Temperature and electric field tuning of the ferroelectric soft mode in a strainedSrTiO3/DyScO3heterostructure

Abstract

We studied THz dielectric spectra of ${\text{SrTiO}}_{3}/{\text{DyScO}}_{3}$ epitaxial multilayers with four 50-nm-thick layers of each compound deposited on ${\text{DyScO}}_{3}$ substrate as a function of temperature and electric field. At room temperature, ${\text{SrTiO}}_{3}$ is in the paraelectric phase and, due to the in-plane tensile strain within the films, the soft-mode frequency is shifted down to $47\text{ }{\text{cm}}^{\ensuremath{-}1}$. The ferroelectric phase transition occurs in the strained ${\text{SrTiO}}_{3}$ layers around 270 K and in the ferroelectric phase the soft-mode hardening upon temperature decrease is observed. The soft mode is linearly coupled to an excitation of relaxation type at $10\text{ }{\text{cm}}^{\ensuremath{-}1}$ at all temperatures. This relaxation is silent in the paraelectric phase, but it becomes coupled to the polarization below the ferroelectric transition temperature and its strength progressively increases. The recently proposed universal model of soft-mode behavior in strained ${\text{SrTiO}}_{3}$ films [C. Kadlec et al., J. Phys.: Condens. Matter 21, 115902 (2009)] was generalized to describe the observed spectra at all temperatures and applied fields.