Relations between structural and superconducting properties of bulk and thin film high-T(sub c) materials

摘要

The structural ordering of oxygen deficient and Co-doped YBCO (YBa(sub 2)Cu(sub 3-y)Co(sub y)O(sub 6+x)) have been studied experimentally, and by computer simulations of the oxygen ordering in the basal plane of the structure. The calculations are based on the two-dimensional ASYNNNI model and its modifications. Good agreement is established between the ASYNNNI calculations and the experimentally observed structural properties of the double cell ortho-II structure and the oxygen disordering process from Co-doping into the basal plane. A model that relates the superconducting transition temperature T(sub c)(x) of undoped YBCO and T(sub c)(y) of Co-doped YBCO to the formation of specific domains of the two orthorhombic ordered oxygen phases, ortho-I and ortho-II, shows a close agreement with experimental T(sub c)(x) and T(sub c)(y) data of samples prepared under equilibrium conditions. The structural changes as a result of metal ion substitutions and oxidation/reduction processes have been studied by neutron powder diffraction in Pb(sub 2)Sr(sub 2)Ln(sub 1-x)Ca(sub x)Cu(sub 3)O(sub 8+y) (Ln = Y and Ho), Nd(sub 1.85)Ce(sub 0.15)CuO(sub 4+y), and chemically oxidized La(sub 2-x)Sr(sub x)CuO(sub 4+y) < x < 0.13. Neutron diffraction studies of oxygen deficient pure (y = 0) and Al-doped LnBa(sub 2)Cu(sub 3-y)Al(sub y)O(sub 6+x) (y < 0.19, Ln = Y and Pr) have related a qualitatively similar high temperature antiferromagnetic phase, AF1. For Ln = Pr the Af1 phase is found for all oxygen stoichiometries, and a two-dimensional antiferromagnetic pr-ordered structure with weak ferromagnetic interplane correlations has been observed for oxidized samples. The structural ordering of epitaxial YBa(sub 2)Cu(sub 3)O(sub 6+x) and Bi(sub 2)Sr(sub 2)CaCu(sub 2)O(sub 8+x) thin films deposited on SrTiO(sub 3) (001), MgO (001), LaAlO(sub 3) (001), and NdGaO(sub 3) (001) substrates has been studied by x-ray diffraction, TEM and RBS, and the structural ordering has been analysed. (Atomindex citation 25:076995)