1,139 research outputs found
Ferromagnetic fluctuation and possible triplet superconductivity in Na_xCoO_2*yH_2O: Fluctuation-exchange study of multi-orbital Hubbard model
Spin and charge fluctuations and superconductivity in a recently discovered
superconductor Na_xCoO_2*yH_2O are studied based on a multi-orbital Hubbard
model. Tight-binding parameters are determined to reproduce the LDA band
dispersions with the Fermi surface, which consist of a large cylindrical one
around the Gamma-point and six hole pockets near the K-points. By applying the
fluctuation-exchange (FLEX) approximation, we show that the Hund's-rule
coupling between the Co t2g orbitals causes ferromagnetic (FM) spin
fluctuation. Triplet f_{y(y^2-3x^2)}-wave and p-wave pairings are favored by
this FM fluctuation on the hole-pocket band. We propose that, in
Na_xCoO_2*yH_2O, the Co t2g orbitals and inter-orbital Hund's-rule coupling
play important roles on the triplet pairing, and this compound can be a first
example of the triplet superconductor in which the orbital degrees of freedom
play substantial roles.Comment: 5 pages, 3 figure
Quantum Melting of Charge Order due to Frustration in Two-Dimensional Quarter-Filled Systems
The effect of geometrical frustration in a two-dimensional 1/4-filled
strongly correlated electron system is studied theoretically, motivated by
layered organic molecular crystals. An extended Hubbard model on the square
lattice is considered, with competing nearest neighbor Coulomb interaction, V,
and that of next-nearest neighbor along one of the diagonals, V', which favor
different charge ordered states. Based on exact diagonalization calculations,
we find a metallic phase stabilized over a broad window at V' ~ V even for
large Coulomb repulsion strengths as a result of frustrating the charge ordered
states. Slightly modifying the lattice geometry relevant to the actual organic
compounds does not alter the results, suggesting that this `quantum melting' of
charge order is a robust feature of frustrated strongly correlated 1/4-filled
systems.Comment: 5 pages, 4 figures, to be published in Phys. Rev.
Gas Stripping of Dwarf Galaxies in Clusters of Galaxies
Many clusters of galaxies contain an appreciable amount of hot gas, the
intracluster medium. As a consequence, gas will be stripped from galaxies that
move through the inter cluster medium, if the ram pressure exceeds the internal
gravitational force. We study the interaction between the intracluster medium
and an extended gas component of dwarf galaxies confined by a surrounding cold
dark matter halo analytically and numerically, using axisymmetric
two-dimensional hydrodynamical simulations at high resolution. The results show
that the gas within the dark matter halo is totally stripped in a typical
galactic cluster. The process of ram pressure stripping therefore must have
played an important role during the chemo-dynamical evolution of dwarf galaxies
in galactic clusters. Our results predict that most of the chemical evolution
of dwarf galaxies must have occurred at high redshift, before the virialized
cluster had formed.Comment: 9 pages, 6 figures; accepted for publication in Ap
Spin-triplet superconductivity due to antiferromagnetic spin-fluctuation in Sr_2RuO_4
A mechanism leading to the spin-triplet superconductivity is proposed based
on the antiferromagnetic spin fluctuation. The effects of anisotropy in spin
fluctuation on the Cooper pairing and on the direction of d vector are examined
in the one-band Hubbard model with RPA approximation. The gap equations for the
anisotropic case are derived and applied to Sr_2RuO_4. It is found that a
nesting property of the Fermi surface together with the anisotropy leads to the
triplet superconductivity with the d=z(sin{k_x}\pm isin{k_y}), which is
consistent with experiments.Comment: 4 pages, 3 eps figures, revte
A crib-shaped triplet pairing gap function for an orthogonal pair of quasi-one dimensional Fermi surfaces in SrRuO
The competition between spin-triplet and singlet pairings is studied
theoretically for the tight-binding - bands in SrRuO,
which arise from two sets of quasi-one dimensional Fermi surfaces. Using
multiband FLEX approximation, where we incorporate an anisotropy in the spin
fluctuations as suggested from experiments, we show that (i) the triplet can
dominate over the singlet (which turns out to be extended s), and (ii) the
triplet gap function optimized in the Eliashberg equation has an unusual, very
non-sinusoidal form, whose time-reversal-broken combination exhibits a
crib-shaped amplitude with dips.Comment: 5 pages, RevTeX, to appear in Phys.Rev.B (Rapid Communications
New Panoramic View of CO and 1.1 mm Continuum Emission in the Orion A Molecular Cloud. I. Survey Overview and Possible External Triggers of Star Formation
We present new, wide and deep images in the 1.1 mm continuum and the
CO (=1-0) emission toward the northern part of the Orion A Giant
Molecular Cloud (Orion-A GMC). The 1.1 mm data were taken with the AzTEC camera
mounted on the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope
in Chile, and the CO (=1-0) data were with the 25 beam receiver
(BEARS) on the NRO 45 m telescope in the On-The-Fly (OTF) mode. The present
AzTEC observations are the widest (\timeform{1.D7}
\timeform{2.D3}, corresponding to 12 pc 17 pc) and the
highest-sensitivity (9 mJy beam) 1.1 mm dust-continuum imaging of
the Orion-A GMC with an effective spatial resolution of 40\arcsec. The
CO (=1-0) image was taken over the northern \timeform{1D.2}
\times\timeform{1D.2} (corresponding 9 pc 9 pc) area with a
sensitivity of 0.93 K in , a velocity resolution of 1.0 km
s, and an effective spatial resolution of 21\arcsec. With these data,
together with the MSX 8 m, Spitzer 24 m and the 2MASS data, we have
investigated the detailed structure and kinematics of molecular gas associated
with the Orion-A GMC and have found evidence for interactions between molecular
clouds and the external forces that may trigger star formation. Two types of
possible triggers were revealed; 1) Collision of the diffuse gas on the cloud
surface, particularly at the eastern side of the OMC-2/3 region, and 2)
Irradiation of UV on the pre-existing filaments and dense molecular cloud
cores. Our wide-field and high-sensitivity imaging have provided the first
comprehensive view of the potential sites of triggered star formation in the
Orion-A GMC.Comment: 32 pages, 20 figures, accepted for publication in PAS
Improving the tensile properties of additively manufactured β-containing tial alloys via microstructure control focusing on cellular precipitation reaction
The effect of a two-step heat treatment on the microstructure and high-temperature tensile properties of β-containing Ti-44Al-4Cr (at%) alloys fabricated by electron beam powder bed fusion were examined by focusing on the morphology of α2/γ lamellar grains and β/γ cells precipitated at the lamellar grain boundaries by a cellular precipitation reaction. The alloys subjected to the first heat treatment step at 1573 K in the α + β two-phase region exhibit a non-equilibrium microstructure consisting of the α2/γ lamellar grains with a fine lamellar spacing and a β/γ duplex structure located at the grain boundaries. In the second step of heat treatment, i.e., aging at 1273 K in the β + γ two-phase region, the β/γ cells are discontinuously precipitated from the lamellar grain boundaries due to excess Cr supersaturation in the lamellae. The volume fraction of the cells and lamellar spacing increase with increasing aging time and affect the tensile properties of the alloys. The aged alloys exhibit higher strength and comparable elongation at 1023 K when compared to the as-built alloys. The strength of these alloys is strongly dependent on the volume fraction and lamellar spacing of the α2/γ lamellae. In addition, the morphology of the β/γ cells is also an important factor controlling the fracture mode and ductility of these alloys.Cho K., Odo H., Okamoto K., et al. Improving the tensile properties of additively manufactured β-containing tial alloys via microstructure control focusing on cellular precipitation reaction. Crystals, 11, 7, 809. https://doi.org/10.3390/cryst11070809
Peculiar microstructural evolution and tensile properties of β-containing γ-TiAl alloys fabricated by electron beam melting
The microstructure and tensile properties of β-containing Ti–44Al–4Cr alloy rods additively manufactured by electron beam melting (EBM) process were examined as a function of input energy density determined by the processing parameters. To the best of our knowledge, this is the first report to demonstrate that two types of fine microstructures have been obtained in the β-containing γ-TiAl alloys by varying the energy density during the EBM process. A uniform α2/β/γ mixed structure containing an α2/γ lamellar region and a β/γ dual-phase region is formed at high energy density conditions. On the other hand, a lower energy density leads to the formation of a peculiar layered microstructure perpendicular to the building direction, consisting of a ultrafine α2/γ lamellar grain layer and a α2/β/γ mixed structure layer. The difference in the microstructures originates from the difference in the solidification microstructure and the temperature distribution from the melt pool, which are dependent on the energy density. Furthermore, it was found that the strength of the alloys is closely related to the volume fractions of the β phase and the ultrafine α2/γ lamellar grains which originates from the massive α grains formed by rapid cooling under low energy density conditions. The alloys with high amounts of these peculiar microstructures exhibit high strength comparable to and higher than the conventional β-containing γ-TiAl at room temperature and 1023 K, respectively.Cho K., Kawabata H., Hayashi T., et al. Peculiar microstructural evolution and tensile properties of β-containing γ-TiAl alloys fabricated by electron beam melting. Additive Manufacturing, 46, 102091. https://doi.org/10.1016/j.addma.2021.102091
Phytochrome-regulated EBL1 contributes to ACO1 upregulation in rice
The 1-aminocyclopropane-1-carboxylate oxidase gene (ACO1) was upregulated in rice (Oryza sativa L.) phyAphyBphyC mutants lacking any phytochrome and containing the GCC box element, a binding site for rice ethylene-responsive element binding protein 1 (OsEREBP1), in its promoter region. Since the OsEREBP1-like gene EBL1 (OsEREBP1-LIKE 1) was significantly downregulated in phyAphyBphyC mutants, EBL1 was suspected to repress ACO1 expression in wild-type plants. However, ACO1 was downregulated in EBL1 RNA interference plants, and the total length of these plants was slightly shorter than that of wild-type plants. This study shows that EBL1 is positively regulated by phytochrome B and associated with ACO1 upregulation
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