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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

Diese Arbeit befasst sich mit der Charakterisierung einer verhältnismäßig wenig beforschten Untergruppe der hochtemperatur-supraleitenden Kuprate (HTSCs-high-temperature superconducting cuprates), den elektronendotierten HTSCs, vermittels kernmagnetischer Resonanz (NMR-nuclear magnetic resonance). Die Untersuchungen umfassen 63Cu und 17O NMR an ausgerichteten Pulverproben und Einkristallen von Pr2−xCexCuO4 (x = 0, 0.05, 0.10, 0.15, 0.20) sowie auch Nd2−xCexCuO4 (x = 0, 0.13) in externen Magnetfeldern von 2.35 bis 17.6 T und Temperaturen zwischen 8 und 400 K. Durch eine Vielzahl von Experimenten wird die erste eindeutige spektrale Analyse für beide Nuklide vorgenommen. Es wird gezeigt, dass die indirekte, homonukleare Kopplung, wie sie beim Hahn-Echo-Zerfall von planarem 63,65Cu in lochdotierten HTSCs und auch im undotierten Pr2CuO4 gefunden wird, durch Elektronendotierung weitestgehend unterdrückt wird. Eine Analyse der Quadrupolaufspaltungen zeigt, dass nicht nur die lokale Verteilung der dotierten Elektronen und Löcher in den CuO2-Schichten quantitativ gemessen werden kann, sondern, dass auch Unterschiede in den 63Cu und 17O Aufspaltungen verschiedener undotierter Kuprate auf eine variable Ladungsverteilung zurückzuführen sind. Somit ist eine quantitative Messung der lokalen Ladungsverteilung in der CuO2 -Schicht der HTSCs möglich, welche ein neues, differenziertes Bild der unterschiedlichen Materialien ergibt.

In the present thesis the superconducting properties of the Iron-based Ba_{1-x}Rb_{x}Fe_{2}As_{2} arsenides, and A_{x}Fe_{2-y}Se_{2} (A = Cs, Rb, K) chalcogenides are investigated by means of Muon Spin Rotation Spectroscopy. The temperature and pressure dependence of the magnetic penetration depth is obtained form muSR experiments and analyzed to probe the superconducting gap-symmetries for each samples. The Ba_{1-x}Rb_{x}Fe_{2}As_{2} system is described within the multi-gap s+s-wave scenario and results are discussed in the light of the suppression of inter-band processes upon hole doping. Due to the lowered upper critical field Bc2 and reduced Tc, a large section of B-T-p phase diagram is studied for the hole-overdoped x=1 case. By applying hydrostatic pressure, the RbFe_{2}As_{2} system exhibits a classical BCS superconducting characteristics. The A_{x}Fe_{2-y}Se_{2} chalcogenide represents a system containing magnetically ordered and superconducting phases simultaneously. In all investigated chalcogenide samples, about 90% of the total volume show the strong antiferromagnetic phase and 10% exhibit a paramagnetic behavior. Magnetization measurements reveal a 100% Meissner effect, while muSR clearly indicates that the paramagnetic phase is a perfect superconductor. Up to now, there is no clear evidence whether the antiferromagnetic phase is also superconducting. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed. Moreover, a new hydrostatic double-wall pressure cell is developed and produced, satisfying the demands of muSR experiments. The designs and characteristics of the new pressure cell are reviewed in the present thesis.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1979. / Vita. / Includes bibliographical references. / by Stuart Forster Cogan. / Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1979.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18779号 / 理博第4037号 / 新制||理||1581(附属図書館) / 31730 / 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 川上 則雄, 教授 佐々 真一, 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(工学) / 甲第19720号 / 工博第4175号 / 新制||工||1644(附属図書館) / 32756 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 川上 養一, 教授 田中 勝久, 准教授 掛谷 一弘 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

We numerically investigate the dynamical properties of driven magnetic flux vortices in disordered type-II superconductors for a variety of temperatures, types of disorder and sample thicknesses. We do so with the aid of Langevin molecular dynamics simulations of a coarsegrained elastic line model of flux vortices in the extreme London limit. Some original findings of this doctoral work include the discovery that flux vortices driven through random point disorder show simple aging following drive quenches from the moving lattice state to both the pinned glassy state (non-universal aging) and near the critical depinning region (universal aging); estimations of experimentally consistent critical scaling exponents for the continuous depinning phase transition of vortices in three dimensions; and an estimation of the boundary curve separating regions of linear and non-linear electrical transport for flux lines driven through planar defects via novel direct measurements of vortex excitations. / Ph. D. / The works contained in this dissertation were undertaken with the goal of better understanding the dynamics of driven magnetic flux lines in type-II superconductors under different conditions of temperature, material defects and sample thickness. The investigations were conducted with the aid of computer simulations of the flux lines which preserve physical aspects of the system relevant to long-time dynamics while discarding irrelevant microscopic details. As a result of this work, we found (among other things) that when driven by electric currents, flux lines display very different dynamics depending on the strength of the current. When the current is weak, the material defects strongly pin the flux lines leaving them in a disordered glassy state. Sufficiently high current overpowers the defect pinning and results in the flux lines forming into a highly ordered crystal-like structure. In the intermediate critical current regime, the competing forces become comparable resulting in very large fluctuations of the flux lines and a critical slowing down of the flux line dynamics.

The thesis examines the prospects of using the superconductor NbN as the gatemetal for an InP HEMT. A HEMT or High Electron Mobility Transistor is aheterostructure transistor engineered to reach very high electron mobility. InPHEMTs are used as cryogenic Low Noise Amplifiers (LNAs), which have increasedin demand as quantum computing is scaling up. A superconducting NbN gate isof interest as it has the potential to decrease the amount of noise generated by theHEMT LNAs.A gate width dependence for both the transconductance (gm) and the large-signal HEMT channel resistance (RON ) of the NbN HEMTs at room temperaturehas been observed, and the first goal pf the thesis is to determine the originof the dependence. Moreover, the measured RF characteristics of the NbNdevices tend to deviate from the norm of a standard HEMT, and the secondgoal is to understand why. The third goal is to determine if the NbN gate stayssuperconducting at cryogenic temperatures or if self-heating from the channelduring DC operations will break superconductivity.In the thesis, it was possible to recreate the observed gate width dependence withnew devices, and additionally, a gate width dependence in the threshold voltageis observed. The origin of width dependence is most likely related to the straincreated by the NbN gate. At DC, extremely high peaks in the transconductanceare observed, which is most likely related to impact ionization and a subsequentincrease in hole trapping caused by the introduction of the NbN gate.Using simulations, it was possible to accurately recreate the observed deviantbehaviour, likely associated with the NbN gate’s high capacitance, inductance andresistance at room temperature. The high capacitance is likely partly related tosome NbN gates of the HEMTs being broken. Finally, the HEMT can operatein DC at 2 K with VG = 0.3 V and a maximum VD = 0.1 V before self-heatingfrom the channel will break the NbN superconductivity of the gate. This is oneof the critical conclusions of the work because it shows that a superconductinggate electrode can be implemented and functional in a high-performance HEMTdevice structure and under realistic operating bias conditions. As long as it can bedemonstrated that the superconductivity does not break when operating in RF, aNbN gate is a promising avenue to increase the noise performance of the cryogenicHEMT. / Detta arbete utforskar möjligheterna att använda supraledaren NbN som metallför gaten av en InP HEMT. En HEMT eller High Electron Mobility Transistor är entyp av transistor som är designad för väldigt hög elektron mobilitet. InP HEMTsanvänds som lågbrusförstärkare eller på engelska Low Noise Amplifiers (LNAs),vilket har ökat i efterfrågan när kvantdatorer nu skalar upp. En supraledande gateär av intresse för att den har potentialen att minska mängden brus som skapas ien HEMT LNA.Ett gatebredd beroende för både transkonduktans och RON har observerats och ettmål i arbetet är försöka fastställa dess ursprung. Utöver detta tenderar beteendet,av HEMTs med en NbN gate, att avvika från normen av en standard HEMT närden drivs i RF, och det andra målet är att förstå varför. Det tredje målet äratt fastställa om en NbN gate kommer supraleda i kryogen temperatur eller omsjälvuppvärmning från kanalen i HEMTen kommer bryta supraledningen.I arbetet var det möjligt att återskapa beroendet av gatebredden för nyaHEMTs och ett gatebredd beroende för HEMTens tröskelström observeradesockså. Gatebredd beroendet kommer troligtvis från spänning i HEMTen skapatav introduktionen av NbN gaten. I DC observerades väldigt höga toppar itranskonduktansen, vilket troligt är relaterat till processen ”impact ionization”och en efterföljande ökning i infångningen av hål som skapats på grund av NbNgaten.Genom simulationer är det möjligt att återskapa det observerade avvikandebeteendet i RF, vilket troligtvis är relaterat till NbN gatens höga resistans,kapacitans och induktans i rumstemperatur. Den höga kapacitansen är möjligtvisrelaterad till att vissa gates är trasiga. Slutligen fastställer vi att HEMTen kanvara i funktion under DC med T = 2 K, VG = 0.3 V och en max source-drainpotential av VD = 0.1 V, innan självuppvärmning bryter supraledningen. Dettaär den viktigaste slutsatsen i arbetet, eftersom den visar att en supraledandegateelektrod kan implementeras i en högpresterande HEMT och fungera underrimliga driftförhållanden. Så länge det går att visa att supraledningen inte brytsunder RF, är en HEMT med en supraledande NbN gate en lovande väg framåt föratt förbättra brusprestandan för en kryogenisk HEMT.

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