EVIDENCE FOR UNPINNED MAGNETIC-FLUX VORTICES ABOVE THE I(C) TRANSITION IN THE 2223-PHASE HIGH-T(C) SUPERCONDUCTOR

A similarity in the dc voltage-current (V-I) curves for both direct an d alternating transport currents is used to propose that unpinned flux vortices are generated above the I(c) transition for dc transport cur rents, when Abrikosov flux vortices begin to penetrate the superconduc tor. Two methods can be used to give a dc voltage drop for an ac trans port current: (1) if there is a slight dc offset voltage in the ac cur rent which favors vortex loop collapse as in a traditional dc I(c) tes t, or (2) if an asymmetric transverse magnetic field is present which favors vortex loop collapse for current in one direction over the reve rse direction.