Mountain trail formation and the active walker model

We extend the active walker model to address the formation of paths on gradients, which have been observed to have a zigzag form. Our extension includes a new rule which prohibits direct descent or ascent on steep inclines, simulating aversion to falling. Further augmentation of the model stops walkers from changing direction very rapidly as that would likely lead to a fall. The extended model predicts paths with qualitatively similar forms to the observed trails, but only if the terms suppressing sudden direction changes are included. The need to include terms into the model that stop rapid direction change when simulating mountain trails indicates that a similar rule should also be included in the standard active walker model.Comment: Introduction improved. Analysis of discretization errors added. Calculations from alternative scheme include

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

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

Predictors of preeclampsia in women in the Metformin in Gestational Diabetes(Mig) study

Research articleBackground: Gestational Diabetes Mellitus (GDM), maternal obesity and pregnancy weight gain are associated with an increased risk of developing Preeclampsia (PE). The aim of this study was to examine the predictors of PE in women commencing pharmacotherapy for GDM in the Metformin in Gestational diabetes trial. Methods: Descriptive and logistic regression analyses examined the relationship between maternal enrolment characteristics and later development of PE. Results: 46 (6.3%) of 703 women developed PE. At enrolment ((30 (SD3.2) weeks gestation), women who later developed PE had higher HbA1c (6.14% (95%CI 5.84, 6.45) vs. 5.73% (95%CI 5.67, 5.78), P=0.003), fasting triglycerides (2.93 mmol/L (95%CI 2.57, 3.29) vs. 2.55mmol/L (95%CI 2.47, 2.62), P=0.03) and blood pressure. Their infants were born 9 days earlier (P<0.001) but were otherwise not different. In univariate analysis, the strongest positive predictors for PE were Polynesian ethnicity (OR 2.75 (95%CI 1.48, 5.09), P=0.001), personal or family history of PE (OR 2.65 (95%CI 1.36, 5.16), P=0.004), maternal HbA1c (OR 1.96 (95%CI 1.35, 2.89), P<0.001), triglycerides (OR 1.45 (95%CI 1.07,1.97), P=0.002), and weight gain from early pregnancy (OR 1.09 (95%CI 1.03,1.17), P=0.01). HDL-C was a negative predictor of PE (OR 0.29 (95%CI 0.09, 0.94), P=0.04). Following adjustment for Polynesian ethnicity and personal or family history of PE, and when further adjusted for HbA1c or early pregnancy BMI, these variables remained significant. Conclusion: Treatment allocation and BMI were not associated with risk of PE. Personal or family history of PE, Polynesian ethnicity, degree of hyperglycemia, maternal triglycerides and weight gain prior to treatment signal increased risk of subsequent PE in women needing pharmacotherapy for GDM.Helen L Barrett, Marloes Dekker Nitert, H David McIntyre, William M Hague, Leonie K Callaway, and Janet Rowa

Glioma Mimics: Magnetic Resonance Imaging Characteristics of Granulomas in Dogs

Granulomas can “mimic” gliomas on magnetic resonance imaging (MRI) in human patients. The goal of this retrospective study was to report canine brain granulomas that were consistent with glioma based upon MRI, report their histologic diagnosis, and identify MRI criteria that might be useful to distinguish granuloma from glioma. Ten granulomas, initially suspected to be glioma based on MRI, were ultimately diagnosed as granulomatous meningoencephalomyelitis (n = 5), infectious granulomas (n = 3) or other meningoencephalitis (n = 2). Age was 1.6–15.0 years and two dogs were brachycephalic breeds. MRI characteristics overlapping with glioma included intra-axial, heterogeneous, T2-weighted hyperintense, T1-weighted hypointense to isointense mass lesions with contrast-enhancement. Signals on fluid attenuation inversion recovery, gradient echo and diffusion weighted imaging also matched glioma. Peri-lesional edema and mass effect were toward the high end of findings reported for glioma. MRI characteristics that would be considered unusual for glioma included dural contact (n = 4), T2-hypointensity (n = 2), concomitant meningeal-enhancement (n = 9), and minor changes in the contralateral brain (n = 2). Cerebrospinal fluid analysis revealed albuminocytological dissociation or mild pleocytosis. These cases show that granulomas can “mimic” glioma on canine brain MRI. In individual cases, certain MRI findings may help increase the index of suspicion for granuloma. Lack of pronounced cerebrospinal fluid pleocytosis does not exclude granuloma. Signalment is very useful in the suspicion of glioma, and many of these dogs with granuloma were of ages and breeds in which glioma is less commonly seen

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

On Active Galactic Nuclei as Sources of Ultra-High Energy Cosmic Rays

We measure the correlation between sky coordinates of the Swift BAT catalogue of active galactic nuclei with the arrival directions of the highest energy cosmic rays detected by the Auger Observatory. The statistically complete, hard X-ray catalogue helps to distinguish between AGN and other source candidates that follow the distribution of local large-scale structure. The positions of the full catalogue are marginally uncorrelated with the cosmic ray arrival directions, but when weighted by their hard X-ray flux, AGN within 100 Mpc are correlated at a significance level of 98 per cent. This correlation sharply decreases for sources beyond ~100 Mpc, suggestive of a GZK suppression. We discuss the implications for determining the mechanism that accelerates particles to these extreme energies in excess of 10^19 eV.Comment: 5 pages, 2 figures. Accepted for publication in MNRA

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