Principles Total Energy Calculations Research Articles

Ab initio pseudopotential calculations for aluminum-rich cobalt compounds.

First principles pseudopotential calculations with a plane wave basis set are performed for a variety of real and hypothetical Al-rich structures in the Al-Co system. The technical issues arising in the application of the plane wave pseudopotential method to these compounds, specifically the transferability and the energy convergence of the pseudopotentials, the broadening schemes for finite k-point sampling, and the implementation of the conjugate gradients algorithm for metallic systems, are discussed. The structural properties of fcc Al and AlCo in the CsCl (B2) structure are studied in detail, and found to be in good agreement with experiment and linear augmented-plane-wave method calculations. Results from first principles total energy calculations for the full set of structures considered are discussed in connection with recent pair potential calculations, trends in the electronic density of states, and the structural energetics of these intermetallic compounds, closely related to quasicrystals.

Metamagnetic transition and magnetic properties of UPt3.

We report fully relativistic band structure and total energy studies of ${\mathrm{UPt}}_{3}$ in the different magnetic states. We find that the ground state of ${\mathrm{Upt}}_{3}$ is nonmagentic, even though stable ferromagnetic and antiferromagnetic solutions are also found. We find that the total energy calculations predict the direction of the ordered moment correctly and for the first time the ``metamagnetic transition'' is found from a first principles total energy calculation for heavy fermion compounds.

First principles calculations of surface phonons for Cu(110)

Using first principles total energy calculations, surface inter- and intraplaner force constants could be determined for a number of high symmetry points in the surface Brillouin zone. These force constants allowed for the parameterfree determination of the eigenmodes and frequencies of surface phonons at X, Y and S points. Results are in good agreement with available data.

First principles calculations of lattice relaxation at low index surfaces of Cu

Using first principles total energy calculations, the lattice relaxation, surface energy and work function of low index surfaces of Cu are calculated. While the (111) and (100) surfaces show only small relaxation effects, the (110) surface exhibits pronounced multilayer relaxation which is typical for all fcc (110) surfaces. The results are in good agreement with experimental LEED and ion scattering studies.

First principles calculations of lattice relaxation at low index surfaces of Cu

Using first principles total energy calculations, the lattice relaxation, surface energy and work function of low index surfaces of Cu are calculated. While the (111) and (100) surfaces show only small relaxation effects, the (110) surface exhibits pronounced multilayer relaxation which is typical for all fcc (110) surfaces. The results are in good agreement with experimental LEED and ion scattering studies.

AB-INITIO BAND STRUCTURE CALCULATION FOR MAGNETIC MULTILAYERS

The magnetic properties of magnetic multilayers are studied using first principles total energy calculations. In this paper we present our results for bcc Fe multilayers with a (001) stacking direction. Using Nb as a spacer material we find an oscillating exchange coupling with a period of about 4.6 Å( ≈3 Nb monolayers). A detailed study of the magetic moments in the Nb spacer is given. Furthermore we discuss the dependence of the exchange coupling strength as a function of the thickness of the Fe slabs.

First principles analysis of vibrational modes in KNbO3

Abstract The vibrational modes in KNbO3 are analyzed using first principles total-energy calculations based on local density functional theory. Normal mode frequencies and eigenvectors are determined for the cubic perovskite structure with wave vectors at the center (Γ) and corner (R) of the Brillouin zone using the frozen-phonon technique. The frequencies are in good agreement with available experimental results with the possible exception of the lowest-frequency zone-center (ferroelectric) mode. Our calculations give a ferroelectric mode which becomes unstable with increasing volume. However, at the equilibrium volume it is stable with a frequency of ∼100 cm−1.

Theoretical investigation of the structure of the [formula omitted] surface

The structure of the ( 3 × 3 ) R30°- Au/Si(111) surface is investigated using first principles total energy calculations. Most models proposed by experiments have been tested. The lowest energy model is a conjugate honeycomb-chained-trimer (CHCT-1) configuration which consists of a top layer of trimers formed by Au atoms lying above a “missing top layer” Si(111) substrate with a honeycomb-chained-trimer structure for its first layer. The electronic charge densities from bands around the Fermi level give a good description of the images observed in STM experiments.

Halogen doping in solid C 60

We have theoretically investigated the possibility of halogen doping in solid buck- minsterfullerene ( C 60 ), assuming that the face-centered cubic lattice structure is unchanged by doping. Based on the first principles total-energy calculations, we have found that bromine atoms are exothermically incorporated into solid C 60, but that hole injection in the valence band of the pristine C 60 is not achieved. Instead, we have found that new states consisting mainly of the bromine 4 p orbitals appear in the midgap of C 60. Since these states are partially filled by electrons, it results in a metallic state. However, the band dispersion is very small so that the electron correlation might play an essential role. In that case a possibility of the Mott insulator may appear.

Transferable Total Energy Parametrization for Metals

ABSTRACTWe have investigated total energy expressions that consist of a term describing the eigenvalue sum and a pair potential term. Such expressions can be used to fit the results of first principles total energy calculations at given structures, and then obtain the total energy of another configuration of atoms avoiding the complexity of further ab initio calculations.In this work we present a method of fitting APW total energy results to a non-orthogonal tight-binding Hamiltonian from which the sum of the eigenvalues is derived and to a pair potential represented by a 5th order polynomial. We fit total energies for the fee and bec structures and we then obtain the elastic constants Cij in good agreement with both full potential LAPW calculations and experiment. We present results of this method for Ir.

On the bilayer growth mode of Au on Ag(110)

The energetics of the growth of Au on the Ag(110) surfave is investigated using first principles total energy calculations. Our results indicate that a recently observed bilayer growth mode is actually energetically unfavorable for this system.

Structure and dynamics at metal surfaces

Using first principles total energy calculations, lattice relaxation, reconstruction and surface phonons have been determined for various metal surfaces. Use of the Hellmann-Feynman theorem allows for a very efficient determination of equilibrium geometries and interplanar force constants. Results for simple metals as well as for noble metals will be presented and compared with available experimental information from LEED, ion channeling, helium scattering and EELS as well as with other theoretical treatments.

Oxygen vacancy ordering in YBa 2 Cu 3 O 7-y

Abstract First principles total energy calculations have been performed for YBa 2 Cu 3 O x with x varying from 6.0 to 7.5. The results of these calculations have been used to determine the effective pair interactions for the oxygen ordering in the basal plane. The phase diagram calculated with these “first principles” pair interactions is in very good agreement with experiment. Comparison of the total energies for the O 6 , O 6.5 and O 7 structures shows that the double-cell phase is thermodynamically stable.

Self-compensation through a large lattice relaxation in p-type ZnSe

The energetics of self-compensation through atomic relaxation around acceptor impurities in ZnSe were examined via first principles total energy calculations. We find large charge state and impurity-dependent lattice relaxations for As and P acceptors which can account for the experimentally observed difficulties in obtaining low-resistivity p-type ZnSe from these dopants. A much smaller relaxation is found for Li.

First principles total energy calculations for the honeycomb model of the [formula omitted] surface

Abstract The structural and electronic properties of √3 × √3 Ag/Si(111) are determined for the honeycomb model ( 2 3 ML Ag) by first-principles total energy calculations. The Ag atoms are found to occupy three-fold hollow sites above the Si surface. The surface is metallic but can be made semiconducting by negatively charging by one electron per surface unit cell.

First principles total energy calculations for the honeycomb model of the √3 × √3 Ag/ Si(111) surface

The structural and electronic properties of √3 × √3 Ag/Si(111) are determined for the honeycomb model ( 2 3 ML Ag) by first-principles total energy calculations. The Ag atoms are found to occupy three-fold hollow sites above the Si surface. The surface is metallic but can be made semiconducting by negatively charging by one electron per surface unit cell.

Ordering in III/V Alloys

AbstractSimple calculations using the delta-lattice-parameter (DLP) model indicate that the enthalpy of mixing is invariably larger than or equal to zero for III/V alloys, and increases with increasing difference in lattice constant for the constituent binary compounds. In terms of the regular solution model this suggests the occurrence of miscibility gaps. Solid phase immiscibility has indeed been observed in a number of systems. Nevertheless, such alloys can be grown by OMVPE, including the highly metastable alloys GaPSb and InPSb. Initially surprising was the occurrence of ordered structures in these same alloys. The regular solution model apparently specifically excludes immiscibility and ordering in the same system. However, when the positive enthalpy of mixing is due to strain energy effects, as in III/V alloys, Hume-Rothery recognized very early that such phenomena should be anticipated. This was later confirmed by detailed first principles calculations. In fact, the tendency for ordering is anticipated to increase as the difference in tetrahedral radii of the elements sharing a common sublattice increases. Thus, it is somewhat surprising that ordering was first observed in the AlGaAs system where Al and Ga have nearly equal sizes. Ordered structures have now been observed in several III/V alloy systems including the ternary systems GaAsSb and GalnP and the quaternaries GalnAsP, GalnAsSb, and AlGalnP. In this paper, ordering in other alloy systems such as GaPSb and InAsSb will be described. Surprisingly, no strong correlation between atomic size difference and the degree of ordering has been observed. Another unexpected observation is that the preferred ordered structure for most ternaries involves ordering along the {111}directions. Both first principles total energy and simple strain energy calculations indicate that such ordered structures are only slightly more stable than the disordered solid solution. Other ordered structures, including L10, where ordering occurs along the {100} directions, are considerably more stable. Both phenomena must be explained in terms of the surface kinetic processes occurring during epitaxial growth. Such an explanation is supported by the importance of kinetic parameters such as growth rate, temperature, and substrate orientation in determining both the degree of order and the specific ordered structures observed.

First principles calculation of lattice relaxation and surface phonons on Al(100)

Using first principles total-energy calculations the geometry of the Al(100) surface has been determined. We found a small expansion of the first interlayer spacing by 1.2 ± 0.4%. Use of the Hellmann-Feynman theorem allowed the calculation of forces and thus the determination of force constants. The modification of the force constants at the surface has significant influence on the surface phonon frequencies near the zone boundary.

First-Principles Theory of Alloy Phase Diagrams

AbstractTemperature-composition phase diagrams of alloys are calculated by a new method combining (i) first principles total energy calculations (at T=0) for ordered structures, using the local density formalism, with (ii) finite-temperature statistical-mechanics approach (the Cluster Variation Method) to the solution of the multi-spin Ising model, using volume-dependent interaction energies obtained from (i). Novel features, including the appearance of metastable long-range ordered compounds at low temperatures are discovered.

Native Defects in Diamond, Sic, and Si: Energetics and Self-Diffusion

AbstractWe have investigated, via first principles total energy calculations, the energetics of elementary native defects in group IV semiconductors. Its implications on the relative abundance of these defects and self-diffusion phenomena are analyzed. The results show that in diamond the self-diffusion is dominated by vacancies, because the interstitial and direct exchange mechanisms have much greater activation energy. In SiC stoichiometry plays an important role. For Si-rich compound, Sic-antisite is the dominant defect in the intrinsic and p-type material, while the carbon vacancy is dominant in the n-type material. For C-rich material, the Csi-antisite is dominant regardless the position of the Fermi level. In Si, it well-known that the vacancy, interstitial and direct exchange mechanisms have very similar activation energies. Our results suggest that self-diffusion experiments carried out at various pressures can determine the relative contribution of each of these mechanisms.