title: Randomness-induced XY ordering in a graphene quantum hall ferromagnet.
abstract: Valley-polarized quantum Hall states in graphene are described by a Heisenberg O(3) ferromagnet model, with the ordering type controlled by the strength and the sign of the valley anisotropy. A mechanism resulting from electron coupling to the strain-induced gauge field, giving a leading contribution to the anisotropy, is described in terms of an effective random magnetic field aligned with the ferromagnet z axis. We argue that such a random field stabilizes the XY ferromagnet state, which is a coherent equal-weight mixture of the K and K' valley states. The implications such as the Berezinskii-Kosterlitz-Thouless ordering transition and topological defects with half-integer charge are discussed.
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Dmitry A
authors.initials: DA
authors.lastname: Abanin
authors.forename: Patrick A
authors.initials: PA
authors.lastname: Lee
authors.forename: Leonid S
authors.initials: LS
authors.lastname: Levitov