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.
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