1,919 research outputs found
Mapping Monte Carlo to Langevin dynamics: A Fokker-Planck approach
We propose a general method of using the Fokker-Planck equation (FPE) to link
the Monte-Carlo (MC) and the Langevin micromagnetic schemes. We derive the
drift and disusion FPE terms corresponding to the MC method and show that it is
analytically equivalent to the stochastic Landau-Lifshitz-Gilbert (LLG)
equation of Langevin-based micromagnetics. Subsequent results such as the time
quantification factor for the Metropolis MC method can be rigorously derived
from this mapping equivalence. The validity of the mapping is shown by the
close numerical convergence between the MC method and the LLG equation for the
case of a single magnetic particle as well as interacting arrays of particles.
We also found that our Metropolis MC is accurate for a large range of damping
factors , unlike previous time-quantified MC methods which break down
at low , where precessional motion dominates.Comment: 4 pages, 4 figures. Accepted for publication in Phys. Rev. Let
Growth of GaN films on porous SiC substrate by molecular-beam epitaxy
Porous SiC (PSiC) substrates were used for the growth of GaN by reactive molecular-beam epitaxy with ammonia as the nitrogen source. Improved quality of GaNfilms has been demonstrated for growth on PSiC substrates, as compared to that on standard 6H–SiC substrates. Cross-sectional transmission electron microscopy and electron diffraction showed a reduction in dislocation density and a higher degree of lattice and thermal relaxation in the GaNfilmsgrown on porous substrates. The submicron GaNfilms exhibit a rocking curve linewidth of 3.3 arcmin for (0002) diffraction and 13.7 arcmin for (101̄2) diffraction. Low-temperature photoluminescence showed an excitonic transition with a full width at half maximum of 9.5 meV at 15 K, as well as high quantum efficiency, on the GaN layer grown on PSiC when the thin skin layer on porous SiC was removed before growth
Patchy Amphiphilic Dendrimers Bind Adenovirus and Control Its Host Interactions and in Vivo Distribution
The surface of proteins is heterogeneous with sophisticated but precise hydrophobic and hydrophilic patches, which is essential for their diverse biological functions. To emulate such distinct surface patterns on macromolecules, we used rigid spherical synthetic dendrimers (polyphenylene dendrimers) to provide controlled amphiphilic surface patches with molecular precision. We identified an,. I optimal spatial arrangement of these patches on certain dendrimers that enabled their interaction with human adenovirus 5 (Ads). Patchy dendrimers bound to the surface of Ads formed a synthetic polymer corona that greatly altered various host interactions of Ads as well as in vivo distribution. The dendrimer corona (1) improved the ability of Ad5-derived gene transfer vectors to transduce cells deficient for the primary Ad5 cell membrane receptor and (2) modulated the binding of Ads to blood coagulation factor X, one of the most critical virus host interactions in the bloodstream. It significantly enhanced the transduction efficiency of Ad5 while also protecting it from neutralization by natural antibodies and the complement system in human whole blood. Ads with a synthetic dendrimer corona revealed profoundly altered in vivo distribution, improved transduction of heart, and dampened vector sequestration by liver and spleen. We propose the design of bioactive polymers that bind protein surfaces solely based on their amphiphilic surface patches and protect against a naturally occurring protein corona, which is highly attractive to improve Ad5-based in vivo gene therapy applications
Angular position of nodes in the superconducting gap of YBCO
The thermal conductivity of a YBCO single crystal has been studied as a
function of the relative orientation of the crystal axes and a magnetic field
rotating in the Cu-O planes. Measurements were carried out at several
temperatures below T_c and at a fixed field of 30 kOe. A four-fold symmetry
characteristic of a superconducting gap with nodes at odd multiples of 45
degrees in k-space was resolved. Experiments were performed to exclude a
possible macroscopic origin for such a four-fold symmetry such as sample shape
or anisotropic pinning. Our results impose an upper limit of 10% on the weight
of the s-wave component of the essentially d-wave superconducting order
parameter of YBCO.Comment: 10 pages, 4 figure
Effectiveness of TiN porous templates on the reduction of threading dislocations in GaN overgrowth by organometallic vapor-phase epitaxy
We report on the reduction of threading dislocations in GaN overlayers grown by organometallic vapor phase epitaxy on micro-porous TiN networks. These networks were obtained by in situannealing of thin Ti layers deposited in a metalization chamber, on the (0001) face of GaN templates. Observations by transmission electron microscopy indicate dislocation reduction by factors of up to 10 in GaN layers grown on TiN networks compared with the control GaN.X-ray diffraction shows that GaNgrown on the TiN network has a smaller (102) plane peak width (4.6 arcmin) than the control GaN (7.8 arcmin). In low temperature photoluminescence spectra, a narrow excitonic full-width-at-half-maximum of 2.4 meV was obtained, as compared to 3.0 meV for the control GaN, confirming the improved crystalline quality of the overgrown GaN layers
Submillimeter emission from the hot molecular jet HH 211
We observed the HH 211 jet in the submillimeter continuum and the CO(3-2) and
SiO(8-7) transitions with the Submillimeter Array. The continuum source
detected at the center of the outflow shows an elongated morphology,
perpendicular to the direction of the outflow axis. The high-velocity emission
of both molecules shows a knotty and highly collimated structure. The SiO(8-7)
emission at the base of the outflow, close to the driving source, spans a wide
range of velocities, from -20 up to 40 km s^{-1}. This suggests that a
wide-angle wind may be the driving mechanism of the HH 211 outflow. For
distances greater than 5" (1500 AU) from the driving source, emission from both
transitions follows a Hubble-law behavior, with SiO(8-7) reaching higher
velocities than CO(3-2), and being located upstream of the CO(3-2) knots. This
indicates that the SiO(8-7) emission is likely tracing entrained gas very close
to the primary jet, while the CO(3-2) is tracing less dense entrained gas. From
the SiO(5-4) data of Hirano et al. we find that the SiO(8-7)/SiO(5-4)
brightness temperature ratio along the jet decreases for knots far from the
driving source. This is consistent with the density decreasing along the jet,
from (3-10)x10^6 cm^{-3} at 500 AU to (0.8-4)x10^6 cm^{-3} at 5000 AU from the
driving source.Comment: 3 pages, 3 figures. Accepted by Astrophysical Journal Letter
Dislocation reduction in GaN grown on porous TiN networks by metal-organic vapor-phase epitaxy
We report on the effectiveness of porous TiN nanonetworks on the reduction of threading dislocations (TDs) in GaN grown by metal-organic vapor-phase epitaxy (MOVPE). The porous TiN networks were formed by in situ annealing of thin-deposited Ti films deposited ex situ on GaN templates within the MOVPE growth chamber. Different annealing parameters in relation to surface porosity of TiN networks were investigated. Transmission electron micrographs indicated dislocation reduction by factors of up to 10 in GaN layers grown on the TiN nanonetwork, compared with a control sample. TiN prevented many dislocations present in the GaN templates from penetrating into the upper layer. Microscale epitaxial lateral overgrowth of GaN above TiN also contributed to TD reduction. The surface porosity of the TiN network had a strong impact on the efficiency of TD reduction. X-ray-diffraction and time-resolved photoluminescence measurements further confirmed the improved GaN quality
Efficacy of single and double SiNx interlayers on defect reduction in GaN overlayers grown by organometallic vapor-phase epitaxy
We report on the growth of and evolution of defects in GaN epilayers having single- and double-layer SiNx nanoporous insertion layers. The SiNx was formed in situ in the growth chamber of an organometallic vapor-phase epitaxy system by simultaneous flow of diluted silane and ammonia. The GaN epilayers and SiNx interlayers were grown on 6H-SiC substrates using three different nucleation layers, namely, low-temperature GaN, high-temperature GaN, and high-temperature AlN nucleation layers. X-ray-diffraction rocking curves and cross-sectional and plan-view transmission electron microscope analyses indicated that a nanoporous SiNx layer can reduce the dislocations density in the GaN overgrown layer to ∼3×108cm−2 range; below this level the defect blocking effect of SiNx would saturate. Therefore the insertion of a second SiNx layer becomes much less effective in reducing dislocations, although it continues to reduce the point defects, as suggested by time-resolved photoluminescence measurements. The insertion of SiNx interlayers was found to improve significantly the mechanical strength of the GaN epilayers resulting in a much lower crack line density
The magnetic field of IRAS 16293-2422 as traced by shock-induced H2O masers
Shock-induced H2O masers are important magnetic field tracers at very high
density gas. Water masers are found in both high- and low-mass star-forming
regions, acting as a powerful tool to compare magnetic field morphologies in
both mass regimes. In this paper, we show one of the first magnetic field
determinations in the low-mass protostellar core IRAS 16293-2422 at volume
densities as high as 10^(8-10) cm^-3. Our goal is to discern if the collapsing
regime of this source is controlled by magnetic fields or other factors like
turbulence. We used the Very Large Array (VLA) to carry out
spectro-polarimetric observations in the 22 GHz Zeeman emission of H2O masers.
From the Stokes V line profile, we can estimate the magnetic field strength in
the dense regions around the protostar. A blend of at least three maser
features can be inferred from our relatively high spatial resolution data set
(~ 0.1"), which is reproduced in a clear non-Gaussian line profile. The
emission is very stable in polarization fraction and position angle across the
channels. The maser spots are aligned with some components of the complex
outflow configuration of IRAS 16293-2422, and they are excited in zones of
compressed gas produced by shocks. The post-shock particle density is in the
range of 1-3 x 10^9 cm^-3, consistent with typical water masers pumping
densities. Zeeman emission is produced by a very strong line-of-sight magnetic
field (B ~ 113 mG). The magnetic field pressure derived from our data is
comparable to the ram pressure of the outflow dynamics. This indicates that the
magnetic field is energetically important in the dynamical evolution of IRAS
16293-2422.Comment: 7 pages, 6 figures, accepted for publication in A&
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All-Fiber Passively Q-Switched Laser Based on Tm<sup>3+</sup>-Doped Tellurite Fiber
We report all-fiber passively Q-switched Tm3+-doped tellurite fiber lasers. The composite tellurite fiber is specially designed to improve the mechanical strength. Both carbon nanotubes (CNTs)
and semiconductor saturable absorber mirror (SESAM) are inserted separately into the laser
cavities as saturable absorbers to demonstrate a fiber integrated setup. In a short, 9 cm tellurite fiber, 1.86 μm pulsed lasers without self-mode-locking effect are demonstrated by in-band pumping at 1.59 μm. An average power of 84 mW is obtained in CNT pulsed laser with 860 ns duration while in SESAM pulsed laser, the average power reaches 21mW with 516 ns pulse width.This work was supported in part by the National Natural Science Foundation of China under Grant 61308084, Natural Science Foundation of Shanghai under Grant 12ZR1451600, and Granted Membership of Youth Innovation Promotion Association, Chinese Academy of Sciences.This is the accepted manuscript. The final version is available from IEEE at http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6998013
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