Exact diagonalization results were presented for an electron-phonon Hamiltonian treatment of the O(4)-Cu(1) cluster. It was found that, for sufficiently large electron-infrared phonon coupling, the motion of holes and ions became strongly correlated. This polaronic behavior then led to a double-well structure for the infra-red mode, as seen in the X-ray absorption fine structure. The associated Raman mode exhibited single-well behavior, even for large electron-Raman phonon coupling. The appearance of polarons accompanied non-adiabatic behavior and led to optical spectroscopic predictions which differed from those which were deduced from harmonic or anharmonic lattice dynamics models.

J.M.De Leon, I.Batistic, A.R.Bishop, S.D.Conradson, S.A.Trugman: Physical Review Letters, 1992, 68[21], 3236-9