Physical Sciences and Mathematics Commons^™

Tachyonic Instability And Dynamics Of Spontaneous Symmetry Breaking, Gary Felder, Lev Kofman, Andrei Linde

Physics: Faculty Publications

Spontaneous symmetry breaking usually occurs due to the tachyonic (spinodal) instability of a scalar field near the top of its effective potential at φ=0. Naively, one might expect the field φ to fall from the top of the effective potential and then experience a long stage of oscillations with amplitude O(v) near the minimum of the effective potential at φ=v until it gives its energy to particles produced during these oscillations. However, it was recently found that the tachyonic instability rapidly converts most of the potential energy V(0) into the energy of colliding classical waves of the scalar field. This …

Terahertz Gain In A Sige/Si Quantum Staircase Utilizing The Heavy-Hole Inverted Effective Mass, Richard A. Soref, Greg Sun

Physics Faculty Publications

Modeling and design studies show that a strain-balanced Si_1−xGe_x/Si superlattice onSi_1−yGe_y-buffered Si can be engineered to give an inverted effective mass HH2 subband adjacent to HH1, thereby enabling a 77 K edge-emitting electrically pumped p–i–pquantum staircase laser for THz emission at energies below the 37 meV Ge–Ge optical phonon energy. Analysis of hole-phonon scattering, lifetimes, matrix elements, and hole populations indicates that a gain of 450 cm⁻¹ will be feasible at f = 7.3 THz during 1.7 kA/cm² current injection.

Interactions Of Ultrashort, Ultrahigh Intensity Laser Pulses With Underdense Plasmas, Xiaofang Wang, Wei Yu, Sterling Backus, Margaret Murnane, Henry Kapteyn, Donald P. Umstadter

Donald Umstadter Publications

The interactions of ultraintense laser pulses with underdense plasmas are studied in a new regime in which the longitudinal spatial extent of the pulse duration is close to both the laser focal spot size and the plasma wavelength.

Laser Acceleration Of Protons From Thin Film Targets, K. Flippo, Sudeep Banerjee, V. Yu. Bychenkov, S. Gu, Anatoly Maksimchuk, G. Mourou, K. Nemoto, Donald P. Umstadter

Donald Umstadter Publications

A collimated beam of fast protons, with energies as high as 10 MeV and total number of 10⁹, confined in a cone angle of 40°±10° has been observed when a 10 TW laser with frequencies either ω₀ (corresponding to 1 μm) or 2ω₀ was focused to an intensity of a few times 10¹⁸ W/cm² on the surface of a thin film target. The protons, which originate from impurities on the front side of the target, are accelerated over a region extending into the target and exit out the backside in a direction normal to …

Status Of The Lilac Experiment, N. Saleh, P. Han, C. Keppel, P. Gueye, V Yanovsky, Donald P. Umstadter

Donald Umstadter Publications

We present the status of the LILAC experiment [1], including results on the propagation of 30-fs duration laser pulses in plasmas of the requisite density, and measurements of the dark current [2]. We also discuss the status of a laser upgrade, an electron beam line and plans for the future.

Magnetite Morphology And Life On Mars, Peter R. Buseck, Rafal E. Dunin-Borkowski, Bertrand Devouard, Richard B. Frankel, Martha R. Mccartney, Paul A. Midgley, Mihály Pósfai, Mathew Weyland

Physics

Nanocrystals of magnetite (Fe₃O₄) in a meteorite from Mars provide the strongest, albeit controversial, evidence for the former presence of extraterrestrial life. The morphological and size resemblance of the crystals from meteorite ALH84001 to crystals formed by certain terrestrial bacteria has been used in support of the biological origin of the extraterrestrial minerals. By using tomographic and holographic methods in a transmission electron microscope, we show that the three-dimensional shapes of such nanocrystals can be defined, that the detailed morphologies of individual crystals from three bacterial strains differ, and that none uniquely match those reported from …

Charged-Particle Multiplicity Pp̅ Collisions At √S = 1.8 Tev, T. Affolder, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration

Kenneth Bloom Publications

We report on a measurement of the mean charged-particle multiplicity of jets in dijet events with dijet masses in the range 80-630 GeV/c², produced at the Tevatron in pp̅ collisions with √s= 1.8 TeV and recorded by the Collider Detector at Fermilab. The data are fit to perturbative-QCD calculations carried out in the framework of the modified leading log approximation and the hypothesis of local parton-hadron duality. The fit yields values for two parameters in that framework: the ratio of parton multiplicities in gluon and quark jets, r=N^g-jet _partons/N^q-jet _partons =1.7 ± …

Micromagnetics Simulation Of Deep-Submicron Supermalloy Disks, Scott L. Whittenburg, Ngocnga Dao, R. P. Cowburn

Chemistry and Biochemistry Faculty Publications

The results of recent micromagnetic simulations of deep submicron supermalloy disks are presented. A recent experimental measurement of the hysteresis and magnetic domain structure in supermalloy disks with diameters ranging from 55 to 500 nm and thickness ranging from 6 to 15 nm has been reported. Our micromagnetic simulations show remarkable agreement with the experimental hysteresis loops. The simulation results show that for thin or small diameter disks a single magnetic domain exists with all spins aligned. The hysteresis loop represents free rotation of these spins. For larger diameter disks or as the thickness increases the hysteresis loops change shape …

Analysis Of Corrosion Of Steel By Lead Bismuth Eutectic, John Farley, Dale L. Perry

Transmutation Sciences Materials (TRP)

We examined stainless samples that were exposed to LBE in experiments conducted by the Russians, under contract to Los Alamos. We examined both corroded and uncorroded samples using the Scanning Electron Microscope (SEM), the Energy Dispersive X-Ray (EDAX) Spectroscopy, and the X-ray Photoelectron Spectrometer (XPS). We found that the surface of the corroded sample is covered by oxygen-containing compounds, presumably mostly iron oxide. In samples exposed for shorter times or lower temperatures, we found that some areas were covered by an oxide layer, and some areas was uncovered. We found that the level of Cr in the uncovered area is …

Design And Analysis For Melt Casting Metallic Fuel Pins Incorporating Volatile Actinides: Quarterly Progress Report 8/16/01- 11/15/01, Yitung Chen, Randy Clarksean, Darrell Pepper

Fuels Campaign (TRP)

An important aspect of the Advanced Accelerator Applications (AAA) program is the development of a casting process by which volatile actinide element (i.e., americium) can be incorporated into metallic alloy fuel pins. The traditional metal fuel casting process uses an inductively heated crucible. The process involves evacuation of the furnace. The evacuation of the furnace also evacuates quartz rods used as fuel pin molds. Once evacuated the open ends of the molds are lowered into the melt; the casting furnace is then rapidly pressurized, forcing the molten metal up into the evacuated molds where solidification occurs.

This process works well …

Ac Susceptibility Studies Of Discontinuous Co80fe20al2o3 Multilayers, Christian Binek

Christian Binek Publications

Discontinuous metal–insulator multilayers [Co80Fe20(t)/Al2O3(3 nm)]10 are prepared by ion-beam sputtering and investigated by SQUID magnetometry and previous AC susceptometry. For 1.0less-than-or-equals, slanttless-than-or-equals, slant1.6 nm, two coexisting subsystems of dipolar coupled nanoparticles are encountered. While a spin-glass-like freezing particulate fraction dominates at t=1.0 nm, a percolating dipolar ferromagnetic fraction becomes increasingly important at higher nominal thickness of t.

Exchange Bias In Fef20-Copt Heterosystems With Perpendicular Anisotropy, Christian Binek

Christian Binek Publications

The previous exchange bias effect is measured in heterosystems with perpendicular anisotropy consisting of Co/Pt multilayers on top of the (0 0 1) face of a previous FeF2 single crystal. The resulting previous exchange field HE exhibits a strong dependence on temperature and the axial freezing field, HF. Within the framework of an Ising-type model, the HE vs. T as well as the HE vs. HF data are explained in terms of the microscopic spin structure at the interface.

Quantum Anomaly In Molecular Physics, Horacio E. Camblong

Physics and Astronomy

The interaction of an electron with a polar molecule is shown to be the simplest realization of a quantum anomaly in a physical system. The existence of a critical dipole moment for electron capture and formation of anions, which has been confirmed experimentally and numerically, is derived. This phenomenon is a manifestation of the anomaly associated with quantum symmetry breaking of the classical scale invariance exhibited by the point-dipole interaction. Finally, analysis of symmetry breaking for this system is implemented within two different models: point dipole subject to an anomaly and finite dipole subject to explicit symmetry breaking.

Anomaly Induced Qcd Potential And Quark Decoupling, Joseph Schechter, Stephen D.H. Hsu, Francesco Sannino

Physics - All Scholarship

We explore the anomaly induced effective QCD meson potential in the framework of the effective Lagrangian approach. We suggest a decoupling procedure, when a flavored quark becomes massive, which mimics the one employed by Seiberg for supersymmetric gauge theories. It is seen that, after decoupling, the QCD potential naturally converts to the one with one less flavor. We study the N_c and N_f dependence of the \eta^{\prime} mass.

Characterization Of The Native Cr2O3 Oxide Surface Of Cro2, Ruihua Cheng, B. Xu, C.N. Borca, Andrei Sokolov, C.S. Yang, Lu Yuan, Sy_Hwang Liou, Bernard Doudin, Peter A. Dowben

Peter Dowben Publications

Using photoemission and inverse photoemission, we have been able to characterize the Cr₂O₃ oxide surface of CrO₂ thin films. The Cr₂O₃ surface oxide exhibits a band gap of about 3 eV, although the bulk CrO₂ is conducting. The thickness of this insulating Cr₂O₃ layer is twice the photoelectron escape depth which is about 2 nm thick. The effective Cr₂O₃ surface layer Debye temperature, describing motion normal to the surface, is about 370 K. From a comparison of CrO₂ films grown by different techniques, with different …

The Formation Of Vortex Loops (Strings) In Continuous Phase Transitions, Mark Bowick, Angelo Cacciuto, Alex Travesset

Physics - All Scholarship

The formation of vortex loops (global cosmic strings) in an O(2) linear sigma model in three spatial dimensions is analyzed numerically. For over-damped Langevin dynamics we find that defect production is suppressed by an interaction between correlated domains that reduces the effective spatial variation of the phase of the order field. The degree of suppression is sensitive to the quench rate. A detailed description of the numerical methods used to analyze the model is also reported.

Transverse Magnetism Of The Diluted Antiferromagnet Fe1-Xmgxbr2 (X=0.15), Christian Binek

Christian Binek Publications

Neutron diffraction and magnetization measurements of the diluted crystal Fe1−xMgxBr2 (xnot, vert, similar0.15) were performed. Successive phase transitions at zero applied field at TN1not, vert, similar12 K and TN2not, vert, similar10 K were found. A previous transverse term ferromagnetic moment was detected just below TN2.

Comments On “Errors In The Estimate Of The Fractal Correlation Dimension Of Raindrop Spatial Distribution”, A. R. Jameson, Alexander Kostinski

Department of Physics Publications

No abstract provided.

Classical Density Functional Theory Of Freezing In Simple Fluids: Numerically Induced False Solutions, M. Valera, F. J. Pinski, Duane D. Johnson

Duane D. Johnson

Density functional theory (DFT) has provided many insights into the freezing of simple fluids. Several analytical and numerical solution have shown that the DFT provides an accurate description of freezing of hard spheres and their mixtures. Compared to other techniques, numerical, grid-based algorithms for solving the DFT equations have more variational freedom and are capable of describing subtle behavior, as that seen in mixtures with multipeaked density profiles. However the grid-based approach is sensitive to the coarseness of the mesh employed. Here we summarize how the granularity of the mesh affects the freezing point within the DFT. For coarse meshes, …

Climatology Of Mid- And Low-Latitude F Region Disturbance Winds Measured By Windii, J. T. Emmert, Bela G. Fejer, C. G. Fesen, G. G. Shepherd, B. H. Solheim

Bela G. Fejer

No abstract provided.

Three-Body Coulomb Problem Probed By Mapping The Bethe Surface In Ionizing Ion-Atom Collisions, Robert Moshammer, Appathurai N. Perumal, Michael Schulz, V. D. Rodriguez, Holger Kollmus, Rido Mann, Siegbert Hagmann, Joachim Hermann Ullrich

Physics Faculty Research & Creative Works

The three-body Coulomb problem has been explored in kinematically complete experiments on single ionization of helium by 100 MeV / u C 6 + and 3.6 MeV / u u 5 3 + impact. Low-energy electron emission (E e < 150 eV ) as a function of the projectile deflection ϑ p (momentum transfer), i.e., the Bethe surface [[15]], has been mapped with Δ ϑ p ± 25 nanoradian resolution at extremely large perturbations (3.6 MeV / uAu⁵³⁺ ) where single ionization occurs at impact parameters of typically 10 times the He K -shell radius. The experimental data are not in agreement with state-of-the-art continuum distorted wave-eikonal initial state theory.

Electron-Impact Excitation To The 4p⁵5s And 4p⁵5p Levels Of Kr | Using Different Distorted-Wave And Close-Coupling Methods, Arati K. Dasgupta, Klaus Bartschat, D. Vaid, Alexei N. Grum-Grzhimailo, Don H. Madison, Milan Blaha, John L. Giuliani

Physics Faculty Research & Creative Works

Electron-impact excitation of the 4p55s and 4p55p levels of Kr I has been investigated in detail by calculating cross sections using distorted-wave and close-coupling approaches. The results are presented from the excitation thresholds up to 50 eV incident energy. They are contrasted among the different calculations and compared with other theoretical predictions and experimental data. Significant disagreement is found with many of the recent experimental data of Chilton et al. [Phys. Rev. A 62, 032714 (2000)].

Local Impurity-Assisted Conductance In Magnetic Tunnel Junctions, Evgeny Y. Tsymbal, David G. Pettifor

Evgeny Tsymbal Publications

Using a simple tight-binding model and the Kubo formula we have calculated the lateral distribution of the tunneling conductance across a magnetic tunnel junction probed by STM. We find that the presence of an isolated impurity within the barrier layer can cause a spike in the conductance distribution, which is in agreement with recent experiments. We show that the local tunneling magnetoresistance (TMR) is very sensitive to the electronic state of the impurity and to the lateral position of the tip. The latter dramatic variation in TMR could be detected by STM.

Electrical Characterization Of 1.8 Mev Proton-Bombarded Zno, F. D. Auret, S. A. Goodman, M. Hayes, M. J. Legodi, H. A. Van Laarhoven, David C. Look

Physics Faculty Publications

We report on the electrical characterization of single-crystal ZnO and Au Schottky contacts formed thereon before and after bombarding them with 1.8 MeV protons. From capacitance–voltage measurements, we found that ZnO is remarkably resistant to high-energy proton bombardment and that each incident proton removes about two orders of magnitude less carriers than in GaN. Deep level transient spectroscopy indicates a similar effect: the two electron traps detected are introduced in extremely low rates. One possible interpretation of these results is that the primary radiation-induced defects in ZnO may be unstable at room temperature and anneal out without leaving harmful defects …

Thermal Stability Of Isolated And Complexed Ga Vacancies In Gan Bulk Crystals, K. Saarinen, T. Suski, I. Grzegory, David C. Look

Physics Faculty Publications

We have applied positron annihilation spectroscopy to show that 2-MeV electron irradiation at 300 K creates primary Ga vacancies in GaN with an introduction rate of 1 cm^-1. The Ga vacancies recover in long-range migration processes at 500–600 K with an estimated migration energy of 1.5 (2) eV. Since the native Ga vacancies in as-grown GaN survive up to much higher temperatures (1300–1500 K), we conclude that they are stabilized by forming complexes with oxygen impurities. The estimated binding energy of 2.2 (4) eV of such complexes is in good agreement with the results of theoretical calculations.

Multiwavelength Monitoring Of The Narrow-Line Seyfert 1 Galaxy Arakelian 564. Iii. Optical Observations And The Optical--Uv--X-Ray Connection, O. Shemmer, P. Romano, R. Bertram, W. Brinkmann, S. Collier, K. A. Crowley, E. Detsis, A. V. Filippenko, C. Martin Gaskell, T. A. George, M. Gliozzi, M. E. Hiller, T. L. Jewell, Shai Kaspi, Elizabeth S. Klimek, M. H. Lannon, W D. Li, P. Martini, Smita Mathur, H. Negoro, Hagai Netzer, Iossif Papadakis, Ioannis Papamastorakis, Bradley M. Peterson, Bradley W. Peterson, Richard William Pogge, V. I. Pronik, Kenneth S. Rumstay, S. G. Sergeev, G. M. Stirpe, C. J. Taylor, R. R. Treffers, T. J. Turner, P. Uttley, Marianne Vestergaard, Kaspar Von Braun, R. M. Wagner, Z. Zheng

Department of Physics and Astronomy: Faculty Publications

We present the results of a 2 yr long optical monitoring program of the narrow-line Seyfert 1 galaxy Ark 564. The majority of this monitoring project was also covered by X-ray observations (RXT E), and for a period of ~50 days, we observed the galaxy in UV (HST) and X-rays (RXTE and ASCA) simultaneously with the ground-based observations. Rapid and large-amplitude variations seen in the X-ray band, on a daily and hourly timescale, were not detected at optical and UV wavelengths, which in turn exhibited much lower variability either on short (1 day) or long (several months) timescales. The only …

Search For Narrow Diphoton Resonances And For Γγ+W/Z Signatures In Pp̅ Collisions At √S=1.8 Tev, T. Affolder, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration

Kenneth Bloom Publications

We present results of searches for diphoton resonances produced both inclusively and also in association with a vector boson (W or Z) using 100 pb^-1 of pp̅ collisions using the CDF detector. We set upper limits on the product of cross section times branching ratio for both pp̅→ γγ+X and pp̅→ γγ+ W/Z. Comparing the inclusive production to the expectations from heavy sgoldstinos we derive limits on the supersymmetrybreaking scale √F in the TeV range, depending on the sgoldstino mass and the choice of other parameters. Also, using a NLO prediction for the associated …

Energetic Particles And Magnetohydrodynamic Activity In The Swarthmore Spheromak Experiment, G. Qin, Vyacheslav S. Lukin , '00, C. D. Cothran, Michael R. Brown, W. H. Matthaeus

Physics & Astronomy Faculty Works

Results from the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] indicate that formation and partial merging of two spheromak plasmas can be described well by a magnetohydrodynamic (MHD) picture in which there is substantial evolution towards force free states within each vessel, while reconnection activity, also described reasonably well by MHD, occurs in the region of interaction. MHD simulations [V. S. Lukin , Phys. Plasmas 8, 1600 (2001)] support and provide further detail to this interpretation. In the present study, test particle equations are integrated using MHD data from SSX simulations to further understand the …

Design And Construction Of A Space-Borne Optical Tweezer Apparatus, Andrew Resnick

Physics Faculty Publications

A compact optical tweezer package has been developed for use on a microscope to be flown on the International Space Station as part of a series of experiments in colloid crystallization. A brief introduction to the principles of single-beam optical tweezer operation will be presented, after which a detailed system layout will be shown. Special design requirements due to the spaceflight nature of the hardware will also be discussed. The tweezer apparatus is capable of trapping many particles through use of a two-axis acousto-optical deflector. The trap strength is sufficient to perform the required science (50 pN at Δn=0.2). The …

Concentration Gradient, Diffusion, And Flow Through Open Porous Medium Near Percolation Threshold Via Computer Simulations, Ras B. Pandey, J.F. Gettrust, D. Stauffer

Faculty Publications

The interacting lattice gas model is used to simulate fluid flow through an open percolating porous medium with the fluid entering at the source-end and leaving from the opposite end. The shape of the steady-state concentration profile and therefore the gradient field depends on the porosity (p). The root mean square (rms) displacements of fluid and its constituents (tracers) show a drift power-law behavior, R∝t in the asymptotic regime (t→∞). The flux current density (j) is found to scale with the porosity according to, j∝(Δp)^β with Δp …