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Article Dans Une Revue Physical Review B: Condensed Matter and Materials Physics (1998-2015) Année : 2013

Unconventional superconductivity in ultrathin superconducting NbN films studied by scanning tunneling spectroscopy

Yves Noat
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Christophe Brun
C. Carbillet
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K. Ilin
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M. Siegel
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A. Semenov
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H. -W. Huebers
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Résumé

Using scanning tunneling spectroscopy, we address the problem of the superconductor-insulator phase transition in homogeneously disordered ultrathin (2-15 nm) films of NbN. Samples thicker than 8 nm, for which the Ioffe-Regel parameter k(F)l >= 5.6, manifest a conventional superconductivity: a spatially homogeneous BCS-like gap, vanishing at the critical temperature, and a disordered vortex lattice in magnetic field. Upon thickness reduction, however, while k(F)l lowers, the STS reveals striking deviations from the BCS scenario, among which a progressive decrease of the coherence peak height and small spatial inhomogeneities. In addition, the gap below T-C develops on a spectral background, which becomes more and more ``V-shaped'' approaching the localization. The thinnest film (2.16 nm), while not being exactly at the superconductor-insulator transition (SIT) (T-C approximate to 0.4T(C)(bulk)), showed unconventional signatures such as the vanishing of the coherence peaks and the absence of vortices. This behavior suggests a weakening of long-range phase coherence, when approaching the SIT in this quasi-2D limit.
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hal-01233579 , version 1 (25-11-2015)

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Yves Noat, V. Cherkez, Christophe Brun, Ten Cren, C. Carbillet, et al.. Unconventional superconductivity in ultrathin superconducting NbN films studied by scanning tunneling spectroscopy. Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2013, 88 (1), pp.014503. ⟨10.1103/PhysRevB.88.014503⟩. ⟨hal-01233579⟩
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