576 research outputs found
Half-metallic diluted antiferromagnetic semiconductors
The possibility of half-metallic antiferromagnetism, a special case of
ferrimagnetism with a compensated magnetization, in the diluted magnetic
semiconductors is highlighted on the basis of the first principles electronic
structure calculation. As typical examples, the electrical and magnetic
properties of II-VI compound semiconductors doped with 3d transition metal ion
pairs--(V, Co) and (Fe, Cr)--are discussed
Auctions with Endogenous Price Ceiling:Theoretical and Experimental Results
This paper analyzes an auction mechanism that excludes overoptimistic bidders inspired by the rules of the procurement auctions adopted by several Japanese local governments. Our theoretical and experimental results suggest that the endogenous exclusion rule reduces the probability of suffering a monetary loss induced by winning the auction, and also mitigates the problem of the winnerfs curse in the laboratory. However, this protection comes at the price of a lower revenue for the seller.
Coordination Dependence of Hyperfine Fields of 5sp Impurities on Ni Surfaces
We present first-principles calculations of the magnetic hyperfine fields H
of 5sp impurities on the (001), (111), and (110) surfaces of Ni. We examine the
dependence of H on the coordination number by placing the impurity in the
surfaces, on top of them at the adatom positions, and in the bulk. We find a
strong coordination dependence of H, different and characteristic for each
impurity. The behavior is explained in terms of the on-site s-p hybridization
as the symmetry is reduced at the surface. Our results are in agreement with
recent experimental findings.Comment: 4 pages, 3 figure
Sirt1 expression is associated with CD31 expression in blood cells from patients with chronic obstructive pulmonary disease
Background: Cigarette smoke induced oxidative stress has been shown to reduce silent information regulator 1 (Sirt1) levels in lung tissue from smokers and patients with COPD patients. Sirt1 is known to inhibit endothelial senescence and may play a protective role in vascular cells. Endothelial progenitor cells (EPCs) are mobilized into circulation under various pathophysiological conditions, and are thought to play an important role in tissue repair in chronic obstructive lung disease (COPD). Therefore, Sirt1 and EPC-associated mRNAs were measured in blood samples from patients with COPD and from cultured CD34+ progenitor cells to examine whether these genes are associated with COPD development. Methods: This study included 358 patients with a smoking history of more than 10 pack-years. RNA was extracted from blood samples and from CD34+ progenitor cells treated with cigarette smoke extract (CSE), followed by assessment of CD31, CD34, Sirt1 mRNA, miR-34a, and miR-126-3p expression by real-time RT-PCR. Results: The expression of CD31, CD34, Sirt1 mRNAs, and miR-126-3p decreased and that of miR-34a increased in moderate COPD compared with that in control smokers. However, no significant differences in these genes were observed in blood cells from patients with severe COPD compared with those in control smokers. CSE significantly decreased Sirt1 and increased miR-34a expression in cultured progenitor cells. Conclusion: Sirt1 expression in blood cells from patients with COPD could be a biomarker for disease stability in patients with moderate COPD. MiR-34a may participate in apoptosis and/or senescence of EPCs in smokers. Decreased expression of CD31, CD34, and miR-126-3p potentially represents decreased numbers of EPCs in blood cell from patients with COPD
The local magnetic moments and hyperfine magnetic fields in disordered metal-metalloid alloys
The local magnetic moments and hyperfine magnetic fields (HFF) in the ordered
alloys Fe_{15}Sn and Fe_{15}Si are calculated with the first-principles
full-potential linear augmented plane wave (FP LAPW) method. The results are
compared with the experimental data on Fe-M (M=Si, Sn) disordered alloys at
small metalloid concentration. The relaxation of the lattice around the
impurity and its influence on the quantities under consideration are studied.
The mechanism of the local magnetic moment formation is described. It is proved
that the main distinction between these alloys is connected with the different
lattice parameters. Three contributions to the HFF are discussed: the
contributions of the core and valence electron polarization to the
Fermi-contact part, and the contibution from the orbital magnetic moment.Comment: 3 pages, 3 figures, submitted to Phys. Rev.
Disorder effects in diluted ferromagnetic semiconductors
Carrier induced ferromagnetism in diluted III-V semi-conductor is analyzed
within a two step approach. First, within a single site CPA formalism, we
calculate the element resolved averaged Green's function of the itinerant
carrier. Then using a generalized RKKY formula we evaluate the Mn-Mn long-range
exchange integrals and the Curie temperature as a function of the exchange
parameter, magnetic impurity concentration and carrier density. The effect of
the disorder (impurity scattering) appears to play a crucial role. The standard
RKKY calculation (no scattering processes), strongly underestimate the Curie
temperature and is inappropriate to describe magnetism in diluted magnetic
semi-conductors. It is also shown that an antiferromagnetic exchange favors
higher Curie temperature.Comment: tex file + 4 .eps figures are included. submited to PR
Optical Conductivity of Ferromagnetic Semiconductors
The dynamical mean field method is used to calculate the frequency and
temperature dependent conductivity of dilute magnetic semiconductors.
Characteristic qualitative features are found distinguishing weak,
intermediate, and strong carrier-spin coupling and allowing quantitative
determination of important parameters defining the underlying ferromagnetic
mechanism
Transition temperature of ferromagnetic semiconductors: a dynamical mean field study
We formulate a theory of doped magnetic semiconductors such as
GaMnAs which have attracted recent attention for their possible use
in spintronic applications. We solve the theory in the dynamical mean field
approximation to find the magnetic transition temperature as a function
of magnetic coupling strength and carrier density . We find that
is determined by a subtle interplay between carrier density and magnetic
coupling.Comment: 4 pages, 4 figure
Carrier induced ferromagnetism in diluted magnetic semi-conductors
We present a theory for carrier induced ferromagnetism in diluted magnetic
semi-conductor (DMS). Our approach treats on equal footing quantum fluctuations
within the RPA approximation and disorder within CPA. This method allows for
the calculation of , magnetization and magnon spectrum as a function of
hole, impurity concentration and temperature. It is shown that, sufficiently
close to , and within our decoupling scheme (Tyablicov type) the CPA for
the itinerant electron gas reduces to the Virtual Crystal Approximation. This
allows, in the low impurity concentration and low density of carriers to
provide analytical expression for . For illustration, we consider the case
of and compare our results with available experimental data.Comment: 5 figures included. to appear in Phys. Rev. B (brief report
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