Seminars:Fall 2008
Department of Physics and Astronomy
George Mason University, Fairfax, VA 22030
Location: Room 204, Innovation Hall
Day and Time: Friday, 11:30-12:30 (unless otherwise indicated)
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Tues, Sept 2. 11:00, 301 Res I: Carlos A. R. Sa de Melo, Department of Physics, Georgia Institute of Technology
When condensed matter met atomic physics: The story of their wedding cake…
Over the last few years, condensed matter and atomic/molecular physics have been merging as many-body phenomena involving bosonic or fermionic atoms and their mixtures begun to be observed experimentally. A new field of physics called condensed atomic and molecular physics (CAMP) is emerging as interesting quantum phases are being discovered for systems of ultracold atoms produced in the laboratory. After a general overview of the state of the field and where it is heading, I will discuss the story of the wedding between condensed matter and atomic physics corresponding to the case of ultracold bosonic atoms trapped in harmonically confined optical lattices. In standard condensed matter lattice systems two very distinct quantum phases can exist at low temperatures: one is insulating and the other is superfluid. However, for harmonically confined optical lattices produced in atomic physics, an alternating shell structure of insulating and superfluid regions emerges producing a wedding cake structure for the atomic density. After presenting characteristic properties of each type of shell, I will conclude by describing novel techniques for the detection of superfluid and insulating regions.
Sept. 12: Andrew Shabaev, Department of Computational & Data Sciences, GMU
Mode-locked Coherence of Electron Spin in Quantum Dots
Recent technological advances make it possible to control the size, shape and composition of quantum dots - nanometer scale structures with a discrete energy spectrum of charge carriers. The use of not only the charge, but also the spin degree of freedom opens broad opportunities for development of spin-based electronics, or spintronics. The spin of the confined electron can be used for information technology including quantum information processing. In particular, a single electron localized in a quantum dot represents a promising qubit for a scalable spin-based quantum computer. The ability to maintain and control spin coherence for a substantially long period of time is extremely important for applications in hardware for information technology. Fast dephasing of electron spins in an ensemble of quantum dots is detrimental to applications in quantum-information processing. We show that dephasing can be overcome by using a periodic train of light pulses to synchronize the phases of the precessing spins. Randomly fluctuating nuclear spins are considered the main source of decoherence of a single spin and dephasing of spins in an array of quantum dots. Recently, however, we demonstrated that the same nuclei can act constructively and help control the electron spin coherence by tuning the precession frequency of an electron to one of the locked modes, leading to the frequency-focusing effect. The electron spin precession frequency is preserved by the nuclei for a strikingly long period of time. The nuclei-induced frequency focusing drives the electron spin precession into dephasing-free subspaces with the potential to realize single frequency precession of the quantum-dot ensemble.
Sept. 19: Karen Leighly, University of Oklahoma
The Quasar Continuum Spectral Energy Distribution and Broad-line Region Emission and Kinematics
Quasars are the most luminous persistently emitting objects in the Universe. Powered by accretion onto a supermassive black hole, many of their observable properties should be determined by the fundamental parameters of accretion: the black hole mass and accretion rate. The black hole mass and accretion rate should, in turn, determine the shape of broad-band continuum emission arising from the accretion disk in the central engine. That broad-band continuum is responsible for powering the strong, broad emission lines that are prominent in optical and UV spectra, and are an identifying feature of active galaxies and quasars. Thus, the line emission may be used as a probe of the fundamental intrinsic properties, if we can break the code. I will present recent work investigating the role of the spectral energy distribution in determining AGN broad-line properties.
Oct. 10: Merav Opher, Department of Physics & Astronomy, GMU
Squashed Solar System: Pinning Down the Direction and Orientation of the Interstellar Magnetic Field
Magnetic effects are ubiquitous and known to be crucial in space physics and astrophysical media. We have now the opportunity to probe these effects in the outer heliosphere with the two spacecraft Voyager 1 and 2. Voyager 1 crossed, in Dec 2004, the termination shock and is now in the heliosheath. On August 30, 2007 Voyager 2 crossed the termination shock, providing us for the first time in-situ measurements of the subsonic solar wind in the heliosheath. With the recent in-situ data from Voyager 1 and 2 the numerical models are forced to confront their models with observational data. Our recent results indicate that magnetic effects, in particular the interstellar magnetic field, are very important in the interaction between the solar system and the interstellar medium. We summarize here our recent work that shows that the interstellar magnetic field affects the symmetry of the heliosphere that can be detected by different measurements. We combined radio emission and energetic particle streaming measurements from Voyager 1 and 2 with extensive state-of-the art 3D MHD modeling, to constrain the direction of the local interstellar magnetic field. The orientation derived is a plane ~ 60°-90° from the galactic plane. This indicates that the field orientation differs from that of a larger scale interstellar magnetic field, thought to parallel the galactic plane. As a result of the interstellar magnetic field the solar system is asymmetric being pushed in the southern direction. We present results from recently developed 5 fluids MHD model (4 neutral fluids and 1 ionized fluid). The presence of neutral H, has the effect of diminishing the global heliospheric symmetries. With a stronger interstellar field, however, the symmetries are increased. We discuss these results and compare with our previous work (Opher et al. 2006, 2007).
Oct. 17: Joe Weingartner, Department of Physics & Astronomy, GMU
The Alignment of Grains with the Interstellar Magnetic Field
Observations of starlight polarization reveal that interstellar grains are non-spherical and systematically aligned with respect to the Galactic magnetic field. Despite over fifty years of effort, no successful alignment theory has been fully elaborated. I will review the most promising mechanisms and describe ongoing work to clarify their roles.
Oct. 24: Dmitri, Klimov, Department of Bioinformatics & Computational Biology, GMU
Replica exchange simulations of the thermodynamics of amyloid fibril growth
Replica exchange molecular dynamics and all-atom implicit solvent model are used to probe the thermodynamics of deposition of Alzheimer's Abeta monomers on preformed amyloid fibrils. Two deposition transitions, docking and locking, are identified. Docking appears to be continuous and occurs without free energy barriers or intermediates. During docking incoming Abeta monomers adopt disordered structure on the fibril edges. Locking transition is characterized by rugged free energy landscape and takes places, when incoming Abeta peptides form ordered beta-sheet structure on the fibril edge. We found that the binding affinities of two distinct fibril edges with respect to incoming Abeta peptides are different. This observation raises the possibility of unidirectional fibril elongation. Comparison with the available experimental data is discussed.
Oct. 31: Mihaela Tanase, Materials Science Division, Argonne National Laboratory
Magnetization behavior in exchange-biased patterned nanostructures
Magnetization processes in patterned magnetic heterostructures are of fundamental scientific interest and have important applications in information storage such as in non-volatile magnetoresistive random access memories (MRAM). Continuous miniaturization causes materials defects to play an increasingly important role in the magnetization switching behavior of these devices. Transmission electron microscopy (TEM) is the technique of choice for high spatial resolution characterization of non-ideal magnetic behavior and its relationship with nanoscale structure. In combination with novel phase retrieval techniques, Lorentz TEM has the potential of becoming an in situ quantitative technique for mapping magnetization reversal processes.
We have used a combination of Lorentz TEM, magneto-optical Kerr magnetometry and micromagnetic simulations to characterize the behavior of micron-size exchange-biased magnetic nanostructures exhibiting vortex magnetization and imprinted with circular exchange bias. Circular exchange bias promotes a reversible vortex behavior and it controls the chirality of the vortex during reversal. It also stabilizes the vortex structure as a low energy state, acting against magnetocrystalline anisotropy which favors the formation of domain walls. Exchange bias suppresses stochastic processes due to thermal activation and cause the magnetization reversal to be reproducible over time, an important feature in applications.
Phase imaging based on the Transport–of-Intensity Equation (TIE) is emerging as a novel method for mapping magnetization phenomena in situ in the TEM. The phase shift of the electrons containing the magnetic information is obtained from the intensity of the wave and its derivative along the optical axis alone, and does not require a reference beam as conventional interferometry techniques do. Examples of application of TIE to patterned magnetic heterostructures will be shown and the requirements for becoming a quantitative technique as well as its limitations will be described. TIE-based phase retrieval has potential applications in novel systems such as multilayered magnetic and multiferroic heterostructures for data storage and logic applications. Furthermore, TIE is not limited to magnetic systems, as it offers opportunities in mapping electric fields and charge transport processes at the nanoscale.
Nov 7: Harley Thronson, Associate Director for Advanced Concepts and Planning Astrophysics Science Division, NASA Goddard Space Flight Center
Future Large Space Astronomy Missions: Lessons from HST and ISS
The National Academy of Sciences has begun the new Decade Review for astronomy and astrophysics. Several teams around the country are developing new (or updating existing) large observatory concepts for consideration for launch 10 ¬ 20 years in the future. NASA and other organizations have enormous experience in astronaut and robot operations in free space. From this experience, we may draw conclusions about how to design, construct, operate, and service the extremely expensive observatories that astronomers will advocate to NASA and other space agencies. In this presentation, I will summarize the past history of and major lessons learned from major servicing missions in space and discuss future concepts and operations that take advantage especially of our experience with HST and ISS.
Nov. 21: Alex Lazarian, Department of Astronomy, University of Wisconsin-Madison
Magnetic Reconnection of Weakly Stochastic Magnetic field
I shall discuss magnetic reconnection problem, explain the limitations of the known solutions and present our model of reconnection in the presence of turbulence. The model employs magnetic fields what are weakly perturbed by ambient turbulence. I shall show numerical simulations testing the idea and discuss a few implications of the turbulent reconnection.
Department of Physics & Astronomy,
Science and Technology 1, Room 303, MSN 3F3,
Fairfax, VA 22030 USA
703-993-1280,
703-993-1269 (FAX)
physics@gmu.edu
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