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hubie [soylentnews.org] writes:

Physicists confirm 67-year-old prediction of massless, neutral composite particle [illinois.edu]:

In 1956, theoretical physicist David Pines predicted that electrons in a solid can do something strange. While they normally have a mass and an electric charge, Pines asserted that they can combine to form a composite particle that is massless, neutral, and does not interact with light. He called this particle a "demon." Since then, it has been speculated to play an important role in the behaviors of a wide variety of metals. Unfortunately, the same properties that make it interesting have allowed it to elude detection since its prediction.

Now, a team of researchers led by Peter Abbamonte [illinois.edu], a professor of physics at the University of Illinois Urbana-Champaign, have finally found Pines' demon 67 years after it was predicted. As the researchers report in the journal Nature [nature.com], they used a nonstandard experimental technique that directly excites a material's electronic modes, allowing them to see the demon's signature in the metal strontium ruthenate.

"Demons have been theoretically conjectured for a long time, but experimentalists never studied them," Abbamonte said. "In fact, we weren't even looking for it. But it turned out we were doing exactly the right thing, and we found it."

One of the most important discoveries of condensed matter physics is that electrons lose their individuality in solids. Electric interactions make the electrons combine to form collective units. With enough energy, the electrons can even form composite particles called plasmons with a new charge and mass determined by the underlying electric interactions. However, the mass is usually so large that plasmons cannot form with the energies available at room temperature.

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

Demons' neutrality means that they do not leave a signature in standard condensed matter experiments. "The vast majority of experiments are done with light and measure optical properties, but being electrically neutral means that demons don't interact with light," Abbamonte said. "A completely different kind of experiment was needed."

[...] According to Abbamonte, it was no accident that his group discovered the demon "serendipitously." He emphasized that he and his group were using a technique that is not widely employed on a substance that has not been well studied. That they found something unexpected and significant is a consequence of simply trying something different, he believes.

"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."

Journal Reference:
Husain, A.A., Huang, E.W., Mitrano, M. et al. Pines' demon observed as a 3D acoustic plasmon in Sr₂RuO₄. Nature 621, 66–70 (2023). https://doi.org/10.1038/s41586-023-06318-8 [doi.org]

Original Submission