"Demon particle" accidentally discovered, solving 67-year mystery

 

In the process of researching a substance that may help reveal the mysteries of superconductors, researchers have inadvertently uncovered a "demon" particle, which was first proposed about 70 years ago but had never been verified empirically.

Electrons are peculiar atoms. Their mutual interactions can result in collective excitations that behave like completely new particles with distinct properties when they pass through substances. We refer to these as quasiparticles.

Plasma oscillations give rise to a specific type of quasiparticle known as a plasmon, which has a new charge and mass apart from the constituent electrons. However, due to insufficient energy levels for the masses needed to produce plasmons, it was previously believed that they could not exist at ambient temperature.

However, theoretical physicist David Pines predicted in 1956 that there would be an exception to the norm: electrons in different energy bands may gather together in an out-of-phase manner to produce a massless, chargeless plasmon. They can develop at any temperature since they have no mass and can form at any energy level. After being dubbed the "Pines' demon," this hypothetical particle has remained undetected—at least until recently.

The demon has now been directly detected for the first time by researchers at Kyoto University and the University of Illinois Urbana-Champaign in a metal known as strontium ruthenate. It turned out that not even trying to look for it directly was the key.

According to study lead author Peter Abbamonte, "the great majority of experiments are conducted with light and measure optical properties, but being electrically neutral means that demons don't interact with light." "An entirely distinct type of experiment was required."

Even though strontium ruthenate isn't a high-temperature superconductor, it is an interesting metal that displays certain of its characteristics. Using a technique known as momentum-resolved electron energy-loss spectroscopy, the researchers examined the material's electrical characteristics in the hopes of uncovering any hints about that intriguing phenomenon. In order to observe the properties of the metal, including any quasiparticles that may arise, it is necessary to blast it with electrons. In the process, the scientists found a mysterious massless plasmon.

"We didn't know what it was at first," the study's co-author Ali Husain said. "Demons are not commonplace. We essentially shrugged it off when the prospect was raised early on. However, as we began eliminating possibilities, we began to worry that we may have actually discovered the devil.

The researchers next investigated strontium ruthenate's electrical structure in an attempt to pinpoint the offender. And sure enough, after 67 years in hiding, the devil was eventually revealed.

Edwin Huang, a study co-author, stated, "Pines' prediction of demons necessitates rather specific conditions, and it was not clear to anyone whether strontium ruthenate should have a demon at all." To understand what was happening, a tiny calculation had to be made. When we performed this, we discovered a particle that was identical to what Pines had described: it was made up of two-electron bands oscillating out of phase at almost the same magnitude.

The group thinks that a variety of metals' electrical properties may be significantly influenced by demons.

The study was released in the Nature Journal.