4,414 research outputs found
Muon-spin-rotation measurements of the penetration depth in Li_2Pd_3B
Measurements of the magnetic field penetration depth in the ternary
boride superconductor LiPdB ( K) have been carried out by
means of muon-spin rotation (SR). The absolute values of , the
Ginzburg-Landau parameter , and the first and the second
critical fields at T=0 obtained from SR were found to be
nm, , mT, and
T, respectively. The zero-temperature value of the
superconducting gap 1.31(3) meV was found, corresponding to the
ratio . At low temperatures saturates and
becomes constant below , in agreement with what is expected for
s-wave BCS superconductors. Our results suggest that LiPdB is a s-wave
BCS superconductor with the only one isotropic energy gap.Comment: 6 pages, 7 figure
Dissecting magnetar variability with Bayesian hierarchical models
Neutron stars are a prime laboratory for testing physical processes under
conditions of strong gravity, high density, and extreme magnetic fields. Among
the zoo of neutron star phenomena, magnetars stand out for their bursting
behaviour, ranging from extremely bright, rare giant flares to numerous, less
energetic recurrent bursts. The exact trigger and emission mechanisms for these
bursts are not known; favoured models involve either a crust fracture and
subsequent energy release into the magnetosphere, or explosive reconnection of
magnetic field lines. In the absence of a predictive model, understanding the
physical processes responsible for magnetar burst variability is difficult.
Here, we develop an empirical model that decomposes magnetar bursts into a
superposition of small spike-like features with a simple functional form, where
the number of model components is itself part of the inference problem. The
cascades of spikes that we model might be formed by avalanches of reconnection,
or crust rupture aftershocks. Using Markov Chain Monte Carlo (MCMC) sampling
augmented with reversible jumps between models with different numbers of
parameters, we characterise the posterior distributions of the model parameters
and the number of components per burst. We relate these model parameters to
physical quantities in the system, and show for the first time that the
variability within a burst does not conform to predictions from ideas of
self-organised criticality. We also examine how well the properties of the
spikes fit the predictions of simplified cascade models for the different
trigger mechanisms.Comment: accepted for publication in The Astrophysical Journal; code available
at https://bitbucket.org/dhuppenkothen/magnetron, data products at
http://figshare.com/articles/SGR_J1550_5418_magnetron_data/129242
CDW photogrammetry of low relief fluvial features: accuracy and implications for reach-scale sediment budgeting
CDW photogrammetry of low relief fluvial features: accuracy and implications for reach-scale sediment budgeting
Territorial Tactics: The Socio-spatial Significance of Private Policing Strategies in Cape Town
This paper analyses the policing strategies of private security companies operating in urban space. An existing literature has considered the variety of ways that territory becomes of fundamental importance in the work of public police forces. However, this paper examines territory in the context of private security companies. Drawing on empirical research in Cape Town, it examines how demarcated territories become key subjects in private policing. Private security companies are responsible for a relatively small section of the city, while in contrast the public police ultimately have to see city space as a whole. Hence, private policing strategy becomes one of displacement, especially of so-called undesirables yielding a patchworked public space associated with private enclaves of consumption. The conclusions signal the historical resonances and comparative implications of these political-legal-security dynamics. © 2013 Urban Studies Journal Limited
Transport of strong-coupling polarons in optical lattices
We study the transport of ultracold impurity atoms immersed in a
Bose-Einstein condensate (BEC) and trapped in a tight optical lattice. Within
the strong-coupling regime, we derive an extended Hubbard model describing the
dynamics of the impurities in terms of polarons, i.e. impurities dressed by a
coherent state of Bogoliubov phonons. Using a generalized master equation based
on this microscopic model we show that inelastic and dissipative phonon
scattering results in (i) a crossover from coherent to incoherent transport of
impurities with increasing BEC temperature and (ii) the emergence of a net
atomic current across a tilted optical lattice. The dependence of the atomic
current on the lattice tilt changes from ohmic conductance to negative
differential conductance within an experimentally accessible parameter regime.
This transition is accurately described by an Esaki-Tsu-type relation with the
effective relaxation time of the impurities as a temperature-dependent
parameter.Comment: 25 pages, 6 figure
On the topology of adiabatic passage
We examine the topology of eigenenergy surfaces characterizing the population
transfer processes based on adiabatic passage. We show that this topology is
the essential feature for the analysis of the population transfers and the
prediction of its final result. We reinterpret diverse known processes, such as
stimulated Raman adiabatic passage (STIRAP), frequency-chirped adiabatic
passage and Stark-chirped rapid adiabatic passage (SCRAP). Moreover, using this
picture, we display new related possibilities of transfer. In particular, we
show that we can selectively control the level which will be populated in
STIRAP process in Lambda or V systems by the choice of the peak amplitudes or
the pulse sequence
On the possibility to supercool molecular hydrogen down to superfluid transition
Recent calculations by Vorobev and Malyshenko (JETP Letters, 71, 39, 2000)
show that molecular hydrogen may stay liquid and superfluid in strong electric
fields of the order of . I demonstrate that strong local
electric fields of similar magnitude exist beneath a two-dimensional layer of
electrons localized in the image potential above the surface of solid hydrogen.
Even stronger local fields exist around charged particles (ions or electrons)
if surface or bulk of a solid hydrogen crystal is statically charged.
Measurements of the frequency shift of the photoresonance transition
in the spectrum of two-dimensional layer of electrons above positively or
negatively charged solid hydrogen surface performed in the temperature range 7
- 13.8 K support the prediction of electric field induced surface melting. The
range of surface charge density necessary to stabilize the liquid phase of
molecular hydrogen at the temperature of superfluid transition is estimated.Comment: 5 pages, 2 figure
Evaluating the cost-effectiveness of TB diagnostic strategies in HIV-positive patients in Lusaka, Zambia
Neuroimaging measurements derived from magnetic resonance imaging provide important information required for detecting changes related to the progression of mild cognitive impairment (MCI). Cortical features and changes play a crucial role in revealing unique anatomical patterns of brain regions, and further differentiate MCI patients from normal states. Four cortical features, namely, gray matter volume, cortical thickness, surface area, and mean curvature, were explored for discriminative analysis among three groups including the stableMCI (sMCI), the convertedMCI (cMCI), and the normal control (NC) groups. In this study, 158 subjects (72 NC, 46 sMCI, and 40 cMCI) were selected from the Alzheimer's Disease Neuroimaging Initiative. A sparse-constrained regression model based on the l2-1-norm was introduced to reduce the feature dimensionality and retrieve essential features for the discrimination of the three groups by using a support vector machine (SVM). An optimized strategy of feature addition based on the weight of each feature was adopted for the SVM classifier in order to achieve the best classification performance. The baseline cortical features combined with the longitudinal measurements for 2 years of follow-up data yielded prominent classification results. In particular, the cortical thickness produced a classification with 98.84% accuracy, 97.5% sensitivity, and 100% specificity for the sMCI-cMCI comparison; 92.37% accuracy, 84.78% sensitivity, and 97.22% specificity for the cMCI-NC comparison; and 93.75% accuracy, 92.5% sensitivity, and 94.44% specificity for the sMCI-NC comparison. The best performances obtained by the SVM classifier using the essential features were 5-40% more than those using all of the retained features. The feasibility of the cortical features for the recognition of anatomical patterns was certified; thus, the proposed method has the potential to improve the clinical diagnosis of sub-types of MCI and predict the risk of its conversion to Alzheimer's diseas
An analysis of integrative outcomes in the Dayton peace negotiations
The nature of the negotiated outcomes of the eight issues of the Dayton Peace Agreement was studied in terms of their integrative and distributive aspects. in cases where integrative elements were Sound, further analysis was conducted by concentrating on Pruitt's five types of integrative solutions: expanding the pie, cost cutting, non-specific compensation, logrolling, and bridging. The results showed that real world international negotiations can arrive at integrative agreements even when they involve redistribution of resources tin this case the redistribution of former Yugoslavia). Another conclusion was that an agreement can consist of several distributive outcomes and several integrative outcomes produced by different kinds of mechanisms. Similarly, in single issues more than one mechanism can be used simultaneously. Some distributive bargaining was needed in order to determine how much compensation was required. Finally, each integrative formula had some distributive aspects as well
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