Nonlinear Conductance of Monolayer WTe₂

Monolayer WTe₂ exhibits edge conduction consistent with most of the expected properties of a 2D topological insulator, including a strong and anisotropic response to a magnetic field explained by the expected spin-momentum locking in the edge states. Although at temperatures above about 10 K the edge conductance is roughly temperature independent, as expected from single-particle theory, at lower temperatures it eventually freezes out in linear response. This behavior presents a challenge to theories of conduction in a helical quantum wire. To provide more input for theory, we have made measurements of the nonlinear conductance of monolayer WTe₂ down to 50 mK in a vector magnetic field. We find that the differential conductance rises quickly to close to the higher temperature value above a particular level of voltage bias which varies strongly with gate voltage, and the magnetic field anisotropy of the threshold bias is similar to that of the linear-response conductance, suggesting that some kind of disorder- and spin-related gap controls the conductance at low temperatures.