Magnetic excitations in the quantum spin system KCuCl

An inelastic neutron scattering study of the magnetic properties in the spin 1/2 double chain insulator KCuCl3 is presented. Above a spin gap of $\sim 2.6 { meV}$ well-defined excitations are observed which are dispersive in all directions of reciprocal space. Both the energy and the spectral weight of the excitations can be quantitatively described by a model based on a singlet nonmagnetic ground state and dispersive singlet-triplet modes. The relevant feature in the model description are weakly coupled dimers. Confirmation of the triplet nature of the excitations and of the robustness of the theoretical approach is provided by the observed three-fold splitting of the modes in a magnetic field. The information deduced from the measurements is discussed and compared to earlier works on the subject