Vibronic pairing mechanism for La,2-xSrxCuO4. The role of local structural distortions

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Abstract
Our vibronic pairing model for high-T c superconductivity is now revisited and applied to interpreting experimental optical energies of La,2-xSrxCuO4. An electronic optical band at 1·3 eV is presumed to arise from transitions at a local double-well oscillator associated with an itinerant off-centre polaron. Jahn-Teller and hole bandgap energies are thereby extracted from the experimental data. Another optical band at 0·44 eV is ascribed to bipolarons. The experimental T c is obtained by assuming a small interlayer coupling and using calculated two-dimensional translational bipolaron masses. In as much as off-centredness violates the inversion symmetry of CuO6 octahedra, the model naturally explains the observed dual occurrence of Raman and infrared activities in doped or photoexcited La2CuO4. A dipolar mode-enhancing mechanism is proposed to explain just how the local symmetry is broken by the electron-phonon interaction. Anomalously large amplitudes of vertex-oxygen vibrations along the c axis are predicted, such as those observed in YBa2Cu3O7, as a dynamic effect arising from the configurational tunnelling between off-centre sites.