[en] The rearrangement of the acoustic phonon spectrum under the increase of isotope impurity concentration is studied in a one-dimensional system. The analysis is based on the method of the expansion of the single-particle Green's function in terms clusters of the indirectly interacting impurity centers. It is demonstrated that the criterion for the validity of the coherent potential approximation(CPA) coincides exactly with the conventional Ioffe-Regel-Mott criterion. Approximate solutions for the dispersion relation and density of states are obtained for concentrations larger and smaller than the critical concentration of the spectrum rearrangement

[en] The surface electronic structures of superconducting MgB₂ were investigated using the all-electron full-potential linearized augmented plane-wave method. Hexagonal (0001) surfaces with both B terminated (B-Term) and Mg terminated (Mg-Term) were considered. Due to the nearly-free-electron nature of the Mg surface layer, the vacuum screening range of Mg-Term is shorter than that of B-Term, which shows the covalent bonding nature of the B surface layer. Considerably enhanced densities of states near the Fermi level are found at the surface layers especially for B-Term, which is expected to yield an enhanced superconductivity in the surface of thin film MgB₂ over that in bulk -- assuming no large changes in the electron-ion matrix elements and phonon frequency contributions. While this expectation is contrary to the weakened superconductivity observed in surface-oriented experiments, we attribute this discrepancy to extrinsic surface effects

[en] Physical properties of colossal magnetoresistance phenomena observed in doped manganese oxides are investigated. Half-metallic electronic structures of metallic phase of doped manganese oxides are examined by using the virtual crystal approximation. Taking into account the strong electron-phonon interaction together with the double exchange, we have investigated transport and lattice properties in the low doping metallic phase and in the high doping charge ordered phase of manganese oxides

[en] The charge ordering phase transition is studied based on the mean field model Hamiltonian with the intersite Coulomb interaction. The charge ordering transition temperature and the charge gap are determined as a function of the band width for 1D and 3D model systems. To address the problem of charge ordering from the point of view of mean field approximation, it is possible to double the size of lattice of the system under consideration by introducing formally two identical sublattices. The procedure automatically implies that the charge ordered (CO) state can do only of a checkerboard type and certain restrictions are put on the lattice symmetry. Also, for simplicity, the electron hopping will be permitted one between sites belonging to different sublattices

[en] We investigated the effects of the bandwidth change on the magnetocapacitance (MC) of a metal-oxide-metal junction. The bandwidth change is induced by an electron-electron interaction on the metallic side of the capacitance system. We employed a generalized Hamiltonian incorporating the on-site and the inter-site Coulomb interactions, the exchange interaction, and pair-hopping and obtained a quadratic dependence of the MC on the magnetic field. We also found that the effects of the bandwidth change could be substantial near the phase boundary between the paramagnetic and the ferromagnetic phases.

[en] Employing the self-consistent local approach, the tight-binding linear-muffin-tin orbital recursion method, we have investigated the electronic structures and the magnetic properties of 3d and 4d transition-metal (TM) impurities in ferromagnetic bcc Fe. In both 3d and 4d TM impurities, virtual bound states appear and are characterized by a high density of states in the energy spectrum. The characters of the states are studied by calculating the bond order between interaction orbitals. For early TM impurities, the states at the impurity sites have more antibonding characters, while the states at neighboring Fe sites have more bonding characters. For late TM impurities, the situation is reversed. late TM impurities of both the 3d and the 4d TM series have the same magnetic ordering as the host Fe atoms whereas early TM impurities have magnetic moments antiparallel to that of the host. As for the Mn impurity, an inward relaxation of neighboring Fe atoms stabilizes the antiferromagnetic ordering with respect to the host. In the case of the 4d TM impurities, we must consider a hybridization of longer range interaction to determine the magnetic moments and the charge redistributions, in contrast to the case of 3d TM impurities

[en] We have revisited metallic ferromagnetism to examine the effects of band broadening due to electron-electron interaction on the magnetic phase diagram. Based on the generalized single band tight-binding Hamiltonian, we have explored the condition when the magnetization jumps discontinuously, by comparing results obtained by the local minima search and the total energy minimization. We have also investigated how the shape of the density of states (DOS) affects the magnetic phase diagram. We have found that a system with the DOS shape of a concave-type could have the discontinuous magnetization jump as the effective Stoner parameter increases

[en] Employing the microscopic superexchange model incorporating the effect of spin-orbit interaction, we have investigated the Dzyaloshinsky-Moriya (DM) interaction in perovskite transition-metal (TM) oxides and explored the interplay between the DM interaction and the TM-3d orbital symmetry. For d³ and d⁵ systems with isotropic orbital symmetry, the DM vectors are well described by a simple symmetry analysis considering only the bond geometry. In contrast, the DM interaction for d⁴ systems with anisotropic orbital symmetry shows slightly different behavior, which does not obey simple symmetry analysis. The direction as well as the strength of the DM vector varies depending on the occupied orbital shape. We have understood this behavior based on the orbital symmetry induced by local crystal field variation.

[en] We have investigated the electronic structures and magnetic properties of Li-intercalated Co-doped rutile TiO₂. For non-intercalated Ti_0.9375Co_0.0625O₂, the half-metallic and low-spin (∼ 0.94 μ_B/Co) ground state is obtained. By Li intercalation, Ti_0.9375Co_0.0625O₂ becomes a paramagnetic insulator at the concentration of Li/Ti = 0.067. At the higher concentration of Li/Ti = 0.133, it becomes a paramagnetic metal. Hence, as in the transition metal doped anatase TiO₂ case, we expect that the magnetic and transport properties of Co-doped rutile TiO₂ can be controlled by an electric field

[en] To investigate the effects of A-site cations on the magnetic interaction in highly distorted perovskite ACuO₃ (A=Se, Te), we have set the microscopic Hamiltonian for the Cu-O-Cu-A cluster, taking into account the s-tilde p or p-tilde p hybridization between A- s-tilde or A- p-tilde and O-2p orbitals. We have found that (i) the superexchange interaction of the simple Cu-O-Cu triad without considering the A-site effect is inadequate to explain the ferromagnetism of SeCuO₃, (ii) the p-tilde p hybridization weakens the superexchange interactions and (iii) the s-tilde p hybridization induces the ferromagnetic superexchange interaction along the c-axis. We have also verified that the s-tilde p hybridization affects the magnetic interaction through anisotropic charge transfer energy shift in the Cu-O-Cu interaction path.