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

Quantum materials associated with correlated energy gaps may show unusual electronic, magnetic and optical properties, whose careful examinations can help reveal the nature of the underlying phases. The optical responses in the far infrared (FIR) regime of interesting moiré materials including twisted bilayer graphene, remain largely unexplored due to experimental challenges in performing FIR spectroscopy on small samples and low temperatures. In this talk, we explore the characterization of low energy optical excitations in twisted bilayer graphene, using our recently developed milli-kelvin FIR spectroscopy instrument. We report such measurements and analyses at electron temperatures that are below the superconducting transition temperatures of the magic angle graphene.