Thermoelectric effects in correlated quantum dots and molecules

We investigate thermoelectric properties of correlated quantum dots and molecules, described by a single level Anderson model coupled to conduction electron leads, by using Wilson's numerical renormalization group method. In the Kondo regime, the thermopower, S(T), exhibits two sign changes, at temperatures T = T1 and T = T2 > T1. We find that T2 is of order the level width Γ and T1 > Tp ∼ TK, where Tp is the position of the Kondo induced peak in the thermopower and TK is the Kondo scale. No sign change is found outside the Kondo regime, or, for weak correlations, making a sign change in S(T) a particularly sensitive signature of strong correlations and Kondo physics. For molecules, we investigate the effect of screening by conduction electrons on the thermoelectric transport. We find that a large screening interaction enhances the figure of merit in the Kondo and mixed valence regimes.