Physicists have confirmed the existence of quasiparticles that were predicted 16 years ago.

Quantum interactions between electrons within materials can give rise to unusual phenomena. For instance, 16 years ago, physicists predicted the existence of so-called Majorana fermions—quasiparticles that alter their mass depending on their direction of movement. However, they were only recently discovered in layered materials, and by chance. This breakthrough could pave the way for the development of new materials with unique optical properties.

2024-12-12 10:00:07https://naked-science.ru/article/physics/fiziki-podtverdili-sushhe

The existence of exotic quasiparticles in quantum materials is not news to scientists. For instance, previously discovered Dirac fermions in graphene (a single layer of carbon) are quasiparticles (or, in theoretical physics, solutions to the Dirac equation) that behave like massless or nearly massless particles moving at speeds close to the speed of light, serving as a starting point for the search for other unusual states of matter.

Specifically, in 2008-2009, physicists proposed that under certain conditions, they could observe semi-Dirac fermions, which would behave as massless particles when moving in one direction, and gain mass when moving perpendicular to that direction. However, detecting them in practice has been challenging, and no one has succeeded so far.

Now, the authors of a new study, published in the journal Physical Review X, have observed them in the topological semimetal zirconium-silicon-sulfide (ZrSiS). Initially, physicists from the University of Pennsylvania and Columbia University (both in the USA) were studying the optical properties of the material, but the data turned out to be so strange that a new interpretation was required.

“It was completely unexpected. We were not searching for semi-Dirac fermions and stumbled upon the first experimental evidence of the existence of these unusual quasiparticles,” said the lead author of the study, Yinming Shao.

To obtain a detailed picture of the quantum states within ZrSiS (which, like graphene, can be divided into extremely thin layers), the team led by Shao applied magneto-optical spectroscopy: the sample was cooled to a temperature just above absolute zero and placed in a powerful magnetic field (approximately 900,000 times stronger than Earth's). Then, infrared light was directed at the sample.

It turned out that when the magnetic field was applied, the energy levels of the electrons were quantized into so-called Landau levels (the energy levels of charged particles in a magnetic field) — a characteristic signature of semi-Dirac fermions (specifically, a unique dependence of transitions between Landau levels on the magnetic field following the law B^(2/3)).

The key point is that within ZrSiS, so-called nodal lines form — intricate chains of intersections of energy levels in momentum space. When two such lines intersect, unique points with distinct energy structures (Dirac points) arise, transforming local electronic states into semi-Dirac fermions: along one coordinate, their energy spectrum is linear (massless), while along the other, it is quadratic (massive).

Just as graphene promises to revolutionize electronics, this discovery could lead to the creation of new quantum devices. The authors of the study noted that their results are a first step towards understanding how these quasiparticles interact with one another. Further research will help clarify how to utilize semi-Dirac fermions for the development of fundamentally new electronic components.