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

E. Khestanova, J. Birkbeck, M. Zhu, Y. Cao, G. L. Yu, D. Ghazaryan, J. Yin, H. Berger, L. Forro, T. Taniguchi, K. Watanabe, R. V. Gorbachev, A. Mishchenko, A. K. Geim, I. V. Grigorieva

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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 TC 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 TC. 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 TC. 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 TC 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.
Original languageEnglish
Pages (from-to)2623-2629
Number of pages7
JournalNano Letters
Issue number4
Early online date12 Mar 2018
Publication statusPublished - 11 Apr 2018

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  • National Graphene Institute


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