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Polarons and axial oxygen anharmonicity in YBCO

We present exact diagonalization results of an electron-phonon model Hamiltonian for the O(4)-Cu(1)-O(4) cluster in YBCO. For large enough electron-infrared phonon coupling the motion of holes aud 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-Ramian phonon coupling, in agreentent with band structure calculations. The appearance of polarons accompanies nonadiabatic behavior yielding optical spectroscopy predictions differing from those obtained from anharmonic lattice dymamics models.

polarons, anharmonicity

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