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

TL;DR

This study compares tunneling and optical conductivity gaps in granular aluminum films near the metal-insulator transition, revealing insights into the nature of the transition and superconducting pairing.

Contribution

It demonstrates that disorder is not the primary cause of the transition and suggests a BCS to BEC crossover in granular aluminum.

Findings

Excellent agreement between tunneling and optical gaps up to transition

Disorder not responsible for the transition in granular aluminum

Evidence of BCS to BEC crossover near the transition

Abstract

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.