Breaking the universal T_c-P relation to higher T_c of cuprate HTS

By investigating the bulk superconducting state via dc magnetization measurements, we have discovered the resurgence of the superconductive transition temperature T_cs of the monolayer Bi₂Sr₂CuO_6+δ and bilayer Bi₂Sr₂CaCu₂O_8+δ to beyond the maximum T_c-maxs predicted by the universal T_c-P relation at higher pressures. We have attributed the resurgence to a possible pressure-induced electronic transition in the compounds, associated with a charge transfer between the Cu 3d_x²-y² and the O 2p bands, leading to an increase of the density of states at the Fermi level, in agreement with our density functional theory calculations. Similar T_c-P behavior has also been reported in the trilayer Br₂Sr₂Ca₂Cu₃O_10+δ. The observations suggest that higher T_cs in the layered cuprate high temperature superconductors than those previously reported can be achieved by breaking away from the universal T_c-P by the application of higher pressures.