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In this Wikipedia link, the definition of exciton is given by

An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and in some liquids. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge.

Why such a bound state (which I believe is a Bosonic state) cannot exist in metals? Coulomb attraction between electron-hole pairs should be present in metals too. The link I cited, doesn't explain this point.

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Excitons certainly exist in metals as you say. But excitons in metals had been able to be not observed so far, since the life time is so short. That is about a few femtoseconds.

Recently, it has been reported that excitons could be observed in silver. That report is http://www.nature.com/nphys/journal/v10/n7/full/nphys2981.html.

The group has estimated that at the surface of silver, the life time of excitons is 100 times longer than other metals. They proved experimentally it and observed the existence of excitons.

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  • $\begingroup$ Is there a reason why excitons are so short-lived in metals? In other words, what makes the excitons in a semiconductor/insulator more common than in metals? $\endgroup$
    – SRS
    Commented Jan 11, 2017 at 7:31
  • $\begingroup$ Thank you for your comment. When holes were created, the coulomb interaction of electron-hole pairs is reduced by the shielding effect. So the exciton state of electron-hole pairs can not be maintained. Such reason is thought. $\endgroup$
    – Tanaike
    Commented Jan 11, 2017 at 7:48

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