If you thought that identifying all of the states of matter was "old science," you might want to think again. Matter continues to surprise us, as a team of researchers from the University of Illinois at Urbana-Champaign recently demonstrated. They have proven the existence of a whole new form of matter called excitonium, a finding that legitimizes a theory first posited nearly 50 years ago.
Excitonium is a material made up of particles known, appropriately, as excitons, which are themselves composed of escaped electrons and the holes they leave behind after they escape. This unusual condensate exhibits macroscopic quantum phenomena and has the power to get matter to act as a superfluid, superconductor, or even as an insulating electronic crystal.
“This result is of cosmic significance,” affirmed Professor of Physics Peter Abbamonte, who led the research, in a press release. “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.”
Abbamonte's team was able to get their definitive proof by studying crystals doped with dichalcogenide titanium diselenide (1T-TiSe2), a transition metal. In their experiments, they were able to observe a soft plasmon phase that is the precursor to the exciton condensation for the first time-- what they described as "smoking gun proof" of excitonium.
Basically, excitonium is what you get when an electron becomes so excited (such as when a photon is absorbed by a semiconductor) that it jumps over to another energy band. Upon jumping over, it leaves a "hole," or empty space from whence it came. Calling these absences "holes" is a bit of a misnomer, however, because the holes are hardly passive, empty spaces. Rather, they end up acting much like positively charged electrons do, attracting the electrons that escaped. They are holes that desperately miss what once filled them.
This resultant attraction between escaped electrons and their holes is what forms excitons.
It's all a bit of a mind-bender, but it does have real world significance. For one, the discovery promises to unlock further quantum mechanical mysteries (as if we need more quantum mysteries!), which will ultimately enlighten our understanding of how the cosmos operates. Although possible technological applications of excitonium are purely speculative at this point, this is the kind of finding that could point the way toward the holy grail of condensed-matter physics: a room-temperature superconductor.
The study has been published in the journal Science.