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

We present far infrared transmission measurements of α'-NaV2O5 single crystals. This material is known to undergo a spin-Peierls like transition at TSP=35K. Below the transition temperature we observed together with expected new sharp phonon lines a strong gap-like depression of absorption occurring in the low energy part of a continuum likely connected with two-magnon excitations. The optical gap temperature dependence and its value closely match the spin gap values measured by neutron scattering experiments. Taking the optical gap as a fingerprint of the spin gap, we found that the relationship between the spin gap value and the dimerisation lattice parameter do not follow the scaling relation expected for the SP systems