The relationship between ethnic classroom composition and Turkish-origin and German students’ reading performance and sense of belonging

Past research on ethnic composition effects on migrant and ethnic majority students’ performance has reported inconclusive results: Some studies have found no relationship between the proportion of migrant students in school and students’ performance, some revealed positive effects, whereas others showed negative effects of the proportion of migrant students. Most of the studies did not consider whether an increase in the proportion of migrant students in the classroom has different effects on migrant and ethnic majority students’ performance. For this reason, the present study (N = 9215) extends previous research by investigating the cross-level interaction effect of the proportion of Turkish-origin students in classrooms on Turkish-origin and German students’ reading performance with data based on the German National Assessment Study 2008/2009 in the school subject German. In addition, we examined the cross-level interaction effect of Turkish-origin students’ proportion on sense of belonging to school of Turkish-origin and German students, as sense of belonging has been shown to be an important predictor of well-being and integration. No cross-level interaction effect on performance emerged. Only a small negative main effect of the Turkish-origin students’ proportion on all students’ performance was found. As predicted, we showed a cross-level interaction on sense of belonging. Only Turkish-origin students’ sense of belonging was positively related to the proportion of Turkish-origin students: The more Turkish-origin students there were in a classroom, the higher Turkish-origin students’ sense of belonging. German students’ sense of belonging was not related to the ethnic classroom composition. Implications of the results in the educational context are discussed

A microfabricated sensor for thin dielectric layers

We describe a sensor for the measurement of thin dielectric layers capable of operation in a variety of environments. The sensor is obtained by microfabricating a capacitor with interleaved aluminum fingers, exposed to the dielectric to be measured. In particular, the device can measure thin layers of solid frozen from a liquid or gaseous medium. Sensitivity to single atomic layers is achievable in many configurations and, by utilizing fast, high sensitivity capacitance read out in a feedback system onto environmental parameters, coatings of few layers can be dynamically maintained. We discuss the design, read out and calibration of several versions of the device optimized in different ways. We specifically dwell on the case in which atomically thin solid xenon layers are grown and stabilized, in cryogenic conditions, from a liquid xenon bath

Dealing with negative stereotypes in sports: the role of cognitive anxiety when multiple identities are activated in sensorimotor tasks

Based on research on stereotype threat and multiple identities, this work explores the beneficial effects of activating a positive social identity when a negative identity is salient on women’s performance in sports. Further, in line with research on the effects of anxiety in sports, we investigate whether the activation of a positive social identity buffers performance from cognitive anxiety associated with a negative stereotype. Two experiments tested these predictions in field settings. Experiment 1 (N = 83) shows that the simultaneous activation of a positive (i.e., member of a soccer team) and a negative social identity (i.e., woman) led to better performance than the activation of only a negative social identity for female soccer players. Experiment 2 (N = 46) demonstrates that identity condition moderated the effect of cognitive anxiety on performance for female basketball players. Results are discussed concerning multiple identities’ potential for dealing with stressful situations

Dynamical Properties of small Polarons

On the basis of the two-site polaron problem, which we solve by exact diagonalization, we analyse the spectral properties of polaronic systems in view of discerning localized from itinerant polarons and bound polaron pairs from an ensemble of single polarons. The corresponding experimental techniques for that concern photoemission and inverse photoemission spectroscopy. The evolution of the density of states as a function of concentration of charge carriers and strength of the electron-phonon interaction clearly shows the opening up of a gap between single polaronic and bi-polaronic states, in analogy to the Hubbard problem for strongly correlated electron systems. The crossover regime between adiabatic and anti-adiabatic small polarons is triggered by two characteristic time scales: the renormalized electron hopping rate and the renormalized vibrational frequency becoming equal. This crossover regime is then characterized by temporarily alternating self- localization and delocalization of the charge carriers which is accompanied by phase slips in the charge and molecular deformation oscillations and ultimately leads to a dephasing between these two dynamical components of the polaron problem. We visualize these features by a study of the temporal evolution of the charge redistribution and the change in molecular deformations. The spectral and dynamical properties of polarons discussed here are beyond the applicability of the standard Lang Firsov approach to the polaron problem.Comment: 31 pages and 23 figs.(eps), accepted in the Phys. Rev.

Adiabatic limit and the slow motion of vortices in a Chern-Simons-Schr\"odinger system

We study a nonlinear system of partial differential equations in which a complex field (the Higgs field) evolves according to a nonlinear Schroedinger equation, coupled to an electromagnetic field whose time evolution is determined by a Chern-Simons term in the action. In two space dimensions, the Chern-Simons dynamics is a Galileo invariant evolution for A, which is an interesting alternative to the Lorentz invariant Maxwell evolution, and is finding increasing numbers of applications in two dimensional condensed matter field theory. The system we study, introduced by Manton, is a special case (for constant external magnetic field, and a point interaction) of the effective field theory of Zhang, Hansson and Kivelson arising in studies of the fractional quantum Hall effect. From the mathematical perspective the system is a natural gauge invariant generalization of the nonlinear Schroedinger equation, which is also Galileo invariant and admits a self-dual structure with a resulting large space of topological solitons (the moduli space of self-dual Ginzburg-Landau vortices). We prove a theorem describing the adiabatic approximation of this system by a Hamiltonian system on the moduli space. The approximation holds for values of the Higgs self-coupling constant close to the self-dual (Bogomolny) value of 1. The viability of the approximation scheme depends upon the fact that self-dual vortices form a symplectic submanifold of the phase space (modulo gauge invariance). The theorem provides a rigorous description of slow vortex dynamics in the near self-dual limit.Comment: Minor typos corrected, one reference added and DOI give

The interaction of vertical collectivism and stereotype activation on the performance of Turkish-origin high school students

The activation of negative stereotypes in achievement situations can lead to decreased performance (i.e., stereotype threat effect). Research has shown that stereotype threat effects increase when performance outcomes become more important. Cultural value orientations such as vertical collectivism can influence the perceived importance of performance outcomes. For this reason, we investigate the interaction between vertical collectivism and the activation of negative stereotypes on the academic performance of migrant students. More precisely, we argue that the more Turkish-origin students endorse vertical collectivism (i.e., the willingness to sacrifice individual interests for their group's interests), the more they try to meet their group's high achievement expectations after negative stereotype activation. This increases performance pressure and thus impairs their performance. We further argue that vertical collectivism increases migrants students' motivation to temporarily join a high performing outgroup in order to achieve high performance outcomes in the future. An experiment conducted in classrooms (N = 94) with Turkish-origin students confirmed our hypotheses. Implications for dealing with stereotype threat in educational contexts are discusse

Quantum Monte Carlo and variational approaches to the Holstein model

Based on the canonical Lang-Firsov transformation of the Hamiltonian we develop a very efficient quantum Monte Carlo algorithm for the Holstein model with one electron. Separation of the fermionic degrees of freedom by a reweighting of the probability distribution leads to a dramatic reduction in computational effort. A principal component representation of the phonon degrees of freedom allows to sample completely uncorrelated phonon configurations. The combination of these elements enables us to perform efficient simulations for a wide range of temperature, phonon frequency and electron-phonon coupling on clusters large enough to avoid finite-size effects. The algorithm is tested in one dimension and the data are compared with exact-diagonalization results and with existing work. Moreover, the ideas presented here can also be applied to the many-electron case. In the one-electron case considered here, the physics of the Holstein model can be described by a simple variational approach.Comment: 18 pages, 11 Figures, v2: one typo correcte

Indication of a deeply bound compact K-pp state formed in the pp -> p Lambda K+ reaction at 2.85 GeV

We have analyzed data of the DISTO experiment on the exclusive pp -> p Lambda K+ reaction at 2.85 GeV to search for a strongly bound compact K-pp (= X) state to be formed in the pp -> K+ + X reaction. The observed spectra of the K+ missing-mass and the p Lambda invariant-mass with high transverse momenta of p and K+ revealed a broad distinct peak with a mass M_X = 2265 +- 2 (stat) +- 5 (syst) MeV/c2 and a width Gamma_X = 118 +- 8 (stat) +- 10 (syst) MeV.Comment: 4 pages, 4 figure

Development of a PbWO4 Detector for Single-Shot Positron Annihilation Lifetime Spectroscopy at the GBAR Experiment

We have developed a PbWO4 (PWO) detector with a large dynamic range to measure the intensity of a positron beam and the absolute density of the ortho-positronium (o-Ps) cloud it creates. A simulation study shows that a setup based on such detectors may be used to determine the angular distribution of the emission and reflection of o-Ps to reduce part of the uncertainties of the measurement. These will allow to improve the precision in the measurement of the cross-section for the (anti)hydrogen formation by (anti)proton-positronium charge exchange and to optimize the yield of antihydrogen ion which is an essential parameter in the GBAR experiment