Colloquium Fall 2005



August 29
Novel quantum phases of antiferromagnets on random lattices
Tommaso Roscilde
Condensed Matter Theory, Physics & Astronomy, USC


The possibility of inducing novel quantum phases in low-dimensional quantum systems through lattice disorder has challenged both theory and experiment for decades. Low-dimensional quantum antiferromagnets offer an ideal testbed for the investigation of randomness effects, given the high level of control on the doping of the magnetic lattice achieved in recent experiments. In this talk I would like to review our numerical work evidencing a rich interplay between quantum fluctuations and geometric randomness in two-dimensional antiferromagnets. The general picture is the emergence of a novel quantum-disordered phase between the conventional geometric phases divided by the percolation threshold of the lattice, or between the conventional insulator/superfluid phases of the system's quasiparticles. In such a phase rare events anomalously dominate the response of the system to an external field, giving rise to so-called Griffiths' phenomena, and/or localization effects prevent quasiparticles from forming a superfluid condensate.

September 5
Labor Day, University Holiday

September 12

September 19
What's New in Cosmology?
Edward (Ned) L. Wright
Astronomy, Astronomy and Astrophysics, UCLA

September 26
Crossing the P's (and T's) and dotting the a: an update on fundamental-symmetry tests at Berkeley
Dmitry Budker
Atomic Experiment, Physics, UC Berkeley


I will discuss the progress towards a measurement of parity nonconservation in atomic ytterbium, the laboratory search for a temporal variation of the fine-structure "constant" using radio-frequency E1 transitions in atomic dysprosium, and will briefly mention exploring a possibility of a nuclear electric-dipole-moment search using a condensed-matter system (PbTiO_3).

October 3
Topological Quantum Computation
Chetan Nayak
Condensed Matter Physics, Department of Physics and Astronomy, UCLA


The computational power of a quantum-mechanical Hilbert space is potentially far greater than that of any classical device. However, it is difficult to harness it because much of the quantum information contained in a system is encoded in phase relations which one might expect to be easily destroyed by its interactions with the outside world (`decoherence'). Therefore, one must keep the error rate low and represent information redundantly so that errors can be diagnosed and corrected. Remarkably, there are phases of electrons (`topological phases') in which this can occur automatically. Topological phases occur in the quantum Hall regime and may occur in other correlated electronic materials. In these phases, the low-energy states are sensitive only to the topology of the system, so interactions with the environment, which are presumably local, cannot cause errors. Furthermore, excitations have non-trivial braiding statistics (`anyons'), so computations can be carried out by performing topologically-protected operations. Some examples of such phases will be discussed as well as some ideas about how quantum information could be stored and manipulated in them.

October 10
    -Location is unusual this week. The event will be in SAL 101. There will be a reception at 3:30pm, and then the event will start at 4:15pm.
Scientific Challenges in Sustainable Energy Technology
Nathan S Lewis
Lewis Research Group, Caltech


This presentation will describe and evaluate the challenges, both technical, political, and economic, involved with widespread adoption of renewable energy technologies. First, we estimate the available fossil fuel resources and reserves based on data from the World Energy Assessment and World Energy Council. In conjunction with the current and projected global primary power production rates, we then estimate the remaining years of supply of oil, gas, and coal for use in primary power production. We then compare the price per unit of energy of these sources to those of renewable energy technologies (wind, solar thermal, solar electric, biomass, hydroelectric, and geothermal) to evaluate the degree to which supply/demand forces stimulate a transition to renewable energy technologies in the next 20-50 years. Secondly, we evaluate the greenhouse gas buildup limitations on carbon-based power consumption as an unpriced externality to fossil-fuel consumption, considering global population growth, increased global gross domestic product, and increased energy efficiency per unit of globally averaged GDP, as produced by the Intergovernmental Panel on Climate Change (IPCC). A greenhouse gas constraint on total carbon emissions, in conjunction with global population growth, is projected to drive the demand for carbon-free power well beyond that produced by conventional supply/demand pricing tradeoffs, at potentially daunting levels relative to current renewable energy demand levels. Thirdly, we evaluate the level and timescale of R&D investment that is needed to produce the required quantity of carbon-free power by the 2050 timeframe, to support the expected global energy demand for carbon-free power. Fourth, we evaluate the energy potential of various renewable energy resources to ascertain which resources are adequately available globally to support the projected global carbon-free energy demand requirements. Fifth, we evaluate the challenges to the chemical sciences to enable the cost-effective production of carbon-free power on the needed scale by the 2050 timeframe. Finally, we discuss the effects of a change in primary power technology on the energy supply infrastructure and discuss the impact of such a change on the modes of energy consumption by the energy consumer and additional demands on the chemical sciences to support such a transition in energy supply.

October 17
    -Special Place and Time: SAL 101, 4:15pm. a special reception begins at 3:30pm on lawn in front of SAL
Toward Nanosystems Biology
Michael L. Roukes
Roukes Research Group, Kavli Nanoscience Institute, Caltech


The grand vision of Systems Biology requires instrumentation that will make it possible to follow the natural logic of complex cellular biochemical processes within interacting cellular systems -- in real time with single-cell resolution. Nanobiotechnology is poised to deliver this; it is beginning to enable the creation of ultrasmall electronic devices offering unprecedented opportunities for genomic and proteomic sensing applications. In this talk I will attempt to outline an (admittedly) long-term view of how cellular systems in hybrid devices might ultimately be harnessed for applications such as early disease detection, drug discovery, and fundamental medical and biological research. Micro- and nanotechnology are poised to provide the requisite tools for this and, indeed, a number of laboratories (including ours) are now taking the first steps toward this vision by embedding nanoscale biosensor arrays into microfluidic systems to form chip-based electronic laboratories for cell biology. When fully realized, such approaches will permit observation and control of multiple intra- and inter-cellular interactions. This technology will extend the opportunity to observe and ultimately reverse-engineer biochemical networks, through the techniques of systems biology carried out at the level of the circuitry" of the individual cell. I will conclude the talk by hazarding guesses as to time scale over which elements of this technology will likely come to fruition.

October 24
Quantum Phase Transitions
Omid Nohadani
Computational Condensed Matter Theory, Department of Physics and Astronomy, USC


In this talk, I aim to explain what a quantum phase transition is.This topic is stimulated by a plethora of recent experiments, such as magnetic-field-induced Bose-Einstein condensation in TlCuCl_3. To illustrate the quantum nature of quantum phase transitions, I will introduce examples of lattices made of spins with antiferromagnetic Heisenberg interactions. The universality class of these phase transitions is discussed based on the extracted set of critical exponents. I will demonstrate that the collective spin excitation modes undergo a Bose-Einstein condensation. This provides a novel explanation of previous experimental and theoretical reports. Furthermore, I will discuss the effects of geometrical randomness as they change the phase diagram significantly, leading to a novel Bose-Glass phase.

October 31
The electron liquid in the age of spintronics
Giovanni Vignale
University of Missouri


The electron liquid model has historically provided the starting point for understanding the role of electron-electron interaction in metals, semiconductors, superconductors etc. It continues to play a central role in the age of spintronics, when one attempts to design electronic devices that make essential use of the electron spin. In this talk I show how the correlation between electrons of opposite spin orientations in an electron liquid leads to an unusual form of internal friction, known as spin Coulomb drag, which in turn affects virtually all aspects of spin transport in metals and semiconductors.

November 7
Discovering the Quantum Universe: The Role of Particle Accelerators
JoAnne L. Hewett
High Energy Physics, Stanford Linear Accelerator


We are on the verge of a revolution in our understanding of what the universe is made of and how it works. Today, a special opportunity is at hand to address the fundamental nature of the quantum universe through astrophysical observations, in underground experiments, and at particle accelerators. Here, I will focus on the special role of particle colliders, which recreate the conditions in the first instants after the Big Bang. The Large Hadron Collider, under construction in Geneva Switzerland, will begin operations in 2007 and will provide the first clear look at a region of energy beyond the reach of today's colliders. Physicists expect that the LHC experiments will find new particles never before observed. These particles will be messengers, telling profound stories about the universe and their discovery will be the opening chapter of the story. The proposed International Linear Collider will allow us to listen very carefully to these stories and consequently discover the corresponding new laws and symmetries that govern the new particles. I will highlight the roles of these two colliders in three scenarios: solving the mysteries of the Terascale, shedding light on Dark Matter, hunting for ultimate unification.

November 14
    - There is no colloquium today, but a special presentation by one of our undergraduate students. Please attend, since he would like feedback on this talk. He will present this material to the January AAS meeting
Special Lecture: Solar Flares, Magnetic Fields, and Subsurface Vorticity. A survey of GONG data
Douglas Mason
Physics and Astronomy, USC


A major goal of solar physics has been to find observable connections to solar flares and magnetic fields. Work at the National Solar Observatory (NSO) with the Global Oscillation Network Group (GONG) has focused on using local helioseismology techniques to extract details of plasma flow beneath the surface of the sun. For the first time, we have been able to search out connections between these flows with solar flare events and magnetic fields using statistically rigorous methods that encompass data from many years. For this purpose, we determine the solar subsurface flows from high-resolution GONG data using the ring-diagram technique. We have found that the curl of the flow field below the solar surface, specifically the maximum flow vorticity within each active region, correlates well with the X-ray flare intensity data for the region, which is provided by the Geostationary Operation Environmental Satellite (GOES). One surprising result which we will discuss shows that above a certain threshold of flare activity, vorticity values exhibit a linear relationship with flare activity that is dependent on the magnetic flux of the active region

November 21
Lighting up the Dark: Connecting Galaxies and Dark Matter
Risa Wechsler
Kavli Institute for Cosmological Physics, Astronomy & Astrophysics Department, University of Chicago


Many of the most vital questions in cosmology require a detailed understanding of a simple question: How does mass connect to light? Making this connection, often described by the galaxy "bias", is essential both to use galaxy clustering to constrain cosmological models and the physics of dark energy, and to develop a physical understanding of the origin of galaxy properties and their correlations. I will describe several advances towards making this connection, which combine the results of numerical simulations with observations of galaxy clustering and of galaxy clusters. I will present results from a simple yet accurate model which connects dark matter subhalos to galaxies, and is able to reproduce nonlinear galaxy clustering as a function of luminosity from z~5 to the present. This indicates that gravity and dynamical evolution of the dark matter halos are the main factors responsible for the spatial distribution of galaxies. Finally, I will describe a method of connecting galaxies to the mass distribution which is applicable to very large volume simulations, and show how it can be used in combination with large cluster surveys to pin down cosmological parameters and the nature of dark energy.

November 28
    - Special Colloquium in Celebration of Einstein Year and World Year of Physics
Einstein's discovery of general relativity, 1905-1915
Tilman Sauer
Einstein Papers Project, Caltech


Ninety years ago, on 25 November 1915, Einstein published the gravitational field equations of general relativity, the so-called Einstein equations. This event marks one of Einstein's most significant achievements, even in comparison to his three most famous papers of his miracle year 1905. It also presents the end of a long and winding path that began soon after Einstein published his theory of special relativity as an unknown patent expert in 1905. At the end of this path he had risen through the ranks of academic hierarchy to being a member of the Prussian Academy of Sciences in Berlin. The experimental confirmation of his theory by a British eclipse expedition in 1919 then irreversibly catapulted Einstein to world fame, making him the first celebrity in the history of science. In the talk I will give an account of Einstein's search for a theory of gravitation and a generalized theory of relativity in those years. I will show how an analysis of some of Einstein's research notes helps us understand his heuristics, and will also comment on his competition with the mathematician David Hilbert in the final days of the discovery of general relativity.

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  • University of Southern California
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