Competing Ground States of the New Class of Halogen-Bridged Metal Complexes

Based on a symmetry argument, we study the ground-state properties of halogen-bridged binuclear metal chain complexes. We systematically derive commensurate density-wave solutions from a relevant two-band Peierls-Hubbard model and numerically draw the the ground-state phase diagram as a function of electron-electron correlations, electron-phonon interactions, and doping concentration within the Hartree-Fock approximation. The competition between two types of charge-density-wave states, which has recently been reported experimentally, is indeed demonstrated.Comment: 4 pages, 5 figures embedded, to appear in J. Phys. Soc. Jp

Elementary Excitations of Heisenberg Ferrimagnetic Spin Chains

We numerically investigate elementary excitations of the Heisenberg alternating-spin chains with two kinds of spins 1 and 1/2 antiferromagnetically coupled to each other. Employing a recently developed efficient Monte Carlo technique as well as an exact diagonalization method, we verify the spin-wave argument that the model exhibits two distinct excitations from the ground state which are gapless and gapped. The gapless branch shows a quadratic dispersion in the small-momentum region, which is of ferromagnetic type. With the intention of elucidating the physical mechanism of both excitations, we make a perturbation approach from the decoupled-dimer limit. The gapless branch is directly related to spin 1's, while the gapped branch originates from cooperation of the two kinds of spins.Comment: 7 pages, 7 Postscript figures, RevTe

ΛΛ\Lambda\Lambda-ΞN\Xi N-ΣΣ\Sigma\Sigma coupling in ΛΛ 6^{~6}_{\Lambda\Lambda}He with the Nijmegen soft-core potentials

The $\Lambda\Lambda$-$\Xi N$-$\Sigma\Sigma$ coupling in $^{~6}_{\Lambda\Lambda}$He is studied with the [$\alpha$ + $\Lambda$ + $\Lambda$] + [$\alpha$ + $\Xi$ + $N$] + [$\alpha$ + $\Sigma$ + $\Sigma$] model, where the $\alpha$ particle is assumed as a frozen core. We use the Nijmegen soft-core potentials, NSC97e and NSC97f, for the valence baryon-baryon part, and the phenomenological potentials for the $\alpha-B$ parts ($B$=$N$, $\Lambda$, $\Xi$ and $\Sigma$). We find that the calculated $\Delta B_{\Lambda\Lambda}$ of $^{~6}_{\Lambda\Lambda}$He for NSC97e and NSC97f are, respectively, 0.6 and 0.4 MeV in the full coupled-channel calculation, the results of which are about half in comparison with the experimental data, $\Delta B^{exp}_{\Lambda\Lambda}=1.01\pm0.20^{+0.18}_{-0.11}$ MeV. Characteristics of the $S=-2$ sector in the NSC97 potentials are discussed in detail.Comment: 18 pages, 4 figure

Alternating-Spin Ladders in a Magnetic Field: New Magnetization Plateaux

We study numerically the formation of magnetization plateaux with the Lanczos method in 2-leg ladders with mixed spins of magnitudes $(S_1,S_2)=(1,1/2)$ located at alternating positions along the ladder and with dimerization $\gamma$. For interchain coupling $J'>0$ and $\gamma=0$, we find normalized plateaux at $m=1/3$ starting at zero field and $m=1$ (saturation), while when $\gamma \ne 0$ is columnar, another extra plateau at $m=2/3$ shows up. For $J'<0$, when $\gamma<\gamma_c(J')$ we find no plateau while for $\gamma>\gamma_c(J')$ we find four plateaux at $m=0,1/3,2/3,1$. We also apply several approximate analytical methods (Spin Wave Theory, Low-Energy Effective Hamiltonians and Bosonization) to understand these findings and to conjeture the behaviour of ferrimagnetic ladders with a bigger number of legs.Comment: REVTEX file, 7 pages, 6 eps Figure

Magnetization cusp singularities of frustrated Kondo necklace model

Magnetization processes of frustrated Kondo necklace model are studied by means of a density matrix renormalization group (DMRG) method and an elementary band theory based on a bond-operator formalism. The DMRG calculations clearly show the cusp singularity in a low-magnetization region ($0<m<1/2$) besides that in a high-magnetization region ($1/2<m<1$) which is expected from previous studies on the magnetization curve of the Majumdar-Ghosh model. An appearance mechanism of the low-magnetization cusp is interpreted in terms of a double-well shape of a low-energy band arising from frustrations between nearest- and next-nearest-neighbor interactions. We also discuss critical behaviors of magnetization near the cusp and obtain a phase diagram showing whether the cusp appears in the magnetization curve or not.Comment: 8 pages, 7 figures. to be published in J. Phys. Soc. Jp