TcSUH
Bi-Weekly Seminar
Condensed Matter Physics Phenomena in Biological Systems
Date: Thursday April 21, 2005
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
This talk will give a brief overview of condensed matter physics phenomena in biological systems, including diamagnetism, quantum tunneling in electron transfer reactions, excitons, charge density waves, and (albeit speculative) proposals of biological superconductivity. I will then, time permitting, discuss some of our own research, such as the production of nonlinear harmonics by enzyme complexes and motor proteins in the plasma membrane and the inner membranes of mitochondria. At low frequencies, we use a high-[Tc] SQUID to directly probe the current response, which greatly reduces electrode polarization effects. We have been studying, in vivo, budding yeast (Saccharomyces cerevisie) and, in vitro, cytochrome c, a mitochondrial membrane protein in the respiratory chain. Also of interest are the electric and magnetic properties of tubulin, which self-assembles to form microtubules in the cytoskeleton.
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Bi-Weekly Seminar
Theory of Inhomogeneous High-Temperature Superconductivty
by: Prof. W. P. Su
Date: Thursday January 27, 2005
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Inhomogeneity is a hallmark of the high-temperature superconductors as evidenced by many experiments. A natural interpretation of that can be found in a phenomenological model of d-wave superconductivity, which is an extended Hubbard model with onsite repulsion and nearest-neighbor attractive interaction. This model gives rise to a phase diagram which is strikingly similar to the observed one in the cuprates. A central result of the model is that below a critical doping concentration, the system is unstable with respect to phase separation between the antiferromagetic state and the d-wave superconducting state. Such a state has a vanishing compressibility, therefore it is easily rendered inhomogeneous by the random dopant potentials.
As a microscopic origin of the intersite attractive force, a tight-binding version of the Little’s exciton model has been examined. Quantum Monte Carlo calculations indicate that the purely repulsive interaction between conduction electrons and exciton electrons (electronic polarization) can indeed induce phase separation and superconductivity, where are manifestations of the intersite attractive force.
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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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