1,097 research outputs found
A quantum Monte Carlo study on the superconducting Kosterlitz-Thouless transition of the attractive Hubbard model on a triangular lattice
We study the superconducting Kosterlitz-Thouless transition of the attractive
Hubbard model on a two-dimensional triangular lattice using auxiliary field
quantum Monte Carlo method for system sizes up to sites.
Combining three methods to analyze the numerical data, we find, for the
attractive interaction of , that the transition temperature stays almost
constant within the band filling range of , while it is found to
be much lower in the region.Comment: RevTeX 6 page
Investigation of passive shock wave-boundary layer control for transonic airfoil drag reduction
The passive drag control concept, consisting of a porous surface with a cavity beneath it, was investigated with a 12-percent-thick circular arc and a 14-percent-thick supercritical airfoil mounted on the test section bottom wall. The porous surface was positioned in the shock wave/boundary layer interaction region. The flow circulating through the porous surface, from the downstream to the upstream of the terminating shock wave location, produced a lambda shock wave system and a pressure decrease in the downstream region minimizing the flow separation. The wake impact pressure data show an appreciably drag reduction with the porous surface at transonic speeds. To determine the optimum size of porosity and cavity, tunnel tests were conducted with different airfoil porosities, cavities and flow Mach numbers. A higher drag reduction was obtained by the 2.5 percent porosity and the 1/4-inch deep cavity
Critical Temperature Tc and Charging Energy Ec between B-B layers of Superconducting diboride materials MgB2 in 3D JJA model
The diboride materials MB2 (M = Mg, Be, Pb, etc.) are discussed on the basis
of the 3D Josephson junction array (JJA) model due to Kawabata-Shenoy-Bishop,
in terms of the B-B layers in the diborides analogous to the Cu-O ones in the
cuprates.
We propose a possibility of superconducting materials with the MgB2-type
structure which exhibit higher critical temperature Tc over 39K of MgB2.
We point out a role of interstitial ionic atoms (e.g., Mg in MgB2) as
capacitors between the B-B layers, which reduce the charging coupling energy in
JJA.Comment: 3 pages, 1 figure included; to be published in J. Phys. Soc. Jpn. 70,
No.10 (2001
Two-Band-Type Superconducting Instability in MgB2
Using the tight-binding method for the -bands in MgB, the Hubbard
on-site Coulomb interaction on two inequivalent boron -orbitals is
transformed into expressions in terms of -band operators. For scattering
processes relevant to the problemin which a wave vector {\bf q} is parallel to
, it is found to take a relatively simple form consisting of
intra-band Coulomb scattering, interband pair scattering etc. with large
constant coupling constants. This allows to get a simple expression for the
amplitude of interband pair scattering between two -bands, which diverges
if the interband polarization function in it becomes large enough.The latter
was approximately evaluated and found to be largely enhanced in the band
structure in MgB. These results lead to a divergent interband pair
scattering, meaning two-band-type superconducting instability with enhanced
. Adding a subsidiary BCS attractive interaction in each band into
consideration, a semi-quantitative gap equation is given, and and isotope
exponent are derived. The present instability is asserted to be the
origin of high in MgB.Comment: 4 pages, to be published in J. Phys. Soc. Jpn. vol. 70, No.
Very fast formation of superconducting MgB2/Fe wires with high Jc
In this paper we have investigated the effects of sintering time and
temperature on the formation and critical current densities of Fe-clad MgB2
wires. MgB2 wires were fabricated using the powder-in-tube process and sintered
for different periods of time at predetermined temperatures. All the samples
were examined using XRD, SEM and magnetisation measurements. In contrast to the
common practice of sintering for several hours, the present results show that
there is no need for prolonged heat treatment in the fabrication of Fe-clad
MgB2 wires. A total time in the furnace of several minutes is more than enough
to form nearly pure MgB2 with high performance characteristics. The results
from Tc, Jc and Hirr show convincingly that the samples which were sintered for
3 minutes above 800 oC are as good as those sintered for longer times. In fact,
the Jc field performance for the most rapidly sintered sample is slightly
better than for all other samples. Jc of 4.5 times 10 ^5 A/cm2 in zero field
and above 10 ^5 A/cm2 in 2T at 15 K has been achieved for the best Fe-clad MgB2
wires. As a result of such a short sintering there is no need for using high
purity argon protection and it is possible to carry out the heat treatment in a
much less protective atmosphere or in air. These findings substantially
simplify the fabrication process, making it possible to have a continuous
process for fabrication and reducing the costs for large-scale production of
MgB2 wires.Comment: 15 pages, one table, 9 figures, submitted to Physica C on June 8,
200
Electronic Structures of CaAlSi with Different Stacking AlSi Layers by First-Principles Calculations
The full-potential linear augmented plane-wave calculations have been applied
to investigate the systematic change of electronic structures in CaAlSi due to
different stacking sequences of AlSi layers. The present ab-initio calculations
have revealed that the multistacking, buckling and 60 degrees rotation of AlSi
layer affect the electronic band structure in this system. In particular, such
a structural perturbation gives rise to the disconnected and cylindrical Fermi
surface along the M-L lines of the hexagonal Brillouin zone. This means that
multistacked CaAlSi with the buckling AlSi layers increases degree of
two-dimensional electronic characters, and it gives us qualitative
understanding for the quite different upper critical field anisotropy between
specimens with and without superstructure as reported previously.Comment: 4 pages, 4 figures, to be published in J. Phys. Soc. Jp
Soft x-ray spectroscopy measurements of the p-like density of states of B in MgB2 and evidence for surface boron oxides on exposed surfaces
Soft X-ray absorption and fluorescence measurements are reported for the
K-edge of B in MgB2. The measurements confirm a high density of B
pxy(sigma)-states at the Fermi edge and extending to approximately 0.9 eV above
the edge. A strong resonance is observed in elastic scattering through a
core-exciton derived from out-of-plane pz(pi*)-states. Another strong
resonance, observed in both elastic and inelastic spectra, is identified as a
product of surface boron oxides.Comment: 7 pages total, 4 figures, submitted to Phys. Rev. Let
Two-Dimensional Sigma-Hole Systems in Boron Layers: A First-Principles Study on Mg_{1-x}Na_xB_2 and Mg_{1-x}Al_xB_2
We study two-dimensional sigma-hole systems in boron layers by calculating
the electronic structures of Mg_{1-x}Na_xB_2 and Mg_{1-x}Al_xB_2. In
Mg_{1-x}Na_xB_2, it is found that the concentration of sigma holes is
approximately described by (0.8 + 0.8 x) * 10^{22} cm^{-3} and the largest
attainable concentration is about 1.6 * 10^{22} cm^{-3} in NaB_2. In
Mg_{1-x}Al_xB_2, on the other hand, it is found that the concentration of sigma
holes is approximately described by (0.8 - 1.4 x) * 10^{22} cm^{-3} and sigma
holes are disappeared at x of about 0.6. These relations can be used for
experimental studies on the sigma-hole systems in these materials.Comment: 5 pages, 5 figure
Flux Jumping and a Bulk-to-Granular Transition in the Magnetization of a Compacted and Sintered MgB2 Superconductor
The recent discovery of intermediate-temperature superconductivity (ITC) in
MgB2 by Akimitsu et al. and its almost simultaneous explanation in terms of a
hole-carrier-based pairing mechanism by Hirsch, has triggered an avalanche of
studies of its structural, magnetic and transport properties. As a further
contribution to the field we report the results of field (H) and temperature
(T) dependent magnetization (M) measurements of a pellet of uniform,
large-grain sintered MgB2. We show that at low temperatures the size of the
pellet and its critical current density, Jc(H) - i.e. its M(H) - ensure low
field flux jumping, which of course ceases when M(H) drops below a critical
value. With further increase of H and T the individual grains decouple and the
M(H) loops drop to lower lying branches, unresolved in the usual full M(H)
representation. After taking into account the sample size and grain size,
respectively, the bulk sample and the grains were deduced to exhibit the same
magnetically determined Jc s (e.g. 105 A/cm2, 20 K, 0T) and hence that for each
temperature of measurement Jc(H) decreased monotonically with H over the entire
field range, except for a gap within the grain-decoupling zone.Comment: 7 pages, 6 figures, Changes: Fig 6 Vertical scale an order of
magnitude out (changed figure and associated text). Also corrected typo in
last sectio
Direct visualization of iron sheath shielding effect in MgB_2 superconducting wires
Local magneto-optical imaging and global magnetization measurement techniques
were used in order to visualize shielding effects in the superconducting core
of MgB_2 wires sheathed by ferromagnetic iron (Fe). The magnetic shielding can
provide a Meissner-like state in the superconducting core in applied magnetic
fields up to ~1T. The maximum shielding fields are shown to correlate with the
saturation fields of magnetization in Fe-sheaths. The shielding has been found
to facilitate the appearance of an overcritical state, which is capable of
achieving a critical current density (J_c) in the core which is larger than J_c
in the same wire without the sheath by a factor of ~2. Other effects caused by
the magnetic interaction between the sheath and the superconducting core are
discussed.Comment: 4 pages, 3 figure
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