The Wiedemann-Franz law in doped Mott insulators without quasiparticles

Many metallic quantum materials display anomalous transport phenomena violating Fermi liquid and Boltzmann theory. We discuss transport in the doped Hubbard model where we observe a cross-over separating high- and low-temperature behavior in both thermal and charge transport. Distinct from the incoherent behavior at high temperatures, as T→0, the Lorenz number obeys the Wiedemann-Franz law, even in a doped Mott insulator that lacks well-defined quasiparticles. By analyzing contributions to thermal diffusivity, we find that the potential energy strongly affects thermal transport at high temperatures, while at low temperatures, the behavior of both thermal and charge transport is controlled primarily by the electronic kinetic energy. Our result helps to clarify the interpretation of transport experiments beyond Boltzmann theory in non-Fermi liquid strange metals.