160 research outputs found
Experimental evidence for the role of cantori as barriers in a quantum system
We investigate the effect of cantori on momentum diffusion in a quantum
system. Ultracold caesium atoms are subjected to a specifically designed
periodically pulsed standing wave. A cantorus separates two chaotic regions of
the classical phase space. Diffusion through the cantorus is classically
predicted. Quantum diffusion is only significant when the classical phase-space
area escaping through the cantorus per period greatly exceeds Planck's
constant. Experimental data and a quantum analysis confirm that the cantori act
as barriers.Comment: 19 pages including 9 figures, Accepted for publication in Physical
Review E in March 199
Magnetic structure and critical behavior of GdRhIn: resonant x-ray diffraction and renormalization group analysis
The magnetic structure and fluctuations of tetragonal GdRhIn5 were studied by
resonant x-ray diffraction at the Gd LII and LIII edges, followed by a
renormalization group analysis for this and other related Gd-based compounds,
namely Gd2IrIn8 and GdIn3. These compounds are spin-only analogs of the
isostructural Ce-based heavy-fermion superconductors. The ground state of
GdRhIn5 shows a commensurate antiferromagnetic spin structure with propagation
vector tau = (0,1/2, 1/2), corresponding to a parallel spin alignment along the
a-direction and antiparallel alignment along b and c. A comparison between this
magnetic structure and those of other members of the Rm(Co,Rh,Ir)n In3m+2n
family (R =rare earth, n = 0, 1; m = 1, 2) indicates that, in general, tau is
determined by a competition between first-(J1) and second-neighbor(J2)
antiferromagnetic (AFM) interactions. While a large J1 /J2 ratio favors an
antiparallel alignment along the three directions (the so-called G-AFM
structure), a smaller ratio favors the magnetic structure of GdRhIn5 (C-AFM).
In particular, it is inferred that the heavy-fermion superconductor CeRhIn5 is
in a frontier between these two ground states, which may explain its
non-collinear spiral magnetic structure. The critical behavior of GdRhIn5 close
to the paramagnetic transition at TN = 39 K was also studied in detail. A
typical second-order transition with the ordered magnetization critical
parameter beta = 0.35 was experimentally found, and theoretically investigated
by means of a renormalization group analysis.Comment: 22 pages, 4 figure
P13-05. Factors associated with pregnancy during the HVTN 503/Phambili trial, a phase IIB HIV trial of the Merck Ad-5 multi-clade HIV vaccine
Changes in Floquet state structure at avoided crossings: delocalization and harmonic generation
Avoided crossings are common in the quasienergy spectra of strongly driven
nonlinear quantum wells. In this paper we examine the sinusoidally driven
particle in a square potential well to show that avoided crossings can alter
the structure of Floquet states in this system. Two types of avoided crossings
are identified: on type leads only to temporary changes (as a function of
driving field strength) in Floquet state structure while the second type can
lead to permanent delocalization of the Floquet states. Radiation spectra from
these latter states show significant increase in high harmonic generation as
the system passes through the avoided crossing.Comment: 8 pages with 10 figures submitted to Physical Review
P15-13. HVTN 503(Phambili) trial discontinuation of enrolment/vaccination: the impact on trial participant attitudes to vaccine trials and scientific research
Background: The HVTN 503/Phambili trial, a phase IIB study of the Merck Ad-5 HIV vaccine, suspended enrollment and vaccinations following results of the HVTN502/STEP study. Participants were notified of their treatment allocation and continue to be followed. We investigated the impact of the suspension on trial participants' attitudes towards HIV vaccine trials and scientific research. Methods: A self-administered questionnaire investigating knowledge and attitudes utilising a five-point likert scale, was administered 12–18 months post suspension at the 4 Phambili trial sites implementing the latest protocol. Results: Of 544 participants enrolled at 4 sites, 490 completed the questionnaire. 51% understood that vaccination, and 31% that enrolment had been permanently discontinued. Although 85% knew Phambili DSMB recommendations were based on results of the STEP study, only 51% knew that the results showed more infections in the vaccine arm. Although 90% knew they should continue protecting themselves against HIV, only 33% acknowledged trialists' concerns that the vaccine may make them more susceptible to HIV. Knowledge was similar across sites. 12% had tested for HIV antibodies outside of the trial before unblinding to establish their treatment allocation. 46% felt disappointed, angry or afraid, primarily because the vaccine would not prevent HIV in their community. 79% said they would engage in less risk behaviour. 94% thought the study team had made the correct decision by unblinding them and 96% thought site-participant communication was appropriate. 91% felt they made the right decision to enrol in Phambili and 91% planned to continue attending scheduled visits. 86% support scientific research and 80% would want to participate in future HIV vaccine. Conclusion: Despite site communications, knowledge levels were low. Importantly, most knew they should continue to protect themselves against HIV. Although almost half of participants reported negative feelings, most attitudes towards trial sites, scientific research and HIV vaccine trials were positive
P14-07. Offering new prevention modalities in HIV vaccine trials: experience with male circumcision in the Phambili trial
Background: New prevention options will be added to the 'standard of prevention' offered in HIV vaccine efficacy trials as new methods prove effective. The HVTN503/Phambili trial was initiated in January 2007, shortly after results from 3 randomized controlled trials of male circumcision (MC) demonstrated that MC reduces the risk of HIV acquisition. Thus, HVTN503 investigators made plans to offer MC at no cost to enrolled male participants. Methods: All participants were informed of the benefits of MC as a component of HIV risk reduction counseling, including how and where MC could be accessed. One site offered on-site MC and the others provided referral to local services for men who requested MC. We present data on uptake of MC post-enrollment. Results: Prior to discontinuation of enrolment, 441 men joined the trial, of whom 312 (70.7%) were uncircumcised. Of these, 82 (26.3% of uncircumcised men) requested MC after enrolment. Uptake varied by site, being lower (70%) at the eThekwini site, the site with lowest baseline MC prevalence. Among 3 sites with intermediate baseline MC prevalence, uptake varied from 3.3 to 37.6%, being highest at the site providing MC on site. Uptake was similar in vaccine and placebo arms of the trial [42 (26.1%) vs. 40 (26.5%)]. There was no significant difference by arm in the timing of circumcision relative to randomization assignment being provided to participants following release of the STEP trial results [post-unblinding, vaccine 18 (42.9%) vs. placebo 13 (32.5%), p = 0.37]. Conclusion: MC, a new prevention modality, was offered as part of HIV prevention services in HVTN503. Uptake varied by provision of care model and inversely with baseline MC prevalence, but did not differ between treatment arms, and remained similar even after provision of treatment
Wavelet analysis of epileptic spikes
Interictal spikes and sharp waves in human EEG are characteristic signatures
of epilepsy. These potentials originate as a result of synchronous,
pathological discharge of many neurons. The reliable detection of such
potentials has been the long standing problem in EEG analysis, especially after
long-term monitoring became common in investigation of epileptic patients. The
traditional definition of a spike is based on its amplitude, duration,
sharpness, and emergence from its background. However, spike detection systems
built solely around this definition are not reliable due to the presence of
numerous transients and artifacts. We use wavelet transform to analyze the
properties of EEG manifestations of epilepsy. We demonstrate that the behavior
of wavelet transform of epileptic spikes across scales can constitute the
foundation of a relatively simple yet effective detection algorithm.Comment: 4 pages, 3 figure
Population-Level Benefits from Providing Effective HIV Prevention Means to Pregnant Women in High Prevalence Settings
Background:HIV prevalence among pregnant women in Southern Africa is extremely high. Epidemiological studies suggest that pregnancy increases the risk of HIV sexual acquisition and that HIV infections acquired during pregnancy carry higher risk of mother-to-child transmission (MTCT). We analyze the potential benefits from extending the availability of effective microbicide to pregnant women (in addition to non-pregnant women) in a wide-scale intervention.Methods and Findings:A transmission dynamic model was designed to assess the impact of microbicide use in high HIV prevalence settings and to estimate proportions of new HIV infections, infections acquired during pregnancy, and MTCT prevented over 10 years. Our analysis suggests that consistent use of microbicide with 70% efficacy by 60% of non-pregnant women may prevent approximately 40% and 15% of new infections in women and men respectively over 10 years, assuming no additional increase in HIV risk to either partner during pregnancy (RRHIV/preg = 1). It may also prevent 8-15% MTCT depending on the increase in MTCT risk when HIV is acquired during pregnancy compared to before pregnancy (RRMTCT/preg). Extending the microbicides use during pregnancy may improve the effectiveness of the intervention by 10% (RRHIV/preg = 1) to 25% (RRHIV/preg = 2) and reduce the number of HIV infections acquired during pregnancy by 40% to 70% in different scenarios. It may add between 6% (RRHIV/preg = 1, RRMTCT/preg = 1) and 25% (RRHIV/preg = 2, RRMTCT/preg = 4) to the reduction in the residual MTCT.Conclusion:Providing safe and effective microbicide to pregnant women in the context of wide-scale interventions would be desirable as it would increase the effectiveness of the intervention and significantly reduce the number of HIV infections acquired during pregnancy. The projected benefits from covering pregnant women by the HIV prevention programs is more substantial in communities in which the sexual risk during pregnancy is elevated. © 2013 Dimitrov et al
P15-08. Did unblinding affect HIV risk behaviour and risk perception in the HVTN503/Phambili study?
Effects of a nonlinear perturbation on dynamical tunneling in cold atoms
We perform a numerical analysis of the effects of a nonlinear perturbation on
the quantum dynamics of two models describing non-interacting cold atoms in a
standing wave of light with a periodical modulated amplitude . One model
is the driven pendulum, considered in ref.\cite{raiz1}, and the other is a
variant of the well-known Kicked Rotator Model. In absence of the nonlinear
perturbation, the system is invariant under some discrete symmetries and
quantum dynamical tunnelling between symmetric classical islands is found. The
presence of nonlinearity destroys tunnelling, breaking the symmetries of the
system. Finally, further consequences of nonlinearity in the kicked rotator
case are considered.Comment: 10 pages, 15 figure
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