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Student Symposium

32nd Semiannual TcSUH Student Symposium and Holiday Buffet

by: TcSUH Administration

Date: Friday December 15, 2006

Time: 9:00 am – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

All members of TCSUH are invited to attend the 32nd Semiannual Student Symposium for presentations of original student work.

The Student Symposium will begin at 9:00 a.m. with a brief review of the Center’s progress during 2006 and an outline of plans for the future. The afternoon will then continue with a series of 15-minute presentations representing novel work by undergraduate and graduate students from each laboratory. A Symposium Program, including the schedule and abstracts for each presentation, will be provided at the door.

The Holiday Buffet will follow at 1:15 p.m., and symposium winners will be announced.

RSVP for Holiday Buffet: Office of Public Affairs, x-38210 by Friday, December 8, 2006.

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Special Seminar

Hybrid Physical-Chemical Vapor Deposition for MgB2 Coated-Conductors

by: Prof. Xiaoxing Xi

Date: Tuesday December 12, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

MgB2 thin films and coated-conductor fibers, grown by Hybrid Physical-Chemical Vapor Deposition (HPCVD), show extraordinarily high values of upper critical field Hc2 (over 60 T) and irreversibility field Hirr (over 40 T), which are much higher than those of Nb-based superconductors. This is the result of two-band superconductivity, a unique feature of MgB2, and the modification of the inter-band and intra-band scattering of the two bands. The carbon alloying also dramatically increases the critical current density in magnetic field. The carbon content-dependence of the lattice constants in the HPCVD films are found to be different from those in bulk carbon-doped MgB2, which may explain their superior high field properties. Coated conductor wires and tapes are a potentially viable approach to turn such properties into practical high-field conductors for applications such as magnetic resonance imaging.

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Special Seminar

Half-Metallic Oxides for Spintronics

by: Prof. Arunava Gupta

Date: Friday December 08, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The emerging field of spintronics aims to exploit the electron spin, in addition to its charge, to create a new class of devices that scale down to much smaller dimensions with possibly added functionalities. Of particular relevance are magnetic tunnel junctions (MTJs), consisting of two ferromagnetic thin film electrodes separated by an insulating barrier, that exhibit large tunneling magnetoresistance (TMR) at relatively low fields. The MTJs are promising for a host of applications including magnetic memory (MRAM), sensors, and storage devices. Most of the studies on MTJs have thus far focused on using transition metal ferromagnets (Fe, Ni, Co) and their alloys—typically with spin polarization values less than 50%—where the maximum observed TMR is limited to about 40–50% at room temperature using an amorphous aluminum oxide barrier. There is obvious interest in further enhancing the TMR by using materials with a higher degree of spin polarization. Half-metallic systems, which contain a gap in one spin band at the Fermi level and no gap in the other spin band, are expected to have a spin polarization value approaching 100%. We have fabricated MTJ devices using half-metallic oxides, such as the mixed-valence manganites (La1-xAxMnO3, A=Ba, Sr, or Ca) and chromium dioxide (CrO2), that exhibit reproducible tunneling characteristics with high TMR values. However, the TMR enhancement has thus far been limited to low temperature. I will present an overview of MTJs, particularly related to the MRAM application, and then focus on the fabrication and properties of tunnel junctions using the half-metallic oxides and the challenges.

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Special Seminar

YBCO and MgB2 conductors for ac and space applications

by: Dr. Bartek A. Glowacki

Date: Wednesday November 22, 2006

Time: 10:00 am – 11:00 am

Location: Houston Science Center – Building 593 — Room 102

Overview

Advances in multifilamentary YBa2Cu3O7 coated conductor technology such as mechanical patterning, laser grooving, and ink-jet patterning may allow the manufacture of ac conductors with narrow filaments on a thin non-magnetic metal alloy or flexible ceramic substrate which is separated from the superconductor by thin dielectric, conductive, or even magnetic buffers. The role of percolative paths in bridged patterned conductors is discussed. It was found that ac losses of striated samples with multiple bridges are higher than those of the samples with no bridges due to significant filament coupling, but, even so, the losses are still substantially lower than those of a monolayer sample. The possibility of using ink-jet printing technology in the manufacture of complex 3D superconducting structures is assessed. The Adiabatic Demagnetisation Refrigerator (ADR) is the preferred technology for cooling cryogenic detectors for space applications. MgB2 has excellent potential for these applications, and an ADR with MgB2 magnet coils is in development to meet the requirements of the European Space Agency’s XEUS project. These include the production of 3 T at 15–20 K with current not exceeding 15 A, placing considerable demands on MgB2 powder-in-tube conductor technology. Practical options for wire design, matrix material, starting powders, dopants, and thermo-mechanical processing are quantitatively reviewed, and experimental results presented, to support design proposals for these conductors. Some numerical results on critical currents and thermal stability of the future MgB2 multifilamentary coated conductors with magnetic cladding of their filaments are presented and discussed.

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Bi-Weekly Seminar

Nanoscale Self-Assembly: A Theoretical Analysis

 Gemunu  Gunaratne

by: Gemunu Gunaratne

Date: Friday November 10, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We use linear and nonlinear stability analysis on a paradigmatic model to extract general characteristics of nano-scale self-assembly. In particular, we identify the order of occurrence of hexagonal and striped arrays, and show that square arrays cannot form when the elastic forces between the substrate and the monolayer are isotropic. In addition, we introduce a method that can be used to estimate hard-to-extract material properties of the monolayer using characteristics of the self-assembled patterns. Finally, we will discuss a technique that can be used to help generate patterns with long-range order.

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