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Comment on 'Polaron origin for anharmonicity of the axial oxygen in YBa_2Cu_3O_7' (and reply)

Mustre de Leon et al. (ibid. vol.68, p.3236, (1992)) used exact diagonalization methods to study the anharmonicity associated with the motion o f the apical oxygens in YBa_2Cu_3O_7. They argued that, for the range of interest of the electron-phonon coupling constant, the motion of holes and ions becomes strongly correlated, leading to features in the optical absorption spectrum which they ascribed entirely to nonadiabatic, polaronic effects. If, as concluded, nonadiabatic effects play a crucial role in the physics of the system, then the problem requires an exact treatment. Thus, only very small clusters would be tractable. Instead, we studied the same cluster by means of the standard adiabatic approximation. Unlike Mustre de Leon et al., we found that a frozen- phonon approximation, together with the exact treatment of the electronic degrees of freedom, leads to essentially the same results. Mustre et al. respond.

high temperature superconductor ; anharmonicity ; axial oxygen ; exact diagonalization methods ; electron-phonon coupling constant ; optical absorption spectrum ; polaronic effects ; standard adiabatic approximation ; YBa_2Cu_3O_7

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