Optical phonon softening in strained SrTiO3 thin film: First-principles study

Abstract

The dielectric response and optical phonon of hydrostatically and anisotropically strained SrTiO3 lattices were studied using density functional theory. A structural analysis was performed on the strained SrTiO3 lattice. Following the structural analysis, the optical phonon frequency was calculated for a wide range of lattice distortion. The dielectric constant was calculated from the optical phonon frequency and the Börn effective charges. It was found that significant optical phonon softening occurs at a certain degree of lattice distortion and a certain amount of volume change. In other words, the optical phonon frequency decreased and reached a minimum (50cm−1) at a specific lattice distortion (c∕a=0.985) as the SrTiO3 lattice was progressively distorted starting from the unstrained state. The phonon frequency increased as the lattice distortion was further increased. This phonon behavior, i.e., optical phonon softening, leads to a maximum dielectric constant at a certain degree of lattice distortion. The first-principles calculation on the strained SrTiO3 shows that the strain affects the dielectric constant of the strained SrTiO3 through the optical phonon softening (or hardening).