P-31-NMR study of low-energy spin excitations in spin ladder (VO)(2)P2O7 and spin dimer VO(HPO4)0.5H(2)O systems

Using P-31-nuclear magnetic resonance (NMR), low-energy spin excitations are investigated in both (VO)(2)P2O7 and VO(HPO4)0.5H(2)O, 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 (T1TK)(-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 Delta is independent of wave vector q in the case of the dimer system, while Delta depends on q for the spin ladder system. Resonance frequency dependence of T-1 in the temperature regions of both T < Delta and T > Delta is also discussed.