Positron distribution contribution to changes in annihilation characteristics across Tc in high-temperature superconductors

In this paper we present detailed calculations of the positron distribution in a host of high-temperature superconductors using the electron densities and potentials obtained from self-consistent orthogonalized linear combination of atomic orbitals band-structure calculations. The positron and electron densities obtained from the calculations are used to evaluate the electron-positron overlap function, which reveals that the major contribution to positron annihilation in these materials is from the oxygen atoms. A systematic correlation between the nature of this overlap function within the Cu-O cluster and the experimentally observed temperature dependence of the annihilation characteristics in the superconducting state is established: A decrease in positron annihilation parameters, below Tc, is observed when the overlap is predominantly from the apical oxygen atom, whereas an increase is observed if the overlap is predominantly from the planar oxygen atom. The observed temperature dependence of the positron parameters below Tc in all the high-Tc superconductors is understood in terms of an electron density transfer from the planar oxygen atoms to the apical oxygen atoms. These results are discussed in the light of charge-transfer models of superconductivity in the cuprate superconductors.