title: Topological insulators and nematic phases from spontaneous symmetry breaking in 2D fermi systems with a quadratic band crossing.
abstract: We investigate the stability of a quadratic band-crossing point (QBCP) in 2D fermionic systems. At the noninteracting level, we show that a QBCP exists and is topologically stable for a Berry flux +/-2pi if the point symmetry group has either fourfold or sixfold rotational symmetries. This putative topologically stable free-fermion QBCP is marginally unstable to arbitrarily weak short-range repulsive interactions. We consider both spinless and spin-1/2 fermions. Four possible ordered states result: a quantum anomalous Hall phase, a quantum spin Hall phase, a nematic phase, and a nematic-spin-nematic phase.
authors: Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801-3080, USA.
authors: Kai
authors: K
authors: Sun
authors: Hong
authors: H
authors: Yao
authors: Eduardo
authors: E
authors: Fradkin
authors: Steven A
authors: SA
authors: Kivelson