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Exact diagonalization of a small strongly coupled electron-phonon system in HTSC

A three-level model is proposed for the IR and Raman response of coupled phonons and charge transfer fluctuations between the conducting planes and chain in YBa_2Cu_3O_7. The model is solved by exact diagonalization with a variational Hilbert space. For this simple model we find that 1000 quantum states are enough even for very strong electron-phonon coupling. The obtained results are compared with analytical ones, particularly with the random phase approximation valid only for small electron-phonon coupling constants. Raman and IR spectra are also discussed.

exact diagonalization, three-level model

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