quasiparticlelike

Quasiparticlelike is an adjective used in physics to describe excitations or states in a many-body system that resemble a quasiparticle, an emergent particle-like excitation arising from collective interactions. Such a mode is often characterized by a relatively well-defined energy-momentum relationship and a coherent, propagating nature within a limited range of energies and times.

In practice, a mode is termed quasiparticlelike when it shows features such as a peak with finite height in the spectral function A(k, ω), a long lifetime relative to its oscillation period, and an effective dispersion relation that can be assigned to an emergent particle with parameters like an effective mass. The description is typically valid within a certain energy or time window where interactions can be treated perturbatively or through an effective single-particle picture.

Quasiparticlelike does not require the excitation to be a true non-interacting quasiparticle. It indicates that, within a given regime, many-body interactions yield particle-like behavior that can be captured by an approximate single-particle description. Outside that regime, strong damping, incoherent continua, or breakdown of the dispersion can occur, and the quasiparticlelike picture may fail.

Contexts where quasiparticlelike behavior is discussed include Fermi liquids, where electron-like excitations near the Fermi surface behave as quasiparticles; polarons, where electrons dressed by lattice vibrations act as quasiparticlelike carriers; and various collective modes in solids and ultracold gases observed via spectroscopy or transport measurements.

The concept is often used with tools such as Green's functions, self-energy, and spectral functions to quantify lifetimes and renormalized parameters. It provides a practical language for describing complex many-body dynamics without asserting a fully fundamental particle.

See also: quasiparticle; spectral function; effective mass; Fermi liquid theory; polarons.