candidate EtMe3Sb[Pd(dmit)2]2
Yamashita GroupThe presence or absence of itinerant gapless excitations in the organic quantum spin liquid (QSL) candidate
EtMe3Sb[Pd(dmit)2]2, probed by a finite residual linear
term in the thermal conductivity κ0 ∕ T ≡ κ ∕ T (T → 0), hasbeen controversial hot topic. We find that κ0 ∕ T strongly
depends on the cooling process. A finite κ0 ∕ T is observed ina slow-cooled sample, but not in a rapid-cooled sample [1].
These results provide evidence that the true ground state of
EtMe3Sb[Pd(dmit)2]2 is a QSL with itinerant gapless excitations.
The organic Mott insulator EtMe 3 Sb[Pd(dmit) 2 ] 2
possesses a nearly isotropic triangular lattice of localized
spin-1/2 on the Pd(dmit)2 dimers. This geometrical frustration effect caused by the triangular lattice and a proximity
effect of the Mott transition are believed to enhance the zero-Fig. 1. Temperature dependence of the thermal conductivity divided
by the temperature of EtMe3Sb[Pd(dmit)2]2. The data of the previous
reports [2, 3] are also plotted.

Fig. 2. Low-temperature data of κ ⁄ T plotted as a function of T. The
dashed lines show an extrapolation of the data.

ISSP Activity Report 20205

References[1] M. Yamashita et al., Phys. Rev. B 101, 140407(R) (2020) * Editor’ssuggestion.
[2] M. Yamashita et al., Science 328, 1246 (2010).
[3] M. Yamashita, J. Phys. Soc. Jpn. 88, 083702 (2019).
[4] P. Bourgeois-Hope et al., Phys. Rev. X 9, 041051 (2019)[5] J. M. Ni et al., Phys. Rev. Lett. 123, 247204 (2019).
Authors
M. Yamashita, Y. Satoa, T. Tominagaa, Y. Kasaharaa, S. Kasaharaa,
H. Cuib, R. Katob, T. Shibauchic, and Y. Matsudaa
aKyoto University
bRIKENcThe University of Tokyo

Sample Dependence of the Half-Integer
Quantized thermal Hall Effectin a Kitaev Candidate α-RuCl3
Yamashita and Uwatoko Groups
Non-trivial topology in a condensed-matter state realizes
a quantization of a physical quantity. One of the most fundamental examples is the quantized Hall conductivity in aquantum Hall system, where the quantized Hall conductivity
is given by the Chern number determined by the topology ofthe system.
A new intriguing case of this topological quantization
is now found in a magnetic insulator, a Kitaev magnet.
In the Kitaev model, localized spin-1/2 moments on a
two-dimensional (2D) honeycomb structure are coupled eachother by bond-dependent Ising interactions. The frustration

Fig. 1. (a) A setup picture of the thermal conductivity (κxx) and thethermal Hall (κxy) of α-RuCl3 (sample A). The sample was tilted to thebase plate so that the magnetic field oriented to 45 degree to the c axis.
(b) The temperature dependence of κxx of different samples (A, B, andC) at zero field.

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Fig. 2. The in-plane field dependence of κxy ⁄ T of all the samples. The
dotted lines show the value corresponding to the half-integer quantized2
thermal Hall effect κ2D
xy / T = qt C / 2, where qt = (π ⁄ 6) k B / ħ . The sign ofκxy of sample A is inverted for a clarity.

effect of this Kitaev Hamiltonian prevents the spins fromordering even at the zero temperature, realizing a quantum
spin liquid state. Remarkably, this ground state of the Kitaev
Hamiltonian is exactly solvable. The ground state has beenshown to be characterized by the two kinds of elementary
excitations; itinerant Majorana fermions and localized Z2
fluxes. In a magnetic field, this itinerant Majorana fermions
have topologically non-trivial gapped bands with the Chern
number C = ±1, giving rise to a quantized chiral edge current
protected by the field-induced gap. In contrast to a quantizedchiral edge current of electrons in a quantum Hall system,
this chiral edge current is carried by the charge neutral
Majorana fermions. Therefore, this quantized chiral edge
current has been predicted to appear in the 2D thermal Hall
2
conductivity as κ2D
xy / T = qt C / 2, where qt = (π ⁄ 6) k B / ħ .
In this work [1], we reveal that it is necessary to protect
the chiral edge current from severe scattering effects onphonons to stabilize the quantized thermal current. We investigate the sample dependence of the longitudinal (κxx) andtransverse (κxy) thermal conductivity of three single crystalsof α-RuCl3 – a promising candidate realizing the Kitaev spin
liquid [2]. We find the half-integer quantized thermal Hall
effect in a sample showing the largest κxx among the three
crystals (sample A in Fig. 1). On the other hand, the othersamples with smaller κxx show κxy much smaller than thevalue expected for the quantization (Fig. 2). Given that κxx ismostly given by phonons, the different magnitudes of κxx indifferent samples reflect the different scattering strengths onphonons; phonons in a sample with smaller κxx are exposedin stronger scattering effects. Therefore, the dependence ofκxy on κxx shows that the chiral edge current in the Kitaev
magnet is sensitive to scattering effects on phonons.
Further, we find that a sample with a larger κxx showsa larger decrease of the magnetic susceptibility below the
Néel temperature, in addition to a larger field-increase effectof κxx, showing that magnetic scattering effects are morestrongly suppressed by magnetic fields in a sample with abetter quality. These results suggest that suppressing thismagnetic scattering effect plays an important role to realize
the quantized thermal Hall effect.
References[1] M. Yamashita et al., Phys. Rev. B 102, 220404(R) (2020).
[2] Y. Kasahara et al., Nature 559, 227 (2018).

Authors
M. Yamashita, J. Gouchi, Y. Uwatoko, N. Kuritaa, and H. Tanakaa
aTokyo Institute of Technology

Ultralow Temperature NMR of CeCoIn5
Yamashita and Takigawa Groups
We have established the experimental procedures tomeasure high-field spin-echo NMR signals down to ultralow
temperatures (~1 mK) with ensuring the thermal equilibrium
between the sample and the cryostat [1]. This techniqueallows us to observe a peak in 1 ∕ T1T of 59Co NMR signals
at ~20 mK, showing a magnetic transition in CeCoIn5.
Understanding the role of enhanced quantum fluctuations near a magnetic quantum critical point (QCP) has
been a central issue in condensed-matter physics, becausesuch quantum fluctuations are believed to mediate various
exotic phenomena, such as non-Fermi liquid behaviors,
enhancements of effective mass, and unconventional Cooper
pairings. Many studies of unconventional superconductivity
near a magnetic QCP have been performed in heavy-fermion
materials. Among the various heavy-fermion materials,
CeCoIn5 has been attracting broad attention because ofits d-wave superconducting state with the high transition temperature of 2.3 K and the proximity to a putative
magnetic QCP. However, no antiferromagnetic (AFM) statecorresponding to the QCP has been observed, shrouding theorigin of the QCP in mystery.
Recently, we have reported anomalous decrease in thede Haas–van Alphen (dHvA) amplitudes of CeCoIn5 below
20 mK for 6–10 T [2]. An appearance of a field-induced
AFM phase has been put forward to explain the decreaseof the dHvA amplitude, because additional dissipation by

Fig. 2. The temperature dependence of 1 ∕ T1T at 5 T. The data of theprevious report [3] is also plotted. The dashed line shows a guide to theeyes.

magnetic breakdowns in the AFM phase can provide themost plausible explanation for the decrease of the dHvA
amplitude. However, direct evidence of magnetic order has
yet to be observed.
In this work, we performed spin-echo NMR measurements down to 5 mK to find the magnetic order. It is a very
challenging issue to observe NMR signals down to ultralow
temperatures without heating the sample. First, we madea strong thermal connection between the sample and thecryostat by indium soldering a silver wire to the sample(Fig. 1). Second, we find that the temperature dependence ofthe spin-echo intensity ISE ( T ) provides a good measure of
the sample temperature. By taking into account the population difference of the nuclear spins in different energy levels
which have the Boltzmann distribution, one can find thetemperature dependence of ISE ( T ) as shown by the solidline in Fig. 1d. We confirm that the NMR signal follows
the Boltzmann curve when the heating caused by the NMR
pulses is negligible (Fig. 1d), allowing us to find a pulse
condition without heating the sample.
Having established the NMR pulse conditions without
heating of the sample, we investigate the temperature dependence of the longitudinal relaxation time (T1). We find apronounced peak in 1 ∕ T1T at 5 T, implying an appearance ofmagnetic order as suggested by our previous dHvA measurements [2]. On the other hand, the NMR spectrum shows nochange below 20 mK. Moreover, the peak in 1 ∕ T1T disappears at 6 and 8 T in contrast to the results of the quantum
oscillation. We discuss that an antiferromagnetic state witha moment lying in the a–b plane can be a possible origin forthe peak in 1 ∕ T1T at 5 T.
References[1] M. Yamashita et al., Phys. Rev. B 102, 165154 (2020).
[2] H. Shishido et al., Phys. Rev. Lett. 120, 177201 (2018).
[3] T. Taniguchi et al., JPS Conf. Proc. 30, 011107 (2020).

Fig. 1. (a) A picture of the sample with a silver wire (100 μm diameter)soldered by indium. (b) Cu wires were wound around the sample for
NMR measurements. The silver wire was thermally anchored to thecryostat. (c) Crystal structure of CeCoIn5. NMR measurements were
performed at 59Co (I = 7 ∕ 2) nuclei. (d) The temperature dependence ofthe spin-echo intensity at 8 T. The solid line shows the calculation fromthe Boltzmann distribution of the nuclear spins.

Authors
M. Yamashita, M. Tashiro, K. Saiki, S. Yamada, M. Akazawa,
M. Shimozawaa, T. Taniguchib, H. Takeda, M. Takigawa, and H.
Shishidoc
aOsaka University
bIMR, Tohoku University
cOsaka Prefecture University

ISSP Activity Report 20207

Dynamics of Composite Domain Walls
in Multiferroics FeGdO3
Tsunetsugu Group
Domain Walls (DW) are topological defects withmesoscopic size and understanding their dynamics is important for both experimental and theoretical interests. We havetheoretically studied the steady state dynamics of DW driven
by magnetic field in a system with multiple orders coexist[1].
Our target system is the multiferroic compound FeGdO3
and its low-temperature phase is characterized by three orderparameters: antiferromagnetic moments of Fe spins N withbiaxial anisotropy and isotropic Gd spins n and one component of electric polarization P. Their coupling cN∙nP leads
to composite DWs, i.e. two order parameters need to flip
together in each DW, and therefore there exist 3 different
types of composite DWs: N-n, N-P, and n-P.
We have studied the dynamics of these composite DWs
driven by magnetic field based on the phenomenologicaltime-dependent Ginzburg-Landau theory. Neglecting fluctuations inside the DW, we have investigated by numericalcalculations the time evolution of their one-dimensionalmotion. With the help of anisotropic exchange couplings,
the applied magnetic field couples to N and thus drives
N-P and N-n DWs. One important finding is the difference between these two types in the scaling of DW
steady state velocity υ(c,H). In its weak field expansion
υ(c,H) = α(c2)H+β(c2)H3+…, the coefficients exhibit a
singularity at c = 0 for N-n DW, and this is related to n’s
internal isotropy.
As a big surprise, it turns out that a composite DW

Fig. 1. Velocity of composite DWs driven by magnetic field expandedin H. α is the linear mobility. c is the interaction strength.

Fig. 2. Stability phase diagram of composite DWs driven by magnetic
field H. B: N-n DW is unstable, C: N-P DW is unstable, A: both DWs
are stable.

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becomes unstable in strong magnetic field and splits into apair of different types: N-P → N-n + n-P and N-n → N-P+ n-P. In both cases, n-P DW is immobile but its partner
succeeds to the moving part after splitting. The possibilityof this type of splitting was pointed out as an impurity
effect [2], but we have found that it may occur in a purebulk as a dynamical instability. An important point is thatthe non-flipping component is also dragged by the flipping
ones due to the coupling c-term. With increasing the driving
field, its modulation grows and finally creates a pair of kink
and anti-kink, which is DW splitting. This type of dynamical
instability may appear, in principle, in any systems withmore than two types of order parameters. It is an interesting challenge to observe in experiments this splitting ofcomposite DW.
References[1] K. Kawahara and H. Tsunetsugu, J. Phys. Soc. Jpn. 90, 014703(2021).
[2] Y. Tokunaga et al., Nat. Mater. 8, 558 (2009).
Authors
H. Tsunetsugu, K. Kawahara

Fermi-Liquid Correction in Current Noisethrough a Correlated Quantum Dot
Kato Group
Artificially generating, controlling, and measuring
quantum states on semiconductor substrates is the key torealizing quantum information technology, such as quantum
computers and quantum communications. On the other hand,the development of technologies to generate and control
artificial quantum states has brought about a paradigm shift
in material science. As a result, it has become possible tothoroughly explore the physical phenomena that give rise to
nontrivial features of matter by artificially reproducing thequantum states in matter, rather than simply investigating
the properties of natural matter as has been done in the past.
Furthermore, through advanced control technology, it has
become possible to precisely investigate the behavior ofquantum many-body states in complex environments, suchas nonequilibrium states, which were previously not easy toaccess.
Along with superconductivity and the quantum Hall
effect, the Kondo effect is one of the fundamental manybody effects of electrons. The primary mechanism of
the Kondo effect was elucidated in 1964 and has beenthoroughly investigated since then. The Kondo effect occurs
not only in dilute magnetic materials, where the phenomenon
was originally discovered, but also in a variety of physicalsystems. Recently, many studies on the Kondo effect havebeen carried out in various physical fields, such as condensed
states of light-trapped cooled atoms and quarks. However,
the actual state of these systems often deviates from the ideal
state assumed in the mathematical analysis. In other words,
it is known that the symmetries possessed by physical states(time-reversal symmetry and particle-hole symmetry) arebroken and play a significant role in the low energy states.
However, the correction on the local-Fermi-liquid statebeyond the linear response had remained a difficult problem
until recently [1].
Consider a situation in which electrons are confinedin a quantum dot, which is a device of several tens ofRecently, our collaborators have succeeded in an experiment to verify the local-Fermi-liquid correction effect using
current measurement on a carbon nanotube-based quantum
dot [3].
As described so far, the results of this research are fundamental to the physics of condensed matter. However, it isexpected that they will lead to the development of materials
science and quantum information technology.
References[1] C. Mora, C. P. Moca, J. von Delft, and G. Zarand, Phys. Rev. B 92,
075120 (2015).
[2] Y. Teratani, R. Sakano, and A. Oguri, Phys. Rev. Lett. 125, 216801(2020).
[3] T. Hata, Y. Teratani, T. Arakawa, S. Lee, M. Ferrier, R. Deblock, R.
Sakano, A. Oguri, and K. Kobayashi, Nat. Commun. 12, 3233 (2021).
Fig. 1. Bias-voltage nonlinear coefficient of current noise
as afunction of magnetic field B normalized by the Kondo energy scale(Kondo temperature) TK for several choices of the Coulomb interaction U ∕ (πΔ) is plotted. Here the current noise is given in the form

Authors
R. Sakano, Y. Teratania, and A. Oguria

aOsaka city university. It is found that all the data
points are on a universal curve.

nanometers in size made of a semiconductor or othermaterial. In a quantum dot made of carbon nanotubes, forexample, the shape of the electron confinement region ishighly symmetric. As a result, in addition to the spin of theelectrons, a magnetic moment is generated in the quantum
dot due to the orbit of the electron motion. By attaching
electrodes to this quantum dot, the number of confinedelectrons can be tuned, and the local-Fermi-liquid state canbe controlled in a state of broken electron-hole symmetry.
Using quantum field theory and mathematical methods, wehave clarified that the local-Fermi-liquid corrections, which
are caused by the three-electron correlation, play an essential
role in the correlated quantum dot beyond the conventional
theory on the local Fermi liquid. This three-electron correlation is particularly pronounced in quantum dots with multiple
orbitals. For example, in the Kondo effect caused by electron
spin alone, it is difficult to observe the local-Fermi-liquid
corrections because the Kondo effect itself disappears when
the number of electrons is changed.
Exploiting numerical simulations and field theory
techniques, we have clarified that the local-Fermi-liquid
corrections can be observed in actual experiments based on
the transport quantities such as nonlinear current, current
noise, and thermal conductivity [2]. We have also found that
there is a scaling universality in the nonlinear current noise
behavior when a magnetic field is applied to a quantum
dot. Application of magnetic field breaks the time-reversal
symmetry of the quantum dot and gives rise to the localFermi-liquid corrections in nonlinear transport quantities. It
is also found that the local-Fermi-liquid corrections have auniversal scaling property independent of the details of thenanoscale materials, similar to the conventional local-Fermiliquid (see Figure).
This research has significantly advanced our understanding of the physical mechanisms that give materials
their nontrivial features through the interaction of electrons.
The Kondo effect is the most fundamental quantum manybody effect, and it appears in the properties of various
materials. Therefore, it is expected to be helpful in designing
new materials and exploring new materials in the future. In
addition, the study of artificially controlled quantum manybody states in nanoscale materials requires highly sensitive
observation techniques, which will contribute to the development of technologies to generate and control quantum states.

Sub-Atomic Lattice Distortionof an Atomic Layer Compound
Visualized by Moiré Pattern
Komori Group
Chemical and physical properties are greatly affectedby lattice distortion through the change of the electronic
structures. At solid surfaces, the local distortion induced by
the adsorbates, subsurface impurities and hetero-interface
modify reactivity, growth and other atomic processes. By
mapping the amplitude and direction of the local distortion,
we can elucidate the details of the atom processes including
surface reactivity. However, it has been technically difficult
to observe the local distortion directly using microscopy
methods. While scanning tunneling microscopy (STM) withatomic resolution is one of the ideal tools for studying the
local structure, its lateral resolution is still insufficient tomeasure the distortion of the order of a few % of the lattice
constant [1]. One of the ways to improve the lateral resolution is a use of the moiré pattern in the STM images [2]. In
the present study, we demonstrate that the local lattice distortion of a hexagonal iron-nitride monatomic film grown overa flat and wide Cu terrace can be visualized by analyzing the
moiré pattern. [3]Fig. 1. (a,b) Topographic STM images of the hexagonal iron-nitride
monolayer on Cu(111). Cyan line in (a) is drawn along the moiré
pattern. The lattice orientation of the iron-nitride monolayer is indicatedby green circles in (a). The red circle in (a) indicates the point protrusion. (c) Schematic model of the hexagonal iron-nitride monolayer on
Cu(111). Green, blue and orange balls indicate Fe, N and Cu atoms,
respectively. The Cu(111) lattice is rotated by 0.28° from the ironnitride lattice. (d) Moiré pattern between the hexagonal iron-nitride
monolayer and Cu(111) obtained by the schematic model in (c). Cyan
rhombuses in (b), (c) and (d) show the unit cell of the moiré pattern [3].

ISSP Activity Report 20209

protrusions are preferentially formed in the regions withlarge lattice distortion of the iron nitride monolayer.
References[1] S. Ohno et al., Surf. Sci. 554, 183 (2004).
[2] M. Ritter et al., Phys. Rev. B 57, 7240 (1998).
[3] T. Hattori et al., Nano Lett. 21, 2406 (2021).
Authors
T. Hattori, N. Kawamura,T. Miyamachi, and F. Komori

Steps as a Disorder on
Atomic-Layer Superconductors
Hasegawa Group

Fig. 2 (a) Two-dimensional histogram of the displacement of the moiré
pattern center and the distance from the center to the nearest point
protrusion. (b) Proposed schematic model of the point protrusion. (c−e)
Histograms of the distance from the centers to the nearest point protrusions for three regions of the displacement 0- 25 pm in (c), 25−50 pmin (d), and 50−75 pm in (e). The counts are normalized by the total
counts in each region [3].

Observed topographic STM images of the surface areshown in Fig. 1(a. b). A uniform iron-nitride film grows
over the Cu(111) terrace larger than 100 × 100 nm2, and ahexagonal moiré pattern with the periodicity of ~ 1.3 nm canbe seen in the images. The green circles in Fig. 1(a) show
the positions of the surface atoms along the [101] direction,and the cyan line is plotted along the moiré pattern. The
latter is tilted from the lattice orientation of the hexagonaliron nitride monolayer by 1.1 ± 0.1°. A schematic modelof the iron nitride monolayer on Cu(111) is displayed inFig. 1(c). We assume that Fe atoms are observed as protrusions by STM. This model can reproduce the overall characteristics of the moiré pattern in the image including the 1.1°
tilt as in Fig. 1(d). In the STM images, we notice larger andhigher protrusions, which randomly distribute on the surface,than the atomic protrusions as indicated by the red circle inFig. 1(a). We call them point protrusions.
A detailed analysis of the moiré pattern reveals that thecenter of the moiré pattern does not form a regular periodic
lattice. The center positions randomly fluctuate from ahypothetical regular lattice points. Figure 2(a) shows a twodimensional histogram of the displacement of the moiré
pattern center from the lattice point and the distance fromthe center of the moiré pattern to the nearest point protrusion. Our structural model of the point protrusion is given
in Fig. 2(b). The histograms of the distance from the centersto the nearest point protrusions are made depending on
the displacement to see the correlation more clearly as inFig. 4(c−e). For the centers with a large displacement, thedistance to the nearest point protrusion is relatively shortcompared to those with a small displacement. The point

10

ISSP Activity Report 2020

Two-dimensional (2D) superconductors, whose thickness is thinner than the coherence length, exhibit peculiar
properties. Compared with bulk superconductors, 2D superconductors are susceptible to disorder since induced electron
localization enhances repulsive electron-electron interaction, which suppresses the Cooper pairing. As demonstrated
with thickness-dependent critical temperature of amorphous
superconducting thin films, changing parameters such as
disorder, thickness, and perpendicular magnetic field lead toa superconductor-insulator transition (SIT). The zero-temperature transition is one of the typical examples of quantum
phase transitions, and has been investigated extensively toelucidate its universal properties.
Recent technical advancements have allowed us to fabricate highly crystalline 2D superconductors, some of which
has only mono-atomic layer thickness. In the case of suchhighly-ordered 2D superconductors it has been reported thatduring the SIT induced by the magnetic field application
metallic phase appears in between. So far, the properties ofthis peculiar quantum metallic phase have been investigatedmainly by transport measurements but the mechanism is still
controversial. In order to understand its details atomistic
real-space measurements are crucial. In the present study,
we studied superconducting properties of striped-incommensurate (SIC) phase of the Pb/Si(111) system; one of themonoatomic-layer superconductors, formed on vicinal Si
substrates. Since the substrate is tilted, uniformly distributed
steps, whose spacing is equivalent or even smaller than thecoherence length, should behave as a dispersed disorder on
the ultimately thin superconductor. We investigated how thesteps affect the superconducting properties and the magnetically induced SIT by using low-temperature scanningtunneling microscopy (STM).

Fig. 1. (left) 3D-rendered STM image of striped incommensurate (SIC)
phase of Pb/Si(111) system formed on a 1.1°-tilted vicinal substrate.
(right) tunneling spectrum taken on a terrace of vicinal SIC phase taken
at 0.36 K showing a superconducting gap of 0.30 meV.

Carrier Density Control
in Oxide Quantum Wells
Lippmaa group
Fig. 2. STM image of the vicinal SIC superconducting phase andzero-bias-voltage maps, which correspond to the spatial distribution of
the density of states (DOS) at the Fermi level, taken under the perpendicular magnetic field of 120, 210, 300, and 400 mT.

Figure 1 shows a 3D-rendered STM image of the SIC
phase formed on a 1.1°-tilted vicinal Si(111) substrate taken
at 0.36 K. A characteristic SIC stripe pattern can be seemon the terraces. In the image, the vertical scale is severely
exaggerated; the step height corresponds to 0.31 nm whereas
the averaged terrace width is 16 nm. A tunneling spectrum
taken on a terrace shows a superconducting gap that canbe well fitted with the Dynes function. The fitted superconducting gap, 0.30 meV, is almost same as that taken on
SIC phase formed on a flat substrate, indicating the Cooper
pairing is not affected by the presence of the steps.
In our previous studies on the role of steps on superconductivity, we found that the steps basically behave asa Josephson junction connecting the neighboring terraces,
and the strength of the decoupling strongly depends on thephases [1, 2]. Among the Pb monolayer phases, the steps of− −the √ 3 × √ 43 phase decouple terraces significantly to makethe terraces superconducting nano stripes [2]. On the other
hand, the decoupling of the SIC phase is rather weak sothat vortices are just pinned at step edges. In the case of thevicinal one, as the terrace width (16 nm) is much shorter thanthe coherence length of the flat one (38 nm), more than one
steps are involved on the formation of a vortex.
Figure 2 shows an STM image and zero-bias-voltage
maps, which correspond to the spatial distribution of thedensity of states (DOS) at the Fermi level, taken undervarious magnetic fields. At 120 mT, the vortices are observed
elongated along the step direction. The DOS at the coreof the vortices is not fully recovered to that of the normal
metal. These are the characteristic features of an AbrikosovJosephson vortex. The oval shape of the vortices is due to the
reduced coherence length along the step-crossing direction
caused by the limited mean free path due to the presenceof the steps. Because of the reduced coherence length, thecritical magnetic field, which is inversely proportional to thecoherence length, is enhanced on the vicinal superconductor.
At the magnetic field of 210 mT, the number of vortices
increases as well as the background DOS. The vortices are
no longer distinct, and significant corrugation in DOS isdiscernible. By further increasing the magnetic field, DOS
gradually increases and its distribution becomes homogeneous. Around 330 mT, DOS exhibits saturation, indicating
the breaking of superconductivity. According to the transportmeasurement performed on the same phase the vortex liquid
phase and quantum metallic phase are observed at thesemagnetic fields. The observed blur vortices and inhomogeneous DOS contrast presumably corresponds to thermally
activated or quantum diffusing vortex liquid phase.

Two-dimensional carrier confinement in a quantum
well at an oxide heterointerface offers new ways of controlling the transport behavior of oxide materials by appliedexternal fields. Magnetic tunability has been attributed to
Rashba-type spin-orbit coupling in an asymmetric electron
distribution, the enhancement of spin-orbit coupling close
to a filling-dependent Lifshitz transition, or a change in theeffective disorder experienced by carriers when the quantum
well width changes. Electric fields, on the other hand, can beused to change the quantum well filling, by accumulating or
depleting carriers at an interface, or to modify the width ofthe electron distribution in the depth direction.
Oxide quantum wells are usually formed by growing athin epitaxial LaAlO3 layer on SrTiO3, in which case thequantum well forms at the surface of the SrTiO3 substrate
due to, at least in part, the change of lattice polarity at theinterface. Instead of using the polar LaAlO3 layer, it is also
possible to form quantum wells by depleting oxygen fromthe surface or by delta-doping SrTiO3 with a (La,Sr)Ox
monolayer. All these techniques produce a metallic quantum
well with qualitatively similar properties, but the LaOx deltadoping technique has several advantages in allowing a broad
range of carrier densities to be explored. In contrast, thecarrier number is essentially fixed by the polarity discontinuity at LaAlO3/SrTiO3 interfaces, while oxygen-deficientsurfaces can only be maintained in ultra-high vacuum conditions.
In our recent work [1], we have studied the possibilities
of constructing oxide quantum wells based on embedded(La,Sr)Ox layers in SrTiO3. The heterostructures were grownby Pulsed Laser Deposition on SrTiO3 single crystals as
illustrated in Fig. 1. The carrier number in these heterostructures is controlled by two parameters: the number of
La atoms in the delta-doping layer, and the thickness of a
SrTiO3 capping layer. Since SrTiO3 surfaces exposed toair are strongly depleted, a space-charge layer forms in thecap layer and the thickness of the capping layer can be used
to partially deplete the carriers in from the quantum well.
Almost complete carrier depletion occurs when the capping

References[1] S. Yoshizawa et al., Phys. Rev. Lett. 113, 247004 (2014).
[2] F. Oguro et al., Phys. Rev B 103, 085416 (2021).
Authors
Y. Sato, M. Haze, S. Yoshizawaa, T. Uchihashia, and Y.Hasegawa
aNational Institute for Materials ScienceFig. 1. Formation of a quantum well in SrTiO3 around a LaTiO3 delta
doping layer. Due to surface depletion, carriers are depleted from thequantum well if the distance is small (c). Saturated metallic behavior is
obtained only for capping layers thicker than 10 nm (a).

ISSP Activity Report 202011

Sandwich Structure Formation of
BN-Covered Silicene on ZrB2/Si(111) by
Adsorption of NO and Thermal Processes
Yoshinobu Group

Fig. 2. Variation of the sheet resistance of SrTiO3/LaTiO3/SrTiO3 heterostructures with the LaTiO3 delta-doping layer thickness (a) and SrTiO3
capping layer thickness (b). Layer thicknesses are given in unit cells(4Å).

layer thickness drops below 4 nm (10 unit cells). Above 10
nm capping thickness, surface depletion no longer affects thecarrier density in the quantum well and the carrier density
is set by the number of La atoms in the delta-doping layer.
The built-in carrier density in the quantum well is thus set by
two layer thickness parameters that are easy to control with
precision in the growth process. When the heterostructures
are formed into gated field-effect transistors, additional finetuning of the carrier density over a narrow range is possible
by applying a suitable gate voltage. The electrostaticallygated heterostructure devices are thus particularly well
suited for studying filling-dependent Lifshitz transitions, i.e.
varying the Fermi energy in the quantum well across the Ti
3d conduction band dxz,yz - dxy level crossing.
The effects of the doping and capping layer thicknesseson the transport behavior of the heterostructures are shownin Fig. 2. When a thick capping layer is used and the surface
depletion effect can be ignored, metallic conductivity is
obtained from the LaOx layer coverage exceeds about 0.3
monolayers and a gradual reduction of the sheet resistanceis observed as the La coverage is increased. For a single unit
cell of LaTiO3, the quantum well carrier density was on theorder of 1013 cm-2. In all cases, however, the Hall resistance
was nonlinear, indicating that a significant depth distribution of carriers exists. Approximately 1/4 of the carriers are
distributed to a distance of 10 nm or more from the interface.
These ‘deep’ carriers do not suffer from interfacial scatteringand reach a mobility of over 104 cm2/Vs at 4K.
The built-in carrier concentration can be further adjusted
by changing the cap layer thickness, as shown in Fig. 2b.
As the cap layer thickness is reduced to 10 unit cells or less,the heterostructures become semiconducting and finallyinsulating. These experiments show that it is possible to tune
the carrier density continuously close to the lowest densities
where metallic conductivity can be sustained.

Atomic layer materials have attracted much attention inphysics, chemistry and materials science. Silicene is a siliconanalogue of graphene. A honeycomb silicene layer has been
prepared on suitable substrates such as Ag, ZrB2, Ir, ZrC
surfaces. It has been reported that a silicene layer grownon these metallic surfaces has relatively strong interaction
with the substrate because of the orbital mixing between
metal bands and Si sp-bands. Towards the functionalizationof silicene for device applications, several theoretical andexperimental investigations have been carried out concerning
the interaction between the silicene and other atoms/
molecules. Here, we investigated the adsorption and thermal
reaction processes of NO with the a silicene monolayer on
the ZrB2/Si(111) substrate using synchrotron radiation X-ray
photoelectron spectroscopy (SR-XPS) and density-functional
theory (DFT) calculations. SR-XPS experiments werecarried out at BL13 of KEK-PF.
Figure 1 shows a series of O 1s, N 1s, Si 2p and B 1s
XPS spectra of these processes. The initial surface is a clean
silicene monolayer on ZrB2/Si(111) (see each XPS for theclean surface). NO is dissociatively adsorbed on the silicene
surface at 300 K. At an early stage of NO exposure, an
atomic nitrogen is bonded to three Si atoms most probably
by a substitutional adsorption with a Si atom of silicene(N≡Si3). An atomic oxygen is inserted between two Si
atoms of the silicene (Si-O-Si). After a large amount of NO
exposure (1500 L), the two-dimensional honeycomb silicene
structure becomes destroyed judging from the decay oftypical Si 2p spectra for silicene (see Fig. 1c). The oxidation
state of Si becomes Si4+ predominantly and the XPS peaks

Reference[1] J. N. Lee, X. Hou, R. Takahashi, and M. Lippmaa, Appl. Phys.
Lett. 116, 171601 (2020).
Authors
J. Lee, X. Hou, R. Takahashia, and M. Lippmaa
aNihon University

Fig. 1. A series of XPS spectra of the 1500 L NO/silicene/ZrB2/Si(111)
surface as a function of heating temperature. (a) O 1s spectra (hn =700 eV), (b) N 1s spectra (hn = 560 eV), (c) Si 2p spectra (hn = 260eV), and (d) B 1s spectra (hn = 700 eV). After quick heating to eachtemperature, the sample was cooled down to room temperature, and
XPS measurements were carried out. Note that at 1053 K, the heating
duration time was 5 min and 25 min.

12

ISSP Activity Report 2020

Functional Characterization andStudy on the Molecular Mechanismof Ion-transporting Rhodopsins
Inoue Group

Fig. 2. (a) top view and (b) side view of the optimized structure ofa sandwich structure of a hexagonal BN sheet on the silicene/ZrB2
substrateof ZrB2 substrate are decreased in intensity, indicating thatcomplicated silicon oxinitride species have developed threedimensionally on the ZrB2/Si(111) substrate at 300 K. By
heating above 900 K, the oxide species start to desorb fromthe surface probably as SiO, but nitrogen-bonded species
still exist (Figs. 1a and 1b). After heating at 1053 K for 5
min., no oxygen species is observed on the surface (Fig. 1a);
SiN species are temporally formed as a metastable species
and BN species also start to develop (Fig.1b). In addition,
the silicene structure is partly restored on the ZrB2/Si(111)substrate (Fig. 1c).
After the prolonged annealing at 1053 K for 25 min.,the XPS spectra show a single peak at 397.5 eV for N 1s
(Fig. 1b) and an emerging peak at 190.1 eV for B 1s state(Fig. 1d). Since the B 1s peak at 190.1 eV can be ascribed to
the B atom in the hexagonal BN structure according to thereference XPS spectra of BN, we conclude that the hexagonal BN structure has developed on the silicene on ZrB2.
By DFT calculations, the geometrical structure has been
optimized as shown in Fig. 2; a hexagonal BN sheet weaklyinteracts with the silicene layer on the ZrB2 substrate. The
calculated binding energy of the N 1s state in the hexagonal
BN is 397. 61 eV on average, while there is a variation of
the binding energies depending on the relative position of
N atoms with respect to the Si atoms of silicene, and theminimum and maximum binding energies are 397.41 and
397.81 eV, respectively. The good agreement between theexperimental and calculated binding energies supports theformation of the hexagonal BN structure after the prolonged
annealing at 1053 K. Thus, the sandwich structure of
BN-covered silicene on the ZrB2/Si(111) substrate is formedby adsorption of NO at 300 K and prolonged heating at
1053 K.

Rhodopsin is photo-receptive heptahelical transmembrane protein in which a retinal chromophore is covalently
bound to a conserved lysine residue in the seventh transmembrane helix (TM7), and animal and microbial rhodopsin
families are known so far. Animal rhodopsins are present
in animal retina to transfer visual signals to brain and arealso related to non-visual light sensing in various tissues.
Microbial rhodopsins show diverse functions: light-driven
ion pump, light-gated ion channel, phototactic sensor, lightdependent regulation of gene expression and enzyme, andso on. Both types of rhodopsins are being widely used in
optogenetics to control various cellular events, such as neuralactivity, gene expression, dynamic protein localization, andso on, by light.
In 2020, we reported the functional characterization ofschizorhodopsin (SzR) which was found in metagenome
assembled genomes of Asgard archaea which is thought tobe the closest living prokaryote to the last common ancestorof eukaryotes [1]. The ion-transport activity assays revealed
that SzRs function as light-driven inward proton (H+) pumps[2]. The inward H+ transport of SzRs are significantly
higher than that of another inward H+ pumping rhodopsin,
xenorhodopsins (Fig. 1). The high-speed atomic force
microscopy revealed the trimeric structure of SzR in lipid
bilayer. The photocycle of SzR in which K, L, L/M and M
intermediates are included was determined by the laser flash
photolysis spectroscopy. The transient absorption change
of a pH indicator (cresol red) indicated that the substrate
H+ is released to the cytoplasmic milieu and taken up fromthe extracellular side on the M-accumulation and decay,
respectively. This suggests that no metastable trapping of
H+ released from retinal Schiff base (RSB) occurs on any
amino acid residues, and it was supported by the low-temperature Fourier transform infrared spectroscopy. This is quite

Reference[1] J. Yoshinobu et al., J. Chem. Phys. 153, 064702 (2020). doi:
10.1063/5.0011175
Authors
J. Yoshinobu, K. Mukai, T. Ozaki, Y. Yamada-Takamuraa et al.
aJapan Advanced Institute of Science and Technology

Fig. 1. Inward H+ transport activity of six SzRs (top and middle)
compared with outward H+ pumping GPR and inward H+ pumping
XeR (PoXeR) (bottom).

ISSP Activity Report 202013

Omnidirectional Controlof Large Electrical Output
in a Topological Antiferromagnet
Nakatsuji Group

Fig. 2. Trp163 of C1C2 plays a role regulating the branching ratio of H+
transfers between RSB and counterions, Glu162 and Asp292.

different from XeR, in which protonation of the proton
accepting aspartate on the cytoplasmic side is involvedin H+ transfer from the RSB to the cytoplasmic milieu. A
comprehensive amino-acid mutation experiment suggested
that Cys75 and Glu81 in the transmembrane helix (TM) 3
and Asp184 in TM7 is critical for the inward H+ transport ofSzR.
Channelrhodopsins (ChRs) are light-gated cation
channels which nonspecifically transports various cations
according to the electrochemical potentials of cells. In orderto reveal the cation transport function of ChR, we investigated a mutant of a member of ChR, C1C2, in which a
tryptophane residue in TM3 was substituted with phenylalanine (C1C2 W163F). Trp163 of C1C2 is highly conservedin most microbial rhodopsins, but its relationship to thechannel function is not known. C1C2 W163F showed greatly
attenuated channel activity, while outward proton pumping
activity became more prominent. This result indicates that
Trp163 plays a critical role in suppressing the outward
proton pumping function and increasing channel activity by
regulating the branching ratio of proton transfer to the twocounterions Glu162 and Asp292 respectively (Fig. 2) [3].

The net magnetization defines the signal size of memory
devices and sensors, and thus the manipulation of magnetization is essential for the broad application of ferromagnets.
However, the magnetization itself causes a dilemma. Whileits size matters to obtain strong responses upon its reversal,
the large magnetization simultaneously suppresses the rangeof its directional control due to the demagnetizing field. To
overcome the demagnetization/shape barrier, one needs tocarefully tune the crystalline and interfacial effects [1]. Thus,
it would be ideal to find alternative magnets that carry nomagnetization but strong responses.
The antiferromagnet Mn­3Sn hosts a large Berry curvature in momentum space owing to the topological metallic
state. The Berry curvature can be manipulated by the ferroicordering of the cluster magnetic octupole and remains large
even with negligible magnetization. This feature enables
disproportionately large transverse responses such as anomalous Hall effect (AHE)[2] and anomalous Nernst effectWe also reported the theoretical and experimental studies on
the Cotton effect observed in the visible CD spectrum of sodium
pump rhodopsin (KR2) [4]. KR2 shows a visible CD spectrum
whose peak is significantly red-shifted from that of its absorption spectrum. The transition density fragment interaction (TDFI)
method quantitatively revealed that this shift is caused by the
excitonic coupling between retinal chromophores in pentamer of
KR2 [4].
References[1] P.-A. Bulzu, A.- . Andrei, M. M. Salcher, M. Mehrshad, K. Inoue,
H. Kandori, O. Béjà, R. Ghai, and H. L. Banciu. Nat. Microbiol. 4,1129 (2019)[2] K. Inoue, S. P. Tsunoda, M. Singh, S. Tomida, S. Hososhima,
M. Konno, R. Nakamura, H. Watanabe, P.-A. Bulzu, H. L. Banciu,
A.- . Andrei, T. Uchihashi, R. Ghai, O. Béjà, and H. Kandori, Sci.
Adv. 6, eaaz2441 (2020)[3] Y. Nagasaka, S. Hososhima, N. Kubo, T. Nagata, H. Kandori, K.
Inoue, and H. Yawo. Biophys. Physicobiol. 17, 59 (2020)[4] K. J. Fujimoto and K. Inoue, J. Chem. Phys. 153, 045101(2020).
Authors
K. Inoue, T. Nagata, H. Yawo

14

ISSP Activity Report 2020

Fig. 1. (a) Schematic illustration of the rotation planes. θ and φ are theangles from the normal direction within the planes parallel and perpendicular to the electric current direction, respectively. The orange arrow
represents the direction of the external magnetic ﬁeld. (b) Field dependence of the Hall resistivity obtained for the Mn3Sn ﬁlm with various
angles θ at 300 K. The black arrows indicate the ﬁeld sweep direction. (c) Schematic illustrations ofthe 3D omnidirectional property for
randomly oriented multi-grains and the 2D omnidirectional property
for a single crystal/grain. The red arrow represents the direction of themagnetic order parameter. (d) Field dependence of the Hall resistivity
obtained for the Mn3Sn single crystal with various angles θ at 300
K. Inset: Schematic illustrations of the rotation planes for the single
crystal. (e) Schematic illustration of heat flux sensor. (f) Field dependence of the Nernst voltage VANE for the Mn3Sn thermopile made of
120 pairs of Mn3Sn/Au wires under several values of the heat ﬂux
qheater at 300 K.

(ANE)[3], opening up the possibilities for replacing ferromagnets with antiferromagnets in magnetic devices. As thefirst step for spintronics applications, we fabricated Mn3Sn
thin films [4]. And more recently, we demonstrated electricalmanipulation of the magnetic ordering in the Mn3Sn /heavy
metal bilayer devices [5], which has triggered extensive
research activities into the interface properties of Mn3Sn thin
film.
In this study [6], we report the omnidirectional behavior
in Mn3Sn due to negligible shape anisotropy. Moreover, wedemonstrate a new method for multi-level data recording anda magnetically robust ANE-type heat flux sensor.
We have fabricated the polycrystalline Mn3Sn thin films
on the thermally oxidized Si substrate by dc magnetron
sputtering methods [4]. With these thin films, we measured
the anomalous Hall effect as a function of magnetic ﬁeld
angles, θ and ϕ (Fig. 1(a)). As shown in Fig. 1(b), the ﬁeld
dependence of AHE obtained for various θ exhibits a multistable signal. Namely, the spontaneous Hall signal gradually
decreases with increasing θ, indicating that the polarization direction of the magnetic octupole essentially follows
the ﬁeld direction rather than being restricted within the xy
plane. Notably, the rotation for both θ and ϕ angles shows
the same angular dependence, indicating that the magnetic
octupole can be freely (omnidirectionally) orientated in all
directions (Fig. 1(c)) as a result of the negligibly small shapeanisotropy and polycrystalline arrangement.
Interestingly, single-crystal/grain samples also show themulti-stable signals as a function of the ﬁeld angle, but only
for field rotations within the kagome layer (Fig. 1(d)). Ferromagnets with magneto-crystalline anisotropy sufficiently
larger than the shape anisotropy can show similar multistable behavior in their polycrystalline form. However, inthe single crystal samples, the magnetization is restricted
to one direction due to the substantial magneto-crystalline
anisotropy, and therefore only a binary signal can occur.
The new multi-level recording method found in Mn3Sn may
allow writing and reading beyond two units of information(0 and 1), namely, a device comprising a single grain of
nanometer scale can hold three or more units of information.
Taking advantage of the small shape anisotropy and theomnidirectional property, we developed a ANE heat ﬂux
sensor consisting of micro-fabricated Mn3Sn wires (Fig.
1(e)). This sensor exhibits the heat ﬂux sensitivity comparable to that made of ferromagnetic materials while being
resistant to magnetic ﬁeld disturbances (Fig. 1(f)).
References[1] R. Skomski and J. M. D. Coey, Scr. Mater. 3, 112 (2016).
[2] S. Nakatsuji et al., Nature 527, 212 (2015).
[3] M. Ikhlas, T. Tomita et al., Nat. Phys. 13, 1085 (2017).
[4] T. Higo et al., Appl. Phys. Lett. 113, 202402 (2018).
[5] H. Tsai, T. Higo et al., Nature 608, 580 (2020).
[6] T. Higo et al., Adv. Funct. Mater. 31, 2008971 (2021).
Authors
T. Higo, Y. Lia, K. Kondoub, D. Qu, M. Ikhlas, R. Uesugi, D. NishioHamane, C. L. Chiena, Y. Otani, and S. Nakatsuji
aJohns Hopkins University
bRIKEN-CEMS

Large Hall Signal due to Electrical
Switching of an Antiferromagnetic
Weyl Semimetal State
Nakatsuji, Otani, and Miwa Groups
The topologically protected properties in Weyl
semimetals have been extensively studied in recent years.
The Weyl points act as sources and drains of Berry curvature, leading to significant responses such as anomalous Hall
effect (AHE) and anomalous Nernst effect (ANE) [1-3].
In nano-scale technology, such large topological responses
may play a significant role in developing next-generationelectronic devices.
Recently, the electrical manipulation of the topologicalstate has been demonstrated in antiferromagnetic Weyl
semimetal Mn3Sn [4]. Mn3Sn is a metallic antiferromagnet
with the hexagonal D019 structure and ABAB-stacking of
Mn kagome lattice along [0001]. The geometrical frustration leads to a 120-degree structure made of Mn magnetic
moments with a negative vector chirality (Fig 1(a)). This
non-collinear structure can be viewed as a ferroic order ofa cluster magnetic octupole made of six neighboring Mn
moments. The polarization of the magnetic octupole determines the distribution of Weyl points in momentum space
and thus the polarity of AHE (Fig 1(b)). Figure. 1(c) shows
the schematic image of the current switching in Mn3Sn/
heavy metal heterostructures. Taking Mn 3Sn/W as an
example, with an applied electrical current in the multilayer,
a spin current generated by the spin Hall effect in the W

Fig. 1. (a) Crystal and spin structures of Mn3Sn. Blue and light blue
spheres represent Mn atoms on the kagome bilayers, forming anoctahedron. Black and gray spheres represent Sn atoms. The noncollinear spin structure consisting six Mn magnetic moments (yellow
and green arrows) can be viewed as a ferroic ordering of an magnetic
octupole (purple arrow). (b) Hall voltage measured in Mn3Sn/W
devices. The sign of readout voltage is determined by the out-of-planecomponent in the polarization direction of the magnetic octupole of
Mn3Sn (purple and blue arrows), namely, “0” or “1” state. (c) Bottom:
Schematic image of Mn3Sn/heavy metal devices and the setup ofelectrodes. The samples are fabricated in 16 μm × 96 μm Hall bar
structure with Au/Ti electrodes. Top: Schematic image of the current
switching in Mn3Sn/W bilayer. An electrical current flowing in the W
layer generates a spin current by the spin Hall effect of W. This spincurrent induces a spin-orbit torque in the Mn3Sn layer and switch thepolarization direction of the magnetic octupole. Here, red and blue thinarrows represent the spin moments of the spin current. Purple and light
purple thick arrows represent the polarization direction of the magnetic
octupole.

ISSP Activity Report 202015

layer exerts the spin-orbit torque on the Mn3Sn spin texture,causing the switching of the polarization direction of themagnetic octupole. This manipulation of AHE enables us tostore the information from the electrical signal, “0 ” or “1 ”
state, into the magnetic octupole of Mn3Sn. Thus, Mn3Sn
can work as a non-volatile memory, in which the information
can be written and read by current. Moreover, the antiferromagnetic structure of Mn3Sn allows electronic devices
with higher density and computing speed than those made of
conventional ferromagnets [5]. For applications, it is essential to enhance the readout signal of the switching. This study
demonstrates that the switching signal of Mn3Sn can growdramatically by adjusting the crystal orientation and interface
condition of the Mn3Sn/heavy metal structures. Moreover, byincreasing the read current, the readout voltage may go wellbeyond 1 mV, a milestone for future memory technology.
To find the largest readout signal, we prepared four
different multilayer stacks: (i) Ru(2)/Mn3Sn(40)/Pt(5), (ii)
Mn3Sn(40)/Pt(5), (iii) Ru(2)/Mn3Sn(40)/W(5), and (iv)
Mn3Sn(40)/W(5), all units in nm, deposited on a Si/SiO2
substrate. For the sample (i)-(iii), the stacks are annealed
at 450 °C for 0.5 h after the fabrication of the Mn 3Snlayer. The samples (iv) Mn3Sn(40)/W(5) is annealed at
450 °C for 0.5 h after the fabrication of the W layer. Using
these thin films, a Hall bar structure (16 μm × 96 μm)
is fabricated, with Ti/Au electrodes attached (Fig 1(c)).
Figure. 2(a) shows the AHE signal in W devices with andwithout the Ru layer. By removing the Ru layer, the AHE
becomes 1.6 times larger than that observed in devices with
the Ru layer. Similar enhancement of the AHE also occurs in
Pt devices. This enhancement comes from the crystal orientation change of Mn3Sn. The X-ray diffraction analysis showsthat the kagome plane of Mn3Sn tends to align in-plane in
the presence of the Ru underlayer. After removing the Ru
layer, Mn3Sn layer becomes completely polycrystalline. The
out-of-plane component of the magnetic octupole is morethan one with the Ru layer, resulting in a larger AHE. Next,
we investigated the electrical switching in W devices. In
these experiments, both electrical current and bias field areapplied along the longitudinal direction (x-direction) andthe transverse Hall voltage VH is detected in the y-direction,
as shown in Fig. 1(c). A strikingly large switching Hall
voltage, ΔVHcurrent ~70 μV, is observed in Mn3Sn/W devices(Fig. 2(b)). This Hall signal is 3 times larger than thatobserved in the Ru/Mn3Sn/W devices, more pronounced than
the 1.6 times difference of the AHE caused by removing the
Ru layer. We attribute this enhancement in current switching
to the different annealing process because only the Mn3Sn/
W samples is once annealed after the W deposition. Thisadditional annealing process may affect the interface condition. Finally, since the Hall voltage is proportional to theFig. 2. (a) Hall voltage VH as a function of a perpendicular magnetic
field Hz for Ru/Mn3Sn/W and Mn3Sn/W devices, respectively. (b) VH
as a function of the write-current Iwrite under a bias field μ0Hx = 0.1
T applied along the electrical current direction for Ru/Mn3Sn/W and
Mn3Sn/W devices, respectively. (c) Switching Hall voltage ΔVH as afunction of the read current Iread in the Mn3Sn(40)/W(5) sample. Inset:
Hall resistance ΔRH as a function of the read current Iread.

16

ISSP Activity Report 2020

read current within a suitable temperature range, we tried toenhance the read current to maximize the readout signal (Fig.
2(c)). We found that the readout switching Hall voltage ΔVH
can exceed 1 mV when applying 3 mA read current. Thisobserved mV order Hall voltage is a significant advance inthe electrical switching technology, and our results may pave
the path for designing a new type of memory device using
the robust topological properties of Weyl antiferromagnets.
References[1] H. B. Nielsen and M. Ninomiya, Phys. Lett. B, 130, 389 (1983).
[2] S. Nakatsuji, N. Kiyohara, and T. Higo, Nature, 527, 212 (2015).
[3] M. Ikhlas, T. Tomita, T. Koretsune, M. Suzuki, D. Nishio-Hamane,
R. Arita, Y. Otani, and S. Nakatsuji, Nat. Phys. 13, 1085 (2017).
[4] H. Tsai, T. Higo, K. Kondou, T. Nomoto, A. Sakai, A. Kobayashi,
T. Nakano, K. Yakushiji, R. Arita, S. Miwa, Y. Otani, and S. Nakatsuji,
Nature 580, 608 (2020).
[5] T. Jungwirth et al., Nat. Nanotechnol. 11, 231 (2016).
Authors
H. Tsai, T. Higo, K. Kondoua, S. Sakamoto, A. Kobayashi, T. Matsuo,
S. Miwa, Y. Otani, and S. Nakatsuji
aRIKEN, Center for Emergent Matter Science (CEMS)

A Tunable Stress Dilatometer and
Measurement of the Thermal Expansion
under Uniaxial Stress of Mn3Sn
Nakatsuji GroupThe hexagonal antiferromagnet Mn3Sn with triangularspin structure exhibits large spontaneous electronic responses
associated with Weyl fermions [1], which has attracted
tremendous attention due to its potential for spintronic
applications [2]. The single crystals of this compound are
known to be stable in their off-stoichiometric compositions. Depending on the composition, three distinct kinds ofmagnetic phases can emerge as a function of temperature,
namely the high-temperature triangular spin structure phase(q = 0) [3], the intermediate-temperature incommensurate
spin spiral phase (q ≠ 0) [4], and the low-temperature cluster
spin glass phase [5]. The topological transport properties
vanish in the incommensurate phase, whose origin remains
elusive. A theoretical study suggests that out-of-plane antiferromagnetic exchange interaction may destabilize the triangular spin structure [6].

Fig. 1. Uniaxial stress dilatometeris consistent with that estimated from heat capacity data.
Above -σz ~ 0.5 GPa, ∂TH/∂σz does not deviate appreciably
from its ambient pressure value; this information cannot
be extrapolated solely based on the zero pressure ∂TH/∂σz.
Overall, we found that the transition is insensitive to stresscompared with its response to chemical doping, suggestingthat the lattice is not the primary driver of the transition. This
device is compatible with other probes, such as transportand spectroscopy, opening a new avenue to study phases ofmatter under extreme conditions [9].
References[1] T. Chen, T. Tomita, S. Minami, M. Fu et al., Nat. Comm. 12, 572(2021).
[2] H. Tsai, T. Higo et. al., Nature 580 (2020).
[3] T. Nagamiya, S. Tomiyoshi, and Y. Yamaguchi, Solid State
Commun. 42 (1982).
[4] J.W. Cable et. al., Solid State Commun. 88 (1983).
[5] W. J Feng, D. Li et. al., Phys. Rev. B 73 (2006).
[6] S. Dasgupta and O. Tchernyshyov, Phy. Rev. B 102 (2020).
[7] M. E. Fischer and W. Selke, Phys. Rev. Lett. 44 (1980).
[8] A. Andrianov et. al., Phys. Rev. B 62 (2000).

[9] M. Ikhlas, K. Shirer et. al., Appl. Phys. Lett. 117 (2020).

Fig. 2. Thermal expansion of Mn3Sn under uniaxial pressure. (a)Sample’s c-axis length change L-L0 vs. temperature under various
applied uniaxial forces. Inset shows the sample used for the measurements. (b) Derivative of c-axis length change -d(L-L0)/dT vs. temperature (bottom panel) under various applied uniaxial forces.

A long-wavelength incommensurate spin structure canoriginate from various mechanisms, such as competition
between short-range and long-range exchanges in CeSb [7],and Fermi surface nesting in hexagonal rare-earth elemental
metals [8]. In the latter case, Fermi surface nesting is largely
determined by the lattice constant ratio c/a, which is tunable
by alloying or uniaxial pressure. Uniaxial pressure is a clean
tuning parameter compared to chemical doping. Here, we
explore the evolution of the magnetic phase diagram of
Mn3Sn under c-axis uniaxial pressure.
In collaboration with Dr. Clifford Hicks and Dr. Kent
Shirer from Max Planck Institute for Chemical Physics ofSolids (MPI CPfS), Dresden, Germany, we developed apiezoelectric-based uniaxial stress dilatometer that provides
in-situ stress tunability, as shown in Fig 1. This device allows
high-sensitivity thermal expansion measurements, offering acompelling route to thermodynamic information on magnetic
phase evolution under large and homogenous applied stress.
The sample is sculpted into a paddle using a focused ion
beam and then epoxied across the central gap of the sample
carrier, as seen in the inset of Fig 1a. Applying voltage to thepiezo stacks displaces a spring and delivers uniaxial stress tothe sample, while the sample’s length change was estimatedfrom the displacement of the parallel plate capacitor attachedto the carrier.
Figure 2a shows the temperature dependence of c-axis
length change L-L0 of Mn3Sn at various applied uniaxial
forces. At zero force, the sample exhibits a first-order transition from triangular to spiral spin structure on cooling below
TH = 267 K, accompanied by a discontinuity in L-L0. Underuniaxial force, the transition splits into two, as shown in
Fig. 2b. The peak in –dΔL/dT that shifts to lower temperatures with an increasing uniaxial force corresponds to the
response of the central narrow portion of the sample. Withan elastic compressive stress of σz ~-1.5 GPa, the transitionis suppressed by ∆T ~ -50 K. Below -σz ~ 0.5 GPa, TH hasa linear dependence on σz, with ∂TH/∂σz = 33.3 K at zerostress. We estimate an entropy change of ΔS ~ 0.1 mJ mol-1
K-1 from Clausius-Clapeyron relation, using the magnitudeof the discontinuity in ΔL and ∂TH/∂σz. This value of ΔS

Authors
M. Ikhlas, K. Shirera, P.-Y. Yanga, A. P. Mackenziea, C.W. Hicksa, and
S. Nakatsuji
aMax Planck Institute for Chemical Physics of Solids

Large Transverse Thermoelectric Effectin Iron-Based Binary Ferromagnets
Nakatsuji and Miwa Groups
Thermoelectricity provides vital technology for versatileenergy harvesting and heat current sensors. So far, thermoelectric technologies are focused on the longitudinal Seebeck
effect. Its transverse counterpart, the anomalous Nernst effect(ANE), has recently gained significant attention, owing
to several potential benefits [1,2]. Namely, the transverse
geometry of the Nernst effect enables simplified structuresof thermoelectric generators with enhanced conversion
efficiency once the suitable materials become readily available. Moreover, the transverse geometry is hypothetically
better suited for thermoelectric conversion, as the Ettingshausen heat current should support the Nernst voltage while
the Peltier heat current may suppress the Seebeck voltage[3]. A server obstacle here is that the anomalous Nernst
effect is too small compared to the Seebeck effect for reallife thermoelectric applications. Thus, it is essential to design
a new class of materials that exhibit a large ANE without anexternal magnetic field.
To find candidate compounds efficiently, we carried out
a high-throughput calculation to screen materials and found
two candidate iron-based cubic compounds, Fe3X (X = Ga,

Fig. 1. (a) Crystal structures of Fe3X (X = Ga, Al) (b) Temperaturedependence of the Nernst signal −Syx in Fe3X (X = Ga, Al) under a[100] magnetic field of 2T. (c) The nodal web in momentum space
formed by nodal lines (yellow) of nearly flat energy dispersion aroundthe L point.

ISSP Activity Report 202017

Al) (Fig. 1a). We discovered record high spontaneous transverse thermoelectric effects at room temperature in thesematerials, reaching about 6 μVK-1 in Fe3Ga and 4 μVK-1 in
Fe3Al (Fig. 1b). We then succeeded in fabricating Fe3Ga andFe3Al thin films that display an ANE of about 4 μVK-1 and
2 μVK-1 at room temperature with an applied in-planetemperature gradient. For an out-of-plane temperature
gradient, these thin films exhibit a zero-field ANE, with acoercivity BC of the in-plane magnetization being about 40
Oe in Fe3Ga and 20 Oe in Fe3Al, respectively. These featuresare suitable for designing low-cost, flexible thermoelectric
generators.
The comparison between experiment and theory indicatesthat the Fermi energy tuning to the nodal web is the key tothe substantial enhancement in the transverse thermoelectriccoefficient. Figure 1(c) shows a schematic of the nodal web,a flat band structure made of interconnected nodal lines. The
Berry curvature is particularly enhanced at the momenta
connecting the edge of the nodal web around the L point at
the Brillouin zone boundary, extending over a quasi-2D areaspanned by the web. The strongly enhanced Berry curvature
occurs near the momenta originally belonging to the flat
nodal web, such as the one around the L point.
Our innovative iron-based thermoelectric material represents a significant step toward commercializing thermoelectric generators that can power small devices such as
remote sensors or wearable devices. It is vital to enhance thecoercivity further to achieve stable performance in daily use.
Finally, it would be an interesting future direction to look
for an enhanced output by combining the ANE with the spin
Seebeck effect, both of which occur in the same transverse
geometry. [4]References[1] A. Sakai et al., Nat. Phys. 14, 1119 (2018).
[2] M. Ikhlas, T. Tomita et al., Nat. Phys. 13, 1085 (2017).
[3] M. Mizuguchi, and S. Nakatsuji, Sci. Technol. Adv. Mater. 20, 262(2019).

state using Mn3Sn/nonmagnetic metals (NM = Pt, W, Cu)bilayer devices and the spin-orbit torque (SOT) switching
method [5]. The inverse triangular spin structure of Mn3Sn
can be viewed as a ferroic ordering of cluster magnetic
octupoles. The magnetic octupole’s polarization direction
determines the location of Weyl nodes and the associateddistribution of the Berry curvature in the momentum space(Fig. 1(b) right) [2]. With an electrical current applied to
the Pt layer, a spin polarized current generated by the spin
Hall effect of Pt induces the SOT on the Mn3Sn layer andcauses the switching of the polarization direction of themagnetic octupole (Fig. 2a). The measurement configuration and optical micrograph of the fabricated bilayer Hallbar devices are shown in Fig. 2(b). We first perform themagnetic field switching of the Hall voltage VH by using
an out-of-plane magnetic field Hz and a read current ofFig. 1. (a) Mn3Sn crystal structure and inverse triangular spin (ITS)
structure. The Mn magnetic moments (light blue and pink cylindricalarrows) lie within the kagome layer with the ABAB stacking sequenceand form the ITS structure at room temperature. This spin structure
can be viewed as a ferroic ordering of cluster magnetic octupoles.
(b) Bottom-left: cluster magnetic octupole (orange cylindrical arrow)consisting of the six spins on the kagome bilayer. Right: schematic of
the Weyl point distribution near the Fermi energy in momentum space(kx-ky plane at kz = 0); the corresponding magnetic structure is shownin the left-side figure. The red and blue spheres correspond to Weylnodes acting as sources (+) and drains (-) of the Berry curvature (green
arrows). Inset: three-dimensional schematic of a pair of Weyl nodes.

[4] A. Sakai, S. Minami, T. Koretsune, T. Chen, T. Higo et al.,Nature 581, 53 (2020).
Authors
A. Sakai, S. Minamia, T. Koretsuneb, T. Chen, T. Higo, Y. Wang,
T. Nomotoc, M. Hirayamad, S. Miwa, D. Nishio-Hamane, F. Ishiia,
R. Aritac,d, and S. Nakatsuji
aKanazawa University
bTohoku University
cUniversity of Tokyo
dRIKEN CEMS

Electrical Manipulation of a Topological
Antiferromagnetic State
Nakatsuji, Otani, and Miwa Groups
The Weyl semimetal features nodal points formedby two linearly dispersive non-degenerate bands. These
touching points or Weyl nodes act as topologically protectedmonopoles or antimonopoles of underlying Berry curvature, resulting in large anomalous Hall effect (AHE) andchiral anomaly. The electrical manipulation of Weyl nodes isamong the key topics of technological innovation utilizing anovel topological state. Moreover, electrical manipulation ofan antiferromagnetic Weyl metal is considered a significant
advancement, given the prospects of antiferromagnetic (AF)spintronics for realizing high-density, ultrafast devices [1].
We demonstrate the electrical switching of the AF Weyl

18

ISSP Activity Report 2020

Fig. 2. (a) Schematic image for the spin-orbit torque switching. Undera write current and a bias field along the x-direction, the spin-polarized
current in Pt (green cylindrical arrows on yellow spheres) exerts aspin-orbit torque, causing the polarization axis of the cluster magnetic
octupole to switch (orange cylindrical arrow) in the polycrystalline
Mn3Sn (b) Measurement configuration and optical micrograph of
the Mn3Sn-nonmagnet bilayer Hall bar devices. (c) Hall voltage VH
vs. magnetic field along the z-direction Hz obtained for the Mn3Sn/
Pt 7.2 nm device at room temperature. (d) Hall voltage VH vs. write
current Iwrite for the Pt 7.2 nm, Cu 5 nm, and W 7.2 nm devices atroom temperature. The Hall voltage is normalized by the zero-field
Hall voltage |ΔVHfield| obtained from the magnetic field dependence
measurements for each sample.

0.2 mA, and a clear hysteresis of VH with the zero-fieldchange of ΔVHfield (= VH(+Hz→0) − VH(−Hz→0)) is observed(Fig. 2(c)). To examine the possible SOT switching of
Mn3Sn, we pass a 100 ms write-pulse current Iwrite along thex-direction; then, we wait for 600 ms after the pulse current
and measure VH with a 0.2 mA read current. Figure 2(d)shows VH as a function of Iwrite in Pt, W, and Cu devices. For
the NM = Pt (spin Hall angle θSH > 0) and a bias field Hx
parallel to the current direction, a clear negative (positive)
jump appeared in the Hall voltage for a positive (negative)
current larger than a critical threshold write-current Ic. The
magnitude of the jump reaches ~30 % of the total Hall
voltage change |ΔVHfield | in the field sweep measurements.
To confirm that the observed switching of VH is related to the
SOT due to the spin Hall effect in the NM layer, we prepared
NM = W (θSH < 0) and Cu (θSH ~ 0). For NM = W, theswitching polarity is opposite to the Pt case, while for NM =Cu, hysteresis is absent with the electrical current cycle. The
difference in switching polarity between Pt and W devices
and the absences of switching in Cu devices cannot be
explained by the current-induced Oersted field but agree wellwith the sign of θSH of the NM layer. These results demonstrate that SOT from the NM layer induces the perpendicularswitching of the AF domain and thus manipulates the direction of the Weyl nodes.
In recent years, utilizing antiferromagnets in spintronics
devices have attracted significant attention as its vanishingsmall stray fields and much faster spin dynamics than ferromagnets [1]. We achieved the electrical switching of the AF
Weyl metal using the same protocol as the one used for ferromagnetic metals [3, 4]. The critical write current density in
the NM layer is considerably smaller than that in the NM/FM
devices. These results indicate that topological antiferromagnets offer the unmatched potential for spintronic applications
that may surpass today’s ferromagnetic counterparts [5].
References[1] T. Jungwirth et al., Nat. Nanotechnol. 11, 231 (2016).
[2] S. Nakatsuji, N. Kiyohara, and T. Higo. Nature 527, 212 (2015).
[3] I. M. Miron et al., Nature 476, 189 (2011).
[4] L. Liu et al. Science 336, 555 (2012).

[5] H. Tsai, T. Higo, K. Kondou, T. Nomoto, A. Sakai, A.
Kobayashi, T. Nakano, K. Yakushiji, R. Arita, S. Miwa, Y. Otani,and S. Nakatsuji, Nature 580, 608 (2020).
Authors
H. Tsai, T. Higo, K. Kondou a , Takuya Nomoto b , A. Sakai,
A. Kobayashi, T. Nakanoc, K. Yakushijic, R. Aritaa, S. Miwa, Y. Otani,and S. Nakatsuji
aRIKEN CEMS
bThe University of Tokyo
cAIST

bringing the study on strongly correlated topological phasesto the main stage of basic science and technological innovation. These anomalous transport properties are consideredfingerprints of Weyl fermions in the momentum-space band
structure [4]. Therefore, understanding the role of magnetic
Weyl fermions in driving the transport properties of chiralantiferromagnets is all the more important for making reallife applications with topological states of matter.
In this work, we establish Weyl fermions as the origin of
the large anomalous Hall and Nernst effects in Mn3X systems
by combining our previous reports with a new, comprehensive set of transport measurements and theoretical analysis
of high-quality Mn3Sn and Mn3Ge single crystals [5]. In
particular, Mn3Ge provide an ideal experimental platform
for identifying transport signatures stemming from Weyl
fermions; in this compound, the inverse triangular covers
a wide temperature range down to at least 0.3 K, approximately one-thousandth of the Néel temperature. This featureallows a clear-cut detection of the Weyl-fermion-induced
transport anomalies at sufficiently low temperatures, where
spin fluctuations are essentially suppressed.
The fingerprint of Weyl points in magnetotransport is
known as the chiral anomaly. With a magnetic field, the Weyl
crossings are quantized into Landau levels (LLs), and thelowest LL are divided into left- and right-movers of opposite
chirality. These two groups do not mix, leading to separate
number conservation of the three-dimensional left- and righthanded Weyl fermions. Once an electric field parallel to themagnetic field is applied, the conservation of chirality nolonger holds due to mixing the left-and-right movers. In sucha case, charge pumping between the Weyl nodes of opposite
chirality generates the positive longitudinal magnetoconductivity (LMC) or the negative magnetoresistivity (LMR)
only when the electric current and magnetic field are parallel
to each other. This strongly anisotropic behavior of LMC
provides transport evidence for Weyl fermions.
Our findings of magnetotransport signatures specific tochiral anomalies in Mn3Ge and planar Hall effect in Mn3Sn
provide concrete evidence for the presence of Weyl fermionsin both materials. Figure 1a shows the field dependence ofthe magnetoconductivity observed in Mn3Ge at 0.3 K. For
applied fields B > 2T, the magnetoconductivity becomes
positive for the parallel configuration and is nearly quadratic
in magnetic field. The B2 dependence of the positive LMC
is likely a result of strongly titled type-II Weyl cones withthe applied magnetic field perpendicular to the tilt axis. We

Weyl-Fermion-Induced Anomalous
Transport in the Chiral Antiferromagnets
Mn3X, (X = Sn, Ge)
Nakatsuji GroupThe blooming field of spintronics has bridged astonishingly complex antiferromagnetic structures to the realm ofenergy-efficient memory technologies. However, a significant obstacle to overcome is the tiny response of the antiferromagnetic structure to external magnetic fields [1]. The
recent discoveries of large room-temperature anomaloustransport effects in antiferromagnets Mn3X (X = Sn, Ge)[2,3] signifies an experimental breakthrough in this direction,

Fig. 1. (a) The magnetoconductivity as a function of the magnetic field
B for the parallel and perpendicular configurations with the current
I || [0001] measured at 0.3 K in Mn3Ge. The dashed line is a fit to thequadratic magnetic field dependence. (b) Angle dependence of thelongitudinal magnetoconductivity (top) and the planar Hall effect(PHE) (bottom) in Mn3.06Sn0.94 obtained at 300 and 100 K underB = 3 T. (c, d) Contour plots of the calculated Berry-curvature spectrum|Ωz| in the kx–ky-plane under B || [01-10] at kz = 0 (Å−1) (c) and at
kz = 0.137 (Å−1) (d). The arrows indicate the Berry curvature inducedby the pairs of Weyl points WP1 and WP3 (WP'3) near the Fermi level.

ISSP Activity Report 202019

further verified that the angular dependence of the magnetoconductivity and the planar Hall signal observed in Mn3Sn
agree well with the theoretical forms for the chiral anomaly(Fig. 1b), another transport evidence for magnetic Weyl
fermions.
Moreover, we compare our results on the anomalous
Hall and Nernst effects with the first-principles calculations
that consider the substantial bandwidth renormalization andquasiparticle damping due to the strong electronic correlation in Mn3X systems (Fig. 1c, d). The calculated transverse thermoelectric coefficients well reproduce the overalltemperature dependence of the experimental data, suggestingthat the large Berry curvature stemming from the Weyl nodes
leads to significantly enhanced AHE and ANE in the absenceof net magnetization.
Strongly correlated topological states have become acentral focus for quantum materials research, and Mn3X
(X = Sn, Ge) is widely considered a textbook example ofthis kind. Our findings provide concrete evidence for thepresence of Weyl fermions in both materials, which may
trigger further exploration of their rich physics and usefulproperties emerging from the interplay between magnetismand band topology.
References[1] T. Jungwirth et al., Nat. Nanotechnol. 11, 231 (2016).
[2] S. Nakatsuji et al., Nature 527, 212 (2015).
[3] M. Ikhlas, T. Tomita et al., Nat. Phys. 13, 1085 (2017).
[4] K. Kuroda et al., Nat. Mater. 16, 1090 (2017).
[5] T. Chen, T. Tomita, S. Minami, M. Fu et al., Nat. Comm. 12, 572(2021).
Authors
T. Chen, T. Tomita, S. Minami, M. Fu, T. Koretsunea, M. Kitatani,
I. Muhammad, D. Nishio-Hamane, R. Ishii, F. Ishiib, R. Aritac,d, and
S. Nakatsuji
aTohoku University
bKanazawa University
cRIKEN CEMS
dThe University of Tokyo

Giant Effective Damping ofOctupole Oscillation in a Chiral
Antiferromagnet Mn3Sn

Fig. 1. (a) Spin and crystal structures of Mn3Sn. Different colors areused to denote Mn and Sn atoms in the z = 0 and z = 1/2 planes. The
inverse triangular spin-structure can be viewed as a ferroic ordering ofcluster magnetic octupole, which possesses the same symmetry as the
spin-magnetization (e.g. as indicated by a mirror plane (orange line)).

Mn atoms along the [0001] axis. Red (blue) circles inFig. 1 indicate Mn atoms in A- (B-) plane of the kagome
lattice. An inverse triangular spin structure is stabilized byexchange and Dzyaloshinskii-Moriya interactions [1,2]. The
inverse triangular spin structure is made of a ferroic orderingof cluster magnetic octupoles [3].
Figure 2(a) shows typical TR-MOKE results, where amagnetic field of 2 T was applied normal to the surface.
First, a pump light induces rapid decrease in the polar
MOKE intensity of a probe light, similar to ultrafast demagnetization observed in ferromagnetic metals [4]. Consecutively, the MOKE intensity start to recover with delay time
more than 0.3 ps, exhibiting a small but clear oscillation
during the recovery. To further characterize the oscillating
component, a non-oscillating component was estimated asa background (black solid curve) and subtracted from theraw data. The lower panel of Fig. 1(b) shows the TR-MOKE
signals after subtracting the background. Figure 1(c) also
shows typical TR-MOKE results (upper panel) and theanalysis for the oscillating component (lower panel) in a
relatively long-time range. From the fits in the lower panelof Figs. 1(b) (orange curve) and 1(c) (blue curve), oscillation frequencies (ωI, ωII) are determined as a function of an
external magnetic field, namely, ωI = 0.86 THz and ωII =18 GHz. The oscillating behaviour is most significant whenthe magnetic field is normal to the Mn3Sn surface. Because

Miwa, Nakatsuji, and Otani Groups
Understanding of spin dynamics forms the basis of
spintronic application. For the ultrafast THz control ofstaggered moments in antiferromagnets, the time-resolved
observation of spin dynamics is essential for developing theassociated technology. However, such observation has been
limited to insulators to date. Recently, the metallic anttiferromagnets Mn3X have attracted significant attention for its
strong response (e.g. anomalous Hall effect) comparable
to ferromagnets owing to the hidden ferroic order, which
configures large Berry curvature originated from Weyl points
in a momentum space. Such ferroic order can be characterized by cluster magnetic octupoles based on neighbouring
magnetic moments, and thus it is highly important to clarify
the dynamics of cluster octupole for designing the spintronic
application. In this research, we conduct time-resolved
observation of spin-dynamics in a chiral antiferromagnet
Mn3Sn.
Bulk single-crystal D019-Mn3Sn has been employed.
Mn3Sn has the hexagonal Ni 3Sn-type crystal structure
consisting of an ABAB stacking of the kagome lattice of

20

ISSP Activity Report 2020

Fig. 2. (a) Schematic diagram of the time-resolved polar MOKE
measurements (TR-MOKE). (b), (c) TR-MOKE signals of Mn3Sn
under a magnetic field of 2 T in the [2-1-10] direction. Backgrounds,
indicated by black curves in upper panels, were subtracted from the raw
data and plotted in the lower panels.

TR-MOKE signal is proportional to the surface normal
polarization of the cluster magnetic octupoles, the oscillating
Kerr signal should come from a change of size of the cluster
magntic octupole order.
Two oscillation modes I and II are found through theanalyses shown in Figs. 1(b) and 1(c), which should comefrom the dynamics of the cluster magnetic octupoles. The
mode I and II are assigned to the optical and collectiveprecession-like modes. From Fig. 1(c), we find that the
dynamics of the cluster octupole in Mn3Sn is enhanced byexchange interaction, and it is feasible to conduct an ultrafast
switching at < 10 ps, a hundred times faster than the case ofspin-magnetization in a ferromagnet. Moreover, our theoretical analysis with the aforementioned experimental resultsshows that the Néel type domain wall velocity in Mn3Sn canbe estimated as > 10 km/s, which is greater than the recently
reported value (~2 km/s) for the ferrimagnetic systems in thevicinity of the angular momentum compensation temperature[5]. Significantly, our study provides the first demonstrationof the time-resolved observation of the spin-wave oscillationin an antiferromagnetic metal.
References[1] S. Nakatsuji, N. Kiyohara, and T. Higo, Nature 527, 212 (2015).
[2] S. Tomiyoshi and Y. Yamaguchi, J. Phys. Soc. Jpn. 51, 2478 (1982).
[3] M.-T. Suzuki, T. Koretsune, and M. Ochi, Phys. Rev. B 95, 094406(2017).
[4] E. Beauprepaire et al., Phys. Rev. Lett. 76, 4250 (1996).
[5] K.-J. Kim et al., Nat. Mater. 16, 1187 (2017).
Authors
S. Miwa, S. Iihamaa, T. Nomotob, T. Tomita, T. Higo, M. Ikhlas, S.
Sakamoto, Y. Otani, S. Mizukamia, R. Aritab, and S. Nakatsuji
aTohoku University
bThe University of Tokyo

Low Gilbert Damping in Epitaxial
Thin Films of the Nodal-Line
Semimetal D03-Fe3Ga
Miwa and Nakatsuji Groups
D03-ordered Fe3Ga [Fig. 1(a)] is a nodal-line semimetaland shows a giant anomalous Nernst effect (ANE). The
origin of the ANE was attributed to the large density ofstates (DOS) and the large Berry curvature near the Fermi
level. The large Berry curvature emerges as a result of
spin-orbit-coupling-induced gap opening along the nodal
lines consisting of nearly flat valence and conduction bands
touching each other. This peculiar band structure may be
useful for spintronics device application because a gate
voltage may effectively modulate Berry-curvature-associatedresponses as well as magnetic anisotropy. In this regard, it
is important to characterize the magnetization dynamics of
D03-Fe3Ga. For such a purpose, we fabricated epitaxial thinfilms of D03-Fe3Ga and bcc-Fe and performed ferromagnetic
resonance (FMR) measurements to evaluate their Gilbert
damping constants.
50-nm-thick D03-Fe3Ga and Fe thin films were grown by
dc magnetron sputtering and molecular beam epitaxy, respectively. We confirmed that the D03-Fe3Ga layer was grown
epitaxially by performing high-energy electron diffraction and x-ray diffraction measurements. We conducted
FMR measurements using a vector network analyzer anda coplanar waveguide. We employed a field-modulation
technique [2], which measures the magnetic-field (H) deriva-

tive of the transmission coefficient ΔS21, to minimize fieldindependent background signals.
Figure 1(b) shows typical FMR spectra of the Fe3Ga
thin film taken with magnetic fields applied along the [001]direction. Here, a FMR spectrum refers to ΔS21 as a functionof magnetic field at a fixed frequency. From the FMR
spectra, we deduced the peak positions and peak widths,
which are shown in Figs. 1(c) and 1(d), respectively. We
fitted the Kittel formula to the FMR peak positions, and thefits are shown by solid curves in Fig. 1(c). We found that thein-plane cubic anisotropy H4// of Fe3Ga favors the magnetization along the [110] direction (H4// ~ −15 mT), while thoseof Fe and disordered Fe-Ga alloys favor the [100] direction(H4// ~ + 60 mT for Fe and H4// ~ + 75-100 mT for the Fe-Ga
alloys [3]). This magnetic-anisotropy change indicates thatthe D03 atomic ordering significantly alters the electronic
band structure of Fe-Ga alloys and induces the giant ANE asa result.
The FMR peak widths of the Fe and Fe3Ga thin films[Fig. 1(d)] showed linear behavior, the slope of whichcorresponds to the Gilbert damping constant. The linear
fits yielded the Gilbert damping constants of α = 0.0023 ±0.0002 for Fe and α = 0.0060 ± 0.0002 for D03-Fe3Ga. These
values are somewhat consistent with what is expected fromthe theoretical DOSs, indicating that Ga atoms do not significantly increase the spin-orbit coupling strength. Although theGilbert damping constant of D03-Fe3Ga is larger than that of
bcc-Fe, it is still low compared to typical ferromagnets suchas CoFeB (α ~ 0.004-0.006) [4] and Permalloy (α ~ 0.007)[5]. In other words, the nodal lines near the Fermi level
do not hinder the magnetization dynamics much. The low
Gilbert damping may be useful for spintronics device applications because one may be able to efficiently manipulate
the magnetization direction by exploiting the peculiar band
structure of D03-Fe3Ga.
In summary, we have investigated the magnetization dynamics of the epitaxial thin films of the nodal-line
semimetal D03-Fe3Ga by ferromagnetic resonance measurements. We have deduced the intrinsic Gilbert damping
constant of D03-Fe3Ga to be (6.0 ± 0.2) × 10−3, which isFig. 1. (a) Crystal structure of D03-Fe3Ga. (b) Typical Ferromagnetic
resonance (FMR) spectra of D03-Fe3Ga. (c) Resonance frequency
plotted against resonance magnetic fields. The solid curves are thefits using the Kittel formula. (d) FMR peak widths of D03-Fe3Ga andbcc-Fe. The solid lines represent the linear fits, the slope of which
represents the intrinsic Gilbert damping constant.

ISSP Activity Report 202021

larger than the Fe Gilbert damping constant of (2.3 ± 0.2)
× 10−3 but as low as other typical ferromagnets. The low
Gilbert damping and the peculiar band structure with a large
Berry curvature make D03-Fe3Ga attractive as a building
block of spintronics devices.
References[1] A. Sakai, S. Minami, T. Koretsune, T. Chen, T. Higo, Y. Wang,
T. Nomoto, M. Hirayama, S.Miwa, D. Nishio-Hamane, F. Ishii,
R. Arita, and S. Nakatsuji, Nature 581, 53 (2020).
[2] S. Tamaru, S. Tsunegi, H. Kubota, and S. Yuasa, Rev. Sci. Instrum.
89, 053901 (2018).
[3] S. Budhathoki, A. Sapkota, K. M. Law, B. Nepal, S. Ranjit,
S. KC, T. Mewes, and A. J. Hauser, J. Magn. Magn. Mater. 496,
165906 (2020).
[4] A. Conca, J. Greser, T. Sebastian, S. Klingler, B. Obry, B. Leven,and B. Hillebrands, J. Appl. Phys. 113, 213909 (2013).
[5] Y. Zhao, Q. Song, S.-H. Yang, T. Su, W. Yuan, S. S. P. Parkin, J.
Shi, and W. Han, Sci. Rep. 6, 22890 (2016).
Authors
S. Sakamoto, T. Higo, S. Tamarua, H. Kubotaa, K. Yakushijia, S.
Nakatsuji, and S. Miwa
aNational Institute of Advanced Industrial Science (AIST)

Textbook Renormalization Group
Prescription in Tensor-Network Language
Kawashima Group
Kadanoff’s block-spin method gives a simple physicalpicture for the general RG transformations. Combined with
Migdal’s bond-moving trick, it serves as the first approximation that allows us direct estimation of the critical exponents,
though the accuracy may not be satisfactory in many cases.
However, it is not obvious how we can systematically
improve on it so that eventually we can obtain, at least in
principle, estimates as accurate as we like. Modern tensornetwork (TN)-based methods, such as tensor renormalization group (TRG), tensor network renormalization (TNR),
loop-TNR, high-order TRG (HOTRG), etc, opened up new
possibilities for the RG transformations. Generalizing theconventional real-space RG (RSRG) transformations, they
are more versatile and, more importantly, systematically
improvable. The free energy, the energy, the magnetization, or any other locally defined order parameters can becomputed by one of those methods. As for the estimationof the scaling dimensions, the CFT-based transfer-matrix
method based on the conformal-field theory has been widelyused for two-dimensional classical systems. For higher
dimensions, however, the only method generally applicable

Fig. 2. The scaling dimensions of the 2D Ising model obtained directlyfrom the linearized RG plotted against the number of RG steps appliedat the critical point. The flat region represents the fixed point values.

was to esimate the correlation functions and apply the finitesize scaling to them. This indirect method may be good for
obtaining one or two scaling dimensions, but it is obviously
not a systematic way for obtaining a series of scaling dimensions.
Conceptually, it is straight-forward to construct a method
for obtaining the series of scaling dimensions from any
RSRG method. Simply we can expand the renormalized
tensor with respect to the preceding tensor, i.e., the tensor
at one RG step before the current one (Fig. 1). At the fixed
point, the linear term would produce the scaling dimensions as its eigenvalues. This is exactly what we learn fromtextbooks for obtaining the scaling dimensions, in manycases with the Migdal-Kadanoff (MK) approximation as
an example. In the MK approximation for the Ising model,
we have only two independent fields, and it is impossible
to obtain more than two scaling dimensions, whereas in
the general TN-based method, we have as many fields as
we like and, accordingly, we can in principle obtain manyscaling dimensions systematically from the same calculation.
However, it has been hard to carry out this straight-forward
textbook program in practice because of the influence of theshort-range entanglement.
In the paper [1], we adopted the graph-independent
local truncation (GILT) method to eliminate the short-range
entanglement. This made it possible to fully exploit the RG
interpretation of various tensor-network-based methods
and show a way to carry out the above-mentioned textbook
RG prescription in the TN language: identify a fixed point,
linearize the RG equation around this fixed point, anddiagonalize it to obtain scaling dimensions. For the RSRG,
we used HOTRG though it is not the only possible choice.
As shown in Fig. 2, our benchmark calculation on the 2Dclassical Ising model shows that the proposed method works
equally well compared with the above-mentioned CFT-basedtransfer matrix. The advantage of the present scheme is its
potential applications to 3D systems, where the CFT-based
method is inapplicable.
Reference[1] X. Lyu, R. G. Xu, and N. Kawashima, Phys. Rev. Research 3,023048 (2021)

Fig. 1. Schematic diagram of the linearized tensor RG equation "R". In
the proposed method, it can be generated using automatic differentiation once the coarse graining is implemented.

22

ISSP Activity Report 2020

Authors
X. Lyu, R. G. Xua, and N. KawashimaaThe University of Tokyo

Weak Ferromagnetism and Possible
Non-Fermi Liquid Behavior in Itinerant
Electronic Material Co3SnC
Uwatoko GroupIn Fermi-liquid (FL) theory, the quasiparticle (QP)
approximation and the generalized Boltzmann equation
can describe the collective behaviors of strongly correlated
multi-electron systems. For magnetic materials, strong
magnetic correlations usually induce large changes in the
QP-QP interactions, which make the QPs behave abnormally. Especially, when approaching the magnetic quantum
critical point (QCP), various pronounced deviations from theLandau FL behaviors appear, e.g., non-Fermi liquid (NFL)and strong enhancement of the QP effective mass. On theseissues, many correlated electronic materials including theheavy fermion (HF) systems have been studied throughtuning the magnetic orders by strengthening the hybridizations between the 4f and conduction electrons and/or
enhancing the magnetic correlations via external parameters.
These studies are helpful to reveal the intrinsic characteristics of the competing electronic orders near QCPs.
Cobalt-contained materials are such alternatives to
explore exotic QCPs owing to the various spin statesof cobalt ions and their strong response to the external
stimuli. Especially for those materials with both localized
and itinerant electrons, the varieties of magnetic transition and phase diagram are more interesting. Metallic
perovskite M3M’X (M is transition metals; M’ is the maingroup elements; X is C, N) is such an example with diversified crystal structures and magnetic phase transitions. But
the intrinsic magnetic interaction in M3M’X is controversial although various classical models were proposed. One
possible reason for the absence of the universal theoreticalmodel is that the system is not limited to the simple localized
and/or itinerant interactions. Some reports have argued thatthese materials are close to the boundary of the localized anditinerant ferromagnetism and thus can be taken as an excellent platform to study the crossover between them. Amongthem, Co3M’X could be a good example to clarify theseissues considering its unstable magnetic ground state andFig. 2. Phase diagram and field-dependence of characteristic parameters for Co3SnC: (a) TFL; (b) the color shows its change tendency ofd(M2)/d(H/M) vs. H which is proportional to the magnitude of magnetic
fluctuations.

the abnormal quantum behaviors are expected by applying
non-thermodynamic parameters.
In this study, weak ferromagnetism and NFL behaviors
were observed in Co3SnC via measurements of magnetic
properties, electrical transport and specific heat undermagnetic fields. Magnetic measurements suggest that it
undergoes a paramagnetic to itinerant ferromagnetic phasetransition with NFL behaviors including the power-law
temperature dependence of resistivity, the -T logT-dependent
upturn in specific heat, and the T 4/3-dependence of inverse
susceptibility. With increasing the magnetic fields, thetemperature coefficient of T 2-term in resistivity follows a(H-Hc)-0.5-divergence upon approaching the critical field Hc,
apart from the 1/(H-Hc)-dependence with the whole Fermi
surface under the singular scattering. The exponent n in thetemperature dependence of resistivity shows an increase
from n = 1.0 to 2.0 with increasing the field above Hc as
the evidence for the field-induced crossover from NFL to
FL behavior; the relative mass enhancement factor λ(H)/λ(0) reduce to nearly 60% at 9.0 T, indicating the gradualsuppression of magnetic fluctuations and electron-electron
scattering. The results indicate that Co3SnC is a good candidate for exploring itinerant quantum magnetic QCP.
Reference[1] B. S.Wang, Y. Uwatoko, J. -G. Cheng, and Y. P. Sun, Phys. Rev. B.
102, 085153 (2020)
Authors
B. S. Wanga, Y. Uwatoko, J.-G. Chenga and Y. P. Sunb,caInstitute of Physics, CAS
bHigh Magnetic Field Laboratory, CAS
cInstitute of Solid State Physics, CAS

Pattern Formation Induced by
Mechanochemical Coupling of ReactionDiffusion and Membrane Deformation
Noguchi Group

Fig. 1. (a) Δρ(T) vs. T 2 in a log-log scale; the arrow shows the crossover from T- to T 2-dependence. (b) Field-dependence of TFL; thearrow shows the critical field; field dependence of the parameters: (c)ρ0; (d) A and the exponential fitting results by the (H-Hc)-0.5 (the redsolid line); (e) n; the arrows in (c)-(e) indicate the change tendency andguides to the eye.

Shapes of biological membranes are dynamicallyregulated in living cells. Although membrane shape deformation by proteins at thermal equilibrium has been extensively studied, nonequilibrium dynamics have been muchless explored. Recently, chemical reaction propagation has
been experimentally observed in plasma membranes. Thus,
it is important to understand how the reaction-diffusion
dynamics are modified on deformable curved membranes.

ISSP Activity Report 202023

Structures and Dynamics of Hydrogen
Cluster Materials Li5MoH11 andLi6NbH11
Yamamuro Group

Fig. 1. Snapshots of vesicles with Turing patterns. Top panel: multispindle shapes. Bottom panel: budded vesicles. Color indicates theconcentration of the curvature-inducing protein.

Fig. 2. Sequential snapshots of a shape-oscillation vesicle. Budding
repeatedly occurs, accompanied by the propagation wave. Color
indicates the concentration of the curvature-inducing protein.

In this study, we present nonequilibrium pattern formation
on vesicles induced by mechanochemical feedback between
membrane deformation and chemical reactions, using
dynamically triangulated membrane simulations combinedwith a modified Brusselator model that is one of the simplest
reaction-diffusion models. Two proteins (curvature-inducing
and regulatory proteins) bind to the membrane. We foundthat membrane deformation changes stable patterns relative
to those that occur on a non-deformable curved surface, as
determined by linear stability analysis [1]. Temporal oscillation of the protein concentration can be changed into Turing
pattern (stable spatial patterns). Budding and multi-spindle
shapes are also induced by Turing patterns as shown in Fig. 1.
We also found that the propagating wave patterns change
into non-propagating patterns and spiral wave patterns dueto the mechanochemical effects [2]. Moreover, the wave
speed is positively or negatively correlated with the local
membrane curvature depending on the spontaneous curvature and bending rigidity. In addition, self-oscillation of thevesicle shape occurs, associated with the reaction-diffusion
waves of curvature-inducing proteins as shown in Fig. 2.
This agrees with the experimental observation of GUVs witha reconstituted Min system, which plays a key role in thecell division of Escherichia coli. Our results demonstrate theimportance of mechanochemical feedback in pattern formation on deforming membranes.

In the current research field of batteries, there are strong
demands for high-performance hydrogen storage materials
and ionic (H+, Li+, etc.) conductors. Recently, a series ofnovel materials Li5MoH11, Li5WH11, Li6NbH11, Li6TaH11
which contain unusual ninefold hydrogen-coordinatedclusters MH9 (M= Mo, W, Nb, Ta), whose structure is shownin the inset of Fig. 1, were synthesized by Orimo group in
Tohoku University [1]. These are of interest since they havehigh hydrogen density and possible high Li ionic conductivity. The dynamics of MH9 cluster may play importantroles in determining the physical properties of the materials.
We have performed the neutron powder diffraction (NPD)
and quasielastic neutron scattering (QENS) experiments of
Li5MoH11 and Li6NbH11.
Both samples were synthesized from solid-state reactions
under high pressure (ca. 5 GPa) and high temperature (ca.
700 K). Owing to these difficulties, the sample amounts of
Li5MoH11 and Li6NbH11 were 33 mg and 20 mg, respectively. The NPD and QENS measurements were conductedusing high intensity total diffractometer NOVA (BL21) andbiomolecular dynamic spectrometer DNA (BL02) in MLF,
J-PARC, respectively. The DNA data were collected withboth high and low energy resolution modes.
From the Rietveld analyses for the NPD data at room
temperature, we have checked the purity of the samples(Li5MoH11: 75 %, Li6NbH11: 57 %) and confirmed that theMH9 clusters are orientationally disordered. Figure 1 shows
the intermediate scattering functions of Li5MoH11. Similar
data were obtained also in Li6NbH11. Relaxation phenomena
were observed in a wide temperatures range between 150 K
and 300 K. It is quite unusual that the relaxation occurs ina wide time range over 3 orders of magnitude as shown inthe data at 300 K. Therefore, the data were fitted with not an
exponential but Kohlraush-Williams-Watts (KWW) function
I(Q,t) = a(Q) + b(Q)exp[–(t/τKWW)β],where Q is magnitude of a scattering vector, t is a time,
τKWW is a relaxation time, β is a non-exponential parameter,
and a(Q) and b(Q) are elastic and relaxation components,
respectively. The β parameter (0 < β < 1) reflects the width

References[1] N. Tamemoto and H. Noguchi, Sci. Rep. 10, 19582 (2020).
[2] N. Tamemoto and H. Noguchi, Soft Matter 17, 6589 (2021).
Authors
N. Tamemoto and H. Noguchi
Fig. 1. Intermediate scattering functions (Q = 1.53 Å-1) of Li5MoH11
measured at several temperatures between 150 K and 300 K. The solid
curves are fitting curves to the KWW functions with β = 0.38. The inset
shows the structure of the MH9 cluster.

24

ISSP Activity Report 2020

Fig. 2. Elastic Incoherent Structure Factor (EISF) obtained at 300 K for
Li5MoH11 and Li6NbH11. The solid curves are the fitting curves to theequation in the figure.

of the relaxation time distribution; no distribution at β = 1.
The fittings were satisfactory as shown by solid curves inFig. 1. The determined β were 0.38 for Li5MoH11 and 0.33
for Li6NbH11, indicating that the distribution of the relaxation time is very wide; cf. β is around 0.5 for glass forming
liquids. The activation energies obtained from the temperature dependence of the relaxation times were 15 kJmol–1 for
Li5MoH11 and 27 kJmol–1 for Li6NbH11. The magnitude of
the activation energy may depend on the ionic valence andsize (M-H distance) of the hydrogen cluster (MoH93-: 1.76
Å, NbH94-: 1.86 Å) .
Figure 2 shows the Q dependence of EISF (Elastic
Incoherent Scattering Factor) which is defined by EISF =a(Q)/[a(Q) + b(Q)]. As shown by the solid curves in Fig. 2,
EISF was well fitted by
EISF = 1 – A + Aj02(QR),
where j0(QR) is a 0th order sphere Bessel function with aradius R. This indicates that MH9 clusters are sphericallyrotating at 300 K, which is consistent with the structural
information. The rotational states of the MH9 clusters are
now investigated with a first-principle MD simulation
technique. We guess that the wide distribution of the relaxation time is associated with the positional disorder of Liions conducting in the crystals.
This work is financially supported by JSPS KAKENHI
No. JP 18H05513 and JP18H05518.
Reference[1] S. Takagi, Y. Iijima, T. Sato, H. Saitoh, K. Ikeda, T. Otomo,
K. Miwa, T. Ikeshoji, and S. Orimo, Sci. Rep. 7, 44253 (2017).
Authors
Y. Ohmasa, H. Akiba, S. Takagia, S. Orimoa, H. Saitob, T. Yamadac,
Y. Kawakitad, K. Ikedae, T. Otomoe and O. YamamuroaTohoku University
bQST
cCROSSdJAEA
eKEK

magnetic anisotropy, exchange randomness, and site dilution.
These thus play key roles in determining ground states infrustrated magnets. The equilateral triangular spin tube
antiferromagnet CsCrF4 [1,2] is a candidate for realizing
a ground state sensitive to small perturbations. Magnetic
Cr3+ ions having S = 3/2 form triangular spin tubes along
the crystallographic c axis, as illustrated in Fig. 1. The tubes
couple magnetically with one another and form the kagometriangular lattice in the ab plane [3]. Neutron powder diffraction study conducted by our group identified long-range
magnetic order below 2.8 K [4]. The magnetic moments
form a quasi-120° structure in the ab plane. It is unique thatthe 120° structure propagates antiferromagnetically along
the a and c axes with a magnetic propagation vector kmag =(1/2, 0, 1/2). Discussion of ground states in the kagometriangular lattice model suggested that the ground stateof CsCrF4 is close to the boundary on the magnetic phase
diagram in the kagome-triangular lattice model [4]. This
proposes that small perturbations may induce various typesof magnetic states in CsCrF4.
To introduce small perturbations into CsCrF4, we investigate chemical substitution effect. A magnetic Fe-substituted CsCr1-xFexF4 showed long-range magnetic order andenhancement of magnetic anisotropy [5]. In a nonmagnetic
Al-substituted CsCr1-xAlxF4, a weak feature of magnetic
ordering was observed [5]. Hereby, we report chemical
substitution effects on the magnetic ground state in CsCrF4[6].
We carried out neutron powder diffraction experiments on CsCr0.94Fe0.06F4 and CsCr0.98Al0.02F4. In bothcompounds, magnetic Bragg peaks indicative of thelong-range magnetic order are clearly observed, as
shown in Fig. 2. Most strikingly, the magnetic peaks in
CsCr0.94Fe0.06F4 are indexed by a propagation vector kmag =(0, 0, 1/2), which is different symmetry from kmag = (1/2, 0,1/2) in the parent CsCrF4. Magnetic structure analysis identifies that the 120° structure does not alternate in the ab plane,
whereas the intratube structure is the same as in CsCrF4.
Based on the Goodenough-Kanamori rule on superexchangeinteractions [7,8], it is predicted that ferromagnetic (antiferromagnetic) interactions between the Cr3+ ions via the Fions turn into antiferromagnetic (ferromagnetic) interactions
by the Fe3+ ion substitution, namely bond substitution. The
identified magnetic structure, thus, means that the bondsubstitution by the magnetic Fe3+ ion modifies the intertube
structure, but the intratube structure is intact.
On the contrary, in CsCr 0.98 Al 0.02 F 4 the observed
magnetic peaks are indexed by the same propagation vector
as in the parent CsCrF4. Analyzing the magnetic structure,

Magnetic Order in the Chemically
Substituted Frustrated
Antiferromagnet CsCrF4
Masuda GroupIn geometrically frustrated magnets, macroscopicallydegenerated ground-states in low-energy regime can besolved by small perturbations. This tells us that magnetic
states at low temperatures highlights small effects such as

Fig. 1. Schematic view of the crystal structure of CsCrF4. Red lines
a indicate the nearest-neighbor interaction along the c axis. Blue andyellow lines indicate the nearest- and second-neighbor interactions in
the ab plane.

ISSP Activity Report 202025

Zero-Energy Excitation in the ClassicalKagome Antiferromagnet NaBa2Mn3F11
Masuda Group

Fig. 2. Neutron diffraction profiles for (a) CsCr0.94Fe0.06F4 and (b)CsCr0.98Al0.02F4 at 1.5 K. Red circles and black curves show the experimental data and simulations, respectively. Vertical bars and triangles
indicate the position of the nuclear and magnetic Bragg peaks. Blue
curves show the difference between the data and simulations. Insets are
the determined magnetic structures.

we find that the spin configuration in CsCr0.98Al0.02 is notchanged drastically, even though a relative angle between
three spins in the 120° structure deviates more from 120°
than that in CsCrF4. Since substituting the Cr3+ ion with the
Al3+ ion creates a spin vacancy, trigonal symmetry of the120° structure is locally broken. However, the spin vacancy
only produces a small effect on the ground state of CsCrF4,
and therefore the quasi-120° structure is still realized
globally in CsCr0.98Al0.02F4.
In conclusion, we have studied magnetic orders inmagnetic Fe- and nonmagnetic Al-substituted CsCrF 4
through the neutron powder diffraction experiment. The
magnetic structure analysis reveals that the Fe-substituted
sample exhibits a 120° structure having kmag = (0, 0, 1/2),
and the Al-substituted one has a quasi-120° structure having
kmag = (1/2, 0, 1/2). Importantly, the magnetic structure in
CsCr0.94Fe0.06F4 differs from that in the parent CsCrF4. This
result concludes that the ground state in CsCrF4 is moresensitive to magnetic substitution rather than nonmagnetic
one on the Cr site.

Localization of spin-wave excitations, so-called zeroenergy mode, is one of the unique phenomena in geometrically frustrated magnets [1]. Largely degenerate groundstates allow a continuous rearrangement of spins with noenergy cost, generating a dispersionless mode in a spin-wave
excitation spectrum. The classical kagome antiferromagnet
is a prototypical model for the dispersionless mode in the
spin-wave excitations. It has an infinite degeneracy of 120°
structures in the ground state. This degeneracy allows acontinuous change of the spin arrangement with no energy
cost, as illustrated in Fig. 1(a).
In real kagome antiferromagnets, the macroscopic
degeneracy of the 120° structures is solved by some typesof magnetic anisotropy such as the Dzyaloshinskii-Moriya
interaction, single-ion anisotropy, and magnetic dipoledipole interaction. Then, the zero-energy mode becomesvisible as an excited state lifted by those anisotropies.
Although the zero-energy mode is one of the most characteristic phenomena in frustrated kagome antiferromagnets, it
has been reported only in KFe3(OH)6(SO4)2 [2]. Therefore,
further study in different materials is highly desirable.
Our target material, a classical kagome antiferromagnet
NaBa2Mn3F11 [3], is a promising candidate exhibiting thezero-energy mode. Mn2+ ions having spin S = 5/2 are responsible for magnetic properties. Neutron powder diffraction
demonstrated that antiferromagnetic order below TN = 2 K
forms the 120° structure selected by the magnetic dipolarinteraction [4], as shown in Fig. 1(a). According to theoretical studies, the zero-energy mode in the classical kagome
antiferromagnet with the magnetic dipolar interaction ispredicted to emerge at its lowest excited state [5], as shownin Fig. 1(b). Hereby, we report the zero-energy mode excitation in NaBa2Mn3F11 through a combination of inelastic
neutron scattering technique and linear spin-wave calculations [6].
Figures 2(a)-2(d) show a temperature variation of theinelastic neutron scattering spectra for a powder sample of
NaBa2Mn3F11. Magnetic diffuse scattering indicative ofshort-range correlation is observed at 30 K in the paramagnetic phase [Fig. 2(d)]. The diffuse scattering is suppressedwith decreasing temperature, and the spectrum splits into
two parts: strong intensity at 0.2 meV and weak intensity at
1.5 meV [Figs. 2(a)-2(c)]. In the magnetic ordered state at
1.5 K, spin-wave excitations having a magnetic anisotropy

References[1] H. Manaka et al., J. Phys. Soc. Jpn. 78, 093701 (2009).
[2] H. Manaka et al., J. Phys. Soc. Jpn. 80, 084714 (2011).
[3] K. Seki and K. Okunishi, Phys. Rev. B 91, 224403 (2015).
[4] M. Hagihala et al., npj Quantum Mater. 4, 13 (2019).
[5] H. Manaka et al., J. Phys. Soc. Jpn. 88, 114703 (2019).
[6] S. Hayashida, M. Hagihala, M. Avdeev, Y. Miura, H. Manaka, andT. Masuda, Phys. Rev. B 102, 174440 (2020).
[7] J. B. Goodenough, Phys. Rev. 100, 564 (1955).

[8] J. Kanamori, J. Phys. Chem. Solids 10, 87 (1959).

Authors
S. Hayashida, M. Hagihalaa, M. Avdeevb, Y. Miurac, H. Manakad, andT. MasudaaHigh Energy Accelerator Research Organization (KEK)
bAustralian Nuclear Science and Technology Organisation (ANSTO)
cSuzuka National College of Technology
dKagoshima University

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ISSP Activity Report 2020

Fig. 1. (a) Magnetic structure having k = 0 of NaBa2Mn3F11. The redarrows represent directions of spins. Dashed loops illustrate the zeroenergy mode as described in the text. Solid and dashed lines are thenearest-neighbor and second-neighbor paths, respectively. (b) Spinwave excitation having the nearest-neighbor exchange interaction J1
and magnetic dipole-dipole interaction JMDD = J1/100. The energy isnormalized by the magnitude of J1 and spin S.

[5] M. Maksymenko, R. Moessner, and K. Shtengel, Phys. Rev. B 96,
134411 (2017).

[6] S. Hayashida, H. Ishikawa, Y. Okamoto, T. Okubo, Z. Hiroi,
G. J. Nilsen, H. Mutka, and T. Masuda, Phys. Rev. B 101, 214409(2020).
Authors
S. Hayashida, H. Ishikawa, Y. Okamotoa, T. Okubob, Z. Hiroi, G. J.
Nilsenc, H. Mutkad, and T. Masuda
aNagoya University
bThe University of Tokyo
cISIS Neutron and Muon Source
dInstitute Laue LangevinFig. 2. Inelastic neutron scattering spectra of NaBa2Mn3F11 at (a)1.5 K, (b) 2.0 K, (c) 5 K and (d) 30 K. The incident neutron energy is
Ei = 3.1 meV. Calculated spin-wave spectra with (e) J1 = 0.28 meV,J2 = 0, and JMDD = 4.9 μeV and with (f) J1 = 0.27 meV, J2 = J1/10, andJMDD = 4.9 μeV. Solid and dashed curves in (e) and (f) are spin-wave
dispersions along [1 0 0] and [1 1 0] directions, respectively.

gap of 0.21 meV is observed, as shown in Fig. 2(a). Notably,
the excitation at 0.21 meV does not shift on varying wave
number Q. This suggests that the excitation is a dispersionless zero-energy mode lifted by the magnetic dipolar interaction.
To identify if the observed dispersionless mode indeed
originate from the magnetic dipolar interaction, we simulated
the spin-wave excitation spectra in the linear spin-wave
theory with considering the nearest-neighbor exchange interaction J1, the second neighbor exchange interaction J2, andthe nearest-neighbor magnetic dipolar interactions JMDD.
Figures 2(e) and 2(f) show powder-averaged spin-wave
spectra in J1-JMDD and J1-J2-JMDD models, respectively.
The calculated spectra semi-qualitatively reproduce thelow-energy strong excitation and high-energy weak excitation. Importantly, the calculated spectrum shows that thesecond-neighbor interaction J2 makes the zero-energy excitation dispersive (Fig. 2(f)), because further neighboring interactions suppress the continuous rearrangement of the spinswith no energy cost. This means that the observed dispersionless excitation at 0.2 meV indicates that J2 is negligible
in NaBa2Mn3F11.
The most remarkable finding is that the energy positionof the dispersionless mode is reproduced solely by themagnetic dipolar interaction. We note that the magnetic
dipolar interaction is ubiquitous in every real magnet eventhough the qualitative behavior of most kagome antiferromagnet can be explained by models including only thenearest-neighbor interaction. Therefore, we come to theconclusion that the observed dispersionless excitation in
NaBa2Mn3F11 is the ideal zero-energy mode in the realisticclassical kagome antiferromagnet.
In summary, we discovered a dispersionless zero energy
mode in the classical kagome antiferromagnet NaBa2Mn3F11
by the inelastic neutron scattering technique. The calculations based on the linear spin-wave theory reveal that theexcitation is described by the zero-energy mode lifted solely
by the magnetic dipole-dipole interactions. NaBa2Mn3F11 isa unique classical kagome antiferromagnet exhibiting a truly
dispersionless lifted zero-energy excitation.
References[1] A. B. Harris, C. Kallin, and A. J. Berlinsky, Phys. Rev. B 45, 2899
(1992).
[2] K. Matan et al., Phys. Rev. Lett. 96, 247201 (2006).
[3] H. Ishikawa et al., J. Phys. Soc. Jpn. 83, 043703 (2014).
[4] S. Hayashida et al., Phys. Rev. B 97, 054411 (2018).

Spin Density Wave Induced by Bound
Two-Magnon in S = 1/2 Frustrated
Chain Compound NaCuMoO4(OH)
Masuda Group
Frustrated magnetism has attracted great attention
because of nontrivial phases induced by the lift of macroscopic degeneracy. One of the interesting examples isfound in S = 1/2 one-dimensional spin system with nearestneighbor ferromagnetic and next-nearest-neighbor antiferromagnetic interactions [1]. A theory predicts that a boundtwo-magnon in high magnetic field induces exotic phasescharacterized by the spin-density-wave (SDW) and spin
multipole correlations in the frustrated ferromagnetic chain[2]. So far, these states have not been fully investigated byspectroscopic method because of high saturation field and/or difficulty of obtaining a large single crystal for existingmodel compounds. NaCuMoO4(OH) is an experimental
realization of the system having decent energy scale, saturation field of 26 T [3] which is available on the pulse magnet
for neutron scattering [4]. The synthesis of a single crystal

Fig. 1. Contour maps of neutron intensities in the (0kl) plane around thereciprocal lattice point (0, δ, 0) at 0.1 K under the magnetic field of (a)0 and (b) 4.5 T.

ISSP Activity Report 202027

aInstituteof Multidisciplinary Research for Advanced Materials,
Tohoku University
bResearch Center for Neutron Science and Technology, Comprehensive
Research Organization for Science and Society
cInstitute of Quantum Beam Science, Ibaraki University
dHigh Energy Accelerator Research Organization

Neutron Resonance Spin Echo
Measurements on the Magnetic
Skyrmion Lattice State in MnSi
Nakajima Group
Fig. 2. Value of δ as a function of field

was reported [5]. The NMR and heat capacity study suggest
a magnetic transition at Hc of 1.5 - 1.8 T [3]. For the full
identification of the magnetic states in zero and finite fields
neutron diffraction technique is indispensable. We hereby
report the magnetic structure analysis by using the time-offlight neutron diffractometer SENJU installed at J-PARC [6]on a single crystal NaCuMoO4(OH).
Figure 1(a) and 1(b) shows the contour maps of neutron
intensities in the (0kl) plane around the reciprocal lattice
point (0, δ, 0) at 0.1 K under the magnetic field of (a) 0 and(b) 4.5 T. We found that the peak indicated by the blue arrow
in Fig. 1(a) appear at (0, δ, 0) (δ ~ 0.48) below the transition
temperature of 0.6 K. It means that the peak comes fromthe magnetic origin. We further found peaks at (1 δ 1) and(0 δ 2). Magnetic structure analysis reveals that the properscrew structure with the ferromagnetic interchain couplingwith the propagation vector of (0, δ, 0) is realized at zero
field. Furthermore, we found that the magnetic peak at (0 δ 0)
was shifted to smaller k direction by applying a magnetic
field as shown in Fig. 1(b). This behavior suggests that thefield induces SDW state, in which the density correlation ofbound magnon is developed and its incommensurability isrepresented in terms of magnetization. In Fig. 2 the value ofδ is independent of the field below Hc, and it decreases withincreasing the field above Hc. The negative slope of the line
above Hc indicated by red line is consistent with the theoretical prediction for the SDW2 state [7]. The result demonstrates that the bound two-magnon leads to the SDW state inhigh magnetic field in the compound.

Magnetic skyrmions are vortex-like spin objects, which
can be regarded as topological defects composed of magnetic
moments in a magnetic material [1]. They were discovered in a small phase pocket in the vicinity of the criticaltemperature in chiral cubic magnets; previous small-angle
neutron scattering (SANS) [2] and Lorentz transmission
electron microscopy [3] measurements have revealed thatthe magnetic vortices are arranged to form a triangular lattice
on a plane perpendicular to an applied magnetic field. It isalso reported that the magnetic skyrmions can be driven byflowing an electric current [4]. These observations showthat magnetic skyrmions behave like particles once they areformed, while it remains unclear how the particle character
emerges upon the phase transition from the paramagnetic(PM) phase to the skyrmion lattice (SkL) phase.
In the present study, we investigate the PM-to-SkL transition in an archetypal skyrmion compound MnSi by means ofneutron spin echo (NSE) spectroscopy [5], which is suitedfor probing diffusive dynamics with the time scales of theorder of pico- to submicroseconds. Actually, spin fluctuations in MnSi were studied by means of polarimetric andferromagnetic NSE measurements [6], in which the Larmor
precession of polarized neutron spin is utilized for detecting
very small changes in neutron velocity upon quasi-elastic
scattering processes. However, neither the polarimetric nor
ferromagnetic NSE is suited for the SkL phase. The former
is limited to zero-field measurements. Although the latter
is applicable for in-field measurements, the non-collinear

References[1] S. Furukawa, M. Sato, and S. Onoda, Phys. Rev. Lett. 105, 257205(2010).
[2] T. Hikihara, L. Kecke, T. Momoi, and A. Furusaki, Phys. Rev. B 78,
144404 (2008).
[3] K. Nawa, Y. Okamoto, A. Matsuo, K. Kindo, Y. Kitahara,
S. Yoshida, S. Ikeda, S. Hara, T. Sakurai, S. Okubo, H. Ohta, and
Z. Hiroi, J. Phys. Soc. Jpn. 83, 103702 (2014).
[4] H. Nojiri, S. Yoshii, M. Yasui, K. Okada, M. Matsuda, J. -S. Jung,
T. Kimura, L. Santodonato, G. E. Granroth, K. A. Ross, J. P. Carlo, andB. D. Gaulin, Phys. Rev. Lett. 106, 237202 (2011).
[5] K. Nawa, Y. Okamoto, and Z. Hiroi, J. Phys.: Conf. Ser. 828,
012005 (2017).
[6] T. Ohhara, R. Kiyanagi, K. Oikawa, K. Kaneko, T. Kawasaki,
I. Tamura, A. Nakao, T. Hanashima, K.Munakata, T. Moyoshi,
T. Kuroda, H. Kimura, T. Sakakura, C.-H. Lee, M. Takahashi, K.-i.
Ohshima, T. Kiyotani, Y. Noda, and M. Arai, J. Appl. Cryst. 49, 120(2016).
[7]. M. Sato, T. Momoi, and A. Furusaki, Phys. Rev. B 79, 060406(R)(2009).
Authors
S. Asai, T. Oyama, K. Nawa a , A. Nakao b , K. Munakata b , K.
Kuwaharac, M. Hagihalad, S. Itohd, Z. Hiroi, and T. Masuda

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ISSP Activity Report 2020

Fig. 1. [(a)-(c)] Intensities of scattered neutrons projected on the Qx-Qy
plane measured at (a) 29.00, (b) 28.75, and (c) 27.50 K. on cooling.
[(d)-(f)] I(Q,t) profiles at each temperature.

spin arrangement of the SkL leads to mixing of spinflip and non-spin-flip scatterings, which depolarizes theneutron spins. We thus employ neutron resonance spinecho technique, specifically the modulation-of-intensitywith-zero-effort (MIEZE) method, in which the intermediate
spin correlation function I(Q,t) = S(Q,t)/S(Q,0), where S(Q,t)
is the temporally Fourier transformed dynamical structure
factor, is obtained not from the final neutron spin polarization after experienced the precessions mentioned above, butfrom beating patterns appearing in intensities of scatteredneutrons as a function of time. Since the MIEZE signals
are independent of the spin state of scattered neutrons, thistechnique is suited for measuring magnetic scattering in amagnetic field [7].
In Figs. 1(a)-1(c), we show typical SANS patterns
measured in a magnetic field of 0.21 T at the VIN ROSE
instrument in the materials and life science experiment
facility in J-PARC. We observed a ring-like diffuse scattering
pattern at 29 K, which is slightly above the critical temperature of 28.8 K (Fig. 1(a)). This indicates that a short-range
isotropic spin correlation is developing in the sample. As thetemperature is lowered, a Bragg peak emerges and coexists
with the ring-like diffuse scattering (Fig. 1(b)). Finally,
the diffuse scattering completely disappears and only the
Bragg peak remains at 27.5 K (Fig. 1(c)). We obtained I(Q,t)
profiles of the diffuse and Bragg scattering components asshown in Figs. 1(d)-1(f). The I(Q,t) profile at 29 K shows adecay, which is well described by a single-exponential decayfunction, revealing that the magnetic moments involved in
the diffuse scattering are fluctuating with a characteristic
time of 1 ns. By contrast, no temporal decay of the spincorrelation is found at 27.5 K (Fig. 1(f)). At the intermediate temperature where the Bragg and diffuse scattering
coexist, the I(Q,t) profile for the diffuse component stillshows a decay, while that for the Bragg peak is approaching
unity. This demonstrates that the isotropic spin fluctuations
with short-range spin correlations coexists with static SkL
domains near the boundary between the PM and SkL phases.
We also analyzed the I(Q,t) values as a function of azimuthal
angle, and concluded that an aggregation of the magnetic
skyrmions upon cooling leads to orientational disorderof the small SkL domains. This picture is consistent withthe particle-like character of the magnetic skyrmions. The
present results also show that the MIEZE-type NSE at VIN
ROSE is a powerful tool to study spin dynamics of magnetic
skyrmion systems, for which measuring spin dynamics in
low-Q regions in a magnetic field is essential.

Dynamics Fracture Behaviorsof Slide-Ring Gels
Mayumi Group
Tough hydrogels have been attracting a lot of interest
because of their potential application for biomaterials. To
improve the mechanical strength of hydrogels, various
chemical design of polymer networks have been proposedin the last two decades. One of the promising strategies totoughen gels is to introduce weak “sacrificial” crosslinks
into polymer networks [1]. The sacrificial bonds in toughpolymer gels are broken under deformation, and a part of
input mechanical energy is dissipated, which leads to thelarge fracture energy of the dissipative gels. A problem of thedissipative tough polymer gels is their low mechanical recoverability. When reversible bonds (e.g. hydrogen bonds, ionicbonds, etc.) are utilized as sacrificial bonds, the broken sacrificial bonds can be recovered, but it takes a relatively longtime (at least minutes) for complete recovery. For the applications as implant biomaterials such as artificial ligaments
and prosthetic joints, the mechanical instant recoverability
is needed because they should show constant mechanicalresponses under repeated deformations at a high frequency(e.g. 1 Hz).
Another tough polymer gel is slide-ring (SR) gel, in
which polymer chains are cross-linked by ring molecules(Fig.1) [2]. The sliding of the movable cross-links composedof rings homogenizes network structure and reduces stressconcentration in polymer networks, which results in highdeformability and softness of SR gels. We have been investigating toughening mechanism of SR gels composed ofpolyethylene glycol (PEG) and cyclodextrin (CD). For
conventional polymer gels cross-linked by fixed covalentbonds, the fracture energy is dominated by the strand length
between cross-links. In the case of SR gels, the sliding of
the cross-links enlarges the strand length and enhances thefracture toughness [3].
A unique feature of SR gels is their high mechanicalrecoverability. SR gels show no mechanical hysteresis under
repeated stretching. This suggests that the sliding movementof the cross-links is faster than breaking/reforming dynamics
of the reversible sacrificial cross-links. We systematicallyinvestigated the crack propagation behavior of SR gels at

References[1] N. Nagaosa and Y. Tokura, Nat. Nanotechnol. 8, 899 (2013).
[2] S. Muhlbauer et al., Science 323, 915 (2009).
[3] X. Z. Yu et al., Nature 465, 901 (2010).
[4] X. Z. Yu et al., Nat. Mater. 10, 106 (2011).
[5] T. Nakajima et al., Phys. Rev. Research 2, 043393 (2020).
[6] C. Pappas et al., Phys. Rev. Lett. 102, 197202 (2009).
[7] J. Kindervater et al., Phys. Rev. X 9, 041059 (2019).
Authors
T. Nakajima, T. Odaa, M. Hinoa, H. Endob, K. Ohishic, K. Kakuraic,d,A. Kikkawad, Y. Taguchid, Y. Tokurad,e, and T. Arimad,eaKyoto University.
bIMSS, High Energy Accelerator Research Organization.
cComprehensive Research Organization for Science and Society(CROSS).
dRIKEN Center for Emergent Matter Science (CEMS)
eThe University of Tokyo.
Fig. 1. Strain rate dependence of fracture energy of slide-ring and fixedcross-link gels.

ISSP Activity Report 202029

various strain rates to evaluate the sliding dynamics of the
movable cross-links [4]. At a certain strain rate and crack
propagation velocity, a drastic decrease of the fractureenergy was observed (Fig.1). For lower strain rates andcrack propagation velocities, the slide-ring cross-links haveenough time to slide along the polymer axes, which leads
to the enhanced fracture energy. At fast fracture, however,the sliding of cross-links cannot catch up with the network
deformation, and the fracture energy decreases to the samelevel as that of a fixed cross-link gel. From the critical crack
propagation velocity at the transition point, we estimatedthe time scale of the sliding dynamics to be micro-seconds.
This result is consistent with the diffusion coefficient of thesliding dynamics for CDs on PEG axis evaluated by a fullatomistic molecular dynamics simulation [5]. The fast sliding
motion of the cross-links is the origin of the high mechanicalrecoverability of SR gels under repeated deformations. SRgels exhibits high toughness and mechanical recoverability,
which are needed for implant biomaterials and elastomer
actuators for soft robots.
References[1] J. Y. Sun, X. Zhao, W. K. Illeperuma, O. Chaudhuri, K. H. Oh, D. J.
Mooney, J. J. Vlassak, and Z. Suo, Nature 489, 133 (2012).
[2] K. Ito, Polym. J. 39, 489 (2007).
[3] C. Liu, H. Kadono, K. Mayumi, K. Kato, H. Yokoyama, and K. Ito,
ACS Macro Lett. 6, 1409 (2017).
[4] C. Liu, H. Yokoyama, K. Mayumi, and K. Ito, Mech. Mater. 156,
103784 (2021).

[5] Y. Yasuda, Y. Hidaka, K. Mayumi, T. Yamada, K. Fujimoto, S.
Okazaki, H. Yokoyama, and K. Ito, J. Am. Chem. Soc. 141, 9655
(2019).
Authors
K. Mayumi. C. Liu. K. Itoa
aThe University of Tokyo

Charge-Fluctuation-MediatedSuperconductivity in Organic Complexes
Kindo, Kohama, and Tokunaga Groups
Electron pairing in unconventional superconductors
is driven by quantum fluctuations of electronic degrees
of freedom. For various unconventional superconductors, such as high-Tc cuprates and heavy-fermion systems,
their magnetic degrees of freedom often play an essential
role in the pairing mechanisms. Although charge degrees
of freedom can also mediate the Cooper-pair formation as
exotic pairings, experimental investigation of the chargefluctuation-mediated superconductivity has been insufficientfor detailed discussion because only a few materials showsuperconductivity with pure charge instability.
The βʹʹ-type organic charge-transfer complexes are
known as the candidates showing superconductivity
mediated by charge fluctuations. As shown in Fig. 1, theorganic molecules in the βʹʹ-type structure are uniformly
arranged. Since the valence of each molecule is +0.5, theenergy band of the electronic system is 1/4-filled. In thiscase, it is known that charge density exhibits inhomogeneous distribution in neighboring sites to reduce the intersite
Coulomb repulsion when electron correlations are strong.
Therefore, it is expected that this charge instability can
trigger a superconducting transition. In the present research,
we investigated the low-temperature electronic states ofsome βʹʹ-type materials, βʹʹ-(BEDT-TTF)4[(H3O)M(C2O4)3]
G (M=metal ions, G=guest molecules), using various

30

ISSP Activity Report 2020

Fig. 1. Molecular arrangement of organic molecules in the conducting
plane of the βʹʹ-type organic materials. The dashed lines show predominant transfer integrals between the molecules, forming the distorted
triangular lattice.

measurements and unveiled that the superconductivity in thiscompound is possibly mediated by charge fluctuations [1,2].
Figure 2 shows the high-magnetic-field quantum oscillations in magnetoresistance ΔR/R of βʹʹ-(BEDT-TTF)4[(H3O)
Ga(C2O4)3]PhNO2 at various temperatures. As shown by
the dashed envelopes, the amplitude of the oscillations ismodulated below 8 K. Detailed analyses with the LifshitsKosevich formula indicate that a transition occurs at 8 K,
leading to a split of the Fermi pocket and an enhancementof the effective mass. This transition is observed in ultrasonic properties whereas a low-temperature x-ray diffraction
measurement detects no significant anomaly at the temperature. Also, this transition has no magnetic field dependence.
As orbital degrees of freedom are quenched in the present
material, these results mean that a local lattice modulation
is induced by charge disproportionation. Just below thistransition temperature, this compound shows a transition
from the charge disproportionation to the superconducting
state at Tc~7 K. By replacement of the guest molecules, Tc is
reduced with a decline of the charge disproportionation. Thistendency implies that the charge fluctuations enhanced inthis compound govern the pairing of the superconductivity.
Besides, we performed heat capacity measurements todetermine the gap symmetry of the superconductivity. The
temperature, magnetic field, and field-angle dependence ofthe heat capacity indicates that the gap function is aniso-

Fig. 2. High-magnetic-field Shubnikov-de Haas oscillations ofβʹʹ-(BEDT-TTF)4[(H3O)Ga(C2O4)3]PhNO2 shown as ΔR/R. The dottedcurves on the data are fits to a two-component Lifshitz-Kosevich
formula. The dashed envelopes are guides for the eye to emphasize theamplitude change.

tropic and has no node on the Fermi surface, distinct fromthat of nodal d-wave symmetry arising from antiferromagnetic spin fluctuations observed in many unconventional
superconductors. This full-gapped nature suggests thatintersite repulsion, leading to the charge disproportionation,
works as an attractive force for the on-site Cooper pairing.
These results indicate that the present superconductivity
is formed by the charge fluctuations, and also, elucidate thedetailed features of the charge-fluctuation-mediated superconductivity.
References[1] S. Imajo, H. Akutsu, A. Akutsu-Sato, A. L. Morritt, L. Martin, and
Y. Nakazawa, Phys. Rev. Res. 1, 033184 (2019).

[2] S. Imajo, H. Akutsu, R. Kurihara, T. Yajima, Y. Kohama, M.
Tokunaga, K. Kindo, and Y. Nakazawa, Phys. Rev. Lett. 125,
177002 (2020).
Authors
S. Imajo, R. Kurihara, Y. Kohama, M. Tokunaga, and K. Kindo

Magnetic-Field-Induced Insulator-Metal
Transition in a Strongly Correlated
Insulator V1-xWxO2 at 500 T
Y. H. Matsuda and Takeyama Groups
The magnetic ground state of most of strongly correlated insulators is antiferromagnetic ordered state. Rolesof electron spins in realizing the insulating nature incorrelated systems is well understood in the single-band
Hubbard model; the magnetic energy determined by theexchange interaction J is much smaller than the Coulomb
energy U, and thus the electrical insulating nature is essentially insensitive to the magnetic state. In some of actual
materials, however, spin degree of freedom couples to crystal
lattice through the orbital degree of freedom and results in
nontrivial electronic states. Actually, in some of the strongly
correlated insulators, the ground state exhibits spin-singlet
state instead of the antiferromagnetic state. One of the moststudied correlated insulators with the singlet ground state isVO2 that undergoes a very sharp metal-insulator transition at
340 K. It has been debated for more than 60 years whether

Fig. 2. Schematic of the magnetic-field-induced insulator-metal (IM)transition. The left side shows that the potential barrier ∆ is lowered by
the Zeeman energy. The middle part schematically shows the collapseof the V–V dimers. The right part shows that applying a magnetic field
induces the dissociation of the dimer owing to the destabilization offormation of the bonding state Ψ of the molecular orbital, where φa andφb are the wave functions of the independent vanadium ions.

the electron correlation or the structural instability is moreessential driving force behind the transition.
In the present work, we have demonstrated that atungsten (W)-doped VO2 undergoes insulator-metal (IM)transition by applying a ultrahigh magnetic field of 500 T.
The field is produced by the electromagnetic flux compression 1000-T generator at the ISSP [1]. Figure 1 shows
the magneto-transmission of a V0.94W0.06O2 thin film at
1.977 μm [2]. A reduction of the transmission with magnetic
field when the field is greater than 100 T is a direct evidenceof the field induced metallization. One of the most probableexplanation for the field-induced IM transition is thatthe singlet-dimer state is suppressed by such a ultrahighmagnetic field, and which results in breaking of structural
V-V dimers as well. The molecular orbital occupied by twoelectrons with different spin directions is collapsed by forcedalignment of the two spins by the strong magnetic field of
500 T. The discovered spin-controlled IM transition suggeststhat the principal driving force to the insulating state at a lowtemperature is structural instability (formation of dimers)
rather than the electron correlation.
From another view point, the observed collapse of theV-V dimer is a similar phenomenon with the so-called thechemical catastrophe where a chemical bond is destroyed by
the Zeeman effect of a super-strong magnetic field greaterthan 106 T in the cosmic space [3]. An energy to collapse
the molecular orbital of the V-V dimer is scaled down by
3-4 orders of magnitude in a solid compared to a normal
molecule, which enable us to observe the chemical catastrophe in a solid. Figure 2 is a schematic of the field-induced
IM transition of W-doped VO2; the V-V dimers are dissociated by forced alignment of spins in a 500-T-field [2].
References[1] D. Nakamura, Akihiko Ikeda, H. Sawabe, Y. H. Matsuda, and
S. Takeyama, Rev. Sci. Instrum. 89, 095106 (2018).
[2] Y. H. Matsuda, D. Nakamura, A. Ikeda, S. Takeyama, Y. Suga,
H. Nakahara, and Y. Muraoka, Nat. Commun. 11 3591 (2020).
[3] M. Date, Australian Journal of Physics 48, 187 (1995).
Authors
Y. H. Matsuda, D. Nakamura, A. Ikeda, S. Takeyama, Y. Sugaa,
H. Nakaharaa, and Y. Muraokaa
aOkayama University

Fig. 1. Magnetic-field dependence of the optical transmission at 14
and 131 K. The transmission is plotted before the breaking point of thedestructive measurement. The shaded area indicates transmission lower
than that in the high-temperature metallic phase at 291 K. Inset: transmission change ∆Trans. as a function of the magnetic field. The dashedyellow line corresponds to ∆Trans.= 0. The red and purple arrow
indicate the field positions (B*) where ∆Trans. deviates from zero.

ISSP Activity Report 202031

Capacitive Detection of PhysicalQuantities in Pulsed Magnetic Fields
Tokunaga Group
Capacitance is known as a physical quantity that can bemeasured with high precision. We have developed a system
to precisely measure various physical quantities that change
in pulsed high magnetic fields using high-speed capacitancemeasurements [1].
First, we measured magnetostriction. We attach a pairof electrode plates on top of a sample and on the probewith keeping a small gap between them. We can detect
small change in the sample length through the change in thecapacitance of this setup. Using this method, we observed
magnetostriction in a Heusler alloy caused by a significant
magneto-volume effect [2]. The entropy change associatedwith this magneto-volume effect is regarded as the originof the giant inverse magneto-caloric effect observed in thisclass of materials [2,3]. Therefore, quantitative evaluationof the structural change can be essential to understand themagnetocaloric effect. In non-magnetic metals, magnetostriction can be used to detect the field-induced changes inband filling. Utilizing this technique, we achieved thermodynamic evidence of complete valley polarization in elemental
bismuth in the ultra-quantum limit state [4].
Capacitance measurements are also valuable for studying
multiferroic materials in which magnetic order correlateswith the dielectric property. If the change in magnetic
symmetry makes the crystal polar, we can detect it throughelectric polarization measurements. On the other hand, thechange is hardly discernible in the polarization if the spinorder does not break global inversion symmetry, such as in
the antiferroelectric state. Our capacitive measurement ofdielectric constants enabled to detect this kind of change
in symmetry at high magnetic fields. Figures 1 show themagnetization and dielectric constant of single crystals of

Fig. 1. Field dependence of (a) magnetization and (b) real part of therelative dielectric constant of Pb(TiO)Cu4(PO4)4. Successive transitions of cluster spin multipole order show up as multiple anomalies inboth physical quantities. Broad hump structure in magnetization around
26 T becomes prominent in the dielectric constant measured using thecapacitance method.

32

ISSP Activity Report 2020 
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Fig. 2. (a) Temperature variation of the capacitance of KTa1-xNbxO3 atzero magnetic field. (b) Magnetic field dependence of the capacitanceof KTa1-xNbxO3 at various temperatures. These two results indicate that
this material can be used as a capacitance thermometer insensitive toa magnetic field. (c) Magnetocaloric effects in Gd3Ga5O12 measured
using this capacitance thermometer. The inset shows schematics ofthe experimental setup. Our fast capacitance measurements enable toevaluate the magnetocaloric effect in this material in a pulsed magnetic
field.

Pb(TiO)Cu4(PO4)4. In contrast to the broad hump structurein the magnetization curve, dielectric constant shows a steep
change around 26 T with field hysteresis, which unambiguously indicates the change in magnetic symmetry at thisfield. This technique provides us with a novel tool to identify
the symmetry of magnetic phases emerging in high magnetic
fields.
By measuring dielectric constants of some non-magnetic
materials attached to a sample, we can evaluate the temperature change of a sample in magnetic fields: the so-called
magneto-caloric effect. We show an example of the measurements of the magneto-caloric effect using KTa1-xNbxO3(x = 0.02) as a capacitance thermometer, whose dielectric
constant has a temperature dependence as shown in Fig. 2(a).
We have previously developed a similar system to measure
the magneto-caloric effect using a thin-film resistancethermometer [5]. Although this method works effectively,
we have to calibrate the effect of magnetoresistance in thethermometer in advance at each temperature. On the other
hand, in the present capacitance thermometer, the capacitance is almost insensitive to the applied magnetic field, as
shown in Fig. 2(b). Using this thermometer, we can successfully measure the magneto-caloric effect of a magnetic
refrigerant Gd3Ga5O12 in a pulsed magnetic field (Fig. 2(c)).
Due to the non-magnetic nature of this thermometer, we
can also measure magnetocaloric effects simultaneouslywith magnetization. Such simultaneous measurements will
enable us to precisely determine the field-temperature phase
diagram of all the kind of magnetic materials and to discuss
more precise thermodynamics.
High precision measurements of capacitance improve
evaluation of the structure, symmetry, and entropy changesof the sample in pulsed magnetic fields. We expect that sucha technological innovation will open up new horizons inscience at high magnetic fields.

References[1] A. Miyake et al., Rev. Sci. Instrum. 91, 105103 (2020).
[2] T. Kihara et al., Phys. Rev. Mater. 5, 034416 (2021).
[3] T. Kihara et al., Phys. Rev. B 90, 214409 (2014).
[4] A. Iwasa et al., Sci. Rep. 9, 1672 (2019).
[5] T. Kihara et al., Rev. Sci. Instrum. 84, 074901 (2013).
Authors
A. Miyake, H. Mitamura, S. Kawachi, K. Kimuraa, T. Kimuraa,
T. Kiharab, M. Tachibanac, and M. Tokunaga
aThe University of Tokyo
bIMR, Tohoku University
cNational Institute for Material Science

Generation of Long Pulsed-Magnetic
Fields by Using Electric-DoubleLayer-Capacitors
Kohama GroupThe generation of pulsed magnetic fields was firstachieved by Pyotr L. Kapitsa approximately a century ago.
After his pioneering work, various power supplies were
tested for pulsed field generation. The most common power
supply for generating a pulsed magnetic field is the highvoltage and high-speed capacitor bank. However, such acapacitor bank has small value of capacitance (C = 1 – 100mF) and consequently restricts the available range of pulsed
duration up to ~0.1 s. The short pulse duration makes it difficult to put pulsed magnets to widespread use. Developing analternative power supply for generating pulsed high magnetic
fields is a demanding task in high magnetic field science.
In this study, we have reported a power supply basedon an electric-double-layer-capacitor (EDLC) which iscomposed of a series connection of 336 EDLC cells [1].
This system has a total capacitance of 10.7 F and can store
the energy of 3.87 MJ with the charged voltage of 860 V.
We find that the EDLC based power supply can generatehigh magnetic field up to 30 T with pulse duration of ~1 s.
Because of the low-cost and compact design of developed
EDLC based power supply, our system can be used in many
research and industrial fields.

Figure 1a and b show the pulsed field profiles generated
by our EDLC system. The arrows in Fig. 1 indicates thetiming of the closing an energy bifurcation thyristor, whichleads to an intentional reduction of the pulsed current. Dueto the large capacitance of EDLC bank, we could observe theoverdamped-like waveform for the setup A (Fig. 1a), which
has long pulse duration of ~ a few seconds. The peak valuesof the fields (currents) are 16.2 T and 23.9 T (5.91 kA and4.39 kA) for setup A and setup B (Fig.1b). The peaks of the
fields shift to later times with decreasing charged voltage
and indicate the strong influence of Joule heating on thepulsed field profile. Indeed, the calculated temperature of thepulsed magnet as shown in the right axes in Fig. 1(b) clearlyshows the large Joule heating effect. In the paper, we havealso reported how the pulsed field profile is influenced by
the capacitance, internal resistance, coil resistance, charged
voltage, coil inductance (not shown in this material) andfound that a suitable choice of the coil design could increase
accessible field strength up to 30 T and enhance pulsed
duration up to a few seconds.
In summary, we have developed the EDLC based power
supply which is cost-effective and compact discharge systemand can generate strong magnetic fields with long duration.
These systems can be used for many research fields such as
nuclear magnetic resonance and neutron scattering experiments, which typically require long accumulation times
to achieve good signal/noise ratios. In addition, we haveconfirmed that the generated pulsed field profile is welldescribed by a series-connected RLC model with taking
Joule heating into account. Finally, we note that connecting
several EDLC units in parallel and series can generate
a strong pulsed field of more than 30 T with a long time
duration of a few seconds.
Reference[1] K. Matsui, T. Kanda, Y. Ihara, K. Kindo, and Y. Kohama, Rev.
Sci. Instrum. 92, 024711 (2021).
Authors
K. Matsui, T. Kanda, Y. Iharaa, K. Kindo, and Y. Kohama
aHokkaido University

Observation of the Quantum Shift ofa Backward Rescattering Caustic by
Carrier-Envelope Phase Mapping
Itatani Group

Fig. 1. Pulsed Field Profile generated by the EDLC based power
supply. (a) Magnetic field profile generated with low-impedance pulsed
magnet. (b) Magnetic field profile generated with high-impedance
pulsed magnet. The right axis represents the calculated magnet temperature.

When an atom or molecule is exposed to linearly-polarized intense optical fields, an electron wavepacket is freed
from the atom, accelerated by the field, then collides withthe parent ion. This process, recollision, is the fundamentalof attosecond science. When recollision becomes recombination, the ionizing atoms emits a high energy photon(high harmonic generation). Meanwhile, it is well known
that a part of recolliding events becomes elastic scattering.
Especially the backward rescattering leads to the generation of high-energy electrons with a drift kinetic energy up
to 10Up, where Up is the ponderomotive potential. Recent
theoretical progress of the adiabatic theory of rescattering
gives an analytical formula for photoelectron momentum
distributions (PEMDs) near the 10Up cutoff which is scaledby the cutoff energy. Although the theoretical treatment of
rescattering is fully quantum mechanical and straightfor-

ISSP Activity Report 202033

ward, the experimental verification has been very difficult
because it is not easy to realize systematic experiments to
extract well-defined single rescattering events with multicycle laser pulses.
We hereby used intense few-cycle pulses in infrared
to measure the backward PEMDs with controlled carrierenvelope phases (CEPs). The extremely short duration of
the optical pulse allows to extract the single rescattering
events of the three highest energies, and the CEP control
enables to systematically change the maximum intensity inthe focused volume without affecting the other parameterssuch as spatial distribution of the field intensity. Figure 1
shows the CEP dependent PEMDs with a pulse energy of
210 μJ. Dashed and solid curves represent classical andquantum caustics. The CEP dependent PEMD assures the
one-to-one correspondence of the electron momentum at the
instance of rescattering and the CEP, which we call the CEP
mapping method. From this CEP mapping, we successfully
determined the field intensity and the pulse duration to be
2.6 × 1013 W/cm2 and 13.8 fs, respectively. Using theseparameters, the observed PEMDs are deconvolved to three
PEMDs that correspond to the rescattering events of different
intensities within a single optical pulse, as shown in Fig. 2.
The laser parameters, elastic scattering differential crosssections, and tunnel ionization rate of the taget are encodedin PDMDs. Such information can be extracted from theFig. 3. Extracted DCSs of Xe+ for estimated field amplitudes of
0.02722 (circle) and 0.03028 a. u. (closed triangle) as a function of therescattering momentum. The black curve represents calculated DCSs.

observed PEMDs based on the adiabatic theory. Based on
the quantitative agreement of the observed cutoff behaviorof PEMDs with the adiabatic theory, elastic scattering crosssection as a function of scattering momenta is successfully
determined as shown in Fig. 3.
In summary, we experimentally and theoretically
explored the rescattering PEMDs of a Xe atoms near thecutoff using CEP-stable few-cycle pulses [1]. The observed
PEMDs are well reproduced by the analytical formula ofthe adiabatic theory including the quantum shift of thecaustics. The results not only demonstrate the fact that the
CEP mapping method with the adiabatic rescattering theory
allows to extract well-defined elastic scattering, but also
opens an opportunity to use rescattering to observe novel
electron states such as vortex electrons [2].
References[1] T. Mizuno, N. Ishii, T. Kanai, P. Rosenberger, D. Zietlow, M. F.
Kling, O. Tolstikhin, T. Morishita, and J. Itatani, Phs. Rev. A 103,
043121 (2021).
[2] O. I. Tolstikhin and T. Morishita, Phys. Rev. A 99, 063415 (2019).

Fig. 1. Measured PEMDs as a function of CEP at a pulse energy of 190
μJ. Dashed and solid curves represent classical and quantum causitcs at
the intensity of 2.6 × 1013 W/cm2 and pulse duration of 13.8 fs.

Authors
T. Mizno, N. Ishii, T. Kanai, P. Rosenbergera, D. Zietlowa, M. F.
Klinga, b, O. Tolstikhinc, T. Morishitad, and J. Itatani
aLudwig-Maximillians-Universität München
bMax Planck Institute for Quantum Optics
cMoscow Institute of Physics and Technology
dThe University of Electro-Communications

Observation of Small Fermi Pockets in a
Copper Oxide High-Tc Superconductor
Kondo and Kohama Groups

Fig. 2. Measured PEMDs at CEP=0 and 0.4π at an estimated intensityof 2.6 × 1013 W/cm2 and a pulse duration of 13.8 fs. The thick red, thinblue, and thin green curves show calculated PEMDs corresponding toa half cycle of the electric field as depicted in the top figures. Thin andthick arrows indicate the classical and quantum caustics, respectively.

34

ISSP Activity Report 2020

High-temperature superconductivity is one of the mostsignificant discoveries in physics in the latter half of the20th century. Since the discovery of high-temperature superconductivity in 1986, their physical properties have been
intensively studied from various aspects. However, a unified
understanding of the mechanism for high-temperature superconductivity has not yet been established. The most basic
and important issue relevant to it is the relationship between
antiferromagnetic state forming Mott insulator and hightemperature superconductivity induced by carrier doping.

around the Fermi pockets, providing spectroscopic evidence
for the coexistence between antiferromagnetism and superconductivity in the same CuO2 sheet.
Our results are expected to facilitate the experimental andtheoretical research pursuing the mechanism of high-temperature superconductivity in cuprates. In particular, these willcontribute to solving the long-standing puzzle of the Fermi
arc phenomena and urge the need for revisiting the Mott
physics leading to the electron pairing in cuprates, which, up
to now, has been based mainly on the research on single- anddouble-layered compounds with inhomogeneous electronic
states.
References[1] S. Kunisada et al., Science 369, 833 (2020).

Fig. 1. (a) Crystal structure of single-layer cuprate superconductor(Bi2Sr2CuO6+δ is presented as an example). (b) Crystal structure of
5-layer cuprate superconductor, Ba2Ca4Cu5O10(F,O)2, we have focusedon in this study. (c) Fermi surfaces of Ba2Ca4Cu5O10(F,O)2 observedby laser-ARPES. (d) Quantum oscillation measurements for the same
compound.

In this study, we focused on a multi-layered cuprate high-Tc
superconductor (Fig. 1b) with a clean superconducting
plane (CuO2 layer) that is structurally flat and has charges
uniformly distributed along the plane. Here we perform
laser-based angle-resolved photoelectron spectroscopy(ARPES) with high energy resolution and quantum oscillation measurement at a strong magnetic field and present thefirst observation of small Fermi pockets formed in the close
vicinity of the Mott insulating phase. Furthermore, we findthe coexistence of antiferromagnetism and high-temperature
superconductivity in the same CuO2 sheet.
Superconductivity of cuprate superconductors occurs
via carrier doping to the CuO2 layers from the dopant layers
sandwiching them. The main target in previous studies wasfor structurally simple compounds with one or two CuO2
planes per unit cell (Fig. 1a). While these have the advantage to be easy in synthesizing, it has been pointed outthat the direct contact of the dopant layers to the superconducting layers causes structural distortion and non-uniform
charge distribution along the CuO2 plane, leading to disordered electronic states. To solve this problem, we selecteda multilayer system with five CuO2 planes per unit cell,
Ba2Ca4Cu5O10(F,O)2, for a study (Fig. 1b).
This compound has inner CuO2 planes that can avoid
direct contact to the dopant layers. These inner CuO2 planes
are structurally flat, and at the same time, protected by theouter CuO2 plane from the dopant layers causing the effectsof spatially non-uniform carrier doping and defects. Therefore, this multilayered system gives us an excellent opportunity to study an extremely clean superconducting layer that isclose to the ideal case treated in theoretical studies.
By performing high-resolution laser-ARPES andquantum oscillation measurements for the clean superconducting layers (Fig. 1c), we successfully observed small
Fermi pockets, which had been theoretically expected yet
elusive for more than 30 years of research for cuprates. The
spectra of Fermi pockets are much sharper in energy thanthat of Fermi arc, indicating that the pockets we observed
indeed come from the inner CuO2 plane protected from therandom potential induced by dopant layers. Moreover, wehave confirmed the existence of d-wave superconducting gap

Authors
S. Kunisada, S. Isonoa, Y. Kohama, S. Sakaib, C. Bareille, S. Sakuragi,
R. Noguchi, K. Kurokawa, K. Kuroda, Y. Ishida, S. Adachi c, R.
Sekinea, T.K. Kimd, C. Cachod, S. Shin, T. Tohyamaa, K. Tokiwaa andT. Kondo
aTokyo University of SciencebRIKEN Center for Emergent Matter Science (CEMS)
cMANA, National Institute of Material SciencedDiamond Light Source

Higher-Order Topological Insulator
Built from van der Waals Stackingof Bismuth-Halide Chains
Kondo GroupA Z2 weak topological insulator (WTI) state can beformed in a material with stacked quantum spin Hall insulators (QSHIs), where the topological surface states appear
only on side surfaces by accumulating helical edge states of
QSHIs [1]. Similarly, a higher-order topological insulator(HOTI) is expected to be built from stacking QSHIs so that
1D helical hinge states are formed by accumulating the edge
states. Utilizing symmetry-based indicators, the HOTI phases
have been predicted in materials which were previously
regarded as trivial insulators [2]. So far, only bulk bismuth
has been experimentally shown to be in the HOTI phase[3]. However, bismuth is a semimetal, whose bulk cannot
become insulating by carrier doping. Therefore, the experimental realization of a HOTI in an insulating material has
been awaited.
In this study, we propose that quasi-1D bismuth halides
offer a novel platform to investigate various topologicalphases that depend on the stacking sequences of Bi4X4(where X is I or Br) chains. A WTI state has been observedin β-Bi4I4 with single-layered chains per unit cell, while anormal insulator state has been obtained in α-Bi4I4, whichtakes a double-layered structure (Fig. 1) [1]. Although Bi4Br4
also consists of double layers, each layer is alternately
rotated by 180 degrees, leading to the prediction of a HOTI
phase due to the slight shift of the inversion center in the unit
cell. Based on this, we present angle-resolved photoemission spectroscopy (ARPES) results showing that Bi4Br4 is a
HOTI for the first time as a real 3D material [4].
An advantage of employing quasi-1D materials lies in
the formation of a number of steps aligned parallel to thechain direction on the cleavage surface. Figure 2(a) shows aphotographic image of ribbon-like Bi4Br4 crystals used in theexperiments. While the cleaved surface is relatively rough,as shown in Fig. 2(b), Grazing incidence Small-angle X-ray

ISSP Activity Report 202035

energy of the roughly estimated Dirac point (middle panel ofFig. 2(c)). The in-gap states are found to form a quasi-1D
intensity distribution in the Fermi surface mapping. Theseobservations are in stark contrast to the case for a trivial
insulator α-Bi4I4, where no indication for in-gap states isfound in ARPES image or the corresponding momentum
distribution curve (Fig. 2(d)). Therefore, the 1D Dirac state
in Bi4Br4 is attributed to the topological states, demonstrating the realization of the HOTI phase in Bi4Br4. Our
comprehensive study on bismuth halides also shows that thedifference in chain stacking is the origin of the different band
topologies among Bi4Br4, α-Bi4I4, and β-Bi4I4. The quasi-1D
HOTI state we have verified in this study will bring the newcapability of designing and engineering functional materials
utilizing the stacking-dependence of the topological phases.

Fig. 1. Schematic of the various topological properties realized bystacking quasi-1D bismuth halide chains.

Scattering (GISAXS) measurements has revealed that thesurface consists only of the top (001) surface and the side(100) surface. This result indicates that a huge number ofhinges between the two planes are exposed on the cleaved
surface, offering the possibility of detecting the topologicalhinge states by ARPES measurements.
Figure 2(c) presents the ARPES result of Bi4Br4 taken
by high-resolution laser-ARPES. The strong photoemission signals observed at ~0.6 eV below the Fermi level (EF)are assigned to the bulk valence band. Interestingly, we
also observed metallic in-gap states with Dirac-like dispersions around EF (bottom panel of Fig. 2(c)). This is further
confirmed in the momentum distribution curves at theReferences[1] R. Noguchi et al., Nature 566, 518 (2019).
[2] F. Tang, H. C. Po, A. Vishwanath, and X. Wan, Nat. Phys. 15, 470(2019).
[3] F. Schindler et al., Nat. Phys. 14, 918 (2018).
[4] R. Noguchi et al., Nat. Mater. (2021). https://doi.org/10.1038/
s41563-020-00871-7
Authors
R. Noguchi, M. Kobayashia, Z. Jiangb, K. Kuroda, T. Takahashia, Z.
Xub, D. Leeb, M. Hirayamac, M. Ochid, T. Shirasawad, P. Zhang, C.
Lin, C. Bareille, S. Sakuragi, H. Tanaka, S. Kunisada, K. Kurokawa,
K. Yaji, A. Harasawa, V. Kandybaf, A. Giampietrif, A. Barinovf, T. K.
Kimg, C. Cachog, M. Hashimotoh, D. Luh, S. Shin, R. Aritac,i, K. Laib,
T. Sasagawaa, and T. Kondo
aTokyo Institute of Technology
bUniversity of Texas at Austin
cRIKEN Center for Emergent Matter Science (CEMS)
dOsaka University
eNational Metrology Institute of Japan, National Institute of Advanced
Industrial Scienceand Technology
fElettra Synchrotron Trieste
gDiamond Light Source
hStanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory
iThe University of Tokyo

Observation and Control of the Weak
Topological Insulator State in ZrTe5
Kondo Group

Fig. 2 (a) Photograph of Bi4Br4 crystals. (b) Laser microscope imageof a cleaved surface. (c) ARPES band map of Bi4Br4 around the Γpoint (bottom) and the corresponding momentum distribution curve at
E-EF=-60 meV (middle). Schematic of the cleaved surface of Bi4Br4
with topological hinge states (top). (d) Similar results to (c), but for
α-Bi4I4.

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ISSP Activity Report 2020

A quantum spin Hall insulator hosts topological statesat the one-dimensional (1D) edge, along which backscattering by nonmagnetic impurities is strictly prohibited. Its
3D analogue, a weak topological insulator (WTI), possesses
similar quasi-1D topological states confined at side surfaces.
The enhanced confinement could provide a route for dissipationless current and better advantages for applications
relative to strong topological insulators (STIs) [1]. However,
the topological side surface is usually not cleavable and isthus hard to observe.
ZrTe5 was predicted to be a WTI candidate at the earlystage of exploring WTIs [2]. This compound exhibits very
high mobility, thus it has been regarded as a promising
platform for devices. Nonetheless, the topological propertyof ZrTe5 has not been experimentally identified to datebecause the observation of band structure on the side surface
has not been successful so far. All the previous surfacesensitive studies were carried out on the top surface, which
only confirms the lack of surface states; whether or not thismaterial is topological, therefore, has not been determinedbeyond speculation. Another difficulty for this study is that

ZrTe5 is in proximity to multiple topological phases, whereas
this feature could bring an attractive functionality of controlling bulk topology by fine-tuning a physical parameter.
In this study, we visualize the topological states of ZrTe5
by spin- and angle- resolved photoemission spectroscopy(ARPES): a quasi-1D band with spin-momentum locking
was revealed on the side surface. We further demonstrate thatthe bulk band gap is controlled by external strain, realizing
a more stable WTI state or an ideal Dirac semimetal (DS)state. The highly directional spin-current and the tunableband gap in ZrTe5 will provide an excellent platform forapplications [3].
ZrTe5 has a quasi-1D crystal structure. In principle, it
should be possible to cleave both top and side surfaces.
Along the lattice b direction, the layers are stacked by van
der Waals forces, and a clean surface can be easily obtainedby cleavage. Instead, Te–Te bonding exists between adjacentlayers along the lattice c direction, causing much moredifficulty in cleaving the side a-b surface. In this work, we
successfully cleaved the side a-b surface with a top post andsilver epoxy. The problem, however, is that the cleaved areas
are very small and inhomogeneous. With a synchrotronbased nano-ARPES (spot size < 1 μm), we took real spacemaps of photoemission intensity on several samples, as
sketched in Fig. 1(a). It is found that the typical dimension ofcleaved areas is around or smaller than 50 μm (Fig. 1(b)).
In k-space, a quasi-1D Fermi surface is exhibited byplotting ARPES intensities about the Fermi level, whichshows no dispersion along lattice b direction (Fig. 1(d)).
Along the lattice a direction, the band shows a hole-like
dispersion (Fig. 1(c)). With different photon energies (74eV and 7 eV), we did not find any change for this band,
which support its surface origin (no kz dispersion). For the
topological surface state, spin polarization is expected. To
confirm this, we have used spin-resolved ARPES to check

Fig. 1. ARPES measurements with nano-ARPES on the side surface.
(a) Schematic of the experimental setup. (b) Real space intensity
mapping with nano- ARPES. The signal is clear only within an area of
50 μm. (c) Band structure along the chain direction. The hole-like bandis one of the topological surface band. (d) Fermi surface of the surface
band in (c). Its spin-texture is indicated by red and blue arrows.

Fig. 2. ARPES measurements with laser-ARPES on the top surface
with strain. (a) Schematic of the experimental setup. (b) Topological phase diagram of ZrTe5 with different strain along chain
direction. (c) Band structure observed from the top surface withdifferent strain.

the spin texture of the surface band. A clear spin polarization
is detected along the lattice b direction. In contrast, the spinpolarization is almost zero along the lattice a and c directions, as indicated by red and blue arrows in Fig. 1(d). Therefore, the band is spin-polarized along the b-direction with
spin-momentum locking, consistent with the spin-texture of
WTIs with a quasi-1D dispersion.
On the top surface (Fig. 2(a)), we observe a small bulk
gap (~ 28 meV), as shown in the middle panel of Fig. 2(c),which indicates that the WTI state in ZrTe5 is protected only
marginally from external perturbations. Theoretical calculations proposed that the band gap changes with externalstrain. Thus, we further designed a strain device and checked
the strain dependence of band structure in ZrTe5. Both
compressive strain and tensile strain are applied along thechain direction of ZrTe5. The band structures under different
strains are displayed in Fig. 2(c). The sample with no strainshows a band gap of ~28 meV. With tensile strain, the band
gap is enlarged to ~ 51 meV. In contrast, we could reducethe band gap by compressive strain, even realizing a Diracsemimetal state with no band gap. In Fig. 2(b), we summarized the topological phases of ZrTe5 with different strains.
Our results demonstrate the existence and tunabilityof the WTI states in ZrTe5. The spin-momentum locked
quasi-1D surface states and the strain-tunable bulk gap make
ZrTe5 an ideal platform for 2D devices and spin engineering.
Reference[1] R. Noguchi et al., Nature, 566, 518 (2019).
[2] H. Weng, X. Dai, and Z. Fang, Phys. Rev. X 4, 011002 (2014).
[3] P. Zhang et al., Nature Commun. 12, 406 (2021).

ISSP Activity Report 202037

Authors
P. Zhang, R. Noguchi, K. Kuroda, C. Lin, K. Kawaguchi, K. Yajia,
A. Harasawa, M. Lippmaa, S. Nieb, H. Wengc, V. Kandybad, A.
Giampietrid, A. Barinovd, Q. Lie, G.D. Gue, S. Shinf, and T. Kondo
aNational Institute for Materials SciencebStanford University
cChinese Academy of Sciences
dElettra - Sincrotrone Trieste
eBrookhaven National Laboratory
fThe University of Tokyo

Bose-Einstein Condensation
Superconductivity Induced by
Disappearance of the Nematic State
Okazaki GroupThe crossover from the superconductivity of the BardeenCooper-Schrieffer (BCS) regime to the Bose-Einsteincondensation (BEC) regime holds a key to understanding
the nature of pairing and condensation of fermions. Figure
1a shows a canonical phase diagram of BCS-BEC crossover.
Weak-coupling Bardeen-Cooper-Schrieffer (BCS) pairingand strong-coupling Bose-Einstein condensation (BEC)
are connected continuously through the BCS-BEC crossover regime. This regime holds a key to understanding thenature of pairing and condensation of particles. In the weakcoupling BCS regime, the dispersion of the Bogoliubov
quasiparticle (BQP) shows the characteristic back-bending
near kF, which is downward convex around k = 0 in the caseof a hole band below EF, as shown in Fig. 1b, where k is the
crystal momentum of electrons. On the other hand, in thestrong-coupling BEC regime, the dispersion with a minimum

Fig. 1. Phase diagram of the BCS-BEC crossover and BQP band dispersions. (a) Canonical phase diagram of BCS-BEC crossover. Tc andT* are condensation and pairing temperatures, respectively. As thepairing strength increases, T* and Tc become separate, and a pseudogap
is expected at T* > T > Tc. (b)-(d) BQP band dispersions in the BCS
regime, the crossover regime, and the BEC regime, respectively. The
dispersion is downward convex around k = 0 for the BCS regime,
becomes flat in the crossover regime, and then becomes upward convex
in the BEC regime, because of the chemical potential shift. Δ/εF isreferred to as a measure of the pairing strength for single-band superconductors.

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ISSP Activity Report 2020

Fig. 2. ARPES intensity plots of FeSe1−xSx along the Γ−M direction measured in the SC state. (a)-(e) ARPES intensity plots of x = 0,0.04, 0.13, 0.16, and 0.21 along the Γ-M direction taken at 2 K withs-polarized incident light. (f)-(j) Enlarged plots of (a)-(e) around EF.
The red markers represent the BQP band dispersions determined fromthe energy distribution curves.

gap at k = 0 is expected, which is upward convex aroundk = 0 in the case of a hole band below EF, as shown in
Fig. 1d. In the crossover regime, the BQP dispersion isintermediate between two extreme cases and the flat dispersion appears. In the crossover regime, a pseudogap is also
expected to open at a temperature higher than Tc as a resultof preformed bosonic pairs, as shown in Fig. 1a. Here, we
provide systematic evidence for the BCS-BEC crossover iniron-based superconductors FeSe1−xSx from laser-excitedangle-resolved photoemission spectroscopy, and show thatthe system enters the BEC regime with x = 0.21, where
the nematic state that breaks the orbital degeneracy is fully
suppressed.
Figures 2a-2e show the ARPES intensity plots of
FeSe1-xSex (where x = 0, 0.04, 0.13, 0.16, and 0.21) takenin the SC state with s-polarized light. To see the BQP band
dispersions more clearly, we show enlarged plots around EF
in Figs. 2f-2j. We have plotted the BQP band dispersions
determined from the energy distribution curves (EDCs) ofeach composition as indicated by red markers. From theseplots, we can see that the BQP band dispersion, which is
downward convex at x = 0, systematically changes andbecomes flat at x = 0.16; it then, eventually, becomes upward
convex at x = 0.21. This systematic change is consistent
with that expected from the chemical potential shift in the
BCS-BEC crossover, as shown in Figs. 1b-1d, and thus
strongly suggests that FeSe1−xSx is tuned from the BCS-BEC
crossover regime (x = 0) to the BEC regime (x = 0.21). To
obtain complete evidence for the BCS-BEC crossover, wehave investigated the existence of a pseudogap, and foundthat the pseudogap appears at T* ~15 K for x = 0.21. Thus,
from the BQP band and the existence of the pseudogap, we
have concluded that BEC superconductivity is realized for
x = 0.21. We have also investigated the substitution dependence of ∆/εF, where where ∆ and εF are the SC gap and the
Fermi energy, respectively, and from the obtained substitution dependence, we have also concluded that the multiband
nature is important for the realization of BEC superconductivity in this system, and that it is induced by the suppressionof electronic nematicity.
Reference[1] T. Hashimoto, Y. Ota, A. Tsuzuki, T. Nagashima, A. Fukushima,
S. Kasahara, Y. Matsuda, K. Matsuura, Y. Mizukami, T. Shibauchi,
S. Shin, and K. Okazaki, Sci. Adv. 6, eabb9052 (2020).
Authors
T. Hashimoto, Y. Ota, A. Tsuzuki, T. Nagashima, A. Fukushima,
S. Kasaharaa, Y. Matsudaa, K. Matsuurab, Y. Mizukamib, T. Shibauchib,S. Shinb, and K. Okazaki
aKyoto University
bThe University of Tokyo

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