Quasiparticle

Under a quasiparticle refers to a suggestion of a many-body system, or solid, which has an energy - momentum relation ( dispersion relation ) as a particle. In a quasiparticle is usually a collective state of many particles, an elementary excitation, or sometimes to the bound state of a Teilchenpaars. A characteristic feature of quasi-particles, however, is that they can not occur outside of their many-body system.

A well-known example is the defect electrons in a semiconductor, in which the negatively charged valence electrons so move collectively in one direction, when a positively charged particle would move in the opposite direction. Other examples are phonons, magnons, Cooper pairs.

The term quasi-particles goes back to Lev Davidovich Landau. He developed a theory of the interaction between the conduction electrons in a metal ( the theory of the Fermi liquid). The basic idea is to describe the interaction of a conduction electron with its environment by allowing " enhanced" the electron to this interaction. He called this extended quasi- particle electron, since it can be treated as a free electron ( in first approximation ) in theory.

Properties

The quasiparticles, properties normal particles such as mass, momentum, energy, wavelength, and spinning can be attributed, for example, the effective mass for a conduction electron having interaction with the other electrons ( quasi electron) instead of the actual mass of an electron. To express that these are properties of a quasi-particle, one also speaks of quasi- mass, etc.

Quasiparticles behave like normal particles and can therefore scatter each other and exchange momentum and energy. You may also be created and destroyed, for quasiparticles therefore applies no conservation of particle number. This means in other words that the chemical potential vanishes for quasiparticles: μ = 0

Systems of quasi-particles (photons gas, phonon, etc.) can not be described classically. The classical limit is impossible in such systems, as always, because of the fugacity.

Quasiparticles, which have half-integer spin or an integer, behave as bosons or fermions and obey accordingly, the Bose -Einstein or Fermi -Dirac statistics.

Sometimes the quasiparticles have a discrete energy spectrum, and the solid body can absorb or emit specific amounts of energy much better than others.

Example

Sees a crystal as a system of atoms, which are bonded to one another by elastic forces, the interaction of the ions with each other can be described by an elastic panel. The quasiparticles of this field are called phonons. They are plane elastic waves in the solid. The phonons correspond to the eigenstates of a harmonic oscillator in the single-particle case.

Beam is an electromagnetic wave in a crystal, it mainly takes place elastic scattering, the magnitude of the wave vector does not change. The shaft may however also be sprinkled with the simultaneous creation or annihilation of a phonon. Here, the magnitude of the wave vector of the light changes. This process is called Raman scattering. From the change of the wave vector one can determine the wave vector of the phonon and calculate the binding energies between the atoms of the crystal.

List of quasiparticles

• Phonons: elastic waves in crystals.
• Magnons: electron spins, which are coupled together by the exchange interaction.
• Plasmons: collective fluctuations of the carrier density in metals and semiconductors.
• Excitons: neutral particles, for example, composed of an electron and a positive hole associated with the dielectric polarization field.
• Polaritons: interaction between a photon and a quasiparticle.

• Polarons: charged particles that are related to the dielectric polarization field.
• Quasi Electron: A conduction electron in a metal.
• Hole: A "missing " electron in semiconductor physics.

• Cooper pairs: two coupled electrons in the BCS theory of superconductivity.
• Roton: suggestions in superfluid 4He.
• Triones
• Amplitudon, phason: elementary excitation of the amplitude and the phase of a inkommensuraten superstructure of a solid or a solid surface.
• Anyons
• Dropleton