Spin Dynamics in Low Dimensional Quantum Spin System

量子自旋效应是低维镁物理中最有趣的现象之一。通过核磁共振实验，研究了一维磁体的S=1和S=1/2量子自旋效应。S=1量子效应(霍尔丹效应)霍尔丹预言，在自旋为整数的一维海森堡反铁磁体中，基态为单重态。这一预言推动了S=1准一维反铁磁体的实验。在S=1的几种反铁磁体中，NENP[Ni(C_2H_8N_2)_2NO_2(ClO_4)]被认为是研究霍尔丹能隙的最佳候选者之一。通过质子核磁共振实验，我们在NENP中观察到，即使在临界场(9.8T)以下，也有一个大的横矩。我们将横向力矩归因于场诱导的交错场和大的交错磁化率的共同作用产生的交错力矩。横向交错力矩的出现不是…的特性更多的是Haldane态，但这是NENP所特有的，其中Ni^&lt；2+&gt；位的局域对称轴与晶轴倾斜。因此，在磁场作用下，必须特别注意处理NEP的自旋态。然而，我们发现质子的自旋-晶格驰豫速率仍然与霍尔丹间隙一致。S=1/2量子效应在ABX_3型三角晶格磁体CsCuCl3中，报道了磁化强度在12T附近的有序相中的反常跃变。材料为准一维海森堡铁磁体，S=1/2。在经典自旋模型中，这种磁化跃变是不能预料的。提出了通过量子自旋起伏引起自旋结构的跃迁而产生磁化跃迁的理论模型。通过核磁共振实验，我们观察到在对应于磁化跃迁的磁场中~(133)Cs的谱不连续。临界磁场清晰可见，磁场扫描方向无滞后现象。光谱的不连续可以分析为自旋结构的突变所致。临界场上下的光谱与理论模型的自旋结构是一致的。较少

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