A 67-year-old particle physicist’s bizarre prediction has finally been confirmed

Researchers have discovered Devil Pines, a collection of electrons in a metal that behaves like a massless wave. Credit: Grainger College of Engineering, University of Illinois Urbana-Champaign

Sixty-seven years after David Baines’ theoretical prediction, the elusive “devil” particle, a massless and neutral entity in solids, has been discovered in strontium ruthenates, underlining the value of innovative research approaches.

In 1956, theoretical physicist David Baines predicted that electrons in solid matter could do something strange. Although electrons usually have mass and an electric charge, Baines asserted that they can combine to make a compound particle that is massless, neutral, and does not interact with light. He called this theoretical particle the “Satan”. Since then, it has been hypothesized to play an important role in the behavior of a wide variety of minerals. Unfortunately, the same characteristics that make him so interesting have allowed him to elude detection since he expected it.

After 67 years, a research team led by Peter Abamonte, a professor of physics at the University of Illinois Urbana-Champaign (UIUC), has finally discovered the elusive Devil of Pines. The researchers also reported in the journal natureThey used a non-standard experimental technique that directly excites the electronic patterns of matter, allowing them to see the devil’s signature in the mineral strontium ruthenate.

“Devils have been speculated theoretically for a long time, but they have never been studied by empiricists,” Abamonte said. “Actually, we weren’t even looking for it. But it turns out we were doing exactly the right thing, and we found it.”

Satan is elusive

One of the most important discoveries of condensed matter physics is that electrons lose their individuality in solids. Electrical interactions cause electrons to combine to form collective units. With enough energy, electrons can form complex particles called plasmons with new charge and mass determined by fundamental electrical interactions. However, the mass is usually too great for plasmons to form with the energies available at room temperature.

Baines found an exception. If a solid contains electrons in more than one energy band, as is the case with many metals, he argued that their plasmons can combine in an out-of-phase pattern to form a new, massless, neutral plasmon: a demon. Since demons are massless, they can form with any energy, so they can exist at all temperatures. This has led to speculation that they have important effects on the behavior of multiscale minerals.

The demons’ neutrality means that they leave no signature in standard condensed matter experiments. “The vast majority of experiments are done using light and measuring optical properties, but being electrically neutral means demons don’t interact with light,” Abbamonte said. “A completely different kind of experiment was needed.”

An unexpected discovery

Abbamonte recalls that he and his collaborators were studying strontium ruthenite for an unrelated reason: the metal is a high-temperature superconductor without being a superconductor. Hoping to find clues as to why this phenomenon occurs in other systems, they conducted the first survey of the metal’s electronic properties.

The research group headed by Yoshi Maeno, a professor of physics at Kyoto University, collected high-quality samples of the metal that Abamonte and former graduate student Ali Hussein examined using momentum-resolved electron energy loss spectroscopy. It is a non-standard technique, and uses the energy from the electrons that are released into the metal to directly observe the properties of the mineral, including the plasmons that form. As the researchers sifted through the data, they found something unusual: an electron mode without mass.

“At first, we had no idea what it was,” recalls Hussain, now a research scientist at Quantinum. Demons are not in the mainstream. This possibility appeared early on, and we basically laughed at it. However, as we began to rule things out, we began to suspect that we had actually found Satan.

Edwin Huang, a UIUC postdoctoral researcher and condensed matter theorist, was eventually asked to calculate features of the electronic structure of strontium ruthenite. “Baines’ prediction of the existence of demons entails fairly specific conditions, and it was not clear to anyone whether strontium ruthenite should have a demon at all,” he said. “We had to perform a microscopic calculation to show what was going on. When we did, we found a particle consisting of two bands of electrons that oscillate out of phase by about the same amount, just as Baines described.

serendipity to search

According to Abbamont, it was no accident that his group discovered Satan “by accident”. He asserted that he and his group were using a technique not widely used on material that had not been well studied. It is believed that their discovery of something unexpected and significant is the result of trying something different, not just luck.

“He talks about the importance of just measuring things,” he said. “Most great discoveries are unplanned. Go find a new place and see what’s out there.”

Reference: “Note the Demon Pines as a 3D acoustic plasmon in Sr2RuO4Written by Ali A. Hussain, Edwin W. Huang, Matthew Mitrano, Melinda S. Rack, Samantha I. Rubik, Ziofi Gu, Hongbin Yang, Chanchal Su, Yoshiteru Maino, Bruno Ochoa, Tai Si Chiang, Philip E. Batson, Philip W. Phillips and Peter Abamonte, Aug. 9, 2023, Available Here. nature.
doi: 10.1038/s41586-023-06318-8

Abamonte is a member of the Materials Research Laboratory at UIUC. Huang is a member of the Institute for Condensed Matter Theory at UIUC.

Professors Philip Phillips of UIUC, Matteo Mitrano of Harvard University, Bruno Ochoa of the University of Oklahoma, and Philip Paston of Rutgers University contributed to this work.

Support has been provided by the US Department of Energy, the Japan Association for the Advancement of Science, the National Science Foundation, and the Gordon and Betty Moore Foundation.

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