Signatures for Berezinskii-Kosterlitz-Thouless critical behavior in the planar antiferromagnet BaNi2V2O8

We investigate the critical properties of the spin-1 honeycomb antiferromagnet BaNi2V2O8, both below and above the ordering temperature T-N using neutron diffraction and muon spin rotation measurements. Our results characterize BaNi2V2O8 as a two-dimensional (2D) antiferromagnet across the entire temperature range, displaying a series of crossovers from 2D XY to 2D XXZ and then to 2D Heisenberg behavior with increasing temperature. In particular, the extracted critical exponent of the order parameter reveals a narrow temperature regime close to T-N, in which the system behaves as a 2D XY antiferromagnet. Above T-N, evidence for Berezinskii-Kosterlitz-Thouless behavior driven by vortex excitations is obtained from the scaling of the correlation length. Our experimental results are in agreement with classical and quantum Monte Carlo simulations performed using microscopic magnetic model Hamiltonians for BaNi2V2O8.