Quasiparticle

In physics, a quasiparticle refers to a 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 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 single 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 spin density waves.

The idea of quasiparticles originated in Landau's theory of Fermi liquids, which was originally invented for studying liquid helium-3. For these systems a strong similarity exists between the notion of quasi-particle and dressed particles in quantum field theory.

The dynamics of Landau's theory is defined by a mean-field theory, involves a quasi-particle concept.

Note that the use of term quasiparticle seems to be ambiguous. Some authors use the term in order to distinguish them from real particles, others (including author of the above passage) to describe an excitation similar to a single particle excitation as opposed to a collective excitation. Both definitions mutually exclude each other as with the former definition collective excitations which are no "real" particles are considered to be quasiparticles.^{[citation needed]} The problems arising from the collective nature of quasiparticles have also been discussed within the philosophy of science, notably in relation to the identity conditions of quasiparticles and whether or not they should be considered "real" by the standards of, for example, entity realism.^[1][2]

Polarons are the quanta of the oscillating polarization in a lightly doped semiconductor and also do not need elementary charge or mass.