Experimental evidence of the topological obstruction in twisted bilayer graphene

The rich physics of magic angle twisted bilayer graphene (TBG) results from the Coulomb interactions of electrons in flat bands of non-trivial topology. While the bands’ dispersion is well characterized, accessing their topology remains an experimental challenge. Recent measurements established the local density of states (LDOS) as a topological observable. Here, we use scanning tunnelling microscopy to investigate the LDOS of TBG near a defect. We observe characteristic patterns resulting from the Dirac cones having the same chirality within a moiré valley. At higher energies, we observe the Lifshitz transition associated with the Dirac cones mixing. Our measurements provide a full characterization of TBG’s band structure, confirming the main features of the continuum model including the renormalization of the Fermi velocity, the role of emergent symmetries and the topological obstruction of the wavefunctions. Each valley of the mini-Brillouin zone ("mini valley") of twisted bilayer graphene (TBG) contains two Dirac cones that hybridize to form flat bands. Theory predicts that these two Dirac cones have the same chirality, leading to topological obstruction. Here, the authors confirm this prediction experimentally.