Van-der-Waals epitaxy and proximity-induced superconductivity in the atomically thin quantum spin Hall insulator 1T'-WTe2

• Main Author: Ho, Duc Quan
• Other Authors: Bent Weber
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Physics::Atomic physics::Solid state physics
• Online Access: https://hdl.handle.net/10356/161898
• Institution: Nanyang Technological University

Ever since the emergence and increased theoretical understanding of topological states of matter, the interplay of superconductivity and topology has become an intensely investigated subject in condensed matter research. An intrinsically normal conducting material can inherit superconductivity from a parent superconductor via the proximity effect, usually understood as Andreev reflection at the interface between the distinct electronic structures of two separate conductors. However, at high interface transparency, strong coupling inevitably leads to changes in the band structure locally, owing to hybridization of electronic states across the heterointerface. In this thesis, we investigate heterostructures of the 2D topological (quantum spin Hall) insulator 1T’-WTe2 by van-der-Waals (vdW) epitaxy, comparing normal and superconducting vdW platforms. Epitaxy of 1T’-WTe2 on HOPG and NbSe2 reveals high-quality crystals on clean substrates, offering atomically sharp vdW interfaces. We confirm a quantum spin Hall (QSH) gap of ∼ 70 meV and metallic boundary states in both heterostructures, but find that strong interface hybridization in WTe2/NbSe2 weakens these topological signatures. The superconducting local density of states (LDOS), resolved in scanning tunneling spectroscopy down to 500 mK, reflects a hybrid electronic structure, well-described by a multi-band framework based on the McMillan equations. Our extended three-band McMillan model captures multi-band superconductivity inherent to the NbSe2 substrate and that induced by proximity in WTe2, self-consistently, and confirms that strong inter-layer coupling gives rise to a semi-metallic density of states in the 2D WTe2 bulk, even for nominally band-insulating crystals. Our model allows us to extract the induced order parameter ∆ ≃ 0.6 meV in the WTe2 monolayer bulk, stable beyond a 2 T magnetic field. We believe that our detailed multi-band analysis of the hybrid electronic structure provides a useful tool for sensitive spatial mapping of induced order parameters in proximitized atomically thin topological materials, and thus may encourage further experimental and theoretical studies of topological quantum devices in vdW QSH heterostructures.