Infrared Spectroscopy\nfor Diagnosing Superlattice\nMinibands in Twisted Bilayer Graphene near the Magic Angle

Twisted bilayer graphene (TBG) represents a highly tunable,\nstrongly\ncorrelated electron system. However, understanding the single-particle\nband structure alone has been challenging due to a lack of spectroscopic\nmeasurements over a broad energy range. Here, we probe the band structure\nof TBG around the magic angle using infrared spectroscopy and reveal\nspectral features that originate from interband transitions. In combination\nwith quantum transport, we connect spectral features over a broad\nenergy range (10–700 meV) and track their evolution with the\ntwist angle. We compare our data with calculations of the band structures\nobtained via the continuum model and find good agreement only when\nconsidering a variation of interlayer/intralayer tunneling parameters\nwith the twist angle. Our analysis suggests that the magic angle also\nshifts due to lattice relaxation and is better defined for a wide\nangular range of 0.9–1.1°. Additionally, our measurements\noffer an optical fingerprint of the magic angle for screening heterostructures\nbefore nanofabrication.