Bistable superlattice switching in a quantum spin Hall insulator.

Bistable switching typically arises from ferroic orders, such as ferroelectricity and ferromagnetism, in which the bistable states are encoded in charge or spin degrees of freedom. Here we report the observation of bistable superlattice switching in monolayer TaIrTe, a dual quantum spin Hall insulator. Switching occurs between two lattice configurations with sharply contrasting periodicities. In particular, in a pristine monolayer, we observe the spontaneous emergence of a long-period superlattice that can be programmed on and off in a non-volatile manner by electrostatic tuning of low-energy electronic states. This switching toggles the system between two structural configurations with unit cell areas differing by two orders of magnitude. Mechanistically, our results reveal two independent and distinct instabilities, one in the lattice and the other in the quantum spin Hall electrons. These instabilities are coupled, leading to electrostatic control of lattice configurations with non-volatile memory. This finding is enabled by combining linear and nonlinear transport measurements, Raman spectroscopy and scanning tunnelling microscopy, which probe complementary aspects of the underlying orders. Notably, this non-volatile memory stabilizes a spontaneous superlattice with a periodicity on the few-nanometre scale that remains robust across a wide doping range, persists over days and survives above 70 K. Our preliminary data also show the emergence of new insulating states at fractional superlattice fillings, which can be switched on and off together with the superlattice.