Detecting low energy optical responses of twisted bilayer graphene via millikelvin far-infrared spectroscopy - Part B

Following the identification of optical excitations magic angle twisted bilayer graphene (MATBG) presented in the first part, we deepen our study by applying external magnetic fields. In this talk, we present the modifications of the excitation spectrum of MATBG by magnetic fields. Tuning the Fermi level to the gap of the band insulator (ν = 4) leads to the observations of new excitations developed in magnetic fields associated with Landau level transitions. These results allow us to directly reveal the allowed optical transitions and to extract parameters associated with the moiré bands. When the Fermi level is inside the flat bands, we also identify interesting modifications of low energy resonances in magnetic fields. Our study demonstrates a new detection scheme for examining excitations in flat band materials at millikelvin temperatures.