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Encyclopedia > Exciton

This page is about the quasiparticle. Exciton is also the title of a single by IDM composer Squarepusher. In physics, a quasiparticle refers to a particle-like entity arising in certain systems of interacting particles. ... Intelligent dance music (commonly IDM) is a genre of electronic music derived from dance music of the 1980s and early 1990s which puts an emphasis on novel processing and sequencing. ... Squarepusher, the performing pseudonym of Tom Jenkinson, is an English electronic music artist signed to Warp Records. ...


An exciton is a bound state of an electron and an imaginary particle called an electron hole in an insulator or semiconductor, and such is a Coulomb-correlated electron-hole pair. It is an elementary excitation, or a quasiparticle of a solid. e- redirects here. ... For the following two reasons the electron hole was introduced into calculations: If an electron is excited into higher state it leaves a hole in its old state. ... This article or section is in need of attention from an expert on the subject. ... A semiconductor is a solid whose electrical conductivity can be controlled over a wide range, either permanently or dynamically. ... The coulomb (symbol: C) is the SI unit of electric charge. ... Electronic correlation refers to the interaction between electrons in a quantum system whose electronic structure is being considered. ... In physics, a quasiparticle refers to a particle-like entity arising in certain systems of interacting particles. ...


A vivid picture of exciton formation is as follows: a photon enters a semiconductor, exciting an electron from the valence band into the conduction band. The missing electron in the valence band leaves a hole behind, of opposite electric charge, to which it is attracted by the Coulomb force. The exciton results from the binding of the electron with its hole; as a result, the exciton has slightly less energy than the unbound electron and hole. The wavefunction of the bound state is hydrogenic (an "exotic atom" state akin to that of a hydrogen atom). However, the binding energy is much smaller and the size much bigger than a hydrogen atom because of the effects of screening and the effective mass of the constituents in the material. The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, microwaves, radio waves, and visible light are all forms of light. ... In solids, the valence band is the highest range of electron energies where electrons are normally present at zero temperature. ... In semiconductors and insulators, the conduction band is the range of electron energy, higher than that of the valence band, sufficient to make the electrons free to accelerate under the influence of an applied electric field and thus constitute an electric current. ... In physics, Coulombs law is an inverse-square law indicating the magnitude and direction of electrical force that one stationary, electrically charged substance of small volume (ideally, a point source) exerts on another. ... This article discusses the concept of a wavefunction as it relates to quantum mechanics. ... An exotic atom is the anologue of a normal atom in which one or more of the electrons are replaced by other negative particles, such as a muon or a pion, or the positively charged nucleus is replaced by other positively charged elementary particles, or both. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Properties In chemistry and physics, an atom (Greek ἄτομος or átomos meaning indivisible) is the smallest particle still characterizing a chemical element. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... In solid state physics, a particles effective mass is the mass it seems to carry in the semiclassical model of transport in a crystal. ...


In a hydrogen atom the core and the electron can have parallel or antiparallel spin, the same is true for the exciton, and also for positronium, but not for the two electrons in the He-atom. Often excitons were given names which look like hydrogen orbital names, but have the wrong numbering for angular momentum, or other quantum numbers. Positronium (Ps) is a quasi-stable system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom. The orbit of the two particles and the set of energy levels is similar to that of the hydrogen atom (electron and proton). ... This gyroscope remains upright while spinning due to its angular momentum. ... A quantum number describes the energies of electrons in atoms. ...

Contents

Subtypes

Excitons can be treated in two limiting cases, which depend on the properties of the material in question. In semiconductors, the dielectric constant is generally large, and as a result, screening tends to reduce the Coulomb interaction between electrons and holes. The result is a Mott-Wannier exciton, which has a radius much larger than the lattice spacing. As a result, the effect of the lattice potential can be incorporated into the effective masses of the electron and hole, and because of the lower masses and the screened Coulomb interaction, the binding energy is usually much less than a hydrogen atom, typically on the order of 0.1 eV. This type of exciton was named for Sir Nevill Francis Mott and Gregory Wannier. The relative dielectric constant of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ... Sir Nevill Francis Mott (September 30, 1905 – August 8, 1996) was a British physicist. ... Gregory Hugh Wannier(1911 - 1983) was a Swiss physicist. ...


When a material's dielectric constant is very small, the Coulomb interaction between electron and hole become very strong and the excitons tend to be much smaller, of the same order as the unit cell (or on the same molecule as with fullerenes), so the electron and hole sit on the same cell. This Frenkel exciton, named after Yakov Frenkel, is typically on the order of 1.0 eV. The fullerenes, discovered in 1985 by researchers at Rice University, are a family of carbon allotropes named after Richard Buckminster Fuller and are sometimes called buckyballs. ... Yakov Frenkel Yakov Ilich Frenkel, Russian: (February 10, 1894, Rostov-on-Don – January 23, 1952, St. ...


Alternatively, an exciton may be thought of as an excited state of an atom or ion, the excitation wandering from one cell of the lattice to another. “Multivalent” redirects here. ...


Often there is more than one band to choose from for the electron and the hole leading to different types of excitons in the same material. Even high lying bands can be used as is seen in femtosecond two-photon experiments. A femtosecond is the SI unit of time equal to 10-15 of a second. ...


At surfaces so called image states may occur, where the hole is inside the solid and the electron is in the vacuum. These electron hole pairs can only move along the surface.


Dynamics

The probability of the hole disappearing (the electron occupying the hole) is limited by the difficulty of losing the excess energy, and as a result excitons can have a relatively long lifetime. (Lifetimes of up to several milliseconds have been observed in copper (I) oxide) Another limiting factor in the recombination probability is the spatial overlap of the electron and hole wavefunctions (roughly the probability for the electron to run into the hole). This overlap is smaller for lighter electrons and holes and for highly excited hydrogenic states. Copper(I) oxide or cuprous oxide (Cu2O) is an oxide of copper. ...


The whole exciton can move through the solid. With this additional kinetic energy the exciton may lie above the band-gap.



The exciton propagating through molecular crystal is one that is of greatest concern. Several mechanisms have been proposed in the literature. Two are important. The first one is exciton energy dissipated due to interaction with phonon bath. The other one is energy carried away by radiation. Combination of the two has also been studied. Normals modes of vibration progression through a crystal. ...


Features

1.Since an exciton is a bound state of a electron and a hole, the overal charge for this quasiparticle is zero. Hence it carries no electric current.


Interaction

With other particles

Excitons are thus the main mechanism for light emission in semiconductors at low temperatures (where kT is less than the exciton binding energy), replacing the free electron-hole recombination at higher temperatures. This page is a list of sources of light. ... Fig. ... Ludwig Boltzmann The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ... Binding energy is the energy required to disassemble a whole into separate parts. ...


The existence of exciton states may be inferred from the absorption of light associated with their excitation. Typically, excitons are observed just below the band gap. This article or section does not adequately cite its references or sources. ...



Excitons may also interact with phonons and lattice distortions to form polarons. In that case, the excitons are called dressed excitons. In solid-state physics, a polaron is formed when a moving charge (typically an electron or a hole) in a crystal with some ionic character polarizes (by its electric field) the lattice around it. ...


With each other

Provided the interaction is attractive, an exciton can bind with other excitons to form a 'biexciton', analogous to a hydrogen molecule. If a large density of excitons is created in a material, they can interact with one another to form an electron-hole liquid, a state observed in k-space indirect semiconductors. In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ...


Additionally, excitons are integer-spin particles obeying Bose statistics in the low-density limit. In some systems, where the interactions are repulsive, a Bose-Einstein condensed state is predicted to be the ground state, and indeed such condensate has been already observed in recent experiments AIP Update 800. The inference was obtained by cooling an exciton state below 5 Kelvin and further observing coherent light emission (with interference patterns) from it. Boson (game) Bosons, named after Satyendra Nath Bose, are particles which form totally-symmetric composite quantum states. ... A Bose–Einstein condensate is a phase of matter formed by bosons cooled to temperatures very near to absolute zero (0 kelvins or -273. ...



Particles in physics - quasiparticles
Phonons | Excitons | Plasmons | Polaritons | Polarons | Magnons

  Results from FactBites:
 
Exciton - Wikipedia, the free encyclopedia (844 words)
An exciton is a bound state of an electron and an imaginary particle called an electron hole in an insulator or semiconductor, and such is a Coulomb correlated electron-hole pair.
Alternatively, an exciton may be thought of as an excited state of an atom or ion, the excitation wandering from one cell of the lattice to another.
Excitons are thus the main mechanism for light emission in semiconductors at low temperatures (where kT is less than the exciton binding energy), replacing the free electron-hole recombination at higher temperatures.
Text of MIJ-NSR Volume 8, Article 3 (10375 words)
Physically, the exciton Bohr radius is analogous to the hydrogen atom's Bohr radius, it is proportional to the average distance between electron and hole for the ground exciton state.
The clearest signature of exciton Bose condensation should be the emission of a coherent light by spontaneous recombination of condensed excitons [40].
The polariton dispersion relation of the microcavity for small wave vectors and for zero detuning between the exciton and the cavity mode is obtained by the scattering state method as shown on figure 3-b.
  More results at FactBites »

 
 

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