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Winter 2010 Colloquium Series

View Departmental Calendar

Colloquia are at 4pm, Thursdays, in 100 Willamette Hall and are preceded by coffee, tea, and cookies at 3:40 in the Willamette Atrium.

The organizer of the Fall Term Colloquia is: Physics Search Committees

Jan 7, 2011

Eugene Lim

Department of Astronomy &
Columbia University

Non-Gaussian Probe of the Inflationary Universe

The Inflationary Paradigm has become our de facto standard picture of the Early Universe. In recent years, it has passed a series of observational tests with flying colors. More remarkably, it generically predicts the spectrum of perturbations that we see in the Cosmic Microwave Background. Despite its successes however, we do not possess a fundamental theory of Inflation, say, one that can be derived from String Theory. In fact, as I will argue in this talk, the simplest models that have been used as phenomenological placeholders are under threat from being ruled out from the latest Cosmic Microwave Background results — leading us to begin exploration of a much wider landscape of models, in particular those that are physically motivated from String Theory. This will require additional probes of Inflation. I will talk about an exciting new probe that will allow us to distinguish between the myriad models of Inflation — namely non-Gaussian statistics encoding the non-linear dynamics of the primordial perturbations.

Host: Hsu

Jan 14, 2011

Naomi Ginsberg

Lawrence Berkeley National Lab, CA

Ultrafast physics in photosynthesis: Mapping sub-nanometer energy flow

In the first picoseconds of photosynthesis, photoexcitations of chlorophyll molecules are passed through a network of chlorophyll-binding proteins to a charge transfer site, initiating the conversion of absorbed energy to chemical fuels. The remarkably high quantum efficiency of this energy transfer relies on near-field coupling between adjacent chlorophyll molecules and their interaction with protein phonon modes. Using two-dimensional electronic spectroscopy, we track the time-evolution of energy flow in a chlorophyll-protein complex, CP29, found in green plants. The results from these nonlinear four-wave mixing experiments elucidate the role of CP29 as a light harvester and energy conduit by drawing causal relationships between the spatial and electronic configurations of its chlorophyll molecules. Through independent control of experimental light pulse polarizations, we have furthermore developed a technique to determine the relative angles between the transition dipole moments responsible for energy transfer. This work not only yields tools for structural and spectral molecular characterization, but also deepens our understanding of how photosynthetic systems have evolved to optimize the conversion of light to biomass.

Host: Taylor

Jan 19, 2011

Amy C. Rowat

Department of Physics and School of Engineering & Applied Sciences,
Harvard University

Note Special Date: Tuesday

Single cell studies using microfluidic devices

Cells that are genetically identical can exhibit differences in phenotype, however, such variation remains masked in bulk measurements. To capture variability among individual cells, as well as the behavior of subpopulations of cells, requires studies with single cell resolution. Here I will describe a new class of microfluidic devices that enables studies at the single cell level. First, I will describe a microfluidic device that enables measurements of the mechanical properties of individual cells. The ability of cells to deform through narrow spaces is central in physiological contexts ranging from immune response to metastasis. To elucidate the effect of nuclear shape on the deformability of neutrophil cells, I manipulate levels of a structural protein in the nucleus, and show this alters both nuclear shape and the ability of cells to deform through the narrow channels. These results help to elucidate the mechanism underlying nuclear shape transitions, and have implications for understanding changes in the physical properties of aging cells. Second, I will describe a microfluidic device that enables tracking lineages of single cells. In most cases, heritable phenotypic variation arises from differences in DNA sequence, yet even cells that are genetically identical can exhibit variation in phenotype, which are critical during differentiation and development, and possibly in response to environmental stress. By studying the expression patterns of three, naturally regulated proteins in lineages deriving from single yeast cells, I show that the timescale of phenotypic variation differs markedly among the observed proteins; this is an essential step towards understanding the timescales of phenotypic variation, and correlations in phenotype among single cells within a population.

Host: Taylor

Jan 21, 2011

Zhiqiang (Jason) Li

Columbia University, NY

Exploring one layer of carbon atoms – graphene

Graphene, a single atomic layer of graphite, is a unique material characterized by Dirac quasiparticles with linear energy-momentum dispersion. This exotic property gives rise to numerous intriguing phenomena such as room temperature quantum Hall effects and perfect tunneling through any energy barriers. The electronic band structure of this material can be readily tailored by many methods, thus providing a wide range of opportunities for device applications. In this talk, I will present our recent investigations on graphene employing synchrotron based infrared spectroscopy. Our measurements directly demonstrated the Dirac nature of the charge carriers in graphene, and revealed several signatures of many body interactions. We further found that the coupling of two graphene sheets leads to entirely new properties in bilayer graphene. These studies have broad implications in the fundamental understanding of graphene as well as its future applications.

Host: Taylor

Jan 28, 2011

Paul Wiggins

Whitehead Institute, Cambridge MA

The physics of structuring a prokaryotic chromosome

The genetic instructions which program our cells are linearly encoded in a long polymer: DNA. This genetic information must be at once protected from damage, accessible to enzymes for the production of the protein machines which control the function of the cell, and replicated before each cell division. To achieve these often competing demands, even the simplest organisms must elaborately structure and organize their genomes. In this talk I will describe the surprising role the physical structure and the mechanics of DNA plays in the life of a virus, and in the control of gene expression in eukaryotic and prokaryotic cells.

Host: Taylor

Feb 4, 2011

Christopher Lee

UC Berkeley, CA

The Generation of Matter

The Big Bang and Inflation produced a Universe with equal amounts of matter and antimatter. How and when our Universe came to be made dominantly of matter is one of the great mysteries in modern physics.
In this talk I explore the possibility that the excess matter was generated during the electroweak phase transition, the latest possible time when such an excess could have arisen. Such a scenario requires new particles beyond those in the Standard Model, such as those predicted by supersymmetry, which may soon be discovered in the Large Hadron Collider. I also describe my collaboration’s recent work toward a more intuitive and consistent theoretical description of the evolution of particle densities in the hot and non-equilibrium environment created by the phase transition, revealing the crucial role of flavor oscillations in generating a baryon asymmetry.

Host: Hsu

Feb 8, 2011

Eric Corwin

Center for Soft Matter Research
New York University

Note Special Date: Monday

Confocal measurements of emulsions leading to a statistical model for frictionless, polydisperse packings

Jammed systems are ubiquitous, yet understanding and predicting the structure of even the simplest, monodisperse packing remains elusive. By studying a seemingly more complicated system, polydisperse packings, we have discovered a simple organizing principle for particulate packing which gives rise to quantitatively accurate predictions. We use confocal microscopy to image a frictionless, polydisperse emulsion in 3D. Using a deconvolution technique we determine the position and radius of every droplet. This information allows us to calculate the network of nearest neighbors and the local packing fraction around each droplet. Additionally, we exploit an enhanced fluorescence at the points of droplet contacts to determine the contact network. These measurements yield a complete description of the packing structure. Based on our observations we build a simple statistical model in which the complexity of the global packing is distilled into a local stochastic process. We show that, locally, the packing problem may be reduced to the random assembly of nearest neighbors, followed by a random choice of contacts among them. Our model is based on only two parameters, the available solid angle around each particle and the ratio of contacts to neighbors, which are both directly obtained from experiments. We find that this “granocentric” view captures the essential properties of the polydisperse emulsion packing, ranging from the microscopic distributions of nearest neighbours and contacts to local density fluctuations and all the way to the global packing density. This model suggests a general principle of organization for random packing.

Host: Taylor

Feb 11, 2011

Jim Schombert & Steve Hsu

University of Oregon

The Value of Hard Work: College GPA
Predictions From SAT Scores

We analyze a data set comprised of the academic records of all undergraduates entering the University of Oregon from 2000-2004. We find correlations of roughly .3 to .5 between SAT scores and upper division, in-major GPA (henceforth, GPA). Interestingly, low SAT scores do not preclude high performance in most majors. That is, the distribution of SAT scores after conditioning on high GPA (e.g., > 3.5 or even 4.0) typically extends below 1000 (the average among test takers). We hypothesize that overachievers overcome cognitive deficits through hard work, and discuss to what extent they can be identified from high school records. Only a few majors seem to exhibit a cognitive threshold — i.e., such that high GPA (mastery of the subject matter) is very unlikely below a certain SAT threshold (i.e., no matter how dedicated or hard working the student). Our results suggest that almost any student admitted to university can achieve academic success, if they work hard enough. We find that the best predictor of GPA is a roughly equally weighted sum of SAT and high school GPA, measured in standard deviation units. We also analyze the performance of UO honors college students, a selected population which resembles that of elite private colleges. Finally, we observe that 1. SAT scores fluctuate little on retest (very high reliability), 2. SAT and GRE scores (where available) correlate at roughly .75, consistent with the notion that both tests measure a relatively stable general cognitive ability, and 3. the SAT distribution of students that obtained a degree does not differ substantially from that of the entering class.

Host: Hsu

Feb 18, 2011

Susumu Takahashi

UC Santa Barbara, CA

Spin decoherence at high magnetic fields

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 present the development of the first ever free-electron laser (FEL)-based pulsed electron paramagnetic resonance (EPR) spectrometer that promises to provide unprecedented nanosecond time resolution.

Host: Taylor

Mar 2, 2011

Johan Alwall

SLAC/Stanford University, Menlo Park, CA

Please note special date: Tuesday

Dark Matter and Missing Energy Signals at Tevatron and the LHC

One of the most exciting discoveries that could be made by the LHC is weak scale Dark matter. Since Dark matter does not interact with the detector material, it would manifest itself through signals of missing energy. I will here introduce some scenarios for Dark matter production at the LHC, and techniques to help overcome the special difficulties in simulation and analysis of missing energy signals at hadron colliders.

Host: Hsu

Mar 4, 2011

Spencer Chang

UC Davis, CA

Exploring the Dark Matter Sector at Direct Detection Experiments

Dark matter direct detection experiments aim to directly discover the existence of dark matter by observing its scattering off targets in underground detectors. In this talk, I discuss how beyond discovery, these experiments can reveal much more about the dark matter sector, potentially giving evidence for a rich structure in its spectra and interactions with the Standard Model. I will review the past, present and future of the experimental effort and highlight how they impact different scenarios of dark matter. In particular, models that explain the purported dark matter signal at the DAMA experiment can be tested and discovered in the near future, which would lead to a groundbreaking insight into the fundamental nature of dark matter.

Host: Hsu

Mar 11, 2011

Leo van Hemmen

Technical University of Munich

Infrared Vision of Snakes

Two groups of snakes, boids and pit vipers, possess an infrared detection system that is used to create a heat image of their environment. We present an explicit reconstruction model, a kind of ‘‘virtual lens,’’ which explains how a snake can overcome the optical limitations of a wide-aperture pinhole camera. In plain English, the aperture is so wide that the direct image of a point source such as a soldering iron on the pit membrane is a big blob, of no practical use whatsoever. Nevertheless these snakes can localize and, more importantly, hit their warm-blooded prey quite precisely. Our model is easy to implement neuronally and agrees well with available neuronal, physiological, and behavioral data on the snake infrared system.

Host: Belitz