Excitation hierarchy of the quantum sine Gordon spin chain in a strong magnetic field

The magnetic excitation spectrum of copper pyrimidine dinitrate, a material containing S antiferromagnetic chains with alternating g tensor and the Dzyaloshinskii Moriya interaction and exhibiting a field induced spin gap, is probed using submillimeter wave electron spin resonance spectroscopy. Ten excitation modes are resolved in the low temperature spectrum, and their frequency field diagram is systematically studied in magnetic fields up to 25 T. The experimental data are sufficiently detailed to make an extremely accurate comparison with predictions based on the quantum sine Gordon field theory. Signatures of three breather branches and a soliton, as well as those of several multi particle excitation modes are identified