Quantum-fluctuation effects on the thermopower of a single-electron transistor

We study thermal conductance and thermopower of a metallic single-electron transistor beyond the limit of weak tunnel coupling. Employing both a systematic second-order perturbation expansion and a nonperturbative approximation scheme, we find, in addition to sequential and cotunneling contributions, terms that are associated with the renormalization of system parameters due to quantum fluctuations. The latter can be identified by their logarithmic temperature dependence that is typical for many-channel Kondo correlations. In particular, the temperature dependence of thermopower, which provides a direct measure of the average energy of transported particles, reflects the logarithmic reduction of the Coulomb-blockade gap due to quantum fluctuations.