Anisotropy of the Lower Critical Field in YBa sub 2 Cu sub 3 O sub 7 Minus delta

摘要

We report the first careful measurements of the two dimensional anisotropy of the lower critical field H/sub c1/. We compare the anisotropy in our values of H/sub c1/ with the anisotropy observed in H/sub c2/ and derive values for the anisotropy of the effective mass. The single crystals used in this study were grown by a solid state annealing method. Y sub 2 O sub 3 , BaCO sub 3 , and CuO powders were mixed in the stoichiometric 1:2:3 ratio, ground with a mortar and pestle, placed in a platinum crucible and heated to 975/degree/C for 3--5 days. The reaction product contained many small crystals up to typical dimensions of 0.5 mm. The crystals showed a sharp inductive onset at 91.5/degree/K as measured in a dc shielding experiment in a field of 10 Oe. In general the crystals were not oxygen annealed before measurement. Three crystals were selected for measurements of H/sub c1/. One of the crystals was used as-grown, one was annealed for 24 hours at 500/degree/C in a pure oxygen environment, and one was irradiated with neutrons to a fluence of 8.16 /times/ 10 sup 17 neutronscm sup 2 . Results are discussed. 10 refs., 4 figs., 1 tab. (ERA citation 13:036854)