Optical pump-probe techniques are used to generate and measure electron spin polarization in a gallium arsenide epilayer in which the electron spin coherence time exceeds the mode-locked laser repetition period. Resonant spin amplification occurs at magnetic fields at which the electron spin polarization excited by successive laser pulses add constructively. Measurements of Kerr rotation as a function of applied magnetic field reveal nuclear spin polarization that aligns either with or against the external magnetic field depending on whether the applied magnetic field is being decreased or increased. Furthermore, the nuclear spin polarization magnitude varies in proportion to the perpendicular net electron spin polarization as the latter changes due to resonant spin amplification and other causes. We also report an experimental technique that reveals a minutes-long memory of precise field history in the electron-nuclear spin system.
We fabricate gallium arsenide-based devices with a wedge-shaped tapering region connected to a rectangular-shaped region and measure the threshold voltage required to trigger the Gunn effect. The threshold voltage reduction is attributed to the focusing of the electric field toward the narrower end of the device and is effective when the device has a steep enough tapering. We also model the electric field profile for the tapered devices using an intuitive graphical approach and the finite element method and provide estimates for the threshold voltages of tapered devices. Finally, we compare the estimates to the measured values and provide possible reasons for the discrepancies. We believe the capability of threshold voltage reduction with the wedge-shaped tapering design could be useful in device applications.
Emerging in the learning sciences field in the early 1990s, qualitative design-based research (DBR) is a relatively new methodological approach to social science and education research. As its name implies, DBR is focused on the design of educational innovations, and the testing of these innovations in the complex and interconnected venue of naturalistic settings. As such, DBR is an explicitly interventionist approach to conducting research, situating the researcher as a part of the complex ecology in which learning and educational innovation takes place. With this in mind, DBR is distinct from more traditional methodologies, including laboratory experiments, ethnographic research, and large-scale implementation. Rather, the goal of DBR is not to prove the merits of any particular intervention, or to reflect passively on a context in which learning occurs, but to examine the practical application of theories of learning themselves in specific, situated contexts. By designing purposeful, naturalistic, and sustainable educational ecologies, researchers can test, extend, or modify their theories and innovations based on their pragmatic viability. This process offers the prospect of generating theory-developing, contextualized knowledge claims that can complement the claims produced by other forms of research. Because of this interventionist, naturalistic stance, DBR has also been the subject of ongoing debate concerning the rigor of its methodology. In many ways, these debates obscure the varied ways DBR has been practiced, the varied types of questions being asked, and the theoretical breadth of researchers who practice DBR. With this in mind, DBR research may involve a diverse range of methods as researchers from a variety of intellectual traditions within the learning sciences and education research design pragmatic innovations based on their theories of learning, and document these complex ecologies using the methodologies and tools most applicable to their questions, focuses, and academic communities. DBR has gained increasing interest in recent years. While it remains a popular methodology for developmental and cognitive learning scientists seeking to explore theory in naturalistic settings, it has also grown in importance to cultural psychology and cultural studies researchers as a methodological approach that aligns in important ways with the participatory commitments of liberatory research. As such, internal tension within the DBR field has also emerged. Yet, though approaches vary, and have distinct genealogies and commitments, DBR might be seen as the broad methodological genre in which Change Laboratory, design-based implementation research (DBIR), social design-based experiments (SDBE), participatory design research (PDR), and research-practice partnerships might be categorized. These critically oriented iterations of DBR have important implications for educational research and educational innovation in historically marginalized settings and the Global South.
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