255 research outputs found
Mesures de vies moyennes par effet Doppler
Des vies moyennes de niveaux nucléaires ont été déterminées par la méthode de l'effet Doppler. Les rayonnements gamma de désexcitation ont été détectés dans des compteurs Ge(Li). Des vies moyennes ont été obtenues pour les niveaux 0,953 MeV de 12B et 6,44 MeV de 14N
Dopant-dependent impact of Mn-site doping on the critical-state manganites: R0.6Sr0.4MnO3 (R=La, Nd, Sm, and Gd)
Versatile features of impurity doping effects on perovskite manganites,
SrMnO, have been investigated with varying the doing
species as well as the -dependent one-electron bandwidth. In
ferromagnetic-metallic manganites (=La, Nd, and Sm), a few percent of Fe
substitution dramatically decreases the ferromagnetic transition temperature,
leading to a spin glass insulating state with short-range charge-orbital
correlation. For each species, the phase diagram as a function of Fe
concentration is closely similar to that for SrMnO
obtained by decreasing the ionic radius of site, indicating that Fe doping
in the phase-competing region weakens the ferromagnetic double-exchange
interaction, relatively to the charge-orbital ordering instability. We have
also found a contrastive impact of Cr (or Ru) doping on a spin-glass insulating
manganite (=Gd). There, the impurity-induced ferromagnetic magnetization is
observed at low temperatures as a consequence of the collapse of the inherent
short-range charge-orbital ordering, while Fe doping plays only a minor role.
The observed opposite nature of impurity doping may be attributed to the
difference in magnitude of the antiferromagnetic interaction between the doped
ions.Comment: 7 pages, 6 figure
Fast 3D super-resolution ultrasound with adaptive weight-based beamforming
Objective: Super-resolution ultrasound (SRUS) imaging through localising and tracking sparse microbubbles has been shown to reveal microvascular structure and flow beyond the wave diffraction limit. Most SRUS studies use standard delay and sum (DAS) beamforming, where high side lobes and broad main lobes make isolation and localisation of densely distributed bubbles challenging, particularly in 3D due to the typically small aperture of matrix array probes. Method: This study aimed to improve 3D SRUS by implementing a new fast 3D coherence beamformer based on channel signal variance. Two additional fast coherence beamformers, that have been implemented in 2D were implemented in 3D for the first time as comparison: a nonlinear beamformer with p-th root compression and a coherence factor beamformer. The 3D coherence beamformers, together with DAS, were compared in computer simulation, on a microflow phantom and in vivo. Results: Simulation results demonstrated that all three adaptive weight-based beamformers can narrow the main lobe suppress the side lobes, while maintaining the weaker scatter signals. Improved 3D SRUS images of microflow phantom and a rabbit kidney within a 3-second acquisition were obtained using the adaptive weight-based beamformers, when compared with DAS. Conclusion: The adaptive weight-based 3D beamformers can improve the SRUS and the proposed variance-based beamformer performs best in simulations and experiments. Significance: Fast 3D SRUS would significantly enhance the potential utility of this emerging imaging modality in a broad range of biomedical applications
Enhancement of Critical Current Density in low level Al-doped MgB2
Two sets of MgB2 samples doped with up to 5 at. % of Al were prepared in
different laboratories using different procedures. Decreases in the a and c
lattice parameters were observed with Al doping confirming Al substitution onto
the Mg site. The critical temperature (Tc) remained largely unchanged with Al
doping. For 1 - 2.5 at.% doping, at 20K the in-field critical current densities
(Jc's) were enhanced, particularly at lower fields. At 5K, in-field Jc was
markedly improved, e.g. at 5T Jc was enhanced by a factor of 20 for a doping
level of 1 at.% Al. The improved Jcs correlate with increased sample
resistivity indicative of an increase in the upper critical field, Hc2, through
alloying.Comment: 17 pages, 4 figures, to be published in Superconductor Science and
Technolog
Singular magnetic anisotropy in the nematic phase of FeSe
FeSe is arguably the simplest, yet the most enigmatic, iron-based
superconductor. Its nematic but non-magnetic ground state is unprecedented in
this class of materials and stands out as a current puzzle. Here, our NMR
measurements in the nematic state of mechanically detwinned FeSe reveal that
both the Knight shift and the spin-lattice relaxation rate 1/T_1 possess an
in-plane anisotropy opposite to that of the iron pnictides LaFeAsO and
BaFe2As2. Using a microscopic electron model that includes spin-orbit coupling,
our calculations show that an opposite quasiparticle weight ratio between the
d_xz and d_yz orbitals leads to an opposite anisotropy of the orbital magnetic
susceptibility, which explains our Knight shift results. We attribute this
property to a different nature of nematic order in the two compounds,
predominantly bond-type in FeSe and onsite ferro-orbital in pnictides. The T_1
anisotropy is found to be inconsistent with existing neutron scattering data in
FeSe, showing that the spin fluctuation spectrum reveals surprises at low
energy, possibly from fluctuations that do not break C_4 symmetry. Therefore,
our results reveal that important information is hidden in these anisotropies
and they place stringent constraints on the low-energy spin correlations as
well as on the nature of nematicity in FeSe.Comment: Submitted version. More recent version and Supplementary Information
(24 pages) available upon reques
Energy deposition by heavy ions: Additivity of kinetic and potential energy contributions in hillock formation on CaF2
The formation of nano-hillocks on CaF2 crystal surfaces by individual ion
impact has been studied using medium energy (3 and 5 MeV) highly charged ions
(Xe19+ to Xe30+) as well as swift (kinetic energies between 12 and 58 MeV)
heavy ions. For very slow highly charged ions the appearance of hillocks is
known to be linked to a threshold in potential energy while for swift heavy
ions a minimum electronic energy loss is necessary. With our results we bridge
the gap between these two extreme cases and demonstrate, that with increasing
energy deposition via electronic energy loss the potential energy threshold for
hillock production can be substantially lowered. Surprisingly, both mechanisms
of energy deposition in the target surface seem to contribute in an additive
way, as demonstrated when plotting the results in a phase diagram. We show that
the inelastic thermal spike model, originally developed to describe such
material modifications for swift heavy ions, can be extended to case where
kinetic and potential energies are deposited into the surface.Comment: 12 pages, 4 figure
Reversible melting and equilibrium phase formation of (Bi,Pb)2Sr2Ca2Cu3O10+d
The decomposition and the reformation of the (Bi,Pb)2Sr2Ca2Cu3O10+d
(?Bi,Pb(2223)?) phase have been investigated in-situ by means of
High-Temperature Neutron Diffraction, both in sintered bulk samples and in
Ag-sheathed monofilamentary tapes. Several decomposition experiments were
performed at various temperatures and under various annealing atmospheres,
under flowing gas as well as in sealed tubes, in order to study the appropriate
conditions for Bi,Pb(2223) formation from the melt. The Bi,Pb(2223) phase was
found to melt incongruently into (Ca,Sr)2CuO3, (Sr,Ca)14Cu24O41 and a
Pb,Bi-rich liquid phase. Phase reformation after melting was successfully
obtained both in bulk samples and Ag-sheathed tapes. The possibility of
crystallising the Bi,Pb(2223) phase from the melt was found to be extremely
sensitive to the temperature and strongly dependent on the Pb losses. The study
of the mass losses due to Pb evaporation was complemented by thermogravimetric
analysis which proved that Pb losses are responsible for moving away from
equilibrium and therefore hinder the reformation of the Bi,Pb(2223) phase from
the melt. Thanks to the full pattern profile refinement, a quantitative phase
analysis was carried out as a function of time and temperature and the role of
the secondary phases was investigated. Lattice distortions and/or transitions
were found to occur at high temperature in Bi,Pb(2223), Bi,Pb(2212),
(Ca,Sr)2CuO3 and (Sr,Ca)14Cu24O41, due to cation diffusion and stoichiometry
changes. The results indicate that it is possible to form the Bi,Pb(2223) phase
from a liquid close to equilibrium conditions, like Bi(2212) and Bi(2201), and
open new unexplored perspectives for high-quality Ag-sheathed Bi,Pb(2223) tape
processing.Comment: 45 pages (including references,figures and captions), 13 figures
Submitted to Supercond. Sci. Techno
Anomalous dependence of the c-axis polarized Fe B phonon mode with Fe and Se concentrations in FeTeSe
We report an investigation of the lattice dynamical properties in a range of FeTeSe compounds, with special emphasis on the c-axis polarized vibration of Fe with B symmetry, a Raman active mode common to all families of Fe-based superconductors. We have carried out a systematic study of the temperature dependence of this phonon mode as a function of Se and excess Fe concentrations. In parent compound FeTe, we observe an unconventional broadening of the phonon between room temperature and magnetic ordering temperature . The situation smoothly evolves towards a regular anharmonic behavior as Te is substituted for Se and long range magnetic order is replaced by superconductivity. Irrespective to Se contents, excess Fe is shown to provide an additional damping channel for the B phonon at low temperatures. We performed Density Functional Theory (DFT) ab-initio calculations within the local density approximation (LDA) to calcuate the phonon frequencies including magnetic polarization and Fe non-stoichiometry in the Virtual Crystal Approximation (VCA). We obtained a good agreement with the measured phonon frequencies in the Fe-deficient samples, while the effects of Fe excess are poorly reproduced. This may be due to excess Fe-induced local magnetism and low energy magnetic fluctuations that can not be treated accurately within these approaches. As recently revealed by neutron scattering and -SR studies, these phenomena occur in the temperature range where anomalous decay of the B phonon is observed, and suggests a peculiar coupling of this mode with local moments and spin fluctuations in FeTeSe
High temperature superconductivity (Tc onset at 34K) in the high pressure orthorhombic phase of FeSe
We have studied the structural and superconducting properties of tetragonal
FeSe under pressures up to 26GPa using synchrotron radiation and diamond anvil
cells. The bulk modulus of the tetragonal phase is 28.5(3)GPa, much smaller
than the rest of Fe based superconductors. At 12GPa we observe a phase
transition from the tetragonal to an orthorhombic symmetry. The high pressure
orthorhombic phase has a higher Tc reaching 34K at 22GPa.Comment: 15 pages, 4 figure
- âŠ