Spin gap state in α'-NaV2O5 studied by far infrared spectroscopy D. Smirnov, J. Leotin, O. Drachenko, P. Millet, J. Jegoudez et A. Revcolevschi J. Phys. IV France, 09 PR10 (1999) Pr10-279-Pr10-280 DOI: 10.1051/jp4:19991073