arXiv subjects

Yoshikazu Suzumura

Publications and source records attributed to Yoshikazu Suzumura.

At least 19 recordsLinked to original sources

arXiv2025

Spin-Orbit Coupling Effect on the Seebeck Coefficient in Dirac Electron Systems in $α$-(BETS)$_2$I$_3$

The Seebeck coefficient, $S=L_{12}/(TL_{11})$, which is proportional to a ratio of the thermoelectric conductivity $L_{12}$ to the electric conductivity $L_{11}$ with $T$ being temperature is examined for two-dimensional Dirac electrons in the three-quarter filled organic conductor, $α$-(BETS)$_2$I$_3$, [BETS = BEDT-TSeF = bis(ethylenedithio)tetraselenafulvalene] at ambient pressure.Using a tight-binding model obtained with the first-principles relativistic density-functional theory method [Tsumuraya and Suzumura, Eur. Phys. J. B 94, 17 (2021)], we calculate $S$ in the presence of the impurity and electron--phonon scatterings. We show that $S_x < 0$ and $S_y >0$ at high temperatures, where $S_x$ ($S_y$) denotes $S$ perpendicular (parallel) to the molecular stacking axis. There is a sign change of $S_y$ with increasing $T$. We find that, at low temperatures the absolute value of $S$ is enhanced by the spin-orbit coupling. The Seebeck coefficient is examined by dividing it into components of the conduction and valence bands; we find that the electron and hole contributions compete with each other. Such $T$ dependence of $S$ is clarified using the spectral conductivity, which determines $L_{12}$ and $L_{11}$

cond-mat.mes-hall↗

arXiv2024

Seebeck Effect of Dirac Electrons in Organic Conductors under Hydrostatic Pressure Using a Tight-Binding Model Derived from First Principles

The Seebeck coefficient is examined for two-dimensional Dirac electrons in the three-quarter filled organic conductor alpha-(BEDT-TTF)_2I_3 under hydrostatic pressure, where the Seebeck coefficient is proportional to the ratio of the thermoelectric conductivity to the electrical conductivity. We present an improved tight-binding model in two dimensions with transfer energies determined from first-principles density functional theory calculations with an experimentally determined crystal structure. The temperatutre dependence of the Seebeck coefficient is calculated by adding impurity and electron-phonon scatterings. Noting a zero-gap state due to the Dirac cone, which results in a competition from contributions between the conduction and valence bands, we show positive S_x and S_y at finite temperatures and analyze them in terms of spectral conductivity. The relevance of the calculated S_x (perpendicular to the molecular stacking axis) to the experiment is discussed.

cond-mat.mes-hall↗

arXiv2023

Seebeck Coefficient of Two-dimensional Dirac Electrons in Organic Conductor under Pressure

The Seebeck coefficient, which is proportional to a ratio of the thermoelectric conductivity to electrical conductivity has been examined for Dirac electrons in the organic conductor $α$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF denotes a molecule given by bis(ethylenedithio)tetrathiafulvalene] under a uniaxial pressure using a two-dimensional tight-binding model with both impurity and electron--phonon (e--p) scatterings. We calculate an anomalous temperature ($T$) dependence of the Seebeck coefficient $S_ν$ with $ν= x$ (perpendicular to the molecular stacking axis) and $y$, which shows $S_ν> 0$ with a maximum at high temperatures and $S_ν < 0$ with a minimum at low temperatures.The microscopic mechanism of such a sign change of $S_ν$ is clarified in terms of the spectral conductivity. The result is compared with experiments on $α$-(BEDT-TTF)$_2$I$_3$.

cond-mat.mtrl-sci↗

arXiv2022

Conductivity of Two-dimensional Dirac Electrons Close to Merging in Organic Conductor $α$-STF$_2$I$_3$ at Ambient Pressure

The electric conductivity of Dirac electrons in the organic conductor $α$-STF$_2$I$_3$ (STF = bis(ethylenedithio)diselenadithiafulvalene), which has an isostructure of ET(=bis(ethylenedithio)tetrathiafulvalene), has been theoretically studied using a two-dimensional tight-binding model in the presence of both impurity and electron-phonon (e-p) scatterings.In contrast to ET, which has a Dirac cone with almost isotropic velocity, STF provides a large anisotropy owing to a Dirac point that is close to merging. As a result, $σ_{x}$ becomes much larger than $σ_{y}$, where $σ_y$ and $σ_x$ are diagonal conductivities parallel and perpendicular to a stacking axis of molecules, respectively. With increasing temperature ($T$), $σ_x$ takes a broad maximum because of e-p scattering and $σ_y$ remains almost constant. The ratio $σ_x/σ_y$ is analyzed in terms of the band structure. Such an exotic conductivity of STF is compared with that of an experiment showing a good correspondence. Finally, $σ_x/σ_y$ values of ET and BETS(= bis(ethylenedithio)tetraselenafulvalene) are shown to demonstrate the dissimilarity with STF.

cond-mat.mes-hall↗

arXiv2021

Electric and Magnetic Responses of Two-dimensional Dirac Electrons in Organic Conductor $α$-(BETS)$_2$I$_3$

Effect of spin-orbit coupling (SOC) on Dirac electrons in the organic conductor $α$-(BETS)$_2$I$_3$ [BETS = bis(ethylenedithio)tetraselenafulvalene] has been examined by calculating electric conductivity and spin magnetic susceptibility. A tight-binding (TB) model with real and imaginary transfer energies is derived using first-principles density-functional theory method. The conductivity without the SOC depends on both anisotropies of the velocity of the Dirac cone and the tiling of the cone. Such conductivity is suppressed by the SOC, which gives rise to the imaginary part of the transfer energy. Due to the SOC, we find at low temperatures that the reduction of the conductivity becomes large and that the anisotropy of the conductivity is reduced. A nearly constant conductivity at high temperatures is obtained by an electron--phonon (e--p) scattering. Further, the property of the Dirac cone is examined for the spin susceptibility, which is mainly determined by the density of states (DOS). The result is compared with the case of the organic conductor $α$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene], which provides the Dirac cone without the SOC. The relevance to experiments is discussed.

cond-mat.mtrl-sci↗

arXiv2021

Anomalous conductivity of two-dimensional Dirac electrons in organic conductor under pressures

The electric conductivity of Dirac electrons in the organic conductor $α$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] under pressure has been examined using a two-dimensional tight-binding (TB) model with both impurity and electron--phonon (e--p) scatterings. We study an anomalous temperature dependence of the conductivity, which shows a crossover from $σ_{x} < σ_{y}$ at low temperatures [region (I)] to $σ_{x} > σ_{y}$ at high temperatures [region (II)]. $σ_y$ and $σ_x$ are diagonal conductivities parallel and perpendicular to a stacking axis of molecules, respectively. The effect of Dirac cone tilting is dominant in region (I), whereas the anisotropy of the velocity of the Dirac cone is dominant in region (II). Such behavior is further examined by calculating the deviation of principal axes due to the off-diagonal conductivity $σ_{xy}$ and a nearly constant conductivity at high temperatures due to the e--p scattering, which is the extension of the previous result of the simple two-band model [Phys. Rev. B {\bf 98},161205 (2018)]. The relevance to experiments of organic conductors is discussed.

cond-mat.mes-hall↗

arXiv2020

First-principles study of the effective Hamiltonian for Dirac fermions with spin-orbit coupling in two-dimensional molecular conductor $α$-(BETS)$_2$I$_3$

We employed first-principles density-functional theory (DFT) calculations to characterize Dirac electrons in quasi-two-dimensional molecular conductor $α$-(BETS)$_2$I$_3$ [= $α$-(BEDT-TSeF)$_2$I$_3$] at a low temperature of 30K. We provide a tight-binding model with intermolecular transfer energies evaluated from maximally localized Wannier functions, where the number of relevant transfer integrals is relatively large due to the delocalized character of Se $p$ orbitals. The spin-orbit coupling gives rise to an exotic insulating state with an indirect band gap of about 2 meV. We analyzed the energy spectrum with a Dirac cone close to the Fermi level to develop an effective Hamiltonian with site-potentials, which reproduces the spectrum obtained by the DFT band structure.

cond-mat.str-el↗

arXiv2020

Electronic Structure of a Single-Component Molecular Conductor [Pd(dddt)$_2$] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) under High Pressure

We examined high-pressure electronic structure of a single-component molecular conductor [Pd(dddt)$_2$] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) at room temperature, based on the crystal structure determined by single crystal synchrotron X-ray diffraction measurements at 5.9 GPa. The monoclinic unit cell contains four molecules that form two crystallographically independent molecular layers. A tight-binding model of 8 $\times$ 8 matrix Hamiltonian gives an electronic structure as a Dirac electron system. The Dirac point describes a loop within the first Brillouin zone, and a nodal line semimetal is obtained. The noticeable property of the Dirac cone with a linear dispersion is shown by calculating density of states (DOS). The Dirac cone in this system is associated with the crossing of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) bands, which originates from the direct interaction between different molecular layers. This is a newly found mechanism in addition to the indirect one [J. Phys. Soc. Jpn., {\bf 86}, 064705 (2017)]. The Dirac points emerge as a line, when the HOMO and LUMO bands meet on the surface and the HOMO-LUMO couplings are absent. Such a mechanism is verified using a reduced model of 4 $\times$ 4 matrix Hamiltonian. The deviation of the band energy ($δE$) at the Dirac point from the Fermi level is very small ($δE < $ 0.4meV). The nodal line is examined by calculating the parity of the occupied band eigen states at TRIM (Time Reversal Invariant Momentum) showing that the topological number is 1.

cond-mat.mtrl-sci↗

arXiv2020

Electric Transport of Nodal Line Semimetal in Single-Component Molecular Conductor

We examine an effect of acoustic phonon scattering on an electric conductivity of single-component molecular conductor [Pd(dddt)$_2$] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) with a half-filled band by applying the previous calculation in a two-dimensional model with Dirac cone [Phys. Rev. B {\bf 98},161205 (2018)], where the electric transport by the impurity scattering exhibits the noticeable interplay of the Dirac cone and the phonon scattering,resulting in a maximum of the conductivity with increasing temperature. The conductor shows a nodal line semimetal where the band crossing of HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) provides a loop of Dirac points located close to the Fermi energy followed by the density of states (DOS) similar to that of two-dimensional Dirac cone. Using a tight-binding (TB) model [arXiv:2008.09277], which was obtained using the crystal structure observed from a recent X ray diffraction experiment under pressure, it is shown that the obtained conductivity explains reasonably the anomalous behavior in [Pd(dddt)$_2$] exhibiting almost temperature independent resistivity at finite temperatures. This paper demonstrates a crucial role of the acoustic phonon scattering at finite temperatures in the electric conductivity of Dirac electrons. The present theoretical results of conductivity are compared with those of experiments.

cond-mat.mtrl-sci↗

arXiv2020

Role of Velocity Field and Principal Axis of Tilted Dirac Cones in Effective Hamlitonan of Non-Coplanar Nodal Loop

Nodal line in single-component molecular conductor [Pd(dddt)_2] has been examined to understand the tilted Dirac cone on the non-coplanar loop. In the previous work [J. Phys. Soc. Jpn. 87, 113701 (2018)], the velocity of the cone was calculated at respective Dirac points on the nodal loop based on our first-principles band structure calculations, which was a new method to derive an effective Hamiltonian with a 2 x 2 matrix. However, the Dirac cones on the nodal line are fully reproduced only at symmetric points. In the present paper, we show that our improved method well reproduces reasonable behaviors of all the Dirac cones and a very small energy dispersion of 6~meV among the Dirac points. The variation of velocities along the nodal line are shown by using principal axes of the gap function between the conduction and valence bands. Further, the density of states close to the chemical potential and orbital magnetic susceptibility are calculated using such an effective Hamiltonian.

cond-mat.mes-hall↗

arXiv2020

A tight-binding model of an ambient-pressure molecular Dirac electron system

By deriving a tight-binding model, we demonstrate a mechanism of forming a nodal line of Dirac points in a single-component molecular conductor [Pt(dmtd)$_2$] [Zhou {\it et al.}, Chem. Commun. {\bfseries 55}, 3327 (2019)], consisting of HOMO and LUMO. The nodal line is obtained as the intersection of two surfaces, where one corresponds to the HOMO-LUMO band crossing and another is vanishing of the HOMO-LUMO couplings due to their different symmetries. The latter property is essential for the Dirac electron in molecular conductors. The nature of the open nodal line is discussed in terms of the parity of the wavefunctions at eight TRIMs (time reversal invariant momenta).

cond-mat.mtrl-sci↗

arXiv2020

Exotic Dirac Cones on the Band Structure of $α$-STF$_2$I$_3$ at Ambient Temperature and Pressure

The quasi-two-dimensional molecular semimetallic conductor $α$-STF$_2$I$_3$ is isostructural with $α$-ET$_2$I$_3$.The latter possesses a unique band structure showing a zero-gap state with Dirac cones under high pressure, whereas the band structure of the former has been elusive because of heavy disorder at all the donor sites. To elucidate the band structure of $α$-STF$_2$I$_3$, a theoretical method based on the observed atomic parameters at 296 K and 1 bar has been proposed. The results suggest that the STF salt should have a band structure with Dirac cones under ambient pressure and temperature, which should promote future experimental studies on this system.Using the extended Hückel method, we demonstrate that the Dirac points of $α$-STF$_2$I$_3$ are aligned to be symmetric with respect to a time reversal invariant momentum (TRIM). Such novel Dirac cones, where the energy difference between the conduction and valence bands has considerable anisotropy, are clarified in terms of the parity of the wavefunction at the TRIM. We propose the conductivity measurement on the present $α$-STF$_2$I$_3$, which is expected to show a large anisotropy as a characteristic of the present Dirac electrons.

cond-mat.mtrl-sci↗

arXiv2018

Anisotropic Conductivity of Nodal Line Semimetal in Single-Component Molecular Conductor [Pd(dddt)$_2$]

Using a tight-binding model, we theoretically examine the anisotropic conductivity of the nodal line semimetal of a three-dimensional Dirac electron in a single-component molecular conductor [Pd(dddt)$_2$], which consists of four molecules with HOMO and LUMO orbitals per unit cell. The conductivity shows an anisotropy given by $σ_y > σ_x > σ_z$ in accordance with that of the velocity of the Dirac cone where $z$ is the interlayer direction and $y$ is the molecular stacking direction. With increasing pressure, the nodal line semimetal emerges, followed by a loop of the Dirac point where $σ_x$ takes its maximum at a pressure. Such a pressure dependence is studied by calculating the density of states and chemical potential. The temperature dependence of anisotropic conductivity is examined at low temperatures to obtain a constant behavior, which is ascribed to the Dirac electron. The relevance of the present calculation to the experiment is discussed.

cond-mat.mtrl-sci↗

arXiv2018

Analysis of Dirac Point in the Organic Conductor $α$-(BEDT-TTF)$_2$I$_3$

The Dirac electron in the organic conductor $α$-(BEDT-TTF)$_2$I$_3$ under pressure is analyzed using a tight-binding model with nearest-neighbor transfer energies and four molecules per unit cell. By noting that the Dirac point between the first and second energy bands emerges or merges followed by the level crossing at a time-reversal invariant momentum (TRIM), an effective Hamiltonian is derived on the basis of these two wave functions at the TRIM, which have different parities associated with an inversion symmetry around the inversion center. We demonstrate that the Dirac point is determined by an intersection of two kinds of lines originating from the Hamiltonian described by symmetric and antisymmetric functions around the TRIM. The present method quantitatively gives a reasonable location of the Dirac point of $α$-(BEDT-TTF)$_2$I$_3$ in a wide pressure range.

cond-mat.mtrl-sci↗

arXiv2018

Effective Hamiltonian of Topological Nodal Line Semimetal in Single-Component Molecular Conductor [Pd(dddt)$_2$] from First-Principles

Using first-principles density-functional theory calculations, we obtain the non-coplanar nodal loop for a single-component molecular conductor [Pd(dddt)$_2$] consisting of HOMO and LUMO with different parity. Focusing on two typical Dirac points, we present a model of an effective 2 $\times$ 2 matrix Hamiltonian in terms of two kinds of velocities associated with the nodal line. The base of the model is taken as HOMO and LUMO on each Dirac point, where two band energies degenerate and the off diagonal matrix element vanishes. The present model, which reasonably describes the Dirac cone in accordance with the first-principles calculation, provides a new method of analyzing electronic states of a topological nodal line semimetal.

cond-mat.mtrl-sci↗

arXiv2018

Role of acoustic phonons in exotic conductivity of two-dimensional Dirac electrons

We examine the effect of acoustic phonon scattering on the conductivity of two-dimensional Dirac electrons. The temperature ($T$) dependence of the conductivity ($σ$) is calculated using the electron Green's function with damping by both the impurity ($Γ_0$) and phonon ($Γ_{\rm ph}$).For zero or small doping, on which the present Rapid Communication focuses, $σ(T)$ increases and becomes almost constant due to the competition between the Dirac electrons and the phonon scattering. Such strange behavior of $σ(T)$ is ascribed to an exotic mechanism of phonon scattering, whose momentum space is strongly reduced in the presence of a Dirac cone. For large doping, $σ$ decreases due to the interplay of the Fermi surface and the phonon. The unconventional $T$ dependence of the resistivity $ρ(=1/σ)$ for small doping is compared with that of the experiment of Dirac electrons in an organic conductor.

cond-mat.mes-hall↗

arXiv2018

Berry Phase of Dirac Nodal Line Semimetal in Single-Component Molecular Conductor

The Berry phase and curvature are studied for the Dirac nodal line semimetal of single-component molecular conductor [Pd(dddt)$_2$]. Using two-band model on the basis of a tight-binding model, it is shown that the Berry curvature, $\bm{B}(\bm{k})$, exists along a loop of the nodal line, on which the Berry phase is obtained from surface integral of $\bm{B}(\bm{k})$. The Berry phase, which is called Zak phase, is also calculated from one-dimensional integral of the Berry connection along a line between two equivalent points of boundaries of Brillouin zone. The possible experiment is discussed in terms of the Berry phase.

cond-mat.mes-hall↗

arXiv2018

Conductivity and Resistivity of Dirac Electrons in Single-Component Molecular Conductor [Pd(dddt)_2]

Dirac electrons, which have been found in the single-component molecular conductor [Pd(dddt)_2] under pressure, are examined by calculating the conductivity and resistivity in terms of a tight-binding model for several pressures of P GPa, which give a nodal line semimetal or insulator. The temperature (T) dependence of the conductivity shows that the conductivity increases linearly under pressure at low T due to the Dirac cone but stays almost constant at high T. Further, at lower pressures, the conductivity is suppressed due to an unconventional gap, which is examined by calculating the resistivity. The resistivity exhibits a pseudogap-like behavior even in the case described by the Dirac cone. Such behavior originates from a novel role of the nodal line semimetal followed by a pseudogap that is different from a band gap. The present result reasonably explains the resistivity observed in the experiment.

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