Physics Colloquium Series
(Refreshments at 3:15 p.m. in 245 Fronczak)
Contact: Prof. Dejan Stojkovic, (716) 645-2017 x258;
Prof. Hao Zeng, (716) 645-2017 x243;
Prof. Xudong Hu, (716) 645-2017 x174
January Go to Feb '08 Mar '08 Apr '08 May '08
01/24/2008, Thursday 3:30 p.m., 220 NSC
FERROELECTRICITY AND SPINTRONICS AT NANOSCALE
Dr. Renat F. Sabirionov
University of Nebraska-OmahaFerroelectric materials are very promising for various technological applications such as dynamic random access memories and non-volatile binary data storage media. A continuing demand to further miniaturize electronic devices brings up a problem of the existence of ferroelectricity on a nanometre scale. Recent experimental results demonstrate that in thin films ferroelectricity persists down to film thickness of a few unit cells, which opens an avenue for novel electronic devices based on ultrathin ferroelectrics. These findings raise a question about factors controlling ferroelectricity and the nature of the ferroelectric state at the nanoscale. I will discuss a first-principles study of ferroelectric thin films placed between two metal electrodes, either SrRuO3 or Pt. The results show that bonding at the ferroelectric-metal interface imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode in thin ferroelectric films. This does not, however, quench local polarization. Although the critical thickness for the net polarization of the KNbO3 film, for example, is finite - about 1 nm for Pt and 1.8 nm for SrRuO3 electrodes - local polarization persists down to thickness of a unit cell.[1] The oppositely-oriented dipole moments at the two interfaces have opposite directions for Pt and SrRuO3 electrodes, making the potential profile asymmetric in ferroelectric junctions with different electrodes. These metal/ferroelectric/metal junctions may exhibit bi-stable resistive switching with resistances in low and high resistance states differ by orders of magnitude.[2] These results are encouraging in view of potential applications of ferroelectric tunnel junctions as a two-terminal non-volatile memory device. Using magnetic electrodes gives a possibility of forming 4-state logic devices. I will discuss the similarities and differences of such devices based on ferroelectric and electroresistive oxide tunnel junctions.[3]
1. Chun-gang Duan, R. F. Sabiryanov, W.-N. Mei, S. S. Jaswal,and E. Y. Tsymbal," Interface-Controlled Ferroelectricity at the Nanoscale", Nano Letters, 6, 483 (2006)
2. M. Ye. Zhuravlev, R. F. Sabirianov, S. S. Jaswal,and E. Y. Tsymbal, "Giant Electroresistance in Ferroelectric Tunnel Junctions", Phys. Rev. Lett. 94, 246802 (2005)
3. A. Sokolov, R.F. Sabirianov, I.F. Sabirianov, B. Doudin, "Magnetic nano-filaments as model systems for electro-resistance switching with magneto-resistance signature", submitted
01/31/2008, Thursday 3:30 p.m., 220 NSC
AUTONOMOUS NANOSCALE MOTION THROUGH CATALYSIS
Dr. Ayusman Sen
Chemistry Department, Penn. StateNanoscale moving systems are currently the subject of intense interest due in part to their potential applications in nanomachinery, nanoscale assembly, robotics, tribology, fluidics, and chemical/biochemical sensing. Most of the research in this area has focused on using biological motor proteins in artificial systems, or using "molecular motors" such as rotaxanes that are powered externally. We will demonstrate that one can build nanomotors "from scratch" that mimic biological motors by using catalytic reactions to create forces based on chemical gradients. These motors are autonomous in that they do not require external electric, magnetic, or optical fields as energy sources. Instead, the input energy is supplied locally and chemically. By appropriate design, the chemical gradients can be translated into anisotropic body and/or surface forces. Depending on the shape of the object and the placement of the catalyst, different kinds of motion can be achieved. The resulting nanomotors can, in principle, be tethered or coupled to other objects to act as the "engines" of nanoscale assemblies. Additionally, an object that moves by generating a continuous surface force in a fluid can, in principle, be used to pump the fluid by the same catalytic mechanism. Thus, by immobilizing these nanomotors, we have developed micro/nanofluidic pumps that transduce energy catalytically.
February Go to Jan '08 Mar '08 Apr '08 May '08
02/07/2008, Thursday 3:30 p.m., 220 NSC
Coaxing cells to cajoling condensates: The many uses of optical forces
Dr. Kristian Helmerson
NIST GaithersburgOptical forces, the force arising from the momentum of light, has found a wide range of applications from laser cooling of atoms to trapping of viruses and cells. My talk will describe the basic physics and techniques in the manipulation of matter with light, drawing on specific examples from my research in the use of optical forces to manipulate and study Bose-Einstein condensates, as well as cell membranes and biomolecules to address problems in biophysics and biotechnology.
02/14/2008, Thursday 3:30 p.m., 220 NSC
SEEING A WORLD IN A GRAIN OF DUST: Dust Secrets Revealed
Dr. Michael Ram
University at BuffaloI will review how my work with dust in polar ice has revealed a very strong connection to solar effects and, hence, why I believe the sun is the main engine behind climate change. As a brief appetizer, will also relate how I got into the dust business.
02/21/2008, Thursday 3:30 p.m., 220 NSC
Is the Universe Out of Tune?
Dr. Glenn Starkman
CWRUThe Cosmic Microwave Background Radiation is our most important source of information about the early universe. Many of its features are in good agreement with the predictions of the so-called standard model of cosmology -- the Lambda Cold Dark Matter Inflationary Big Bang. However, the large-angle correlations in the microwave background exhibit several closely related statistically significant anomalies compared to the predictions of the standard model. Not only is there a lack of large angle correlations, but the lowest multipoles seem to be correlated with each other, rather than statistically independent. Indeed, they also seem to be correlated with the geometry of the solar system, suggesting that what little power there is on large scales is locally not cosmologically produced. The ripples in the CMB may be the sound of the cosmic symphony, but why are the tuba and bass quietly playing the wrong music?
02/28/2008, Thursday 3:30 p.m., 220 NSC
Twistor Spinoffs for Collider Physics
Dr. Lance Dixon
SLACOver the next decade, the search for the Higgs boson and for new phenomena at short distances will be carried out by colliding protons at the Large Hadron Collider at CERN. Each proton is a complicated bound state of quarks and gluons. Feynman once described proton collisions as "smashing two Swiss watches together to figure out how they work." In recent decades, we have learned much better how the Swiss watches work, using the theory of quark-gluon interactions, quantum chromodynamics. We can predict the results of collisions at the LHC within the Standard Model, and see whether the predictions hold up, or whether new particles are required by the data. But a major bottleneck is simply in adding up Feynman diagrams. The rules are well known, yet thousands of extremely complicated diagrams have to be evaluated. The sum of all diagrams is often much simpler than the typical diagram, suggesting hidden symmetries and better ways to compute. Over the past three years, spinoffs from "twistor string theory", related to the general analytic properties of quantum-mechanical amplitudes, have led to very efficient alternatives to Feynman diagrams, which promise to help improve our ability to find new physics at the LHC.
March Go to Jan '08 Feb '08 Apr '08 May '08
03/06/2008, Thursday 3:30 p.m., 220 NSC
Conformational changes of biological macromolecules: observations and calculations
Dr. Sebastian Doniach
StanfordConnecting structure to function at resolutions approaching the atomic scale is one of the major goals of contemporary biophysicists. The observation of these changes using x-ray scattering and their interpretation in terms of the forces involved will be discussed for small functional RNA molecules. Understanding the time dependence of such changes will be discussed using a simplified description which allows for a mathematically complete representation of the resulting kinetics, with applications to enzymatic reactions
03/20/2008, Thursday 3:30 p.m., 220 NSC
Picking Apart a Complex Catalyst: Defects and Islands on Titania
Dr. Steven Lewis
University of Georgia
Solid-state catalysts are at the center of countless natural and engineered processes. They make possible numerous multi-billion dollar industries. In most cases, the catalyst itself is a complex heterogeneous material, often involving a solid substrate supporting nanostructured components. In order to better understand how these complex systems function at the microscopic level, we have selected a prototypical system, consisting of copper on the surface of titanium dioxide. Using state-of-the-art first-principles quantum calculations based on Density Functional Theory, we explore the structural phase diagram for lattice defects in this system and show how these defects are crucial for understanding how copper atoms cluster and form islands on the titania surface. These metal islands and their interface with the oxide substrate give rise to the strong catalytic activity of this class of heterogeneous materials.
03/27/2008, Thursday 3:30 p.m., 220 NSC
NANOSCALE SPECTROSCOPY WITH OPTICAL ANTENNAS
Dr. Lukas Novotny
The Institute of Optics and Department of Physics and Astronomy, University of RochesterAntennas are devices that efficiently convert localized energy to free propagating radiation, and vice versa. They are a key enabling technology in the microwave and radiowave regime but their optical counterpart is greatly unexplored.
In order to understand antenna-coupled light emission and absorption we use a single molecule as an elementary light emitting device. With an optical antenna in the form of a simple gold particle we are able to increase the emission efficiency by more than a factor of 10. However, for very short distances between particle and molecule the fluorescence yield drops drastically because of nonradiative energy transfer. A simple gold particle is not an efficient optical antenna and it can be expected that favorably designed nanoplasmonic structures will yield much higher enhancement.
Optical antennas can be employed as light sources for high-resolution optical microscopy and spectroscopy. We demonstrate vibrational (Raman scattering) and nonlinear imaging with spatial resolutions down to 10nm. Enhancing the emission efficiency of a single molecule with a particle antenna. (a) Fluorescence enhancement as a function of distance. (b) Fluorescence imaging of single molecules.
April Go to Jan '08, Feb '08 Mar '08 May '08
04/03/2008, Thursday 3:30 p.m., 220 NSC
THE SECONDARY ROLE of CO2 and CH4 FORCING in CLIMATE CHANGE: PAST, PRESENT, and FUTURE. Al Gore's CO2-Global Warming Theory and Real Facts You Are Not Even Supposed to Find Out!
Dr. Willie Soon
Harvard-Smithsonian Center for AstrophysicsA review of the recent refereed literature fails to confirm quantitatively that carbon dioxide (CO2) radiative forcing was the prime mover in the changes in temperature, ice-sheet volume, and related climatic variables in the glacial and interglacial periods of the past 650,000 years, even under the "fast response" framework where the convenient, if artificial, distinction between forcing and feedback is assumed. Atmospheric CO2 variations generally follow changes in temperature and other climatic variables rather than preceding them. Likewise, there is no confirmation of the often-posited significant supporting role of methane (CH4) forcing, which despite its faster atmospheric response time is simply too small, amounting to less than 0.2 W/m2 from a change of 400 ppb. We cannot quantitatively validate the numerous qualitative suggestions that the CO2 and CH4 forcings that occurred in response to the Milankovich orbital cycles accounted for more than half of the amplitude of the changes in the glacial/interglacial cycles of global temperature, sea level, and ice volume. Consequently, we infer that natural climatic variability notably, the persistence of insolation forcing at key seasons and geographical locations, taken with closely-related thermal, hydrological, and cryospheric changes (such as the water vapor, cloud, and ice-albedo feedbacks) suffices in se to explain the proxy-derived, global and regional, climatic and environmental phase-transitions in the paleoclimate. If so, it may be appropriate to place anthropogenic greenhouse-gas emissions in context by separating their medium-term climatic impacts from those of a host of natural forcings and feedbacks that may, as in paleoclimatological times, prove equally significant.
04/10/2008, Thursday 3:30 p.m., 220 NSC
Some facts on global warming and a likely human role
Dr. Jason P. Briner
Department of Geology, University at Buffalo
04/17/2008, Thursday 3:30 p.m., 220 NSC
Controlling light with spin coherences in semiconductors
Dr. Hailin Wang
University of OregonAn intriguing aspect of quantum mechanics is that a quantum system can be prepared in a coherent superposition state. The induced quantum coherence can lead to quantum interference. A remarkable phenomenon of quantum interference is electromagnetically induced transparency (EIT), which exploits destructive quantum interference induced by a spin coherence to render an otherwise opaque medium transparent. EIT enables one to use spin coherences to manipulate and control light. Recent successes include the realization of slow light, photon storage, and entanglement between distant quantum systems. In this talk, I will discuss recent experimental studies to explore and realize EIT by adapting concepts originally developed in atomic physics to various semiconductor systems.
04/24/2008, Thursday 3:30 p.m., 220 NSC
Connecting Cosmology and Fundamental Physics
Dr. Mark Trodden
Syracuse UniversityThe universe is composed of normal matter, dark matter and a component that is causing cosmic acceleration. The existence of all three of these components poses a challenge to fundamental physics; the nature of dark matter remains unknown, dark energy or its equivalent is a complete mystery and even baryons, which we see all around us, should have annihilated with their antiparticles long before galaxies formed. In this talk I will briefly review the evidence for each of these components before discussing the associated problems and their possible resolutions. The focus will on the importance of connecting cosmic observations with both theoretical and experimental progress in fundamental physics. In the last part of the talk I will describe some speculative ideas on the origin of cosmic acceleration.
May Go to Jan '08, Feb '08 Mar '08 Apr '08
05/01/2008, Thursday 3:30 p.m., 220 NSC
Thoughts about Animal Motion from a Physicist's Perspective: Epidemics Spread and Related Problems
Dr. V. M. Kenkre (Distinguished Professor of Physics)
University of New MexicoSimple thoughts about the motion of living objects ranging from bacteria to rodents as it affects phenomena such as the spread of epidemics, e.g., the Hantavirus and the West Nile virus, will be presented from a physicist's perspective. Spatiotemporal patterns in bacteria in a Petri dish, refugia of epidemics in a landscape, and patterns arising from nonlinear competitive interactions will be among the subjects discussed. The work has been done in collaboration with a number of field biologists and theoretical physicists and is supported by the NSF, NIH and DARPA.