Effect of central mode on the dielectric tunability of ferroelectrics near room temperature: a first-principle-based study.

First-principles-based effective Hamiltonian molecular dynamics simulations are performed to investigate GHz-THz dynamical properties of bulk and epitaxially strained film made of SrTiO3 near room temperature. Our simulations confirm the huge dielectric tunability recently observed in films. Moreover, universal phenomenological laws, with bulk-like parameters, are found to describe reasonably well the dielectric tunability-versus-dc electric field curves in both systems at low and high electric fields, except for the sole case of the STO film in the low-field regime. Such deviation originates from the presence of a central mode in this low-dimensional system. A revised equation, arising from an original analysis of the simulations, is proposed for modeling this latter situation.