1,422 research outputs found
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
First-principles study of ternary fcc solution phases from special quasirandom structures
In the present work, ternary Special Quasirandom Structures (SQSs) for a fcc
solid solution phase are generated at different compositions,
and , ,
whose correlation functions are satisfactorily close to those of a random fcc
solution. The generated SQSs are used to calculate the mixing enthalpy of the
fcc phase in the Ca-Sr-Yb system. It is observed that first-principles
calculations of all the binary and ternary SQSs in the Ca-Sr-Yb system exhibit
very small local relaxation. It is concluded that the fcc ternary SQSs can
provide valuable information about the mixing behavior of the fcc ternary solid
solution phase. The SQSs presented in this work can be widely used to study the
behavior of ternary fcc solid solutions.Comment: 20 pages, 7 figure
Using bond-length dependent transferable force constants to predict vibrational entropies in Au-Cu, Au-Pd, and Cu-Pd alloys
A model is tested to rapidly evaluate the vibrational properties of alloys
with site disorder. It is shown that length-dependent transferable force
constants exist, and can be used to accurately predict the vibrational entropy
of substitutionally ordered and disordered structures in Au-Cu, Au-Pd, and
Cu-Pd. For each relevant force constant, a length- dependent function is
determined and fitted to force constants obtained from first-principles
pseudopotential calculations. We show that these transferable force constants
can accurately predict vibrational entropies of L1-ordered and disordered
phases in CuAu, AuPd, PdAu, CuPd, and PdAu. In
addition, we calculate the vibrational entropy difference between
L1-ordered and disordered phases of AuCu and CuPt.Comment: 9 pages, 6 figures, 3 table
Role of defects in the electronic properties of amorphous/crystalline Si interface
The mechanism determining the band alignment of the amorphous/crystalline
Si heterostructures is addressed with direct atomistic simulations of the
interface performed using a hierarchical combination of various computational
schemes ranging from classical model-potential molecular dynamics to ab-initio
methods. We found that in coordination defect-free samples the band alignment
is almost vanishing and independent on interface details. In defect-rich
samples, instead, the band alignment is sizeably different with respect to the
defect-free case, but, remarkably, almost independent on the concentration of
defects. We rationalize these findings within the theory of semiconductor
interfaces.Comment: 4 pages in two-column format, 2 postscript figures include
Muonium as a hydrogen analogue in silicon and germanium; quantum effects and hyperfine parameters
We report a first-principles theoretical study of hyperfine interactions,
zero-point effects and defect energetics of muonium and hydrogen impurities in
silicon and germanium. The spin-polarized density functional method is used,
with the crystalline orbitals expanded in all-electron Gaussian basis sets. The
behaviour of hydrogen and muonium impurities at both the tetrahedral and
bond-centred sites is investigated within a supercell approximation. To
describe the zero-point motion of the impurities, a double adiabatic
approximation is employed in which the electron, muon/proton and host lattice
degrees of freedom are decoupled. Within this approximation the relaxation of
the atoms of the host lattice may differ for the muon and proton, although in
practice the difference is found to be slight. With the inclusion of zero-point
motion the tetrahedral site is energetically preferred over the bond-centred
site in both silicon and germanium. The hyperfine and superhyperfine
parameters, calculated as averages over the motion of the muon, agree
reasonably well with the available data from muon spin resonance experiments.Comment: 20 pages, including 9 figures. To appear in Phys. Rev.
First Principles Phase Diagram Calculations for the Octahedral-Interstitial System ZrO,
First principles based phase diagram calculations were performed for the
octahedral-interstitial solid solution system \alpha ZrOX (\alpha Zr[
]_(1-X)OX; [ ]=Vacancy; 0 \leq X \leq 1/2). The cluster expansion method was
used to do a ground state analysis, and to calculate the phase diagram. The
predicted diagram has four ordered ground-states in the range 0 \leq X \leq
1/2, but one of these, at X=5/12, is predicted to disproportionate at T \approx
20K, well below the experimentally investigated range T \approx 420K. Thus, at
T \succeq 420K, the first-principles based calculation predicts three ordered
phases rather than the four that have been reported by experimentalists
Anion vacancies as a source of persistent photoconductivity in II-VI and chalcopyrite semiconductors
Using first-principles electronic structure calculations we identify the
anion vacancies in II-VI and chalcopyrite Cu-III-VI2 semiconductors as a class
of intrinsic defects that can exhibit metastable behavior. Specifically, we
predict persistent electron photoconductivity (n-type PPC) caused by the oxygen
vacancy VO in n-ZnO, and persistent hole photoconductivity (p-type PPC) caused
by the Se vacancy VSe in p-CuInSe2 and p-CuGaSe2. We find that VSe in the
chalcopyrite materials is amphoteric having two "negative-U" like transitions,
i.e. a double-donor transition e(2+/0) close to the valence band and a
double-acceptor transition e(0/2-) closer to the conduction band. We introduce
a classification scheme that distinguishes two types of defects (e.g., donors):
type-alpha, which have a defect-localized-state (DLS) in the gap, and
type-beta, which have a resonant DLS within the host bands (e.g., conduction
band). In the latter case, the introduced carriers (e.g., electrons) relax to
the band edge where they can occupy a perturbed-host-state (PHS). Type alpha is
non-conducting, whereas type beta is conducting. We identify the neutral anion
vacancy as type-alpha and the doubly positively charged vacancy as type-beta.
We suggest that illumination changes the charge state of the anion vacancy and
leads to a crossover between alpha- and beta-type behavior, resulting in
metastability and PPC. In CuInSe2, the metastable behavior of VSe is carried
over to the (VSe-VCu) complex, which we identify as the physical origin of PPC
observed experimentally. We explain previous puzzling experimental results in
ZnO and CuInSe2 in the light of this model.Comment: submitted to Phys. Rev.
Electric fields and valence band offsets at strained [111] heterojunctions
[111] ordered common atom strained layer superlattices (in particular the
common anion GaSb/InSb system and the common cation InAs/InSb system) are
investigated using the ab initio full potential linearized augmented plane wave
(FLAPW) method. We have focused our attention on the potential line-up at the
two sides of the homopolar isovalent heterojunctions considered, and in
particular on its dependence on the strain conditions and on the strain induced
electric fields. We propose a procedure to locate the interface plane where the
band alignment could be evaluated; furthermore, we suggest that the
polarization charges, due to piezoelectric effects, are approximately confined
to a narrow region close to the interface and do not affect the potential
discontinuity. We find that the interface contribution to the valence band
offset is substantially unaffected by strain conditions, whereas the total band
line-up is highly tunable, as a function of the strain conditions. Finally, we
compare our results with those obtained for [001] heterojunctions.Comment: 18 pages, Latex-file, to appear in Phys.Rev.
Ferromagnetism in Mn doped GaAs due to substitutional-interstitial complexes
While most calculations on the properties of the ferromagnetic semiconductor
GaAs:Mn have focussed on isolated Mn substituting the Ga site (Mn), we
investigate here whether alternate lattice sites are favored and what the
magnetic consequences of this might be. Under As-rich (Ga-poor) conditions
prevalent at growth, we find that the formation energies are lower for
Mn over interstitial Mn (Mn).As the Fermi energy is shifted towards
the valence band maximum via external -doping, the formation energy of
Mn is reduced relative to Mn. Furthermore, under epitaxial growth
conditions, the solubility of both substitutional and interstitial Mn are
strongly enhanced over what is possible under bulk growth conditions. The high
concentration of Mn attained under epitaxial growth of p-type material opens
the possibility of Mn atoms forming small clusters. We consider various types
of clusters, including the Coulomb-stabilized clusters involving two Mn
and one Mn. While isolated Mn are hole killers (donors), and therefore
destroy ferromagnetism,complexes such as Mn-Mn-Mn) are found
to be more stable than complexes involving Mn-Mn-Mn. The
former complexes exhibit partial or total quenching of holes, yet Mn in
these complexes provide a channel for a ferromagnetic arrangement of the spins
on the two Mn within the complex. This suggests that ferromagnetism in
Mn doped GaAs arises both from holes due to isolated Mn as well as from
strongly Coulomb stabilized Mn-Mn-Mn clusters.Comment: 7 figure
Proof of the thermodynamical stability of the E' center in SiO2
The E' center is a paradigmatic radiation-induced defect in SiO2 whose
peculiar EPR and hyperfine activity has been known since over 40 years. This
center has been traditionally identified with a distorted, positively-charged
oxygen vacancy V_O+. However, no direct proof of the stability of this defect
has ever been provided, so that its identification is still strongly
incomplete. Here we prove directly that distorted V_O+ is metastable and that
it satisfies the key requirements for its identification as E', such as thermal
and optical response, and activation-deactivation mechanisms.Comment: RevTeX 4 pages, 2 figure
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