Experimental confirmation of the existence of a new universality class for stacked triangular lattices

Neutron scattering studies of CsMnBr 3 , a three-dimensional XY antiferromagnet on a stacked triangular lattice, have been carried out in the critical region. Measurements of three magnetic Bragg peak intensities as a function of temperature have shown that the transition to an antiferromagnetically ordered state at T N =8.3 K takes place with critical exponent beta =0.22+or-0.02. This value of beta does not correspond to any standard model of a universality class characterised by the real-space dimensionality and the dimensionality of the order parameters. It does, however, agree well with the predictions of Kawamura (1986, 1988) for a universality class described by the symmetry of the order parameter rather than just the dimensionality. For CsMnBr 3 this symmetry is V=Z 2 *S 1 , and Kawamura has predicted beta =0.25+or-0.02, in agreement with the experimental data.