Acoustic mode instabilities in ferroelectrics: Anharmonic mode-mode coupling induced by electron-phonon interactions

Abstract

Various reports on large anomalies in the ferroelectric acoustic mode at small wave vectors have been evidenced as an instability to a new coherent quantum state. Especially in SrTiO₃ this anomaly is found to occur at a characteristic temperature which has been detected by EPR and Brillouin scattering experiments. The present analysis shows that the observed anomaly can find an alternative explanation, i.e. it can originate from electron-phonon-driven anharmonic mode-mode coupling, which leads to giant q-dependent renormalization in the effective electron-phonon interaction. These renormalizations do not only occur in SrTiO₃, but also in other oxide perovskite ferroelectrics, e.g., in PbTiO₃, and stem from highly anharmonic oxygen ion displacements, which are present even for temperatures far away from the ferroelectric transition temperature.

Key Words:

• Nonlinear lattice dynamics