Microscopic charging and in-gap states in superconducting granular aluminum*

Granular aluminum (grAl) is a viable material for high-impedance quantum circuits [1], thanks to its high kinetic inductance, low microwave frequency losses [2], and fabrication by deposition of pure aluminum in an oxygen atmosphere. The material’s microstructure – pure aluminum grains in non stoichiometric aluminum-oxide – can be modeled as a Josephson junction array [3], in which the oxide thickness is controlled by the oxygen partial pressure during deposition. Here we present scanning tunneling microscope measurements of the local electronic structure of superconducting grAl and we confirm an increased superconducting gap in the grains of films with ρ ≈ 300 μΩcm and ρ ≈ 2000 μΩcm. In the high resistivity film, we find Coulomb charging effects, a first indication for grain decoupling, and in-gap states on individual grains, which could contribute to flux noise and dielectric loss not only in devices employing grAl, but also in pure aluminum circuits, where such states could form in Josephson junctions or in oxidized aluminum surfaces.

[1] L. Grünhaupt & M. Spiecker et al., Nat. Mat. 18 (2019)
[2] L. Grünhaupt et al., PRL 121 (2018)
[3] N. Maleeva et al., Nat. Commun. 9 (2018)