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My understanding is that solid matter is made out of fermions (which are subject to the Pauli exclusion principle, which is why they're solid) and bosons (which are not subject to Pauli exclusion and are not solid) and are force-carrying particles.

In the news today: Scientists Have Discovered a New Form of Matter Called “Excitonium”

Researchers at the University of Illinois have announced an exciting finding — the discovery of a new form of matter: excitonium. This material is made up of a kind of boson, a composite particle that could allow the matter to act as a superfluid, superconductor, or even as an insulating electronic crystal.

Excitonium is a condensate made up of excitons, which are what you get when you combine escaped electrons and the “holes” they left. This quirky quantum-mechanical pairing is possible because, in semiconductors, electrons on the edge of one energy level in an atom are able, when excited, to jump into the next energy level, leaving behind a “hole” in the previous level. This hole acts like a positively charged particle, attracting the negatively charged electron that escaped.

Until now, scientists had not had the experimental tools needed to distinguish with certainty whether they were detecting excitonium or another similar phase of matter. Using a novel technique, however, this research team was able to definitively measure, for the first time, the collective excitations of the low-energy bosonic particles, the paired electrons, and the holes, no matter what their momentum might be.

How can excitonium be made of bosons if it's made of electrons?

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    $\begingroup$ Exciton droplets are nothing new en.wikipedia.org/wiki/Electron-hole_droplets $\endgroup$
    – user137289
    Dec 10, 2017 at 16:57
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    $\begingroup$ Any bound state of an even number of fermions (e.g. mesons, oxygen-16 atoms) is a boson because it has integer spin. $\endgroup$
    – dukwon
    Dec 10, 2017 at 17:28

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My understanding is that solid matter is made out of fermions (which are subject to the Pauli exclusion principle, which is why they're solid) and bosons (which are not subject to Pauli exclusion and are not solid) and are force-carrying particles.

Your understanding is incomplete. This is true for fundamental particles, but for composite particles the total spin determines whether it is a boson or fermion*. For example, an atom of Rubidium-87 is made up of protons, neutrons, and electrons, which are all individually fermions, but the atom itself is a boson, which is why it was able to create a Bose-Einstein condensate in the Nobel-prize winning work of Cornell and Wieman.

Similarly, excitonium is a bound state of two quasiparticles that are individually fermions, but this makes the composite particle a boson.

*Technically this is only true in three dimensions, in lower dimensionalities there is the exotic possibility of so-called 'anyonic statistics.'

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