Unconventional Superconducting Quantum Criticality in Monolayer WTe2

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Abstract
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. The presence of an anomalous metal state near the transition in various systems further enriches the field and challenges existing theories. Using the Nernst effect, 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 very large in the millikelvin regime, with a coefficient of ~ 4,100 uV/KT at zero magnetic field, an indication of the proliferation of vortices. However, the Nernst signal abruptly disappears when the doping is decreased below the gate-tuned QPT, in striking conflict with expectations for a conventional QPT. Our findings establish a 2D model platform for studying unconventional superconducting QPTs and quantum critical matter.

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