Monday, 1 June 2020

electromagnetism - Physical cause of Negative Permittivity


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=1ω2Pω2+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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