Physics Forum Schedule

2017, Fall Semester

This semester, Physics Forum will be held every other Thursday at 3:45pm, in the Interaction Zone (Howey S105).  The planned speakers will be updated below.

11-30  Greg Richards – A Brief Guide to Pulsar Astro(particle) Physics
 Pulsars are highly magnetized, rapidly rotating neutron stars that radiate across the entire electromagnetic spectrum. Since the first pulsar detection in the late 1960s, these odd stars have been the subject of intense scientific study in order to better understand the structure of the magnetosphere and the specifics of the particle acceleration leading to the observed radiation. In this talk, I will start by covering the basic understanding of what pulsars are, followed by a synopsis of the headaches involved in detecting and timing pulsars via measurement of their periodic emission signature. Subsequently, I will discuss the (simple) methods used to deduce various pulsar properties, including density limits, rotational energies, surface magnetic field strength limits, and approximate ages. In the remainder of the talk, I will focus on the gamma-ray emission from pulsars, which was the subject of my PhD thesis work.

11-16  Bo Broadwater – DNA Strand Displacement at the Base Pair Level
 Strand displacement is a swapping reaction involving 3 nucleic acid strands where a single strand invades a duplex and removes a similar strand through a process called branch migration. This characteristic switching behavior has been widely adopted in nanotechnology to build DNA walkers, DNA sorting robots, and structures that self assemble. In biology, strands exchange in many fundamental reactions including homologous recombination (responsible for DNA repair and a source of genetic diversity) and the revolutionary gene editing tool, the CRISPR/Cas9 system. Despite its wide ranging importance, there have been few single molecule studies that seek to answer basic questions about the biophysics of strand displacement and branch migration. In this week’s physics forum, I will discuss my work to uncover the effect of base pair mismatches on strand displacement. Further, I will present the first direct measurement of branch migration first passage times as well as surprising sequence dependence. Also, I will show how these behaviors can be described by physically intuitive models. Finally, I will present plans for future study that will provide deep insight to the biophysics of branch migration.

11-02  Kevin Driscoll – Long Wavelength Theory of Strongly Interacting Cold Atoms in Few- and Many-body Systems
Using Feshbach resonance to tune the scattering length, cold atom experiments today can access strongly interacting regimes where typical perturbative theories fail. However, since the scattering length becomes large compared to all of the microscopic length scales, we expect that the scattering length dominates the physics at wavelengths much longer than these length scales, irrespective of the microscopic details. Thus, we seek effective theories that replace the true microscopic interactions with analytically tractable alternatives and yet remain approximately valid for all wavelengths much larger than the range of the microscopic interactions. Typically, we first investigate few-body systems both analytically and numerically and determine the prevalence and character of their eigenstates near zero energy. Then, we identify aspects of the corresponding many-body system which inherit a structure similar to their few-body counterparts. Our goal is to develop both intuitive characterizations and quantitative relations for strongly interacting many-body systems.

10-19  Grant Nunn – Magnetic surface anisotropy in nickel nanoparticles
In this talk, we discuss the study of Tunneling Magnetoresistance and spin transfer torque in single magnetic nanoparticles using electron tunneling at 6K and 30mK. In an unexplored size range, these particles, made from transition metal ferromagnets, contain approximately 100-1000 magnetic atoms. Due to the size, the magnetic properties of the nanoparticles are fundamentally different than the properties of the bulk. Analyzing current as a function of magnetic field, we observe an abundance of magnetic hysteresis in Co but not Ni at 6K. Analytical methodologies yield large errors because the energy levels are given by the difference between two much larger energies. By conducting tunneling spectroscopy of the energy levels, and measuring the magnetic properties of those levels as a function of the magnetic field, we can obtain information unavailable analytically. Using the Neél model of surface anisotropy and numerical methods, we find a strong correlation of measured Tunneling Magnetoresistance with calculated surface anisotropy of the magnetic nanoparticles.

10-05  Ravi Pallantla – A new approach to understand turbulence, “the greatest unsolved problem of classical physics.”
Turbulence in fluid flows is omnipresent. It needs to be understood and (if possible) controlled, for example, to have a smooth flight or survive a heart attack. Humankind has been trying to understand it not for years or decades, but for centuries, to limited success. Nonetheless, scientists persistently try new methods until the science is understood. As a result of the tireless efforts of many genius minds of the 20th century, there has emerged a new direction that seems promising to understanding turbulence: viewing it using a dynamical systems perspective. This perspective is being tested, and is showing encouraging results in both numerics and experiments. The last decade in particular has seen a lot of progress in part due to advancements in computers, development of sophisticated algorithms, and carefully designed experiments. I will (try to) give a broad and brief view of this new approach, and the recent successes, failures and contributions from our group.
09-21  Michael Tennenbaum – Rheology of active matter: fire ants as a model system
Fire ant aggregations are an inherently active system. Each ant has its own energy source and can convert this energy into motion. I work on exploring the effect this activity has on the mechanical properties of aggregations. We have found that the properties of ant aggregations change cyclically in time. These cycles are connected to the activity level of the aggregation. We monitor the mechanics by measuring the normal force, oscillatory rheology, and real space imaging. With these measurements we can connect activity level with normal force, spacial distribution, fluctuations in the aggregation, and viscous and elastic moduli.

2017, Spring Semester

03-31 Scott DouglasPeer Assessment and Scientific Communication

The Schatz Physics Education Research group has, over the last few years, developed and implemented an inquiry-based introductory mechanics curriculum for use online and in on-campus “flipped” courses. The centerpiece of this curriculum—named “Your World is Your Lab” (YWYL)—is a set of structured laboratory activities designed to be conducted by students with their own equipment in their own homes, culminating with each student producing their own colloquium-style video lab report. These laboratories include not only real-world observations and computer modeling (building on our group’s previous work on computation in the classroom) but also include a calibrated peer-assessment system of these video lab reports, an altogether new feature for Georgia Tech physics courses. This talk will describe our laboratories, our peer assessment system, our motivations for designing each, and our analysis of the development of student behavior and attitudes regarding peer assessment and lab report production.

02-06 Michael Dimitriev – Large deflections of a hydrogel rod caused by internal phase separation
Hydrogels are soft materials that consist of a cross-linked polymer matrix capable of undergoing large volume changes via absorption of a solvent. As with binary mixtures, hydrogels can undergo a macroscopic phase separation transition to create a more swollen region and a less swollen one. We address this transition in the case of an initially swollen hydrogel, in a slender-rod geometry, possibly curved, which is heated to a temperature at which one would expect deswelling of the entire sample. However, the rapidity of the rise in temperature inhibits the system from expelling solvent through the rod’s surface, so that re-equilibration takes place at fixed solvent volume. Owing to this constraint and the system’s elasticity, the solvent-poor region fails to fully deswell, and the hydrogel partitions into an incompletely deswollen region and an excessively swollen one, determined by stress balance and a lever rule. Because the polymer network remains contiguous the rod undergoes a macroscopic shape change. When the partitioning is constant along the rod, the interface-orientation is a Goldstone mode that couples to the rod’s bending and twisting degrees of freedom and as a result, a large deflection of the rod occurs.

2016, Fall Semester

12-01 Perrin SchiebelMechanical Diffraction in a Sand-Specialist Snake

Limbless locomotors such as snakes move by pressing the trunk against terrain heterogeneities. Our laboratory studies of the desert-dwelling Mojave Shovel-nosed snake (C. occipitalis, ~40 cm long, N=9) reveal that these animals use a stereotyped sinusoidal traveling wave of curvature. However, this snake also encounters rigid obstacles in its natural environment, and the tradeoff between using a cyclic, shape controlled gait versus one which changes shape in response to the terrain is not well understood. We challenged individuals to move across a model deformable substrate (carpet) through a row of 6.4 mm diameter force-sensitive pegs—a model of terrain heterogeneities such as grass—oriented perpendicular to the direction of motion. Instead of forward-directed reaction forces, reaction forces generated by the obstacles were more often perpendicular to the direction of motion. Distributions of post-peg travel angles displayed preferred directions revealing a diffraction-like pattern with a central peak at zero and symmetric peaks at 19 +\-  3 degrees and 41 +/-  5 degrees. We observed similar dynamics in a robotic snake using shape-based control. This suggests that this sand-specialist snake adheres to its preferred waveform as opposed to changing the shape in response to the terrain.

11-03 Bharath Hebbe MadhusudhanaNon-Abelian Geometric Phases For Quantum Spin Systems

Geometric phase in classical and quantum mechanical systems has its origin in the geometry of the path traversed by the system in the phase space or the Hilbert space. As a non-relativistic analogue of Wilson loop operators and as a key tool to explore the deep relationship between geometry and physics, geometric phase remains an active area of research.Here we formulate a non-abelian geometric phase for spin systems. When the spin vector of a quantum system is transported along a closed loop inside the solid spin sphere (i.e., the unit ball), the tensor of second moments picks up a geometric phase in the form of an SO(3) operator. Considering spin-1 quantum systems, we formulate this phase. Geometrically interpreting this holonomy is tantamount to defining a steradian angle for loops inside a unit ball, including the ones that pass through the center. We accomplish this  by projecting  the loop onto the real projective plane. We show that the SO(3) holonomy of a loop inside the unit ball is equal to the steradian angle of the projected path in the real projective plane. This can be generalized to any spin system.

10-20 Omid Khosravani – How to use quantum mechanics to track down a single phonon in a thermal bath
Abstract omitted upon the request of the speaker.
10-06 Noah David DeTal – Numerical Harmonic Oscillator Expansions
The raising and lowering operators commonly associated with the harmonic oscillator Hamiltonian can be used to numerically find the quantum eigenvalues of other Hamiltonians. 1-D results can be compared to the WKB approximation to analyze issues of convergence associated with matrix truncation. The method for generalizing to higher dimensional systems can be derived but ultimately does not generate useful eigenvalues.
09-22 Pranav Dave – Production of Stable Iron Peak Elements in Type Ia SNR 3C 397

Thermonuclear, or Type Ia Supernovae, are white dwarf stars composed of carbon and oxygen which undergo explosive nuclear burning. The end products of this nuclear burning include radioactive nickel, which powers a highly luminous optical transient as bright as an entire galaxy for several weeks. Type Ia supernovae also play a crucial role in enriching a galaxy with intermediate mass elements such as silicon and calcium as well as heavier elements including nickel and iron. Since an isolated white dwarf is inherently stable, such an explosion requires the presence of a companion. However, while the nature of the white dwarf and its companion star, as well as the explosion mechanism itself have been active subjects of research for decades, until very recently there has been little concrete evidence directly connecting a given explosion to a specific stellar progenitor.

In 2015, astronomers found strong spectral signatures of stable iron-peak elements, including manganese, nickel, and chromium, in the interior of the supernova remnant 3C 397 using the X-ray telescope Suzaku. The nucleosynthesis of these elements require high density stellar environments, and consequently point towards a very specific white dwarf progenitor for 3C 397, namely one which is close to the Chandrasekhar mass. In my research, I have undertaken an investigation into this system using multidimensional numerical simulations, and have addressed some key questions about the progenitor white dwarf of the supernova which gave rise to 3C 397, as well as the nature of the explosion mechanism.

2016, Spring Semester

This semester, Physics Forum will be held every other Thursday at 4pm, in the Interaction Zone (Howey S105).  See below for the planned speakers.

04-21 Thanassis Stavropoulos – Gravitational wave asteroseismology with rapidly rotating neutron stars

During their evolution, relativistic stars may undergo oscillations which can become unstable under certain conditions. Scenarios that can lead to such oscillations include crust- or core-quakes and binary mergers. Also, relativistic stars are expected to intensely oscillate during their creation after a supernovae collapse or when they are members of binary systems, due to the tidal interaction or mass and angular momentum transfer from their companion star. The presence of rotation can strongly affect these oscillations. During these phases of their lives such compact objects emit large amounts of gravitational waves which along with viscosity tend to suppress these oscillations. Studying the characteristics of gravitational waves emitted by such compact objects can lead to conclusions about the various types of oscillations within the compact star and consequently to information about the stellar parameters of the source.

The main interest of this project is to use numerical relativistic hydrodynamic simulations to study the characteristics of rapidly rotating neutron stars as sources of gravitational waves. The final goal is to acquire theoretical information of such systems that can later be used as comparison to actual observations of gravitational waves.

04-07 Gable Wadsworth – Single-probe fluorescence in situ hybridization (FISH) in budding yeast
Quantitative determination of the copy number of RNA transcripts in single cells is crucial to understand the genotype-phenotype connection.  Current single-molecule Fluorescence In Situ Hybridization (FISH) protocols commonly used for this purpose require a large number of fluorophores per target RNA for single RNA detection, thus limiting the length range of RNA that can be probed.  We have developed a FISH protocol for budding yeast, which can detect RNA molecules with a singly labeled 24-nucleotide DNA probe.  Our single-probe protocol features highly inclined illumination and methanol fixation. We demonstrated high signal-to-noise and specificity of our protocol when tested against both constitutive and inducible genes in budding yeast. The technique presented offers a cost effective and efficient means of quantifying short RNA transcripts at the single cell level.                                                                                                                                                                                 
02-25 Matthew Johnson – Novel Sub-Doppler Multi-Photon Cooling in K40
Standard methods of getting dilute atomic ensembles down to micro-kelvin temperatures involve cooling by laser-light.  So-called sub-Doppler techniques are employed wildly in ultracold Atomic experiments as a precursor to the lossy evaporative cooling stage in order to minimise atomic loss. Many of these techniques have been well understood for a long time now, but the majority work poorly for Fermionic species of atoms.   As many ultracold atomic experiments today are moving toward Fermionic systems, effective and powerful cooling techniques are a must.  In this talk I will detail a discovery, some of the theory and a proposed ‘new’ sub-doppler laser cooling technique for fermionic Potassium (K40). (A SUPA research collaboration from the University of St. Andrews and Strathclyde, Scotland.)
02-11 John Indergaard – Atomic origins of bulk ferromagnetism in Fe, Co, and Ni
Magnetic phenomena have been utilized by human beings for hundreds of years with tremendous success in advancing technologies, and in the realm of physics the deliberate manipulation of magnetic systems is widely utilized to probe for underlying physical properties or theoretical validations. The theory of magnetism itself, however, has been shown to have major limitations in describing bulk magnetic orderings. In particular, ferromagnetism is largely incompatible with band theory. Cluster physics investigates how the physical properties of an element (or molecule) change with size, from the atomic scale to clusters of hundreds or thousands of atoms. In this talk I will discuss magnetism in general, with focus on Fe, Co, and Ni in and between the atomic and bulk size regime. Recent experimental data will be presented that may have implications for the understanding of the atomic origins of bulk ferromagnetism.
01-28 Conner Herndon – Theoretical and experimental quantification of a period doubling cascade to chaos in cardiac tissue and its relation to arrhythmias
Abstract: The heart is an excitable system through which electrical waves of depolarization propagate in a coordinated manner to initiate mechanical contraction. A fundamental characteristic of cardiac cells is a shortening of the depolarization time, known as the action potential duration (APD), with increasing electrical stimulation rates. However, at high stimulation frequencies many cardiac cells exhibit alternans, a beat-to-beat alternation in APD between long and short pulses. Experiments and models strongly suggest that this desynchronized depolarization can lead to fibrillation and sudden cardiac death, taking the lives of over 350,000 Americans each year — half of all heart disease deaths in the US. Alternans has been proposed as the of the first period doubling bifurcation along a cascade under which the system enters a state of complex spatiotemporal dynamics: fibrillation, the manifestation of chaos in the heart.

2015, Fall Semester

This semester, Physics Forum will be held every other Wednesday at 4pm, in the Interaction Zone (Howey S105).  See below for the planned speakers.

10-7 Elaine Chambers – Measuring Mechanical Properties of Thin Optical Coatings
Abstract: The Laser Interferometer Gravitational Wave Observatory is very sensitive to coherent mechanical vibrations of the surface of the coatings on its cavity mirrors. These vibrations are related to the mechanical properties of thin films; in particular, the vibration amplitude depends on the Young’s modulus and Poisson’s ratio of the coatings. However, many of the materials that are being considered for the mirror coatings do not have well-known mechanical properties. These properties can be measured using a number of different techniques. Two methods were used: ultrasonic at Embry-Riddle Aeronautical University and nanoindentation at California Institute of Technology. Results of these studies confirm that the Young’s modulus and Poisson’s ratio can be measured successfully. The methods developed can be used to compare multiple materials, leading to a final recommendation on coating material for the next production of mirrors.
Supplementary Material:
Stephen Spitz – Timely Talk Tying Topology To Topical Techniques in physics
Abstract: Topology provides a useful tool for physical analysis. Despite its intimidating reputation, many basic principles are easy to understand. I will give a brief overview of topology in physics with application to condensed matter.
Supplementary Material:
10-21 Michael Tennenbaum – Mechanics of Fire Ants
Abstract: Fire ants, Solenopsis invicta, form aggregations that are able to drip and spread like simple liquids, but that can also store energy and maintain a shape like elastic solids. They are an active material where the constituent particles constantly transform chemical energy into work. We probe the material properties of ant aggregations using rheology. We find that fire ant aggregations shear thin when forced to flow. And below a threshold stress both flow with and resist the applied stress. Unusually, we find that when no force is applied ant aggregations will still rotate.  All of our results for live ant aggregations are compared to those of dead ant aggregations to assure that we are seeing the effects of activity.
Supplementary Material: http://www.antphysics.gatech.edu/
11-4 Logan Kageorge – Simulating the Quantum Nature and Foil Fragmentation of Antiprotons and the Creation of Antiprotonic Lithium via GEANT4
Abstract: Scientists have long been able to measure the de Broglie wavelength of matter. To compare this property with that of antimatter, three gratings, assembled in such a way as to create a Moiré pattern, yields a more focused diffraction pattern than Young’s double slit, thus giving a more accurate measurement. The gratings are assembled into the ACE beam line, an adjunct to CERN’s AEgIS project. This one-meter pipe takes 200 nanosecond packets of 107 antiprotons at 5.3 MeV from the Antiproton Decelerator, cools them via foil degraders to the order of 1 keV, delivers them to the grating system, and then fires them into a nucleon-sensitive emulsion detector. By substituting foils of different atomic number for the gratings, fragmentation patterns created by antiproton-nucleus annihilations can be tailored in computer simulations by studying the same emulsion detectors. Rather than annihilating with a lithium nucleus, an antiproton can ionize the lithium, then orbit the nucleus in a Rydberg (high-n) state. Such a state would allow the newly created antiprotonic lithium to exist as a metastable atom with a lifetime of 106 seconds. Detailed analysis using the simulation program GEANT4 verifies that the signal (at a silicon pixel detector rather than emulsion) from this decay will not be convoluted with that of annihilations elsewhere in the chamber.
Supplementary Material:
Forrest Kieffer – Star-disk collisions in the Galactic Center
Abstract: Observations of the Galactic Center (GC) have revealed that there is a relative paucity of Red Giant (RG) stars within the central parsec. However, these observations conflict with our current theoretical understanding. We would expect the GC to have formed a segregated cusp of late-type stars. A recent explanation for this theoretical issue is that the outer envelopes of RG stars may have been stripped due to collisions with a fragmenting accretion disk in the GC. Both numerical and analytic models of star-disk collisions have been considered by several authors prior to this work, but a majority of the literature has focused on either the envelope stripping of a Main-Sequence (MS) star or other phenomena associated with this particular interaction. Here we investigate the envelope stripping of a RG star of radius R* = 10 R and mass M* = 1 M colliding with the dense regions of a fragmenting disk. From our simulations, we are able to conclude that a RG star is likely to be stripped of its outer envelope and, occasionally, disrupted.
Supplementary Material:
11-18 Bharath Hebbe Madhusudhan – Classical simulation of entangled states
Abstract: Characterization of nonclassicality or quantumness of a state is fundamental to foundations of quantum mechanics and quantum information. At the heart of the problem is the question whether there exist classical systems—howsoever complicated—that can mimic a given quantum state. Whilst this has been traditionally addressed through the violation of Bell inequality or nonseparability, we show that it is possible to go beyond them, by introducing the concept of classical simulation. Focusing on the two-qubit case, we show that, while for pure states, classical simulability is equivalent to existence of a local hidden variable (LHV) model, the conditions for simulability can be weaker for mixed states, demanding what we call only a generalized LHV description. Consequently, quantum states which defy a classical simulation—which we call exceptional—may require conditions which are more stringent than violation of Bell inequalities. We illustrate these features with a number of representative examples and discuss the underlying reasons, by employing fairly simple arguments.
Supplementary Material: http://journals.aps.org/pra/abstract/10.1103/PhysRevA.89.062110
12-2 Chris Crowley – Searching for Exact Coherent Structures in turbulence: a new approach to solving the most important problem in classical physics
Abstract: Almost every fluid flow that you experience in your life will be a turbulent one.  From blood flowing through your heart to air rushing over an airplane wing to the snow storm that shut Atlanta down for almost a week two years ago – all turbulent.  Despite the 500 years that man has been studying this ubiquitous phenomenon, it is still considered “the most important unsolved problem in classical physics” (quote attributed to Feynman).  Here at Tech, you walk among some of the pioneers of an entirely new way of approaching turbulence.  Using recently developed tools from chaos theory and nonlinear dynamics, we are developing a fundamentally deterministic model of turbulence that, if valid, will both offer predictive power as well as an easier way of controlling turbulent flows.  In this discussion, I will briefly summarize the problem with turbulent modeling as it stands now, describe the theory behind our approach, and present some promising preliminary results.
Supplementary Material:

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