Scientists spot Pines 'demon', 67 years after it was predicted

Why demons have remained elusive

Theoretical physicist Pines, however, argued that if atoms have multiple energy bands, as most metals do, plasmons can even form in an out-of-phase pattern where it is massless and neutral in charge. He called them 'demons' in honor of James Clerk Maxwell, who conceived Maxwell's Demon.

Pines suggested that since demons are massless, they can form with any energy and can exist at all temperatures. However, since they do not have any charge, they do not leave a signature in conventional condensed matter experiments, where light is used, and optical properties are measured. Since demons do not interact with light, they have remained elusive for all these years.

A 'serendipitous' discovery?

Back in 2018, a team of researchers led by Peter Abbamonte, a professor of physics at the University of Illinois Urbana-Champaign, was studying strontium ruthenate (Sr₂RuO₄) to determine the material's electronic properties.

They used a method called momentum-resolved electron energy-loss spectroscopy, where electrons are shot into the metal to observe its features. The method also allows for the detection of plasmons, but the researchers found something more: an electronic mode with no mass.

This piqued the interest of the researchers, who even laughed off the possibility that they had found Pines' demon. However, as they began pouring over their data to discover the reasons for their observations, they found two electron bands oscillating out of phase, much like Pines described nearly seven decades ago.

Abbamonte's team was not explicitly looking for Pines's demon, but the physics professor does not believe it was all a "serendipitous" discovery. In the university press release, he emphasized that his team used a not-so-widely employed method on a not-so-widely studied substance. They were bound to find something new and significant because they were doing something different.

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

The research findings were published in the journal Nature.

Abstract

The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. 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 carrier1. Consisting of out-of-phase movement of electrons in different bands, demons are acoustic, electrically neutral and do not couple to light, so have never been detected in an equilibrium, 3D metal. Nevertheless, demons are believed to be critical for diverse phenomena including phase transitions in mixed-valence semimetals2, optical properties of metal nanoparticles3, soundarons in Weyl semimetals4 and high-temperature superconductivity in, for example, metal hydrides3,5,6,7. Here, we present evidence for a demon in Sr2RuO4 from momentum-resolved electron energy-loss spectroscopy. Formed of electrons in the β and γ bands, the demon is gapless with critical momentum qc = 0.08 reciprocal lattice units and room-temperature velocity v = (1.065 ± 0.12) × 105 m s−1 that undergoes a 31% renormalization upon cooling to 30 K because of coupling to the particle–hole continuum. The momentum dependence of the intensity of the demon confirms its neutral character. Our study confirms a 67-year old prediction and indicates that demons may be a pervasive feature of multiband metals.