Performance of a two-junction array SIS-mixer operating around 345GHz

The authors have made a detailed study of the gain and noise of a SIS (superconductor-insulator-superconductor) heterodyne receiver at 345 GHz. An array of two Nb-Al₂O₃-Nb SIS junctions in series are used as the mixing element. The array is operated in a waveguide mount with a backshort and an E-plane tuner. The best receiver noise temperature achieved is 140 K DSB (double sideband). The embedding impedances were determined by fitting theory to the measured pumped curves. High-quality fits were obtained, providing the first detailed test of the Tucker-theory at frequencies above 300 GHz. The impedances found by this method are in very good agreement with impedances measured in a scale model at 3.3 GHz. From these embedding impedances, the gain and noise of the mixer were calculated over a full bias range using the Tucker theory in the three-port low-IF approximation. The measured dependence of mixer gain and noise on bias voltage, pump power and embedding impedance is in good agreement with theory. However the absolute values show discrepancies that appear to be independent of the bias parameters of the mixer     

Preserving knowledge in software projects

Up-to-date preservation of project knowledge like developer communication and design documents is essential for the successful evolution of software systems. Ideally, all knowledge should be preserved, but since projects only have limited resources, and software systems continuously grow in scope and complexity, one needs to prioritize the subsystems and development periods for which knowledge preservation is more urgent. For example, core subsystems on which the majority of other subsystems build are obviously prime candidates for preservation, yet if these subsystems change continuously, picking a development period to start knowledge preservation and to maintain knowledge for over time become very hard. This paper exploits the time dependence between code changes to automatically determine for which subsystems and development periods of a software project knowledge preservation would be most valuable. A case study on two large open source projects (PostgreSQL and FreeBSD) shows that the most valuable subsystems to preserve knowledge for are large core subsystems. However, the majority of these subsystems (1) are continuously foundational, i.e., ideally for each development period knowledge should be preserved, and (2) experience substantial changes, i.e., preserving knowledge requires substantial effort.     

Roles as a structuring tool in online discussion groups: The differential impact of different roles on social knowledge construction

This study focuses on stimulating social knowledge construction in e-discussions and examines the introduction of five roles: starter, summariser, moderator, theoretician, and source searcher. Asynchronous discussion groups of 10 1st-year students Instructional Sciences were organised to foster students’ processing of the learning content. Four successive authentic tasks of three weeks each were presented. Taking into account the moment of introduction of the role assignment (at the start of the discussions versus at the end), the differential impact of the roles on knowledge construction is studied through quantitative content analysis based on the interaction analysis model of Gunawardena, Lowe, and Anderson (1997). The results show a positive effect of role assignment on students’ social knowledge construction at the start of the discussions. This implies that roles should be introduced at the start of the discussions and can be faded out towards the end. With respect to the differential impact of the roles, the results show that messages from moderators, theoreticians, and summarisers reflect higher levels of knowledge construction at the start of the discussions. Even students without a role in role-supported groups benefit from the role introduction.     

Continuously mining distributed version control systems: an empirical study of how Linux uses Git

Distributed version control systems (D-VCSs —such as git and mercurial) and their hosting services (such as Github and Bitbucket) have revolutionalized the way in which developers collaborate by allowing them to freely exchange and integrate code changes in a peer-to-peer fashion. However, this flexibility comes at a price: code changes are hard to track because of the proliferation of code repositories and because developers modify (“rebase”) and filter (“cherry-pick”) the history of these changes to streamline their integration into the repositories of other developers. As a consequence, researchers and practitioners, who typically only consider the (cleaned up) history in the official project repository, are unaware of important elements and activities in the collaborative software development process. In this paper, we present a method that continuously mines all known D-VCSs of a software project to uncover the complete development history of a project. We use this method to (1) show the divergence between the code history development in the official Linux kernel repository and the complete kernel development history, and (2) to investigate the characteristics of the ecosystem of git repositories of the Linux kernel. Finally, we discuss how continuous mining could be adopted by current D-VCS hosting services.     

An empirical study of the impact of modern code review practices on software quality

Software code review, i.e., the practice of having other team members critique changes to a software system, is a well-established best practice in both open source and proprietary software domains. Prior work has shown that formal code inspections tend to improve the quality of delivered software. However, the formal code inspection process mandates strict review criteria (e.g., in-person meetings and reviewer checklists) to ensure a base level of review quality, while the modern, lightweight code reviewing process does not. Although recent work explores the modern code review process, little is known about the relationship between modern code review practices and long-term software quality. Hence, in this paper, we study the relationship between post-release defects (a popular proxy for long-term software quality) and: (1) code review coverage, i.e., the proportion of changes that have been code reviewed, (2) code review participation, i.e., the degree of reviewer involvement in the code review process, and (3) code reviewer expertise, i.e., the level of domain-specific expertise of the code reviewers. Through a case study of the Qt, VTK, and ITK projects, we find that code review coverage, participation, and expertise share a significant link with software quality. Hence, our results empirically confirm the intuition that poorly-reviewed code has a negative impact on software quality in large systems using modern reviewing tools.     

Electric Current-Assisted Sintering of 8YSZ: A Comparative Study of Ultrafast High-Temperature Sintering and Flash Sintering

Electric current-assisted sintering (ECAS) is a promising powder consolidation technique that can achieve short-term sintering with high heating rates. Currently, main methods of performing ECAS are indirect heating of the powder compact in a conductive tool or direct heating with current flowing through the powder compact. Various influencing factors have been identified to explain the rapid densification during ECAS, such as ultrahigh heating rates, extra-high temperatures, and electric field. However, the key consolidation-enhancing factor is still under debate. This study aims at understanding the role of heating rate on the enhanced densification during ECAS of 8 mol% Y₂O₃-stabilized ZrO₂ (8YSZ) by experimental and numerical methods. Two different heating modes, ultrafast high-temperature sintering (UHS, indirect heating) and flash sintering (FS, direct heating), are studied. The novel UHS technique is successfully applied to consolidate the 8YSZ samples. Additionally, finite element methods (FEM) combined with a constitutive model is adopted to predict the densification and grain growth. Furthermore, a comparison of UHS and FS is performed to investigate the thermal effect (heating rate) and athermal effect (electric field) individually. The results indicate that the high heating rate is the key factor of the rapid densification during UHS and FS of 8YSZ.     

Enhancement of superconductivity by quasiparticle injection. I. Model analysis

Nonequilibrium phenomena in the two electrodes of high-current-density superconducting tunnel junctions are analyzed. The quasiparticle injection-induced enhancement of the energy gaps as well as of the critical current of one of the electrodes are considered. A graphic approach is employed to solve the coupled gap equations for both films. Multiple-gap states are found at voltages close to the sum and difference of the two gaps. If a large transport current is passed through one of the electrodes, multiple stable solutions of the gap equations occur at most bias voltages. Because each solution has its own critical current level, the nonequilibrium critical current is no longer a simple single-valued quantity.