The Mixed State of Thin Films in Parallel Fields

Many interesting phenomena and regimes occur when the magnetic field B is oriented parallel to a superconducting film of thickness d much less than the penetration depth λ. The negligible demagnetization effects, together with the nearly complete field penetration, lead to a very large threshold field B c1 ∥ for vortex entry, which is much larger than the bulk B c1. Furthermore, orbital pair breaking is suppressed in parallel fields so that overlap of vortex cores is not the determining condition for the parallel upper critical field B c2 ∥ , but Pauli paramagnetic pair breaking is. In this case B c2 becomes very large and you have novel possibilities such as the Fulde–Ferrell–Larkin–Ovchinnikov phase (FFLO). Finally, when the cross section facing the magnetic field is reduced below a threshold value, Abrikosov vortex cores cease to exist. These features make the determination of the coherence length in the direction of the film thickness problematic. This work demonstrates transport techniques that allow a determination of this important fundamental length through experimental confirmation of the condition when Abrikosov vortices cease to exist.