Spin-orbit coupling effect on the Seebeck coefficient in Dirac electron systems in α − ( BETS ) 2 I 3

The Seebeck coefficient, S=L₁₂/(TL₁₁), which is proportional to a ratio of the thermoelectric conductivity L₁₂ to the electric conductivity L₁₁ with T being temperature is examined for two-dimensional Dirac electrons in the three-quarter filled organic conductor, $α$-(BETS)₂I₃, [BETS = BEDT-TSeF = bis(ethylenedithio)tetraselenafulvalene] at ambient pressure.Using a tight-binding model obtained with the first-principles relativistic density-functional theory method [Tsumuraya and Suzumura, Eur. Phys. J. B 94, 17 (2021)], we calculate S in the presence of the impurity and electron--phonon scatterings. We show that Sₓ < 0 and Sy >0 at high temperatures, where Sₓ (Sy) denotes S perpendicular (parallel) to the molecular stacking axis. There is a sign change of Sy with increasing T. We find that, at low temperatures the absolute value of S is enhanced by the spin-orbit coupling. The Seebeck coefficient is examined by dividing it into components of the conduction and valence bands; we find that the electron and hole contributions compete with each other. Such T dependence of S is clarified using the spectral conductivity, which determines L₁₂ and L₁₁