Phonon and Electronic Non-radiative Decay of Excitons in Carbon Nanotubes

We investigate theoretically the rates of non-radiative decay of excited semiconducting nanotubes by a variety of decay mechanisms and compare with experimental findings. We find that the multi-phonon decay (MPD) of free excitons is too slow to be responsible for the experimentally observed lifetimes. However, MPD lifetimes of localized excitons could be 2-3 orders of magnitude shorter. We also propose a new decay mechanism that relies on a finite doping of nanotubes and involves exciton decay into an optical phonon and an intraband electron-hole pair. The resulting lifetime is in the range of 5 to 100 ps, even for a moderate doping level.Comment: 4 pages, 3 figures, + 7 pages, 3 figures in attached appendi

A study of low density, high strength high modulus filaments and composites

Filament and whisker reinforcement of low density, high strength, high modulus composites - metallic and ceramic layers alternated in multilaminar composite

Analysis of the Tapered Transition Waveguide

The tapered transition waveguide is used in waveguide measurement systems for characterizing biaxial electromagnetic properties of materials, but its complex geometry does not support an analytic field solution. To ensure single-mode field behavior, the system includes sections of standard waveguides that only support the dominant mode. As a result, full-wave modeling and simulation of the system is exceedingly large. Using the finite-element method to analyze the high-order modes at the junctions and to explore field configurations within the transition altering geometry, it is shown that besides the TE10 mode, the TE11 mode is significant. Then, two methods are proposed for using multi-mode excitation in the model as a way to simulate the scattering parameters of a material without the feed and transition section

Spectroscopy of the optical Einstein ring 0047-2808

We present optical and near-infrared spectroscopic observations of the optical Einstein ring 0047-2808. We detect both [OIII] lines 4959, 5007 near 2.3 micron, confirming the redshift of the lensed source as z=3.595. The Ly-a line is redshifted relative to the [OIII] line by 140+-20 km/s. Similar velocity shifts have been seen in nearby starburst galaxies. The [OIII] line is very narrow, 130 km/s FWHM. If the ring is the image of the centre of a galaxy the one-dimensional stellar velocity dispersion sigma=55 km/s is considerably smaller than the value predicted by Baugh et al. (1998) for the somewhat brighter Lyman-break galaxies. The Ly-a line is significantly broader than the [OIII] line, probably due to resonant scattering. The stellar central velocity dispersion of the early-type deflector galaxy at z=0.485 is 250+-30 km/s. This value is in good agreement both with the value predicted from the radius of the Einstein ring (and a singular isothermal sphere model for the deflector), and the value estimated from the D_n-sigma relation.Comment: 7 pages, 3 figures, accepted for publication in MNRA

The Impact of Accreditation on the Teacher Education Curricula at Two Historically Black Colleges from 1920-1940

Problem. Many historically Black colleges and universities (HBCUs) are experiencing major challenges in meeting accreditation requirements. Without good accreditation status they are in jeopardy of losing student enrollment and facing closure. Concern about accreditation has become a growing topic of interest within the African American higher education community. This study explored the topic by looking at some of the historical origins of the accreditation movement in HBCUs. It uses various experiences of two higher education organizations to detail some of the original challenges and the impact on HBCUs from regional accreditation. It focuses on HBCUs through the time period 1920-1940. This time period was selected as it is considered a crucial time for two premier colleges selected for this study. Tuskegee Institute and Atlanta University modeled the industrial and classical curriculum, respectively. These two curriculum models were the most common models utilized in HBCUs. Understanding the role that accreditation may have played on HBCUs and their curricula, particularly teacher education programs, is critical when one considers the far-reaching influence African American teachers had on their community. Analyzing the interaction of the regional accreditation organization, the Southern Association of Colleges and Schools (SACS), and the approval processes of these schools highlights the forces shaping these schools, these teachers, and later their community. The role of accreditation in this process, primarily regional accreditation, will be specifically detailed. Method. This study used a problem-oriented historical approach to research. It formulated a question generally prompted by reading secondary sources in a historiography, then evaluated primary sources in order to draw conclusions for this initial reading. While the purpose, statement, and objectives emerged from an extensive review of the literature, the historical method was utilized and the research stayed close to primary and secondary data sources related specifically to HBCUs and higher education. Results. The accreditation process was a catalyst for curricula changes in the teacher education programs at Tuskegee Institute. Furthermore, social, economic, political, and racial hegemonic forces were apparent in these curricula changes at the institution. The impetus for curriculum changes at Atlanta University ranged from meeting student and African American community needs to remaining current with national educational trends. Although governmental, state, and private organizations urged HBCUs toward the vocational curriculum, regional accreditors such as the Southern Association of Colleges and Schools (SACS) did not endorse a particular curriculum model. Conclusions. First, accreditation standards influenced the Tuskegee Institute’s teacher education curriculum. Second, Tuskegee employed the accreditation movement to strengthen the academic curriculum of the teacher education program which allowed them to avoid pejorative repercussions by not strictly adhering to the Tuskegee-Hampton curriculum model, an education that combined cultural uplift with moral and manual training. Third, accreditation did not influence schools to adopt any particular curriculum model. Fourth, economic, social, and political realities provided an impetus for the adoption and maintenance of the Tuskegee-Hampton curriculum model. Significant federal, state, and private funding directly impacted the teacher education program at Tuskegee. Finally, social motivations such as the needs of the African American community and norms of educational practices influenced teacher education curriculum changes at Atlanta University

Development of dispersion strengthened chromium alloys Summary report

Dispersion strengthened chromium alloys with minimal quantities of interstitial impuritie

Redshifted 21cm Signatures Around the Highest Redshift Quasars

The Ly-alpha absorption spectrum of the highest redshift quasars indicates that they are surrounded by giant HII regions, a few Mpc in size. The neutral gas around these HII regions should emit 21cm radiation in excess of the Cosmic Microwave Background, and enable future radio telescopes to measure the transverse extent of these HII regions. At early times, the HII regions expand with a relativistic speed. Consequently, their measured sizes along the line-of-sight (via Ly-alpha absorption) and transverse to it (via 21 cm emission) should have different observed values due to relativistic time-delay. We show that the combined measurement of these sizes would directly constrain the neutral fraction of the surrounding intergalactic medium (IGM) as well as the quasar lifetime. Based on current number counts of luminous quasars at z>6, an instrument like LOFAR should detect >2 redshifted 21cm shells per field (with a radius of 11 degrees) around active quasars as bright as those already discovered by SDSS, and >200 relic shells of inactive quasars per field. We show that Ly-alpha photons from the quasar are unable to heat the IGM or to couple the spin and kinetic temperatures of atomic hydrogen beyond the edge of the HII region. The detection of the IGM in 21cm emission around high redshift quasars would therefore gauge the presence of a cosmic Ly-alpha background during the reionization epoch.Comment: 11 pages, 6 figures. Submitted to Ap