Research: Field-tunable Berezinskii-Kosterlitz-Thouless correlations

Two-dimensional (2D) spin systems with an XY anisotropy are known to undergo a Berezinskii-Kosterlitz-Thouless (BKT) phase transition, which marks the binding of topological defects in vortex-antivortex pairs. A group of researchers from Germany, the United Kingdom and the USA investigated the concept of a magnetic field-driven tuning of quasi-2D spin systems from the Heisenberg to the XY limit. As a model case, the spin correlations in the material CuPOF were investigated by means of nuclear magnetic resonance measurements and quantum Monte Carlo simulations. The results of the
critical temperatures of the BKT transition as well as that of the onset of long-range order provide clear evidence that the magnetic phase diagram of CuPOF is determined by the field-tuned XY anisotropy and the concomitant BKT physics under the influence of small interlayer interactions. The findings of this study are of importance for the research of a number of materials that realize 2D spin systems, with an emerging low-temperature phenomenology driven by BKT correlations.

D. Opherden, M. S. J. Tepaske, F. Bärtl, M. Weber, M. M. Turnbull, T. Lancaster, S. J. Blundell, M. Baenitz, J. Wosnitza, C. P. Landee, R. Moessner, D. J. Luitz, H. Kühne,
Field-Tunable Berezinskii-Kosterlitz-Thouless Correlations in a Heisenberg Magnet,
Phys. Rev. Lett. 130, 086704 (2023) (arXiv)