  The capability to switch electrically between superconducting and insulating
states of matter represents a novel paradigm in the state-of-the-art
engineering of correlated electronic systems. An exciting possibility is to
turn on superconductivity in a topologically non-trivial insulator, which
provides a route to search for non-Abelian topological states. However,
existing demonstrations of superconductor-insulator switches have involved only
topologically trivial systems, and even those are rare due to the stringent
requirement to tune the carrier density over a wide range. Here we report
reversible, in-situ electrostatic on off switching of superconductivity in a
recently established quantum spin Hall insulator, namely monolayer tungsten
ditelluride (WTe2). Fabricated into a van der Waals field effect transistor,
the monolayer's ground state can be continuously gate-tuned from the
topological insulating to the superconducting state, with critical temperatures
Tc up to ~ 1 Kelvin. The critical density for the onset of superconductivity is
estimated to be ~ 5 x 10^12 cm^-2, among the lowest for two-dimensional (2D)
superconductors. Our results establish monolayer WTe2 as a material platform
for engineering novel superconducting nanodevices and topological phases of
matter.
