• Friday, January 27, 1:30 pm
Dr. Maria Gherasimova
Yale University
Controlled synthesis of III-nitride nanostructures for device applications
abstract
I will discuss the issues and challenges pertaining to the synthesis of III-nitride semiconductor heterostructures by metalorganic chemical vapor deposition (MOCVD) for ultraviolet optoelectronics, illustrating with the examples of the light emitting diode (LED) performance and prototype application demonstrations.  In addition, I will focus on the dimensionality reduction of the nitride structures and report on the morphology and properties of nanowires and quantum dots obtained by MOCVD, including the discussion of the pathways towards novel applications for the nanostructured active media and integrated nanowire networks.
Host: Likharev

•     Friday, February 3, 1:30 pm

          Dr. Jue Wang
          MIT
          Control of DNA replication in Bacillus subtilis
abstract
Control of DNA replication is essential for the faithful propagation of genetic information.  Using DNA microarrays, I systematically profiled replication fork progression in the bacterium Bacillus subtilis.  I characterized a novel mechanism by which Bacillus subtilis regulates replication elongation in response to nutrient availability.  The relative simplicity of bacterial replication makes it an ideal model system for studying DNA replication quantitatively.
Host: Allen/Jacobsen
• Friday, February 10, 1:30 pm
Dr. Robert Endres
Princeton
Precise adaptation in bacterial chemotaxis through ``assistance neighborhoods''
abstract
Adaptation is ubiquitous in biological sensory systems. Specifically, precise adaptation allows bacteria such as Escherichia coli to efficiently chemotax,  i.e. to swim up gradients of attractants such as amino acids or sugars. The adaptation mechanism relies on methylation and demethylation (or deamidation) of specific modification sites of the membrane-bound chemoreceptors by the enzymes CheR and CheB, respectively. These enzymes can assist modifying 5-7 nearby receptors when tethered to a particular receptor. Using a free-energy based model for signalling by clusters of chemoreceptors, we show that these ``assistance neighborhoods'' are necessary for precise adaptation. In agreement with experiment, clusters of receptors of different type exhibit high sensitivity and precise adaptation over a wide range of concentrations, and the response of adapted clusters to addition/removal of attractant scales with the free-energy change of the receptors. We predict two limits of precise adaptation at large attractant concentrations: either receptors become fully methylated and turn off, or receptors become saturated and cease to respond to attractant, but retain their adapted activity.
Host: Allen/Shrock
• Wednesday March 29, 5:00 pm, Rm. B-131
Dr. Szabolcs Csonka
Budapest University of Technology and Economics, Hungary
Molecular adsorption of hydrogen on nanojunctions
abstract
The ultimate goal of molecular transport studies is to construct electronic devices at the molecular level. At the actual stage of this field there is still a lot to do even to understand and control the transport properties of single molecules. The experimental results are usually not completely reproducible and the theoretical models are also far from being satisfactory, which motivate a systematic analysis of the molecular transport beginning from simple molecules. This talk will focus on experimental studies of the interaction of hydrogen with metallic nanojunctions (Au, Pd, Pt, Nb). The presented results will show that even this simple molecule can interact with the metallic electrodes in many different ways resulting in various atomic configurations. The hydrogen can bridge two platinum electrodes providing a completely open conductance channel, it can dissolve inside the electrodes modifying their electronic structure in palladium, and the hydrogen can even form a clamp which is strong enough to pull an atomic chain from gold electrodes.
Host: Likharev
• Friday, March 31, 1:30 pm
Michael Pustilnik
Georgia Tech
Dynamic response of one-dimensional interacting fermions.
abstract
Much of our current understanding of interacting fermions in one  dimension is based on the exactly solvable Tomonaga-Luttinger (TL)  model. The model plays the same role for the Luttinger liquid concept  as the free Fermi gas does for the Fermi liquid. The crucial  ingredient of the TL model is the assumption of strictly linear  fermionic dispersion relation. With this assumption, the elementary  excitations are bosons, the density waves quanta. Nonlinearity of  spectrum generates interaction between the bosons, which results in a  finite lifetime of the Luttinger liquid quasiparticles. The finite  lifetime is encoded in broadening of the peak in the dynamic  structure factor (density-density correlation function) $S(q,\omega) $. However, unlike in the Fermi liquid, where interaction between the  quasiparticles is weak and can be accounted for in the second order  of perturbation theory, here the peak in $S(q,\omega)$ is not  described by a simple Lorentzian. It turns out that the main spectral  weight of $S(q,\omega)$ is confined to a narrow interval of  frequencies of the width $\propto q2/m$. At the borders of this  interval the structure factor develops power-law singularities with  exponents depending on the interaction strength. The singularities  have the same origin as the well-known Fermi-edge singularity in the  X-ray absorption spectra of metals. Besides being of fundamental interest for its own sake, detailed  knowledge of the structure factor is crucial for the description of  the variety of effects that owe their existence to particle-hole  asymmetry.
Host: Abanov
• Friday, April 7, 1:30 pm
Valerii Vinokour
Argonne National Lab
Hopping Conductivity in Granular Metals
abstract
Arrays of metallic granules, granular metals, are in the insulating phase at low temperatures if the coupling between granules is weak and exhibit stretched exponential conductivity behavior exp[-(T0/T)1/2] resembling Efros-Shklovskii hopping conductivity in semiconductors.   This temperature dependence observed not only in numerous experiments on disordered granular metals but, strikingly, also in perfectly periodic arrays of metallic granules and periodic arrays of semiconductor quantum dots without visible traces of disorder posed a fundamental problem that resisted almost three decade long intense attacks of theorists.   We explain the observed dependence in terms of the Mott-Efros-Shklovskii-like variable range hopping mechanism.   Our approach includes two key ingredients: (i) electrostatic disorder induced on the granules by the charged centers imbedded in dielectric matrix of granular conductors and (ii) the theory of coherent co-tunneling through the chain of granules providing mechanism for the electron hopping over several granules.
Host: Likharev
• Friday, April 28, 1:30 pm
Kevin Beach
Boston University
Simulating quantum spins systems in the valence bond basis
abstract
It has long been believed that systems of interacting spins can support, in addition to the usual collinear Néel state, a variety of paramagnetic ground states with resonating or static valence bond order. Confirmation of their existence in candidate models has been complicated by the fact that the frustrating interactions that might support these exotic phases are generally sign problematic and not amenable to exact numerical simulation.  Recent advances in projector valence bond Monte Carlo [A. W. Sandvik, Phys. Rev. Lett. 95, 207203 (2005)], however, have expanded the limited class of models that can be simulated. It is now possible to study SU(2) invariant four- and higher-spin interactions that suppress antiferromagnetic order. I will give examples in two and three dimensions where such interactions drive a phase transition to a valence bond solid ground state.
Host: Durst

• Wednesday, May 17,  3:30 pm

Send comments to Phil Allen (substituting for Laszlo Mihaly); created 5/24/2005