Armchair graphene nanoribbons: PT-symmetry breaking and exceptional   points without dissipation
  We consider a single layer graphene nanoribbon with armchair edges in a longitudinally constant external potential and point out that its transport properties can be described by means of an effective non-Hermitian Hamiltonian. We show that this system has some features typical of dissipative systems, namely the presence of exceptional points and of PT-symmetry breaking, although it is not dissipative. 
Maurizio Fagotti, Claudio Bonati, Demetrio Logoteta, Paolo Marconcini,   Massimo Macucci
cond-mat.mes-hall math-ph math.MP quant-ph