Pseudogap state in strongly disordered conventional superconductor, NbN

We present experimental evidence of the formation of a "Pseudogap" state in a disordered conventional s-wave superconductor, NbN, as the system is driven towards the Anderson Metal-Insulator transition. Series of scanning tunnelling spectroscopy measurements done on films with increasing disorder shows that for strongly disordered samples the dip in the tunnelling spectra at Fermi level persists much above the superconducting transition temperature. We propose that the gap like feature at Fermi level is associated with superconductivity based on the observation of BCS like spectra with dip at Fermi level and diffused coherence peaks after correcting them with Altshuler-Aronov background. We propose a scenario based on phase fluctuations to understand the pseudogap state in strongly disordered NbN films