Microscopic charging and in-gap states in superconducting granular aluminum
Fang Yang, Thomas Gozlinski, Tim Storbeck, Lukas Grünhaupt, Ioan M. Pop, Wulf Wulfhekel
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.