Electric-field-gradient analysis of high-Tc superconductors

The qualitative and quantitative features of the electronic charge distribution in cuprate superconductors are reexamined on the basis of a comprehensive set of nuclear quadrupole resonance and NMR measurements. A systematic analysis of measured electric-field gradients is performed within the tight-binding approach, commonly used for electronic models in cuprates. Both in-plane and out-of-plane sites and orbitals are considered. Special attention is given to the generic pd model involving only the sigma orbitals in the CuO2 planes. This model is checked against the experimental data. The physical origin of several material-dependent features is clarified. It is shown that in La2-xSrxCuO4 the 3d(3z2-r2) orbitar of the in-plane copper and the 2p(z) apex oxygen orbital are occupied by a substantially smaller fraction of holes than the in-plane sigma orbitals, at variance with proposals lending importance to copper-apex oxygen hybridization. At the in-plane oxygen site a sizable 2p(z) admixture is found at the Fermi level of YBa2Cu3O7. The in-plane charge distribution of Tl2Ba2CuO4 is found to be qualitatively similar to that of the high-temperature tetragonal phase of La2-xSrxCuO4. It appears that the additional holes are shared among copper and oxygen sites in similar proportions. This is shown to be compatible with the large U-d Emery model provided that the difference between the p and d atomic energies is comparable to the first neighbor overlap energy.