Quasiparticle

In condensed matter physics, a quasiparticle is a concept used to describe a collective behavior of a group of particles that can be treated as if they were a single particle. Formally, quasiparticles and collective excitations are closely related phenomena that arise when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in vacuum.

For example, as an electron travels through a semiconductor, its motion is disturbed in a complex way by its interactions with other electrons and with atomic nuclei. The electron behaves as though it has a different effective mass travelling unperturbed in vacuum. Such an electron is called an electron quasiparticle.^[1] In another example, the aggregate motion of electrons in the valence band of a semiconductor or a hole band in a metal^[2] behave as though the material instead contained positively charged quasiparticles called electron holes. Other quasiparticles or collective excitations include the phonon, a quasiparticle derived from the vibrations of atoms in a solid, and the plasmons, a particle derived from plasma oscillation.

These phenomena are typically called quasiparticles if they are related to fermions, and called collective excitations if they are related to bosons,^[1] although the precise distinction is not universally agreed upon.^[3] Thus, electrons and electron holes (fermions) are typically called quasiparticles, while phonons and plasmons (bosons) are typically called collective excitations.

The quasiparticle concept is important in condensed matter physics because it can simplify the many-body problem in quantum mechanics. The theory of quasiparticles was started by the Soviet physicist Lev Landau in the 1930s.^[4]^[5]

^ ^a ^b Cite error: The named reference Kaxiras was invoked but never defined (see the help page).
^ Ashcroft; Mermin (1976). Solid State Physics (1st ed.). Holt, Rinehart, and Winston. pp. 299–302. ISBN 978-0030839931.
^ Mattuck, Richard D. (1 January 1992). A Guide to Feynman Diagrams in the Many-body Problem. Courier Corporation. p. 10. ISBN 978-0-486-67047-8. As we have seen, the quasiparticle consists of the original real, individual particle, plus a cloud of disturbed neighbors. It behaves very much like an individual particle, except that it has an effective mass and a lifetime. But there also exist other kinds of fictitious particles in many-body systems, i.e. 'collective excitations'. These do not center around individual particles, but instead involve collective, wavelike motion of all the particles in the system simultaneously.
^ "Ultracold atoms permit direct observation of quasiparticle dynamics". Physics World. 18 March 2021. Retrieved 26 March 2021.
^ Kozhevnikov, A. B. (2004). Stalin's great science : the times and adventures of Soviet physicists. London, England: Imperial College Press. ISBN 1-86094-601-1. OCLC 62416599.

[Kaxiras-1] Cite error: The named reference Kaxiras was invoked but never defined (see the help page).

[ashcroftandmermin-2] Ashcroft; Mermin (1976). Solid State Physics (1st ed.). Holt, Rinehart, and Winston. pp. 299–302. ISBN 978-0030839931.

[Mattuck-3] Mattuck, Richard D. (1 January 1992). A Guide to Feynman Diagrams in the Many-body Problem. Courier Corporation. p. 10. ISBN 978-0-486-67047-8. As we have seen, the quasiparticle consists of the original real, individual particle, plus a cloud of disturbed neighbors. It behaves very much like an individual particle, except that it has an effective mass and a lifetime. But there also exist other kinds of fictitious particles in many-body systems, i.e. 'collective excitations'. These do not center around individual particles, but instead involve collective, wavelike motion of all the particles in the system simultaneously.

[4] "Ultracold atoms permit direct observation of quasiparticle dynamics". Physics World. 18 March 2021. Retrieved 26 March 2021.

[5] Kozhevnikov, A. B. (2004). Stalin's great science : the times and adventures of Soviet physicists. London, England: Imperial College Press. ISBN 1-86094-601-1. OCLC 62416599.

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