13,832 research outputs found
Deuterium site occupancy and phase boundaries in ZrNiDx (0.87<=x<=3.0)
ZrNiDx samples with compositions between x=0.87 and x=3.0 were investigated by 2H magic-angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR), powder x-ray diffraction (XRD), neutron vibrational spectroscopy (NVS), and neutron powder diffraction (NPD). The rigid-lattice MAS-NMR spectrum for a ZrNiD0.88 sample in the triclinic beta phase shows a single phase with two well-resolved resonances at +11.5 and −1.7 ppm, indicating that two inequivalent D sites are occupied, as was observed previously in ZrNiD1.0. For ZrNiD0.88, the ratio of spectral intensities of the two lines is 1:0.76, indicating that the D site corresponding to the +11.5 ppm line has the lower site energy and is fully occupied. Similarly, the neutron vibrational spectra for ZrNiD0.88 clearly confirm that at least two sites are occupied. For ZrNiD1.0, XRD indicates that ~5% of the metal atoms are in the gamma phase, corresponding to an upper composition for the beta phase of x=0.90±0.04, consistent with the MAS-NMR and neutron vibrational spectra indicating that x=0.88 is single phase. The MAS-NMR and NVS of ZrNiD1.87 indicate a mixed-phase sample (beta+gamma) and clearly show that the two inequivalent sites observed at x=0.88 cannot be attributed to the sites normally occupied in the gamma phase. For ZrNiD2.75, NPD results indicate a gamma-phase boundary of x=2.86±0.03 at 300 K, increasing to 2.93±0.02 at 180 K and below, in general agreement with the phase boundary estimated from the NVS and MAS-NMR spectra of ZrNiD1.87. Rigid-lattice 2H MAS-NMR spectra of ZrNiD2.75 and ZrNiD2.99 show a ratio of spectral intensities of 1.8±0.1:1 and 2.1±0.1:1 (Zr3Ni:Zr3Ni2), respectively, indicating complete occupancy of the lower-energy Zr3Ni2 site, consistent with the NPD results. For each composition, the correlation time for deuterium hopping was determined at the temperature where resolved peaks in the MAS-NMR spectrum coalesce due to motion between inequivalent D sites. The measured correlation times are consistent with previously determined motional parameters for ZrNiHx
Characterization of the nitrogen split interstitial defect in wurtzite aluminum nitride using density functional theory
We carried out Heyd-Scuseria-Ernzerhof hybrid density functional theory plane
wave supercell calculations in wurtzite aluminum nitride in order to
characterize the geometry, formation energies, transition levels and hyperfine
tensors of the nitrogen split interstitial defect. The calculated hyperfine
tensors may provide useful fingerprint of this defect for electron paramagnetic
resonance measurement.Comment: 5 pages, 3 figure
Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach
We study spin and charge currents pumped by precessing magnetization of a
single ferromagnetic layer within F|I|N or F|I|F (F-ferromagnet; I-insulator;
N-normal-metal) multilayers of nanoscale thickness attached to two normal metal
electrodes with no applied bias voltage between them. Both simple
one-dimensional model, consisting of a single precessing spin and a potential
barrier as the "sample," and realistic three-dimensional devices are
investigated. In the rotating reference frame, where the magnetization appears
to be static, these junctions are mapped onto a four-terminal dc circuit whose
effectively half-metallic ferromagnetic electrodes are biased by the frequency
of microwave radiation driving magnetization precession at the
ferromagnetic resonance (FMR) conditions. We show that pumped spin current in
F|I|F junctions, diminished behind the tunnel barrier and increased in the
opposite direction, is filtered into charge current by the second layer to
generate dc pumping voltage of the order of V (at FMR frequency
GHz) in an open circuit. In F|I|N devices, several orders of
magnitude smaller charge current and the corresponding dc voltage appear
concomitantly with the pumped spin current due to barrier induced asymmetry in
the transmission coefficients connecting the four electrodes in the rotating
frame picture of pumping.Comment: 8 pages, 5 figure
Combined electrical transport and capacitance spectroscopy of a field effect transistor
We have measured both the current-voltage (-)
and capacitance-voltage (-) characteristics of a
field effect transistor. From the measured capacitance
we calculate the electron surface density and show that its gate voltage
dependence follows the theoretical prediction resulting from the
two-dimensional free electron model. This model allows us to fit the measured
- characteristics over the \emph{entire range} of
. Combining this experimental result with the measured
current-voltage characteristics, we determine the field effect mobility as a
function of gate voltage. We show that for our device this improved combined
approach yields significantly smaller values (more than a factor of 4) of the
electron mobility than the conventional analysis of the current-voltage
characteristics only.Comment: to appear in Applied Physics Letter
Decay of scalar turbulence revisited
We demonstrate that at long times the rate of passive scalar decay in a
turbulent, or simply chaotic, flow is dominated by regions (in real space or in
inverse space) where mixing is less efficient. We examine two situations. The
first is of a spatially homogeneous stationary turbulent flow with both viscous
and inertial scales present. It is shown that at large times scalar
fluctuations decay algebraically in time at all spatial scales (particularly in
the viscous range, where the velocity is smooth). The second example explains
chaotic stationary flow in a disk/pipe. The boundary region of the flow
controls the long-time decay, which is algebraic at some transient times, but
becomes exponential, with the decay rate dependent on the scalar diffusion
coefficient, at longer times.Comment: 4 pages, no figure
Structure of the silicon vacancy in 6H-SiC after annealing identified as the carbon vacancy–carbon antisite pair
We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S=1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisite–carbon vacancy pair (CSi-VC) in the doubly positive charge state can explain all experimental findings. We propose that the (CSi-VC) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy
Regression of tricuspid regurgitation after two-stage arterial switch operation for failing systemic ventricle after atrial inversion operation
AbstractThe cases of five patients with previous Senning ( n = 4) or Mustard ( n = 1) operations and failing systemic ventricles in whom banding of the pulmonary artery was performed as an interim step toward an arterial switch procedure are reported. The rise in the ratio of left to right mean systolic ventricular pressure, from 0.35 before operation to 0.90 during operation and 0.80 on the first postoperative day, caused a shift of the ventricular septum from a leftward to a midline or nearly midline position. This shift was associated with a reduction of tricuspid regurgitation. At a median interval of 5.1 months after pulmonary artery banding, the mean left ventricular posterior wall thickness had increased to 8.2 mm, versus 5 mm before operation, and the mean left ventricular myocardial mass index had increased to 90 gm/m2, versus 55.6 gm/m2 before operation. After the arterial switch operation, which was performed in four patients, the tricuspid regurgitation decreased to a trivial amount ( n = 1) or disappeared completely ( n = 3). (J THORAC CARDIOVASC SURG 1996;111:342-7
Initial State: Theory Status
I present a brief discussion of the different approaches to the study initial
state effects in heavy ion collisions in view of the recent results from Pb+Pb
and p+p collisions at the LHC.Comment: 8 pages, 6 figures. Contribution to the proceedings of the XXII
International Conference on Ultrarelativistic Nucleus-Nucleus Collisions,
QM2011. Annecy, France, 22-28 May 201
- …