Charge Distribution in La2-xSrxCuO4

Abstract

The electronic structure of La 2 CuO 4 has been studied by first-principles cluster calculations before and after doping. Clusters containing up to five planar copper atoms were investigated using the density functional method. At variance with band-structure calculations, we have been able to determine the charge and spin distributions from molecular orbitals expressed as linear combinations of atomic orbitals localised at the nuclear sites. Doping is achieved by subtracting an electron from the cluster which, although there is a change of spin state, produces a charge distribution which is remarkably similar to the charge distribution of the peripheral charge method which does not involve changes in spin state. The peripheral charge method is designed to simulate doping by placing a carefully chosen set of point charges beyond the periphery of the cluster and relies on the supposition that the hole distribution is closely related to the charge distribution. More importantly the peripheral charge method enables charge distribution changes to be obtained for fractional changes in doping. The results show that with fixed nuclear positions, doping not only depopulates the 2pσ orbitals but also affects other orbitals. This redistribution of charges influences the electric field gradients and hyperfine coupling parameters. The theoretical values for the field gradients are compared to values derived from experiments.