230 research outputs found
High resolution astronomical imaging through the turbulent atmosphere
technical reportThis research is principally concerned with the digital reconstruction of star images observed with large ground-based telescopes, although the techniques developed here will have application to a broad class of reconstruction problems. Since the work of Labeyrie, the difficulty in producing accurate and detailed reconstructions of stars has stemmed primarily from the extreme degradation of the phase spectrum caused by the atmospheric turbulence, and secondarily from the low-pass filter characteristic of the telescope itself. In this research, we describe solutions to both problems. Our phase estimator is based on the Knox-Thompson phase difference estimator, which we have extended and modified to produce more accurate estimates. The performance of this estimator is evaluated by simulation at various signal-to-noise ratios. We also describe a new non-linear super-resolution algorithm which appears to exhibit the best accuracy and convergence characteristics of any such algorithm proposed to date. It is also evaluated empirically. These two techniques were then used to restore images of the stars Betelgeuse and Capella. In the latter restoration resolution is realized well beyond the diffraction-limit of the telescope. Both reconstructions are consistent with known astrophysical facts, and both appear to be of high quality
Spin-dependent phenomena and device concepts explored in (Ga,Mn)As
Over the past two decades, the research of (Ga,Mn)As has led to a deeper
understanding of relativistic spin-dependent phenomena in magnetic systems. It
has also led to discoveries of new effects and demonstrations of unprecedented
functionalities of experimental spintronic devices with general applicability
to a wide range of materials. In this article we review the basic material
properties that make (Ga,Mn)As a favorable test-bed system for spintronics
research and discuss contributions of (Ga,Mn)As studies in the general context
of the spin-dependent phenomena and device concepts. Special focus is on the
spin-orbit coupling induced effects and the reviewed topics include the
interaction of spin with electrical current, light, and heat.Comment: 47 pages, 41 figure
Control of Coercivities in (Ga,Mn)As Thin Films by Small Concentrations of MnAs Nanoclusters
We demonstrate that low concentrations of a secondary magnetic phase in
(Ga,Mn)As thin films can enhance the coercivity by factors up to ~100 without
significantly degrading the Curie temperature or saturation magnetisation.
Magnetic measurements indicate that the secondary phase consists of MnAs
nanoclusters, of average size ~7nm. This approach to controlling the coercivity
while maintaining high Curie temperature, may be important for realizing
ferromagnetic semiconductor based devices.Comment: 8 pages,4 figures. accepted for publication in Appl. Phys. Let
Anisotropic Magnetoresistance components in (Ga,Mn)As
Our experimental and theoretical study of the non-crystalline and crystalline
components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As is aimed at
exploring the basic physical aspects of this relativistic transport effect. The
non-crystalline AMR reflects anisotropic lifetimes of the holes due to
polarized Mn impurities while the crystalline AMR is associated with valence
band warping. We find that the sign of the non-crystalline AMR is determined by
the form of spin-orbit coupling in the host band and by the relative strengths
of the non-magnetic and magnetic contributions to the impurity potential. We
develop experimental methods directly yielding the non-crystalline and
crystalline AMR components which are then independently analyzed. We report the
observation of an AMR dominated by a large uniaxial crystalline component and
show that AMR can be modified by local strain relaxation. We discuss generic
implications of our experimental and theoretical findings including predictions
for non-crystalline AMR sign reversals in dilute moment systems.Comment: 4 pages, 3 figures. Phys. Rev. Lett. in pres
Surface morphology and magnetic anisotropy in (Ga,Mn)As
Atomic Force Microscopy and Grazing incidence X-ray diffraction measurements
have revealed the presence of ripples aligned along the direction
on the surface of (Ga,Mn)As layers grown on GaAs(001) substrates and buffer
layers, with periodicity of about 50 nm in all samples that have been studied.
These samples show the strong symmetry breaking uniaxial magnetic anisotropy
normally observed in such materials. We observe a clear correlation between the
amplitude of the surface ripples and the strength of the uniaxial magnetic
anisotropy component suggesting that these ripples might be the source of such
anisotropy.Comment: 3 pages, 4 figures, 1 table. Replaced with published versio
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