442 research outputs found
Space telescope phase B definition study. Volume 2A: Science instruments, astrometer
The analysis and design of an astrometer instrument for the space telescope are discussed. The design concepts utilize the astrometric multiplexing area scanner and the OTA fine guidance sensor
Study of alternate optical and fine guidance sensor designs for the space infrared telescope facility (SIRTF)
A unique optical design was developed that compensates for the coma degraded images caused by field chopping in SIRTF. The conic constants of a Cassegrain telescope were altered to compensate for the coma induced by the secondary mirror tilt. The modulation transfer function is essentially independent of secondary mirror tilt, and diffraction limited image quality is maintained over a several arcminute field during chopping. With an untilted secondary mirror, the coma compensated (CC) design has a smaller field than the unchopped Ritchey-Chretien design; but use of relay optics, such as the inverted Cassegrain design developed for the fine guidance sensor (FGS), can increase the CC telescope's field size. A reactionless secondary mirror chopper mechanism that uses superconducting magnets was studied. The heart producing elements are confined to a reaction plate that is not directly viewed by the IR focal plane. A design was also developed for a low moment of inertia, reticulated HIP beryllium secondary mirror consistent with blank fabrication technology and optical finishing requirements
Theoretical efficiency of the Princeton two-element echelle spectrograph
Echelle spectrometer for use with spaceborne stellar telescope in Advanced Princeton Satellite Stud
Suppression of spin-state transition in epitaxially strained LaCoO_{3}
Epitaxial thin films of LaCoO_{3} (E-LCO) exhibit ferromagnetic order with a
transition temperature T_c = 85 K, while polycrystalline thin LaCoO_{3} films
(P-LCO) remain paramagnetic. The temperature-dependent spin-state structure for
both E-LCO and P-LCO was studied by x-ray absorption spectroscopy at the Co
L_{2,3} and O K edges. Considerable spectral redistributions over temperature
are observed for P-LCO. The spectra for E-LCO, on the other hand, do not show
any significant changes for temperatures between 30 K and 450 K at both edges,
indicating that the spin state remains constant and that the epitaxial strain
inhibits any population of the low-spin (S = 0) state with decreasing
temperature. This observation identifies an important prerequisite for
ferromagnetism in E-LCO thin films.Comment: 5 pages, 5 figures, submitted to Physical Review
Giant Spin Relaxation Anisotropy in Zinc-Blende Heterostructures
Spin relaxation in-plane anisotropy is predicted for heterostructures based
on zinc-blende semiconductors. It is shown that it manifests itself especially
brightly if the two spin relaxation mechanisms (D'yakonov-Perel' and Rashba)
are comparable in efficiency. It is demonstrated that for the quantum well
grown along the [0 0 1] direction, the main axes of spin relaxation rate tensor
are [1 1 0] and [1 -1 0].Comment: 3 pages, NO figure
Effect of bulk inversion asymmetry on the Datta-Das transistor
A model of the Datta-Das spin field-effect transistor is presented which, in
addition to the Rashba interaction, takes into account the influence of bulk
inversion asymmetry of zinc-blende semiconductors. In the presence of bulk
inversion asymmetry, the conductance is found to depend significantly on the
crystallographic orientation of the channel. We determine the channel direction
optimal for the observation of the Datta-Das effect in GaAs and InAs-based
devices.Comment: 4 pages, Revtex4, 4 EPS figure
Electron Spin Decoherence in Bulk and Quantum Well Zincblende Semiconductors
A theory for longitudinal (T1) and transverse (T2) electron spin coherence
times in zincblende semiconductor quantum wells is developed based on a
non-perturbative nanostructure model solved in a fourteen-band restricted basis
set. Distinctly different dependences of coherence times on mobility,
quantization energy, and temperature are found from previous calculations.
Quantitative agreement between our calculations and measurements is found for
GaAs/AlGaAs, InGaAs/InP, and GaSb/AlSb quantum wells.Comment: 11 pages, 3 figure
Spin Orientation and Spin Precession in Inversion-Asymmetric Quasi Two-Dimensional Electron Systems
Inversion asymmetry induced spin splitting of the electron states in quasi
two-dimensional (2D) systems can be attributed to an effective magnetic field B
which varies in magnitude and orientation as a function of the in-plane wave
vector k||. Using a realistic 8x8 Kane model that fully takes into account spin
splitting because of both bulk inversion asymmetry and structure inversion
asymmetry we investigate the spin orientation and the effective field B for
different configurations of a quasi 2D electron system. It is shown that these
quantities depend sensitively on the crystallographic direction in which the
quasi 2D system was grown as well as on the magnitude and orientation of the
in-plane wave vector k||. These results are used to discuss how spin-polarized
electrons can precess in the field B(k||). As a specific example we consider
GaInAs-InP quantum wells.Comment: 10 pages, 6 figure
Anisotropic splitting of intersubband spin plasmons in quantum wells with bulk and structural inversion asymmetry
In semiconductor heterostructures, bulk and structural inversion asymmetry
and spin-orbit coupling induce a k-dependent spin splitting of valence and
conduction subbands, which can be viewed as being caused by momentum-dependent
crystal magnetic fields. This paper studies the influence of these effective
magnetic fields on the intersubband spin dynamics in an asymmetric n-type
GaAs/AlGaAs quantum well. We calculate the dispersions of intersubband spin
plasmons using linear response theory. The so-called D'yakonov-Perel'
decoherence mechanism is inactive for collective intersubband excitations,
i.e., crystal magnetic fields do not lead to decoherence of spin plasmons.
Instead, we predict that the main signature of bulk and structural inversion
asymmetry in intersubband spin dynamics is a three-fold, anisotropic splitting
of the spin plasmon dispersion. The importance of many-body effects is pointed
out, and conditions for experimental observation with inelastic light
scattering are discussed.Comment: 8 pages, 6 figure
- âŠ