記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Review Research  

In this paper, the optical response to a terahertz pulse was investigated in the transition metal chalcogenide Ta2NiSe5, a candidate excitonic insulator. First, by irradiating a terahertz pulse with a relatively weak electric field (0.3 MV/cm), the spectral changes in reflectivity near the absorption edge due to third-order optical nonlinearity were measured, and the absorption peak characteristic of the excitonic phase just below the interband transition was identified. Next, by irradiating a strong terahertz pulse with a strong electric field of 1.65 MV/cm, the absorption of the excitonic phase was found to be reduced, and a Drude-like response appeared in the mid-infrared region. These responses can be interpreted as carrier generation by exciton dissociation induced by the electric field, resulting in the partial melting of the excitonic phase and metallization. The presence of a distinct threshold electric field for carrier generation indicates exciton dissociation via quantum-tunneling processes. The spectral change due to metallization by the electric field is significantly different from that due to the strong optical excitation across the gap, which can be explained by the different melting mechanisms of the excitonic phase in the two types of excitations.

DOI： 10.1103/PhysRevResearch.6.043187

Open Access

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Communications Materials  

A strong coupling between electric polarization and elastic deformation in solids is an important　factor in creating useful electromechanical nanodevices. Such coupling is typically allowed in insulating materials with inversion symmetry breaking as exemplified by the piezoelectric effect in ferroelectric materials. Therefore, materials with metallicity and centrosymmetry have tended to be out of scope in this perspective. Here, we report the observation of giant elastic deformation by the application of an alternating electric current in topological semimetals (V,Mo)Te2, regardless of the centrosymmetry. Considering the crystal and band structures and the asymmetric measurement configurations in addition to the absence of the electromechanical effect in a trivial semimetal TiTe2, the observed effect is discussed in terms of a Berry-phase-derived converse flexoelectric effect in metals. The observation of the flexoelectric effect in topological semimetals paves a way for a new type of nanoscale electromechanical sensors and energy harvesting.

DOI： 10.1038/s43246-024-00677-z

Open Access

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Review B  

We performed inelastic x-ray and neutron scattering (IXS and INS) experiments for (Ba,Ca)TiO3 single crystals to elucidate the effect of Ca doping on the atomic dynamics. Dispersion relations of transverse phonons in BaTiO3, (Ba0.9Ca0.1)TiO3, and (Ba0.8Ca0.2)TiO3 were determined. The A-site-O vibrational mode as well as the acoustic mode were observed both by IXS and INS, whereas the Ti-O soft mode was detected only by INS. It was found that the A-site-O vibration exhibits significant softening with increasing Ca concentration, while the Ti-O soft mode is almost unaffected by the Ca doping. This finding is discussed in comparison with the polarization hysteresis loops and piezoelectric coefficients of (Ba,Ca)TiO3. It is indicated that the softening of the A-site-O vibration contributes to the enhancement of the piezoelectricity of Ca-doped BaTiO3-based materials.

DOI： 10.1103/PhysRevB.110.134314

Web of Science

Scopus

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of the Physical Society of Japan  

We have investigated the Nernst effect of the ternary transition-metal chalcogenide semimetal Ta2PdSe6, which has recently attracted much attention due to its giant Peltier conductivity at cryogenic temperatures. We observed that Nernst signals increase linearly with magnetic fields across the temperature and magnetic range investigated. The Nernst coefficient is higher than some kind of superconductors, and heavy fermion systems around 20 K. We discuss the origin of the large Nernst effect in relation to the small Fermi energy and high carrier mobility.

DOI： 10.7566/JPSJ.93.103703

Web of Science

Scopus

担当区分：責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/apxr.202400063

Open Access

Web of Science

記述言語：英語   出版者・発行元：npj Quantum Materials  

Given the rarity of metallic systems that exhibit ferroelectric-like transitions, it is apparently challenging to find a system that simultaneously possesses superconductivity and ferroelectric-like structural instability. Here, we report the observation of superconductivity at 2.4 K in a layered semimetal SrAuBi characterized by strong spin–orbit coupling (SOC) and ferroelectric-like lattice distortion. Single crystals of SrAuBi have been successfully synthesized and found to show a polar-nonpolar structure transition at 214 K, which is associated with the buckling of Au-Bi honeycomb lattice. On the basis of the band calculations considering SOC, we found significant Rashba-type spin splitting and symmetry-protected multiple Dirac points near the Fermi level. We believe that this discovery opens up new possibilities of pursuing exotic superconducting states associated with the semimetallic band structure without space inversion symmetry and the topological surface state with the strong SOC.

DOI： 10.1038/s41535-023-00612-4

Web of Science

Scopus

担当区分：筆頭著者,　責任著者   記述言語：英語   出版者・発行元：Journal of the Ceramic Society of Japan  

We have prepared a dense ceramic sample of a novel thermoelectric sulfide Ta2PdS6 by means of spark plasma sintering technique under a uniaxial pressure. The Rietveld analyses of the X-ray diffraction patterns have clarified occurrence of weak preferred orientation in the sintering process. Resistivity, thermopower, and Hall resistivity measurements on the ceramic sample have indicated that it shows higher carrier density and lower carrier mobility than a single crystal. The inferior carrier mobility in the ceramic sample may partly come from increased point-defect and/or grain-boundary scattering, to which poor power factor is attributed.

DOI： 10.2109/jcersj2.23038

Web of Science

Scopus

CiNii Research

担当区分：筆頭著者,　責任著者   記述言語：英語   出版者・発行元：Journal of Materials Chemistry C  

We have investigated the physical properties of an oxygen-deficient perovskite-type Sr10Ga6Sc4O25 by means of dielectric permittivity, pyroelectric current, and X-ray diffraction (XRD) measurements using a polycrystalline sample. The real part of the dielectric permittivity shows a clear anomaly around 310 K. Switchable pyroelectric current is also observed around this temperature, indicating a ferroelectric phase transition in the title compound. Synchrotron powder XRD measurements reveal that no discontinuous jump takes place in the temperature dependent cell volume around 310 K, implying that the phase transition is of second order. The candidates of the low and high temperature space groups are discussed based on the group theory.

DOI： 10.1039/d2tc05118d

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of the Physical Society of Japan  

We have measured and analyzed the thermopower of single-crystal samples of the Mott insulator Ca2RuO4 in high pressure up to 0.9 GPa. We find that the thermopower rapidly changes at a metal-insulator transition temperature, which is explained in terms of a percolation theory. In contrast, the thermopower in the insulator phase slightly decreases with increasing pressure well below the transition temperature, and is associated with thermodynamic quantities through the Kelvin formula. The pressure dependence is compared with the current-density dependence, and a possible origin for the current-enhanced thermopower is proposed.

DOI： 10.7566/JPSJ.91.033703

Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of the Physical Society of Japan  

An excitonic insulator, in which an optical gap is formed by exciton condensation, is expected to exhibit large photoresponses. Here, we report the results of femtosecond reflection spectroscopy covering a wide infrared region on a typical excitonic insulator, Ta2NiSe5, which gives direct information not only about a change of excitonic gap but also a metallization. With a weak excitation, the excitonic gap decreases due to the weakening of excitonic effect by photocarriers. With a strong excitation, a Drude-like high-reflection band appears, showing an excitonic-insulator-to-metal transition. Analyses of spectral and temporal changes of reflectivity revealed that the metallization is accompanied by large structural changes. This demonstrates an important role of electron–lattice interaction on the stabilization of excitonic phase.

DOI： 10.7566/JPSJ.91.023701

Web of Science

Scopus

出版者・発行元：Journal of the Physical Society of Japan  

We have prepared a set of polycrystalline samples of Ba3Ca(Ru1−xNbx)2O9 consisting of the face-shared (Ru,Nb)2O9 dimer block from x = 0 to 0.12, and have measured the x-ray diffraction and magnetization. We find that all the samples of x > 0 show Curie–Weiss-like susceptibility below 30 K with the Weiss temperature θNb of 2–7 K, which suggests that the Nb-substituted NbRuO9 dimer interacts the Ru2O9 dimers magnetically. We also find the NbRuO9 dimer effectively works as S = 1 in the dilution limit, which is also suggested by the first-principle calculation. The Ru2O9 dimer lattice is nontrivial in the sense that S = 1 moment in the NbRuO9 dimer is screened for large x without any magnetic long range order. A possible origin is discussed.

DOI： 10.7566/JPSJ.90.114706

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1039/d1tc03897d

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

We have prepared a set of polycrystalline samples of PbPd 1 - x - y Fe y Li x O 2 (x = 0, 0.01, 0.02, 0.03, and 0.04; y = 0, 0.02, and 0.05) and have measured synchrotron x-ray diffraction and magnetization systematically. We have found high-temperature ferromagnetism in Fe- and Li-substituted samples and the largest magnetic moment of 0.023 μ B per formula unit in PbPd 0.93 Fe 0.05 Li 0.02 O 2. The ferromagnetism survives at 700 K, and the Curie temperature will be far above 800 K. Although similar but controversial high-temperature ferromagnets have been reported in many thin-film samples thus far, the discovered ferromagnetism is macroscopic in the sense that bulk samples stick to a permanent magnet at room temperature. The Fe and Li dependence of the ferromagnetism is complicated, implying that the ferromagnetism is truly unconventional.

DOI： 10.1063/5.0051283

Web of Science

Scopus

担当区分：筆頭著者,　責任著者   記述言語：英語   出版者・発行元：Physical Society of Japan  

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：日本物理学会  

We have measured the dielectric constant and thermopower of single crystal samples of the excitonic insulator candidate Ta2NiSe5. The dielectric constant below 50 K shows relaxor-like relaxation, implying the existence of randomly distributed electric dipoles. We have evaluated the concentration of the dipoles to be 10^19 cm^−3 at 30 K, which gives a rough estimate of the exciton concentration in this compound. A large thermopower of 600 µV/K at 100 K suddenly drops toward zero down to 50 K, and this zero thermopower evidences the neutral particles flow against the applied temperature gradient. We have semi-quantitatively explained the above two anomalies in terms of spontaneously generated excitons at low temperatures where thermally excited holes are quenched.ファイル公開：2020/11/15

記述言語：日本語   出版者・発行元：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.74.1.0_2550

CiNii Research

記述言語：日本語   出版者・発行元：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.74.1.0_2549

CiNii Research

記述言語：日本語   出版者・発行元：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.74.1.0_2551

CiNii Research

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevB.98.045139

Web of Science

Scopus

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1107/S2052252517018334

Web of Science

Scopus

PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Science and Technology of Advanced Materials  

Computational approaches using theoretical calculations and data scientific methods have become increasingly important in materials science and technology, with the development of relevant methodologies and algorithms, the availability of large materials data, and the enhancement of computer performance. As reviewed herein, we have developed computational methods for the design and prediction of inorganic materials with a particular focus on the exploration of semiconductors and dielectrics. High-throughput first-principles calculations are used to systematically and accurately predict the local atomic and electronic structures of polarons, point defects, surfaces, and interfaces, as well as bulk fundamental properties. Machine learning techniques are utilized to efficiently predict various material properties, construct phase diagrams, and search for materials satisfying target properties. These computational approaches have elucidated the mechanisms behind material functionalities and explored promising materials in combination with synthesis, characterization, and device fabrication. Examples include the development of ternary nitride semiconductors for potential optoelectronic and photovoltaic applications, the exploration of phosphide semiconductors and the optimization of heterointerfaces toward the improvement of phosphide-based photovoltaic cells, and the discovery of ferroelectricity in layered perovskite oxides and the theoretical understanding of its origin, all of which demonstrate the effectiveness of our computer-aided materials research.

DOI： 10.1080/14686996.2024.2423600

Open Access

Web of Science

Scopus

PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ACS Nano  

Domains in a crystal, which have crystallographic uniformity and are geometrically segmented, typically arise from various phase transitions. The physical properties within individual domains are inherently the same as those in the homogeneous bulk. As a result, sufficiently large domains have little influence on the bulk properties. However, as the domains decrease in size to the nanoscale, for instance, due to multiple phase instabilities or spatial inhomogeneities, then the materials often acquire exceptional functionalities that are unattainable without these domains. This effect is exemplified by the ultrahigh dielectric and piezoelectric responses observed in ferroelectric oxides with nanoscale polar domains as well as in ferroelectric relaxors with polar nanoclusters. Here, we demonstrate that hashed nanoscale domains in an antiferroelectric material are also capable of boosting dielectric permittivity in an unconventional way. This discovery has been made in an antiferroelectric titanite-type oxide, CaTi(Si1-xGex)O5, in which the permittivity significantly increases when the antiferroelectric order becomes short-range. Our transmission electron microscopy observations have clarified that polar regions simultaneously appear around antiphase boundaries in the antiferroelectric phase of CaTi(Si1-xGex)O5. As the concentration of the antiphase boundary increases, the polar regions become denser and play a crucial role in boosting the permittivity. At the composition of x = 0.5, the value of the permittivity finally reaches double that in the bulk and shows excellent linearity, at least until an electric field of 500 kV/cm is applied. The present findings highlight the promise of domain engineering for boosting the permittivity in antiferroelectrics as a way to develop materials with excellent dielectric properties.

DOI： 10.1021/acsnano.4c02168

Web of Science

Scopus

PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Review B  

The metal-insulator transition (MIT) in vanadium dioxide (VO2) due to V-V dimerization has been extensively discussed for decades. While it is widely acknowledged that electron correlations, Peierls instabilities, and molecular orbital formations are crucial for understanding the MIT of VO2, the primary origin of the MIT remains controversial. In this study, we delve into the crystal structure and orbital state of VO2 through synchrotron x-ray diffraction experiments. The molecular orbital formation corresponding to the V-V dimerization is directly observed in the low-temperature insulating monoclinic phase, called the M1 phase, as indicated by the valence electron density distribution. Moreover, diffuse scattering observed in the high-temperature metal phase of rutile structure suggests the presence of short-range correlation of V displacements, which is not directly attributed to the structural fluctuation of the M1 phase. The short-range order in the rutile phase will be the key to understanding the MIT in this system.

DOI： 10.1103/PhysRevB.109.L100101

Web of Science

Scopus

その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.109.L100101/fulltext

掲載種別：研究論文（学術雑誌）   出版者・発行元：ACS Applied Electronic Materials  

Thermoelectric energy conversion has attracted attention as an energy-harvesting technology for converting waste heat into electricity via the Seebeck effect. Conducting oxide-based thermoelectric materials that exhibit a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Among many conducting oxides, Ba1/3CoO2 epitaxial films exhibit the highest figures of merit. For the practical use of Ba1/3CoO2, bulk ceramics or single-crystalline Ba1/3CoO2 is necessary. Here, we show that freestanding Ba1/3CoO2 single-crystalline films can be fabricated by peeling Ba1/3CoO2 epitaxial films from the substrate. We fabricated Ba1/3CoO2 epitaxial films and immersed them in 40 °C hot water for several tens of minutes. Subsequently, the Ba1/3CoO2 epitaxial film spontaneously peeled off and floated on the surface of the water like seaweed. We measured and analyzed the crystal structure, chemical composition, and thermoelectric properties before and after peeling and realized that there was no significant difference. The present results provide a useful method for fabricating freestanding single-crystalline oxide films for thermoelectrics.

DOI： 10.1021/acsaelm.3c01129

Web of Science

Scopus

掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Review Materials  

A phase variation of a pseudo-Roddlesden-Popper-type ferroelectric oxide Li2Sr1-xCax(Nb1-xTax)2O7 is systematically investigated using dielectric measurements, second harmonic generation, and x-ray diffraction experiments. We find an exotic x-T phase diagram, including paraelectric Cmcm, antiferroelectric Pmcn, and in-plane antiferroelectric and out-of-plane ferroelectric P21cn phases. At low x, we observe large and divergent dielectric anomalies associated with phase transitions from Cmcm to P21cn, whereas these become small kinks at high x. Structural analyses reveal an internal distortion, and the rotation of octahedra strongly depends on x, which is consistent with the lattice dynamics obtained by a density functional theory calculation. These results demonstrate the great tunability of dielectric properties in layered perovskite-type oxides by tuning the chemical bonding state in the octahedron.

DOI： 10.1103/PhysRevMaterials.6.044412

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of the Physical Society of Japan  

The excitonic insulator Ta2NiSe5 experiences a first-order structural transition under pressure from rippled to flat layer-structure at Ps ~ 3 GPa, which drives the system from an almost zero-gap semiconductor to a semimetal. The pressure-induced semimetal, with lowering temperature, experiences a transition to another semimetal with a partial-gap of ~0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Nivalence bands due to the lattice distortion, indicative of the importance of electron-lattice coupling. The transition is suppressed with increasing pressure to Pc ~ 8 GPa. Superconductivity with a maximum Tsc ~ 1.2K emerges around Pc, likely mediated by strongly electron-coupled soft phonons. The electron-lattice coupling is as important ingredient as the excitonic instability in Ta2NiSe5.

DOI： 10.7566/JPSJ.90.074706

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of the Physical Society of Japan  

We have measured and analyzed the in-plane thermal diffusivity of the Mott insulator Ca2RuO4 around room temperature under external currents with an infrared camera as a contactless thermometer. We find that the thermal diffusivity decreases by 40% for an external current density of 14 A=cm2. This large impact on lattice properties highly nontrivial in the sense that the external current primarily affects the electronic states. A possible explanation for this is discussed.

DOI： 10.7566/JPSJ.90.063601

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Open Ceramics  

The aluminum-sodalite-type oxide, Ca8[AlO2]12(MoO4)2, shows an improper ferroelectricity below a ferroelectric phase transition temperature around 630 ​K. Here we present effects of a Ga-substitution for Al on ferroelectric properties of Ca8[AlO2]12(MoO4)2. Systematic investigations with synchrotron x-ray powder diffractions and dielectric/pyroelectric measurements have clarified that the Ga-substitution suppresses the ferroelectricity and markedly enhances a thermal hysteresis of the ferroelectric phase transition. In contrast to the case of Sr-substitution for Ca, with which the ground state results in a relaxor-like state, the polar ordering of macroscopic scale has been found to survive under the Ga-substitution. The present results shed light on intuitive designing of ferroelectricity and the phase transition properties in the aluminum-sodalite-type oxide.

DOI： 10.1016/j.oceram.2021.100080

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Chemistry of Materials  

DOI： 10.1021/acs.chemmater.0c04032

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Materials Chemistry C  

DOI： 10.1039/d0tc03539d

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of the Physical Society of Japan  

DOI： 10.7566/JPSJ.89.093707

Web of Science

Scopus

記述言語：英語   出版者・発行元：Physical Review B  

DOI： 10.1103/PhysRevB.101.094108

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Chemistry of Materials  

DOI： 10.1021/acs.chemmater.9b04022

Web of Science

Scopus

出版者・発行元：公益社団法人 日本表面真空学会  

<p>Ta₂NiSe₅は励起子絶縁体の候補である。既に価電子帯端の低温での低エネルギーシフトが報告されており、伝導帯の観測が切望されている。本研究では低エネルギー逆光電子分光法で伝導帯を直接観測した。フェルミ準位から0.55 eVに観測されたバンドは80 Kでもエネルギーが変化しなかった。これは、従来の励起子絶縁体とは異なり、バンドギャップが強い電子正孔相互作用により開いた状態であるという描像と整合する。</p>

DOI： 10.14886/jvss.2020.0_135

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Chemistry of Materials  

DOI： 10.1021/acs.chemmater.9b02373

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevB.99.104109

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1039/c7cc08202a

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1021/acsami.7b14132

Web of Science

Scopus

PubMed