634 research outputs found
Neel order in doped quasi one-dimensional antiferromagnets
We study the Neel temperature of quasi one-dimensional S=1/2 antiferromagnets
containing non-magnetic impurities. We first consider the temperature
dependence of the staggered susceptibility of finite chains with open boundary
conditions, which shows an interesting difference for even and odd length
chains. We then use a mean field theory treatment to incorporate the three
dimensional inter-chain couplings. The resulting Neel temperature shows a
pronounced drop as a function of doping by up to a factor of 5.Comment: 4 pages in revtex4 format including 2 epsf-embedded figures. The
latest version in PDF format is available from
http://fy.chalmers.se/~eggert/papers/staggered.pd
The effect of genetic relatedness and diversity on parasite load in the North American raccoon (procyon lotor) [abstract]
Abstract only availableThe influence of genetic relatedness and diversity on parasite transmission and diversity was examined in raccoons (Procyon lotor) from mid-Missouri. Relatedness and diversity of individuals was measured using 12 polymorphic microsatellite loci that were amplified using the polymerase chain reaction (PCR). Fragment analysis was used to determine the alleles of the individuals at each locus. Using the computer analysis program Kinship, relationship probabilities were calculated between individuals and populations. Heterozygosity, internal relatedness, and d2 values were also calculated. Parasite similarity indices did not differ among pairwise comparisons of animals with first order, second order, and no detected relationships. We are currently investigating if correlations exist between ectoparasite or endoparasite loads and heterozygosity, internal relatedness and d2 values. The data generated in this preliminary study will be used in a larger study of patterns of disease/parasite occurrences in artificially aggregating raccoons.Life Sciences Undergraduate Research Opportunity Progra
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Crystal structure and equation of state of Fe-Si alloys at super-Earth core conditions
The high-pressure behavior of Fe alloys governs the interior structure and dynamics of super-Earths, rocky extrasolar planets that could be as much as 10 times more massive than Earth. In experiments reaching up to 1300 GPa, we combine laser-driven dynamic ramp compression with in situ x-ray diffraction to study the effect of composition on the crystal structure and density of Fe-Si alloys, a potential constituent of super-Earth cores. We find that Fe-Si alloy with 7 weight % (wt %) Si adopts the hexagonal close-packed structure over the measured pressure range, whereas Fe-15wt%Si is observed in a body-centered cubic structure. This study represents the first experimental determination of the density and crystal structure of Fe-Si alloys at pressures corresponding to the center of a ~3–Earth mass terrestrial planet. Our results allow for direct determination of the effects of light elements on core radius, density, and pressures for these planets
Genomic Dissection of an Icelandic Epidemic of Respiratory Disease in Horses and Associated Zoonotic Cases.
Iceland is free of the major infectious diseases of horses. However, in 2010 an epidemic of respiratory disease of unknown cause spread through the country's native horse population of 77,000. Microbiological investigations ruled out known viral agents but identified the opportunistic pathogen Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) in diseased animals. We sequenced the genomes of 257 isolates of S. zooepidemicus to differentiate epidemic from endemic strains. We found that although multiple endemic clones of S. zooepidemicus were present, one particular clone, sequence type 209 (ST209), was likely to have been responsible for the epidemic. Concurrent with the epidemic, ST209 was also recovered from a human case of septicemia, highlighting the pathogenic potential of this strain. Epidemiological investigation revealed that the incursion of this strain into one training yard during February 2010 provided a nidus for the infection of multiple horses that then transmitted the strain to farms throughout Iceland. This study represents the first time that whole-genome sequencing has been used to investigate an epidemic on a national scale to identify the likely causative agent and the link to an associated zoonotic infection. Our data highlight the importance of national biosecurity to protect vulnerable populations of animals and also demonstrate the potential impact of S. zooepidemicus transmission to other animals, including humans.IMPORTANCE An epidemic of respiratory disease affected almost the entire native Icelandic horse population of 77,000 animals in 2010, resulting in a self-imposed ban on the export of horses and significant economic costs to associated industries. Although the speed of transmission suggested that a viral pathogen was responsible, only the presence of the opportunistic pathogen Streptococcus zooepidemicus was consistent with the observed clinical signs. We applied genomic sequencing to differentiate epidemic from endemic strains and to shed light on the rapid transmission of the epidemic strain throughout Iceland. We further highlight the ability of epidemic and endemic strains of S. zooepidemicus to infect other animals, including humans. This study represents the first time that whole-genome sequencing has been used to elucidate an outbreak on a national scale and identify the likely causative agent
Crystal structure and equation of state of Fe-Si alloys at super-Earth core conditions
The high-pressure behavior of Fe alloys governs the interior structure and dynamics of super-Earths, rocky extrasolar planets that could be as much as 10 times more massive than Earth. In experiments reaching up to 1300 GPa, we combine laser-driven dynamic ramp compression with in situ x-ray diffraction to study the effect of composition on the crystal structure and density of Fe-Si alloys, a potential constituent of super-Earth cores. We find that Fe-Si alloy with 7 weight % (wt %) Si adopts the hexagonal close-packed structure over the measured pressure range, whereas Fe-15wt%Si is observed in a body-centered cubic structure. This study represents the first experimental determination of the density and crystal structure of Fe-Si alloys at pressures corresponding to the center of a ~3–Earth mass terrestrial planet. Our results allow for direct determination of the effects of light elements on core radius, density, and pressures for these planets
Single Hit Energy-resolved Laue Diffraction
In-situ white light Laue diffraction has been successfully used to interrogate the structure of single crystal materials undergoing rapid (nanosecond) dynamic compression up to megabar pressures. However, information on strain state accessible via this technique is limited, reducing its applicability for a range of applications. We present an extension to the existing Laue diffraction platform in which we record the photon energy of a subset of diffraction peaks. This allows for a measurement of the longitudinal and transverse strains in-situ during compression. Consequently, we demonstrate measurement of volumetric compression of the unit cell, in addition to the limited aspect ratio information accessible in conventional white light Laue. We present preliminary results for silicon, where only an elastic strain is observed. VISAR measurements show the presence of a two wave structure and measurements show that material downstream of the second wave does not contribute to the observed diffraction peaks, supporting the idea that this material may be highly disordered, or has undergone large scale rotation
Simultaneous Bright- and Dark-Field X-ray Microscopy at X-ray Free Electron Lasers
The structures, strain fields, and defect distributions in solid materials
underlie the mechanical and physical properties across numerous applications.
Many modern microstructural microscopy tools characterize crystal grains,
domains and defects required to map lattice distortions or deformation, but are
limited to studies of the (near) surface. Generally speaking, such tools cannot
probe the structural dynamics in a way that is representative of bulk behavior.
Synchrotron X-ray diffraction based imaging has long mapped the deeply embedded
structural elements, and with enhanced resolution, Dark Field X-ray Microscopy
(DFXM) can now map those features with the requisite nm-resolution. However,
these techniques still suffer from the required integration times due to
limitations from the source and optics. This work extends DFXM to X-ray free
electron lasers, showing how the photons per pulse available at these
sources offer structural characterization down to 100 fs resolution (orders of
magnitude faster than current synchrotron images). We introduce the XFEL DFXM
setup with simultaneous bright field microscopy to probe density changes within
the same volume. This work presents a comprehensive guide to the multi-modal
ultrafast high-resolution X-ray microscope that we constructed and tested at
two XFELs, and shows initial data demonstrating two timing strategies to study
associated reversible or irreversible lattice dynamics
Modeling Structure-Function Relationships in Synthetic DNA Sequences using Attribute Grammars
Recognizing that certain biological functions can be associated with specific DNA sequences has led various fields of biology to adopt the notion of the genetic part. This concept provides a finer level of granularity than the traditional notion of the gene. However, a method of formally relating how a set of parts relates to a function has not yet emerged. Synthetic biology both demands such a formalism and provides an ideal setting for testing hypotheses about relationships between DNA sequences and phenotypes beyond the gene-centric methods used in genetics. Attribute grammars are used in computer science to translate the text of a program source code into the computational operations it represents. By associating attributes with parts, modifying the value of these attributes using rules that describe the structure of DNA sequences, and using a multi-pass compilation process, it is possible to translate DNA sequences into molecular interaction network models. These capabilities are illustrated by simple example grammars expressing how gene expression rates are dependent upon single or multiple parts. The translation process is validated by systematically generating, translating, and simulating the phenotype of all the sequences in the design space generated by a small library of genetic parts. Attribute grammars represent a flexible framework connecting parts with models of biological function. They will be instrumental for building mathematical models of libraries of genetic constructs synthesized to characterize the function of genetic parts. This formalism is also expected to provide a solid foundation for the development of computer assisted design applications for synthetic biology
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