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Bi-Weekly Seminar

Electropolymerizable Dendrimer and Hybrid Nanomaterials

Prof. Rigoberto  Advincula

by: Prof. Rigoberto Advincula

Date: Thursday March 26, 2009

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In this talk, we describe the investigation of dendrimeric precursor and hybrid nanomaterials towards the formation of conjugated polymer nanoparticles, networks, nanopatterning, and nanobjects. Electropolymerizable precursor polymer materials have been widely reported by our group and have been used to modify electrode surfaces with the formation of conjugated polymer network films. Most of these materials are based on linear polymers and block/graft copolymers. Very few reports have been given on dendritic precursor materials utilized for their electrochemical activity. In this talk, we will describe several strategies in which we have synthesized dendritic precursor polymers. These materials have applications for conducting polymer-based energy transfer materials, nanoelectronics, and sensing.

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Bi-Weekly Seminar

Pseudogap State of High-Temperature Superconductors

by: Dr. Kim Wonkee

Date: Friday February 06, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

One of the most defining features of high-temperature superconductors is the pseudogap state. It is important not only because high-temperature superconductors are significantly different from conventional superconductors above the superconducting transition temperature (Tc) but also because it is related with the natures of the superconducting transition of cuprate superconductors. In this talk, I briefly review experimental observations of the pseudogap state of high-Tc superconductors. Various experiments seem to indicate that the pseudogap state exists in all high-Tc superconductors regardless of the doping level. This is an important fact for the phase diagram of the cuprates. There are two commonly believed phase diagrams. These phase diagrams lead to quite different theoretical models for the pseudogap state of high-Tc superconductors. In general, the theoretical models can be categorized into two pictures. One is the preformed-pair scenario and the other the competing gap model. I compare the two scenarios based on the basic ideas of the models. Nonetheless, the emphasis in the talk goes to the preformed-pair model because growing experimental evidence appears to favor this picture. The preformed pair model is partially motivated by a two-dimensional nature of copper-oxide planes. Unlike the conventional BCS theory, this model does not assume that the Cooper pair formation and the phase coherence take place at the same time. The pairs form above Tc while the phase is locked in via the Kosterliz-Thouless (KT) transition at the KT transition temperature. Consequently, this temperature is identified as Tc. Since the KT phenomenon can be described within the classical XY model, I explain it pictorially. Recent experiments on spatial variations of the order parameters visualized in topographic images revel local structures of the order parameters in the pseudogap state of cuprate superconductors. Within the preformed-pair scenario (also known as phase fluctuation model), we incorporate the phase fluctuations generated by the classical XY model with the Bogoliubov-de Gennes formalism utilizing a field-theoretical method. This picture delineates the inhomogeneous characteristics of local order parameters observed in high-Tc superconductors above Tc. I also present the local density of states near a non-magnetic impurity with a strong scattering potential computed based on the model. The resonance peak smoothly evolves as temperature increases through Tc as observed in a recent experiment. Possible application of the theoretical framework would be discussed.

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Bi-Weekly Seminar

Raman Spectroscopy of Ferroelectric Co3B7O13X (X=Cl,Br,I) Boracites (Phonons, X-Sublattice Instability, Raman Imaging of Twin Transformations)

Prof. Milko N. Iliev

by: Prof. Milko N. Iliev

Date: Friday January 23, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The boracites with general formula M3B7O13X (M=divalent metal, X=Cl,Br,I), shortly denoted as M-X, are the first known multiferroic materials. They exhibit a sequence of transitions from the high temperature paraelectric cubic phase to ferroelectric orthorhombic, monoclinic, trigonal phases, and finally to a monoclinic phase at low temperatures, where both ferroelectric and magnetic orders coexist. The lattice dynamics of boracites has been scarcely studied, the main problem with non-cubic phases being the coexistence of twin variants with different crystallographic and polarization orientation. We will present results of our detail temperature-dependent Raman study of Co-X and Ni-Br boracites. The spectra in the paraelectric cubic phase are analyzed in close comparison with results of ab initio (DFT) calculations of lattice dynamics. The analysis provides clear evidence for structural instability of the halogen sublattice, which triggers the ferroelectric cubic-to-orthorhombic transition. The spectra of the non-cubic ferroelectric phases of Co-Cl and Co-Br were obtained after Raman visualization of the twin variants. Using Raman microscopy imaging we were able to follow the twin-domain transformations through the crystallographic transitions, obtain Raman spectra from untwined domains in exact scattering configurations, determine the Raman mode symmetries, and assign Raman lines to definite atomic motions. The effect of elemental substitution at the X and M sites is also discussed.

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