9,281 research outputs found
Short range repulsive interatomic interactions in energetic processes in solids
The repulsive interaction between two atoms at short distances is studied in
order to explore the range of validity of standard first-principles simulation
techniques and improve the available short-range potentials for the description
of energetic collision cascades in solids. Pseudopotentials represent the
weakest approximation, given their lack of explicit Pauli repulsion in the
core-core interactions. The energy (distance) scale realistically accessible is
studied by comparison with all-electron reference calculations in some binary
systems. Reference calculations are performed with no approximations related to
either core (frozen core, augmentation spheres) or basis set. This is important
since the validity of such approximations, even in all-electron calculations,
rely on the small core perturbation usual in low-energy studies. The expected
importance of semicore states is quantified. We propose a scheme for improving
the electronic screening given by pseudopotentials for very short distances.
The results of this study are applied to the assessment and improvement of
existing repulsive empirical potentials.Comment: 10 pages, 7 figure
Ab initio wavefunction based methods for excited states in solids: correlation corrections to the band structure of ionic oxides
Ab initio wavefunction based methods are applied to the study of electron
correlation effects on the band structure of oxide systems. We choose MgO as a
prototype closed-shell ionic oxide. Our analysis is based on a local
Hamiltonian approach and performed on finite fragments cut from the infinite
solid. Localized Wannier functions and embedding potentials are obtained from
prior periodic Hartree-Fock (HF) calculations. We investigate the role of
various electron correlation effects in reducing the HF band gap and modifying
the band widths. On-site and nearest-neighbor charge relaxation as well as
long-range polarization effects are calculated. Whereas correlation effects are
essential for computing accurate band gaps, we found that they produce smaller
changes on the HF band widths, at least for this material. Surprisingly, a
broadening effect is obtained for the O 2p valence bands. The ab initio data
are in good agreement with the energy gap and band width derived from
thermoreflectance and x-ray photoemission experiments. The results show that
the wavefunction based approach applied here allows for well controlled
approximations and a transparent identification of the microscopic processes
which determine the electronic band structure
Strong-correlation effects in Born effective charges
Large values of Born effective charges are generally considered as reliable
indicators of the genuine tendency of an insulator towards ferroelectric
instability. However, these quantities can be very much influenced by strong
electron correlation and metallic behavior, which are not exclusive properties
of ferroelectric materials. In this paper we compare the Born effective charges
of some prototypical ferroelectrics with those of magnetic, non-ferroelectric
compounds using a novel, self-interaction free methodology that improves on the
local-density approximation description of the electronic properties. We show
that the inclusion of strong-correlation effects systermatically reduces the
size of the Born effective charges and the electron localization lengths.
Furthermore we give an interpretation of the Born effective charges in terms of
band energy structure and orbital occupations which can be used as a guideline
to rationalize their values in the general case.Comment: 10 pages, 4 postscript figure
Towards first-principles understanding of the metal-insulator transition in fluid alkali metals
By treating the electron-ion interaction as perturbation in the
first-principles Hamiltonian, we have calculated the density response functions
of a fluid alkali metal to find an interesting charge instability due to
anomalous electronic density fluctuations occurring at some finite wave vector
{\bi Q} in a dilute fluid phase above the liquid-gas critical point. Since
|{\bi Q}| is smaller than the diameter of the Fermi surface, this instability
necessarily impedes the electric conduction, implying its close relevance to
the metal-insulator transition in fluid alkali metals.Comment: 11 pages, 5 figure
Coupling and Dissociation in Artificial Molecules
We show that the spin-and-space unrestricted Hartree-Fock method, in
conjunction with the companion step of the restoration of spin and space
symmetries via Projection Techniques (when such symmetries are broken), is able
to describe the full range of couplings in two-dimensional double quantum dots,
from the strong-coupling regime exhibiting delocalized molecular orbitals to
the weak-coupling and dissociation regimes associated with a Generalized
Valence Bond combination of atomic-type orbitals localized on the individual
dots. The weak-coupling regime is always accompanied by an antiferromagnetic
ordering of the spins of the individual dots. The cases of dihydrogen (H,
) and dilithium (Li, ) quantum dot molecules are discussed in
detail.Comment: 7 pages. Latex with 4 GIF and 1 EPS figures. Based on an invited talk
at the ISSPIC10 conference (see http://www.physics.gatech.edu/isspic10/) A
version of the manuscript with high quality figures incorporated in the text
is available at http://calcite.physics.gatech.edu/~costas/qds_isspic10.html
For related papers, see http://www.prism.gatech.edu/~ph274c
Tuning biexciton binding and anti-binding in core/shell quantum dots
We use a path integral quantum Monte Carlo method to simulate excitons and
biexcitons in core shell nanocrystals with Type-I, II and quasi-Type II band
alignments. Quantum Monte Carlo techniques allow for all quantum correlations
to be included when determining the thermal ground state, thus producing
accurate predictions of biexciton binding. These subtle quantum correlations
are found to cause the biexciton to be binding with Type-I carrier localization
and strongly anti-binding with Type-II carrier localization, in agreement with
experiment for both core shell nanocrystals and dot in rod nanocrystal
structures. Simple treatments based on perturbative approaches are shown to
miss this important transition in the biexciton binding. Understanding these
correlations offers prospects to engineer strong biexciton anti-binding which
is crucial to the design of nanocrystals for single exciton lasing
applications.Comment: 10 pages, 11 figure
Coherent view of crystal chemistry and ab initio analyses of Pb(II) and Bi(III) Lone Pair in square planar coordination
The stereochemistry of 6s2 (E) lone pair of divalent Pb and trivalent Bi
(PbII and BiIII designated by M*) in structurally related PbO, PbFX (X= Cl, Br,
I), BiOX (X= F, Cl, Br, I) and Bi2NbO5F is rationalized. The lone pair LP
presence determined by its sphere of influence E, equal to those of oxygen or
fluorine anions, was settled by its center then giving M*-E directions and
distances. Detailed description of structural features of both elements in the
title compounds characterized by [PbEO]n and [BiEO]n layers allowed to show the
evolution of M*-E distance versus the changes with the square pyramidal SP
coordination polyhedra. All are different, in red PbO one finds {PbEO4E4}
square antiprism, a {[Bi.E]O4X4Xapical} monocapped square antiprism in PbFX and
BiOX and {BiEO4F4}square antiprism in Bi2NbO5F. To analyze the crystal
chemistry results, the electronic structures of these compounds were calculated
within density functional theory DFT. Real space analyses of electron
localization illustrate a full volume development of the lone pair on PbII
within {PbEO4E4} in PbOE, {PbEF4X4} in PbFXE and Bi(III) within {BiEO4X4}
square antiprisms, contrary to Bi(III) within {[Bi.E]O4F4Fapical} monocapped
square antiprism. Larger hardness (larger bulk modules B0) and band gap
characterize BiOF versus PbO due to the presence of F which brings antibonding
Bi-F interactions oppositely to mainly bonding Bi-O. In PbFX and BiOX series
there is a systematic decrease of B0 with the increasing volume following the
nature and size of X which is decreasingly electronegative and increasingly
large. The electronic densities of states mirror these effects through the
relative energy position and relative electronegativities of F/X and O/X
leading to decrease the band gap.Comment: 20 text pages/ 10 multifigures/large review article, J. Progress
Solid State Chemistry, under production MAY 201
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