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特性引起了人们的极大兴趣。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。在本研究中，我们研究了KCuCl3、TlCuCl3和NH4CuCl3的双链新的量子效应。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最近邻相互作用的交替链。根据最近的理论，预计目前系统的基态是具有激发能隙的自旋单重态。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KCuCl3和TlCuCl3的基态如预期的那样是具有激发带隙的自旋单态。我们通过高场磁化测量估计了激发间隙的大小。为了…为了进一步研究磁激发，我们在KCuCl3和TlCuCl3中进行了高频高场ESR测量，观察到了来自单重态基态的激发。我们还进行了中子非弹性散射测量，测定了它们在KCuCl3和TlCuCl3中的色散关系。NH4CuCl3在室温下与KCuCl3和TlCuCl3具有相同的晶体结构。然而，与KCuCl3和TlCuCl3相比，其零场基态是磁性的。NH4CuCl3的磁化过程的显著特征是在饱和磁化强度的四分之一和四分之三处出现两个平台。高原出现在每个外场方向上。在磁化平台之间，有一些斜坡。因此，不能用类伊辛自旋系统中的变磁跃迁来理解这个平台。高原的起源应该归因于量子效应。结果表明，高原与激发间隙相伴出现。我们进行了ESR测量，发现在高原地区存在激发带。据作者所知，NH4CuCl3显示了自旋1/2系统中磁化的第一个例子。NH_4CuCl_3中磁化平台的发现是本研究最重要的成果。然而，NH4CuCl3中量子磁化过程的微观机制尚不清楚。因此，阐明这一机制是未来最重要的问题之一。较少

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