Advancing crystal quality and quantum phenomena in thin film WTe2 (Part 2)

Monolayer WTe₂ has been found to host a number of electronic phases including a topologically nontrivial correlated insulator state below 100 K and superconductivity when electrostatically doped. Bilayers show a similar but less insulating state that is topologically trivial (has no edge conduction), no signs of superconductivity, and sliding ferroelectricity. The recent development of horizontal flux growth for WTe₂ yields crystals with an order of magnitude higher mobility than before, indicating a substantially reduced defect density. In monolayers of these improved crystals, we find that the insulator-metal transition occurs at lower doping and Shubnikov-de Haas oscillations are visible in the metallic regime for both electron and hole doping, allowing analysis of the Fermi surface for the first time. Also, the critical temperature and magnetic field in the electron-doped superconducting state are enhanced. In bilayers, the insulating state near neutrality is more robust than in the lower mobility crystals, and more closely resembles the state found in the monolayer with absence of edge conduction. We discuss the implications of these findings for the nature of the insulating and superconducting states.