Dynamics of low dimensional quantum spin systems and effects of carrier doping.

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Since the high temperature superconductivity was discovered in 1986, various attempts were made to dope carriers into low dimensional quantum spin systems, which are the Mott-insulators and to find superconductivity. In such a trend of research, the first objectives of this project was set to clarify the dynamics of the low dimensional quantum spin systems from a microscopic point of view using nuclear magnetic resonance as the principal tool. Second objective was to understand the change in magnetic dynamics upon elemental substitution or doping carriers or impurities. The former was achieved by the experiments on the hole-doped spin-ladder materials and the two-dimensional orthogonal dimer spin systems, which yielded variety of different behavior of spin excitations for the singket ground state. Concerning the latter, experiments on hole-doped ladder materials have shown that interaction between carriers and the magnetic excitations induces paring of holes. The thermally depaired carriers are shown to cause significant damping of magnetic excitations. Furthermore, extremely slow charge fluctuations associated with partial charge ordering were observed in a few quasi-one-dimensional conducting systems. On the other hand, in Kondo insulators, which is another example of spin gap material, we found that local moments are induced near impurities similar to the case of quantum spin systems.