Theory of thermopower in 2D graphene

Motivated by recent experiments [1-3] we calculate the thermopower of graphene incorporating the energy dependence of various transport scattering times. We find that scattering by screened charged impurities gives a reasonable explanation for the measured thermopower. The calculated thermopower behaves as 1/\sqrt{n} at high densities, but saturates at low densities. We also find that the thermopower scales with normalized temperature T/T_F and does not depend on the impurity densities, but strongly depends on the fine structure constant r_s and the location of the impurities. We discuss the deviation from the Mott formula in graphene thermopower, and use an effective medium theory to calculate thermopower at low carrier density regimes where electron-hole puddles dominate.