Optical Conductivity in a Two-Dimensional Extended Hubbard Model for an Organic Dirac Electron System α-(BEDT-TTF)2I3

Daigo Ohki,Yukiko Omori,Genki Matsuno,Akito Kobayashi

doi:10.3390/cryst8030137

Daigo Ohki, Yukiko Omori + Show 2 more

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https://doi.org/10.3390/cryst8030137

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Abstract

The optical conductivity in the charge order phase is calculated in the two-dimensional extended Hubbard model describing an organic Dirac electron system α -(BEDT-TTF) 2 I 3 using the mean field theory and the Nakano-Kubo formula. Because the interband excitation is characteristic in a two-dimensional Dirac electron system, a peak structure is found above the charge order gap. It is shown that the peak structure originates from the Van Hove singularities of the conduction and valence bands, where those singularities are located at a saddle point between two Dirac cones in momentum space. The frequency of the peak structure exhibits drastic change in the vicinity of the charge order transition.

Highlights

An organic conductor α-(BEDT-TTF)2 I3 has attracted much interest, since it exhibits a transition between the charge order (CO) [1,2,3,4,5,6,7] and the massless Dirac electron (DE) [8,9,10,11,12,13,14,15] as hydrostatic pressure, P, increases
It is a future problem to investigate the optical conductivity in the non-uniform CO with the edges or the domain walls [26,27] using the real space dependent mean field theory, since it has been shown that the non-uniform CO may be important to explain the metallic CO near the CO transition [23]
It has been found that a peak structure above the CO gap emerges due to the two-dimensional Dirac cones

Summary

Introduction

An organic conductor α-(BEDT-TTF) I3 has attracted much interest, since it exhibits a transition between the charge order (CO) [1,2,3,4,5,6,7] and the massless Dirac electron (DE) [8,9,10,11,12,13,14,15] as hydrostatic pressure, P, increases. The CO gap induced by the inversion symmetry breaking exists in both the massive DE phase and the trivial CO, a pair of Dirac cones with a finite gap at incommensurate momentum, ±kD , merges at a time reversal invariant momentum (TRIM) at the transition between these two phases [12,15,28,29]. Such a drastic change in the band structure is expected to give rise to a characteristic in the optical conductivity

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Discussion

Conclusion