Search for Ferromagnetism in doped semiconductors in the absence of transition metal ions

In contrast to semiconductors doped with transition metal magnetic elements, which become ferromagnetic at temperatures below ~ 100K, semiconductors doped with non-magnetic ions (e.g. silicon doped with phosphorous) have not shown evidence of ferromagnetism down to millikelvin temperatures. This is despite the fact that for low densities the system is expected to be well modeled by the Hubbard model, which is predicted to have a ferromagnetic ground state at T=0 on 2- or 3-dimensional bipartite lattices in the limit of strong correlation near half-filling. We examine the impurity band formed by hydrogenic centers in semiconductors at low densities, and show that it is described by a generalized Hubbard model which has, in addition to strong electron-electron interaction and disorder, an intrinsic electron-hole asymmetry. With the help of mean field methods as well as exact diagonalization of clusters around half filling, we can establish the existence of a ferromagnetic ground state, at least on the nanoscale, which is more robust than that found in the standard Hubbard model. This ferromagnetism is most clearly seen in a regime inaccessible to bulk systems, but attainable in quantum dots and 2D heterostructures. We present extensive numerical results for small systems that demonstrate the occurrence of high-spin ground states in both periodic and positionally disordered 2D systems. We consider how properties of real doped semiconductors, such as positional disorder and electron-hole asymmetry, affect the ground state spin of small 2D systems. We also discuss the relationship between this work and diluted magnetic semiconductors, such as Ga_(1-x)Mn_(x)As, which though disordered, show ferromagnetism at relatively high temperatures.Comment: 47 page

Introduction to dynamical horizons in numerical relativity

This paper presents a quasi-local method of studying the physics of dynamical black holes in numerical simulations. This is done within the dynamical horizon framework, which extends the earlier work on isolated horizons to time-dependent situations. In particular: (i) We locate various kinds of marginal surfaces and study their time evolution. An important ingredient is the calculation of the signature of the horizon, which can be either spacelike, timelike, or null. (ii) We generalize the calculation of the black hole mass and angular momentum, which were previously defined for axisymmetric isolated horizons to dynamical situations. (iii) We calculate the source multipole moments of the black hole which can be used to verify that the black hole settles down to a Kerr solution. (iv) We also study the fluxes of energy crossing the horizon, which describes how a black hole grows as it accretes matter and/or radiation. We describe our numerical implementation of these concepts and apply them to three specific test cases, namely, the axisymmetric head-on collision of two black holes, the axisymmetric collapse of a neutron star, and a non-axisymmetric black hole collision with non-zero initial orbital angular momentum.Comment: 20 pages, 16 figures, revtex4. Several smaller changes, some didactic content shortene

Niche shifts and energetic condition of songbirds in response to phenology of food-resource availability in a high-elevation sagebrush ecosystem

Seasonal fluctuations in food availability can affect diets of consumers, which in turn may influence the physiological state of individuals and shape intra- and inter-specific patterns of resource use. High-elevation ecosystems often exhibit a pronounced seasonal “pulse” in productivity, although few studies document how resource use and energetic condition by avian consumers change in relation to food-resource availability in these ecosystems. We tested the hypothesis that seasonal increases (pulses) in food resources in high-elevation sagebrush ecosystems result in 2 changes after the pulse, relative to the before-pulse period: (1) reduced diet breadth of, and overlap between, 2 sympatric sparrow species; and (2) enhanced energetic condition in both species. We tracked breeding-season diets using stable isotopes and energetic condition using plasma metabolites of Brewer\u27s Sparrows (Spizella breweri), Vesper Sparrows (Pooecetes gramineus), and their food resources during 2011, and of only Brewer\u27s Sparrows and their food resources during 2013. We quantify diet breadth and overlap between both species, along with coincident physiological consequences of temporal changes in resource use. After invertebrate biomass increased following periods of rainfall in 2011, dietary breadth decreased by 35% in Brewer\u27s Sparrows and by 48% in Vesper Sparrows, while dietary overlap decreased by 88%. Energetic condition of both species increased when dietary overlap was lower and diet breadth decreased, after the rapid rise of food-resource availability. However, energetic condition of Brewer\u27s Sparrows remained constant in 2013, a year with low precipitation and lack of a strong pulse in food resources, even though the species\u27 dietary breadth again decreased that year. Our results indicate that diet breadth and overlap in these sparrow species inhabiting sagebrush ecosystems generally varied as predicted in relation to intra- and interannual changes in food resources, and this difference in diet was associated with improved energetic condition of sparrows at least in one year

Resonating Valence Bond Theory of Coupled Heisenberg Chains

Using numerical results from a density matrix renormalization group study as a guide, we develop a resonating valence bond (RVB) theory for coupled Heisenberg chains. We argue that simple topological effects mandate a short-range RVB description of systems with an even number of chains $n_c$, with a spin gap, short-range correlations, and confinement of topological spin defects. Odd-$n_c$ systems have long-range RVB ground states, no gap, and power-law correlations.Comment: 11 pages and 6 postscript figures, RevTeX 3.0, UCI-CMTHE-94-02 (Revised version

Social and socioecological studies of ecological restoration: a review of the literature at global and Iberoamerican scales

La restauración ecológica (RE) se ha consolidado como campo académico y política pública. No obstante, podría presentar sesgos que obstaculicen el abordaje integral de sus causas y consecuencias naturales y sociales. Por ende, se propuso analizar cómo se están abordando los aspectos sociales y socioecológicos sobre RE en la literatura científica a escala global e iberoamericana. Se esperaba que el cambio paradigmático en las ciencias ecológicas, que empezó a incorporar la dimensión humana de los ecosistemas, se viera reflejado en la literatura sobre RE, y se predijo un incremento en estudios sociales y socioecológicos de RE en la última década. Se realizó una revisión bibliográfica sistemática entre 1900-2016 a escala global (Web of Science, n=550) y de Iberoamérica (Scientific Electronic Library Online, n=290). Se cuantificaron patrones temporales, geográficos, metodológicos y disciplinares en publicaciones con marcos conceptuales ecológico, social o socioecológico. Para Iberoamérica, se profundizó con un análisis cualitativo que incluyó conceptos sociopolíticos (i.e., gobierno, políticas públicas, participación ciudadana). Se encontró que la productividad científica sobre RE aumentó en general, pero la tasa de crecimiento fue mayor para estudios ecológicos que sociales y socioecológicos. Estos trabajos se publican principalmente (>50%) en revistas de ciencias naturales o aplicadas en ambas escalas. A su vez, las publicaciones sociales y socioecológicas contribuyen más a la literatura global que a la regional (37.5% y 15.3%, respectivamente), y hubo una dominancia de publicaciones proveniente de países del Norte Global. La mirada interdisciplinaria está parcialmente reflejada en la literatura científica sobre RE, y la articulación entre la ciencia y la gestión fue escasa en los estudios regionales. Además de pensar en incrementar la producción en investigación social y socioecológica de RE, se debería reforzar el “diálogo de saberes” entre regiones y disciplinas para alcanzar experiencias exitosas de RE en la práctica.. Ecological restoration (RE) has become established as an academic field and public policy. However, it could have biases that hinder a comprehensive approach to its natural and social causes and consequences. Therefore, we proposed to analyze how the social and socioecological aspects of RE are addressed in the scientific literature at global and Iberoamerican scales. It was expected that the paradigmatic change in the ecological sciences, which has begun to incorporate the human dimension of ecosystems, would be reflected in the RE literature, and we predicted an increase in social and socioecological studies of RE in the last decade. A systematic literature review was conducted between 1900-2016 on a global scale (Web of Science, n=550) and for Iberoamerica (Scientific Electronic Library Online, n=290). Temporal, geographic, methodological and disciplinary patterns were quantified in publications with ecological, social or socioecological conceptual frameworks. For Iberoamerica, the assessment was deepened with a qualitative analysis of sociopolitical concepts (i.e., government, public policy, citizen participation). We found that scientific productivity about ER has increased overall, but the growth rate was greater for ecological studies than social and socioecological studies. These works are published mainly (>50%) in natural and applied science journals at both scales. In turn, social and socioecological publications were found more in the global literature than regional (37.5% and 15.3%, respectively), and there was a dominance of publications from countries in the Global North. An interdisciplinary view is only partially reflected in the scientific literature on ER, and the linkage between science and management is scarce in regional studies. In addition to thinking about increasing social and socioecological research production of RE, “knowledge dialogue” between regions and disciplines should be re-enforced to achieve successful RE experiences in practice.Fil: Roulier, Catherine Solange. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina. Universidad Nacional de Tierra del Fuego. Instituto de Cultura, Sociedad y Estado; ArgentinaFil: Anderson, Christopher Brian. Universidad Nacional de Tierra del Fuego, Antártida e Islas del Atlántico Sur. Instituto de Ciencias Polares, Ambientales y Recursos Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; ArgentinaFil: Ballari, Sebastián A.. Ministerio de Ambiente y Desarrollo Sustentable. Administración de Parques Nacionales. Parque Nacional "Nahuel Huapi". Departamento Conservación y Educación Ambiental; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Nielsen, Erik A.. Northern Arizona University; Estados Unido

Plasma sTNFR1 and IL8 for prognostic enrichment in sepsis trials: a prospective cohort study.

BackgroundEnrichment strategies improve therapeutic targeting and trial efficiency, but enrichment factors for sepsis trials are lacking. We determined whether concentrations of soluble tumor necrosis factor receptor-1 (sTNFR1), interleukin-8 (IL8), and angiopoietin-2 (Ang2) could identify sepsis patients at higher mortality risk and serve as prognostic enrichment factors.MethodsIn a multicenter prospective cohort study of 400 critically ill septic patients, we derived and validated thresholds for each marker and expressed prognostic enrichment using risk differences (RD) of 30-day mortality as predictive values. We then used decision curve analysis to simulate the prognostic enrichment of each marker and compare different prognostic enrichment strategies.Measurements and main resultsAn admission sTNFR1 concentration > 8861 pg/ml identified patients with increased mortality in both the derivation (RD 21.6%) and validation (RD 17.8%) populations. Among immunocompetent patients, an IL8 concentration > 94 pg/ml identified patients with increased mortality in both the derivation (RD 17.7%) and validation (RD 27.0%) populations. An Ang2 level > 9761 pg/ml identified patients at 21.3% and 12.3% increased risk of mortality in the derivation and validation populations, respectively. Using sTNFR1 or IL8 to select high-risk patients improved clinical trial power and efficiency compared to selecting patients with septic shock. Ang2 did not outperform septic shock as an enrichment factor.ConclusionsThresholds for sTNFR1 and IL8 consistently identified sepsis patients with higher mortality risk and may have utility for prognostic enrichment in sepsis trials

Scanned Probe Microscopy of Electronic Transport in Carbon Nanotubes

We use electrostatic force microscopy and scanned gate microscopy to probe the conducting properties of carbon nanotubes at room temperature. Multi-walled carbon nanotubes are shown to be diffusive conductors, while metallic single-walled carbon nanotubes are ballistic conductors over micron lengths. Semiconducting single-walled carbon nanotubes are shown to have a series of large barriers to conduction along their length. These measurements are also used to probe the contact resistance and locate breaks in carbon nanotube circuits.Comment: 4 page

Criticality in one dimension with inverse square-law potentials

It is demonstrated that the scaled order parameter for ferromagnetic Ising and three-state Potts chains with inverse-square interactions exhibits a universal critical jump, in analogy with the superfluid density in helium films. Renormalization-group arguments are combined with numerical simulations of systems containing up to one million lattice sites to accurately determine the critical properties of these models. In strong contrast with earlier work, compelling quantitative evidence for the Kosterlitz--Thouless-like character of the phase transition is provided.Comment: To appear in Phys. Rev. Let