VORTEX MOVEMENT INTO POLYCRYSTALLINE YBA2CU3O7-X LOOPS

We have studied the fluxoid quantization and the critical state of a YBa2Cu3O7-x (YBCO) loop, and vortex movement into the loop using a weakly coupled low-T(c) superconducting quantum interference device (SQUID). The YBCO loop was made from a low-current-density polycrystalline film so that the critical-state behavior could be observed in modest magnetic fields at 4.2 K. As the magnetic field was swept, we observed a drastic change in the field responsivity of the SQUID when the loop went into and out of the critical state at a certain field H*. At fixed magnetic fields above and below H*, we measured the flux in the loop as a function of time by operating the SQUID in a flux-locked loop. Above H* we observed flux-creep-like behavior as well as the movement of individual flux quanta into the loop. Below H* we observed negligible dissipation in the form of flux quanta jumping into the loop, but did observe occasional hopping of individual vortices to other pinning sites in the YBCO.