Anomalous conductivity of two-dimensional Dirac electrons in organic conductor under pressures

TL;DR

This study investigates the unusual temperature-dependent electrical conductivity in a 2D Dirac electron system within an organic conductor under pressure, highlighting the roles of Dirac cone tilting and anisotropy.

Contribution

It provides a detailed theoretical analysis of the anomalous conductivity crossover in organic Dirac electron systems, incorporating impurity and electron-phonon scatterings, extending previous models.

Findings

Conductivity shows a crossover from σ_x < σ_y to σ_x > σ_y with increasing temperature.

Dirac cone tilting dominates low-temperature behavior, while velocity anisotropy dominates at high temperatures.

Nearly constant high-temperature conductivity is attributed to electron-phonon scattering.

Abstract

The electric conductivity of Dirac electrons in the organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] under pressure has been examined using a two-dimensional tight-binding (TB) model with both impurity and electron--phonon (e--p) scatterings. We study an anomalous temperature dependence of the conductivity, which shows a crossover from $\sigma_{x} < \sigma_{y}$ at low temperatures [region (I)] to $\sigma_{x} > \sigma_{y}$ at high temperatures [region (II)]. $\sigma_y$ and $\sigma_x$ are diagonal conductivities parallel and perpendicular to a stacking axis of molecules, respectively. The effect of Dirac cone tilting is dominant in region (I), whereas the anisotropy of the velocity of the Dirac cone is dominant in region (II). Such behavior is further examined by calculating the deviation of principal axes due to the off-diagonal conductivity $\sigma_{xy}$ and a nearly constant conductivity at high temperatures due to the e--p scattering, which is the extension of the previous result of the simple two-band model [Phys. Rev. B {\bf 98},161205 (2018)]. The relevance to experiments of organic conductors is discussed.