[Paper Review] Unconventional Superconducting Quantum Criticality in Monolayer WTe2

Tiancheng Song, Yanyu Jia+13 more|arXiv (Cornell University)|Mar 12, 2023

Quantum and electron transport phenomenaPhysics and Astronomy3 citations

TL;DR

This study identifies an unconventional superconducting quantum critical point (QCP) in monolayer WTe2 using ultra-low-temperature Nernst measurements. It reveals anomalously large quantum fluctuations and vortex proliferation in the normal state above the transition, with a Nernst coefficient of ~4,100 μV/KT at zero field, disappearing abruptly below the critical doping—challenging conventional QPT models and suggesting a novel continuous transition between distinct ordered phases.

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 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.

Motivation & Objective

• To investigate quantum phase transitions in two-dimensional superconductors, particularly near a superconductor-insulator or superconductor-metal transition.
• To probe the nature of quantum criticality in monolayer WTe2, a gate-tuned excitonic quantum spin Hall insulator.
• To determine whether the observed quantum fluctuations indicate a continuous quantum phase transition between distinct ordered phases.
• To challenge conventional theoretical models of quantum criticality by identifying non-universal behavior in a 2D system.

Proposed method

• Conducting ultra-low-temperature (millikelvin) Nernst effect measurements on exfoliated monolayer WTe2 devices.
• Applying gate voltage to tune electron doping and drive the system through a quantum critical point.
• Measuring the Nernst signal in the resistive normal state above the superconducting transition temperature.
• Analyzing the Nernst coefficient to quantify quantum fluctuations and vortex proliferation in the absence of long-range order.
• Comparing the behavior of the Nernst signal across doping levels, especially near the critical doping where it abruptly vanishes.
• Using magnetic field and temperature sweeps to isolate the contribution of quantum fluctuations from classical effects.

Experimental results

Research questions

• RQ1 What is the nature of the quantum critical point in monolayer WTe2, and does it conform to established fermionic or bosonic localization scenarios?
• RQ2 Why does the Nernst signal—indicative of quantum fluctuations—disappear abruptly below the critical doping, contrary to conventional expectations?
• RQ3 Can the observed large Nernst coefficient (~4,100 μV/KT) at zero field be attributed to vortex proliferation in the normal state?
• RQ4 Does the system exhibit a continuous quantum phase transition between two distinct ordered phases, as suggested by the anomalous behavior?
• RQ5 What mechanisms underlie the unconventional superconducting quantum criticality observed in this 2D system?

Key findings

• The Nernst signal in monolayer WTe2 reaches an exceptionally large value of ~4,100 μV/KT at zero magnetic field in the millikelvin regime, indicating strong quantum fluctuations.
• These fluctuations persist in the resistive normal state well above the superconducting transition temperature, signaling the presence of preformed Cooper pairs or vortex-like excitations.
• The Nernst signal abruptly vanishes when doping falls below the critical value, a behavior inconsistent with conventional quantum critical models.
• The observed phenomena suggest a continuous quantum phase transition between two distinct ordered phases, challenging existing theoretical frameworks.
• The results point to a novel form of unconventional superconducting quantum criticality not explained by standard fermionic or bosonic localization theories.
• The findings open a new pathway for studying quantum critical matter in two-dimensional systems with strong electron correlations.