Universal thermopower of bad metals

“Bad metals” are known to have a large linear resistivity at highT that is universally seen near the Mott-Hubbard insulating phase. Less well known is that the thermopower α(T ) of the Mott-Hubbard systems also exhibits simple universal features: (i) close to the insulating phase, where the resistivity has a low-temperature upturn, α(T ) has a pronounced low-T peak that shifts to higher T with doping; (ii) when the resistivity is nearly linear, which occurs at moderate doping, α(T ) has a small low-T peak that shifts to lower T with doping and has a high-T sign change; and (iii) at the highest doping, where the resistivity acquires a T 2 term, α(T ) is negative and depends monotonically on T . The universality α(T ) can be understood using the Kelvin formula and the fact that the chemical potential for doped Mott insulators displays similar behavior at high T . The universality is illustrated with the exact solution of the simplest model for a doped Mott insulator at high T .