In insulating and semiconductor crystals, a quantized ripple in electron energy that moves about the crystal transferring energy but not charge. A type of quasi-particle, it is important in understanding optical reflection and transmission properties.
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 electron hole in an insulator or semiconductor, and such is a Coulomb correlated electron-hole pair.
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 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.
In a hydrogen atom the core and the electron can have parallel or antiparallel spin, the same is true for the exciton and for the positronium, but not for the two electrons in the He-atom.
Subtypes
Excitons can be treated in two limiting cases, which depend on the properties of the material in question. 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.
When a material's dieletric 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 Buckminster Fullerene), so the electron and hole sit on the same cell.
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.
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).
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.
The existence of exciton states may be inferred from the absorption of light associated with their excitation.
With each other
Provided the interaction is attractive, an exciton can bind with other excitons to form a 'biexciton', analogous to a hydrogen molecule. In some systems, where the interactions are repulsive, a Bose-Einstein condensed state is predicted to be the ground state, but has yet to be observed due to the influence of factors such as material disorder, short exciton lifetimes (less than the re-thermalization times) and low exciton densities.
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