Physics Colloquium

Fall 2008

Thursdays 4:00 p.m., Room 104 Physics
Refreshments served at 3:40 p.m.
  Colloquium organizer: Alexey Yamilov
(Link to main colloquium page)

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DateHostSpeakerTitle of the talkAbstract
Aug. 21DuboisAntonio Carlos Fontes dos Santos,
Universidade Federal do Rio de Janeiro
Ionization and Fragmentation of Molecules by ion and photon impact Ionization and fragmentation of molecules by charged particles or photons has many applications in many scientific areas such as plasma physics, astrophysics and upper planetary atmosphere physics. In this talk, we present results on ionizing collisions of intermediate energy charged particle impact and UV or soft X-rays photons with molecules. For the photon impact case, a new and unexpected resonant-like structure was observed in the fragmentation of water molecule, which is interpreted as a dynamical electron correlation.   
Aug. 28 Yamilov  Gary Yao,
 Optical Diffuse Reflectance in Skeletal Muscle Normal biomechanical and physiological functions of striated muscles are facilitated by the repeating sarcomere units. Light scattering technique has been used in studying single extracted muscle fibers. However, few studies, if any have been conducted to investigate the possibility of using optical detection to examine sarcomere structure changes in whole muscles. We conducted a series of experiments to demonstrate that the sarcomere structure plays a significant role in modulating light propagation in whole muscle. The diffuse reflectance patterns measured in whole muscle are related to changes in the sarcomere structure. These results suggest that photon migration technique has a potential for characterizing in vivo tissue ultrastructure changes in whole muscle. 
Sept. 4  Yamilov Maksim Skorobogatiy
Ecole Polytechnique de Montréal
 Color-on-demand photonic textiles using plastic photonic bandgap fibers We present the fabrication and use of plastic Photonic Band Gap Bragg fibers in photonic textiles for applications in interactive cloths, sensing fabrics, signage and art. In their cross section Bragg fibers feature periodic sequence of layers of two distinct plastics. Under ambient illumination the fibers appear colored due to optical interference in their microstructure. Additionally, Bragg fibers guide light in the low refractive index core by photonic bandgap effect, while emitting a portion of guided color without the need of mechanical perturbations such as surface corrugation or microbending. Intensity of side emission is controlled by varying the number of layers in the Bragg reflector. Emitted color is very stable over time as it is defined by the fiber geometry rather than by spectral content of a light source. Moreover, Bragg fibres can be designed to reflect one color when side illuminated, and to emit another color while transmitting the light. By controlling the relative intensities of the ambient and guided light the overall fibre color can be varied, thus opening an opportunity for the passive color changing textiles. Finally, we argue that all-plastic Bragg fibres offer economical solution demanded by textile applications.
Sept. 11Waddill Faculty presentations:
Research opportunities
Sept. 18Wilemski   William Klein
Boston U 
 A Physicist's View of EarthquakesEarthquakes can cause considerable loss of life and property. Unlike hurricanes, fires and floods, earthquakes strike with almost no warning. To forecast earthquakes will require considerable insight into their physics. Tantalizing clues lie in the fact that fault systems appear to be operating near critical points. The evidence for a critical point is in the appearance of scaling laws such as the Gutenburg-Richter power law distribution of the number of earthquakes of magnitude m and the Omori law for the number of aftershocks as a function of time. However, there is also clustering of earthquake events and evidence of an earthquake cycle in which large events cause a lessening of earthquake activity until the system reloads the stress dissipated in the large event.
    In order to understand these processes we have studied simple models of earthquake fault systems with some surprising results. In this talk I will discuss the physical properties of a certain class of models and the relation of these models to real fault systems. The models indicate a mechanism that can produce both scaling and an earthquake cycle and can be seen, in the right range of parameters that control the physics, to be in the same universality class as Ising models and simple fluids.
Sept. 25Waddill  Faculty presentations:
Research opportunities 
Oct. 2Schulz
Zaal Machavariani,
Tbilisi State University
 Single and double ionization of two electron atomic and molecular systems by charged particles impact Single ionization, double ionization and simultaneous ionization–excitation of atoms and molecules by charged particles have been a subject of intensive theoretical and experimental investigations during the last decade. The reason is that the treatment of such four-body processes is the effective tool for the studies of atomic electronic structure and ionization mechanisms.
  In my talk I will consider electron impact double ionization (DI) of helium atom and proton impact single ionization of hydrogen molecule. Both targets are two electron systems and represent the unique samples for studying correlation effects as in initial target state as well as in the final continuum state. As concern DI of helium our approach is based on the possibility of including in the consideration the angular correlation between the target electrons, repulsion between the ejected electrons (in an approximate way), and at the same time, to calculate the fully differential cross section (FDCS) analytically. In order to calculate total DI cross section this is an essential step forward which facilitates the numerical calculation of TCS. Based on dynamical variational principal the effective, momentum-dependent screened charges have been calculated for both outgoing electrons. Hence, the asymptotic behavior of the DI TCS has been explained.
  Doubly differential cross sections (DDCS) for single ionization of molecular hydrogen by 75 keV proton impact have been calculated in the so called one independent center approximation. In this approximation DDCS for hydrogen molecule can be represented as twice corresponding atomic cross section multiplied on some phase factor describing molecular nature of the target. For qualitative explanation of the interference structure observed in a scattering angular dependence of the DDCS the projectile-target interaction has been taken into account for calculation of independent atomic scattering cross sections.
Oct. 9 DuboisChristian Buth, Louisiana State University Atoms and molecules interacting with ultrafast x-ray lasers X-ray science is undergoing one of its greatest revolutions to date with the construction of intense x-ray free electron lasers in Stanford, USA (LCLS), Hamburg, Germany (XFEL), and Harima Science Garden City, Japan (SCSS). These are vast, several-hundred-million dollar machines that will provide  x-ray pulses that are many million times brighter than current sources. Similarly groundbreaking are the emerging attosecond light sources based on intense, pulsed lasers; they are relatively inexpensive laboratory-size instruments. These two emerging radiation sources will enable radically new research and have unnumbered potential applications in materials science, chemistry, biology, AMO, condensed-matter, and plasma physics.

My work contributes to a theoretical understanding of atoms and molecules in gas phase which are exposed to light from these x rays and laser sources. This lays the foundation for future studies using such new resources. So far my work has been based on x rays from a third-generation synchrotron facility synchronized to pulsed, intense, optical lasers. Specifically, I discuss in my talk:
(i) How laser-dressed atoms interact with x rays:  the novel effect electromagnetically induced transparency (EIT) for x rays; provides opportunities for ultrafast x-ray pulse shaping.
(ii) how laser-aligned molecules can be probed with x rays: properties of laser-alignment of molecules; probing molecular dynamics with x rays.

I will also give an outlook on proposed experiments on double core holes in laser-aligned molecules at Stanford's LCLS and the control of Auger decay on an electronic time scale using attosecond light pulses. 
Oct. 17Alexander  Homecoming:
Ray Vandiver
Oregon Museum of Science and Industry
Oct. 24Yamilov  MST&UMSL
Fred Schlachter
Lawrence Berkeley National Laboratory
Americal Physical Society
Over a barrel: America’s energy crisis
America’s energy security and economic stability are in danger due to our reliance on imported oil. We face the additional threat of global climate change caused by combustion of fossil fuels. America has less than 5% of the world’s population, yet we consume fully 25% of the world’s energy. Reducing our energy consumption without changing our quality of life will require political will, significant changes in our use of energy, reducing our “carbon footprint,” and major improvements in technology. We as scientists and engineers have a responsibility to discover new cost-effective energy sources and means of reducing our energy use. This talk will address the above issues and propose research directions in science and technology.
Oct. 30 Medvedeva Renat Sabirianov
U of Nebraska, Omaha
Novel effects in spin-dependent transport Spin dependent transport in a synergistic combination with novel nanoscale magnetic and ferroelectric structures can be used to manipulate spin-dependent currents yielding new paradigms for device operation. I will discuss recent advances in spintronics concentrating on the discovery of new magnetoresistive phenomena:
(1) ballistic anisotropic magnetoresistance (BAMR) [1], a quantized change in the ballistic conductance of the nanowires and nanocontact with magnetization direction.
(2) voltage induced magnetoresistance(MR)  sign reversal.[4] Transition metal oxides exhibit large changes in conductivity when an appropriate bias is applied [2], which make them promising for memory devices [3]. The change in the MR associated with this transition has not been, however, explored, but may uncover fascinating physics. We have recently observed a MR sign reversal associated with the bias dependent on/off state in a Ni/NiO/CoNi/NiO/Co magnetic tunnel junction (MTJ) in nanowire geometry at T=1.5K [4]. This behavior may be explained by a metal-insulator transition driven by a charge accumulation controlled by applied bias voltage.

[1] J. Velev, R. F. Sabirianov, S. S. Jaswal and E. Y. Tsymbal, “Ballistic anisotropic magneto-resistance”, Phys. Rev. Lett. 94, 127203 (1-4) (2005).
[2] G. Dearnaley, A. M. Stoneham, and D. V. Morgan, “Electrical phenomena in oxide films”, Rep. Prog. Phys. 33, 1129-1191 (1970).
[3] R. Waser, Nanoelectronics and Information Technology (Wiley, Weinheim, 2003).
[4] A. Sokolov, R. Sabirianov, I. Sabirianov, and B. Doudin, “Resistive switching of tunneling magnetoresistance in artificial nano-filaments”, submitted paper.
Nov. 6VojtaMichel Gingras
U of Waterloo
 Geometric Frustration in Magnetic Pyrochlore Oxides   Geometric frustration arises in a magnetic system whenever the interacting magnetic moments cannot minize their total classical energy by minimizing their pairwise magnetic interactions, pair by pair. The pyrochlore oxides, of generic chemical formula R2M2O7 (R = Y, Gd, Tb, Ho, Dy, Er, Yb; M = Mo, Mn, Ti, Sn), are highly prone to magnetic frustration effects and have attracted wide theoretical and experimental attention over the past fifteen years. In these systems, both the trivalent rare-earth ions (R or yttrium) and the tretravalent transition metal ions M reside on distinct interpenetrating three-dimensional lattices of corner-sharing tetrahedra. It has become standard practice in the field to simply refer to this lattice structure as the pyrochlore lattice. The pyrochlore lattice leads to extremely high level of geometric magnetic frustration when the nearest-neighbor R-R or M-M exchange interaction between Heisenberg spins is antiferromagnetic. Interestingly, in the case of extreme local Ising anisotropy, as realized in Dy2Ti2O7 and Ho2Ti2O7, ferromagnetic interactions are also frustrated, giving rise to a so-called (spin ice) phenomenology analogous to the problem of entropic proton disorder in common water ice. Because of the high level of frustration, the ground state of magnetic pyrochlore oxides is highly sensitive to small perturbations of the spin Hamiltonian beyond the nearest-neighbor coupling. As a result, these materials exhibit a plethora of very interesting thermodynamic and magnetic phenomena. Examples of phenomenologies include long range order, spin glass, spin ice, collective paramagnetism, anomalous Hall effect, persistent spin dynamics, order by disorder, magnetization plateau, and seemingly hidden order. In this talk, I will review some of the salient problems associated with the collective thermodynamic and magnetic phenomena offered by these fascinating materials. 
Nov. 13Yamilov  Lev Deych
City U of New York
 Whispering gallery modes in microspherical resonators:
coupling and
 Whispering gallery modes (WGM) are high-Q resonances arising in optical microresonators such as microspheres or microdisks. In this talk I will present a unified theoretical approach to description of evanescent coupling between several resonators arranged in a linear chain and Rayleigh scattering of WGMs due to an individual subwavelength scatterer.
Nov. 20 YamilovThomas Vojta2008 Nobel Prize in Physics: Spontaneous Broken Symmetry This year's Nobel Prize in Physics was awarded jointly to Yoichiro Nambu for "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics" and to Makoto Kobayashi and Toshihide Maskawa "for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature". This talk gives an elementary introduction into the physics of spontaneous symmetry breaking, and its consequences for shaping the world as we know it.
Dec. 4ParrisFifteenth Annual Laird D. Schearer Prize Colloquium  
Dec. 11 Yamilov Ping Yu
Physics of Emerging Biomedical Optical Imaging
 In this talk I will present two new optical techniques for biological tissue imaging and discuss the physics behind these imaging modalities. The first technique is coherence domain imaging based on dynamic holography for high-speed and high-resolution biomedical imaging. The holograms are recorded in vivo in a photorefractive device and reconstructed in a conventional CCD camera. This technique can be used to monitor interactions between targeted fluorescence peptide conjugates and cancer cells at a tissue level. The second technique is fluorescence mediated tomographic imaging using an image-intensified CCD camera. The photon density waves are modulated in radio frequency and the phases of the waves are used for the tomography. Instead of measuring the phase in radio frequency, the new technique measures the phase in the “beat” of two radio frequency waves, which is a sound frequency. This work is expected to lead to the development of a new biomedical optical imaging technique which will greatly improve the specificity and accuracy of anatomical and functional information for early cancer detection and therapy-based pharmaceutical development.
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