Longitudinal magnetic dynamics and dimensional crossover in the quasi-one-dimensional spin-1/2 Heisenberg antiferromagnet KCuF3

The spin dynamics of coupled spin-1/2 antiferromagnetic Heisenberg chains is predicted to exhibit a longitudinal mode at low energies and temperatures below the Neel temperature. This mode is a dimensional crossover effect and reveals the presence of a limited amount of long-range antiferromagnetic order coexisting with quantum fluctuations. In this paper the existence of such a mode is confirmed in the model material KCuF3 using polarized and unpolarized inelastic neutron scattering, and the longitudinal polarization of the mode is definitively established. The line shape is broadened, suggesting a reduced lifetime due to decay into spin waves. In addition, the data show evidence of continuum scattering with a lower edge greater than the longitudinal mode energy. A detailed comparison is made with theoretical predictions and experimental work on other model materials.