Abstract
Observing a persistent current in a superconducting ring is a classic superconductivity experiment that can explore the magnetic behavior of type II superconductors. Using an exchange gas cryostat immersed in liquid nitrogen, a 12-mT variable magnet, a low cost Hall sensor, and basic electronic instrumentation, we have performed persistent current experiments on polycrystalline YBa2Cu3O7−δ (YBCO) rings at various magnetic fields. The relatively small critical fields of polycrystalline YBCO make it possible to explore a range of type II magnetic behavior for B<12 mT. Experiments described here measure the critical current density and lower critical field of a YBCO ring and investigate qualitative differences between the Meissner and vortex state. Time-dependent measurements observe logarithmic time decay of persistent currents. This paper describes the practical implementation of this experiment, including descriptions of sample fabrication as well as experimental apparatus and techniques. It also provides an elementary description of the theory related to critical currents and magnetic effects in type II superconductors.
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