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Fall 2012 Colloquium Series

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Colloquia are at 4pm, Thursdays, in 100 Willamette Hall and are preceded by coffee, tea, and cookies at 3:40 in the Willamette Atrium.

The organizer of the Fall Term Colloquia is: Dietrich Belitz

Sep 27, 2012

Ray Frey

UO Physics

State of the Department, and Prospects for Gravitational-Wave Astronomy

Host: Belitz

Oct 4, 2012

Sharon J. Gerbode

Harvey Mudd College

Mechanics, motion, and morphosis in plants

The burgeoning field of image for colloquium abstractplant biomechanics examines the physical underpinnings of plant form and function. Though frequently thought of as sessile, plants do indeed move, and their organs can change shape in dramatic transformations. The fundamental mechanical principles that underlie these motions offer a view into how nature has solved challenging physics problems through generations of evolution. Highlighting two recent studies in this field that build on original botanical observations by Charles Darwin, I’ll first discuss the role of plant cell anisotropy in shaping the Columbine flower to match its pollinators’ beaks, and then quantitatively describe the unusual nonlinear springs formed by cucumber tendrils.

Host: Raghu Parthasarathy

Oct 11, 2012

Miriam Deutsch

UO Department of Physics

Plasmons in Nanocrystalline Silver Films – Shedding New Light on an Old Material

In recent years there has been growing interest in harnessing surface-plasmon resonances of nanoscale metal structures to efficiently concentrate electromagnetic fields into sub-wavelength volumes. One of the mainstay model materials for these studies is the rough silver film, favored for its relatively low loss in the visible frequency range as well as its chemical compatibility with many biologically and chemically relevant materials. The intense plasmon fields, localized by disorder in randomly aggregated silver films, have been invaluable in studying surface enhanced Raman scattering, nonlinear optical interactions, and enhanced light emission phenomena. One of the biggest challenges in this field remains the formulation of an accurate theory linking multiscale structure to the observed optical and electronic characteristics in these materials. In this talk I will present results from several studies we conducted using nanostructured silver films, grown as quasi-2D nanoshells on colloidal silica spheres and also as thin films on planar substrates. I will show how the plasmonic resonances of these macroscopic systems are determined by their nanoscale structure, in particular when the metal films are close to the percolation threshold, and discuss our modeling of these phenomena.

Host: Belitz

Oct 18, 2012

Ursula Keller

ETH Zurich

Attoclock and electron tunneling time

The attoclock is a powerful, new, and unconventional tool to study fundamental attosecond dynamics on an atomic scale. We established its potential by using the first attoclock to measure the tunneling delay time in laser-induced ionization of helium and argon atoms, with surprising results [1-2].
Tunneling probabilities and escape rates are well defined and widely accepted, however, the question how long it takes for an electron to tunnel through an energetically forbidden region has been subject to ongoing debate for the last sixty years.
So far we could set an upper limit on the tunnel delay time in strong laser field ionization of <30 as for both helium and argon over a large intensity range.
This is for example significantly shorter than the tunneling time predicted by Keldysh or by Büttiker-Landauer which are in the range of a few hundred attoseconds for our experimental conditions.

[1] P. Eckle et. al., Science, vol. 322, pp. 1525-1529, 2008
[2] A. N. Pfeiffer et al., Nature Phys., vol. 8, pp. 76-80, 2012

Host: Graduate students

Oct 25, 2012

Ronny Thomale

Stanford Institute of Theoretical Physics (SITP)

A Kohn Luttinger perspective on topological superconductivity

We argue that electron-driven pairing fluctuations, from the viewpoint of Fermi surface instabilities, generically provide a propensity towards topological superconductivity when the irreducible lattice representation associated with the Cooper pairs is multi-dimensional. For the square lattice, the px and py-type degenerate Cooper pairs yield p+ip triplet superconductivity, where we link its topological universality class to non-Abelian chiral spin liquid states recently introduced by us. In particular, in the case of water-intercalated cobaltates as well as graphene doped to van Hove filling, we illustrate this generation recipe for d+id topological singlet superconductivity on the triangular and honeycomb lattice. For the cobaltates, we can reconcile both the measured Knight shift data and excitation profile in the superconducting phase via an anisotropic d+id gap. For graphene, we predict a subtle competition between a spin density wave as well as d+id-wave and f-wave superconducting phases as a function of tight binding parameters, doping, and range of Coulomb interactions.

Host: Belitz

Nov 1, 2012

David Strom

UO Department of Physics

Finding the Higgs Boson at ATLAS?

The recent discovery of a particle in the search for the Higgs Boson at the LHC using the ATLAS detector will be discussed. The new particle has been observed in its decays to pairs of Z bosons, pairs of photons and pairs of W bosons. This result is based on years of work and preparation. The experimental techniques crucial for the discovery will be reviewed. Constraints on the properties of the new particle and the experimental prospects for improving these constraints will be examined. The implications of the results for future new physics searches at ATLAS will also be covered.

Host: Belitz

Nov 8, 2012

Nasim Alem

University of California, Berkeley

Defects in two dimensional crystals: An ultra-high resolution aberration-corrected electron microscopy study

Graphene and hexagonal boron nitride (h-BN) are considered new emerging materials with potential applications in sensing, hydrogen storage, and electronics. The configuration of defects and edges in these crystals can have a significant impact on their resulting physical, chemical and electronic properties. In addition, small distortions in the atomic structure of such crystal membranes can lead to peculiar physical, chemical and electronic properties at the bulk. During the past decade, aberration-corrected transmission electron microscopy (TEM) has revolutionized our understanding of nanoscale phenomena by opening up the possibility of imaging every single atom within a crystal. This study investigates the atomic structure of h-BN and graphene using the aberration-corrected electron microscope, TEAM, located at Lawrence Berkeley National Laboratory. This study presents in situ formation, growth and migration of defects in h-BN and their interaction dynamics with adatoms and molecules under the electron beam. This talk also addresses the atomic scale structural transformations in graphene under electrical bias.

Host: Benjamin McMorran

Nov 15, 2012

Rana Adhikari

California Institute of Technology

Turning on and tuning in the gravitational radiation antennae

In this decade, the detection of gravitational radiation from astrophysical sources will become commonplace. The 2nd generation interferometers in the U.S., Italy, and Japan will be the components of a global network capable of pointing towards sources on the sky; this will allow us to form a pictures of the most violent events in the universe with both electromagnetic and gravitational radiation. I will describe how these interferometers can measure space-time strains which are 100 million times smaller than a proton. Finally, I will show how the next version detectors can reach back to cosmological distances.

Host: Ray Frey

Nov 29, 2012

Daniel Holz

University of Chicago

Cosmology from gravitational waves

We discuss the use of gravitational wave sources as probes of cosmology. The inspiral and merger of a binary system, such as a pair of black holes or neutron stars, is extraordinarily bright in gravitational waves. By observing such systems it is possible to directly measure a self-calibrated absolute distance to these sources out to very high redshift. When coupled with independent (electromagnetic) measures of the redshift, these “standard sirens” enable precision estimates of cosmological parameters. We review potential standard sirens for the LIGO and LISA gravitational wave observatories, including gamma-ray bursts and supermassive black-hole inspirals. Percent-level measurements of the Hubble constant and the dark energy equation-of-state may be feasible with these instruments. In addition, measurements of the beaming angles and inner engines of gamma-ray bursts will be possible. We also discuss a proposal for the “ultimate” cosmology mission, through the gravitational-wave observations of stellar-mass compact binaries from space.

Host: Ray Frey