10,167 research outputs found
Reversable heat flow through the carbon nanotube junctions
Microscopic mechanisms of externally controlled reversable heat flow through
the carbon nanotube junctions (NJ) are studied theoretically. Our model
suggests that the heat is transfered along the tube section by
electrons () and holes () moving ballistically in either in parallel or
in opposite directions and accelerated by the bias source-drain voltage (Peltier effect). We compute the Seebeck coefficient , electric
and thermal conductivities and find that their magnitudes
strongly depend on and . The sign reversal of
versus the sign of formerly observed experimentally is interpreted
in this work in terms of so-called chiral tunneling phenomena (Klein paradox)
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
Design of helicopter rotor blades for optimum dynamic characteristics
The mass and stiffness distributions for helicopter rotor blades are tailored in such a way to give a predetermined placement of blade natural frequencies. The optimal design is pursued with respect of minimum weight, sufficient inertia, and reasonable dynamic characteristics. Finite element techniques are used as a tool. Rotor types include hingeless, articulated, and teetering
The congruence of the social and conventional entrepreneur: An examination of goal split distribution, emergence by age, and antecedent model congruence
Defined social and conventional entrepreneurs enjoy very different levels of support and educational offerings in the United States and abroad. With the launch of more and more nonprofit organizations being spurred on by the entrepreneur, the extent to which those that seek a distributed bottom line for their intended organization differ from those with economic goals becomes an important line of inquiry.
The study of the entrepreneur began, in many ways, with Schumpeter in that late 1930’s, and the echoes of his economic background remain implicit in the definition of the breed to this day. This work seeks to determine the extent to which the defined social and conventional entrepreneur share a common process, predeliction, and mind set, in an effort to determine if their commonalities warrant generally congruent classification and treatment.
This work examines the extent of congruence through a study of the goal splits, emergence levels by age, and classification rates of antecedent composites of both defined types of entrepreneur. It uncovers interesting similarities that bring about an alternate conceptualization of what it means to be an entrepreneur, and challenges how they should be best educated and incubated
Pristine CNO abundances from Magellanic Cloud B stars II. Fast rotators in the LMC cluster NGC 2004
We present spectroscopic abundance analyses of three main-sequence B stars in
the young Large Magellanic Cloud cluster NGC 2004. All three targets have
projected rotational velocities around 130 km/s. Techniques are presented that
allow the derivation of stellar parameters and chemical abundances in spite of
these high v sin i values. Together with previous analyses of stars in this
cluster, we find no evidence among the main-sequence stars for effects due to
rotational mixing up to v sin i around 130 km/s. Unless the equatorial
rotational velocities are significantly larger than the v sin i values, this
finding is probably in line with theoretical expectations. NGC 2004/B30, a star
of uncertain evolutionary status located in the Blue Hertzsprung Gap, clearly
shows signs of mixing in its atmosphere. To verify the effects due to
rotational mixing will therefore require homogeneous analysis of statistically
significant samples of low-metallicity main-sequence B stars over a wide range
of rotational velocities.Comment: 12 pages, 5 figures, 2 tables; accepted for publication in ApJ (vol.
633, p. 899
Controlling hole spin dynamics in two‐dimensional hole systems at low temperatures
With the recent discovery of very long hole spin decoherence times in GaAs/AlGaAs heterostructures of more than 70 ns
in two-dimensional hole systems, using the hole spin as a viable alternative to electron spins in spintronic applications seems
possible. Furthermore, as the hyperfine interaction with the nuclear spins is likely to be the limiting factor for electron spin
lifetimes in zero dimensions, holes with their suppressed Fermi contact hyperfine interaction due to their p-like nature should
be able to show even longer lifetimes than electrons. For spintronic applications, electric-field control of hole spin dynamics
is desirable.
Here, we report on time-resolved Kerr rotation and resonant spin amplification measurements on a two-dimensional hole
system in a p-doped GaAs/AlGaAs heterostructure. Via a semitransparent gate, we tune the charge density within the sample.
We are able to observe a change in the hole g factor, as well as in the hole spin dephasing time at high magnetic fields
Resonant spin amplification of hole spin dynamics in two‐dimensional hole systems: experiment and simulation
Spins in semiconductor structures may allow for the realization of scalable quantum bit arrays, an essential
component for quantum computation schemes. Specifically, hole spins may be more suited for this purpose than electron
spins, due to their strongly reduced interaction with lattice nuclei, which limits spin coherence for electrons in quantum dots.
Here, we present resonant spin amplification (RSA) measurements, performed on a p-modulation doped GaAs-based quantum
well at temperatures below 500 mK. The RSA traces have a peculiar, butterfly-like shape, which stems from the initialization
of a resident hole spin polarization by optical orientation. The combined dynamics of the optically oriented electron and hole
spins are well-described by a rate equation model, and by comparison of experiment and model, hole spin dephasing times of
more than 70 ns are extracted from the measured data
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
Design of helicopter rotor blades for optimum dynamic characteristics
The possibilities and limitations of tailoring blade mass and stiffness distributions to give an optimum blade design in terms of weight, inertia, and dynamic characteristics are discussed. The extent that changes in mass of stiffness distribution can be used to place rotor frequencies at desired locations is determined. Theoretical limits to the amount of frequency shift are established. Realistic constraints on blade properties based on weight, mass, moment of inertia, size, strength, and stability are formulated. The extent that the hub loads can be minimized by proper choice of E1 distribution, and the minimum hub loads which can be approximated by a design for a given set of natural frequencies are determined. Aerodynamic couplings that might affect the optimum blade design, and the relative effectiveness of mass and stiffness distribution on the optimization procedure are investigated
Scanning Raman spectroscopy of graphene antidot lattices: Evidence for systematic p-type doping
We have investigated antidot lattices, which were prepared on exfoliated
graphene single layers via electron-beam lithography and ion etching, by means
of scanning Raman spectroscopy. The peak positions, peak widths and intensities
of the characteristic phonon modes of the carbon lattice have been studied
systematically in a series of samples. In the patterned samples, we found a
systematic stiffening of the G band mode, accompanied by a line narrowing,
while the 2D mode energies are found to be linearly correlated with the G mode
energies. We interpret this as evidence for p-type doping of the nanostructured
graphene
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