Colloquium Fall 2011
Labor Day Holiday
Atom Interferometry for Fundamental Physics and Gravity Measurements in Space
Dr. Nan Yu
Technical Group Supervisor
Jet Propulsion Laboratory
Matter-wave interferometers have shown great promises for precision measurements in fundamental physics and inertial sensing. Advances in laser cooling and manipulation of atoms have brought a new generation of atom-wave interferometers using cold atoms. The cold atom-based atom interferometers can achieve much enhanced performance when operating under the microgravity environment in space. In this talk, we will review the basic concept of light-pulse atom interferometer and its application as inertial force sensors, discuss some of possible atom interferometer applications in space, and describe JPL's efforts in developing atom interferometer gravity gradiometer towards a space-based system.
Spin Decoherence at High Magnetic Fields
Prof. Susumu Takahashi
Department of Chemistry
Spin decoherence is the process by which spins interact with their surrounding environments. In quantum science, spin decoherence is often considered an unbidden guest in the spin system that destroys quantum information. In this talk, I will show that spin decoherence is a useful tool to probe physical and chemical environments. In particular, investigation at high magnetic fields provides information on a nanometer scale with extraordinary sensitivity. I will discuss our recent demonstrations of quenching spin decoherence in nitrogen-vacancy (NV) center in diamond and S=10 Fe8 single-molecule magnets, as well as introduce a new method for distance measurements based on measurement of spin decoherence time (T2). In addition, I will talk about the development of the first ever free-electron laser (FEL)-based pulsed electron paramagnetic resonance (EPR) spectrometer.
Third Generation Photovoltaics Based on Multiple Exciton Generation in Quantum Dots
Dr. Arthur Nozik
Associate Director, NREL/Los Alamos Energy Frontier Research Center on Advanced Solar Photophysics
National Renewable Energy Lab
One potential, long-term approach to more efficient future generation solar cells is to utilize the unique properties of quantum dots (QDs) and unique molecular chromophores to control the relaxation pathways of excited states to produce enhanced conversion efficiency through efficient multiple electron-hole pair generation from single photons. We have observed efficient multiple exciton generation (MEG) in PbSe, PbS, PbTe, and Si QDs and efficient singlet fission (SF) in molecules that satisfy specific requirements for their excited state energy level structure.. We have studied MEG in close-packed QD arrays where the QDs are electronically coupled in the films and thus exhibit good transport. We have developed simple, all-inorganic QD solar cells that produce large short-circuit photocurrents and power conversion efficiencies in the 3-5% range via both nanocrystalline Schottky junctions and nanocrystalline p-n junctions. These solar cells also show QYs for photocurrent that exceed 100% in the photon regions where MEG is possible; the photocurrent MEG QYs as a function of photon energy match those determined via time-resolved spectroscopy. We have also observed very efficient SF in thin films of molecular crystals of 1,3 diphenylisobenzofuran with quantum yields of 200% , reflecting the creation of two excited triplet states from the first excited singlet state. Various possible configurations for novel solar cells based on MEG in QDs and SF in molecules that could produce high conversion efficiencies will be presented, along with progress in developing such new types of solar cells. Recent analyses of the effect of MEG or SF combined with solar concentration on the conversion efficiency of solar cells will be discussed.
Toward a Hippocampal Neural Prosthesis: Implantable Biomimetic Microelectronics to Restore Lost Memory
Dr. Theodore W. Berger
David Packard Chair of Engineering
Department of Biomedical Engineering and Neurobiology
Dr. Berger leads a multi-disciplinary collaboration with Dr. Sam Deadwyler (Wake Forest Univ.), Dr. John Granacki (USC), Dr. Vasilis Marmarelis (USC), and Dr. Greg Gerhardt (Univ. of Kentucky), that is developing a microchip-based neural prosthesis for the hippocampus, a region of the brain responsible for long-term memory. Damage to the hippocampus is frequently associated with epilepsy, stroke, and dementia (Alzheimer's disease), and is considered to underlie the memory deficits characteristic of these neurological conditions. The essential goals of Dr. Berger’s multi-laboratory effort include: (1) experimental study of neuron and neural network function -- how does the hippocampus encode information?, (2) formulation of biologically realistic models of neural system dynamics -- can that encoding process be described mathematically to realize a predictive model of how the hippocampus responds to any event?, (3) microchip implementation of neural system models -- can the mathematical model be realized as a set of electronic circuits to achieve parallel processing, rapid computational speed, and miniaturization?, and (4) creation of hybrid neuron-silicon interfaces -- can structural and functional connections between electronic devices and neural tissue be achieved for long-term, bi-directional communication with the brain? By integrating solutions to these component problems, the team is realizing a microchip-based model of hippocampal nonlinear dynamics that can perform the same function as part of the hippocampus. Through bi-directional communication with other neural tissue that normally provides the inputs and outputs to/from a damaged hippocampal area, the biomimetic model can serve as a neural prosthesis. A proof-of-concept will be presented using rats that have been chronically implanted with stimulation/recording micro-electrodes throughout the dorso-ventral extent of the hippocampus, and that have been trained using a delayed, non-match-to-sample task. Normal hippocampal functioning is required for successful delayed non-match-to-sample memory. Memory/behavioral function of the hippocampus is blocked pharmacologically, and then in the presence of the blockade, hippocampal memory/behavioral function is restored by a multi-input, multi-output model of hippocampal nonlinear dynamics that interacts bi-directionally with the hippocampus. The model is used to predict output of the hippocampus in the form of spatio-temporal patterns of neural activity in the CA1 region; electrical stimulation of CA1 cells is used to “drive” the output of hippocampus to the desired (predicted) state. These results show for the first time that it is possible to create “hybrid microelectronic-biological” systems that display normal physiological properties, and thus, may be used as neural prostheses to restore damaged brain regions.
Problems of Probalistic Evidence
School of Philosophy
Much evidence we have to compare various scientific theories is merely probabilistic, contrary to Karl Popper's dictum that falsification is the only means of gaining scientific knowledge. In most cases this doesn't cause serious problems - we know just how to apply the probabilities to determine which theory is better supported by our data. However, some controversial arguments in cosmology turn on evidence about the probability of our own existence. I will show how all three are related to a toy case much discussed recently by philosophers of probability (the "Sleeping Beauty" problem), and show that properly understanding this simpler case will shed some light on these controversial issues. Along the way, I will also discuss how the interpretation of probability relates to all of these matters.
Heavy Electrons and Superconductivity
Physics & Astronomy
Heavy electron materials are the only class of materials in which we know where to look for superconductivity. The discussion will include heavy electron physics, what sets the energy scale in these materials, and the ways in which their low temperature exotic superconductivity is a prototype for all highly correlated electron superconductivity.
Functionalizing Materials by Pulsed Laser Radiation
Materials are commonly functionalized by fashioning, alloying or doping if they are to be efficiently utilized for an application. The development of advanced lithography by the microelectronics industry has proven the power of locally functionalizing materials. As a directed energy source, lasers also have the ability for site-selective modification, with the modern counterparts having sufficient finesse and reliability to be useful in a manufacturing environment. The presentation will explore the application of controlled laser material processing to the site-selective modification of materials to imbue selected properties such as plasmonic in metals and metal-insulator-metal systems and chemical solubility, optical transmission change, the inducement of specific crystalline phase formations and the alteration of the speed of sound in a specific doped photosensitive glass ceramic. While lasers can induce these transformations, the current understanding of the ongoing photophysical processes is quite poor and is further compounded by the complexities that arise in many species systems that evolve with applied energy. Our approach has been to develop laser techniques that enhance the finesse of the exposure process and thus enable the preparation of functionalized materials over a large enough area to enable cogent analysis. Under controlled exposure conditions we attempt to correlate the resulting material transformations as a function of the initial conditions. Some photo physical evidence that this approach works will be presented albeit it is only a first order attempt to gaining insight. The ultimate goal is to be able to apply specific laser pulse script doses (i.e. photon distributions) by direct-write (i.e. mask less) patterning, onto specially developed protean materials, and thereby induce localized transformations of selected properties to allow the development of a fully integrated device that is ensconced in its host package. As a national laboratory of the US Government, The Aerospace Corporation is conducting research on means to mass produce nanosatellites (i.e. spacecraft < 10 kg mass) where the major structural material is a glass ceramic rather than metal. Lasers, direct writing patterning techniques and localized functionalization is to be used to transform the material to suite. The experimental technique, the current understanding of the photophysics and some of the consequent applications will be presented.
One Hundred Years of Superconductivity
Dr. Chandra Varma
Department of Physics & Astronomy
University of California, Riverside
Superconductivity was discovered in 1911 in Leiden and since then thousands
of metals and their compounds have shown the phenomena. Understanding the phenomena became one of the prime occupations of the leading theoretical
physicists of the times. The microscopic theory of the phenomena began only in
1957 through the work of Bardeen, Cooper and Schrieffer and the fundamentals
could be said to be all understood when Josephson predicted his effect. The
new concepts that the theory has given rise to are perhaps the most remarkable
development in Science since the discovery of Quantum-mechanics. Then twenty five years ago, superconductivity at temperatures an order of magnitude higher than previous materials was discovered in the Cuprate family of compounds raising new questions and hopes for the basis of a new technology. I will explain the basic concepts developed in understanding superconductivity both new and old in a very simple and non-technical fashion.
Synaptic Defects in Spinal Muscular Atrophy
Dr. Chien-Ping Ko
Synapses are essential to neuronal communication in the nervous system. Increasing evidence suggests that defects in synapses could contribute to many neurological diseases, including Spinal Muscular Atrophy (SMA), a leading genetic cause of infant mortality. I will first give a brief overview on how neuromuscular synapses work and then discuss our recent work showing how synaptic defects in the neuronal circuitry contribute to the motor impairment in a SMA mouse model, as well as how neuromuscular synapses could be a therapeutic target for drug development.