Spectroscopy of magic-angle twisted bilayer graphene via planar tunneling

Magic-angle twisted bilayer graphene (MATBG) exhibits a variety of quantum phases, including correlated insulating states and superconductivity, that were revealed through transport, STM/STS and inverse compressibility measurements. Here we report on results obtained with an alternative technique — planar tunneling - in which the graphene sample and a graphite probe are separated by a few-layer hBN sheet acting as a tunneling barrier. The planar tunneling junction size is adjustable and can cover length scales down to several moire periods, that are not accessible to either transport or STM devices. The ultra-thin hBN tunneling layer guarantees highly stable and non-invasive tunneling measurements. By using the planar tunneling geometry on MATBG, we observed spectroscopic features of flat bands, a cascade of phase transitions at integer fillings, the emergence of Chern insulators, and Landau levels near the charge neutrality point. The ability of the planar tunneling geometry to combine access to both the density of states and transport signals in a single device, provides a new handle for gaining insight into the physics of the interplay between strong correlations and non-trivial topology responsible for the emergence of novel correlated quantum states in MATBG.