NMR Study of Spin Dynamics of Ladder Compounds and High-Tc Superconductors

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Hole-doping effects on the magnetism of the chain and the ladder site are investigated by Cu-NMR and NQR for single crystal of Sr_<14-x>A_xCu_<24>O_<41> (A=Ca and La). Antiferromagnetic (AF) order of Cu moments is confirmed in the chain for 3≦x≦6 of Sr_<14-x>La_xCu_<24>O_<41>. The magnetic order of Cu spins is ferromagnetic within the chain, and is antiferromagnetic for inter-chains. The field-induced staggered moments at the ladder Cu site appear fin above T_N, and its temperature dependence obeys the Curie-Weiss behavior, suggesting that this staggered field is attributed to the magnetic nature of the ladder Cu spins themselves. A possible origin is ascribed to the disturbance of the coherence of the RVB state by doping centers in the ladder. Finally a long-range AF order in the highly-doped compound of Sr_<2.5>Ca_<11.5>Cu_<24>O_<41> is discussed based on the Cu-NMR/NQR results. This AF order is responsible for the chain Cu spins.