Spin-cluster model for the low-temperature phase of alpha '-NaV2O5

We discuss magnetic excitations of a spin-cluster model which has been sugg ested to describe the low-temperature phase of alpha'-NaV2O5. This model fu lfills all symmetry criteria proposed by recent x-ray investigations. We fi nd that this model is not able to describe the occurrence of two well separ ated magnon lines perpendicular to the ladder direction as observed in inel astic neutron scattering experiments. We suggest further experimental analy sis to generally distinguish between models with double reflection or inver sion symmetry.