NMR investigations of high-Tc superconductors

This article reviews insights on high-Tc superconductors from the nuclear magnetic resonance (NMR) technique, with emphasis on local hole density, d-wave superconducting gap and its consequences, and the nature of the pseudogap. In addition to the total hole density in the CuO2 plane, the partition of the holes between Cu and O orbitals is an important parameter in determining Tc. The temperature dependence of the spin-lattice relaxation rate and the Knight shift indicate d-wave pairing, which brings about the extended quasiparticle states outside the vortex cores. The existence of nodes in the gap function also results in strong Tc reduction by non-magnetic impurity or crystal disorder. The zero-temperature normal state has been revealed by the application of strong magnetic fields up to 45T to be a “Fermi arc” metallic state, which suggests that the pseudogap and the superconducting states are coexisting states of matter.