Electronically Induced Anomaly in LO Phonon Dispersion of High - Tc Superconductors

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The strong, electronically induced anomaly in the spectrum of the longitudinal optical (LO) phonons propagating along the main axes of the CuO₂ plane is tentatively attributed to the oxygen-oxygen charge transfer between the two oxygens in the plane. It is argued that this charge transfer can be large and that it is strongly coupled to the zone boundary LO phonons. The corresponding negative contribution to the free energy is quartic in the LO phonon amplitude, making the LO phonon unstable through the first order phase transition, with a concomitant domain structure.