Neutron scattering studies on quantum spin systems - Spin excitations in KCuCl3 and NaV2O5

We present the results of inelastic neutron scattering experiments on quant um spin systems KCuCl3 and NaV2O5. Both compounds show non-magnetic singlet ground state behaviour at low temperature. An important insight into the o rigin of the spin excitation gap can be obtained by looking at the wave vec tor dependence of the magnetic excitations in these systems without a Neel order. KCuCl3 turns out to be a weakly coupled dimer system, in contrast to the initially assumed zig-zag chain or ladder system. NaV2O5 on the other hand was first thought to be a linear chain system undergoing a spin-Peierl s transition, but it turns out to be most likely a zig-zag charge ordered l adder system at low temperature.