2,127 research outputs found
Fractional Langevin equation
We investigate fractional Brownian motion with a microscopic random-matrix
model and introduce a fractional Langevin equation. We use the latter to study
both sub- and superdiffusion of a free particle coupled to a fractal heat bath.
We further compare fractional Brownian motion with the fractal time process.
The respective mean-square displacements of these two forms of anomalous
diffusion exhibit the same power-law behavior. Here we show that their lowest
moments are actually all identical, except the second moment of the velocity.
This provides a simple criterion which enables to distinguish these two
non-Markovian processes.Comment: 4 page
Interleukin-3Rα+ Myeloid Dendritic Cells and Mast Cells Develop Simultaneously from Different Bone Marrow Precursors in Cultures with Interleukin-3
The distinct developmental routes of dendritic cells and mast cells from murine bone marrow cultures with interleukin-3 are unclear. We found that short-term bone marrow cultures with interleukin-3 after 8–10 d consist of about 10%–30% dendritic cells and 70%–90% mast cell precursors, and only after 4–6 wk do homogeneous populations of mast cells emerge. Phenotypical and functional analysis of interleukin-3/dendritic cells revealed a high similarity with myeloid dendritic cells generated with granulocyte-macrophage colony stimulating factor in the expression of myeloid dendritic cell markers (CD11c+ B220– CD8α– CD11b+), major histocompatibility complex II and costimulatory molecules, endocytosis, maturation potential, interleukin-12 production, and T cell priming. Interleukin-3/dendritic cells expressed higher levels of interleukin-3 receptor, however. To dissect the interleukin-3/dendritic cell and mast cell development, we sorted fresh bone marrow cells into six subsets by the antibodies ER-MP12 (CD31) and ER-MP20 (Ly-6C). Both interelukin-3/dendritic cells and granulocyte-macrophage colony stimulating factor/dendritic cells develop from the same bone marrow populations, including the ER-MP12neg, ER-MP20high bone marrow monocytes. In contrast, mast cells only developed from ER-MP12int+high, ER-MP20neg bone marrow cell subsets, indicating that different precursors exist for interleukin-3/dendritic cells and mast cells. Established mast cell cultures could not be converted to dendritic cells or stimulated to express major histocompatibility complex II molecules in vitro or home to lymph node T cell areas in vivo. In summary, we show that dendritic cells generated from bone marrow precursors with interleukin-3 are clearly myeloid and develop via a different pathway compared to bone marrow mast cells
Universal fluctuations in subdiffusive transport
Subdiffusive transport in tilted washboard potentials is studied within the
fractional Fokker-Planck equation approach, using the associated continuous
time random walk (CTRW) framework. The scaled subvelocity is shown to obey a
universal law, assuming the form of a stationary Levy-stable distribution. The
latter is defined by the index of subdiffusion alpha and the mean subvelocity
only, but interestingly depends neither on the bias strength nor on the
specific form of the potential. These scaled, universal subvelocity
fluctuations emerge due to the weak ergodicity breaking and are vanishing in
the limit of normal diffusion. The results of the analytical heuristic theory
are corroborated by Monte Carlo simulations of the underlying CTRW
Solar-Type Post-T Tauri Stars in the Nearest OB Subgroups
I discuss results from the recent spectroscopic survey for solar-type pre-MS
stars in the Lower Centaurus-Crux (LCC) and Upper Centaurus-Lupus (UCL) OB
subgroups by Mamajek, Meyer, & Liebert (2002, AJ, 124, 1670). LCC and UCL are
subgroups of the Sco-Cen OB association, and the two nearest OB subgroups to
the Sun. In the entire survey of 110 pre-main sequence stars, there exists only
one Classical T Tauri star (PDS 66), implying that only ~1% of ~1 Msun stars
are still accreting at age 137 (1) Myr. Accounting for
observational errors, the HRD placement of the pre-MS stars is consistent with
the bulk of star-formation taking place within 5-10 Myr. In this contribution,
I estimate conservative upper limits to the intrinsic velocity dispersions of
the post-T Tauri stars in the LCC and UCL subgroups (<1.6 km/s and <2.2 km/s,
respectively; 95% CL) using Monte-Carlo simulations of Tycho-2 proper motions
for candidate subgroup members. I also demonstrate that a new OB subgroup
recently proposed to exist in Chamaeleon probably does not.Comment: 8 pages, 2 figures, to appear in proceedings for "Open Issues in
Local Star Formation and Early Stellar Evolution", eds. J. Gregorio-Hetem &
J. Lepine. Minor edits (5/30/03
Noise-assisted classical adiabatic pumping in a symmetric periodic potential
We consider a classical overdamped Brownian particle moving in a symmetric
periodic potential. We show that a net particle flow can be produced by
adiabatically changing two external periodic potentials with a spatial and a
temporal phase difference. The classical pumped current is found to be
independent of the friction and to vanish both in the limit of low and high
temperature. Below a critical temperature, adiabatic pumping appears to be more
efficient than transport due to a constant external force.Comment: six pages, 3 figure
Neutron star properties in the quark-meson coupling model
The effects of internal quark structure of baryons on the composition and
structure of neutron star matter with hyperons are investigated in the
quark-meson coupling (QMC) model. The QMC model is based on mean-field
description of nonoverlapping spherical bags bound by self-consistent exchange
of scalar and vector mesons. The predictions of this model are compared with
quantum hadrodynamic (QHD) model calibrated to reproduce identical nuclear
matter saturation properties. By employing a density dependent bag constant
through direct coupling to the scalar field, the QMC model is found to exhibit
identical properties as QHD near saturation density. Furthermore, this modified
QMC model provides well-behaved and continuous solutions at high densities
relevant to the core of neutron stars. Two additional strange mesons are
introduced which couple only to the strange quark in the QMC model and to the
hyperons in the QHD model. The constitution and structure of stars with
hyperons in the QMC and QHD models reveal interesting differences. This
suggests the importance of quark structure effects in the baryons at high
densities.Comment: 28 pages, 10 figures, to appear in Physical Review
Weak Localization Effect in Superconductors by Radiation Damage
Large reductions of the superconducting transition temperature and
the accompanying loss of the thermal electrical resistivity (electron-phonon
interaction) due to radiation damage have been observed for several A15
compounds, Chevrel phase and Ternary superconductors, and in
the high fluence regime. We examine these behaviors based on the recent theory
of weak localization effect in superconductors. We find a good fitting to the
experimental data. In particular, weak localization correction to the
phonon-mediated interaction is derived from the density correlation function.
It is shown that weak localization has a strong influence on both the
phonon-mediated interaction and the electron-phonon interaction, which leads to
the universal correlation of and resistance ratio.Comment: 16 pages plus 3 figures, revtex, 76 references, For more information,
Plesse see http://www.fen.bilkent.edu.tr/~yjki
Synthesis, Structure and Reactivity of a Mononuclear N,N,O-Bound Fe(II) α-Keto-Acid Complex
A bulky, tridentate phenolate ligand (ImPh2NNOtBu) was used to synthesise the first example of a mononuclear, facial, N,N,O-bound iron(II) benzoylformate complex, [Fe(ImPh2NNOtBu)(BF)] (2). The X-ray crystal structure of 2 reveals that the iron centre is pentacoordinate (τ=0.5), with a vacant site located cis to the bidentate BF ligand. The Mössbauer parameters of 2 are consistent with high-spin iron(II), and are very close to those reported for α-ketoglutarate-bound non-heme iron enzyme active sites. According to NMR and UV-vis spectroscopies, the structural integrity of 2 is retained in both coordinating and non-coordinating solvents. Cyclic voltammetry studies show that the iron centre has a very low oxidation potential and is more prone to electrochemical oxidation than the redox-active phenolate ligand. Complex 2 reacts with NO to form a S=3/2 {FeNO}7 adduct in which NO binds directly to the iron centre, according to EPR, UV-vis, IR spectroscopies and DFT analysis. Upon O2 exposure, 2 undergoes oxidative decarboxylation to form a diiron(III) benzoate complex, [Fe2(ImPh2NNOtBu)2(μ2-OBz)(μ2-OH)2]+ (3). A small amount of hydroxylated ligand was also observed by ESI-MS, hinting at the formation of a high-valent iron(IV)-oxo intermediate. Initial reactivity studies show that 2 is capable of oxygen atom transfer reactivity with O2, converting methyl(p-tolyl)sulfide to sulfoxide
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