Calendar for Summer 2017

Talks take place on Wednesdays throughout the summer in 340 West Hall. Lunch is provided at 11:45 in Don Meyer and the presentations are from 12:10 to 1:00pm.

  • May 10
  • Carrier Multiplication in Semiconductor Nanocrystals - Does it Exist?
    Albert Liu, Physics
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    Although p-n junction based solar cells were first demonstrated almost 70 years ago, the efficiencies of mass-produced solar cells have struggled to reach even 30%. The main reason for this low efficiency is so-called spectrum loss, in which the high-energy portion of the solar spectrum has much of its energy wasted from thermal relaxation of excited carriers. The Shockley-Queisser limit, which is the theoretical efficiency limit for a single p-n junction solar cell, is only ~32% for silicon, the most popular solar cell material. However, almost 20 years ago it was proposed that a process called impact ionization (now generally termed carrier multiplication) in semiconductor nanocrystals may be a possible way to circumvent this limit. I will give an overview of the controversy surrounding carrier multiplication and, if time allows, go over an experimental technique called 2D spectroscopy which may allow us to provide the first direct observation of carrier multiplication.

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  • May 17
  • Searching for Sterile Neutrinos with JSNS^2
    Johnathon Jordan, Physics, Advisor: Josh Spitz
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    In the past several decades, experiments have provided revolutionary insight into the properties of neutrinos. While our understanding of neutrino interactions and behavior has improved substantially, many questions still remain. The Standard Model picture of neutrinos only includes 3 species, but there are experimental hints of at least one additional flavor which does not participate in the weak interaction: a so-called sterile neutrino. In this talk, I give an overview of experimental evidence (and null results) related to the sterile neutrino hypothesis. I then describe JSNS^2, an upcoming search for sterile neutrinos in Japan.

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  • May 24
  • AdS/CFT and applied string theory
    Dr. Jim Liu, Physics
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    The advent of AdS/CFT has led to a quiet revolution of applying string theory beyond its traditional realm of quantum gravity and grand unification. In particular, recent developments have led to a breakthrough in our understanding of many strongly interacting systems that have been challenging to study using standard perturbative methods. In this talk, I will give examples of what string theory can tell us about QCD and physics of the quark-gluon plasma as well as about condensed matter systems including models of high-temperature superconductivity.

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  • May 31
  • Measurement of the muon anomalous magnetic moment
    Alec Tewsley-Booth, Physics
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    Precise measurements of the anomalous magnetic moment, a = (g - 2)/2, of the muon provide strong tests of the Standard Model, and are more sensitive to physics beyond the Standard Model than measurements of the electron anomalous magnetic moment. The most recent measurement of the muon magnetic moment at Brookhaven E821 has hinted at new physics, with its result differing from theoretical calculations by over three standard deviations, with an uncertainty of 540 ppb. The new Fermilab E989 experiment seeks to improve on both the statistical and systematic errors of the measurement with a projected uncertainty of 140 ppb, which represents a four-fold improvement on the Brookhaven result. The experiment will use the high intensity muon beam at the new Fermilab muon campus, and store polarized muons in a magnetic storage ring. The magnetic field with be monitored by an array of calibrated NMR probes; calorimeters will measure muon decays as they travel around the ring, which indicates the spin direction. The combined measurements of the magnetic field and muon precession rate can be used to calculate the anomalous magnetic moment. A general overview of the theoretical motivation, experimental techniques, and possible implications of the experiment will be presented.

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  • June 7
  • Understanding the strongly correlated topological insulator SmB6
    Alexa Rakoski, Physics
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    Topological insulators (TIs) are a subject of recent interest in the condensed matter community due to their unusual electronic properties. Physically, TIs are characterized by the presence of protected metallic surface states located within the bulk gap that are manifested at low temperatures. A recent TI candidate is samarium hexaboride (SmB6), which is especially intriguing because it is the first strongly correlated TI. Instead of a bulk gap as in “conventional” TIs, SmB6 exhibits the Kondo effect, in which conduction and localized bands hybridize at low temperatures to open a gap at the Fermi energy. An overview of this system will be presented to highlight its unique characteristics, and experimental results with an emphasis on transport will be discussed. Specifically, I will focus on the role of impurities in the band structure, as well as the experimental implications of this detected in recent transport experiments.

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  • June 14
  • Using two-dimensional electronic spectroscopy to study energy transfer in photosynthesis
    Libby Maret, Applied Physics
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    Plants have long since figured out a means of transforming solar energy into long-term storable chemical energy, with oxygen gas as its only major byproduct. As energy consumption increases across the globe, so too does emission of environmentally harmful greenhouse gases, and it is ever more important that we find a way to effectively and sustainably harvest solar energy. On a macroscopic scale, photosynthesis is well understood in terms of the protein complexes involved, their functionality, and their byproducts. What is less understood is the structure/function relationship in photosynthetic systems: how the physical arrangement of light-absorbing pigments in their protein matrix harvest solar energy with high quantum efficiency on ultrafast time scales. Experimental methods like ultrafast pump-probe spectroscopy attempt to uncover the energy transfer mechanisms in various photosynthetic complexes, but struggle to overcome the limitations of spectral congestion to resolve pigment-pigment electronic coupling and energy transfer pathways. In this presentation I will show how two-dimensional electronic spectroscopy (2DES) can be a powerful tool in overcoming the barriers of traditional spectroscopies and how we are applying it in our studies on the photosynthetic pigment chlorophyll a.

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  • June 21
  • Torque Differential Magnetometry Using Qplus-mode of Quartz Tuning Fork
    Lu Chen, Physics
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    Quartz tuning fork is the key component of high-resolution atomic force microscope. Because of its high quality factor, quartz tuning fork can also be used for high sensitivity magnetometry. Herein, we developed a highly sensitive torque differential magnetormetry using the Qplus-mode of a quartz tuning fork. The tuning fork is driven by an AC voltage and its deflection is measured by the resultant AC current. We observed a sharp resonance of the quartz tuning fork at low temperature down to 1.6K. We calibrated our torque differential magnetometry by measuring the angular dependence of the hysteresis loop in single crystalline Fe0.25TaS2. Furthermore, we demonstrated the high sensitivity of the torque differential magnetormetry by measuring the quantum oscillations of bismuth single crystal. The extracted Fermi surface cross sections are consistent with those of bismuth crystals.

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  • June 28
  • Looking at ghosts---Optical methods for studying light-matter interaction
    Glenn Leung, Physics
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    The interaction between photons and excitons in a semiconductor microcavity are described as quasiparticles, known as exciton-polaritons. They have been the subject of much research interest, largely due to their ability to form non-equilibrium condensates at relatively high temperatures, and the possibility of making lasers which do not require population inversion. When polaritons decay, they produce a photon that carries information about its phase and energy (the ghost). This is how many experimentalists study the properties of these polaritons. I will go over the basic physics of exciton-polaritons, the various optical methods used to study these polaritons, as well as some of the recent exciting developments in this field, including a few words about my own research.

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  • July 5
  • Wavequide Quantum Electrodynamics (waveguide QED): A unique platform for long distance quantum communication with strong light-matter interactions
    Imran Mirza, Physics
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    Photons are excellent carriers of information. They are fast and their interation with the environment is weak. However, their interaction with each other is also very weak. This poses a considerable challenge on performing several quantum optics based information processing protocols, which require strong photon-photon interactions. As a solution, commonly atoms or atomic media are used as photon-photon interaction mediator, but in vacuum atom-light interaction also turns out to be quite weak. In this talk I'll discuss how guided photons coupled to quantum emitters (waveguide QED platforms) may be used to establishy strong light-matter interaction. Additionally, I'll present how preferential emissions in the waveguide (chirality) can be used to enhance qubit-qubit entanglement. Finally, I'll also show how disordered waveguide QED architectures can influence the transport of single photons.

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  • July 12
  • Searching for gravitational waves from spinning neutron stars with LIGO
    Ansel Neunzert, Physics
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    The detection of gravitational waves (GWs) from two inspiralling black holes put LIGO in the news, and marked the beginning of a new era of GW astronomy. But there's more to GW detection than meets the eye -- or the headlines! In this talk, I'll give some background on GWs, the LIGO detectors, and different types of GW searches, before zooming in on the focus of the Michigan GW group: the search for GWs from spinning neutron stars.

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  • July 26
  • Searching for Trans-Neptunian Objects (and a New Planet) with the Dark Energy Survey
    Stephanie Hamilton, Physics
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    The discovery of 1992 QB1, the second known trans-Neptunian object (TNO) after Pluto, came as a bit of a shock to solar system astronomers after 60 years of believing Pluto was alone. Today, we know Pluto is far from alone, with over 1700 objects discovered beyond Neptune in the past 25 years. These small bodies act as a unique tracer of the solar system's dynamical history, preserving the effects of both long and short term gravitational interactions. In particular, the peculiar apparent clustering of the longest-period objects beyond Neptune has recently been used to argue for an additional planet in the distant solar system -- Planet Nine. In my talk, I'll discuss why the outer solar system has once again become a hot topic of research in addition to how my research with the Dark Energy Survey is in a unique position to significantly contribute to our knowledge of this region and to search for Planet Nine.

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  • August 2
  • Hidden Symmetries of the S-matrix: From Goldstone Modes to Supermembranes
    Callum Jones, Physics
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    Over the past decade-and-a-half there has emerged an exciting new subfield in high-energy theory challenging the primacy of the role of quantized fields in the description of the scattering of fundamental particles. Powerful new techniques allow scattering amplitudes to be recursively "bootstrapped" from the most primitive interactions and symmetry principles, entirely avoiding the apparatus of Feynman diagrammatics. These ideas have lead to new insights into the dynamics of fundamental particls and quantum gravity. In this talk, I will give an overview of recent work describing the recursive construction of scattering amplitudes for Goldstone modes of spontaneously broken symmetries. In particular, I will discuss the spontaneous breaking of spacetime symmetries, including supersymmetry, and argue that the landscape of possibel self-consistent models which emerge should be understood in terms of the effective description of the dynamics of extended objects called supermembranes.

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  • August 9
  • Constraining fundamental physics with the Dark Energy Survey
    Jessie Muir, Physics
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    Decades of cosmological observations have allowed us to build a standard cosmological model in which the mass-energy density of our universe is dominated by a mysterious component which drives the accelerating expansion of the universe, but which we don't understand at a fundamental level. We call this component dark energy. The Dark Energy Survey (DES) is able to measure dark energy's properties to an unprecedented precision and just released its first competititve cosmological constraints last week. In my talk, I will put these results into context by giving an overview of our standard cosmological model, and the observations which support it. I'll also discuss some of the steps we need to take to leverage DES' precision towards better understanding fundamental physics, focusing on two projects I'm working on: checking whether the relationship between expansion history and cosmological structure growth agree with the predictions of general relativity, and preventing experimenters' bias by developing a method to blind cosmological analyses which use multiple obervational probes.

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  • August 16
  • Generalized statistical mechanics and applications to distributed system design problems
    Andrei Klishin, Physics
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    Our life is filled with complex engineered products, each consisting of many subsystems offering distinct functionalities. In designing such a product, one is torn between optimizing each component or subsystem, or optimizing the overall system objectives. Overall system objectives give rise to competing design pressures, whose effects can be difficult to trace in subsystem design. Here, we analyze a problem of routing connections between functional units of a naval battleship, subject to competing pressures of cost reduction, design flexibility and performance. To tackle this problem, we re-examine the axioms of statistical mechanics and generalize it beyond the usual language of energies and pressures. We show that the simple routing problem example breakes into different architecture classes, separated by finite-size phase transitions. The methods developed are easily generalizable to other types of design problems.

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  • August 23
  • Radio Frequency Pulse Design for Magnetic Resonance Imaging
    Sydney Williams, Biomedical Engineering
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    Magnetic resonance imaging (MRI) works by transmitting radio frequency (RF) pulses to a patient inside an external magnetic field (B0) that alignes the net magentic moment of the atomic nuclear spins along the direction of B0. The RF pulses act by tilting the magnetization into the perpendicular transverse place where is precesses, eventually recovering to its initial direction parallel to B0. Detectable voltage signals are recorded and processed to form an MR image when the precession of spins induces a voltage across RF receive coils. During the scan, linear gradient magnetic fields are manipulated to spatially localize the magnetization. In the human body, H atoms found in water are susceptible to nuclear magnetic resonance, making MRI a valuable noninvasive medical imaging tool. Furthermore, by manipulating the RF pulse delivery and gradient fields in an image acquisition, images can be weighted to emphasize tissue contrast of interest. Radio frequency pulse excitation is critical to MR imaging because the magnetization signal can be detected only if a component lies in the transverse plane. For more sophisticated imaging schemes, there is a need for RF pulses that are tailored to their applications. In my talk I will discuss my work on RF pulse design for advanced MRI applications, including simultaneous multislice imaging for fast acquisitions and spectral-spatial prewinding pulses for counteracting field inhomogeneity.

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Talks from Summer 2016 are here.

Maintained by Elizabeth Drueke (edrueke[at]umich[dot]edu)
PGSS at the University of Michigan, Summer, 2017