Covalency in La2CuO4: A study of O-17 hyperfine couplings in the paramagnetic phase - art. no. 014404

O-17 nuclear magnetic resonance spectra from single crystals of La2CuO4 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 incl udes spin-orbit coupling effects. The results show a 7.7% hybridization eff ect of the oxygen 2p(sigma) orbital from a single copper neighbor, in good agreement with recent density-functional (DF) calculations by Husser ed al. (HSSM). A large, positive isotropic shift component is also reported, pres umably originating from the contact interaction with a hybridized 2s orbita l component. First-order quadrupolar-splitting data lead to complete charac terization of the electric-field gradient (EFG) tensor, which varies only s lightly with temperature up to 800 K. EFG tensors for both doped and undope d La2CuO4 are fitted with a two-component model, which incorporates a subst antial anisotropy in (r(-3)) for the 2p(sigma) wave functions, an effect th at originated in the DF calculations of HSSM. This analysis reveals an incr eased charge density on the planar oxygens for the superconducting phase, i n accord with the original Zhang-Rice model. However, the increase is found to correspond to only similar to 80% of the nominal doped-hole density, co rroborating a similar conclusion reached recently by Hammel et al. Regardin g the anomalous spin HF interaction reported in a previous paper for the we akly ferromagnetic state, the present results show that its effects extend all the way to and slightly beyond the orthorhombic-tetragonal phase bounda ry (TO-T similar or equal to 550 K). Further, the predominant 2s contact HF interaction reported here supports the notion, suggested earlier, that a 2 s admixture underlies the anomaly. However, the basic mechanism of the anom aly remains obscure.