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An understanding of the dynamic behavior of complex systems such as supercooled liquids requires a knowledge of the microscopic processes occurring in them. Correlation functions, which can be derived from experimental measurements of macroscopic properties or from computer simulations, cannot distinguish between particles that have never moved and those that have changed their position or state and then returned to the original one. Such a distinction can be obtained from the comparison of a correlation function with the transition function, i.e., the fraction of particles that have never moved up to time t. As the author demonstrates by an extension of his recently published results for the ferromagnetic Potts model [J. Chem. Phys. 124, 214508 (2006)], valuable insights into the microscopic behavior of a system can be obtained from such a comparison. A generalization of the transition function to a first passage function can provide useful extra insights. [ABSTRACT FROM AUTHOR]

Among the outstanding problems in the theory of supercooled liquids are the reasons for the rapid increase in their viscosity and relaxation times as the temperature is lowered towards the glass transition temperature Tg, the nonexponential time dependence of the relaxation, and the possible connection between these two properties. The ferromagnetic Potts model on a square latice is a simple system that is found to exhibit these properties. Our calculations show that in this system the connection between them is associated with the dependence on temperature and time of the average environment of the sites. Some of the consequences of this for understanding the behavior of supercooled liquids are discussed. [ABSTRACT FROM AUTHOR]

We present a model for the ac conduction in ionically conducting solids that takes into account, in a simple way, the interaction between carriers. The Coulomb force forms an “ionic atmosphere” that exerts a restoring force on a central ion, whose motion corresponds to an overdamped oscillator. We consider the effect of the relaxation of the ionic atmosphere by introducing an additional equation for the displacement of the potential toward the particle position. The general behavior of the ac conductivity can be understood in terms of two types of motions: motion of the bound ion at high frequencies determined by microscopic friction, and a much slower motion coupled to the surrounding carriers relaxation at low frequencies. [ABSTRACT FROM AUTHOR]

Objective: To determine the effectiveness and safety of repeated precoital and postcoital use of levonorgestrel for pregnancy prevention.Data Sources: We searched eight computerized databases for studies that evaluated oral hormones taken for contraception immediately before or after each coital act during one or more menstrual cycles. We reviewed reference lists and contacted experts.Methods Of Study Selection: Of 31 studies identified, we excluded 16 from this review because they contained incomplete information on the drug regimen, number of pregnancies, or duration of follow-up or because they did not evaluate levonorgestrel. The remaining 15 studies collectively included 20 groups of women treated with levonorgestrel: 10 groups received 0.75 mg and 10 received other doses.Tabulation, Integration, and Results: Two authors independently extracted data from the included studies. We calculated Pearl indices from individual studies and pooled data and estimated confidence intervals (CIs) using a Poisson distribution. Most studies had methodological limitations. The studies of the 0.75-mg levonorgestrel dose included 2,628 patients who used the method for 1,033 woman-years and yielded a pooled Pearl index of 5.1 pregnancies per 100 woman-years (95% CI 3.8-6.7). The studies of other doses added 5,787 patients who used the method for 53,347 cycles. The pooled Pearl index for all studies was 4.9 pregnancies per 100 woman-years (95% CI 4.3-5.5). The main side effect was frequent menstrual irregularities. No serious adverse events were reported. Most women liked the pericoital method.Conclusion: The data suggested that pericoital oral levonorgestrel is safe and moderately effective. However, the quality of the studies was suboptimal. A pressing need exists for rigorous research to evaluate pericoital use of levonorgestrel as a primary means of contraception. [ABSTRACT FROM AUTHOR]

A theoretical model is presented in this Letter, that gives a qualitative and scale-estimation description of anomalous thermal properties of glasses exhibiting the experimentally observed boson peak and following “high frequency sound”, at moderately low temperatures corresponding to THz scale frequencies. It is shown that the observed “bump” of the reduced specific heat, as well as “plateau” and following quasi-linear increase of the thermal conductivity, with increasing temperature, have a common origin with each other and with the glass vibrational, THz frequency, dynamics and reasonably characterize the related experimental data. [Copyright &y& Elsevier]

A resonant mechanism is suggested for absorption of ultrasonic and electromagnetic waves of THz frequencies in glasses exhibiting both the boson peak and high frequency sound. The absorption is due to resonant transitions for single harmonic soft-mode related excitations determining the THz frequency dynamics of such glasses. The frequency dependence of the absorption coefficient is found to exhibit a broad peak similar to the boson peak in scattering spectra. It is argued that, under appropriate conditions, the predicted qualitative behavior of the absorption coefficient can be observed in experiment. [Copyright &y& Elsevier]

In the usual random energy model for supercooled liquids, the particles can occupy traps of various depths E, which they can leave by thermal excitation. An extension of this model is considered in which the particles can leave the traps by two possible routes, corresponding to direct thermal excitation of single particles to the energy E = 0 and to the correlated motion of a group of particles with a lower activation energy and smaller prefactor. In the model studied here, the traps for the particles are grouped into sets with discrete depths E[sub j] and with two different sets of densities g(E[sub j]). Our calculations show that, as the temperature is lowered, the second mechanism starts to dominate the system's dynamics, and that with the appropriate densities of states the combined mechanism can lead to meaningful results. [ABSTRACT FROM AUTHOR]

In order to investigate the importance of cooperative effects in the cooling of systems into a supercooled liquid or glassy state, a kinetic Ising model is considered with all transition rates thermally activated, so that cooling can lead to the system being trapped in a long-lived nonequilibrium glassy state. A difference in energy between the states with up and down spin permits the definition of a fictive temperature T[sub f] that defines the state of the system, and in this paper the effects of different cooling rates on the evolution of T[sub f] are examined. Two types of model system are considered, one in which the transitions of the different spins are independent and the other in which they need to be facilitated by the presence of adjacent up spins (the Fredrickson-Andersen model), so that cooperative effects are involved. It is found that the requirement of facilitation not only slows down the transition rates but also affects the apparent activation energy deduced from the variation of the final fictive temperature with the cooling rate, and a theoretical analysis of the results is presented. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract Background Lower-than-expected incidence of HIV undermines sample size calculations and compromises the power of a HIV prevention trial. We evaluated the effectiveness of interim monitoring of HIV infection rates and on-going modification of recruitment strategies to enroll women at higher risk of HIV in the Cellulose Sulfate Phase III study in Nigeria. Methods We analyzed prevalence and incidence of HIV and other sexually transmitted infections, demographic and sexual behavior characteristics aggregated over the treatment groups on a quarterly basis. The site investigators were advised on their recruitment strategies based on the findings of the interim analyses. Results A total of 3619 women were screened and 1644 enrolled at the Ikeja and Apapa clinics in Lagos, and at the Central and Peripheral clinics in Port Harcourt. Twelve months after study initiation, the overall incidence of HIV was less than one-third of the pre-study assumption, with rates of HIV that varied substantially between clinics. Due to the low prevalence and incidence rates of HIV, it was decided to close the Ikeja clinic in Lagos and to find new catchment areas in Port Harcourt. This strategy was associated with an almost two-fold increase in observed HIV incidence during the second year of the study. Conclusion Given the difficulties in estimating HIV incidence, a close monitoring of HIV prevalence and incidence rates during a trial is warranted. The on-going modification of recruitment strategies based on the regular analysis of HIV rates appeared to be an efficient method for targeting populations at greatest risk of HIV infection and increasing study power in the Nigeria trial. Trial Registration The trial was registered with the ClinicalTrials.gov registry under #NCT00120770 http://clinicaltrials.gov/ct2/show/NCT00120770