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Polaron tunnelling as a mechanism for the double-well anharmonicity of the axial oxygen in YBa_2Cu_3O_7

The authors present exact diagonalization results of an electron-phonon model Hamiltonian for the O(4)-Cu(1)-O(4) cluster in YBa/sub 2/Cu/sub 3/O/sub 7/. For large enough electron-infrared phonon coupling the motion of holes and ions becomes strongly correlated, i.e., polaronic, leading to a double-well structure for the infrared mode, as encountered in X-ray-absorption fine structure. In contrast, the associated Raman mode shows a single-well behavior even for large electron-Raman phonon coupling. The appearance of polarons accompanies nonadiabatic behavior yielding optical spectroscopy predictions differing from those obtained from anharmonic lattice dynamics models.

high temperature superconductivity; polaron tunnelling; infrared mode; double-well anharmonicity; exact diagonalizatio

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