366 research outputs found
The Relationship Between Beam Power and Radio Power for Classical Double Radio Sources
Beam power is a fundamental parameter that describes, in part, the state of a
supermassive black hole system. Determining the beam powers of powerful
classical double radio sources requires substantial observing time, so it would
be useful to determine the relationship between beam power and radio power so
that radio power could be used as a proxy for beam power. A sample of 31
powerful classical double radio sources with previously determined beam and
radio powers are studied; the sources have redshifts between about 0.056 and
1.8. It is found that the relationship between beam power, Lj, and radio power,
P, is well described by Log(Lj) = 0.84 Log(P) + 2.15, where both L_j and P are
in units of 10^(44) erg/s. This indicates that beam power is converted to radio
power with an efficiency of about 0.7%. The ratio of beam power to radio power
is studied as a function of redshift; there is no significant evidence for
redshift evolution of this ratio over the redshift range studied. The
relationship is consistent with empirical results obtained by Cavagnolo et al.
(2010) for radio sources in gas rich environments, which are primarily FRI
sources, and with the theoretical predictions of Willott et al. (1999).Comment: 6 pages, 2 figures, 2 tables; accepted for publication in MNRA
Tuning the electron-phonon coupling in multilayer graphene with magnetic fields
Magneto Raman scattering study of the E optical phonons in multi-layer
epitaxial graphene grown on a carbon face of SiC are presented. At 4.2K in
magnetic field up to 33 T, we observe a series of well pronounced avoided
crossings each time the optically active inter Landau level transition is tuned
in resonance with the E phonon excitation (at 196 meV). The width of the
phonon Raman scattering response also shows pronounced variations and is
enhanced in conditions of resonance. The experimental results are well
reproduced by a model that gives directly the strength of the electron-phonon
interaction.Comment: 4 pages, 3 figure
Effect of a magnetic field on the two-phonon Raman scattering in graphene
We have studied, both experimentally and theoretically, the change of the
so-called 2D band of the Raman scattering spectrum of graphene (the two-phonon
peak near 2700 cm-1) in an external magnetic field applied perpendicular to the
graphene crystal plane at liquid helium temperature. A shift to lower frequency
and broadening of this band is observed as the magnetic field is increased from
0 to 33 T. At fields up to 5--10 T the changes are quadratic in the field while
they become linear at higher magnetic fields. This effect is explained by the
curving of the quasiclassical trajectories of the photo-excited electrons and
holes in the magnetic field, which enables us (i) to extract the electron
inelastic scattering rate, and (ii) to conclude that electronic scattering
accounts for about half of the measured width of the 2D peak.Comment: 11 pages, 7 figure
Symmetry breaking in commensurate graphene rotational stacking; a comparison of theory and experiment
Graphene stacked in a Bernal configuration (60 degrees relative rotations
between sheets) differs electronically from isolated graphene due to the broken
symmetry introduced by interlayer bonds forming between only one of the two
graphene unit cell atoms. A variety of experiments have shown that non-Bernal
rotations restore this broken symmetry; consequently, these stacking varieties
have been the subject of intensive theoretical interest. Most theories predict
substantial changes in the band structure ranging from the development of a Van
Hove singularity and an angle dependent electron localization that causes the
Fermi velocity to go to zero as the relative rotation angle between sheets goes
to zero. In this work we show by direct measurement that non-Bernal rotations
preserve the graphene symmetry with only a small perturbation due to weak
effective interlayer coupling. We detect neither a Van Hove singularity nor any
significant change in the Fermi velocity. These results suggest significant
problems in our current theoretical understanding of the origins of the band
structure of this material.Comment: 7 pages, 6 figures, submitted to PR
The structural properties of the multi-layer graphene/4H-SiC(000-1) system as determined by Surface X-ray Diffraction
We present a structural analysis of the multi-layer graphene-4HSiC(000-1})
system using Surface X-Ray Reflectivity. We show for the first time that
graphene films grown on the C-terminated (000-1}) surface have a
graphene-substrate bond length that is very short (0.162nm). The measured
distance rules out a weak Van der Waals interaction to the substrate and
instead indicates a strong bond between the first graphene layer and the bulk
as predicted by ab-initio calculations. The measurements also indicate that
multi-layer graphene grows in a near turbostratic mode on this surface. This
result may explain the lack of a broken graphene symmetry inferred from
conduction measurements on this system [C. Berger et al., Science 312, 1191
(2006)].Comment: 9 pages with 6 figure
Air-Combat Strategy Using Approximate Dynamic Programming
Unmanned Aircraft Systems (UAS) have the potential to perform many
of the dangerous missions currently own by manned aircraft. Yet, the
complexity of some tasks, such as air combat, have precluded UAS from
successfully carrying out these missions autonomously. This paper presents
a formulation of a level flight, fixed velocity, one-on-one air combat maneuvering problem and an approximate dynamic programming (ADP) approach for computing an efficient approximation of the optimal policy. In the version of the problem formulation considered, the aircraft learning the
optimal policy is given a slight performance advantage. This ADP approach
provides a fast response to a rapidly changing tactical situation, long planning horizons, and good performance without explicit coding of air combat tactics. The method's success is due to extensive feature development, reward shaping and trajectory sampling. An accompanying fast and e ffective rollout-based policy extraction method is used to accomplish on-line implementation. Simulation results are provided that demonstrate the robustness of the method against an opponent beginning from both off ensive and defensive situations. Flight results are also presented using micro-UAS own at MIT's Real-time indoor Autonomous Vehicle test ENvironment
(RAVEN).Defense University Research Instrumentation Program (U.S.) (grant number FA9550-07-1-0321)United States. Air Force Office of Scientific Research (AFOSR # FA9550-08-1-0086)American Society for Engineering Education (National Defense Science and Engineering Graduate Fellowship
Localization of Dirac electrons by Moire patterns in graphene bilayers
We study the electronic structure of two Dirac electron gazes coupled by a
periodic Hamiltonian such as it appears in rotated graphene bilayers. Ab initio
and tight-binding approaches are combined and show that the spatially periodic
coupling between the two Dirac electron gazes can renormalize strongly their
velocity. We investigate in particular small angles of rotation and show that
the velocity tends to zero in this limit. The localization is confirmed by an
analysis of the eigenstates which are localized essentially in the AA zones of
the Moire patterns.Comment: 4 pages, 5 figure
Patterning graphene nanostripes in substrate-supported functionalized graphene: A promising route to integrated, robust, and superior transistors
It is promising to apply quantum-mechanically confined graphene systems in
field-effect transistors. High stability, superior performance, and large-scale
integration are the main challenges facing the practical application of
graphene transistors. Our understandings of the adatom-graphene interaction
combined with recent progress in the nanofabrication technology indicate that
very stable and high-quality graphene nanostripes could be integrated in
substrate-supported functionalized (hydrogenated or fluorinated) graphene using
electron-beam lithography. We also propose that parallelizing a couple of
graphene nanostripes in a transistor should be preferred for practical
application, which is also very useful for transistors based on graphene
nanoribbon.Comment: Frontiers of Physics (2012) to be publishe
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