Negative phase velocity in isotropic dielectric-magnetic mediums via   homogenization
  We report on a strategy for achieving negative phase velocity (NPV) in a homogenized composite medium (HCM) conceptualized using the Bruggeman formalism. The constituent material phases of the HCM do not support NPV propagation. The HCM and its constituent phases are isotropic dielectric-magnetic mediums; the real parts of their permittivities/permeabilities are negative-valued whereas the real parts of their permeabilities/permittivities are positive-valued. 
Tom G. Mackay (University of Edinburgh), Akhlesh Lakhtakia   (Pennsylvania State University)