BEGIN:VCALENDAR VERSION:2.0 CALSCALE:GREGORIAN PRODID:UW-Madison-Physics-Events BEGIN:VEVENT SEQUENCE:0 UID:UW-Physics-Event-4352 DTSTART:20170126T160000Z DURATION:PT1H0M0S DTSTAMP:20260313T150057Z LAST-MODIFIED:20170119T133132Z LOCATION:5310 Chamberlin hall SUMMARY:Observation of new fractional quantum Hall states in graphene heterostructures\, R. G. Herb Condensed Matter Seminar\, Andrea Young\ , University of California Santa Barbara DESCRIPTION:I will describe recent experiments probing the many body g round states of a new generation of ultra-clean graphene heterostructu res. 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 substantia lly improved\, rivaling that of long-studied semiconductor quantum wel ls. I will discuss two recent results that have emerged from these ul tra-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 cont rollably moving occupation between these two orbitals\, we find that a new phase emerges\, characterized by charge gap but large inter-orbit al polarization. Numerical modeling suggests that the resulting phase hosts a Fermi surface of neutral excitons\, and I will discuss experi mental schemes to directly probe these unusual particles.
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\nIn the second part of the talk\, I will describe the effects of a s uperlattice on the nature of the fractional quantum Hall effect. Rece nt theoretical work\, largely motivated by efforts to engineer fractio nal quantum Hall states in optical lattice systems\, has suggested tha t new kinds of fractional quantum Hall states--termed fractional Chern insulators--can exist in lattice systems with intrinsically finite ba ndwidth. At high magnetic field in our devices\, a substrate-induced moire superlattice gives rise to a variety of Hofstadter bands with di fferent Chern numbers. We find a wide variety of incompressible state s at fractional filling of these bands characterized by fractionally q uantized 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 inte raction strength and bandwidth required to realize these phases. URL:https://www.physics.wisc.edu/events/?id=4352 END:VEVENT END:VCALENDAR