TcSUH
Bi-Weekly Seminar
Optical Spectroscopy of Manganese Oxides and Advanced Semiconducting Materials and Structures
by: Dr. Alexander P. Litvinchuk
Date: Thursday October 14, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Part of research activities within the Raman & Infrared Research Lab. of TcSAM will be overviewed. Using ultra-short period InAs/A1Sb superlattices and doped GaAs:N semiconducting films as examples we will demonstrate capabilities of optical pectroscopic techniques in the non-destructive characterization of advanced materials, which yields information on structure stability, spatial distribution and/or depth profile of dopants and impurities, bound and free charge carriers, their mobility, etc. Further, we will examine optical properties, charge and lattice dynamics of La1/2Ca1/2MnO3, which exhibits a rich phase diagram and a variety of intriguing properties due the delicate interplay of spin, charge, lattice, and orbital degrees of freedom. We will present the experimental evidence for the existence of an insulating ground state, development of the charge density waves, and opening of a gap in the excitation spectrum at low temperatures. Phonon and crystal-field excitations of hexagonal HoMnO3 single crystals will also be analyzed with the emphases on the anomalies due to antiferromagnetic Mn ordering.
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Bi-Weekly Seminar
Electromagnetic Properties of Live Cells and Proteins
Date: Friday September 17, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
A live cell in an electrolyte or other extracellular medium has a finite membrane potential due to a net negative charge in the interior, and can thus be polarized by an applied electric field. In addition, most proteins in their native (folded) state are either electrically charged (e.g. actin, which self assembles into 8-nm diameter filaments) or have a net electrical dipole moment (e.g. the a-b tubulin heterodimer). These properties lead to enormous dielectric responses at low frequencies, which can be probed non-invasively at various length scales. We observe changes with time in the dielectric properties of a-b tubulin heterodimers as they self assemble to form 25-nm diameter microtubules, a major component of the cellular cytoskeleton. In addition, we have been studying live cells, and, for example, have observed substantial reductions in the dielectric response of eucaryotic cells when exposed to respiratory inhibitors, such as cyanide, that attack the mitochondria. This is significant because it shows the technique can non-invasively probe the metabolic states of these internal organelles. More recently, our group has found possible evidence for novel phase transitions in the temperature-dependent dielectric responses of some biological systems, such as E. coli.
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Bi-Weekly Seminar
A Complete Charge Model of the Under-doped La2-xSrxCuO4 Superconductors
by: Prof. Pei Hor
Date: Friday May 21, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Based on the transport and far-infrared (far-IR) reflectivity measurements in the direction parallel (ab-plane) and perpendicular (c-axis) to the CuO2 planes, we present a complete picture of the electronic structure of La2-xSrxCuO4 (LSCO) superconductors in under-doped regime. Contrary to the common belief, we found conclusive experimental evidence that the c-axis charge transport is intrinsically coherent and the c-axis scattering rate (?c) is extremely small and temperature (T)-independent. Our findings suggest that the normal state of LSCO in the under-doped regime is an unconventional anisotropic metal built upon a three-dimensional (3D) Wigner hole lattice resulting from the 3D ordering of the two-dimensional (2D) square hole lattices formed in the ab-planes.
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Bi-Weekly Seminar
Nanoscale Modulations in Optimally Doped and Underdoped YBCO: X-ray Evidence and Theory for Atomic Displacements in the Higher-Order Ortho Phases
Date: Friday April 30, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
High intensity x-ray measurements at the undulator line of the Advanced Photon Source have shown a distinct pattern of diffuse satellites about the normal Bragg peaks which possess a characteristic asymmetry and have a correlation range of only 4-5 unit cells in all directions. Measurements made at 7K show an intensity distribution about the Bragg peaks which strongly resemble those found in totally disordered tetragonal YBCO doped with Al, indicating substantial disorder within these modulated regions. An intensity ?t for the optimally doped crystal reveals a pattern of modulations along the a-axis with a strong c-axis component of the Ba atoms. In the underdoped crystal, an ab initio calculation of the relaxed atoms in the modulated Ortho-V phase gives nearly perfect agreement with former experiments, leading to the unavoidable conclusion that these crystals show essentially diffusion-limited formation of imperfect Ortho phases predicted some years ago by de Fontaine and co-workers. They are thus of static origin, and have little to do with stripes, CDW’s, or soft optic phonons.
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Bi-Weekly Seminar
Higher Jc at Reduced Pinning Potential
Date: Friday April 16, 2004
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
In a decade of study it has been frequently noted that continuous columnar pinning centers provide the largest pinning potential, Upin. It has been assumed therefore that such pinning, e.g., created by ions, provides the highest values of Jc, and pinned field, Bpin. Generally, studies of columnar pinning are discontinued at pinning center densities beyond which Jc is observed to decrease. It has also been broadly noted that such decrease is due to degradation of the order parameter and the critical current, Tc. We previously reported theoretical findings that Jc and Bpin were limited by the pinning center microstructure, not Tc, and could be markedly improved by reducing the damage incurred in the formation of pinning centers, even though Upin is reduced in the process. We now report experimental results. Spatially discontinuous melting occurs for ions having Se dE/dx 0.7 ke. This becomes continuous for Se 3.5 ke. We studied the interval 1.4 Se < 4.0 ke, using parallel ions (no splay). We find Jc is higher by a factor of 6 at Se ~ 2.0 ke (intermittent columns) than at Se ~ 3.5 ke (continuous columns), despite the fact that Upin is reduced by ~ 75% at the lower Se. In addition, pinnable field is increased by factors > 10 at the lower Se. Future basic studies and application to current problems of interest will be discussed.
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