5/16/2019 12:00 PM 340 West Hall - Quantum oscillations in electrical resistivity in Kondo insulators
Lu Li, Professor, Department of Physics
In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here we report a notable exception in an insulator — ytterbium dodecaboride (YbB12). The resistivity of YbB12exhibits distinct quantum oscillations despite having a much larger magnitude than in metals . This unconventional oscillation is shown to arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conventional Fermi liquid theory of metals. The large effective masses indicate the presence of a Fermi surface consisting of strongly correlated electrons. Quantum oscillations are also observed in the magnetization of YbB12 . Our result reveals a mysterious dual nature of the ground state in YbB12: it is both a charge insulator and a strongly correlated metal. Z. Xiang et al., Science 362, 65 (2018).
5/23/2019 12:00 PM 340 West Hall - Status on the Search for the Rare Kaon Decay, KL→ π0νν
Melissa Hutcheson, Ph.D. Candidate, Department of Physics
The KOTO experiment at the J-PARC research facility in Tokai, Japan aims to observe and measure the rare decay of the neutral kaon, KL→π0νν. This decay has a very small Standard Model predicted branching ratio of 3 x 10-11 which is why it has never been experimentally observed. While this decay is extremely rare, it is one of the best decays for studying charge-parity violation, which can tell us about the matter and antimatter asymmetry that we see in the universe today. In this talk, I will explain the details of how KOTO searches for this rare decay and present new results from the collaboration published in January 2019 as well as preliminary results from the current analysis.
5/30/2019 12:00 PM 1324 East Hall - Multi-scale problems in Quantum Chromodynamics
Jordan Roth, Ph.D. Candidate, Department of Physics
The origin of structure in the proton still evades a detailed description by first-principles calculations. Instead, the structure is extracted from global fits to its data. In proton-proton collisions, the current extraction procedure relies on our ability to independently describe each proton. It has been predicted, however, that correlations between two protons prohibit an independent description of each proton in certain scattering processes. These correlations may provide a powerful source of insight into the origin of collective structures in strongly-bound few-body systems. In this talk, I will explain how to probe these correlations and present measurements by the PHENIX experiment at Brookhaven National Lab in Long Island, New York. Measurements are also planned by the LHCb experiment at CERN in Geneva, Switzerland.
6/6/2019 12:00 PM 1324 East Hall - Nonlinear Optical Effects in Weyl Semimetals
Elizabeth Drueke, Ph.D. Candidate, Department of Physics
Weyl semimetals lie at the intersection of strongly correlated materials and materials with nontrivial spin-orbit coupling. These topological materials have attracted a lot of interest in the last several years because of their wide variety of novel properties and resulting potential applications. In this talk, I will begin by presenting a brief overview of the unique band structure and topology of these materials. Then I will go on to examine a couple of their nonlinear optical properties and highlight past and proposed experiments to further explore this novel state of matter.
6/13/2019 12:00 PM 1324 East Hall - The Role of Cell-Cell Contacts in Pattern Formation in Tissues:
from Juvenile Zebrafish to Mammalian Embryos
Hayden Nunley, Ph.D. Candidate, Department of Biophysics
Many physicists see biology as very complex and messy, and often it is. Certain problems in biology, though, serve as an elegant playground for physicists to develop quantitative AND predictive models. For example, problems in biology in which cells generate forces to perform some function allow physicists to make ourselves useful to biologists, our collaborators. In this talk, I will take you on a journey from the retinae of juvenile zebrafish to the outer tissue layer of developing mammalian embryos. In juvenile zebrafish, the cone photoreceptors in retinae form a precise crystalline lattice based on subtype (i.e., sensitivity to different wavelengths of light). We find that the defects in this lattice form lines, called grain boundaries, as the pattern forms, not by subsequent defect motion. Based on this observation, we propose a model in which cells of fixed fate (i.e., subtype) contact their neighbors of the same subtype, generating active forces for building the crystal. From there, I will take you to an example in which cell fate is not fixed. In this stem cell culture system, without any imposed chemical gradients and in the absence of many known endogenous gradients, cells of initially unspecified fate differentiate into two types, with one type localized to a ring at the boundary. We propose a model for this system in which mechanical stress biases fate and fate determines contractility. The role of cell-cell contacts and mechanics in pattern formation in developing tissues remains poorly understood. Luckily for us physicists, these problems provide endless intellectual stimulation.
6/20/2019 12:00 PM 1324 East Hall - Information scrambling in Quantum Phases
Ceren Burcak Dag, Ph.D. Candidate, Department of Physics
Out-of-time-order correlators (OTOCs) have become a widely-appreciated tool to measure the correlation build-up in space and time, and hence quantitatively characterize information scrambling in interacting many-body systems. Started off as a theoretical tool to understand quantum information in a black hole its impact quickly expanded to a wide variety of subjects including quantum chaos, many-body localization, quantum integrability and recently symmetry-breaking quantum phase transitions. After giving a short introduction to information scrambling and out-of-time-order correlators, I will talk about the emergent relation between symmetry breaking quantum phase transitions and the information scrambling. I will introduce a new theoretical tool to study the physics encoded in an OTOC: dynamical decomposition method. I will show how this tool lets us analytically see the reasons and the mechanism of dynamical detection of symmetry-broken quantum phases via OTOCs. Based on the studies in literature and our numerical results in XXZ-model, our method seems to be universal in explaining the reasoning behind the relation between scrambling and the quantum criticality. If time permits, I will talk about an interesting numerical observation that led us to find a relation between the topological order (in 1D superconductor) at zero temperature and the OTOCs at infinite temperature.
6/27/2019 12:00 PM 1324 East Hall - The MicroBooNE Neutrino Experiment
Christopher Barnes, Ph.D. Candidate, Department of Physics
Despite its postulation in the 1930s and discovery in the 1950s, very little is known about the neutrino, a neutral fundamental particle with thousands of times less mass than the electron that can potentially answer some of the biggest questions in physics. MicroBooNE, an 85-active-ton Liquid Argon Time Projection Chamber (LArTPC) experiment located at Fermilab in Batavia, IL, seeks to answer one such question: whether more than three types of neutrinos exist. Additionally, MicroBooNE is a means to study neutrino-argon scattering and perform R&D for the Deep Underground Neutrino Experiment (DUNE), a large-scale LArTPC set to take data in the mid-2020s. In this talk, I will give a brief overview of neutrinos before describing MicroBooNE and its public physics results to date.
7/11/2019 12:00 PM 340 West Hall - Rapid scanning AOM modulation-based linear measurements to derive the linear absorption spectra of purple bacteria
Ariba Javed, Ph.D. Candidate, Materials Science and Engineering Department
Photosynthesis is a vital process that forms the basis of most life and energy sources on the planet. The knowledge of the underlying mechanisms of charge and energy transfer involved in this process can be used to develop artificial light-harvesting systems and biofuels, helping us to meet our own energy needs. In this talk, I will discuss how we use fluorescence-detection-based two-dimensional electronic spectroscopy (F-2DES) to study the energy transfer in light-harvesting (LH2, in particular) complexes present in photosynthetic purple bacteria. Due to long acquisition times, photobleaching effects during the 2D measurements can distort the features of the acquired spectra. Motivated by the desire to reduce these effects without sacrificing the signal-to-noise ratio (SNR), we have adapted a rapid-scanning approach to record the linear spectra of the complexes in question. I will discuss the technique and results from the same. Extending this rapid-scanning technique to F-2DES promises reduced acquisition times and improved SNR for the 2D spectra.
7/18/2019 12:00 PM 340 West Hall - A Phase Transition in Network Community Inference
Alec Kirkley, Ph. D. Candidate, Department of Physics
Decomposing a network into communities (a partition of the vertices such that there is a significantly higher density of connections within groups than between groups) has been a subject of great interest in the network science community due to its numerous applications in data compression and machine learning. For many real networks, however, we do not know the "true" community labels, and so one way of assessing whether a community detection algorithm works well or not is to frame the task as an inference problem: there is a set of nodes with artificially assigned `"ground truth" community labels, from which a network is created through some probabilistic generative process, and the goal is to recover this structure using only the network and the algorithm of interest. Intuitively, if a graph is too sparsely connected or it is generated from a noisy process, we should fail to recover partitions that are correlated with our artificial ground truth. In this talk I discuss an interesting phenomenon in which it suddenly (in terms of a control parameter) becomes impossible to recover the true communities in a graph, even when they are explicitly planted in its topology! This abrupt qualitative change in the difficulty of the community detection problem is characterized by a phase transition analogous to that in a generalized Potts model in statistical mechanics, which can be derived from a statistical physics perspective using a free energy approximation and the cavity method. I will also discuss future work in this area and its implications for nonconvex optimization.
7/25/2019 12:00 PM 340 West Hall - Constraining Neutrino Properties with the Cosmic Microwave Background
Joey Golec, Ph. D. Candidate, Department of Physics
Neutrinos are one half of the leptons included in the standard model of particle physics yet some of their properties are the most poorly constrained aspects of the standard model. Neutrinos are also important in the cosmological standard model due to their suppression of the growth of structure at small angular scales and their influence on the evolution of early universe. The cosmic microwave background (CMB) is one of the best probes we have at observing the effects of neutrinos on the growth of large scale structure and by observing those effects we in turn can place tight constraints on two elusive properties of neutrinos, the sum of their masses and the number of different species. In my talk I’ll introduce both properties of the neutrino and the CMB, the effects neutrinos have on large scale structure that leave imprints on the CMB, current and future missions to observe those effects, and my experimental contributions to those missions.
8/1/2019 12:00 PM 340 West Hall - The Spin Polarization History Mystery; or, History-Dependent Dynamic Nuclear Polarization in Gallium Arsenide
Joseph Iafrate, Ph. D. Candidate, Department of Applied Physics
Electron spin has great potential for use in electronic device applications. To that end, our research group focuses on using optical pump-probe techniques to study electron spin dynamics in semiconductor materials. My current project began with an observation of an unexpected dependence of electron spin polarization in gallium arsenide on external magnetic field history. In this talk, I will recount this mystery and how we have set out to solve it. Join me as we search for clues and interrogate the prime suspect, dynamic nuclear polarization. Along the way, I will introduce the key concepts vital to understanding our experiments. Together, we will unravel the mystery of an unexpected spin phenomenon in gallium arsenide as I present a tale of intrigue and spin dynamics.
8/8/2019 12:00 PM 260 Weiser Hall - A Hearty Higgs Boson: Exploring Higgs Boson Properties Through the Refined Palette of the ATLAS Detector
Rachel Hyneman, Ph. D. Candidate, Department of Physics
The Higgs Boson is a newly introduced cuisine in the world of particle physics. We can now recognize it on the menu card of the Standard Model, but the details of its production, decay, and interactions are not yet precisely understood. I'll discuss the various recipes for creating a Higgs Boson with the Large Hadron Collider, and how these different methods affect the flavors we detect within the ATLAS detector. I'll also explore how refining our palette for Higgs Bosons can impact our broader understanding of fundamental physics.
8/15/2019 12:00 PM 340 West Hall - High performance micro-sensors for navigation-grade MEMS gyroscope
Sajal Singh, Ph. D. Candidate, EECS
GPS navigation is commonly used in many applications including defense, autonomous vehicles, and robotics. However, absolute dependence on GPS is unreliable due to its limited reachability and susceptibility to interference. For example, a jammer or even a simple and cheap device can be used to spoof GPS signal. As a result, for navigation of high-end vehicles like that of defense and military, one can’t rely entirely on GPS. To make navigation more secure and reliable, inertial sensors are used for navigation when GPS signal is unavailable. Inertial sensors consist of primarily three accelerometers and three gyroscopes in the three perpendicular axes to measure acceleration (or velocity or position) or rate (or angle) of rotation respectively for navigation. Gyroscopes are used to measure the rotation rate and angle of rotation with high precision. Commercial gyroscopes which are used in commercial flights as well as space missions are very precise in their measurement. However, their large sizes, high costs and power requirements limit their use in many applications. MEMS or Microelectromechanical systems consists of a range of mechanical structures which can be used for various applications. They have an inherent advantage of low cost (C), weight (W), size (S) and power (P) or low CWSaP. They, however, are limited in performance due to large noise. This is a major hurdle which has been limiting the entry of MEMS inertial sensors in navigation-grade performance applications. Our research is focused on bridging this gap and making an ultra-low noise MEMS gyroscope using the microfabrication technologies. In this talk, I will talk about the design and fabrication of miniaturized 3D shell resonators for gyroscopes. These resonators have exhibited quality factor as high as 10 million leading to very low noise gyroscope at their small size. The achieved performance matrices would enable the use of MEMS sensors as a navigation-grade gyroscope at a cost lower by several orders of magnitude than the existing commercial gyroscopes. Only this would enable each one of us to own a self-driving car and autonomous robots at our homes!
8/22/2019 12:00 PM 340 West Hall - Miniaturized Frequency Combs Enable Advanced Spectroscopies to Leave the Lab and (Maybe) Enter Orbit
Matthew Day, Ph. D. Candidate, Department of Physics
Frequency Combs, or pulsed lasers which are capable of emitting many narrow and closely spaced spectral lines (teeth) with fixed phase relationships between adjacent teeth, are an essential tool in precision metrology and spectroscopy. Their usefulness comes from the fact that their entire spectrum can be controlled by just adjusting the time between pulses and the pulse-to-pulse phase slip of their electric field. This means that, using relatively simple control schemes, frequency combs enable the most precise measurements of time and frequency possible, among a plethora of other applications. Typically, however, these light sources are roughly the size of a kitchen table and require the high stability of a lab environment to maintain the controllability of their output. Miniaturized combs exist, in the form of microscopic ring resonators, but these light sources are not very tunable, typically require large and powerful pump lasers to operate, and are expensive to manufacture. These drawbacks are all showstoppers when it comes to allowing frequency comb enabled precision measurement and spectroscopy to leave the lab. We have demonstrated a new, extremely cheap, simple, and low power laser diode-based frequency comb which is roughly the size of a grain of rice. This laser can be battery powered, and its spectrum is highly controllable, making it an ideal light source to allow advanced precision measurement and spectroscopy to leave the lab. In my talk, I will give a brief overview of frequency comb-based measurements, demonstrate the stability and tunability of our new sources, and outline their prospect for future ground- and space-based applications.