Item

Abstract

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 tau > 10(-3) and tau > 7 x 10(-3),
respectively. Their values, beta(C) = 0.44(2) and gamma(C) = 0.85(3),
together with the scaling relation alpha + 2 beta(C) + gamma(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
beta(C) - 2 beta = 0.14(4), where beta is the staggered magnetisation
exponent. The scaling relation alpha + 2 beta + gamma = 2 could be
fulfilled with a reasonable alpha = 0.23(4), although for the chiral
critical exponents beta(C) = 0.90(2) and gamma(C) = 0.69(5) one needs
alpha = -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 beta = 0.22(2) is found to be in agreement with the
XY chiral scenario for a TLA. Chiral scattering above T-N 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. (c) 2006 Elsevier B.V. All rights reserved.