Exciton, New Form of Matter, Discovered

S u m m a r y :
Exciton, a new form of matter, has been discovered by physicists, a team that has become the first to measure excitonium collective modes. The findings are published in the journal Science.

Excited At Exciton

Scientists are excited at the discovery of new form of matter, exciton. The latter has left researchers clueless for nearly 5 decades now—but now, thanks to the new study, conducted by physicists from the University of Illinois at Urbana-Champaign, the theoretical matter has been proved.

An artist’s rendition of what excitons look like collectively in an excitonic solid. The yellow region represents propagating domain walls, and the blue, an ordered solid exciton background. Photo Credits: Peter Abbamonte, U. of I. Department of Physics and Frederick Seitz Materials Research Laboratory.

The Brains Behind the Discovery

Led by Physics Professor, Peter Abbamonte, graduate students Anshul Kogar and Mindy Rak, and researchers from the University of California, Berkeley, and University of Amsterdam, pooled in their minds, and resources to study non-doped crystals of transition metal dichalcogenide titanium diselenide (1T-TiSe2). They repeated the experiments, and their results were reproduced 5 times from different cleaved crystals.

Formation of An Exciton

What is an exciton? Basically, it is made up of excitons, which are particles formed when an escaped electron leaves behind a hole, a pair that is most queer in the quantum world. And, because it displays macroscopic quantum traits like being a superconductor, superfluid, and insulating electronic crystal, it is categorised as a condensate. What makes it so special? It is the hole produced by the electron: the latter gets excited, and leaves a valence band of electrons in a semiconductor, moving over the energy gap to the empty conduction band, creating this hole in the valence band.

The hole, then, behaves like a particle (because of the collective behaviour of the electrons in its surroundings), one with a positive charge, thereby attracting the runaway electron. As a result, the electron fills up the hole, a composite particle is formed (a boson), an exciton.

Tracking the Path of An Electron

Abbamonte and his colleagues were able to achieve what none has before thanks to a technique of theirs known as momentum-resolved electron energy-loss spectroscopy (M-EELS). This allowed them to measure valence band excitations in a more sensitive manner. With an EEL spectrometer retrofitted by Kogar, and a goniometer, they could measure the path of an electron, while also detecting the amount of energy and momentum it lost, together with the precise momentum in real space. Thus was the team able to measure the collective excitations of the paired electrons and holes.

“This result is of cosmic significance,” says 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.”


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