Ground States and Excitation Gaps the Double Spin Chain Systems ACuX_3 and KCrCl_3

近年来，具有空间结构的低维量子自旋系统的磁性研究引起了人们极大的兴趣。特别是一维系统中磁化强度的量子化是一个新的重要问题。本文研究了含双链的KCuCl_3、TlCuCl_3和NH_4CuCl_3体系的新量子效应。双链中的交换相互作用被描述为具有对角相互作用的海森堡自旋梯或具有次近邻相互作用的交替链。从最近的理论，它预计目前的系统的基础是一个自旋单态与激发间隙。我们首先制备了大单晶，并测量了这三种化合物的磁化率。我们发现KCuCl_3和TlCuCl_3的基态都是自旋单态，并有激发能隙。通过高场磁化测量，我们估计了激发间隙的大小。为了 关于我们 为了研究磁激发，我们对KCuCl_3和TlCuCl_3进行了高频高场ESR测量，观察到了单重基态的磁激发。NH_4CuCl_3与KCuCl_3和TlCuCl_3在室温下具有相同的晶体结构。与KCuCl_3和TlCuCl_3相比，它的零场基态是磁性的。NH_4CuCl_3的磁化过程的显著特征是在四分之一和四分之三饱和磁化强度处出现两个平台。高原出现在每个外部场方向。在磁化平台之间，存在斜坡。因此，在类伊辛自旋系统中的变磁跃迁不能理解平台。高原的起源应归因于量子效应。这表明，平台是伴随着激发间隙。我们进行了ESR测量，发现在平台区存在激发能隙。据作者所知，NH_4CuCl_3是自旋为1/2的体系中磁化强度量子化的第一个例子。NH_4CuCl_3磁化平台的发现是本研究中最重要的成果。然而，NH_4CuCl_3量子磁化过程的微观机制尚未阐明。因此，阐明其作用机理是今后研究的重要课题之一。少

Recently, magnetic properties of the low-dimensional quantum spin systems with spatial structures have been of great interest. In particular, the quantization of the magnetization in the one-dimensional system is a new important problem. In the present study, we have investigated new quantum effect in KCuCl_3, TlCuCl_3 and NH_4CuCl_3 with the double chain. The exchange interactions in the double chain are described as a Heisenberg spin ladder with the diagonal interaction or an alternating chain with the next-nearest-neighbor interaction. From the recent theories, it is expected that the ground of the present systems is a spin singlet with an excitation gap. We first prepared large single crystals, and measured the magnetic susceptibilities of these three compounds. We found that the ground states of KCuCl_3 and TlCuCl_3 are spin singlets with excitation gaps as expected. We estimated the magnitude of the excitation gap by means of the high-field magnetization measurements. In order to … More investigate the magnetic excitations, we carried out the high-frequency high-field ESR measurements in KCuCl_3 and TlCuCl_3, and observed the excitations from the singlet ground states. We also performed neutron inelastic scattering measurements, and determined the dispersion relations in KCuCl_3 and TlCuCl_3.NH_4CuCl_3 has the same crystal structure as those of KCuCl_3 and TlCuCl_3 at room temperature. However, its zero-field ground state is magnetic in contrast with those of KCuCl_3 and TlCuCl_3. The remarkable feature of the magnetization process in NH_4CuCl_3 is two plateaus at one-quarter and three-quarters of the saturation magnetization. The plateaus appear in every external field direction. Between the magnetization plateaus, there are slopes. Thus, the plateaus cannot be understood in terms of the metamagnetic transitions in the Ising-like spin systems. The origin of the plateaus should be attributed to the quantum effect. It is suggested that the plateaus are accompanied with the excitation gap. We carried out the ESR measurements, and found that there are excitation gaps in the plateau regions. To the best of the authors' knowledge, NH_4CuCl_3 shows the first example of the quantization of the magnetization in a spin-1/2 system. The discovery of the magnetization plateaus in NH_4CuCl_3 is the most important fruit in the present study. However, the microscopic mechanism of the quantum magnetization process in NH_4CuCl_3 has not not clarified. Thus, to clarify the mechanism is one of the most important future problem. Less