Potential Signature of a Kosterlitz-Thouless Transition inBaNi2V2O8

ESR measurements are reported for the quasi-two-dimensional honeycomb antiferromagnet ${\mathrm{B}\mathrm{a}\mathrm{N}\mathrm{i}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. Planar anisotropic properties are confirmed by angular dependent investigations of resonance field and linewidth. The divergence of the temperature-dependent linewidth on approaching $T_{\mathrm{N}}$ from above is described in terms of the Kosterlitz-Thouless transition, the critical behavior close to long-range magnetic order, and the 2D Heisenberg antiferromagnet. We provide arguments that the Kosterlitz-Thouless scenario is compatible with the observed critical exponent and suggest ${\mathrm{B}\mathrm{a}\mathrm{N}\mathrm{i}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ is a weakly anisotropic 2D Heisenberg antiferromagnet.