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

TL;DR

This study explores how the Seebeck coefficient's sign changes across the metal-insulator transition in the doped Fermi Hubbard model, revealing that charge gap formation, not pseudogap, controls this behavior, with strong U and temperature effects.

Contribution

It demonstrates that the sign change in the thermoelectric coefficient is governed by charge gap opening, not pseudogap, and highlights the role of local moment formation in this phenomenon.

Findings

Sign change controlled by charge gap, not pseudogap.

Significant U and temperature dependence of the sign change.

Local moment formation drives anomalous thermoelectric behavior.

Abstract

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 $n_s$ (and correspondingly a hole density $p_s$) 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 $n_s(T,U)$ depends strongly on the interaction $U$ and shows an unusual non-monotonic dependence on temperature with a maximum at a temperature $T\approx t$, 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.