The transport properties of ultrathin 2H–NbSe₂

The competing orders arising in a two-dimensional limit are the key to understanding unconventional superconductivity. The monolayer NbSe₂ possessing a charge density wave (CDW) and superconducting orders provides a playground to study unconventional superconductivity. Here we fabricate an ultrathin 2H–NbSe₂ device based on the mechanical exfoliation method and our electrode transfer technique. Detailed four-lead and two-lead transport measurements are performed in the ultrathin sample. The superconducting transition (T _c) at 4.3 K and CDW transition at 70 K are found in the sample, consistent with the literature report for ultrathin 2H–NbSe₂. The superconducting gap (Δ) of ultrathin 2H–NbSe₂ is estimated by fitting the two-lead transport spectroscopy with the Blonder–Tinkham–Klapwijk (BTK) theory. We found that Δ(T) exhibits a BCS-like temperature dependence with Δ $\approx$ 1.1 meV at 0 K and $\Delta /k_B{T}_c\approx \text{3}$. Meanwhile the gap-like features persist in the normal state and become unresolvable with the increasing temperature and magnetic field, suggesting a possible precursor superconductivity similar to the cuprate superconductors. Our results provide a new insight into superconductivity in ultrathin 2H–NbSe₂. (paper)