Pressure-induced effects in high-T_c superconductors: Raman scattering as a probe of charge-lattice dynamics under high pressure

The Raman spectra of Bi₂Sr₂CaCu₂O_8+d (Bi2212) and YBa₂Cu₄O₈ (Y124) have been investigated as a function of hydrostatic pressure. In both the overdoped Bi2212 and the underdoped Y124, the 140 cm^-1 Cu mode exhibits a pronounced nonlinear pressure dependence which correlates to the dependence of T_c versus pressure. We argue that this behavior is a signature of charge-redistribution effects induced by compression. For Bi2212, we also find that the mode-specific Grüneisen parameters scale inversely with the corresponding phonon frequencies, a behavior which is reminiscent of phonon pressure dependences in molecular solids.