Universality of Upper Critical Field in the TMD Superconductor Family

In transition metal dichalcogenides (TMDs) such as H-NbSe₂ an H-TaS₂ , superconducting properties are retained down to a single layer, making them useful platforms for studying thickness-dependent effects. Specifically, NbSe₂ exhibits a reduction in its T_C from 7.2 K in the bulk to approximately 3 K in the single-layer limit. In TaS₂ , conversely, T_C increases from 0.8 K in the bulk to approximately 3 K in the single layer limit. This contradicting behavior, which long puzzled researchers, could be related to a thickness-dependent suppression of superconductivity by the competing charge density wave (CDW) phase. I will present measurements of device-based high-resolution tunneling spectra in TaS₂ , where we track the gap structure from the bulk all the way to a single layer. Our devices allow for simultaneous evaluation of the gap ∆, T_C , and the upper critical field H_C2 . Although TaS₂ is considered as a dirty superconductor, we find that H_C2 is proportional to ∆ ₂ , a relation expected for clean superconductors. Even more curiously, we find that the same ratio between H_C2 and ∆₂ holds for other TMDs: NbSe₂ of all thicknesses, and bulk TaSe₂ ,  covering 4 orders of magnitude in H_C2 and covering both clean and dirty limits.