Joint Research Highlights
Heavy Fermion State of YbNi2Si3
without Local Inversion Symmetry
S. Nakamura, T. Sakakibara, and M. Yamashita
Various fascinating phenomena often appear in thematerials without the global inversion symmetry. It istheoretically suggested that both global and local inversionsymmetry-breaking host an exotic superconductivity. This
study focuses on the materials in which only the local inversion symmetry is broken. There are not much examples
showing superconductivity in the materials without the local
inversion symmetry, but the local inversion symmetry is
broken in high temperature superconductors such as ironbased and copper-oxide superconductors.
We discover a new heavy-fermion compound YbNi2Si3
in which the local inversion symmetry at ytterbium site is
broken (Fig. 1(a) and (b)), whereas the global inversion
symmetry is retained. The space group of this material is I4/
mmm, and the site symmetry at Yb-ion is 4mm. This crystal
structure is partially similar to tetragonal ThCr2Si2-type
crystal structure; CeCu2Si2 and URu2Si2 are well known as
heavy-fermion superconductors. Rare-earth metal, transition
metal, and silicone atoms form a square lattice on the c planeboth in these two crystal structures.
The electrical resistivity, the magnetization, the magnetic
torque and the heat capacity measurements have been
performed in YbNi2Si3, and we check if there is a superconducting state without the local inversion symmetry. We

Fig. 2. (a) The temperature dependence of the heat capacity of YbNi2Si3
at zero magnetic field. (b) The magnetic field dependence of themagnetic torque of YbNi2Si3 at 208 and 2 mK.

determine the H–T phase diagram of YbNi2Si3 by the magnetization and the heat capacity measurements (Fig. 1(c)). We
find an antiferromagnetic state appears below 0.35 K at zeromagnetic field, and it is suppressed by applying a magnetic
field of 0.7 T. The temperature dependence of the electricalresistivity shows a typical heavy-fermion behavior with the
Kondo temperature of ~ 2 K. Remarkably, a large electronic
specific heat coefficient of γ = 5 J/mol K2 is found to remain
down to 0.08 K in the antiferromagnetic state (Fig. 2(a)),
demonstrating a heavy-fermion state in YbNi2Si3. Such large
entropy is often released by a superconducting transition ata lower temperature. To this end, we have carried out themagnetic torque measurements down to ultralow temperatures. However, no discernible change is observed down to
2 mK (Fig. 2(b)), showing the persistence of the heavyfermion state. We further check the valence state of the Yb
ions via hard X-ray photoemission spectroscopy, and findthat almost all the Yb-ions are in the Yb3+ state, showing theabsence of the valence fluctuation between Yb2+ and Yb3+.
This absence would suggest that the origin of the heavyfermion state is a magnetic fluctuation rather than a valencefluctuation, which often expected in Yb compounds. It
remains as a important future issue to reveal the origin of theheavy-fermion state in YbNi2Si3 by microscopic measurements such as NMR.
Reference[1] S. Nakamura, K. Hyodo, Y. Matsumoto, Y. Haga, H. Sato, S. Ueda,
K. Mimura, K. Saiki, K. Iso, M. Yamashita, S. Kittaka, T. Sakakibara,
and S. Ohara, J. Phys. Soc. Jpn. 89, 024705 (2020).
Authors
S. Nakamuraa, T. Sakakibara, and M. YamashitaaNagoya Institute of Technology

Fig. 1. (a) The crystal structure of YbNi2Si3 in which the local inversion
symmetry around the Yb ions is broken as shown in (b). (c) The H-T
phase diagram, where TN and Tmax denote the Néel temperature and acrossover line between paramagnetic and forced ferromagnetic regions,
respectively.

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ISSP Activity Report 2019

Patterning of TopologicalInsulator on Thin Film
H. Mine, J. Haruyama and T. Nakamura
Because topological insulators (TIs) have theirconducting channels at the sample edges, shapes of themplay crucial roles in device applications. Hence it can be avery convenient tool if we can “draw” spatial patterns of TI
just like drawing a picture. Members of two-dimensional(2D) transition metal dichalcogenides (TMDCs) have been
predicted and experimentally confirmed as a new class of
2D TI materials though the edge conduction is fragile inmost cases. Here, we realize the controlled patterning of a TI
phase embedded in an insulating phase of thin molybdenumdisulfide (MoS2) by laser beam irradiation.
MoS2 is known to have various metastable lattice structures according to the freedom in the stacking angle of thechalcogen layer. In so called H-phase, the chalcogens form
triangle poles or trigonal prism. In single crystal, two-layers
form a unit and called 2H phase. This is semiconducting.
In so called T-phase, the two-triangles are in the twisted
relation. The stacking unit is single and called 1T phase. Thisis metallic. These two are the representative phases but there
are many other metastable phases.
An interesting is the one called 1T’ phase, which issomewhat intermediate between these two. In this 1T’ phase,for example this upper chalcogen atoms shift in this way andstop at some intermediate position. Some theories predict
this 1T’ phase should be a topological phase. So if we cancause local phase transition between these three, we canmake up desired circuits with semiconductors, metals andtopological insulators. We showed that 1T’ phase can be
actually created in 2H MoS2 and also it works as a TI [1].
In order to obtain 1T’, we used the excitation by Ar laser
beam of energy 2.5 eV and simply drew some patterns on
2H MoS2. From an atomic force microscope topography, it

Fig. 2. Resistance of laser irradiated pattern on MoS2 as a functionof back gate voltage. In the inset we illustrate the setup of resistancemeasurement.

turned out that the irradiated region is a bit thinner than thesurrounding region. This is probably due to some evaporation of atoms and the phase transition is not due to the directenergy transfer from photon to lattice but some heating
effect causes the transition. The appearance of 1T’ phase wasconfirmed in the Raman and the XPS spectra as shown in
Fig. 1 (a) and (b).
The pattern of 1T’ phase we used for the measurement ofedge channel conduction is a simple H-shape as illustrated in
the inset of Fig. 2. The size is about 2 μm, which is comparatively large compared with ordinary reported coherence
length of edge conductions in TIs. The flake of MoS2 wasplaced on a metallic doped Si substrate covered with SiO2,
hence we can tune the position of Fermi energy in MoS2
wtih the back gate voltage. Figure 2 shows the resistance ofH-shaped sample in the measurement geometry drawn in
the inset, as a function of the back gate voltage. The resistance R took a peak structure around at R = Rq / 2 = h / 2e2 =12.6 kΩ. This is the value calculated for ideal helical edge
states and clearly tells that the patterned 1T’ was working asa TI. The resistance quantization persisted surprisingly up to
room temperature manifesting an opening of large spin-orbit
energy gap. The gap closing around the sample edges was
also confirmed with a scanning tunneling spectroscopy.
Reference[1] H. Mine, A. Kobayashi, T. Nakamura, T. Inoue, S. Pakdel,
D. Marian, E. Gonzalez-Marin, S. Maruyama, S. Katsumoto, A.
Fortunelli, J. J. Palacios, and J. Haruyama, Phys. Rev. Lett. 123,
146803 (2019).
Authors
H. Minea, J. Haruyamaa, T. Nakamura, and S. Katsumoto
aAoyama Gakuin University

Electronic Structures of 4º-Twisted Bilayer
Graphene Fabricated in a Vacuum
S. Tanaka and F. Komori

Fig. 1. (a) X-ray photoelectron spectroscopy taken at a laser-irradiatedregion (black line). Red and blue lines are those expected for 1T’and 2H phases respectively.(b) Raman shift spectra. Blue: 2H phase
before laser irradiation. Red: mixture of 1T’ phase and 2H phase after
irradiation.

Twisted bilayer graphene (TBG) is a stacked twographene sheets with mutually in-plane rotation, and exhibits
electronic states different from those observed in a typicalbilayer graphene with Bernal stacking. Its Fermi velocity inthe vicinity of the Dirac point strongly depends on the twistangle θ below 10º [1], and vanishes around a magic angle ofθ ~ 1.1º [2]. At this magic angle, a flat band with extremely
sharp density of states is thus realized, and its electronic

ISSP Activity Report 201941

Fig. 1. (a) LEED pattern at the area with TBG. Bright 12 spots are
identified as two sets of 6-fold symmetric graphene diffraction spots
from the lower (gCVD) and upper (gtr ) layers. Inset shows the magnified
image of the graphene diffraction including satellite spots (circles) due
to the moiré structure. (b) STM image taken at the TBG area, showing
graphene atomic image and moiré lattices. Moiré unit cell of the lattice
is indicated by arrows. The image was taken at the sample bias voltage
0.8 V and tunneling current 0.5 nA at 80 K [3].

properties have attracted much attention. TBG is usually
prepared by stacking mechanically-exfoliated graphenesheets in air, and thus it is difficult to prepare a millimeterscale samples with clean interface between the sheets forusing standard surface analysis techniques such as various
diffraction methods and high-resolution photoemission
spectroscopy. To prepare wide TBG’s with clean interface,
we have developed a new method of directly bonding clean
graphene sheets in a vacuum without any chemical adhesion
and transmission media. For this purpose, we use monolayer
graphene that can be easily exfoliated from the SiC substrate[3]. The band structure observed by angle resolved photoemission spectroscopy (ARPES) exhibits changes of theelectronic states due to the interlayer interaction [1].
Graphene sheets are made on a 4H-SiC (0001) substrateby chemical vapor deposition (CVD) using ethylene. A
buffer layer with a 3 × 3 periodicity is formed at the interface between the graphene and substrate, and allows us to
exfoliate the graphene layer easily. It is noted that graphene−−on the thermally decomposed 6√3 × 6√3 -R30º buffer layer(6R3) coexists. The TBG samples are prepared at 200 ºC
in a vacuum by pressing together for 1 hour and detachedat the same temperature. The samples were then evaluatedby optical microscope, μ-Raman spectroscopy, low energy
electron diffraction (LEED), scanning tunneling microscopy(STM) and ARPES. Sub-millimeter areas with TBG wereconfirmed on the substrate.
Observed LEED and STM images of a 4º-rotated TBG
are shown in Fig. 1. Two sets of 6-fold bright graphene spots
with surrounding satellite spots are seen in Fig. 1(a), andcorrespond to lower-layer (gCVD) and upper-layer transferred (gtr) graphene. In the STM image, a moiré pattern
due to the twisted layers is observed with the graphenelattice. The twist angle θ is estimated from the satellite-spot
periodicity and the moiré image. Graphene band structures
measured by ARPES at 300 K using 52 eV synchrotron light(KEK-PF, BL13) are shown in Fig. 2. Two graphene Diraccones appear at Fermi energy (EF) as in Fig. 2(a). The band
structure shown in Fig. 2(b) indicates the both graphene
layers are p-doped. The cone at negative ky originates fromthe transferred graphene on the 3 × 3 substrate. In addition,an n-doped Dirac band is seen in Fig. 2(b), which originatesfrom the transferred graphene on the 6R3 substrate. Energy
gap of the bands due to the interlayer interaction is found at
ky = 0 in the same figure. The Dirac band shown in Fig. 2(b,right) has a kind structure at 0.2 eV.
References[1] J. M. B. Lopes dos Santos et al., Phys. Rev. Lett. 99, 256802(2007).
[2] R. Bistritzer, and A.H. MacDonald, Proc. Natl. Acad. Sci. U. S. A.
108, 12233 (2011).
[3] H. Imamura et al., Appl. Phys. Express 13, 075004 (2020).
Authors
H. Imamuraa, A. Visikovskiya, R. Uotania, T. Kajiwaraa, H. Andoa, T.
Iimori, K. Iwata, T. Miyamachi, K. Nakatsujib, K. Masec, T. Shirasawad, S. Tanakaa, F. Komori
aKyushu University
bTokyo Institute of Technology
cKEK & SOKENDAIdAIST

Pulsed Laser Deposition with
Rapid Target Switching
Y. Matsumoto and M. Lippmaa

Fig. 2. ARPES constant energy and band dispersion maps. The kx axis
is on the line between the middle of the two K points of the graphene
layers (KCVD, Ktr) and Γ point. (a) Constant energy maps at EF without(left) and with (right) image enhancement. Replica bands are seen at
this energy as in left. (b) Band dispersions along ky axis at kx = 1.695
Å-1 (left) and along kx axis at ky = - 0.069 Å-1 (right). Here, the ARPES
intensity is normalized by the Fermi-Dirac distribution function at 300
K. Two Dirac bands of the transferred graphene are seen at negative ky
in the left figure [3].

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ISSP Activity Report 2019

Pulsed laser deposition (PLD) is one of the most common
techniques for growing complex oxide thin films. The
biggest merit of PLD is the near-stoichiometric transfer ofmaterial from a polycrystalline ceramic target to a singlecrystalline thin film, which means that it is usually notnecessary to tune individual cation ratios during the crystal
growth process. Indeed, PLD became the method of choicefor developing high-temperature superconductor thin films
and devices due to the simplicity of phase and stoichiometry
control.
The PLD process is based on the use of a high-intensity
pulsed laser that momentarily heats the surface of a ceramicsource material pellet, forming a rapidly expanding plasma
cloud in vacuum or in a low-pressure reactive gas ambient.

Due to the pulsed nature of the process, the surface of a
growing film reaches a high level of supersaturation when
the plasma plume arrives, but this very high instantaneousdeposition rate is followed by a long relaxation period of up
to about a second. The dynamic aspect of the crystal growthis therefore also important in determining the growth modeand crystallinity of a film.
In recent years, there is increasing interest in materials
property mapping and the use of combined computationaland experimental materials data to feed machine learningmodels for aiding materials discovery and optimization.
In thin film property mapping experiments, it is necessary
to grow large numbers of thin film samples with slightly
different compositions. In a traditional PLD process, thiswould require the synthesis of a large number of bulk
compositions for use as PLD ablation targets, which istechnically difficult. Multi-target PLD has been used to
avoid this problem by using just two ablation targets to growmixed-composition phases. For example, by alternating
ablation from SrTiO3 and BaTiO3 targets, it is possible togrow any intermediate composition in the Ba 1-xSrxTiO3
phase space. Since each unit cell layer usually requires up toa hundred ablation pulses to grow, it is easy to adjust compositions on a percent scale. However, switching between twotargets generally requires mechanical motion of the target
stage, which is a slower process than the normal ablation
pulse period. Since adatom surface migration during theinterpulse period affects the film microstructure, the longer
time delay of target exchange may lead to a nanoscale multiphase film of, e.g., SrTiO3 and BaTiO3 domains instead of amixed Ba1-xSrxTiO3 crystal.
In this project, supported by the Joint Use program,a rapid beam deflection system was developed to switch
ablation between two target materials on a millisecond scale.
[1] The experimental setup is illustrated in Figs. 1(a,b). A
galvanometer mirror is used to direct ablation laser pulses on
one of two targets without having to move the target stage.
The mirror motion is fast enough to interleave depositions

from two targets in a 1:1 pulse sequence at laser pulse rates
of up to 200 Hz.
Various film synthesis experiments were used to test thebenefits of rapid sequential deposition of various mixedcomposition oxide films. The experiments clearly showed thebenefit of rapid target switching in synthesizing (Ba,Sr)TiO3,(Bi,Sm)FeO3, (Mg,Zn)O, Ni-Ge, and Ni-Si films. A particularly interesting capability of this new rapid target switching
PLD system is depth direction grading of film composition for tuning the lattice parameter, doping level, crystal
symmetry, or other properties without sacrificing the deposition rate or risking phase segregation when slower mechanical target stage motion is used for multi-target deposition.
An example of a depth-graded Ba1-xSrxTiO3 film composition is shown in Fig. 1(c). The Ba content was adjusted
over the 100 nm thickness of the film from 0 to 30% asshown by the design plot (blue) and verified by Secondary
Ion Mass Spectrometry (SIMS) as shown in red. The continuous change of the lattice parameter can be seen in the x-ray
diffraction pattern for a gradient film grown on a SrRuO3electrode layer on a SrTiO3 substrate. The spread of the(Ba,Sr)TiO3 film peak corresponds to the continuous composition gradient in the film. This type of gradient films can beused in novel types of photoelectrochemical electrodes andfor constructing polar films with an internal electric field
gradient related to the depth-variable polarization of thelattice.
References[1] S. Maruyama, N. Sannodo, R. Harada, Y. Anada, R. Takahashi,
M. Lippmaa, and Y. Matsumoto, Rev. Sci. Instrum. 90, 093901 (2019).
Authors
S. Maruyamaa, N. Sannodoa, R. Haradaa, Y. Anadaa, R. Takahashi,
M. Lippmaa, and Y. Matsumotoa
aTohoku University

Density Functional Theory
with Machine Learned Functional
R. Nagai, R. Akashi, and O. Sugino

Fig. 1. (a) Photograph of the ablation target stage, showing the ablation
spot on the lower target. (b) Schematic diagram of the rapid galvanometer beam switching between two targets. (c) Design (blue) andmeasured (red) composition of a (Ba,Sr)TiO3 film. The x-ray diffraction pattern in (d) shows that the film lattice parameter varied continuously in the film.

Modern electronic structure theory of materials is mostfrequently based on the Kohn-Sham density functional
theory (KS-DFT), where the accuracy depends on thequality of the density-to-energy mapping called exchangecorrelation (xc) functional. The mapping has extremely
large degrees of freedom that are too complex to determine
manually. In this context, we have proposed a machinelearning approach [1]. We have prepared the atomizationenergy (Ea) and electron density n(r) of reference molecules
using a quantum chemical simulation and let neural network(NN) learn the data. In the learning, the energy Ea and thedensity n(r) are related via a functional of the density called
the xc potential εxc[n](r), and the parameters characterizing the functional are optimized to best reproduce theinput data. The mapping thus established depends on theform of the functional, such as the point-to-point mapping
called local density approximation (LDA) and the mapping
reinforced using the density gradient called generalized
gradient approximation (GGA) or meta-GGA. Quality of
the functionals was tested for a ten dozen of unreferenced
molecules and was found superior to existing ones foreach form, suggesting possibility to systematically develop
accurate functionals.

ISSP Activity Report 201943

Fig. 1. Machine learning scheme for developing exchange correlation (xc) functional. Referring to electron density n(r) and atomizationenergy Ea of a few molecules, the xc energy was related to the density
as a functional εxc[n](r). With this machine learned functional, n(r) and
Ea of unrefernced materials were found to be predicted more accurately
than with conventional functionals.

In our study, we have used three molecules, H2O, NH3,
and NO, that contain different bonding property and spinpolarization, as the reference. The NN successfully determined the parameters without overtraining owing large
number of data contained in the density, which amounts
to 100,000. We further tried to include nonlocality into thefunctional beyond the gradient approximations by considering the electron density averaged in the vicinity of anelectron. The resulting functional form, called near region
approximation (NRA), improves the accuracy equivalent tothat achieved using the nonlocal exchange, allowing thereby
significantly to reduce the computational complexity.
In view of the ability of artificial intelligence to recognize a picture, it is likely to be possible to learn the chemicalbonds without explicitly indicating a descriptor. Suchadvance in the method will enable a researcher to develop ahigh-quality functional dedicated to the target materials ofinterest with an effect of speed-up of material design. Impact
of systematic functional development is thus expected to belarge in the computational materials science.

ture. So far, first-principles calculations based on density
functional theory (DFT) have been performed extensively for
identifying (meta-)stable structures at T = 0 K. On the other
hand, predicting finite-temperature structures based on DFT
is still challenging because it requires a proper treatment ofentropic effects of phonons and magnons. In particular, thevibrational entropy plays a central role in realizing structural phase transitions of nonmagnetic materials, including
perovskite oxides such as BaTiO3. However, the conventional phonon calculation method based on the harmonic
approximation (HA) often yields unstable phonon modes forhigh-temperature phases of phase change materials, whichhinders an estimation of vibrational entropy.
To overcome the limitation of the conventional phonon
calculation method, we have recently developed a new
approach that is applicable to high-temperature phases [1].
Our method is based on the self-consistent phonon (SCP)
theory, which incorporates the anharmonic renormalizationof phonons at the mean-field level. We have so far employed
the developed SCP scheme to compute finite-temperaturephonons of cubic SrTiO3 [1], thermoelectric clathrate [2],and ScF3 [3], where we have obtained an excellent agreementwith the experimental data. Also, it can be used to computethe vibrational free energy of solid whose structure is dynamically unstable within the HA, as has been demonstrated for
ScF3 [3] and hydrogen-rich superconductors [4].
To further test and improve the prediction accuracy of
our approach, we have recently applied the SCP method toan all-inorganic halide perovskite CsPbBr3, which has been
actively studied in recent years as an efficient photovoltaic material. CsPbBr3 shows a successive structural phasetransition with cooling from cubic α phase (Pm3̄m) to thetetragonal β phase (P4/mbm) at 403 K and then to the orthorhombic γ phase (Pnma) at 361 K. When the HA was used,
we obtained unstable phonon modes for the high-temperaturephases as shown in Fig. 1. These phonon modes involve a
distortive motion of the bromine octahedra; therefore, thesesoft modes, particularly those at the M (1/2,1/2,0) point of
the α phase and the Z (0,0,1/2) point of the β phase, may
act as the soft modes of the structural phase transition. After
including the effect of finite-temperature renormalization by
the SCP theory, all phonon modes were stabilized, as shownby solid lines in Fig.1. After that, we calculated the vibra-

Reference[1] R. Nagai, R. Akashi, and O. Sugino, npj Compt. Mater. 6, 43(2020).
Authors
R. Nagai, R. Akashia and O. Sugino
aThe University of Tokyo

Predicting and Understanding Structural
Phase Transition of Solids fromFinite-Temperature Phonon Calculations
T. Tadano and W. A. Saidi
The study of structural phase transition has been an active
research subject in condensed matter physics and materials
science because a variety of materials’ properties, including
but not limited to transport, optical, and superconducting
properties, often changes sensitively with the crystal struc-

44

ISSP Activity Report 2019

Fig. 1. (Top) Crystal structures of α, β, and γ phases of halide
perovskite CsPbBr3. (Bottom) Phonon dispersion curves of threedifferent phases of CsPbBr3 calculated within the harmonic approximation (dotted lines) and self-consistent phonon method at 500 K (solidlines). The unstable phonon modes in the high-temperature phases(β, γ) are stabilized by anharmonic renormalization at finite temperature.

tional free energies with including the effect of the quartic
anharmonicity [3] and predicted the phase transition temperature (Tc) based on the calculated Helmholtz free energies.
When we used the PBEsol exchange-correlational functional,
the Tc values of the cubic-to-tetragonal and tetragonal-toorthorhombic phase transitions were predicted as Tc ~ 280
K and Tc ~ 180 K, respectively, both of which underestimate
the experimental results. To improve the prediction accuracy,
we have further considered the higher-order correction to thevibrational free energy that originates from the cubic anharmonicity. Then, the Tc value of the cubic-to-tetragonal phasetransition changed as Tc ~ 400 K, which shows better agreement with the experimental value.
We are also interested in how the structural phase transition affects thermal transport properties. To investigate thispoint quantitatively, we have calculated the lattice thermal
conductivity of CsPbBr3 for the three different phases. To
this end, we calculated the phonon lifetimes by considering the three-phonon scattering process on top of the SCP
calculations. After that, the lattice thermal conductivity
was estimated, where both of the particle-like transport andcoherent transport terms were calculated following Ref. [5].
For all phases, the thermal conductivity values were smaller
than 1 W/mK, indicating the strong lattice anharmonicity of
CsPbBr3. We found that the lattice thermal conductivity of
the high-temperature α phase was larger than those of the
low-temperature phases due to the longer phonon lifetimes in
the highly symmetric α phase, which can be attributed to thesmaller scattering phase space of the three-phonon process.
Interestingly, unlike the conventional semiconductors wheremost of the thermal transport is accounted for by the highly
dispersive acoustic phonons, more than ~60% of the total
thermal conductivity was carried by optical phonons above
30 cm-1 in CsPbBr3.
The computational results on CsPbBr3 again highlightthe versatility and efficiency of the SCP theory for studying
lattice dynamics in solids. While it is tempting to state that
the SCP-based approaches are already reliable enough tomake a quantitative prediction of structural phase transitions, many effects are yet to be investigated, including
thermal expansion and choice of the exchange-correlation
functionals. Investigating these effects on a Tc value would
be an important future direction for our forthcoming studies.

Neutron Spin Resonance andHorizontal Line Nodes in Sr2RuO4
K. Iida, K. Suzuki, and T. Masuda
Sr2RuO4 with Tc = 1.5 K has attracted a great deal ofinterest as the prime candidate for the chiral p-wave superconductor. Recently, however, experimental and theoreticalstudies under an application of uniaxial pressure along<100> reported a factor of 2.3 enhancement of Tc, raising
the possibility of an even-parity spin-singlet order parameterin Sr2RuO4 [1]. More recently, new NMR results demonstrated that the spin susceptibility substantially drops below
Tc, ruling out the chiral-p spin-triplet superconductivity [2].
As such, these recent works turn the research on Sr2RuO4
towards a fascinating new era. So far, various experimental
techniques reported that the superconducting gaps of
Sr2RuO4 have line nodes, but the details of the line nodes,
e.g. of the vertical or horizontal nature, are not uncovered
yet. Since the complete information of the superconducting
gaps can shed light on the symmetry of the pairing, exclusive
determination of the direction of the line nodes in Sr2RuO4 ishighly desirable. To investigate in detail the superconducting
gap, we performed the high-resolution inelastic neutron
scattering (INS) measurements and random phase approximation (RPA) calculations.
Three single crystals were co-aligned with the (HHL)
plane been horizontal to the scattering plane using the highenergy x-ray Laue diffractometer installed at ISSP. We
performed INS measurements at 0.3 and 1.8 K using the disk
chopper time-of-flight neutron spectrometer AMATERAS
installed at J-PARC.
The most pronounced magnetic signal in the normal
state of Sr2RuO4 is nearly two-dimensional incommensurate (IC) magnetic fluctuations at QIC = (0.3, 0.3, L) owing
to the Fermi surface nesting between (or within) the quasione-dimensional α and β sheets [3]. We focused on the

References[1] T. Tadano and S. Tsuneyuki, Phys. Rev. B 92, 054301 (2015);
J. Phys. Soc. Jpn. 87, 041015 (2018); J. Ceram. Soc. Jpn. 127, 404(2019).
[2] T. Tadano and S. Tsuneyuki, Phys. Rev. Lett. 120, 105901 (2018).
[3] Y. Oba, T. Tadano, R. Akashi, and S. Tsuneyuki, Phys. Rev.
Materials 3, 033601 (2019).
[4] I. Errea, F. Belli, L. Monacelli, A. Sanna, T. Koretsune, T. Tadano,
R. Bianco, M. Calandra, R. Arita, F. Mauri, and J. A. Flores-Livas,
Nature 578, 66-69 (2020).
[5] M. Simoncelli, N. Marzari, and F. Mauri, Nature Phys. 8, 809-813(2019).
Authors
T. Tadanoa, W. A. Saidib
aNational Institute for Materials SciencebUniversity of Pittsburgh

Fig. 1. (a) Low-energy INS intensity maps regarding the IC magnetic
fluctuations in Sr2RuO4 along (H, H, 0.5). (b) I(ћω) cuts at Q = (0.3,
0.3, 0.5) below and above Tc. (c) I(Q) cuts along Q = (H, H, 0.5) at 0.3
and 1.8 K with the energy window of 0.56 meV. (d) I(Q) cut along Q =(0.3, 0.3, L) at 0.3 K with the energy window of 0.56 meV.

ISSP Activity Report 201945

Large Enhancement of Thermoelectric
Efficiency Due to Pressure-Induced
Lifshitz Transition in SnSe
H. Sakai, M. Tokunaga, and Y. Uwatoko

Fig. 2. Calculated density maps of the dynamical spin susceptibility
assuming the horizontal line nodes in Sr2RuO4 for energy ћω = 2Δ0.

low-energy IC magnetic fluctuations in Sr2RuO4 below
and above Tc to search a spin resonance as the feedback
effect from the superconducting gaps. We observed a spinresonance with energy of ћωres = 0.56 meV centered ata characteristic wavevector Qres = (0.3, 0.3, 0.5) (Figs.
1(a)–1(c)). The resonance energy corresponds well to thesuperconducting gap 2Δ = 0.56 meV estimated by thetunneling spectroscopy. In addition, the spin resonanceshows the L modulation with a maximum at around L = 0.5(Fig. 1(d)), indicating the three-dimensional gap structure.
To theoretically elucidate the origin of the L modulated
intensity of the spin resonance in Sr2RuO4, RPA calculations
assuming the horizontal line nodes at the superconducting
gaps were also performed. Figure 2 illustrates the calculated dynamical spin susceptibilities for energy ћω = 2Δ0.
The dynamical spin susceptibility shows the maximum at
Q = (1/3, 1/3, 0.5) and (2/3, 2/3, 0.5). The superconducting
gaps with the horizontal line nodes give such the feature
along L, which is indeed observed in our INS measurements(Fig. 1(d)).
The L modulation of the spin resonance and our RPA
calculations indicate that the superconducting gaps regarding
the quasi-one-dimensional α and β sheets at the Fermi
surfaces have the horizontal line nodes [4]. These results may
set a strong constraint on the pairing symmetry of Sr2RuO4.
References[1] A. Steppke et al., Science 355, eaaf9398 (2017).
[2] A. Pustogow et al., Nature (London) 574, 72 (2019).
[3] K. Iida et al., Phys. Rev. B 84, 060402(R) (2011).
[4] K. Iida et al., J. Phys. Soc. Jpn. 89, 053702 (2020).

Control of the number of Fermi pockets, i.e., electronic
valleys, in conductive materials has recently attracted agreat deal of attention in terms of the emerging electronics
utilizing valley degrees of freedom. An important physicalaspect of the valley engineering is that the emergence andannihilation of the valleys are described by a topologicalelectronic transition known as Lifshitz transition. The
Lifshitz transition is totally different from the conventional
Landau-type phase transition, because it is not accompanied
by any symmetry breaking. Despite many predictions of its
important roles on various quantum phenomena, the experimental verification has remained challenging.
In this study [1], we have revealed a marked influenceof the pressure-induced Lifshitz transition on thermoelectric
efficiency in SnSe (Fig. 1(a)), which has recently become
the focus of attention for its high thermoelectric performance[2]. Thermopower, which is proportional to the energy derivative of density of states, is anticipated to be an ideal probe
for the Lifshitz transition. However, it has been overlookedso far owing to the lack of a model thermoelectric material,
where the valley state is controllable. Here we have discovered that the band structure as well as thermoelectric performance for SnSe is highly sensitive to the external pressure
by measuring the quantum oscillation of magneto-resistivity
concurrently with the thermopower in a piston cylinder typehigh pressure cell (Fig. 2).
The obtained pressure dependence of the quantum oscillation frequencies clearly indicates that the number of thevalleys increases from two to four at ~1 GPa with increasingpressure (Fig. 1(b) upper panel), as is semi-quantitatively
reproduced by the first-principles calculations. At around 1.6
GPa, the four valleys are almost equivalent in size, where
the thermoelectric power factor is more than 100% higher
than that at ambient pressure over a wide temperature range(10–300 K) (Fig. 1(b) lower panel).
The present result demonstrates the multi-valley state isof great importance for achieving high thermoelectric performance, providing a guide for band engineering for the SnSebased thermoelectric materials. Furthermore, since atomically thin SnSe films have recently attracted much attention

Authors
K. Iidaa,b, M. Kofua,c, K. Suzukid, N. Muraic, S. Ohira-Kawamurac,
R. Kajimotoc, Y. Inamurac, M. Ishikadob, S. Hasegawa, T. Masuda, Y.
Yoshidae, K. Kakuraib, K. Machidad, and S.-H LeeaaUniversity of Virginia
bCROSS
cJ-PARC CenterdRitsumeikan University
eAIST

Fig. 1. (a) Schematic of the layered crystal structure of SnSe. (b) (upper
panel) A schematic illustration of the pressure variation in the valley
topology for SnSe. (lower panel) Pressure dependence of the thermoelectric power factor at 150 and 300 K for SnSe.

46

ISSP Activity Report 2019

Fig. 2. Setup of the thermopower and resistivity measurements underpressure.

as a new 2D material, the controllability of valley structure
presented here may help for exploring their novel functionsin the future electronics.
References[1] T. Nishimura, H. Sakai, H. Mori, K. Akiba, H. Usui, M. Ochi,
K. Kuroki, A. Miyake, M. Tokunaga, Y. Uwatoko, K. Katayama, H.
Murakawa, and N. Hanasaki, Phys. Rev. Lett. 122, 226601 (2019).
[2] L.-D. Zhao, S.-H. Lo, Y. Zhang, H. Sun, G. Tan, C. Uher, C.
Wolverton, V. P. Dravid, and M. G. Kanatzidis, Nature (London) 508,
373 (2014).
Authors
T. Nishimuraa, H. Sakaia,b, H. Moria, K. Akiba, H. Usuia, M. Ochia,
K. Kurokia, A. Miyake, M. Tokunaga, Y. Uwatoko, K. Katayamaa, H.
Murakawaa, and N. Hanasakia
aOsaka University
bPRESTO, JSTcoil (STC) at the ISSP for magnetic field generation and an
infrared CO2 gas laser as the light source. The wavelengths
used were 9.22, 10.48, and 10.6 μm. The laser light shines
the sample and the transmitted light intensity was measured
as a function of time during a pulse of a magnetic field (B)generated by the STC. A high speed HgCdTe PIN photo
diode was used for detection of the light signal. The samples
were grown on a semi-insulating InP (100) substrate, using
a molecular beam epitaxy system. The InxAl1−xAs buffer isgrown at low temperature to help mitigate formation of dislocations originating from the lattice mismatch between the
InP substrate and higher levels of the heterostructure [2]. The
indium content of InxAl1−xAs is step-graded from x = 0.52
to 0.81. Next, a delta-doped Si layer of ~ 7.5 × 1011 cm−2
density is placed here followed by 6 nm of In0.81Al0.19As.
The quantum well is grown next, consisting of a 4 nm thicklayer of In0.81Ga0.19As layer, a 4 nm thick layer of InAs, andfinally a 10 nm thick top layer of In0.81Ga0.19As.
Figure 1 shows the CR spectra at 300 and 20 K. In theupper panel, the CR peaks around 40 T indicated by solid
allows are interpreted as the spin-split resonances : They are
the inter-Landau level transitions between n = 0 and n = 1
with different spin polarization, where n is the Landau levelindex. Another CR peak at around 55 T is, on the other hand,due to the transition between n =1 and n = 2 levels. Whenthe temperature is lowered to 20 K, only one of the spin-split
CR peaks is observed, which can be explained in terms of the
population of carriers. They populate only in the lowest levelat a low temperature. Remaining 55-T CR peak confirm that
this resonance is due to non-equilibrium distributed carriers.
The effective mass of electrons (m*) is directly deduced
from the peak position Bc. The left-panel of Fig. 2 shows themagnetic field dependence of m*. It is found that they are
considerably larger than the band-edge effective mass of abulk InAs 0.023m0, where m0 is the mass of a free electron.
The large effective mass is due to the quantum confinement

Cyclotron Resonance in Near Surfaceof InAs Quantum Well
G. A. Khodaparast, B. A. Magill,
and Y. H. Matsuda
Electronic states of semiconductor nanostructure nearsurface has been attracting much attention in terms ofcontrolling quantum effects that can emerge through correlation with a superconductor. Josephson junctions made from
Al-InAs have been used to create tunable superconducting
qubits [1]. The key feature of the superconductor (Al) andsemiconductor (InAs) nanostructure is that the two-dimensional gas (2DEG) is confined near the surface, in close
proximity to the superconductor.
Although the electronic state of a bulk InAs is wellunderstood, it is expected that the quantum confinement
effect modifies the electronic state. Moreover, a narrowgap semiconductor exhibits small effective mass and large
g factor, and the non-parabolicity of the band due to theconfinement affects these fundamental parameters. Investigation of the electronic state of the 2DEG in InAs quantum
well is essential for development of the novel semiconductorsuperconductor device. In the present study, the cyclotron resonance (CR) of an InAs quantum well has been
investigated. Clear spin-split cyclotron resonance has been
observed and the effective mass and the g factor are deduced
as a function of magnetic field.
The CR experiment was performed with the single-turnFig. 1. Cyclotron resonance (CR) in the InAs quantum well. The upper
panel shows the resonance spectrum at 300 K. The two solid arrows
suggest the spin-split CR. The broad CR indicated by the dashed arrowis due to the inter-Landau level transition between n = 1 and n = 2. The
lower panel shows the CR at 20 K. Observation of only single peaknear 40 T indicates effect of the thermal population of carriers. Since
the CR around 55 T is considered to be due to the non-equilibrium ofelectrons, it remains at a low temperature.

ISSP Activity Report 201947

Fig. 2. Left-side panel: The obtained effective mass of electrons. The
rather large field variation indicates the strong non-parabolicity ofthe energy band. The right-side panel: The g-factor deduced fromthe experiment and numerical analysis. It is clear that g-factor is also
affected by the non-parabolicity.

effect and the cyclotron energy. The enhancement of m*with increasing magnetic field is interpreted as a result of
the non-parabolicity of the band due to the cyclotron energy.
The g factor can also be obtained by comparison between
the CR peak position and the calculated inter-Landau level
energies. It has been found that the g factor becomes small
with increasing cyclotron energy. The g factor of a bulk InAs
is known to be -14.
The magnetic field dependences of the m* and g leads us
to the values at zero magnetic fields and they are found tobe 0.027m0 and -12, respectively [3]. Determination of thesequantum parameters will significantly contribute development of the novel Josephson-junction devices.
References[1] T. W. Larsen, K. D. Petersson, F. Kuemmeth, T. S. Jespersen,
P. Krogstrup, J. Nygard, and C. M. Marcus, Phys. Rev. Lett. 115,
127001 (2015).
[2] J. Shabani, S. Das Sarma, and C. J. Palmstrøm, Phys. Rev. B 90,
161303 (2014).
[3] J. Yuana, M. Hatefipoura, B. A. Magillb, W. Mayera, M. C.
Dartiailha, K. Sardashtia, K. S. Wickramasinghea, G. A. Khodaparastb,
Y. H. Matsuda, Y. Kohama, Z. Yang, S. Thapac, C. J. Stantonc, andJ. Shabania, Phys, Rev. B 101, 205310 (2020).

These criteria were found to be realized in the spin-1/2
Heisenberg-Ising chain antiferromagnetic material
BaCo2V2O8. Our recent work has revealed a unique quantum
critical behaviors under a strong transverse magnetic field(B⊥c) of 40 T [1], which is characteristic for the transversefield Ising quantum critical point. In contrast, in an applied
longitudinal field (B//c), the phase diagram of the Heisenberg-Ising antiferromagnetic chain is completely different.
Two phase transitions can be induced, and the different
phases are characterized by different quantum spin excitations. In this work, we performed high-resolution terahertz
spectroscopy and magnetocaloric-effect (MCE) measurements in BaCo2V2O8 as a function of temperature andapplied longitudinal magnetic field [2]. We found characteristic features revealing the field-induced quantum phasetransitions, and observed the characteristic spin dynamics forthe various phases. In particular, many-body string excitations as well as low-energy fractional excitations [3] wereidentified in the field-induced gapless phase, by comparing
to the Bethe-Ansatz calculations [2]. Moreover, approaching
the field-induced quantum phase transition from higher field,
we observed a dominant contribution of the higher-energy
string states than the lower-lying fractional excitations, incontrast to the conventional belief.
Figure 1(a) shows the MCE data, T(B), measured at
different initial temperatures. The T(B) starting from 1.7
K at zero field reaches a minimum at the critical field (Bc)of 3.8 T, following by a slight increase towards higher
fields. Before the temperature jump above the saturation
fields of 22.9 T, the MCE detect a weak minimum at about
19.5 T. The weak minimum corresponds to the half-saturated
magnetization as seen in Figure 1(b) and possibly reflects
commensurate fluctuations. In down-sweeping data, theT(B) curve matches with the up-sweeping curve, except
for a (irreversible) heating effect at the phase boundary.
At higher temperatures, the anomalies at the critical andthe half-saturated fields broaden and smear out, whereas
the minimum at saturation fields remains and becomes thedominant feature. These observations qualitatively agree withthe calculated phase diagram for the 1D Heisenberg-Ising

Authors
J. Yuana, M. Hatefipoura, B. A. Magillb, W. Mayera, M. C. Dartiailha,
K. Sardashtia, K. S. Wickramasinghea, G. A. Khodaparastb, Y. H.
Matsuda, Y. Kohama, Z. Yang, S. Thapac, C. J. Stantonc, and J.
Shabania
aNew York UniversitybVirginia TechcUniversity of Florida

Many-Body String Excitations in the
Antiferromagnetic Ising Spin Chain
Compound BaCo2V2O8
Z. Wang, Y. Kohama, and K. Kindo
The one-dimensional (1D) spin-1/2 Heisenberg-Isingmodel is the paradigmatic model for the study of quantum
phase transitions and spin dynamics. However, the realization of this spin model in a solid-state material and its study
are quite challenging. Several criteria must be fulfilled simultaneously: 1. the intrachain coupling should be dominant
over the interchain coupling. 2. A significant Ising-likeanisotropy should be present. 3. The required experimental
conditions, such as high magnetic fields, should be available.

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ISSP Activity Report 2019

Fig. 1. (a) Magnetocaloric effect T(B), (b) magnetization of BaCo2V2O8
measured in the longitudinal configuration (B//c). The solid red line in(b) is calculated from the Bethe ansatz solution of the Heisenberg-Isingmodel with a paramagnetic background.

model, confirming that the BaCo2V2O8 is a suitable system
for investigating the string and the additional fractional
magnetic excitations. This conclusion is also checked by themagnetization data, which shows quantitatively agreementwith the 1D Heisenberg-Ising model (Red curve in Fig.1(b)).
We investigated the dynamical properties with THz transmission spectra in the applied longitudinal magnetic field.
These data are available in Ref. [2] which tracked the fielddependence of exotic spin excitations, i.e., many-body string
and fractional excitations. We have revealed their selection
rule in the vicinity of a field induced QCP in the Ising-likespin-½ chain compound. While the gapped fractional excitations are dominant in the 3D ordered state, we found thatthe high-energy string excitations plays an important role inquantum critical dynamics, which is consistent with BetheAnsatz calculations [2].
References[1] Zhe Wang et al., Phys. Rev. Lett. 120, 207205 (2018).
[2] Zhe Wang et al., Phys. Rev. Lett. 123, 067202 (2019).
[3] Zhe Wang et al., Nature 554, 219 (2018).
AuthorsZhe Wanga,b,c, A. Loidla, Y. Kohama and K. Kindo
aUniversity of Augsburg
bHelmholtz-Zentrum Dresden-Rossendorf
cUniversity of Cologne

Operando Soft X-ray Emission Spectrosopy
of Fe2O3 Anode for Li-Ion Battery
D. Asakura, E. Hosono, J. Miyawaki,
and Y. Harada
Improving the energy and power density of electrodematerials for Li-ion batteries (LIBs) is highly importantto further develop electric vehicles. For the improvementsof battery performance, understanding the charge(Li-extraction) and discharge (Li-insertion) mechanisms
from the viewpoint of the electronic structure isindispensable. Here, we demonstrate operando Fe L3-edge
resonant XES for Fe2O3 which is an anode material of LIB[1]. It is known that the discharge/charge mechanism isbased on conversion reaction denoted as Fe2O3 + 6Li+ + 6e⇔ 3Li2O + 2Fe [2,3]. To confirm the conversion reaction
and further investigate the electronic-structure charge, we
analyzed the operando XES spectra using full-multiplet
calculation [4,5].
The operando cell consists of Fe2O3 poly-crystalline
thin film (~100 nm), a Li-metal counter electrode, andan electrolyte solution [1,6]. The operando XES experiments were carried out with the HORNET XES spectrometer at BL07LSU of SPring-8 [7]. The electrochemical
discharge/charge was performed by cyclic voltammetry.
The operando XES measurements were at an open-circuit
voltage (OCV), discharged state and charged state on thesecond discharge-charge cycle.
Figure 1 shows the XES results [1]. The XES spectrum
for the initial state is attributed to Fe3+ state because the XESline shape is almost the same as that for Fe2O3 in previous
reports [8]. On the other hand, the valence of Fe at the
OCV before the second cycle was not Fe3+, but Fe2+ witha weak p-d hybridization, suggesting considerable irreversibility on the first discharge-charge cycle and a weakened
Fe-O bond after the first cycle. Moreover, we revealed that

Fig. 1. Operando Fe L3-edge resonant XES spectra for the Fe2O3 on thesecond cycle taken with the excitation energy Ein = 708.2 eV [1]. The
Fe L3-edge XES spectrum for the initial state is also displayed. All the
spectra are plotted as a function of energy loss (Eloss).

the Fe 3d electronic-structure change during the second
cycle was to some extent reversible as Fe2+ (2.7 V vs.
Li/Li+: OCV) → Fe0 (0.1 V vs. Li/Li+: discharged) → Fe(2+d)+(3.0 V vs. Li/Li+: charged). This operando Fe L3-edge
resonant XES in combination with the full-multiplet calculation provides detailed information for redox chemistryduring a discharge-charge operation that cannot be obtainedby other methods such as crystal-structure and morphology
analyses. XES is thus very powerful to investigate the origin
and limitation of the lithiation function of anodes involving
conversion reaction.
References[1] D. Asakura et al., Phys. Chem. Chem. Phys. 21, 26351 (2019).
[2] S. Morzilli et al., Electrochem. Acta 30, 1271 (1985).
[3] J. Chen et al., Adv. Mater. 17, 582 (2005).
[4] D. Asakura et al., J. Phys. Chem. Lett 5, 4008 (2014).
[5] D. Asakura et al., Phys. Chem. Chem. Phys. 21, 18363 (2019).
[6] D. Asakura et al., Electrochem. Commun. 50, 93 (2015).
[7] Y. Harada et al., Rev. Sci. Instrum. 83, 013116 (2012).
[8] J. Miyawaki et al., Phys. Rev. B 96, 214420 (2017).
Authors
D. Asakuraa, Y. Nanbaa, M. Okuboa, H. Niwa, H. Kiuchib, J. Miyawaki,
M. Oshimab, E. Hosonoa, and Y. HaradaaNational Institute of Advanced Industrial Science and Technology(AIST)
bThe University of Tokyo

Mass Transport in the PdCu Phase Structuresduring Hydrogen Adsorption andAbsorption Studied by Ambient-Pressure
XPS under Hydrogen Atmosphere
J. Tang, I. Matsuda, and J. Yoshinobu
Pd-based alloys have been the attractive materials forthe hydrogen permeation process, because they possess
higher hydrogen permeability and suppress the hydrogen
embrittlement compared to pure Pd bulk. It was reported
that the PdCu alloy, especially so-called “PdCu40 alloy”,
has a relatively higher hydrogen diffusivity than the other
Pd-alloys (PdAg, PdAu, PdRh, etc.). Industrial usages of

ISSP Activity Report 201949

the PdCu40 alloys with the high Cu content also improvethe economic perspectives because of the lower Cu cost.
Although there are advantages of PdCu alloys in the application fields, it requires further improvements in microscopic
aspects. For example, hydrogen solubility of the facecentered cubic (fcc) structure of a PdCu alloy is approximately one order of magnitude higher than that of bodycentered cubic (bcc) structure.
In the present research, we traced Pd and Cu atoms
during hydrogen adsorption and absorption processes atthe elemental steps, dissociative adsorption of H atoms on
the surface and dissolution of H atoms into the bulk, by theindividual core-level photoelectron spectroscopies under thehydrogen atmosphere [1]. The absorption process is investigated by the Ambient Pressure (AP)-XPS measurements
at the soft X-ray undulator beamline BL07LSU at SPring-8.
The PdCu40 sample, provided from Tanaka Kikinzoku Kogyo
K.K., was annealed at 673 K to form the B2 phase structure.
Then, the B2/fcc phase structure was prepared by subsequent annealing at 1000 K. Figures 1(a) and (b) show theFig. 1. XRD patterns of (a) B2 and (b) fcc/B2 mixed-phase PdCu
alloys.

XRD patterns of the B2- and fcc/bcc- phased PdCu alloys,
respectively. Figures 2 and 3 show the temperature dependence of hydrogen absorption reactions on the B2 and fcc/
bcc phases, observed by measuring the Pd 3d5/2 and Cu
2p3/2 XPS spectra during the hydrogen exposure at 1.5 Torr
at 300-573 K. The temperature-dependent data show thata number of the Pd-H bond increased accompanied withthe decrease of the Cu-H bond with temperature. The Pd
hydride formation in the B2 and fcc/bcc structures show thesame behavior at 300-373 K, however, more Pd hydride wasformed in the B2 structure than in the fcc/bcc structure at473-573 K. The decrease rate of Cu-H component is higher
in the B2 phase than fcc/bcc phase. These results indicatethat the interstitial hydrogen occupied the different sites in
the PdCu alloy bulk at different temperature and confirms thedifference in absorption between the two phases. Comparing
with the previous calculation [2], the significant enhancement of the hydride formation at 473 K can consistently bedescribed in terms of variation of the hydrogen diffusion
paths from the Cu-rich octahedral site to the energeticallyfavorable Pd-rich octahedral site. Furthermore, the activationenergy of the hydrogen diffusion is found to be lower in theB2 phase than that in the fcc/B2 phase. Our elucidation ofhydrogen diffusion mechanisms on the different phase structures gives a new understanding of the hydrogen absorption
process, which could be the critical point for the rate-limiting
process in the hydrogen permeation through the PdCu alloy.
Similar investigation on hydrogen permeation process
was carried out for the PdAg alloy[3]. The measurements of
AP-XPS revealed that Pd atoms were also found to reactive
sites during hydrogen absorption. On the other hand, theprocess is likely promoted by the Pb segregation. It is ofinterest to make systematic investigations of individual
Pd-alloys to capture the whole picture.
References[1] J. Tang, S. Yamamoto, T. Koitaya, A. Yoshigoe, T. Tokunaga, K.
Mukai, I. Matsuda, and J. Yoshinobu, Appl. Surf. Sci. 480, 419 (2019).
[2] L. C. Liu, J. W. Wang, Y. H. He, and H. R. Gong, J. Membr. Sci.
542 , 24 (2017).
[3] J. Tang, S. Yamamoto, T. Koitaya, Y. Yoshikura, K. Mukai, S. Yoshimoto, I. Matsuda, and J. Yoshinobu, Appl. Surf. Sci. 463, 1161 (2019) .
Authors
J. Tanga, I. Matsuda, and J. Yoshinobu
aUniversity of Hyogo

Sub-Cycle Spectroscopy of Proton
Transfer in Ferroelectric Cocrystals
Driven by Intense THz and MIR Fields
T. Umanodan, Y. Okimoto, and J. ItataniFig. 2. Pd 3d5/2 spectra measured on (a) clean surface, and under H2
exposure at 1.5 Torr at (b) 300 K, (c) 373 K, (d) 473 K, (e) 573 K withphoton energy of 680 eV on PdCu(B2) alloy. Pd 3d5/2 spectra measuredon (f) clean surface, and under H2 exposure at 1.5 Torr at (g) 300 K, (h)373 K, (i) 473 K, (j) 573 K with photon energy of 680 eV on PdCu(fcc/
B2) alloy. Surface Pd component, bulk Pd component, CO adsorption
component and Pd-H component are presented with green, blue, purple,
and yellow solid lines, respectively.

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Ferroelectricity in organic materials is one of the mostimportant topics in condensed matter physics. It has a wide
range of practical applications because organic systems
possess advantages such as mechanical flexibility, printability, disposability, and cost-effectiveness. Very recently,
proton-mediated co-crystals have been developed as novel
organic ferroelectric materials [1]. In these co-crystals,
proton ordering along the hydrogen bonds between the
proton-donor and -acceptor molecules leads to the emergenceof ferroelectricity, leading to the lack of inversion symmetry.
We focused on a proton-mediated co-crystal [H-dppz][Hca](dppz = 2,3-di(2-pyridinyl)pyrazine, Hca = chloranilate) that

Fig. 1. (a) Crystal structure of Hdppz-Hca cocrystal. (b, c) Schematic
illustrations of the (b) ferroelectric and (c) paraelectric phases.

shows ferroelectricity at room temperature (Tc = 402 K).
Figure 1 shows the crystal structure (a), the ferroelectric andparaelectric origins due to the position of protons (b, c), andthe imaginary part of dielectric constant at room temperature(d).
In this study [2], we investigated the ultrafast dielectric
response of [H-dppz][Hca] by sub-cycle spectroscopy where
carrier-envelope phase-stable intense optical pulses at 1 or
40 THz drove the ferroelectricity, then sub-cycle visible
pulses probes the field-driven dynamics.
First, we irradiated the co-crystal with a single-cycle
THz pulse at a center frequency around 1 THz with amaximum electric field of ~1 MV/cm, and measured thesecond harmonic (SH) of 100-fs laser pulses at 800 nm as afunction of the relative delay between the THz and 800-nm
pulses. The modulation of SH intensity exactly followed the
waveform of the applied THz electric field, and the magnitude of the relative SH change reached up to 10%. This result
indicates that the ultrafast change of the ferroelectricity was
directly driven by the THz electric field.
Second, we irradiated the crystal with an intense
mid-infrared (MIR) pulse (center frequency ~40 THz,maximum electric field ~30 MV/cm), and found that theinduced SH change occurred very differently. The time
profile of observed change of SH intensity was not scaled by
the field profile of the 40 THz pulse as shown in Fig. 2(b).
The SH change contained a component proportional to thesquare of the applied electric field in addition to the linear
component that is proportional to the field as observed in
the 1-THz case. These results suggest that the change of thepolarity of the co-crystal cannot follow the oscillation of theelectric field at 40 THz.
To understand these field effects, we analyzed thefollowing nonlinear equation of motion that contains the
C-O- stretching mode,

Fig. 2. (a-d) Calculated amplitudes of C-O- stretching mode for various
anharmonic parameter a. The black circles in (c) denote the observed
time profile of the SH intensity. (e, f) Schematic illustrations of the
proton motion in the binding potential of the cocrystal when a THz
or MIR pulse is applied. X0, X' denote the equilibrium position and ametastable position with excitation pulses, respectively.

equation, and found that the observed signal ΔISH /ISH were
well reproduced when a ≈ −18 Å-1fs-2, γ ≈ 0.75 eV, and k ≈26 Å−1 as shown by the solid line in Fig. 2. This result
reveals the importance of phonon excitation in our observation of unconventional positive shift of ΔISH /ISH.
Figures 2(e, f) summarize the origins of the observed SHchanges for THz and MIR excitation. In the presence of astrong MIR field that resonantly excites the C-O- stretchingmode, dynamical repositioning of protons occurs, whichleads to the asymmetric polarization. These results can beviewed as an example of ultrafast control of organic ferroelectric cocrystals using intense ultrafast laser pulses at room
temperature.
References[1] S. Horiuchi, F. Ishii, R. Kumai, Y. Okimoto, H. Tachibana, N.
Nagaosa, and Y. Tokura, Nat. Mater. 4, 163 (2005).
[2] T. Umanodan, K. Kaneshima, K. Takeuchi, N. Ishii, J. Itatani, H.
Hirori, Y. Sanari, K. Tanaka, Y. Kanemitsu, T. Ishikawa, S. Koshihara,S.
Horiuchi, and Y. Okimoto, J. Phys. Soc. Jpn. 88, 013705 (2019).
Authors
T. Umanodana, K. Kaneshima, K. Takeuchi, N. Ishii, J. Itatani, H.
Hirorib, Y. Sanarib, K. Tanakab, Y. Kanemitsub, T. Ishikawaa, S. Koshiharaa, S. Horiuchic, and Y. Okimotoa
aTokyo Institute of Technology
bKyoto University
cNational Institute of Advanced Industrial Science and Technology(AIST)d 2X(t) μγ d X(t) μΩ2
μ ⎯⎯⎯+ ⎯⎯− +
X(t) + aμX2(t) = aEMIR(t),dt2dt
where χ(t) is the change in amplitude of the normal mode,
which can be viewed as an index of the displacement of
the atoms including the protons caused by the intense MIR
electric field EMIR(t). We used the experimental data forEMIR(t) that was obtained by EO sampling with 6-fs visible
pulses. The values μ, Ω, q, are the reduced mass, frequency,
and effective charges of the C-O- mode derived from a
DFT calculation and reflectivity data. We fit the observed
time profile of ΔISH /ISH by numerically solving the above

ISSP Activity Report 201951