NMR investigation of ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ in alternating-chain and dimer-chain models

${}^{51}\mathrm{V}$ NMR measurements were performed on the quasi-one-dimensional compound ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. Two ${}^{51}\mathrm{V}$ 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 $T_{\mathit{max}}\simeq 280\mathrm{K}$. Below $T_{\mathit{max}}$, the NMR shifts and spin-lattice relaxation rates clearly indicate activated behavior, confirming the spin gap formation in ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. 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.

Figure 1

Temperature dependence of the ${}^{51}\mathrm{V}$ NMR shifts for both V-I and V-II sites in ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. Inset: the ${}^{51}\mathrm{V}$ NMR spectrum for ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ measured at 180 K.

Figure 2

Plots of $K_{\mathit{\text{spin}}}\sqrt{T}$ against $1∕T$ for ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. The solid lines indicate the activation law with ${\Delta }_K^{\left(c\right)}$ of $360\pm 20$ and $370\pm 20\mathrm{K}$ for sites I and II, respectively. The inset illustrates the linear relationship between $K_{\mathit{obs}}$ and $\chi$ (deduced from Ref. 6).

Figure 3

Plots of $K_{\mathit{\text{spin}}}T$ against $1∕T$ for ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ . The solid curves are fitted to the function described by the dimer model with ${\Delta }_K^{\left(d\right)}$ of $460\pm 25$ and $470\pm 25\mathrm{K}$ for sites I and II, respectively.

Figure 4

Inverted-temperature dependence of $1∕T_1$ for the V-II site in ${\mathrm{BaCu}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. The dashed line is obtained by fitting to a simple exponential relation. The solid curve is the fitted function based on the dimer model. Inset: a representative recovery curve taken at 85 K. Solid line: a fit to the theoretical recovery behavior from Eq. (1).