Physicists Confirm Existence of Massless “Demon” Particle

Physicists studying an exotic metal have uncovered new evidence proving the existence of the so-called “demon” particle – a previously unknown quasiparticle that is speculated to play an important role in determining the electronic behavior of a wide variety of metals and superconductors.

The research is published in the journal Nature.

Demons in physics

A core tenet of condensed matter physics is the idea that electrons lose their individuality when they are in solids. With enough energy – normally at very high temperatures – these electrons oscillate together in such a way that they give rise to quasiparticles called plasmons.

Just like phonons are the quantum mechanical description for physical vibrations, plasmons describe the oscillation of delocalized electrons in a material. Depending on the electrons involved, plasmons will have a new charge and mass. This new mass is usually so large that plasmons will not form at room temperature.

But in 1956, theoretical physicist David Pines made a prediction. He believed that if a solid has more than one energy band, as many metals do, then its plasmons could combine in an out-of-phase pattern to form a new plasmon that is both massless and chargeless. Pines would name this new neutral plasmon the “demon” particle, a play on the DEM acronym for “distinct electron motion”.

Since this particle is massless, Pines believed that it could form even at room temperature or below. With such a particle able to exist at all temperatures, researchers hypothesized that these demon particles could play an important role in the electronic transitions in superconductors, semimetals and other interesting materials.

For the past 67 years, Pines’ prediction has been just that – a prediction. This is largely due to the fact that the demon particle’s unique properties also make it perfectly suited to eluding detection by traditional means.

“The vast majority of experiments are done with light and measure optical properties, but being electrically neutral means that demons don’t interact with light,” said Peter Abbamonte, a professor of physics at the University of Illinois Urbana-Champaign. “A completely different kind of experiment was needed.”

67-year-old prediction confirmed during unrelated study

Recently, Abbamonte was the lead researcher on a project investigating strontium ruthenate (Sr₂RuO₄). This metal is interesting as it bears a strong similarity to high-temperature superconductors without actually being one.

Hoping to explain why strontium ruthenate lacks this property, Yoshi Maeno, a professor of physics at Kyoto University, synthesized some high-quality samples of the metal which were passed on to Abbamonte and former graduate student Ali Husain for study.

To do this, the team used the nonstandard technique of momentum-resolved electron energy-loss spectroscopy (M-EELS), which uses energy from electrons being fired at metal samples to directly observe the metal’s features, including any plasmons that form.

Upon studying the data, the researchers found something unusual: an electronic mode had been observed, but it had no measurable mass.

A serendipitous discovery?

Far from being a eureka moment, the researchers at first could not understand what might be causing these readings. There are other known quasiparticles, but the velocity of this quasiparticle was too fast to be an acoustic phonon and too slow for a surface phonon.

“At first, we had no idea what it was. Demons are not in the mainstream. The possibility came up early on, and we basically laughed it off,” recalled Husain. “But, as we started ruling things out, we started to suspect that we had really found the demon.”

For further investigation, the researchers contacted Edwin Huang, a condensed matter theorist at the University of Illinois Urbana-Champaign, to calculate the features of strontium ruthenate’s electronic structure.

“Pines’ prediction of demons necessitates rather specific conditions, and it was not clear to anyone whether strontium ruthenate should have a demon at all,” Huang said. “We had to perform a microscopic calculation to clarify what was going on. When we did this, we found a particle consisting of two electron bands oscillating out-of-phase with nearly equal magnitude, just like Pines described.”

“Demons have been theoretically conjectured for a long time, but experimentalists never studied them,” Abbamonte said. “In fact, we weren’t even looking for it. But it turned out we were doing exactly the right thing, and we found it.”

Demons may be a “pervasive feature” of some metals

While his group did not set out to prove the existence of such a quasiparticle, Abbamonte firmly states that it is no accident how the team “serendipitously” came to discover the demon particle. The M-EELS technique is not very widely used and here it was being deployed on a material that has also not been extensively studied.

“It speaks to the importance of just measuring stuff,” Abbamonte said. “Most big discoveries are not planned. You go look somewhere new and see what’s there.”

It is also important to note that demon particles have been conjectured to play a role in mediating superconductivity, which is likely to attract attention from researchers in other areas of study. Superconductors allow the passage of electricity through them with no resistance, and so could greatly impact the efficiency of electronic and energy transfer technologies.

Now that their existence has been confirmed, the study authors are encouraging further research into these quasiparticles. As they write in the paper, there is the potential for scanning electron microscopy measurements to provide even more information about the demon particles. The study of other metals could also lead to unique insights, they say.

Reference: Husain AA, Huang EW, Mitrano M, et al. Pines’ demon observed as a 3D acoustic plasmon in Sr₂RuO₄. Nature. 2023. doi: 10.1038/s41586-023-06318-8

This article is a rework of a press release issued by the University of Illinois Urbana-Champaign. Material has been edited for length and content.