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

Yoshikazu Suzumura, Masao Ogata

Published 2020-09-30Version 1

Dirac electrons in organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ under pressures, which exhibit anomalous conductivity being nearly constant at high temperatures, have been examined using a two-dimensional tight-binding model (TB) model with both the impurity and electron-phonon (e-p) scatterings. A crucial role of scattering by acoustic phonon is shown based on the previous study for a model with simple Dirac cone [Phys. Rev. B {\bf 98},161205 (2018)]. In addition to diagonal conductivity $\sigma_{x}=\sigma_{xx}$ and $\sigma_{y}=\sigma_{yy}$, off-diagonal conductivity $\sigma_{xy}$ exists due to a tilted Dirac cone, where $y$ ($x$) corresponds to a direction parallel (perpendicular) to a stacking axis of molecules. This $\sigma_{xy}$ results in a rotation of axis of the principal value $\sigma_{\pm}$. Since the conductivity at high temperatures is suppressed by the e-p scattering on the Dirac cone, the increase of temperature results in a broad maximum for $\sigma_y$ and a nearly constant $\sigma_x$ for a moderate choice of the e-p coupling constant. Further a correlation effect is examined employing a mean-field for the on-site and nearest-neighbor-site interactions. Anisotropic behavior of $\sigma_{\nu}$, ($\nu$ = $x$ and $y$) is discussed by comparing with experiments of organic conductors, which present nearly constant resistivity at high temperatures.