1,645 research outputs found
Non-linear Evolution of Matter Power Spectrum in Modified Theory of Gravity
We present a formalism to calculate the non-linear matter power spectrum in
modified gravity models that explain the late-time acceleration of the Universe
without dark energy. Any successful modified gravity models should contain a
mechanism to recover General Relativity (GR) on small scales in order to avoid
the stringent constrains on deviations from GR at solar system scales. Based on
our formalism, the quasi non-linear power spectrum in the
Dvali-Gabadadze-Porratti (DGP) braneworld models and gravity models are
derived by taking into account the mechanism to recover GR properly. We also
extrapolate our predictions to fully non-linear scales using the Parametrized
Post Friedmann (PPF) framework. In gravity models, the predicted
non-linear power spectrum is shown to reproduce N-body results. We find that
the mechanism to recover GR suppresses the difference between the modified
gravity models and dark energy models with the same expansion history, but the
difference remains large at weakly non-linear regime in these models. Our
formalism is applicable to a wide variety of modified gravity models and it is
ready to use once consistent models for modified gravity are developed.Comment: 25 pages, 8 figures, comparison to N-body simulations in DGP added,
published in PR
Evaluation of Trapped Magnetic Field Properties in Superconducting MgB Bulk Magnets of Various Shapes by Finite Element Method
The trapped magnetic field properties of superconducting MgB2 bulk magnets with various shapes such as a triangular, a quadrangular, a hexangular bulk were calculated by the Finite Elements Method (FEM). The effect for the combination of several numbers of bulks was also investigated for several kinds of shapes to obtain large area of bulk surface in spite of one large bulk. In this calculation, the simple magnetization process replaced by the field-cool magnetization was used to obtain the equivalent distribution of the magnetic field, and the thermal equation in FEM was omitted. The trapped magnetic field for the triangular bulk by FEM was compared with the experimental result. It was found that the calculated results agreed well with the experimental result. The maximum trapped magnetic field was obtained in the cylindrical shape among several kinds of shapes. The trapped magnetic field was increased by the combination of multi-bulks. It was confirmed that the trapped magnetic field of the multi-bulks was larger than that of the single bulk. The trapped magnetic field increases with increasing the number of the bulks.28th International Symposium on Superconductivity(ISS 2015), November 16-18, 2015, Tokyo, Japa
Advantageous grain boundaries in iron pnictide superconductors
High critical temperature superconductors have zero power consumption and
could be used to produce ideal electric power lines. The principal obstacle in
fabricating superconducting wires and tapes is grain boundaries-the
misalignment of crystalline orientations at grain boundaries, which is
unavoidable for polycrystals, largely deteriorates critical current density.
Here, we report that High critical temperature iron pnictide superconductors
have advantages over cuprates with respect to these grain boundary issues. The
transport properties through well-defined bicrystal grain boundary junctions
with various misorientation angles (thetaGB) were systematically investigated
for cobalt-doped BaFe2As2 (BaFe2As2:Co) epitaxial films fabricated on bicrystal
substrates. The critical current density through bicrystal grain boundary
(JcBGB) remained high (> 1 MA/cm2) and nearly constant up to a critical angle
thetac of ~9o, which is substantially larger than the thetac of ~5o for YBCO.
Even at thetaGB > thetac, the decay of JcBGB was much smaller than that of
YBCO.Comment: to appear in Nature Communication
Evaluation of Magnetic Cutting and Polishing with Superconducting Bulks
In this paper, magnetic levitation tool with superconducting bulks is introduced as a new hollow machining technology. Magnetic levitation tool is the machine that magnet levitates above superconducting bulks and driving force of rotating magnet shaves the object. This tool is expected to use for a grinding machine and machining device because of hollow machining and micromachining by strong fixing. For using magnetic levitation tool, the attractive force, the repulsive force and rotating torque are important for grinding machine, machining outer surface and both, respectively. These forces are calculated by FEM, and compared with experimental results. The experimental results are agreed well with calculated results. However, the attractive force is one order smaller than that required in chemical mechanical polishing
Scalar perturbations in braneworld cosmology
We study the behaviour of scalar perturbations in the radiation-dominated era
of Randall-Sundrum braneworld cosmology by numerically solving the coupled bulk
and brane master wave equations. We find that density perturbations with
wavelengths less than a critical value (set by the bulk curvature length) are
amplified during horizon re-entry. This means that the radiation era matter
power spectrum will be at least an order of magnitude larger than the
predictions of general relativity (GR) on small scales. Conversely, we
explicitly confirm from simulations that the spectrum is identical to GR on
large scales. Although this magnification is not relevant for the cosmic
microwave background or measurements of large scale structure, it will have
some bearing on the formation of primordial black holes in Randall-Sundrum
models.Comment: 17 pages, 7 figure
Transport and magnetic properties of Co-doped BaFe_{2}As_{2} epitaxial thin films
We report resistivity, Hall coefficient, current-voltage characteristics, and
magneto-optical imaging measurements of epitaxial Co-doped BaFe_{2}As_{2} thin
films deposited on MgO(001) substrate. The Hall resistivity of the films has a
substantial contribution arising from anomalous Hall effect of ferromagnetic
components. The critical current density (J_{c}) of the films is ~2 MA/cm^{2}
at low temperatures. Differential magneto-optical images of the remanent state
give similar J_{c} values and also exhibit presence of extended defects in the
film.Comment: 9 pages, 4 figure
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