Fall 2006 Physics Colloquium Schedule

University of Colorado Department of Physics

Colloquia are Wednesdays at 4 PM, Duane G1B20 unless otherwise noted

Coffee, tea, and cookies before regular colloquia at 3:45 PM in Duane G1B31

Colloquia that have already occurred:

• August 30
  • Alex Levine, UCLA
  • Host: Meredith Betterton
  • Title: Mechanics of semiflexible networks: Implications for the cytoskeleton
  • Abstract: Cross-linked networks of filamentous proteins making up the cytoskeleton play a crucial role in cell mechanics. The microstructure of these cross-linked networks is unlike those of many synthetic polymer gels in that the constituent filaments are remarkably stiff on the length scale of the typical distance between cross-links. Because of this stiffness the mechanical properties of such semiflexible networks differs dramatically from better understood polymer gels such as rubber. In this talk I discuss new research on the mechanics of semiflexible networks and show that they admit an abrupt cross-over from a system that stores elastic energy heterogeneously in the bending of a few filaments to one that stores it homogeneously in the stretching of all the filaments.
    A consequence of this cross-over from homogeneous stretching to heterogeneous bending is an equally dramatic change in both the linear and nonlinear elasticity of these materials. It appears to be possible that cells use this cross-over to amplify the effect of small biochemical changes in their cytoskeleton to produce large changes in their response to external stress.
• September 6
  • William Marciano, Brookhaven National Laboratory
  • Host: Uriel Nauenberg
  • Title: Future Neutrino Physics Deep Underground
  • Abstract: The National Science Foundation is in the process of establishing a deep underground laboratory with the Henderson and Homestake mines as candidate site finalists. A very large water Cherenkov detector (500 kton) at either site would search for proton decay, supernova neutrinos, etc and precisely measure atmospheric neutrino properties. In addition, as I will discuss, such a massive detector used in conjunction with an intense neutrino beam originating from either Brookhaven or Fermilab could be used to study all aspects of neutrino oscillations, including CP violation and thereby, perhaps, elucidate the origin of the matter-antimatter asymmetry of our Universe. The merits and status of this ambitious program will be described.
• September 13
  • Tilman Sauer, California Institute of Technology
  • Host: Allan Franklin
  • Title: Einstein and the Early Theory of Superconductivity
  • Abstract: The history of superconductivity constitutes an example of conceptual change in physics where unexpected experimental discoveries have preceded theoretical analyses more than once. Thus, by 1920, the infinite conductivity of some metals at very low temperatures was an anomalous and isolated, albeit well-established phenomenon of cutting-edge technology. These experimental data challenged contemporary theories of electric conductivity that were based on the paradigm of Drude's electron theory of metals. In the talk, I will examine Einstein's thoughts and ideas on the subject in the early twenties and their historical context. I will argue that Einstein was well informed about the experimental data produced at Kamerlingh Onnes's cryogenic laboratory through his strong ties to the Leyden physicists. Some letters and manuscripts that have recently been published in the Collected Papers of Albert Einstein document considerations of Einstein that aim at a theoretical understanding of the phenomenon of superconductivity. These considerations proceeded both on a phenomenological level in an exploration of implicit consequences of perfect conductivity and on the level of speculations about microscopic models of charge transport. I will conclude with some comments on Einstein's only paper on the phenomenon of superconductivity, published in 1922, and discuss his own methodological reflections on the nature of theoretical physics as put forward in that publication.
• September 20
  • Vassili Papavassiliou, New Mexico State University
  • Host: Eric Zimmerman
  • Title: How strange is that? Probing the strangeness content of the proton
  • Abstract: The naive view of the proton is that of a composite object consisting almost exclusively of up and down quarks; while a non-zero fraction of its mass has been known to be due to strange quark-antiquark pairs, these were in general regarded as vacuum fluctuations, thus not contributing to any other of the proton's quantum numbers. However, this picture was challenged by the realization that strange quarks almost certainly account for a non-trivial fraction of the proton's spin.
    A program is underway to probe the strange-quark contributions to the electric and magnetic properties of the nucleon, as well as establish the extent of the contribution to its spin. Elastic electron-nucleon scattering, making use of the parity-violating electroweak interference, provides the bulk of the information, while neutrino scattering affords a different viewpoint. 
    The G0 experiment at Jefferson Lab is discussed in detail and recent results, from this and other similar experiments, are presented. A comprehensive picture is attempted, based on all the available information, and the current status of the experiments and future prospects are reviewed.
• September 27
  • Eva Silverstein, SLAC and Stanford University
  • Host: Victor Gurarie
  • Title: Cosmological Physics and String Theory
  • Abstract: I will begin by reviewing the motivations and evidence for accelerated expansion of the scale factor of the universe at early times (inflation) and late times (dark energy). A compelling phenomenological description of these processes is available using general relativity and scalar field theory, but these descriptions break down in two important ways -- strong gravity becomes important at an initial singularity in the spacetime, and later-time inflationary model building is sensitive to corrections introduced by high scale physics. After discussing string theory as a candidate UV completion of general relativity, I will describe recent progress on the initial singularity. This physics being screened away from our view by inflation, we next turn to an overview of observationally accessible models of inflation in string theory, including new mechanisms with distinctive predictions. The talk will conclude with a brief discussion of models of the cosmological constant obtained by stabilizing the extra dimensions of string theory via a balance of forces, and the challenges that ensue.
• October 4
  • David DeMille, Yale University
  • Host: John Bohn
  • Title: Fundamental physics with diatomic molecules: probing the Standard Model, and beyond
  • Abstract: Our group is using certain properties of diatomic molecules to obtain unique leverage on 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--a CP-violating effect predicted by many particle physics models such as supersymmetry, grand unified theories, etc. Similarly, we use close spacing between rotational levels to amplify parity-violating effects. This experiment will yield unique information on the couplings of Z^0 bosons to ordinary matter. Finally, we have developed a scheme for enhance sensitivity to possible variations of the fundamental constants--an effect suggested by the observation of dark energy in the universe, and by models of extra dimensions.
• October 11
  • Eric Adelberger, University of Washington
  • Host: John Price
  • Title: Tests of the Gravitational Inverse-Square Law at the Dark-Energy Length Scale
  • Abstract: It is remarkable that small-scale experiments can address important open issues in fundamental science such as: "why is gravity so weak?" and "why is the cosmological constant so small?" String theory ideas (new scalar particles and extra dimensions) and other notions hint that Newton's Inverse-Square Law could break down at distances less than 1 mm. Furthermore, the observed dark energy density corresponds to a length scale of 85 microns that may have  fundamental significance. I will review the motivations for testing the Inverse-Square Law, and discuss recent experiments with torsion balances, small oscillators and micro-cantilevers. Our torsion-balance experiments at separations down to 60 micrometers exclude gravitational-strength Yukawa interactions with length scales greater than about 57 micrometers (approximately the diameter of a human hair), and set a robust 95% confidence upper limit of 46 micrometers on the size of an extra dimension. Problems encountered in probing these short length scales will be discussed. 

• October 18 Joint Geological Sciences/Physics Department Colloquium
  • Raymond Jeanloz, University of California Berkeley
  • Host: Mike Ritzwoller
  • Title: From Earth to Stars: Toward Gigabar Pressures and Kilovolt Chemistry
  • Abstract: Materials can be experimentally characterized to TPa pressures  - conditions at which the Periodic Table is profoundly altered - by sending a laser-induced shock wave through a sample that is pre-compressed inside a diamond-anvil cell. This combination of static- and dynamic-compression methods provides access to the 10-100 TPa (~ Gbar) pressure range that is relevant to planetary science, to testing first-principles theories of condensed matter, and to experimentally studying a new regime of chemical bonding.

• October 25
  • Antoine van Oijen, Harvard University
  • Host: Meredith Betterton
  • Title: Watching single enzymes on DNA
  • Abstract: Advances in optical imaging and molecular manipulation techniques have made it possible to observe individual proteins and record “molecular movies” that provide new insight into their dynamics and reaction mechanisms. I will discuss results of single-molecule experiments on enzymes that process DNA. We stretch individual DNA molecules and use their elastic properties to obtain dynamic information on the proteins that unwind the double helix and copy its genetic information.
• November 1
  • Birgitta Whaley, University of California Berkeley
  • Host: Margaret Murnane
  • Title: Cat States and Bose Condensates in Double Well Potentials
  • Abstract: How macroscopic is a quantum superposition state in an interacting many-body system? Recent experiments claim to have observed quantum superposition states in near-macroscopic systems with very different physical constituents.  How can we define the ``size'' of a superposition state in general, so as to allow comparison between diverse physical realizations?  We propose a measure of size for such Schrödinger cat-like states that is based fundamentally on what measurements can be performed to probe and distinguish the different branches of the cat-state, allowing comparison of sizes of superposition states in very different physical systems.   Applications to quantum superposition state of Bose gases trapped in a three-dimensional potential with a double well in one coordinate are described.  We summarize the behavior of these condensates with both repulsive and attractive scattering lengths as a function of the strength of the interaction parameters and number of particles, using quantum Monte Carlo calculations to examine conditions under which superposition states are formed. The new measurement-based measure of cat size is evaluated explicitly for this system and compared with the effective size predicted by other measures such as disconnectivity.

     * Work performed together with J. Korsbakken, J. DuBois at the University of California, Berkeley and with I. Cirac at the Max-Planck Institute for Quantum Optics, Garching, Germany.

• November 8
  • Mark Raizen, University of Texas Austin
  • Host: Jun Ye
  • Title: Experiments with a "Particle in a Box"
  • Abstract: The "particle in a box" is at the heart of quantum mechanics and is a paradigm for many problems in physics.  In this talk, I will describe recent experiments conducted by my group which confine a Bose-Einstein condensate to a one-dimensional optical box.  These conditions have enabled  the experimental realization of atomic number squeezing, toward the production of number states.  More generally, we have demonstrated the capability to measure atom statistics by single-atom counting, paving the way for the new field of quantum atom optics.

    The concept of a "particle in a box" is also widely used in thermodynamics.  The historic paradox of Maxwell's demon suggested a way to cause particles to accumulate in one side of the box, in an apparent violation of the Second Law.  Motivated by these ideas we have developed a method to form an optical "one-way" barrier for atoms.  This device would allow atoms coming from one side of the barrier to pass through, but those coming from the other side to be reflected.  I will show how this idea can be used for phase space compression and cooling, as an optical realization of Maxwell's demon.  

• November 15
  • Steve Simon, Bell Labs, Lucent Technologies
  • Host: Victor Gurarie
  • Title: The Unexpected Physics of Modern Wireless Communication: Replicas, Diffusons, and Supersymmetry for Fun and Profit
  • Abstract: In the modern information era, where the demand for higher bit-rate seems to be increasing without bound, it is essential to understand the physical limits on communications. I will start by reviewing the concept of a Shannon limit of how many bits per second can be conveyed from a transmitter to a receiver. We then discover that in a disordered environment (in a city, for example) it becomes essential to understand wave interference in order to find this Shannon limit for modern communication systems. It becomes advantageous to make an analogy between radio waves in cities and  electron waves in disordered metals. Using our understanding of mesoscopics we can understand more about how information capacity is limited. We find the question of information can be reduced to a random matrix problem which is attacked with traditional condensed matter field theory methods. Finally we bring the story full-circle by deducing new insights about mesoscopics from our results on wireless communications.

• November 22
  • Fall Break, no Colloquium
• November 29
  • John Goree, University of Iowa
  • Host: Scott Robertson
  • Title: Two-dimensional liquids at an atomistic scale: dusty plasma experiments and numerical simulations.
  • Abstract: In a liquid, transport coefficients such as the diffusion coefficient and viscosity are usually measured on a macroscopic scale. In this talk, I explore something different: transport observed at a much smaller atomistic scale. This is done using both laboratory experiments and numerical simulations. The system that is studied is a dusty plasma, which is a suspension of charged microspheres interacting with a Yukawa potential. These particles are constrained to move on a 2D plane, and when they have sufficient temperature, they constitute a 2D liquid. Experimental results are compared to molecular dynamics simulations. We track the motion of individual particles, allowing us to compute transport coefficients. The experiments are typically non-equilibrium, and are often anisotropic as well. We find that in 2D liquids, random motion usually doesn't obey the laws of diffusion that are common in 3D; instead, we observe "superdiffusion" in simulations of equilibrium liquids, and a multi-rate diffusion in the experiments with non-equilibrium liquids. In addition to diffusion, we have also studied viscosity at an atomistic level, where we find violations of the Stokes Einstein relation in 2D liquids.
• December 6
  • Philip Kim, Columbia University
  • Host: Victor Gurarie
  • Title: Relativistic Quantum Physics at Your Pencil Tips: Dirac Fermion in Graphitic Carbon
  • Abstract: The massless Dirac particle moving at the speed of light has been a fascinating subject in relativistic quantum physics. Graphene, an isolated single atomic layer of graphite, now provides us an opportunity to investigate such exotic effect in low-energy condensed matter systems. The unique electronic band structure of graphene lattice provides a linear dispersion relation where the Fermi velocity replaces the role of the speed of light in usual Dirac Fermion spectrum. In this presentation I will discuss experimental consequence of Dirac Fermion spectrum in charge transport, realized in two representative low dimensional graphitic carbon systems: 1-dimensional carbon nanotubes and 2-dimensional graphene. Combined with semiconductor device fabrication techniques and the development of new methods of nanoscaled material synthesis/manipulation enables us to investigate mesoscopic transport phenomena in these materials. The exotic quantum transport behavior discovered in these materials, such as room temperature ballistic transport, unusual half-integer quantum Hall effect, and a non-zero Berry's phase in magneto-oscillations will be discussed in the connection to Dirac Fermion description in graphitic systems.
• December 13
  • David Weiss, Penn State University
  • Host: Eric Cornell
  • Title: 1D Bose gases and the quantum Newton's cradle
  • Abstract: I will review the physics of 1D Bose gases, and describe experiments that confirm the longstanding exact theory across all coupling regimes. I will then describe how we create 1D Bose gases far from equilibrium. These oscillating gases are quantum versions of the well-known Newton's cradle momentum lecture demonstration. Whether or not a real 1D Bose gas will thermally equilibrate has been an open theoretical question. We observe negligible approach to equilibrium even after each atom has undergone thousands of collisions. We also show that even a slight relaxation of the criteria that make the gas one dimensional allows it to thermally equilibrate.

Colloquium schedules from previous semesters can be found here

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