16,568 research outputs found
Out of plane effect on the superconductivity of Sr2-xBaxCuO3+y with Tc up to 98K
A series of new Sr2-xBaxCuO3+y (0 x 0.6) superconductors were prepared using
high-pressure and high-temperature synthesis. A Rietveld refinement based on
powder x-ray diffraction confirms that the superconductors crystallize in the
K2NiF4-type structure of a space group I4/mmm similar to that of La2CuO4 but
with partially occupied apical oxygen sites. It is found that the
superconducting transition temperature Tc of this Ba substituted Sr2CuO3+y
superconductor with constant carrier doping level, i.e., constant d, is
controlled not only by order/disorder of apical-O atoms but also by Ba content.
Tcmax =98 K is achieved in the material with x=0.6 that reaches the record
value of Tc among the single-layer copper oxide superconductors, and is higher
than Tc=95K of Sr2CuO3+y with optimally ordered apical-O atoms. There is
Sr-site disorder in Sr2-xBaxCuO3+y which might lead to a reduction of Tc. The
result indicates that another effect surpasses the disorder effect that is
related either to the increased in-plane Cu-O bond length or to elongated
apical-O distance due to Ba substitution with larger cation size. The present
experiment demonstrates that the optimization of local geometry out of the Cu-O
plane can dramatically enhance Tc in the cuprate superconductors.Comment: 23 Pages, 1 Table, 5 Figure
Preasymptotic Convergence of Randomized Kaczmarz Method
Kaczmarz method is one popular iterative method for solving inverse problems,
especially in computed tomography. Recently, it was established that a
randomized version of the method enjoys an exponential convergence for
well-posed problems, and the convergence rate is determined by a variant of the
condition number. In this work, we analyze the preasymptotic convergence
behavior of the randomized Kaczmarz method, and show that the low-frequency
error (with respect to the right singular vectors) decays faster during first
iterations than the high-frequency error. Under the assumption that the inverse
solution is smooth (e.g., sourcewise representation), the result explains the
fast empirical convergence behavior, thereby shedding new insights into the
excellent performance of the randomized Kaczmarz method in practice. Further,
we propose a simple strategy to stabilize the asymptotic convergence of the
iteration by means of variance reduction. We provide extensive numerical
experiments to confirm the analysis and to elucidate the behavior of the
algorithms.Comment: 20 page
Numerical study of a sphere descending along an inclined slope in a liquid
The descending process of a sphere rolling and/or sliding along an inclined
slope in a liquid involves interactions between the hydrodynamic forces on the sphere
and the contact forces between the sphere and the plane. In this study, the descending
process of sphere in a liquid was examined using coupled LBM-DEM technique. The
effects of slope angle, viscosity and friction coefficient on the movement of a sphere
were investigated. Two distinct descending patterns were observed: (a) a stable
rolling/sliding movement along the slope, and (b) a fluctuating pattern along the slope.
Five dimensionless coefficients (Reynolds number (Re), drag coefficient, lift
coefficient, moment coefficient and rolling coefficient) were used to analyze the
observed processes. The vortex structure in the wake of the sphere gives a lift force to
the sphere, which in turn controls the different descending patterns. It is found that the
generation of a vortex is not only governed by Re, but also by particle rotation.
Relationships between the forces/moments and the dimensionless coefficients are
established
Tuning Jeff = 1/2 Insulating State via Electron Doping and Pressure in Double-Layered Iridate Sr3Ir2O7
Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting
between Jeff=1/2 and 3/2 bands due to spin-orbit interaction. We report a
metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+
for Sr2+) or application of high pressure up to 35 GPa. Our study of
single-crystal Sr3Ir2O7 and (Sr1-xLax)3Ir2O7 reveals that application of high
hydrostatic pressure P leads to a drastic reduction in the electrical
resistivity by as much as six orders of magnitude at a critical pressure, PC =
13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35
GPa produces no fully metallic state at low temperatures, possibly as a
consequence of localization due to a narrow distribution of bonding angles
{\theta}. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7
readily induces a robust metallic state in the resistivity at low temperatures;
the magnetic ordering temperature is significantly suppressed but remains
finite for (Sr0.95La0.05)3Ir2O7 where the metallic state occurs. The results
are discussed along with comparisons drawn with Sr2IrO4, a prototype of the
Jeff = 1/2 insulator.Comment: five figure
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