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ItemTransforming the preparation of physics graduate teaching assistants(Georgia Institute of Technology, 2019-12-02) Alicea-Munoz, EmilyGraduate Teaching Assistants (GTAs) are key partners in the education of undergraduate students. In large-enrollment intro physics classes, students spend roughly half of their in-class hours in labs and recitations under the supervision of GTAs. Since GTAs can have a large impact on their students' learning, it is important to provide them with appropriate preparation for teaching. But GTAs are also students themselves -- they have many demands on their time, and not all of them want to become professors after grad school. Therefore, it is crucial that GTA preparation not be a burden but rather be fully integrated into their professional development. The School of Physics at Georgia Tech has been offering a GTA prep course for first-year Ph.D. students since 2013. The majority of these first-time GTAs have no prior teaching experience but consider teaching to be an important part of their professional development as physicists. Through a cycle of implementation and revision, and guided by the 3P Framework we developed (Pedagogy, Physics, Professional Development), the course has evolved into a robust and comprehensive professional development program that is well-received by physics graduate students. We assessed the effectiveness of the course with a combination of surveys, pre/post tests, and student evaluations. We found that GTAs feel better prepared for teaching after going through the Orientation. GTAs consider most useful the course activities in which they can practice and get feedback on their teaching ("Microteaching", "Lab Simulation") and the lessons in which we discuss the pedagogical content knowledge necessary to teach intro physics labs and recitations ("Teaching Physics"). GTAs who participate in the GTA prep course adopt more learner-centered teaching approaches and increase their pedagogical knowledge. They also receive higher end-of-semester student evaluations than GTAs whose first teaching experience predated the establishment of the GTA prep course.
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ItemTransport in low-dimensional interacting systems(Georgia Institute of Technology, 2019-11-20) Sharma, KamalOne-dimensional quantum many-body systems have been an interesting area of theoretical research since last 90 years. However, advances in fabrication technologies has led to influx of real materials and devices that are one-dimensional or quasi one-dimensional. These devices have brought back a renewed interest in understating the physics of such systems. However, the established Luttinger liquid theory has some limitations due to absence of scattering processes at finite temperatures. Further, any arbitrarily weak interaction potential between Luttinger liquid bosons leads to divergences already in the first order calculation. We adopt the low energy one-dimensional Wigner crystal as the strongly interacting regime of Luttinger liquid. We show that the violation of the Wiedemann-Franz law can be demonstrated by calculating correction to thermal conductance of a non-linear interacting Wigner crystal.
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ItemMechanics of hierarchical, filamentous tissues(Georgia Institute of Technology, 2019-11-12) Michel, Jonathan A.Structural hierarchy, the property of possessing spatial organization on multiple, distinct length scales, is omnipresent in biological tissues, and is increasingly popular as a means of pursuing designer properties of human-made materials. Hierarchy can offer economy of material, resilience against fracture, and novel mechanical response; however, the apparent opportunity for errors in assembly at multiple stages na\"ively seems to present an imposing obstacle to the evolution of hierarchical tissue. Nonetheless, many organisms, from many evolutionary lineages, exhibit structural hierarchy. In this work, we build upon previous efforts to model tissue as spring networks. We create networks with a nested, self-similar structure, whose geometrical attributes can be independently varied at each scale. Following previous researchers, we focus upon the mean coordination number, which gives the typical number of nearest neighbors to which a vertex in a network is connected, as a parameter for controlling the elastic properties of structures. We extend this idea, defining separate coordination numbers for the network architecture, and find a simple scaling law relating a material's stiffness to its structural attributes at each length scale. We validate this scaling law with simulations, and find it to hold for structures derived from crystalline lattices and triangulations of random point sets. From this scaling law, we predict that the variability in the stiffness of a network resulting from variability in its structural attributes at each length scale diminishes with increasing levels of hierarchy, up to some threshold. Our results suggest that robustness to errors in assembly may be a generic benefit of a modular assembly process. Finally, we elucidate the role of large-scale and small-scale structural attributes. We find the small scale structure sets the vibrational density of states of our model systems at large frequency, while the large-scale structure is important in coordinating a system-wide, percolating force network to stiffen the material.
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ItemCoherent structures in incompressible fluid flows(Georgia Institute of Technology, 2019-11-08) Short, Kimberly YovelThe work is broadly related to the transition to turbulence in pipe at intermediate Reynolds numbers and includes a discussion of two classes of structures observed during the transition to turbulence: numerically-extracted solutions of the Navier-Stokes equations (the ``exact'') and localized/patterned turbulent spots that are not themselves solutions of the Navier-Stokes equation but are nonetheless pervasive during the transition (the ``inexact''). High-dimensional descriptions of turbulence is predicted by periodic orbit theory (POT) which expects to describe turbulence exactly, as opposed to approximately. The search for relative periodic orbits and traveling waves in pipe---the constituents solutions to periodic orbit theory---are discussed. The successful search for relative periodic orbits at transitional Reynolds numbers gave a catalogue of invariant solutions; many of these solutions were continued in parameter space to find that solutions coexist with the transitional regime. In addition to collecting solutions to eventually test periodic orbit theory, an investigation into the Barkley's pipe model---a system that successfully models the transition to turbulence in pipe---was undertaken.
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ItemEmergent collective phenomena in V. Cholerae biofilms, mixed human-autonomous fleets, and nascently multicellular bodies(Georgia Institute of Technology, 2019-11-07) Yanni, DavidHere I focus on the physics of the evolution of multicellularity and division of labor, the cyberphysical risks of hacked internet connected vehicles, and diffusive motion that arises from cell reproduction and lysis (cell death) in biofilms.
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ItemNeutron Scattering and Quantitative Modeling of Magnetic Excitations in Frustrated Materials(Georgia Institute of Technology, 2019-11-06) Bai, XiaojianThe basic theme of my Ph.D. research is understanding exotic magnetic phases of matter and investigate their collective low-energy excitations using neutron-scattering and quantitative modeling. In this thesis, I start with an attempt to answer a list of questions that I had in the beginning of my Ph.D. study, such as why we can use a simple effective model to describe this complex world, how to synthesize and characterize samples, how to analyze the data and find a good theoretical model and many more. There is no unique answer to these questions. I speak from experience and hope to provide a road map to whoever read my thesis and is interested in starting condensed matter research using neutron-scattering. Next, I present two material projects that I assume a major role. In both projects, high resolution single-crystal inelastic neutron-scattering data enables me and my collaborators to make significantly advances in understanding complex dynamical responses of magnetic materials. In Chapter 2, I present our study on a canonical frustrated magnet MgCr2O4 in the deep cooperative paramagnetic regime. In experiment, we observe a highly structured elastic scattering pattern with continuous excitation spectrum. Using analytic and computational methods, we reveal the highly correlated spin state is proximate to a "spiral spin-liquid" phase and the collective excitations are predominantly fast harmonic precessions of spin on a slow-varying disordered background. In Chapter 3, I present our study on an enigmatic compound with prior investigations dated back to 1970s – FeI2. In experiment, we observe a bright and dispersive band with "quadrupolar" character, apparently at odds with the dipole selection rule. Using advance numerical techniques, we are able to fully account for this band via a novel hybridization mechanism involving off-diagonal symmetric exchange interactions. In Chapter 4, I introduce detailed implementations of spin dynamics simulations and application to a realistic diamond-lattice system. This technique provides a simple framework to study finite temperature and non-linear effects of complex magnetic materials and has increasingly been used to study disordered and strongly-correlated spin systems. I close this thesis in Chapter 5 with an outlook of future directions.
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ItemCube-Shaped Poo and Georgia Tech's Second Ig Nobel Prize(Georgia Institute of Technology, 2019-10-08) Hu, David L.How does a wombat produce cube-shaped feces? How long does it take an elephant to urinate? Answering these two questions have landed David Hu two Ig Nobel Prizes, awards given at Harvard University for research that makes people laugh, and then think. Hu will talk about his lab's latest adventures catching elephant pee in trash cans, inflating wombat intestines with clown balloons, and dressing up as a gigantic piece of cubed poo at this year's Ig Nobel Ceremony.
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ItemAll-microwave control of hyperfine states in ultracold spin-1 rubidium(Georgia Institute of Technology, 2019-10-04) Boguslawski, MatthewThe manipulation of quantum spin states in a spinor Bose-Einstein condensate is critical for nearly all types of studies in these systems. State control methods are used to initialize the state of the system, apply Hamiltonian terms to modify the dynamics, and to measure properties of the quantum states. This thesis details the implementation of circularly polarized microwaves to selectively drive hyperfine transitions in the context of a spin-1 Bose-Einstein condensate of rubidium. This provides a new powerful tool for addressing specific transitions in the presence of frequency-degenerate transitions, allowing for new possibilities in state control. With this tool, we demonstrate a factor of 1/45.3 reduction in the coupling strength between polarization selected and blocked transitions by the application of a circularly polarized microwave field. This newly-developed tool is used to explore a couple of important applications. First, this polarized field is used to couple only three levels, out of all eight levels in the F=1, 2 hyperfine structure of ground-state rubidium-87, to drive an otherwise degenerate lambda system with 99.5% fidelity in state transfer from one base state of the lambda to the other. This is comparable to two-level transition fidelities measured in our system. This lambda transition has applications such as in implementing a non-adiabatic holonomic gate within the spin-1 states and could be extended to give full SU(2) control over two of the spin-1 states. Second, the circularly polarized field is applied to selectively drive hyperfine transitions in low bias fields, where the Zeeman splitting between the spin-1 states is small and comparable to the spectral linewidth of the driving field. In such low fields, microwave transitions without polarization selection scramble the state, as there are couplings between multiple levels within the hyperfine structure. This thesis demonstrates the selection of transitions using polarization control of the microwave field to solve this problem. These measurements imply the utility of circular polarization selected transitions for more rapid manipulations than otherwise possible.
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ItemEmergent Nonequilibrium Statistical Mechanics from Death and Birth in Biofilms(Georgia Institute of Technology, 2019-10-04) Kalziqi, Arben L.This thesis experimentally explores the statistical mechanics which emerge in the study of bacterial biofilms, highly nonequilibrium communities in which a vast number of bacteria make their homes, and which are of tremendous importance in ecology, medicine, and the economy. In the first set of experiments, we found that local, contact-based killing between cells results. We inoculated multiple, well-mixed strains of V. cholerae on agar pads, then incubated them for 24 hours. When we chose a strain pairing where cells could not kill each other, we found that the strains remained well-mixed regardless of temperature. However, when we mixed together two strains which could kill each other on contact via the Type VI Secretion System (“T6SS”), we found that they underwent an order-disorder transition reminiscent to that seen in the Ising model of an electron spin lattice, with higher temperatures corresponding to later timepoints in this transition. Because spatial assortment is a common means by which bacteria solve public goods dilemmas, we hypothesized that bacteria which could kill non-kin might be more cooperate with their kin. Though a phylogenetic analysis, we found that the number of different T6SS toxins strongly correlated with the number of genes dedicated to the production of external goods, a proxy for cooperativity. Thus, intercellular killing leads to Model A coarsening and (possibly) to the evolution of cooperation. In the next set of experiments, we used genetically modified strains of V. cholerae which secreted no exopolysaccharides (“EPSes”), and thus formed tissue-like (“Matrix-”) biofilms resembling simple stacks of cells. We inoculated biofilms with “nonkiller” or “mutual killer” pairings, and used a white-light interferometer to measure their surface topographies with ~nanometer precision. Surprisingly, we found that surface of biofilms with killing were significantly rougher than those without. A 2015 paper by Risler, Peilloux, and Prost suggested that in the homeostatic limit, the surface fluctuation spectra of a tissue surface may resemble those of a thermal permeable membrane, with an activity- mediated effective temperature. Our biofilm measurements served as experimental support for this theory, and provide further evidence of an effective fluctuation-response relationship driven by birth and death which may exist in cellular solids. Further, we performed minimal simulations which both recapitulated the aforementioned topographical difference and suggested the killing serves to fluidize biofilms. The final set of experiments served as a theoretical and experimental expansion of the previous set. We grew biofilms that could produce EPSes (“Matrix+”), and were thus less tissue-like and more similar to the typical biofilms which are found in nature. We tested the mechanical properties of Matrix- and Matrix+ biofilms, and found that the latter had a higher viscosity by a factor of roughly three. Next, we measured surface topographies and found that while the topographies of Matrix- and Matrix+ biofilms looked similar, Matrix+ biofilms had an effective temperature that was roughly three times higher. To probe whether the effective temperature derived in the second set of experiments had a kinetic interpretation, we used the generalized Stokes-Einstein relation to convert extracted effective temperatures into effective diffusivities, and found that cellular diffusion inside Matrix- and Matrix+ biofilms occurred at the same (viscosity-independent) rate. Simulations, analytical results, and experimental PIV all agree with this result, lending yet more credence to the effective fluctuation-response relationship suggested by Risler et al.
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ItemUnraveling Nanoscale Thermal Transport in Multilayered Semiconductors(Georgia Institute of Technology, 2019-09-09) Kothari, KartikA thorough comprehension and control of thermal transport in nanoscale thermoelectric, microelectronic and optoelectronic devices is crucial since it is paramount to their optimum performance. The fundamental understanding of how phonons move and the physical mechanisms behind nanoscale thermal transport, however, remain poorly understood. In this thesis, we move beyond thermal conductivity calculations and provide a rigorous and comprehensive physical description of thermal phonon transport in layered nanostructures by solving the Boltzmann transport equation and extending the Beckman-Kirchhoff surface scattering theory with shadowing to precisely describe phonon-interface interactions. We commence with analyzing periodic layered nanostructures called superlattices. We explicate in-plane thermal energy distribution in Si-Ge, GaAs/AlAs and their alloy-based superlattices by segregating it into different modes based on their trajectories. We provide a rigorous structural, microscopic, spectral and finite-sized analysis of thermal transport characteristics. Next, we examine cross-plane thermal conduction in GaAs/AlAs and their alloy-based superlattices. We present a comprehensive study of superlattice thermal transport, including structure-property relations, spectral and modal descriptions, and contrast it with in-plane heat conduction thereby explaining the resultant anisotropy in III-V semiconductor superlattices. We uncover the phonon injection mechanism which provides novel pathways in modulating thermal conduction of specific layers within layered nanostructures. We use that to examine thin film-on-substrate, a ubiquitously found architecture in nanostructured optoelectronic devices. We observe an unconventional behavior of thermal conductivity variation with film thickness i.e. for Al0.1Ga0.9As thin films grown over GaAs substrate and Ge thin film on Si substrate, we find an increased thermal conductivity with decreasing thickness. We fundamentally investigate interfacial coupling through analyzing the transmission coefficient and its variation on the phonon frequency and interfacial roughness. We study thermal conductivity enhancement through a 2D visualization of the spatial distribution of heat through mapping phonon MFPs and their corresponding thermal conductivity contributions. We also include an elementary discussion on thermal phonon wave effects and classify the kinds of wave effects that can occur in a superlattice, namely, quantum confinement and thermal band gaps. We elaborate upon the mechanisms which cause them and provide strategies to enhance the possibility of their occurrence in layered nanostructures. The results and insights in this thesis advance the fundamental understanding of heat transport in layered nanostructures and the prospects of rationally designing thermal systems with tailored phonon transport properties and providing inputs for thermal management which is a crucial component in enhancing the performance of nanostructured devices.