Physicists confirm existence of “demon” particle

A “demon” particle that has been haunting physicists for nearly 70 years has been found in an experiment by American researchers.

It is not a particle in the traditional sense like a proton or electron. It is a “composite” particle made up of a combination of electrons, in a solid.

In 1956, theoretical physicist David Pines predicted that electrons in a solid could do something strange. Electrons have both mass and charge. But Pines asserted that combinations of electrons in a solid could form a composite particle that is massless, has no charge and does not interact with light.

He called this a “demon” particle.

Physicists have speculated that Pines’s demon plays an important role in the characteristics of many metals. But they’ve never been able to confirm its existence until now. The research from a team at the University of Illinois, Urbana-Champaign, is published in Nature.

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

Abbamonte’s team was using an unusual experimental setup to excite the electronic modes of a material when they spotted the demon in the metal strontium ruthenate (Sr₂RuO₄).

Condensed matter physics has revealed the collective behaviour of electrons in solids. Interactions between the particles can lead to the formation of units which act and interact collectively. If enough energy is supplied to the system, the electrons can form composite particles known as plasmons.

The plasmons have a new charge and mass which arises out of the underlying interactions between the electrons. But the energy required to produce plasmons is so high that they cannot be produced at room temperature.

Pines’s demon is an exception to this.

He argued that, the arrangement of electron energies in a metal would allow the formation of a massless plasmon. And, because it is massless, it would require no extra energy to be formed.

Abbamonte explains that, because the demon particle doesn’t interact with light, a nonstandard apparatus was required to detect it. The strontium ruthenate’s properties were studied by shooting electrons into it, rather than using light.

In their data, they found an electron mode with no mass.

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

“Pines’ prediction of demons necessitates rather specific conditions, and it was not clear to anyone whether strontium ruthenate should have a demon at all,” says co-author Dr Edwin Huang. “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.”

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