Electron spin resonance in a model S=1/2 chain antiferromagnet with a uniform Dzyaloshinskii-Moriya interaction

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 Neel temperatures. This type of magnetic resonance absorption corresponds well to the two-spinon continuum of excitations in S = 1/2 antiferromagnetic spin chainwith 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.