Graphene, with its linear band dispersion at low energy and massless Dirac-like fermions, allows the Quantum Hall effect (QHE) to be observed at room temperature under a strong magnetic field[1]. In the QHE, electrons will travel in quantized cyclotron orbits with discrete energy levels called Landau levels (LLs). Yet, the massless Dirac-like behaviour of graphene provides a potential avenue to realise LLs without the need for strong magnetic fields as it allows electronic band structure modification by lattice deformation. If a non-uniform distortion is applied to the graphene lattice it shifts the graphene Dirac cones at K and K’ in two opposite directions and thus generates a pseudo magnetic field (PMF)[2]. Strain induced pseudo magnetic fields offer the possibility of generating zero magnetic field Quantum Hall effect in graphene, possibly up to room temperature. Strain engineering on graphene is usually achieved via random nanobubbles or artificial nanostructures on the substrate[3,4]. Heterostructure engineering offers an alternative approach, by stacking graphene on top of another van der Waals material with large lattice mismatch at a desired twist angle it is possible to generate large strain induced pseudo magnetic fields over the entire heterostructure[5].
In this study, we used nano-angle resolved photoemission spectroscopy (nano-ARPES) to probe the electronic bandstructure of a graphene – black phosphorus heterostructure. ARPES is a momentum and energy resolved technique that has proven to be a powerful tool in directly studying the electronic bandstructure of 2D materials and heterostructures. By directly measuring the iso-energy contours of graphene and black phosphorus we determine our heterostructure has a twist angle of 20-degrees. This twist angle, together with the large lattice mismatch between graphene and black phosphorus creates a shear strained superlattice which gives rise to a periodic PMF across the entire heterostructure. High-resolution nano-ARPES of the graphene bands near the Fermi level reveals a large increase in the Fermi velocity and the emergence of flat bands located within the Dirac cone. The spacing of the flat bands is consistent with Landau level formation in graphene, and corresponds to a pseudo-field of 10 T.
Our work provides a new way to study quantum Hall phases induced by strain in 2D materials and heterostructures.
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About the presenter
Emily Vu is an affiliate PhD student at Monash University with AI Mark Edmonds and CI Michael Fuhrer.