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Description
Magnetic, thermodynamic, and magnetocaloric properties of the spin-$1/2$ quantum Heisenberg octahedral chain with three distinct exchange interactions in an external magnetic field are investigated. The exact results derived for the one-magnon energy spectrum suggest the existence of three flat bands among five one-magnon energy bands, which allow the application of the theory of localized magnons for the description of low-temperature magnetic and magnetocaloric properties, independently verified through numerical exact diagonalization. One of the flat bands corresponds to a magnon bound to a single square plaquette, whereas the other two flat bands correspond to a two-fold degenerate dimer-singlet state residing on two opposite corners of the square plaquette. The case in which the next-nearest-neighbor interaction on the square plaquette is absent has been previously studied [1]. In this work, the influence of this interaction is analyzed in detail alongside the nearest-neighbor interaction on the square plaquette, leading to a more complete understanding of the system's properties. The magnetization process exhibits characteristic jumps near the saturation magnetic field, which result from a level crossing between the magnon-crystal eigenstate and fully polarized ferromagnetic state. The temperature-dependent interception point of magnetization curves is shown to be significantly influenced by the degree of degeneracy of the respective flat one-magnon state. A similar analysis is conducted for the heat capacity, which displays significant variations near phase transitions between different quantum ground states. Furthermore, the magnetocaloric effect is examined, with a particular focus on cooling via the adiabatic demagnetization process. The results indicate that the system facilitates efficient temperature reduction upon a gradual decrease of the external magnetic field, making it a promising candidate for magnetocaloric applications in cryogenic cooling.
Acknowledgements
This work was financially supported by The Ministry of Education, Research, Development and Youth of the Slovak Republic under the grant No. VEGA 1/0298/25, by the Slovak Research and Development Agency under Contract No. APVV-20-0150, and by the internal grant of Faculty of Science of Pavol Jozef Šafárik University in Košice under the contract No. VVGS-2025-3497.
References
[1] J. Strečka et al., “Spectacular diversity of quantum ground states and quantum phase transitions of a spin-1/2 Heisenberg octahedral chain”, Physical Review B, vol. 95, no. 22. American Physical Society (APS), Jun. 13, 2017. https://doi.org/10.1103/physrevb.95.224415
