Universal thermopower of bad metals

"Bad metals" are known to have a large linear resistivity at high T that is universally seen near the Mott-Hubbard insulating phase. Less well known is that the thermopower alpha (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, alpha (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, alpha (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 super(2) term, alpha (T) is negative and depends monotonically on T. The universality alpha (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.