Swarthmore College Department of Physics and Astronomy

How Old Is That Star?  The Importance of 'Boring' Stellar Properties for Understanding Extrasolar Planets

Eric Jensen, Department of Physics and Astronomy, Swarthmore College

Fri., Apr. 23, 2010, 12:45 PM

Despite being one of the most fundamental stellar properties, the age of a star is surprisingly difficult to determine, yet essential for understanding planet formation and stellar evolution.  I will talk about my research on using the abundance of the element lithium in young stars to determine their ages, and how this can help us better understand the formation and early evolution of planetary systems.  I will also talk briefly about how precise measurements of another seemingly mundane stellar property - the radii of stars - are increasingly important for studies of extrasolar planets.

More information is available at Prof. Jensen's website. 

Better Living Through Supersymmetry

Brian Wecht , School of Natural Sciences, Institute for Advanced Study

Fri., Feb. 19, 2010, 12:45 PM

Despite having no direct experimental evidence in its favor, supersymmetry is still the leading candidate for new physics being sought by the Large Hadron Collider. In this talk, I will review the basics of supersymmetry and discuss why many people take this idea seriously from both theoretical and phenomenological perspectives. In particular, I will focus on the use of supersymmetry as a toolbox for understanding hard problems in quantum field theory and mathematics. No background beyond first-year undergraduate physics is assumed.

Applying Quantum Mechanics to an Entire Electrical Circuit; or, How an Atomic Orbital Can Flow Through a Resistive Wire

Jack Harris, Department of Physics, Yale University

Fri, Feb. 5, 2010, 12:45 PM

One of the most remarkable predictions of the quantum theory of electronic circuits is that a small loop of resistive metal can have a perpetual current flowing in it in the absence of any applied voltage. This "persistent" current is directly analogous to the motion of electrons around the nucleus of an atom (albeit on a much larger scale!), and the prediction that it could be observed generated considerable excitement --- twenty-five years ago. Since then, experiments in this area have produced confusing results at odds with theory and even with other experiments. To address this long-standing controversy we developed a new type of detector for persistent currents which offers greater sensitivity and a less-invasive measurement than was previously possible. Our approach has made possible a painfully detailed comparison with theory. I will describe these results, which seem to give the clearest picture yet of persistent currents in normal metals.

More information is available at Prof. Harris's website.

Pre-colloquium talk by Frank Moscatelli on Friday, Jan. 29, 2010, 12:45 in Cunniff

Chromic Liquid Crystals: Molecular Aggregation in an Extremely Strong Magnetic Field

Peter Collings, Department of Physics, Swarthmore College

Fri., Dec. 4, 2009, 12:45 PM

Chromonic liquid crystals are ordered fluids in which the basic units are molecular aggregates that spontaneously form in aqueous solutions.  They differ drastically from the type of liquid crystals used in LCDs, and as a result open up new areas of science and technology.  Investigating such complex systems is difficult and multiple techniques must be employed in order to make progress.  So in addition to x-ray diffraction experiments and optical studies using polarizing microscopy and absorption spectroscopy, experiments done in the extremely strong magnetic fields available at the National High Magnetic Field Laboratory are useful in understanding the behavior of these systems.

More information is available at Prof. Collings's website.

All Things Pyramids: A New Nanostructure for Diagnostics and Therapeutics

Teri W. Odom, Chemistry and Materials Science & Engineering, Northwestern University

Thu., Nov. 19, 2009, 4:30 PM

(Note date and time)

Plasmonics is an exciting and emerging area that uses metal nanostructures to manipulate light on the nanoscale. Depending on their size, shape, and materials properties, noble metal nanoparticles can scatter and absorb light to produce colors ranging from the ultra-violet to the near-infrared. The physical basis behind these interesting properties is the interaction between surface conduction electrons and light; these collective excitations are surface plasmons (SPs). In general, there are two types of SPs: localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs).

This talk will focus on the optical properties of two different but complementary systems that can control light on the nanometer scale: (i) nanofabricated pyramidal particles and (ii) nanopyramidal plasmonic crystals. Both types of nanostructures can be fabricated by our large-area, nanoscale patterning tools. For the nanoparticle case, pyramids can be generated with variable sizes, thicknesses, and multi-material compositions and designed with blunt, ultra-sharp, or even without tips. Applications of the LSPs of these particles include bioimaging and localized cancer therapeutics. For the plasmonic crystal case, these artificially structured materials exhibit band structure analogous to crystalline solids. The SPPs generated by these lattices can be used for real-time, label-free sensing of biomolecules and other proteins.

More information is available at Prof. Odom's website.  As are two relevant papers of Dr. Odom's: "Screening plasmonic materials using pyramidal gratings" and "Pyramids: a platform for designing multifunctional plasmonic particles."

Pre-colloquium talk by Catherine Crouch on Friday, Nov. 13, 2009, 12:45 in SC 128

Loopy Stabilization of RNA Loops: How Entropy Creates Order

Daniel Aalberts, Department of Physics, Williams College

Fri., Oct. 30, 2009, 12:45 PM

Predicting nucleic acid folds has been a topic of intense investigation because structure implies function and as an analog to the protein folding problem. We show, surprisingly, that loop entropy actually promotes tertiary order. By including this overlooked source of stabilization, we improve the accuracy in predicting tRNA and other folds.

More information is available at Prof. Aalberts's website.

Pre-colloquium talk by Catherine Crouch on Friday, Oct. 23, 2009, 12:45 in Cunniff

String Theory, The Final Frontier?

Vatche Sahakian, Department of Physics, Harvey Mudd College

Fri, Sep. 18, 2009, 12:45 PM

Since the 1970’s, we’ve known of a major inconsistency in the laws of modern physics: quantum mechanics is seemingly incompatible with gravitational physics…String theory purports to be the leading framework to address and resolve this issue. But, does it really succeed, or is it simply a clever scheme to keep theoretical physicists like me employed? We will explore this question by looking at exotic phenomena involving black holes and implications from the cosmic microwave background radiation.

More information is available at Prof. Sahakian's website.

The "Main Sequence" of Galaxy Formation

Sandra Faber ('66), Department of Astronomy and Astrophysics, University of California, Santa Cruz

Fri., April 24, 2009, 12:45 PM

Major surveys have now thoroughly sampled the galaxy population at redshifts below z~1.4. This talk will summarize broadly what has been learned about galaxy evolution over these epochs, focusing on results from DEEP2 and Sloan. The main conclusion is that galaxy evolution is highly correlated with stellar mass, which in turn is plausibly correlated with halo mass. A useful working model is that galaxy evolution is driven principally by halo mass and that the star formation rate is given uniquely by halo mass at each redshift over these epochs.

More information is available at Prof. Faber's website.

The DRIFT Dark Matter Detector

Daniel Snowden-Ifft, Department of Physics, Occidental College

Fri., Feb. 27, 2009, 12:45 PM

The Directional Recoil Identification From Tracks (DRIFT) is designed to detect the dark matter thought to make up 85% of the mass of the universe.  Unlike other dark matter detectors its design allows it to conclusively determine the galactic origin of the dark matter events created within it.  I will describe the detector, its capabilities and our recent efforts to detect dark matter in the Boulby salt mine in England.

More information is available at Prof. Snowden-Ifft's website.

Time Keeping with Ultracold Atoms

Gretchen Campbell , JILA/University of Colorado

Fri, Feb. 6, 2009, 12:45 PM

Precise timekeeping is important for a number of everyday applications. Precise clocks are used for synchronizing telecom networks and GPS. They are also essential for navigation and for communicating with deep-space probes and rovers. It is predicted that with even higher precision, clocks will lead to new types of gravity sensors, and will allow for the testing of fundamental physical laws. Currently, the most precise clocks are made with cesium atoms, where microwave radiation is used to probe and measure an atomic resonance. At JILA, we’ve developed a new atomic clock which uses optical light to probe ultra-cold strontium atoms. By using higher frequency optical light instead of microwaves, the clocks divide time into smaller units, offering record precision. In our clock, a few thousand atoms of strontium are held very tightly in an optical lattice. The lattice is formed by standing waves of intense laser light. By carefully controlling the environment and interactions between our atoms, we’ve been able surpass the accuracy of the best cesium clocks. The JILA strontium clock would neither gain nor lose a second in more than 200 million years

Pre-colloquium talk by Frank Moscatelli on Friday, Jan. 30, 2009, 12:45 PM in Cunniff

Protoplanetary Disks: The Accretion/Outflow Connection

Suzan Edwards, Department of Astronomy, Smith College

Fri., Dec. 5, 2008, 12:45 PM

The early evolution of a protoplanetary disk is characterized by active accretion and the ejection of collimated mass outflows. The outflows are almost certainly accretion-powered since there is a robust correlation between mass accretion and outflow rates over many orders of magnitude, although how they are launched remains a mystery. Establishing their origin is critical for determining the angular momentum evolution of star-disk systems, as they may spin down the star during the accretion phase and/or aid in transporting angular momentum in the disk during the era of planet formation. They likely also play a role in disk clearing, pumping energy into molecular clouds, and terminating the infall of material from the cloud to the disk. Probing the wind launch and acceleration region in the inner AU of the accretion disk via high resolution spectroscopy offers a promising means of identifying the origin of these ubiquitous but mysterious outflows.

Universality and Intermittency in Hydrodynamical Turbulence

Nick Ouellette ('02), Department of Mechanical Engineering, Yale University

Fri., Nov. 14, 2008, 12:45 PM

Fluid flow has been studied by scientists and engineers for hundreds of years, and yet remains a problem without a general solution. Most flows in nature are, to some degree, turbulent, characterized by wild fluctuations and nearly complete unpredictability. Nevertheless, there are aspects of turbulence that appear to be independent of how the flow is generated. With the development of new theoretical, numerical, and experimental tools over the past two decades, significant progress has been made towards characterizing these universal features. I will discuss the current state of our understanding of turbulence in the context of my own measurements of the dynamics of small tracer particles transported by the flow; along the way, we will touch on several of the big ideas of modern physics, including universality, symmetry and symmetry breaking, and self-similarity.

More information about Prof. Ouellette's research is available on his website.

Experiments at Extreme States of Matter - High Energy Density Physics at the National Ignition Facility

Gail Glendinning, Lawrence Livermore National Laboratory

Fri., Oct. 31, 2008, 12:45 PM

The National Ignition Facility is a 1.8 Megajoule laser currently under construction at the Lawrence Livermore National Laboratory. When complete, it will be used to research fundamental questions about matter in extreme states. In 2003, the National Research Council of the NAS published a report titled Frontiers in High Energy Density Physics: The X-Games of Contemporary Science. Their conclusion was that experimental facilities such as the NIF, Sandia's Z machine, and SLAC (among others) are now capable of reaching regimes of high energy density allowing unprecedented insight into the behavior of matter under extreme conditions. We describe in this presentation experiments planned for the NIF and current experiments at existing facilities addressing some of the questions posed in this report:

• How does matter behave under conditions of extreme temperature, pressure, and density?
• Can the transition to turbulence, and the turbulent state, in high energy density systems be understood experimentally and theoretically?
• Will measurements of the equation of state and opacity of materials at high temperatures and pressures change models of stellar and planetary structure?

David Cohen gave a pre-colloquium talk on Friday, October 24

Interactions Between Laser Light and Matter: What happens when you dye liquid crystals?

David Statman, Department of Physics, Allegheny College

Fri., Sep. 26, 2008, 12:30 PM (note slightly early start time)

During the past several years, I have been investigating the physics and chemistry of nonlinear optical systems.  My current work involves studying the effects of dye structure on the dye-induced enhancement of the optical nonlinearities in liquid crystals. The interaction of dyes with liquid crystal samples is of much interest in chemical physics and materials science, whereas the nonlinear optical interaction of laser light with these dye-doped samples is of much interest in optical physics.  In this talk I will discuss how photo-excitation of azo dyes leads to optical reorientation of the nematic liquid crystal director.  Bulk and surface interactions will be described.  This work has impact on a broad range of technologies involving communications and image reconstruction.

More information is available on Prof. Statman's website.

Pre-colloquium talk by Peter Collings on Friday, Sep. 19, 2008, 12:45 PM in Cunniff

X-ray Emission from Massive Stars

David Cohen, Swarthmore College, Dept. of Physics and Astronomy

Fri, Sep. 12, 2008, 12:45 PM

The strong X-ray emission from the most massive stars in the Galaxy was a surprising discovery of the first generation of X-ray telescopes in the late 1970s. Massive stars, unlike the Sun, are not supposed to have a magnetic dynamo, and thus were not expected to be X-ray sources at all. Now, nearly 30 years later, the newest generation of X-ray telescopes, with high spectral resolution, have shown that the supersonic stellar winds of these massive stars are the site of their X-ray production. In this talk I will show how my group has used X-ray spectroscopy combined with theoretical modeling to characterize the production of X-rays in these unstable stellar winds, and to develop a new way of measuring the stellar wind mass-loss rates.

More information on Professor Cohen's website.

Dynamics on the Nanoscale: Light Emission from Single Semiconductor Nanorods

Catherine Crouch, Department of Physics and Astronomy, Swarthmore College

Friday, February 22, 2008, 12:50 PM

How do single quantum objects, such as molecules and semiconductor nanoparticles, emit light? One intriguing feature of the fluorescence observed from a wide variety of single fluorophores is intermittency, colloquially called “blinking”. Under steady excitation, single fluorophores do not emit light steadily, but turn on and off, remaining on or off for milliseconds to minutes at a time. Intermittency is fairly well understood in many molecular systems, but it is still poorly understood in semiconductor nanocrystals, tiny crystalline particles of a semiconductor such as cadmium selenide that are only a few nanometers across. Such nanocrystals are the subject of extensive study, both for the prospect of optoelectronic applications and for the fundamental physics of light emission from quantum particles. This talk will introduce the general field of single-nanocrystal fluorescence and present our results on fluorescence intermittency in single rod-shaped nanocrystals.

More information on Professor Crouch's research can be found on her website.

Novel Shaped Traps for Ultracold Atoms Using the Dipole Force

Frank Moscatelli, Department of Physics and Astronomy, Swarthmore College

Wednesday, November 28, 2007, 4:30 PM

Ultra cold atoms in two-dimensional traps have attracted much research interest lately. They have potential applications in rotational inertial sensing (gyroscopes), quantum computing, and studies of exotic atom interferometry using BEC excitations.  The talk will show that micron sized traps can be created using the optical dipole force between the ends of two single-mode optical fibers carrying counter-propagating light beams of two wavelengths.  It will emerge that a static ring-shaped trap can be formed with the proper choice of beam parameters. Furthermore, the ring can be made to split into two longitudinally adjacent traps.  Recent advances in the photonic industry that make the realization of such traps will also be discussed.

Electrical Transport through Spin Channels in a Silicon Double-Quantum Dot

Mark Eriksson, Department of Physics, University of Wisconsin

Friday, November 16, 2007, 12:45 PM

It is intriguing that silicon, the material at the heart of modern classical electronics, also has properties well suited to quantum electronics.  The roots of quantum electronics in silicon stretch all the way back to the first transistors developed in the middle of the last century.  Such transistors have always depended on quantum mechanics for their underlying properties.  What is new is that these devices are now being reinvented in ways that bring quantum effects to the forefront of the device operation.  The underlying motivation is the possibility of quantum computation in a semiconductor system.

I will describe the roots of silicon quantum devices in the transistors we all make use of in modern electronic products.  I will then discuss how silicon-germanium technology research over the last ten years – much of it originally aimed at wireless technology – has led, in fact, to new opportunities in basic physics research.  As an example of the type of physics one can now study, I will present results of our measurements of electrical current through spin channels in coupled quantum dots.  These results provide evidence that silicon quantum devices display interesting new effects that are enhanced by the underlying physics of the host material.

More information is available at Dr. Eriksson's website.

Catherine Crouch will give a pre-colloquium talk covering some of the background material for this talk on Friday, November 9, at 12:45 PM in Science Center room 199. Food will be served starting at 12:30 PM.

Computer Simulation of Glasses: Jumps and Self-Organized Criticality

Katharina Vollmayr-Lee, Department of Physics and Astronomy, Bucknell University

Friday, November 2, 2007, 12:45 PM

Glasses can be described as between a crystal and a liquid: The molecules forming the glass are disordered like in a glass and the molecules are frozen in as in a crystal. We study via computer simulation the resulting complex motion (dynamics) of the molecules/particles. We focus on events when a particle jumps out of its cage of neighboring particles and present the resulting jump statistics. We then present how these single particle jumps are correlated in space and time. Surprisingly, we find self-organized criticality, a phenomenon known for very different systems than glasses, such as earthquakes or sand piles.

More information is available at Dr. Vollmayr-Lee's website.

Amy Bug will give a pre-colloquium talk covering some of the background material for this talk on Friday, October 26, at 12:45 PM in Science Center room 199. Food will be served starting at 12:30 PM.

Animating the Glowing Magnetospheres of Massive Stars

Richard Townsend, Bartol Research Institute, University of Delaware

Wednesday, September 26, 2007, 4:30 PM

Massive, luminous stars are not expected to harbor magnetic fields, owing to their lack of envelope convection zones and associated field-generating dynamos. Puzzlingly, however, it has been known since the 1970s that a small subset of chemically peculiar massive stars harbor strong, global-scale fields. Moreover, recent advances in instrumentation have revealed hitherto-undetected fields in a number of 'ordinary' stars. By channeling the hypersonic winds of massive stars into violent shocks, magnetic fields appear to play a key role in generating these stars' characteristically hard X-ray emission. Furthermore, by confining the post-shock gas into stable circumstellar clouds, the fields also furnish an explanation for the glowing magnetospheres seen around these objects at wavelengths from the UV all the way through to the radio. In my talk, I will present recent highlights in the my ongoing theoretical and observational studies of wind channeling and confinement in magnetic massive stars. Topics to be covered include the new 'Rigidly Rotating Magnetosphere' and 'Rigid Field Hydrodynamics' models for optical, UV and X-ray emission from magnetospheres, which will be presented with the aid of extensive animations.

David Cohen will give a pre-colloquium talk covering some of the background material for this talk on Friday, September 21, at 12:45 PM in Science Center room 199. Food will be served starting at 12:30 PM.

Exploring the High-Energy Frontier with the Large Hadron Collider

David Tucker-Smith, Williams College

Wednesday, April 7, 2007, 4:30 PM

The Large Hadron Collider (LHC) at CERN will soon begin exploring a new energy regime, colliding protons at a center-of-mass energy of 14 TeV. This experiment will almost certainly shed light on the nature of electroweak symmetry breaking -- one of the most mysterious aspects of the standard model -- and may very well provide evidence for new physics beyond the standard model. In this talk Dr. Tucker-Smith will describe some possible experimental signatures of new physics at the LHC, and explain why so many expect that we'll actually get to see some of them. In the process he will also touch upon some of the leading theoretical proposals for physics beyond the standard model.

Quantum Computing with Single Photons

Todd Pittman , Department of Physics, University of Maryland, Baltimore County

Wednesday, February 21, 2007

Quantum computers are expected to be able to solve mathematical problems that cannot be solved using conventional computers. We are developing an optical approach to quantum computing in which the bits of information, or "qubits," are represented by single photons. This talk will focus on some of our recent experimental work on the basic building blocks of this approach, including a periodic source of single-photons, a prototype quantum memory device for single-photon qubits, and a two-qubit quantum logic gate. Many of these experiments rely on modern photonics technology as well as fundamental quantum interference effects.

How Quickly Do Planets Form?

Eric Jensen , Department of Physics and Astronomy, Swarthmore College

Friday, February 9, 2007

Planets form in disks of gas and dust that orbit young stars.
Recent observations have established that such disks, the raw material
for planet formation, are present around more than 80% of young stars. However,
these disks---or at least their dusty component---are largely absent by six
million years after the stars form. This timescale is uncomfortably short
compared to the time necessary to form planets like Earth under most current
models of planet formation, raising questions about the frequency of extrasolar
planets and/or our understanding of the formation process. I will discuss recent
observations related to our calibration of the ages of young stars (which could
help reconcile the two timescales), and also our attempts to detect gaseous planet-forming
material that may still be present (and important for forming planets) around many young
stars after the dust in disks is no longer detectable.

Brown Dwarfs: Young and Old

Kelle Cruz , Department of Astrophysics, American Museum of Natural History

Wednesday, December 6, 2006

Brown Dwarfs are very low-mass star-like objects that continually cool with time. In this talk, I will describe the nature of brown dwarfs and discuss the role they play in the Solar Neighborhood. I will also describe the discovery of young, low-mass brown dwarfs found in the field through the analysis of optical and near-infrared spectra. Intriguingly, many of these new-found young objects lie in the direction of the recently discovered nearby loose associations such as the TW Hya association and the Beta Pictoris moving group. I will describe our efforts to confirm cluster membership and to further investigate this likely new intermediate-age population of brown dwarfs.

Molecular Dynamics in Monolayers as Measured with Sensitive Dielectric Spectroscopy

Laura Clarke , Department of Physics, North Carolina State University

Friday, November 17, 2006

Self-assembled monolayers (SAMs) have become a ubiquitous tool in research and have current or proposed uses in corrosion prevention, tribology, and chromatography applications. These monomolecular coatings provide the ability to transform substrate characteristics (for instance, from a strongly hydrophilic hydroxyl-terminated surface to hydrophobic sheet of methyl groups) with the addition of a film only ~ 1 nm in thickness, which is easily grown from solution phase. SAM-covered substrates have become a standard surface upon which electrochemical, scanning microscopy, and solid-state NMR studies are conducted. However, despite their wide-spread use, few experimental investigations have addressed molecular motion within such films, which offer the possibility of studying fundamental science, such as dipolar and physical glass transitions in two-dimensions. In addition, relaxations such as "rotator" phases where molecular groups rotate in a plane parallel to the surface have been correlated with film conductivity, adhesive, and wetting properties. I'll discuss investigations of various SAM molecular motions using cryogenic, surface-specific, sensitive dielectric spectroscopy, and how these motions change as a function of film quality and type.

Atom Interferometry using Bose-Einstein Condensates

Charles Sackett , Department of Physics, University of Virginia

Wednesday, November 1, 2006

Atom interferometers are devices that manipulate quantum-mechanical matter waves in the same way that regular interferometers manipulate light waves. And just as a laser is a coherent light wave, a Bose-Einstein condensate can be thought of as a coherent matter wave. Extending this analogy, it can be hoped that Bose-Einstein condensation will permit great advances in atom interferometry just as the laser revolutionized optical interferometry. Potential applications include inertial navigation, oil exploration, and measurements of chemical interactions. However, BEC interferometry also presents substantial challenges. In order to be useful for precision measurements, experiments must exhibit long coherence times and/or large arm separations. This can be difficult, especially in the presence of interactions between the atoms. We have implemented a BEC interferometer designed to minimize the effects of interactions and other noise sources, which has been able to operate with wave packets separated by as much as 0.26 mm, approaching a truly macroscopic scale.

High Velocity Flows and High Temperatures in the SSX Experiment

Michael Brown , Department of Physics and Astronomy, Swarthmore College

Friday, September 22, 2006

I'll talk about several new experimental results related to flow dynamics and heating in rings of plasma called spheromaks in the SSX device here at Swarthmore. First, recent high-resolution velocity measurements of impurity ions using ion Doppler spectroscopy (IDS) show bi-directional outflow jets at 40 km/s. High flow speeds are corroborated using an in situ diagnostic called a Mach probe. Second, ion heating to nearly 10^6 K is observed after merging two spheromaks in a low density regime. Transient electron heating is inferred from bursts on a 4-channel soft x-ray array as well as a vacuum ultraviolet (VUV) spectrometer. Light analyzed by the IDS, soft x-ray, and VUV spectrometer is largely due to line radiation from highly stripped carbon ions. Each of these measurements will be related to and compared with similar observations in a solar or space context.

Non-linear Multi-photon Microscopy in Biosciences

Watt Webb, Dept. of Biomedical Engineering, Cornell University

Friday, April 14, 2006

We fluoresce, and we are composed of many optically nonlinear, non-centrosymmetric tissue structures. And hundreds of square miles of our cell membranes contain electric fields of ~ 250,000 volts/cm; that’s ~10 times the dielectric breakdown strength of air, but we want to be able to measure variations in those fields too. Nonlinear optics does provide powerful diagnostic microscopy to observe the dynamics of our biomolecular life processes in action within living tissues. Here some descriptions of the biophysical methods and some of the applications aim to illustrate the research and its discoveries. Now one of our challenges is to apply these discoveries and the tools of nonlinear optical microscopy in human clinical medicine.

Multidisciplinary Applications of State-Selected Atoms

Ronald Walsworth, Harvard University and Smithsonian Institution

Friday, March 24, 2006

State-selected atoms provide powerful tools to attack a wide range of problems. Examples I will discuss include: atomic population inversion, which enables masers that can be used as high-stability clocks and for precision tests of relativity; spin polarization of noble gases, which enables high-sensitivity gas-phase NMR, with applications in biomedical imaging and materials science; (and if time permits) coherent superposition of atomic states, which can be crafted to enable "slow" and "stopped" light, with applications in quantum information processing and elsewhere.

Diamondoids are forever:Using quantum mechanics and supercomputers to calculate the properties of real materials

Steven Richardson, Department of Electrical and Computer Engineering, Howard University

Friday, February 24, 2006

Diamondoids are cage-like saturated hydrocarbon molecules that possess a rigid carbon framework which is superimposable upon the crystal structure of diamond. While lower-order diamondoids (e.g. adamantane (C10H16), diamantane (C14H20), triamantane (C18H24), and anti-tetramantane (C22H28)) have been synthesized in the lab attempts to make even larger diamondoids have not been successful to date.

This field has recently been rejuvenated with the fascinating report by Dahl et al. of Chevron-Texaco (J. E. Dahl, S. G. Liu, and R. M. K. Carlson, Science 299 (2003) 96) of the isolation of new diamondoids from petroleum oil. Given their rigid structures and unique shapes of diamondoids they might be potential building blocks for various applications in nanotechnology. Recently, the Chevron-Texaco group also isolated and characterized a novel, disc-shaped lower-order diamondoid named cyclohexamantane (C26H30).

In this talk we will discuss density-functional theory (DFT) which is a very successful approximation for solving the Schrödinger wave equation for real materials. DFT has been used in physics and chemistry for the last forty years and it can be implemented on modern massively parallelized supercomputers to compute the structural, electronic, and vibrational properties of real materials and molecules from first-principles, that is without any experimental input. In particular, we will show that DFT calculations are capable of providing important information about cyclohexamantane and that our results are in very good agreement with recent experimental vibrational data. We are confident that DFT could help experimentalists in identifying more complicated diamondoids either isolated from natural products or made by rational synthesis.

The Development of Order in Ultra-Thin PS-PMMA Diblock Copolymer Films

Ward Lopes, James Franck Institute and Consortium for Nanoscience Research, University of Chicago

Thursday, February 23, 2006

Knowledge of how two dimensional systems order is important for techniques like hierarchical self-assembly or diblock copolymer lithography. The applicability of these techniques can be limited by the defects which influence the late stage of ordering. Further, one would like to know whether or not the qualitative features of ordering depend only on the symmetry of the system. I address these concerns by studying the growth of order in weakly-segregated, cylindrical-phase, PS-PMMA diblock copolymer films. My samples have smectic (striped) symmetry and form a single layer of half-cylinders with more than 10^5 repeat spacings. I have found qualitative differences between our results and results reported on strongly segregated cylindrical-phase diblock copolymer films. I find, for example, that the number of dislocations and disclinations are approximately equal and that grain boundaries persist for long times. I will report on progress in comparing weakly-segregated and strongly-segregated diblock films with stripped symmetry. I will compare my results with numerical simulations of the Swift-Hohenberg Model.

Jamming

Andrea Liu, Department of Physics and Astronomy, University of Pennsylvania

Friday, January 27, 2006

All around us things seem to be getting jammed. Before breakfast, coffee grounds and cereal jam as they refuse to flow into our filters and bowls. On the way to work, we are caught in traffic jams. In factories, powders jam as they clog in the conduits that were designed to have them flow smoothly from one side of the factory floor to the other. Our recourse in all these situations is to pound on our containers, dashboards and conduits until the jam miraculously disappears. We are usually so irritated by the jam that we have not really noticed that the approach to jamming and the jammed state, in all of these situations, have common properties and similar behaviors that are quite different from those in systems near the liquid-solid transition. I will discuss recent progress in understanding these phenomena from a more unified point of view.

An Atom Michelson Interferometer and Other Stories of Atom Chips

Dana Anderson, Joint Institute for Laboratory Astrophysics, Colorado University

Friday, November 18, 2005

Atom "chip" technology seeks to simplify ultracold atom systems and make practical applications of ultracold atoms more feasible.  I begin with the specific example of an atom Michelson interferometer that we have implemented on an "atom chip." The chip uses lithographically patterned conductors and external magnetic fields to produce and guide a Bose-Einstein condensate. Splitting, retroreflecting, and recombining of condensate atoms are achieved within a magnetic waveguide by a standing-wave light field having a wave vector aligned along the guide.  We have demonstrated interference in the recombined atoms by varying a magnetic field gradient applied during propagation.

The Michelson interferometer is one instance of a potentially useful atom optical device that can be used, for example, as an inertial sensor.  Atom optical technology is now as lasers were in the early 1960's: exciting, full of potential, but complicated and relegated to a small number of laboratories around the world.  I will review our efforts to develop compact atom chip systems intended to speed the development of atom chip technology and bring it into the hands of those who want to use it without necessarily needing all of the expertise currently required to do even a "simple" ultracold atom experiment.  In particular we have developed (and still are developing) a compact and portable atom chip vacuum system that we believe will make atom chip experiments much easier to carry out. 

How Crystals  Melt: What We Can Learn From Colloids

Peter Collings, Dept. of Physics and Astronomy, Swarthmore College

Friday, November 4, 2005

The importance of melting in nature can hardly be overestimated, yet a detailed understanding of the mechanisms that drive this transformation is still evolving.  Recent experiments and theory have shown that atomic crystal surfaces at equilibrium below the bulk melting point often form melted layers.  Many theories have suggested that a similar premelting should occur at defects such as grain boundaries, stacking faults, and dislocations with the bulk crystal, but these effects have not been observed.  In the work to be described, premelting at grain boundaries and dislocations within bulk colloidal crystals is observed using real-time video microscopy.  The crystals are equilibrium close-packed, three-dimensional colloidal structures made from thermally responsive micro gel spheres.  Particle tracking reveals increased disorder in crystalline regions bordering defects, the amount of which depends on the type of defect, distance from the defect, and particle volume fraction.  These observations suggest that interfacial free energy is the crucial parameter for premelting in colloidal and atomic-scale crystals.

Further information can be found on Peter Collings's website.

Greenland's Glacial Earthquakes

Göran Ekström, Dept. of Earth and Planetary Sciences, Harvard University

Friday, October 21, 2005

Earthquakes and underground explosions generate elastic waves that propagate through the Earth and can be recorded by sensitive seismometers worldwide. High-frequency (around 1 Hz) waves are traditionally used to detect and locate earthquakes and explosions, since these  frequencies are strongly excited by such impulsive events. In this study, we explore the possible existence of other seismic phenomena that do not radiate high-frequency waves, but do generate detectable long-period (around 100 s) waves. We use continuous seismic recordings from a global network of seismic stations for the last 12 years and array-processing techniques to search for seismic sources in the long-period band. We find hundreds of previously undetected sources with seismic magnitudes around 5. Close to 200 of the new events occur along the edge of the Greenland Icesheet and reflect a previously unknown glacial phenomenon. In our interpretation, large volumes of ice (perhaps 10 cubic kilometers) suddenly lurch forward by a significant distance (on the order of 10 meters) in a matter of 30-60 seconds. These glacial earthquakes show a strong seasonality, with many more events in the summer, suggesting a connection with widespread melting on the surface of the Greenland Icesheet. The Greenland glacial earthquakes have doubled in frequency since 2002, potentially in response to significant general warming in the Arctic and more widespread summer melting of the icesheet.

Further information can be found on Göran Ekström's website.

Women in Academic Science

Andrea Liu, Department of Physics and Astronomy, University of Pennsylvania

Thursday, September 22, 2005

note: room L26, 4:30 PM

I will present some statistics on the progress of women in the physical sciences and speculate as to why the progress is slower than one might have expected.    I will also suggest ways in which the system could be made more hospitable for women.

Further information can be found on Andrea Liu 's website.

Planets Orbiting Distant Stars or Touring the Galaxy with Car and Camera

Tim Brown, National Center for Atmospheric Research/High Altitude Observatory

Friday, September 9, 2005

The last decade has seen an explosion in our knowledge about planets circling stars other than our Sun.  I will attempt to explain in general terms what we know about these planets, how we know it, and what this new information implies about our own solar system.  Along the way I will address such urgent and life-defining questions as:  What is a planet (anyhow)?   How do distant solar systems resemble or differ from our own?  Which methods for detecting and characterizing extrasolar planets have been most successful in the past, and which hold the most promise for the future?  Will salsa dancing ever become popular on the planets of the star upsilon Andromedae?  And how may we extend present observational methods to study planets that are similar to the Earth?  I will show how the answers to these questions bear on but do not yet answer the fundamental question concerning other solar systems:  Is our own solar system typical, or is it weird?

Further information about Tim Brown's work can be found in this article.

Investigating Quantum Coherence: Mapping Electron Wavefunctions in Nanostructures

Mark Topinka, Department of Physics, Stanford University

Friday, April 1, 2005

In this talk I will be presenting the results of using a new technique that allows one to spatially image electronic wavefunctions in two-dimensional electron gas nanostructures. A 1/2 Fermi-Wavelength rippling of the quantum mechanical wavefunctions is clearly visible in these images, as is the overall envelope of the wavefunction. I will be both discussing previous work we performed involving imaging coherent electron wave flow through a quantum point contact (a 1-d constriction of the 2-d electron gas), as well as talking about more recent developments in our current work - directly imaging the decoherence of an electronic wavefunction's interference pattern as it passes through 2-d electron gas Youngs double slit apparatus.

Fundamental Physics with Diatomic Molecules: from Supersymmetry to Quantum Computation

David DeMille, Department of Physics, Yale University

Friday, March 18, 2005

Certain properties of diatomic molecules can be used to give unique types of leverage on several problems of fundamental interest in physics. For example, we use molecules as amplifiers of electric field, to increase the sensitivity of our search for a small electric dipole moment of the electron--an effect predicted by many theories of particle physics such as supersymmetry, grand unified theories, etc. We are also developing sources of ultracold molecules for use in many experiments, such as for the bits in a proposed large-scale quantum computer. This talk will describe the basic ideas and techniques used in these experiments.

Futher information can be found on Dave DeMille's website.

A Brief History of Dark Matter

Vera Rubin, Department of Terrestrial Magnetism, Carnegie Institution of Washington

Friday, February 25, 2005 (Note: Room 199)

As early as 1784, discussions of dark stars were in the literature, although attempts to evaluate the density of these non-luminous objects were rare before the early part of 20th Century. In the second half of the 20th Century, observations with large optical and radio telescopes and spectrographs led to the conclusion that most of the matter in the universe is dark. I will discuss the early history and the observations that led to this conclusion. I will also mention the curious history of detecting gravitational lenses.

Futher information can be found on Vera Rubin's website.

Lipid-Protein Interactions at Interfaces: From Lung Surfactant to Poloxamer

Ka Yee Lee, Department of Chemistry, University of Chicago

Friday, February 4, 2005

Many functions crucial to life are carried out by membrane proteins bound to or embedded in lipid bilayers. Conversely, a wide variety of diseases result from deficient or abnormal lipid-protein interactions. Study of these interactions can, therefore, help elucidate the normal functions of these proteins, and the mechanisms by which toxicity is introduced in the case of a disease. Using two-dimensional monolayers as well as supported bilayers as model systems, we have applied isotherm measurements, optical microscopy, scanning probe microscopy, x-ray and neutron scattering techniques to address fundamental questions concerning lipid-protein or lipid-polymer interactions: What is the effect of the protein or polymer on the stability of the phases of the lipid film? How does the protein alter the surface morphology of the system? How does the protein or polymer change the ordering of the host lipid layer? To what extent and how does the protein or polymer associate with membrane lipids? How are the observed phenomena related to biological functions? To illustrate the capability of these techniques, their applications to the understanding of (1) the collapse mechanism in lung surfactant, and (2) the use of poloxamer as a membrane sealant will be discussed.

Futher information can be found on Ka Yee Lee's website.

Nanotubes and the Electronics of Small-Scale Structures

Nadya Mason, Department of Physics, Harvard University

Friday, November 19, 2004

What happens to physics when we shrink materials down? Can we measure the transition from classical to quantum behavior, or use quantum properties to make novel devices? In this talk, I will approach these questions by discussing electrical transport experiments on carbon nanotubes. These one-dimensional wires made of carbon have been considered leading candidates for the study and application of nanoscale electronics, but have so far been limited by a lack of control over device parameters, particularly in the quantum regime. In this talk, I will discuss recent progress in controlling the quantum properties of nanotubes, focusing on new fabrication techniques that allow us to measure properties such as quantized conductance steps, and the behavior of confined regions of electrons (“quantum dots”) within nanotubes.

Global Warming: State of the Science

Rich Wolfson, Department of Physics, Middlebury College

Friday, October 1, 2004

The years 2002, 2003, and 2004 are among the warmest Earth has seen since record keeping began in the mid-nineteenth century--and almost certainly in the past two millennia. The climate of recent years is only one of many new pieces of evidence for an unprecedented warming of our planet in the past few decades. Both observations and modeling suggest independently that the dominant cause of this warming is human activity--in particular, our fossil-fuel consumption and the resulting increase in atmospheric carbon dioxide. This talk reviews the latest evidence for human-induced global warming, explores the science behind our changing climate, and outlines scenarios for possible climatic futures.

Futher information can be found on Rich Wolfson's website.

Like a Work of Shakespeare: Reality and Metaphor in Modern Physics

Rich Wolfson, Department of Physics, Middlebury College

Thursday, September 30, 2004

cholars of the humanities thrive on metaphor. So do the rest of us, in our everyday use of language. In fact, metaphors often shape our perception and understanding of reality. Surely, though, science is beyond the ambiguities of metaphorical language. But no! In fact, modern physics abounds with metaphor. The very language we use to describe reality at the atomic level itself affects what we observe. And a host of physical phenomena, from the wave-particle duality to the recently discovered Bose-Einstein condensate, and on to such far-out ideas as parallel universes and time travel, all admit metaphorical description or outright links to concepts from literature and the humanities. This talk will explore metaphorical connections between modern physics and the humanities. The talk is aimed at an audience of nonscientists and scientists together.

Futher information can be found on Rich Wolfson's website.

Measuring the Magnetic Field in the Solar Corona

Steve Spangler, Department of Physics and Astronomy, University of Iowa

Friday, September 24, 2004

The corona is the outer layer of the Sun's atmosphere, and is obviously visible at times of total solar eclipse. To the naked eye, it is clearly structured by magnetic fields, and it is generally believed that magnetic fields play a role in heating the corona to a temperature of 1 - 2 million degrees. I will discuss ways to measure the vector magnetic field in the corona, and thereby test theories of coronal heating. I will emphasize radioastronomical remote sensing observations, in which a distant radio source is "eclipsed" by the corona. These observations are made with the Very Large Array radiotelescope in New Mexico. With these observations, one can measure the Faraday rotation due to the corona, and infer properties of the magnetic field. I will discuss what we know about the corona, astrophysical theories of its heating, the physics of Faraday rotation and its measurement with radio telescopes, and present observational results.

Futher information can be found on Steve Spangler's website.