In a recent Russian-Italian study, new, unusual properties of quasiparticles - polaritons - were discovered at ultralow temperatures, which was done thanks to the use of a waveguide with an unusual geometry in the experiment. The leading role in the joint study was played by young scientists from the Institute for Laser and Plasma Technologies (LaPlas) of the MEPhI - postgraduate student of the Department of Theoretical Nuclear Physics Anna Grudinina and associate professor Nina Voronova.

In this case, exciton polaritons became the subject of research by scientists. So in physics they call quasiparticles, which are systems of interacting photons (that is, light quanta) and excitons. An exciton, in turn, is an electronic excitation in a dielectric or semiconductor that migrates along the crystal lattice of a substance. The system, "consisting" of the mutual transformation of excitons into photons and vice versa, forms exciton polaritons, which are also called "liquid light particles". Exciton polaritons have a complete set of properties of light: they are characterized by phase, polarization, wavelength, they can move quickly, but at the same time they also have the properties of ordinary material particles: they interact with the crystal lattice, repel each other, accelerate, slow down, react to external fields.

According to the classification accepted in physics, exciton polaritons belong to the class of particles and quasiparticles called bosons, and, accordingly, they have all the properties common to bosons. One is that, at low temperatures, bosons form a Bose condensate, a “community” of particles, most of which are characterized by “collective behavior”— they are in the same quantum state, but at a minimum energy level. Most – but still not all. Those particles that, at a temperature of the order of 4 kelvins (that is, minus 269 degrees Celsius), do not follow the collective, and have other, not like most quantum states, are called supercondensate.

It is very difficult to study above-condensate particles, precisely because there are few of them, and they are literally “blocked” by the activity of ordinary condensate particles. In particular, if we talk about exciton polaritons, which were the subject of research at the MEPhI, then their age is short, such a quasiparticle lives less than a few tens of trillionths of a second, after which it decays, turning into light. The Bose condensate, which consists of polaritons, constantly glows, but this light is emitted by decaying particles that were in the lowest quantum state, that is, belonging to the "majority" - and in this light flux it is impossible to detect those few photons that were obtained during the decay of above-condensate particles. The light of the "majority" literally overshadows the information that could be obtained from the "minority".

That is why the study of supercondensate particles is a non-banal scientific problem.

Scientists from the MEPhI were able to solve it by changing the medium in which the Bose condensate exists, or rather, by changing the geometry of the waveguide in which it is formed.

In a gallium arsenide waveguide with periodic notches, the properties of polaritons change. In particular, if usually a three-dimensional graph that describes the dependence of the energy of an elementary particle on its momentum has the shape of a paraboloid, then in a notched waveguide it acquires a saddle shape, while the energy of particles in such a Bose condensate depends on the direction of their movement - that is, as physicists say, such a Bose condensate has anisotropy. But the main thing is not even this, but the fact that in a waveguide with notches, the “most” particles that make up the Bose condensate suddenly acquire “immortality”, they stop decaying, which means they stop glowing. Thus, their light does not block the glow of above-condensate particles, and it can be measured and studied.

The properties of supercondensate particles in this experiment also turned out to be very unusual; graphs that describe the relationship between the energy and momentum of particles in such systems have not yet been encountered in such studies. Such a graph already has the shape not of a paraboloid and not of a saddle, but of a complex structure with “grooves”, in two-dimensional sections of this graph, zones that are flat in energy and other unusual features, such as spectrum linearization, are found.

  “The results obtained can be used as a tool to impart new fundamental properties to the Bose condensates themselves, such as, for example, anisotropic superfluidity, and once again emphasize the unimaginable richness of exciton-polariton systems,” says the initiator of the study, Ph.D. Sciences Nina Voronova.

Initially, the properties of above-condensate polaritons were theoretically calculated at the MEPhI (calculations were carried out by graduate student Anna Grudinina), and then the theoretical predictions were experimentally verified at the Italian Advanced Photonics Laboratory of the CNR-NANOTEC Institute of Nanotechnology. The experimental observations practically coincided with the predictions of the theory.

The results of the study are important not only in themselves, but also have methodological significance - they showed that, by working with the geometry of waveguides, it is possible to vary the experimental conditions and discover new properties of elementary particles. Thus, the study is a contribution to the engineering of future physical experiments.

The results of the scientists' work were published in the journal Nature Communications . The project was supported by the Priority 2030 program within the framework of the Relativistic Quantum Engineering strategic