15,984 research outputs found
Origin of positive magnetoresistance in small-amplitude unidirectional lateral superlattices
We report quantitative analysis of positive magnetoresistance (PMR) for
unidirectional-lateral-superlattice samples with relatively small periods
(a=92-184 nm) and modulation amplitudes (V_0=0.015-0.25 meV). By comparing
observed PMR's with ones calculated using experimentally obtained mobilities,
quantum mobilities, and V_0's, it is shown that contribution from streaming
orbits (SO) accounts for only small fraction of the total PMR. For small V_0,
the limiting magnetic field B_e of SO can be identified as an inflection point
of the magnetoresistance trace. The major part of PMR is ascribed to drift
velocity arising from incompleted cyclotron orbits obstructed by scatterings.Comment: 12 pages, 9 figures, REVTe
Evaporation of a Kerr black hole by emission of scalar and higher spin particles
We study the evolution of an evaporating rotating black hole, described by
the Kerr metric, which is emitting either solely massless scalar particles or a
mixture of massless scalar and nonzero spin particles. Allowing the hole to
radiate scalar particles increases the mass loss rate and decreases the angular
momentum loss rate relative to a black hole which is radiating nonzero spin
particles. The presence of scalar radiation can cause the evaporating hole to
asymptotically approach a state which is described by a nonzero value of . This is contrary to the conventional view of black hole
evaporation, wherein all black holes spin down more rapidly than they lose
mass. A hole emitting solely scalar radiation will approach a final asymptotic
state described by . A black hole that is emitting scalar
particles and a canonical set of nonzero spin particles (3 species of
neutrinos, a single photon species, and a single graviton species) will
asymptotically approach a nonzero value of only if there are at least 32
massless scalar fields. We also calculate the lifetime of a primordial black
hole that formed with a value of the rotation parameter , the minimum
initial mass of a primordial black hole that is seen today with a rotation
parameter , and the entropy of a black hole that is emitting scalar or
higher spin particles.Comment: 22 pages, 13 figures, RevTeX format; added clearer descriptions for
variables, added journal referenc
PATENTS, R&D AND LAG EFFECTS: EVIDENCE FROM FLEXIBLE METHODS FOR COUNT PANEL DATA ON MANUFACTURING FIRMS
Hausman, Hall and Griliches (1984) and Hall, Griliches and Hausman (1986) investigated whether there was a lag in the patent-R&D relationship for the U.S. manufacturing sector using 1970Âżs data. They found that there was little evidence of anything but contemporaneous movement of patents and R&D. We reexamine this important issue employing new longitudinal patent data at the firm level for the U.S. manufacturing sector from 1982 to 1992. To address unique features of the data, we estimate various distributed lag and dynamic multiplicative panel count data models. The paper also develops a new class of count panel data models based on series expansion of the distribution of individual effects. The empirical analyses show that, although results are somewhat sensitive to different estimation methods, the contemporaneous relationship between patenting and R&D expenditures continues to be rather strong, accounting for over 60% of the total R&D elasticity. Regarding the lag structure of the patents-R&D relationship, we do find a significant lag in all empirical specifications. Moreover, the estimated lag effects are higher than have previously been found, suggesting that the contribution of R&D history to current patenting has increased from the 1970Âżs to the 1980Âżs.Innovative activity, Patents and R&D, Individual effects, count panel data methods.
Social Disorganization Outside the Metropolis: An Analysis of Rural Youth Violence
In order to extend the study of community social disorganization and crime beyond its exclusive focus on large urban centers, we present an analysis of structural correlates of arrest rates for juvenile violence in 264 nonmetropolitan counties of four states. Findings support the generality of social disorganization theory: Juvenile violence was associated with rates of residential instability, family disruption, and ethnic heterogeneity. Though rates of poverty were not related to juvenile violence, this is also in accord with social disorganization theory because, unlike urban settings, poverty was negatively related to residential instability. Rates of juvenile violence varied markedly with population size through a curvilinear relationship in which counties with the smallest juvenile populations had exceptionally low arrest rates. Analyses used negative binomial regression (a variation of Poisson regression) because the small number of arrests in many counties meant that arrest rates would be ill suited to least-squares regression
Community Correlates of Rural Youth Violence
Rates of crime and delinquency vary widely across communities, and research going back many decades provides a good understanding of the nature, correlates, and probable causes of these community differences. Unfortunately, previous studies have been limited in an important way. Virtually all studies of communities and crime are based on large urban areas, almost totally excluding nonmetropolitan areasâthat is, rural areas and smaller cities and towns. The findings in this Bulletin help to fill some gaps in the research by examining variations in rates of juvenile violence across nonmetropolitan communities in Florida, Georgia, Nebraska, and South Carolina.
Social disorganization is the primary theory by which criminologists account for rates of crime in urban communities. If this theory also applies to rural settings, then what is known about crime in urban areas can provide a basis for developing programs that address the problem of delinquency in smaller communities. The research presented in this Bulletin indicates that the principles of social disorganization theory hold up quite well in rural settings. As in urban areas, rates of juvenile violence are considerably higher in rural communities that have a large percentage of children living in single-parent households, a high rate of population turnover, and significant ethnic diversity. These factors, it should be noted, are statistical correlates and not causes of such violence; nor are they the only correlates
Damping of field-induced chemical potential oscillations in ideal two-band compensated metals
The field and temperature dependence of the de Haas-van Alphen oscillations
spectrum is studied for an ideal two-dimensional compensated metal. It is shown
that the chemical potential oscillations, involved in the frequency
combinations observed in the case of uncompensated orbits, are strongly damped
and can even be suppressed when the effective masses of the electron- and
hole-type orbits are the same. When magnetic breakdown between bands occurs,
this damping is even more pronounced and the Lifshits-Kosevich formalism
accounts for the data in a wide field range.Comment: 11 pages, 10 figures, to appear in PR
Drawing Planar Graphs with a Prescribed Inner Face
Given a plane graph (i.e., a planar graph with a fixed planar embedding)
and a simple cycle in whose vertices are mapped to a convex polygon, we
consider the question whether this drawing can be extended to a planar
straight-line drawing of . We characterize when this is possible in terms of
simple necessary conditions, which we prove to be sufficient. This also leads
to a linear-time testing algorithm. If a drawing extension exists, it can be
computed in the same running time
Applications of polymer optical fibre grating sensors to condition monitoring of textiles
Fibre Bragg gratings (FBGs) in polymer optical fibres (POFs) have been used to measure the strain in a woven textile.
FBGs in both POFs and silica optical fibres were attached to a woven textile specimen, and their performance
characterised. It was demonstrated that the POF FBGs provide improved strain transfer coefficients and reduce local
structural reinforcement compared to silica FBGs and therefore make a more suitable proposition for textile monitoring
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