We report measurements of the energy gap of granular aluminum films by THz spectroscopy. We find that as the grains progressively decouple, the coupling ratio 2∆(0)/kBTc increases above the BCS weak coupling ratio 3.53, and reaches values consistent with an approach to BCS-BEC crossover for the high resistivity samples, expected from the short coherence length. The Mattis-Bardeen theory describes remarkably well the behavior of σ1,s/σ1,n for all samples up to very high normal state resistivities. arXiv:1901.02814v2 [cond-mat.supr-con]

No abstract

Granular aluminum is a promising material for high kinetic inductance devices such as qubit circuits. It has the advantage over atomically disordered materials such as NbNx, to maintain a high kinetic inductance concomitantly with a high quality factor. We show that high quality nano-scale granular aluminum films having a sharp superconducting transition with normal state resistivity values on the order of 1×105 μΩ cm and kinetic inductance values on the order of 10 nH/◻ can be obtained, surpassing the state-of-the-art values. We argue that this is a result of the different nature of the metal-to-insulator transition, being electronic correlations driven (Mott type) in the former and disorder driven (Anderson type) in the latter.