Structural and phonon anomalies in the superconducting Bi₂Sr₂CaCu₂O_8+δ with varying doping level at high pressures

Abstract We examined the effect of the lattice compression on the crystal structure of the Bi2Sr2CaCu2O8+δsuperconductor with nearly optimally doped and overdoped composition using x-ray diffraction technique. Our studies show that at high pressures (up to 30 GPa) the doping level does not affect the crystal structure of this superconductor. Along with this, structural anomalies in c/a ratio appear at a pressure that corresponds to that at which Tc begins to decrease. This fact indicates a close connection between the observed anomalies and superconductivity. We also studied the effect of doping and pressure on the Tc(P) and on the Raman active lattice modes of the Bi2Sr2CaCu2O8+δ superconductors. We find universal suppression of the Tc by the pressure starting from the critical pressure Pc in the range from 9 to 16 GPa, depending on the doping level of the Bi2Sr2CaCu2O8+δsamples. Concomitantly, we observe phonon anomalies around 10 to 20 GPa, which indicate possible pressure-induced charge redistribution in BiO layers. These newly detected anomalies may be related to the changes in the electronic structure which compete with the superconductivity.