Imaging electronic states in van der Waals heterostructures

The ability to create arbitrary stacking configurations of layered two-dimensional materials has opened the way to the creation of designer band structures.  Twisted bilayer graphene and graphene on hexagonal boron nitride (hBN) are two of the simplest examples of such a van der Waals heterostructure where the electronic properties of the composite material can be fundamentally different from either individual material.   These van der Waals heterostructures can be formed using a wide variety of layered materials including transition metal dichalcogenides, graphene and topological insulators.  This talk will mostly focus on creating topologically protected states in graphene devices by breaking inversion symmetry.  The lattice mismatch and twist angle between layers in the heterostructure produces a moiré pattern which affects its electronic properties.  For graphene on hBN, the moiré pattern creates a new set of superlattice Dirac points.  In small twist angle bilayer graphene, the combination of a long-wavelength moiré pattern and an electric field leads to the formation of an array of topologically protected states on the domain walls of the moiré pattern.  In this talk, I will discuss our fabrication of these heterostructures and measurements using scanning probe microscopy.