Terahertz ferroelectric soft mode in weakly doped SrTiO3: M thin films (M=Mn, Ni, Fe, Co)

Systematic study of the terahertz soft-mode in SrTiO3: M thin (150 nm) films doped (2 at.%) with transition metals M = Mn, Fe, Ni, Co is performed at frequencies 7–1000 cm−1 and temperatures 5–300 K. The soft mode dielectric strength and frequency follow the Barrett-like temperature behaviours with unusually high quantum temperatures and large in magnitude negative Curie temperatures, indicating strong destabilization of the ferroelectric state due to a combined effect of quantum fluctuations, dead layers at the boundaries of crystallites, misfit strains within crystallites and disorder induced by doping. Dopant-dependent variation in the asymmetry of the Fano resonance describing coupling of the TO2 phonon and continuum states of polarization fluctuations is observed suggesting presence of polar nanoregions in the films. Clear correlation between the asymmetry parameter and the Barrett quantum temperature is observed that highlights the complex character of the local structure-dielectric properties relationships in the films.