Vortex trapping and expulsion in thin-film YBa2 Cu3 O7-δ strips

Abstract

A scanning superconducting quantum interference device microscope was used to image vortex trapping as a function of the magnetic induction during cooling in thin-film YBa2 Cu3 O7-δ (YBCO) strips for strip widths W from 2 to 50 μm. We found that vortices were excluded from the strips when the induction Ba was below a critical induction Bc. We present a simple model for the vortex exclusion process which takes into account the vortex-antivortex pair production energy as well as the vortex Meissner and self-energies. This model predicts that the real density n of trapped vortices is given by n= (Ba - BK) / Φ0 with BK =1.65 Φ0 / W2 and Φ0 =h/2e the superconducting flux quantum. This prediction is in good agreement with our experiments on YBCO, as well as with previous experiments on thin-film strips of niobium. We also report on the positions of the trapped vortices. We found that at low densities the vortices were trapped in a single row near the centers of the strips, with the relative intervortex spacing distribution width decreasing as the vortex density increased, a sign of longitudinal ordering. The critical induction for two rows forming in the 35 μm wide strip was (2.89+1.91-0.93) Bc, consistent with a numerical prediction. © 2008 The American Physical Society.