How pressure enhances the critical temperature of superconductivity in YBa₂Cu₃O₆₊

High-temperature superconducting cuprates respond to doping with a dome-like dependence of their critical temperature (T_c). But the family-specific maximum T_c can be surpassed by application of pressure, a compelling observation known for decades. We investigate the phenomenon with high-pressure anvil cell NMR and measure the charge content at planar Cu and O, and with it the doping of the ubiquitous CuO₂ plane with atomic-scale resolution. We find that pressure increases the overall hole doping, as widely assumed, but when it enhances T_c above what can be achieved by doping, pressure leads to a hole redistribution favoring planar O. This is similar to the observation that the family-specific maximum T_c is higher for materials where the hole content at planar O is higher at the expense of that at planar Cu. The latter reflects dependence of the maximum T_c on the Cu-O bond covalence and the charge-transfer gap. The results presented here indicate that the pressure-induced enhancement of the maximum T_c points to the same mechanism.