The phase stability, electronic properties and crystal structure of the Pb based 1212 cuprate superconductor (Pb[1+x]/2Cu[1-x]/2)Sr2(Y1-xCax)Cu2O7+/-s were studied using a number of techniques. Since synthetic difficulties limited study of these materials, sol-gel techniques incorporating mixed metal nitrate precursors were applied. Novel synthesis techniques improved reaction rate but did not entirely overcome synthesis difficulties. The effects of Ca content; reaction temperature, and oxygen content in controlling product phase formation and phase stability under synthesis conditions were investigated. X-ray and neutron diffraction were used to probe structural disorder within samples as Ca levels increased and sample oxygen content was varied. The subsequent Rietveld refinements revealed that the levels of cation site disorder increase in line with increasing Ca levels. Under oxidising conditions, annealing temperature and thus oxygen content of the sample plays a critical role in controlling superconducting properties. Optimal Tc and superconducting volume are only achieved over the oxygen content range 6.99-7.00. Low pO2 annealing conditions were used to control sample oxygen content; however, these were not significantly better than standard techniques. Low pO2 environments facilitate the removal of oxygen from samples and allowed the study of phase stability. It was possible to show that decomposition of the 1212 phase leads to formation of new superconducting phases that might well account for contentious literature claims. EXAFS and NMR measurements show that on a local scale, the structure of the 1212 phase rocksalt layer is quite different to the average structure generated from diffraction measurements. The Pb atoms tend to form a PbO4+x molecular type cluster, in which x can increase up to 0.5 to accommodate the inclusion of an excess 0.1 oxygens into the structure. Solid-state NMR measurements reveal in Ca containing samples that up to 20% of the Y within the structure is dimerised.

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.

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The optical properties of the 10.4 K organic superconductor, κ-{BEDT—TTF)₂[CU-(NCS)₂], are the subject of this thesis. This organic superconductor is a crystalline solid consisting of planes of BEDT-TTF molecules separated by planes of Cu{NCS)₂ anions. The arrangement of molecules results in electrical two-dimensionality, with high conductivity within the planes of BEDT-TTF molecules and poor conductivity in the perpendicular direction. Until the recent discovery of an organic superconductor with TC = 11.5 K, the 10.4 K transition of (BEDT-TTF)₂[Cu(NCS)₂] was the highest in the class of organic metals. Optical properties of superconductors have played an important role in analysis of their unusual properties. Using single crystals of (BEDT—TTF)₂[Cu(NCS)₂] and light polarized within the highly conducting plane, I have measured both specular reflectivity and bolometric absorption over a wide region of the infrared spectrum. The reflectivity measurements cover most of the far and mid-infrared for both protonated and deuter-ated compounds, with sample temperatures between 300 K and 6.5 K. The resulting spectra show contributions from electronic interband transitions, intraband transitions, and molecular phonon vibrations. A search for the superconducting energy gap was undertaken using the bolometric technique to measure the sample absorption at far-infrared energies. These measurements were performed at temperatures below Tc and cover the part of the spectrum where condensation of electrons into superconducting Cooper pairs should be directly observable optically. On other materials, measurements in this part of the spectrum have directly revealed the binding energy for the superconducting electron pairs, and have been instrumental in showing that details of the electron-phonon interactions are very important in superconducting systems. In the spectra which we have measured, any effect of the superconducting transition is overwhelmed by other contributions to the absorption in the far-infrared energy region. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A study has been made of some energy and momentum properties associated with the electrons of superconducting indium. In the experiments, an electric current was induced in the indium sample and measurements were made to detect any effects of an increasing super-state electron density on the current. By lowering the sample's temperature appropriately, the electron density could be controlled. The indium sample studied was in the form of a thin film, constructed by vacuum evaporation onto a glass substrate. Two indium wires connected the ends of the thin film to a copper wafer, forming a complete electric circuit. This circuit was electrically isolated and measurements of the current through the thin film were made with a search-coil coupled to the current's magnetic field. A relationship between the current changes observed and some momentum properties of superstate electrons was then established. The assumption that all "virtual pairs" in a superconductor have a common momentum proved consistent with the experimental results. Theoretical calculations are given which suggest that only those normal electrons with a preferred momentum can take part in increasing the superconductive electron density. This implies that a superstate electron system may effect the transformation of internal energy into work. However, the experiments carried out have not been sufficiently sensitive to show this conclusively. The inference stated above is based on the experimental results in conjunction with the required agreement of theoretical calculations with accepted theory. The indium-copper junction resistance had a resistance value which was markedly temperature dependent in the region below 3.4°K (the transition temperature of pure indium). A variation of resistance between the extremes 9.4 x 10⁻¹º Ω at 3.1°K and 8 x 10⁻¹º Ω at 1.2°K was found. The cause of this large resistance change is ascribed to various effects but probably the most important one is the diffusion of copper impurity into the indium. Arguments in favor of this explanation are given. A number of suggestions are also included which may be helpful to the design of experiments similar to the one reported in this thesis. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The Y₁Ba₂Cu₃0x superconducting thin films were made by both sequential electron-beam evaporation and co-evaporation of Cu, BaF₂ and Y, followed with high temperature post-deposition annealing. The possibility of making Y₁Ba₂Cu₃0x superconducting film in situ with BaF₂ as the Ba source was investigated by studying the conditions of in situ conversion of BaF₂ to BaO. The BaO concentration in the film as function of the substrate temperature was estimated with an equilibrium thermodynamic model and found to agree with the experimental results. A brief experimental and theoretical exploration of BaCl₂ also showed that BaCl₂ does not have any advantage over BaF2 as the Ba source for in situ growth of Y-Ba-Cu-0 films from the growth temperature point of view. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

In this thesis, we study stripe states in underdoped cuprate superconductors using the Bogoliubov-de Gennes (BdG) method with appropriate models. In non-self-consistent calculations, the nature of stripe states with coexisting unidirectional Pair Density Wave (PDW), Charge Density Wave (CDW), and Spin Density Wave(SDW) order is studied, and it was found that among these coexisting stripe orders, the SDW order plays a key role in reconstructing the Fermi Surface (FS), and the magnitude of SDW order needs to be relatively small in order for Fermi arcs to be realized, while strong SDW order necessarily leads to completely gapped spectra. In self-consistent calculations, we mapped out parameter regions where stripe SC states can be stabilized, which are in qualitative agreement with the generic phase diagram of cuprate superconductors. In addition, self-consistent PDW solutions with small SDW order are stabilized in some parameter region, forming small FS pockets in antinodal regions. This parameter region is a promising one to explore in future work, where self-consistent BdG calculation is to be carried out with a magnetic field in order to study the effect of magnetic field on the stability of stripes along with the possible quantum oscillations. / Thesis / Master of Science (MSc)

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