Field dependence of resistance transitions in thin films of YBA₂CU₃O₇ and YBA₂CU₄O₈

Preferentially oriented thin films of YBa₂Cu₃O₇ (123) and YBa₂Cu₄O₈ (248) with the c-axis perpendicular to the substrate have been prepared by e-gun evaporation from Y, BaF₂ and Cu. Critical current and resistance transition measurements have been carried out in magnetic fields parallel to the c-axis. The results will be analysed in terms of a model for thermally activated flux flow combined with two models for flux pinning, one in which the pinning is predominantly caused by twin boundaries and a second in which pinning by dislocations is considered. The resistance broadening in the 248 is well described by the model with dislocation pinning. The B_c2 (T) values obtained from the fits compare well with estimates from the fluxflow resistivity and a kink in the resistance vs field curves. The slope of B_c2 at T_c is 0.37 T/K, a factor four smaller than in the 123.