5,553 research outputs found
Optimization of a Langmuir-Taylor detector for lithium
This paper describes the construction and optimization of a Langmuir-Taylor
detector for lithium, using a rhenium ribbon. The absolute detection
probability of this very sensitive detector is measured and the dependence of
this probability with oxygen pressure and surface temperature is studied.
Sources of background signal and their minimization are also discussed in
details. And a comparison between our data concerning the response time of the
detector and literature values is given. A theoretical analysis has been made:
this analysis supports the validity of the Saha-Langmuir law to relate the
ionization probability to the work function. Finally, the rapid variations of
the work function with oxygen pressure and temperature are explained by a
chemical equilibrium model.Comment: 11 pages, 7 figures, to appear in Rev. Sci. Instru
Microstructure and superconducting properties of hot isostatically pressed MgB2
Bulk samples of MgB2 have been formed by hot isostatic pressing (HIPping) of
commercial powder at 100MPa and 950=B0C. The resulting material is 100% dense
with a sharp superconducting transition at 37.5K. Microstructural studies have
indicated the presence of small amounts of second phases within the material,
namely MgO and B rich compositions, probably MgB4. Magnetisation measurements
performed at 20K have revealed values of Jc=1.3 x 106A/cm2 at zero field, and
9.3 x 105A/cm2 at 1T. Magneto optical (MO) studies have shown direct evidence
for the superconducting homogeneity and strong intergranular current flow in
the material.Comment: 3 pages, 6 figures, text updated, new references included and
discussed. Submitted to Superconductor Science and Technolog
Absorption-Fluctuation Theorem for Nuclear Reactions: Brink-Axel, Incomplete Fusion and All That
We discuss the connection between absorption, averages and fluctuations in
nuclear reactions. The fluctuations in the entrance channel result in the
compound nucleus, Hauser-Feshbach, cross section, the fluctuations in the
intermediate channels, result in modifications of multistep reaction cross
sections, while the fluctuations in the final channel result in hybrid cross
sections that can be used to describe incomplete fusion reactions. We discuss
the latter in details and comment on the validity of the assumptions used in
the develpoment of the Surrogate method. We also discuss the theory of
multistep reactions with regards to intermediate state fluctuations and the
energy dependence and non-locality of the intermediate channels optical
potentials.Comment: 9 pages. Contribution to the International Workshop on
Compound-Nuclear Reactions and Related Topics (CNR*2007), October 22-26,
2007, Fish Camp, California. To be published in AIP Proceedings (Editor Jutta
Escher
Dispersive Gap Mode of Phonons in Anisotropic Superconductors
We estimate the effect of the superconducting gap anisotropy in the
dispersive gap mode of phonons, which is observed by the neutron scattering on
borocarbide superconductors. We numerically analyze the phonon spectrum
considering the electron-phonon coupling, and examine contributions coming from
the gap suppression and the sign change of the pairing function on the Fermi
surface. When the sign of the pairing function is changed by the nesting
translation, the gap mode does not appear. We also discuss the suppression of
the phonon softening of the Kohn anomaly due to the onset of superconductivity.
We demonstrate that observation of the gap dispersive mode is useful for
sorting out the underlying superconducting pairing function.Comment: 7 pages, 12 figures, to be published in J. Phys. Soc. Jp
Note on Triangle Anomalies and Assignment of Singlet in 331-like Model
It is pointed out that in the like model which uses both fundamental
and complex conjugate representations for an assignment of the representations
to the left-handed quarks and the scalar representation to their corresponding
right-handed counterparts, the nature of the scalar should be taken into
account in order to make the fermion triangle anomalies in the theory
anomaly-free, i.e. renormalizable in a sense with no anomalies, even after the
spontaneous symmetry breaking.Comment: 8 page no figures, acknowledgments adde
Terminal velocity and drag reduction measurements on superhydrophobic spheres
Super water-repellent surfaces occur naturally on plants and aquatic insects and are created in the laboratory by combining micro- or nanoscale surface topographic features with hydrophobic surface chemistry. When such types of water-repellent surfaces are submerged they can retain a film of air (a plastron). In this work, we report measurements of the terminal velocity of solid acrylic spheres with various surface treatments settling under the action of gravity in water. We observed increases in terminal velocity corresponding to drag reduction of between 5% and 15% for superhydrophobic surfaces that carry plastrons
Interplay of charge and orbital ordering in manganese perovskites
A model of localized classical electrons coupled to lattice degrees of
freedom and, via the Coulomb interaction, to each other, has been studied to
gain insight into the charge and orbital ordering observed in lightly doped
manganese perovskites. Expressions are obtained for the minimum energy and
ionic displacements caused by given hole and electron orbital configurations.
The expressions are analyzed for several hole configurations, including that
experimentally observed by Yamada et al. in La_{7/8}Sr_{1/8}MnO_3. We find
that, although the preferred charge and orbital ordering depend sensitively on
parameters, there are ranges of the parameters in which the experimentally
observed hole configuration has the lowest energy. For these parameter values
we also find that the energy differences between different hole configurations
are on the order of the observed charge ordering transition temperature. The
effects of additional strains are also studied. Some results for
La_{1/2}Ca_{1/2}MnO_3 are presented, although our model may not adequately
describe this material because the high temperature phase is metallic.Comment: 12 pages in RevTex, 5 figures in PS files, to appear in Phys. Rev. B
(New paragraphs and references added, typos corrected
The Rewards of Patience: An 822 Day Time Delay in the Gravitational Lens SDSS J1004+4112
We present 107 new epochs of optical monitoring data for the four brightest
images of the gravitational lens SDSS J1004+4112 observed between October 2006
and June 2007. Combining this data with the previously obtained light curves,
we determine the time delays between images A, B and C. We confirm our previous
measurement finding that A leads B by dt_BA=40.6+-1.8 days, and find that image
C leads image A by dt_CA=821.6+-2.1 days. The lower limit on the remaining
delay is that image D lags image A by dt_AD>1250 days. Based on the
microlensing of images A and B we estimate that the accretion disk size at a
rest wavelength of 2300 angstrom is 10^{14.8+-0.3} cm for a disk inclination of
cos{i}=1/2, which is consistent with the microlensing disk size-black hole mass
correlation function given our estimate of the black hole mass from the MgII
line width of logM_BH/M_sun=8.44+-0.14. The long delays allow us to fill in the
seasonal gaps and assemble a continuous, densely sampled light curve spanning
5.7 years whose variability implies a structure function with a logarithmic
slope of gamma = 0.35+-0.02. As C is the leading image, sharp features in the C
light curve can be intensively studied 2.3 years later in the A/B pair,
potentially allowing detailed reverberation mapping studies of a quasar at
minimal cost.Comment: Submitted to ApJ, 12 pages, 3 figure
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