Vortex exclusion transition

Ordinarily, Abrikosov vortices penetrate a type-II superconductor in a magnetic field larger than the lower critical field. However if the sample dimensions/s transverse to the applied magnetic field become smaller than some limit (found by Likharev to be $d<4.4 \xi$ for a film of thickness $d$ where $\xi$ is the coherence length), the formation of a parallel vortex in a film is excluded. We measured the transport response of a Molybdenum-Germanium superconducting film in parallel magnetic field and observed a sharp change in the V-I characteristics at a temperature which corresponds to the condition that $d \approx 4.4 \xi(T)$. We present the evolution of the observed transport behavior as a function of temperature and magnetic field.