714 research outputs found
Crossover and scaling in a two-dimensional field-tuned superconductor
Using an analysis similar to that of Imry and Wortis, it is shown that the
apparent first order superconductor to metal transition, which has been claimed
to exist at low values of the magnetic field in a two-dimensional field-tuned
system at zero temperature,can be consistentlyinterpreted as a sharp crossover
from a strong superconductor to an inhomogeneous state, which is a weak
superconductor. The true zero-temperature superconductor to insulator
transition within the inhomogenous state is conjectured to be that of randomly
diluted XY model. An explaination of the observed finite temperature
approximate scaling of resistivity close to the critical point is speculated
within this model.Comment: 5 pages, 2 figures, corrected and modified according to referee
Report
Measurement of the motional sidebands of a nanogram-scale oscillator in the quantum regime
We describe measurements of the motional sidebands produced by a mechanical oscillator (with effective
mass 43 ng and resonant frequency 705 kHz) that is placed in an optical cavity and cooled close to its quantum
ground state. The red and blue sidebands (corresponding to Stokes and anti-Stokes scattering) from a single laser
beam are recorded simultaneously via a heterodyne measurement. The oscillatorâs mean phonon number ÂŻn is
inferred from the ratio of the sidebands, and reaches a minimum value of 0.84 ± 0.22 (corresponding to a mode
temperature T = 28 ± 7ÎŒK). We also infer ÂŻn from the calibrated area of each of the two sidebands, and from the
oscillatorâs total damping. The values of ÂŻn inferred from these four methods are in close agreement. The behavior
of the sidebands as a function of the oscillatorâs temperature agrees well with theory that includes the quantum
fluctuations of both the cavity field and the mechanical oscillator
Uniform in bandwidth exact rates for a class of kernel estimators
Given an i.i.d sample , taking values in \RRR^{d'}\times \RRR^d,
we consider a collection Nadarya-Watson kernel estimators of the conditional
expectations \EEE(+d_g(z)\mid Z=z), where belongs to a
compact set H\subset \RRR^d, a Borel function on \RRR^{d'} and
are continuous functions on \RRR^d. Given two
bandwidth sequences h_n<\wth_n fulfilling mild conditions, we obtain an exact
and explicit almost sure limit bounds for the deviations of these estimators
around their expectations, uniformly in g\in\GG,\;z\in H and h_n\le h\le
\wth_n under mild conditions on the density , the class \GG, the kernel
and the functions . We apply this result to prove
that smoothed empirical likelihood can be used to build confidence intervals
for conditional probabilities \PPP(Y\in C\mid Z=z), that hold uniformly in
z\in H,\; C\in \CC,\; h\in [h_n,\wth_n]. Here \CC is a Vapnik-Chervonenkis
class of sets.Comment: Published in the Annals of the Institute of Statistical Mathematics
Volume 63, p. 1077-1102 (2011
Understanding emission reductions in the freight transport sector through system dynamics
Paper presented at the 33rd Annual Southern African Transport Conference 7-10 July 2014 "Leading Transport into the Future", CSIR International Convention Centre, Pretoria, South Africa.The National Climate Change Response White Paper presents the governmentâs
commitment to moving to a lower carbon economy in South Africa. The White Paper
requires significant greenhouse gas (GHG) emitting sectors, including transport, to define
âbottom upâ carbon budgets. This work forms part of the WWF low carbon frameworks
transport project and explores the implications of strategies for GHG emission reductions
in the freight transport sub-sector within this context. The study uses system dynamics
modelling to interrogate mitigation opportunities in this sector by looking specifically at the
transport of processed food along the Cape Town-Gauteng corridor. Stakeholder input
from business, labour and government is used to guide the modelling process. The
objectives are to explore the implications of strategies for GHG emission reductions in the
freight transport sub-sector as well as its impacts on the wider economy and environment;
determine the barriers to change, with a specific focus on the mode switch from road to
rail; recommend appropriate measures that could ensure developments towards achieving
sustainability in transportation planning; and identify possible opportunities for further
research and innovation. The methodology behind the model as well as the initial casual
loop diagrams for the system dynamics model are presented in this paper.This paper was transferred from the original CD ROM created for this conference. The material was published using Adobe Acrobat 10.1.0 Technology. The original CD ROM was produced by CE Projects cc. Postal Address: PO Box 560 Irene 0062 South Africa. Tel.: +27 12 667 2074 Fax: +27 12 667 2766 E-mail: [email protected]
Numerical approximation of the Euler-Poisson-Boltzmann model in the quasineutral limit
This paper analyzes various schemes for the Euler-Poisson-Boltzmann (EPB)
model of plasma physics. This model consists of the pressureless gas dynamics
equations coupled with the Poisson equation and where the Boltzmann relation
relates the potential to the electron density. If the quasi-neutral assumption
is made, the Poisson equation is replaced by the constraint of zero local
charge and the model reduces to the Isothermal Compressible Euler (ICE) model.
We compare a numerical strategy based on the EPB model to a strategy using a
reformulation (called REPB formulation). The REPB scheme captures the
quasi-neutral limit more accurately
Development of Morphologically Discrete PEGâPDLLA Nanotubes for Precision Nanomedicine
Precise control over the morphological features of nanoparticles is an important requisite for their application in nanomedical research. Parameters such as size and shape have been identified as critical features for effective nanotherapeutic technologies due to their role in circulation, distribution, and internalization in vivo. Tubular PEG-PDLLA polymersomes (nanotubes) exhibit an interesting morphology with potential for immunotherapeutics, as the elongated shape can affect cell-particle interactions. Developing methodologies that permit control over the precise form of such nanotubes is important for their biomedical implementation due to the stringent physicochemical constraints for efficacious performance. Through careful control over the engineering process, we demonstrate the generation of well-defined nanotubes based on polymersomes as small as 250 and 100 nm, which can be successfully shape transformed. The quality of the resulting nanostructures was established by physical characterization using AF4-MALS and cryo-TEM. Moreover, we show the successful loading of such nanotubes with model payloads (proteins and drugs). These findings provide a promising platform for implementation in biomedical applications in which discrete structure and functionality are essential features
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Synthesis of accelerograms compatible with the Chinese GB 50011-2001 design spectrum via harmonic wavelets: artificial and historic records
A versatile approach is employed to generate artificial accelerograms which satisfy the compatibility criteria prescribed by the Chinese aseismic code provisions GB 50011-2001. In particular, a frequency dependent peak factor derived by means of appropriate Monte Carlo analyses is introduced to relate the GB 50011-2001 design spectrum to a parametrically defined evolutionary power spectrum (EPS). Special attention is given to the definition of the frequency content of the EPS in order to accommodate the mathematical form of the aforementioned design spectrum. Further, a one-to-one relationship is established between the parameter controlling the time-varying intensity of the EPS and the effective strong ground motion duration. Subsequently, an efficient auto-regressive moving-average (ARMA) filtering technique is utilized to generate ensembles of non-stationary artificial accelerograms whose average response spectrum is in a close agreement with the considered design spectrum. Furthermore, a harmonic wavelet based iterative scheme is adopted to modify these artificial signals so that a close matching of the signalsâ response spectra with the GB 50011-2001 design spectrum is achieved on an individual basis. This is also done for field recorded accelerograms pertaining to the May, 2008 Wenchuan seismic event. In the process, zero-phase high-pass filtering is performed to accomplish proper baseline correction of the acquired spectrum compatible artificial and field accelerograms. Numerical results are given in a tabulated format to expedite their use in practice
Continuous variable entanglement and quantum state teleportation between optical and macroscopic vibrational modes through radiation pressure
We study an isolated, perfectly reflecting, mirror illuminated by an intense
laser pulse. We show that the resulting radiation pressure efficiently
entangles a mirror vibrational mode with the two reflected optical sideband
modes of the incident carrier beam. The entanglement of the resulting
three-mode state is studied in detail and it is shown to be robust against the
mirror mode temperature. We then show how this continuous variable entanglement
can be profitably used to teleport an unknown quantum state of an optical mode
onto the vibrational mode of the mirror.Comment: 18 pages, 10 figure
Maternal iodine status in a multi-ethnic UK birth cohort:Associations with child cognitive and educational development
Background: Maternal iodine requirements increase during pregnancy to supply thyroid hormones critical for fetal neurodevelopment. Iodine insufficiency may result in poorer cognitive or child educational outcomes but current evidence is sparse and inconsistent. Objectives: To quantify the association between maternal iodine status and child educational outcomes. Methods: Urinary iodine concentrations (UIC) and iodine/creatinine ratios (I:Cr) were measured in 6971 mothers at 26-28 weeks' gestation participating in the Born in Bradford cohort. Maternal iodine status was examined in relation to child school achievement (early years foundation stage (EYFS), phonics, and Key Stage 1 (KS1)), other learning outcomes, social and behavioural difficulties, and sensorimotor control in 5745 children aged 4-7 years. Results: Median (interquartile range) UIC was 76 ”g/L (46, 120), and I:Cr was 83 ”g/g (59, 121). Overall, there was no strong or consistent evidence to support associations between UIC or I:Cr and neurodevelopmental outcomes. For instance, predicted EYFS and phonics scores (primary outcomes) at the 25th vs 75th I:Cr percentiles (99% confidence intervals) were similar, with no evidence of associations: EYFS scores were 32 (99% CI 31, 33) and 33 (99% CI 32, 34), and phonics scores were 34 (99% CI 33, 35) and 35 (99% CI 34, 36), respectively. Conclusions: In the largest single study of its kind, there was little evidence of detrimental neurodevelopmental outcomes in children born to pregnant women with iodine insufficiency as defined by World Health Organizationâoutlined thresholds. Alternative functional biomarkers for iodine status in pregnancy and focused assessment of other health outcomes may provide additional insight.</p
Nonperturbative renormalization group approach to frustrated magnets
This article is devoted to the study of the critical properties of classical
XY and Heisenberg frustrated magnets in three dimensions. We first analyze the
experimental and numerical situations. We show that the unusual behaviors
encountered in these systems, typically nonuniversal scaling, are hardly
compatible with the hypothesis of a second order phase transition. We then
review the various perturbative and early nonperturbative approaches used to
investigate these systems. We argue that none of them provides a completely
satisfactory description of the three-dimensional critical behavior. We then
recall the principles of the nonperturbative approach - the effective average
action method - that we have used to investigate the physics of frustrated
magnets. First, we recall the treatment of the unfrustrated - O(N) - case with
this method. This allows to introduce its technical aspects. Then, we show how
this method unables to clarify most of the problems encountered in the previous
theoretical descriptions of frustrated magnets. Firstly, we get an explanation
of the long-standing mismatch between different perturbative approaches which
consists in a nonperturbative mechanism of annihilation of fixed points between
two and three dimensions. Secondly, we get a coherent picture of the physics of
frustrated magnets in qualitative and (semi-) quantitative agreement with the
numerical and experimental results. The central feature that emerges from our
approach is the existence of scaling behaviors without fixed or pseudo-fixed
point and that relies on a slowing-down of the renormalization group flow in a
whole region in the coupling constants space. This phenomenon allows to explain
the occurence of generic weak first order behaviors and to understand the
absence of universality in the critical behavior of frustrated magnets.Comment: 58 pages, 15 PS figure
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