4,164 research outputs found
Half-Metallic Ferromagnetism and the spin polarization in CrO
We present electronic structure calculations in combination with local and
non-local many-body correlation effects for the half-metallic ferromagnet
CrO. Finite-temperature Dynamical Mean Field Theory results show the
existence of non-quasiparticle states, which were recently observed as almost
currentless minority spin states near the Fermi energy in resonant scattering
experients. At zero temperatures, Variational Cluster Approach calculations
support the half-metallic nature of CrO as seen in superconducting point
contact spectroscopy. The combination of these two techniques allowed us to
qualitatively describe the spin-polarization in CrO.Comment: 5 pages, 3 figure
A Generalized Preferential Attachment Model for Business Firms Growth Rates: II. Mathematical Treatment
We present a preferential attachment growth model to obtain the distribution
of number of units in the classes which may represent business firms
or other socio-economic entities. We found that is described in its
central part by a power law with an exponent which depends on
the probability of entry of new classes, . In a particular problem of city
population this distribution is equivalent to the well known Zipf law. In the
absence of the new classes entry, the distribution is exponential. Using
analytical form of and assuming proportional growth for units, we derive
, the distribution of business firm growth rates. The model predicts that
has a Laplacian cusp in the central part and asymptotic power-law tails
with an exponent . We test the analytical expressions derived using
heuristic arguments by simulations. The model might also explain the
size-variance relationship of the firm growth rates.Comment: 19 pages 6 figures Applications of Physics in Financial Analysis,
APFA
The Growth of Business Firms: Theoretical Framework and Empirical Evidence
We introduce a model of proportional growth to explain the distribution of
business firm growth rates. The model predicts that the distribution is
exponential in the central part and depicts an asymptotic power-law behavior in
the tails with an exponent 3. Because of data limitations, previous studies in
this field have been focusing exclusively on the Laplace shape of the body of
the distribution. In this article, we test the model at different levels of
aggregation in the economy, from products to firms to countries, and we find
that the model's predictions agree with empirical growth distributions and
size-variance relationships.Comment: 22 pages, 5 Postscript figures, uses revtex4. to be published in
Proc. Natl. Acad. Sci. (2005
A Generalized Preferential Attachment Model for Business Firms Growth Rates: I. Empirical Evidence
We introduce a model of proportional growth to explain the distribution
of business firm growth rates. The model predicts that is Laplace
in the central part and depicts an asymptotic power-law behavior in the tails
with an exponent . Because of data limitations, previous studies in
this field have been focusing exclusively on the Laplace shape of the body of
the distribution. We test the model at different levels of aggregation in the
economy, from products, to firms, to countries, and we find that the its
predictions are in good agreement with empirical evidence on both growth
distributions and size-variance relationships.Comment: 8 pages, 4 figure
Ka-band Ga-As FET noise receiver/device development
The development of technology for a 30 GHz low noise receiver utilizing GaAs FET devices exclusively is discussed. This program required single and dual-gate FET devices, low noise FET amplifiers, dual-gate FET mixers, and FET oscillators operating at Ka-band frequencies. A 0.25 micrometer gate FET device, developed with a minimum noise figure of 3.3 dB at 29 GHz and an associated gain of 7.4 dB, was used to fabricate a 3-stage amplifier with a minimum noise figure and associated gain of 4.4 dB and 17 dB, respectively. The 1-dB gain bandwidth of this amplifier extended from below 26.5 GHz to 30.5 GHz. A dual-gate mixer with a 2 dB conversion loss and a minimum noise figure of 10 dB at 29 GHz as well as a dielectric resonator stabilized FET oscillator at 25 GHz for the receiver L0. From these components, a hybrid microwave integrated circuit receiver was constructed which demonstrates a minimum single-side band noise figure of 4.6 dB at 29 GHz with a conversion gain of 17 dB. The output power at the 1-dB gain compression point was -5 dBm
Analysis of Absorbing Times of Quantum Walks
Quantum walks are expected to provide useful algorithmic tools for quantum
computation. This paper introduces absorbing probability and time of quantum
walks and gives both numerical simulation results and theoretical analyses on
Hadamard walks on the line and symmetric walks on the hypercube from the
viewpoint of absorbing probability and time.Comment: LaTeX2e, 14 pages, 6 figures, 1 table, figures revised, references
added, to appear in Physical Review
Quantum computation with devices whose contents are never read
In classical computation, a "write-only memory" (WOM) is little more than an
oxymoron, and the addition of WOM to a (deterministic or probabilistic)
classical computer brings no advantage. We prove that quantum computers that
are augmented with WOM can solve problems that neither a classical computer
with WOM nor a quantum computer without WOM can solve, when all other resource
bounds are equal. We focus on realtime quantum finite automata, and examine the
increase in their power effected by the addition of WOMs with different access
modes and capacities. Some problems that are unsolvable by two-way
probabilistic Turing machines using sublogarithmic amounts of read/write memory
are shown to be solvable by these enhanced automata.Comment: 32 pages, a preliminary version of this work was presented in the 9th
International Conference on Unconventional Computation (UC2010
Therapeutic target-site variability in α1-antitrypsin characterized at high resolution
The intrinsic propensity of [alpha]1-antitrypsin to undergo conformational transitions from its metastable native state to hyperstable forms provides a motive force for its antiprotease function. However, aberrant conformational change can also occur via an intermolecular linkage that results in polymerization. This has both loss-of-function and gain-of-function effects that lead to deficiency of the protein in human circulation, emphysema and hepatic cirrhosis. One of the most promising therapeutic strategies being developed to treat this disease targets small molecules to an allosteric site in the [alpha]1-antitrypsin molecule. Partial filling of this site impedes polymerization without abolishing function. Drug development can be improved by optimizing data on the structure and dynamics of this site. A new 1.8 Ă
resolution structure of [alpha]1-antitrypsin demonstrates structural variability within this site, with associated fluctuations in its upper and lower entrance grooves and ligand-binding characteristics around the innermost stable enclosed hydrophobic recess. These data will allow a broader selection of chemotypes and derivatives to be tested in silico and in vitro when screening and developing compounds to modulate conformational change to block the pathological mechanism while preserving function
Super-Eddington outburst of V4641 Sgr
X-ray transients provide unique opportunity to probe accretion regimes of at
a vastly different accretion rates. We analyze a collection of the RXTE
observations (Galactic Center scans, ASM monitoring and a pointed observation)
of enigmatic transient source high mass X-ray binary V4641 Sgr and argue that
they broadly support the hypothesis that giant September 1999 outburst was
associated with an episode of super-Eddington accretion onto the black hole.
During the outburst an extended optically thick envelope/outflow has been
formed around the source making the observational appearance of V4641 Sgr in
many aspects very similar to that of SS433. These results suggest that objects
like V4641 Sgr and SS433 indeed represent the class of objects accreting matter
at a rate comparable or above Eddington value and the formation of an
envelope/outflow is a generic characteristic of supercritical accretion.
When the accretion rate decreased the envelope vanished and the source short
term variability and spectral properties started to resemble those of other
galactic black hole candidates accreting at a rate well below the Eddington
value. Interestingly that during this phase the source spectrum was very
similar to the Cygnus X-1 spectrum in the low state inspite of more than order
of magnitude larger X-ray luminosity.Comment: 10 pages, 12 figures. Accepted in Astronomy & Astrophysic
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