Critical-temperature enhancement in thin superconducting films due to field dependence of the coherence length

Nucleation of superconductivity in a thin film situated in a parallel magnetic field is considered. The field dependence of the coherence length is taken into account. In particular, the known result for the nucleation field H near the critical temperature Tc is shown to hold only in the dirty limit. If a certain condition imposed upon the thickness and the effective mean free path of the film is met (the film should be clean enough and sufficiently thin), the phase boundary H(T) bends to temperatures T>Tc. A criterion for the anomalous phase boundary to occur is obtained. The method to calculate the whole curve H(T) is developed. Possible enhancement of the critical temperature, Tc(H)-Tc(0), is evaluated and shown to be substantial for a proper set of film parameters.