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Bi-Weekly Seminar

Magnetic Nanowires

Dr. Li  Sun

by: Dr. Li Sun

Date: Friday November 07, 2003

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Nanoscience and technology as an emerging interdisciplinary research area has caught a lot of attention in recent years. Nanostructured materials exhibit novel properties bulk samples do not possess, however, fabrication of well-controlled nanostructures, understanding physics at the reduced dimensionality and device application of individual nanomaterial still remain challenging. Here we use nanowires as an example to show how magnetic properties of materials can be designed and tuned by nanofabrication. Magnetic shape anisotropy, finite size effects and magnetization switching in these quasi-one dimensional structures has been studied. Manipulation and potential application of individual single elemental and multi-component magnetic nanowires will be discussed.

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Bi-Weekly Seminar

Dielectric Properties of Biological Cell and Protein Suspensions

Prof. John H. Miller

by: Prof. John H. Miller

Date: Friday October 31, 2003

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The charges in live cells can be polarized by an electric field, creating a dipole moment for each cell. As a result, a suspension of live cells in water has an enormous dielectric response at low frequencies, and this response decreases rapidly with increasing frequency. The magnitude of the low-frequency dielectric response has been shown theoretically to correlate with the cell’s membrane potential. Recently, we have been exploring new applications, such as sensors designed to detect biological warfare agents inside sealed containers. Another ongoing study is the dielectric response of tubulin dimers, in an attempt to determine their intrinsic dipole moment. Tubulin is the protein that comprises microtubules -- remarkable structures that form much of the cellular cytoskeleton. In addition, due to their high concentration in neuronal axons, microtubules have been proposed to play a role in information processing. We are investigating several approaches to performing linear and nonlinear dielectric spectroscopy, ranging from simple direct electrode methods to techniques that employ SQUIDs.

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Bi-Weekly Seminar

Superconducting MRI Coils for Clinical and Research Applications

Dr. Jaroslow  Wosik

by: Dr. Jaroslow Wosik

Date: Friday October 10, 2003

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Magnetic Resonance Imaging (MRI) is related to the phenomenon of nuclear magnetic resonance (NMR), which is based on the excitation and relaxation of nuclei (most frequently protons) within living tissues in a dc magnetic field. In a MRI set-up, a receiver probe detects such a signal. For selected applications, high-Tc superconductor MRI receiver coils and coil arrays have superior properties to those of comparable copper coils. In this talk, a brief review of MRI physics will also include design issues such as the signal-to-noise ratio dependence on frequency, body size and coil size. Fundamentals of design processes of MRI coils, including issues pertaining to cryo-packaging will be discussed together with examples of practical coils made of normal metal and superconductors. New theoretical and practical concepts for significant shortening of the MRI acquisition time by using parallel processing with phased array probes will be presented. Utilization of superconducting coils in cardiovascular medicine will be discussed for examination of aortic walls and identification of atherosclerotic plaques responsible for acute coronary syndromes (vulnerable plaque). Carotids images will show that superconducting coils can overcome limitations of routine MRI, which does not provide enough resolution for plaque investigation. Application of the HTS resonators in research on small animals will be addressed.

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Bi-Weekly Seminar

Raman Spectroscopy of Structural Disorder

Prof. Milko N. Iliev

by: Prof. Milko N. Iliev

Date: Friday September 19, 2003

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Mostly unwanted, the static or dynamical lattice disorder in many cases is either desirable or unavoidable. Static disorder is always present in non-stoichiometric, microtwinned and nano-size materials. Dynamical disorder governs such phenomena as ion diffusion and ion conductivity. It is also substantial for the unusual properties of CMR compounds. Raman spectroscopy is an efficient tool for study of structural disorder, including the dynamical one. This will be demonstrated for three different classes of materials. The doped rare earth manganites R1-xAxMnO3 (R=rare earth, A=Sr,Ca,Ba,Pb) provide an example of partial disorder due to non-coherent Jahn-Teller distortions of oxygen sublattice. In the insulating paramagnetic or antiferromagnetic phases the Raman spectrum is dominated by broad bands reflecting the partial (oxygen) phonon density-of-states (PDOS). These bands disappear in the metallic ferromagnetic phase, where the Jahn-Teller distortions cannot develop. The second example will illustrate the Raman monitoring of structural disorder in NaxCoO2 crystals. It will be shown that the disorder in this material is surface-dependent and develops with ageing. As a third example, the evolution with temperature of the Raman spectra of nanoporous Na2Nb2-x MxO6-xOH)x .H2O will be discussed. Raman spectroscopy in this case allows monitoring of the dehydration above 250C and the transition to a disordered perovskitelike structure above 550C.

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Bi-Weekly Seminar

Intrinsic Electronic Instabilities, Magic Doping Levels, Wigner Lattices and High Temperature Superc(...)

Prof. Pei  Hor

by: Prof. Pei Hor

Date: Friday June 27, 2003

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We present a systematic study of the super-oxygenated La2-xSrxCuO4+ system. We show (i) that around room temperature, there are energetically favored intrinsic electronic instabilities of doped holes occur at some magic doping levels p = 1/N2 where p is the number of holes per copper atom and N = 2, 3, 4 | in the CuO2 plane, (ii) that at low temperature, there are only two intrinsic Tc's, Tc ~15K and Tc ~ 30K, in the under-doped La2-xSrxCuO4+. We argue that these instabilities are the manifestations of 2D electronic Wigner lattices and further show that they are intimately related to the occurrences of the intrinsic Tc ~15K and Tc ~ 30K superconducting transitions in La2-xSrxCuO4 single crystals. Based on (i), (ii) and our detailed studies of the far-infrared charge dynamics focusing on samples near N = 4, we find that only a very small (< 1 %) amount of the total holes participated in the nearly dissipationless normal state charge transport and superconductivity. These free carriers are riding on and massively screened by the rest of the holes organized in 2D Wigner lattice state. This unique composite system of charge carriers provides further insights into the understanding of the cuprate physics.

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