Observation of the Quantum Spin Hall Effect up to 100 Kelvin in a Monolayer Crystal (1711.03584v1)

Abstract: The field of topological insulators (TI) was sparked by the prediction of the quantum spin Hall effect (QSHE) in time reversal invariant systems, such as spin-orbit coupled monolayer graphene. Ever since, a variety of monolayer crystals have been proposed as two-dimensional (2D) TIs exhibiting the QSHE, possibly even at high temperatures. However, conclusive evidence for a monolayer QSHE is still lacking, and systems based on semiconductor heterostructures operate at temperatures close to liquid helium. Here we report the observation of the QSHE in monolayer WTe2 at temperatures up to 100 Kelvin. The monolayer exhibits the haLLMark quantized transport conductance, ~ e2/h per edge, in the short edge limit. Moreover, a magnetic field suppresses the conductance, and the observed Zeeman-type gap indicates the existence of a Kramers degenerate point, demonstrating the importance of time reversal symmetry for protection from elastic backscattering. Our results establish the high-temperature QSHE and open a new realm for the discovery of topological phases based on 2D crystals.

• The paper demonstrates quantized edge conductance of e²/h in monolayer WTe₂ at up to 100 K, confirming the high-temperature QSHE in 2D materials.
• The authors employ hexagonal boron nitride encapsulation to fabricate devices with atomic flatness and stability, ensuring accurate conductance measurements.
• Magneto-conductance experiments reveal the role of time-reversal symmetry in protecting helical edge modes, highlighting the impact of magnetic fields on QSHE.

Quantum Spin Hall Effect in Monolayer WTe₂ at Elevated Temperatures

The paper "Observation of the Quantum Spin Hall Effect up to 100. Kelvin in a Monolayer Crystal" presents a significant advancement in the field of two-dimensional (2D) topological insulators (TIs). This paper primarily focuses on the demonstration of the quantum spin Hall effect (QSHE) in monolayer tungsten ditelluride (WTe₂), a monolayer transition metal dichalcogenide (TMD), at remarkably high temperatures reaching up to 100 Kelvin. This temperature range significantly surpasses the previously established low-temperature domain essential for observing QSHE in traditional semiconductor heterostructures.

Key Findings

1. QSHE Observation: The research effectively demonstrates the haLLMark characteristic of the QSHE, which is the quantized transport conductance of $e^2/h$ per edge at elevated temperatures, an observation confirmed through the fabrication and analysis of monolayer WTe₂ devices.
2. Device Fabrication Strategies: To achieve this, devices were fabricated with key considerations to maintain atomic flatness and chemical stability, features achieved through the encapsulation of the TMD flake in hexagonal boron nitride. The strategic design ensures minimized contact resistance and facilitates a significant length-dependence paper on the conductance, which is crucial to validate the QSHE’s intrinsic nature through saturation experiments at short edge limits.
3. Magnetic Field Influence: Additional magneto-conductance measurements confirmed the Zeeman-type energy gap, evidencing the significance of time-reversal (TR) symmetry in protecting against backscattering and thereby supporting the observed helical edge modes. A noteworthy decrease in conductance when a perpendicular magnetic field is applied suggests the breaking of such TR symmetry, further underpinning the QSHE occurrence.
4. Temperature Resilience: The edge conductance dominance is upheld up to 100 K, with only minor disruptions observed due to increased bulk conduction at even higher temperatures. This aspect is particularly compelling as it opens pathways toward leveraging QSHE in practical applications such as electronic devices operating above liquid nitrogen temperatures.

Implications and Future Directions

The implications of this work extend broadly within the field of condensed matter physics and device engineering, mainly because of the demonstration of a substantial bandgap in 2D TMD monolayers that supports high-temperature QSHE. Future explorations could delve into understanding the interaction between such topological phases with superconductors and magnets in novel van der Waals heterostructures. Additionally, engineering TMD-based devices with improved quality might stretch the observable QSHE to temperatures beyond the current 100 K limit, enhancing their utility for quantum information processing and spintronic applications.

Conclusion

In conclusion, this paper robustly illustrates the presence of QSHE in monolayer WTe₂ at high temperatures and sets the stage for further experimental and theoretical pursuits to uncover and manipulate such exotic phases in atomically thin materials. The work propels the understanding of topological insulators in 2D systems and anticipates the potential of incorporating these materials into advanced technological infrastructures.