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This article comes from Weixin Official Accounts: SF Chinese (ID: kexuejiaodian), author: SF

(Photo source: Mai-Linh Doan) From the discovery of room temperature superconducting materials by American scientists in March this year to the recent LK-99 superconducting material proposed by Korean scientists at room temperature and pressure, many people may feel that the era of superconductivity is coming. But reality has poured cold water on us. The discovery of American scientists has not been replicated by other research teams, and LK-99 has been proved by other scientists not to be a superconductor. Time and again, the frustration with superconductivity has led us to believe that it is too difficult to achieve superconductivity, but a recent discovery has raised hopes that scientists will find real superconductors. This is the "demon" particle.

Wen| Merlin

The "demon" particle, with its sci-fi name, makes people feel that it should be a magical prop in Marvel comics. In fact, it is a particle that exists in science. Let's first get to know this amazing particle.

What is a demon particle? The "demon" particle is actually a plasmon. When energy is sufficient, electrons "float" out of atoms. These freed electrons can also exhibit the motion characteristics of ordinary plasma and are also regarded as plasma. Ordinary plasmas are quasi-neutral because they carry roughly equal positive and negative charges. These plasmas, which consist of free electrons with only negative charges, are called nonneutral plasmas.

Plasmas are the most common form of matter in the universe, mostly found in thin intergalactic space and stars. On Earth, we also see plasmas, such as lightning. When lightning strikes, a powerful current breaks through the air, turning it into plasma. (Photo credit: United States Air Force)

Simply put, a plasmon is a collective vibration within a plasma that behaves like a particle. In 1956, American physicist David Pines predicted that plasmons produced by two plasmas of different energies oscillating at different frequencies exist in solid metals. These two plasmons merge into a single particle with various characteristics, such as massless, invisible to light, electrically neutral, and behaving like sound. Such particles are "demon" particles. It was named demon because Pines thought these features were too unusual for solid metal.

Because demon particles behave so strangely, and because scientists have not observed real demon particles experimentally for decades, many people suspect that Pines 'prediction is wrong. But the latest research shows that things are not that simple.

A team led by physicist Peter Abbamonte of the University of Illinois at Urbana-Champaign has discovered the first "demon" particles in a superconducting crystal, strontium ruthenate. The study was published in Nature on August 9, 2023.

In the study, the scientists bounced electrons off the crystal and measured the energy gained or lost by the crystal with great precision, and then after a series of studies, they found that their results were in good agreement with Pines 'predictions.

Using microscopic calculations, the scientists characterized the electronic structure of strontium ruthenate and found a particle composed of two electron bands-oscillating out of phase with nearly equal amplitudes-consistent with Pines 'description. After further measurement, calculation and analysis, the scientific community has proved the electric neutrality and acoustic characteristics of particles. Based on these characteristics, the scientists determined that what they observed was the "demon" particle predicted by Pines.

As mentioned earlier, the demon particle offers hope for scientists looking for real superconductors. And why is that? This starts with how superconductivity arises. According to the Bardeen-Cooper-Schrieffer theory (named for the initials of the three discoverers), some electrons in metals can pair with each other to form Coulomb pairs, and the Coulomb pairs move freely through the metal without loss to form superconducting currents. Nikolay Bogolyubov, a Soviet theoretical physicist, argued that the electrons that make up Coulomb pairs need to interact with phonons in order to pair. Some scientists also believe that paired electrons interact with "demon" particles to form Coulomb pairs, which in turn form superconducting currents.

Thus, demon particles are crucial for scientists to discover the cause of superconductivity, and the observation of signs of demon particles has become a determining factor in the possibility of metals becoming superconductors.

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