Spinon, soliton, and breather in the spin-1/2 antiferromagnetic chain compound KCuGaF6

Elementary excitations of the S = 1/2 one-dimensional antiferromagnet KCuGaF6 were investigated by inelastic neutron scattering in zero and finite magnetic fields perpendicular to the (1,1,0) plane combined with specific heat measurements. KCuGaF6 exhibits no long-range magnetic ordering down to 50 mK despite the large exchange interaction J/k(B) = 103 K. At zero magnetic field, well-defined spinon excitations were observed. The energy of the des Cloizeaux and Pearson [J. des Cloizeaux and J. J. Pearson, Phys. Rev. 128, 2131 (1962)] mode of the spinon excitations is somewhat larger than that calculated with the above exchange constant. This discrepancy is mostly ascribed to the effective XY anisotropy arising from the large Dzyaloshinsky-Moriya [T. Moriya, Phys. Rev. 120, 91 (1960)] interaction with an alternating D vector. KCuGaF6 in a magnetic field is represented by the quantum sine-Gordon model, for which low-energy elementary excitations are composed of solitons and antisolitons and their bound states called breathers. Unlike the theoretical prediction, it was found that the energy of a soliton is smaller than that of the first breather, although the energy of the first breather coincides with that observed in a previous electron spin resonance measurement.