What is the physical cause behind a material having a negative real part of its dielectric function? Given the complex permittivity, ϵ(ω)=ϵ(ω)′+iϵ(ω)″, the Drude model gives ϵ′=1−ω2Pω2+ω2τ
where ω is the frequency of the incoming light, ωP=√Ne2mϵ0 is the plasma frequency, N is the electron density, m is the electron's mass, e is the electronic charge, and ωτ is the frequency of collisions between conduction electrons and the ion lattice.
If ω is small enough, then ϵ′<0. But how does this physically happen?
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