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 mono-layer Bi ₂ Sr ₂ CuO ₆ + _δ (Bi2201) and bilayer Bi ₂ Sr ₂ CaCu ₂ O ₈ + _δ (Bi2212) to beyond the maximum T _c s (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 _x 2 _−y 2 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 Br ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ + _δ (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.