585 research outputs found
An optical threshold function based on polarization rotation in a single semiconductor optical amplifier
Optical threshold functions are a basic building block for alloptical signal processing, and this paper investigates a threshold function design reliant on a single active element. An optical threshold function based on nonlinear polarization rotation in a single semiconductor optical amplifier is proposed. It functions due to an induced modification of the birefringence of a semiconductor optical amplifier caused by an externally injected optical control signal. It is shown that switching from both the TE to the TM mode and vice versa is possible. The measured results are supported by simulation results based on the SOA rate equations. ©2007 Optical Society of Americ
Neoclassical tearing modes in DIII-D and calculations of the stabilizing effects of localized electron cyclotron current drive
Neoclassical tearing modes are found to limit the achievable beta in many high performance discharges in DIII-D. Electron cyclotron current drive within the magnetic islands formed as the tearing mode grows has been proposed as a means of stabilizing these modes or reducing their amplitude, thereby increasing the beta limit by a factor around 1.5. Some experimental success has been obtained previously on Asdex-U. Here the authors examine the parameter range in DIII-C in which this effect can best be studied
Which blazars are neutrino loud?
Protons accelerated in the cores of active galactic nuclei can effectively
produce neutrinos only if the soft radiation background in the core is
sufficiently high. We find restrictions on the spectral properties and
luminosity of blazars under which they can be strong neutrino sources. We
analyze the possibility that neutrino flux is highly beamed along the rotation
axis of the central black hole. The enhancement of neutrino flux compared to
GeV gamma-ray flux from a given source makes the detection of neutrino point
sources more probable. At the same time the smaller open angle reduces the
number of possible neutrino-loud blazars compared to the number of gamma-ray
loud ones. We present the table of 15 blazars which are the most likely
candidates for the detection by future neutrino telescopes.Comment: 9 pages, 5 figures, version to be published in PR
Organization of atomic bond tensions in model glasses
In order to understand whether internal stresses in glasses are correlated or
randomly distributed, we study the organization of atomic bond tensions (normal
forces between pairs of atoms). Measurements of the invariants of the atomic
bond tension tensor in simulated 2D and 3D binary Lennard-Jones glasses, reveal
new and unexpected correlations and provide support for Alexander's conjecture
about the non-random character of internal stresses in amorphous solids
Anomalous Hopping Exponents of Ultrathin Films of Metals
The temperature dependence of the resistance R(T) of ultrathin
quench-condensed films of Ag, Bi, Pb and Pd has been investigated. In the most
resistive films, R(T)=Roexp(To/T)^x, where x=0.75. Surprisingly, the exponent x
was found to be constant for a wide range of Ro and To in all four materials,
possibly implying a consistent underlying conduction mechanism. The results are
discussed in terms of several different models of hopping conduction.Comment: 6 pages, 5 figure
Growth and characterization of ZnO films on (11-20) sapphire substrates by atomic layer deposition using DEZn and N2O
Zinc oxide (ZnO) films were grown on (11-20) sapphire substrates at 600 A degrees C by atomic layer deposition (ALD) using diethylzinc (DEZn) and nitrous oxide (N2O). A ZnO buffer layer was deposited at low temperature (LT) prior to the growth of a bulk ZnO film for a typical growth run. In some cases, buffer-layer annealing or post-annealing treatments were employed to optimize ZnO growth. Based on the experimental results of X-ray diffractometry (XRD) and transmission electron microscopy (TEM), all the as-grown ZnO films were found to show c-axis preferred orientation with co-existence of (ZnO)ayen (sapphire) and (ZnO)ayen (sapphire) relationships in the (0001)ZnO/(11-20)sapphire hetero-interface. Typical room temperature (RT) photoluminescence (PL) spectrum of the as-grown ZnO film shows only near band edge (NBE) emissions without defect luminescence. ZnO films with improved quality were achieved by post-annealing or buffer-layer annealing treatments. In particular, buffer-layer annealing was found to improve the crystalline and optical properties of a ZnO film substantially
Non Linear Current Response of a Many-Level Tunneling System: Higher Harmonics Generation
The fully nonlinear response of a many-level tunneling system to a strong
alternating field of high frequency is studied in terms of the
Schwinger-Keldysh nonequilibrium Green functions. The nonlinear time dependent
tunneling current is calculated exactly and its resonance structure is
elucidated. In particular, it is shown that under certain reasonable conditions
on the physical parameters, the Fourier component is sharply peaked at
, where is the spacing between
two levels. This frequency multiplication results from the highly nonlinear
process of photon absorption (or emission) by the tunneling system. It is
also conjectured that this effect (which so far is studied mainly in the
context of nonlinear optics) might be experimentally feasible.Comment: 28 pages, LaTex, 7 figures are available upon request from
[email protected], submitted to Phys.Rev.
Quantum walks: a comprehensive review
Quantum walks, the quantum mechanical counterpart of classical random walks,
is an advanced tool for building quantum algorithms that has been recently
shown to constitute a universal model of quantum computation. Quantum walks is
now a solid field of research of quantum computation full of exciting open
problems for physicists, computer scientists, mathematicians and engineers.
In this paper we review theoretical advances on the foundations of both
discrete- and continuous-time quantum walks, together with the role that
randomness plays in quantum walks, the connections between the mathematical
models of coined discrete quantum walks and continuous quantum walks, the
quantumness of quantum walks, a summary of papers published on discrete quantum
walks and entanglement as well as a succinct review of experimental proposals
and realizations of discrete-time quantum walks. Furthermore, we have reviewed
several algorithms based on both discrete- and continuous-time quantum walks as
well as a most important result: the computational universality of both
continuous- and discrete- time quantum walks.Comment: Paper accepted for publication in Quantum Information Processing
Journa
Ion radial transport induced by ICRF waves in tokamaks
The wave-induced fluxes of energetic-trapped ions during ICRF heating of tokamak plasmas are calculated using quasilinear equations. A simple single particle model of this transport mechanism is also given. Both a convective flux proportional to k/sub phi/vertical bar E/sub +/vertical bar/sup 2/ and a diffusive flux proportional to k/sub phi//sup 2/vertical bar E/sub +/vertical bar/sup 2/ are found. Here, k/sub phi/ is the toroidal wave number and E/sub +/ is the left-hand polarized wave field. The convective flux may become significant for large k/sub phi/ if the wave spectrum is asymmetric in k/sub phi/. But for the conditions of most previous experiments, these calculations indicate that radial transport driven directly by the ICRF wave is unimportant
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