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Special Seminar

Investigations of Static and Dynamic Structural Distortions and Their Correlation with the Complex Magnetic Behavior in CMR Manganites

by: Dr. Rajeshwar P. Sharma

Date: Friday June 04, 1999

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The role of lattice distortions in the transport property of doped perovskite-type manganites, exhibitting colossal magnetoresistance (CMR), is becoming one of the cutting edge problems in condensed matter research today. Phenomena such as Jahn-Teller distortions, charge and orbital ordering, which strongly influence the phase diagram of these materials, tend to be accompanied by either or both of static and dynamic lattice distortions. Ion channeling is a unique method which provides a direct real space probe of small (< picometer) uncorrelated displacements (static and dynamic) of atoms in single crystalline materials. Using this technique a direct correlation between the dynamic structural distortions and the ferromagnetic transition in CMR doped manganite films as well as in layered double sheet (n=2) perovskite La1.2Sr1.8Mn2O7 (hole concentration x=0.4 and 0.3) systems has been found. The transport properties in these materials depend strongly on the hole concentration and R-site ionic radius. The Nd1.2Sr1.8Mn2O7 (n=2, x=0.4) system has not shown any significant structural anomaly, as in this case the ferromagnetic transition is suppressed by possible charge ordering around 90 K, rendering the material insulating with an antiferromagnetic ordering. It appears that the incoherent atomic displacements have important participation in the transport process and at the onset of the ferromagnetic phase transitions. The importance of dynamic lattice effects in the phenomena of colossal magnetoresistance is discussed.

Special Seminar

Exploring Complex Material Systems Using Continuous Phase-Diagrams and Materials Chips

by: Dr. Xiao-Dong Xiang

Date: Thursday May 27, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Conventional approach to mapping phase diagrams or exploring new materials is to make and characterize samples of discrete composition one at a time. In an effort to speed up this process, “integrated materials chips” (IMCs) and “continuous phase diagrams” (CPDs) are fabricated by thin film deposition of elemental precursors through “combinatorial masks” or linear shutters. Followed by proper annealing processes, thousands of distinct compounds or continuous ternary phase-diagrams are formed, in either polycrystalline or more often epitaxial thin film format, on a small (e.g. inch2) substrate. Various physical properties, including electrical impedance, optical and magnetic properties, of these compounds are then mapped using various imaging instruments. We are routinely applying this approach to explore and optimize existing function materials and to study materials phase diagrams. Application areas include exploring superconductors, ferroelectrics/dielectrics, electro-optical, luminescent, piezoelectric and magnetic materials. I will discuss a recent study of spin & charge stripe phases in doped Mott insulators with CPDs. I will show evidence of smetic phase and commensurate static charge orderings in perovskite manganites.

Special Seminar

Synthesis of New Organic-Inorganic Molecular Composites - A Route to Novel Functional Materials

Prof. Arnold Mejia Guloy

by: Prof. Arnold Mejia Guloy

Date: Friday April 16, 1999

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The syntheses of low-dimensional organic-inorganic hybrid materials with novel magnetic, electronic and optical properties represent new directions in solid state chemistry. These are motivated by the notion that complex systems consisting of organic and inorganic components have great potential for the creation of functional materials utilizing the wide variety of properties associated with each component. Due to the limitations presented by differences in synthesis conditions for organic molecules and inorganic solid state materials, formation of crystalline hybrid compounds by self-assembly of molecular and ionic components provides new challenges in chemical synthesis. The preparation of these materials is essentially via self-assembly of the organic and inorganic moieties into crystalline hybrid compounds. Self-assembly techniques take advantage of weak intermolecular interactions to create more complex crystal structures between organic and inorganic moieties while preserving the unique characteristics of the individual components. Studies on crystalline hybrid compounds will allow important structure property relationships among low dimensional self-assembled structures to be established. Our current research focuses on the synthesis and characterization of low dimensional crystalline organic-inorganic metal (Au, Sn, Pb, Bi) iodides with the possibility of incorporating interesting properties associated with each of the organic and inorganic moieties.

Special Seminar

A Backdoor Approach to Cuprate Superconductivity

by: Prof. Anthony J. Leggett

Date: Wednesday April 07, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

A question which has been asked with insufficient persistence concerning cuprate superconductivty is: If we assume that the driving mechanism for the transition is primarily electronic rather than phononic and is associated with a saving of repulsive Coulomb energy, then exactly where in the space of wave vector and frequency does the saving occur? I first point out that, quite irrespective of any theoretical preconceptions, this question can in principle be answered by differential EELS experiments. I then conjecture the answer: In the region of small q and midinfrared omega! This conjecture, if correct, explains in a natural way the systematics of [Tc] within the various homologous series, and moreover predicts changes, on the transition, in both the EELS and the optical spectra in the midinfrared region which are one or two orders of magnitude larger than would follow from a simple BCS picture and hence of an observable order. Implications for our general theoretical understanding of cuprate superconductivity are discussed.

Special Seminar

X-Ray Emission Spectra as a Tool for Study of Local and Electronic Structure and Characterization of Materials

by: Prof. Ernst Z. Kourmaev

Date: Tuesday April 06, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The results of study of the local and electronic structure of advanced materials by means of X-ray emission spectroscopy (XES) are presented. The small-spot XES with variable electron excitation is used for the characterization of phase distribution in depth in buried solid-solid interfaces after heat treatment. The site selective X-ray fluorescence (excited with tuneable synchrotron radiation) is used for the band mapping of in-plane and apical oxygen 2p-states in the valence band of Sr2CuO4 superconductor. The XES of impurity atoms are measured in HTSC: B, C, F, P and Ni3Al:B and their local structure is determined. The chemical reactions in polymer films induced by ion irradiation are studied with help of X-ray fluorescence measurements. The results of XES measurements of organic superconductors: (SN)x, k-(ET)2Cu[N(CN)2]Br and k-(ET)2Cu(NCS)2 are compared with UPS (XPS) spectra and band structure calculations and orbital composition of energy bands is determined.

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