Scientists have solved a 67-year-old mystery by accidentally discovering a "demon particle."

Scientists studying a material that could help unlock the secrets of superconductors have accidentally discovered a "demon" particle that was first theorized nearly 70 years ago but never experimentally confirmed. It's a strange particle. As they pass through a solid, their interactions create collective excitations that behave like entirely new particles with different properties, called quasiparticles.

Scientists have discovered a long-rumored "demon" particle hidden in the plasma oscillations of a metal

Plasma oscillations produce a special form of quasiparticles, called plasmas, that have new charges and masses different from the electrons that make them up. However, it was thought that plasmas could not exist at room temperature because the energy levels were not sufficient for the mass needed to form plasmas.

But in 1956, theoretical physicist David Pines predicted an exception - electrons in multiple energy bands could clump together in out-of-phase patterns to form plasmons with no mass or charge. Without any mass, they can form at any energy level and therefore at any temperature. This theoretical particle is known as the "demon of the pine tree" and has eluded detection ever since, until now.

Now, for the first time, scientists at the University of Illinois at Urbana-Champaign and Kyoto University have directly detected the particle in a metal called strontium ruthenate.

Peter Abbamonte, lead author of the study, said: "Most experiments use light to measure optical properties, but electrical neutrality means that the demon does not interact with light. We need to design a completely different kind of experiment."

Strontium ruthenate is an intriguing metal that has some of the properties of a high-temperature superconductor, even though it is not itself a high-temperature superconductor. The team hopes to find some clues about this valuable phenomenon by studying the material's electronic properties through a method called momentum-resolved electron energy loss spectroscopy. This involves bombarding the metal with electrons to observe its properties, including any quasiparticles that may form. In the process, the researchers discovered a puzzling massless plasma.

Ali Husain, co-author of the study, said: "At first, we had no idea what it was. As we started to rule out the possibilities, we began to suspect that we really had found the demon particle."

Scientists continue to explore the electronic structure of strontium ruthenate. Sure enough, the secret that had been hidden for 67 years finally came into view.

Edwin Huang, co-author of the study, said: "Pines' prediction of the devil requires quite special conditions, and it is not clear whether strontium ruthenate should have demon particles. We had to do microscopic calculations to figure out what was going on. When we did this, we found a particle consisting of two bands of electrons that oscillated out of phase with an almost equal amplitude, just as Pines described.

The team believes that demons may play a key role in the electronic behavior of a variety of metals.

The research is in the journal Nature.

Scientists have solved a 67-year-old mystery by accidentally discovering a "demon particle."

Scientists studying a material that could help unlock the secrets of superconductors have accidentally discovered a "demon" particle that was first theorized nearly 70 years ago but never experimentally confirmed. It's a strange particle. As they pass through a solid, their interactions create collective excitations that behave like entirely new particles with different properties, called quasiparticles.

Scientists have discovered a long-rumored "demon" particle hidden in the plasma oscillations of a metal

Plasma oscillations produce a special form of quasiparticles, called plasmas, that have new charges and masses different from the electrons that make them up. However, it was thought that plasmas could not exist at room temperature because the energy levels were not sufficient for the mass needed to form plasmas.

But in 1956, theoretical physicist David Pines predicted an exception - electrons in multiple energy bands could clump together in out-of-phase patterns to form plasmons with no mass or charge. Without any mass, they can form at any energy level and therefore at any temperature. This theoretical particle is known as the "demon of the pine tree" and has eluded detection ever since, until now.

Now, for the first time, scientists at the University of Illinois at Urbana-Champaign and Kyoto University have directly detected the particle in a metal called strontium ruthenate.

Peter Abbamonte, lead author of the study, said: "Most experiments use light to measure optical properties, but electrical neutrality means that the demon does not interact with light. We need to design a completely different kind of experiment."

Strontium ruthenate is an intriguing metal that has some of the properties of a high-temperature superconductor, even though it is not itself a high-temperature superconductor. The team hopes to find some clues about this valuable phenomenon by studying the material's electronic properties through a method called momentum-resolved electron energy loss spectroscopy. This involves bombarding the metal with electrons to observe its properties, including any quasiparticles that may form. In the process, the researchers discovered a puzzling massless plasma.

Ali Husain, co-author of the study, said: "At first, we had no idea what it was. As we started to rule out the possibilities, we began to suspect that we really had found the demon particle."

Scientists continue to explore the electronic structure of strontium ruthenate. Sure enough, the secret that had been hidden for 67 years finally came into view.

Edwin Huang, co-author of the study, said: "Pines' prediction of the devil requires quite special conditions, and it is not clear whether strontium ruthenate should have demon particles. We had to do microscopic calculations to figure out what was going on. When we did this, we found a particle consisting of two bands of electrons that oscillated out of phase with an almost equal amplitude, just as Pines described.

The team believes that demons may play a key role in the electronic behavior of a variety of metals.

The research is in the journal Nature.