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

Nano Letters. 2018. Vol. 18. No. 4. P. 2623–2629.

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

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

Keywords: graphene