Single crystal NMR of high-temperature superconductors

The author reports Cu NMR studies of the high temperature superconductor YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$ (T$\sb{\rm c}$ = 90K) using a 1 mg single crystal.Three types of measurements have been made for both the "plane" and "chain" Cu crystallographic sites and with the static magnetic field placed successively along each of the three crystal axes a, b, and c: NMR lineshapes, spin-lattice relaxation, and transverse relaxation.From these measurements four NMR tensors are deduced: the magnetic shift tensor, the electric field gradient tensor, the spin-lattice relaxation rate tensor, and the transverse relaxation rate tensor.As a preliminary matter the author uses the symmetries of the magnetic shift and electric field gradient tensors to assign the two families of NMR lines to their respective crystallographic sites.From an analysis of the electric field gradient tensors it is shown that both the plane and chain Cu have valences Cu$\sp{2+}$, giving a single hole in the Cu atomic d-shell.The author then analyzes the spin-lattice relaxation rate and magnetic shift of the plane and chain Cu; included are both analyses in which only normal state data are considered, and in which superconducting state measurements of magnetic shifts are considered as well. These analyses show that the plane Cu atom behaves very nearly as a local moment. The chain Cu is probably also in the local moment limit; however, its NMR properties are largely determined by its interaction with oxygen holes in the chain.From transverse relaxation measurements a strong spin-spin coupling of plane Cu nuclei is observed; this coupling results from an indirect, electron-mediated mechanism involving exchange coupling between neighboring plane Cu local moments. The exchange coupling energy is found to be J$\sb{\rm eff}$ = 1700cm$\sp{-1}$.U of I OnlyETDs are only available to UIUC Users without author permissio