A new quasi-particle has been discovered: the pi-ton

The quasi-particle (called π-ton) is composed of two electrons and two holes.

Vienna University of Technology

A team of scientists from the Vienna University of Technology (Austria) has discovered a new quasi-particle, in a system made up of many particles, which in many respects behave like a particle, called π-ton (pi-ton) in computer simulations. They hope to be able 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 an environment that behaves like a particle and can conveniently be considered as one.

A hole is almost a particle

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

How was it discovered?

It’s not easy. "The dividing line between particles and quasi-particles is not as clear 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, the excitations of particle holes 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 inside the samples. “Imagine, for example, that many atoms are arranged in a regular pattern on a crystal and that there is an electron in motion on each atom. If an electron is missing in a particular atom, it is what we call a hole,” clarified Held. Electron holes allow electrons to pass through semiconductors, for example. They had found something totally different in their calculations: a completely new kind of quasi-particle.


"The name pi-ton comes from the fact that the two electrons and the two holes are held together by fluctuations in load density or rotational fluctuations that always invert their character at 180 degrees from one grid point of the crystal to the next, i.e., In the angle pi, measured in radians,” clarified Anna Kauch, co-author of the study. “As if it were a change from black to white on a chessboard. Pi-ton is created spontaneously by absorbing a photon. When it disappears, a photon is emitted again".

Although we are constantly surrounded by countless quasi-particles, the discovery of a new kind of quasi-particle is something really special. Its finding contributes to a better understanding of the link 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 photovoltaic energy.

A sea of particles

Dozens of particles have already been identified, and about 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 electron spinning structure in a crystalline network) and polarons (electrons covered by a cloud of phonons).

The next step will be to identify pi-ton in situ rather than in a computer simulation, which can be really difficult, due to the fact that observing and measuring any subatomic particle is a delicate and unique matter to begin with, and quasi-particles are a product of context as well as their 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.

Reference: 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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