16,330 research outputs found
Thick silicon growth techniques
Hall mobility measurements on a number of single crystal silicon ribbons grown from graphite dies have shown some ribbons to have mobilities consistent with their resistivities. The behavior of other ribbons appears to be explained by the introduction of impurities of the opposite sign. Growth of a small single crystal silicon ribbon has been achieved from a beryllia dia. Residual internal stresses of the order of 7 to 18,000 psi have been determined to exist in some silicon ribbon, particularly those grown at rates in excess of 1 in./min. Growth experiments have continued toward definition of a configuration and parameters to provide a reasonable yield of single crystal ribbons. High vacuum outgassing of graphite dies and evacuation and backfilling of growth chambers have provided significant improvements in surface quality of ribbons grown from graphite dies
A search for rotating radio transients and fast radio bursts in the Parkes high-latitude pulsar survey
Discoveries of rotating radio transients and fast radio bursts (FRBs) in
pulsar surveys suggest that more of such transient sources await discovery in
archival data sets. Here we report on a single-pulse search for dispersed radio
bursts over a wide range of Galactic latitudes (|b| < ) in data
previously searched for periodic sources by Burgay et al. We re-detected 20 of
the 42 pulsars reported by Burgay et al. and one rotating radio transient
reported by Burke-Spolaor. No FRBs were discovered in this survey. Taking into
account this result, and other recent surveys at Parkes, we corrected for
detection sensitivities based on the search software used in the analyses and
the different backends used in these surveys and find that the all-sky FRB
event rate for sources with a fluence above 4.0 Jy ms at 1.4 GHz to be FRBs day sky, where the
uncertainties represent a confidence interval. While this rate is lower
than inferred from previous studies, as we demonstrate, this combined event
rate is consistent with the results of all systematic FRB searches at Parkes to
date and does not require the need to postulate a dearth of FRBs at
intermediate latitudes.Comment: Accepted, 10 pages, 6 figure
SIRT1 Activity Is Linked to Its Brain Region-Specific Phosphorylation and Is Impaired in Huntington’s Disease Mice
Huntingtons disease (HD) is a neurodegenerative disorder for which there are no disease-modifying treatments. SIRT1 is a NAD+-dependent protein deacetylase that is implicated in maintaining neuronal health during development, differentiation and ageing. Previous studies suggested that the modulation of SIRT1 activity is neuroprotective in HD mouse models, however, the mechanisms controlling SIRT1 activity are unknown. We have identified a striatum-specific phosphorylation-dependent regulatory mechanism of SIRT1 induction under normal physiological conditions, which is impaired in HD. We demonstrate that SIRT1 activity is down-regulated in the brains of two complementary HD mouse models, which correlated with altered SIRT1 phosphorylation levels. This SIRT1 impairment could not be rescued by the ablation of DBC1, a negative regulator of SIRT1, but was linked to changes in the sub-cellular distribution of AMPK-α1, a positive regulator of SIRT1 function. This work provides insights into the regulation of SIRT1 activity with the potential for the development of novel therapeutic strategies
Thick film silicon growth techniques
One inch wide silicon ribbons up to 14 inches long have been produced from graphite dies. Several different techniques have been employed to improve the semiconductor purity of silicon. This has resulted in a general increase in quality although the techniques involved have not been optimized. The power factor of uncoated ribbon solar cells produced for material evaluation has increased to approximately 75% of those evaluation cells made from commercial silicon. The present limitation is believed due to low lifetime. Additional work has continued with new die materials; however, only composite dies of SiO2 and C show significant potential at this time
Entropy-induced Microphase Separation in Hard Diblock Copolymers
Whereas entropy can induce phase behavior that is as rich as seen in
energetic systems, microphase separation remains a very rare phenomenon in
entropic systems. In this paper, we present a density functional approach to
study the possibility of entropy-driven microphase separation in diblock
copolymers. Our model system consists of copolymers composed of freely-jointed
slender hard rods. The two types of monomeric segments have comparable lengths,
but a significantly different diameter, the latter difference providing the
driving force for the phase separation. At the same time these systems can also
exhibit liquid crystalline phases. We treat this system in the appropriate
generalization of the Onsager approximation to chain-like particles. Using a
linear stability (bifurcation) analysis, we analytically determine the onset of
the microseparated and the nematic phases for long chains. We find that for
very long chains the microseparated phase always preempts the nematic. In the
limit of infinitely long chains, the correlations within the chain become
Gaussian and the approach becomes exact. This allows us to define a Gaussian
limit in which the theory strongly simplifies and the competition between
microphase separation and liquid crystal formation can be studied essentially
analytically. Our main results are phase diagrams as a function of the
effective diameter difference, the segment composition and the length ratio of
the segments. We also determine the amplitude of the positional order as a
function of position along the chain at the onset of the microphase separation
instability. Finally, we give suggestions as to how this type of
entropy-induced microphase separation could be observed experimentally.Comment: 16 pages, 7 figure
Lower semicontinuity of attractors for non-autonomous dynamical systems
This paper is concerned with the lower semicontinuity of attractors for semilinear
non-autonomous differential equations in Banach spaces. We require the unperturbed
attractor to be given as the union of unstable manifolds of time-dependent hyperbolic
solutions, generalizing previous results valid only for gradient-like systems in which
the hyperbolic solutions are equilibria. The tools employed are a study of the continuity
of the local unstable manifolds of the hyperbolic solutions and results on the continuity of
the exponential dichotomy of the linearization around each of these solutions
Strong Shift Equivalence of -correspondences
We define a notion of strong shift equivalence for -correspondences and
show that strong shift equivalent -correspondences have strongly Morita
equivalent Cuntz-Pimsner algebras. Our analysis extends the fact that strong
shift equivalent square matrices with non-negative integer entries give stably
isomorphic Cuntz-Krieger algebras.Comment: 26 pages. Final version to appear in Israel Journal of Mathematic
RISE: a fast-readout imager for exoplanet transit timing
By the precise timing of the low amplitude (0.005 - 0.02 magnitude) transits
of exoplanets around their parent star it should be possible to infer the
presence of other planetary bodies in the system down to Earth-like masses. We
describe the design and construction of RISE, a fast-readout frame transfer
camera for the Liverpool Telescope designed to carry out this experiment. The
results of our commissioning tests are described as well as the data reduction
procedure necessary. We present light curves of two objects, showing that the
desired timing and photometric accuracy can be obtained providing that
autoguiding is used to keep the target on the same detector pixel for the
entire (typically 4 hour) observing run.Comment: Published in PROC SPIE, vol 7014, 70416
Probing a non-biaxial behavior of infinitely thin hard platelets
We give a criterion to test a non-biaxial behavior of infinitely thin hard
platelets of symmetry based upon the components of three order
parameter tensors. We investigated the nematic behavior of monodisperse
infinitely thin rectangular hard platelet systems by using the criterion.
Starting with a square platelet system, and we compared it with rectangular
platelet systems of various aspect ratios. For each system, we performed
equilibration runs by using isobaric Monte Carlo simulations. Each system did
not show a biaxial nematic behavior but a uniaxial nematic one, despite of the
shape anisotropy of those platelets. The relationship between effective
diameters by simulations and theoretical effective diameters of the above
systems was also determined.Comment: Submitted to JPS
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