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An [[electron]] and an [[electron hole]] that are attracted to each other by the [[Coulomb's law|Coulomb force]] can form a [[bound state]] called an '''exciton'''. It is an electrically neutral [[quasiparticle]] that exists mainly in [[condensed matter]], including [[Electrical insulation|insulators]], [[semiconductor]]s, some metals, but also in certain atoms, molecules and liquids. The exciton is regarded as an elementary excitation that can transport energy without transporting net electric charge.<ref>R. S. Knox, ''Theory of excitons, Solid state physics'' (Ed. by Seitz and Turnbul), New York, New York: Academic, v. 5, 1963.</ref><ref name=":0">{{Cite journal |last1=Mueller |first1=Thomas |last2=Malic |first2=Ermin |date=2018-09-10 |title=Exciton physics and device application of two-dimensional transition metal dichalcogenide semiconductors |journal=npj 2D Materials and Applications |language=en |volume=2 |issue=1 |pages=1–12 |doi=10.1038/s41699-018-0074-2 |s2cid=119537445 |issn=2397-7132|doi-access=free |arxiv=1903.02962 }}</ref><ref>Monique Combescot and Shiue-Yuan Shiau, "Excitons and Cooper Pairs: Two Composite Bosons in Many-Body Physics", Oxford University Press. {{ISBN|9780198753735}}.</ref><ref>{{cite book |last1=Fox |first1=Mark |title=Optical Properties of Solids |date=2010-03-25 |publisher=Oxford Master Series in Physics |edition=2nd |language=en-uk}}</ref>
An exciton can form when an electron from the valence band of a crystal is promoted in energy to the [[conduction band]] e.g., when a material absorbs a photon. Promoting the electron to the conduction band leaves a positively charged [[hole]] in the valence band. Here 'hole' represents the unoccupied quantum mechanical electron state with a positive charge, an analogue in crystal of a [[positron]]. Because of the attractive [[coulomb force]] between the electron and the hole, a bound state is formed, akin to that of the electron and proton in a hydrogen atom or the electron and positron in [[positronium]]. Excitons are composite bosons since they are formed from two fermions which are
The concept of excitons was first proposed by [[Yakov Frenkel]] in 1931,<ref>{{cite journal|doi=10.1103/PhysRev.37.17|title=On the Transformation of light into Heat in Solids. I|year=1931|last1=Frenkel|first1=J.|journal=Physical Review|volume=37|issue=1|page=17|bibcode = 1931PhRv...37...17F }}</ref> when he described the excitation of an atomic lattice considering what is now called the tight-binding description of the [[band structure]]. In his model the electron and the hole bound by the coulomb interaction are located either on the same or on the nearest neighbouring sites of the lattice, but the exciton as a composite quasi-particle is able to travel through the lattice without any net transfer of charge, which lead to many propositions for [[optoelectronic devices]].
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