Can Photo- and Cathodoluminescence be Regarded as Complementary Techniques?

Photoluminescence (PL) usually provides macroscopic, high quality spectroscopic data. Cathodoluminescence (CL), on the other hand, offers the same information with microscopic imaging. However, replicating PL signatures in a CL system is not straightforward since matching experimental conditions, such as temperature and excitation density, is difficult. The matter is further exacerbated by inherent differences in the nature of excitation: electrons versus photons. Our work with high purity semiconductors suggests that CL is generally more sensitive to excitation circumstance than PL. For example, electrons can cause sample charging and contamination-related phenomena that dramatically affect CL. Changes in surface attributes (e.g., by chemical passivation) also affect PL and CL signals differently. Here, we extend previous work on GaAs by exploring the role of surface topography (by atomic force microscopy) and temperature (1.8K-100K) on excitonic lineshapes. We find that topographic subtleties strongly influence the character of exciton-polariton luminescence. We interpret these changes in terms of non-classical scattering phenomena derived from microscopic roughness. These microscopic changes also influence the temperature behaviour of excitons in crystals. Specifically, we find that passivated samples are brighter partly because there is a corresponding reduction in the (Arrhenius) activation energy for excitonic processes. In summary, the changes in surface topography and corresponding recombination physics seem well correlated

Quasiperiodic functions theory and the superlattice potentials for a two-dimensional electron gas

We consider Novikov problem of the classification of level curves of quasiperiodic functions on the plane and its connection with the conductivity of two-dimensional electron gas in the presence of both orthogonal magnetic field and the superlattice potentials of special type. We show that the modulation techniques used in the recent papers on the 2D heterostructures permit to obtain the general quasiperiodic potentials for 2D electron gas and consider the asymptotic limit of conductivity when $\tau \to \infty$. Using the theory of quasiperiodic functions we introduce here the topological characteristics of such potentials observable in the conductivity. The corresponding characteristics are the direct analog of the "topological numbers" introduced previously in the conductivity of normal metals.Comment: Revtex, 16 pages, 12 figure

Quantum Transport in Semiconductor Nanostructures

I. Introduction (Preface, Nanostructures in Si Inversion Layers, Nanostructures in GaAs-AlGaAs Heterostructures, Basic Properties). II. Diffusive and Quasi-Ballistic Transport (Classical Size Effects, Weak Localization, Conductance Fluctuations, Aharonov-Bohm Effect, Electron-Electron Interactions, Quantum Size Effects, Periodic Potential). III. Ballistic Transport (Conduction as a Transmission Problem, Quantum Point Contacts, Coherent Electron Focusing, Collimation, Junction Scattering, Tunneling). IV. Adiabatic Transport (Edge Channels and the Quantum Hall Effect, Selective Population and Detection of Edge Channels, Fractional Quantum Hall Effect, Aharonov-Bohm Effect in Strong Magnetic Fields, Magnetically Induced Band Structure).Comment: 111 pages including 109 figures; this review from 1991 has retained much of its usefulness, but it was not yet available electronicall

Teach the kids to code switch... which is a very easy thing to say: Heterosystemic pedagogies for racial justice within a field of (im)possibilities

University of Minnesota Ph.D. dissertation.December 2018. Major: Education, Curriculum and Instruction. Advisor: Cynthia Lewis. 1 computer file (PDF); xii, 272 pages.This study employs ethnographic field methods to investigate the ways Eric, a ninth-grade English language arts teacher committed to racial justice, enacted literacy education in his classroom. With a background in critical literacy, I entered the field with preconceived notions of what a justice-oriented classroom would look like. So, I was surprised to observe a teacher-centric, basic-skills-focused classroom that offered few opportunities for student voice. By observing staff development and committee meetings, learning that Eric’s school, Wayside Junior High, also expressed commitments to racial justice, but engaged in behavior management practices (PBIS) that are generally understood be in tension with racial justice. The purpose of this study is to better understand how Eric and the Staff at Wayside came to take up these apparently contradictory practices. To achieve this purpose, I developed a Foucauldian interpretation of Activity Systems Analysis (FASA), which combines elements of Engestrom’s Activity Systems Analysis (objects, rules, tools, subjectivities, and contradictions/tensions), with several Foucauldian concepts (problematization, freedom, and the field of possibilities). The resulting analytical framework can be expressed in the question: How do subjects construct and enact agency within the field of possibilities produced by the problems, rules, tools, and subjectivities in their environment? An analysis of the data had several implications. Justice-oriented teacher educators 1) must engage with the real and practical problem of organizing a group of individuals for a collective learning activity, what many call “behavior management” and 2) must avoid conveying an ethic of moral purity, and instead encourage teachers to cultivate practices of hybridity—to inventory the multiplicity of problems, objects, rules, and tools in their environment and creatively assemble new justice-oriented learning activities that we, as researchers and teacher educators, could not have imagined. Furthermore, justice-oriented teacher educators in the field of literacy and English language arts 1) must devote more resources and more credit hours to preparing educators to teach the craft of writing, and 2) must identify, teach, and conduct research on strategies for teaching so called “basic skills” through a critical and justice-oriented lens in order to provide minoritized students with codes of power (Delpit, 1988)

Zum Kampf der westdeutschen Arbeiterklasse um die Herstellung der Aktionseinheit zur Verhinderung der Remilitarisierung der BRD in den Jahren 1951/1952

SIGLEMilitaerbibliothek Dresden - 80 14 0264/0265, T. 1, 2 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Bioabbaubare Kunststoffe: Untersuchung zur mikrobiologischen Abbaubarkeit von Kunststoffen Entwicklung eines Schnelltestes unter praxisnahen Bedingungen. Abschlussbericht

Because of a shortage in testing methods to evaluate true biodegradation of plastics that go further than just showing changes in material properties, the goal of this project was to find methods, that will show biodegradation leading to natural metabolic endproducts. Following testing methods have been evaluated; (a) Screening methods by following the oxygen consumption or CO_2-production (aerobic) and biogas- and CH_4-production (anaerobic); (b) longterm-systems like aquarium and soil burial; (c) disposal-like systems in a landfill-simulation and a static laboratory composting. It became evident, that in screening systems a C-balance is necessary to show complete biodegradation of a complex plastic material. Longterm-systems like an aquarium- or soil burial-test help to evaluate the biodegradation potential of plastics degrading in longer time periods, especially when combined with polymer-specific analytics. The results from the screeing-systems were backed up by those under environmental and/or disposal conditions. The project lead to standardizable testing methods and a step by step programm to assess biodegradability of plastics. (orig.)Testmethoden zur Untersuchung der Bioabbaubarkeit von Kunststoffen die ueber eine unerwuenschte Beeinflussung der Materialeigenschaften hinausgingen, lagen nicht vor. Ziel war somit, Methoden zum Nachweis der biologischen Abbaubarkeit, die zu natuerlichen Stoffwechselendprodukten fuehrt, zu finden. Als Testmethoden wurden eingesetzt: (a) Verfolgung des O_2-Verbrauchs bzw. der CO_2-Entwicklung (aerob) und der Biogas- und CH_4-Freisetzung (anaerob) als Screeningmethode; (b) Aquarium- und Erdeingrabeversuche als Langzeitversuch; (c) Deponiesimulation und statische Kompostierung als entsorgungsrelevante Testmethode. Fuer die Screeningversuche zeigte sich, dass bei der Komplexitaet der Kunststoffe ein e C-Bilanzierung des Abbaus anzustreben ist, um eine vollstaendige Abbaubarkeit aufzeigen zu koennen. Aquarien- und Erdeingrabeversuche sind bei den Materialien, die nur ueber laengere Zeitraeume abbauen, sinnvoll und koennen mit polymerchemischer Analytik verbunden das Abbaugeschehen beleuchten. Die Ergebnisse aus den Screeningversuchen konnten unter umwelt- und entsorgungsrelevanten Bedingungen bestaetigt werden. Das Vorhaben fuehrte zu standardisierbaren Screening-Testmethoden und einem Stufenprogramm zur Untersuchung der Bioabbaubarkeit von Kunststoffen. (orig.)Available from TIB Hannover: F94B0851+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman