Localization of the Helical Edge States in the Absense of External Magnetic Field

TL;DR

This study reports unexpected localization of helical edge states in a 2D topological insulator at millikelvin temperatures without magnetic field, revealing complex field-dependent localization behavior and implications for edge state protection.

Contribution

It provides the first experimental evidence of zero-field localization of helical edge states in HgTe quantum wells at very low temperatures.

Findings

Localization occurs at zero magnetic field at millikelvin temperatures.

Field-dependent localization length follows a power law with magnetic field.

Luttinger liquid parameters inferred from localization behavior.

Abstract

Theoretically, the helical edge states of two-dimensional topological insulators are protected from coherent backscattering due to nonmagnetic disorder provided electron interactions are not too strong. Experimentally, the edges typically do not demonstrate the systematic and robust quantization, at the same time little is known about the sub-Kelvin temperature behavior. Here, we report the surprising localization of the edge states in an 8 nm HgTe quantum well in zero magnetic field at millikelvin temperatures. Additionally, the magnetoresistance data at 0.5 K for the edges few micrometers long suggests the field-dependent localization length $l_B\propto B^{-\alpha}$, with $\alpha$ ranging approximately from $1.6$ to $2.8$ at fields $B\lesssim0.1\,\text{T}$ and $\alpha\approx1.1$ at higher fields up to $0.5\,\text{T}$. In the frame of disordered interacting edge, these values of $\alpha$ correspond to the Luttinger liquid parameters $K\approx 0.9-1.1$ and $K\approx 0.6$, respectively. We discuss possible scenarios which could result in the zero magnetic field localization.