Revised NMR Shift Phenomenology for Cuprate Superconductors

We have compiled an unbiased overview of the planar Cu NMR shifts of the superconducting cuprates based on all available NMR literature data from the last three decades. Resulting plots reveal rather astonishing features. First of all, they show that the hitherto employed models of the cuprate hyperfine scenario must be wrong. Without invoking a particular model, we point to a number of salient features and trends in the data. For example, a large isotropic shift component must be present in the strongly overdoped cuprates and disappears at lower doping levels; while the low-temperature shift vanishes for all materials if the magnetic field lies in the plane, the hallmark of spin singlet pairing, a substantial shift remains at the lowest temperatures if the magnetic field is perpendicular to the planes; revised orbital shifts become apparent, which seem to solve the old rift between experimentally deduced orbital shifts and first-principle calculations. We also discuss the data in the context of recent insights from NMR, like the material-specific charge distribution that controls the maximum T_c and superfluid density, or the material-specific nuclear relaxation rate 1/T₁.