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Colloquia History

ics Colloquium
Friday, November 8, 2019
3:30 PM
Physics Building, Room 204
Professor Leo Stein [Host: Kent Yagi]
University of Mississippi
"Testing Einstein with numerical relativity: theories beyond general relativity, and the precision frontier"
ABSTRACT:

Advanced LIGO and Virgo have already detected black holes crashing into each other at least ten times. With their upgrades we anticipate a rate of about 1 gravitational-wave detection per week. More signals and higher precision will take the dream of testing Einstein's theory of gravity, general relativity, and make it a reality. But would we know a correction to Einstein's theory if we saw it? How do we make predictions from theories beyond GR? And do current numerical relativity simulations have enough precision that we could be confident in any potential discrepancy between observations and predictions? I will discuss (i) how to perform simulations in beyond-GR theories of gravity, and (ii) how numerical relativity simulations need to improve to be ready for the precision frontier of gravitational wave astrophysics.

 

ics Colloquium
Friday, November 1, 2019
3:30 PM
Physics Building, Room 204
Sherry Yennello [Host: Simonetta Liuti]
Texas A&M University
"Studying the stars here on earth: Experimental investigations of the nuclear equation-of-state "
ABSTRACT:

Heavy-ion collisions can produce nuclear material over a range of densities and proton fractions to study the nuclear equation-of-state. These measurements are enabled by accelerating nuclei to – in some cases – GeV energies and detecting the fragments that are produced from the collisions.  The detectors are multi-detector arrays capable of measuring dozens of particles simultaneously from a single collision.  Data rates can range up to many hundreds of collisions per second. One can either explore the characteristics of the individual fragments that are produced, often extracting particle ratios or double ratios, or correlations between the fragments – in particular transverse collective flow.  From very low density to about three times normal nuclear density measurements have been made of the density dependence of the asymmetry energy.  I will present an overview of how these measurements have been made and the constraints they have set on the nuclear equation-of-state.

ics Colloquium
Friday, October 25, 2019
3:30 PM
Physics Building, Room 204
Rajveer Nehra
University of Virginia - Physics
"Phase space characterization of optical quantum states and quantum detectors"
ABSTRACT:

We are in the midst of a second quantum revolution fueled by “quantumness” of physical systems and the sophisticated measurement devices or detectors to produce and characterize these exotic systems. Thus, characterization of quantum states and the detectors is a key task in optical quantum science and technology. The Wigner quasi-probability distribution function provides such a characterization. In this talk, I present our recent results on quantum state tomography of a single-photon Fock state using photon-number-resolving measurements using superconducting transition-edge sensor [1]. We directly probe the negativity of the Wigner function in our raw data without any inference or correction for decoherence, which is also an important indicator of the “quantum-only” nature of a physical system. For the second part of the talk, we discuss a method to characterize quantum detectors by experimentally identifying the Wigner functions of the detector positive-operator-value-measures (POVMs), a set of hermitian operators completely describing the detector [2]. The proposed scheme uses readily available thermal mixtures and probes the Wigner function point-by-point over the entire phase space from the detector’s outcome statistics. In order to make the reconstruction robust to the experimental noise, we use techniques from convex quadratic optimizations.

References
1. R. Nehra, A. Win, M. Eaton, R. Shahrokhshahi, N. Sridhar, T. Gerrits,A. Lita, S. W. Nam, and O. Pfister, “State-independent quantum state tomography by photon-number-resolving measurements,” Optica 6,1356–1360 (2019). 2. R. Nehra and K. Valson Jacob (2019), “Characterizing quantum detectors by Wigner functions,” [arXiv:1909.10628].
 

ics Colloquium
Friday, October 4, 2019
3:30 PM
Physics Building, Room 204
Professor Xiaoyang Zhu [Host: Seunghun Lee]
Columbia University
"Ferroelectric Polarons, Belgian Waffles, and Principles for “Perfect” Semiconductors"
ABSTRACT:

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids.  Here we explain the essential physics in this class of materials based on their dielectric functions and dynamic symmetry breaking on nano scales. We show that the dielectric function in the THz region may lead to dynamic and local ordering of polar nano domains by an extra electron or hole, resulting a quasiparticle which we call a ferroelectric large polaron, a concept similar to solvation in chemistry. Compared to a conventional large polaron, the collective nature of polarization in a ferroelectric large polaron may give rise to order(s)-of-magnitude larger reduction in the Coulomb potential. We show that the shape of a ferroelectric polaron resemble that of a Belgian waffle. Using two-dimensional coherent phonon spectroscopy, we directly probe the energetics and local phonon responses of the ferroelectric large polarons. We find that that electric field from a nascent e-h pair drives the local transition to a hidden ferroelectric order on picosecond time scale.  The ferroelectric or Belgian waffle polarons may explain the defect tolerance and low recombination rates of charge carriers in lead halide perovskites, as well as providing a design principle of the “perfect” semiconductor for optoelectronics.

ics Colloquium
Friday, September 13, 2019
3:30 PM
Physics Building, Room 204
Israel Klich [Host: Bob Jones]
University of Virginia - Physics
"Quantum states, walks, tiles, and tensor networks"
ABSTRACT:

A major challenge of physics is the complexity of many-body systems. While true for classical systems, the difficulty is exasperated in quantum systems, due to entanglement between system components and thus the need to keep track of an exponentially large number of parameters. In particular, this complexity places a challenge to numerical methods such as quantum Monte Carlo and tensor networks. Here, exactly solvable models are of crucial importance:  we use these to test numerical procedures, to develop intuition, and as a starting point for approximations.

In this talk, I will explain our current understanding of a new solvable "walk" model, the area deformed Motzkin model. The model shows that entanglement may be more acute than previously thought, in particular, it features a novel quantum phase transition between a non-entangled phase and extensively entangled “rainbow” phase. Most remarkably, the model motivated the construction of a new tensor network, providing, after many years, the first example for an exact tensor network description of a critical system. Finally, I will remark on open problems, and on exciting connections to other fields such as the notion of holography in field theory, and a famous problem in non-equilibrium statistical mechanics.

ics Colloquium
Friday, September 6, 2019
2:30 PM
Physics Building, Room 204
Professor Or Hen [Host: Nilanga Liyanage]
MIT - Massachusetts Institute of Technology
"Neutron stars droplets and the quarks within"
ABSTRACT:

Neutron stars are one of the densest strongly-interacting many-body systems in our universe. A main challenge in describing the structure and dynamics of neutron stars steams from our limited understanding of the nuclear interaction at high-densities (i.e. short-distances) and its relation to the underlaying quark-gluon substructure of nuclei.

In this talk I will present new results from high-energy electron scattering experiments that probe the short-ranged part of the nuclear interaction via the hard breakup of Short-Range Correlations (SRC) nucleon pairs. As the latter reach densities comparable to those existing in the outer core of neutron stars, they represent ’neutron stars droplets’ who’s study can shed new light to the dynamical structure of neutron stars. Special emphasis will be given to the effect of SRCs to the behavior of protons in neutron-rich nuclear systems and how it can impact the cooling rates and equation of state of neutron stars.  Pursuing a more fundamental understanding of such interactions, I will present new measurements of the internal quark-gluon sub-structure of nucleons and show how its modification in the nuclear medium relates to SRC pairs and short-ranged nuclear interactions. 

Given time I will also discuss the development of new effective theories for describing short-ranged correlations, the way in which they relate to experimental observables, and the emerging universality of short-distance and high-momentum physics in nuclear systems.

ics Colloquium
Friday, August 30, 2019
3:30 PM
Physics Building, Room 204
Jeffrey Teo [Host: Bob Jones]
University of Virginia - Physics
"From interacting Majorana to universal fractional quasiparticles"
ABSTRACT:

Ising anyons, Majorana fermions (MF) and zero energy Majorana bound states have promising prospects in topological quantum computing (TQC) because of their ability to store quantum states non-locally in space and insensitivity to local decoherence. Unfortunately, these objects are not powerful enough to assemble a TQC that can perform universal operations using topological braiding operations alone. On the other hand, there are anyonic quasiparticles, like the Fibonacci anyon in a Read-Rezayi quantum Hall state, that are universal in the braiding-based TQC sense. However, these are quantum dynamical excitations, which can be challenging to spatially manipulate and susceptible to temperature fluctuations in a thermodynamic system. We propose and define a new notion of universal fractional quasiparticles, which are semi-classical static topological defects, supported by many-body interacting MFs in a superconducting spin-orbit coupled topological electronic system.

ics Joint Colloquium with Physics and Astronomy/NRAO


Friday, April 26, 2019
3:30 PM
Physics Building, Room 204
Andrew Steiner [Host: Kent Yagi]
University of Tennessee
"From Multimessenger Astronomy to Neutrons and Protons"
ABSTRACT:

Of course, multimessenger astronomy promises to revolutionize

astronomy and our understanding of nucleosynthesis. My

research shows that it goes further: astronomical observations

(via both photons and gravitational waves) provides a unique

laboratory to deepen our understanding of QCD and the

nucleon-nucleon interaction. Most current work is focused

on the equation of state. While the equation of state is

indeed important, in this talk, I show how we can

go beyond energy density and pressure. I present the first

large-scale Bayesian inference of neutron star observations

and nuclear structure data to obtain novel results on the

composition of dense matter and the nature of nucleonic

superfluidity.

ics Colloquium
Friday, April 19, 2019
3:30 PM
Physics Building, Room 204
Dr. Craig D. Roberts [Host: Nilanga Liyanage]
Argonne National Laboratory
"Emergence of Mass in the Standard Model"
ABSTRACT:

Quantum Chromodynamics (QCD), the nuclear physics part of the Standard Model, is the first theory to demand that science fully resolve the conflicts generated by joining relativity and quantum mechanics.  Hence in attempting to match QCD with Nature, it is necessary to confront the innumerable complexities of strong, nonlinear dynamics in relativistic quantum field theory.  The peculiarities of QCD ensure that it is also the only known fundamental theory with the capacity to sustain massless elementary degrees-of-freedom, gluons (gauge bosons) and quarks (matter fields); and yet gluons and quarks are predicted to acquire mass dynamically so that the only massless systems in QCD are its composite Nambu-Goldstone bosons.  All other everyday bound states possess nuclear-size masses, far in excess of anything that can directly be tied to the Higgs boson.  These points highlight the most important unsolved questions within the Standard Model, namely: what is the source of the mass for the vast bulk of visible matter in the Universe and how is this mass distributed within hadrons?  This presentation will provide a contemporary sketch of the strong-QCD landscape and insights that may help in answering these questions.

 

ics Colloquium
Friday, April 12, 2019
3:30 PM
Physics Building, Room 204
Dr. Vivek Goyal [Host: MIller Eaton]
Boston University
"Computing Images from Weak Optical Signals"
ABSTRACT:

In conventional imaging systems, the results are poor unless there is a physical mechanism for producing a sharp image with high signal-to-noise ratio.  In this talk, I will present two settings where computational methods enable imaging from very weak signals:  range imaging and non-line-of-sight (NLOS) imaging.

Lidar systems use single-photon detectors to enable long-range reflectivity and depth imaging.  By exploiting an inhomogeneous Poisson process observation model and the typical structure of natural scenes, first-photon imaging demonstrates the possibility of accurate lidar with only 1 detected photon per pixel, where half of the detections are due to (uninformative) ambient light.  I will explain the simple ideas behind first-photon imaging and lightly touch upon related subsequent works that mitigate the limitations of detector arrays, withstand 25-times more ambient light, allow for unknown ambient light levels, and capture multiple depths per pixel.

NLOS imaging has been an active research area for almost a decade, and remarkable results have been achieved with pulsed lasers and single-photon detectors.  Our work shows that NLOS imaging is possible using only an ordinary digital camera.  When light reaches a matte wall, it is scattered in all directions.  Thus, to use a matte wall as if it were a mirror requires some mechanism for regaining the one-to-one spatial correspondences lost from the scattering.  Our method is based on the separation of light paths created by occlusions and results in relatively simple computational algorithms.

Related paper DOIs:
10.1126/science.1246775
10.1109/TSP.2015.2453093
10.1109/LSP.2015.2475274
10.1364/OE.24.001873
10.1038/ncomms12046
10.1109/TSP.2017.2706028
10.1038/s41586-018-0868-6

ics Colloquium
Friday, March 29, 2019
3:30 PM
Physics Building, Room 204
Dr. Ho Nyung Lee [Host: Seunghun Lee]
Oak Ridge National Laboratory
"Interfaces in oxide quantum heterostructures"
ABSTRACT:

Complex oxides are known to possess the full spectrum of fascinating properties, including magnetism, colossal magneto-resistance, superconductivity, ferroelectricity, pyroelectricity, piezoelectricity, multiferroicity, ionic conductivity, and more. This breadth of remarkable properties is the consequence of strong coupling between charge, spin, orbital, and lattice symmetry. Spurred by recent advances in the synthesis of such artificial materials at the atomic scale, the physics of oxide heterostructures containing atomically smooth layers of such correlated electron materials with abrupt interfaces is a rapidly growing area. Thus, we have established a growth technique to control complex oxides at the level of unit cell thickness by pulsed laser epitaxy. The atomic-scale growth control enables to assemble the building blocks to a functional system in a programmable manner, yielding many intriguing physical properties that cannot be found in bulk counterparts. In this talk, examples of artificially designed, functional oxide heterostructures will be presented, highlighting the importance of heterostructuring, interfacing, and straining. The main topics include (1) charge transfer induced interfacial magnetism and topologically non-trivial spin textures in SrIrO3-based heterostructures and (2) lattice and chemical potential control of oxygen stability and associated electronic and magnetic properties in nickelate-and cobaltite-based heterostructures.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

ics Colloquium
Friday, March 22, 2019
3:30 PM
Physics Building, Room 204
Jim Gates, Ph.D. [Host: Diana Vaman]
Brown University
"A Mathematical Journey Thru SUSY, Error-Correcting Codes, Evolution, and a Sustainable Reality "
ABSTRACT:

This presentation describes an arc in mathematical/theoretical physics traversing concepts from equations, graphs, error-correction, and pointing toward an evolution-like process acting on the  mathematical laws that sustain reality.

ics Colloquium
Friday, March 1, 2019
3:30 PM
Physics Building, Room 204
Genya Kolomeisky [Host: Israel Klich]
University of Virginia - Physics
"Kelvin-Froude wake patterns of a traveling pressure disturbance"
ABSTRACT:

Water wave patterns behind ships fuel human curiosity because they are both beautiful and easily observed.  These patterns called wakes were famously described in 1887 by Lord Kelvin.  According to Kelvin, the feather-like appearance of the wake is universal and the entire wake is confined within a 39 degree angle.  While such wakes have been observed, deviations from Kelvin’s predictions have also been reported.  In this talk summarizing my work with UVA alumnus Jonathan Colen I will present a quantitative reasoning based on classical surface water wave theory that explains why some wakes are similar to Kelvin’s prediction, and why others are less so.  The central result is a classification of wake patterns which all can be understood in terms of the problem originally treated by Kelvin.

ics Special Colloquium


Wednesday, February 20, 2019
3:30 PM
Physics Building, Room 204
David Nichols [Host: Diana Vaman]
University of Amsterdam
"Gravitational waves and fundamental properties of matter and spacetime"
ABSTRACT:

Gravitational waves from the mergers of ten binary black holes and one binary neutron star were detected in the first two observing runs by the Advanced LIGO and Virgo detectors. In this talk, I will discuss the eleven gravitational-wave detections and the electromagnetic observations that accompanied the neutron-star merger. These detections confirmed many of the predictions of general relativity, and they initiated the observational study of strongly curved, dynamical spacetimes and their highly luminous gravitational waves. One aspect of these high gravitational-wave luminosities that LIGO and Virgo will be able to measure is the gravitational-wave memory effect: a lasting change in the gravitational-wave strain produced by energy radiated in gravitational waves. I will describe how this effect is related to symmetries and conserved quantities of spacetime, how the memory effect can be measured with LIGO and Virgo, and how new types of memory effects have been recently predicted. I will conclude by discussing the plans for the next generation of gravitational-wave detectors after LIGO and Virgo and the scientific capabilities of these new detectors. These facilities could detect millions of black-hole and neutron-star mergers per year, and they can provide insights on a range of topics from the population of short gamma-ray bursts to the presence of dark matter around black holes.

 

ics Special Colloquium


Wednesday, February 13, 2019
3:30 PM
Physics Building, Room 204
Eliu Huerta [Host: Diana Vaman]
University of Illinois at Urbana-Champaign
"Frontiers in Multi-Messenger Astrophysics at the interface of gravitational wave astrophysics, large scale astronomical surveys and data science "
ABSTRACT:

The next decade promises fundamental new scientific insights and discoveries from Multi-Messenger Astrophysics, enabled through the convergence of large scale astronomical surveys, gravitational wave astrophysics, deep learning and large scale computing. In this talk I describe a Multi-Messenger Astrophysics science program, and highlight recent accomplishments at the interface of gravitational wave astrophysics, numerical relativity and deep learning. I discuss the convergence of this program with large scale astronomical surveys in the context of gravitational wave cosmology. Future research and development activities are discussed, including a vision to leverage data science initiatives at the University of Virginia to spearhead, maximize and accelerate discovery in the nascent field of Multi-Messenger Astrophysics.

 

ics Special Colloquium


Wednesday, February 6, 2019
3:30 PM
Physics Building, Room 204
Robert Penna [Host: Diana Vaman]
Columbia University
"Black Hole Bridges"
ABSTRACT:

Black holes are bridges between astrophysics and fundamental physics.  I will describe three examples of this theme.  First, I will explain how contemporary theoretical ideas deriving from the holographic principle have proven useful for interpreting numerical simulations of electromagnetic outflows from spinning black holes.  These models are currently being tested against X-ray and radio observations of galactic black holes.  Second, I will describe a correspondence between black holes and lower dimensional fluids and discuss the possibility of probing this correspondence with gravitational wave memory experiments.  Finally, I will describe how gravitational wave observations of black hole tidal interactions might be used to find new symmetries acting on the event horizon.

ics Special Colloquium


Wednesday, January 30, 2019
3:30 PM
Physics Building, Room 204
Jeremy Sakstein [Host: Diana Vaman]
University of Pennsylvania
"Testing Gravity with Cosmology and Astrophysics"
ABSTRACT:

We are entering a golden age of cosmology and astrophysics. In the coming decade we will have cosmological data for over a billion galaxies, a census of objects in the Milky Way, and a network of gravitational detectors spanning the globe that will detect thousands of events per year. This presents us with the unprecedented opportunity to learn how gravity behaves at the largest distances, and in the most extreme environments. In this talk I will describe how we can use current and upcoming data to understand the unexplained mysteries of the Universe, such as why the expansion of the Universe accelerating (dark energy). I will also discuss how to connect physics in these disparate regimes and how to test cosmology on small scales. To maximize the discovery potential of the data requires us to construct robust theoretical models, identify novel probes, and connect theory with observation, and I will describe projects where I have attempted to accomplish this. I will conclude the talk by discussing how this interdisciplinary effort will continue into the next decade and beyond.    

 

ics Special Colloquium


Wednesday, January 23, 2019
3:30 PM
Physics Building, Room 204
Sarah Vigeland [Host: Diana Vaman]
University of Wisconsin Milwaukee
"Probing Massive and Supermassive Black Holes with Gravitational Waves"
ABSTRACT:

Observations have shown that nearly all galaxies harbor massive or supermassive black holes at their centers. Gravitational wave (GW) observations of these black holes will shed light on their growth and evolution, and the merger histories of galaxies. Massive and supermassive black holes are also ideal laboratories for studying strong-field gravity. Pulsar timing arrays (PTAs) are sensitive to GWs with frequencies ~1-100 nHz, and can detect GWs emitted by supermassive black hole binaries, which form when two galaxies merge. The Laser Interferometer Space Antenna (LISA) is a planned space-based GW detector that will be sensitive to GWs ~1-100 mHz, and it will see a variety of sources, including merging massive black hole binaries and extreme mass-ratio inspires (EMRIs), which consist of a small compact object falling into a massive black hole. I will discuss source modeling and detection techniques for LISA and PTAs, as well as present limits on nanohertz GWs from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration.

 

ics Colloquium
Friday, January 18, 2019
3:30 PM
Physics Building, Room 204
Markus Diefenthaler [Host: Simonetta Liuti]
Jefferson Lab
"The Electron Ion Collider Science "
ABSTRACT:

Quantum Chromodynamics (QCD), the theory of the strong interaction, is a cornerstone of the Standard Model of modern physics. It explains all nuclear matter as bound states of point-like fermions, known as quarks, and gauge bosons, known as gluons. The gluons bind not only quarks but also interact with themselves. Unlike with the more familiar atomic and molecular matter, the interactions and structures are inextricably mixed up, and the observed properties of nucleons and nuclei, such as mass and spin, emerge out of this complex system. To precisely image the quarks and gluons and their interactions and to explore the new QCD frontier of strong color fields in nuclei, the Nuclear Physics community proposes an US-based Electron Ion collider (EIC) with high-energy and high-luminosity, capable of a versatile range of beam energies, polarizations, and ion species. The community is convinced that the EIC is the right tool to understand how matter at its most fundamental level is made.

ics Special Colloquium


Thursday, January 17, 2019
3:30 PM
Physics Building, Room 204
Prem Kumar [Host: Bob Jones]
Northwestern University
"Quantum Engineering: A Transdisciplinary Vision"
ABSTRACT:

A global quantum revolution is currently underway based on the recognition that the subtler aspects of quantum physics known as superposition (wave-like aspect), measurement (particle-like aspect), and entanglement (inseparable link between the two aspects) are far from being merely intriguing curiosities, but can be transitioned into valuable, real-world technologies with performances that can far exceed those obtainable with classical technologies. The recent demonstration by the Chinese scientists of using a low-earth-orbit satellite to distribute entangled photons to two ground stations that are over a thousand kilometers apart is a stunning technological achievement—direct entanglement distribution over the best available fiber links is limited to a few hundred kilometers—and a harbinger of future possibilities for globally secure communications guaranteed by the power of quantum physics.

Harnessing the advantages enabled by superposition, measurement, and entanglement (SME)—the three pillars of quantum physics—for any given application is what is termed quantum engineering in general. In many instances, however, the details of the underlying science (high-temperature superconductivity, photosynthesis, avian navigation, are some examples) is still not fully understood, let alone how to turn the partially understood science into a potentially useful technology. Nevertheless, it has become clear in the last few decades that quantum engineering will require a truly concerted effort that will need to transcend the traditional disciplinary silos in order to create and sustain new breeds of science and technology communities that will be equally versed in quantum physics as they would be in their chosen area of technology. In this talk, I will present my vision for unleashing the potential of quantum engineering, taking some examples from ongoing and proposed research.

ics Special Colloquium


Wednesday, January 16, 2019
3:30 PM
Physics Building, Room 204
Stephen Taylor [Host: Diana Vaman ]
California Institute of Technology
"Frontiers of Multi-Messenger Black-Hole Physics"
ABSTRACT:

The bounty of gravitational-wave observations from LIGO and Virgo has opened up a new window onto the warped Universe, as well as a pathway to addressing many of the contemporary challenges of fundamental physics. I will discuss how catalogs of stellar-mass compact object mergers can probe the unknown physical processes of binary stellar evolution, and how these systems can be harnessed as standard distance markers (calibrated entirely by fundamental physics) to map the expansion history of the cosmos. The next gravitational-wave frontier will be opened within 3-6 years by pulsar-timing arrays, which have unique access to black-holes at the billion to ten-billion solar mass scale. The accretionary dynamics of supermassive black-hole binaries should yield several tell-tale signatures observable in upcoming synoptic time-domain surveys, as well as gravitational-wave signatures measurable by pulsar timing. Additionally, pulsar-timing arrays are currently placing compelling constraints on modified gravity theories, cosmic strings, and ultralight scalar-field dark matter. I will review my work on these challenges, as well as in the exciting broader arena of gravitational-wave astrophysics, and describe my vision for the next decade of discovery. 

ics Colloquium
Friday, December 7, 2018
3:30 PM
Physics Building, Room 204
Zohar Komargodski [Host: Marija Vucelja]
Stony Brook University
" Using Topology to Solve Strongly Coupled Quantum Field Theories"
ABSTRACT:

I will begin by describing an interacting model in Quantum Mechanics where exact results about the ground state can be established by using tools from topology. I will then argue that such tools are also useful for tackling interesting problems in Quantum Field Theory. In particular, I will review Yang-Mills theory and argue that using topology one can make several predictions about its possible phases. We will then also extend the considerations to Quantum Chromodynamics and discuss possible connections with particle physics phenomenology and with condensed matter physics.

ics Colloquium
Friday, November 30, 2018
3:30 PM
Physics Building, Room 204
Valery Nesvizhevsky [Host: Stefan Baessler]
Institut Laue Langevin, France
"A new approach to search for neutron-antineutron oscillations, and a couple of other phenomena based on neutron reflection from surface: gravitational and whispering-gallery quantum states of neutrons"
ABSTRACT:

“An observation of neutron-antineutron oscillations (n-n ̅), which violate both B and B — L conservation, would constitute a scientific discovery of fundamental importance to physics and cosmology. A stringent upper bound on its transition rate would make an important contribution to our understanding of the baryon asymmetry of the universe by eliminating the post-sphaleron baryogenesis scenario in the light quark sector. We show that one can design an experiment using slow neutrons that in principle can reach the required sensitivity of 1010 s in the oscillation time, an improvement of 104 in the oscillation probability relative to the existing limit for free neutrons. This can be achieved by allowing both the neutron and antineutron components of the developing superposition state to coherently reflect from mirrors. We present a quantitative analysis of this scenario and show that, for sufficiently small transverse momenta of n/n ̅ and for certain choices of nuclei for the n/n ̅ guide material, the relative phase shift of the n and n ̅ components upon reflection and the n ̅ annihilation rate can be small. While the reflection of n ̅ from surface looks exotic and counterintuitive and seems to contradict to the common sense, in fact it is fully analogous to the reflection of n from surface. The later phenomenon is well known and used in neutron research from its first years. We illustrate it with two selected example of gravitational and whispering-gallery quantum states of neutrons.”


[V.V. Nesvizhevsky, A.Yu. Voronin, Surprising Quantum Bounces, Imperial College Press, London, 2015]  

ics Colloquium
Friday, November 16, 2018
3:30 PM
Physics Building, Room 204
Mark Stiles [Host: Joe Poon & Avik Ghosh]
NIST
"Energy-efficient neuromorphic computing with magnetic tunnel junctions"
ABSTRACT:

Human brains can solve many problems with orders of magnitude more energy efficiency than traditional computers.  As the importance of such problems, like image, voice, and video recognition, increases, so does the drive to develop computers that approach the energy efficiency of the brain.  Magnetic devices, especially tunnel junctions, have several properties that make them attractive for such applications.  Their conductance depends on the state of the ferromagnets making it easy to read information that is stored in their magnetic state.  In addition, current can manipulate the magnetic state.  Based on this electrical control of the magnetic state, magnetic tunnel junctions are actively being developed for integration into CMOS integrated circuits to provide non-volatile memory.  This development makes it feasible to consider other geometries that have different properties.  I describe two of the computing primitives that have been constructed based on the different functionalities of magnetic tunnel junctions.  The first of these uses tunnel junctions in their superparamagnetic state as the basis for a population coding scheme.  The second uses them as non-linear oscillators in the first nanoscale “reservoir” for reservoir computing.

ics Colloquium
Friday, November 9, 2018
3:30 PM
Physics Building, Room 204
Marija Vucelja [Host: Bob Jones]
UVA-Physics
"Thermal relaxations, the Mpemba effect, and adaptation of bacteria "
ABSTRACT:

Most of my talk will be about anomalous thermal relaxations, such as the Mpemba effect. Towards the end of my talk, I will also highlight a few topics in population dynamics that I have been working on. 

 

The Mpemba effect is a phenomenon when "hot can cool faster than cold" - a “shortcut” in relaxation to thermal equilibrium. It occurs when a physical system initially prepared at a hot temperature, cools down faster than an identical system prepared at a colder temperature. The effect was discovered as a peculiarity of water. Despite following observations in granular gasses, magnetic alloys, and spin glasses, the effect is still most often referred to as an “oddity” of water, although it is widespread and general.  I will describe how to define a Mpemba effect for an arbitrary physical system, and show how to quantify and estimate the probability of the Mpemba effect on a few examples. 

 

In the remaining time, I will briefly talk about the adaptation of bacterial populations and the immune system of bacteria with CRISPR.  Besides being the biology's newest buzzword and favorite gene editing tool, CRISPR is also a mechanism that allows bacteria to defend adaptively against phages and other invading genomic material. From the standpoint of physics and biology, the coevolution of bacteria and phages yields fascinating open questions. 

ics Colloquium
Friday, October 26, 2018
3:30 PM
Physics Building, Room 204
Susan Coppersmith [Host: Despina Louca]
University of Wisconsin - Madison
""Building a Quantum Computer Using Silicon Quantum Dots""
ABSTRACT:

The steady increase in computational power of information processors over the past half-century has led to smart phones and the internet, changing commerce and our social lives.  Up to now, the primary way that computational power has increased is that the electronic components have been made smaller and smaller, but within the next decade feature sizes are expected to reach the fundamental limits imposed by the size of atoms.  However, it is possible that further huge increases in computational power could be achieved by building quantum computers, which exploit in new ways of the laws of quantum mechanics that govern the physical world.  This talk will discuss the challenges involved in building a large-scale quantum computer as well as progress that we have made in developing a quantum computer using silicon quantum dots.  Prospects for further development will also be discussed.

ics Colloquium
Friday, October 5, 2018
3:30 PM
Physics Building, Room 204
Dragana Popovic [Host: Despina Louca]
Florida State University
"Unveiling the Normal State of Cuprate High-Temperature Superconductors: Hidden Order of Cooper Pairs"
ABSTRACT:

Many unusual properties of strongly correlated materials have been attributed to the proximity of quantum phase transitions (QPTs), where different types of orders compete and coexist, and may even give rise to novel phases.  In two-dimensional (2D) systems, the nature of the magnetic-field-tuned QPT from a superconducting to a normal state has been widely studied, but it remains an open question.  Underdoped copper-oxide high-temperature superconductors are effectively 2D materials and thus present a promising new platform for exploring this long-standing problem.  Although in cuprates the normal state is commonly probed by applying a perpendicular magnetic field (H) to suppress superconductivity, the identification and understanding of the H-induced normal state has been a challenge because of the complex interplay of disorder, temperature and quantum fluctuations, and the near-universal existence of charge-density-wave correlations. 

 

This talk will describe recent experimental advances in identifying and characterizing a full sequence of ground states as a function of H in underdoped cuprates.  In both the absence and the presence of charge order, the results demonstrate the key role of disorder in the H-tuned suppression of 2D superconductivity, giving rise to an intermediate regime with large quantum phase fluctuations, in contrast to the conventional scenario.  Most strikingly, the interplay of the “striped” charge order with high-temperature superconductivity leads to the emergence of an unanticipated, insulatinglike ground state with strong superconducting phase fluctuations, suggesting an unprecedented freezing (i.e. “the hidden order”) of Cooper pairs.  Possible scenarios will be discussed, including the implications of the results for understanding the physics of the cuprate pseudogap regime, as well as other 2D superconductors.

ics Colloquium
Thursday, October 4, 2018
3:30 PM
Physics Building, Room 204
Utpal Chatterjee [Host: Bob Jones]
University of Virginia - Department of Physics
"Charge density wave phase transitions in transition metal dichalcogenides"
ABSTRACT:

Layered transition-metal dichalcogenides (TMDs) are well known for their rich phase diagrams, which
encompass diverse quantum states including metals, semiconductors, Mott insulators, superconductors, and
charge density waves (CDWs). For instance, 2H-NbSe2 and 2H-TaS2 are canonical incommensurate CDW
systems, while 1T-TiSe2 harbors a commensurate CDW order. There is a coexistence/competition of CDW
and superconductivity in 2H-NbSe2 and 2H-TaS2, though this is not the case for pristine 1T-TiSe2. A subtle
interplay of CDW and superconducting orders, however, appears in each of these materials via chemical
intercalation or under pressure. Such a competition between or coexistence of proximate broken-symmetry
phases resembles many aspects of the phase diagram of cuprate high temperature superconductors
(HTSCs)—particularly, in the underdoped regime where the enigmatic pseudogap phase exists. The origin
of the CDW order in these compounds is an intriguing puzzle despite decades of research. We will present
our experimental data, which combine Angle Resolved Photoemission Spectroscopy, Scanning Tunneling
Spectroscopy, scattering and transport measurements, to provide new insights into the relative importance
of lattice and Coulomb effects in the CDW transitions of these compounds. These studies will also highlight
the distinctive impacts of disorder and doping in commensurate and incommensurate CDW systems.
Finally, comparing spectroscopic features of the CDW state of the TMDs with those of the normal state
underdoped HTSCs, we will discuss whether a CDW order can possibly be the origin of the pseudogap
phase in the cuprates.

ics Colloquium
Friday, September 28, 2018
3:30 PM
Physics Building, Room 204
Roxanne Springer [Host: Simonetta Liuti]
Duke University
"Feynmanʼs Footprints: Quantum Field Theory in Nuclear and Particle Physics"
ABSTRACT:

2018 is the 100th Anniversary of the birth of Richard Feynman.
His discoveries and new formalisms, and the way he thought about
solving problems, transformed the way we think
about physics. I will talk about examples of how these impacted present results
in nuclear and particle physics.

I will also expand upon what might be called Feynmanʼs Scientific Method,
and how by following that method we can become better scientists ourselves
and nurture the next generation of scientists.

ics Colloquium
Monday, September 24, 2018
3:30 PM
Physics Building, Room 203
Diana Vaman [Host: Bob Jones]
University of Virginia - Physics
"Emergent Gravity"
 
 Slideshow (PDF)
ABSTRACT:

Is gravity a fundamental force? I will discuss a few scenarios in which gravity emerges from the dynamics of some underlying field theory. In holography (or AdS/CFT correspondence), Einstein's equations for the bulk gravity dual are linked to entanglement in the boundary field theory. In another example, the graviton emerges as a composite spin two massless particle in a scalar field theory.

SLIDESHOW:
Colloquia and Special Lectures Committee
Brad Cox (Chair)
P.Q. Hung (Member)
Israel Klich (Member)
Seunghun Lee (Member)
Peter Schauss (Member)
Jeffrey Teo (Member)
Marija Vucelja (Member)

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To add a speaker, send an email to bbc2x@Virginia.EDU Include the seminar type (e.g. Colloquia), date, name of the speaker, title of talk, and an abstract (if available). [Please send a copy of the email to phys-speakers@Virginia.EDU.]