Plasmons are about to be brought up in the lecture material, but I stumbled across them in some reading of the Jellium model. They are a quasi-particle brought about by oscillations of the free-electron density against the positive ions in a metal.
An interesting consequence of plasmons is the optical properties of a metal. The frequency of the density oscillation is known as the plasma frequency. When light above the plasma frequency is incident on a metal it is transmitted, whereas light below this frequency is reflected due to the collective screening behavior of the electrons.
This optical effect was first predicted in the drude model, as a result of maxwells equations and also dependent on the density of electrons. This is the first mention of it since then. It's interesting how this effect can be described well in both models given the differentces between them.
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