Fall Colloquium Series
Physics Department, University of Oregon

3:30 Thursdays, 100 Willamette (except week of 10/31)
(Reception at 3:15 in the Atrium)

October 3 Dietrich Belitz University of Oregon	State of the Department and Nature of the Quantum Phase Transition in Itinerant Ferromagnets After the traditional State of the Department address, I will discuss a theory for the quantum, or zero-temperature, transition from a metallic paramagnet to a metallic ferromagnet. I will focus on the order of the transition, which is observed to be of first order in some materials (e.g., MnSi, UGe2), but of second order in others (e.g., ZrZn2, NiPd). (Host: N/A)
October 10 Joe Niemela University of Oregon	Beyond the Borders of Disorder: New Results and Insights in Turbulent Thermal Convection Turbulent thermal convection is ubiquitous in nature and in engineering, and plays a prominent role in the energy transport within stars, atmospheric and oceanic circulations, the generation of the earth's magnetic field, and innumerable engineering processes in which heat transport is an important factor. In this talk, I will discuss and review recent results of laboratory and numerical experiments that have significant bearing on the contact between observations and theory. In particular, the talk will discuss the interplay between large-scale coherent structures, their self-organization from random fluctuations, and their coupling to the geometry and lateral boundary conditions of the container.
October 17 Bob Pelcovits Brown University	Numerical Simulations of Liquid Crystals Ever increasing computer power has made simulations of liquid crystal systems a feasible enterprise. Larger, more complex systems can now be simulated for longer times and many interesting phenomena can be studied for the first time. With the aid of computers it is possible to begin to study the effects of molecular shapes, sizes and interactions on macroscopic behavior. While present day computers are not powerful enough to simulate a large system of liquid crystal molecules, each modeled with atomic detail, it is possible to study simple models which capture the essential physics. In this talk I will describe how liquid crystal systems are modeled on computers and discuss the results of a number of simulation studies of liquid crystal physics. (Host: J. Toner)
October 24 Henri Jansen Oregon State University	Magnetic Anisotropy in Transition Metals Magnetism is a complex phenomenon, since it has both a magnitude and a direction. In magnetic materials the direction of the magnetization is determined by either the shape of the sample or by the crystalline arrangement of the atoms. All applications depend on this magnetic anisotropy. In this talk I will discuss the basic idea of magnetic anisotropy, how one can calculate the magnetic anisotropy from first principles, and which numerical problems play a role. This is a very interesting problem in computational physics. In addition, I will show the results of our work relating changes in the magnetic anisotropy to changes in the environment of the atoms. These results are still very puzzling. (Host: D. Belitz)
MONDAY, October 28, 4PM, 177 Lawrence (NOTE: Unusual day of week, time, and place) Rocky Kolb University of Chicago/Fermilab	Superheavy Dark Matter In the last few years, a new scenario for the cosmic origin of dark matter has emerged. Rather than resulting from collisions of particles in the hot primordial soup, dark matter would be produced by the conversion of virtual particles to real particles caused by the rapid expansion of the universe during the inflationary epoch. Implications of the new source of dark matter for present dark matter searches will be discussed. (Host: N. Deshpande)
November 7 Tom Steiman-Cameron NASA Ames Laboratory	Spiral Structure in the Milky Way Galaxy The geometry of the Milky Way's spiral arms provides important constraints on the underlying physics of star formation, the production of spiral arms, the long-term maintenance of these structures, and galaxy evolution. Many approaches have been pursued to decipher the geometry of the arms, often with conflicting results. While general consensus exists that a ``global'' pattern is present, considerable disagreement remains in the details. Even the number of spiral arms remains open to debate. I will review models proposed for over the past two decades in the light of several recent all-sky surveys and present a new model which maximizes agreement with recent satellite data. (Host: J. Imamura)
November 14 Daniel Chemla Department of Physics, Unversity of California at Berkeley, Advanced Light Source and Materials Sciences Division,Lawrence Berkeley National Laboratory, Berkeley	Title Macroscopic Coherence in Degenerate Exciton gases More than thirty years ago Keldysh and Kozlov [1] have shown that in the dilute limit, excitons should undergo Bose-Einstein condensation (BEC). Because of their very small mass at experimentally accessible densities the 3D critical temperature for exciton--BEC should be about five orders of magnitude higher than for atom--BEC. To date, however, exciton--BEC has not been observed. We report on two sets of experiments where we have observed non-classical effects in highly degenerate exciton--gases. We exploit the properties of quasi--2D indirect excitons (i-X) in GaAs/AlGaAs coupled quantum wells (CQW) [2]: (i) long lifetime, (ii) efficient cooling via emission of bulk LA phonons, (iii) repulsive interaction, r-3, which favors condensation, limits screening and prevents collapse toward molecules and eventually droplets or plasmas. Inspired by atom--BEC which is observed for atomic gases confined in potential traps, we collected i-Xs in an in-plane natural potential trap [3]. Spectrally and spatially resolved photoluminescence (PL) under uniform and localized excitation far away from the traps reveal i-X transport over distances l 300 mm and collection the trap at densities, NXtrap 10 11cm-2, corresponding a Bose occupation number of the lowest energy state 0.3 0.5. Exploring the i-X PL away from any trap, we have observed a concentric-rings structure and a macroscopically ordered state of i-X appearing in the ring the most remote from the excitation spot [4]. The most interesting feature of that ring is its abrupt fragmentation at temperatures T<3K into a periodic array of circular fragments. The existence of this periodic ordering shows that the i-X state formed in the ring has a macroscopic coherence of length scale Lcoh 1mm. This coherence appears spontaneously is not driven by a laser excitation. Macroscopically ordered arrays of vortices in quantum liquids, such as superconductors, He-II, and atom BEC, demonstrate quantum macroscopic coherence in flowing superfluids. We note however that spontaneous macroscopic flow organization with periodic vortical structures is a general property of thermodynamically open systems described by nonlinear partial differential equations, including classical ones [5]. [1] Keldysh, L.V. and Kozlov, A.N. Zh. Eksp. Teor. Fiz. 54, 978 (1968) {Sov. Phys. JETP 27, 521 (1968)}. [2] Butov, L.V. and Filin, A.I. Phys. Rev. B 58, 1980 (1998). [3] Butov, L.V., Lai, C. W., Ivanov, A.L., Gossard, A. C., and Chemla D. S. Nature, 417, 47 (2002). [4] Butov, L.V. Gossard, A. C., and Chemla D. S. Nature.418, 751 (2002) [5] Taylor, G.I. Philos. Trans. R. Soc. London Ser. A-223, 289 (1923). (Host: S. Kevan)
November 21 Steve Kevan University of Oregon	How Magnets Forget Magnetic hysteresis is fundamental to all magnetic storage technology, and this technology is one cornerstone of our present information age. Yet, despite decades of intense study, we still do not have a fully satisfactory microscopic understanding of magnetic hysteresis. An interesting issue in magnetic hysteresis, return point memory, was the subject of Madelung's 1905 dissertation. This pertains to a magnet's ability to return to the same point on the major magnetization loop after having been driven onto a minor loop inside the major loop. We have developed a speckle metrology technique that combines the magneto-optical contrast of soft x-rays with the high optical brightness of undulator radiation to test Madelung's hypothesis microscopically. That is, when the system returns to the point it left on the major loop, does it remember (i.e., return to) the same original microscopic configuration of magnetic domains, or does it remember only the ensemble average? We find that the loss of memory is gradual and seems to follow an exponential behavior.
November 28 Thanksgiving Holiday	No Colloquium
December 5 Laura Peticolas University of California at Bezerkeley	Time-dependent Transport of Electrons in the Aurora Studying the aurora provides a unique opportunity to understand non-thermal plasma physics, much of which cannot be studied in laboratory plasma experiments. The light of the aurora is caused by high energy (.5-30 keV) electrons from the magnetosphere which collide with the upper atmosphere. At times, processes at the ionosphere-magnetosphere boundary cause the aurora to turn on and off at frequencies from 10-100 Hz. This type of aurora is known as flickering aurora. In order to understand the atmospheric and emission response to such high frequency changes, a time-dependent transport equation must be solved. After a general overview on the physics of the aurora and the types of aurora one observes from the ground, I will discuss the physics of the electron transport equation and the first solution to the time-dependent equation. (Host: S. Micklavzina)

Send comments to the colloquium chairman, John Toner

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