Raman modes and strong intraplanar oxygen-oxygen charge fluctuations in YBa2Cu3O7

It is shown that symmetric and antisymmetric (340 cm-1) Raman modes are strongly coupled respectively to the intracell, intraplanar oxygen-copper and oxygen-oxygen charge fluctuations in YBa2Cu3O7. The dominant coupling arises from the variations of the crystal field with deformations. The anomalous behaviour of the 340 cm-1 mode is thus attributed to the corresponding behaviour of the in-plane oxygen-oxygen charge correlation function. The symmetric modes are continuous at Tc, and this contrast is examined within the existing theories. It appears that the Emery model is a suitable starting point for the explanation of the Raman mode behaviour. The increase of the ep coupling from one high Tc family to another turns out to correlate with increase of Tc. (author). 33 refs, 3 figs, 2 tabs