Synthesis of the two-dimensional orthogonal dieter compound with an exact dieter ground state and investigation of its magnetic properties

本课题的目的是通过实验实现Shastry-Sutherland模型，即我在五年前发现的二维正交Dieter体系SrCu_2（BO_3）_2，阐明与其特殊拓扑结构相关的磁性，并利用一些化学和物理方法诱导量子相变。结果发现，物理压力对于调节基态是非常有效的。在10 kbar左右的压力作用下，自旋能隙减小了一半，当进一步加压时，发现有相分离的趋势。这一结果与理论上认为该化合物中的Dieter自旋单重态非常接近于其他一些态的相边界的观点是一致的。与其他团队的合作也很成功：使用我生长的优质晶体进行的高场NMR研究揭示了1/8平台状态下的磁性超结构以及作为温度和磁场函数的奇异相变的性质，其结果发表在Science上。这项研究引发了理论家重新考虑他们的理论，因为现有的理论无法重现实验确定的上层建筑。我们还寻找自旋化合物，这是拓扑等价的Shastry-Suhherland模型。发现Nd_2BaZnO_5对应于这种模型的另一个例子，但具有经典自旋。我们用中子衍射实验测定了这种材料的磁结构。在SrCu_2（BO_3）_2中寻找基于正方晶格的新型磁阻挫系统。利用离子交换反应，我们新合成了（CuCl）Ca_2Ta_3O_3和（FeCl）Ca_2Ta_3O_3等低维磁性材料<10><10>，其中一些材料表现出有趣的磁性，推测它们可能是由磁性离子的低维网络所引起的。

The purpose of this project on the experimental realization of Shastry-Sutherland model, the two-dimensional orthogonal dieter system SrCu_2(BO_3)_2 that I discovered about 5 years ago, is to clarify the magnetic properties which are related to its special topology and to induce quantum phase transition, utilizing some chemical as well as physical method. As a result, physical pressure was found to be very effective for tuning the ground state. Even the application of pressure about 10kbar results in the reduction of spin gap which is as half as that without pressure, and when further pressure is applied, we have found a tendency of phase separation. This result is in consistent with the theoretical suggestion that the dieter spin-singlet state in this compound resides very close to the phase boundaries to some other states. The collaboration with other groups was also successful: the high-field NMR study using the good-quality crystal grown by me has revealed the magnetic superstructure at the 1/8 plateau state and nature of exotic phase transitions as a function of temperature and magnetic field, the result of which was published in Science. This study triggered theoreticians to reconsider their theory, since existing theories could not reproduce experimentally determined superstructure. We also search for the spin compound which is topologically equivalent to the Shastry-Suhherland model. Nd_2BaZnO_5 was found to correspond to another example of such model but with classical spins. We determined the magnetic structure of this material by neutron diffraction experiment. The search for the new magnetically frustrated systems based on the square lattice as in SrCu_2(BO_3)_2 has been conducted. By utilizing the ion-exchange reaction, we newly synthesized the low-dimensional magnetic materials such as (CuCl)Ca_2Ta_3O_<10> and (FeCl)Ca_2Ta_3O_<10>, some of which show interesting magnetic properties presumably arising from the low-dimensional network of magnetic ions.