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Event Number 3239

  Thursday, February 13th, 2014

Faculty Candidate Seminar
Engineering New Electronic States in Graphene Heterostructures: Massive Dirac Fermions, Hofstadter’s Butterfly and the Quantum Spin Hall Effect
Time: 4:00 pm
Place: 4274 Chamberlin Hall
Speaker: Benjamin Hunt, MIT
Abstract: Van der Waals heterostructures represent a new and surprising direction in nanoscale device engineering: we stack isolated two-dimensional crystals to create layered structures with atomic precision. The layer-by-layer assembly allows us to introduce a new design parameter - the interlayer twist angle - which can have profound consequences for the engineering of electronic states based on tunable interactions between adjacent layers. In this talk, I will discuss recent experiments at MIT in which we have used a hexagonal boron nitride (hBN) layer to modify the electronic bands of monolayer graphene in a van der Waals heterostructure, inducing a sizable bandgap at the charge neutrality point and imparting a mass to the normally massless Dirac charge carriers. The bandgap occurs only in samples in which the twist angle between the graphene and hBN crystals is small, resulting in a long-wavelength moirA(c) that acts as a superlattice potential; by adjusting the twist angle the bandgap can be tuned. The moirA(c) superlattice potential also allows us to study the problem of a charged particle in a periodic potential and magnetic field aEuro" the so-called Hofstadter problem aEuro" whose theoretical solution exhibits a rare instance of fractal behavior in a quantum-mechanical energy spectrum.

I will also discuss our recent studies of weakly-coupled graphene-hBN heterostructures in which massless Dirac fermions in graphene exhibit a quantum spin Hall effect, a fascinating example of a aEurooesymmetry-protected topological phaseaEuro of which the more familiar contemporary examples are the edge and surface states of the topological insulators.
Host: Mark Eriksson/Sridhara Dasu
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