Magnetic Brightening and Nanoscale Imaging of Spin-Polarized Helical Edge Modes

TL;DR

This study visualizes and characterizes spin-polarized helical edge modes in topological materials using cryogenic magneto-infrared microscopy, revealing magnetic field effects and layer-dependent responses relevant for nanoelectronics.

Contribution

It introduces a novel nanoscale imaging technique to observe magnetic field-induced edge states and their layer-dependent electrodynamic responses in topological materials.

Findings

Magnetic field induces near-field conductivity at step edges.

Infrared edge response scales with atomic layer number.

Magnetic gaps do not disrupt single-layer edge states at ~100 meV.

Abstract

Efficient sub-10 nm electric transport remains a major challenge for nanoelectronics due to high losses and impedance mismatches in conventional Drude metals. Despite their promise of dissipationless, reflection-free conduction, topologically protected chiral edge modes remain little explored in their nanoscale spin polarized transport-particularly regarding real-space visualization, magnetic field tunability, and high-frequency edge conductivity. Here, we report magnetic brightening and nanoscale visualization of highly spin-polarizable infrared helical edge states using cryogenic magneto-infrared scattering-type scanning near-field optical microscopy (cm-IR-sSNOM). Our measurements reveal magnetic field-induced near-field conductivity at step edges, uncovering quantum spin Hall spin-splitting modes with enhanced infrared polarizability and slightly narrowed near-field profiles. In addition, the infrared edge electrodynamic response scales nearly linearly with atomic layer number, providing compelling evidence that magnetic-field-induced gaps do not disrupt individual-layer edge states at energies of around 100 meV. These results sharply contrast with microwave and DC transport, where even small magnetically induced gaps decrease edge conduction. Magnetically tunable, topologically robust high-frequency edge modes open a pathway toward ultralow-loss nanoscale interconnects and quantum logic architectures for next-generation microelectronics, spintronics and quantum information science.