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 $S=\frac{1}{2}$ chain compound $\mathrm{Cu}{\mathrm{Cl}}_{2}∙2({(\mathrm{C}{\mathrm{D}}_{3})}_{2}\mathrm{S}\mathrm{O})$. At zero field the system acquires magnetic long-range order below ${T}_{N}=0.93\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with an ordered moment of $0.44{\ensuremath{\mu}}_{B}$. An external field along the $\mathbf{b}$ axis strengthens the zero-field magnetic order, while fields along the $\mathbf{a}$ and $\mathbf{c}$ axes lead to a collapse of the exchange stabilized order at ${\ensuremath{\mu}}_{0}{H}_{c}=6\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ and ${\ensuremath{\mu}}_{0}{H}_{c}=4\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ (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 $\mathbf{a}$ and $\mathbf{c}$ axes. Competition between anisotropy, interchain interactions, and staggered fields leads to a succession of three phases as a function of field applied along the $\mathbf{c}$ axis. For fields greater than ${\ensuremath{\mu}}_{0}{H}_{c}$, we find a magnetic structure that reflects the symmetry of the staggered fields. The critical exponent, $\ensuremath{\beta}$, 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 $\ensuremath{\beta}$.