Higher superconducting transition temperature by breaking the universal pressure relation

By investigating the bulk superconducting state via dc magnetization measurements, we have discovered a common resurgence of the superconducting transition temperatures (T(c)s) of the monolayer Bi2Sr2CuO6+delta (Bi2201) and bilayer Bi2Sr2CaCu2O8+delta (Bi2212) to beyond the maximum Tcs (T(c-max)s) predicted by the universal relation between T-c and doping (p) or pressure (P) at higher pressures. The T-c of underdoped Bi2201 initially increases from 9.6 K at ambient to a peak at 23 K at 26 GPa and then drops as expected from the universal T-c-P relation. However, at pressures above 40 GPa, T-c rises rapidly without any sign of saturation up to 30 K at 51 GPa. Similarly, the T-c for the slightly overdoped Bi2212 increases after passing a broad valley between 20 and 36 GPa and reaches 90 K without any sign of saturation at 56 GPa. We have, therefore, attributed this T-c resurgence to a possible pressure-induced electronic transition in the cuprate compounds due to a charge transfer between the Cu 3d(x2-y2) and the O 2p bands projected from a hybrid bonding state, 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 Br2Sr2Ca2Cu3O10+delta (Bi2223). These observations suggest that higher T(c)s than those previously reported for the layered cuprate high-temperature superconductors can be achieved by breaking away from the universal T-c-P relation through the application of higher pressures.