Events

## SMARTech Video Archive

Oct

31

*Albert Fert, Nobel Prize in Physics 2007*

*Joint colloquium between the School of Physics and MRSEC*

Spintronics exploits the influence of the electron spin orientation on electronic transport. It is mainly known for the “giant magnetoresistance” (GMR) and the large increase of the hard disc capacity obtained with read heads based on the GMR, but it has...

Oct

28

A quantum computer uses superposition states to accomplish tasks (e.g., database search and factoring of integers) more efficiently than any known classical computing strategy. In conventional registers for quantum information processing, quantum bits are associated with individual two-level quantum systems. Separate addressing and interaction with these systems permit one-bit gates, while an interaction between systems is needed

to accomplish two-bit gates.

The seminar will review recent theoretical proposals to implement quantum computing in collective excitation degrees of freedom in ensembles of

identical quantum systems. In these proposals one does not address...

Oct

26

Computational models of the Earth system lie at the heart of modern climate science. Concerns about their predictions have been illegitimately used to undercut the case that the climate is changing and this has put dynamical modelling in an awkward position. I will discuss ways that we, as a community, can contribute by highlighting some of the major outstanding questions that drive climate science, and I will outline their mathematical dimensions. I will put a particular focus on the issue of simultaneously handling the information coming from data and models, and argue that this balancing act will impact the way in...

Oct

25

At the small scale of a cell swimming in water, inertial effects are unimportant. Therefore, the motion of the fluid is governed by Stokes equations, which are linear. Nevertheless, there are many situations in which nonlinear effects are important. In this talk I will describe two such situations. The first is swimming in a viscoelastic material, which is motivated by the fact that many microorganisms move in non-Newtonian media such as mucus. I will present a simple model that shows how fading memory affects swimming speed. We will also present experimental results for a helix swimming in a viscoelastic fluid. The second situation I will consider is the...

Oct

19

Neutron stars are observed to rotate as fast as 716 Hz. Astrophysicists believe that they are spun-up by accretion of matter and angular momentum in binary star systems. However, the "r-mode" instability of rotating neutron stars, which is driven by gravitational radiation reaction, appears to prevent spin up via accretion to rotation frequencies above about 350 Hz.

Oct

13

We all know that modern science is undergoing a profound transformation as it aims to tackle the complex problems of the 21st Century. It is

becoming highly collaborative; problems as diverse as climate change, renewable energy, or the origin of gamma-ray bursts require understanding

processes that no single group or community alone has the skills to address. At the same time, after centuries of little change, compute, data, and network environments have grown by 9-12 orders of magnitude in the last few decades. Moreover, science is not only compute-intensive but is dominated now by data-intensive methods. This dramatic change in the culture and methodology of...

Oct

07

In the framework of the Fitzhugh-Nagumo kinetics and the oscillatory recovery in excitable media, we present a new type of meandering of the spiral waves, which leads to spiral break up and spatiotemporal chaos. The tip of the spiral follows an outward spiral-like trajectory and the spiral core expands in time. This type of destabilization of simple rotation is attributed to the effects of curvature and the wave-fronts interactions in the case of oscillatory damped recovery to the rest state. This model offers a new route to and caricature for cardiac fibrillation, and when we apply the feedback resonant drift method, for defibrillation all wave activity gets eliminated at the...

Oct

05

Gamma-ray bursts have been detected at photon energies up to tens of GeV, and there are reasons to believe that the sources emit at least up to TeV energies, via leptonic or/and hadronic mechanisms. I review some recent developments in the GeV photon phenomenology in the light of Fermi observations, as well as recent related theoretical work. I discuss then the expected production of gravitational waves, the possibility of accelerating cosmic rays resulting in high energy neutrinos, and recent observational constraints.

Sep

29

We are going to present new results related to the dynamics and the associated instabilities of strong magnetic fields in neutron stars. The results are the first of their kind in general relativistic magneto-hydrodynamics (GR-MHD). We verify and extend earlier Newtonian results produced using either perturbation theory or Newtonian MHD codes. Finally, we will present estimations of the possibility that the giant flares observed in magnetars can be associated with significant emission of detectable gravitational waves.

Sep

28

"From Cardiac Cells to Genetic Regulatory Networks"

R. Grosu, G. Batt, F. Fenton, J. Glimm, C. Le Guernic, S.A. Smolka, and E. Bartocci

A fundamental question in the treatment of cardiac disorders, such as tachycardia and fibrillation, is under what circumstances does such a disorder arise? To answer to this question, we develop a multiaffine hybrid automaton (MHA) cardiac-cell model, and restate the original question as one of identication of the parameter ranges under which the MHA model accurately reproduces the disorder. The MHA model is obtained from the minimal cardiac model of Fenton by first bringing it into the form of a canonical, genetic regulatory...

Sep

28

Particle scattering processes at experiments such as the Large Hadron Collider at CERN are described by scattering amplitudes. In quantum field

theory classes, students learn to calculate amplitudes using Feynman diagram methods. This is a wonderful method for a process like electron +

positron -> muon^- + muon^+, but it is a highly challenging for a process like gluon+gluon -> 5 gluons, which requires 149 diagrams even at the leading order in perturbation theory. It turns out, however, that the result for such gluon scattering processes is remarkably simple, in some cases it is just a single term! This has lead to new methods for...

Sep

27

We study wetting and filling of patterned surfaces by a nematic liquid crystal. We focus on three important classes of periodic surfaces: saw-toothed, sinusoidal and stepwise, which have been considered in the literature as promising candidates to develop less-consuming zenithal bistable switches for practical applications. For saw-toothed substrates, geometry induces the nucleation of disclination lines on the wedges and apexes of the substrate, so the nematic surface free energy density develops an elastic contribution which scales as qlnq (with q being the wavenumber associated with the substrate periodicity). This leads to a large departure from...

Sep

21

Bose-Einstein condensates (BECs) have revolutionized atomic physics, a revolution which, sixteen years after their discovery, shows little sign of stopping. The attention of the quantum gases community has increasingly shifted from studies of broad features of the many-body condensed state to more specific realizations based upon control of spin state, trapping geometry, dimensionality and temporal behavior. In many regards quantum gases have no direct counterpart in condensed matter, although many parallels do exist, and these serve to guide efforts at the interface between disciplines....

Sep

12

As a student of numerical relativity who planned to work the rest of his life in academia, I had never envisioned the possibility of finding myself working in the commercial sector. The perception exists that opportunities to do novel research or to direct one's own career path are limited in industry, but I have found that this is largely not true. Rather, different constraints on your time and resources are imposed, with different challenges and rewards. In this talk, I will describe how I came to find myself leading the development of a cloud-scale information extraction and retrieval application for a customer within the Intelligence Community, share my experiences working with...

Sep

07

Measuring an event in time seems to require a shorter one. As a result, the development of a technique for measuring ultrashort laser pulses—the shortest events ever created—has been particularly difficult. We have, however, developed simple methods for fully characterizing these events, that is, for measuring a pulse's intensity and phase vs. time. One involves making an optical spectrogram of the pulse by using nonlinear optic. The mathematics involved is equivalent to the two-dimensional phase-retrieval problem—a problem that’s solvable because the Fundamental Theorem of Algebra fails for polynomials of two variables. We call...