1,951 research outputs found
First principles determination of the Peierls stress of the shuffle screw dislocation in silicon
The Peierls stress of the a/2 screw dislocation belonging to the shuffle
set is calculated for silicon using density functional theory. We have checked
the effect of boundary conditions by using two models, the supercell method
where one considers a periodic array of dislocations, and the cluster method
where a single dislocation is embedded in a small cluster. The Peierls stress
is underestimated with the supercell and overestimated with the cluster. These
contributions have been calculated and the Peierls stress is determined in the
range between 2.4 x 10-2 and 2.8 x 10-2 eV {\AA}-3. When moving, the
dislocation follows the {111} plane going through a low energy metastable
configuration and never follows the 100 plane, which includes a higher energy
metastable core configuration
Slip line growth as a critical phenomenon
We study the growth of slip line in a plastically deforming crystal by
numerical simulation of a double-ended pile-up model with a dislocation source
at one end, and an absorbing wall at the other end. In presence of defects, the
pile-up undergoes a second order non-equilibrium phase transition as a function
of stress, which can be characterized by finite size scaling. We obtain a
complete set of critical exponents and scaling functions that describe the
spatiotemporal dynamics of the slip line. Our findings allow to reinterpret
earlier experiments on slip line kinematography as evidence of a dynamic
critical phenomenon.Comment: 4 pages, 4 figure
Dislocation core field. I. Modeling in anisotropic linear elasticity theory
Aside from the Volterra field, dislocations create a core field, which can be
modeled in linear anisotropic elasticity theory with force and dislocation
dipoles. We derive an expression of the elastic energy of a dislocation taking
full account of its core field and show that no cross term exists between the
Volterra and the core fields. We also obtain the contribution of the core field
to the dislocation interaction energy with an external stress, thus showing
that dislocation can interact with a pressure. The additional force that
derives from this core field contribution is proportional to the gradient of
the applied stress. Such a supplementary force on dislocations may be important
in high stress gradient regions, such as close to a crack tip or in a
dislocation pile-up
Mesoscopic Analysis of Structure and Strength of Dislocation Junctions in FCC Metals
We develop a finite element based dislocation dynamics model to simulate the
structure and strength of dislocation junctions in FCC crystals. The model is
based on anisotropic elasticity theory supplemented by the explicit inclusion
of the separation of perfect dislocations into partial dislocations bounding a
stacking fault. We demonstrate that the model reproduces in precise detail the
structure of the Lomer-Cottrell lock already obtained from atomistic
simulations. In light of this success, we also examine the strength of
junctions culminating in a stress-strength diagram which is the locus of points
in stress space corresponding to dissolution of the junction.Comment: 9 Pages + 4 Figure
Dislocation Core Energies and Core Fields from First Principles
Ab initio calculations in bcc iron show that a screw dislocation
induces a short-range dilatation field in addition to the Volterra elastic
field. This core field is modeled in anisotropic elastic theory using force
dipoles. The elastic modeling thus better reproduces the atom displacements
observed in ab initio calculations. Including this core field in the
computation of the elastic energy allows deriving a core energy which converges
faster with the cell size, thus leading to a result which does not depend on
the geometry of the dislocation array used for the simulation.Comment: DOI: 10.1103/PhysRevLett.102.05550
Visualizing Quantum Well State Perturbations of Metallic Thin Films near Stacking Fault Defects
We demonstrate that quantum well states (QWS) of thin Pb films are highly
perturbed within the proximity of intrinsic film defects. Scanning Tunneling
Spectroscopy (STM/STS) measurements indicate that the energy of these states
have a strong distance dependence within 4 nm of the defect with the strongest
energetic fluctuations equaling up to 100 meV. These localized perturbations
show large spatially-dependent asymmetries in the LDOS around the defect site
for each corresponding quantum well state. These energetic fluctuations can be
described by a simple model which accounts for fluctuations in the confinement
potential induced by topographic changes.Comment: Updated Versio
Modelling two-dimensional Crystals with Defects under Stress: Superelongation of Carbon Nanotubes at high Temperatures
We calculate analytically the phase diagram of a two-dimensional square
crystal and its wrapped version with defects under external homogeneous stress
as a function of temperature using a simple elastic lattice model that allows
for defect formation. The temperature dependence turns out to be very weak. The
results are relevant for recent stress experiments on carbon nanotubes. Under
increasing stress, we find a crossover regime which we identify with a cracking
transition that is almost independent of temperature. Furthermore, we find an
almost stress-independent melting point. In addition, we derive an enhanced
ductility with relative strains before cracking between 200-400%, in agreement
with carbon nanotube experiments. The specific values depend on the Poisson
ratio and the angle between the external force and the crystal axes. We give
arguments that the results for carbon nanotubes are not much different to the
wrapped square crystal.Comment: 12 pages, 6 eps figures, section VI added discussing the
modifications of our model when applied to tube
Ground state of a large number of particles on a frozen topography
Problems consisting in finding the ground state of particles interacting with
a given potential constrained to move on a particular geometry are surprisingly
difficult. Explicit solutions have been found for small numbers of particles by
the use of numerical methods in some particular cases such as particles on a
sphere and to a much lesser extent on a torus. In this paper we propose a
general solution to the problem in the opposite limit of a very large number of
particles M by expressing the energy as an expansion in M whose coefficients
can be minimized by a geometrical ansatz. The solution is remarkably universal
with respect to the geometry and the interaction potential. Explicit solutions
for the sphere and the torus are provided. The paper concludes with several
predictions that could be verified by further theoretical or numerical work.Comment: 9 pages, 9 figures, LaTeX fil
Theoretical study of dislocation nucleation from simple surface defects in semiconductors
Large-scale atomistic calculations, using empirical potentials for modeling
semiconductors, have been performed on a stressed system with linear surface
defects like steps. Although the elastic limits of systems with surface defects
remain close to the theoretical strength, the results show that these defects
weaken the atomic structure, initializing plastic deformations, in particular
dislocations. The character of the dislocation nucleated can be predicted
considering both the resolved shear stress related to the applied stress
orientation and the Peierls stress. At low temperature, only glide events in
the shuffle set planes are observed. Then they progressively disappear and are
replaced by amorphization/melting zones at a temperature higher than 900 K
Voltage from mechanical stress in type-II superconductors: Depinning of the magnetic flux by moving dislocations
Mechanical stress causes motion of defects in solids. We show that in a
type-II superconductor a moving dislocation generates a pattern of current that
exerts the depinning force on the surrounding vortex lattice. Concentration of
dislocations and the mechanical stress needed to produce critical depinning
currents are shown to be within practical range. When external magnetic field
and transport current are present this effect generates voltage across the
superconductor. Thus a superconductor can serve as an electrical sensor of the
mechanical stress.Comment: 3 pages, 1 figure
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