In physics, a quasiparticle refers to a particle-like entity arising in certain systems of interacting particles. It can be thought of as a single particle moving through the system, surrounded by a cloud of other particles that are being pushed out of the way or dragged along by its motion, so that the entire entity moves along somewhat like a free particle. The quasiparticle concept is one of the most important in condensed matter physics, because it is one of the few known ways of simplifying the quantum mechanical many-body problem, and is applicable to an extremely wide range of many-body systems.

In the language of many-body quantum mechanics, a quasiparticle is a type of low-lying excited state of the system (a state possessing energy very close to the ground state energy) that is known as an elementary excitation . This means that most of the other low-lying excited states can be viewed as states in which multiple quasiparticles are present. It turns out that the interactions between quasiparticles become negligible at sufficiently low temperatures, in which case we can obtain a great deal of information about the system as a whole, including the flow properties and heat capacity, by investigating the properties of individual quasiparticles.

Actually, most many-body systems possess two types of elementary excitations. The first type, the quasiparticles, correspond to sìngle particles whose motions are modified by interactions with the other particles in the system. The second type of excitation corresponds to a collective motion of the system as a whole. These excitations are called collective modes , and they include phenomena such as zero sound, plasmons, and spin density waves .

The idea of quasiparticles originated in Landau's theory of Fermi liquids, which was originally invented for studying liquid helium-3

This is pretty much the same thing as dressed particles in quantum field theory.

References

• L. D. Landau, Soviet Phys. JETP. 3, 920 (1957)
• L. D. Landau, Soviet Phys. JETP. 5, 101 (1957)