Correlation between local oxygen disorder and electronic properties in superconducting RESr/sub 2/Cu/sub 3/O/sub 6+X/ (RE=Y, Yb)

This work aims at understanding the large reduction of superconducting critical temperature T/sub c/ observed in YSr/sub 2/Cu/sub 3/O/sub 6+x/ as compared to its YBa/sub 2/Cu/sub 3/O/sub 6+x/ counterpart ( Delta T/sub c/=- 30 K). We report on a combined study of structural and electronic properties of RESr/sub 2/Cu/sub 3/O/sub 6+x/ (RE=Y, Yb) polycrystalline samples. Neutron diffraction data and Cu NQR spectra show that, contrary to REBa/sub 2/Cu/sub 3/O/sub 6+x/ RESr/sub 2/Cu/sub 3/O/sub 6+x/ is locally tetragonal and no CuO chains are formed. This arises from the random occupancy of oxygen along the a-or b-direction in the basal planes. Ab-initio calculations of the electronic structure using the full-potential linearized-augmented-plane-wave method (FLAPW) in the local density approximation (LDA) suggest that the CuO chains are not formed because of the large elastic strain associated with the orthorhombic distortion produced by the chain formation. In addition, by using ! a square root (2a) * square root (2a) supercell simulating the absence of chains, we find that oxygen disorder greatly alters the band structure near the Fermi level. Our analysis indicates that this alteration leads to a reduction of hole transfer from the CuO chains to the CuO/sub 2/ planes, which accounts for the reduction of T/sub c/ experimentally observed.