Study of Pseudogap State in NbN using Scanning Tunneling Spectroscopy (original) (raw)
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Tunneling studies in a homogeneously disordered s-wave superconductor: NbN
Physical Review B, 2009
We report the evolution of superconducting properties as a function of disorder in homogeneously disordered epitaxial NbN thin films grown on (100) MgO substrates, studied through a combination of electrical transport, Hall Effect and tunneling measurements. The thickness of all our films are >50nm much larger than the coherence length ξ 0~5 nm. The effective disorder in different films encompasses a large range, with the Ioffe-Regel parameter varying in the range k F l~1. 38-8.77. Tunneling measurements on films with different disorder reveals that for films with large disorder the bulk superconducting transition temperature (T c ) is not associated with a vanishing of the superconducting energy gap, but rather a large broadening of the superconducting density of states. Our results provide strong evidence of the loss of superconductivity via phase-fluctuations in a disordered s-wave superconductor.
Pseudogap state in strongly disordered conventional superconductor, NbN
2012
Abstract. 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.
2013
The notion of spontaneous formation of an inhomogeneous superconducting state is at the heart of most theories attempting to understand the superconducting state in the presence of strong disorder. Using scanning tunneling spectroscopy and high resolution scanning transmission electron microscopy, we experimentally demonstrate that under the competing effects of strong homogeneous disorder and superconducting correlations, the superconducting state of a conventional superconductor, NbN, spontaneously segregates into domains. Tracking these domains as a function of temperature we observe that the superconducting domains persist across the bulk superconducting transition, Tc, and disappear close to the pseudogap temperature, T*, where signatures of superconducting correlations disappear from the tunneling spectrum and the superfluid response of the system.
Journal of Physics: Condensed Matter, 2021
The superconducting transition temperature (T C) of rock-salt type niobium nitride (δ − NbN) has been reported to vary in a large range (between 9 to 17 K) and the theoretically predicted value of 18 K has not achieved hitherto. Such a variation in the T C has been assigned to disorder present in δ − NbN irrespective of microstructure (polycrystalline or epitaxial), methods or conditions applied during the growth of NbN thin films. In this work, we investigate the atomic origin such suppression of T C in δ − NbN thin film by employing combined methods of experiments and abinitio simulations. Sputtered δ − NbN thin films with different disorder were studied using N and Nb K-edge x-ray absorption spectroscopy. A strong correlation between the superconductivity and the atomic distortion induced electronic reconstruction was observed. The theoretical analysis revealed that under N-rich conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously. As a result, the electronic densities of states around the Fermi level get smeared leading to a suppression of the T C in δ − NbN.
Phase diagram and upper critical field of homogenously disordered epitaxial 3-dimensional NbN films
2010
We report the evolution of superconducting properties with disorder, in 3 dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal-insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder we observe that as k F l→1 the superconducting transition temperature (T c ) and normal state conductivity in the limit T 0 (σ 0 ) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal-insulator transition and the superconductor-insulator transition occur at a single quantum critical point, k F l~1.
Phase Diagram and Upper Critical Field of Homogeneously Disordered Epitaxial 3-Dimensional NbN Films
… and novel magnetism, 2011
We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal–insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder, we observe that as kF l→1 the superconducting transition temperature (Tc) and normal state conductivity in the limit T →0 (σ0) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal–insulator transition and the superconductor–insulator transition occur at a single quantum critical point, kF l ∼ 1.
arXiv: Superconductivity, 2020
Rock-salt type niobium nitride ($\delta$-NbN) is a well-known superconductor having superconducting transition temperature (Tc) approx\approxapprox 18\,K and a large superconducting gap approx\approxapprox3\,meV. The Tc of delta\deltadelta-NbN thin film exhibits a large scattering irrespective of the growth conditions and lattice parameter. In this work, we investigate the atomic origin of suppression of Tc in delta\deltadelta-NbN thin film by employing combined methods of experiments and ab-initio simulations. Sputtered delta\deltadelta-NbN thin films with different disorder were analyzed through electrical resistivity and x-ray absorption spectroscopy. A strong correlation between the superconductivity and the atomic distortion induced electronic reconstruction was observed. The theoretical analysis revealed that under N-rich growth conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously and are responsible for the suppression of Tc in delta\deltadelta-NbN by smearing its electronic d...