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Special Seminar

Braided Rivers and Superconducting Vortex Avalanches

Prof. Kevin E. Bassler

by: Prof. Kevin E. Bassler

Date: Thursday January 21, 1999

Time: 3:00 pm – 4:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Magnetic vortices have experimentally been found to intermittently flow through preferred channels when they are forced in or out of a superconductor. This behavior is studied using a simple cellular model. The vortex flow is found to make braided rivers strikingly similar to aerial photographs of braided fluvial rivers, such as the Brahmaputra. The scaling properties of the braided vortex rivers in the model are quantitatively compared with those of naturally occurring braided fluvial rivers, giving general agreement. This suggests that a common dynamical mechanism exists for braiding. This mechanism is avalanches of stick-slip events, either of sediment or vortices, which organize the system into a critical braided state.

Special Seminar

Scattering Studies of Staging and Phase Transitions in Layered Graphite Intercalation Compounds and Superconductors (La2CuO4+δ)

Prof. Simon C. Moss

by: Prof. Simon C. Moss

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.

Special Seminar

A Study of the Electrochemically Oxidized La2CuO4+δ

Prof. Pei  Hor

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.

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.

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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