Critical current density and dynamical relaxation rate below the matching field in thin films of YBa2Cu3O7-d

Abstract

We studied the superconducting current density js(B, T) and dynamical relaxation rate Q(B, T) of laser ablated thin films of YBa2Cu3O7−δwith various dislocation densities (ndisl∼ 7-70 μm−2). In these films dislocations act as strong pinning centers like columnar defects in heavy ion-irradiated crystals. The matching field clearly manifests itself in a constant js(B) ∼ 5·1011- 1·1012Am−2at T = 4.2 K and μ0H ≲ 0.7 BΦand a constant Q(B). In contrast to irradiated crystals, in films we do not observe a peak in the relaxation rate Q versus T below the matching field BΦ≡ ndisl·Φ0. Instead, in thin films Q(T) increases monotonously with increasing temperature. We conclude that the influence of vortex-vortex interactions in thin films is greatly reduced due to the non-random distribution of pinning sites and the low matching field.