Potential signature of a kosterlitz-thouless transition in [formula presented]

ESR measurements are reported for the quasi-two-dimensional honeycomb antiferromagnet [Formula presented]. Planar anisotropic properties are confirmed by angular dependent investigations of resonance field and linewidth. The divergence of the temperature-dependent linewidth on approaching [Formula presented] 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 [Formula presented] is a weakly anisotropic 2D Heisenberg antiferromagnet.