Spin gap state in alpha '-NaV2O5 studied by far infrared spectroscopy

We present far infrared transmission measurements of alpha'-NaV2O5 single c rystals. This material is known to undergo a spin-Peierls like transition a t T-SP=35K. Below the transition temperature we observed together with expe cted new sharp phonon lines a strong gap-like depression of absorption occu rring in the low energy part of a continuum likely connected with two-magno n excitations. The optical gap temperature dependence and its value closely match the spin gap values measured by neutron scattering experiments. Taki ng the optical gap as a fingerprint of the spin gap, we found that the rela tionship between the spin gap value and the dimerisation lattice parameter do not follow the scaling relation expected for the SP systems.