Colloquium Fall 2008
Labor Day, University Holiday
Chemical Routes to Graphene for Applications
Prof Richard Kaner
Graphene, a zero band gap semiconductor, is a promising candidate for the next generation of nano-electronic devices. While many theoretical studies of graphene promote exciting properties, experimental results have been far less forthcoming likely due to the difficulty in producing single layer specimens. Despite tremendous efforts made to develop a scalable production method, bulk processing has not yet been achieved. Single layer samples are currently made by either a laborious drawing method, highly oriented pyrolytic graphite (HOPG) is repeatedly pealed using Scotch tape and deposited onto a silicon substrate. Alternatively, the reduction of silicon carbide can be used to produce very small domains of single layer samples, however, temperatures of greater than 1100oC are needed and producing large domains remains a challenge. We suggest an alternative method for creating single sheets starting from graphite oxide (GO). Graphite can be oxidized to produce GO and then exfoliated to create stable aqueous dispersions of individual sheets. After deposition, GO may be reduced to graphene either chemically or via thermal annealing. The scale of single sheets produced allows for characterization by scanning electron and atomic force microscopies (SEM and AFM), Raman spectroscopy, and field effect measurements. It is through the combination of these diagnostics that we able to confirm the presence of single sheets. Such specimens show step heights of ~0.6 nm in AFM and exhibit strong G and 2D peaks in Raman spectroscopy, both good indications of their similarities to peeled graphene. Field effect measurements display p-type behavior, with currents in the mili-amp range with source-drain voltages of 1 V under ambient conditions. The lack of ambipolar effect is likely due to the presence of residual oxygen or sp3 hybridized carbons. With additional chemical preparation, reduced samples may indeed exhibit electrical properties approaching those of native graphene. A chemical approach toward graphene provides several advantages, including high scalability and solution processing. The method also allows for a variety of chemical modifications before deposition, something yet to be seriously investigated. As the demand for graphene increases, graphite oxide may well play an increasing role in production of samples.
Beating the diffraction limit--Don't believe everything you hear in Physics 163!
Prof Alex Small
Physics Department, Cal Poly Pomona
Standard microscopes cannot resolve features smaller than the wavelength of light, due to diffraction of light by the microscope aperture. This same phenomenon also limits the features that can be formed in conventional photolithography. In recent years, several techniques have been proposed for beating the diffraction limit in fluorescence microscopy, enabling the detection of cellular features down to 30 nm scales. I will describe two of these techniques and calculations that I've been performing for extending their capabilities. I will propose that one fluorescence imaging (Stimulated Emission Depletion) technique can be "inverted" to perform optical nanolithography with a conventional microscope, and show preliminary simulations of subwavelength nanolithography. I will also discuss the limits on image acquisition rates in a related technique (Stochastic Reconstruction Microscopy), and the dependence of the image acquisition rate on detection errors and image analysis algorithms.
Einstein, Nanoscience, and Superconductivity
Prof Marvin Cohen
Department of Physics, University of California, Berkeley
I participated in activities related to the World Year of Physics in 2005 when I served as President of the American Physical Society. We celebrated the centennial anniversary of Einstein's great accomplishments and generally promoted and celebrated physics. I would like to share a few observations about these activities and how they were related to Einstein. I'll then go on to discuss some of the history and conceptual underpinnings of condensed matter physics with examples related to research on the electronic structure of solids. Finally, I'll focus on some recent work in nanoscience and superconductivity that my colleagues and I have been doing.
Can the Superconductor-Insulator Transition be a Duality Quantum Phase Transition?
Professor Wenhao Wu
Physics, Texas A&M
Duality transformations are mathematical tricks theoreticians use to find solutions for a variety of problems. Using duality transformations, one can relate the physical properties of one system to that of another, seemingly different one. Now, experimentalists are also getting into the action, presenting evidence for some potentially important consequences of duality symmetry in condensed matter systems. A particularly interesting example is two-dimensional (2D) superconductors at absolute zero of temperature. The superconducting state can be viewed as a Bose condensate of electron Cooper pairs. The condensate wave function has a topological point defect: the vortex. By a duality transformation, the roles of particles and vortices may be interchanged. The 2D system can be in two thermodynamic phases: superconducting (a superfluid of Copper pairs) or insulating (a superfluid of vortices). A duality transformation maps one phase into the other. A direct consequence of this duality argument is that the superconductor-insulator transition should occur at a universal critical resistance h/(2e)^2, where 2e is the charge for a pair of electrons. However, this universal critical resistance has been elusive in experimental searches in quite a number of 2D superconducting systems. In this talk, our recent experimental results on the superconductor-insulator transition in ultrathin beryllium films will be presented. Remarkably, the critical resistance is found to be h/(2e)^2 in a quite broad parameter space. The implication of this observation on a possible duality transition will be discussed.
The Mysterious Beauty of the Kondo Effect
Professor Gerd Bergmann
Prof. Bergmann gives no abstract in order to guard the beauty of the mystery.
Evolution of Substructure in Clusters of Galaxies as Observed in X-Rays
B C Hart
Clusters of galaxies are the largest gravitationally-bound objects in the Universe, having diameters on order of Mpc. Our work asked whether their shapes (morphologies) change over time as the Universe ages. We observed a sample of 165 galaxy clusters, at 0.1 < z < 1.3. A variety of measures were used to quantify the shapes of galaxy clusters. Archive observations from the Chandra X-Ray Observatory were used. Morphology evolution was probed at two different distances from clusters' centers. ¨C 300 kpc and 500 kpc -- for comparison. In almost all cases, we were able to rule out that clusters retain their morphology over the history of the Universe, which is in agreement with our current picture of large-scale structure formation. In addition, we found that ellipticities, as a means of quantifying morphologies, are of limited use in studies such as this work.
Carbon nanoelectronics: from correlated electrons to sensors and devices
Prof Marc Bockrath
In my talk I will discuss a number of our ongoing research projects on carbon nanotubes and graphene, with the goals of studying fundamental physics in nanostructures as well as developing sensing and device applications. In particular, I will highlight our recent results demonstrating the following: (1) strongly correlated electron behavior in ultra-clean carbon nanotubes, specifically, one-dimensional (1D) Wigner crystallization of dilute holes in semiconducting nanotubes, and the formation of a 1D Mott insulator in nominally metallic nanotubes, indicating that carbon nanotubes are never truly metallic. Our results underscore nanotubes' promise for studying a variety of tunable correlated electron phenomena in 1D; (2) individual carbon nanotube nanomechanical resonators as atomic-scale resolution inertial mass sensors, with the prospects for single atomic mass unit sensitivity and chemical or isotope discrimination; and (3) non-volatile graphene atomic switches, which we understand by a model of electric field driven motion of single-atom wide chains of carbon. These devices have the potential for high density, long term storage of information.
Two and Three dimensional Integration of Nanowires for Flexible Electronics
Dr Zhiyong Fan
Electrical Engineering, UC Berkeley
Semiconductor nanowires have been extensively explored as the potential building blocks for a variety of electronic and optoelectronic applications due to the continuous increased demand for miniaturized devices and circuits. However, controlled and uniform assembly of "bottom-up" nanowire (NW) materials with high scalability is one of the major bottleneck challenges towards the integration of nanowires for circuit applications. In the first part of my talk, I will present wafer-scale assembly of highly ordered arrays of NWs through a simple contact printing method, which utilizes van de Waals interaction between nanowires and substrates. By tuning receiver substrate surface chemistry, density and alignment of nanowires can be readily modulated and the optimum printing is achieved with a lubricant to minimize NW-NW mechanical interactions. With this generic approach, a wide range of semiconductor NWs have been successfully assembled at large-scale and configured as a variety of functional electronic and optoelectronic devices, including field-effect transistors, Schottky diodes and photodiodes on rigid and flexible substrates. Furthermore, these functional components are integrated together and an all-nanowire image sensing circuit is realized. In parallel to contact nanowire printing, we have also developed roll printing method. This method demonstrates a proof-concept for a roll-to-roll process towards nanowire based flexible electronics. In the second part of my presentation, I will talk about the approach to directly assemble nanowires into 3D array with density ~1010/cm2 in porous dielectric templates. The nanowire arrays have large scale high regularity with tunable inter-nanowire spacing; they demonstrate unique optical absorption property. We have fabricated photovoltaic devices based on the nanowire arrays utilizing single crystalline nature of the nanowires and unique structure to facilitate photo-carrier separation and collection. More importantly, such nanowire arrays can be transferred to plastic substrates which open up a wide range of applications as low weight, low cost and flexible solar cells.
Complexity of the Quantum Adiabatic Algorithm
Prof Peter Young
Physics, UC Santa Cruz
There is considerable interest in knowing the possible uses of a quantum computer, in case one can eventually be built. There are a small number of quantum algorithms for specific problems, such as Shor's algorithm for integer factorization, which are known to be more efficient than the best known classical algorithm. However, there is particular interest in knowing whether a quantum computer could solve a wider class of problems more efficiently than a classical computer. I will discuss the ``Quantum Adiabatic Algorithm'' which has been proposed for solving a general class of NP-hard optimization problems. In the absence of a quantum computer, it is necessary to study the efficiency of this algorithm by simulating it on a classical computer. In this talk I will review the quantum adiabatic algorithm, and describe calculations which investigate how its running time, or ``complexity'', varies with system size for much larger sizes than had been undertaken before. Larger sizes are possible by using Quantum Monte Carlo simulations, a technique which is well known to people in statistical physics but not to the quantum computing community.
Carbon and Non-Carbon Nanotubes: Growth, Properties, and Potential Applications
Professor Yoke K Yap
Inorganic nanotubes represent a unique class of nanomaterials in which all atoms are located near the surface. Since electron flows on nanotubes are confined near the surface, nanotubes are especially attractive for electronics and sensing applications. In addition, their tubular structures enable nanofluidic devices. In this colloquium, controlled growth of a series of inorganic nanotubes and the possible growth mechanisms will be discussed, in particular, vertically-aligned carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs), ZnO nanotubes (ZnO NTs), and Si nanotubes (SiNTs). The roles of dissociative adsorption, tri-layer catalyst, reactive plasmas, nucleation controls, and growth vapor controls on the formation of these nanotubes will be emphasized. Structural, optical, and electronic properties of these nanotubes as well as their potential applications will be discussed. Professor Yoke Khin Yap received his Ph.D. in 1999 from Osaka University as a "Monbusho" scholar. He was a fellow of the Japan Society for the Promotion of Science (JSPS) before joining Michigan Tech in 2002. Professor Yap received the National Science Foundation CAREER Award in 2005. He has published more than 140 articles including book and encyclopedia chapters, review papers, peer-reviewed articles, and conference proceedings. His research program at Michigan tech is supported by DOA, NSF, DARPA, DOE, USDA, Argonne National Laboratory, and multiple DOE Nanoscale Science Research Centers. Professor Yap is the first elected Chair of the user group of the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory.
The Numerical Study of Quantum Chromodynamics
Prof Robert Sugar
Quantum chromodynamics (QCD) is the component of the Standard Model of high energy physics that describes the strong interactions. It has proven difficult to extract many of the most interesting predictions of QCD, those that depend on the strong coupling regime of the theory; however, in recent years significant progress has been made in doing so through the use of large scale numerical simulations. I will describe how these simulations are carried out, and present results of recent calculations that are aimed at making precise tests of the Standard Model, and determining some of its basic parameters.
Preserving and extending quantum coherence: from the spin echo effect to fault tolerant quantum computation
Prof Daniel Lidar
Dynamical decoupling pulse sequences have been used to extend coherence times in quantum systems ever since the discovery of the spin-echo effect. But while for good reasons the nuclear magnetic resonance (NMR) community has typically been content with moderate line narrowing, in quantum computing extremely high levels of coherence are required in order to perform meaningful computational tasks. In this talk I will describe a method of recursively concatenated dynamical decoupling pulses, designed to overcome both decoherence and operational errors . For bounded-strength, non-Markovian environments, such as for the spin-bath that arises in electron- and nuclear-spin based solid-state quantum computer proposals, it is strictly advantageous to use concatenated, as opposed to standard periodic dynamical decoupling pulse sequences. Namely, the concatenated scheme is both fault-tolerant and super-polynomially more efficient, at equal cost [2,3]. Preliminary experimental results on NMR of 13C in adamantene (due to Dieter Suter, Dortmund), and NMR of the 31P donor in Si (due to Steve Lyon, Princeton), demonstrating the advantages of concatenated decoupling, will also be presented. Time permitting, I will describe our recent results on the construction of a universal set of quantum logic gates whose fidelity can be kept arbitrarily high for essentially arbitrarily long times in the presence of coupling to a spin bath, by use of concatenated decoupling. References:  K. Khodjasteh and D.A. Lidar, "Fault-Tolerant Quantum Dynamical Decoupling", Phys. Rev. Lett. 95, 180501 (2005).  K. Khodjasteh and D.A. Lidar, "Performance of Deterministic Dynamical Decoupling Schemes: Concatenated and Periodic Pulse Sequences", Phys. Rev. A 75, 062310 (2007).  K. Khodjasteh and D.A. Lidar, "Rigorous Bounds on the Performance of a Hybrid Dynamical Decoupling-Quantum Computing Scheme", Phys. Rev. A 78, 012355 (2008).
Tsunami in a Nanomagnet: Nonlinear Spin Waves Excited by Spin Current
Professor Ilya Krivorotov
Spin-polarized current injected into a metallic ferromagnet can excite persistent spin waves of very large amplitude. We use this unique property of spin current to study new types of magnetization dynamics in strongly nonlinear regimes. I will describe our recent measurements of spin-current-driven dynamics in ferromagnetic nanomagnets and nanowires. In these systems, we observe several unusual magneto-dynamic phenomena such as intrinsic spin wave self-localization, magnetic stochastic resonance, diffusion of magnetization and size quantization of nonlinear magnon scattering. I will also discuss emerging technologies that utilize interactions between spin currents and ferromagnets such as universal magnetic memory and microwave nanooscillators.