Events Archive

Mar 24

Aeons Before the Big Bang?

There is much impressive observational evidence, mainly from the cosmic microwave background (CMB), for an enormously hot and dense early stage of the universe referred to as the Big Bang. Observations of the CMB are now very detailed, but this very detail presents new puzzles of various kinds, one of the most blatant being an apparent paradox in relation to the second law of thermodynamics. The hypothesis of inflationary cosmology has long been argued to explain away some of these puzzles, but it does not resolve some key issues, including that raised by the second law. In this talk, I describe a quite different proposal, which posits a succession of universe aeons prior to our...

Mar 25

Twistor Theory, Then and Now

Twistor theory is now over 45 years old. In December 1963, I proposed the initial ideas of this scheme, based on complex-number geometry, which presents an alternative perspective to that of standard 4-dimensional space-time, for the basic arena in which (quantum) physics takes place. Over the succeeding years, there were numerous intriguing developments. But many of these were primarily mathematical, and there was little interest expressed by the physics community. Things changed rather dramatically, in December 2003, when E.Witten produced a 99-page article initiating the subject of “twistor-string theory” this providing a novel approach to high-energy scattering...

May 05

Atomic and molecular strong-field physics at ultrafast x-ray sources

X-ray science is undergoing one of its greatest revolutions to date with the construction of intense x-ray free electron lasers in Stanford, USA (LCLS), Hamburg, Germany (XFEL), and Harima Science Garden City, Japan (SCSS). These are vast, several-hundred-million dollar machines that will provide x-ray pulses that are many million times brighter than current sources. Similarly groundbreaking are the emerging attosecond light sources based on intense, pulsed lasers; they are relatively inexpensive laboratory-size instruments. These two emerging radiation sources will enable radically new research and have unnumbered potential applications in materials science, chemistry, biology, AMO,...

May 26

Strands of Superconductivity at the Nanoscale

Superconducting circuitry can now be fabricated at the nanoscale by depositing suitable materials on to single molecules, such as DNA or carbon nanotubes. I shall discuss various themes that arise when superconductivity is explored in this new regime, including the thermal passage over and quantum tunneling through barriers by the superconducting condensate, as well as the hormetic impact that magnetism can have on nanosuperconductivity. I shall also describe circuits that realize nanoscale superconducting quantum interference devices, exploring their sensitivity to magnetic fields and spatial patterns of supercurrent. These features hint at possible uses of nanoscale superconducting...

Oct 06

"Neutron Stars: Cosmic laboratories for theoretical and computational physics" by Kostas Kokkotas

Neutron stars were discovered accidentally in 1967 although their existence was predicted 65 years earlier.  These exotic objects are the remnants from the deaths of massive stars, a death marked by one of the most spectacular pyrotechnic events in the cosmos, a supernovae explosion. Neutron stars have a solid crust overlying a sea of neutrons that can flow without friction (superfluidity). Their unique, yet not fully understood, internal structure, together with their immense gravitational field, makes them the perfect laboratory where the physics of the macro-cosmos meets microphysics phenomena. Explosive thermonuclear processes on neutron stars and their colossal...

Oct 13

Black holes in the centers of galaxies

The massive black holes found at the centers of most nearby galaxies, including our own, are believed to be the ashes of the fuel that powered quasars early in the history of the universe. I will review the observational evidence for these objects and describe some of the exotic dynamical phenomena that originate in their vicinity, including hypervelocity stars, resonant relaxation, phase transitions, and lopsided stellar disks.

Oct 20

"Normal Modes and Density of States of Disordered Colloidal Solids" by Mohammad Islam

The normal modes and the density of states (DOS) of any material provide a basis for understanding its thermal and mechanical transport properties. In perfect crystals, normal modes take the form of planewaves, but they can be complex in disordered systems. I will show our recent experimental measurement of the normal modes, the DOS and dynamical structure factor (DSF) in disordered colloidal solids: disordered colloidal crystals composed of thermally sensitive micron‐sized hydrogel particles at several different particle volume fractions, φ. Particle positions are tracked over long times using optical microscopy and particle tracking algorithms in a single two dimensional (2D) [111]...

Oct 27

"Theoretical investigations of the rheology of fluid membranes" by George Biros

Fluid membranes (vesicles) are area-preserving interfaces that resist bending. They are  models of cell membranes, intracellular organelles, and viral particles.  We are interested in developing simulation tools for dilute suspensions of deformable vesicles. These tools should be computationally efficient, that is, they should scale well as the number of vesicles increases.  For very low Reynolds numbers, as it is often the case in mesoscopic length scales, the Stokes approximation can be used for the background fluid.  We use a boundary integral...

Nov 03

"Epitaxial Graphene: Designing a New Electronic Material" by Walt deHeer

Graphene has been known for decades in many forms (exfoliated, epitaxial, isolated) and a number of its properties were measured or inferred from related materials, like graphite and carbon nanotubes. Yet, only recently was its potential as an electronic material recognized.  Epitaxial graphene on silicon carbide (EG) has played a pivotal role in this development: it was the first to be proposed as a platform for graphene-based electronics [1]; the first measurements on graphene monolayers were made on EG; and the graphene-electronic band structure was first measured on EG. The epitaxial graphene program, initiated in 2001 at the Georgia Institute...

Nov 10

"Advanced Polymeric Materials for Electronic Applications" by Elsa Reichmanis

Since the invention of the point contact transistor over 50 years ago, semiconductor technologies have become a ubiquitous mainstay of our Society.  Continued advancements in these technologies rely heavily on materials research spanning many areas including polymer and organic materials which play significant roles as sacrificial, passive and active layers in electronic and photonic devices.   The research outlined in this talk will identify fundamental materials parameters that will allow for the definition of materials architectures leading to sub-nanometer scale dimensional control of features for future semiconductor fabrication...

Nov 17

"Turbulence: a stroll through 61,506 dimensions" by Predrag Cvitanovic

In the world of moderate Reynolds number, everyday turbulence of fluids flowing across planes and down pipes a velvet revolution is taking place. Experiments are almost as detailed as the numerical simulations, Numerical simulations are yielding exact numerical solutions that one dared not dream about a decade ago, and dynamical systems visualization of turbulent fluid's state space geometry is unexpectedly elegant. We shall take you on a tour of this newly breached, hitherto inaccessible territory. Mastery of fluid mechanics is no prerequisite, and perhaps a hindrance: the talk is aimed at anyone who had ever wondered why - if no cloud is ever seen twice - we know a cloud when we...

Dec 01

"Condensed Matter Physics Opportunities in Semiconductor Nanowires from Bottom-up" by Xuan Gao

Semiconductor nanowires synthesized in the bottom up approach have shown promising potential for a variety of applications in nanoelectronics, optoelectronics, biosensing and energy conversion. With the small length scale and a variety of material choices, nanowires also offer a versatile playground to explore mesoscopic and quantum physics. I will discuss our recent studies of magneto-transport phenomena in InAs and Bi2Se3 nanowires. In nanowires of InAs, a conventional low band-gap semiconductor, quantum interference and spin-orbit coupling lead to one-dimensional localization or anti-localization of electrons. For nanowires/ribbons of Bi2Se3, a 3D...

Dec 06

"Multiple Time Scale Dynamics in Chemical Oscillators" by Chris Scheper

(Nonlinear Science Webinar) Dynamical systems with multiple time scales have invariant geometric objects that organize the dynamics in phase space. The slow-fast structure of the dynamical system leads to phenomena such as canards, mixed-mode oscillations, and bifurcation delay. We'll discuss two projects involving chemical oscillators. The first is the analysis of a simple chemical model that exhibits complex oscillations. Its bifurcations are studied using a geometric reduction  of the system to a one-dimensional induced map. The second investigates the slow-fast mechanisms generating mixed-mode oscillations in a model of the Belousov-Zhabotinsky (BZ)...

Dec 08

"Nearly perfect flows" by Wendy Zhang

In school, we learned that fluid flow becomes simple in two limits.  Over long lengthscales and at high speeds, inertia dominates and the motion can approach that of a perfect fluid with zero viscosity.  On short lengthscales and at slow speeds, viscous dissipation is important.  Fluid flows that correspond to the formation of a finite-time singularity in the continuum description involve both a vanishing characteristic lengthscale and a diverging velocity scale.  These flows can therefore evolve into final limits that defy expectations derived from properties of their initial states.  This talk focuses on 3 familiar processes that...

Dec 09

"2D turbulence - Where do we stand?" by Robert Ecke

Two dimensional turbulence is an idealization of real 3D systems with anisotropy caused by geometric confinement or body forcing.  I will review the current state of understanding of 2D turbulent flows including specific theoretical predictions, numerical simulation results and experimental realizations of quasi-2D turbulent systems.  Relevance to geophysical systems will be discussed.