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The interplay of superconductivity and disorder is one of the most intriguing problems of quantum many body physics. Quite early on, it was argued by Anderson that since BCS superconductors respect time reversal symmetry, superconductivity is robust against nonmagnetic impurities and the critical temperature TC is not affected by such disorder. However experiments showed that strong disorder reduces TC and ultimately drives the system into an insulator.In recent times numerous theories and numerical simulations have been carried out in order to understand real space evolution of superconductivity in presence of strong disorder. It has been argued that in the intermediate disorder limit the effect of disorder is to decrease the pairing amplitude which results in decrease in TC whereas strong disorder which is homogeneously distributed over atomic length scales makes the superconducting state inhomogeneous. Here the system segregates into domains of large order parameter embedded in the insulating sea which is predicted to have profound consequences, including the formation of a pseudogap state above TC similar to the underdoped High-TC Cuprates.
In my talk I will present scanning tunneling spectroscopy (STS) measurements on strongly disordered s-wave superconductor, NbN, close to superconductor insulator transition. Our results show that in presence of strong disorder NbN forms pseudogapped state above TC. Spatially resolved STS data at lowest temperature shows the emergence of inhomogeneity in the form of domains of large superconducting order separated by regions where superconducting coherence is completely suppressed. With increasing temperature these domains continue to persist above TC giving rise to pseudogapped state. I will also present our statistical analysis on superconducting order parameter (OP) where we observe universality in the OP distributionirrespective of the strength of disorder present in the system. |