Superlight small bipolarons

Recent angle-resolved photoemission spectroscopy (ARPES) has identified that a finite-range Fr\"ohlich electron-phonon interaction (EPI) with c-axis polarized optical phonons is important in cuprate superconductors, in agreement with an earlier proposal by Alexandrov and Kornilovitch. The estimated unscreened EPI is so strong that it could easily transform doped holes into mobile lattice bipolarons in narrow-band Mott insulators such as cuprates. Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the total energy, effective mass, pair radius, number of phonons and isotope exponent of lattice bipolarons in the region of parameters where any approximation might fail taking into account the Coulomb repulsion and the finite-range EPI. The effects of modifying the interaction range and different lattice geometries are discussed with regards to analytical strong-coupling/non-adiabatic results. We demonstrate that bipolarons can be simultaneously small and light, provided suitable conditions on the electron-phonon and electron-electron interaction are satisfied. Such light small bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids. The light bipolaron mass is shown to be universal in systems made of triangular plaquettes, due to a novel crab-like motion. Another surprising result is that the triplet-singlet exchange energy is of the first order in the hopping integral and triplet bipolarons are heavier than singlets in certain lattice structures at variance with intuitive expectations. Finally, we identify a range of lattices where superlight small bipolarons may be formed, and give estimates for their masses in the anti-adiabatic approximation.Comment: 31 pages. To appear in J. Phys.: Condens. Matter, Special Issue 'Mott's Physics

Determining the phonon DOS from specific heat measurements via maximum entropy methods

The maximum entropy and reverse Monte-Carlo methods are applied to the computation of the phonon density of states (DOS) from heat capacity data. The approach is introduced and the formalism is described. Simulated data is used to test the method, and its sensitivity to noise. Heat capacity measurements from diamond are used to demonstrate the use of the method with experimental data. Comparison between maximum entropy and reverse Monte-Carlo results shows the form of the entropy used here is correct, and that results are stable and reliable. Major features of the DOS are picked out, and acoustic and optical phonons can be treated with the same approach. The treatment set out in this paper provides a cost-effective and reliable method for studies of the phonon properties of materials.Comment: Reprint to improve access. 10 pages, 6 figure

National interest to global reform: patterns of reasoning in British foreign policy discourse

Discussion of the national interest often focuses on how Britain's influence can be maximized, rather than on the goals that influence serves. Yet what gives content to claims about the national interest is the means-ends reasoning which links interests to deeper goals. In ideal-typical terms, this can take two forms. The first, and more common, approach is conservative: it infers national interests and the goals they advance from existing policies and commitments. The second is reformist: it starts by specifying national goals and then asks how they are best advanced under particular conditions. New Labour's foreign policy discourse is notable for its explicit use of a reformist approach. Indeed, Gordon Brown's vision of a 'new global society' not only identifies global reform as a key means of fulfilling national goals, but also thereby extends the concept of the national interest well beyond a narrow concern with national security

Centaurus A at Ultra-High Energies

We review the importance of Centaurus A in high energy astrophysics as a nearby object with many of the properties expected of a major source of very high energy cosmic rays and gamma-rays. We examine observational techniques and the results so far obtained in the energy range from 200 GeV to above 100 EeV and attempt to fit those data with expectations of Centaurus A as an astrophysical source from VHE to UHE energies.Comment: 11 pages, 4 figures, accepted for publication in PAS

Recruitment strategies in randomised controlled trials of men aged 50 years and older: a systematic review

Objectives: To identify and review evaluations of strategies to recruit men aged 50 years and over to randomised controlled trials (RCTs). Design: Systematic review and narrative synthesis. Data Sources: MEDLINE, EMBASE, CINAHL and ORRCA databases were searched to 1 December 2017. Eligibility Criteria: Studies using quantitative methods to evaluate recruitment strategies to RCTs of men aged 50 years and older. Data Extraction and Synthesis: A single reviewer extracted data (for each strategy, number of participants approached, screened and randomised, and cost). Study quality was assessed using National Heart, Lung and Blood Institute Quality Assessment Tools and considered study design, description of interventions, description and measurement of outcomes, completeness of outcome reporting, performance of statistical testing and consideration of confounders. Recruitment strategies were categorised by the recruitment stage they addressed. Results: Sixteen studies (n >14 000) were included: one good quality, ten fair quality and five poor quality. Studies evaluated strategies to identify prospective participants, and to improve the processes for assessing participant eligibility, providing participant information and seeking consent. In good and fair quality studies, the most effective strategies for identifying participants were referral from an affiliated health service provider (two studies), mass mailing (five studies) and media coverage (two studies). Community outreach activities such as displaying posters and attending local community events were not effective (two studies). Trial-specific training of site recruitment staff, developed using qualitative analysis of recruitment visits (two studies), and provision of study information to prospective participants at a multidisciplinary, group information session (one study) both improved recruitment. Conclusion: Improved engagement of men aged 50 years and older in RCTs is needed. A gender-sensitised approach to RCT recruitment may help to address this need. We have identified several promising recruitment strategies that merit further evaluation.Karen Bracken, Lisa Askie, Anthony C Keech, Wendy Hague, Gary Witter

Modification of a charged-Bose-gas model for observed room-temperature superconductivity in narrow channels through films of oxidised atactic polypropylene

Reasons have been found for thinking that the minimum diameter of channels of a given length to support superconductivity at room temperature through films of oxidised atactic polypropylene (OAPP) is considerably larger than found in a model for Bose condensation in an array of nanofilaments [D.M. Eagles, Phil. Mag. 85, 1931 (2005)] used previously. This model was introduced to interpret experimental results dating from 1988 on OAPP. The channels are thought to be of larger diameter than believed before because, for an N-S-N system where the superconductor consists of an array of single-walled carbon nanotubes, the resistance, for good contacts, is R_Q/2N, where N is the number of nanotubes and R_Q=12.9 kOhm [See e.g. M. Ferrier et al., Solid State Commun. 131, 615 (2004)]. We assume this would be 2R_Q/N for a triplet superconductor with all spins in the same direction and no orbital degeneracy, which may be the case for nanofilaments in OAPP. Hence one may infer a minimum number of filaments for a given resistance. In the present model, the E(K) curve for the bosons is taken to be of a Bogoliubov form, but with a less steep initial linear term in the dispersion at T_c than occurs at low T. This form is different from the simple linear plus quadratic dispersion, with a steeper initial slope, used in my 2005 paper. A combination of theory and experimental data has been used to find approximate constraints on parameters appearing in the theory.Comment: Publication details added. Correction to bracketing in Eq. (1), which was incorrect in v1 of preprint and in the published articl

Observation of magnetic circular dichroism in Fe L_{2,3} x-ray-fluorescence spectra

We report experiments demonstrating circular dichroism in the x-ray-fluorescence spectra of magnetic systems, as predicted by a recent theory. The data, on the L_{2,3} edges of ferromagnetic iron, are compared with fully relativistic local spin density functional calculations, and the relationship between the dichroic spectra and the spin-resolved local density of occupied states is discussed

Homogeneous Fermion Superfluid with Unequal Spin Populations

For decades, the conventional view is that an s-wave BCS superfluid can not support uniform spin polarization due to a gap $\Delta$ in the quasiparticle excitation spectrum. We show that this is an artifact of the dismissal of quasiparticle interactions $V_{qp}^{}$ in the conventional approach at the outset. Such interactions can cause triplet fluctuations in the ground state and hence non-zero spin polarization at "magnetic field" $h<\Delta$. The resulting ground state is a pairing state of quasiparticles on the ``BCS vacuum". For sufficiently large $V_{qp}$, the spin polarization of at unitarity has the simple form $m\propto \mu^{1/2}$. Our study is motivated by the recent experiments at Rice which found evidence of a homogenous superfluid state with uniform spin polarization.Comment: 4 pages, 3 figure

Effect of metformin in addition to dietary and lifestyle advice for pregnant women who are overweight or obese: the GRoW randomised, double-blind, placebo-controlled trial

BACKGROUND:Maternal overweight and obesity are associated with well recognised pregnancy complications. Antenatal dietary and lifestyle interventions have a modest effect on gestational weight gain without affecting pregnancy outcomes. We aimed to assess the effects on maternal and infant outcomes of antenatal metformin given in addition to dietary and lifestyle advice among overweight and obese pregnant women. METHODS:GRoW was a multicentre, randomised, double-blind, placebo-controlled trial in which pregnant women at 10-20 weeks' gestation with a BMI of 25 kg/m2 or higher were recruited from three public maternity units in Adelaide, SA, Australia. Women were randomly assigned (1:1) via a computer-generated schedule to receive either metformin (to a maximum dose of 2000 mg per day) or matching placebo. Participants, their antenatal care providers, and research staff (including outcome assessors) were masked to treatment allocation. All women received an antenatal dietary and lifestyle intervention. The primary outcome was the proportion of infants with birthweight greater than 4000 g. Secondary outcomes included measures of maternal weight gain, maternal diet and physical activity, maternal pregnancy and birth outcomes, maternal quality of life and emotional wellbeing, and infant birth outcomes. Outcomes were analysed on an intention-to-treat basis (including all randomly assigned women who did not withdraw consent to use their data, and who did not have a miscarriage or termination of pregnancy before 20 weeks' gestation, or a stillbirth). The trial is registered with the Australian New Zealand Clinical Trials Registry, number ACTRN12612001277831. FINDINGS:Of 524 women who were randomly assigned between May, 28 2013 and April 26, 2016, 514 were included in outcome analyses (256 in the metformin group and 258 in the placebo group). Median gestational age at trial entry was 16·29 weeks (IQR 14·43-18·00) and median BMI was 32·32 kg/m2 (28·90-37·10); 167 (32%) participants were overweight and 347 (68%) were obese. There was no significant difference in the proportion of infants with birthweight greater than 4000 g (40 [16%] with metformin vs 37 [14%] with placebo; adjusted risk ratio [aRR] 0·97, 95% CI 0·65 to 1·47; p=0·899). Women receiving metformin had lower average weekly gestational weight gain (adjusted mean difference -0·08 kg, 95% CI -0·14 to -0·02; p=0·007) and were more likely to have gestational weight gain below recommendations (aRR 1·46, 95% CI 1·10 to 1·94; p=0·008). Total gestational weight gain, pregnancy and birth outcomes, maternal diet and physical activity, and maternal quality of life and emotional wellbeing did not differ significantly between groups. Similar numbers of women in both treatment groups (76% [159/208] in the metformin group and 73% [144/196] in the placebo group) reported side-effects including nausea, diarrhoea, and vomiting. Two stillbirths (placebo group) and one neonatal death (metformin group) occurred; none of the perinatal deaths were determined to be attributable to participation in the trial. INTERPRETATION:For pregnant women who are overweight or obese, metformin given in addition to dietary and lifestyle advice initiated at 10-20 weeks' gestation does not improve pregnancy and birth outcomes. FUNDING:Australian National Health and Medical Research Council.Jodie M Dodd, Jennie Louise, Andrea R Deussen, Rosalie M Grivell, Gustaaf Dekker, Andrew J McPhee and William Hagu

Suppression of Mott–Hubbard states and metal–insulator transitions in the two-band Hubbard model

I investigate band and Mott insulating states in a two-band Hubbard model, with the aim of understanding the differences between the idealized one-orbital model and the more realistic multi-band case. Using a projection ansatz I show that additional orbitals suppress the metal–insulator transition, leading to a critical coupling of approximately eight times the bare bandwidth. I also demonstrate the effects of orbital ordering, which hinder Mott–Hubbard states and open a bandgap. Since multi-band correlations are common in real materials, this work suggests that very strongly correlated band insulators may be more common than Mott–Hubbard insulators