[en] In this work, the superconducting transition temperature of hole-doped BC3 was studies. The total energy, phonon frequencies, and electron-phonon couplings were calculated for different hole doping levels using the ab initio pseudopotential method within the local density approximation. The harmonic and anharmonic phonon frequencies were calculated by using the frozen-phonon approximation. As in MgB2, the electron-phonon coupling between the electronic states in the sigma bands and phonon modes associated with bond stretching was found to be very strong. The calculation predicts that the superconducting temperature will increase as a function of doping level

[en] Ab initio pseudopotential total-energy calculations on infinite monatomic chains of Au, Al, Ag, Pd, Rh, and Ru were performed within the local-density approximation. We used the frozen phonon approximation to study the stability of these chains as a function of strain. Within a window of strains the au, Al, Ag, Pd, and Rh linear chains are stable with respect to q=pi/a deformations. For large strains all the chains dimerize. All the chains exhibit at least one zero-strain zigzag stable equilibrium configuration, and Au, Al, and Rh zigzag chains exhibit two. The ideal strengths of the different chains were calculated. The stability of the chains is discussed in connection with the electronic structure

[en] We show that for specially designed linear dispersive media with one absorption line and one gain line the Sommerfeld precursors of a pulse can be amplified leading to an earlier detection of the signal. Also, we show that in some systems with one strong absorption line, a carefully placed gain resonance must induce a discontinuity in the imaginary part of the frequency dependent index of refraction and in the first derivative of its real part

[en] Ab initio pseudopotential total energy density-functional theory--local-density approximation calculations were performed to study the crystalline structures of Ge under pressure. Following the well established sequence of structural phases (diamond→β-Sn→Imma→sh) under increasing pressure, we predict a transition into a new phase, with Cmca space-group symmetry, at 90±2 GPa. We estimate the superconducting transition temperature T_c for this phase to be in the range 2 to 7 K, the same range obtained previously by detailed calculations for the Ge-sh phase. The Cmca phase should remain stable up to 137±10 GPa where a transition to the hcp structure is predicted to occur. The same path is followed by Si although at lower pressures

[en] Ab initio pseudopotential total energy calculations of MoSe nanowires were performed within the local density approximation. The Li2Mo6Se6 crystal is composed of molecular chains, which can be separated from one another to form individual nanowires approximately 3 Angstroms in diameter. In this study we consider three systems: the quasi-one-dimensional bulk crystal Li2Mo6Se6, one isolated MoSe nanowire, and one isolated MoSe nanowire with Li adsorbates. The equilibrium structures and the electronic structures of the three systems were calculated and compared to each other. The calculated density of states of an isolated MoSe wire is compared with experimental tunneling spectroscopy measurements of the local density of states. The binding energy of a Li atom to an isolated wire was calculated and the effects of Li adsorption are discussed. In addition, the calculated value for the ideal tensile strength of a single MoSe nanowire is presented and compared with estimated values for carbon nanotubes

[en] In this paper we report on ab initio pseudopotential density functional calculations of some possible high-pressure phases of carbon. The total energies of several hybrid diamond-graphite structures were calculated as a function of volume using density-functional theory and the local density approximation. The lowest calculated transition pressures between hexagonal-graphite and hybrid structures were 17 and 20 GPa, which compare well with the experimental value of 14 GPa for the transition at low temperatures between graphite and a still unidentified hard transparent phase. The electronic densities of states for the different structures are presented. Also, the x-ray powder diffraction patterns for a few structures were simulated and qualitatively compared to published experimental diffraction patterns

[en] Different stacking arrangements of BC3 layered crystals are studied with the use of the ab initio pseudopotential density-functional method. The total energies, lattice constants, electron energy band structures and density of states, as well as phonon frequencies are calculated for the possible bulk BC3 structures obtained by full relaxations starting from different initial atomic configurations of ABAB (or ABCABC)... layer stacking. Two stable BC3 structures, one semiconductor and the other metal, are obtained, which have lower total energies compared with those of the structures proposed previously. Our calculations show that except for these two BC3 structures, all the structures we studied, including the BC3 structures proposed previously, have imaginary phonon frequencies corresponding to the relative, parallel motion of the adjacent BC3 layers, indicating the instability of the layer stacking in these structures