Editing Exciton (section)

=== Charge-transfer exciton ===
An intermediate case between Frenkel and Wannier excitons is the ''charge-transfer (CT) exciton''. In molecular physics, CT excitons form when the electron and the hole occupy adjacent molecules.<ref>{{cite book |author=Wright |first=J. D. |url={{Google books|7sroAgMASIEC|Molecular Crystals|page=108|plainurl=yes}} |title=Molecular Crystals |publisher=Cambridge University Press |year=1995 |isbn=978-0-521-47730-7 |edition=2nd |page=108 |language=en-uk |orig-year=First published 1987}}</ref> They occur primarily in organic and molecular crystals;<ref>{{cite book |author=Lanzani |first=Guglielmo |url={{Google books|RVyvgKo0lGQC|The Photophysics Behind Photovoltaics and Photonics|page=82|plainurl=yes}} |title=The Photophysics Behind Photovoltaics and Photonics |publisher=Wiley-VCH Verlag |year=2012 |page=82 |language=en}}</ref> in this case, unlike Frenkel and Wannier excitons, CT excitons display a static [[electric dipole moment]]. CT excitons can also occur in transition metal oxides, where they involve an electron in the transition metal 3''d'' orbitals and a hole in the oxygen 2''p'' orbitals. Notable examples include the lowest-energy excitons in correlated cuprates<ref>{{cite journal|doi=10.1103/PhysRevB.77.060501|title=Charge-transfer exciton in La<sub>2</sub>CuO<sub>4</sub> probed with resonant inelastic x-ray scattering|year=2008|journal=Physical Review B|volume=77|issue=6|pages=060501(R)|last1=Ellis|first1=D. S.|last2=Hill|first2=J. P.|last3=Wakimoto|first3=S.|last4=Birgeneau|first4=R. J.|last5=Casa|first5=D.|last6=Gog|first6=T.|last7=Kim|first7=Young-June|arxiv=0709.1705|bibcode=2008PhRvB..77f0501E|s2cid=119238654}}</ref> or the two-dimensional exciton of TiO<sub>2</sub>.<ref>{{cite journal|doi=10.1038/s41467-017-00016-6|title=Strongly bound excitons in anatase TiO<sub>2</sub> single crystals and nanoparticles|year=2017|journal=Nature Communications|volume=8|issue=13|last1=Baldini|first1=Edoardo|last2=Chiodo|first2=Letizia|last3=Dominguez|first3=Adriel|last4=Palummo|first4=Maurizia|last5=Moser|first5=Simon|last6=Yazdi-Rizi|first6=Meghdad|last7=Aubock|first7=Gerald|last8=Mallett|first8=Benjamin P P|last9=Berger|first9=Helmuth|last10=Magrez|first10=Arnaud|last11=Bernhard|first11=Christian|last12=Grioni|first12=Marco|last13=Rubio|first13=Angel|last14=Chergui|first14=Majed|page=13|pmid=28408739|pmc=5432032|arxiv=1601.01244|bibcode=2017NatCo...8...13B|doi-access=free}}</ref> Irrespective of the origin, the concept of CT exciton is always related to a transfer of charge from one atomic site to another, thus spreading the wave-function over a few lattice sites.