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Search Results: 1 - 10 of 457107 matches for " J. N. Andersen "
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Experimentally feasible quantum erasure-correcting code for continuous variables
J. Niset,U. L. Andersen,N. J. Cerf
Physics , 2007, DOI: 10.1103/PhysRevLett.101.130503
Abstract: We devise a scheme that protects quantum coherent states of light from probabilistic losses, thus achieving the first continuous-variable quantum erasure-correcting code. If the occurrence of erasures can be probed, then the decoder enables, in principle, a perfect recovery of the original light states. Otherwise, if supplemented with postselection based on homodyne detection, this code can be turned into an efficient erasure-filtration scheme. The experimental feasibility of the proposed protocol is carefully addressed.
Biogeography of Timor and Surrounding Wallacean Islands: Endemism in Ants of the Genus Polyrhachis Fr. Smith
Alan N. Andersen,Rudolf J. Kohout,Colin R. Trainor
Diversity , 2013, DOI: 10.3390/d5010139
Abstract: The Wallacean island of Timor is of particular biological interest due to its relatively large size and transitional location between the Indo-Malayan and Australasian biogeographic realms. However, the origins and levels of endemism of its invertebrate fauna are poorly known. A recent study of Timorese ants revealed a diverse fauna with predominantly Indo-Malayan affinities, but species-level taxonomy was considered to be too poorly understood for an analysis of levels of endemism. The highly diverse Old World tropical genus Polyrhachis represents a notable exception, and here we analyse levels of endemism in the Polyrhachis fauna of Timor and surrounding islands. We supplement the species listed in the previous study with additional collections to record a total of 35 species of Polyrhachis from Timor and surrounding islands. Only 14 (40%) of the 35 species could be named ( P. constricta, P. costulata, P. gab, P. sokolova, P. hera, P. illaudata, P. rixosa, P. acantha chrysophanes, P. saevissima, P. bicolor, P. cryptoceroides, P. dives, P. longipes and P. olybria), and the large majority of the remaining species have not previously been collected. These are very likely to be endemic to Timor and surrounding islands, and point to remarkably high levels (>50%) of endemism in the regional ant fauna.
Impurity states and cooperative magnetic order in Fe-based superconductors
Maria N. Gastiasoro,P. J. Hirschfeld,Brian M. Andersen
Physics , 2013, DOI: 10.1103/PhysRevB.88.220509
Abstract: We study impurity bound states and impurity-induced order in the superconducting state of LiFeAs within a realistic five-band model based on the band structure and impurity potentials obtained from density functional theory (DFT). In agreement with recent experiments, we find that Co impurities are too weak produce sub-gap bound states, whereas stronger impurities like Cu do. We also obtain the bound state spectrum for magnetic impurities, such as Mn, and show how spin-resolved tunnelling may determine the nature of the various defect sites in iron pnictides, a prerequisite for using impurity bound states as a probe of the ground state pairing symmetry. Lastly we show how impurities pin both orbital and magnetic order, providing an explanation for a growing set of experimental evidence for unusual magnetic phases in doped iron pnictides.
A common column density threshold for scattering at 3.6 mum and water-ice in molecular clouds
M. Andersen,W-F. Thi,J. Steinacker,N. Tothill
Physics , 2014, DOI: 10.1051/0004-6361/201424011
Abstract: Context: Observations of scattered light in the 1-5 $\mu$m range have revealed dust grains in molecular cores with sizes larger than commonly inferred for the diffuse interstellar medium. It is currently unclear whether these grains are grown within the molecular cores or are an ubiquitous component of the interstellar medium. Aims: We investigate whether the large grains necessary for efficient scattering at 1-5 mum are associated with the abundance of water-ice within molecular clouds and cores. Methods: We combined water-ice abundance measurements for sight lines through the Lupus IV molecular cloud complex with measurements of the scattered light at 3.6 mum for the same sight lines. Results: We find that there is a similar threshold for the cores in emission in scattered light at 3.6 mum (tau_9.7=0.15pm0.05, A_K=0.4pm0.2 as water-ice (tau_9.7=0.11pm0.01, A_K=0.19pm0.04) and that the scattering efficiency increases as the relative water-ice abundance increases. The ice layer increases the average grain size, which again strongly increases the albedo. Conclusions: The higher scattering efficiency is partly due to layering of ice on the dust grains. Although the layer can be relatively thin it can enhance the scattering substantially.
Origin of electronic dimers in the spin-density wave phase of Fe-based superconductors
Maria N. Gastiasoro,P. J. Hirschfeld,Brian M. Andersen
Physics , 2013, DOI: 10.1103/PhysRevB.89.100502
Abstract: We investigate the emergent impurity-induced states arising from point-like scatterers in the spin-density wave phase of iron-based superconductors within a microscopic five-band model. Independent of the details of the band-structure and disorder potential, it is shown how stable magnetic (pi,pi) unidirectional nematogens are formed locally by the impurities. Interestingly, these nematogens exhibit a dimer structure in the electronic density, are directed along the antiferromagnetic a-axis, and have typical lengths of order 10 lattice constants in excellent agreement with recent scanning tunnelling experiments. These electronic dimers provide a natural explanation of the dopant-induced transport anisotropy found e.g. in the 122 iron pnictides.
Crystal Structure and Magnetic Properties of the Ba3TeCo3P2O14, Pb3TeCo3P2O14, and Pb3TeCo3V2O14 Langasites
J. W. Krizan,C. de la Cruz,N. H. Andersen,R. J. Cava
Physics , 2013, DOI: 10.1016/j.jssc.2013.04.035
Abstract: We report the structural and magnetic characterizations of Ba3TeCo3P2O14, Pb3TeCo3P2O14, and Pb3TeCo3V2O14, compounds that are based on the mineral Dugganite, which is isostructural to Langasites. The magnetic part of the structure consists of layers of Co2+ triangles. Nuclear and magnetic structures were determined through a co-refinement of synchrotron and neutron powder diffraction data. In contrast to the undistorted P321 Langasite structure of Ba3TeCo3P2O14, a complex structural distortion yielding a large supercell is found for both Pb3TeCo3P2O14 and Pb3TeCo3V2O14. Comparison of the three compounds studied along with the zinc analog Pb3TeZn3P2O14, also characterized here, suggests that the distortion is driven by Pb2+ lone pairs; as such, the Pb compounds crystallize in a pyroelectric space group, P2. Magnetic susceptibility, magnetization, and heat capacity measurements were performed to characterize the magnetic behavior. All three compounds become antiferromagnetic with N\'eel temperatures TN ~ 21 K (Ba3TeCo3P2O14), ~ 13 K (Pb3TeCo3P2O14), and ~ 8 K (Pb3TeCo3V2O14), and they exhibit magnetic transitions at high applied magnetic fields, suggesting intrinsically complex magnetic behavior for tetrahedrally coordinated d7 Co2+ in this structure type.
New Abundances From Very Old Stars
T. Hansen,C. J. Hansen,N. Christlieb,D. Yong,T. C. Beers,J. Andersen
Physics , 2015,
Abstract: Metal-poor stars provide the fossil record of Galactic chemical evolution and the nucleosynthesis processes that took place at the earliest times in the history of our Galaxy. From detailed abundance studies of low mass, extremely metal-poor stars ([Fe/H] < -3), we can trace and help constrain the formation processes which created the first heavy elements in our Galaxy. Here we present the results of a ~25-star homogeneously analysed sample of metal-poor candidates from the Hamburg/ESO survey. We have derived abundances for a large number of elements ranging from Li to Ba, covering production processes from hydrostatic burning to neutron-capture. The sample includes some of the most metal-poor stars ([Fe/H] < -4) studied to date, and also a number of stars enhanced in carbon. The so called CEMP (carbon enhanced metal-poor) stars, these stars make up ~20% of the stars with [Fe/H] < -3, and 80% of the stars with [Fe/H] < -4.5. The progenitors of CEMP stars are still not fully constrained; they could be a result of binary mass transfer or high-mass explosive events in the early universe.
Experimental investigation of the Landau-Pomeranchuk-Migdal effect in low-Z targets
K. K. Andersen,S. L. Andersen,J. Esberg,H. Knudsen,R. E. Mikkelsen,U. I. Uggerh?j,T. N. Wistisen,P. Sona,A. Mangiarotti,T. J. Ketel
Physics , 2013, DOI: 10.1103/PhysRevD.88.072007
Abstract: In the CERN NA63 collaboration we have addressed the question of the potential inadequacy of the commonly used Migdal formulation of the Landau-Pomeranchuk-Migdal (LPM) effect by measuring the photon emission by 20 and 178 GeV electrons in the range 100 MeV - 4 GeV, in targets of LowDensityPolyEthylene (LDPE), C, Al, Ti, Fe, Cu, Mo and, as a reference target, Ta. For each target and energy, a comparison between simulated values based on the LPM suppression of incoherent bremsstrahlung is shown, taking multi-photon effects into account. For these targets and energies, we find that Migdal's theoretical formulation is adequate to a precision of better than about 5%, irrespective of the target substance.
Determining Vaccination Frequency in Farmed Rainbow Trout Using Vibrio anguillarum O1 Specific Serum Antibody Measurements
Lars Holten-Andersen, Inger Dalsgaard, J?rgen Nylén, Niels Lorenzen, Kurt Buchmann
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0049672
Abstract: Background Despite vaccination with a commercial vaccine with a documented protective effect against Vibrio anguillarum O1 disease outbreaks caused by this bacterium have been registered among rainbow trout at Danish fish farms. The present study examined specific serum antibody levels as a valid marker for assessing vaccination status in a fish population. For this purpose a highly sensitive enzyme-linked immunosorbent assay (ELISA) was developed and used to evaluate sera from farmed rainbow trout vaccinated against V. anguillarum O1. Study Design Immune sera from rainbow trout immunised with an experimental vaccine based on inactivated V. anguillarum O1 bacterin in Freund’s incomplete adjuvant were used for ELISA optimisation. Subsequently, sera from farmed rainbow trout vaccinated with a commercial vaccine against V. anguillarum were analysed with the ELISA. The measured serum antibody levels were compared with the vaccine status of the fish (vaccinated/unvaccinated) as evaluated through visual examination. Results Repeated immunisation with the experimental vaccine lead to increasing levels of specific serum antibodies in the vaccinated rainbow trout. The farmed rainbow trout responded with high antibody levels to a single injection with the commercial vaccine. However, the diversity in responses was more pronounced in the farmed fish. Primary visual examinations for vaccine status in rainbow trout from the commercial farm revealed a large pool of unvaccinated specimens (vaccination failure rate = 20%) among the otherwise vaccinated fish. Through serum analyses using the ELISA in a blinded set-up it was possible to separate samples collected from the farmed rainbow trout into vaccinated and unvaccinated fish. Conclusions Much attention has been devoted to development of new and more effective vaccines. Here we present a case from a Danish rainbow trout farm indicating that attention should also be directed to the vaccination procedure in order to secure high vaccination frequencies necessary for optimal protection with a reported effective vaccine.
Association between Plasma Antibody Response and Protection in Rainbow Trout Oncorhynchus mykiss Immersion Vaccinated against Yersinia ruckeri
Martin K. Raida, J?rgen Nylén, Lars Holten-Andersen, Kurt Buchmann
PLOS ONE , 2011, DOI: 10.1371/journal.pone.0018832
Abstract: A key hallmark of the vertebrate adaptive immune system is the generation of antigen-specific antibodies from B cells. Fish are the most primitive gnathostomes (jawed vertebrates) possessing an adaptive immune system. Vaccination of rainbow trout against enteric redmouth disease (ERM) by immersion in Yersinia ruckeri bacterin confers a high degree of protection to the fish. The immune mechanisms responsible for protection may comprise both cellular and humoral elements but the role of specific immunoglobulins in this system has been questioned and not previously described. The present study demonstrates significant increase in plasma antibody titers following immersion vaccination and significantly reduced mortality during Y. ruckeri challenge. Rainbow trout were immersion-vaccinated, using either a commercial ERM vaccine (AquaVac? ERM vet) or an experimental Y. ruckeri bacterin. Half of the trout vaccinated with AquaVac? ERM vet received an oral booster (AquaVac? ERM Oral vet). Sub-groups of the fish from each group were subsequently exposed to 1x109 CFU Y. ruckeri/ml either eight or twenty-six weeks post vaccination (wpv). All vaccinated groups showed 0% mortality when challenged, which was highly significant compared to the non-vaccinated controls (40 and 28% mortality eight and twenty-six weeks post vaccination (wpv), respectively) (P<0.0001). Plasma samples from all groups of vaccinated fish were taken 0, 4, 8, 12, 16 and 26 wpv. and Y. ruckeri specific IgM antibody levels were measured with ELISA. A significant increase in titers was recorded in vaccinated fish, which also showed a reduced bacteremia during challenge. In vitro plasma studies showed a significantly increased bactericidal effect of fresh plasma from vaccinated fish indicating that plasma proteins may play a role in protection of vaccinated rainbow trout.
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