S. Bağcı

Structural, elastic, electronic, and phonon properties of zinc-blende and wurtzite BeO

The results of ab initio calculations of the structural, elastic, electronic, and phonon properties of BeO in both zinc-blende and wurtzite structures are presented. Our calculations are based on the application of plane-wave basis, pseudopotentials, and the generalized gradient approximation of the density functional scheme. Our total energy calculations indicate that the wurtzite phase has lower energy (around 5.8 meV) than zinc-blende phase. It is found that for the two crystal phases the values of the equilibrium atomic volume and of the bulk modulus and its pressure derivative are almost identical. The zinc-blende and wurtzite phases are predicted to be characterized with indirect (and smaller) and direct band gaps, respectively. The maximum longitudinal optical and transverse optical phonon frequencies in the zinc-blende phase compare well to the average of the corresponding E1 and A1 modes in the wurtzite phase.

Electronic, elastic and phonon properties of the rock-salt LaSb and YSb

The electronic structures and elastic constants of LaSb and YSb are calculated using an ab initio pseudopotential scheme within the generalized gradient approximation. In agreement with experimental studies, it has been found that the upper valence bands in LaSb are characterized by Sb-5p and La-5d states. Our calculated elastic constants for LaSb are compared with available experimental data. A linear-response approach to density functional theory is used to derive phonon dispersion curves and the density of states for LaSb and YSb. Our phonon results for LaSb are in good agreement with experimental data. Frequency differences between phonon modes in these materials are discussed in terms of differences involving total mass, reduced mass and the lattice constant.

Ab initio calculations of the electronic and phonon states on the HfC(001)-(1×1) surface

We have investigated the structural and electronic properties of the HfC(001)-(1×1) surface using the density functional theory and the pseudopotential method. The most important feature of the HfC(001) surface relaxation is that both in the surface and subsurface layers C atoms move out of the surface and Hf atoms move inward. In general, our structural results are in good agreement with previous experimental and theoretical works. We have identified surface electronic states on the HfC(001) surface in the bulk gap pocket around the Fermi energy. Using our calculated atomic and electronic structures, zone-centre and X point surface phonon modes are calculated by employing a linear response approach based on the density functional perturbation theory. Good agreement has been observed between our calculated surface phonons and available experimental results. A hitherto unidentified surface localized mode has been predicted and its polarization characterists has been examined.

Ab initio calculations of the structure, electronic states and phonon dispersion of the BSb(110) surface

We have studied the structural and electronic properties of BSb(110) by employing plane-wave pseudopotential method within density functional theory. The relaxed geometry shows a tilt of 26.5° of the top-layer B-Sb atomic chain, a result typical of non-nitride III-V(110) surfaces. Four surface electronic states have been identified. Several surface phonon branches have been determined from the application of a density-functional linear response theory. The eigensolutions of some of the surface modes are explained in terms of the large cation-anion mass difference and the small B core without any p electrons.

Structural, electronic and lattice dynamical properties of the BeS(110) surface

We present results for the relaxed atomic geometry, electronic states, and lattice dynamics of the BeS(110) surface. Our investigations are based on first principles pseudopotential calculations within the local density approximation of the density functional theory. The relaxed geometry of this surface follows the general trend observed for traditional III-V(110) and II-VI(110) surfaces, with a surface tilt of 26.3°. Surface localised phonon modes below and above the bulk continuum, and within the acoustic-optical gap region have been dtermined.

Theoretical studies of electronic structure, phonon spectrum and electron-phonon interaction in AlCNi3

We report results of first-principles calculations for structural properties, electronic structure, phonon spectrum and electron-phonon interaction for the antiperovskite compound AlCNi3. The structural properties are calculated using a plane-wave-pseudopotential method and the density functional theory within the generalised gradient approximation. The electronic structure and density of states for AlCNi3 are presented and compared with previous theoretical calculations. Our structural and electronic results are used, within the implementation of a linear response technique, for calculations of phonon states. We have observed that all phonon modes are stable along the [100] direction while unstable phonon modes are found in the [110] and [111] symmetry directions. At the Brillouin zone edge point X, the electron-phonon coupling parameters for phonon modes in AlCNi3 are calculated to be smaller than their corresponding values for MgCNi3. This result indicates that the electron-phonon interaction is not very strong in AlCNi3.