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

本研究计划研究包含梯形晶格和三角形晶格的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，我们研究了杂质和载流子掺杂对两臂自旋梯子系统的影响。结果表明：1.Sr2Cu3O5在-60K以下表现出长程磁序，并且具有高于200K的量子自旋起伏。在磁化率测量中，我们发现该化合物的磁有序度低于-120K。具有自旋能隙的自旋液体态和磁性有序态之间可能存在竞争。3.S=1/2三角形晶格LiNiO_2在低温下没有表现出长程有序，尽管磁化率测量表明Ni自旋之间存在铁磁交换作用，这意味着没有受挫。提出了三角晶格结构使两个简并轨道态受挫，导致一些轨道液体基态，阻碍了对数范围的自旋有序化。4.在空穴掺杂的两臂自旋梯系统中，自旋隙的大小：从Ca9到Ca11.5，自旋能隙保持不变。7.空穴载流子局域化与梯形平面中准一维导电沟道有关。从而阐明了S=1/2低维量子自旋系统固有的量子自旋涨落和反常磁化的新特征。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.