In the quest to study a material that could unlock the secrets of superconductivity, scientists have unexpectedly discovered a “demon particle”—a type of particle that theoretically appeared nearly 70 years ago but has never been experimentally confirmed.
“Demon particle” refers to a peculiar type of electron particle. As they move through solids, their interactions can lead to the formation of collective excitations that behave like entirely new particles with distinct characteristics. Thus, they are also known as quasiparticles.
A specific quasiparticle form of the demon particle is called plasmon, which arises from plasma oscillations and possesses charge and mass distinct from the electrons that constitute them. However, it is believed that plasmons cannot exist at room temperature.
In the 1950s, theoretical physicist David Pines made a prediction. He believed that if a solid had more than one energy band, like many metals, its plasmons could combine in a phase-shifted manner to form a new plasmon that would be both massless and chargeless. Pines named this neutral plasmon “demon particle.”
Because this particle has no mass, Pines believed it could form even at room temperature or lower. With such a particle potentially existing at any temperature, researchers hypothesized that these demon particles could play a significant role in electronic transitions in superconductors, semimetals, and other interesting materials.
Physicists studying a peculiar metal have found new evidence supporting the existence of the so-called “demon particle”—a previously unknown quasiparticle believed to play a crucial role in determining the electronic behavior of various metals and superconductors. (Photo: Technology Networks)
For the past 67 years, Pines’ prediction remained just that—a prediction. This was largely due to the unique properties of the demon particle, which made it difficult to detect using traditional methods.
Currently, scientists at the University of Illinois Urbana-Champaign and Kyoto University have made the first direct discoveries of the demon particle in a metal known as strontium ruthenate.
Peter Abbamonte, the lead author of the study, stated: “Most experiments are conducted with light and measure optical properties, but electrically neutral particles won’t interact with light.”
Strontium ruthenate (Sr2RuO4) is a very intriguing metal because it has strong similarities with high-temperature superconductors without actually being one. Hoping to explain why strontium ruthenate lacks this property, Yoshi Maeno, a physics professor at Kyoto University, synthesized several high-quality metal samples and sent them to Abbamonte and graduate student Ali Husain for study.
To do this, the research team employed a non-standard momentum-resolved electron energy loss spectroscopy (M-EELS) technique, using energy from electrons fired into the metal samples to directly observe the characteristics of the metal, including any forming plasmons.
While analyzing the data, researchers encountered an anomaly: an electronic mode was observed, but it had no measurable mass.
Just as phonons describe quantum mechanical vibrations, plasmons describe the oscillations of electrons localized within materials. Depending on the participating electrons, plasmons will possess new charge and mass. This new mass is typically so large that plasmons do not form at room temperature. (Photo: New Atlas).
“At first, we didn’t know what it was,” Ali Husain, a co-author of the research, remarked. “But as we began to rule out every possible outcome, we started to suspect that we had truly found the demon particle.”
To confirm this hypothesis, the scientists reached out to Edwin Huang, a condensed matter theorist at the University of Illinois Urbana-Champaign, to calculate the electronic structure characteristics of strontium ruthenate.
“Pines’ prediction about the demon particle requires quite specific conditions, and no one knew whether strontium ruthenate actually contained a demon particle,” Huang explained. “We had to perform a microscopic calculation to clarify what was happening. When we did this, we found a particle consisting of two phase-shifted electron bands of nearly equal magnitude, just as Pines described.”
“The demon particle has been theoretically postulated for a long time, but experimentalists have never studied them,” Abbamonte added.
And indeed, the demon particle, which had only existed theoretically for the last 67 years, has finally revealed itself.
Although Peter Abbamonte’s team did not conduct the research with the aim of proving the existence of such a quasiparticle, Abbamonte firmly asserted that it was no coincidence that the team “accidentally” discovered the demon particle.
The research team believes that the demon particle could play an important role in the electronic behaviors of various metals.
The study was published in the journal Nature.
The demon particle is postulated to mediate superconductivity phenomena, which could attract the attention of researchers in other fields. Superconductors allow electricity to flow through them without resistance, potentially greatly impacting the efficiency of energy transmission and electronic technologies. (Photo: Technology Networks).
