Field-Induced Phenomena and Its Frustration Effect in Quantum Spin System

各种新的量子自旋系统已经从实验和理论上发展和研究了受挫效应。最显著的成就是通过各种实验和理论分析，阐明了具有Shastry-Sutherland晶格的正交二聚体--SrCu2(BO_3)_2的磁化平台和1/8平台的磁性超结构的起源。这种量子化磁化平台现象可以看作是三重态(磁子)的维格纳结晶。我们还在其他不同的量子自旋系统中发现了磁化平台现象，如NH_4CnCl3中的1/4和3/4平台，Cs_2CuBr4中的1/3和2/3平台，Ba3Mn2O_8中的1/2平台，BIP-Teno中的1/4平台，BIPNNBNO中的1/3和2/3平台。任何磁化受挫都可以被认为是这些磁化平台现象的重要原因。在自旋-二聚体体系T1CuCl3中观察到了压力和杂质诱导的反铁磁相变，在F_2PNNNO中发现了场诱导的反铁磁相变，即所谓的磁子玻色-爱因斯坦凝聚，并发现了1/2的磁化平台。在三维受挫尖晶石系统中，成功地合成了自旋1/2(d^1)化合物--MgTi3O4，并发现了类Peierls转变。轨道自由度不仅在过渡过程中起着重要的作用。以及自旋为1的ZNV_2O_4的结构转变。这些结果说明了如何解除磁阻体系中的受挫。研究发现，无轨道自由度的含铬化合物如ZnCr2O4、CdCr2O4、HgCr2O4等发生了变磁相变，并伴随着结构的变化，表现出磁化平台。

Various new quantum spin systems have been developed and investigated in the light of frustration effect, both experimentally and theoretically. The most remarkable achievement is that the origin of magnetization plateau and the magnetic superstructure at 1/8 plateau in the orthogonal dimer system, SrCu_2(BO_3)_2 with the Shastry-Sutherland lattice have been elucidated from various experiments and theoretical analyses. This phenomenon of quantized magnetization plateau can be regarded as a Wigner crystallization of triplets (magnons). We also found phenomena of magnetization plateaus in other various quantum spin systems, for examples, 1/4 and 3/4 plateaus in NH_4CnCl_3, 1/3 and 2/3 plateaus in Cs_2CuBr_4, 1/2 plateau in Ba_3Mn_2O_8, 1/4 plateau in BIP-TENO, 1/3 and 2/3 plateaus in BIPNNBNO. Any magnetic frustration can be considered to play an important role for these phenomena of magnetization plateau. The pressure-and impurity-induced antiferromagnetic transitions have been observed in spin-dimer system TlCuCl3 and the field-induced antiferromagnetic transition, so-called Bose-Einstein condensation of magnons, has been found in F_2PNNNO as well as 1/2 magnetization plateau. In the three dimensional frustrated spinel systems, a spin-1/2 (d^1) compound, MgTi_3O_4 was successfully synthesized and a Peierls like transition has been found. The orbital degree of freedom plays an important role not only in the transition of. MgTi_2O_4 but also in the structural transition of spin-1 ZnV_2O_4. These results present how to lift the frustration in the magnetically frustrated systems. The Cr compounds such as ZnCr_2O_4, CdCr_2O_4 and HgCr_2O_4 without orbital degree of freedom have been found to exhibit metamagnetic transitions accompanied by the structural changes, showing the magnetization plateau.