Giant spin rotation in the normal metal/quantum spin Hall junction

TL;DR

This paper theoretically investigates spin rotation at the interface between a normal metal and a quantum spin Hall insulator, revealing large spin rotations near the quantum critical point due to helical edge mode resonances, with potential experimental implications.

Contribution

It introduces a detailed theoretical analysis of spin rotation in N/QSH junctions, highlighting the resonance effects of helical edge modes and their impact on spin dynamics.

Findings

Spin rotation angle can reach up to π near the quantum critical point.

Resonant enhancement of spin rotation due to helical edge modes.

Potential for mapping edge mode energy dispersion through spin rotation measurements.

Abstract

We study theoretically reflection problem in the junction between a normal metal and an insulator characterized by a parameter $M$, which is a usual insulator for $M>0$ or a quantum spin Hall system for $M<0$. The spin rotation angle $\alpha$ at the reflection is obtained in the plane of $M$ and the incident angle $\theta$ measured from the normal to the interface. The $\alpha$ shows rich structures around the quantum critical point M=0 and $\theta=0$, i.e., $\alpha$ can be as large as $\sim \pi$ at an incident angle in the quatum spin Hall case $M<0$ because the helical edge modes resonantly enhance the spin rotation, which can be used to map the energy dispersion of the helical edge modes. As an experimentally relevant system, we also study spin rotation effect in quantum spin Hall/normal metal/quantum spin Hall trilayer junction.