Elementary excitations in S = 1/2 Heisenberg spin chains with alternating g-tensor and the Dzyaloshinskii-Moriya interaction

The magnetic-excitation spectrum of copper pyrimidine dinitrate, a material containing S = 1/2 antiferromagnetic chains with alternating g-tensor and the Dzyaloshinskii-Moriya interaction, and exhibiting a field-induced spin gap, is probed using tunable-frequency electron spin resonance spectroscopy in magnetic fields up to 25 T. The data are interpreted in frame of the sine-Gordon quantum-field theoretical concept proposed recently by Oshikawa and Affleck. The field-induced gap is measured directly; signatures of soliton and three breather branches are identified.