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Bi-Weekly Seminar

Novel Bioelectric Phenomena in Cellular Organelles

Prof. John H. Miller

by: Prof. John H. Miller

Date: Friday February 03, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We have been investigating unusual bioelectric phenomena using novel, generally noninvasive, sensors. Recent topics of interest include: (1) studies of harmonic response of active membrane pumps (P-type ATPases), which we have observed in budding yeast cells using superconducting quantum interference devices (SQUIDs); (2) nonlinear harmonics, in response to sinusoidal electric fields, produced by electron transport chain complexes and a molecular turbine, known as ATP synthase, in mitochondria; (3) similar studies of light-activated harmonics, which we have recently discovered in chloroplasts; and (4) studies of the dielectric and conducting properties of cytoskeletal proteins, such as tubulin heterodimers, which self-assemble to form microtubules.

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Bi-Weekly Seminar

MOD Development of Coated Conductors at TcSUH

by: Prof. Kamel Salama

Date: Friday December 02, 2005

Time: 1:00 pm – 2:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Currently, first-generation high temperature superconducting (HTS) wires/tapes are commercially available for practical applications. However, second-generation HTS wires/tapes (coated conductors) exhibit the capability of carrying a larger current operating at higher temperatures and stronger magnetic fields. Development of the second generation will accelerate the applications of HTS products into the marketplace. Highlights of our research on YBCO coated conductors at TcSUH include results on textured substrates, metal-organic deposition (MOD) buffer layers and YBCO films. The sharpest cube textured non-ferromagnetic Ni-9at%W alloy substrates were successfully achieved for the first time using the powder metallurgy process and give promise for coated conductors with reduced AC losses. Also, new MOD buffer layers have been developed to simplify coated conductor architectures, leading to a lower cost/performance ratio. In addition, chemically doped MOD YBCO films with enhanced critical current density (Jc) were developed and Jc exceeding 5 MA/cm2 at 77 K was obtained. Finally, we will present results of electric-mechanical properties of SuperPower IBAD coated conductors as part of the collaboration between TcSUH and SuperPower.

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Bi-Weekly Seminar

Magnetoelectric Effects, Spin Frustration, and Ferroelectricity in Multiferroic Manganites

 Bernd  Lorenz

by: Bernd Lorenz

Date: Friday November 18, 2005

Time: 1:00 pm – 2:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The interaction between electric and magnetic fields in matter and/or between dielectric and magnetic orders is one of the fundamental problems in condensed matter physics. The magnetoelectric effect that allows the control of magnetic (dielectric) properties by electric (magnetic) fields is of principal physical interest and it bears the potential for the development of a new type of magnetoelectric memory. The topic has attracted renewed interest quite recently with the discovery of the coexistence of ferroelectricity and magnetic orders in multiferroic rare earth manganites. We discuss the complex physical properties of multiferroic RMnO3 and RMn2O5 (R=rare earth, Y) and show that magnetic frustration as well as strong spin-lattice coupling are the origin of a wealth of interesting phenomena such as incommensurate magnetic orders, frustration-induced ferroelectricity, magnetic field control of ferroelectric polarization, etc. The interactions between the Mn spins, the rare earth magnetic moments, and the ferroelectric polarization in these compounds give rise to an unprecedented phase complexity, e.g. as observed in hexagonal HoMnO3. In orthorhombic RMn2O5, our high-resolution thermal expansion measurements provide unambiguous proof that the ferroelectric transitions are accompanied by strong structural anomalies resulting in anisotropic lattice strain along the principal crystallographic directions.

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Bi-Weekly Seminar

Magnetic-Ordering-Related Phonon and Crystal Field Anomalies in Rare Earth Manganites

Prof. Milko N. Iliev

by: Prof. Milko N. Iliev

Date: Friday October 28, 2005

Time: 1:00 pm – 2:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The complex relationships among the lattice distortions, magnetism, and dielectric and transport properties of rare earth manganites RMnO3 (R = rare earth, Y, Sc) with both orthorhombic and hexagonal structure are attracting increasing interest. The role of structural distortions is widely recognized, but there are only a few studies on their variation with R and how this affects the spin-phonon and electron-phonon coupling. The results of recent experiments on the variations with R of the Raman spectra of orthorhombic RMnO3 (R=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) will be reported. In this series, with decreasing radius rR of R (R=La to Eu), the magnetic transition temperature TN to A-type antiferromagnetic (A-AFM) ordering of Mn3+ decreases from ~140 K to ~40 K. With further decrease of rR (R=Gd to Ho), however, the magnetic structure below TN changes from A-AFM to an incommensurate antiferromagnetic one (IC-AFM) with sine-wave ordering of the Mn3+ moments. It will be shown that the change of magnetic structure correlates with strong mixing of phonon modes involving in-plane Mn-O stretchings and bendings of MnO6 octahedra. The strong spin-phonon coupling is evidenced by phonon softening at T<TN in A-AFN, but not in IC-AFM manganites.Another promising experimental approach&nash;temperature-dependent crystal field IR spectroscopy–will be discussed and the first results on crystal field anomalies near TN in hexagonal RMnO3 (R = Ho, Er, Tm, Yb) will be reported.

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Bi-Weekly Seminar

First Experimental Test of the Incorrect Assumption that Continuous Columnar Pinning Centers Produce the Highest Jc in Superconductors

by: Dr. Alberto Gandini

Date: Friday September 30, 2005

Time: 1:00 pm – 2:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The improvement of critical current in HTS by optimization of pinning center morphology has been a crucial area for research since the HTS's discovery. In the past decade it has been stated in numerous papers that the optimum pinning centers are provided by continuous columnar defects. This conventional wisdom has never been questioned, nor experimentally tested. This has led several researchers to believe that the highest Jc could only be achieved by means of continuous columnar defects. Columnar defects have been assumed to provide the highest Jc because theoretically they have been shown to maximize the pinning potential. However, pinning theory completely neglects that, as the pinning center density increases, the current percolation is reduced, and hence Jc decreases. Recently we argued that percolation has a larger effect on Jc than previously expected. We proposed that discontinuous pinning centers, which reduce the loss of current percolation, would result in a higher Jc. An experiment was performed to directly compare continuous and discontinuous pinning, using high-energy ions. We now present the surprising experimental result that, in clear contrast with the conventional belief, Jc for discontinuous pinning is much higher than for continuous. This experiment indicates that the superior percolation achieved by discontinuous pinning outweighs the decrease in pinning potential. Record Jc ~ 300 KA/cm2 at 77 K was achieved in melt-textured YBCO for pinning which was 67% discontinuous. This work stands as the first experimental test of the postulate that continuous columnar pinning centers produce the highest Jc, and shows that the postulate is incorrect.

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