期刊
PHYSICAL REVIEW B
卷 88, 期 1, 页码 -出版社
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.88.014503
关键词
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资金
- UPMC emergence project [11-272]
- DFG Center for Functional Nanostructures [A4.3]
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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