A first-principles comparison of the electronic properties of MgC_{y}Ni_{3} and ZnC_{y}Ni_{3} alloys

First-principles, density-functional-based electronic structure calculations are employed to study the changes in the electronic properties of ZnC_{y}Ni_{3} and MgC_{y}Ni_{3} using the Korringa-Kohn-Rostoker coherent-potential approximation method in the atomic sphere approximation (KKR-ASA CPA). As a function of decreasing C at%, we find a steady decrease in the lattice constant and bulk modulus in either alloys. However, the pressure derivative of the bulk modulus displays an opposite trend. Following the Debye model, which relates the pressure derivative of the bulk modulus with the average phonon frequency of the crystal, it can thus be argued that ZnCNi_{3} and its disordered alloys posses a different phonon spectra in comparison to its MgCNi_{3} counterparts. This is further justified by the marked similarity we find in the electronic structure properties such as the variation in the density of states and the Hopfield parameters calculated for these alloys. The effects on the equation of state parameters and the density of states at the Fermi energy, for partial replacement of Mg by Zn are also discussed.Comment: 19 pages, 15 figure

Magnetic hardening in rapidly quenched Fe-Pr and Fe-Nd alloys

We report studies of high-field magnetization and thermomagnetic effects in rapidly quenched and heat treated alloys based on Fe-Pr and Fe-Nd. Coercivities up to ~40 kOe and large energy products result from the precipitation of a finely dispersed crystalline phase. Studies of varying the alloy composition and heat treatment are reported. Journal of Applied Physics is copyrighted by The American Institute of Physics

Movies of cellular and sub-cellular motion by digital holographic microscopy

BACKGROUND: Many biological specimens, such as living cells and their intracellular components, often exhibit very little amplitude contrast, making it difficult for conventional bright field microscopes to distinguish them from their surroundings. To overcome this problem phase contrast techniques such as Zernike, Normarsky and dark-field microscopies have been developed to improve specimen visibility without chemically or physically altering them by the process of staining. These techniques have proven to be invaluable tools for studying living cells and furthering scientific understanding of fundamental cellular processes such as mitosis. However a drawback of these techniques is that direct quantitative phase imaging is not possible. Quantitative phase imaging is important because it enables determination of either the refractive index or optical thickness variations from the measured optical path length with sub-wavelength accuracy. Digital holography is an emergent phase contrast technique that offers an excellent approach in obtaining both qualitative and quantitative phase information from the hologram. A CCD camera is used to record a hologram onto a computer and numerical methods are subsequently applied to reconstruct the hologram to enable direct access to both phase and amplitude information. Another attractive feature of digital holography is the ability to focus on multiple focal planes from a single hologram, emulating the focusing control of a conventional microscope. METHODS: A modified Mach-Zender off-axis setup in transmission is used to record and reconstruct a number of holographic amplitude and phase images of cellular and sub-cellular features. RESULTS: Both cellular and sub-cellular features are imaged with sub-micron, diffraction-limited resolution. Movies of holographic amplitude and phase images of living microbes and cells are created from a series of holograms and reconstructed with numerically adjustable focus, so that the moving object can be accurately tracked with a reconstruction rate of 300ms for each hologram. The holographic movies show paramecium swimming among other microbes as well as displaying some of their intracellular processes. A time lapse movie is also shown for fibroblast cells in the process of migration. CONCLUSION: Digital holography and movies of digital holography are seen to be useful new tools for visualization of dynamic processes in biological microscopy. Phase imaging digital holography is a promising technique in terms of the lack of coherent noise and the precision with which the optical thickness of a sample can be profiled, which can lead to images with an axial resolution of a few nanometres

Application of Autologous Bone Marrow Derived Mesenchymal Stem Cells to an Ovine Model of Growth Plate Cartilage Injury

Injury to growth plate cartilage in children can lead to bone bridge formation and result in bone growth deformities, a significant clinical problem currently lacking biological treatment. Mesenchymal stem/stromal cells (MSC) offer a promising therapeutic option for regeneration of damaged cartilage, due to their self renewing and multi-lineage differentiation attributes. Although some small animal model studies highlight the therapeutic potential of MSC for growth plate repair, translational research in large animal models, which more closely resemble the human condition, are lacking. Our laboratory has recently characterised MSCs derived from ovine bone marrow, and demonstrated these cells form cartilage-like tissue when transplanted within the gelatin sponge, Gelfoam, in vivo. In the current study, autologous bone marrow MSC were seeded into Gelfoam scaffold containing TGF-β1, and transplanted into a surgically created defect of the proximal ovine tibial growth plate. Examination of implants at 5 week post-operatively revealed transplanted autologous MSC failed to form new cartilage structure at the defect site, but contributed to an increase in formation of a dense fibrous tissue. Importantly, the extent of osteogenesis was diminished, and bone bridge formation was not accelerated due to transplantation of MSCs or the gelatin scaffold. The current study represents the first work that has utilised this ovine large animal model to investigate whether autologous bone marrow derived MSC can be used to initiate regeneration at the injured growth plate

The energy spectrum of cosmic rays beyond the turn-down around 10¹⁷ eV as measured with the surface detector of the Pierre Auger Observatory

We present a measurement of the cosmic-ray spectrum above 100 PeV using the part of the surface detector of the Pierre Auger Observatory that has a spacing of 750 m. An inflection of the spectrum is observed, confirming the presence of the so-called second-knee feature. The spectrum is then combined with that of the 1500 m array to produce a single measurement of the flux, linking this spectral feature with the three additional breaks at the highest energies. The combined spectrum, with an energy scale set calorimetrically via fluorescence telescopes and using a single detector type, results in the most statistically and systematically precise measurement of spectral breaks yet obtained. These measurements are critical for furthering our understanding of the highest energy cosmic rays

Reconstruction of Events Recorded with the Water-Cherenkov and Scintillator Surface Detectors of the Pierre Auger Observatory

With the knowledge and statistical power of over a decade and a half of measurements, the Auger Collaboration has developed, assessed, and refined robust methods for reconstructing the energies and arrival directions of the highest-energy cosmic rays from the signal and timing measurements of its surface detector array. Concurrently, the unearthing of an increasingly complex astrophysical scenario and tensions with hadronic interaction models have demanded the addition of primary mass as an observable measurable using the surface detector. Access to information on the mass hinges on the disentanglement of the electromagnetic and muonic components of extensive air showers. Consequently, an upgrade to the Observatory, AugerPrime, is being carried out by equipping existing water-Cherenkov stations with a 3.8 m2 Scintillator Surface Detector (SSD). The SSDs, with their high sensitivity to electrons and positrons, will provide samples of the lateral distribution of particles at the ground that complement those of the water-Cherenkov detectors, which are significantly more sensitive to muons. When used together, the two measurements enable extraction of the number of incident muons, which is a quantity that strongly correlates with primary mass. We describe the reconstruction methods being developed for application to measurements of the surface detector of the Observatory with a particular focus on the enhancement of these methods with data of the SSDs of AugerPrime. Results from the reconstruction of thousands of high-energy events already measured with deployed SSDs are also shown

Monte Carlo simulations for the Pierre Auger Observatory using the VO auger grid resources

The Pierre Auger Observatory, located near Malargüe, Argentina, is the world’s largest cosmic-ray detector. It comprises a 3000 km2 surface detector and 27 fluorescence telescopes, which measure the lateral and longitudinal distributions of the many millions of air-shower particles produced in the interactions initiated by a cosmic ray in the Earth’s atmosphere. The determination of the nature of cosmic rays and studies of the detector performances rely on extensive Monte Carlo simulations describing the physics processes occurring in extensive air showers and the detector responses. The aim of the Monte Carlo simulations task is to produce and provide the Auger Collaboration with reference libraries used in a wide variety of analyses. All multipurpose detector simulations are currently produced in local clusters using Slurm and HTCondor. The bulk of the shower simulations are produced on the grid, via the Virtual Organization auger, using the DIRAC middleware. The job submission is made via python scripts using the DIRAC-API. The Auger site is undergoing a major upgrade, which includes the installation of new types of detectors, demanding increased simulation resources. The novel detection of the radio component of extensive air showers is the most challenging endeavor, requiring dedicated shower simulations with very long computation times, not optimized for the grid production. For data redundancy, the simulations are stored on the Lyon server and the grid Disk Pool Manager and are accessible to the Auger members via iRODS and DIRAC, respectively. The CERN VM-File System is used for software distribution where, soon, the Auger Offline software will also be made available

The ultra-high-energy cosmic-ray sky above 32 EeV viewed from the Pierre Auger Observatory

The region of the toe in the cosmic-ray spectrum, located at about 45 EeV by the Pierre Auger Collaboration, is of primary interest in the search for the origin of ultra-high energy cosmic rays (UHECRs). The suppression of the flux with increasing energy can be explained by the interaction of UHECRs with intergalactic photons, resulting in a shrinking of the observable universe, and/or by cut-offs in acceleration potential at the astrophysical sources, yielding a high-rigidity sample of single (or few) UHECR species around the toe. The predominance of foreground sources combined with reduced deflections could thus offer a path towards localizing ultra-high energy accelerators, through the study of UHECR arrival directions. In this contribution, we present the results of blind and astrophysically-motivated searches for anisotropies with data collected above 32 EeV during the first phase of the Pierre Auger Observatory, i.e. prior to the AugerPrime upgrade, for an exposure of over 120,000 km2 yr sr. We have conducted model-independent searches for overdensities at small and intermediate angular scales, correlation studies with several astrophysical structures, and cross-correlation analyses with catalogs of candidate extragalactic sources. These analyses provide the most important evidence to date for anisotropy in UHECR arrival directions around the toe as measured from a single observatory

A tau scenario application to a search for upward-going showers with the Fluorescence Detector of the Pierre Auger Observatory

Recent observations of two coherent radio pulses with the ANITA detector can be interpreted as steeply upward-going cosmic-ray showers with energies of a few tenths of an EeV and remain unexplained. The Pierre Auger Observatory has a large exposure to such upward propagating shower-like events, and has used 14 years of its Fluorescence Detector (FD) data to perform a generic search for such events with elevation angles greater than 20◦ from the horizon. Here this search is recast to constrain models generating high energy τ-leptons. For maximal flexibility, only the propagation, decay, and interactions of τ-leptons are treated in this analysis, meaning that the results are independent of the τ-production scenario. This treatment allows for the application of these results to the wide range of models producing τ-leptons that have been proposed to describe the "anomalous" ANITA events. The goal of this study is accomplished by generating τ-leptons within the Earth and its atmosphere with an intensity dependent on the media density. The zenith angle, location and calorimetric energy of any resulting τ-induced air showers are then used to calculate the exposure of the FD of the Pierre Auger Observatory to τ primaries. Differential limits as low as 10−9 GeV s−1cm−2sr−1 to the flux of τ-leptons produced with less than a 50 km path length below the Earth’s surface are reported for several zenith angle ranges and primary energy spectra. Full exposure and sensitivity information is provided, facilitating the application of these results to different τ-lepton production models