Hidden Universal Metal in Cuprate Superconductors

Abstract

Nuclear relaxation is a powerful probe of electronic excitations in superconducting materials. Their emergence from a condensed state near the critical temperature, $T_\mathrm{c}$, is of particular interest. In cuprate superconductors, the behavior is not yet understood. Here, based on planar Cu and O relaxation data available in the literature, a universal metal is uncovered that reigns in the pseudogap phase, characterised by an average $1/{^{63}T}_{1\perp} T \approx 25$/Ks, i.e.\@ cuprates condense at $T_\mathrm{c}$ out of this universal metallic density of states. The metal exists up to $T^*$, above which Cu relaxation lags behind the universal metal rate. It is the Cu relaxation anisotropy, temperature independent but doping and family dependent, set by this metal that correlates with the maximum critical temperature, $T_\mathrm{c,max}$, of the cuprates. It appears to be formed from two metal components, A and B. A is known from planar O shift and relaxation and loses low-temperature states in the pseudogap. B is doping dependent and isotropically coupled to planar Cu only. Comparison with recent NMR shift analyses of the cuprates suggest that the hidden metal describes the pseudogap matter that has, in addition, a significantly lower uniform response compared to the normal cuprate metal, presumably due to antiferromagnetic coupling. The new phenomenology will be discussed and should give a better foundation for the understanding of the cuprates.