Epitaxial Ferromagnetic Nanoislands of Cubic GdN in Hexagonal GaN

Periodic structures of GdN particles encapsulated in a single crystalline GaN matrix were prepared by plasma assisted molecular beam epitaxy. High resolution X-ray diffractometery shows that GdN islands, with rock salt structure are epitaxially oriented to the wurtzite GaN matrix. Scanning transmission electron microscopy combined with in-situ reflection high energy electron diffraction allows for the study of island formation dynamics, which occurs after 1.2 monolayers of GdN coverage. Magnetometry reveals two ferromagnetic phases, one due to GdN particles with Curie temperature of 70K and a second, anomalous room temperature phase.Comment: 4 pages, 3 figure

Systematic study of d-wave superconductivity in the 2D repulsive Hubbard model

The cluster size dependence of superconductivity in the conventional two-dimensional Hubbard model, commonly believed to describe high-temperature superconductors, is systematically studied using the Dynamical Cluster Approximation and Quantum Monte Carlo simulations as cluster solver. Due to the non-locality of the d-wave superconducting order parameter, the results on small clusters show large size and geometry effects. In large enough clusters, the results are independent of the cluster size and display a finite temperature instability to d-wave superconductivity.Comment: 4 pages, 3 figures; updated with version published in PRL; added values of Tc obtained from fit

Where’s the Data? Using Data Convincingly in Transdisciplinary Doctoral Research

Aim/Purpose The aim of this paper is to identify some of the issues in writing a transdisciplinary doctoral thesis and to develop strategies for addressing them, particularly focusing on the presentation of data and data analysis. The paper, based on the authors’ own experience, offers guidance to, and invites further comment from, transdisciplinary doctoral candidates, their supervisors and their examiners, as well as the broader field of interdisciplinary and multidisciplinary researchers. Background The paper uses the authors’ experience of writing four very different transdisciplinary doctoral theses to examine the diverse responses received from examiners and what this means for the thesis writing process. The theses and examiners’ reports span an array of disciplinary and transdisciplinary epistemologies, ontologies, and world views. Methodology A preliminary review of the examiners’ reports revealed a common concern with the definition of ‘data’ and with ‘data analysis’. The examiners’ reports were then more formally coded and thematized. These themes were then used to reflect critically on the four theses, within a broad interpretive framework based on the idea of writing ‘convincingly’, and in light of current literature on the meaning of ‘data’ and the idea and aims of transdisciplinarity. Contribution The paper offers specific strategies for doctoral candidates, their supervisors, and examiners in working with the burgeoning number of doctoral research projects that are now taking place in the transdisciplinary space. Findings Doctoral candidates engaged in transdisciplinary research need to define what they mean by data and make data visible in their research, be creative in their conceptions of data and in how they communicate this to examiners, specify the quality criteria against which they wish their work to be assessed and hold discussions with their supervisors about examiner appointments and briefing, and communicate to examiners the special value of transdisciplinary research and the journey on which it takes the researcher. Our conclusion connects these findings to the development of an emerging concept of transdisciplinary research writing. Recommendations for See below under ‘Recommendations for Researchers’ (For the purpose of Practitioners this paper, practitioners are the researchers). Recommendations The paper makes the following recommendations for transdisciplinary for Researchers doctoral researchers: • Make the data visible and argue for the unique or special way in which the data will be used • Make clear the quality criteria against which you expect the work to be judged • Be creative and explore the possibilities enabled by a broad interpretation of ‘data’ • Transdisciplinary research is transformative. Communicate this to your examiner. Impact on Society As more complex and ‘wicked’ problems in the world are increasingly addressed through transdisciplinary research, it is important that doctoral research in this area be encouraged, which continues to develop transdisciplinary theoretical frameworks, methodologies and applications. The strategies proposed in this paper will help to ensure the development of high quality transdisciplinary researchers and a greater understanding of the value of transdisciplinary research in the wider research community. It also draws attention to the potential benefits of similar strategies in multidisciplinary and interdisciplinary research. Future Research Further exploration is needed of how researchers across disciplines can ‘talk’ to one another to resolve complex problems, and how the solitary transdisciplinary scholar, such as the doctoral student, can effectively communicate their research contribution to others. These issues could also be explored for multidisciplinary and interdisciplinary research teams

Combined density-functional and dynamical cluster quantum Monte Carlo calculations for three-band Hubbard models for hole-doped cuprate superconductors

Using a combined local density functional theory (LDA-DFT) and quantum Monte Carlo (QMC) dynamic cluster approximation approach, the parameter dependence of the superconducting transition temperature Tc of several single-layer hole-doped cuprate superconductors with experimentally very different Tcmax is investigated. The parameters of two different three-band Hubbard models are obtained using the LDA and the downfolding Nth-order muffin-tin orbital technique with N=0 and 1 respectively. QMC calculations on 4-site clusters show that the d-wave transition temperature Tc depends sensitively on the parameters. While the N=1 MTO basis set which reproduces all three $pd\sigma$ bands leads to a d-wave transition, the N=0 set which merely reproduces the LDA Fermi surface and velocities does not

Resonance-like piezoelectric electron-phonon interaction in layered structures

We show that mismatch of the piezoelectric parameters between layers of multiple-quantum well structures leads to modification of the electron-phonon interaction. In particular, short-wavelength phonons propagating perpendicular to the layers with wavevector close to $2\pi n/d$, where $d$ is the period of the structure, induce a strong smoothly-varying component of the piezo-potential. As a result, they interact efficiently with 2D electrons. It is shown, that this property leads to emission of collimated quasi-monochromatic beams of high-frequency acoustic phonons from hot electrons in multiple-quantum well structures. We argue that this effect is responsible for the recently reported monochromatic transverse phonon emission from optically excited GaAs/AlAs superlattices, and provide additional experimental evidences of this.Comment: 6 pages, 7 figure

Theory and Application of Dissociative Electron Capture in Molecular Identification

The coupling of an electron monochromator (EM) to a mass spectrometer (MS) has created a new analytical technique, EM-MS, for the investigation of electrophilic compounds. This method provides a powerful tool for molecular identification of compounds contained in complex matrices, such as environmental samples. EM-MS expands the application and selectivity of traditional MS through the inclusion of a new dimension in the space of molecular characteristics--the electron resonance energy spectrum. However, before this tool can realize its full potential, it will be necessary to create a library of resonance energy scans from standards of the molecules for which EM-MS offers a practical means of detection. Here, an approach supplementing direct measurement with chemical inference and quantum scattering theory is presented to demonstrate the feasibility of directly calculating resonance energy spectra. This approach makes use of the symmetry of the transition-matrix element of the captured electron to discriminate between the spectra of isomers. As a way of validating this approach, the resonance values for twenty-five nitrated aromatic compounds were measured along with their relative abundance. Subsequently, the spectra for the isomers of nitrotoluene were shown to be consistent with the symmetry-based model. The initial success of this treatment suggests that it might be possible to predict negative ion resonances and thus create a library of EM-MS standards.Comment: 18 pages, 7 figure

Zinc-blende and wurtzite AlxGa1-xN bulk crystals grown by molecular beam epitaxy

There is a significant difference in the lattice parameters of GaN and AlN and for many device applications AlxGa1-xN substrates would be preferable to either GaN or AlN. We have studied the growth of free-standing zinc-blende and wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick (similar to 10 mu m) zinc-blende and wurtzite AlxGa1-xN films were grown by PA-MBE on 2-in. GaAs (0 0 1) and GaAs (1 1 1)B substrates respectively and were removed from the GaAs substrate after the growth. We demonstrate that free-standing zinc-blende and wurtzite AlxGa1-xN wafers can be achieved by PA-MBE for a wide range of Al compositions. (C) 2011 Elsevier B.V. All rights reserved

Social Media Interaction as Informal Science Learning: a Comparison of Message Design in Two Niches

Social media provides science learners opportunities to interact with content-specific messages. However, most science-specific social media content is designed to disseminate information instead of encouraging dialog. In this novel, ex post facto exploratory study of a science social media community, we sought to understand the relationships among community member interaction, design elements of messages, and post type on two digital niches (i.e., Facebook and Twitter). Framed by the theory of symbolic interactionism, we conducted a content analysis of 1370 messages that were systematically created by an informal science learning project and found that usage frequency of messaging elements varied by niche; interaction within each niche differed, varying by messaging element; and differential interaction was found to be associated with post types within Facebook only. This study suggests a pathway for developing and examining social media as an educational component of informal science learning

Profiles in Practice: Stories of Paleontology Within an Online, Scientific Community

The Community of Practice framework has been used theoretically and empirically to describe the diverse ways people learn from one another through social interaction in a variety of specific contexts. To date, most research of this genre has favored investigating the community and domain constructs of the theory over the authentic practice construct. Those interested in recognizing and supporting science learning in non-school contexts across a lifespan are then limited in efforts to delineate how communities engage in domain-specific practice. This is especially relevant in the study of online environments which afford more democratic forms of participation. With the goal of adding to both theoretical and practical knowledge, this study explored practices that members enacted on a community-based website specific to the domain of paleontology. We used a multiple case study approach to provide comparative and contrasting narratives concerning the development of practice within an online, scientific community. Methods consisted of downloading data from the website, including members’ self-described attributes within member profiles, followed by their contributions to three of the website’s features: the forums, activity feed, and messages. An analytical framework which typified members based on their self-described attributes was applied, then members’ contributions were coded using an empirically based Communities of Practice framework. For one of the first times, we identified practice within an online, scientific community through comparing the contributions of three community members, finding that practice consisted of providing social support to other community members and having domain-based conversations