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Spring 2008 Physics Colloquium Schedule

Unless otherwise noted, the physics colloquia are held in Room 307 of the Science and Engineering Research Facility. Refreshments are served at 3:00 p.m. with the talk following at 3:30. The ORNL Physics Division Seminar Schedule and the UTK Nuclear Physics Seminar Schedule might also be of interest. Professor John Quinn is chair of the colloquium program. He may be contacted via e-mail at: jjquinn@utk.edu.

Physics Colloquium Webcasts





Date Speaker Title
January 14 Tony Mezzacappa, UT/ORNL Towards the Core Collapse Supernova Mechanism: An Emerging Picture? [Abstract]
January 21 MLK Holiday/No Talk  
January 28 Richard Furnstahl, Ohio State Atomic Nuclei at Low Resolution [Abstract]
February 4 Alex Gurevich, FSU New effects of the phase coherence in superconductors: from SQUIDS to fractional vortices and phase textures in high-Tc oxides and MgB2 [Abstract]
February 11 Gregory Gabadadze, NYU The Cosmological Constant Problem [Abstract] [Talk]
February 18 Fred Wietfeldt, Tulane Precision Neutron Decay Measurements at NIST [Abstract]
February 25 Collin Broholm, The Johns Hopkins University and NIST Center for Neutron Research Scattering neutrons from magnons, spinons, solitons, and breathers [Abstract]
March 3 Sacha Kopp, University of Texas Quantum Mechanics 101: Neutrino Oscillations as a Demonstration of what is Knowable and what is Uncertain [Abstract]
March 10 Andrew Boston, Liverpool University Teaching a SmartPET new tricks, medical imaging with semiconductor detectors [Abstract]
March 17 Spring Break/No Talk  
March 24 Stefano Marchesini, LBNL Ultrafast imaging and holography with x-ray lasers and synchrotrons [Abstract]
March 31 John J. Quinn The Hierarchy of Incompressible Fractional Quantum Hall States [Abstract]
April 7 John Singleton, National High Magnetic Field Laboratory, Los Alamos National Laboratory Our latest understanding of the cuprates- some very recent quantum oscillation experiments using the 75 and 85 T magnets at the National High Magnetic Field Laboratory [Abstract]
April 14 Peter Maksymovych, ORNL Center for Nanophase Materials Sciences Imaging hot-electron transport using chemical reactions on metal surfaces [Abstract]
April 21 Honors Day/Talk by Dr. Lee Riedinger  

Abstracts

January 14
Tony Mezzacappa, UT/ORNL Adjunct Professor
Towards the Core Collapse Supernova Mechanism: An Emerging Picture?

Core collapse supernovae are the death throes of massive stars, 8-10 times the mass of our sun. They are the dominant source of elements in the Universe between oxygen and iron and are believed responsible for half the elements heavier than iron. Knowing how they occur is key to understanding how we came to be in the Universe. The core collapse supernova explosion mechanism is not at present known, at least not definitively. We still work within the neutrino heating paradigm discovered by Wilson more than two decades ago, and recent results from our group and others show promise we are closing in on the mechanism. Nonetheless, the extension of the present-day simulations to three dimensions and the addition of new, not-well-explored physics, such as the evolution of magnetic fields in supernova cores, will likely hold significant surprises. I will discuss the results of one-, two-, and three-dimensional core collapse supernova simulations performed by the ORNL-centered supernova effort and discuss the ramifications of each for the future of core collapse supernova modeling. I will also discuss the requirements of three-dimensional multi-physics core collapse supernova modeling and hopefully demonstrate the daunting but nonetheless surmountable computational challenges that lie ahead over the next five to ten years as we take on one of Nature¹s Grand Challenge computational problems.

January 28
Richard Furnstahl, Ohio State University
Atomic Nuclei at Low Resolution

For digital televisions and cameras, higher resolution is considered better. Progress in particle physics requires higher resolution (shorter wavelengths) using higher energy accelerators. But for many-body problems, such as the microscopic description of nuclei, lowering the resolution can be a big advantage. Evolving Hamiltonians to low resolution involves only basic principles of quantum mechanics, but with less familiar features such as nonlocality and many-body forces, which can run counter to intuition about wave functions and potentials. I will illustrate the machinery and consequences of lowering the resolution (using colored graphics and movies!) and outline the benefits for major nuclear structure initiatives such as the SciDAC project to develop a Universal Nuclear Energy Density Functional (UNEDF).

February 4
Alex Gurevich, National High Magnetic Field Laboratory, FSU
New effects of the phase coherence in superconductors: from SQUIDS to fractional vortices and phase textures in high-Tc oxides and MgB2

Macroscopic phase coherence of the condensate of strongly overlapping Cooper pairs is one of the key features of superconductivity resulting in the Meissner effect, quantized magnetic vortices, and the superconducting quantum interference due to Josephson’s effect. The discovery of the d-wave high-temperature superconductors, other superconductors with exotic pairing symmetries, and the two-gap superconductor MgB2 have brought to focus new effects in which nontrivial intrinsic symmetries of the Cooper pairs can result in fractional vortices, spontaneous p junctions on grain boundaries and current-induced phase textures in two-gap or nanostructured superconductors. This fascinating physics brings new challenges and opportunities for the superconducting electronics and improving performance of high-current superconducting wires.

February 11
Gregory Gabadadze, Department of Physics, NYU
The Cosmological Constant Problem [Talk]

Dr. Gabadadze will review the cosmological constant problem -- one of the main enigmas of quantum theory, gravity and cosmology -- and discuss an approach to this problem within the framework of large distance modification of gravity.

February 18
F. E. Wietfeldt, Department of Physics, Tulane University
Precision Neutron Decay Measurements at NIST

The decay of the free neutron is the simplest nuclear beta decay and the prototype charged current semileptonic weak interaction. The observables of neutron decay, such as the lifetime and angular correlations, are directly related to the fundamental weak coupling constants and can be used to test the completeness and validity of the Electroweak Standard Model. This includes searches for supersymmetry, scalar and tensor forces, right-handed currents, T and CP symmetry violation, and tests of the unitarity of the CKM matrix. This program contributes to the "low energy frontier" of particle physics. The neutron lifetime is also an important input for theoretical predictions of light element abundances in Big Bang nucleosynthesis. I will describe the neutron decay program at the NIST Center for Neutron Research with an emphasis on recent and upcoming experiments.

February 25
Collin Broholm, The Johns Hopkins University and NIST Center for Neutron Research
Scattering neutrons from magnons, spinons, solitons, and breathers

I describe neutron scattering experiments that provide evidence for exotic quasi-particles in long chains of antiferromagnetically interacting electronic spins [1]. Such chains exist in certain coordination polymer magnets where partially filled 3d shells of transition metal ions such as copper and nickel provide the spin. While most of the phenomena to be discussed were theoretically predicted, the presentation focuses on conclusions that can be inferred directly from scattering data.

In spin-1 chains (NENP and Y2BaNiO5) neutron scattering predominantly occurs along a sharp dispersion relation in energy-momentum space. This is evidence for magnons that propagate as massive spin-1 particles through the vacuum of the singlet ground state. In uniform spin-1/2 chains (CDC, CuPzN, and copper benzoate) a bounded continuum indicates that neutrons scatter from a half filled band of mass-less spin-1/2 particles called spinons. The dynamic scaling behavior as a function of temperature indicates that this is a quantum critical state.

The quantum critical spin-1/2 chain undergoes dramatic changes in response to perturbations. Bond alternation creates massive magnons as in the spin-1 chain, a uniform field retains a critical state though shifting the chemical potential. Finally, a staggered field causes spinon binding into massive soliton and breather particles, the relative mass of which is perfectly accounted for by a sine-Gordon quantum field theory.

The goal of the talk is to demonstrate for the non-expert the emergence of new particles in quantum correlated systems and to illustrate through neutron scattering, that these are indistinguishable from fundamental particles at low energies and within the confines of the material they inhabit.

[1] "Magnetized States of Quantum Spin Chains", C. Broholm, G. Aeppli, Y. Chen, D. C. Dender, M. Enderle, P. R. Hammar, Z. Honda, K. Katsumata, C. P. Landee, M. Oshikawa, L. P. Regnault, D. H. Reich, S. M. Shapiro, M. Sieling, M. B. Stone, M. M. Turnbull, I. Zaliznyak, and A. Zheludev, p 211-234 in “High Magnetic Fields – applications in condensed matter physics and spectroscopy” C. Berthier, L. P. Lévy, and G. Martinez, Eds. Springer Verlag (2002).

March 3
Sacha E. Kopp, Department of Physics, University of Texas
Quantum Mechanics 101: Neutrino Oscillations as a Demonstration of what is Knowable and what is Uncertain

The three known types of neutrinos are unique in that they are nearly massless particles. Being nearly massless, they are distinguished from one another by an obscure quantum number called lepton 'flavor'. This near-degeneracy permits, in quantum mechanics, a mixing of these states, and thus a pure beam of one type of neutrino may transmute to species over time. Measurements of this effect from the Fermilab Main Injector will be presented, as well as potential implications for neutrino mass, dark matter and the baryon asymmetry in the universe.

March 10
Andrew Boston, Liverpool University
Teaching a SmartPET new tricks, medical imaging with semiconductor detectors

The development of highly segmented germanium detectors for Nuclear Medical Imaging applications is potentially the most exciting development in applied nuclear physics in recent years. The University of Liverpool Department of Physics has lead an international collaboration to develop a small animal Positron Emission Tomography (PET) scanner utilising segmented germanium detectors. The prototype system has been used to image point sources and phantoms producing high quality images in both PET and Single Photon Emission Computed Tomography (SPECT) mode. Imaging of multi-nuclide sources has demonstrated how the combination of excellent energy resolution and cone beam reconstruction techniques can be used to distinguish between gamma-ray energies and reconstruct individual locations. These results show how such semiconductor systems have the potential to find application not only in the field of medical imaging but also in homeland security and environmental assay. Such performance has allowed the system to be deployed as a multi-modality medical imaging system, which should enable many new and exciting clinical studies to become possible particularly in the fields of Cardiology, Oncology, Neurology and Genetics. This presentation aims to provide an overview of the SmartPET system including the use of Pulse Shape Analysis and digital electronics. Operation of the system in Compton Camera mode will be discussed and some imaging results will be presented.

March 24
Stefano Marchesini, LBNL
Ultrafast imaging and holography with x-ray lasers and synchrotrons

Recent breakthroughs in the development of free-electron lasers now offer the realistic prospect, for the first time, of imaging on the time-scale of atomic motion. Combining this with near-atomic spatial resolution and a non-invasive probe suitable for the life sciences is a huge challenge. The recent development of lensless ("diffractive") X-ray imaging techniques appears to cut this gordian knot by offering diffraction-limited resolution at femtosecond speeds, while taking advantage of the elemental specificity and phase-constrast possibilities of X-ray imaging, combined with adequate pentration. By replacing the necessary lens with a computer algorithm, which solves the famous phase problem to recombine the scattered photons, an aberration-free image may be reconstructed whose resolution is limited in principle only by the X-ray wavelength. These methods have been applied to image object as complex as biological cells, quantum dots, nanaocrystals, and nanoscale aerogel structures. Other test patterns were captured in the fastest flash image ever recorded at suboptical resolution. The simple geometry of lensless imaging however shifts the reconstruction problem to the computational methods. Novel experimental geometries such as massively parallel x-ray holography will be discussed.

March 31
John J. Quinn, University of Tennessee
The Hierarchy of Incompressible Fractional Quantum Hall States

The fractional quantum Hall (FQH) effect is the paradigm for all strongly interacting systems. The only energy scale in the problem, the interaction energy of a pair of Fermions, determines everything. Laughlin explained the simplest FQH states in the lowest Landau level (LL0) in terms of the avoidance of electron pair states with the strongest repulsion. These Laughlin correlated electrons (LCEs) give an effective LCE filling factor of unity and an electron filling factor equal to the reciprocal of an odd integer. Jain extended Laughlin’s picture to include all states which resulted in the LCEs filling an integral number of such levels. LCEs occur only when the interaction energy V(L') of the pairs of Fermions is "superhmonic", i.e. it rises with increasing pair angular momentum L' faster than L'(L'+l), as the value of the pair angular momentum avoided in the LCE state is approached. This requirement is not satisfied at all values of L' for higher Landau levels (LLN, with N > O), nor is it satisfied for all L' for the pseudopotential VQP(L') describing the interaction between Laughlin quasiparticles (QPs). The electron correlations in LLI and the correlations of Laughlin QPs in LL0 can be very different from Laughlin correlations. For example, pairing occurs at certain filling factors even though V(L') is repulsive. Correlations of the most stable FQH states in LLI and of some daughter FQH states of Laughlin QPs in LL0 will be discussed and compared to LCE states in LL0. This work was supported in part by Basic Energy Science of DOE.

April 7
John Singleton, National High Magnetic Field Laboratory, Los Alamos National Laboratory
Our latest understanding of the cuprates- some very recent quantum oscillation experiments using the 75 and 85 T magnets at the National High Magnetic Field Laboratory

Pulsed magnetic fields of up to 85 T and temperatures down to 0.40 K have been used to study single crystals of various cuprate superconductors for several different hole dopings. The samples are measured using a MHz technique that is sensitive to small changes in penetration depth in the superconducting state, and to changes in the skin depth in the normal state. Some experiments also employ torque magnetometers. In the normal state, two or three series of clear magnetic quantum oscillations are observed, periodic in inverse field. The observed frequencies are low (e.g. in YBa2Cu4O8, the frequencies are 200±20 T, 660±15 T and approximately 2400 T) suggesting that the predicted large Fermi surface is broken into smaller pockets due to nesting. The temperature dependence of the oscillations also provides the quasiparticle masses for the Fermi surface sections of the various compounds. In contrast to some predictions, no outstandingly heavy quasiparticles are found; instead, observed masses lie in the approximate range from 2 to 4 times that of the free electron. Our data reveal some general features of the bandstructure of the cuprates and provide information about the doping dependence of the Fermi surface. Consequently, we suggest a nesting scheme that accounts for the evolution of the Fermi-surface topology.

Magnetic quantum oscillations are generally recognized to be the most reliable method for deducing the Fermi-surface topology of metals. It is therefore instructive to consider how such data can be reconciled with the results of techniques such as ARPES. By considering the effect of a short antiferromagnetic correlation length ? on the electronic bandstructure and a Fermi-surface topology consistent with the magneticquantum- oscillation experiments, it can be shown that a reduced ? gives an asymmetric broadening of the quasiparticle dispersion, resulting in simulated ARPES data very similar to those observed in experiment. Predicted features include the presence of “Fermi arcs” close to ak = (p/2, p/2), where a is the in-plane lattice parameter, without the need to invoke a d-wave pseudogap order parameter.

Finally, based on the Fermi-surface measurements and neutron scattering data from many different sources, it is possible to propose a magnetically-mediated mechanism for superconductivity in all the cuprates, driven by the topological mapping of the d-wave Cooper-pair wavefunction onto the antiferromagnetic fluctuations that are observed across the whole cuprate phase diagram.

April 14
Peter Maksymovych, ORNL Center for Nanophase Materials Sciences

We have investigated a new regime of single-molecule excitation in the scanning tunneling microscope, where hot electrons locally injected from the STM tip spread out via surface resonances over length scales of up to 100 nm and electronically excite surrounding molecules causing chemical reactions. Such non-local reactions were observed for several different molecules on the (111), (110) and (100) terminated surfaces of gold and copper. The hot-electron origin of these reactions was differentiated from the possible electric field effect in the tip-surface junction on the basis of the statistical analysis of the dissociation yield as well as the non-local excitation in the presence of artificially fabricated nanoclusters. One of the new opportunities provided by the non-local excitation is a direct measurement of hot-electron transport on a metal surface. Using a phenomenological kinetic model for the statistical analysis of the non-local reactions, it is shown that the reaction rate increases linearly with tunneling current and decays exponentially with the distance from the excitation pulse. The angular distribution of the reaction events is isotropic on the Au(111) surface, which is consistent with the symmetry of its surface resonances in the energy range of the non-local reaction. Since the attenuation length of the non-local reaction has little dependence on the STM-tip and the parameters of the excitation, we argue that it is proportional to the inelastic mean-free path of hot-electrons in the surface resonance. It is also shown that the total yield of the non-local reaction provides a measure of hot-electron transport across single-atom steps. The reflectance of the hot-electrons by single atom steps on Au(111) was directly measured to be less than 20% at energies above 1.5 V.

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