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October 13, 2016

Spinning, Swirling, Twisting: Adventures with Structured Electrons

Physics colloquium preview: http://physics.uoregon.edu/current_events/fall-2016-colloquium-preview/

 

Date: Thursday, October 20th, 2016

Speaker: Ben McMorran, University of Oregon

Abstract:

Electron wavefunctions with sculpted phase and distribution can provide new insights into quantum behaviour, as well as novel capabilities for electron microscopy. A new tool – nanofabricated diffractive optics – can be used to coherently manipulate electron wavefunctions, analogous to optical wavefront engineering using computer-generated holograms. We use fork grating holograms to shape freely propagating electrons into unique topological states, such as quantum vortices  [1,2]. This electron vortex state is remarkable for its helical wavefront structure and quantized orbital angular momentum (OAM). Free electrons with OAM evolve in magnetic fields in unique ways [3], can be used for nanoscale imaging [4], and provide a new probe for angular momentum at the nanoscale [5]. In addition to producing electrons with OAM, we are developing ways to measure OAM in incoherent superpositions of scattered electrons [6,7]. Investigations of topology, angular momentum, and spatial phase of free electrons has recently lead us to observations of magnetic skyrmion, topologically non-trivial distributions of electron spin, within materials [8,9].

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Click here for colloquium archive: http://physics.uoregon.edu/colloquium-archive/

 

References

[1]      J. Verbeeck, H. Tian, and P. Schattschneider, Nature 467, 301 (2010).

[2]      B. J. McMorran, A. Agrawal, I. M. Anderson, A. A. Herzing, H. J. Lezec, J. J. McClelland, and J. Unguris, Science 331, 192 (2011).

[3]      G. M. Gallatin and B. McMorran, Phys. Rev. A 86, 12701 (2012).

[4]      P. Ercius, T. Harvey, J. Pierce, J. Chess, M. Linck, and B. McMorran, Microsc. Microanal. 20, 84 (2014).

[5]      T. R. Harvey, J. S. Pierce, J. J. Chess, and B. J. McMorran, Phys. Rev. Lett. under review (2015).

[6]      T. R. Harvey and B. J. McMorran, ArXiv160603631 quant-Ph (2016).

[7]      B. J. McMorran, T. R. Harvey, and M. P. J. Lavery, ArXiv160909124 Phys. (2016).

[8]      J. C. T. Lee, J. J. Chess, S. A. Montoya, X. Shi, N. Tamura, S. K. Mishra, P. Fischer, B. J. McMorran, S. K. Sinha, E. E. Fullerton, S. D. Kevan, and S. Roy, Appl. Phys. Lett. 109, 22402 (2016).

[9]      J. J. Chess, S. A. Montoya, T. R. Harvey, C. Ophus, S. Couture, V. Lomakin, E. E. Fullerton, and B. J. McMorran, ArXiv160806000 Cond-Mat (2016).

 

October 7, 2016

Theories and Signals of a “Dark” Photon

Physics colloquium preview: http://physics.uoregon.edu/current_events/fall-2016-colloquium-preview/

 

Date: Thursday, October 13th, 2016

Speaker: Spencer Chang, University of Oregon

Abstract:

Recently, a new force mediated by “Dark” photons has been the subject of intense interest in high energy physics, both theoretical and experimental, leading to a resurgence in their experimental search with several fixed target experiments operating in the near future.  On the other hand, complementary dark photon signals at colliders are model-dependent, requiring theoretical input to determine the most interesting signals.  In this talk, after a review, I describe a class of theories where the mechanism in which the dark photon mixes with the photon predicts a new particle accessible at the LHC.  After describing the theory, I will discuss our analyses on simulated experimental data, which suggest substantial improvements on existing collider searches for dark photons that decay to electrons.

Time: 4:00-5:00pm
Location: 100 Willamette Hall

 

Click here for colloquium archive: http://physics.uoregon.edu/colloquium-archive/

 

September 30, 2016

State of the Department and LIGO Update

Physics colloquium preview: http://physics.uoregon.edu/current_events/fall-2016-colloquium-preview/

 

Date: Thursday, October 6th, 2016

Speaker: Ray Frey, University of Oregon

Time: 4:00-5:00pm
Location: 100 Willamette Hall

 

Click here for colloquium archive: http://physics.uoregon.edu/colloquium-archive/

 

 

September 14, 2016

Optical Nanofibers: A Platform for Quantum Optics

Physics colloquium preview: http://physics.uoregon.edu/current_events/fall-2016-colloquium-preview/

 

Date: Thursday, September 29th, 2016

Speaker: Luis A. Orozco, University of Maryland

Abstract:

Nanofibers produced by tapering an ordinary single mode optical fiber to diameters of half a micron are interesting optical objects. Evanescent fields, with large gradients, develop as the radius reaches less than the wavelength of light posing puzzles, questions, and opportunities. The geometry of the nanofiber mode allows for study of quantum optical effects in a one-dimensional configuration, with the preferential evanescent mode of fiber accessible when good adiabatic geometry allows high coupling back into the single mode fiber.

Recent experiments with cold Rb atoms around the nanofiber include the modification of the lifetime of the D2 line in the presence of the nanofiber and its relation to the single atom coupling. We find modification of the lifetime that depends on the alignment of the dipole with respect to the nanofiber: parallel or perpendicular. We also explore collective effects in the decay from the atomic excited that depend on the number of atoms interacting in the evanescent mode of the nanofiber. Polarimetry measurements with trapped atoms show dynamics of the trap atoms with distinctive frequencies in the radial and azimuthal direction.

Host: Michael Raymer

Time: 4:00-5:00pm
Location: 100 Willamette Hall

 

Click here for colloquium archive: http://physics.uoregon.edu/colloquium-archive/

 

 

June 3, 2016

UO Physics Colloquium Series will Resume Fall Term 2016

Fall 2016 Colloquium Preview

Colloquium Archive

 

 

May 27, 2016

Active Matter: from Colloids to Living Cells

Date: Thursday, June 2nd, 2016

Speaker: Cristina Marchetti, Syracuse

Abstract:

Systems ranging from bird flocks to bacterial suspensions to colloids propelled by self-catalytic reactions are examples of active matter – individually driven, dissipative units that self-organize in collectives with coordinated motion at large scales. In this talk I will highlight common properties of these diverse systems and describe recent progress in understanding and classifying their complex behavior using modeling and simulations.

Hosts: Eric Corwin/Marina Guenza

Time:4:00-5:00pm
Location: 100 Willamette Hall

Colloquium Archive

 

May 20, 2016

Temperature-Like Variables in Granular Materials

Date: Thursday, May 26th, 2016

Speaker:  Karen Daniels, North Carolina State University

Abstract:

Statistical mechanics has provided a powerful tool for understanding the thermodynamics of materials. Because granular materials exhibit reproducible statistical distributions which depend in simple ways on macroscopic parameters such as volume and pressure, it is tempting to create a statistical mechanics of athermal materials. I will describe a suite of experiments on two-dimensional granular materials which investigate to what extent these ideas are meaningful. For example, under agitated conditions, we measure both bulk and particle-scale dynamics, and find a number of thermal-like behaviors including diffusive dynamics, a granular Boyle’s Law with a van der Waals-like equation of state, and energy equipartition for rotational and translational degrees of freedom. However, the scarcity of free volume within a granular material provides a crucial control on the dynamics, and each of the above thermal-like behaviors is accompanied by interesting caveats. In an apparatus designed to generate a large number of static configurations, we test whether or not various temperature-like variables are able to equilibrate between a subsystem and a bath. We find that while a volume-based temperature known as “compactivity” fails to equilibrate, a stress-based temperature succeeds (with some unfortunate side-effects). This points to the importance of interparticle forces in controlling the mechanics of granular materials.

Host: Eric Corwin

Time:4:00-5:00pm
Location: 100 Willamette Hall

Colloquium Archive

May 17, 2016

Collective Dynamics, Deadly Competition, and Shape Switching in Bacterial Colonies

Date: Thursday, May 19th, 2016

Speaker:  Harry Swinney, University of Texas at Austin

Abstract:

Bacillus subtilis bacteria growing in a colony are found to exhibit large (non-thermal) number fluctuations. Also, the swimming bacteria are observed to form dynamic clusters where the orientational correlations of bacteria within a cluster are scale invariant. Studies of another rod-shaped swimming bacterium found commonly in soil, Paenibacillus dendritiformis, reveal that neighboring colonies secrete a previously unknown toxic protein, Slf, which is not secreted by the bacteria in isolated colonies. A mathematical model gives insight in to this deadly competition. Some bacteria within a colony survive by switching their shape from a long motile rod to an immobile Slf-resistant spherical shape. If these spherical bacteria later encounter sustained favorable conditions, they secrete a signaling molecule that induces a switch back to the motile rod-shaped form. The genes that encode the switching pathway are widespread among bacterial species, suggesting that this survival mechanism is not unique to P. dentritiformis.

 Host: Tristan Ursell
Time: 4:00-5:00pm
Location: 100 Willamette Hall
May 6, 2016

Vibration Propagation in Spider Webs

Date: Thursday, May 12th, 2016

Speaker:  Ross Hatton, Oregon State University

Abstract:

Due to their poor eyesight, spiders rely on web vibrations for situational awareness. Web-borne vibrations are used to determine the location of prey, predators, and potential mates. The influence of web geometry and composition on web vibrations is important for understanding spider’s behavior and ecology. Past studies on web vibrations have experimentally measured the frequency response of web geometries by removing threads from existing webs. The full influence of web structure and tension distribution on vibration transmission; however, has not been addressed in prior work. We have constructed physical artificial webs and computer models to better understand the effect of web structure on vibration transmission. These models provide insight into the propagation of vibrations through the webs, the frequency response of the bare web, and the influence of the spider’s mass and stiffness on the vibration transmission patterns.

 Host: Jim Remington
Time: 4:00-5:00pm
Location: 100 Willamette Hall
May 2, 2016

Structure and Stability in Canonical Cortical Computations

Date: Thursday, May 5th, 2016

Speaker:  Yashar Ahmadian,UO, Institute of Neuroscience, Departments of Biology and Mathematics

Abstract:

The cerebral cortex, or the gray matter, is evolutionarily the newest part of the brain, and underlies most of our intelligent behavior. After an introduction to biological neural networks and theoretical approaches to studying their dynamics, I will present my work on the dynamics of local neural circuits in the cortex. The vast majority of cortical neurons are of the excitatory type and they are highly interconnected: a typical neuron receives thousands of excitatory inputs and in turn excites other neurons. How does such a network prevent runaway activity despite this strong positive feedback? Single-neuronal biology provides one stabilizing mechanism: neurons cannot activate at indefinitely high rates for biophysical reasons.  But this still leaves open the following question: can cortical networks self-organize into a stable state with moderate activity, without relying on single-neuronal saturation? I will show that fast feedback from the minority of inhibitory neurons is generically sufficient to dynamically stabilize cortical networks even when single-neuronal nonlinearities are of the expansive, non-saturating type.
I will then explore the computational consequences of this collective inhibitory stabilization, and show that it accounts for a wide range of “contextual modulation” effects (modulations of responses of neurons to their preferred stimuli by contextual stimuli). Contextual modulation is a ubiquitous and canonical brain computation, and is an early manifestation of the global integration of sensory information that underlies higher level perception and object recognition.
I will also discuss some of the transient and time-dependent properties of such networks, and how our theory accounts for modulations of cortical oscillations and correlated ‘noise’ in the visual cortex. Time allowing, I will briefly mention my work using random matrix theory and large-N expansions to study the interplay of disordered and structured network connectivity in the dynamics of neural networks.

Host: Raghu Parthasarathy

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Colloquium Archive

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