Properties of the electronic fluid of superconducting cuprates from $^{63}$Cu NMR shift and relaxation

We use available $^{63}$Cu nuclear magnetic resonance (NMR) data of the high-temperature superconducting cuprates and show that one must resort to a coupled electronic spin scenario that leads to a suppression of the shifts, except for a few heavily overdoped systems where the coupling is weak. We uncover universal, Fermi liquid-like nuclear relaxation that is independent of material and doping in the normal state, and that is even in quantitative agreement with what one calculates from Korringa's law for the maximum shifts. Contrary to the common interpretation of NMR that invokes enhanced electronic spin fluctuations we argue that suppressed shifts explain the lacking Korringa behavior. Shift and relaxation in the condensed state support this view, as well. A simple model of two coupled electronic spin components, one with $3d(x^2-y^2)$ orbital symmetry and the other with an isotropic $s$-like interaction can explain the data. The negative coupling between the two spins must be related to the pseudogap behavior of the cuprates. Its interplay with the spin components determines the dependence of the shifts as function of material and temperature. We can also explain the negative shift conundrum and the long-standing orbital shift discrepancy for NMR in the cuprates. We hint at consequences for other experiments and theory.