Study of Elementary Excitations and Microscopic Process of Phase Transitions in Triangular Antiferromagnets

在这个项目中，我们主要研究了六方ABX_3化合物，它们在顺磁状态下表现为一维自旋系统，在低温下由于-BX_3-链之间的反铁磁交换耦合而表现为TAF。主要研究结果如下：(1)在样品制备方面做了一定的工作，得到了许多优良的单晶。利用这些晶体，我们与名古屋大学内外的许多人合作，取得了许多丰硕的结果。(2)在平均场理论的框架下，推导了不能用传统的反铁磁共振理论来理解的六方ABX_3反铁磁体的ESR模。对CsNiCl3，CsNiBr3，RbNiCl3，CsMnI3，CsMnBr3等的ESR测量证实了这一结果。我们表明，在不缺乏与自旋结构和自旋波色散测量…一致性的情况下，斜线链间交换耦合对于理解ESR模式是必要的(3)用磁化率、扭矩、光学双折射和中子散射等方法研究了六方ABX_3反铁磁体的相变和临界行为，得到了不同的结果。例如，由于交换作用在c面上的不均匀，RbVBr3经历了两个连续的相变。中间相的自旋有序为部分无序(PD)结构。据作者所知，RbVBr3是第一个在Rb_&lt；1-chi&gt；K_&lt；chi&gt；NiCl3中具有Pd相的例子。(4)用中子散射精确地研究了六方ABX_3反铁磁体的自旋波激发。结果表明，线性自旋波理论很好地描述了AMnX_3体系的色散关系，而对AniX_3和AVX_3体系则不能很好地描述。我们得到了CsCuCl3的ESR模，它在c面上具有120ﾟ的自旋结构，沿c轴具有长周期螺旋结构。我们的结果与Palme等人的实验结果吻合得很好。对于H||c，CsCuCl3在H()SYAPPRXEQ()12T处发生相变。最近Shiba和Nikuni提出了一种理论，认为相变是由自旋的量子涨落引起的。Motokawa和他的合作者在KEK上用脉冲磁铁进行了中子散射实验，使用的是我们制备的大单晶。所得结果与理论相符。还测量了高场相的ESR模。这一结果可以用理论提出的自旋结构来解释。较少

In this project we have studied mainly hexagonal ABX_3 compounds which behave as the one-dimensional spin systems in the paramagnetic state and as TAF at low-temperatures because of the antiferromagnetic exchange coupling between -BX_3- chains. Results obtained are summarized as follows :(1) We made an effort for sample preparation and obtained many good single crystals. With the crystals we collaborated with many people inside Nagoya University and out, so that many fruitful results were produced.(2) The ESR modes in hexagonal ABX_3 antiferromagnets which cannot be understood in terms of the conventional theory of antiferromagnetic resonance were derived within the framework of the mean field theory. The results were confirmed by ESR measurements on CsNiCl_3, CsNiBr_3, RbNiCl_3, CsMnI_3 CsMnBr_3 etc. We showed that a diagonal interchain exchange coupling is essential to understand the ESR modes without lacking the consistency with the spin structure and the spin wave dispersion measur … More ed by neutron scattering.(3) We investigated the phase transitions and the critical behavior of hexagonal ABX_3 antiferromagnets by means of susceptibility, torque, optical birefringence and neutron scattering measurements and obtained various results. For example, RbVBr_3 undergoes two successive phase transitions due to the inequality of exchange interactions in the c-plane. The spin ordering in the intermediate phase is the partially disordered (PD) structure. To the author's knowledge, RbVBr_3 is the first example having the PD phase in the Heisenberg spin system In Rb_<1-chi>K_<chi>NiCl_3 a multicritical point occurs on the diagram for temperature versus chi.(4) Spin wave excitations of hexagonal ABX_3 antiferromagnets were investigated precisely by neutron scattering. It was found that the linear spin wave theory gives good description of the despersion relation for AMnX_3 systems, while not for ANiX_3 and AVX_3 systems. The deviation arises from the quantum effect.(5) We have derived the ESR modes of CsCuCl_3 which has the 120ﾟ spin structure in the c-plane and a long period helical structure along the c-axis. Our results agrees well with experimental ones obtained by Palme et al. CsCuCl_3 undergoes a phase transition at H(〕SY.apprxeq.〔)12 T for H||c. Recently Shiba and Nikuni presented a theory that the transitionis caused by the quantum fluctuation of spins. Motokawa and his collaborators performed neutron scattering experiment with pulse magnet in KEK using a large single crystal prepared by us. The results obtained is consistent with the theory. The ESR modes in the high-field phase were also measured. The results can be interpreted in terms of the spin structure proposed by the theory. Less