Energy Versus Magnetic-Field Diagram of the Spin-1 Haldane System with an Impurity

Energy versus magnetic-field diagram of the spin-$1$ Haldane system with an impurity bond is studied in terms of spin-1/2 degree of freedom at the sites neighboring the impurity bond by means of analytical method. We examine the equivalence between the realistic Hamiltonian and the phenomenological Hamiltonian which is composed two spin-1/2 spins representing the spin-1/2 degree of freedom. It is proved that when the strength of the impurity bond is sufficiently weak, the two Hamiltonians are equivalent to each other, as far as the energies of the low-lying states are concerned. We determine the correspondence between the interaction constants in the phenomenological Hamiltonian and those in the realistic Hamiltonian.Comment: 10 pages, plain TeX (Postscript figures are included), KU-CCS-93-00

Phase Diagram of Adsorbate-Induced Row-Type-Alignments

The phase diagram of adsorbate-induced row-type-alignments, such as missing-row reconstructions induced by adsorbate-atoms on the FCC(110) surface, is calculated by the Blume-Emmery-Griffiths (BEG) model. In the model, we introduce adatom-adatom and dipole-dipole interactions between nearest-neighbor (NN) and next-nearest-neighbor (NNN) rows. The calculation of the temperature versus adatom chemical potential phase diagram is performed using mean-field approximation. It is indicated that when NN and NNN interactions are competitive, there appear either dipole or coverage modulated (incommensurate) phases at high temperatures for wide regime of the interactions.Comment: 5 pages, 6 figures, ICSOS'99. to appear in Surf. Rev. and Let

Ground-State Phase Diagram of Frustrated Anisotropic Quantum Spin Chains

Recent studies on the frustrated quantum spin chains with easy-plane anisotropy are reviewed. We are particularly interested in novel "chiral" phases characterized by the spontaneous breaking of the parity symmetry. The ground-state phase diagrams of the chains are discussed.Comment: 6 pages (ptptex.sty), 3 figures, to appear in Prog. Theor. Phys. Suppl. (Proc. of the 16th Nishinomiya-Yukawa Symposium and YITP International Workshop, Nov. 2001

Observation of zigzag and armchair edges of graphite using scanning tunneling microscopy and spectroscopy

The presence of structure-dependent edge states of graphite is revealed by both ambient- and ultra-highvacuum- (UHV) scanning tunneling microscopy (STM) / scanning tunneling spectroscopy (STS) observations. On a hydrogenated zigzag (armchair) edge, bright spots are (are not) observed together with (SQRT(3) by SQRT(3))R30 superlattice near the Fermi level (V_S = −30 mV for a peak of the local density of states (LDOS)) under UHV, demonstrating that a zigzag edge is responsible for the edge states, although there is no appreciable difference between as-prepared zigzag and armchair edges in air. Even in hydrogenated armchair edge, however, bright spots are observed at defect points, at which partial zigzag edges are created in the armchair edge.Comment: 4 pages, 4 figures, contents for experimental/theoretical reseachers, accepted as an issue of Physical Review B (PRB

Haldane to Dimer Phase Transition in the Spin-1 Haldane System with Bond-Alternating Nearest-Neighbor and Uniform Next-Nearest-Neighbor Exchange Interactions

The Haldane to dimer phase transition is studied in the spin-1 Haldane system with bond-alternating nearest-neighbor and uniform next-nearest-neighbor exchange interactions, where both interactions are antiferromagnetic and thus compete with each other. By using a method of exact diagonalization, the ground-state phase diagram on the ratio of the next-nearest-neighbor interaction constant to the nearest-neighbor one versus the bond-alternation parameter of the nearest-neighbor interactions is determined. It is found that the competition between the interactions stabilizes the dimer phase against the Haldane phase