Vortex-persistent current interaction, Meissner effect, and diamagnetic shielding in a ring of YBa2Cu3O7.

Abstract

The vortex-persistent current interaction, Meissner effect, and diamagnetic shielding in a ring of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ have been studied by the measurement of the magnetic-field distributions across the sample and their time dependence at 77 K over an applied magnetic-field range 0--1000 G. The measurements were performed both for zero-field-cooling (ZFC) and field-cooling (FC) cases. Drilling a hole in a disk results in a time-dependent decay of the diamagnetic shielding and a time-dependent increase of the Meissner field being observed. The dependence of a trapped magnetic field on an applied magnetic field shows a maximum at an applied field of 150--200 G for the ZFC case and of 60--80 G for the FC case. For the ZFC case the magnetic field is trapped at applied magnetic fields as low as 10 G. The trapped flux (intergranular vortices) and macroscopic persistent current circulating around the ring contribute to the trapped field. The logarithmic decay rate of the trapped field is a linear function of an initial trapped field for the disk of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The decay rate of the trapped field for the ring of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ was measured both at its center and its edge. The results revealed a jump in the decay rate at trapped fields in the range 5--10 G for the ZFC case. These results are interpreted as being due to the Josephson vortex-persistent current interaction in the ring. The Josephson vortex motion in the ring is the combination of flux-creep- and flux-flow-like processes.