Microwave reflectometry for van der Waals 2D materials and heterostructures (Part-3): Measurement of the dielectric response of graphene quantum Hall insulators

Microwave conductivity measurements can provide deep insights into the nature of quantum Hall insulators including information about the disorder landscape, the quantum geometry of Landau level wavefunctions, and the Wigner crystallization of electrons. Here, we use a microwave resonator method to measure the real and imaginary parts of the microwave conductance of graphene quantum Hall insulators in a Corbino device geometry. We fabricate the Corbino device with capacitively coupled electrodes to a monolayer of graphene encapsulated by hBN crystals. The real part of the measured AC conductivity shows a well-developed insulating bulk of quantum Hall states tuned by density and magnetic fields. The imaginary part of the AC conductivity, tracked by the resonance frequency shift in the reflectometry measurement, probes the dielectric response of the quantum Hall insulator. We present experiments for several device geometries and discuss the behavior of the complex conductivity over a range of Landau level filling factors, magnetic fields, and sample temperature.