Thermoelectric performance of strongly-correlated quantum impurity models

We derive asymptotically exact expressions for the thermopower and figure of merit of a quantum impurity connecting two noninteracting leads in the linear response regime where the chemical potential and temperature differences between the leads are small. Based on sum rules for the single-particle impurity spectral function, these expressions become exact at high temperatures as well as in the very strongly correlated regime, where the impurity Coulomb repulsion is much larger than the temperature. Although modest interactions impede thermoelectric performance, a very large Coulomb scale restores the optimal transport properties of noninteracting electrons, albeit renormalized to account for the absence of double occupancy in the impurity. As with noninteracting electrons, the electronic contribution to the figure of merit is limited only by the spectral broadening that arises from the coupling between the impurity and the leads.