Tunable Berezinskii-Kosterlitz-Thouless correlations in a quasi-2d Heisenberg magnet

We discuss the manifestation of field- and pressure-tuned Berezinskii-Kosterlitz-Thouless (BKT) correlations in the weakly-coupled spin-1/2 Heisenberg layers of the material [Cu(pz)₂(2-HOpy)₂](PF₆)₂ (CuPOF). Due to the moderate intralayer exchange coupling of J/k_B = 6.8 K, laboratory magnetic fields induce a substantial XY anisotropy of the spin correlations. This provides a significant BKT regime, as the tiny interlayer exchange J′/k_B ≈ 1 mK only induces 3d correlations upon close approach to the BKT transition. We employed NMR and μ⁺SR measurements to probe the spin correlations that determine the critical temperatures of the long-range order and the BKT transition for various applied magnetic fields and hydrostatic pressures. Further, we performed stochastic series expansion QMC simulations based on the experimentally determined model parameters. Finite-size scaling of the spin stiffness yields an excellent agreement of the critical temperatures between theory and experiment.