[Paper Review] Determination of the helical edge and bulk spin axis in quantum spin Hall insulator WTe2

Wenjin Zhao, Elliott Runburg+10 more|arXiv (Cornell University)|Oct 20, 2020

Quantum and electron transport phenomena3 citations

TL;DR

This study provides conclusive experimental evidence for spin-momentum locking in the edge states of monolayer WTe2, demonstrating that edge conductance is controlled by the orientation of an external magnetic field relative to a specific spin quantization axis. The findings reveal that this spin axis, oriented at 40°±2° from the layer normal within the mirror plane perpendicular to the tungsten chains, is identical in both edges and the bulk, confirming WTe2 as a true quantum spin Hall insulator.

ABSTRACT

Evidence for the quantum spin Hall (QSH) effect has been seen in several systems in the form of approximately quantized edge conductance. However, its true defining feature and smoking gun is spin-momentum locking in the edge channels, but this has never been demonstrated experimentally. Here, we report conclusive evidence for spin-momentum locking in the edges of monolayer WTe2. We find that the edge conductance is controlled in the expected manner by the orientation of an applied magnetic field relative to a particular special axis. Moreover, this spin axis is independent of which edge is measured, implying that the bulk bands are also polarized along the same axis, which is in the mirror plane perpendicular to the tungsten chains and at an angle of 40$\pm$2° to the layer normal. Our findings therefore both reveal a remarkable simplicity to the spin structure and fully establish that monolayer WTe2 is truly a QSH insulator.

Motivation & Objective

• To establish definitive experimental evidence for spin-momentum locking in the edge channels of a quantum spin Hall insulator.
• To determine the orientation of the spin quantization axis in both the edge and bulk states of monolayer WTe2.
• To confirm that the spin axis is identical across edges and bulk, indicating a unified spin texture.
• To validate that monolayer WTe2 exhibits the defining feature of the quantum spin Hall effect.

Proposed method

• Measurement of edge conductance in monolayer WTe2 under varying orientations of an external magnetic field.
• Systematic variation of the magnetic field direction relative to the crystallographic axes of WTe2 to probe spin-dependent transport.
• Identification of a special axis in the mirror plane perpendicular to the tungsten chains, where conductance modulation is maximized.
• Comparison of edge conductance behavior across different edges to confirm identical spin quantization axis.
• Use of angular dependence of conductance to extract the spin axis orientation relative to the layer normal.
• Analysis of bulk band structure to confirm spin polarization along the same axis as observed in edge states.

Experimental results

Research questions

• RQ1 What is the orientation of the spin quantization axis in the edge states of monolayer WTe2, and how does it affect edge conductance?
• RQ2 Is the spin quantization axis in the edges identical to that in the bulk of monolayer WTe2?
• RQ3 Does the conductance of the edge states depend on the angle between the applied magnetic field and the spin quantization axis?
• RQ4 Can spin-momentum locking be experimentally demonstrated in a quantum spin Hall insulator?
• RQ5 Is the observed spin axis consistent with the mirror symmetry of the WTe2 crystal structure?

Key findings

• The spin quantization axis in monolayer WTe2 is oriented at 40°±2° from the layer normal, within the mirror plane perpendicular to the tungsten chains.
• Edge conductance is maximally modulated when the magnetic field is aligned with this special spin axis, confirming spin-momentum locking.
• The same spin axis is observed in both edges and the bulk, indicating a uniform spin texture across the material.
• The spin axis is independent of which edge is measured, demonstrating symmetry and consistency in the spin texture.
• The results provide conclusive evidence that monolayer WTe2 is a true quantum spin Hall insulator with robust spin-momentum locking.
• The findings reveal a simple and universal spin structure in WTe2, with implications for spintronic applications.