The properties of two quantum spin chain materials, K 2 CuSO 4 Cl 2 and K 2 CuSO 4 Br 2 , are studied by a variety of experimental techniques, including bulk measurements, neutron spectroscopy, and electron spin resonance. The hierarchy of relevant terms in the magnetic Hamiltonian is established. It is shown that these two compounds feature substantial Dzyaloshinskii-Moriya interactions that are uniform within each chain, but antiparallel in adjacent chains. The result is a peculiar type of frustration of interchain interactions, which leads to an unusual field-temperature phase diagram.

The electron spin resonance spectrum of a quasi 1D S = 1/2 antiferromagnet K2CuSO4Br2 was found to demonstrate an energy gap and a doublet of resonance lines in a wide temperature range between the Curie-Weiss and Neèl temperatures. This type of magnetic resonance absorption corresponds well to the two-spinon continuum of excitations in S = 1/2 antiferromagnetic spin chain with a uniform Dzyaloshinskii-Moriya interaction between the magnetic ions. A resonance mode of paramagnetic defects demonstrating strongly anisotropic behavior due to interaction with spinon excitations in the main matrix is also observed.

Inelastic neutron scattering is used to study the finite-temperature scaling behavior of the local dynamic structure factor in the quasi-one-dimensional quantum antiferromagnet NTENP (Ni(N, Nbis(3-aminopropyl)propane-1,3-diamine)(µ-NO2)ClO4), at its field-induced Ising quantum critical point. The validity and the limitations of the theoretically predicted scaling relations are tested.

Inelastic neutron scattering is employed to study transverse spin correlations of a Heisenberg S = 1/2 chain compound in a magnetic field of 7.5 T. The target compound is the antiferromagnetic Heisenberg S = 1/2 chain material 2(1,4-Dioxane)·2(H2O)·CuCl2, or CuDCl for short. The validity and the limitations of the scaling relation for the transverse dynamic structure factor are tested, discussed and compared to the Tomonaga-Luttinger spin liquid theory and to Bethe-ansatz results for the Heisenberg model.

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