Dimensional crossover of polaron dynamics in Nb:SrTiO3/SrTiO3 superlattices: Possible mechanism of thermopower enhancement

Using optical spectroscopy, we investigated the electrodynamic properties of Nb:SrTiO3/SrTiO3 superlattices. In these superlattices, a large enhancement of the Seebeck coefficient (S) has been reported with decreasing Nb:SrTiO3 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 Nb: SrTiO3 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.