Events: Departmental Colloquia

• Perovskite Chalcogenides: New Semiconductors for Visible to Infrared Optoelectronics

  

  Guest: Dr. Jayakanth Ravichandran, University of Southern California/Georgia Institute of Technology
  Thursday, March 28, 2019 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  Perovskite Chalcogenides are a new class of semiconductors, which have tunable band gap in the visible to infrared part of the electromagnetic spectrum. Besides this band gap tunability, they offer a unique opportunity to realize large density of states semiconductors with high carrier mobility. In this talk, I will discuss some of the advances made both in my research group and in the research community on the theory, synthesis of these materials and understanding their optoelectronic properties. First, I will discuss how structure and chemical composition in Zr-containing perovskite sulfides can tune the optical properties in the visible spectrum, with an eye towards solar energy conversion. Second, I will discuss how the band gap can be further reduced to the infrared region for Ti-containing perovskite sulfides. I will also discuss about their anisotropic optical properties and large linear optical response. Finally, I will provide a general outlook for future studies on these exciting new class of materials.

  References:
  Nature Photonics, 12, 392-396 (2018).
  Advanced Materials 29, 1604733 (2017).
  Chemistry of Materials, 30 (15), 4897-4901 (2018).
  Chemistry of Materials, 30 (15), 4882-4886 (2018).

• Statistical Learning in Modern Physics

  

  Guest: Dr. Ping Ma, UGA Department of Statistics
  Thursday, March 14, 2019 3:30 pm - 4:30 pm
  Location: CSP Conference Room (322)

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  The rapid advance in science and technology in the past decade brings an extraordinary amount of data that were inaccessible just a decade ago, offering researchers an unprecedented opportunity to tackle much larger and more complex research challenges. The opportunity, however, has not yet been fully utilized, because effective and efficient statistical and computing tools for analyzing super-large dataset are still lacking. One major challenge is that the advance of computing technologies still lags far behind the exponential growth of database.
  In this talk, I will present an emerging family of statistical methods, called leveraging methods to facilitate scientific discoveries using limited computing resources. Leveraging methods are designed under a subsampling framework, in which one samples a small proportion of the data (subsample) from the full sample, and then performs intended computations for the full sample using the small subsample as a surrogate. The key to the success of the leveraging methods is to construct nonuniform sampling probabilities so that influential data points are sampled with high probabilities. These methods stand as a unique development of their type in big data analytics and allow pervasive access to massive amounts of information without resorting to high-performance computing and cloud computing.

• Quantum Chemistry and Computer Science: A Historical Account of Their Tightly Coupled Parallel Development

  

  Guest: Prof. Henry Schaefer, UGA Department of Chemistry
  Thursday, February 21, 2019 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  TBD

• Unraveling Multicritical Phenomena in Lattice Models

  

  Guest: Prof. Joao Antonio Plascak, Federal University of Paraíba, Center for Exact and Nature Sciences, João Pessoa, Brazil
  Thursday, February 14, 2019 2:30 pm - 3:30 pm
  Location: Physics Auditorium (202)

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  The "era of phase transitions" began with the discovery of the heat capacity and latent heat in the eighteenth century. A half century after that, critical phenomena emerged and had its heyday with the critical opalescence seen in carbon dioxide. It took, however, another 100 years to obtain an explanation of the microscopic ingredients underlying these intriguing phenomena. This achievement lead to Wilson's noble prize. Possible generalizations, sometimes not so easily reproduced in experiments, could be foreseen from theoretical grounds, leading to the discovery of multicritical behavior. We will present some models defined on regular lattices, mainly magnetic spin systems and geometrical polymers, which exhibit a variety of multicritical phenomena that can be studied via computer simulations as well as theoretical approaches.

• Black Hole Masses in Active Galaxies

  

  Guest: Prof. Misty C. Bentz, Department of Physics and Astronomy, Georgia State University
  Thursday, February 7, 2019 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  One of the legacy results of the Hubble Space Telescope is the discovery that supermassive black holes inhabit the centers of all massive galaxies, and these black holes appear to have a symbiotic relationship with their host galaxies. One of the keys to understanding this relationship involves constraining the masses of the black holes. However, black hole mass measurements are difficult to achieve because they require direct observations of the invisible black hole's gravitational influence on luminous tracers (stars or gas). A few different techniques have been developed over the last 25 years to meet this goal. One technique, known as reverberation mapping, is exclusively applicable to active black holes but may be used for even the most distant quasars in our universe, providing a way to study black holes across history. However, the most widely used technique in the local universe requires exquisite spatial resolution and is based on observations of the bulk motions of stars deep in the nucleus of a (usually inactive) galaxy. I will introduce these techniques and describe our ongoing program to identify a small sample of galaxies where multiple black hole mass techniques can be applied to each galaxy. This effort includes our recently-approved JWST ERS program, as well as programs carried out on multiple moderate- and large-aperture ground-based telescopes. The results of this work will allow us to directly test these independent mass measurement techniques against each other, investigating whether the masses of all black holes, both near and far, are on the same scale and thus having implications for our understanding of the evolution of galaxies across the ~13 billion year history of the universe.

• Quantum Coherent Electronic Technologies

  Guest: Prof. Michael E. Flatte’, Department of Physics and Astronomy, University of Iowa
  Thursday, January 24, 2019 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  Electrons in most materials experience dramatic and frequent scattering from other electrons, phonons, and a variety of other excitations. Such scattering events often rapidly dissipate any memory the electron had of its quantum state, so the electrons can be described as an ensemble that is near local thermal equilibrium. If the electrons can retain a good memory of their quantum state, however, then they are quantum coherent and can be used for very unusual and exciting tasks such as quantum computing. Realizing these quantum technologies has traditionally been expected to require very special elements such as superconducting devices or very high mobility transistors, as well as very low temperatures, in order to avoid rapid loss of quantum coherence (decoherence). Over the past fifteen years we and others have identified remarkable examples of room-temperature quantum coherent behavior in condensed matter electronic systems, usually involving spin coherence. Predicting the behavior of these spin coherent systems requires integrating theoretical techniques to cope with energy scales ranging from far smaller than the thermal energy to far larger. I will describe some examples of quantum coherent technologies and identify some of the features they share.

• Attosecond Probing of Solid-State Dynamics

  Guest: Prof. Stephen R. Leone, Departments of Chemistry and Physics and Lawrence Berkeley National Laboratory
  Thursday, January 17, 2019 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  A new method of probing solid-state materials involves laser pump-probe measurements with attosecond light pulses produced by the process of high harmonic generation. Such radiation in the extreme ultraviolet or x-ray region interrogates 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 that can be explored from a new viewpoint, on ever-shorter timescales. These include the extremely fast processes of core-level screening and broadening, coherences, and scattering, as well as electron configuration rearrangements. Topics in this presentation include semiconductor band gap excitation, charge transfer in metal oxides, core-level excitons in 2D metal dichalcogenides, and strong-field-induced Floquet Bloch bands. Lifetimes, scattering, and electronic coherences, as well as theoretical comparisons, will be considered. Coherent dynamics measurements in the extreme ultraviolet provide a novel and powerful probe for nonequilibrium states of matter.

• A Song of Ice and Fire --- Dynamics of Planets Hot and Cold

  Guest: Professor Gongjie Li, Department of Physics, Georgia Tech
  Thursday, January 10, 2019 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  The unexpected diversity of planetary systems has posed challenges to our classical understanding of planetary formation. For instance, Jupiter sized planets have been detected with short orbital periods of a few days, misaligned with respect to the spin- axis of their host stars. I will first describe the dynamical interactions between an outer perturber and the inner planet, which naturally leads to the formation of such misaligned hot Jupiters. Next, I will discuss a similar dynamical process in the outer Solar System, far away from our Sun, which causes the observed clustering of extreme trans-Neptunian objects. This can constrain properties of a possible outer planet, Planet Nine, in our own Solar System.

• Infrared Spectroscopy of Cold Organic Ions in the Gas Phase: Potential Interstellar Molecules

  Guest: Prof. Michael A. Duncan, Department of Chemistry, UGA
  Thursday, November 29, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  Cold cations of small hydrocarbon molecules, i.e., carbocations, are produced in pulsed supersonic molecular beams by a pulsed discharge source. These ions are mass-selected and studied with infrared photodissociation spectroscopy in a time- of-flight mass spectrometer using the method of rare gas atom complex predissociation. Infrared spectra are compared to the predictions of theory (DFT, MP2, CCSD(T)) to elucidate the structures of these ions, their isomers and the potential energy surfaces connecting them. The carbocation species studied include C₂H₃⁺, C₃H₅⁺, C₃H₃⁺, protonated benzene, and protonated naphthalene. Several of these exhibit more than one isomer, allowing investigation of the multiple minima on their potential surfaces. Protonated naphthalene has spectral lines relevant for the three of the main Unassigned Infrared Bands (UIR's) seen in emission from interstellar gas clouds. Oxocations studied with the same methodology include protonated formaldehyde, the acetyl cation, methanol cation and formaldehyde cation.

• Prebiotic Astrochemistry in the "THz-Gap"

  Guest: Prof. Susanna L. Widicus Weaver, Chemistry Director of Graduate Studies, Emory University
  Thursday, November 15, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  Small reactive organic molecules are key intermediates in interstellar chemistry, leading to the formation of biologically-relevant species as stars and planets form. These molecules are identified in space via their pure rotational spectral fingerprints in the far-IR or terahertz (THz) regime. Despite their fundamental roles in the formation of life, many of these molecules have not been spectroscopically characterized in the laboratory, and therefore cannot be studied via observational astronomy. The reason for this lack of fundamental laboratory information is the challenge of spectroscopy in the THz regime combined with the challenge of studying unstable molecules. Ions, radicals, and small reactive organics tend to be produced in trace quantities, often at high energies, and therefore have weak laboratory spectra. In addition, THz spectrometers have historically lagged behind those in other wavelength regimes because of a lack of sources and detectors that provide the power and sensitivity needed for such studies. The laboratory astrochemistry portion of my research program combines innovative spectroscopic approaches that seek to increase spectral sensitivity in the THz regime with novel chemical production mechanisms for species of astrochemical interest. Our laboratory work involves characterization of astrophysically-relevant unstable species, including small radicals that are the products of photolysis reactions, organic ions formed via plasma discharges, and small reactive organics that form via O(¹D) insertion reactions. In this seminar, I will present recent results from our laboratory studies, and discuss these results in the broader context of my integrative research program that encompasses laboratory spectroscopy, observational astronomy, and astrochemical modeling.

• Optical Spectroscopy for Sensing and Imaging Applications

  Guest: Prof. Zhiwen Liu, Department of Electrical Engineering, The Pennsylvania State University
  Thursday, November 8, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  In this talk I will discuss our work on the application of optical spectroscopy to sensing and label-free imaging. A compact spectrometer is developed by using a G-Fresnel device with dual functionalities of focusing and wavelength dispersion, which can enable miniaturization of optical spectrometers and thus the integration of optical spectroscopy with smartphone technology. The use of smartphone based diffuse reflectance spectroscopy for hemoglobin sensing and for detecting plant d i s e a s e s w i l l b e d i s c u s s e d . M e r g i n g t h e c ap a b i l i t i e s o f v i b r a t i o n a l spectroscopy and holography can lead to new opportunities for bio- imaging. The vibrational response of target molecules provides a means for chemical contrast, while holographic detection captures both the amplitude and the phase of a complex signal field to enable label-free three-dimensional imaging. Vibrational spectroscopic holography, including holographic coherent anti-Stokes Raman scattering imaging and holographic sum frequency generation imaging, will be discussed.

• What are CubeSats? A look at how UGA is advancing the new realm of space research

  Guest: Caleb Adams, Katie Summey & Nicholas Heavner, Department of Physics and Astronomy, UGA
  Thursday, November 1, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  The Small Satellite Research Laboratory began in 2016 with the funding of two small satellite, also known as CubeSats, missions through a joint effort between faculty and undergraduate students. Caleb Adams led the lab from the beginning as an undergraduate and now a graduate student. He will discuss the specifics and needs of CubeSats in the new era of space exploration as well as our current Air Force Research Lab (AFRL) funded mission. The AFRL mission, named the Multi-view Onboard Computational Imager (MOCI), will change the way data is used in the space environment by advancing high performance computing capabilities. Katie Summey, a Computer Science Engineering Major, will describe the goals of our NASA funded project, the Spectral Ocean Color (SPOC) mission and focus on the unique optical setup required for such a small space based imager. Finally Nicholas Heavner, a Mechanical Engineering and Physics/Astronomy student, will cover the thermal issues that arise on such a small platforms and the methods used to mitigate this risk.

• Computational Topological Photonics Using the Finite-Difference Time-Domain Method

  Guest: Prof. William Dennis and Jim Heneghan, Department of Physics and Astronomy, UGA
  Thursday, October 25, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  In this talk we describe our efforts to use the Finite-Difference Time-Domain (FDTD) method to model nanostructures that may have topological photonics applications. Our starting point is an attempt to reproduce some of the results of Raghu and Haldane [ Phys. Rev. A. 78, 033834 (2008)]. We will describe our approach, some of the technical challenges of using the FDTD method for this application, their resolution as well as discuss our progress to date. We will conclude with a discussion of next steps, future directions and how this work can support experimental work in this area.

• Leveraging Optical Phase Change Materials on Silicon Photonic Devices

  Guest: Prof. Sharon Weiss, Department of Electrical Engineering, Vanderbilt University
  Thursday, October 18, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  Silicon photonics is considered to be the leading platform to achieve faster data transfer speeds on-chip. However, the weak electro-optic coefficient of silicon limits the maximum achievable single channel data rates. A hybrid solution consisting of a silicon photonic backbone and an incorporated optical phase change material that provides improved optical functionality may provide the solution for realizing broadband, low power, small footprint on-chip photonic devices capable of achieving record modulation speed. In this presentation, we discuss theoretical and experimental work integrating vanadium dioxide in electro-optic and all-optical silicon photonic devices. Future directions will also be discussed.

• Exploring Complex Free Energy Landscapes with Innovative Monte Carlo Simulations

  Guest: Dr. David Landau, Center for Simulational Physics, UGA
  Thursday, October 11, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  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).

• The Third Woman to Ever Win Nobel Prize in Physics for Laser Breakthrough: Informal Discussion Over Coffee

  Guest: Prof. Melanie Reber, Prof. Susanne Ullrich, and Prof. Yohannes Abate, Department of Chemistry (Reber, Ullrich), Department of Physics (Ullrich, Abate), UGA
  Thursday, October 4, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  Three scientists from the United States, France and Canada have been awarded the Nobel Prize in physics for advances in laser physics. The Swedish Royal Academy of Sciences on Tuesday awarded half the 9-million-kronor ($1.01 million) prize to Arthur Ashkin of the United States and the other half will be shared by Gerard Mourou of France and Canada's Donna Strickland. The academy says Ashkin developed "optical tweezers" that can grab tiny particles such as viruses without damaging them. The awarding of the Nobel Prize in physics to Strickland has ended a drought for women winning any of the prestigious prizes. Strickland is the first woman to be named a Nobel laureate since 2015. She is also only the third to have won the physics prize — the first was Marie Curie in 1903. [http://time.com/5412361/nobel-physics-2018-laser/]

  We will discuss the significance of the discovery and the role of women in physics in an informal setting. The fact that Strickland's Wikipedia page has only appeared today is telling. It took a Nobel prize for Donna Strickland to be noticed enough to have a (short) Wikipedia page written about her. Another example of how womens' contributions to science go unnoticed and uncelebrated. [www.theguardian.com]

• Using "Shadows" of Galaxies to Probe Interstellar and Circumgalactic Gas

  Guest: Prof. Varsha P. Kulkarni, Department of Physics and Astronomy, University of South Carolina
  Thursday, September 27, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  Some of the key open questions in galaxy evolution are how gas flows into and out of galaxies, and how this gas gets converted into stars over time. A primary challenge in observing distant galaxies is that the light emitted by them is often too faint to allow detailed studies. Absorption lines in the spectra of background quasars can be used to probe gas in and around galaxies at various stages of evolution, and thus provide powerful probes of the history of star formation and chemical enrichment in galaxies. We have used this technique to track the build up of "metals" over the past ~11.5 billion years, uncovering both, a discrepancy relative to the models in the observed amount of metals in the more gas-rich galaxies, and a very high level of metals in the less gas-rich galaxies. We are now pushing the limits of this technique to reach galaxies in the first ~1 billion years after the Big Bang. Measurements of element abundances in this early epoch can help to constrain the initial mass function of the early generations of stars. Furthermore, spectra of gravitationally lensed quasars allow us to measure spatial variations in element abundances in the foreground galaxy. We have also succeeded in obtaining "3-D spectra" of some galaxies and measuring the flows of gas in and around them. We will discuss how all of these observations are helping to shed light on various aspects of galaxy evolution.

• Probing the Quark Gluon Plasma

  Guest: Prof. Megan E. Connors, Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia
  Thursday, September 20, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  In normal nuclear matter, quarks and gluons are confined within particles such as protons. However, under extreme temperatures and densities, quarks and gluons may behave as free particles in a state known as the Quark Gluon Plasma (QGP). Such extreme conditions existed immediately after the Big Bang and can be recreated in high energy collisions of heavy nuclei at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab in New York and the Large Hadron Collider (LHC) at CERN in Switzerland. The properties of the QGP can be studied by colliding heavy ions such as gold and lead at relativistic speeds and comparing measurements in such events to those in proton-proton collisions. In addition to bulk properties such as temperature and flow, high momentum particles produced in the early stages of the collisions can serve as probes of the QGP. Quantifying the properties of the QGP enhances our understanding of Quantum Chromodynamics, the theory of the strong force which binds the nucleus together.

• Phonon-photon entanglement in WSe₂ quantum dots

  Guest: Prof. Ajit Srivastava, Department of Physics, Emory University, Atlanta, GA
  Thursday, September 6, 2018 3:30 pm - 4:30 pm
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  Monolayer transition metal dichalcogenides (TMDs), such as WSe₂, are atomically thin semiconductors with a "valley" degree of freedom, which can be optically addressed, thus opening up exciting possibilities for "valleytronics". Recently, naturally occurring single quantum emitters, believed to be excitons trapped in shallow potentials, were reported in TMDs. They seem to inherit the valley degree of freedom from the host TMD and owing to their longer lifetimes, appear promising for quantum information processing applications.

  In this talk, I will begin by highlighting some unique properties of TMDs excitons which result from the off-Gamma-point origin of the constituent single particle electronic states. After describing the basic properties of quantum dots in TMDs, I will present evidence for quantum entanglement between chiral phonons of the 2D host and single photons emitted from the quantum dots. Finally, I will discuss our future plans for implementing a dynamically tunable array of qubits in pristine TMDs which can serve as an ideal platform for quantum information processing applications and also for understanding fundamental many-body physics.

• Improving Stroke Treatment by Magnetic Nanomotors

  Guest: Prof. Yiping Zhao, UGA Department of Physics and Astronomy
  Thursday, August 30, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  The treatment of ischemic stroke remains a daunting task as few therapeutic strategies have proven to be effective. Systemic thrombolysis with intravenous tissue plasminogen activator (tPA) remains the only proven treatment to improve clinical outcome of patients with acute ischemic stroke. But because of an increased risk of hemorrhage beyond 4.5 hours after onset of stroke, only certain stroke patients (1-2%) can benefit from tPA treatment. Current limitations of intravascular thrombolysis are primarily due to the inefficient/ineffective penetration of the systemically circulating tPA into the thrombus core. To improve tPA-induced thrombolysis and recanalization rates, we have developed a new strategy by incorporating rotary magnetic iron oxide (Fe3O4)-nanorods powered by an external magnetic field with the tPA delivery. Once the nanorods encounter the blood clot in the artery, it could not only improve the mass transport of the tPA-clot reaction, but also could mechanically disrupt the clot network to make a larger opening in the clot. Additionally, when magnetic nanorods are covalently bound to tPA with retained enzyme activity, the tPA functionalized magnetic nanorods can target to the blood clot in vivo under the guidance of an external magnet and tPA can subsequently be efficiently delivered at the site of embolism at high concentration to facilitate thrombolysis. Thus, an efficient tPA delivery system to the brain in combination with magnetic nanorods overcomes the limitations of current therapy with tPA alone. In a mouse model, we show that with two orders of less concentration of tPA on tPA-magnetic nanorod injection, it takes less than 1/3 time to lyse the blood clot, compared to that of tPA injection alone. In this talk, I will discuss the synthesis, functionalization of the magnetic nanorods, the physical and chemical mechanism of the improved thrombolysis, and the results from both the in vitro and animal tests as well as future challenges.

• Ultrafast Spectroscopy with Frequency Combs: the What, Why, and How

  Guest: Prof. Melanie Reber, Department of Chemistry, University of Georgia, Athens, GA
  Thursday, August 23, 2018 3:30 pm - 4:30 pm
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  Ultrafast spectroscopy continues to be a powerful tool for observing dynamics in quantum mechanical systems on the femtosecond timescale, a chemically important time window corresponding to molecular vibrations and many electronic transitions. Exploiting the characteristics of frequency comb lasers, we develop new techniques to overcome limitations of current methods of ultrafast spectroscopy. Frequency combs were developed as a tool for metrology and their use for precision spectroscopy has been widely recognized. However, the utility of a frequency comb for ultrafast spectroscopy is not as obvious. We exploit the properties of frequency combs to improve the sensitivity and broadband detection of ultrafast spectroscopies. One such technique we are developing uses ultrafast fiber-laser frequency combs coupled to external enhancement cavities to increase the sensitivity of ultrafast transient absorption spectroscopy. The enhancement cavities increase the laser power and effective absorption path length, thus giving signal enhancements of several orders of magnitude over traditional transient absorption spectroscopy. Noise reduction techniques improve the sensitivity of transient absorption by more than four orders-of-magnitude over previous best techniques. We can now study dilute samples in molecular beams on the femtosecond timescale with transient absorption spectroscopy, opening up new areas of chemical research. I will discuss this technique and others we are developing that use frequency combs to make significant improvements in ultrafast spectroscopies.

• The Upside Down World of Random Quantum Systems

  Guest: Prof. Michael Geller, UGA Department of Physics and Astronomy
  Thursday, August 16, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  I will explain the idea that random quantum states and systems have universal physical attributes that make them all look alike. This is a consequence of a rigorous geometric property of hyperspheres, and is in stark contrast to what we are used to, where the wave functions of a free particle, atom, and oscillator are all different. I will use this idea to motivate a challenge for quantum computers to test for these universal physical attributes, thereby testing whether or not a given simulated quantum system is "typical“.

• Undergraduate Day Colloquium

  Guest: Dr. Whitney Ingram, Sandia National Laboratory
  Thursday, April 19, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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• Visualizing Charge Carrier Dynamics in Nanowires with Pump-Probe Microscopy

  Guest: Prof. John Papanikolas, UNC Chapel Hill
  Thursday, April 5, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  A detailed understanding of the factors that govern the motion of mobile charge carriers through nanostructures is critical to many emerging nanotechnologies in electronics, optoelectronics and solar energy conversion. While the motion of charge carriers at low carrier densities is uncorrelated and easy to understand, many active electronic components operate at high carrier concentrations resulting from heavy doping or high injection. In this regime, carrier-carrier interactions and other many body effects (e.g. dopant/carrier interactions, electron screening, and electron-hole scattering) must be considered. We have combined ultrafast pump-probe spectroscopy with optical microscopy to directly image the charge carrier dynamics in individual Si nanowires (NWs) with both spatial and temporal resolution. In these experiments, an individual NW is excited by a femtosecond pump pulse that has been focused to a diffraction limited spot by a microscope objective, producing photogenerated carriers (electrons and holes) in a localized region of the structure. Motion of the photogenerated carriers is observed using a configuration in which carriers are created in one location and detected in another. In this configuration the pump beam is held fixed and the position of the probe beam is scanned by varying the angle of the probe beam as it enters the objective, resulting in a spatial map of the free carriers that provides a direct visualization of carrier diffusion.

• MOOC Research and Big Data in Education: A Physicist’s Journey in Digital Learning

  Guest: Dr. Daniel Seaton, Sr. Research Scientist, Harvard University, Cambridge, MA
  Thursday, March 29, 2018 3:30 pm - 4:30 pm
  Location: Physics Auditorium (202)

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  The Massive Open Online Course (MOOC) movement has catalyzed discussions of digital learning on campuses around the world and highlighted the increasingly large, complex datasets related to learning. This talk will highlight the MOOC movement from the perspective of a researcher studying learner behavior, a developer of big data pipelines capable of handling billions of data points, and above all, as a physicist working in the education field. The talk will begin with a MOOC research retrospective, reviewing our understanding of the nearly 7 million unique users that have led to over 13 million enrollments across more than 300 open online courses from Harvard and MIT. From there, I will share preliminary efforts to better inform instructors how design choices impact learning outcomes. By abstracting - or coarse-graining - MOOC content into simple linear sequences of course components (videos, problems, pages), we are able to cluster sequences and analyze variance in learning outcomes. Lastly, in order to further motivate the impact of big data in education, I will highlight new projects at Harvard aiming to transform current models of teaching and learning. My hope for this talk is that listeners will better understand the current wave of digital education and the opportunities it provides for data-driven teaching and learning.

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