The existence of a “demon” quasiparticle predicted 70 years ago is confirmed by science

The existence of a “demon” quasiparticle predicted 70 years ago is confirmed by science

A devilish quasiparticle… David Baines predicted relatively strange behavior of electrons in some metals. Electrons inherently have mass and electrical charge. Thus, the physicist confirmed that they could combine to form a composite particle that is massless, neutral, and does not interact with light. However, scientists believe that this particle plays an important role in the optical properties of metals and semiconductors

This devilish particle would be particularly responsible for phase transitions in certain types of metalloids, the optical properties of metal nanoparticles, or even high-temperature superconductivity in metal hydrides. Since they are electrically neutral, demons are very difficult to detect. ” We are aware ofThe vast majority of experiments are with light, where we measure Optical properties. However, the electrical neutrality of demons means that they do not interact with light “, Peter Abamonte explains. He is a professor of physics at the University of Illinois Urbana-Champaign and was involved in this discovery.

Collective, neutral and non-college mode

Inside a solid, electrons lose their character. Electrical interactions mean that they tend to combine to form collective units. With enough energy, electrons can form what are called plasmons. These are plasma oscillatory quanta. Just as light (optical oscillation) consists of photons, plasma oscillation consists of plasmons.

Plasmons have a new charge and a new mass. These are determined by basic electrical interactions. However, the mass is generally too large to be formed from the energies available at room temperature… except in one particular case, according to David Baines. If a solid contains electrons in more than one energy band — as many metals do — its plasmons can combine to form a new massless neutral plasmon.

A conceptual demon in strontium ruthenate (Sr2RuO4), which is a modification of the γ and β band fillings, keeping the total electron density constant. Credits: Hussein et al., Nature (2023)

[Ce] A new collective mode, called the “devil,” appears when the electrons of different bands move out of phase. This does not result in any net transfer of fees but rather an adjustment to the band occupancy “, Explanation of the researchers in nature. Demons have no mass. They can therefore theoretically form with any energy, and can therefore exist at all temperatures. This would explain in particular the special behavior of some minerals.

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Demons have been speculated on in theory for a long time. However, it has never been studied by experimenters Abamonte said. The specific properties of demons are also the reasons why they cannot yet be detected in 3D minerals. So it was necessary to design a completely different experience.

Discovering the devil by chance

The neutrality of demons means that they leave no form of signature in traditional condensed matter experiences. Fortunately, Abamonte and his collaborators were using a non-standard experimental technique. It turns out that they were studying the electronic properties of strontium ruthenate (Sr2RuO4). However, this metal is structurally very similar to cuprate-type high-temperature superconductors. It is also the first identified perovskite superconductor that does not contain copper.

To analyze the mineral sample, they used electron energy loss spectroscopy, a different type of electron microscope. This technique consists of bombarding a material with a beam of electrons with precisely defined and limited kinetic energies. Strontium ruthenate (Sr2RuO4) has three entangled energy bands, α, β and γ, which pass through the Fermi energy, i.e. the energy of the highest occupied quantum level.

Demon, quasiparticle, superconductivity

Devilish mode dispersion in the (1,0) direction at T = 30 K (a, in blue) and 300 K (b, in red), compared to the dispersion expected by Random Phase Approximation (RPA) (in grey). The weakly dispersive excitation at 63 meV is an optical phonon. Credits: Hussein et al., Nature (2023)

This non-standard technique uses the energy of electrons injected into a metal to directly observe its properties, including the plasmons that form. Some electrons undergo inelastic diffusion. This means that they lose energy and their path is slightly skewed. The researchers examined the data. And so they found something unusual. A massless electron mode, which was none other than the devil's signature in strontium ruthenite.

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In fact, the speed of this quasiparticle can correspond neither to an acoustic phonon nor to a surface plasmon. ” At first we had no idea what it was. Demons are not part of the mainstream. This possibility was brought up very early on and we laughed about it. But as we began to rule things out, we began to doubt that we had actually found the devil », narrates Ali Hussein, first author of the study.

This devilish quasiparticle could lead us to a better understanding of superconductors

To verify their theory and confirm that strontium ruthenite could indeed represent a demon, the team asked Edwin Huang, a condensed matter theorist, to calculate the properties of the metal's electronic structure. ” We had to do a microscopic calculation to explain what was happening. We then discovered a particle consisting of two bands of electrons oscillating out of phase with approximately equal amplitude, just as Baines had described “, explains the physicist.

It seems that serendipity has struck again and Abamonte is happy about it. He emphasizes that he and his team used a technique that is not widely used, on a material that has been little studied. The fact that they ended up finding something exceptional shows how important innovation is in research. ” We don't planMost great discoveries. We'll look for something new and see what's out there “, he stated.

This result suggests that devils may be a ubiquitous feature of multiscale minerals. It could help explain how superconductors work. Pines Demon could help fabricate room-temperature superconductors. The standard theory, called BCS theory, explains superconductivity by the formation of pairs of electrons (called Cooper pairs) under the influence of an attractive interaction between electrons resulting from the exchange of phonons. It is possible that Devil Baines also contributes to bringing the electrons together. This will then lead to better superconductors.

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