Upper critical fields near unitarity at a BCS–BEC crossover in granular aluminum


  Aviv Glezer Moshe [1,2]  ,  Guy Deutscher [1]  ,  Eli Farber [2]  
[1] Tel Aviv University
[2] Ariel University

The temperature dependence of the upper critical field of superconducting granular aluminum has been measured for films approaching the metal to insulator transition, down to 50 mK. The results show a progressive transition from an orbital limited critical field in the BCS side to a Pauli limited critical field close to a possible BCS-BEC crossover regime. Their ratio is in agreement with recent theoretical predictions for the behavior of the gap and of the coherence length in a Bardeen-Cooper-Schrieffer/Bose-Einstein condensate (BCS-BEC) crossover region, associated with strong coupling effects.