2,902 research outputs found
Bragg spectroscopy of an accelerating condensate with solitary-wave behaviour
We present a theoretical treatment of Bragg spectroscopy of an accelerating
condensate in a solitary-wave state. Our treatment is based on the
Gross-Pitaevskii equation with an optical potential representing the Bragg
pulse and an additional external time-dependent potential generating the
solitary-wave behaviour. By transforming to a frame translating with the
condensate, we derive an approximate set of equations that can be readily
solved to generate approximate Bragg spectra. Our analytic method is accurate
within a well defined parameter regime and provides physical insight into the
structure of the spectra. We illustrate our formalism using the example of
Bragg spectroscopy of a condensate in a time-averaged orbiting potential trap.Comment: 9 pages, 3 figure
The ‘Interior World’ of the Nineteenth-Century Maloti-Drakensberg Mountains
Over the last four decades researchers have cast the Maloti-Drakensberg Mountains as a marginal refuge for ‘Bushmen’ amidst constricting nineteenth-century frontiers. Rock art scholarship has expanded on this characterisation of mountains as refugia, focusing on heterogeneous raiding bands forging new cultural identities. Here, we propose another view of the Maloti-Drakensberg: a dynamic political theatre in which polities that engaged in illicit or ‘heterodox’ activities like cattle raiding and hunter-gatherer lifeways set the terms of colonial encounters. We employ the concept of the ‘interior world’ to refigure the region as one fostering subsistence and political behaviours that did not conform to the expectations of colonial authority. Paradoxically, such heterodoxies over time constituted widespread social logics within the Maloti-Drakensberg, and thus became commonplace and meaningful. We synthesise historical and archaeological evidence (new and existing) to illustrate the significance of the nineteenth-century Maloti-Drakensberg, offering a revised southeast-African colonial landscape and directions for future research
Gut-seeded α-synuclein fibrils promote gut dysfunction and brain pathology specifically in aged mice
Parkinson’s disease is a synucleinopathy that is characterized by motor dysfunction, death of midbrain dopaminergic neurons and accumulation of α-synuclein (α-Syn) aggregates. Evidence suggests that α-Syn aggregation can originate in peripheral tissues and progress to the brain via autonomic fibers. We tested this by inoculating the duodenal wall of mice with α-Syn preformed fibrils. Following inoculation, we observed gastrointestinal deficits and physiological changes to the enteric nervous system. Using the AAV-PHP.S capsid to target the lysosomal enzyme glucocerebrosidase for peripheral gene transfer, we found that α-Syn pathology is reduced due to the increased expression of this protein. Lastly, inoculation of α-Syn fibrils in aged mice, but not younger mice, resulted in progression of α-Syn histopathology to the midbrain and subsequent motor defects. Our results characterize peripheral synucleinopathy in prodromal Parkinson’s disease and explore cellular mechanisms for the gut-to-brain progression of α-Syn pathology
Solitary-wave description of condensate micro-motion in a time-averaged orbiting potential trap
We present a detailed theoretical analysis of micro-motion in a time-averaged
orbiting potential trap. Our treatment is based on the Gross-Pitaevskii
equation, with the full time dependent behaviour of the trap systematically
approximated to reduce the trapping potential to its dominant terms. We show
that within some well specified approximations, the dynamic trap has
solitary-wave solutions, and we identify a moving frame of reference which
provides the most natural description of the system. In that frame eigenstates
of the time-averaged orbiting potential trap can be found, all of which must be
solitary-wave solutions with identical, circular centre of mass motion in the
lab frame. The validity regime for our treatment is carefully defined, and is
shown to be satisfied by existing experimental systems.Comment: 12 pages, 2 figure
An ultraviolet excess in the superluminous supernova Gaia16apd reveals a powerful central engine
Since the discovery of superluminous supernovae (SLSNe) in the last decade,
it has been known that these events exhibit bluer spectral energy distributions
than other supernova subtypes, with significant output in the ultraviolet.
However, the event Gaia16apd seems to outshine even the other SLSNe at
rest-frame wavelengths below \AA. Yan et al (2016) have recently
presented HST UV spectra and attributed the UV flux to low metallicity and
hence reduced line blanketing. Here we present UV and optical light curves over
a longer baseline in time, revealing a rapid decline at UV wavelengths despite
a typical optical evolution. Combining the published UV spectra with our own
optical data, we demonstrate that Gaia16apd has a much hotter continuum than
virtually any SLSN at maximum light, but it cools rapidly thereafter and is
indistinguishable from the others by -15 days after peak. Comparing
the equivalent widths of UV absorption lines with those of other events, we
show that the excess UV continuum is a result of a more powerful central power
source, rather than a lack of UV absorption relative to other SLSNe or an
additional component from interaction with the surrounding medium. These
findings strongly support the central-engine hypothesis for hydrogen-poor
SLSNe. An explosion ejecting M, where
is the opacity in cmg, and forming a magnetar with spin
period ms, and G (lower than other SLSNe with
comparable rise-times) can consistently explain the light curve evolution and
high temperature at peak. The host metallicity, Z, is
comparable to other SLSNe.Comment: Updated to match accepted version (ApJL
Transformation as Development: Southern Africa Perspectives on Capacity Building and Heritage
As an agenda for development, ‘transformation’ invokes the long-standing and (in South Africa) constitutionally supported struggle for redistributive socioeconomic rights. This chapter brings experiences from two different sorts of heritage management programs to bear on discussions of transformation as development: the Metolong Cultural Resource Management Project associated with Lesotho’s Metolong Dam, and the Matatiele Archaeology and Rock Art program as a National Research Foundation-funded academic project, both of which included capacity building components. Tracing their paths—and the expectations for heritage that they entailed—reveals where invoking heritage as a platform for capacity building too often works against the cause of empowerment. In this chapter, we disarticulate received narratives of transformation, community engagement, and development, identifying tensions and concerns that emerge in practical examples. We highlight issues surrounding credentialing trainees, knowledge production and the creation of expert/technician divides, and recommend policies for the southern African heritage sector to address these
Bragg scattering of Cooper pairs in an ultra-cold Fermi gas
We present a theoretical treatment of Bragg scattering of a degenerate Fermi
gas in the weakly interacting BCS regime. Our numerical calculations predict
correlated scattering of Cooper pairs into a spherical shell in momentum space.
The scattered shell of correlated atoms is centered at half the usual Bragg
momentum transfer, and can be clearly distinguished from atoms scattered by the
usual single-particle Bragg mechanism. We develop an analytic model that
explains key features of the correlated-pair Bragg scattering, and determine
the dependence of this scattering on the initial pair correlations in the gas.Comment: Manuscript substantially revised. Version 2 contains a more detailed
discussion of the collisional interaction used in our theory, and is based on
three-dimensional solution
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