Simple model of the static exchange-correlation kernel of a uniform electron gas with long-range electron-electron interaction

A simple approximate expression in real and reciprocal spaces is given for the static exchange-correlation kernel of a uniform electron gas interacting with the long-range part only of the Coulomb interaction. This expression interpolates between the exact asymptotic behaviors of this kernel at small and large wave vectors which in turn requires, among other thing, information from the momentum distribution of the uniform electron gas with the same interaction that have been calculated in the G0W0 approximation. This exchange-correlation kernel as well as its complement analogue associated to the short-range part of the Coulomb interaction are more local than the Coulombic exchange-correlation kernel and constitute potential ingredients in approximations for recent adiabatic connection fluctuation-dissipation and/or density functional theory approaches of the electronic correlation problem based on a separate treatment of long-range and short-range interaction effects.Comment: 14 pages, 14 figures, to be published in Phys. Rev.

Disorder-Induced Resistive Anomaly Near Ferromagnetic Phase Transitions

We show that the resistivity rho(T) of disordered ferromagnets near, and above, the Curie temperature T_c generically exhibits a stronger anomaly than the scaling-based Fisher-Langer prediction. Treating transport beyond the Boltzmann description, we find that within mean-field theory, d\rho/dT exhibits a |T-T_c|^{-1/2} singularity near T_c. Our results, being solely due to impurities, are relevant to ferromagnets with low T_c, such as SrRuO3 or diluted magnetic semiconductors, whose mobility near T_c is limited by disorder.Comment: 5 pages, 3 figures; V2: with a few clarifications, as publishe

Overcoming Assessment Challenges - Tipping the Balance

It is well known to primary teachers that effective assessment of children requires a multi-faceted approach (Sparks Linfield 1994). Equally, written feedback on a piece of work is often not understood by the pupils themselves (Sparks Linfield 1995). As one proceeds through secondary and tertiary education, this situation changes little, with the best attempts to set ‘perfect' assessments still leading to misinterpretation by students. It is also true that students often do not always recognise what is meant by the term ‘feedback' and have difficulty in interpreting and understanding the feedback that they receive, even with the most careful and targeted advice in advance. (Sutcliffe et al 2014) In 2010 the National Union of Students released a ‘Charter for Assessment and Feedback' which outlined ten principles for effective assessment and feedback. Despite this charter, the National Student Survey (NSS) in 2014 still showed twenty-eight percent of students were not satisfied. ‘Assessment and feedback was again rated the lowest by students, with just seventy-two percent saying they were satisfied with this, the same level as last year.' (Grove 2014) This poster considers research carried out in 2014 when the Year 2 cohort of students on a Bachelor of Arts Primary Education course were asked to complete a questionnaire inviting views on feedback on assessment they found most helpful in clarifying things they did not understand. Analysis of completed questionnaires revealed that although students' experiences of feedback and assessment within their first year of study had broadly matched the principles outlined within the NUS Charter, twenty-five percent of students still were not satisfied. Results from the cohort showed a desire for a range of types of feedback including a wish for face-to-face discussion to enable them to both assess their understanding of feedback comments and feed-forward actions. In addition, a common theme emerged: a lack of perception by students of their own roles and responsibilities within the assessment/feedback cycle. Recommendations are made for ways to overcome the challenge to provide assessment feedback that aims to give total satisfaction

Metal–insulator transition in 2D as a quantum phase transition

We discuss the metal–insulator transition phenomenon in two dimensions in terms of a quantum critical point that controls a range of the low temperature insulator region as well as the usual quantum critical sector. We show that this extended range of criticality permits a determination of both the dynamical critical exponent z and the correlation length critical exponent ν from published data from a single experiment in the insulator critical region. The resulting value of the product zν is consistent with the temperature dependence of the resistance in the quantum critical sector. This provides strong quantitative evidence for the presence of a quantum critical point

Conserving Approximations in Time-Dependent Density Functional Theory

In the present work we propose a theory for obtaining successively better approximations to the linear response functions of time-dependent density or current-density functional theory. The new technique is based on the variational approach to many-body perturbation theory (MBPT) as developed during the sixties and later expanded by us in the mid nineties. Due to this feature the resulting response functions obey a large number of conservation laws such as particle and momentum conservation and sum rules. The quality of the obtained results is governed by the physical processes built in through MBPT but also by the choice of variational expressions. We here present several conserving response functions of different sophistication to be used in the calculation of the optical response of solids and nano-scale systems.Comment: 11 pages, 4 figures, revised versio

Statistical characterization of the forces on spheres in an upflow of air

The dynamics of a sphere fluidized in a nearly-levitating upflow of air were previously found to be identical to those of a Brownian particle in a two-dimensional harmonic trap, consistent with a Langevin equation [Ojha {\it et al.}, Nature {\bf 427}, 521 (2004)]. The random forcing, the drag, and the trapping potential represent different aspects of the interaction of the sphere with the air flow. In this paper we vary the experimental conditions for a single sphere, and report on how the force terms in the Langevin equation scale with air flow speed, sphere radius, sphere density, and system size. We also report on the effective interaction potential between two spheres in an upflow of air.Comment: 7 pages, experimen