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 excitatio ns 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) configuratio ns, 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 appear s in the T-dependence of (T1TK)(-1) at low temperature, i.e., the T-in dependent behavior in the dimer system and the large decrease below th e order of the spin gap in the ladder system. These experimental resul ts 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 la dder system. Resonance frequency dependence of T-1 in the temperature regions of both T < Delta and T > Delta is also discussed.