On mediums with negative phase velocity: a brief overview
  Several issues relating to oppositely directed phase velocity and power flow are reviewed. A necessary condition for the occurrence of this phenomenon in isotropic dielectric-magnetic mediums is presented. Ramifications for aberration-free lenses, homogenization approaches, and complex mediums are discussed. 
Akhlesh Lakhtakia, Martin W. McCall, Werner S. Weiglhofer, Jaline   Gerardin, and Jianwei Wang