Parametric resonance of a two-dimensional electron gas under bichromatic irradiation

In an ultrahigh mobility 2D electron gas, even a weak nonparabolicity of the electron dispersion, by violating Kohn's theorem, can have a drastic effect on dc magnetotransport under ac drive. We study theoretically the manifestation of this effect in the dc response to the combined action of two driving ac-fields (bichromatic irradiation). Compared to the case of monochromatic irradiation, which is currently intensively studied both experimentally and theoretically, the presence of a second microwave source provides additional insight into the properties of an ac-driven 2D electron gas. In particular, we find that nonparabolicity, being the simplest cause for a violation of Kohn's theorem, gives rise to new qualitative effects specific to bichromatic irradiation. Namely, when the frequencies $\omega_1$ and $\omega_2$ are well away from the cyclotron frequency, $\omega_c$, our simple classical considerations demonstrate that the system becomes parametrically unstable with respect to fluctuations with frequency $(\omega_1+\omega_2)/2$. As an additional effect of nonparabolicity, this parametric instability can manifest itself in the dc properties of the system. This happens when $\omega_1$, $\omega_2$ and $\omega_c$ are related as 3:1:2, respectively. Even for weak detuning between $\omega_1$ and $\omega_2$, the effect of the bichromatic irradiation on the dc response in the presence of nonparabolicity can differ dramatically from the monochromatic case. In particular, the equations of motion can acquire multistable solutions. As a result, the diagonal dc-conductivity can assume several stable negative values at the same magnetic field.Comment: 11 pages, 10 figure

Geometric characterization of nodal domains: the area-to-perimeter ratio

In an attempt to characterize the distribution of forms and shapes of nodal domains in wave functions, we define a geometric parameter - the ratio $\rho$ between the area of a domain and its perimeter, measured in units of the wavelength $1/\sqrt{E}$. We show that the distribution function $P(\rho)$ can distinguish between domains in which the classical dynamics is regular or chaotic. For separable surfaces, we compute the limiting distribution, and show that it is supported by an interval, which is independent of the properties of the surface. In systems which are chaotic, or in random-waves, the area-to-perimeter distribution has substantially different features which we study numerically. We compare the features of the distribution for chaotic wave functions with the predictions of the percolation model to find agreement, but only for nodal domains which are big with respect to the wavelength scale. This work is also closely related to, and provides a new point of view on isoperimetric inequalities.Comment: 22 pages, 11 figure

Dominant cognitive frames and the innovative power of social networks

In this paper, we illustrate the link between social network structures, dominant cognitive frames on network purpose and the innovative power of a network, through a mixed-method comparative analysis of two Knowledge Translation Networks (KTNs)in the English National Health Service (NHS). Our findings illustrate several challenges for networked forms of organization linked to different manifestations of social networks (centralized/decentralized) and dominant cognitive frames (polarizing/loosely clustered). Our paper contributes a better understanding of how dominant frames on network purpose emerge alongside the development of network structure itself, and explores how this interplay between dominant frames and social networks impacts upon the collaborative work that supports the networks’ innovative power

Decentring emotion regulation: from emotion regulation to relational emotion

YesThis article takes a critical approach to emotion regulation suggesting that the concept needs supplementing with a relational position on the generation and restraint of emotion. I chart the relational approach to emotion, challenging the ‘two-step’ model of emotion regulation. From this, a more interdisciplinary approach to emotion is developed using concepts from social science to show the limits of instrumental, individualistic and cognitivist orientations in the psychology of emotion regulation, centred on appraisal theory. Using a social interactionist approach I develop an ontological position in which social relations form the fundamental contexts in which emotions are generated, toned, and restrained, so that regulation is decentred and seen as just one moment or aspect in the relational patterning of emotion

Relational agency: Relational sociology, agency and interaction

yesThis article explores how the concept of agency in social theory changes when it is conceptualised as a relational rather than an individual phenomenon. I begin with a critique of the structure/agency debate, particularly of how this emerges in the critical realist approach to agency typified by Margaret Archer. It is argued that this approach, and the critical realist version of relational sociology that has grown from it, reifies social relations as a third entity to which agents have a cognitive, reflexive relation, playing down the importance of interaction. This upholds the Western moral and political view of agents as autonomous, independent, and reflexive individuals. Instead of this I consider agency from a different theoretical tradition in relational sociology in which agents are always located in manifold social relations. From this I create an understanding of agents as interactants, ones who are interdependent, vulnerable, intermittently reflexive, possessors of capacities that can only be practiced in joint actions, and capable of sensitive responses to others and to the situations of interaction. Instead of agency resting on the reflexive monitoring of action or the reflexive deliberation on structurally defined choices, agency emerges from our emotional relatedness to others as social relations unfold across time and space

Fish consumption patterns and hair mercury levels in children and their mothers in 17 EU countries

The toxicity of methylmercury (MeHg) in humans is well established and the main source of exposure is via the consumption of large marine fish and mammals. Of particular concern are the potential neurodevelopmental effects of early life exposure to low-levels of MeHg. Therefore, it is important that pregnant women, children and women of childbearing age are, as far as possible, protected from MeHg exposure.Within the European project DEMOCOPHES, we have analyzed mercury (Hg) in hair in 1799 mother–child pairs from 17 European countries using a strictly harmonized protocol for mercury analysis. Parallel, harmonized questionnaires on dietary habits provided information on consumption patterns of fish and marine products. After hierarchical cluster analysis of consumption habits of the mother–child pairs, the DEMOCOPHES cohort can be classified into two branches of approximately similar size: one with high fish consumption (H) and another with low consumption (L). All countries have representatives in both branches, but Belgium, Denmark, Spain, Portugal and Sweden have twice as many or more mother–child pairs in H than in L. For Switzerland, Czech Republic, Hungary, Poland, Romania, Slovenia and Slovakia the situation is the opposite, with more representatives in L than H.There is a strong correlation (r=0.72) in hair mercury concentration between the mother and child in the same family, which indicates that they have a similar exposure situation. The clustering of mother–child pairs on basis of their fish consumption revealed some interesting patterns. One is that for the same sea fish consumption, other food items of marine origin, like seafood products or shellfish, contribute significantly to the mercury levels in hair. We conclude that additional studies are needed to assess and quantify exposure to mercury from seafood products, in particular. The cluster analysis also showed that 95% of mothers who consume once per week fish only, and no other marine products, have mercury levels 0.55 µg/g. Thus, the 95th percentile of the distribution in this group is only around half the US-EPA recommended threshold of 1 µg/g mercury in hair. Consumption of freshwater fish played a minor role in contributing to mercury exposure in the studied cohort.The DEMOCOPHES data shows that there are significant differences in MeHg exposure across the EU and that exposure is highly correlated with consumption of fish and marine products. Fish and marine products are key components of a healthy human diet and are important both traditionally and culturally in many parts of Europe. Therefore, the communication of the potential risks of mercury exposure needs to be carefully balanced to take into account traditional and cultural values as well as the potential health benefits from fish consumption. European harmonized human biomonitoring programs provide an additional dimension to national HMB programs and can assist national authorities to tailor mitigation and adaptation strategies (dietary advice, risk communication, etc.) to their country’s specific requirements

Reculturing schools in England: how ‘cult’ values in education policy discourse influence the construction of practitioner identities and work orientations

The imperative of continuous improvement has now become normative in education policy discourse, typically framed as setting ‘aspirational’ targets for pupil performance as a prerequisite for gaining competitive advantage in the global economy. In this context, teachers, leaders, teacher assistants and other practitioners working in schools across England have been under increasing pressure to raise standards. This article focuses on how values are deployed in reculturing and regulating practitioners to develop identities and work orientations which are congruent with the policymakers’ agendas. G.H. Mead’s concept of ‘cult’ values illuminates the process of fostering homogeneity with the dominant policy discourse through an inclusion/exclusion dynamic. Interview data collected in two primary schools revealed a significant convergence of practitioner discourse with policy objectives. Delivering improvement affects how practitioners talk about their work and see themselves as educators. The ‘cult’ of continuous improvement appears to inhibit a critical approach to the implementation of education policies by school practitioners in their everyday work

Symbolic meanings and e-learning in the workplace: The case of an intranet-based training tool

This article contributes to the debate on work-based e-learning, by unpacking the notion of ‘the learning context’ in a case where the mediating tool for training also supports everyday work. Users’ engagement with the information and communication technology tool is shown to reflect dynamic interactions among the individual, peer group, organizational and institutional levels. Also influential are professionals’ values and identity work, alongside their interpretations of espoused and emerging symbolic meanings. Discussion draws on pedagogically informed studies of e-learning and the wider organizational learning literature. More centrally, this article highlights the instrumentality of symbolic interactionism for e-learning research and explores some of the framework’s conceptual resources as applied to organizational analysis and e-learning design. </jats:p

Random-phase approximation and its applications in computational chemistry and materials science

The random-phase approximation (RPA) as an approach for computing the electronic correlation energy is reviewed. After a brief account of its basic concept and historical development, the paper is devoted to the theoretical formulations of RPA, and its applications to realistic systems. With several illustrating applications, we discuss the implications of RPA for computational chemistry and materials science. The computational cost of RPA is also addressed which is critical for its widespread use in future applications. In addition, current correction schemes going beyond RPA and directions of further development will be discussed.Comment: 25 pages, 11 figures, published online in J. Mater. Sci. (2012