High-Resolution Spectroscopic Studies of Magic-Angle Twisted Bilayer Graphene at Millikelvin Temperatures

Magic-angle twisted bilayer graphene (MATBG) has a complex gate-accessible electronic phase diagram characterized by a variety of exotic insulating, superconducting, and topological phases. Using a homebuilt dilution-fridge scanning tunneling microscope (STM), we probe the spectroscopic properties of MATBG to gain insight into the underlying mechanisms of its phases. At partial filling of the flat bands, consistent with previous studies, our measurements at high temperatures (< 12K) show significant deviations from the single-particle density of states (DOS), indicating the presence of strong electronic interactions [1]. We extend these STM measurements to millikelvin temperatures, where we explore how the “parent phase” develops into correlated insulating and superconducting phases. Our measurements have uncovered a series of gate-dependent energy gaps in the DOS at the Fermi level corresponding to electron fillings at which electrical transport measurements reveal these phases. These spectra, along with their magnetic field and spatial dependencies, provide key information about the microscopic origin of each energy gap and their connection to the various electronic phases in MATBG.

[1] Xie et al. Nature 572, 101 (2019).