24,004 research outputs found
Maximum thickness of amorphous NiZr interlayers formed by a solid-state reaction technique
Formation of the equilibrium intermetallic compound NiZr in sputter deposited Ni/Zr diffusion couples is suppressed by the formation of a metastable amorphous NiZr alloy until a critical thickness of the amorphous NiZr interlayer is reached. The temperature dependence of this critical thickness is studied experimentally. A phenomenological model based on the premise of interfacial heterogeneous nucleation is proposed to understand the evolution of Ni/Zr diffusion couples
Superconductivity in pure and electron doped MgB2: Transport properties and pressure effects
The normal state and superconducting properties of MgB2 and Mg1-xAlxB2 are
discussed based on structural, transport, and high pressure experiments. The
positive Seebeck coefficient and its linear temperature dependence for Tc<T<160
K provide evidence that the low-temperature transport in MgB2 is due to
hole-like metallic carriers. Structural and transport data show the important
role of defects as indicated by the correlation of Tc, the residual resistance
ratio, and the microstrain extracted from x-ray spectra. The decrease of Tc
with hydrostatic pressure is well explained by the strong-coupling BCS theory.
The large scatter of the pressure coefficients of Tc for different MgB2
samples, however, cannot be explained within this theory. We speculate that
pressure may increase the defect density, particularly in samples with large
initial defect concentration.Comment: Presented at NATO Advanced Research Workshop "New Trends in
Superconductivity", Yalta (Ukraine) 16-20 September, 200
Multiple crossings of a very thin plasma sheet in the Earth's magnetotail
High resolution magnetic field, plasma and energetic particle data from the IMP-8 spacecraft were studied for multiple crossings of the Earth's magnetotail plasma sheet when it becomes thin during magnetospheric substorms. Traversals recur on a time scale of several minutes and they are associated with high velocity plasma flows that are usually directed tailward but are occasionally directed earthward for brief intervals. Observations are explained by rapid oscillations of a plasma sheet that is only a few thousand km thick, a dimension comparable to the gyroradius of energetic protons. Differences in the angular distributions of the two energies indicate that the higher energy protons are preferentially located on field lines deeper in the tail lobe. A neutral line acceleration model is supported tailward streaming energetic electrons which are occasionally present at the lobe plasma sheet interface
Pseudospin symmetry and its approximation in real nuclei
The origin of pseudospin symmetry and its broken in real nuclei are discussed
in the relativistic mean field theory. In the exact pseudospin symmetry, even
the usual intruder orbits have degenerate partners. In real nuclei, pseudospin
symmetry is approximate, and the partners of the usual intruder orbits will
disappear. The difference is mainly due to the pseudo spin-orbit potential and
the transition between them is discussed in details. The contribution of
pseudospin-orbit potential for intruder orbits is quite large, compared with
that for pseudospin doublets. The disappearance of the pseudospin partner for
the intruder orbit can be understood from the properties of its wave function.Comment: 10 pages, 3 figure
Water Content and Superconductivity in Na0.3CoO2*yH2O
We report here the correlation between the water content and
superconductivity in Na0.3CoO2*yH2O under the influences of elevated
temperature and cold compression. The x-ray diffraction of the sample annealed
at elevated temperatures indicates that intergrowths exist in the compound at
equilibrium when 0.6 < y < 1.4. Its low-temperature diamagnetization varies
linearly with y, but is insensitive to the intergrowth, indicative of quasi-2D
superconductivity. The Tc-onset, especially, shifts only slightly with y. Our
data from cold compressed samples, on the other hand, show that the water-loss
non-proportionally suppresses the diamagnetization, which is suggestive of weak
links.Comment: 10 pages, 10 figures; submitted to Physica C (August 13, 2003
Magic numbers for superheavy nuclei in relativistic continuum Hartree-Bogoliubov theory
The magic proton and neutron numbers are searched in the superheavy region
with proton number =100 - 140 and neutron number = (+30) - (2+32)
by the relativistic continuum Hartree-Bogoliubov (RCHB) theory with
interactions NL1, NL3, NLSH, TM1, TW99, DD-ME1, PK1, and PK1R. Based on the
two-nucleon separation energies and , the two-nucleon gaps
and , the shell correction energies
and , the pairing energies and ,
and the pairing gaps and , =120, 132, and 138 and
=172, 184, 198, 228, 238, and 258 are suggested to be the magic numbers
within the present approach. The -decay half-lives are also discussed.
In addition, the potential energy surfaces of possible doubly magic nuclei are
obtained by the deformation-constrained relativistic mean field (RMF) theory,
and the shell effects stabilizing the nuclei are investigated. Furthermore, the
formation cross sections of 120 and 120 at the
optimal excitation energy are estimated by a phenomenological cold fusion
reactions model with the structure information extracted from the constrained
RMF calculation.Comment: 37 pages, 14 figure
Unusual Field-Dependence of the Intragrain Superconductive Transition in RuSr2EuCu2O8
A narrow intragrain phase-lock transition was observed in RuSr2EuCu2O8 under
a magnetic field H up to a few Tesla. The corresponding transition temperature,
T2, decreases rapidly (about 100 K/T at low fields) with H indicating that the
grains of RuSr2EuCu2O8 behave like a Josephson-junction-array instead of a
homogeneous bulk superconductor. Our data suggest that the bulk superconducting
transition may occur on a length scale well below the grain size of 2 to 6
micrometer
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