This thesis is concerned with measurements of alternating field losses in cylindrical rods of niobium in its superconducting state. Six samples have been investigated each with a different surface condition, Magnetization measurements of the samples have been studied which show the effect of the surface condition on the first critical magnetic field H[sub]c1. The effect of the surface on flux pinning are discussed. Power losses have been studied experimentally for single crystal niobium and for polycrystalline niobium each with three different surface conditions in order to study the effect of the surface properties on these ac losses. A wattmeter technique was developed for measurements of power losses in the samples. Experiments were conducted over a range of applied fields at 50 Hz and at 4.2 K. This technique will be described together with the experimental results. The results will be compared with predictions from a modified critical state theory and their significance discussed. The critical current density has been measured for each sample using a method based on the measurements of the power dissipation in a small modulating field, in the presence of a steady bias field. The technique will be described together with experimental results. The shielding field △H in the mixed state has been studied for the samples. The technique and the results will be discussed for each sample.

We have carried out an experimental investigation of the resistance transition in Indium and Thallium wires when superconductivity is destroyed by a current. Our results, as well as those obtained previously by other workers, do not agree with the theories put forward by London (1937) and Gorter (1957) and the consideration of secondary effects does not satisfactorily account for the discrepancies. We present a new model of the intermediate state in Type-I current-carrying superconductors. In addition to predicting a resistance transition in reasonable agreement with experimental observations, the model gives good agreement with experimental values of the radius of the intermediate state core as obtained by Rinderer (1956). A treatment of secondary effects is also given and together with the basic resistance transition predicted by the model, they provide a better understanding of the destruction of superconductivity in Type-I wires by a current.

Made available in DSpace on 2015-09-17T15:25:59Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-04-13. Added 1 bitstream(s) on 2015-09-17T15:46:17Z : No. of bitstreams: 1 000847789.pdf: 6861157 bytes, checksum: bcac54e0f580964955acb7c0540ca401 (MD5) / Um dos fatores ainda pouco tratados na literatura a respeito do sistema supercondutor BSCCO é o processo de dopagem e/ou substituição química da fase 2223. Isto se deve às dificuldades de síntese dessa fase. Neste ínterim, no presente trabalho foram feitas análises da influência do tratamento térmico nas propriedades estruturais, elétricas e morfológicas de amostras sem substituição química (BPSCCO) e com substituições de La nos sítios de Sr e de Gd nos sítios de Sr e Ca. Estas foram preparadas de acordo com as composições nominais Bi 1,60 Pb 0,40 Sr 2,00- x La x Ca 2,00 Cu 3,00 O 10+δ, x Gd x Ca 2,00 Cu 3,00 O 10+δ, x Gd x Cu 3,00 O 10+δ, para x=0, 0,25, 0,50, 0,75 e 1,00 mol/g; Bi 1,60 Pb 0,40 Sr 2,00- para x= 0,25 e 0,50 mol/g e Bi 1,60 Pb 0,40 Sr 2,00 Ca 2,00- para x= 0,25 e 0,50 mol/g. Foram efetuadas, ainda, análises térmicas dos exemplares produzidos e a influência dos vários elementos substituintes na obtenção da fase 2223 foi investigada. Os resultados indicaram que as substituições realizadas provocam a degradação das propriedades de supercondução tais como a diminuição da T c e J c . Além disso verificou-se o favorecimento da formação da fase 2212 e de fases secundárias quando comparadas à 2223 nas amostras com substituição química. Por outro lado, entre tais amostras, as que apresentaram melhoras nas propriedades estruturais, elétricas e morfológicas (aumento de T c e melhor conectividade entre os grãos) foram aquelas tratadas em maiores temperaturas. Isso indica que amostras com substituição química podem apresentar boas características supercondutoras se receberem tratamentos térmicos adequados / One of the issues that is not well studied in the literature is the doping process and/or the chemical substitution in BSCCO-2223. The reasons for that is related to the difficulties to synthesize such phase. Thus, in the present study we carried out analysis of the influence of the heat treatment in samples with and without (BPSCCO) chemical substitutions. The elements used in the substitutions were La in Sr sites and Gd in Sr and Ca sites. The analyzes were focused on the structural, electrical and morphological properties of the materials. The samples were prepared according to the nominal composition Bi 1.60 Pb 0.40 Sr 2.00-x La x Ca 2.00 Cu 3.00 O 10+δ, with x = 0, 0.25, 0.50, 0.75 and 1.00 mol/g, Bi 1.60 Pb 0.40 Sr 2.00-x Gd x Ca 2.00 Cu 3.00 O 10+δ with x = 0.25 and 0.50 mol/g and Bi 1.60 Pb 0.40 Sr 2.00 Ca 2.00-x Gd x Cu 3.00 O 10+δ, with x = 0.25 and 0.50 mol/g. We also carried out thermal analyzes of the samples, for which the influence of the various substitutions in the obtainment of the 2223 phase were studied. The results indicated that the substitutions allow the deterioration of the superconducting properties, such as a decrease in the T c and J c, as well as the formation of 2212 and secondary phases when compared to the 2223 phase. On the other hand, processes with different heat treatments lead to improvements in the structural, electrical and morphological properties (increase of T c and better connectivity between the grains). This indicates that samples with chemical substitution can have good superconducting characteristics if they receive the adequate heat treatment

Studies have been undertaken trying to produce new high Tc, superconducting cuprates. As a mechanism for superconductivity in these materials is sought, it is important to produce new materials with new structural features. Studies were carried out on three systems: bismuthates, quadruple perovskites and ruthenates. Bismuthate work was aimed at trying to intergrow Sillen type phases with layered cuprate phases. This work yielded four new Sillen related structures: CaBiO2Cl [P21/m : a = 7.7311(1)Å, b = 4.1234(1)Å, c = 6.3979(2)Å and ß = 105.21(1)0] has a puckered halide layer due to the lower co-ordination preference of the small Ca2+ ion. SrBiO2C1 [Cmcm : a = 5.7109(2)Å, b = 12.4081(5)Å, c = 5.5888(2)Å] is a Sillen X1 type material with an ordering of Sr and Bi in the double fluorite layers as previously observed in BaBiO2C1. Bi2LaO4C1 [I4/mmm: a = 3.9547(1)Å and c = 9.1275(3)Å] has triple fluorite layers with ordering such that La occupies the central layer only. These layers are separated by planar Cl layers. Isostructural Bi2NdO4C1 and Bi2YO4CI have also been prepared. Bi6Ca5O11Cl6[Cmcm: a = 3.865(6)Å, b = 12.57(2)Å and c = 43.05(7)Å] has an X1 related, Arppe type structure, with undulating (M2O2) layers, determined from microcrystal diffraction data. Microcrystals formed on reaction of the stoichiometry Bi1.7Ca2.7O4-yCl4 were shown to possess a Sillen X1X3 type structure, confirming a previous report. Work on perovskites involved doping studies of the Quadruple Perovskite series Ln2Ba2Cu2Ti2O11 (Ln = Nd, Gd and Tb). Investigation of Sr doping for Ba in this series is reported, as is Cu substitution for Ti in the Nd system. Work on trying to extend the observation of both superconductivity and ferromagnetism in the ruthenate RuSr2GdCu2O8 involved investigation of RuSr2TbCu2O8 1-2-1-2 and quadruple perovskite Ln2Ba2Cu2Ru2O11 (Ln = La, Nd, Gd and Tb) and NdBa3Cu2TiNb1-xRuxO11 systems.

This thesis details recent work on an innovative new approach to Josephson junction fabrication. These junctions are created in low TC superconductor-normal metal bilayer tracks on a deep submicron scale using a Focused Ion Beam Microscope (FIB). The FIB is used to mill away a trench 50_nm wide in the upper layer of niobium superconductor (125 nm thick), weakening the superconducting coupling and resulting in a Josephson junction. With the aid of a newly developed in situ resistance measurement technique it is possible to determine the cut depth to a high degree of accuracy and hence gain insight into how this affects the resulting device parameters. Devices fabricated over a wide range of cut depths and copper normal metal layer thicknesses (0-175 nm) have been thoroughly characterized at 4.2 K in terms of current-voltage (I-V) characteristics, magnetic field- and microwave-response. In selected cases I-V characteristics have been studied over the full temperature range from TC down to 300 mK. Devices with resistively-shunted (RSJ) I-V characteristics and ICRN products above 50 μV at 4.2 K have been fabricated reproducibly. This work has been complemented by Transmission Electron Microscopy (TEM) studies that have allowed the microstructure of the individual devices to be inspected and confirm the validity of the in situ resistance measurement. The individual junction devices are promising candidates for use in the next generation of Josephson voltage standards. In collaboration with Dr. Sam Benz at the National Institute of Standards and Technology (NIST) in the U.S., series arrays of junctions have been fabricated and characterized. Phase-locking behaviour has been observed in arrays of 10 junctions of spacings 0.2 to 1.6 μm between 4.2 K and TC in spite of the relatively large spread in individual critical currents. Strategies for minimizing junction parameter spread and producing large-scale arrays are discussed. The opportunities offered by the FIB for the creation of novel device structures has not been overlooked. By milling a circular trench in the Nb Cu bilayer a Corbino geometry SNS junction is created. In this unique device the junction barrier is enclosed in a superconducting loop, implying that magnetic flux can only enter the barrier as quantized vorticies. This gives rise to a startling magnetic field response - with the entry of a vortex the critical current is suppressed from its maximum value to zero. Experimental results and theoretical modeling are reported. Possible future applications of this novel device geometry (which may be of relevance to Quantum Computing and to studies of Berry's phase effects) are considered.

No description available.

The possibility of creating superconductivity in Highly Oriented Pyrolytic Graphite (HOPG) by means of doping was investigated. Bulk HOPG samples were doped with phosphorous using either ion-implantation or by Chemical Vapor Deposition growth with phosphine in the gas mixture. The methods for testing the graphene samples, once doped, were done by performing R vs. T measurements, and determining via observation suppressed superconductive characteristics signaling the presence of the Meissner Effect in a strong applied magnetic field. Before doping, the resistance vs. temperature (R vs. T) characteristic of the HOPG was measured. The R vs. T characteristic was again measured after doping, and for surface multilayers of graphene exfoliated from the post doped bulk sample. A 100 to 350 mT magnetic field was supplied, and the R vs. T characteristic was re-measured on a number of samples. Phosphorous-implanted HOPG samples exhibit deviations from the expected rise in resistance as the temperature is reduced to some point above 100 K. The application of a modest magnetic field reverses this trend. A step in resistance at a temperature of approximately 50-60 K in all of the samples is clearly observed, as well as a second step at 100-120 K, a third at a temperature range of 150-180 K and a fourth from about 200-240 K. A response consistent with the presence of magnetic field flux pancake vortices in phosphorous implanted HOPG and in phosphorous-doped exfoliated multilayer graphene has been observed. The lack of zero resistance at low temperatures is also consistent with pancake vortex behaviour in the flux-flow regime. The presence of magnetic vortices requires, and is direct evidence of superconductivity.

The developments over the last decade in Focused Ion Beam (FIB) instrument technology have reached a point where there is sufficient control of an ion beam to make cuts, trenches, and other shapes in a sample on a scale of tens of nanometers. This work concentrates on the use of an FIB instrument for making superconducting devices. It is shown for the first time that planar-bridge (Nb/Cu/Nb) Superconductor/Normalmetal/Superconductor (SNS) junctions can be reliably fabricated using a standard FIB instrument. This is demonstrated by the responses of junctions to microwaves and magnetic fields; the junctions display the appropriate Josephson behaviour demanded by current technological applications. In addition, the reproducibility in junction behaviour (the variation of critical current is approximately 10%) is the best so far observed for this type of junction. The SNS junction fabrication method has been successfully extended for making high-density SNS junction arrays, dc-SQUIDs, and related devices. A simple model is devised to explain the normal-state resistance and critical current of a junction. The model is based on the geometry of a junction as defined by the FIB instrument and the film deposition. The model is mostly successful in qualitatively explaining many of the geometrical factors that affect the electrical properties of the junction. Nb/Cu/Nb junction series arrays, made using an FIB instrument, are also successfully fabricated. The yield of the junctions forming small arrays is found to be similar to the yield of single junctions. For the series arrays studied here, new observations have been made: the electrical properties of an array have been found to be dependent on the spacing of the junctions and the number of junctions in the array. This work also investigates the thermal properties of SNS and micron-scale superconductor/insulator/normal-metal junction based devices for use in bolometer device based applications. It is shown that self-heating raises the temperature of the junctions significantly above their operating temperatures. For a device sitting on a low thermally conductive membrane, it is found that the effects of heating, or cooling, in the junctions are exaggerated.

This dissertation describes the investigation of the normal-state properties of high-angle grain boundaries in YBa2Cu3O7-d (YBCO) and Y1-xCaxBa2Cu3O7-d (calcium-doped YBCO). YBCO is a high-temperature superconducting material with a superconducting transition temperature up to 93 K. Grain boundaries are interfaces between two crystals or grains, and severely reduce the attainable currents in practical, polycrystalline material. A grain boundary is characterised by the misorientation angle between the two adjacent crystals, which determines the atomic structure of the interface. The structure of low-angle grain boundaries (misorientation angles < ~7°) is well understood; it consists of a regular array of dislocations. For higher misorientation angles the dislocations merge and form a continuously distorted zone. The structure of these high-angle grain boundaries, and, hence, the mechanism for charge transport across the interface, is less well understood. The current-voltage behaviour of grain boundaries below the transition temperature of the YBCO has been investigated extensively, but less data is available of the resistive behaviour of the grain boundary in the normal state above the transition temperature. The doping of YBCO with calcium is known to decrease its transition temperature, but it can simultaneously improve the charge transport properties of grain boundaries in polycrystalline material. YBCO and calcium-doped YBCO thin films were fabricated on bicrystalline substrates. The grain boundaries had misorientation angles between 18° and 45°. The films were processed in order to obtain microscopic devices that made it possible to determine the resistance of the grain boundary below and above the transition temperature. A measurement system was used with which the voltage across the grain boundary can be measured as a function of applied current between 5 K and room temperature. A detailed model for charge transport by tunnelling across a grain boundary was used to interpret the results of the measurements of grain boundaries. An algorithm based on this model was formulated that made it possible to calculate a shape for the potential barrier at the grain boundary from the temperature dependence of its resistance. The microstructure of the grain boundary was investigated using Transmission Electron Microscopy. It was found that the grain boundary can show considerable deviations from a straight path in line with the substrate grain boundary, but there are also locations where such deviations are not observed. Extensive measurements showed that the resistance of the grain boundary decreases with increasing temperature above the transition temperature, and that resistance and the extent of resistance variation increases with misorientation angle. The resistance below the transition temperature was for certain misorientation angles observed to be independent of temperature at voltages sufficiently high to exclude the influence of the superconducting behaviour of YBCO. A reduction of the oxygen content of YBCO (higher value of d) increases the resistance and the temperature dependence of the resistance. The doping of YBCO with calcium decreases the resistance of the grain boundary below and above the transition temperature. The shape of the potential barrier at the grain boundary was calculated on the basis of most resistance-temperature measurements. The shape and size of potential barriers are used to explain the variation of the grain boundary resistance with misorientation angle, oxygen content and calcium doping percentage. The model shows little validity for grain boundaries with a higher misorientation angle and resistance, which indicates that charge transport across the grain boundary does not necessarily take place according the tunnelling with the assumptions made.

The problem of design optimisation of thin film direct current Superconducting QUantum Interference Device (SQUID) magnetometers made of YBCO (YBa2Cu3O7-x) was considered. The inductances and effective areas were calculated using the software package 3D-MLSI. Resolution and reliability issues were first tested on simple superconducting systems, showing good agreement with analytical formulae and experimental results, and demonstrating that a remarkable precision can be obtained though at the expense of CPU time and memory. The software was then used to simulate a SQUID magnetometer fabricated in the Device Materials Group of the Department of Materials Science and Metallurgy, proving that 3D-MLSI can be used to predict the parameters of real systems with acceptable accuracy.