By allowing to measure the magnetic field distribution inside a material, muon spin rotation experiments have the potential to provide valuable information about microscopic properties of high-temperature superconductors. Nevertheless, information about the intrinsic superconducting properties of the material is masked by random thermal and static fluctuations of the magnetic field which penetrates the material in the form of vortices of quantized magnetic flux. A good understanding of the fluctuations of those vortices is needed for the correct determination of intrinsic properties, notably the coherence length &xi;, and the field penetration depth &lambda;. We develop a simulation based on the Metropolis algorithm in order to understand the effect, on the magnetic field distribution, of disorder- and thermally-induced fluctuations of the vortex lattice inside a layered superconductor. <br /><br /> Our model correctly predicts the melting temperatures of the YBa₂Cu₃O_{6. 95} (YBCO) superconductor but largely underestimates the observed entropy jump. Also we failed to simulate the high field disordered phase, possibly because of a finite size limitation. In addition, we found our model unable to describe the first-order transition observed in the highly anisotropic Bi₂Sr₂CaCu₂O_8+<em>y</em>. <br /><br /> Our model predicts that for YBCO, the effect of thermal fluctuations on the field distribution is indistinguishable from a change in &xi;. It also confirms the usual assumption that the effect of static fluctuations at low temperature can be efficiently modeled by convolution of the field distribution with a Gaussian function. However the extraction of &xi; at low fields requires a very high resolution of the field distribution because of the low vortex density.

By allowing to measure the magnetic field distribution inside a material, muon spin rotation experiments have the potential to provide valuable information about microscopic properties of high-temperature superconductors. Nevertheless, information about the intrinsic superconducting properties of the material is masked by random thermal and static fluctuations of the magnetic field which penetrates the material in the form of vortices of quantized magnetic flux. A good understanding of the fluctuations of those vortices is needed for the correct determination of intrinsic properties, notably the coherence length &xi;, and the field penetration depth &lambda;. We develop a simulation based on the Metropolis algorithm in order to understand the effect, on the magnetic field distribution, of disorder- and thermally-induced fluctuations of the vortex lattice inside a layered superconductor. <br /><br /> Our model correctly predicts the melting temperatures of the YBa₂Cu₃O_{6. 95} (YBCO) superconductor but largely underestimates the observed entropy jump. Also we failed to simulate the high field disordered phase, possibly because of a finite size limitation. In addition, we found our model unable to describe the first-order transition observed in the highly anisotropic Bi₂Sr₂CaCu₂O_8+<em>y</em>. <br /><br /> Our model predicts that for YBCO, the effect of thermal fluctuations on the field distribution is indistinguishable from a change in &xi;. It also confirms the usual assumption that the effect of static fluctuations at low temperature can be efficiently modeled by convolution of the field distribution with a Gaussian function. However the extraction of &xi; at low fields requires a very high resolution of the field distribution because of the low vortex density.

This thesis presents the design, fabrication and characterization of high performance and high fill factor surface micromachined uncooled infrared resistive microbolometer detectors which can be used in large format focal plane arrays (FPAs). The detector pixels, which have a pixel pitch of 25 &mu / m, are designed and fabricated as two-level structures using the enhanced sandwich type resistor while the active material is selected as Yttrium Barium Copper Oxide (YBCO). First level of the pixel structure is allocated for the formation of the support arms in order to obtain longer support arms hence lower thermal conductance values to get the desired high performance levels. The pixel body is built in the second level such that the fill factor and absorption of the detector is maximized. Structural and sacrificial layer thicknesses are also optimized in order to increase the absorption coefficient of the pixel in the 8-12 &mu / m wavelength range. The thermal simulations are conducted using finite element method (FEM) by CoventorWare software. The designed pixel has a fill factor of 92 % together with the thermal conductance and thermal time constant values calculated as 16.8 nW/K and 19.3 ms in the simulations, respectively. The pixels are fabricated at METU MEMS facilities after the design of a CMOS compatible process flow. All process steps are optimized individually to obtain the expected high performance. Characterization step of the pixels includes the measurements of temperature coefficient of resistance (TCR), noise and thermal conductance value together with the thermal time constant. Effective TCR of the pixel is measured as -2.81 %/K for a pixel with a support arm resistance of 8 k&Omega / and total resistance of 55 k&Omega / . The corner frequency of 1/f noise in the pixel is 9.5 kHz and 1.4 kHz under 20 &mu / A and 10 &mu / A current bias, respectively. The total rms noise is 192 pA within 8.4 kHz bandwidth for a current bias of 20 &mu / A. Thermal conductance, Gth, of the pixel is measured as 17.4 nW/K with a time constant of 17.5 ms. The measurement results indicate that the single pixels designed and fabricated in the scope of this thesis are applicable to large format FPAs in order to obtain a high performance imager. The expected NETD values are 33 mK and 36 mK for 384x288 and 640x480 format FPAs, respectively.

<p>High-temperature superconductors have been intensely studiedsince the discovery, almost 20 years ago. Their layeredstructure, extreme type-II behavior, large anisotropy, andstrong fluctuations have led to a large number of new andinteresting problems. In this work the resistive transitionhave been studied in YBa₂Cu₃O_7-δsingle crystals, from the superconductingfluctuations above T<i>c</i>down to the vortex matter near the onset of resistivity.Superconducting fluctuations above T<i>c</i>are suitably studied by measuring themagnetoconductivity Δσ = 1/ρ(B)-1/ρ(0).Such experiments were performed on untwinned, optimally dopedsamples, for<b>I</b>||<b>ab</b>. For<b>H</b>||<b>c</b>, fluctuations in the CuO-chains was not important, andfluctuations in the CuO2 planes seemed isotropic. The in-planecoherence length anisotropy was determined to be close tounity. For<b>H</b>||<b>ab</b>, Δσ<i>b</i>depended on the field direction, indicating an effect ofthe chains on the magnetoconductivity in thisconfiguration.</p><p>The nature of the vortex phase below T<i>c</i>can be examined through vortex-correlation measurements.Using a modified pseudo-flux transformer (PFT) geometry forin-plane experiments, the magnetic field could be appliedparallel to both the a, b, and c axis. A strong Lorentz forcewas detected, and observed to be a requirement for the meltingstep feature. The resistive anisotropy close to T<i>c</i>was strongly field and temperature dependent, which wasexplained by an almost isotropic fluctuation contribution.</p><p>The effect of anisotropy on vortex-liquid correlation infields<b>H</b>||<b>ab</b>, was studied by<i>c</i>-axis transport measurements in oxygen-deficientsamples, using a PFT configuration. Increasing disordersuppressed both longitudinal and transverse correlation. Inheavily underdoped samples, vortex correlation was enhanced formagnetic fields exactly aligned with the<i>ab</i>plane because of the strong anisotropy. Also, thesolid-to-liquid transition temperature became nearly fieldindependent.</p><p>Attempts were also made to connect the fluctuation regime ofthe resistive transition with the vortex region,by accountingfor critical fluctuations close to the transition.</p><p><b>Keywords:</b>high-temperature superconductors, YBa₂Cu₃O_7-δ, untwinned YBCO, oxygen deficiency,superconducting fluctuations, magnetoconductivity, in-planeanisotropy, vortex liquid, vortex solid, vortex correlation</p>

With the rapid increase in electrical loads, a considerable percentage of global power systems have been stretched to their designed capacity, leading to a sharp rise in the fault current. High fault current levels have severe impact on power systems. They can increase the likelihood of overcurrent damage, which may destroy electrical facilities or even cause system blackout. The resistive-type superconducting fault current limiter (RSFCL) is a device that can help to mitigate the increasing level of fault currents. It can also contribute to the performance, stability and efficiency of electricity grids. In order to promote the RSFCL more effectively, it is essential to study the device itself and conduct simulations regarding the performance and applicability from the system point of view. Chapter 1 and Chapter 2 of this thesis introduced fundamentals of superconductivity and RSFCLs, respectively. In Chapter 3, a power system model was built, and the transient analysis of short-circuit currents was given. Then, the operating principle of RSFCL was explained, and a step-resistance RSFCL model was introduced. To validate the current limiting performance of the SFCL module, wind farm protection schemes were studied under various fault scenarios. After thorough analysis it was concluded that the optimal allocating strategy of SFCLs was the installation of one SFCL at the integrating point of the system model. Chapter 4 presented a comprehensive study on the performance and optimal allocation strategy of RSFCLs. The two power system models used in this chapter were built based on the UK network standard. To assess the impact of incorporating SC material properties on the performance of SFCLs, three different models were compared throughout the study. Although computing time can be reduced when step-resistance and exponential equation models were used, such simplifications led to strong overestimations of the SFCL performance and resulted in wrong conclusion of optimal installing strategies. For both power system models, the simultaneous use of three SFCLs was the best protection strategy in terms of the performance, economic efficiency and reliability of the overall grids. To draw this conclusion, all the potential combinations of two, three, four, and five SFCLs were studied under a wide number of fault scenarios and measuring strategies. In Chapter 5, a series of experiments were performed to study the magnetic field-angular dependence of the critical current of different commercial YBCO samples. We selected ten 2G-HTS tapes with broad differences in width, fabrication process, and laminar structure. The obtained I_c (B,θ) characteristics of HTS samples were applied in the simulation of RSFCLs, showing unneglectable effect on the first peak limiting performance during faults. This study is helpful to the design and simulation of RSFCLs and other HTS applications which require superconducting wires operating inside magnetic fields. In Chapter 6, we presented a High Frequency (HF) AC-assisted quench study of the YBCO conductor. The differences in quench processes triggered by HF AC field were recorded and studied. We found that AC signals of 10 kHz can trigger quench of the HTS tape. Also, the device proved to be effective at guaranteeing uniform quench of the sample. After then, we placed the experimental device in a magnet, to study whether the quench process can be accelerated by external DC magnetic field. It was found that the DC field can reduce the quench time, but its performance was dependent on the amplitude of transport current and the characteristics of HF AC signals. In addition, the angle between the DC magnetic field and the tape surface showed a huge impact on the quench time. Finally, a comprehensive field-angular dependence study about the quench time of the YBCO sample was conducted, revealing the same pattern as J_c (θ,B) dependence of the tape, but with a greater anisotropy. To summarize, this technique showed outstanding performance regarding quench acceleration and tape protection, and therefore has huge potential to be applied in RSFCLs.

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Die für technische Anwendungen attraktivsten Eigenschaften der Hochtemperatur-Supraleiter beruhen auf der Wechselwirkung der Materialien mit einem externen Magnetfeld. Für hochleistungsfähige Supraleiteranwendungen werden großvolumige Funktionselemente mit eindomänigem Magnetisierungsverhalten benötigt. Zur Vergrößerung der magnetischen Domäne schmelztexturierter YBa2Cu3O7-δ-Kompaktsupraleiter wurde anhand der Multi-Seeding-Technik, die Rekristallisation unter Verwendung mehrerer SmBa2Cu3O7-δ-Keimpräparate untersucht. Als Schlüsseltechnologie zur Herstellung anwendungsorientierter Supraleitererzeugnisse wurde zu Beginn der Arbeit die Keimkristallherstellung optimiert. Gemäß den Zielvorgaben stehen zukünftig quasi-einkristalline Keimkörper definierter Form und Orientierung in hoher Stückzahl zur Verfügung. Die Supraleiter betreffend ist es gelungen, für den wechselseitigen Abstand und die Ausrichtung der Keimpräparate ein Optimum zu finden sowie tolerierbare Winkel der Verschwenkung angrenzender Kristalle zu ermitteln. Es wurde festgestellt, dass das Auftreten isolierender Korngrenzeneinschlüsse mit den magnetischen Materialeigenschaften korreliert und vom Keimabstand und der Korngrenzenorientierung abhängig ist. Mit Einsatz von bis zu 16 Keimkristallen gelang es, eindomänige Supraleiterhalbzeuge mit Höchstmaßen von (79 x 39 x 20) mm3 und Remanenzflussdichten von bis zu 1,3 T zu erzeugen. Im Chargenprozess konnten abschließend Multi-Seeding-Funktionselemente mit anwendungskonformen geometrischen und magnetischen Materialeigenschaften zum Aufbau eines hochdynamischen Supraleitermotors reproduzierbar gefertigt werden. In Leistungstests wurde für die Dynamik des Motors mit 200.000 rpm/s ein Rekordwert erzielt – nun können die Motoren in der Anwendung erprobt werden.

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