Physics Colloquia Fall 2010

(usually Fridays 2:00 PM in PS 109)

Titles link to the abstracts.

Date Speaker Title
Sep 3
Mahmoud Madani (FAU)
Sep 17
Bronson Messer (ORNL)
Sep 24
James Mcguire (FAU)
Oct 8
Christian Binek (University of Nebraska)
Oct 15
Olga Korotkova (University of Miami)
Oct 22
Thomas Angelini (University of Florida)
Nov 5
Aniket Bhattacharya (UCF and FAU)
Nov 19
Enrique Del Barco (UCF)

Colloquium Abstracts

Free Volume Study of Water Uptake in Vinylester-Carbon fiber Composite by PALS
Mahmoud Madani (Dept of Ocean and Mechanical Engineering at FAU), September 3
Carbon fiber composite consisting of vinylester resins, Derakane 8084 and 510A, were studied dry and after water exposure. Several conventional methods such as Positron Annihilation Lifetime Spectroscopy (PALS), and Scanning Electron Microscope (SEM) were used to investigate above materials. Positron annihilation lifetime spectroscopy (PALS) was used to investigate the free volume fraction and the size of the free volume voids within the polymer matrix, the results are presented here. Two method of analysis will be discussed. The second method will be a continues method of analysis that tested with simulation in comparison with natural specimens. The relative volume fraction in VE8084 resin was 35.2% and in VE510A resin was 13.8%. The free volume lifetime and intensities were determined as a function of the polymer thickness and a significant difference was observed in both resins with and without post-curing. Significant changes in the sizes of free volume cavities were observed after post-curing which were not observed in epoxy resins. The effects of water uptake in these materials were also determined by PALS. Water uptake showed a 2% change in intensity of the longer lifetime (1.85 ns) in VE8084 resin and in VE510A about 1.8%. The longer lifetime intensity in the wet composites were 17.1% in the 8084 resin and its carbon fiber composite and 7.1% in the 510A resin and its carbon fiber composite. In comparison with neat polymers resin one can estimate the percentage of fiber filled in composite (i.e.; 48% carbon fiber). For composite with 8084 resin saturated (0.33% mass gain) with seawater at 40 or 60 C, no change in the longer lifetime intensity was observed which indicates no water entered the free volume voids. For 510A resin the third lifetime intensity dropped from 7.1% to 3.9% indicating 52% of free volume filled with water in the composite only after saturation with seawater
Thermonuclear Supernovae: Understanding Turbulence and Nuclear Burning via Simulation
Bronson Messer (ORNL, National Center for Computational Sciences), September 17
Type Ia supernovae (SNe) are a class of stellar explosions that are distinguished by a lack of hydrogen in the observed spectra. Several observational campaigns for Type Ia SNe have produced strong evidence that the universe is undergoing an accelerated expansion caused by a mysterious "dark energy." This result depends on the notion that Type Ia SNe are standardizable candles -- that their intrinsic brightness can be determined using a single parameter. This calibration requires a thorough knowledge of the explosion mechanism, something which is not yet in hand, even down to the identification of the progenitor systems. I will discuss some of the issues associated with modeling these explosions and matching these simulations with observations.
Wittgenstein, Wheeler and Wallace (IYI)}
James Mcguire (FAU), September 24
A discussion of the curious relation of philosophy, mathematics and physics. IYI: Ludwig Wittgenstein, John Archibald Wheeler and David Foster Wallace. IFI = If You are Interested, as per DFW.
Isothermal electric switching of interface magnetization: A route to voltage-controlled spintronics
Christian Binek (University of Nebraska), October 8
Spintronics exploits the electronic spin degree of freedom of electrons for an advanced generation of electronic devices. Voltage-controlled spintronics is of particular importance to continue progress in information technology through reduced power consumption, enhanced processing speed, integration density, and functionality in comparison with present day CMOS electronics. Solid-state spintronic devices rely on tailored interface magnetism, enabling spin-selective transmission or scattering of electrons. Hence, controlling magnetism at thin-film interfaces is a key challenge. In this talk, I report on our interest in magnetoelectric Cr 2O 3. Evidence for a roughness-insensitive and electrically controllable ferromagnetic surface state of antiferromagnetic chromia is given with stunning consequences for electrically controlled exchange bias. After a brief introduction of exchange bias and a reminder to the linear magnetoelectric effect, I show our progress in reversible, isothermal, and global electric control of exchange bias at room temperature. It became possible by combining theoretical insights with spin-resolved ultraviolet photoemission, and magnetometry at Cr 2O 3 (0001) surfaces and interfaces. Our results promise a new route towards purely voltage-controlled spintronics and electrically controlled magnetism.
Interaction of random optical fields with natural environments
Olga Korotkova (University of Miami), October 15
The recently developed unified treatment of polarization and coherence of light has made it possible to predict the changes in statistical properties of electromagnetic waves, such as spectrum, polarization, coherence and scintillation, on interaction with any deterministic and random media. Our main point of interest in this talk is the interaction of waves with natural environments, such as atmospheric turbulence, oceanic turbulence and biological tissues. We consider how waves are transmitted through thin random layers, propagate in continuous random media and scatter from particulate media (collections of particles) on a somewhat unified manner. The applications of our theoretical foundations for communications through and sensing of natural random media are outlined.
Forces in Collective Cell Motion
Thomas Angelini (University of Florida, Dept of Mechanical and Aerospace Engineering), October 22
Individual living cells generate forces and direct their motion in well known ways. For example, planktonic bacteria swim through fluids by rapidly turning their flagella, and individual tissue cells migrate across surfaces in a cyclic process of expansion, adhesion, and retraction. These canonical types of motion, however, are not characteristic of cells within large, dense aggregates, such as bacterial colonies or the tissues of complex organisms. In this talk I will discuss tools and concepts of condensed matter physics that we have adapted to study the collectively generated forces that control multi-cellular motion within enormous cell aggregates. I will present research on bacterial biofilms, showing how they can spread by generating molecular gradients throughout the colony. I will also discuss collective motion within two-dimensional confluent sheets of mammalian tissue cells, showing how sub-cellular motions as well as multi-cellular forces, transmitted across long distances, each influence collective migration in different ways.
Application of Quantum Monte Carlo methods to many electron systems
Aniket Bhattacharya (UCF and FAU), November 5
Quantum Monte Carlo(QMC) methods are used to solve Many-Body Schr dinger equation numerically "exactly" in the sense that in certain cases no further approximations are made so that the errors in the final calculations are due to statistical uncertainties only. With faster chips, development in parallel algorithms, and new architecture of computing machines, such as, CUDA, it is likely that in the next decade QMC methods will be used more often especially when many body effects (such as electron correlations in narrow band systems) are particularly strong and QMC calculations will be possible for systems with several thousand electrons. In this talk I will provide an overview of several QMC methods such as, Finite Temperature Path Integral QMC with auxiliary Bosonic fields, Variational QMC applied to states constructed from mean field non-interacting states and applying Gutzwiller projection operators onto these states, and "exact" Green's function QMC method applied to specific condensed matter systems.
Spintronics at the single-molecule level
Enrique Del Barco (UCF), November 19
Electrical transport properties of individual molecules have received considerable attention over the last several years due to the introduction of single-electron transistor (SET) devices. The potential of single-electron transport spectroscopy for the understanding of the fundamental physics and chemistry of individual molecules has been recently demonstrated by various breakthrough experimental discoveries. For example, transport excitations associated with fundamental vibrational modes of an individual C 60 molecule in a SET have been reported. More recently, groups at Cornell and Harvard have observed the Kondo effect in individual paramagnetic molecules. Along this line, single-molecule magnets (SMMs) will provide a unique venue for probing novel aspects of the interplay between conduction electrons and molecular spin levels. I will discuss some of the phenomena expected to be observed in the conductance through a SMM-based SET. I will also describe the fabrication procedure and characterization of three terminal single-electron transistor devices utilizing Al/Al 2O 3 gate electrodes developed for these studies. The devices were patterned via multiple layers of optical and electron beam lithography. Electromigration induced breaking of the nanowires reliably produces 1-3 nm gaps between which the SMM can be situated. Preliminary results of the conductance through a Mn 4 SMM displaying the coulomb blockade effect with several excitations that bend with the magnetic field have already been obtained.