Probing Spin Helical Surface States in Topological HgTe Nanowires

TL;DR

This study demonstrates that high-mobility strained HgTe nanowires host topological helical surface states with phase-coherent transport over several micrometers, confirmed by conductance oscillations and numerical modeling, advancing topological quantum device research.

Contribution

It provides experimental evidence and theoretical analysis showing that HgTe nanowires preserve topological surface states with phase coherence, even under inhomogeneous gating, highlighting their potential for topological quantum applications.

Findings

Phase-coherent surface states extend up to 5 μm.

Conductance oscillations show h/e periodicity.

Topological surface states persist despite inhomogeneous gating.

Abstract

Nanowires with helical surface states represent key prerequisites for observing and exploiting phase-coherent topological conductance phenomena, such as spin-momentum locked quantum transport or topological superconductivity. We demonstrate in a joint experimental and theoretical study that gated nanowires fabricated from high-mobility strained HgTe, known as a bulk topological insulator, indeed preserve the topological nature of the surface states, that moreover extend phase-coherently across the entire wire geometry. The phase-coherence lengths are enhanced up to 5 $\mu$m when tuning the wires into the bulk gap, so as to single out topological transport. The nanowires exhibit distinct conductance oscillations, both as a function of the flux due to an axial magnetic field, and of a gate voltage. The observed $h/e$-periodic Aharonov-Bohm-type modulations indicate surface-mediated quasi-ballistic transport. Furthermore, an in-depth analysis of the scaling of the observed gate-dependent conductance oscillations reveals the topological nature of these surface states. To this end we combined numerical tight-binding calculations of the quantum magneto-conductance with simulations of the electrostatics, accounting for the gate-induced inhomogenous charge carrier densities around the wires. We find that helical transport prevails even for strongly inhomogenous gating and is governed by flux-sensitive high-angular momentum surface states that extend around the entire wire circumference.