Direct probe of valley and orbital order in bilayer graphene

Direct probe of valley and orbital order in bilayer graphene
Andrea Young, Dept of Physics, UC Santa Barbara
Date and time: Thu, Apr 21, 2016 - 11:30am
Refreshments at 11:15am
Location: LGRT 1033
Category: Condensed Matter Seminar
Abstract:
Two dimensional electronic systems combine massive degeneracy with strong interactions, hosting a multitude of broken symmetry and topologically ordered ground states. In the bilayer graphene zero energy Landau level (ZLL), the degeneracy of cyclotron orbits is augmented by spin, valley, and orbital quantum numbers, whose intricate interplay contributes generates a complex hierarchy of gapped states but complicates accurate theoretical treatment. Here we introduce the first direct experimental probe of an order parameter by using a layer-resolved capacitive measurement technique to directly map both valley and orbital polarization throughout the ZLL. We observe a panoply of phase transitions, which allow us to put tight constraints on a theoretical model that accounts for both Coulomb interactions as well as band structure anisostropies and the Zeeman effect, all of which play comparable roles in symmetry breaking. Direct visualization of phase transitions across a wide range of density, moreover, allows us to directly quantify the correlation energy in states with different orbital character, providing a new window into the energetics of strongly correlated low dimensional systems. Our technique is readily extendable to other strongly coupled van der Waals bilayers as well as to the fractional quantum Hall and Wigner crystal regimes.