Novel Longitudinal Mode in the Coupled Quantum Chain CompoundKCuF3

Abstract

Inelastic neutron scattering measurements are reported that show a new longitudinal mode in the antiferromagnetically ordered phase of the spin- $1/2$ quasi-one-dimensional antiferromagnet ${\mathrm{KCuF}}_3$. This mode signals the crossover from one-dimensional to three-dimensional behavior and indicates a reduction in the ordered spin moment of a spin- $1/2$ antiferromagnet. The measurements are compared with recent quantum field theory calculations and are found to be in excellent agreement. A feature of the data not predicted by theory is a damping of the mode by decay processes to the transverse spin-wave branches.