Quasiparticle

In <a href="/facts/Condensed_matter_physics/EBVI5nmL">condensed matter physics</a>, 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 <a href="/facts/Solid/YU0hJq6f">solid</a> behaves as if it contained different weakly interacting <a href="/facts/Particle/JHKr2w6h">particles</a> in <a href="/facts/Vacuum/BTk3goyV">vacuum</a>.
For example, as an <a href="/facts/Electron/L0KBo5cW">electron</a> travels through a <a href="/facts/Semiconductor/zFf3mGTE">semiconductor</a>, its motion is disturbed in a complex way by its interactions with other electrons and with <a href="/facts/Atomic_nucleus/orpIm5Xl">atomic nuclei</a>. The electron behaves as though it has a different <a href="/facts/Effective_mass_(solid-state_physics)/I8acHGPd">effective mass</a> travelling unperturbed in vacuum. Such an electron is called an electron quasiparticle. In another example, the aggregate motion of electrons in the <a href="/facts/Valence_band/2NUALlHJ">valence band</a> of a <a href="/facts/Semiconductor/zFf3mGTE">semiconductor</a> or a hole band in a metal behave as though the material instead contained positively charged quasiparticles called <a href="/facts/Electron_hole/YymdTv9d">electron holes</a>. Other quasiparticles or collective excitations include the <a href="/facts/Phonon/tflWW6vw">phonon</a>, a quasiparticle derived from the vibrations of atoms in a solid, and the <a href="/facts/Plasmon/kuCMxx5f">plasmon</a>, a particle derived from <a href="/facts/Plasma_oscillation/B7lKhaMT">plasma oscillation</a>.
These phenomena are typically called quasiparticles if they are related to <a href="/facts/Fermion/LEckdDsR">fermions</a>, and called collective excitations if they are related to <a href="/facts/Boson/XtPFcjb5">bosons</a>, although the precise distinction is not universally agreed upon. 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 <a href="/facts/Condensed_matter_physics/EBVI5nmL">condensed matter physics</a> because it can simplify the <a href="/facts/Many-body_problem/n2rIxgwv">many-body problem</a> in <a href="/facts/Quantum_mechanics/BRc3Mzgr">quantum mechanics</a>. The theory of quasiparticles was started by the Soviet physicist <a href="/facts/Lev_Landau/lNXQTC49">Lev Landau</a> in the 1930s.

Quasiparticle open-in-new

Quasiparticle