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

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 NbSe, 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 in high-quality monocrystals of few-layer NbSe, 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. 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. We attribute the observed behavior to changes in the electronic band structure predicted for mono- and bi- layer NbSe 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 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.

Electron-phonon coupling and the coexistence of superconductivity and charge-density wave in monolayer NbSe2

“…2(b) shows that the Fermi surface is partially or fully gapped when the CDW is formed [7], which is in qualitatively agreement with experimental observation that a striking dip at Fermi energy (but does not reach to zero) was seen in the dI/dV spectrum [2]. The experimental striking dip is mainly due to the extensive Fermi surface gapping in K and K pockets according to the computed Fermi surface We now show that the absence of two-gap feature in few-layer NbSe 2 experimentally [16] is possibly due to the extensive Fermi surface gapping in K and K pockets induced by CDW. The two-gap feature is found in the non-CDW phase with 3 intact Fermi pockets and we further assign the large gap to K and K pockets dominated by the in-plane orbitals of Nb, the small gap to the Γ pocket (out-of-plane orbitals of Nb).…”

Robust parity-mixed superconductivity in disordered monolayer transition metal dichalcogenides

“…In principle, one can adjust U such that the order parameters vanish at the system's superconducting critical temperature T c . In monolayer 1H-NbSe 2 , T c ≈ 2 K [5,6]. This yields a BCS zero temperature gap ∆(0) = 1.76k B T c ≈ 0.3 meV, and the ratio between the SOC and superconducting energy scales is estimated to be of order λ SO /∆(0) ∼ 200 [35].…”

Phase slip lines in superconducting few-layer NbSe 2 crystals

“…The most studied van der Waals SC is the 2H polytype of NbSe 2 (in the following simply indicated as NbSe 2 ). [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] This material can be easily exfoliated in few-layer-thick flakes and then encapsulated in hBN. 25 Typical devices for transport measurements have a size of several micrometers.…”