Tech UPTechnologyThey discover a new quasi-particle: the pi-ton

They discover a new quasi-particle: the pi-ton

A team of scientists from the Vienna University of Technology (Austria) has observed a new quasi-particle -excitations in a system made up of many particles, which in many respects behave like one particle- called π-ton (pi-ton) in simulations By computer. They hope to identify it in the future.

In physics, there are many types of particles: elementary particles, which are the fundamental building blocks of matter, atoms, which consist of several smaller constituents, and “quasi-particles”: a disturbance in a medium that behaves like a particle and that can conveniently be considered as one.

A hole is almost a particle

In relation to solid state physics and nuclear physics, we study them because they play a very important role in determining the properties of matter. “The simplest quasiparticle is a hole,” explained Karsten Held, a researcher at the Institute of Solid State Physics at the Vienna University of Technology and co-author of the study in the journal Physical Review Letters.

How did they discover it?

It is not easy at all. “The dividing line between particles and quasi-particles is not as clear-cut as you might imagine,” says Held. “Strictly speaking, even ordinary particles can only be understood in the context of their environment. Even in a vacuum, particle hole excitations occur constantly, albeit for a short time. Without them, the mass of an electron, for example, would be completely different ”.

The experts explain in their work how they began to notice a different pattern in the holes observed within the samples. “Imagine, for example, that many atoms are arranged in a regular pattern in a crystal and that there is an electron in motion in each atom. If an electron is missing in a particular atom, it is what we call a hole, ” Held clarified. Electron holes allow electrons to pass through semiconductors, for example . And they had found something totally different in their calculations: a completely new type of quasiparticle.


“The name pi-ton comes from the fact that the two electrons and the two holes are held together by fluctuations in charge density or fluctuations of rotation that always reverse their character by 180 degrees from one lattice point of the crystal to the next, that is that is to say, in the angle of pi, measured in radians, “explained Anna Kauch, co-author of the study. As if it were a change from black to white on a chessboard. ” The pi-ton is created spontaneously by absorbing a photon. When it disappears, a photon is emitted again. “

Despite the fact that we are constantly surrounded by countless quasiparticles, the discovery of a new type of quasi-particle is something really special. Their finding contributes to a better understanding of the coupling between light and solids, a field that plays a key role not only in basic research but also in many technical applications, from semiconductor technology to photovoltaics.


A sea of particles

Dozens of particles have already been identified, and some 20 quasi-particles and collective excitations: excitons, phonons (particles derived from the vibrations of atoms in a solid), plasmons (particles derived from the oscillations of plasma), magnons (collective excitations of the spinning structure of electrons in a crystal lattice) and polarons (electrons covered by a cloud of phonons).

The next step will be to identify pi-ton in situ rather than in computer simulation, something that can be really difficult, because observing and measuring any subatomic particle is a delicate and unique business to begin with, and quasi-particles are both a product of context. as well as its specific composition. However, for the research team, there is no doubt about the existence of this quasi-particle and that an observable version will be possible very soon.


Referencia: A. Kauch et al. Generic Optical Excitations of Correlated Systems: π -tons, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.124.047401

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