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Old 09-12-2017, 01:53 PM
gary
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Excitonium - a form of matter theorized 50 years ago - finally discovered

In an article today at phys.org, Siv Schwink reports on the discovery
of a new form of matter that was first theorized 50 years ago.

Quote:
Originally Posted by Siv Schwink, phys.org
Excitonium has a team of researchers at the University of Illinois at Urbana-Champaign... well... excited! Professor of Physics Peter Abbamonte and graduate students Anshul Kogar and Mindy Rak, with input from colleagues at Illinois, University of California, Berkeley, and University of Amsterdam, have proven the existence of this enigmatic new form of matter, which has perplexed scientists since it was first theorized almost 50 years ago.

....

So what exactly is excitonium?

Excitonium is a condensate—it exhibits macroscopic quantum phenomena, like a superconductor, or superfluid, or insulating electronic crystal. It's made up of excitons, particles that are formed in a very strange quantum mechanical pairing, namely that of an escaped electron and the hole it left behind.

It defies reason, but it turns out that when an electron, seated at the edge of a crowded-with-electrons valence band in a semiconductor, gets excited and jumps over the energy gap to the otherwise empty conduction band, it leaves behind a "hole" in the valence band. That hole behaves as though it were a particle with positive charge, and it attracts the escaped electron. When the escaped electron with its negative charge, pairs up with the hole, the two remarkably form a composite particle, a boson—an exciton.

In point of fact, the hole's particle-like attributes are attributable to the collective behavior of the surrounding crowd of electrons. But that understanding makes the pairing no less strange and wonderful.

...

"This result is of cosmic significance," affirms Abbamonte. "Ever since the term 'excitonium' was coined in the 1960s by Harvard theoretical physicist Bert Halperin, physicists have sought to demonstrate its existence. Theorists have debated whether it would be an insulator, a perfect conductor, or a superfluid—with some convincing arguments on all sides. Since the 1970s, many experimentalists have published evidence of the existence of excitonium, but their findings weren't definitive proof and could equally have been explained by a conventional structural phase transition."
Article here :-
https://phys.org/news/2017-12-physic...xcitonium.html

Further reading "Spontaneous Bose-Einstein condensation of excitons":-
https://phys.org/news/2017-12-sponta...-excitons.html

Paper published in Science by Kogar et.al. "Signatures of exciton condensation in a transition metal dichalcogenide" (subscription required) :-
http://science.sciencemag.org/content/358/6368/1314
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