Conductivity of Two-dimensional Dirac Electrons Close to Merging in Organic Conductor $α$-STF$_2$I$_3$ at Ambient Pressure

Abstract

The electric conductivity of Dirac electrons in the organic conductor α-STF2I3 (STF = bis(ethylenedithio)diselenadithiafulvalene), which has an isostructure of α-(BEDT-TTF)2I3, has been theoretically studied using a two-dimensional tight-binding model in the presence of both impurity and electron-phonon (e-p) scatterings. In contrast to α-(BEDT-TTF)2I3, which has a Dirac cone with almost isotropic velocity, α-STF2I3 provides a large anisotropy owing to a Dirac point that is close to merging. As a result, σx becomes much larger than σy, where σy and σx are diagonal conductivities parallel and perpendicular to a stacking axis of molecules, respectively. With increasing temperature (T ), σx takes a broad maximum because of e-p scattering and σy remains almost constant. The ratio σx/σy is analyzed in terms of the band structure. Such an exotic conductivity of α-STF2I3 is compared with that of an experiment showing a good correspondence. Finally, σx/σy values of α-ET2I3 and α-BETS2I3 are shown to demonstrate the dissimilarity with α-STF2I3.