Fumiyuki Ishii

First-principles predictions of giant electric polarization

Giant electric polarization of more than 150 µC/cm2 is predicted for PbVO3 and BiCoO3 on the basis of the first-principles Berry-phase method. The stable crystal structure is tetragonally distorted with a large c/a ratio and significant ionic displacements breaking centrosymmetry. In PbVO3, the key factor that stabilizes such a highly distorted structure and realizes an insulating electronic structure, leading to giant polarization, is the coexistence of ferro-orbital and antiferro-spin orderings in the V4+ d1 configuration. It is shown that the same electronic mechanism works also for BiCoO3 with Co3+ d6 configuration.

Phase Control of Graphene Nanoribbon by Carrier Doping: Appearance of Noncollinear Magnetism

The zigzag graphene nanoribbon (ZGNR) has an antiferromagnetic property, that is, the relative spin angle theta between the two edges is 180 degrees . By using noncollinear first-principles calculations, we find that the magnetic phase of the ZGNR can be controlled by injecting either electrons or holes: as the carrier density increases, theta continuously decreases from 180 to 0 degrees , which indicates that the net magnetization is possible. Either FET doping or chemical doping is found to be possible.

Electronic States of Perovskite-Type Oxides and Ferroelectricity.

A relation between B-site transition metal and ferroelectricity is investigated in ABO3 perovskite-type oxides. We focus on intensity of the covalent bonds between the B-site atom and the oxygen octahedron and evaluate Harrisons covalency parameter α. The density functional theory within the local density approximation is adopted to obtain the valence electron energy levels of free atoms from which the covalency α is calculated. Strong correlation between the covalency α and the Curie temperature (TC) is found. As the covalency α increases, TC of ferroelectricity arises. It is also noted that the hybridization between Pb 6p and O 2p is crucial for the high ferroelectricity of PbTiO3.

First-principles study on the electronic structure of bismuth transition-metal oxides

The electronic structure, magnetic and electric properties, and lattice stability of multiferroic BiMnO 3 as a typical system in perovskite Bi transition-metal oxides (BiMO 3 ) are studied from first principles. It is demonstrated theoretically for the first time that the orbital ordering within the Mn eg orbitals is actually realized in BiMnO 3 , being consistent with crystallographic data, and plays a crucial role in the appearance of ferromagnetism. Total-energy calculation shows the ferromagnetic state is indeed stabilized. Electrical polarization of BiMnO 3 is also estimated based on the Berry phase theory. Lattice instability to off-centred displacement, which is driven by strong covalent bonding between Bi 6p and O 2p states, is found to be rather common in the BiMO 3 series.

Electronic and Thermoelectric Properties of the Intermetallic Compounds MNiSn (M=Ti, Zr, and Hf)

We have performed first-principles calculations for the half-Heusler type ternary compounds M NiSn ( M =Ti, Zr, and Hf). To discuss the relationship between the electronic and thermoelectric properties, we evaluate Seebeck coefficients from the calculated band structure within the Boltzmann transport theory. The temperature and carrier concentration dependences of the Seebeck coefficients are discussed in terms of the density of states and Fermi velocity near the band edges. The calculated Seebeck coefficients for all these materials are found to be about -300 µV/K at a low n -type carrier concentration at room temperature, being in good agreement with experiments.

Electronic band structure of the pyrochlore ruthenium oxides A2Ru2O7 (A=Bi, Tl and Y)

A local-density electronic-band-structure calculation was performed for pyrochlore ruthenium oxides A 2 Ru 2 O 7 ( A = Bi, Tl and Y). As a general feature, these pyrochlores have band structure com...

Relationship between Lattice Deformation and Polarization in BaTiO3

The relationship between lattice deformation and electrical polarization in tetragonal BaTiO3 is investigated. The density functional theory within the local density approximation using the full-potential-linear-augmented-plane-wave (FLAPW) method is adopted to obtain internal atom positions and one-electron wave functions. Electric polarization is calculated using the Berry-phase theory. We have found that a lattice strain of the order of 1% along the c-axis enhances polarization considerably. However, for that of the order of 0.1%, polarization hardly changes. We assume that these responses of the polarization to lattice strain are related to the stress sensitivity of the polarization in ferroelectric-thin films through nanoscale domains, especially ferroelectric-90-degree domains. We have also found that the polarization of BaTiO3 can be scaled linearly by the distance between Ti and its nearest-neighbor oxygen (apical site in oxygen octahedron). This indicates that the covalency between Ti and the apical oxygen is the only driving force for the ferroelectricity in BaTiO3. We suggest that this covalency softens Youngs modulus of BaTiO3 in the ferroelectric states compared to the paraelectric states through the increase of the degree of freedom for atomic displacements in a unit cell.

Two-staged magnetoresistance driven by the ising-like spin sublattice in SrCo6O11

A two-staged, uniaxial magnetoresistive effect has been discovered in SrCo6O11 having a layered hexagonal structure. Conduction electrons and localized Ising spins are in different sublattices but their interpenetration makes the conduction electrons sensitively pick up the stepwise field dependence of magnetization. The stepwise field dependence suggests two competitive interlayer interactions between ferromagnetic Ising-spin layers, i.e., a ferromagnetic nearest-layer interaction and an antiferromagnetic next-nearest-layer interaction. This oxide offers a unique opportunity to study nontrivial interplay between conduction electrons and Ising spins, the coupling of which can be finely controlled by a magnetic field of a few Tesla.

First-Principles Calculation of Spontaneous Polarization and Phase Stability in NaNO2

Ferroelectricity in sodium nitrite NaNO 2 has been studied from first principles. The spontaneous polarization calculated with Berry-phase theory is 0.116 C/m 2 , being in excellent agreement with experimental values, 0.117 C/m 2 and 0.119 C/m 2 . In order to discuss the phase transitions, we have estimated coupling parameters J 1 and J 2 in the axial next-nearest-neighbor Ising model from total-energy and force calculations. As results, J 1 and J 2 are quite sensitive to relative displacement of NO 2 molecules. The transition temperature and a role of molecular displacement in the phase transitions are discussed in detail.

Rashba Effect on the Structure of the Bi One-Bilayer Film: Fully Relativistic First-Principles Calculation

Using first-principles calculations, we study the spin–orbit interactions and spin textures of a Bi one-bilayer film, which attracts scientific interest because of the topological insulator and so on. The substrate effect is successfully mimicked by applying on electric field in the perpendicular direction of the film, which breaks the inversion symmetry. We study the highest occupied band around the Γ point. Although the vortex of the in-plane spin component is well explained on the basis of the conventional Rashba effect, we find a substantial out-of-plane component which cannot be explained by the conventional Rashba model. This spin texture is similar to that of a multi-bilayer Bi film, which has recently been observed using a spin-resolved angle-resolved photoemission spectroscopy experiment. We also find a spin vortex around the K point although this point has no time-reversal symmetry. We expect that a similar vortex appears in materials having the p3m1 symmetry, whose spin–orbit interactions have recently attracted scientific interest.