Simultaneous negative- and positive-phase-velocity propagation in an   isotropic chiral medium
  The phase velocity of plane waves propagating in an isotropic chiral medium can be simultaneously positive for left-circularly polarized light and negative for right-circularly polarized light(or vice versa). The constitutive parameter regimes supporting this phenomenon are established for nondissipative and dissipative mediums. The boundary between positive and negative phase velocity is characterized by infinite phase velocity. 
Tom G. Mackay (University of Edinburgh), Akhlesh Lakhtakia   (Pennsylvania State University)