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Previous CIRCS seminars (2003-2002) |
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Previous Seminars
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Thursday, May 27th, 1999 "Quantum computation with mesoscopic Josephson junctions" byProf. D. Daverin, Joint Circs and Physics Colloquium, State University of New York at Stony Brook ABSTRACT: In the talk, I will give brief introduction to quantum computation and quantum dynamics of mesoscopic Josephson junctions, and discuss qubits and quantum logic gates based on these junctions. Various dephasing mechanism that limits junction performance as qubits but present interesting physics, will also be discussed.
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Tuesday, May
25, 1999 "Quantum Chaos: Ergodicity, Localization, Transport, and the Theory of Wavefunction Scarring" by Dr. Lev Kaplan, Physics Department, Harvard University ABSTRACT: The scar phenonmenon is the anomalous enhancement or suppression of quantum wavefunction intensity near the unstable periodic orbits of a classically chaotic system. It is a dramatic and visually striking example of the influence of classical structures on quantum wavefunctions and transport. We review recent progress in attaining a quantitative understanding of scarring, leading to predictions about wavefunction intensity statistics, conductances and conductance fluctuations, resonance widths and decay rates, and other experimentally relevant quantities in quantum systems which exhibit classical chaos. Strong deviations from the naive predictions of random matrix theory are predicted and observed. |
| Tuesday, May 18, 1999 "Quantum simulations on a quantum computer" by Dr. Ching Hua-Tseng Harvard-Smithsonian Center for Astrophysics, Harvard University ABSTRACT: A general scheme is presented for performing a simmulation of the dynamics of one quantum system using another. This scheme is used to experimentally simulate the dynamics of truncated quantum harmonic and anharmonic oscillators using nuclear magnetic resonance. We believe this to be the first explicit physical realization of such a simulation. The relation to decoherence will also be discussed. |
Tuesday, May 11, 1999 ABSTRACT: Alzheimer's disease (AD) is just one example of a group of biochemically diverse conditions that are disorders of protein metabolism, or more generally, of protein conformation, in which soluble polypeptides polymerize to form insoluble fibrils that accumulate as heterogeneous deposits. The amyloid fibrils associate with or may be nucleated by other proteins in the extracellular matrix or plasma of blood or cerebrospinal fluid. Not only are normal tissue structure and function compromised by the deposits, but the aggregated peptides may also be cytotoxic. The deposits may be local, confined to single organs or tissues, or distributed throughout the body. In addition to AD, examples of such disorders include AA and AL amyloidoses, the transmissible spongiform encephalopathies ("mad cow disease", scrapie in sheep, Creutzfeldt-Jakob in humans), and HuntingtonŐs disease in humans. The amyloidoses often share certain common features Đ e.g., a correlation with aging; an extracellular accumulation of the amyloid; a structural variant of a precursor protein; defective proteolytic processing; a high local concentration of the precursor protein or fragment; an amyloidogenic enhancing factor. The most common characteristic of amyloid, however, is the structural organization of its constituent polypeptide chains: namely, the (-pleated sheet conformation, and specifically the cross-( arrangement. In this type of structure, the extended polypeptide chains are running approximately perpendicular to rather than parallel to the fiber axis. There has been substantial progress in understanding the biochemistry, cell biology, and molecular biology of different amyloidoses; however, the precise mechanism of amyloid formation and details on the molecular organization and side chain arrangement within the fibrils are still unclear. To understand the formation and molecular organization of the amyloid fibrils, we have pioneered a structural neurochemical approach based on studying synthetic peptide homologues. By correlating results from fiber diffraction and electron microscopy, our approach is concerned with characterizing the interrelationships among primary sequence, peptide conformation, and morphology of the assembly. |
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Tuesday, April 27, 1999 ABSTRACT: Large nuclear spin polarizations can be created in dense samples of the spin 1/2 noble gases (3He and 129Xe) using laser optical pumping techniques. Such large spin-polarization greatly enhances the nuclear magnetic resonance (NMR) detection sensitivity of these noble gases, enabling high precision tests of fundamental symmetries, practical gas-space magnetic resonance imaging (MRI), and probes of complex system structure through measurements of restricted gas diffusion and flow. Laser polarized noble gas NMR is a powerful new tool for biomedical imaging, materials science, and fundamental physics. I will discuss the research activities of my group and collaborators in each of these areas. |
Tuesday, April 20, 1999 ABSTRACT: In a superconductor, the phase of the order parameter and the number of Cooper pairs are non-commuting conjugate variables. This leads to special phenomena in circuits of small Josephson junctions where the Josephson coupling energy (addressing phase differences) and the Coulomb charging energy for a single excess Cooper pair (addressing the number) are of comparable magnitude. We have experimentally studied fluxoids (vortex-like phase excitations) that behave as quantum particles, exhibiting interference and various localization effects. We have also demonstrated the Heisenberg uncertainty relation between phase and number, and observed quantum superpositions of charges and fluxoids. We are now investigating the possibility to use Josephson circuits for quantum computing. |
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Wednesday, April 14, 1999 ABSTRACT: Electromigration, that is, the forced motion of ions under an electric current is one of failure mechanisms in microelectronic devices. This process may induce the propagation of voids in thin metal films, whose dynamics is often described by a continuous model similar to the classical Hele-Shaw problem. As a consequence, both the shape and the velocity of voids are solutions of a free boundary problem remisniscent of the well-known Saffman-Taylor viscous instability. We present here analytical results in the strip geometry which represents an interconnect but is also the geometry of the linear Hele-Shaw cell. Two cases are investigated: first, the void is assumed to travel steadily along the axis of symmetry of the metal strip,second the void propagates along the border. Although perhaps unrealistic the first situation is very similar to viscous flow and the comparison between analogous results concerning fingers or bubbles will be explained. One notes in particular that the breakdown of analyticity of one boundary condition restricts the space of allowed solutions in the absence of surface self-diffusion. A more realistic situation concerns the distortion of borders whose asymmetry is at the origin of the complete splitting of the strip. |
Tuesday, March 30, 1999 ABSTRACT: Both in atomic physics and in mesoscopic physics it is sometimes interesting to consider the energy time-dependence of a parametrically-driven chaotic system. We assume an Hamiltonian H(Q,P;x(t)) where x(t)=Vt. The velocity V is slow in the classical sense but not necessarily in the quantum-mechanical sense. We study the crossover (in time) from ballistic to diffusive energy-spreading, and the associated irreversible growth of the average energy. The latter has the meaning of dissipation. We define a dimensionless velocity v_{PR} that determines the nature of the dynamics, and controls the rout towards quantal-classical correspondence. We distinguish between a perturbative regime and a semiclassical regime. |
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Tuesday, March 30, 1999 ABSTRACT: In the mixed state of cuprate superconductors the magnetic field is concentrated in an array of flexible flux bundles that, much like ordinary matter, can form crystalline, liquid and glassy phases. The dynamics of flux-line arrays determines the resistive properties of the materials and has therefore been the focus of much attention in recent years. Various types of glasses are also possible because of pinning in disordered samples. In particular, the introduction of columnar damage tracks by heavy ions irradiation yields a low-temperature "Bose-glass" phase, in which every vortex is trapped on a columnar defect. After reviewing the rich phase diagram of vortex arrays, I will focus on the properties of viscous flux-line liquids, where dissipation is controlled by the liquid shear viscosity. This important transport coefficient is predicted to diverge at the continuous Bose-glass transition with a universal scaling exponent. Patterned irradiation of cuprate superconductors with columnar defects allows for a new generation of experiments that probe flux flow in narrow geometries, where viscous effects are very important. I will show how an analysis of flux flow in such confined geometries that combines inhomogeneous scaling theory with the hydrodynamic of viscous vortex liquids can be used to extract viscous critical behavior near the Bose glass transition.
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Friday, March 16, 1999 ABSTRACT: Many organisms, including mammals, use small peptides as neurotransmitters. The family of FMRFamide (Phe-Met-Arg-Phe-NH2)-like neuropeptides, which all share a C-terminal RFamide sequence, have been shown to have diverse functions, including neuromodulation, cardioexcitation, and stimulation or inhibition of muscle contraction, in many organisms. In the nematode Caenorhabditis elegans, FMRFamide-like peptides (FaRPs) are expressed in over 50% of the cells in the nervous system; immunoreactive neurons include motor, sensory, and interneurons that are involved in movement, feeding, defecation, and reproduction. At least 19 genes, designated flp-1 through flp-19, encode FaRPs in C. elegans. Each flp gene encodes a different set of FaRPs, yielding a predicted total of 55 distinct FaRPs. At least 16 of these flp genes are transcribed in C. elegans, and several of these genes are alternatively spliced. Using staged RNA for reverse-transcription/polymerase chain reactions (PCR), we determined that most flp genes are expressed throughout development. To determine the cellular localization of specific flp genes, reporter constructs (with the lacZ or green fluorescent protein genes) are being made for each of the flp genes. To characterize the function of specific flp genes, we are using PCR-based screening methods to isolate deletion mutants. Thus far, animals carrying deletions in four flp genes have been isolated. However, we have only seen behavioral defects in animals in which the flp-1 gene was disrupted. flp-1 deletion animals show multiple sensory and motor deficits that can be rescued by germline transformation with the wild-type copy of the gene. Our results suggest that a complex family of FaRPs acting in different subsets of neurons have varied and sometimes overlapping functions through all stages of development and in adulthood in C. elegans. |
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Wednesday, March 10, 1999 Abstract: Current risk-management practices are based on probabilities of extreme dollar losses, e.g., Value-at-Risk measures, but these measures capture only part of the story. There are two other important factors that any complete risk-management system must address: prices and preferences. Together with probabilities, these comprise the three P's of risk management. In this talk, I will review the three P's and discuss new research directions that focus on each of them and how they interact to determine the risks that individuals and corporations bear.
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March 9, 1999 Abstract: Alzheimer's disease (AD) is just one example of a group of biochemically diverse conditions that are disorders of protein metabolism, or more generally, of protein conformation, in which soluble polypeptides polymerize to form insoluble fibrils that accumulate as heterogeneous deposits. The amyloid fibrils associate with or may be nucleated by other proteins in the extracellular matrix or plasma of blood or cerebrospinal fluid. Not only are normal tissue structure and function compromised by the deposits, but the aggregated peptides may also be cytotoxic. The deposits may be local, confined to single organs or tissues, or distributed throughout the body. In addition to AD, examples of such disorders include AA and AL amyloidoses, the transmissible spongiform encephalopathies ("mad cow disease", scrapie in sheep, Creutzfeldt-Jakob in humans), and Huntington's disease in humans. The amyloidoses typically share certain common features ‚ e.g., a correlation with aging; an extracellular accumulation of the amyloid; a structural variant of a precursor protein; defective proteolytic processing; a high local concentration of the precursor protein or fragment; an amyloidogenic enhancing factor. The most common characteristic of amyloid, however, is the structural organization of its constituent polypeptide chains: namely, the ß-pleated sheet conformation, and specifically the cross-ß arrangement. In this type of structure, the extended polypeptide chains are running approximately perpendicular to rather than parallel to the fiber axis. There has been substantial progress in understanding the biochemistry, cell biology, and molecular biology of different amyloidoses; however, the precise mechanism of amyloid formation and details on the molecular organization and side chain arrangement within the fibrils are still unclear. To understand the formation and molecular organization of the amyloid fibrils, we have pioneered a structural neurochemical approach based on studying synthetic peptide homologues. By correlating results from x-ray fiber diffraction and electron microscopy, our approach is concerned with characterizing the interrelationships among primary sequence, peptide conformation, and morphology of the assembly. In this seminar, I will focus on the polymorphism of amyloid structure. |
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March 2, 1999 ABSTRACT: Dopamine neurotransmission, particularly in the nucleus accumbens has long been thought to be the critical neural event for the expression of reward in the brain and a biological site of action for most if not all drugs of abuse. Much data supports this view. However, in the last few years, strongly contradictory data has been generated. My laboratory has contributed to both sides of this debate. A potential resolution comes from a relatively new concept borrowed from the psychobiology drug addiction research: craving. This is linked to an old concept in psychology of incentive motivation. All three parts of this emerging story will be discussed. |
February 24, 1999
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February 23, 1999 ABSTRACT: Zeolite and zeo-type materials form the backbone of the modern chemical process industry. A zeolite's effectiveness is determined by its size, cation type, and location, and the type and concentration of defects present. The ability to control zeolite chemistry to optimize effectiveness has yet to be achieved. The low-gravity environment (10-6 g) of low-earth orbit provides a unique laboratory virtually free of sedimentation and convective flows. Fifty-eight experiments crystallizing five different zeolites have been performed on the space shuttle. An initial experiment (STS-40) involved a premixed zeolite A solution. Triethanolamine (a chelating agent) was used to "stabilize" the "gel" as well as to time release alumina during nucleation and growth. Electromicrographs of both the flight and control crystals indicated extensive intergrowths, andt heir particle size distributions were identical. Terrestrial experiments on similar batch compositions suggest that nucleation had occurred prior to launch even though it was undetectable using standard XRD. When mixed on orbit (STS-50/STS-57), in all cases where the initial nucleation event was controlled, all zeolites crystallized grew larger than their ground-based controls, typically 25 percent larger for zeolite A and 50 percent larger for zeolite X. Mordenite crystals were not different in size, but they did not agglomerate (typical on earth), and they had a different morphology. Using a combination of IR, XRD, BET, XPS, and TEM, the flight crystals were generally found to have fewer lattice defects. Some zeolite A crystals appeared to be lattice defect free (SiAl = 1. 00). Particle size distributions indicated that 30 to 40 percent fewer crystals were nucleated on orbit. It was hypothesized that fewer nuclei were produced because reduced convective flows reduced the rate of solubilization of the aluminosilicate "gel" phase. To test this hypothesis terrestrially, different silica source materials of varying pore sizes were physically modified to have different surface areas and pore volumes. For the same overall mordenite batch composition, reduction in surface area (affecting the rate of solubilization) for large-pore silicas (150 A) resulted in a six-fold increase in the time until crystallization is first seen by XRD, and a concomitant increase in crystal size. These physically altered silicas were used in the next flight experiments on STS-73. Five different zeolites were crystallized on STS-73: zeolite A, zeolite X, ZSM-5, Silicalite, and zeolite P. Data from these experiments have indicated that the zeolite A and zeolite X crystals are twice as large in linear dimension as those grown on STS-50/STS-57, while their degree of "perfection" appears to be similar to the ground controls. The flight ZSM-5, Silicalite and zeolite P appeared to be free flowing crystals of various morphologies. High magnification TEM indicates that the flight zeolite P has significantly fewer point and line defects than does its ground control. Preliminary surface studies have indicated apparent differences in both the morphology and OH concentration on the space-grown crystals. These results have both increased understanding and posed new questions on zeolite nucleation and growth. |
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January 19, 1999 Abstract: Using the Gay-Berne model for liquid crystal intermolecular pair potentials, we have performed a molecular dynamics simulation of the isotropic-nematic transition following a temperature quench and investigated the resulting defect structures. Disclination loops are clearly evident, although the time behavior of the line density deviates significantly from theoretical and experimental results. The large-k exponent of the structure factor also deviates significantly from the Porod Law prediction. We believe that finite-size effects are a major cause of these deviations, and our results indicate a lower limit on system size for future simulations. |
February 16, 1999 ABSTRACT: Recent experimental
data indicate that the strengthening or weakening of synaptic connections
between neurons depends on the relative timing of pre- and postsynaptic
action potentials. A Hebbian synaptic modification rule based on these
data leads to a stable state in which the excitatory and inhibitory
inputs to a neuron are balanced, producing an irregular pattern of firing.
It has been proposed that neurons in vivo operate in such a mode. |
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