Physicists excited by discovery of new form of matter, excitonium

December 8, 2017 by Siv Schwink, University of Illinois at Urbana-Champaign
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). Credit: Peter Abbamonte, U. of I. Department of Physics and Frederick Seitz Materials Research Laboratory

Excitonium has a team of researchers at the University of Illinois at Urbana-Champaign... well... excited! Professor of Physics Peter Abbamonte and graduate students Anshul Kogar and Mindy Rak, with input from colleagues at Illinois, University of California, Berkeley, and University of Amsterdam, have proven the existence of this enigmatic new form of matter, which has perplexed scientists since it was first theorized almost 50 years ago.

The team studied non-doped crystals of the oft-analyzed transition metal dichalcogenide titanium diselenide (1T-TiSe2) and reproduced their surprising results five times on different cleaved crystals. University of Amsterdam Professor of Physics Jasper van Wezel provided crucial theoretical interpretation of the experimental results.

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.

The relationship between energy and momentum for the excitonic collective mode observed with M-EELS. Credit: Peter Abbamonte, U. of I. Department of Physics and Frederick Seitz Materials Research Laboratory

Abbamonte and his team were able to overcome that challenge by using a novel technique they developed called momentum-resolved electron energy-loss spectroscopy (M-EELS). M-EELS is more sensitive to valence band excitations than inelastic X-ray or neutron scattering techniques. Kogar retrofit an EEL spectrometer, which on its own could measure only the trajectory of an electron, giving how much energy and momentum it lost, with a goniometer, which allows the team to measure very precisely an electron's momentum in real space.

With their new technique, the group was able for the first time to measure collective excitations of the low-energy bosonic particles, the paired electrons and holes, regardless of their momentum. More specifically, the team 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 three-dimensional solid and the first-ever definitive evidence for the discovery of excitonium.

"This result is of cosmic significance," affirms 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."

Rak recalls the moment, working in the Abbamonte laboratory, when she first understood the magnitude of these findings: "I remember Anshul being very excited about the results of our first measurements on TiSe2. 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."

U of I Professor of Physics Peter Abbamonte (center) works with graduate students Anshul Kogar (right) and Mindy Rak (left) in his laboratory at the Frederick Seitz Materials Research Laboratory. Credit: L. Brian Stauffer, University of Illinois at Urbana-Champaign.

"The excitement generated by this discovery remained with us throughout the entire project," she continues. "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."

Kogar admits, discovering excitonium was not the original motivation for the research—the team had set out to test their new M-EELS method on a crystal that was readily available—grown at Illinois by former graduate student Young Il Joe, now of NIST. But he emphasizes, not coincidentally, excitonium was a major interest:

"This discovery was serendipitous. But Peter and I had had a conversation about 5 or 6 years ago addressing exactly this topic of the soft electronic mode, though in a different context, the Wigner crystal instability. So although we didn't immediately get at why it was occurring in TiSe2, we did know that it was an important result—and one that had been brewing in our minds for a few years."

The team's findings are published in the December 8, 2017 issue of the journal Science in the article, "Signatures of exciton condensation in a transition metal dichalcogenide."

This fundamental research holds great promise for unlocking further quantum mechanical mysteries: after all, the study of macroscopic quantum phenomena is what has shaped our understanding of quantum mechanics. It could also shed light on the metal-insulator transition in band solids, in which exciton condensation is believed to play a part. Beyond that, possible technological applications of excitonium are purely speculative.

Explore further: Spontaneous Bose-Einstein condensation of excitons

More information: Anshul Kogar et al. Signatures of exciton condensation in a transition metal dichalcogenide, Science (2017). DOI: 10.1126/science.aam6432

Related Stories

Spontaneous Bose-Einstein condensation of excitons

December 8, 2017

Excitons are pairs of electrons and holes inside a solid material that together behave like a single particle. It has long been suspected that when many such excitons exist in the same piece of matter, they can form a single ...

When an exciton acts like a hole

August 27, 2014

(Phys.org) —When is an electron hole like a quasiparticle (QP)? More specifically, what happens when a single electron hole is doped into a two-dimensional quantum antiferromagnet? Quasiparticle phenomena in such a system ...

Recommended for you

X-rays reveal chirality in swirling electric vortices

January 16, 2018

Scientists used spiraling X-rays at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) to observe, for the first time, a property that gives handedness to swirling electric patterns – dubbed ...

Slow 'hot electrons' could improve solar cell efficiency

January 16, 2018

Photons with energy higher than the band gap of the semiconductor absorbing them give rise to what are known as hot electrons. The extra energy in respect to the band gap is lost very fast, as it is converted into heat and ...

Quan­tum physics turned into tan­gi­ble re­al­ity

January 16, 2018

ETH physicists have developed a silicon wafer that behaves like a topological insulator when stimulated using ultrasound. They have thereby succeeded in turning an abstract theoretical concept into a macroscopic product.

13 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Steve 200mph Cruiz
5 / 5 (5) Dec 08, 2017
Where does the spin come from that lets the combination of the electron and hole to behave as a boson?
ursiny33
1 / 5 (4) Dec 08, 2017
Its quit possible the electron was bound magnetically to a poistron which can nor be measured in the pairing by its smaller mass charge in a electron combination as such do to the larger electron charge mass the seperation of this construction left the hole in which the position still magnetically bound in the electron chain in the conductor attracts an electron to fill the void on the position bound magnetic position
ursiny33
1 / 5 (4) Dec 08, 2017
A line of electrons would repel each other a line of electron positron electron positron electron that you could not measure because the position is a minority particle mass charge in the chain of moving current of electrons, when you remove an electron from the chain of electron current leaves two positrons repelling each other you alleged hole in the chain
Da Schneib
4.4 / 5 (7) Dec 08, 2017
Good question, @Steve.

Electrons are fermions and have spin +/- 1/2. Having spin 1/2 is the definition of a fermion. Quantum particles have intrinsic spins, for reasons that have to do with Special Relativity; the explanation of why is quite complex, but if you want to push further into this just ask and I'll see if I can try to explain it for you.

Because of the spin of the electron, a place where an electron should be, but isn't, called a "hole," has spin 1/2 too. In semiconductors, like the titanium diselenide used in this experiment, holes behave like they're real particles, and they're not unique. Plasmons and phonons behave like they're particles too, and are even more esoteric than holes. I can explain those for you, but it's even more difficult than showing how spin is a consequence of SRT; but again, ask, and I'll see what I can do.

So with a pair of spin 1/2 particles, there are two possibilities: they are equal or opposite.
[contd]
Da Schneib
4 / 5 (5) Dec 08, 2017
[contd]
If they are equal, either +1/2 or -1/2, then you get a composite spin 1 particle, and that's a boson because bosons have integer spins. If they're opposite, then you get spin 0, which is also a boson.

So the direct answer to your question, where does the spin come from, is that it's intrinsic to the particles. It's like asking where a particle's mass, or its charge, "comes from." It's just there. That's how particles are. But I think the above is a bit better answer than that for you.
axemaster
5 / 5 (3) Dec 08, 2017
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.

"It defies reason"? To who? All of this is perfectly reasonable and expected.
mackita
2.3 / 5 (3) Dec 08, 2017
The vibrations of lattice concentrate at the edges of crystal, where they get geometrically frustrated, which supports their condensation even more (they're shielded from thermal fluctuations of lattice here). This observation is the solid state evidence of lattice gauge theory, which considers, that the boson particles exchanged between pair of fermions may condense and serve as another generation of fermions exchanging another bosons, and so on, recursively. The nuclear physics analogy is for example formation of glueball fermions from gluon bosons, which itself exchange an energy between another nucleon fermions. We can find many other analogies of this materialization in both particle physics, both biology and social sciences or economy.
mackita
2.3 / 5 (3) Dec 08, 2017
For example money are serving like virtual bosons exchanging the value between material goods (fermions). But once the financial market gets sufficiently intensive and separated from price fluctuations, the the money itself can become a subject of independent exchange - well, in form of various monetary derivatives. In particular, the excitonium resembles the bill exchange market (someone grants a loan to hole - debtor) and the bills collected from creditors can be sold independently. Try to find and propose some meaningful social analogy for it.
NoStrings
not rated yet Dec 09, 2017
It is all very exciting. Except for the claim of it never being produced before. There is not much difference between exciton and soliton . Solitons were experimented with since 1970th. I was in a research group that produced solitons in layered semiconductors in early 1980th. We knew it was just another case of a soliton - a non-dispersing wave of electrons and holes - in a different physical material, which was 'exciting' but not anything to claim 'never before'. I guess, we could, never before in that material, but not a material for a major headline. And it has no practical application since.
savvys84
5 / 5 (2) Dec 11, 2017
Wow time for sexitonium.

On topic tho, there are more ways that one to produce materials that dont belong to our universe
Whydening Gyre
not rated yet Dec 27, 2017
Wow time for sexitonium.

On topic tho, there are more ways that one to produce materials that dont belong to our universe

If they can be produced, they belong...
and... sexitonium is the boson produced when "Led Zeppelin" or "Lords of Acid" (or any host of others) fire up their guitars...
Whydening Gyre
5 / 5 (1) Dec 27, 2017
Wow time for sexitonium.

On topic tho, there are more ways that one to produce materials that dont belong to our universe

If they can be produced, they belong...
and... sexitonium is the boson produced when "Led Zeppelin" or "Lords of Acid" (or any host of others) fire up their guitars...

(Or when Salma Hayek takes off her shirt...)
savvys84
5 / 5 (1) Dec 28, 2017
@whydening gyre
Lol nice one. yo Salma

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.