Events

## SMARTech Video Archive

Aug

28

I discuss "simple" dynamical systems on networks and examine how network structure affects dynamics of processes running on top of networks. I consider results based on "locally tree-like" and/or mean-field and pair approximations and examine when they seem to work well, what can cause them to fail, and when they seem to produce accurate results even though their hypotheses are violated fantastically. I'll also present a new model for multi-stage complex contagions--in which fanatics produce greater influence than mere followers--and examine dynamics on networks with hetergeneous...

Aug

21

Well-controlled experiments in pipe flow began at least as early as those by Reynolds himself (1883). Forming a model for transition to turbulence, however, has taken a long time to develop. The first nonlinear solutions to the equations governing fluid flow in a pipe were discovered only 10 years ago (Faisst and Eckhardt 2003), but since this time our understanding of the underlying nonlinear dynamics has developed thick and fast. I begin by reviewing some of the progress that followed the discovery of travelling wave solutions. For the future, it will be...

Jul

09

DNA coils undergo striking conformational transitions when it is confined to volumes with dimensions smaller than one of the characteristic lengths of the molecule. We are particularly interested in confinement to channels less than two persistence length wide, and hundreds of microns long. In these channels, DNA extends to 50 % of its contour length and more, thus establishing a clear connection between location and the linear "genetic address" expressed in base pairs. We can fabricate nanochannel systems with arbitrary configurations in two dimensions using fused silica, and thus are...

Jun

25

Characterization of the mechanical properties of cells, as well as the tissues and extracellular matrices (ECM) in which they reside, requires microscale manipulation platforms that allow precise measurement of their local rheology. To achieve this, my laboratory has developed a suite of NdFeB-based magnetic tweezers devices optimized for biomaterials characterization. In this talk, I will present the design and construction of three new microscope-mounted magnetic tweezers devices that allow controlled forces to be applied locally to networks, cells, and tissues while their deformation is...

May

24

Textbooks on colloidal phenomena teach us to describe the electrostatic and dispersion interactions at interfaces using single parameters (Hamaker constants, surface potentials or charge densities), an approach which treats surface as uniform. Real surfaces, be they mineral, polymeric, or biological, present heterogeneous surface chemistry that complicates their interactions. It is recognized that clustered rather than uniform presentation of attractive chemistries, for instance peptide sequences, enhances biological interactions. Likewise, heterogeneity in charge and acid-base...

May

07

We will present a simple non-relativistic model to describe the low energy excitations of graphene. Our model is based on a deformation of the Heisen-

berg algebra in such a way that the commutator of momenta is proportional to the pseudo-spin. We solve the Landau problem for the resulting Hamil-

tonian, which reduces in the large mass limit, while keeping constant the Fermi velocity, to the usual linear one employed to describe these excitations

as massless Dirac fermions. Extending this model to negative mass we re-produce the leading mass term in the low energy expansion of...

Apr

30

One of the long term challenges in human health and disease is the control of pathogens, such as antibiotic-resistant forms of bacteria. In this talk, we will briefly describe two directions where soft condensed matter physics based approaches have been useful.

Apr

29

Recently, there is revived interest in non-equilibrium dynamics of the nuclear spins partially due to the decoherence issue of the electron spin qubit in semiconductor quantum dots for quantum computation. In this talk, I will first introduce a microscopic theory for the non-equilibrium nuclear spin dynamics controlled by a closed feedback loop mediated by the electron and/or the hole under continuous wave pumping in a quantum dot [W. Yang and L. J. Sham, Phy. Rev. B 85, 235319(2012)], and then present a study on the nuclear-spin-fluctuation induced spin decoherence of an electron (SDE) in an optically pumped quantum dot....

Apr

25

Cosmic rays are predominantly nuclei, in particular protons. However, the less abundant cosmic-ray electrons and positrons are also important probes of open questions in astrophysics and particle physics. The Fermi Gamma Ray Space Telescope, designed to study the high-energy universe with gamma rays, is also an excellent electron and positron detector. Ground-based imaging atmospheric Cherenkov telescopes have also measured cosmic-ray electrons and positrons up to several TeV. PAMELA, Fermi, and AMS have discovered a surprising excess of positrons between 10 GeV and 350 GeV. I will describe these measurements and their...

Apr

25

Atoms in optical lattices are very versatile experimental systems. They can be used to study many-body quantum physics, to aid

precision measurements and tests of fundamental symmetries, and for quantum computation. I will describe ongoing efforts along these lines

in my research group.

Apr

24

Most organisms live in aqueous environments and propel themselves by swimming. A large subclass is micro-organisms that have slender rod-like shapes, e.g. sperm. These organisms propel themselves using undulations that follow certain waveforms depending on the type of desired...

Apr

23

Nature and technology abound with fluid interfaces such as the surfaces of oil droplets in water or the membrane surfaces of living cells. These interfaces are typically crowded with adsorbed particles, proteins or other large molecules, which are effectively confined to a two-dimensional fluid. This two-dimensional system, though, has a twist: it can spontaneously change its curvature and thereby substantially alter the interactions among the bound particles or proteins. ...

Apr

22

In addition to providing vital clues as to the formation and evolution of black holes, the spin of black holes may be an important energy source in the Universe. Over the past couple of years, tremendous progress has been made in the realm of observational measurements of spin. I will describe these efforts with particular focus on the use of X-ray spectroscopy to probe the spin of supermassive black holes in active galactic nuclei (AGN). I shall describe results from the Suzaku AGNSpin Survey, a Suzaku Key Project that targets five bright and well-known AGN...

Apr

18

Understanding dynamics of pattern formation, following a symmetry breaking quantum phase transition is an area of active interest. Spontaneous spin domain is formed in sodium Bose-Einstein condensates that are quenched, i.e. rapidly tuned, through a quantum phase transition from polar to antiferromagnetic phases. A microwave ``dressing'' field globally shifts the energy of the m_{F}= 0 level below the average of the m_{F}= ±1 energy levels, inducing a dynamical instability . We use local spin measurements to quantify the spatial ordering kinetics in the vicinity...