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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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Distinguished Lecture Series

Are all HTS Josephson Junctions the Same?

Dr. Cathy  Foley

by: Dr. Cathy Foley

Date: Wednesday July 16, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Josephson junctions are the basis of all active superconducting electronics. Since the discovery of high temperature superconducting (HTS) materials and YBCO, in particular, a number of different methods of junction fabrication have been devised on a range of different substrates; grain boundary step-edges, bi-epitaxial, bi-crystal and ramp junctions are common examples. The properties of these junctions vary with differences in the range of critical currents and normal resistance that are achievable, their response to magnetic fields and the amount of s-or d-wave phase shifting across the junction. Superconducting electronic applications are broad ranging including SQUIDs for magnetometry, gradiometry, macroscopic quantum state formation for quantum computer qubits, and microwave and terahertz resonators and detectors. However the requirements of the Josephson junction for each of these applications are quite different. This paper will review four different Josephson junction types and what their properties are by considering the impact of the junction morphology and the substrate material on their demonstrated characteristics. We will report on various devices fabricated at CSIRO and use some data from the literature. We will show that these different junctions have different s- and d-wave contributions as well as other properties that make different junctions more appropriate for each specific application in superconducting electronics.

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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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Special Seminar

Current Carrying State and Its Implication in a D-Wave Superconductor

by: Dr. Jian-Xin Zhu

Date: Tuesday July 08, 2008

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

A fundamental property of all known superconductors is the formation of Cooper pairs in the superconducting state. A far-reaching implication of this fact is the quantization of magnetic flux in multiply connected superconducting geometries. In this talk, I will discuss the magnetic flux dependence of order parameter and supercurrent in a hollow $d$-wave superconducting cylinder. It is shown that the existence of line nodal quasiparticles in a pure $d_{x^2-y^2}$ pairing state gives rise to an $hc/e$ periodicity in the order parameter and a first-order quantum phase transition for a large system size. We demonstrate that the flux periodicity in the supercurrent is sensitive to the detailed electronic band structure and electron filling factor. In particular, we find that, in cooperation with the increase of the cylinder circumference, the $hc/2e$ periodicity can be restored significantly in the supercurrent by avoiding the particle-hole symmetry point. A similar study of a $d_{x^2-y^2}+id_{xy}$ pairing state verifies the peculiarity of unconventional superconductors with nodal structure. I will also discuss the possibility of an impurity quantum phase transition as driven by the supercurrent.

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Bi-Weekly Seminar

Bioconjugation onto Silicon Surfaces

 Chengzhi  Cai

by: Chengzhi Cai

Date: Friday June 27, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We have recently prepared robust monolayers on silicon or silicon carbide surfaces by surface hydrosilylation. We have demonstrated that these monolayers are the most protein-resistant and stable monolayers reported to date. We have also developed a method for nanopatterning on the above monolayers with 10 nm resolution, and introduced handles on the monolayers for bioconjugation. Meanwhile, we are developing "click" reaction based methods for efficient bioconjugation onto surfaces and nanoparticles. We have used the method to functionalize silicon and silicon carbide surfaces with carbohydrates. The research is relevant to the development of efficient silicon-neuron interfaces.

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