^{31P-NMR Study of Low-Energy Spin Excitations in Spin Ladder (VO)₂P₂O₇ and Spin Dimer VO₄(HPO)0.5H₂O Systems}

Yuji FURUKAWA, Azusa IWAI, Ken-ichi KUMAGAI and Andrey YAKUBOVSKY^(*)

Department of Physics, Graduate School of Science, Hokkaido University, Sapporo 060

^{(*)Russian Research Center, Kurchatov Institute, Moscow, Russia}

Keywords: NMR, Knight shift, nuclear spin-lattice relaxation time, spin ladder, spin dimer, (VO)₂P₂O₇, VO₄(HPO)0.5H₂O

Abstruct

Using ³¹P-nuclear magnetic resonance ( NMR ), low-energy spin excitations are investigated in both (VO)₂P₂O₇ and VO₄(HPO)0.5H₂O, in which V²⁺(S=1/2) ions take ladder and dimer (isolated pair) configurations, respectively. Temperature (T) dependences of Knight shift (K) and nuclear spin-lattice relaxation time (T₁) in the two compounds show a thermally activated behavior originating from the spin gap formation in the low energy spin excitation. The most distinct difference appears in the T-dependence of (1/T₁T) at low temperature, i.e., the T-independent behavior in the dimer system and the large decrease below the order of the spin gap in the ladder system. These experimental results suggest that the spin gap Δ is independent of wave vector q in the case of the dimer system, while Δ depends on q for the spin ladder system. Resonance frequency dependence of T₁ in the temperature regions of both T<Δ and T>Δ is also discussed.