Transport Study in Graphene with High Frequency Measurements

Abstract

In atomically thin van der Waals layered materials such as graphene, electrons are confined in two dimensions and interact more strongly with each other, giving rise to a rich variety of many-body quantum phenomena. Thermal transport offers a probe of all energy-carrying excitations in these systems. We developed a novel and versatile experimental technique based on radio-frequency Johnson noise thermometry and extended it from a two-terminal to a multi-terminal device geometry. This technique is applicable in a broad range of temperatures, carrier densities, and magnetic fields, enabling measurements of thermal transport in various electronic states regardless of their electrical conductivity and transport regime. Using this technique, we study the electron-phonon interaction in monolayer and bilayer graphene at zero magnetic field. Measurements reveal clear signatures of heat transport from electrons to phonons, with a substantially stronger electron-phonon coupling in bilayer graphene than that in monolayer graphene. We also study thermal transport in quantum Hall ferromagnetism in bilayer graphene at the filling factor $\nu=0$, where competing spin and layer polarizations produce a rich phase diagram of broken-symmetry states. Multi-terminal thermal conductance measurements across magnetic field, electric field, and carrier density reveal clear signatures of phase transitions between insulators with different symmetry breaking. By comparing the measured thermal conductance with the quantized contribution of quantum Hall edge states, thermal transport of gapless magnon excitations is measured, providing a direct probe of magnon heat transport in quantum Hall graphene. Finally, microwave reflectometry technique is developed and used to probe the ac conductivity of quantum Hall insulating states in monolayer graphene. Preliminary experimental results show ac conductivity consistent with recent theoretical predictions based on quantum geometry. Together, these results demonstrate the versatility of radio frequency experimental techniques for probing thermal and electronic properties of two-dimensional materials, especially for accessing collective phenomena that are challenging to resolve through conventional electronic transport.