The transition-metal-dichalcogenide family as a superconductor tuned by charge density wave strength

Metallic transition metal dichalcogenides (TMDs), consisting of H-NbSe₂, H-NbS₂, H-TaSe₂ and H-TaS₂, remain superconducting down to a thickness of a single layer. In these materials, thickness affects a variety of properties, including Ising protection, two-band superconductivity, and the critical temperature TC, which decreases for the Nb-based, and increases for the Ta-based materials. This contradicting trend is puzzling, and has precluded the development of a unified theory. We approach the question of thickness-evolution of TC and the superconducting gap Δ by measuring high-resolution tunneling spectra in TaS₂-based stacked devices. Our measurements allow for simultaneous evaluation of Δ, TC, and the upper critical field HC2. The latter, we find, is strongly enhanced towards the single-layer limit, following a HC2 ∝ Δ² proportionality ratio. Our main finding is that the same ratio holds for the entire family of metallic TMDs: TaS₂ and NbSe₂ of all thicknesses, bulk TaSe₂ and bulk NbS₂, extending over 4 orders of magnitude in HC2 and covering both clean and dirty limits. We propose that this tunability across the TMD family is controlled by the competing charge density wave (CDW) phase. Using Gor'kov's theory, we calculate how a CDW order affects the quasiparticle density of states and the resulting TC and HC2. Our results suggest that CDW is the key determinant factor limiting TC in the TMD family. They also show that HC2 is universally enhanced by a factor of two orders of magnitude above the expected value, an effect that remains an open question.