Physics

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1/f noise and Luttinger liquid phenomena in carbon nanotubes
(2007-08-03) Tobias, David; Lobb, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Carbon nanotubes (CNTs) provide an ideal medium for testing the behavior of one-dimensional electron systems and are promising candidates for electronic applications such as sensors or field-effect transistors. This thesis describes the use of low frequency resistance fluctuations to measure both the properties of the one-dimensional electron system in CNTs, and the sensitivity of CNT devices to their environment. Low frequency noise was measured in CNTs in field effect transistor (FET) geometry. CNTs have a large amount of surface area relative to their volume and are expected to be strongly affected by their environment, leading to speculation that CNTs should have large amounts of 1/f noise. My measurements indicate that the noise level is in the same range as that of traditional FETs, an encouraging result for possible electronic applications. The temperature dependence of 1/f noise from 1.2 K to 300 K can be used to extract the characteristic energies of the fluctuators responsible for the noise. The characteristic energies allows for the elimination of structural and electronic transitions within the CNT itself as possible sources of 1/f noise in CNTs, leaving the motion of defects in the gate dielectric, or possibly strongly physisorbed species, as the likely culprits. Another form of low frequency noise found in CNTs is random telegraph signal (RTS), which manifests as the alternation between two current states at a stable voltage bias. In CNTs, this phenomenon occurs due to the tunneling of electrons into and out of the CNT from a nearby defect, and thus provides a way to probe the tunneling density of states of the CNT itself. The tunneling density of states in turn provides information on the strength of the electron-electron interaction in CNTs. Due to the one-dimensional structure of CNTs their electronic state is expected to be a Luttinger liquid, which should manifest as a power-law suppression of the tunneling density of states at the Fermi energy. The power law exponent is measured in both the temperature dependence and energy dependence of the tunneling rates. In agreement with theory, the power-law exponent is significantly larger in semiconducting CNTs than found in previous experiments on metallic CNTs. The RTS can also be used as a "defect thermometer" to probe the electron temperature of the CNT. The effect of the bias voltage on the electron temperature provides a means to determine the energy relaxation length for the electrons in the CNT.
3D Ionospheric Effects on HF Propagation and Heating
(2015) Zawdie, Katherine A.; Papadopoulos, Konstantinos; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The thesis uses a three-dimensional, first-principles model of the ionosphere in combination with High Frequency (HF) raytracing model to address key topics related to the physics of HF propagation and artificial ionospheric heating. In particular: 1. Explores the effect of the ubiquitous electron density gradients caused by Medium Scale Traveling Ionospheric Disturbances (MSTIDs) on high-angle of incidence HF radio wave propagation. Previous studies neglected the all-important presence of horizontal gradients in both the cross- and down-range directions, which refract the HF waves, significantly changing their path through the ionosphere. The physics-based ionosphere model SAMI3/ESF is used to generate a self-consistently evolving MSTID that allows for the examination of the spatio-temporal progression of the HF radio waves in the ionosphere. 2. Tests the potential and determines engineering requirements for ground- based high power HF heaters to trigger and control the evolution of Equatorial Spread F (ESF). Interference from ESF on radio wave propagation through the ionosphere remains a critical issue on HF systems reliability. Artificial HF heating has been shown to create plasma density cavities in the ionosphere similar to those that may trigger ESF bubbles. The work explores whether HF heating may trigger or control ESF bubbles. 3. Uses the combined ionosphere and HF raytracing models to create the first self-consistent HF Heating model. This model is utilized to simulate results from an Arecibo experiment and to provide understanding of the physical mechanism behind observed phenomena. The insights gained provide engineering guidance for new artificial heaters that are being built for use in low to middle latitude regions. In accomplishing the above topics: (i) I generated a model MSTID using the SAMI3/ESF code, and used a raytrace model to examine the effects of the MSTID gradients on radio wave propagation observables; (ii) I implemented a three- dimensional HF heating model in SAMI3/ESF and used the model to determine whether HF heating could artificially generate an ESF bubble; (iii) I created the first self-consistent model for artificial HF heating using the SAMI3/ESF ionosphere model and the MoJo raytrace model and ran a series of simulations that successfully modeled the results of early artificial heating experiments at Arecibo.
3D Magnetic Imaging using SQUIDs and Spin-valve Sensors
(2016) Jeffers, Alex; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
We have used 2 µm by 4 µm thin-film Cu-Mn-Ir spin-valve sensors and high Tc YBa2Cu3O7-x dc SQUIDs to take magnetic images of test samples with current paths that meander between 1 and 5 metallization layers separated by 1 µm to 10 µm vertically. I describe the development and performance of a 3D magnetic inverse for reconstructing current paths from a magnetic image. I present results from this inverse technique that demonstrate the reconstruction of the 3D current paths from magnetic images of samples. This technique not only maps active current paths in the sample but also extracts key parameters such as the layer-to-layer separations. When imaging with 2 µm by 4 µm spin-valve sensors I typically applied currents of 1 mA at 95 kHz and achieved system noise of about 200 nT for a 3 ms averaging time per pixel. This enabled a vertical resolution of 1 µm and a lateral resolution of 1 µm in the top layers and 3 µm in the bottom layer. For our roughly 30 µm square SQUID sensors, I typically applied currents of 1 mA at 5.3 kHz, and achieved system noise of about 200 pT for a 3 ms averaging time per pixel. The higher sensitivity compared to the spin-valve sensor allowed me to resolve more deeply buried current paths.
A facet is not an island: step-step interactions and the fluctuations of the boundary of a crystal facet
(Elsevier, 2005) Pimpinelli, Alberto; Degawa, M.; Einstein, Theodore L.; Williams, Ellen D.
Ab initio Lattice Dynamics and Infrared Dielectric Response
(2004-11-24) lawler, hadley Mark; Shirley, Eric L.; Drew, H. Dennis; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Methods for theoretically evaluating lattice dynamics, anharmonic effects and related optical properties from first principles are designed and implemented. Applications of density-functional theory and the pseudopotential approximation are adapted, via the Born-Oppenheimer approximation, the Hellmann-Feynman force theorem, and wave-commensurate supercells, to a direct calculation of the Born-von Karman force constants. With a symmetry analysis and interpolation of Born-von Karman force constants, the complete phonon spectra are obtained for the cubic systems Ar, Si, Ge, and diamond, and for the stacked hexagonal system, graphite. The phonon spectra for the polar materials GaAs and GaP, in which the degeneracy between longitudinal and transverse optical modes is lifted, are also calculated. The splitting is a consequence of the macroscopic field associated with long-range Coulombic interactions and longitudinal displacements. Diagramatically-derived expressions for the finite lifetime of the Raman mode arising from phonon-phonon interactions are calculated for Si, Ge, and diamond from first principles, and agree with experiment to within uncertainty. The infrared absorption spectra of GaAs and GaP are calculated from first principles through the phonon anharmonic self-energy (phonon-phonon interaction) and the Born effective charges (photon-phonon interaction). Several aspects of the spectra are in detailed agreement with the experimental spectra, including the strong temperature dependence of the far-infrared absorption due to the onset of difference processes; the linewidth and asymmetric lineshape of the reststrahlen; the spectral structure of the absorption by two-phonon modes, and overall oscillator strengths. The theory allows for the identification of narrow spectral transmission bands with an ionic mass mismatch in the case of GaP. Analytic and complete calculations are performed for the ion-ion displacement correlation function in solid Ar, and agree well. The correlations are evaluated for arbitrary lattice vector and Cartesian displacement directions, and their pressure dependence leads to the conjecture that anharmonic effects are less prominent at higher pressures.
Accounting for variability in a teacher’s epistemology: Resources and framing
(2018-05-31) Lau, Matty; Elby, Andrew
Adenylyl cyclase mRNA localizes to the posterior of polarized DICTYOSTELIUM cells during chemotaxis
(Springer Nature, 2017-05-25) Das, Satarupa; Parker, Joshua M.; Guven, Can; Wang, Weiye; Kriebel, Paul W.; Losert, Wolfgang; Larson, Daniel R.; Parent, Carole A.
In Dictyostelium discoideum, vesicular transport of the adenylyl cyclase A (ACA) to the posterior of polarized cells is essential to relay exogenous 3′,5′-cyclic adenosine monophosphate (cAMP) signals during chemotaxis and for the collective migration of cells in head-to-tail arrangements called streams. Using fluorescence in situ hybridization (FISH), we discovered that the ACA mRNA is asymmetrically distributed at the posterior of polarized cells. Using both standard estimators and Monte Carlo simulation methods, we found that the ACA mRNA enrichment depends on the position of the cell within a stream, with the posterior localization of ACA mRNA being strongest for cells at the end of a stream. By monitoring the recovery of ACA-YFP after cycloheximide (CHX) treatment, we observed that ACA mRNA and newly synthesized ACA-YFP first emerge as fluorescent punctae that later accumulate to the posterior of cells. We also found that the ACA mRNA localization requires 3′ ACA cis-acting elements. Together, our findings suggest that the asymmetric distribution of ACA mRNA allows the local translation and accumulation of ACA protein at the posterior of cells. These data represent a novel functional role for localized translation in the relay of chemotactic signal during chemotaxis.
Adjoint methods for stellarator shape optimization and sensitivity analysis
(2020) Paul, Elizabeth; Dorland, William; Landreman, Matthew; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Stellarators are a class of device for the magnetic confinement of plasmas without toroidal symmetry. As the confining magnetic field is produced by clever shaping of external electro-magnetic coils rather than through internal plasma currents, stellarators enjoy enhanced stability properties over their two-dimensional counterpart, the tokamak. However, the design of a stellarator with acceptable confinement properties requires numerical optimization of the magnetic field in the non-convex, high-dimensional spaces describing their geometry. Another major challenge facing the stellarator program is the sensitive dependence of confinement properties on electro-magnetic coil shapes, necessitating the construction of the coils under tight tolerances. In this Thesis, we address these challenges with the application of adjoint methods and shape sensitivity analysis. Adjoint methods enable the efficient computation of the gradient of a function that depends on the solution to a system of equations, such as linear or nonlinear PDEs. Rather than perform a finite-difference step with respect to each parameter, one additional adjoint PDE is solved to compute the derivative with respect to any parameter. This enables gradient-based optimization in high-dimensional spaces and efficient sensitivity analysis. We present the first applications of adjoint methods for stellarator shape optimization. The first example we discuss is the optimization of coil shapes based on the generalization of a continuous current potential model. We optimize the geometry of the coil-winding surface using an adjoint-based method, producing coil shapes that can be more easily constructed. Understanding the sensitivity of coil metrics to perturbations of the winding surface allows us to gain intuition about features of configurations that enable simpler coils. We next consider solutions of the drift-kinetic equation, a kinetic model for collisional transport in curved magnetic fields. An adjoint drift-kinetic equation is derived based on the self-adjointness property of the Fokker-Planck collision operator. This adjoint method allows us to understand the sensitivity of neoclassical quantities, such as the radial collisional transport and self-driven plasma current, to perturbations of the magnetic field strength. Finally, we consider functions that depend on solutions of the magneto-hydrodynamic (MHD) equilibrium equations. We generalize the well-known self-adjointness property of the MHD force operator to include perturbations of the rotational transform and the currents outside the confinement region. This self-adjointness property is applied to develop an adjoint method for computing the derivatives of such functions with respect to perturbations of coil shapes or the plasma boundary. We present a method of solution for the adjoint equations based on a variational principle used in MHD stability analysis.
Adsorbates in a Box: Titration of Substrate Electronic States
(American Physical Society, 2010) Cheng, Zhihai; Wyrick, Jonathan; Luo, Miaomiao; Sun, Dezheng; Kim, Daeho; Zhu, Yeming; Lu, Wenhao; Kim, Kwangmoo; Einstein, Theodore L.; Bartels, Ludwig
Nanoscale confinement of adsorbed CO molecules in an anthraquinone network on Cu(111) with a pore size of ?4??nm arranges the CO molecules in a shell structure that coincides with the distribution of substrate confined electronic states. Molecules occupy the states approximately in the sequence of rising electron energy. Despite the sixfold symmetry of the pore boundary itself, the adsorbate distribution adopts the threefold symmetry of the network-substrate system, highlighting the importance of the substrate even for such quasi-free-electron systems.
Advanced Lagrangian Simulation Algorithms for Magnetized Plasmas Turbulence
(2008-08-05) Broemstrup, Ingmar; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Nonlinear processes in hot, magnetized plasma are notoriously difficult to understand without the use of numerical simulations. In recent decades, first principles, kinetic simulations have been widely and successfully used to study plasma turbulence and reconnection in weakly collisional systems. In this thesis, extensions of well-known, Lagrangian, particle-in-cell (PIC) simulation algorithms for problems such as these are derived and implemented. The algorithms are tested for multiple species (electrons and ions, with the physical mass ratio) in non-trivial magnetic geometry (cylindrical/toroidal). The advances presented here address two major shortcomings of conventional gyrokinetic PIC algorithms, with demonstrated excellent performance on large, parallel supercomputers. Although the gyrokinetic formalism rigorously describes the evolution of fluctuations which are small compared to a typical Larmor radius, most existing algorithms use low-order approximations of the gyroaveraging operator, and cannot accurately describe small scale fluctuations. The gyroaveraging algorithm presented here accurately and uniquely treats a wide range of fluctuation scales, above and below the thermal gyroradius. The second shortcoming of traditional algorithms relates to the slow loss of accuracy that is associated with the build-up of noise. In this thesis, a PIC pitch-angle scattering collision operator is developed. This collision operator is physically motivated and controls the growth of noise without introducing non-physical dissipation. Basic tests of the new algorithms are presented in linear and nonlinear regimes, using one to thousands of processors simultaneously.
An All-Sky Search for Bursts of Very High Energy Gamma Rays with HAWC
(2016) Wood, Joshua Randall; Goodman, Jordan; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
A new ground-based wide-field extensive air shower array known as the High-Altitude Water Cherenkov (HAWC) Observatory promises a new window to monitoring the ~100 GeV gamma-ray sky with the potential for detecting a high energy spectral cutoff in gamma-ray bursts (GRBs). It represents a roughly 15 times sensitivity gain over the previous generation of wide-field gamma-ray air shower instruments and is able to detect the Crab Nebula at high significance (>5 sigma) with each daily transit. Its wide field-of-view (~2 sr) and >95% uptime make it an ideal instrument for detecting GRB emission at ~100 GeV with an expectation for observing ~1 GRB per year based on existing measurements of GRB emission. An all-sky, self-triggered search for VHE emission produced by GRBs with HAWC has been developed. We present the results of this search on three characteristic GRB emission timescales, 0.2 seconds, 1 second, and 10 seconds, in the first year of the fully-populated HAWC detector which is the most sensitive dataset to date. No significant detections were found, allowing us to place upper limits on the rate of GRBs containing appreciable emission in the ~100 GeV band. These constraints exclude previously unexamined parameter space.
All-Sky Search for Neutrinos Correlated with Gamma-Ray Bursts in Extended Time Windows Using Eight Years of IceCube Data
(2021) Friedman, Elizabeth A.; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
GRBs have long been considered as potential sources of hadronic acceleration of ultra-high energy cosmic rays and, more recently, as potential sources of the diffuse neutrino flux measured by IceCube. This thesis presents a search for neutrinos correlated with 2,091 gamma-ray bursts (GRBs) in prompt and extended time windows using data from the IceCube Neutrino Observatory ranging from May 2011 to October 2018. Ten time windows, ranging from 10 seconds to -1/+14 days around the time of the GRB's gamma-ray emission, were searched for coincident neutrino emission and a p-value was assigned based on the most significant time window. The results for all 2,091 GRBs were divided by region of the sky and observed gamma-ray emission time, and were evaluated with a Binomial test, which was found to be consistent with background. The 23 most significant GRBs were examined in more detail and they were also found to be consistent with background. Limits were set assuming two flux models: equal neutrino flux at Earth for every GRB and standard candle neutrino emission. The equal flux at Earth model led to similar constraints as previous IceCube prompt GRB studies, namely that GRBs are responsible for, at most, a few percent of the diffuse flux. The results of the standard candle analysis, however, indicate GRBs may be contributing up to 11% of the diffuse neutrino flux up to 1,000 second timescales, which leaves the door open to GRBs as neutrino sources and hadronic accelerators of at least some of the ultra-high energy cosmic rays.
All-Sky Search for Very-High-Energy Emission from Primordial Black Holes and Gamma-Ray Bursts with the HAWC Observatory
(2023) Engel, Kristi Lynne; Goodman, Jordan A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Transient sources of very-high-energy gamma rays are short-lived astrophysical phenomena often associated with catastrophic events that change their brightness over relatively short timescales. The search for and study of such objects, especially in the TeV energy regime, has the possibility to shed light not only on the physics at play within the enigmatic, chaotic environments that produce such emission, but also to answer several remaining questions in fundamental physics. In this dissertation, we leverage the sensitivity and characteristics of the High-Altitude Water Cherenkov (HAWC) Observatory in pursuit of gaining insight into these areas. The HAWC Observatory, located on the side of the side of the Sierra Negra volcano in Puebla, Mexico at an altitude of 4,100 m above sea level, is an extensive-air-shower array sensitive to gamma rays from ~0.1 to >100 TeV that has been in operation since March of 2015. It has a wide field of view of ~2 sr at any one time and, combined with its large operational duty cycle (>95%), observes 2/3 of the sky every day. HAWC operates using the water-Cherenkov detection technique with 1,200 photomultiplier tubes (PMTs) in two different sizes to detect Cherenkov emission from secondary air-shower particles. Herein, we present an improved characterization for the larger of these two PMT models for inclusion within the Monte Carlo simulation of the HAWC Observatory, as well as the custom testing apparatus designed and constructed for this purpose. With HAWC's wide field of view, near-continuous uptime, and large archival dataset, it serves as an ideal observatory with which to search for transient sources of all kinds. We apply these advantages to perform searches for two types of transient sources--- Primordial Black Holes (PBHs) and Gamma-Ray Bursts (GRBs). The first of these, a search for emission signatures of evaporating PBHs, is performed on 959 days of HAWC data for remaining lifetimes of 0.2, 1, 10, and 100 s, assuming radiation development according to the Standard Emission Model. We show that previous attempts to perform searches for transient searches similar to PBHs with HAWC were oversampling at detrimental levels and improve upon that method to achieve greater statistical rigor. Finding no significant emission for any duration, we place upper limits at the 99% confidence level on the local burst rate density. For the second of these source types, we apply the low-energy improvements recently made to the HAWC data reconstruction procedure to search for very-high-energy emission within the first 0.1, 1, 10, and 100 s of emission for 93 GRBs within HAWC's field of view at their reported T₀ over the first 7 years of HAWC operations. This search is performed using permutations of four different assumed redshift values and four different assumed spectral indices. Finding no significant emission for any duration under any set of assumption parameters, we place upper limits at the 95% confidence level on the intrinsic flux for all GRBs. For those GRBs with external flux models available from other gamma-ray detectors, we compare the HAWC limits to those models in order to constrain the possible emission in the TeV regime with respect to that at lower energy values. We also perform a follow-up execution of this analysis with start times shifted to match external model start times which differed from T₀. Again finding no significant emission, we place upper limits at the 95% confidence level on the intrinsic flux for all parameter sets and for all external start times for those GRBs HAWC was most likely to have seen. Finally, we speculate about the future of searches for PBHs and GRBs with the next-generation wide-field-of-view instrument, the Southern Wide-field Gamma-ray Observatory (SWGO), presenting projected performance for these two types of transient sources.
An All-Sky, Three-Flavor Search for Neutrinos from Gamma-Ray Bursts with the IceCube Neutrino Observatory
(2015) Hellauer, Robert Eugene; Sullivan, Gregory; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Ultra high energy cosmic rays (UHECRs), defined by energy greater than 10^18 eV, have been observed for decades, but their sources remain unknown. Protons and heavy ions, which comprise cosmic rays, interact with galactic and intergalactic magnetic fields and, consequently, do not point back to their sources upon measurement. Neutrinos, which are inevitably produced in photohadronic interactions, travel unimpeded through the universe and disclose the directions of their sources. Among the most plausible candidates for the origins of UHECRs is a class of astrophysical phenomena known as gamma-ray bursts (GRBs). GRBs are the most violent and energetic events witnessed in the observable universe. The IceCube Neutrino Observatory, located in the glacial ice 1450 m to 2450 m below the South Pole surface, is the largest neutrino detector in operation. IceCube detects charged particles, such as those emitted in high energy neutrino interactions in the ice, by the Cherenkov light radiated by these particles. The measurement of neutrinos of 100 TeV energy or greater in IceCube correlated with gamma-ray photons from GRBs, measured by spacecraft detectors, would provide evidence of hadronic interaction in these powerful phenomena and confirm their role in ultra high energy cosmic ray production. This work presents the first IceCube GRB-neutrino coincidence search optimized for charged-current interactions of electron and tau neutrinos as well as neutral-current interactions of all neutrino flavors, which produce nearly spherical Cherenkov light showers in the ice. These results for three years of data are combined with the results of previous searches over four years of data optimized for charged-current muon neutrino interactions, which produce extended Cherenkov light tracks. Several low significance events correlated with GRBs were detected, but are consistent with the background expectation from atmospheric muons and neutrinos. The combined results produce limits that place the strongest constraints thus far on models of neutrino and UHECR production in GRB fireballs.
Almost Symmetric Spaces and Gravitational Radiation
(1967) Matzner, Richard Alfred; Misner, Charles W.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
Alphas and Surface Backgrounds in Liquid Argon Dark Matter Detectors
(2017) Stanford, Chris; Meyers, Peter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Current observations from astrophysics indicate the presence of dark matter, an invisible form of matter that makes up a large part of the mass of the universe. One of the leading theories for dark matter is that it is made up of Weakly Interacting Massive Particles (WIMPs). One of the ways we try to discover WIMPs is by directly detecting their interaction with regular matter. This can be done using a scintillator such as liquid argon, which gives off light when a particle interacts with it. Liquid argon (LAr) is a favorable means of detecting WIMPs because it has an inherent property that enables a technique called pulse-shape discrimination (PSD). PSD can distinguish a WIMP signal from the constant background of electromagnetic signals from other sources, like gamma rays. However, there are other background signals that PSD is not as capable of rejecting, such as those caused by alpha decays on the interior surfaces of the detector. Radioactive elements that undergo alpha decay are introduced to detector surfaces during construction by radon gas that is naturally present in the air, as well as other means. When these surface isotopes undergo alpha decay, they can produce WIMP-like signals in the detector. We present here two LAr experiments. The first (RaDOSE) discovered a property of an organic compound that led to a technique for rejecting surface alpha decays in LAr detectors with high efficiency. The second (DarkSide-50) is a dark matter experiment operated at LNGS in Italy and is the work of an international collaboration. A detailed look is given into alpha decays and surface backgrounds present in the detector, and projections are made of alpha-related backgrounds for 500 live days of data. The technique developed with RaDOSE is applied to DarkSide-50 to determine its effectiveness in practice. It is projected to suppress the surface background in DarkSide-50 by more than a factor of 1000.
Amoeboid Shape Dynamics on Flat and Topographically Modified Surfaces
(2012) Driscoll, Meghan Katrien; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
I present an analysis of the shape dynamics of the amoeba Dictyostelium discoideum, a model system for the study of cellular migration. To better understand cellular migration in complicated 3-D environments, cell migration was studied on simple 3-D surfaces, such as cliffs and ridges. D. discoideum interact with surfaces without forming mature focal adhesion complexes. The cellular response to the surface topography was characterized by measuring and looking for patterns in cell shape. Dynamic cell shape is a measure of the interaction between the internal biochemical state of a cell and its external environment. For D. discoideum migrating on flat surfaces, waves of high boundary curvature were observed to travel from the cell front to the cell back. Curvature waves are also easily seen in cells that do not adhere to a surface, such as cells that are electrostatically repelled from the coverslip or cells that are extended over the edge of micro-fabricated cliffs. At the leading edge of adhered cells, these curvature waves are associated with protrusive activity, suggesting that protrusive motion can be thought of as a wave-like process. The wave-like character of protrusions provides a plausible mechanism for the ability of cells to swim in viscous fluids and to navigate complex 3-D topography. Patterning of focal adhesion complexes has previously been implicated in contact guidance (polarization or migration parallel to linear topographical structures). However, significant contact guidance is observed in D. discoideum, which lack focal adhesion complexes. Analyzing the migration of cells on nanogratings of ridges spaced various distances apart, ridges spaced about 1.5 micrometers apart were found to guide cells best. Contact guidance was modeled as an interaction between wave-like processes internal to the cell and the periodicity of the nanograting. The observed wavelength and speed of the oscillations that best couple to the surface are consistent with those of protrusive dynamics. Dynamic sensing via actin or protrusive dynamics might then play a role in contact guidance.
An Extended Detailed Investigation of First and Second Order Supersymmetries for Off-Shell N = 2 and N = 4 Supermultiplets
(MDPI, 2015-06-16) Gates, Sylvester James Jr.; Parker, James; Rodgers, Vincent G. J.; Rodriguez, Leo; Stiffler, Kory
This paper investigates the d = 4, 𝒩=4 Abelian, global Super-Yang Mills system (SUSY-YM). It is shown how the 𝒩=2 Fayet Hypermultiplet (FH) and 𝒩=2 vector multiplet (VM) are embedded within. The central charges and internal symmetries provide a plethora of information as to further symmetries of the Lagrangian. Several of these symmetries are calculated to second order. It is hoped that investigations such as these may yield avenues to help solve the auxiliary field closure problem for d = 4, 𝒩=4, SUSY-YM and the d = 4, 𝒩=2 Fayet-Hypermultiplet, without using an infinite number of auxiliary fields.
Analog quantum simulation and quantum many-body dynamics in atomic, molecular and optical systems
(2021) Liu, Fangli; Gorshkov, Alexey; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In recent decades, the rapid development of quantum technologies has led to a new era of programmable platforms, enabling the realizations of quantum simulation and quantum computation. This dissertation is motivated by recent experimental progress on controlling individual quantum degrees of freedom in systems such as trapped ions and Rydberg atom arrays. By tailoring the interactions in these quantum systems, we study analog quantum simulations of various physical phenomena, including non-equilibrium quantum dynamics and nontrivial topological physics. In the first part of the dissertation, we study slow quantum many-body dynamics in trapped-ion systems and Rydberg atom arrays. We first show that either the long-range interactions or an additional symmetry-breaking field can give rise to a confining potential. Such a potential can couple domain wall quasiparticles into mesonic or baryonic bound states. These confined quasiparticles strongly suppress the quantum information dynamics and lead to slow thermalization. In thelimit of strict domain-wall confinement, the full Hilbert space is fragmented into exponentially many disconnected subspaces. Furthermore, we demonstrate that thermalization can be halted by quantum engineering a uniformly increasing field in the trapped-ion quantum simulator. The second part of the dissertation focuses on topologically relevant phenomena in quantum simulators. We first study the effect of experimentally relevant disorder in 2D Rydberg atom arrays. We find that there are three distinct localization regimes due to the presence of nontrivial topological bands. We further study the non-equilibrium dynamics of Abelian anyons in a one-dimensional system. We show that the interplay of anyonic statistics and interactions can give rise to spatially asymmetric quantum dynamics. Finally, we use Nielsen’s geometric approach to quantify circuit complexity in topological models. We find that the circuit complexities of ground states and non-equilibrium steady states both exhibit nonanalytical behavior at topological transition points.
Analog-Digital Quantum Simulations with Trapped Ions
(2023) Collins, Katherine Sky; Monroe, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Since its inception in the early 1920s, the theory of quantum mechanics has provided a framework to describe the physics of nature; or at least our interpretations about systems in nature. However, even though quantum theory works, the unsettling question of “why?’ still remains. The field of quantum information science and technology (QIST) has brought together a collection of disciplines forming a united multidisciplinary collaborative effort towards realizing a large-scale quantum processor as an attempt to understand quantum mechanics better. It has been established in the field that the most efficient architectural design of this quantum processor would be composed of numerous individual quantum computers, quantum simulators, quantum networks, quantum memories, and quantum sensors that are “wired” together creating just the hardware layer in the full stack of the machine. Realizing a module-based quantum processor on such a macroscopic scale is an ongoing and challenging endeavor in itself. However, existing noisy intermediate-scale quantum (NISQ) devices across all the quantum applications above are still worth building, running, and studying. NISQ quantum computers can still provide quantum advantages over classical computation for given algorithms, and quantum simulators can still probe complex many-body dynamics that remain improbable to consider even on the best supercomputer. One such system is the trapped-ion quantum simulator at the center of this dissertation. Using 171Yb+ ions, we expand our “analog” quantum simulation toolbox by incorporating “digital” quantum computing techniques in each of the three experiments presented in this work. In the first experiment, we perform a quantum approximate optimization algorithm (QAOA) to estimate the ground-state energy of a transverse-field antiferromagnetic Ising Hamiltonian with long-range interactions. For the second project, we develop and demonstrate dynamically decoupled (DD) quantum simulation sequences in which the coherence in observed dynamics evolving under the unitary operator of the target Hamiltonian is extended while the known noise is suppressed. Finally, in the third project, we implement an experimental protocol to measure the spectral form factor (SFF) and its generalization, the partial spectral form factor (PSFF), in both an ergodic many-body quantum system and in a many-body localized (MBL) model. As a result, a quantum simulator can be utilized to test universal random matrix theory (RMT) predictions, and simultaneously, probe subsystem eigenstate thermalization hypothesis (ETH) predictions of a quantum many-body system of interest.