arXiv subjects

Akitoshi Nakano

Publications and source records attributed to Akitoshi Nakano.

17 recordsLinked to original sources

arXiv2026

Valley-dependent electron-phonon scattering in thermoelectric semimetal Ta$_2$PdSe$_6$

Quasi-one-dimensional transition-metal chalcogenide Ta$_2$PdSe$_6$ is a promising thermoelectric semimetal due to the strong electron-hole asymmetry in the carrier lifetime. However, the microscopic origin of such a strong asymmetry remains unclear. In this study, we theoretically investigate electron-phonon scattering in Ta$_2$PdSe$_6$. There is a soft phonon mode mainly consisting of atomic displacements in PdSe$_4$ chains. This soft mode is strongly coupled with the highest valence band at the $\Gamma$ point, which lies slightly below the Fermi energy, and causes strong electron-phonon scattering. The bottom of the electron pocket energetically overlapped with that band also suffers from strong intervalley scattering, by which the imaginary part of the electron self-energy exhibits a sharp change near the Fermi level. On the other hand, the imaginary part of the self-energy for carriers in the hole pocket shows a moderate energy dependence. Thus, we find that electron-phonon scattering is strongly valley-dependent. Our finding will help us to understand the distinctive transport properties observed in Ta$_2$PdSe$_6$.

cond-mat.mtrl-sci↗

arXiv2025

Band-selective Plasmonic Polaron in Thermoelectric Semimetal Ta$_2$PdSe$_6$ with ultra-high power factor

We report the electronic structure of the thermoelectric semimetal Ta$_2$PdSe$_6$ with a large thermoelectric power factor and giant Peltier conductivity by means of angle-resolved photoemission spectroscopy (ARPES). The ARPES spectra reveal the coexistence of a sharp hole band with a light electron mass and a broad electron band with a relatively heavy electron mass, which originate from different quasi-one-dimensional (Q1D) chains in Ta$_2$PdSe$_6$. Moreover, the electron band around the Brillouin-zone (BZ) boundary shows a replica structure with respect to the energy originating from plasmonic polarons due to electron-plasmon interactions. The different scattering effects and interactions in each atomic chain lead to asymmetric transport lifetimes of carriers: a large Seebeck coefficient can be realized even in a semimetal. Our findings pave the way for exploring the thermoelectric materials in previously overlooked semimetals and provide a new platform for low-temperature thermoelectric physics, which has been challenging with semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Enhanced thermopower in a magnetic semiconductor EuTe4 with multiple charge-density-wave instabilities/

We investigate the layered magnetic semiconductor EuTe4, focusing on its intricate charge-density wave (CDW) states near and above room temperature through single-crystal X-ray diffraction (XRD), magnetic, and thermoelectric measurements.The XRD measurement revealed that the CDW state inducing the polar lattice distortion persists even at 650 K, demonstrating its remarkable thermal stability. Notably, the Seebeck coefficient near room temperature reaches values exceeding 500 uVK-1. This large Seebeck coefficient, not fully captured by a simple band calculation, is comparable to those observed in heavy-electron semiconducting oxides, suggesting the importance of electron correlation and spin/lattice instabilities. Furthermore, potentially reflecting the competition of two types of CDW states, the thermal conductivity near room temperature is as low as 0.02 Wcm-1K-1. As a result, the thermoelectric figure of merit zT reaches 0.22 at 460 K. These findings establish EuTe4 as a compelling platform to explore novel types of thermoelectric materials with multiple electronic instability.

cond-mat.mtrl-sci↗

arXiv2025

A rutile-based homologous series Na(PtO$_2$)$_{2\it{n}+1}$ discovered by computationally assisted high-pressure synthesis

Layered transition metal oxides typified by the Ruddlesden-Popper phase have been extensively studied for its applications in high-temperature superconductivity, catalysis, and battery technologies. Despite the remarkable structural diversity and catalytic functionality of platinum oxides, the exploration of layered polymorphs has remained significantly constrained mainly due to the high inertness of platinum. Here, we discover a new homologous series of layered ternary oxides, Na(PtO$_2$)$_{2\it{n}+1}$, by a combination of highly oxidizing high-pressure methods and density functional theory (DFT) calculations. This series features unprecedented layered structural motifs, rutile-based PtO$_6$ octahedra and one-dimensional PtO$_4$ square-planar columns, which enables systematic control of dimensionality. Furthermore, we demonstrate a computationally-assisted identification of isomeric and putative members of this homologous series as confirmed by controlled synthesis and quantitative analysis of diffuse scattering data. This approach provides an effective platform for the exhaustive exploration of metastable transition metal oxides with rich structural variations.

cond-mat.mtrl-sci↗

arXiv2025

Current-induced successive structural phase transitions beyond thermal equilibrium in single-crystal VO2

Nonequilibrium systems driven by external energy sources host unexplored physics; yet phase transitions beyond thermal equilibrium remain elusive. Here, we demonstrate that electric current induces structural phase transitions in single-crystal VO2, a prototypical strongly correlated material. At room temperature, synchrotron X-ray diffraction shows that a current density of 6.5 A/cm2 disrupts V-V dimers, driving a monoclinic-to-tetragonal insulator-to-metal transition, independent of Joule heating. Increasing the current to 10 A/cm2 triggers a discontinuous isotropic lattice expansion, stabilizing a novel tetragonal structure that does not exist in thermal equilibrium. Optical microscopy and microscopic Raman spectroscopy reveal dynamic domain motion and metastable phases, reminiscent of dissipative structures. These findings establish direct pathways to access hidden phases and symmetry changes beyond thermal equilibrium, broadening the frontiers of nonequilibrium thermodynamics.

cond-mat.str-el↗

arXiv2025

Current-induced Nonequilibrium Phase Transition Accompanied by Giant Gap Reduction in Vanadium Dioxide

We investigated nonlinear conduction in bulk single crystals of VO2 with precise temperature control. Two distinct nonequilibrium phenomena were identified: a gradual reduction of the charge gap and a current-induced insulator-metal transition. The electric field required to drive the nonlinear conduction is two to three orders of magnitude smaller than that reported for VO2 thin films or nanobeams, strongly indicating an intrinsic electronic origin rather than a temperature increase due to self heating. Notably, our results suggest that the application of a steady current to the frozen insulating state can induce a nonequilibrium steady-state metallic phase -- effectively melting the electronic ice. This highlights a novel route to controlling electronic states via nonthermal, current-driven mechanisms.

cond-mat.str-el↗

arXiv2024

Pressure-induced anomalous enhancement in superconducting critical temperature of transition-metal chalcogenide Ta$_2$PdS$_6$ and Ta$_2$PdSe$_6$

The emergence of a second dome in the superconducting phase through pressure-driven manipulation of crystal structures in materials has attracted considerable attention. Transition metal chalcogenides (TMCs) represent a highly promising platform, as the second dome has been observed in several binary compounds. Recently, ternary TMCs such as Ta$_2$PdS$_6$ and Ta$_2$PdSe$_6$ have exhibited pressure-induced superconducting domes. In this study, we perform electrical transport measurements of Ta$_2$PdS$_6$ and Ta$_2$PdSe$_6$ under extremely high pressures exceeding 100 GPa, namely uninvestigated regions in previous reports, to reveal the emergence of the second dome. The superconducting critical temperatures (T$_c$) in both Ta$_2$PdS$_6$ and Ta$_2$PdSe$_6$ initially decrease with increasing pressure. Subsequently, the Tcs tend to enhance drastically above 100 GPa. Notably, the maximum T$_c$ in Ta$_2$PdS$_6$ is 11.2 K at 130.0 GPa, which is a relatively high record among the TMCs. The emergence of the second dome in Ta$_2$PdS$_6$ and Ta$_2$PdSe$_6$ opens further motivation for the investigation under extreme conditions beyond the first dome to find hidden ordered phases.

cond-mat.supr-con↗

arXiv2024

Observation of converse flexoelectric effect in topological semimetals

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.

cond-mat.mtrl-sci↗

arXiv2024

Molecular orbital formation and metastable short-range ordered structure in VO$_2$

The metal-insulator transition (MIT) in vanadium dioxide VO$_2$ 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 VO$_2$, the primary origin of the MIT remains controversial. In this study, we delve into the crystal structure and orbital state of VO$_2$ 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.

cond-mat.str-el↗

arXiv2023

Superconductivity in a ferroelectric-like topological semimetal SrAuBi

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.

cond-mat.supr-con↗

arXiv2023

Melting of excitonic insulator phase by an intense terahertz pulse in Ta$_2$NiSe$_5$

In this study, the optical response to a terahertz pulse was investigated in the transition metal chalcogenide Ta$_2$NiSe$_5$, 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-tunnelling 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.

cond-mat.str-el↗

arXiv2021

Giant Peltier Conductivity in an Uncompensated Semimetal Ta2PdSe6

Thermoelectric properties of single crystal Ta2PdSe6 is investigated by means of transport measurements, and a density functional calculation. We found a giant Peltier conductivity of 100 Acm-1K-1 at 10 K and successfully explained it by means of conventional semiconductor theory. We concluded that an uncompensated semimetal, high mobility, and heavy effective mass are responsible for the giant Peltier conductivity. Our finding opens a new ground in the field of thermoelectrics to explore much better semimetals for a new possible application such as an electric current generator for a superconducting magnet.

cond-mat.mtrl-sci↗

arXiv2021

Hybridization-gap Formation and Superconductivity in the Pressure-induced Semimetallic Phase of the Excitonic Insulator Ta$_2$NiSe$_5$

The excitonic insulator Ta$_2$NiSe$_5$ 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 Ni-valence 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 = 1.2 K 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.

cond-mat.str-el↗

arXiv2021

Room Temperature Thermoelectric Properties of Isostructural Selenides Ta2PdS6 and Ta2PdSe6

We have measured thermoelectric properties of Ta2PdX6 (X=S, Se) around room temperature using single crystal samples. We find that the power factor of Ta2PdX6 is relatively high from middle-low to room temperatures, and notably Ta2PdSe6 shows the largest power factor among thermoelectric materials with an electrical conductivity of 10-2 Ωcm at 300 K. Ta2PdS6 will be a possible candidate for a Peltier cooling material if the lattice thermal conductivity is reduced by chemical substitution.

cond-mat.mtrl-sci↗

arXiv2019

Weak Ferroelectricity in n = 2 pseudo Ruddlesden-Popper-type niobate Li2SrNb2O7

Li2SrNb2O7 (LSNO) crystallizes in a structure closely related to n = 2 Ruddlesden-Popper-type compounds, which is gen-erally formed by intergrowth of 2-dimensional perovskite-type blocks and rocksalt-type layers. The present study demonstrates a coexistence of spontaneous polarization and anti-ferroelectric-like nonlinear response in LSNO at 80 K, suggesting a weak ferroelectricity below the phase transition temperature of 217 K. A combination of first-principles cal-culations and single crystal x-ray diffractions clarifies a polar P21cn structure for the ground state of LSNO, where an in-plane anti-ferroelectric displacement and an out-of-plane polar shift simultaneously take place. The present study offers a new perspective to design ferroelectric and antiferroelectric materials with Ruddlesden-Popper-type compounds.

cond-mat.mtrl-sci↗

arXiv2019

Effect of Cu intercalation and pressure on excitonic interaction in 1T-TiSe2

1T-TiSe2 has a semimetallic band structure at room temperature and undergoes phase transition to a triple-q charge density wave (CDW) state with a commensurate superlattice structure (2a * 2a * 2c) below Tc ~ 200 K at ambient pressure. This phase transition is caused by cooperative phenomena involving electron-phonon and electron-hole (excitonic) interactions, and cannot be described by a standard CDW framework. By Cu intercalation or the application of pressure, this phase transition temperature is suppressed and superconductivity (SC) appears. However, it is not clear what kind of order parameters are affected by these two procedures. We investigated the crystal structure of CuxTiSe2 and pressurized 1T-TiSe2 around the SC state by synchrotron x-ray diffraction on single crystals. In the high-temperature phase, the variation of structural parameters for the case of Cu intercalation and application of pressure are considerably different. Moreover, the relationship between the critical points of the CDW phase transition and the SC dome are also different for the two cases. The excitonic interaction appears to play an important role in the P-T phase diagram of 1T-TiSe2, but not in the x-T phase diagram.

cond-mat.str-el↗

arXiv2018

Electron-Phonon Coupling Mode in Excitonic Insulator

Ta2NiSe5 is considered a promising excitonic insulator (EI) candidate with slight phonon contributions, since it exhibits a tiny orthorhombic-to-monoclinic structural distortion at 328 K without any superlattice structure. Our synchrotron inelastic x-ray scattering measurements reveal strong electron-optical-phonon coupling occurring at temperatures higher than the transition temperature. Density functional theoretical calculations indicate that two coupled optical modes arise due to the vibration of Ta and Se ions. Further, the two modes are frozen such that Ta and Se approach each other, forming atomic-displacement-type electric dipoles in the monoclinic phase. The characteristic of electronic toroidal moment formation by the antiferro-arrangements of electric dipoles is the universality of EI between Ta2NiSe5 and 1T-TiSe2.

cond-mat.str-el↗