26,089 research outputs found
The quantum metrology triangle and the re-definition of the SI ampere and kilogram; Analysis of a reduced set of observational equations
We have developed a set of seven observational equations that include all of
the physics necessary to relate the most important of the fundamental constants
to the definitions of the SI kilogram and ampere. We have used these to
determine the influence of alternative definitions being considered for the SI
kilogram and ampere on the uncertainty of three of the fundamental constants
(h, e and mu). We have also reviewed the experimental evidence for the
exactness of the quantum metrology triangle resulting from experiments
combining the quantum Hall effect, the Josephson effects and single-electron
tunnelling.Comment: 16 pages, 3 figures & 5 table
Widespread HCN maser emission in carbon-rich evolved stars
Context. HCN is a major constituent of the circumstellar envelopes of
carbon-rich evolved stars, and rotational lines from within its vibrationally
excited states probe parts of these regions closest to the stellar surface. A
number of such lines are known to show maser action. Historically, in one of
them, the 177 GHz line in the -doubled bending mode has
been found to show relatively strong maser action, with results only published
for a single object, the archetypical high-mass loss asymptotic giant branch
(AGB) star IRC+10216. Aims. To examine how common 177 GHz HCN maser emission
is, we conducted an exploratory survey for this line toward a select sample of
carbon-rich asymptotic giant branch stars that are observable from the southern
hemisphere. Methods. We used the Atacama Pathfinder Experiment 12 meter
submillimeter Telescope (APEX) equipped with a new receiver to simultaneously
observe three HCN rotational transitions, the and -doublet components, and the line from
the (0,0,0) ground state. Results. The maser line is
detected toward 11 of 13 observed sources, which all show emission in the
(0,0,0) transition. In most of the sources, the peak intensity of the
line rivals that of the (0,0,0) line; in two sources,
it is even stronger. Except for the object with the highest mass-loss rate,
IRC+10216, the line covers a smaller velocity range
than the (0,0,0) line. Conclusions. Maser emission in the 177 GHz
line of HCN appears to be common in
carbon-rich AGB stars. (Abbreviated)Comment: 12 pages (including appendix), 3 figures / Astronomy & Astrophysics
(in press
An analytical analysis of vesicle tumbling under a shear flow
Vesicles under a shear flow exhibit a tank-treading motion of their membrane,
while their long axis points with an angle < 45 degrees with respect to the
shear stress if the viscosity contrast between the interior and the exterior is
not large enough. Above a certain viscosity contrast, the vesicle undergoes a
tumbling bifurcation, a bifurcation which is known for red blood cells. We have
recently presented the full numerical analysis of this transition. In this
paper, we introduce an analytical model that has the advantage of being both
simple enough and capturing the essential features found numerically. The model
is based on general considerations and does not resort to the explicit
computation of the full hydrodynamic field inside and outside the vesicle.Comment: 19 pages, 9 figures, to be published in Phys. Rev.
Energy Gaps and Kohn Anomalies in Elemental Superconductors
The momentum and temperature dependence of the lifetimes of acoustic phonons
in the elemental superconductors Pb and Nb was determined by resonant spin-echo
spectroscopy with neutrons. In both elements, the superconducting energy gap
extracted from these measurements was found to converge with sharp anomalies
originating from Fermi-surface nesting (Kohn anomalies) at low temperatures.
The results indicate electron many-body correlations beyond the standard
theoretical framework for conventional superconductivity. A possible mechanism
is the interplay between superconductivity and spin- or charge-density-wave
fluctuations, which may induce dynamical nesting of the Fermi surface
Ptychographic reconstruction of attosecond pulses
We demonstrate a new attosecond pulse reconstruction modality which uses an
algorithm that is derived from ptychography. In contrast to other methods,
energy and delay sampling are not correlated, and as a result, the number of
electron spectra to record is considerably smaller. Together with the robust
algorithm, this leads to a more precise and fast convergence of the
reconstruction.Comment: 12 pages, 7 figures, the MATLAB code for the method described in this
paper is freely available at
http://figshare.com/articles/attosecond_Extended_Ptychographyc_Iterative_Engine_ePIE_/160187
Ultrafast Molecular Imaging by Laser Induced Electron Diffraction
We address the feasibility of imaging geometric and orbital structure of a
polyatomic molecule on an attosecond time-scale using the laser induced
electron diffraction (LIED) technique. We present numerical results for the
highest molecular orbitals of the CO2 molecule excited by a near infrared
few-cycle laser pulse. The molecular geometry (bond-lengths) is determined
within 3% of accuracy from a diffraction pattern which also reflects the nodal
properties of the initial molecular orbital. Robustness of the structure
determination is discussed with respect to vibrational and rotational motions
with a complete interpretation of the laser-induced mechanisms
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