Editing Exciton (section)

==Spatially direct and indirect excitons==
Normally, excitons in a semiconductor have a very short lifetime due to the close proximity of the electron and hole. However, by placing the electron and hole in spatially separated quantum wells with an insulating barrier layer in between so called 'spatially indirect' excitons can be created. In contrast to ordinary (spatially direct), these spatially indirect excitons can have large spatial separation between the electron and hole, and thus possess a much longer lifetime.<ref>{{Cite journal |doi = 10.1038/s41563-019-0337-0|pmid = 30962556|title = Ultrafast transition between exciton phases in van der Waals heterostructures|journal = Nature Materials|volume = 18|issue = 7|pages = 691–696|year = 2019|last1 = Merkl|first1 = P.|last2 = Mooshammer|first2 = F.|last3 = Steinleitner|first3 = P.|last4 = Girnghuber|first4 = A.|last5 = Lin|first5 = K.-Q.|last6 = Nagler|first6 = P.|last7 = Holler|first7 = J.|last8 = Schüller|first8 = C.|last9 = Lupton|first9 = J. M.|last10 = Korn|first10 = T.|last11 = Ovesen|first11 = S.|last12 = Brem|first12 = S.|last13 = Malic|first13 = E.|last14 = Huber|first14 = R.|arxiv = 1910.03890|bibcode = 2019NatMa..18..691M|s2cid = 104295452}}</ref> This is often used to cool excitons to very low temperatures in order to study Bose–Einstein condensation (or rather its two-dimensional analog).<ref>{{Cite journal | doi=10.1038/nature10903| pmid=22437498| title=Spontaneous coherence in a cold exciton gas| journal=Nature| volume=483| issue=7391| pages=584–588| year=2012| last1=High| first1=A. A.| last2=Leonard| first2=J. R.| last3=Hammack| first3=A. T.| last4=Fogler| first4=M. M.| last5=Butov| first5=L. V.| last6=Kavokin| first6=A. V.| last7=Campman| first7=K. L.| last8=Gossard| first8=A. C.| bibcode=2012Natur.483..584H| arxiv=1109.0253| s2cid=3049881}}</ref>