8,014 research outputs found
Spin characterization and control over the regime of radiation-induced zero-resistance states
Over the regime of the radiation-induced zero-resistance states and
associated oscillatory magnetoresistance, we propose a low magnetic field
analog of quantum-Hall-limit techniques for the electrical detection of
electron spin- and nuclear magnetic- resonance, dynamical nuclear polarization
via electron spin resonance, and electrical characterization of the nuclear
spin polarization via the Overhauser shift. In addition, beats observed in the
radiation-induced oscillatory-magnetoresistance are developed into a method to
measure and control the zero-field spin splitting due to the Bychkov-Rashba and
bulk inversion asymmetry terms in the high mobility GaAs/AlGaAs system.Comment: IEEE Transactions in Nanotechnology (to be published); 10 pages, 10
color figure
Radiation-induced zero-resistance states with resolved Landau levels
The microwave-photoexcited high mobility GaAs/AlGaAs two-dimensional electron
system exhibits an oscillatory-magnetoresistance with vanishing resistance in
the vicinity of magnetic fields , where , m is an the effective mass, e is the charge,
\textit{f} is the microwave frequency, and =1,2,3... Here, we report
transport with well-resolved Landau levels, and some transmission
characteristics.Comment: 4 pages, 3 color figures, published versio
Compressibility effects in the Kemp-Sears Problem
The effects of including compressibility in the Kemp-Sears problem of aerodynamic interference between moving blade rows are examined. Methods of linearized, subsonic, plane, unsteady flow are adopted. The major new effect is that a resonance appears at certain combinations of flow Mach number, tip Mach number, and blade vane ratios. The resonance is at exactly the Tyler-Sofrin cutoff condition for rotor-stator interaction. At such conditions the unsteady lift on a blade row due to externally imposed nonstationary upwash vanishes. However, the resonance appears to be very sharp and seems to be more significant as an indication that around this condition the unsteady lift changes very rapidly
Photo-excited zero-resistance states in the GaAs/AlGaAs system
The microwave-excited high mobility two-dimensional electron system exhibits,
at liquid helium temperatures, vanishing resistance in the vicinity of , where , m is an
effective mass, e is the charge, and \textit{f} is the microwave frequency.
Here, we summarize some experimental results.Comment: 7 color figures, 5 page
Temperature effects on microwave-induced resistivity oscillations and zero resistance states in 2D electron systems
In this work we address theoretically a key issue concerning
microwave-induced longitudinal resistivity oscillations and zero resistance
states, as is tempoerature. In order to explain the strong temperature
dependence of the longitudinal resistivity and the thermally activated
transport in 2DEG, we have developed a microscopic model based on the damping
suffered by the microwave-driven electronic orbit dynamics by interactions with
the lattice ions yielding acoustic phonons. Recent experimental results show a
reduction in the amplitude of the longitudinal resistivity oscillations and a
breakdown of zero resistance states as the radiation intensity increases. In
order to explain it we have included in our model the electron heating due to
large microwave intensities and its effect on the longitudinal resistivity.Comment: 4 pages and 4 figures. Accepted in Phys Rev
Theory of low frequency noise transmission through turbines
Improvements of the existing theory of low frequency noise transmission through turbines and development of a working prediction tool are described. The existing actuator-disk model and a new finite-chord model were utilized in an analytical study. The interactive effect of adjacent blade rows, higher order spinning modes, blade-passage shocks, and duct area variations were considered separately. The improved theory was validated using the data acquired in an earlier NASA program. Computer programs incorporating the improved theory were produced for transmission loss prediction purposes. The programs were exercised parametrically and charts constructed to define approximately the low frequency noise transfer through turbines. The loss through the exhaust nozzle and flow(s) was also considered
Experimental and theoretical studies of subsonic fan noise
The noise generated by inlet turbulence impinging on a subsonic axial flow fan was studied as a function of tip speed, flow coefficient, and intensity and scale of turbulence was carried out. Both turbulence and far field acoustic measurements were made. The new elements introduced in the theoretical analysis were accounting for blade loading dependent noise mechanisms and consideration of anisotropic turbulence impinging on the rotor because of inlet flow contraction effects. Experimentally an unexplained increase of noise at about 1/2 and 1 1/2 times blade passsing frequency was observed at low flow coefficients even though there was no evidence of compressor surge. In the final version the theory does a fair job of predicting variations of noise with blade loading and tip speed. Alteration of inlet turbulence length scales produced some but not very pronounced changes in the far field PWL spectra. Some degree of eddy contraction and resulting anisotropy were essential to explain the concentration of energy around blade passing frequencies
Demonstration of a 1/4 cycle phase shift in the radiation-induced oscillatory-magnetoresistance in GaAs/AlGaAs devices
We examine the phase and the period of the radiation-induced
oscillatory-magnetoresistance in GaAs/AlGaAs devices utilizing in-situ magnetic
field calibration by Electron Spin Resonance of DiPhenyl-Picryl-Hydrazal. The
results confirm a -independent 1/4 cycle phase shift with respect to the condition for , and they also suggest a small
( 2%) reduction in the effective mass ratio, , with respect
to the standard value for GaAs/AlGaAs devices.Comment: 4 pages, 4 color figure
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