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_c 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 TaS₂ heterostructures. The upper critical field H_c2 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_c2. Using Gor'kov's theory, we calculate the suppression of T_c by the competing charge density wave (CDW) order, which affects the quasiparticle density of states and the resulting T_c and H_c2. The latter is found to be universally enhanced by two orders of magnitude. Our results substantiate CDW as the key determinant factor limiting T_c across the TMD family.