Soliton Pairing and Magnon Bound States in DSG Spin Chains: NMR and ESR Experiments in TMMC and TMNC

Abstract

Nonlinear excitations in the form of kink solitons play an important role in understanding the low-temperature properties of quasi-one-dimensional (1D) magnets and have been widely studied, both theoretically and experimentally, within the last 15 years^1–6. Experimental investigations mainly refer to sine-Gordon-like spin chains, such as CsNiF₃ ^{3, 6}or (CH₃)₄NMnCl₃ (TMMC)^4–6. The crystal structure of these two prototypes (and of many other related compounds) is isomorphous: It consists of a hexagonal array of magnetic chains with the magnetic nickel or manganese ions coupled ferro- or antiferromagnetically along the crystallographic c-direction. Intrachain dipolar and crystal field interactions provide for an easy plane, so that the spins tend to align perpendicularly to the chain direction. Applying an external magnetic field within the easy plane, or considering the effect of some symmetry-breaking anisotropy of easy-axis type, we arrive at a Hamiltonian which, using some standard approximations¹, can be mapped to a sine-Gordon (SG) model. Thus, most of the experimental investigations on magnetic solitons refer to SG type systems even though the theoretical interpretations applied have generally proceeded beyond that limit⁷.