Geometrical Frustration and Static Correlations in Hard-Sphere Glass Formers

We analytically and numerically characterize the structure of hard-sphere fluids in order to review various geometrical frustration scenarios of the glass transition. We find generalized polytetrahedral order to be correlated with increasing fluid packing fraction, but to become increasingly irrelevant with increasing dimension. We also find the growth in structural correlations to be modest in the dynamical regime accessible to computer simulations.Comment: 21 pages; part of the "Special Topic Issue on the Glass Transition

Brave Forms of Mentoring Supported by Technology in Teacher Education

Indexación: Web of ScienceQuality education is undoubtedly a global concern, tied closely to preoccupations with economic and social development. Increasingly, the adoption and effective use of current technology tools are being recognized as visible signs of that quality. Scholars are providing increasing evidence of the kinds of empowered teacher identities that will adopt the effective use of technology tools in teaching. Less is being discussed about how technology can support the processes needed to mediate such identities. The context of Teacher Education is a strategic place to begin to initiate such processes. Our aim in this article is twofold: 1) to describe two recent examples of innovative, technology - supported mentoring processes that were conducted in the context of an EFL Teacher Education program in Chile; 2) to revisit the findings of these studies in light of new evidence from participants who have moved on in their careers. This evidence is viewed in the framework of recent scholarship on the responsibilities that Teacher Education plays in their development. The first 16-month study examined the influences of a guided reading program involving e-readers on the identities and literacy skills of pre-service teachers. The second was a student-conceived study. That inquiry sought to determine the influence of upper year students' peer mentoring, made available partly through a social media site (SMS), on the identities and investment in learning of 12 firs-year students in the pedagogy program. The initial evidence from ethnographic tools used in both studies indicated that the participants were struggling with confidence and doubting themselves as knowledgeable, effective future teachers - not predictive of a potential for quality teaching. Positive signs at the end of both studies and more recent reports from participants suggest that the mentoring had longitudinal benefits for some, although not uniformly. The potential of apprenticeship and mentoring in a technology-supported environment requires rethinking Teacher Education mandates if we are to empower emerging teachers to be quality teachers.http://www.ejel.org/issue/download.html?idArticle=48

Systematic characterization of thermodynamic and dynamical phase behavior in systems with short-ranged attraction

In this paper we demonstrate the feasibility and utility of an augmented version of the Gibbs ensemble Monte Carlo method for computing the phase behavior of systems with strong, extremely short-ranged attractions. For generic potential shapes, this approach allows for the investigation of narrower attractive widths than those previously reported. Direct comparison to previous self-consistent Ornstein-Zernike approximation calculations are made. A preliminary investigation of out-of-equilibrium behavior is also performed. Our results suggest that the recent observations of stable cluster phases in systems without long-ranged repulsions are intimately related to gas-crystal and metastable gas-liquid phase separation.Comment: 10 pages, 8 figure

The Spitzer search for the transits of HARPS low-mass planets - I. No transit for the super-Earth HD 40307b

We have used Spitzer and its IRAC camera to search for the transit of the super-Earth HD 40307b. The transiting nature of the planet could not be firmly discarded from our first photometric monitoring of a transit window because of the uncertainty coming from the modeling of the photometric baseline. To obtain a firm result, two more transit windows were observed and a global Bayesian analysis of the three IRAC time series and the HARPS radial velocities was performed. Unfortunately, any transit of the planet during the observed phase window is firmly discarded, while the probability that the planet transits but that the eclipse was missed by our observations is nearly negligible (0.26%).Comment: Submitted to A&

Phase Transformations in Binary Colloidal Monolayers

Phase transformations can be difficult to characterize at the microscopic level due to the inability to directly observe individual atomic motions. Model colloidal systems, by contrast, permit the direct observation of individual particle dynamics and of collective rearrangements, which allows for real-space characterization of phase transitions. Here, we study a quasi-two-dimensional, binary colloidal alloy that exhibits liquid-solid and solid-solid phase transitions, focusing on the kinetics of a diffusionless transformation between two crystal phases. Experiments are conducted on a monolayer of magnetic and nonmagnetic spheres suspended in a thin layer of ferrofluid and exposed to a tunable magnetic field. A theoretical model of hard spheres with point dipoles at their centers is used to guide the choice of experimental parameters and characterize the underlying materials physics. When the applied field is normal to the fluid layer, a checkerboard crystal forms; when the angle between the field and the normal is sufficiently large, a striped crystal assembles. As the field is slowly tilted away from the normal, we find that the transformation pathway between the two phases depends strongly on crystal orientation, field strength, and degree of confinement of the monolayer. In some cases, the pathway occurs by smooth magnetostrictive shear, while in others it involves the sudden formation of martensitic plates.Comment: 13 pages, 7 figures. Soft Matter Latex template was used. Published online in Soft Matter, 201

On the penetration of meridional circulation below the solar convection zone

Meridional flows with velocities of a few meters per second are observed in the uppermost regions of the solar convection zone. The amplitude and pattern of the flows deeper in the solar interior, in particular near the top of the radiative region, are of crucial importance to a wide range of solar magnetohydrodynamical processes. In this paper, we provide a systematic study of the penetration of large-scale meridional flows from the convection zone into the radiative zone. In particular, we study the effects of the assumed boundary conditions applied at the convective-radiative interface on the deeper flows. Using simplified analytical models in conjunction with more complete numerical methods, we show that penetration of the convectively-driven meridional flows into the deeper interior is not necessarily limited to a shallow Ekman depth but can penetrate much deeper, depending on how the convective-radiative interface flows are modeled.Comment: 13 pages, 5 figures. Subitted to Ap