Systematic study of terahertz response of SrTiO3 based heterostructures: Influence of strain, temperature, and electric field

Epitaxial heterostructures consisting of a variable number of SrTiO3/DyScO3 bilayers deposited on DyScO3 substrates were investigated using time-domain terahertz spectroscopy down to helium temperatures. Interdigitated electrodes deposited on top of the structures allowed probing of the terahertz response upon an applied electric field. The phase transition into a ferroelectric state is observed in SrTiO3 films in all samples close to room temperature (between 250 and 310 K) due to in-plane epitaxial strain induced by the substrate and intercalated layers of DyScO3. Evolution of the dielectric spectra with temperature and external electric field is described by a general model which involves a damped harmonic oscillator (soft mode) coupled to a Debye relaxation (central mode). Both modes are connected with the soft mode eigenvector, as recently shown by molecular dynamics simulations, and they reflect a strong anharmonicity of the soft-mode potential. At high temperatures the soft-mode frequency variation drives all the changes observed in the spectra with temperature and applied field. At low temperatures, deep in the ferroelectric phase, the soft mode significantly hardens and loses its importance for the terahertz dynamics; the central mode becomes stronger and it almost completely determines the shape of the measured spectra. The observed variation of phase transition temperature and of the dielectric response among the structures is ascribed to a partial epitaxial strain relaxation confirmed also by x-ray diffraction.