High-pressure Raman study of Bi2Sr2CaCu2O8+d: indications of strong bond-strength hierarchy and pressure-induced charge transfer

Phonon Raman scattering from slightly overdoped Bi2Sr2CaCu2O8+d (Bi2212) sa mples have been investigated under hydrostatic pressure up to 7.2 GPa. The mode-specific Gruneisen parameters scale inversely with the corresponding p honon frequencies, the behavior of which is reminiscent of phonon pressure dependences in molecular solids. We also find that the 140 cm(-1) Cu mode e xhibits a pronounced nonlinear pressure dependence which correlates to the dependence of T-c versus pressure. The data are interpreted in terms of a n on-uniform structural compressibility that results in a redistribution of c harge carriers within the Bi2212 unit-cell.