930 research outputs found
Preliminary design of a short-medium range windowless aircraft
This paper describes a new aircraft concept, where all windows, except those for emergency exits, are replaced with simulated windows, which consist of monitors connected to external cameras to overcome the discomfort for the passenger due to the absence of real windows. This concept is developed through an analytical method to estimate the potential advantages for the environment and for airline companies deriving from a windowless configuration for a short-medium range aircraft, within the boundaries of the preliminary design. Actually, the reduction in weight is directly linked to the reduction in fuel consumption, providing advantages in terms of operating costs and emissions of carbon dioxide. The method is applied to four models of short and medium range aircraft, namely Boeing 737\u2013800, Airbus 320, ATR72 and Embraer 190. The results show the benefits of a windowless configuration that become very positive for the operating life of an aircraft and the total fleet, potentially leading to the saving of millions of tons of carbon dioxide every year when applied to the whole fleet of the analyzed aircraft
Narrow structure in the coherent population trapping resonances in rubidium and Rayleigh scattering
The measurement of the coherent-population-trapping (CPT) resonances in
uncoated Rb vacuum cells has shown that the shape of the resonances is
different in different cells. In some cells the resonance has a complex shape -
a narrow Lorentzian structure, which is not power broadened, superimposed on
the power broadened CPT resonance. The results of the performed investigations
on the fluorescence angular distribution are in agreement with the assumption
that the narrow structure is a result of atom interaction with Rayleigh
scattering light. The results are interesting for indication of the vacuum
cleanness of the cells and building of magnetooptical sensors
Rashba Effect at Magnetic Metal Surfaces
We give experimental and theoretical evidence of the Rashba effect at the
magnetic rare-earth metal surface Gd(0001). The Rashba effect is substantially
enhanced and the Rashba parameter changes its sign when a metal-oxide surface
layer is formed. The experimental observations are quantitatively described by
ab initio calculations that give a detailed account of the near-surface charge
density gradients causing the Rashba effect. Since the sign of the Rashba
splitting depends on the magnetization direction, the findings open up new
opportunities for the study of surface and interface magnetism.Comment: 4 Fig
Current-induced magnetization dynamics in disordered itinerant ferromagnets
Current-driven magnetization dynamics in ferromagnetic metals are studied in
a self-consistent adiabatic local-density approximation in the presence of
spin-conserving and spin-dephasing impurity scattering. Based on a quantum
kinetic equation, we derive Gilbert damping and spin-transfer torques entering
the Landau-Lifshitz equation to linear order in frequency and wave vector.
Gilbert damping and a current-driven dissipative torque scale identically and
compete, with the result that a steady current-driven domain-wall motion is
insensitive to spin dephasing in the limit of weak ferromagnetism. A uniform
magnetization is found to be much more stable against spin torques in the
itinerant than in the \textit{s}-\textit{d} model for ferromagnetism. A dynamic
spin-transfer torque reminiscent of the spin pumping in multilayers is
identified and shown to govern the current-induced domain-wall distortion
Detailed studies of non-linear magneto-optical resonances at D1 excitation of Rb-85 and Rb-87 for partially resolved hyperfine F-levels
Experimental signals of non-linear magneto-optical resonances at D1
excitation of natural rubidium in a vapor cell have been obtained and described
with experimental accuracy by a detailed theoretical model based on the optical
Bloch equations. The D1 transition of rubidium is a challenging system to
analyze theoretically because it contains transitions that are only partially
resolved under Doppler broadening. The theoretical model took into account all
nearby transitions, the coherence properties of the exciting laser radiation,
and the mixing of magnetic sublevels in an external magnetic field and also
included averaging over the Doppler profile. Great care was taken to obtain
accurate experimental signals and avoid systematic errors. The experimental
signals were reproduced very well at each hyperfine transition and over a wide
range of laser power densities, beam diameters, and laser detunings from the
exact transition frequency. The bright resonance expected at the F_g=1 -->
F_e=2 transition of Rb-87 has been observed. A bright resonance was observed at
the F_g=2 --> F_e=3 transition of Rb-85, but displaced from the exact position
of the transition due to the influence of the nearby F_g=2 --> F_e=2
transition, which is a dark resonance whose contrast is almost two orders of
magnitude larger than the contrast of the bright resonance at the F_g=2 -->
F_e=3 transition. Even in this very delicate situation, the theoretical model
described in detail the experimental signals at different laser detunings.Comment: 11 pages, 9 figure
Interplay of Chemical Bonding and Magnetism in Fe_4N, Fe_3N, Fe_2N
Using spin density functional theory we have carried out a comparative study
of chemical bonding and magnetism in Fe_4N, Fe_3N and Fe_2N. All of these
compounds form close packed Fe lattices, while N occupies octahedral
interstitial positions. High spin fcc Fe and hypothetical FeN with rock salt
structure have been included in our study as reference systems. We find strong,
covalent Fe-N bonds as a result of a substantial \sigma-type p-d hybridisation,
with some charge transfer to N. Those Fe d orbitals which contribute to the p-d
bonds, do no longer participate in the exchange splitting of the Fe d bands.
Because of the large exchange fields, the majority spin d bands are always
fully occupied, while the minority spin d bands are close to half-filling, thus
optimizing the Fe d-d covalent bonding. As a consequence, in good approximation
the individual Fe moments decrease in steps of 0.5 \mu_B from fcc iron (2.7
\mu_B) via Fe_4N (2.7 and 1.97 mu_B}), \chem{Fe_3N} (1.99 \mu_B) to \zeta -
Fe_2N (1.43 \mu_B).Comment: 16 pages, 15 figure
Four-terminal resistance of an interacting quantum wire with weakly invasive contacts
We analyze the behavior of the four-terminal resistance, relative to the
two-terminal resistance of an interacting quantum wire with an impurity, taking
into account the invasiveness of the voltage probes. We consider a
one-dimensional Luttinger model of spinless fermions for the wire. We treat the
coupling to the voltage probes perturbatively, within the framework of
non-equilibrium Green function techniques. Our investigation unveils the
combined effect of impurities, electron-electron interactions and invasiveness
of the probes on the possible occurrence of negative resistance.Comment: 10 pages, 7 figure
Spin Screening and Antiscreening in a Ferromagnet/Superconductor Heterojunction
We present a theoretical study of spin screening effects in a
ferromagnet/superconductor (F/S) heterojunction. It is shown that the magnetic
moment of the ferromagnet is screened or antiscreened, depending on the
polarization of the electrons at the Fermi level. If the polarization is
determined by the electrons of the majority (minority) spin band then the
magnetic moment of the ferromagnet is screened (antiscreened) by the electrons
in the superconductor. We propose experiments that may confirm our theory: for
ferromagnetic alloys with certain concentration of Fe or Ni ions there will be
screening or antiscreening respectively. Different configurations for the
density of states are also discussed.Comment: 5 pages; 4 figures. to be published in Phys. Rev,
Efficient nonlinear room-temperature spin injection from ferromagnets into semiconductors through a modified Schottky barrier
We suggest a consistent microscopic theory of spin injection from a
ferromagnet (FM) into a semiconductor (S). It describes tunneling and emission
of electrons through modified FM-S Schottky barrier with an ultrathin heavily
doped interfacial S layer . We calculate nonlinear spin-selective properties of
such a reverse-biased FM-S junction, its nonlinear I-V characteristic, current
saturation, and spin accumulation in S. We show that the spin polarization of
current, spin density, and penetration length increase with the total current
until saturation. We find conditions for most efficient spin injection, which
are opposite to the results of previous works, since the present theory
suggests using a lightly doped resistive semiconductor. It is shown that the
maximal spin polarizations of current and electrons (spin accumulation) can
approach 100% at room temperatures and low current density in a nondegenerate
high-resistance semiconductor.Comment: 7 pages, 2 figures; provides detailed comparison with earlier works
on spin injectio
Spin-orbit coupling in ferromagnetic Nickel
We use the Gutzwiller variational theory to investigate the electronic and
the magnetic properties of fcc-Nickel. Our particular focus is on the effects
of the spin-orbit coupling. Unlike standard relativistic band-structure
theories, we reproduce the experimental magnetic moment direction and we
explain the change of the Fermi-surface topology that occurs when the magnetic
moment direction is rotated by an external magnetic field. The Fermi surface in
our calculation deviates from early de-Haas--van-Alphen (dHvA) results. We
attribute these discrepancies to an incorrect interpretation of the raw dHvA
data.Comment: 4 pages, 3 figures, submitted to PR
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