MOLECULAR-ORBITAL CALCULATIONS OF POSSIBLE PINNING CENTERS FOR MAGNETIC-FLUX IN COPPER-OXIDE SUPERCONDUCTORS

Cluster calculations using the molecular orbital method were performed to search for impurities which would act as pinning centers for magnetic flux in copper oxide superconductors. Electronic structures were first investigated for interstitial helium atoms and for He atoms (including He clusters) substituted for oxygen and copper atoms. Calculations were then done for F atoms substituted for O atoms, and C, Mg, Al, Zn, Ga, and Cd atoms substituted for Cu. With these impurities, the energies of orbitals mainly attributable to the impurities differ from the Fermi energy. The Mulliken populations show that most charge carriers do not exist on the impurity sites (though some charge carriers enter the C sites), so it can be expected that the impurities considered here will act as pinning centers.