Transition Temperature of a Magnetic Semiconductor with Angular Momentum j

We employ dynamical mean-field theory to identify the materials properties that optimize Tc for a generalized double-exchange (DE) model. We reach the surprising conclusion that Tc achieves a maximum when the band angular momentum j equals 3/2 and when the masses in the 1/2 and 3/2 sub-bands are equal. However, we also find that Tc is significantly reduced as the ratio of the masses decreases from one. Consequently, the search for dilute magnetic semiconductors (DMS) materials with high Tc should proceed on two fronts. In semiconductors with p bands, such as the currently studied Mn-doped Ge and GaAs semiconductors, Tc may be optimized by tuning the band masses through strain engineering or artificial nanostructures. On the other hand, semiconductors with s or d bands with nearly equal effective masses might prove to have higher Tc's than p-band materials with disparate effective masses.Comment: 5 pages, 4 figure

Thermodynamic Consistency of the Dynamical Mean-Field Theory of the Double-Exchange Model

Although diagrammatic perturbation theory fails for the dynamical-mean field theory of the double-exchange model, the theory is nevertheless Phi-derivable and hence thermodynamically consistent, meaning that the same thermodynamic properties are obtained from either the partition function or the Green's function. We verify this consistency by evaluating the magnetic susceptibility and Curie temperature for any Hund's coupling.Comment: 9 pages, 1 figur

Decision Gate Process for Assessment of a Technology Development Portfolio

The NASA Dust Management Project (DMP) was established to provide technologies (to TRL 6 development level) required to address adverse effects of lunar dust to humans and to exploration systems and equipment, which will reduce life cycle cost and risk, and will increase the probability of sustainable and successful lunar missions. The technology portfolio of DMP consisted of different categories of technologies whose final product is either a technology solution in itself, or one that contributes toward a dust mitigation strategy for a particular application. A Decision Gate Process (DGP) was developed to assess and validate the achievement and priority of the dust mitigation technologies as the technologies progress through the development cycle. The DGP was part of continuous technology assessment and was a critical element of DMP risk management. At the core of the process were technology-specific criteria developed to measure the success of each DMP technology in attaining the technology readiness levels assigned to each decision gate. The DGP accounts for both categories of technologies and qualifies the technology progression from technology development tasks to application areas. The process provided opportunities to validate performance, as well as to identify non-performance in time to adjust resources and direction. This paper describes the overall philosophy of the DGP and the methodology for implementation for DMP, and describes the method for defining the technology evaluation criteria. The process is illustrated by example of an application to a specific DMP technology

The role of quasi-momentum in the resonant dynamics of the atom-optics kicked rotor

We examine the effect of the initial atomic momentum distribution on the dynamics of the atom-optical realisation of the quantum kicked rotor. The atoms are kicked by a pulsed optical lattice, the periodicity of which implies that quasi-momentum is conserved in the transport problem. We study and compare experimentally and theoretically two resonant limits of the kicked rotor: in the vicinity of the quantum resonances and in the semiclassical limit of vanishing kicking period. It is found that for the same experimental distribution of quasi-momenta, significant deviations from the kicked rotor model are induced close to quantum resonance, while close to the classical resonance (i.e. for small kicking period) the effect of the quasi-momentum vanishes.Comment: 10 pages, 4 figures, to be published in J. Phys. A, Special Issue on 'Trends in Quantum Chaotic Scattering

Monitoring Cen X-3 with BATSE

The eight uncollimated BATSE Large Area Detectors (LAD's) provide the ability to monitor pulsed hard x ray sources on a nearly continuous basis. Using data from the LAD's, the pulse timing and pulsed flux of the 4.8 second period binary x ray pulsar Centaurus X-3 was analyzed over a two month period. The methods and initial results of this analysis, which includes both data folded onboard GRO and 1.024 second resolution discriminator rates folded on the ground, are presented

Comparison of Tropospheric Ozone Columns Calculated from MLS, OMI, and Ozonesonde Data

This poster shows a comparison of three derived tropospheric ozone residual (TOR) products with integrated tropospheric ozone columns from ozonesonde profile: (1) the method of Ziemke et al. (2006), (2) a modified version of Fishman et al. (2003), and (3) a trajectory mapping approach. In each case, MLS ozone profiles are integrated to the tropopause and subtracted from OMI (TOMS retrieval) total column ozone. The effectiveness of each of these techniques is examined as a function of latitude, time, and geographic region. In general, we find good agreement between the derived products and the ozonesondes, with the Fishman et al. TOR (labeled “Amy”) generally high and the Schoeberl trajectory mapping (labeled “Mark”) product generally low as compared to the integrated ozonesonde profiles (labeled “Sonde”) as computed using the WMO tropopause definition. Differences in TOR results are due, at least in part, to non-uniform tropopause height definitions between the three approaches

Achieving a Prioritized Research and Technology Development Portfolio for the Dust Management Project

Mission architectures for human exploration of the lunar surface continue to advance as well as the definitions of capability needs, best practices and engineering design to mitigate the impact of lunar dust on exposed systems. The NASA DMP has been established as the agency focal point for dust characterization, technology, and simulant development. As described in this paper, the DMP has defined a process for selecting and justifying its R&T portfolio. The technology prioritization process, which is based on a ranking system according to weighted criteria, has been successfully applied to the current DMP dust mitigation technology portfolio. Several key findings emerged from this assessment. Within the dust removal and cleaning technologies group, there are critical technical challenges that must be overcome for these technologies to be implemented for lunar applications. For example, an in-situ source of CO2 on the moon is essential to the CO2 shower technology. Also, significant development effort is required to achieve technology readiness level TRL 6 for the electrostatic cleaning system for removal of particles smaller than 50 pm. The baseline materials related technologies require considerable development just to achieve TRL 6. It is also a nontrivial effort to integrate the materials in hardware for lunar application. At present, there are no terrestrial applications that are readily adaptable to lunar surface applications nor are there any obvious leading candidates. The unique requirements of dust sealing systems for lunar applications suggest an extensive development effort will be necessary to mature dust sealing systems to TRL 6 and beyond. As discussed here, several alternate materials and technologies have achieved high levels of maturity for terrestrial applications and warrant due diligence in ongoing assessment of the technology portfolio. The present assessment is the initial step in an ongoing effort to continually evaluate the DMP technology portfolio and external non-NASA relevant technology developments efforts to maintain an optimal investment profile. At the same time, there is an ongoing review of agency-wide dust-related R&T activities. The results of these ongoing assessments will be reported in future publications