abstract: Niobium is the primary material for fabricating superconducting radio-frequency (SRF) cavities. However, presence of impurities and defects degrade the superconducting behavior of niobium twofold, first by nucleating non-superconducting phases and second by increasing the residual surface resistance of cavities. In particular, niobium absorbs hydrogen during cavity fabrication and promotes precipitation of non-superconducting niobium hydride phases. Additionally, magnetic flux trapping at defects leads to a normal conducting (non-superconducting) core which increases surface resistance and negatively affects niobium performance for superconducting applications. However, undelaying mechanisms related to hydride formation and dissolution along with defect interaction with magnetic fields is still unclear. Therefore, this dissertation aims to investigate the role of defects and impurities on functional properties of niobium for SRF cavities using first-principles methods. Here, density functional theory calculations revealed that nitrogen addition suppressed hydrogen absorption interstitially and at grain boundaries, and it also decreased the energetic stability of niobium hydride precipitates present in niobium. Further, hydrogen segregation at the screw dislocation was observed to transform the dislocation core structure and increase the barrier for screw dislocation motion. Valence charge transfer calculations displayed a strong tendency of nitrogen to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen leading to a very unstable state for interstitial hydrogen and hydrides. Thus, presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties. First-principles methods were further implemented to gain a theoretical perspective about the experimental observations that lattice defects are effective at trapping magnetic flux in high-purity superconducting niobium. Full-potential linear augmented plane-wave methods were used to analyze the effects of magnetic field on the superconducting state surrounding these defects. A considerable amount of trapped flux was obtained at the dislocation core and grain boundaries which can be attributed to significantly different electronic structure of defects as compared to bulk niobium. Electron redistribution at defects enhances non-paramagnetic effects that perturb superconductivity, resulting in local conditions suitable for flux trapping. Therefore, controlling accumulation or depletion of charge at the defects could mitigate these tendencies and aid in improving superconductive behavior of niobium. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2019

PhD (Physics) / Department of Physics / Understanding how and why superconductivity (SC) occurs in a given material has been very challenging for physicists for more than a hundred years, notwithstanding the major milestones, such as the London theory, the Landau-Ginzburg theory, and the BCS theory. The extreme challenge to predict the occurrence of SC is symbolized by the long string of unanticipated but breathtaking advances, i.e., the unexpected discoveries of cuprates and Fe-pnictides being the dramatic modern examples. Because of their incompatibility, the nucleation of SC near a ferromagnet is di cult and has never been realized except for the case that another superconductor provides proximity-boosted Cooper pairs. This perceived necessity to start with another superconductor is engrained in the exten- sive study of the proximity e ect in superconductor/ferromagnet (S/F) powder sample, where all the structures involve a superconductor with either stable or metastable struc- ture. Compounding the di culty, it is also generally recognized that SC with substantial Tc is favourable in low dimensionality because of strong quantum uctuation. In this thesis, we report a serendipitous nding of SC that emerges under the most implausible circumstances in low eld microwave absorption measurement. This new revelation may lead to unconventional avenues to explore novel SC for applications in superconducting spintronics. By means of a varienty of techniques, including EPR, SEM, FTIR, PPMS/VSM and XRD, nanonickel incorporated YBCO in di erent weighting factors have been studied. With its complex chemical structure and magnetic properties, Ni-YBCO is far from well understood and the magentic behavior of the system under di erent conditions is investi- gated. From the dilute mixture of nanonickel particles, it is found that groups of normal Josephson junctions (JJs) and JJs due to YBCO-nickel-YBCO interparticle weaklinks form as nickel is ferromagnetic. We experimentally show, for the rst time multiple phase reversals in the non-resonant microwave absorption (NRMA) spectra from Ni-YBCO pos- sibly, due to the formation of JJs. We also show that these multiple phase reversals then vii depend on microwave power and temperature. We argue that microwave power induced coherence among some groups of JJs and breaking of some of the weaker JJs can then lead to the disappearance of multiple phase reversals at higher microwave power levels. Further, we also report a role of pair breaking e ects that shall give a linear eld de- pendence of the derivative microwave absorption signal, which is essentially the NRMA signal. This pair-breaking e ect dominates at temperatures closer to Tc as expected thermodynamically. The presence of two peaks in the system, results in high permeability ferromagnet which acts as a magnetic short circuit for magnetic ux density and creates low reluctance path. A transition from normal to anomalous does not occur in this work, because of the possibility of junction in the sample. As predictable at the region around the origin where the weaklinks are supposed to be very strong for a very low doping or low nanonickel addition ( 0.5 % wt), not much e ect was observed. However, when the nanonickel addition is increased to 2 % and 3% we see a signi cant change in the magnetization and the associated hysteresis, indicating ux pinning. / NRF

Strongly correlated systems represent one of the major topics in modern solid-state physics. The rare-earth intermetallic compounds belonging to this class provide rich grounds for investigation of various phenomena. They show one of the most fascinating types of ground states in condensed-matter physics. Among them are: Kondolattice effects, heavy fermion behavior, superconductivity, magnetic order, non-Fermi liquid behavior, and quantum phase transition. Those properties occur mainly due to two competing interactions, the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida interaction. The study of unconventional superconductivity in heavy fermion systems attracted great interest over the last two decades. The exotic pairing mechanism (e.g. mediated by spin fluctuations) and the symmetry of the order parameter have been intensively discussed especially for superconducting Ce- and U-based compounds. The discovery of superconductivity below 0.65 K in the heavy-electron system CeCu2Si2 appeared unexpected as magnetic moments were known to destroy superconductivity. The pronounced anomaly of the electronic specific heat at Tc, however, strongly suggests that the unusual low temperature properties of heavy-electron systems indicate an unconventional origin of the superconducting phase. Since the discovery of superconductivity in CeCu2Si2, the question of the exact nature and origin of this phenomenon has been the subject of great interest in research. It has been postulated, that the superconductivity in these materials is not caused primarily by the usual electronphonon mechanism but rather by some magnetic interaction. CeCu2Si2 shows a rich phase diagram with different phases competing, depending on slight changes of the interactions. These properties are also strongly sample dependent. Small changes in composition eventually lead to changes in the electron interactions. These unique properties make this compound a fascinating subject of study. On the other hand it is difficult to synthesis the single crystals with defined physical properties. During the last three decades CeCu2Si2 has been an active research topic, from single crystal growth to sophisticated experiments like high-pressure measurements, neutron experiments etc. This thesis involved systematic investigations of the phase diagram, starting with the single crystal growth of different ground state and catheterized their physical properties including neutron experiments. The second part of the thesis contains, for the first time (to our knowledge), detailed investigations of the very interesting physical properties on YbRu2Ge2, which shows a quasiquartet crystalelectric-field ground state with quadrupolar ordering at 10 K. The first chapter is an overview of the underlying physics of heavy- fermion systems, including a description of the Doniach phase diagram. The second part of this chapter gives a brief introduction of crystalline-electric-field effect in rare-earth intermetallic compounds. Chapter 2. describes the experimental methods and crystal growth details. This chapter provides the main focus of this dissertation, presenting detailed experimental results for the different types of CeCu2Si2 crystals. Magnetic, thermodynamic and transport measurements on the new generation of large highquality single crystals were conducted by our research group. Furthermore, complimentary neutron investigations have been performed, which allowed to conclude that both magnetic and superconducting phases compete with each other. The effect of Ge doping on the Si site and possible coexistence of magnetic and superconducting phase is discussed in chapter 4. Chapter 5 provides a detailed investigation of the physical properties of YbRu2Ge2 single crystals. In addition, neutron experiments as well as the determination the magnetic structure and crystalline-electric-field scheme of YbRu2Ge2 are presented. The μSR experiments were also performed as a complimentary method to the neutron experiments. Chapter 6 ends the dissertation with a conclusion and summary.

Thesis: M.S., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 1979 / Includes bibliographical references. / by Kohji Tanaka. / M.S. / M.S. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

abstract: This dissertation details the development of an open source, frequency domain multiplexed (FDM) readout for large-format arrays of superconducting lumped-element kinetic inductance detectors (LEKIDs). The system architecture is designed to meet the requirements of current and next generation balloon-borne and ground-based submillimeter (sub-mm), far-infrared (FIR) and millimeter-wave (mm-wave) astronomical cameras, whose science goals will soon drive the pixel counts of sub-mm detector arrays from the kilopixel to the megapixel regime. The in-flight performance of the readout system was verified during the summer, 2018 flight of ASI's OLIMPO balloon-borne telescope, from Svalbard, Norway. This was the first flight for both LEKID detectors and their associated readout electronics. In winter 2019/2020, the system will fly on NASA's long-duration Balloon Borne Large Aperture Submillimeter Telescope (BLAST-TNG), a sub-mm polarimeter which will map the polarized thermal emission from cosmic dust at 250, 350 and 500 microns (spatial resolution of 30", 41" and 59"). It is also a core system in several upcoming ground based mm-wave instruments which will soon observe at the 50 m Large Millimeter Telescope (e.g., TolTEC, SuperSpec, MUSCAT), at Sierra Negra, Mexico. The design and verification of the FPGA firmware, software and electronics which make up the system are described in detail. Primary system requirements are derived from the science objectives of BLAST-TNG, and discussed in the context of relevant size, weight, power and cost (SWaP-C) considerations for balloon platforms. The system was used to characterize the instrumental performance of the BLAST-TNG receiver and detector arrays in the lead-up to the 2019/2020 flight attempt from McMurdo Station, Antarctica. The results of this characterization are interpreted by applying a parametric software model of a LEKID detector to the measured data in order to estimate important system parameters, including the optical efficiency, optical passbands and sensitivity. The role that magnetic fields (B-fields) play in shaping structures on various scales in the interstellar medium is one of the central areas of research which is carried out by sub-mm/FIR observatories. The Davis-Chandrasekhar-Fermi Method (DCFM) is applied to a BLASTPol 2012 map (smoothed to 5') of the inner ~1.25 deg2 of the Carina Nebula Complex (CNC, NGC 3372) in order to estimate the strength of the B-field in the plane-of-the-sky (B-pos). The resulting map contains estimates of B-pos along several thousand sightlines through the CNC. This data analysis pipeline will be used to process maps of the CNC and other science targets which will be produced during the upcoming BLAST-TNG flight. A target selection survey of five nearby external galaxies which will be mapped during the flight is also presented. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2019

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18060号 / 理博第3938号 / 新制||理||1567(附属図書館) / 30918 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 石田 憲二, 教授 前野 悦輝, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19310号 / 工博第4107号 / 新制||工||1633(附属図書館) / 32312 / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 山川 宏, 教授 松尾 哲司, 准教授 中村 武恒 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・論文博士 / 博士(理学) / 乙第12996号 / 論理博第1552号 / 新制||理||1604(附属図書館) / 32924 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 教授 前野 悦輝, 教授 石田 憲二 / 学位規則第4条第2項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20166号 / 理博第4251号 / 新制||理||1611(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 教授 川上 則雄, 教授 前野 悦輝 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21247号 / 理博第4417号 / 新制||理||1634(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 石田 憲二, 教授 前野 悦輝, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM