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

In metallic transition metal dichalcogenides (TMDs), which remain superconducting down to single-layer thickness, the critical temperature T decreases for Nb-based, and increases for Ta-based materials. This contradicting trend is puzzling, impeding the development of a unified theory. Here we study the thickness-evolution of superconducting tunneling spectra in TaSheterostructures. The upper critical field H is strongly enhanced towards the single-layer limit - following . The same ratio holds for the entire family of intrinsically metallic 2H-TMDs, covering 4 orders of magnitude in H. Using Gor'kov's theory, we calculate the suppression of T by the competing charge density wave (CDW) order, which affects the quasiparticle density of states and the resulting T and H. The latter is found to be universally enhanced by two orders of magnitude. Our results substantiate CDW as the key determinant factor limiting T across the TMD family.