TcSUH EventS
Special Seminar
Scattering Studies of Staging and Phase Transitions in Layered Graphite Intercalation Compounds and Superconductors (La2CuO4+δ)
Date: Friday November 20, 1998
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
In intercalation systems a second species of atom or molecule is inserted within the layers of the host. The most highly studied materials are the graphite intercalation compounds (GIC's) which incorporate both alkali donors and molecular acceptors in between the graphite sheets. Via a repulsive interaction (both Coulomb and elastic) these intercalated species tend to stage, i.e. to form a periodic stacking . In La2CuO4+δ, the extra oxygen also goes into random interstitial sites in layers of the Fmmm phase and with decreasing temperature, the oxygen phase separates into δ ~ 0 and δ ~ 0.05 phases. The O-rich phase then undergoes a periodic layering (staging) which depends on both temperature and composition. The phase diagram will be discussed and the (neutron) scattering evidence for staging via periodic tilts of the CuO6 octahedra will be shown. The mechanism for the continuous change in staging period [en] the Daumas/Herold model [en] will be discussed as will the gradual evolution of a striped phase in La2CuO4+δ at low temperature.
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Special Seminar
A Study of the Electrochemically Oxidized La2CuO4+δ
by: Prof. Pei Hor
Date: Friday October 30, 1998
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
It is recognized that the single most important and universal parameter in high [Tc] is the “nominal” hole concentration p. We argue that the La2CuO4+δ system exhibits real intrinsic behaviors of doped holes around room temperature. Using the La2CuO4+δ as a model system and taking advantage of precise control of p by electrochemical intercalation technique, we will address some critical issues concerning the universal electronic phase diagram of high [Tc]. We will first focus on the metal-insulator transition and the onset of superconductivity at the critical hole concentration pc = 0.0625. It is found that the metallic state of high [Tc] is composed of two types of holes: Free holes on top of an underlying more localized condense. The existence and physical significance of some special hole concentrations will also be discussed.
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Special Seminar
The National Technology Roadmap for Semiconductors and SEMATECH's Doping Program
by: Dr. Lawrence A. Larson
Date: Wednesday October 28, 1998
Time: 2:30 pm – 3:30 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
SEMATECH sets its strategy and project directions through a series of advisory boards consisting of technical experts from the member companies. Early in the 90's the SIA (Semiconductor Industry Association) requested a longer term strategic plan covering the directions of the US semiconductor industry. This document is now in its fourth revision and is called the NTRS (National Technology Roadmap for Semiconductors). The NTRS has become the longer term strategic plan for SEMATECH and for the SRC.
This talk will describe the part of the roadmap that concerns doping technology and device characteristics. The connection of this document to the needs of the member companies gives the SEMATECH Doping Program its project directions. These are cross-checked through technical Working Groups and advisory boards. Presently, topics of interest include alternate boron dopants, molecular implants such as Decaborane, low energy implant, alternative doping/implant techniques (PLAD, PIII and PGILD), and alternative annealing technologies such as Fast Ramp Rate RTP and Laser Thermal Processing.
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Special Seminar
Molecule and Crystal Engineering of High-Tc Structures and Devices
by: Prof. John T. McDevitt
Date: Friday September 25, 1998
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
In an attempt to foster more rapid developments in the areas of high-[Tc] superconductivity and macromolecular electronic devices, our group has recently initiated a series of studies designed to explore fundamental issues related to the chemistry of high-[Tc] compounds. Knowledge in this regard is particularly important because the poor reactivity problems and materials processing limitations have hampered developments in this area. From our studies, we have identified lattice stress and strain as important variables which influence the environmental reactivity of the most popular high-[Tc] superconductor compound, YBa2Cu3O7. Using solid-state substitution reactions, we have tailored the high-[Tc] lattice so as to relieve the internal stress and strain. Consequently, formulations such as Y0.6Ca0.4Ba1.6La0.4Cu3O7 have been prepared that exhibit both excellent stability and good superconducting properties. Armed with detailed fundamental knowledge of the surface and interface properties of the high-[Tc] materials, our group has completed the initial studies in a new area of research which involves the study of superconductor elements that are derivatized with molecular compounds. Consequently, we have designed, fabricated and tested the first two classes of molecule/superconductor devices. Through the combination of carefully chosen photoactive and electroactive molecular assemblies with high-[Tc] electronic devices, we have shown that it is possible to engineer the molecular layer so that a number of structures can be prepared to serve a variety of applications.
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Special Seminar
Fabrication and Characterization of Superconducting Bi-2223/Ag Tapes with High Critical Current Densities in km Lengths
by: Dr. Zhenghe Han
Date: Monday September 21, 1998
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
High critical current density, long-length Bi-2223/Ag tapes are needed for large-scale applications. I will describe our recent results reaching critical current densities of 23 kA/cm2 and engineering critical current densities of 5.2 kA/cm2 at 77 K over the whole length of 1250 m Bi-2223/Ag tapes. To our knowledge, this is the highest critical current density reported for Bi-2223/Ag tapes longer than 1 km. Detailed measurements of the critical current over the 1250 m tape by both the conventional four-probe method and a specially developed continuous measurement of the remanent field by Hall probes will be reported. Various Ag alloy sheathed Bi-2223 tapes have been made by our standard production line. The approach for improving the homogeneity and considerations to be made in processing and handling the long tape will be presented. Electrical, mechanical, and thermal properties of our Ag alloy sheathed tapes, including Ag-Au alloy sheathed tapes, will also be described.
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