Studies of new low-dimensional quantum spin systems with the spin gap

我们研究了Rb_2Cu_2Mo_3O_<12>的磁性。通过对实验结果和计算结果的比较，我们发现该铜酸盐可以被认为是具有铁磁第一近邻（1NN）和反铁磁第二近邻（2NN）竞争相互作用（竞争系统）的一维自旋1/2海森堡系统的模型化合物。估计1NN和2NN交换相互作用的值分别为J_1 = -138 K和J_2= 51 K （α = J_2/J_1 = -0.37）。这个值表明基态是一个自旋单重态不相称态。我们还研究了Cu_2CdB_2O_6的磁性。存在两种相邻的Cu位点[Cu(1)和Cu(2)]，以及三种交换相互作用（J_1、J_2和J_3）。Cu(1)自旋的AF二聚体和Cu(2)自旋的AF均匀链分别由J_1和J_3相互作用形成。J_2相互作用连接Cu(1)和Cu(2)自旋。从磁化率和比热的温度依赖关系中，我们发现了反铁磁体的出现，在转变温度T_N= 9.8 K时出现了更多的ic长程序（AFLRO）。在23t以上的磁场中，我们还发现了一个1/2的磁化平台。实验结果与J_1 = 160 K, J_2= 38.8 K, J_3= 9.7 K的自旋模型的量子蒙特卡罗结果一致。自旋态可以理解如下。在23t以下的磁场中，Cu(1)和Cu(2)自旋分别形成近自旋单重态和AFLRO态。相反，在23t以上的磁场中，Cu(1)自旋的近自旋单重态几乎保持不变，Cu(2)自旋几乎呈平行于磁场的极化。结果，我们发现近自旋单重态和AFLRO共存，尽管Cu(1)和Cu(2)自旋之间的相互作用不可忽视。Cu_2CdB_2O_6是量子自旋系统中第一个在原子尺度上表现出这种共存的材料。少

We have studied magnetism of Rb_2Cu_2Mo_3O_<12>. From the comparison between experimental and calculated results, we found that this cuprate can be considered as a model compound of a one-dimensional spin-1/2 Heisenberg system with ferromagnetic first-nearest-neighbor (1NN) and antiferromagnetic (AF) second-nearest-neighbor (2NN) competing interactions (competing system). Values of the 1NN and 2NN exchange interactions are estimated as J_1 = -138 K and J_2= 51 K (α = J_2/J_1 = -0.37). This value of a suggests that the ground state is a spin-singlet incommensurate state.We have also studied magnetism of Cu_2CdB_2O_6. There are two kinds of Cu sites [Cu(1) and Cu(2)], which are located adjacent to each other, and three kinds of exchange interactions (J_1,J_2, and J_3). AF dimers of the Cu(1) spins and AF uniform chains of the Cu(2) spins are formed by J_1 and J_3 interactions, respectively. The J_2 interaction connects the Cu(1) and Cu(2) spins. We found the appearance of antiferromagnet … More ic long-range order (AFLRO) with the transition temperature T_N= 9.8 K from the temperature dependence of the magnetic susceptibility and specific heat. We also found a 1/2 magnetization plateau in the magnetic fields above 23 T from the magnetic-field dependence of the magnetization. Experimental results agree with the quantum Monte Carlo results for the spin model with J_1 = 160 K, J_2= 38.8 K, and J_3= 9.7 K. Spin states can be understood as follows. In the magnetic fields below 23 T, the Cu(1) and Cu(2) spins form a nearly spin-singlet state and AFLRO, respectively. In the magnetic fields above 23 T, on the contrary, the nearly spin-singlet state of the Cu(1) spins are almost unchanged and the Cu(2) spins are almost polarized parallel to the magnetic field. As a result, we discovered that the nearly spin-singlet state and AFLRO coexist, although the interaction between the Cu(1) and Cu(2) spins cannot be ignored. Cu_2CdB_2O_6 is the first material in quantum spin systems that shows such coexistence in an atomic scale. Less