365 research outputs found
Population polygons of tektite specific gravity for various localities in australasia
Comparison of specific gravity of tektites from australia, asia, texas, and czechoslovaki
Magnetism and d-wave superconductivity on the half-filled square lattice with frustration
The role of frustration and interaction strength on the half-filled Hubbard
model is studied on the square lattice with nearest and next-nearest neighbour
hoppings t and t' using the Variational Cluster Approximation (VCA). At
half-filling, we find two phases with long-range antiferromagnetic (AF) order:
the usual Neel phase, stable at small frustration t'/t, and the so-called
collinear (or super-antiferromagnet) phase with ordering wave-vector
or , stable for large frustration. These are separated by a phase with
no detectable long-range magnetic order. We also find the d-wave
superconducting (SC) phase (), which is favoured by frustration if
it is not too large. Intriguingly, there is a broad region of coexistence where
both AF and SC order parameters have non-zero values. In addition, the physics
of the metal-insulator transition in the normal state is analyzed. The results
obtained with the help of the VCA method are compared with the large-U
expansion of the Hubbard model and known results for the frustrated J1-J2
Heisenberg model. These results are relevant for pressure studies of undoped
parents of the high-temperature superconductors: we predict that an insulator
to d-wave SC transition may appear under pressure.Comment: 12 pages, 10 figure
Aluminum depletion induced by complex co-segregation of carbon and boron in a {\Sigma} 5 [3 1 0] bcc-iron grain boundary
The local variation of grain boundary atomic structure and chemistry caused
by segregation of impurities influences the macroscopic properties of
poylcrystalline materials. Here, the effect of co-segregation of carbon and
boron on the depletion of aluminum at a tilt
grain boundary in a Fe-Al bicrystal was studied by combining
atomic resolution scanning transmission electron microscopy, atom probe
tomography and density functional theory calculations. The atomic grain
boundary structural units mostly resemble kite-type motifs and the structure
appears disrupted by atomic scale defects. Atom probe tomography reveals that
carbon and boron impurities are co-segregating to the grain boundary reaching
levels of >1.5 at.\%, whereas aluminum is locally depleted by approx. 2~at.\%.
First-principles calculations indicate that carbon and boron exhibit the
strongest segregation tendency and their repulsive interaction with aluminum
promotes its depletion from the grain boundary. It is also predicted that
substitutional segregation of boron atoms may contribute to local distortions
of the kite-type structural units. These results suggest that the
co-segregation and interaction of interstitial impurities with substitutional
solutes strongly influences grain boundary composition and with this the
properties of the interface.Comment: 26 pages, 10 Figures, 1 Tabl
A petri nets based design of cognitive radios using distributed signal processing
AbstractReconfigurability for transceivers for wireless access networks like Bluetooth, WiMAX and W-LAN will become increasingly important. An appropriately flexible and reliable software architecture, allowing the concurrent processing of different controlling tasks for wireless terminals will hence be an important asset. Already during the 1980s reconfigurable receivers were developed for radio intelligence in the short wave range and the concept of software radio (SR) was born. A software defined radio (SDR) is a practical version of an SR: The received signals are sampled after a suitable band selection filter, usually in the base band or a low intermediate frequency band. The signal processing in both SR and SDR requires a considerable amount of concurrent processes. Since Petri nets (PNs) are both simple and strong tools for the description and the design of such concurrent processes, it is recommendable to deploy them for SDR. SDRs have paved the way towards cognitive radios (CRs), which are based on SDRs that additionally sense their environments, track changes, and react upon their findings. A CR is an autonomous unit in a communications environment that frequently exchanges information with the networks it is able to access as well as with other CRs. In this communication, the authors will introduce a realization concept for a CR which forms the basis of a hardware/firmware demonstrator developed by the authors. This demonstrator makes use of a digital signal processor (DSP) which forms the core of the design and flexibly programmable hardware accelerators based on field programmable gate arrays (FPGAs). The authors will describe the solution also in view of the recent developments of IEEE 802.2
Interstitial segregation has the potential to mitigate liquid metal embrittlement in iron
The embrittlement of metallic alloys by liquid metals leads to catastrophic
material failure and severely impacts their structural integrity. The weakening
of grain boundaries by the ingress of liquid metal and preceding segregation in
the solid are thought to promote early fracture. However, the potential of
balancing between the segregation of cohesion-enhancing interstitial solutes
and embrittling elements inducing grain boundary decohesion is not understood.
Here, we unveil the mechanisms of how boron segregation mitigates the
detrimental effects of the prime embrittler, zinc, in a
tilt grain boundary in Fe ( Al). Zinc forms nanoscale
segregation patterns inducing structurally and compositionally complex grain
boundary states. Ab-initio simulations reveal that boron hinders zinc
segregation and compensates for the zinc induced loss in grain boundary
cohesion. Our work sheds new light on how interstitial solutes intimately
modify grain boundaries, thereby opening pathways to use them as dopants for
preventing disastrous material failure.Comment: 29 pages, 6 figures in the main text and 10 figures in the
supplementar
Intravenous iron increases labile serum iron but does not impair forearm blood flow reactivity in dialysis patients
Intravenous iron increases labile serum iron but does not impair forearm blood flow reactivity in dialysis patients.BackgroundThere are concerns about adverse vascular effects of intravenous iron by inducing oxidative stress. We therefore examined the effect of a single high dose of intravenous iron on endothelial function and biochemical markers of iron homeostasis.MethodsIn a randomized, placebo-controlled, double-blind, parallel-group study, forearm blood flow (FBF) was assessed by strain-gauge plethysmography in 38 peritoneal dialysis patients before and after a single intravenous infusion of 300mg iron sucrose.ResultsIron infusion increased total (Δ 601 μg/100mL, CI 507, 696) and non-transferrin-bound iron (Δ 237.2 μmol/L, CI 173.6, 300.8) approximately 10-fold, as well as redox-active iron nearly five-fold (Δ 0.76 μmol/L, CI 0.54, 0.98). After iron infusion basal FBF was 59% higher than after placebo. FBF response to acetylcholine before and after iron infusion was 263 ± 32% and 310 ± 33%, corresponding to 304 ± 43% and 373 ± 29% in the placebo group, respectively. Before and after iron or placebo infusion, glyceryl-trinitrate increased resting FBF to 232 ± 22% and 258 ± 21% in the iron group, and to 234 ± 18% and 270 ± 30% in the placebo group. L-N-monomethyl-arginine decreased FBF to 70 ± 4% and 72 ± 3% before and after iron, and to 74 ± 4% and 73 ± 4% before and after placebo infusions, respectively. Despite higher basal FBF after iron infusion, absolute and relative FBF changes in response to vasoactive substances were not significantly different between iron and placebo groups.ConclusionOur data suggest that 300mg intravenous iron sucrose has a vasodilatory effect, but does not impair vascular reactivity in dialysis patients, despite a significant increase in non-transferrin-bound and redox-active iron
Pressure Dependence of Born Effective Charges, Dielectric Constant and Lattice Dynamics in SiC
The pressure dependence of the Born effective charge, dielectric constant and
zone-center LO and TO phonons have been determined for -SiC by a linear
response method based on the linearized augmented plane wave calculations
within the local density approximation. The Born effective charges are found to
increase nearly linearly with decreasing volume down to the smallest volume
studied, , corresponding to a pressure of about 0.8 Mbar. This
seems to be in contradiction with the conclusion of the turnover behavior
recently reported by Liu and Vohra [Phys.\ Rev.\ Lett.\ {\bf 72}, 4105 (1994)]
for -SiC. Reanalyzing their procedure to extract the pressure dependence of
the Born effective charges, we suggest that the turnover behavior they obtained
is due to approximations in the assumed pressure dependence of the dielectric
constant , the use of a singular set of experimental data
for the equation of state, and the uncertainty in measured phonon frequencies,
especially at high pressure.Comment: 25 pages, revtex, 5 postscript figures appended, to be published in
Phys. Rev.
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