Vortex trapping and expulsion in thin-film YBa(2)Cu(3)O(7-delta) strips

Artikel i vetenskaplig tidskrift, 2008

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 YBa(2)Cu(3)O(7-delta) (YBCO) strips for strip widths W from 2 to 50 mu m. We found that vortices were excluded from the strips when the induction B(a) was below a critical induction B(c). 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=(B(a)-B(K))/Phi(0) with B(K)=1.65 Phi(0)/W(2) and Phi(0)=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 mu m wide strip was (2.89+1.91-0.93)B(c), consistent with a numerical prediction.