Session F1: Astrophysics

An Alternative Explaination to CMBR based on Curvature

The author would like to propose an alternative explaination to the CMBR based on curvature instead of temperature. When one looks at the horizon of a curved surface such as the earth, that horizon is non-magnifiable due to the fact that objects not only shrink but tilt away from the observer as well. This clue applied to horizons in 3 dim. of curved space and one perp. time dimension (which would tilt backward, away from the observer and result in a nonmagnifiable 2 dim. surface, in all directions). The temperature of the resulting area (similar to the look of the resulting line of the earth's horizon) would tell the observer the mass and the curvature of the surface. The reason that the time vectors on the other side of the Universe do not contribute is that they are backward in time and the parallel displaced vectors would result in a particle that we would not interpret as a photon (it would appear to be a photon moving backward in time - and thus adding a correction to QED/QCD). The implications of this may shake the foundations of Physics in that it gives a direct connection between the shape of the Universe and spin. Pilots use the Horizon to determine up and down, in a 4 space this would give absolute meaning to spin and tell the correct number of dim. of the manifold. The author submits that there may be enough evidence to conclude that this is the correct interpretation of the CMBR and the Big-Bang in its simple form is not.   

Fermi-LAT Sensitivity to Dark Matter Annihilation in Via Lactea II Substructure

We present a study of the ability of the Fermi Gamma-ray Space Telescope to detect dark-matter annihilation signals from the Galactic subhalos predicted by the Via Lactea~II N-body simulation. We implement an improved formalism for estimating the boost factor needed to account for the effect of dark-matter clumping on scales below the resolution of the simulation, and we incorporate a detailed Monte Carlo simulation of the response of the Fermi-LAT telescope, including a simulation of its all-sky observing mode integrated over a ten year mission. The results are less optimistic than previous estimates that did not fully account for the variations of the LAT effective area and point-spread function. Nevertheless, for WIMP masses up to about 150~GeV/$c^2$ in standard supersymmetric models a few subhalos could be detectable with $>5$ standard deviations significance and would likely deviate significantly from the appearance of a point source.   

The Search for Ultra High-Energy Neutrinos With The ANITA Experiment

The ANITA (ANtarctic Impulsive Transient Antenna) experiment is an innovative balloon-borne radio telescope, designed to detect coherent Cherenkov emission from cosmogenic ultra high-energy neutrinos with energy greater than $10^{18}$ eV. The second flight of the ANITA experiment launched on December 21\textsuperscript{st} 2008, and collected data for 30 days. This large data set allows for the most sensitive investigation into the exciting GZK (Greisin-Zatsepin-Kuzmin) neutrino flux regime to date. I will present the status of the first pass analysis of the ANITA-II data set. I will discuss calibration techniques, analysis methods, and background rejection.   

Observation of UHE Cosmic Rays from a Balloon-borne Neutrino Telescope

The ANtarctic Impulsive Transient Antenna (ANITA) is a balloon-borne array of radio antennas designed to detect coherent radio Cherenkov radiation from ultra-high energy (UHE) neutrino-induced particle showers in the Antarctic ice sheet. The first flight of ANITA has produced limits on the UHE neutrino flux, and analysis of data from the second flight is underway. I will describe the neutrino search and concurrent observations of geosynchrotron radio emissions from UHE cosmic ray extensive air showers.   

Can Dark Matter explain the WMAP Haze?

There is currently a significant effort to observe indirect evidence of dark matter annihilation in our galaxy. One interesting finding was an unexpected synchrotron haze (the ``WMAP haze'') with a similar intensity and morphology to those predicted by dark matter models. This might also be connected to another recent puzzle in cosmic ray physics: the excess of high energy positrons reported by the Pamela satellite. We evaluate a wide variety of well motivated dark matter particle physics setups as well as cosmic ray propagation models, and compare the simulated dark matter driven synchrotron signal against the observed WMAP haze. Further analyzing several reasonable dark matter fits to the WMAP haze, we determine the expected inverse-Compton scattering and positron/electron signals which should be detectable by cutting edge Fermi and Pamela observations.   

Waves in an accretion disk: nodal superhumps versus permanent superhumps in V378 Pegasi

First science results from a new observatory, Fresno State's station at Sierra Remote Observatories, are presented.~ The nova-like cataclysmic variable V378 Pegasi (PG 2337+300) is discovered to show variable, often sawtooth-wave variations in its light curve, which have a period of 3.22 hours.~ These variations are present in light curves taken in 2001, 2008, and 2009, and have amplitudes between 0.2 and 0.4 magnitudes, as detected through a clear filter.~ We identify these as waves in this close binary star system's accretion disk, which are related to the superhump phenomenon shown by SU UMa stars.~ We also present the results of a radial velocity study to measure the orbital period, and discuss physical models for the variation in V378 Peg: either permanent superhumps, in which the disk is made elliptical and precessed by tidal forces from the stars' pronounced mass ratio, or nodal superhumps, from a tilted disk.~ We also discuss the evolutionary status of V378 Peg: at just above the period gap, this system may provide a critical test of cataclysmic variable evolution theory.   

Quantum Universe Theory

The Initial Condition (that which existed prior to the universe) is compared as an infinite thermodynamic system (reservoir and system) to a two-component blackbody system, where one component, composed of unbound bosons, contained a symmetry breaking potential. Symmetry breaking resulted in the moment of inflation in a subsystem (small part) of one component, which in turn ignited an unloading wave. The ensuing Big Bang Unloading Wave created a continuously expanding cavity in that component. The cavity is the universe. Within the expanding unloading wave, the first energy cascade has continuously produced intense plasma effects, superelectric fields, and supermagnetic effects. The intense plasma produces violent pinch effects propelling superelectric-magnetic particles to the speed of light c impacting them within the other component (bound boson Fermi-Dirac particles) as original energy particles representing the apex of the spectral ladder and the beginning of the second energy cascade. Here quench factors freeze persistent superconducting current vibrations into place prior to application of the algorithmic ladder of the quantum field theory time line. Energies evolve to include the formation of std model physics (QM,QED,QCD) general theory of relativity (GRT), special theory (SRT), linear momentum, and angular momentum, etc.   

Video Crosstalk in Kepler CCDs

Kepler is the first mission capable of detecting Earth-size planets in the habitable zone of solar-like stars, and is at the forefront of the exciting field of extrasolar planets. Kepler looks for planetary transits in F-M main-sequence stars, ranging from 7$^{th}$ to 14$^{th}$ magnitude. I investigate video crosstalk, which is the noise due to coupled readout of CCDs. Video crosstalk is modeled as a linear coefficient multiplied by the signal from the source CCD, and can be as large as 0.066{\%} of the signal. The transit of an Earth-size planet in Kepler's field of view is a 0.01{\%} drop in flux, so video crosstalk can significantly skew planet mass estimations. It produces both positive and negative images, and is not symmetric between two CCDs.~ While the exact cause for this phenomenon remains to be clarified, the undesirable effects of video crosstalk have been mitigated, and specific stars in Kepler's field of view with high amounts of crosstalk have been flagged for more thorough analysis of flux measurements.   

An Overview of Hubble's Newest Addition: WFC3

The newest camera on the Hubble Space Telescope (HST), Wide Field Camera 3 (WFC3), opens a new window into the Universe. WFC3's two channels (UVIS and IR) provide HST with improved imaging capabilities in the ultraviolet and near-infrared wavelengths and allow for clearer and more detailed images of the universe compared to previous generations of instruments. A review of instrument specifications, features, and operation of the IR and UVIS channels work is provided. Advancements of WFC3 are described, including enhancements of CCD and IR detector technology. WFC3 has many benefits and few drawbacks compared to past and future instruments. With improved spatial resolution, a large field of view, and reduced noise, WFC3 is capable of reaching many scientific goals for the first time. New science goals include studies of galactic evolution and improved resolution of high redshift galaxies. I will illustrate the capabilities of WFC3 with specific examples relating to the study of galaxy formation.   

Theoretical Derivation of Equations Governing the Coupled Distributions of Dark and Baryonic Matters

A dark matter (DM) particle is yet to be detected. Milgrom's modified Newton dynamics (MOND) successfully models much of the observed DM phenomena. Unfortunately, the modification conflicts with general relativity. Exploring here an alternative model of DM leads to the sequential derivation of an equation of state (EoS) for such DM in a gravitation field, of an equation governing the coupled distributions of DM and baryonic matter around galaxies, of galactic flat rotation curves, of the Pioneer anomaly, of a Tully-Fisher relation, of a possible mechanism of black hole formation at the center of a large galaxy, and of Milgrom's MOND relation. The conflict between the PA and the Viking ranging data is resolved in an appendix. Results are discussed.   

A Different Reason Why Black Holes are Black

Although it is true that black holes appear to be black on the outside due to the fact that the escape velocity from the event horizon is even higher than that of light, black holes may be black on the inside as well. A recent paper by Zach Adams (2009) presents a new model which provides evidence of gravitons actually being a result of a fusion of 2 photons, which manifests in 4-D space. In fact, the duality between gravitons and photons has been suggested in earlier works as well. Falling Photon Experiment shows that as photons approach a massive body, their energies increase, and their wavelengths decrease. Photon-graviton conversions occur when the wavelengths of photons decrease to Planck's length. As a result, the photons approaching the event horizon of a black hole may gain energy enough for photon pairs to fuse and become gravitons. Therefore, as we will discuss in this work, there exists a probability that photons cannot survive within the event horizon of a black hole. It is true that nothing can escape a black hole, which is the reason why it looks black on the outside, but also the possibility that photons may not be able to survive on a black hole means that black holes may be black on the inside as well.   

Quantum trajectories for entanglement phenomena

Quantum trajectories are used to investigate entangled systems. Herein, we present a procedure or recipe for applying the quantum trajectory representation to entanglement. We may synthesize the wave function for an entangled system from the wave functions of the individual anyons comprising the entangled system. The reduced action (generator of the motion) for the entire entangled system may be extracted from entangled system's wave function rather than from solving the quantum Hamilton-Jacobi equation if, for sufficiency, all the wave functions for the individual anyons are complex. Applying Jacobi's theorem to reduced action renders the quantum trajectory for the entangled system. We exhibit quantum trajectories for entangled systems that give insight into EPR, wave packet spreading, and the quantum Young's experiment. Dissection of the trajectory equation for the entangled system reveals the emergence of an ``entangalon" that maintains entanglement within the system.   

Tutorial Device Illustrating A Spin One-Half Object

Hawkins (The Universe in a Nutshell) illustrates a spin 1 and a spin 2 object with the ace of spades and queen of spades playing cards. Rotation by 360 and 180 degrees respectively reproduce the original appearance of these two objects. A spin 1/2 object requires a rotation of 720 degrees to reproduce the original appearance. Apart from a Moebius band such object can be constructed. However, it differs from the above mentioned playing cards by the fact that it has an internal mechanism operating on the original picture. It is thus somewhat different than the original two illustrations.   

Acoustic Attraction

A sound source of finite size produces a diverging traveling wave in an unbounded fluid. A rigid body that is small compared to the wavelength experiences an attractive radiation force (toward the source). An attractive force is also exerted on the fluid itself. The effect can be demonstrated with a styrofoam ball suspended near a loudspeaker that is producing sound of high amplitude and low frequency (for example, 100 Hz). The behavior can be understood and roughly calculated as a time-averaged Bernoulli effect. A rigorous scattering calculation yields a radiation force that is within a factor of two of the Bernoulli result. For a spherical wave, the force decreases as the inverse fifth power of the distance from the source. Applications of the phenomenon include ultrasonic filtration of liquids and the growth of supermassive black holes that emit sound waves in a surrounding plasma. An experiment is being conducted in an anechoic chamber with a 1-inch diameter aluminum ball that is suspended from an analytical balance. Directly below the ball is a baffled loudspeaker that exerts an attractive force that is measured by the balance.   

Effect of Dynamic Compression on Accelerating and Sustaining Over-Driven Explosive Detonation

A novel circumferential initiation technique is used to create pseudo-steady-state convergence conditions at rates faster than those attainable by conventional means. Once established, the convergent front envelops and pre-compresses un-reacted explosive to a continuum of higher von-Neumann spike condition prior to chemical reaction. The mechanism will be described along with specific experiments with high-energy and extremely insensitive explosives. Measured velocity increases achieved to-date range between 35 and 65 percent faster than Chapman-Jouguet: Predicted peak pressures increases are greater than 300 percent and well-beyond the detection range of traditionally employed piezo-electric gauges. Very good correlation between experimentally observed detonation front geometry and computational modeling will also be shown. The background of work leading to these accomplishments and details of the experimentation and simulation will be reported. The Office of Navel Research supported this work.   

Session F2: High Energy/Nuclear Accelerators and Plasma Physics

Development of Mirrors for the CLAS12 High Threshold Cerenkov Counter

The Thomas Jefferson National Accelerator Facility (JLab) has begun an ambitious program to upgrade its beam energy from 6~GeV to 12~GeV. CLAS, one of the detectors at JLab, is being upgraded (to ``CLAS12'') to accommodate the higher energy. The existing \v{C}erenkov counter in CLAS will be unable to distinguish electrons from pions at the higher beam energy, which necessitates the construction of a new, High-Threshold \v{C}erenkov Counter (HTCC). An important part of the HTCC is the light collection system, which utilizes high-quality, extremely lightweight mirrors to reflect the \v{C}erenkov light to a set of photomultiplier tubes located at the back of the detector. To ensure uniformity in performance, it is important to simplify as much as possible the construction of these mirrors. This talk will discuss the properties of CLAS12 and the HTCC, and will describe the technique to be used in the construction of the mirrors.   

LHC Status and Upgrade Challenges

The Large Hadron Collider has had a trying start-up and a challenging operational future lays ahead. Critical to the machine's performance is controlling a beam of particles whose stored energy is equivalent to 80 kg of TNT. Unavoidable beam losses result in energy deposition throughout the machine and without adequate protection this power would result in quenching of the superconducting magnets. A brief overview of the machine layout and principles of operation will be reviewed including a summary of the September 2008 accident. The current status of the LHC, startup schedule and upgrade options to achieve the target luminosity will be presented.   

Identifying non-photonic electrons in Pb+Pb collisions with ALICE at the LHC

One useful method for probing a quark-gluon plasma is through analysis of partonic energy loss, which is a direct indicator of the color charge density of the plasma medium. Electrons coming from the decays of heavy quarks, the so-called ``non- photonic'' electrons, should be sensitive to the differences in partonic energy loss for heavy and light quarks, and yet observations at RHIC suggest that they are as suppressed as light hadrons. This talk will overview the capabilities of the ALICE Experiment at CERN to detect non-photonic electrons, particularly emphasizing the performance of the Electromagnetic Calorimeter (EMCal), which will be used to investigate the flavor-dependence of partonic energy loss in Pb+Pb collisions at the LHC.   

Identification of bottom quark jets in Pb+Pb collisions in ALICE at the LHC

Partonic energy loss of high transverse momentum (pT) quarks and gluons in the Quark-Gluon Plasma has been inferred from the suppression of high pT hadrons observed in heavy ion collisions at RHIC. In order to learn more about the details of this energy loss, one would like to separately measure the effects on quarks and gluons. One way to do this is by identifying heavy quark jets through the semileptonic decay to electrons of bottom hadrons produced in the jet. This talk will show how results from the ALICE tracking system and electromagnetic calorimeter (EMCAL) can be used to identify heavy quark jets by identifying candidates containing electrons that satisfy a displaced vertex cut or those with large transverse impact parameter significance.   

Charged hadron spectra for Cu+Cu collisions at $\sqrt{S}=22.4 GeV$ with the STAR detector at RHIC

Identified charged particle spectra of $\pi\pm$, $K\pm$, $p$ and $\bar{p}$ measured using energy loss in the STAR TPC are reported for $|y|<0.1$ for Cu+Cu collisions at $\sqrt{S} = 22.4 GeV$. Total particle production, particle ratios, and average transverse momenta, are presented for different collision centralities. These results are compared with previously published results from collisions of different systems at similar collision energies.   

STAR as a fixed target experiment?

Collisions between gold or copper ions in the RHIC beam with aluminum nuclei in the beam pipe allow us to analyze fixed target interactions with the STAR detector at RHIC. These lower energy fixed target collisions may allow us to extend the low energy reach of the RHIC beam energy scan and possibly improve the chance of finding the critical point of the hadronic to quark matter phase boundary. In this talk, we will present preliminary results of spectra analysis for the fixed target collisions from various low energy test runs. Moreover, the viability of doing fixed target experiments with a collider detector will be discussed.   

Exact treatment of confinement in the semirelativistic Faddeev approach to three-quark problems

The spin=1/2 elementary particles, the baryons, are mostly described as three-quark configurations. The quarks obey relativistic quantum mechanics. Their mutual interaction is modeled by infinitely rising potentials whose short-range nature is mediated by the exchange of Goldstone bosons. We solve the relativistic three-quark problem by using the Faddeev method. In the Faddeev method we break the wave function into components, and the components satisfies somewhat better integral equations. Nevertheless, the solution was not possible without approximating and violating the asymptotically rising potential. In this work we overcome this problem. We devised an approximation method, which allows the exact calculation of the Green's operator of an asymptotically rising potential with semirelativistic kinetic energy operator by using matrix continued fractions.   

Semi-Analytical Approach to the Gravitational Wave Signal From the Electroweak Phase Transition in General Standard Model-like Effective Potentials

Upcoming gravitational wave (GW) detectors might detect a stochastic background of GWs possibly arising from bubble collisions and turbulence from a strongly first-order electroweak phase transition (EWPT). We investigate whether it is possible to connect, via a semi-analytical approximation to the tunneling rate of scalar fields with quartic potentials, the GW signal with the parameters entering the potential that drives the EWPT. We consider a finite temperature effective potential similar to the Higgs potential in the Standard Model (SM). In the context of a semi-analytic approximation to the three dimensional Euclidean action, we derive a general approximate form for the tunneling temperature and the relevant GW parameters. We explore the GW signal across the parameter space of the potential. We comment on the potential detectability of a GW signal with future experiments, and physical relevance of the associated potential parameters from extensions to the SM. In particular we consider singlet, triplet, higher dimensional operators, and top-flavor extensions to the Higgs sector of the SM. We find that the addition of a temperature independent cubic term in the potential, arising from a gauge singlet for instance, can greatly enhance the GW power. The other parameters have milder, but potentially noticeable, effects.   

CUORE: Cryogenic Maintenance

CUORE (Cryogenic Underground Observatory for Rare Events) will be the largest detector used to investigate neutrinoless double beta decay in tellurium-130 (Te-130). Neutrinoless double beta decay has never been observed in nature. If detected, it would be a major scientific discovery indicating that the neutrino is its own antiparticle; this breakthrough would signal a fundamental revision to the Standard Model of physics. Located in Assergi, Italy at the Gran Sasso National Laboratory (LNGS), CUORE will be a cryogenic bolometer composed of 988 tellurium dioxide (Te02) crystals with a total mass of 750 kg. Over the summer of 2009, we traveled to the LNGS to assist the CUORE Collaboration by performing standard shifts for the Three Towers Test, a diagnostic experiment used to determine optimal hardware cleaning methods. This involved refilling the cryogenics system with liquid helium coolant to keep the crystal bolometers at an operating temperature of approximately 10 mK, and other routine tasks. This work was supported in part by the NSF RUI grant PHY-0653284.   

New results from ADMX -- an ultra sensitive axion detection experiment

Axions are hypothetical pseudoscalar particles that exist as a consequence of the Peccei-Quinn solution to the strong-CP problem. Light axions (ueV-meV) are also a natural cold dark matter candidate. One important detection technique is via resonant conversion to microwave photons in a high-Q cavity immersed in a strong magnetic field. In this class of experiment, the signal from the cavity is amplified by an ultralow noise amplifier, and mixed down to the audio frequency range using a double-heterodyne receiver. The power spectrum results by a Fast Fourier Transform, with the putative axion appearing as a narrow line at a frequency corresponding to its rest mass. This detection strategy provides the basis for the Axion Dark Matter eXperiment (ADMX) which has been taking data at Lawrence Livermore National Laboratory (LLNL) since 1996. ADMX has established limits in two distinct data channels - a medium resolution channel configured to search for ``thermalized'' axions and a high resolution channel for detecting axions that have recently fallen into the gravitational well of our galaxy. This talk will present an overview of the newly reconfigured experiment featuring an ultralow-noise first stage cryogenic SQUID amplifiers and present latest results from the two data channels.   

Development of a Compact Neutron Generator to be Used For Associated Particle Imaging Utilizing a RF-Driven Ion Source

The development of a prototype compact neutron generator for the application of associated particle imaging (API) to be used for explosive and contraband detection will be presented. The API technique makes use of the 3.5 MeV alpha particles that are produced simultaneously with the 14 MeV neutrons in the deuterium-tritium ($^{2}$D($^{3}$T,n)$^{4}\alpha )$ fusion reaction to determine the direction of the neutrons and reduce background noise. This method determines the spatial position of each neutron interaction and requires the neutrons to be generated from a small spot in order to achieve high spatial resolution. In this work an axial type neutron generator was designed and built with a predicted neutron yield of 10$^{8}$ n/s for a 50 $\mu $A D/T ion beam current accelerated to 80 kV. It was shown that the measured yield for a D/D gas filled generator was 2x10$^{5}$n/s, which scales to 2x10$^{7}$ n/s if a D/T gas fill is used. The generator utilizes an RF planar spiral antenna at 13.56 MHz to create a highly efficient inductively coupled plasma at the ion source. Experimental results show that beams with an atomic ion fraction of $>$ 80{\%} can be obtained with only 100 watts of RF power in the ion source. A single acceleration gap with a secondary electron suppression electrode is used in the acceleration column, to suppress secondary backscattered electrons produced at the target. Initial measurements of the neutron generator performance including the beam spot size and neutron yield under sealed operation will be discussed, along with suggestions for future improvements.   

Bound-Compton profiles for inelastic x-ray scattering in warm, dense matter

Inelastic x-ray scattering has recently been developed as a powerful diagnostic method for determining the densities and temperatures of warm dense matter. Accurate measurements require determination of the spectral bound-free Compton profile. Thus, improved models of bound-free transitions are of great interest to correctly infer the inelastic scattering component from bound and free electrons, particularly in mid-Z systems. We present inelastic scattering spectra taken from un-shocked samples of Boron at the Advanced Photon Source (APS) synchrotron and laser-driven samples at LLNL's Titan laser. These spectra are compared with profiles calculated within the impulse approximation. These measured profiles provide an important tool for analyzing scattering in warm, dense systems. Additionally, they will be used as a benchmark for an improved self-consistent-field model of bound-free transitions currently in development.   

Spectrally Resolved X-ray Scattering from Implosion Targets

Spectrally resolved x-ray Thomson scattering has been applied at the Omega Laser Facility to investigate the capsule adiabat of cone-in-shell targets. The technique of scattering from implosion targets was developed and tested for use as a diagnostic at the National Ignition Facility (NIF), LLNL. Measurement of the adiabat is applied to test low-adiabat pulse shaping methods, designed for optimum compressibility and stability. Theoretical equation of state models (EOS) can also be studied for conditions encountered during implosion. In these experiments, the noncollective, or microscopic behavior of the plasma, was probed with a Zn He-alpha x-ray source at a scattering angle of 113$^{\circ}$. For these degenerate plasmas, the width of the inelastic scattering peak is proportional to the Fermi energy, and thus the electron density. The electron temperature is obtained from the measured intensities of the elastic and inelastic features. In-flight scattering measurements yielded electron densities of $\sim $ 1.2x10$^{24}$cm$^{-3}$, temperatures of $\sim $10 eV, and an ionization state of C(+4)H(+1). This work was performed under the auspices of the DOE by LLNL under Contract No. DE-AC52-07NA27344, LDRD 08-ERI-003, and the Nat. Lab. User Fac. Prog.   

Evolution of Elastic X-ray Scattering in Laser-Shocked Warm Dense Li

Li foils were heated and compressed using shock waves driven by 4 ns long laser pulses. Separate 1 ns long laser pulses were used to generate a bright source of 2.96 keV Cl Ly-$\alpha$ photons for near-elastic x-ray scattering. Comparison with radiation hydrodynamics simulations shows that the plasma is highly coupled during the first several nanoseconds, then relaxes to a moderate coupling state at later times. Our main finding is that the near-elastic scattering amplitudes are quite sensitive to the mean ionization state $\overline{Z}$, and by extension to the choice of ionization model in the radiation-hydrodynamics simulations used to predict plasma properties within the shocked Li.   

The Search for Neutron Oscillations at Super-Kamiokande

Neutron oscillations are predicted by theories that attempt to unify the fundamental forces of nature. This knowledge along with the enhanced sensitivity of the detectors used to observe neutron oscillations has increased interest in a search for this phenomenon. I, Mark Gregg, will be presenting research that is being conducted at California State University, Dominguez Hills involving the Super-Kamiokande nucleon decay detector and neutrino observatory and collaboration under the supervision of Dr. Kenneth Ganezer. This research consists of the work I have conducted along with my colleagues on the Monte Carlo program that simulates the physical events expected to be seen in the detector as a result of neutron oscillations. I will also describe the overall experiment and the latest results obtained by this experiment for lifespan and oscillation time lower limits of neutrons bound in oxygen nuclei and free neutrons, respectively.   

Session F3: Materials/Nanomaterials Science

Atomic and electronic structures of GaN:ZnO Alloys

GaN:ZnO is a new class of alloy which currently holds the record for the efficiency of water photo-splitting. The mechanism of the large band gap bowing of this alloy and its detailed atomic structure, which are essential to understand the remarkable performance, however, are still not clear. We developed a model Hamiltonian describing the ab initio energies of different alloy atomic configurations and used it in Monte Carlo simulations to study the atomic structures of systems containing thousands of atoms. The equilibrium atomic structures from the MC simulations at different temperatures are then used to calculate their electronic structures. We found that at the experimental synthesis temperature of 1100 K, uniform alloy can be formed, albeit with a strong short range ordering. Consequently, their electronic structure is very different from the completely random alloy. Based on our calculation, we also predict that higher synthesis temperature can yield even lower energy band gap.   

GW study of the half metallic band gap of zinc blende CrAs

We determined the semiconducting gap of zinc blende (ZB) CrAs within the $GW$ approximation ($GW$A). This is the first $GW$ calculation of a half-metal. Previous calculations using density functional theory within the generalized gradient approximation (GGA) determined a gap of 1.8 eV, but the GGA is known to give too small of a value for this quantity in semiconductors. Additionally, since ZB CrAs is a half metal, one of its spin channels behaves like a metal and changes the quasiparticle screening compared to the insulating case. Due to the local field effect, we only included the $\Gamma$-point term in the metallic channel calculation of the polarizability while keeping the full set of terms in the insulating channel $GW$ calculation. Preliminary results suggest these terms from the polarizability produce little change in the value of the semiconducting gap when compared to the ``full'' $GW$A calculation.   

Creating wide-band negative-index-of-refraction metamaterials with fractal-based geometry

A burgeoning topic of modern research in electrodynamics and antenna design is the design and fabrication of ``left-handed'' metamaterials. This ``left-handedness'' is often created through use of an array of conductive structures with geometry appropriate for coupling on the wavelength scale with incident radiation to produce a phase-shifted reflected wave that cancels out incoming radiation and prevents transmission. This property has been demonstrated in several papers published in the last decade. In every instance, though the ``left-handed'' response is only exhibited in a small bandwidth centered about a specific frequency (bandwidth typically less that 0.1 GHz). I will show that through use of tessellated, fractal-based structures, one can create a repeatable geometry that exhibits a negative index of refraction (NIR) for multiple frequency bands, limited only by fabrication precision, with the ultimate goal being a wide-band absorptive response.   

Influence of Nanostructuring and Heterogeneous Nucleation on the Thermoelectric Figure of Merit in AgSbTe2

Thermoelectric materials directly interconvert heat and electricity in the solid state. In some cases, nanoscale microstructures improve thermoelectric efficiency, but this phenomenon has rarely been studied systematically for precipitates in bulk materials. We quantified the influence of nanostructuring on the thermoelectric figure of merit (zT) by embedding Sb2Te3 inclusions, from nanometer to micron sizes, in an Sb-rich AgSbTe2 matrix through solid-state precipitation. Nucleation/growth and coarsening regimes of precipitate formation had a clear effect on transport properties, which could be understood using the effective medium theory of a two-phase composite. The majority of precipitates nucleated heterogeneously at grain boundaries and at planar defects found in the matrix phase, forming a complex interconnected network. This heterogeneous nucleation causes the precipitate/matrix system to follow effective medium theory even at small precipitate sizes, thus lowering zT. Therefore, heterogeneous nucleation is a major obstacle to zT improvement using nanoscale precipitates in bulk thermoelectrics.   

Electro-optical properties of quantum dots dispersed in chiral nematic liquid crystal

The electro-optical properties of quantum dots can be significantly altered if they are assembled in close proximity to each other. The partial ordering of liquid crystal molecules can be utilized to form directed quantum dot assemblies.~ Typically, this results in a red shift in the emission spectrum of the dots as the induced order leads to enhanced dipolar interactions, resulting in electronically coupled states. Spherical cadmium selenide quantum dots of different diameters are dispersed in various concentrations in a chiral nematic liquid crystal phase.~ The quantum dots are seen to aggregate, the sizes of the aggregates depend on the size and concentration of the dots as well as the mixing time. Optimum mixing times and quantum dot concentrations are determined for dots of different sizes to achieve a uniform quantum dot dispersion. Quantum dots with emission peaks ranging from 490 nm to 640 nm were studied using polarized optical microscopy and scanning microscopy photoluminescence measurements.   

Comparative analysis of the hydrogen-vacancy interaction in Mg and Al based on density functional theory

The interactions of vacancies (V) with atomic hydrogen (H) in the bulk of the metal are expected to play an important role in H-storage as well as H-embrittlement. Using density functional theory we have studied the H-V interactions in hcp-Mg and fcc-Al, two prototypic systems for H storage. We show that a single V can in principle host up to 9 H atoms in Mg and 10 in Al. In going beyond previous theoretical studies we further evaluate the concentration of the H-V complexes for different H loading conditions -- ranging from low pressures to high pressures of H2 gas. We find significant differences between Mg and Al. In the case of Al, up to 15 {\%} of H atoms are trapped in single vacancies even for very low H pressures, which strongly slows down the diffusion of H atoms. In the case of Mg, these trapping effects are negligible for low H pressures. However, vacancies containing multiple H atoms and H-induced superabundant vacancy formation are predicted to occur in Mg at much lower H loading pressures (about 1 GPa) than in Al (about 10 GPa).   

Strain-induced isosymmetric phase transition in multiferroic BiFeO$_3$

We examine the effect of large epitaxial strain on multiferroic bismuth ferrite, BiFeO$_3$, using density functional calculations. We investigate a previously unidentified phase transition induced by experimentally accessible values of compressive strain. The transition occurs between phases that are isosymmetric yet have dramatically different structures and properties, the most notable of which is a strong enhancement and rotation of the electric polarization. This presents the opportunity to shift the transition boundary with an applied electric field, similar to a morphotropic phase boundary. Our work contributes to the limited body of knowledge about isosymmetric transitions and explains recent experimental reports of morphotropic phase boundary-like behavior in highly strained films of BiFeO$_3$ (Zeches {\em et al.}, {\em to appear} Science (2009)).   

Superparamagnetic Magnetite Nanoparticles for Optical Modulation/Chopping

We demonstrate proof of concept operation of superparamagnetic magnetite nanoparticles and magnetite-TiO$_{2}$ peapod-superstructures for laser intensity optical modulation and chopping. The frequency of the modulation is shown to be twice that of the driving signal and a function of the size of the particles. Specifically, optical modulation with round nanoparticles of sizes 80, 130, 200 nm is compared with optical modulation with magnetite-TiO$_{2}$ peapod-superstructures of sizes of around 1 $\mu$m. The former gave rise to modulations of up to 2 kHz of frequency--a number comparable to that of the commercial optical choppers--, the latter up to 100 Hz. We also show that particle shape asymmetry and anisotropy enhance optical modulation.   

Phonon Transport in Graphene: Umklapp Quenching and Heat Conduction

Since its exfoliation, graphene attracted tremendous attention of the research community. Graphene, which consists of a single atomic plane of carbon atoms, revealed many unique properties including extremely high electron mobility. In this talk I will show that unusual properties of graphene are not limited to electrons alone. Phonons also behave differently in two-dimensional (2D) system such as graphene. We have recently discovered experimentally that thermal conductivity of suspended graphene layers is extremely high and exceeds that of diamond or graphite [2-3]. We explained our results theoretically by considering the Umklapp and edge scattering of phonons in graphene [3]. Unlike in bulk graphite, the phonon transport in graphene is pure 2D for all phonon energies. As a result, the thermal conductivity of graphene can become extremely high. The extraordinary high thermal conductivity of graphene can be used for thermal management of nanoscale electronic devices. This work was supported by SRC-DARPA Functional Engineered Nano Architectonics (FENA) center and Interconnect Focus Center (IFC). [1] A.A. Balandin, et al. Nano Letters, 8, 902 (2008); S. Ghosh, et al., Appl. Phys. Lett., 92, 151911 (2008). [2] D.L. Nika, et al., Phys. Rev. B, 79, 155413 (2009); D.L. Nika et al., Appl. Phys. Lett., 94, 203103 (2009)   

Computational Approach for Quantifying Structural Disorder in Biomolecular Lattices

We have developed a novel computational approach for quantifying structural disorder in biomolecular lattices with nonlinear susceptibility based on analysis of polarization-modulated second harmonic signal. Transient, regional disorder at the level of molecular organization is identified using a novel signal-processing algorithms sufficiently compact for near real-time analysis with a desktop computer. Global disorder and regional disorder within the biostructure are assessed and scored using a multiple methodologies. Experimental results suggest our signal processing method represents a robust, scalable tool that allows us to detect both regional and global alterations in signal characteristics of biostructures with a high degree of discrimination.   

Studies of singly doping of Me and Fe in Si to deduce simple guidelines in selecting transition metal elements for Si-based spintronic materials

Single dopings of Mn and Fe in Si are investigated using 8-, 64-, and 216-atom supercells and a first-principles method based on density functional theory. Atomic sizes play an essential role in determining the contraction or the expansion of neighboring atoms around the transition metal element at a substitutional site. At a tetrahedral interstitial site, there is only expansion. Magnetic moments/transition- metal-element at the two sites are calculated. Physical reasons for these properties are given. Some guidelines for selecting transition metal elements doped in Si for future Si-based effective spintronic materials are proposed.   

Faraday Effect in Magnetic and Non-Magnetic Colloidal Nanoparticles in Water

We have investigated Faraday Effect in a variety of nanoparticle solutions. Verdet constant of superparamagnetic nanocrystal clusters of magnetite (Fe3O4), diluted in water, is measured as a function of particle size. Particle sizes ranging from 3 to 210 nm, resulted in a nonlinear size dependence in Verdet constant. The relationship between Verdet constant and particle size is possibly due to variation in magnetic domain sizes within the particles. Domain size evolution investigations are underway using X-ray diffraction. Non-magnetic nanoparticle solutions investigated consisted of silver, silver oxide, magnesium oxide, nickel oxide, and carbon nanotubes. Solutions demonstrated diamagnetic and paramagnetic properties, as expected. We believe that Faraday Effect is an efficient method of investigating magnetic properties of nanoparticles.   

Computational study of the adsorption of methanol, formic acid, and formaldehyde on the $\beta $-SiC(100)-3x2 surface

The absorption of methanol, formic acid, and formaldehyde on the Si-rich $\beta $-SiC(100)-(3x2) surface has been studied using density functional theory (DFT) computational methods and small clusters to model the surface reactivity. A single cluster dimer model is used to calculate energies after the interaction of adsorbates on the surface. The dissociative adsorption of methanol on the SiC(100)-3x2 surface is predicted to take place facilely, giving rise to Si-OCH$_{3}$ and Si-H surface species and followed a path similar to that predicted for Si(100)-2x1 surface. The reaction is highly exothermic and predicted to occur with essentially no barrier. Formaldehyde is also predicted to adsorb with essentially no barrier on the SiC(100)-3x2 surface with formation of a 4-member ring on the surface. This adsorption is also exothermic and similar to the corresponding Si(100)-2x1 surface. This result shows that the carbonyl group can undergo cycloaddition onto the SiC(100) surface. Formic acid is also predicted to undergo dissociative chemisorption on the SiC(100) surface with the formation of Si-OCOH and Si-H surface species. This process is also highly exothermic (-283.1 kJ/mol) and essentially barrierless.   

Band-gap bowing, band offsets, and electron affinities for AlN, GaN, InN and InGaN: A DFT study

AlN, GaN, and InN and their alloys are successfully being used in optical, electronic, and photovoltaic devices; a novel application is for photochemical water splitting. In order to further improve nitride-based devices a detailed understanding of the materials properties as a function of alloy composition is needed. To obtain such insight we have investigated the band gap and absolute band positions of AlN, GaN, InN and InGaN using density functional theory. The HSE exchange correlation functional has been used in order to accurately calculate the electronic band structure [1]. Detailed surface calculations have been performed that, combined with bulk calculations for alloys, yield information about the positions of valence and conduction bands on an absolute energy scale. We will discuss bowing effects, band offsets, and electron affinities in light of the application for photochemical hydrogen production. \\[4pt] [1] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003)   

Strain effect in group-III nitride semiconductors and their alloys

Strain plays a crucial role in group-III nitride semiconductor based devices since it affects the band structure near the valence- and conduction-band edges and thus the optical properties and the device characteristics. However, the deformation potentials that describe the change in band structure under strain have not yet been reliably determined. We present a systematic study of the strain effects in AlN, GaN and InN in the wurtzite phase. We apply density functional theory and hybrid functionals to address the band-gap problem. We observe nonlinearities of transition energies under realistic strain condition that may, in part, explain the appreciable scatter in previous theoretical work on deformation potentials of group-III-nitrides. For the linear regime around the experimental lattice parameters, we present a complete set of deformation potentials. Applying our deformation potentials, we study strain effects in InGaN alloys (including c-, m-, and semi-polar planes) grown on GaN substrates. We make predictions for the transition energies in these systems and their dependence on In composition.   

Session F4: Condensed Matter I

Imaging Transport in Nanowires with NSOM

A novel system has been developed for the imaging of carrier transport within semiconductor nanostructures by operating a near field scanning optical microscopy (NSOM) within a scanning electron microscope. Luminescence associated with carrier recombination is collected with high spatial resolution to monitor the motion and recombination of charge generated by use of an electron beam as an independent point source. Light is collected in the near field from a scanning fiber using tuning fork feedback in an open architecture combined AFM/NSOM system allowing for independent motion of sample and tip. From a single image, it is possible to obtain a direct measure of minority carrier diffusion length. This technique has been used in the near-field collection mode to image the diffusion of holes in n-type GaN-AlGaN core-shell nanowires, grown via Ni-catalyzed MOCVD. Measurements were made on tapered nanowires ranging in diameter from 500 to 800 nm, with lengths up to $\sim 30 \mu m$. The average 1-dimensional carrier diffusion length was measured to be 1.2 +/- 0.2 $\mu$m in the low injection limit. In addition, it is possible to map the luminescence that is wave-guided to the end of the structure, imaging waveguide modes   

Little and Large:Topological Defects in Cosmology and Condensed Matter Theory

Cosmology and condensed matter theory seem to be worlds apart, and yet are ubiquitously linked. Testing our current understanding of phenomena that occur on galactic scales can now be realized in the laboratory. The coming-together of cosmology and condensed matter theory is facilitated by the phase transitions and defect formation that is common to both areas. A recurring question in cosmology has concerned whether the vacuum is empty or contains vortex-strings or other topological defects. Understanding the formation and evolution of these topological defects plays a significant role in our understanding of cosmology and the early universe. Condensed matter systems provide an important starting point to studying the phenomena of phase transitions and the formation of topological defects. In both the cosmological and condensed matter scenarios, symmetry breaking causes a change to a degenerate vacuum manifold with non-trivial topology. This occurrence will be discussed along with experimental results in superfluid Helium and superconductors.   

Search for anomalous spin-mass coupling with a rubidium magnetometer

We report on progress of our experiment using a dual-isotope rubidium magnetometer to search for a hypothetical long-range coupling between Rb nuclear spins and the mass of the Earth. The valence electron dominates magnetic interactions and serves as a precise co-magnetometer for the nuclei in a simultaneous measurement of Rb-85 and Rb-87 spin precession frequencies, enabling accurate subtraction of magnetic perturbations. The construction and optimization of the apparatus is nearly complete, and we are now addressing several technical sources of noise and studying potential sources of systematic error. The optimized dual-isotope Rb magnetometer has sufficient shot- noise-projected sensitivity to improve experimental limits on long-range spin-mass couplings by an order of magnitude in general and by two orders of magnitude for the proton spin in particular.   

Light Propagation in Liquid Crystals with a Chiral Dopant

This project will investigate the design and feasibility of a novel liquid crystal sensor that could be used to detect the presence and amount of foreign biological and/or chemical airborne agents. Such a sensor would have the advantage of being very portable. As such could have particular value in detecting biological or chemical weapons in the field of military operations. It would also be of use in a rapid response to a chemical or biological terrorist attack. The device would operate on the basic principal that when certain types of molecules bind to a liquid crystal molecule, the conformation of the liquid crystal molecule changes. This would in turn lead to a change in the overall arrangement of the liquid crystal, which could be detected using polarized light. In the absence of a contaminant the average molecular direction (optical axis, $\hat{n}$ ) is constant throughout the liquid crystal. The dopant adds a chirality or twist so that $\hat{n}$ precesses as a function of depth. We first solve for the reflected and transmitted light off of the air-liquid crystal boundary in the simplified case where there is linear chirality or a spiral configuration which repeats itself over some fixed interval (or pitch). We then generalize for cases in which this repeat distance varies with crystal depth. Finally we will obtain an expression for the contaminated crystal configuration which should depend on time and a diffusion constant and examine how the light properties change with respect to intensity and duration of exposure to the contaminant.   

Towards microwave modulation in a wavelength-tuned magneto-optical trap

In this project, I present a new method for trapping Rubidium-87 atoms. The method proposed is microwave modulation of an external cavity diode laser. The modulation is designed so as to produce frequency sidebands for hyperfine pumping in addition to the main cooling frequency. It is designed for use in magneto-optical trapping.   

La-139 NMR in La$_{4}$Ni$_{3}$O$_{8}$: a possible analog to the cuprate high temperature superconductors

The Ni$^{1+}$/Ni$^{2+}$ states in the nickelates have identical electronic configurations as Cu$^{2+}$/Cu$^{3+}$ in the high temperature superconducting cuprates (3d$^{9}$/3d$^{8})$, and may exhibit similar properties. However, the Ni$^{1+}$ state is rare and cannot be easily stabilized. Recently, Martha Greenblatt and collaborators at Rutgers University have succeeded in growing a family of such compounds, Ln$_{n+1}$Ni$_{n}$O$_{2n+2 }$with a layered structure similar to the cuprates. The La$_{4}$Ni$_{3}$O$_{8}$ compound is particularly interesting as it undergoes an antiferromagnetic transition at T$_{N}$ = 100 K. We have done La NMR on powder samples to investigate the nature of this phase. Our spin lattice relaxation rate measurements clearly reveal a second order electronic phase transition similar to that observed in other antiferromagnets. Although we found clear signatures of changes to the spectra below T$_{N}$, we are unable to assign these changes to the presence of an internal field from the antiferromagnetic structure, or changes to the electric field gradient at the La site.   

Quantum Phases of Atom-Molecule Mixtures of Fermionic Atoms

Cold atom experiments have observed atom-molecule mixtures by tuning the interactions between particles.\footnote{M.L. Olsen, J. D. Perreault, T. D. Cumby, and D. S. Jin, Phys. Rev. A 80, 030701(R) (2009)} We study many particle interactions by examaning a simple model that describes the destruction of fermionic atom pairs to form single bosonic molecules and vice versa. A set of functional Renomalization Group equations\footnote{R. Shankar, Rev. Mod. Phys., Vol 66 No. 1, January 1994}$^,$\footnote{S.W. Tsai, A.H. Castro Neto, R. Shankar, D.K. Campbell, Phys. Rev. B 72, 054531 (2005)} describing these processes are set up and solved numerically. The Self Energy of the fermions are attained as a function of frequency and we search for frequency dependent instabilities that could denote a transition from a disordered liquid to a BCS phase. (Financial support from NSF DMR-084781 and UC-Lab Fees Research Program.)   

Knight Shift Probe of Onset of Coherence in Heavy Electron Superconductor CeIrIn5

The CeMIn$_{5}$ compounds, where M = Co, Ir, Rh, are a novel class of superconductors discovered about 10 years ago. The 115 compounds are Kondo lattice materials: the compound's conduction electrons are coupled to an ordered lattice of local moments through the Kondo effect. These materials have a large effective electronic mass at low temperatures. We present new NMR Knight shift data in single crystals of CeIrIn5 between 2K and 120K. We find that the Knight shift of the In(1) site in this material is proportional to the bulk magnetic susceptibility above a temperature T* $\sim $ 30(?)K. Below this temperature, the Knight shift fails to track the susceptibility. We interpret these results in terms of the two-fluid model, in which the susceptibility of the heavy electron component, chi{\_}cf, grows in intensity with decreasing temperature. We find that K{\_}cf $\sim $ chi{\_}cf $\sim $ log(T/T*), in agreement with other heavy fermion compounds. Our results confirm the predictions of the dynamical mean field theory calculations of Haule et al. for the onset of coherence in this compound.   

Cyclotron Resonance Vanishing effect in Correlated 2D Electron Systems

``Cyclotron Resonance - Vanishing effect'' (CRV) arise on magnetospectra of cyclotron resonance line (CR) as a well-defined gap that reduce to zero CR effect. CRV have been discovered due to experimental study of terahertz radiation transmission and photoresistivity magnetospectra at CR conditions in two-dimensional electron system (2DES) of GaAs/AlGaAs nanostructures with higher electron mobility at low (non-quantized Hall effect) magnetic fields. Unique experimental approach based on study of 2DES with photoresistivity and transmission techniques allows to get complementary data. One of the more significant results is that CRV-line shape (and consequently CRV effect) independent from testing THz power. We will discuss experimental study of ``CR- Vanishing effect'' and theoretical analysis that indicates on appearance of new fundamental correlated states of electrons at CRV conditions. To study CRV effect in detail we are working to create new model taking into account models for quantum Hall effect, magneto-plasma waves, non-linear zero-resistance states, and others that was develop for comparable experimental conditions.   

Steady-State and Transient Photoconductivity in the Poly(2,7-Carbazole) Copolymer PCDTBT, and in Bulk Heterojunction Composites with PC$_{70}$BM

We have studied the nature of carrier generation in an alternating donor-acceptor low bandgap copolymer and in composites of that polymer with a soluble fullerene derivative, using steady-state and transient photoconductivity. The Poly(2,7-Carbazole) copolymer PCDTBT that we studied represents a class of donor-acceptor copolymers that hold promise for photovoltaic applications because of the ability to tune the electronic energy levels by changing the acceptor unit (see Blouin, N.; Michaud, A.; Leclerc, M. \textit{Adv. Mater.} \textbf{2007}, $19$, 2295 - 2300). Photovoltaic devices fabricated from PCDTBT in composites with the soluble fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC$_{70}$BM) have exhibited a higher solar cell power conversion efficiency than has been achieved in P3HT based devices. In PCDTBT, the absorption extends out to 700 nm, with two distinct but broad absorption bands that are centered at $\sim $400 nm and $\sim $600 nm. We have used steady-state and transient photoconductivity to investigate the carrier generation and collection efficiency of PCDTBT and its composite with PC$_{70}$BM after photoexcitation at each of its distinct absorption bands.   

Local density of states and scanning tunneling currents in graphene

Graphene consists of an atom-thick layer of carbon atoms arranged in a honeycomb lattice and has been intensively studied due to its fascinating properties. We calculate the local density of states in graphene with different chemical substitution impurities, such as boron and nitrogen atoms, as well as for vacancies. We give exact analytical expressions for the local density of states for the whole energy range including energies beyond the Dirac cone approximation. The momentum maps of the local density of states for different impurities and discussion of their interpretation are given. We also present exact analytical calculations of scanning tunneling currents in locally disorded graphene using a multimode description of the microscope. [N.~M.~R. Peres, L. Yang, and S. - W. Tsai, New J. Phys. {\bf 11}, 095007(2009)]   

Arsenic nuclear magnetic resonance in CaFe2As2

We present $^{75}$As nuclear magnetic resonance measurements in the paramagnetic and antiferromagnetic states of CaFe$_{2}$As$_{2}$. Single crystals were produced using a Sn flux method and characterized via powder X-Ray diffraction, susceptibility, and specific heat measurements. The NMR data show that the internal hyperfine field and the electric field gradient change discontinuously at T$_{0}$ = 169 K. The observed hyperfine field is consistent with stripe antiferromagnetic ordering of the Fe spins in the a-b plane. Spin lattice relaxation data show metallic T$_{1}$$^{-1}$ $\sim$ T for T $\leq$ T$_{0}$/3. However, T$_{1}$$^{-1}$ shows a small peak at 10 K attributed to slow spin fluctuations that could indicate the emergence of antiferromagnetic domain wall motion.   

Growth of Iridium on Ge(111) Studied by STM and LEEM

Iridium on germanium is a system which is useful for understanding the interaction of 5d metals with semiconductors, with potential applications to electronic contacts. We have used both scanning tunneling microscopy (STM) and low energy electron microscopy (LEEM) to characterize the submonolayer growth of iridium onto Ge(111) as a function of coverage, deposition temperature, and annealing temperature. Ir deposited onto the Ge(111) c(2x8) surface forms a ($\surd $3x$\surd $3)R30$^{\circ}$ phase with the island size dependent upon substrate temperature during deposition. Deposition at a sample temperature of 670 C yields large micron-sized regions of continuous ($\surd $3x$\surd $3)R30$^{\circ}$ coverage, as seen by LEEM. Deposition at 400 C produces Ir islands of three different sizes, all of which are too small to be resolved in LEEM but can be easily observed in room temperature STM images: large islands of roughly 10 to 20 nm diameter, consisting of multiple layers; medium-sized islands of roughly 4 nm in diameter, and small islands about 1 nm in diameter. Heating the sample over 640 C yields islands of large enough size to be resolved with LEEM, with the island size dependent upon annealing temperature. Ostwald ripening was observed in LEEM movies.   

A model of electron spin relaxation momentum time in GaAs cyclindrical quantum dots: including the Dresshaus effect

A model of GaAs quantum dots embedded in a quantum wire is studied. We show that how the electron spin relaxation momentum time (SRT) is varying with some physical parameters. Under this model, a general conclusion is given : SRT decreases while the four parameters (external magnetic field, surrounding temperatures, quantum wire width and thickness) are increasing. The phenonmena is understood by more and more phonon modes resulted in a higher electron-phonon scattering probability when the system is under high magnetic field and high temperature. Thus the SRT is reduced. The most important reason for us to study such topics is that it is related with quantum information processing ability. In the present model, we deal with rectangular acoustic strain with deformation potential theory including the Dresshaus effect. Next step, we want to deal with how the SRT varies of quantum dots under very low temperature. A model of piezoelectric scattering with cyclindrical acoustic strain is considered in which the ionic displacement field (IDF) based on Born-Huang equation is shown. We are working on this line.   

Scanning Conductive Force Microscopy for Characterization of Model Molecular Devices

We have used scanning conductive atomic force microscopy as a tool to characterize molecular devices. Using self-assembled monolayers such as n-decanethiolate or n-octadecanethiolate as a matrix, we inlaid device components such as dendrimers or gold nanoparticles. All model systems were studied under constant force mode in air, while topography, lateral force, and current images were acquired. This configuration enables high resolution atomic force microscopy imaging, such as revealing of etch pits among the self-assembled monolayers, which is otherwise difficult to attain. In addition, the local conductivity can be correlated with the topographic features such as thiolate domains or surface defects. Both the technical development associated with this method and the detailed results will be discussed.