Molecular dynamics simulations of porous silica in the density range 2.2–0.1 g/cm3 are carried out on a 41 472 particle system using a multiple instruction multiple data computer. The internal surface area, pore surf...
Molecular dynamics simulations of porous silica in the density range 2.2–0.1 g/cm3 are carried out on a 41 472 particle system using a multiple instruction multiple data computer. The internal surface area, pore surface-to-volume ratio, pore size distribution, fractal dimension, correlation length, and mean particle size are determined as a function of the density. Structural transition between a condensed amorphous phase and a low-density porous phase is characterized by these quantities. Various dissimilar porous structures with different fractal dimensions are obtained by controlling the preparation schedule and temperature.
Effects of atomic sizes on structural correlations in AX2-type disordered systems such as SiO2, GeSe2, and Ag2Se are studied using a charged-hard-sphere model and the hypernetted-chain scheme. Structural change is elu...
Effects of atomic sizes on structural correlations in AX2-type disordered systems such as SiO2, GeSe2, and Ag2Se are studied using a charged-hard-sphere model and the hypernetted-chain scheme. Structural change is elucidated as a function of the size ratio through the first sharp diffraction peak in the number-number structure factor (medium-range correlations), the A-X coordination [formation of A(X1/2)4 tetrahedra], and the principal peak in the charge-charge structure factor (charge ordering). We find a gradual change from SiO2-type to Ag2Se-type disordered structure in the range of 0.5
The structure of n-fold rings in vitreous SiO2 is investigated using molecular-dynamics configurations. A recently developed interaction potential for SiO2 consisting of long-range Coulomb interactions, the effect of ...
The structure of n-fold rings in vitreous SiO2 is investigated using molecular-dynamics configurations. A recently developed interaction potential for SiO2 consisting of long-range Coulomb interactions, the effect of electronic polarizability, and three-body covalent forces is used in the molecular-dynamics study of the vitreous state. Results for the statistics of rings and distribution of interatomic distances and bond angles in the rings are presented for the vitreous state. The statistics of rings for the molten state is also discussed.
Algorithms are designed to implement molecular‐dynamics simulations on emerging concurrent architectures. For systems with finite‐range interactions, a domain decomposition algorithm is used to implement the multipl...
A quantum dynamical simulation method is developed to investigate coupled many electron-phonon systems. Both electron and phonon wave functions are numerically propagated in time. The method is applied to the study of...
A quantum dynamical simulation method is developed to investigate coupled many electron-phonon systems. Both electron and phonon wave functions are numerically propagated in time. The method is applied to the study of resonant tunneling of an electron through double quantum dots. Phonon-induced electron localization is observed. The space-splitting Schrödinger solver and dynamical-simulated-annealing Poisson solver are implemented on an 8,192-node MP-1 computer from MasPar.
We study the atomic vibrational dynamics in silica glass (a-SiO2) using molecular-dynamics (MD) simulations and classical lattice dynamics method. The SiO2 glasses were generated by molecular-dynamics and steepest-des...
We study the atomic vibrational dynamics in silica glass (a-SiO2) using molecular-dynamics (MD) simulations and classical lattice dynamics method. The SiO2 glasses were generated by molecular-dynamics and steepest-descent quench (SDQ) using an effective interatomic potential consisting of two-body and three-body interactions. The frequency and eigenvectors of vibrational normal modes are obtained by diagonalization of the dynamical matrix. The partial and total vibrational density of states (DOS), bond-projected DOS, participation ratio (PR), and neutron-weighted dynamic structure factor are calculated. The results are compared with inelastic neutron scattering experiments on SiO2 glass.
Molecular dynamics (MD) method is used to investigate structural transformation and the loss of intermediate range order in SiO2 glass at very large positive pressures and the modification of SiO2 glass network at ver...
Molecular dynamics (MD) method is used to investigate structural transformation and the loss of intermediate range order in SiO2 glass at very large positive pressures and the modification of SiO2 glass network at very large negative pressures. The nature of molecular vibrations in solid C60 has been studied with tight binding molecular dynamics (TBMD) method. Implementations of simulation algorithms on parallel computers are also discussed. In a-SiO2 at high pressures, the height of the first sharp diffraction peak in S(q) is considerably diminished and its position shifts to larger wave vectors. At twice the normal density, Si-O bond length increases, Si-O coordination changes from 4 to 6, and O-Si-O band-angle changes from 109° to 90°. This is clearly a tetrahedral to octahedral transformation, which is observed recently by Meade, Hemley, and Mao in their diffraction experiments using synchrotron radiation. MD simulations of porous silica are carried out in the density range 2.2 - 0.1 g/cm3Internal surface area, pore surface-to-volume ratio, gyration radius, and fractal dimension are studied as a function of density. simulations are in good agreement with the experimental results obtained by x-ray scattering. The results reveal a crossover of the structural correlations between short- to intermediate-range (< 8 Å) and fractal- to large-scale-regime (10~ 100 Å). Dispersion and density of states (DOS) of inter- and intra-molecular phonons are calculated for orientationally ordered and disordered solid C60 using the TBMD method. Inter-molecular phonon DOS extends up to 7.6 meV and shows libron peaks at 2.4 meV and 3.7 meV in the orientationally ordered phase. Orientational disorder softens libron modes. Intra-molecular phonons below 70 meV also show significant dispersion. Our results are in good agreement with the recent inelastic neutron scattering experiments. MD is a numerical approach which involves the solution of Newton's equations for particles in the system
Superconductivity in Ba1−xKxBiO3 is studied within the framework of Eliashberg theory using a model of the electron-phonon interaction, α2F(ω). The model is based upon molecular-dynamics calculations of the phonon d...
Superconductivity in Ba1−xKxBiO3 is studied within the framework of Eliashberg theory using a model of the electron-phonon interaction, α2F(ω). The model is based upon molecular-dynamics calculations of the phonon density of states, which are in good agreement with inelastic-neutron-scattering experiments. The function α(ω) is constructed using information from electron-tunneling experiments. Our model is constructed with the premise that the electron-phonon coupling constant λ≊1 and that strong electron-phonon coupling exists for high-energy (30–60 meV) phonon modes. Superconducting properties of the material at zero and finite temperatures are calculated and compared with experiments.
The nature of tetrahedral molecular fragments is investigated in SiSe2 glasses using the molecular-dynamics method. The glass consists of both edge-sharing (ES) and corner-sharing tetrahedra. The ES tetrahedra are the...
The nature of tetrahedral molecular fragments is investigated in SiSe2 glasses using the molecular-dynamics method. The glass consists of both edge-sharing (ES) and corner-sharing tetrahedra. The ES tetrahedra are the building blocks of chain-like-molecular fragments. The two-edge-sharing tetrahedra are the nucleus, and corner-sharing configurations provide connecting hinges between fragments. Statistics of rings and fragments reveals that threefold and eightfold rings are most abundant, chainlike fragments that are typically 10–15 Å long occur mostly in eightfold rings, and the longest fragments occur in elevenfold rings.
We study the tunneling-current characteristics of a double-barrier resonant-tunneling diode connected to an electron reservoir. The dynamics of electrons in the reservoir is treated explicitly in the framework of the ...
We study the tunneling-current characteristics of a double-barrier resonant-tunneling diode connected to an electron reservoir. The dynamics of electrons in the reservoir is treated explicitly in the framework of the time-dependent density-functional theory. It is found that Coulomb correlations give rise to plasmon-assisted tunneling.
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