A particle called Pines's demon has been seen inside a superconductor, decades after it was first predicted.
A mysterious particle has been discovered inside a superconducting crystal, more than 60 years after it was first predicted. The particle, called Pines’s demon, could explain why some materials superconduct, meaning they have zero electrical resistance, and help guide the search for new superconducting materials.
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| It is unclear exactly how superconductors work SeniMelihat/Shutterstock |
The particle is a type of plasmon, so-called because it arises from a collection of charged particles called a plasma. Such plasmas can form when electrons float freely from an atom, like in metallic materials. Plasmons are collective vibrations in this plasma that behave as if they are a particle, making them a quasiparticle, and play a key role in the way metals reflect and absorb light.
A specific kind of plasmon made when two plasmas of different energies oscillate out of sync was first proposed by physicist David Pines in 1956. He called it a demon for “distinct electron motion” and the suffix -on, which normally denotes a particle, and predicted that it would have unusual properties, such as being invisible to light, being electrically neutral and act similarly to sound. These features might help explain a range of poorly understood phenomena like superconductivity and how metallic nanoparticles interact with light, but the demon’s transparent nature made it elusive.
Now, Peter Abbamonte at the University of Illinois Urbana-Champaign and his colleagues have detected the hallmarks of Pines’s demon in a superconducting crystal called strontium ruthenate. “This fundamental, basic excitation in materials probably should have been studied for the last half a century, but no one had ever seen one,” says Abbamonte.
To find the demon, Abbamonte and his team bounced electrons off the crystal and measured how much energy they gained or lost to a very high precision. They could then use this small energy change to calculate the momentum of the demon within the crystal, finding it closely matched Pines’s predictions.
Such demons should exist in countless other metallic materials too, says Abbamonte. The only requirement is that a metal must have two populations of electrons of different energies vibrating at different frequencies – a feature that exists in many materials, including the lanthanum hydrides, a class of high-temperature superconductors. “Demons are not rare,” says Abbamonte. “I think they are in many things, we just haven’t seen them because we didn’t do the right kind of measurement.”
It may also be a potential explanation for how superconductivity itself arises. The conventional idea, called BCS theory, suggests that electrons can pair up and move around with zero resistance. These paired electrons are thought to interact through quasiparticles of sound, called phonons, but some people think that they might also interact through Pines’s demon. The fact that strontium ruthenate is also a superconductor could be a hint that this is the case, says Abbamonte.
Unfortunately, the discovery of Pines’s demon probably won’t shed any light on LK-99, a claimed room-temperature superconductor that physicists are currently scrambling to understand. That is because LK-99 doesn’t seem to be a BCS superconductor, and Pines’s demon is only proposed as an explanation for paired electrons within this theory. “I don’t know if [LK-99] is superconducting or not, but my inner child wants it to be,” says Abbamonte.
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