18,892 research outputs found
Electromotive forces and the Meissner effect puzzle
In a voltaic cell, positive (negative) ions flow from the low (high)
potential electrode to the high (low) potential electrode, driven by an
`electromotive force' which points in opposite direction and overcomes the
electric force. Similarly in a superconductor charge flows in direction
opposite to that dictated by the Faraday electric field as the magnetic field
is expelled in the Meissner effect. The puzzle is the same in both cases: what
drives electric charges against electromagnetic forces? I propose that the
answer is also the same in both cases: kinetic energy lowering, or `quantum
pressure'
Wages, racial composition, and quality sorting in labor markets
This paper examines the relationship between wage rates and the racial composition of jobs, using large cross-sectional and longitudinal samples constructed from monthly Current Population Surveys for 1983-92. Support is found for a "quality sorting" model that posits an equilibrium in which the racial composition of jobs serves as a skill index of unmeasured labor quality. Estimation of standard wage-level equations shows that wages of both black and white workers are substantially lower in occupations with a high density of blacks. Consistent with the quality sorting hypothesis, the magnitude of the relationship is reduced sharply after accounting for occupational skill measures. Longitudinal wage-change estimates controlling for person-specific quality indicate little if any causal effect of racial composition on wages. Estimates of racial discrimination are reduced only moderately after accounting for racial composition; unexplained differentials occur within occupations or reflect inter-occupational differences uncorrelated with racial composition and occupational skill measures.
Antiferromagnetism in NiO Observed by Transmission Electron Diffraction
Neutron diffraction has been used to investigate antiferromagnetism since
1949. Here we show that antiferromagnetic reflections can also be seen in
transmission electron diffraction patterns from NiO. The diffraction patterns
taken here came from regions as small as 10.5 nm and such patterns could be
used to form an image of the antiferromagnetic structure with a nanometre
resolution.Comment: 10 pages, 7 figures. Typos corrected. To appear in Physical Review
Letter
The analysis of temporal variations in regional models of the Sargasso Sea from GEOS-3 altimetry
The dense coverage of short pulse mode GEOS-3 altimeter data in the western North Atlantic provides a basis for studying time variations in the sea surface heights in the Sargasso Sea. Two techniques are utilized: the method of regional models, and the analysis of overlapping passes. An 88 percent correlation is obtained between the location of cyclonic eddies obtained from infrared imagery and sea surface height minima in the altimeter models. This figure drops to 59 percent in the case of correlations with maxima and minima of surface temperature fields. The analysis of overlapping passes provides a better picture of instantaneous sea state through wavelengths greater than 30 km. The variability of the Sargasso Sea through wavelengths between 150 km and 5000 km is estimated at + or - 28 cm. This value is in reasonable agreement with oceanographic estimates and is compatible with the eddy kinetic energy of a wind driven circulation
Spherical agglomeration of superconducting and normal microparticles with and without applied electric field
It was reported by R. Tao and coworkers that in the presence of a strong
electric field superconducting microparticles assemble into balls of
macroscopic dimensions. Such a finding has potentially important implications
for the understanding of the fundamental physics of superconductors. However,
we report here the results of experimental studies showing that (i) ball
formation also occurs in the absence of an applied electric field, (ii) the
phenomenon also occurs at temperatures above the superconducting transition
temperature, and (iii) it can also occur for non-superconducting materials.
Possible origins of the phenomenon are discussed.Comment: Small changes in response to referee's comments. To be published in
Phys. Rev.
Determining R-parity violating parameters from neutrino and LHC data
In supersymmetric models neutrino data can be explained by R-parity violating
operators which violate lepton number by one unit. The so called bilinear model
can account for the observed neutrino data and predicts at the same time
several decay properties of the lightest supersymmetric particle. In this paper
we discuss the expected precision to determine these parameters by combining
neutrino and LHC data and discuss the most important observables. We show that
one can expect a rather accurate determination of the underlying R-parity
parameters assuming mSUGRA relations between the R-parity conserving ones and
discuss briefly also the general MSSM as well as the expected accuracies in
case of a prospective e+ e- linear collider. An important observation is that
several parameters can only be determined up to relative signs or more
generally relative phases.Comment: 13 pages, 13 figure
Invisible Higgs Boson Decays in Spontaneously Broken R-Parity
The Higgs boson may decay mainly to an invisible mode characterized by
missing energy, instead of the Standard Model channels. This is a generic
feature of many models where neutrino masses arise from the spontaneous
breaking of ungauged lepton number at relatively low scales, such as
spontaneously broken R-parity models. Taking these models as framework, we
reanalyze this striking suggestion in view of the recent data on neutrino
oscillations that indicate non-zero neutrino masses. We show that, despite the
smallness of neutrino masses, the Higgs boson can decay mainly to the invisible
Goldstone boson associated to the spontaneous breaking of lepton number. This
requires a gauge singlet superfield coupling to the electroweak doublet
Higgses, as in the Next to Minimal Supersymmetric Standard Model (NMSSM)
scenario for solving the -problem. The search for invisibly decaying Higgs
bosons should be taken into account in the planning of future accelerators,
such as the Large Hadron Collider and the Next Linear Collider.Comment: 24 pages, 10 figures; typos corrected, published versio
Atmosphere Selection for Long-duration Manned Space Missions
This viewgraph reviews the spacecraft environment for future human space exploration missions. The choice of a atmosphere mix will play a critical role in the ultimate safety, productivity, and cost. There are a multitude of factors involved in selection of spacecraft environments
Analysis of hydrogen-rich magnetic white dwarfs detected in the Sloan Digital Sky Survey
Context
A large number of magnetic white dwarfs discovered in the SDSS have so far only been analyzed by visual comparison of the observations with relatively simple models of the radiation transport in a magnetised stellar atmosphere.
Aims We model the structure of the surface magnetic fields of the hydrogen-rich white dwarfs in the SDSS.
Methods
We calculated a grid of state-of-the-art theoretical optical spectra of hydrogen-rich magnetic white dwarfs (WDs) with magnetic field strengths of between 1 MG and 1200 MG for different angles between the magnetic field vector and the line of sight,and for effective temperatures between 7000 K and 50 000 K. We used a least squares minimization scheme with an evolutionary algorithm to find the best-fit magnetic field geometry of the observed data. We used centered dipoles or dipoles that had been shifted
along the dipole axis to model the coadded SDSS fiber spectrum of each object.
Result
We analyzed the spectra of all known magnetic hydrogen-rich (DA) WDs from the SDSS (97 previously published, plus 44 newly discovered) and also investigated the statistical properties of the magnetic field geometries of this sample.
Conclusions
The total number of known magnetic white dwarfs has already been more than tripled by the SDSS and more objects are expected after more systematic searches. The magnetic fields have strengths of between ≈1 and 900 MG. Our results further support the claims that Ap/Bp population is insufficient in generating the numbers and field strength distributions of the observed MWDs, and that of either another source of progenitor types or binary evolution is needed. Clear indications of non-centered dipoles exist in about ∼50%, of the objects which is consistent with the magnetic field distribution observed in Ap/Bp stars
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