Spin ice under pressure: symmetry enhancement and infinite order multicriticality

We study the low-temperature behaviour of spin ice when uniaxial pressure induces a tetragonal distortion. There is a phase transition between a Coulomb liquid and a fully magnetised phase. Unusually, it combines features of discontinuous and continuous transitions: the order parameter exhibits a jump, but this is accompanied by a divergent susceptibility and vanishing domain wall tension. All these aspects can be understood as a consequence of an emergent SU(2) symmetry at the critical point. We map out a possible experimental realisation

A Three Dimensional Kasteleyn Transition: Spin Ice in a [100] Field

We examine the statistical mechanics of spin-ice materials with a [100] magnetic field. We show that the approach to saturated magnetisation is, in the low-temperature limit, an example of a 3D Kasteleyn transition, which is topological in the sense that magnetisation is changed only by excitations that span the entire system. We study the transition analytically and using a Monte Carlo cluster algorithm, and compare our results with recent data from experiments on Dy2Ti2O7.Comment: 4 pages, 5 figure

Review of food-based dietary guidelines in Africa: Opportunities to enhance the healthiness and environmental sustainability of population diets

Food-based dietary guidelines (FBDGs) are important tools for promoting healthy eating and nutrition education at the population level. Currently,  more than 100 countries worldwide have developed FBDGs with majority of existing FBDGs in highincome countries. However, there are a few  countries in Africa which have developed FBDGs. This review describes and compares the characteristics of existing FBDGs in Africa. Data were  extracted from all existing FBDGs from African countries which have been archived in the Food and Agriculture Organization’s (FAO) online  repository of FBDGs. In addition, supporting documentation from other sources linked to the identified FBDGs were also reviewed. Extracted data  were coded and synthesized to describe the purpose of the FBDGs, the process for developing the FBDGs, and how healthy diets were expressed in  the FBDGs. In addition, the FBDGs were examined for content on considerations for planetary health, and non-dietary recommendations. A checklist  was used to extract the evidence in the identified documents. Of the 47 African countries in the WHO Africa region, only eight FBDGs were  identified from seven countries. South Africa had two FBDGs (including a FBDG for young children). Multi-disciplinary technical working groups were  convened in developing majority of the FBDGs. The working groups utilised scientific evidence on diet-related diseases and nutrient  requirements as a basis for recommendations. All the FBDGs were intended as a tool for promoting healthy diets among the lay population. The  FBDGs that were reviewed included between six and sixteen dietary messages. Diet diversification was promoted as the most common  recommendation across African country FBDGs. The recommendations often promoted consumption of four to six food groups communicated  using text as well as images (food guide). Local availability and cultural acceptability were important values promoted as part of an optimal diet in  some of the countries. However, none of the recommendations addressed environmental sustainability. Apart from South Africa, none of the FBDGs  had been evaluated or revised. Across Africa, there is a need for increased focus on developing new FBDGs or revising existing ones as a tool for  meeting the dietary information needs of populations at risk of malnutrition in all its forms.&nbsp

A review of food-based dietary guidelines in Africa: opportunities to enhance the healthiness and environmental sustainability of population diets

Food-based dietary guidelines (FBDGs) are important tools for promoting healthy eating and nutrition education at the population level. Currently, more than 100 countries worldwide have developed FBDGs with majority of existing FBDGs in highincome countries. However, there are a few countries in Africa which have developed FBDGs. This review describes and compares the characteristics of existing FBDGs in Africa. Data were extracted from all existing FBDGs from African countries which have been archived in the Food and Agriculture Organization’s (FAO) online repository of FBDGs. In addition, supporting documentation from other sources linked to the identified FBDGs were also reviewed. Extracted data were coded and synthesized to describe the purpose of the FBDGs, the process for developing the FBDGs, and how healthy diets were expressed in the FBDGs. In addition, the FBDGs were examined for content on considerations for planetary health, and non-dietary recommendations. A checklist was used to extract the evidence in the identified documents. Of the 47 African countries in the WHO Africa region, only eight FBDGs were identified from seven countries. South Africa had two FBDGs (including a FBDG for young children). Multi-disciplinary technical working groups were convened in developing majority of the FBDGs. The working groups utilised scientific evidence on diet-related diseases and nutrient requirements as a basis for recommendations. All the FBDGs were intended as a tool for promoting healthy diets among the lay population. The FBDGs that were reviewed included between six and sixteen dietary messages. Diet diversification was promoted as the most common recommendation across African country FBDGs. The recommendations often promoted consumption of four to six food groups communicated using text as well as images (food guide). Local availability and cultural acceptability were important values promoted as part of an optimal diet in some of the countries. However, none of the recommendations addressed environmental sustainability. Apart from South Africa, none of the FBDGs had been evaluated or revised. Across Africa, there is a need for increased focus on developing new FBDGs or revising existing ones as a tool for meeting the dietary information needs of populations at risk of malnutrition in all its forms

Origin of the approximate universality of distributions in equilibrium correlated systems

We propose an interpretation of previous experimental and numerical experiments, showing that for a large class of systems, distributions of global quantities are similar to a distribution originally obtained for the magnetization in the 2D-XY model . This approach, developed for the Ising model, is based on previous numerical observations. We obtain an effective action using a perturbative method, which successfully describes the order parameter fluctuations near the phase transition. This leads to a direct link between the D-dimensional Ising model and the XY model in the same dimension, which appears to be a generic feature of many equilibrium critical systems and which is at the heart of the above observations.Comment: To appear in Europhysics Letter

Severe acute malnutrition in children aged under 5 years can be successfully managed in a non-emergency routine community healthcare setting in Ghana

This study investigated the performance of community-based management of severe acute malnutrition (CMAM) within routine healthcare services in Ghana. This was a retrospective cohort study of n = 488 children (6–59 months) who had received CMAM. Data for recovery, default, and mortality rates were obtained from enrolment cards in 56 outpatient centres in Upper East region, Ghana. Satisfactory rates of recovery of 71.8% were reported. Children who were enrolled with higher mid-upper arm circumference (MUAC) ≥11.5 cm had seven times greater chance of recovery compared with children who were enrolled with lower MUAC <11.5 cm, OR = 7.35, 95% CI [2.56, 21.15], p < .001. Children who were diagnosed without malaria at baseline were 30 times, OR = 30.39, 95% CI [10.02, 92.13], p < .001, more likely to recover compared with those with malaria (p < .001). The average weight gain was 4.7 g−1·kg−1·day−1, which was influenced by MUAC status at baseline, β = .78, 95% CI [0.46, 1.00], p < .001, presence of malaria, β = −1.25, 95% CI [−1.58, 0.92], p < .001, and length of stay, β = 0.13, 95% CI [0.08, 0.18], p < .001. The default rate (28.5%) was higher than international standards recommendations by Sphere. Mortality rate (1.6%) was lower than international standards. Our findings suggest that community-based management of SAM can achieve similar success when delivered in routine non-emergency settings. However, this success can be diluted by a high default rate, and the factors contributing to this need to be explored to improve programme effectiveness within communities

Magnetic Monopole Dynamics in Spin Ice

One of the most remarkable examples of emergent quasi-particles, is that of the "fractionalization" of magnetic dipoles in the low energy configurations of materials known as "spin ice", into free and unconfined magnetic monopoles interacting via Coulomb's 1/r law [Castelnovo et. al., Nature, 451, 42-45 (2008)]. Recent experiments have shown that a Coulomb gas of magnetic charges really does exist at low temperature in these materials and this discovery provides a new perspective on otherwise largely inaccessible phenomenology. In this paper, after a review of the different spin ice models, we present detailed results describing the diffusive dynamics of monopole particles starting both from the dipolar spin ice model and directly from a Coulomb gas within the grand canonical ensemble. The diffusive quasi-particle dynamics of real spin ice materials within "quantum tunneling" regime is modeled with Metropolis dynamics, with the particles constrained to move along an underlying network of oriented paths, which are classical analogues of the Dirac strings connecting pairs of Dirac monopoles.Comment: 26 pages, 12 figure

Spin-wave analysis of the transverse-field Ising model on the checkerboard lattice

The ground state properties of the S=1/2 transverse-field Ising model on the checkerboard lattice are studied using linear spin wave theory. We consider the general case of different couplings between nearest neighbors (J1) and next-to-nearest neighbors (J2). In zero field the system displays a large degeneracy of the ground state, which is exponential in the system size (for J1=J2) or in the system's linear dimensions (for J2>J1). Quantum fluctuations induced by a transverse field are found to be unable to lift this degeneracy in favor of a classically ordered state at the harmonic level. This remarkable fact suggests that a quantum-disordered ground state can be instead promoted when non-linear fluctuations are accounted for, in agreement with existing results for the isotropic case J1=J2. Moreover spin-wave theory shows sizable regions of instability which are further candidates for quantum-disordered behavior.Comment: 12 pages, 13 figure

Classical Topological Order in Kagome Ice

We examine the onset of classical topological order in a nearest-neighbor kagome ice model. Using Monte Carlo simulations, we characterize the topological sectors of the groundstate using a non-local cut measure which circumscribes the toroidal geometry of the simulation cell. We demonstrate that simulations which employ global loop updates that are allowed to wind around the periodic boundaries cause the topological sector to fluctuate, while restricted local loop updates freeze the simulation into one topological sector. The freezing into one topological sector can also be observed in the susceptibility of the real magnetic spin vectors projected onto the kagome plane. The ability of the susceptibility to distinguish between fluctuating and non-fluctuating topological sectors should motivate its use as a local probe of topological order in a variety of related model and experimental systems.Comment: 17 pages, 9 figure

Analysis of a 290-Year Net Accumulation Time Series from Mt. Logan, Yukon

A 102.5-m mechanically continuous firn and ice core sequence retrieved from the Northwest Col of Mt. Logan (latitude 60°30\u27N; longitude 140°35\u27W; site location 5340 m a.s.l.) in the Yukon Territory, Canada, has been analyzed continuously for stable isotopes, pH and liquid electrolytic conductivity. Specific sections of the core have been analyzed for total β-activity (0-22 m) and trace ion concentrations (across major volcanic events) in order to date the core. In the lower half of the core, nitrate and some other ionic species are used to identify annual increments except between AD 1693 and AD 1720 and between AD 1729 and AD 1735 where only average annual increments are given. Annual increments were converted to water equivalents, then corrected for ice flow thinning as well as for origin, since a significant net accumulation gradient exists across the borehole site. The time series was subjected to cross correlation analysis, using instrumental data for the last 80 years, and to spectral analyses, using a 250-year sequence