252 research outputs found
Two Approaches to Dislocation Nucleation in the Supported Heteroepitaxial Equilibrium Islanding Phenomenon
We study the dislocation formation in 2D nanoscopic islands with two methods,
the Molecular Static method and the Phase Field Crystal method. It is found
that both methods indicate the same qualitative stages of the nucleation
process. The dislocations nucleate at the film-substrate contact point and the
energy decreases monotonously when the dislocations are farther away from the
island-wetting film contact points than the distance of the highest energy
barrier.Comment: 4 page
Equilibrium Shape and Size of Supported Heteroepitaxial Nanoislands
We study the equilibrium shape, shape transitions and optimal size of
strained heteroepitaxial nanoislands with a two-dimensional atomistic model
using simply adjustable interatomic pair potentials. We map out the global
phase diagram as a function of substrate-adsorbate misfit and interaction. This
phase diagram reveals all the phases corresponding to different well-known
growth modes. In particular, for large enough misfits and attractive substrate
there is a Stranski-Krastanow regime, where nano-sized islands grow on top of
wetting films. We analyze the various terms contributing to the total island
energy in detail, and show how the competition between them leads to the
optimal shape and size of the islands. Finally, we also develop an analytic
interpolation formula for the various contributions to the total energy of
strained nanoislands.Comment: 9 pages, 7 figure
R&D and productivity in OECD firms and industries: A hierarchical meta-regression analysis
The relationship between R&D investment and firm/industry productivity has been investigated widely following seminal contributions by Zvi Griliches and others from late 1970s onwards. We aim to provide a systematic synthesis of the evidence, using 1253 estimates from 65 primary studies that adopt the so-called primal approach. In line with prior reviews, we report that the average elasticity and rate-of-return estimates are positive. In contrast to prior reviews, however, we report that: (i) the estimates are smaller and more heterogeneous than what has been reported before; (ii) residual heterogeneity remains high among firm-level estimates even after controlling for moderating factors; (iii) firm-level rates of return and within-industry social returns to R&D are small and do not differ significantly despite theoretical predictions of higher social returns; and (iv) the informational content of both elasticity and rate-of-return estimates needs to be interpreted cautiously. We conclude by highlighting the implications of these findings for future research and evidence-based policy
Splitting fields and general differential Galois theory
An algebraic technique is presented that does not use results of model theory
and makes it possible to construct a general Galois theory of arbitrary
nonlinear systems of partial differential equations. The algebraic technique is
based on the search for prime differential ideals of special form in tensor
products of differential rings. The main results demonstrating the work of the
technique obtained are the theorem on the constructedness of the differential
closure and the general theorem on the Galois correspondence for normal
extensions..Comment: 33 pages, this version coincides with the published on
Minimum energy paths for dislocation nucleation in strained epitaxial layers
We study numerically the minimum energy path and energy barriers for dislocation nucleation in a two-dimensional atomistic model of strained epitaxial layers on a substrate with lattice misfit. Stress relaxation processes from coherent to incoherent states for different transition paths are determined using saddle point search based on a combination of repulsive potential minimization and the Nudged Elastic Band method. The minimum energy barrier leading to a final state with a single misfit dislocation nucleation is determined. A strong tensile-compressive asymmetry is observed. This asymmetry can be understood in terms of the qualitatively different transition paths for the tensile and compressive strains.Peer reviewe
Global transition path search for dislocation formation in Ge on Si(001)
Global optimization of transition paths in complex atomic scale systems is
addressed in the context of misfit dislocation formation in a strained Ge film
on Si(001). Such paths contain multiple intermediate minima connected by
minimum energy paths on the energy surface emerging from the atomic
interactions in the system. The challenge is to find which intermediate states
to include and to construct a path going through these intermediates in such a
way that the overall activation energy for the transition is minimal. In the
numerical approach presented here, intermediate minima are constructed by
heredity transformations of known minimum energy structures and by identifying
local minima in minimum energy paths calculated using a modified version of the
nudged elastic band method. Several mechanisms for the formation of a 90{\deg}
misfit dislocation at the Ge-Si interface are identified when this method is
used to construct transition paths connecting a homogeneously strained Ge film
and a film containing a misfit dislocation. One of these mechanisms which has
not been reported in the literature is detailed. The activation energy for this
path is calculated to be 26% smaller than the activation energy for half loop
formation of a full, isolated 60{\deg} dislocation. An extension of the common
neighbor analysis method involving characterization of the geometrical
arrangement of second nearest neighbors is used to identify and visualize the
dislocations and stacking faults
Quantum corrections in the Boltzmann conductivity of graphene and their sensitivity to the choice of formalism
Semiclassical spin-coherent kinetic equations can be derived from quantum
theory with many different approaches (Liouville equation based approaches,
nonequilibrium Green's functions techniques, etc.). The collision integrals
turn out to be formally different, but coincide in textbook examples as well as
for systems where the spin-orbit coupling is only a small part of the kinetic
energy like in related studies on the spin Hall effect. In Dirac cone physics
(graphene, surface states of topological insulators like Bi_{1-x}Sb_x, Bi_2Te_3
etc.), where this coupling constitutes the entire kinetic energy, the
difference manifests itself in the precise value of the electron-hole coherence
originated quantum correction to the Drude conductivity . The leading correction is derived analytically for single and multilayer
graphene with general scalar impurities. The often neglected principal value
terms in the collision integral are important. Neglecting them yields a leading
correction of order , whereas including them can give a
correction of order . The latter opens up a counterintuitive
scenario with finite electron-hole coherent effects at Fermi energies
arbitrarily far above the neutrality point regime, for example in the form of a
shift that only depends on the dielectric constant. This residual
conductivity, possibly related to the one observed in recent experiments,
depends crucially on the approach and could offer a setting for experimentally
singling out one of the candidates. Concerning the different formalisms we
notice that the discrepancy between a density matrix approach and a Green's
function approach is removed if the Generalized Kadanoff-Baym Ansatz in the
latter is replaced by an anti-ordered version.Comment: 31 pages, 1 figure. An important sign error has been rectified in the
principal value terms in equation (52) in the vN & NSO expression. It has no
implications for the results on the leading quantum correction studied in
this paper. However, for the higher quantum corrections studied in
arXiv:1304.3929 (see comment in the latter) the implications are crucia
The functional state of the kidneys and endothelial dysfunction in patients with arterial hypertension
Endothelial dysfunction (ED) is the leading pathogenetic link connecting the development of atherosclerosis, diabetes mellitus (DM) type 2, arterial hypertension (AH) and chronic kidney disease (CKD). In this regard, the integrated assessment of markers of endothelial dysfunction in patients at the early stages of hypertension based on kidney function seems important. The survey included 67 patients with hypertension (mean age 48.2±2.4 years) and 30 matched for sex and age healthy volunteers. The levels of markers of endothelial dysfunction of blood VEGF, endothelin-1 and renal damage - MAU, VEGF in the urine - were significantly higher in the group with AH. Correlation analysis showed the existence of a high degree of direct connection between the level of VEGF in the blood and the arterial pressure value, the degree of obesity, cholesterol, creatinine, MAU and negative correlation with VEGF and glomerular filtration rate (GFR). We also found a positive correlation of endothelin-1 and VEGF in the blood. Similarly, there was a positive relationship between the levels of VEGF in the urine and the systolic AH, MAU, uric acid, and the negative one - with GFR. The value of the MAU is directly dependent on the level of diastolic blood pressure and creatinine, and it was inversely proportional to GFR value. Thus, in patients with hypertension endothelin-1 and VEGF in the blood are markers of endothelial dysfunction responsible for the adverse cardiovascular prognosis, and the level of MAU and VEGF in urine correlating with the degree of decrease in kidney function and high blood pressure are markers of adverse cardiorenal relationship
Energetics and atomic mechanisms of dislocation nucleation in strained epitaxial layers
We study numerically the energetics and atomic mechanisms of misfit
dislocation nucleation and stress relaxation in a two-dimensional atomistic
model of strained epitaxial layers on a substrate with lattice misfit.
Relaxation processes from coherent to incoherent states for different
transition paths are studied using interatomic potentials of Lennard-Jones type
and a systematic saddle point and transition path search method. The method is
based on a combination of repulsive potential minimization and the Nudged
Elastic Band method. For a final state with a single misfit dislocation, the
minimum energy path and the corresponding activation barrier are obtained for
different misfits and interatomic potentials. We find that the energy barrier
decreases strongly with misfit. In contrast to continuous elastic theory, a
strong tensile-compressive asymmetry is observed. This asymmetry can be
understood as manifestation of asymmetry between repulsive and attractive
branches of pair potential and it is found to depend sensitively on the form of
the potential.Comment: 11 pages, 9 figures, to appear in Phys. Rev.
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