THE PHASE-DIAGRAM OF A QUASI-1D S=1 2 ANTIFERROMAGNET/

The magnetic structure and excitations of the quasi-one-dimensional S = 1/2 Heisenberg antiferromagnet (HAF) Cs2CuCl4 have been measured usi ng neutron scattering. At low temperatures and zero field the spins ar e arranged in a cycloid close to the (b,c) plane and the ordering is i ncommensurate along the chain direction. When a held B > 1.66 T is app lied along the c-axis a transition occurs to a phase in which the orde r disappears and the excitations are in agreement with the theoretical predictions for the quantum 1D S = 1/2 HAF chain. When the field is a pplied along the a-axis the incommensurate ordering persists up to the critical field approximate to 8.5 T, above which the system is fully aligned by the field. (C) 1998 Elsevier Science B.V. All rights reserv ed.