<p> Collective magnetic behavior in nanostructures is a phenomenon commonly observed in various magnetic systems. It arises due to competing inter/intra&ndashparticle; interactions and size distribution and can manifest in phenomena like magnetic freezing, magnetic aging, and exchange bias (EB) effect. In order to probe these rather complex phenomena, conventional DC and AC magnetic measurements have been performed along with radio&ndashfrequency; transverse susceptibility (TS) measurements. We also demonstrate the magnetic entropy change as a parameter sensitive to subtle changes in the magnetization dynamics of nanostructures. The focus of this dissertation is to study the collective magnetic behavior in core-shell nanostructures of Fe/&gamma;&ndashFe;₂O₃ and Co/CoO, La_0.5Sr_0.5MnO₃ nanowires, and LaMnO₃ nanoparticles.</p><p> In the case of core/shell Fe/&gamma;&ndashFe;₂O₃, we found the particles to critically slow down below the glass transition temperature, below which they exhibit aging effects associated with a superspin glass (SSG) state. We demonstrate that it is possible to identify individual magnetic responses of the Fe core and the &gamma;&ndashFe;₂O₃ shell. Consistently, a systematic study of the magnetocaloric effect (MCE) in the Fe/&gamma;&ndashFe;₂O₃ system reveals the development of inverse MCE with peaks associated with the individual magnetic freezing of the core and the shell. From these obtained results, we establish a general criterion for EB to develop in core/shell nanostructures, that is when the core is in the frozen state and the magnetic moments in the shell begin to block. This criterion is shown to be valid for both ferromagnetic/ferrimagnetic (FM/FIM) Fe/&gamma;&ndashFe;₂O₃ and ferromagnetic/antiferromagnetic (FM/AFM) Co/CoO core&ndashshell; nanostructures. We also elucidate the physical origin of the occurrence of asymmetry in field-cooled hysteresis loops and its dependence on magnetic anisotropy in the Co/CoO system by performing a detailed TS study.</p><p> We have performed a detailed magnetic study on hydrothermally synthesized single crystalline La_0.5Sr_0.5MnO₃ nanowires. The temperature and field dependent evolution of the different magnetic phases leading to development of the inverse MCE and EB in the nanowires is discussed. Finally, we have studied the collective magnetic behavior of LaMnO₃ nanoparticles synthesized by the sol&ndashgel; technique. The nanoparticle ensemble shows the unusual co&ndashexistence; of super-ferromagnetism (SFM), as well as the SSG state, which we term the &lsquoferromagnetic; superglass&rsquo; (FSG) state. The existence of FSG and the characteristics of its magnetic ground state are discussed.</p>

O protocolo conhecido como Dynamic Clamp consiste em utilizar um computador para introduzir condutâncias artificiais em um neurônio biológico. O modo como estas condutâncias dependem da voltagem da membrana ou do tempo são modelado por equações diferenciais que são integradas em tempo real por um computador conectado ao neurônio biológico. Resumidamente, o computador tem acesso ao potencial de membrana dos neurônios através de eletrodos intracelulares conectados a conversores analógico-digitais (ADCs), calcula as correntes a serem injetadas nos neurônios e produz os sinais de saída através de conversores digitalanalógicos (DACs) que produzem a injeção das correntes nos eletrodos intracelulares. De um certo modo, o Dynamic Clamp utiliza os neurônios como simuladores, permitindo investigar a importância de um tipo de condutância para a atividade elétrica de um neurônio, assim como determinar o efeito produzido pelas sinapses em uma rede, combinando o controle e flexibilidade de uma simulação no computador com a acurácia e o realismo de um experimento em eletrofisiologia. Descrevemos a implementação de um protocolo de Dynamic Clamp utilizando um computador pessoal tipo IBM-PC que permitiu contornar 3 das principais limitações que apresentam alguns dos programas de Dynamic Clamp comerciais/gratuitos disponíveis atualmente: (a) Garantia de que o sistema roda em tempo real - nossa implementação é baseada em um programa de Dynamic Clamp que roda em uma plataforma Linux Real-Time que além de controlar os experimentos em tempo real consiste em software livre com codigo fonte aberto e que pode ser instalado gratuitamente; (b) Não necessita de hardware de aquisição de dados dedicado para eletrofisiologia - utilizamos uma placa ADC/DAC comercial comum marca National Instruments modelo PCIMIO16E4. Com o driver COMEDI instalado para placas de aquisição de dados Linux, a maioria das placas ADC/DAC tipo PCI disponíveis no mercado podem ser utilizadas em implementações futuras; (c) Aumentar o número de neurônios que podem ser conectados simultaneamente - desenvolvemos um circuito demultiplex analógico que permite controlar até 8 neurônios biológicos/artificiais a partir das duas saídas analógicas que as placas DAC comerciais possuem e ainda atingir frequências de atualização da corrente de até 3 kHz (para 8 correntes de saída). Apresentamos os resultados de diversos testes que fizemos usando o programa adaptado e o circuito demultiplex para produzir sinapses em tempo real e conectar diversos neurônios artificiais em pequenas redes. Também mostramos alguns resultados preliminares obtidos com a primeira implementação de um modelo de neurônio estocástico tipo Hodgkin-Huxley em um programa de Dynamic Clamp. / The Dynamic Clamp protocol consists in using a computer to introduce artificial conductances in a biological neuron. The voltage- and time-dependency of each conductance is modeled by differential equations integrated in real-time by the computer connected to the biological neurons. In short, the computer executes a 3-phase cycle in which the membrane potential of the neurons is measured by intracellular electrodes and digitized by analog-to-digital converters (ADCs), the currents are calculated based in the digitized membrane potentials and current signals are generated by digital-to-analog converters (DACs). These currents are actually injected in the neurons by other intracellular electrodes. In some extent the Dynamic Clamp uses the neurons as simulators, allowing one to investigate the role of a specific conductance in the intrinsic activity of a neuron as well as to look for the effects of a synapse in the behavior of a small network. The Dynamic Clamp combines the control and flexibility of a computer simulation with the reality of an experiment in electrophysiology. We describe an implementation of a Dynamic Clamp protocol that allowed us to surmount 3 of the main drawbacks present in some commercial/freely available Dynamic Clamp programs: (a) Runs in real time - our implementation is based in a program that runs in a Real-Time Linux platform. This operating system not only ensures the experiments will be controlled in real time but also consists in open source software that can be freely downloaded and installed; (b) No need of special electrophysiology acquisition hardware - we used a commercial ADC/DAC acquisition board model PCI-MIO16E4 from National Instruments. With the COMEDI Linux package driver that is used most of the PCI commercial ADC/DAC boards can be used in future implementations with no change needed in the program itself. (c) We can connect more than two neurons with artificial synapses - we developed an analog demultiplex circuit that allowed us to control simultaneously up to 8 biological/artificial neurons from the two analog outputs available in most of the commercial ADC/DAC boards and we could still reach current update rates of about 3 kHz (for 8 current outputs enabled). We present the results of several tests we performed using the program adapted to control the analog demultiplex to establish synapses and to connect several artificial neurons in small neural networks. Preliminary results from the first implementation of a stochastic whole cell Hodgkin-Huxley model neuron in a real time Dynamic Clamp program are also shown.

Neptune| An astrophysical smooth particle hydrodynamics code for massively parallel computer architectures

<p>Beauty quarks are pair-produced by strong interactions in multi-TeV proton-proton (pp) collisions at the CERN Large Hadron Collider (LHC). Such interactions allow for a test of perturbative Quantum Chromodynamics (QCD) in a new energy regime. The primary beauty-antibeauty quark b<span style="text-decoration:overline"> b</span> pair production mechanisms in perturbative QCD are referred to as flavor creation, flavor excitation, and gluon splitting. These three mechanisms produce b<span style="text-decoration:overline">b</span> pairs with characteristic kinematic behavior, which contribute differently to the shape of the differential b<span style="text-decoration:overline"> b</span> production cross section with respect to the difference in the azimuthal angle &Delta;&phis; and the combined separation variable &Delta;<i> R</i> = [special characters omitted] between the beauty and antibeauty quarks (b and <span style="text-decoration:overline">b</span>, respectively); with &Delta;&eta; being the change in the pseudorapidity &eta; = &mdash; ln (<i>tan</i> (&thetas;/2)), &thetas; being the polar angle. These &Delta;&phis; and &Delta;^R variables are collectively referred to as angular correlation variables and hence forth referred to as &Delta;<i> A</i>. By measuring the shape and absolute normalization of the differential production cross section distributions with respect to &Delta;<i>A</i> a test of the predictions of perturbative QCD can be performed. </p><p> This dissertation describes a measurement of the differential production cross sections with respect to the &Delta;<i>A</i> between two hadronic jets arising from the hadronization and decay of b or <span style="text-decoration:overline">b</span> (referred to as <i>b</i> hence forth) produced in pp collisions at the LHC observed with the Compact Muon Solenoid (CMS) detector. Hadronic jets are identified as originating from b quarks, i.e. b-tagged, based on the presence of high impact parameter tracks with respect to the primary pp interaction point in events in which a muon is also produced. The study presented in this dissertation corresponds to an integrated luminosity of 3 pb^-1 collected in 2010 by the CMS experiment at a center-of-mass energy of 7 TeV. </p><p> The visible kinematic phase-space of the differential production cross sections probed in this study is given by the requirement of two b-tagged hadronic jets with [special characters omitted] > 30 GeV and &par;&eta;^jet&par; &lt; 2.4, with an angular separation of &Delta;R > 0.6 between them, one of these jets has a muon within its constituents with [special characters omitted] > 8 GeV and &par;&eta;^&mu;&par; &lt; 2.1. The results obtained in data are compared with predictions based on perturbative QCD calculations given by CASCADE, MADGRAPH/MADEVENT, and PYTHIA Monte Carlo event generators. The predictions of perturbative QCD are found to be in agreement the measured differential cross sections within uncertainties. </p>

<p> Organic photovoltaics (OPV) offers a potential for solar-electric power generation to be affordable. Crucial to OPV device performance is the incorporation of interlayers, ultra-thin films deposited between the photoactive material and the electrical contacts. These interlayers have various, targeted functionalities: optical window, encapsulation, or electronic bridge. The last category is known as "transport layers'', and is the focus of this thesis. </p><p> In this thesis, we explore and investigate the physics that leads to improvements in OPV device performance when a transport layer is employed. We focus on the use of non-stoichiometric nickel oxide (NiO_x) as a hole transport layer (HTL) in poly(3-hexylthiophene):phenyl-C₆₁-butryric acid methyl ester (P3HT:PCBM) solar cells. NiO_x deposited by physical vapor deposition is chosen for this study because of its successful use as HTL, the ease to engineer its electronic properties by varying deposition parameters, and it leading to improved device lifetime. </p><p> Our initial studies indicate that the well-known "high work-function'' rule is not adequate to explain the trends observed in the devices. More in-depth studies is required to fully understand the impact of HTL electronic properties on device performance. These series of investigations reveal that band-offsets at the NiO_x / P3HT:PCBM interface need to be taken into account in order to explain the observed trends. Non-optimal band-offsets lead to either sigmoidal current-voltage characteristics or reduced photocurrent. </p><p> The optimal energy level alignment depends on the energy levels of the photo-active material, which are measurable. This means that an HTL material must be optimized for different photoactive material. A simple and practical set of rules are proposed to achieve this optimal energy level alignment for a given photoactive material. The rules not only include the pervasively-used "high work-function'' rule, but also the impacts of band-offsets investigated in this work.</p>

<p> In the past three decades, spintronic devices have played an important technological role. Half-metallic alloys have drawn much attention due to their special properties and promised spintronic applications. This dissertation describes some theoretical techniques used in first-principal calculations of alloys that may be useful for spintronic device applications with an emphasis on half-metallic ferromagnets. I consider three types of simple spintronic materials using a wide range of theoretical techniques. They are (a) transition metal based half-Heusler alloys, like CrMnSb, where the ordering of the two transition metal elements within the unit cell can cause the material to be ferromagnetic semiconductors or semiconductors with zero net magnetic moment, (b) half-Heusler alloys involving Li, like LiMnSi, where the Li stabilizes the structure and increases the magnetic moment of zinc blende half-metals by one Bohr magneton per formula unit, and (c) zinc blende alloys, like CrAs, where many-body techniques improve the fundamental gap by considering the physical effects of the local field. Also, I provide a survey of the theoretical models and numerical methods used to treat the above systems.</p>

<p>Recent results at RHIC seem to confirm T.D.Lee's hypothesis that a new form of matter, the quark-gluon plasma (QGP), could be formed in heavy-ion collisions at high energies. Heavy quarks, being formed in the early stages of heavy-ion collisions, form a good probe for the properties of the QGP. The energy loss of heavy quarks as they traverse the medium is predicted to be less than that of the lighter quarks. However; previous measurements of the nuclear modification factor at RIHC indicate that the energy loss of heavy and light quarks is comparable. Thus measurements of the in-medium energy loss of heavy-quarks are of particular interest. In this thesis, a measurement of the differential production cross-section of electrons from the semi-leptonic decay of heavy-flavor quarks in [special characters omitted] = 2.76 TeV <i>pp</i> is presented. This provides a stringent test of perturbative QCD in a new energy regime, and forms a crucial baseline for Pb-Pb collisions where the in-medium energy loss mechanism can be studied. </p>

<p> The electric (<i>G_E</i>) and magnetic (<i> G_M</i>) form factors of the proton are fundamental observables which characterize its charge and magnetization distributions. There are two methods to measure the proton form factors: the Rosenbluth separation method and the polarization transfer technique. However, the ratio of the electric and magnetic form factors measured by those methods significantly disagree at momentum transfer <i>Q</i>² > 1 GeV². The most likely explanation of this discrepancy is the inclusion of two-photon exchange (TPE) amplitude contributions to the elastic electron-proton cross section which significantly changes the extraction of <i>G_E</i> from the Rosenbluth separation measurement. The Jefferson Lab CLAS TPE experiment determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections. The primary electron beam was used to create an intense bremsstrahlung photon beam. Some of the photons were then converted to a mixed <i>e</i>⁺/<i> e</i>^- beam which then interacted with a liquid hydrogen target. The <i>e</i>⁺<i>p</i> and <i> e</i>^-<i>p</i> events were detected by the CLAS (CEBAF Large Acceptance Spectrometer). The elastic cross section ratios ((&sigma;(<i> e</i>⁺<i>p</i>)/(&sigma;(<i>e</i>^-<i>p</i>)) were measured over a wide range of virtual photon polarization <i>&epsiv;</i> and <i>Q</i>². The cross section ratios displayed a strong <i>&epsiv;</i> dependence at <i>Q</i>² = 1.45 GeV². There is no significant <i>Q</i>² dependence observed at <i> &epsiv;</i> = 0.45. The results are consistent with a recent measurement at the VEPP-3 lepton storage ring in Novosibirsk and with the hadronic calculation by Blunders, Melnitchouk and Tjon. The hadronic calculation resolves the disagreement between the Rosenbluth separation and polarization transfer extractions of <i> G_E/G_M</i> at <i>Q</i>² up to 2 &ndash; 3 GeV². Applying the GLAS TPE correction to the Rosenbluth cross section measurements significantly decreases the extracted value of <i>G_E</i> and brings it into good agreement with the polarization transfer measurement at <i>Q</i>²&sim;1.75 GeV². Thus, these measurements appear to resolve the proton electric form factor discrepancy for <i>Q</i>² &lt; 2 GeV².</p>

<p> Coherent electron cooling (CeC) offers a potential new method of cooling hadron beams in colliders such as the Relativistic Heavy Ion Collider (RHIC) or the future electron ion collider eRHIC. A 22 MeV linear accelerator is currently being built as part of a proof of principle experiment for CeC at Brookhaven National Laboratory (BNL). In this thesis we present a simulation of electron beam dynamics including space charge in the 22 MeV CeC proof of principle experiment using the program ASTRA (A Space charge TRacking Algorithm). </p>

Thesis (M.S.); Submitted to the Univ. of Texas A and M, College Station, TX (US); 4 Oct 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "UCRL-TH-208163" Schilling, O; Mueschke, N J. 10/04/2004. Report is also available in paper and microfiche from NTIS.