Determining the Berezinskii-Kosterlitz-Thouless coherence length inBaNi2V2O8byV51NMR

The magnetic properties of ${\mathrm{BaNi}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ are studied by electron paramagnetic resonance (EPR) and $^{51}\mathrm{V}$ nuclear magnetic resonance (NMR). The divergence of the $^{51}T_{1}$ rate on approaching ${T}_{\mathrm{N}}$ in the paramagnetic phase displays the exponential correlation length expected for the Berezinskii-Kosterlitz-Thouless transition. The critical power-law behavior can be restored by applying a strong magnetic field perpendicular to the hexagonal axis, destroying the $XY$ anisotropy. The comparison of the EPR and NMR measurements at several magnetic fields yields a consistent description in terms of the Berezinskii-Kosterlitz-Thouless scenario in the temperature range above the antiferromagnetic phase transition of ${\mathrm{BaNi}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$.