Using optical spectroscopy, we investigated the electrodynamic properties of $\text{Nb}:{\text{SrTiO}}_3/{\text{SrTiO}}_3$ superlattices. In these superlattices, a large enhancement of the Seebeck coefficient $\left(S\right)$ has been reported with decreasing $\text{Nb}:{\text{SrTiO}}_3$ layer thickness [H. Ohta et al., Nature Mater. 6, 129 (2007)]. By analyzing the optical spectra, we found that the polaron plays an important role in determining the electrodynamic properties of the superlattices. With decreasing $\text{Nb}:{\text{SrTiO}}_3$ layer thickness from 11 to 1 unit cell, we observed a threefold enhancement of the polaron effective mass and relaxation time. Such increases were attributed to a dimensional crossover of the polaron from three-dimensional to quasi-two-dimensional. Moreover, the modified nature of the polaron at low dimensions enhanced the thermoelectric properties of the oxide superlattice, by simultaneously increasing $S$ and preventing the decrease in carrier mobility. Our results indicate that strong electron-phonon coupling can provide an alternative pathway in searching for efficient thermoelectric materials.

• Received 12 April 2010

DOI:https://doi.org/10.1103/PhysRevB.82.024301