Condensed Matter & Biological Physics Seminar Condensed Matter & Biological Physics Seminar| Date | Speaker | Affiliation | Title | Host | |||
|---|---|---|---|---|---|---|---|
| September 1st | |||||||
| September 8th | |||||||
| September 11th Monday |
Prof. Susan Coppersmith | University of Wisconsin, Madison | Computational complexity and complex systems | Schwarz | |||
| In this talk I will discuss how physics concepts can be useful for understanding issues arising in the field of computational complexity, the study of the amount of computational resources needed to solve different problems. In particular, I will use a renormalization group construction similar to those used to provide insight into phase transitions in physical systems to provide new insight into how to distinguish computational problems that can and cannot be solved efficiently. | |||||||
| September 22nd | Prof. Steve Girvin | Yale University | Quantum Mechanics and Quantum Computation with Electrical Circuits: Coupling a Single Photon to a Single 'Atom' | Plourde | |||
| Recent experimental breakthroughs have led to the construction of artificial superconducting ‘atoms’: nanoscale objects containing approximately one billion aluminum atoms coherently acting in concert. When placed inside a resonant cavity, these ‘atoms’ can strongly interact with single microwave photons. This talk will give an elementary introduction to recent remarkable experiments in the labs of Rob Schoelkopf and Michel Devoret at Yale. In addition to being a new test bed for quantum mechanics and quantum optics, this system has many promising features for quantum computation. | |||||||
| September 29th 10:30 A.M. |
Prof. Jene Golovchenko | Harvard University | Single-molecule detection with solid-state nanopores | Movileanu | |||
| October 6th | Gilman Toombes | Cornell University | Holey Sheets and Crossed Concertinas: Sculpting Silica with ABC Block Copolymers | Schwarz | |||
| Organisms such as diatoms and sponges produce organic-silica composites with complex, hierarchical structures and handy material properties. Some aspects of biological ceramics can be mimicked using surfactants or amphipathic molecules to structure inorganic precursors. In a block copolymer, chemically dissimilar polymer chains (blocks) are covalently linked together and these blocks micro-phase separate into nanometer-scale domains. When a silica sol is mixed with a block copolymer, the silica assumes the structure of the block copolymer domains and several silica morphologies have been formed using AB di-block copolymers. For ABC tri-block copolymers, the third distinct block leads to a zoo of interesting structures, both for the pure copolymer and copolymer/silica hybrids. | |||||||
| October 13th | |||||||
| October 17th Tuesday, 4-5 PM | Prof. Abraham Stroock | Cornell | Microfluidic control of mass transfer for micro-fuel cells and tissue engineering | Schwarz | |||
| The advancement of microfluidic technology calls for the application of chemical engineering principles, the development of new experimental methods, and the definition of new target applications. I will discuss two efforts in my research group in which we have attempted to address this multiplicity of challenges. In a first theme, we have focused on a fundamental question regarding the role of three-dimensional laminar flows in controlling the rate of mass transfer to solid reactive boundaries in ducts. This question is relevant to the design of efficient microreactors that involve heterogeneous catalysts, and sensing technologies that involve binding at surfaces, as in protein- and DNA-chips. I will present results from simulation and theory that clarify the importance of chaotic advection in defining the rate of interfacial mass transfer from liquids. In particular, I will explain how the appropriate choice of a chaotic flow guarantees transition after a short entrance length into an asymptotic regime characterized by high, Peclet number-dependent values of the Sherwood number. I will conclude on this theme by presenting our experimental application of these concepts to the optimization of power density and fuel efficiency in a micro-fuel cell. In a second theme, we have developed microfluidics as a mimic of the vascular system for the control mass transfer within biological materials for applications such as wound healing and tissue engineering. Using in vitro growth of cartilage as an example, I will discuss engineering principles for the design of microfluidic systems that provide spatial and temporal control of the chemical environment of cells (primary chondrocytes or cartilage cells) within three-dimensional scaffolds. I will then present experimental methods we have developed to embed functional microfluidic structure directly within cell-seeded hydrogels, and quantitative characterization of the mass transfer provided by these structures. I will demonstrate the use of these microfluidic scaffolds for the culture of thick sections of tissue and for the induction of spatially varying cellular behavior within single, monolithic scaffolds. I will conclude on this theme with discussion of the opportunities and challenges for the application of microfluidics to biomedical applications. | |||||||
| October 24th Tuesday, 4-5 PM | Prof. Xiao-Gang Wen | MIT | Quantum liquids of polymers and origin of light and electrons | Schwarz | |||
| We discuss a new quantum state of matter -- string-net condensed state. Such a state is beyond Landau symmetry breaking paradigm. The collective modes in the string-net condensed state are shown to satisfy Maxwell equation and correspond to (artificial) photons. The ends of string correspond to electrons. String-net condensation unifies the Coulomb interaction and Fermi statistics. | |||||||
| October 27th | Prof. Bulbul Chakraborty | Brandeis University | Jamming as a critical phenomenon: a field theory of zero-temperature grain packings | Marchetti | |||
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In a remarkably diverse range of systems, the transition from a flowing, liquid state to a jammed, solid state is heralded by a dramatic slowing down of relaxations. Does an equilibrium critical point underlie this glassy dynamics? Experiments on weakly sheared granular media indicate that at a critical packing fraction there is a transition accompanied by slow dynamics, vanishing of mean stress, increasing stress fluctuations and a change in the distribution of contact forces. Simulations indicate a critical point occuring at zero temperature and a packing fraction close to the random close packing value. At this critical point the grain packing is isostatic, havin reached the special coordination where all contact forces are completely determined by the packing geometry. In order to understand the occurrence of critical points in granular assemblies, we need to generalize the framework of thermal equilibrium to these athermal systems. In this talk, I will show how to construct a statistical ensemble for grain packings and present a framework which is a generalized Edwards' ensemble. We have used this statistical ensemble to construct a field theory of two dimensional, zero-temperature, frictionless grain packings and have shown that there is a critical point that separates a disordered phase from an "ordered" one characterized by two order parameters: (i) the magniutde of the force per contact, <&phi> and |
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| November 3rd 10:45 A.M. | Prof. Dacheng Ren | Syracuse University | Inhibiting Bacterial Multicellular Behavior: Challenges and Opportunities | Schwarz | |||
| Bacteria have evolved complex multicellular behaviors, such as biofilm formation and quorum sensing, to initiate infections and to survive in competitive environments. These phenotypes lead to serious problems in medical and engineering environments; e.g., bacterial drug resistance, contamination of medical devices and biocorrosion. Such systems involve coordinated gene expression and cell-cell signaling. In this presentation, we will discuss our progress in understanding the genetic basis of bacterial multicellular behavior and development of novel control strategies. | |||||||
| November 10th Normal time | Prof. Bob Behringer | Duke University | Statistical Properties of Dense Granular Materials | Schwarz | |||
| This seminar will explore the properties of dense granular materials through a series of experiments. The first of these will describe experiments which probe the interior states of granular systems through the use of photoelastic particles. These studies yield vector contact forces and related contact inforamtion. An interesting observation from these studies is that sheared and compressed states are characterized by long and short range correlations, respectively. Using a similar technique, we characterize the jamming transition. In particular, we test and largely confirm recent predictions for the jamming transition in granular-like systems. We next turn to plastic failure, which typically occurs when the system is subject to large enough shear deformation. In steady Coutte shear, we measure the local diffusivity and relate the meso-scopic behavior to shear transformation zone models. We also consider the local behavior of material that is subject to pure shear. In this case, we observe individual plastic failure events, and a localization of motion in a vortex-filled shear band. | |||||||
| November 10th 1:30-2:30 P.M. | Prof. Ken Segall | Colgate University | Vortices, Ratchets and Breathers: Nonlinear Dynamics in Josephson Arrays | Plourde | |||
| In 1962, in work for which he was awarded the Nobel Prize, Brian Josephson made the remarkable prediction that if two pieces of superconductor were separated by an insulator, the superconducting electrons in the two electrodes would be able to tunnel back and forth. Such devices, called Josephson Junctions, are interesting in many respects, one of which is the nonlinear relation between the phase of the wave function and the current flowing though the junction. This nonlinearity manifests itself in a nonlinear, pendulum-like equation for the dynamics of the phase of the wave function when the junction is placed in circuit. When many such junctions are coupled together into an array, complex nonlinear phenomena can result. This talk will focus on our recent work to study different nonlinear phenomena that can result from coupling Josephson Junctions together in different topologies. The first type of array emulates a "Ratchet" type of device, where random noise can move a particle in a preferred direction. In another type of array, two different kinds of nonlinear excitations, one called a Discrete Breather and another called a Vortex, can be collided and their interaction can be studied. Both of these circuits need to be described by complex nonlinear theory, and may contain new physical effects that have yet to be observed. We will present an introduction along with recent measurements which aim to explore these ideas. | |||||||
| November 17th | Dr. Vincenzo Vitelli | U. Penn. | Curved space crystallography | Bowick | |||
| We present a theoretical and numerical study of the static and dynamical properties that distinguish two dimensional curved crystals from their flat space counterparts. Experimental realizations include block copolymer mono-layers on lithographically patterned substrates and self-assembled colloidal particles on a curved interface. | |||||||
| At the heart of our approach lies a simple observation: the packing of interacting spheres constrained to lie on a curved surface is necessarily frustrated even in the absence of defects. As a result, whenever lattice imperfections or topological defects are introduced in the curved crystal they couple to the pre-stress of geometric frustration giving rise to elastic potentials. | |||||||
| These geometric potentials are non-local functions of the Gaussian curvature and depend on the position of the defects. They play an important role in stress relaxation dynamics, elastic instabilities and melting. | |||||||
| November 24th | Thanksgiving | ||||||
| December 1st | Prof. George Thurston | Rochester Institute of Technology | Liquid-liquid phase separation of aqueous mixtures of eye lens proteins | Foster | |||
| In cataract, the leading cause of blindness worldwide, scattering of light in the lens of the eye degrades vision. One possible source of this light scattering is a phase transition within the eye lens cytoplasm. Concentrated protein mixtures of the cytoplasm create a high index of refraction within the lens, which helps focus light on the retina. However, these concentrated solutions can also scatter light and can even separate to form coexisting liquid phases that differ in refractive index, and thereby cloud the lens. | |||||||
| This lecture will present studies of the aqueous phase diagram of mixtures of the lens proteins, B crystallin and crystallin. Concentrated mixtures of B crystallin and crystallin can separate reversibly to form two liquid phases at body temperature, although B crystallin alone separates only below ~10 degrees Celsius, and crystallin alone does not phase separate. | |||||||
| We also study light scattering, X-ray scattering and neutron scattering of B crystallin and crystallin mixtures. In combination with modeling and simulation, the scattering data are consistent with the following key molecular features as determining the phase diagram and other mixture properties: size disparity between crystallin and B crystallin, net attractive B-B interactions, and net attractive B-crystallin interactions. | |||||||
| December 8th | Dr. Arvind Gopinath | Harvard | CANCELLED | Foster | |||
| CANCELLED | |||||||
| December 15th | Prof. Sidney R. Nagel | University of Chicago | Jamming and the Low-Temperature Properties of Glasses | Bowick | |||
| Both glasses and granular materials are amorphous systems of particles in which the dynamics is perched precariously near a transition between a flowing and a static state. Is there something generic about such transitions so that the freezing of a liquid into a glass can profitably be compared to the arrest of a flowing granular material or a colloidal suspension as external stresses are reduced below the yield stress? | |||||||
| In the present talk, I will consider properties of the marginally jammed state at zero temperature and zero applied shear stress. The transition into this state has many unusual features [1] and the solid that is formed is equally strange. In particular, there is a dramatic contribution to an excess density of vibrational states at low frequencies which is reminiscent of the Boson peak seen in glasses. An analysis of the origin of these modes suggests a new approach to jammed materials and to the low-temperature excitations of glasses. | |||||||
| [1] J. M. Schwarz, A. J. Liu, L. Chayes, Europhys. Lett. 73, 560-566 (2006). Jamming as the sudden emergence of an infinite k- core cluster. | |||||||
| Date | Speaker | Affiliation | Title | Host | |||
|---|---|---|---|---|---|---|---|
| January 19th | |||||||
| January 31st Wednesday, 11:00 A.M. |
Dr. Matthew Doty | Naval Research Laboratory | Spins in Quantum Dot Molecules | ||||
| In this talk, I will present recent results from optical studies of quantum dot molecules, which are formed by growing two self-assembled quantum dots on top of one another with a small spacer layer. We find that the combination of spin exchange interactions and tunneling lead to complex photoluminescence spectra that can be understood with relatively simple models of the molecular states. I will present and explain some of these spectra, which tell us about the detailed spin interactions of particles localized in spatially distinct dots. We also find that there are uniquely molecular spin properties. As an example, I will discuss our discovery that the g-factor of spins confined within a quantum dot molecule depends on both the molecular state and the applied electric field. Electrically tunable g-factors are of great interest for the development of spintronics, and our results suggest that there may be new opportunities to control spins by engineering the wavefunctions of quantum dot molecules. | |||||||
| February 2nd, Normal time | Dr. Alberto Fernandez-Nieves | Harvard University | Drops and jets in co-flowing liquids | ||||
| We study the dripping-to-jetting transition of a liquid that is injected into a second, immiscible, coaxially flowing liquid; a slightly more complicated problem than dripping and jetting in a conventional kitchen faucet. We show that these transitions are controlled by two non-dimensional numbers: The capillary number of the outer liquid (Caout) and the Weber number of the inner liquid (Wein). These numbers establish the relative importance of viscous and inertial forces with respect to surface tension forces, respectively. When jetting is forced by Caout, the diameter of the jet narrows in the downstream direction. For these jets, the drop diameter and jet length scaling are well predicted assuming that the Rayleigh-Plateau instability is convected downstream. However, when jetting is forced by Wein, the diameter of the jet widens in the downstream direction. In this case, the resultant drop size can not be predicted assuming that the Rayleigh-Plateau instability drives the break-up of the jet. Instead, we believe that these jets break-up due to an absolute instability: Perturbations grow in time at a fixed spatial location irrespective of external noise. Overall, our results highlight the importance of the instability nature for producing stable monodisperse emulsions using microfluidic-based devices. | |||||||
| February 2nd,
2:30-3:30 P.M. |
Prof. Igor Sokolov | Clarkson University | Mechanics of human cells at nanoscale studied with AFM | Movileanu | |||
| Recent advance of modern nanotechnology has been stimulated a lot by the development of scanning probe techniques, in particular, the Atomic Force Microscopy (AFM). AFM is especially useful for studying biological systems because it can be used on viable cells directly in physiological media. Recently we have developed an AFM method to reliably measure rigidity of cells. Here, I will focus on AFM study of mechanics of human epithelial cell, cells that cover the surface of the body and line its cavities. To demonstrate the range of what can specifically be learned with AFM, I will present two examples: aging skin cells and cervical cancer vs normal cells. | |||||||
| In the first example, we found a considerable increase in rigidity of cells while their ageing in-vitro. Such a phenomenon has not been reported before. It is important because the loss in elasticity of epithelial tissues with ageing is associated with many aging problems. Developing a novel high resolution method to study cytoskeleton, we found correlation between the cell rigidity and the density of microfilament cytoskeletal fibers. Using drugs that inhibit polymerization of microfilaments, we managed to restore rigidities of old cells back to the young level while preserving cells vitality. Some potential in-vivo implications of such treatment will be presented. | |||||||
| "Mechanics" of cancer cells considered in the second example is still in controversy in the literature. Studying cervical cell, we found that physics of mechanical properties of these cells is more complicated than scientists thought before. I will demonstrate that the surface layer of cancer cells is rather different from the surface of normal cells. Based on these measurements we developed two methods for easy identification of cancer cells using fluorescent silica particles. This is still ongoing research aiming at developing a method for fast screening of precancerous epithelial tissue without biopsy. | |||||||
| February 7th, Wednesday, 11:00 A.M. |
Dr. Daniel Blair | Harvard University | Pushing on colloids: How a gel can compress a glass | ||||
| This talk will discuss recent experimental results on localized restructuring of colloidal glasses and suspensions under uniaxial compression. We employ a novel technique for the application of external osmotic pressure on dense suspensions of colloidal particles near their glass transition. Thermally expandable NIPAM hydrogels are used as 'permeable pistons' that confine and compress a sediment of colloidal particles. We are able to compress the colloids relative to their solvent, thus creating a overall increase in particle volume fraction above the glass transition. Using laser scanning confocal microscopy and three dimensional image analysis, we obtain time resolved information on both the dynamics and structure of the colloidal particles. The local strain tensor for each particle is determined and reveals that volume changes in colloidal glasses are mediated through correlated shear transformations. I will discuss how these results relate to both granular materials and metallic glasses. | |||||||
| February 9th | Dr. Masa Ishigami | University of Maryland | Impact of individual atoms/molecules in nanoscale transport physics | ||||
| Nanoscale materials allow for unprecedented experimental control over their composition, atomic structure and environment, and provide opportunities to explore physics which would otherwise be merely theoretical curiosities. I have been studying how individual atoms and molecules influence the transport properties through nanoscale materials. I will present my recent results using a new high-resolution microscopy technique capable of imaging atomic structures of nanoscale devices. Using the unprecedented capability of this new technique, I show the mechanical properties of a monolayer of graphite, graphene. I will also present the impact of individual adsorbates on nanoscale devices fabricated from individual carbon nanotubes and show that these devices are capable of .listening. to atomic-scale fluctuations. | |||||||
| February 16th | |||||||
| February 21st Wednesday, 11:00 A.M. |
Dr. Ivan Smalyukh | University of Illinois Urbana-Champaign | Elasticity-Mediated Colloidal Interactions and Controlled Self-Assembly in Liquid Crystals | ||||
| Self-assembly of colloidal particles and molecules into ordered structures is a fascinating phenomenon of both fundamental and applied interest. This lecture will demonstrate that the self-assembly phenomena in elastic liquid crystal (LC) media are particularly rich and can be controlled. In LCs the embedded spherical particles can cause elastic distortions of either dipolar or quadrupolar symmetry because of the anisotropic molecular interactions at the colloid-LC interfaces. I will show that the surface treatment of the colloidal particles and the LC confinement determine the nature of inter-particle interactions as well as the formation of colloidal structures. For example, the particles with the dipolar elastic distortions interact similar to the electrostatic dipoles and form chains along the far-field LC director. The particles with tangential surface anchoring and quadrupolar elastic distortions aggregate into chains directed at about 30 degrees to the average orientation of the LC molecules far from the spheres. When the elasticity-mediated colloidal interactions between particles take place at the surfaces of anisotropic fluids, the particle arrangements strongly depend on the boundary conditions at the confining surfaces and can vary from linear chains to hexagonal colloidal structures. Moreover, both in the LC bulk and at the surfaces, the inter-particle interactions and the structures are strongly altered by adding tiny amounts of chiral additives, resulting in twisted colloidal chains and spirals. I will show that even live biological cells (such as Pseudomonas aeruginosa) can be orientationally ordered when placed into the elastic liquid-crystalline matrices of DNA biopolymers. The particle motions are monitored using video-rate optical imaging and the interactions are explored using laser tweezers. The inter-particle colloidal forces exhibit rich angular and distance dependencies, consistent with the symmetry of the elastic distortions around the particles. The experimental observations are modeled considering the LC elastic properties and correctly reproduce the experimentally measured parameters. These findings impinge broadly on understanding of phenomena as diverse as colloidal interactions in anisotropic media and biofilm formation in the presence of extracellular biopolymers, as well as demonstrate the possibility of controlled colloidal self-assembly into tunable photonic crystal structures. | |||||||
| February 23rd | Dr. Maria Iavarone | Argonne National Laboratory | Two-band Superconductivity: the Case of Magnesium Diboride | ||||
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The salient feature in MgB2 is the existence of two distinct gaps on two different portions of the Fermi surface. The two sets of electrons are only weakly interacting and the high superconducting critical temperature of ~40 K, is due to a strong coupling between a small fraction of phonons and a portion of the Fermi surface. Tunneling spectroscopy is a powerful tool to directly probe the density of states on the surface and when it is combined with scanning capability it allows studying the detailed nature of superconductivity on a local scale. The highly anisotropic MgB2 band structure acts as a momentum filter in a tunneling experiment. Therefore scanning tunneling spectroscopy can be used to selectively probe the two different types of bands in this material and to measure the two superconducting gaps. One of the open issues is the role played by the disorder in a two-band superconductor. Two-band models predict that nonmagnetic scattering causes pair breaking as the magnetic scattering does in a one band superconductor. According to the theory, increasing disorder should increase the interband scattering and mix the Cooper pairs from different bands causing an isotropization of the Fermi surface. Due to the possible simultaneous occurrence of several sources of disorder, quantitative evidence of the role played by the interband scattering has not been given yet. A systematic study of Scanning Tunneling Spectroscopy (STS) performed on high quality thin films, pellets and single crystals of MgB2 will be presented. Moreover, the field scale for the gap filling in the presence of a magnetic field is set by the ratio of the diffusivities of the quasiparticles in the two bands. A combined analysis of the magnetic field dependence of the tunneling spectra and upper critical field measurements in disordered films has been used to gain understanding into the effect of disorder in this material. |
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| March 2nd | |||||||
| March 9th | |||||||
| March 16th | Spring Break | ||||||
| March 21st Wednesday, 11:00 A.M. |
Dr. Matt Hastings | Los Alamos National Laboratory | Community Detection as an Inference Problem | Middleton | |||
| Community detection is a well-studied problem with applications to biological, social, and other networks. Remarkably, however, there is no good definition of this problem. In this talk, we show that one of the standard tests of community detection algorithms, the "four groups" test due to Newman and Girvan, provides a precise definition of community detection as a problem in Bayesian inference. We then apply techniques used for other Bayesian inference problems, in particular the belief propagation algorithm developed for decoding error-correcting codes in communication over a noisy channel, to develop a very accurate and fast community detection algorithm. | |||||||
| March 30th | Dr. Arvind Gopinath | Harvard | Oscillations of Eukaryotic Cilia and Flagella | Foster | |||
| The undulating beat of eukaryotic flagella and cilia produces forces that move cells and cause locomotion. The timing mechanisms that generate these periodic undulations are still mysterious and the question of how these oscillations arise is still a subject of much research - both experimental and theoretical. Recent experimental results on paralyzed and reconstituted flagella offer new insight into the dynamical mechanisms that could result in sustained waveform generation. Motivated by these we propose a model that mimics the flagellar structure as elastic, inextensible filaments driven by the active sliding of dynein motors. The equations describing the evolution of the populations of attached motors is actively coupled to the local configuration as well as local sliding velocity via strain and configuration dependent kinetic reaction rates. At the same time, the filament configuration is actively coupled to the motor densities via the dependence of the active internal torque densities on the motor populations as well as their internal state. Appropriate ensemble averaged force-velocity relationships for the motors completes the set of equations. Numerical solutions reveal the possible onset of dynamical instabilities via Hopf-bifurcations with oscillatory waveforms emerging from a trivial base state corresponding to a straight, non-moving flagellum. | |||||||
| April 6th | Dr. Robert McDermott | University of Wisconsin-Madison | SQUID-Detected NMR and MRI in Microtesla Magnetic Fields | Plourde | |||
| We have performed a series of liquid-state nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) experiments involving detection with a low-Tc dc Superconducting QUantum Interference Device (SQUID) in magnetic fields from microtesla to tens of microtesla, corresponding to proton Larmor frequencies from tens of Hz to kHz. At these field strengths, it is possible to achieve NMR linewidths of the order of 1 Hz even in grossly inhomogeneous magnetic fields. Narrowing of the NMR signal bandwidth through reduction of the measurement field strength leads to an enhancement in both spectral resolution and, in the case of detection with an untuned SQUID magnetometer, signal-to-noise ratio. I describe a low-Tc SQUID system for liquid-state NMR spectroscopy in microtesla fields. Heteronuclear scalar couplings have been resolved in 1H-31P and 1H-13C systems. I next describe a SQUID system for MRI in microtesla fields. I present images acquired using only weak encoding gradients of the order of 10 uT/m; the spatial resolution is a few mm. Finally, I discuss the prospect for hybrid SQUID systems designed for both biomagnetic measurements and MRI. | |||||||
| April 13th | |||||||
| April 18th Wednesday, 11:00 A.M. |
Dr. Gergely T. Zimanyi | University of California Davis | Long range interactions at work: the stripe glass and the dislocation glass | ||||
| Long range interactions can introduce frustration without any quenched randomness. In dislocation systems the logarithmic interaction has a dipole prefactor, changing sign depending on the relative orientation of the dislocations, giving rise to a dislocation glass. In perpendicularly magnetized films the dipolar interaction is antiferromagnetic, i.e. the triangle of any three spins is frustrated, resulting in a stripe glass. We analyze these two models with the concepts of modern glass physics. We analyze the aging behavior of the models through various correlation functions. We also identify growing length scales associated with spatial inhomogeneities. | |||||||
| April 27th | Dr. Cris Moore | University of New Mexico | Fearful Symmetries: The Hunt for a Quantum Algorithm for Graph Isomorphism | Schwarz | |||
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In 1994, Peter Shor showed that quantum computers can solve two basic problems which appear to be very hard for classical computers: factoring large integers and finding the discrete logarithm. These results began the field of quantum computing in earnest. In particular, they showed that quantum computers can break RSA public- key encryption, on which most of electronic commerce is based, and Diffie-Helman key exchange, which is used by programs like Skype to encrypt communication. Because of some tempting mathematical analogies, researchers in the field guessed that an approach similar to Shor's could be brought to bear on another problem believed to be hard for classical computers: Graph Isomorphism, the problem of telling whether two graphs have the same topological structure up to a relabeling of their nodes. However, a series of negative results have shown that these approaches cannot work. In particular, I will present joint work with Alex Russell (Connecticut) and Leonard J. Schulman (Caltech) showing that one class of algorithms would require an exponential number of measurements to tell whether two graphs are isomorphic. On the way, I will give an accessible introduction to nonabelian Fourier analysis and the representation theory of finite groups. | |||||||
| May 4th | Dr. Stephen Teitel | University of Rochester | Critical Scaling at the Jamming Transition | ||||
| In granular materials, or other spatially disordered systems such as colloidal glasses, gels, and foams, in which thermal fluctuations are believed to be negligible, a jamming transition has been proposed: upon increasing the volume density (or "packing fraction") of particles above a critical value, the sudden appearance of a finite shear stiffness signals a transition between flowing liquid and rigid (but disordered) solid states. We carry out numerical simulations of a soft sphere model of a granular material in two dimensions at zero temperature, computing the shear viscosity of the flowing state as a function of both particle volume density and applied shear stress. About the jamming transition we find an excellent scaling collapse of our data to a function of a single scaling variable. By considering velocity correlations we extract a correlation length and show that it too obeys a scaling collapse, diverging at the jamming transition. From these scaling collapses, we estimate critical exponents. Our results confirm that jamming is a true second order critical phenomenon that, as originally proposed by Liu and Nagel, extends to driven steady states along the non-equilibrium axis of applied shear stress. | |||||||
| May 25th | Dr. Austin Fowler | University of Waterloo | Scalable quantum computer architecture for superconducting flux qubits | Plourde | |||
| For a quantum computer architecture to be called scalable, it should in principle be possible to construct an arbitrarily large number of qubits, including all necessary classical control circuitry and devices, and the number of simultaneous measurements and gates should grow linearly with the number of qubits. Furthermore, where heat dissipation is an issue, the cooling power of the computer should also grow linearly with the number of qubits. Lastly, and most importantly, the physics of each individual measurement or gate should not depend on the total number of qubits. In this talk, we present a quantum computer architecture for superconducting flux qubits that satisfies the above definition of scalability. We also show that despite optimising the design to permit simple and efficient error correction, the threshold two-qubit gate error rate of the architecture is $6.25\times 10^{-6}$ due to the lack of arbitrarily long-range interactions and the limited dimensionality of the qubit layout -- both general features of real quantum computer architectures. | |||||||