Superconductivity In A Disordered Metal With Coulomb Interactions

We study electronic densities of states (DOS) of strongly disordered superconducting thin films of TiN. We find through scanning tunneling microscopy (STM) measurements that the DOS decreases towards the Fermi level in the normal phase obtained by applying magnetic fields. The DOS shows spatial fluctuations whose length scale is related to the energy-dependent decrease in the DOS close to the Fermi level and is similar in normal and superconducting phases. This shows that Coulomb interaction in disordered metals strongly influences the normal DOS and suggests that reduced Coulomb screening leads to spatial variations in the superconducting DOS.