Many properties of the thin films are different from the bulk value and in many cases, depend dramatically on the film thickness. In the metallic ultra-thin films epitaxially grown on the semiconductor substrate, the conduction electrons are confined by the vacuum and metal-semiconductor interface. When the film thickness is comparable to the electron Fermi wavelength, this confinement will produce discrete energy levels known as quantum well states (QWS), which dramatically modify the electronic structures of the thin film and this is called quantum size effect (QSE). QSE will have a profound effect on a lot of physical properties of the thin films. Among various systems exhibiting QSE, Pb/Si (111) is the most widely studied one and exhibits the richest phenomena in QSE. In this study, a home made low temperature Scanning Tunneling Microscopy/Spectroscopy (LT-STM/S) was used to study the superconductivities of the Pb thin films. Quantum oscillations of the superconductivity have been observed for the films down to 4 monolayer and the oscillation amplitude increases as the film gets thinner. To resolve the discrepancies between the superconductivities measured with ex-situ transport and in-situ STS. We also studied the influence of Au overlay on the Pb thin films with LT-STM/S, and found out the deposition of Au on Pb dramatically roughened the Pb films. Finally, we successfully grew large scale near perfect 2ML Pb films. There are two types of films which exhibit different Moiré patterns. LT-STS studies revealed there is big difference in the superconductivity Tc of these two films, both of which decreased dramatically from that of the 4ML film. / text

Over the past twenty-five years, condensed matter physics has been developing materials with novel electronic characteristics for a wide range of future applications. Two research directions have shown particular promise: topological insulators, and high temperature copper based superconductors (cuprates). Topological insulators are a newly discovered class of materials that can be manipulated for spintronic or quantum computing devices. However there is a poor spectroscopic understanding of the current topological insulators and emerging topological insulator candidates. In cuprate superconductors, the challenge lies in raising the superconducting transition temperature to temperatures accessible in non-laboratory settings. This effort has been hampered by a poor understanding of the superconducting mechanism and its relationship with a mysterious pseudogap phase. In this thesis, I will describe experiments conducted on topological insulators and cuprate superconductors using scanning tunneling microscopy and spectroscopy, which provide nanoscale spectroscopic information in these materials. / Physics

The class of high Tc superconductors share one common structural aspect, the existence of planes of copper and oxygen ions. These planes are thought to be the source of the superconducting behaviour. They can be represented as a two-dimensional lattice of ions, which facilitates their study using numerical models. One such model is the t − J model. In most studies utilising numerical models, the planes have been considered isotropic. However, recent analysis of cuprate structure has illustrated that this may not be representative of the copper oxide planes. A number of cuprate structures exhibit different phases in which the planes are not isotropic, such as the low temperature orthorhombic and low temperature tetragonal phases. This work will examine the effects of introducing anisotropy into the t − J model in order to understand how these phases affect the results gained from numerical studies. / x, 89 leaves : ill. ; 29 cm.

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.

The stoichiometry of the Nd_1+xBa_2-xCu₃O_7-δ solid solution has been investigated using XRD and EPMA. At 980°C in air, an essentially continuous solid solution forms with limiting compositions x_min = 0.03(1) and x_max = 0.92(2). The solid solution limits are independent of temperature over the range 300 to 1050°C, i.e. stoichiometric Nd123 does not form. Preliminary studies show that annealing in an Ar atmosphere does not affect x_min. Three structurally distinct polymorphs of Nd123ss exist - tetragonal Nd123ss, orthorhombic Nd123ss and orthorhombic Nd123ss. The stability range of each in air has been determined. Quenched samples with 0.03 ≤ x ≤ 0.6 have the tetragonal Nd123ss structure. On oxygenation, samples with 0.03 ≤ x <˜0.2 are orthorhombic. The orthorhombic-tetragonal phase transition is second order, both with increasing temperature and increasing x. Samples with 0.7-0.9 have the orthorhombic Nd213ss structure at all oxygen contents. Tetragonal Nd123ss is isostructural with tetragonal Y123 and orthorhombic Nd123ss is isostructural with orthorhombic Y123. Orthorhombic Nd213ss has the ideal stoichiometry Nd₂BaCu₃O_7-δ and is based on a 2a x b x 2c superstructure of the Nd123ss structure. The supercell is due to ordering of the Nd and Ba atoms, which leads to ordering of the oxygen atoms. Melting temperatures decrease with x. Two distinct regions of melting behaviour are observed; the first for 0.03 ≤ x ≤ 0.6 with a thermal minimum at x˜0.4, and the second for 0.7 ≤ x ≤ 0.9. Oxygen contents increase with x. Samples with larger x values have a smaller range of oxygen contents. High pressure oxygen annealing results in a constant Cu valence state of ˜2.35 for all values of x. Average copper valence states <2 are only obtained readily for x ≤ 0.3. T_c decreases with x and samples become non-superconducting at x˜0.5. For samples annealed in 1 bar O₂, 'double plateau' behaviour is observed.

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No description available.

Fabrication of nanoscale Josephson junctions and Superconducting Quantum Interference Devices (SQUID) is very promising but challenging topic in the superconducting electronics and device technology. In order to achieve best sensitivity of SQUIDs and to reproduce them easily with a straightforward method, new fabrication techniques for realization of nanoSQUIDs needs to be investigated. This study concentrates on investigation of new fabrication methodology for manufacturing nanoSQUIDs with High Temperature Bi-Crystal Grain Boundary Josephson Junctions fabricated onto SrTiO3 bi-crystal substrates using YBa2Cu3O7-δ (YBCO) thin-films. In this process nanoscale patterning of YBCO was realized by using electron beam patterning and physical dry etching of YBCO thin films on STO substrates. YBCO thin films were deposited using RF magnetron sputtering technique in the mixture of Ar and O2 gases and followed by annealing at high temperatures in O2 atmosphere. Structural characterization of YBCO thin films was done by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). Superconducting properties of thin films was characterized by AC magnetic susceptibility measurements. Nanoscale structures on YBCO thin films were fabricated by one E-Beam Lithography (EBL) step followed by Reactive Ion Etching (RIE) and physical dry etching. First SiO2 thin film were deposited on YBCO by RF magnetron sputtering and it was patterned by EBL using Polystyrene (PS) as resist material and RIE. Then SiO2 was used as an etch mask for physical dry etching of YBCO and nanoscale structures on YBCO were formed.

A high resolution pressure gauge has been developed for use in thermodynamic measurements along the lambda line in liquid helium. The gauge was designed to operate at cryogenic temperatures and provide pressure measurements up to 30bar, with an accuracy of 3 × 10¯¹ºbar in a 1Hz bandwidth. Experiments reported here show the gauge to have met these specifications; at least for measurements close to zero pressure, at temperatures close to 4.5K. It is expected that operation at higher pressures, and at temperatures closer to the lambda transition, will result in similar or even improved performance. The gauge consists of a titanium-alloy diaphragm with a superconducting position transducer read-out. Compensation techniques internal to the superconducting circuit were used to eliminate any significant sensitivity to temperature fluctuations and in-line acceleration. For high values of common-mode rejection, thermal compensation revealed a non-linear temperature characteristic which was exploited to provide a further reduction in the temperature sensitivity. Acceleration compensation was achieved up to a common-mode rejection of more than 78dB. Present performance appears to be limited by thermal gradient fluctuations at low frequencies and at higher frequencies by a noise source which appears to originate beyond the superconducting transducer. It is expected that some further improvement may be gained in this higher frequency band simply by trapping a larger persistent current in the superconducting circuit. In the course of development and characterization of the gauge several anomalous effects were discovered and investigated. In response to changes in temperature, the gauge was found to exhibit irreversible behaviour in a variety of ways. These phenomena were fully investigated and found to be complex in nature. A critical state model was employed which was successful in explaining many of the observed effects. Other authors have observed apparently related behaviour in samples of niobium and some have developed similar critical state models which give results generally consistent with those reported here. However, these latter works have not investigated the presence of such effects within superconducting wires; neither have they considered the implications for devices based upon superconducting wire circuits. It appears this anomalous behaviour may be relevant to a broad range of instruments employing superconducting wire circuits similar to that used here. If this is the case, the results presented here have significant consequences for the performance of such devices

Thesis (Ph.D.)--McMaster University, 2005. / Supervisor: J. S. Preston. Includes bibliographical references (leaves 111-117).