ABSTRACTS and PRESENTATIONS: 2007 APS-CA section meeting

Abstracts and Presentations
for the 2007 Annual Meeting of the California Section
October 26-27, 2007

Lawrence Berkeley National Laboratory

Baxter Armstrong

On the Physical Interpretation of the Fine Structure Constant

A characteristic longitudinal Coulomb energy of a charged particle is defined that is equal to its relativistic mass energy. By quantization of this characteristic Coulomb energy, it is shown that the product of its quantum frequency and the classical period characteristic of the particle is equal to the (low energy) fine structure constant divided by 2π. The same procedure carried out for the photon yields the same result. This derivation also obtains the invariant constant of e²/(h/2π) (independently of the H atom).

Vladislav Bevc

Need for an Energy Primer for the General Public

In the current global warming hysteria it is necessary for the general public to have a readily accessible manual with which almost any lay person can assess the various claims advanced by special interest groups. The Primer will show where to find the necessary data (Physics Handbook, Statistical Abstracts of the USA, etc) and how to make overall systems oriented calculations. For example: how much additional electric power plant would be necessary to produce enough hydrogen for powering all US automobiles, or how much energy is required to produce biofuels, etc, as well as where could the energy for such projects come from. Possibilities of involving the physics community in the project are discussed.

Rajan Bista

Near-Infrared (NIR) Spectroscopic Analysis of Newly Developed Self-Forming Synthetic Lipid Vesicles

In this work, we present the first spectroscopic data in the near infrared (NIR) for synthetic lipid vesicles in the spectral range of 11000-4700 cm^-1. The NIR spectra are recorded by a novel dual-detector micro spectrometer which is based on micro-opto-electromechanical systems (MOEMS) technology. We have examined the NIR spectra of lipid in pure form as well as in aqueous environment with different solvent. In addition, we have established specific band structures as molecular fingerprints corresponding to overtones and combinations vibrational modes involving C-H, O-H and N-H functional groups for sample analysis of self-forming nano vesicles trademarked as QuSomes developed by BioZone Laboratories Inc. In particular, we have provided evidence that these types of nano particles are stable in aqueous environment and obey Beer's law at low concentration. This information may be of great importance for the development of new substance delivery system.

Noah Bray-Ali

Scaling Analysis of Magnetic Nanoparticles

The magnetic properties of single-domain nanoparticles depend on geometric shape, crystalline anisotropies and lattice structures. We use a recently proposed scaling approach to obtain their phase diagrams, which are found to feature universal characteristics, such as in-plane and out-of-plane ferromagnetism competing with vortex formation. In particular, magnetic nanorings are found to stabilize the vortex configuration. Three-dimensional phase diagrams are obtained for cylindrical nanorings, depending on their height, outer and inner radius. The triple point in these phase diagrams is shown to be in linear relationship with the inner radius of the ring, which offers a new way to estimate the critical inner radius above which there exists no in-plane ferromagnetism.

Presentation

Korana Burke

Homoclinic Tangle Approach to Kicked Hydrogen

Kicked hydrogen atom consists of an electron moving in a Coulomb potential and subjected to periodic forcing by an external electric field. This system as such exhibits chaotic behavior. We study the geometry of homoclinic tangles that arise in phase space and use the knowledge we gain from the transport of the electron through the "turnstile" to draw conclusions about the ionization rate. We apply square-shaped kicks to the electron which mimic the laboratory setup. Since all the calculations are classical in nature this approach can be applied to the study of ionization rates of highly excited Rydberg atoms.

John Joseph Carrasco

Cancellations in Gravity Theories

I will present recent results through three loops demonstrating that maximally supersymmetric N=8 supergravity is surprisingly well behaved in the ultraviolet as a result of unexpected cancellations between contributing terms. These cancellations first manifest at one loop in the form of the "no-triangle hypothesis," with all-loop order implications through unitarity. I will conclude by mentioning similar novel cancelations recently identified in pure Einstein gravity, at one loop, which suggest a possible explanation for the unexpectedly tame high energy behavior of N=8 beyond the limited UV protection of SUSY.

Elizabeth Carroll

Ultrafast Charge Carrier Dynamics in Nanostructured Calcium Niobate Photocatalysts in Water and Methanol

We have investigated photoexcited carrier trapping, recombination, and interfacial charge transfer dynamics in calcium niobate nanosheets using transient absorption spectroscopy. Exfoliated Ca₂Nb₃O₁₀ photocatalytically generated H₂ from water with 0.22% quantum yield. In methanol, hole scavenging was observed within 100 ps in direct competition with electron-hole recombination, and resulted in 15% quantum yield of H₂. The nanosheets are easily functionalized with metal and semiconductor nanoparticles to form novel multi-component photocatalysts. Ca₂Nb₃O₁₀ nanostructures generated H₂ from water with 7.5% quantum yield as a result of sub-ps interfacial electron transfer.

Presentation

Jeng-Da Chai

Systematic Optimization of Long-Range Corrected Hybrid Density Functionals

A general scheme for systematically modeling long-range corrected (LC) hybrid density functionals is proposed. Our resulting functionals are shown to be quite accurate in thermochemistry, kinetics, and non-covalent systems, when compared with other hybrid density functionals. The qualitative failures of the commonly used hybrid density functionals in some "difficult problems", such as dissociation of symmetric radical cations and long-range charge-transfer excitations, are significantly reduced by the present LC hybrid density functionals.

Chuan Chen

A Survey of EGRET Sources by Milagro Observatory

The Milagro gamma-ray observatory employs a water-Cherenkov technique to continuously monitor the northern sky for TeV gamma-ray emission from astrophysical sources. Milagro has a high duty-cycle (~90%) and wide aperture (~2 sr). About seven years of Milagro data are used to search for gamma-ray emission from the EGRET sources in the northern sky. Constraints on the fluxes at 5 TeV and 20 TeV, assuming various power law spectra, will be presented. Different background rejection variables are used for different energy ranges. We compare Milagro fluxes with the fluxes measured by EGRET and Whipple and their extrapolation to Milagro energies.

Yong Choi

The Actual Crack Size and Shape of Electro-deposited Chromium Layers

Small angle neutron scattering (SANS) was applied to non-destructively evaluate the crack size and distribution of eco-friendly thin trivalent chromium layers. The deposits were prepared in a trivalent chromium sulfate bath by various pulsing conditions. Nano-size cracks less than 40 nm in size increase with plating voltage at constant current density. Such small round nano-size cracks interconnect to form calabash-shaped micro-size cracks, both of which are the result of hydrogen gas evolution. From this study, small angle neutron scattering is a useful technique to evaluate defect size and shape of thin deposit.

Slim Chourou

Dissociative Electron Attachment to HCN and HNC

HCN and its isomer HNC are known to be among the initial species that drive synthesis of amino acid and protein in interstellar media. Dissociative electron attachment (DEA) to those molecules may thus have an impact on these chemical processes of relevance in astrophysics. Previous experimental and theoretical studies have indicated both σ and π low-lying resonances. These resonant states are expected to depend on stretching and bending of the molecule and to lead to competing (CN^- + H) and (CN + H^-) products. In this work, we present a comparative study of the dissociation mechanism. We carried out electron scattering calculations using the Complex Kohn Variational Method as a function of the three internal degrees of freedom to obtain the resonance energy surface and autoionization widths. We use this as input to a dynamics calculation using the multiconfiguration time-dependant Hartree (MCTDH) approach. We finally compare our DEA cross sections and branching ratios to available findings.

Robert Close

Canonical Variables, Metrics, and Parity

Two theoretical problems with Dirac theory are resolved. First, the Dirac Hamiltonian is not in proper canonical form because the spin and velocity matrices are not independent. Second, the assumption that the product of Dirac gamma matrices should yield the Minkowski metric tensor misinterprets relativity. The resultant parity operator is incorrect because it does not invert axes associated with velocity rotations. Correcting these problems yields a theory of matter consistent with parity conservation and a soliton wave interpretation of particles. The canonical form of the Dirac equation is derived by extending the one-dimensional wave equation to three dimensions. The co-existence of forward- and backward-propagating waves along a single axis is the basis of half-integer spin. Electromagnetic potentials represent spatial and temporal variations of wave velocity rotation. Wave interference produces both the Lorenz force and the Pauli exclusion principle. The coupled radial equations describing a Dirac electron are obtained by choosing an appropriate mass term. This mass is associated with rotation of wave velocity such as occurs in a soliton. Matter and anti-matter are related by spatial inversion, consistent with parity conservation.

Joshua Coleman

Neutralized Compression and Focusing of an Intense Ion Beam for Target Heating Experiments

Future target heating experiments with space-charge dominated ion beams require simultaneous longitudinal bunching and transverse focusing. An experiment to focus transversely and simultaneously axially bunch a space charge neutralized K+ ion beam has been carried out at LBNL. The principal objectives of the simultaneous bunching and focusing experiments are to control the beam envelope, demonstrate effective neutralization of the beam space-charge, control the velocity tilt on beam, understand effects of net defocusing, field imperfections, and limitations on minimal spot size such as emittance and aberrations. A demonstration of increased axial compression (> 100x axial compression, < 2 ns pulses) and a reduction in spot size compared to earlier measurements is presented.

Sudarshan Dhungana

Atomic Systems in Screening Environments

In calculations of atomic systems in screening environments both the electron nucleus attraction and the electron-electron repulsion has to be replaced by screened potentials. Hence, the well -known Legendre expansion of the Coulomb interaction operator 1/r₁₂ needs to be generalized to encompass screened interactions. This has been done in nuclear physics for harmonic Oscillator wave functions. Here it is being done for the use with Slater type orbitals in order to facilitate the analytic evaluation of two- electron matrix elements.

Aleksandar Donev

An Event-Driven Hybrid Molecular Dynamics and Direct Simulation Monte Carlo Algorithm

Authors: Aleksandar Donev, Alejandro L. Garcia and Berni J. Alder
A novel Stochastic Event-Driven Molecular Dynamics (SEDMD) algorithm is developed for the simulation of polymer chains suspended in a solvent. The polymers are represented as chains of hard spheres tethered by square wells and interact with the solvent particles with hard core potentials. The algorithm uses Event-Driven Molecular Dynamics (EDMD) for the simulation of the polymer chain and the interactions between the chain beads and the surrounding solvent particles. The interactions between the solvent particles themselves are not treated deterministically as in event-driven algorithms, rather, the momentum and energy exchange in the solvent is determined stochastically using the Direct Simulation Monte Carlo (DSMC) method. The coupling between the solvent and the solute is consistently represented at the particle level, however, unlike full MD simulations of both the solvent and the solute, the spatial structure of the solvent is ignored. The algorithm is described in detail and applied to the study of the dynamics of a polymer chain tethered to a hard wall subjected to uniform shear. The algorithm closely reproduces full MD simulations with two orders of magnitude greater efficiency. Results do not confirm the existence of periodic (cycling) motion of the polymer chain.
Note: This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. (UCRL-ABS-233205).

Presentation

Yonatan Dubi - Charles Kittel Award for Best Theoretical Research - First Place

Superconducting Islands, Phase Fluctuations and the Superconductor-Insulator Transition

Highly disordered superconducting thin films undergo a magnetic-field (B)tuned superconductor-insulator transition (SIT), which is believed to be due to the interplay of disorder and superconductivity. The nature of the SIT is still under debate, and the subject has become even more relevant with the realization that high-Tc superconductors are intrinsically disordered, as observed in local tunneling experiments. We present the first numerical simulations of the superconductor SIT in two-dimensional disordered superconductors, starting from a microscopic description. We demonstrate explicitly that disorder leads to formation of islands where the SC order is high. For weak disorder, or high density, increasing B results in the eventual vanishing of the amplitude of the SC order parameter, thereby forming an insulating state. On the other hand, at lower densities or higher disorder, increasing B suppresses the correlations between the phases of the SC order parameter in different islands, giving rise to a different type of a SIT. One of the important predictions of this work is that in the latter regime there are still SC islands in the sample even on the insulating side. This result, which is consistent with experiments, explains the recently observed huge magneto-resistance peak in disordered thin films and may be relevant to the observation of pseudo-gap in underdoped high-Tc superconductors.

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Alice Durand

Low Temperature Heat Capacity of the Frustrated Pyrochlore Magnets Gd₂Hf₂O₇ and Gd₂Zr₂O₇

We present heat capacity and susceptibility data for the frustrated pyrochlore magnets Gd₂Hf₂O₇ and Gd₂Zr₂O₇. Both compounds show antiferromagnetic interactions with a Weiss constant of approximately -7K. Gd₂Hf₂O₇ exhibits a sharp peak in the heat capacity at roughly 0.75K, consistent with long range magnetic order. The peak is superimposed on a broad maximum due to short range order. Contrasting results for Gd₂Zr₂O₇ will also be presented, together with estimates for the amount of structural disorder present in the two compounds. Comparison will be made, where possible, with earlier published results in the isomorphic compounds Gd₂Ti₂O₇ and Gd₂Sn₂O₇.

Matilda Fernandez

Theoretical Structure and Spectra of Gallium Arsenide Clusters

In an extension of our work on nanostructures, [1-2] we will now create models to study the optical properties GanAsn clusters (n = 1 thru 15) and of gallium arsenide nanostructures. In the first phase of the calculations, we derive the optimal geometries of the larger of these Ga_nAs_n clusters for n going from 10 to 15, just as we did for n going from 1 to 9 in our previous work. So far the largest value of binding energy calculated is 2.0908 eV, for the Ga₈As₈ cluster. We will also calculate binding energies, bondlengths, ionization potentials and charge distributions for these geometries. The hybrid ab initio methods of quantum chemistry will be used to incorporate electron correlation effects in the computations for binding energies and optimal intermolecular bondlengths. The second phase of the investigation will focus on the derivation of the optical properties of all the GaAs clusters up to Ga₁₅As₁₅. Advances in techniques for the synthesis of cluster-engineered materials containing controlled nanostructures provide the capability of preparing new classes of materials with enhanced optical, magnetic, chemical sensor and photo-catalytic properties.[3] The third phase of the investigation will examine the effects of confinement on Ga_nAs_n clusters.
References: [1] A. S. Hira and A. K. Ray, Phys. Rev. A 52, 141(1995); A 54, 2205 (1996). [2] Ajit Hira and Matilda Fernandez, Interactions of GanAsn Clusters with CGaAs Cages: Possible Nanostructures, Bull. Am. Phys. Soc. 52, 1006 (March 2007). [3] Xianmao Lu and Younan Xia, Buckling down for flexible electronics, Nature Nanotechnology vol. 1, 163 (December 2006).

Jacob George

Mitigation of Aero-Optical Effects and Subsonic Cavity Noise Using Fluid Actuators

This state of the art experiment demonstrates the ability to correlate the flow field effects with the resultant wavefront distortions. The distortions are due to wavefront aberrations that occur during the passage of an optical wavefront through a turbulent and varying index of refraction flow field. The diagnostic flow field was a subsonic cavity flow field and the results of particle image velocimetry diagnostics synchronous with those from wavefront measurements showed a distinct correlation between large scale structures and wavefront distortions. A flow control strategy using fluidic actuators was implemented to mitigate the aero-optic distortions which also resulted in significant noise suppression of cavity tones.

Maria Gonzalez

Impedance Matching Networks for UWB Systems

With the FCC's allocation of spectrum bandwidth for ultra-wideband (UWB) applications in 2002, new markets are being developed for devices and instruments using signals with fractional bandwidth greater than 20 percent. With this type of signal, power transfer from the signal generator to other components is greatly affected by the load characteristics and intermediate connections. For instance, impedance measurement for broadband antennas and ultra-wideband applications has resistance and reactance values that vary in the frequency band. Therefore the design of feeding techniques or matching networks is important for reliable and efficient communications systems. Although impedance matching techniques for resistive loads have been developed for use with both a single frequency and a small band, the optimal design of a matching network for complex loads over a large bandwidth is a mathematical challenge. In this paper, we develop matching networks for UWB antennas whose impedance is modeled as frequency dependent. In addition, we assess the impact of different matching circuit results on the antenna efficiency.

Peter Greene - Steven Chu Award for Best Research - Second Place

Synthesis and Spin-Transport Properties of Co/Cu Multilayered Nanowires

Using pulsed electrodeposition, we have fabricated high density arrays of Co/Cu multilayered nanowires in nuclear-track etched, polycarbonate membranes. The dimensions of the magnetic components can be tuned to achieve Co discs or pillars. The Cu spacer layer thicknesses can also be adjusted independently. The confined geometry of the nanowires enables study of both nanomagnetism and current perpendicular-to-plane giant magnetoresistance (CPP-GMR). In 50 nm diameter nanowires of [Co(5nm)/Cu(8nm)]400, a 7% GMR effect is observed at 300 K. In 200 nm diameter nanowires, by tuning the Co disc aspect ratio, we have realized single domain and vortex states which have distinct magnetic "fingerprints". Correspondingly the magnetoresistance effect shows different characteristics. This work has been supported in part by NSF-REU (PHY-0649297) and the Alfred P. Sloan Foundation.

Howard D. Greyber

Origin of Large-scale Primordial Magnetic Fields in the Big Bang Universe Model

Applying the known physics of plasma and the known physics of the instability called Spinodal Decomposition, a unique "Strong" Magnetic Field model (SMF) explains the origin of a large-scale primordial magnetic field in each DeVaucoleurs Supercluster, and thus the existence of significant magnetic fields that are observed in stars, galaxies, quasars and clusters of galaxies. The SMF processes, occur at and soon after Combination Time (when the famous CMB is released) in the Big Bang Model. SMF leads to a physical model where Gravity continues to attract matter into relatively thin sheets of matter around huge voids, which matches the observations of John Huchra and Margaret Geller (Harvard/SAO). Eventually, critical density for gravitational collapse of a cloud is reached at some local volumes, and stars, galaxies, and quasars begin forming. SMF suggests that galaxies/quasars can be classified by the Ratio of magnetic energy to rotational energy in that object. (However the activity observed is also a function of the matter accretion rate at the time observed) Also, applying SMF, a specific physical model for the "central engine" (AGN) of galaxies is created.

Madhu Gyawali

Atmospheric Radiation Transfer: Sun Photometer

Radiation transfer in the atmosphere is the phenomenon of energy transfer in the form of electromagnetic radiation .This phenomenon is affected by absorption, scattering as well as the emission by various atmospheric constituents .This talk considers the nature of solar radiation and its interaction with gases and particles in the Earth's atmosphere. A discussion will be given for the construction and use of sun photometers to measure important radiation transfer parameters like optical depth, and the Angstrom coefficient, as well as for instrument calibration by the Langley plot method. Experimental observations from our simple 6-channel sun photometer and the spectrometer will be presented.

Alison Hatt - Kennedy Reed Award for Best Theoretical Research - Second Place

First-Principles Investigation of Trilayer Superlattices as a Route to Magnetoelectric Multiferroics

We present results from a first-principles study of trilayer superlattices as a route to creating mangnetoelectric multiferrroics. In our model system of La(Al,Fe,Cr)O₃ we find that the effect of trilayering on the superlattice polarization is small and contributes a negligible switchable component. However, epitaxial strain in two and three dimensions induces a large polarization which is modified by the trilayering geometry, and which is compatible with the coexistence of magnetism.

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Roland Henry

Mapping Brain Connections with Magnetic Resonance Imaging

The self-diffusion of water in the brain can be detected using Magnetic Resonance Imaging. However, tissue barriers to diffusive motion results in detection of an apparent diffusivity that is dependent on the tissue microstructure. This technique therefore yields novel information about the brain microstructure and estimation of the direction of white matter axonal bundles that connect the different parts of the brain. Diffusion MRI has thus revolutionized our ability to visualize brain structure and is being used to infer the presence and nature of axonal connections supporting functional networks.

Presentation

Mikel Holcomb - Margaret Burbidge Award for Best Experimental Research - Second Place

Determination of Magnetic Directions in Multiferroic BiFeO₃/FM Thin Films

BiFeO₃, a ferroelectric and an antiferromagnet, is the only single phase room temperature multiferroic that is currently known. Multiferroics are interesting materials not only because of their exciting order parameters, but for the potential for parameter coupling. In order to understand the magnetoelectric coupling, the individual order parameters must first be understood. A combination of in-plane and out-of-plane piezoresponse force microscopy (PFM) allows 3D mapping of the ferroelectric polarization directions in micron-sized regions of the films. The magnetic order was obtained by using x-ray linear and circular dichroism images using a high spatial resolution photoelectron emission microscope (PEEM). Temperature and angle dependent structural measurements allow decoupling of the two order parameters, ferroelectric and magnetic, contributing to the photoemission signal. Careful analysis of linear and circular dichroism images at critical angles allows determination of magnetic directions in BiFeO₃. These studies reveal a strain-driven reduction in magnetic symmetry in thin films, leading to the formation of an easy magnetic axis along the [110] as opposed to the observed easy plane for bulk films. This simplification is promising for the exciting field of electric control of magnetism. The effect of an applied electric field on the magnetic domains of a ferromagnet grown on BFO was examined and the observed effects show the potential for this control.

Hope Ishii

Comet Dust

Nearly 9 years ago, a solar-powered NASA spacecraft was launched into space with an ambitious mission. Its goals were to travel to a comet, a small body that has remained cryogenically frozen since the beginnings of our solar system, to fly through its tail capturing dusty debris, and to return to Earth with the first-ever sample collected from a specific comet. Today, scientists the world over are analyzing the micro- and nano-"rocks" making up this comet dust with a variety of powerful micro- and nano-beam techniques. I will discuss our interest in comets, the NASA Stardust mission, comet dust collection, some surprising results from the analysis to date and their impact on our understanding of our solar system's infancy.

Presentation

Louis Jacome

Shot-Noise-Limited Polarimetry for Precision Measurement of Optical Rotation due To Zeeman Sublevel Shifts

Measurement of the polarization of light propagating through a vapor of atoms contained in an antirelaxation-coated cell can be used to detect very small shifts of the Zeeman sublevels (sensitivity = 1 microHertz in 1 second of measurement time). We are using this technique to search for an anomalous coupling of rubidium nuclear spins to the earth's gravitational field. An important first step in the experiment is to be able to perform shot-noise-limited measurement of the light polarization. We will discuss our progress toward this goal and future directions of our research.

Yong Jiang

A Theoretical Study on Interface Adhesion of gamma-Ni(Al)/Al₂O₃: Al Activity, Dopant, and Impurity Effects

We report a systematic first-principles investigation of the interrelationships between structure, composition and adhesion for a technologically important metal/oxide interface. The chosen interface is that between gamma-Ni(Al) and alpha-Al₂O₃, which governs the thermo-structural characteristics of hot section components used in such applications as aero-turbines. Calculations are used to examine the effects on the interfacial adhesion of stoichiometry, Al activity, impurities, and dopants. By performing computations at realistic temperatures, the equilibrium for this interface is revealed to be at the transition between the Al-rich and stoichiometric phases. We demonstrate that the Al-rich interface has significantly stronger adhesion. Moreover, S impurities can segregate and substantially decrease the adhesion of both. Further calculations reveal that doping with (the reactive-element) Hf substantially improves adhesion, even in the presence of S, by simultaneously enabling three mechanisms.

Hirofumi Kakemoto

Figure of Merit for Thermoelectric Power Generation Estimated from Beta-FeSi₂ Film

beta-FeSi₂ has been attracted to be applied to the thermoelectric device, for instance, the Seebeck coefficient shows the maximum value about 500 ºC that it is good for thermoelectric power generation. However low figure of merit (Z) has been reported about 5x10^−4 K^−1. The Z is represented as m*^2/3(μ/kph), where m*, μ and kph are effective mass, mobility and thermal conductivity, respectively. Although kph is good for thermoelectric power, low μ has been reported as polaronic-conduction in beta-FeSi₂ crystal. In 3D electron density distribution of beta-FeSi₂ crystal, Si layer in the crystal shows covalent bonding network with Si atoms, and it suggests the new possibility for enhancement of μ. In this report, the objective is to exhibit the possibility for enhancement of Z in order to control the crystallographic orientation of beta-FeSi₂crystal by means of film formation. beta-FeSi₂ thin film was prepared on Si(100) substrate using molecular beam epitaxy method. The crystallographic orientation of sample showed about 80% of [100] direction from x-ray diffraction pattern. The transport properties were investigated using Hall measurement with van der Pauw electrode configuration. The resistivity and μ were also measured, and they were compared with beta-FeSi₂ polycrystal. In addition, enhancement of Z was estimated using above formula.

Piyush Kar

Phase Transformation Studies of Doped Titania Nanotubes

Phase transformation analysis of phosphate containing and carbon doped titania nanotubes reveal complete transformation from amorphous to anatase phase in air between 360 and 400 ºC. Activation energy, enthalpy and rate constant for formation of anatase phase was evaluated and compared for the two titania nanotube types. X-ray diffraction data indicated onset of the rutile phase at 400 ºC in the carbon doped nanotubes but no rutile phase was noticed in the phosphate containing nanotubes. This is a first time study that will lead to deeper understanding of the transformation and stability of the anatase phase in the titania nanotubes conditioned with carbon and phosphate ion.

Peter Kimani

Electron Correlation in Weakly Confining Quantum Dot Potentials

We present results obtained from Hartree-Fock (HF), second-order Green's function (GF), and Tamm-Dancoff approximation (TDA) calculations of the electronic structure of spherical quantum dots with 2, 4, 8, 10, 18, and 20 active electrons in weakly confining potentials. The ground state energies for these quantum dots compare well with energies reported by others for similar nanoscale systems. Beyond such ground state data, the GF and TDA calculations provide correlated energies of N 1 particle systems both for ground states and low-lying excited states associated with addition and removal spectra. The investigated systems exhibit shell filling sequences which resemble the ones known for atomic systems but are different in binding higher angular momentum electrons more tightly. In the extended version of the TDA used here, the self-energy operator is complete through third order of many-body perturbation theory.

Derek Kimball

New Search for a Spin-Gravity Interaction

We are beginning an experiment to search for a new long-range coupling between nuclear spins and the mass of the Earth. If interpreted as a limit on a spin-gravity interaction of the form S.g between nuclear spins S and the gravitational field of the Earth g, the experiment would improve present experimental limits by over two orders of magnitude. The presence of such an interaction would be evidence that gravity violated parity and time-reversal symmetries to a small degree, as well as being a breakdown of the equivalence principle which underlies the theory of general relativity. The experiment would set new experimental limits on hypothetical scalar and vector components of gravitational fields. This new experimental search is motivated by recently developed techniques in the field of atomic magnetometry enabling significant improvement in sensitivity to atomic spin precession. The experiment will use nonlinear optical rotation of near-resonant laser light to measure the spin-precession frequency of alkali atoms in the presence of a magnetic field B. The difference between the precession frequencies for the two different ground state hyperfine levels yields a signal proportional only to anomalous interactions that do not scale with the magnetic moments of the atoms. The sum of the precession frequencies enables ultra-precise determination of B to correct for associated systematic errors.

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Yerzhanov Koblandy

3- Brane in 5-Dimension Universe

Current higher dimensional cosmological "brane" world models are receiving much attention. The simpliest such model involves matter, trapped to a 3D spatial brane embedded in a 5D bulk. This 5D model may explain the cosmological expansion dark matter, dark energy and other open questions in cosmology. A key feature of such higher dimension cosmological models is a time dependent fifth component of the metric and a time dependent scale factor.

Inga Kuznetsova

Non-Equilibrium Heavy Flavored Hadrons from Strangeness-Rich QGP

We study b, c quarks hadronization from QGP. We obtain the yields of charm and bottom flavored hadrons within the statistical hadronization model. The important novel feature of this study is that we take into account the high strangeness and entropy content of QGP, conserving strangeness and entropy yields at hadronization.

Krishna Lamichhane

The use of the virial theorem and sum-rules in atomic structure calculations

Dilatation transformations have been used successfully to obtain eigenvalues and eigenfunctions of metastable states such as resonances in electron atom scattering. The application of dilatation theory requires in general the expansion of the eigenfunctions and energies of dilated Hamiltonians in powers of the dilatation parameter. These expansions yield a set of sum rules which supply stationary and stability conditions which exact solutions will satisfy automatically. For approximate wave functions these expansions provide tools for optimization in a given parameter space. The first member in the set of sum rules is the quantum virial theorem which is particularly valuable to obtain the correct balance of potential and kinetic energy. Applied to resonances these expectation values are both complex numbers. Accurate calculations of properties of few electron systems are of interest for astrophysical plasma diagnostics. So far, most calculations have been performed on isolated atoms and molecules.

Florence J. Lin

Overall rotation due to internal motion in the three-body problem: Applications in molecular dissociation and collisions

As a result of the conservation of total rotational angular momentum in an N-body system, an internal motion with nonzero orbital angular momentum produces a net overall rotation of a generalized Eckart frame of a polyatomic molecular system in the center-of-mass frame, regardless of whether or not the total rotational angular momentum vanishes. Examples appear in a net rotation of 20 degrees in the recoil angle of an atom departing from a dissociating triatomic molecule and in a net overall rotation of 42 degrees in 100,000 reduced time units in the computational dynamics of a protein. While the diatomic molecule in atom-diatomic molecule scattering has previously been treated as a point mass, this approach describes the contribution to the deflection angle of the atom due to rotation of the diatomic target molecule. When an N-body system returns to its original shape over the time interval, the net rotation due to internal motion is an example of a classical geometric phase. Other applications appear in the dissociation of polyatomic molecules, in the separation of overall rotation and internal motions of N-body systems, and in the dynamics of molecular rotors and machines.
References: F. J. Lin, Hamiltonian dynamics of atom-diatomic molecule complexes and collisions, Discrete and Continuous Dynamical Systems, Supplement, vol. 2007, 655 - 666 (2007). J. E. Marsden, R. Montgomery, and T. Ratiu, Reduction, symmetry, and phases in mechanics, Memoirs of the American Mathematical Society, vol. 88, no. 436, (American Mathematical Society, Providence, RI, 1990).

Dan Liu

Frontier Orbital Band Study on Superconducting Mechanism of K₃C₆₀

Department of Physics, Atmospheric Sciences and General Science, Jackson State University, Jackson, MS, 39217

Numerous theoretical and experimental studies have been conducted on the superconducting mechanism of the fcc K₃C₆₀ . It has reviewed that although K₃C₆₀ is a s-wave BCS-like superconductor, driven by the coupling to the Hg phonon and probably with some strong-coupling effects, there is no conclusive evidence that this picture is correct or that an electronic mechanism is excluded [1]. Moreover, even though the good agreement between theoretical models and experimental observations has been achieved, an essential question is still opening to be answered that how the three-dimensional fcc K₃C₆₀ appears as a superconductor, that is to say, whether or not the fcc K₃C₆₀ structure is against the Heisenberg's model that superconduction exclusively appears in the low-dimension systems. Our frontier orbital band study will provide a satisfactory answer to the opening questions. Moreover, in application of the frontier orbital band approach, we discover, at the orbital and spin levels, the superconducting mechanism of K₃C₆₀. The mechanism is combination of the intramolecular resonance-structure and the intermolecular noninteracting-electron tunneling mechanisms. Our picture provides explanations to the experimental observations and theoretical models.
Reference [1] O. Gunnarsson, Mod. Phys. Rev. 69, 575 (1997).

Ernesto Marinero

Magnetotransport Studies of III-V Mesoscopic Heterostructures

We report on magneto-transport studies in lithographically patterned InAs 2DEG mesoscopic devices. Electron transport studies in a variety of sample geometries were studied as a function of temperature, applied magnetic field and device size dimensions. Negative bend resistance and quantum Hall effect measurements were conducted to investigate the influence of sample material properties and scattering effects on magnetotransport.

Presentation

Alexander Mayer

Wave Energy in Quantum Mechanics

The definition of energy established by Einstein in 1905 was enhanced by Hermann Minkowski's 1908 mathematical formalization of the special theory of relativity in terms of the complex numbers, yet this has not been previously appreciated. There are profound consequences for quantum mechanics and other areas of fundamental physics.

Alan McCone, Jr.

Brownian Motion, The Oscillator, Kinetic Gas Space Medium, And The Photon

The bell-shaped quantum oscillator is fit as an exact sum of dwell times in an array of classical orbits of increasing energy and position extremes. The quantum particle moves in an orbit until jostled by a Brownian collision into another orbit. Orbits are weighted by a declining Maxwellian exponential factor, a fit pointing to a granular kinetic gas space medium with Boltzmann distributed granule energy abundances, allowing a model of the photon accounting for photon inertial properties and explaining Planck's constant. The Weinberg super -dense medium consists of tiny identical balls, each in a free volume 30 times ball volume, moving at energies where the speed of sound in the medium is our speed of light. The photon is a weak shock N-wave with momentum proportional to energy E. Transport properties smear the wave into a single cycle sinusoid of length inverse to E.

Stephen Minter - Margaret Burbidge Award for Best Experimental Research - First Place

Production and Detection of Gravitational Radiation by a Two-Body Superconducting System

A two-body superconducting system can convert electromagnetic radiation to gravitational radiation and vice versa by means of magnetic fields generated by the London moment. As charge currents create the familiar magnetic field, mass currents can be used to create a gravito-magnetic field which is reflected from the boundary of a superconductor due to the single-valuedness of the Cooper pairs. The physics of production and detection of gravitational radiation will be discussed, as well as the experimental procedures which will be used to verify the theory.

Sang Ki Nam

Effect of Electron Energy Distribution Function on the Global Model for High Power Microwave Breakdown at High Pressures

Most global models assume the Maxwellian distribution for the electron energy distribution function (EEDF) in the plasma discharge. The electrons, however, are not in the equilibrium in most discharges unless the electron-electron collision is dominant and EEDF differs from the Maxwellian. The assumption of the Maxwellian EEDF can result in the inaccurate reaction rate coefficients for the plasma discharge and lead to the wrong breakdown behavior prediction. Since the plasma parameters strongly depend on those coefficients, they are important to predict the plasma breakdown properly. This presentation will talk about the method to approximate the EEDF that can gives the proper reaction rate coefficients to the volume-averaged global model for the High Power Microwave breakdown at high pressure regimes.

Geoffrey Nelson - Steven Chu Award for Best Research - First Place

Facts from the Fallout

The purpose of this project was to make a documentary summarizing the findings, methodology, and implications of the research conducted at Chernobyl. The author participated with researchers on their current projects at the Research Center for Radiation Medicine which is the leading group studying long term effects of the Chernobyl accident. The author interviewed researchers about the projects conducted from the time of the Chernobyl accident up to the current research. Clips of selected interviews were posted online for a group of students and professors to watch, evaluate, and comment on. The author is currently editing the footage into a documentary about what the world has learned from Chernobyl.

Ingrid Neumann

Enhanced Friction for Moving Vortex when interacting with Vortex-Mesh on Wall

Considering the two fluid model of supefluidity the bulk mutual friction can be calculated. This theoretical value is however eight magnitudes of order smaller than the friction observed in experiments. We consider that a mesh of microscopic vortices, left after annihilation of most of the bulk vortices in the superfluid after rotation ceases, as a possible mechanism altering the tangential forces near the wall. This additional effect, parallel to local superfluid velocity, may well explain the increased friction observed experimentally. We consider a partially trapped vortex on a wire in the center of a cylindrical cell. The core of the free end of this vortex moves along the wall of the container at the local superfluid velocity. We use periodic boundary conditions for the mesh of vortices on the wall and allow the moving vortex to recombine with the mesh vortices when approaching within in certain distance. Our simulations use a fifth order Runge-Kutta methods to calculate the change in velocity/fluid flow rate/circulation and the fluid flow field is calculated using the Biot-Savart law and boundary conditions. To keep our simulations stable and their run time efficient we use a friction coefficient representing a force perpendicular to the direction of local superfluid flow. We find that, even for artificially high friction coefficients, the energy dissipation rate of the moving vortex is still between 1.5 to 2 times larger than what it would be not considering interaction of the moving vortex with the mesh.

Roger Newcomb

Magnetic Shield Efficiency Tests for the CLAS HTCC

An upgrade of the accelerator at Jefferson Lab has made necessary an upgrade of the CEBAF Large Acceptance Spectrometer (CLAS) detector. In order to maintain good pion/electron separation, and to add the additional capability of pion/kion separation, a new Cerenkov counter, the high threshold Cerenkov counter (HTCC), is currently being built. The HTCC uses photomultiplier tubes (PMTs) to convert the light detected into electrical signals that can be read out for later analysis. Due to its internal geometry and the space allocated for the HTCC, the PMTs are in a fringe magnetic field of 50 Gauss, approximately two orders of magnitude too high for proper PMT operation. Shielding must therefore be applied around the PMTs. Using a calibrated Helmholtz coil apparatus, the residual magnetic field inside a prototype shield was measured for both an axial and a transverse orientation of the shield relative to the field. This talk will present the major aspects of this project, including the physical construction of the apparatus, the acquisition of the experimental data, and its offline analysis. The expected implications of this work on the final design of the magnetic shields will also be presented.

Hendrik Ohldag

X-rays and Magnetism - A Perfect Match

Today's fundamental and applied magnetism research is particularly focused on magnetic materials that are suitable as magnetic sensors, spin valves, spin transistors or magnetic media consisting of complex magnetic multilayer structures. Scientific investigations in this area are concerned with the origin of magnetic coupling, spin transport across interfaces, magnetic properties of magnetic oxides and the complex magnetic structures which evolve when different kind of magnets for example antiferromagnets (AF) and ferromagnets (FM) are brought into contact. Dichroism x-ray absorption spectroscopy (XAS) represents a unique tool to understand complex nanomagnetic samples. The power of XAS is that it provides a possibility to address individual magnetic properties of different elements in a sample and a way to distinguish between different magnetic order like AF and FM order at the same time. It can furthermore be used to study the magnetism of buried interfaces, diluted magnetic systems like FM semiconductors or other exotic new magnets. To perform these studies a source of tunable soft x rays with high brilliance and full polarization control is required which is available at today's state of the art synchrotron radiation sources. The pulsed nature of the synchrotron as x-ray source allows for studying the time dependent behavior of a sample with a temporal resolution of a few tens of picoseconds. Dichroism soft x-ray absorption spectroscopy can furthermore be used to obtain spatially resolved information with less than 50nm lateral resolution in a modern full field or scanning x-ray microscopes.

Presentation

JGO Ojwang

Trapping the Hydrogen Genie: a Reactive Force Field for NaH

Ojwang JGO(1), Adri van Duin(2), William Goddard III(2), Rutger van Santen(1), Gert Jan Kramer(1) (1) Eindhoven University of Technology, Postbus 513, 5600 MB, Den Dolech 2, Eindhoven, The Netherlands. (2) Material Research Center, California Institute of Technology(Caltech) , 1200 East California Boulevard Pasadena, California 91125, U.S.A.
Bogdanovic [1] pioneered the interest in the complex metal alanates by establishing that NaAlH₄ could be rehydrogenated by doping with metal catalysts of which, titanium is the most thoroughly investigated. However, there are still unresolved problems in understanding of H₂ dissociation process in the structure. Among these include the role of titanium (is it a dopant or a catalyst?) and long range transport mechanism of Al during the dissociation process. In this work we are developing a reactive force field(ReaxFFNaAlH₄)[2] to study the structural and dynamical details of hydrogen absorption/desorption processes in NaAlH4 system. This will be done by parameterizing a force field, ReaxFF, to simulate large clusters containing NaH, Na₃AlH₆, NaAlH₄ and Al phases plus catalysts atoms with a view to understanding the dynamics governing hydrogen absorption/desorption. We have already sufficiently parameterized ReaxFFNaH to adequately describe H2 desorption process in NaH. Parameterization of the reactive force field for NaH is done using density functional theory (DFT) data. Parameterizations of the energy expressions in ReaxFFNaH were done by fitting into the training set the ab initio derived equations of state (EoS) of pure Na and NaH condensed phases, reaction energies and bond dissociation profiles on small finite clusters. Phase transformations/crystal modifications in both Na and NaH systems during desorption process was accounted for by adding the high pressure phases of Na and NaH, in addition to the groundstate phases, to the quantum calculations. In the case of Na we considered four phases: bcc-Na(8-coordinate), sc-Na(6-coordinate), fcc-Na(12-coordinate) and hcp-Na(12-coordinate). For NaH, the high pressure (CsCl-type) and NaCl-type phases were considered. The parameterized force field, ReaxFFNaH, is used to study the dynamics governing hydrogen desorption in NaH. During the abstraction process of surface molecular hydrogen charge transfer is found to be well described by the parameterized force field. A molecular dynamics run is done, which shows that a clear signature of hydrogen desorption is the fall in potential energy surface during heating.
[1] B. Bogdanovic, M. Schwickardi, J. Alloys Compd., 1997, 1, 253-254. [2] Sam Cheung et al, J. Phys. Chem. A, 2005, 109, 851-859.

Tommaso Pardini - Kennedy Reed Award for Best Theoretical Research - First Place

Ground State Phase Diagram of the Heisenberg Model on Anisotropic Triangular Lattice

We study the spin-half and spin-one Heisenberg model on the anisotropic triangular lattice with interaction J1 and J2. Tha lattice interpolates between the limits of the square lattice (J1=0), the triangular lattice (J1=J2) and the one dimensional linear chain (J2=0). Results have been obtained by means of linked-cluster series expansion around the collinear antiferromagnetic phase (CAF) and the non collinear antiferromagnetic phase (NCAF). We find that the CAF phase is not the ground state of the model, since the NCAF phase always has a lower energy. The study of the spin-one model dramatically brings out their differences, where we find a transition from the Haldane gap phase to a NCAF phase as a function of J1/J2. The CAF phase does not appeared to be favored for any value of J1/J2.

Staci Pearson

A Theoretical Model for an Unusual New Liquid Crystal

Liquid crystals are a class of phases intermediate between liquid and crystal and possess interesting characteristics. I will discuss the properties associated with conventional phase transitions in liquid crystals and contrast these with those occurring in de Vries type liquid crystals. The de Vries liquid crystals have unusual features such as an order parameter that decreases as a more ordered phase is approached. This talk will focus on a theoretical model that explains these observations.

Cecile Portello-Roucelle - Luis Alvarez Award for Best Experimental Research - First Place

High Energy Neutrinos from the Cold: Status and Perspectives of the IceCube Experiment

The observation of high energy neutrinos from cosmic objects is expected to bring us key information about the most energetic processes known in the universe, such as gamma ray bursts and events in the surroundings of supermassive black holes. Their observation could also help us to understand the mechanism for cosmic ray acceleration, a long-standing puzzle. High energy neutrinos may also elucidate the nature of the dark matter, via the observation of WIMPs annihilation into neutrinos. In recent years, several projects aiming at the observation of high energy neutrinos have been developed. The most ambitious, and most advanced of these is the IceCube Neutrino Observatory, currently under construction at the geographic South Pole. When completed in 2011, IceCube will consist of an instrumented ice volume of about one cubic kilometer, together with a surface air shower array of matching dimensions. Twenty two out of the eighty foreseen strings are already taking data and the first physics analyses using IceCube data are being developed within the collaboration. An overview of high energy neutrino astronomy will be given, with special emphasis on expectations for IceCube. We will also present the status of the experiment and some recent results obtained with the 9-strings detector that was running in 2006.

Elizabeth Read

Uncovering Hidden Signals in Two-dimensional Electronic Spectroscopy

Two-dimensional (2D) ultrafast Fourier transform electronic spectroscopy yields frequency maps of molecular coupling and dynamical processes with femtosecond time resolution. The technique is therefore ideally suited to the study of the first steps of photosynthesis, wherein energy from the sun is absorbed and funneled to the reaction center through a network of pigment-protein complexes with high efficiency. Two-dimensional spectra of photosynthetic complexes have revealed pathways of energy flow and signatures of excitonic coherence, and enabled quantitative determination of energy transfer rates, electronic states, coupling energies, and disorder. This information, along with knowledge of protein crystal structures, is crucial to understanding the design principles governing natural light-harvesting. Although 2D spectra contain information often unavailable or obscured in other femtosecond laser experiments, broad linewidths can still complicate interpretation of spectra. Two-dimensional spectra contain signals arising from many different energetic processes, and separation of individual contributions is desirable for better characterization of excited state dynamics. This can be partly achieved by separating rephasing (photon echo) and nonrephasing contributions to the signal. Additionally, rotating the laser pulse polarizations has been shown to suppress diagonal peaks, revealing cross peaks connecting different excited states. The information contained in these "hidden" signals and the mechanistic insight they afford will be discussed. Separating rephasing and nonrephasing contributions in 2D spectra of Light-Harvesting Complex III (LH3) from purple bacteria reveals coherent excitonic beating among strongly coupled bacteriochlorophyll pigments that is obscured in the full signal. Additionally, the nonrephasing spectra reveal more detailed excitonic structure in both LH3 and the Fenna-Matthews-Olson complex (FMO), a light-harvesting protein from green sulfur bacteria, due to interference between positive and negative signals. The nonrephasing signal under certain polarization schemes for FMO yields a wealth of cross-peaks that are concealed in the conventional 2D spectrum, enabling refinement of the current model of FMO excited state and coupling energies. These extensions of the 2D technique will be especially useful for interpretation of 2D spectra of photosynthetic protein complexes at physiological temperatures.

Chris Seck

High Resolution bBthymetric LIDAR Measurements at San Luis Obispo Bay, CA

We have installed a bathymetric Light Detection And Ranging (LIDAR) system on the Cal Poly Center for Costal Marine Sciences Pier in Port San Luis Obispo, California. Using a 1 Watt Nd:YAG laser operating at 532 nm and a single photon detector, we have observed photons scattered off the ocean floor (benthic boundary layer or BBL). The photon detector is attached to a high resolution, multiple-stop timer, with 15 picoseconds of event-to-event resolution. This yields an approximate 1 cm resolution of LIDAR ranging and hence sediment transport dynamics. In this talk, we will present preliminary results of our work including evidence of surface- and BBL-scattered photons.

Michael Shaughnessy

A Si-based Half-Metal from Transition Metal Heterostructures

A Si-based half-metal from transition metal heterostructures We determine the influence of defects on the electronic properties of a proposed Si-based digital ferromagnetic heterostructure (DFH) for future use in spintronic applications. Using first principles, spin-polarized density functional methods we investigate the robustness of the predicted half-metallic behavior with vacancies and substitutional defects by Si in the Mn layer as well as with extra Mn atoms outside the layer. Half metallicity is robust for substitutional defects and imperfect delta layer, in which the Mn is spread in more than one layer. A general physical picture of half-metallic behavior in the presence of d-p hybridization and an exchange interaction (molecular field effect) will be presented to explain the results.

Jaspinder Singh

Chaotic Crystallography

Determining the structure of materials from their x-ray diffraction spectra is a common problem in condensed matter science. When these materials are complex and disordered, the challenge is even greater. A new technique, called Epsilon Machine Spectral Reconstruction, has been developed that efficiently characterizes the structure of complex materials entirely from their x-ray diffraction spectra, with no other assumed knowledge. I will demonstrate the success of this procedure on several spectra of varying amounts of disorder, as well as touch on the broader implications in the field of crystallography.

Douglas Singleton

Relationship between Unruh and Sokolov-Ternov Effects

We show that if one uses a circularly orbiting electron in a constant external magnetic field as the Unruh--DeWitt detector, then the Unruh effect physically coincides with the experimentally verified Sokolov--Ternov effect.

David Sivak

The Meltable Wormlike Chain

Double-stranded DNA (dsDNA) is often tightly bent in vivo, and indeed the details of DNA bending elasticity crucially influence many cellular processes, such as transcriptional regulation. We provide a statistical mechanical treatment of the meltable wormlike chain (WLC), a discrete WLC model with thermally activated melts, that properly treats the conformational entropy. This model makes qualitatively different predictions for cyclization experiments than previous treatments. We use this model to examine novel dsDNA bending experiments that report the entire distribution of bending fluctuations, and find that the meltable WLC predicts these distributions quite well with one free parameter, capturing features entirely unpredicted by the standard WLC.

Daniel Tennant

Novel Effects of Nonlinear Electrodynamics

I will be exploring two non-linear extensions of classical electrodynamics, precisely QED and Born-Infeld. These two theories lead to two different predictions for a relatively low energy phenomenon, the birefringence of light.

Cynthia Trevisan

Binding Mechanisms of Metal Ions in Prion Proteins

General consensus is that a key factor in many neurodegenerative diseases is the breakdown of metal homeostasis. In the particular case of Creutzfeldt-Jacob Disease (CJD) and other diseases associated with prion proteins, there is much debate about whether the binding of metals such as copper or zinc plays a neuroprotective or neurodegenerative role. A strong copper binding site of the infectious form of the prion protein (PrPSc) is in the region that contains the sequence 92-96 GGGTH in humans. Structural models that are in agreement with experimental constraints from electron spin resonance data show that the bending of the peptide backbone concomitant of the copper binding is in conflict with the straight strand structure associated with PrPSc, which can be indicative of a protective role of metal ion binding in prion proteins [1]. We are currently exploring the role of metal ions in two other possible binding sites in the C-terminal region of the prion protein, namely, sequences 175-177 FVH and 185-187 KQH in humans. We are performing quantum mechanical (QM) calculations on systems composed of a Cu₂⁺ ion and the residues in each one of these two sequences: systems Cu₂⁺ - FVH and Cu₂⁺ - KQH. The QM calculations of local structure (geometry) and energetics on the above fragments of the prion protein are then embedded in Molecular Dynamics (MD) simulations of the entire protein to study the Cu₂⁺- protein system binding and solvation energetics. We will present preliminary results of the binding of Cu₂⁺ to the 175-177 FVH sequence of the human prion protein and discuss both the predictive capabilities and limitations of our approach.
[1] Cox, D. L., Pan, J and Singh, R. R. P. A Mechanism for Copper Inhibition of Infectious Prion Conversion. Biophys. J. 2006 91: L11- L13

Justin Vandenbroucke

Pop Goes the Neutrino: Acoustic Detection of Astrophysical Neutrinos

Interest has grown recently in neutrinos of astrophysical origin, in particular the ~10¹⁸ eV (EeV) neutrinos generated by interactions of the highest energy cosmic rays with the cosmic microwave background. Detecting ~100 of these neutrinos in order to build sky maps and energy spectra would contribute significantly to resolving the mystery of the highest energy cosmic rays and would probe fundamental particle physics at ~100 TeV center of mass energy. However, new techniques are necessary to achieve the desired effective volume of ~100 km³. In dense media such as ice, water, and salt, neutrino-induced particle showers heat the medium locally causing it to expand and emit a shock wave that is detectable as acoustic radiation in the 10-60 kHz band. South Pole ice in particular is predicted to have low acoustic attenuation and background noise, making the method significantly more sensitive than the optical Cherenkov technique in the EeV range. The South Pole Acoustic Test Setup (SPATS) was installed in three IceCube holes in 2007 to determine the feasibility of acoustic neutrino detection in South Pole ice. With SPATS we have measured the noise level and sound speed, and work is underway to measure the attenuation length.

Presentation and Sound File

Joel Varley

Passivation of Group III-A Acceptor Impurities in SnO₂

Using first-principles calculations we investigate the role of hydrogen in the passivation of p-type dopants in SnO₂ belonging to the group III-A elements, including Al, Ga, and In. The stability of these defects when substituting Sn are discussed and compared in both a hydrogen-free and hydrogen-rich environment, as would be similar to a realistic growing conditions. We also calculate the stretch-mode vibrational frequencies associated with the O-H bond in all relevant cases, providing an experimental signature to detect in the vibrational spectroscopy of these doped SnO₂ systems. We find that although the group III-A elements studied are suitably shallow acceptors for p-type doping, the stability of interstitial H inevitably present will passivate a large concentration of the desired hole population for p-type conductivity in all cases.

Presentation

Jigang Wang - Luis Alvarez Award for Best Experimental Research - Second Place

Femtosecond Detection of Magnetic Memory States in a Ferromagnetic Semiconductor

Magnetic materials displaying carrier-mediated spin-spin exchange interaction are ideal for non-thermal, potentially fast spin manipulation and detection. Prominent examples of such materials are Mn doped III-V semiconductors. The steady-state magneto-optical/transport measurements reveal rich magnetic memory effects. However, no time-resolved experiments in (III,Mn)V semiconductors and their nanostructures have shown these collective magnetic phenomena, and hence their time scales are completely unknown. We present femtosecond detection of magnetic memory states in a 70 nm GaMnAs film. Our time-resolved technique reveals a laser-induced collective spin reorientation, distinctly depending on the initial magnetic state. Our measurements reveal new fundamental collective magnetic process at ultrafast time scales, which may represent as-yet-undiscovered universal features in all carrier-mediated ferromagnetic materials.

Presentation

Chia Wang

Imaging Photoelectron Dynamics in Doped Helium Nanodroplets

Photoelectron dynamics of doped Helium nanodropelts has been investigated by utilizing the photoelectron imaging technique at the Advance Light Source. Significant ionization of dopants was observed at 21.6 eV, the absorption maximum of the first optically-allowed excited state of Helium nanodroplets, sugguesting an indirect ion formation mechanism via excitation energy transfer. By examining the correlations between all observed dynamical features and the size-controlled Helium droplets environment, the photoexcitation transfer mechanism as well as the charge transport and escape processes in this finite-sized quantum fluid can be realized.

Ying Wang

Non-Harmonic Behavior Induced by PIC Scheme for Cold Plasma Oscillations

In a cold plasma, particles exhibit harmonic motion about their equilibrium positions at the plasma frequency. A 1D electrostatic model is used to investigate the non-harmonic behaviors caused by PIC scheme. Simulations in two 1D PIC codes, XES1 and OOPD1, with varying numbers of particles and different numbers of grid cells indicate that for small number of particles, non-harmonic behavior can be caused numerically when particles approach within two grid cells. The numerical errors increase as the distance of closest approach between particles decreases. A shift of the plasma frequency is also observed. When a large number of particles are considered, the numerical errors are reduced if enough particles exist in one grid cell.

Steve Whitelam - Charles Kittel Award for Best Theoretical Research - Second Place

Entropy-driven Hysteresis in a Model of DNA Overstretching

When pulled along its axis, double-stranded DNA elongates abruptly at a force of about 65 pN. Two physical pictures have been developed to describe this overstretched state. The first proposes that strong forces induce a phase transition to a molten state consisting of unhybridized single strands. The second picture instead introduces an elongated hybridized phase, called S-DNA, structurally and thermodynamically distinct from standard B-DNA. Little thermodynamic evidence exists to discriminate directly between these competing pictures. Here we show that within a microscopic model of DNA we can distinguish between the dynamics associated with each. In experiment, considerable hysteresis in a cycle of stretching and shortening develops as temperature is increased. Since there are few possible causes of hysteresis in a system whose extent is appreciable in only one dimension, such behavior offers a discriminating test of the two pictures of overstretching. Most experiments are performed upon nicked DNA, permitting the detachment ('unpeeling') of strands. We show that the long-wavelength progression of the unpeeled front generates hysteresis, the character of which agrees with experiment only if we assume the existence of S-DNA.

Peter Winkler

Collective Dipole Excitations in C₆₀ Molecules

We show in a semiclassical approach ("local current approximation") that the coupling of the surface dipole plasmon of the Buckminster fullerene - a purely translational mode of excitation - with compressional volume modes yields good agreement with earlier time-dependent density functional calculations. Both theories reproduce the most significant features of recent experimental results equally well.

Friedwardt Winterberg

Super-Thermite Explosives for Fast Ignition

The combustion of aluminum with liquid oxygen releases ten times more energy per unit volume, than the combustion of liquid hydrogen with liquid oxygen. The same is true for thermite, a mixture of iron-oxide and aluminum powders, where the energy released is in the form of photons. It burns better the finer are the iron oxyd and aluminum powders. This suggests to search for super-thermite explosives, consisting of nano-powders of different chemicals,where inner electronic shells react with the release of X-rays,if the powders are suddenly put under high pressure, of the order 100 megabar or more. These pressures can be reached with chemical convergent shock waves, but also with electric pulse power driven exploding wires. These X-rays can then be used for the fast ignition of thermonuclear targets.

Megumi Yamamoto

Study of Grain Boundaries Using Transport and Scanning Laser Microscopy

Grain boundary of high temperature superconducting YBa₂Cu₃O₇ (YBCO) and YBCO-based multilayers is studied using transport and scanning laser microscopy (SLM) techniques. In the light of current progress in coated conductors, there are scientific and technological interests in understanding the relation between critical current (Ic) measured by transport and local dissipation by SLM in various superconducting layers across a grain boundary. We used thermoelectric SLM (TE-SLM) to locate the grain boundary, low temperature SLM (LTSLM) to map the superconducting dissipation, and variable temperature SLM (VTSLM) to map the Tc variation and current distribution in a grain boundary. The transport measurements reveal step-like features in I-V from most samples. We will discuss the relation between SLM and transport measurements and our understanding of the origin of the step-like features in I-V.

Ding-Shyue Yang

Transitional Structures during Phase Transformations Probed by Ultrafast Electron Diffraction

Time-resolved electron diffraction has been successfully used for the direct probing of photoinduced structural dynamics in condensed matter. For conventional semiconductors (e.g., gallium arsenide and silicon) and metals (e.g., gold and aluminum), following excitation by a femtosecond optical pulse, the crystal lattice exhibits nonthermal vibrations and expansion due to the photogenerated carriers and their subsequent energy transfer to the optical/acoustic phonons [1]. Such knowledge of structural dynamics provides the foundation for the investigation of more complex processes (e.g., phase transformations), systems (e.g., interfacial molecular assembly [2]), and materials (e.g., superconductors [3]). Here, we present the visualization of transitional structures in crystalline vanadium dioxide, initiated by a near-infrared laser pulse, from the initial monoclinic to the final tetragonal phase. The spatiotemporal behavior of the observed Bragg diffraction spots reveals the pathway of the phase transition: the femtosecond primary V−V bond dilation, the displacements of atoms within a unit cell in picoseconds, and the sound-wave shear motion in ~100 picoseconds. Such a nonconcerted pathway mechanism, and the minimum excitation fluence necessary to initiate the transformation, provide a clear understanding of this rigorously scrutinized phase transition in vanadium dioxide [4].
References: [1] D.-S. Yang, N. Gedik, A. H. Zewail, J. Phys. Chem. C 111, 4889 (2007). [2] M. T. Seidel, S. Chen, A. H. Zewail, J. Phys. Chem. C 111, 4920 (2007). [3] N. Gedik, D.-S. Yang, G. Logvenov, I. Bozovic, A. H. Zewail, Science 316, 425 (2007). [4] P. Baum, D.-S. Yang, A. H. Zewail, Science (in press).

Presentation-pdf and Presentation-ppt

Quan Yin

Phonon Spectra of UO₂ and PuO₂

The electronic structure and phonon spectra of UO₂ and PuO₂ are studied using a combined density functional theory in the local density approximation and dynamic mean filed theory (LDA+DMFT). UO₂ and PuO₂ are both Mott-insulators but have different magnetic properties. Their phonon spectra are very similar and show a softened optical branch, which is observed in experiment of UO₂ and reproduced in our calculation. The compressed phonon softening indicates structural instability in these two materials.