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

Abstract

Since the discovery of the high T_c superconducting oxides, the electronic structure of these compounds in the normal state (T > T_c) 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-_xSr_xCuO₄ as well as Cu NMR spectroscopy ^4-6 and neutron inelastic scattering^7,8 in YBa₂Cu₃O_6+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 T_c 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 T_c materials ^12-15.¹⁷O NMR in these oxides is one of the most powerful tool to address this problem^16,23. 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* > T_c should behave differently from χCu(q,ω).