STRONG CORRELATIONS, FROZEN-PHONONS AND DOUBLE-WELL POTENTIAL IN THE CU-O CHAINS OF YBA2CU3O7-DELTA - AN EXACT DIAGONALIZATION APPROACH

We studied the infrared and Raman modes in the Cu - O chain of the sys tem YBa3O7-delta by means of a Peierls-Hubbard model. We performed an exact Lanczos-diagonalization on a [Cu2 - O6] cluster (2-cells in the chain including two apex -oxygen ions per unit cell). In contrast to p revious calculations, we found that the electronic correlations combin ed with dynamical phonons are not crucial for the existence of a doubl e-well potential in the system. We also discuss the importance of the coupling between the planes and the chains as well as the magnitude of the electron-phonon coupling.