298 research outputs found
First-principles Calculation of the Formation Energy in MgO-CaO Solid Solutions
The electronic structure and total energy were calculated for ordered and
disordered MgO-CaO solid solutions within the multiple scattering theory in
real space and the local density approximation. Based on the dependence of the
total energy on the unit cell volume the equilibrium lattice parameter and
formation energy were determined for different solution compositions. The
formation energy of the solid solutions is found to be positive that is in
agreement with the experimental phase diagram, which shows a miscibility gap.Comment: 11 pages, 3 figure
Primate innate immune responses to bacterial and viral pathogens reveals an evolutionary trade-off between strength and specificity
Despite their close genetic relatedness, apes and African and Asian monkeys (AAMs) differ in their susceptibility to severe bacterial and viral infections that are important causes of human disease. Such differences between humans and other primates are thought to be a result, at least in part, of interspecies differences in immune response to infection. However, because of the lack of comparative functional data across species, it remains unclear in what ways the immune systems of humans and other primates differ. Here, we report the whole-genome transcriptomic responses of ape species (human and chimpanzee) and AAMs (rhesus macaque and baboon) to bacterial and viral stimulation. We find stark differences in the responsiveness of these groups, with apes mounting a markedly stronger early transcriptional response to both viral and bacterial stimulation, altering the transcription of ~40% more genes than AAMs. Additionally, we find that genes involved in the regulation of inflammatory and interferon responses show the most divergent early transcriptional responses across primates and that this divergence is attenuated over time. Finally, we find that relative to AAMs, apes engage a much less specific immune response to different classes of pathogens during the early hours of infection, up-regulating genes typical of anti-viral and anti-bacterial responses regardless of the nature of the stimulus. Overall, these findings suggest apes exhibit increased sensitivity to bacterial and viral immune stimulation, activating a broader array of defense molecules that may be beneficial for early pathogen killing at the potential cost of increased energy expenditure and tissue damage. Copyrigh
Ab initio study of ferroelectric domain walls in PbTiO3
We have investigated the atomistic structure of the 180-degree and 90-degree
domain boundaries in the ferroelectric perovskite compound PbTiO3 using a
first-principles ultrasoft-pseudopotential approach. For each case we have
computed the position, thickness and creation energy of the domain walls, and
an estimate of the barrier height for their motion has been obtained. We find
both kinds of domain walls to be very narrow with a similar width of the order
of one to two lattice constants. The energy of the 90-dergree domain wall is
calculated to be 35 mJ/m^2, about a factor of four lower than the energy of its
180-degree counterpart, and only a miniscule barrier for its motion is found.
As a surprising feature we detected a small offset of 0.15-0.2 eV in the
electrostatic potential across the 90-degree domain wall.Comment: 12 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/bm_dw/index.htm
Unscreened Hartree-Fock calculations for metallic Fe, Co, Ni, and Cu from ab-initio Hamiltonians
Unscreened Hartree-Fock approximation (HFA) calculations for metallic Fe, Co,
Ni, and Cu are presented, by using a quantum-chemical approach. We believe that
these are the first HFA results to have been done for crystalline 3d transition
metals. Our approach uses a linearized muffin-tin orbital calculation to
determine Bloch functions for the Hartree one-particle Hamiltonian, and from
these obtains maximally localized Wannier functions, using a method proposed by
Marzari and Vanderbilt. Within this Wannier basis all relevant one-particle and
two-particle Coulomb matrix elements are calculated. The resulting
second-quantized multi-band Hamiltonian with ab-initio parameters is studied
within the simplest many-body approximation, namely the unscreened,
self-consistent HFA, which takes into account exact exchange and is free of
self-interactions. Although the d-bands sit considerably lower within HFA than
within the local (spin) density approximation L(S)DA, the exchange splitting
and magnetic moments for ferromagnetic Fe, Co, and Ni are only slightly larger
in HFA than what is obtained either experimentally or within LSDA. The HFA
total energies are lower than the corresponding LSDA calculations. We believe
that this same approach can be easily extended to include more sophisticated
ab-initio many-body treatments of the electronic structure of solids.Comment: 11 papes, 7 figures, 5 table
Formulae for zero-temperature conductance through a region with interaction
The zero-temperature linear response conductance through an interacting
mesoscopic region attached to noninteracting leads is investigated. We present
a set of formulae expressing the conductance in terms of the ground-state
energy or persistent currents in an auxiliary system, namely a ring threaded by
a magnetic flux and containing the correlated electron region. We first derive
the conductance formulae for the noninteracting case and then give arguments
why the formalism is also correct in the interacting case if the ground state
of a system exhibits Fermi liquid properties. We prove that in such systems,
the ground-state energy is a universal function of the magnetic flux, where the
conductance is the only parameter. The method is tested by comparing its
predictions with exact results and results of other methods for problems such
as the transport through single and double quantum dots containing interacting
electrons. The comparisons show an excellent quantitative agreement.Comment: 18 pages, 18 figures; to appear in Phys. Rev.
First-principles study of the ferroelastic phase transition in CaCl_2
First-principles density-functional calculations within the local-density
approximation and the pseudopotential approach are used to study and
characterize the ferroelastic phase transition in calcium chloride (CaCl_2). In
accord with experiment, the energy map of CaCl_2 has the typical features of a
pseudoproper ferroelastic with an optical instability as ultimate origin of the
phase transition. This unstable optic mode is close to a pure rigid unit mode
of the framework of chlorine atoms and has a negative Gruneisen parameter. The
ab-initio ground state agrees fairly well with the experimental low temperature
structure extrapolated at 0K. The calculated energy map around the ground state
is interpreted as an extrapolated Landau free-energy and is successfully used
to explain some of the observed thermal properties. Higher-order anharmonic
couplings between the strain and the unstable optic mode, proposed in previous
literature as important terms to explain the soft-phonon temperature behavior,
are shown to be irrelevant for this purpose. The LAPW method is shown to
reproduce the plane-wave results in CaCl_2 within the precision of the
calculations, and is used to analyze the relative stability of different phases
in CaCl_2 and the chemically similar compound SrCl_2.Comment: 9 pages, 6 figures, uses RevTeX
Statistical Theory of Spin Relaxation and Diffusion in Solids
A comprehensive theoretical description is given for the spin relaxation and
diffusion in solids. The formulation is made in a general
statistical-mechanical way. The method of the nonequilibrium statistical
operator (NSO) developed by D. N. Zubarev is employed to analyze a relaxation
dynamics of a spin subsystem. Perturbation of this subsystem in solids may
produce a nonequilibrium state which is then relaxed to an equilibrium state
due to the interaction between the particles or with a thermal bath (lattice).
The generalized kinetic equations were derived previously for a system weakly
coupled to a thermal bath to elucidate the nature of transport and relaxation
processes. In this paper, these results are used to describe the relaxation and
diffusion of nuclear spins in solids. The aim is to formulate a successive and
coherent microscopic description of the nuclear magnetic relaxation and
diffusion in solids. The nuclear spin-lattice relaxation is considered and the
Gorter relation is derived. As an example, a theory of spin diffusion of the
nuclear magnetic moment in dilute alloys (like Cu-Mn) is developed. It is shown
that due to the dipolar interaction between host nuclear spins and impurity
spins, a nonuniform distribution in the host nuclear spin system will occur and
consequently the macroscopic relaxation time will be strongly determined by the
spin diffusion. The explicit expressions for the relaxation time in certain
physically relevant cases are given.Comment: 41 pages, 119 Refs. Corrected typos, added reference
Screened Coulomb interactions in metallic alloys: I. Universal screening in the atomic sphere approximation
We have used the locally self-consistent Green's function (LSGF) method in
supercell calculations to establish the distribution of the net charges
assigned to the atomic spheres of the alloy components in metallic alloys with
different compositions and degrees of order. This allows us to determine the
Madelung potential energy of a random alloy in the single-site mean field
approximation which makes the conventional single-site density-functional-
theory coherent potential approximation (SS-DFT-CPA) method practically
identical to the supercell LSGF method with a single-site local interaction
zone that yields an exact solution of the DFT problem. We demonstrate that the
basic mechanism which governs the charge distribution is the screening of the
net charges of the alloy components that makes the direct Coulomb interactions
short-ranged. In the atomic sphere approximation, this screening appears to be
almost independent of the alloy composition, lattice spacing, and crystal
structure. A formalism which allows a consistent treatment of the screened
Coulomb interactions within the single-site mean-filed approximation is
outlined. We also derive the contribution of the screened Coulomb interactions
to the S2 formalism and the generalized perturbation method.Comment: 28 pages, 8 figure
Structural, Electronic, and Magnetic Properties of MnO
We calculate the structural, electronic, and magnetic properties of MnO from
first principles, using the full-potential linearized augmented planewave
method, with both local-density and generalized-gradient approximations to
exchange and correlation. We find the ground state to be of rhombohedrally
distorted B1 structure with compression along the [111] direction,
antiferromagnetic with type-II ordering, and insulating, consistent with
experiment. We show that the distortion can be understood in terms of a
Heisenberg model with distance dependent nearest-neighbor and
next-nearest-neighbor couplings determined from first principles. Finally, we
show that magnetic ordering can induce significant charge anisotropy, and give
predictions for electric field gradients in the ground-state rhombohedrally
distorted structure.Comment: Submitted to Physical Review B. Replaced: regenerated figures to
resolve font problems in automatically generated pd
Interatomic potentials for atomistic simulations of the Ti-Al system
Semi-empirical interatomic potentials have been developed for Al, alpha-Ti,
and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large
database of experimental as well as ab-initio data. The ab-initio calculations
were performed by the linear augmented plane wave (LAPW) method within the
density functional theory to obtain the equations of state for a number of
crystal structures of the Ti-Al system. Some of the calculated LAPW energies
were used for fitting the potentials while others for examining their quality.
The potentials correctly predict the equilibrium crystal structures of the
phases and accurately reproduce their basic lattice properties. The potentials
are applied to calculate the energies of point defects, surfaces, planar faults
in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al
system, the proposed potentials provide reasonable description of the lattice
thermal expansion, demonstrating their usefulness in the molecular dynamics or
Monte Carlo studies at high temperatures. The energy along the tetragonal
deformation path (Bain transformation) in gamma-TiAl calculated with the EAM
potential is in a fairly good agreement with LAPW calculations. Equilibrium
point defect concentrations in gamma-TiAl are studied using the EAM potential.
It is found that antisite defects strongly dominate over vacancies at all
compositions around stoichiometry, indicating that gamm-TiAl is an antisite
disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press
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