Electrically Tunable Low Density Superconductivity in a Monolayer Topological Insulator (1809.04637v1)

Abstract: The capability to switch electrically between superconducting and insulating states of matter represents a novel paradigm in the state-of-the-art engineering of correlated electronic systems. An exciting possibility is to turn on superconductivity in a topologically non-trivial insulator, which provides a route to search for non-Abelian topological states. However, existing demonstrations of superconductor-insulator switches have involved only topologically trivial systems, and even those are rare due to the stringent requirement to tune the carrier density over a wide range. Here we report reversible, in-situ electrostatic on off switching of superconductivity in a recently established quantum spin Hall insulator, namely monolayer tungsten ditelluride (WTe2). Fabricated into a van der Waals field effect transistor, the monolayer's ground state can be continuously gate-tuned from the topological insulating to the superconducting state, with critical temperatures Tc up to ~ 1 Kelvin. The critical density for the onset of superconductivity is estimated to be ~ 5 x 10^12 cm^-2, among the lowest for two-dimensional (2D) superconductors. Our results establish monolayer WTe2 as a material platform for engineering novel superconducting nanodevices and topological phases of matter.

• The paper establishes a superconductor-insulator transition in monolayer WTe₂ at a critical electron density of approximately 5×10¹² cm⁻², confirming its low-density superconducting behavior.
• The study shows that superconductivity is reversible via gate voltage with critical temperatures near 1 K, indicative of a Berezinskii-Kosterlitz-Thouless transition in a 2D system.
• The authors reveal that in-plane magnetic fields exceed the Pauli limit by fourfold due to spin-orbit scattering, emphasizing the material's potential for topological superconducting applications.

Electrically Tunable Low-Density Superconductivity in a Monolayer Topological Insulator

The paper "Electrically Tunable Low-Density Superconductivity in a Monolayer Topological Insulator" explores novel superconductivity phenomena in monolayer tungsten ditelluride (WTe₂), a quantum spin Hall insulator. This monolayer transition metal dichalcogenide exhibits electrically tunable superconductivity at remarkably low charge carrier densities, a feature promising for applications in the development of superconducting nanodevices and the paper of unique quantum phenomena such as non-Abelian topological states.

Key Findings and Analysis

The authors present several critical insights into the tunable superconducting nature of monolayer WTe₂:

1. Superconductor-Insulator Transition: The paper demonstrates the field effect transistor-like ability of monolayer WTe₂ to switch between insulating and superconducting states. The critical electron density at which superconductivity emerges is estimated to be approximately $5 \times 10^{12} \text{cm}^{-2}$, notably low among 2D superconductors.
2. Temperature and Carrier Density Dependence: Superconductivity in WTe₂ was fully tunable by gate voltage, exhibiting a reversible transition with critical temperatures reaching up to around 1 K in some devices. This temperature-dependent transition aligns with the properties of a Berezinskii-Kosterlitz-Thouless (BKT) transition, typical of 2D systems.
3. Magnetic Field Effects: The paper reports on the response of superconductivity to magnetic fields, revealing significant anisotropy. The in-plane critical field exceeds the Pauli paramagnetic limit by fourfold, attributed to factors such as spin-orbit scattering—a topic warranting further exploration due to its implications in unconventional superconductivity.
4. Interplay with Topological Features: The coexistence of superconductivity and the quantum spin Hall effect within the same monolayer suggests WTe₂ as an effective platform for exploring interactions between superconductivity and topology. This includes the potential for engineering superconducting edge states via the proximity effect, relevant for Majorana mode studies.

Implications and Future Prospects

The findings underscore the potential for monolayer WTe₂ to function as a hub for exploring and fine-tuning superconducting and topological phases in 2D systems. The combination of gate-tunability, low-density superconductivity, and nontrivial topological characteristics in a single-material platform presents favorable conditions for developing advanced quantum devices and enhancing our understanding of topologically protected states in superconducting materials.

Future work could capitalize on this material's unique properties by optimizing the electronic phase space, exploring potential superconducting domes, and investigating effects arising from WTe₂'s inherent structural anisotropy. Additionally, further scrutiny of superconducting edge states in WTe₂ may pave the way for innovations in topological quantum computation.

In summary, the research provides an essential step toward harnessing low-dimensional superconductors for complex quantum environments and applications. Monolayer WTe₂ emerges as a distinguished material with promise for novel device architectures that marry superconductivity with topological phases.