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Articles 661 - 690 of 880

Full-Text Articles in Physics

The Higgs Sector On A Two-Sheeted Space Time, Cosmin Macesanu, Kameshwar C. Wali Mar 2005

The Higgs Sector On A Two-Sheeted Space Time, Cosmin Macesanu, Kameshwar C. Wali

Physics - All Scholarship

We present a general formalism based on the framework of non-commutative geometry, suitable to the study the standard model of electroweak interactions, as well as that of more general gauge theories. Left- and right-handed chiral fields are assigned to two different sheets of space-time (a discretized version of Kaluza-Klein theory). Scalar Higgs fields find themselves treated on the same footing as the gauge fields, resulting in spontaneous symmetry breaking in a natural and predictable way. We first apply the formalism to the Standard Model, where one can predict the Higgs mass and the top Yukawa coupling. We then study the …

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Linking Physics And Algorithms In The Random-Field Ising Model, Alan Middleton, Jan H. Meinke Feb 2005

Linking Physics And Algorithms In The Random-Field Ising Model, Alan Middleton, Jan H. Meinke

Physics - All Scholarship

The energy landscape for the random-field Ising model (RFIM) is complex, yet algorithms such as the push-relabel algorithm exist for computing the exact ground state of an RFIM sample in time polynomial in the sample volume. Simulations were carried out to investigate the scaling properties of the push-relabel algorithm. The time evolution of the algorithm was studied along with the statistics of an auxiliary potential field. At very small random fields, the algorithm dynamics are closely related to the dynamics of two-species annihilation, consistent with fractal statistics for the distribution of minima in the potential (``height''). For $d=1,2$, a correlation …

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Flux Qubits And Readout Device With Two Independent Flux Lines, Britton Plourde, T. L. Robertson Jan 2005

Flux Qubits And Readout Device With Two Independent Flux Lines, Britton Plourde, T. L. Robertson

Physics - All Scholarship

We report measurements on two superconducting flux qubits coupled to a readout Superconducting QUantum Interference Device (SQUID). Two on-chip flux bias lines allow independent flux control of any two of the three elements, as illustrated by a two-dimensional qubit flux map. The application of microwaves yields a frequency-flux dispersion curve for 1- and 2-photon driving of the single-qubit excited state, and coherent manipulation of the single-qubit state results in Rabi oscillations and Ramsey fringes. This architecture should be scalable to many qubits and SQUIDs on a single chip.

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Exploring Optimization For The Random-Field Ising Model, Alan Middleton, D. Clay Hambrick, Jan H. Meinke Jan 2005

Exploring Optimization For The Random-Field Ising Model, Alan Middleton, D. Clay Hambrick, Jan H. Meinke

Physics - All Scholarship

The push-relabel algorithm can be used to calculate rapidly the exact ground states for a given sample with a random-field Ising model (RFIM) Hamiltonian. Although the algorithm is guaranteed to terminate after a time polynomial in the number of spins, implementation details are important for practical performance. Empirical results for the timing in dimensions d=1,2, and 3 are used to determine the fastest among several implementations. Direct visualization of the auxiliary fields used by the algorithm provides insight into its operation and suggests how to optimize the algorithm. Recommendations are given for further study of the RFIM.

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The Btev Rich Front End Electronics, Marina Artuso Jan 2005

The Btev Rich Front End Electronics, Marina Artuso

Physics - All Scholarship

We report on the design and testing of novel mixed analog and digital front end ASICs custom made for the single photon detectors considered for the BTeV RICH system. The key features are reviewed, as well as results achieved using electronics bench tests and beam studies.

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Conducting Polymer And Hydrogenated Amorphous Silicon Hybrid Solar Cells, Evan L. Williams, Ghassan E. Jabbour, Qi Wang, Sean E. Shaheen, Eric A. Schiff Jan 2005

Conducting Polymer And Hydrogenated Amorphous Silicon Hybrid Solar Cells, Evan L. Williams, Ghassan E. Jabbour, Qi Wang, Sean E. Shaheen, Eric A. Schiff

Physics - All Scholarship

An organic-inorganic hybrid solar cell with a p-i-n stack structure has been investigated. The p-layer was a spin coated film of PEDOT:PSS poly 3,4-ethylenedioxythiophene polystyrenesulfonate. The i-layer was hydrogenated amorphous silicon a-Si:H, and the n-layer was microcrystalline silicon c-Si. The inorganic layers were deposited on top of the organic layer by the hot-wire chemical vapor deposition technique at 200 °C. These hybrid devices exhibited open circuit voltages VOC as large as 0.88 V and solar conversion efficiencies as large as 2.1%. Comparison of these devices with those incorporating a-SiC:H:B p-layers indicates that the organic layer is acting as an electrically …

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Hole Drift-Mobility Measurements In Microcrystalline Silicon, T. Dylla, F. Finger, Eric A. Schiff Jan 2005

Hole Drift-Mobility Measurements In Microcrystalline Silicon, T. Dylla, F. Finger, Eric A. Schiff

Physics - All Scholarship

We have measured transient photocurrents on several p-i-n solar cells based on microcrystalline silicon. For two of these samples, we were able to obtain conclusive hole drift-mobility measurements. Despite the predominant crystallinity of these samples, temperature-dependent measurements were consistent with an exponential-bandtail trapping model for transport, which is usually associated with noncrystalline materials. We estimated valence bandtail widths of about 31 meV and hole band mobilities of 1–2 cm2 /V s. The measurements support mobility-edge transport for holes in these microcrystalline materials, and broaden the range of materials for which mobility-edge transport corresponds to an apparently universal band mobility of …

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Temperature-Dependent Open-Circuit Voltage Measurements And Light-Soaking In Hydrogenated Amorphous Silcon Solar Cells, Jianjun Liang, Eric A. Schiff, S. Guha, Baojie Yan, Jeff Yang Jan 2005

Temperature-Dependent Open-Circuit Voltage Measurements And Light-Soaking In Hydrogenated Amorphous Silcon Solar Cells, Jianjun Liang, Eric A. Schiff, S. Guha, Baojie Yan, Jeff Yang

Physics - All Scholarship

We present temperature-dependent measurements of the open-circuit voltage VOC(T) in hydrogenated amorphous silicon nip solar cells prepared at United Solar. At room-temperature and above, VOC measured using near-solar illumination intensity differs by as much as 0.04 V for the as-deposited and light-soaked states; the values of VOC for the two states converge below 250 K. Models for VOC based entirely on recombination through deep levels (dangling bonds) do not account for the convergence effect. The convergence is present in a model that assumes the recombination traffic in the as-deposited state involves only bandtails, but which splits the recombination traffic fairly …

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Light-Soaking Effects On The Open-Circuit Voltage Of A-Si:H Solar Cells, Jianjun Liang, Eric A. Schiff, S. Guha, Baojie Yan, J. Yang Jan 2005

Light-Soaking Effects On The Open-Circuit Voltage Of A-Si:H Solar Cells, Jianjun Liang, Eric A. Schiff, S. Guha, Baojie Yan, J. Yang

Physics - All Scholarship

We present measurements on the decline of the open-circuit voltage VOC in a-Si:H solar cells during extended illumination (light-soaking) at 295 K. We used a near-infrared laser that was nearly uniformly absorbed in the intrinsic layer of the cell. At the highest photogeneration rate (about 2x1021 cm-3), a noticeable decline (0.01 V) occurred within about 10 minutes; VOC stabilized at 0.04 V below its initial value after about 200 hours. We found that both the kinetics and the magnitudes of VOC are reasonably consistent with the predictions of a calculation combining a bandtail+defect picture for recombination and a hydrogen-collision model …

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Prototype Of Fault Adaptive Embedded Software For Large-Scale Real-Time Systems, Derek Messie, Mina Jung, Jae C. Oh, Shweta Shetty, Steven Nordstrom, Michael Haney Jan 2005

Prototype Of Fault Adaptive Embedded Software For Large-Scale Real-Time Systems, Derek Messie, Mina Jung, Jae C. Oh, Shweta Shetty, Steven Nordstrom, Michael Haney

Electrical Engineering and Computer Science - All Scholarship

This paper describes a comprehensive prototype of large-scale fault adaptive embedded software developed for the proposed Fermilab BTeV high energy physics experiment. Lightweight self-optimizing agents embedded within Level 1 of the prototype are responsible for proactive and reactive monitoring and mitigation based on specified layers of competence. The agents are self-protecting, detecting cascading failures using a distributed approach. Adaptive, reconfigurable, and mobile objects for reliability are designed to be self-configuring to adapt automatically to dynamically changing environments. These objects provide a self-healing layer with the ability to discover, diagnose, and react to discontinuities in real-time processing. A generic modeling environment …

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Leptonic Cp Violation In A Two Parameter Model, Joseph Schechter, Samina S. Masood, Salah Nasri Dec 2004

Leptonic Cp Violation In A Two Parameter Model, Joseph Schechter, Samina S. Masood, Salah Nasri

Physics - All Scholarship

We further study the "complementary" Ansatz, Tr(M_\nu)=0, for a prediagonal light Majorana type neutrino mass matrix. Previously, this was studied for the CP conserving case and the case where the two Majorana type CP violating phases were present but the Dirac type CP violating phase was neglected. Here we employ a simple geometric algorithm which enables us to "solve" the Ansatz including all three CP violating phases. Specifically, given the known neutrino oscillation data and an assumed two parameter (the third neutrino mass m_3 and the Dirac CP phase \delta) family of inputs we predict the neutrino masses and Majorana …

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Fixed-Connectivity Membranes, Mark Bowick Dec 2004

Fixed-Connectivity Membranes, Mark Bowick

Physics - All Scholarship

The statistical mechanics of flexible surfaces with internal elasticity and shape fluctuations is summarized. Phantom and self-avoiding isotropic and anisotropic membranes are discussed, with emphasis on the universal negative Poisson ratio common to the low-temperature phase of phantom membranes and all strictly self-avoiding membranes in the absence of attractive interactions. The study of crystalline order on the frozen surface of spherical membranes is also treated.

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What Are The Building Blocks Of Our Universe?, Kameshwar C. Wali Nov 2004

What Are The Building Blocks Of Our Universe?, Kameshwar C. Wali

Physics - All Scholarship

We are told that we are living in a Golden Age of Astronomy. Cosmological Parameters are found with un precedented accuracy. Yet, the known form of matter forms only a small fraction of the total energy density of the universe. Also, a mysterious dark energy dominates the universe and causes acceleration in the rate of expansion.

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A Geometrical Approach To N=2 Super Yang-Mills Theory On The Two Dimensional Lattice, Simon Catterall Nov 2004

A Geometrical Approach To N=2 Super Yang-Mills Theory On The Two Dimensional Lattice, Simon Catterall

Physics - All Scholarship

We propose a discretization of two dimensional Euclidean Yang-Mills theories with N=2 supersymmetry which preserves exactly both gauge invariance and an element of supersymmetry. The approach starts from the twisted form of the continuum super Yang Mills action which we show may be written in terms of two real Kahler-Dirac fields whose components transform into each other under the twisted supersymmetry. Once the theory is written in this geometrical language it is straightforward to discretize by mapping the component tensor fields to appropriate geometrical structures in the lattice and by replacing the continuum exterior derivative and its adjoint by appropriate …

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Stabilizing Moduli With String Cosmology, Scott Watson Oct 2004

Stabilizing Moduli With String Cosmology, Scott Watson

Physics - All Scholarship

In this talk I will discuss the role of finite temperature quantum corrections in string cosmology and show that they can lead to a stabilization mechanism for the volume moduli. I will show that from the higher dimensional perspective this results from the effect of states of enhanced symmetry on the one-loop free energy. These states lead not only to stabilization, but also suggest an alternative model for cold dark matter. At late times, when the low energy effective field theory gives the appropriate description of the dynamics, the moduli will begin to slow-roll and stabilization will generically fail. However, …

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Lattice Supersymmetry Via Twisting, Simon Catterall Sep 2004

Lattice Supersymmetry Via Twisting, Simon Catterall

Physics - All Scholarship

We describe how the usual supercharges of extended supersymmetry may be {\it twisted} to produce a BRST-like supercharge Q. The usual supersymmetry algebra is then replaced by a twisted algebra and the action of the twisted theory is shown to be generically Q-exact. In flat space the twisting procedure can be regarded as a change of variables carrying no physical significance. However, the twisted theories can often be transferred to the lattice while preserving the twisted supersymmetry. As an example we construct a lattice version of the two-dimensional supersymmetric sigma model

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Entropy And The Approach To The Thermodynamic Limit In Three-Dimensional Simplicial Gravity, Simon Catterall, John B. Kogut, R. Renken Aug 2004

Entropy And The Approach To The Thermodynamic Limit In Three-Dimensional Simplicial Gravity, Simon Catterall, John B. Kogut, R. Renken

Physics - All Scholarship

We present numerical results supporting the existence of an exponential bound in the dynamical triangulation model of three-dimensional quantum gravity.Both the critical coupling and various other quantities show a slow power law approach to the infinite volume limit.

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Study Of Leptonic Cp Violation, Joseph Schechter, Salah Nasri, Sherif Moussa Jun 2004

Study Of Leptonic Cp Violation, Joseph Schechter, Salah Nasri, Sherif Moussa

Physics - All Scholarship

The "complementary" Ansatz, Tr(M_\nu)=0, where M_\nu is the prediagonal neutrino mass matrix, seems a plausible approximation for capturing in a self-contained way some of the content of Grand Unification. We study its consequences in the form of relations between the neutrino masses and CP violation phases.

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Bridging The Microscopic And The Hydrodynamic In Active Filament Solutions, Tanniemola B. Liverpool, M. Cristina Marchetti Jun 2004

Bridging The Microscopic And The Hydrodynamic In Active Filament Solutions, Tanniemola B. Liverpool, M. Cristina Marchetti

Physics - All Scholarship

Hydrodynamic equations for an isotropic solution of active polar filaments are derived from a microscopic mean-field model of the forces exchanged between motors and filaments. We find that a spatial dependence of the motor stepping rate along the filament is essential to drive bundle formation. A number of differences arise as compared to hydrodynamics derived (earlier) from a mesoscopic model where relative filament velocities were obtained on the basis of symmetry considerations. Due to the anisotropy of filament diffusion, motors are capable of generating net filament motion relative to the solvent. The effect of this new term on the stability …

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Simulations Of Dynamically Triangulated Gravity -- An Algorithm For Arbitrary Dimension, Simon Catterall May 2004

Simulations Of Dynamically Triangulated Gravity -- An Algorithm For Arbitrary Dimension, Simon Catterall

Physics - All Scholarship

Recent models for discrete euclidean quantum gravity incorporate a sum over simplicial triangulations. We describe an algorithm for simulating such models in general dimensions. As illustration we show results from simulations in four dimensions

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Could Dark Energy Be Vector-Like?, Christian Armendariz-Picon May 2004

Could Dark Energy Be Vector-Like?, Christian Armendariz-Picon

Physics - All Scholarship

In this paper I explore whether a vector field can be the origin of the present stage of cosmic acceleration. In order to avoid violations of isotropy, the vector has be part of a ``cosmic triad'', that is, a set of three identical vectors pointing in mutually orthogonal spatial directions. A triad is indeed able to drive a stage of late accelerated expansion in the universe, and there exist tracking attractors that render cosmic evolution insensitive to initial conditions. However, as in most other models, the onset of cosmic acceleration is determined by a parameter that has to be tuned …

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Lattice Sigma Models With Exact Supersymmetry, Simon Catterall, Sofiane Ghadab May 2004

Lattice Sigma Models With Exact Supersymmetry, Simon Catterall, Sofiane Ghadab

Physics - All Scholarship

We show how to construct lattice sigma models in one, two and four dimensions which exhibit an exact fermionic symmetry. These models are discretized and {\it twisted} versions of conventional supersymmetric sigma models with N=2 supersymmetry. The fermionic symmetry corresponds to a scalar BRST charge built from the original supercharges. The lattice theories possess local actions and in many cases admit a Wilson term to suppress doubles. In the two and four dimensional theories we show that these lattice theories are invariant under additional discrete symmetries. We argue that the presence of these exact symmetries ensures that no fine tuning …

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Moduli Stabilization With The String Higgs Effect, Scott Watson May 2004

Moduli Stabilization With The String Higgs Effect, Scott Watson

Physics - All Scholarship

We review the notion of the Higgs effect in the context of string theory. We find that by including this effect in time dependent backgrounds, one is led to a natural mechanism for stabilizing moduli at points of enhanced gauge symmetry. We consider this mechanism for the case of the radion (size of the extra dimensions) and find that as decompactification of the large spatial dimensions takes place the radion will remain stabilized at the self dual radius. We discuss how this mechanism can be incorporated into models of string cosmology and brane inflation to resolve some outstanding problems. We …

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Decoherence In Josephson-Junction Qubits Due To Critical Current Fluctuations, Britton Plourde, D. J. Van Harlingen Apr 2004

Decoherence In Josephson-Junction Qubits Due To Critical Current Fluctuations, Britton Plourde, D. J. Van Harlingen

Physics - All Scholarship

We compute the decoherence caused by 1/f fluctuations at low frequency f in the critical current I_0 of Josephson junctions incorporated into flux, phase, charge and hybrid flux-charge superconducting quantum bits (qubits). The dephasing time \tau_{\phi} scales as I_0/ \Omega \Lambda S_{I_0}^{1/2}(1 Hz),where \Omega / 2\pi is the energy level splitting frequency, S_{I_0}(1 Hz) is the spectral density of the critical current noise at 1 Hz, and \Lambda \equiv |I_0 d \Omega / \Omega d I_0| is a parameter computed for given parameters for each type of qubit that specifies the sensitivity of the level splitting to critical current fluctuations. …

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Improved Extremal Optimization For The Ising Spin Glass, Alan Middleton Mar 2004

Improved Extremal Optimization For The Ising Spin Glass, Alan Middleton

Physics - All Scholarship

A version of the extremal optimization (EO) algorithm introduced by Boettcher and Percus is tested on 2D and 3D spin glasses with Gaussian disorder. EO preferentially flips spins that are locally ``unfit''; the variant introduced here reduces the probability to flip previously selected spins. Relative to EO, this adaptive algorithm finds exact ground states with a speed-up of order $10^{4}$ ($10^{2}$) for $16^{2}$- ($8^{3}$-) spin samples. This speed-up increases rapidly with system size, making this heuristic a useful tool in the study of materials with quenched disorder.

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Effective Field Theory Approach To String Gas Cosmology, Scott Watson, Thorsten Battefeld Mar 2004

Effective Field Theory Approach To String Gas Cosmology, Scott Watson, Thorsten Battefeld

Physics - All Scholarship

We derive the 4D low energy effective field theory for a closed string gas on a time dependent FRW background. We examine the solutions and find that although the Brandenberger-Vafa mechanism at late times no longer leads to radion stabilization, the radion rolls slowly enough that the scenario is still of interest. In particular, we find a simple example of the string inspired dark matter recently proposed by Gubser and Peebles.

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Leptonic Cp Violation Phases Using An Ansatz For The Neutrino Mass Matrix And Application To Leptogenesis, Joseph Schechter, Salah Nasri, Sherif Moussa Feb 2004

Leptonic Cp Violation Phases Using An Ansatz For The Neutrino Mass Matrix And Application To Leptogenesis, Joseph Schechter, Salah Nasri, Sherif Moussa

Physics - All Scholarship

We further study the previously proposed Ansatz, Tr(M)=0, for a prediagonal light Majorana type neutrino mass matrix. If CP violation is neglected this enables one to use the existing data on squared mass differences to estimate (up to a discrete ambiguity)the neutrino masses themselves. If it is assumed that only the conventional CP phase is present, the ansatz enables us to estimate this phase in addition to all three masses. If it is assumed that only the two Majorana CP phases are present, the Ansatz enables us to obtain a one parameter family of solutions for the masses and phases. …

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Non-Adiabatic Perturbations From Single-Field Inflation, Christian Armendariz-Picon Feb 2004

Non-Adiabatic Perturbations From Single-Field Inflation, Christian Armendariz-Picon

Physics - All Scholarship

If the inflaton decays into several components during reheating, and if the corresponding decay rates are functions of spacetime-dependent quantities, it is possible to generate entropy perturbations after a stage of single-field inflation. In this paper, I present a simple toy example that illustrates this possibility. In the example, the decay rates of the inflaton into ``matter'' and ``radiation'' are different functions of the total energy density. In particular cases, one can exactly solve the equations of motion both for background and perturbations in the long-wavelength limit, and show that entropy perturbations do indeed arise. Beyond these specific examples, I …

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Uv Perturbations In Brane Gas Cosmology, Scott Watson Feb 2004

Uv Perturbations In Brane Gas Cosmology, Scott Watson

Physics - All Scholarship

We consider the effect of the ultraviolet (UV) or short wavelength modes on the background of Brane Gas Cosmology. We find that the string matter sources are negligible in the UV and that the evolution is given primarily by the dilaton perturbation. We also find that the linear perturbations are well behaved and the predictions of Brane Gas Cosmology are robust against the introduction of linear perturbations. In particular, we find that the stabilization of the extra dimensions (moduli) remains valid in the presence of dilaton and string perturbations.

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Hole Drift-Mobility Measurements And Multiple-Trapping In Microcrystalline Silicon, T. Dylla, F. Finger, Eric A. Schiff Jan 2004

Hole Drift-Mobility Measurements And Multiple-Trapping In Microcrystalline Silicon, T. Dylla, F. Finger, Eric A. Schiff

Physics - All Scholarship

We present photocarrier time-of-flight measurements of the hole drift-mobility in microcrystalline silicon samples with a high crystalline volume fraction; typical roomtemperature values are about 1 cm2/Vs. Temperature-dependent measurements are consistent with the model of multiple-trapping in an exponential bandtail. While this model has often been applied to amorphous silicon, its success for predominantly crystalline samples is unexpected. The valence bandtail width is 31 meV, which is about 10-20 meV smaller than values reported for a-Si:H, and presumably reflects the greater order in the microcrystalline material. The hole band-mobility is about 1 cm2/Vs – essentially the same magnitude as has been …

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