Realization of Quantum Spin Hall State in Monolayer 1T'-WTe2 (1703.03151v1)

Abstract: A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin-orbit coupling. By investigating electronic structure of epitaxially grown monolayer 1T'-WTe2 using angle-resolved photoemission (ARPES) and first principle calculations, we observe clear signatures of the topological band inversion and the band gap opening, which are the haLLMarks of a QSH state. Scanning tunneling microscopy measurements further confirm the correct crystal structure and the existence of a bulk band gap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe2 as a new class of QSH insulator with large band gap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).

• The paper confirms the QSH state by detecting band inversion and a 55 meV gap using high-resolution ARPES measurements.
• Material quality was verified through meticulous molecular beam epitaxy on bilayer graphene and corroborated by diffraction and core level photoemission data.
• STM analyses and first-principles calculations together revealed conductive edge states, underscoring potential applications in spintronic devices.

Realization of Quantum Spin Hall State in Monolayer 1T'-WTe

This paper presents a comprehensive experimental paper on the realization of the Quantum Spin Hall (QSH) state in a monolayer of 1T'-WTe, a transition metal dichalcogenide (TMDC). By employing advanced techniques such as angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM), the paper offers substantive evidence supporting the manifestation of a QSH phase in monolayer 1T'-WTe.

Key Findings and Methodology

• QSH State Confirmation: The paper successfully observes the signatures characteristic of the QSH state, most notably the band inversion and the opening of a sizable band gap, through detailed ARPES measurements. The measured band gap is notably significant at approximately 55 meV, far surpassing the gaps found in existing quantum well structures in three-dimensional semiconductors.
• Material Characterization: The monolayer 1T'-WTe was grown using molecular beam epitaxy on a bilayer graphene substrate, ensuring high-quality samples for investigation. The reflected high-energy electron diffraction patterns and core level photoemission spectra confirm the expected crystal structure, supporting the accurate realization of the 1T' phase.
• First-Principles Calculation: Theoretical calculations using first-principles methods reinforce the experimental findings. The juxtaposition of theoretical and experimental data demonstrates consistency, especially in terms of band inversion and band gap formation due to strong spin-orbit coupling (SOC).
• Edge State Identification: STM measurements provided further validation by revealing the differential electronic structure at the edges compared to the bulk, indicative of conductive edge states—a haLLMark of non-trivial topology in QSH insulators.

Implications and Future Directions

The successful realization of the QSH state in 1T'-WTe adds a significant candidate to the existing family of 2D QSH insulators. This achievement is pivotal, particularly because 2D materials like TMDCs provide robust, surface/interface complication-free platforms necessary for advancing QSH studies and applications. The findings suggest potential applications in spintronic devices, given the time-reversal symmetry-protected edge states that could facilitate dissipationless transport.

Furthermore, the robustness and inert nature of TMDCs not only make them advantageous for experimental studies but also enable them to serve as a viable platform for constructing van der Waals heterostructures and multi-layered devices. Such flexibility may lead to further exploration into the interplays between QSH states and other emergent phenomena like superconductivity and topological superconductors.

Conclusion

This paper provides compelling experimental evidence supporting the emergence of the QSH state in monolayer 1T'-WTe, establishing it as a superior candidate among 2D QSH insulators due to its large band gap and robust material properties. As TMDCs continue to be a focus for material science and condensed matter physics, the insights gained from this paper will likely spur future research endeavors and technological advancements leveraging the unique properties of QSH insulators.