10,545 research outputs found
Rate theory for correlated processes: Double-jumps in adatom diffusion
We study the rate of activated motion over multiple barriers, in particular
the correlated double-jump of an adatom diffusing on a missing-row
reconstructed Platinum (110) surface. We develop a Transition Path Theory,
showing that the activation energy is given by the minimum-energy trajectory
which succeeds in the double-jump. We explicitly calculate this trajectory
within an effective-medium molecular dynamics simulation. A cusp in the
acceptance region leads to a sqrt{T} prefactor for the activated rate of
double-jumps. Theory and numerical results agree
Partly Occupied Wannier Functions
We introduce a scheme for constructing partly occupied, maximally localized
Wannier functions (WFs) for both molecular and periodic systems. Compared to
the traditional occupied WFs the partly occupied WFs posses improved symmetry
and localization properties achieved through a bonding-antibonding closing
procedure. We demonstrate the equivalence between bonding-antibonding closure
and the minimization of the average spread of the WFs in the case of a benzene
molecule and a linear chain of Pt atoms. The general applicability of the
method is demonstrated through the calculation of WFs for a metallic system
with an impurity: a Pt wire with a hydrogen molecular bridge.Comment: 5 pages, 4 figure
Computational Design of Chemical Nanosensors: Metal Doped Carbon Nanotubes
We use computational screening to systematically investigate the use of
transition metal doped carbon nanotubes for chemical gas sensing. For a set of
relevant target molecules (CO, NH3, H2S) and the main components of air (N2,
O2, H2O), we calculate the binding energy and change in conductance upon
adsorption on a metal atom occupying a vacancy of a (6,6) carbon nanotube.
Based on these descriptors, we identify the most promising dopant candidates
for detection of a given target molecule. From the fractional coverage of the
metal sites in thermal equilibrium with air, we estimate the change in the
nanotube resistance per doping site as a function of the target molecule
concentration assuming charge transport in the diffusive regime. Our analysis
points to Ni-doped nanotubes as candidates for CO sensors working under typical
atmospheric conditions
Monte Carlo Study of Short-Range Order and Displacement Effects in Disordered CuAu
The correlation between local chemical environment and atomic displacements
in disordered CuAu alloy has been studied using Monte Carlo simulations based
on the effective medium theory (EMT) of metallic cohesion. These simulations
correctly reproduce the chemically-specific nearest-neighbor distances in the
random alloy across the entire Cu\$_x\$Au\$_{1-x}\$ concentration range. In the
random equiatomic CuAu alloy, the chemically specific pair distances depend
strongly on the local atomic environment (i.e. fraction of like/unlike nearest
neighbors). In CuAu alloy with short-range order, the relationship between
local environment and displacements remains qualitatively similar. However the
increase in short-range order causes the average Cu-Au distance to decrease
below the average Cu-Cu distance, as it does in the ordered CuAuI phase. Many
of these trends can be understood qualitatively from the different neutral
sphere radii and compressibilities of the Cu and Au atoms.Comment: 9 pages, 5 figures, 2 table
A real-space grid implementation of the Projector Augmented Wave method
A grid-based real-space implementation of the Projector Augmented Wave (PAW)
method of P. E. Blochl [Phys. Rev. B 50, 17953 (1994)] for Density Functional
Theory (DFT) calculations is presented. The use of uniform 3D real-space grids
for representing wave functions, densities and potentials allows for flexible
boundary conditions, efficient multigrid algorithms for solving Poisson and
Kohn-Sham equations, and efficient parallelization using simple real-space
domain-decomposition. We use the PAW method to perform all-electron
calculations in the frozen core approximation, with smooth valence wave
functions that can be represented on relatively coarse grids. We demonstrate
the accuracy of the method by calculating the atomization energies of twenty
small molecules, and the bulk modulus and lattice constants of bulk aluminum.
We show that the approach in terms of computational efficiency is comparable to
standard plane-wave methods, but the memory requirements are higher.Comment: 13 pages, 3 figures, accepted for publication in Physical Review
Dynamics of Ordering in Alloys with Modulated Phases
This paper presents a theoretical model for studying the dynamics of ordering
in alloys which exhibit modulated phases. The model is different from the
standard time-dependent Ginzburg-Landau description of the evolution of a
non-conserved order parameter and resembles the Swift-Hohenberg model. The
early-stage growth kinetics is analyzed and compared to the Cahn-Hilliard
theory of continuous ordering. The effects of non-linearities on the growth
kinetics are discussed qualitatively and it is shown that the presence of an
underlying elastic lattice introduces qualitatively new effects. A lattice
Hamiltonian capable of describing these effects and suitable for carrying out
simulations of the growth kinetics is also constructed.Comment: 18 pages, 3 figures (postscript files appended), Brandeis-BC9
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 and alloys
and results of simulations of and alloys.Comment: 12 page
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