Phase transition in one-dimensional mixed state in thin superconducting films

The temperature dependence of the field corresponding to the formation of a one-dimensional mixed state is determined for vanadium thin films in a magnetic field H‖ parallel to their surface. The phase diagram of the films is studied by measuring the temperature and field dependences of the critical current Ic. The field corresponding to the penetration of a vortex chain is determined by the kinks on the Ic (H‖) curves. For samples with rough surfaces, the experimental Hc1 (T) curves are in good agreement with the theoretical ones. The values of Hcl for films with d ≲ 1000 Å are approximately one order of magnitude higher than the typical values for type II bulk superconductors. For films with smooth boundaries, the vortex penetration is inhibited by a surface barrier, and the corresponding field Hs is appreciably higher than Hcl. The influence of the Bean-Livingston barrier increases in the temperature interval where the relation between the film thickness and the coherence length ξ (T) satisfies the condition d ≫ 2 ξ (T).

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