Spin gap in the quasi-one-dimensional S=12 antiferromagnet K2CuSO4Cl2

Electron spin resonance (ESR) experiments in the quasi-one-dimensional (quasi-1D) $S=\frac{1}{2}$ antiferromagnet ${\mathrm{K}}₂{\mathrm{CuSO}}₄{\mathrm{Cl}}₂$ reveal the opening of a gap in the absence of magnetic ordering, as well as an anisotropic shift of the resonance magnetic field. These features of the magnetic excitation spectrum are explained by a crossover between a gapped spinon-type doublet ESR formed in a 1D antiferromagnet with uniform Dzyaloshinskii-Moriya interaction and a Larmor-type resonance of a quasi-1D Heisenberg system.