On a Thomas-Fermi model of "hollow" atom
A Thomas-Fermi model of a spherical shell of positive charge is investigated, under various boundary conditions. The electron distribution and the ionization charge are given particular attention.
chem-ph↗
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Many-body and Covalence Effects in the Polarization of Ferroelectric Perovskites
The ferroelectric polarization of perovskite oxides is much larger than implied by displacement of static ionic charges. We use an explicitly correlated scheme to investigate the phenomenon; charge transport is evaluated as a geometric quantum phase. Both covalence and electron-electron interaction enhance polarization in the weakly correlated regime. At higher values of the electron-electron interaction, the system undergoes a transition from a band insulator to a Mott insulator: the static ionic charge is continuous across the transition, whereas the polarization is discontinuous. Above the transition, oxygen transports a positive charge.
cond-mat↗
Model for the thermal conductivity of a quasicrystalline alloy
We present a simple model for the observed temperature dependence of the thermal conductivity of AlPdMn (Chernikov et al, Phys.Rev.B vol.51, (1995),153). We account for the low temperature data, upto and including the plateau of the thermal conductivity, and discuss the similarities and the differences between the quasicrystal and amorphous solids.
cond-mat↗
On thermoelastic phenomena around orientational ordering transition in crystalline C60
Thermoelastic phenomena associated with orientational phase transition in solid C60 are considered. Coupling of the order parameter with elastic strain is analyzed in the Landau theory of phase transitions. Wide range of possible coexistence of the FCC phase with SC one in C60 has been found from 260 K down to critical temperature 160 K of the instability of high-symmetry phase. The specific heat, thermal expansion coefficient and isothermal compressibility for low-symmetry phase are calculated as function of temperature and pressure in an agreement with reported data. Applicability of the Landau theory to ordering transition in fullerenes is discussed.
cond-mat↗
Exciton effects and nonlinear optical response in soliton lattice states of doped conjugated polymers
Exciton effects on conjugated polymers are investigated in the soliton lattice system. We use the Su-Schrieffer-Heeger model with long-range Coulomb interactions treated by the single-excitation configuration-interaction method. The soliton band is present in the Peierls gap of the doped system. There appears a new kind of the exciton where an electron-hole pair is excited between the soliton band and the continuum states. We find that the oscillator strengths accumulate rapidly at this exciton as the soliton concentration increases. The contribution from the lowest exciton is more than 90% at the 10% doping. The third-harmonic generation (THG) at off-resonance frequencies is calculated as functions of the soliton concentration and the chain length of the polymer. The optical nonlinearity by the THG at the 10% doping increases by the factor about 10^2 from that of the neutral system.
cond-mat↗
A fractal model for the first stages of thin film growth
In this paper we briefly review a model that describes the diffusion-controlled aggregation exhibited by particles as they are deposited on a surface. This model allows to understand many experiments of thin film deposition. In the first part, we describe the model, which incorporates deposition, particle and cluster diffusion, and aggregation. In a second part, we study the dynamical evolution of the model. Finally, we analyze the effects of small cluster mobility, and we show that the introduction of cluster diffusion dramatically affects the dynamics of film growth. Some of these effects can be tested experimentally.
cond-mat↗
Effect of Crack Blunting on Subsequent Crack Propagation
Theories of toughness of materials depend on an understanding of the characteristic instabilities of the crack tip, and their possible interactions. In this paper we examine the effect of dislocation emission on subsequent cleavage of a crack and on further dislocation emission. The work is an extension of the previously published Lattice Greens Function methodology. We have developed a Cavity Greens Function describing a blunt crack and used it to study the effect of crack blunting under a range of different force laws. As the crack is blunted, we find a small but noticeable increase in the crack loading needed to propagate the crack. This effect may be of importance in materials where a dislocation source near the crack tip in a brittle material causes the crack to absorb anti-shielding dislocations, and thus cause a blunting of the crack. It is obviously also relevant to cracks in more ductile materials where the crack itself may emit dislocations.
cond-mat↗
Electronic Structures in C60-Polymers (review)
Variations in the band structures of C60-polymers are studied, when conjugation conditions and the electron number are changed. We use a semiempirical model with the Su-Schrieffer-Heeger type electron-phonon interactions. In the neutral one-dimensional C60-polymer, electronic structures change among direct-gap insulators and the metal, depending on the degree of conjugations. High pressure experiments could observe such pressure-induced metal-insulator transitions. The C60-polymer doped with one electron per one molecule is always a metal. The energy difference between the highest-occupied state and the lowest-unoccupied state of the neutral system becomes smaller upon doping owing to the polaron effects. When the C60-polymer is doped with two electrons per one C60, the system is insulating. When the conjugation in the direction of the polymer chain is smaller, it is a direct-gap insulator. The energy gap becomes indirect when the conjugation is stronger. We also study the antiferromagnetic phase of AC60 by using a tight-binding model with long-range Coulomb interactions. The antiferromagnetism is well described by the model. The comparison with the photoemission studies shows that the new band around the Fermi energy of the AC60 phase can be explaind by the extremely large intrusion of an energy level into the gap of the neutral system. This indicates that the interaction effects among electrons are important in doped C60-polymers.
cond-mat.mtrl-sci↗
Electrostriction of Polar Glasses
We develop a finite temperature theory for the susceptibility and electrostriction of isotropic substances in which permanent electric dipoles are restrained from free rotation by elastic forces. All parameters are constrained by the measured susceptibility and elastic constants. When applied to polyurethane, the predicted electrostriction is approximately consistent wiht some of the wide range of data. The saturation of the electrostriction at high field may be explained qualitatively if the dipoles consist of several amide groups locked together by crystallization of the hard segments of the polymer.
cond-mat↗
Competing structural instabilities in cubic perovskites
We study the antiferrodistortive instability and its interaction with ferroelectricity in cubic perovskite compounds. Our first-principles calculations show that coexistence of both instabilities is very common. We develop a first-principles scheme to study the thermodynamics of these compounds when both instabilities are present, and apply it to SrTiO$_3$. We find that increased pressure enhances the antiferrodistortive instability while suppressing the ferroelectric one. Moreover, the presence of one instability tends to suppress the other. A very rich $P$--$T$ phase diagram results.
mtrl-th↗
Scalable Parallel Numerical Methods and Software Tools for Material Design
A new method of solution to the local spin density approximation to the electronic Schrödinger equation is presented. The method is based on an efficient, parallel, adaptive multigrid eigenvalue solver. It is shown that adaptivity is both necessary and sufficient to accurately solve the eigenvalue problem near the singularities at the atomic centers. While preliminary, these results suggest that direct real space methods may provide a much needed method for efficiently computing the forces in complex materials.
mtrl-th↗
Ab-initio Calculations of Energies and Self-Diffusion on Flat and Stepped Surfaces of Al and their Implications on Crystal Growth
Using density-functional theory we investigate several properties of Al(111), Al(100), Al(110), and stepped Al(111) surfaces. We report results of formation energies of surfaces, steps, adatoms, and vacancies. For the adsorption and diffusion of Al on flat regions of Al(111) surfaces we find the hcp site energetically slightly preferred over the fcc site. The energy barrier for self-diffusion on Al(111) is very low (0.04eV). Coming close to one of the two sorts of close packed, monoatomic steps on Al(111), labeled according to their {111} and {100} micro-facets, Al adatoms experience an attraction of <~ 0.1eV already before direct contact with the edge of the step. This attraction has a range of several atomic spacings and is of electronic origin. Upon arrival at the lower step edge, the adatom attaches with no barrier at a low energy five-fold coordinated site. Coming from the upper terrace, it incorporates into the step by an atomic exchange process, which has a barrier below 0.1eV for both sorts of close packed steps. The barrier for diffusion ...
mtrl-th↗
Frustrated H-induced Instability of Mo(110)
The large interest in the properties of transition metal surfaces has been recently fostered by inelastic helium atom scattering experiments carried out by Hulpke and Luedecke. Sharp and giant anomalies in the surface phonon dispersion curves along Gamma-H and Gamma-S have been detected on W(110) and Mo(110) at a coverage of one monolayer of hydrogen. At the same critical wave vectors a smaller second indentation is present in the experimental phonon branches. Recently we have proposed a possible interpretation which is able to explain this and other experiments. In this paper we discuss results of our recent ab initio calculations of the atomic and electronic properties of the clean and H-covered Mo(110) surface in more details. For the full monolayer coverage the calculated Fermi-surface contours are characterized by strong nesting features which originate the observed anomalies.
mtrl-th↗
Electrostatic Energy Calculations for Molecular Dynamics
The evaluation of Coulomb forces is a difficult task. The summations that are involved converge only conditionally and care has to be taken in selecting the appropriate procedure to define the limits. The Ewald method is a standard method for obtaining Coulomb forces, but this method is rather slow, since it depends on the square of the number of atoms in a unit cell. In this paper we have adapted the plane-wise summation method for the evaluation of Coulomb forces. The use of this method allows for larger computational cells in molecular dynamics calculations.
mtrl-th↗
The Influence of Gradient Corrections on Bulk and Surface Properties of TiO2 and SnO2
First-principles calculations based on density functional theory and the pseudo\-potential method have been used to investigate the influence of gradient corrections to the standard LDA technique on the equilibrium structure and energetics of rutile TiO$_2$ and SnO$_2$ perfect crystals and their (110) surfaces. We find that gradient corrections increase the calculated lattice parameters by roughly 3~\%, as has been found for other types of material. Gradient corrections give only very minor changes to the equilibrium surface structure, but reduce the surface energies by about 30~\%.
mtrl-th↗
Multigrid Methods in Electronic Structure Calculations
We describe a set of techniques for performing large scale ab initio calculations using multigrid accelerations and a real-space grid as a basis. The multigrid methods provide effective convergence acceleration and preconditioning on all length scales, thereby permitting efficient calculations for ill-conditioned systems with long length scales or high energy cut-offs. We discuss specific implementations of multigrid and real-space algorithms for electronic structure calculations, including an efficient multigrid-accelerated solver for Kohn-Sham equations, compact yet accurate discretization schemes for the Kohn-Sham and Poisson equations, optimized pseudo\-potentials for real-space calculations, efficacious computation of ionic forces, and a complex-wavefunction implementation for arbitrary sampling of the Brillioun zone. A particular strength of a real-space multigrid approach is its ready adaptability to massively parallel computer architectures, and we present an implementation for the Cray-T3D with essentially linear scaling of the execution time with the number of processors. The method has been applied to a variety of periodic and non-periodic systems, including disordered Si, a N impurity in diamond, AlN in the wurtzite structure, and bulk Al. The high accuracy of the atomic forces allows for large step molecular dynamics; e.g., in a 1 ps simulation of Si at 1100 K with an ionic step of 80 a.u., the total energy was conserved within 27 microeV per atom.
mtrl-th↗
The equilibrium shape of InAs quantum dots grown on a GaAs(001) substrate
The equilibrium shape of strained InAs quantum dots grown epitaxially on a GaAs(001) substrate is derived as a function of volume. InAs surface energies are calculated within density-functional theory, and a continuum approach is applied for the elastic relaxation energies.
mtrl-th↗
Surface Effects in Ferroelectrics: Periodic Slab Computations for BaTiO_3
Total energies, electronic structure, surface energies, polarization, potentials and charge densities were studied for slabs of BaTiO_3 using the Linearized Augmented Plane Wave (LAPW) method. The depolarization field inhibits ferroelectricity in the slabs, and the macroscopic field set up across a ferroelectric slab is sufficient to cause electronic states to span the gap and give a metallic band structure, but the band shifts are not rigid and O p states tend to pile up at the Fermi level. There are electronic surface states, especially evident on TiO_2 surfaces. The dangling bonds bond back to the surface Ti's and make the surface stable and reactive. The BaO surfaces are more ionic than the bulk.
mtrl-th↗