NMR-STUDY ON THE SPIN GAP IN (VO)(2)P2O7 AND VO(HPO4)(0.5)H2O

We have carried out P-31-nuclear magnetic resonance (NMR) to investiga te low energy spin excitations in both spin ladder (VO)(2)P2O7 and spi n dimer VO(HPO4)0.5H(2)O systems. Temperature (7) dependences of NMR s hift, K and nuclear spin-lattice relaxation time, T-1 in the both comp ounds show a thermal activation-type behavior originating from spin ga p formations in the spin excitation. The most distinct difference appe ars in the T-dependence of (T1TK)(-1), namely, the T-independent behav ior in the dimer system, but the large decrease below the order of the spin gap in the ladder system. These experimental results suggest tha t a spin gap Delta is independent of wave vector q in dynamical suscep tibility x (q, omega) for the dimer system, while Delta depends on q i n x (q, omega) for the ladder system.