6,789 research outputs found
EVS: Head-up or Head Down? Evaluation of Crew Procedure and Human Factors for Enhanced Vision Systems
Feasibility of an EVS head-down procedure is examined that may provide the same operational benefits under low visibility as the FAA rule on Enhanced Flight Visibility that requires the use of a head-up display (HUD). The main element of the described EVS head-down procedure is the crew procedure within cockpit for flying the approach. The task sharing between Pilot-Flying and Pilot-Not-Flying is arranged such that multiple head-up/head-down transitions can be avoided. The pilot-flying is using the head-down display for acquisition of the necessary visual cues in the EVS image. The pilot-not-flying is monitoring the instruments and looking for the outside visual cues
Spinning Q-balls in the complex signum-Gordon model
Rotational excitations of compact Q-balls in the complex signum-Gordon model
in 2+1 dimensions are investigated. We find that almost all such spinning
Q-balls have the form of a ring of strictly finite width. In the limit of large
angular momentum M_z their energy is proportional to |M_z|^(1/5).Comment: 10 page
The Case of HCO Isomers, Revisited: Solving the Mystery of the Missing Propadienone
To date, two isomers of HCO have been detected, namely, propynal
(HCCCHO) and cylclopropenone (c-HCO). A third, propadienone
(CHCCO), has thus far eluded observers despite the fact that it is the
lowest in energy of the three. This previously noted result is in contradiction
of the minimum energy principle, which posits that the abundances of isomers in
interstellar environments can be predicted based on their relative stabilities
- and suggests, rather, the importance of kinetic over thermodynamic effects in
explaining the role of such species.
Here, we report results of \textit{ab initio} quantum chemical calculations
of the reaction between H and (a) HCO, (b) HCO (both propynal and
propadienone), and (c) CHCHCO. We have found that, among all possible
reactions between atomic hydrogen and either propadienone or propynal, only the
destruction of propadienone is barrierless and exothermic. That this
destruction pathway is indeed behind the non-detection of CHCCO is further
suggested by our finding that the product of this process, the radical
CHCHCO, can subsequently react barrierlessly with H to form propenal
(CHCHCHO) which has, in fact, been detected in regions where the other two
HCO isomers are observed. Thus, these results not only shed light on a
previously unresolved astrochemical mystery, but also further highlight the
importance of kinetics in understanding the abundances of interstellar
molecules.Comment: ApJ, accepted: 14 pages, 2 figure
Axion Dark Matter and Cosmological Parameters
We observe that photon cooling after big bang nucleosynthesis (BBN) but
before recombination can remove the conflict between the observed and
theoretically predicted value of the primordial abundance of Li. Such
cooling is ordinarily difficult to achieve. However, the recent realization
that dark matter axions form a Bose-Einstein condensate (BEC) provides a
possible mechanism, because the much colder axions may reach thermal contact
with the photons. This proposal predicts a high effective number of neutrinos
as measured by the cosmic microwave anisotropy spectrum.Comment: 4 pages, one figure. Version to appear in Phys. Rev. Lett.,
incorporating useful comments by the referees and emphasizing that photon
cooling by axion BEC is a possibility, not a certaint
Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells
Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells is studied
via the fully microscopic kinetic spin Bloch equation approach where all the
scatterings, such as the electron-impurity, electron-phonon, electron-electron
Coulomb, electron-hole Coulomb, electron-hole exchange (the Bir-Aronov-Pikus
mechanism) and the - exchange scatterings, are explicitly included. The
Elliot-Yafet mechanism is also incorporated. From this approach, we study the
spin relaxation in both -type and -type Ga(Mn)As quantum wells. For
-type Ga(Mn)As quantum wells where most Mn ions take the interstitial
positions, we find that the spin relaxation is always dominated by the DP
mechanism in metallic region. Interestingly, the Mn concentration dependence of
the spin relaxation time is nonmonotonic and exhibits a peak. This behavior is
because that the momentum scattering and the inhomogeneous broadening have
different density dependences in the non-degenerate and degenerate regimes. For
-type Ga(Mn)As quantum wells, we find that Mn concentration dependence of
the spin relaxation time is also nonmonotonic and shows a peak. Differently,
this behavior is because that the - exchange scattering (or the
Bir-Aronov-Pikus) mechanism dominates the spin relaxation in the high Mn
concentration regime at low (or high) temperature, whereas the DP mechanism
determines the spin relaxation in the low Mn concentration regime. The
Elliot-Yafet mechanism also contributes the spin relaxation at intermediate
temperature. The spin relaxation time due to the DP mechanism increases with Mn
concentration due to motional narrowing, whereas those due to the spin-flip
mechanisms decrease with Mn concentration, which thus leads to the formation of
the peak.... (The remaining is omitted due to the space limit)Comment: 12 pages, 8 figures, Phys. Rev. B 79, 2009, in pres
Extreme laser pulses for possible development of boron fusion power reactors for clean and lasting energy
Extreme laser pulses driving non-equilibrium processes in high density
plasmas permit an increase of the fusion of hydrogen with the boron isotope 11
by nine orders of magnitude of the energy gains above the classical values.
This is the result of initiating the reaction by non-thermal ultrahigh
acceleration of plasma blocks by the nonlinear (ponderomotive) force of the
laser field, in addition to the avalanche reaction that has now been
experimentally and theoretically manifested. The design of a very compact
fusion power reactor is scheduled to produce then environmentally fully clean
and inexhaustible generation of energy at profitably low costs. The reaction
within a volume of cubic millimetres during a nanosecond can only be used for
controlled power generation.Comment: 10 pages, 5 fugure
Efficiency of initiating cell adhesion in hydrodynamic flow
We theoretically investigate the efficiency of initial binding between a
receptor-coated sphere and a ligand-coated wall in linear shear flow. The mean
first passage time for binding decreases monotonically with increasing shear
rate. Above a saturation threshold of the order of a few 100 receptor patches,
the binding efficiency is enhanced only weakly by increasing their number and
size, but strongly by increasing their height. This explains why white blood
cells in the blood flow adhere through receptor patches localized to the tips
of microvilli, and why malaria-infected red blood cells form elevated receptor
patches (knobs).Comment: 4 pages, Revtex, 4 Postscript figures included, to appear in PR
On worst-case investment with applications in finance and insurance mathematics
We review recent results on the new concept of worst-case portfolio optimization, i.e. we consider the determination of portfolio processes which yield the highest worst-case expected utility bound if the stock price may have uncertain (down) jumps. The optimal portfolios are derived as solutions of non-linear differential equations which itself are consequences of a Bellman principle for worst-case bounds. They are by construction non-constant ones and thus differ from the usual constant optimal portfolios in the classical examples of the Merton problem. A particular application of such strategies is to model crash possibilities where both the number and the height of the crash is uncertain but bounded. We further solve optimal investment problems in the presence of an additional risk process which is the typical situation of an insurer
Solvable Examples of Drift and Diffusion of Ions in Non-uniform Electric Fields
The drift and diffusion of a cloud of ions in a fluid are distorted by an
inhomogeneous electric field. If the electric field carries the center of the
distribution in a straight line and the field configuration is suitably
symmetric, the distortion can be calculated analytically. We examine the
specific examples of fields with cylindrical and spherical symmetry in detail
assuming the ion distributions to be of a generally Gaussian form. The effects
of differing diffusion coefficients in the transverse and longitudinal
directions are included
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