Events Archive

Feb
26
2013
Unsteadiness and dynamical heterogeneities in dense granular materials
The physics of granular flow is of widespread practical and fundamental interest, and is also important in geology and astrophysics. One challenge to understanding and controlling behavior is that the mechanical response is nonlinear, with a forcing threshold below which the medium is static and above which it flows freely. Furthermore, just above threshold the response may be intermittent even though the forcing is steady. Two familiar examples are avalanches on a heap and clogging in a silo. Another example is dynamical heterogeneities for systems...
Feb
25
2013
Fate of the Kinetic Ising Model
What could possibly be new in the Ising model, arguably the most-studied model of statistical physics?  Plenty!  Consider the Ising model initially at infinite temperature that is suddenly cooled to zero temperature and evolves by single spin-flip dynamics.  What happens?  In one dimension, the ground state is always reached and the evolution can be solved exactly.  In two dimensions, the ground state is reached only about 2/3 of the time, and the long-time evolution is characterized by two distinct time scales, the longer of which arises from topological defects...
Feb
20
2013
Revival-echoes and Localization of Interacting Wave Packets in Anharmonic Potentials
PLEASE NOTE: This is a WEBINAR We investigate the effect of anharmonicity and interactions on the dynamics of an initially Gaussian wavepacket in a weakly anharmonic potential. We find that repeated perturbations can create revivals, echoes, and revival-echoes, with properties that can be controlled via the strength and symmetry of the perturbations.  We also find that depending on the strength and sign of interactions and anharmonicity, the quantum state can be either...
Feb
19
2013
Colloidal Discotic Liquid Crystals
Feb
18
2013
Fractionalized Quasi-particles in Frustrated Quantum Magnets
I present inelastic neutron scattering data from one- two- and three-dimensional insulating magnetic materials at low temperatures that do not display a coherent resonant mode of excitation. Instead, momentum resolved spectra take the form of bounded continua. I interpret the spectra as evidence for fractionalization of a spin flip into distinct quasi-particles.
Feb
18
2013
Neural and mechanical determinants of stability and maneuverability in animal locomotion
Feb
14
2013
An Ensemble of Atomic Fountains: Timekeeping and Local Position Invariance
The U.S. Naval Observatory provides the master clock for the DoD.  To support this mission, we have built and fielded 4 rubidium atomic fountain clocks at our Washington D.C. site.  This ensemble of clocks has been running continuously for slightly less than two years and is contributing to our larger ensemble of atomic clocks.
Feb
12
2013
Unraveling the Biofluidynamics of Flight as an Inspiration for Design
Many organisms fly in order to survive and reproduce. I am fascinated by the mechanics of flying birds, insects, and autorotating seeds. Their development as an individual and their evolution as a species are shaped by the physical interaction between organism and surrounding air. It is critical that the organism’s architecture is tuned for propelling itself and controlling its motion. Flying macroscopic animals and plants maximize performance by generating and manipulating vortices. These vortices are created close to the body as it is driven by the action of muscles or gravity, then are...
Feb
11
2013
Computation with Quanta
A quantum processor,  using quantum states of light and matter, holds the promise of performing calculations and simulations that are not practical by a classical processor. Many of the key components for a quantum processor have been demonstrated by various research groups and we can expect these components to be integrated into basic quantum processors in the near future. However, there remain significant technical challenges in scaling the system size and making the system robust and flexible. In GTRI ‘s Quantum Information Systems (QIS)  Group, we use...
Feb
07
2013
A New Channel for Low-Luminosity GRBs: Tidal Disruptions of White Dwarfs by Intermediate Mass Black Holes
Low-luminosity GRBs or X-ray flashes (XRFs), which often accompany supernovae, are typically ascribed to either the supernova shock breakout or weak GRBs powered by the central engine of stellar mass. We propose the tidal disruption of a white dwarf (WD) by an intermediate-mass black hole (IMBH) as another channel for XRFs. Such disruptions last for 100-5000 seconds.  The release of gravitational energy over short time generates a powerful flare. The magnetic field is quickly amplified in the fallback material, and then the BH launches a slow uncollimated jet. The emission from jet photosphere dominates X-...

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