Phase diagram and spin Hamiltonian of weakly-coupled anisotropic S=1/2 chains in CuCl₂∙2((CD₃)₂SO)

Field-dependent specific heat and neutron scattering measurements were used to explore the antiferromagnetic S=1/2 chain compound CuCl₂∙2((CD₃)₂SO). At zero field the system acquires magnetic long-range order below T_N=0.93 K with an ordered moment of 0.44μ_B. An external field along the b axis strengthens the zero-field magnetic order, while fields along the a and c axes lead to a collapse of the exchange stabilized order at μ₀H_c=6 T and μ₀H_c=4 T (extrapolated to zero temperature) and the formation of an energy gap in the excitation spectrum. We relate the field-induced gap to the presence of a staggered g-tensor and Dzyaloshinskii-Moriya interactions, which lead to effective staggered fields for magnetic fields applied along the a and c axes. Competition between anisotropy, interchain interactions, and staggered fields leads to a succession of three phases as a function of field applied along the c axis. For fields greater than μ₀H_c, we find a magnetic structure that reflects the symmetry of the staggered fields. The critical exponent, β, of the temperature driven phase transitions are indistinguishable from those of the three-dimensional Heisenberg magnet, while measurements for transitions driven by quantum fluctuations produce larger values of β.