Ab-initio density functional studies of stepped TaC surfaces

We report on density functional total energy calculations of the step formation and interaction energies for vicinal TaC(001) surfaces. Our calculations show that double and triple-height steps are favored over single-height steps for a given vicinal orientation, which is in agreement with recent experimental observations. We provide a description of steps in terms of atomic displacements and charge localization and predict an experimentally observable rumpled structure of the step-edges, where the Ta atoms undergo larger displacements compared to the C atoms.Comment: 4 pages, 4 figure

‘Building a Humane Society’: An Intellectual Sketch of H. Odera Oruka

This paper discusses Odera Oruka’s philosophical work from the perspective of its emphasis on the ‘practical’ impetus that Oruka himself underlined. In different ways, his various projects - his sage philosophy, his philosophy of liberty, his environmental philosophy and, perhaps most importantly, his critiques of African (and implicitly Kenyan) social and political realities - can be seen as manifestations of his commitment to the practical relevance and social significance of knowledge, and his conviction about the potentially liberating force of philosophical critique. Here, I try to provide an overall sketch of this agenda, seeking to initiate more thorough and detailed discussion for the future. As a main reference point for discussion, I look at how the term ‘humanism’ has been used (and can be used) to describe Oruka’s work, in contrast to the invocation of this term by some nationalist political ideologies, in particular Moi’s so-called ‘nyayo philosophy’. Oruka’s work could be more explicitly appreciated and explored, I argue, for the ways in which he observed and actively criticized instances of inhumanity and ‘false humanism’ in post-colonial Africa.   Key Words Odera Oruka, humanism, liberty, freedom, ideolog

Intrinsic hole localization mechanism in magnetic semiconductors

The interplay between clustering and exchange coupling in magnetic semiconductors for the prototype (Ga_{1-x},Mn_x)As with manganese concentrations x of 1/16 and 1/32 in the interesting experimental range is investigated. For x ~ 6 %, when all possible arrangements of two atoms within a large supercell are considered, the clustering of Mn atoms at nearest-neighbour Ga sites is energetically preferred. As shown by spin density analysis, this minimum energy configuration localizes further one hole and reduces the effective charge carrier concentration. Also the exchange coupling constant increases to a value corresponding to lower Mn concentrations with decreasing inter Mn distance.Comment: Accepted for publication in Journal of Physics: Condensed Matte

Binding energies and electronic structures of adsorbed titanium chains on carbon nanotubes

We have studied the binding energies and electronic structures of metal (Ti, Al, Au) chains adsorbed on single-wall carbon nanotubes (SWNT) using first principles methods. Our calculations have shown that titanium is much more favored energetically over gold and aluminum to form a continuous chain on a variety of SWNTs. The interaction between titanium and carbon nanotube significantly modifies the electronic structures around Fermi energy for both zigzag and armchair tubes. The delocalized 3d electrons from the titanium chain generate additional states in the band gap regions of the semiconducting tubes, transforming them into metals.Comment: 4 pages, 3 figure

CO adsorption on metal surfaces: a hybrid functional study with plane wave basis set

We present a detailed study of the adsorption of CO on Cu, Rh, and Pt (111) surfaces in top and hollow sites. The study has been performed using the local density approximation, the gradient corrected functional PBE, and the hybrid Hartree-Fock density functionals PBE0 and HSE03 within the framework of generalized Kohn-Sham density functional theory using a plane-wave basis set. As expected, the LDA and GGA functionals show a tendency to favor the hollow sites, at variance with experimental findings that give the top site as the most stable adsorption site. The PBE0 and HSE03 functionals reduce this tendency. In fact, they predict the correct adsorption site for Cu and Rh but fail for Pt. But even in this case, the hybrid functional destabilizes the hollow site by 50 meV compared to the PBE functional. The results of the total energy calculations are presented along with an analysis of the projected density of states.Comment: 32 pages, 6 tables, 3 figures. (Re)Submitted to Phys. Rev. B; LDA results added in the tables; minor changes in the tex

Electronic structure of the (111) and (-1-1-1) surfaces of cubic BN: A local-density-functional ab initio study

We present ab initio local-density-functional electronic structure calculations for the (111) and (-1-1-1) surfaces of cubic BN. The energetically stable reconstructions, namely the N adatom, N3 triangle models on the (111), the (2x1), boron and nitrogen triangle patterns on the (-1-1-1) surface are investigated. Band structure and properties of the surface states are discussed in detail.Comment: 8 pages, 12 figure

Three-dimensional MgB2_{2}-type superconductivity in hole-doped diamond

We substantiate by calculations that the recently discovered superconductivity below 4 K in 3% boron-doped diamond is caused by electron-phonon coupling of the same type as in MgB$_2$, albeit in 3 dimensions. Holes at the top of the zone-centered, degenerate $\sigma$-bonding valence band couple strongly to the optical bond-stretching modes. The increase from 2 to 3 dimensions reduces the mode-softening crucial for $T_{c}$ reaching 40 K in MgB$_{2}.$ Even if diamond had the same \emph{bare} coupling constant as MgB$_{2},$ which could be achieved with 10% doping, $T_{c}$ would only be 25 K. Superconductivity above 1 K in Si (Ge) requires hole-doping beyond 5% (10%).Comment: revised version, accepted by PR

The structural analysis of Cu(111)-Te (√3 × √3) R30° and (2√3 × 2√3)R30° surface phases by quantitative LEED and DFT,

The chemisorption of tellurium on atomically clean Cu(111) surface has been studied under ultra-high vacuum conditions. At room temperature, the initial stage of growth was an ordered 23×23R30° phase (0.08 ML). An ordered 3×3R30° phase is formed at 0.33 ML coverage of Te. The adsorption sites of the Te atoms on the Cu(111) surface at 0.08 ML and 0.33 ML coverages are explored by quantitative low energy electron diffraction (LEED) and density functional theory (DFT). Our results indicate that substitutional surface alloy formation starts at very low coverages

Achirality in the low temperature structure and lattice modes of tris(acetylacetonate)iron(iii)

Tris(acetylacteonate) iron(III) is a relatively ubiquitous mononuclear inorganic coordination complex. The bidentate nature of the three acetylacteonate ligands coordinating around a single centre inevitably leads to structural isomeric forms, however whether or not this relates to chirality in the solid state has been questioned in the literature. Variable temperature neutron diffraction data down to T = 3 K, highlights the dynamic nature of the ligand environment, including the motions of the hydrogen atoms. The Fourier transform of the molecular dynamics simulation based on the experimentally determined structure was shown to closely reproduce the low temperature vibrational density of states obtained using inelastic neutron scattering