Theory of Dirac Electrons in Organic Conductors

Yoshikazu Suzumura,Akito Kobayashi

doi:10.3390/cryst2020266

Abstract

The dynamical property of electrons with the tilted Dirac cone was examined using the tilted Weyl equation. The polarization function exhibits cusps and nonmonotonic structures by varying both the frequency and the momentum. A pair of tilted Dirac cones exhibits a new plasmon for the intermediate magnitude of momentum owing to the combined effects of two tilted cones. Dirac electrons with the zero-gap state (ZGS) in organic conductor α-(BEDT-TTF)2I3 are examined by calculating the Berry curvature, which displays the peak structure for a pair of Dirac particles between the conduction band and the valence band. The ZGS is theoretically predicted for α-(BEDT-TTF)2NH4Hg(SCN)4 under uniaxial pressure. Examining the band structure of the stripe charge ordered state of α-(BEDT-TTF)2I3 under pressure, we have found a topological transition from a conventional insulator to a new phase of a pair of Dirac electrons with a finite mass. Further, investigating the zero-energy (N = 0) Landau level under a strong magnetic field, we propose ferromagnetism breaking the SU(2) valley-pseudo-spin symmetry, and the phase fluctuations of the order parameters leading to Kosterlitz-Thouless transition at lower temperatures.

Highlights

The organic conductor α-(BEDT-TTF)2I3 (BEDT-TTF=bis(ethylene-dithio)tetrathiafulvalene) [1]exhibits a band structure of two-dimensional semimetal or a narrow gap semiconductor as claimed byMori et al [2] based on the extended Huckel molecular orbital calculation
We have shown the effect of tilting of Dirac cone and the characteristic behavior of the Berry curvature in organic conductors
The tilting effect is examined by calculating the polarization function of the massless Dirac particle for a finite doping

Summary

Introduction

The organic conductor α-(BEDT-TTF)2I3 (BEDT-TTF=bis(ethylene-dithio)tetrathiafulvalene) [1]. A similar behavior was found under uniaxial pressure [5], but at rather a lower pressure where the crystal structure under uniaxial strain and ambient pressure, and at room and low temperatures has been determined [6] Using these transfer integrals between molecules [6], massless Dirac fermion has been found [7,8] in the band calculation. Ferromagnetism breaking the SU(2) valley-pseudo-spin symmetry in the N = 0 Landau level is proposed by noting the fact that the scattering processes between valleys becomes non-zero in the case of the tilted Dirac cones In this case, the phase fluctuations of the order parameters can be described by the XY model leading to a Kosterlitz-Thouless transition at lower temperatures.

Dynamical Polarization Function

Summary