Low-Temperature Microwave Impedance Microscopy of Topological Materials

Quantum-relativistic materials often host electronic phenomena with exotic spatial distributions. We employ microwave impedance microscopy (MIM), which characterizes the local complex conductivity of a material, to directly visualize chiral edge modes and phase transitions in a quantum anomalous Hall insulator. This near-field imaging technique detects the unique fingerprint of topological edge modes in the GHz regime and spatially disentangles them from trivial states in the bulk of the material. Motivated by these experimental findings, we model the microwave response of topological edge states in a Chern insulator and demonstrate an enhanced MIM response can appear at the crystal boundaries due to collective edge magnetoplasmon excitations. The resonance frequency of these plasmonic modes should depend quantitatively on the topological invariant of the Chern insulator state and on the sample’s circumference, which highlights their non-local, topological nature. Looking forward, we will also discuss progress on the construction of a new milliKelvin MIM, which will support spatially-resolved detection of topological states at low temperatures.