2,043 research outputs found
Role of defects and impurities in doping of GaN
We have calculated formation energies and position of the defect levels for
all native defects and for a variety of donor and acceptor impurities employing
first-principles total-energy calculations. An analysis of the numerical
results gives direct insight into defect concentrations and impurity solubility
with respect to growth parameters (temperature, chemical potentials) and into
the mechanisms limiting the doping levels in GaN. We show how compensation and
passivation by native defects or impurities, solubility issues, and
incorporation of dopants on other sites influence the acceptor doping levels.Comment: 8 pages, 3 figures, to appear in "The Physics of Semiconductors
Modeling carbon black reinforcement in rubber compounds
One of the advocated reinforcement mechanisms is the formation by the filler of a network interpenetrating the polymer network. The deformation and reformation of the filler network allows the explanation of low strain dynamic physical properties of the composite. The present model relies on a statistical study of a collection of elementary mechanical systems, This leads to a mathematical approach of the complex modulus G* = G' + iG". The storage and loss modulus (G' and G", respectively), are expressed in the form of two integrals capable of modeling their Variation with respect to strain
First-principles calculation of phase equilibrium of V-Nb, V-Ta, and Nb-Ta alloys
In this paper, we report the calculated phase diagrams of V-Nb, V-Ta, and Nb-Ta alloys computed by combining the total energies of 40–50 configurations for each system (obtained using density functional theory) with the cluster expansion and Monte Carlo techniques. For V-Nb alloys, the phase diagram computed with conventional cluster expansion shows a miscibility gap with consolute temperature T_c=1250 K. Including the constituent strain to the cluster expansion Hamiltonian does not alter the consolute temperature significantly, although it appears to influence the solubility of V- and Nb-rich alloys. The phonon contribution to the free energy lowers T_c to 950 K (about 25%). Our calculations thus predicts an appreciable miscibility gap for V-Nb alloys. For bcc V-Ta alloy, this calculation predicts a miscibility gap with T_c=1100 K. For this alloy, both the constituent strain and phonon contributions are found to be significant. The constituent strain increases the miscibility gap while the phonon entropy counteracts the effect of the constituent strain. In V-Ta alloys, an ordering transition occurs at 1583 K from bcc solid solution phase to the V_(2)Ta Laves phase due to the dominant chemical interaction associated with the relatively large electronegativity difference. Since the current cluster expansion ignores the V_(2)Ta phase, the associated chemical interaction appears to manifest in making the solid solution phase remain stable down to 1100 K. For the size-matched Nb-Ta alloys, our calculation predicts complete miscibility in agreement with experiment
Self-driven lattice-model Monte Carlo simulations of alloy thermodynamic
Monte Carlo (MC) simulations of lattice models are a widely used way to
compute thermodynamic properties of substitutional alloys. A limitation to
their more widespread use is the difficulty of driving a MC simulation in order
to obtain the desired quantities. To address this problem, we have devised a
variety of high-level algorithms that serve as an interface between the user
and a traditional MC code. The user specifies the goals sought in a high-level
form that our algorithms convert into elementary tasks to be performed by a
standard MC code. For instance, our algorithms permit the determination of the
free energy of an alloy phase over its entire region of stability within a
specified accuracy, without requiring any user intervention during the
calculations. Our algorithms also enable the direct determination of
composition-temperature phase boundaries without requiring the calculation of
the whole free energy surface of the alloy system
Nuclear structure study around Z=28
Yrast levels of Ni, Cu and Zn isotopes for have been
described by state-of-the-art shell model calculations with three recently
available interactions using Ni as a core in the model
space. The results are unsatisfactory viz. large for very neutron rich
nuclei, small B(E2) values in comparison to experimental values. These results
indicate an importance of inclusion of and orbitals
in the model space to reproduce collectivity in this region.Comment: 12 pages, 14 figure
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