Polarized inelastic neutron scattering in the quasi-one-dimensional antiferromagnet KCuF3

The material KCuF3 is a prototypical example of a weakly coupled S=1/2 Heisenberg antiferromagnetic chain system. The inter-chain coupling leads to antiferromagnetic order at low temperatures. In the ordered state theory predicts that the low-energy spin dynamics exhibits an unusual gapped longitudinal mode. Here we report polarized inelastic scattering measurements of the excitations in KCuF3. A weak magnetic field is utilized to prepare a single domain state with the spins aligned in the scattering plane but perpendicular to the chain direction. This configuration enables a clean measurement of the longitudinal magnetic response as spin-flip scattering.