Physicists confirm the 67-year-old prediction of massless, neutral composite particle

Demon hunting.

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A metal’s plasmon is an oscillation of its electron density in a quantized collective manner, its distinctive excitation. In 1956, David Pines predicted that a distinct type of plasmon, dubbed a ‘demon,’ could exist in three-dimensional (3D) metals containing more than one species of charge carrier.

The demons consist of out-of-phase movement of electrons in different bands. They are acoustic, electrically neutral, and do not couple to light. That’s why they have never been detected in an equilibrium, 3D metal.

Sixty-seven years after Pines’ demon was predicted to exist, a team of researchers under the direction of Peter Abbamonte, a professor of physics at the University of Illinois Urbana-Champaign, has now discovered the monster. They observed the demon’s signature in the metal strontium ruthenate using an unconventional, experimental method directly activating a material’s electronic states.

If they have enough energy, the electrons can even combine to produce composite particles known as plasmons with new charges and masses defined by the underlying electric interactions. However, the mass is typically so big that plasmons cannot form at ambient temperature using the available energies.

Pines discovered an anomaly. If a solid contains electrons in more than one energy band, as many metals do, the plasmons from each band can merge in an out-of-phase pattern to create a new, massless, neutral plasmon called a demon. Demons may create with any energy since they have no mass, allowing them to exist at any temperature. This has given rise to the theory that they significantly influence the behavior of multi-band metals.

Abbamonte said, “Demons’ neutrality means they do not leave a signature in standard condensed matter experiments. 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. A completely different kind of experiment was needed.”

Scientists were studying strontium ruthenate. They were performing the first study of the metal’s electrical characteristics to uncover hints as to why the event occurs in other systems.

With the help of momentum-resolved electron energy-loss spectroscopy, Abbamonte and former doctoral student Ali Husain analyzed the metal they had produced in high-quality samples. It is an unconventional method that employs the energy of electrons fired into the metal to observe the properties of the metal, particularly the plasmons that arise. However, as scientists were poring over the data, they discovered something peculiar: an electrical mode devoid of mass.

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

Edwin Huang, a Moore Postdoctoral Scholar at UIUC and condensed matter theorist, was eventually asked 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. 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.”

Journal Reference:

  1. Husain, A.A., Huang, E.W., Mitrano, M. et al. Pines’ demon observed as a 3D acoustic plasmon in Sr2RuO4. Nature (2023). DOI: 10.1038/s41586-023-06318-8
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