Extended Data Fig. 8: Ruling out a first-order transition.
From: Unconventional superconducting quantum criticality in monolayer WTe2
a, Rxx as a function of ng, swept back and forth, at base temperature. The green and orange curves correspond to decreasing and increasing ng, respectively. b, Nernst signal as a function of ng measured at 5 mT. The dual-heater power is 128 pW. No hysteresis in the sweeps, the characteristic of a first-order transition, is found for both resistance and Nernst signal. Note that data in a and b were taken from a new cooldown after about a year (stored in an Ar-filled glovebox) after the first cooldown, in which we took most of the data presented in the manuscript. The device and data remain of high quality with negligible changes. c, Rxx normalized to its value at 900 mK as a function of ng (red) or T (blue), along the two dashed lines shown in d. The data is extracted from Fig. 1b. Rxx(ng) and Rxx(T) display an excellent overlap under linear scaling which suggests that the two transitions share the same characteristic. Started from the same superconducting state, the T-induced transition is a continuous BKT transition in nature, whereas the ng-induced transition is also induced by the proliferation of vortices and antivortices (driven by quantum fluctuations). d, Nernst signal as a function of ng and T, taken at B = 2 mT and Ph = 46 pW (the same data from Fig. 4a). The ng-induced QPT manifests as the zero-temperature limit of the continuous BKT transition. All these observations speak against a first-order transition and demonstrate that indeed the ng-induced 2D superconducting transition is BKT-like, in terms of how the superconducting state is destroyed (the proliferation of vortices and antivortices). The sudden disappearance of the Nernst signal below the critical density (Fig. 2 & 3) is unexpected, raising intriguing questions regarding the behaviors of the vortices near the QPT.
