What is it?
"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.
Why has excitonium taken 50 years to be discovered in real materials?
Until now, scientists have not had the experimental tools to positively distinguish whether what looked like excitonium wasn't in fact a Peierls phase. Though it's completely unrelated to exciton formation, Peierls phases and exciton condensation share the same symmetry and similar observables—a superlattice and the opening of a single-particle energy gap."
snip
"defies reason" until more is learned. IOW just like most knowledge and learning. "Science"
