arXiv subjects

Yoshikazu Suzumura

Publications and source records attributed to Yoshikazu Suzumura.

At least 37 records · Page 2Linked to original sources

arXiv2018

Effect of Long-Range Coulomb Interaction on NMR Shift in Massless Dirac Electrons of Organic Conductor

The nuclear magnetic resonance (NMR) with the site-dependent shift, at low temperatures is examined for a massless Dirac electrons in the organic conductor, alpha-(BEDT-TTF)_2I_3, where the sites of the four molecules in the unit cell are given by A (= A'), B, and C. The Dirac cone exists within an energy of 0.01 eV between the conduction and valence bands. The magnetic response function is calculated by taking account of the long-range Coulomb interaction and electron doping. Calculating the interaction within the first order in the perturbation, the chemical potential is determined self-consistently, and the self-energy and vertex corrections are taken to satisfy the Ward identity. The site-dependent shift is calculated at low temperatures of 0.0002 < T < 0.002 (T is temperature in the unit of eV) by correctly treating the wave function of the Dirac cone. At lower (higher) temperatures the self-energy (vertex) correction of the shift at all sites except for B is dominant and the sign is negative (positive), while the sign of the correction at the B site is always negative. For moderate doping, the shift as a function of T takes a minimum. The relevance of the shift to the experiment is discussed.

cond-mat.mtrl-sci↗

arXiv2017

Novel Dirac Electron in Single-Component Molecular Conductor [Pd(dddt)$_2$]

Dirac electrons in a single-component molecular conductor [Pd(dddt)$_2$] under pressure have been examined using a tight-binding model which consists of HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) functions in four molecules per unit cell. The Dirac cone between the conduction and valence bands comes from the property that HOMO has ungerade symmetry and LUMO has gerade symmetry. The Dirac point forms a loop in the three-dimensional Brillouin zone, which is symmetric with respect to the plane of $k_y$ = 0 where $k_y$ is the intralayer momentum along the molecular stacking direction, i.e., with the largest (HOMO-HOMO, LUMO-LUMO) transfer energy. The parity at TRIM (time reversal invariant momentum) is calculated using the inversion symmetry around the lattice point of the crystal. It is shown that such an exotic Dirac electron is understood from the parity of the wave function at the TRIM and also from an effective Hamiltonian.

cond-mat.mtrl-sci↗

arXiv2015

Longitudinal conductivity of massless fermions with tilted Dirac cone in magnetic field

We investigate a longitudinal conductivity of a two-dimensional relativistic electron gas with a tilted Dirac cone in magnetic field. It is demonstrated that the conductivity behaves differently in the directions parallel and perpendicular to the tilting of the cone. At high magnetic fields the conductivity at non-zero Landau levels in the direction perpendicular to the tilting modifies non-trivially, in contrast to the parallel case. At zero temperature the crossover of the conductivity at the Dirac point from high to low magnetic field is studied numerically. It is found that that the tilting produces anisotropy of the conductivity which changes with the magnetic field which is different from the anisotropy coming from the Fermi velocity. We also discuss the conductivity at finite temperatures and finite magnetic fields which can be directly compared with the experiments in $α$-(BEDT-TTF)$_2$I$_3$ organic conductor. We find that the tilting does not affect so much the magnetic-filed dependence of the conductivity except for the prefactor. We discuss the interpretation of recent experimental data and make some proposals to detect the effect of the tilting in future experiments.

cond-mat.mes-hall↗

arXiv2015

Plaquette chirality patterns for robust zero-gap states in $α$-type organic conductor

Dirac electrons with a zero-gap state (ZGS) in organic conductor $α$-(BEDT-TTF)$_2$I$_3$ result from a fine tuning of the seven nearest neighbors transfer integrals ($a_1, a_2, a_3, b_1, b_2, b_3, b_4$) between the four molecules of the unit cell. In this work we show that for given moduli $|a_1|,...|b_4|$, the possibility of having Dirac electron with a ZGS at $3/4$ (or $1/4$) filling strongly depends on the specific configurations of signs of the seven transfer integral. More precisely it is possible to classify the sign configurations into essentially four classes determined by $χ_{a}={\rm sign} (a_2a_3)$ and $χ_b={\rm sign} (b_1b_2b_3b_4)$. Using extended numerics, we show that for both weak and large inhomogeneity in the moduli, the class $(χ_a,χ_b)=(-,-)$ is the most favorable to find Dirac electrons with ZGS at $3/4$ (or $1/4$) filling. For the class $(χ_a,χ_b)=(+,+)$ in the opposite case, we never found any ZGS at either $1/4$ or $3/4$ filling. The last two classes \suzum{given by} $(χ_a,χ_b)=(+,-)$ and $(χ_a,χ_b)=(-,+)$ corresponding to an intermediate situation; they allow for ZGS at $3/4$ (resp. $1/4$) filling but are much less favorable than class $(χ_a,χ_b)=(-,-)$. As a matter of fact, all previous numerical studies of Dirac electrons and ZGS in $α$-(BEDT-TTF)$_2$I$_3$ correspond to class $(χ_a,χ_b)=(-,+)$.

cond-mat.str-el↗

arXiv2013

Inversion Symmetry and Wave-Function-Nodal-Lines of Dirac Electrons in Organic Conductor alpha-(BEDT-TTF)_2I_3

By examining organic conductor alpha-(BEDT-TTF)_2I_3 which is described by a nearest neighbors tight-binding model it is shown that because of inversion symmetry, each component of a wave function (WF) exhibits nodal lines (NLs) in the Brillouin zone. In the absence of any band crossing, each NL connects two time reversal invariant momenta (TRIM) as partners. In the presence of a pair of Dirac points (band crossing), for each band that crosses and for each WF component there is a NL that connects the pair of Dirac points via a TRIM without partner. This second kind of NL leads to a discontinuous sign change for non vanishing components of the WF. Such a property is at the origin of the +/- pi Berry phase accumulated on a contour integral encircling one Dirac point. The results are examplified by numerical calculation of WFs components for the above conductor with a 3/4 filled band.

cond-mat.mes-hall↗

arXiv2013

Existence and merging of Dirac points in $α$-(BEDT-TTF)$_2$I$_3$ organic conductor

We reexamine the existence and stability conditions of Dirac points between valence and conduction bands of 3/4 filled $α$-(BEDT-TTF)$_2$I$_3$ conducting plane. We consider the usual nearest neigbhor tight binding model with the seven transfer energies that depend on the applied pressure. Owing to the four distinct molecules (A, A', B, C) per unit cell of the Bravais lattice, the corresponding Bloch Hamiltonian is a $4\times4$ matrix ${\cal H}({\bm k})$ for each wave vector $\mathbf{k}$ of the Brillouin zone. In most previous works the study of Dirac points was achieved through direct numerical diagonalization of matrix ${\cal H}({\bm k})$. In this work we develop a novel analytical approach which allows to analyze the existence and stability conditions of Dirac points from the knowledge of their merging properties at time reversal points. Within this approach we can discuss thoroughly the role of each transfer integrals and demonstrate that inversion symmetry is convenient but not necessary.

cond-mat.mes-hall↗

arXiv2012

Algebraic structure of Dirac fermion state in α-(BDET-TTF)_2I_3

We study the algebraic structure of the Dirac fermion state in the organic conductor, α-(BDET-TTF)_2I_3. We find a pair of generators for the Hamiltonian of α-(BDET-TTF)_2I_3 that describes the chirality of Dirac fermions. The phase parameters associated with those generators have an intimate and simple relationship with the positions of the Dirac points in the Brillouin zone. By making use of the form of the generators, a reduced form of the Hamiltonian is constructed that shares some characteristic features of the Dirac fermions with α-(BDET-TTF)_2I_3. For the reduced Hamiltonian we present the analytic expression for the Dirac point position that sits at arbitrary point in the Brillouin zone determined by the hopping parameters.

cond-mat.mes-hall↗

arXiv2012

Effects of Zeroline and Ferrimagnetic Fluctuation on Nuclear Magnetic Resonance for Dirac Electrons in Molecular Conductor alpha-(BEDT-TTF)2I3

We re-examine the wave function of two-dimensional massless Dirac electron in alpha-(BEDT-TTF)2I3 consisting of four molecules A, A', B and C in a unit cell, using a tight-binding model. We find zerolines in the Brillouin zone, on which the component of the wave function becomes zero for B or C sites. The zerolines, which are bounded by two Dirac points at k0 and pass through the M- or Y-points, result in a fact that the density of states of the B site exhibits no the Van Hove singularity near the energy of the Dirac points. By taking account of the on-site Coulomb interaction within the random phase approximation, we examine the spin fluctuation in order to investigate properties of the nuclear magnetic resonance for temperatures T > 50K. In the region for 100 < T < 300K, it is shown that the Knight sift for B-site monotonously decreases with decreasing temperature, owing to lack of the Van Hove singularity, while it shows a maximum for the other sites (A, A' and C sites). In the region for 50 < T < 100K, it is shown that the Knight sift is convex downward and the Korringa ratio increases with decreasing temperature for B-site. Such a behavior originates from the ferrimagnetic spin fluctuation related to the zerolines. These results are consistent with those of the nuclear magnetic resonance experiments.

cond-mat.mes-hall↗

arXiv2012

Ab initio derivation of multi-orbital extended Hubbard model for molecular crystals

From configuration interaction (CI) ab initio calculations, we derive an effective two-orbital extended Hubbard model based on the gerade (g) and ungerade (u) molecular orbitals (MOs) of the charge-transfer molecular conductor (TTM-TTP)I_3 and the single-component molecular conductor [Au(tmdt)_2]. First, by focusing on the isolated molecule, we determine the parameters for the model Hamiltonian so as to reproduce the CI Hamiltonian matrix. Next, we extend the analysis to two neighboring molecule pairs in the crystal and we perform similar calculations to evaluate the inter-molecular interactions. From the resulting tight-binding parameters, we analyze the band structure to confirm that two bands overlap and mix in together, supporting the multi-band feature. Furthermore, using a fragment decomposition, we derive the effective model based on the fragment MOs and show that the staking TTM-TTP molecules can be described by the zig-zag two-leg ladder with the inter-molecular transfer integral being larger than the intra-fragment transfer integral within the molecule. The inter-site interactions between the fragments follow a Coulomb law, supporting the fragment decomposition strategy.

cond-mat.mtrl-sci↗

arXiv2011

Spectral functions of two-band spinless fermion and single-band spin-1/2 fermion models

We examine zero-temperature one-particle spectral functions for the one-dimensional two-band spinless fermions with different velocities and general forward-scattering interactions. By using the bosonization technique and diagonalizing the model to two Tomonaga-Luttinger-liquid Hamiltonians, we obtain general expressions for the spectral functions which are given in terms of the Appell hypergeometric functions. For the case of identical two-band fermions, corresponding to the SU(2) symmetric spin-1/2 fermions with repulsive interactions,the spectral functions can be expressed in terms of the Gauss hypergeometric functions and are shown to recover the double-peak structure suggesting the well-known "spin-charge" separation. By tuning the difference in velocities for the two-band fermions, we clarify the crossover in spectral functions from the "spin-charge" separation to the decoupled fermions. We discuss the relevance of our results to the spin-1/2 Hubbard model under a magnetic field which can be mapped onto two-band spinless fermions.

cond-mat.str-el↗

arXiv2011

Berry Curvature of the Dirac Particle in $α$-(BEDT-TTF)$_2$I$_3$

We examine several properties of the Berry curvature for the organic conductor $α$-(BEDT-TTF)$_2$I$_3$ consisting of four bands, which exhibits a zero-gap state with Dirac cones. By adding a small potential acting on two molecular sites, which breaks the inversion symmetry, it is shown that the curvature for the Dirac particles displays a pair of peaks with opposite signs and that each peak increases with decreasing potential. The Berry curvature originating from the property of the wave function is analyzed using a reduced Hamiltonian with a 2x2 matrix based on the Luttinger-Kohn representation, which describes a pair of Dirac particles between the conduction band and the valence band. Two types of velocity fields in the reduced Hamiltonian, whose vector product gives the Berry curvature, rotate around the Dirac point as a vortex. It is also shown that the other bands exhibit another pair of peaks of Dirac particles with a tendency toward merging.

cond-mat.mes-hall↗

arXiv2011

Dirac Electrons in Molecular Solids

Electrons in solids are characterized by the energy bands, which indicate that electrons are considered to be "elementary particles" with specific effective masses and g-factors reflecting features of each solid. There are cases where these particles obey dispersion relationship similar to those of Dirac electrons. Examples include graphite and bismuth both of which are known for many years, together with graphene, a single layer of graphite, recently addressed intensively after its realization. Another recent example is a molecular solid, alpha-ET2I3, which is described by an equation similar to Weyl equation with massless Dirac cones but the coordinate axis is tilted because of the location of cones at off-symmetry points. Orbital susceptibility of such Dirac electrons in graphite and bismuth has been known to have striking features not present in ordinary band electrons but resulting from the inter-band matrix elements of magnetic field. Results of theoretical studies on not only orbital susceptibility but also Hall effect of such Dirac electrons in molecular solids with tilting are introduced in this paper.

cond-mat.mes-hall↗

arXiv2011

Role of charge fluctuation in Q1D organic superconductor (TMTSF)2ClO4

We have theoretically investigated the spin and charge fluctuations in the quasi-one dimensional organic superconductor (TMTSF)2ClO4. Using the extended multi-site Hubbard model, which contains four sites in a unit cell and the transfer energies obtained by the extended Hückel method, we calculate the linearized gap equation with the random phase approximation, to find novel order parameters of superconductivity due to several kinds of charge fluctuations induced by the anisotropic intersite repulsive interactions. For the singlet state, the order parameter with line nodes appears in the case of the strong charge fluctuation, while the order parameter with anisotropic gap suggested by Shimahara is reproduced in the spin fluctuation. The triplet state is also obtained for the wide parameter range of repulsive interactions due to a cooperation between charge and spin fluctuations.

cond-mat.supr-con↗

arXiv2010

Multi-Orbital Molecular Compound (TTM-TTP)I_3: Effective Model and Fragment Decomposition

The electronic structure of the molecular compound (TTM-TTP)I_3, which exhibits a peculiar intra-molecular charge ordering, has been studied using multi-configuration ab initio calculations. First we derive an effective Hubbard-type model based on the molecular orbitals (MOs) of TTM-TTP; we set up a two-orbital Hamiltonian for the two MOs near the Fermi energy and determine its full parameters: the transfer integrals, the Coulomb and exchange interactions. The tight-binding band structure obtained from these transfer integrals is consistent with the result of the direct band calculation based on density functional theory. Then, by decomposing the frontier MOs into two parts, i.e., fragments, we find that the stacked TTM-TTP molecules can be described by a two-leg ladder model, while the inter-fragment Coulomb energies are scaled to the inverse of their distances. This result indicates that the fragment picture that we proposed earlier [M.-L. Bonnet et al.: J. Chem. Phys. 132 (2010) 214705] successfully describes the low-energy properties of this compound.

cond-mat.mtrl-sci↗

arXiv2010

Tilted-Cone Induced Cusps and Nonmonotonic Structures in Dynamical Polarization Function of Massless Dirac Fermions

The polarization function of electrons with the tilted Dirac cone found in organic conductors is studied using the tilted Weyl equation. The dynamical property is explored based on the analytical treatment of the particle-hole excitation. It is shown that the polarization function as the function of both the frequency and the momentum exhibits cusps and nonmonotonic structures. The polarization function depends not only on the magnitude but also the direction of the external momentum. These properties are characteristic of the tilted Dirac cone, and are contrast to the isotropic case of grapheme. Further, the results are applied to calculate the optical conductivity, the plasma frequency and the screening of Coulomb interaction, which are also strongly influenced by the tilted cone.

cond-mat.mes-hall↗

arXiv2010

Intramolecular charge ordering in the multi molecular orbital system (TTM-TTP)I_3

Starting from the structure of the (TTM-TTP)I_3 molecular-based material, we examine the characteristics of frontier molecular orbitals using ab initio (CASSCF/CASPT2) configurations interaction calculations. It is shown that the singly-occupied and second-highest-occupied molecular orbitals are close to each other, i.e., this compound should be regarded as a two-orbital system. By dividing virtually the [TTM-TTP] molecule into three fragments, an effective model is constructed to rationalize the origin of this picture. In order to investigate the low-temperature symmetry breaking experimentally observed in the crystal, the electronic distribution in a pair of [TTM-TTP] molecules is analyzed from CASPT2 calculations. Our inspection supports and explains the speculated intramolecular charge ordering which is likely to give rise to low-energy magnetic properties.

cond-mat.mtrl-sci↗

arXiv2010

Mechanism for the Singlet to Triplet Superconductivity Crossover in Quasi-One-Dimensional Organic Conductors

Superconductivity of quasi-one-dimensional organic conductors with a quarter-filled band is investigated using the two-loop renormalization group approach to the extended Hubbard model for which both the single electron hopping t_{\perp} and the repulsive interaction V_{\perp} perpendicular to the chains are included. For a four-patches Fermi surface with deviations to perfect nesting, we calculate the response functions for the dominant fluctuations and possible superconducting states. By increasing V_{\perp}, it is shown that a d-wave (singlet) to f-wave (triplet) superconducting state crossover occurs, and is followed by a vanishing spin gap. Furthermore, we study the influence of a magnetic field through the Zeeman coupling, from which a triplet superconducting state is found to emerge.

cond-mat.str-el↗

arXiv2009

Tilted-Cone-induced easy-plane pseudo-spin ferromagnet and Kosterlitz-Thouless transition in massless Dirac fermions

The possible quantum Hall ferromagnet at a filling factor $ν=0$ is investigated for the zero-energy (N=0) Landau level of the two dimensional massless Dirac fermions in $α$-(BEDT-TTF)$_2$I$_3$ under pressure with tilted cones and a twofold valley degeneracy resulting from time-reversal symmetry. In the case of the Dirac cones without tilting, the long-range Coulomb interaction in the N=0 Landau level exhibits the SU(2) valley-pseudo-spin symmetry even to the order $O(a/l_{\rm H})$, in contrast to $N \ne 0$ Landau levels, where $a$ and $l_{\rm H}$ represent the lattice constant and the magnetic length, respectively. Such a characteristic comes from a fact that zero-energy states in a particular valley are restricted to only one of the spinor components, whereas the other spinor component is necessarily zero. In the case of the tilted Dirac cones as found in $α$-(BEDT-TTF)$_2$I$_3$, one obtains a non-zero value of the second component and then the ackscattering processes between valleys becomes non-zero. It is shown that this fact can lead to easy-plane pseudospin ferromagnetism (XY-type). In this case, the phase fluctuations of the order parameters can be described by the XY model leading to Kosterlitz-Thouless transition at lower temperature. In view of these theoretical results, experimental findings in resistivity of $α$-(BEDT-TTF)$_2$I$_3$ are discussed.

cond-mat.mes-hall↗