69,810 research outputs found
The measurement of velocity gradients in laminar flow by homodyne light-scattering spectroscopy
A technique for measuring velocity gradients in laminar flows by homodyne light
scattering is presented. A theory which describes the light-scattering spectrum is
derived that includes the effects of different types of linear flow fields, particle diffusion
and the intensity profile in the scattering volume. The conditions which must be
satisfied in order that the theory describe the experimental situation are outlined and
complementary experiments are performed which both verify the theory and apply
the technique. Verification is provided using the flow in a Couette device, and the flow
due to single rotating cylinder in a large bath of fluid. The technique is then applied
to measure the spatial variation of the shear rate in a four-roll mill
Sun direction detection system
One of the detectors is an illumination detector consisting of two spaced apart elongated strips with a strip of cadmium sulphide (Cds) deposited therebetween. Whenever the line image impinges the CdS strip, the resistance between the two other strips is relatively low, while being high when the line image is outside the field of view of the illumination detector. Also included is a sun angle detector which consists of a vapor deposited resistor strip connected at one end to plus 10v and at the other end to minus 10v. Spaced apart from the resistor strip is an elongated strip of low resistance material acting as an output strip, with a CdS strip between the two strips. When the line image is within the field of view of the sun angle detector, the output voltage at the output strip depends on the position of the line image across the sun angle detector
Bound hole states in a ferromagnetic (Ga,Mn)As environment
A numerical technique is developed to solve the Luttinger-Kohn equation for
impurity states directly in k-space and is applied to calculate bound hole wave
functions in a ferromagnetic (Ga,Mn)As host. The rich properties of the band
structure of an arbitrarily strained, ferromagnetic zinc-blende semiconductor
yields various features which have direct impact on the detailed shape of a
valence band hole bound to an active impurity. The role of strain is discussed
on the basis of explicit calculations of bound hole states.Comment: 9 pages, 10 figure
Voltage-Controlled Spin Selection in a Magnetic Resonant Tunnelling Diode
We have fabricated all II-VI semiconductor resonant tunneling diodes based on
the (Zn,Mn,Be)Se material system, containing dilute magnetic material in the
quantum well, and studied their current-voltage characteristics. When subjected
to an external magnetic field the resulting spin splitting of the levels in the
quantum well leads to a splitting of the transmission resonance into two
separate peaks. This is interpreted as evidence of tunneling transport through
spin polarized levels, and could be the first step towards a voltage controlled
spin filter.Comment: To be published in Phys. Rev. Let
Large magnetoresistance effect due to spin-injection into a non-magnetic semiconductor
A novel magnetoresistance effect, due to the injection of a spin-polarized
electron current from a dilute magnetic into a non-magnetic semiconductor, is
presented. The effect results from the suppression of a spin channel in the
non-magnetic semiconductor and can theoretically yield a positive
magnetoresistance of 100%, when the spin flip length in the non-magnetic
semiconductor is sufficiently large. Experimentally, our devices exhibit up to
25% magnetoresistance.Comment: 3 figures, submitted for publicatio
Development and evaluation of a Kalman-filter algorithm for terminal area navigation using sensors of moderate accuracy
Translational state estimation in terminal area operations, using a set of commonly available position, air data, and acceleration sensors, is described. Kalman filtering is applied to obtain maximum estimation accuracy from the sensors but feasibility in real-time computations requires a variety of approximations and devices aimed at minimizing the required computation time with only negligible loss of accuracy. Accuracy behavior throughout the terminal area, its relation to sensor accuracy, its effect on trajectory tracking errors and control activity in an automatic flight control system, and its adequacy in terms of existing criteria for various terminal area operations are examined. The principal investigative tool is a simulation of the system
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