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Special Seminar

Bi2Sr2CaCu2O8+δ c-Axis Twist Josephson Junctions: The Superior Test of Order Parameter Symmetry

by: Prof. Richard A. Klem

Date: Tuesday December 07, 2004

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Three c-axis twist Josephson junction experiments to test the symmetry of the superconducting order parameter have been performed on Bi2Sr2CaCu2O8+d. These are the experiments performed on bicrystals [Q. Li et al., Phys. Rev. Lett. 83, 4160 (1999)], on artificially prepared cross whiskers [Y. Takano et al., Phys. Rev. B 65, 140513 (2002); Physica C 408-410, 296 (2004)], and on naturally-formed cross whiskers [Yu. I. Latyshev et al., Phys. Rev. B 70, 094517 (2004)]. These experiments are analyzed in terms of the twist theorem for weak, first-order c-axis tunneling between two d-wave superconductors twisted 45- about the c axis, with treatments of weak, first-order c-axis tunneling matrix elements with varying amounts of coherent and incoherent components and order parameter forms, including the effects of the Fermi surface details, strong coherent tunneling, orthorhombicity, nanoscale disorder, and order parameter twisting near the physical twist junction. It is shown that in all three experiments, essentially all of the c-axis Josephson tunneling arises from an s-wave component to the superconducting order parameter, and that the overall order parameter must contain a substantial s-wave component for T & TC. In addition, the c-axis tunneling in the bicrystal and natural cross-whisker experiments must be strongly incoherent. New calculations for in-plane (001) tilt grain boundary junctions relevant to the IBM tricrystal and related phase-sensitive experiments will also be presented.

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Special Seminar

Heteroepitaxial Oxide Materials Obtained by MOCVD

by: Prof. Andrey Kaul

Date: Monday December 06, 2004

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

A talk will give a review of recent activity of the group at the Department of Chemistry of Moscow State Lomonosov University, specializing in MOCVD of oxide thin films. The spectrum of materials under investigation covers the HTSC cuprates, CMR manganites, perovskites with high electric conductivity and metal-insulator transition, oxygen permeable mixed conducting perovskites and related phases. Also a short overview of the MOCVD technical facilities and approaches of the group will be presented. Particular attention will be paid to epitaxial stabilization phenomena in complex oxides. The synthesis of new phases, unusual phase relations and other thermodynamic consequences of epitaxial stabilization will be regarded in application to the above mentioned materials.

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Special Seminar

Magneto-Optical Imaging of Vorticesand Flux Avalanches in Superconductors

by: Prof. Tom H. Johansen

Date: Thursday November 18, 2004

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Magneto-optical imaging (MOI) is a technique which is based on the large Faraday effect in Bi:YIG films, and is now becoming widely used to study space and time-resolved magnetic behavior of superconductors. Recently, we succeeded to bring MOI to a level where individual Abrikosov vortices can be observed and their motion visualized in real-time. I will report on this latest development, and illustrate by VIDEO clips how MOI can reveal the detailed dynamical behavior of vortex matter. An interesting side effect of using the Bi:YIG sensor is that Bloch walls, commonly present in such films, can be used to actively manipulate vortices. It is shown that such a wall, which basically is a movable nanoscopic bar magnet, can act either as a “vortex brush” or “vortex shovel”. On a larger scale, MOI recently discovered that flux penetration in thin film superconductors often occurs in abrupt bursts of sample spanning (and beautiful!) dendritic flux structures. Various aspects of this avalanche behavior, which manifests also in large noise in M-H loops, will be reported for the cases of MgB2, YBCO and NbN films. It will also be shown that coexisting with the flux dendrites (typically involving 106-107 vortices), one finds well-defined mesoscopic avalanches where 20-10000 vortices participate. Avalanche distributions, their morphology, and their origin will be discussed.

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Special Seminar

Hydrogen Economy: Status of Science & Technology and R&D Opportunities

by: Prof. U. Balachandran

Date: Friday November 12, 2004

Time: 2:00 pm – 3:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Hydrogen is considered the fuel of choice for both the electric power and transportation industries because of concerns over global climate change. Dependence on depleting oil reserves found in politically unstable regions of the world is forcing many nations to look into the so-called hydrogen economy - a solution that holds the potential to provide sustainable clean, secure, affordable, and reliable energy. At present, petroleum refining and the production of ammonia and methanol collectively consume 95% of all deliberately produced hydrogen in the U.S. Most of the demands for hydrogen are currently met by fossil-based technologies such as steam reforming of methane, naphtha reforming, and coal gasifcation. New cost-effcient production pathways will be needed as we move into the hydrogen-based transportation system. Present needs include economically viable and environmentally benign sources for hydrogen, safe and efficient storage, infrastructure for delivery, and utilization technologies. Also needed are establishment of safety codes and standards, and public training/acceptance. Materials science will play a major role in addressing the challenges of the hydrogen economy. The current status of the hydrogen production, storage, distribution, and utilization technologies will be reviewed. Topics addressed will include membranes for hydrogen production/separation, thermo-chemical water splitting, and technical barriers/research opportunities.

* Work supported by the U.S. Department of Energy.

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Special Seminar

Analysis of Surface Nano-structures by XPS

by: Prof. Sven M. Tougaard

Date: Thursday November 11, 2004

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The accuracy and capability of NPS for analysis of surface nano-structures has improved significantly in the past decade. This was achieved by modeling the change in energy distribution with the path the XPS electrons travel to reach the surface. When this is compared to measured energy spectra, the depth distribution of atoms can readily be determined. The method, which is now widely used, provides analysis with sub nano-meter depth resolution in the 0 - ~10 nm depth range. We discuss the underlying physics as well as the prospects of a newly proposed model for automatic data analysis intended for XPS imaging. We will also discuss several practical applications of the technique as for example to thin metal-oxide films, SiO-nitrides, low energy ion implants in SiO2, surface segregation phenomena in PS-PDMS diblock copolymers, etc.

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