Special AEP Seminar: Hadar Steinberg (Hebrew University of Jerusalem)

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 these materials useful platforms for studying thickness-dependent effects. Specifically, NbSe2 exhibits a reduction in its TC from 7.2K in the bulk to approximately 3K in the single-layer limit. In TaS2, conversely, TC increases from 0.8K in the bulk to approximately 3K 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 ∆, TC, and the upper critical field HC2. Although TaS2 is considered as a dirty superconductor, we find that HC2 is proportional to ∆2, a relation expected for clean superconductors. Even more curiously, we find that the same ratio between HC2 and ∆2 holds for other TMDs: NbSe2 of all thicknesses, and NbS2 and TaSe2, covering 4 orders of magnitude in HC2 and covering both clean and dirty limits.