Trap-recharging waves versus damped,forced charge-density oscillations in hexagonal silicon carbide

The structural parameters and hydrostatic pressure coefficients of CdS_xTe_1-x in the two phases, namely zinc-blende and NaCl as well as the transition pressures from zinc-blende to NaCl structures at various S concentrations are presented. The calculations are performed using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the local density approximation (LDA), and two developed refinements, namely the generalized gradient approximation (GGA) of Perdew et al. for the structural properties and Engel-Vosko for the band structure calculations. Detailed comparisons are made with published experimental and theoretical data and show generally good agreement. The present results regarding the studied quantities for compositions x in the 0–1 range (0 < x < 1) and for the NaCl phase are predictions and may serve as a reference for experimental work.     

Structural, electronic and optical properties of AgI under pressure

We report results of first-principles total-energy calculations for structural properties of the group I-VII silver iodide (AgI) semiconductor compound under pressure for B1 (rocksalt), B2 (cesium chloride), B3 (zinc-blende) and B4 (wurtzite) structures. Calculations have been performed using all-electron full-potential linearized augmented plane wave plus local orbitals FP-LAPW + lo method based on density-functional theory (DFT) and using generalised gradient approximation (GGA) for the purpose of exchange correlation energy functional. In agreement with experimental and earlier ab initio calculations, we find that the B3 phase is slightly lower in energy than the B4 phase, and it transforms to B1 structure at 4.19 GPa. Moreover, we found AgI has direct gap in B3 structure with a band gap of 1.378 eV and indirect band gap in B1 phase with a bandgap around 0.710 eV. We also present results of the effective masses for the electrons in the conduction band (CB) and the holes in the valence band (VB). To complete the fundamental characteristics of this compound we have analyzed their linear optical properties such as the dynamic dielectric function and energy loss function for a wide range of 0-25 eV.     

First-principles study of the electronic and optical properties of rutile TiO2

The electronic, structural properties and optical properties of the rutile TiO₂ have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our results including lattice parameter, bulk modulus, density of states, the reflectivity spectra, the refractive index and band gap are compared with the experimental data. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0).     

A theoretical study of Fe adsorption along Bi-nanolines on the H/Si(0 0 1) surface

We have investigated the energetic stability and equilibrium geometry of the adsorption of transition metal Fe atoms near the self-organized Bi lines on hydrogen passivated Si(0 0 1) surface. Our total energy results show that there is an attractive interaction between Fe adatoms along the Bi-nanolines. For the energetically most stable configuration, the Fe adatoms are seven-fold coordinated, occupying the subsurface interstitial sites aside the Bi-nanolines. With increased coverage, Fe atoms are predicted to form two parallel lines, symmetrically on both sides of the Bi line. Within our local spin-density functional calculations, we find that for the most stable geometries the Fe adatoms exhibit an antiferromagnetic coupling.     

Optical properties of the alkaline-earth fluorohalides matlockite-type structure SrFX (X=Cl, Br, I) compounds

The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0). We find that the value of ε₁(0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data.     

Study of arsenic for antimony exchange at the Sb-stabilized GaSb(0 0 1) surface

In this paper we present a first-principle study on the energetics of a single As₂ molecule on GaSb(0 0 1) reconstructed surface. In order to shed light into the mechanisms of anion exchange at the Sb-rich GaSb(0 0 1) surface, we studied firstly As₂ adsorption and then As for Sb exchange. We identify a surface region where both the processes are energetically favored. The results of this twofold analysis can be combined to derive possible reaction paths for the anion exchange process.     

The influences of the local impact site and incident energy on the transport behaviors of single copper atom onto Cu (0 0 1) surface

Molecular dynamics (MD) simulation is carried out to study the transport behaviors of a single deposited atom in Cu film homoepitaxy. We consider the normal Cu incident atoms impinging on the Cu (0 0 1) surface at four possible local impact sites (top, bridge, hollow and general). The observed transport behaviors of the deposited atom onto the surface include: direct adsorption (DA), penetration by atomic exchange, and transient penetration (TP), which a deposited atom penetrates the interstitial site and then rapidly migrates to a stable site on the surface. The results show that transport behaviors of the deposited atom are closely related to both the local impact site and the incident energy. The maximum increment of kinetic energy at every impact site approaches to a certain value except for the incident energy below 2.0 eV. Furthermore, as the incident energy is higher than the penetration threshold, TP behavior could be observed again in some energy ranges. This interesting phenomenon, which cannot be explained by the existing theories, is possibly attributed to the dynamical competition between the deposited atom and substrate atoms.     

First-principles study of the optical properties of PbFX (X = Cl,Br, I) compounds in its matlockite-type structure

We present the results of the ab initio theoretical study of the optical properties for PbFX (X = Cl, Br, I) compounds in its matlockite-type structure using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Z resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε( ω) and its zero-frequency limit ε₁ ( 0 ). We find that the values of ε₁ ( 0 ) increases with decreasing the energy gap. The reflectivity spectra and absorption coefficient has been calculated and compared with the available experimental data. The optical properties are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure.     

Theoretical study of CuxAg1−xI alloys

We have performed first-principles calculations using full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT) to investigate the fundamental properties of Cu_xAg_1−xI alloys. We used both GGA96 [J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.] and EVGGA [E. Engel, S.H. Vosko, Phys. Rev. B. 47 (1993) 13164.] generalized gradient approximations of the exchange-correlation energy that are based on the optimization of total energy and corresponding potential. Quantities such as lattice constants, bulk modulus, band gap, density of occupied states and effective mass were calculated as a function of copper molar fraction x. These parameters were found to depend non-linearly on alloy composition x, except the lattice parameter, which follows Vegard's law. The microscopic origins of the gap bowing were explained using the approach of Zunger and co-workers; we have concluded that the band-gap energy bowing was mainly caused by the chemical charge-transfer effect and the volume deformation , while the structural relaxation contribute to the gap bowing parameter at smaller magnitude. The calculated phase diagram shows a broad miscibility gap for this alloy with a high critical temperature.     

Generalized-stacking-fault energy and surface properties for HCP metals: A first-principles study

We present first-principles calculations on the generalized-stacking-fault (GSF) energies and surface properties for several HCP metals on Mg, Be, Ti, Zn, and Zr, employing density functional theory (DFT) within generalized-gradient-approximation (GGA) and spin-polarized GGA (SGGA) using the Vienna ab initio simulation package (VASP). Using a supercell approach, stacking fault energies for the [1 1  0] and [1 0  0] slip systems, and surface properties on basal plane (0 0 0 1) have been determined. Our results show that GSF energy is sensitive to the primitive cell volumes and the ratio c/a for HCP metals. A spin-polarized calculations should be considered for transition-metal Ti, Zn, and Zr. The results for Mg from this work are good with ones from the previous ab initio and the experiments.     

Ab initio study of pressure-induced magnetic transition in manganese pnictides

We report a density functional calculation on the NiAs-type Mn-based pnictides. The total energy as a function of volume is obtained by means of self-consistent tight-binding linear muffin–tin orbital method by performing spin and non-spin polarized calculation. From the present study, we predict a magnetic-phase transition from ferromagnetic (FM) to non-magnetic (NM) around 49 and 35.7 GPa for MnAs and MnSb, respectively. The pressure-induced transition is found to be a second-order transition. The band structure and density of states (DOS) are plotted for FM and NM states. Apart from this the ground-state properties like magnetic moment, lattice parameter and bulk modulus are calculated and are compared with the available results. Under large volume expansion these compounds exist in zinc-blende (ZB) structure, which shows half metallicity. The magnetic moment and equilibrium lattice constants for ZB structure are obtained as well as band structure and DOS are presented.     

Adsorption structure of germanium on the Ru(0 0 0 1) surface

Coverage-dependent adsorption energy of the Ge/Ru(0 0 0 1) growth system and the geometrical distortions of the most stable adsorption structure are investigated through first-principles calculations within density functional theory. A local minimum in adsorption energy is found to be at a Ge coverage of 1/7 monolayer with a Ru(0 0 0 1)- symmetry. Based on this stale superstructure, the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) images are simulated by means of surface local-density of states (LDOS). The results are consistent well with the STM measurements on the phase for Ge overlayer on Ru(0 0 0 1). From this stimulation, the relations between the STM images and the lattice distortion are also clarified.     

First principles calculations of the adsorption and diffusion of Y on the Si(0 0 1)-c(4 × 2) surface

We have studied the adsorption and diffusion of yttrium on the Si(0 0 1)-c(4 × 2) surface in the early stages of growth. Our first principles total energy calculations are based on the density functional theory as implemented in the SIESTA code. The exchange and correlation energies are treated within the generalized gradient approximation according to the Perdew, Burke and Ernzerhof parametrization. Our results demonstrate that the most favorable adsorption site is in the trench between two silicon dimer rows, identified as valley-bridge (V). Our studies show that the diffusion of an Y adatom on Si(0 0 1)-c(4 × 2) surface presents an anisotropic behavior. We found two values for the barriers along the valley (0.54 and 1.07 eV) and one of 1.24 eV in the perpendicular direction, showing that diffusion along the valley is more probable. The analysis of the Mulliken overlap populations shows that the bonding between an Y adatom and the surface is partially covalent. Two Y atoms on the surface do not form dimers instead they are adsorbed as adatoms.     

First-principles investigations of iridium low index surfaces

We report density functional theory (DFT) calculations for the surface energy, work function, and interlayer spacings for Ir(1 1 1), Ir(1 0 0), and Ir(1 1 0) surfaces using pseudopotential method and plane waves basis set. We investigate the convergence of the surface energy as a function of the number of layers in the slab for the Ir(1 0 0) surface. The results show that the surface energies calculated using the bulk total energies obtained by a fit to a series of slab total energies converge within 0.01 J/m². We also investigate the convergence of the work function and interlayer spacings as a function of the number of layers in the slab, for the Ir(1 0 0) surface. Employing the local-density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange correlation functional, we obtained a very good agreement of the calculated surface energies and work functions with experimental results. For the studied surfaces, the calculations give interlayer relaxations that are in an excellent agreement with available low-energy electron diffraction (LEED) analysis. Furthermore, we discuss the performance of the LDA and GGA for the exchange correlation functional in describing the various surface properties. The results show that calculations using GGA give results that are in a better agreement with experiment than the LDA.     

Formation and ordering of local magnetic moments in Fe–Al alloys

The density functional theory is used to study the local magnetic moments in Fe–Al alloys depending on concentration (from 29 to 44 at% Al) and the Fe nearest environment. We have found three different solutions for the system: a spin-spiral wave (SSW) which has a minimum energy and two collinear states, a ferromagnetic one and a state with both positive and negative Fe magnetic moments (the Fe atoms with many neighboring Al atoms around them have negative magnetic moments, while the other Fe atoms—positive). Both the SSW and the negative Fe moments agree with the experiments. Magnetization curves taken from the literature are analyzed. The assumption of percolation character of the size distribution of magnetic clusters describes well the experimental superparamagnetic behavior above 150 K.     

First principle study of SrTiO3 (0 0 1) surface and adsorption of NO on SrTiO3 (0 0 1)

The results of first-principles calculations about the two possible terminations of (0 0 1) surface of SrTiO₃ perovskite and adsorption of NO on SrTiO₃ (0 0 1) surface were presented. Both surface parameters (atomic structures and electronic configurations) and adsorption parameters (bond, energy and charge) of NO on SrTiO₃ (0 0 1) surface, which have never been investigated before as far as we know were investigated using density functional theory calculations with the local-density approximation (DFT-LDA). It was found that the two possible terminations of SrTiO₃ (0 0 1) surface have large surface relaxation, which leads to surface polarization and exhibits different reactivity toward the dissociative adsorption of NO. The electronic states of TiO₂-terminated surface have larger difference than that of bulk, so it is more favorable for adsorption of NO than SrO-terminated surface.     

First-principles calculations of hydrogen molecule adsorption on Ti (0 0 0 1)-(2 × 1) surface

Adsorption of H₂ molecule on the Ti (0 0 0 1)-(2 × 1) surface was studied by density functional theory with generalized gradient approximation (GGA). The parallel and vertical absorption cases were investigated in detail by adsorption energy and electronic structure analysis, we obtained three stable configurations of FCC-FCC (the two H atoms adsorption on the two adjacent fcc sites of Ti (0 0 0 1) surface, respectively), HCP-HCP (the two H atoms adsorption on the two adjacent hcp sites of Ti (0 0 0 1) surface, respectively) and FCC-HCP (the one H atom adsorption on the fcc site and the other adsorption on the near hcp site) based on the six different parallel adsorption sites after the H₂ molecule dissociates. However, all the end configurations of four vertical adsorption sites were unstable, H₂ molecule was very easy to desorb from Ti surface. The H-H bond breaking and Ti-H bond forming result from the H₂ molecule dissociation. H-H bond breaking length ranges from 1.9 Å to 2.3 Å for different adsorption configurations due to the strong Ti-H bond forming. The H₂ dissociative approach and the end stable configurations formation in parallel adsorption processes are attributed to the quantum mechanics steering effects.     

Experimental and theoretical studies of polaron optical properties in KNbO3 perovskite

Time-resolved absorption and luminescence spectra have been measured in KNbO₃ perovskite crystals after pulsed band-gap excitation by 200 fs laser pulses and 10 ns electron pulses. Quantum chemical calculations using the large unit cell periodic model support the interpretation of the observed transient absorption bands at 0.8 and 1.1 eV as the self-trapped electron polarons and bound hole polarons, respectively. The activation energy for the 2.2 eV green luminescence quenching is 0.05 eV. We suggest that the short lifetime (<15 ns) of the luminescence at RT is caused by the radiative recombination of nearest electron and hole polarons.     

Structural and elastic properties under pressure effect of the cubic antiperovskite compounds ANCa3 (A = P, As, Sb, and Bi)

Using first-principles density functional calculations, the effect of high pressures, up to 40 GPa, on the structural and elastic properties of ANCa₃, with A = P, As, Sb, and Bi, were studied by means of the pseudo-potential plane-waves method. Calculations were performed within the local density approximation and the generalized gradient approximation for exchange-correlation effects. The lattice constants are in good agreement with the available results. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's modulus, Poisson's ratio and Lamé's constants for ideal polycrystalline ANCa₃ aggregates. By analysing the ratio between the bulk and shear moduli, we conclude that ANCa₃ compounds are brittle in nature. We estimated the Debye temperature of ANCa₃ from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of PNCa₃, AsNCa₃, SbNCa₃, and BiNCa₃ compounds, and it still awaits experimental confirmation.     

Molecular dynamics investigation of deposition and annealing behaviors of Cu atoms onto Cu(0 0 1) substrate

The deposition growth and annealing behaviors of Cu atoms onto Cu(0 0 1) are investigated in atomic scale by molecular dynamics (MD) simulation. The results indicate that the film grows approximately in a layer-island mode as the incident energy is from 1 to 5 eV, while surface intermixing can be significantly observed at 10 eV. The surface roughness of the film decreases with increasing the incident energy, and the film after annealing becomes smoother and more ordered. These phenomena may be attributed to the enhanced atomic mobility for higher incident energy and thermal annealing. It also indicates that atomic mixing is more significant with increasing both the incident energy and substrate temperature. In addition, the peak-to-peak distances of radial distribution function (RDF) clearly indicate that the films before and after annealing are still fcc structure except for that at the melting temperature of 1375.6 K. After annealing, the film at the melting temperature returns to fcc structure instead of amorphous. Moreover, the residual stress and Poisson ratio of the film are remarkably affected by the thermal annealing. Furthermore, the density of thin film is obviously affected by the substrate temperature and annealing process. Therefore, one can conclude that high incident energy, substrate temperature and thermal annealing could help to enhance the surface morphology and promote the microstructure of the film.