High-pressure studies on pristine and Pb-substituted Bi-based high-temperature superconductor

Abstract

Transport measurements were performed on over-doped samples of pristine and Pb-substituted Bi–Sr–Ca–Cu–O as a function of applied pressure up to 7 GPa. A monotonic decrease in the superconducting transition temperature accompanied by a change in the normal-state behaviour from metallic to semiconducting as a function of pressure is observed in the pristine sample. In the Pb-substituted sample, a monotonic decrease in the superconducting transition temperature up to ~4.5 GPa followed by an increase after attaining a valley is observed. The pressure required to achieve this increase in T_C in the Pb-substituted Bi–Sr–Ca–Cu–O was found to be much lower than that reported in the literature. High-pressure X-ray diffraction measurements performed on these samples reveal the absence of any structural phase transition. Furthermore, the values of the bulk moduli are found to increase in the Pb-substituted sample. The density functional theory calculations used to substantiate the obtained results indicate a pressure-induced buckling of the Cu–O plane in the Bi-2212 phase of the pristine sample. In the Pb-substituted sample, an increase in the number of electronic states available for conduction at the Fermi level and a pressure-dependent increase in the contribution of Pb towards the density of states are revealed from the calculations of the density functional theory.