NMR Studies of S=1/2 Quantum Spin Systems with either Ladder or Triangular Lattice

本研究计划研究S=1/2量子自旋系统中的量子自旋涨落与反常磁性之间的关系。在此基础上，我们进一步研究了杂质和载流子掺杂对两条腿自旋梯结构的影响，得到以下结果：1.Sr_2Cu_3O_5在-60 K以下表现出长程磁有序，在200 K以上表现出三条腿自旋梯结构所固有的量子自旋<2.5>涨落。3.S =1/2三角晶格LiNiO_2在低温下不存在长程有序，但磁化率测量表明Ni自旋间存在铁磁交换作用，这意味着没有阻挫。我们认为三角形晶格结构使两个简并轨道态受挫，导致一些轨道液态基态，阻碍了logn范围的自旋有序化. 4.在空穴掺杂的两腿自旋梯系统中，Sr_<14-x>Ca_xCu_<24>O_<41>，自旋能隙的大小，对于未掺杂Ca的Δ =510 K，对于Ca 11.5.5，Δ = 270 K。从Ca 9到Ca 11.5.6，自旋隙保持不变。自旋动力学由一维量子临界自旋涨落很好地表征，7.空穴载流子在低T下局域化，并与梯形平面中的准一维导电沟道相关联，从而阐明了S=1/2的量子自旋涨落和反常磁性的新特征。2个低维量子自旋系统。目前的研究项目使我们能够在这一领域开辟新的领域。

This research project planed to investigate the relationship between the quantum spin fluctuations and anomalous magnetism in S=1/2 quantum spin systems containing either the ladder or triangular lattice. Furthermore, we addressed the impurity and carrier doping effects into two-leg spin ladder systems.We listed the following results.1.Sr_2Cu_3O_5 exhibits the long-range magnetic order below -60 K and is characterized by quantum spin fluctuations above 200 K inherent to three-leg ladder.2.Two-leg ladder compound LaCuO_<2.5> was suggested from the susceptibility measurement to have seemingly a spin gap as in SrCu_2O_3. From the Cu-NMR study, we, however, found that this compound was magnetically ordered below-120 K.A possible competition takes place between the spin liquid state with a spin gap and the magnetic ordered states.3.S=1/2 triangular lattice LiNiO_2 exhibis no long-range order at low-T,although the ferromagnetic exchange interaction is expected among Ni spins from the susceptibility measurement, meaing no frustration. We proposed that the triangular lattice structure make two degenerated orbital states frustrated, leading to some orbital liquid ground states, which prevents the logn-range spin ordering.4.In hole-doped two-leg spin ladder systems, Sr_<14-x>Ca_xCu_<24>O_<41>, magnitude of the spin gap, DELTA=510 K for Ca-undoped one decreases to 270 K for the Ca11.5.5.The spin gap stays constant in going from the Ca9 to the Ca11.5.6.In DELTA <= T,spin dynamics are well characterized by the one-dimensional quantum critical spin fluctuations, despite two chains are exchange-coupled to one another and holes are doped.7.hole carrieres are localized at low T associated with quasi one-dimensional conducting channel in the ladder plane.We have thus clarified novel characteristics of quantum spin fluctuations and anomalous magentism inherent to S=1/2 low-dimesional quantum spin systems. The present reaserch project enabled us to open new frontiers in the field.