Tunneling study in granular aluminum near the Mott metal-to-insulator transition

We find excellent agreement between tunneling and optical conductivity gap values in superconducting. granular aluminum films, up to the metal-to-insulator transition. This behavior, in strong contrast with that recently reported in atomically disordered samples for which the optical gap becomes smaller than the tunneling gap, suggests that disorder is not at the origin of the transition. The large increase seen in the strong-coupling ratio and a finite value of the gap at ${T}_c$ near the metal-to-insulator transition are consistent with a BCS-to-BEC crossover.