Critical current density and dynamical relaxation rate below the matching field in thin films of YBa₂Cu₃O_7-δ

We studied the superconducting current density j_s (B, T) and dynamical relaxation rate Q(B, T) of laser ablated thin films of YBa₂Cu₃O_7-δ with various dislocation densities (n_disl ∼ 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 j_s(B) ∼ 5.10¹¹ - 1.10¹² Am^-2 at T = 4.2 K and μ₀H ≲ 0.7·B_Φ and 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 B_Φ = n_disl · 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.