Magnetoresistance and electronic structure of asymmetric GaAs/AlGaAs double quantum wells in the in-plane/tilted magnetic field

Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs buffer of a standard heterostructure were investigated by magnetotransport measurements. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and the field-induced transition into a decoupled bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields allow to reconstruct the evolution of the electron concentration in the individual subbands as a function of the in-plane magnetic field. The characteristics of the system derived experimentally are in quantitative agreement with numerical self-consistent-field calculations of the electronic structure.Comment: 6 pages, 5 figure

Remote terminal system evaluation

An Earth Resources Data Processing System was developed to evaluate the system for training, technology transfer, and data processing. In addition to the five sites included in this project two other sites were connected to the system under separate agreements. The experience of these two sites is discussed. The results of the remote terminal project are documented in seven reports: one from each of the five project sites, Purdue University, and an overview report summarizing the other six reports

The Location of the Nucleus of NGC 1068 and the Three-dimensional Structure of Its Nuclear Region

The HST archival UV imaging polarimetry data of NGC 1068 is re-examined. Through an extensive estimation of the observational errors, we discuss whether the distribution of the position angles (PAs) of polarization is simply centrosymmetric or not. Taking into account the effect of a bad focus at the time of the observation, we conclude that, within the accuracy of HST/FOC polarimetry, the PA distribution is completely centrosymmetric. This means that the UV polarization originates only from scattering of the radiation from a central point-like source. However, our analysis shows that the most probable location of the nucleus is only ~0.''08 (~6pc) south from the brightest cloud called ``cloud B''. The error circle of 99% confidence level extends to cloud B and to ``cloud A'' which is about 0.''2 south of cloud B. By this FOC observation, Cloud B is only marginally rejected as the nucleus. Assuming that the UV flux is dominated by electron-scattered light, we have also derived a three-dimensional structure of the nuclear region. The inferred distribution suggests a linear structure which could be related to the radio jet.Comment: 19 pages, 14 figures, to be published in the Astrophysical Journa

A breakthrough curve analysis of unstable density-driven flow and transport in homogenous porous media

2000 FLORIDA AVE NW, WASHINGTON, USA, DC, 2000

Zener Tunneling Between Landau Orbits in a High-Mobility Two-Dimensional Electron Gas

Magnetotransport in a laterally confined two-dimensional electron gas (2DEG) can exhibit modified scattering channels owing to a tilted Hall potential. Transitions of electrons between Landau levels with shifted guiding centers can be accomplished through a Zener tunneling mechanism, and make a significant contribution to the magnetoresistance. A remarkable oscillation effect in weak field magnetoresistance has been observed in high-mobility 2DEGs in GaAs-AlGa$_{0.3}$As$_{0.7}$ heterostructures, and can be well explained by the Zener mechanism.Comment: 5 pages, 4 figures. Text slightly shortened, figures resize

Physical Properties of a Set of Sandstones, III: the Effects Of Fine Grained Pore Filling Material on Compressional Wave Velocity

We have used aspect ratio modeling to explain the measured compressional wave velocities of twenty different dry sandstone samples with varying clay contents at a single confining pressure of 0.5 kbar. Velocities of the sandstones range between 3.1 km/sec and 5.7 km/sec. Measured porosities are between 6% and 33%, clay contents between 2% and 30%. Pores were described using three simple type classifications. The pore type distributions of the samples were quantified by point counting polished impregnated thin sections using a scanning electron microscope. A representative aspect-ratio was assigned to each of the three categories of pore type. Velocities were modeled using these aspect ratios weighted by the observed distribution of the porosity types. Agreement between theoretical and measured velocities is generally within 10%. The modeling suggests that the effects of clays in sandstone pores is to reduce the sample porosity without reducing the non-framework (void + clay) volume. Thus, for a given porosity, clay rich samples contain greater non-framework volume, which in turn lowers velocity. The model derived from the dry measurements can be used to successfully approximate empirical relationships for saturated samples of velocity-porosity-clay content taken from the literature.Schlumberger-Doll Research CenterSchlumberger Foundation. Post-Doctoral Fellowshi

Polarimetry of the Type Ia Supernova SN 1996X

We present broad-band and spectropolarimetry of the Type Ia SN 1996X obtained on April 14, 1996 (UT), and broad-band polarimetry of SN 1996X on May 22,1996, when the supernova was about a week before and 4 weeks after optical maximum, respectively. The Stokes parameters derived from the broad-band polarimetry are consistent with zero polarization. The spectropolarimetry, however, shows broad spectral features which are due intrinsically to an asymmetric SN atmosphere. The spectral features in the flux spectrum and the polarization spectrum show correlations in the wavelength range from 4900 AA up to 5500 AA. The degree of this intrinsic component is low (<0.3 %). Theoretical polarization spectra have been calculated. It is shown that the polarization spectra are governed by line blending. Consequently, for similar geometrical distortions, the residual polarization is smaller by about a factor of 2 to 3 compared to the less blended Type II atmosphere, making it intrinsically harder to detect asphericities in SNIa. Comparison with theoretical model polarization spectra shows a resemblance to the observations. Taken literally, this implies an asphericity of about 11 % in the chemical distribution in the region of partial burning. This may not imperil the use of Type Ia supernovae as standard candles for distance determination, but nontheless poses a source of uncertainty. SN 1996X is the first Type Ia supernova for which spectropolarimetry revealed a polarized component intrinsic to the supernova and the first Type Ia with spectropolarimetry well prior to optical maximum.Comment: 7 pages, 5 figures, macros 'aas2pp4.sty,psfig.tex'. LaTeX Style. Astrophysical Journal Letters, submitted September 199