Joint Research Highlights
Quadrupole Orders in Pr-BasedCompounds with fcc Lattice Structure
H. Tsunetsugu, T. Ishitobi, and K. Hattori
Recently, physical properties of electric quadrupoles in
PrMgNi4 were studied experimentally and their antiferroordering has been discussed [1]. Praseodym ions constitute afrustrated structure of fcc lattice, and little had been known
for its ordered quadrupolar pattern. Studying this issue
theoretically, we have identified various ordered patterns anddetermined the phase diagram [2].
A non-Kramers doublet is the ground state of Pr3+ ion
in a crystal field with cubic symmetry, and two components of electric quadrupole are active therein: O20 and O22
with symmetry z2 and x2–y2, respectively. Despite the same
number of degrees of freedom, ordering of those quadrupoles may show different features from those in easyplane magnets due to two reasons. First, while magnetic
moments are axial vectors, electric quadrupoles are secondrank tensors, and therefore their effective interactions havea different form. We have determined its minimal form
with two parameters J and K corresponding to isotropic andanisotropic couplings. The second reason is the presence ofa third order Z3 anisotropy which time-reversal symmetry
prohibits for magnetic moments. This is induced by a hybridization of a singlet excited state, and its excitation energy Δ isanother control parameter of this system.
We have employed a mean field approach with a foursite unit and determined the phase diagram at zero andfinite temperatures. In the limit of Δ → ∞, the Z3 anisotropy
vanishes and three ordered phases appear at low temperatures depending on the ratio of J and K: O20 ferro, O20
antiferro, and O22 antiferro. For Δ < ∞, the Z3 anisotropy
takes effects and this leads to a phase diagram with richFig. 2. Structure of electric quadrupoles in the zyox phase. Quadrupoles
are not ordered on the corner sites.

structure. Related to induced canting of O22 components,
the most important result is the instability of the two-site
antiferro orders to more complex four-site structures. It isalso interesting that two of the new phases (zyox and zoxy)
exhibit a partial order in which one sublattice remains disordered and quadrupole is polarized as x2, y2, and z2 in eachof the other three sublattices. They are a triple-q order inwhich three components with different polarizations andwave vectors are coupled by the Z3 anisotropy, and we haveconfirmed their stability by a phenomenological Landau
theory. Thus, we have established the basic picture of thephase structure for quadruple orders, and hope that thisencourages further experimental studies as well as moreadvanced theoretical analysis on this issue.
References[1] Y. Kusanose et al., J. Phys. Soc. Jpn. 88, 083703 (2019);
Y. Kusanose et al., JPS Conf. Proc. 30, 011160 (2020).
[2] H. Tsunetsugu, T. Ishitobi, and K. Hattori, J. Phys. Soc. Jpn. 90,
043701 (2021).
Authors
H. Tsunetsugu, T. Ishitobia, and K. Hattoria
aTokyo Metropolitan University

Microscopic Theory of the Spin Hall
Magnetoresistance
M. Matsuo and T. Kato

Fig. 1. Phase diagram of the J-K-Δ model for electric quadrupoles on
the fcc lattice at temperature T = 0.1Δ. See Ref.2 for details of orderedphases.

40

ISSP Activity Report 2020

Magnetoresistance is one of the fundamental phenomenain the research field of spintronics. Recently, a novel type ofmagnetoresistance called spin Hall magnetoresistance (SMR)
has been observed in a bilayer system composed of a normal
metal (NM) and a ferromagnetic insulator (FI) [1]. SMR is
explained by the combination of charge-spin conversions
in NM and loss of spins at the NM/FI interface [1,2] (seeFig. 1 (a)). When an in-plane charge current is applied to thewhich is induced by creation or annihilation of magnons,
depends on the temperature, and has opposite magnetization
dependence, i.e., takes a maximum when the magnetization (or the Néel vector) is parallel to accumulated spins ina normal metal. The dynamic part becomes larger when theamplitude of the localized spin, S0, is smaller. This indicatesthat the sign of SMR changes at a specific temperature if
S0 is sufficiently small. In Fig. 1 (b), we show the measured
temperature dependence of SMR in Pt/NiO/YIG bilayer
systems [3]. The sign of SMR changes at 80, 140, and
180 K for NiO thickness of 2.0, 2.2, and 2.7 nm. In addition,
we show the fitted curve in the figure using quadratic
temperature dependence that is predicted by our theory. Thisfitting indicates that the quadratic temperature dependence
explains well the experimental data at low temperatures. We
also derived the Onsager relation between spin conductanceand thermal spin-current noise.
This study has been performed as a joint study withMamoru Matsuo, who was a visiting professor of ISSP in theacademic year 2016.

Fig. 1. (a) Schematic diagram of the normal-metal(NM)/ferromagneticinsulator(FI) bilayer system for the spin Hall magnetoresistance (SMR)
measurement. (b) Experimental data of SMR measured in Pt/NiO/YIG
bilayer systems. The data is normalized as −1 at zero temperature using
the extrapolated value by the fitting.

NM layer with a large spin-orbit interaction, spin accumulation is caused near the NM/FI interface by the spin Hall
effect. The amount of spin accumulation is affected by theorientation of FI magnetization because it changes the rate ofspin loss at the interface. A backward spin current owing tospin diffusion is converted into the charge current again by
the inverse spin Hall effect and induces longitudinal magnetoresistance, which depends on the orientation of FI magnetization. Recently, SMR has been reported for the bilayer
system composed of NM and an antiferromagnetic insulator(AFI) [3], where the orientation of the Néel vector of AFI has
been changed by the orientation of magnetization of FI using
the NM/AFI/FI trilayer structure.
SMR can be described theoretically by combining thespin diffusion theory with the boundary condition at theinterface in terms of the spin-mixing conductance [2].
However, in this theory, the spin-mixing conductance at theinterface is a phenomenological parameter that has to be
determined experimentally; therefore, its temperature dependence cannot be predicted. Furthermore, the magnetizationorientation dependence of the spin-mixing conductanceis assumed phenomenologically by its definition. This
semiclassical description of SMR seems to be insufficient forstudying quantum features of magnetic insulators such as theeffect of thermally excited magnons.
In our work, we constructed a microscopic theory of
SMR and derived a general formula of a spin conductance at
the interface for both NM/FI and NM/AFI bilayer systems
in terms of spin susceptibilities. We clarified that the spincurrent is composed of static and dynamic parts. The staticpart of spin current originates from spin flip owing to aneffective magnetic field induced by an interfacial exchange
coupling. This part is almost independent of the temperature,and takes a maximum when the magnetization (or the Néelvector) is perpendicular to accumulated spins in a normal
metal, which is consistent with intuitive discussions inprevious experimental studies. However, the dynamic part,

References[1] H. Nakayama et al., Phys. Rev. Lett. 110, 206601 (2013).
[2] Y. T. Chen et al., Phys. Rev. B 87, 144411 (2013).
[3] D. Hou et al., Phys. Rev. Lett. 118, 147202 (2017).
[4] T. Kato, Y. Ohnuma, and M. Matsuo, Phys. Rev. B 102, 094437(2020).
Authors
T. Kato, Y. Ohnumaa, and M. Matsuoa
aKavli Institute for Theoretical Sciences

Controlling Magnetic Anisotropyof Nano-Size Islands by Alloying
Y. Hasegawa and W. C. Lin
Atomically-thin Fe and Co layers and nano-size island
structures formed on the Cu(111) substrate usually exhibit
magnetization in the in-plane and out-of-plane directions,
respectively. The preferred magnetization directions is due tomagnetic anisotropy, and controlling the energy barrier due
to the anisotropy are fundamentally important for preserving
the magnetization direction and switching it from one ofthe preferred directions to the other for magnetic storage/
recording and spintronics devices.
The magnetic anisotropy basically originates from thespin-orbit interaction, and in the case of the 3d magnetic
elements, the magnetic anisotropic energy (MAE) is
described with its perturbation. MAE is, therefore, closely
related with the occupation/un-occupation of 3d valencebands near the Fermi energy. In the present study using
spin-polarized scanning tunneling microscopy (SP-STM)
we investigated how the anisotropy and MAE of the 3dmagnetic islands can be controlled by adjusting the valenceoccupation, that is, by alloying the two elements: Fe and Co,
which exhibit the contrasting anisotropic behaviors [1].
Figure 1 shows one of the Co50Fe50 islands pseudomorphically grown on the Cu(111) substrate. The thicknessof the triangular island is 2 monolayer, and the white barin the STM image corresponds to 10 nm. Spin-polarizedtunneling conductance spectra taken at the marked sitewith a Fe-coated W tip exhibits a strong peak at the sample
bias voltage of -0.3 V (-0.3 eV below the Fermi level). The
peak just below the Fermi level has been assigned to 3d

ISSP Activity Report 202041

Fig. 1. Spin-polarized tunneling spectra taken at the marked site of a
Co50Fe50 island structure presented in the inset STM image. Spectra
taken under magnetic fields of 0 T and ±1 T applied in the perpendicular direction to the island are presented. A Fe-coated W tip was used asa spin-polarized probe.

minority states of pure Co and the alloy islands. The peakheight varies depending on the magnetic field applied in
the perpendicular direction. Since the Fe layer coating theprobe tip is in-plane magnetized, the applied magnetic fieldtilts the probe magnetization toward the field direction. The
reduction in the -0.3 V peak height, that is, tunneling fromthe minority states, by a positive (upward) magnetic fieldindicates upward magnetization of the alloy island. The
magnetic field dependence of the tunneling conductance isalso found around -0.5 V. The states there are majority ones
as they react to the applied field in the manner opposite to
the minority ones.
The magnetic response of the tunneling conductancespectra indicates that the alloy island exhibits perpendicularmagnetic anisotropy (PMA). In order to make it clear we
swept the applied magnetic field and measured the tunnelingconductance at -0.3 V. The obtained result is shown in Fig. 2.
Gradual increase (decrease) with the increment (decrement)of the magnetic field is due to the tilt of the probe magnetization by the applied field. Then sudden drops observed
around ±0.12 T are due to the flip of the magnetization of thealloyed island from anti-parallel to parallel directions to theapplied field. From the positions the drops, we can measureswitching magnetic field or coercivity, which are closely
related with the energy barrier between the two PMA configurations.
The PMA energy depends on the size of the islands;
basically proportional to the area or the number of thecomposing atoms. We measured the switching field on

Fig. 3. Plot of switching magnetic field as a function of the number ofthe constituent atoms for both pure Co and CoFe islands. Coercivity(right vertical axis) was estimated from the switching fields taken withan antiferromagnetic tip, whose stray field is negligible.

various sizes of the alloy islands, and the results are summarized in Fig. 3 and compared with those taken on pure Co
islands. One can clearly find that the switching field of thealloy islands is significantly reduced from pure Co islands
by a factor of 3-4. For both Co and CoFe alloy islands,
the switching field gradually increases with the size of theislands or the number of the composing atoms, but around8000 atoms it exhibits saturation or even reduction with theisland size. The peculiar behavior of the switching field is
explained with a crossover of two magnetization reverse
processes; for small islands the magnetization flips at atime whereas large islands are reversed through a process of
domain wall nucleation and propagation. It should be noted
here that since a ferromagnetic tip is used stray field fromthe tip significantly reduces the switching field; throughthe comparison with the switching filed measured with an
antiferromagnetic Co-coated tip, which directly provides
coercivity, we estimated coercivity of our samples as
depicted in the right axis of Fig. 3. Nevertheless, the resultspresented in Fig. 3 clearly indicate that the switching field
is significantly reduced by the alloying, and suggest that thecrossover same as pure Co islands also occur on the alloy
islands.
Reference[1] H.-H. Yang, C.-C. Hsu, W.-C. Lin, and Y. Hasegawa, Phys, Rev.

B 104, 035422 (2021).

Authors
H.-H. Yang, C.-C. Hsua, W.-C. Lina, and Y. Hasegawa
aNational Taiwan Normal University

Free-Standing Oxide
Thin Film Membranes
Fig. 2. Tunneling conductance at the sample bias voltage of -0.3 V
taken at the center of the CoFe alloy island presented in the right STM
image during a sweep of the applied perpendicular magnetic field. The
sweeping directions are marked with red dashed arrows. Sudden dropsof the tunneling conductance indicate switching of magnetization of theisland.

42

ISSP Activity Report 2020

R. Takahashi and M. Lippmaa
A fundamental parameter that determines the physicalproperties of crystalline materials is the lattice parameter,
which is generally determined by the chemical composition

and the lattice type of a crystal. Options for tuning materials
properties or electronic phases by changing the lattice parameters is generally limited to the use of thermal expansion,
the application of mechanical pressure, or ionic substitutions— also known as chemical pressure. Thin films offer an
additional option of using epitaxial strain for lattice parameter control, since nanometer-scale films grown on singlecrystal substrates tend to assume the same in-plane lattice
parameter as the substrate material. Even in mechanicallyhard materials, such as oxides, epitaxial tensile or compressive strains of several percent can be routinely obtained.
Larger strain states, however, are difficult to obtain without
mechanical failure of a crystal. Likewise, it can be difficult
to fabricate entirely strain free thin films. An interesting
technique that can be used to apply very high local or longrange strain fields on thin layers is the use of free-standing
thin films. In our recent work, we have developed a newoxide thin film lift-off process based on the use of latticematched BaO sacrificial layers to grow thin films that canbe released from a substrate and transferred to a secondary
carrier [1]. The technique can be used to extend the achievable range of mechanical strain in oxide materials and is thebasis for constructing novel vibrational energy harvesting
devices.
The process of fabricating free-standing BaTiO3 films is
illustrated in Fig. 1. A sacrificial BaO layer is first grown on
a SrTiO3 substrate. The BaO layer is nearly lattice matchedwith titanate lattices and can be grown epitaxially on thesubstrate surface. The membrane material, in this case
BaTiO3, is grown on top of the sacrificial BaO layer. The
heterostructure is then removed from the thin film growthchamber and attached with an adhesive layer to a flexible
secondary carrier, such as a glass sheet or even a polymer
backing. When the whole structure is immersed in water, theBaO layer dissolves, detaching the single-crystal substratefrom the membrane layer, as illustrated in Fig. 1c, whichshows the detached membrane on a secondary flexible
carrier and the detached substrate. In principle, the substrate
can be recycled for growing another thin film.
One interesting application of the membrane process
is the development of piezoelectric energy converters for
harvesting vibrational energy. The use of a membrane device
is benefitial because the piezoelectric film, such as BaTiO3,
can be grown epitaxially under optimal growth conditions on a high-quality single crystal substrate, yielding thehighest piezoelectric coefficient. However, the bulk substrateprevents the application of large strains on the film, as isrequired for low-frequency vibrational energy harvesting.
Much higher strains can be achieved in a membrane device
where the piezoelectric film is detached from the substrateand sandwiched between flexible conducting electrodes,

Fig. 2. Construction of a vibrational energy harvesting device
consisting of a BaTiO3 membrane embedded between two ITO/PET
electrodes (a). The actual test device is shown in the inset of (b). The
voltage signal obtained by vibrating the test device at 5 Hz is shown in(b).

as illustrated in Fig. 2a. An energy harvester device can beconstructed by supported a flexible membrane cantileverfrom one end and attaching a test mass on the free end. Whenmechanical vibration is applied, a periodic current can begenerated in an external circuit, powering, for example, alow-power environmental sensor device. The output signal ofa test cantilever using a BaTiO3 film is shown in Fig.2b.
The demonstration of the process used a BaTiO3 film,
but the technique itself is applicable to any material that canbe grown epitaxially on the sacrificial BaO layer. For energy
harvesting devices, a flexible backing is generally required tomaintain the integrity of a millimeter-scale membrane.
An obvious use of the technique is to fabricate films thatare truly strain free and for this reason, the technique hasmuch broader potential applications than flexible piezoelectric energy conversion devices.
Reference[1] R. Takahashi and M. Lippmaa, ACS Appl. Mater. Interfaces 12,25042 (2020).
Authors
R. Takahashia, and M. Lippmaa
aNihon University

Lasing Dynamics in
Perovskite Microcavities
S. Chen and H. Akiyama

Fig. 1. Free-standing oxide thin film membrane fabrication process:(a) growth of a sacrificial BaO layer and the functional membrane
layer, (b) attaching the heterostructure with a poly-dimethylpolysiloxane (PDMS) adhesion layer to a indium tin oxide-coated polyethylene terephthalate (PET) carrier, and (c) removing the substrate bydissolving the BaO layer in water.

Perovskite materials have shown remarkable progressin both the field of solar cell and light emitting devices,
for solar cell, conversion efficiencies more than 25% havebeen achieved, and for light emitting devices, perovskitebased vertical cavity surface emitting lasers (VCSEL) anddistributed feedback (DFB) lasers have been fabricated, andvery fascinating single-mode lasing properties have been
demonstrated, however, the detailed lasing mechanism is still
unclear.

ISSP Activity Report 202043

We have fabricated various kinds of CePbBr3 perovskite
microcavities with a low-temperature liquid-phase growthmethod, such as microplate, microrods, and microcubes,
as shown in Figure 1, with a size from several tens to 200
micrometers. A clear substrate dependence of the morphologies of the perovskite microstructures in this growth method
has been observed [1]. With the excitation of a femtosecond
laser beam from a Ti: sapphire ultrafast laser system, all the
microcavities show lasing behavior under elevated excitation densities, the higher carrier density more that Mott
density at lasing threshold indicates the lasing behavior inthese perovskite microcavities should be mainly related toelectron-hole plasma rather exciton.
The lasing dynamics of the microcavities were further
studied with time-resolved photoluminescence system.
Figure 2 shows the spectrally and temporally resolved lasing
images recorded by a streak camera. With increasing excitation fluences, two importance features have been observed,
one is the redshift of the dominant lasing spectra to the low
energy side due to the band gap renormalization (BGR)
effect, another one is the blueshift of each lasing mode
to higher energy side due to the reduction of the refractive index caused by the high injection carrier density. At acertain excitation density, for example 180 μJ/cm2, we cansee a redshift of each mode to low energy side with timegoing on after excitation, which indicates a recovery of index
in microcavity, and the blueshift of the total lasing spectrawith time going on indicates a recovery of BGR, therefore
the lasing dynamics is merged with the transient BGR effectand the transient change of refractive index in perovskite
microcavities, detailed explanations can be found in the
published paper [2].
We also fabricated CsPbBr3 perovskite nanocrystals witha ligand-assisted recrystallization method, and sandwiched
the perovskite nanocrystals with two high-reflectivity distributed Bragg reflectors (DBRs) to form a VCSEL device[3]. The lasing performances of the perovskite nanocrystal

Fig. 2. (a) TEM image of CsPbBr3 perovskite nanocrystals with anaverage size around 12 nm, (b) Schematic of the CsPbBr3 nanocrystal
based VCSEL, where the CsPbBr3 nanocrystals are sandwiched with
two high-reflectivity DBRs. (c) Emission spectra of the VCSEL withdifferent excitation densities. The inset shows a waveform of the output
pulse with a pulse width of 23 ps from the VCSEL with an excitation
density 2 times of the threshold.

based VCSEL have been characterized with femtosecond
pulse excitations, and single-mode lasing of the VCSEL has
been observed. The threshold excitation density was around
8 μJ/cm2. The cavity quality factor Q of the VCSEL is
estimated to be around 1425. With the excitation powerover 2 times of the threshold, the output pulse width of theperovskite based VCSEL is measured to be around 23 ps,demonstrating a potential application in short pulse generation of perovskite based VCSELs. Investigations on the
carrier dynamics in the VCSEL is still in progress.
References[1]. G. Weng, J. Yan, S. Chen, C. Zhao, H. Zhang, J. Tian, Y. liu, X.
Hu, J. Tao, S. Chen, Z. Zhu, H. Akiyama, and J. Chu, Photonics Res. 9,54 (2021).
[2]. G. Weng, J. Tian, S. Chen, J. Yan, H. Zhang, Y. Liu, C. Zhao, X.
Hu, X.Luo, J.Tao, S. Chen, Z. Zhu, and J. Chu, H. Akiyama, ACS
Photonics 8, 787 (2021).
[3] C. Zhao, J. Tao, J. Tian, Y. Bai, G. Weng, H. Liu, Y. Liu, J. Yan, S.
Chen, Y. Pan, X. Hu, S. Q. Chen, H. Akiyama, L.Wang, and J. Chu,
Chemical Engineering Journal, 2020, in press, https://doi.org/10.1016/
j.cej.2020.127660
Authors
S. Chena and H. Akiyama
aEast China Normal University

Second Hydrogen-Ordered
Phase of Ice VI: Ice XIX
R. Yamane, K. Komatsu, and Y. Uwatoko

Fig. 1. SEM micrograph of CsPbBr3 single-crystal microcavities onsubstrates of (a) single-crystal Si, (b) sapphire, and (c) amorphousquartz. Streak-camera images (d) of the emissions from microplate withvarious excitation densities. In the vertical dimension, a time range of
40 ps for each slice is shown. The distinct lasing modes are labeled asMode A (with the highest photon energy) to Mode G (with the lowest
photon energy) above the threshold. In each slice, a redshift to low
energy with time for each mode is clearly demonstrated. Inset is alasing image of a microplate under the excitation of femtosecond lasing
beam, showing a whispering-gallery mode (WGM) lasing.

44

ISSP Activity Report 2020

Ice exhibits extraordinary structural variety in its
polymorphic structures. To date, more than 20 crystalline andamorphous phases of ice have been reported.
The existence of a new form of diversity in ice polymorphism has recently been debated in both experimental andtheoretical studies, questioning whether hydrogen-disordered
ice can transform into multiple hydrogen-ordered phases,
contrary to the known one-to-one correspondence between
disordered ice and its ordered phase. Recent experiments on
a high-pressure hydrogen-disordered phase, ice VI, revealed
an unknown hydrogen-ordered form besides the known
ordered phase, ice XV. However, due to experimental difficulties under the high-pressure condition, the intriguingmanner of hydrogen ordering in ice VI has been investigatedonly using decompressed sample to ambient pressure. Also,Fig. 3. Proposed crystal structure of ice XIX based on Pcc2 structure
model.

Fig. 1. Representative experimental paths of dielectric and neutron
diffraction experiments described in the phase diagram of ice obtained
herein. Phase boundaries among ice VI, ice XV, and ice XIX aredescribed by black solid lines, based on dielectric experiments (red andblue squares correspond to phase transition temperatures from ice VI to
ice XV and XIX, respectively). The dotted line shows the provisional
phase boundary between ice XV and ice XIX.

no previous study reports direct experimental evidence
for the multiplicity of hydrogen-ordered phase of ice VI.
We report a second hydrogen-ordered phase for ice VI, ice
XIX, unambiguously demonstrated by in-situ dielectric andneutron diffraction measurements under high pressure originally developed by our group [1]. Based on a newly obtainedphase diagram of ice VI, XV and XIX (Fig. 1), we revealed
that ice XIX is thermodynamically stabilized by pressurecompared to ice XV.
To investigate hydrogen ordering of ice VI in its stablepressure region, comprehensive dielectric experiments werefirst carried out in the pressure range 0.88–2.2 GPa, using a
newly developed pressure cell. Ice VI was initially obtainedat room temperature and its dielectric properties were determined in both cooling and heating runs in the temperature
range 100–150 K. Neutron diffraction experiments wereconducted at 1.6 and 2.2 GPa, in which ice XIX appears ashydrogen-ordered phase of ice VI based on the dielectric
experiments, using Mito-system at J-PARC [2]. Both coolingand heating runs were conducted at each pressure in thetemperature range 80–150 K.

Phase transitions from ice VI to its hydrogen-orderedphases were observed at around 120–130 K, along withsudden weakening of the dielectric response of ice VI
with decreasing temperature (Fig. 2a). The reason for thisweakening is that hydrogen ordering of ice suppresses
reorientation of water molecules which induces the dielectric response of ice. The transition from ice VI to ice XIX
was also observed in the neutron diffraction experiments,
as appearance of new peaks due to symmetry lowering(Fig. 2b). Some of the new peaks, e.g. those at 2.20 Å and2.26 Å (indicated by blue triangles in Fig. 2b), cannot be
explained by the structure of ice XV. This is unambiguous
evidence that ice VI has a hydrogen-ordered phase distinct
from ice XV, and we named this unknown phase as ice XIX.
Based on the phase boundary of ice VI and its hydrogenordered phases, ice VI contracts upon hydrogen ordering,which thermodynamically stabilizes ice XIX in the higherpressure region compared to ice XV because of its smaller
volume than ice XV.
In addition, from our structural analysis, two structuralmodels, whose space groups are P 4 and Pcc2, are the mostplausible for the crystal structure of ice XIX (the Pcc2 structural model is shown in Fig. 3). It should be noted that the
proposed structure models intrinsically include hydrogenatoms whose site occupancy is 50 %; in other words, themodels exhibit that ice XIX is partially ordered state.
This indicate a possibility that there exists another fullyhydrogen-ordered phase of ice VI instead of the partiallyhydrogen-ordered ice XIX considering the third law ofthermodynamics. Considering the suggested space group of
ice XV, P 1 or Pmmn, centrosymmetry of hydrogen configurations is the most significance difference in hydrogenconfiguration between ice XIX and ice XV. Such symmetry
difference would induce intriguing difference of physicalproperties between ice XIX and XV in terms of such as optic
and dielectric.
References[1] R. Yamane, K. Komatsu, J. Gouchi, Y. Uwatoko, S. Machida,
T. Hattori, H. Ito, and H. Kagi, Nat. Commun. 12, 1129 (2021).
[2] K. Komatsu, M. Moriyama, T. Koizumi, K. Nakayama, H. Kagi,
J. Abe, and S. Harjo, High Press. Res. 33, 208 (2013).

Fig. 2. a Dielectric constant and dielectric loss of HCl-doped ice VI and
its hydrogen-ordered phase (ice XIX) obtained at 1.9 GPa upon cooling.
b Neutron diffraction patterns of ice VI and XIX obtained at 1.6 GPa in
the cooling run. Only an expanded area showing new peaks of ice XIX
is displayed. The blue and black ticks represent all the peak positions
expected from the unit cells of ice XIX and ice XV, respectively. Blue
triangles indicate new peaks at 2.20 Å and 2.26 Å, which do not appear
from the unit cell of ice XV.

Authors
R. Yamanea, K. Komatsub, J. Gouchi, Y. Uwatoko, S. Machidac,
T. Hattorid, H. Itob, and H. Kagib
aTohoku University
bThe University of Tokyo
cNeutron Science and Technology Center, CROSSdJ-PARC Center, Japan Atomic Energy Agency

ISSP Activity Report 202045

Fabrication of a Compact
Cubic-Anvil Pressure cell
N. Fujiwara and Y. Uwatoko
We have succeeded in fabricating the smallest cubic-anvilpressure cell (CAPC) to date. This miniaturization allowed
us to investigate physical properties at high pressures of over
4 GPa under a magnetic field [1, 2].
In strongly correlated electron systems, one of the mostintriguing topics is the quantum phase transition. Unlike
a conventional thermally induced phase transition, thisphase transition is driven via quantum fluctuations at zerotemperature, leading to an unconventional ground state evenat finite temperatures. Pressure is a representative parameter as well as carrier doping in this phase transition. At low
pressures below 3-4 GPa, a piston-cylinder pressure cell
(PCPC) has been most widely used for various experiments
because of its large sample space. However, this pressure
cell is unavailable at high pressures above 4 GPa. The CAPC
marks high hydrostaticity among pressure cells available atpressures above 4 GPa. Conventionally, the CAPC has notbeen suitable for measurements that require a magnetic field
due to its large body size. In fact, the CPAC has been usedmainly for resistivity or alternating-current susceptibility andrecently for neutron-scattering measurements at zero field[3].
We have fabricated the smallest PCAC to date using ϕ60
NiCrAl guide blocks and investigated pressure homogeneityfrom 63Cu nuclear-quadrupole-resonance (NQR) linewidthof Cu2O. The NQR frequency is proportional to the electricfield gradient at a 63Cu site and the spectral shape is sensitive to local disorder or inhomogeneity. Therefore, the NQR
measurements are available for investigating hydrostaticity
inside the pressure cell.
Figure 1(a) shows a sectional view of the CAPC, and Fig.
1(b) shows a bird’s-eye view of three WC anvils mounted
on the ϕ60 NiCrAl guide block. The four WC anvils are
fitted in the NiCrAl sliding blocks covered with Teflon
sheets. The four anvils move toward the 6.0-mm pyrophyllite cube without friction when the load is imposed on the
NiCrAl guide blocks. The cube is made of lower and upper
pyrophyllite gaskets and contains a ϕ3.0 Teflon capsule (seethe inset of Fig. 2(a)). A single coil wound around Cu2O
power samples is inserted into the Teflon capsule filled

Fig. 1. (a) Sectional view of a cubic anvil pressure cell (CAPC) with
ϕ60 NiCrAl guide blocks. (b) A bird’s-eye view of three WC anvils anda pyrophyllite cube mounted on the lower NiCrAl guide block.

46

ISSP Activity Report 2020

Fig. 2. (a) Bird’s-eye view of a ϕ3.0 Teflon capsule mounted on apyrophyllite gasket. A nuclear-quadrupole-resonance (NQR) coilwrapped around powder samples of Cu2O is inserted into the capsule.
The inset shows upper and lower gaskets and a Teflon capsule. (b)Overview of the ϕ3.0 Teflon capsule on the gasket. The inset shows theϕ2.0 Teflon capsule used for the resistivity measurements.

with pressure transmitting media, the mixture of fluorinate
FC-70 and FC-77 (see Fig. 2(a)). An overview of the ϕ3.0
Teflon capsule on the gasket is shown in Fig. 2(b). The
Teflon capsule has a ϕ2.5 × 1.5 mm sample space and issignificantly larger than that used for conventional resistivity
measurements (see the inset of Fig. 2(b)).
Figure 3(a) shows the 63Cu-NQR spectra of Cu2O
measured at room temperature with a conventional spectrometer and the ϕ60 CAPC. The spectra were obtained using thefast Fourier transform of the spin-echo signal. Figure 3(b)shows the pressure dependence of the NQR linewidth. The
linewidth for the CAPC is compared with that for the PCPC.
The NQR spectra for the PCPC exhibited double-peaks
structure above 2 GPa [1], and the data below 2 GPa wereplotted in Fig. 3(b). The linewidth for the PCPC increases
with increasing pressure above 0.8 GPa where the mixture of
the FC-70 and FC-77 begins to freeze, whereas that for the
CAPC saturates at high pressures above 4 GPa. The pressure
homogeneity for the CAPC is better than that for the PCPC
in a high-pressure regime. This result suggests that pressure
application from multiple directions suppress the inhomogeneity much more effectively than uniaxial pressure application.
In conclusion, we have fabricated a CAPC having a
diameter of 60 mm, the smallest cubic-anvil cell to date,
which allowed us to measure physical properties under amagnetic field utilizing a superconducting magnet witha large-bore sample space greater than 100 mm. In thispressure cell, a large sample space having ϕ2.5 × 1.5 mm
was secured. This pressure cell would open a new avenue forthe quest for novel pressure-induced phenomena in strongly
correlated systems.

Fig. 3. (a) 63Cu-NQR spectra of Cu2O measured with the CAPC atroom temperature. (b) Half width at half maximum of the NQR spectra
measured using the CAPC (black open circles) and NiCrAl pistoncylinder pressure cell (blue open circles). The curve is a guide for theeye.

References[1] S. Nakagawa, J. Gochi, T. Kuwayama, S. Nagasaki, T. Takahashi,
J. Cheng, Y. Uwatoko, and N. Fujiwara, Rev. Sci. Instru. 91 073907(2020).
[2] N. Fujiwara and Y. Uwatoko, Butsuri 76 295 (2021).
[3] S. E. Dissanayake, M. Matsuda, K. Munakata, H. Kagi, J. Gouchi,
and Y. Uwatoko, J. Phys.: Condens. Matter 31 384001 (2019).
Authors
N. Fujiwaraa and Y. Uwatoko
aKyoto University

Magnetoelectric Effect in the
Antiferromagnetic Ordered Stateof Ce3TiBi5 with Ce zig-zag Chains
M. Shinozaki, G. Motoyama, and Y. Uwatoko
Some magnetoelectric effects, including the Edelsteineffect, often occur in the materials without some inversion
symmetry. Materials exhibiting such cross-correlation are
expected as various new functional materials. Recently,
a new magnetoelectric effect is attracting attention in themetallic antiferromagnetic compound, in which the spatial
inversion symmetry is broken at the magnetic site. This new
cross-correlation is understood as a phenomenon that appears
as a result of the realization of odd parity multipole [1, 2].
We discovered a new antiferromagnetic heavy fermion
compound Ce3TiBi5 in which the local inversion symmetryat cerium site is broken (Figs. 1(a) and (b)). The crystal
structure of Ce3TiBi5 is P63 /mcm, and the global inversion
symmetry is maintained. Therefore, the current-induced
magnetization phenomenon in the sense of the Edelsteineffect does not occur. Ce3TiBi5 has only one cerium site, andthe cerium forms zigzag chains along the c-axis. The antiferromagnetic ordering temperature of TN is 5.0 K, electricalresistivity, magnetic susceptibility, and specific heat show a

Fig. 1. (a) Top view of the crystal structure of Ce3TiBi5. (b) Schematicside view of the crystal structure, where only Ce ions on a front side
are drawn for clarity. Green line denotes a unit cell. Black circles
indicate the position of the inversion center. (c) Magnetic structureon the Ce-zigzag chain, where the unit cell does not change even at
the ordered state. Orange arrows on the Ce ions indicate the expectedordered magnetic moments below TN. The red mark denotes a toroidal
moment on a Ce ion at the ordered state, which is a candidate as arealized odd multipole state on Ce3TiBi5.

distinct anomaly, respectively. Some odd parity multipoles
are expected in the antiferromagnetic state, although themagnetic structure has not yet been determined (Fig. 1(c)).
We performed the magnetization measurements with
applying several constant electric currents parallel to thec-axis of Ce3TiBi5 at around TN using commercial magnetometer of MPMS. In this measurement, slight static
magnetic field was applied to determine the sample position.
Current-induced magnetization (MME) was obtained fromsubtracting magnetization induced by applying slight static
magnetic field and zero electric current as blank magnetization.
We have succeeded in measuring the current-induced
magnetization only on the antiferromagnetic state [3, 4].
Figure 2(a) shows the temperature (T) dependence of MME
on Ce3TiBi5. It is found that MME(T) starts to appear justbelow TN with decreasing T, although it is almost independent of temperature and is approximately zero above TN.
This T dependence was also observed at several electricalcurrent (i), and then we show plots of MME versus i at 3.0 K
in Fig. 2(b) to discuss the i dependence of the magnitude of
MME. MME shows the linear i dependence, which suggeststhat MME on Ce3TiBi5 linearly increases with an increase in
the electric field.
Next, T dependence of MME at several magnetic field(H) is shown in Fig. 2(c). MME(T) exhibits an identical curve
despite a difference in the magnitude of H. The magnitude of
MME at each magnetic field and T = 3.0 K was decided from
the MME(T) at several magnetic fields (5 ≤ H ≤ 40 Oe) and isplotted in Fig. 2(d). The magnitude of MME is independent of
H in this field range, which indicates that MME has a similarvalue even in zero magnetic field.
The current-induced magnetization was observed only
below the antiferromagnetic transition temperature of 5.0 K.
This current-induced magnetization exhibits linear dependence with respect to the DC electric current and demonstrates little dependence on the magnetic field, i.e., thebehavior persists even in zero magnetic field. Based on theseFig. 2. (a) Comparison of the T dependence of the MME between several
i for sample A. (b) i dependences of MME at 3 K for samples A and B.
(c) Comparison of the T dependence of MME between different H. (d) H
dependence of MME at 3 K.

ISSP Activity Report 202047

results, we suggest that the current-induced magnetization
originated from the magnetoelectric effect of the odd multipole state on the Ce-zigzag chain structure. Meanwhile, thesign and magnitude of the slope of i dependence of MME has
large sample dependence as shown in Fig. 2(b). We consider
the sample dependence to be explained by the imbalance ofthe domain structure of the AFM state. However, we do notknow what kind of domain structure is realized in the antiferromagnetic state. We are conducting experiments with aninterest in the consistency between the realized odd parity
multipole state on Ce3TiBi5 and the current-induced magnetization measurements with various geometry.
References[1] S. Hayami, H. Kusunose, and Y. Motome, Phys. Rev. B 90, 024432(2014).
[2] H. Saito, K. Uenishi, N. Miura, C. Tabata, H. Hidaka, T. Yanagisawa, and H. Amitsuka, J. Phys. Soc. Jpn. 87, 033702 (2018).
[3] M. Shinozaki, G. Motoyama, M. Tsubouchi, M. Sezaki, J. Gouchi,
S. Nishigori, T. Mutou, A. Yamaguchi, K. Fujiwara, K. Miyoshi, and Y.
Uwatoko, J. Phys. Soc. Jpn. 89, 033703 (2020).
[4] H. Saito, H. Amitsuka, G. Motoyama, and M. Shinozaki, Solid
State Physics 55, 611 (2020).
Authors
M. Shinozakia, G. Motoyamaa, J. Gouchi, S. Nishigoria, T. Mutoua, A.
Yamaguchib, K. Fujiwaraa, K. Miyoshia, and Y. Uwatoko
aShimane University
bUniversity of Hyogo

High Precision Studyof the Anderson Transition
T. Ohtsuki and K. Slevin
More than six decades have passed since Anderson’s
seminal paper on localization. During the subsequent
decades, there have been numerous important discoveries including weak localization, universal conductancefluctuations, and the scaling theory of localization. Since
the proposal of the scaling theory, determining the criticalbehavior of the localization-delocalization transition, which
is usually referred to as the Anderson transition (AT), hascontinued to attract considerable attention.
One of the most widely used approaches to study the
AT quantitatively is to calculate the Lyapunov exponents(LE’s) and to analyze them using finite size scaling. Large
scale computations using the supercomputer System B at
ISSP have enabled us to calculate LE’s with high precisions
for very large system sizes. We consider a system of size
Lx × L × L, which we divide into layers of an L × L square.
The transfer matrix M relates the wave functions on the 1st
layer to Lxth layer, and the LE’s are the eigenvalues of thematrix. From the smallest positiveLE, γ, we define a quantity Γ = γL.
The conventional method requires the simulation ofa single very long sample. While this method has been
employed very successfully in numerous simulations overthe preceding decades [1], the calculations are inherently
serial and do not allow us to take advantage of massively
parallel computing. We devised an alternative method tosimulate an ensemble of much shorter samples and take anensemble average, allowing us to make full use of System B.
We have analyzed the LE’s using finite size scaling,and estimated the critical exponent for the divergence ofthe correlation length, ν = 1.572 [1.566,1.577] for three
dimensional (3D) Anderson model of localization [1,2],

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ISSP Activity Report 2020

Fig. 1. Γ as a function of the disorder strength W in the 3D Andersonmodel of localization for various cross section size L = 12.18, 24, 32,
48 and 64. L = 96 is underway using new System B.

H = Σi εi | i〉〈i | − Σ〈i,j〉| i 〉〈 j | , where εi is the onsite random
potential distributed uniformly in [-W ⁄ 2, W ⁄ 2]. We fit the W
and L dependence of Γ using finite size scaling (Fig. 1).
The critical exponent changes when a magnetic field isapplied. This can be realized by inserting Peierls phase in thenearest neighbor transfer. We obtain ν = 1.443 [1.437,1.449]
which is distinctly different from the case without a magnetic
field [3].
At the critical point, the wave functions exhibit multifractality, hence the local density of states fluctuates significantly. If we consider dilute magnetic impurities, the Kondo
temperature TK varies from one magnetic impurity to anotherdue to the large fluctuations of wave functions (Fig. 2).
We have calculated the local density of states at the criticalpoint using the kernel polynomial method and calculated the
Kondo temperature distribution [4]. The massively parallel
calculations on System B enabled us to reach the small TK
needed to check the analytic prediction.
In addition to the cases of Anderson’s model of localization with and without magnetic fields, depending on whether
we have spin-rotation symmetry, chiral symmetry as wellas particle-hole symmetry, we have 10 symmetry classes
for random quantum systems. In recent years, thenon-Hermitian systems with randomness, which are classified into 38 symmetry classes, are attracting renewed interests. We have studied the critical behaviors of AT with chiralFig. 2. Schematic of a magnetic impurity and a multifractal wave
function.

and/or particle-hole symmetries [5], and most of the criticalexponents for the AT in 3D are now determined. In addition,
we have verified that the transfer matrix method and thefinite size scaling work for non-Hermitian systems. We are
now studying the localization-delocalization transition for
non-Hermitian systems [6,7] and determining the criticalexponents, which might explain the critical behavior of light
localization.
References[1] K. Slevin and T. Ohtsuki, New J. Phys. 16, 015012 (2014).
[2] K. Slevin and T. Ohtsuki, J. Phys. Soc. Jpn. 87, 094703 (2018).
[3] K. Slevin and T. Ohtsuki, J. Phys. Soc. Jpn. 85, 104712 (2016).
[4] K. Slevin, S. Kettemann, and T. Ohtsuki, Eur. Phys. J. B 92, 281(2019).
[5] X. Luo, B. Xu, T. Ohtsuki, and R. Shindou, Phys. Rev. B 101,
020202 (2020).
[6] X. Luo, T. Ohtsuki, and R. Shindou, Phys. Rev. Lett. 126, 090402(2021).
[7] X. Luo, Z. Xiao, K. Kawabata, T. Ohtsuki, and R. Shindou,
arXiv:2105.02514.
Authors
T. Ohtsukia and K. Slevinb
aSophia University
bOsaka University

spectrometer of HRC at J-PARC. Our temperature-dependent measurements show that CoPS3 does not show the
spin-orbit exciton expected near 30 meV at high temperature in stark contrast with initial expectations. Figure 1
shows the temperature dependence of magnetic excitations. Above TN = 122 K, there is no sign of excitations
near 30 meV. This can only be interpreted as evidence forthe ground state of CoPS3 being an S = 3/2 state, not the
spin-orbital entangled Jeff = 1/2 state. Figure 2 shows
the spin-wave spectrum of CoPS3 with incident neutronenergy Ei = 71 meV. The measured spin-wave spectrum
shows two magnon branches over 40 meV with a massive
13 meV spin gap. Also, the spin-wave spectrum shows
extra gap-like features at 24~27 meV. Based on the former
analysis about temperature dependence of magnetic excitations, we use magnetic Hamiltonian with S = 3/2 system toexplain the spin-waves of CoPS3. This magnon was wellfitted using an anisotropic Heisenberg (XXZ) model with areasonable anisotropy coefficient α ≡ Jz/Jx = 0.6 and strong
easy-axis single-ion anisotropy K = 2.4 meV along the aSpin Waves in the Two-Dimensional
Honeycomb Lattice XXZ-Type van der
Waals Antiferromagnet CoPS3
C. Kim, J. G. Park and T. Masuda
The recent introduction of magnetic van der Waals(vdW) materials has opened new and novel opportunities to
examine the low-dimensional magnetism in real materials.
In particular, TMPS3 (TM = Mn, Fe, Co, Ni) family has
attracted special interests in the community as a class of
antiferromagnetic 2D vdW materials. The crystal structureof TMPS3 is a monoclinic structure with a C 2/m spacegroup, where a weak vdW force couples honeycomb layerswith edge-shared TMS6 octahedra on the ab-plane along
the c-axis. Since magnetic structure and exchange interactions in TMPS­3 depend on the TM elements, they provide an
excellent playground to validate spin dynamics theory in low
dimensions experimentally.
CoPS3 has been less studied among them due to thedifficulty in synthesizing high-purity samples [1]. Therefore,
the magnetic Hamiltonian of CoPS3 has been unknown so
far and needs investigation. CoPS3 has antiferromagnetic
phase transition at TN = 122 K with zig-zag magnetic order.
The spins are aligned along the a axis with a small canting
to the c axis. The magnetic susceptibility shows a difference
between H // ab and H // c in the paramagnetic state, which
implies XY-like anisotropy [1].
Moreover, Co2+ magnetic systems have recently drawn
significant attention because Co compounds may host Kitaev
interactions [2]. A spin-orbital entangled state is an essential
ingredient to realize the Kitaev interactions, which can beverified by measuring spin excitations. Typically, a spinorbit exciton, the crystal field excitation between Jeff = 1/2 to
3/2 state, is strong evidence of a spin-orbital entangled state.
Most cobalt compounds have such excitation at 20~30meV,
proportional to the strength of spin-orbit coupling of cobalt[3].
In this study [4], we examined such a possibility
using inelastic neutron scattering at the time-of-flight

Fig. 1. (a-f) Magnon spectra of CoPS3 at different temperatures with
Ei = 71.3 meV.

Fig. 2. The experimental INS data of CoPS3 measured at T = 8 K with
Ei = 71.3 meV is shown in (b). (a), (c) The best-fit magnon spectra with
the XXZ model and the isotropic Heisenberg model. An instrumental
energy resolution of 3 meV was used to convolute the theoreticalresults shown in (a) and (c). Horizontal and vertical white boxes
denote the integration range for the constant-E and constant-Q cuts in(d-g), respectively. (d), (e) Constant-Q cut at the momentum range of
Q = [1.7 1.8] and Q = [2.2 2.3] Å-1 for the measured data with thebest-fit simulations. (f), (g) Constant-E cut with the energy range of
E = [13 16] and E = [24 27] meVISSP Activity Report 2020

49

axis. The magnetic structure from our magnetic Hamiltonian also consistent with the reported magnetic structure.
In summary, our experiment and theoretical analysis suggest
CoPS3 as another exciting platform to study anisotropic
XXZ-type spin Hamiltonian in the honeycomb antiferromagnet. Moreover, it provides an excellent playground for
future investigation of low-dimensional magnetism withmagnetic van der Waals materials.
References[1] A. R. Wildes et al., J. Phys.: Condens. Matter 29, 455801 (2017).
[2] H. Liu et al., Phys. Rev. B 97, 014407 (2018).
[3] K. Tomiyasu et al., Phys. Rev. B 84, 054405 (2011).
[4] C. Kim et al., Phys. Rev. B 102, 184429 (2020).
Authors
C. Kima, J. Jeonga, P. Parka, T. Masuda, S. Asai, S. Itohb, H. S. Kimc,
A. Wildesd, and J. G. ParkaaSeoul National University
bKEK
cKangwon National University
dInstitut Laue-Langevin

Quantum Spin Liquid Statein a Square-Kagome Antiferromagnet
M. Fujihala, K. Morita, and T. Masuda
New insight into the spin behavior in an exotic state ofmatter puts us closer to next-generation spintronic devices.
The quantum spin liquid (QSL) state is an exotic state ofmagnet where the spin of electrons, which generally exhibitsorder at low temperatures, remains disordered. Observation of a QSL state is one of the most important goals incondensed-matter physics. However, the QSL state in
two-dimensional (2D) systems has not been clearly observedin real materials owing to the presence of site-mixing
disorder or deviations from ideal models. The lack of asuitable model material exhibiting the QSL hinders observations of the QSL state in the 2D spin-1/2 systems.
Square-kagome lattice (SKL) magnet is known as thehighly frustrated 2D system. There is a possibility that the
QSL ground states are realized in the antiferromagnetic S= 1/2 Heisenberg model on SKL [1]. However, the lackof a model compound for the SKL system has obstructeda deeper understanding of its spin state. Motivated by
the present status on the study of the SKL system, we

Fig. 1. (a) Crystal structure of KCu6AlBiO4(SO4)5Cl. Square-kagome
lattice consisting of the Cu2+ ions with nearest-neighbor exchange
couplings J1, J2, J3 and next-nearest-neighbor exchange coupling JX.
(b) Crystal structure of KCu6AlBiO4(SO4)5Cl featuring a large interlayer spacing.

50

ISSP Activity Report 2020

Fig. 2. (a) INS spectra at 5 K observed using HRC with an incident
neutron energy of 45.95 meV. (b) INS spectra at 0.3 K observed using
AMATERAS with incident neutron energy of 3.14 meV.

searched for compounds with the SKL containing Cu2+
spins, and synthesized the first compound of a SKL
antiferromagnet, KCu 6 AlBiO 4 (SO 4 ) 5 Cl, successfully[2]. As shown in Fig. 1(a), the SKL in the crystal structure of KCu6AlBiO4(SO4)5Cl comprises the Cu2+ ions.
In each SK unit, the square is enclosed by four scalene
triangles. From this crystal structure, it is recognized that
KCu6AlBiO4(SO4)5Cl has three types of first neighborinteractions, J1, J2 and J3, as shown in Fig. 1(a). By comprehensive experimental studies via magnetic susceptibility,
magnetization, heat capacity, and muon spin relaxation, we
confirm that the absence of magnetic long-range order down
to 58mK, roughly three orders of magnitude lower than the
NN interactions.
The quantum statistics of quasiparticle excitations dependon the type of QSL, in particular, the nature of their excitation. To grasp the whole picture of the spin excitation, we
performed the inelastic neutron scattering (INS) experiments using the high-resolution chopper spectrometer HRC
and cold-neutron disk chopper spectrometer AMATERAS
installed in the MLF at J-PARC. As shown in Fig. 2(a), flat
signals at around E = 10 and 7 meV are observed at 5 K.
The signal due to magnetic excitation is generally enhanced
at low-Q values, whereas phonon excitation is dominant at
high-Q. The signals at around 10 and 7 meV increase withdecreasing with Q, indicating that it comes from magnetic
excitation. As shown in Fig. 2(b), streak-like excitation at
Q = 0.8, 1.25, and 1.58 Å-1 is clearly visible down to theelastic line, and its intensity increases continuously without
signature of energy gap at least within the instrumental
resolution. These INS data are consistent with a gapless
continuum of spinon excitations. From the above, the flat
signals at 10 and 7 meV probably indicate a van Hove singularity of spinon continuum edges at this energy.
Our experimental results strongly suggest the formation of a gapless QSL in KCu6AlBiO4(SO4)5Cl at very low
temperature close to the ground state; however, they areinconsistent with the theoretical studies based on the J1-J2-J3
SKL Heisenberg model. We calculate the dynamical spin
structure factor S(q, ω) in the SKL model with various valuesof the parameters, but we could not reproduce the INS experimental results. Therefore, in order to realize the QSL statein the SKL, we must impose an additional condition such as
longer-range exchange interactions. Further experimental
and theoretical study would reveal the conditions inducing
the QSL state in SKL antiferromagnets.
References[1] K. Morita et al., J. Phys. Soc. Jpn. 87, 043704 (2018).
[2] M .Fujihala et al., Nat. Commun. 11, 3429 (2020).
Authors
M. Fujihalaa, K. Moritaa, R. A. Moleb, S. Mitsudaa, T. Tohyamaa, S.

Yanoc, D.H. Yub, S. Sotad, T. Kuwaie, A. Kodaf, H. Okabef, H. Leef, S.
Itohf, T. Hawaif, T. Masuda, H. Sagayamaf, A. Matsuo, K. Kindo, S.
Ohira-Kawamurag, and K. Nakajimag
aTokyo University of SciencebANSTOcNSRRCdRIKENeUniversity of Toyama
fKEK
gJ-PARC CenterTwo-step Magnetic Anomaly
in the High Field Magnetization
Process in Yb1+xIn1−xCu4
S. Yamanaka, A. Matsuo, and C. Michioka

electrical resistivity ρ of Yb1+xIn1-xCu4. In all compositions,ρ rapidly decreases below the valence transition temperature due to decrease in magnetic scattering accompanied
by decrease in the localized moment of Yb3+. The valencetransition temperature seems to shift to higher temperature with increasing x, consistent with behaviors of χ. For
x = 0.21 and x = 0.25, ρ reaches a minimum value at approximately 40 K and then increases with decreasing temperature. These behaviors are typical in a metallic state with an
impurity-Kondo-effect in which ρ shows -lnT dependence. It
is natural to think that in the Yb-rich phase such as x = 0.21
or 0.25, all the Yb ions could not behave homogeneously anda part of Yb ions remain trivalent with an impurity moment.
Figure 3 shows the magnetization curve of Yb1+xIn1-xCu4
measured at 4.2 K up to 72 T. For x = 0.16, 0.15, and 0.20,
two-step magnetic anomalies are observed in the high fieldmagnetization process. If this originates from phase separation, three or more steps would frequently appear. However,no such a behavior was observed in the results of morethan 20 samples. In addition, the magnetization value for
x = 0.16, 0.15, and 0.20 reaches 2 μB after the first step andreaches 4 μB after the second step. The fact that the similar
behavior was observed in all three compounds suggests thatthe magnetic moments of the plateaux may be meaningful.
Although the changes in the magnetic transition field bysubstitutions in YbInCu4 have been reported, their magnetictransitions are all one step. The present observation of a
successive phase transition in this system is the first time.
The temperature dependences of χ and ρ do not show a
behavior corresponding to the two-step anomaly. An intermediate phase is induced only by the magnetic field. The degen-

The intermetallic compound YbInCu4 exhibits a firstorder valence transition at 42 K under ambient pressure;Ytterbium ions have trivalent local moments in the hightemperature phase, and they become a mixed-valence stateof an average valence of 2.9 in the low-temperature phase [1,2]. In addition, a magnetic field-induced valence transition
also occurs below 42 K with the transition field of ~30 T [3,4]. To clarify the mechanism of the valence transition, substitution effects in Yb or In site have been extensively studied[3, 5]. Although this system was originally discovered asa solid solution RCu2 without site-ordering, there are few
reports on the substitution of In site by Yb itself on YbInCu4.
In this study, the magnetic and transport properties of
Yb1+xIn1-xCu4 are investigated using polycrystalline samples
synthesized by the arc melting method, and the magnetic
phase diagram in Yb1+xIn1-xCu4 is investigated.
Figure 1 shows the temperature dependence of themagnetic susceptibility χ of Yb1+xIn1-xCu4 measured in amagnetic field of 1 T. In a parent material, Yb1.01In0.99Cu4, χ
rapidly decreases below 42 K due to decrease in the effective
valence of Yb ion from trivalent, which is consistent withthe previous report [1]. The sharp magnetic anomaly accompanied by the valence transition in Yb1.01In0.99Cu4 seems tochange to a broad shape and shift to a higher temperature
with increasing x. In all compositions, χs obey the CurieWeiss law above 150 K, and the values of the effective
magnetic moment estimated from Curie-Weiss fits roughly
agree with the calculated value for free Yb3+ ion, 4.54 μB.
This fact confirms that the valence state of Yb ions in thehigh-temperature phase is trivalent.
Figure 2 shows the temperature dependence of theFig. 2. Temperature dependence of the electrical resistivity ρ ofYb1+xIn1-xCu4..

Fig. 1. Temperature dependence of the magnetic susceptibility χ ofYb1+xIn1-xCu4 measured in a field of 1 T.

Fig. 3 Magnetization curves M -H of Yb1+xIn1-xCu4 measured at 4.2 K
up to 72 T.

ISSP Activity Report 2020

51

erated ground state of Yb3+ (2F7/2) is split by the crystal
electric field (CEF) effects, and each state has a different
saturation magnetization. In x = 0.16, 0.15, and 0.20, thelower transition field induces the trivalent state however themagnetic moment is limited by CEF. Subsequently in thehigher transition field, the Zeeman- energy exceeds CEF andinduces a full saturation magnetization of Yb3+ of 4 μB. For
x = 0.01 and 0.10, the magnetization reaches about 4 μB ina one-step transition without an intermediate phase. This isprobably because CEF of small substituted compounds issmaller than the magnetic transition field which induces thetrivalent state.
In conclusion, we discovered the successive magnetictransition in the magnetization process in Yb1+xIn1-xCu4(x = 0.15, 0.16, and 0.20) and proposed that it originatesfrom competition and corporation of the Zeeman energy,
CEF effect and magnetic interactions.
This study was selected for the 22nd distinguished paper
award sponsored by Japan Society of Powder and Powder
Metallurgy.
References[1] I. Felner and I. Nowik, Phys. Rev. B 33, 617 (1986).
[2] B. Kindler, D. Finsterbusch, R. Graf, F. Ritter, W. Assmus, and B.
Lüthi, Phys. Rev. B 50, 704 (1994).
[3] K. Yoshimura, T. Nitta, M. Mekata, T. Shimizu, T. Sakakibara, T.
Goto, and G. Kido, Phy. Rev. Lett. 60, 851 (1988).
[4] T. Shimizu, K. Yoshimura, T. Nitta, T. Sakakibara, T. Goto, and M.
Mekata, J. Phys. Soc. Jpn. 57, 405 (1988).
[5] J. He, N. Tsujii, M. Nakanishi, K. Yoshimura, and K. Kosuge, J.
Alloys Compd. 240, 261 (1996).
Authors
S. Yamanakaa, T. Nakahigashia, C. Michiokaa, H. Uedaa, A. Matsuo,
K. Kindo, and K. Yoshimuraa
aKyoto Unuversity

magnetocaloric effect, and magnetostriction of Ni-Co-MnGa alloys in pulsed high magnetic fields up to 56 T [4, 5].
As shown in Fig. 1(a), metamagnetic transitions similar
to those in the Ni-Co-Mn-In alloy were observed in thismaterial. Magnetostriction measured on a polycrystalline sample using the capacitance method [7] showed asignificant lattice distortion associated with this transition[Fig. 1(b)], suggesting that this magnetic transition is accompanied by a structural phase transition from the martensite to
the austenite phase.
The change in the sample temperature at the magnetic
field-induced phase transition shows a negative magnetocaloric effect, as shown in Fig. 2. In contrast to the Ni-Co-MnIn alloy, the negative magnetocaloric effect shows up evenafter completing the field-induced magnetic and structuraltransition. The experimental results of heat capacity andtheoretical results of first-principles calculation show thatthe electronic specific heat coefficient increases significantly
at the transition to the austenite phase: the entropy of theelectron system plays an essential role in the magnetocaloric
effect of this system. The increase in lattice and electronic
entropy associated with the magnetic field-induced phasetransition dominates over the decrease in the spin entropychange, which shows up as a negative magnetocaloric effectthat decreases the sample temperature in a magnetic field. If
we can incorporate these entropy changes in the same sign,
we expect magnetocaloric effects beyond the limit of thespin system. Microscopic discussion in the present study will
pave the way for this giant magnetocaloric effect.
Most of the previous studies on magnetocaloric materials
have focused on their properties in the magnetic field region
below 5 T. In the future society where hydrogen energy andhigh-temperature superconducting cables are put to practicaluse. In this situation, large-scale application of the magneto-

Origin of Inverse Magnetocaloric Effects
in Ni-Mn-based Heusler Alloys
T. Kihara, X. Xu, and M. Tokunaga
The development of heat-exchange systems using themagnetocaloric effect is a topic where material sciencecan solve energy problems. The magnetocaloric effecthas long been used to realize ultra-low temperature using
the adiabatic demagnetization procedure. This effect wasrecently expected for heat exchangers with higher efficiency
than conventional gas-compression/expansion systems[1]. However, most of the materials that exhibit significant
magnetocaloric effect contain heavy rare-earth elements, sothat the high cost of the materials hinders their practical use.
We have studied thermodynamic properties of Heusler alloys
without rare-earth elements to realize the giant magnetocaloric effect involving the entropy of the lattice and electronicsystems [2-5].
Ni-Mn-based Heusler alloys have attracted considerable attention as magnetic shape memory alloys [6]. In ourprevious work on Ni-Co-Mn-In alloys, we directly observed
prominent negative magnetocaloric effects at the fieldinduced phase transitions from paramagnetic martensite toferromagnetic austenite phase. From the systematic highfield studies of magnetocaloric effect, magnetization, heat
capacity, and microscopy, we concluded that this effect originated from the increase of lattice entropy associated with thestructural phase transition [2].
In the present study, we measured the magnetization,

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ISSP Activity Report 2020

Fig. 1. (a) Field-dependence of magnetization in a polycrystal of
Ni41Co9Mn31.5Ga18.5 at several temperatures. Steep increase in magnetization indicates emergence of metamagnetic transitions. (b) Longitudinal magnetostriction of Ni41Co9Mn31.5Ga18.5 measured at 290 K. The
inset shows schematics of the capacitance method. Prominent increaseof the sample length corresponds to the structural transition from themartensite to the austenite phase simultaneously with the metamagnetictransition.

Fig. 2. Magnetocaloric effect of Ni 41Co9Mn31.5Ga18.5 studied inadiabatic magnetization process realized in pulsed magnetic fields.
Instantaneous change in sample temperature is detected using a thinfilm resistive thermometer attached to the sample. In addition to steep
reduction of temperature at the metamagnetic transition (ΔTIMT),
further negative magnetocaloric effect is observed at higher fields(ΔT*).

caloric effects using large superconducting magnets, such as
gas liquefaction projects, will also be important. The evaluation of the magnetocaloric effects in Heusler alloys at highmagnetic fields provides an essential hint for solving energy
problems in near future.
References[1] K. A. Gschneidner Jr., V. K. Pecharsky, and A. O. Tsokol, Rep.
Prog. Phys. 68, 1479 (2005).
[2] T. Kihara et al., Phys. Rev. B 90, 214409 (2014).
[3] X. Xu et al., MATEC Web of Conf. 33, 01004 (2015).
[4] T. Kihara et al., Scr. Mater. 181, 25 (2020).
[5] T. Kihara et al., Phys. Rev. Mater. 5, 034416 (2021).
[6] R. Kainuma et al., Nature 439, 957 (2006).

[7] A. Miyake et al., Rev. Sci. Instrum. 91, 105103 (2020).

Authors
T. Kiharaa, T. Roya, X. Xua, A. Miyake, M. Tsujikawaa, H. Mitamura,
M. Tokunaga, Y. Adachib, T. Etoc, and T. KanomatadaTohoku University
bYamagata University
cKurume Institute of Technology
dTohoku Gakuin University

field HP ≈ 15.6 T (HP = 1.84 Tc), and the Zeeman energy
at Hc2 becomes comparable with the superconducting gap
ΔSC (μBHc2 ~ ΔSC). In addition, high-quality FeSe single
crystals are found to be in the clean limit with a much longer
mean free path l than the superconducting coherence lengthξ (l >> ξ). More importantly, FeSe is in the so-called
BCS-BEC (Bose-Einstein condensate) crossover regime(ΔSC~EF) with exceptionally small Fermi energies. The spin
imbalance reaches up to ~ 40% for one of the Fermi surfaces(FSs) at Hc2ab ≈ 25 T, much larger than the typical value ofa few % in other superconductors. Therefore, FeSe can bea model system to study whether, and if so how, competing
magnetic or superconducting instabilities trigger exotic fieldinduced phases in multiband superconductors with a large
spin imbalance.
In this work, using torque magnetometry, specific heat,and magnetocaloric measurements, we identified successive
anomalies with increasing in-plane magnetic field, below
Hc2 at low temperatures and in the clean limit. The signatures of unusual phase transitions below Hc2ab and below
T *≈ 2 K are observed in the field-dependent torque magnetometry τ (H). The torque magnetometry in superconductors usually exhibits a typical saw-tow shaped curve of the
field derivative τ (H), dτ/dH due to vortex pinning. In FeSe,
however, we observed two additional anomalies at H1~15 T
and H2~22 T, well below the irreversible field Hirr at which
the hysteresis in τ (H) starts to develop (Fig. 1a and 1b).
The transition fields H1 and H2 follow the distinct dependence on temperature or field angle from those of Hirr ≈ Hc2(Figs. 1a and 1c). Upon increasing temperature or the tilting
angle of the magnetic field (θ) from H || ab, Hirr systematically decreases, similar to Hc2. In contrast, the anomalies
are pronounced only at low temperatures below T * and near
H || ab with θ ≤ 15o. Also, the transition fields H1 and H2
remain almost the same with variation of temperature or
field angle (Figs. 2a and 2b). The anomalies seen in torquemagnetometry are further confirmed by magnetocaloric
and specific heat measurements [1]. These findings clearly
evidence the presence of field-induced phases near thein-plane magnetic fields below Hc2ab.

Observation of In-Plane Magnetic Field
Induced Phase Transitions in FeSe
J. S. Kim, Y. Kohama, and J. M. Ok
In conventional superconductors, spin polarization can
destroy superconductivity when Zeeman energy surpasses
the binding energy of Cooper pairs, known as the Pauliparamagnetic limit. Beyond the Pauli limit, exotic superconducting phases, often coexisting with complex magnetic
orders, have been observed in some unconventional superconductors. Even in these cases, however, field-induced spin
imbalance is usually at most a few %. So far, realization of
the highly spin-imbalanced superconductivity and possible
field-induced exotic phases have been remained elusive,
partly due to the lack of the suitable material candidates.
FeSe is a promising candidate hosting a strongly spin
imbalanced state at high magnetic fields. The observed
in-plane upper critical field of FeSe is Hc2ab ≈ 25 T, well
above the conventional BCS value of the Pauli limiting

Fig. 1. (a) Magnetic field dependent torque τ (H) close to H || ab atvarious temperatures. The irreversibility field at Hirr ~ 25 T and anomalies at H1 ~ 15 T and H2 ~ 22 T are indicated by black, red and blue
arrows, respectively. (b) The field-derivative curve dτ(H)/dH showing
clear anomalies at H1 and H2. (c) Magnetic field dependent torque τ (H)at various field angles.

ISSP Activity Report 202053

Gurevich, and J. S. Kim, Phys. Rev. B 101, 224509 (2020).
AuthorsJ. M. Oka, C. I. Kwona, J. S. Youa, S. K. Parka, K.-S. Kima, J. S. Kima,
Y. Kohama, K. Kindo, J.-H. Kimb, Y. J. Job, E. S. Choic, W. Kangd, E.
G. Moone, and A. GurevichfaPohang University of Science and Technology
bKyungpook National University
cNational High Magnetic Field Laboratory
dEwha Womans University
eKorea Academy Institute of Science and Technology
fOld Dominion University

Hydrogen-Bonded Structure of
Water Responsible for Ion Selective
Permeability of Self-Assembled Liquid
Crystalline Polymer Membranes
Fig. 2. (a,b) Magnetic phase diagram of FeSe for H || ab as a functionof temperature (a) and field orientation (θ) with respect to the abplane (b). The upper critical field Hc2ab (T) (open symbols) are determined by resistivity (black open circle), TDO frequency (black open
diamond), and torque magnetometry (black open star) measurements,
while the field-induced phase transitions at H1 and H2 are obtained bytorque magnetometry (colored open star) and the magnetocaloric effect(colored solid triangle). The inset shows schematics of the possible
nesting effect via q = (0, π) or (π, π) between the spin-split Fermi
surfaces (FSs) with anisotropic superconducting gap (gray shade). Near
H1, the nesting effect is expected between two FSs with opposite spins
via a SDW momentum q = (π, π) (bottom), while the nesting with q =(0, π) is expected near H2 between two FSs with the same spins (top).

One possible candidate for the high field anomalies isthen the FFLO phase. In this case, the Cooper-pair state(k↑, −k+q↓) is formed with a momentum mismatch (q)between the spin-split FSs. In multiband superconductors, each FS has its own FFLO instability with a different
modulation length qi−1 (i, band index), and these instabilities
compete with each other, inducing the multiple FFLO orders
at different magnetic fields. Alternatively, the spin-density
wave (SDW) phase of field-induced quasiparticles can be
another promising candidate. When the superconductivity issuppressed by Pauli pair breaking near the superconducting
gap nodes or minima, the SDW order can be triggered by
the nesting effect by field-induced quasiparticles. Due toan exceptionally small EF, the Zeeman effect results in the
spin-split FSs much different in size, which significantly
affects the nesting condition in FeSe. We found that thenesting via q = (π, π) nicely matches the two different FSs
with opposite spins at H~16 T, and with the same spins at
H~22 T (Fig. 2b). in good agreement with the observed
transition fields, H1 and H2. The incipient spin fluctuations
and strong coupling between the field-induced quasiparticles
may allow the coexisting phase with the SDW and superconductivity.
In the current stage, the detailed nature of the fieldinduced phase transitions, including possible coexistenceof the SDW and the FFLO orders, remains to be clarified.
Nevertheless, these observations clearly demonstrate that
FeSe offers a unique system, in which field-induced phasetransitions occur in the superconducting state. This posesa challenge to our understanding on the complex interplay
between anisotropic superconducting order, incipientmagnetic instabilities, and the multi-band effect in the largely
spin-imbalanced superconducting systems.
Reference[1] J. M. Ok, C. I. Kwon, Y. Kohama, J. S. You, S. K. Park, J. Kim,
Y. J. Jo, E. S. Choi, K. Kindo, W. Kang, K.-S. Kim, E. G. Moon, A.

54

ISSP Activity Report 2020

T. Kato and Y. Harada
Liquid crystals (LC) are appealing for nanofiltration
membranes with uniform subnanopores which enableefficient and thermally stable filtration and ion selectivity[1]. Many principles of water filtration are intuitive: particles
bigger than the pore size cannot pass through a membrane,
and electrostatic interactions between ions, water molecules
and the membrane can aid the process. However, the permeability of these LC membranes is higher for larger, dianionicSO42− ions than it is for monoanionic, smaller Cl− ions,which could not be explained using the common filtration
principles, which implies the presence of another driving
force for ion selectivity. One of such possibility is the structure of the water surrounding the ions
Here we investigated the hydrogen-bonded configurationof water molecules in the subnanopores of a bicontinuous
cubic LC membrane formed by in-situ polymerization ofcompound 1 (Fig. 1a), and how it affects the permeation of
dianionic SO42− and monoanionic Cl−[2]. We used highresolution X-ray emission spectroscopy (XES) [3] to learn

Fig. 1. (a) Molecular structure of an LC monomer (1). The monomers
were self-assembled to form a bicontinuous cubic (Cubbi) LC structure
through which ions are selectively transported. (b) Schematic image ofsoft X-ray emission detection from water in the LC membrane.

Fig. 2. Valence electronic structure of H2O in (a) the LC membrane,(b) 1.0 mol/l and 2.5 mol/l MgSO4, (c) 1.0 mol/l and 3.0 mol/l NaClaqueous solutions.

how water arranges itself in the subnanopores.
XES spectra of pure liquid H2O absorbed in the LC
membrane and hydrating water of permeating solutes,
MgSO4 and NaCl were collected at the SPring-8 BL07LSU
HORNET station using a high-resolution XES spectrometer [4]. A custom-made ambient pressure cell was used to
expose the LC membrane to humidity-controlled moistureand nitrogen as a carrier gas. (Fig. 1b). To explore the originof the selective ion permeation (73±5% for MgSO4 and30±6% for NaCl), hydrogen-bonded structures of H2O in the
LC membrane were compared with those in the MgSO4 andNaCl hydrations. Spectra of water within the subnanopores(Fig. 2a) revealed that its hydrogen-bonded network, when
confined, loses some structural features characteristic of bulk
water in favor of new structural features. Very similar structural features are observed in the spectra of the hydration
shell of SO42− (Fig. 2b) but not in that of Cl− (Fig. 2c). We
interpret this as the reason for the greater membrane permeability of SO42− than that of Cl−. In the near future computational studies might help understand the exact structure of
the hydrogen-bonded network within the subnanopores. The
nature of hydrogen-bonded networks becomes an additionalparameter that should be considered and controlled to obtainhighly efficient water filtrations.
References[1] (a) T. Kato et al., Nat. Rev. Mater. 2, 17001 (2017); (b) T. Ichikawa
et al., J. Am. Chem. Soc. 134, 2634 (2012). (c) M. Henmi et al., Adv.
Mater. 24, 2238 (2012). (d) T. Sakamoto et al., Adv. Sci. 5, 1700405(2018). (e) K. Hamaguchi et al., ACS Macro Lett. 8, 24 (2019). (f) M.
Gupta et al., ACS Macro Lett. 8, 1303 (2019).
[2] R. Watanabe, T Sakamoto, K. Yamazoe, J. Miyawaki, T. Kato, and
Y. Harada, Angew. Chem. Int. Ed. 59, 23461 (2020).
[3] T. Fransson et al., Chem. Rev. 116, 7551 (2016).
[4] Y. Harada et al., Rev. Sci. Instrum. 83, 013116 (2012).
Authors
R. Watanabea, T Sakamotoa, K. Yamazoe, J. Miyawaki, T. Katoa, and Y.
HaradaISSP Activity Report 202055