13,827 research outputs found
Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V2O3 and Ti2O3
The electronic structure of the corundum-type transition-metal oxides V2O3
and Ti2O3 is studied by means of the augmented spherical wave method, based on
density-functional theory and the local density approximation. Comparing the
results for the vanadate and the titanate allows us to understand the peculiar
shape of the metal 3d a_{1g} density of states, which is present in both
compounds. The a_{1g} states are subject to pronounced bonding-antibonding
splitting due to metal-metal overlap along the c-axis of the corundum
structure. However, the corresponding partial density of states is strongly
asymmetric with considerably more weight on the high energy branch. We argue
that this asymmetry is due to an unexpected broadening of the bonding a_{1g}
states, which is caused by hybridization with the e_g^{pi} bands. In contrast,
the antibonding a_{1g} states display no such hybridization and form a sharp
peak. Our results shed new light on the role of the a_{1g} orbitals for the
metal-insulator transitions of V2O3. In particular, due to a_{1g} - e_g^{pi}
hybridization, an interpretation in terms of molecular orbital singlet states
on the metal-metal pairs along the c-axis is not an adequate description.Comment: 7 pages, 3 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
Nutritional Requirements for Space Station Freedom Crews
The purpose of this report was to set preliminary nutritional requirements for crewmembers flying from 90 to 180 day missions on Space Station Freedom. Specific recommendations included providing crewmembers with in flight feedback on nutritional intake, weight and strength, and incorporating issues of energy intake, body weight, body composition, strength, and protein intake in the flight medicine program. Exercise must be considered an integral part of any plan to maintain nutritional status, especially those modes that stress the skeleton and maintain body weight. Nutrient intake, amount of exercise, and drugs ingested must be recorded daily; high priority should be given to development of fully automated record systems that minimize astronauts' effort. A system of nutritional supplements should be developed to provide a method for reducing intake deficits that become apparent. Finally, post flight monitoring should include bone density, muscle mass and function, and iron status at three and six months after landing
Studies of auroral X-ray imaging from high altitude spacecraft
Results of a study of techniques for imaging the aurora from a high altitude satellite at X-ray wavelengths are summarized. The X-ray observations allow the straightforward derivation of the primary auroral X-ray spectrum and can be made at all local times, day and night. Five candidate imaging systems are identified: X-ray telescope, multiple pinhole camera, coded aperture, rastered collimator, and imaging collimator. Examples of each are specified, subject to common weight and size limits which allow them to be intercompared. The imaging ability of each system is tested using a wide variety of sample spectra which are based on previous satellite observations. The study shows that the pinhole camera and coded aperture are both good auroral imaging systems. The two collimated detectors are significantly less sensitive. The X-ray telescope provides better image quality than the other systems in almost all cases, but a limitation to energies below about 4 keV prevents this system from providing the spectra data essential to deriving electron spectra, energy input to the atmosphere, and atmospheric densities and conductivities. The orbit selection requires a tradeoff between spatial resolution and duty cycle
Affective bias and current, past and future adolescent depression: A familial high risk study.
Affective bias is a common feature of depressive disorder. However, a lack of longitudinal studies means that the temporal relationship between affective bias and depression is not well understood. One group where studies of affective bias may be particularly warranted is the adolescent offspring of depressed parents, given observations of high rates of depression and a severe and impairing course of disorder in this group
Spurious free dynamic range for a digitizing array
The paper addresses the problem of improving the spurious free dynamic range (SFDR) for digitization by use of sensor arrays. Nonlinearities in the analog-to-digital conversion process give rise to spurious signals (harmonics and intermodulation products) that limit the overall SFDR of the digitization process. When the signal of interest arises from a sensor, such as an antenna or hydrophone, the paper addresses the question of whether array processing (i.e., use of multiple sensors) can improve the resulting SFDR at the beamformer output. The paper argues that significant improvements can be obtained using linear, or more effectively, optimal (minimum variance distortionless response) beamforming.White, L.B.; Feng Rice and Angus Massi
Placing Confidence Limits on Polarization Measurements
The determination of the true source polarization given a set of measurements
is complicated by the requirement that the polarization always be positive.
This positive bias also hinders construction of upper limits, uncertainties,
and confidence regions, especially at low signal-to-noise levels. We generate
the likelihood function for linear polarization measurements and use it to
create confidence regions and upper limits. This is accomplished by integrating
the likelihood function over the true polarization (parameter space), rather
than the measured polarization (data space). These regions are valid for both
low and high signal-to-noise measurements.Comment: 8 pages, 3 figures, 1 table, submitted to PAS
Moving Toward a Unified Effort to Understand the Nature and Causes of Language Disorders
DOI: 10.1017.S014271640505002
Coarse-Grained Picture for Controlling Complex Quantum Systems
We propose a coarse-grained picture to control ``complex'' quantum dynamics,
i.e., multi-level-multi-level transition with a random interaction. Assuming
that optimally controlled dynamics can be described as a Rabi-like oscillation
between an initial and final state, we derive an analytic optimal field as a
solution to optimal control theory. For random matrix systems, we numerically
confirm that the analytic optimal field steers an initial state to a target
state which both contains many eigenstates.Comment: jpsj2.cls, 2 pages, 3 figure files; appear in J. Phys. Soc. Jpn.
Vol.73, No.11 (Nov. 15, 2004
The circumbinary disk of HD 98800B: Evidence for disk warping
The quadruple young stellar system HD 98800 consists of two spectroscopic binary pairs with a circumbinary disk around the B component. Recent work by Boden and collaborators using infrared interferometry and radial velocity data resulted in a determination of the physical orbit for HD 98800B. We use the resulting inclination of the binary and the measured extinction toward the B component stars to constrain the distribution of circumbinary material. Although a standard optically and geometrically thick disk model can reproduce the spectral energy distribution, it cannot account for the observed extinction if the binary and the disk are coplanar. We next constructed a dynamical model to investigate the influence of the A component, which is not in the BaâBb orbital plane, on the B disk. We find that these interactions have a substantial impact on the inclination of the B circumbinary disk with respect to the BaâBb orbital plane. The resulting warp would be sufficient to place material into the line of sight and the noncoplanar disk orientation may also cause the upper layers of the disk to intersect the line of sight if the disk is geometrically thick. These simulations also support that the dynamics of the BaâBb orbit clear the inner region to a radius of~3 AU. We then discuss whether the somewhat unusual properties of the HD 98800B disk are consistent with material remnant from the star formation process or with more recent creation by collisions from larger bodies
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