High tunability of the soft mode in strained SrTiO3/DyScO3 multilayers

SrTiO3/DyScO3 epitaxial multilayers with variable number and thickness (10-100 nm) of bilayers deposited on DyScO3 substrates were investigated by means of time-domain terahertz spectroscopy at room temperature. A tensile strain develops in the SrTiO3 films and shifts the eigenfrequency of the ferroelectric soft mode down by ~25-45 cm-1 with respect to the value found for single crystals. In all films the soft mode strongly hardens upon the electrical bias and a linear coupling to a silent excitation of relaxation type at 10 cm-1 is observed. We show that the change in the THz and sub-THz response of the layers with an increasing field is determined solely by the soft mode eigenfrequency and we propose a phenomenological model describing the origin of the tunability and the peculiar properties of the ferroelectric soft mode in the terahertz spectral range.