Discussion of a Spin-Cluster Model for the Low Temperature Phase of alpha' -NaV sub 2 O sub 5

We discuss magnetic excitations of a spin-cluster model which has been suggested to describe the low temperature phase of alpha' -NaV sub 2 O sub 5. This model fulfills all symmetry criteria proposed by recent x-ray investigations. We find that this model is not able to describe the occurrence of two well separated magnon lines perpendicular to the ladder direction as observed in INS experiments. We suggest further experimental analysis to generally distinguish between models with double reflection or inversion symmetry.