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We present a computational study of the adhesive and structural properties of the Al/Al2O3 interfaces as building blocks of the Metal-Insulator-Metal (MIM) tunnel devices, where electron transport is accomplished via tunnelling mechanism through the sandwiched insulating barrier. The main goal of this paper is to understand, on the atomic scale, the role of the geometrical details in the formation of the tunnel barrier profiles. To provide reliable results, we carefully assess the accuracy of the traditional methods used to examine Al/Al2O3 interfaces. These are the most widely employed exchange-correlation functionals, LDA, PBE and PW91, the Universal Binding Energy Relation (UBER) for predicting equilibrium interfacial distances and adhesion energies, and the ideal work of separation as a measure of junction stability. Finally, we perform a detailed analysis of the atomic and interplanar relaxations in each junction. Our results imply that the structural irregularities on the surface of the Al film have a significant contribution to lowering the tunnel barrier height, while interplanar relaxations in the Al film, away from the immediate interface do not have a notable impact on the tunnelling properties. On the other hand, up to 5-7 layers of Al2O3 may be involved in shaping the tunnel barriers. Interplanar relaxations of these layers depend on the geometry of the interface and may result in the net contraction by 13% relative to the corresponding thickness in the bulk oxide. This is a significant amount as the tunnelling probability depends exponentially on the barrier width., Comment: 13 pages, 5 figures

We investigate how different interface geometries of an Al/Al$_2$O$_3$ junction, a common component of modern tunnel devices, affect electron transport through the tunnel barrier. We study six distinct Al/Al$_2$O$_3$ interfaces which differ in stacking sequences of the metal and the oxide surface atoms and the oxide termination. To construct model potential barrier profiles for each examined geometry, we rely on first-principles density-functional theory (DFT) calculations for the barrier heights and the shapes of the interface regions as well as on experimental data for the barrier widths. We show that even tiny variations in the atomic arrangement at the interface cause significant changes in the tunnel barrier parameters and, consequently, in electron transport properties. Especially, we obtain that variations in the crucial barrier heights and widths can be as large as 2 eV and 5 \AA, respectively. Finally, to gain information about the average properties of the measured junction, we fit the conductance calculated within the WKB approximation to the experimental data and interpret the fit parameters with the help of the DFT results., Comment: 9 pages, 8 figures

We present two approaches for studying the uniformity of a tunnel barrier. The first approach is based on measuring single-electron and two-electron tunneling in a hybrid single-electron transistor. Our measurements indicate that the effective area of a conduction channel is about one order of magnitude larger than predicted by theoretical calculations. With the second method, transmission electron microscopy, we demonstrate that variations in the barrier thickness are a plausible explanation for the larger effective area and an enhancement of higher order tunneling processes., Comment: 4 pages, 2 figures

We investigate the effect of spin-orbit coupling on the behavior of magnetic impurity at the edge of a zigzag graphene ribbon by means of quantum Monte Carlo simulations. A peculiar interplay of Kane-Mele type spin-orbit and impurity-host coupling is found to affect local properties such as the impurity magnetic moment and spectral densities. The special helical nature of the topological insulator on the edge is found to affect nonlocal quantities, such as the two-particle and spin-spin correlation functions linking electrons on the impurity with those in the conduction band., Comment: 12 figures. And a shorter version will be published in PRB

We apply a range of density-functional-theory-based methods capable of describing van der Waals interactions to weakly bonded layered solids in order to investigate their accuracy for extended systems. The methods under investigation are the local density approximation, semi-empirical force fields, non-local van der Waals density functionals and the random-phase approximation. We investigate the equilibrium geometries, elastic constants and the binding energies of a large and diverse set of compounds and arrive at conclusions about the reliability of the different methods. The study also points to some directions of further development for the non-local van der Waals density functionals., Comment: Accepted for publication in Journal of Physics: Condensed Matter

We use computational and experimental methods to study d0 ferromagnetism at a charge- imbalanced interface between two perovskites. In SrTiO3/KTaO3 superlattice calculations, the charge imbalance introduces holes in the SrTiO3 layer, inducing a d0 ferromagnetic half-metallic 2D electron gas at the interface oxygen 2p orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a gen- eral property of hole-type d0 perovskite interfaces. Atomically sharp epitaxial d0 SrTiO3/KTaO3, SrTiO3 /KNbO3 and SrTiO3 /NaNbO3 interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest the behavior is due to high density of states and exchange coupling at the oxygen t1g band in comparison with the more studied d band t2g symmetry electron gas., Comment: 5 pages, 5 figures

Although the precise microscopic knowledge of van der Waals interactions is crucial for understanding bonding in weakly bonded layered compounds, very little quantitative information on the strength of interlayer interaction in these materials is available, either from experiments or simulations. Here, using many-body perturbation and advanced density-functional theory techniques, we calculate the interlayer binding and exfoliation energies for a large number of layered compounds and show that, independent of the electronic structure of the material, the energies for most systems are around 20 meV/\AA$^2$. This universality explains the successful exfoliation of a wide class of layered materials to produce two-dimensional systems, and furthers our understanding the properties of layered compounds in general., Comment: Accepted for publication in Physical Review Letters. Includes Supplementary Material

In piezoelectric ceramics the changes in the phase stabilities versus stress and temperature in the vicinity of the phase boundary play a central role. The present study was dedicated to the classical piezoelectric, lead-zirconate-titanate (PZT) ceramic with composition Pb(Zr$_{0.54}$Ti$_{0.46}$)O$_3$ at the Zr-rich side of the morphotropic phase boundary at which both intrinsic and extrinsic contributions to piezoelectricity are significant. The pressure-induced changes in this two-phase (rhombohedral $R3c$+monoclinic $Cm$ at room temperature and $R3c+P4mm$ above 1 GPa pressures) system were studied by high-pressure neutron powder diffraction technique. The experiments show that applying pressure favors the $R3c$ phase, whereas the $Cm$ phase transforms continuously to the $P4mm$, which is favored at elevated temperatures due to the competing entropy term. The $Cm\rightarrow R3c$ phase transformation is discontinuous. The transformation contributes to the extrinsic piezoelectricity. An important contribution to the intrinsic piezoelectricity was revealed: a large displacement of the $B$ cations (Zr and Ti) with respect to the oxygen anions is induced by pressure. Above 600 K a phase transition to a cubic phase took place. Balance between the competing terms dictates the curvature of the phase boundary. After high-pressure experiments the amount of rhombohedral phase was larger than initially, suggesting that on the Zr-rich side of the phase boundary the monoclinic phase is metastable., Comment: 6 figures

With quantum Monte Carlo methods, we investigate the consequences of placing a magnetic adatom adjacent to a vacancy in a graphene sheet. We find that instead of the adatom properties depending on the energy of the adatom orbital, as in a single impurity problem, they develop a dependence on the energy of the split localized state associated with the single vacancy problem. Shifting the chemical potential through this experimentally more accessible energy scale reveals novel behavior in the spectral density, magnetic susceptibility, and the correlations of the adatom spin and charge with those of the conduction electrons. In general, the behavior of the adatom in the presence of a vacancy differs significantly from its behavior in the absence of a vacancy., Comment: 5 pages and 5 figures, accepted by Physical Review B

Motivated by recent in situ studies of carbon nanotube growth from large transition-metal nanoparticles, we study various alpha-iron (ferrite) facets at different carbon concentrations using ab initio methods. The studied (110), (100) and (111) facets show qualitatively different behaviour when carbon concentration changes. In particular, adsorbed carbon atoms repel each other on the (110) facet, resulting in carbon dimer and graphitic material formation. Carbon on the (100) facet forms stable structures at concentrations of about 0.5 monolayer and at 1.0 monolayer this facet becomes unstable due to a frustration of the top layer iron atoms. The stability of the (111) facet is weakly affected by the amount of adsorbed carbon and its stability increases further with respect to the (100) facet with increasing carbon concentration. The exchange of carbon atoms between the surface and sub-surface regions on the (111) facet is easier than on the other facets and the formation of carbon dimers is exothermic. These findings are in accordance with a recent in situ experimental study where the existence of graphene decorated (111) facets is related to increased carbon concentration.

In an attempt to understand why catalytic methods for the growth of boron nitride nanotubes work much worse than for their carbon counterparts, we use first-principles calculations to study the energetics of elemental reactions forming N2, B2 and BN molecules on an iron catalyst. We observe that in the case of these small molecules, the catalytic activity is hindered by the formation of B2 on the iron surface. We also observe that the local morphology of a step edge present in our nanoparticle model stabilizes the boron nitride molecule with respect to B2 due to the ability of the step edge to offer sites with different coordination simultaneously for nitrogen and boron. Our results emphasize the importance of atomic steps for a high yield chemical vapor deposition growth of BN nanotubes and may outline new directions for improving the efficiency of the method., Comment: submitted to physical review B

Fullerene functionalized carbon nanotubes -- NanoBuds -- form a novel class of hybrid carbon materials, which possesses many advantageous properties as compared to the pristine components. Here, we report a theoretical study of the electronic transport properties of these compounds. We use both ab initio techniques and tight-binding calculations to illustrate these materials' transmission properties, and give physical arguments to interpret the numerical results. Specifically, above the Fermi energy we find a strong reduction of electron transmission due to localized states in certain regions of the structure while below the Fermi energy all considered structures exhibit a high-transmission energy band with a geometry dependent width.

Our first-principles computations show that the ground state of PbTiO$_3$ under hydrostatic pressure transforms discontinuously from $P4mm$ to $R3c$ at 9 GPa. Spontaneous polarization decreases with increasing pressure so that the $R3c$ phase transforms to the centrosymmetric $R\bar{3}c$ phase at around 30 GPa. The first-order phase transition between tetragonal and rhombohedral phase is exceptional since there is no evidence for a bridging phase. The essential feature of the $R3c$ and $R\bar{3}c$ phases is that they allow the oxygen octahedron to increase its volume $V_B$ at the expense of cuboctahedral volume $V_A$ around a Pb ion. This is further supported by the fact that neither the $R3m$ nor $Cm$ phase, which keep the $V_A/V_B$ ratio constant, is a ground state within the pressure range between 0 and 40 GPa. Thus tetragonal strain is dominant up to 9 GPa, whereas at higher pressures efficient compression through oxygen octahedra tilting plays the central role for PbTiO$_3$. Previously predicted pressure induced colossal enhancement of piezoelectricity in PbTiO$_3$ corresponds to unstable $Cm$ and $R3m$ phases. This suggests that the phase instability, in contrast to the polarization rotation, is responsible for the large piezoelectric properties observed in systems like Pb(Zr,Ti)O$_3$ in the vicinity of the morphotropic phase boundary.

We study effects of electron-electron interactions and confinement potential on the magneto-optical absorption spectrum in the far-infrared range of lateral quantum dot molecules. We calculate far-infrared (FIR) spectra for three different quantum dot molecule confinement potentials. We use accurate exact diagonalization technique for two interacting electrons and calculate dipole-transitions between two-body levels with perturbation theory. We conclude that the two-electron FIR spectra directly reflect the symmetry of the confinement potential and interactions cause only small shifts in the spectra. These predictions could be tested in experiments with nonparabolic quantum dots by changing the number of confined electrons. We also calculate FIR spectra for up to six noninteracting electrons and observe some additional features in the spectrum., Comment: For better quality Figs download manuscript from http://www.fyslab.hut.fi/~mma/FIR/Helle_qdmfir.ps.gz

Laterally coupled quantum dot molecules are studied using exact diagonalization techniques. We examine the two-electron singlet-triplet energy difference as a function of magnetic field strength and investigate the magnetization and vortex formation of two- and four-minima lateral quantum dot molecules. Special attention is paid to the analysis of how the distorted symmetry affects the properties of quantum-dot molecules., Comment: 18 pages, 26 figures

Atomic oxygen adsorption on the Mo(112) surface has been investigated by means of first-principles total energy calculations. Among the variety of possible adsorption sites it was found that the bridge sites between two Mo atoms of the topmost row are favored for O adsorption at low and medium coverages. At about one monolayer coverage oxygen atoms prefer to adsorb in a quasi-threefold hollow sites coordinated by two first-layer Mo atoms and one second layer atom. The stability of a structural model for an oxygen-induced $p(2\times 3)$ reconstruction of the missing-row type is examined., Comment: 6 pages, 6 postscript figures, RevTeX

We study ground states and far-infrared spectra (FIR) of two electrons in four-minima quantum-dot molecule in magnetic field by exact diagonalization. Ground states consist of altering singlet and triplet states, whose frequency, as a function of magnetic field, increases with increasing dot-dot separation. When the Zeeman energy is included, only the two first singlet states remain as ground states. In the FIR spectra, we observe discontinuities due to crossing ground states. Non-circular symmetry induces anticrossings, and also an additional mode above $\omega_+$ in the spin-triplet spectrum. In particular, we conclude that electron-electron interactions cause only minor changes to the FIR spectra and deviations from the Kohn modes result from the low-symmetry confinement potential., Comment: 4 pages, 3 figures, QD2004 conference paper, accepted in Physica E

We study electronic structures of two-dimensional quantum dots in high magnetic fields using the density-functional theory (DFT) and the exact diagonalization (ED). With increasing magnetic field, beyond the formation of the totally spin-polarized maximum density droplet (MDD) state, the DFT gives the ground-state total angular momentum as a continuous function with well-defined plateaus. The plateaus agree well with the magic angular momenta of the ED calculation. By constructing a conditional wave function from the Kohn-Sham states we show that vortices enter the quantum dot one-by-one at the transition to the state with the adjacent magic angular momentum. Vortices are also observed outside the high-density region of the quantum dot. These findings are compared to the ED results and we report a significant agreement. We study also interpretations and limitations of the density functional approach in these calculations., Comment: 4 pages, 5 figures, QD2004 conference paper

We have studied the single-electron transport spectrum of a quantum dot in GaAs/AlGaAs resonant tunneling device. The measured spectrum has irregularities indicating a broken circular symmetry. We model the system with an external potential consisting of a parabolic confinement and a negatively charged Coulombic impurity placed in the vicinity of the quantum dot. The model leads to a good agreement between the calculated single-electron eigenenergies and the experimental spectrum. Furthermore, we use the spin-density-functional theory to study the energies and angular momenta when the system contains many interacting electrons. In the high magnetic field regime the increasing electron number is shown to reduce the distortion induced by the impurity., Comment: 6 pages, 8 figures, submitted to PRB

We study electronic structures of two-dimensional quantum dots in strong magnetic fields using mean-field density-functional theory and exact diagonalization. Our numerically accurate mean-field solutions show a reconstruction of the uniform-density electron droplet when the magnetic field flux quanta enter one by one the dot in stronger fields. These quanta correspond to repelling vortices forming polygonal clusters inside the dot. We find similar structures in the exact treatment of the problem using a new type of operator for the analysis. We discuss important differences and limitations of the methods used., Comment: 4 pages, 4 figures

We use density-functional methods to study the effects of an external magnetic field on two-dimensional quantum dots with a rectangular hard-wall confining potential. The increasing magnetic field leads to spin polarization and formation of a highly inhomogeneous maximum-density droplet at the predicted magnetic field strength. At higher fields, we find an oscillating behavior in the electron density and in the magnetization of the dot. We identify a rich variety of phenomena behind the periodicity and analyze the complicated many-electron dynamics, which is shown to be highly dependent on the shape of the quantum dot., Comment: 6 pages, 6 figures, submitted to Phys. Rev. B

We present a detailed analysis of the broken-symmetry mean-field solutions using a four-electron rectangular quantum dot as a model system. Comparisons of the density-functional theory predictions with the exact ones show that the symmetry breaking results from the single-configuration wave function used in the mean-field approach. As a general cure we present a scheme that systematically incorporates several configurations into the density-functional theory and restores the symmetry. This cure is easily applicable to any density-functional approach., Comment: 4 pages, 4 figures, submitted to PRL

We study the effects of electron-electron correlations and confinement potential on the far-infrared spectrum of a lateral two-electron quantum dot molecule by exact diagonalization. The calculated spectra directly reflect the lowered symmetry of the external confinement potential. Surprisingly, we find interactions to drive the spectrum towards that of a high-symmetry parabolic quantum dot. We conclude that far-infrared spectroscopy is suitable for probing effective confinement of the electrons in a quantum dot system, even if interaction effects cannot be resolved in a direct fashion., Comment: 4 pages, 2 figures

The interplay between clustering and exchange coupling in magnetic semiconductors for the prototype (Ga_{1-x},Mn_x)As with manganese concentrations x of 1/16 and 1/32 in the interesting experimental range is investigated. For x ~ 6 %, when all possible arrangements of two atoms within a large supercell are considered, the clustering of Mn atoms at nearest-neighbour Ga sites is energetically preferred. As shown by spin density analysis, this minimum energy configuration localizes further one hole and reduces the effective charge carrier concentration. Also the exchange coupling constant increases to a value corresponding to lower Mn concentrations with decreasing inter Mn distance., Comment: Accepted for publication in Journal of Physics: Condensed Matter

Ferromagnetic \nmg has unique magnetoelastic properties. These are investigated by detailed computational studies of the phonon dispersion curves for the non-modulated cubic \Ltw and tetragonal structures. For the \Ltw\ structure, a complete softening of the transverse acoustic mode has been found around the wave vector $\mathbf{q}=[1/3,1/3,0](2 \pi/a)$. The softening of this \TA{2} phonon mode leads to the premartensitic modulated super-structure observed experimentally. Further phonon anomalies, related to other structural transformations in \nmg, have also been found and examined. These anomalies appear to be due to the coupling of particular acoustic phonon modes and optical modes derived from Ni., Comment: 4 pages, 4 figures

We study the ground state properties of rectangular quantum dots by using the spin-density-functional theory and quantum Monte Carlo methods. The dot geometry is determined by an infinite hard-wall potential to enable comparison to manufactured, rectangular-shaped quantum dots. We show that the electronic structure is very sensitive to the deformation, and at realistic sizes the non-interacting picture determines the general behavior. However, close to the degenerate points where Hund's rule applies, we find spin-density-wave-like solutions bracketing the partially polarized states. In the quasi-one-dimensional limit we find permanent charge-density waves, and at a sufficiently large deformation or low density, there are strongly localized stable states with a broken spin-symmetry., Comment: 8 pages, 9 figures, submitted to PRB

We study a model quantum dot system in an external magnetic field by using both the spin-density-functional theory and the current-spin-density-functional theory. The theories are used with local approximations for the spin-density and the vorticity. The reliabilities of different parametrizations for the exchange-correlation functionals are tested by comparing the ensuing energetics with quantum Monte Carlo results. The limit where the vorticity dependence should be used in the exchange-correlation functionals is discussed., Comment: 18 pages, 3 figures, 5 eps-figure files

Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy showing large magnetic field induced strains. We present here results for the magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and theory. Experimental trends for the composition dependence of the magnetization are measured by a vibrating sample magnetometer (VSM) in magnetic fields of up to several tesla and at low temperatures. The saturation magnetization has a maximum near the stoichiometric composition and it decreases with increasing Mn content. This unexpected behaviour is interpreted via first-principles calculations within the density-functional theory. We show that extra Mn atoms are antiferromagnetically aligned to the other moments, which explains the dependence of the magnetization on composition. In addition, the effect of Mn doping on the stabilization of the structural phases and on the magnetic anisotropy energy is demonstrated., Comment: 4 pages, 3 figures

We report here non-collinear magnetic configurations in the Heusler alloys Ni2MnGa and Ni2MnAl which are interesting in the context of the magnetic shape memory effect. The total energies for different spin spirals are calculated and the ground state magnetic structures are identified. The calculated dispersion curves are used to estimate the Curie temperature which is found to be in good agreement with experiments. In addition, the variation of the magnetic moment as a function of the spiral structure is studied. Most of the variation is associated with Ni, and symmetry constraints relevant for the magnetization are identified. Based on the calculated results, the effect of the constituent atoms in determining the Curie temperature is discussed., Comment: 7 pages, 10 figures

We investigate the properties of many-electron systems in two-dimensional polygonal (triangle, square, pentagon, hexagon) potential wells by using the density functional theory. The development of the ground state electronic structure as a function of the dot size is of particular interest. First we show that in the case of two electrons, the Wigner molecule formation agrees with the previous exact diagonalization studies. Then we present in detail how the spin symmetry breaks in polygonal geometries as the spin density functional theory is applied. In several cases with more than two electrons, we find a transition to the crystallized state, yielding coincidence with the number of density maxima and the electron number. We show that this transition density, which agrees reasonably well with previous estimations, is rather insensitive to both the shape of the dot and the electron number., Comment: 8 pages, 11 figures

A general real-space multigrid algorithm MIKA (Multigrid Instead of the K-spAce) for the self-consistent solution of the Kohn-Sham equations appearing in the state-of-the-art electronic-structure calculations is described. The most important part of the method is the multigrid solver for the Schr\"odinger equation. Our choice is the Rayleigh quotient multigrid method (RQMG), which applies directly to the minimization of the Rayleigh quotient on the finest level. Very coarse correction grids can be used, because there is in principle no need to represent the states on the coarse levels. The RQMG method is generalized for the simultaneous solution of all the states of the system using a penalty functional to keep the states orthogonal. Special care has been taken to optimize the iterations towards the self-consistency and to run the code in parallel computer architectures. The scheme has been implemented in multiple geometries. We show examples from electronic structure calculations employing nonlocal pseudopotentials and/or the jellium model. The RQMG solver is also applied for the calculation of positron states in solids., Comment: To appear in a special issue of Int J. Quant. Chem. devoted to the conference proceedings of 9th International Conference on the Applications of the Density Functional Theory in Chemistry and Physics

We examine the conductance properties of a chain of Na atoms between two metallic leads in the limit of low bias. Resonant states corresponding to the conductance channel and the local charge neutrality condition cause conductance oscillations as a function of the number of atoms in the chain. Moreover, the geometrical shape of the contact leads influences the conductivity by giving rise to additional oscillations as a function of the lead opening angle., Comment: 5 pages, 7 figures

We study a two-electron quantum dot molecule in a magnetic field by the direct diagonalization of the Hamiltonian matrix. The ground states of the molecule with the total spin S=0 and S=1 provide a possible realization for a qubit of a quantum computer. Switching between the states is best achieved by changing the magnetic field. Based on an analysis of the wave function, we show that the system consists of composite particles formed by an electron and flux quanta attached to it. This picture can also be used to explain the spin phase diagram., Comment: 4 pages, 6 figures

Structural properties of a finite number ($N = 2 - 20$) of point charges (classical electrons) confined laterally in a two-dimensional two-minima potential are calculated as a function of the distance ($d$) between the minima. The particles are confined by identical parabolic potentials and repel each other through a Coulomb potential. Both ground state and metastable electron configurations are discussed. At zero distance previous results of other calculations and experiments are reproduced. Discontinuous transitions from one configuration to another as a function of $d$ are observed for $N = 6, 8, 11, 16, 17, 18, 19$., Comment: Submitted to Phys. Rev. B

Using variational quantum Monte Carlo method, the effect of Landau-level mixing on the lowest-energy--state diagram of small quantum dots is studied in the magnetic field range where the density of magnetic flux quanta just exceeds the density of electrons. An accurate analytical many-body wave function is constructed for various angular momentum and spin states in the lowest Landau level, and Landau-level mixing is then introduced using a Jastrow factor. The effect of higher Landau levels is shown to be significant; the transition lines are shifted considerably towards higher values of magnetic field and certain lowest-energy states vanish altogether., Comment: 4 pages, 2 figures. Submitted to Phys. Rev. B

We study two-dimensional quantum dots using the variational quantum Monte Carlo technique in the weak-confinement limit where the system approaches the Wigner molecule, i.e., the classical solution of point charges in an external potential. We observe the spin-polarization of electrons followed by a smooth transition to a Wigner-molecule-like state as the confining potential is made weaker., Comment: 7 pages, 11 figures

We investigate electronic structures of Al quantum wires, both unsupported and supported on the (100) NaCl surface, using the density-functional theory. We confirm that unsupported nanowires, constrained to be linear, show magnetization when elongated beyond the equilibrium length. Allowing ions to relax, the wires deform to zig-zag structures with lower magnetization but no dimerization occurs. When an Al wire is deposited on the NaCl surface, a zig-zag geometry emerges again. The magnetization changes moderately from that for the corresponding unsupported wire. We analyse the findings using electron band structures and simple model wires., Comment: submitted to PHys. Rev. B

A general real-space multigrid algorithm for the self-consistent solution of the Kohn-Sham equations appearing in the state-of-the-art electronic-structure calculations is described. The most important part of the method is the multigrid solver for the Schroedinger equation. Our choice is the Rayleigh quotient multigrid method (RQMG), which applies directly to the minimization of the Rayleigh quotient on the finest level. Very coarse correction grids can be used, because there is no need to be able to represent the states on the coarse levels. The RQMG method is generalized for the simultaneous solution of all the states of the system using a penalty functional to keep the states orthogonal. The performance of the scheme is demonstrated by applying it in a few molecular and solid-state systems described by non-local norm-conserving pseudopotentials., Comment: 9 pages, 3 figures

We present first-principles approaches based on density functional theory for calculating positron states and annihilation characteristics in condensed matter. The treatment of the electron-positron correlation effects (the enhancement of the electron density at the positron with respect to mean-field density) is shown to play a crucial role when calculating the annihilation rates. A generalized gradient approximation (GGA) takes the strong inhomogeneities of the electron density in the ion core region into account and reproduces well the experimental total annihilation rates (inverses of the positron lifetimes) by suppressing the rates given by a local density approximation (LDA). The GGA combined with an electron-state-dependent enhancement scheme gives a good description for the momentum distributions of the annihilating positron-electron pairs reproducing accurately the trends observed in the angular correlation (ACAR) or Doppler broadening measurements of the annihilation radiation. The combination of the present positron lifetime and momentum density calculations with the corresponding measurements yields a unique tool for defect identification. Especially, the investigation of various vacancy-type defects in semiconductors able to trap positrons will be an important field for these methods. We will show that the identification of vacancy-impurity complexes in highly n-Type Si and the study of the SiO$_2$/Si interface are particularly interesting applications., Comment: 6 pages, 3 figures, MRS style (enclosed), to appear in MRS 1999 Fall Meeting Proc. 579; added acknowledgments

We present a study of the effects of Nitrogen and Boron doping on the growth of CVD diamond in the (100) mechanism. These are the first calculations of this type and show that, in accordance with recent experimental results, the presently accepted growth mechanism, that due to Harris, is cataylsed by the presence of subsurface Boron impurities. In contrast, we find that the Harris Mechanism cannot explain growth in the presence of subsurface N and suggest an alternative mechanism., Comment: 6 pages, 5 figures, 1 table

Zn in GaN forms an efficient radiative center and acts as a deep acceptor which can make the crystal insulating. Four different Zn-related centers have been by now identified, leading to light emission in the range between 1.8 eV and 2.9 eV. We present a first-principles investigation total energy and electronic structure calculations for Ga-substitutional and hetero-antisite N-substitutional Zn in wurtzite GaN, using ultrasoft pseudopotentials and a conjugate-gradient total energy minimization method. Our results permit the identification of the blue-light emission center as the substitutional acceptor, while contrary to a common belief the Zn_N heteroantisite has a very high formation energy and donor behavior, which seems to exclude it as the origin of the other centers., Comment: 4 pages, to appear in "The Physics of Semiconductors"