Experimental evidence for a spin-Peierls-transition in $α'-NaV_2O_5$

We present the first measurements of magnetisation and Raman light scattering on $\alpha'-NaV_2O_5$ single crystals. Below 34 K, we observe a pronounced isotropic decrease of the susceptibility indicating the opening of a spin gap. The transition temperature is slightly field dependent. Raman experiments reveal a crystallographic distortion at the transition. Our results clearly establish $\alpha'-NaV_2O_5$ to be the second inorganic spin-Peierls system.