Hidden Universal Metal in Cuprate Superconductors

Abstract

Nuclear relaxation is a very robust probe of electronic excitations in superconducting materials, above and below the critical temperature of superconductivity, $T_\mathrm{c}$. Here, a phenomenology of it in cuprate superconductors is established based on essentially all cuprate data available in the literature from the CuO$_2$ plane. A universal 'hidden metal' with $1/T_1 T = const$ reigns below the pseudogap line, and, similar to usual superconducting metals, all cuprates condense at $T_\mathrm{c}$ out of this metal and relaxation ceases rapidly. There is no Hebel-Slichter peak. Above the hidden metal, a renormalized two-component metal is found. Therefore, the hidden metal is identified as the pseudogap matter, which has other important properties. It predominantly lacks a uniform response, unlike the normal metal above $T^*$, and it does not significantly relax planar O. However, for planar Cu it exhibits a special relaxation anisotropy. With the field in the plane, it causes a relaxation rate that is very similar for all cuprates, $1/{^{63}T}_{1\perp}T\approx 25/$Ks, while with the field parallel to the $c$-axis, $1/{^{63}T}_{1\parallel}T$, the proportional rate (in the pseudogap range) changes as a function of doping and material. This relaxation anisotropy is strictly correlated with the maximum critical temperature, $T_\mathrm{c,max}$, of all cuprates. It is stipulated that two partially independent spin components are needed to understand this behavior. The new phenomenology will be discussed and should give a better foundation for the understanding of the cuprates