Quasiparticles: Physicists find "demon" in superconducting crystal
It was predicted more than 60 years ago that massless, electrically neutral quasiparticles can form in certain materials. This phenomenon could help to explain high-temperature superconductivity.
In 1956 the American theoretical physicist David Pines predicted that electrons in a solid could do something strange. While they normally have a mass and an electrical charge, Pines predicted that under certain circumstances they could also produce a quasiparticle that is massless, electrically neutral and does not interact with light. Now a research group led by physicist Peter Abbamonte from the University of Illinois Urbana-Champaign has discovered this phenomenon inside a superconducting crystal.
As the scientists in the journal Nature, they used a rarely used experimental technique called electron energy loss spectroscopy (electron energy loss spectroscopy), which directly excites the electronic modes of a material and enabled the researchers to recognise the signature of the mysterious particle in strontium ruthenate (Sr2RuO4). The particle is a type of plasmon. This is what physicists call collective oscillations in a collection of charged particles, a so-called plasma, which behave like a single particle. In terms of quantum mechanics, they are treated as bosonic quasiparticles. They play a key role in how metals reflect and absorb light.
According to David Pines' prediction, this special type of plasmon is created when two plasmas with different energies do not oscillate synchronously. He called the particle a "demon" - made up of the first letters of "distinct electron motion" and the suffix "-on", which indicates that it is a particle. Such a Pines' demon could help to explain a number of poorly understood phenomena such as high-temperature superconductivity and the interaction of metallic nanoparticles with light.
Not as rare as you might think
To find the demon, Abbamonte and his team bounced electrons off a strontium ruthenate crystal and measured with very high precision how much energy they gained or lost in the process. Using this small change in energy, they were then able to calculate the momentum of the demon in the crystal and found that it matched David Pines' predictions quite closely.
Such demons should also exist in countless other metallic materials, said Peter Abbamonte according to the science magazine "New Scientist". The only requirement is that a metal has two populations of electrons with different energies that oscillate at different frequencies - a property that occurs in numerous materials, including lanthanum hydrides, a class of high-temperature superconductors. "Such demons are not uncommon," Abbamonte is quoted as saying. "I think they occur in many materials, we just haven't seen them yet because we haven't done the right kind of measurement."
Pines' demon could also be a possible explanation for the origin of superconductivity itself. The conventional idea, known as the BCS theory, states that electrons can join together in so-called Cooper pairs and move through a material without resistance. It is assumed that these paired electrons interact with each other through quasiparticles of sound, known as phonons. However, so far the theory only seems to explain low-temperature superconductivity beyond doubt. In the case of high-temperature superconductivity, the mechanism of pair formation is still unclear; direct electron-phonon interaction is out of the question. There is therefore a theory that the electrons could interact via Pines' demons. The fact that strontium ruthenate behaves like a superconductor could be an indication that this is the case, Abbamonte said.
Spectrum of Science
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