18,374 research outputs found
Sudden approximation applied to rotational excitation of molecules by atoms. ii- scat- tering of polar diatomics
Sudden approximation applied to computation of rotational transition probability and inelastic total cross sections for scattering of polar and nonpolar diatomic molecules by atom
Operator Inference for Non-Intrusive Model Reduction of Systems with Non-Polynomial Nonlinear Terms
Maxwell Fields in Spacetimes Admitting Non-Null Killing Vectors
We consider source-free electromagnetic fields in spacetimes possessing a
non-null Killing vector field, . We assume further that the
electromagnetic field tensor, , is invariant under the action of the
isometry group induced by . It is proved that whenever the two
potentials associated with the electromagnetic field are functionally
independent the entire content of Maxwell's equations is equivalent to the
relation \n^aT_{ab}=0. Since this relation is implied by Einstein's equation
we argue that it is enough to solve merely Einstein's equation for these
electrovac spacetimes because the relevant equations of motion will be
satisfied automatically. It is also shown that for the exceptional case of
functionally related potentials \n^aT_{ab}=0 implies along with one of the
relevant equations of motion that the complementary equation concerning the
electromagnetic field is satisfied.Comment: 7 pages,PACS numbers: 04.20.Cv, 04.20.Me, 04.40.+
Small Footprint Multilayered Millimeter-Wave Antennas and Feeding Networks for Multi-Dimensional Scanning and High-Density Integrated Systems
This paper overviews the state-of-the-art of substrate integrated waveguide (SIW) techniques in the design and realization of innovative low-cost, low-profile and low-loss (L3) millimeter-wave antenna elements, feeding networks and arrays for various wireless applications. Novel classes of multilayered antenna structures and systems are proposed and studied to exploit the vertical dimension of planar structures to overcome certain limita-tions in standard two-dimensional (2-D) topologies. The developed structures are based on two techniques, namely multi-layer stacked structures and E-plane corners. Differ-ent E-plane structures realised with SIW waveguide are presented, thereby demonstrating the potential of the proposed techniques as in multi-polarization antenna feeding. An array of 128 elements shows low SLL and height gain with just 200g of the total weight. Two versions of 2-D scanning multi-beam are presented, which effectively combine frequency scanning with beam forming networks. Adding the benefits of wide band performance to the multilayer structure, two bi-layer structures are investigated. Different stacked antennas and arrays are demonstrated to optimise the targeted antenna performances in the smallest footprint possible. These structures meet the requirement for developing inexpensive compact millimeter-wave antennas and antenna systems. Different structures and architectures are theoretically and experimentally studied and discussed for specific space- and ground-based appli-cations. Practical issues such as high-density integration and high-volume manufacturability are also addressed
Anderson transitions in three-dimensional disordered systems with randomly varying magnetic flux
The Anderson transition in three dimensions in a randomly varying magnetic
flux is investigated in detail by means of the transfer matrix method with high
accuracy. Both, systems with and without an additional random scalar potential
are considered. We find a critical exponent of with random
scalar potential. Without it, is smaller but increases with the system
size and extrapolates within the error bars to a value close to the above. The
present results support the conventional classification of universality classes
due to symmetry.Comment: 4 pages, 2 figures, to appear in Phys. Rev.
On three topical aspects of the N=28 isotonic chain
The evolution of single-particle orbits along the N=28 isotonic chain is
studied within the framework of a relativistic mean-field approximation. We
focus on three topical aspects of the N=28 chain: (a) the emergence of a new
magic number at Z=14; (b) the possible erosion of the N=28 shell; and (c) the
weakening of the spin-orbit splitting among low-j neutron orbits. The present
model supports the emergence of a robust Z=14 subshell gap in 48Ca, that
persists as one reaches the neutron-rich isotone 42Si. Yet the proton removal
from 48Ca results in a significant erosion of the N=28 shell in 42Si. Finally,
the removal of s1/2 protons from 48Ca causes a ~50% reduction of the spin-orbit
splitting among neutron p-orbitals in 42Si.Comment: 12 pages with 5 color figure
Single pulse and profile variability study of PSR J1022+1001
Millisecond pulsars (MSPs) are known as highly stable celestial clocks.
Nevertheless, recent studies have revealed the unstable nature of their
integrated pulse profiles, which may limit the achievable pulsar timing
precision. In this paper, we present a case study on the pulse profile
variability of PSR J1022+1001. We have detected approximately 14,000 sub-pulses
(components of single pulses) in 35-hr long observations, mostly located at the
trailing component of the integrated profile. Their flux densities and
fractional polarisation suggest that they represent the bright end of the
energy distribution in ordinary emission mode and are not giant pulses. The
occurrence of sub-pulses from the leading and trailing components of the
integrated profile is shown to be correlated. For sub-pulses from the latter, a
preferred pulse width of approximately 0.25 ms has been found. Using
simultaneous observations from the Effelsberg 100-m telescope and the
Westerbork Synthesis Radio Telescope, we have found that the integrated profile
varies on a timescale of a few tens of minutes. We show that improper
polarisation calibration and diffractive scintillation cannot be the sole
reason for the observed instability. In addition, we demonstrate that timing
residuals generated from averages of the detected sub-pulses are dominated by
phase jitter, and place an upper limit of ~700 ns for jitter noise based on
continuous 1-min integrations.Comment: 13 pages, 20 figures, 3 tables, accepted for publication in MNRA
Comparison and Appraisal of Approximation Formulas for Total Elastic Molecular Scattering Cross Sections
The MasseyâMohr (MM), Schiff (S), and LandauâLifshitz (LL) approximations for the total elastic cross section (Q) are intercompared. All can be shown to follow from the same assumption, (i.e., the classical smallâangle deflection function, thence the JeffreysâBorn phases via the semiclassical equivalence relationship), sufficing to determine the velocity dependence of Q. Thus, for V=±C(s)/rs, Q(s)=p(s)[C(s)/hv ]2/(sâ1) The coefficient p(s) is the same for the S and LL approximations; the ratio pSLL(s)/pMM(s)â„1, (<1.075); it is 1.0709 and 1.0458 for s=6 and 12, respectively.A numerical calculation for a repulsive (s=12) interaction shows that the SLL formula reproduces the partialâwave calculated Q to within â%. A graphical presentation suggests the generality of this result; it also indicates the source of bias in the MM approximation. For a ``realistic'' intermolecular potential, (restricting consideration to collisions in the ``thermal'' energy range), the influence of the repulsion is only to produce undulations in Q(v); the correct value of C(6) may be obtained by velocity averaging the ``apparent'' CSLL(6).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70541/2/JCPSA6-38-10-2507-1.pd
Low-Frequency Spectral Turn-Overs in Millisecond Pulsars Studied from Imaging Observations
Measurements of pulsar flux densities are of great importance for
understanding the pulsar emission mechanism and for predictions of pulsar
survey yields and the pulsar population at large. Typically these flux
densities are determined from phase-averaged "pulse profiles", but this method
has limited applicability at low frequencies because the observed pulses can
easily be spread out by interstellar effects like scattering or dispersion,
leading to a non-pulsed continuum component that is necessarily ignored in this
type of analysis. In particular for the class of the millisecond pulsars (MSPs)
at frequencies below 200MHz, such interstellar effects can seriously compromise
de- tectability and measured flux densities. In this paper we investigate MSP
spectra based on a complementary approach, namely through investigation of
archival con- tinuum imaging data. Even though these images lose sensitivity to
pulsars since the on-pulse emission is averaged with off-pulse noise, they are
insensitive to effects from scattering and provide a reliable way to determine
the flux density and spectral indices of MSPs based on both pulsed and unpulsed
components. Using the 74MHz VLSSr as well as the 325MHz WENSS and 1.4GHz NVSS
catalogues, we investigate the imaging flux densities of MSPs and evaluate the
likelihood of spectral turn-overs in this population. We determine three new
MSP spectral indices and identify six new MSPs with likely spectral turn-overs.Comment: 10 pages, 4 figures, 3 tables, accepted for publication in MNRA
A New Spinning-test Method
This report contains a description of a new spinning-test arrangement wherein the otherwise customary rotation of the model about a fixed axis is abandoned in favor of a corresponding rotation of the air stream. The advantage of this method lies in the fact that the model is at rest while the spin is recorded. In this manner it is possible to secure systematic results with little loss of time while employing 3- or 6-component wind-tunnel balances. The troublesome equalization of the mass forces is eliminated and the flow phenomena are accessible to direct observation
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