New Electronic States in Graphene Heterostructures
A few years ago, researchers at Columbia University and the University of Manchester developed methods for producing layered structures of graphene, insulating hexagonal boron nitride (hBN), and other "exfoliatable" materials. The idea of stacking layers of these different materials opens the door to a new class of heterostructures with novel capabilities, perhaps eventually comparable to those that have developed with MBE grown semiconductor heterostructures. Recently, we have found two remarkable new phenomena arising in graphene stacked on top of hBN. First, transport and capacitance measurements show the development of a band-gap in these systems whose energy width depends on the relative angular orientation of the graphene on the hBN. The hBN breaks the sub-lattice symmetry in the graphene, allowing for the development of a band-gap. Along with this band-gap, we observe features in magnetic field that match predictions of the long sought "Hofstadter Butterfly" for electrons in a periodic potential in high magnetic fields. Secondly, we find that in strong magnetic fields applied parallel to the graphene plane, we can create a novel "quantum spin Hall effect" similar to that seen in topological insulator materials. The talk will also describe our efforts of using novel tunneling method, using an exfoliated and stacked tunnel barrier, for probing the density of states in graphene with very high resolution and precision.