563 research outputs found
Built-in bleeder system in laminated plastic structures
Method is described for interplying resin absorbing, glass bleeder cloth with the layup for fabricating laminated nonflammable plastic structures with wrinkle-free surfaces
Molecules in clusters: the case of planar LiBeBCNOF built from a triangular form LiOB and a linear four-center species FBeCN
Krueger some years ago proposed a cluster LiBeBCNOF, now called periodane.
His ground-state isomer proposal has recently been refined by Bera et al. using
DFT. Here, we take the approach of molecules in such a cluster as starting
point. We first study therefore the triangular molecule LiOB by coupled cluster
theory (CCSD) and thereby specify accurately its equilibrium geometry in free
space. The second fragment we consider is FBeCN, but treated now by restricted
Hartree-Fock (RHF) theory. This four-center species is found to be linear, and
the bond lengths are obtained from both RHF and CCSD calculations. Finally, we
bring these two entities together and find that while LiOB remains largely
intact, FBeCN becomes bent by the interaction with LiOB. Hartree-Fock and CCSD
theories then predict precisely the same lowest isomer found by Bera et al.
solely on the basis of DFT.Comment: to appear in Phys. Lett.
Unscreened Coulomb repulsion in the one dimensional electron gas
A tight binding model of electrons interacting via bare Coulomb repulsion is
numerically investigated by use of the Density Matrix Renormalization Group
method which we prove applicable also to very long range potentials. From the
analysis of the elementary excitations, of the spin and charge correlation
functions and of the momentum distribution, a picture consistent with the
formation of a one dimensional "Wigner crystal" emerges, in quantitative
agreement with a previous bosonization study. At finite doping, Umklapp
scattering is shown to be ineffective in the presence of long range forces.Comment: RevTex, 5 pages with 8 eps figures. To be published on Phys. Rev.
Operator Transformations Between Exactly Solvable Potentials and Their Lie Group Generators
One may obtain, using operator transformations, algebraic relations between
the Fourier transforms of the causal propagators of different exactly solvable
potentials. These relations are derived for the shape invariant potentials.
Also, potentials related by real transformation functions are shown to have the
same spectrum generating algebra with Hermitian generators related by this
operator transformation.Comment: 13 pages with one Postscript figure, uses LaTeX2e with revte
Developing and Verifying the Psychometric Integrity of the Certification Examination for Imaging Informatics Professionals
The American Board of Imaging Informatics (ABII) was founded in 2005 by the Society of Imaging Informatics in Medicine (SIIM) and the American Registry of Radiologic Technologists (ARRT). ABII’s mission is to enhance patient care, professionalism, and competence in imaging informatics. This is accomplished primarily through the development and administration of a certification examination. The creation of the exam has been an exercise in open community involvement with SIIM providing access to the PACS community and ARRT providing skilled psychometric support to ensure a balanced and comprehensive examination. The process to generate the exam required several years and the efforts of dozens of subject matter experts active who volunteered to submit and validate questions for the examination. This article describes the organizational and statistical processes used to generate test items, assemble test forms, set performance standards, and validate test scores
Point defect structure of La-doped SrTiO3 ceramics with colossal permittivity
Sr1-xLaxTiO3 (SLTO) ceramics with colossal permittivity were fabricated by conventional solid-state reaction method. The point defects of pure STO and SLTO ceramics were analyzed by Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening (CDB). The charge compensation mechanisms and dielectric properties of ceramics were investigated. The results indicated that the intrinsic defects in pure STO ceramics were mainly V00 00 Ti . The charge compensation mechanism of SLTO ceramics was predominantly formation of V00 Sr . With increasing La content, εr of SLTO ceramics increased up to ~70000 at room temperature. The results of first-principle calculations indicated that the colossal permittivity came from a sharp polarization increase caused by dipole structure of defects. tand of SLTO ceramics showed obvious Debye relaxation at high temperatures and the relaxation showed a multiple relaxation times derived from different kinds of polarization mechanism. The main polarization mechanism of SLTO ceramics gradually changed from ion displacement polarization to defect dipole polarization influenced by the concentration of La dopants
Time evolution of the Rabi Hamiltonian from the unexcited vacuum
The Rabi Hamiltonian describes a single mode of electromagnetic radiation
interacting with a two-level atom. Using the coupled cluster method, we
investigate the time evolution of this system from an initially empty field
mode and an unexcited atom. We give results for the atomic inversion and field
occupation, and find that the virtual processes cause the field to be squeezed.
No anti-bunching occurs.Comment: 25 pages, 8 figures, RevTe
Influence of severe plastic deformation on the precipitation hardening of a FeSiTi steel
The combined strengthening effects of grain refinement and high precipitated
volume fraction (~6at.%) on the mechanical properties of FeSiTi alloy subjected
to SPD processing prior to aging treatment were investigated by atom probe
tomography and scanning transmission electron microscopy. It was shown that the
refinement of the microstructure affects the precipitation kinetics and the
spatial distribution of the secondary hardening intermetallic phase, which was
observed to nucleate heterogeneously on dislocations and sub-grain boundaries.
It was revealed that alloys successively subjected to these two strengthening
mechanisms exhibit a lower increase in mechanical strength than a simple
estimation based on the summation of the two individual strengthening
mechanisms
Outstanding Thermal Conductivity of Single Atomic Layer Isotope-Modified Boron Nitride
Materials with high thermal conductivities (k) is valuable to solve the
challenge of waste heat dissipation in highly integrated and miniaturized
modern devices. Herein, we report the first synthesis of atomically thin
isotopically pure hexagonal boron nitride (BN) and its one of the highest k
among all semiconductors and electric insulators. Single atomic layer (1L) BN
enriched with 11B has a k up to 1009 W/mK at room temperature. We find that the
isotope engineering mainly suppresses the out-of-plane optical (ZO) phonon
scatterings in BN, which subsequently reduces acoustic-optical scatterings
between ZO and transverse acoustic (TA) and longitudinal acoustic (LA) phonons.
On the other hand, reducing the thickness to single atomic layer diminishes the
interlayer interactions and hence Umklapp scatterings of the out-of-plane
acoustic (ZA) phonons, though this thickness-induced k enhancement is not as
dramatic as that in naturally occurring BN. With many of its unique properties,
atomically thin monoisotopic BN is promising on heat management in van der
Waals (vdW) devices and future flexible electronics. The isotope engineering of
atomically thin BN may also open up other appealing applications and
opportunities in 2D materials yet to be explored
Summing Divergent Perturbative Series in a Strong Coupling Limit. The Gell-Mann - Low Function of the \phi^4 Theory
An algorithm is proposed for determining asymptotics of the sum of a
perturbative series in the strong coupling limit using given values of the
expansion coefficients. Operation of the algorithm is illustrated by test
examples, method for estimating errors is developed, and an optimization
procedure is described. Application of the algorithm to the theory
gives a behavior at large for its Gell-Mann
-- Low function. The fact that the exponent is close to unity can be
interpreted as a manifestation of the logarithmic branching of the type
(with ), which is
confirmed by independent evidence. In any case, the theory is
internally consistent. The procedure of summing perturbartive series with
arbitrary values of expansion parameter is discussed.Comment: 23 pages, PD
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