Observation of solitons in the S=12 antiferromagnetic chain [CuPM(NO3)2(H2O)2]n by 13C-NMR

We present 13C-NMR investigations of the S = 12 antiferromagnetically coupled Heisenberg chain system [CuPM(NO3)2(H2O)2]n, (PM=C4N2H4=pyrimidine). For this material, as result of the Dzyaloshinskii-Moriya interaction and an alternating g-tensor, the ground state is characterized by a giant static spin canting and an anisotropic magnetic field-induced spin excitation gap. We measured the spin-lattice relaxation rate 1/T1 at three inequivalent carbon sites in the pyrimidine molecule as function of temperature. From these studies we deduce the magnetic excitation spectrum of copper pyrimidine dinitrate at a constant applied magnetic field of 9.3 T. The experimental data are discussed in context with recent ESR investigations, which have been interpreted in terms of breather, soliton and multi-particle excitations based on the quantum sine-Gordon field theory.