65,356 research outputs found
The Excess Sensitivity of Layoffs and Quits to Demand
Excessive layoffs in bad times and excessive quits in good times both stem from the same weakness in practical employment arrangements: the specific nature of worker-firm relations creates a situation of bilateral monopoly. Institutions which have arisen to avert the associated inefficiency cannot mimic the separation decisions of a perfect-information, first-best allocation rule. Simple employment rules based on predetermined or indexed wages are in many cases the most desirable among the class of feasible employment arrangements. More complicated contracts which seem to deal more effectively with turnover issues are either infeasible because of informational requirements or create adverse incentives on some other dimension.
Discovery of hydroxyl and water masers in R Aquarii and H1-36 Arae
We present the first results from an all-sky maser-line survey of symbiotic
Miras. Interferometric spectral-line observations of R Aqr and H1-36 Arae have
revealed a 22-GHz water maser in the former and 1612-MHz hydroxyl and weak
22-GHz water maser emission from the latter. H1-36 has thus become the first
known symbiotic OH/IR star. We have also detected weak OH line emission from
the vicinity of R Aqr, but we note that there are small discrepencies between
the OH- and H2O-line velocities and positions. These detections demonstrate
unequivocally that dust can shield some circumstellar hydroxyl and water
molecules from dissociation, even in systems which possess intense local
sources of UV. Finally, we discuss some of the implications of these
observations. The narrow profile of the water maser in R Aqr means that there
may finally be an opportunity to determine the system's orbital parameters. We
also point out that high resolution synthesis observations may trace the
distribution of dust in H1-36 and R Aqr, possibly throwing light on the
mass-loss process in symbiotic Miras and placing constraints on the amount of
collimation experienced by UV radiation from their hot, compact companions.Comment: 7 pages; no figures attached; LaTex (MN style); postscript figures
via anonymous ftp in /users/ers on astro.caltech.edu; University of Toronto
pre-print; ERSRJI
The evaluation of failure detection and isolation algorithms for restructurable control
Three failure detection and identification techniques were compared to determine their usefulness in detecting and isolating failures in an aircraft flight control system; excluding sensor and flight control computer failures. The algorithms considered were the detection filter, the Generalized Likelihood Ratio test and the Orthogonal Series Generalized Likelihood Ratio test. A modification to the basic detection filter is also considered which uses secondary filtering of the residuals to produce unidirectional failure signals. The algorithms were evaluated by testing their ability to detect and isolate control surface failures in a nonlinear simulation of a C-130 aircraft. It was found that failures of some aircraft controls are difficult to distinguish because they have a similar effect on the dynamics of the vehicle. Quantitative measures for evaluating the distinguishability of failures are considered. A system monitoring strategy for implementing the failure detection and identification techniques was considered. This strategy identified the mix of direct measurement of failures versus the computation of failure necessary for implementation of the technology in an aircraft system
Anomalous Hall Effect in three ferromagnets: EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30
The Hall resistivity (Rho_xy), resistivity (Rho_xx), and magnetization of
three metallic ferromagnets are investigated as a function of magnetic field
and temperature. The three ferromagnets, EuFe4Sb12 (Tc = 84 K), Yb14MnSb11 (Tc
= 53 K), and Eu8Ga16Ge30 (Tc = 36 K) are Zintl compounds with carrier
concentrations between 1 x 10^21 cm^-3 and 3.5 x 10^21 cm^-3. The relative
decrease in Rho_xx below Tc [Rho_xx(Tc)/Rho_xx(2 K)] is 28, 6.5, and 1.3 for
EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30 respectively. The low carrier
concentrations coupled with low magnetic anisotropies allow a relatively clean
separation between the anomalous (Rho_'xy), and normal contributions to the
measured Hall resistivity. For each compound the anomalous contribution in the
zero field limit is fit to alpha Rho_xx + sigma_xy rho_xx^2 for temperatures T
< Tc. The anomalous Hall conductivity, sigma_xy, is -220 +- 5 (Ohm^-1 cm^-1),
-14.7 +- 1 (Ohm^-1 cm^-1), and 28 +- 3 (Ohm^-1 cm^-1) for EuFe4Sb12,
Yb14MnSb11, and Eu8Ga16Ge30 respectively and is independent of temperature for
T < Tc if the change in spontaneous magnetization (order parameter) with
temperature is taken into account. These data are consistent with recent
theories of the anomalous Hall effect that suggest that even for stochiometric
ferromagnetic crystals, such as those studied in this article, the intrinsic
Hall conductivity is finite at T = 0, and is a ground state property that can
be calculated from the electronic structure.Comment: 22 pages, 13 figures Submitted to PR
Anomalous Hall effect in superconductors with spin-orbit interaction
We calculate the anomalous Hall conductance of superconductors with
spin-orbit interaction and with either uniform or local magnetization. In the
first case we consider a uniform ferromagnetic ordering in a spin triplet
superconductor, while in the second case we consider a conventional s-wave spin
singlet superconductor with a magnetic impurity (or a diluted set of magnetic
impurities). In the latter case we show that the anomalous Hall conductance can
be used to track the quantum phase transition, that occurs when the spin
coupling between the impurity and electronic spin density exceeds a certain
critical value. In both cases we find that for large spin-orbit coupling the
superconductivity is destroyed and the Hall conductance oscillates strongly.Comment: 10 pages, 6 figure
Suppression of intrinsic neutron background in the Multi-Grid detector
One of the key requirements for neutron scattering instruments is the
Signal-to-Background ratio (SBR). This is as well a design driving requirement
for many instruments at the European Spallation Source (ESS), which aspires to
be the brightest neutron source of the world. The SBR can be effectively
improved with background reduction. The Multi-Grid, a large-area thermal
neutron detector with a solid boron carbide converter, is a novel solution for
chopper spectrometers. This detector will be installed for the three
prospective chopper spectrometers at the ESS. As the Multi-Grid detector is a
large area detector with a complex structure, its intrinsic background and its
suppression via advanced shielding design should be investigated in its
complexity, as it cannot be naively calculated. The intrinsic scattered neutron
background and its effect on the SBR is determined via a detailed Monte Carlo
simulation for the Multi-Grid detector module, designed for the CSPEC
instrument at the ESS. The impact of the detector vessel and the neutron
entrance window on scattering is determined, revealing the importance of an
optimised internal detector shielding. The background-reducing capacity of
common shielding geometries, like side-shielding and end-shielding is
determined by using perfect absorber as shielding material, and common
shielding materials, like BC and Cd are also tested. On the basis of the
comparison of the effectiveness of the different shielding topologies and
materials, recommendations are given for a combined shielding of the Multi-Grid
detector module, optimised for increased SBR.Comment: 26 pages, 18 figures, revise
Theoretical Evidence for the Berry-Phase Mechanism of Anomalous Hall Transport: First-principles Studies on CuCrSeBr
To justify the origin of anomalous Hall effect (AHE), it is highly desirable
to have the system parameters tuned continuously. By quantitative calculations,
we show that the doping dependent sign reversal in
CuCrSeBr, observed but not understood, is nothing but
direct evidence for the Berry-Phase mechanism of AHE. The systematic
calculations well explain the experiment data for the whole doping range where
the impurity scattering rates is changed by several orders with Br
substitution. Further sign change is also predicted, which may be tested by
future experiments.Comment: 4 page
Radio-frequency dressing of multiple Feshbach resonances
We demonstrate and theoretically analyze the dressing of several proximate
Feshbach resonances in Rb-87 using radio-frequency (rf) radiation. We present
accurate measurements and characterizations of the resonances, and the dramatic
changes in scattering properties that can arise through the rf dressing. Our
scattering theory analysis yields quantitative agreement with the experimental
data. We also present a simple interpretation of our results in terms of
rf-coupled bound states interacting with the collision threshold.Comment: 4+ pages, 3 figures, 1 table; revised introduction & references to
reflect published versio
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