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January 18, 2019

Cyclic Memories in Disordered Matter

Date:  Thursday, January 24th, 2019

Time: 4:00pm

Location: 100 Willamette Hall

Speaker: Nathan Keim,  Polytechnic State University

Abstract:

Cyclic driving happens all around us. Buildings and bridges are repeatedly loaded and unloaded, and temperatures change between day and night. This kind of driving can change a material, but in some cases it also forms memories that can be recalled later. I present two examples of materials that, when deformed repeatedly, can “learn” and report the magnitudes of those deformations: a suspension of particles in liquid, and a jammed solid made of closely packed particles. Their memories follow different rules, with jammed solids approximating the return-point behavior best known in magnetic materials. These materials’ disordered structure, and sometimes even the presence of noise, are essential for the fidelity of their memories.

Host: Eric Corwin

All attendees are invited to attend a colloquium reception in the Willamette Hall, Paul Olum atrium at 3:40pm.

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January 11, 2019

Exploring the Mystery of Dark Matter

Date:  Thursday, January 17th, 2019

Time: 4:00pm

Location: 100 Willamette Hall

Speaker: Rakshya Khatiwada, Fermilab

Abstract:  Very few mysteries in our current picture of the universe are bigger than the puzzle of dark matter. Recently the QCD axion — a weakly interacting, sub-eV particle — has been in the limelight as a cold dark matter candidate which also enjoys compelling theoretical motivation as a possible solution to the strong CP problem. This talk will give an overview of modern axion searches along with a detailed discussion of the most sensitive experiment to probe the QCD axion to date, ADMX. In particular, it will focus on ADMX’s recent success in reaching the so-called DFSZ sensitivity—a decade long goal sought by axion experimenters — and its newest limits covering axion mass ranges of 2.66 to 2.81 μeV. The limits ADMX has placed in this range have crucial implications for the future direction of on-going dark matter searches as I will outline.

Host: Stephanie Majewski

All attendees are invited to attend a colloquium reception in the Willamette Hall, Paul Olum atrium at 3:40pm.

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November 30, 2018

Dynamics of Colloids in Liquid Crystals

Date:  Thursday, January 10th, 2019

Time: 4:00pm

Location: 100 Willamette Hall

Speaker: Oleg D. Lavrentovich, Department of Physics, Advanced Materials and Liquid Crystal Institute, Kent State University

Abstract:

Dynamics of microparticles in fluids has fascinated scientists for centuries.  Phenomena such as electrokinetics of Janus spheres and swimming of microorganisms continue to inspire research and innovation.  The fluid in which the particles move is typically isotropic, such as water, and the dynamics is usually chaotic.  If one could learn how to control and streamline dynamics at microscale, it would open many technological opportunities.  This presentation describes how one can use an anisotropic fluid, namely, a nematic liquid crystal, to command the dynamics of living and inanimate microparticles. In nematics, molecules align along a preferred direction called the director.  A spatially-varying director field enables new mechanisms of transport and allows one to control many aspects of microscale dynamics. Three examples are considered: (i) liquid crystal-enabled electrokinetics; (ii) command of bacterial dynamics by a patterned director field; (iii) electrically powered 3D particle-like solitary waves. The main results are published in Nature 467, 947-950 (2010), Science 342, 1351-1354 (2013), Nature Comm. 5, 5033 (2014) and 9, 2912 (2018). Current work is supported by NSF grant DMREF DMS-1507637 and by the Office of Sciences, DOE, grant DE-SC0019105.

Host: John Toner

All attendees are invited to attend a colloquium reception in the Willamette Hall, Paul Olum atrium at 3:40pm.

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November 16, 2018

Spectroscopy with Josephson Junctions

Date:  Thursday, November 29th, 2018

Time: 4:00pm

Location: 100 Willamette Hall

Speaker: Çağlar Girit, Collège de France

Abstract:

Spectroscopy, by providing a direct measurement of the energy spectrum, is a powerful tool to probe matter. Conventional spectroscopy techniques suffer several drawbacks when applied to mesoscopic systems, or artificial quantum coherent atoms. I present an on-chip, Josephson-junction based spectrometer which surpasses state-of-the-art instruments, works between 2-200 GHz, and is ideally suited for probing elementary excitations in mesoscopic systems. I describe the operating principle and design of the spectrometer, show spectra of several superconducting quantum circuits, and outline experiments to investigate single quasiparticles in superconductors.

Host: Benjamín J. Alemán

All attendees are invited to attend a colloquium reception in the Willamette Hall, Paul Olum atrium at 3:40pm.

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Happy Thanksgiving

The Physics colloquium series will resume on Thursday, November 29th, 2018.

November 9, 2018

Forces and Mechanosensing in Immune Cells

Date:  Thursday, November 15th, 2018

Time: 4:00pm

Location: 100 Willamette Hall

Speaker: Arpita Upadhyaya, University of Maryland

Abstract:

Cells need to sense and adaptively respond to their physical environment in diverse biological contexts such as development, cancer and the immune response. In addition to chemical signals and the genetic blueprint, cellular function and dynamics are modulated by the physical properties of their environment such as stiffness and topography. In order to probe and respond to these environmental attributes, cells exert forces on their surroundings and generate appropriate biochemical and genetic responses. These forces arise from the spatiotemporal organization and dynamics of the cell cytoskeleton, a network of entangled biopolymer filaments that is driven out of thermal equilibrium by enzymes that actively convert chemical energy to mechanical energy. Understanding how cells generate forces and sense the mechanical environment (mechanosensing) is an important challenge with implications for physics and biology. We have investigated the principles of cellular force generation, the statistical properties of these forces, and their role in stiffness and topography sensing by immune and cancer cells. During activation, immune cells interact with structures possessing a diverse range of physical properties and respond to physical cues such as stiffness, topography and ligand mobility. We have used traction force microscopy to measure the forces exerted by T cells during activation on elastic substrates. I will discuss the distinct roles of the actin and microtubule cytoskeleton in the exertion of mechanical stresses that support signaling activation, microcluster assembly and receptor movement in T cells. We found two spatially distinct regimes of force generation, potentially arising from different actin-based structures. Furthermore, T cells are mechanosensitive, as cytoskeletal dynamics, force generation and signaling are modulated by substrate stiffness. Our recent studies have also shown that actin dynamics and signaling in B cells is modulated by subcellular topography of the antigen-presenting surface. Our work highlights the importance of cytoskeletal forces in immune cell receptor activation.

Host: Raghu Parthasarathy

All attendees are invited to attend a colloquium reception in the Willamette Hall, Paul Olum atrium at 3:40pm.

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November 2, 2018

Quantum Diamond Sensors for the Life & Physical Sciences

Date:  Thursday, November 8th, 2018

Time: 4:00pm

Location: 100 Willamette Hall

Speaker: Ronald Walsworth, Harvard University and Smithsonian Institution

Abstract:
In recent years, optically probed nitrogen–vacancy (NV) quantum defects in diamond have become a leading modality for magnetic, electrical, and temperature sensing at short length scales (nanometers to millimeters) under ambient conditions.  This technology has wide-ranging application across the physical and life sciences — from NMR spectroscopy at the scale of individual cells to improved biomedical diagnostics to understanding the formation of the solar system to the search for dark matter.  I will provide an overview of quantum diamond sensors and their diverse applications.

Host: Michael Raymer

All attendees are invited to attend a colloquium reception in the Willamette Hall, Paul Olum atrium at 3:40pm.

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October 26, 2018

Why is everything squishy?

Date:  Thursday, November 1st, 2018

Time: 4:00pm

Location: 100 Willamette Hall

Speaker: John Crocker, University of Pennsylvania

Abstract:

A broad variety of different biomaterials, foods and personal products are found to have remarkably similar mechanical properties when deformed–which might typically be called ‘squishy’.  Technically termed soft-glassy materials (SGMs), these materials include such examples as soap foams, mayonnaise, ketchup, toothpaste, as well as remarkably, the actin cytoskeleton and the chromatin in cells’ nuclei. When gently activated by internal energy sources, these SGMs display dynamic shear moduli that have a power-law frequency dependence, super-diffusive particle motion, and large cooperative particle rearrangements, or avalanches, all phenomena which are essentially unexplained. I will report recent micromechanical studies of the cytoskeleton of fibroblast that show such behavior, as well as three-dimensional tracking experiments that show similar behavior in a transparent dense emulsion–essentially clear mayonnaise. We recently constructed a minimal computational model for SGMs whose physics was determined solely by energy minimization on an energy landscape spanning a high-dimensional configuration space (Nature Materials, 15, 1031-1036, 2016). The model is essentially a wet soap foam consisting of compressible spherical bubbles, whose sizes slowly evolve due to ripening. Surprisingly, we find that the steepest-descent configuration space path is a self-similar fractal curve, resembling a river cascading down a tortuous mountain canyon. The previously unexplained SGM rheology and Lévy-like super-diffusive motion in our model stem directly from these paths’ fractal dimension and energy function. In the clear mayonnaise, we are able to show experimentally that the high-dimensional configuration space path is a fractal. The cell finding suggests that the cytoskeleton is an active network that robustly self-organizes into marginally stable mechanical state akin to that in an SGM or jammed solid.

Host: Eric Corwin

All attendees are invited to attend a colloquium reception in the Willamette Hall, Paul Olum atrium at 3:40pm.

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October 19, 2018

Quantum optimal measurements for clocks and microscopes

Date:  Thursday, October 25th, 2018

Time: 4:00pm

Location: 100 Willamette Hall

Speaker: Mark Kasevich, Stanford

Abstract:

When and how can quantum entanglement be exploited as a resource to improve measurement precision?

This talk will discuss this question in the context of two sensing scenarios:  i) precision atomic clocks [1] and ii) phase contrast optical and electron microscopy [2].  In the first case, massively entangled atomic states have been exploited to realize nearly 20 dB metrological improvement in atomic clock precision.  In the second, quantum optimal performance is obtained without entanglement, but rather through the use of a mulit-pass measurement protocol.   In either case, quantum optimal measurements show promise of enabling a new generation of sensors with at least 10-fold improved performance.  Future applications, which include include low-damage imaging of nm-scale proteins and new tests of quantum mechanics and gravitation, will be described.

[1]  Hosten, O., et al.  “Measurement Noise 100 Times Lower than the Quantum-Projection Limit Using Entangled Atoms.” Nature 529 (Jan. 2016).

[2]  Juffmann, T., et al., “Multi-Pass Microscopy.” Nature Communications 7 (Sept. 2016).

Host:  Benjamin McMorran

All attendees are invited to attend a colloquium reception in the Willamette Hall, Paul Olum atrium at 3:40pm.

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October 12, 2018

Exploring Complex Free Energy Landscapes with Innovative Monte Carlo Simulations

Date:  Thursday, October 18th, 2018

Time: 4:00pm

Location: 100 Willamette Hall

Speaker: David P. Landau, Center for Simulational Physics, The University of Georgia

Abstract:

Complexity is everywhere in nature, and it often manifests itself in the existence of a rough free energy landscape that is extraordinarily difficult to investigate.  Computer simulations have become the method of choice for studying a wide variety of systems, but traditional algorithms fail when the free energy has multiple minima and maxima that may be widely separated in phase space.  We will introduce a generic, parallel Wang-Landau Monte Carlo sampling method[1] that is naturally suited for implementation on massively parallel, petaflop supercomputers. The approach introduces a replica-exchange framework involving densities of states that are determined iteratively for overlapping windows in energy space, each via traditional Wang-Landau sampling.  The framework is valid for models of soft and hard condensed matter, including systems of biological interest.  The significant scalability, performance advantages, and general applicability of the method are demonstrated using thousands of computing cores for several quite different models of interacting particles.  Systems studied include those possessing discrete as well as those with continuous degrees of freedom, including those with complex free energy landscapes and topological constraints.

[1] T. Vogel, Y. W. Li, T. Wüst, and D. P. Landau, Phys. Rev. Lett. 110, 210603 (2013); Phys. Rev. E 90, 023302 (2014).

Host:  Jayanth Banavar

All attendees are invited to attend a colloquium reception in the Willamette Hall, Paul Olum atrium at 3:40pm.

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