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

本研究计划探讨S=1/2量子自旋系统中包含阶梯晶格或三角晶格的量子自旋涨落与异常磁性之间的关系。此外，我们还研究了杂质和载流子掺杂对两腿自旋阶梯体系的影响。我们列出了以下结果。Sr_2Cu_3O_5在- 60k以下表现出长程磁序，在200k以上表现出三脚阶梯固有的量子自旋涨落。两脚梯状化合物LaCuO_<2.5>的磁化率测定表明，其自旋间隙与SrCu_2O_3相似。然而，从Cu-NMR研究中，我们发现该化合物在- 120k以下是有序的。具有自旋间隙的自旋液态和磁有序态之间可能发生竞争。S=1/2三角形晶格LiNiO_2在低温度下没有长程有序，尽管从磁化率测量中可以预期Ni自旋之间存在铁磁交换相互作用，这意味着没有挫败感。我们提出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，从而阻碍了长距离自旋有序化。在双脚自旋阶梯体系中，当Sr_<14-x>Ca_xCu_<24>O_<41>时，自旋间隙的大小从δ =510 K降至Ca11.5.5时的270 K。从Ca9到Ca11.5.6，自旋间隙保持不变。在DELTA <= T的情况下，自旋动力学可以很好地表征为一维量子临界自旋涨落，尽管两条链彼此交换耦合并且有空穴掺杂。空穴载流子局域于与梯面上准一维导电通道相关的低T。因此，我们阐明了量子自旋涨落的新特征和S=1/2低维量子自旋系统固有的异常磁性。目前的研究项目使我们在这个领域开辟了新的前沿。

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.