37,300 research outputs found
Evaluation and study of advanced optical contamination, deposition, measurement, and removal techniques
A program is described to design, fabricate and install an experimental work chamber assembly (WCA) to provide a wide range of experimental capability. The WCA incorporates several techniques for studying the kinetics of contaminant films and their effect on optical surfaces. It incorporates the capability for depositing both optical and contaminant films on temperature-controlled samples, and for in-situ measurements of the vacuum ultraviolet reflectance. Ellipsometer optics are mounted on the chamber for film thickness determinations, and other features include access ports for radiation sources and instrumentation. Several supporting studies were conducted to define specific chamber requirements, to determine the sensitivity of the measurement techniques to be incorporated in the chamber, and to establish procedures for handling samples prior to their installation in the chamber. A bibliography and literature survey of contamination-related articles is included
Rabi oscillations under ultrafast excitation of graphene
We study coherent nonlinear dynamics of carriers under ultrafast interband
excitation of an intrinsic graphene. The Rabi oscillations of response appear
with increasing of pumping intensity. The photoexcited distribution is
calculated versus time and energy taking into account the effects of energy
relaxation and dephasing. Spectral and temporal dependencies of the response on
a probe radiation (transmission and reflection coefficients) are considered for
different pumping intensities and the Rabi oscillations versus time and
intensity are analyzed.Comment: 6 pages, 6 figure
Accelerations in Flight
This report deals with the accelerations obtained in flight on various airplanes at Langley Field for the purpose of obtaining the magnitude of the load factors in flight and to procure information on the behavior of an airplane in various maneuvers. The instrument used in these tests was a recording accelerometer of a new type designed by the technical staff of the National Advisory Committee for Aeronautics. The instrument consists of a flat steel spring supported rigidly at one end so that the free end may be deflected by its own weight from its neutral position by any acceleration acting at right angles to the plane of the spring. This deflection is measured by a very light tilting mirror caused to rotate by the deflection of the spring, which reflected the beam of light onto a moving film. The motion of the spring is damped by a thin aluminum vane which rotates with the spring between the poles of an electric magnet. Records were taken on landings and takeoffs, in loops, spins, spirals, and rolls
Control in Circling Flight
This investigation was undertaken for the purpose of developing instruments that would record the forces and positions of all three controls, and to obtain data on the behavior of an airplane in turns. All the work was done on a standard rigged JN4H (airplane no. 2 of National Advisory Committee for Aeronautics, report no. 70). It was found that the airplane was longitudinally unstable and nose heavy; that it was laterally unstable, probably due to too little dihedral; and that it was directionally unstable, due to insufficient fin area, this last being very serious, for in case of a loss of rudder control the airplane immediately whips into a spin from which there is no way of getting it out. On the other hand, it was found possible to fly quite satisfactorily with the rudder locked, and safely, though not so well, with the ailerons locked
Effects of energy dependence in the quasiparticle density of states on far-infrared absorption in the pseudogap state
We derive a relationship between the optical conductivity scattering rate
1/\tau(\omega) and the electron-boson spectral function \alpha^2F(\Omega) valid
for the case when the electronic density of states, N(\epsilon), cannot be
taken as constant in the vicinity of the Fermi level. This relationship turned
out to be useful for analyzing the experimental data in the pseudogap state of
cuprate superconductors.Comment: 8 pages, RevTeX4, 1 EPS figure; final version published in PR
Coherent oscillations of electrons in tunnel-coupled wells under ultrafast intersubband excitation
Ultrafast intersubband excitation of electrons in tunnell-coupled wells is
studied depending on the structure parameters, the duration of the infrared
pump and the detuning frequency. The temporal dependencies of the photoinduced
concentration and dipole moment are obtained for two cases of transitions: from
the single ground state to the tunnel-coupled excited states and from the
tunnel-coupled states to the single excited state. The peculiarities of
dephasing and population relaxation processes are also taken into account. The
nonlinear regime of the response is also considered when the splitting energy
between the tunnel-coupled levels is renormalized by the photoexcited electron
concentration. The dependencies of the period and the amplitude of oscillations
on the excitation pulse are presented with a description of the nonlinear
oscillations damping.Comment: 8 pages, 12 figure
Quantifying river form variations in the Mississippi Basin using remotely sensed imagery
Geographic variations in river form are often estimated using the framework
of downstream hydraulic geometry (DHG), which links spatial changes in
discharge to channel width, depth, and velocity through power-law models.
These empirical relationships are developed from limited in situ data and do not
capture the full variability in channel form. Here, we present a data set of
1.2 ×106 river widths in the Mississippi Basin measured from the
Landsat-derived National Land Cover Dataset that characterizes width
variability observationally. We construct DHG for the Mississippi drainage
by linking digital elevation model (DEM)-estimated discharge values to each width measurement.
Well-developed DHG exists over the entire Mississippi Basin, though
individual sub-basins vary substantially from existing width–discharge
scaling. Comparison of depth predictions from traditional depth–discharge
relationships with a new model incorporating width into the DHG framework
shows that including width improves depth estimates by, on average, 24%.
Results suggest that channel geometry derived from remotely sensed imagery
better characterizes variability in river form than do estimates based on
DHG
Functional requirements for the man-vehicle systems research facility
The NASA Ames Research Center proposed a man-vehicle systems research facility to support flight simulation studies which are needed for identifying and correcting the sources of human error associated with current and future air carrier operations. The organization of research facility is reviewed and functional requirements and related priorities for the facility are recommended based on a review of potentially critical operational scenarios. Requirements are included for the experimenter's simulation control and data acquisition functions, as well as for the visual field, motion, sound, computation, crew station, and intercommunications subsystems. The related issues of functional fidelity and level of simulation are addressed, and specific criteria for quantitative assessment of various aspects of fidelity are offered. Recommendations for facility integration, checkout, and staffing are included
Core-Level X-Ray Photoemission Satellites in Ruthenates: A New Mechanism Revealing the Mott Transition
Ru 3d core-level x-ray photoemission spectra of various ruthenates are
examined. They show in general two-peak structures, which can be assigned as
the screened and unscreened peaks. The screened peak is absent in a Mott
insulator, but develops into a main peak in the metallic regime. This spectral
behavior is well explained by the dynamical mean-field theory calculation for
the single-band Hubbard model with on-site core-hole potential using the exact
diagonalization method. The new mechanism of the core-level photoemission
satellite can be utilized to reveal the Mott transition phenomenon in various
strongly correlated electron systems, especially in nano-scale devices and
phase-separated materials.Comment: 4 pages, 5 figures, submitted to PR
Clearing algorithms and network centrality
I show that the solution of a standard clearing model commonly used in
contagion analyses for financial systems can be expressed as a specific form of
a generalized Katz centrality measure under conditions that correspond to a
system-wide shock. This result provides a formal explanation for earlier
empirical results which showed that Katz-type centrality measures are closely
related to contagiousness. It also allows assessing the assumptions that one is
making when using such centrality measures as systemic risk indicators. I
conclude that these assumptions should be considered too strong and that, from
a theoretical perspective, clearing models should be given preference over
centrality measures in systemic risk analyses
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