Physicists from the University of Illinois have discovered a new form of matter called excitonium. This condensate, made up of excitons, was first theorized almost 50 years ago, and although evidence of this form of matter has been observed in that time it was difficult to be sure of what was happening. Now, the Illinois researchers have found a "smoking gun" that they claim is the first definitive proof that excitonium exists.

The laws of physics at the quantum level are very different than at the macro scale, but a form of matter called a Bose-Einstein condensate somewhat bridges the gap. This state is formed when particles or quasiparticles clump together and begin to behave as one entity, known as a boson.

Excitons are a type of boson formed in a semiconductor. When an electron on the edge of a semiconductor's valence band gets excited, it can cross the band gap into the conduction band, which is empty. When it does, it leaves a "hole" in the valence band, which itself becomes a quasiparticle with a positive charge. The positively-charged hole and the negatively-charged electron are attracted to each other and together form a kind of boson known as an exciton.

Like other bosons, excitons have long been believed to have a "ground state," which was named excitonium and, until now, was largely theoretical.

"Ever since the term 'excitonium' was coined in the 1960s by Harvard theoretical physicist Bert Halperin, physicists have sought to demonstrate its existence," says Peter Abbamonte, lead researcher on the new study. "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."

The team made their observations using a new technique called momentum-resolved electron energy-loss spectroscopy (M-EELS). Made up of a combination of other instruments, this system allowed the researchers to precisely measure the collective excitations of the excitons, regardless of their momentum.

Using that process, the team examined non-doped crystals of a transition metal called dichalcogenide titanium diselenide (1T-TiSe2), cooling it to 190 Kelvin (-83° C, -118° F).

As it approached that critical temperature, the material entered a soft plasmon phase, which has never been seen in any material before. This phase marks the precursor to exciton condensation, and the researchers call it a "smoking gun" for the existence of excitonium.

"I remember Anshul (Kogar, co-author) being very excited about the results of our first measurements on TiSe2," says Mindy Rak, co-author of the study. "We were standing at a whiteboard in the lab as he explained to me that we had just measured something that no one had seen before: a soft plasmon. The work we did on TiSe2 allowed me to see the unique promise our M-EELS technique holds for advancing our knowledge of the physical properties of materials and has motivated my continued research on TiSe2."

While it's hard to project any possible technological applications of excitonium, the researchers say the discovery helps shed more light on the weird world of quantum mechanics.

The research was published in the journal Science.

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