Colloquia are held on Thursdays at 4pm in room 4327 (building 4) of the Stevenson Science Center unless otherwise noted. Click here for directions, or phone the department. A reception with the speaker is held at 3:30pm in Stevenson 6333.
Thursday August 30th 2018 4:00 PM
Mahmoud Parvizi, Department of Physics and Astronomy, Vanderbilt University
See One, Do One, Teach One: Graduate Physics Preparation and Conditioning via Adult Learning Principles
A student's advance from an undergraduate to a graduate physics program generally includes a subtle, yet often challenging, transition from the standard pedagogical approach of undergraduate studies to the andragogy intrinsic to graduate physics, i.e. trading the techniques of rote memorization and the Socratic method for self-directed and self-contained practices of adult learners. In this talk, a senior physics graduate student and core course tutor reviews his experience with both the traditional and nontraditional path through the physics pipeline in order to discuss hard-won lessons from his transition that remain applicable at all phases. The aim is to align these lessons with adult learning principles and impart a practical adaptation of Halsted's famous See One, Do One, Teach One" model, developed to prepare students in surgical residency programs, to one that prepares and conditions graduate students for core physics coursework requirements.
Wednesday September 5th 2018 4:00 PMNOTE, DATE CHANGE
Christopher White, Illinois Institute of Technology
A Brief History of Neutrino Physics, with an Emphasis on the Search for Sterile Neutrinos using the PROSPECT Experiment.
While much has been learned about neutrinos in the 80 years since they were first postulated to exist, many mysteries (experimental anomalies) remain. One of the open questions is whether additional neutrino species exist, specifically, a type of neutrino referred to as "sterile”. In this talk, I will provide a brief history of neutrino physics, followed by results and puzzles from recent neutrino experiments, and conclude with a description of the PROSPECT experiment. PROSPECT is a multi-phased short-baseline reactor antineutrino experiment located at the High Flux Isotope Reactor at Oak Ridge National Laboratory with a primary goal of performing a search for sterile neutrinos.
Thursday September 13th 2018 4:00 PMWENDELL HOLLADAY LECTURE
Brad Roth, Department of Physics, Oakland University
The Physics of Mechanotransduction: How Biological Tissue Responds to Mechanical Forces
Mechanotransduction is the mechanism by which mechanical forces cause biological tissue to grow and remodel. Mechanotransduction can arise from the coupling of the intracellular cytoskeleton to the extracellular matrix by integrin proteins in the cell membrane. A complete description of mechanotransduction requires this idea be expressed using a mathematical model. The mechanical bidomain model treats tissue as a macroscopic continuum, yet accounts for microscopic forces acting on integrins. The model’s central hypothesis is that forces on integrins arise from differences between intracellular and extracellular displacements. This model provides a different view of mechanotransduction than do traditional biomechanics models that do not differentiate between the intra- and extracellular spaces and do not predict forces on integrins. In this talk, I will introduce the bidomain model and use it to interpret experimental data. The model describes the growth of engineered tissue, the remodeling of cardiac tissue around a region of ischemia in the heart, and the differentiation of stem cells in growing cell colonies. This model may impact fields as diverse as development, wound healing, and tumor growth. It is an example of how a simple model grounded in fundamental physics can provide new insights into biological phenomena.
Thursday September 20th 2018 4:00 PM
Michael McCracken, Washington and Jefferson College
Repackaging the physics major for inclusion
Data recently presented by the American Institute of Physics suggest that though undergraduate and graduate programs have made some gains in attracting and promoting students from under-represented minorities, the significant gap in representation remains. In response, the Physics Department at Washington and Jefferson College has spent the last five years on a comprehensive repackaging of its curriculum to meet the expectations, needs, and preparations of students from a variety of backgrounds. I will present the motivation and framework for these changes, which are characterized by an increased emphasis on experiential learning, development of professional skills, and promotion of invisible dimensions of student diversity. The largest revisions appear in the second-year courses, emphasizing technical writing and scientific computation. I will also describe several shifts in departmental culture that have attended this curriculum review, and present preliminary enrollment and outcome results.
Thursday September 27th 2018 4:00 PM
Kenneth Brown, Duke University
Quantum Computation with Trapped Ions
Quantum computers promise to solve certain mathematical and scientific problems exponentially faster than standard computers. The challenge is building a device that is sufficiently well-controlled to achieve this goal. In this talk, I will describe the basics of ion trap quantum computation and the prospects for constructing an error corrected qubit from trapped ions.
Thursday October 4th 2018 4:00 PM
Chong-Yu Ruan, Department of Physics and Astronomy, Michigan State University
Imaging thermal and quantum phase transitions with femtosecond coherent electron beams
The self-organization of matters close to a critical point of a continuous phase transition is relatively well understood at near equilibrium conditions. Studying the non-statistical responses of matters driven towards a thermal or quantum phase transition is an outstanding problem. Such problem has implications in the evolution towards quark-gluon plasma following big bang in the early universe and in the heavy-ion collision experiments. The associated nonequilibrium processes could also be directly responsible for the creation of hidden phases discovered recently in several quantum materials. We show that with femtosecond coherent electron pulses created with a new type of ultrafast electron microscope, we can image the macroscopic thermal and interaction-driven phase transitions of correlated electron phases where the dynamical scale-invariant behavior signifies the presence of nonthermal critical points on the excited energy landscape. Such new light-induced critical phenomena provide an alternative platform for studying nonequilibrium many-body quantum physics besides the so elegantly demonstrated recent cold-atom quantum microscopy experiments in a similar context. We will discuss several nontrivial non-statistical physical processes thus obtained, involving prethermalization, noncollinear symmetry breaking, and the formation of novel topological phases in 2D quantum materials with technological implications.
Hosts:N. Tolk and K. Varga
Thursday October 11th 2018 4:00 PM
Qi Zhang, Argonne National Laboratory
Exploring quantum optics and spintronics with terahertz light
As the last frontier of the electromagnetic spectrum, terahertz (THz, 1012Hz) radiation becomes an emergent powerful tool in probing various collective excitations in condensed matter systems. Novel light-matter interaction properties in the THz range make it possible to realize unconventional quantum optics phenomena. Meanwhile, by probing the spin and charge dynamics down to sub-ps time scale, THz spectroscopy also provides valuable insights on ultrafast spintronics. In this talk, we will introduce our recent progress in applying THz spectroscopy to quantum optics and spintronics studies of two-dimensional (2D) systems. First, we will demonstrate the collective Rabi splitting with 2D Landau polaritons inside terahertz cavities. We achieved ultrastrong light-matter interaction between Landau level transitions and THz cavity photons. Large vacuum Bloch Siegert shift is unambiguously observed. In the second part, we utilized THz emission spectroscopy to demonstrate and further control sub-ps spin-charge conversion processes at various 2D spintronic interfaces. Its application will be discussed.
Thursday October 18th 2018 4:00 PMFall Break
Thursday October 25th 2018 4:00 PMFRANCIS G. SLACK LECTURE
Paul Corkum, University of Ottawa and National Research Council of Canada
Attosecond pulses generated in gases and solids
Attosecond pulses are generated by electrons that are extracted from a quantum system by an intense light pulse and travel through the continuum under the influence of the electric field of the light. Portions of each electron wave packet are forced to re-collide with its parent ion after the field reverses direction. Upon re-collision, the electron and ion can recombine, emitting soft X-ray radiation that can be in the form of attosecond pulses. This highly nonlinear process occurs in atoms, molecules and solids and offers unique measurement opportunities – for measuring the attosecond pulse itself; the orbital(s) from which it emerged; and the band structure of material in which the wave packets moved.
Host: K. Varga
Thursday November 1st 2018 4:00 PM
Kandice Tanner, Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health
Probing the role of tissue biophysics in metastasis
Tumor latency and dormancy are obstacles in effective treatment of cancer. In the event of metastastic disease, emergence of a lesion can occur at varying intervals from diagnosis and in some cases following successful treatment of the primary tumor. Genetic factors that drive metastatic progression have been identified, such as those involved in cell adhesion, signaling, extravasation and metabolism. Is there a difference in strategy to facilitate outgrowth? Why is there a difference in latency? One missing cue may be the role of tissue biophysics of the organ microenvironment on the infiltrated cells. Here, I discuss optical tweezer based active microrheology in efforts to study the mechanical cues that may influence disseminated tumor cells in different organ microenvironments. I further discuss in vitro and in vivo preclinical models such as 3D culture systems and zebrafish in efforts of understanding the earliest stage of organ colonization.
Thursday November 8th 2018 4:00 PM
Stanislav Y. Shvartsman, Department of Molecular Biology, Princeton University
Collective dynamics of growing cell trees
Clusters of cells connected by stable intercellular cytoplasmic bridges played a key role during the emergence of multicellularity and continue to serve critical functions in present day organisms. Our research uses Drosophila egg development as an experimental model that provides unmatched opportunities for quantitative studies of this important class of multicellular systems. Drosophila oogenesis relies on two types of cell clusters with stable cytoplasmic bridges: a germline-derived cluster containing the future oocyte and 15 nurse cells, and somatic cell clusters in the epithelium that envelops the germline cluster. We identified collective dynamics in both of these clusters. First, we discovered that cells in the germline cluster grow in groups defined by the cluster’s connectivity. Second, we showed that somatic cell clusters display strong clonal dominance, a commonly observed, yet poorly understood effect during developmental tissue growth. Our experimental and theoretical studies suggests that both of these effects may be described using mathematical models that take the form of dynamical systems on tree-like networks.
Host: W. Holmes
Thursday November 15th 2018 4:00 PM
Jamie Nagle, Department of Physics, University of Colorado
Pushing out of our comfort zone for quark-gluon plasma formation
The quark-gluon plasma is a high temperature (> a trillion Kelvin) state of matter where quarks and gluons are no longer confined inside hadrons. This plasma is studied in the laboratory via nuclear collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider, and displays some remarkable properties including near perfect fluidity. Recent experiments have revealed similar fluidity signatures in collisions of small systems, including proton-proton and proton/deuteron/helium-nucleus reactions. These challenge our understanding of the requirements necessary for plasma formation and the applicability of hydrodynamic modeling.
Thursday November 22nd 2018 4:00 PMThanksgiving
Thursday November 29th 2018 4:00 PM
Gianmarco Pinton, University of North Carolina at Chapel Hill, and North Carolina State University
Ultrasound imaging and nonlinear propagation: applications to traumatic brain injury and super-resolution
The soft tissue of the human body supports both fast acoustic waves (1540 m/s) and slow shear waves (2 m/s). At large amplitudes these waves exhibit nonlinear behavior, such as harmonic development and shock formation. We develop models and simulation tools that describe the physics of nonlinear acoustic propagation, attenuation, and scattering in highly realistic representations of the human body. We use these models to develop new ultrasound imaging methods. For example, to understand brain motion during the rapid events associated with traumatic injury, we have developed a new high frame-rate (10,000 images/second) imaging technique that meaures image brain motion down to the micron level. By interrogating this spatio-temporal regime, we have discovered that destructive shear shock waves form and propagate deep inside the brain. High ultrasound frame-rates, in combination with micro-bubble contrast agents, can also be used to generate super-resolved images. We demonstrate how this technique can image vessels as small as 200 microns transcranially at a depth of several centimeters and how blood velocity can be mapped within these microvessels.
Thursday December 6th 2018 4:00 PM
Stephen Leone, Departments of Chemistry and Physics and Lawrence Berkeley National Laboratory
Attosecond Probing of Core-Level Dynamics in Solids
A new method of probing solid-state materials involves laser pump-probe measurements with extreme ultraviolet attosecond light pulses, which interrogate core level transitions. The simple act of charge transfer from one atom to another or excitation of the band gap in a solid unveils many fundamental aspects to be explored, in the quest to measure ever-shorter time processes. These include the extremely fast processes of core-level screening and broadening, coherences, and scattering, as well as electron configuration rearrangements. Examples in this presentation include charge transfer in metal oxides, core-level excitons in 2D metal dichalcogenides, insulator-to-metal transitions, and strong-field induced Floquet Bloch bands. Lifetimes, scattering, and electronic coherences, as well as theoretical comparisons, will be discussed. Coherent dynamics measurements in the extreme ultraviolet provide a novel and powerful probe for nonequilibrium states of matter.
Host: K. Varga
Thursday January 10th 2019 4:00 PM
Sohrab Ismail-Beigi, Department of Applied Physics, Physics, and Mechanical Engineering and Materials Science, Yale University
Two examples of picoscale materials engineering in transition metal oxides
The atomic-scale structure and the bonding topology in a material determines its resulting physical properties. Alterable or reversible bond distortions at the picometer length scale in turn modify a material’s electronic configuration and can create interesting physical and functional properties. Picoscale bond perturbations represent the ultimate length scale for materials engineering:* any smaller, and the effects are too small to matter; any larger, and the bonds are completely broken so one is describing a different material. I will describe, using first principles theory together with parallel experimental results from my Yale collaborators, two examples where we can understand and/or design picoscale distortions in 3d transition metal oxides in order to control electron transport or relative orbital energies and occupancies. The first system is an oxide/oxide ferroelectric mobility-effect device (not field effect), while the second is an artificially designed oxide superlattice that achieves strong orbital polarization and strong antiferromagnetic inter-layer coupling. * Ismail-Beigi, Walker, Disa, Rabe, and Ahn, “Picoscale materials engineering,” Nature Reviews Materials 2, 17060 (2017).
Thursday January 17th 2019 4:00 PM
Piran R. Kidambi, Department of Chemical and Biomolecular Engineering, Vanderbilt University
Atomically thin membranes and barriers from 2D materials
Atomically thin 2D materials have been extensively researched for electronic applications and synthesis efforts have focused on minimizing defects and obtaining larger single crystals. However, 2D materials offer transformative opportunities as ultra-thin barriers and membranes for molecular separations. Pristine graphene and h-BN are impermeable to species larger than protons but the introduction of nanoscale defects in the 2D material lattice allows for the creation of size-selective nanoporous atomically thin membranes. Here, I will discuss advances in 2D material synthesis and integration/processing routes to realize i) large-area atomically thin gas barriers, ii) fully functional nanoporous atomically thin membranes for dialysis based molecular separations, iii) novel approaches for in-situ growth of nanopores in 2D materials, and iv) the development of methods to probe sub-nanometer to nanometer defects over centimeter scale single crystalline 2D materials. Specifically, I will focus on the role of defects and associated engineering challenges with quality and scalability for electronics vs membrane applications.
Tuesday January 22nd 2019 4:00 PMSpecial Colloquium
Jonathan Trump, University of Connecticut
Charting a Course for Multimessenger Astronomy: Mapping the Census of Supermassive Black Holes
The past 20 years have revealed that supermassive black holes play an essential role in the formation and growth of galaxies. But a reliable census of supermassive black holes over cosmic time has remained elusive, and it is this census that future gravitational wave missions need to interpret the gravitational map of the sky. With the advent of two new emphases in astronomical surveys: industrial-scale time-domain monitoring, and massively multiplexed spatially resolved spectroscopy, a supermassive black hole census is within reach. The pioneering new SDSS-RM project is now vastly expanding the number of supermassive black holes with reliable mass measurements through time-domain echo-mapping. Beyond mass, SDSS-RM is also starting to enable the first survey measurements of the other two fundamental black hole quantities: accretion rate and spin. I will also show how Hubble WFC3 grism spectroscopy spatially resolves a population of nuclear black holes that are otherwise missed due to host galaxy dilution. CANDELS/3D-HST grism data uniquely reveal the black hole content of low-mass hosts, discriminating between models of black hole formation at cosmic dawn. I will conclude by looking forward to the next generation of observatories: SDSS-V and LSST for a new time-domain frontier of black hole mass, accretion, and spin, JWST / CEERS and WFIRST for a new spatially resolved frontier of black hole seeds, and LISA and 3G for a brand-new gravitational wave window onto black hole formation and evolution.
Thursday January 24th 2019 4:00 PM
Jessie Runnoe, University of Michigan
Quasars in the age of time-domain astronomy
Black hole feeding, visible as quasars, is a critical ingredient in many fields from galaxy evolution to multi-messenger gravitational wave astrophysics. Despite their prodigious luminosities, the important emission regions surrounding the supermassive black hole – the accretion disk and broad line region – cannot be imaged directly because of their small angular sizes. Because quasars are intrinsically variable phenomena, time-domain spectroscopy is a powerful tool for revealing their nature. Single-epoch spectroscopy has been a workhorse for building the modern picture of quasar central engines. Extending this to the time domain promises new insights and exotic discoveries. I will describe two examples from my work on quasars in the time domain: an observational search for supermassive black hole binaries, an expected but unobserved product of galaxy evolution, and changing-look quasars, newly observed rapid transitions between "quasar-like" and "galaxy-like" spectral states. With ongoing and new facilities like the Sloan Digital Sky Survey and Large Synoptic Survey Telescope, the future is bright for our understanding of quasars in the upcoming era of time-domain astronomy.
Thursday January 31st 2019 4:00 PM
Chiara Mingarelli, Center for Computational Astrophysics at the Flatiron Institute
Probing supermassive black hole mergers with pulsar timing
Galaxy mergers are a standard aspect of galaxy formation and evolution, and most (likely all) large galaxies contain supermassive black holes. As part of the merging process, the supermassive black holes should in-spiral together and eventually merge, generating a background of gravitational radiation in the nanohertz regime. An array of precisely timed pulsars spread across the sky can form a galactic-scale gravitational wave detector in this band. I describe the current efforts to develop and extend the pulsar timing array concept, together with recent limits which have emerged from international efforts to constrain astrophysical phenomena at the heart of supermassive black hole mergers.
Tuesday February 5th 2019 4:00 PM
Stephen Taylor, CalTech
Frontiers Of Multi-Messenger Gravitational-Wave Astrophysics
The bounty of gravitational-wave observations from LIGO and Virgo has opened up a new window onto the warped Universe, as well as a pathway to addressing many of the contemporary challenges of fundamental physics. I will discuss how catalogs of stellar-mass compact object mergers can probe the unknown physical processes of binary stellar evolution, and how these systems can be harnessed as standard distance markers (calibrated entirely by fundamental physics) to map the expansion history of the cosmos. The next gravitational-wave frontier will be opened within 3-6 years by pulsar-timing arrays, which have unique access to black-holes at the billion to ten-billion solar mass scale. The accretionary dynamics of supermassive black-hole binaries should yield several tell-tale signatures observable in upcoming synoptic time-domain surveys (like the Large Synoptic Survey Telescope), as well as gravitational-wave signatures measurable by pulsar timing. Additionally, pulsar-timing arrays are currently placing compelling constraints on modified gravity theories, cosmic strings, and ultralight scalar-field dark matter. I will review my work on these challenges, as well as in the exciting broader arena of gravitational-wave astrophysics, and describe my vision for the next decade of discovery.
Thursday February 7th 2019 4:00 PM
Carl Rodriguez, MIT
From Stellar Dynamics to Compact Binaries: Unlocking the Future of Gravitational Waves
Since the first detection over three years ago, gravitational waves have promised to revolutionize our understanding of compact objects, binary evolution, general relativity, and cosmology. But to make that a reality, we need to understand how and where these relativistic binaries form. In this talk, I will describe the various astrophysical pathways for creating the binary mergers detected by LIGO/Virgo, and how specific features of the gravitational waves (such as the binary eccentricities and black hole spins) can shed light on the formation of these dark remnants. I will show how simple gravitational dynamics makes the centers of dense star clusters, particularly globular clusters, uniquely efficient at producing merging binaries. Finally, I will talk about the future of the field, and how gravitational-wave astronomy is poised to offer us unprecedented insights into physics, astrophysics, and cosmology over the coming years and decades.
Friday February 15th 2019 2:00 PMTO BE HELD IN 4309 STEVENSON CENTER-NOTE DATE, TIME AND PLACE CHANGE
Tana Joseph, University of Manchester
Multi-messenger studies of binary stellar systems
Xray binary systems are excellent probes of accretion physics and late stage binary stellar evolution, but until recently, they could only be detected in the nearby universe. Gravitational wave observations of merging binary neutron stars and black holes with LIGO have dramatically expanded the volume in which binaries can be found, and also showcased new types of binary systems. By combining population information from electromagnetic Xray binary studies we can learn more about the progenitors of LIGO sources. This multi-messenger approach can serve to constrain major unknowns in binary population synthesis, such as the distribution of mass ratios and binary formation history. We will discuss how future flagship radio surveys with the Square Kilometer Array can uncover potential gravitational wave sources of interest to the next generation of gravitational wave observatories.
Thursday February 21st 2019 4:00 PM
David Furbish, Department of Earth and Environmental Sciences, Vanderbilt University
Rarefied granular gas behavior and the sport of boulder rolling (trundling)
We describe the probabilistic physics of rarefied particle motions and disentrainment on rough hillslope surfaces. The particle energy balance involves gravitational heating with conversion of potential to kinetic energy, frictional cooling associated with particle-surface collisions, and an apparent heating associated with preferential deposition of low energy particles. Deposition probabilistically occurs with frictional cooling in relation to the distribution of particle energy states. The dimensionless Kirkby number Ki, the ratio of gravitational heating to frictional cooling sets the basic deposition behavior and the form of the probability distribution fr(r) of travel distances r. With non-isothermal conditions this distribution may be truncated with rapid thermal collapse at small Ki, it may possess finite mean and variance with moderate Ki, or it may be heavy-tailed with large Ki. For isothermal conditions where frictional cooling matches gravitational heating plus the apparent heating due to deposition, the distribution fr(r) is exponential. The formulation provides key elements of the entrainment forms of the particle flux and the Exner equation of mass conservation, and it clarifies the mechanisms of particle-size sorting on large scree slopes. Future Martians likely will have far more fun than Earthlings in the sport of boulder rolling (trundling.
Thursday February 28th 2019 4:00 PMGUY and REBECCA FORMAN LECTURE
Dianna Cowern,The Physics Girl
Physics Girl: Where Physics Education Meets Cat Videos
YouTube was originally perceived as an entertainment medium to watch pets, gaming, and music videos. In recent years, educational channels have gained momentum on the platform, some garnering millions of subscribers and billions of views. The Physics Girl YouTube channel is an educational series with PBS Digital Studios created by Dianna Cowern. Using Physics Girl as an example, this talk will examine what it takes to start a short-form educational video series. We will look at the channel’s demographical reach, best practices for effective physics outreach, and survey how online media and technology can facilitate good and bad learning. This talk will show how videos are a unique way to share science and enrich the learning experience, in and out of a classroom.
Thursday March 7th 2019 4:00 PMSpring Holidays
Thursday March 14th 2019 4:00 PM
Michael Lubell, City College of New York
Navigating the Maze
Science and the technologies it has spawned have been the principal drivers of the American economy since the end of World War II. Today, economists estimate that a whopping 85 percent of gross domestic product (GDP) growth traces its origin to science and technology. The size of the impact should not be a surprise, considering the ubiquity of modern technologies. Innovation has brought us the consumer products we take for granted: smart phones and tablets, CD and DVD players, cars that are loaded with electronics and GPS navigating tools and that rarely break down, search engines like Google and Yahoo, the Internet and the Web, money-saving LED lights, microwave ovens and much more. Technology has also made our military stronger and kept our nation safer. It has made food more affordable and plentiful. It has provided medical diagnostic tools, such as MRIs, CT scanners and genomic tests; treatments for disease and illness, such as antibiotics, chemo-therapy, immunotherapy and radiation; minimally-invasive procedures, such as laparoscopy, coronary stent insertion and video-assisted thoracoscopy; and artificial joint and heart valve replacements. None of those technological developments were birthed miraculously. They owe a significant part of their realization to public and private strategies and public and private investments. Collectively the strategies and investments form the kernel of science and technology policy. Navigating the Maze is a narrative covering more than 230 years of American science and technology history. It contains stories with many unexpected twists and turns, illustrating how we got to where we are today and how we can shape the world of tomorrow.
Thursday March 21st 2019 4:00 PM
Steven Yalisove, University of Michigan
Driving extraordinary diffusion with an ultrafast laser
Ultrafast laser irradiation can push a material into very extreme states, far from equilibrium. During the rarefied time scales of hundreds of femtoseconds to 1-20 picoseconds a semiconductor can become metallic, atoms can exhibit rms excursions from their lattice positions approaching half of a unit cell, and the electron temperatures can easily rise to over 15,000 degrees Kelvin. What is exciting is that these phenomena can occur at fluences below the melt threshold. The vast majority of studies to date have neither studied, nor observed any permanent structural changes after a single irradiation. We will present evidence, in both single crystal GaAs and Si, that significant numbers of point defects are generated during these single exposures. We will further show how the population density of these defects can build with subsequent irradiation leading to a number of mechanisms that drive the evolution of surface morphology. The mass transport is accomplished by extraordinary diffusion that is enabled by the near collapse of the attractive part of the interatomic potential. This collapse occurs because we excite ~10% of the valence electrons into unoccupied states in the first 10-100 fs after irradiation. The bond softening of all of the atoms (ions now) permits just about all attempts to hop to be successful. This represents an increase of more than 12 orders of magnitude in diffusion. The morphology will be shown to be driven by dissociated Frenkel pairs where the self interstitials diffuse to the free surface and form epitaxial islands. Further morphological evolution is driven by surface plasmon polarititons and finally a strain induced morphological transformation. This talk will review what ultrafast lasers are and how they interact with materials. Our earlier results from GaAs will be used as an introduction to the subject and then we will present our very recent work with Si.
Thursday March 28th 2019 4:00 PM
Andrew Leifer, Princeton University
Probing neural dynamics and behavior of a simple animal
The human brain is an immensely complex electrochemical network with roughly 10^11 neurons and orders of magnitude more connections between them. How does the brain's neural network process signals and generate actions and movements? We take a reductionist approach and tackle this question in a simpler animal, the small roundworm Caenorhabditis elegans. The nematode C. elegans has only 302 neurons, yet it performs sophisticated functions like learning, memory and sensory-guided navigation. To study how its brain generates behavior, we developed a suite of instruments that measure and manipulate the animal's neural dynamics as it crawls. We leverage advances from the emerging field of optogenetics to optically read out, turn on or turn off individual neurons’ activity throughout the brain during movement. We combine this approach with quantitative measures of the animal’s posture dynamics to seek out relations between sensory signals, neural activity and behavior. In this talk I will share results from two recent investigations. The first probes how the animal decides to respond to a mechanical “touch” stimulation. In that work we find evidence that the animal integrates information about the sensory stimulus with its own behavior state in making a decision. In the second investigation we address the question of where and how locomotion related signals are represented in the brain. We perform “mindreading” and show that a linear combination of a subset of neurons’ activity is sufficient to predict the animal’s current velocity and body posture.
Thursday April 4th 2019 4:00 PMFRANCIS G. SLACK LECTURE
Barbara Jacak, UC Berkeley and LBNL
Thursday April 11th 2019 4:00 PM
Professor Rafael Lopez-Mobilia, Department of Physics and Astronomy, The University of Texas at San Antonio
Hosts:L. Vega and R. Scherrer
Thursday April 18th 2019 4:00 PM
Michael Murrell, Yale University
Work and Dissipation in the Cell Cytoskeleton
Living cells generate and transmit mechanical forces over diverse timescales and lengthscales to determine the dynamics of cell and tissue shape during both homeostatic and pathological processes, from early embryonic development to cancer metastasis. These forces arise from the cell cytoskeleton, a scaffolding network of entangled protein polymers driven out of equilibrium by enzymes that convert chemical energy into mechanical work. However, how molecular interactions within the cytoskeleton lead to the accumulation of mechanical stresses that determine the dynamics of cell shape is unknown. Furthermore, how cellular interactions are subsequently modulated to determine the shape of the tissue is also unclear. To bridge these scales, our group in collaboration with others, uses a combination of experimental, computational and theoretical approaches. On the molecular scale, we use active gels as a framework to understand how mechanical work is produced and dissipated within the cell cytoskeleton. On the scale of cells and tissues, we abstract mechanical stresses to surface tension in a liquid film and draw analogies between the dynamics of wetting and the dynamics of simple tissues. Together, we attempt to develop comprehensive description for how cytoskeletal stresses translate to the physical behaviors of cells and tissues with significant phenotypic outcomes such as epithelial wound healing.