TcSUH
Bi-Weekly Seminar
Superconducting MRI Coils for Clinical and Research Applications
by: Dr. Jarek 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
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(...)
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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