Microscopic charging and in-gap states in superconducting granular aluminum

Abstract

We present scanning tunneling microscope (STM) measurements of the local electronic structure of superconducting granular aluminium films. The STM spectra show a homogeneously increased superconducting gap compared to that of aluminum, both near and above the Mott resistivity ${\ensuremath{\rho}}_{\mathrm{M}}\ensuremath{\approx}400\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$. Above ${\ensuremath{\rho}}_{\mathrm{M}}$ we find Coulomb charging effects, a first indication of electrical decoupling, and in-gap states on individual grains, which could contribute to flux noise and dielectric loss in quantum devices. We also observe multiple low-energy states outside the gap, which indicate bosonic excitations of an energy below twice the superconducting gap.