31P-NMR Study of Low-Energy Spin Excitations in Spin Ladder (VO)_2P_2O_7 and Spin Dimer VO(HPO_4)0.5H_2O Systems

Abstract

Using ³¹P-nuclear magnetic resonance (NMR), low-energy spin excitations are investigated in both (VO)_2P_2O_7 and VO(HPO_4)0.5H_2O, in which V⁴⁺ (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 (T_1) 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 (T_1TK)^-1 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 T_1 in the temperature regions of both T < Δ and T > Δ is also discussed.