Ab initio modeling study of boron diffusion in silicon

We present investigations of boron diffusion in crystalline silicon using ab initio calculations (W. Windl et al., Phys. Rev. Lett. 83 (1999) 4345). Based on these results, a new mechanism for B diffusion mediated by Si self-interstitials was proposed. Rather than kick-out of B into a mobile channel-interstitial, one- or two-step diffusion mechanisms have been found for the different charge states. The predicted activation energy of 3.5–3.8 eV, migration barrier of 0.4–0.7 eV, and diffusion-length exponent of −0.6 to −0.2 eV are in excellent agreement with experiment. We also present results of ab initio calculations for the structure and energetics of boron-interstitial clusters in Si. We show how these first-principles results can be used to create a physical B diffusion model within a continuum simulator which has strongly enhanced predictive power in comparison to traditional diffusion models.     

Energetics of NP and NB complexes in silicon

Using ab initio (Hartree–Fock and local density approximation) and semiempirical (Austin Model 1) calculations, we studied the energetics and electronic structures of NB and NP complexes. We found that these complexes are electrically inactive. The formation energies are 1.6 eV for the NB coupling and 2.4 eV for the NP pairing. The N–P and N–B interatomic equilibrium distances are about 3.5 Å for both complexes.     

Quantum chemical modelling of point defects in KNbO3 perovskite crystals

We present results of semi-empirical quantum chemical calculations for several perovskite KNb_xTa_1−xO₃ (KTN) solid solutions, as well as point intrinsic defects – F centers and hole polarons bound to K vacancy – in KNbO₃. Method of the intermediate neglect of the differential overlap (INDO) was combined with typically 320-atom supercells and atomic geometry optimization. Analysis of the optimized atomic and electronic structure has clearly demonstrated that several nearest Nb atoms substituting for Ta in KTaO₃ – unlike Ta impurities in KNbO₃ – reveal the self-ordering effect, which probably triggers the ferroelectricity observed in KTN. We predict co-existence of one-site (atomic) and two-site (molecular) polarons with close absorption energies (≈1 eV). When available, the INDO results are compared with ab initio calculations. The relevant experimental data are discussed.     

A cluster approach to hydrogen chemisorption on the GaAs(1 0 0) surface

Chemisorption properties of atomic hydrogen on the Ga-rich GaAs(1 0 0), (2×1) and β(4×2) surfaces are investigated using ab initio self-consistent restricted open shell Hartree–Fock (ROHF) total energy calculations with Hay–Wadt (HW) effective core potentials. The effects of electron correlation have been included using many-body perturbation theory through second order with the exception of β(4×2) symmetry due to computational limitations. The semiconductor surface is modeled by finite sized hydrogen saturated clusters. The effects of surface reconstruction have been investigated in detail. We report on the energetics of chemisorption on the (1 0 0) surface layer, including adsorption beneath the surface layer at an interstitial site, and also report on the possible dimer bond breaking at the bridge site. Chemisorption energies, bond lengths, and charge population analysis are reported for all considered sites of chemisorption.     

Modifying the buckyball

Structural and electronic properties of carbon clusters, in particular the C₆₀ “buckyball” molecule as well as structurally and chemically modified fullerenes, are calculated using a combination of predictive ab initio techniques and parametrized total energy schemes. These calculations indicate that single- and multi-shell fullerenes are the most stable C_n isomers at T = 0 for n < 20. More open structures are favored by entropy at higher temperatures. Upon interaction with donor elements, C₆₀ molecules form stable M@C₆₀ endohedral complexes; analogous acceptor-based complexes are unstable. Solid C₆₀ reacts with alkali metals and forms a stable intercalation compound which shows superconducting behavior. The relatively high value of the critical temperature for superconductivity can be explained quantitatively within the Bardeen-Cooper-Schrieffer formalism.     

Zinc-blende GaN: ab initio calculations

The purpose of this paper is to contribute, on a theoretical basis, an understanding of future wide-gap device concepts and applications based on III–V nitride semiconductors. The electronic properties of zinc-blende structure GaN and their (110), (100) and (111) surfaces are investigated using ab initio calculations based on the full potential linear augmented plane-wave (FPLAPW) method within the large unit cell approach, and on the molecular Gaussian-92 code. Lattice constant, cohesive energy, bulk modulus are obtained from total energy calculations. Light-hole and heavy-hole effective masses along (100), (111) and (110) directions and electron masses at Γ point are extracted from band structure calculations and compared with previous ones based on pseudopotential methods. The hydrostatic pressure dependence of the ΓΓ, ΓX and ΓL energy gaps are also obtained. Comparing our band structure and ‘molecular cluster' calculations, the relaxations of the surfaces are found to be mostly determined by local rehybridization or valence effects and are basically independent of energy band features.     

Calculations of the atomic and electronic structure for SrTiO3 perovskite thin films

The results of calculations of SrTiO₃ (100) surface relaxation and rumpling with two different terminations (SrO and TiO₂) are presented and discussed. We have used the ab initio Hartree–Fock (HF) method with electron correlation corrections and the density functional theory (DFT) with different exchange–correlation functionals, including hybrid exchange techniques. All methods agree well on surface energies and on atomic displacements, as well as on the considerable increase of covalency effects near the surface. More detailed experiments on surface rumpling and relaxation are necessary for further testing of theoretical predictions.     

Influence of disorder on electronic structure and magnetic properties in Fe-rich Fe–Si alloys

We present the results of ab initio calculations of the spin-polarized electronic structure of disordered bcc and fcc Fe_1 − xSi_x alloys from dilute solid solutions of Si on iron up to FeSi intermetallic compound composition (x=0.01..0.5) and ordered intermetallic phases FeSi in B20 structure and Fe₃Si in DO₃ structure. The self-consistent calculations were performed with the coherent potential approximation within the Korringa–Kohn–Rostoker method (KKR–CPA) for disordered case and by using the tight-binding linear muffin–tin orbital (TB LMTO) method for intermetallic compounds. In the last case the supercell approach has been utilized in order to take into account the structural defects for this ordered phase. In particular, we have performed a series of calculations in the BCC Fe_1 − xSi_x alloys with x=0.01, 0.015, 0.1, 0.15, 0.25.     

A theoretical study on the second-order nonlinear optical susceptibilities of lithium formate monohydrate crystal, HCOOLi·H2O

Ab initio calculations of the first hyperpolarizabilities of (HCOOLi·H₂O)_2n supermolecules, as the building-blocks of lithium formate monohydrate (LFM) crystal with extended system, were performed for the first time. The dependence of the static β_ijk⁰ values on chain length was explored, and the frequency dependence of β_ijk(−2ω;ω,ω) was measured, and the influences of electron correlation and basis set on β_ijk⁰ were evaluated. Finally, we predicted the second-order nonlinear optical coefficients of LFM crystal. The β_ijk⁰ value of (HCOOLi·H₂O)_2n is linearly dependent on the chain length of supermolecule, which is quite unusual for an extended system connected by the O–Li bonds with ionic characters. Although the static component of β_zzz⁰ tensor is the static largest in these three components under study, the absolute value of frequency-dependent β_zyy(−2ω;ω,ω) element, transforming the smallest into the largest, is the most sensitive to frequency. After the fundamental wavelength is smaller than 500 nm, it is found that the β_ijk(−2ω;ω,ω) value is resonantly enhanced to a great extent due to the double frequency lies in the region of resonance. In addition, the β_zxx⁰ value goes from negative to positive with changes of electron correlation and basis set. Obviously, it is very necessary to take into account the effect of electron correlation, if the hyperpolarizability tensor components must be accurately calculated. Moreover, it is also very important whether it is adopted a complete basis set with diffuse and polarization functions. The calculated nonlinear coefficients at high level suggest that the scaled set reported by Robert seem more reasonable.     

Electronic structure of the ladder-chain compound Sr14−xCaxCu24O41

The electronic structure of the ladder-chain compound Sr_14−xCa_xCu₂₄O₄₁ is studied by ab initio calculations within the local density approximation. The effects of Ca substitution and structure modulation on electronic structure are discussed. It is found that 0.05 holes per copper atom are on the ladder layers for fully substituted compound, Ca₁₄Cu₂₄O₄₁.     

Elastic constants predicted from sintered porous MgB2 via micromechanics modeling

Elastic constants of MgB₂ from acoustic measurements using sintered porous samples are much different from the values by ab initio calculations. This paper considers the discrepancy in terms of the pore shape with micromechanics modeling. The model calculations showed that ab initio calculations are consistent with the values corrected for the oblate ellipsoidal pores from the measured elastic constants, and the extremely small values can be measured with sintered samples.     

Atomistic study of vibrational properties of γ-Al2O3

A study of the vibrational density of states (DOS) of γ-Al₂O₃ is presented. Four structural models from the recent literature are considered: vacant spinel model and three nonspinel models. The vacant spinel and one of the nonspinel models have unit cells with 40 atoms, while the other two models have 160 atoms. The interatomic interactions are computed using classical force fields that include Coulomb and van der Waals attractive interactions, short range repulsive interactions, as well as three-body terms. The oxygen polarizability is included via a core-shell potential. The DOS is compared with ab initio calculations recently published for the vacant spinel model. The classical and ab initio DOS show some differences for frequencies higher than 200 cm⁻¹, the ab initio having more peaks and having a frequency cutoff 100 cm⁻¹ lower than the classical DOS. The DOS of all models present some small differences. While the 160-atoms nonspinel models present a rather structureless DOS, 40-atoms models present peaks and dips relative to the 160-atoms models. The elastic constants of polycrystalline γ-Al₂O₃ are also estimated using several force fields. In general, the classical force field predict higher elastic moduli than the ab initio method. The infrared spectra of the four models are calculated.     

Predicted modifications in the direct and indirect gaps of Ge

The charge density of Ge was studied at various k-points and for various bands, by the ab initio pseudopotential method, using additionally the interstitial sites. The lowest X_c conduction-band points were found to be unique in having a high charge density in the interstitial site. It has been therefore predicted and verified that the X_c points move up in energy relative to the Γ_c point when closed-shell atoms (like H) are substituted at the interstitial sites. The calculations also indicate the change of the band-gap for HGeH.     

Computer simulation of the quartz surface: a combined ab initio and empirical potential approach

We investigate several possible reconstructions of the (0 0 1) α-quartz surface by using a combinatory approach of classical and ab initio molecular dynamics (MD). Configurational space is efficiently explored by fast classical MD simulations with a semi-empirical three-body potential, which has been shown to be accurate in simulations of the bulk. These runs generate initial structures for further refinement by the accurate quantum MD method of Car–Parrinello [P. Car, M. Parrinello, Phys. Rev. Lett. 55 (1985) 2471]. Stable reconstructions of the quartz surface have been produced. They show the same pattern: formation of six- and three-membered rings. The same type of reconstruction was found in earlier full ab initio calculations [J.M. Rignanese, Ph.D. thesis, 1998]. The obtained structures were found to be local minima in the classical potential, thereby demonstrating the usefulness of the empirical potential for surface calculations. This allows for a fast characterisation of the surface reconstructions and their dynamical properties.     

First-principles calculations for vacancy formation energies in Cu and Al; non-local effect beyond the LSDA and lattice distortion

We show ab initio calculations for vacancy formation energies in Cu and Al. The calculations are based on density-functional theory and the full-potential Korringa–Kohn–Rostoker Green's function method for impurities. The non-local effect beyond the local-spin-density approximation (LSDA) for density-functional theory is taken into account within the generalized-gradient approximation (GGA) of Perdew and Wang. The lattice relaxation around a vacancy is also investigated using calculated Hellmann–Feynman forces exerted on atoms in the vicinity of a vacancy. We show that the GGA calculations reproduce very well the experimental values of vacancy formation energies and bulk properties of Cu and Al, as they correct the deficiency of LSDA results (underestimation of equilibrium lattice parameters, overestimation of bulk moduli, and vacancy formation energies). It is also shown that the GGA calculations reduce the LSDA results for the lattice relaxation energy for a vacancy in Cu.     

Lattice dynamics of Ga1−xAlxAs studied by ab initio calculations

We have calculated the phonon dispersion and phonon density of states of superlattices of Ga_1−xAl_xAs for x=0.0, 0.25, 0.75 and 1.0 using the ab initio method within the supercell approach and calculating the Hellmann–Feynman forces. A noticeable difference of force constants between Ga–As and Al–As atomic pairs has been found. In any case, Al atoms vibrate in well-separated high-frequency optic modes.     

Elastic tensor of the forsterite (Mg2SiO4) under pressure

A complete elastic tensor of the low-pressure structure of the magnesium orthosilicate (Mg₂SiO₄, forsterite) is determined by an ab initio technique for the pressure range P=0–240 kB. The geologically important quantities: density, sound velocity, Young's modulus, Poisson's ratio, crystal anisotropy, are derived from the calculated data. A systematic increase of crystal's anisotropy with pressure has been noticed. The results agree well with the available experimental data.     

Ab Initio and Monte Carlo Approaches for the Magnetocaloric Effect in BaMnO<Subscript>3</Subscript> Oxide Perovskite

The self-consistent ab initio calculations, based on density functional theory approach (DFT) and using full potential linearised augmented plane wave (FLAPW) method, have been used to investigate both electronic and magnetic properties of the BaMnO₃ perovskite. Spin-polarised calculations, including the spin-orbit interaction, are used to determine the energy of the ferromagnetic (FM) and antiferromagnetic (AFM) states of BaMnO₃ perovskite. Obtained data from ab initio calculations are used as input for the Monte Carlo simulations to compute other magnetic parameters. Magnetisation, specific heat and magnetic entropy change have been given using the Monte Carlo simulations. The adiabatic temperature change, transition temperature and relative cooling power have been established.     

Ideal-gas properties of new refrigerants from quantum mechanical ab initio calculations

Theoretical predictions of ideal-gas properties from molecular data such as structure, vibrational frequencies, and the barrier of internal rotation are compared to recent experimental data on heat capacities of new refrigerants. It is demonstrated that the required molecular data can be obtained from quantum-mechanical ab initio calculations with sufficient accuracy to provide heat capacities with an accuracy of ±2%. Further improvement of the approach appears feasible. This is of great practical significance, since molecular data obtained from experimental spectra tend to be inaccurate for systems of technical interest with somewhat larger molecules, like the new refrigerants.