Hidden Universal Metal in Cuprate Superconductors

Nuclear relaxation, 1/T1, is a very robust probe of electronic excitations in superconducting materials above and below the critical temperature of superconductivity, Tc. Here, a relaxation phenomenology of hole-doped cuprate superconductors is presented based on the majority of the available literature data from the CuO2 plane, without assumptions with respect to a hyperfine scenario, form factors, or particular theoretical models. Below a temperature similar to the pseudogap temperature, Heitler–Teller-type relaxation is uncovered universally; i.e., the nuclear spin relaxation above Tc is only determined by the absolute temperature, 1/T1∝T. All materials condense out of this metal at Tc, below which relaxation drops even faster, as expected from conventional superconductors, albeit without a Hebel–Slichter peak. It is a ’hidden metal’ in the sense that it has a vanishing uniform response and thus hardly affects the NMR shifts; it is also not seen in planar O relaxation. The hidden metal causes a temperature-independent but material-dependent planar Cu relaxation anisotropy that is strongly correlated with the size of Tc. Moreover, the rate measured with the field in the CuO2 plane is nearly the same for all cuprates: 1/T631⊥T≈25/Ks, where 1/T631‖ is mainly responsible for the change in anisotropy. Above the hidden metal, the relaxation behavior changes and can be described by an ordinary but renormalized metal, with a reduced Cu relaxation anisotropy. The relaxation phenomenology, which should hold clues to the so-called strange metal, is also discussed in the context of the two spin components previously uncovered in the shifts, as well as the pseudogap and relation to other probes. This new phenomenology should give a better foundation for the understanding of the cuprates.