139,574 research outputs found
Critical gravity with a scalar field in four dimensions
We consider the critical gravity theory with a scalar field in four
dimensions. We find that this theory has the solution corresponding to the de
Sitter (dS), anti-de Sitter (AdS), and Minkowski background depending on
whether the action includes the cosmological term or not. The Minkowski
background is the solution which cannot be obtained in the model without a
scalar field. At the critical point, we show that the Abbott-Deser (AD) mass of
the Schwarzschild-de Sitter (SdS) black hole and the energy for the massless
graviton vanish, whose situation is not changed from the model without a scalar
field.Comment: 6 page
Subtropical middle atmosphere dynamics observed by the Chung Li radar
The Chung Li Radar (24.91 N; 121.24 E) has been operating since 1986. A five beam observational configuration was used on a regular basis to study the various dynamics processes in the atmosphere-lower stratosphere height region. Due to its geographical location, the annual Typhoon and Mei-Yu seasons provide good opportunities to study the various interesting dynamic processes such as instabilities, generation of gravity waves, wave mean field interaction, etc. Three dimensional air motions due to these fronts are presented. Special cases of gravity wave generation, propagation and their effects on the turbulent layers are discussed
Cold electron beams from cryo-cooled, alkali antimonide photocathodes
In this letter we report on the generation of cold electron beams using a
Cs3Sb photocathode grown by co-deposition of Sb and Cs. By cooling the
photocathode to 90 K we demonstrate a significant reduction in the mean
transverse energy validating the long standing speculation that the lattice
temperature contribution limits the mean transverse energy or thermal emittance
near the photoemission threshold, opening new frontiers in generating
ultra-bright beams. At 90 K, we achieve a record low thermal emittance of 0.2
m (rms) per mm of laser spot diameter from an ultrafast (sub-picosecond)
photocathode with quantum efficiency greater than using a
visible laser wavelength of 690 nm
Single top or bottom production associated with a scalar in \gamma p collision as a probe of topcolor-assisted technicolor
In the framework of the topcolor-assisted technicolor (TC2) models, we study
the productions of a single top or bottom quark associated with a scalar in
\gamma-p collision, which proceed via the subprocesses c\gamma -> t\pi_t^0,
c\gamma -> t h_t^0 and c\gamma -> b\pi^+_t mediated by the anomalous top or
bottom coupling tc\pi_t^0, tch_t^0 and bc\pi_t^+. These productions, while
extremely suppressed in the Standard Model, are found to be significantly
enhanced in the large part of the TC2 parameter space, especially the
production via c\gamma -> b\pi^+ can have a cross section of 100 fb, which may
be accessible and allow for a test of the TC2 models.Comment: 13 pages, 4 figures, comments and references adde
First principles theoretical studies of half-metallic ferromagnetism in CrTe
Using full-potential linear augmented plane wave method (FP-LAPW) and the
density functional theory, we have carried out a systematic investigation of
the electronic, magnetic, and cohesive properties of the chalcogenide CrTe in
three competing structures: rock-salt (RS), zinc blende (ZB) and the NiAs-type
(NA) hexagonal. Although the ground state is of NA structure, RS and ZB are
interesting in that these fcc-based structures, which can possibly be grown on
many semiconductor substrates, exhibit half-metallic phases above some critical
values of the lattice parameter. We find that the NA structure is not
half-metallic at its equilibrium volume, while both ZB and RS structures are.
The RS structure is more stable than the ZB, with an energy that is lower by
0.25 eV/atom. While confirming previous results on the half-metallic phase in
ZB structure, we provide hitherto unreported results on the half-metallic RS
phase, with a gap in the minority channel and a magnetic moment of 4.0
per formula unit. A comparison of total energies for the
ferromagnetic (FM), non-magnetic (NM), and antiferromagnetic (AFM)
configurations shows the lowest energy configuration to be FM for CrTe in all
the three structures. The FP-LAPW calculations are supplemented by linear
muffin-tin orbital (LMTO) calculations using both local density approximation
(LDA) and LDA+U method. The exchange interactions and the Curie temperatures
calculated via the linear response method in ZB and RS CrTe are compared over a
wide range of the lattice parameter. The calculated Curie temperatures for the
RS phase are consistently higher than those for the ZB phase.Comment: 11 pages, 14 figure
Weak antilocalization and zero-field electron spin splitting in AlGaN/AlN/GaN heterostructures with a polarization induced two-dimensional electron gas
Spin-orbit coupling is studied using the quantum interference corrections to
conductance in AlGaN/AlN/GaN two-dimensional electron systems where the carrier
density is controlled by the persistent photoconductivity effect. All the
samples studied exhibit a weak antilocalization feature with a spin-orbit field
of around 1.8 mT. The zero-field electron spin splitting energies extracted
from the weak antilocalization measurements are found to scale linearly with
the Fermi wavevector with an effective linear spin-orbit coupling parameter
5.5x10^{-13} eV m. The spin-orbit times extracted from our measurements varied
from 0.74 to 8.24 ps within the carrier density range of this experiment.Comment: 16 pages, 4 figure
Original stopping criteria associated tomultilevel adaptive mesh refinement to dealwith local singularities
International audienceThis paper introduces a local multilevel mesh refinement strat-egy that automatically stops relating to a user-defined tolerance even incase of local singular solutions. Refinement levels are automatically gener-ated thanks to a criterion based on the direct comparison of the a posteriorierror estimate with the prescribed error. Singular solutions locally increase with the mesh step (e.g. load discontinuities, point load or geometric in-duced singularities) and are hence characterized by locally large element-wise error whatever the mesh refinement. Then, the refinement criterionmay not be self-sufficient to stop the refinement process. Additional stop-ping criteria are required to avoid an infinite refinement process while stillrespecting the desired threshold. Two original geometry-based stopping cri-teria are proposed that consist in determining the critical region for whichthe mesh refinement becomes inefficient. Numerical examples show the effi-ciency of the methodology for stress tensor approximation in L 2 -relative orL et8734; -absolute norms
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