TcSUH
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 shovelâ€Â. 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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Special Seminar
A Global Odyssey: From Science to Administration; From the U.S. to the Orient
by: Dr. Ching Wu Chu
Date: Thursday October 14, 2004
Time: 4:00 pm – 5:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Life is a strange encounter. Three years ago, a difficult-to-refuse opportunity dogged me repeatedly and finally has helped expand my horizon from that of a scientist in the trenches at the University of Houston to that of the President of Hong Kong University of Science and Technology (HKUST) in the Orient, a young and rising premier university in the most dynamic region of our 21st century world. The vision, generosity, and hard work of colleagues on both sides of the Pacific these past three years have demonstrated once again that exceptions do exist to the expression "one cannot have his cake and eat it too." Science in our Houston lab has continued to flourish in the past three years, while new heights have been scaled by HKUST on her way to becoming a world-class university. Synergistic collaboration is being developed between UH and HKUST in the area of the almighty nanoscience and technology. In this talk, I will share with you my experience on this unusual odyssey and point out the opportunities and challenges ahead for us all as citizens of the global village.
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Special Seminar
Collective Modes in Unconventional Superconductors
by: Prof. Peter Brusov
Date: Friday September 24, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
After the recent discovery of collective modes in unconventional superconductors (USC), their study becomes very important. Collective modes (CM) in HTSC exhibit themselves in many experiments: ultrasound attenuation (UA) and microwave absorption (MWA), neutron scattering, photoemission and Raman scattering, etc. The large peak in the dynamical spin susceptibility in HTSC arises from a weakly damped spindensity- wave CM. This gives rise to a dip between the sharp low energy peak and the higher binding energy hump in the ARPES spectrum. Also, the CM of amplitude fluctuation of the d-wave gap yields a broad peak above the pairbreaking threshold in the B1g Raman spectrum. The contribution of collective modes to UA and MWA may be substantial. We consider two-dimensional and three-dimensional models of p- and d-pairing for superconductors built by the path integration technique. Within these models we calculate the collective excitations in different unconventional superconductors [high temperature superconductors (HTSC), heavy fermion superconductors (HFSC), etc.] under p- and d-pairing. We consider both bulk and 2D systems. Some recent ideas concerning realization in HTSC of the mixtures of different states are investigated. In particular, we consider the mixture of two d-wave states in HTSC (of dx2-y2 and dXY states). Obtained results could be used for interpretation of the sound attenuation and microwave absorption data as well as for identi?cation of the type of pairing and order parameter in unconventional superconductors. They allow us to distinguish pure d-wave state from the mixture of two d-wave states in HTSC.
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