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At least 253 records · Page 14Linked to original sources

arXiv1996

Local Isoelectronic Reactivity of Solid Surfaces

The quantity w^N(r) = ( 1/ k^2 T_el)[partial n(r, T_el) / partial T_el]_(v(r),N) is introduced as a convenient measure of the local isoelectronic reactivity of surfaces. It characterizes the local polarizability of the surface and it can be calculated easily. The quantity w^N(r) supplements the charge transfer reactivity measured e.g. by the local softness to which it is closely related. We demonstrate the applicability and virtues of the function w^N(r) for the example of hydrogen dissociation and adsorption on Pd(100).

mtrl-th↗

arXiv1996

Structure and stability of a high-coverage (1x1) oxygen phase on Ru(0001)

The formation of chemisorbed O-phases on Ru(0001) by exposure to O_2 at low pressures is apparently limited to coverages Theta <= 0.5. Using low-energy electron diffraction and density functional theory we show that this restriction is caused by kinetic hindering and that a dense O overlayer (Theta = 1) can be formed with a (1x1) periodicity. The structural and energetic properties of this new adsorbate phase are analyzed and discussed in view of attempts to bridge the so-called "pressure gap" in heterogeneous catalysis. It is argued that the identified system actuates the unusually high rate of oxidizing reactions at Ru surfaces under high oxygen pressure conditions.

mtrl-th↗

arXiv1996

Anharmonic Decay of Vibrational States in Amorphous Silicon

Anharmonic decay rates are calculated for a realistic atomic model of amorphous silicon. The results show that the vibrational states decay on picosecond timescales and follow the two-mode density of states, similar to crystalline silicon, but somewhat faster. Surprisingly little change occurs for localized states. These results disagree with a recent experiment.

cond-mat↗

arXiv1996

Gross and Scheffler Reply

Among other things (e.g. steering and steric effects in dissociative adsorption) we had predicted that the initial sticking probability of H_2 molecules impinging at clean Pd(100) exhibits oscillations, reflecting the quantum nature of the scattering process. In the preceding comment Rettner and Auerbach (RA) analyze experimental results and conclude that these oscillations are not detectable and thus either not existing or at least very small. In this reply we argue that the experimental study of RA is not conclusive to rule out the existence of quantum oscillations in the scattering of H_2 and note several problems and incongruities in their study.

mtrl-th↗

arXiv1996

Event-based relaxation of continuous disordered systems

A computational approach is presented to obtain energy-minimized structures in glassy materials. This approach, the activation-relaxation technique (ART), achieves its efficiency by focusing on significant changes in the microscopic structure (events). The application of ART is illustrated with two examples: the structure of amorphous silicon, and the structure of Ni80P20, a metallic glass.

cond-mat↗

arXiv1996

Modeling of Covalent Bonding in Solids by Inversion of Cohesive Energy Curves

We provide a systematic test of empirical theories of covalent bonding in solids using an exact procedure to invert ab initio cohesive energy curves. By considering multiple structures of the same material, it is possible for the first time to test competing angular functions, expose inconsistencies in the basic assumption of a cluster expansion, and extract general features of covalent bonding. We test our methods on silicon, and provide the direct evidence that the Tersoff-type bond order formalism correctly describes coordination dependence. For bond-bending forces, we obtain skewed angular functions that favor small angles, unlike existing models. As a proof-of-principle demonstration, we derive a Si interatomic potential which exhibits comparable accuracy to existing models.

mtrl-th↗

arXiv1996

Reconstruction and thermal stability of the cubic SiC(001) surfaces

The (001) surfaces of cubic SiC were investigated with ab-initio molecular dynamics simulations. We show that C-terminated surfaces can have different c(2x2) and p(2x1) reconstructions, depending on preparation conditions and thermal treatment, and we suggest experimental probes to identify the various reconstructed geometries. Furthermore we show that Si-terminated surfaces exhibit a p(2x1) reconstruction at T=0, whereas above room temperature they oscillate between a dimer row and an ideal geometry below 500 K, and sample several patterns including a c(4x2) above 500 K.

mtrl-th↗

arXiv1996

Substrate-adsorbate coupling in CO-adsorbed copper

The vibrational properties of carbon monoxide adsorbed to the copper (100) surface are explored within density functional theory. Atoms of the substrate and adsorbate are treated on an equal footing in order to examine the effect of substrate--adsorbate coupling. This coupling is found to have a significant effect on the vibrational modes, particularly the in-plane frustrated translation, which mixes strongly with substrate phonons and broadens into a resonance. The predicted lifetime due to this harmonic decay mechanism is in excellent quantitative agreement with experiment.

mtrl-th↗

arXiv1996

Structure and Stability of Si(114)-(2x1)

We describe a recently discovered stable planar surface of silicon, Si(114). This high-index surface, oriented 19.5 degrees away from (001) toward (111), undergoes a 2x1 reconstruction. We propose a complete model for the reconstructed surface based on scanning tunneling microscopy images and first-principles total-energy calculations. The structure and stability of Si(114)-(2x1) arises from a balance between surface dangling bond reduction and surface stress relief, and provides a key to understanding the morphology of a family of surfaces oriented between (001) and (114).

mtrl-th↗

arXiv1996

Inhibited Al diffusion and growth roughening on Ga-coated Al (100)

Ab initio calculations indicate that the ground state for Ga adsorption on Al (100) is on-surface with local unit coverage. On Ga-coated Al (100), the bridge diffusion barrier for Al is large, but the Al$\rightarrow$Ga {\it exchange barrier is zero}: the ensuing incorporation of randomly deposited Al's into the Ga overlayer realizes a percolation network, efficiently recoated by Ga atoms. Based on calculated energetics, we predict rough surface growth at all temperatures; modeling the growth by a random deposition model with partial relaxation, we find a power-law divergent roughness $w\sim t^{\,0.07\pm0.02}$.

mtrl-th↗

arXiv1996

Size, Shape and Low Energy Electronic Structure of Carbon Nanotubes

A theory of the long wavelength low energy electronic structure of graphite-derived nanotubules is presented. The propagating $π$ electrons are described by wrapping a massless two dimensional Dirac Hamiltonian onto a curved surface. The effects of the tubule size, shape and symmetry are included through an effective vector potential which we derive for this model. The rich gap structure for all straight single wall cylindrical tubes is obtained analytically in this theory, and the effects of inhomogeneous shape deformations on nominally metallic armchair tubes are analyzed.

cond-mat↗

arXiv1996

Strained tetragonal states and Bain paths in metals

Paths of tetragonal states between two phases of a material, such as bcc and fcc, are called Bain paths. Two simple Bain paths can be defined in terms of special imposed stresses, one of which applies directly to strained epitaxial films. Each path goes far into the range of nonlinear elasticity and reaches a range of structural parameters in which the structure is inherently unstable. In this paper we identify and analyze the general properties of these paths by density functional theory. Special examples include vanadium, cobalt and copper, and the epitaxial path is used to identify an epitaxial film as related uniquely to a bulk phase.

mtrl-th↗

arXiv1997

Ab initio molecular dynamics study of the desorption of D_2 from Si(100)

Ab initio molecular dynamics calculations of deuterium desorbing from Si(100) have been performed in order to monitor the energy redistribution among the various D$_2$ and silicon degrees of freedom during the desorption process. The calculations show that a considerable part of the potential energy at the transition state to desorption is transferred to the silicon lattice. The deuterium molecules leave the surface vibrationally hot and rotationally cold, in agreement with thermal desorption experiments; the mean kinetic energy, however, is larger than found in a laser-induced desorption experiment. We discuss possible reasons for this discrepancy.

mtrl-th↗

arXiv1995

The Markoff-Automaton - a New Algorithm for Simulating the Time--Evolution of Large Stochastic Dynamic Systems

We describe a new algorithm for simulating complex Markoff-processes. We have used a reaction-cell method in order to simulate arbitrary reactions. It can be used for any kind of RDS on arbitrary topologies, including fractal dimensions or configurations not being related to any spatial geometry. The events within a single cell are managed by an event handler which has been implemented independently of the system studied. The method is exact on the Markoff level including the correct treatment of finite numbers of molecules. To demonstrate its properties, we apply it on a very simple reaction-diffusion-systems (RDS). The chemical equations A+A -> inert and A+B -> inert in 1 to 4 dimensions serve as models for systems whose dynamics on an intermediate time scale are governed by fluctuations. We compare our results to the analytic approach by the scaling ansatz. The simulations confirm the exponents of the A+B system within statistical errors, including the logarithmic corrections in the dimension d=2. The method is capable to simulate the crossover from the reaction to diffusion limited regime, which is defined by a crossover time depending on the system size.

chem-ph↗

arXiv1995

Theory of alkali metal adsorption on close-packed metal surfaces

Results of recent density functional theory calculations for alkali metal adsorbates on close-packed metal surfaces are discussed. Single adatoms on the (111) surface of Al and Cu are studied with the self-consistent surface Green-function method by which the pure adsorbate-substrate interaction may be analyzed. Higher coverage ordered adlayers of K on Al(111), Na on Al(111), and Na on Al(001) are treated using the ab-initio pseudopotential plane wave method which affords the prediction of coverage dependent stable and metastable adsorbate geometries and phase transitions of the adsorbate layers. Together, these studies give insight and understanding into current key issues in alkali metal adsorption, namely, the nature of the adsorbate-substrate bond at low coverage and the occurrence of hitherto unanticipated adsorbate geometries, and the associated electronic properties.

mtrl-th↗

arXiv1996

Theory of Alkali Induced Reconstruction of the Cu(100) Surface

LEED experiments show that Li adsorbed at Cu(100) surfaces at room temperature induces a (2x1) missing row substrate reconstruction while adsorption at lower temperatures, T=180 K, results in an unreconstructed Cu(100)+c(2x2)--Li overlayer structure. Substrate reconstruction has not been observed for Na nor for K adsorption. In order to study the specific reconstruction behavior of the Li adsorbate ab initio DFT calculations have been performed on Cu(100)+Ad, Ad = Li, Na, K systems at coverages Theta_Ad=0.25-0.5 with and without reconstruction. The calculations show that the (2x1) MR reconstructed surface lies energetically above the ideal (1x1) surface by 0.2 eV per unit cell. However, alkali binding is stronger in the MR geometry as compared to that of the ideal surface where the increase in bond strength becomes smaller in going from Li to Na to K. As a result, the MR reconstructed and the overlayer adsorbate systems are energetically very close for Cu(100)+Li while for Na and K the overlayer geometry is always favored.

mtrl-th↗

arXiv1997

Long-Range Excitons in Optical Absorption Spectra of Electroluminescent Polymer Poly(para-phenylenevinylene)

The component of photoexcited states with large spatial extent is investigated for poly(para-phenylenevinylene) using the intermediate exciton theory. We find a peak due to long-range excitons at the higher-energy side of the lowest main feature of optical spectra. The fact that the onset of long-range excitons is located near the energy gap is related to the mechanisms of large photocurrents measured in such energy regions. We show that a large value of the hopping integral is realistic for characterizing optical excitations.

cond-mat↗

arXiv1995

Static displacements and chemical correlations in alloys

Recent experiments in metallic solid solutions have revealed interesting correlations between static pair-displacements and the ordering behavior of these alloys. This paper discusses a simple theoretical model which successfully explains these observations and which provides a natural framework for analyzing experimental measurements of pair-displacements and chemical correlations in solid solutions. The utility and scope of this model is demonstrated by analyzing results of experiments on $Ni-Fe$ and $Cr-Fe$ alloys and results of simulations of $Cu-Au$ and $Cu-Ag$ alloys.

mtrl-th↗