Nuclear Magnetic Resonance Study of High Temperature Superconductivity.

Three compounds of high temperature superconductor have been studied with nuclear magnetic resonance (NMR) technique on ^{203,205}Tl and ^{63}Cu nuclei. The experiments include measurements of NMR spectrum, spin -lattice relaxation, and spin-spin lattice relaxation of the three nuclei. The Tl NMR spectra of Tl_2Ba _2Ca_2Cu _3O_{10+delta} in the normal state provide a characterization of the c-axis alignment of the powder sample. The spectra in the superconducting state were found to be significantly broadened by the magnetic field inhomogeneity originating from the flux lattice. An angular study of the spectrum indicated a strong anisotropy in the flux lattice and the angular dependence of the saddle point field was found to be consistent with the anisotropic London theory. With such a measurement, we developed a new method to measure the penetration depth of superconductor. Our measurements of the temperature dependence of the penetration depth suggest a non-s-wave pairing state as the ground state for superconductivity in this compound. Our NMR spectra of Cu in La_{ rm 2-x}Sr_{x} CuO_4 have indicated two electronically distinct sites. We obtained the symmetry of the electric-field-gradient tensors for the two sites to be uniaxial with the principal axis along the crystal c-axis. Our measurements of the transverse shift of copper suggest a partial oxidation model for the second copper site. Our copper NMR on YBa_2Cu _3O_7 was focused on the transverse spin-spin relaxation process of Cu(2). We obtained the indirect coupling between copper nuclear spins and found a substantial reduction in this indirect coupling constant in the superconducting state. This indicates that the real part of the spin susceptibility is reduced or its structure in the momentum space is greatly changed below T_{c}. Within the phenomenological model of Millis, Monien, and Pines, our results indicate a significant reduction of this susceptibility at the antiferromagnetic wave vector.