On the inapplicability of a negative-phase-velocity condition as a   negative-refraction condition for active materials
  A negative-phase-velocity condition derived by Depine and Lakhtakia [Microwave Opt Technol Lett 41 (2004) 315] for isotropic, homogeneous, passive, dielectric-magnetic materials is inapplicable as a negative-refraction condition for active materials. 
A. Lakhtakia, T.G. Mackay, and J.B. Geddes III