Yoshihiro Gohda

Impact of rattlers on thermal conductivity of a thermoelectric clathrate: a first-principles study.

We investigate the role of rattling guest atoms on the lattice thermal conductivity of a type-I clathrate Ba_{8}Ga_{16}Ge_{30} by first-principles lattice dynamics. Comparing phonon properties of filled and empty clathrates, we show that rattlers cause tenfold reductions in the relaxation time of phonons by increasing the phonon-phonon scattering probability. Contrary to the resonant scattering scenario, the reduction in the relaxation time occurs in a wide frequency range, which is crucial for explaining the unusually low thermal conductivities of clathrates. We also find that the impact of rattlers on the group velocity of phonons is secondary because the flattening of phonon dispersion occurs only in a limited phase space in the Brillouin zone.

Influence of water on elementary reaction steps in electrocatalysis

We studied simple reaction pathways of molecules interacting with Pt(111) in the presence of water and ions using density functional theory within the generalized gradient approximation. We particularly focus on the dissociation of H2 and O2 on Pt(111) which represent important reaction steps in the hydrogen evolution/ oxidation reaction and the oxygen reduction reaction, respectively. Because of the weak interaction of water with Pt(111), the electronic structure of the Pt electrode is hardly perturbed by the presence of water. Consequently, processes that occur directly at the electrode surface, such as specific adsorption or the dissociation of oxygen from the chemisorbed molecular oxygen state, are only weakly influenced by water. In contrast, processes that occur further away from the electrode, such as the dissociation of H2, can be modified by the water environment through direct molecule-water interaction.

Anharmonic force constants extracted from first-principles molecular dynamics: applications to heat transfer simulations

A systematic method to calculate anharmonic force constants of crystals is presented. The method employs the direct-method approach, where anharmonic force constants are extracted from the trajectory of first-principles molecular dynamics simulations at high temperature. The method is applied to Si where accurate cubic and quartic force constants are obtained. We observe that higher-order correction is crucial to obtain accurate force constants from the trajectory with large atomic displacements. The calculated harmonic and anharmonic force constants are, then, combined with the Boltzmann transport equation (BTE) and non-equilibrium molecular dynamics (NEMD) methods in calculating the thermal conductivity. The BTE approach successfully predicts the lattice thermal conductivity of bulk Si, whereas NEMD shows considerable underestimates. To evaluate the linear extrapolation method employed in NEMD to estimate bulk values, we analyze the size dependence in NEMD based on BTE calculations. We observe strong nonlinearity in the size dependence of NEMD in Si, which can be ascribed to acoustic phonons having long mean-free-paths and carrying considerable heat. Subsequently, we also apply the whole method to a thermoelectric material Mg2Si and demonstrate the reliability of the NEMD method for systems with low thermal conductivities.

Direct observation of ferromagnetism in grain boundary phase of Nd-Fe-B sintered magnet using soft x-ray magnetic circular dichroism

We have investigated the magnetism of the grain boundary (GB) phase in a Nd14.0Fe79.7Cu0.1B6.2 sintered magnet using soft x-ray magnetic circular dichroism (XMCD) at the Fe L2,3-edges. Soft XMCD spectra were measured from the fractured surface that was confirmed to be covered with a thin GB phase by Auger electron spectroscopy. The magnetic moment of Fe in the GB phase was estimated to be mGB=1.4 μB at 30 °C using the sum rule analysis for XMCD spectra, which is 60% of that of Fe in the Nd2Fe14B compound. The temperature dependence of mGB evaluated with reference to Fe in the Nd2Fe14B phase indicated that the Curie temperature of the GB phase is more than 50 °C lower compared to that of Nd2Fe14B.

Ab Initio Calculation of the Electric Properties of Al Atomic Chains under Finite Bias Voltages

We have analyzed electric conduction through Al atomic chains between two jellium electrodes by ab initio calculations, focusing on their dependences on bias voltages and the number of atoms. We have found that the electric current increases more slowly at high bias voltages (Vbias>3 V) as the number of atoms increases, whereas the current is roughly independent of the number of atoms at low bias voltages. This result can be understood if we assume that resonant conduction through states spreading over the entire chain is destroyed in the cases of Al4 and Al5 at high bias voltages.

Diversity of hydrogen configuration and its roles in SrTiO3−δ

As a source of carrier electron, various configurations of hydrogen in SrTiO3 are searched by using first-principles calculations. The most stable form of hydrogen is found to be H−, where doubly charged oxygen vacancy VO2+ changes into singly charged HO+. Most importantly, an additional H− is found to be weakly trapped by HO+, which completely neutralizes carrier electrons by forming (2H)O0. These unexpected behaviors of hydrogen, which can explain reported experimental results, expand the role of the hydrogen in carrier-control technology in transition-metal oxides.

Structural phase transition of graphene caused by GaN epitaxy

We report first-principles predictions, where the structure of graphene changes drastically with the epitaxial growth of GaN (which has been performed experimentally). We identify GaN-3×3/graphene-2 × 2 superstructure as the most probable interface atomic structure, where three C-C bonds are replaced with C-N-C bonds preserving the Dirac cones. As the GaN epitaxy proceeds expanding graphene gradually, the tensile strain for graphene is released suddenly by partial breaking of the C-bond network, attributable to the two-dimensionality of graphene. In contrast, graphene retains its honeycomb structure at the AlN-graphene interface. Both of GaN- and AlN-graphene interfaces exhibit spin polarization.

Theoretical analysis of field emission from metallic nanostructures on Si(100) surfaces

We have analysed effects of submonolayer aluminium adsorption on field emission from Si(100) surfaces using ab initio density functional calculations incorporating scattering states. We have clarified that the electron transfer from aluminium atoms to silicon atoms plays an important role in reducing the local barrier height in front of aluminium atoms, resulting in large emission current. We have also found that, when nanostructures having comparable minimum local barrier height are considered, the relative efficiency of field emission can be explained by the difference in the density of emission sites and the surface local density of states at the Fermi energy.

Strain effects on the magnetic anisotropy of Y2Fe14B examined by first-principles calculations

We investigate strain effects on the magnetic anisotropy energy (MAE) and the magnetic moment of Y2Fe14B on the basis of density functional theory. We find that the MAE is significantly enhanced upon compression of the lattice. By applying second-order perturbation theory, the coupling among orbitals that is the most significant in enhancing the perpendicular magnetic anisotropy by the compression is identified to be the 3dx2−y2↓−3dxy↓ coupling at the Fe j2 site, thereby we emphasize importance of both the effect of the local density of states and the orbital couplings.

Stabilization Mechanism of Vacancies in Group-III Nitrides: Exchange Splitting and Electron Transfer

We report first-principles calculations on mono-, di-, and tri-vacancies in group-III nitrides with clarifying two distinctive mechanisms in stabilization of the vacancy: Spin polarization due to exchange splitting of nitrogen-dangling bond states and electron transfer caused by breathing relaxation of cations. We also find that the significance of the two mechanisms strongly depends on the charge state of the vacancy and thus the Fermi-level position in the gap at which the charge state changes (the thermodynamic charge-state level) cannot be determined from single-electron levels at a certain charge state.