Colloquium Fall 2009


August 31
Tangling with Entanglement: Area Laws in Condensed Matter Physics
Roger G Melko
Department of Physics & Astronomy, University of Waterloo


Entanglement is perhaps the quintessential feature of quantum mechanics that distinguishes it from classical physics. The concept is familiar to the fields of quantum information science and quantum computing, but is also increasingly playing a role in other subfields of physics. For example, it is now generally believed that most conventional quantum groundstates of condensed matter systems have entanglement properties that obey an "area law" - scaling as the boundary between entangled regions, something first discussed in the context of black hole event horizons. I will discuss the area law from a condensed matter theory perspective, including what deviations from it tell us about exotic phases and quantum critical points. The adherence of a quantum many-particle wavefunction to the area law is also known to be closely related to the ability to simulate it using classical computers - a new understanding that has lead to significant progress in addressing old problems related to hi-Tc superconductivity and frustrated magnetism.

September 7
Labor Day, University Holiday

September 14
Quantum fluctuations in small laser diodes
A.F.J. Levi
Departments of Electrical Engineering and Physics, University of Southern California


Fluctuations in electron and photon number play an important role in determining the behavior of a laser diode. For example, in large devices containing many particles, conventional Landau-Ginzburg theory of phase transitions predicts fluctuations in photon number enhance lasing below threshold. These fluctuations are of practical importance because, among other things, they contribute to the temperature dependence of laser threshold. The modern ability of nanophotonics to scale laser diode dimensions suggests one should study the role of particle fluctuations in small laser diodes. To capture the physics dominating such meso-scale devices, master equations can be used to quantize particle number. In the meso-scale regime, correlated quantum fluctuations, and the fact that a lowest energy state of the system exists, can result in suppression of lasing, enhancement of spontaneous emission, and creation of a non-Poisson probability distribution for n discrete excited electronic states and s discrete photons. Dynamic switching between two characteristic system states dominates these fluctuations.

September 21
Magnetometry and magnetic resonance on a small scale
Louis Bouchard
Department of Chemistry and Biochemistry, University of California, Los Angeles


Recent advances in magnetic measurements have allowed interrogation of volumes on smaller scales. I will give an overview of the applications of nuclear magnetic resonance technology from biomedicine to heterogeneous catalysis, and I will discuss the possibilities for nanoscale measurements. If time permits, I will discuss our latest results from an ongoing solid-state EDM experiment.


September 28
Superatoms: A Third Dimension to the Periodic Table Nanoscale Materials Using New Building Blocks
Shiv N. Khanna
Department of Physics, Virginia Commonwealth University


Size selected stable clusters have the potential to mimic the chemistry of elements in the periodic table and can be regarded as "superatoms" forming a third dimension to the periodic table. The talk will focus on this intriguing discovery and its implication for creating nanoscale materials with tunable characteristics. Specific examples illustrating how the clusters of the most easily oxidized solids can display extreme resistance to oxidation, clusters with specific geometry that can split water to generate hydrogen on demand, and clusters that display novel magnetic properties will be presented. Recent protocols that enable us to synthesize materials from the new building blocks will be discussed. Finally, the talk will present our recent applications of nanoparticles to medical areas, specifically contrast imaging and treatment of brain tumors.



October 5
Surprises at Strong Coupling and How to Cope
Clifford V. Johnson
Department of Physics and Astronomy, University of Southern California


The last six years have seen some remarkable phenomena emerging from experimental systems in diverse areas of physics - nuclear, atomic, and condensed matter - over an impressive range of temperatures (from micro Kelvin to several trillion Kelvin - 19 orders of magnitude), densities (26 orders of magnitude), and experimental scales (desktop experiments to giant particle accelerators 4 km around). These experiments have uncovered a new type of behaviour for matter, that appears at extremely strong coupling, in a variety of systems. The traditional techniques for understanding these systems have proven unequal to the task of modelling or predicting many of the properties uncovered. Perturbation theory fails, and various strong coupling techniques also fail. This talk will describe an exciting new set of powerful tools that seem to capture much of the new strongly coupled physics, and may point the way to new phenomena and insights into various important emergent phenomena at strong coupling, such as high-temperature superconductivity and quantum phase transitions.


October 12
Theoretical Investigations of Nanostructures and Biosystems at Surfaces
Rosa Di Felice
Theoretical Nanoscience, National Center for nanoStructures and bioSystems at Surfaces (S3), Istituto Nazionale per la Fisica della Materia (INFM)


The interaction of nanoscale objects with surfaces plays a paramount role in several contexts, ranging from nanomedicine (implants, toxicology) to nanotechnology (bottom-up self-assembly of integrated circuits). Relevant systems include organic matter of diverse nature adsorbed on inorganic surfaces, as well as hybrid inorganic nanoparticles composed of different materials. Theoretical investigations can substantially aid the understanding of adsorption mechanisms, in particular to answer some critical open questions: Is there an electronic coupling between the adsorbate and the substrate? How does the interaction modify the properties of the reagents? Is there any recognition and specificity? How strong is the interaction and how stable is the hybrid system? These questions are at the basis of manipulating the properties of hybrid systems at the nanoscale and of controlling the interaction of inorganic nanoparticles with living organisms. After tracing the motivations for research in this field, illustrating few systems of current interest with their potential applications and the specific open issues, I will focus on selected examples investigated in my group. I will present results obtained with different approaches (density functional theory, classical molecular dynamics and empirical modeling) and will emphasize the importance of applying a variety of complementary methods. I will also outline the current theoretical limitations and the needs for further improvements in the methods and facilities.


October 19
Shedding light on the origin of life
Mattanjah de Vries
Department of Chemistry and Biochemistry, University of California, Santa Barbara


We use a combination of laser desorption mass spectrometry and double resonant laser spectroscopy to study the properties of and the interactions between compounds that may have made up the primordial soup on an early earth. IR-UV double resonance spectroscopy allows us to explore the photophysics as well as the hydrogen bonded structures of nucleobases. Amongst our findings is the observation that the structures of nucleobases and base pairs that occur in life as we know it today exhibit photophysical properties distinctly different from those of other possible structures. The excited state dynamics of these compounds depends on electronic structure in subtle ways. Prebiotic conditions presumably included more energetic UV irradiation than is the case today. Selective UV stability may have affected the eventual makeup of critical biomolecules and the photophysics we observe today may be a relic of a prebiotic past.


October 26
Gamma-Ray Bursts as Cosmological Tools
Rosalba Perna
Department of Astrophysical and Planetary Sciences, University of Colorado at Boulder


Gamma-Ray Bursts (GRBs) are the brightest light sources in the Universe, as well as the most distant sources known. These characteristics, combined with their powerlaw spectra, make them ideal cosmological probes. In this talk I will discuss how GRBs are impacting several areas of cosmology. In particular, I will show how they can be used to trace the evolution of the mean density and clumpiness of the interstellar medium with redshift, and the properties of dust in high-z galaxies. Detection of GRBs at very high redshifts can help set constraints on the small-scale power spectrum of density fluctuations. High-resolution observations of long GRBs allow to shed light on the properties of their massive star progenitors. Statistical studies of short GRBs can improve our understanding of evolutionary binary scenarios.


November 2
Harry A. Atwater, Jr.
Applied Physics and Materials Science, California Institute of Technology

November 9
Gelation of stomach mucus and its relevance to motility of the ulcer causing bacterium
Rama Bansil
Physics Department, Boston University


In this talk, I will describe the underlying biophysical mechanisms involved in the remarkable ability of the mucus lining of the stomach for protecting the stomach from being digested by the acidic gastric juices that it secretes. These remarkable physical properties can be attributed to the presence of a high molecular weight glycoprotein found in mucus, called mucin, which forms a gel under acidic pH preventing the acid from diffusing back. A model of gelation based on the interplay of hydrophobic and electrostatic interactions will be discussed. Molecular Dynamics simulation studies of folding and aggregation of mucin domains provide further support for this model. In the second part of the talk I will address the question, "How does H. pylori, the bacterium that causes ulcers, move across the mucus layer?" Stay tuned for the surprising answer.


November 16
-WiSE Speaker. SGM 101 (Note Different Location)
Bringing our Galaxy's Supermassive Black Hole and its Environs into Focus with Laser Guide Star Adaptive Optics
Andrea Ghez
UCLA Division of Astronomy and Astrophysics, University of California, Los Angeles


The proximity of our Galaxy's center presents a unique opportunity to study a galactic nucleus with orders of magnitude higher spatial resolution than can be brought to bear on any other galaxy. After more than a decade of astrometry from diffraction-limited speckle imaging on large ground-based telescopes, the case for a supermassive black hole at the Galactic center has gone from a possibility to a certainty, thanks to measurements of individual stellar orbits. The advent of adaptive optics technology has significantly expanded the scientific reach of our high-spatial-resolution infrared studies of the Galactic center. In this talk, I will present the results of several new adaptive optics studies on (1) our current understanding of the galaxy's central gravitational potential, (2) the puzzling problem of how young stars form in the immediate vicinity of the central black hole, (3) the surprising, apparent absence of the predicted central stellar cusp around the central supermassive black hole (an essential input into models for the growth of nuclear black holes), and (4) how future large ground-based telescope may allow these studies to test general relativity and cosmological models.


November 23
Close Encounters with the Quantum Berry Phase
Hari Manoharan
Manipulation of the Atom, Department of Physics, Stanford University


If we deform a material and restore it precisely back to its starting point, our everyday intuition tells us that the material before and afterwards is identical. This is true classically, and was believed to be true quantum mechanically until recently. Even if all the atoms, electrons, and other ingredients are returned exactly to where they started, we now know that the restored material can differ from the undeformed material by nontrivial quantum mechanical phase factors. The importance of these so-called geometric or Berry phases has garnered increasing appreciation and attention in recent years. The quantum Berry phase can fundamentally alter the ground state of a system, lead to new states of quantum matter, and be exploited in quantum devices and topological quantum computing strategies. This talk will overview new experiments from our lab, employing scanning tunneling microscopy and atomic manipulation, that directly visualize Berry's phase in nanostructures, graphene, and topological insulators.


November 30
Exploring the quantum vacuum with the Casimir force
Umar Mohideen
Department of Physics & Astronomy, University of California, Riverside


The intriguing properties of the quantum vacuum and its mechanical manifestation through the Casimir force will be introduced. Our precision measurements of the normal Casimir force will be discussed. Our experiments on the geometry dependence of the Casimir force through observation of diffraction-like effects of zero point photons will also be presented. Its role in present day micro electro mechanical machines will be explained.


December 7
No Colloquium

USC Physics & Astronomy / Colloquia /

  • Department of Physics and Astronomy
  • University of Southern California
  • 825 Bloom Walk
  • ACB 439
  • Los Angeles, CA 90089-0484