816 research outputs found
Eureka and beyond: mining's impact on African urbanisation
This collection brings separate literatures on mining and urbanisation together at a time when both artisanal and large-scale mining are expanding in many African economies. While much has been written about contestation over land and mineral rights, the impact of mining on settlement, notably its catalytic and fluctuating effects on migration and urban growth, has been largely ignored. African nation-states’ urbanisation trends have shown considerable variation over the past half century. The current surge in ‘new’ mining countries and the slow-down in ‘old’ mining countries are generating some remarkable settlement patterns and welfare outcomes. Presently, the African continent is a laboratory of national mining experiences. This special issue on African mining and urbanisation encompasses a wide cross-section of country case studies: beginning with the historical experiences of mining in Southern Africa (South Africa, Zambia, Zimbabwe), followed by more recent mineralizing trends in comparatively new mineral-producing countries (Tanzania) and an established West African gold producer (Ghana), before turning to the influence of conflict minerals (Angola, the Democratic Republic of Congo and Sierra Leone)
Surface superconductivity and order parameter suppression in UPt
We show that a recent measurement of surface superconductivity in UPt
(Keller {\it et. al.}, Phys. Rev. Lett. {\bf 73}, 2364 (1994)) can be
understood if the superconducting pair wavefunction is suppressed
anisotropically at a vacuum to superconductor interface. Further measurements
of surface superconductivity can distinguish between the various
phenomenological models of superconducting UPt.Comment: 4 pages, latex, 2 Figures available upon request
([email protected]
Effect of diffusive boundaries on surface superconductivity in unconventional superconductors
Boundary conditions for a superconducting order parameter at a diffusive
scattering boundary are derived from microscopic theory. The results indicate
that for all but isotropic gap functions the diffusive boundary almost
completely suppresses surface superconductivity in the Ginzburg-Landau regime.
This indicates that in anisotropic superconductors surface superconductivity
can only be observed for surface normals along high symmetry directions where
atomically clean surfaces can be cleaved.Comment: Latex File, 12 pages, 2 Postscript figures, to appear in Phys. Rev. B
(June 1 1996
The PL calibration for Milky Way Cepheids and its implications for the distance scale
The rationale behind recent calibrations of the Cepheid PL relation using the
Wesenheit formulation is reviewed and reanalyzed, and it is shown that recent
conclusions regarding a possible change in slope of the PL relation for
short-period and long-period Cepheids are tied to a pathological distribution
of HST calibrators within the instability strip. A recalibration of the
period-luminosity relation is obtained using Galactic Cepheids in open clusters
and groups, the resulting relationship, described by log L/L_sun =
2.415(+-0.035) + 1.148(+-0.044)log P, exhibiting only the moderate scatter
expected from color spread within the instability strip. The relationship is
confirmed by Cepheids with HST parallaxes, although without the need for
Lutz-Kelker corrections, and in general by Cepheids with revised Hipparcos
parallaxes, albeit with concerns about the cited precisions of the latter. A
Wesenheit formulation of Wv = -2.259(+-0.083) - 4.185(+-0.103)log P for
Galactic Cepheids is tested successfully using Cepheids in the inner regions of
the galaxy NGC 4258, confirming the independent geometrical distance
established for the galaxy from OH masers. Differences between the extinction
properties of interstellar and extragalactic dust may yet play an important
role in the further calibration of the Cepheid PL relation and its application
to the extragalactic distance scale.Comment: Accepted for Publication (Astrophysics & Space Science
Complete solutions to the metric of spherically collapsing dust in an expanding spacetime with a cosmological constant
We present semi-analytical solutions to the background equations describing
the Lema\^itre-Tolman-Bondi (LTB) metric as well as the homogeneous Friedmann
equations, in the presence of dust, curvature and a cosmological constant
Lambda. For none of the presented solutions any numerical integration has to be
performed. All presented solutions are given for expanding and collapsing
phases, preserving continuity in time and radius. Hence, these solutions
describe the complete space time of a collapsing spherical object in an
expanding universe. In the appendix we present for completeness a solution of
the Friedmann equations in the additional presence of radiation, only valid for
the Robertson-Walker metric.Comment: 23 pages, one figure. Numerical module for evaluation of the
solutions released at
http://web.physik.rwth-aachen.de/download/valkenburg/ColLambda/ Matches
published version, published under Open Access. Note change of titl
Mid- And Late-Life Leisure-Time Physical Activity and Global Brain Amyloid Burden- And Atherosclerosis Risk in Communities (ARIC)-PET Study
Background: Physical activity (PA) may slow the development of dementia by reducing the accumulation of amyloid. Objective: We tested the hypothesis that higher levels of leisure-time PA in mid- or late-life were associated with lower brain amyloid burden in late-life among 326 non-demented participants from the Atherosclerosis Risk in Communities Study of brain florbetapir positron emission tomography (ARIC-PET) ancillary. Methods: Self-reported PA was quantified using a past-year recall, interviewer-administered questionnaire in mid-life (1987-1989, aged 45-64 years) and late-life (2011-2013, aged 67-89 years). Continuous PA estimates were classified as 1) any leisure-time PA participation (yes/no); 2) meeting the 2018 United States' PA guidelines (yes/no); and 3) per 1 standard deviation (SD) higher metabolic equivalent of task (MET) minutes per week (MET min wk-1). A brain magnetic resonance imaging scan with Florbetapir PET was performed in late-life. Adjusted odds ratios (OR) of elevated amyloid burden, defined as a global cortical standardized uptake value ratio (>1.2), compared to no elevated amyloid burden were estimated according to PA measures. Results: Among the 326 participants (mean age: 76 years, 42% male, 41% Black), 52% had elevated brain amyloid burden. Mid-life leisure-time PA did not show a statistically significant lower odds of elevated late-life amyloid burden (OR = 0.71, 95% CI: 0.43-1.18). A 1 SD (970 MET. min. wk-1) higher PA level in mid-life was also not significantly associated with elevated amyloid burden (OR = 0.89, 95% CI: 0.69-1.15). Similar estimates were observed for meeting versus not meeting PA guidelines in both mid- and late-life. Conclusion: Self-reported higher mid- and late-life leisure-time PA were not significantly associated with lower amyloid burden. Data show a trend of an association, which is, however, imprecise, suggesting replication in larger studies
Feedback-control of quantum systems using continuous state-estimation
We present a formulation of feedback in quantum systems in which the best
estimates of the dynamical variables are obtained continuously from the
measurement record, and fed back to control the system. We apply this method to
the problem of cooling and confining a single quantum degree of freedom, and
compare it to current schemes in which the measurement signal is fed back
directly in the manner usually considered in existing treatments of quantum
feedback. Direct feedback may be combined with feedback by estimation, and the
resulting combination, performed on a linear system, is closely analogous to
classical LQG control theory with residual feedback.Comment: 12 pages, multicol revtex, revised and extende
Qualitative theory testing as mixed-method research
While the concept of mixed-methods research is more usually associated with combining quantitative and qualitative approaches, this paper outlines a study that mixed methods by undertaking qualitative theory testing and derivation when examining the relationship between health promotion theory and hospital nursing practice. Thus, it is concerned with relating the metatheoretical aspects of the debate and not with the pragmatic aspects of the research and concomitant methods. A deductive–inductive–deductive design, based on the theory–research–theory strategy of Meleis (1985), tested, revised and developed for nursing established health promotion theory using theory-testing criteria. To complement the methodological mix, the study also used the theory (i.e. a health-promotion taxonomy) as a framework to contextualise the findings rather than generate theory in the way associated with interpretative inquiry. While inconsistent with the traditional view linking theory testing with quantitative, objective epistemology, the process enabled a theoretically robust health-promotion taxonomy to be synthesised and advanced for use in nursing in relation to a paradigm of social thought
Modeling magnetospheric fields in the Jupiter system
The various processes which generate magnetic fields within the Jupiter
system are exemplary for a large class of similar processes occurring at other
planets in the solar system, but also around extrasolar planets. Jupiter's
large internal dynamo magnetic field generates a gigantic magnetosphere, which
is strongly rotational driven and possesses large plasma sources located deeply
within the magnetosphere. The combination of the latter two effects is the
primary reason for Jupiter's main auroral ovals. Jupiter's moon Ganymede is the
only known moon with an intrinsic dynamo magnetic field, which generates a
mini-magnetosphere located within Jupiter's larger magnetosphere including two
auroral ovals. Ganymede's magnetosphere is qualitatively different compared to
the one from Jupiter. It possesses no bow shock but develops Alfv\'en wings
similar to most of the extrasolar planets which orbit their host stars within
0.1 AU. New numerical models of Jupiter's and Ganymede's magnetospheres
presented here provide quantitative insight into the processes that maintain
these magnetospheres. Jupiter's magnetospheric field is approximately
time-periodic at the locations of Jupiter's moons and induces secondary
magnetic fields in electrically conductive layers such as subsurface oceans. In
the case of Ganymede, these secondary magnetic fields influence the oscillation
of the location of its auroral ovals. Based on dedicated Hubble Space Telescope
observations, an analysis of the amplitudes of the auroral oscillations
provides evidence that Ganymede harbors a subsurface ocean. Callisto in
contrast does not possess a mini-magnetosphere, but still shows a perturbed
magnetic field environment. Callisto's ionosphere and atmospheric UV emission
is different compared to the other Galilean satellites as it is primarily been
generated by solar photons compared to magnetospheric electrons.Comment: Chapter for Book: Planetary Magnetis
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