Unusual Suppression of the Superconducting Energy Gap and Critical Temperature in Atomically Thin NbSe2

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...

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Bibliographic Details
Published in:	Nano letters 2018-04, Vol.18 (4), p.2623-2629
Main Authors:	Khestanova, E, Birkbeck, J, Zhu, M, Cao, Y, Yu, G. L, Ghazaryan, D, Yin, J, Berger, H, Forró, L, Taniguchi, T, Watanabe, K, Gorbachev, R. V, Mishchenko, A, Geim, A. K, Grigorieva, I. V
Format:	Article
Language:	eng
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Summary:	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 Δ 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 Δ and T C expected in the latter case while the effect of band-structure reconstruction is evidenced by a stronger suppression of Δ 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.
ISSN:	1530-6984 1530-6992