⁵¹V-NMR Studies on the Spin Dynamics of S=1/2 Zig-Zag Chains of VO₅ Pyramids

Abstract

NMR Knight shifts K and nuclear relaxation rates 1/T₁ have been studied on the V⁴⁺ sites of two compounds [N(CH₃)₄]V₃O₇ and (en)₂ZnV₆O₁₄ which are known to exhibit spin-gap behavior from their susceptibility data. The activation-type behavior has been observed in K and 1/T₁, confirming the existence of the spin-gap in these systems. The frequency (ω _N) dependence of 1/T₁ of [N(CH₃)₄]V₃O₇ has been found to be 1/√ ω _N-type one and 1/T₁ of (en)₂ZnV₆O₁₄ is linear in -ln (ω _N), indicating the diffusive and ballistic characters of the triplet excitations, respectively. The results are discussed in relation to the theory recently developed by Sachdev and Damle [Phys. Rev. B 57 (1998) 8307].