A new class of quantum bound states: diprotons in extreme magnetic fields

This paper considers the possibility that two charged particles with an attractive short-ranged potential between them which is not strong enough to form bound states in free space, may bind in uniform magnetic fields. It is shown that in the formal limit where Coulomb repulsion is negligible (q -> 0 and B_0 -> \infty with q B_0 fixed where q is the charge and B_0 the field strength) there always exists a bound state for a system of two identical charged particles in a constant magnetic field, provided that there is a short-range uniformly attractive potential between them. Moreover, it is shown that in this limit {\it any} potential with an attractive s-wave scattering length will posses bound states provided that the range of the potential is much smaller than the characteristic magnetic length, r_0 = (\frac{q B_0}{4})^{-1/2}. For this case, the binding is computed numerically. We estimate the size of the magnetic field needed to approximately reach a regime where the formal limit considered becomes a good approximation to the dynamics. These numerical estimates indicate that two protons in an extremely strong magnetic field such as might be found in a magnetar will bind to form a diproton

The Schwinger mechanism and graphene

The Schwinger mechanism, the production of charged particle-antiparticle pairs in a macroscopic external electric field, is derived for 2+1 dimensional theories. The rate of pair production per unit area for four species of massless fermions, with charge $q$, in a constant electric field $E$ is given by $\pi^{-2} \hbar^{-3/2} \tilde{c}^{-1/2} (q E)^{3/2}$ where $\tilde{c}$ is the speed of light for the two-dimensional system. To the extent undoped graphene behaves like the quantum field-theoretic vacuum for massless fermions in 2+1 dimensions, the Schwinger mechanism should be testable experimentally. A possible experimental configuration for this is proposed. Effects due to deviations from this idealized picture of graphene are briefly considered. It is argued that with present day samples of graphene, tests of the Schwinger formula may be possible.Comment: Extensive revisions. The distinction between the vacuum decay rate and the pair production rate in the Schwinger mechanism is now stressed. The discussion of quality of sample needed for a viable experimental test has been significantly expanded. References adde

Schwinger pair creation in multilayer graphene

The low energy effective field model for the multilayer graphene (at ABC stacking) in external Electric field is considered. The Schwinger pair creation rate and the vacuum persistence probability are calculated using the semi - classical approach.Comment: Latex, 5 pages, accepted for publication in JETP let

Quantum electrodynamics with anisotropic scaling: Heisenberg-Euler action and Schwinger pair production in the bilayer graphene

We discuss quantum electrodynamics emerging in the vacua with anisotropic scaling. Systems with anisotropic scaling were suggested by Horava in relation to the quantum theory of gravity. In such vacua the space and time are not equivalent, and moreover they obey different scaling laws, called the anisotropic scaling. Such anisotropic scaling takes place for fermions in bilayer graphene, where if one neglects the trigonal warping effects the massless Dirac fermions have quadratic dispersion. This results in the anisotropic quantum electrodynamics, in which electric and magnetic fields obey different scaling laws. Here we discuss the Heisenberg-Euler action and Schwinger pair production in such anisotropic QEDComment: 5 pages, no figures, JETP Letters style, version accepted in JETP Letter

Planar QED at finite temperature and density: Hall conductivity, Berry's phases and minimal conductivity of graphene

We study 1-loop effects for massless Dirac fields in two spatial dimensions, coupled to homogeneous electromagnetic backgrounds, both at zero and at finite temperature and density. In the case of a purely magnetic field, we analyze the relationship between the invariance of the theory under large gauge transformations, the appearance of Chern-Simons terms and of different Berry's phases. In the case of a purely electric background field, we show that the effective Lagrangian is independent of the chemical potential and of the temperature. More interesting: we show that the minimal conductivity, as predicted by the quantum field theory, is the right multiple of the conductivity quantum and is, thus, consistent with the value measured for graphene, with no extra factor of pi in the denominator.Comment: 27 pages, no figures. Minor misprints corrected. Final version, to appear in J. Phys. A: Math. Ge

Magnetodielectric effect of Graphene-PVA Nanocomposites

Graphene-Polyvinyl alcohol (PVA) nanocomposite films with thickness $120 \mu m$ were synthesized by solidification of PVA in a solution with dispersed graphene nanosheets. Electrical conductivity data were explained as arising due to hopping of carriers between localized states formed at the graphene-PVA interface. Dielectric permittivity data as a function of frequency indicated the occurrence of Debye-type relaxation mechanism. The nanocomposites showed a magnetodielectric effect with the dielectric constant changing by 1.8% as the magnetic field was increased to 1 Tesla. The effect was explained as arising due to Maxwell-Wagner polarization as applied to an inhomogeneous two-dimensional,two-component composite model. This type of nanocomposite may be suitable for applications involving nanogenerators.Comment: 13 pages, 11 figure

School-based interventions modestly increase physical activity and cardiorespiratory fitness but are least effective for youth who need them most: an individual participant pooled analysis of 20 controlled trials

Dette er siste tekst-versjon av artikkelen, og den kan inneholde små forskjeller fra forlagets pdf-versjon. Forlagets pdf-versjon finner du her: http://dx.doi.org/10.1136/bjsports-2020-102740 / This is the final text version of the article, and it may contain minor differences from the journal's pdf version. The original publication is available here: http://dx.doi.org/10.1136/bjsports-2020-102740Objectives: To determine if subpopulations of students benefit equally from school-based physical activity interventions in terms of cardiorespiratory fitness and physical activity. To examine if physical activity intensity mediates improvements in cardiorespiratory fitness. Design: Pooled analysis of individual participant data from controlled trials that assessed the impact of school-based physical activity interventions on cardiorespiratory fitness and device-measured physical activity. Participants: Data for 6621 children and adolescents aged 4-18 years from 20 trials were included. Main outcome measures: Peak oxygen consumption (VO2Peak mL/kg/min) and minutes of moderate and vigorous physical activity. Results: Interventions modestly improved students' cardiorespiratory fitness by 0.47 mL/kg/min (95% CI 0.33 to 0.61), but the effects were not distributed equally across subpopulations. Girls and older students benefited less than boys and younger students, respectively. Students with lower levels of initial fitness, and those with higher levels of baseline physical activity benefitted more than those who were initially fitter and less active, respectively. Interventions had a modest positive effect on physical activity with approximately one additional minute per day of both moderate and vigorous physical activity. Changes in vigorous, but not moderate intensity, physical activity explained a small amount (~5%) of the intervention effect on cardiorespiratory fitness. Conclusions: Future interventions should include targeted strategies to address the needs of girls and older students. Interventions may also be improved by promoting more vigorous intensity physical activity. Interventions could mitigate declining youth cardiorespiratory fitness, increase physical activity and promote cardiovascular health if they can be delivered equitably and their effects sustained at the population level.acceptedVersionInstitutt for idrettsmedisinske fag / Department of Sports Medicin