Akito Kobayashi

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...

Hall Effect and Orbital Diamagnetism in Zerogap State of Molecular Conductor α-(BEDT-TTF)2I3

The apparent zerogap fermions in α-(BEDT-TTF) 2 I 3 are recently proposed to obey the tilted Weyl equation which is very unusual. The Hall conductivity, the conductivity and the orbital susceptibility are investigated theoretically for electrons described by this tilted Weyl equation. It is found that these quantities are enhanced due to tilting but keeping the same dependence on chemical potential as in the absence of tilting. Based on these results and expected temperature dependence of the chemical potential in α-(BEDT-TTF) 2 I 3 , it is found that there will be the strong enhancement of the Hall coefficient toward low temperatures. Moreover, it is proposed that the sharp but continuous reversal of the sign of the Hall coefficient is possible at low temperatures if the extremely small amount of electron doping, about 1 ppm, exists. These results may explain very anomalous features of Hall coefficient observed in α-(BEDT-TTF) 2 I 3 and can be an unambiguous indication of the existence of the zerogap fer...

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.

Charge-fluctuation-induced superconducting state in two-dimensional quarter-filled electron systems

We investigate superconducting (SC) states in a two-dimensional extended Hubbard model at quarter-filling with on-site repulsive interaction ( U ) and nearest-neighbor repulsive interaction ( V ), which lead to spin fluctuation and charge fluctuation, respectively. The effect of charge fluctuation on the onset of the SC state is estimated by calculating pairing interaction within the random phase approximation. Solving a linearized Eliashberg equation with an anomalous self-energy as a function of both momentum and frequency, we examine the stability of a singlet SC state in a phase diagram of U and V with fixed temperatures. We obtain a novel result that the obtained SC state has d x y symmetry in the vicinity of both spin density wave (SDW) phase (\(U \gtrsim V\)) and charge density wave (CDW) phase (\(V \gtrsim U\)). The CDW-SC and SC-metal phase boundaries exhibit reentrant structures on the V – T (temperature) plane.

Molecular Dirac Fermion Systems — Theoretical and Experimental Approaches —

The transport phenomena of zero-gap conductors are one of the central subjects in condensed matter physics. α-(BEDT-TTF)2I3 [BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene] is a typical zero-gap conductor consisting of two-dimensional multiple two-dimensional layers. Under high pressures of above 1.5 GPa, it undergoes a phase transition to a zero-gap state, in which it exhibits unusual transport phenomena. This paper reviews the recent progress in clarifying the physics of α-(BEDT-TTF)2I3. In particular, the effect of a magnetic field on the behavior of electrons is investigated. On the basis of the interlayer magnetoresistance, evidence of the Landau level with the index N = 0 was obtained. The collaboration of experiments and theory has opened a new field of exploring the interlayer electron transport in two-dimensional layered zero-gap conductors. Furthermore, these phenomena have been observed by NMR and specific heat measurements in contrast to the case of graphene. The paper also reviews some rec...

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.