From orbital to Pauli-limited critical fields in granular aluminum films

The temperature dependence of the perpendicular upper critical field of superconducting granular aluminum films has been measured for samples approaching the metal-to-insulator transition. Analysis of the results shows a shift from an orbital to a Pauli-limited critical field, which we propose is made possible by electron mass renormalization. In that regime, the critical field transition becomes of the first order, as predicted by Fulde, Ferrel, Larkin, and Ovchinikov. The phase-coherence length ξ_{phase} and the superconducting gap Δ obtained from the analysis are consistent with a Bardeen-Cooper-Schrieffer–Bose-Einstein condensate crossover region, which we propose is triggered by the Mott transition of the granular films.