期刊
NANO LETTERS
卷 18, 期 4, 页码 2623-2629出版社
AMER CHEMICAL SOC
DOI: 10.1021/acs.nanolett.8b00443
关键词
Two-dimensional superconductors; transition temperature; tunneling spectroscopy; energy gap; NbSe2
类别
资金
- U.K. Engineering and Physical Sciences Research Council (EPSRC)
- Graphene Flagship
- Lloyd's Register Foundation
- University of Manchester President's Doctoral Scholarships
- EPSRC CDT in Science and Applications of Graphene and Related Nanomaterials (Graphene NOW NANO)
- EPSRC [EP/N007131/1]
- EPSRC [EP/K005014/1, EP/N010345/1] Funding Source: UKRI
- Engineering and Physical Sciences Research Council [EP/N007131/1, EP/N010345/1] Funding Source: researchfish
It is well-known that superconductivity in thin films is generally suppressed with decreasing thickness. This suppression is normally governed by either disorder-induced localization of Cooper pairs, weakening of Coulomb screening, or generation and unbinding of vortex-antivortex pairs as described by the Berezinskii-Kosterlitz-Thouless (BKT) theory. Defying general expectations, few-layer NbSe2, an archetypal example of ultrathin superconductors, has been found to remain superconducting down to monolayer thickness. Here, we report measurements of both the superconducting energy gap Delta and critical temperature T-c in high-quality monocrystals of few-layer NbSe2, using planar-junction tunneling spectroscopy and lateral transport. We observe a fully developed gap that rapidly reduces for devices with the number of layers N <= 5, as does their T-c. We show that the observed reduction cannot be explained by disorder, and the BKT mechanism is also excluded by measuring its transition temperature that for all N remains very close to T-c. We attribute the observed behavior to changes in the electronic band structure predicted for mono- and bi- layer NbSe2 combined with inevitable suppression of the Cooper pair density at the superconductor-vacuum interface. Our experimental results for N > 2 are in good agreement with the dependences of Delta and T-c expected in the latter case while the effect of band-structure reconstruction is evidenced by a stronger suppression of Delta and the disappearance of its anisotropy for N = 2. The spatial scale involved in the surface suppression of the density of states is only a few angstroms but cannot be ignored for atomically thin superconductors.
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