9,514 research outputs found
Implications of Particle Acceleration in Active Galactic Nuclei for Cosmic Rays and High Energy Neutrino Astronomy
We consider the production of high energy neutrinos and cosmic rays in
radio-quiet active galactic nuclei (AGN) or in the central regions of
radio-loud AGN. We use a model in which acceleration of protons takes place at
a shock in an accretion flow onto a supermassive black hole, and follow the
cascade that results from interactions of the accelerated protons in the AGN
environment. We use our results to estimate the diffuse high energy neutrino
intensity and cosmic ray intensity due to AGN. We discuss our results in the
context of high energy neutrino telescopes under construction, and measurements
of the cosmic ray composition in the region of the ``knee'' in the energy
spectrum at GeV.Comment: 37 pages of compressed and uuencoded postscript; hardcopy available
on request; to be published in Astroparticle Physics; ADP-AT-94-
Quantum Black Holes, Elliptic Genera and Spectral Partition Functions
We study M-theory and D-brane quantum partition functions for microscopic
black hole ensembles within the context of the AdS/CFT correspondence in terms
of highest weight representations of infinite-dimensional Lie algebras,
elliptic genera, and Hilbert schemes, and describe their relations to elliptic
modular forms. The common feature in our examples lie in the modular properties
of the characters of certain representations of the pertinent affine Lie
algebras, and in the role of spectral functions of hyperbolic three-geometry
associated with q-series in the calculation of elliptic genera. We present new
calculations of supergravity elliptic genera on local Calabi-Yau threefolds in
terms of BPS invariants and spectral functions, and also of equivariant D-brane
elliptic genera on generic toric singularities. We use these examples to
conjecture a link between the black hole partition functions and elliptic
cohomology.Comment: 42 page
Hierarchic plate and shell models based on p-extension
Formulations of finite element models for beams, arches, plates and shells based on the principle of virtual work was studied. The focus is on computer implementation of hierarchic sequences of finite element models suitable for numerical solution of a large variety of practical problems which may concurrently contain thin and thick plates and shells, stiffeners, and regions where three dimensional representation is required. The approximate solutions corresponding to the hierarchic sequence of models converge to the exact solution of the fully three dimensional model. The stopping criterion is based on: (1) estimation of the relative error in energy norm; (2) equilibrium tests, and (3) observation of the convergence of quantities of interest
Induced Dilaton in Topologically Massive Quantum Field Theory
We consider the conformally-invariant coupling of topologically massive
gravity to a dynamical massless scalar field theory on a three-manifold with
boundary. We show that, in the phase of spontaneously broken Lorentz and Weyl
symmetries, this theory induces the target space zero mode of the vertex
operator for the string dilaton field on the boundary of the three-dimensional
manifold. By a further coupling to topologically massive gauge fields in the
bulk, we demonstrate directly from the three-dimensional theory that this
dilaton field transforms in the expected way under duality transformations so
as to preserve the mass gaps in the spectra of the gauge and gravitational
sectors of the quantum field theory. We show that this implies an intimate
dynamical relationship between T-duality and S-duality transformations of the
quantum string theory. The dilaton in this model couples bulk and worldsheet
degrees of freedom to each other and generates a dynamical string coupling.Comment: 26 pages RevTeX, 1 figure, uses epsf.st
Octopamine increases the excitability of neurons in the snail feeding system by modulation of inward sodium current but not outward potassium currents
Background: Although octopamine has long been known to have major roles as both transmitter and modulator in arthropods, it has only recently been shown to be functionally important in molluscs, playing a role as a neurotransmitter in the feeding network of the snail Lymnaea stagnalis. The synaptic potentials cannot explain all the effects of octopamine-containing neurons on the feeding network, and here we test the hypothesis that octopamine is also a neuromodulator. Results: The excitability of the B1 and B4 motoneurons in the buccal ganglia to depolarising current clamp pulses is significantly (P << 0.05) increased by (10 mu M) octopamine, whereas the B2 motoneuron becomes significantly less excitable. The ionic currents evoked by voltage steps were recorded using 2-electrode voltage clamp. The outward current of B1, B2 and B4 motoneurons had two components, a transient I-A current and a sustained I-K delayed-rectifier current, but neither was modulated by octopamine in any of these three buccal neurons. The fast inward current was eliminated in sodium - free saline and so is likely to be carried by sodium ions. 10 mu M octopamine enhanced this current by 33 and 45% in the B1 and B4 motoneurons respectively (P << 0.05), but a small reduction was seen in the B2 neuron. A Hodgkin-Huxley style simulation of the B1 motoneuron confirms that a 33% increase in the fast inward current by octopamine increases the excitability markedly. Conclusion: We conclude that octopamine is also a neuromodulator in snails, changing the excitability of the buccal neurons. This is supported by the close relationship from the voltage clamp data, through the quantitative simulation, to the action potential threshold, changing the properties of neurons in a rhythmic network. The increase in inward sodium current provides an explanation for the polycyclic modulation of the feeding system by the octopamine-containing interneurons, making feeding easier to initiate and making the feeding bursts more intense
Thermal measurement and modeling of multi-die packages
Thermal measurement and modeling of multi-die packages became a hot topic
recently in different fields like RAM chip packaging or LEDs / LED assemblies,
resulting in vertical (stacked) and lateral arrangement. In our present study
we show results for a mixed arrangement: an opto-coupler device has been
investigated with 4 chips in lateral as well as vertical arrangement. In this
paper we give an overview of measurement and modeling techniques and results
for stacked and MCM structures, describe our present measurement results
together with our structure function based methodology of validating the
detailed model of the package being studied. Also, we show how to derive
junction-to-pin thermal resistances with a technique using structure functions.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Radiation Generated by Charge Migration Following Ionization
Electronic many-body effects alone can be the driving force for an ultrafast
migration of a positive charge created upon ionization of molecular systems.
Here we show that this purely electronic phenomenon generates a characteristic
IR radiation. The situation when the initial ionic wave packet is produced by a
sudden removal of an electron is also studied. It is shown that in this case a
much stronger UV emission is generated. This emission appears as an ultrafast
response of the remaining electrons to the perturbation caused by the sudden
ionization and as such is a universal phenomenon to be expected in every
multielectron system.Comment: 5 pages, 4 figure
The Bravyi-Kitaev transformation for quantum computation of electronic structure
Quantum simulation is an important application of future quantum computers
with applications in quantum chemistry, condensed matter, and beyond. Quantum
simulation of fermionic systems presents a specific challenge. The
Jordan-Wigner transformation allows for representation of a fermionic operator
by O(n) qubit operations. Here we develop an alternative method of simulating
fermions with qubits, first proposed by Bravyi and Kitaev [S. B. Bravyi, A.Yu.
Kitaev, Annals of Physics 298, 210-226 (2002)], that reduces the simulation
cost to O(log n) qubit operations for one fermionic operation. We apply this
new Bravyi-Kitaev transformation to the task of simulating quantum chemical
Hamiltonians, and give a detailed example for the simplest possible case of
molecular hydrogen in a minimal basis. We show that the quantum circuit for
simulating a single Trotter time-step of the Bravyi-Kitaev derived Hamiltonian
for H2 requires fewer gate applications than the equivalent circuit derived
from the Jordan-Wigner transformation. Since the scaling of the Bravyi-Kitaev
method is asymptotically better than the Jordan-Wigner method, this result for
molecular hydrogen in a minimal basis demonstrates the superior efficiency of
the Bravyi-Kitaev method for all quantum computations of electronic structure
3D-2D crossover in the naturally layered superconductor (LaSe)1.14(NbSe2)
The temperature and angular dependencies of the resistive upper critical
magnetic field reveal a dimensional crossover of the superconducting
state in the highly anisotropic misfit-layer single crystal of
(LaSe)(NbSe) with the critical temperature of 1.23 K. The
temperature dependence of the upper critical field for
a field orientation along the conducting -planes displays a
characteristic upturn at 1.1 K and below this temperature the angular
dependence of has a cusp around the parallel field orientation. Both
these typical features are observed for the first time in a naturally
crystalline layered system.Comment: 7 pages incl. 3 figure
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