Unidirectional Nano-modulated Binding and Electron Scattering in Epitaxial Borophene.

A complex interplay between the crystal structure and the electron behavior within borophene renders this material an intriguing 2D system, with many of its electronic properties still undiscovered. Experimental insight into those properties is additionally hampered by the limited capabilities of the established synthesis methods, which, in turn, inhibits the realization of potential borophene applications. In this multimethod study, photoemission spectroscopies and scanning probe techniques complemented by theoretical calculations have been used to investigate the electronic characteristics of a high-coverage, single-layer borophene on the Ir(111) substrate. Our results show that the binding of borophene to Ir(111) exhibits pronounced one-dimensional modulation and transforms borophene into a nanograting. The scattering of photoelectrons from this structural grating gives rise to the replication of the electronic bands. In addition, the binding modulation is reflected in the chemical reactivity of borophene and gives rise to its inhomogeneous aging effect. Such aging is easily reset by dissolving boron atoms in iridium at high temperature, followed by their reassembly into a fresh atomically thin borophene mesh. Besides proving electron-grating capabilities of the boron monolayer, our data provide comprehensive insight into the electronic properties of epitaxial borophene which is vital for further examination of other boron systems of reduced dimensionality.

First-principles determination of intergranular atomic arrangements and magnetic properties in rare-earth permanent magnets.

Development of high-performance permanent magnets relies on both the main-phase compound with superior intrinsic magnetic properties and the microstructure effect for the prevention of magnetization reversal. In this article, the microstructure effect is discussed by focusing on the interface between the main phase and an intergranular phase and on the intergranular phase itself. First, surfaces of main-phase grains are considered, where a general trend in the surface termination and its origin are discussed. Next, microstructure interfaces in SmFe12-based magnets are discussed, where magnetic decoupling between SmFe12 grains is found for the SmCu subphase. Finally, general insights into finite-temperature magnetism are discussed with emphasis on the feedback effect from magnetism-dependent phonons on magnetism, which is followed by explanations on atomic arrangements and magnetism of intergranular phases in Nd-Fe-B magnets. Both amorphous and candidate crystalline structures of Nd-Fe alloys are considered. The addition of Cu and Ga to Nd-Fe alloys is demonstrated to be effective in decreasing the Curie temperature of the intergranular phase.

First-principles study of the adsorption of 3d transition metals on BaO- and TiO2-terminated cubic-phase BaTiO3(001) surfaces.

The deposition of transition metals (TM) on barium titanate (BaTiO3, BTO) surfaces is involved in the development of several BTO-based devices, such as diodes, catalysts, and multiferroics. Here, we employ density functional theory to investigate the adsorption of 3d TM on both BaO- (type-I) and TiO2-terminated (type-II) surfaces of cubic BaTiO3(001) at low levels of surface coverage, which is important to comprehend the initial stages of the formation and growth of TM overlayers on BTO. The most stable adsorption site is identified for each adatom on both surfaces. Our discussion is based on analyses of structural distortions, Bader charge, electron density difference, magnetic moments, work function, density of states, and adsorption energies. For the type-I surface, most of the adatoms bind covalently on top of the surface oxygens, except for Sc, Ti, and V atoms, which adsorb preferentially on the bridge site, between O ions, to form two polar TM-O bonds. On the type-II surface, the TM are located at the fourfold hollow site, which allows the formation of four TM-O interactions that are predominantly ionic. Upon the adsorption, we noticed the formation of in-gap states originated mostly from the adatom. When electrons are transferred to the substrates, their conduction bands become partially occupied and metallic. We observed a decrease in the work function of the type-II surface that is fairly proportional to the charge gained, which suggests that the BTO work function can be manipulated by the controlled deposition of TM.

Hidden order in amorphous structures: Extraction of nearest neighbor networks of amorphous Nd-Fe alloys with Gabriel graph analyses.

Using the scheme of Delaunay and Gabriel graphs, we analyzed the amorphous structures of computationally created Nd-Fe alloys for several composition ratios based on melt quench simulations with finite temperature first-principles molecular dynamics. By the comparison of the radial distribution functions of the whole system and those derived from the Delaunay and Gabriel graphs, it was shown that the Gabriel graphs represent the first nearest neighbor networks well in the examined amorphous systems. From the Gabriel graph analyses, we examined the coordination structures of amorphous Nd-Fe alloys statistically. We found that the ranges of distributions of coordination numbers are wider at the lower Nd composition ratios. The angular distributions among three adjacent atoms were also analyzed, and it was found that the steeper the angular distributions become the higher the Nd composition ratios are. These features mean that the orders in the amorphous system become stronger as the Nd ratio increases, which corresponds to the appearance of crystalline grain boundary phases at high Nd composition ratios [T. T. Sasaki et al., Acta Mater. 115, 269-277 (2016)].

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.