Field-tunable Berezinskii-Kosterlitz-Thouless correlations in a Heisenberg magnet

TL;DR

This study demonstrates field-induced Berezinskii-Kosterlitz-Thouless (BKT) correlations in a layered Heisenberg magnet, revealing how magnetic fields induce XY anisotropy and lead to a BKT regime confirmed by experiments and simulations.

Contribution

It provides the first experimental and theoretical evidence of field-tuned BKT physics in a Heisenberg magnet with weak interlayer coupling, highlighting the role of field-induced XY anisotropy.

Findings

Field-induced BKT correlations observed via NMR and μ+SR.

Quantum Monte Carlo simulations match experimental critical temperatures.

Nonmonotonic phase diagram explained by field-tuned XY anisotropy and BKT physics.

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)$_2$(2-HOpy)$_2$](PF$_6$)$_2$. Due to the moderate intralayer exchange coupling of $J/k_\mathrm{B} = 6.8$ K, the application of laboratory magnetic fields induces a substantial $XY$ anisotropy of the spin correlations. Crucially, this provides a significant BKT regime, as the tiny interlayer exchange $J^\prime / k_\mathrm{B} \approx 1$ 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 $\mu^{+}$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)$_2$(2-HOpy)$_2$](PF$_6$)$_2$ is determined by the field-tuned $XY$ anisotropy and the concomitant BKT physics.