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Colloquium

During the 2020 Fall semester, all Physics and Astronomy Colloquia will be presented remotely using Zoom on Thursdays at 4pm CT.  For details please contact Ashley E. Brammer by email (ashley.e.brammer@vanderbilt.edu) or phone (615-343-7389).

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Spring 2016

Thursday January 14th 2021 12:00 AM

Yuri Oganessian, Scientific Leader of the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Russia

Synthesis of Super Heavy Nuclei: a look at new possibilities    (show abstract)

The discovery of super heavy elements has posed many questions in terms of further investigations of heaviest nuclei near the borders of their existence. We will consider the processes of complete and incomplete fusion of complex nuclei at the strong Coulomb interaction. We will discuss also the advantages of hot fusion reactions and the perspectives of synthesis of heavy nuclei with large neutron excess. It is expected that launching the SHE-Factory in Dubna will significantly increase the luminosity of the experiment. This will help in the exploring of rare reaction channels and rare decays that will expand our knowledge of the structure of the heaviest nuclei. We will also show the current construction of a new complex at the SHE-Factory in figures and photographs.

Host: J. Hamilton

Thursday January 21st 2021 12:00 AM

Alfredo Gurrola, Department of Physics & Astronomy, Vanderbilt University

Searching for Dark Matter at the LHC: finding the weak amongst the strong    (show abstract)

The nature and identity of the dark matter of the universe is one of the most challenging problems in science today. Multiple study groups have identified the nature of dark matter as one of the important questions of our time. Only 5% of the energy density of the universe can be associated with known forms of matter and a host of new precision measurements have provided clear and surprising evidence that the universe is composed of approximately 23% dark matter. The Large Hadron Collider (LHC) was built to probe what are believed to be two intimately connected pillars in our understanding of the universe: the Higgs Boson and dark matter. Since the discovery of the Higgs Boson with Run 1 data from the LHC, the new “holy grail” of collider physics is to be able to produce and study dark matter in the laboratory. The current status of the searches for dark matter at the LHC will be described. In doing so, the ideas and strategies behind the searches, channels considered, and the experimental techniques used by scientists at the LHC will be outlined. Some focus will be placed on the role Vanderbilt has played in the search for dark matter and the role it will play in the years to follow. In particular, the Vanderbilt group has championed a new idea to probe dark matter with vector boson fusion (VBF) events. By tagging LHC events produced by VBF processes, it is possible to reduce standard model backgrounds sufficiently to probe unexplored regions of “new physics” phase space, including regions which were previously thought to be accessible only at future facilities.

Host: R. Scherrer

Thursday January 28th 2021 12:00 AM

William Holmes, Department of Physics & Astronomy, Vanderbilt University

A Unified Model for a Diverse Array of Migratory Cell Behaviors    (show abstract)

How cells make key decisions is an important, if mysterious question that has generated sustained investigation in molecular biology, mathematical modeling, and biopyhsics. Many types of migratory cells react in different ways to changes in their environment. Typical responses might include spreading to form greater contact area with the substratum, contracting to avoid contact, polarizing to prepare for motility, or generating highly dynamic waves of intracellular activity. Each of these behaviors is initiated by the spatial reorganization of a complex network of regulatory proteins (most notably for this discussion, the Rho GTPases) that promote biophysical remodeling of the cell. Here I will discuss how a simple and highly evolutionarily conserved set of molecular machinery can give rise to this diverse array of cellular behaviors. As part of this discussion, I will also describe a new perturbation technique, the “Local Perturbation Analysis”, that has proven immensely useful in investigating this question.

Host: R. Scherrer

Thursday February 11th 2021 12:00 AM

Frank van den Bosch, Department of Astronomy, Yale University

Coming of Age in the Dark Sector    (show abstract)

The assembly of dark matter halos is a highly non-linear process that is typically addressed using N-body simulations. However, since the initial conditions are Gaussian, and gravity is scale-free, it is also possible to construct simple analytical models that capture the essentials of structure formation in a CDM cosmology. In this talk I present a new, analytical model for the assembly of dark matter halos that can be used to predict their structure, their substructure population, and their detailed assembly history. I show that the model is as accurate as N-body simulations, but with the advantage that it is orders of magnitude faster, and gives powerful insights. I present two applications; a statistical assessment of the "Too-Big-To-Fail" problem, and a demonstration that the growth history of dark matter potential wells is universal. Finally, I present some preliminary results of a detailed study aimed at understanding the stripping, heating and disruption of dark matter subhalos.

Host: K. Holley-Bockelmann

Thursday February 18th 2021 12:00 AM

Szabolcs Marka, Imre Bartos and Zsuzsa Marka, Department of Physics, Columbia University

The Discovery of Gravitational-Waves and the First Observation of a Binary Black Hole System    (show abstract)

On September 14th 2015 the gravitational wave signature of a binary black hole merger was detected by the LIGO observatories. This marks the beginning of a completely new era of modern physics, the dawn of gravitational-wave astrophysics. We will discuss the discovery, its impact and its consequences.

Host: T. Weiler

Thursday February 25th 2021 12:00 AM

Sharon Weiss, Department of Electrical Engineering, Vanderbilt University

Light-matter interaction in silicon    (show abstract)

Silicon has traditionally been associated with being the most favorable material platform for most modern microelectronics technologies due to its electronic properties, compatibility with lithographic patterning, and earth abundance. However, silicon is also a favorable material platform for supporting light propagation due in part to the large refractive index contrast between silicon and air or a covering oxide. In this talk, I will discuss design approaches for enhancing light-matter interaction on a silicon platform, many of which take advantage of the same lithographic patterning approaches used to fabricate silicon microelectronics components. Several applications of silicon photonic components exhibiting strong light-matter interaction in optical communication and optical biosensing will be discussed.

Host: S. Hutson

Thursday March 4th 2021 12:00 AM

Caleb Scharf, Director of Astrobiology, Columbia University

Astrobiology: The Science of Life In The Universe    (show abstract)

For a very long time our species has asked the questions: Are we alone? Where do we come from? And for a very long time we've not had many good answers. This is starting to change. With the exploration of our solar system, the discovery of thousands of exoplanets, and a renewed quest to understand the origins of life, there is real progress. I'll review some of this science, including work on building virtual worlds at Columbia University, and will try to take a peek into the near future of astrobiology.

Host: R. Scherrer

Thursday March 18th 2021 12:00 AM

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Thursday March 25th 2021 12:00 AM

Savvas Koushiappas, Department of Physics, Brown University

Dark matter in the cosmos — are we close to a discovery?    (show abstract)

Searching for something that we don’t really understand is very difficult. This is certainly the case with dark matter, a problem that has been around in physics for more than 30 years. After many years of predictions, experimental developments and innovations, we are now at an epoch where we begin to test the hypothesis of dark matter that interacts with gravity and could potentially interact via other forces. I will give an overview of the status of the field and discuss the complementarity of current experimental probes. In addition, I will show that cosmology and the intricate particle physics underlying large scale structure play a key role in any potential interpretation of a dark matter signal.

Host: R. Scherrer

Thursday April 8th 2021 12:00 AM

Susan Stewart, Astronomer, U.S. Naval Observatory

Celestial Navigation: Looking to the Stars in the Age of Modern Navigation    (show abstract)

Techniques employing stellar observations for terrestrial navigation have been used effectively for many centuries. As navigation methods have advanced, the practice of celestial navigation has largely been overshadowed by the use of GNSS (Global Navigation Satellite Systems, aka GPS). However, celestial navigation is still quite relevant and of particular interest to the Department of Defense, as the rising role of cyberwarfare and the proliferation of cheap but powerful "jammers” has raised concerns that satellite navigational methods may be potentially compromised. As such, redundant, independent backup navigation systems are required on US warships and aircraft to eliminate a single point of failure. The vital role of celestial navigation is underscored by the recent development and improvement of systems which rely on implicit knowledge of the principles of celestial navigation and prediction of precise stellar positions. In this talk, I will illustrate the increased need for precise stellar astrometry in light of the development of these new systems. I will also discuss the vulnerabilities of GNSS and outline both the principles and techniques of celestial navigation. The vital need for celestial navigation instruction and Vanderbilt’s role in developing an online learning module which will have a high impact in this effort will be presented.

Host: R. Scherrer

Thursday April 15th 2021 12:00 AM

Jin Wang, Professor of Chemistry and Physics, SUNY Stony Brook

The potential and flux landscape for physical and biological nonequilibrium systems    (show abstract)

The complex systems are everywhere around us ranging from the physical to the biological objects. These are often open systems with inputs of energy and information from outside. Uncovering the organization principles and physical quantification of the complex non-equilibrium open systems are essential for understanding the global function and stability. This presents us a great challenge. In this talk, we summarized our recent efforts in this direction. We found that the dynamics of the complex nonequilibrium systems can be determined by the two driving forces. One is the gradient of the underlying landscape and the other is from the curl flux in an analogy to the force experienced of an electron moving in the electric and magnetic field. The underlying landscape is linked to the probability distribution of the steady state and provides a global quantification for describing the complex nonequilibrium system. We found that the landscape can be used to quantify the global stability and robustness of the system. The non-zero flux breaks the detailed balance and therefore gives a quantitative measure of how far away the system is from the equilibrium state, reflecting the degree of the energy/material/information input to the system. Our decomposition of the driving forces of the complex nonequilibrium systems into landscape gradient and curl flux establishes the link between the dynamics and the underlying thermodynamic non-equilibrium natures. We further have uncovered the non-equilibrium versions of the optimal kinetic paths, the transition state theory, and the fluctuation-dissipation theorem, which are all deviated from their equilibrium counter-parts. We applied our theory to several physical and biological systems such as cell cycle, stem cell differentiation and reprograming, cancer, neural networks, evolution, ecology and chaos [J. Wang, Landscape and flux theory of non-equilibrium dynamical systems with application to biology, Advances in Physics, 64:1, 1-137. (2015)]. For cell cycle oscillations, we found the underlying landscape has a Mexican hat ring shape topology. The height of the Mexican hat determines the global stability. The landscape gradient attracts the system down to the oscillation ring. The curl flux is the driving force for coherent oscillation on the ring. The flux and landscape together determines the global function (such as period and amplitude) of the cell cycle oscillation. The global sensitivity analysis according to the landscape and flux reveals the key genes and regulations controlling the cell cycle, important for anti-cancer strategy.

Host: E. Rericha

Thursday April 22nd 2021 12:00 AM

Ilias E. Perakis, Department of Physics, University of Alabama, Birmingham (UAB)

Exploring Non-equilibrium Phases of Complex Materials via Femtosecond-Quantum-Tuning    (show abstract)

Understanding the physical properties of complex materials ranging from high temperature superconductors to biological systems is complicated by the strong coupling of many different degrees of freedom. Learning how to dissect such couplings is important not only for understanding perplexing physical phenomena such as high temperature superconductivity or colossal magneto-resistance but also for learning how to manipulate materials far away from equilibrium. The doped perovskite manganites of interest here display an extreme sensitivity to small perturbations such as magnetic or optical fields, as well as a variety of different phases and phase transitions that remain mysterious. Although strong interactions of electronic, magnetic, and lattice degrees of freedom are believed to lead to coexisting insulating/lattice distorted/antiferromagnetic and metallic/undistorted/ferromagnetic phases, the relevant quasi-particles have not yet been characterized. By exciting these complex phases with intense laser pulses shorter than the fundamental timescales, new metastable states can arise that are not accessible close to equilibrium. In this talk I will discuss how pump-probe optical and magneto-optical spectroscopy with femtosecond time resolution can dissect a complex order parameter by “suddenly” bringing the system away from equilibrium and then monitoring the time evolution by taking snapshots in a way analogous to a stroboscope. I will discuss recent femtosecond-resolved pump-probe measurements that show the emergence of two distinct fs and ps charge relaxation times, but only when the photo-excitation intensity exceeds a threshold for antiferro-to-ferromagnetic switching during the 100 fs laser pulse. We interpret the observation of such intensity-dependent bi-exponential relaxation correlated with femtosecond magnetism as the signature of a new non-equilibrium phase, where fast, metallic quasi-electrons dressed by quantum spin fluctuations coexist with the slow, localized polaronic carriers that dominate in the ground state. Such coexistence favors laser-driven ferromagnetic correlations created within femtoseconds, i.e. faster than the period of lattice oscillations and prior to any thermalization. The possibility of complex order parameter “vivisection” in real time suggests new directions for manipulating materials phases far from equilibrium using terahertz, mid-infrared, and X-ray pulses. At the same time, it may facilitate a more profound understanding of a broader class of complex materials that remain perplexing after many decades of studies.

Host: N. Tolk

Fall 2015

David Awschalom, Institute for Molecular Engineering, University of Chicago

Beyond electronics: abandoning perfection for quantum technologies    (show abstract)

Our technological preference for perfection can only lead us so far: as traditional transistor-based electronics rapidly approach the atomic scale, small amounts of disorder begin to have outsized negative effects. Surprisingly, one of the most promising pathways out of this conundrum may emerge from recent efforts to embrace defects and construct 'quantum machines’ to enable new information technologies based on the quantum nature of the electron. Recently, individual defects in diamond and other materials have attracted interest as they possess an electronic spin state that can be employed as a solid state quantum bit at and above room temperature. Research at the frontiers of this field includes creating and manipulating these unusual states in a new generation of nanometer-scale structures. These developments have launched technological efforts aimed at developing applications ranging from secure data encryption to radical improvements in computation speed and complexity. We will describe recent advances towards these goals, including the surprising ability to control atomic-scale spins for communication and computation within materials surrounding us for generations.

Host: R. Scherrer

Thursday September 16th 2021 12:00 AM

Greg Tarlé, Department of Physics, University of Michigan

Exploring the Fabric of Space-Time with DESI    (show abstract)

It has been seventeen years since the discovery of the accelerating universe. While most have attributed the acceleration to a type of vacuum energy called Dark Energy, others have suggested that modifications to General Relativity may be required. An ambitious new experiment, the Dark Energy Spectroscopic Instrument (DESI), will see first light on the Mayall Telescope early in 2019. In an attempt to reveal the underlying mechanism of cosmic acceleration, DESI will utilize a combination of Baryon Acoustic Oscillations and Redshift Space Distortions to map the expansion history of the universe and the growth of structure to unprecedented precision. In doing do, DESI may ultimately reveal to us the true nature of the “fabric” of empty space.

Host: R. Scherrer

Mario Affatigato, Fran Allison and Francis Halpin Professor of Physics, Coe College

Becoming a teacher-scholar at a small liberal arts college    (show abstract)

The myths surrounding physics professors at small liberal arts colleges range from the idea that they only engage in (many hours) of teaching, to the belief that such schools are ideal for people who want to quit research after graduate school. Nothing could be further from the truth. Small colleges can be wonderful environments on which to build highly successful research programs while maintaining a strong teaching relationship with undergraduate students. In this talk we will explore the rewards and pitfalls of establishing such a program, and spend significant time presenting the research projects undertaken by the speaker at the Coe College Physics Department. From developing glasses for CERN detectors to bactericidal glass, from levitation of molten beads to industrial applications and time of flight mass spectrometry, the variety of scientific questions and challenges has helped develop many undergraduates into strong researchers, and kept the speaker happily involved in materials science. The talk will be of special interest to graduate and undergraduate students considering careers at smaller institutions, and also for professors who are constantly mentoring the next generation of doctoral candidates for the workforce.

Host: R. Scherrer

Thursday September 30th 2021 12:00 AM

Andrew Steigerwald, Senior Advisor in the Advanced Manufacturing Office (AMO), Department of Energy

How many watts are in a gigawatt? One perspective from the intersection of science and public policy    (show abstract)

The diffusion of advanced technologies into daily life has led to increasing calls for deeper engagement between science and policymakers, both to communicate the value of the scientific endeavor while aiding in the creation of sound public policy. This raises two important questions to consider: What does it mean for the scientific community to engage with policymakers, and what does an effective engagement look like in reality? This talk will answer neither of these questions, but—through an illustrative example of how the National Network for Manufacturing Innovation (NNMI) initiative evolved through the legislative process from idea to reality—hopes to provide context for others to explore these questions and their potential role in addressing them.

Host: N. Tolk

Thursday October 7th 2021 12:00 AM

Lisa Manning, Department of Physics, Syracuse University

Understanding asthma and developing embryos using insights from mayonnaise    (show abstract)

Biological tissues involved in important processes such as embryonic development, lung function, wound healing, and cancer progression have recently been shown to be close to a liquid-to-solid or "jamming" transition, similar to the one that occurs when oil and water are mixed to make mayonnaise. In mayonnaise and materials like it, a disordered liquid-to-solid transition occurs when the packing density of oil droplets increases past a critical threshold, and over the past 20 years physicists have made great progress in understanding the universal nature of this transition. However, existing theories cannot explain observations of jamming transitions in confluent biological tissues, where there are no gaps between cells and the packing density is always unity. I will discuss a new theoretical framework for predicting rates of cell migration in biological tissues, and show that this model predicts a novel type of critical rigidity transition, which takes place at constant packing density and depends only on single cell properties such as the number of adhesion molecules expressed. I will show that our a priori theoretical predictions with no fit parameters are precisely realized in cell cultures from human patients with asthma, and discuss how these ideas might also be applied to understand other processes in embryonic development and cancer progression.

Host: S. Hutson

Thursday October 14th 2021 12:00 AM

Gregory Benford, Department of Physics & Astronomy, University of California, Irvine

Our Next Century in Space    (show abstract)

This century may well see the opening of the solar system to the productive use of near-Earth space and far beyond. Manufacturing using existing mass in Earth orbit, plus 3D printing, can greatly lower the costs of operating in space. Already companies are prospecting for asteroid mining. Ability to move large masses will come from nuclear thermal rockets, already developed. Some beam-powered sails and high-specific-impulse ion rockets may be useful for fast, light loads. Asteroids are rich in volatiles and thus propellants, making transport cheaper. Platinum group metals and rare earths seem prominent assets to be gained, especially when Earthside supplies begin to run out. Other efficiencies in solar system economics will doubtless appear as the frontier expands.

Host: R. Scherrer

Thursday October 28th 2021 12:00 AM

James Peebles, Professor of Physics and Albert Einstein Professor of Science Emeritus, Princeton University

The Renaissance of General Relativity   (show abstract)

In the mid 1950s, applications of Einstein's theory of gravitation, general relativity, were still limited to the classical three tests, the measurement of the precession of the perihelion of the planet Mercury, and the marginal detections of deflection of light by the Sun and gravitational redshift of light from white dwarf stars. This meager relation between theory and observation began improving in the decade that followed as new technology drove and was driven by new ideas. I will discuss three examples: precision measurements of the acceleration of gravity on Earth, the orbit of the Moon, and the properties of the sea of thermal radiation left from the early stages of expansion of the universe. These and other experiments have produced a rich network of evidence that now makes the empirical case for general relativity theory about as convincing as it gets in physical science.

Host: R. Scherrer

Thursday November 4th 2021 12:00 AM

Pasquale L. Iafelice, Director of Forensic Investigations, Italian State Police - Department of Public Security

A physicist at the crime scene    (show abstract)

Entering into a crime scene appears to be the most natural consequence of being engaged into a law enforcement agency. Many television series, books and novels are inspired by crime phe-nomenology and by the enigma arising from a crime scene. This place may be considered as the contact point of two worlds: the criminal world and the world of justice, naively representing the point in the space-time where the criminal intent finishes (sadly, it only stops for a while) and investigation begins. The history of Criminalistic and Forensic Science represent the constant effort that scientists have devoted over the the past centuries to help in investigating and solving crimes. As a result, a plethora of forensic disciplines are present today: fingerprints, ballistics, tool marks, gunshot residues, bloodstain patterns, drugs, explosives, genetic and digital analysis, just to name a few. All of these are based on techniques borrowed from natural sciences and they evolve, but yet slowly, according to our degree of knowledge. However, they represent a very peculiar sector of applied science for two main reasons: the phase of collecting data (evidence) from a crime scene is not repeatable; the results of the analysis are required to be validated in Court, in addition to being scientifically correct. How does a physicist handle the enigma of a crime and how can the issue of reproducibility be taken into account to ensure scientific validity of the results? Based on real cases, we will discuss forensic analysis performed using physical techniques, pointing out their contribution to the investigation, and we will address some open questions, particularly the problem of micro-traces on fired bullets recovered at crime scene, in a context where the amount of uncertainty can make the difference between a verdict of guilty or innocent.

Host: T. Kephart

Tuesday November 9th 2021 12:00 AM

Catherine Espaillat, Department of Astronomy, Boston University

Tracking Planet Footprints in Dusty Disks    (show abstract)

We know that most stars were once surrounded by protoplanetary disks. How these young disks evolve into planetary systems is a fundamental question in astronomy. Observations of T Tauri stars (TTS) may provide insights, particularly a subset of TTS with “transitional disks” that contain holes or gaps in their dust disk. Many researchers have posited that these holes and gaps are the “footprints” of planets given that theoretical simulations predict that a young, forming planet will clear the material around itself, leaving behind a cavity in the disk. In this talk, I will review the key observational constraints on the dust and gas properties of transitional disks and examine these in the context of theoretical planet-induced disk clearing models. I will also discuss possibilities for future work in this field in the era of ALMA observations.

Host: R. Scherrer

Thursday November 11th 2021 12:00 AM

David Furbish, Department of Earth & Environmental Sciences, Vanderbilt University

An Earthy Version of Classical Statistical Mechanics Applied to Sediment Particles    (show abstract)

The movement of sediment on Earth’s hillslopes and within its rivers is a fundamental ingredient of the evolution of landscapes, the construction of river floodplains and deltas, and the intermittent transport and redistribution of sediment-borne substances, including nutrients, over the landscape. Much of the work on this topic over the past 50 years has focused on predicting the rate of sediment movement in response to fluid motions, and it has mostly adopted a deterministic, continuum based approach, despite the fact that sediment motions are patchy, intermittent and mostly rarefied — conditions that are at odds with continuum formulations. The work that I will describe in this talk involves stepping back, and taking a fresh look. Our approach involves building a framework, informed by experiments and advanced computations, that appeals to ideas and methods of classical statistical mechanics. This involves reconfiguring our thinking around the idea of a statistical ensemble of sediment particle systems in a manner similar to that envisioned by Gibbs for gas particle systems, then focusing on statistically expected behavior. I will focus on the example of bed load sediment transport within turbulent shear flows — outlining ‘our’ version of a Gibbs-like ensemble — then present a formulation of the sediment particle velocity distribution (analogous to the Maxwell-Boltzmann distribution for gas particles), touch on the connection between this formulation and Edwin Jaynes’s principle of maximum entropy, and finish with the idea of sediment particle diffusion, including anomalous diffusion.

Host: S. Hutson

Alan Calder, Department of Physics & Astronomy, SUNY Stony Brook

Stellar Explosions Known as Supernovae    (show abstract)

Supernovae are bright astronomical explosions that blazon the violent deaths of stars. Supernovae fall into two principal theoretical classifications, the collapse of the core of a massive star and a thermonuclear runaway occurring in a compact star known as a white dwarf. These events produce and disseminate heavy elements and the thermonuclear class serves as the premier distance indicators for cosmological studies. I will present highlights from research into these fascinating events, focusing mainly on recent studies of systematic effects on the brightness of thermonuclear supernovae. The results show reasonable agreement with observed correlations between brightness and the age or composition of the host galaxy and thereby offer insight into a source of significant uncertainty in cosmological studies.

Host: R. Scherrer

Thursday November 25th 2021 12:00 AM

Warren Skidmore, Thirty Meter Telescope Observatory

Thirty Meter Telescope: The Next Generation of Ground Based Optical/Infra Red Observatory    (show abstract)

After a short construction status update I will discuss some of the observational capabilities that the Thirty Meter Telescope will provide and some of the areas of study that will benefit from the TMT's capabilities. I'll describe how the telescope design was developed to support a broad range of observing capabilities and how the observatory is being engineered. Finally I'll describe the avenues through which astronomers can actively participate in the project; in the planning for a potential TMT/NSF partnership, preparing for the development of 2nd generation instruments and directing the scientific aims for the observatory.

Host: R. Scherrer