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

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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