Thermoelectric properties of a double quantum dot out of equilibrium in Kondo and intermediate valence regimes

We study a system composed of two quantum dots connected in series between two leads at different temperatures, in the limit of large intratomic repulsion. Using the noncrossing approximation, we calculate the spectral densities at both dots ${\ensuremath{\rho}}_i(\ensuremath{\omega})$, the thermal and thermoelectric responses, thermopower, and figure of merit in different regimes. The interatomic repulsion leads to finite heat transport even if the hopping between the dots $t=0$. The thermopower can be very large compared to single-dot systems in several regimes. The changes in sign of the thermoelectric current can be understood from the position and magnitude of the Kondo and charge-transfer peaks in ${\ensuremath{\rho}}_i(\ensuremath{\omega})$. The figure of merit can reach values near 0.7. The violation of the Wiedemann-Franz law is much more significant than in previously studied nanoscopic systems. An analysis of the widths of ${\ensuremath{\rho}}_i(\ensuremath{\omega})$ indicates that the dots are at effective temperatures ${T}_i$ intermediate between those of the two leads, which tend to be the same for large $t$.