31,634 research outputs found
A test of adverse selection in the market for experienced workers
We show that in labor market models with adverse selection, otherwise observationally equivalent workers will experience less wage growth following a period in which they change jobs than following a period in which they do not. We find little or no evidence to support this prediction. In most specifications the coefficient has the opposite sign, sometimes statistically significantly so. When consistent with the prediction, the estimated effects are small and statistically insignificant. We consistently reject large effects in the predicted direction. We argue informally that our results are also problematic for a broader class of models of competitive labor markets.Othe
The critical O() -model at dimension : Hardy-Ramanujan distribution of quasi-primary fields and a collective fusion approach
The distribution of quasiprimary fields of fixed classes characterized by
their O representations and the number of vector fields from which
they are composed at in dependence on their normal dimension
is shown to obey a Hardy-Ramanujan law at leading order in a
-expansion. We develop a method of collective fusion of the
fundamental fields which yields arbitrary \qps and resolves any degeneracy.Comment: KL-TH-94/2, 21 pages (uuencoded Postscript file
The Rocketdyne Multifunction Tester. Part 2: Operation of a Radial Magnetic Bearing as an Excitation Source
The operation of the magnetic bearing used as an excitation source in the Rocketdyne Multifunction Tester is described. The tester is scheduled for operation during the summer of 1990. The magnetic bearing can be used in two control modes: (1) open loop mode, in which the magnetic bearing operates as a force actuator; and (2) closed loop mode, in which the magnetic bearing provides shaft support. Either control mode can be used to excite the shaft; however, response of the shaft in the two control modes is different due to the alteration of the eigenvalues by closed loop mode operation. A rotordynamic model is developed to predict the frequency response of the tester due to excitation in either control mode. Closed loop mode excitation is shown to be similar to the excitation produced by a rotating eccentricity in a conventional bearing. Predicted frequency response of the tester in the two control modes is compared, and the maximum response is shown to be the same for the two control modes when synchronous unbalance loading is not considered. The analysis shows that the response of this tester is adequate for the extraction of rotordynamic stiffness, damping, and inertia coefficients over a wide range of test article stiffnesses
Sub-millimeter nuclear medical imaging with high sensitivity in positron emission tomography using beta-gamma coincidences
We present a nuclear medical imaging technique, employing triple-gamma
trajectory intersections from beta^+ - gamma coincidences, able to reach
sub-millimeter spatial resolution in 3 dimensions with a reduced requirement of
reconstructed intersections per voxel compared to a conventional PET
reconstruction analysis. This '-PET' technique draws on specific beta^+
- decaying isotopes, simultaneously emitting an additional photon. Exploiting
the triple coincidence between the positron annihilation and the third photon,
it is possible to separate the reconstructed 'true' events from background. In
order to characterize this technique, Monte-Carlo simulations and image
reconstructions have been performed. The achievable spatial resolution has been
found to reach ca. 0.4 mm (FWHM) in each direction for the visualization of a
22Na point source. Only 40 intersections are sufficient for a reliable
sub-millimeter image reconstruction of a point source embedded in a scattering
volume of water inside a voxel volume of about 1 mm^3 ('high-resolution mode').
Moreover, starting with an injected activity of 400 MBq for ^76Br, the same
number of only about 40 reconstructed intersections are needed in case of a
larger voxel volume of 2 x 2 x 3~mm^3 ('high-sensitivity mode'). Requiring such
a low number of reconstructed events significantly reduces the required
acquisition time for image reconstruction (in the above case to about 140 s)
and thus may open up the perspective for a quasi real-time imaging.Comment: 17 pages, 5 figutes, 3 table
Interaction induced Dirac fermions from quadratic band touching in bilayer graphene
We revisit the effect of local interactions on the quadratic band touching
(QBT) of Bernal stacked bilayer graphene models using renormalization group
(RG) arguments and quantum Monte Carlo simulations of the Hubbard model. We
present an RG argument which predicts, contrary to previous studies, that weak
interactions do not flow to strong coupling even if the free dispersion has a
QBT. Instead they generate a linear term in the dispersion, which causes the
interactions to flow back to weak coupling. Consistent with this RG scenario,
in unbiased quantum Monte Carlo simulations of the Hubbard model we find
compelling evidence that antiferromagnetism turns on at a finite , despite
the hopping problem having a QBT. The onset of antiferromagnetism takes
place at a continuous transition which is consistent with a dynamical critical
exponent as expected for 2+1 d Gross-Neveu criticality. We conclude that
generically in models of bilayer graphene, even if the free dispersion has a
QBT, small local interactions generate a Dirac phase with no symmetry breaking
and that there is a finite-coupling transition out of this phase to a
symmetry-broken state
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