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Aspects of Quasi-Single Field Inflation
Get PDFA simple extension of single-field slow roll inflation is called quasi-single field inflation (QSFI). In quasi-single field inflation, the inflaton is coupled to one or more scalar fields with masses of order Hubble constant during inflation, H, called isocurvatons. Depending on the interactions between the inflaton and the isocurvatons, the observable consequences of such a theory can vary dramatically. This thesis is primarily concerned with analyzing how these interactions affect cosmological observables.
We begin by discussing QSFI with one isocurvaton. In particular, we study the non-perturbative limit of a kinetic interaction coupling the inflaton to the isocurvaton. In this non-perturbative limit, the kinetic interaction results in isocurvaton mode functions that oscillate yet decay quickly after horizon crossing. This oscillatory, decaying behavior is reflected in the primordial non-Gaussianity and other observables.
Then, we study the perturbative limit of the aforementioned kinetic interaction. Instead of quickly decaying, oscillating mode functions as in the non-perturbative limit, the mode functions in the perturbative limit decay slowly and do not oscillate. Due to this slow decay, the primordial bispectrum is enhanced in the squeezed limit. This enhanced squeezed limit can result in large contributions to galactic halo correlations at long wavelengths. We explore these long wavelength enhancements to galactic halo correlations in detail.
We then discuss how quantum loops of isocurvatons in QSFI can significantly contribute to galactic halo correlations at long wavelengths. In fact, we show that loops can give the most important contributions at long wavelengths in certain models.
Finally, we consider theories with multiple isocurvatons. Such theories may consist of interactions coupling the isocurvatons to eachother. These interactions can result in slowly decaying yet oscillating late-time behavior for the isocurvaton mode functions. This combines the two characteristic features from the non-perturbative (oscillating) and perturbative (slowly-decaying) single isocurvaton QSFI models discussed above. These features could result in an oscillatory bispectrum that is enhanced in the squeezed limit. Oscillations in the galactic halo power spectrum at long wavelengths may then be indicative of such multi-isocurvaton models.</p
Clocked Atom Delivery to a Photonic Crystal Waveguide: Simulations and Experiments
Get PDFIntegrating atomic physics with nanophotonics devices provides a new research platform for quantum optics and many-body physics. The robustness and scalability of advanced lithographic fabrication technology provide powerful tools to enhance and control atom-photon interaction. Dispersion engineered photonic crystal waveguides (PCWs), such as the alligator photonic crystal waveguides (APCWs) described in this thesis, allow stable trapping and probing of atoms via guided modes (GMs). By tuning the photonic band-edges of the PCWs, the photon-mediated interactions between atoms can be modified. This thesis describes simulations and experiments that develop a quantitative understanding of atomic motion near the surfaces of APCWs. The atoms are delivered to APCWs using optical lattice. Synchronous with the moving lattice, transmission spectra for a guided-mode probe field are recorded as functions of lattice transport time and frequency detuning of the probe beam. With these 2D "clocked" spectra, we have been able to validate quantitatively our numerical simulations, which are based upon a detailed understanding of atomic trajectories that pass around and through nanoscopic regions of the APCW under the influence of optical and surface forces. By introducing auxiliary GMs of various polarizations and intensities, we have begun to control the atomic trajectories to some degree. For example, atoms can be guided to the central vacuum gap of the APCW at predetermined times and with known AC-Stark shifts. The applications of combining clocked atom delivery and numerical simulation include enabling high fractional filling of optical trap sites within PCWs, calibration of optical fields within PCWs, and utilization of the time-dependent, optically dense atomic medium for novel nonlinear optical experiments
Numerical Methods for Many-Body Quantum Dynamics
Get PDFThis thesis describes two studies of the dynamics of many-body quantum systems with extensive numerical support.
In Part I we first give a new algorithm for simulating the dynamics of one-dimensional systems that thermalize (that is, come to local thermal equilibrium). The core of this algorithm is a new truncation for matrix product operators, which reproduces local properties faithfully without reproducing non-local properties (e.g. the information required for OTOCs). To the extent that the dynamics depends only on local operators, timesteps interleaved with this truncation will reproduce that dynamics.
We then apply this to algorithm to Floquet systems: first to clean, non-integrable systems with a high-frequency drive, where we find that the system is well-described by a natural diffusive phenomenology; and then to disordered systems with low-frequency drive, which display diffusion — not subdiffusion — at appreciable disorder strengths.
In Part II, we study the utility of many-body localization as a medium for a thermodynamic engine. We first construct a small ("mesoscale") engine that gives work at high efficiency in the adiabatic limit, and show that thanks to the slow spread of information in many body localized systems, these mesoscale engines can be chained together without specially engineered insulation. Our construction takes advantage of precisely the fact that MBL systems do not thermalize. We then show that these engines still have high efficiency when run at finite speed, and we compare to competitor engines.</p
Earth-Abundant Metal Oxides for Anodic Reactions in Acidic Electrolytes
Get PDFThe development of electrochemical systems such as electrolyzers and photoelectrochemical devices in corrosive electrolytes has been limited by the lack of earth-abundant materials that are both stable in acidic electrolytes and efficiently utilize energy for electrochemical reactions. Chapter 1 introduces several of the challenges in developing earth-abundant materials for electrochemical systems in acidic electrolytes, such as electrocatalysts for the oxygen and the chlorine evolution reactions, and protective layers for photoanodes. Chapter 2 reports the electrochemical behavior of crystalline transition metal antimonates consisting of solid solutions of MnSb2O6 with NiSb2O6 for the oxygen evolution reaction in strongly acidic electrolytes. In Chapter 3, the crystalline transition metal antimonates NiSb2O6, CoSb2O6, and MnSb2O6 are investigated for the chlorine evolution reaction, and CoSb2O6 is found to exhibit activity and stability comparable to noble metal oxide electrocatalysts. Chapter 4 describes the development of earth-abundant SnOx coatings as protective heterojunctions for planar Si photoanodes in corrosive electrolytes. Chapter 5 focuses on the development of conformal SnOx coatings that form protective heterojunctions on Si microcone photoanodes. The work presented herein demonstrates several strategies towards the development of stable earth-abundant materials for efficient electrochemical and photoelectrochemical energy conversion in acidic electrolytes.</p
Imaging and Control of Engineered Cells using Magnetic Fields
Get PDFMaking cells magnetic is a long-standing goal of synthetic biology, aiming to enable the separation of cells from complex biological samples and their non-invasive visualization in vivo using Magnetic Resonance Imaging (MRI). Previous efforts towards this goal, focused on engineering cells to biomineralize superparamagnetic or ferromagnetic iron oxides, have largely been unsuccessful due to the stringent required chemical conditions. In this thesis, we introduce an alternative approach to making cells magnetic, focusing on biochemically maximizing cellular paramagnetism. Here, we show that a novel genetic construct combining the functions of ferroxidation and iron-chelation enables engineered bacteria to accumulate iron in 'ultraparamagnetic' macromolecular complexes, which subsequently allows for these cells to be trapped using strong magnetic field gradients and imaged using MRI in vitro and in vivo. We characterize the properties of these cells and complexes using magnetometry, an array of spectroscopic techniques, biochemical assays, and computational modeling to elucidate the unique mechanisms and implications of this 'ultraparamagnetic' concept.
In addition to making cells magnetic, remote control of cellular localization in deep tissue is another long-standing goal of synthetic biology. Such an ability to non-invasively direct cells to sites of interest will not only improve therapeutic outcomes by minimizing off-target activity, but more broadly enable new research on complex cellular communities, such as the gut microbiome, in living animals. Given their deep penetrance through tissues, magnetic fields are ideally suited for facilitating non-invasive targeting of cells; however, the rapid decay of magnetic flux density from its source currently limits the depths to which magnetic targeting can be employed to within 1-2 mm from the surface. Here, we demonstrate a new approach wherein the retention of orally-administered and synthetically magnetized cell-like-particles is selectively enhanced within the murine intestinal tract to depths of up to 13 mm from the surface. Our cellular localization assisted by magnetic particles (CLAMP) strategy can potentially be generalized to any cell (bacterial, mammalian) or drug-containing nanoparticle of interest, and can be combined with existing non-invasive imaging modalities thereby facilitating remote environmental sensing at sites of interest.
Finally, while magnetic fields in MRI scanners are widely used today to safely and non-invasively image anatomical structures in living animals, much of the image contrast in MRI is the result of microscale magnetic-field variations in tissues. However, the connection between these microscopic patterns and the appearance of macroscopic MR images has not been the subject of direct experimental studies due to a lack of methods to map microscopic fields in biological samples under ambient conditions. Here, we optically probed magnetic fields in mammalian cells and tissues with submicron resolution and nanotesla sensitivity using nitrogen-vacancy (NV) diamond magnetometry and combined these measurements with simulations of nuclear-spin precession to predict the corresponding MRI contrast. Additionally, we demonstrate the broad utility of this technology for imaging an in vitro model of cellular iron uptake, as well as imaging histological samples from a mouse model of hepatic iron overload. Taken together, our approach bridges a fundamental intellectual gap between a macroscopic MRI voxel and its microscopic constituents.</p
Light Emission and Ultrafast Carrier Dynamics in III-V Semiconductors from First Principles
Get PDFThe III-V semiconductors are a broad class of technologically important materials which have seen immense research interest in academia and industry due to their electronic, optoelectronic, and photovoltaic properties. In particular, GaN and the III-nitride family of wide bandgap semiconductors have emerged as promising candidates for the next generation of high-efficiency power electronics and light-emitting devices. Their device operation and macroscopic properties are governed by the dynamics of charge carriers and their microscopic scattering processes. Near room temperature, the carriers are scattered by lattice vibrations (phonons) at ultrafast timescales of order fs-ps. Microscopic understanding of carrier dynamics is challenging due to both the ultrafast time scale at play and to the presence of defects, interfaces, and impurities affecting transport and spectroscopy measurements. Typical theoretical treatments of carrier dynamics and light emission employ empirical models to interpret and fit experimental results. Over the last few years, so-called first-principles (or "ab initio") methods to accurately compute ultrafast carrier dynamics, transport, and light emission have seen a rapid rise. These approaches do not employ parameters from experiments, and using only the structure of the material as input, together with quantum mechanics and condensed matter theory, are enabling accurate predictions of carrier dynamics in a wide range of materials and are shedding light on microscopic details such as which electronic states, phonon modes and dissipative processes are responsible for the observed charge transport and light emission properties.
Here, we present first-principles calculations of different aspects of ultrafast carrier dynamics and light emission in III-V semiconductors of technological relevance, focusing on GaN, a key material for solid-state light emission technology. We first present a study of the ultrafast nonequilibrium dynamics of excited (so-called "hot") carriers in GaN, with a focus on electron-phonon scattering and the nanometer scale transport of carriers in GaN light emitting devices (LEDs). Using cutting-edge first-principles methods developed in this work, we find an asymmetry between the time scale of hot electron and hole thermalization which provides a possible explanation on a major open problem in the efficiency and energy losses of GaN LEDs. We then develop and apply a new rigorous first-principles approach for computing light emission and the radiative recombination lifetimes in bulk crystals, nanomaterials and isolated systems. Our approach is based on the Bethe-Salpeter equation (BSE), and it accurately includes excitons, namely electron-hole states bound by the Coulomb interaction that play a key role in light-matter interactions. Using this method, we carry out benchmark calculations of radiative lifetimes in GaAs and GaN. In GaN, our computed radiative lifetimes are in excellent agreement with experiment (within a factor of two), and our calculations further highlight the importance of including excitonic effects and spin-orbit coupling to obtain accurate radiative. We also employ a model to account for exciton thermal dissociation at high temperature, finding excellent agreement with spectroscopic measurements. Lastly, we discuss ongoing work on computing the intrinsic (phonon-limited) mobility in bulk GaN from first principles, focusing on efforts to include piezoelectric electron-phonon interactions, which are important for acoustic phonon modes in GaN. We compute the electron and hole mobilities in GaN and obtain excellent agreement with experiment. Our calculations shed light on which phonon modes scatter the carriers, providing new microscopic insight into charge carrier dynamics in GaN and related III-V semiconductors.</p
Impassioned Love and Marriage in Shakespeare’s Comedies
No full textImpassioned Love and Marriage in Shakespeare’s Comedies
Throughout the history of theater, marriage and love have been depicted in several different manners, from a function of duty to country or duty to family, to a choice that can only be determined by the desires of the heart. In several of his comedies, Shakespeare raises doubts about archetypal notions of love and marriage, while still maintaining a thin facade that all would end with a “happily ever after”
On the Dynamics of Flat Plates in a Fluid Environment: A Study of Inverted Flag Flapping and Caudal Fin Maneuvering
Get PDFDespite serving analogous functions, the mechanical designs conceived by human engineering and those that result from natural evolution often possess fundamentally differing properties. This thesis explores the use of principles that stem from natural evolution to improve the performance of engineered mechanisms, focusing on systems whose role is to interact with a fluid environment. Two different principles are considered: the use of compliance, abundant in nature's structures, and the use of flapping propulsion, prevalent among nature's swimmers.
The first part of this thesis is dedicated to investigating the physics that govern the behavior of an inverted-flag energy harvester; an unactuated flexible cantilever plate that is clamped at its trailing edge and submerged in a flow. The resonance between solid motion and fluid forcing generates large-amplitude unsteady deformations of the structure that may be used for energy harvesting purposes. The effect of the flag's aspect ratio on its stability is first evaluated. Flags of very small aspect ratio are demonstrated to undergo a saddle-node bifurcation instead of a divergence instability. The angle of attack of the flag is then modified to reveal the existence of dynamical regimes additional to those present at zero angle of attack. A side-by-side flag configuration is finally explored, highlighting the presence of an energetically favorable symmetric flapping mode among other coupled dynamics.
The second part of this thesis delves into the analysis of underwater flapping propellers and the optimization of their three-dimensional motion to generate desired maneuvering forces, with the objective of obtaining an appendage for use in autonomous underwater vehicles that can perform both fast maneuvering and efficient propulsion. An experimental optimization procedure is employed to obtain the most efficient trajectory that generates a specified side force. The effect of increasing the fin's aspect ratio is examined, and a highly efficient trajectory, that makes use of high three-dimensionality and rotation angles, is obtained for a fin of AR=4. The use of a flexible fin is then analyzed and shown to be detrimental to the maneuvering efficiency of the system.</p
Memory and Decoding in Signaling Transduction Pathways
Get PDFIntercellular communication allows cells to broadcast and receive necessary information for decision making, and is essential for development, growth, and maintenance of a community of cells in a multicellular organism. Signaling pathways are highly conserved systems of communication between cells, each composed of a distinct network of protein interactions that detect extracellular signal and transduce the signal information for cellular response. A signaling pathway typically encodes information from signaling events into dynamics of second messengers, intracellular molecules in the signaling pathway that activate in response to signal and initiate cellular response. Therefore, understanding how information is encoded in second messenger dynamics, and how transcriptional machinery decode and generate output response is an important aspect in investigating how signaling information is transduced inside a cell. In the first chapter, we investigate the timescales of memory in endogenous β-catenin and Smad3, second messengers in the Wnt and Tgf-β pathways, through single cell timelapse microscopy. The findings demonstrate that both second messengers have short memory and high cell-to-cell variability, and that their memory is tunable through modulating cellular contexts. In the second chapter, we investigate decoding of information from β-catenin in the Wnt pathway. We identify a novel 11-bp DNA element that recruit β-catenin for transcriptional suppression. This negative regulatory element is shown to act in conjunction with the canonical Wnt responsive element to form an incoherent feedforward loop (IFFL). Through mathematical simulations, we present how the IFFL circuit can generate complex output functions in decoding β-catenin dynamics, which include those that confer robustness against perturbations in signaling response such as band-pass filtering and fold change detection.</p
Using the Force: Applications and Implications of Turbulence Forcing Terms in Direct Numerical Simulations
Get PDFMost energy requirements of modern life can be fulfilled by renewable energy sources, but it is impossible in the near future to provide an alternative energy source to combustion for airplanes. That being said, combustion in aviation can be made more sustainable by using alternative jet fuels, which are made from renewable sources like agricultural wastes, solid wastes, oils, and sugars. These alternative fuels can be used in commercial flights only after a long certification process by the Federal Aviation Agency (FAA) and ASTM International. Unfortunately, in over 50 years of fuel research, only five fuels have been certified.
This research project aims to speed up the certification process with quicker testing of alternative fuels. Engine testing and even laboratory testing require large amounts of time and fuel. Simulations can make the process much more efficient, but accurately simulating highly turbulent flames in such complex geometries would need large amounts of computational resources. The goal of this thesis is to create an efficient computational framework, that can replicate different engine-like turbulent flow conditions in simple geometries with numerical tractability.
The central idea is to decompose the flow field into ensemble mean and fluctuating quantities. The simulations then resolve only the fluctuations using simple computational domains, while emulating the effect of the mean flow using "forcing" terms. These forcing terms are calculated first for incompressible turbulence, and this method is later extended to turbulent reacting flows. In incompressible turbulence, Direct Numerical Simulations (DNS) performed on simple triply periodic cubic domains reasonably capture the statistically stationary shear turbulence, that is observed in free shear flows. The simulations are also performed in cuboidal domains, that are longer in one direction and with an inflow/outflow along it. Both changes are observed to not have a significant impact on the turbulence statistics. Finally, shear convection is applied to the turbulence simulations with inflow/outflow, which has a significant impact on the turbulence. These simulations accurately capture the turbulence anisotropy in free-shear flows.
The study is extended to DNS of highly turbulent n-heptane-air flames performed under different flow conditions. Turbulent flames involve two-way coupling between fluid mechanics and combustion. The effects of the flame on the turbulence and the impact of the turbulent flow conditions on the flame behavior are analyzed. The focus is placed on the effects of turbulence production, shear convection, and pressure gradients. The anisotropy produced in the turbulence due to the different flow conditions and the flame are also compared and contrasted. While the global behavior and flow anisotropy were affected by these conditions, the local chemistry effects were unaffected, and depend only on the laminar flame properties and turbulence intensity. These findings can help predict turbulent flame behavior, and can expedite the search and testing of sustainable alternatives to conventional jet fuels.</p