Spring 2006 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. Abstracts are included below the schedule. The ORNL Physics Division 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.

Abstracts

January 30

Dr. Steven Moszkowski, UCLA

Personal Recollections of Albert Einstein

My grandparents were good friends of Albert Einstein in Berlin. Later my parents also were on friendly terms with him. I had the opportunity to meet Einstein four times after my parents and I came to the United States in 1940. My parents and I, on occasion, had correspondence with Einstein and took a few photos of him. Albert Einstein had considerable influence on my development and style of doing physics, as I will discuss.

February 20

Dr. C.O. Reinhold, ORNL Physics Division

Manipulating Rydberg Wavepackets

Manipulation and control of the electronic states of atoms provide an exciting area of research with important potential applications. Such control can be achieved using electromagnetic pulses whose strengths are comparable to the Coulomb electric field, whose durations are of the order of the classical orbital period of the atom, and whose shapes can be tailored at will.

Possibilities for tailoring atomic wavefunctions of very high-n Rydberg atoms will be discussed and various examples will be presented for creating specific wavepackets. Since the energy levels in atoms are not equispaced, Rydberg wavepackets are transient entities (even without external influence). However, wavepackets can be "trapped" for extended periods using trains of short unidirectional electric field pulses, termed half-cycle pulses (HCPs), and later "released" simply by turning off the pulses. Trapping results because the combination of the Coulomb interaction and the external periodic driving field gives rise to sizable stable islands in an otherwise chaotic phase space from which the electron cannot escape. Trapping islands can also be used as tweezers capable of picking up the electron in a given region of phase space and moving it to a different region by using a carefully tailored train of HCPs with varying frequency and strength.

When a Rydberg wavepacket interacts with an environment (e.g. an ambient gas), the interaction leads to irreversible dephasing, referred to as decoherence. Given recent developments, it is possible to envision studies of decoherence of Rydberg wavepackets interacting with "partially controlled" environments (e.g. an ambient gas whose density can be varied). This opens up the possibility of performing new fundamental studies of how the classical world emerges from the quantum world as well as new potential applications. For example, decoherent dephasing could be used as a tool to measure cross sections for quasi-elastic electron-atom (or molecule) collisions at energies extending down to micro electron volts.

February 27

Dr. Robert Grzywacz, UT Physics

Life on the Edge: Experiments with Single Nuclei

The main thrust in modern day nuclear physics is the study of nuclei with unusual combinations of protons and neutrons. Some of the key questions are: What are the possible particle stable combinations of nucleons? Will a New Order emerge for nuclei with large excesses of either protons or neutrons? What is the effect of the nuclear medium on inter-nucleon interactions? How relevant are properties of the exotic short-lived nuclei to the elemental compostion of our Universe? In recent years, I have conducted research at heavy ion accelerator facilities in the US, France, and Germany, chasing the quickly decaying single ions of the most exotic nuclei. In my talk, I shall explain the challanges of experiments addressing the nature of our world's femtostructures.

March 27

Dr. Hanno Weitering, UT Physics

Hard Superconductivity in Soft Quantum Films

Superconductivity is inevitably suppressed in reduced dimensionality. Questions of how thin superconducting wires or films can be before they lose their superconducting properties have important technological ramifications and go to the heart of understanding coherence and robustness of the superconducting state in quantum-confined geometries. In this talk, I will show how quantum confinement of itinerant electrons in a soft metal, Pb, can be exploited to stabilize superconductors with lateral dimensions of the order of a few millimeters and vertical dimensions of only a few atomic layers. These extremely thin superconductors show no indication of defect- or fluctuation-driven suppression of superconductivity and sustain enormous supercurrents of up to 10% of the theoretical depairing current density. Their magnetic hardness implies a superconducting critical state with strong vortex pinning that is attributed to quantum trapping of vortices. Our study paints a conceptually appealing, elegant picture of a model nanoscale superconductor with calculable critical state properties and surprisingly strong phase coherence. It indicates the intriguing possibility of exploiting robust superconductivity at the nanoscale.

March 6

Dr. Daniel Khomskii, University of Cologne

Multiferroics: Different Ways to Combine Magnetism and Ferroelectricity

Multiferroics - materials which are simultaneously (ferro)magnetic and ferroelectric, and often also ferroelastic, attract now considerable attention, both because of the interesting physics involved and as they promise important practical applications. In this talk I will give a survey of microscopic factors determining the coexistence of these properties, and will discuss different possible routes to combine them in one material. In particular the role of the occupation of d-states in transition metal perovskites will be discussed, possible role of spiral magnetic structures will be stressed and the novel mechanism of ferroelectricity in magnetic systems due to combination of site-centered and bond-centered charge ordering will be presented. Microscopic nature of multiferroic behavior in several particular materials, including magnetite Fe3O4, will be also discussed.

April 3

Dr. Robert Brandenberger, McGill University

String Theory and the Birth of the Universe

In this colloquium I will explain why the inflationary universe scenario, our current paradigm for early universe cosmology, is incomplete, in spite of its phenomenological successes. A better theory of fundamental physics is required to understand the earliest stages in the evolution of the universe - and string theory is a candidate for such a theory. I will explore new cosmological scenarios which emerge from the confluence of string theory and cosmology, models which may not contain a big bang singularity.

April 10

Dr. Aron Pinczuk, Columbia University and Bell Labs

"Shining Light" on Quantum Hall Fluids

Electron fluids in the quantum Hall regimes support low-energy excitation modes that are linked to remarkable behaviors that emerge from fundamental interactions in two-dimensions. Inelastic light scattering methods at very low temperatures (below 1 Kelvin) offer unique experimental venues to study excitations in the charge and spin degrees of freedom of the fluids.

The light scattering experiments access directly low-lying “quasiparticle” excitations above the fluid ground states. These are the excitations that express distinct quantum phases of the electron liquids. This talk presents an overview of recent results that reveal physics of the electron fluids that is not accessible by other methods.

April 19: Special Colloquim

Dr. Russell A. Hulse, 1993 Nobel Prize Winner, The University of Texas at Dallas and Princeton University Plasma Physics Laboratory

The Discovery of the Binary Pulsar

Pulsars are rapidly rotating neutron stars which are observed using radio telescopes as pulsed, short-duty-cycle radio sources with typical periods on the order of 1 second or less. The mechanism underlying this pulsed emission is analogous to that of a lighthouse beacon, in that the observed pulses are the result of a tightly beamed pattern of radio radiation from the star which periodically sweeps across the earth as the star rotates. In 1974 a high sensitivity search to discover previously unknown pulsars using the 1000' Arecibo radio telescope discovered 40 new pulsars in our galaxy, including the 59 ms period pulsar PSR 1913+16. Puzzling and unexpected variations in the observed pulsation period of this object were ultimately understood to be the result of doppler shifts associated with the orbital motion of this pulsar around a companion star. Discovery of this "binary pulsar" has provided an almost textbook-perfect laboratory for studying effects predicted by Einstein's theories of relativity, by combining high orbital velocities and gravitational fields with the measurement capabilities afforded by use of the pulsar as an extremely precise clock. Continued high precision observations by Prof. Joseph Taylor and his colleagues of this orbiting pulsar clock over the 30 years since its discovery have led to ever more accurate confirmation of these relativistic effects, including decay of the pulsar's orbit due to the emission of gravitational radiation.

April 17

Dr. Armand A. Lucas
Honorary Prof. at the Facultés N.-D. de la Paix in Namur, Belgium

Diffraction of X-rays and of Electrons by Helical Molecules: Determination of the Structure of DNA and Carbon Nanotubes

The ubiquitous and beautiful helical organization of the biological world at the molecular level of DNA [1], protein alpha-helices [2], etc... as well as at the mesoscopic scale of viruses [3], cellular fibers [4], etc... has been investigated mostly by diffraction methods. More recently Carbon nanotubes were discovered [5] by high resolution electron microscopy and their detailed atomic structure has again been determined by electron diffraction [5], [6]. In this lecture I will use optical simulation experiments (the optical transform method) to explain just how X-ray fiber diffraction and electron diffraction have been used to reveal the helical arrangements of DNA and Carbon nanotubes [7]-[10].

The audience will have the opportunity to take part, hands on, in the optical simulation experiments.

[1] Franklin R.E. and Gosling R.G., Nature 171, 740, 1953; Watson J.D. and Crick F.H.C., Nature 171, 737, 1953
[2] Cochran W., Crick F.H.C. and Vand V., Acta Crystallogr. 5, 581, 1952
[3] Klug A. and Finch J.T., J. Mol. Biol. 31, 1, 1968
[4] Amos L.A. and Baker T.S., Nature 279, 607, 1979
[5] Iijima S., Nature 354, 56, 1991
[6] Lambin Ph. and Lucas A.A., Phys. Rev. B56, 3571, 1997
[7] Lucas A.A., Lambin Ph., Mairesse R. and Mathot M., J. Chem. Educ. 76, 378 (1999)
[8] Lucas A.A., Moreau F. and Lambin Ph., Rev. Mod. Physics 74, 1, 2002.
[9] Lucas A.A., Int. J. Quantum Chemistry 90, 1491-1504 (2002).
[10] Lucas A.A. and Lambin Ph., Rep. Prog. Physics 68, 1181-1249, 2005

Previous Physics Department Colloquia:

• Fall 2005
• Spring 2005
• Fall 2004
• Spring 2004
• Fall 2003
• Spring 2003
• Fall 2002