933 research outputs found
Strong Pinning Enhancement in MgB2 Using Very Small Dy2O3 Additions
0.5 to 5.0 wt.% Dy2O3 was in-situ reacted with Mg + B to form pinned MgB2.
While Tc remained largely unchanged, Jc was strongly enhanced. The best sample
(only 0.5 wt.% Dy2O3) had a Jc of 6.5 x 10^5 A/cm^2 at 6K, 1T and 3.5 x 10^5
A/cm^2 at 20K, 1T, around a factor of 4 higher compared to the pure sample, and
equivalent to hot-pressed or nano-Si added MgB2 at below 1T. Even distributions
of nano-scale precipitates of DyB4 and MgO were observed within the grains. The
room temperature resistivity decreased with Dy2O3 indicative of improved grain
connectivity.Comment: 13 pages, 4 figures and 1 tabl
Observation of Josephson coupling through an interlayer of antiferromagnetically ordered chromium
The supercurrent transport in metallic Josephson tunnel junctions with an
additional interlayer made up by chromium, being an itinerant antiferromagnet,
was studied. Uniform Josephson coupling was observed as a function of the
magnetic field. The supercurrent shows a weak dependence on the interlayer
thickness for thin chromium layers and decays exponentially for thicker films.
The diffusion constant and the coherence length in the antiferromagnet were
estimated. The antiferromagnetic state of the barrier was indirectly verified
using reference samples. Our results are compared to macroscopic and
microscopic models.Comment: Phys. Rev. B (2009), in prin
Improved Current Densities in MgB2 By Liquid-Assisted Sintering
Polycrystalline MgB2 samples with GaN additions were prepared by reaction of
Mg, B, and GaN powders. The presence of Ga leads to a low melting eutectic
phase which allowed liquid phase sintering and produces plate-like grains. For
low-level GaN additions (5% at. % or less), the critical transition
temperature, Tc, remained unchanged and in 1T magnetic field, the critical
current density, Jc was enhanced by a factor of 2 and 10, for temperatures of
\~5K and 20K, respectively. The values obtained are approaching those of hot
isostatically pressed samples.Comment: 12 pages, 1 table, 4 figures, accepted in Applied Physics Letter
X-ray absorption study of Ti-activated sodium aluminum hydride
Ti K-edge x-ray absorption near edge spectroscopy (XANES) was used to explore
the Ti valence and coordination in Ti-activated sodium alanate. An empirical
relationship was established between the Ti valence and the Ti K-edge onset
based on a set of standards. This relationship was used to estimate oxidation
states of the titanium catalyst in 2 mol% and 4 mol% Ti-doped NaAlH4. These
results demonstrate that the formal titanium valence is zero in doped sodium
alanate and nearly invariant during hydrogen cycling. A qualitative comparison
of the edge fine structure suggests that the Ti is present on the surface in
the form of amorphous TiAl3.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns
We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films
grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures
(Tg) ranging from 80deg C to 200deg C, with average Mn contents between 1 % and
11 %. Their crystalline structure, morphology and composition have been
investigated by transmission electron microscopy (TEM), electron energy loss
spectroscopy and x-ray diffraction. In the whole range of growth temperatures
and Mn concentrations, we observed the formation of manganese rich
nanostructures embedded in a nearly pure germanium matrix. Growth temperature
mostly determines the structural properties of Mn-rich nanostructures. For low
growth temperatures (below 120deg C), we evidenced a two-dimensional spinodal
decomposition resulting in the formation of vertical one-dimensional
nanostructures (nanocolumns). Moreover we show in this paper the influence of
growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns
size and density. For temperatures higher than 180deg C, we observed the
formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns
and nanoclusters coexist. Combining high resolution TEM and superconducting
quantum interference device magnetometry, we could evidence at least four
different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn
atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc
nanocolumns (120 K 400 K) and
(iv) Ge3Mn5 clusters.Comment: 10 pages 2 colonnes revTex formatte
Ohmic contacts to n-type germanium with low specific contact resistivity
A low temperature nickel process has been developed that produces Ohmic contacts to n-type germanium with specific contact resistivities down to (2.3 ± 1.8) x10<sup>-7</sup> Ω-cm<sup>2</sup> for anneal temperatures of 340 degC. The low contact resistivity is attributed to the low resistivity NiGe phase which was identified using electron diffraction in a transmission electron microscope. Electrical results indicate that the linear Ohmic behaviour of the contact is attributed to quantum mechanical tunnelling through the Schottky barrier formed between the NiGe alloy and the heavily doped n-Ge.<p></p>
Speculations about plessite
Metallographic examination and microprobe analysis of plessite areas in meteorites - iron and nickel concentration in meteorite
Pt-induced nanowires on Ge(001): a DFT study
We study formation of the nanowires formed after deposition of Pt on a
Ge(001) surface. The nanowires form spontaneously after high temperature
annealing. They are thermodynamically stable, only one atom wide and up to a
few hundred atoms long. Ab initio density functional theory calculations are
performed to identify possible structures of the Pt-Ge (001) surface with
nanowires on top. A large number of structures is studied. With nanowires that
are formed out of Pt or Ge dimers or mixed Pt-Ge dimers. By comparing simulated
scanning tunneling microscopy images with experimental ones we model the
formation of the nanowires and identify the geometries of the different phases
in the formation process. We find that the formation of nanowires on a
Pt-Ge(001) surface is a complex process based on increasing the Pt density in
the top layers of the Ge(001) surface. Most remarkably we find the nanowires to
consist of germanium dimers placed in troughs lined by mixed Pt-Ge dimer rows.Comment: 22 pages, 24 figure
Structure and giant magnetoresistance of granular Co-Cu nanolayers prepared by cross-beam PLD
A series of Co_xCu_{100-x} (x = 0, 40...75, 100) layers with thicknesses
in-between 13 nm and 55 nm were prepared on silicon substrates using cross-beam
pulsed laser deposition. Wide-angle X-ray diffraction (WAXRD), transmission
electron microscopy (TEM) and electrical transport measurements revealed a
structure consisting of decomposed cobalt and copper grains with grain sizes of
about 10 nm. The influence of cobalt content and layer thickness on the grain
size is discussed. Electron diffraction (ED) indicates the presence of an
intermetallic Co-Cu phase of Cu3Au structure-type. Thermal treatment at
temperatures between 525 K and 750 K results in the progressive decomposition
of Co and Cu, with an increase of the grain sizes up to about 100 nm. This is
tunable by controlling the temperature and duration of the anneal, and is
directly observable in WAXRD patterns and TEM images. A careful analysis of
grain size and the coherence length of the radiation used allows for an
accurate interpretation of the X-ray diffraction patterns, by taking into
account coherent and non-coherent scattering. The alloy films show a giant
magnetoresistance of 1...2.3 % with the maximum obtained after annealing at
around 725 K.Comment: 9 pages, 9 figure
Hybridization Mechanism for Cohesion of Cd-based Quasicrystals
Cohesion mechanism of cubic approximant crystals of newly discovered binary
quasicrystals, CdM (M=Yb and Ca), are studied theoretically. It is found
that stabilization due to alloying is obtained if M is an element with
low-lying unoccupied states. This leads to conclusion that the cohesion of
the Cd-based compounds is due to the hybridization of the states of Yb and
Ca with a wide band. %unlike known stable quasicrystals without transition
elements %such as Al-Li-Cu and Zn-Mg-RE (RE:rare earth). Although a diameter of
the Fermi sphere coincides with the strong Bragg peaks for Cd-Yb and Cd-Ca, the
Hume-Rothery mechanism does not play a principal role in the stability because
neither distinct pseudogap nor stabilization due to alloying is obtained for
isostructural Cd-Mg. In addition to the electronic origin, matching of the
atomic size is very crucial for the quasicrystal formation of the Cd-based
compounds. It is suggested that the glue atoms, which do not participate in the
icosahedral cluster, play an important role in stabilization of the compound.Comment: 4 pages, 2 figure
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