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The researchers studied the ferroelectric material Germanium telluride (GeTe), which is known, for example, as the active layer in rewritable DVDs or CDs. It is in it that the phenomenon of the "triple point", ie a very unusual crossing of three fermions, can be experimentally verified.

"Today, we distinguish thousands of particles, which we divide into two large groups according to their behavior: fermions and bosons. Fermions can exist either as individual particles, ie as electrons, protons, and neutrons or as so-called quasiparticles. These are composed of many components but behave like a single fermion particle. Fermions are characterized by intolerable behavior, where two particles with the same rotation cannot occupy the same state and place. Either four (Weyl) or two fermions (Dirac) can intersect," explains Ján Minár from the NTC research center.

However, NTC scientists have predicted triple fermions. They can occur in different crystals and only in special circumstances. In addition, for the first time ever, experts were able to identify spin windings around triple fermions, ie their internal magnetic moment. The study was published by the renowned journal Physical Review Letters of the American Physical Society. The authors of its theoretical part are scientists from the NTC, the experimental part is based on experts from the Swiss Federal Institute of Technology in Lausanne in cooperation with other contributors.

“Our study confirms that the special symmetries available in crystals leads to the existence of fermions that are not permitted as free particles,” says Professor Hugo Dil at  the Swiss Federal Institute of Technology in Lausanne. Laurent Nicolaï, who did theoretical calculations in Ján Minář's group at the NTC Research Center adds: "We expect these triple and other related fermions will play an important role in future quantum computers, built on the basis of new topological materials." 

On the Czech side, the project was financed from the Operational Program of the Ministry of Education, Youth and Sports - Research, Development and Education: Support for Excellent Research Teams CZ.02.1.01 / 0.0 / 0.0 / 15_003 / 0000358.

Study:

Laurent Nicolaï, Juraj Krempaský, Martin Gmitra, Houke Chen, Mauro Fanciulli, Eduardo B. Guedes, Marco Caputo, Milan Radović, Valentine V. Volobuev, Ondřej Caha, Gunther Springholz, Jan Minár, J. Hugo Dil. Phys. Rev. Lett. 126, 206403, 17 May 2021. DOI: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.206403

Leader of the theoretical part:

University of West Bohemia – NTC (New Technologies – Research Centre)

Leader of the experimental part:

École polytechnique fédérale de Lausanne ‐ EPFL

Other contributors:

Univerzita P. J. Šafárika v Košicích (Slovensko), Univerzita Tsinghua (Peking), CY Cergy Paris University (Francie), Polská akademie věd, Národní technická univerzita „KhPI“ (Ukrajina), Masarykova univerzita (Česká republika), Univerzita Johanna Keplera (Rakousko)

Financing:

Swiss National Science Foundation (SNSF), Austrian Science Fund (FWF), Ministry of Education, Youth and Sports of the Czech Republic, Grant Agency of the Czech Republic (GAČR), Grant System for Internal Research, Foundation for Polish Science (IRA EU Program), European Regional Development Fund development (CEITEC Nano +)