Spin Dynamics in Low Dimensional Quantum Spin System

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The quantum spin effect is one of the most interesting phenomena in the physics of the low dimensional magnetium. The study is focussed on the S=1 and S=1/2 quantum spin effects of the1D magnets through the experiments of NMR in high magnetic fields up to 15T.1. S=1 Quantum Effect (Haldane Effect)Haldane has predicted that in 1D Heisenberg antiferromagnet with integer spin the ground state is singlet. The prediction has given an impetus to experiments on S=1, quasi-1D antiferromagnets. Among several S=1 antiferromagnets, NENP [Ni(C_2H_8N_2)_2NO_2(CIO_4)] has been reported to be one of the best candidates to study the Haldane gap.In NENP we have observed, even below the critical field (9.8T), a large transverse moment through the proton NMR experiment. We have attributed the transverse moment to the staggered moment induced by the cooperation of the field induced staggered field and the large staggered susceptibility. The appearance of the transverse staggered moment is not a property o … More f the Haldane state, but is peculiar to NENP where the local symmetry axis at the site of Ni^<2+> tilts from the crystalline axis. Thus, under the magnetic field the special attention must be paid to treat the spin states of NENP.Nevertheless, we found that the proton spin-lattice relaxation rate was still consistent with the Haldane gap.2. S=1/2 Quantum EffectIn CsCuCl_3, one of the ABX_3-type triangular-lattice magnets, an anomalous jump of the magnetization has been reported in the ordered phase at the magnetic field around 12 T.The material is quasi-1D Heisenberg ferromagnet with S = 1/2. In the classical spin model such a magnetization jump can not be expected. A theoretical model has been proposed that the magnetization jump will appear through the transition of the spin structure caused by the quantum spin fluctuation.We have observed through the NMR experiment a discontinuity in the spectrum of^<133>Cs at the magunetic field corresponding to the magnetization jump. The critical field was clear-cut and no hysteresis was observed for the sweep direction of the magnetic field. The discontinuity of the spectum can be analyzed as due to the abrupt change in the spin structure. The spectra above and below the critical field are compatible with the spin structure of the theoretical model. Less