12,227 research outputs found
Evidence for the absence of regularization corrections to the partial-wave renormalization procedure in one-loop self energy calculations in external fields
The equivalence of the covariant renormalization and the partial-wave
renormaliz ation (PWR) approach is proven explicitly for the one-loop
self-energy correction (SE) of a bound electron state in the presence of
external perturbation potentials. No spurious correctio n terms to the
noncovariant PWR scheme are generated for Coulomb-type screening potentia ls
and for external magnetic fields. It is shown that in numerical calculations of
the SE with Coulombic perturbation potential spurious terms result from an
improper treatment of the unphysical high-energy contribution. A method for
performing the PWR utilizing the relativistic B-spline approach for the
construction of the Dirac spectrum in external magnetic fields is proposed.
This method is applied for calculating QED corrections to the bound-electron
-factor in H-like ions. Within the level of accuracy of about 0.1% no
spurious terms are generated in numerical calculations of the SE in magnetic
fields.Comment: 22 pages, LaTeX, 1 figur
PANIC: A Near-infrared Camera for the Magellan Telescopes
PANIC (Persson's Auxiliary Nasmyth Infrared Camera) is a near-infrared camera
designed to operate at any one of the f/11 folded ports of the 6.5m Magellan
telescopes at Las Campanas Observatory, Chile. The instrument is built around a
simple, all-refractive design that reimages the Magellan focal plane to a plate
scale of 0.125'' pixel^{-1} onto a Rockwell 1024x1024 HgCdTe detector. The
design goals for PANIC included excellent image quality to sample the superb
seeing measured with the Magellan telescopes, high throughput, a relatively
short construction time, and low cost. PANIC has now been in regular operation
for over one year and has proved to be highly reliable and produce excellent
images. The best recorded image quality has been ~0.2'' FWHM.Comment: 8 pages, 5 figures. To appear in "Astronomical Telescopes and
Instrumentation," Proc SPIE (Glasgow), June 2004. Version with higher
resolution figures is available at
http://cfa-www.harvard.edu/~pmartini/professional/publications/panic.pd
Rubber friction: role of the flash temperature
When a rubber block is sliding on a hard rough substrate, the substrate
asperities will exert time-dependent deformations of the rubber surface
resulting in viscoelastic energy dissipation in the rubber, which gives a
contribution to the sliding friction. Most surfaces of solids have roughness on
many different length scales, and when calculating the friction force it is
necessary to include the viscoelastic deformations on all length scales. The
energy dissipation will result in local heating of the rubber. Since the
viscoelastic properties of rubber-like materials are extremely strongly
temperature dependent, it is necessary to include the local temperature
increase in the analysis. At very low sliding velocity the temperature increase
is negligible because of heat diffusion, but already for velocities of order
0.01 m/s the local heating may be very important. Here I study the influence of
the local heating on the rubber friction, and I show that in a typical case the
temperature increase results in a decrease in rubber friction with increasing
sliding velocity for v > 0.01 m/s. This may result in stick-slip instabilities,
and is of crucial importance in many practical applications, e.g., for the
tire-road friction, and in particular for ABS-breaking systems.Comment: 22 pages, 27 figure
The Y-Band at 1.035 um: Photometric Calibration and the Dwarf Stellar/Sub-Stellar Color Sequence
We define and characterize a photometric bandpass (called "Y") that is
centered at 1.035 um, in between the traditionally classified ``optical'' and
``infrared'' spectral regimes. We present Y magnitudes and Y-H and Y-K colors
for a sample consisting mostly of photometric and spectral standards, spanning
the spectral type range sdO to T5V. Deep molecular absorption features in the
near-infrared spectra of extremely cool objects are such that the Y-H and Y-K
colors grow rapidly with advancing spectral type especially from late M through
mid L, substantially more rapidly than J-H or H-K which span a smaller total
dynamic range. Consistent with other near-infrared colors, however, Y-H and Y-K
colors turn blueward in the L6-L8 temperature range with later T-type objects
having colors similar to those of warmer M and L stars. Use of the Y-band
filter is nonetheless promising for easy identification of low-mass stars and
brown dwarfs, especially at young ages. The slope of the interstellar reddening
vector within this filter is A_Y = 0.38 x A_V. Reddening moves stars nearly
along the YHK dwarf color sequence making it more difficult to distinguish
unambiguously very low mass candidate brown dwarf objects from higher mass
stars seen, e.g. through the galactic plane or towards star-forming regions.
Other diagrams involving the Y-band may be somewhat more discriminating.Comment: accepted at PAS
Theory of friction: contribution from fluctuating electromagnetic field
We calculate the friction force between two semi-infinite solids in relative
parallel motion (velocity ), and separated by a vacuum gap of width . The
friction force result from coupling via a fluctuating electromagnetic field,
and can be considered as the dissipative part of the van der Waals interaction.
We consider the dependence of the friction force on the temperature , and
present a detailed discussion of the limiting cases of small and large and
.Comment: 15 pages, No figure
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propnet: A Knowledge Graph for Materials Science
Discovering the ideal material for a new application involves determining its numerous properties, such as electronic, mechanical, or thermodynamic, to match those of its desired application. The rise of high-throughput computation has meant that large databases of material properties are now accessible to scientists. However, these databases contain far more information than might appear at first glance, since many relationships exist in the materials science literature to derive, or at least approximate, additional properties. propnet is a new computational framework designed to help scientists to automatically calculate additional information from their datasets. It does this by constructing a network graph of relationships between different materials properties and traversing this graph. Initially, propnet contains a catalog of over 100 property relationships but the hope is for this to expand significantly in the future, and contributions from the community are welcomed
Probing Electron Correlation via Attosecond XUV Pulses in the Two-Photon Double Ionization of Helium
Recent experimental developments of high-intensity, short-pulse XUV light
sources are enhancing our ability to study electron-electron correlations. We
perform time-dependent calculations to investigate the so-called "sequential"
regime (photon energy above 54.4 eV) in the two-photon double ionization of
helium. We show that attosecond pulses allow to induce and probe angular and
energy correlations of the emitted electrons. The final momentum distribution
reveals regions dominated by the Wannier ridge break-up scenario and by
post-collision interaction.Comment: 4 pages, 5 figure
Self-energy correction to the bound-electron g factor in H-like ions
The one-loop self-energy correction to the 1s electron g factor is evaluated
to all orders in Z\alpha with an accuracy, which is essentially better than
that of previous calculations of this correction. As a result, the uncertainty
of the theoretical prediction for the bound-electron g factor in H-like carbon
is reduced by a factor of 3. This improves the total accuracy of the recent
electron-mass determination [Beier et al. Phys. Rev. Lett. 88, 011603 (2002)].
The new value of the electron mass is found to be m_e = 0.000 548 579 909 3(3)
u
On the semi-classical analysis of the groundstate energy of the Dirichlet Pauli operator in non-simply connected domains
We consider the Dirichlet Pauli operator in bounded connected domains in the
plane, with a semi-classical parameter. We show, in particular, that the ground
state energy of this Pauli operator will be exponentially small as the
semi-classical parameter tends to zero and estimate this decay rate. This
extends our results, discussing the results of a recent paper by
Ekholm--Kova\v{r}\'ik--Portmann, to include also non-simply connected domains.Comment: 15 pages, 4 figure
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