Covalency in (formula presented) A study of (formula presented) hyperfine couplings in the paramagnetic phase

(formula presented) nuclear magnetic resonance spectra from single crystals of (formula presented) are reported for temperatures ranging from 285 to 800 K. Hyperfine tensor data for the planar sites are analyzed using a spin Hamiltonian model that includes spin-orbit coupling effects. The results show a 7.7% hybridization effect of the oxygen (formula presented) orbital from a single copper neighbor, in good agreement with recent density-functional (DF) calculations by Hüsser et al. (HSSM). A large, positive isotropic shift component is also reported, presumably originating from the contact interaction with a hybridized (formula presented) orbital component. First-order quadrupolar-splitting data lead to complete characterization of the electric-field gradient (EFG) tensor, which varies only slightly with temperature up to 800 K. EFG tensors for both doped and undoped (formula presented) are fitted with a two-component model, which incorporates a substantial anisotropy in (formula presented) for the (formula presented) wave functions, an effect that originated in the DF calculations of HSSM. This analysis reveals an increased charge density on the planar oxygens for the superconducting phase, in accord with the original Zhang-Rice model. However, the increase is found to correspond to only (formula presented) of the nominal doped-hole density, corroborating a similar conclusion reached recently by Hammel et al. Regarding the anomalous spin HF interaction reported in a previous paper for the weakly ferromagnetic state, the present results show that its effects extend all the way to and slightly beyond the orthorhombic-tetragonal phase boundary (formula presented) Further, the predominant (formula presented) contact HF interaction reported here supports the notion, suggested earlier, that a (formula presented) admixture underlies the anomaly. However, the basic mechanism of the anomaly remains obscure.