Colloquia Archive: 2002

February 1, 2002, 3:00pm

Mark B. Hoffman, University of Chicago
Testing Inflation
Location: Physics 152

Measurements of the the Cosmic Microwave Background (CMB) have recently begun to probe inflation, the leading candidate for a description of the Universe for the first 10-32 seconds after the Big Bang. So far, the idea has passed the observational tests. Due to the influx of data, theorists today are looking for new and more restrictive tests of the inflationary paradigm; however, since there is no preferred model of inflation, new general tests are difficult to come by. In this talk, I will describe some of the theoretical challenges involved in testing inflation along with an attempt to produce a test that could potentially rule out a wide class of models.

February 8, 2002, 3:00pm

Jiufeng Tu, Brookhaven National Laboratory
Probing Superconductors with Infrared — Electron-boson coupling in high-temperature superconductors
Location: Physics 152

High-temperature superconductivity in an oxide containing quasi-two-dimensional copper-oxygen planes was observed by Bednorz and Mueller in 1986. Recently, several high-temperature superconductors without copper or oxygen have been discovered including MgB2 with a Tc of 39K and electric field doped C60 with a Tc as high as 117K. Infrared spectroscopy has emerged as one of the most powerful experimental tools for the study of correlated electron systems and for high-Tc superconductors in particular. This talk will be focused on the infrared studies of two representative high-Tc superconductors: MgB2 (Tc =39.6 K) and optimally doped Bi2Sr2CaCu2O8+d (Tc = 91.5 K). Effects of electron-boson coupling are observed in optical conductivities for both systems and their significance with respect to superconductivity will be discussed. In general, having a small free carrier plasma frequency (< 3 eV) seems to be an universal characteristic shared by almost all high-temperature superconductors with a Tc > 30 K which means that the issue of reduced screening should be treated carefully in all of these systems.

February 15, 2002, 3:00pm

Masashi Yamaguchi, University of California, Riverside
Light scattering and nonlinear spectroscopy in disordered systems and crystals near structural phase transitions
Location: Physics 152

Results of recent light scattering and ultrafast nonlinear spectroscopic studies of phonon dynamics in condensed matter systems are presented. In this talk, I will focus on the following: (1) Quantum paraelectrics and structural phase transitions in ferroelectrics and related materials: Optical spectroscopy has been used to study structural phase transitions and related phenomena in condensed matter systems. Perovskite-type crystals provide a good model system for fundamental research on phase transition dynamics and have properties applicable to non-voltaic memories (FeRAM), actuators, and DRAM. Here I discuss in detail the results of light scattering studies of hexagonal BaTiO3 near the successive structural phase transitions and SrTiO3 in the quantum paraelectric regime. (2) Anharmonicirty of strongly localized modes and low temperature dynamics in glasses: Vibrational dynamics in disordered systems are examined using low-frequency light scattering and photon echo spectroscopy. Strongly disordered materials such as glasses exhibit distinctly different behavior from crystals at low temperature because of their lack of translational symmetry and inclusion of intrinsic internal stress. Two level tunneling systems (TLS) and strongl localized vibrational states above the mobility edge energy are considered to characterize amorphous and glassy structures. The combination of light scattering experiments and high-pressure diamond anvil cell technique revealed a strong anharmonic character of the low-energy vibrations. The results are discussed in connection with the low temperature thermal conductivity above so called 'plateau' region, universally observed in glasses. (3) Low frequency modes in molecular liquids probed by ultrafast optical Kerr effect spectroscopy: Recent development of ultrafast techniques has opened a new field for numerous applications of nonlinear optical spectroscopy. Ultrafast optical Kerr effect (OKE) is one of these techniques, wherein an ultrashort pump pulse creates a time-dependent optical anisotropy. Although the total OKE response is described by a third-order dielectric susceptibility tensor, the response of the nuclear part can be described by a product of first-order dielectric constants, similarly to the response function obtained from light scattering. Time-domain experiments are particularly suited for probing low frequency modes in disordered materials. Femtosecond OKE results were compared experimentally with the results of high-resolution light scattering in molecular liquids, demonstrating complete agreement in a frequency region extending over 3 orders of magnitude.

February 22, 2002, 3:00pm

Jan Chaloupka, Brookhaven National Laboratory
Strong-field double ionization of rare gases
Location: Physics 152

Photoionization with a weak field can occur only when the single-photon energy is greater than the electron binding energy. In strong fields however, ionization can occur even for very low photon energies. Since the photons in the field can stack on top of one another, a single atom may absorb N quanta of energy, thereby liberating deeply bound electrons. This so-called multiphoton ionization has been extensively studied for over two decades. Observed ion yields and electron energy spectra from the single ionization process have helped to characterize the strong-field laser-matter interaction. Until recently, the electron energy distributions from the double ionization process have remained a mystery. Through the use of an electron-ion coincidence scheme, we have compiled the double ionization electron spectra from several of the rare gases. Of particular interest are the results from helium and xenon. Helium is the simplest two-electron system and its ionization falls squarely within the tunneling description of ionization, while the more complex ionization of xenon enters the multiphoton regime. We will present our latest results from these studies.

March 1, 2002, 3:00pm

Sarah K. Patch, GE Medical Systems
Computational Methods for Volumetric Computerized Tomography
Location: Physics 152

In computed tomography (CT) imaging, an x-ray beam penetrates the object, and transmitted beam intensity is measured by an array of detectors. A 3D image of the object is reconstructed from a series of the resulting 2D cross-sectional images. CT was introduced in the early 1970s as a neurological examination technique, and later extended to industrial applications. We will describe work on computing unmeasured cone beam projections from measured projections in volumetric computerized tomography (VCT). We do this by solving a characteristic boundary value problem for an ultrahyperbolic differential equation. One potential use for this technique is reduction of cone-angle artifacts suffered by approximate volumetric reconstruction techniques, including Feldkamp. By working in the Fourier domain, we convert the 2nd order PDE into a family of 1st order ODE's. A simple 1st order integration is used to solve the ODEs.

March 4, 2002, 3:00pm

Benjamin Owen, University of Wisconsin-Milwaukee
The r-mode instability, or: How the Coriolis force makes gravitational waves
Location: Physics 152

In the past four years there has been considerable excitement in the relativistic astrophysics community over the prospect that some fluid oscillations of rapidly rotating neutron stars are driven unstable by gravitational radiation reaction in astrophysically realistic conditions. This instability has been proposed as an explanation for the relatively slow spins of young neutron stars and those in low-mass x-ray binaries, and has been predicted to produce gravitational waves detectable by LIGO at a rate comparable to that of binary inspirals.

March 6, 2002 , 3:00pm

Alan G Wiseman, University of Wisconsin-Milwaukee
Memory effects in gravitational radiation
Location: Physics 152

There is a mysterious phenomenon in gravitational wave detection: the detector “remembers” that a wave has passed through it. Although seldom thought of in this way, this effect has a simple electromagnetic counter part. I will exploit the electromagnetic analogy to present an intuitive understanding of the origins of memory. I will also explore a memory effect that is unique to gravitational theory: the Christodoulou non-linear memory. I will also examine a number of astrophysical phenomena, such as gravitational scattering, gamma-ray bursts and stellar collisions, that may plant memories in a gravitational wave detector.

March 8, 2002, 3:00pm

Jolien Creighton, University of Wisconsin-Milwaukee
To hear a black hole with LIGO
Location: Physics 152

Gravitational waves must be radiated from systems that produce oscillating tidal fields. Waves from inspiralling massive bodies, such as binary black holes, will be detectable out to cosmological distances using the new laser interferometer detectors such as LIGO. The detection of these waves will give us our first observations of strong field gravity, and will open up a new field of observational astronomy. To detect these waves we need to develop data analysis strategies that make optimal use of our knowledge of the expected waveforms in order to extract the weak signals from the interferometer noise.

March 13, 2002, 3:00pm

Dennis Ugolini, California Institute of Technology
Plans and Progress at the Caltech LIGO 40-Meter Prototype
Location: Physics 152

The Caltech LIGO 40-meter gravitational wave interferometer prototype is being rebuilt to investigate resonant sideband extraction (RSE), a new optical technique to improve sensitivity to gravitational waves at high frequencies. In this technique a signal recycling mirror is added to the asymmetric port of the interferometer, and the compound cavity formed by this mirror and the interferometer arms is tuned to resonate at the signal sidebands, the beats between the carrier frequency and the gravitational-wave signal. The 40-meter will be an engineering prototype for RSE, testing the electronics and controls scheme before delivery to the main LIGO sites, as well as a shot-noise-limited testbed to reveal the effects or RSE on the high-frequency sensitivity curve. The recommissioning of the interferometer is on schedule for completion in early 2003, with significant progress in several systems: seismic isolation, vacuum controls, environmental monitoring, data acquisition, pre-stabilized laser commissioning, and suspended optic and control scheme design.

April 5, 2002, 3:00pm

Robert Olsson, Milwaukee School of Engineering
The effect of defects on the melting transition and critical points in YBa2Cu3O7-d single crystals
Location: Physics 152

Within the mixed state of YBa2Cu3O7-d (YBCO), a first-order vortex phase transition exists between the vortex lattice and liquid states. Since vortex lines can be pinned by defects, we have investigated the transition via the introduction of defect tracks created by high energy (> 1GeV) heavy ions. The resultant columnar defects not only increase critical currents by arresting vortex motion, but also affect the nature and position of the phase transition. Dependent on the defect density, we see either a vortex liquid-to-lattice or a liquid-to-Bose glass transition. For large defect densities a kink in the Bose glass transition is observed at fields in which the defect density and vortex density are equal. Both high and low defect density data will be presented, and the role of defects on the upper and lower critical points will be discussed.

April 12, 2002, 3:00pm

Mark D. Pauli, University of Wisconsin-Milwaukee
Investigation of Surface Structure with X-ray Absorption and Emission
Location: Physics 152

The use of electron spectromicroscopy for the study of the chemical composition and electronic properties of surfaces, overlayers, and interfaces has become widely accepted. Improvements to the optics of instruments such as the X-ray photoelectron emission microscope have pushed spectroscopic microscopies into the realm of very high spatial resolution, at and below 1 micrometer. Coupled with the high spectral resolution available from third generation synchrotron sources, this spatial resolution allows the measurement of micro-X-ray absorption near-edge spectra in addition to the more typical electron emission spectra and diffraction patterns. Complementary to the experimental developments is the development of improved theoretical methods for computational modeling of X-ray absorption and emission spectroscopies. In particular, a new path-reversal formalism is expounded for the calculation of photoelectron and Auger electron diffraction.

April 19, 2002, 3:00pm

Alex L. Burin, Northwestern University
Understanding and Controlling of Random Lasing
Location: Physics 152

Random lasing has attracted attention because of its fundamental significance for understanding coherent phenomena in disordered media and for its potential applications in optoelectronics (easy preparation, no need for mirrors, and small size down to several microns). Since the first experimental demonstration by Cao and coworkers in 1998 of the lasing emission from ZnO nanopowder, remarkable progress in studying the material, geometry, and external pumping dependences of laser properties and efficiency has been made. I will demonstrate that lasing occurs from special random cavities of high quality formed within the active medium. They can be described as the decaying eigen optical modes within the medium and the optical mode having the minimum decay rate is responsible for lasing. Numerical and analytical studies of the properties of these modes permit nterpretation of existing experiments and suggest ways to optimize the performance of lasers.

April 26, 2002, 3:00pm

Juan Carlos Campuzano, U. Illinois at Chicago
Spontaneous time reversal symmetry breaking in high temperature superconductors
Location: Physics 152

Within the mixed state of YBa2Cu3O7-d (YBCO), a first-order vortex phase transition exists between the vortex lattice and liquid states.� Since vortex lines can be pinned by defects, we have investigated the transition via the introduction of defect tracks created by high energy (> 1GeV) heavy ions. The resultant columnar defects not only increase critical currents by arresting vortex motion, but also affect the nature and position of the phase transition. Dependent on the defect density, we see either a vortex liquid-to-lattice or a liquid-to-Bose glass transition. For large defect densities a kink in the Bose glass transition is observed at fields in which the defect density and vortex density are equal. Both high and low defect density data will be presented, and the role of defects on the upper and lower critical points will be discussed.

September 27, 2002, 3:00pm

Carol Hirschmugl, University of Wisconsin-Milwaukee
Frontiers of Infrared Spectroscopy: Applications in Physics, Chemistry and Biology
Location: Physics 152

I will introduce the basics of infrared spectroscopy, and then give two examples of state-of-the art research ongoing in my laboratory. In the first example, infrared images of single cells of algae will be shown. Measurements were made on algae that were provided sufficient nutrients, and deprived of nutrients. Results for two algae cells show the effect on the chemical pools within the cells. In the second example, electronic friction at the interface between one layer of molecules and a metal surface can be probed. I will describe what electronic friction is and show the effects of different molecules.

October 21, 2002, 3:00pm

Professor Peter Dyson, La Trobe University
Opportunities for Study in Australia: The University of Wisconsin – La Trobe University Exchange Program in Physics
Location: Physics 152

Over the last few years a link has developed between the Physics Departments at University of Wisconsin-Milwaukee and La Trobe University in Australia. One result is that the La Trobe Physics Department courses are now clearly specified in terms of their US system credit point equivalents, making it straightforward for University of Wisconsin-Milwaukee students to study for one or more semesters in Australia and be certain of gaining appropriate credit towards their US degree. This talk will describe the La Trobe Physics Department, its undergraduate course structure as it relates to the US system, its postgraduate programs and research areas, course fees and what to expect when living in Australia.

November 15, 2002, 3:00pm

Professor Ellen Zweibel, University of Wisconsin-Madison
Magnetic Fields in Galaxies and Beyond
Location: Physics 152
Not yet available

November 22, 2002, 3:00pm

Professor Patrick Brady, University of Wisconsin-Milwaukee
Ripples in spacetime: gravitational wave astronomy and what it might tell us
Location: Physics 152

A fundamental prediction of General Relativity, Einstein's theory of gravity, is the existence of gravitational waves—ripples in the fabric of spacetime itself. The direct observation of gravitational waves is a realistic goal for kilometer-scale interferometers, such as LIGO, which are now under construction at various sites around the world. After reviewing some basic properties of gravitational waves, I will briefly describe the laser interferometers which will be used as gravitational wave detectors, paying particular attention to the ultimate sensitivity of these instruments. Gravitational waves are produced by bulk motions of matter, and therefore carry information about the universe in the large. The remainder of the talk will be devoted to a discussion of potential sources and what we may learn by observing them gravitationally; the emphasis will be on compact coalescing binary systems, black-hole/black-hole mergers, and rapidly rotating neutron stars.