BGU Physics Department

Colloquium, June 20th, 2013

Tailoring the properties of low dimensional electronic systems in van der Waals heterostructures

Andrea Yang, MIT
Graphene and related atomically thin layered materials can be controllably stacked layer by layer to create van der Waals heterostructures with properties that are difficult or impossible to achieve in single, isolated layers. In the first part of the talk I will show how the electronic structure of graphene, in which the charge carriers are described by the two dimensional massless Dirac equation, can be modified by placing it in close contact with a rotationally aligned hexagonal boron nitride substrate. The staggered potential arising from the substrate breaks the sublattice symmetry in graphene, turning it into a high mobility semiconductor with massive charge carriers. At the same time, the interplay between the two slightly mismatched lattices generates a long wavelength moiré pattern, whose wavelength is such more than one magnetic flux quantum can be threaded through a single superlattice unit cell. In this regime, we observe the transport signatures of the Hofsta dter butterfly—the fractal energy spectrum of a quantum particle moving under the simultaneous influence of a periodic potential and magnetic field. In the second part, I will show how to generate symmetry protected topological edge states in graphene. The Landau levels spectrum of graphene is distinguished from that in conventional semiconductors by honeycomb lattice structure, which leads to fourfold spin- and valley- degenerate LLs hosting a variety of isospin ferromagnetic states. By careful heterostructure design, we can experimentally tune the energetic balance between these states, culminating in the realization of a quantum spin Hall effect, the hallmark of a two dimensional topological insulator, in charge neutral graphene at very high magnetic fields. Unlike the topological insulators based on time reversal symmetry, the QSH state presented here is protected by a spin-rotation symmetry that emerges as spins are polarized by a large in-plane magnetic field. The properties of the resulting helical edge states can be modulated by controllably breaking the spin rotation symmetry by balancing the in-plane field wi th an intrinsic antiferromagnetic coupling in the graphene. In the resulting canted antiferromagnetic state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with tunable band gap and associated spin-texture.
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