An anomalous decrease in the elastic moduli of Cu/Nb multilayers has been observed via acoustic wave measurements. The decrease occurs for (111) fcc Cu and (110) bcc Nb layered structures with repeat periods between 1 and 5 nm. The coherency strain model has been used to simulate modulus enhancement in noble/transition metal multilayers. This approach addresses the atomic displacements corresponding with the lattice distortions of biaxially stressed layers. Elastic moduli are derived with respect to higher order differentials of a Born-Mayer type potential for nearest neighbor ions. The elastic moduli anomalies of Cu/Nb multilayers will be modelled within this conceptual framework.

A first-principles local-density based algorithm which employs optimized Gaussian-type orbitals is used to carry out calculations on a large variety of lithium clusters consisting of one to twenty-seven atoms. Bulk moduli, bond lengths and cohesive energies for the isolated clusters are presented and the results are extrapolated so as to predict the bulk (BCC) cohesive energy as well. Vacancy formation energies and vacancy induced lattice relaxation are also presented for three BCC fragments and compared to the bulk experimental results. For our largest cluster, we obtain a vacancy formation energy of 0.36 eV which is in good agreement with the experimental result of 0.34 eV.

We have calculated the total energy of a simple zeolite structure, sodalite, Na8Cl2Si6Al6O24 using the Modified Electron Gas (MEG) technique for the static lattice and for spherical ions. Comparison with the constituent oxides and chlorides gives a ΔGf of -0.394 au. Calculated bond lengths in the aluminosilicate framework are within 4% of the values obtained from experiment, but bond angles have error up to 12%.

Electron-phonon matrix elements, phonon linewidths and mode coupling strengths are being calculated for La2-xMxCuO4 (M-divalent cation, for paramagnetic x-0.0 and for x-0.15 in a rigid band picture) from first principles local density calculations. The change in potential due to a particular phonon mode is calculated from the difference of self-consistent one-electron potentials, and appropriate Fermi surface averages are carried out for selected modes, allowing us to obtain the phonon linewidth due to the electron-phonon interaction, and the corresponding coupling strength λQ. Here we establish the numerical accuracy within the dual representation of the potential used in the Linearized Augmented Plane Wave (LAPW) method. Evaluations of phonon linewidths and mode coupling strengths are presented for Al and Nb and compared with previous information on these modes. We present preliminary results for the full matrix elements and coupling of the La2CuO4 oxygen planar X-point breathing mode, and compare with a simpler approximation.

Full potential linearized augmented plane wave (LAPW) total energy calculations predict that the breathing mode in La2CuO4 is a high frequency mode, in agreement with experiment and contrary to previously published results. We also find the experimentally observed tilt mode to be unstable within the local density approximation (LDA). Eight other modes were studied, as well as the static structural parameters, and excellent agreement with experiment was found, indicating that LDA gives accurate total energies and static density response for the high temperature superconductors. Our results also suggest that phonons in the high Tc superconductors are not heavily dressed by magnetic or excitonic fluctuations, and thus that the isotope effect observed in the oxide superconductors is probably due to direct involvement of phonons in the superconductivitymechanism.

Temperature-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.

A new empirical potential model is proposed which can be used for a full fit to second and third order elastic moduli. The lattice stability of the β-phase of Cu-based martensitic alloys is known to depend strongly on these moduli. The pair potential is then used to compute the eigenvalues of the effective elastic moduli tensor for different deformations such as uniaxial z-distortion and (110)[110] shear. The results of these simulations are compared with the predictions of linear elasticity theory.

AB2C4 defect semiconductors can be thought of as generalized zincblende compounds, in which the presence of two different cations and of vacant sites favours the formation of several crystalline phases. In this work we present a theoretical study of the structural stability of the ZnxCdl-xIn2S4 solid solutions. The end compounds crystallize in a layer (x = 1) and in a spinel structure (x = 0). Total energy first principle calculations have been performed for both phases and for various values of x. The theoretical structural stability diagram compares very well with experiment.

We present microscopic calculations of the cohesive energy of amorphous metallic alloys. Our method is based on a tight-binding and Monte Carlo simulation approaches to calculate the equilibrium atomic structure. The same model tight-binding Hamiltonian is then used to calculate electronic structure and energy using a Bethe-Cluster approximation.

An empirical three-body potential, suitable for molecular dynamics (MD) simulations, has been developed to model the natural covalency of the Si-O bond in vitreous silica and silicate glass systems. Through the addition of a small directional-dependent three-body term to a previously used modified ionic pair interaction, a narrow distribution of tetrahedral angles and a low concentration of defects were obtained, in good agreement with experiment. The structure of bulk silica resulting from the MD technique also contained a larger average ring size, no edge-sharing tetrahedra, and a calculated static structure factor in good agreement with neutron diffraction results. The simulated sodium silicate glass was also largely improved over previous simulations using pair interactions alone. All silicon atoms were found to be exactly four coordinated while the number of non-bridging oxygen nearly equaled the number of sodium ions present with a reasonable distribution of Qi species.

Amorphous silicon structures have been generated by quenching liquid silicon configurations using molecular-dynamics simulations. Localized vibrational modes have been identified in these models. The presence of under-coordinated atoms in these a-Si models leads to extra resonant modes at low frequencies. The vibrational densities of states, and dynamic structure factors for localized, resonant and extended modes, are discussed and compared with neutron scattering data. The amorphous networks have also been adapted to model amorphous silicon-germanium systems. Densities of states and localization characteristics have been calculated for a-SixGe1-x alloys and a-Si/a-Ge superlattices, and are compared to Raman measurements.

Vibrations of amorphous Si are represented by three available 216 atom models and the forces of Stillinger and Weber. It is shown how the Kubo formula for thermal conductivity can be evaluated “exactly” assuming harmonic vibrations, making only finite system-size errors. The results are rather insensitive to the particular model for the coordinates of the atoms and support a “shunt resistor model” for the thermal conductivity which fits data for glasses very well.

Simulations of unhydrogenated amorphous silicon carbide, formed by condensing a vapor of silicon and carbon atoms, indicate partial chemical ordering, with heteronuclear bonds predominating. We find no evidence of phase separation. Carbon atoms are predominantly three-fold coordinated while silicon atoms have four-fold coordination. By combining these results with results of experimental studies, we speculate on the role of hydrogenation in determining the structure of amorphous silicon carbide.

The structural and electronic properties of liquid arsenic are calculated using density-functional quantum theory to calculate forces and trajectories of atoms. A semiconducting gap of 0.4 eV is found, and a coordination number of 2.8, close to the experimental values of 0.5 eV and 3. Our results support the existence of a Peierls-type distortion in liquid arsenic.

A first-principles approach is reviewed for calculating the total energy of chain polymers using a linear combination of atomic orbitals local-density functional approach. The geometry for the all-trans conformation of polysilane is optimized by finding the minimum energy structure using this method.

We have carried out systematic state-of-the-art calculations using density-functional theory, norm-conserving pseudopotentials, and large supercells in order to investigate the diffusion mechanisms of B, P, As, and Sb in Si under both equilibrium and non-equilibrium concentrations of intrinsic point defects. In addition, we have developed a theory for the non-equilibrium concentrations of the relevant diffusing species from which expressions for the activation energies may be derived. In equilibrium, we find that vacancies and self-interstitials mediate the diffusion of B, P, and As with comparable activation energies, but we show from our non-equilibrium diffusion calculations that these impurities have a dominant interstitial component. Sb diffusion, on the other hand, is mediated primarily by vacancies. We also find that the direct exchange mechanism plays only a minor role in all cases.

We 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.

The oxygen vacancy in silicon dioxide is currently believed to explain one of the most important defects observed in this material, the E1 ' center. To get a better understanding of the oxygen vacancy, we have investigated the equilibrium geometric structures, total energies, and charge densities of several charge states of the defect in alpha cristobalite. We also present some preliminary results for alpha quartz.

Interstitial oxygen precipitates in silicon during thermal treatment. The amount precipitated increases in an S-shaped fashion as a function of increasing initial interstitial oxygen concentration. A likely hypothesis for this behavior is that the number of nucleation sites that develop into precipitates (successful sites) varies with the initial interstitial oxygen concentration as well as with the precipitation rate at each site. In this paper, a deterministic precipitate growth model is first used to show that a fit to the present data requires the precipitate density to increase by more than a factor of 10 when the oxygen concentration goes from 24 to 40 ppma.

Three-dimensional Monte Carlo calculations are then used to show how the nucleation site survival probability depends on the initial number of oxygen atoms at the site and the oxygen concentration. The program treats oxygen diffusion, growth at nucleation sites by the addition of oxygen atoms, and loss at nucleation sites by the escape of oxygen atoms.

A three-dimensional Monte Carlo computer program has been developed to study the heterogeneous nucleation and growth of oxide precipitates during the thermal treatment of crystalline silicon. In the simulations, oxygen atoms move on a lattice with randomly selected lattice points serving as nucleation sites. The change in free energy that the oxygen cluster would experience in gaining or losing one oxygen atom is used to govern growth or dissolution of the cluster. All the oxygen atoms undergo a jump or a growth decision during each time step of the anneal. The growth and decay kinetics of each nucleation site display interesting fluctuation phenomena. The time dependence of the cluster size generally differs from the expected 3/2 power law due to the fluctuations in oxygen arrival at and incorporation in a precipitate. Competition between growing sites and coarsening are observed.