26,285 research outputs found
Inter-valley plasmons in graphene
The spectrum of two-dimensional (2D) plasma waves in graphene has been
recently studied in the Dirac fermion model. We take into account the whole
dispersion relation for graphene electrons in the tight binding approximation
and the local field effects in the electrodynamic response. Near the
wavevectors close to the corners of the hexagon-shaped Brillouin zone we found
new low-frequency 2D plasmon modes with a linear spectrum. These "inter-valley"
plasmon modes are related to the transitions between the two nearest Dirac
cones.Comment: 4 pages, 2 figures; submitted in PR
Deconfinement in the Quark Meson Coupling Model
The Quark Meson Coupling Model which describes nuclear matter as a collection
of non-overlapping MIT bags interacting by the self-consistent exchange of
scalar and vector mesons is used to study nuclear matter at finite temperature.
In its modified version, the density dependence of the bag constant is
introduced by a direct coupling between the bag constant and the scalar mean
field. In the present work, the coupling of the scalar mean field with the
constituent quarks is considered exactly through the solution of the Dirac
equation. Our results show that a phase transition takes place at a critical
temperature around 200 MeV in which the scalar mean field takes a nonzero value
at zero baryon density. Furthermore it is found that the bag constant decreases
significantly when the temperature increases above this critical temperature
indicating the onset of quark deconfinement.Comment: LaTeX/TeX 15 pages (zk2.tex)+ 6 figures in TeX forma
Composite nucleons in scalar and vector mean-fields
We emphasize that the composite structure of the nucleon may play quite an
important role in nuclear physics. It is shown that the momentum-dependent
repulsive force of second order in the scalar field, which plays an important
role in Dirac phenomenology, can be found in the quark-meson coupling (QMC)
model, and that the properties of nuclear matter are well described through the
quark-scalar density in a nucleon and a self-consistency condition for the
scalar field. The difference between theories of point-like nucleons and
composite ones may be seen in the change of the -meson mass in nuclear
matter if the composite nature of the nucleon suppresses contributions from
nucleon-antinucleon pair creation.Comment: 10 page
Muon spin relaxation and rotation study on the solid solution of the two spin-gap systems (CH3)2CHNH3-CuCl3 and (CH3)2CHNH3-CuBr3
Muon-spin-rotation and relaxation studies have been performed on
(CH)CHNHCu(ClBr) with =0.85 and 0.95, which are
solid solutions of the two isomorphic spin-gap systems
(CH)CHNHCuCl and (CH)CHNHCuBr with different
spin gaps. The sample with =0.85 showed a clear muon spin rotation under
zero-field below =11.65K, indicating the existence of a long-range
antiferromagnetic order. A critical exponent of the hyperfine field was
obtained to be =0.33, which agrees with 3D-Ising model. In the other
sample with =0.95, an anomalous enhancement of the muon spin relaxation was
observed at very low temperatures indicating a critical slowing down due to a
magnetic instability of the ground state
TARGET: Rapid Capture of Process Knowledge
TARGET (Task Analysis/Rule Generation Tool) represents a new breed of tool that blends graphical process flow modeling capabilities with the function of a top-down reporting facility. Since NASA personnel frequently perform tasks that are primarily procedural in nature, TARGET models mission or task procedures and generates hierarchical reports as part of the process capture and analysis effort. Historically, capturing knowledge has proven to be one of the greatest barriers to the development of intelligent systems. Current practice generally requires lengthy interactions between the expert whose knowledge is to be captured and the knowledge engineer whose responsibility is to acquire and represent the expert's knowledge in a useful form. Although much research has been devoted to the development of methodologies and computer software to aid in the capture and representation of some types of knowledge, procedural knowledge has received relatively little attention. In essence, TARGET is one of the first tools of its kind, commercial or institutional, that is designed to support this type of knowledge capture undertaking. This paper will describe the design and development of TARGET for the acquisition and representation of procedural knowledge. The strategies employed by TARGET to support use by knowledge engineers, subject matter experts, programmers and managers will be discussed. This discussion includes the method by which the tool employs its graphical user interface to generate a task hierarchy report. Next, the approach to generate production rules for incorporation in and development of a CLIPS based expert system will be elaborated. TARGET also permits experts to visually describe procedural tasks as a common medium for knowledge refinement by the expert community and knowledge engineer making knowledge consensus possible. The paper briefly touches on the verification and validation issues facing the CLIPS rule generation aspects of TARGET. A description of efforts to support TARGET's interoperability issues on PCs, Macintoshes and UNIX workstations concludes the paper
NMR characterization of spin-1/2 alternating antiferromagnetic chains in the high-pressure phase of (VO)2P2O7
Local-susceptibility measurements via the NMR shifts of P and V
nuclei in the high-pressure phase of (VO)PO confirmed the
existence of a unique alternating antiferromagnetic chain with a zero-field
spin gap of 34 K. The P nuclear spin-lattice relaxation rate scales with
the uniform spin susceptibility below about 15 K which shows that the
temperature dependence of both the static and dynamical spin susceptibilities
becomes identical at temperatures not far below the spin-gap energy.Comment: 6 pages, 5 figures; To be published in J. Phys. Condens. Matte
Energy gap tuning in graphene on hexagonal boron nitride bilayer system
We use a tight binding approach and density functional theory calculations to
study the band structure of graphene/hexagonal boron nitride bilayer system in
the most stable configuration. We show that an electric field applied in the
direction perpendicular to the layers significantly modifies the electronic
structure of the whole system, including shifts, anticrossing and other
deformations of bands, which can allow to control the value of the energy gap.
It is shown that band structure of biased system may be tailored for specific
requirements of nanoelectronics applications. The carriers' mobilities are
expected to be higher than in the bilayer graphene devices.Comment: 10 pages, 7 figures, submitted to Physical Review
Phase-resolved Crab pulsar measurements from 25 to 400 GeV with the MAGIC telescopes
We report on observations of the Crab pulsar with the MAGIC telescopes. Our
data were taken in both monoscopic (> 25GeV) and stereoscopic (> 50GeV)
observation modes. Two peaks were detected with both modes and phase-resolved
energy spectra were calculated. By comparing with Fermi- LAT measurements, we
find that the energy spectrum of the Crab pulsar does not follow a power law
with an exponential cutoff, but has an additional hard component, extending up
to at least 400 GeV. This suggests that the emission above 25 GeV is not
dominated by curvature radiation, as suggested in the standard scenarios of the
OG and SG models.Comment: 4 pages, 2 figures, Proc. TAUP 2011, submitted for publication in
JCP
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