Unusual suppression of the superconducting energy gap and critical temperature in atomically thin NbSe2

TL;DR

This study investigates the suppression of superconductivity in atomically thin NbSe2, revealing that changes in electronic band structure and surface effects, rather than disorder or BKT physics, cause the observed reduction in energy gap and critical temperature.

Contribution

The paper provides experimental evidence that band-structure changes and surface suppression effects explain superconductivity behavior in monolayer NbSe2, challenging previous assumptions about disorder and BKT mechanisms.

Findings

Superconducting gap and Tc decrease rapidly for N < 5 layers.

Disorder and BKT do not account for the suppression observed.

Band-structure reconstruction explains the suppression effects.

Abstract

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 and critical temperature 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 ctitical temperature. 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 the gap and Tc expected in the latter case while the effect of band-structure reconstruction is evidenced by a stronger suppression of the gap 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.