Quasiparticle

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 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 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. 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 kinetic equation of the mean-field type. A similar equation, the Vlasov Equation, is valid for a plasma in the so-called plasma approximation, in which charged particles are considered moving in the electromagnetic field collectively generated by all other particles, and hard collisions between the charged particles are neglected. When a kinetic equation of the mean-field type is a valid first-order description of a system, second-order corrections determine the entropy production, and generally take the form of a Boltzmann-type collision term, in which figure only "far collisions" between virtual particles. In other words, every type of mean-field kinetic equation, and in fact every 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]

Phonons are the quanta of classical sound waves and sound waves do not need the notion of atoms. Magnons are the quanta of classical spinwaves, which also do not need elementary spins. Photons inside an isolator are the quanta of classical dressed electromagnetic waves and do not need the notion of electrons for the definition of the refractive index. Plasmons are the quanta of the plasma oscillations and they only need charge density and mass density and no electrons or ions. Polarons are the quanta of the oscillating polarization in a lightly doped semiconductor and also do not need elementary charge or mass.

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Examples

• Landau quasiparticles in normal metals
• Stoner excitations in ferromagnetic metals
• Bogoliubov quasiparticles in superconductors
• Electron holes in semiconductors and conventional current

See also

• Mean field theory

References

Further reading

• L. D. Landau, Soviet Phys. JETP. 3, 920 (1957)
• L. D. Landau, Soviet Phys. JETP. 5, 101 (1957)
• A. A. Abrikosov, L. P. Gorkov, and I. E. Dzyaloshinski, Methods of Quantum Field Theory in Statistical Physics (Prentice-Hall, New Jersey, 1963); (Dover Publications, New York, 1975)
• D. Pines, and P. Nozières, The Theory of Quantum Liquids, Volume I: Normal Fermi Liquids (W.A. Benjamin, New York, 1966); (Westview Press, Boulder, 1999)
• J. W. Negele, and H. Orland, Quantum Many-Particle Systems (Westview Press, Boulder, 1998)