Theorem on the Distribution of Short-Time Particle Displacements with Physical Applications

The distribution of the initial short-time displacements of particles is considered for a class of classical systems under rather general conditions on the dynamics and with Gaussian initial velocity distributions, while the positions could have an arbitrary distribution. This class of systems contains canonical equilibrium of a Hamiltonian system as a special case. We prove that for this class of systems the nth order cumulants of the initial short-time displacements behave as the 2n-th power of time for all n>2, rather than exhibiting an nth power scaling. This has direct applications to the initial short-time behavior of the Van Hove self-correlation function, to its non-equilibrium generalizations the Green's functions for mass transport, and to the non-Gaussian parameters used in supercooled liquids and glasses.Comment: A less ambiguous mathematical notation for cumulants was adopted and several passages were reformulated and clarified. 40 pages, 1 figure. Accepted by J. Stat. Phy

Phase behavior of a system of particles with core collapse

The pressure-temperature phase diagram of a one-component system, with particles interacting through a spherically symmetric pair potential in two dimensions is studied. The interaction consists of a hard core plus an additional repulsion at low energies. It is shown that at zero temperature, instead of the expected isostructural transition due to core collapse occurring when increasing pressure, the system passes through a series of ground states that are not triangular lattices. In particular, and depending on parameters, structures with squares, chains, hexagons and even quasicrystalline ground states are found. At finite temperatures the solid-fluid coexistence line presents a zone with negative slope (which implies melting with decreasing in volume) and the fluid phase has a temperature of maximum density, similar to that in water.Comment: 11 pages, 15 figures included. To appear in PRE. Some figures in low quality format. Better ones available upon request from [email protected]

Studies on optimizing potential energy functions for maximal intrinsic hyperpolarizability

We use numerical optimization to study the properties of (1) the class of one-dimensional potential energy functions and (2) systems of point charges in two-dimensions that yield the largest hyperpolarizabilities, which we find to be within 30% of the fundamental limit. We investigate the character of the potential energy functions and resulting wavefunctions and find that a broad range of potentials yield the same intrinsic hyperpolarizability ceiling of 0.709.Comment: 9 pages, 9 figure

Propensity score matching in estimating the effect of managerial education on academic planning behavior. Study design: a cross-sectional study

<p>Abstract</p> <p>Background</p> <p>In many academic settings teaching a particular topic is applied to every student enrolled in the same academic year, it is a difficult task for researchers to design a randomized control group study. This research aimed to estimate the effect of teaching management and planning on increasing academic planning behavior (APB), using propensity score matching (PSM).</p> <p>Methods</p> <p>In a cross-sectional survey utilizing a self-reported structured questionnaire on a systematic random sample of 421 students in Hanoi Medical University, one of the eight medical schools in Vietnam, this evaluation study adopted regression procedures to assess model fit, then PSM to create a matched control group in order to allow for evaluating the effect of management education.</p> <p>Results</p> <p>The study showed both direct and indirect effects of the education on behavior. After PSM to adjust for the possible confounders to balance statistically two groups - with and without management education, there is statistically a significant difference in APB between these two groups, making a net difference of 18.60% (p < .05). The estimated 18.6 percentage point increase can be translated into the practice of APB by 670 students in the population. This number of academic planners can be attributed to a high recall of important management and planning education.</p> <p>Conclusions</p> <p>The study provided theoretical as well as practical implications to guide the design of the education and evaluation of teaching.</p

The Explanatory Value of Abstracting Away from Idiosyncratic and Messy Detail

Some explanations are relatively abstract: they abstract away from the idiosyncratic or messy details of the case in hand. The received wisdom in philosophy is that this is a virtue for any explanation to possess. I argue that the apparent consensus on this point is illusory. When philosophers make this claim, they differ on which of four alternative varieties of abstractness they have in mind. What's more, for each variety of abstractness there are several alternative reasons to think that the variety of abstractness in question is a virtue. I identify the most promising reasons, and dismiss some others. The paper concludes by relating this discussion to the idea that explanations in biology, psychology and social science cannot be replaced by relatively micro explanations without loss of understanding.This work has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no 284123.This is the author accepted manuscript. The final version is available from Springer via http://dx.doi.org/10.1007/s11098-015-0554-

Sound-propagation gap in fluid mixtures

We discuss the behavior of the extended sound modes of a dense binary hard-sphere mixture. In a dense simple hard-sphere fluid the Enskog theory predicts a gap in the sound propagation at large wave vectors. In a binary mixture the gap is only present for low concentrations of one of the two species. At intermediate concentrations sound modes are always propagating. This behavior is not affected by the mass difference of the two species, but it only depends on the packing fractions. The gap is absent when the packing fractions are comparable and the mixture structurally resembles a metallic glass.Comment: Published; withdrawn since ordering in archive gives misleading impression of new publicatio

The effect of participatory community communication on HIV preventive behaviors among ethnic minority youth in central Vietnam

<p>Abstract</p> <p>Background</p> <p>In Vietnam, socially marginalized groups such as ethnic minorities in mountainous areas are often difficult to engage in HIV research and prevention programs. This intervention study aimed to estimate the effect of participatory community communication (PCC) on changing HIV preventive ideation and behavior among ethnic minority youth in a rural district from central Vietnam.</p> <p>Methods</p> <p>In a cross-sectional survey after the PCC intervention, using a structured questionnaire, 800 ethnic minority youth were approached for face-to-face interviews. Propensity score matching (PSM) technique was then utilized to match these participants into two groups-intervention and control-for estimating the effect of the PCC.</p> <p>Results</p> <p>HIV preventive knowledge and ideation tended to increase as the level of recall changed accordingly. The campaign had a significant indirect effect on condom use through its effect on ideation or perceptions. When intervention and control group statistically equivalently reached in terms of individual and social characteristics by PSM, proportions of displaying HIV preventive knowledge, ideation and condom use were significantly higher in intervention group than in matched control counterparts, accounting for net differences of 7.4%, 12.7% and 5%, respectively, and can be translated into the number of 210; 361 and 142 ethnic minority youth in the population.</p> <p>Conclusions</p> <p>The study informs public health implications both theoretically and practically to guide effective HIV control programs for marginalized communities in resources-constrained settings like rural Vietnam and similar contexts of developing countries.</p

Synchrotron radiation from a charge moving along a helical orbit inside a dielectric cylinder

The radiation emitted by a charged particle moving along a helical orbit inside a dielectric cylinder immersed into a homogeneous medium is investigated. Expressions are derived for the electromagnetic potentials, electric and magnetic fields, and for the spectral-angular distribution of radiation in the exterior medium. It is shown that under the Cherenkov condition for dielectric permittivity of the cylinder and the velocity of the particle image on the cylinder surface, strong narrow peaks are present in the angular distribution for the number of radiated quanta. At these peaks the radiated energy exceeds the corresponding quantity for a homogeneous medium by some orders of magnitude. The results of numerical calculations for the angular distribution of radiated quanta are presented and they are compared with the corresponding quantities for radiation in a homogeneous medium. The special case of relativistic charged particle motion along the direction of the cylinder axis with non-relativistic transverse velocity (helical undulator) is considered in detail. Various regimes for the undulator parameter are discussed. It is shown that the presence of the cylinder can increase essentially the radiation intensity.Comment: 18 pages, 8 EPS figure