Tracing out Correlated Chern Insulators in Magic Angle Twisted Bilayer Graphene
Abstract
Magicangle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electronelectron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when $ \pm 1, \pm 2, \pm 3$ electrons occupy each moir\' e unit cell and lead to the formation of correlated insulating, superconducting and ferromagnetic phases. While some phases have been shown to carry a nonzero Chern number, the local microscopic properties and topological character of many other phases remain elusive. Here we introduce a set of novel techniques hinging on scanning tunneling microscopy (STM) to map out topological phases in MATBG that emerge in finite magnetic field. By following the evolution of the local density of states (LDOS) at the Fermi level with electrostatic doping and magnetic field, we visualize a local Landau fan diagram that enables us to directly assign Chern numbers to all observed phases. We uncover the existence of six topological phases emanating from integer fillings in finite fields and whose origin relates to a cascade of symmetrybreaking transitions driven by correlations. The spatially resolved and electrondensitytuned LDOS maps further reveal that these topological phases can form only in a small range of twist angles around the magicangle value. Both the microscopic origin and extreme sensitivity to twist angle differentiate these topological phases from the Landau levels observed near charge neutrality. Moreover, we observe that even the chargeneutrality Landau spectrum taken at low fields is considerably modified by interactions and exhibits an unexpected splitting between zero Landau levels that can be as large as ${\sim }\,35$ meV. Our results show how strong electronic interactions affect the band structure of MATBG and lead to the formation of correlationenabled topological phases.
 Publication:

arXiv eprints
 Pub Date:
 August 2020
 DOI:
 10.48550/arXiv.2008.11746
 arXiv:
 arXiv:2008.11746
 Bibcode:
 2020arXiv200811746C
 Keywords:

 Condensed Matter  Strongly Correlated Electrons;
 Condensed Matter  Mesoscale and Nanoscale Physics