Topological defects in crystals: A density-wave theory

A new approach for describing dislocations and other topological defects in crystals, based on the density wave theory of Ramakrishnan and Yussouff is presented. Quantitative calculations are discussed in brief for the order parameter profiles, the atomic configuration and the free energy of a screw dislocation with Burgers vector b = (a/2, a/2,a/2 ) in a bcc solid. Our results for the free energy of the dislocation in a crystal of sizeR, when expressed as (λb 2/4π) ln (αR/|b|) whereλ is the shear elastic constant, yield, for example, the valueα ⋍ 1·85 for sodium at its freezing temperature (371°K). The density distribution in the presence of the dislocation shows that the dislocation core has a columnar character. To our knowledge, this study represents the first calculation of dislocation structure, including the core, within the framework of an order parameter theory incorporating thermal effects.

Management of Routine in Two Chinese Kindergartens - An Ethnographic Study

Based on a one-month long ethnographic study conducted in two Chinese kindergartens, this study aims to understand the issue of discipline in the Chinese preschool setting through an examination of practices teachers use to manage everyday routines in the kindergarten. It also seeks to understand teachers’ values behind their choices of practices. Data of this study are comprised of three parts - notes of participant observation in eight classrooms with a focus on teacher-child interaction; interviews with nine teachers and directors of the two kindergartens; and written accounts of four teachers collected after the fieldwork in order to understand the particular practices of teachers’ praising and criticizing children. A grounded theory approach is applied to code and analyze data. Results of analysis are structured as followed. First the concept of routine is clarified based on teachers’ definition of it and observation notes on its main components, namely the timetable of everyday activities; general behavioral rules in the kindergarten; and detailed rules and procedures for various activities in the kindergarten. Then practices for managing routines are examined – how teachers organize children in activities, enforce routines, and restore routines when they are not followed well. After that, the matter of self-control is examined in relation with external control. Then teachers’ perception of their roles as the manager, director and executor of routines is presented in a discourse of control in which the values behind practices are found to be embedded. The next section of analysis examines the role of routine in relation with other activities in the kindergarten. Results indicate that routine which is supposed to be the foundation of other activities is in conflict with other activities. The last section of analysis provides some reflections on the rational of routine management in Chinese kindergartens in relation with the overall goals of Chinese preschool education. It also provides some reflections on the effect current mode of teacher-child interactions may have on children's self construction and their understanding of self in relation with the society. As a conclusion, this study suggests that the current mode of routine management in Chinese kindergartens relies heavily on teachers' control, leaving great room for better acknowledging children's agency.

Eigenvalues of kinematical conservation laws (KCL) based 3-D weakly nonlinear ray theory (WNLRT)

System of kinematical conservation laws (KCL) govern evolution of a curve in a plane or a surface in space, even if the curve or the surface has singularities on it. In our recent publication K. R. Arun, P. Prasad, 3-D kinematical conservation laws (KCL): evolution of a surface in R-3-in particular propagation of a nonlinear wavefront, Wave Motion 46 (2009) 293-311] we have developed a mathematical theory to study the successive positions and geometry of a 3-D weakly nonlinear wavefront by adding an energy transport equation to KCL. The 7 x 7 system of equations of this KCL based 3-D weakly nonlinear ray theory (WNLRT) is quite complex and explicit expressions for its two nonzero eigenvalues could not be obtained before. In this short note, we use two different methods: (i) the equivalence of KCL and ray equations and (ii) the transformation of surface coordinates, to derive the same exact expressions for these eigenvalues. The explicit expressions for nonzero eigenvalues are important also for checking stability of any numerical scheme to solve 3-D WNLRT. (C) 2010 Elsevier Inc. All rights reserved.