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This article comes from the official account of Wechat: ID:fanpu2019, with the original title: "is it a particle?" Not particles? Oh, it's quasiparticles! ", compiled by: Crow Youth

Quasiparticles are different from the so-called elementary particles, but are similar to the physical entities of elementary particles, resulting from the interaction of a large number of elementary particles. However, the line between particles and quasiparticles is becoming blurred, and those so-called elementary particles may also be derived, and there may be a larger framework to unify them.

Seventeen elementary particles in the standard model of particle physics. Quasiparticles are not elementary particles. | Fermilab Visual Media Services. When it comes to creating new particles, particle physicists may think of bombarding existing particles in increasingly powerful colliders, hoping to collide with unknown particles. Condensed matter physicists may consider placing materials in different environments, such as extremely low temperatures, or flat two-dimensional planes, to create quasiparticles with novel properties.

According to the standard model of particle physics, 17 elementary particles make up all the matter in our world. So what are quasiparticles (quasiparticle)? A quasiparticle is also a kind of particle, but a quasiparticle is not an elementary particle, but is produced from the complex interaction of a large number of elementary particles and behaves like a particle.

Physicists can expose solids, liquids or plasmas made up of a large number of particles to extreme temperatures and pressures, and then describe the entire system as particle-like entities, that is, quasiparticles. Quasiparticles can be quite stable and, like elementary particles, have inherent properties such as mass and charge.

Quasiparticles are not elementary particles, but are generated from the complex interactions of a large number of elementary particles. | one of the common quasiparticles from network solids is phonon. In crystals, atoms are arranged regularly to form a three-dimensional lattice structure, which oscillates collectively as if connected by springs, carrying heat and sound through the material. The phonon is the quasiparticle corresponding to this oscillation mode. In solid materials, electrons determine the electrical conductivity of the material, and phonons determine the speed at which sound travels through the material. Just as photons are quantized electromagnetic waves, we can think of phonons as quantized sound waves.

Phonon is one of the most common quasiparticles. | NIST, for example, Landau proposed a quasiparticle called polaron in 1933. When the electron moves in the solid material, it will attract the surrounding positive charge and repel the negative charge, causing the atom to deviate from the original equilibrium position to shield the electron effectively, which makes the electron behave more like a quasiparticle with a larger effective mass, that is, the polaron.

A schematic diagram of the polaron. | Wikipedia there are other more peculiar quasiparticles in condensed matter physics. For example, all elementary particles can be divided into two types according to their spin properties: one is semi-integer spin fermions, such as electrons, and the other is integer spin bosons, such as photons. But a quasiparticle called anyon can go beyond this limit, which is different from fermions and bosons. (for related introduction, see "looking for anyons in the miniature Particle Collider + Wen Xiaogang answering questions | the door of many wonderful things")

Physicists can almost magically create quasiparticles with any precise fractional electron charge or spin, even if we don't know why quasiparticles have such strange properties. Through intuitive guessing, theoretical derivation and computer simulation, condensed matter physicists have been able to calculate which quasiparticles may exist in theory. At the same time, in the laboratory, experimental physicists put new materials in a more extreme environment, making the quasiparticle family grow rapidly and become more and more bizarre.

The process of discovering quasiparticles is like a remarkable intellectual challenge. Recently discovered quasiparticles include pions (pi-ton), immovable fractals (fracton) and twisted folds (wrinklon). Steve Simon, a condensed matter theory physicist at the University of Oxford, said: "the quasiparticles we are considering now have properties that we have never really dreamed of before."

Here are some of the most curious and perhaps most useful quasiparticles.

01. One of the first quasiparticles discovered by the invisible Mayorana particle was a "hole", in which there were no electrons where they were supposed to exist. In the 1940s, physicists discovered that holes in solids would jump around like positively charged particles. A more exotic and possibly useful quasiparticle is the Mayorana fermion (Majorana fermion), whose antiparticle is itself in theory. However, from the point of view of particle physics, this kind of particle has not been found so far, but in condensed matter physics, it is half an electron and half a hole at the same time, which can be used as a quasiparticle.

In 2010, Sankar Das Sarma, a physicist at the University of Maryland and his collaborators, believed that Mayorana particles could be used to make quantum computers. When electrons and holes move around each other, they can store information, just as two ropes are woven into a pattern. Different rope twisting methods correspond to the superposition of 0, 1, and 0 and 1, which are bits in quantum computing.

So far, efforts to build effective quantum computers have been difficult because the quantum superposition states of most particles fall apart when they get too hot or collide with other particles. But Mayorana quasiparticles don't have this problem.

Their unusual properties give themselves zero energy and zero charge, which theoretically allows them to exist in a particular type of superconductor. There can be no other particles there, which creates a "gap" that makes it impossible for the Mayorana particles to decay. In other words, this superconducting gap protects the Mayorana particle.

Since 2010, experimenters have been competing to use a complex combination of superconductors, nanowires and magnetic fields to construct real Mayorana particles. In 2018, a team of researchers reported in the journal Nature that they had observed key features of the Mayorana particle. But some experts questioned some of the data analysis, and the paper was withdrawn in March 2021.

It is one thing to imagine a possible quasiparticle, another to observe it in an experiment close to absolute zero. The sample is made up of one atom after another, and tiny impurities can destroy everything. But Das Sarma is not deterred, saying, "the Mayorana particle is bound to be observed because its theory is pure. This is an engineering problem, not a physical problem."

02. The family of quasi-particles in black holes simulated by electromagnetic polarons is growing, and a series of unusual characteristics provide physicists with rich weapons, and can even be used to simulate systems such as black holes. to help us explore physics that we can't touch.

In some areas of the universe, black holes form when gravity is so strong that even light cannot escape. Popularly speaking, the scene near the black hole is a bit similar to what happens when the bathtub is unplugged: the water revolves around the outlet and is inevitably sucked into the whirlpool when it is too close to the center. However, a quasiparticle called a polaron (polariton) can be used to better simulate black holes.

The rotating electromagnetic polaron fluid can be used to simulate rotating black holes. The image on the left shows the fluid density in different regions, and the edge of the middle dark region is like the event horizon of a black hole. The fluid phase diagram on the right shows its eddy current. | Maxime Jacquet polaron is a quasi-particle similar to half-light and half-matter. When the particle simulation lattice is cooled to near absolute zero, it will produce electromagnetic polaron. In the experiment, the researchers used two mirrors to trap a photon in a cage with an exciton (exciton). Excitons are also quasiparticles, made up of a pair of electrons and holes that revolve around each other, but unlike Mayorana particles, where half electrons and half holes exist in the same place.

Excitons are quasiparticles formed by the mutual attraction of a pair of electrons and holes. | Wikipedia photons reflect back and forth between two mirrors about 1 million times, and then escape. In the process of reflection, photons mix with excitons to form polarons. Many photons and excitons are trapped in this way and combine to form polarons, which generally behave like liquid light (liquid light), frictionless and non-scattering. The researchers designed the flow of the electromagnetic polaron to simulate the movement of light around the black hole.

Liquid light is a superfluid formed by the interaction between light and matter, which can show the behavior of liquid. The picture shows the liquid light encounters an obstacle. | Polytechnique Montreal liquid light is unstable and photons will eventually escape. It is this cage that leaks photons that allows physicists to study how black holes evolve over time.

Roger Penrose, the 2020 Nobel laureate in physics, put forward the theory that rotating black holes lose energy and gradually slow down. Maxime Jacquet, a physicist at the University of the Sorbonne in Paris, used the electromagnetic polaron to test the idea. Astrophysics cannot do this, he says, but experiments that simulate black holes with electromagnetic polarons can be a leap from experiments to real black holes.

Never decay magnon if quasiparticles can decay, it will eventually decay. For example, a quasiparticle called a magnon (Magnon) can decay into two other magnon, as long as the energy of these generated particles is no more than the original magnon. Magnon is a quasiparticle excited collectively by the electron spin structure in the lattice, which can be regarded as a quantitative spin wave. They spin 1 and obey the behavior of bosons.

The animation shows electron spin waves propagating in antiferromagnetic materials, in which neighboring atoms (spheres) have opposite spins. When a photon or particle of light (golden ball) of the right wavelength hits an atom and disturbs its spin, the disturbance spreads like ripples in the water. | | SLAC National Accelerator Laboratory |

However, quasiparticles are generally quite stable, probably for two reasons: first, they appear in very low-temperature systems, so the energy is very low in the first place; and the interaction between quasiparticles is so weak that few disturbances trigger them to decay.

If there are many disturbances in the system, are quasiparticles more likely to decay? Physicists were naive enough to think so. But Ruben Verresen, a condensed matter physicist at Harvard University, subverts this view with research.

In a paper published in 2019, Verresen and colleagues described how they could theoretically simulate quasiparticle decay and then gradually increase the intensity of the interaction between quasiparticles to see what happens. At first, quasiparticles did decay faster as expected. But then surprisingly, when the interaction is very strong, quasiparticles become stable and their lifetimes become infinitely long.

The team then used a computer to simulate the behavior of an ultra-cold magnet and did see magnon that did not decay. This enhances physicists' understanding of quasiparticles with strong interactions and can help explain some of the puzzling characteristics of magnon. These never-decay magnon are not only theoretical ideas, but also realized in nature.

All kinds of strange quasiparticles. | Atoms and Sporks results show that quasiparticles are much more powerful than people once thought. The line between particles and quasiparticles becomes blurred. Verresen even believes that there is no fundamental difference between the two.

Quasiparticles are produced by a combination of many particles. But the elementary particles we are talking about, such as quarks, photons and electrons, may not be as basic as we think. Some physicists suspect that these seemingly basic particles are also derived-although no one can tell exactly what they are derived from.

After all, so far, we do not have a basic theory to explain how electrons, photons and other particles are produced, and there is likely to be a larger framework to unify everything. In this framework, we think that elementary particles may not be basic, but quasiparticles in some other theory.

Reference source

[1] https://www.quantamagazine.org/like-magic-physicists-conjure-curious-quasiparticles-20210324/

[2] Giovanni Lerario et al. Room-temperature superfluidity in a polariton condensate. Nature Physics (2017). DOI: 10.1038/NPHYS4147

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