Advanced memory effects in the aging of a polymer glass

A new kind of memory effect on low frequency dielectric measurements on plexiglass (PMMA) is described. These measurements show that cooling and heating the sample at constant rate give an hysteretic dependence on temperature of the dielectric constant $\epsilon$. A temporary stop of cooling produces a downward relaxation of $\epsilon$. Two main features are observed i) when cooling is resumed $\epsilon$ goes back to the values obtained without the cooling stop (i.e. the low temperature state is independent of the cooling history) ii) upon reheating $\epsilon$ keeps the memory of all the cooling stops({\it Advanced memory}). The dependence of this effect on frequency and on the cooling rate is analyzed. The memory deletion is studied too. Finally the results are compared with those of similar experiments done in spin glasses and with the famous experiments of Kovacs.Comment: to be published in the European Physical Journa

Electrical noise properties in aging materials

The electric thermal noise has been measured in two aging materials, a colloidal suspension (Laponite) and a polymer (polycarbonate), presenting very slow relaxation towards equilibrium. The measurements have been performed during the transition from a fluid-like to a solid-like state for the gel and after a quench for the polymer. For both materials we have observed that the electric noise is characterized by a strong intermittency, which induces a large violation of the Fluctuation Dissipation Theorem (FDT) during the aging time, and may persist for several hours at low frequency. The statistics of these intermittent signals and their dependance on the quench speed for the polymer or on sample concentration for the gel are studied. The results are in a qualitative agreement with recent models of aging, that predict an intermittent dynamics.Comment: SPIE Proceeding Journa

Formes d'organisation de maraîchers organiques périurbains de São Paulo.

Le développement de l'agriculture organique au Brésil prend des formes multiples. Au travers de leur expérience de l'AO, dans une communauté proche de trois métropoles, de petits maraîchers d'Ibiúna (São Paulo) créent des entités collectives et expérimentent de nouvelles pratiques sociales. À partir d'entretiens et d'observations sur le terrain, nous montrons comment ces minifundios se sont, ou ont été, organisés pour développer l'AO dans un même territoire Nous identifions quatre formes sociales d'organisation, leurs dynamiques, leurs valeurs et leurs inter-relations. Nous discutons des éléments susceptibles de sécuriser leur reproduction sociale et de soutenir de nouvelles voies de développement

Off equilibrium fluctuations in a polymer glass

The fluctuation-dissipation relation (FDR) is measured on the dielectric properties a polymer glass (polycarbonate). It is observed that the fluctuation dissipation theorem is strongly violated for a quench from above to below the glass transition temperature. The amplitude and the persistence time of this violation are decreasing functions of frequency. Around $1Hz$ it may persist for several hours. The origin of this violation is a highly intermittent dynamics characterized by large fluctuations a strongly non-Gaussian statistics. The intermittent dynamics depends on the quenching rate and it disappears after slow quenches. The relevance of these results for recent models of aging are discussed.Comment: submitted to Physica

Cross-cultural peer-mentoring: Mentor outcomes and perspectives

Limited relationships with peers and faculty create barriers for retaining racial or ethnic minority students through the academic path to SPHS professions. As such, university programs must pay particular attention to formalized avenues of social support to decrease attrition and increase retention (Cokley, 2000). Mentoring is one such avenue (Wright-Harp & Cole, 2008). The purpose of this paper is to describe mentor outcomes and perspectives of a cross-cultural peer-mentoring program entitled LEAD: Leadership, Education, Advocacy, and Diversity. Graduate students from SLP and AUD graduate programs were recruited to serve as peer mentors to potential undergraduate candidates representing diverse ethnicities who might be interested in a career in SLP or AUD. Using a qualitative-dominant concurrent mixed method design, goals, values, and barriers as well as differences in cultural constructs perceived by graduate students as a result of participating in LEAD are examined. Outcomes and future direction of the peer-mentoring program are discussed