Sign changes of the thermoelectric transport coefficient across the metal-insulator crossover in the doped Fermi Hubbard model

We investigate the doping-dependence of the Seebeck coefficient, as calculated from the Kelvin formula, for the Fermi Hubbard model using determinantal quantum Monte Carlo simulations. Our key findings are: (1) Besides the expected hole to electron-like behavior change around half filling, we show that the additional sign change at an electronic density nₛ (and correspondingly a hole density pₛ) is controlled by the opening of a charge gap in the thermodynamic density of states or compressibility and not by the pseudogap scale in the single particle density of states. (2) We find that nₛ(T,U) depends strongly on the interaction U and shows an unusual non-monotonic dependence on temperature with a maximum at a temperature T≈ t, on the order of the hopping scale. (3) We identify local moment formation close to half filling as the main driver for the anomalous behavior of the thermoelectric transport coefficient.