52 research outputs found
Coarse Molecular-Dynamics Determination of the Onset of Structural Transitions: Melting of Crystalline Solids
Using a coarse molecular-dynamics (CMD) approach with an appropriate choice
of coarse variable (order parameter), we map the underlying effective
free-energy landscape for the melting of a crystalline solid. Implementation of
this approach provides a means for constructing effective free-energy
landscapes of structural transitions in condensed matter. The predictions of
the approach for the thermodynamic melting point of a model silicon system are
in excellent agreement with those of ''traditional'' techniques for
melting-point calculations, as well as with literature values
Coarse Stability and Bifurcation Analysis Using Stochastic Simulators: Kinetic Monte Carlo Examples
We implement a computer-assisted approach that, under appropriate conditions,
allows the bifurcation analysis of the coarse dynamic behavior of microscopic
simulators without requiring the explicit derivation of closed macroscopic
equations for this behavior. The approach is inspired by the so-called
time-step per based numerical bifurcation theory. We illustrate the approach
through the computation of both stable and unstable coarsely invariant states
for Kinetic Monte Carlo models of three simple surface reaction schemes. We
quantify the linearized stability of these coarsely invariant states, perform
pseudo-arclength continuation, detect coarse limit point and coarse Hopf
bifurcations and construct two-parameter bifurcation diagrams.Comment: 26 pages, 5 figure
Opening and tuning of band gap by the formation of diamond superlattices in twisted bilayer graphene
We report results of first-principles density functional theory calculations, which introduce a new class of carbon nanostructures formed due to creation of covalent interlayer C-C bonds in twisted bilayer graphene (TBG). This interlayer bonding becomes possible by hydrogenation of the graphene layers according to certain hydrogenation patterns. The resulting relaxed configurations consist of two-dimensional (2D) superlattices of diamondlike nanocrystals embedded within the graphene layers, with the same periodicity as that of the Moiré pattern corresponding to the rotational layer stacking in TBG. The 2D diamond nanodomains resemble the cubic or the hexagonal diamond phase. The detailed structure of these superlattice configurations is determined by parameters that include the twist angle, ranging from 0° to ∼15°, and the number of interlayer C-C bonds formed per unit cell of the superlattice. We demonstrate that formation of such interlayer-bonded finite domains causes the opening of a band gap in the electronic band structure of TBG, which depends on the density and spatial distribution of interlayer C-C bonds. We have predicted band gaps as wide as 1.2 eV and found that the band gap increases monotonically with increasing size of the embedded diamond nanodomain in the unit cell of the superlattice. Such nanostructure formation constitutes a promising approach for opening a precisely tunable band gap in bilayer graphene
Molecular-dynamics simulations of stacking-fault-induced dislocation annihilation in pre-strained ultrathin single-crystalline copper films
We report results of large-scale molecular-dynamics (MD) simulations of
dynamic deformation under biaxial tensile strain of pre-strained
single-crystalline nanometer-scale-thick face-centered cubic (fcc) copper
films. Our results show that stacking faults, which are abundantly present in
fcc metals, may play a significant role in the dissociation, cross-slip, and
eventual annihilation of dislocations in small-volume structures of fcc metals.
The underlying mechanisms are mediated by interactions within and between
extended dislocations that lead to annihilation of Shockley partial
dislocations or formation of perfect dislocations. Our findings demonstrate
dislocation starvation in small-volume structures with ultra-thin film
geometry, governed by a mechanism other than dislocation escape to free
surfaces, and underline the significant role of geometry in determining the
mechanical response of metallic small-volume structures.Comment: 28 pages, 3 figure
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Electronic structure of electron-irradiated graphene and effects of hydrogen passivation
We report results for the electronic structure of irradiated and irradiation-induced amorphized graphene based on first-principles density functional theory calculations, using models of irradiated graphene sheets that were initially relaxed structurally according to molecular-dynamics simulations. We find that localized states appear at the Fermi level upon irradiation damage and the corresponding local density of states increases with increasing defect density. Electronic structure calculations show that band flattening occurs due to electron localization in the vicinity of irradiation-induced defects and reduces the charge carrier mobility. This band flattening effect becomes stronger with increasing defect density due to a greater degree of carrier localization at irradiation-induced carbon dangling bonds. Passivating these dangling bonds with hydrogen atoms delocalizes the charge density, reduces the density of states at the Fermi level, and increases the band dispersion. Hydrogen passivation also has the additional effect of quenching any localized magnetic moments at dangling bonds. Our studies show that defect engineering of graphene—even at a gross level without atomic-scale precision—can be employed to tune its properties for additional electronic functionality
Coarse molecular-dynamics analysis of stress-induced structural transitions in crystals
We present a coarse molecular-dynamics (CMD) approach for the study of
stress-induced structural transformations in crystals at finite temperatures.
The method relies on proper choice of a coarse variable (order parameter,
observable), which parameterizes the changes in effective free energy during
the transformation. Results are reported for bcc-to-hcp lattice transitions
under pressure. We explore coarse-variable space to reconstruct an effective
free-energy landscape quantifying the relative stability of different
metastable basins and locate the onset, at a critical pressure, of the
bcc-to-hcp transformation.Comment: 3 page
Tuning the band structure of graphene nanoribbons through defect-interaction-driven edge patterning
We report a systematic analysis of pore-edge interactions in graphene nanoribbons (GNRs) and their outcomes based on first-principles calculations and classical molecular-dynamics simulations. We find a strong attractive interaction between nanopores and GNR edges that drives the pores to migrate toward and coalesce with the GNR edges, which can be exploited to form GNR edge patterns that impact the GNR electronic band structure and tune the GNR band gap. Our analysis introduces a viable physical processing strategy for modifying GNR properties by combining defect engineering and thermal annealing
Coarse Bifurcation Diagrams via Microscopic Simulators: A State-Feedback Control-Based Approach
The arc-length continuation framework is used for the design of state
feedback control laws that enable a microscopic simulator trace its own
open-loop coarse bifurcation diagram. The steering of the system along solution
branches is achieved through the manipulation of the bifurcation parameter,
which becomes our actuator. The design approach is based on the assumption that
the eigenvalues of the linearized system can be decomposed into two well
separated clusters: one containing eigenvalues with large negative real parts
and one containing (possibly unstable) eigenvalues close to the origin
Hydrogen in Si–Si bond center and platelet-like defect configurations in amorphous hydrogenated silicon
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