High-pressure Raman study of Bi₂Sr₂CaCu₂O_8+d: indications of strong bond-strength hierarchy and pressure-induced charge transfer

Phonon Raman scattering from slightly overdoped Bi₂Sr₂CaCu₂O_8+d (Bi2212) samples have been investigated under hydrostatic pressure up to 7.2 GPa. The mode-specific Gruneisen parameters scale inversely with the corresponding phonon frequencies, the behavior of which is reminiscent of phonon pressure dependences in molecular solids. We also find that the 140 cm^-1 Cu mode exhibits a pronounced nonlinear pressure dependence which correlates to the dependence of T_c versus pressure. The data are interpreted in terms of a non-uniform structural compressibility that results in a redistribution of charge carriers within the Bi2212 unit-cell.