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

The superconducting transition temperature of amorphous thin films often decreases dramatically as their thickness is reduced. This suppression can be attributed to either disorder-induced localization of Cooper pairs or generation of free vortices during Berezinskii-Kosterlitz-Thouless (BKT) transition. In the case of crystalline superconductors such as exfoliated NbSe₂, superconductivity is preserved down to monolayer thickness, however, the transition temperature is also found to be suppressed.
Here we present measurements of both the superconducting energy gap Δ and critical temperature T_c in 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 ≥ 2 in the same way as their T_c. We attribute the observed behavior to inevitable depletion in the Cooper pair density in the immediate vicinity of the superconductor-vacuum interface. Our experimental data are in good agreement with the 1/N dependence of Δ and T_c predicted in this case. The involved spatial scale is only a few angstroms but cannot be ignored for atomically thin superconductors.