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A short history of solar and celestial high-energy observations
Dr. Gerald Share of U. Maryland, Naval Research Lab.
May 4, 2007 4pm
TBA
Dr. Ted Einstein of Univ. of Maryland
April 30, 2007 4pm
(Host: Karsten Pohl)
The Neutron, the Big Bang, and the "Left-Handed" Universe.
Dr. Geoff Greene of U. of Tennessee, Oak Ridge Nat. Lab.
April 23, 2007 4pm
(Host: John Calarco)
Abstract
Measurements of the properties of the free neutron shed light on a number of interesting problems including Big Bang nucleosynthesis, the origin of parity violation, and the cosmic matter -antimatter asymmetry. Of particular interest are the measurement of parameters that describe the decay of the free neutron and the search for a possible neutron electric dipole moment. An overview of these issues will be given along with a discussion of the program of research planned at the newly operational Spallation Neutron Source at Oak Ridge National Laboratory.
Electrical manipulation of single spins in semiconductors
Dr. Jian-Ming Tang of Univ. of Iowa
April 16, 2007 1pm
(Host: Jim Connell )
Abstract
The ability to control and monitor single spins in a solid-state environment provides a new pathway to study the fundamental physics of magnetism in solids. In this talk I will present our studies of the spin-spin interactions between two magnetic atoms in a semiconductor. These interactions are unusual and long-ranged because they are mediated through the charge carriers loosely bound to the magnetic atoms. The highly extended carrier wavefunction is susceptible to external perturbation. I will describe our predictions that single spins associated with individual magnetic atoms can be controlled using electric or strain fields. From a technological point of view an all-electrical spin manipulation scheme is highly desirable for building high-density spin-based electronic devices or scalable quantum computers. I will discuss our joint efforts with experimental groups to probe the local electronic structure near a magnetic atom (Mn) substituted into a semiconductor (GaAs) using scanning tunneling microscopy. Our predictions of the strong spatial anisotropy of the charge carrier bound to the magnetic atom, and the anisotropic magnetic interaction between two magnetic atoms have been verified by these measurements. These results improve our understanding of ferromagnetism in semiconductors, and suggest new approaches for building atomic quantum spin systems in a solid.
Elusive Massless Particles and Condensed Matter Physics.
Dr. Ivan Sergienko of Oak Ridge National Lab.
April 12, 2007 1pm
Abstract
Magnetic reconnection is the process by which magnetic field flux in plasmas is annihilated and the released energy is converted to plasma kinetic energy. It is now recognized as a universal process that occurs in space plasmas around the Sun, the Earth, and magnetized astrophysical objects as well as within laboratory plasma confinement devices, such as future fusion reactors. Forty years ago, it was generally believed to be impossible; today, the observational evidence is overwhelming that it occurs universally. No one knows how it works, although there are lots of opinions. As the highest priority for heliophysical research for both the National Academy of Sciences and the NASA Science plan, NASA has undertaken the Magnetospheric MultiScale (MMS) mission to resolve the mysteries of reconnection. This talk will describe the dilemma of reconnection in space plasmas and how MMS is designed to answer them.
Who Needs Space Weather Forecasts?
Terry Onsager of NOAA, Space Environment Center
April 9, 2007 4pm
Abstract
Space weather disturbances occur due to a complex chain of interactions involving the sun, the solar wind, the magnetosphere, the ionosphere, and the upper atmosphere. This presentation will summarize the different aspects of space weather and the commercial and government activities that are affected, including impacts on commercial airline communication, GPS navigation accuracy, satellite failures, and electric power generation and transmission. An important challenge for solar-terrestrial physics research is to advance our understanding of the Sun-Earth system and to develop models that give timely and accurate predictions of space weather that result in benefits to society. For this to occur, the information provided by these models must be directly usable and must enable decisions to be made that have positive economic consequences. The current overlap between emerging scientific capabilities and space weather needs will also be discussed.
TBA
Dr. Predrag Nikolić of Harvard
February 28, 2008 4pm
Abstract
Charge Transport Characteristics
of Molecular Devices
Dr. Emil Prodan of Princeton, Center for Complex Materials
February 26, 2007 4pm
Abstract
Charge transport across molecules and nano-structures is a subject of great interest these days. Understanding, predicting and controlling this physical process can revolutionize the electronic devices. Charge transport, however, is one of the most challenging problems in physics since it involves out-of-equilibrium, open, interacting many- body systems. Better and better measurements of the transport characteristics and the gradual down-scaling of the devices to the molecular level, where the quantum effects are important and measurable, lead to re-consideration of several fundamental issues, like what is really measured and how to interpret such experiments? What is conductance and how can it be rigorously defined? Does the conductance depend on how electrons are injected in the device?
The current, most successful theoretical approaches involve Time Dependent Density Functional Theory (TDFT) at different levels of approximation. In the first part of my talk, I will discuss my efforts of going beyond TDFT and give a rigorous definition and formally exact expression of the two-terminal conductance based on the new and promising approach of Time Dependent Current-Density Functional Theory. This answers some of the above questions.
For long molecular chains, I was able to analytically manipulate the above formal expression to a point where one can explicitly see which properties of the chain and contacts directly influence the two- terminal conductance. For insulating chains, for example, I found the well known exponential decay behavior of the conductance with the chain's length, but, in addition, I obtained an explicit expression for the amplitudes of the exponentially decaying factors. These will be discussed in the second part of my talk, which will also include explicit numerical examples.
In the third part of the talk I will discuss how these new results enhance our understanding of several experiments, such as those on transport characteristics of alkyl chains self-assembled on silicon substrates (in both thermionic and tunneling regimes) or those on tunneling magneto-resistance.
Acknowledgments: This work is in collaboration with Prof. Roberto Car. Some of the analytical tools used in the project were derived during my previous postdoc with Prof. Walter Kohn. On the computational side, I am also strongly interacting with Prof. Leeor Kronik. On the experimental side, I am collaborating with Prof. Antoine Kahn and Prof. David Cahen.
Phase fluctuations and charge ordering in two-dimensional superfluids
Anton Burkov of Harvard, Condensed Matter Theory Group
February 19, 2007 4pm
Abstract
In this talk I will discuss the interplay between charge ordering and superfluidity in two-dimensional superfluids. Understanding the physics of two-dimensional superfluids and superconductors with low superfluid density is an important problem in modern condensed matter theory. This physics is at play in a wide variety of experimental systems: high-temperature superconductors, thin amorphous superconducting films, excitonic superfluids in quantum Hall bilayers, and many others. I will discuss a theory of charge ordering in two-dimensional superfluids, which is based on the idea that quantum mechanics of low-energy vortex excitations of the superfluid determines the "most natural" charge ordered states, proximate to it. I will discuss the situations in which this theory may be relevant and illustrate this with numerical simulations of specific models of charge ordering.
"Discovering the nanoworld", NOTE: Colloquium is on Thursday 1pm.
Francesca Baletto of DMSE-MIT
February 15, 2007 4pm>
Abstract
The emergence and spectacularly rapid evolution of the field of atomic and molecular nanoparticles are among the most exciting developments in the recent history of materials science. The pursuit of nanoscience and nanotechnology is to understand, control, and manipulate ob jects of a few nanometer size (say 1-100 nm). The structure is one of the most fundamental properties of a cluster and plays an important role to understand all aspects of its chemical and physical behavior. Here, I present an overview of the structural properties of nanoclusters, with the aim of defining 'magicity' in the nanoworld, showing the interplay of energetic, thermodynamic and kinetic factors in building up the structures. Several examples have shown that this interplay is crucial, and that a satisfactory explanation of the experimental outcomes is very often impossible on the basis of energetic considerations alone. This can have deep consequences on the very important issue of controlling the shapes of the produced nanoclusters, which is of relevant technological importance.
Orbital magnetization in periodic solids
Timo Thonhauser of MIT, Physics
February 12, 2007 4pm
Abstract
A complete description of magnetism in solids requires not only the spin degrees of freedom, but also the ``orbital magnetization.'' Despite the recent surge of interest in magnetic materials, it is quite surprising that the theory of orbital magnetization has remained in a condition similar to that of the polarization before the early 1990s, when the problem of computing finite polarization changes was solved [R.D. King-Smith and D. Vanderbilt, Phys. Rev. B 47, 1651 (1993)]. The essential difficulty, that the matrix elements of the position operator "r" are not well-defined in the Bloch representation, could be overcome by reformulating the problem in the Wannier representation. In order to derive an analogous theory for the orbital magnetization, we again work in the Wannier representation and assume a periodic insulator with broken time-reversal symmetry and vanishing magnetic field. We show that a naive replacement of the dipole operator "r" by the circulation operator "r x v" in the expectation value of a bulk Wannier function gives only one contribution to the magnetization, i.e., the magnetization associated with the internal circulation of bulk-like Wannier functions. The missing contribution arises from net currents carried by the Wannier functions at the boundary of the sample. We prove that both contributions can be expressed as bulk properties in terms of Bloch functions in a gauge-invariant way [T. Thonhauser, Davide Ceresoli, David Vanderbilt, and R. Resta, Phys. Rev. Lett. 95, 137205 (2005)].
James Drake of University of Maryland
2006-12-04
(Host: Amitava Bhattacharjee )
Magnetic Fields at the Large Cosmic Scales
Steven Cowley of UCLA
2006-11-27
Magnetic Reconnection and the Magnetospheric MultiScale Mission
Roy Torbert
2006-11-20
Abstract:
Magnetic reconnection is the process by which magnetic field flux in plasmas is annihilated and the released energy is converted to plasma kinetic energy. It is now recognized as a universal process that occurs in space plasmas around the Sun, the Earth, and magnetized astrophysical objects as well as within laboratory plasma confinement devices, such as future fusion reactors. Forty years ago, it was generally believed to be impossible; today, the observational evidence is overwhelming that it occurs universally. No one knows how it works, although there are lots of opinions. As the highest priority for heliophysical research for both the National Academy of Sciences and the NASA Science plan, NASA has undertaken the Magnetospheric MultiScale (MMS) mission to resolve the mysteries of reconnection. This talk will describe the dilemma of reconnection in space plasmas and how MMS is designed to answer them.
Hyperpolarized Xenon and Functional Lung Imaging
Bill Hersman of UNH
2006-11-13
Jeff Yepez
2006-11-06
Recent Results from the Spitzer Sapce Telescope: A New View of the Infrared Universe
Giovanni Fazio of Harvard
2006-10-23
(Host: Edward Chupp )
Scott Thomas of Rutgers
2006-10-16
(Host: Per Berglund )
Problem Solving and the Use of Math in Physics Courses
Joe Redish of UMD
2006-10-02
(Host: Dawn Meredith )
Abstract:
Mathematics is an essential element of physics problem solving, but as professionals, we often fail to appreciate exactly what we are doing with it. Math may be the language of science, but math-in-physics is a distinct dialect of that language that requires both more subtlety and more skills than are typically taught in math courses. Research with students in classes ranging from algebra-based physics to graduate quantum mechanics indicates that (1) we sometimes don't appreciate the skills students need to solve the problems we assign, and (2) students' problems are sometimes with their expectations about what they are supposed to be doing rather than with their math skills. Implications for instruction will be discussed.
The Earth's Plasma Sheet
Dick Kaufmann
2006-09-25
Viscous Energy Dissipation by Flux Pile-Up Merging in the Solar Corona
Yuri Litvinenko of UNH
2006-09-18
Abstract:
Magnetic field annihilation in resistive viscous incompressible plasmas is analyzed. Anisotropic viscous transport is modeled by the dominant terms in the Braginskii viscous stress tensor. An analytical solution for steady-state magnetic merging, driven by vortical plasma flows in two dimensions, is derived. Resistive and viscous energy dissipation rates are calculated. It is shown that, except in the limiting case of zero vorticity, viscous heating can significantly exceed Joule heating at the merging site. The results strongly suggest that viscous dissipation can provide a significant fraction of the total energy release in solar flares, which may have far-reaching implications for flare models.
Recent Developments in Astronomical Gamma-Ray Polarimetry
Mark McConnell
2006-09-11
Nathaniel Fisch of Princeton
to be determined
(Host: Amitava Bhattacharjee )
The Future of Gravity
James Hartle of University of California, Santa Barbara
2006-05-08
(Host: Per Berglund )
Abstract:
Of the four fundamental forces, gravity has been studied the longest, yet gravitational physics is one of the most rapidly developing areas of science today. This talk will give a broad brush survey of the past achievements and future prospects of general relativistic gravitational physics. Gravity is a two frontier science being important on both the very largest and smallest length scales considered in contemporary physics. Recent advances and future prospects will be surveyed in precision tests of general relativity, gravitational waves, black holes, and cosmology. The aim will be an overview of a subject that is becoming increasingly integrated with experiment and other branches of physics.
The strong nuclear force on a space-time lattice
Silas Beane of UNH
2006-04-24
Abstract:
I will discuss recent work which uses large computers in conjunction with effective quantum field theories to compute properties of hadrons (like the proton) and simple nuclear systems (like the deuteron) directly from Quantum Chromo-dynamics (QCD), the underlying theory of the strong nuclear force.
Ultra-sensitive Atomic Magnetometers
Michael Romalis of Princeton
2006-04-17
(Host: Bill Hersman )
Abstract:
IElectron spins in alkali-metal atoms can be easily polarized with lasers and caused to precess around a magnetic field. The frequency of their precession is directly proportional to the strength of the magnetic field and can be used for accurate magnetometery. With recently developed techniques for suppressing various sources of spin relaxation such atomic magnetometers have reached unprecedented sensitivity, surpassing even SQUID magnetometers operating in liquid helium. I will discuss the physics of atomic magnetic field sensors and several applications explored in our group, including detection of nuclear magnetic resonance and magnetic fields generated by the brain. In other applications of atomic magnetometers it is often desirable to cancel magnetic field noise while retaining sensitivity to other sources of spin precession. For such applications we developed a co-magnetometer consisting of noble gas and alkali-metal spins occupying the same volume. Using spin-exchange interaction between the two spin ensembles one can arrange for an effective cancellation of magnetic field drifts. Such co-magnetometer can serve as a sensitive gyroscope, measuring spin precession due to microradian level rotation of the apparatus. The co-magnetometer is also utilized for tests of fundamental symmetries, in particular, search for a violation of Local Lorentz invariance manifested by spin coupling to a background field, as recently predicted in some theories of quantum gravity.
Dynamics and Excitations in Time-Dependent Density Functional Theory: Successes and Challenges
Neepa Maitra of Hunter College
2006-04-10
(Host: James Harper )
Abstract:
Density functional theory (DFT) is recognized as a powerful alternative to traditional wavefunction methods for studying the properties of atoms, molecules, clusters, solids, and biomolecules. Its extension to electron dynamics in time-dependent fields (TDDFT) has become increasingly popular for calculating excitation spectra, frequency-dependent (hyper)polarizabilities, magneto-optical response, van der Waals coefficients, and, most recently, molecular electronics. The promise to describe electron correlation in strong-field phenomena, including high-harmonic generation, above-threshold ionization, and electronic quantum control theory, is only beginning to be tapped; the non-perturbative regime is where TDDFT is the only feasible scheme, as wavefunction methods for even a few electrons are prohibitively expensive. Although in principle exact, the functionals of TDDFT must in practice be approximated. There is intense activity concerning functional development, and when and why the usual approximations fail. In this talk, I begin with an introduction to DFT and TDDFT, and then present current challenges for the theory, with some partial solutions: including quantum control, double excitations and long-range charge transfer.
Professor Maitra's background:
BSc (Hons) from University of Otago in New Zealand, 1993. Came to Harvard University for PhD (with Frank Knox fellowship). Studied semiclassical methods and quantum chaos under Rick Heller there, with my thesis title "Topics in Semiclassics: Non-Classical Phenomena in Integrable and Non-Integrable Systems". Graduated in 1998. Postdoc'ed in Bill Miller's chemical physics group at Berkeley for a year before joining Kieron Burke's density functional theory group at Rutgers as a postdoc. Joined Hunter CUNY as assistant professor in January 2004, where I conduct research in time-dependent density functional theory for excitations and dynamics of atoms, molecules, and systems of chemical interest. External Grants: ACS PRF (2004-2006), NSF Career (2006-2011).
Probing Fundamental Physics with Ultracold Neutrons: Following the Bouncing Ball
Albert R. Young of NCSU
2006-04-03
(Host: Bill Hersman )
Abstract:
Ultracold neutrons (UCN) are neutrons with energies below about 400 neV that can be stored in material bottles for hundreds of seconds and poured or guided into a variety of experiments. The goal of the UCNA experiment is to improve our knowledge of the angular correlation between the emitted electron and the initial spin of the neutron in beta-decay (the beta-asymmetry) using ultracold neutrons. The precision of this correlation defines the current limits to which fundamental electroweak data can be extracted from neutron decay and also the limits which can be placed on new physics from neutron decay. In the process of developing their experimental approach, the UCNA collaboration developed the first functioning solid deuterium superthermal source of UCN coupled to a spallation target, initiated a unique program to develop new technnology to transport UCN and to understand the systematics of UCN depolarization, initiated the first detailed studies of the backscattering of electrons in the energy range of beta-decay, and developed new technology for low energy proton detection. The experiment is now fully constructed and is in the process of commissioning. The motivation and status of this experiment will be reviewed, and recent results of its research and development program presented.
The properties of matter a micro-second after the Big Bang: RHIC tells all
< Raju Venugopalan of Brookhaven
2006-03-27
(Host: Silas Beane )
Abstract:
Ultrarelativistic nuclear collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory create the hottest and densest matter on earth - comparable to those existing 0.000001 seconds after the Big Bang. We review the remarkable and unexpected results from RHIC and what they may tell us about the properties of this matter.
Helium Isotopes in the Solar wind and in the Interstellar Gas
Constraints for Solar and Galactic Evolution Models
Peter Bochsler
2006-02-20
(Host: Eberhard Möbius )
Abstract:
With the foil experiments, which were deployed on the Moon during the Apollo missions, and with several in-situ experiments we have been measuring the helium isotopic composition of the solar wind. More recently, we have also been able to successfully use the foil collection technique onboard the Mir space station to measure the isotopic composition of helium in the local interstellar medium (ISM). 4He in the Sun and in the ISM is mostly a remnant from the cosmological big bang, and some 4He has also been recycled into the ISM from stars during galactic evolution. 3He originates partly from cosmological D and 3He. Furthermore, it is built up in low-mass stars (such as the Sun) at the fringes of their nuclear burning zones. The solar wind helium isotopic composition can be used to infer the isotopic composition of the outer convective zone of the Sun. The low solar 3He abundance proves that the Sun never was never mixed after ignition of nuclear reactions, and it can also be used as a reference for the local galactic composition at the time of the formation of the solar system, 4.6 Gy ago. The composition of the contemporary ISM indicates, whether low-mass stars have altered the interstellar gas during the last 4.6 Gy of galactic evolution. I will present our results in the context of recent galactic models.
John Dorelli of UNH, EOS Space Science Center
2006-01-30
(Host: Bill Hersman )