Observation of Pseudogap State in NbN using Scanning Tunneling Spectroscopy

In case of high temperature superconductors the superconducting gap continuously merges into a gap like feature around Fermi level much above the superconducting transition temperature (TC) called as Pseudogap and the state is called pseudogap state. The origin of this pseudo-gap state has been strongly debated for the past two decades. Several alternative scenarios based either peculiarities specific to High Tc cuprates (namely, an order parameter competing with superconductivity) or a superconducting transition driven by phase fluctuations have been suggested as possible origin of this feature. In this talk, I will present experimental evidence of the formation of a pseudogapped state in a disordered conventional s-wave superconductor, NbN, as the system is driven towards the Anderson Metal-insulator transition. Series of scanning tunneling spectroscopy (STS) measurements carried out on films with increasing disorder shows that as one goes from the moderate to very high disorder, NbN starts forming a pseudogapped state above TC,  where we observe the dip in the tunneling spectra at the Fermi level. We propose that this dip is due to the superconducting correlations persisting above TC based on observation of BCS like spectra with diffused coherence peak after correcting it for Altshuler-Aronov background. Based on the temperature at which the pseudogap vanishes, called as T*, we establish the phase diagram highlighting pseudogap state and superconducting state near Anderson metal insulator transition.

The observation of pseudogapped state can be explained based on the phase fluctuation. The superconducting order is characterized by the complex order parameter consisting of amplitude and phase. The loss of superconductivity can be because of either vanishing of this amplitude as described by mean field theories like BCS, or because of phase fluctuations. Recent theories shows that in presence of strong disorder superconductor segregates into phase disconnected islands where superconductivity exists locally, but the global superconducting state is destroyed because of phase incoherence. The phase fluctuation scenario is further supported by the magneto-transport measurements where we observe strong positive magneto-resistance component that persist upto the pseudogap temperature T*.

Reference:  arXiv:1006.4143.