Polarized neutron scattering studies of chiral criticality, and new universality classes of phase transitions
Polarized neutron scattering studies of chiral criticality, and new universality classes of phase transitions
Plakhty, V. P.; Wosnitza, J.; Kulda, J.; Brückel, T.; Schweika, W.; Visser, D.; Gavrilov, S. V.; Moskvin, E. V.; Kremer, R. K.; Banks, M. G.
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>7x10^{-3} respectively. Their values, β_{c}= 0.44(2) and γ_{c}= 0.85(3), together with the scaling relation α+2β_{c}+γ_{c}=2.13(9), including the critical exponent where a 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 β_{c} - 2β = 0.14(4), where β is the staggered magnetisation exponent. The scaling relation α+2β+γ=2 could be fulfilled with a reasonable α= 0.23(4), although for the chiral critical exponents β_{c}= 0.90(2) and γ_{c}=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 DzyaloshinskiiMoriya interaction with the lattice.
- Physica B: Condensed Matter 385-386(2006), 288-294
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Publ.-Id: 9170