Optical spectroscopic investigation in Nb:SrTiO$_{3}$/SrTiO$_{3}$ oxide heterostructures

SrTiO$_{3}$ based heterostructures show various fascinating properties ranging from anomalous magnetism to field effect controlled superconductivity. In this talk, we used optical spectroscopy to study electron-phonon coupling in Nb:SrTiO$_{3}$/SrTiO$_{3}$ heterostructures, which governs the electrodynamics of the system. First, we will discuss on the heavily doped SrTiO$_{3}$ superlattices (SrTi$_{0.8}$Nb$_{0.2}$O$_{3}$/SrTiO$_{3})$, which show large enhancement of thermoelectric Seebeck coefficient.[1] As the conducting Nb:SrTiO$_{3}$ layer thickness decreases, the polaron effective mass and relaxation time increase due to the dimensional crossover of polaron. It is shown that such changes due to the electron-phonon coupling can benefit the thermoelectric properties. Second, we will discuss on electrodynamics of the \textit{$\delta $}-doped SrTiO$_{3}$ (SrTiO$_{3}$/SrTi$_{0.99}$Nb$_{0.01}$O$_{3}$/SrTiO$_{3})$ heterostructures.[2] As the temperature decreases, these heterostructures show a large enhancement of mobility, while charge carrier density remains almost constant. The role of strong electron-phonon coupling in the quasi 2-dimensional SrTiO$_{3}$ heterostructure will be also discussed. [1] H. Ohta, \textit{et al}., Nature materials \textbf{6}, 129 (2007) [2] Y.Kozuka, \textit{et al}., unpublished