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.

We investigate the role of rattling guest atoms on the lattice thermal-conductivity of a type-I clathrate Ba8Ga16Ge30 by first-principles lattice dynamics. Comparing phonon properties of filled and empty clathrates, we show that rattlers cause 10-fold 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 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. Here, T is the absolute temperature, σ is the electrical conductivity, S is the Seebeck coefficient, and κ c(L) is the thermal conductivity by electrons (phonons), respectively. Since κ L of a clathrate is intrinsically low without introducing micro/nanostructures such as grain boundaries or nanoscale precipitates, semiconductor clathrates are one of prototype materials which follow the phonon glass/electron crystal (PGEC) concept proposed by Slack [3].The origin of low κ L of host-guest structures such as clathrates and skutterudites has commonly been attributed to the "rattlers", i.e. guest atoms loosely bound inside oversized cages [4][5][6][7]. However, its actual role is not fully understood. In the simple kinetic theory, the lattice thermal conductivity is given bywhere C is the lattice specific heat, v is the average group velocity, and τ is the average relaxation time of phonons, respectively. Historically, the reduction in κ L has been attributed to resonant scatterings by localized rattling modes [4], which reduce τ of heat-carrying acoustic modes in a limited energy region near avoided-crossing points. This mechanism was originally introduced to explain κ L observed in a solid solution of KCl and KNO 2 [8], and it was also applied to a clathrate hydrate assuming the guest as an isolated point defect [5]. However, although the resonant phonon scattering has repeatedly been employed to explain low κ L 's of host-guest structures [4-6], little attention has been paid to the validity of that mechanism itself. Recently, the validity of resonant scattering was questioned by experimental and theoretical studies in skutterudites [9,10]. Another important role of rattlers which has recently been recognized is its impact on the group velocity [11,12]. On the basis of inelastic neutron scattering (INS) of a type-I clathrate Ba 8 Ga 16 Ge 30 (BGG), Christensen et al. [11] claimed that a major effect of rattlers is to reduce v of acoustic phonons at the avoided-crossing points rather than to reduce τ . This mechanism clearly conflicts with the resonant scattering scenario, where τ is the main source of low κ L . It is still an open question that which of the group velocity v or the relaxation time τ is mainly affected by rattlers, which should be understood precisely for further redu...

We improve the linear tetrahedron method to overcome systematic errors due to overestimations (underestimations) in integrals for convex (concave) functions, respectively. Our method is applicable to various types of calculations such as the total energy, the charge (spin) density, response functions, and the phonon frequency, in contrast with the Blöchl correction, which is applicable to only the first two. We demonstrate the ability of our method by calculating phonons in MgB2 and fcc lithium.

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.

We report total-energy electronic-structure calculations based on density-functional theory that clarify magnetism of Gd-doped GaN. We find that Ga vacancies with the magnetic moment of 3 B formed upon Gd doping interact ferromagnetically with each other and thus cause gigantic magnetic moments per Gd atom. Our detailed analyses are indicative of intrinsic ferromagnetism due to cation vacancies rather than magnetic dopants in nitride semiconductors.

We have developed a fully self-consistent method which is suitable to examine field emission currents, on the basis of the density functional theory. In our method, the nearby counterelectrode is not necessary. By using this method, we have investigated field emission currents from a biased metallic surface represented by the jellium model. We have found that the energy barrier between the jellium and vacuum becomes lower than the Fermi energy under strong electric fields (e.g., 10 V/nm for r(s) = 4 bohr). In this situation, the slope of the Fowler-Nordheim plot becomes flatter than that under a weaker field.

No abstract

No abstract