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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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Bi-Weekly Seminar

Mechanisms and Detection of Biological Molecular Motors

Prof. John H. Miller

by: Prof. John H. Miller

Date: Friday October 31, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Rotary motors, including ATP synthase and the bacterial flagellar motor, play critical roles in living organisms. ATP synthase produces ATP, life's chemical currency of energy, in all three domains of life - bacteria, archaea, and eukarya. In humans, ATP synthase operates in the inner membranes of mitochondria. I will describe our recently developed electric field driven torque model of ion-driven rotary motors. The model predicts a scaling law that relates torque to the number of ion-carrying subunits in the rotor, the number of stators, and the ion motive force across the membrane. When the F0 complex of ATP synthase is coupled to F1, the model predicts a critical proton motive force below which ATP production drops to zero. In a human, such a drop in ATP would lead to unconsciousness and, eventually, death. We have also been measuring electromagnetic properties, such as impedance and harmonic responses, of live cells, mitochondria, and chloroplasts, in an effort to detect activity of active enzymes and changes in membrane potential. Dysfunction of mitochondrial enzymes has been implicated in type-2 diabetes, cancer, heart disease, Alzheimer's disease, and numerous specific mitochondrial disorders. Therefore, improved understanding of ATP synthase and other enzymes of mitochondrial respiratory chain is broadly significant to human health.

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Bi-Weekly Seminar

Electronic States and Dynamics at Semiconducting Polymer Heterojunction Interfaces

Prof. Eric  Bittner

by: Prof. Eric Bittner

Date: Friday September 05, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The optical-electronic properties of conjugated polymer-based electronic devices are acutely sensitive to the details of the intermolecular interactions and local environment. This is especially true at the interface between different semiconducting materials. In my talk I shall discuss our recent theoretical studies of OLEDS and solar cell materials based on polymer heterojunctions. Our theoretical approach combines modern quantum chemical methods based upon time-dependent density functional theory, projection operator techniques, and state of the art quantum dynamical methods for studying coupled electron/phonon systems. In my talk I shall discuss exciton breakup and recombination at interfaces as driven by phonons. I shall also talk about the possibility that interfacial triplet states may actually enhance the conversion efficiency of a heterojunction OLED device.

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Bi-Weekly Seminar

Quantum Measurement on Nano-Mechanical Resonators

Dr. Haibing H. Peng

by: Dr. Haibing H. Peng

Date: Friday July 11, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Harmonic oscillator has been well described in both classical mechanics and quantum mechanics. Recent advances in nano-fabrication technology make nano-mechanical resonator a model macroscopic system for investigating quantum behaviors in experiment, e.g., zero-point motion fluctuation. Studying the measurement (interaction) on quantum states of such macroscopic systems may lead to the achievement of ultimate sensitivity for many physical variables limited by quantum interactions. I will describe recent progress in pursuing the position detection limit governed by Heisenberg uncertainty principle and quantum back-action effects, in nano-mechanical resonators coupled to mesoscopic detectors such as single-electron transistors. I will also talk on the potentials of carbon-nanotube based devices in pushing the mechanics into quantum regime.

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