Are There Negative Conductivities/Resistivities?

[CanIExplore]

Are there such things as negative conductivities and/or resistivities in the real world? If so, what would these materials' electric characteristics be? This question is coming from my reading in an EM textbook where the EM wave equations are solved for a source-free, lossy medium (finite conductivity).

In the dispersion equation,

$\gamma$=$\pm$$\sqrt{j\omega\mu(\sigma+j\omega\epsilon)}$

the textbook states that the conductivity $\sigma$ can be allowed to take on positive or negative values but they don't explain what a negative conductivity would mean.

Thanks!

 

[UltrafastPED]

Yes, but then it doesn't obey Ohm's law.

For details see http://en.wikipedia.org/wiki/Negative_resistance