4,356 research outputs found
Constraints on SN Ia progenitor time delays from high-z SNe and the star formation history
We re-assess the question of a systematic time delay between the formation of
the progenitor and its explosion in a type Ia supernova (SN Ia) using the
Hubble Higher-z Supernova Search sample (Strolger et al. 2004). While the
previous analysis indicated a significant time delay, with a most likely value
of 3.4 Gyr, effectively ruling out all previously proposed progenitor models,
our analysis shows that the time-delay estimate is dominated by systematic
errors, in particular due to uncertainties in the star-formation history. We
find that none of the popular progenitor models under consideration can be
ruled out with any significant degree of confidence. The inferred time delay is
mainly determined by the peak in the assumed star-formation history. We show
that, even with a much larger Supernova sample, the time delay distribution
cannot be reliably reconstructed without better constraints on the
star-formation history.Comment: accepted for publication in MNRA
EuO and Eu on metal crystals and graphene: interface effects and epitaxial films
Growth of the ferromagnetic semiconductor EuO was studied on the metal crystals Ni(100) and Ir(111) and on graphene. Primarily, characterisation was done by means of in-situ scanning tunnelling microscopy (STM) and low energy electron diffraction. The epitaxy on the metal crystals is strongly influenced by interface effects which lead to a complicated growth behaviour in the sub-monolayer regime, especially on Ni(100). Therefore, also films of sub-monolayer thickness were analysed in detail for these substrates.
Eu oxide on Ni(100) shows a variety of different surface phases in the sub-monolayer regime, depending on the growth temperature and the ratio of the Eu and O fluxes. Hence, a careful selection of the initial growth parameters is decisive to obtain a surface oxide suitable for subsequent epitaxy of single phase EuO(100). After creation of a 3 layer thick coalesced oxide film, for subsequent growth a distillation technique can be applied. Ex-situ X-ray adsorption spectroscopy and magneto-optical Kerr effect microscopy measurements of thicker films on Ni(100) are consistent with stoichiometric single phase EuO with bulk properties.
On Ir(111) initially only islands of polar EuO(111) grow, but formation of EuO(100) sets in before the first oxide layer is completed. The ratio of EuO(100) to EuO(111) is thereby influenced by the ratio of the Eu and O fluxes. Thus, the EuO films on Ir(111) consist of a phase mixture of EuO(111) and three rotational domains of EuO(100). The thinnest structure of the EuO(111) is a bilayer. Field emission resonances revealed a work function increase of 6 eV for this structure compared to EuO(100). Despite the polarity, the bilayer shows no obvious reconstruction which could reduce the high electric field. Triangular reconstruction motifs were found for the third EuO(111) layer.
On graphene EuO can be grown as thin film of distinct, {100}-faceted grains which are oriented to the substrate at a sufficiently high growth temperature. As the EuO on graphene is not affected by interface effects, the initial growth stage is not crucial. Thus, the growth of these grains is far less sensitive to the ratio of Eu and O fluxes than the EuO growth on Ni(100).
Appropriate annealing of EuO(100) films generates sufficient conductivity for STM and electron spectroscopies, even for films of 100 nm thickness. Oxygen vacancies were directly imaged by STM. They are of decisive importance for the metal-to-insulator transition of EuO around the temperature of the ferromagnetic-to-paramagnetic transition. Tunnelling spectra of EuO were recorded for the first time. For EuO(100) with 1% O vacancies in the topmost layer they exhibit states about 500 meV above the Fermi level which are most probably related to O vacancies.
On all substrates, monolayer high EuO(100) films have a contracted lattice which expands with increasing film thickness. Even if the substrate applies compressive biaxial stress, the EuO bulk lattice constant is almost reached for 5 nm film thickness. This leaves little hope for an increase of the Curie temperature through epitaxial compression.
During the investigation of the EuO on graphene, intercalation of Eu between the graphene and its Ir(111) substrate was observed and analysed further. For Eu deposition at 720 K a variety of equilibrium intercalate structures occur, dependent on the deposited Eu amount, all of which have a height of one monolayer. The dimensions and orientations of these structures are determined by binding energy differences within the unit cell of the graphene moiré on Ir(111). The energetically preferred lattice of the intercalated Eu is a p(2x2) structure, but intercalation continues until a denser (1.73x1.73)R30° structure is saturated. Angular resolved photoemission spectroscopy finds a shift of the graphene's Dirac cone by -1.5 eV for both of these structures.
For closed graphene films, intercalation is hindered by a penetration barrier for temperatures below 400 K. The adsorption and equilibrium surface phases of Eu on graphene were investigated in the temperature range from 35 K to 400 K and for coverages ranging from a small fraction of a saturated monolayer to the second layer. Using density functional theory, including the 4f-shell Coulomb interactions and modelling of the electronic interactions, excellent agreement with the experimental results for the equilibrium adsorbate phase, adsorbate diffusion, and work function was obtained. Most remarkable, at 300 K in an intermediate coverage range a phase of uniformly distributed Eu clusters coexists in two dimensional equilibrium with large Eu-islands in a (1.73x1.73)R30° structure. The formation of the cluster phase is driven by the interplay of three effects: First, the metallic Eu-Eu binding leads to the local stability of (1.73x1.73)R30° structures. Second, electrons lower their kinetic energy by leaving the Eu clusters, thereby doping graphene. Third, the Coulomb energy penalty associated with the charge transfer from Eu to graphene is strongly reduced for smaller clusters
Mol. Cell. Proteomics
Chemical cross-linking in combination with mass spectrometric analysis offers the potential to obtain low-resolution structural information from proteins and protein complexes. Identification of peptides connected by a cross-link provides direct evidence for the physical interaction of amino acid side chains, information that can be used for computational modeling purposes. Despite impressive advances that were made in recent years, the number of experimentally observed cross-links still falls below the number of possible contacts of cross-linkable side chains within the span of the cross-linker. Here, we propose two complementary experimental strategies to expand cross-linking data sets. First, enrichment of cross-linked peptides by size exclusion chromatography selects cross-linked peptides based on their higher molecular mass, thereby depleting the majority of unmodified peptides present in proteolytic digests of cross-linked samples. Second, we demonstrate that the use of proteases in addition to trypsin, such as Asp-N, can additionally boost the number of observable cross-linking sites. The benefits of both SEC enrichment and multiprotease digests are demonstrated on a set of model proteins and the improved workflow is applied to the characterization of the 20S proteasome from rabbit and Schizosaccharomyces pombe
Detection of TeV emission from the intriguing composite SNR G327.1-1.1
The shock wave of supernova remnants (SNRs) and the wind termination shock in
pulsar wind nebula (PWNe) are considered as prime candidates to accelerate the
bulk of Galactic cosmic ray (CR) ions and electrons. The SNRs hosting a PWN
(known as composite SNRs) provide excellent laboratories to test these
hypotheses. The SNR G327.1-1.1 belongs to this category and exhibits a shell
and a bright central PWN, both seen in radio and X-rays. Interestingly, the
radio observations of the PWN show an extended blob of emission and a curious
narrow finger structure pointing towards the offset compact X-ray source
indicating a possible fast moving pulsar in the SNR and/or an asymmetric
passage of the reverse shock. We report here on the observations, for a total
of 45 hours, of the SNR G327.1-1.1 with the H.E.S.S. telescope array which
resulted in the detection of TeV gamma-ray emission in spatial coincidence with
the PWN.Comment: Proceeding of the 32nd ICRC, August 11-18 2011, Beijing, Chin
Spin fluctuations with two-dimensional XY behavior in a frustrated S = 1/2 square-lattice ferromagnet
The spin dynamics of the layered square-lattice vanadate Pb2VO(PO4)2 is
investigated by electron spin resonance at various magnetic fields and at
temperatures above magnetic ordering. The linewidth divergence towards low
temperatures seems to agree with isotropic Heisenberg-type spin exchange
suggesting that the spin relaxation in this quasi-two dimensional compound is
governed by low-dimensional quantum fluctuations. However, a weak easy- plane
anisotropy of the g factor points to the presence of a planar XY type of
exchange. Indeed, we found that the linewidth divergence is described best by
XY-like spin fluctuations which requires a single parameter only. Therefore,
ESR-probed spin dynamics could establish Pb2VO(PO4)2 as the first frustrated
square lattice system with XY-inherent spin topological fluctuations.Comment: 5 pages, 3 figure
Current-voltage correlations in interferometers
We investigate correlations of current at contacts and voltage fluctuations
at voltage probes coupled to interferometers. The results are compared with
correlations of current and occupation number fluctuations at dephasing probes.
We use a quantum Langevin approach for the average quantities and their
fluctuations. For higher order correlations we develop a stochastic path
integral approach and find the generating functions of voltage or occupation
number fluctuations. We also derive a generating function for the joint
distribution of voltage or occupation number at the probe and current
fluctuations at a terminal of a conductor. For energy independent scattering we
found earlier that the generating function of current cumulants in
interferometers with a one-channel dephasing or voltage probe are identical.
Nevertheless, the distribution function for voltage and the distribution
function for occupation number fluctuations differ, the latter being broader
than that of former in all examples considered here.Comment: 23 pages, 10 figures, minor changes, additional appendix, added
reference
Central Acceptance Testing for Camera Technologies for CTA
The Cherenkov Telescope Array (CTA) is an international initiative to build
the next generation ground based very-high energy gamma-ray observatory. It
will consist of telescopes of three different sizes, employing several
different technologies for the cameras that detect the Cherenkov light from the
observed air showers. In order to ensure the compliance of each camera
technology with CTA requirements, CTA will perform central acceptance testing
of each camera technology. To assist with this, the Camera Test Facilities
(CTF) work package is developing a detailed test program covering the most
important performance, stability, and durability requirements, including
setting up the necessary equipment. Performance testing will include a wide
range of tests like signal amplitude, time resolution, dead-time determination,
trigger efficiency, performance testing under temperature and humidity
variations and several others. These tests can be performed on fully-integrated
cameras using a portable setup at the camera construction sites. In addition,
two different setups for performance tests on camera sub-units are being built,
which can provide early feedback for camera development. Stability and
durability tests will include the long-term functionality of movable parts,
water tightness of the camera housing, temperature and humidity cycling,
resistance to vibrations during transport or due to possible earthquakes,
UV-resistance of materials and several others. Some durability tests will need
to be contracted out because they will need dedicated equipment not currently
available within CTA. The planned test procedures and the current status of the
test facilities will be presented.Comment: 8 pages, 3 figures. In Proceedings of the 34th International Cosmic
Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions
at arXiv:1508.0589
Theory and simulations of rigid polyelectrolytes
We present theoretical and numerical studies on stiff, linear
polyelectrolytes within the framework of the cell model. We first review
analytical results obtained on a mean-field Poisson-Boltzmann level, and then
use molecular dynamics simulations to show, under which circumstances these
fail quantitatively and qualitatively. For the hexagonally packed nematic phase
of the polyelectrolytes we compute the osmotic coefficient as a function of
density. In the presence of multivalent counterions it can become negative,
leading to effective attractions. We show that this results from a reduced
contribution of the virial part to the pressure. We compute the osmotic
coefficient and ionic distribution functions from Poisson-Boltzmann theory with
and without a recently proposed correlation correction, and also simulation
results for the case of poly(para-phenylene) and compare it to recently
obtained experimental data on this stiff polyelectrolyte. We also investigate
ion-ion correlations in the strong coupling regime, and compare them to
predictions of the recently advocated Wigner crystal theories.Comment: 32 pages, 15 figures, proceedings of the ASTATPHYS-MEX-2001, to be
published in Mol. Phy
K+ and K- production in heavy-ion collisions at SIS-energies
The production and the propagation of K+ and of K- mesons in heavy-ion
collisions at beam energies of 1 to 2 AGeV have systematically been
investigated with the Kaon Spectrometer KaoS at the SIS at the GSI. The ratio
of the K+ production excitation function for Au+Au and for C+C reactions
increases with decreasing beam energy, which is expected for a soft nuclear
equation-of-state. At 1.5 AGeV a comprehensive study of the K+ and of the K-
emission as a function of the size of the collision system, of the collision
centrality, of the kaon energy, and of the polar emission angle has been
performed. The K-/K+ ratio is found to be nearly constant as a function of the
collision centrality. The spectral slopes and the polar emission patterns are
different for K- and for K+. These observations indicate that K+ mesons
decouple earlier from the reaction zone than K- mesons.Comment: invited talk given at the SQM2003 conference in Atlantic Beach, USA
(March 2003), to be published in Journal of Physics G, 10pages, 7 figure
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