Free healthy breakfasts in primary schools: A cluster randomised controlled trial of a policy intervention in Wales, UK

Objective: The present study evaluated the impact of a national school programme of universal free healthy breakfast provision in Wales, UK. Design: A cluster randomised controlled trial with repeated cross-sectional design and a 12-month follow-up. Primary outcomes were breakfast skipping, breakfast diet and episodic memory. Secondary outcomes were frequency of eating breakfast at home and at school, breakfast attitudes, rest-of-day diet and class behaviour. Setting: Primary schools in nine local education authority areas. Subjects: A total of 4350 students (aged 9–11 years) at baseline and 4472 at follow-up in 111 schools. Results: Students in intervention schools reported significantly higher numbers of healthy food items consumed at breakfast and more positive attitudes towards breakfast eating at 12 months. Parents in intervention schools reported significantly higher rates of consumption of breakfast at school and correspondingly lower rates of breakfast consumption at home. No other significant differences were found. Conclusions: The intervention did not reduce breakfast skipping; rather, pupils substituted breakfast at home for breakfast at school. However, there were improvements in children’s nutritional intake at breakfast time, if not the rest of the day, and more positive attitudes to breakfast, which may have implications for life-course dietary behaviours. There was no impact on episodic memory or classroom behaviour, which may require targeting breakfast skippers

Do stronger school smoking policies make a difference? Analysis of the health behaviour in school-aged children survey

Background: Associations of the strength of school smoking policies with cigarette, e-cigarette and cannabis use in Wales were examined. Methods: Nationally representative cross-sectional survey of pupils aged 11–16 years (N=7376) in Wales. Senior management team members from 67 schools completed questionnaires about school smoking policies, substance use education and tobacco cessation initiatives. Multi-level, logistic regression analyses investigated self-reported cigarette, e-cigarette and cannabis use, for all students and those aged 15–16 years. Results: Prevalence of current smoking, e-cigarette use and cannabis use in the past month were 5.3%, 11.5% and 2.9%, respectively. Of schools that provided details about smoking policies (66/67), 39.4% were strong (written policy applied to everyone in all locations), 43.9% were moderate (written policy not applied to everyone in all locations) and 16.7% had no written policy. There was no evidence of an association of school smoking policies with pupils’ tobacco or e-cigarette use. However, students from schools with a moderate policy [OR = 0.47; 95% (confidence interval) CI: 0.26–0.84] were less likely to have used cannabis in the past month compared to schools with no written policy. This trend was stronger for students aged 15–16 years (moderate policy: OR = 0.42; 95% CI: 0.22–0.80; strong policy: OR = 0.45; 95% CI: 0.23–0.87). Conclusions: School smoking policies may exert less influence on young people’s smoking behaviours than they did during times of higher adolescent smoking prevalence. Longitudinal studies are needed to examine the potential influence of school smoking policies on cannabis use and mechanisms explaining this associatio

Coulomb scattering lifetime of a two-dimensional electron gas

Motivated by a recent tunneling experiment in a double quantum-well system, which reports an anomalously enhanced electronic scattering rate in a clean two-dimensional electron gas, we calculate the inelastic quasiparticle lifetime due to electron-electron interaction in a single loop dynamically screened Coulomb interaction within the random-phase-approximation. We obtain excellent quantitative agreement with the inelastic scattering rates in the tunneling experiment without any adjustable parameter, finding that the reported large ($\geq$ a factor of six) disagreement between theory and experiment arises from quantitative errors in the existing theoretical work and from the off-shell energy dependence of the electron self-energy.Comment: 11 pages, RevTex, figures included. Also available at http://www-cmg.physics.umd.edu/~lzheng

Lifetime of Two-Dimensional Electrons Measured by Tunneling Spectroscopy

For electrons tunneling between parallel two-dimensional electron systems, conservation of in-plane momentum produces sharply resonant current-voltage characteristics and provides a uniquely sensitive probe of the underlying electronic spectral functions. We report here the application of this technique to accurate measurements of the temperature dependence of the electron-electron scattering rate in clean two-dimensional systems. Our results are in qualitative agreement with existing calculations.Comment: file in REVTEX format produces 11 pages, 3 figures available from [email protected]

Tunneling from a correlated 2D electron system transverse to a magnetic field

We show that, in a magnetic field parallel to the 2D electron layer, strong electron correlations change the rate of tunneling from the layer exponentially. It results in a specific density dependence of the escape rate. The mechanism is a dynamical Mossbauer-type recoil, in which the Hall momentum of the tunneling electron is partly transferred to the whole electron system, depending on the interrelation between the rate of interelectron momentum exchange and the tunneling duration. We also show that, in a certain temperature range, magnetic field can enhance rather than suppress the tunneling rate. The effect is due to the magnetic field induced energy exchange between the in-plane and out-of-plane motion. Magnetic field can also induce switching between intra-well states from which the system tunnels, and a transition from tunneling to thermal activation. Explicit results are obtained for a Wigner crystal. They are in qualitative and quantitative agreement with the relevant experimental data, with no adjustable parameters.Comment: 16 pages, 9 figure

Electron-electron interactions and two-dimensional - two-dimensional tunneling

We derive and evaluate expressions for the dc tunneling conductance between interacting two-dimensional electron systems at non-zero temperature. The possibility of using the dependence of the tunneling conductance on voltage and temperature to determine the temperature-dependent electron-electron scattering rate at the Fermi energy is discussed. The finite electronic lifetime produced by electron-electron interactions is calculated as a function of temperature for quasiparticles near the Fermi circle. Vertex corrections to the random phase approximation substantially increase the electronic scattering rate. Our results are in an excellent quantitative agreement with experiment.Comment: Revtex style, 21 pages and 8 postscript figures in a separate file; Phys. Rev. B (in press

Has the Universe always expanded ?

We consider a cosmological setting for which the currently expanding era is preceded by a contracting phase, that is, we assume the Universe experienced at least one bounce. We show that scalar hydrodynamic perturbations lead to a singular behavior of the Bardeen potential and/or its derivatives (i.e. the curvature) for whatever Universe model for which the last bounce epoch can be smoothly and causally joined to the radiation dominated era. Such a Universe would be filled with non-linear perturbations long before nucleosynthesis, and would thus be incompatible with observations. We therefore conclude that no observable bounce could possibly have taken place in the early universe if Einstein gravity together with hydrodynamical fluids is to describe its evolution, and hence, under these conditions, that the Universe has always expanded.Comment: 11 pages, LaTeX-ReVTeX, no figures, submitted to PR

Inelastic lifetimes of confined two-component electron systems in semiconductor quantum wire and quantum well structures

We calculate Coulomb scattering lifetimes of electrons in two-subband quantum wires and in double-layer quantum wells by obtaining the quasiparticle self-energy within the framework of the random-phase approximation for the dynamical dielectric function. We show that, in contrast to a single-subband quantum wire, the scattering rate in a two-subband quantum wire contains contributions from both particle-hole excitations and plasmon excitations. For double-layer quantum well structures, we examine individual contributions to the scattering rate from quasiparticle as well as acoustic and optical plasmon excitations at different electron densities and layer separations. We find that the acoustic plasmon contribution in the two-component electron system does not introduce any qualitatively new correction to the low energy inelastic lifetime, and, in particular, does not produce the linear energy dependence of carrier scattering rate as observed in the normal state of high-$T_c$ superconductors.Comment: 16 pages, RevTeX, 7 figures. Also available at http://www-cmg.physics.umd.edu/~lzheng

Primordial perturbations in a non singular bouncing universe model

We construct a simple non singular cosmological model in which the currently observed expansion phase was preceded by a contraction. This is achieved, in the framework of pure general relativity, by means of a radiation fluid and a free scalar field having negative energy. We calculate the power spectrum of the scalar perturbations that are produced in such a bouncing model and find that, under the assumption of initial vacuum state for the quantum field associated with the hydrodynamical perturbation, this leads to a spectral index n=-1. The matching conditions applying to this bouncing model are derived and shown to be different from those in the case of a sharp transition. We find that if our bounce transition can be smoothly connected to a slowly contracting phase, then the resulting power spectrum will be scale invariant.Comment: 11 pages, RevTeX 4, 8 figures, submitted to Phys. Rev.

Phenomenology of the Lense-Thirring effect in the Solar System

Recent years have seen increasing efforts to directly measure some aspects of the general relativistic gravitomagnetic interaction in several astronomical scenarios in the solar system. After briefly overviewing the concept of gravitomagnetism from a theoretical point of view, we review the performed or proposed attempts to detect the Lense-Thirring effect affecting the orbital motions of natural and artificial bodies in the gravitational fields of the Sun, Earth, Mars and Jupiter. In particular, we will focus on the evaluation of the impact of several sources of systematic uncertainties of dynamical origin to realistically elucidate the present and future perspectives in directly measuring such an elusive relativistic effect.Comment: LaTex, 51 pages, 14 figures, 22 tables. Invited review, to appear in Astrophysics and Space Science (ApSS). Some uncited references in the text now correctly quoted. One reference added. A footnote adde