Superconductivity in high-mobility monolayer WTe₂

The monolayer excitonic insulator candidate WTe₂ undergoes a transition to a gate-tunable superconducting state when electron-doped above a low threshold density. It has been speculated that the superconductivity is unconventional, topological, and/or influenced by excitons. We have recently employed a horizontal-flux crystal growth technique that produces WTe₂ crystals with several times higher mobility than before, exhibiting record magnetoresistance. In the highest quality samples, we find that the threshold doping is as low as 10¹² cm^-2 and the superconductivity shows a dome with a maximum T_c approaching 1.8 K. However, in other samples the threshold doping is higher and T_c several times lower. This strong sensitivity to disorder is indicative of non-s-wave pairing. Nevertheless, and although the Fermi surface is small, the cleanest samples appear to be in the weak coupling regime, and the dependence of the coherence length on  is consistent with BCS theory in the clean limit.