  In low magnetic field, the stacked, triangular antiferromagnet CsCuCl3 has a
helical structure incommensurate (IC) in the chain direction. The IC wavenumber
(from neutron-diffraction experiments) decreases with increasing field
transverse to the chains, as predicted by classical theory, but then it has a
plateau almost certainly caused by quantum fluctuations. Linear spin-wave
theory fails because fluctuations have particularly large effects in the IC
phase. An innovative phenomenological treatment of quantum fluctuations yields
a plateau at approximately the observed value and the observed fields; it
predicts a transition to the commensurate phase so far not observed. Results
depend sensitively on a weak anisotropy.
