Resistively Shunted NbN/AlN/NbN Tunnel Junctions for Single Flux Quantum Circuits

AbstractWe have developed resistively shunted NbN junctions to realize superconducting single flux quantum circuits operating at 10K and/or high speed. The junctions consist of epitaxial NbN/AlN/NbN tunnel junctions and molybdenum (Mo) resistors fabricated on single-crystal MgO substrates. The junction qualities are systematically investigated in a wide range of critical current density (Jc). The gap voltage and the ratio of Rsg/RN were about 5.6mV and 11 for the junctions with the Jc of 10 kA/cm2, respectively. The overdamped Josephson junctions with parallel Mo resistors having a nominal sheet resistance showed non-hysteretic current-voltage characteristics for the junctions with Jc of 10 kA/cm2

Origin of intrinsic dark count in superconducting nanowire single-photon detectors

The origin of the decoherence in superconducting nanowire single-photon detectors, the so-called dark count, was investigated. We measured the direct-current characteristics and bias-current dependencies of the dark count rate in a wide range of temperatures from 0.5 K to 4 K, and analyzed the results by theoretical models of thermal fluctuations of vortices. Our results indicate that the current-assisted unbinding of vortex-antivortex pairs is the dominant origin of the dark count.Comment: 10 pages, 2 figure

脱分化ないし異所性分化を示す肉腫におけるMDM2増幅とH3K27me3消失の診断的意義に関する研究

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 間野 博行, 東京大学連携教授 中釜 斉, 東京大学教授 柴田 龍弘, 東京大学准教授 國松 聡, 東京大学准教授 齋藤 琢University of Tokyo(東京大学

〈研究論文〉県立高校再編過程における教育ガバナンス : 鹿児島県曽於地区3校再編の事例分析

The purpose of this research is to clarify the role of actors inside the municipality and the scope of exercise of influence on the policy process of the consolidation of prefectural high schools from the viewpoint of educational governance. This paper focusses on the case of the restructuring of three schools in Soo city, Kagoshima prefecture. First, in this case, the existence of alumni association actors who had not paid attention to previous research was significant. One reason is because the previous research focused on the case study of elementary and junior high schools. Moreover, it showed that alumni chairmen did not only just persist in order to continue the existence of their respective schools, but also laid claim to consideration of the implicit power of many alumni members. Second, the educational administrative actors, who had been required to provide appropriate information and to support active discussions or direct discussions in Soo local area meetings, were unable to observe the exercise of authority from this case. Compared to other areas under the Oosumi conference, Soo city totally entrusted policy decision-making to local area meetings. In other words, it did not function strategically as a policy maker. Third, the mayor as a political actor did not clearly demonstrate his position and did not take initiative proactively at both Soo local area meetings and the Oosumi conference. This was found to be from his complicated position in the background, and resulted in the wrong impression at the Oosumi conference. Based on the above three points, the difference in educational governance from the case of the consolidation of municipal schools in contrast to high schools was observed. Issues regarding the leadership of political actors who assume the role of negotiators inside local governments and externally and the qualifications of the administrative actors who provide appropriate information to participating actors are of significant concern.本研究は科学研究費助成事業・研究課題番号17J00021の成果の一部である

Microwave characterization of tantalum superconducting resonators on silicon substrate with niobium buffer layer

Tantalum thin films sputtered on unheated silicon substrates are characterized with microwaves at around 10 GHz in a 10 mK environment. We show that the phase of tantalum with a body-centered cubic lattice ($\alpha$-Ta) can be grown selectively by depositing a niobium buffer layer prior to a tantalum film. The physical properties of the films, such as superconducting transition temperature and crystallinity, change markedly with the addition of the buffer layer. Coplanar waveguide resonators based on the composite film exhibit significantly enhanced internal quality factors compared with a film without the buffer layer. The internal quality factor approaches $2\times 10^7$ at a large-photon-number limit. While the quality factor decreases at the single-photon level owing to two-level system (TLS) loss, we have identified the primary cause of TLS loss to be the amorphous silicon layer at the film-substrate interface, which originates from the substrate cleaning before the film deposition rather than the film itself. The temperature dependence of the internal quality factors shows a marked rise below 200 mK, suggesting the presence of TLS-TLS interactions. The present low-loss tantalum films can be deposited without substrate heating and thus have various potential applications in superconducting quantum electronics.Comment: 6 pages, 4 figures + Supplementary Material (7 pages, 5 figures