1,160 research outputs found
Direct observation of growth and collapse of a Bose-Einstein condensate with attractive interactions
The dynamical behavior of Bose-Einstein condensation (BEC) in a gas with
attractive interactions is striking. Quantum theory predicts that BEC of a
spatially homogeneous gas with attractive interactions is precluded by a
conventional phase transition into either a liquid or solid. When confined to a
trap, however, such a condensate can form provided that its occupation number
does not exceed a limiting value. The stability limit is determined by a
balance between self-attraction and a repulsion arising from position-momentum
uncertainty under conditions of spatial confinement. Near the stability limit,
self-attraction can overwhelm the repulsion, causing the condensate to
collapse. Growth of the condensate, therefore, is punctuated by intermittent
collapses, which are triggered either by macroscopic quantum tunneling or
thermal fluctuation. Previous observation of growth and collapse has been
hampered by the stochastic nature of these mechanisms. Here we reduce the
stochasticity by controlling the initial number of condensate atoms using a
two-photon transition to a diatomic molecular state. This enables us to obtain
the first direct observation of the growth of a condensate with attractive
interactions and its subsequent collapse.Comment: 10 PDF pages, 5 figures (2 color), 19 references, to appear in Nature
Dec. 7 200
Paging through history: parchment as a reservoir of ancient DNA for next generation sequencing.
Parchment represents an invaluable cultural reservoir. Retrieving an additional layer of information from these abundant, dated livestock-skins via the use of ancient DNA (aDNA) sequencing has been mooted by a number of researchers. However, prior PCR-based work has indicated that this may be challenged by cross-individual and cross-species contamination, perhaps from the bulk parchment preparation process. Here we apply next generation sequencing to two parchments of seventeenth and eighteenth century northern English provenance. Following alignment to the published sheep, goat, cow and human genomes, it is clear that the only genome displaying substantial unique homology is sheep and this species identification is confirmed by collagen peptide mass spectrometry. Only 4% of sequence reads align preferentially to a different species indicating low contamination across species. Moreover, mitochondrial DNA sequences suggest an upper bound of contamination at 5%. Over 45% of reads aligned to the sheep genome, and even this limited sequencing exercise yield 9 and 7% of each sampled sheep genome post filtering, allowing the mapping of genetic affinity to modern British sheep breeds. We conclude that parchment represents an excellent substrate for genomic analyses of historical livestock
Collapse of a Bose gas: kinetic approach
We have analytically explored temperature dependence of critical number of
particles for the collapse of a harmonically trapped attractively interacting
Bose gas below the condensation point by introducing a kinetic approach within
the Hartree-Fock approximation. The temperature dependence obtained by this
easy approach is consisted with that obtained from the scaling theory.Comment: Brief Report, 4 pages, 1 figure, Accepted in Pramana-Journal of
Physic
A candidate redshift z ~ 10 galaxy and rapid changes in that population at an age of 500 Myr
Searches for very-high-redshift galaxies over the past decade have yielded a
large sample of more than 6,000 galaxies existing just 900-2,000 million years
(Myr) after the Big Bang (redshifts 6 > z > 3; ref. 1). The Hubble Ultra Deep
Field (HUDF09) data have yielded the first reliable detections of z ~ 8
galaxies that, together with reports of a gamma-ray burst at z ~ 8.2 (refs 10,
11), constitute the earliest objects reliably reported to date. Observations of
z ~ 7-8 galaxies suggest substantial star formation at z > 9-10. Here we use
the full two-year HUDF09 data to conduct an ultra-deep search for z ~ 10
galaxies in the heart of the reionization epoch, only 500 Myr after the Big
Bang. Not only do we find one possible z ~ 10 galaxy candidate, but we show
that, regardless of source detections, the star formation rate density is much
smaller (~10%) at this time than it is just ~200 Myr later at z ~ 8. This
demonstrates how rapid galaxy build-up was at z ~ 10, as galaxies increased in
both luminosity density and volume density from z ~ 8 to z ~ 10. The 100-200
Myr before z ~ 10 is clearly a crucial phase in the assembly of the earliest
galaxies.Comment: 41 pages, 14 figures, 2 tables, Nature, in pres
Formation and Propagation of Matter Wave Soliton Trains
Attraction between atoms in a Bose-Einstein-Condensate renders the condensate
unstable to collapse. Confinement in an atom trap, however, can stabilize the
condensate for a limited number of atoms, as was observed with 7Li, but beyond
this number, the condensate collapses. Attractive condensates constrained to
one-dimensional motion are predicted to form stable solitons for which the
attractive interactions exactly compensate for the wave packet dispersion. Here
we report the formation or bright solitons of 7Li atoms created in a quasi-1D
optical trap. The solitons are created from a stable Bose-Einstein condensate
by magnetically tuning the interactions from repulsive to attractive. We
observe a soliton train, containing many solitons. The solitons are set in
motion by offsetting the optical potential and are observed to propagate in the
potential for many oscillatory cycles without spreading. Repulsive interactions
between neighboring solitons are inferred from their motion
Production of a chromium Bose-Einstein condensate
The recent achievement of Bose-Einstein condensation of chromium atoms [1]
has opened longed-for experimental access to a degenerate quantum gas with
long-range and anisotropic interaction. Due to the large magnetic moment of
chromium atoms of 6 {}B, in contrast to other Bose- Einstein condensates
(BECs), magnetic dipole-dipole interaction plays an important role in a
chromium BEC. Many new physical properties of degenerate gases arising from
these magnetic forces have been predicted in the past and can now be studied
experimentally. Besides these phenomena, the large dipole moment leads to a
breakdown of standard methods for the creation of a chromium BEC. Cooling and
trapping methods had to be adapted to the special electronic structure of
chromium to reach the regime of quantum degeneracy. Some of them apply
generally to gases with large dipolar forces. We present here a detailed
discussion of the experimental techniques which are used to create a chromium
BEC and alow us to produce pure condensates with up to {} atoms in an
optical dipole trap. We also describe the methods used to determine the
trapping parameters.Comment: 17 pages, 9 figure
Finite element modeling and in vivo analysis of electrode configurations for selective stimulation of pudendal afferent fibers
<p>Abstract</p> <p>Background</p> <p>Intraurethral electrical stimulation (IES) of pudendal afferent nerve fibers can evoke both excitatory and inhibitory bladder reflexes in cats. These pudendovesical reflexes are a potential substrate for restoring bladder function in persons with spinal cord injury or other neurological disorders. However, the complex distribution of pudendal afferent fibers along the lower urinary tract presents a challenge when trying to determine the optimal geometry and position of IES electrodes for evoking these reflexes. This study aimed to determine the optimal intraurethral electrode configuration(s) and locations for selectively activating targeted pudendal afferents to aid future preclinical and clinical investigations.</p> <p>Methods</p> <p>A finite element model (FEM) of the male cat urethra and surrounding structures was generated to simulate IES with a variety of electrode configurations and locations. The activating functions (AFs) along pudendal afferent branches innervating the cat urethra were determined. Additionally, the thresholds for activation of pudendal afferent branches were measured in α-chloralose anesthetized cats.</p> <p>Results</p> <p>Maximum AFs evoked by intraurethral stimulation in the FEM and in vivo threshold intensities were dependent on stimulation location and electrode configuration.</p> <p>Conclusions</p> <p>A ring electrode configuration is ideal for IES. Stimulation near the urethral meatus or prostate can activate the pudendal afferent fibers at the lowest intensities, and allowed selective activation of the dorsal penile nerve or cranial sensory nerve, respectively. Electrode location was a more important factor than electrode configuration for determining stimulation threshold intensity and nerve selectivity.</p
Who should be prioritized for renal transplantation?: Analysis of key stakeholder preferences using discrete choice experiments
Background
Policies for allocating deceased donor kidneys have recently shifted from allocation based on Human Leucocyte Antigen (HLA) tissue matching in the UK and USA. Newer allocation algorithms incorporate waiting time as a primary factor, and in the UK, young adults are also favoured. However, there is little contemporary UK research on the views of stakeholders in the transplant process to inform future allocation policy. This research project aimed to address this issue.
Methods
Discrete Choice Experiment (DCE) questionnaires were used to establish priorities for kidney transplantation among different stakeholder groups in the UK. Questionnaires were targeted at patients, carers, donors / relatives of deceased donors, and healthcare professionals. Attributes considered included: waiting time; donor-recipient HLA match; whether a recipient had dependents; diseases affecting life expectancy; and diseases affecting quality of life.
Results
Responses were obtained from 908 patients (including 98 ethnic minorities); 41 carers; 48 donors / relatives of deceased donors; and 113 healthcare professionals. The patient group demonstrated statistically different preferences for every attribute (i.e. significantly different from zero) so implying that changes in given attributes affected preferences, except when prioritizing those with no rather than moderate diseases affecting quality of life. The attributes valued highly related to waiting time, tissue match, prioritizing those with dependents, and prioritizing those with moderate rather than severe diseases affecting life expectancy. Some preferences differed between healthcare professionals and patients, and ethnic minority and non-ethnic minority patients. Only non-ethnic minority patients and healthcare professionals clearly prioritized those with better tissue matches.
Conclusions
Our econometric results are broadly supportive of the 2006 shift in UK transplant policy which emphasized prioritizing the young and long waiters. However, our findings suggest the need for a further review in the light of observed differences in preferences amongst ethnic minorities, and also because those with dependents may be a further priority.</p
- …