$\pi$ Spin Berry Phase in a Quantum-Spin-Hall-Insulator-Based Interferometer: Evidence for the Helical Spin Texture of the Edge States

TL;DR

This paper demonstrates the detection of a $$ Berry phase in a quantum spin Hall insulator, providing direct evidence for the helical spin texture of edge states through interference experiments involving Rashba spin-orbit coupling.

Contribution

It introduces a method to observe the geometric  Berry phase in edge states, confirming their helical spin texture via conductance phase shifts controlled by spin-orbit coupling.

Findings

Detection of a  Berry phase via conductance oscillations.

Evidence for the helical spin texture of edge states.

Control of spin rotation using Rashba spin-orbit coupling.

Abstract

Quantum spin Hall insulator is characterized by the helical edge states, with the spin polarization of electron being locked to its direction of motion. Although the edge-state conduction has been observed, unambiguous evidence of the helical spin texture is still lacking. Here, we investigate the coherent edge-state transport in an interference loop pinched by two point contacts. Due to the helical character, the forward inter-edge scattering enforces a $\pi$ spin rotation. Two successive processes can only produce a nontrivial $2\pi$ or trivial $0$ spin rotation, which can be controlled by the Rashba spin-orbit coupling. The nontrivial spin rotation results in a geometric $\pi$ Berry phase, which can be detected by a $\pi$ phase shift of the conductance oscillation relative to the trivial case. Our results provide a smoking gun evidence for the helical spin texture of the edge states. Moreover, it also provides the opportunity to all-electrically explore the trajectory-dependent spin Berry phase in condensed matter.