17O and 63Cu NMR Investigation of Spin Fluctuations in High Tc Superconducting Oxides

Abstract

Since the discovery of the high Tc superconducting oxides, the electronic structure of these compounds in the normal state (T > Tc) is still a matter of controversy. It was reported early on by Emery 1 that the holes introduced in these materials by doping were primarly going into the O(2p) orbitals, leaving exactly one d-hole at the Cu site (Cu2+) in the CuO2 plane. Neutron inelastic scattering 2 and La NQR 3 in La2-xSrxCuO4 as well as Cu NMR spectroscopy 4-6 and neutron inelastic scattering7,8 in YBa2Cu3O6+x, support this scheme. In YBa2Cu3O7 at least, however the temperature dependence of the Nuclear Spin Lattice Relaxation Rate (NSLRR)9 and the Magnetic Hyperfine Shift (MHS)10,11 of the Cu nuclei below Tc imply that these quasi- localized d-holes must be involved in the quasi-particles associated with superconductivity. Thus the nature of the coupling between these copper d-spins and the oxygen p-holes remains a central issue for the theoretical understanding of these high Tc materials 12-15.17O NMR in these oxides is one of the most powerful tool to address this problem16,23. In the framework of the so- called t-J model 12,14coupiing is so strong that a knowledge of the Cu(2) spin dynamical susceptibility χCu(q,ω) should allow one to predict all NMR observables at O(2,3) and Y sites in the crystal. In contrast, two spin fluids models imply 15 or strongly suggest 13 the existence of a second spin susceptibility associated with the oxygen p-holes χh(q,ω) which at least above some temperature T* > Tc should behave differently from χCu(q,ω).