Spin excitations in $S=1/2$ AFM chains with alternating $g$-tensor and the Dzyaloshinskii-Moriya interaction.

The magnetic excitation spectrum of copper pyrimidine dinitrate (Cu-PM), an $S=1/2$ antiferromagnetic chain system with alternating $g$-tensor and the Dzyaloshinskii-Moriya interaction, is probed using ESR spectroscopy in magnetic fields up to 25 T. The field-induced gap is measured $directly$. Signatures of a soliton and three breather branches are identified, and their frequency-field dependences are described in frame of the quantum field sine-Gordon model. Furthermore, the ESR linewidth and shift behavior in Cu-PM in the $perturbative~spinon$ regime is studied as function of temperature and field. Excellent quantitative agreement between theoretical predictions (Phys. Rev. Lett. 82, 5136 (1999)) and experiment is obtained. The results are published in: S.A. Zvyagin $et~al.$, Phys. Rev. Lett. 93, 027201 (2004); $ibid.$ 95, 012707 (2005).