Condensed Matter Seminar Series

Composite Fermions and Broken Symmetries in Graphene

Francois Amet

Stanford University

Thursday April 24,  11:30 am,  Room 298,  Physics Building

Abstract: The electronic properties of graphene are well described by a non-interacting Dirac Hamiltonian with a fourfold symmetry associated with spin and valley, an additional degree of freedom due to the hexagonal crystal lattice of graphene. As a result, graphene exhibits a variety of peculiar phenomena such as an anomalous quantum Hall effect. At high magnetic fields, the electron kinetic energy is quenched by the Landau quantization, and Coulomb interactions become the dominant energy scale of the system. This results in a variety of new electronic phases, whose ground states depend on the competition between symmetry breaking interactions.
      Observing these so-called fractional quantum Hall (FQH) phases is challenging in graphene because potential fluctuations induced by disorder blur out transport signatures of FQH states. I will describe fabrication techniques to overcome these difficulties and obtain devices with carrier mobility exceeding one million cm2/Vs in FET or bipolar geometries. This quality allows for the observation of a plethora of fractional quantum Hall phases at filling factors following the composite fermion theory. The sequence of fractions, as well as the magnetic field dependence of their activation gaps, informs us about the spin and valley polarization of the ground state in each phase.