303 research outputs found
Self-Consistent Effective-Medium Approximations with Path Integrals
We study effective-medium approximations for linear composite media by means
of a path integral formalism with replicas. We show how to recover the
Bruggeman and Hori-Yonezawa effective-medium formulas. Using a replica-coupling
ansatz, these formulas are extended into new ones which have the same
percolation thresholds as that of the Bethe lattice and Potts model of
percolation, and critical exponents s=0 and t=2 in any space dimension d>= 2.
Like the Bruggeman and Hori-Yonezawa formulas, the new formulas are exact to
second order in the weak-contrast and dilute limits. The dimensional range of
validity of the four effective-medium formulas is discussed, and it is argued
that the new ones are of better relevance than the classical ones in dimensions
d=3,4 for systems obeying the Nodes-Links-Blobs picture, such as
random-resistor networks.Comment: 18 pages, 6 eps figure
Shock-driven nucleation and self-organization of dislocations in the dynamical Peierls model
Dynamic nucleation of dislocations caused by a stress front ('shock') of
amplitude moving with speed is investigated by solving
numerically the Dynamic Peierls Equation with an efficient method. Speed
and amplitude are considered as independent variables, with
possibly exceeding the longitudinal wavespeed . Various
reactions between dislocations take place such as scattering, dislocation-pair
nucleation, annihilation, and crossing. Pairs of edge dislocation are always
nucleated with speed (and likewise for screws with replaced by , the shear wavespeed). The plastic wave exhibits
self-organization, forming distinct `bulk' and `front' zones. Nucleations occur
either within the bulk or at the zone interface, depending on the value of .
The front zone accumulates dislocations that are expelled from the bulk or from
the interface. In each zone, dislocation speeds and densities are measured as
functions of simulation parameters. The densities exhibit a scaling behavior
with stress, given by , where
represents the nucleation threshold and .Comment: 16 pages and 14 figures + 3 appended pages and 6 figures of
Supplemental Materia
Cycler Orbits and the Solar System Pony Express
In this work, we explore the concept of a secondary “data mule” consisting of a small satellite used to ferry data from a Mars mission to Earth for downlink. The concept exploits the fact that two nearby optical communicators can achieve extremely high data rates, and that a class of trajectories called “cyclers” can carry a satellite between Mars and Earth regularly. By exploiting cycler orbits, the courier needs minimal onboard propulsion. However, cycler orbits have long periodicity, as it can take years for the satellite, Mars, and Earth to repeat their relative geometry. Therefore, we propose the use of a network of such cycler “couriers” on phase-shifted trajectories to achieve a regular cadence of downlink trips. We design a series of search and optimization steps that can output a set of trajectories that at first approximation have low onboard propulsion requirements and can be used for any regular logistics network to and from Mars, then derive the link budget for proximity optical communications to show that this network can ferry large amounts of data
Loss of Bim Increases T Cell Production and Function in Interleukin 7 Receptor–deficient Mice
Interleukin (IL)-7 receptor (R) signaling is essential for T and B lymphopoiesis by promoting proliferation, differentiation, and survival of cells. Mice lacking either IL-7 or the IL-7Rα chain have abnormally low numbers of immature as well as mature T and B lymphocytes. Transgenic expression of the apoptosis inhibitor Bcl-2 rescues T cell development and function in IL-7Rα–deficient mice, indicating that activation of a proapoptotic Bcl-2 family member causes death of immature and mature T cells. BH3-only proteins such as Bim, which are distant proapoptotic members of the Bcl-2 family, are essential initiators of programmed cell death and stress-induced apoptosis. We generated Bim/IL-7Rα double deficient mice and found that loss of Bim significantly increased thymocyte numbers, restored near normal numbers of mature T cells in the blood and spleen, and enhanced cytotoxic T cell responses to virus infection in IL-7Rα−/− mice. These results indicate that Bim cooperates with other proapoptotic proteins in the death of IL-7–deprived T cell progenitors in vivo, but is the major inducer of this pathway to apoptosis in mature T cells. This indicates that pharmacological inhibition of Bim function might be useful for boosting immune responses in immunodeficient patients
FOXO3a-dependent regulation of Puma in response to cytokine/growth factor withdrawal
Puma is an essential mediator of p53-dependent and -independent apoptosis in vivo. In response to genotoxic stress, Puma is induced in a p53-dependent manner. However, the transcription factor driving Puma up-regulation in response to p53-independent apoptotic stimuli has yet to be identified. Here, we show that FOXO3a up-regulates Puma expression in response to cytokine or growth factor deprivation. Importantly, dysregulated Akt signaling in lymphoid cells attenuated Puma induction upon cytokine withdrawal. Our results suggest that Puma, together with another BH3 only member, Bim, function as FOXO3a downstream targets to mediate a stress response when PI3K/Akt signaling is down-regulated
Performance of neutron-irradiated 4H-Silicon Carbide diodes subjected to Alpha radiation
The unique electrical and material properties of 4H-silicon-carbide (4H-SiC)
make it a promising candidate material for high rate particle detectors. In
contrast to the ubiquitously used silicon (Si), 4H-SiC offers a higher carrier
saturation velocity and larger breakdown voltage, enabling a high intrinsic
time resolution and mitigating pile-up effects. Additionally, as radiation
hardness requirements grow more demanding, wide-bandgap materials such as
4H-SiC could offer better performance. In this work, the detector performance
of 50 micron thick 4H-SiC p-in-n planar pad sensors was investigated at room
temperature, using an 241Am alpha source at reverse biases of up to 1100 V.
Samples subjected to neutron irradiation with fluences of up to 1e16/cm^2 were
included in the study in order to quantify the radiation hardness properties of
4H-SiC. The obtained results are compared to previously performed UV-TCT
studies. Samples exhibit a drop in charge collection efficiency (CCE) with
increasing irradiation fluence, partially compensated at high reverse bias
voltages far above full depletion voltage. A plateau of the collected charges
is observed in accordance with the depletion of the volume the alpha particles
penetrate for an unirradiated reference detector. For the neutron-irradiated
samples, such a plateau only becomes apparent at higher reverse bias. For the
highest investigated fluence, CCE behaves almost linearly with increasing
reverse bias. Compared to UV-TCT measurements, the reverse bias required to
deplete a sensitive volume covering full energy deposition is lower, due to the
small penetration depth of the alpha particles. At the highest reverse bias,
the measured CCE values agree well with earlier UV-TCT studies, with
discrepancies between 1% and 5%.Comment: 10 pages (8 without references), 6 figures, 1 table, to be published
in the Proceedings Section of Journal of Instrumentation (JINST) as a
proceeding of iWoRiD202
Chemical chaperone TUDCA prevents apoptosis and improves survival during polymicrobial sepsis in mice
Sepsis-induced lymphopenia is a major cause of morbidities in intensive care units and in populations with chronic conditions such as renal failure, diabetes, HIV and alcohol abuse. Currently, other than supportive care and antibiotics, there are no treatments for this condition. We developed an in vitro assay to understand the role of the ER-stress-mediated apoptosis process in lymphocyte death during polymicrobial sepsis, which was reproducible in in vivo mouse models. Modulating ER stress using chemical chaperones significantly reduced the induction of the pro-apoptotic protein Bim both in vitro and in mice. Furthermore, in a ‘two-hit’ pneumonia model in mice, we have been able to demonstrate that administration of the chemical chaperone TUDCA helped to maintain lymphocyte homeostasis by significantly reducing lymphocyte apoptosis and this correlated with four-fold improvement in survival. Our results demonstrate a novel therapeutic opportunity for treating sepsis-induced lymphopenia in humans
1.26 m intersubband transitions in InGaAs/AlAs quantum wells
We observed room-temperature intersubband transitions at 1.26 microns in
n-doped type-II InGaAs/AlAs strained quantum wells. An improved
tight-binding model was used to optimize the structure parameters in order to
obtain the shortest wavelength intersubband transition ever achieved in a
semiconductor system. The corresponding transitions occur between the first
confined electronic levels of the well following mid-infrared optical pumping
of electrons from the barrier X-valley into the well ground state.Comment: To appear in Applied Physics Letter
The Assembly History of Field Spheroidals: Evolution of Mass-to-light Ratios and Signatures of Recent Star Formation
We present a comprehensive catalog of high signal-to-noise spectra obtained
with the DEIMOS spectrograph on the Keck II telescope for a sample of
F850LP<22.43 (AB) field spheroidal (E+S0s; 163) and bulge dominated disk (61)
galaxies in the redshift range 0.2<z<1.2. We examine the zero point, tilt and
scatter of the Fundamental Plane (FP) as a function of redshift and
morphological properties, carefully accounting for luminosity-dependent biases
via Montecarlo simulations. The evolution of the overall FP can be represented
by a mean change in effective mass-to-light ratio given by <d \log (M/L_{\rm
B})/dz>=-0.72^{+0.07}_{-0.05}\pm0.04. However, this evolution depends
significantly on the dynamical mass, being slower for larger masses as reported
in a previous letter. In addition, we separately show the intrinsic scatter of
the FP increases with redshift as d(rms(M/L_{\rm B}))/dz=0.040\pm0.015.
Although these trends are consistent with single burst populations which formed
at for high mass spheroidals and z_{f}~1.2 for lower mass systems, a
more realistic picture is that most of the stellar mass formed in all systems
at z>2 with subsequent activity continuing to lower redshifts (z<1.2). The
fraction of stellar mass formed at recent times depend strongly on galactic
mass, ranging from <1% for masses above 10^{11.5} M_{\odot} to 20-40% below
10^{11} M_{\odot}. Independent support for recent activity is provided by
spectroscopic ([\ion{O}{2}] emission, H\delta) and photometric (blue cores and
broad-band colors) diagnostics. Via the analysis of a large sample with many
independent diagnostics, we are able to reconcile previously disparate
interpretations of the assembly history of field spheroidals. [Abridged]Comment: 26 pages including 24 figures, submitted to ApJ. Complete and compact
version with full resolution images available at
http://www.astro.ucla.edu/~ttreu/ms.pd
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