Seebeck effect of Dirac electrons

We study the Seebeck effect in the three-dimensional Dirac electron system based on the linear response theory with Luttinger's gravitational potential. The Seebeck coefficient $S$ is defined by $S = L_{12} / L_{11} T$, where $T$ is the temperature, and $L_{11}$ and $L_{12}$ are the longitudinal response coefficients of the charge current to the electric field and to the temperature gradient, respectively; $L_{11}$ is the electric conductivity and $L_{12}$ is the thermo-electric conductivity. It is confirmed that $L_{11}$ and $L_{12}$ are related through Mott's formula in low temperatures. The dependences of the Seebeck coefficient on the chemical potential $\mu$ and the temperature $T$ when the chemical potential lies in the band gap ($|\mu| 0$ as in semiconductors. The Seebeck coefficient takes the relatively large value $|S| \simeq 1.7 \,\mathrm{m V/K}$ at $T \simeq 8.7\,\mathrm{K}$ for $\Delta = 15 \,\mathrm{m eV}$ by assuming doped bismuth.