Theory of thermopower in two-dimensional graphene

Motivated by recent experiments by Yuri M. Zuev et al. Phys. Rev. Lett. 102, 096807 (2009), Peng Wei et al. Phys. Rev. Lett. 102, 166808 (2009), and Joseph G. Checkelsky et al. Phys. Rev. B 80, 081413 (R) (2009), 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/n at high densities, but saturates at low densities. We also find that the thermopower scales with the normalized temperature T/T₅ and does not depend on the impurity densities, but strongly depends on the fine-structure constant rₒ and on 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.