Field-tunable Berezinskii-Kosterlitz-Thouless correlations in a Heisenberg magnet
arXiv:2209.11085 · doi:10.1103/PhysRevLett.130.086704
Abstract
We report the manifestation of field-induced Berezinskii-Kosterlitz-Thouless (BKT) correlations in the weakly coupled spin-1/2 Heisenberg layers of the molecular-based bulk material [Cu(pz)(2-HOpy)](PF). Due to the moderate intralayer exchange coupling of K, the application of laboratory magnetic fields induces a substantial anisotropy of the spin correlations. Crucially, this provides a significant BKT regime, as the tiny interlayer exchange mK only induces 3D correlations upon close approach to the BKT transition with its exponential growth in the spin-correlation length. We employ nuclear magnetic resonance and SR measurements to probe the spin correlations that determine the critical temperatures of the BKT transition as well as that of the onset of long-range order. Further, we perform stochastic series expansion quantum Monte Carlo simulations based on the experimentally determined model parameters. Finite-size scaling of the in-plane spin stiffness yields excellent agreement of critical temperatures between theory and experiment, providing clear evidence that the nonmonotonic magnetic phase diagram of [Cu(pz)(2-HOpy)](PF) is determined by the field-tuned anisotropy and the concomitant BKT physics.
arXiv comment10 pages, 7 figures
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