Nature of finite-temperature transition in anisotropic pyrochlore Er2Ti2O7

We study the finite-temperature transition in a model XY antiferromagnet on a pyrochlore lattice, which describes the pyrochlore material Er2Ti2O7. The ordered magnetic structure selected by thermal fluctuations is six-fold degenerate. Nevertheless, our classical Monte Carlo simulations show that the critical behavior corresponds to the three-dimensional XY universality class. We determine an additional critical exponent nu_6=0.75>nu characteristic of a dangerously irrelevant scaling variable. Persistent thermal fluctuations in the ordered phase are revealed in Monte Carlo simulations by the peculiar coexistence of Bragg peaks and diffuse magnetic scattering, the feature also observed in neutron diffraction experiments.Comment: 5+5 pages (including supplemental material

Quantum order by disorder and accidental soft mode Er2Ti2O7

Motivated by recent neutron scattering experiments, we derive and study an effective "pseudo-dipolar" spin-1/2 model for the XY pyrochlore antiferromagnet Er2Ti2O7. While a bond-dependent in-plane exchange anisotropy removes any continuous symmetry, it does lead to a one-parameter `accidental' classical degeneracy. This degeneracy is lifted by quantum fluctuations in favor of the non-coplanar spin structure observed experimentally -- a rare experimental instance of quantum order by disorder. A non-Goldstone low-energy mode is present in the excitation spectrum in accordance with inelastic neutron scattering data. Our theory also resolves the puzzle of the experimentally observed continuous ordering transition, absent from previous models.Comment: 5 pages, 4 figures, final versio

SALT HRS Capabilities for Time Resolved Pulsation Analysis: A Test with the roAp Star α Circini

Spectroscopy is a powerful tool for detecting variability in the rapidly oscillating Ap (roAp) stars. The technique requires short integrations times and high resolution, and so is limited to only a few telescopes and instruments. To test the capabilities of the High Resolution Spectrograph (HRS) at the Southern African Large Telescope (SALT) for the study of pulsations in roAp stars, we collected 2.45 hr of high-resolution data of the well studied roAp star α Cir in a previously unused instrument configuration. We extracted radial velocity measurements using different rare earth elements, and the core of Hα, via the cross correlation method. We performed the same analysis with a set of α Cir data collected with the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph to provide a benchmark for our SALT HRS test. We measured significant radial velocity variations in the HRS data and show that our results are in excellent agreement between the two data sets, with similar signal-tonoise ratio detections of the principal pulsation mode. With the HRS data, we report the detection of a second mode, showing the instrument is capable of detecting multiple and low-amplitude signals in a short observing window. We concluded that SALT HRS is well-suited for characterising pulsations in Ap stars, opening a new science window for the telescope. Although our analysis focused on roAp stars, the fundamental results are applicable to other areas of astrophysics where high temporal and spectral resolution observations are required

May 8, 1860

Volume: 1860 ; Start Page: 243 ; End Page: 26

The nature and regional significance of structures in the Gala Group of the Southern Uplands terrane, Berwickshire coast, southeastern Scotland

Structures deforming Llandovery turbidites of the Gala Group in the Southern Uplands terrane are spectacularly exposed in the Berwickshire coastal section, southeastern Scotland. The upward-facing, upright to NW-vergent folds and associated structures appear to record a single regional phase of subhorizontal NW-SE contractional deformation, with a steeply dipping direction of bulk finite extension. These structures are markedly different from those developed in rocks correlated with the Upper Llandovery Hawick Group exposed some 5 km to the south in the Eyemouth-Burnmouth coastal section. Here a highly domainal system of sinistral transpressional strain occurs, with zones of steeply plunging curvilinear folds, clockwise cleavage transection and bedding-parallel sinistral detachment faults. The markedly different bulk strain patterns in the Berwickshire coastal sections are thought to reflect the regionally diachronous nature of transpressional deformation in the Southern Uplands terrane. There are striking similarities in the structures recognized in the Berwickshire coastal sections and those developed in stratigraphically equivalent units along strike in southwestern Scotland and Northern Ireland. This confirms the lateral structural continuity and correlation of tracts and tract boundaries along the entire length of the Southern Uplands terrane. The regional structure suggests that a phase of top-to-the-NW backtbrusting and backfolding associated with the southern margin of the Gala Group outcrop marks the transition from orthogonal contraction to sinistral transpression in the Southern Upland thrust wedge during late Llandovery times

Dense colloidal suspensions under time-dependent shear

We consider the nonlinear rheology of dense colloidal suspensions under a time-dependent simple shear flow. Starting from the Smoluchowski equation for interacting Brownian particles advected by shearing (ignoring fluctuations in fluid velocity) we develop a formalism which enables the calculation of time-dependent, far-from-equilibrium averages. Taking shear-stress as an example we derive exactly a generalized Green-Kubo relation, and an equation of motion for the transient density correlator, involving a three-time memory function. Mode coupling approximations give a closed constitutive equation yielding the time-dependent stress for arbitrary shear rate history. We solve this equation numerically for the special case of a hard sphere glass subject to step-strain.Comment: 4 page

Neutron scattering from fragmented frustrated magnets

The fragmentation description is used to analyse calculated neutron scattering intensities from kagom\'e ice and spin ice systems. The longitudinal, transverse and harmonic fragments produce independent contributions to the neutron scattering intensity. This framework is used to analyse the ordering due to quantum fluctuations in the topologically constrained phase of kagom\'e ice and the monopole crystal phase of spin ice. Here, quantum fluctuations are restricted to the transverse fragment and they drive the system into a double-$q$ structure in which longitudinal and transverse fragments have a different ordering wave vector. The intensity reduction of the Bragg peaks for the transverse fragments, compared with known classical limits can be used as a diagnostic tool for quantum fluctuations. Published quantum Monte Carlo data for spin ice in a $[111]$ field are consistent with the proposed protocol.Comment: 13 pages, 8 figure

Tensorial Constitutive Models for Disordered Foams, Dense Emulsions, and other Soft Nonergodic Materials

In recent years, the paradigm of `soft glassy matter' has been used to describe diverse nonergodic materials exhibiting strong local disorder and slow mesoscopic rearrangement. As so far formulated, however, the resulting `soft glassy rheology' (SGR) model treats the shear stress in isolation, effectively `scalarizing' the stress and strain rate tensors. Here we offer generalizations of the SGR model that combine its nontrivial aging and yield properties with a tensorial structure that can be specifically adapted, for example, to the description of fluid film assemblies or disordered foams.Comment: 18 pages, 4 figure

Drivers of dietary behaviours in women living in urban Africa: a systematic mapping review.

OBJECTIVE: To (i) systematically review the literature to determine the factors influencing diet and dietary behaviour in women living in urban Africa; (ii) present these in a visual map; and (iii) utilize this to identify potentially important areas for future research. DESIGN: Systematic mapping review. The review protocol was registered at PROSPERO (http://www.crd.york.ac.uk/PROSPERO/; registration number CRD42015017749). Six databases were systematically searched, followed by reference and citation searching. Eligibility criteria included women aged 18-70 years living in urban Africa, any design/methodology, exploring any driver, using any measure of dietary behaviour. Quality appraisal occurred parallel with data extraction. Twelve predominantly cross-sectional quantitative studies were included; reported in seventeen publications. Determinants were synthesized narratively and compiled into a map adapted from an existing ecological model based on research in high-income countries. SETTING: Urban Africa. SUBJECTS: African women aged 18-70 years. RESULTS: Determinants significantly associated with unhealthy dietary behaviour ranged from the individual to macro level, comprising negative body image perception, perceptions of insufficient food quantity and poorer quality, poorer food knowledge, skipping meals, snacking less, higher alcohol consumption, unhealthy overall lifestyle, older age, higher socio-economic status, having an education, lower household food expenditure, frequent eating outside the home and media influence. Marital status and strong cultural and religious beliefs were also identified as possible determinants. CONCLUSIONS: Few studies have investigated drivers of dietary behaviours in urban African settings. Predominantly individual-level factors were reported. Gaps in the literature identified a need for research into the neglected areas: social, physical and macro-level drivers of food choice

Universal Fluctuations of the Danube Water Level: a Link with Turbulence, Criticality and Company Growth

A global quantity, regardless of its precise nature, will often fluctuate according to a Gaussian limit distribution. However, in highly correlated systems, other limit distributions are possible. We have previously calculated one such distribution and have argued that this function should apply specifically, and in many instances, to global quantities that define a steady state. Here we demonstrate, for the first time, the relevance of this prediction to natural phenomena. The river level fluctuations of the Danube are observed to obey our prediction, which immediately establishes a generic statistical connection between turbulence, criticality and company growth statistics.Comment: 5 pages, 1 figur