Unconventional superconducting quantum criticality in monolayer WTe₂

The transitions from a superconductor to a resistive state in two dimensions provide a valuable platform for studying continuous quantum phase transitions and critical phenomena. Here we uncover anomalous quantum fluctuations and identify an unconventional superconducting quantum critical point in a gate-tuned excitonic quantum spin Hall insulator, the monolayer tungsten ditelluride. We do this by extending Nernst experiments down to millikelvin temperatures. The vortex Nernst effect that we observe reveals singular superconducting fluctuations in the resistive normal state induced by magnetic fields or temperature, even well above the transition. Near the doping-induced quantum critical point, the Nernst signal driven by quantum fluctuations is large in the millikelvin regime, an indication of the proliferation of vortices. Unexpectedly, the Nernst signal abruptly disappears when the doping falls below the critical value, in contrast with conventional expectations. This series of phenomena calls for careful examination of the mechanism of the quantum critical point in the monolayer.