RNDr. Katarína Karľová, PhD. a Assoc. Prof. RNDr. Jozef Strečka, PhD. from the Institute of Physics at the Faculty of Science, P. J. Šafárik University in Košice, contributed to the theoretical discovery of a new type of behavior in quantum magnets. They demonstrated that a certain class of quantum magnetic materials can exhibit an unusual Kasteleyn phase transition, which until now had been associated primarily with classical physical systems. The findings were published in the prestigious physics journal *Physical Review Letters*.
The uniqueness of the Kasteleyn phase transition lies in the fact that the system behaves completely differently on its two sides. Below the critical temperature, it remains practically “frozen,” but once that temperature is exceeded, long chains of excitations suddenly begin to form, propagating throughout the entire magnetic system (Fig. 1). This dramatic change is accompanied by a highly unusual temperature response of the magnetic system in the vicinity of the critical temperature. For example, the heat capacity approaches a step-like change upon reaching the critical temperature from the low-temperature region, whereas when approaching from the high-temperature region, it follows a power-law relationship (Fig. 2).
The authors have shown that the Kasteleyn phase transition can also occur in quantum magnets, where it had not previously been identified. Moreover, this is not limited to a single specific magnetic system. The results suggest the existence of a broader class of frustrated quantum magnets with similar behavior. This theoretical research also holds promise for experimental verification. The scientists have proposed a method for preparing quantum magnetic materials with the desired magnetic structure. This proposal is based on the targeted chemical synthesis of structural analogs of already known Cu- and Fe-based bimetallic polymeric coordination compounds.
This publication is the main outcome of Kataríny Karľova’s MSCA postdoctoral fellowship at CY Cergy Paris Université in France. The research was conducted in international collaboration with scientists from CY Cergy Paris Université in France, École Polytechnique Fédérale de Lausanne in Switzerland, RWTH Aachen University in Germany, the Yukhnovskii Institute for Condensed Matter Physics in Lviv, Ukraine, and Sorbonne Université in France.
K. Karľová, A. Rufino, T. Verkholyak, N. Caci, S. Wessel, J. Strečka, F. Mila, A. Honecker, Approaching Kasteleyn Transition in Frustrated Quantum Heisenberg Antiferromagnets. Phys. Rev. Lett. 137 (2026) 116703, doi: https://doi.org/10.1103/4cn4-fymq
