Talk:Quasiparticle

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Is phonon a quasiparticle, or is it something different? Samohyl Jan 08:56, 5 Apr 2005 (UTC)

It's a "collective mode" excitation, not a quasiparticle. -- CYD

It's not as simple as CYD makes it sound. In some cases one can make a clear distinction between quasiparticles and collective modes on the basis of their quantum numbers. For example, electrons (charge -1, spin 1/2) are clearly different from plasmons (charge 0, spin 0). An electron is a quasiparticle similar to a free electron, whereas a plasmon is a collective mode involving oscillations of density in an electron gas. In other cases collective excitations and quasiparticles are not that different and one can, in fact, go continuously from one to the other. A well-known example is a weakly interacting Bose gas. Its excitations are phonons at the lowest energies and momenta. But at higher energies and momenta these excitations are indistinguishable from free particles. Here quasiparticles and collective modes form a single branch of excitations and can be "deformed" gradually into each other. -- Oleg Tchernyshyov 02:57, 27 January 2006 (UTC)

It can be even worse: sometimes the same object can be viewed as a quasiparticle or as a collective excitation. For instance, a magnon in a ferromagnet can be considered in one of two perfectly equivalent ways: (a) as a mobile defect (a misdirected spin) in a perfect alignment of magnetic moments or (b) as a quantized spin wave that involves the precession of many spins. In the first case the magnon is treated as a quasiparticle, in the second as a collective excitation. I stress that both descriptions are equivalent to each other. -- Oleg Tchernyshyov 03:25, 27 January 2006 (UTC)