Exciton

Exciton - Wikipedia, the free encyclopedia
Exciton is also the title of a single by IDM composer Squarepusher. An exciton is a bound state of an electron and an imaginary particle called an ...
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exciton: Definition from Answers.com
exciton n. An electrically neutral excited state of an insulator or semiconductor, often regarded as a bound state of an electron and a hole
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This page is about the quasiparticle. Exciton is also the title of a single by intelligent dance music composer Squarepusher.

An exciton is a bound state of an electron and an imaginary particle called an electron hole in an Electrical insulation or semiconductor, and such is a Coulomb-Electronic correlation electron-hole pair. It is an elementary excitation, or a quasiparticle of a solid.

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.

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.

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 electron volt. This type of exciton was named for Sir Nevill Francis Mott and Gregory Wannier.

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 fullerene), 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.

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.

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.

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.

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. The other one is energy carried away by radiation. Combination of the two has also been studied.

Features

Since an exciton is a bound state of an electron and a hole, the overall 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 Boltzmann constantT is less than the exciton binding energy), replacing the free electron-hole recombination at higher temperatures.

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.

Excitons may also interact with phonons and lattice distortions to form polarons. In that case, the excitons are called dressed excitons.

With each other Provided the interaction is attractive, an exciton can bind with other excitons to form a 'biexciton', analogous to a dihydrogen 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.

Additionally, excitons are integer-spin particles obeying Bosons statistics in the low-density limit. In some systems, where the interactions are repulsive, a Bose-Einstein condensate 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 kelvins and further observing coherent light emission (with interference patterns) from it.

Exciton - Wikipedia, the free encyclopedia
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.

Exciton
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e-Prints Soton - Exciton-light coupling in quantum wells: from ...
We show numerically that spatial delocalization of excition-polariton states in multiple quantum wells leads to efficient narrowing of exciton resonances with respect to the single ...

e-Prints Soton - Theory of propagation and scattering of exciton ...
A novel semi-classical theory of exciton-polaritons in quantum wells (QWs) is presented. Time-resolved coherent optical spectra of QWs are described by the light coupling with ...

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exciton -- Britannica Online Encyclopedia
Britannica online encyclopedia article on exciton: the combination of an electron and a positive hole (an empty electron state in a valence band), which is free to move through a ...

University of Exeter: Strong exciton–photon coupling in a low-Q all ...
Copyright © 2002 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American ...

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Exciton-light coupling in quantum wells: From motional narrowing to ...
We show numerically that spatial delocalization of excition-polariton states in multiple quantum wells leads to efficient narrowing of exciton resonances with respect to the single ...