Quantum fluctuations in the incommensurate phase of CsCuCl 3 in a transverse magnetic field

In zero magnetic field, the stacked, triangular antiferromagnet ${\mathrm{CsCuCl}}_{3}$ has a helical structure incommensurate (IC) in the chain direction (normal to the planes). A magnetic field applied transverse to the chains distorts the helix, but the IC structure persists up to at least $0.43$ times the saturation field. The IC wave number $q$ (from neutron-diffraction experiments) decreases with increasing field, but then it has an unexpected plateau. Classical theory explains the behavior at small fields, including the temperature dependence, but it fails to explain the plateau, which we ascribe to quantum fluctuations. We find that linear spin-wave (LSW) theory also fails to explain the plateau; in fact, LSW theory fails more severely than classical theory in describing the IC phase. We introduce a phenomenological treatment of quantum fluctuations. After verifying that it describes well some known results, we apply the phenomenological theory to the IC phase of ${\mathrm{CsCuCl}}_{3}$, finding that it yields a plateau at approximately the observed value of $q$ and the observed fields; in addition, it predicts a transition to the commensurate phase so far not observed. Results depend sensitively on a weak anisotropy: A deviation of less than 1% from isotropy in the intrachain ferromagnetic exchange changes the phase diagram completely at fields above about half the saturation value.