Hannes Kühne, HLD Dresden.

Two-dimensional (2D) spin systems with an XY anisotropy are known to undergo a Berezinskii-Kosterlitz-Thouless (BKT) phase transition at a finite temperature TBKT, which marks the binding of topological defects in vortex-antivortex pairs. So far, experimental efforts to probe a genuine BKT transition in bulk materials were compromised by the onset of 3D long-range order (LRO). Still, if the perturbative terms relative to a purely 2D XY model are small enough, magnetic properties associated with BKT correlations are still observable in the transition regime. A group of researchers from Germany, the United Kingdom, and the USA investigated the concept of magnetic-field-driven tuning of a quasi-2D square-lattice spin-1/2 antiferromagnet from the Heisenberg to the XY limit. As a model system of this study, the molecularbased bulk material [Cu(pz)2(2-Hopy)2](PF6)2 (CuPOF hereafter) yields a moderate intralayer nearest-neighbor exchange coupling of J/kB = 6.80 K, determined from pulsed-field magnetometry (Figure 1), and a small interlayer interaction of about 1 mK. The researchers investigated the spin correlations in CuPOF by means of various techniques, including nuclear magnetic resonance (NMR) and quantum Monte Carlo (QMC) 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 (Figure 2). The findings of this study are of importance for the research of a number of materials that realize quasi-2D spin systems, with an emerging low-temperature phenomenology driven by BKT correlations.

                                   

Figure 1: Pulsed-field magnetometry of CuPOF. The nearestneighbor exchange coupling between the spin-1/2 moments is determined by comparison of the data (triangles) with QMC simulations (red solid line).

Figure 2: Magnetic phase diagram of CuPOF extracted from NMR, μSR, magnetometry, and QMC simulations.

Field-tunable Berezinskii-Kosterlitz-Thouless correlations in a Heisenberg magnet, 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, and H. Kühne, Phys. Rev. Lett. 130, 086704 (2023).

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.086704

Contact: h.kuehne@hzdr.de