Sixty-seven years after the theoretical prediction by David Pines, the elusive ‘devil’ particle, a massless, neutral entity present in solids, has been detected in strontium ruthenate, highlighting the value of an innovative research approach. it was done.
In 1956, theoretical physicist David Pines predicted that electrons in solids could do something strange. Electrons typically have mass and charge, but Pines argued that electrons could combine to create composite particles that are massless, neutral, and do not interact with light. He named this theoretical particle “devil”. It has since been theorized to play an important role in the behavior of various metals. Unfortunately, the same properties that make it interesting have allowed it to evade detection since its prediction.
Sixty-seven years later, a team of researchers led by Peter Abamonte, a professor of physics at the University of Illinois at Urbana-Champaign (UIUC), has finally discovered Pines’ elusive demon.As researchers report in the journal NatureThey used non-standard experimental techniques to directly excite the electronic modes of matter, allowing them to see demonic imprints in strontium ruthenate metal.
“Devils have been speculated in theory for a long time, but experimentalists had never studied them,” says Abamonte. “Actually, we weren’t looking for it. But it turned out that what we were doing was exactly right, and we found it.”
elusive devil
One of the most important discoveries in condensed matter physics is that electrons lose their individuality in solids. Electrons combine to form collective units through electrical interactions. With enough energy, electrons can even form composite particles called plasmons with new charges and masses determined by the underlying electrical interactions. However, the mass is usually so large that the energies available at room temperature cannot form plasmons.
Pines found an exception. If a solid has electrons in multiple energy bands, as in many metals, each plasmon will combine in out-of-phase patterns to form a new massless, neutral plasmon, the devil. could form, he argued. Since demons have no mass, they can form with any energy and therefore can exist at any temperature. This has led to the speculation that they may have important influences on the behavior of multiband metals.
The demon’s neutrality means that standard condensed matter experiments leave no trace. “Most of the experiments are done with light, measuring optical properties, but being electrically neutral means the devil doesn’t interact with light,” Abamonte says. . “It required a completely different kind of experiment.”
unexpected discovery
Abamonte recalls that he and his collaborators were working on strontium ruthenate. The reason was that the metal is similar, though not a high-temperature superconductor. Hoping to find clues as to why this phenomenon occurs in other systems, they conducted a first survey of the electronic properties of metals.
A research group led by Yoshi Maeno, a physics professor at Kyoto University, synthesized high-quality samples of the metal that were examined by momentum-resolved electron energy-loss spectroscopy by Abamonte and former graduate student Ali Husain. A non-standard technique that uses energy from electrons fired into the metal to directly observe metal features such as the plasmons that form. However, as the researchers examined the data, they discovered something unusual: a massless electronic mode.
Husain, now a researcher at Quantinuum, recalls: The devil is not mainstream. That possibility surfaced early on, but we basically laughed it off. But as I started to eliminate things, I began to doubt that I really had found the devil. “
Moore postdoctoral fellow at UIUC and condensed matter theorist Edwin Huang was eventually asked to calculate the electronic structure features of strontium ruthenate. “Mr. Pines’ demonic prophecies require quite specific conditions, but it was not clear to anyone whether demons should exist in strontium ruthenate in the first place,” he said. “We had to perform microscopic calculations to find out what was going on. When we did this, we found that, as Pines described, it oscillated about the same magnitude but out of phase. Then he found a particle consisting of two electronic bands.”
research contingency
Abamonte said his group’s “accidental” discovery of the devil was no accident. He emphasized that he and his group are using techniques that have not been widely adopted for substances that have not been well studied. He believes that their discoveries of unexpected significance were simply a result of trying something different rather than pure luck.
“This speaks to the importance of simply measuring things,” he said. “Most big discoveries aren’t planned. Let’s go somewhere new and see what’s there.”
Reference: “Devil of Pines Observed as 3D Acoustic Plasmon of Sr”2LuoFourAli A. Husain, Edwin W. Huang, Matteo Mitrano, Melinda S. Luck, Samantha I. Lubek, Guo Shufei, Yang Hongbin, Chanchal Sou, Yoshiteru Maeno, Bruno Uchoa, Tai Ho C. Chang, Philip E. Batson, Philip W. Phillips and Peter Abamonte, 9 Aug. 2023, Nature.
DOI: 10.1038/s41586-023-06318-8
Abamonte is a member of UIUC’s Materials Lab. Huang is a member of the UIUC Condensate Theory Laboratory.
Professor Philip Phillips of UIUC, Professor Matteo Mitrano of Harvard University, Professor Bruno Uchoa of the University of Oklahoma, and Professor Philip Baston of Rutgers University contributed to this study.
Support was provided by the U.S. Department of Energy, Japan Society for the Promotion of Science, the National Science Foundation, and the Gordon and Betty Moore Foundation.