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R. G. Herb Condensed Matter Seminars

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Events on Thursday, January 26th, 2017

Observation of new fractional quantum Hall states in graphene heterostructures
Time: 10:00 am
Place: 5310 Chamberlin hall
Speaker: Andrea Young, University of California Santa Barbara
Abstract: I will describe recent experiments probing the many body ground states of a new generation of ultra-clean graphene heterostructures. By replacing conventional gate metallization based on deposition of amorphous films with single crystal graphite flakes, we find that the electronic quality of graphene heterostructures can be substantially improved, rivaling that of long-studied semiconductor quantum wells. I will discuss two recent results that have emerged from these ultra-clean electronic devices. In the first part of the talk, I will describe the fate of the half filled Landau level in bilayer graphene, where different orbital Landau levels host composite fermion liquid (for N=0) and a gapped Pfaffian quantum Hall phase (for N=1). By controllably moving occupation between these two orbitals, we find that a new phase emerges, characterized by charge gap but large inter-orbital polarization. Numerical modeling suggests that the resulting phase hosts a Fermi surface of neutral excitons, and I will discuss experimental schemes to directly probe these unusual particles.

In the second part of the talk, I will describe the effects of a superlattice on the nature of the fractional quantum Hall effect. Recent theoretical work, largely motivated by efforts to engineer fractional quantum Hall states in optical lattice systems, has suggested that new kinds of fractional quantum Hall states--termed fractional Chern insulators--can exist in lattice systems with intrinsically finite bandwidth. At high magnetic field in our devices, a substrate-induced moire superlattice gives rise to a variety of Hofstadter bands with different Chern numbers. We find a wide variety of incompressible states at fractional filling of these bands characterized by fractionally quantized Hall conductance. These results demonstrate that fractional Chern insulators are indeed a generic phenomenon. Going forward, we anticipate being able to place approximate limits on the required interaction strength and bandwidth required to realize these phases.
Host: Levchenko
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