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 Tc 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 Hc2 is strongly enhanced towards the single-layer limit – following $${H}_{c2}\propto {T}_{c}^{2}$$ . The same ratio holds for the entire family of intrinsically metallic 2H-TMDs, covering 4 orders of magnitude in Hc2. Using Gor'kov's theory, we calculate the suppression of Tc by the competing charge density wave (CDW) order, which affects the quasiparticle density of states and the resulting Tc and Hc2. The latter is found to be universally enhanced by two orders of magnitude. Our results substantiate CDW as the key determinant factor limiting Tc across the TMD family. By mapping the superconducting gap, the critical field, and temperature in the metallic H-TMD family of superconductors, the authors find a universal relationship that holds over two orders of magnitude of the critical temperature across materials.