Seebeck Effect of Dirac Electrons in Organic Conductors under Hydrostatic Pressure Using a Tight-Binding Model Derived from First Principles

The Seebeck coefficient is examined for two-dimensional Dirac electrons in the three-quarter filled organic conductor α-(BEDT-TTF)₂I₃ [BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene] under hydrostatic pressure, where the Seebeck coefficient S is proportional to the ratio of the thermoelectric conductivity L₁₂ to the electrical conductivity L₁₁, i.e., S = L₁₂/TL₁₁ with T being the temperature. We present an improved tight-binding model in two dimensions with transfer energies determined from first-principles density functional theory calculations with an experimentally determined crystal structure. The T dependence of the Seebeck coefficient is calculated by adding impurity and electron–phonon scatterings. Noting a zero-gap state due to the Dirac cone, which results in a competition from contributions between the conduction and valence bands, we show positive S_x and S_y at finite temperatures and analyze them in terms of spectral conductivity. The relevance of the calculated S_x (perpendicular to the molecular stacking axis) to the experiment is discussed.