Magnetic excitations in the quantum spin system KCuCl3

An inelastic neutron scattering study of the magnetic properties in the spi n 1/2 double chain insulator KCuCl3 is presented. Above a spin gap of simil ar to 2.6 meV well-defined excitations are observed which are dispersive in all directions of reciprocal space. Both the energy and the spectral weigh t 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. Confir mation of the triplet nature of the excitations and of the robustness of th e theoretical approach is provided by the observed three-fold splitting of the modes in a magnetic field. The information deduced from the measurement s is discussed and compared to earlier works on the subject.