Sympathetic cooling and self-oscillations in a hybrid atom-membrane system

Hybrid systems combining mechanical oscillators and ultracold atoms provide novel opportunities for cooling, detection and quantum control of mechanical motion with applications in precision sensing, quantum-level signal transduction and for fundamental tests of quantum mechanics. In this thesis I present experiments performed with a hybrid atom-membrane system, in which the vibrations of a Si_3N_4 membrane in an optical cavity are coupled to the motion of laser-cooled atoms in an optical lattice. The interactions are mediated by the lattice light over a macroscopic distance and enhanced by the cavity. Via the coupling to the cold atoms, the fundamental vibrational mode of the membrane at 2π x 276 kHz is cooled sympathetically from room temperature to 0.4(2) K, even though the mass of the mechanical oscillator exceeds that of the atomic ensemble by a factor of 4 x 10^10. In other systems, sympathetic cooling of molecules with cold atoms or ions has been limited to mass ratios of up to 90. Previous theoretical work has shown that our coupling mechanism is able to cool the membrane vibration into the ground state and to perform coherent state transfers between atomic and membrane motion. Under certain experimental conditions, the atom-membrane system shows self-oscillations, which arise from an effective delay in the backaction of the atoms onto the light. This retardation drives the system into limit-cycle oscillations if the coupling is large. I study the dependence of this instability on several system parameters and find that a larger atom number and a smaller atom-light detuning make the system less stable. Further, the stability of the coupled system in presence of a delay is investigated theoretically and a modified expression for the sympathetic cooling rate is derived. This model allows to fit the measured atom number dependence with a delay of τ = 88(1) ns. Moreover, direct measurements of the atomic backaction onto the lattice light are presented. These show phase lags exceeding 180° in parameter regimes where the instability is observed, proving that the retardation arises within the atomic ensemble. Finally, I present the results of numerical simulations, which show that collective atomic effects within the atomic ensemble in an asymmetric lattice are able to induce the observed phase lag in the atomic backaction

Self-Assembly and Its Inhibition in Bolaform Amphiphilic Dendrimers

Self-assembly is scientifically interesting, technologically important, and sometimes life-threatening. A striking example: Alzheimer’s disease is caused by the self-assembly of the â-amyloid fragment of amyloid precursor protein. Such natural amphiphiles normally are very expensive and complicated to isolate. Synthetic mimics that are much cheaper and easier to prepare can facilitate the development of efficient characterization methods, even as they suggest new materials science applications. Two-directional arborols are synthetic bolaform amphiphilic dendrimers. They dissolve in hot water and form reversible gels at room temperature. Bundles of fibrils, which are made by self-assembly of arborols, are found in the gels. The properties of the fibrils are characterized with scattering and microscopy from one hundred micron meters scale to molecular level at nano meter size. The porous nature inside the gels is investigated by comparing the diffusion of fluorescently labeled dextrans in arborol gels with that in water. The time dependence of the self-assembly of two-directional arborols from short fibrils to long ones is tracked by microscopy and light scattering. Addition of a one-directional inhibitor slows the self-assembly. To understand the mechanism, the properties of the one-directional arborols at the air-water interface are addressed. The self-associated arborol fibrils can be disassembled by growing gold nanoparticles in the arborol gels or solutions. In a basic arborol solution or gel, gold nanoparticles with an average diameter below 50 nm are formed after adding a diluted gold ion solution. The arborols act as both reducing agents and stabilizers in the formation of gold nanoparticles. Additionally, the long and thin fibrils in the arborol gel/solution are dismantled. As applications, an attempt to build nano-tunnels in the cross-linked poly(N-isopropyl acrylamide), a temperature sensitive hydrogel, PNIPAM gel, by replacing the preloaded arborol fibrils with water is discussed. Finally, encapsulation of the self-assembly fibrils in giant unilamellar vesicles (GUVs) is tried. As supplemental information, the size variance of liposomes at different lipids/arborols ratios is studied by fluorescence photobleaching recovery (FPR) and light scattering

Tree-Based Radiative Transfer of Diffuse Sources: A Novel Scheme and its Application in Massive Star Formation

Electromagnetic radiation plays an essential role in the evolution of astrophysical phenomena. Solving radiative transfer in numerical radiation hydrodynamics simulations is expensive and challenging. This work presents a novel method called \textsc{TreeRay/RadPressure} employing a tree-based backwards ray-tracing approach to compute radiative transfer of non-ionizing radiation on diffuse sources such as interstellar dust. The novel scheme allows for every computational entity to be a source of radiation and a contributor towards extinction. The scheme is implemented and tested for a finite volume method and can be generalized to work with particles of, for example, a Smoothed-Particle-Hydrodynamics method. The scheme developed in this thesis is suitable to compute radiation pressure on dust and gas. Coupled to a chemical network, the method allows to accurately model radiative cooling and heating of dust. An application of the scheme in the context of massive star formation shows that fragmentation induced by self-gravity is reduced by the scheme's more accurate teatmennt of cooling and heating. In particular, self-shielding of dust allows efficient cooling only from the surface of optically thick regions. With the help of \textsc{TreeRay/RadPressure}, the role of metallicity in the collapse of a subvirial turbulent core forming a massive stars is investigated. The core has a radius of 0.1 parsec and a mass of 150 solar masses. Along with ionizing radiation computed by \textsc{TreeRay/OnTheSpot}, the relative strength of radiation pressure and ionizing feedback are examined. Radiation pressure is the dominant feedback mechanism during the formation of the massive star while it hosts an ultra-compact HII region. Radiation pressure does not halt accretion onto the massive star and its strength is reduced once a disk establishes. Greater metallicities favour fragmentation by enhancing cooling. This results in a greater multiplicity with increased metallicity. Radiation pressure manages to launch bipolar outflows along the rotational axis of the disk. \textsc{TreeRay/RadPressure} is compared to a multi-wavelength scheme based on RADMC-3D to compute radiation pressure. The comparison is done using non-trivial density distributions generated by radiation hydrodynamics simulations with \textsc{TreeRay/RadPressure}. The comparison shows the radiation pressure calculations of both schemes to agree for the majority of density and luminosity configurations. Simplfied theoretical modelling using the optical depth as a proxy for the momentum boost fails to estimate the radiation pressure

Effect of precious metal particle size and support type on catalytic activity as revealed by X-ray methods

For supported nanoparticulate catalysts, the effect of their properties, such as size and interaction with support, on a catalytic process is still in debate. This is partially caused by the limited control that typical nanoparticle preparation methods offer which may lead to the presence of nanoparticles possessing a range of sizes and shapes, leading to a lack of clarity in the correlation between properties and catalysis. The work presented in this thesis focuses on producing size-selected monometallic supported nanoparticles and their detailed characterisation, both ex situ and in situ (under reaction conditions) mainly through the application of the technique of X-ray absorption spectroscopy (XAFS). Chapter 3 of this thesis focuses on understanding the mechanism of formation of Au nanoparticles produced by reverse micelle encapsulation, through a combination of various techniques (transmission electron microscopy, dynamic light scattering, UV-vis, combined small angle X-ray scattering/XAFS). From the combined data, it is possible to rationalise the synthesis process, which can be divided into three steps: fast reduction of Au (III) species to Au (I), slow reduction and agglomeration of Au atoms in sub-nanometric clusters and a final agglomeration to form the nanoparticles. Chapter 4 and 5 focus on the application of monometallic nanoparticles to the catalytic hydrogenation of 1,3-butadiene, used as a model reaction, using in situ XAFS to correlate size and support effect with their catalytic activities. For Au nanoparticles (chapter 4) it appears that a restructuring process takes places under reaction conditions. Depending on the sample, this process can be favourable (Au/SiO2) or detrimental (Au/Al2O3) and is highly dependent on particle size. For Pd nanoparticles (chapter 5) it was possible to identify the active species necessary (Pd0), and detrimental (PdH and PdC) for the selective hydrogenation of 1,3-butadiene in Pd as well as the role of support and size in promoting the presence of each phase

Introduction to Particle and Astroparticle Physics : Multimessenger Astronomy and its Particle Physics Foundations -2/E

This book introduces particle physics, astrophysics and cosmology starting from experiment. It provides a unified view of these fields, which is needed to answer our questions to the Universe–a unified view that has been lost somehow in recent years due to increasing specialization. This is the second edition of a book we published only three years ago, a book which had a success beyond our expectations. We felt that the recent progress on gravitational waves, gamma ray and neutrino astrophysics deserved a new edition including all these new developments: multimessenger astronomy is now a reality. In addition, the properties of the Higgs particle are much better known now than three years ago. Thanks to this second edition we had the opportunity to fix some bugs, to extend the material related to exercises, and to change in a more logical form the order of some items. Last but not least, our editor encouraged us a lot to write a second edition. Particle physics has recently seen the incredible success of the so-called standard model. A 50-year long search for the missing ingredient of the model, the Higgs particle, has been concluded successfully, and some scientists claim that we are close to the limit of the physics humans may know

Graduate Catalog, 1996-1999, New Jersey Institute of Technology

https://digitalcommons.njit.edu/coursecatalogs/1003/thumbnail.jp

Multiscattering on the cube-connected cycles

International audienc

Optimal Total Exchange in Cayley Graphs

Consider an interconnection network and the following situation: every node needs to send a different message to every other node. This is the total exchange problem, one of a number of information dissemination problems known as collective communications. Under the assumption that a node can send and receive only one message at each step (single-port model) it is seen that the minimum time required to solve the problem is governed by the status (or total distance) of the nodes in the network. We present here a time-optimal solution for any Cayley network. Rings, hypercubes, cube-connected cycles, butterflies are some well-known Cayley networks which can take advantage of our method. The solution is based on a class of algorithms which we call node-invariant algorithms and which behave uniformly across the network. Keywords: Cayley graphs, collective communications, interconnection networks, total exchange (multiscattering) AMS(MOS) subject classifications: 94C15, 68M10, 68R10, 68Q22, ..