A Raman view of local lattice distortions and charge transfer in cuprates

Small changes in doping or the application of a hydrostatic pressure can induce non-linear lattice distortions, lattice instabilities, and phase separation in the cuprates. In YBa2Cu3Ox, oxygen doping above optimal induces a separation into microphases originating from a modification in the buckling of the CuO2 planes. In the same compound, the excess nominal doping by Ca creates changes in the CuO2 buckling at some critical concentrations. These are correlated with the formation of domains where the additional carriers brought by Ca remain isolated surrounded by yttrium atoms. Furthermore, it was observed that hydrostatic pressure induces non-linear modifications in the spectral characteristics of the Ag-symmetry phonons in all cuprates studied (YBa2Cu3O6.5, YBa2Cu3Oover, YBa2Cu4O8, and Bi2Sr2CaCu2O8). A coexistence of phases appears at a critical pressure where changes in the superconducting transition temperature have been detected. We have also observed similar correlations between the transition temperature and spectral modifications by chemical doping in the Raman spectra of La2−xSrxCuO4. Finally, soft modes were found in the cuprates studied, with energies scaling with the maximum transition temperature of the compound.