Colloquia / Seminars
Colloquia are held Monday afternoons in 265 Phillips Hall at 4:00pm. Tea and coffee are served at the speaker’s reception in the Chapman meeting area, 3:30pm. The student Questions and Answers will be held in Phillips 277 after the colloquium from 5:15-6:00pm.
“Electronic Stopping in Condensed Matter: Ab Initio Understanding of Electronic Excitation Dynamics under Proton Irradiation”
Yosuke Kanai, University of North Carolina
Transfer of the kinetic energy from a highly-energetic ion to electrons in condensed matter is described by the so-called electronic stopping. The projectile ions bear highly localized electric field that is quite heterogeneous at the atomistic scale, and massive electronic excitations are produced in the electronic stopping process. Understanding this phenomenon= in condensed matter systems under proton and other ion irradiation has implications in various modern technologies, ranging from nuclear fission/fusion reactors, to semiconductor devices for aerospace missions, to cancer therapy based on proton beam radiation. Electronic stopping has been long studied within linear response theory framework (e.g. Bethe theory), but recent advances in high-performance computers allow us to study the phenomena beyond such simplified treatment through the use of numerical simulations. In this talk, I will discuss how non-equilibrium dynamics simulations based on our recently-developed large-scale real-time time-dependent density functional theory enable us to study this electronic excitation process, using an important case of liquid water under proton irradiation as an example. In addition to determining the energy transfer rate (i.e. electronic stopping power), our work reveals several key features in the excitation dynamics at the mesoscopic and molecular levels for deciphering water radiolysis mechanism under proton irradiation.
“The Cosmological Quest for Evidence of the Birth of the Universe out of the Multiverse Landscape”
Grant Mathews, Notre Dame University
One expects that the universe was born out of a complicated string-theory landscape near the Planck epoch. Although the energy scale of the birth of the universe is not accessible in terrestrial experiments, the energy scale of such trans-Plankian physics might have been obtained during the early instants of accelerated chaotic inflation. This talk will summarize the quest for cosmological evidence of this birth of space-time out of the string-theory landscape. We will explore the possibility that a set of superstring excitations may have made itself known via their coupling to the field of inflation. This may have left an imprint of “dips” in the power spectrum of temperature fluctuations in the cosmic microwave background. The identification of this as due to a superstring is possible because there may be evidence for different oscillator states of the same superstring that appear on different scales on the sky. Similarly, as the universe emerged it is possible that the interaction with other nascent universes led to the formation of cold spots and/or large-scale curvature in the cosmic microwave background. Such curvature might appear as a cosmic “dark flow” with respect to the frame of the big bang. This talk will summarize current constraints on the existence of such dark flow and prospects for its identification in the future. The existence of extra dimensions during inflation can also impact impact the cosmic expansion after the inflation epoch through the projection of curvature and/or mass-energy from a higher dimension. This can be constrained by the ratio of tensor to scalar fluctuations in the cosmic microwave background and via the effects of modified expansion on the light elements produced during big bang nucleosynthesis.
“The QCD Critical Point”
Thomas Schaefer, North Carolina State University
I summarize arguments that suggest that the phase diagram of QCD (the theory of quarks and gluons) has a critical endpoint which is analogous to the endpoint of the water-vapor transition. I will argue that this point can be searched for in collisions of relativistic heavy ions. The main observables are fluctuation measurements, and the expected signatures are related to critical opalescence. I summarize the ongoing theoretical and experimental efforts devoted to observing signatures of critical fluctuations. I argue that along the way, we have gained new insights into an old theory of fluid dynamics.
“The Maps Inside Your Head”
Vijay Balasubramanian, University of Pennsylvania
How do our brains make sense of a complex and unpredictable world? In this talk, I will discuss a physicist’s approach to the neural topography of information processing in the brain. First I will review the brain’s architecture, and how neural circuits map out the sensory and cognitive worlds. Then I will describe how highly complex sensory and cognitive tasks are carried out by the cooperative action of many specialized neurons and circuits, each of which has a simple function. I will illustrate my remarks with one sensory example and one cognitive example. For the sensory example, I will consider the sense of smell (“olfaction”), whereby humans and other animals distinguish vast arrays of odor mixtures using very limited neural resources. For the cognitive example, I will consider the “sense of place”, that is, how animals mentally represent their physical location. Both examples demonstrate that brains have evolved neural circuits that exploit sophisticated principles of mathematics – principles that scientists have only recently discovered.
“What is the MUSCEL behind Low Surface Brightness Galaxies?”
Rachel Kuzio de Naray, Georgia State University
Low surface brightness galaxies are disk galaxies that are characterized by their gas richness, low gas surface densities, low past and current star formation rates, blue colors, low metal abundances, and kinematics that are dominated by dark matter. While they span the same range of masses as their high surface brightness “normal” counterparts, they have clearly taken a different evolutionary path. Why is this? Why do they have so few stars? Why do their dark matter halos look the way they do? I will present spectroscopic and photometric observations that explore the properties of these galaxies. I will also discuss our efforts to answer these questions as part of the MUSCEL (Multiwavelength observations of the Structure, Chemistry, and Evolution of LSB galaxies) Program.
“Perforations, Curvature and Thermal Fluctuations in Free-Standing Graphene”
David Nelson, Lyman Laboratory of Physics, Harvard University
Understanding deformations of macroscopic thin plates and shells has a long and rich history, culminating with the Foeppl-von Karman equations in 1904, characterized by a dimensionless coupling constant (the “Foeppl-von Karman number”) that can easily reach vK = 10^7 in an ordinary sheet of writing paper. However, thermal fluctuations in thin elastic membranes fundamentally alter the long wavelength physics. We discuss the remarkable properties of free-standing graphene sheets (with vK = 10^13!) at room temperature, where enhancements of the bending rigidity by factors of ~4000 compared to T = 0 values have now been observed. Thermalized elastic membranes can undergo a crumpling transition when the microscopic bending stiffness is comparable to kT. We argue that the crumpling temperature can be dramatically reduced by inserting a regular lattice of laser-cut perforations. These expectations are confirmed by extensive molecular dynamics simulations, which also reveal a remarkable “frame crumpling transition” triggered by a simple large hole inserted into a graphene sheet. We show finally that thin amorphous spherical shells with a background Gaussian curvature are inevitably (in the absence of a stabilizing pressure difference) crushed by thermal fluctuations beyond a critical size, of order 160nm for graphene at room temperature.
“Path integrals, complexified fields and machine learning”
Paulo Bedaque, University of Maryland
The Feynman path integral is a tool applicable to all fields of physics. Recently, the possibility of changing the “path” of integration to lie on a complex manifold has been explored in both theoretical and numerical contexts. We review recent work using this ideas directly attacking the famous “sign problem” appearing in numerical studies of finite density systems and in the computation of the subtle phase leading to quantum interference. We will provide a short introduction to artificial intelligence techniques that were used in this work.
“Title to be Announced”
Raphael Buosso, University of California, Berkeley
The study of black holes has revealed a deep connection between quantum information and spacetime geometry. Its origin must lie in a quantum theory of gravity, so it offers a valuable hint in our search for a unified theory. Precise formulations of this relation recently led to new insights in Quantum Field Theory, some of which have been rigorously proven. An important example is our discovery of the first universal lower bound on the local energy density. The energy near a point can be negative, but it is bounded below by a quantity related to the information flowing past the point.
“Black Holes, Quantum Information, and Unification”
Lindley Winslow, Massachusetts Institute of Technology
Abstract to be announced.
“Earth’s Field NMR to Detect Spilled Oil Trapped under Arctic Ice”
Mark Conradi, ABQMR, Albuquerque, New Mexico
Oil production from wells in the arctic sea must face the possibility of leakage. Spilled oil would try to rise to the surface, but would be blocked by the 1-2 meters of ice coverage. ExxonMobil wants a detection method in-place before drilling or production begins.
Nuclear magnetic resonance can detect the abundant hydrogen nuclear spins of oil (and, unfortunately, water). The device uses a detection coil large enough to reach through the ice, 6 meters in diameter. The apparatus is flown by helicopter and then set onto the ice for detection.
Three physics issues arose in implementing the NMR solution. (1) Pre-polarization is used to align the nuclear spins to a greater extent than the earth’s field can. This field must be intense and the stored energy must be removed rapidly, leading to innovative switching circuitry. (2) The resonant pulses are frequency swept and must be adiabatic. Pulses were designed that avoid interference from the counter-rotating field component. (3) The signal of the oil must be distinguished from the much greater signal from sea water; this relies on the differences in the oil relaxation times compared to water.
“Title to be Announced”
Kate Scholberg, Duke University
Abstract to be announced.
“Modified Dark Matter: Relating Dark Energy, Dark Matter and Baryonic Matter”
Doug Edmonds, Penn State Hazleton
Modified dark matter (MDM) is a phenomenological model of dark matter, inspired by gravitational thermodynamics. For an accelerating Universe with positive cosmological constant ($\Lambda$), such phenomenological considerations lead to the emergence of a critical acceleration parameter related to $\Lambda$. Such a critical acceleration is an effective phenomenological manifestation of MDM, and is found in tight correlations between dark matter and baryonic matter in galaxy rotation curves. The resulting MDM mass profiles are consistent with observational data at both the galactic and cluster scales. In this talk, we will discuss the model and fits to data, where we find that the same critical acceleration appears at both galactic and galaxy cluster scales.
“Toward Quantum Control of Mechanical Motion”
Hailin Wang, Oregon State University
Quantum fluctuations or zero-point fluctuations of a macroscopic mechanical oscillator are many orders-of-magnitudes smaller than the size of an atom. The control of mechanical motion at this length scale opens a new frontier in quantum science and technology, but poses formidable experimental challenges. In this talk, I will discuss two closely-related experimental approaches aimed at achieving quantum control of mechanical motion. For the opto-mechanical approach, a mechanical oscillator is coupled to an optical microcavity via radiation pressure. The opto-mechanical interactions can cool the mechanical oscillator to its motional ground state and can generate entanglement between optical and mechanical systems. For the spin-mechanical approach, a mechanical oscillator is coupled to an electron spin via strain induced by the mechanical vibration. Phonon-assisted optical or spin transitions enable the control of mechanical motion. Complete quantum control of both the spin and mechanical degrees of freedom in this system can provide a promising experimental platform for quantum computers.