Phonon anomalies in ferroelectrics and superconductors

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Abstract
Many ferroelectrics and superconductors exhibit unusual features in the low-frequency phonon spectrum. In superconductors the microscopic origin of phonon anomalies (“generalized Kohn anomalies”) is known to be an electron-one-phonon potential, while in displacive ferroelectrics different mechanisms for soft modes are presently discussed. In this paper we point out that similar physical mechanisms drive both ferroelectricity and superconductivity in many compounds. An analysis of the electronic band structure in both cubic ferroelectrics and superconductors exhibits high density-of-states near the Fermi level due to nonbonding nonmetal orbitals. Their phonon-induced hybridization with the metal ion states (“dynamical covalency”) is responsible for the strong electron-phonon interactions in both phenomena. This mechanism relates the phonon anomalies for both cases to cluster deformabilities, but with different symmetries. A nonlinear polarizability mechanism is discussed which quantitatively accounts for the temperature dependence of the soft modes in many ferroelectrics.