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Comparison of the occupation numbers n_d in La_(2-x)Sr_xCuO_4 estimated by NMR/NQR and ARPES probes

The analysis of electric-field-gradients (EFG's) measured in La_(2-x)Sr_xCuO_4 is performed to estimate the intracell distribution of charge among one copper ion and two oxygen ions in the conducting planes of the high-Tc cuprates. Using the essentially model independent expression for EFGs, with the lattice contributions to EFGs treated self-consistently, we obtain that in the x=0 parent compound the average number of holes on Cu is n_d=0.72. It changes to n_d=0.75 in the x=0.3 compound, with the linear dependence on the hole doping in the metallic phase. The occupation numbers n_d are then calculated using the Emery three-band model for conduction electrons, with two first-neighbour hopping integrals (t_(pd), between two Cu-O neighbours, and t_(pp), between two O-O neighbours) and the Cu-O splitting energy Delta_(pd). For the values of these parameters estimated from ARPES and optical conductivity measurements, we obtain that the calculated n_d's are about 20 percent smaller than that estimated by the EFG analysis. It is possible to overcome this discrepancy by including next-to-nearest hopping integrals, which contribute to the Fermi surface curvature, but do not result in the substantial intracell charge redistributions. Alternatively, such a large difference between two estimations of n_d may be due to the omission of incoherent contributions to n_d.

NMR; NQR; ARPES

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