3,985 research outputs found
In-situ growth of superconducting MgB2 thin films by molecular beam epitaxy
The in-situ growth of superconducting MgB2 thin films was examined from
various perspectives. The paper discusses (1) growth temperature, (2) the
effect of excess Mg, (3) the effect of residual gas during growth, (4) the
effect of in-situ annealing, (5) thickness dependence and (6) the effect of
substrates. Our results provide a guide to the preparation of high-quality
superconducting MgB2 films for potential electronics applications.Comment: 26 pages, 16 figures, submitted to J. Appl. Phy
Cosmic-ray Acceleration at Ultrarelativistic Shock Waves: Effects of a "Realistic" Magnetic Field Structure
First-order Fermi acceleration processes at ultrarelativistic shocks are
studied with Monte Carlo simulations. The accelerated particle spectra are
derived by integrating the exact particle trajectories in a turbulent magnetic
field near the shock. ''Realistic'' features of the field structure are
included. We show that the main acceleration process at superluminal shocks is
the particle compression at the shock. Formation of energetic spectral tails is
possible in a limited energy range only for highly perturbed magnetic fields,
with cutoffs occuring at low energies within the resonance energy range
considered. These spectral features result from the anisotropic character of
particle transport in the downstream magnetic field, where field compression
produces effectively 2D perturbations. Because of the downstream field
compression, the acceleration process is inefficient in parallel shocks for
larger turbulence amplitudes, and features observed in oblique shocks are
recovered. For small-amplitude turbulence, wide-energy range particle spectra
are formed and modifications of the process due to the existence of long-wave
perturbations are observed. In both sub- and superluminal shocks, an increase
of \gamma leads to steeper spectra with lower cut-off energies. The spectra
obtained for the ``realistic'' background conditions assumed here do not
converge to the ``universal'' spectral index claimed in the literature. Thus
the role of the first-order Fermi process in astrophysical sources hosting
relativistic shocks requires serious reanalysis.Comment: submitted to Ap
Instability and Periodic Deformation in Bilayer Membranes Induced by Freezing
The instability and periodic deformation of bilayer membranes during freezing
processes are studied as a function of the difference of the shape energy
between the high and the low temperature membrane states. It is shown that
there exists a threshold stability condition, bellow which a planar
configuration will be deformed. Among the deformed shapes, the periodic curved
square textures are shown being one kind of the solutions of the associated
shape equation. In consistency with recent expe rimental observations, the
optimal ratio of period and amplitude for such a texture is found to be
approximately equal to (2)^{1/2}\pi.Comment: 8 pages in Latex form, 1 Postscript figure. To be appear in Mod.
Phys. Lett. B. 199
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Flux jumps in high-J <inf>c</inf> MgB<inf>2</inf> bulks during pulsed field magnetization
Pulsed field magnetization (PFM) of a high-J_c MgB_2 bulk disk has been investigated at 20 K, in which flux jumps frequently occur for high pulsed fields. Using a numerical simulation of the PFM procedure, we estimated the time dependence of the local magnetic field and temperature during PFM. We analyzed the electromagnetic and thermal instability of the high-J_c MgB_2 bulk to avoid flux jumps using the time dependence of the critical thickness, d_c(t), which shows the upper safety thickness to stabilize the superconductor magnetically, and the minimum propagation zone length, l_m(t), to obtain dynamical stability. The values of d_c(t) and l_m(t) change along the thermally-stabilized direction with increasing temperature below the critical temperature, T_c. However, the flux jump can be qualitatively understood by the local temperature, T(t), which exceeds T_c in the bulk. Finally, possible solutions to avoid flux jumps in high-J_c MgB_2 bulks are discussed.This work was supported by Open Partnership Joint Projects of Japan Society for the Promotion of Science (JSPS) Bilateral Joint Research Projects, and JSPS KAKENHI grant number 23560002 and 15K04646. Dr Mark Ainslie would like to acknowledge the support of a Royal Academy of Engineering Research Fellowship and a Royal Society International Exchanges Scheme grant, IE131084.This is the author accepted manuscript. The final version is available from IOP Publishing via http://dx.doi.org/10.1088/0953-2048/29/3/03400
Shielding and Trapped Field Properties of Large MgB2 Bulk
AbstractWe have studied the magnetic shielding properties of MgB2 bulk fabricated by a reactive liquid Mg infiltration (Mg-RLI) method. The magnetic shielding profiles indicated partially field-penetrated areas at the bulk periphery with weaker pinning force at 38K. We discuss about the superconductive inhomogeneity in the bulk by comparing the shielding profiles with trapped field ones by the subsequent zero-field-cooled magnetization (ZFCM) and the pulsed-field magnetization (PFM). The present method is advantageous to detect the inhomogeneity at the periphery of a large superconducting bulk
Cosmic Ray Acceleration at Relativistic Shock Waves with a "Realistic" Magnetic Field Structure
The process of cosmic ray first-order Fermi acceleration at relativistic
shock waves is studied with the method of Monte Carlo simulations. The
simulations are based on numerical integration of particle equations of motion
in a turbulent magnetic field near the shock. In comparison to earlier studies,
a few "realistic" features of the magnetic field structure are included. The
upstream field consists of a mean field component inclined at some angle to the
shock normal with finite-amplitude sinusoidal perturbations imposed upon it.
The perturbations are assumed to be static in the local plasma rest frame.
Their flat or Kolmogorov spectra are constructed with randomly drawn wave
vectors from a wide range . The downstream field structure
is derived from the upstream one as compressed at the shock. We present
particle spectra and angular distributions obtained at mildly relativistic sub-
and superluminal shocks and also parallel shocks. We show that particle spectra
diverge from a simple power-law, the exact shape of the spectrum depends on
both the amplitude of the magnetic field perturbations and the wave power
spectrum. Features such as spectrum hardening before the cut-off at oblique
subluminal shocks and formation of power-law tails at superluminal ones are
presented and discussed. At parallel shocks, the presence of finite-amplitude
magnetic field perturbations leads to the formation of locally oblique field
configurations at the shock and the respective magnetic field compressions.
This results in the modification of the particle acceleration process,
introducing some features present in oblique shocks, e.g., particle reflections
from the shock. We demonstrate for parallel shocks a (nonmonotonic) variation
of the particle spectral index with the turbulence amplitude.Comment: revised version (37 pages, 13 figures
Optical conductivity and superconductivity in LaSb
We have measured the resistivity, optical conductivity, and magnetic
susceptibility of LaSb to search for clues as to the cause of the
extraordinarily large linear magnetoresistance and to explore the properties of
the superconducting state. We find no evidence in the optical conductivity for
the formation of a charge density wave state above 20 K despite the highly
layered crystal structure. In addition, only small changes to the optical
reflectivity with magnetic field are observed indicating that the MR is due to
scattering rate, not charge density, variations with field. Although a
superconducting ground state was previously reported below a critical
temperature of 0.4 K, we observe, at ambient pressure, a fragile
superconducting transition with an onset at 2.5 K. In crystalline samples, we
find a high degree of variability with a minority of samples displaying a full
Meissner fraction below 0.2 K and fluctuations apparent up to 2.5 K. The
application of pressure stabilizes the superconducting transition and reduces
the anisotropy of the superconducting phase.Comment: 4 pages with 4 figure
Dispersive photoluminescence decay by geminate recombination in amorphous semiconductors
The photoluminescence decay in amorphous semiconductors is described by power
law at long times. The power-law decay of photoluminescence at
long times is commonly observed but recent experiments have revealed that the
exponent, , is smaller than the value 1.5 predicted from a
geminate recombination model assuming normal diffusion. Transient currents
observed in the time-of-flight experiments are highly dispersive characterized
by the disorder parameter smaller than 1. Geminate recombination rate
should be influenced by the dispersive transport of charge carriers. In this
paper we derive the simple relation, . Not only the
exponent but also the amplitude of the decay calculated in this study is
consistent with measured photoluminescence in a-Si:H.Comment: 18pages. Submitted for the publication in Phys. Rev.
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