Significant Enhancement of Electronic Thermal Conductivity of Two-Dimensional Zero-Gap Systems by Bipolar-Diffusion Effect

Electrical conductivity (sigma), thermopower (S), electronic thermal conductivity (kappa(e)), and Lorentz ratio (L = kappa(e)/sigma T) are examined for two-dimensional (2D) zero-gap systems. The bipolar-diffusion (BD) effect in 2D Dirac cones is discussed for the first time. Semiclassical calculations show that L is enhanced by a factor of 200-400% as compared with the conventional Sommerfeld value L-0 = 2.45 x 10(-8) W.Omega.K-2 due to the BD effect. It is, however, shown that this enhancement is not due to the linear dispersion of the Dirac cones but due to the zero band gap and is widely expected to occur in all zero-gap systems under its charge neutrality condition. Calculations taking into account electron-hole puddles revealed that the BD effect is suppressed in pristine graphene devices affected by charged-impurity potentials.