Site-Selective NMR in the Quasi One Dimensional Conductor ѓА-Sr0.33V2O5

Site-Selective NMR in the Quasi One Dimensional Conductor ѓА-S_r0.33V₂O₅

The family of quasi-one-dimensional mixed valent vanadium oxides ѓА-A _0.33V₂O₅ (A = Li, Na, Ag, Ca, Sr, and Pb) exhibits a variety of exotic phenomena such as metal-insulator transition, charge ordering, and pressure-induced superconductivity [1], providing fascinating opportunities to study competing electronic orders in one series of compounds. There are three types of one-dimensional V-O network extending along the most conducting b -axis (Fig. 1): two types of zigzag chains formed by the V1 and the V3 sites and a two-leg ladder formed by the V2 sites. The ordering of A ions doubles the unit cell along the b -axis and generates two inequivalent V sites in each of the three 1D structural units denoted as Vn a and Vn b (n = 1 - 3) in the monoclinic space group P2₁ /a.

Fig. 1. Crystal structure of ѓА-S_r0.33V₂O₅ with the space group P2₁ /a viewed along the (a) b -, (b) a* -, and (c) c -directions.

Fig. 2. ⁵¹V NMR spectra at T = 190 K obtained from a needle-shape single crystal (0.5x1.8x0.2 mm³). The magnetic field of 10.5 T was rotated in the ac -plane (parallel to the a -axis at 0 degree). A single V site yields seven resonance lines split by the electric quadupole interaction.

We have performed nuclear magnetic resonance (NMR) experiments on ⁵¹V nuclei in ѓА-S_r0.33V₂O₅, which shows a metal-insulator transition at T = 170 K (see the article in this report by Y. Ueda's group). We have resolved NMR line from all six V sites in the high temperature metallic phase (Fig. 2) and made site-selective measurements of the Knight shift (K) (Figs. 3 and 4a) and the nuclear spin-lattice relaxation rate 1/(T₁T ) (Figs. 4b - 4e) [2].
We found that different sites show contrasting temperature dependences of K and 1/(T₁T ) (Fig. 4). Thus the magnetic properties are remarkably heterogeneous not only among the three 1D structural units but also among the two V sites in the same unit. In particular, the V2 ladder unit shows pronounced charge disproportionation of d -electrons among the two (V2a and V2b) sites as evidenced by the large difference in angular variation of K for the C and D sites (Fig. 3). Only one of them (D site) shows strong increase of |K | and 1/(T₁T ) with decreasing temperature (Fig. 4), suggesting development of ferromagnetic correlation. The strong heterogeneity even in the metallic phase should be a key feature to understand the metal-insulator transition in this family of compounds.

The Knight shift (K ) is plotted against the field direction for all six sites (labeled as A - F) at T = 190 K. The angles for the maximum of |K | match precisely with the directions of the shortest V-O bonding in the ac -plane, leading to the assignment of the pairs of lines (A, B), (C, D) and (E, F) to (V1a, V1b), (V2a, V2b) and (Va3, V3b) sites, respectively.

Fig. 4. (a): Temperature (T ) dependence of K at various sites normalized by the value at T = 300 K. (b) - (d) : T -dependences of 1/(T₁T ). (e): T -dependences of 1/(T₁T ) and 1/(T₁TK ²) at the D sites (one of the V2 sites). All measurements were done for the field oriented along one of the "magic angles" shown in the figures, where the quadrupole splitting vanishes.

References
[1] T. Yamauchi, M. Isobe and Y. Ueda, Solid State Science 7 (2005) 874.
[2] T. Waki, M. Takigawa, T. Yamauchi, J. Yamaura, H. Ueda and Y. Ueda, J. Phys. Chem Solid 68 (2007) 2143.