Seebeck Coefficient of Two-dimensional Dirac Electrons in Organic Conductor under Pressure

The Seebeck coefficient, which is proportional to a ratio of the thermoelectric conductivity to electrical conductivity has been examined for Dirac electrons in the organic conductor $\alpha$-(BEDT-TTF)${}_2$I${}_3$ [BEDT-TTF denotes a molecule given by bis(ethylenedithio)tetrathiafulvalene] under a uniaxial pressure using a two-dimensional tight-binding model with both impurity and electron–phonon (e–p) scatterings. We calculate an anomalous temperature ($T$) dependence of the Seebeck coefficient $S_{\nu }$ with $\nu =x$ (perpendicular to the molecular stacking axis) and $y$, which shows $S_{\nu }>0$ with a maximum at high temperatures and $S_{\nu }<0$ with a minimum at low temperatures.The microscopic mechanism of such a sign change of $S_{\nu }$ is clarified in terms of the spectral conductivity. The result is compared with experiments on $\alpha$-(BEDT-TTF)${}_2$I${}_3$.