Local Density Functional Calculations Research Articles

Theoretical study of ground state and high-pressure phase of platinum carbide

We report local density-functional calculations using the full-potential linearized muffin-tin orbital method (FP-LMTO) for platinum carbide (PtC) in the, rock-salt (B1), zinc-blende (B3), wurtzite (B4), nickel-arsenide (B8) and PbO (B10) structures. The ground state properties such as the equilibrium lattice constant, elastic constants, the bulk modulus and its pressure derivative of PtC in these phases are determined and compared with available experimental and theoretical data. Our calculations show that the ground state phase of PtC to be zinc-blende (B3) structure at zero pressure and the nickel-arsenide (B8) structure is a high-pressure phase. The transition pressures at which this compound undergoes the structural phase transition from (B3) to (B8) and from (B3) to (B1) are found to be 34.25 and 51.28 GPa, respectively. The highest bulk modulus values in the nickel-arsenide (B8), zinc-blende (B3), rock-salt (B1) and PbO (B10) structures indicate that PtC is a hard material.

Density functional theory calculation of theDIoptical center in SiC

The ${\mathrm{D}}_{I}$ center is a prominent defect which is detected in as-grown or irradiated SiC. It is unusual in that its intensity grows with heat treatments and survives anneals of $1700\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. It has been assigned recently to either a close-by antisite pair or to the close-by antisite pair adjacent to a carbon antisite. We show here using local density functional calculations that these defects are not stable enough to account for ${\mathrm{D}}_{I}$. Instead, we assign ${\mathrm{D}}_{I}$ to an isolated Si antisite and the four forms of the close-by antisite pair in $4H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ to the $a$, $b$, $c$, and $d$ members of the alphabet series. The assignments allow us to understand the concentration of ${\mathrm{D}}_{I}$ following growth, the recombination enhanced destruction of these alphabet defects and the annealing behavior of the remaining members of the series.

Formation energy and migration barrier of a Ge vacancy from ab initio studies

Here we present local density functional calculations of the formation and migration energies of a vacancy in large Ge supercells and hydrogen-terminated Ge clusters. Migration barriers for neutral ( V 0 ), negatively charged ( V - ) and double negatively charged ( V = ) vacancies were calculated by using symmetry-constrained atomic relaxations, as well as a nudged elastic band scheme. The formation energy of the neutral vacancy is estimated at 2.6 eV, whereas 0.4, 0.1 and 0.04 eV are obtained for migration barriers of V 0 , V - and V = , respectively. These figures account well for the formation kinetics of vacancy-impurity complexes in Ge at cryogenic temperatures, and are also in line with measured self-diffusion activation barriers obtained at elevated temperatures.

First-principles investigation of a bistable boron-oxygen interstitial pair in Si

Local density functional calculations are used to predict and compare the properties of the two distinct interstitial boron-interstitial oxygen $({\mathrm{B}}_{i}{\mathrm{O}}_{i})$ complexes recently reported in the literature. The electronic and free energies, as well as the small transformation barrier, suggest that both forms of the defect are present at the temperature at which the defect forms. The vibrational spectra of the defects are predicted. The electrical levels of the defect are calculated and compared to experimental data. The existence of two forms of the ${\mathrm{B}}_{i}{\mathrm{O}}_{i}$ defect may have implications for the lifetime degradation of space-based Czochralski-silicon solar cells.

Theoretical study of the phosphorus vacancy inZnGeP2

First-principles calculations are presented for the phosphorus vacancy ${V}_{\mathrm{P}}$ in $\mathrm{Zn}\mathrm{Ge}{\mathrm{P}}_{2}$, using full-potential linearized muffin-tin orbital supercell local density functional theory calculations. We find the ${V}_{\mathrm{P}}$ to have a high energy of formation compared to the ${\mathrm{Ge}}_{\mathrm{Zn}}$ antisite donor in Zn-poor material. Including a band gap correction to the Hamiltonian, the defect is predicted to be amphoteric with a donor +/0 level at $0.6\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and an acceptor 0/\ensuremath{-} level at $1.25\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence band maximum.

On the bonding situation in TlCo2Se2

The electronic structure of TlCo2Se2 has been investigated by means of band structure calculations and x-ray photoelectron andemission spectroscopy (XPS and XES). The formation of the valence band is described inconnection with calculations of partial densities of states of the valence band and CoLα,β x-ray emission () aligned to the binding energy scale. The experimental results are in agreement with thecalculated distribution of Co 3d states, which lie close to the Fermi level. Theeffect of Co–Se bonding is seen as satellite structures in the XPS Co 2p signals.Thermoelectric and Hall effect data as well as resistivity measurements show thatTlCo2Se2 is metallic with electron holes as charge carriers with broadbandmobility characteristics. The observed anisotropy of the resistivity in theab planeand along c remains constant as a function of temperature, i.e. without signs of the magnetic ordering that occurs(TN∼85 K). The results of local spin density functional calculations show ferromagnetic couplingwithin the cobalt plane as a result of direct magnetic interactions between the magneticmoments.

Mn-dopedScN: A dilute ferromagnetic semiconductor with local exchange coupling

We present local spin density functional calculations for Mn-doped $\mathrm{ScN}$ using the linear muffin-tin orbital method in a supercell approach, including structural relaxation. Mn is found to induce a localized state in middle of the band gap of $\mathrm{ScN}$ with ${t}_{2g}$ character which spin splits and leads to a net magnetic moment of about $2--3{\ensuremath{\mu}}_{B}$ per Mn. The defect is a deep acceptor with 0/- transition state at about $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the Fermi level. The magnetic moment increases when in the negative charge state by acquiring more ${e}_{g}$ majority spin electrons and will thus persist even in the presence of the commonly found unintentional background $n$-type doping of $\mathrm{ScN}$. Up to fourth nearest neighbors exchange interactions are found to be ferromagnetic. The experimentally observed high solubility of Mn in $\mathrm{ScN}$, presumably related to their common crystal structure, is favorable for this dilute magnetic semiconductor. Within mean field theory the Curie temperature is estimated to be above $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for 3% Mn. While this may be overly optimistic because of the neglect of disorder, the $n$-type character of $\mathrm{ScN}$ adds further interest to this unusual magnetic semiconductor because the spin-split conduction bands are nondegenerate and thus essentially free from spin-orbit coupling induced spin relaxation.

Density-functional study of small interstitial clusters in Si: Comparison with experiments

Local density functional calculations are carried out on models of tri- and tetra-self-interstitial clusters in Si. Electrical levels and local vibrational modes (LVMs) of the defects are found. The latter are compared with recent experiments detailing the isotopic shifts of LVM frequencies associated with phonon replicas of the W- and X-photoluminescence lines, attributed to ${\mathrm{I}}_{3}$ ad ${\mathrm{I}}_{4}$, respectively. While the ground state ${\mathrm{I}}_{4}$ structure accounts well for the measured optical and electrical data, the most stable ${\mathrm{I}}_{3}$ aggregates either show no gap states, or the lack for LVMs that could be linked with the W-line. Instead, this optical center can be linked to a metastable form of ${\mathrm{I}}_{3}$.

Density-functional study of the mechanical and electronic properties of narrow carbon nanotubes under axial stress

The behavior of some narrow single-wall carbon nanotubes under uniaxial stress was studied. The first principles local density-functional calculations were carried out using helical repeat units and periodic boundary conditions for a single infinite chain. The mechanical response of the tube, the geometrical deformations, and the axial elastic modulus are calculated. The mechanical deformations are reflected also in the electronic properties. The change of the band gaps of the chiral tubules under axial stress was investigated. The indirect gaps of $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ can be closed applying small $(l5%)$ deformations which results in semiconductor-metal transitions.

Cohesive and magnetic properties of Ni, Co, and Fe on W(100), (110), and (111) surfaces: A first-principles study

Local density functional calculations are used to investigate the structural, cohesive, and magnetic properties of Ni, Co, and Fe on various orientations of the W surface. We found that all these elements wet all the W surface orientations for at least one monolayer so that thermodynamically stable ultrathin coatings of these elements can be formed. However, the magnetic properties are strongly orientation dependent. We will see that thermodynamically stable and ferromagnetic ultrathin coatings with large differences between the majority- and minority-spin density of states near the Fermi level may be realizable for some of these $3d$ metals on some orientations of the tungsten surface.

Electronic behavior of rare-earth dopants in AlN: A density-functional study

Local density functional calculations are carried out on Er, Eu, and Tm rare-earth (RE) dopants in hexagonal AlN. We find that the isolated impurities prefer to substitute for Al and, in contrast with isolated RE dopants in GaAs and GaN, ${\mathrm{RE}}_{\mathrm{Al}}$ defects are electrically active and introduce deep donor levels around ${E}_{v}+0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. RE complexes with oxygen and vacancies are discussed; some of these have deep levels in the upper third of the gap and could account for a threshold excitation energy around $4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ observed for intra-$f$ transitions at 465 and $478\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in AlN:Tm.

Spin-dependent transport in Fe andFe∕Aumultilayers

In situ resistance measurements of epitaxial Fe layers and $\mathrm{Au}∕\mathrm{Fe}$ bilayers were used to quantify the scattering in giant magnetoresistance (GMR) spin valve structures. The semiclassical Boltzmann transport equation, incorporating first-principles local density functional calculations, fitted the thickness dependence of the conductivity. Fits to the data indicate that Fe has a large spin asymmetry with bulk relaxation times ${\ensuremath{\tau}}_{\ensuremath{\downarrow}}=3.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ and ${\ensuremath{\tau}}_{\ensuremath{\uparrow}}=2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. These give a conductivity equal to that of bulk Fe. The interface scattering from the $\mathrm{Fe}∕\mathrm{GaAs}$, the $\mathrm{Fe}∕\text{vacuum}$, and the $\mathrm{Au}∕\text{vacuum}$ interfaces is purely diffuse. This is in contrast to the high electron reflection coefficients determined from kinematical calculations using scanning tunneling microscope images. Fits to conductivity measurements of $\mathrm{Au}∕\mathrm{Fe}∕\mathrm{GaAs}(001)$ indicate that the Au films have the conductivity of bulk material modified only by interface scattering. The GMR of $\mathrm{Au}∕\mathrm{Fe}∕\mathrm{Au}∕\mathrm{Fe}∕\mathrm{GaAs}(001)$ structures is 1.8% at room temperature and 2.9% at 10 K. The magnetoresistance is reduced by the presence of partial diffuse scattering at the inner interfaces, as indicated by the fits to both the GMR and the Au conductivity. The GMR in $\mathrm{Fe}∕\mathrm{Au}$ structures is intrinsically low due to a large electron band mismatch between Au and Fe band structures.

Nematic–smectic-Aphase boundary of ideally oriented Gay-Berne system: Local density functional versus isothermal-isobaric Monte Carlo simulation

The main focus of the present paper is on studying the nematic-smectic- A phase boundary of an ideally oriented Gay-Berne system. The phase diagram is determined by means of an isothermal-isobaric Monte Carlo simulation. The results are compared with predictions of the local density functional expanded up to second and third order in the one-particle distribution function. It is shown that generally the second-order expansion does not give satisfactory predictions for smectics. Going beyond the leading order yields good quantitative agreement at moderate densities. With increasing density the relative error of the local density functional calculations increases, but usually does not exceed 10% in densities. We conclude that the density functional approach could be competitive to time-consuming simulations in determining phase diagrams of spatially and orientationally ordered liquid crystalline structures.

Electrical and optical properties of rod-like defects in silicon

Self-interstitials in silicon can aggregate to form rod-like defects (RLDs) having both electrical and optical activity. We carry out local density functional calculations for both {113} and {111} RLDs to determine their structures and electrical activity. We find that small {113} RLDs are more stable than {111} RLDs but this reverses for larger defects. We attribute the electrical activity of {113} RLDs found in deep level transient spectroscopy studies with the bounding dislocations and the 0.903 eV photoluminescence to vacancy point defects lying on the habit plane.

Hydrogen-related photoluminescent centers in SiC

Local density functional calculations are used to investigate models of the center responsible for a prominent set of luminescent lines with zero-phonon lines around $3.15\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in hydrogen rich $4H\ensuremath{-}\mathrm{Si}\mathrm{C}$ and previously attributed to ${V}_{\mathrm{Si}}\text{\ensuremath{-}}\mathrm{H}$. We find that the electronic structure of this defect and the character of its vibrational modes are inconsistent with this assignment. In contrast, a ${\mathrm{H}}_{2}^{*}$ center, bound to a carbon anti-site, is more stable than the isolated molecule and possesses a donor level close to that observed for the H-lines. Moreover, its vibrational modes are in good agreement with experiment. A possible mechanism for the radiation enhanced quenching of the defect is discussed.

Theoretical investigation of the valence-band offset between Si(001) andSiO2

The valence band offset at device quality interfaces between Si(001) and $\mathrm{Si}{\mathrm{O}}_{2}$ are investigated using local density functional electronic structure calculations. Several model interfaces have been examined. One includes a relaxed amorphous oxide and several hundred atoms. Combining the results of several calculations, one can draw the following conclusions regarding band offset formation. Reduced oxide strain within an amorphous oxide results in a small reduction in the valence band offset. The electrostatic dipole is important while the actual sub-oxide distribution is found to be a minor effect. Also, hydrogen and other defects normally are insignificant. However, the many body corrections to the local density single particle band edges are significant. Finally, the overall theoretical valence band offset is found to be $4.2\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The experimental band offset results are re-examined in the context of these calculations.

Calculations of optical properties in strongly correlated materials

We present a method to calculate optical properties of strongly correlated systems. It is based on dynamical mean-field theory and it uses as an input realistic electronic structure obtained by local density-functional calculations. Numerically, tractable equations for optical conductivity, which show a correct noninteracting limit, are derived. Illustration of the method is given by computing optical properties of the doped Mott

Effects of intersite Coulomb interactions on ferromagnetism: Application to Fe, Co, and Ni

We reanalyze the condition for metallic ferromagnetism in the framework of the tight-binding approximation and investigate the consequences of the inter-site Coulomb interactions using the Hartree-Fock approximation. We first consider a non-degenerate $s$ band and we show that the inter-site interactions modify the occurrence of ferromagnetism, and we derive a generalized Stoner criterion. We analyze the main effects due to the renormalization of the hopping integrals by the inter-site Coulomb interactions. These effects are strongly dependent on the relative values of the inter-site electron-electron interactions and on the shape of the density of states as illustrated by a study of cubic crystals from which we establish general trends. We then investigate a realistic $spd$ tight-binding model, including intra (Coulomb and exchange) and inter-site charge-charge Coulomb integrals. This model is used to study the electronic structure (band structure, densities of states, magnetic moment) of bulk ferromagnetic 3d transition metals Fe(bcc), Co(hcp and fcc) and Ni(fcc). An excellent agreement with local spin density functional calculations is obtained for the three metals, in particular concerning the relative widths of the majority and minority spin bands. Thus our tight-binding Hartree-Fock model provides a consistent interpretation of this effect.

Generalized Kubas Complexes as a Novel Means for Room Temperature Molecular Hydrogen Storage

ABSTRACTWe propose that generalized Kubas complexes of molecular hydrogen with light metal elements, such as B and Be embedded in carbon nanostructures, or related Be and B materials, could offer breakthrough performance in room temperature hydrogen storage. First-principles local-density functional calculations show that hydrogen bound to these materials are intact, in similarity to physisorbed H2, but with a greatly enhanced adsorption energy in the range of 0.2–0.7 eV. The metal-H2 binding is attributed to the Coulombic interaction between holes created at the metal sites and Σ electrons of the H2. Management of the hole density and electron-hole orbital overlap thus enables us to control the binding strength of H2 for optimal storage properties.

THE METALLIC NATURE OF BORON LAYERS IN MAGNESIUM DIBORIDE

We performed ab initio, local density functional calculations for the electronic structure of MgB 2. The Fermi level of MgB 2 cuts through relatively narrow electron bands which have a dominant contribution from B (2p) states. The above metallic behavior of boron layers can also be seen from the calculated partial density of states. The metallic nature, strong covalent bonds, and the light atomic mass in the boron layers could lead to a strong electron-phonon (e–ph) interaction. The strong e–ph interaction is particularly important for the high energy phonons that are associated with boron layers in MgB 2.