V 51 NMR study of the spin-gap compound BiCu₂ VO₆: A microscopic probe of magnetic interactions

We report the results of a V 51 nuclear-magnetic-resonance (NMR) study on the quasi-one-dimensional compound BiCu₂ VO₆ at temperatures between 4 and 300 K. Three V 51 NMR resonance lines that are associated with three nonequivalent crystallographic sites have been resolved. For each vanadium site, the temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a broad maximum. At low temperatures, all NMR shifts and spin-lattice relaxation rates clearly indicate thermally activated behavior, confirming the spin-gap formation in BiCu₂ VO₆. We further analyze the spin shift data based on a proposed spin-ladder scenario with three different types of interactions along the rungs. With this accordance, the spin gaps and the couplings along the rungs of the ladder are properly estimated.