FLUCTUATION-INDUCED PHASE IN CSCUCL3 IN A TRANSVERSE MAGNETIC-FIELD -THEORY

CsCuCl3 is a quantum triangular antiferromagnet. ferromagnetically sta cked, with an incommensurate (IC) structure due to a Dzyaloshinskii-Mo riya interaction. Because of the classical degeneracy caused by the fr ustration, fluctuations in CsCuCl3 have extraordinarily large effects, such as the phase transition in longitudinal magnetic field (normal t o the planes, parallel to the IC wavenumber q) and the plateau in q in transverse field (perpendicular to q). We argue that fluctuations are responsible also for the new IC phase discovered in transverse field near the Neel temperature T-N, by Werner et al (Werner T, Weber H B, W osnitza J, Kelnberger A, Meschke M, Schotte U, Stusser N, Ding Y and W inkelmann M 1997 Solid State Commun. 102 609). We develop and analyse the corresponding minimal Landau theory; the ground-state reconstructi on due to fluctuations is described phenomenologically, by means of a biquadratic term. The Landau theory gives two IC phases, one familiar from previous studies; the properties of the new IC phase, which occup ies a pocket of the temperature-held phase diagram near T-N, agree qua litatively with those of the new phase found experimentally.