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

本项目主要研究了六方ABX_3化合物，由于- bx_3 -链之间的反铁磁交换偶联，它们在顺磁状态下表现为一维自旋体系，在低温状态下表现为TAF体系。所得结果总结如下：(1)我们在样品制备方面做出了努力，获得了许多良好的单晶。在水晶方面，我们与名古屋大学内外的许多人合作，因此产生了许多卓有成效的成果。(2)在平均场理论的框架内推导出了六方ABX_3反铁磁体中不能用常规反铁磁共振理论理解的ESR模式。用ESR对CsNiCl_3、CsNiBr_3、RbNiCl_3、CsMnI_3、CsMnBr_3等材料进行了实验验证。我们证明了对角链间交换耦合对于理解ESR模式是必不可少的，同时又不缺乏与自旋结构和自旋波色散测量的一致性。(3)通过磁化率、转矩、光学双折射和中子散射等测量方法研究了六方ABX_3反铁磁体的相变和临界行为，得到了不同的结果。例如，RbVBr_3由于c面交换相互作用的不平等，经历了两次连续的相变。中间相的自旋有序为部分无序（PD）结构。据作者所知，RbVBr_3是海森堡自旋体系中第一个具有PD相的例子。在Rb_<1-chi>K_<chi>NiCl_3中，温度与chi的关系图上出现了一个多临界点。(4)用中子散射法精确地研究了六方ABX_3反铁磁体的自旋波激发。发现线性自旋波理论能很好地描述AMnX_3体系的绝望关系，而不能很好地描述ANiX_3和AVX_3体系的绝望关系。偏差是由量子效应引起的。(5)我们得到了CsCuCl_3在c平面上具有120个<s:1>自旋结构和沿c轴的长周期螺旋结构的ESR模式。我们的结果与Palme等人的实验结果吻合得很好。CsCuCl_3在H（] SY.apprxeq）处发生相变。[)12 T为H b| b| c。最近，Shiba和Nikuni提出了一个理论，认为这种转变是由自旋的量子涨落引起的。Motokawa和他的合作者利用我们制备的大单晶在KEK进行了脉冲磁体中子散射实验。所得结果与理论相符。测量了高场相位的ESR模式。结果可以用该理论提出的自旋结构来解释。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