Studies of the paramagnetic Meissner effect in niobium disks.

摘要

The paramagnetic Meissner effect (PME) is exemplified by a positive field-cooled magnetization below the transition temperature T{dollar}sb{lcub}rm c{rcub}{dollar} of a superconductor. The recent discovery of the PME in certain high-T{dollar}sb{lcub}rm c{rcub}{dollar} cuprate superconductors has been suggested to be a signature for d-wave pairing in the cuprate superconductors. Since not all cuprate super-conductors exhibit the PME, there remains the question: what is unique about the samples that exhibit the PME? As a means to answer this question, a study of Nb disks which also can exhibit the PME has been pursued.; While the PME was exhibited with fields applied perpendicular to untreated Nb disks with t = 0.127 mm, only diamagnetic magnetizations were observed with disks oriented parallel to applied fields. The anisotropy in the disk orientation with respect to field in order to observe the PME in Nb disks is consistent with a recent report that a YBCO 123 single crystal exhibited the PME only for fields applied parallel to the crystalline c-axis, i.e., perpendicular to the crystalline platelet surface and not with H parallel to the crystalline-platelet. The effect of the surface microstucture was investigated by altering the surface through abrading, oxygen annealing, chemical etching, and ion implantation. Whereas surface abrading and chemical etching reduced and even eliminated the PME, oxygen annealing and ion implantation enhanced the magnitude of the PME in disks already exhibiting a positive FCM below T{dollar}sb{lcub}rm c{rcub}{dollar}. In fact, surface implanting of Kr{dollar}sp+{dollar} ions induced the PME in disks with t = 0.25 mm, which had previously shown only negative FCMs below T{dollar}rmsb{lcub}c{rcub}{dollar}.; This investigation has determined that the PME in Nb disks is the result of extrinsic characteristics of the samples and not an intrinsic property related to mechanisms of super-conductivity. A phenomenological model has been developed using two key elements for explaining the PME in the Nb disks: (i) the presence of strong flux pinning due to a T{dollar}sb{lcub}rm c{rcub}{dollar} variation giving rise to an inhomogeneous local field distribution and (ii) the resulting temperature dependence of the local field which for decreasing temperatures decreases or reverses the shielding current direction in accordance with Lenz's Law.