Hiroki Nakano

Abstract The spin- S Heisenberg antiferromagnet on the two-dimensional lattice is investigated for S = 1/2 and S = 1. We consider interaction at isolated dimers ( J _d ) and interaction bonds that form the bounce lattice ( J _b ). For J _d = J _b , the system is reduced to the maple-leaf-lattice antiferromagnet. We primarily conduct highly parallelized numerical diagonalization to examine the spin excitation gap above the ground state for various J _b / J _d cases. For S = 1/2, we report calculations for a 42-site cluster that has not been previously treated. The S = 1 case is examined for the first time for clusters up to 24 sites. Regardless of whether S = 1/2 or 1, we find that the system has a gapped nature for small J _d / J _b and becomes gapless at J _d / J _b ∼ 1.4. For S = 1, we also find that another gapped region appears between the gapless case at J _d / J _b ∼ 1.4 and the boundary of the exact-dimer phase.

Abstract The magnetization process of the S = 1/2 distorted diamond spin chain with ferromagnetic interactions is investigated using the numerical diagonalization of finite-size clusters. The level spectroscopy analysis applied for the model with the spin anisotropy indicates that two different magnetization plateau phases appear at 1/3 of the saturation magnetization. The phase diagrams for some typical interaction parameters are presented. In addition the magnetization curves for several typical parameters are obtained.

Abstract The magnetization process of the S = 1/2 anisotropic spin ladder with the ferromagnetic rung interaction is investigated using the numerical diagonalization of finite-size clusters. It is found that the translational symmetry broken magnetization plateau would appear at half the saturation magnetization, when the competing anisotropies are sufficiently large. The phase diagram with respect to the anisotropies and several magnetization curves are presented.