Topologically Protected Zero Modes in Twisted Bilayer Graphene
  We show that the twisted graphene bilayer can reveal unusual topological properties at low energies, as a consequence of a Dirac-point splitting. These features rely on a symmetry analysis of the electron hopping between the two layers of graphene and we derive a simplified effective low-energy Hamiltonian which captures the essential topological properties of twisted bilayer graphene. The corresponding Landau levels peculiarly reveal a degenerate zero-energy mode which cannot be lifted by strong magnetic fields. 
R. de Gail, M. O. Goerbig, F. Guinea, G. Montambaux, A. H. Castro Neto
cond-mat.mes-hall