There is growing awareness that established structures of higher education are often predicated on problematic assumptions about merit, excellence, and rigor. Doctoral qualifying exams, for example, ...are required to advance to candidacy in many Ph.D. programs despite decades of documented concerns about the implications of standard modes for student equity and well-being. As more Ph.D. programs move to reform these exams and candidacy requirements, it is important to understand how Ph.D. programs, as academic organizations, construct the significance of the qualifying exam. A sociocultural lens suggests qualifying exams and the learning that enables their passage are symbolic rituals that move doctoral students from legitimate peripheral participation toward full membership and belonging in academic communities of practice. We conducted a comparative case study to understand how two Ph.D. programs in the physical sciences that have reformed their candidacy requirements--one elite and one middle ranked but striving for respect--constructed the significance and purpose of their qualifying exam and the broader transition to candidacy. Our inquiry included the contexts and mechanisms that mediated student learning. Through interviews with faculty, staff, and students, we found that the Ph.D. programs' recognition of their status within their respective disciplines emerged as a crucial component in constructions about the significance of exams and candidacy. The middle-ranked Ph.D. program changed the exam and candidacy structure to reflect legitimate practices in their discipline. The elite Ph.D. program created multiple pathways toward candidacy to mitigate long-standing concerns about gender equity and student well-being. Despite the structural changes, the Ph.D. programs left intact cultural understandings of merit, excellence, and rigor that maintain inequity in doctoral socialization. Our findings suggest that researchers and practitioners should pay more attention to designing and implementing structures that facilitate faculty assessments of doctoral student learning.
The temperature dependence of the stability of bulk BaZrO3 (BZO) and of the vacancies in this material are investigated by considering phonon contributions to the free energy. The stability diagram ...of BZO is determined for different chemical environments. With increasing temperature the stability region becomes smaller which is particularly caused by the strong temperature dependence of the chemical potential of gaseous oxygen. The free formation energy of Ba, Zr, and O vacancies in BZO is calculated for all possible charge states and for different atomic reservoirs. While the free formation energy of Zr vacancies is strongly influenced by temperature a weaker dependence is found for Ba and O vacancies. This also has an effect on the charge transition levels at different temperatures. The present results demonstrate that O poor reservoir conditions and a Fermi level close to the valence band maximum favour a high concentration of doubly positively charged O vacancies which is a prerequisite to get a large number of protonic defects and good proton conductivity. In such a chemical environment the number of Ba and Zr vacancies is low so that Ba and Zr deficiencies are not an important issue and BZO remains sufficiently stable.
The electronic, thermodynamic, and optical properties of a new type of two-dimensional Janus layer (JL) consisting exclusively of chalcogens are investigated using first-principles calculations. The ...permutations on atomic sites provide increased stability due to the multi-valency of chalcogens, and a heavier central atom further stabilizes the layer due to the increased coordination number. The investigated JLs are indirect bandgap materials with a bandgap larger than 1.23 eV, making them suitable for photocatalytic activity. Different feasible chemical potentials are analyzed, and chalcogens’ poor limits are proposed to fabricate the JLs. Based on the comparison of the formation energy, the energetic profile of the JLs is identified as E f TeSeS < E f SSeTe < E f SeSTe, irrespective of the chemical potentials of chalcogen. Hence, TeSeS is more stable than the JL arrangements SSeTe and SeSTe. The flat bands around the Fermi energy level and the reduction in path length between the maximum of conduction and minimum of valence bands explain the magnitude of multiple peaks observed in the optical spectra of the JLs. These absorptions turn the studied JLs into potential candidates for water splitting. The optimized bandgap reveals that the band edges efficiently straddle the water redox potentials at different pH levels. In addition, the positive vibrational frequencies depict the stability of these layers. Because of the minimal formation energy requirement, higher density of states around the Fermi level, as well as enhanced optical absorption compared to other JL, TeSeS JLs may lead to enhanced performance in photovoltaic and photocatalytic applications. These results add new members to the JL family of pure chalcogens and pave the way toward novel materials for respective applications.
The temperature dependence of the stability of bulk BaZrO
(BZO) and of the vacancies in this material are investigated by considering phonon contributions to the free energy. The stability diagram of ...BZO is determined for different chemical environments. With increasing temperature the stability region becomes smaller which is particularly caused by the strong temperature dependence of the chemical potential of gaseous oxygen. The free formation energy of Ba, Zr, and O vacancies in BZO is calculated for all possible charge states and for different atomic reservoirs. While the free formation energy of Zr vacancies is strongly influenced by temperature a weaker dependence is found for Ba and O vacancies. This also has an effect on the charge transition levels at different temperatures. The present results demonstrate that O poor reservoir conditions and a Fermi level close to the valence band maximum favour a high concentration of doubly positively charged O vacancies which is a prerequisite to get a large number of protonic defects and good proton conductivity. In such a chemical environment the number of Ba and Zr vacancies is low so that Ba and Zr deficiencies are not an important issue and BZO remains sufficiently stable.
The competitive dynamics that sustain stratification among postsecondary institutions have reinforced racial inequality in selective college enrollment between 1972 and 2004. Using a data set ...constructed from four nationally representative surveys (National Longitudinal Survey 1972, High School & Beyond 1980, National Educational Longitudinal Survey 1988, and Educational Longitudinal Survey 2002), the authors model how escalating admissions standards—including academic preparation and the growing importance of SAT scores and extracurricular leadership—effectively maintain racial inequality in selective college enrollment over time. Black and Latino students have made strides in their pre-collegiate academic preparation. Nevertheless, although access to postsecondary education has expanded since 1972 for all ethnic groups, Black and Latino students' odds of selective college enrollment have declined relative to White and Asian American students.
Clinical islet allotransplantation has been successfully regulated as tissue/organ for transplantation in number of countries and is recognized as a safe and efficacious therapy for selected patients ...with type 1 diabetes mellitus. However, in the United States, the FDA considers pancreatic islets as a biologic drug, and islet transplantation has not yet shifted from the experimental to the clinical arena for last 20 years. In order to transplant islets, the FDA requires a valid Biological License Application (BLA) in place. The BLA process is costly and lengthy. However, despite the application of drug manufacturing technology and regulations, the final islet product sterility and potency cannot be confirmed, even when islets meet all the predetermined release criteria. Therefore, further regulation of islets as drugs is obsolete and will continue to hinder clinical application of islet transplantation in the US. The Organ Procurement and Transplantation Network together with the United Network for Organ Sharing have developed separately from the FDA and BLA regulatory framework for human organs under the Human Resources & Services Administration to assure safety and efficacy of transplantation. Based on similar biologic characteristics of islets and human organs, we propose inclusion of islets into the existing regulatory framework for organs for transplantation, along with continued FDA oversight for islet processing, as it is for other cell/tissue products exempt from BLA. This approach would reassure islet quality, efficacy and access for Americans with diabetes to this effective procedure.
We report experiments on the impact of 2.5 MeV proton irradiation on self-diffusion and dopant diffusion in germanium (Ge). Self-diffusion under irradiation reveals an unusual depth independent ...broadening of the Ge isotope multilayer structure. This behavior and the observed enhanced diffusion of B and retarded diffusion of P demonstrates that an interstitial-mediated diffusion process dominates in Ge under irradiation. This fundamental finding opens up unique ways to suppress vacancy-mediated diffusion in Ge and to solve the donor deactivation problem that hinders the fabrication of Ge-based nanoelectronic devices.
Silicon chips containing arrays of single dopant atoms can be the material of choice for classical and quantum devices that exploit single donor spins. For example, group‐V donors implanted in ...isotopically purified 28Si crystals are attractive for large‐scale quantum computers. Useful attributes include long nuclear and electron spin lifetimes of 31P, hyperfine clock transitions in 209Bi or electrically controllable 123Sb nuclear spins. Promising architectures require the ability to fabricate arrays of individual near‐surface dopant atoms with high yield. Here, an on‐chip detector electrode system with 70 eV root‐mean‐square noise (≈20 electrons) is employed to demonstrate near‐room‐temperature implantation of single 14 keV 31P+ ions. The physics model for the ion–solid interaction shows an unprecedented upper‐bound single‐ion‐detection confidence of 99.85 ± 0.02% for near‐surface implants. As a result, the practical controlled silicon doping yield is limited by materials engineering factors including surface gate oxides in which detected ions may stop. For a device with 6 nm gate oxide and 14 keV 31P+ implants, a yield limit of 98.1% is demonstrated. Thinner gate oxides allow this limit to converge to the upper‐bound. Deterministic single‐ion implantation can therefore be a viable materials engineering strategy for scalable dopant architectures in silicon devices.
On‐chip electrodes are employed to signal near‐surface single‐ion implantation in silicon. A physical model of the signals provides insights into the ion stopping trajectories within the silicon chip and reveals greater than 99.8% single‐ion‐detection confidence. The study demonstrates the feasibility of controlled scalable dopant architecture engineering for classical and quantum silicon devices.
We studied the periodicity of the multilamellar membrane system of granal chloroplasts in different isolated plant thylakoid membranes, using different suspension media, as well as on different ...detached leaves and isolated protoplasts-using small-angle neutron scattering. Freshly isolated thylakoid membranes suspended in isotonic or hypertonic media, containing sorbitol supplemented with cations, displayed Bragg peaks typically between 0.019 and 0.023Å(-1), corresponding to spatially and statistically averaged repeat distance values of about 275-330 Å⁻¹. Similar data obtained earlier led us in previous work to propose an origin from the periodicity of stroma thylakoid membranes. However, detached leaves, of eleven different species, infiltrated with or soaked in D2O in dim laboratory light or transpired with D2O prior to measurements, exhibited considerably smaller repeat distances, typically between 210 and 230 Å⁻¹, ruling out a stromal membrane origin. Similar values were obtained on isolated tobacco and spinach protoplasts. When NaCl was used as osmoticum, the Bragg peaks of isolated thylakoid membranes almost coincided with those in the same batch of leaves and the repeat distances were very close to the electron microscopically determined values in the grana. Although neutron scattering and electron microscopy yield somewhat different values, which is not fully understood, we can conclude that small-angle neutron scattering is a suitable technique to study the periodic organization of granal thylakoid membranes in intact leaves under physiological conditions and with a time resolution of minutes or shorter. We also show here, for the first time on leaves, that the periodicity of thylakoid membranes in situ responds dynamically to moderately strong illumination. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.
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