How pressure enhances the critical temperature of superconductivity in YBa2Cu3O6+y

High-temperature superconducting cuprates respond to doping with a dome-like dependence of their critical temperature (Tc). But the family-specific maximum Tc 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 Tc 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 Tc 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 Tc on the Cu-O bond covalence and the charge-transfer gap. The results presented here indicate that the pressure-induced enhancement of the maximum Tc points to the same mechanism.

Automated summary

High-temperature superconducting cuprates exhibit a dome-like dependence of their critical temperature (Tc) with doping, but this maximum Tc can be enhanced by applying pressure, a well-known observation for decades. In this study, we used high-pressure anvil cell NMR to investigate this phenomenon and found that pressure increases overall hole doping and leads to a redistribution of holes favoring planar O, similar to the observation of higher maximum Tc for materials with higher hole content at planar O. These results indicate that the pressure-induced enhancement of the maximum Tc points to the same mechanism.