  The superconductor to insulator or metal transition in two dimensions (2D)
provides a valuable platform for studying continuous quantum phase transitions
(QPTs) and critical phenomena. Distinct theoretical models, including both
fermionic and bosonic localization scenarios, have been developed, but many
questions remain unsettled despite decades of research. Extending Nernst
experiments down to millikelvin temperatures, we uncover anomalous quantum
fluctuations and identify an unconventional superconducting quantum critical
point (QCP) in a gate-tuned excitonic quantum spin Hall insulator (QSHI), the
monolayer tungsten ditelluride (WTe2). The observed vortex Nernst effect
reveals singular superconducting fluctuations in the resistive normal state
induced by magnetic fields or temperature, even well above the transition. Near
the doping-induced QCP, the Nernst signal driven by quantum fluctuations is
exceptionally large in the millikelvin regime, with a coefficient of ~ 4,100
uV/KT at zero magnetic field, an indication of the proliferation of vortices.
Surprisingly, the Nernst signal abruptly disappears when the doping falls below
the critical value, in striking conflict with conventional expectations. This
series of phenomena, which have no prior analogue, call for careful
examinations of the mechanism of the QCP, including the possibility of a
continuous QPT between two distinct ordered phases in the monolayer. Our
experiments open a new avenue for studying unconventional QPTs and quantum
critical matter.
