351 research outputs found
Effects of Capping on the (Ga,Mn)As Magnetic Depth Profile
Annealing can increase the Curie temperature and net magnetization in
uncapped (Ga,Mn)As films, effects that are suppressed when the films are capped
with GaAs. Previous polarized neutron reflectometry (PNR) studies of uncapped
(Ga,Mn)As revealed a pronounced magnetization gradient that was reduced after
annealing. We have extended this study to (Ga,Mn)As capped with GaAs. We
observe no increase in Curie temperature or net magnetization upon annealing.
Furthermore, PNR measurements indicate that annealing produces minimal
differences in the depth-dependent magnetization, as both as-grown and annealed
films feature a significant magnetization gradient. These results suggest that
the GaAs cap inhibits redistribution of interstitial Mn impurities during
annealing.Comment: 12 pages, 3 figures, submitted to Applied Physics Letter
Modification of titanium and titanium dioxide surfaces by ion implantation: combined XPS and DFT study
The results of XPS measurements (core levels and valence bands) of P+, Ca+,
P+Ca+ and Ca+P+ ion implanted (E=30 keV, D=1x1017 cm-2) commercially pure
titanium (cp-Ti) and first-principles density functional theory (DFT)
calculations demonstrates formation of various structural defects in titanium
dioxide films formed on the surface of implanted materials. We have found that
for double implantation (Ti:P+,Ca+ and Ti:Ca+,P+) the outermost surface layer
formed mainly by Ca and P, respectively, i.e. the implantation sequence is very
important. The DFT calculations show that under P+ and Ca+P+ ion implantation
the formation energies for both cation (P-Ti) and anion (P-O) substitutions are
comparable which can induce the creation of [PO4]3- and Ti-P species. For Ca+
and P+Ca+-ion implantation the calculated formation energies correspond to
Ca2+-Ti4+ cation substitution. This conclusion is in agreement with XPS Ca 2p
and Ti 2p core levels and valence band measurements and DFT calculations of
electronic structure of related compounds. The conversion of implanted ions to
Ca2+ and [PO4]3- species provides a good biocompatibility of cp-Ti for further
formation of hydroxyapatite.Comment: 18 pages, 6 figures, 3 tables, accepted phys. stat. solidi (b
Definitive Evidence of Interlayer Coupling Between (Ga,Mn)As Layers Separated by a Nonmagnetic Spacer
We have used polarized neutron reflectometry to study the structural and
magnetic properties of the individual layers in a series of
(Al,Be,Ga)As/(Ga,Mn)As/GaAs/(Ga,Mn)As multilayer samples. Structurally, we
observe that the samples are virtually identical except for the GaAs spacer
thickness (which varies from 3-12 nm), and confirm that the spacers contain
little or no Mn. Magnetically, we observe that for the sample with the thickest
spacer layer, modulation doping by the(Al,Be,Ga)As results in (Ga,Mn)As layers
with very different temperature dependent magnetizations. However, as the
spacer layer thickness is reduced, the temperature dependent magnetizations of
the top an bottom (Ga,Mn)As layers become progressively more similar - a trend
we find to be independent of the crystallographic direction along which spins
are magnetized. These results definitively show that (Ga,Mn)As layers can
couple across a non-magnetic spacer, and that such coupling depends on spacer
thickness.Comment: Submitted to Physical Review
CuO-CeO2 nanocomposite catalysts produced by mechanochemical synthesis
Mechanochemical synthesis based on ball-milling of individual oxides was applied as a one-step preparation technique for CuO-CeO2 catalyst for preferential CO oxidation in H2 excess. The mechanical energy dose transferred to the original powder mixture determines both the catalyst composition and activity. It is found that after 90 min of milling (corresponding to a dose of 372 kJ mol-1), a mixture of 10 wt.% CuO-CeO2 powder exhibits a CO conversion of 97% at 423 K. Four active oxygen states, which are not observed in case of pure CeO2, were detected in the nanocomposite lattice and attributed to the presence of Cu in surface sites as well as in subsurface bulk sites of CeO2, in nearest neighbor and next nearest neighbor positions. Correspondingly, oxidation of CO to CO2 was found to occur in a two-stage process with Tmax = 395/460 K, and oxidation of H2 to H2O likewise in a four-stage process with Tmax = 426/448/468/516 K. The milled powder consists of CeO2 crystallites sized 8-10 nm agglomerated to somewhat larger aggregates, with CuO dispersed on the surface of the CeO2 crystallites, and to a lesser extent present as Cu2O. © 2019 Author(s).This work was partially supported by Russian Foundation for Basic Research [Projects n.n. 16-03-00330a and 16-03-00178a] in theoretical studies and part of experimental research and by FASO [program no. AAAA-A18-118012390374-3]. XPS measurements were supported by FASO (Theme “Electron”). The Alexander von Humboldt foundation is gratefully acknowledged for funding. We also would like to thank N. Berezkina for SEM measurements. We acknowledge support by the Open Access Publication Funds of the Göttingen University. Declarations of interest: none
Reorientation of Spin Density Waves in Cr(001) Films induced by Fe(001) Cap Layers
Proximity effects of 20 \AA thin Fe layers on the spin density waves (SDWs)
in epitaxial Cr(001) films are revealed by neutron scattering. Unlike in bulk
Cr we observe a SDW with its wave vector Q pointing along only one {100}
direction which depends dramatically on the film thickness t_{Cr}. For t_{Cr} <
250 \AA the SDW propagates out-of-plane with the spins in the film plane. For
t_{Cr} > 1000 \AA the SDW propagates in the film plane with the spins
out-of-plane perpendicular to the in-plane Fe moments. This reorientation
transition is explained by frustration effects in the antiferromagnetic
interaction between Fe and Cr across the Fe/Cr interface due to steps at the
interface.Comment: 4 pages (RevTeX), 3 figures (EPS
Continuous Spectrum of Automorphism Groups and the Infraparticle Problem
This paper presents a general framework for a refined spectral analysis of a
group of isometries acting on a Banach space, which extends the spectral theory
of Arveson. The concept of continuous Arveson spectrum is introduced and the
corresponding spectral subspace is defined. The absolutely continuous and
singular-continuous parts of this spectrum are specified. Conditions are given,
in terms of the transposed action of the group of isometries, which guarantee
that the pure-point and continuous subspaces span the entire Banach space. In
the case of a unitarily implemented group of automorphisms, acting on a
-algebra, relations between the continuous spectrum of the automorphisms
and the spectrum of the implementing group of unitaries are found. The group of
spacetime translation automorphisms in quantum field theory is analyzed in
detail. In particular, it is shown that the structure of its continuous
spectrum is relevant to the problem of existence of (infra-)particles in a
given theory.Comment: 31 pages, LaTeX. As appeared in Communications in Mathematical
Physic
Heat treatment of cold-sprayed C355 Al for repair: microstructure and mechanical properties
Cold gas dynamic spraying of commercially pure aluminum is widely used for dimensional repair in the aerospace sector as it is capable of producing oxide-free deposits of hundreds of micrometer thickness with strong bonding to the substrate, based on adhesive pull-off tests, and often with enhanced hardness compared to the powder prior to spraying. There is significant interest in extending this application to structural, load-bearing repairs. Particularly, in the case of high-strength aluminum alloys, cold spray deposits can exhibit high levels of porosity and microcracks, leading to mechanical properties that are inadequate for most load-bearing applications. Here, heat treatment was investigated as a potential means of improving the properties of cold-sprayed coatings from Al alloy C355. Coatings produced with process conditions of 500 °C and 60 bar were heat-treated at 175, 200, 225, 250 °C for 4 h in air, and the evolution of the microstructure and microhardness was analyzed. Heat treatment at 225 and 250 °C revealed a decreased porosity (~ 0.14% and 0.02%, respectively) with the former yielding slightly reduced hardness (105 versus 130 HV0.05 as-sprayed). Compressive residual stress levels were approximately halved at all depths into the coating after heat treatment, and tensile testing showed an improvement in ductility
Photon Production from a Quark--Gluon Plasma
In-medium interactions of a particle in a hot plasma are considered in the
framework of thermal field theory. The formalism to calculate gauge invariant
rates for photon and dilepton production from the medium is given. In the
application to a QED plasma, astrophysical consequences are pointed out. The
photon production rate from strongly interacting quarks in the quark--gluon
plasma, which might be formed in ultrarelativistic heavy ion collisions, is
calculated in the previously unaccessible regime of photon energies of the
order of the plasma temperature. For temperatures below the chiral phase
transition, an effective field theory incorporating dynamical chiral symmetry
breaking is employed, and perturbative QCD at higher temperatures. A smooth
transition between both regions is obtained. The relevance to the soft photon
problem and to high energy heavy ion experiments is discussed.Comment: Paper in ReVTeX. Figures and complete paper available via anonymous
ftp, ftp://tpri6c.gsi.de/pub/phenning/hq95ga
A new perspective of the structural complexity of HCMV-specific T-cell responses
Background: In studies exploring the effects of HCMV infection on immune system aging (‘immunosenescence’), after organ transplantation or in other settings, HCMV-specific T-cell responses are often assessed with respect to purportedly ‘immunodominant’ protein subunits. However, the response structure in terms of recognized antigens and response hierarchies (architecture) is not well understood and actual correlates of immune protection are not known.
Methods: We explored the distribution of T-cell response sizes and dominance hierarchies as well as response breadth in 33 HCMV responders with respect to >200 HCMV proteins.
Results: At the individual responder level HCMV-specific T-cell responses were generally arranged in clear dominance hierarchies; interestingly, the number of proteins recognized by an individual correlated closely with the size of their biggest response. Target-specificity varied considerably between donors and across hierarchy levels with the presence, size, and hierarchy position of responses to purportedly ‘immunodominant’ targets being unpredictable.
Conclusions: Predicting protective immunity based on isolated HCMV subunit-specific T-cell responses is questionable in light of the complex architecture of the overall response. Our findings have important implications for T-cell monitoring, intervention strategies, as well as the application of animal models to the understanding of human infection
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