26,050 research outputs found
Magnification relations of quad lenses and applications on Einstein crosses
In this work, we mainly study the magnification relations of quad lens models
for cusp, fold and cross configurations. By dividing and ray-tracing in
different image regions, we numerically derive the positions and magnifications
of the four images for a point source lying inside of the astroid caustic.
Then, based on the magnifications, we calculate the signed cusp and fold
relations for the singular isothermal elliptical lenses. The signed fold
relation map has positive and negative regions, and the positive region is
usually larger than the negative region as has been confirmed before. It can
also explain that for many observed fold image pairs, the fluxes of the Fermat
minimum images are apt to be larger than those of the saddle images. We define
a new quantity cross relation which describes the magnification discrepancy
between two minimum images and two saddle images. Distance ratio is also
defined as the ratio of the distance of two saddle images to that of two
minimum images. We calculate the cross relations and distance ratios for nine
observed Einstein crosses. In theory, for most of the quad lens models, the
cross relations decrease as the distance ratios increase. In observation, the
cross relations of the nine samples do not agree with the quad lens models very
well, nevertheless, the cross relations of the nine samples do not give obvious
evidence for anomalous flux ratio as the cusp and fold types do. Then, we
discuss several reasons for the disagreement, and expect good consistencies for
more precise observations and better lens models in the future.Comment: 12 pages, 11 figures, accepted for publication in MNRA
Properties of superconducting MgB_2 wires: "in-situ" versus "ex-situ" reaction technique
We have fabricated a series of iron-sheathed superconducting wires prepared
by the powder-in-tube technique from (MgB_2)_{1-x}:(Mg+2B)_x initial powder
mixtures taken with different proportions, so that x varies from 0 to 1. It
turned out that "ex-situ" prepared wire (x = 0) has considerable disadvantages
compared to all the other wires in which "in-situ" assisted (0 < x < 1) or pure
"in-situ" (x = 1) preparation was used due to weaker inter-grain connectivity.
As a result, higher critical current densities J_c were measured over the
entire range of applied magnetic fields B_a for all the samples with x > 0.
Pinning of vortices in MgB_2 wires is shown to be due to grain boundaries.
J_c(B_a) behavior is governed by an interplay between the transparency of grain
boundaries and the amount of "pinning" grain boundaries. Differences between
thermo-magnetic flux-jump instabilities in the samples and a possible threat to
practical applications are also discussed.Comment: To be published in Supercond. Sci. Technol. (2003), in pres
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Microwave-Induced Interfacial Nanobubbles
A new method for generating nanobubbles via microwave irradiation was verified and quantified. AFM measurement showed that nanobubbles with diameters ranging in 200 - 600 nm were generated at water-HOPG surface by applying microwave to aqueous solutions with 9.0 - 30.0 mg/L of dissolved oxygen. Graphite displays strong microwave absorption and transmits high thermal energy to surface. Due to high dielectric constant (20 °C, 80 F/m) and dielectric loss factor, water molecule has strong absorption ability for microwave. The thermal and non-thermal effects of microwave both had contributions to decrease gas solubility and that facilitated nanobubble nucleation. The yield of nanobubbles increased about ten times when irradiation time increased from 60 s to 120 s at 200 W microwave. The nanobubbles density increased from 0.8 to 15 numbers/μm2 by improving working power from 200 to 600 W. An apparent improvement of nanobubbles yield was obtained between 300 and 400 W, and the resulting temperature was 34 - 52 °C. When the initial dissolved oxygen increased from 11.3 to 30.0 mg/L, the density of nanobubbles increased from 1.2 to 13 numbers/μm2. The generation of nanobubbles could be well controlled by adjusting gas concentration, microwave power or irradiation time. The method maybe valuable in preparing surface nanobubbles quickly and conveniently for various applications, such as catalysis, hypoxia/anoxia remediation or as templates to prepare nanoscale materials
Various Correlations in Anisotropic Heisenberg XYZ Model with Dzyaloshinski-Moriya Interaction
Various thermal correlations as well as the effect of intrinsic decoherence
on the correlations are studied in a two-qubit Heisenberg XYZ spin chain with
the Dzyaloshinski--Moriya (DM) interaction along the z direction, i.e. Dz. It
is found that tunable parameter Dz may play a constructive role on the
concurrence (C), classical correlation (CC) and quantum discord (QD) in thermal
equilibrium while it plays a destructive role on the correlations in the
intrinsic decoherence case. The entanglement and quantum discord exhibit
collapse and revival under the phase decoherence. With a proper combination of
the system parameters, the correlations can effectively be kept at high steady
state values despite the intrinsic decoherence.Comment: 4 pages, 4 figure
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