In the previous chapter we saw that in the long-wavelength limit kℓ ≪ 1 (ℓ = υ f τ) one has to distinguish the low-frequency (ωτ ≪ 1) and high-frequency (ωτ ≫1) limits. In the first case one has local equilibrium with a hydrodynamic behaviour (diffusion in the case of electrons). In the second case one can neglect collisions. Nevertheless one has collective excitations, the so-called plasma vibrations. They occur as a consequence of the self-consistent Hartree field.
KeywordsAttenuation Helium Compressibility
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