Physicists Prove Existence of New Form of Matter: Excitonium

New research led by a University of Illinois physicist has proved the existence of a new form of matter — ‘excitonium,’ a solid crystal of excitons. The research, led by Professor Peter Abbamonte, is published in the December 8 issue of the journal Science.

Artist’s depiction of the collective excitons of an excitonic solid. These excitations can be thought of as propagating domain walls (yellow) in an otherwise ordered solid exciton background (blue). Image credit: Peter Abbamonte, University of Illinois.

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.

“Our result is of cosmic significance. Ever since the term ‘excitonium’ was coined in 1968 by Harvard theoretical physicist Bert Halperin, physicists have sought to demonstrate its existence,” Professor Abbamonte said.

“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.”

Professor Abbamonte and his co-authors from the University of Illinois, the University of Amsterdam in the Netherlands and the University of Oxford in the UK studied non-doped crystals of the transition metal dichalcogenide titanium diselenide (1T-TiSe₂) and reproduced their surprising results five times on different cleaved crystals.

“Until now, physicists have not had the experimental tools to positively distinguish whether what looked like excitonium wasn’t in fact a Peierls phase,” they explained.

“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.”

The team was able to overcome that challenge by using a novel technique they developed called momentum-resolved electron energy-loss spectroscopy.

“With this new technique, we were able for the first time to measure collective excitations of the low-energy bosonic particles, the paired electrons and holes, regardless of their momentum,” the authors said.

“More specifically, we achieved the first-ever observation in any material of the precursor to exciton condensation, a soft plasmon phase that emerged as the material approached its critical temperature of 190 Kelvin.”

“This soft plasmon phase is ‘smoking gun’ proof of exciton condensation in a 3D solid and the first-ever definitive evidence for the discovery of excitonium.”

_____

Anshul Kogar et al. 2017. Signatures of exciton condensation in a transition metal dichalcogenide. Science 358 (6368): 1314-1317; doi: 10.1126/science.aam6432