Quenched chirality in RbNiCl₃

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The critical behaviour of stacked-triangular antiferromagnets has been intensely studied since Kawamura predicted new universality classes for triangular and helical antiferromagnets. The new universality classes are linked to an additional discrete degree of freedom, chirality, which is not present on rectangular lattices, nor in ferromagnets. However, the theoretical as well as experimental situation is discussed controversially, and generic scaling without universality has been proposed as an alternative scenario. Here we present a careful investigation of the zero-field critical behaviour of RbNiCl₃, a stacked-triangular Heisenberg antiferromagnet with very small Ising anisotropy. From linear birefringence experiments we determine the specific heat exponent α as well as the critical amplitude ratio A+/A-. Our high-resolution measurements point to a single second order phase transition with standard Heisenberg critical behaviour, contrary to all theoretical predictions. From a supplementary neutron diffraction study we can exclude a structural phase transition at Tₙ. We discuss our results in the context of other available experimental results on RbNiCl₃ and related compounds. We arrive at a simple intuitive explanation which may be relevant for other discrepancies observed in the critical behaviour of stacked-triangular antiferromagnets. In RbNiCl₃ the ordering of the chirality is suppressed by strong spin fluctuations, yielding to a different phase diagram, as compared to e.g. CsNiCl₃, where the Ising anisotropy prevents these fluctuations.