38 research outputs found
Superlattice structures in twisted bilayers of folded graphene
The electronic properties of bilayer graphene strongly depend on relative
orientation of the two atomic lattices. Whereas Bernal-stacked graphene is most
commonly studied, a rotational mismatch between layers opens up a whole new
field of rich physics, especially at small interlayer twist. Here we report on
magnetotransport measurements on twisted graphene bilayers, prepared by folding
of single layers. These reveal a strong dependence on the twist angle, which
can be estimated by means of sample geometry. At small rotation, superlattices
with a wavelength in the order of 10 nm arise and are observed by friction
atomic force microscopy. Magnetotransport measurements in this small-angle
regime show the formation of satellite Landau fans. These are attributed to
additional Dirac singularities in the band structure and discussed with respect
to the wide range of interlayer coupling models
Replica Higher-Order Topology of Hofstadter Butterflies in Twisted Bilayer Graphene
The Hofstadter energy spectrum of twisted bilayer graphene is found to have
recursive higher-order topological properties. We demonstrate that higher-order
topological insulator (HOTI) phases, characterized by localized corner states,
occur as replicas of the original HOTIs to fulfill the self-similarity of the
Hofstadter spectrum. We show the existence of the exact flux translational
symmetry of twisted bilayer graphene at all commensurate angles. Based on this
result, we carefully identify that the original HOTI phase at zero flux is
re-entrant at a half-flux periodicity, where the effective twofold rotation is
preserved. In addition, numerous replicas of the original HOTIs are found for
fluxes without protecting symmetries. Similar to the original HOTIs, replica
HOTIs feature both localized corner states and edge-localized real-space
topological markers. The replica HOTIs originate from the different interaction
scales, namely, intralayer and interlayer couplings, in twisted bilayer
graphene. The topological aspect of Hofstadter butterflies revealed in our
results highlights symmetry-protected topology in quantum fractals.Comment: 6 pages, 4 figures + Supplemental Materia
Correlation-driven topological phases in magic-angle twisted bilayer graphene
Magic-angle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electron–electron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when ±1, ±2 and ±3 electrons occupy each moiré unit cell, and lead to the formation of various correlated phases. Although some phases have been shown to have a non-zero Chern number, the local microscopic properties and topological character of many other phases have not yet been determined. Here we introduce a set of techniques that use scanning tunnelling microscopy to map the topological phases that emerge in MATBG in a finite magnetic field. By following the evolution of the local density of states at the Fermi level with electrostatic doping and magnetic field, we create a local Landau fan diagram that enables us to assign Chern numbers directly to all observed phases. We uncover the existence of six topological phases that arise from integer fillings in finite fields and that originate from a cascade of symmetry-breaking transitions driven by correlations. These topological phases can form only for a small range of twist angles around the magic angle, which further differentiates them from the Landau levels observed near charge neutrality. Moreover, we observe that even the charge-neutrality Landau spectrum taken at low fields is considerably modified by interactions, exhibits prominent electron–hole asymmetry, and features an unexpectedly large splitting between zero Landau levels (about 3 to 5 millielectronvolts). Our results show how strong electronic interactions affect the MATBG band structure and lead to correlation-enabled topological phases