A Spectroscopic Study of Electronic Correlations in Twisted Bilayer Graphene by Scanning Tunneling Microscopy

Twisted bilayer graphene around the magic angle has shown variety of correlated phases such as superconductivity, correlated insulators, and magnetism due to its flat band structure. The unconventional nature of the superconductivity and its pos- sible relation to high temperature superconductors have sparked a lot of theoretical and experimental efforts to understand the properties of the magic angle twisted bilayer graphene. While electrical transport measurements revealed the interesting phases, spectroscopic understanding is strongly needed to connect the phases with theoretical calculations. We present the spectroscopic studies of gate-tunable magic angle twisted bilayer graphene using scanning tunneling microscopy. We report that the band structure is significantly modified even at charge neutrality due to exchange interaction. We apply a perpendicular magnetic field and develop a novel method that enables scanning tunneling microscopy to reveal Landau fan diagrams. We discover topologically non-trivial states appearing at finite magnetic field, and from spectroscopy we are able to identify the mechanism. Finally, we verify inter- action driven band flattening experimentally in twisted bilayer graphene, which is responsible for creating strong correlations.