1,141 research outputs found
Suppression of spin-density-wave transition and emergence of ferromagnetic ordering of Eu moments in EuFeNiAs
We present a systematic study on the physical properties of
EuFeNiAs (0\emph{x}0.2) by electrical
resistivity, magnetic susceptibility and thermopower measurements. The undoped
compound EuFeAs undergoes a spin-density-wave (SDW) transition
associated with Fe moments at 195 K, followed by antiferromagnetic (AFM)
ordering of Eu moments at 20 K. Ni doping at the Fe site simultaneously
suppresses the SDW transition and AFM ordering of Eu moments. For
0.06, the magnetic ordering of Eu moments evolves from
antiferromagnetic to ferromagnetic (FM). The SDW transition is completely
suppressed for 0.16, however, no superconducting transition was observed
down to 2 K. The possible origins of the AFM-to-FM transition and the absence
of superconductivity in EuFeNiAs system are discussed.Comment: 5 pages, 5 figures, accepted for publication in PR
Superconductivity in SrNi2As2 Single Crystals
The electrical resistivity \rho(T) and heat capacity C(T) on single crystals
of SrNi2As2 and EuNi2As2 are reported. While there is no evidence for a
structural transition in either compound, SrNi2As2 is found to be a bulk
superconductor at T_c=0.62 K with a Sommerfeld coefficient of \gamma= 8.7
mJ/mol K^2 and a small upper critical field H_{c2} \sim 200 Oe. No
superconductivity was found in EuNi2As2 above 0.4 K, but anomalies in \rho and
C reveal that magnetic order associated with the Eu^{2+} magnetic moments
occurs at T_m = 14 K.Comment: 8 pages, 5 figure
Spin density wave anomaly at 140 K in the ternary iron arsenide BaFe2As2
The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type
structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to
LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is
a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase
transition at 140 K, accompanied by strong anomalies in the specific heat,
electrical resistance and magnetic susceptibility. In the course of this
transition, the space group symmetry changes from tetragonal (I4/mmm) to
orthorhombic (Fmmm). 57Fe Moessbauer spectroscopy experiments show a single
signal at room temperature and full hyperfine field splitting below the phase
transition temperature (5.2 T at 77 K). Our results suggest that BaFe2As2 can
serve as a new parent compound for oxygen-free iron arsenide superconductors.Comment: 4 pages, 6 figures, submitted to PR
PNR in 2011: Recalling Ten Years of Transatlantic Cooperation in PNR Information
In Fall 2011, U.S. and EU negotiators agreed on new parameters for the collection, processing, use, storage and crossborder transfer of Passenger Name Record (PNR) data. 2011 also marks the tenth anniversary of the September 11, 2001 terrorist attacks on the World Trade Center in New York City and the Pentagon in Washington D.C., which provides the historic reason for the cooperation in this area. These two events thus provide a timely basis and background against which to review the ten year history of the cooperation between the U.S. and the EU in PNR information management.
This article maps the evolution of collaboration between the U.S. and the EU with respect to the PNR program by presenting the major dimensions involved. Moreover, it provides a comprehensive framework with a particular focus on the constant struggle for a consistent EU policy as well as the creation of the U.S.-EU Agreements in 2004 and 2007. It furthermore sketches major legal and political developments that have most likely had a significant impact on the negotiations and are, as a consequence, reflected in the concrete design of the new PNR Agreement. All this leads the author to the conclusion that—as thoroughly as it may have been designed and as complete as it may seem—the new PNR Agreement will not be the end of the transatlantic PNR saga, but rather the beginning of another intriguing chapter
PNR in 2011: Recalling Ten Years of Transatlantic Cooperation in PNR Information
In Fall 2011, U.S. and EU negotiators agreed on new parameters for the collection, processing, use, storage and crossborder transfer of Passenger Name Record (PNR) data. 2011 also marks the tenth anniversary of the September 11, 2001 terrorist attacks on the World Trade Center in New York City and the Pentagon in Washington D.C., which provides the historic reason for the cooperation in this area. These two events thus provide a timely basis and background against which to review the ten year history of the cooperation between the U.S. and the EU in PNR information management.
This article maps the evolution of collaboration between the U.S. and the EU with respect to the PNR program by presenting the major dimensions involved. Moreover, it provides a comprehensive framework with a particular focus on the constant struggle for a consistent EU policy as well as the creation of the U.S.-EU Agreements in 2004 and 2007. It furthermore sketches major legal and political developments that have most likely had a significant impact on the negotiations and are, as a consequence, reflected in the concrete design of the new PNR Agreement. All this leads the author to the conclusion that—as thoroughly as it may have been designed and as complete as it may seem—the new PNR Agreement will not be the end of the transatlantic PNR saga, but rather the beginning of another intriguing chapter
Non-invasive quantification of exercise-induced changes in regional left ventricular function in normals and patients with one vessel coronary artery disease using radionuclide ventriculography
To quantitate changes in regional left ventricular function induced by ischemia or scar, rest and exercise equilibrium radionuclide studies of 26 patients with one vessel coronary artery disease and 12 normal individuals were analysed with a new method for regional ejection fraction determination. A computer algorithm provided observer-independent segmental analysis from a centre of gravity of the left ventricular activity at end-diastole (left anterior oblique projection). Special segments were assigned for anteroseptal, inferoapical and posterolateral areas corresponding to the three main coronary arteries. Reproducibility using an unchanged camera positioning was excellent even for 2 min acquisition studies (r=0.93) and still good after repositioning (r=0.80 to 0.87). In normal areas, regional ejection fraction increased or showed no change during exercise. In contrast, it decreased significantly in regions supplied by stenosed coronary arteries (ischemia) and remained depressed in scar zones. The method proved to be valid for regional changes induced by left anterior descending and left circumflex coronary obstructions, but less for right coronary artery lesions. Global ejection fraction reflected a sum of all regional changes implying that regional analysis should be more sensitive in detecting coronary artery diseas
Pressure induced superconductivity in CaFeAs
CaFeAs has been found to be exceptionally sensitive to the
application of hydrostatic pressure and superconductivity has been found to
exist in a narrow pressure region that appears to be at the interface between
two different phase transitions. The pressure - temperature () phase
diagram of CaFeAs reveals that this stoichiometric, highly ordered,
compound can be easily tuned to reveal all the salient features associated with
FeAs-based superconductivity without introducing any disorder. Whereas at
ambient pressure CaFeAs does not superconduct for K and
manifests a first order structural phase transition near K, the
application of kbar hydrostatic pressure fully suppresses the
resistive signature of the structural phase transition and instead
superconductivity is detected for K. For kbar a different
transition is detected, one associated with a clear reduction in resistivity
and for kbar superconductivity is no longer detected. This higher
pressure transition temperature increases rapidly with increasing pressure,
exceeding 300 K by kbar. The low temperature, superconducting dome
is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This
superconducting phase appears to exist when the low pressure transition is
suppressed sufficiently, but before the high pressure transition has reduced
the resistivity, and possibly the associated fluctuations, too dramatically
Strong coupling between Eu2+ spins and Fe2As2 layers in EuFe1.9Co0.1As2 observed with NMR
A combination of x-ray diffraction, magnetization, and 75As nuclear magnetic
resonance (NMR) experiments were performed on single-crystal EuFe1.9Co0.1As2.
The strength of the hyperfine interaction between the 75As nuclei and the
Eu^(2+) 4f states suggests a strong coupling between the Eu^(2+) moments and
the Fe1.9Co0.1As2 layers. Such a strong interlayer coupling may be due to an
indirect exchange interaction between the localized Eu^(2+) 4f moments,
mediated by the Fe 3d conduction electrons. Magnetic susceptibility as well as
75As-NMR measurements reveal a decrease of the SDW transition temperature to
T_SDW = 120 K as a result of Co doping. A change of the slope in the
temperature dependence of the NMR frequency of the 75As lower-satellite line
was observed at 225 K. At the same temperature also a change of the satellite
line shape was found. These changes of the NMR spectra may be caused by the
formation of a nematic phase below 225 K in EuFe1.9Co0.1As2.Comment: 8 pages, 7 figure
Strong coupling between magnetic and structural order parameters in SrFe2As2
X-ray and Neutron diffraction as well as muon spin relaxation and M\"ossbauer
experiments performed on SrFeAs polycrystalls confirm a sharp first
order transition at ,K corresponding to an orthorhombic phase
distortion and to a columnar antiferromagnetic Fe ordering with a propagation
vector (1,0,1), and a larger distortion and larger size of the ordered moment
than reported for BaFeAs. The structural and the magnetic order
parameters present an remarkable similarity in their temperature dependence
from down to low temperatures, showing that both phenomena are intimately
connected. Accordingly, the size of the ordered Fe moments scale with the
lattice distortion when going from SrFeAs to BaFeAs.
Full-potential band structure calculations confirm that the columnar magnetic
order and the orthorhombic lattice distortion are intrinsically tied to each
other.Comment: 10 pages, 4 figure
Prolonged myocardial stunning after thrombolysis: can left ventricular function be assessed definitely at hospital discharge?
To assess whether myocardial dysfunction after acute reperfusion (‘stunning') may show delayed recovery, 33 patients of the European Cooperative Study (rtPA vs placebo) had radionuclide angiocardiography on day 9 and after 3-6 months. Sixteen patients (13 inferior, three anterior infarcts) had a normal left ventricular ejection fraction (LVEF) which remained unchanged (55.4 vs 53.9%). In contrast, LVEF of 17 patients (10 inferior, seven anterior infarcts) with depressed values on day 9 improved during follow-up from 38.8 to 45.2% (P<0.01). Improvement was only observed in patients with early reperfusion defined a priori as peak creatine kinase valuè ≤ 15 h of pain onset (from 40.9 to 49.3%; P<0.05) in contrast to patients without reperfusion (from 34.0 to 35.2%; ns). Accordingly, LVEF increased in patients with open infarct-related arteries at hospital discharge (n = 8; P = 0.053) but not with persistent occlusion (n = 7; P = 0.11). Thus, a depressed LVEF observed 9 days after reperfusion may show delayed recovery due to prolonged stunning. Therefore, after thrombolysis, left ventricular function may not be evaluated definitively at hospital discharge; results of such studies should be interpreted with cautio
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