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A Novel Mott-Hall Insulator and Spin Liquid Phase in Neutral Graphene in a Magnetic Field


Neutral graphene in a magnetic field exhibits an unexpected insulating  state and a missing quantum Hall plateau. Electrons in graphene in a  magnetic field has 2N_B fold degenerate n = 0 single particle Landau  levels from two valleys and exactly 2N_B spinful electrons. Here N_B  is the number of elementary magnetic flux quanta threading the sample.  We develop a microscopic theory where, at low energy scales, a  complete set of 2N_B single particle Landau-Gaussian orbitals forming  a \texit{Von Neuman lattice} have single electron occupancy and  dangling spins. Certain novel superexchange interactions among spins  destabilise the exchange induced quantum Hall ferromagnetism and  decouples spin from charge and organize spins into a chiral spin  liquid state. Charged excitations have a Mott gap and spin excitations  are either gapless or have a smaller gap. Spin charge separation  removes edge channels for spinful electrons and prevents Hall plateau formation.