Temperature and electric field tuning of the ferroelectric soft mode in a strained SrTiO3/DyScO3 heterostructure

We studied THz dielectric spectra of SrTiO3/DyScO3 epitaxial multilayers with four 50-nm-thick layers of each compound deposited on DyScO3 substrate as a function of temperature and electric field. At room temperature, SrTiO3 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 cm(-1). The ferroelectric phase transition occurs in the strained SrTiO3 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 cm(-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 SrTiO3 films [C. Kadlec , J. Phys.: Condens. Matter 21, 115902 (2009)] was generalized to describe the observed spectra at all temperatures and applied fields.