1,641 research outputs found
f(R) Gravity, relic coherent gravitons and optical chaos
We discuss the production of massive relic coherent gravitons in a particular
class of f(R) gravity which arises from string theory and their possible
imprint in Cosmic Microwave Background. In fact, in the very early universe
these relic gravitons could have acted as slow gravity waves. They may have
then acted to focus the geodesics of radiation and matter. Therefore, their
imprint on the later evolution of the universe could appear as filaments and
domain wall in the Universe today. In that case, the effect on Cosmic Microwave
Background should be analogous to the effect of water waves, which, in focusing
light, create optical caustics which are commonly seen on the bottom of
swimming pools. We analyze this important issue by showing how relic massive
GWs perturb the trajectories of Cosmic Microwave Background photons
(gravitational lensing by relic GWs). The consequence of the type of physics
discussed is outlined by illustrating an amplification of what might be called
optical chaos.Comment: 32 pages, 1 figure, invited paper to appear in "Beyond Standard
Gravity and Cosmology", special issue of "Galaxies" edited by Antonaldo
Diaferi
Flight investigation of the effect of tail boom strakes on helicopter directional control
A joint U.S. Army/NASA flight investigation was conducted utilizing a single-rotor helicopter to determine the effectiveness of horizontally mounted tail boom strakes on directional controllability and tail rotor power during low-speed, crosswind operating conditions. Three configurations were investigated: (1) baseline (strakes off), (2) single strake (strake at upper shoulder on port side of boom), and (3) double strake (upper strake plus a lower strake on same side of boom). The strakes were employed as a means to separate airflow over the tail boom and change fuselage yawing moments in a direction to improve the yaw control margin and reduce tail rotor power. Crosswind data were obtained in 5-knot increments of airspeed from 0 to 35 knots and in 30 deg increments of wind azimuth from 0 deg to 330 deg. At the most critical wind azimuth and airspeed in terms of tail rotor power, the strakes improved the pedal margin by 6 percent of total travel and reduced tail rotor power required by 17 percent. The increase in yaw control and reduction in tail rotor power offered by the strakes can expand the helicopter operating envelope in terms of gross weight and altitude capability. The strakes did not affect the flying qualities of the vehicle at airspeeds between 35 and 100 knots
Spin injection from perpendicular magnetized ferromagnetic -MnGa into (Al,Ga)As heterostructures
Electrical spin injection from ferromagnetic -MnGa into an (Al,Ga)As
p-i-n light emitting diode (LED) is demonstrated. The -MnGa layers show
strong perpendicular magnetocrystalline anisotropy, enabling detection of spin
injection at remanence without an applied magnetic field. The bias and
temperature dependence of the spin injection are found to be qualitatively
similar to Fe-based spin LED devices. A Hanle effect is observed and
demonstrates complete depolarization of spins in the semiconductor in a
transverse magnetic field.Comment: 4 pages, 3 figure
Hyperfine Interactions and Spin Transport in Ferromagnet-Semiconductor Heterostructures
Measurements and modeling of electron spin transport and dynamics are used to
characterize hyperfine interactions in Fe/GaAs devices with -GaAs channels.
Ga and As nuclei are polarized by electrically injected electron spins, and the
nuclear polarization is detected indirectly through the depolarization of
electron spins in the hyperfine field. The dependence of the electron spin
signal on injector bias and applied field direction is modeled by a coupled
drift-diffusion equation, including effective fields from both the electronic
and nuclear polarizations. This approach is used to determine the electron spin
polarization independently of the assumptions made in standard transport
measurements. The extreme sensitivity of the electron spin dynamics to the
nuclear spin polarization also facilitates the electrical detection of nuclear
magnetic resonance.Comment: Submitted to Phys. Rev.
- …