2,423 research outputs found
Comparative study of hormonal counterregulation during GCIIS-guided hypoglycemia tests using human Proinsulin and Human Insulin (recombinant DNA)
Schlechte Scanvorlage
Circular polarization dependent study of the microwave photoconductivity in a two-dimensional electron system
The polarization dependence of the low field microwave photoconductivity and
absorption of a two-dimensional electron system has been investigated in a
quasi-optical setup in which linear and any circular polarization can be
produced in-situ. The microwave induced resistance oscillations and the zero
resistance regions are notedly immune to the sense of circular polarization.
This observation is discrepant with a number of proposed theories. Deviations
only occur near the cyclotron resonance absorption where an unprecedented large
resistance response is observed.Comment: 5 pages, 3 figure
Observation of Quantized Hall Drag in a Strongly Correlated Bilayer Electron System
The frictional drag between parallel two-dimensional electron systems has
been measured in a regime of strong interlayer correlations. When the bilayer
system enters the excitonic quantized Hall state at total Landau level filling
factor \nu_T=1 the longitudinal component of the drag vanishes but a strong
Hall component develops. The Hall drag resistance is observed to be accurately
quantized at h/e^2.Comment: 4 pages, 3 figures. Version accepted for publication in Physical
Review Letters. Improved discussion of experimental and theoretical issues,
added references, correction to figure
Computing the shortest elementary flux modes in genome-scale metabolic networks
This article is available open access through the publisher’s website through the link below. Copyright @ The Author 2009.Motivation: Elementary flux modes (EFMs) represent a key concept to analyze metabolic networks from a pathway-oriented perspective. In spite of considerable work in this field, the computation of the full set of elementary flux modes in large-scale metabolic networks still constitutes a challenging issue due to its underlying combinatorial complexity.
Results: In this article, we illustrate that the full set of EFMs can be enumerated in increasing order of number of reactions via integer linear programming. In this light, we present a novel procedure to efficiently determine the K-shortest EFMs in large-scale metabolic networks. Our method was applied to find the K-shortest EFMs that produce lysine in the genome-scale metabolic networks of Escherichia coli and Corynebacterium glutamicum. A detailed analysis of the biological significance of the K-shortest EFMs was conducted, finding that glucose catabolism, ammonium assimilation, lysine anabolism and cofactor balancing were correctly predicted. The work presented here represents an important step forward in the analysis and computation of EFMs for large-scale metabolic networks, where traditional methods fail for networks of even moderate size.
Contact: [email protected]
Supplementary information: Supplementary data are available at Bioinformatics online (http://bioinformatics.oxfordjournals.org/cgi/content/full/btp564/DC1).Fundação Calouste Gulbenkian, Fundação para a Ciência e a Tecnologia (FCT) and Siemens SA
Portugal
Highly Anisotropic Transport in the Integer Quantum Hall Effect
At very large tilt of the magnetic (B) field with respect to the plane of a
two-dimensional electron system the transport in the integer quantum Hall
regime at = 4, 6, and 8 becomes strongly anisotropic. At these filling
factors the usual {\em deep minima} in the magneto-resistance occur for the
current flowing {\em perpendicular} to the in-plane B field direction but
develop into {\em strong maxima} for the current flowing {\em parallel} to the
in-plane B field. The origin of this anisotropy is unknown but resembles the
recently observed anisotropy at half-filled Landau levels.Comment: 4 pages, 4 figure
Challenging fear: Chemical alarm signals are not causing morphology changes in crucian carp (Carassius carassius)
Crucian carp develops a deep body in the presence of chemical cues from predators, which makes the fish less vulnerable to gape-limited predators. The active components originate in conspecifics eaten by predators, and are found in the filtrate of homogenised conspecific skin. Chemical alarm signals, causing fright reactions, have been the suspected inducers of such morphological changes. We improved the extraction procedure of alarm signals by collecting the supernatant after centrifugation of skin homogenates. This removes the minute particles that normally make a filtered sample get turbid. Supernatants were subsequently diluted and frozen into ice-cubes. Presence of alarm signals was confirmed by presenting thawed ice-cubes to crucian carp in behaviour tests at start of laboratory growth experiments. Frozen extracts were added further on three times a week. Altogether, we tested potential body-depth-promoting properties of alarm signals twice in the laboratory and once in the field. Each experiment lasted for a minimum of 50 days. Despite growth of crucian carp in all experiments, no morphology changes were obtained. Accordingly, we conclude that the classical alarm signals that are releasing instant fright reactions are not inducing morphological changes in this species. The chemical signals inducing a body-depth increase are suspected to be present in the particles removed during centrifugation (i.e., in the precipitate). Tissue particles may be metabolized by bacteria in the intestine of predators, resulting in water-soluble cues. Such latent chemical signals have been found in other aquatic organisms, but hitherto not reported in fishe
Nonlinear effects in microwave photoconductivity of two-dimensional electron systems
We present a model for microwave photoconductivity of two-dimensional
electron systems in a magnetic field which describes the effects of strong
microwave and steady-state electric fields. Using this model, we derive an
analytical formula for the photoconductivity associated with photon- and
multi-photon-assisted impurity scattering as a function of the frequency and
power of microwave radiation. According to the developed model, the microwave
conductivity is an oscillatory function of the frequency of microwave radiation
and the cyclotron frequency which turns zero at the cyclotron resonance and its
harmonics. It exhibits maxima and minima (with absolute negative conductivity)
at the microwave frequencies somewhat different from the resonant frequencies.
The calculated power dependence of the amplitude of the microwave
photoconductivity oscillations exhibits pronounced sublinear behavior similar
to a logarithmic function. The height of the microwave photoconductivity maxima
and the depth of its minima are nonmonotonic functions of the electric field.
It is pointed to the possibility of a strong widening of the maxima and minima
due to a strong sensitivity of their parameters on the electric field and the
presence of strong long-range electric-field fluctuations. The obtained
dependences are consistent with the results of the experimental observations.Comment: 9 pages, 6 figures Labeling of the curves in Fig.3 correcte
Performance of the CMS Cathode Strip Chambers with Cosmic Rays
The Cathode Strip Chambers (CSCs) constitute the primary muon tracking device
in the CMS endcaps. Their performance has been evaluated using data taken
during a cosmic ray run in fall 2008. Measured noise levels are low, with the
number of noisy channels well below 1%. Coordinate resolution was measured for
all types of chambers, and fall in the range 47 microns to 243 microns. The
efficiencies for local charged track triggers, for hit and for segments
reconstruction were measured, and are above 99%. The timing resolution per
layer is approximately 5 ns
Magnetotransport in a pseudomorphic GaAs/GaInAs/GaAlAs heterostructure with a Si delta-doping layer
Magnetotransport properties of a pseudomorphic
GaAs/Ga0.8In0.2As/Ga0.75Al0.25As heterostructure are investigated in pulsed
magnetic fields up to 50 T and at temperatures of T=1.4 K and 4.2 K. The
structure studied consists of a Si delta-layer parallel to a Ga0.8In0.2As
quantum well (QW). The dark electron density of the structure is n_e=1.67x
10^16 m^-2. By illumination the density can be increased up to a factor of 4;
this way the second subband in the Ga0.8In0.2As QW can become populated as well
as the Si delta-layer. The presence of electrons in the delta-layer results in
drastic changes in the transport data, especially at magnetic fields beyond 30
T. The phenomena observed are interpreted as: 1) magnetic freeze-out of
carriers in the delta-layer when a low density of electrons is present in the
delta-layer, and 2) quantization of the electron motion in the two dimensional
electron gases in both the Ga0.8In0.2As QW and the Si delta-layer in the case
of high densities. These conclusions are corroborated by the numerical results
of our theoretical model. We obtain a satisfactory agreement between model and
experiment.Comment: 23 pages, RevTex, 11 Postscript figures (accepted for Phys. Rev. B
Evolutionary and pulsational properties of white dwarf stars
Abridged. White dwarf stars are the final evolutionary stage of the vast
majority of stars, including our Sun. The study of white dwarfs has potential
applications to different fields of astrophysics. In particular, they can be
used as independent reliable cosmic clocks, and can also provide valuable
information about the fundamental parameters of a wide variety of stellar
populations, like our Galaxy and open and globular clusters. In addition, the
high densities and temperatures characterizing white dwarfs allow to use these
stars as cosmic laboratories for studying physical processes under extreme
conditions that cannot be achieved in terrestrial laboratories. They can be
used to constrain fundamental properties of elementary particles such as axions
and neutrinos, and to study problems related to the variation of fundamental
constants.
In this work, we review the essentials of the physics of white dwarf stars.
Special emphasis is placed on the physical processes that lead to the formation
of white dwarfs as well as on the different energy sources and processes
responsible for chemical abundance changes that occur along their evolution.
Moreover, in the course of their lives, white dwarfs cross different
pulsational instability strips. The existence of these instability strips
provides astronomers with an unique opportunity to peer into their internal
structure that would otherwise remain hidden from observers. We will show that
this allows to measure with unprecedented precision the stellar masses and to
infer their envelope thicknesses, to probe the core chemical stratification,
and to detect rotation rates and magnetic fields. Consequently, in this work,
we also review the pulsational properties of white dwarfs and the most recent
applications of white dwarf asteroseismology.Comment: 85 pages, 28 figures. To be published in The Astronomy and
Astrophysics Revie
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