Polarized neutron scattering studies of chiral criticality, and new universality classes of phase transitions

Using a novel polarised neutron scattering technique, the critical exponents for the spin chirality and chiral susceptibility are determined for the triangular lattice antiferromagnet (TLA) CsMnBr3 in the ranges of reduced temperature τ>10−3 and τ>7×10−3, respectively. Their values, βC=0.44(2) and γC=0.85(3), together with the scaling relation α+2βC+γC=2.13(9)α+2βC+γC=2.13(9), including the critical exponent where α for the specific heat, prove that the spin-ordering transition belongs to the XY   chiral universality class. In the case of helimagnet Ho, it is found that βC-2β=0.14(4)βC-2β=0.14(4), where β   is the staggered magnetisation exponent. The scaling relation α+2β+γ=2α+2β+γ=2 could be fulfilled with a reasonable α=0.23(4), although for the chiral critical exponents βC=0.90(2) and γC=0.69(5) one needs α=−0.49(5) in contradiction with any experimental data. As the scaling relation always holds, we assume that the spin-ordering transition in Ho is of the first order. In the quantum antiferromagnet CsCuCl3, a triangular spin order coexists with a long-period Dzyaloshinskii helix. The Dzyaloshinskii axial vector should remove the helix chiral degeneracy, which has not been observed in reality. The critical exponent β=0.22(2) is found to be in agreement with the XY chiral scenario for a TLA. Chiral scattering above TN is very weak, probably being masked by zero-point quantum fluctuations. A modulation of the crystal structure with the periodicity of the helix is observed, indicating strong coupling of the Dzyaloshinskii–Moriya interaction with the lattice.