Quantum fluctuations in the incommensurate phase of CsCuCl3 under transverse magnetic field

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.