664 research outputs found
First-principles investigation of magnetism and electronic structures of substitutional transition-metal impurities in bcc Fe
The magnetic and electronic structures of impurity atoms from Sc to Zn
in ferromagnetic body-centered cubic iron are investigated using the
all-electron full-potential linearized augmented plane-wave method based on the
generalized gradient approximation (GGA). We found that in general, the GGA
results are closer to the experimental values than those of the local spin
density approximation. The calculated formation enthalpy data indicate the
importance of a systematic study on the ternary Fe-C- systems rather than
the binary Fe- systems, in steel design. The lattice parameters are
optimized and the conditions for spin polarization at the impurity sites are
discussed in terms of the local Stoner model. Our calculations, which are
consistent with previous work, imply that the local spin-polarizations at Sc,
Ti, V, Cu, and Zn are induced by the host Fe atoms. The early transition-metal
atoms couple antiferromagnetically, while the late transition-metal atoms
couple ferromagnetically, to the host Fe atoms. The calculated total
magnetization () of bcc Fe is reduced by impurity elements from Sc to Cr as
a result of the antiferromagnetic interaction, with the opposite effect for
solutes which couple ferromagnetically. The changes in are attributed to
nearest neighbor interactions, mostly between the impurity and host atoms. The
atom averaged magnetic moment is shown to follow generally the well-known
Slater-Pauling curve, but our results do not follow the linearity of the
Slater-Pauling curve. We attribute this discrepancy to the weak ferromagnetic
nature of bcc Fe. The calculated Fermi contact hyperfine fields follow the
trend of the local magnetic moments. The effect of spin-orbit coupling is found
not to be significant although it comes into prominence at locations far from
the impurity sites.Comment: 26 pages, 11 figure
First-principles prediction of spin-density-reflection symmetry driven magnetic transition of CsCl-type FeSe
Based on results of density functional theory (DFT) calculations with the
local spin density approximation (LSDA) and the generalized gradient
approximation (GGA), we propose a new magnetic material, CsCl-type FeSe. The
calculations reveal the existence of ferromagnetic (FM) and antiferromagnetic
(AFM) states over a wide range of lattice constants. At 3.12\,{\AA} in the GGA,
the equilibrium state is found to be AFM with a local Fe magnetic moment of
. A metastable FM state with Fe and Se local magnetic
moments of and , respectively,
lies 171.7\,{meV} above the AFM state. Its equilibrium lattice constant is
\,{\%} smaller than that of the AFM state, implying that when the
system undergoes a phase transition from the AFM state to the FM one, the
transition is accompanied by volume contraction. Such an AFM-FM transition is
attributed to spin-density -reflection symmetry; the symmetry driven AFM-FM
transition is not altered by spin-orbit coupling. The relative stability of
different magnetic phases is discussed in terms of the local density of states.
We find that CsCl-type FeSe is mechanically stable, but the magnetic states are
expected to be brittle.Comment: LaTeX,16 pages, 6 figure
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Photoreversible interconversion of a phytochrome photosensory module in the crystalline state.
A major barrier to defining the structural intermediates that arise during the reversible photointerconversion of phytochromes between their biologically inactive and active states has been the lack of crystals that faithfully undergo this transition within the crystal lattice. Here, we describe a crystalline form of the cyclic GMP phosphodiesterases/adenylyl cyclase/FhlA (GAF) domain from the cyanobacteriochrome PixJ in Thermosynechococcus elongatus assembled with phycocyanobilin that permits reversible photoconversion between the blue light-absorbing Pb and green light-absorbing Pg states, as well as thermal reversion of Pg back to Pb. The X-ray crystallographic structure of Pb matches previous models, including autocatalytic conversion of phycocyanobilin to phycoviolobilin upon binding and its tandem thioether linkage to the GAF domain. Cryocrystallography at 150 K, which compared diffraction data from a single crystal as Pb or after irradiation with blue light, detected photoconversion product(s) based on Fobs - Fobs difference maps that were consistent with rotation of the bonds connecting pyrrole rings C and D. Further spectroscopic analyses showed that phycoviolobilin is susceptible to X-ray radiation damage, especially as Pg, during single-crystal X-ray diffraction analyses, which could complicate fine mapping of the various intermediate states. Fortunately, we found that PixJ crystals are amenable to serial femtosecond crystallography (SFX) analyses using X-ray free-electron lasers (XFELs). As proof of principle, we solved by room temperature SFX the GAF domain structure of Pb to 1.55-Ă
resolution, which was strongly congruent with synchrotron-based models. Analysis of these crystals by SFX should now enable structural characterization of the early events that drive phytochrome photoconversion
Ab initio prediction of pressure-induced structural phase transition of superconducting FeSe
External pressure driven phase transitions of FeSe are predicted using
\textit{ab initio} calculations. The calculations reveal that -FeSe
takes transitions to NiAs-type, MnP-type, and CsCl-type FeSe. Transitions from
NiAs-type to MnP-type and CsCl-type FeSe is also predicted. MnP-type FeSe is
also found to be able to transform to CsCl-type FeSe, which is easier from
-FeSe than the transition to MnP-type FeSe, but comparable to the
transition from NiAs-type FeSe. The calculated electronic structures show that
all phases of FeSe are metallic, but the ionic interaction between Fe-Se bonds
becomes stronger and the covalent interaction becomes weaker when the
structural phase transition occurs from -FeSe to the other phases of
FeSe. The experimentally observed decrease in of superconducting
-FeSe at high pressure may be due to a structural/magnetic instability,
which exists at high pressure. The results suggest us to increase the
of -FeSe if such phase transitions are frustrated by suitable methods.Comment: Accepted for publications in J. Phys.: Condens. Matter (2012
Assessment of Esophageal Reconstruction via Bioreactor Cultivation of a Synthetic Scaffold in a Canine Model
Objectives Using tissue-engineered materials for esophageal reconstruction is a technically challenging task in animals that requires bioreactor training to enhance cellular reactivity. There have been many attempts at esophageal tissue engineering, but the success rate has been limited due to difficulty in initial epithelialization in the special environment of peristalsis. The purpose of this study was to evaluate the potential of an artificial esophagus that can enhance the regeneration of esophageal mucosa and muscle through the optimal combination of a double-layered polymeric scaffold and a custom-designed mesenchymal stem cell-based bioreactor system in a canine model. Methods We fabricated a novel double-layered scaffold as a tissue-engineered esophagus using an electrospinning technique. Prior to transplantation, human-derived mesenchymal stem cells were seeded into the lumen of the scaffold, and bioreactor cultivation was performed to enhance cellular reactivity. After 3 days of cultivation using the bioreactor system, tissue-engineered artificial esophagus was transplanted into a partial esophageal defect (5Ă3 cm-long resection) in a canine model. Results Scanning electron microscopy (SEM) showed that the electrospun fibers in a tubular scaffold were randomly and circumferentially located toward the inner and outer surfaces. Complete recovery of the esophageal mucosa was confirmed by endoscopic analysis and SEM. Esophagogastroduodenoscopy and computed tomography also showed that there were no signs of leakage or stricture and that there was a normal lumen with complete epithelialization. Significant regeneration of the mucosal layer was observed by keratin-5 immunostaining. Alpha-smooth muscle actin immunostaining showed significantly greater esophageal muscle regeneration at 12 months than at 6 months. Conclusion Custom-designed bioreactor cultured electrospun polyurethane scaffolds can be a promising approach for esophageal tissue engineering
Association of Angiotensin-Converting Enzyme and Angiotensinogen Gene Polymorphisms with Preeclampsia
We tested the hypothesis that angiotensin-converting enzyme (ACE) and angiotensinogen gene polymorphism influence the incidence, development and outcome of preeclampsia. Subjects were recruited from 90 Korean patients with preeclampsia during pregnancy and 98 age-matched controls. After isolation of DNA, polymerase chain reactions (PCR) were carried out to detect polymorphism of the ACE and angiotensinogen. M235T and T174M genotypes of angiotensinogen were determined by digestion with restriction enzyme endonuclease Tth 111-I and NCo I, respectively. The frequency of DD genotype was significantly greater in preeclampsia (0.36) than in controls (0.14) (p<0.05). The frequency of D allele was 0.55 in preeclampsia and 0.40 in controls (p<0.05). There were no differences in the onset of preeclampsia and pregnancy outcomes according to the ACE genotypes. There was no difference in the frequency of a allele of angiotensinogen M235T between the groups (0.79:0.78 in preeclampsia : controls). The frequency of T allele of angiotensinogen T174M gene was slightly increased, but not significantly, in preeclampsia (0.11) than in controls (0.07). In a multivariate analysis, only ACE genotype was associated with the development of preeclampsia (ÎČ=0.27, p=0.05). In conclusion, a molecular variant of ACE, but not angiotensinogen, gene is associated with preeclampsia in Korean women
Measurement of the cross-section of high transverse momentum vector bosons reconstructed as single jets and studies of jet substructure in pp collisions at âs = 7 TeV with the ATLAS detector
This paper presents a measurement of the cross-section for high transverse momentum W and Z bosons produced in pp collisions and decaying to all-hadronic final states. The data used in the analysis were recorded by the ATLAS detector at the CERN Large Hadron Collider at a centre-of-mass energy of âs = 7 TeV;{\rm Te}{\rm V}4.6\;{\rm f}{{{\rm b}}^{-1}}{{p}_{{\rm T}}}\gt 320\;{\rm Ge}{\rm V}|\eta |\lt 1.9{{\sigma }_{W+Z}}=8.5\pm 1.7$ pb and is compared to next-to-leading-order calculations. The selected events are further used to study jet grooming techniques
Observation of associated near-side and away-side long-range correlations in âsNN=5.02ââTeV proton-lead collisions with the ATLAS detector
Two-particle correlations in relative azimuthal angle (ÎÏ) and pseudorapidity (Îη) are measured in âsNN=5.02ââTeV p+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1ââÎŒb-1 of data as a function of transverse momentum (pT) and the transverse energy (ÎŁETPb) summed over 3.1<η<4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2<|Îη|<5) ânear-sideâ (ÎÏâŒ0) correlation that grows rapidly with increasing ÎŁETPb. A long-range âaway-sideâ (ÎÏâŒÏ) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small ÎŁETPb, is found to match the near-side correlation in magnitude, shape (in Îη and ÎÏ) and ÎŁETPb dependence. The resultant ÎÏ correlation is approximately symmetric about Ï/2, and is consistent with a dominant cosâĄ2ÎÏ modulation for all ÎŁETPb ranges and particle pT
Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at â s = 8 TeV with the ATLAS detector
Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fbâ1 of â s = 8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT > 120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between Emiss T > 150 GeV and Emiss T > 700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presente
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