Friday Afternoon Seminar
|March 8 2019||Thomas K. Allison||Angle-resolved photoelectron spectroscopy (ARPES)||Angle-resolved photoelectron spectroscopy (ARPES) is the best way to determine the ground states of the electrons in condensed matter systems, and every synchrotron has a beamline devoted to ARPES. However, synchrotron radiation is not suitable for studying excited states because the light pulses from synchrotrons are too long and excited electronic states are short lived. In this talk I will describe a new instrument we have developed for performing ARPES measurements from excited states with < 100 fs resolution, but synchrotron data rates. Combined with a new time-of-flight momentum microscope, this instrument surpasses what was previously possible by several orders of magnitude, enabling many experiments previously considered impossible. A wide range of research directions for students are available for students who wish to use this unique and extraordinary tool, in addition to other opportunities in the Allison lab.|
|March 1 2019||Michael R. Douglas||Physics and Machine Learning|| Over the last decade there has been revolutionary progress in machine learning and artificial intelligence, and breakthroughs in applications such as machine translation, image recognition and competitive games such as Go. Physicists have made many contributions to the field, such as the Hopfield model and the Boltzmann machine. Another important contribution from statistical physics is replica symmetry breaking, which has led to many theoretical results on combinatorial optimization and machine learning.
In this talk we will give an introduction to feedforward neural networks and demonstrate some readily available software which makes it easy to experiment with them. We then survey a few research topics in this area with connections to physics, such as the relation between stochastic gradient descent and the Langevin equation, applications of random matrix theory, and physics-inspired multi-agent models.
|February 22 2019||Ken Dill||Dillgroup research: (1) The physics of biological cells. And (2) foundations of nonequilibrium statistical physics.||I will describe our current labgroup interests. (1) Biological cells perform very purpose-like actions, seeking food, deciding on reproductive actions depending on available food, repairing themselves, and many others. How are these actions encoded in purposeless physical chemistry? We are looking at the forces that drive cell decision-making and actions, and the forces that drive their evolution. (2) Since Boltzmann and Gibbs, there has been a search for a variational principle that might underlie nonequilibrium statistical physics, resembling the way the Second Law principle of Maximum Entropy predicts equilibria. Over the past decade, we have been exploring a principle called Maximum Caliber. It looks increasingly promising as the principle for nonequilibrium.|
|February 8 2019||Roy Lacey||Explorations of Quark Gluon Matter: a fascinating world governed by the strong force||High-energy proton-proton, proton-nucleus, and nucleus-nucleus collisions create an extended and strongly-coupled quark-gluon matter, whose properties and dynamics are controlled by Quantum ChromoDynamics (QCD) at or near the non-perturbative regime. The STAR experiment at RHIC@BNL and the ATLAS experiment at CERN@LHC provide unprecedented opportunities to create and study this matter using multiple collision systems and probes. After a brief overview of our experimental program at RHiC and the LHC, we will discuss some recent results and new opportunities to use multi-particle correlation measurements to study the emergent properties of strongly coupled QCD matter.|
|February 1 2019||Marilena Loverde||The Universe as a Laboratory for Fundamental Physics||While we have learned much about the evolution and matter contents of our Universe fundamental questions remain unanswered: what is the origin of structure? what are dark matter and dark energy? I will give a general overview of how we can use observations of the cosmic microwave background and maps of the large-scale distribution of galaxies to test this physics. In doing so I will describe recent results from my research group, including new methods we have developed and new observables we have discovered. I will also aim to convey how we do this research, e.g. what we actually do all day.|
|November 30 2018||Deyu Lu||First-principles modeling of electronic excitations: From basic understanding to materials applications||Electronic excitations are fundamental physical processes. Spectroscopic information, such as valence or core electron absorption spectra, from electron or photon probes is crucial for materials characterization and interrogation, especially in the context of in situ studies of materials or processes under operando conditions. When experimental data are supplemented by first-principles atomic modeling and state-of-the-art data analytics tools, a coherent physical picture can be established containing atomic level details of materials and insights derived from spectral signatures, which eventually allows us to establish the mechanistic understanding of the intriguing structure-property-function relationship. In this talk, the significance of the first-principles modeling of electronic excitations is highlighted in different aspects. First, we introduce a local representation of the non-local microscopic susceptibility, and show that this local representation can be used to partition bulk electronic excitations into quantities associated with local structural motifs. Next, we apply ab intio X-ray absorption near edge structure (XANES) modeling for spinel lithium titanate (Li4/3Ti5/3O4), an appealing lithium ion battery material. We identified key spectral features as fingerprints for quantitative assessment of the structural transformation during lithiation. Finally, we demonstrate that how machine learning algorithms can be combined with theory to extract the atomic structures of materials from their XANES spectra on-the-fly.|
|November 2 2018||Dominik Schneble||Exploring the quantum world with ultracold atoms||By cooling atoms to nanokelvin temperatures, it has become possible to create "artificial" manybody systems that exhibit pure quantum behavior on a macroscopic scale. I will describe how we produce, manipulate and detect ultracold, dilute-gas Bose-Einstein condensates (BECs) in our laboratory, and how we combine them with optical lattices to address fundamental questions in condensed-matter physics and the physics of dissipative quantum systems.|
|October 26 2018||Navid Vafaei-Najafabadi||“Honey, I Shrunk the CERN”- A Plasma Physics Perspective on Accelerating Electrons||Conventional electron accelerators use RF waveguides to accelerate electron bunches with a gradient on the order of tens of MeV/m. The electric field in such a structure cannot exceed ~100 MeV/m, because otherwise the accelerating structure would experience electrical breakdown and damage. In contrast, a plasma is an already ionized medium and therefore is capable of supporting electric fields of over 100 GeV/m, i.e. three orders of magnitude higher than that of the conventional accelerators. Such a dramatic increase in the accelerating gradient could enable table-top and inexpensive particle accelerators and radiation sources. With applications in fields as diverse as medicine and material science, table-top accelerators would greatly benefit society by popularizing a valuable scientific tool that is currently costly and difficult to access. Experimentally, the extreme fields in a plasma are generated by driving a plasma wave with a high-energy-density driver such as a high power laser or a dense electron beam. In this talk, I will discuss the physics principles of operation for such a plasma-wave-based accelerator, otherwise known as a plasma wakefield accelerator. Furthermore, I will discuss current research in understanding the limits of beam quality that can be produced by these accelerators and how the Stony Brook Plasma Accelerator Group is contributing to this research.|
|October 19 2018||Michael Wilking||Beyond the Standard Model of Particle Physics: Neutrino Oscillations, CP Violation, and Nucleon Decay||Since its inception in the 1970's, the "standard model" of particle physics has successfully the interactions of all known matter particles (excluding dark matter) via 3 of the 4 known fundamental forces (excluding gravity), and has made numerous precise predictions that have been confirmed by experiment, culminating with the discovery of the Higgs boson in 2012. The phenomenon of neutrino oscillations, however, is the first "laboratory-confirmed" physics not predicted by the standard model, and physicists are still trying to understand the implications of this discovery, and precisely measure the oscillation parameters. In particular, the current- and next-generation of neutrino oscillation experiments will search for Charge+Parity (CP) violation in the lepton sector, which may be responsible for the matter-antimatter asymmetry of the universe. Neutrino experiments require very large detectors due to the smallness of the neutrino interaction cross section, and such detectors are naturally well-suited to search for nucleon decay, which would be the first direct observation of new physics at the Grand Unified Theory (GUT) scale. This talk will describe the current state of the field, and efforts of the Stony Brook Neutrino and Nucleon decay (NN) group to search for answers to some of the most profound questions in particle physics.|
|October 5 2018||Petrovic Cedomir||Research Opportunities in Exploratory Materials Synthesis and Characterization||I will present Exploratory Materials Synthesis and Characterization at Condensed Matter Physics and Materials Science Department of Brookhaven National Laboratory. I will introduce the concept of correlated electrons, and quasiparticle mass enhancement on real materials examples such as heavy fermions and iron superconductors. This will be followed by brief description of Dirac materials, some experimental techniques, research and career opportunities.|
|September 28 2018||Cyrus Dreyer||Computational materials physics for next generation electronics||The next generation of electronic devices for, e.g., computing, sensing, energy production, and energy storage, require new materials to be discovered and developed, and novel phenomena to be utilized. First-principles computational calculations based on density-functional theory (DFT) play a key role in linking the concepts of condensed matter physics to the properties of real materials and the phenomena for such applications. I will give a brief introduction to DFT and how it can be used to predict and elucidate properties of electronic materials. To illustrate this, I will give examples relating to quantum processes at point defects, as well as electromechanical coupling in insulators.|
|September 21 2018||Jennifer Cano||What are topological phases of matter?||I will first define a topological band structure, using the Chern insulator and time-reversal protected topological insulator as examples. I will then discuss the role of symmetry. Finally, I will outline the past and future research directions of my group.|
|September 14 2018||Axel Drees||Thermal radiation from the QGP: Research Opportunities in Heavy Ion Physics at Stony Brook||In collisions of heavy ions at high energies small droplets of quark-gluon plasma (QGP) are formed in the laboratory at the BNL and CERN accelerator facilities, recreating the state of matter of the universe until about 10 micro seconds after the big bag. My group studies of these droplets of QGP using the PHENIX experiment at BNL’s relativistic heavy ion collider RHIC. In particular, we use the thermal radiation emitted by the QGP droplets to characterize the properties of this hot and dense form of matter. My talk will give an overview of the latest research results, including a sneak preview of results that will be submitted for publication in the next weeks, and future research opportunities.|