Shinya Katayama

Pressure-Induced Zero-Gap Semiconducting State in Organic Conductor α-(BEDT-TTF)2I3 Salt

We show a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 salt, which emerges under uniaxial pressure along the a -axis (the stacking axis of t...

Massless Fermions in Organic Conductor

The electronic states in two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 have been investigated to show the noticeable property of the massless fermions, i.e., the linear dispersion which exists on the contact point between the conduction band and the valence band. These fermions are well known in bismuth and graphite, where the former are described by the Dirac equation and the latter obeys the Weyl equation corresponding to the massless fermion. In the present study, we show that the effective Hamiltonian describing the massless fermions in α-(BEDT-TTF) 2 I 3 contains intrinsically new terms of Pauli matrices σ z and σ 0 in addition to the Weyl equation which consists of σ x and σ y . The new massless fermions are robust against the charge disproportionation, and induce the anomalous momentum-dependence in the charge density.

Superconductivity in Charge Ordered Organic Conductor –α-(ET)2I3 Salt–

We theoretically examine superconductivity in quasi-two-dimensional organic conductor, α-(ET) 2 I 3 salt, which has been found under uniaxial pressure and at temperature below the charge ordering. The interplay of many bands and several repulsive interactions gives rise to novel role of the charge ordering (CO), which leads to the coexistence with the superconducting (SC) state. The hole and electron pockets at Fermi surface, which exist in normal state and at ambient pressure, vanish in the charge ordered insulating state due to the formation of the charge gap. Under pressures, the charge gap disappears and the narrow gap semiconductor (NGS) with CO exhibits the density of state with a sharp peak just below the Fermi energy. Based on the mean field of CO state, charge and spin susceptibilities, and the paring interaction are evaluated by using the random phase approximation. The onset for the SC state is calculated in terms of a linearized Eliashberg equation, where the pairing interaction consists of bo...

Superconductivity in charge ordered metal for quasi-two-dimensional organic conductor

The superconductivity that occurs below the charge ordering temperature in a quasi-two-dimensional (quasi-2D) organic conductor, the α-(ET) 2 I 3 salt, has been examined by applying the mean field theory and the random phase approximation to the extended Hubbard model with four sites in the unit cell and anisotropic nearest-neighbor interactions. It is shown that, under uniaxial pressure, the insulating phase moves to the metal phase with small pockets of electrons and holes, while the charge ordering state with the horizontal stripe pattern is retained. The novel property of the metal phase gives rise to the superconducting (SC) state with the full gap. It is found that the pairing interaction for the superconductivity is given mainly by the spin fluctuation existing between hole-rich sites in the unit cell. The role of the anisotropic repulsive interaction is discussed for the mechanism of the present SC state.

Theoretical study of the zero-gap organic conductor α-(BEDT-TTF)2I3

Abstract The quasi-two-dimensional molecular conductor α-(BEDT-TTF)2I3 exhibits anomalous transport phenomena where the temperature dependence of resistivity is weak but the ratio of the Hall coefficient at 10 K to that at room temperature is of the order of 104. These puzzling phenomena were solved by predicting massless Dirac fermions, whose motions are described using the tilted Weyl equation with anisotropic velocity. α-(BEDT-TTF)2I3 is a unique material among several materials with Dirac fermions, i.e. graphene, bismuth, and quantum wells such as HgTe, from the view-points of both the structure and electronic states described as follows. α-(BEDT-TTF)2I3 has the layered structure with highly two-dimensional massless Dirac fermions. The anisotropic velocity and incommensurate momenta of the contact points, ±k0, originate from the inequivalency of the BEDT-TTF sites in the unit cell, where ±k0 moves in the first Brillouin zone with increasing pressure. The massless Dirac fermions exist in the presence of the charge disproportionation and are robust against the increase in pressure. The electron densities on those inequivalent BEDT-TTF sites exhibit anomalous momentum distributions, reflecting the angular dependences of the wave functions around the contact points. Those unique electronic properties affect the spatial oscillations of the electron densities in the vicinity of an impurity. A marked behavior of the Hall coefficient, where the sign of the Hall coefficient reverses sharply but continuously at low temperatures around 5 K, is investigated by treating the interband effects of the magnetic field exactly. It is shown that such behavior is possible by assuming the existence of the extremely small amount of electron doping. The enhancement of the orbital diamagnetism is also expected. The results of the present research shed light on a new aspect of Dirac fermion physics, i.e. the emergence of unique electronic properties owing to the structure of the material.

Electronic properties close to Dirac cone in two-dimensional organic conductor α-(BEDT-TTF)2I3

A zero-gap state (ZGS) has been found in a bulk system of two-dimensional organic conductor, α-(BEDT-TTF)2I3 salt which consists of four sites of donor molecules in a unit cell. In the present paper, the characteristic of the ZGS is analyzed in detail and the electronic properties are examined in the vicinity of the Dirac point where the conduction and valence bands degenerate to form the zero-gap. The eigenvectors of the energy band have four components of respective sites, where two of them correspond to inequivalent sites and the other two correspond to equivalent sites. It is shown that the former exhibits an exotic momentum dependence around the contact point and the latter shows almost a constant dependence. The density of states of each site close to the Dirac point is calculated to demonstrate the temperature dependence of the local magnetic susceptibility and the local nuclear magnetic relaxation rate. Further, the robust property of the ZGS against the anion potential is also shown by using the second-order perturbation.

Electric Conductivity of the Zero-Gap Semiconducting State in α-(BEDT-TTF)2I3 Salt

Electric conductivity that reveals the zero-gap semiconducting (ZGS) state has been investigated as a function of temperature T and lifetime τ in order to understand the ZGS state in a quarter-filled α-(BEDT-TTF) 2 I 3 salt with four sites in the unit cell. By treating τ as a parameter and employing the one-loop approximation, it is found that the conductivity is proportional to T and τ for \(k_{\text{B}} T\gg\hbar/\tau\) and is independent of T and τ for \(k_{\text{B}} T\ll\hbar/\tau\). Further, the conductivity, which is independent of T in the ZGS state, is examined for the impurity scattering in terms of the Born approximation.

Role of Interlayer Electron Hopping for Spin Density Wave State in the Zero-Gap Organic Conductor

We investigate the formation of density waves in the zero-gap state (ZGS) which has been found in the quasi-two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 salt under the hydrostatic pressure....

Effect of anion potential on the zero-gap state in the two-dimensional organic conductor α-(BEDT-TTF)2I3

The organic conductor α-(BEDT-TTF)2I3 exhibits the zero-gap state (ZGS) described by the massless Dirac fermions, and is in contrast to that of the single layer graphite, i.e. graphene since the former is robust against the site potential compared with the latter. However, the ZGS of the organic conductor is not fully clarified due to the complexity of several kinds of transfer energies arising from four kinds of donor molecules in a unit cell. In the present paper, we analyze the details of the ZGS by focusing on the role of the anion potential, which acts differently on respective site of donor molecules. We show that the anion potential with a small magnitude does not destroy the ZGS but has an effect of varying the location of the contact point between the conduction and valence bands as found in the case of pressure. Such a behavior can be understood by the tilted Weyl equation with a perturbation for the potential. Further, the ZGS is discussed by examining the relation between the anion potential and the charge disproportionation.

Exotic properties of zerogap state in α-(BEDT-TTF)2I3

We investigate effects of a non-magnetic impurity on zerogap state of molecular conductor α-(BEDT-TTF)2I3 and predict theoretically incommensurate oscillations of charge density. These oscillations reflect unique properties of the massless Dirac fermions, which are characterized by the incommensurate momenta ±κo of the contact points and the singular momentum distributions of the charge density. We examine an effective Hamiltonian in the presence of stripe charge ordering, neighboring the zerogap state. The energy bands are divided into two parts, having one-dimensional fermions and two-dimensional fermions. The one-dimensional-like bands exhibit a unique anisotropic dispersion which is linear along b-axis (parallel to the stripe) and parabolic along the a-axis (the stacking axis). Thus we expect anomalous magnetoresistance in stripe charge ordering state, since such a anisotropic dispersion indicates the unconventional Landau quantization.