89,103 research outputs found
Theoretical analysis of dynamic chemical imaging with lasers using high-order harmonic generation
We report theoretical investigations of the tomographic procedure suggested
by Itatani {\it et al.} [Nature, {\bf 432} 867 (2004)] for reconstructing
highest occupied molecular orbitals (HOMO) using high-order harmonic generation
(HHG). Using the limited range of harmonics from the plateau region, we found
that under the most favorable assumptions, it is still very difficult to obtain
accurate HOMO wavefunction, but the symmetry of the HOMO and the internuclear
separation between the atoms can be accurately extracted, especially when
lasers of longer wavelengths are used to generate the HHG. We also considered
the possible removal or relaxation of the approximations used in the
tomographic method in actual applications. We suggest that for chemical
imaging, in the future it is better to use an iterative method to locate the
positions of atoms in the molecule such that the resulting HHG best fits the
macroscopic HHG data, rather than by the tomographic method.Comment: 13 pages, 14 figure
Thermodynamic dislocation theory of high-temperature deformation in aluminum and steel
The statistical-thermodynamic dislocation theory developed in previous papers
is used here in an analysis of high-temperature deformation of aluminum and
steel. Using physics-based parameters that we expect theoretically to be
independent of strain rate and temperature, we are able to fit experimental
stress-strain curves for three different strain rates and three different
temperatures for each of these two materials. Our theoretical curves include
yielding transitions at zero strain in agreement with experiment. We find that
thermal softening effects are important even at the lowest temperatures and
smallest strain rates.Comment: 7 pages, 8 figure
A Vaccine Against Group B Streptococcus: Recent Advances
Group B streptococcus (GBS) causes a high burden of neonatal and infant disease globally. Implementing a vaccine for pregnant women is a promising strategy to prevent neonatal and infant GBS disease and has been identified as a priority by the World Health Organisation (WHO). GBS serotype-specific polysaccharide – protein conjugate vaccines are at advanced stages of development, but a large number of participants would be required to undertake Phase III clinical efficacy trials. Efforts are therefore currently focused on establishing serocorrelates of protection in natural immunity studies as an alternative pathway for licensure of a GBS vaccine, followed by Phase IV studies to evaluate safety and effectiveness. Protein vaccines are in earlier stages of development but are highly promising as they might confer protection irrespective of serotype. Further epidemiological, immunological and health economic studies are required to enable the vaccine to reach its target population as soon as possible
A continuum-microscopic method based on IRBFs and control volume scheme for viscoelastic fluid flows
A numerical computation of continuum-microscopic model for visco-elastic flows based on the Integrated Radial Basis Function (IRBF) Control Volume and the Stochastic Simulation Techniques (SST) is reported in this paper. The macroscopic flow equations are closed by a stochastic equation for the extra stress at the microscopic level. The former are discretised by a 1D-IRBF-CV method while the latter is integrated with Euler explicit or Predictor-Corrector schemes. Modelling is very efficient as it is based on Cartesian grid, while the integrated RBF approach enhances both the stability of the procedure and the accuracy of the solution. The proposed method is demonstrated with the solution of the start-up Couette flow of the Hookean and FENE dumbbell model fluids
Quarkonium above deconfinement as an open quantum system
Quarkonium at temperatures above deconfinement is modeled as an open quantum
system, whose dynamics is determined not just by a potential energy and mass,
but also by a drag coefficient which characterizes its interaction with the
medium. The reduced density matrix for a heavy particle experiencing
dissipative forces is expressed as an integral over paths in imaginary time and
evaluated numerically. We demonstrate that dissipation could affect the
Euclidean heavy-heavy correlators calculated in lattice simulations at
temperatures just above deconfinement.Comment: 13 pages, 1 figur
Using an ontology for interoperability and browsing of museum, library and archive information
Ontologies play an important part in the development of the future ‘semantic web’; the CIDOC conceptual reference model (CRM) is an ontology aimed at the cultural heritage domain. This paper describes a Concept Browser, developed for the EU/IST-funded SCULPTEUR project (semantic and content-based multimedia exploitation for European benefit environment (programme IST-2001-no. 35372); May 2002 to May 2005), which is able to access different museum information systems through a common ontology, the CRM. The development of this Concept Browser has required mappings from the legacy museum database systems to the CRM. The crucial process of creating the mappings is described, using the C2RMF catalogue (EROS) and library databases as a case study
Sum rules in the heavy quark limit of QCD
In the leading order of the heavy quark expansion, we propose a method within
the OPE and the trace formalism, that allows to obtain, in a systematic way,
Bjorken-like sum rules for the derivatives of the elastic Isgur-Wise function
in terms of corresponding Isgur-Wise functions of transitions to
excited states. A key element is the consideration of the non-forward
amplitude, as introduced by Uraltsev. A simplifying feature of our method is to
consider currents aligned along the initial and final four-velocities. As an
illustration, we give a very simple derivation of Bjorken and Uraltsev sum
rules. On the other hand, we obtain a new class of sum rules that involve the
products of IW functions at zero recoil and IW functions at any . Special
care is given to the needed derivation of the projector on the polarization
tensors of particles of arbitrary integer spin. The new sum rules give further
information on the slope and also on the curvature
, and imply, modulo a very natural assumption, the
inequality , and therefore the absolute bound
.Comment: 64 pages, Late
Genesis Mission Bulk Metallic Glass Solar Wind Collector: Characterization of Return Samples Available for Re-Allocation
The Genesis mission collected solar wind atoms for 28 months with a variety of collectors mounted on a spacecraft. A total of fifteen pure materials were selected as collectors based on engineering and science requirements. One of the materials was the bulk metallic glass (BMG). It was intended for collecting noble gases and solar energetic particles (SEP). This material is an amorphous metal which was custom made by C.C. Hays at the California Institute of Technology. The final glass composition is Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 (in atom percent). The BMG was located on top of the wafer array mechanism and was exposed for the entire time the science canister was open (~28 months). Fortunately, the BMG did not suffer any serious damage and was intact after the Genesis canisters hard-landing into the Utah desert (Fig. 1)
On the Floquet Theory of Delay Differential Equations
We present an analytical approach to deal with nonlinear delay differential
equations close to instabilities of time periodic reference states. To this end
we start with approximately determining such reference states by extending the
Poincar'e Lindstedt and the Shohat expansions which were originally developed
for ordinary differential equations. Then we systematically elaborate a linear
stability analysis around a time periodic reference state. This allows to
approximately calculate the Floquet eigenvalues and their corresponding
eigensolutions by using matrix valued continued fractions
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