Editing Quasiparticle (section)

{{Short description|Concept in condensed matter physics}}
{{Use dmy dates|date=October 2020}}

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 [[particle]]s 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 nucleus|atomic nuclei]]. The electron behaves as though it has a different [[effective mass (solid-state physics)|effective mass]] travelling unperturbed in vacuum. Such an electron is called an ''electron quasiparticle''.<ref name=Kaxiras/> In another example, the aggregate motion of electrons in the [[valence band]] of a [[semiconductor]] or a hole band in a metal<ref name="ashcroftandmermin">{{cite book |last1=Ashcroft |url=https://archive.org/details/solidstatephysic00ashc/page/299 |title=Solid State Physics |last2=Mermin |date=1976 |publisher=Holt, Rinehart, and Winston |isbn=978-0030839931 |edition=1st |pages=[https://archive.org/details/solidstatephysic00ashc/page/299 299–302] |language=en |url-access=registration}}</ref> behave as though the material instead contained positively charged quasiparticles called ''[[electron hole]]s''. Other quasiparticles or collective excitations include the ''[[phonon]]'', a quasiparticle derived from the vibrations of atoms in a solid, and the ''[[plasmon]]'', a particle derived from [[plasma oscillation]].

These phenomena are typically called ''quasiparticles'' if they are related to [[fermion]]s, and called ''collective excitations'' if they are related to [[boson]]s,<ref name=Kaxiras/> although the precise distinction is not universally agreed upon.<ref name="Mattuck">{{Cite book |last=Mattuck |first=Richard D. |url={{google books|plainurl=y|id=pe-v8zfxE68C|page=10}}|page=10|title=A Guide to Feynman Diagrams in the Many-body Problem |date=1992-01-01 |publisher=Courier Corporation |isbn=978-0-486-67047-8 |language=en|quote=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.}}</ref> 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.<ref>{{Cite web|date=2021-03-18|title=Ultracold atoms permit direct observation of quasiparticle dynamics|url=https://physicsworld.com/ultracold-atoms-permit-direct-observation-of-quasiparticle-dynamics/|access-date=2021-03-26|website=Physics World|language=en-GB}}</ref><ref>{{Cite book |last=Kozhevnikov |first=A. B. |url=https://www.worldcat.org/oclc/62416599 |title=Stalin's great science : the times and adventures of Soviet physicists |date=2004 |publisher=Imperial College Press |isbn=1-86094-601-1 |location=London, England |language=en-uk |oclc=62416599}}</ref>