Physics Colloquium - Winter 2000

Eric Torrence University of Chicago

TITLE:

Measuring the W mass at LEP II

ABSTRACT:

The LEP accelerator at CERN has been running at collision energies approaching 200 GeV since 1996. Along with searching for new phenomena, one of the primary goals of the LEP II program is a precise measurement of the W boson mass. In this lecture, I will describe the LEP II accelerator at CERN, and detail the analysis performed by the OPAL collaboration to measure the W boson mass. The current status of other precision electroweak measurements will also be presented.

Paul Burrows Center for Photonics and Optoelectronic Materials, Princeton University, Princeton, NJ.

TITLE:

Vacuum Deposited Organic Light Emitting Displays: Colors, Architectures and Efficiency

ABSTRACT:

We review recent results from small molecule organic light emitting devices (OLEDs) and their implications for full color organic displays. The seminar will focus on the unique properties of organic semiconductors which potentially enable new display applications such as foldable or flexible displays. In particular, progress towards a three dimensional stacked pixel architecture for full color display applications will be described, including recent improvements in transparent cathodes using sputter-deposited indium tin oxide (ITO) to achieve > 70% pixel transparency at low operating voltages. Until recently, light emission from OLEDs utilized the radiative recombination of singlet excitons, either formed directly on the dye molecules or transferred from the host molecules by Forster energy transfer. This intrinsically limits the internal quantum efficiency of OLEDs to ~ 25%, since ~ 75% of electrically injected charge carriers form non-emissive triplet excitons. Optical losses in the OLED reduce the maximum achievable external quantum efficiency to ~ 5%. The development of organic electrophosphorescence using organic dyes such as iridium tris(phenylpyridine) (Ir(ppy)3) has led to dramatically increased device efficiencies. Spin-orbit coupling via the heavy metal ion in such molecules mixes the singlet and triplet states, potentially allowing radiative recombination of 100% of the electrically generated excitons. Using Ir(ppy)3-based OLEDs, we have demonstrated phosphorescence at an external quantum efficiency of > 10% and luminous efficiency of > 30 lm/W.

Heiner Linke Physics Department, University of New South Wales, Sydney, Australia.

TITLE:

Muscles, Molecules and a Quantum Version of Maxwell's Demon

ABSTRACT:

Over the last few years it has been realized that a number of biological processes, for instance the contraction of muscles and intracellular transport, may make use of a fascinating physical principle: In periodic, asymmetric potentials, so-called ratchets, the random motion of Brownian particles can be put to use by extracting energy from an environment that is not in thermal nonequilibrium.

In this seminar talk I will introduce the ratchet concept and point out its possible relevance to some molecular motors in living cells. Our own research focuses on experimental realizations of so-called "quantum ratchets" using mesoscopic nano-devices. A quantum dot ratchet, based on electron interference, and a tunnelling ratchet will be described. The tunnelling ratchet turns out to have the intriguing feature to be able to sort electrons by energy, similar to the task assigned to Maxwell's Demon - luckily, without violating the second law.

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Masahiro Morii Stanford Linear Accelerator Center, Stanford, CA.

TITLE:

CP Violation in Experimental Particle Physics

ABSTRACT:

Violation of the CP invariance, considered as responsible for the creation of the matter-dominant universe, was first discovered in 1964. After 36 years of extensive experimental studies, the origin of the CP violation remains one of the least-tested aspects of the Standard Model of Particle Physics. In this talk, I will present an overview of our knowledge of the CP violation from an experimental physicist's perspective.

Cass Sackett NIST, Boulder CO.

TITLE:

Decoherence of Quantum Superpositions Coupled to Engineered Reservoirs

ABSTRACT:

The theory of quantum mechanics applies to closed systems. In such ideal situations, a single atom can exist, for example, in a superposition of being in two different positions at the same time. Real systems, in contrast, always interact with their environment, with the consequence that macroscopic quantum superpositions like Schrodinger's cat are not observed. Moreover, macroscopic superpositions decay so quickly that the dynamics of decoherence can not even be observed. However, mesoscopic systems offer the possibility of observing the decoherence of such quantum superpositions states of the motion of a single trapped atoms. Decoherence is induced by coupling the atoms to engineered reservoirs, where the coupling an state of the environment are under the experimenter's control. We exhibit this with three experiments, finding that the decoherence scales exponentially with the square of the size of the superposition.

Manfred Paulini Lawrence Berkeley National Laboratory, Berkeley, CA.

TITLE:

Physics of CP Violation at CDF

ABSTRACT:

Since the surprising discovery of CP violation, observed in neutral K meson decays in 1964, the kaon system provides the only experimental evidence for CP violation to date. The Standard Model of elementary particles, with its three generations, explains CP violation in quark decays in an elegant way, but the origin of CP violation is still not completely known. The system of neutral B mesons is expected to yield large CP violating asymmetries. We discuss the search for CP violation in B decays using the CDF detector at the Fermilab Tevatron Collider.

Dan Heinzen Physics Department, University of Texas, Austin, TX.

TITLE:

Molecules in a Dilute Gas Bose-Einstein Condensate

ABSTRACT:

Recently, several groups including ours have produced a dilute gas Bose-Einstein condensate. Dilute gas Bose condensates are similar in certain respects to other quantum collective systems such as lasers or superfluid helium, but there are important differences as well. One such difference is that atoms in a dilute gas can combine into molecules. We have studied the coherent conversion of Bose-condensed 87Rb atoms into 87Rb2 molecules. The conversion was accomplished through stimulated Raman free-to-bound transitions induced by two laser fields. We observed extremely narrow transition lineshapes, and found that these lineshapes were shifted and broadened by interactions between the molecules and the atomic condensate. With further work, it should be possible to reversibly convert an atomic Bose-Einstein condensate into a molecular condensate through this mechanism. The process we have studied is a natural matter wave analog to optical frequency doubling and parametric down conversion in nonlinear optics.

Robert Leheny Physics Department, MIT, Cambridge, MA.

TITLE:

The Two-Dimensional Quantum Heisenberg Antiferromagnet

ABSTRACT:

Magnetic systems with reduced dimensionality provide excellent models in which to study the varying roles that thermal and quantum effects play in driving phase transitions. In this talk, I present results of neutron scattering experiments on a large spin (S=5/2) 2D Heisenberg antiferromagnet which characterize its spin correlations over a broad range of temperatures. To construct this isotropic magnet, I exploit the presence of a bicritical point in the phase diagram of Rb_2MnF_4 to tune the effective anisotropy in its spin interaction to zero. The resulting measurements permit stringent tests of various theoretical approaches to the 2D Heisenberg system. Specifically, the instantaneous spin correlations reveal a very broad crossover between high-temperature classical behavior and the low-temperature renormalized classical regime of the quantum non-linear sigma model. Results for the dynamic correlations demonstrate properties of dynamic scaling and illustrate the behavior of remnant propagating modes in the short-range ordered state of the system.

Deborah Jin NIST and JILA, University of Colorado, Boulder, CO.

TITLE:

A Quantum Degenerate Fermi Gas of Atoms

ABSTRACT:

Fermions are ubiquitous in nature and their quantum behavior is responsible for the periodic table and the stability of matter. However low density, weakly interacting Fermi systems are exceedingly rare. Building on the technologies that were used to produce the first dilute gas Bose-Einstein condensates in 1995, we have cooled a million potassium-40 atoms to temperatures below a microKelvin and observed effects of the Fermi-Dirac quantum statistics. The atoms are first laser cooled and trapped out of a room temperature vapor. Following this initial cooling, the atoms are magnetically confined and further cooled via forced evaporation. I will discuss the consequences of the fermionic nature of the atoms on the evaporative cooling process and present measurements of the quantum statistics at our lowest temperatures.

John Bohn JILA, University of Colorado, Boulder, CO.

TITLE:

Cold Collisions of Atoms and Molecules

ABSTRACT:

For nearly five years experimental and theoretical studies have revealed an astonishing variety of many-body phenomena in atomic Bose-Einstein condensates and degenerate Fermi gases. Underlying the physics of these quantum gases is the physics of atomic interactions, which follows in turn from the collision characteristics of atoms and molecules at ultralow energies. In this talk I will explore the physics of cold collisions, citing recent work on atomic potassium and molecular oxygen as case studies. These examples illustrate how we can understand and manipulate ultracold dilute matter at the atomic level.

Miriam Deutsch NEC Research Institute, Princeton, NJ.

TITLE:

Self Assembled Photonic Crystals

ABSTRACT:

Ever since the prediction that photonic crystals can lead to an omnidirectional photonic band gap, the goal has been to create three-dimensional photonic crystals for optical wavelengths. As lithographic methods at these length scales are limited mostly to two-dimensional patterning, an alternative method is necessary. One approach for making these materials utilizes sub-micron sized colloidal spheres, which can be induced to self assemble into crystalline structures. Several methods have been proposed to incorporate high refractive index materials into these structures, so that a full photonic band gap will form. Of particular interest are photonic crystals consisting of active materials, such as semiconductors and organic chromophores. In such structures we expect to obtain strong coupling between photonic states and electronic excitations in the material. The immediate result of this should be enhancement of optical nonlinearities, such as gain and nonlinear polarizabilities. The challenges of this field are dual to produce materials that are three dimensionally patterned on sub-micron length scales, and to conduct reliable experiments in which their photonic behavior can be tested. An ultimate goal still remaining is the incorporation of photonic structures in device applications.

Sally Seidel Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM.

TITLE:

Jets, Partons, and the Search for New Physics

ABSTRACT:

The discovery that local gauge invariance provides a foundation for the theory of the strong force has lead to a beautiful sequence of predictions and confirmations in the latter part of the twentieth century. Among these are the existence of the gluons, the naturalness of asymptotic freedom, and the identity of the gauge symmetry responsible for the strong interaction. One well confirmed prediction by QCD is the presence of jets among the products of high energy particle collisions; these are collimated streams of particles whose momenta are understood to reflect the momenta of the quarks and gluons from which they arise. This link that jets provide between large and small scales makes them a useful experimental probe of new physics at short distance scales. In the 1990's, the CDF experiment obtained an unexpected result for the measurement of the inclusive jet cross section in proton-antiproton collisions. Interpretation of the result highlighted the importance of precision measurements of the parton distribution functions (pdf's) that describe the partitioning of the nucleon's momentum among its constituents. An introduction to the parton distribution functions is provided to place them in the context of jet-based searches for new physics. The role of future hadron collider experiments in improving pdf measurements is described.

Mark Spano Naval Surface Warfare Center, White Oak Laboratory, Silver Spring, MD.

TITLE:

Applied Chaos Control

ABSTRACT:

The publication by Ott, Grebogi and Yorke [1] of their theory of chaos control in 1990 led to an explosion of experimental work applying their theory to mechanical systems and electronic circuits, lasers and chemical reactors, and heart and brain tissue, to name only a few. In this talk the basics of chaos control as implemented in a simple mechanical system will be described, as well as extensions of the method to biological applications. Finally, current advances in the field, including the maintenance of chaos and the control of high dimensional chaos, will be briefly discussed. 1. E. Ott, C. Grebogi and J. A. Yorke, Phys. Rev. Lett. 64, 1196 (1990).

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Bernardo Huberman Xerox Palo Alto Research Center, Palo Alto, CA.

TITLE:

The Laws of the Web

ABSTRACT:

The past decade has witnessed the birth and explosive growth of the World Wide Web, both in terms of content and its user population. From a few thousand pages in 1994 the web has grown to several hundred million pages today. The number of users online has been growing exponentially as well, with the number of internet users expected to be 320 million by the year 2000. There is hidden order behind the apparent arbitrariness of the growth of Web and the preferences of its users. And this hidden order can be explained from dynamical organizing principles. One observed pattern is that there are many small elements contained within the web, but few large ones. A few sites consist of millions of pages, but millions of sites only contain a handful of pages. Also, few pages contain millions of links, but many pages have one or two. In addition, millions of users flock to a few select sites, giving little attention to millions of others. And within those sites users surf according to a mathematical law, while generating predictable congestion patterns.