NMR investigation of BaCu2 V2 O8 in alternating-chain and dimer-chain models

V51 NMR measurements were performed on the quasi-one-dimensional compound BaCu2 V2 O8. Two V51 NMR lines that are associated with two vanadium crystallographic sites have been resolved. For each site the temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a broad maximum at Tmax 280 K. Below Tmax, the NMR shifts and spin-lattice relaxation rates clearly indicate activated behavior, confirming the spin gap formation in BaCu2 V2 O8. While the alternating-chain model has been proposed to account for the observed spin gap characteristics, our NMR results suggest that the nearly isolated dimer picture could be an alternative scenario for the understanding of the gap feature in this material.