• January 22, Monday, 4:00PM
  Yonatan Dubi ( Ben-Gurion University ): On transport and other properties of highly disordered superconducting films
  Experimental studies of magnetoresistance in disordered superconducting (SC) thin films reveal an abundance of unexpected results, such as a huge peak in the magnetoresistance on the insulating side of the super-conductor-insulator transition which evolves as the field is tilted, and traces of superconducting correlations that survive well above the transition. We present a phenomenological theory that accounts for these finding (especially the non-monotonic magnetoresistance) in terms of the formation of SC islands, which leads to a competition between Coulomb-blockaded Cooper-pairs and normal electrons. The theory is accompanied by extensive numerical calculations to support it, in which a locally self-consistent solution of the BdG equations in the presence of disorder and tilted magnetic field is obtained. Simple arguments from percolation theory are then used to explain more experimental findings, such as the non-monotonic magnetoresistance in parallel field and the relation between the critical field and the magnetic field tilt angle.
  Host: Likharev
• January 23, Tuesday, 2:30PM
  Alan Calder
  Host: Lattimer
• January 25, Thursday, 4:00PM
  Maria-Victoria Fernandez-Serra ( Centre Européen de Calcul Atomique et Moléculaire (CECAM) ): First principles simulations of complex bio and nano materials with Linear-Scaling techniques.
  Over the last decade, the field of quantum mechanical simulations of materials and complex systems has experienced an important development in the area of order-N or linear-scaling electronic structure calculations. New algorithms and methods have been developed, aiming to make possible the description of systems with hundreds or thousands atoms. The study of realistic systems is subject to the increasing complexity that always accompanies an increase in the number of atoms, N. I will present our work on two systems for which the use of O(N) methods provide a pathway to significantly more efficient calculations. Systems with a large and stable gap such as water and large biomolecules are excellent candidates to be described using full O(N) techniques; their characterization implies performing extensive ab initio molecular dynamics simulations, which in real time still represent a bottleneck for O(N^3) methods, even with the help of massive parallelization. Secondly, as an example of the use of O(N) techniques to study transport properties of nanostructured systems, I will present an overview of our conductance calculations within the Landauer formalism for doped Silicon Nanowires (SiNWs), showing how dimensionality effects (quantum confinement) can strongly modify transport properties of p- and n-doped SiNWs, as compared to bulk Si.
  Host: Likharev
• January 26, Friday, 1:30PM
  Matthew Dawber ( University of Geneva ): Nano-engineered ferroelectric oxides: Tailored functional materials with exceptional properties
  Ferroelectric materials possess high degrees of functionality, with highly exploitable electrical, mechanical and optical properties. Integration of ferroelectrics into heterostructure devices with other materials with different but equally exciting properties (e.g., magnetism and superconductivity) is a direction that shows enormous potential for both exciting physics and breakthrough devices. At present there is still much to learn regarding the behaviour of ferroelectrics when they are modified on the nanoscale, and the nature of their interactions with other kinds of functional materials. In this seminar I will focus on the lead titanate/strontium titanate superlattice system, which we have experimentally constructed and characterised using a broad range of techniques. In the system we find some behaviour that is readily understandable (and therefore easily exploitable) on the basis of simple electrostatics, but also some aspects that hint at significant modification of the ferroelectric interactions when the length scales are sufficiently short. Looking to the future, I will discuss how the techniques developed and the knowledge gained in this work can be integrated into a broader program of research into nano-engineered functional materials with exceptional properties.
  Host: Mihaly
• January 29, Monday, afternoon
  Jan Kunes ( University of Augsburg ): Bunsenite - Hole Doping and Local Correlations in Charge Transfer Insulator
  Charge-transfer insulators present an important subgroup of transition metal compounds which exhibit phenomena such as metal-insulator transition or high temperature superconductivity. Using a combination of ab initio bandstructure and dynamical mean-field theory we study the single particle spectrum of bunsenite, the prototypical charge-transfer insulator, and compare it with photoemission and inverse-photoemission experiments. Further, we investigate the effect of hole doping which exemplifies the role played by the local correlations and defies the band-structure intuition. I will briefly introduce the ideas behind the dynamical mean-field theory and the concept of charge-transfer insulator. The bulk of the talk will be devoted to the application of the dynamical mean-field theory on a material specific level and correlations in multi-band systems.
  Host: Shrock
• February 1, Thursday, 2:30PM
  Natalia Ivanova
  Host: Zingale
• February 2, Friday, 1:30PM
  Andras Kis ( U.C. Berkeley ): Mechanical Manipulation of Nanotubes
  One of the most widely studied nanoscale structures today are carbon nanotubes. Similarly to graphite, they are highly anisotropic from the mechanical point of view: the stiffness of individual tubes is close to that of diamond, while their ensembles are prone to shearing due to the van der Waals interaction holding them together. I will describe quantitative measurements of this weak intertube coupling, based on mechanical manipulation and show that nanotubes exhibit negligibly low friction and high tolerance to structural damage, making them the ideal component for nanoelectromechanical applications and an interesting system for studying friction at the nanoscale. I will also show how this weak intertube coupling can be defeated, resulting in strong nanotube bundles and overcoming one of the main obstacles in the use of carbon nanotubes in composite materials.
  Host: Mihaly
• February 5, Monday, 1:15PM
  John Jameson ( Stanford University ): Solids behaving badly: How the double wells in glasses are hindering the development of nanoscale transistors
  More than half a century after the invention of the transistor, the continued improvement of computers now seems a lmost as inevitable as death and taxes. Barriers to further improvement do exist, however, one of which is the tunneling of electrons through the SiO2 gate insulator of the transistor. This tunneling current is becoming unacceptably large as transistors are scaled toward nanometer dimensions, and extraordinary effort (and money) has been spent trying to develop a material to replace SiO₂. Transition-metal oxides have been leading candidates almost from the start, but, unexpectedly, all of the transition-metal oxides investigated to date (e.g., HfO₂) were found to produce time-dependent shifts in the electrical properties of transistors. This is also unacceptable, and after more than ten years of work, no satisfactory alternative to SiO₂ is in hand.
      This talk discusses the physics of these electrical instabilities, tracing them back, in part, to the amorphous s tate itself. The primary instability observed in the transition-metal oxide gate dielectrics (as well as other glasses) is a transient current varying as the inverse of time following the application of a step voltage. This current is described as a polarization current resulting from the field-induced motion of atoms, or groups of atoms, between neighboring local equilibrium positions in the glass. It is shown that a simple semiclassical treatment of the traditional double-well description of the structure of glasses leads to a dielectric relaxation current whose properties match those observed experimentally. A constant density of barrier heights and well depths is assumed. Connections are then made to other unusual dielectric phenomena observed in glasses, including a term in the ac susceptibility varying as the log of time, and a recently observed memory effect at low temperature. Since glassy insulators are ubiquitous components of electronic devices, dielectric relaxation is expected to become increasingly problematic as the dimensions of devices get smaller and their electric fields get larger. For the student, this serves as an example of how the line between physics and engineering can be blurry at the edge of high technology.
  Host: Durst
• February 9, Friday, 1:30PM
  Gleb Finkelstein ( Duke University ): Carbon Nanotube Quantum Dots: from the Coulomb Blockade to the SU(4) Kondo and the Mixed Valence regimes.
  Carbon nanotubes present an ideal system for investigating quantum electronic transport at the nanoscale. By controlling the contact transparency within the same nanotube Quantum Dot, we can observe the conductance evolution from the well-developed Coulomb blockade through the Kondo regime to the 'Mixed Valence' regime. We work with high quality nanotubes, where quantum-mechanical orbitals are doubly-degenerate, forming four-electron 'shells', so that the system possesses an SU(4) symmetry. We observe the SU(4) Kondo effect for one, two, and three electrons in a shell. As the contacts are made more transparent, the sample enters the 'Mixed Valence' regime, where different charge states are hybridized by electron tunneling. The hallmark of the new regime is the surprising lack of single-electron features at low temperature, which are uncovered as the temperature is raised.
  Host: Likharev
• February 12, Monday, 1:15PM
  Xuan Gao ( Harvard University ): Pushing the Sensitivity Limit of Nanowire Bio-Sensors
  Nanowire field effect transistors (NWFETs) are emerging as powerful tools for biological applications such as bio-molecule detection, yet their sensitivity limits are not understood at a fundamental level. I will discuss the interplay of device parameters such as gate bias and NW diameter on the operating modality and sensitivity of NWFET sensors. pH and cancer marker detections are studied as silicon-NWFETs are tuned from linear to subthreshold regimes by electrochemical gating. First, pH sensing data show that NWFET has the strongest response and the best signal to noise ratio in the subthreshold regime. Operating in the subthreshold regime also reduces the detection limit for prostate specific antigen down to ~fM for a device with ~pM detection limit in the linear regime. This sensitivity improvement is due to the reduced screening ability of carriers in NW. A quantitative model describing these results and the intrinsic charge detection limit of NWFET sensors will be discussed.
  Host: Goldman
• February 16, Friday, 1:30PM
  Irene Dujovne ( Delft University of Technology ): Optical tracking of molecular processes at high resolution with a novel traveling wave technique
  In order to understand the basic mechanisms of living cells, it is essential to elucidate the complexity of fundamental interactions and dynamical processes between their underlying molecular building blocks, such as DNA, RNA, and proteins. High resolution, non invasive, optical methods are outstanding tools that can provide us with unique insights into such sophisticated molecular machineries.
  In this talk I will present a new optical experimental technique which is highly sensitive and minimally invasive that allows, in particular, study of real time dynamics of molecular motors in the single molecule limit. These dynamical processes are essential to many cell functions, such as intracellular transport, cell division, growth, and locomotion. The extremely high temporal (~10 microsecond) and spatial (subnanometer) resolutions provided by the new method may be key to studying various interactions and dynamical mechanisms of molecular motors. I will describe the basic physical principles of the technique, and will discuss preliminary results from several systems, whose detailed study is currently underway.
  Host: Allen
• February 19, Monday, 1:15PM
  Tim Duty ( Chalmers University of Technology ): Circuit-QED, quantum capacitance and probing multiphoton dressed states using Cooper-pair box qubits
  During the last few years, it has been shown that the non-linearity of the Josephson effect can be harnessed to build quantum bits out of superconducting micro/nano-electronic circuits. One such class of circuits is based upon the Cooper-pair box---a small superconducting island connected to a superconducting lead by an insulating junction. We now know that the Cooper-pair box behaves as a qubit should. (Ok, maybe not such a great qubit due to decoherence, but a qubit nonetheless). This talk will focus on circuit-QED, i.e., qubit-photon interactions using Cooper-pair box qubits. I will address the following questions: How does the Cooper-pair box behave as a circuit element? In particular, how does it behave in a resonant circuit? How can we couple our qubit to single photons? Since they are mesoscopic, Cooper-pair box qubits can be driven hard....very hard indeed. What happens then? Why are these effects interesting, and what can we do with them? And finally, why do Cooper-pair box devices (at the moment) make such lousy qubits?
  Host: Likharev
• March 2, Friday, 1:30PM
  Eugene Sukhorukov ( University of Geneva ): Resonant dephasing of electronic Mach-Zehnder interferometer
  We propose a simple explanation of recently observed [I. Neder et al., PRL 96, 016804 (2006)] unexpected Aharonov Bohm effect in a quantum Hall system. Our explanation, apart from being valuable in the context of this experiment, brings forward a new interesting phenomenon of resonant dephasing, which might be encountered in other physical systems. We show that the accidentally discovered new effect can be used as a powerful instrument for investigating the properties of one-dimensional interacting systems and for discriminating among the effective models.
  Host: Averin
• April 20, Friday, 1:30PM
  Martin Dressel ( Stuttgart University ): Electronic Correlations in Layered Metals: Optical Spectroscopy of Organic Conductors
  Two-dimensional organic conductors serve as model compounds to investigate the influence of electronic correlations on the optical properties. Competing interactions cause various types of ordering phenomena and metal-insulator transitions. Using chemical and hydrostatic pressure as well as chemical modifications, we can tune lattice parameters, the effective correlations and the bandfilling.
  Host: Mihaly
• April 23, Monday, 2:00PM
  Gergely T. Zimanyi ( University of California, Davis ): Freezing, aging, and coarsening in long range glasses
  Gergely was a postdoc in our Department some time ago in the Condensed Matter Theory group. His current research interest covers applied and basic sciences, including nanostructured solar cells, magnetic recording in perpendicular media and disordered vortex matter.
  Host: Mihaly
• May 16, Wednesday, 1:30PM
  Mark S. Hybertsen ( Center for Functional Nanomaterials, Brookhaven National Laboratory & Center for Electron Transport in Molecular Nanostructures, Columbia University ): Amine Linked Single Molecule Circuits: Systematic Measurements & Understanding
  Formation and function of well-defined linkages between organic molecules and metallic electrodes has been a key issue in the field of molecular electronics. We recently discovered that the conductance of single molecule junctions formed using gold-amine linkages can be measured reliably and reproducibly. The conductance of approximately 40 different amine linked molecules from several families has been measured revealing systematic trends with length, conformation, conjugation and substituent. In this talk, I will present results of the experiments performed at the Columbia NSEC together with our present theoretical understanding of the properties and function of the gold-amine link in single molecule conductance.
  Host: Allen

Created by Laszlo Mihaly, 1/1/2007.