Temperature and magnetization-dependent band-gap renormalization and optical many-body effects in diluted magnetic semiconductors

We calculate the Coulomb interaction induced density, temperature and magnetization dependent many-body band-gap renormalization in a typical diluted magnetic semiconductor GaMnAs in the optimally-doped metallic regime as a function of carrier density and temperature. We find a large (about 0.1 eV) band gap renormalization which is enhanced by the ferromagnetic transition. We also calculate the impurity scattering effect on the gap narrowing. We suggest that the temperature, magnetization, and density dependent band gap renormalization could be used as an experimental probe to determine the valence band or the impurity band nature of carrier ferromagnetism.Comment: Revised versio

Possible magnetic-field-induced voltage and thermopower in diluted magnetic semiconductors

In diluted magnetic semiconductors, the carrier concentration and the magnetization of local moments are strongly coupled, since the magnetic interaction is mediated by the carriers. It is predicted that this coupling leads to an electric polarization due to an applied magnetic-field gradient and to the appearance of a magnetic-field-dependent voltage. An expression for this voltage is derived within Landau theory and its magnitude is estimated for (Ga,Mn)As. Furthermore, a large contribution to the thermopower based on the same mechanism is predicted. The role of fluctuations is also discussed. These predictions hold both if the magnetization is uniform and if it shows stripe-like modulations, which are possible at lower temperatures.Comment: 6 pages revtex, 5 figure

Mock Modular Mathieu Moonshine Modules

We construct super vertex operator algebras which lead to modules for moonshine relations connecting the four smaller sporadic simple Mathieu groups with distinguished mock modular forms. Starting with an orbifold of a free fermion theory, any subgroup of Co_0 that fixes a 3-dimensional subspace of its unique non-trivial 24-dimensional representation commutes with a certain N=4 superconformal algebra. Similarly, any subgroup of Co_0 that fixes a 2-dimensional subspace of the 24-dimensional representation commutes with a certain N=2 superconformal algebra. Through the decomposition of the corresponding twined partition functions into characters of the N=4 (resp. N=2) superconformal algebra, we arrive at mock modular forms which coincide with the graded characters of an infinite-dimensional Z-graded module for the corresponding group. The Mathieu groups are singled out amongst various other possibilities by the moonshine property: requiring the corresponding weak Jacobi forms to have certain asymptotic behaviour near cusps. Our constructions constitute the first examples of explicitly realized modules underlying moonshine phenomena relating mock modular forms to sporadic simple groups. Modules for other groups, including the sporadic groups of McLaughlin and Higman--Sims, are also discussed.Comment: 94 pages, including 56 pages of tables; v2: updated references and minor revisions to abstract, introduction and sections 8 and

Mathieu Moonshine and N=2 String Compactifications

There is a `Mathieu moonshine' relating the elliptic genus of K3 to the sporadic group M_{24}. Here, we give evidence that this moonshine extends to part of the web of dualities connecting heterotic strings compactified on K3 \times T^2 to type IIA strings compactified on Calabi-Yau threefolds. We demonstrate that dimensions of M_{24} representations govern the new supersymmetric index of the heterotic compactifications, and appear in the Gromov--Witten invariants of the dual Calabi-Yau threefolds, which are elliptic fibrations over the Hirzebruch surfaces F_n.Comment: 28 pages; v2: minor changes, published versio

A Delphi Study: Expanding K-12 Student Knowledge and Interests in Aviation/Aerospace Recommendations from NASA Aerospace Education Specialists

An aging aerospace workforce coupled with declining student interest and skills beginning at the secondary level and running through the doctoral level has prompted a potential crisis in the aviation/aerospace industry. The Bureau of Labor Statistics estimated that 54 percent of U.S. aerospace workers over age 45 will leave the field by 2008 opening up some six million aerospace jobs. If the existing trends continue, there will not be enough workers to fill those positions which are vital to the U.S. economy and security. Aerospace employment is at its lowest in 50 years (Aerospace Industries Association, 2004). Data from the Commission on the Future of the U.S. Aerospace Industry indicated that math and science performance of U.S. high school seniors ranks near the bottom internationally (2002). The Commission made the following recommendation: The Commission recommends the nation immediately reverse the decline in, and promote the growth of, a scientifically and technologically trained U.S. aerospace workforce. In addition, the nation must address the failure of the math, science and technology education of Americans. The breakdown of America\u27s intellectual and industrial capacity is a threat to national security and our capability to continue as a world leader (p. xvi). The Commission further stressed the importance of taking action to improve math and science instruction across the entire education range--K-12 through graduate school. In an effort to encourage students to pursue scientific interests in aviation/aerospace, Aerospace Education Specialists working for the National Aeronautics and Space Administration (NASA) are involved in a relatively new effort to work with K-12 educators to provide in-classroom education about the field of aviation/aerospace. As former K-12 educators, these specialists are constantly looking for new and creative ways to convey to current K-12 educators the importance of aviation/aerospace careers to the future of the industry. And, as representatives of NASA, these specialists are trained and experienced in matching agency resources with the needs of educators

Doping dependence of the Neel temperature in Mott-Hubbard antiferromagnets: Effect of vortices

The rapid destruction of long-range antiferromagnetic order upon doping of Mott-Hubbard antiferromagnetic insulators is studied within a generalized Berezinskii-Kosterlitz-Thouless renormalization group theory in accordance with recent calculations suggesting that holes dress with vortices. We calculate the doping-dependent Neel temperature in good agreement with experiments for high-Tc cuprates. Interestingly, the critical doping where long-range order vanishes at zero temperature is predicted to be xc ~ 0.02, independently of any energy scales of the system.Comment: 4 pages with 3 figures included, minor revisions, to be published in PR

High-to-low CO2 acclimation reveals plasticity of the photorespiratory pathway and indicates regulatory links to cellular metabolism of Arabidopsis

Background: Photorespiratory carbon metabolism was long considered as an essentially closed and nonregulated pathway with little interaction to other metabolic routes except nitrogen metabolism and respiration. Most mutants of this pathway cannot survive in ambient air and require CO 2-enriched air for normal growth. Several studies indicate that this CO 2 requirement is very different for individual mutants, suggesting a higher plasticity and more interaction of photorespiratory metabolism as generally thought. To understand this better, we examined a variety of high- and low-level parameters at 1% CO 2 and their alteration during acclimation of wild-type plants and selected photorespiratory mutants to ambient air. Methodology and Principal Findings: The wild type and four photorespiratory mutants of Arabidopsis thaliana (Arabidopsis) were grown to a defined stadium at 1% CO 2 and then transferred to normal air (0.038% CO 2). All other conditions remained unchanged. This approach allowed unbiased side-by-side monitoring of acclimation processes on several levels. For all lines, diel (24 h) leaf growth, photosynthetic gas exchange, and PSII fluorescence were monitored. Metabolite profiling was performed for the wild type and two mutants. During acclimation, considerable variation between the individual genotypes was detected in many of the examined parameters, which correlated with the position of the impaired reaction in the photorespiratory pathway. Conclusions: Photorespiratory carbon metabolism does not operate as a fully closed pathway. Acclimation from high to low CO 2 was typically steady and consistent for a number of features over several days, but we also found unexpected short-term events, such as an intermittent very massive rise of glycine levels after transition of one particular mutant to ambient air. We conclude that photorespiration is possibly exposed to redox regulation beyond known substrate-level effects. Additionally, our data support the view that 2-phosphoglycolate could be a key regulator of photosynthetic-photorespiratory metabolism as a whole. © 2012 Timm et al

Absence of skew scattering in two-dimensional systems: Testing the origins of the anomalous Hall effect

We study the anomalous Hall conductivity in spin-polarized, asymmetrically confined two-dimensional electron and hole systems, focusing on skew-scattering contributions to the transport. We find that the skew scattering, principally responsible for the extrinsic contribution to the anomalous Hall effect, vanishes for the two-dimensional electron system if both chiral Rashba subbands are partially occupied, and vanishes always for the two-dimensional hole gas studied here, regardless of the band filling. Our prediction can be tested with the proposed coplanar two-dimensional electron/hole gas device and can be used as a benchmark to understand the crossover from the intrisic to the extrinsic anomalous Hall effect.Comment: 4 pages, 2 figures include

Comparative planning in practice: identifying the cultural and behavioral barriers to learning from international sustainability solutions

This dissertation is composed of three separate but related papers that address if and how practicing planners can identify the cultural and behavioral barriers to learning from international sustainability solutions. Chapter 2 starts by acknowledging that analytical frameworks examining the international flow of concepts, practices, and techniques ("traveling ideas" or "international best practices") most often call for the slow and critical examination of these ideas prior to local adaptation. However, in fast-paced working environments where planners are challenged to urgently address environmental and social issues, this paper uncovers how these analytical frameworks are actually interpreted in practice. Building on existing comparative planning literature, semi-structured interviews with public sector planners from both the City of Chicago and the City of Stockholm were conducted in order to 1) explore the extent to which planners from each city utilize international best practices in relation to sustainability; and 2) when these best practices are used, understand the planners' evaluation process in determining if the practice could or should be adapted for their city’s use. The insights and reflections garnered from this study were used to identify potential factors that influence successful translation of sustainability solutions across contexts. Chapter 3 puts the findings from Chapter 2 in a specific context by aiming to uncover the key behavioral barriers to translating Singapore's high reclaimed water public acceptance rate and successful conservation strategies to different contexts. A national mail survey (n=218) utilizing the Theory of Planned Behavior framework (Ajzen, 1991) was conducted. The survey indicates that 74 percent of Singaporeans generally approve of NEWater. A positive attitude toward NEWater was the most significant variable in predicting respondent’s general approval. Furthermore, of the 7 specific water behaviors the Singaporean Public Utilities Board is trying to encourage, we found that fixing water leaks promptly (80.8%) and monitoring water bills (80.3%) are the most widely adopted, while washing dishes under a filled sink (11.7%) was the least adopted. Path analysis of the data showed that engagement in water conservation behaviors was most influenced by an individual's perceived social norms. Building off of Chapter 3, where we discuss the behavioral barriers to Singapore's integrated water management plan, Chapter 4 explores the cultural barriers to translating Singapore's successful water conservation strategies. Fieldwork in the form of interviews and participant observation was conducted over a period of 9 months in Singapore. This qualitative data was analyzed using Hofstede's dimensions of national culture framework. We find that Singapore's national culture has played a significant role in the overall success of their water planning and management strategies. Differences in specific national cultural dimensions such as power distance and individualism could act as barriers to successfully translating Singapore's success to different countries. We propose simple, straightforward recommendations for identifying and addressing these cultural barriers. Chapter 5 is a general discussion on the entire dissertation with a review of the work, methods and significant findings along with their policy and other implications. I conclude the piece with an exploration of future work

Limitation of spiral microchannels for particle separation in heterogeneous mixtures: impact of particles’ size and deformability

Spiral microchannels have shown promising results for separation applications. Hydrodynamic particle-particle interactions are a known factor strongly influencing focussing behaviours in inertial devices, with recent work highlighting how the performance of bidisperse mixtures is altered when compared with pure components, in square channels. This phenomenon has not been previously investigated in detail for spiral channels. Here, we demonstrate that, in spiral channels, both the proportion and deformability of larger particles (13 μm diameter) impact upon the recovery (up to 47% decrease) of small rigid particles (4 μm). The effect, observed at low concentrations (volume fraction <0.0012), is attributed to the hydrodynamic capture of beads by larger cells. These changes in particles focussing behaviour directly impede the efficiency of the separation – diverting beads from locations expected from measurements with pure populations to co-collection with larger cells – and could hamper deployment of the technology for certain applications. Similar focussing behaviour alterations were noted when working with purification of stem cell end products