Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H1: Electronic Structure of Disordered Graphene
Sponsoring Units: DCMPChair: Allan MacDonald, University of Texas at Austin
Room: Spirit of Pittsburgh Ballroom A
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H1.00001: Scanning Single Electron Transistor Microscopy on Graphene Invited Speaker: We use a scanning single electron transistor to map the local density of states of graphene and the carrier density landscape in the vicinity of the neutrality point. At zero magnetic field our results confirm the existence of electron-hole puddles. These puddles could explain graphene's anomalous non-zero minimal conductivity at zero average carrier density. Moreover, we find that, unlike non-relativistic particles the density of states can be quantitatively accounted for by considering non-interacting electrons and holes. At high magnetic field we investigate the appearance of localized states. Particle localization is an essential ingredient in quantum Hall physics. In conventional high mobility two-dimensional electron systems Coulomb interactions were shown to compete with disorder and to play a central role in particle localization. Surprisingly, despite the stronger disorder in graphene compared to the standard two-dimensional systems, our findings indicate that localization in graphene is also dominated by Coulomb interactions and not single particle physics. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H1.00002: Magnetic Oscillations and Landau Quantization in Decoupled Epitaxial Graphene Multilayers* Invited Speaker: A fundamental challenge to the development of a new electronics based on single atomic sheets of carbon, known as graphene, is to realize a large-area production platform that can produce a carbon system with the same intrinsic properties as a single sheet of graphene. Multi-layer epitaxial graphene (MEG) grown on SiC substrates has been proposed as a possible platform to this end [1]. The central question is, Can MEG \textit{behave} as single layer graphene with the same intrinsic electrical characteristics? In this talk we show that MEG graphene on SiC exhibits single layer graphene properties through new tunneling magnetic measurements. The circular motion of electrons in a magnetic field has historically been a powerful probe of the Fermi surface properties of materials. Oscillations in many measureable properties, such as magnetization, thermal conductivity, and resistance, all reflect the Landau quantization of the electron energy levels. In this talk we show the ability to observe tunneling magneto-conductance oscillations (TMCOs) in the tunneling differential conductance as a function of both magnetic field and electron energy. The TMCO arise from \textit{intense} Dirac quantization of the 2-dimensional Dirac electron and hole quasiparticles in MEG grown on SiC substrates. Spatial profiles of the Landau quantization demonstrate the high quality of MEG on SiC with carrier concentrations that vary less than 10{\%} over hundreds of nm. The single layer quantization observed in these multi-layer samples is attributed to observed rotational stacking domains that effectively decouple the carbon layers in MEG on SiC, thereby yielding single layer graphene properties in a large area carbon production method. *In collaboration with Lee Miller, Kevin Kubista, Gregory M. Rutter, Ming Ruan, Mike Sprinkle, Claire Berger, Walt A. de Heer, and Phillip N. First, Georgia Institute of Technology [1] W.A. de Heer et. al., Solid State Comm. \textbf{143}, 92 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H1.00003: Effect of potential barriers on transport in graphene Invited Speaker: The energy of graphene charge carriers grows linearly with their momentum. This zero-mass behavior, associated with an absence of a forbidden region between electrons and holes, deeply modifies transport properties of electrons across potential steps and barriers. We perform transport measurements in graphene monolayers where the potential profile is tuned by a set of local gates [1,2]. By varying the height and width of potential barriers and the energy of charge carriers, we can test the predictions on the transmissions of the conduction channels across a potential step in graphene. Besides, we observe the effect of disorder and of screening of an external field in graphene. These experiments have a direct consequence in any transport measurement in graphene. We indeed showed that such potential steps naturally develop at the interface between graphene and a metallic electrode [3]. We discuss the effects of these steps in various geometries [4]. In collaboration with N. Stander, J.A. Sulpizio, Physics Department, Stanford University, Stanford, CA 94025, USA; and J. Cayssol, D. Goldhaber-Gordon, CPMOH, UMR5798, Universit\'e de Bordeaux, 33405 Talence, France. \\[4pt] [1] B. Huard, J.A. Sulpizio, N. Stander, K. Todd, B. Yang, D. Goldhaber-Gordon, Phys. Rev. Lett. \textbf{98}, 236803 (2007)\\[0pt] [2] N. Stander, B. Huard, D. Goldhaber-Gordon, condmat/0806.2319\\[0pt] [3] B. Huard, N. Stander, J.A. Sulpizio, D. Goldhaber-Gordon, \textbf{Phys. Rev. B}, \textbf{78}, 121402 (R) (2008)\\[0pt] [4] J. Cayssol, B. Huard, D. Goldhaber-Gordon, (to appear soon) [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H1.00004: Ground-state Properties of Inhomogeneous Graphene Sheets Invited Speaker: When inter-valley scattering is weak and gauge fields due to {\it e.g.} ripples are neglected, doped and gated graphene sheets can be described using an envelope-function Hamiltonian with a new sublattice pseudospin degree-of freedom, an ultrarelativistic massless-Dirac free-fermion term, a pseudospin {\it scalar} disorder potential, and a non-relativistic instantaneous Coulombic interaction term. There is considerable evidence from experiment that this simplified description of a honeycomb lattice of Carbon atoms is usually a valid starting point for theories of those observables that depend solely on the electronic properties of $\pi$-electrons near the graphene Dirac point [1]. Although the use of this model simplifies the physics considerably it still leaves us with a many-body problem without translational invariance, which we do not know how to solve. In this talk we present a Kohn-Sham-Dirac density-functional-theory (DFT) scheme for graphene sheets that treats slowly-varying inhomogeneous scalar external potentials and electron-electron interactions on an equal footing [2]. The theory is able to account for the unusual property that the exchange-correlation contribution to chemical potential increases with carrier density in graphene [3,4]. Consequences of this property, and advantages and disadvantages of using the DFT approach to describe it, are discussed. The approach is illustrated by solving the Kohn-Sham-Dirac equations self-consistently for a model random potential describing charged point-like impurities located close to the graphene plane. The influence of electron-electron interactions on these non-linear screening calculations is discussed at length, in the light of recent experiments [5,6] reporting evidence for the presence of electron-hole puddles in nearly-neutral graphene sheets. \\[4pt] [1] A.K. Geim and K.S. Novoselov, Nature Mater. {\bf 6}, 183 (2007); A.K. Geim and A.H. MacDonald, Phys. Today {\bf 60}, 35 (2007); A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, and A.K. Geim, arXiv:0709.1163v2 (2007).\\[0pt] [2] M. Polini, A. Tomadin, R. Asgari, and A.H. MacDonald, Phys. Rev. B {\bf 78}, 115426 (2008).\\[0pt] [3] Y. Barlas, T. Pereg-Barnea, M. Polini, R. Asgari, and A.H. MacDonald, Phys. Rev. Lett. {\bf 98}, 236601 (2007); M. Polini, R. Asgari, Y. Barlas, T. Pereg-Barnea, and A.H. MacDonald, Solid State Commun. {\bf 143}, 58 (2007). \\[0pt] [4] E.H. Hwang, B.Y.-K. Hu, and S. Das Sarma, Phys. Rev. Lett. {\bf 99}, 226801 (2007).\\[0pt] [5] J. Martin, N. Akerman, G. Ulbricht, T. Lohmann, J.H. Smet, K. von Klitzing, and A. Yacoby, Nature Phys. {\bf 4}, 144 (2008).\\[0pt] [6] V.W. Brar, Y. Zhang, C. Girit, F. Wang, A. Zettl, and M. Crommie, Bull. Am. Phys. Soc. {\bf 53} (2), 443 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 11:00AM |
H1.00005: Ground-state of Two-dimensional Graphene in the Presence of Random Charged Impurities Invited Speaker: The low energy electronic excitations of graphene are described by a massless Dirac fermion model. In clean isolated graphene the Fermi energy lies exactly at the Dirac point where the linear chiral electron and hole bands cross each other. Close to the Dirac point the average carrier density vanishes and the density fluctuations are expected to dominate the physics of graphene. In current experiment the fluctuations are mostly due to quenched disorder. In this talk I present the Thomas-Fermi-Dirac (TFD) theory [1] to calculate the carrier density of graphene in presence of disorder. The TFD theory includes the effects of non-linear screening, exchange and correlation. The approach is independent of the disorder source and very efficient allowing the calculation of disorder-averaged quantities that can be directly compared with experiments. Recent transport results strongly suggest that in current graphene samples charge impurities are the main source of disorder. I then present the results of the TFD theory for this case. I show that close to the Dirac point the carrier density breaks-up in electron-hole puddles and is characterized by two types of inhomogeneities: wide regions of low density and sparse narrow regions of high density and a typical correlation length of ~10 nm. I present detailed results that show how the disordered averaged quantities characterizing the carrier density profile depend on the experimental parameters. I show that at finite voltages the density probability distribution has a bimodal character providing direct evidence for the existence of puddles over a finite range of gate voltages. In graphene the exchange-correlation term increases with density contrary to parabolic-band electron liquids and because of this it tends to suppress density inhomogeneities. I show that this effect becomes very important close to the Dirac point, especially at low impurity densities. [Preview Abstract] |
Session H2: Polymer Physics Prize Symposium Honoring Steve Granick
Sponsoring Units: DPOLYChair: MIchael Rubinstein, University of North Carolina
Room: Spirit of Pittsburgh Ballroom BC
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H2.00001: Polymer Physics Prize Talk: Polymer Mobility at Surfaces and in Confined Environments Invited Speaker: Rich new chemistry and physics emerge when one considers confined fluids, where the environment is distinctly different than in bulk. The intuition of what to expect based on bulk properties is found to break down. This talk will emphasize recent findings using a combination of single-particle imaging and fluorescence correlation spectroscopy of polymers at hard surfaces (mica), soft surfaces (phospholipid bilayers) and random network environments. A surprising dependence is found on the polymer molecular weight and concentration, as well as on the substrate makeup. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H2.00002: Engineered Colloids Having Particles of Controlled Size, Shape, and Chemistry Invited Speaker: This lecture will focus on opportunities for complex particles made using a novel ``top-down'' fabrication method called PRINT (Particle Replication In Non-wetting Templates). PRINT enables the production of monodisperse, shape-specific nano and micro-particles from an extensive range of organic and inorganic liquid precursors. The assembly of colloidal particles has long been a rich and continuously growing area of materials science, with great potential for a broad range of applications including electronics, control systems, optics and biotechnology. Within this field, the bulk of research has been devoted to studying the assembly of isotropic spherical particles. In spite of this, there has been a growing interest in studying the assembly of anisotropic particles due to the more complex and useful structures that these particles can potentially assemble into. There are few reports on the assembly of anisotropic particles, in part because of the lack of effective fabrication processes for the preparation of these particles with the monodispersity, control and range of compositions required for in-depth study. Herein we will discuss the use of PRINT to fabricate monodisperse, nanometer- and micron-sized particles of varying size, shape and composition. PRINT stands out because of the high degree of molding resolution, the broad range of chemistries that can be molded, and the ease with which reel-to-reel technology can be incorporated for scalability. Thus, it is ideally suited to the synthesis of unique, highly anisotropic particles in a wide range of compositions. Herein we discuss the use of dielectrophoresis to study the assembly of highly anisotropic polymer particles: rods, discs, hexnuts and boomerangs, fabricated with the PRINT process. In addition, the discussion will focus on the details and opportunities for loading shape controlled particles with magnetite and their manipulation when dispersed in various liquid media. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H2.00003: Polymers under Cylindrical Confinement Invited Speaker: Anodized alumina oxide (AAO) membranes offer a unique platform to investigate polymers under confinement. AAO membranes have been prepared where the diameters of the nanopores in the membrane have been varied from 8 to 50 nm by varying the anodization conditions. Capillary force is sufficiently large to draw high molecular weight polymers into the membrane, producing either nanotubes or nanorods. Polymer solutions can also be used place a thin film on the walls of the nanopores, forming nanotubes. With pore diameters less than the radius of gyration, a quantitative understanding of perturbations to chain dynamics due to geometric constraints was examined. We found a weak molecular weight-dependent mobility of polymers confined within AAO nanopores having diameters smaller than the dimension of the chains in the bulk. The measured mobility of polymers in the confined geometry was much higher than the mobility of the unconfined chain. Rayleigh instabilities in thin polymer films confined within nanoporous alumina membranes were also found where periodic undulations on the film surface were found to increase with time, eventually bridging across the cylindrical nanopore, resulting in the formation of polymer nanorods with a periodic array of encapsulated holes. With microphase separated block copolymers, where the characteristic period of the BCP morphology is comparable to the pore diameter, significant deviations from the bulk morphology as revealed by electron tomography. Small angle neutron scattering was also used to investigate the influence of cylindrical confinement on the order-to-disordered transition. This work was done in collaboration with T. J. McCarthy (UMass), K. Shin (Seoul National University), H. Jinnai (Kyoto University), D. Chen, J. Chen, H. Xiang, T. Kim, and P. Dobriyal, and was supported by the DOE, NSF MRSEC, NSF CHM. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H2.00004: The Challenge of Understanding the ``Complexity'' of Polymeric Fluids and Solutions Invited Speaker: It is well known that the complexity of polymer conformational shapes makes this class of molecules prone to glass formation and that high molecular mass polymers exhibit rubbery viscoelastic flow properties associated with their topological and packing interactions. Many natural and synthetic polymers also exhibit complex associative interactions arising from the variation of chemical species and the presence of charged and polar groups within the molecule that can give rise to polymer supermolecular organization into a wide range of fragile structures at the nanoscale and larger. There are changes in both the thermodynamics and dynamics of these fluids associated with these general patterns of ``complex fluid'' behavior that provide a fundamental challenge for theoretical understanding so that this field remains at the frontier of materials science. The high level of regularity observed in the relatively high frequency glassy dynamics of polymer fluids, and other glass forming liquids more broadly, and in the viscoelastic properties that define chain ``entanglement'' in high molecular mass polymers, provides some hope for a general theoretical framework describing the complex fluid dynamics of polymeric fluids. Specifically, it is argued, and supported by evidence, that the complex fluid behavior underlying glass formation, entanglement and self-assembly in polymeric fluids all involve emergent collective behavior taking the form of supermolecular polymer structures that form and disintegrate in dynamic equilibrium. This ``dynamic heterogeneity'' paradigm, which is not addressed by conventional mean field theories such as the mode-coupling model of glass formation and the reptation model, provides a framework for understanding many aspects of the linear and non-linear dynamics of polymer complex fluid behavior such as stretched exponential stress relaxation, and shear thinning and ``aging'' following cessation of flow. It also provides a framework for understanding the influence of nanoparticles, and other additives to polymeric fluids, that modify the fluid mesoscale structure, often with significant changes in material properties. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 11:00AM |
H2.00005: Anisotropic Self-Assembly of Nanoparticle Amphiphiles Invited Speaker: It is easy to understand the self-assembly of particles having anisotropic shapes or interactions, such as Co nanoparticles or proteins, into highly extended structures. However, there is no experimentally established strategy for creating anisotropic structures from common spherical nanoparticles. We demonstrate that spherical nanoparticles, uniformly grafted with macromolecules, robustly self-assemble into a range of anisotropic superstructures when they are dispersed in the corresponding homopolymer matrix. This phenomenon is driven by the microphase separation between the inorganic nanoparticles and the (organic) polymeric chains grafted to their surfaces in a fashion similar to block copolymers. This microphase separation driven particle self-assembly provides a unique means of controlling the global nanoparticle dispersion state in polymer nanocomposites. The relationship between the state of particle dispersion and nanocomposite properties can thus be critically examined, and in particular we focus on the mechanical reinforcement afforded when particles are added to polymers. Grafted nanoparticles are thus versatile building blocks for creating tunable and functional particle superstructures with significant practical applications. With Pinar Akcora, Hongjun Liu, Yu Li, Brian Benicewicz, Linda Schadler, Thanos Panagiotopoulos, Jack Douglas, P. Thiyagarajan and Ralph Colby. [Preview Abstract] |
Session H3: New Frontiers in Biomolecular Physics
Sponsoring Units: DBPChair: Dean Astumian, University of Maine
Room: 301/302
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H3.00001: Are DNA transcription factor proteins Maxwellian Demons? Invited Speaker: This abstract is not available. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H3.00002: The mechanism of load detection in the molecular motor myosin VI Invited Speaker: Myosin VI is thought to act as both a molecular transporter and as an anchor in vivo. Recent results demonstrate that a rigid isolated alpha helix extends the myosin VI lever arm, generating an unexpectedly large stroke size of approximately 30 nm. Here we use single-molecule fluorescence, optical trapping, and gold nanoparticle tracking to examine the role of the lever arm extension in both myosin VI translocation and anchoring. Our results suggest that the rigidity of this unusual structural element plays an essential role in load-induced anchoring. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H3.00003: Full Counting Statistics for Brownian Sieves and Brownian Molecular Machines Invited Speaker: A Brownian sieve is a spatially periodic microstructured device that combines the effects of thermal noise, local spatial asymmetry, and external forces to separate particles based on their transport properties. By treating the motion of an individual particle as a cyclical process in which the particle fluctuates away from and returns to the origin of some unit cell I derive generalized fluctuation-dissipation and reciprocal relations for the averages (and for all moments) of the number of periodic displacements that are exact and valid for arbitrary values of the external forces. These relations hold not only for Brownian sieves, but for all molecular machines in which a nanoscale system couples two chemical, mechanical, or transport processes by a cycle in which the molecular machine itself fluctuates away from, and then returns to some arbitrary reference state, in the process doing or receiving work on or from the environment. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H3.00004: Theory of protein misfolding and applications to misfolding diseases Invited Speaker: Physics-based algorithms can predict the misfolding mechanisms of proteins involved in aggregation-related diseases, including ALS and the Prion diseases. Predictions based on such an algorithm that we have developed, which employs both atomistic interactions and surface-area based coarse-graining, have been recently verified by immunological assays and point to diagnostic and therapeutic applications. I will describe the results of our misfolding theory, and discuss future directions towards drug research. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 11:00AM |
H3.00005: Theoretical methods for engineering protein structure and function Invited Speaker: Designing and engineering proteins provides ways to probe the determinants of folding, to facilitate their study, and to arrive at novel molecules, materials and nanostructures. Recent computational methods for identifying the properties of proteins consistent with a desired structure and function will be discussed. Computationally designed protein-based molecular systems will be presented, including proteins tailored to accommodate nonbiological cofactors and their novel functional properties. [Preview Abstract] |
Session H4: New Developments in Heavy Electron Superconductivity
Sponsoring Units: DCMPChair: Joe Thompson, Los Alamos National Laboratory
Room: 306/307
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H4.00001: New developments in our understanding of superconductivity in the 115 materials Invited Speaker: Is a quantum critical point (QCP) pertinent to unconventional superconductivity? There are several heavy-fermion compounds in which unconventional superconductivity emerges in proximity to a spin-density-type quantum-critical point (1). The absence of superconductivity in prime candidates for a local or Kondo-breakdown quantum criticality, however, raises the question of whether this type of criticality could benefit superconductivity (2). Using the heavy-fermion antiferromagnet CeRhIn$_{5}$ as an example (3), we present the first evidence that critical modes associated with the Kondo-breakdown criticality can provide a new route to unconventional superconductivity. At a local QCP, accessed by applied pressure, magnetic and charge fluctuations coexist and produce electronic scattering that is maximal at the optimal pressure for unconventional superconductivity. References: (1) Mathur et al., Nature 394, 39 (1998); Monthoux et al., Nature 450, 1177 (2007). (2) Gegenwart et al., Nat. Phys. 4, 186 (2008). (3) T. Park et al., Nature 440, 65 (2006); T. Park et al. Proc. Nat. Acad. Sci. 105, 6825 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H4.00002: Unconventional superconductivity of NpPd$_5$Al$_2$ Invited Speaker: The 5f electrons in actinide compounds has an intermediate character between the 4f-localized state and the 3d-itinerant state. This leads to a variety of exotic phenomena, such as non-Fermi liquid behavior, multipole ordering, hidden order state and unconventional superconductivity. The discovery of superconductivity in PuCoGa$_5$ and PuRhGa$_5$ with high critical temperatures provides a new perspective on the physics of actinide compounds. It is generally believed that Np compounds have more 5f-itinerant characteristic features like d-electron metals compared with U compounds. In fact, the results of dHvA experiments in Np-115 compounds are in good agreement with the 5f-itinerant band model. However, the complicated magnetic properties in Np-115 are well explained by the mean field theory including the orbital ordering based on the 5f-localized model. This indicates the dual nature of 5f electrons. NpPd$_5$Al$_2$ is the first Np-based heavy fermion superconductor with the ZrNi$_2$Al$_5$-type tetragonal structure. The superconductivity was found below $T_{\rm sc}=5\,{\rm K}$. The non-Fermi liquid behavior and the large specific heat coefficient ($\gamma=200\,{\rm mJ/K^2 mol}$) were detected. The upper critical field $H_{\rm c2}$ at $0\,{\rm K}$ is large and highly anisotropic: $37\,{\rm kOe}$ for $H \parallel [100]$ and $143\,{\rm kOe}$ for $H \parallel [001]$. $H_{\rm c2}$ is strongly suppressed by the magnetic field in the $H_{\rm c2}$--$T$ phase diagram for both field direction, indicating the strong Pauli paramagnetic effect. The d-wave spin-singlet superconductivity is most likely realized. The large specific heat jump $\Delta C/\gamma T_{\rm sc} = 2.33$ suggests the superconductivity with strong coupling. The results are compared with the well known heavy fermion superconductor CeCoIn$_5$. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H4.00003: Heavy electrons and symplectic symmetry of a spin Invited Speaker: Motivated by the recent discovery of the heavy fermion materials NpPd5Al$_2$ [1] and PuCoGa$_5$ [2] which transform directly from Curie paramagnets into superconductors, we have developed a novel theory of these materials based on the idea of composite pairing between local moments and electron pairs. This talk will discuss a simple model of this kind of pairing that can be solved exactly in a large-N limit [3]. The talk will discuss how this concept enables us to understand the giant entropy of condensation, the symmetry of the order parameter as well as an enhancement of the Andreev reflection in tunneling measurements and an upturn in the NMR relaxation rate above Tc. \\[0pt] [1] D. Aoki et al., Jour. Phys. Soc. of Japan 76, 063701 (2007).\\[0pt] [2] J. Sarrao et al., Nature (London) 420, 297 (2002).\\[0pt] [3] R. Flint, M. Dzero and P. Coleman, Nature Physics 4, 643 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H4.00004: Quantum Criticality and Superconductivity in $\beta $-YbAlB$_{4}$ Invited Speaker: Heavy fermion systems have provided a number of prototypical compounds to study unconventional superconductivity and non-Fermi-liquid (NFL) states. A long standing issue in the research of heavy fermion superconductivity in 4$f$ intermetallics is the dramatically different behavior between the electron like Ce (4$f^{1})$ and hole like Yb (4$f^{13})$ compounds. While superconductivity has been found in a number of Ce based heavy fermion compounds, no superconductivity has been reported for the corresponding Yb systems. In this talk, I present our recent finding of the superconductivity in the new heavy fermion system $\beta $-YbAlB$_{4}$ [1-3]. The superconducting transition temperature is 80 mK, and above it, the system exhibits pronounced NFL behavior in the transport and thermodynamic properties [2,3]. Furthermore, the magnetic field dependence of the NFL behavior indicates that the system is a rare example of a pure metal that displays quantum criticality at ambient pressure and under zero magnetic field. Using our latest results, we discuss the detailed properties of superconductivity and quantum criticality. This is the work performed in collaboration with K. Kuga, Y. Matsumoto, T. Tomita, Y. Machida, T. Tayama, T. Sakakibara, Y. Karaki, H. Ishimoto, S. Yonezawa, Y. Maeno, E. Pearson, G. G. Lonzarich, L.Balicas, H. Lee, and Z. Fisk. \\[4pt] [1] Robin T. Macaluso, Satoru Nakatsuji, Kentaro Kuga, Evan Lyle Thomas, Yo Machida, Yoshiteru Maeno, Zachary Fisk, and Julia Y. Chan, Chem. Mater$. $\textbf{19} 1918 (2007). \\[0pt] [2] S. Nakatsuji, K.Kuga, Y. Machida, T. Tayama, T. Sakakibara, Y. Karaki, H. Ishimoto, S. Yonezawa, Y. Maeno, E. Pearson, G. G. Lonzarich, L.Balicas, H. Lee, and Z. Fisk, Nature Phys \textbf{4}, 603-607 (2008). \\[0pt] [3] K. Kuga, Y. Karaki, Y. Matsumoto, Y. Machida, and S. Nakatsuji, Phys. Rev. Lett. \textbf{101}, 137004 (2008). [Preview Abstract] |
Session H5: Physicists as Entreprenuers
Sponsoring Units: FIAP FGSAChair: Venky Venkatesan, Neocera
Room: 401/402
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H5.00001: From a PhD in Physics to Manufacturing Carbon Nanotubes Invited Speaker: |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H5.00002: Building an R\&D Based Company Invited Speaker: Entrepreneurship is the wellspring of innovation and the ability to seek out opportunities and turn them to profitable businesses. It is also closely associated with desire to succeed with taking risk. According to the U.S. Small Business Administration, entrepreneurs are major contributors to our economy. Entrepreneurs are also major employers, as small businesses generate 60-80 percent of all new jobs annually. It is agreed that small businesses are one of the key engines of economy and growth by contributing to employment creation. In this regard, having your own business is regarded as one of the most respectable pathways to prosperity and fulfillment. While some strike success the first time around, most successful entrepreneurs overcome several hurdles before succeeding. In my talk I will present my experience in starting my technology based company, ``Blue Wave Semiconductors, Inc.'' I will focus on opportunities as well as numerous factors faced as the small technology start-up including finance, access to Small Business Innovation Research (SBIR), identifying market, managerial skills, team and infrastructure development. Also, I will highlight some of the important factors such as perseverance, independence, ingenuity, confidence and the determination to overcome barriers for eventual success. If you are sitting on a great idea that can possibly crate a good technology business and you don't know where to start, then this talk will be helpful for getting your dream closer and resolving your risk factors for great rewards. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H5.00003: Lessons Learned as a Serial Technology Entrepreneur Invited Speaker: Starting a new technology company can be an exciting experience, and can, on occasion, be financially lucrative as well. Aside from the obvious requirement to have some new technology to offer, the main impediment to making the leap is usually fear of unknown. This arises from several real issues: a) you don't know how to do it; b) you assume the new situation will be less secure; c) you have to give up progress along your current career path; d) you fear failure itself, and how it will reflect on you as a person. There is no easy way to resolve these concerns, and although talking with others who have done it is helpful, the final decision is always difficult and very personal. Assuming you decide to go forward, there are some simple rules that will help along the way: 1. Have a PRODUCT idea, not just an innovative technology: it's a business, not a way to continue interesting research. 2. Team up with experienced people in certain (not all) key jobs. The specifics depend on what your own skills are. 3. Make sure you have adequate funding at the outset to achieve some significant milestones. 4. Be selective about who funds you; they will be your business partners and will have a lot to say about what happens. 5. Start thinking about the liquidity/exit strategy from day one. The stories of how other companies were started and developed are good background information. Several of these from my own experiences will be discussed. They include a company that struggled for many years but ultimately went public and was successful, one that was quickly a success and was acquired, one that was an outright failure and two that continue on but with dubious prospects of success. Some thoughts on what went right or wrong, and what could have been done better will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H5.00004: My experience with Bandwidth9 Invited Speaker: |
Tuesday, March 17, 2009 10:24AM - 11:00AM |
H5.00005: The Roller Coaster Ride of an Entrepreneur Invited Speaker: |
Session H6: Quench Dynamics and Thermalization in Quantum Systems
Sponsoring Units: DCMP DAMOPChair: Vladimir Gritsev, Harvard University
Room: 406
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H6.00001: Eigenstate Thermalization Hypothesis and Quantum Thermodynamics Invited Speaker: One of the open questions in quantum thermodynamics reads: how can linear quantum dynamics provide chaos necessary for thermalization of an isolated quantum system? To this end, we perform an ab initio numerical analysis of a system of hard-core bosons on a lattice and show [Marcos Rigol, Vanja Dunjko \& Maxim Olshanii, Nature 452, 854 (2008)] that the above controversy can be resolved via the Eigenstate Thermalization Hypothesis suggested independently by Deutsch [J. M. Deutsch, Phys. Rev. A 43, 2046 (1991)] and Srednicki [M. Srednicki, Phys. Rev. E 50, 888 (1994)]. According to this hypothesis, in quantum systems thermalization happens in each individual eigenstate of the system separately, but it is hidden initially by coherences between them. In course of the time evolution the thermal properties become revealed through (linear) decoherence that needs not to be chaotic. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H6.00002: Microscopic diagonal entropy and many-body dynamics Invited Speaker: We define microscopic diagonal entropy to characterize many-body dynamics of systems far from equilibrium. For the systems prepared initially in thermal equilibrium, it increases with time and is related to the heat generated in the dynamics. We illustrate our results with numerical simulations of a toy-BCS model. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H6.00003: Statistics of the Work done in a Quantum Quench Invited Speaker: The quantum quench, i.e. a rapid change in time of a control parameter of a quantum system, is the simplest paradigm of non-equilibrium process, completely analogous to a standard thermodynamic transformation. The dynamics following a quantum quench is particularly interesting in strongly correlated quantum systems, most prominently when the quench in performed across a quantum critical point. In this talk I will present a way to characterize the physics of quantum quenches by looking at the statistics of a basic thermodynamic variable: the work done on the system by changing its parameters [1]. I will first elucidate the relation between the probability distribution of the work, quantum Jarzynski equalities, and the Loschmidt echo, a quantity that emerges usually in the context of dephasing. Using this connection, I will then characterize the statistics of the work done on a Quantum Ising chain by quenching locally or globally the transverse field. I will then show that for global quenches the presence of a quantum critical point results in singularities of the moments of the distribution, while, for local quenches starting at criticality, the probability distribution itself displays an interesting edge singularity. The results of a similar analysis for other systems will be discussed. \\[4pt] [1] A. Silva, Phys. Rev. Lett. 101, 120603 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H6.00004: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 10:24AM - 11:00AM |
H6.00005: ABSTRACT WITHDRAWN |
Session H7: Cellular Imaging at the Nanometer Scale
Sponsoring Units: DBPChair: K.C. Huang, Stanford University
Room: 407
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H7.00001: Irving Langmuir Prize Talk: Single-Molecule Fluorescence Imaging: Nanoscale Emitters with Photoinduced Switching Enable Superresolution. Invited Speaker: In the two decades since the first optical detection and spectroscopy of a single molecule in a solid (Phys. Rev. Lett. \textbf{62}, 2535 (1989)), much has been learned about the ability of single molecules to probe local nanoenvironments and individual behavior in biological and nonbiological materials in the absence of ensemble averaging that can obscure heterogeneity. The early years concentrated on high-resolution spectroscopy in solids, which provided observations of lifetime-limited spectra, optical saturation, spectral diffusion, optical switching, vibrational spectra, and magnetic resonance of a single molecular spin. In the mid-1990's, much of the field moved to room temperature, where a wide variety of biophysical effects were subsequently explored, but it is worth noting that several features from the low-temperature studies have analogs at high temperature. For example, in our first studies of yellow-emitting variants of green fluorescent protein (EYFP) in the water-filled pores of a gel (Nature \textbf{388}, 355 (1997)), optically induced switching of the emission was observed, a room-temperature analog of the earlier low-temperature behavior. Because each single fluorophore acts a light source roughly 1 nm in size, microscopic imaging of individual fluorophores leads naturally to superlocalization, or determination of the position of the molecule with precision beyond the optical diffraction limit, simply by digitization of the point-spread function from the single emitter. Recent work has allowed measurement of the shape of single filaments in a living cell simply by allowing a single molecule to move through the filament (PNAS \textbf{103}, 10929 (2006)). The additional use of photoinduced control of single-molecule emission allows imaging beyond the diffraction limit (superresolution) by several novel approaches proposed by different researchers. For example, using photoswitchable EYFP, a novel protein superstructure can now be directly imaged in a living bacterial cell at sub-40nm resolution (Nat. Meth. \textbf{5}, 947 (2008)). These important advances provide the impetus for the further development of both new imaging schemes with 3-D capability as well as invention of new photoswitchable single-molecule emitters for use in polymers and in biological systems (JACS \textbf{130}, 9204 (2008); J. Phys. Chem. B \textbf{112}, 11878 (2008)). [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H7.00002: Far-Field Fluorescence Nanoscopy Invited Speaker: The resolution of a far-field optical microscopy is usually limited to $d=\lambda \mathord{\left/ {\vphantom {\lambda {\left( {2\,n\sin \alpha } \right)}}} \right. \kern-\nulldelimiterspace} {\left( {2\,n\sin \alpha } \right)} \quad >$ 200 nm, with $n\sin \alpha $ denoting the numerical aperture of the lens and $\lambda $ the wavelength of light. While the diffraction barrier has prompted the invention of electron, scanning probe, and x-ray microscopy, the 3D-imaging of the interior of (live) cells requires the use of focused visible light. I will discuss new developments of optical microscopy that I anticipate to have a lasting impact on our understanding of living matter. Emphasis will be placed on physical concepts that have overcome the diffraction barrier in far-field fluorescence microscopy. To set the scene for future directions, I will show that all these concepts share a common strategy: exploiting selected states and transitions of the fluorescent marker to neutralize the limiting role of diffraction. The first viable concept of this kind was Stimulated Emission Depletion (STED) microscopy where the spot diameter follows$d\approx \lambda \mathord{\left/ {\vphantom {\lambda {\left( {2\,n\sin \alpha \sqrt {1+I \mathord{\left/ {\vphantom {I {I_s }}} \right. \kern-\nulldelimiterspace} {I_s }} } \right)}}} \right. \kern-\nulldelimiterspace} {\left( {2\,n\sin \alpha \sqrt {1+I \mathord{\left/ {\vphantom {I {I_s }}} \right. \kern-\nulldelimiterspace} {I_s }} } \right)}$; $I \mathord{\left/ {\vphantom {I {I_s }}} \right. \kern-\nulldelimiterspace} {I_s }$is a measure of the strength with which the molecule is send from the fluorescent state to the dark ground state. For $I \mathord{\left/ {\vphantom {I {I_s }}} \right. \kern-\nulldelimiterspace} {I_s }\to \infty $ it follows that $d\to 0$, meaning that the resolution that can, in principle, be molecular. The concept underlying STED microscopy can be expanded by employing other transitions that shuffle the molecule between a dark and a bright state, such as (i) shelving the fluorophore in a dark triplet state, and (ii) photoswitching between a `fluorescence activated' and a `fluorescence deactivated' conformational state. Examples for the latter include photochromic organic compounds, and fluorescent proteins which undergo a cis-trans photoisomerizations. Photoswitching provides ultrahigh resolution at ultralow light levels. Switching can be performed in an ensemble or individually in which case the image is assembled molecule by molecule at high resolution. By providing molecular markers with the appropriate transitions, synthetic organic chemistry and protein biotechnology plays a key role in this endeavor. Besides being a fascinating development in physics, far-field optical ``nanoscopy'' is highly relevant to the life sciences. In fact, it has already been a key to answering important questions in biology [1, 2]. Due to its simplicity and improving performance, I expect far-field optical nanoscopy to enter virtually every cell biology laboratory in the near future. \\[4pt] [1] S. W. Hell, Far-field optical nanoscopy, \textit{Science} 316 (2007) 1153. \\[0pt] [2] Westphal, V., S. O. Rizzoli, M. A. Lauterbach, D. Kamin, R. Jahn, S. W. Hell, Video-Rate Far-Field Optical Nanoscopy Dissects Synaptic Vesicle Movement, Science (2008) DOI: 10.1126/science.1154228. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H7.00003: Advances in super-resolution imaging technologies Invited Speaker: Superresolution techniques such as photoactivated localization microscopy (PALM) enable the imaging of fluorescent protein chimeras to reveal the organization of genetically-expressed proteins on the nanoscale with a density of molecules high enough to provide structural context. Various applications of this new technology are now possible. One application is for in cellula pulse-chase analysis to follow protein turnover and diffusion of photoactivated fluorescent proteins. Another approach combines the techniques of PALM and single particle tracking to resolve the dynamics of individual molecules by tracking them in live cells. Called single particle tracking PALM (sptPALM), the technique involves activating, localizing and bleaching many subsets of photoactivatated fluorescent protein chimeras in live cells. Spatially-resolved maps of single molecule motions can be obtained by imaging membrane proteins with this technique, providing several orders of magnitude more trajectories per cell than by traditional single particle tracking. By probing distinct subsets of molecules, including Gag and VSVG, sptPALM can provide a powerful means for exploring the origin of spatial and temporal heterogeneities in membranes. Examples such as these will be presented to illustrate the value of super-resolution imaging in providing quantitative insights into protein organization and dynamics at the nanoscale. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H7.00004: Bacterial cryotomography Invited Speaker: Electron cryotomography (ECT) is an emerging technique that allows thin samples such as small cells, viruses, or tissue sections to be imaged in 3-D in a near-native, ``frozen-hydrated'' state to molecular ($\sim $4 nm) resolution. Thus ECT fills a critical gap between light microscopy and higher resolution structural techniques like X-ray crystallography and NMR. In a combination of technology development and biological application, during the past few years our lab has been studying bacterial ultrastructure through ECT of intact, plunge-frozen cells. We have now collected over a thousand tomograms of more than ten different species. This work has revealed the surprising complexity of the bacterial cytoskeleton as well as the architectures of several important ``supramolecular'' complexes including the chemoreceptor array, the flagellar motor, and the cell wall peptidoglycan. Example results highlighting both the potential and limitations of this technology will be shown. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 11:00AM |
H7.00005: The Role of MreB in Escherichia Coli's Cellular Rigidity Invited Speaker: Bacteria possess homologs of all three classes of eukaryotic cytoskeletal proteins. These filamentous proteins have been shown to localize proteins essential for a number of cell-biological processes in prokaryotes such as cell growth and division. However, to date, there has been no direct evidence that the cytoskeleton in bacteria bears mechanical loads or can generate physical forces than are used by the cell. I will present evidence from combined fluorescence and force microscopy measurements that MreB, an actin homolog, is responsible for half of Escherichia coli's cellular rigidity. These data support an interpretation in which the cytoskeleton, the peptidoglycan cell wall and a large turgor pressure work together to give gram-negative cells their mechanical properties. [Preview Abstract] |
Session H8: The Greening of the City of Pittsburgh: The History, Science and Examples
Sponsoring Units: FPSChair: Brian Schwartz, City University of New York
Room: 414/415
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H8.00001: Devastation and Renewal: Water, Air and Land in Pittsburgh Environmental History Invited Speaker: This talk will focus on the metabolism of cities as a concept through which to view the environmental history of Pittsburgh. In many ways, the history of Pittsburgh, perhaps more than that of any other city in the nation, reflects the impact of industrialism and of urban infrastructure on environmental quality. The talk will explore these effects and attempts at remediation through a slide lecture that will examine three domains: water supply and wastewater disposal; smoke and air pollution; and land contamination. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H8.00002: Air Quality from Early Pittsburgh to the Present: The Science of Change Invited Speaker: Throughout Pittsburgh's history over the past 250 years, coal reserves in the city and nearby have influenced its economy, demographics, and environmental quality. They have also played a major role in determining air quality in the region. For example, Pittsburgh became famous for its high particle loadings as early as the beginning of the nineteenth century, when the first complaints about air quality in the city were recorded. Nevertheless, residents tolerated the high coal smoke levels since jobs depended on the iron works, steel mills, and other industries. When natural gas was discovered just east of the city in the 1870's and replaced coal for some applications, particle concentrations decreased. But the local supplies of natural gas ran short several years later, and as industry continued to expand in the 1890's the city went back to the use of coal as its primary fuel. The return to smoky air was met with resistance that marked the beginning of sustained public outcry and initiation of several air pollution studies. The next half century was marked by periods of occasional high and low concentration, the latter due to events such as the financial panic of 1907 and the depression of the 1930's. It was not until the 1940's that effective regulations were passed to reduce smoky conditions. Particle levels fell throughout the 1950's and 1960's, and eventually the decline of heavy industry in Pittsburgh led to relatively clean air in many parts of the city. Over the past few decades, airborne particle concentrations averaged across the Pittsburgh region have remained below their earlier levels. However, there are still ``hot spots'' of high concentration resulting from regional background coming from upwind areas and emissions of some large sources that have continued to operate in the Pittsburgh region. Furthermore, the composition of airborne particles in the city has changed from earlier times. Such particles are now the result of emissions from sources in upwind states, greater numbers of mobile sources, and the influence of control technologies that remove certain classes of pollutants but not others. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H8.00003: Material Science and Construction Invited Speaker: We will review the new materials and technologies that are being applied in the construction of high performance (green) buildings to improve energy efficiency, Indoor Air and Environmental Quality, water conservation and reclamation, and resource conservation. We present an introduction to state-of-the-art building concepts, including ``Net-Zero'' buildings, which generate as much energy as they use, reclaim water, and minimize waste; and ``Waste as Resource,'' including waste to energy plants, biofuels, materials reclamation and recycling. The role of advanced materials and technologies, such as spectrally selective glazing, photocatalytic concrete, solar heating and cooling, and organic solar collectors will be discussed. We also give an overview of advanced analytic tools used in building design, including Computational Fluid Dynamics, energy, and lighting/daylighting computer-based simulation programs. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H8.00004: The Greening of the David L. Lawrence Pittsburgh Convention Center Invited Speaker: The David L. Lawrence Convention Center is the largest Gold LEED NC (new construction) certified convention center in the USA and the first of its kind in the world. The designation has been awarded by the United States Green Building Council through its Leadership in Energy and Environmental Design (LEED) Green Building Rating System. In this talk we discuss the unique green properties of this 1.5 million square foot Convention Center including the design and use of daylight, natural ventilation and other sustainable design and practices. No other building the size of the Convention Center (1.5 million square feet), uses natural ventilation or can illuminate an exhibition hall entirely through its windows and skylights. Approximately 75\% of the convention center's exhibition space is lit by natural daylight. The use of natural ventilation and extensive day lighting is designed to reduce energy consumption by nearly 35\% compared to traditional ventilated and lit buildings of a similar size. [Preview Abstract] |
Session H9: Complex Networks and Their Applications
Sponsoring Units: GSNPChair: Ira Schwartz, Naval Research Laboratory
Room: 303
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H9.00001: Dependence on Initial Conditions in a Numerical Model of River Network Formation Geoffrey Poore, Susan Kieffer We investigated the effect of initial conditions on river network formation, using a simple model of erosional dynamics. Previous research suggests that river network scaling and geomorphic properties may be sensitive to initial conditions, but this has not been systematically studied. We used simulations of a stream power law, with initial conditions consisting of a flat or sloping surface combined with random fluctuations in elevation, and considered dependence of steady-state solutions on initial slope and randomness. The sinuosity exponent and the sinuosity are sensitive to these initial conditions, while the Hack exponent and hypsometry show little or no sensitivity. The results suggest that initial conditions deserve greater consideration in attempts to understand the emergence of scaling in river networks. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H9.00002: Optimization of Ramified Flow Networks Martin Singleton, Alfred Hubler, Gregor Heiss A class of Ramified graphs (RG) is introduced as Iterated Function Systems (IFS) to optimally design networks for efficient reverse osmosis desalination in deep seawater. Ramified flow networks of absorbers, ranging from simple structures with constant weights, branch angles, and branch ratios, to fully optimized binary networks are considered. A contracting IFS with fixed overall length is presented for the generation of RG's which serve as candidates for optimality in terms of desalination performance criteria. Using the analogy to electrostatics, the diffusion equation is solved for the desalination systems under three different boundary conditions, i) all nodes having the same pressure difference across the absorbers, ii) all nodes producing permeate at identical rates, and iii) each node having the same salinity. Optimal branching angles and branch length ratios will be found by phase-space methods for each boundary condition, which either maximize production of permeate or minimize expenditure of energy for different fixed numbers of absorbers. For constant salinity absorbers, we give the total water production rate as functions of branching angle and branching ratio for up to 10 branching generations. Both optimal angle and optimal ratios are found to be decreasing functions of generation for constant salinity absorbers. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H9.00003: Reaction fronts and synchronization in ordered and disordered vortex networks Tom Solomon, Garrett O'Malley, Justin Winokur We present experimental studies of front propagation and synchronization in an advection-reaction-diffusion system. The reaction is either the excitable or oscillatory Belousov-Zhabotinsky chemical reaction, and the flow is an array or annular chain of vortices forced using magnetohydrodynamic techniques. The reaction in each vortex acts as a node in a complex fluid network, and communication between these nodes is via chaotic mixing. Mixing in this system is either diffusive (enhanced) or superdiffusive, depending on the forcing protocol. We find that the network with the oscillatory reaction synchronizes if the mixing is superdiffusive with long-range connections. We also find that reaction fronts in this system typically pin to moving vortices. We explore any changes in front propagation behavior when the transport changes from normal diffusion to superdiffusion. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H9.00004: Infinitely Robust Order and Local Order-Parameter Tulips in Apollonian Networks with Quenched Disorder C. Nadir Kaplan, Michael Hinczewski, A. Nihat Berker For a variety of quenched random spin systems on an Apollonian network, including ferromagnetic and antiferromagnetic bond percolation and the Ising spin glass, we find the persistence of ordered phases up to infinite temperature over the entire range of disorder.[1] We develop a renormalization-group technique that yields highly detailed information, including the exact distributions of local magnetizations and local spin-glass order parameters, which turn out to exhibit, as function of temperature, complex and distinctive tulip patterns. [1] C.N. Kaplan, M. Hinczewski, and A.N. Berker, arXiv:0811.3437v1 [cond-mat.dis-nn] (2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H9.00005: Correlated Spin Networks in Spin Glasses Thomas E. Stone, Susan R. McKay We introduce a network model for frustrated spin systems based on highly correlated spin fluctuations, which allows us to quantify and visualize their ordering. In this model, individual lattice sites are treated as nodes, with links between any two nodes existing only if the correlated fluctuations between those two nodes are above a threshold value. As a test case, we have implemented this model on the two-dimensional Ising antiferromagnet on a triangular lattice with randomly inserted ferromagnetic bonds, which has a finite temperature spin-glass phase transition.$^{1}$ In the paramagnetic phase, nodes within a network are spatially contiguous and networks are localized to areas of relieved frustration. In the spin-glass phase, a very broad degree distribution physically manifests itself through networks of strongly correlated but non-contiguous spins. This finding is consistent with that predicted via chaotic rescaling.$^{2}$ 1. Grest G.S. and Gabl E.G., Phys. Rev. Lett. \textbf{43}, 1182 (1979). 2. McKay S.R., Berker A.N. and Kirkpatrick S., Phys. Rev. Lett. \textbf{48}, 767 (1982). [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H9.00006: Influence of spatial dependence on the eigenvalue spectrum of complex networks Joris Billen, Mark Wilson, Arlette R.C. Baljon, Avinoam Rabinovitch Many real life networks are known to exhibit a spatial dependence (SD), i.e. the probability to form a link between two nodes in the network, inversely depends on the distance between them. We investigate the influence of SD on the eigenvalue spectrum of networks. By increasing the SD in Erd\"{o}s R\'{e}nyi (ER), scalefree, and small-world networks we find that the eigenvalue spectrum becomes asymmetric, as a result of the increased clustering in the system. We quantify this asymmetry by the skewness, kurtosis, and the change in highest and lowest eigenvalues of the spectrum as a function of SD. Our results show that the eigenvalue spectrum can be used as a tool to detect SD in real-life networks. We illustrate this ability for the eigenvalue spectrum of a spatial dependent polymeric gel. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H9.00007: A family of fat-tail random matrix ensemble Jinmyung Choi, K.A. Muttalib We present a family of novel fat-tail random matrix ensembles characterized by a parameter $\lambda$. We show that the eigenvalue densities of the ensembles exhibit a power law distribution. In particular, for $\lambda>1$, the tail of the distribution is bounded, whereas for $\lambda<1$, the distribution has a fat tail. In the limit $\lambda=1$, the ensemble corresponds to the well-established critical ensemble. We evaluate the eigenvalue correlations in terms of a novel family of orthogonal polynomials that are generalizations of the q-Hermite polynomials. We show that the two-level correlation of the novel fat-tail ensemble is qualitatively different from that of either the Gaussian or the critical ensemble. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H9.00008: Poisson vaccination for epidemic control in adaptive social networks Leah Shaw, Ira Schwartz We study an epidemic model for disease spread on an adaptive network modeling avoidance behavior. Individuals are assumed to adapt their social behavior to minimize their risk of disease. Non-infected nodes rewire their connections away from infected nodes to connect instead to other non-infected nodes, and the disease follows an SIS (susceptible-infected-susceptible) dynamics. We add Poisson distributed vaccination of susceptibles. Effects of the vaccination frequency and amplitude are studied in the full system and compared to a mean field theory. Disease extinction rates using vaccination are found for both adaptive and static networks. We show that vaccine control is much more effective in adaptive networks than in static networks due to an interaction between the rates of adaptive network rewiring and vaccine application. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H9.00009: Epidemics in Complex Networks: The Diversity of Hubs Maksim Kitsak, Lazaros K. Gallos, Shlomo Havlin, H. Eugene Stanley, Hernan A. Makse Many complex systems are believed to be vulnerable to spread of viruses and information owing to their high level of interconnectivity. Even viruses of low contagiousness easily proliferate the Internet. Rumors, fads, and innovation ideas are prone to efficient spreading in various social systems. Another commonly accepted standpoint is the importance of the most connected elements (hubs) in the spreading processes. We address following questions. Do all hubs conduct epidemics in the same manner? How does the epidemics spread depend on the structure of the network? What is the most efficient way to spread information over the system? We analyze several large-scale systems in the framework of of the susceptible/infective/removed (SIR) disease spread model which can also be mapped to the problem of rumor or fad spreading. We show that hubs are often ineffective in the transmission of virus or information owing to the highly heterogeneous topology of most networks. We also propose a new tool to evaluate the efficiency of nodes in spreading virus or information. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H9.00010: Prevalence of and Epidemic Spreading on Hierarchical Networks Jiankui He, Michael Deem Recent studies show that real networks are organized in a modular or even hierarchical fashion. However, there is no clear mathematical definition of hierarchy and current studies do not tell us the degree to which a network is hierarchical. In this talk, we will discuss a quantitative measurement of hierarchy. We find that networks of protein interactions, metabolic pathways, electronic circuits, power grids, and emails display strong hierarchy compared with networks generated at random or scale free networks of the Barab\'asi-Albert model. Further, we investigated the spread of virus in hierarchical networks. Viral spread on hierarchical networks displays quite different pattern from scale free and random networks. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H9.00011: Understanding the spreading patterns of mobile phone viruses Pu Wang, Marta Gonzalez, Cesar Hidalgo, Albert-Laszlo Barabasi Mobile viruses are little more than a nuisance today, but given our increased reliance on wireless communication, in the near future they could pose more risk than their PC based counterparts. Despite of the more than three hundred mobile viruses known so far, little is known about their spreading pattern, partly due to a lack of data on the communication and travel patterns of mobile phone users. Starting from the traffic and the communication pattern of six million mobile phone users, we model the vulnerability of mobile communications against potential virus outbreaks. We show that viruses exploiting Bluetooth and multimedia messaging services (MMS) follow markedly different spreading patterns. The Bluetooth virus can reach all susceptible handsets, but spreads relatively slowly, as its spread is driven by human mobility. In contrast, an MMS virus can spread rapidly, but because the underlying social network is fragmented, it can reach only a small fraction of all susceptible users. This difference affects both their spreading rate, the number of infected users, as well as the defense measures one needs to take to protect the system against potential viral outbreak. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H9.00012: Partitioning Links: A New Approach to Communities in Complex Networks James Bagrow, Yong-Yeol Ahn, Sune Lehmann, Albert-L\'aszl\'o Barab\'asi We propose a new viewpoint for the problem of community detection in complex networks. Rather than defining a community as a set of densely interconnected nodes, we define a community as a set of (related) links. We show how this alternative viewpoint incorporates significant aspects including overlapping communities. A quantitative framework for evaluating the link partitions is also introduced. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H9.00013: Community Structure in Online Collegiate Social Networks Amanda Traud, Eric Kelsic, Peter Mucha, Mason Porter Online social networking sites have become increasingly popular with college students. The networks we studied are defined through ``friendships'' indicated by Facebook users from UNC, Oklahoma, Caltech, Georgetown, and Princeton. We apply the tools of network science to study the Facebook networks from these five different universities at a single point in time. We investigate each single-institution network's community structure, which we obtain through partitioning the graph using an eigenvector method. We use both graphical and quantitative tools, including pair-counting methods, which we interpret through statistical analysis and permutation tests to measure the correlations between the network communities and a set of characteristics given by each user (residence, class year, major, and high school). We also analyze the single gender subsets of these networks, and the impact of missing demographical data. Our study allows us to compare the online social networks for the five schools as well as infer differences in offline social interactions. At the schools studied, we were able to define which characteristics of the Facebook users correlate best with friendships. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H9.00014: Patterns of behavior in an online community Lazaros Gallos, Diego Rybski, Fredrik Liljeros, Shlomo Havlin, Hernan Makse Human behavior can be seen as the expression of inherent motives. Despite the diverse range of these motives, social theorists have long identified a small number of distinct underlying mechanisms, where, consciously or unconsciously, every individual tries to exploit a different aspect of social interactions and/or optimize the efficiency of certain procedures for the benefit of the society or for personal gain. Here we show that users in an online community follow certain behavioral patterns and the choice of their favorite members is far from a random process. More importantly, these patterns are systematically modified with time as a member becomes more involved in such a community. We are able to identify a crossover in the average behavior of the members when their favorites list exceeds roughly 10 favorites. Additionally, this process allows us to identify individuals with a markedly different behavior than the average person. This study can help us understand the process of establishing friendships and the motives behind this process. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H9.00015: Renormalization describes distinct regimes of information flow in complex networks Hernan Rozenfeld, Chaoming Song, Hernan Makse The speed at which information travels from one site to another in a complex system is largely determined by the number of short-cuts within the network topology. It remains an important open question how to optimize the connectivity of the links in the network structure to minimize the travel time. Here we show that ideas taken from renormalization group theory applied to complex self-similar networks are essential to define distinct regimes of information flow within the network. We find that networks that are human decision based such as the WWW are sufficiently randomized to give a topology that is close to optimal. On the other hand, biological evolution-based networks show evidence of clear sign of a modular deterministic structure shaped by evolution showing suboptimal large-world character which may be even so as a mean of protection, preservation and conservation. [Preview Abstract] |
Session H10: Focus Session: Optical Properties of Nanostructures I: Quantum Dots
Sponsoring Units: DCMPChair: Jie Shan, Case Western Reserve University
Room: 304
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H10.00001: Coherent Optical Phenomena in Quantum Dots Invited Speaker: The quantum confinement provided by a semiconductor quantum dot suppresses much of the many body physics associated with the coherent nonlinear optical response observed in higher dimensional systems. This makes them attractive for potential device applications where atomic like properties, such as high Q resonances, strong optical interactions, or long quantum coherence times, could be important. In this talk, we present recent results demonstrating high field effects beyond Rabi oscillations including the Mollow absorption spectrum showing gain without inversion, dark state formation in single electron doped dots, and suppression of nuclear fluctuations by the hyperfine interaction leading to longer electron spin coherence times. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H10.00002: Creating an artificial periodic table using quantum dots Patanjali Kambhampati, Samuel Sewall, Ryan Cooney Confinement of carriers in quantum dots results in hydrogenic like states for the exciton. Thus a single excitation in a quantum dot bears resemblance to a hydrogen atom; these materials are often referred to as ``artificial atoms.'' A pair of excitons will form a four body biexciton, akin to a helium atom. The excitonic `He atom should have an eigenstate spectrum in the vein of atomic orbitals. The eigenstate spectrum of the biexciton has remained elusive due to the ultrafast timescale of relaxation processes in quantum dots which mask observation of the excited states. Here, we show the first, direct observation of spectrum of states of the biexciton, completing the analogy of excitons in quantum dots to atomic and molecular systems. We report on the first observation of a biexciton Stokes shift, which we will discuss in terms of non-Aufbau filling and biexciton fine structure. The observation of biexciton Stokes shift underpins the physics of optical gain in quantum dots. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H10.00003: Selection rules for optical transitions in lead salts semiconductor nanocrystals: Drastic effect of structure inversion asymmetry Serguei Goupalov We resolve a paradox according to which some of the optical transitions in PbSe and PbS semiconductor nanocrystals seemingly forbidden by the parity selection rule exhibit a large oscillator strength in optical absorption. We calculate oscillator strengths for various optical transitions in the framework of the effective mass approximation. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H10.00004: The appearance of localized resonances above the continuum in quantum dots Voicu Popescu, Gabriel Bester, Alex Zunger We investigate the nature of hole and electron states in self-assembled InAs/GaAs and (In,Ga)As quantum dots, using a multi-band atomistic pseudopotential approach. We offer a classification of both hole and electron states based on an analysis of their localization both in the $z$ and $xy$-directions. We show that the coherent dot-matrix strain present in self-assembled quantum dots distorts the electron confining potential, creating ``wings'' in the vicinity of the dot. This results in the appearance of dot-confined electronic states that lie above the continuum of the matrix material. The spectroscopic manifestation of these resonant states is investigated, by calculating the inter-band as well as the intra-band absorption spectra. We find, in both cases, that clear finger-prints of the resonances appear, in the form of sharp, well-defined peaks. In contrast, the previously suggested ``cross-transitions'' between wetting-layer states and dot states are shown to disappear once realistic strain is included. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H10.00005: Effect of Atomic-Scale Alloy Randomness on the Optical Polarization of Semiconductor Quantum Dots Vladan Mlinar, Alex Zunger Alloyed Ga$_{1-x}$In$_x$As system consists of different random assignments $\sigma$ of the Ga and In atoms to the cation sublattice sites; each configuration having, in principle, distinct physical properties. For self-assembled dots made of finite number of cations ($\leq$10$^5$), self-averaging of configurations may not be complete, so single-dot spectroscopy can observe the atomic-scale alloy randomness effects. We examine the effect of such atomic-scale randomness on the fine structure-splitting (FSS) of the exciton observed via the polarization anisotropy of its components. We find: (i) The FSS of the monoexciton X$^0$ changes by more than a factor of 7 with $\sigma$. Thus, finite nanostructure systems provide clear evidence for the effects of atomic-scale randomness on physical properties. (ii) The polarization anisotropy of two X$^0$ transitions is affected both by $\sigma$ variations and from possible QD base elongation. Thus, the polarization anisotropy cannot be used as a measure of geometrical anisotropy alone, (iii) Polarization directions of different multiexciton emission lines are determined by $\sigma$. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H10.00006: Blinking suppression on millisecond-to-minutes time scales in giant nanocrystal quantum dots Anton Malko, David Bussian, Han Htoon, Sid Sampat, Javier Vela, Yongfen Chen, Jennifer Hollingsworth, Victor Klimov Fluorescence intermittency (blinking) is an intrinsic feature of molecular-like fluorophores, including nanocrystal quantum dots (NQDs). The effect complicates applications of NQDs in areas such as quantum informatics, bio-imaging, and real-time tracking. Previously we developed ``giant'' NQDs in which a small emitting core is overcoated with a thick shell of a wider-gap material and observed strong blinking suppression on a time scale of 100s ms and longer. In this work, we employ time-tagged correlated single photon counting to detect photoluminescence (PL) traces from individual ``giant'' CdSe/CdS NQDs with resolution better than 1 ms. We observe a strong dependence of the fluorescence on/off times on shell thickness and almost complete blinking suppression on all measured time scales for NQDs coated with more than $\sim $10 monolayers of CdS. Further systematic analysis of our PL traces reveal a photon statistics that differs significantly from a power-law distribution of on/off times typically observed for ``regular'' NQDs. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 10:12AM |
H10.00007: Photoluminescence and multiexciton dynamics in semiconductor nanoparticles Invited Speaker: |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H10.00008: Scaling of Multiexciton Nonradiative and Radiative Decay Rates with Exciton Number in Semiconductor Nanocrystals John A. McGuire, Victor I. Klimov Rapid multiexciton decay by nonradiative Auger recombination places strong constraints on potential applications of semiconductor nanocrystals (NCs) in lasing and solar energy conversion exploiting carrier multiplication. Hence, it is important to understand the scaling of the Auger recombination rate with exciton number in NCs. Likewise, understanding the scaling of multiexciton radiative rates with exciton number is important both for potential applications of, e.g., ordered multiphoton emission and for interpreting experimental measurements of time-resolved photoluminescence. We report measurements of the scaling of Auger and recombination rates in CdSe and PbSe NCs and of multiexciton radiative rates in PbSe NCs. The more rapid scaling of Auger rates with exciton number N in PbSe compared to CdSe can be understood in terms of the different symmetries of N-excitons with N$>$2 due to the different degeneracies of the lowest-energy excitonic states. The scaling of the multiexciton radiative rates in PbSe can be interpreted in terms of a ``free-carrier'' model. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H10.00009: Carrier Multiplication in PbSe Nanocrystals and Extraneous Processes John A. McGuire, Jin Joo, Jeffrey M. Pietryga, Istvan Robel, Victor I. Klimov Spatial confinement of electronic wave functions in semiconductor nanocrystals (NCs) can enhance the efficiency of carrier multiplication (CM), a process whereby multiexcitons are generated from single absorbed photons. In the last year, a controversy has emerged due to large discrepancies between values of CM efficiencies reported by different groups using different techniques -- transient absorption (TA) and photoluminescence upconversion (uPL). We report studies of CM in solutions of PbSe NCs using measurements of exciton dynamics by both TA and uPL and find excellent agreement between the CM efficiencies extracted by both techniques. Moderate variations in efficiencies are observed for nominally similar samples. More dramatically, measurements of static and stirred solutions can display large differences in dynamics. This indicates that extraneous effects such as NC photoionization can distort the results of CM studies and are a likely contribution to the discrepancies between previously reported CM efficiencies. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H10.00010: Universal size dependence of Auger constants in direct- and indirect-gap semiconductor nanocrystals Istvan Robel, Ryan Gresback, Uwe Kortshagen, Richard D. Schaller, Victor I. Klimov We compare Auger recombination rates in several direct- and indirect-gap semiconductor nanocrystals including Ge, PbSe, InAs, and CdSe. Our size-dependent biexciton lifetime measurements indicate that the most important factor determining recombination rates is nanocrystal size, while details of the materials' electronic structure such as the width of the energy gap or its direct/indirect nature play only a minor role. We observe that the effective Auger constants for all semiconductor nanocrystals in this study exhibit a universal cubic dependence on particle radius (R), C$_{A }\sim $ R$^{3}$. Moreover, absolute values of nanocrystal Auger constants are comparable across different materials despite a dramatic difference (up to 4-5 orders of magnitude) in C$_{A}$ values in the respective bulk solids. Our results can be explained by confinement-induced relaxation of momentum conservation, diminishing the difference between direct- and indirect-gap semiconductors at the nanoscale. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H10.00011: Steady-state fluorescence spectroscopy of multiexcitons in single `giant' nanocrystal quantum dots Han Htoon, David Bussian, Javier Vela-Becerra, Yongfen Chen, Jennifer Hollingsworth, Victor Klimov Due to ultrafast nonradiative Auger recombination, emission of multiexciton states is not pronounced in steady-state spectra of nanocrystal quantum dots (NQDs). Here, we report the first observation of multiexcitonic signatures in steady-state photoluminescence (PL) from single `giant' core/shell NQDs$^{\ast }$, in which a CdSe core is overcoated with a thick ($>$10 monolayers) CdS shell. At low temperature, we observe the emergence of multiple high-energy PL peaks with increasing pump power. Analysis of intensity scaling of these PL peaks with pump power allows us to assign them to bi-, tri- and higher order multiexcitons. Lifetimes of these multiexciton states obtained by single-dot time- and wavelength-resolved PL further corroborate this assignment. These results suggest that in `giant' NQDs Auger recombination is greatly suppressed compared to regular NQDs, which likely stems from their large effective volume and decreased spatial overlap between electrons (occupy entire NQD volume) and holes (localized in CdSe core). $^{\ast }$Y.F. Chen et al. \textit{J. Am. Chem. Soc}. \textbf{130}, 5026 (2008) [Preview Abstract] |
Session H11: Focus Session: Transport Properties of Nanostructures II: Molecular Junctions I
Sponsoring Units: DMPChair: Latha Venkataraman, Columbia University
Room: 305
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H11.00001: Conductance of Conjugated Organic Compounds in Controlled Environments Invited Speaker: We use the mechanical and the electromigration break junction technique, as well as nanoparticle arrays, to measure the electrical conductance of a range of conjugated organic molecules with different end functionalities at room temperature in a liquid cell. We first report on a comparison between oligo(phenylene vinylene) (OPV) oligo(phenylene ethynylene) (OPE). We find that OPV conducts slightly better than OPE. Solubilizing side groups do not prevent the molecules from being anchored within a break junction. With the aim to realize a functional switch, we show preliminary electrical conductance studies of a newly synthesized cruciform molecule. Using the nanoparticle platform we further demon-strate light and electrochemical-induced conductance switching of photochromic and redox molecules. We further discuss OPV and OPE molecules with different end groups, including asymmetric ones. To our surprise, molecules having an anchor group only on one side also gave rise to a pronounced mo-lecular signal. We attribute this effect to the interaction between neighboring molecules in the junction likely induced by $\pi$-$\pi$ stacking. This remarkable property highlights the importance of intermolecu-lar interaction in molecular junctions, an often overlooked aspect. If time permits, a recent study on low- frequency fluctuations in molecular junctions will be mentioned as well. Collaborators are (alphabetic order): J. Agustsson, J. Brunner, M. Calame, T. Gonzalez, S. Grunder, V. Horhoiu, R. Huber, J. Liao, M. Mayor, M. Mangold, S. Oberholzer, M. Steinacher, S. Wu, Z.M. Wu, (all at the Swiss Nanoscience Institute at the Univ. of Basel) and M. R. Bryce (Durham University, UK). [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H11.00002: Externally controlled spin state switching in metal-organic complexes. Alexei Bagrets, Velimir Meded, Mario Ruben, Ferdinand Evers Recent transport experiments have demonstrated that a manipulation of the charge of individual molecules is feasible using electromigrated metal junctions [1] or electrochemical gates in conjunction with the STM [2]. Using elaborated density functional theory calculations, we will discuss a possibility to induce -- by means of charging or applied stress -- a switching between low and high spin states in certain metal-organic systems, [Fe(bpp)$_2$]$^{2+}$ (bpp:\ bispyrazolyl pyridine) and [Mn(tpy)$_2$]$^{2+}$ (tpy:\ terpyridine). Based upon a recent success of the single molecular conduction experiment through Ru(II) complex [3], we anticipate the transport properties of Fe(II) and Mn(II) complexes to be gate controlled via exploiting their spin degree of freedom. \medskip \newline [1] E.\ A.\ Osorio {\it et al.}, J.\ Phys.:\ Condens.\ Matter{\bf 20}, 374121 (2008); [2] F.\ Chen {\it el al.}, Ann.\ Rev.\ Phys.\ Chem.\ {\bf 58}, 535 (2007); Li {\it et al.}, Nanotechnology {\bf 18}, 044018 (2007). [3] M.\ Ruben, A.\ Landa, E.\ L{\"o}rtscher, H. Riel, M. Mayor, H.\ G{\"o}rls, H.\ Weber, A.\ Arnold, and F.\ Evers, Small (online), {\small DOI}: 10.1002/smll.200800390 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H11.00003: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H11.00004: Quantum-Interference-Controlled Molecular Electronics San-Huang Ke, Weitao Yang, H. U. Baranger Quantum interference in coherent transport through single molecular rings may provide a mechanism to control the current in molecular electronics. We investigate its applicability, using a single-particle Green function method combined with \textit{ab initio} electronic structure calculations. We find that the quantum interference effect (QIE) is strongly dependent on the interaction between molecular $\pi $-states and contact $\sigma $-states. It is masked by $\sigma $ tunneling in small molecular rings with Au leads, such as benzene, due to strong $\pi -\sigma $ hybridization, while it is preserved in large rings, such as [18]annulene, which then could be used to realize quantum interference effect transistors. [Nano Letters 8, 3257 (2008)] [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H11.00005: Simultaneous electronic transport and Raman spectroscopy in single-molecule devices Invited Speaker: Over the last decade several techniques have been developed to examine electronic transport through individual small molecules. These include scanned probe methods, mechanical break junctions, and electromigrated junctions. One recurring challenge is the need to confirm that current flow is, indeed, through the molecule of interest rather than a contaminant. We recently discovered (D. R. Ward \textit{et al}., Nano Lett. \textbf{7}, 1396 (2007)) that the same electromigrated Au source and drain electrodes used for transport are tremendously effective optical antennas in the near infrared. Surface plasmon modes localized to the nm-scale interelectrode gap lead to large enhancements of the local electric field relative to that from incident radiation. The result is that these nanoscale gaps are tremendous ``hot spots'' for surface-enhanced Raman scattering (SERS). We perform simultaneous measurements of electronic transport and SERS in junctions incorporating molecules of interest, and find in 10-15\% of devices that the conductance and SERS emission are strongly correlated in time (D. R. Ward \textit{et al}., Nano Lett. \textbf{8}, 919 (2008)). Since the conductance mechanism is tunneling and therefore dominated by a volume comparable to that of a single molecule, this strongly implies that the SERS emission comes from that same molecule. The distinctive SERS spectra allow us to confirm that conduction in these devices is through the molecule of interest. Furthermore, these devices open up many opportunities, including studies of electron-vibrational couplings and dissipation at the single-molecule level. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H11.00006: Simultaneous Measurements of Force and Conductance through Single Molecular Junctions Michael Frei, Maria Kamenetska, Max Koentopp, Mark S. Hybertsen, Latha Venkataraman Simultaneous conductance and force measurements of single molecule junctions are performed by repeatedly forming and breaking junctions between a molecule coated gold substrate and a gold-coated cantilever in a simplified atomic force microscope (AFM). We show that the forces required to break the molecular junctions in the case of 1,4 diaminobutane and 1,4 bis (methyl thiol) butane are significantly smaller than the breaking force of a single atom gold contact. This indicates a breaking of the Au-N and Au-SMe bond respectively. Data for 1,2-bis(dimethyl phosphino) ethane differs significantly. We find that the force required to break the molecular junction is comparable to that required to break a gold-gold bond. We find further that for a significant fraction of the traces measured, the molecular junction conductance often drops by an order of magnitude, while the forces do not change significantly. We will discuss the implications of these findings and show how they relate to detailed simulation of the junction elongation process for these links. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H11.00007: Simulation and Measurement of Single Molecule Junction Evolution Under Stress: Comparison of Amine and Phosphine Link Groups Max Koentopp, Mark Hybertsen, Maria Kamenetska, Adam Whalley, Young Park, Michael Steigerwald, Colin Nuckolls, Latha Venkataraman Reliable measurements of single molecule conductance are performed by repeated breaking of Au point contacts in a solution containing target molecules with link groups that readily form donor-acceptor bonds to specific Au atoms on the electrodes. Measured traces present the junction conductance as a continuous function of junction elongation under applied stress. Our extensive experimental database shows strong correlation between measured conductance step length and molecular backbone length, with phosphine link groups showing longer steps than amine link groups. We model adiabatic junction evolution by discrete steps, with structure determined by energy minimization in a DFT approach and the low bias junction conductance at each step computed using a Green's function approach. We identify different mechanisms whereby the attachment point to the electrode can shift while maintaining similar conductance, explaining why conductance steps can extend over distances of several angstroms. Phosphine and amine link groups sustain different maximum forces, accounting for key differences in junction evolution. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H11.00008: Kondo effect in the electronic transport of magnetic atomic-size contacts M. Reyes Calvo, Joaquin Fernandez-Rossier, Juan Jose Palacios, David Jacob, Douglas Natelson, Carlos Untiedt Low coordination alters dramatically the magnetic properties of materials at the nanoscale. Our results indicate that, in the case of atomic contacts of certain materials, a localized magnetic moment appears at the contact and is screened at low temperatures by means of the Kondo effect. We observe characteristic Fano-Kondo lineshapes in the spectroscopy of atomic contacts of typical ferromagnetic materials (Fe, Co and Ni). The parameters obtained from the fitting of these curves to the Fano equation show statistical distributions that agree with the Kondo theory. The Kondo origin of the measured resonances is confirmed by their temperature dependence and supported by our theoretical calculations. These results are surprising since ferromagnetism and Kondo effect are expected to compete. We have also observed a similar spectroscopy in atomic contacts of palladium and platinum monoatomic chains. In this case, the Kondo resonances would be the signature of an emergent magnetism that is ultimately screened. Partially founded by Spanish MEC (grant nr. MAT2007-65487 and CONSOLIDER CSD2007-0010) and EU (project nr. 211284) [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H11.00009: Magnetic phenomena, ferro and antiferro Kondo, and transport in transition metal break junction nanocontacts Erio Tosatti Transport in transition metal break junctions is of particular interest in connection with magnetism, which may be present either in the leads, and/or in bridging magnetic impurities, or else which may emerge spontaneously at the nanocontact, as expected in pristine Pt or Pd.[1] The standard Landauer ballistic conductance across the locally magnetic atomic contact is modified by Kondo phenomena that are predicted using ab initio electronic structure as a starting point, and the numerical renormalization group as a tool. The case of a magnetic impurity in a nonmagnetic contact[2] is shown to illustrate the competition between antiferro and ferro Kondo screening[3] and its effects on conductance. *) Work in collaboration with A. Smogunov, A. Dal Corso, L. De Leo, P. Gentile, M. Fabrizio, P. Lucignano, and R. Mazzarello. [1] A. Delin, et al., Phys. Rev. Lett. 92, 057201 (2004); A. Smogunov et al., Phys. Rev. B78, 014423 (2008). [2] R. Mazzarello, P. Lucignano, A. Smogunov, M. Fabrizio, and E. Tosatti, in preparation [3] P. Gentile, L. De Leo, M. Fabrizio, and E. Tosatti, in preparation [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H11.00010: Spectra of discrete electrons-in-a-box energy levels in chemically-formed Au and Pt nanoparticles. Su-Fei Shi, F. Kuemmeth, K.I. Bolotin, W. Li, D.C. Ralph We report tunneling spectroscopy measurements of the discrete electron spectra of individual metal nanoparticles formed by chemical synthesis, so that they are well-defined in their composition, size, and shape. The spectra of 5-15 nm diameter gold particles exhibit as many as 40 resolvable electronic excited states for a fixed value of gate voltage. We find excellent agreement between the measured level statistics and random matrix predictions for the regime of strong spin-orbit coupling and ballistic transport. As a function of changing gate voltage, the energy-level spectra in the Au particles are not scrambled by the addition of electrons, indicating that in these particles the variation in the strength of electron-electron interactions between states is negligible. We have also succeeded in fabricating single-electron transistors from individual Pt nanoparticles and hope to present measurements of their discrete spectra. Pt, unlike Au, has a sufficiently strong exchange interaction that it is expected to exhibit non-zero values of the ground state spin in the form of ``mesoscopic magnetism''. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H11.00011: Ballistic transport in gold [110] nanowire Yoshihiko Kurui, Yoshifumi Oshima, Masakuni Okamoto, Kunio Takayanagi Conductance of gold nanowire elongated along the [110] direction (gold [110] nanowire) was measured during many breaking procedures, while simultaneously acquiring transmission electron microscope images. The conductance histogram exhibits a series of peaks whose conductance values increased nearly in steps of the conductance quantum, G0 =2e$^{2}$/h. However thick nanowires above 10G$_{0}$ showed dequantization, where the increment was only 0.9G$_{0}$. The structure for each peak was determined to be either an atomic sheet or a hexagonal prism. The number of conductance channels calculated for each atomic structure by first principles theory, coincided well with the peak index in the conductance histogram. The present study shows that the [110] nanowire behave as ballistic conductors, and a conductance peak appears whenever a conductance channel is opened. [Preview Abstract] |
Session H12: Focus Session: Directed Organization of Molecular Semiconducting Films
Sponsoring Units: DMP DCMPChair: Daniel Dougherty, North Carolina State University
Room: 308
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H12.00001: Structural evolution of the self-assembled layers of funtionalized fullerenes on metal surfaces. Bogdan Diaconescu, Mikael Jazdzyk, Glen Miller, Karsten Pohl Self-assembled organic thin films have a great number of practical applications, ranging from sensors and biological interfaces in medical implants to organic electronics and photovoltaics. Self-assembled monolayers (SAMs) form as a result of a delicate balance between competing molecule-substrate and intermolecular interactions. To control such self-assembly processes, it is mandatory to understand how this balance reflects onto the SAM's final structure. Here, we present a STM study of the self-assembly of C60 functionalized with alkane chains of various lengths (F-C60) on Ag(111). We find that F-C60 molecules lay down on the Ag surface and form a zigzag like pattern with an oblique unit cell of size dependent on the alkyl chain lengths and two molecules per basis. The C60s are placed at a larger than van der Waals distance. The symmetry of the F-C60 SAM is dictated by the alkane-surface interaction while the size of the unit cell is a consequence of the intermolecular interactions. These results show that C60s can be assembled in 2D and non-compact molecular arrays with a surface density controllable via appropriate chemical functionalization. Funded by the NSF Center for High-rate Nanomanufacturing (NSF NSEC-425826). [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H12.00002: Controlled Chemical Morphology in TiOPc - C$_{60}$ Films Yinying Wei, Steve Robey, Janice Reutt-Robey A key strategy for the improvement of organic electronic devices involves the optimization of chemical morphology for efficient charge separation. Fundamental studies of chemical morphology - electronic property relations, particularly along crucial domain boundaries, are needed to realize these goals. We present STM/STS studies of TiOPc: C$_{60}$ films, prepared by vapor deposition on Ag (111). We show how growth conditions can be adjusted to harness anisotropic TiOPc - TiOPc interactions, leading to three dramatically distinct film structures: nanophase segregated TiOPc and C$_{60}$ domains, a co-crystalline TiOPc$_{ (2)}$ C$_{60 (1)}$ honeycomb network, and a quasi-periodic array of triangular TiOPc domains and C$_{60}$ nanoclusters. Electronic transport gaps measured by STS (Z (V)) prove to be sensitive to the phase, varying by up to about 0.5 eV along domain boundaries. We propose structural models for each hetero interface and discuss the physical origin of the observed transport characteristics. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H12.00003: Monte Carlo Study of the Honeycomb Structure of Anthraquinone Molecules on Cu(111) Kwangmoo Kim, T. L. Einstein, Ludwig Bartels Using Monte Carlo calculations of the two-di\-men\-sion\-al (2D) lattice gas model, we demonstrate a mechanism for the spontaneous formation of honeycomb structure of anthraquinone (AQ) molecules on a Cu(111) plane. Unlike the suggestion of long-range sub\-strate-me\-di\-ated repulsion,\footnote{Greg Pawin {\it et al.}, Science {\bf 313}, 961 (2006).} long-range attractions play important roles in our calculations. However, the interplay between attractions and repulsions is still integral to the spontaneous formation of AQ's honeycomb structure. We also compare the critical local coverage rate of AQ's where the honeycomb structure starts to form. Furthermore, we study the diffusion of CO molecules inside AQ honeycombs on the Cu(111) plane. The surface phase transitions of CO molecules between solid, liquid, and gas 2D phases are studied via the specific heat singularity in short-range correlation functions.\footnote{N. C. Bartelt, T. L. Einstein, and L. D. Roelofs, PRB {\bf 32}, 2993 (1985).} [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H12.00004: Patterned molecular nanostructures Invited Speaker: Surfaces and interfaces not only determine to a large extent the properties of small-scale materials due to their high surface-to-volume ratio, they are also an ideal platform for the design, fabrication and device integration of nanostructures. Both, top-down and bottom-up methods have been developed for the handling of matter at the molecular and atomic scale. In the present talk I will demonstrate how the remarkable progress in controlling atomic and molecular interactions at surfaces has provided the unique ability to engineer supramolecular architectures of well-defined size, shape, composition and functionality. Using noncovalent interactions as hydrogen bonding, ionic bonding and metal-ligand interactions, molecular building blocks can be rationally combined into desired functional architectures. The potential functionalities comprise molecular magnetism, novel heterogeneous catalysis, selective host-guest interactions and new concepts of nanoscale mechanics. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H12.00005: Reactions on surfaces for the creation of stable templates M. Matena, M. Wahl, M. Stoehr, T.A. Jung, T.-L. Lee, J. Zegenhagen, T. Riehm, L.H. Gade Molecular assemblies on surfaces can be used as templates that allow the study of host guest interactions and thus provide a starting point for the generation of complex hierarchic structures. An important prerequisite besides the regularity of such structures is their stability. We reported the formation of a molecular network generated by thermal dehydrogenation of a perylene derivative (DPDI) on a Cu(111) surface [1]. By thermal activation, these molecules become autocomplementary H-bond donors/acceptors and form a honeycomb structure. Besides utilizing this network for the incorporation of guest molecules [2], NIXSW (normal incidence x-ray standing wave) experiments were carried out to determine the height of DPDI above the substrate surface before and after the thermal activation. The formation of the network involves a lowering of the height difference between the molecular end groups and the perylene core what is required to enable H-bonding between the molecules. [1] M. St\"{o}hr et al., Angew. Chem. Int. Ed., 44 (2005) 7394; [2] M. St\"{o}hr et al., Small 3 (2007) 1336 [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H12.00006: Meandering C$_{60}$ Chains on Organic Film Substrates Wei Jin, Daniel Dougherty, Qiang Liu, William Cullen, John Weeks, Steven Robey, Janice Reutt-Robey Meandering chains of C$_{60}$ molecules are observed following vapor deposition of C$_{60}$ on a variety of organic molecular films including pentacene and zinc phthalocyanine,as well as previously reported a-sexithiophene. Such filamentous structures are in complete contrast to C$_{60}$'s typical close-packed growth habit, but are reminiscent of dipole fluids. We present STM images and a statistical analysis of chain structures observed for 0.2 - 0.9 mL C$_{60}$ on the square ZnPc overlayer $(a=13.6\mathop A\limits^\circ ,b=14.5\mathop A\limits^\circ ,\alpha =89^\circ )$on Ag (111). Large islands of meandering C$_{60}$ islands indicate $>$50 nm C60 diffusion lengths on this ZnPc film. The C$_{60}$ chain length, $\ell \sim $ 20 nm, depends weakly on coverage, reflecting the $\sim $constant C$_{60}$ density within these islands, and exhibits multiple branches. Chain structures are compared to the molecular dynamics predictions of a 2-d dipole fluid with the C$_{60}$ - C$_{60}$ interaction described by the Girifalco Potential (1 eV attraction) with additional dipole terms. A vertical moment of $\sim $0.8 D simulates filament formation. The structure of the ZnPc support, not included in this simulation, appears to be a secondary consideration in these chain structures. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H12.00007: Molecular dynamics study of polarized C60 on an organic surface Qiang Liu, Wei Jin, John D. Weeks, Janice Reutt-Robey The experimental results of vapor deposition of C60 on a variety of organic molecular films has showed unusual meandering chain structures. Here we use the Langevin molecular dynamics method to mimic the system. In the simulation, we considered interactions including the intermolecular Girifalco potential, dipole-dipole interactions, octupole interactions suggested by W. Losert$^{2}$ and substrate potentials. Choosing different parameters can give us different C60 patterns: the close-packed C60 islands, isolated C60 molecules and meandering chain islands. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H12.00008: Harnessing Surface Dislocation Networks for Molecular Self-Assembly Invited Speaker: The controlled fabrication of functional wafer-based nano-arrays is one of the ultimate quests in current nanotechnologies. Well-ordered misfit dislocation networks of ultrathin metal films are viable candidates for the growth of two- dimensional ordered cluster arrays in the nanometer regime. Such bottom-up processes can be very complex, involving collective effects from a large number of atoms. Unraveling the fundamental forces that drive these self-assembly processes requires detailed experimental information at the atomic level of large ensembles of hundreds to thousands of atoms. The combination of variable temperature measurements from our home-built STM correlated with 2D Frenkel-Kontorova models based on first-principle interaction parameters is used to explain how uniform arrays can form with the strain in the thin film as the driving force responsible for the surface self-assembly process. This process is generally applicable to assemble many molecular species thus opening avenues towards complex self-assembled structures based on a lock-and-key type approach. Moreover, when increasing the molecular coverage and/or decreasing the strain in the thin film the intermolecular interactions will eventually dominate the elastic effects and dictate the self-assembly process via molecular structure and functionality. We will show that controlling this delicate balance leads to a richness of structures, ranging from disperse ordered arrays of molecular clusters to patterned self-assembled monolayers (SAMs) of functionalized fullerenes and methanethiol. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H12.00009: Molecular and electronic structure of organic semiconductors on ultra-thin oxide films Brad Conrad, William Cullen, Ellen Williams We utilize scanning tunneling microscopy (STM) to molecularly image and probe the interactions of organic semiconductors. To mimic a device substrate and growth modes, ultra-thin oxide (UTO) films less than 1 nm thick are grown on Si(111) in ultrahigh vacuum at room temperature. These films are characterized by STM and display a long range RMS roughness of 0.109 nm versus a typical RMS roughness of 0.3 nm for thick SiO2. UTO films are then used as substrates for growth of pentacene, C60, and PCBM. Standing up pentacene is molecularly resolved and described by a thin-film phase unit cell with a=0.76nm and b=0.59nm in the ab-plane. The morphology and electronic structure of co-depositions of pentacene, C60, and PCBM are then deposited on UTO films and will be presented. http://arxiv.org/abs/0811.2515 [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H12.00010: Mapping Molecular States of a Co-Porphyrin Molecule on Au(111) surface Howon Kim, Won Jun Jang, Jong Keun Yoon, Se-Jong Kahng, Won-joon Son, Seungwu Han The biggest factor determining electronic transport property in molecular devices, is the character of molecular electronic states and their interaction with conducting electrodes. While there are tremendous efforts to manifest electronic devices with single molecules, the study of molecular orbitals and the molecule-metal interaction is comparatively small. In this talk, we will present our study of Co-porphyrin on Au(111) performed using low-temperature scanning tunneling microscope (STM). As the sample bias is changed, the STM images show strong spatial variations. With molecular orbital calculation and STM simulation, the bias-dependent images were well-reproduced. At +2V, the molecule shows four lobes, while at -2V, it exhibit two lobes. At intermediate biases, the molecule displays center-concentrated orbitals. Even though the molecules adsorb on Au(111) surface, the calculated orbitals from a free molecule are in good agreement with experimental data, implying marginal substrate effect. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H12.00011: Self-organized Pattern Formation in Dewetting of Elastically Confined Thin Polymer Layer Danish Faruqui, Ashutosh Sharma We report various stages of self-organized, sub-micron, surface directed patterns in a thin polystyrene (PS) layer (thickness $\sim $20nm) sandwiched between a silicon substrate and a cross-linked elastomeric layer (polydimethylsiloxane; PDMS, thickness $\sim $20nm). Morphological evolution of the self-organized surface patterns was recorded both on the elastic PDMS surface and on visco-elastic PDMS-PS interface. The instability patterns could be aligned by placing a micro-stamp (pitch 1500 nm) in conformal contact on the surface of the PDMS-PS bilayer.inducing anisotropic and regular surface pattern very similar to that on the master stamp. AFM scans of the top elastic PDMS surface and optical micrographs confirmed this contact-less transfer of master pattern, at both interfaces, uniformly over a larger area (cm$^{2})$. The anisotropic surface pattern thus formed on the PDMS and PDMS-PS interfaces was employed to explore the subsequent stages of self-organized sub micron structures. Subsequent stages of self organization in this system refers to formation of ordered assembly of sub-micron structures in initial pattern followed by their morphological and topographical evolution in terms of shape, size, separation and aerial density. [Preview Abstract] |
Session H13: Numerical Methods for Strongly Correlated System: Heisenberg and Density Matrix
Sponsoring Units: DCOMPChair: James Chelikowsky, University of Texas at Austin
Room: 309
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H13.00001: Computational Phase Diagrams for Strongly Correlated Quantum Spins Roger Haydock, C.M.M. Nex In an extended system of strongly interacting quantum spins, a single spin flip is an example of a microscopic disturbance whose time-evolution is well behaved and given by Heisenberg's equation. At long times most of the disturbance decays exponentially, leaving behind a few excitations whose decay is slower than exponential. These have energies at which the excitation spectrum is singular, separating bands of qualitatively different excitations. We apply the recursion method [Solid State Physics 35, Academic Press, 2l5-94(1980)] to a generalization of Heisenberg's equation for the evolution of an appropriate microscopic disturbance. This produces a continued fraction whose essential singularities are the desired phase boundaries. Calculations for some Heisenberg spin Hamiltonians illustrate this approach. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H13.00002: Iterative Diagonalization of Inhomogeneous Heisenberg Models Valter Libero, Fabiano Souza The antiferromagnetic Heisenberg model is one of the most important in describing quantum spins coupled by exchange interactions. Difficulties arise especially in presence of broken symmetry, due for instance to impurities or defects. In these cases, even well-established numerical methods like Lanczos or Monte Carlo, encounter limitations. We propose a numerical method which works even in the absence of translational invariance. We diagonalize the Heisenberg model exploiting the conservation of both the z-component of the total spin and the square of the total spin, a much more complicated procedure but that renders an additional block diagonalization. In essence, the N-site Hamiltonian is built using basis-vectors generated from the direct product of the eigenvectors of the (N-1)-site Hamiltonian and the states of the added N-th spin. The procedure is also applied for the two-leg ladder, an experimental relevant system. Results are shown for ground-state energy and temperature dependent specific heat for chains with local spin impurities or with random distributions of spins 1/2, 1 or 3/2. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H13.00003: Entanglement perturbation theory for the excitation spectrum in one dimension Sung Chung A novel many-body method, entanglement perturbation theory, is developed for the excitation spectra in one dimension. Applied to the antiferromagnetic Heisenberg chains with spin one-half and 1, converging and hence exact results are obtained, including known Bethe Ansatz result for spin one-half and DMRG results for spin 1. We have found that the magnons are spread over about 4 lattice sites. An essential ingredient in this theory is the exact, un-renormalized ground state of arbitrary system sizes, which are also calculated by EPT in a simple, general and exact manner. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H13.00004: A Brief Introduction to the Truncated Eigenfermion Decomposition Jonathan E. Moussa, James R. Chelikowsky We present a computational formalism for the approximate unitary transformation of a many-body fermion Hamiltonian with two-body interactions. This work is a further development of the numerical canonical transformation approach of S. R. White [J. Chem. Phys. 117, 7472 (2002)]. The Hamiltonian can be diagonalized in a basis of \textit{eigenfermion} operators, in which case the eigenstates are all single Slater determinants of eigenfermions. The transformation of two-body interactions generates higher-order interactions that can be approximated by effective two-body interactions using a novel generalization of normal ordering. The error in representating a target eigenstate is minimized by performing the generalized normal ordering with respect to that eigenstate. Numerical results are presented for several test cases, including Hubbard model clusters. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H13.00005: The Density Matrix Renormalization Group Algorithm for Strongly Correlated Systems: A Generic Implementation Gonzalo Alvarez I will present DMRG++, a fully functional generic Density-Matrix Renormalization Group (DMRG) code with sample cases for the Hubbard and Heisenberg model, and for one-dimensional chains and n-leg ladders. My talk will include an overview of the core C++ classes, effective symmetry blocking and parallelization found in DMRG++. I will also explain how to add new strongly correlated electron (SCE) models and geometries with minimal code changes. Even if you are not very familiar with the DMRG or C++, you will be able to understand the main motivations and advantages of generic programming applied to SCE systems. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H13.00006: A Renormalization Group for Treating 2D Coupled Arrays of Continuum 1D Systems Robert Konik, Yury Adamov We study the spectrum of two dimensional coupled arrays of continuum one-dimensional systems by wedding a density matrix renormalization group (DMRG) procedure to a renormalization group improved truncated spectrum approach. To illustrate the methodology we study the spectrum of large arrays of coupled quantum Ising chains. We demonstrate explicitly that the method can treat the various regimes of chains, in particular the three dimensional Ising ordering transition the chains undergo as a function of interchain coupling. We argue that the methodology's success is based on finite size corrections being exponentially small which in turn allows the block DMRG entanglement entropy to be kept to a minimum. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H13.00007: Low-temperature density matrix renormalization group using regulated polynomial expansion Shigetoshi Sota, Takami Tohyama We propose a new scheme of density matrix renormalization group (DMRG) for low dimensional strongly correlated electron systems at finite temperatures, which is a straightforward extension of the target-state procedure at zero temperature. In order to investigate thermodynamical properties, we employ the target state that is weighted by a Boltzmann factor [1]. Making use of a regulated polynomial expansion [2] and random sampling, we can calculate static and dynamical quantities at finite temperatures. In order to obtain good convergency in high temperature region, we need a large truncation number of the density matrix, while a necessary truncation number is small at low temperatures. The proposed method is, therefore, suitable for lower temperature region. As a demonstration of the method, we show the specific heat and dynamical current-current correlation function of the 1D Hubbard model at half filling. The DMRG results reproduce the exact digitalization results at low temperatures. [1] S. Sota and T. Tohyama, Phys. Rev. B \textbf{78}, 113101 (2008). [2] S. Sota and M. Itoh, J. Phys. Soc. Jpn. \textbf{76}, 054004 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H13.00008: Screened Coulomb Interactions of Localized Electrons from First-Principles Bi-Ching Shih, Peihong Zhang We present a recently developed, maximally localized Wannier function approach for calculating the screened Coulomb(U) and exchange (J) interactions of localized electrons in solids. The localized orbitals are constructed using the maximally localized Wannier function approach. The dielectric screening is calculated from first-principles within the random phase approximation. Results for several systems containing strongly localized d electrons will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H13.00009: Refinement of a Lanczos-based variational procedure to solve the Holstein model Zhou Li, Devin Baillie, Cindy Blois, Frank Marsiglio We propose a slight refinement to the Trugman variational procedure to more efficiently solve the Holstein model with Lanczos methods. The modified Lanczos method converges much more quickly compared with the usual one in the intermediate- and strong- coupling regimes. To get a 6-digit accuracy of ground state energy at intermediate-strong coupling in the adiabatic region (small phonon frequency), only about 5000 basis states are need to be included. We also construct a variational ground state based on the numerical results. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H13.00010: Plaquette Renormalization Scheme for Tensor Network States Ling Wang, Ying-Jer Kao, Anders Sandvik We present a method for contracting a square-lattice tensor network in two dimensions based on auxiliary tensors accomplishing successive truncation (renormalization) of the effective 8-index tensors for $2\times 2$ plaquettes into 4-index tensors. The schheme is variational, and thus the tensors can be optimized by minimizing the energy. Test results for the quantum phase transition of the transverse-field Ising model confirm that even the smallest possible tensors (two values for each tensor index at each renormalization level) produce much better results than the simple product (mean-field) state. We also discuss several extensions of the scheme. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H13.00011: A Diagrammatic Extension to Dynamical Cluster Approximation based on the Two-Particle Irreducible Vertex at Intermediate Length Scales Cengiz Sen, Cyrill Slezak, Thomas Maier, Karen Tomko, Mark Jarrell We present a non-perturbative multi-scale extension to the Dynamical Cluster Approximation (DCA) based on the two particle irreducible vertex $\Gamma$. The correlations at short length scales are calculated exactly using Quantum Monte Carlo (QMC) on small cluster of size $N_c^{(1)}$, and long length scales are treated at the dynamical mean field level. Intermediate length scales are treated on a second cluster of size $N_c^{(2)}> N_c^{(1)}$ by approximating its two-particle irreducible vertex with that of the smaller cluster, which is calculated by retaining its full momentum and frequency dependence. The resulting self energy of the large cluster is calculated using the Schwinger-Dyson equation. The method is applied to the 2D Hubbard model with cluster sizes $N_c^{(2)}\ge N_c^{(1)}$ and the results are compared with those that are calculated using QMC by increasing the size of the small cluster $N_c^{(1)}$ up to $N_c^{(2)}$. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H13.00012: Nonlinear conductance oscillation in strong correlation limit of molecular quantum dots near zero bias anomaly Jong E. Han Recent experiments on strong correlation effects in molecular junctions have demonstrated that the interplay of electronic coupling to molecular vibrations and the Coulomb interaction produces intriguing oscillatory structures in the nonlinear conductance near zero bias anomaly at voltages in the energy scale of, presumably, the Kondo temperature. Using the imaginary-time quantum Monte Carlo technique recently developed for strongly correlated nonequilibrium, the nonlinear conductance of the Anderson-Holstein model at finite bias has been calculated. We discuss the mapping between the charge- and spin-Kondo limits and their distinctly different transport physics under finite chemical potential bias. We show that the conductance oscillation emerges at finite bias in the vicinity of the Kondo temperature due to strong electron-vibration coupling. The origin of the oscillation is from the bias-induced strong electron density modes as opposed to direct phonon excitations. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H13.00013: Role of phonons and of finite temperature on the spectral function of a single hole in a 2D quantum antiferromagnet Satyaki Kar, Efstratios Manousakis We study thermal broadening of the hole spectral function of the two-dimensional $t-J$ and $t-t^{\prime}-t^{\prime\prime}-J$ modela within the non-crossing approximation (NCA) with and without the contribution of optical phonons. We have also studied the range of validity of the NCA by including the role of vertex corrections. The broadening of the quasiparticle peak as well as the transfer of spectral weight as a function of momentum to higher energy string excitations is found to be in reasonably good agreement with experimental angle resolved photo-emission spectroscopy(ARPES) results using a rather large electron-phonon coupling. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H13.00014: Inelastic Scattering from Local Vibrational Modes Balazs Dora, Miklos Gulacsi We study a nonuniversal contribution to the dephasing rate of conduction electrons due to local vibrational modes. Bosonization allows us to evaluate the full T-matrix. The inelastic scattering rate is strongly influenced by multiphonon excitations, exhibiting oscillatory behaviour. For higher frequencies, it saturates to a finite, coupling dependent value. In the strong coupling limit, the phonon is almost completely softened, and the inelastic cross section reaches its maximal value. This represents a magnetic field insensitive contribution to the dephasing time in mesoscopic systems, in addition to magnetic impurities. [Preview Abstract] |
Session H14: Colloids III: Formation and Control
Sponsoring Units: DFDChair: Itai Cohen, Cornell University
Room: 315
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H14.00001: Connecting structure and rheology in sheared colloidal suspensions Jonathan McCoy, Itai Cohen We investigate the shear properties of colloidal suspensions confined between parallel plates. When the distance between the plates is very small, i.e. approaching the size of the colloidal particles, a number of dramatic phase behaviors are observed under shear, including buckling, banding, jamming, and crystallization. This strongly confined regime is difficult to access using standard rheological techniques. Our experiment explores connections between microstructural behaviors and macroscopic flow by combining confocal microscopy and force measurement techniques in a custom-built thin-film shear cell. Here, we will focus on the interplay between confinement, slip, and order. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H14.00002: Modulation of attractive colloidal interactions by lipid and protein membrane functionalization Yupeng Kong, Raghuveer Parthasarathy The broad technological and scientific importance of colloidal materials has spurred a large body of research into the functionalization of micron-scale particles. Progress towards self-assembled microparticle superstructures remains slow, however, and fundamental mysteries such as the attractions observed between like-charged particles near a confining wall remain unresolved. These difficulties arise in large part due to the lack of experimental systems with tunable, attractive interparticle interactions. Biomembranes are appealing candidates for colloidal functionalization, enabling access to electrostatic and chemical properties that influence inter-particle relations. We describe here the first measurements of the pair interaction energy for membrane-functionalized colloids, using a newly developed optical line trapping technique. Two classes of particles, derivatized with lipid-only and lipid-plus-protein membranes, each show attractive interactions. The two particle types exhibit different relations between the depth and spatial range of the interactions, however. Control of lipid composition allows the first reported decomposition of like-charge interactions into charge-dependent and -independent terms, leading to a striking insight into the long-standing paradox of like-charge attraction: the charge-dependent term in the interaction is purely repulsive, while the attraction is independent of particle charge. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H14.00003: Self-protected interactions in DNA-functionalized colloids: Nano Contact Glue Mirjam Leunissen, Remi Dreyfus, Roujie Sha, Nadrian Seeman, David Pine, Paul Chaikin The ability of single-stranded DNA to form a variety of sequence-dependent secondary structures, such as hairpins, is frequently used in DNA nanotechnology, but has so far not been explored for the directed assembly of (nano)colloidal structures. We will show how mono- and bimolecular hybridization events in the DNA coatings of individual micrometer-sized beads can give rise to unusual, quench-rate dependent aggregation behavior, and how it can give additional control over the colloidal self-assembly process. For example, it provides us with `self-protected' interactions that are activated by temperature or prolonged proximity and that facilitate the formation of finite-sized structures. A simple quantitative model describes the underlying competition between intra- and interparticle hybridization events, based on the known thermodynamic parameters of the DNA sticky ends. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H14.00004: A simple quantitative model for the reversible association of DNA coated colloids Remi Dreyfus, Mirjam Leunissen, Roujie Shah, Alexei Tkachenko, Nadrian Seeman, David Pine, Paul Chaikin We investigate the reversible association of micrometer-sized colloids coated with complementary single-stranded DNA `sticky ends' as a function of the temperature and the sticky end coverage. We find that even a qualitative description of the dissociation transition curves requires the inclusion of an entropic cost. We develop a simple general model for this cost in terms of the configurational entropy loss due to binding and confinement of the tethered DNA between neighboring particles. With this easy-to-use model, we demonstrate for different kinds of DNA constructs quantitative control over the dissociation temperature and the sharpness of the dissociation curve, both essential properties for complex self-assembly processes. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H14.00005: Observation of condensed phases of quasi-planar core-softened colloids Primoz Ziherl, Natan Osterman, Dusan Babic, Igor Poberaj, Jure Dobnikar We experimentally study the condensed phases of repelling core-softened spheres in two dimensions. The dipolar pair repulsion between superparamagnetic spheres trapped in a thin cell is induced by a transverse magnetic field and softened by suitably adjusting the cell thickness. We scan a broad density range and we materialize a large part of the theoretically predicted phases in systems of core-softened particles, including expanded and close-packed hexagonal, square, chain-like, stripe/labyrinthine, and honeycomb phase. Further insight into their structure is provided by Monte Carlo simulations. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H14.00006: In situ real time measurement of temperature responsive nanoparticles. Denis Pristinski, Thomas Q. Chastek, Vivek Prabhu, Kalman Migler In this work, we combine dynamic light scattering (DLS) and diffusing wave spectroscopy (DWS) to evaluate the size of temperature responsive nanoparticles over a broad range of concentrations. A fiber optic probe DLS instrument was previously demonstrated to measure nanoparticle solutions at a relatively high concentration. The incorporation of back-scattering DWS further extends the technique application to highly turbid conditions. The combined setup was designed to have a simplified and compact optical arrangement employing singlemode fiber based components. Data analysis for both methods was carried out using integrated open source cross-platform software. Measurements were conducted to monitor the progress of poly(N-isopropyl acrylamide) nanoparticle syntheses, including a multi-step seeded polymerization, commonly used to prepare core-shell particles. These particles have received a lot of attention due to their potential for use as targeted drug delivery systems. It was found that DLS and DWS were in good quantitative agreement, and able to accurately characterize the samples. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H14.00007: Enhanced particle transport in an oscillating sinusoidal optical potential. Weiqiang Mu, Lan Luan, Gang Wang, Gabriel Spalding, John Ketterson We have studied the delivery of a colloidal particle in the presence of an oscillating, spatially periodic, optical potential. The average particle velocity relative to the fluid velocity in this potential depends greatly on the oscillation amplitude and frequency. The results of both our simulations and experiments show that for some combinations of these parameters, the average particle velocity can be enhanced due to the synchronization of the particle movement with the oscillating potential. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H14.00008: Charge inversion in monovalent ionic solutions Alex Travesset, Alberto Martin-Molina, Carles Calero, Jordi Faraudo, Manuel Quesada-Perez, Roque Hidalgo-Alvarez We present measurements of the mobility of colloids as a function of the concentration of the monovalent salt Tetraphenyl Arsonium Chloride (TACl). The experiments show a decrease of the mobility with increasing salt concentration that flips sign (charge inversion) at mM salt concentrations. A modified version of the O'Brien and White theory taking into account the hydrophobic nature of the phenyl groups describes the experimental data without fitting parameters. Saturation effects in the mobility as well as possible generalizations are also discussed. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H14.00009: Position Control of Particles embedded in Microbeads and Fibers Produced by Electrohydrodynamics. Unyong Jeong, Eun Min Jo, Sungwon Lee, Kyu Tae Kim Electrohydrodynamics is a good approach to produce uniform-sized colloids and fibers in a continuous process. The dimension can be controlled from tens of nanometers to a few micrometers. The structure of the colloids and nanofibers from electrohydrodynamics has been diversified according to the uses. Especially, core-shell structure and hybridization with functional nanomaterials are fascinating due to their possible uses in drug-delivery systems, multifunctional scaffolds, organic/inorganic hybrids with new functions, and highly sensitive gas- or bio-sensors. This talk will present the structural variations by tuning the position of small particles in the colloids and fibers produced from electrohydrodynamics and demonstrate their possible applications. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H14.00010: Using colloids to model atomic thin film growth Rajesh Ganapathy, Mark Buckley, Itai Cohen We epitaxially grow colloidal thin films by sedimenting micron sized colloidal particles on a microfabricated substrate. The attractive interaction between the colloids, induced by a depletant polymer, leads to the nucleation of islands that grow and coalesce with one another. We use confocal microscopy and particle tracking to study the dynamics of the colloidal particles as they diffuse, aggregate and rearrange configurations during deposition. The saturation island density is estimated as a function of the deposition rate and depletant concentration. We find that our results are in excellent agreement with those obtained from atomic deposition experiments suggesting that our system can be used to model various phenomena that occur in atomic thin film growth. Furthermore, we quantify the Ehrlich-Schwoebel step edge barrier by using holographic optical tweezers to create artificial islands and study the dynamics of colloidal monomers placed on the edge of these islands. Owing to the short-range of the attractive interaction in our system, the origin of the step edge barrier in colloids is strikingly different from atoms. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H14.00011: Stochastic Rotational Dynamics Simulations of Nanocolloid Suspensions Jeremy B. Lechman, Matt K. Petersen, Steven J. Plimpton, P. Randall Schunk, Gary S. Grest, Pieter in't Veld The use of nanoparticle suspensions to potentially tailor the functionality of composite devices has broad applicability, but is limited in practice, in part, due to poor understanding of the phenomena at that scale. In order to address this we have implemented a mesoscale fluid technique called Stochastic Rotation Dynamics (SRD). Here, we discuss the use of this method to investigate the behavior of hard sphere like nanocolloids. In particular we will present a direct, ``one-to-one'' comparison of an SRD fluid with an explicit Lennard Jones solvent. For small colloids in this low viscosity fluid no ``telescoping of timescales'' is required for efficiency, which allows us to consider the accuracy of the base numerical scheme without complicating approximations. We present the diffusion and reduced viscosity as a function of volume fraction of colloids and compare to well known results. The efficiency of an SRD simulation relative to an explicit atom simulation is also discussed. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H14.00012: Light-induced structure transformation of colloidal nanocrystals by using generalized Ewald-Kornfeld formulation M.J. Zheng, K.L. Chan, K.W. Yu When metallic nanoparticles are brought close together and they are illuminated by laser light, there will be strongly enhanced forces between these particles [1]. If these particles are suspended in a liquid, the force can promote aggregation. As a result, the cluster size can exceed the wavelength of light and retardation effect must be considered. For this sake, we derived a generalized Ewald- Kornfeld summation [2] which is valid for fully retarded electromagnetic interaction. More importantly, we have extended the formula for a many-point basis in a unit cell. We used the formula to study the colloidal nanocrystal formation and transition driven by surface plasmon resonance enhanced forces. Our results are of fundamental importance to the relevant topics in soft matter physics and can be widely applied in the research of light-induced manipulation. \\[3pt] [1] A. S. Zelenina, R. Quidant, M. N. Vesperinas, Opt. Lett. \textbf{32}, 1156 (2007). \\[0pt] [2] C. K. Lo, K. W. Yu, Phys. Rev. E \textbf{64}, 031501 (2001). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H14.00013: Universal Nanocolloid Deposition Patterns: Harmonics of a Taylor Cone and Separation of DNA-Hybridized Nanocolloids Xinguang Cheng, Hsueh-Chia Chang With judiciously placed far-field electrodes, harmonics of the Laplace equation are selected near a conducting Taylor cone with discrete polar angles for the field maxima. Charged nanocolloids ejected along the discrete electric field lines of these mode maxima are observed to deposit a universal spectrum of rings on an intersecting plane, with particles of different size occupying different spectral lines due to different residue charge. After an affine transformation, nanocolloids ejected into a microslit and deposited onto one substrate exhibit the same universal line spectra. The size-selective deposition pattern is used to quantify DNA hybridization yield onto oligo-functionalized nanocolloids. [Preview Abstract] |
Session H15: Low Dimensional Magnetism (including molecules and surfaces)
Sponsoring Units: GMAGChair: William Ratcliff,, National Institute of Standards and Technology
Room: 316
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H15.00001: Reversible switching of charge states of single TCNE molecules on Cu(111) Taeyoung Choi, Jay Gupta The interplay of electronic structure and magnetic properties is of interest in various organic materials. TCNE (TCNE = tetracyanoethylene) is one component of well-known organic magnets with ferromagnetism up to room temperature. TCNE has a strong electron affinity that facilitates chemical bond formation and charge transfer with metals. We use scanning tunneling microscopy and spectroscopy to study single TCNE molecules on Cu(111) and Cu(100) surfaces. On Cu(111), we find that TCNE can be reversibly switched among three configurations via a controlled voltage pulse. We determine the adsorption sites for these configurations by co-adsorbing CO molecules, which are well known to adsorb atop Cu atoms. We believe these states represent different adsorption configurations and charge states. One of the configurations shows a strong Kondo resonance at low temperature; spectroscopic imaging indicates that this state is strongly localized at the corners of the TCNE molecule. Several features symmetric about $V$=0 suggest a convolution of the Kondo density of states with inelastic electron tunneling spectroscopy of vibrational modes. http://www.physics.ohio-state.edu/$\sim $jgupta [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H15.00002: Spin-Electric Coupling in Molecular Magnets Mircea Trif, Filippo Troiani, Dimitrije Stepanenko, Daniel Loss We study the triangular antiferromagnet Cu$_3$ in external electric fields, using symmetry group arguments and a Hubbard model approach. We identify a spin-electric coupling caused by an interplay between spin exchange, spin-orbit interaction, and the chirality of the underlying spin texture of the molecular magnet. This coupling allows for the electric control of the spin (qubit) states, e.g. by using an STM tip or a microwave cavity. We propose an experimental test for identifying molecular magnets exhibiting spin-electric effects. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H15.00003: Magnetic excitation in artificially designed oxygen molecule magnet Takatsugu Masuda, Satoshi Takamizawa, Kazuma Hirota, Masaaki Ohba, Susumu Kitagawa We performed inelastic neutron scattering experiment to study magnetic excitation of O$_2$ molecules adsorbed in microporous compound. The dispersionless excitation with characteristic intensity modulation is observed at $\hbar \omega = 7.8$ meV at low temperature. The neutron cross section is explained by spin dimer model with intradimer distance of 3.1 \AA . Anomalous behaviour in the temperature dependence is discussed in the context of enhanced magnetoelasticity in the soft framework of O$_2$ molecule. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H15.00004: Influence of the Dzyaloshinskii-Moriya exchange interaction on quantum phase interference of spins Wolfgang Wernsdorfer, T.C. Stamatatos, G. Christou Magnetization measurements of a Mn$_{12}$mda wheel single-molecule magnet (SMM) with a spin ground state of $S = 7$ show resonant tunneling and quantum phase interference, which are established by studying the tunnel rates as a function of a transverse field applied along the hard magnetization axis. We show how the Dzyaloshinskii-Moriya (DM) exchange interaction can affect the tunneling transitions and quantum phase interference of a SMM. Of particular novelty and importance is the phase-shift observed in the tunnel probabilities of some transitions as a function of the DM vector orientation. Such observations are of importance to potential applications of SMMs that hope to take advantage of the tunneling processes that such molecules can undergo. Ref.: W. Wernsdorfer, T.C. Stamatatos, G. Christou, Phys. Rev. Lett., 101, (28 Nov. 2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H15.00005: Coupling between molecular spin cluster qubits Marco Affronte Supramolecular chemistry enables nanoscale engineering of scalable structures, by introducing controlled interactions between well defined molecular building blocks. The ability to assemble weakly-interacting subsystems is a prerequisite for implementing quantum-information processing (QIP) and generating controlled entanglement. In recent years, molecular nanomagnets (MNMs) have been proposed as suitable candidates for the qubit encoding and manipulation. In particular, antiferromagnetic Cr7Ni rings at low temperature behave as effective spin-1/2 systems and exhibit long decoherence times. Here we show that these rings can be linked to each other through supramolecular functional groups, which allow an extensive tuning of the coupling between their spins. We demonstrate that maximally entangled states can be deterministically generated in tripartite supramolecular assemblies, formed by two Cr7Ni rings and a Cu ion, by simulating the system's time evolution under the effect of realistic microwave pulse sequences (under consideration Nature Nanotechnology). [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H15.00006: High Resolution Neutron Scattering Studies of Spin Excitations in the 2D Singlet Ground State Systems of SrCu2(BO3)2, SrCu(2-x)Mg(x)(BO3)2 and Sr(1-x)La(x)Cu2(BO3)2. Sara Haravifard, Sarah Dunsiger, Bruce Gaulin, Hanna Dabkowska, Mark Telling, Toby Perring, Samir El Shawish, Janez Bonca SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state and a realization of the Shastry-Suthrland model. One aspect of the study of SrCu2(BO3)2 for which there is a little information is the influence of impurities on the nature of the singlet ground state. There is much interest in such studies due to the remarkable phenomena associated with doping other quasi-2D copper-oxide quantum magnets with high-T superconductivity. Here we report high resolution time-of-flight neutron scattering studies of single crystals of doped SrCu(2-x)Mg(x)(BO3)2 and Sr(1-x)La(x)Cu2(BO3) 2 and compare these results to the pure SrCu2(BO3)2 neutron scattering measurements. Particular emphasis is placed on the lifetimes of one-triplet excitations as well as the existence of in-gap spin excitations in the presence of Mg and La impurities. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H15.00007: Entanglement Perturbation Theory for Antiferromagnetic Heisenberg Chains. Lihua Wang, Sung Chung We use a novel method, Entanglement Perturbation Theory (EPT) to solve the Heisenberg chain comprehensively for both spin 1/2 and spin 1. A variety of quantities for xxx model and xxz model are calculated, including the ground state energies, the spin-spin correlation functions, and the first excited state energies relevant to a phase transition. EPT allows us to calculate systems with nearly one thousand sites and obtain spin-spin correlation functions over hundreds of sites with unprecedented accuracy. The successful application of EPT to the Heisenberg model shows that it is simple, general and exact for macroscopic quantum systems with translational symmetry. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H15.00008: Magnetoelectric Coupling in a Quantum Spin Ladder Jessica White, Janice Musfeldt, Scott Crooker, John Singleton, Ross MacDonald, Chris Landee, Mark Turnbull, Hans Christen We investigated the optical properties and magnetization of (2,3-dmpyH)$_2$CuBr$_4$, an antiferromagnetic quantum spin ladder with strong rail interactions. It is similar to the copper oxides, yet its modest exchange interactions allows it to be saturated at 29 T, whereas copper oxides require a much higher field. In the end, we were able to see that the field dependent integrated absorption difference tracks the magnetization, demonstrating that the structure is sensitive to the ferromagnetic transition. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H15.00009: Valence-Bond Monte Carlo Study of Random-Singlet Phase Formation Huan Tran, Nicholas Bonesteel In valence-bond Monte Carlo (VBMC)\footnote{A. Sandvik, PRL {\bf 95}, 207203 (2005).} the ground state of a quantum spin system is sampled directly from the valence-bond (VB) basis --- a useful basis for visualizing the properties of singlet ground states. For example, the ground state of the uniform AFM spin-$\frac{1}{2}$ Heisenberg chain is characterized by strongly fluctuating bonds with power-law length distribution, while in the random-singlet phase (RSP) of a {\it random} Heisenberg chain these bonds, while still having a power-law length distribution, lock into a particular VB state on long length scales.\footnote{D. S. Fisher, PRB \textbf{50}, 3799 (1994).} We use VBMC to directly probe the formation of a RSP by calculating both the average number of bonds $n_L$ leaving a block of $L$ spins (the VB entanglement entropy\footnote{F. Alet, et al., PRL \textbf{99}, 117204 (2007).}), and its {\it fluctuations}, $\sigma_{n_L}^2 = \left\langle\langle n_L^2\rangle- \langle n_L\rangle^2\right\rangle$. For the uniform chain they have been calculated exactly\footnote{J. L. Jacobsen and H. Saleur, PRL \textbf{100}, 087205 (2008).} and shown to grow logarithmically with $L$ --- signaling the strong bond fluctuations. For random chains while $n_L$ grows logarithmically with $L$, we find $\sigma_{n_L}^2$ {\it saturate} for large $L$, signaling the ``freezing" of the bonds into a particular random singlet state. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H15.00010: Quantum Monte Carlo simulations of dynamical properties for gapped spin chains Zhaoxin Xu, Juana Moreno, Mark Jarrell We study the dynamical properties of spin-1 antiferromagnetic chains and spin-1/2 ferromagnetic-antiferromagnetic bond alternating chains. We calculate their dynamical structure factors using quantum Monte Carlo simulations combined with the Maximum Entropy Method. We focus on the finite temperature dynamical behavior and impurity effects on these gapped spin chains. We also discuss the connection between our results and recent neutron scattering experiments. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H15.00011: Exploring Possible Magnetic Properties of Ordered Manganese Monolayer on Wurtzite GaN A.R. Smith, A. Chinchore, K. Wang, W. Lin, T. Chen, Y. Liu, J. Pak For future spin-based device technologies, it is crucial to investigate magnetic material systems using techniques having high magnetic resolution. Spin-polarized scanning tunneling microscopy has proven to be extremely powerful for resolving magnetic structure down to even the atomic scale. Of great interest lately are transition-metal-on-semiconductor systems. We have recently discovered a well-ordered Mn monolayer having $\surd $3$\times \surd $3-R30$^{o}$ structure, formed on wurtzite gallium nitride.[1] It is intriguing to explore the possibility of atomic-scale magnetic ordering in this system. For this purpose, we have designed a new SP-STM system combined with a highly flexible, epitaxial nitride growth facility. The new SP-STM has been designed for variable-low-temperature operation within an applied magnetic field of up to +/- 4.5 Tesla. Initial results with the new system are expected within the very near future. Work is supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No. 0730257). Equipment support from ONR is also acknowledged. [1] A. Chinchore et al., Appl. Phys. Lett. \textbf{93(18)}, 181908 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H15.00012: Manganese Atom Ordered Monolayer on Wurtzite Gallium Nitride Abhijit Chinchore, Kangkang Wang, Wenzhi Lin, Jeongihm Pak, Yinghao Liu, Arthur Smith While transition-metal-doped gallium nitride (GaN) thin films have been explored as potential dilute magnetic semiconductor bulk layers, the structural and magnetic effects of various transition metal adatoms on GaN surfaces are not even well understood. In this work, we investigate the sub-monolayer deposition of manganese (Mn) onto the N-polar wurtzite GaN (000-1) 1$\times $1 surface. The growth is monitored in-situ using reflection high energy electron diffraction (RHEED). A fresh GaN(000-1) 1$\times $1 surface is prepared by rf nitrogen plasma-assisted MBE followed by annealing to remove excess gallium adatoms. The atomically flat GaN surface, held at 200$^{o}$ C, is then exposed to submonolayer doses of Mn. The deposition rate is maintained at 0.007 ML per second, and a 3$\times $ pattern develops along [10-10]; whereas, only 1$\times $ is seen along [11-20]. Analysis of the RHEED pattern and subsequent modeling indicates a $\surd $3 $\times \surd $3 R 30$^{o}$ structure consisting of 2/3 ML Mn atoms in a row-like arrangement having spacing $\surd $3a/2 along rows and 3a/2 between rows. Scanning tunneling microscopy/spectroscopy studies are currently underway to explore this surface further. This work is supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No. 0730257). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H15.00013: Real-space pseudopotential method for noncollinear magnetism within density functional theory Doron Naveh, Leeor Kronik We present a real-space pseudopotential method for first principles calculations of noncollinear magnetic phenomena within density functional theory. We demonstrate the validity of the method using the test cases of the Cr$_3$ the a Cr monolayer. The approach retains all the typical benefits of the real-space approach, notably massive parallelization. It can be employed with arbitrary boundary conditions and can be combined with the computation of pseudopotential-based spin-orbit coupling effects. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H15.00014: Properties of weakly coupled 1/2 spin ladders in a magnetic field Pierre Bouillot, Corinna Kollath, Andreas Lauchli, Edmond Orignac, Roberta Citro, Thierry Giamarchi Weakly coupled $1/2$-spin ladders have a very rich physics that can be observed in their $(T,H)$ phase diagram. One of the most interesting particularity, is the possibility to explore the entirety of their Luttinger-Liquid phase by controlling the exponent through the applied magnetic field. We investigate this system that is closely related to the recent experiments on the compound $(Hpip)_2CuBr_4$. From a bosonization interpretation of zero temperature DMRG calculations, we determine the related Luttinger liquid parameters in the incommensurate phase and use them to obtain the transition temperature between the Luttinger-Liquid and the Neel phase. In the latter, we determine the order parameter at zero temperature. From temperature dependent DMRG computations, we obtain the magnetic field dependence of the specific heat and the magnetization. A very good agreement is found between our numerical results and the measurements in the coumpound $(Hpip)_2CuBr_4$ intensively explored in [Phys. Rev. Lett. 101, 137207 (2008)], [arXiv:0808.2715 (PRL in press.)] and [arXiv:0809.0440]. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H15.00015: Spin and exchange coupling in Ti atom and Ti dimers Pushpa Raghani, Jesus Cruz, Barbara Jones It is very important to know the spins on magnetic atoms embedded in a molecular network. Single or multiple magnetic atoms with a large spin can be used as molecular magnets for magnetic storage devices. We use denisty functional theory (DFT) with pseudopotentials and GGA+U to calculate spin on Ti atoms adsorbed on CuN/Cu(100) surface; and compare this spin with that obtained from scanning tunneling microscopy experiments. Then we calculate the exchange coupling for a complete layer of Ti, as well as dimers of Ti on the same CuN/Cu(100) surface. [Preview Abstract] |
Session H16: Matter waves and Quantum Optics
Sponsoring Units: DAMOPChair: Dan Goldbaum, Arizona
Room: 317
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H16.00001: Matter wave interferometry as a tool for molecule metrology Stefan Gerlich, Michael Gring, Hendrik Ulbricht, Klaus Hornberger, Jens Tuexen, Marcel Mayor, Markus Arndt Kapitza-Dirac-Talbot-Lau interferometry (KDTLI) has recently been established as an ideal method to perform quantum matter wave experiments with large, highly polarizable molecules in an unprecedented mass range of beyond 1000 atomic mass units [1]. Since the interference visibility reveals important information on the properties of the examined particles, such as their mass and polarizability, we identified KDTLI as a valuable tool for precision metrology. We demonstrate that quantum interferometry can therefore also serve as a powerful complement to mass spectrometry [2], in particular in cases where fragmentation may occur in the detector. Our new method is applicable to a wide range of molecules and is particularly valuable for characterizing neutral molecular beams.\\ ~[1] S. Gerlich, L. Hackerm\"uller, K. Hornberger, A. Stibor, H. Ulbricht, M. Gring, F. Goldfarb, T. Savas, M. M\"uri, M. Mayor, M. Arndt, Nat. Phys. 2007, 3, 711 – 715.\\ ~[2] Stefan Gerlich, Michael Gring, Hendrik Ulbricht, Klaus Hornberger, Jens T\"uxen, Marcel Mayor, and Markus Arndt, Angew. Chem. Int. Ed. 2008, 47, 6195 - 6198. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H16.00002: Poisson's Spot with Molecules Thomas Reisinger, Amil Patel, Herbert Reingruber, Katrin Fladischer, Wolfgang E. Ernst, Gianangelo Bracco, Henry I. Smith, Bodil Holst In the Poisson-Spot experiment, waves emanating from a source are blocked by a circular obstacle. Due to their positive on-axis interference an image of the source (the Poisson spot) is observed within the geometrical shadow of the obstacle. The Poisson spot is the last of the classical optics experiments to be realized with neutral matter waves. In this paper we report the observation of Poisson's Spot using a beam of neutral deuterium molecules. The wavelength-independence and the weak constraints on angular alignment and position of the circular obstacle make Poisson's spot a promising candidate for applications ranging from the study of large-molecule diffraction and coherence in atom-lasers to patterning with large molecules. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H16.00003: The Two-Mode Approximation in a Realistic Bose-Josephson Junction in a $^{85}$Rb/$^{87}$Rb BEC Mixture Jeffrey Heward, Mark Edwards, Charles Clark We have studied the behavior of an experimentally realistic Bose-Einstein condensate (BEC) mixture subjected to a double-well potential. The mixture studied consists of $^{87}$Rb and $^{85}$Rb held in an optical trap with an external magnetic field that enables tuning of the $^{85}$Rb--$^{85}$Rb scattering length. This system, without the external double--well potential, has been implemented at JILA [S.B.\ Papp, et al, Phys.Rev.Lett.\ {\bf 101}, 040402 (2008)]. A double--well potential can be added to this system by applying another pair of lasers as was done in a previous experiment for single condensates [M.\ Albiez, et al, Phys.\ Rev.\ Lett.\ {\bf 95}, 010402 (2005)]. We have used the Variable Tunneling Model (VTM) within the two--mode approximation to search for novel condensate mixture behavior in this experimentally accessible system. Possible behaviors include Bose--Josephson oscillations with both swapping and non--swapping modes and macroscopic quantum self--trapping with zero and pi modes as described in a recent paper [I.\ Satija, et al, arXiv:0811.1921v1 [quant-ph]]. We compare the behavior as predicted by the two--mode VTM with the solution obtained by integrating the coupled Gross--Pitaevskii equations. We propose some new experiments designed to observe these novel phenomena. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H16.00004: Spin-orbit coupled Bose-Einstein condensates Brandon Anderson, Tudor Stanescu, Victor Galitski We consider a Bose-Einstein condensate (BEC) of cold atoms with an internal pseudo-spin-1/2 degree of freedom that is coupled to momentum. The pseudo-spin degree of freedom emerges from the trapped multi-level atoms moving in the presence of spatially modulated laser fields. Within a so-called tripod scheme, the atom-laser interaction generates a pair of degenerate dark states. Upon adiabatically projecting onto the dark states subspace, an effective Hamiltonian emerges with a spin-orbit coupled pseudo-spin-1/2 degree of freedom. For a symmetric, Rashba-type spin orbit interaction the ground state of the pseudo-spin space is continuously degenerate along a circle in momentum space and may lead to many-body states with nontrivial topological properties. We investigate the Rashba-type spin-orbit BEC in the presence of weak density-density interactions. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H16.00005: Spin Squeezing in Spinor Condensates Sabrina Leslie, Jay Sau, Marvin Cohen, Dan Stamper-Kurn Spin squeezing in spinor condensates enables the control of quantum spin fluctuations in a fascinating multi-mode system. Further, it provides a coherent spin system characterized by sub-shot-noise spin fluctuations, with applications towards sensitive spatially-resolved magnetometry. With the application of appropriate unitary transformations, we show that one may manipulate the spin fluctuation modes atop an arbitrary F=1 coherent state, and in so doing prepare an arbitrary F=1 spin squeezed state. Taking into account experimental limitations to spin squeezing such as atom loss and nonlinear interactions in the condensate, we find that one may achieve roughly 17 dB of spin squeezing in the single mode regime and 10 dB of spin squeezing in the multi mode regime, for reasonable experimental parameters. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H16.00006: Squeezing and entanglement in a Bose-Einstein condensate Christian Gross, Jerome Esteve, Stefano Giovanazzi, Andreas Weller, Markus Oberthaler We report on the observation of spin squeezing and entanglement in a Bose-Einstein condensate trapped in double well and periodic potential [1]. The measurement of two conjugate variables - atom number difference and relative phase between adjacent sites - allows a direct connection to the presence of entanglement. The observations indeed confirm that entanglement is present even at finite temperature. The observed coherent spin squeezing of 3.8 dB implies that a usable quantum resource has been generated which is directly applicable to overcome the standard quantum limit of atom interferometry. The limitations due to experimental imperfections, finite temperature and three body loss will be discussed in detail. Latest results on spin squeezing using hamiltonian dynamics of internal states are presented. [1] J. Esteve, C. Gross, A. Weller, S. Giovanazzi and M. K. Oberthaler: Nature 455, 1216-1219 [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H16.00007: Creation of resilient entangled states and a resource for measurement-based quantum computation with optical superlattices Andreas Nunnenkamp, Benoit Vaucher, Dieter Jaksch We investigate how to create entangled states with ultracold bosonic atoms trapped in optical lattices by dynamical manipulation of the shape of the lattice potential. We consider a period-two superlattice that allows both the splitting of each site into a double-well potential and also the variation of the height of the potential barrier between the sites. We show how to use this array of double-well potentials to entangle neighboring qubits encoded on the Zeeman levels of the atoms, without using the different vibrational states of the atoms. Finally, we present a method of realizing a Bell-pair encoded cluster state, a resource for measurement-based quantum computing which remains resilient to collective dephasing noise throughout the computation [NJP 10, 023005 (2008)]. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H16.00008: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H16.00009: Numerical Model of Polariton Dynamics in~GaAs Quantum Well-Microcavity Structures Vincent Hartwell, David Snoke, Ryan Balili Recent experimental results from GaAs quantum well-microcavity structures show evidence for Bose-Einstein condensation of polaritons. A main indicator of this is a large accumulation of polaritons near k=0 above a critical density threshold. The polariton gas is never in complete equilibrium, however. To model the system, we therefore cannot use an equilibrium model for the momentum distribution; instead, we have developed numerical methods for solving the quantum Boltzmann equation for the polariton momentum distribution, including the effects of polariton-polariton scattering, polariton-phonon scattering, and polariton scattering with free electrons. The model allows direct comparison to experimental results. An unexpected experimental result which we address in our modeling is that at very low densities, the polariton momentum distribution is invariant and independent of density. At high densities, our numerical model predicts the accumulation of polaritons at k=0 is indeed the result of the boson statistics of the particles, and though it is not in complete equilibrium, it has the characteristics of a ``quasicondensate.'' [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H16.00010: Single vortex-antivortex pair in an exciton polariton condensate Georgios Roumpos, Sven Hoefling, Alfred Forchel, Yoshihisa Yamamoto We report the observation of a single vortex-antivortex pair in a two-dimensional exciton polariton condensate. The pairs are evidenced in the time-integrated phase maps acquired using Michelson interferometry. The striped pattern of the sample disorder potential prevents rotation of the pairs, but allows their translational movement. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H16.00011: Rabi oscillations in semiconductor multiwave response Mikhail Erementchouk, Michael Leuenberger We study the semiconductor response with respect to high intensity resonant excitation on short time scale when the contribution of the Fermi statistics of the electrons and holes prevails. Both the single and double pulse excitations are considered in the framework of asymptotically exact description. For the double pulse excitation we consider the time evolution of the multiwave mixing (MWM) exciton polarization. The main difference between the excitation by a single pulse or by two non-collinear pulses is that the Rabi oscillations of the MWM response are characterized by two harmonics. The operator dynamics governed by the external excitation exhibits three invariant spin classes, which do not mix with the evolution of the system. Two classes correspond to the bright exciton states and one contains all dark states. The dynamics of the classes turn is described by six characteristic frequencies and the Rabi frequencies (RF) are only two of them (one for each bright class). We show that if initially the system is in the ground state then the semiconductor Bloch equation preserves the invariant spin classes thus proving absence of the dark excitons in the framework of this description. We found that due to the mass difference between holes of different kind two additional RF's, presenting in the operator dynamics, should appear in the evolution of the exciton polarization. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H16.00012: Single-atom trajectories of intermittent fluorescence with quantum jumps Patrick Coles, Robert Griffiths Understanding the trajectories of single quantum systems is a modern theoretical challenge, given that experiments are no longer restricted to ensemble-averaged dynamics. Here, we present a model for a single 3-level atom driven resonantly on two transitions, resulting in intermittent fluorescence from one transition. The consistent histories formalism provides insight into the intermittent ``shelving'' process and predicts the distribution of dark periods for a given trajectory. We further predict quantum interference that leads to oscillations in the quantum-jump probability and discuss the possibility of observing these oscillations experimentally. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H16.00013: Dynamics and fidelity of entanglement of photons coupled to optical fibers and waveguides Bereket Berhane We investigate the time evolution of the quantum mechanical states of photon-modes in optical fibers and waveguides starting from a multipolar coupling of neutral atoms with a quantized electromagnetic field. We apply our results to the propagation of entangled photons in optical fibers and investigate the fidelity of entanglement for various optical fiber lengths. Furthermore, we obtain a quantum input-output formalism in three dimensions and investigate the dependence of coupling and transfer efficiency on the quantum state of the incident photons. We apply our results to the coupling and propagation of entangled photons for applications in quantum memory transfer over optical fibers and waveguides. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H16.00014: Quantum fluctuations in small lasers Kaushik Roy Choudhury, Stephan Haas, A.F.J Levi Master equations are used to demonstrate the dominant role of quantum fluctuations in determining the steady-state and transient response of a laser when there is a small number of particles in the system. In this regime, quantum fluctuations are found to suppress the lasing threshold and create a non-Poisson probability distribution for $n$ discrete excited electronic states and $s$ discrete photons. The correlation between $n$ and $s$ damps the averaged dynamic response of laser emission. Random walk calculations verify the master equation predictions and are used to connect to systems containing larger numbers of particles. [Preview Abstract] |
Session H17: Focus Session: Semiconducting Qubit Approaches
Sponsoring Units: GQIChair: Jeremy Levy, University of Pittsburgh
Room: 318
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H17.00001: Solid state quantum memory using 31P spins in silicon: advantages of hybrid qubit systems Invited Speaker: |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H17.00002: Radio-frequency single-electron transistor coupled to a few-electron double quantum dot Feng Pan, Joel Sttentheim, Mustafa Bal, Mingyun Yuan, Alex Rimberg, Vladimir Umansky The radio frequency single-electron transistor (rf-SET) has been shown to be an ultra fast and highly sensitive electrometer, and can be potentially operated close to the quantum noise limit as a qubit readout device [1]. The interplay between the rf-SET electrometer and a two-level system offers an interesting system for study. Here we report our progress on investigating rf-SETs capacitively coupled to few-electron double quantum dots (DQDs). We fabricate lateral-defined DQDs from an AlGaAs/GaAs heterostructure and the rf-SET from superconducting aluminum embedded in a tank circuit. The sensitivity and bandwidth of on-chip rf-SET electrometer can be used to probe DQD operated in the few-electron regime. We have observed coupling between SET and DQD and have optimized our device design to enhance coupling in the few-electron limit. Recent experimental results will be discussed. [1] M. H. Devoret and R. J. Schoelkopf, Nature, \textbf{406}, 1039 (2000). [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H17.00003: 3D Acoustic Modes, Shot Noise and Strain Displacements in a Radio Frequency Quantum Point Contact J. Stettenheim, M. Thalakulam, F. Pan, M. Bal, L.N. Pfeiffer, K.W. West, A.J. Rimberg As previously reported, our broadband frequency resolved measurements of shot noise in a radio frequency QPC (RF-QPC) reveal a remarkable frequency dependence absent from theoretical predictions. Based on piezoelectric coupling in GaAs, our data suggest a feedback loop in which shot noise drives resonant acoustic vibrations that in turn create correlations in electron tunneling. The feedback concentrates the initially white noise in a narrow band around the sample's resonant frequency, allowing shot noise spectrum engineering. We solve for the 3D acoustic modes of our samples, finding close correspondence with measured frequencies. We have determined that the geometry and magnitude of the polarization field selects the acoustic mode excited. As polarization fields and strain displacements are linked in GaAs, we estimate the ultimate mechanical displacement sensitivity of our RF-QPC. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H17.00004: Radio-frequency quantum point contact in a silicon/silicon-germanium two-dimensional electron system Madhu Thalakulam, Christie Simmons, Eric Sackmann, Bjorn Van Bael, D.E. Savage, Max Lagally, M.A. Eriksson Radio frequency quantum point contacts (RF{\_}QPC) are sensitive and fast electrometers. The capability to integrate such devices with semiconductor-based quantum dot systems makes them an attractive candidate for fast charge readout, and the increasing interest in spins in group-IV quantum dots motivates the development of such devices in silicon/silicon-germanium two-dimensional electron systems. We report the operation of an RF-QPC fabricated on a silicon/silicon-germanium heterostructure with an on-chip matching network. An on-chip spiral inductor and the capacitance from the bonding pads define the tank circuit. The inductance and capacitance parameters are optimized to achieve a resonant frequency of approximately one GHz. The operation of RF-QPC at milli-Kelvin temperature and the charge readout of a quantum dot system using an RF-QPC will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H17.00005: Observation of tunnel rates of phosphorus dopants using silicon SETs H. Huebl, C.D. Nugroho, A. Morello, C. Escott, A.S. Dzurak, R.G. Clark, C. Yang, J.V. Donkelaar, A. Alves, D. Jamieson, M.A. Eriksson Charge centres, such as donors in semiconductors, have significant potential for quantum information processing. In silicon, which can be produced nuclear-spin free, phosphorus donors are a prime candidate for implementation of a qubit, due to their long spin coherence times. In this presentation we will discuss a hybrid structure, consisting of implanted phosphorus donors controlled by a gate potential in close vicinity to a gate-induced, MOS-based silicon single electron transistor (Si-SET). We study the dual functionality of the nearby Si-SET as a sensitive charge detector as well as a gate-induced electron reservoir. Experimentally, we observe shifts in the position of the Coulomb peaks of the Si-SET corresponding to $\sim$20\% of an electron charge. We attribute these shifts to charge transfers between the Si-SET island reservoir and the nearby phosphorus donors. Pulsed voltage spectroscopy on one of these charge transitions allows us to investigate the capture and emission times of a donor resulting in a capture rate of 3000 s-1 and an emission rate of 1000 s-1 corroborating expectations from device modelling. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H17.00006: Self-Correcting Dynamic Decoupling Pulse Sequences Alexei M. Tyryshkin, Stephen A. Lyon, Wenxian Zhang, Viatcheslav V. Dobrovitski Dynamic decoupling (DD) techniques employ a series of strong refocusing pulses to combat decoherence in quantum systems. However, each DD pulse is imperfect and thus introduces a small instrumental error; the error can accumulate rapidly upon applying many DD pulses, and this error can destroy the quantum state. We have examined several DD pulse sequences, including CPMG, XZXZ, XYXY and their concatenated variants, using electron spin resonance (ESR) of donors electron spins in silicon. While all these DD sequences performed comparably in cancelling the phase noise arising from magnetic field fluctuations, only one sequence (XYXY) demonstrated the ability to protect an arbitrary coherent state, including X, Y, and Z states in the rotating frame. The other sequences (CPMG and XZXZ) were able to store only one state (Y) while destroying other states (X and Z). The superior performance of XYXY arises from its internal ability to correct for pulse amplitude errors, the dominant error in these ESR experiments. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H17.00007: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H17.00008: Hyperfine tolerant triplet-singlet qubit in rotating double quantum dots David Drummond, Leonid P. Pryadko, Kirill Shtengel We examine the triplet-singlet state of pairs of GaAs single electron quantum dots, in which the singlet and m=0 triplet represent the logical 0 and 1. Each electron's spin feels a different magnetic field from the hyperfine coupling to the local nuclei of the GaAs, leading to a relative phase difference between the two electrons, and thus decoherence. Previous methods have delayed decoherence using complicated electrical pulse schemes or nuclear polarization with Overhauser fields, which can only be maintained for short times and require great energy. Instead we propose to eliminate hyperfine decoherence by repeatedly rotating the dots adiabatically, exposing the electrons to the same average magnetic field. In addition, we show that if these rotations are performed repeatedly in a symmetric fashion, the Dresselhaus and Rashba spin-orbit couplings are significantly suppressed. The construction of such a device might also be of interest to topological quantum computers, which depend on similar rotations. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H17.00009: Multiple Nuclear Polarization States in a Double Quantum Dot Jeroen Danon In a double quantum dot under conditions of electron paramagnetic resonance we have observed multiple stable states of nuclear polarization and also switching between those states. The system exhibited strong hysteretic behavior over a large range of magnetic fields, indicating the dynamical buildup of effective nuclear magnetic fields up to 150 mT. We have explained these findings in the framework of an elaborated theoretical model. The results reported enable applications of this nuclear polarization effect, including manipulation and control of the nuclear fields and possible use of this for improving the electron spin coherence time. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H17.00010: Electron spin dephasing by hyperfine-mediated interactions in a nuclear spin bath Lukasz Cywinski, Wayne M. Witzel, Sankar Das Sarma We investigate pure dephasing decoherence (free induction decay and spin echo) of a quantum dot spin qubit interacting with a nuclear spin bath. While for infinite magnetic field $B$ the only decoherence mechanism is spectral diffusion due to dipolar flip-flops of nuclear spins, with decreasing $B$ the hyperfine-mediated interactions between the nuclear spins become important. We give a theory [1] of decoherence due to these interactions which takes advantage of their long range nature. For a thermal uncorrelated bath we show that our theory is applicable down to $B\sim$10 mT, allowing for comparison with recent experiments on spin echo in GaAs quantum dots [2].\\ {[1]} L. Cywinski, W.M. Witzel, and S. Das Sarma, preprint arXiv:0809:0003 (2008).\\ {[2]} F.H.L. Koppens, K.C. Nowack, and L.M.K. Vandersypen, Phys. Rev. Lett. \textbf{100}, 236802 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H17.00011: Nuclear State Preparation via Landau-Zener-St\"uckelberg transitions in Double Quantum Dots Hugo Ribeiro, Guido Burkard We theoretically model a nuclear-state preparation scheme that increases the coherence time of a two-spin qubit in a double quantum dot. The two-electron system is tuned repeatedly across a singlet-triplet level-anticrossing with alternating slow and rapid sweeps of an external bias voltage. Using a Landau-Zener-St\"uckelberg model, we find that in addition to a small nuclear polarization that weakly affects the electron spin coherence, the slow sweeps are only partially adiabatic and lead to a weak nuclear spin measurement and a nuclear-state narrowing which prolongs the electron spin coherence. Based on our description of the weak measurement, we simulate a system with up to n=200 nuclear spins per dot and qualitatively explain recent experimental findings. Scaling in n indicates a stronger effect for larger n, also in qualitative agreement with experiments. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H17.00012: Electromagnetic fluctuations as a source of decoherence for double quantum dot charge-based qubits Diego Valente, Frank Wilhelm, Eduardo Mucciolo Solid-state quantum dots are strong candidates for the physical realization of qubits. They present the ubiquitous advantage of easier scalability, but also couple rather effectively to external degrees of freedom which lead to decoherence phenomena. One such source of decoherence lays in the electromagnetic fluctuations occurring in the circuits utilized for preparation and measurement of these qubits. Here we investigate these sources of decoherence in double quantum dot charge-based qubit systems. We use effective circuit models and estimates of time correlations of such fluctuations to calculate the energy ($T_1$) and phase ($T_2$) relaxation times introduced into the qubit system. We also present ideas on how to suppress some of the destructive effects of these fluctuations and increase the quality factor for quantum oscillations. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H17.00013: Probing the spin structure of $\nu=2$ quantum Hall fluid around an antidot Lee Bassett, Chris Ford, Nigel Cooper, Jonathan Griffiths, David Anderson, Ian Farrer, Geb Jones, Dave Ritchie We experimentally investigate spin and charge excitations in a small closed edge of integer quantum Hall fluid encircling a nano-scale potential island, or antidot (AD), in a two dimensional electron system. Using quantum point contacts to inject and detect spin-polarized currents via edge states, we have measured spin-resolved transport through single ADs at filling factor two. At relatively low magnetic fields ($\approx 1$T), tunneling between the AD states and higher Landau levels in the bulk produces pairs of Coulomb blockade peaks in conductance above the $2e^2/h$ plateau. These transmission resonances were thought to result from spin-polarized tunneling through individual single-particle AD states (of alternating spin), but our experiments show that, while spin is generally conserved during transport, the tunneling current is not spin-polarized. We interpret these results as signatures of interactions within the AD which result in a separation of the energy scales associated with spin and charge excitations. [Preview Abstract] |
Session H18: Block Copolymer Thin Films I
Sponsoring Units: DPOLYChair: Kevin Yager, National Institute of Standards and Technology, Polymers
Room: 319
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H18.00001: Polymer Physics Prize Symposium Break
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Tuesday, March 17, 2009 8:36AM - 8:48AM |
H18.00002: Analysis of block-copolymer thin film ordering through a moving thermal zone Kevin Yager, Nathaniel Fredin, Ronald Jones Block-copolymer thin films self-assemble into well-defined structures at the nanometer lengthscale. It has been shown that the morphology, orientation, and degree of order resulting from assembly is sensitive to a variety of preparation parameters, including annealing time and temperature, solvent exposure, substrate surface energy, application of electric fields, etc. A moving thermal zone can also strongly affect the ordering. We have shown that relatively ``cold'' zone annealing (CZA) conditions (above the glass-transition but well below the disordering temperature) can induce a preferential orientation of the microdomains in thin films. We further analyze this effect by measuring the ordering through the thermal front, using atomic force microscopy and scattering techniques (reflectivity and GI-SAXS), which are combined to quantify the order and 3D orientational distribution. Zone annealing leads to increased grain sizes and substantially faster coarsening kinetics, as compared to oven annealing. Moreover, the evolution of order through the thermal front constrains models which aim to explain the CZA's ability to induce orientational bias. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H18.00003: Neutral Parameter Window for Perpendicularly Oriented Block Copolymer Resists Deposited on Organosilicate Substrates with Tunable Surface Energy Hyoseon Suh, Kookheon Char, Huiman Kang, Paul F. Nealey Balancing the interfacial interactions of a block copolymer (BCP) with a substrate as well as the free surface can induce the perpendicular orientation of microdomains, allowing the BCP films to serve as templates for nanofabrication. However, it is known that such orientation of microdomains is quite sensitive to the film thickness. In this presentation, we investigated the effect of film thickness on the orientation of microdomains in lamellae-forming P(S-$b$-MMA) thin films placed on thermally cured organosilicate (OS) substrates. For the film thickness of a P(S-$b$-MMA) ranging from 1 L$_{0}$ up to 2.5 L$_{0}$, we varied the surface energy of the OS substrate around the value close to the enthalpically neutral condition by controlling the substrate cure temperature. We will demonstrate the origin of the observed thickness effect by taking into account the increase in surface area at the free surface when P(S-$b$-MMA) films make holes or islands depending on the incommensurable conditions of the BCP film. This analysis allows us to define a more accurate neutral window for the P(S-$b$-MMA) in terms of both substrate surface energy and BCP film thickness. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H18.00004: Effect of Surface Energetics on Block Copolymer Thin Film Phase Behavior Julie Lawson, Michael Baney, Thomas Epps The development of block copolymer materials for future nanotechnologies requires an understanding of how surface energetics affect block copolymer thin film phase behavior. In this work, we use combinatorial methods to study these effects and to identify transitions in thin film phase behavior and microstructure orientation. Surface energy gradients were created using a vapor deposition technique developed by our group in which cross-diffusion of functionalized chlorosilanes under dynamic vacuum results in a linear gradient in surface energy on a silicon substrate. These gradients were characterized using x-ray photoelectron spectroscopy (XPS) and contact angle measurements. We then cast a thin film of block copolymer on the modified substrates using a flow coating technique. Finally, we used thermal and solvent annealing conditions to affect the surface energy at the free surface. The surface morphology of the films was examined with atomic force microscopy (AFM), and morphological changes across the gradient were found. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H18.00005: Systematic tunability of self-assembled block copolymer patterns Yeon Sik Jung, Caroline Ross The morphology and length scale of diblock copolymers (BCPs) are determined by the chain lengths, and therefore to obtain different geometries and feature sizes, polymers with different chain lengths or BCP/homopolymer blends have been employed. Here, we report on the solvent vapor induced tunability of pattern dimension and morphology of thin films of polystyrene-polydimethylsiloxane (PS-$b$-PDMS) BCPs, which provide robust patterns with exceptionally good ordering due to their large interaction parameter.$^{[}$\footnote{ Y. S. Jung \textit{et al.}, Nano Letters 7, pp.~2046-2050 (2007); Science 321, pp. 939 - 943 (2008); Nano Letters 8, pp. 2975-2981 (2008)\par }$^{]}$ Vapor pressure can control the interfacial interaction between the two blocks, and a mixed solvent can manipulate the effective volume fraction of each block. We show both coupled and independent control of the microdomain size and the periodicity by changing the vapor pressure and the mixing ratio of a selective (heptane) and a partially selective (toluene) solvent. We also demonstrate the transformations from spheres to cylinders and from cylinders to perforated lamellar structures by increasing the portion of selective solvent in the vapor. These results are supported by a theoretical model. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H18.00006: Solvent annealing of Micropatterned PS-b-PEO copolymer films Tae Hee Kim, Himadri Acharya, Hee June Joeng, Cheolmin Park Solvent annealing of block copolymer thin films have been known as an effective way to control both orientation of microdomains with respect to the surface and their registration into a well ordered periodic lattice structure. We have recently demonstrated hierarchically ordered microdomains in a thin poly(styrene-b-ethylene oxide)(PS-b-PEO) film combined with microcontact printing. The solvent annealing gave rise to well ordered spherical PEO microdomains in large area by the confined dewetting of thin PS-b-PEO films which had been micropatterned on chemically modified surface during solvent annealing. In this presentation, we intentionally prepare a micropatterned dewet film of PS-b-PEO by spincoating a block copolymer solution on a topographic PDMS pre-pattern. Convex lens shaped spherical caps of PS-b-PEO individually located on each PDMS mesa were successfully transferred to a Si substrate by a conventional transfer printing technique. We investigate the effect of solvent on not only film wettability but also formation of hierarchical nanostructures. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H18.00007: Transition Behavior of Block Copolymer Thin Films Du Yeol Ryu, Changhak Shin, Hyungju Ahn, June Huh, Kwang-Woo Kim, Thomas Russell The phase transitions in block copolymers (BCPs), like the order-to-disorder transition, occur when the enthalpic term of free energy of mixing is equal to the entropic term. In thin films, interactions at the substrate/polymer and polymer/air interfaces influence this free energy balance, resulting in a change in the transition behavior. Here, we report on the transition behavior of BCP thin films. The thickness dependence of the transition temperature shows that interfacial interactions enhance the orientation of the lamellar microdomain parallel to the film surface even in 40L0 in thickness, where L0 is the equilibrium period of the BCP in the bulk. In thin film geometry, this phenomenon can be attributed to the fact that a preferential interaction of one component with the substrate leads to an amplification of a periodic variation in the composition and a shift of transition temperature. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H18.00008: Thin Film Morphology of Diblock and Triblock Copolymers with Bulk Order-Order Transitions (OOT) K.E. Sohn, R.C. Coffin, G.C. Bazan, E.J. Kramer, K. Kojio, B.C. Berry, A. Karim, M. Sprung, J. Wang The thin film morphology of SEB and SEBS block copolymers that have an OOT in the bulk from cylinders to spheres as the annealing temperature is increased was studied as a function increasing film thickness using AFM and GISAXS. For both SEB and SEBS, the morphology is the same no matter if the film is annealed above or below the bulk OOT. The SEB morphology is governed by the free energy penalty due to chain stretching, showing spheres when the film thickness is less than that of a monolayer of cylinders. The cylindrical morphology dominates when the film thickness is larger than that of a monolayer of cylinders. On the other hand, the SEBS morphology is governed by the free energy penalty due to looping of the midblock at the surface. Spheres require a lower fraction of midblocks to loop at the surface than cylinders, therefore spheres pay a lower free energy penalty due to chain looping and were found for all film thicknesses studied (up to $\sim$100nm). [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H18.00009: UV-Induced Order-to-Order Transition (OOT) in Thin Films of Supramolecular Diblock Copolymer Assemblies Containing 2-(4'-Hydroxyphenylazo)benzoic Acid Wei Chen, Jia-Yu Wang, Xinyu Wei, Anna Balazs, Thomas Russell Long-range lateral ordering and orientation in block copolymer thin films, which are highly desired for the applications requiring addressability, as in magnetic storage, may be obtained in a controlled way via an order-to-order transition (OOT), i.e. a morphological transition in a microphase-separated system. The photoisomerization of azobenzene results in volume changes that, when integrated into copolymers, can bring about phase transitions that, in turn, by sweeping the light across a surface, will promote long-range lateral ordering, similar to zone-refinement process used to produce large single crystals. We investigated UV-induced OOT in the supramolecule-assembled thin films of 2-(4-hydroxyphenylazo)benzoic acid and polystyrene-\textit{block}-poly(2-vinylpyridine) diblock copolymer. Grazing incidence small angle X-ray scattering demonstrated that phase transition from lamellae to hexagonally packed cylinders occurred at 150 $^{\circ}$C after UV radiation for 1 hour due to a significantly enhanced interfacial fluctuations induced by photoisomerization as evidenced by X-ray Reflectivity. This suggested that UV light can be utilized to control OOT in the supramolecule-assembled thin films and, hence, to fabricate long-range ordered nanostructures, and even smart responsive surfaces. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H18.00010: Dilute Micelle Arrays in Block Copolymer Thin Films John Papalia, Richard Register, Douglas Adamson, Paul Chaikin Thin films of sphere-forming block copolymers are attractive templates for surface patterning and nanofabrication. While the areal density of spheres (micelles) can be adjusted through the diblock's molecular weight, sparse micelle arrays are quite difficult to achieve. Instead, we blend the diblock with matrix homopolymer in the ``dry brush'' regime, which eliminates the ``terracing'' (island/hole formation) present in films of the neat diblock. Furthermore, by choosing a system where the sphere-forming block wets the substrate and/or free surface, we can achieve very sparse micelle arrays without correspondingly extensive homopolymer dilution, by using the film thickness as the control parameter rather than the blend ratio. Specifically, we employ a polystyrene-polyisoprene diblock (PS/PI blocks of 68/12 kg/mol), blended with PS homopolymer; the PI block wets both the free surface and the SiOx substrate. For sufficiently thin films ($<$60 nm for 50 wt{\%} homoPS), all the block copolymer goes to form brush-like layers at the two surfaces, yielding no micelles. For thicker films, sufficient block copolymer remains to form spherical microdomains between the brushes; the areal density of micelles can be continuously tuned via the film thickness. We evaluate this approach by preparing a film with a thickness gradient, and apply a simple model to the measured areal densities of micelles. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H18.00011: Surface Dynamics of Segregation Layer in Blockcopolymer Films Sanghoon Song, Wonsuk Cha, Zhang Jiang, Suresh Narayanan, Adrian Ruehm, Sunil K. Sinha, Hyunjung Kim We have investigated the surface dynamics of supported block copolymer films of poly(styrene)-b-poly(dimethylsiloxane) (PS-b- PDMS) in the spherical phase, i.e., PDMS cores surrounded by PS shells by x-ray photon correlation spectroscopy (XPCS) in grazing angle geometry. The experiment was performed at the beamline 8ID-I in Advanced Photon Source. We found that the PDMS-rich layer near the surface appears at the temperature higher than the glass transition temperature. We applied the modified theory for bilayer model with surface capillary waves on simple viscoelastic liquid films. The viscosity obtained in this study is compared with that from the rheology measurement for bulk. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H18.00012: Block Copolymer Brushes Mark Matsen Using self-consistent field theory (SCFT), we examine dry brushes of AB diblock copolymer, where the B ends are uniformly grafted to a planar substrate. Four different morphologies are predicted, which are conveniently described as the uniform, stripe, hexagonal, and inverted hexagonal phases on the basis of the patterned formed at the air surface by the A-rich domain. Phase diagrams are calculated for different grafting densities and for different A-segment surface affinities. In contrast to unanchored diblock-copolymer films, the brush system has a much greater tendency to form chemically-patterned surfaces. [Preview Abstract] |
Session H19: Solid Amorphous Polymers
Sponsoring Units: DPOLYChair: Sergei Nazarenko, University of Southern Mississippi
Room: 320
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H19.00001: Polymer Physics Prize Symposium Break
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Tuesday, March 17, 2009 8:36AM - 8:48AM |
H19.00002: Paradigms for the Glass Transition Gregory McKenna Several paradigms of the glass transition are commonly used to either characterize the behavior of glass-forming materials or as ``targets'' for theoretical outcomes. The purpose of the work and discussion was motivated by a view that some of the paradigms currently used to frame the glass transition event, while potentially useful, may also have limitations that we often do not fully consider. Discussion focuses on isochoric glass formation paths, thermodynamic and dynamic fragilities and how dynamic fragility in many systems (especially polymers, metals, ionic liquids and hydrogen bonding systems) seems to vary primarily with the glass transition temperature, Tg, itself. This leads to the conclusion that such systems have an apparent activation energy that varies as the square of the glass temperature and consequently a fragility index m that varies linearly in Tg. The work concludes with a presentation of evidence that the apparent super Arrhenius divergence of viscosity or relaxation time as temperature decreases towards the Tg is precarious, suggesting that the common expectations of a diverging relaxation time or viscosity as the glass temperature is approached are not met. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H19.00003: Twinkling Fractal Theory of the Glass Transition: Applications and Insights Richard Wool The new perspective on the Glass Transition of amorphous materials offered by theTwinkling Fractal Theory (TFT). [R. P. Wool, J. Polym. Sci, Part B: Polym Phys. 46, 2765 (2008)] is examined in several applications. The TFT describes Tg in terms of the autocorrelation relaxation function for the spatio-temporal solid-liquid fluctuations which are related to the vibrational frequencies (``twinkles'') described by the Orbach vibrational density of states for a fractal. The twinkling frequencies for solid-liquid interchange are due to Boltzmann energy populations of interatomic oscillators interacting through anharmonic potentials U(x) with energy D$_{o}$ of order 1-5 kcal/mol. T$_{g}$ occurs when the activation energy for the solid-liquid transition goes to zero at the inflection point of U(x) and is given by T$_{g }$= 2D$_{o}$/9k. The applications include: (a) group contributions to Do, (b) the rate and temperature dependence of yielding and fracture, (c) shear thickening fluids, (d) rate dependence of dynamical mechanical properties, particularly the tan delta damping peak used to measure Tg, (e) derivation of the empirical WLF time-temperature superposition empirical relation, (f) thermal expansion and (g) physical aging. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H19.00004: Cooperativity and fragility in glass forming systems: not a simple relationship Liang Hong, Alexander Kisliuk, Alexei Sokolov Understanding the sharp increase of the main structural relaxation time $\tau $ on approaching the glass transition temperature (Tg) remains a great challenge. Traditionally this relaxation is considered as a cooperative process, with larger cooperativity leading to a steeper temperature dependence of $\tau $ around Tg, i.e. higher fragility. On the other hand, the boson peak, a collective vibration in the pico-second time region, is also described as a cooperative motion. In this study we estimate the structural correlation length for various glass forming systems from the collective vibration. The obtained values are in good agreement with the dynamic heterogeneity length estimated by 4 dimensional NMR for the main structural relaxation. Thus the two different motions appear to have very similar length scale for cooperativity. Direct comparison of cooperativity to fragility reveals no correlation. However, we discover that cooperativity correlates with the pure volume contribution to fragility. This result explains why many earlier attempts to find direct relationship between fragility and cooperativity fail. A possible origin for the observed correlation is discussed. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H19.00005: Molecular Interpretations of Observed non-Fickian Penetrant Transport Behavior in Glassy Polymers Adam Ekenseair, Richard Ketcham, Nicholas Peppas The relative rates of the diffusional and relaxational processes during the absorption of penetrant molecules in glassy polymers determine the nature of the transport process and lead to Fickian, Case II, and anomalous absorption behavior. While previous models account for anomalous behavior, there is still a disconnect between theory and experiment, as data must be fit to the model with previously determined independent parameters. With trends leading to smaller device scales and increasingly complex polymer structures, there is a need for a quantitative understanding of the manner in which a polymer's network structure alters both the rate and the mode of penetrant transport. To this end, the effects of the basic network parameters of PMMA, including the degree of crosslinking, polymer mesh size, and the crosslink size, on the integral sorption of methanol were studied utilizing both gravimetric data and \textit{in-situ} ultra-high-resolution X-ray computed tomography studies. The effects of sub-Tg annealing/aging, temperature, and the presence of un-reacted monomer were also investigated. Controlling the relative timescale of the relaxational process by altering the polymer network structure is shown to directly influence the Case II front propagation velocity and the nature of the observed transport behavior. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H19.00006: On the Nature of Gas Transport of Ethylene Vinyl Alcohol Copolymers Sergei Nazarenko, Justin Brandt, Brian Olson, Alexander Jamieson Historically, all the approaches describing gas diffusion in polymers can be roughly divided in two categories, based on free volume models and the activation molecular models, which take into account the cooperative penetrant-polymer chain motions, chain rigidity and intermolecular forces. Although gas transport characteristics exhibit a general correlation with free volume, alone free volume can not adequately describe gas barrier. The chain rigidity and the strength of intermolecular interactions are two additional important factors which are manifested via activation energy. The main objective of this work was to develop fundamental understanding of oxygen transport in a broad range of EVOH copolymers as it is related to free volume characteristics probed by positron annihilation lifetime spectroscopy and hydrogen bonding interaction. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H19.00007: Photochemical Crosslinking of Preformed Glassy and Amorphous Polymers Through bis-Benzophenone Mediated Covalent Bridging Nicholas Carbone, Mary Dickson, Jeffrey Lancaster, Greg Carroll, Jeffrey Koberstein We show that bis-benzophenone (bis-BP) is an effective method to photochemically crosslink essentially any solvent-free glassy or amorphous preformed polymer system that contains abstractable hydrogen atoms. When bis-BP is mixed into a polymer and exposed to UV radiation, it abstracts hydrogen atoms from any chains in proximity, thereby initiating a cascade of free radical reactions that include several mechanisms that can lead to covalent polymer crosslinking. Herein we study the early stages of branching reactions that precede gelation by following molecular weight changes in bis-BP modified glassy polystyrene (PS) and amorphous poly(n-butyl acrylate) (PnBA) thin films on silicon wafers by Gel Permeation Chromatography. Quantitative molecular weight changes in PS:bis-BP and PnBA:bis-BP thin films are studied as a function of irradiation time, polymer:bis-BP molar ratio, and film height. Increases in molecular weight and polydispersity are quantified and model equations are developed. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H19.00008: Evolution of Entanglements During Craze Formation Ting Ge, Mark Robbins, Robert Hoy, Stefanos Anogiannakis, Christos Tzoumanekas, Doros Theodorou Craze formation occurs during fracture of many polymers and leads to a substantial increase in the fracture energy. Models of craze formation usually assume that entanglements act like permanent chemical crosslinks.This model is tested by following the evolution of entanglements using the Contour Reduction Topological Analysis (CReTA) algorithm. The CReTA algorithm shortens each chain until further shortening would require chains to pass through each other. The contacts between chains that limit further shortening are identified as entanglements or topological constraints. Unlike related algorithms, the chain shortening has little effect on the craze structure, allowing the entanglements to be followed in real space, as well as along chains. CReTA is applied to molecular simulations of crazing using a coarse-grained bead-spring polymer model. The number of beads in each chain N and the entanglement length Ne are varied. Our results show that entanglements do not act like fixed chemical crosslinks. There is a systematic loss in entanglements during craze formation that does not occur when chains are deformed affinely and is nearly independent of N/Ne.The role of chain length, N, Ne, interchain friction and other parameters in determining the degree of entanglement loss is discussed. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H19.00009: Dynamics of a polymer nanocomposite during active deformation Robert Riggleman, Gregory Toepperwein, Juan de Pablo, Hau-Nan Lee, M. D. Ediger Recent molecular simulation and experimental studies have explored the effects of stress on the dynamics of polymer glasses and both have demonstrated that relaxation times can decrease by more than two orders of magnitude. However, many questions on the origins of the changes in the dynamics remain unaddressed. In this study, we have performed extensive molecular dynamics and Monte Carlo simulations of a polymer glass and a polymer nanocomposite undergoing active deformation. We measure the dynamics during both constant stress and constant strain rate deformations and provide a detailed comparison of the two modes of deformation. The nanoparticles impart mechanical reinforcement onto the polymer, requiring larger stresses to achieve the same deformation. In both systems, the dynamics correlate very well with the instantaneous strain rate whether we deform at constant stress or constant strain rate. Additionally, we explore the effects of each mode of deformation on the potential energy landscape and find qualitatively different behaviors when we deform at constant stress versus constant strain rate. Finally, we provide a brief comparison of our simulation results to recent experiments and demonstrate that the simulations are capable of reproducing all of the behaviors observed in the experiments. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H19.00010: Rheological Scaling Relation for an Out-of Equilibrium Colloidal Solid H. Henning Winter, X. Wang, G. Xue, P. Sun We explore scaling relations for the slow ripening of an out-of-equilibrium model colloidal solid that consists of clay particles that swell and exfoliate into randomly oriented clay sheets through the action of end-functionalized (``sticky'') polymer molecules. A freshly mixed sample quickly forms a sample-spanning network structure that gradually approaches its equilibrium. The ripening process accelerates at elevated temperature. After rescaling (Rheol Acta 45:331-338, 2006), the complex modulus data $G$',$G$''(\textit{$\omega $}, $t_{r})$ from time-resolved mechanical spectroscopy (Rheol Acta 33:385-397, 1994) shows that, surprisingly, the growth function of the elastic modulus is the inverse of the decaying characteristic relaxation time. Parameter of the isothermal ripening process is the ``ripening time'', $t_{r}$. A single scaling function with two pronounced powerlaw regions, a fast ripening process ($\sim \quad t_{r}^{-2})$ followed by slow ripening ($\sim \quad t_{r}^{-1/2})$, defines the state of ripening and projects the time necessary to reach equilibrium. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H19.00011: Strain Hardening in Bidisperse Polymer Glasses Mark O. Robbins, Robert S. Hoy The connections between glassy and rubbery strain hardening have been a matter of great controversy in recent years. Recent experiments and our earlier simulations have suggested that the hardening modulus $G_R$ is proportional to the entanglement density in glasses, as it is to the crosslink density in rubbers. In this work we present more extensive studies of strain hardening in bidisperse glasses and its relation to microscopic conformational changes. The mixtures contain chains of very different lengths but equivalent chemistry. $G_R$ does not scale simply with the entanglement density. Instead it obeys a simple mixing rule, with $G_R$ equal to the volume fraction weighted average of the moduli of the two pure components. As in recent studies of monodisperse systems (R. S. Hoy and M. O. Robbins, Phys. Rev. Lett. \textbf{99}, 117801 (2007)), the stress is directly correlated to the degree of chain orientation. Chains of a given length undergo almost the same degree of alignment in pure systems and mixtures, explaining why the simple mixing rule applies. The connection to recent analytic theories by K. Chen and K. S. Schweizer (PRL, in press) will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H19.00012: A Model of Glassy Polymers that Includes both Spatial and Temporal Fluctuations Grigori Medvedev, James Caruthers Glass forming polymers near and below Tg are dynamically heterogeneous as has been found via a number of experimental techniques, where the dynamic heterogeneity is the probable cause of the non-exponential decay of the orientation correlation function of probe molecules embedded in polymer matrix as well as ``breaking'' of the Stokes-Einstein relations for rotational and translational diffusion. Although dynamic heterogeneity in glassy polymers is well established, constitutive models for describing the mechanical behavior employ quantities that ignore fluctuations. Consequently, the mechanical implications of dynamic heterogeneity are largely unexplored. In this talk we report on a finite element type model, where the local relaxation times in the material experience fluctuations, i.e. both the temporal and spatial nature of the fluctuations is explicitly acknowledged. The stochastic force between neighboring domains is assumed to be uncorrelated; however, since neighboring domains tile space, there is spatial and temporal correlation in the stochastic response of the system. The mechanical response of the sample under different deformation histories, including constant strain rate tensile and compressive loading as well as creep under constant load, will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H19.00013: Anomalous crack propagation in reinforced natural rubber Paul Sotta, Brice Gabrielle, Didier Long, Loic Vanel, Pierre-Antoine Albouy, Francesca Peditto In reinforced natural rubber, crack propagation in mode I exhibits rotation of the tear in a direction perpendicular to the usual one. Our objective is, first, to understand the impact of this phenomenon on fracture toughness of the material, and, secondly, to understand how this phenomenon is related to the specific properties of reinforced natural rubber. To this aim, we combine measurements of ultimate properties, measurements of the number and length of tear rotations as a function of loading velocity and temperature, and investigation of material heterogeneities at sub-micrometric scales, originating both from fillers and strain-induced crystallites (strain-induced crystallinity is measured up to failure by X ray diffraction), in natural rubber samples reinforced by nanometric aggregates. Observations suggest that tear rotation is related both to the mechanical anisotropy induced by strain-induced crystallinity and to the dissipative properties of the material at high strain. [Preview Abstract] |
Session H20: Electrically and Optically Active Polymers
Sponsoring Units: DPOLYChair: Lynn Loo, Princeton University
Room: 321
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H20.00001: Polymer Physics Prize Symposium Break
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Tuesday, March 17, 2009 8:36AM - 8:48AM |
H20.00002: Soliton and polaron induced 3D conformational changes in conjugated polymers Andre Leitao Botelho, Xi Lin We perform ab initio calculations on polyacetylene (PA), polypyrrole (PPY), and polyaniline (PANI) to examine the 3D conformational change as a function of injected charge. We find that self-localized solitons and polarons in their ground states are dispersed along the chain to minimize the localized charge Coulomb repulsion and the strain repulsion due to chain terminations. Each polaron in PPY or PANI progressively straightens each chain from their neutral bent state, where PPY and PANI are shown to have analogous conformations due to subsequent twisting along non-collinear axes. Solitons in PA are able to fully utilize the zigzag backbone geometry to minimize Coulomb repulsions, by alternating from one side of the chain to the other. This causes bending in alternating directions, leading to a sinusoidal shape, while maintaining a straight chain axis on average. Since PPY and PANI can change from a coil to a straight rod, these polymers can achieve strains about an order of magnitude higher than PA. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H20.00003: Conjugated Polymer based sensor for detecting explosives such as RDX (1,3,5-Trinitroperhydro-1,3,5-triazine) Abhishek Kumar, Robinson Anandkathir, Jayant Kumar Conjugated polymers are an important class of materials and have attracted great scientific interests because of their unique optical and electrical properties. Synthesis of various conjugated polymers has been reported for detecting DNT and TNT. Here, for the first time, we report the synthesis and characterization of a conjugated polymer based on thiophene, Poly[2-(3-thienyl)ethanol $n$-butoxycarbonylmethylurethane] (PURET), for the detection of RDX via fluorescence quenching. The incorporation of judiciously chosen bulky butoxycarbonylmethyl urethane side group in the back bone of polythiophene reduces the aggregation, improves the solubility of the polymer in the organic solvents and quantum yield of fluorescence. We have observed a 15 {\%} decrease of fluorescence in 60 sec in thin films of PURET exposed to the saturated vapor of RDX at room temperature. The large exciton diffusion length and interaction between side group and the quencher molecule are believed to be the reasons for enhanced RDX detection. Sensitivity below 5 parts per trillion (ppt) has been demonstrated for RDX at room temperature. The effect of nanostructures (electrospun nanofibers and stamped 1 D periodic pattern) on sensitivity and response time will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H20.00004: Charge-transfer excitons in strongly coupled organic semiconductors Paul-Ludovic Karsenti, Jean-Francois Glowe, Carlos Silva Time-resolved and temperature-dependent photoluminescence measurements on one-dimensional sexithiophene lattices reveal intrinsic branching of photoexcitations to two distinct species: self-trapped excitons and dark charge-transfer excitons (CTX; $\agt 5$\% yield), with radii spanning 2--3 sites. The significant CTX yield results from the strong charge-transfer character of the Frenkel exciton band due to the large free exciton bandwidth ($\sim400$\,meV) in these supramolecular nanostructures. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H20.00005: Highly Conductive Polymer Films by Post-Processing Solvent Annealing and Their Broad Applications in Organic Electronics Joung Eun Yoo, Kwang Seok Lee, Enrique Gomez, Kimberly Baldwin, Yangming Sun, Chang Su Kim, Hong Meng, Yueh-Lin Loo The electrical conductivity of polyaniline (PANI) that is template synthesized with a polymer acid of poly(2-acrylamido-2-methyl-1-propanesulfonic acid), PAAMPSA, is enhanced by more than two orders of magnitude with post-processing solvent annealing. Such solvent annealing allows the conducting polymer to rearrange from the globular structure that is arrested by strong ionic interactions during synthesis to the conformationally more favorable structure that dramatically enhances charge transport. This solvent annealing is general; we demonstrate conductivity enhancement with this process for at least two common classes of conducting polymers, including poly(3,4-ethylenedioxytheophene), PEDOT, as well as PANI. The treated conducting polymer films make efficient source and drain electrodes and anodes for organic thin-film transistors and organic solar cells, respectively. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H20.00006: Electrochemical Stability of Polyaniline Beyond pH 9 Jacob Tarver, Joung Eun Yoo, Yueh-Lin Loo Conductive polymer films are promising candidates for solution-based biosensor and organic electrochemical transistor devices. For many conducting polymers, however, stable electrochemical activity often demands restrictively acidic solutions. This need has in turn limited the use of conductive polymers in near-neutral and physiological conditions. Using spectroelectrochemical methods, we studied the stability of polyaniline that is template synthesized on poly(2-acrylamido-2-methyl-1-propanesulfonic acid) as a function of pH. Transitions between the different oxidation states of polyaniline are stable and reversible in solutions as high as pH 10. This range of sustained electroactivity far exceeds that of previously reported polyaniline systems. In comparison, polyaniline that is doped with small-molecule acids loses its electroactivity in solutions beyond pH 4. Immobilization of polyaniline within a polymer acid matrix retards dopant diffusivity and reduces proton mobility. The preservation of local acidic conditions within the film greatly extends the pH range of stable electroactivity. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H20.00007: Quadratic Electro-optic Effect in a Novel Nonconjugated Conductive Polymer, iodine-doped Polynorbornene Ananthakrishnan Narayanan, Mrinal Thakur Quadratic electro-optic effect in a novel nonconjugated conductive polymer, iodine-doped polynorbornene has been measured using field-induced birefringence at 633 nm. The electrical conductivity$^{1}$ of polynorbornene increases by twelve orders of magnitude to about 0.01 S/cm upon doping with iodine. The electro-optic measurement has been made in a film doped at the medium doping-level. The electro-optic modulation signal was recorded using a lock-in amplifier for various applied ac voltages (4 kHz) and the quadratic dependence of the modulation on the applied voltage was observed. A modulation of about 0.01{\%} was observed for an applied electric field of 3 V/micron for a 100 nm thick film The Kerr coefficient as determined is about 1.77x10$^{-11}$m/V$^{2}$. This exceptionally large quadratic electro-optic effect has been attributed to the confinement of this charge-transfer system within a sub-nanometer dimension. 1. A. Narayanan, A. Palthi and M. Thakur, J. Macromol. Sci. -- PAC, accepted. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H20.00008: Quadratic Electro-optic Measurements in Nonconjugated Conductive Polymers, iodine-doped Polyisoprene and Poly($\beta $-pinene) at 1.55$\mu $m Ananthakrishnan Narayanan, Jitto Titus, Mrinal Thakur Exceptionally large near-resonant (at 633nm) quadratic electro-optic effects in nonconjugated conductive polymers, iodine-doped poly($\beta $-pinene) and 1,4-cis-polyisoprene have been previously reported. In this report, we discuss the quadratic electro-optic effects in these polymers at 1.55$\mu $m. The measurements were made using the field-induced birefringence technique. A modulation depth of about 0.1{\%} was observed for a 1 $\mu $m thick sample of doped poly($\beta $-pinene) at an applied field of 1V/$\mu $m. The Kerr coefficient as determined was about 1.6$\times $10$^{-10}$ m/V$^{2}$. For polyisoprene samples the modulation was slightly smaller. These exceptionally large Kerr coefficients at a technologically important wavelength make these polymers promising for guided-wave applications in electro-optics. Techniques for longer-term stability of the samples have been established. The large optical nonlinearities as observed have been attributed to the sub-nanometer confinement of these charge-transfer systems [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H20.00009: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H20.00010: Ultrasensitive Solution Processed Polymer Photodetectors Xiong Gong, Minghong Tong, Gang Yu, Chan-Long Shieh, Boo Nilsson, Alan Heeger Semiconducting polymeric optoelectronic and electric devices have evolved as a promising cost-effective alternative to silicon-based devices. Organic photodetectors have been the subject due to several inherent advantages. Some of the important advantages of these so-called ``plastic'' electronics include large-area detection, low cost of fabrication, ease of processing and mechanical flexibility. However, there are few reports on organic photodetectors whose performances are comparable with inorganic countparts. will report ultrasensitive solution processed photodetectors fabricated by different semiconducting polymers as the electron donors and various fullerences derivatives and/or inorganic quantum dots as the electron acceptors. Polymer photodetectors with different photo-response and detectivity were demonstrated. One example is that polymer photodetectors have photo-response from 300nm to 1450nm, the detectivity larger than 10$^{12}$ cm Hz$^{1/2}$/W, and linear dynamic range larger than 120 dB. All these values are comparable to or even better than their inorganic counterparts. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H20.00011: Twin instability of Peierls distortion and its mechanical consequence on conductive polymer actuation Minghai Li, Andre Botelho, Xi Lin We prove analytically that a one-dimensional metallic chain is subject to two coupled spontaneous conformational relaxations, resulting in the well-known Peierls bond length alternation and an overall chain contraction. Using the Su-Schrieffer-Heeger (SSH) Hamiltonian, a tight-binding version of the Peierls theory, we find in a neutral defect-free polyacetylene chain these two coupled distortions work cooperatively against the backbone elastic deformation. The cooperative bond alternation and chain contraction deformations have two effects, allowing bonds alternate and contract less than the case when deformations are independent and breaking the charge conjugation symmetry which would otherwise be conserved. Making such a deformed neutral chain as the reference, we find that creation of self-localized solitons upon dopings results in spontaneous chain contractions within the self-localized domains where the anti-Peierls distortions are enforced. Our numeric results based on the SSH model and first-principles calculations indicate that chain contractions are proportional to the soliton density at low dopings, and the overall chain length varies non-monotonically with respect to the doping level, reaching a maximum contraction of 0.15{\%} at 5{\%} doping. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H20.00012: Soliton migration along \textit{trans}-polyacetylene backbone Minghai Li, Yongwoo Shin, Xi Lin We compute the minimum energy paths and activation barriers for the soliton migration process along trans-polyacetylene backbone via the Su-Schrieffer-Heeger (SSH) model and \textit{ab initio} calculations. Our results confirm the conventional consensus that soliton hops over two CH sites in one single step, maintaining wavefunction nodal structures at intermediate CH sites. Standard SSH parameters give rise to negligible migration barriers, which increases exponentially as the localization width decreases. Favoring the opposite elastic strain as the boundaries, soliton prefers staying at the center of an open chain; this ground state energy increases linearly when the chain length decreases. Starting at the center and moving towards the chain termination, soliton first sees a quadratic wash-board energy landscape when it is far away from the boundary and then smoothly switches to an exponentially increased wash-board energy landscape when it is in the vicinity of the boundary due to its localization width shrinkage. The local minima energy has a larger exponent compared to that of the activation barrier so they cross each other at a certain point, beyond which the barrier disappears and solitons cannot get closer to the boundary. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H20.00013: ABSTRACT WITHDRAWN |
Session H21: Spectroscopic Studies of Semiconductors
Sponsoring Units: FIAPChair: Norman Tolk, Vanderbilt University
Room: 323
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H21.00001: Two- and three-photon absorption of germanium in the mid-infrared Dongmin Seo, Leonard Feldman, Norman Tolk, Philip Cohen We have studied the nonlinear optical response of crystalline germanium using high-power infrared picosecond laser pulses at wavelengths ranging from 2.8 $\mu $m to 5.2 $\mu $m. Transmittance as a function of fluence at 2.8 $\mu $m and 4.4 $\mu $m were fitted by using two- and three-photon absorption, respectively. Data at 3.2, 3.6, and 4.0 $\mu $m, however, required consideration of simultaneous two- and three- photon absorptions in order to fit the experimental data. Transmittance as a function of wavelength further supports the onset of the two- and three-photon absorption at appropriate wavelengths. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H21.00002: Link between optical second-harmonic and reflectance-anisotropy spectroscopy of stepped Si(001) surfaces Robert Ehlert, Jinhee Kwon, Michael C. Downer Optical second-harmonic generation (SHG) and reflectance-anisotropy spectroscopy (RAS) are the two dominant noninvasive optical probes of electronic structure and chemical dynamics at surfaces, but underlying connections between these spectroscopies remain poorly understood. Here we combine spectroscopic SHG and RAS to characterize stepped Si(001) surfaces offcut toward [110] before and after dissociative adsorption of H$_2$ at the D$_B$ step edges. Such stepped surfaces provide attractive templates for self-directed growth of nanoscale structures, while SHG/RAS provide non-invasive in-situ sensors to guide and interpret step edge chemistry. Our major finding is that the broad negative step-induced feature of the RA spectrum around 3eV, and the spectrum of the step induced third order Fourier component of the SHG azimuthal anisotropy, show strikingly similar spectral shapes and dependencies on H$_2$ adsorption, suggesting that these features share a common microscopic origin in the step edges. Separate analysis with a simplified bond hyperpolarizability model indicates that chemically active step dangling bonds are dominant contributors to both SHG and RAS, and that hydrogen termination of the step edges alters both by redistributing oscillator strength from the dangling bond to step back bonds. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H21.00003: Photocurrent measurement on donor bound excitons in Si Na Young Kim, Darin Sleiter, Thaddeus Ladd, Katsuya Nozawa, Yoshihisa Yamamoto Donor bound excitons are formed when free excitons are captured by neutral donor impurities. Due to the spatial localization of exciton at the impurity site, the decay process of donor bound exciton state to neutral donor state features extremely narrow linewidth in energy. Utilizing this inherent feature, it would be feasible to identify nuclear spin states of the donor impurity resulting from the hyperfine interaction between phosphorus nucleus spin and electron spin. We study ensembles of phosphorus donor bound excitons in Si via photocurrent measurements at low temperatures since Auger non-radiative decay process is primarily dominant in an indirect band-gap semiconductor such as Si. We report electric and magnetic field effects on photocurrent signals of phosphorus donor bound excitons. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H21.00004: Polaronic effects in single doped GaAs quantum well . G. Martinez, M. Orlita, C. Faugeras, S. Deutschlander, P.Y. Yu, A. Riedel, R. Hey, K. Friedland Absolute magneto transmission experiments, for magnetic fields B up to 33 T, have been performed on a series of single GaAs quantum well with a width of 13nm, doped at a level Ns ranging from 2 to 7.5 10$^{11}$ cm$^{-2}$ and mobilities exceeding 10$^{6}$ cm$^{2}$/V/sec . The analysis of the spectra with a complete multi-dielectric model allows to extract the imaginary part of the electronic dielectric function which clearly exhibits different features for cyclotron energies higher than the phonon energies of GaAs. For the lower doped samples, the dependence on B of the damping parameter shows a pronounce increase when the energy exceeds the longitudinal optical phonon energy characteristic of polaronic effects. At higher fields this parameter increases once more resonantly showing a new interaction called ``X''. For Ns higher than 6 10$^{11}$cm$^{-2}$, only this new interaction is clearly visible though its amplitude decreases when increasing Ns. The origin of this new interaction is very likely related to mechanisms involving phonons. The different possibilities are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H21.00005: Anisotropy of plasmon-phonon coupling under intense optical excitation of GaAs. Amlan Basak, Hrvoje Petek The dependence of coherent optical processes in time-frequency domain on the polarization of pump and probe beams can shed light on different generation and detection mechanisms. Here, we report the response of (100) oriented n-doped GaAs (n$_{d}$=2x10$^{18 }$cm$^{-3})$ when excited to an e-h pair density n$_{exc}\sim $10$^{18}$-10$^{20}$ cm$^{-3}$ with a 10 fs laser pulse centered at 400 nm. Experiments are performed in the transient reflectivity and reflective electro-optic sampling geometries with various pump and probe orientations. Time domain signal showing plasmon-phonon oscillations has a weak pump polarization dependence indicating isotropic generation mechanism and strong probe polarization dependence revealing symmetry properties of various carrier-phonon interaction mechanisms. Fourier Transform analysis of the time domain signals at different probe orientations show both plasmon and phonon anisotropy. Results are discussed considering different possible carrier-phonon interaction mechanisms with different symmetry dependences. Frequency evolution of plasmon-phonon coupled mode with increasing photoexcited carrier density is consistent with the hole-phonon coupling in the high damping regime. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H21.00006: Photoluminescence of surface InAs quantum layer on GaAs(001) and the carrier dynamics Itaru Kamiya, Kosei Fukui The opto-electronic properties of self-assembled quantum dots (QDs) grown by epitaxial crystal growth, for instance InAs/GaAs(001) by MBE, have attracted great attention during the past few decades. Recently, there have been a number of reports on the properties of surface InAs QDs, where the QDs are not capped by GaAs and instead exposed to ambient, since such structure exhibits luminescence in the 1.5 micron regime. The growth of such QDs accompanies formation of a wetting layer (WL) which is typically a monolayer thick film of InAs. However, in the studies on InAs QDs, the contribution of the WL is often neglected, if not, not distinguished from those of the QDs. However, we find that surface WL exhibits unique properties, also providing information for better understanding those of the QDs. Here, using PL and PLE, we studied surface InAs WL and QDs grown on GaAs(001). We show that signals arising from the WL and QDs can be distinguished, and that they exhibit properties different from those of buried structures. Based on these results, we discuss the carrier dynamics in the near surface regime. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H21.00007: Determining the bandtail shape of highly Si-doped Al$_{0.3}$Ga$_{0.7}$As using persistent photoconductivity Jennifer Misuraca, Stephan von Molnar, Peng Xiong, Jelena Trbovic, Jun Lu, Jianhua Zhao, Hideo Ohno Highly Si-doped Al$_{0.3}$Ga$_{0.7}$As can be driven through the metal-insulator phase transition using persistent photoconductivity [1].~ Owing to the bi-stable nature of the Si donor, samples cooled in the dark are insulating. In the present work, an infrared LED is used to photodope the sample at 5K for a range of illumination times, which populates shallow states and provides a way to change the carrier concentration of the sample\textit{ in situ}.~ Measuring the carrier concentration as a function of temperature allows for the infinite temperature carrier concentrations and Hall activation energies to be extracted for various illumination times as the Fermi energy is tuned systematically.~ Application to Si- doped Al$_{0.3}$Ga$_{0.7}$As prepared by MBE allows one to infer the bandtail shape [2] in the energy range between the Fermi energy of the unilluminated sample and the mobility edge. [1] S. Katsumoto, et al. J. Phys. Soc. Jpn. 56, 2259 (1987) [2] I. Terry, et al. Solid State Commun. 84, 235 (1992) [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H21.00008: Effect of order/disorder near the $\Gamma $-L and L-X crossovers in the conduction band of lattice-mismatched Ga$_{x}$In$_{1-x}$P alloys L. Bhusal, M. Steiner, J. Geisz, A. Mascarenhas In this work we have studied the effect of order/disorder on the $\Gamma $-L and L-X crossover points in the conduction band of Ga$_{x}$In$_{1-x}$P alloys, using polarized photoluminescence and electroreflectance techniques at various temperatures. Ga$_{x}$In$_{1-x}$P samples (x=0.25-0.78) were grown by atmospheric pressure organometallic vapor phase epitaxy (OMVPE). Some samples were grown directly on a miscut GaAs substrate while in other samples a thick GaAsP step-grade was grown first, to reduce the dislocation density. The significance of the crossover point in the conduction band of the alloy for the efficiency of devices such as multijunction high-efficiency solar cells and light emitting diodes will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H21.00009: Intersubband absorption in InAlN/GaN heterostructures O. Malis, C. Edmunds, M. J. Manfra, D. L. Sivco, R. Molnar Nitride superlattices are promising for intersubband light emission and detection in the currently inaccessible near-infrared range (2-3 $\mu $m). Efforts to exploits the nitride properties have been hampered so far by difficulties related to the quality of the materials. Most studies to date have employed AlGaN/GaN heterostructures. However, the large lattice mismatch between AlGaN and GaN limits the total thickness of the structures. We are focusing on lattice-matched InAlN/GaN superlattices. InAlN has been less investigated due to the challenges in growing high-quality In-containing nitrides. Nevertheless, the large conduction band offset (1 eV) and lack of piezoelectric effect make the lattice matched nitrides ideally suited for near-infrared applications. We have performed a detailed intersubband absorption study of InAlN/GaN superlattices grown by MBE on HVPE GaN templates. X-ray diffraction analysis suggests that our samples are among the highest quality ever reported. The band structure of the materials was examined with Fourier-transform infrared spectroscopy. Strong intersubband absorption in the 430-530 meV energy range is reported for the first time for 2-4.5 nm-wide quantum wells. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H21.00010: Beryllium acceptor binding energy in AlN. Ashok Sedhain, T. M. Al Tahtamouni, Jing Li, Jingyu Lin, Hongxing Jiang The acceptor binding energy of an alternative dopant, Be, in AlN epilayers has been probed by time-resolved photoluminescence (PL) spectroscopy. The binding energy of excitons bound to Be acceptors in AlN is determined to be about 33 meV, which implies that the Be acceptor binding energy in AlN is about 0.33 eV in accordance with Haynes' rule. The measured PL decay lifetimes of the acceptor-bound exciton transitions in Be- and Mg-doped AlN (93 and 119 ps, respectively) also indicate that the binding energy of Be acceptor is smaller than that of the most common acceptor dopant in AlN, namely, Mg. The smaller activation energy of Be in AlN has the potential to partly address the critical $p$-type doping issue in AlN- and Al-rich AlGaN by increasing the room temperature free hole concentration by $\sim $10$^{3}$ compared to the case of Mg doping. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H21.00011: Estimation of third-order nonlinear optical susceptibility $\chi ^{(3) }$of synthetic Cu$_{2}$O crystal Shahin Mani, Joon Jang, John Ketterson High-quality crystals of Cu$_{2}$O were prepared by an improved method for thermally oxidizing metallic copper. We report the nonlinear refractive index (n$_{2})$ and the nonlinear two-photon absorption coefficient ($\beta )$ of the resultant crystals. The following techniques were utilized: i) Z-scan, ii) third-harmonic generation, and iii) intensity-dependent interferometry. The third-order susceptibility ($\chi ^{(3)})$ of a material plays important role in optical signal processing including switching, altering the frequency and the transmission characteristics. A comparison between the third-order nonlinear susceptibilities of a standard nonlinear reference material, carbon disulfide (CS$_{2})$ and Cu$_{2}$O will be made. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H21.00012: Anomalous Fresnel coefficients for quadrupole polaritons in Cu$_{2}$O Joon Jang, Yi Sun, Shahin Mani, John Ketterson In a direct-gap semiconductor, a polariton is a quantum superposition of an exciton and a photon, formed near the light cone. Unlike dipole polaritons that have been strongly confined in a 2D microcavity structure, a quadrupole polariton in a bulk Cu$_{2}$O is a coherently propagating bosonic state with an unusually long decoherence time owing to its unusual underlying electronic structure. Therefore, this unique semiconductor provides a model system for studying the theory of so-called additional boundary conditions at the vacuum-crystal boundary. Using resonant two-photon excitation, we create a coherently propagating polariton wave packet at 2 K and measure its reflectance (R) and transmittance (T) at the boundary opposite to the incoming surface. Surprisingly, we find an enhanced reflection of polaritons from sample surfaces such that the ratio T/R deviates significantly from the present theory. This anomalous boundary effect most likely arises from the quadrupole excitonic (matter) component of polaritons. Our experimental results have implications for the design of polariton-based waveguides and resonators in which traveling polaritons are effectively confined in the medium. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H21.00013: Photoluminescence Characteristics of Pulsed Laser Deposited ZnO Thin Films Grown in Nitrogen/Oxygen Ambients M.A. Thomas, J.B. Cui, Y.C. Soo, H. Kandel, T.P. Chen, C.P. Daghlian ZnO thin films were grown by pulsed laser deposition using a Zn target in different atmospheres. The samples were characterized by SEM, XRD, EDX, and temperature dependent photoluminescence (PL) measurements. The growth conditions were varied sequentially from a pure oxygen to a pure nitrogen atmosphere, and the resulting changes of the material properties were investigated. The presence of nitrogen during growth was found to have a strong impact on the materials. Samples grown with higher nitrogen concentrations showed weak PL characteristics at room temperature as well as a small temperature dependence of the near band edge emission. At temperatures below 40 K, a sharp and pronounced emission peak was present at 3.362 eV. In an attempt to understand the PL characteristics, the samples were annealed in both pure oxygen and pure nitrogen environments at 600\r{ }C. The samples grown with large nitrogen ratios exhibited a strong dependence on the annealing atmosphere; those annealed in nitrogen showed a strong increase in emissions in the 3.362 eV range compared to the same samples annealed in oxygen. In addition, the defect emissions of the samples were strongly affected by the presence of nitrogen during annealing. The possible role of nitrogen in ZnO growth and annealing is discussed. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H21.00014: Optical properties of epitaxial ZnGeAs$_{2}$ thin film S.G. Choi, D.E. Aspnes, M. Van Schilfgaarde, T.J. Peshek, T.J. Coutts, A.G. Norman, J.M. Olson, D.H. Levi Chalcopyrite ZnGeAs$_{2}$ lattice-matched to GaAs(001) is a promising 1.1 eV band gap semiconductor for applications in nonlinear photonic devices and multijunction solar cells. Knowledge of the optical functions of a material over a wide photon energy range is of importance to optimize photonic and photovoltaic device structures. We present room-temperature optical properties of a ZnGeAs$_{2}$ thin film grown epitaxially on a GaAs(001) substrate by metalorganic vapor phase epitaxy. Spectroscopic ellipsometry was employed to measure the pseudodielectric function of the ZnGeAs$_{2}$ thin film, and was compared with a theoretical calculation within the quasiparticle self-consistent GW approximation. The interband-transition critical-point energies were obtained from a standard lineshape analysis of the measured spectrum. We will also present a comparison of the optical properties of ZnGeAs$_{2}$ with those of other II-IV-V$_{2}$ chalcopyrite compounds as well as their corresponding III-V zincblende compounds. This abstract is subject to government rights. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H21.00015: Electronic and optical properties of dilute Bismide alloys Rajeev Kini, Angelo Mascarenhas, Ryan France, Aaron Ptak We will present photoluminescence measurements of GaAs$_{(1-x)}$Bi$_{x}$ thin films containing dilute concentration ($x \quad \le $ 0.045{\%}) of isoelectronic impurity Bi. We observe that Bi induces strong perturbation to the host band structure even at these low concentrations and see no spectral evidence for isolated Bi forming a bound state in GaAs. Very similar to the case of Bi in GaP, we observed no Bi-Bi pair states. An `undulation' spectrum is observed which we attribute to the vibronic levels of acceptors. [Preview Abstract] |
Session H22: Focus Session: Optical Generation and Detection of Spins in Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Jay Kikkawa, University of Pennsylvania
Room: 324
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H22.00001: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H22.00002: Coupling Mechanism of Anisotropic Interface and Bulk Magnetism in Thin Epitaxial Fe Film on AlGaAs (001) Yichun Fan, Haibin Zhao, Gunter Lupke, Aubrey Hanbicki, Connie Li, Berry Jonker A systematic study of the coupling mechanism of anisotropic interface and bulk magnetism in Fe/AlGaAs(001) heterostructures is carried out with different Fe film thicknesses. The uniaxial, cubic and out-of-plane anisotropy fields are determined for the interface and the bulk by time-resolved magnetization induced second harmonic generation (MSHG) and magneto-optic Kerr effect (MOKE), respectively. The experimental data show that the interface-induced uniaxial magnetic anisotropy is uniform within the first 5nm, but it decreases fast beyond this thickness. Moreover, the thickness of the Fe interface layer is estimated to be 1.7A (close to half of the Fe lattice constant), which is expected by MSHG theory. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H22.00003: Dynamic polarization of Mn spins coupled to vertical optical cavities G. Calusine, R.C. Myers, D.D. Awschalom Single magnetic spins in semiconductors can exhibit long lifetimes and are electrically controllable due to coupling of their spin states with those of the host semiconductor. In particular, Mn ions doped into GaAs quantum wells have spin lifetimes $\sim$10 nanoseconds and can be optically manipulated in zero magnetic field\footnote{R. C. Myers, M. H. Mikkelsen, J.-M. Tang, A. C. Gossard, M. E. Flatt\'{e}, and D. D. Awschalom, \textit{ Nature Materials} \textbf{7}, 203 (2008).}. To study this system in the low density limit, we incorporate a distributed Bragg reflector optical cavity around the Mn containing wells, which enhances Mn luminescence and allows spatial isolation of a small number of Mn ions imaged using scanning micro-photoluminescence. In such structures, we observe unusually long Mn T$_{2}^{\ast}> 60$ ns. We discuss the effect of cavity coupling on spin dynamics Mn ions in the single ion limit. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H22.00004: Generation and transport of photo-excited carriers in diamond F.J. Heremans, G.D. Fuchs, C.F. Wang, D.D. Awschalom, Ronald Hanson Photo-excited carrier transport in diamond has generated substantial interest in the past few years. The combination of high thermal conductivity, large band-gap, and large dielectric breakdown make diamond attractive in optoelectronic, high-power and high-frequency applications. Here we present measurements of electrical conduction by sub-band gap photo-excited carriers between metallic gates lithographically patterned on the surface of single-crystal type Ib diamond. The time-dependent charging and discharging photo-currents follow a ``stretched exponential'' form, which results from a trap state conduction mechanism mitigated by a local space charge. We also perform photo-excited magneto-transport as well as energy-dependant photo-conduction measurements to investigate the detailed origins of this sub-gap photo-excited conduction. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H22.00005: Excitation and doping dependence of hole-spin relaxation in bulk GaAs Michael Krauss, David Hilton, Hans Christian Schneider We present theoretical and experimental results on ultrafast hole-spin dynamics in bulk GaAs. By combining a sufficiently realistic bandstructure at the level of an 8$\times$8 $\vec{k} \cdot \vec{p}$ theory and a dynamical treatment of the relevant scattering mechanisms [1], we obtain quantitative agreement between the microscopic theoretical results and differential transmission measurements [2] for different excitation conditions. In particular, we examine the dependence of the hole-spin relaxation time on the optically excited carrier density, lattice temperature, and doping concentration. Although the spin relaxation is rather insensitive to changes in the optically excited density and temperature, strong p-doping causes a significantly faster relaxation. \newline [1] M. Krauss, M. Aeschlimann, and H. C. Schneider, Phys.Rev.Lett.\ \textbf{100}, 256601 (2008)\newline [2] D. J. Hilton and C. L. Tang, Phys.\ Rev.\ Lett. \textbf{89}, 146601 (2002) [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H22.00006: Studies of electron spin lifetimes in InGaAs:Al quantum wells T. Ali, I. Khan, M. Yasar, A. Petrou, C. Li, A. Hanbicki, G. Kioseoglou, B. Jonker We have carried out optical pumping, Hanle and longitudinal Hanle studies of InGaAs:Al/GaAs single quantum wells. The circular polarization at zero magnetic field has a maximum around 50 K indicating that at low temperatures the recombination is associated with a bound electron. The measured spin lifetimes at low temperatures are an order of magnitude longer than those measured in reference GaAs/AlGaAs quantum wells. This is attributed to the suppression of the Dyakonov-Perel spin relaxation mechanism in this bound system. As the temperature is increased from 5 to 50 K the spin lifetimes decrease and become comparable to the lifetimes of the reference sample. In the longitudinal Hanle geometry the circular polarization increases with magnetic field and reaches a maximum at B $\approx $ 1.5 tesla. Beyond 1.5 tesla the circular polarization decreases. A series of polarization oscillations superimposed on the decreasing background with a periodicity of approximately 1 tesla is observed. These oscillations are tentatively attributed to the variations in the magnetic flux through the bound electron orbit. Work at SUNY was supported by ONR and NSF [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H22.00007: Spontaneous Loss of Spin Coherence in GaAs/Si Heterostructure Guanglei Cheng, Patrick Irvin, Biqin Huang, Ian Appelbaum, Jeremy Levy We present a possible way to optically inject spins into silicon. In this work, GaAs and Silicon-on-Insulator (SOI) wafers are bonded together by an ultrathin Ag layer using UHV wafer bonding. Standard optical pump-probe Kerr microscopy technique is applied to examine the spin coherence in the GaAs/Si heterostructure. In some areas of the bonded wafer, a relatively long spin coherence time of T2*=0.5 ns is observed in the GaAs. In other parts of the sample, the spin coherence is observed to decay much more rapidly (T2*=140ps). One possible explanation is that the quality of the bond varies across the wafer and that the strongly bonded areas exhibit spin transport from GaAs to silicon. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H22.00008: Coherent optical control of correlation waves of spins in semiconductors Eran Ginossar, Yehoshua Levinson, Shimon Levit We calculate the dynamical fluctuation spectrum of electronic spins in a semiconductor under a steady-state illumination by light containing polarization squeezing correlations. Taking into account quasi-particle lifetime and spin relaxation for this non-equilibrium situation we consider up to fourth order optical effects which are sensitive to the squeezing phases. We demonstrate the possibility to control the spin fluctuations by optically modulating these phases as a function of frequency, leading to a non-Lorentzian spectrum which is very different from the thermal equilibrium fluctuations in n-doped semiconductors. Specifically, in the time-domain spin-spin correlation can exhibit time delays and sign flips originating from the phase modulations and correlations of polarizations, respectively. For higher light intensity we expect a regime where the squeezing correlations will dominate the spectrum. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H22.00009: Time Resolved Spectroscopy of InSb Quantum Wells Using Differential Transmission Technique K. Nontapot, M. Bhowmick, G.A. Khodaparast, S.J. Chung, M.B. Santos The growing interest in spin-related phenomena and devices has prompted intense activity in the science and engineering of narrow gap semiconductors (NGS). NGS offer several scientifically unique features such as small effective masses, large g-factors, high intrinsic mobilities, and large spin- orbit coupling effects. In this work we report the dynamics of photo-excited carrier/spin in several InSb/ Al$_{x}$In$_{1-x} $Sb based quantum wells (QWs) using differential transmission spectroscopy. The InSb QW layers were selectively pumped and probed by mid-infrared pulses to avoid possible contributions from the barrier materials. We compare our results with the earlier measurements using magneto-optical Kerr (MOKE) effect. Our results are important to understand different relaxation mechanisms in NGS with strong-spin orbit interactions. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H22.00010: Spin polarized current in InSb based structures M. Frazier, M. Bhowmick, J.J. Heremans, G.A. Khodaparast, S.J. Chung , M.B. Santos, X. Liu, J. Furdyna Recently, there has been much interest in developing and exploring spin based semiconductor devices and phenomena. One of the key challenges in developing spin based devices is to generate, control, and measure spin currents directly. In this talk, we report interband circular photogalvanic (CPG) effects using pulsed near-infrared radiation in InSb quantum wells and two InSb films grown on GaAs and InP substrates. We observe a CPG current whose direction and magnitude depend on the helicity of the incident light, the angle of incidence, and temperature. Our observation is important to understand zero- field spin splitting mechanisms in a system with strong spin- orbit interaction. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H22.00011: Optically-Induced $^{13}$C Nuclear Spin Polarization through Nitrogen-Vacancy Centers in Diamond Jonathan King, Patrick Coles, Jeffrey Reimer The spin-1 negatively charged nitrogen-vacancy (NV) center in diamond has received much attention for its long spin-coherence times and optical polarization into the m$_{s}$=0 sublevel. These properties make it attractive for applications such as quantum information processing and high-resolution magnetometry. Large nuclear polarizations in diamond may be useful to quench decoherence, as an initialization step for quantum computing, or as a platform for enhancement of nuclear magnetic resonance (NMR) signal in dilute spin systems. In this work, we demonstrate the polarization of the bulk $^{13}$C nuclear spin system by interaction with the optically polarized NV center system at 9.4 Tesla and 5K nominal temperature. Large nuclear polarizations are observed through Faradaic detection of bulk $^{13}$C NMR signals. The signals are opposite in sign to thermally-generated signals, indicating nuclear polarization into the m$_{I}$ = -1/2 sublevel. We model the phenomenon and propose microscopic mechanism for the polarization. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H22.00012: Optical hyperpolarization of the nuclear and electronic spins of $^{31}$P in $^{28}$Si A. Yang, M. Steger, T. Sekiguchi, M. L. W. Thewalt, T. D. Ladd, K. M. Itoh, H. Riemann, N. V. Abrosimov, P. Becker, H.-J. Pohl, J. W. Ager III, E. E. Haller We have recently shown that the donor hyperfine splitting can be resolved for the $^{31}$P donor bound exciton transition in highly enriched $^{28}$Si, enabling either optical or optical-electrical readout of the electronic and nuclear spin state of this promising qubit candidate.[1] Here we show that these same optical transitions can be used to quickly achieve large nuclear and electronic hyperpolarizations of the $^{31}$P donor in $^{28}$Si. This may provide a viable solution to the problem of initializing the nuclear spins, a roadblock for quantum computing schemes involving nuclear spins in Si. We also report on the remarkably narrow homogeneous linewidth of this bound exciton transition, measured by hole burning spectroscopy. This suggests that even higher spin selectivity and hyperpolarization may be achievable in more highly enriched $^{28}$Si, or when dealing with individual $^{31}$P donors. [1] A. Yang et al., Phys. Rev. Lett. \underline {97}, 227401 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H22.00013: Time-resolved Infrared Magnetospectroscopy of GaAs at the NSLS G.L. Carr, J.J. Tu We describe a facility for performing photo-induced time-resolved infrared spectroscopy of materials in magnetic fields up to 10T at beamline U4IR of the National Synchrotron Light Source (NSLS). The facility combines an existing time-resolved capability (based on pulsed synchrotron radiation and a synchronized Ti:sapphire laser to achieve $\sim $100 ps resolution) with a split-coil superconducting solenoid and optical cryostat. We also report a THz study of photocarrier and exciton dynamics in GaAs using this facility. It is found that, for B$>$0, a portion of the photo-induced carrier absorption appears as an electron cyclotron resonance, while the exciton unbinding absorption splits into spin and orbital transitions (Zeeman effect). At low temperatures, we observe that the relaxation of photocarriers toward the band edge involves the breaking of existing excitons, leading to a combination of absorption and bleaching features that evolve on a $\sim $1 ns time scale. [Preview Abstract] |
Session H23: Optical and Spectroscopic Properties and Nano
Sponsoring Units: FIAP DCMPChair: Stefano Curtarolo, Duke University
Room: 325
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H23.00001: Photoemission core level shifts and phonon broadening for Cs film on Cu(100) Xubing Zhou, Kevin Koch, J.L. Erskine Department of Physics, University of Texas at Austin, Austin, TX 78712. We have measured Cs 5p photoemission core level broadening data for Cs film on Cu(100) at different temperatures and have obtained bulk-atom and surface-atom values of the zero-temperature phonon width and the effective Debye temperature, which governs the temperature dependence of the broadening. The coupling constant $C$ for alkali metals in the phonon-broadening theory of Hedin and Rosengren[1] is about 30{\%} higher in the surface than in the bulk. This result is compared with experiment results on other alkali metals[2]. We also studied the temperature dependent binding energy shifts for the Cs 5p core level peaks and our data fit the lattice expansion theory very well except at temperature higher than 220 K. The high temperature deviations are proved to be caused by thermal evaporation of Cs film. [1] L. Hedin and A. Rosengren, J. Phys. F \textbf{7}, 1339 (1977). [2] D.M. Riffe and G.K. Wertheim, Phys. Rev. B 61, 2302 (2000). [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H23.00002: Auger-Photoelectron Coincidence Spectroscopy measurement of the secondary electron distribution from 0 eV to 81 eV, created by the MVV Auger transition in Cu (100) K. Shastry, S. Mukherjee, A.H. Weiss, S.L. Hulbert, R.A. Bartynski In conventional spectroscopic measurements, low energy Auger lines are superimposed upon a large background due to secondary electrons that arise from loss processes that are unrelated to the Auger process. Here we present the results of measurements in which Auger-Photoelectron coincidence techniques were used to eliminate background unrelated to the Auger process and obtain the energy distribution of electrons emitted as a result of the M$_{23}$VV transition in Cu (100) over the full range of emitted energies (0 eV -- 81 eV). The measurements revealed a well formed Auger peak at $\sim $60eV accompanied by a low energy tail (LET) associated with the MVV transition. The LET extends to 0 eV and has a broad maximum at $\sim $ 6eV. The integrated intensity of the LET was $\sim $ 6 times larger than that of the Auger peak itself. The origin of the LET will be discussed in terms of extrinsic mechanisms in which electrons from the peak lose energy as they propagate to the sample surface, as well as intrinsic mechanisms in which multi-electron Auger processes distribute the energy gained by the filling of the core-hole to multiple electrons. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H23.00003: Ultrafast Spectroscopy on Solids at FLASH David Bernstein, Yves Acremann, Andreas Scherz, Martin Beye, Alexander F\"{o}hlisch, William Schlotter, Torbin Beeck, Florian Sorgenfrei, Annette Pietzsch, Wilfried Wurth, Joachim St\"{o}hr X-ray/VUV free electron laser (FEL) facilities such as FLASH, LCLS, and the European X-FEL open the door to a wide variety of exciting experiments in x-ray physics. Due to the random stochastic processes governing FEL radiation and the difficulties in tuning an FEL, it has not been clear whether spectroscopy could be done using such sources. Here we demonstrate the feasibility of doing near edge x-ray absorption fine structure (NEXAFS) spectroscopy on solids. Samples consisting of LaMnO and Al films, respectively, were lithographically fabricated on thin silicon nitride membranes. Ultrafast femtosecond pulses of radiation from the FLASH FEL were dispersed by the monochromator grating at beamline PG2 and impinged upon the samples. Absorption was measured in transmission using a Ce:YAG crystal and imaged by an intensified CCD. The incident intensity was measured through a blank nitride membrane next to the sample. By tuning the FEL to the La N-edge ($\sim{}$102eV) and the Al L-edge ($\sim{}$72eV), respectively, we take an entire NEXAFS absorption spectrum in each shot. Spectra are calculated using many shots in order to reduce statistical uncertainties. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H23.00004: STM study of CeTe$_{3}$: contribution of the subsurface lattice Aleksandra Tomic, Josh Veazey, Zsolt Rak, Christos Malliakas, Mercouri Kanatzidis, S. D. Mahanti, Stuart Tessmer We have studied the nature of the surface charge distribution in CeTe$_{3}$ with scanning tunneling microscopy (STM). At 77 K, the STM topography and Fourier transform show both the atomic lattice of surface Te atoms arranged in a square net and the CDW modulations oriented at 45 degrees with respect to the Te net. In addition, we observe peaks in the Fourier transform that we attribute to atoms lying below the surface Te net. We discuss the possibility of both subsurface Ce and Te as giving rise to this signal; density functional theory calculations indicate that the subsurface Ce atom gives a more significant contribution to the overall tunneling current. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H23.00005: Effect of a Fe substitutional impurity on the geometric and electronic structure of Au$_{13}$ cluster Ghazal Shafai, Talat Rahman We have carried out spin-polarized density functional theory calculations based on the pseudopotential method to determine the changes in the characteristics of the Au$_{13}$ cluster when one Au atom is replaced by Fe. For a pure Au$_{13}$ cluster, the 2D geometry is the lowest-energy isomer, followed closely by a flake structure, while the icosahedron is higher in energy by 2.98 eV and is not stable since it is found to undergo Mackay transition to form a cuboctahedron. When a surface or central Au atom is replaced by Fe, we find dramatic changes in the energy ordering of these nanoparticles, since Fe tries to move inwards so as to be highly coordinated. In fact the distorted icosahedron and a biplanar structure obtain the lowest energy. The structure of the Fe-centered icosahedron is slightly distorted (Jahn-Teller distortion), so that the degeneracy on two bands near Fermi level is removed. The lowest energy isomer in this study has the highest magnetic moment (3.98 $\mu _{B})$ in comparison with that of the other isomers. The magnetic moment of the icosahedron with an Fe atom at the center is 3.1 $\mu _{B}$, which is in in agreement with previous findings. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H23.00006: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H23.00007: Size-dependent crystallinity of nano-Pt/$\gamma $-Al$_{2}$O$_{3}$ Long Li, L.-L. Wang, S. I. Sanchez, J. H. Kang, Q. Wang, Z. Zhang, A. I. Frenkel, R. G. Nuzzo, D. D. Johnson, J. C. Yang Metallic platinum nanoparticles (NP) on $\gamma $-Al$_{2}$O$_{3 }$powders were synthesized with a size range from sub- to several nanometers. High-resolution transmission electron microscopy (HRTEM) studies revealed a size-dependent crystallinity of the Pt NPs, where Pt NPs with size $<$1 nm had a disordered structure, Pt NPs with size $>$2.5 nm all showed a crystalline structure. For Pt NPs with sizes between 1.1 and 2.4 nm, a transition zone exists in which $\sim $85{\%} of NPs appeared disordered and $\sim $15{\%} ordered. X-ray absorption spectroscopy (XAS) measurements support this result where increasing-disorder distribution of Pt-Pt bond lengths was noted with decreasing Pt nanoparticle size. A search for ground state structure of Pt37/$\gamma $-Al$_{2}$O$_{3}$ (100) with density functional theory (DFT) showed that the disordered structure is energetically more favorable than the ordered close-packed structures by 1.53 eV at the size of 1.1 nm. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H23.00008: Size Distribution of Nano-Crystallites in Non-Crystalline Binary Alloys Yong W. Kim, Andrew Abraham, Jerry Kim The factors that affect thermophysical property determination for non-crystalline metallic alloys include non-uniformities in compositional and morphological property within a specimen. A series of measurements have shown that a specimen's thermal history leads to a spatial profile of elemental composition that differ from one specimen to another for the same alloy.(See Y.W. Kim, \textit{Int. J. Thermophysics }\textbf{28, }732 (2007), and references therein.) In order to develop a theoretical model for temperature dependence of thermophysical properties, we consider thermal dissociation of nano-crystallites within a randomly close-packed (RCP) medium. Once the size distribution of the nano-crystallites has been established at a given temperature, a set of coupled dissociation equations can be solved at all other temperatures. Transport properties can then be computed over a range of temperature. In this paper we present the size distribution of crystallites in a bed of RCP spheres. Two different size spheres are mixed at several compositions to simulate non-crystalline binary alloys in 2-D. The distribution is found to be peaked at a crystallite size specific to a given alloy composition. This work is support in part by the NSF-DMR(Metals). [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H23.00009: Irreversibity in Cooling and Heating Processes in the Magnetocaloric MnAs and Alloys A.L. Lima Sharma, S. Gama, A.A. Coelho Irreversibility of adiabatic processes in the magnetocaloric MnAs and alloys are presented here. We used a differential scanning calorimeter in order to record the heat flux as a function of the temperature and applied field for MnAs, Mn$_{0.994}$Fe$_{0.006}$As and Mn$_{0.994}$Cu$_{0.006}$As. From the measured heat flux, we extracted the latent heat and entropy associated to cooling and heating processes. In the cooling curve, we observed that $S^{c}_{Mn} > S^{c}_{Fe} > S^{c}_{Cu}$, the index $c$ refers to cooling process, similarly, for the heating process: $S^{h}_{Fe}$\textit{ $\approx $ S}$^{h}_{Mn} > S^{h}_{Cu}.$ On the doped samples, the thermomagnetic behavior is compatible with a scenary where Zener's $p-d $exchange mechanism dominates, i.e. the interaction range is weaker but long ranged, because the extended valence hole states mediate the ferromagnetic interaction. The difference of the entropy obtained from cooling and heating process was found to be as high as 37{\%}. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H23.00010: Semiconductor behavior in bcc Cr$_{1-x}$Al$_{x}$ thin films Z. Boekelheide, F. Hellman Cr$_{1-x}$Al$_{x}$, with x = 0.15-0.25, has semiconducting electronic properties: extremely high resistivity and a negative temperature coefficient of resistance, along with a gap in the infrared reflectivity. This is unusual for an alloy of two metals, but similar behavior has been observed in Fe$_{2}$VAl and has been attributed to a hybridization-induced band gap. In bulk, Cr$_{1-x}$Al$_{x}$ is known to be inhomogeneous, with two crystal phases coexisting: one, a disordered bcc solid solution, and the other, called the ``X-phase'', with a microstructure consisting of 1-3nm domains. Because of this inhomogeneity, it was previously not known which phase was responsible for the semiconducting behavior. We used epitaxial thin film growth techniques to preferentially nucleate the bcc phase and study the effect of crystal structure on the electronic properties. We found that films grown epitaxially on MgO (100) substrates, expected to grow preferentially in the bcc phase, have high resistivity like bulk Cr$_{1-x}$Al$_{x}$, while polycrystalline films grown on amorphous SiO$_{2}$ have lower resistivity. This suggests that the semiconducting behavior is intrinsic to the bcc structure. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H23.00011: Exciton Recombination in Nanometer-Wide GaN/AlN Quantum Wells Zhenwen Pan, Madalina Furis, Alexander N. Cartwright, William J. Schaff In nitride semiconductor heterostructures, the presence of very strong built-in electric fields, oriented perpendicular to the semiconductor layers, dramatically impacts the electronic states, excitonic recombination, and photoluminescence in these materials. The origins of these fields lie in the non-centro- symmetric character and the strong piezoelectricity of the heterostructures. We investigated electronic states in the presence of strong built-in fields ($\sim$ 5 MV/cm) in very narrow, nanometer-wide GaN/AlN quantum wells via time-resolved photoluminescence spectroscopy. We find that the strong confinement ($\sim$ 2eV in the conduction band) leads to significant overlap in the electron and hole wavefunctions, even in the presence of large built-in fields. The temperature dependence of radiative lifetimes and emission energies indicates the band-edge recombination contributions (i. e. excitonic and/or shallow -acceptor pair) dominate the PL spectrum. Wells narrower than 3 monolayers exhibit temperature- independent emission and 1 ns radiative lifetimes. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H23.00012: Enhancement of Subband Effective Mass in Ag/Ge(111) Thin Film Quantum Wells Shu-Jung Tang, Wen-Kai Chang, Yu-Mei Chiu, Hsin-Yi Chen, Cheng-Maw Cheng, Ku-Ding Tsuei, Tom Miller, Tai-Chang Chiang Subband dispersions of quantum-well states in Ag films on Ge(111) have been determined by angle-resolved photoemission. The effective masses of the subbands at the zone center increase substantially for decreasing film thicknesses. This peculiar behavior is attributed to a kinetic constraint for standing wave formation governed by a momentum-dependent phase shift function. No evidence is found for in-plane electron localization within the confined geometry. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H23.00013: Structure and Electrical Properties of Electrochemical Anodized Valve Metal Wenbin Fan, David Kirkwood, Jiwei Lu, Stuart Wolf Localized electrochemical anodization has been used to prepare metal tunnel junctions. The room-temperature I-V characteristics of anodized Vanadium, Tantalum and Titanium ultra small wires were studied. The nonlinear I-V curves indicate they behave as tunnel junctions. The resistance and tunneling characteristics are strongly determined by the details of anodization process. The High Resolution Transmission Electron Microscope (HRTEM) is used to explore the structure of the anodized Ti and V films and we found there are some crystaline grains on the bottom of the V film and in the center of Ti film. The grain size and the distance between two grains are changed by how the anodization process was terminated. Low temperature electrical properties of anodized films will be reported. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H23.00014: Direct writing of hafnium diboride metallic nanostructures on silicon (100) surfaces using a UHV-STM Wei Ye, Pamela Martin, Navneet Kumar, John Abelson, Greg Girolomi, Angus Rockett, Joseph Lyding The patterning of metallic nanostructures on surfaces is of great interest in fabricating nanoelectronics and quantum devices. In this work, we deposited HfB$_{2}$ nanostructures on silicon surfaces from Hf(BH$_{4})_{4}$ by electron beam induced deposition (EBID). At positive sample bias, the electron beam from a STM probe initiates the local CVD by the decomposition of Hf(BH$_{4})_{4}$ under STM tip. By repeatedly scanning STM tip along a specific path, well-defined HfB$_{2 }$nanostructures can be directly written onto the surface. Scanning tunneling spectroscopy was used to characterize the electronic properties of the nanostructures. We have achieved 4 nm linewidths and complete selectivity relative to adjacent H-Si(100) regions. The thickness of the nanostructures is controlled by the exposing time to the electron beam from STM tip, while the width is controlled only by the geometry of the tip apex and the sample-tip separation. STS data confirm that the HfB$_{2}$ nanostructures deposited are pure metallic, indicating minimum contaminations in the nanostructures, which we attribute to the carbon-free nature of the CVD precursor. To our knowledge this is the first demonstration of sub-5 nm metallic nanostructures in a STM/CVD experiment. [Preview Abstract] |
Session H24: Focus Session: Chemical Modification of Nanotubes
Sponsoring Units: DCMPChair: Traian Dumitrica, University of Minnesota
Room: 326
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H24.00001: Single Molecule Detection in Living Biological Cells using Carbon Nanotube Optical Probes Invited Speaker: Nanoscale sensing elements offer promise for single molecule analyte detection in physically or biologically constrained environments. Molecular adsorption can be amplified via modulation of sharp singularities in the electronic density of states that arise from 1D quantum confinement [1]. Single-walled carbon nanotubes (SWNT), as single molecule optical sensors [2-3], offer unique advantages such as photostable near-infrared (n-IR) emission for prolonged detection through biological media, single-molecule sensitivity and, nearly orthogonal optical modes for signal transduction that can be used to identify distinct classes of analytes. Selective binding to the SWNT surface is difficult to engineer [4]. In this lecture, we will briefly review the immerging field of fluorescent diagnostics using band gap emission from SWNT. In recent work, we demonstrate that even a single pair of SWNT provides at least four optical modes that can be modulated to uniquely fingerprint chemical agents by the degree to which they alter either the emission band intensity or wavelength. We validate this identification method in vitro by demonstrating detection and identification of six genotoxic analytes, including chemotherapeutic drugs and reactive oxygen species (ROS), which are spectroscopically differentiated into four distinct classes. We also demonstrate single-molecule sensitivity in detecting hydrogen peroxide, one of the most common genotoxins and an important cellular signal. Finally, we employ our sensing and fingerprinting method of these analytes in real time within live 3T3 cells, demonstrating the first multiplexed optical detection from a nanoscale biosensor and the first label-free tool to optically discriminate between genotoxins. We will also discuss our recent efforts to fabricate biomedical sensors for real time detection of glucose and other important physiologically relevant analytes in-vivo. The response of embedded SWNT in a swellable hydrogel construct to osmotic pressure gradients will be discussed, as well as its potential as a unique transduction mechanism for a new class of implantable sensors. \\[4pt] [1] Saito, R., Dresselhaus, G. {\&} Dresselhaus, M. S. \textit{Physical Properties of Carbon Nanotubes }(Imperial College Press, London, 1998). \\[0pt] [2] Barone, P. W., Baik, S., Heller, D. A. {\&} Strano, M. S. Near-Infrared Optical Sensors Based on Single-Walled Carbon Nanotubes. \textit{Nature Materials }\textbf{4}, 86-92 (2005). \\[0pt] [3] Jeng, E. S., Moll, A. E., Roy, A. C., Gastala, J. B. {\&} Strano, M. S. Detection of DNA hybridization using the near infrared band-gap fluorescence of single-walled carbon nanotubes. \textit{Nano Letters }\textbf{6}, 371-375 (2006). \\[0pt] [4] Heller, D. A. et al. Optical detection of DNA conformational polymorphism on single-walled carbon nanotubes. \textit{Science }\textbf{311}, 508-511 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H24.00002: Argon and Krypton Adsorption Isotherms on Single Carbon Nanotube Devices Zenghui Wang, Peter Morse, Jiang Wei, Oscar Vilches, David Cobden We have fabricated mass balances each consisting of an individual single-walled carbon nanotube suspended across a micron-sized trench in an oxidized Si wafer. The vibrational resonance frequency of a nanotube, which is in the range 50-500 MHz, is determined by monitoring the current through it while applying an electrostatic driving signal. By tracking changes in the resonance frequency we have measured isotherms of adsorbed mass vs vapor pressure for Ar ot Kr at liquid nitrogen temperatures. The sensitivity of the balances corresponds to just a few atoms. We have compared the monolayer mass shifts due to Ar and Kr, and measured a family of isotherms of Ar below 77 K. From the latter we calculated the isosteric heat of adsorption on the nanotube surface, which is found to be lower than that of Ar on basal plane graphite and only slightly larger than the latent heat of sublimation of bulk Ar at these temperatures. In one device we observed a phase transition in the adsorbed Ar near monolayer completion. In another device, which probably consists of two nanotubes joined in parallel, we observed enhanced adsorption at lower coverages which may be in the groove between the two nanotubes. This work is supported by the NSF, grant number 0606078. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H24.00003: Resonance Raman study of Polyynes encapsulated in single-wall Carbon Nanotubes with different diameters. L.G. Moura, L.M. Malard, D. Nishide, Y. Achiba, H. Shinohara, M.A. Pimenta Polyynes are one of the simplest linear carbon chains and they have been recently encapsulated in single-wall carbon nanotubes. The stability of encapsulated polyynes opens a way to investigate experimentally these sp-hydrized carbon structures and to study of electronic correlation effects in 1D systems with potential applications in nanoelectronics. In this work we present a resonance Raman study of C$_{10}$H$_2$ and C$_{12}$H$_2$ polyynes inside single-wall carbon nanotubes with different diameters, using many different laser energies in visible range. We show that the observed optical resonance energies of the polyynes depends on the diameter of nanotubes, the maximum of the resonances decreasing with increasing diameter of the nanotubes. Moreover, the resonance energy is generally lower for C$_{12}$H$_2$ than C$_{10}$H$_2$. We have also observed a red-shift and strong changes in the shape of Raman G band of the metallic nanotubes when they encapsulate the polyynes, and these results were interpreted in terms charge transfer between these two systems and its effect on the electron-phonon coupling of the nanotubes. We have also observed that the interaction between polyynes and nanotubes is stronger for nanotubes with small diameters. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H24.00004: Alkanes adsorbed on carbon nanotubes: specific surface areas and isothermal adsorption kinetics Dinesh Rawat, Toyohisa Furuhashi, Aldo Migone, Jose Antonio Ramirez-Pastor, Federico Roma, Jose Luis Riccardo We measured the specific surface area of single-wall carbon nanotube substrates using methane, ethane, propane and butane adsorption isotherms. The monolayer capacity for each gas was obtained from the BET equation. We found that the specific surface areas measured decrease as the length of the alkanes used to measure them increases. This trend may be explained by the fact that an increasing fraction of the substrate's surface should be left uncovered as longer adsorbates are used. We also studied the isothermal adsorption kinetics of these alkanes. We monitored the evolution of the gas pressure with time, from the instant at which the gas is added to the sample cell up until the time at which equilibrium is reached. Equilibration times for comparable fractional coverages increase with increasing alkane length. While the equilibration times decrease with increasing fractional coverage for methane and ethane, they increase with increasing coverage for propane and butane (this increase may be due to reorientation of the adsorbed molecules in the film). [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H24.00005: Study of propane films adsorbed on Purified HiPco Carbon Nanotubes Toyohisa Furuhashi, Dinesh Rawat, Aldo Migone We investigated the adsorption characteristics of propane on purified HiPco single-walled carbon nanotubes for coverages limited to the first layer. We found two prominent substeps present in the monolayer; this suggests the presence of at least two distinct groups of binding energy sites in the substrate. The results are similar to those that we have found for ethane, but different from those we measured for butane, on the same substrate. We compare and contrast the characteristics of the isotherms for these three alkanes. A high-binding-energy (low-pressure) adsorption isotherm substep is present for all three alkanes. By contrast, the high- pressure substep (corresponding to adsorption on the outer surface area of the nanotubes bundles) shows a gradual smearing with increasing alkane length; this feature is barely resolvable for butane. We also found a gradual increase in the binding energy of alkane molecules with the increasing carbon chain length. This result confirms findings of simulation results for similar systems. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H24.00006: Adsorption of Nitro Aromatics on Single-Walled Carbon Nanotubes Erik Alldredge, Stefan Badescu, Thomas Reinecke, Navdeep Bajwa, F. Keith Perkins, Eric Snow Recent experiments with arrays of carbon nanotubes reveal a strong conductivity response after exposure to aromatic molecules containing nitro functional groups, such as nitrobenzene and trinitrotoluene. The detection of these compounds is of particular interest in the use of nanotube arrays as chemical sensors. To develop an understanding of the microscopic mechanisms involved, we perform detailed \textit{ab initio} calculations of adsorption geometries, charge configurations, and vibration spectra for these compounds on pristine armchair and zigzag nanotubes. We use density functional theory with localized orbitals in a cluster approach and the M05-2X functional that is appropriate for the weak interactions of physisorption for these systems. We find a strong increase in adsorption energy with the addition of each nitro group to a molecule (around 100 meV) and a gradual increase with nanotube size, in agreement with preliminary experimental results. For most of these compounds, little charge transfer ($<$ 0.1 $e)$ occurs. Finally, these calculations are compared with results for the adsorption at oxidation defects. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H24.00007: Transparent Boron-Doped Nanotube Films Xiaoming Liu, Hugo Romero, Humberto Gutierrez, Peter Eklund We report results of FTIR transmission and temperature-dependent resistance measurements on transparent thin films of bundled single-walled carbon nanotubes exposed to B2O3 at 1000C. This reaction is proposed to B-dope the films. Doping is observed to lower the T=300 K dc sheet resistance by a factor of five without changing the optical transmittance in the visible range and suggests that boron-doped SWNT provide a better approach to transparent electrodes. The optical transmission (T=300K) of SWNT and B- doped SWNT films measured in the range 50-7000 cm-1 show that the doped films have a greater optical density in the mid to far IR, consistent with the B-doping creating new free carriers. This optical result shows that the DC conductivity of the doped tubes is indeed higher, with the interpretation not being entangled with changes in the tube-tube contact resistance within the film, as would be the case in a dc conductivity measurement. Also the dc resistance of both B-doped and annealed SWNT films were measured over 10$<$T$<$300K. The data showed for the doped films the metallic term (linear in T) is $\sim $10 times greater than for the annealed and undoped films. Work supported by NSF NIRT ECS 06-09243. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H24.00008: Kinetic selectivity effects of binary mixtures on nanotube bundles: Internal and external adsorption Seyoum Tsige, Mercedes Calbi, Jared Burde We investigate kinetic selectivity effects that take place during the adsorption of a binary mixture inside a nanotube and on the external surface of a bundle. By using a kinetic Monte Carlo Scheme we allow adsorption on sites with different binding energy (external surface) and we restrict adsorption/desorption only to the end sites in the case of the internal adsorption. In both cases, we analyze the appearance of an overshoot in the fractional coverage of the weaker species (before equilibrium) observed previously for a single, homogeneous, one-dimensional chain. \newline [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H24.00009: Temperature Programmed Desorption on Carbon Nanotube Bundles: a Computer Simulation study Nayeli Zuniga, Mercedes Calbi, Jared Burde We present a study of gas desorption on external surfaces of carbon nanotube bundles by means of a Kinetic Monte Carlo scheme. Starting with an initial coverage, we follow the amount of gas desorbed as the temperature increases for different values of initially occupied sites and initial temperature of the sample. We analyze the spectrum obtained in terms of the different binding energies of the adsorption sites. We also compare our results with some available experimental measurements on nanotube bundles and other nanostructures. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H24.00010: Scattering Process of Gas Molecules on Vertically Aligned Single-Walled Carbon Nanotubes Ikuya Kinefuchi, Yushi Harada, Junpei Kawasaki, Kei Ishikawa, Junichiro Shiomi, Shu Takagi, Shigeo Maruyama, Yoichiro Matsumoto Scattering process of gas molecules on quartz surfaces covered with vertically aligned single-walled carbon nanotubes (VA-SWNTs) was investigated using the molecular beam technique. We found that the surface modification with VA-SWNT films significantly enhances the energy transfer between gas molecules and surfaces at room temperature and makes the energy accommodation coefficient of helium, which tends to be small even for contaminated surfaces because of the large mass mismatch between helium and surface atom, close to unity. Our results demonstrate a potential application of VA-SWNTs as nanoscale fin structures to enhance heat transfer between gas phase and solid surfaces. As the surface temperature increases, however, the energy accommodation becomes less efficient since the small adsorption energy reduces the trapping probability of helium on carbon nanotube (CNT) bundles. The weak dependence of the accommodation coefficient on the film thickness suggests that gas molecules penetrate into the films because of their high porosity and suffer more than one collision with CNT bundles. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H24.00011: Carbon Nanotube CdSe Nanoparticle Hybrid Materials: Synthesis and Optical Properties Austin Akey, Chenguang Lu, Wei Want, Irving Herman Carbon nanotubes present remarkable opportunities for the construction of nanomaterials with unique properties, and for use in sensors and optoelectronic device applications. Chemical attachment of nanoparticles to nanotubes has thus far resulted in low loading; direct nucleation of particles on the tube sidewalls leads to a loss of control over particle size and monodispersity. We report the synthesis of novel heterostructures composed of single-walled carbon nanotubes and chemically attached, monodisperse cadmium selenide nanoparticles. Pyridine is used to strip the ligand shell from the nanoparticles, which are then bound to SWNTs in suspension. The resulting hybrid material is stable and resists aggregation; TEM and SEM characterization shows the nanotubes to be densely covered with nanoparticles. The nanoparticles used range in size from 3.5 to 6.0 nm in diameter, and exhibit strong quantum confinement. Also synthesized were hybrids of carbon nanotubes with core-shell CdSe/ZnS nanoparticles and with CdSe nanorods. The absorption and photoluminescence properties of the hybrid materials are also presented. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H24.00012: Electrochemical and Optical Characterization of Metal-Decorated Carbon Nanotubes Tatyana Sheps, Vaikunth R. Khalap, Alexander A. Kane, Philip G. Collins, Hyunmin Kim, Eric O. Potma Hybrid electrodes combining carbon nanotubes with metal and metal oxide particles are promising for many catalytic applications including energy conversion and energy storage. Understanding the chemical interactions between the nanotube substrate and the catalytic nanopaticle is crucial for optimizing these types of electrodes. Here, we describe techniques for interrogating the metal-nanotube interface on the single-molecule level, using isolated single-walled nanotubes (SWNTs) decorated by single metal particles as the most basic, representative element of a bulk hybrid electrode. The resulting composite and its chemical interface is studied by two complementary techniques, electrochemical voltammetry and Raman spectroscopy. Results comparing the electrochemical behavior with the surface chemistry are presented. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H24.00013: Electrochemistry of single-walled carbon nanotubes (SWCNTs) supporting single palladium nanoparticles Vaikunth R. Khalap, Tatyana Sheps, Alexander A. Kane, Philip G. Collins The capabilities to produce, chemically tailor, and label point defects [1-3] provide a versatile toolkit for studying complex model systems based on SWNCTs and catalytic metals. Here, we describe experiments on individual SWCNTs with and without point defects and Pd nanoparticles. In this single particle limit, the effects of a defect on a supported Pd nanoparticle can be very clearly discerned. Electrochemical voltammetry reveals an important interplay between the Pd catalytic activity and the defect termination chemistry. In addition, conductivity measurements reveal an enormous response to H2 gas, making the devices competitive commercial H2 sensors. This research is partly supported by NSF (CBET-0729630) and a GAANN fellowship (VRK). [1] B. Goldsmith et al, \textit{Science} \textbf{315} 77 (2007) [2] J. Coroneus et al, \textit{ChemPhysChem} \textbf{9 }1053 (2008) [3] Y. Fan et al, \textit{Nature Materials} \textbf{4} 906 (2005) [Preview Abstract] |
Session H25: Focus Session: Probing and Modifying Materials with Lasers I
Sponsoring Units: DMPChair: Michael Kelley, College of William and Mary
Room: 327
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H25.00001: Advances in micro/nano scale materials processing by ultrafast lasers Invited Speaker: Materials processing by ultrafast lasers offers several attractive possibilities for micro/nano scale applications based on surface and in bulk laser induced modifications. The origin of these applications lies in the reduction of undesirable thermal effects, the non-equilibrium surface and volume structural modifications which may give rise to complex and unusual structures, the supression of photochemical effects in molecular substrates, the possibility of optimization of energy dissipation by temporal pulse shaping and the exploitation of filamentation effects. Diverse applications will be discussed, including the development and functionalization of laser engineered surfaces, the laser transfer of biomolecules and the functionalization of 3D structures constructed by multiphoton stereolithography. Two examples will be presented in this context: A new approach for the development of superhydrophobic, self-cleaning surfaces [1,2] and the fabrication of functional scaffolds for tissue engineering applications [3-5]. \\[4pt] References: \\[0pt] [1] V. Zorba et al., ``Biomimetic artificial surfaces quantitatively reproduce the water repellency of a Lotus leaf'', Advanced Materials 20, 4049 (2008).\\[0pt] [2] V. Zorba et al., ``Tailoring the wetting response of silicon surfaces via fs laser structuring'', Applied Physics A 93, 819 (2008).\\[0pt] [3] V. Dinca et al., ``Quantification of the activity of biomolecules in microarrays obtained by direct laser transfer'', Biomedical Microdevices 10, 719 (2008).\\[0pt] [4] B. Hopp et al., ``Laser-based techniques for living cell pattern formation'', Applied Physics A 93, 45 (2008).\\[0pt] [5] V. Dinca et al., ``Directed three-dimensional patterning of self-assembled peptide fibrils'', Nano Letters 8, 538 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H25.00002: Positional Accuracy in Optical Trap-Assisted Nanolithography Craig B. Arnold, Euan McLeod The ability to directly print patterns on size scales below 100 nm is important for many applications where the production or repair of high resolution and density features are important. Laser-based direct-write methods have the benefit of quickly and easily being able to modify and create structures on existing devices, but feature sizes are conventionally limited by diffraction. In this presentation, we show how to overcome this limit with a new method of probe-based near-field nanopatterning in which we employ a CW laser to optically trap and manipulate dispersed microspheres against a substrate using a 2-d Bessel beam optical trap. A secondary, pulsed nanosecond laser at 355 nm is directed through the bead and used to modify the surface below the microsphere, taking advantage of the near-field enhancement in order to produce materials modification with feature sizes under 100 nm. Here, we analyze the 3-d positioning accuracy of the microsphere through analytic modeling as a function of experimental parameters. The model is verified in all directions for our experimental conditions and is used to predict the conditions required for improved positional accuracy. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H25.00003: 3D photofabrication by femtosecond laser pulses and its applications in biomedicine. Aleksandr Ovsianikov, Xiao Shizhou, Mangirdas Malinauskas, Boris Chichkov Two-photon polymerization (2PP) is a novel laser-based microstructuring technique. In this approach, the multiphoton absorption of femtosecond laser pulses is used to induce a highly localised chemical reaction leading to a photopolymerization of the material. By moving the laser focus in 3D the trace of modified material is created. In the next step, the unmodified material is removed by an appropriate developer, and the fabricated structure is revealed. Therefore, fabrication of any computer generated 3D structure by ''direct laser recording'' into the volume of photosensitive material is possible. Nonlinear nature of the interaction allows true 3D microfabrication and realisation of structures with submicrometer resolution. Recently, we studied possible applications of 2PP technique in biomedicine. It is a very attractive technology for the fabrication of drug delivery systems and medical implants. Application of 2PP in tissue engineering provides possibility to fabricate series of exactly identical scaffolds. Therefore, it is possible to conduct systematic studies of cell interactions in 3D environment. In this contribution, our recent advances in two-photon activated laser processing, properties of applied materials, and applications of this technology are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H25.00004: Single-pulse fabrication of deep vertical nano-holes with a microjoule~femtosecond laser. Lloyd M. Davis, Yelena V. White, Xiaoxuan Li, Zbigniew Sikorski, William H. Hofmeister When a single energetic 200-femtosecond laser pulse is tightly focused onto the surface of a dielectric material, Zener (tunnelling) ionization and Zener-seeded saturation avalanche ionization cause a plasma to be formed at the surface. The tail of the pulse reflects from the plasma so that the resultant damage is typically shallow. However, we have found that when the laser pulse is focused with negative spherical aberration, holes exceeding 11 microns in depth and with diameters at the surface of only 200---500 nm may be created. A simple acetate sample replication technique is used to estimate the dimensions of the holes. The validity of the replication technique for characterizing such high-aspect-ratio features is confirmed by focused ion-beam sectioning of holes followed by visualization with a scanning electron microscope. We discuss physical processes that may be responsible for the creation of such deep nano-holes, including the role of self-focusing of the laser beam, and the possibility of acceleration of electrons along the direction of the laser beam into the hole. Applications, including direct laser writing of vertical channels for nanofluidic devices, are discussed. Related research on femtosecond laser machining of diamond is also presented. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H25.00005: Optical Bulk Metamaterials Invited Speaker: Metamaterials are artificially designed subwavelength composites that possess extraordinary properties not existing in naturally occurring materials. In particular, they can alter the propagation of electromagnetic waves resulting in negative refraction, sub-wavelength focusing, and transmission of sub-wavelength information over a finite distance. Such unusual properties can be obtained by a careful design of the metal-dielectric composites on a deep sub-wavelength scale. The metamaterials may have profound impact in wide range of applications such as nano-scale imaging, nanolithography, and integrated nano photonics. I will discuss a few recent experiments demonstrating intriguing phenomena associated with Metamaterials. These include sub-diffraction limit imaging and focusing, low loss negative refraction and imaging in bulk optical metamaterials, and Negative-index Metamaterials (NIM) exhibiting negative phase propagation that can be accessed from free-space. I'll also discuss nano plasmonics for imaging and bio-sensing. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H25.00006: Coloration of single crystal ZnO with ultraviolet laser irradiation Enamul Khan, S.C. Langford, J.T. Dickinson Many dielectric crystals color in the visible region of the spectrum when exposed to intense ultraviolet light such as excimer laser irradiation. In the alkali and alkaline earth halides, the decay of self-trapped excitons produces strongly absorbing defects. We were recently surprised to see single crystal ZnO darken dramatically during exposure to 193-nm ArF excimer laser radiation. ZnO is a wide bandgap (3.44 eV) semiconductor of significant technological interest. The increase in absorption is very broadband, extending from the bandgap into the infrared, and appears nearly black or grey. We present convincing evidence that this color is due to metallic zinc nanoparticles residing on the surface of the irradiated region. As expected, the laser fluence has considerable impact on the size, number, and spatial distribution of these nanoparticles. We propose a model for production of metallic nanoparticles on the surface. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H25.00007: Generation of point defects in femtosecond laser interactions with Cr targets Eaman Tahir, Zhibin Lin, Leonid Zhigilei The mechanisms and driving forces responsible for the generation of point defects (vacancies and interstitials) in femtosecond laser interactions with Cr target are investigated in atomic-scale simulations. Two series of simulations are performed. In the first set of simulations, the processes induced by 200 fs laser pulse irradiation of a bulk Cr target are studied with a computational model that combines the classical molecular dynamic method with a continuum description of the laser excitation of conduction band electrons, electron-phonon coupling, and electron heat conduction. The distribution of point defects in the surface regions of irradiated targets is analyzed for a broad range of laser fluences, covering the regimes of surface melting, photomechanical spallation, and ablation. To investigate the relative contributions of the thermally-activated generation of vacancy-interstitial pairs and the production of the vacancies during the solidification process, the second series of simulations of solidification at fixed temperatures below the melting point is performed. The densities of vacancies generated under different undercooling conditions are related to the distributions observed in laser-irradiated targets. The implications of the computational predictions for atomic mixing and damage accumulation in multi-pulse irradiation regime are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H25.00008: Nanostructuring of thin gold films by femtosecond laser-induced melt dynamics Arseniy Kuznetsov, Jurgen Koch, Boris Chichkov Femtosecond laser irradiation of thin gold films with fluences close to the ablation threshold induces formation of various nanostructures on the film surface [1-3]. These structures are formed due to laser-induced melting of gold and redistribution of the melted material from the edges into the center of the irradiated region. Shape of the created structures can be controlled by varying the laser intensity distribution on the gold film surface. It is shown that the sizes of the produced structures can be below than 100 nm. In this paper, a study of laser-induced molten material dynamics is presented. Analysis of the structural shapes produced with different laser intensity distributions and dynamical reflection measurements allow clarifying mechanisms of the structure formation. Possible applications of this nanostructuring method are discussed. \\[3pt] [1] F. Korte, J. Koch, B.N. Chichkov, Appl. Phys. A 79 (2004), pp. 879-881. \\[0pt] [2] J.Koch, F. Korte, T. Bauer, C. Fallnich, A. Ostendorf, B.N. Chichkov, Appl. Phys. A 81 (2005), pp. 325-328. \\[0pt] [3] A.I. Kuznetsov, J. Koch, B.N. Chichkov, Appl. Phys. A (2008), DOI 10.1007/s00339-008-4859-6. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H25.00009: Laser-diffraction-assisted ordering effects in nano-quasipercolated silver thin films. E. Haro-Poniatowski, J. C. Alonso-Huitron, C. Acosta-Zepeda, M. C. Acosta-Garcia, N. Batina Nanostructured silver thin films in a quasipercolated state are used as the starting morphology for inducing changes in shape and ordering effects by pulsed laser irradiation with a Nd:YAG laser (355 nm) [1,2]. The complex nanostructures are transformed into nanospheres which in turn are ordered in regular patterns when irradiated through a diffractive element such as a slit or a pinhole. The samples are deposited onto TEM grids by the pulsed laser deposition technique. These transformations are subsequently characterized by transmission electron microscopy and by atomic force microscopy. The observed effects are explained using Fresnel diffraction theory. Excellent agreement with the experimental results are obtained. [1] E. Haro-Poniatowski et al. Appl. Phys. Lett. 87, 143103, 2005 [2] E. Haro-Poniatowski et al. Radiation Effects {\&} Defects in Solids 162, 491-499 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H25.00010: Examining Femtosecond Laser Induced Plasma Dynamics via Ultrafast Electron Shadow Imaging Junjie Li, Richard Clinite, Xuan Wang, Jim Cao, Zhaoyang Chen, Samuel Mao We report a study of the dynamics of plasma creation and evolution in real time using a pulsed electron beam to create two-dimensional shadow images of the plasma plume. Due to the electron beam's sensitivity to charge, the dynamics of electric fields and charge motion during the earliest stage of laser ablation of Copper plate were directly measured by taking snapshots of the plasma shadow images. Based on a dipole field assumption, a multiplying magnitude of Q(total charge), d(average electrons' distance from surface), and t(duration time of field) is obtained, Q*d*t $\sim $ 12*10$^{-8}$ e*m*s under 6.8*10$^{12}$W/cm$^{2}$ laser power on 0.3mm diameter area, moreover, most electrons which are initially emitted retract back in 1 to 2 ps after laser pulse. The results provide new information about multi-photon emission and charge motion during intense laser material interaction. [Preview Abstract] |
Session H26: Focus Session: Graphene IV: Electronic and Structural Properties
Sponsoring Units: DMPChair: Stefano Curtarolo, Duke University
Room: 328
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H26.00001: Graphene-based nanomechanical cooling device Wan-Ju Li, Daoxin Yao, Erica Carlson We propose a novel structure for nanomechanical cooling, based upon graphene. Because thermal transport occurs perpendicular to the surface of the substrate, the proposed structure can be used to facilitate thermal transport between two objects in contact. Furthermore, the strength of the Seeback coefficient may be tuned by applying pressure. We calculate the Seebeck coefficient in this geometry, as a function of applied voltage and strain. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H26.00002: Doping Single-Layer Graphene from SiO2 Substrates or Jointed Few-Layer Graphene Lain-Jong Li, Yumeng Shi Electrostatic force microscopy (EFM) measurements reveal that the surface potential of graphene either increases or decreases, depending on the initial surface potential of substrates, to a ``bulk'' value with their thickness. Graphene layers tend to screen of the substrate potential, which drives the charge exchange between graphene and the substrates. The direct consequence is the effective doping of graphene, where the substrate-dependent p- or n-doping of graphene is corroborated by Raman spectroscopy and electrical measurements for the transistor devices. Our results promise a practical method for tailoring the electronic properties of graphene for nanoelectronics. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 9:00AM |
H26.00003: Graphene-based Materials Invited Speaker: Our top-down approaches [Lu et al.] inspired physicists to obtain graphene by micromechanical exfoliation. Another approach to individual layers involves converting graphite to graphite oxide (GO) to generate aqueous colloidal suspensions of `graphene oxide'(GO') sheets. (i) Reduced GO' (RGO') sheets were embedded in polymers such as polystyrene and their dispersion/morphology studied by SEM/TEM, and the conductivity/ percolation threshold of such composites was determined; (ii) individual GO' and RGO' sheets were studied to elucidate their chemical, optical, and electrical properties, (iii) GO' and RGO' sheets were embedded in thin glass films by a sol-gel route yielding conductive/transparent films, (iii) a `paper' material of stacked GO' sheets was made and characterized, (iv) powders composed of RGO' showed exceptional promise for use in ultracapacitors, and (v) C13-labeled GO was made and the detailed chemical structure of GO was determined with SS NMR.~ --Lu,Yu,Huang,Ruoff, ``Tailoring graphite with the goal of achieving single sheets'', Nanotechnology, 10, 269-272 (1999). See also http://bucky-central.me.utexas.edu/publications.htm 139, 146, 150, 155, 160, 164, 166, 168, 169, 174, 179-182, 184 where collaborators are shown as coauthors. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H26.00004: Quantum transport and Klein tunneling in graphene heterojunctions Andrea Young, Philip Kim I will discuss the observation of quantum conductance oscillations in extremely narrow graphene heterostructures where a resonant cavity is formed between two electrostatically created bipolar junctions. From analysis of the observed interference pattern, it can be inferred that individual p-n junctions have a collimating effect on transmitted carriers, leading directly to the observation of resonant oscillations despite the largely diffusive carrier dynamics. The oscillatory part of the conductance is insensitive to scattering within individual p-n junctions, electrons which scatter in the junctions, making them a novel probe of the ballistic physics of graphene at the Dirac point and allowing an estimate of the electric field due to nonlinear screening. In a weak applied magnetic field, the oscillations undergo a phase shift characteristic of reflectionless normal transmission, or ``Klein Tunneling,'' at the individual p-n junctions. Finally, at high magnetic field, graphene heterostructures show modified Shubnikov de Haas oscillations due to the inhomogenous external potential. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H26.00005: Electronic transport properties of graphene irradiated by charged particles Jian-Hao Chen, William Cullen, Chaun Jang, Michael Fuhrer, Ellen Williams We have measured the effect of low energy charged particle irradiation (electrons, He ions, Ne ions or Ar ions) on the electronic transport properties of clean graphene devices on SiO$_{2}$. Charged particle irradiation induces additional scattering which is consistent with adding both short-ranged (i.e. point defect) and long-ranged (i.e. charged) impurities to the device. We also performed temperature-dependent conductivity of the irradiated devices from 9K to 400K in ultra high vacuum. In sharp contrast to graphene with charged impurity disorder, which remains metallic at low temperature, even a small amount of irradiation-induced disorder (one order of magnitude increase in room-temperature resistivity compared to pristine devices) produces a divergence of the resistivity and insulating behavior at low temperature for carrier densities below 4*10$^{12}$cm$^{-2}$. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H26.00006: Fractionalization in graphene-like systems Conan Weeks, Marcel Franz I will discuss the possible interaction-driven instabilities that can arise in a system of massles Dirac fermions modeled by the extended Hubbard model on the $\pi$ flux square lattice and the honeycomb lattice, and their relevance to fractionalization in 2D graphene-like systems. Through numerical studies we have shown that these instabilities can result in a number of interesting phases. In addition to the charge density wave and various stripe phases these include the exotic ``quantum anomalous Hall'' (Haldane) phase and the dimerized ``Kekul\'e'' phase. A self consistent calculation inside the Kekul\'e phase on the $\pi$ flux lattice indicates that a discretized $U(1)$ vortex can be stabilized in this region leading to a zero-energy bound state with fractional charge $e/2$. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H26.00007: Diffraction Symmetries in Epitaxially-Grown Graphene and the SiC Substrate David Siegel, Shuyun Zhou, Farid El Gabaly, Andreas Schmid, Kevin McCarty, Alessandra Lanzara Diffraction patterns in epitaxially-grown graphene and the SiC substrate are studied by observing dark field low energy electron microscopy (LEEM) and low energy electron diffraction (LEED) images. The breaking of 6-fold symmetry is mapped out as a function of the position on the sample for different crystal periodicities. Such observations provide information about the stacking and domain boundaries in epitaxially-grown graphene. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H26.00008: Dynamical conductivity of graphene Oskar Vafek Frequency dependent conductivity of Coulomb interacting massless Dirac fermions coupled to random scalar and random vector potentials is found as a function of frequency in the regime controlled by a line of fixed points. Such model provides a low energy description of a weakly rippled suspended graphene. The main finding is that at the neutrality point the a.c. conductivity is not frequency independent and may either increase or decrease with decreasing frequency, depending on the values of the disorder variances $\Delta_{\phi}$, $\Delta_ {A}$ and the Coulomb coupling $\alpha=e^2/(\epsilon v_F)$. The low frequency behavior is characterized by the values of two dimensionless parameters $\gamma=\Delta_{\phi}/\alpha^2$ and $\Delta_A$ which are RG invariants, and for small values of which the electron-hole ``puddles'' are effectively screened making the results asymptotically exact. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H26.00009: Charge transfer and electronic structure of graphene and graphite intercalation compounds T. Valla, J. Camacho, M. H. Upton, Z.-H. Pan, A. V. Fedorov, A. C. Walters, C. A. Howard, M. Ellerby In graphite intercalation compounds (GIC), layers of different chemical species (intercalants) are introduced between graphene sheets. Due to the charge transfer between the intercalant and graphene layers, intercalation allows a controlled doping of graphene sheets and a broad variation of many physical properties, including the emergence of relatively high transition temperature superconductivity in some GICs. We have studied the changes in the electronic structure of various GICs in angle-resolved photoemission spectroscopy and found that, with the doping of graphene sheets, the electronic correlations become stronger and more anisotropic. In particular, the electron phonon coupling of graphene-derived electronic states with high-frequency graphene-derived vibrations increases dramatically with the amount of charge transfer. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H26.00010: Low Temperature Atomic Layer Deposition of Thin HfO$_{2}$ Film as Top Gate Oxide in Graphene Field Effect Transistors K. Zou, D. Keefer, X. Hong, J. Zhu We explore the possibility of using HfO$_{2}$ as top gate oxide in graphene field effect transistors (FETs). Graphene flakes are obtained by mechanically exfoliating HOPG graphite on SiO$_ {2}$ (300nm)/doped Si substrates. We fabricate graphene FETs using e-beam lithography and metal electrodes deposition. A second e-beam writing is used to define the area of the HfO$_{2} $ over-layer. It is followed by an atomic layer deposition (ALD) of 30 nm HfO$_{2}$ film at low temperature without the use of an adhesion layer. This low-temperature recipe produces smooth HfO$_{2}$ films with RMS roughness of 2-3{\AA} over a 1x1 $\mu$m area. These films exhibit a dielectric constant of $\sim$12-15 and a breakdown field of $\sim$0.8 MV/cm. Carrier mobility in HfO$_{2}$-covered FETs is comparable to that of uncovered graphene. We report and discuss the influence of the HfO$_{2}$ over-layer on the transport properties of graphene. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H26.00011: Graphene nanoribbons and flakes produced from graphite in solution Gabriel Setzler, L.K.K.D Chamath, Zhixian Zhou We developed a method to fabricate graphene nanoribbons and flakes from graphite in solution without covalent chemical functionalization. Atomic force microscopy (AFM) was used to characterize individual graphene samples deposited onto Si/SiO$_{2}$ substrates. Few layer (and possibly monolayer) graphene sheets were observed, including micron size graphene flakes and nanoribbons with very high aspect ratios. Possible mechanisms of graphite exfoliation and graphene nanoribbon formation will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H26.00012: Electronic structure of graphene nanoflakes: band gap evolution with increasing size E. Martinez-Guerra, M.E. Cifuentes-Quintal, I. Rodriguez-Vargas, S. Vlaev, R. de Coss Graphene has unusual electronic properties which make it a promising material for electronic devices. Neverthless, the absence of a band gap sets limitations on its applications. Thus, it is crucial to tune the band gap of systems based on graphene. In this way, we explore to modulate the electronic properties of graphene through the size system. In particular, we studied the evolution of the electronic structure of graphene nanoflakes (C$_{6n2}$H$_{6n})$ as a function of size. The calculations were performed using the pseudopotential LCAO method with a Generalized Gradient Approximation for the exchange-correlation functional. We found that the energy gap decreases as the diameter of the nanoflakes increases. From a tight-binding analysis, we show that the energy gap reduction is due to an HOMO-LUMO bands widening. This effect results from the fact that as nanoflakes size increases the ratio between the number of internal and periferical atoms decreases. Thus, we found that controlling the size of graphene nanoflakes is a useful way for the electronic modulation. Supported from CONACyT Grant 83604. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H26.00013: Fabrication of graphene nanoribbon by local anodic oxidation lithography using atomic force microscope Satoru Masubuchi, Masashi Ono, Kenji Yoshida, Kazuhiko Hirakawa, Tomoki Machida We conducted local anodic oxidation (LAO) lithography in single-layer, bilayer, and multilayer graphene using tapping-mode atomic force microscope (AFM). We found that the width of the insulating oxidized area depends systematically on the number of graphene layers. An 800-nm-wide bar-shaped device fabricated in single-layer graphene exhibits the half-integer quantum Hall effect, which indicates that the conducting channel region of graphene is intact during LAO. We also fabricated a 55-nm-wide graphene nanoribbon (GNR). The conductance of the GNR at the charge neutrality point was suppressed at low temperature, which suggests the opening of an energy gap due to the lateral confinement of charge carriers. These results show that LAO lithography is an effective technique for the fabrication of graphene nanodevices. [Preview Abstract] |
Session H27: Focus Session: Advances in Scanned Probe Microscopy II: Force Methods
Sponsoring Units: DCMPChair: Young Kuk, Seoul National University
Room: 329
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H27.00001: Temperature Dependence of Single-Asperity Friction for Diamond on Diamond and DLC Interfaces C. Dunckle, I.B. Altfeder, P. Taborek A variable temperature, ultrahigh vacuum atomic force microscope with a diamond-coated probe was used to characterize interfacial friction over a temperature range of 30 to 300 Kelvin. A vertical scan was used to measure tip to surface adhesion and contact normal forces. Friction (lateral) force measurements were taken by dragging the tip along the surface. Calibration was done in situ using substrates with known dimensions and angles. Measurements were made on diamond-like carbon surface and a single crystallite in a micro crystalline diamond film. Results were analyzed by fitting into the DMT continuum model. Comparison of friction versus load showed approximately a factor of two increase in the friction at cryogenic temperatures compared to room temperature. Results are qualitatively consistent with MD simulations but are not well described by models of thermally activated friction. Problems associated with temperature gradients at the tip- surface interface will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H27.00002: Small amplitude vibrations of curved atomic force microscope cantilevers: Theory Arvind Narayanaswamy, Carlo Canetta, Ning Gu The shifts in resonance frequencies of cantilevers are used to infer tip--sample interactions in tapping--mode atomic force microscopy as well as in a wide variety of cantilever based sensors. In this work, we investigate theoretically as well as experimentally the effect of curvature on the vibration dynamics of micro--cantilevers to which a micro--sphere is attached at the free end. We show that resonance frequencies of cantilevers to which a tip mass is attached can be altered by controlling the curvature of the cantilever. This control over the resonance frequency spectrum is independent of other causes of resonance frequency variation, such as adsorbed mass on cantilever or variation of material properties due to change in temperature. In the case when the cantilever is a bi--material cantilever, this shift in resonance frequency can be used as to detect changes in the thermal environment of the cantilever. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H27.00003: Intermodulation Atomic Force Microscopy and Spectroscopy Carsten Hutter, Daniel Platz, Erik Tholen, David Haviland, Hans Hansson We present a powerful new method of dynamic AFM, which allows to gain far more information about the tip-surface interaction than standard amplitude or phase imaging, while scanning at comparable speed. Our method, called intermodulation atomic force microscopy (ImAFM), employs the manifestly nonlinear phenomenon of intermodulation to extract information about tip-surface forces. ImAFM uses one eigenmode of a mechanical resonator, the latter driven at two frequencies to produce many spectral peaks near its resonace, where sensitivity is highest [1]. We furthermore present a protocol for decoding the combined information encoded in the spectrum of intermodulation peaks. Our theoretical framework suggests methods to enhance the gained information by using a different parameter regime as compared to Ref. [1]. We also discuss strategies for solving the inverse problem, i.e., for extracting the nonlinear tip-surface interaction from the response, also naming limitations of our theoretical analysis. We will further report on latest progress to experimentally employ our new protocol.\\[3pt] [1] D. Platz, E. A. Tholen, D. Pesen, and D. B. Haviland, Appl. Phys. Lett. 92, 153106 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H27.00004: Independent determination of depth and energy of electronic trap states in dielectric films by Dynamic Tunneling Force Microscopy Jon Paul Johnson, Clayton C. Williams Dynamic Tunneling Force Microscopy (DTFM) is a new scanning probe technique that images electronic states in completely non-conducting films with sub-nanometer spatial resolution$^{1}$. In DTFM, electrons are shuttled via quantum tunneling between a metallic tip and localized electronic states in an insulating dielectric film, while a lock-in amplifier detects an electrostatic force signal that is proportional to the shuttled charge. The DTFM signal provides a map of the available electronic states within tunneling range of the surface. These states are not observable by STM. The depth of the states can be estimated from the dependence of tunneling rate on the tip/sample gap$^{2}$ and also inferred from their apparent lateral size. Images show states below the surface that drop out of the image when the tip/sample gap is increased. A methodology is introduced to independently determine state energy and depth, potentially on a sub-nanometer scale. This work was supported by AFOSR and SRC. [1] J P Johnson and C C Williams, \textit{Nanotechnology} (accepted) [2] N Zheng, et al., \textit{Journ. App. Phys.} \textbf{101}, 093702 [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:24AM |
H27.00005: Applications of AFM for atomic manipulation and spectroscopy Invited Speaker: Since the first demonstration of atom-by-atom assembly [1], atomic manipulation with scanning tunneling microscopy has yielded stunning realizations in nanoscience. A new exciting panorama has been recently opened with the possibility of manipulating atoms at surfaces using atomic force microscopy (AFM) [2-5]. In this talk, we will present two different approaches that enable patterning structures at semiconductor surfaces by manipulating individual atoms with AFM and at room temperature [2, 3]. We will discuss the physics behind each protocol through the analysis of the measured forces associated with these manipulations [3-5]. Another challenging issue in scanning probe microscopy is the ability to disclose the local chemical composition of a multi-element system at atomic level. Here, we will introduce a single-atom chemical identification method, which is based on detecting the forces between the outermost atom of the AFM tip and the atoms at a surface [6]. We demonstrate this identification procedure on a particularly challenging system, where any discrimination attempt based solely on topographic measurements would be impossible to achieve. \\[4pt] \textbf{References}: \\[0pt] [1] D. M. Eigler and E. K. Schweizer, \textit{Nature} \textbf{344}, 524 (1990); \\[0pt] [2] Y. Sugimoto, M. Abe, S. Hirayama, N. Oyabu, O. Custance and S. Morita, \textit{Nature Materials} \textbf{4}, 156 (2005); \\[0pt] [3] Y. Sugimoto, P. Pou, O. Custance, P. Jelinek, M. Abe, R. Perez and S. Morita, \textit{Science} \textbf{322}, 413 (2008); \\[0pt] [4] Y. Sugimoto, P. Jelinek, P. Pou, M. Abe, S. Morita, R. Perez and O. Custance, Phys. Rev. Lett. 98, 106104 (2007); \\[0pt] [5] M. Ternes, C. P. Lutz, C. F. Hirjibehedin, F. J. Giessibl and A. J. Heinrich, \textit{Science} \textbf{319}, 1066 (2008); \\[0pt] [6] Y. Sugimoto, P. Pou, M. Abe, P. Jelinek, R. Perez, S. Morita, and O. Custance, \textit{Nature} \textbf{446}, 64 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H27.00006: Design of a variable temperature scanning force microscope E. Nazaretski, K. S. Graham, J. D. Thompson, J. K. Baldwin, J. A. Wright, D. V. Pelekhov, P. C. Hammel, R. Movshovich We have developed the variable temperature scanning force microscope capable of performing both magnetic resonance force microscopy (MRFM) and magnetic force microscopy (MFM) measurements in the temperature range between 5 and 300 K. Modular design, large scanning area, and interferometric detection of the cantilever deflection make it a sensitive, easy to operate and reliable instrument suitable for studies of the dynamic and static magnetization in various systems. We have verified the performance of the microscope by imaging microfabricated permalloy dots and vortices in Nb thin film in the MFM mode of operation. MRFM spectra in a diphenyl-picryl-hydrazyl film were recorded to evaluate the MRFM mode of operation. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H27.00007: Bimodal atomic force microscopy imaging of isolated antibodies in air and liquids Jose R. Lozano, Elena T. Herruzo, Nicolas F. Martinez, Ricardo Garcia We develop a dynamic atomic force microscopy (AFM) method based on the simultaneous excitation of the first two flexural modes of the cantilever. The instrument, called bimodal AFM, opens up additional channels (amplitude and phase of the 2$^{nd}$ mode) which can be used for imaging with enhanced lateral resolution with respect to amplitude modulation AFM (AM-AFM). Bimodal AFM allows us to resolve the structural components of antibodies in both monomer and pentameric forms. The instrument operates in both high and low quality factor environments, i.e., air an liquids, so that the imaging of biomolecules can be carried out in their natural media. Bimodal AFM is studied in great detail by means of theoretical and numerical methods. Our model allows us to study the material contrast sensitivity of the two additional channels (amplitude and phase of the 2$^{nd}$ mode) that can be used for imaging. The theoretical approach also allows us to estimate the forces applied on the sample during bimodal AFM operation. The calculated forces lie below 120 pN, an essential fact when imaging proteins. This is due to the enhanced sensitivity of 2$^{nd}$ mode phase to detect changes while the cantilever is far away from the sample. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H27.00008: High-Resolution Magnetic Resonance Force Microscopy using Iron Filled Carbon Nanotubes Michael Herman, Palash Banerjee, Kin Chung Fong, Denis Pelekhov, Franziska Wolny, Thomas Muhl, Bernd B\"uchner, Chris Hammel Magnetic Resonance Force Microscopy is able to probe below surfaces to map out spins in a non-destructive manner by measuring the force from the dipolar coupling of a magnetic probe to spins in the sample. We have used low force constant cantilevers with low intrinsic dissipation to obtain 2 spin sensitivity. To obtain better sensitivity one avenue of improvement is to increase the magnetic field gradient from the magnetic probe. ~Iron-filled carbon nanotubes provide a promising route for very high magnetic field gradient micromagnetic probes; we have successfully attached these iron nanowires to IBM style ultrasoft silicon cantilevers. The smaller size of the tip (15 to 25 nm) allows gradients an order of magnitude greater than micron-sized rare-earth magnets. In addition, iron filled carbon nanotubes have the possibility to lower the non-contact friction by reducing the surface area of the probe close to the sample. Iron filled carbon nanotubes also exhibit high anisotropy fields, a result of the shape anisotropy. This work was supported by The Army Research Office under W911NF-07-1-0305 and the National Science Foundation under DMR-0807093. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H27.00009: Electron Spin Magnetic Resonance Force Microscopy of Nitroxide Spin Labels Eric W. Moore, SangGap Lee, Steven A. Hickman, Sarah J. Wright, John A. Marohn Nitroxide spin labels are widely used in electron spin resonance studies of biological and polymeric systems. Magnetic resonance force microscopy (MRFM) is a magnetic resonance technique that couples the high spatial resolution of a scanning probe microscope with the species selectivity of magnetic resonance. We report on our investigations of 4-amino TEMPO, a nitroxide spin label, by force-gradient MRFM. Our microscope operates at high vacuum in liquid helium, using a custom fabricated ultra-soft silicon cantilever in the magnet-on-cantilever geometry. An 18 GHz gap coupled microstripline resonator supplies the transverse field. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H27.00010: A Compact, Wide Temperature Range (300mK-300K) Magnetic Force Microscope using High Resolution Fibre Interferometer and Alignment-Free Cantilevers Ozgur Karci, Munir Dede, Ahmet Oral We describe a design of a Low Temperature Magnetic Force Microscope (LT-MFM) for variable temperatures between milli-Kelvin temperatures to 300 K. The design of LT-MFM is very compact, 23.6mm ODx200mm, flexible and is compatible with almost any cryostat (included PPMS of Quantum Design Inc), even He3 systems or DR, provided that there is enough space. The sensor is mounted on a scan piezo tube which has five electrodes: four quadrants are used for scanning, the fifth electrode is used for dithering the cantilever by means of a digital Phase Lock Loop (PLL) with 5mHz frequency resolution. We employed a fibre interferometer deflection measurement for our LT-MFM. A special alignment holder is designed for this purpose. A 225$\mu $m length MFM cantilever is placed on an Alignment-Free AFM Cantilever holder chip from NanoSensors. Our design can sustain cantilever-fiber alignment down to 300mK without any signal loss. An improved fiber interferometer with $\sim $1x10$^{-3}$ A/$\surd $Hz noise level is designed and used to detect cantilever deflection. LT-MFM also enables us to work under high external magnetic fields. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H27.00011: A novel method to measure 3 components of magnetic fields with submicron resolution using Scanning Hall Probe Microscopy/Gradiometry Ahmet Oral, Munir Dede, Rizwan Akram We present the development of a new 4-lead hall gradiometer and a novel method to measure 3 components( Bx, By {\&} Bz) of magnetic fields on specimen surfaces with submicron resolution using Scanning Hall probe Microscope[1] and gradiometer. We used a 1$\mu $m size P-HEMT Hall sensor, operated in gradiometer configuration to image Bx, By and Bz distribution of a hard disk sample surface at 77K. The SHPM was used in Quartz Crystal AFM tracking mode[2]. This simple and quick novel method shows $\sim $40 better spatial resolution compared to previously developed techniques[3] and can be improved even further, down to sub 50nm resolution. 1. Chang, A.M., et al., \textit{Scanning Hall Probe Microscopy.} Applied Physics Letters, 1992. \textbf{61}(16): p. 1974-1976. 2. Dede, M., et al., \textit{Scanning Hall Probe Microscopy (SHPM) using quartz crystal AFM feedback.} Journal of Nanoscience and Nanotechnology, 2008. \textbf{8}(2): p. 619-622. 3. Gregusova, D., et al., \textit{Fabrication of a vector Hall sensor for magnetic microscopy.} Applied Physics Letters, 2003. \textbf{82}(21): p. 3704-3706. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H27.00012: Development of Superconducting Quantum Interference Device DC Magnetometer for High Magnetic Field and Dilution Refrigerator Applications J.-H. Park, T.P. Murphy, S.W. Tozer, E.C. Palm A commercially available SQUID (Superconducting Quantum Interference Device) DC magnetometer is often limited by its relatively high temperature ($\ge $ 1.9 K) and low magnetic field ($\le $ 7 T) operating environment. The need for the lower temperature and higher field DC magnetization measurements keeps growing as more materials show interesting physical phenomena whose energy scales are relevant to low temperatures ($\sim $ mK). To meet these needs we have developed a probe for a top loading dilution refrigerator in which all the DC magnetometer components including SQUID electronics, detection coil, and sample motion shaft are placed together. The probe was tested in a top loading dilution refrigerator and the results show that the base temperature at 25 mK increased $\sim $ 1.6 {\%} when the sample displacement was 3.2 cm with a speed of 3 cm/min. The moment of the test sample was successfully detected down to 50 mK. Improvement in coil balancing and shielding of the detection coil are planned. [Preview Abstract] |
Session H28: Focus Session: Thermoelectric Materials: Transport Physics
Sponsoring Units: DMP FIAPChair: David Cahill, University of Illinois
Room: 330
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H28.00001: Thermoelectricity: A Bottom-up View Invited Speaker: It is well-known that the nature of electronic transport changes significantly as the length L of the active region of a device is reduced from millimeter down to nanometer dimensions. Historically our understanding of electrical resistance and conduction has progressed top-down: from large macroscopic conductors to small atomic scale conductors. Indeed thirty years ago it was common to argue about what, if anything, the concept of resistance meant on an atomic scale. Since then there has been significant progress in our understanding, spurred by actual experimental measurements made possible by the technology of miniaturization. However, despite this progress in understanding the flow of current on an atomic scale, the standard approach to the problem of electronic transport continues to be top-down and we have argued elsewhere  that an alternative bottom-up viewpoint  can be extremely illuminating [1]. In this talk we will briefly summarize this viewpoint and discuss the unique insights it provides into the subject of thermoelectricity and thermoelectric device design in general and into the possibilities of molecular thermoelectrics in particular. \\[4pt] [1] See for example, S.Datta, ``Nanolectronic Devices: A Unified View,'' to appear in The Oxford Handbook on Nanoscience and Nanotechnology: Frontiers and Advances, eds. A.V. Narlikar and Y.Y. Fu, volume 1, chapter 1, arXiv/0809.4460 [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H28.00002: Nanostructured materials design for thermoelectric applications Keivan Esfarjani, Mona Zebarjadi, Ali Shakouri Nanostructured materials have shown great promise for superior thermoelectric properties. Recently, our collaborators and us have been able to enhance thermoelectric properties of InGaAs by doping it with ErAs nanoparticles. Transport properties are dominated by scattering of electrons with nanoparticles, phonons and impurities. We can design the scattering potential of the former to maximize the power factor, P.is in fact an inverse problem, attempting to solve for the best nanoparticle scattering potential which maximizes P. Using a least square method, we find the potential which minimizes the difference between the actual scattering cross section and its target value. The target value is chosen so as to display energy filtering property. More generally, we also simply maximize the power factor with respect to the nanoparticle potential profile. A simple and fixed model is chosen for other scattering rates, as well as the dispersion relation for the bulk electrons.results between the two approaches will be compared in order to see the effect of electron filtering on the power factor enhancement. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H28.00003: Influence of Dimensionality on Thermoelectric Device Performance Raseong Kim, Supriyo Datta, Mark S. Lundstrom Significant improvements in the thermoelectric figure of merit have recently been demonstrated in low dimensional structures. These improvements have been largely due to the reduced lattice thermal conductivity, so the question of how much additional improvement is possible by engineering the electronic structure has become important. Our goal is to present a clear answer to this question using the Landauer formalism, which applies from the ballistic to diffusive limits. We first relate thermoelectric coefficients to the transmission and the number of conducting channels, M(E). The optimum M(E) is known to be a delta-function. We then compare thermoelectric coefficients in one, two, and three dimensions and show that the channels are utilized more effectively in lower dimensions. The shape of M(E) improves as dimensionality decreases, but lower dimensionality itself does not guarantee better performance because it is controlled by both the shape and the magnitude of M(E). To realize the advantage of lower dimensionality, the packing density must be very high. The benefits of engineering the shape of M(E) appear to be modest, but approaches to increase the magnitude of M(E) could pay large dividends. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H28.00004: Optimized vacuum thermionic energy conversion using diamond materials Joshua Smith, Griff Bilbro, Robert Nemanich The vacuum thermionic energy conversion device (TEC) has been an attractive alternative to other means of energy production for some time due to its potentially high efficiency operation, but practical devices have been difficult to develop as a result of the negative space charge effect. It is well known that a hydrogen termination layer on a diamond material induces a negative electron affinity (NEA). In this study we present a theoretical analysis showing it is possible to tune the parameters of a thermionic device featuring a doped diamond material as the emitter electrode to maximize the output power produced. For example, a TEC operating between $950K$ and $300K$ with an emitter negative electron affinity of $0.5eV$, a collector barrier height of $0.6eV$, Richardson's constant of both electrodes equal to $10A cm^{-2} K^{-2}$, emissivity of both electrode of $0.5$, and interelectrode spacing of $10\mu m$ will have a maximum output power of $1.5W cm^{-2}$ and efficiency of $20\%$ occurring at an emitter barrier height of $1.2eV$. The efficiency calculation includes electronic and blackbody heat transport across the device. The analysis establishes approaches to increase the efficiency to values greater than $20\%$. This work was funded by the Office of Naval Research through the TEC MURI Program. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H28.00005: Nanostructured Silicon as a Thermoelectric Material Invited Speaker: |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H28.00006: Design of thermoelectric composite materials for energy applications. Martin Maldovan, Edwin Thomas Energy supply is becoming a major world-wide problem as fossil energy supplies decrease while energy demands increase. Thermoelectric materials, which reversibly convert thermal and electrical energy, offer the prospect of power generation and cooling by means of the rational transport of electrons and phonons. In nanocomposite materials, both quantum and classical effects provide opportunities to control the transfer of electrons and phonons. The difficulty associated with thermoelectric materials is the need to couple and optimize a variety of physical properties in order to exhibit necessary efficiencies, which are determined by the figure of merit ZT. To exhibit large efficiencies, the best thermoelectric material should possess low thermal conductivity (similar to that of a glass) and high electrical conductivity (similar to that of a perfect crystal material). In this paper we study thermoelectric materials by preparing composite materials that can provide the desired coupled physical properties. Our research concentrates on predicting and designing thermoelectric material properties using theoretical and computational methodologies. We use currently available algorithms and numerical techniques to design thermoelectric materials with increased efficiencies. The ultimate goal of our research is to develop a basic understanding of the coupled physical properties in these materials and to create a framework that allows for the systematic design, optimization, and characterization of their thermal and electric properties. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H28.00007: Thermal conduction mechanisms in isotope-disordered boron nitride and carbon nanotubes Ivana Savic, Natalio Mingo, Derek Stewart We present first principles studies which determine dominant effects limiting the heat conduction in isotope-disordered boron nitride and carbon nanotubes [1]. Using an ab initio atomistic Green's function approach, we demonstrate that localization cannot be observed in the thermal conductivity measurements [1], and that diffusive scattering is the dominant mechanism which reduces the thermal conductivity [2]. We also give concrete predictions of the magnitude of the isotope effect on the thermal conductivities of carbon and boron nitride single-walled nanotubes [2]. We furthermore show that intershell scattering is not the main limiting mechanism for the heat flow through multi-walled boron nitride nanotubes [1], and that heat conduction restricted to a few shells leads to the low thermal conductivities experimentally measured [1]. We consequently successfully compare the results of our calculations [3] with the experimental measurements [1]. [1] C. W. Chang, A. M. Fennimore, A. Afanasiev, D. Okawa, T. Ikuno, H. Garcia, D. Li, A. Majumdar, A. Zettl, Phys. Rev. Lett. 2006, 97, 085901. [2] I. Savic, N. Mingo, D. A. Stewart, Phys. Rev. Lett. 2008, 101, 165502. [3] I. Savic, D. A. Stewart, N. Mingo, to be published. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H28.00008: Phonon relaxation times extracted from first principles thermal conductivity calculations D. A. Broido, A. Ward The lattice thermal conductivity of semiconductors, $\kappa _L $, is a key component in assessing a material's utility for thermoelectric applications. Calculations of $\kappa _L $ commonly employ phonon relaxation times, $\tau _{ph} $. Over the past few decades, a variety of mathematical forms have been used for these$\tau _{ph} $s to represent the phonon-phonon scattering [1], which dominates the behavior of $\kappa _L $ around and above room temperature. However, these forms have typically been obtained in a low frequency/low temperature approximation where umklapp scattering is weak and outside the thermal regime of interest for thermoelectrics. Recently we have developed a first principles approach that accurately calculates $\kappa _L $ using no adjustable parameters [2]. In this presentation, we use our \textit{ab initio} results for Si, Ge and diamond to examine the accuracy of the different models for $\tau _{ph} $, and we identify alternative models. [1] See for example, M. Asen-Palmer et al., Phys. Rev. B 56, 9431 (1997), and references therein. [2] D. A. Broido, M. Malorny, G. Birner, N. Mingo and D. A. Stewart, Appl. Phys. Lett. 91, 231922 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H28.00009: Intrinsic lattice thermal conductivity of diamond from first principles A. Ward, D. A. Broido, D. A. Stewart Predictive theoretical descriptions of the lattice thermal conductivity, $\kappa _L $, are essential in facilitating the design of high efficiency thermoelectric materials. In the thermal regime of interest for thermoelectrics, the $\kappa _L $ of high quality crystalline semiconductors is typically limited by phonon-phonon scattering due to the anharmonicity of the interatomic potential. We have calculated $\kappa _L $ for isotopically pure diamond, combining a first principles approach for the harmonic and anharmonic interatomic force constants with an iterative solution of the full phonon Boltzmann equation. Our adjustable parameter free calculation of $\kappa _L $ for diamond is in excellent agreement with measurements[1-3]. This provides further validation of our \textit{ab initio} approach previously used successfully for Si and Ge [4]. [1] D. G. Onn, et al.,Phys. Rev. Lett. 68, 2806 (1992). [2] L. Wei, et al., Phys. Rev. Lett. 70, 3764 (1993). [3] J. R. Olson, et al., Phys. Rev. B. 47, 14850 (1993). [4] D. A. Broido, et al., Appl. Phys. Lett. 91, 231922 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H28.00010: Thermal boundary resistance of closely-spaced Si/Ge interfaces from lattice dynamics calculations Eric Landry, Alan McGaughey An ability to accurately predict the thermal boundary resistance (TBR) of closely-spaced semiconductor interfaces will allow the design of superlattices with high values of the thermoelectric figure-of-merit. Here, the TBR and phonon transmission coefficients of closely-spaced Si/Ge interfaces are predicted using harmonic lattice dynamics calculations and the scattering boundary method. The atomic interactions are modeled using the Stillinger-Weber potential. The computational domain contains a thin germanium layer sandwiched between two semi-infinite extents of silicon, forming two closely-spaced interfaces. We also consider the opposite situation, where a silicon layer is placed between two large extents of germanium. Due to the harmonic approximation, the calculations are only valid when the phonon scattering is elastic. To examine the assumption of elastic scattering, we compare the lattice dynamics predictions to those obtained using molecular dynamics simulations and the direct method, which require no assumptions about the nature of the phonon transport. We conclude by discussing how the atomic force constants needed in the lattice dynamics calculations can be calculated from density functional theory. This novel approach will allow for the prediction of TBR for interfaces between semiconductors for which a suitable interatomic potential does not exist. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H28.00011: Prediction of phonon transport properties and thermal conductivities in superlattices by anharmonic lattice dynamics calculations Joseph Turney, Alan McGaughey, Cristina Amon Phonon transport in superlattices is investigated using anharmonic and quasi-harmonic lattice dynamics calculations. Within the lattice dynamics framework, we develop a method for predicting the properties of both coherent and incoherent phonons. The method is implemented for test systems consisting of Stillinger-Weber silicon-germanium superlattices. In these systems the mode dependent frequencies, heat capacities, group velocities, transmission coefficients, and relaxation times of the phonons are computed and used to predict the thermal conductivity. We relate changes in the superlattice structure (e.g., period length and interface roughness) to the predicted phonon properties and, for each structure, identify the phonon modes that dominate thermal transport. [Preview Abstract] |
Session H29: The Physics and Astronomy New Faculty Workshops II
Sponsoring Units: FEdChair: Robert Hilborn, University of Texas at Dallas
Room: 333
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H29.00001: How the New Faculty Workshop in 2004 affected my teaching-learning strategies Natalia Dushkina In this presentation, I will discuss how my experience with the New Faculty Workshop in fall 2004 affected my course offering and teaching-learning strategies in PHYS 103, PHYS 132 and PHYS 331: Fundamentals of Optics courses. PHYS 103, Elements of Physics, is a general education conceptual physics lab course with no prerequisites for non-science majors. PHYS 132 is an introductory algebra-based general physics lab course which deals with electricity, magnetism, waves, light atomic and nuclear physics. PHYS 331, Fundamentals of Optics, is a lab-based course required for physics majors, which was offered for the first time at the MU Department of Physics in fall 2007. I will discuss curricular changes, as well as new teaching-learning and assessment methods implemented in these courses for the first time at our department. I will offer examples of peer group discussions, just in time teaching and the effect of chapter summaries on motivating the students and their participation in problem solving. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H29.00002: Taking inquiry to the next level: Tablet PC's to stimulate active learning and unify introductory physics curriculum Tikhon Bykov, Yelena Kosheleva A project has been started in 2005 to design innovative curriculum for the intro physics courses at McMurry University. The project is rooted in science education research and ideas from the NFW. The goal is to achieve better integration of traditional course components by means of instructional design and technology. First, a system of flexible curriculum modules with emphasis on inquiry-based teaching has been introduced. Second, technology is used to improve continuity among module components (lecture/lab/discussion) and stimulate active learning. Enabling technology suite incorporates Tablet PC's and software applications including java-based Physlets, tablet-adapted personal response systems, and PASCO data acquisition systems. Lab curriculum has been modified to accommodate for different learning styles, and levels of baseline knowledge. Activity options and pre-lab Physlet-based assignments were added. To enhance knowledge generalization, multiple experiments are used to illustrate different aspects of the same physics phenomenon. Physlet-based problems were adapted for student peer group discussions. Student feedback showed that modifications were beneficial. Student knowledge assessment, performed with the FCI test, indicated improvement in student learning. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H29.00003: RealTime Physics at UCF Costas Efthimiou, Dan Maronde, Tim McGreevy, Stefanie McCole, Enrique del Barco RealTime Physics (RTP) is an activity-based laboratory curriculum developed by D. Sokoloff, R. Thornton, and P. Laws which has met with success at other universities and colleges. It was recently been adopted at the University of Central Florida (UCF), a large metropolitan university that is rapidly becoming a major research university. A key question is then if successful physics education practices remain effective in an environment as the one present at UCF. By the completion of the Fall 2008 semester, UCF will have three semesters using RTP in some classes while keeping a traditional format in others. Using a pre and post semester test, as well as student interviews, the preliminary data indicate quite a success at UCF during the initial phase of the project. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H29.00004: How Physics Test Scores Reflect the Students' Time Spent Maria Babiuc-Hamilton, Tim Hamilton We found that exam scores in introductory physics courses show a wedge-shaped pattern when plotted against the order they are turned in. The article will explore some of the factors contributing to this pattern and will propose guidelines to help determine an ideal test time limit, aimed at students with good perseverance and average skill. The reason for this pattern appears to stem from a combination of students' skills and perseverance. The first students to finish tend to have either the best or worst grades in the class, with few in between. Students with the highest skills (knowledge, memory, problem-solving skills) need little time to finish, and they have the confidence to turn in the test quickly, before other students. Good students with moderately high skills tend to take longer. The later submissions gradually converge to a grade close the class average. Results of over 200 grades from students in different introductory physics classes, from two universities, taught using a variety of methods, when collected and analyzed, show the same wedge-shaped pattern. This seems to indicate a universal component of the pattern, which does not depend on the teaching methods or the test structure. From our analysis, we aim at deriving what an ideal test time limit would be, at which students hit a point of diminishing returns. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H29.00005: ``Physical Concepts in Cell Biology,'' an upper level interdisciplinary course in cell biophysics/mathematical biology Dimitrios Vavylonis I will describe my experience in developing an interdisciplinary biophysics course addressed to students at the upper undergraduate and graduate level, in collaboration with colleagues in physics and biology. The students had a background in physics, biology and engineering, and for many the course was their first exposure to interdisciplinary topics. The course did not depend on a formal knowledge of equilibrium statistical mechanics. Instead, the approach was based on dynamics. I used diffusion as a universal ``long time'' law to illustrate scaling concepts. The importance of statistics and proper counting of states/paths was introduced by calculating the maximum accuracy with which bacteria can measure the concentration of diffuse chemicals. The use of quantitative concepts and methods was introduced through specific biological examples, focusing on model organisms and extremes at the cell level. Examples included microtubule dynamic instability, the search and capture model, molecular motor cooperativity in muscle cells, mitotic spindle oscillations in \textit{C. elegans}, polymerization forces and propulsion of pathogenic bacteria, Brownian ratchets, bacterial cell division and MinD oscillations. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H29.00006: The Quality Enhancement Plan Matrix (12 years and still tweaking the process!) Gary Mankey As an alumni of the 1997 New Faculty Workshop, I've had plenty of opportunities to experiment with the teaching methods introduced there. Most recently, faculty at UA have been asked to develop a Learner-Centered approach to teaching, where regular, timely and thorough assesments of student learning becomes a key component of the curriculum. This involves four phases: 1) Identifying and defining learning outcomes, 2) Matching assessments to outcomes, 3) Identifying baselines and progression toward benchmarks and 4) Documenting the process using a quality enhancement plan matrix. This has led to the development of learning tools designed to enhance critical thinking and problem solving skills. This will be discussed in the context of setting teaching goals for new faculty, including how to document the process for teaching portfolios. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H29.00007: Pedagogical Reforms from Private Engineering to Public University Lok Lew Yan Voon This talk will document my and associated colleagues' impact on physics pedagogy at the two universities I have been employed at from my initiation at the New Faculty Workshop as an assistant professor in 1998 to being currently a full professor and chair. The present reforms involving our introductory physics courses are connected to our science education program. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H29.00008: Applying Hands-On Activity Concepts to Advanced Mathematical Instruction for Physics William Dieterle While the use of hands-on instruction by constructivist principles has been extensively documented and is covered in great detail in the New Faculty Workshops run by APS, most examples discussed in this series apply to the introductory course level. Many of the same principles can be applied at the upper division level, but topics that are normally considered purely mathematical do not readily lend themselves to such an approach. A common complaint of students first studying vector analysis in an upper division course is the fact that they can't relate the divergence, curl, and gradient to real life. This talk discusses a method of presentation emphasizing the physical significance of these mathematical entities, with laboratory exercises for a two-dimensional gradient and for the divergence theorem in three dimensions. The approach has been successfully utilized in the first weeks of an upper division electricity and magnetism class. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H29.00009: Meeting the Challenges of New Faculty -- My Own Lesson Mirela Fetea A few years ago, I thought that students with different scientific backgrounds posed the biggest challenge for me -- a new assistant professor teaching physics courses at a small liberal-arts private college. I thought that my knowledge of physics, enthusiasm, and willingness to succeed are most of the ingredients needed to become a very good teacher -- how little I knew{\ldots} Now, after being awarded tenure, having received a Distinguished Educator Award, an Outstanding Mentor award, and serving as acting-chair in my department, I am grateful for having had the opportunity to attend The Physics and Astronomy New Faculty Workshop which is responsible for who I am today. This presentation will offer a reflection on how the teaching techniques presented by specialists in pedagogy or educational methodology at The Physics and Astronomy New Faculty Workshop shaped my own performance as a teacher. [Preview Abstract] |
Session H30: Focus Session: Superconducting and Magnetic Oxide Superlattices and Films
Sponsoring Units: DMP GMAGChair: Jak Chakhalian, University of Arkansas
Room: 334
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H30.00001: Magnetism in Complex Oxide Heterostructures Determined with Neutron Scattering Invited Speaker: With the creation of high quality superlattices consisting of complex oxide materials novel materials exhibiting a wide range of interesting phenomena are emerging. Due to the diverse physical properties of complex oxides, (e.g., ferromagnetism, antiferromagnetism, superconductivity), some of which can be varied by doping, the versatility in their applications is large. The physical properties in these new materials, often is tied to the behavior at the interfaces between the different components of the superlattice, and therefore requires detailed knowledge of the relationship between the chemical and electronic composition. Polarized neutron reflectometry (PNR) provides access to the depth-dependent magnitude and orientation of the magnetization and can therefore link the magnetic to the electronic and chemical properties, especially close to these interfaces. Several examples of our work will be presented, including that on La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/ YBa$_{2}$Cu$_{3}$O$_{7-\delta}$/ La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ trilayers which exhibit the inverse superconducting spin switch behavior, and where suppression of the magnetization close to the interface, as well as a varying anisotropy axis have been determined [1]. Another example is work on digitally layered analogs of La$_{1-x}$Sr$_{x}$MnO$_{3}$, where PNR reveals an asymmetric distribution of the magnetization across the two components (antiferromagnetic) LaMnO$_{3}$and SrMnO$_{3}$, which has been linked to structural properties at the interfaces [2]. \\[4pt] [1] V. Pe\~{n}a, Z. Sefrioui, D. Arias, C. Leon, J. Santamaria, J. L. Martinez, S. G. E. te Velthuis, A. Hoffmann, Phys. Rev. Lett. 94 (2005) 057002. \\[0pt] [2] S. J. May, A. B. Shah, S. G. E. te Velthuis, M. R. Fitzsimmons, J. M. Zuo, X. Zhai, J. N. Eckstein, S. D. Bader, and A. Bhattacharya, Phys. Rev. B 77 (2008) 174409. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H30.00002: Spatial Mapping of the Interface Orbital Reconstruction in LaCaMnO$_3$/YBa$_2$Cu$_3$O$_7$ Heterostructures J.W. Freeland, E. Kratsov, S. Grenier, J.-M. Tonnerre, M. Kareev, J. Liu, J. Chakhalian Interfaces between strongly correlated electron materials is an exciting area for exploring new phenomena as these states are altered in the proximity of the interface. In recent work, we have shown that at the interface between the ferromagnetic metal LaCaMnO$_3$ and the superconductor YBa$_2$Cu$_3$O$_7$, the electronic state of Cu undergoes an orbital reconstruction[1,2]. Here we present results using polarization-dependent resonant scattering at the Cu L edge to probe depth dependence of the orbital occupancies in the YBCO layer. By modeling the scattering in the region of the YBCO (001) Bragg peak in LCMO/YBCO heterostructures, we can work to extract a picture of the orbital occupancies in the interface region. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. JC is funded by U.S. DOD-ARO under Contract No. 0402-17291 [1] J. Chakhalian, J.W. Freeland, et. al. Nature Physics {\bf 2}, 244 (2006). [2] J. Chakhalian, J.W. Freeland, et. al. Science {\bf 318}, 1114 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H30.00003: Electronic and Magnetic Properties of the Interface Between a High-Tc Cuprate and CMR Manganite Jian Liu, J. Freeland, B. Kirby, M. Kareev, H.U. Habermeier, G. Cristiani, J. Chakhalian Atomically controlled interfaces between two materials can give rise to novel physical phenomena and functionalities. Modern synthesis methods have yielded high-quality hetero-junctions of oxide materials with competing order parameters. Orbital reconstructions and covalent bonding has been shown to be important factors in the rational design of oxide heterostructures$^{1}$. To clarify the role of superconductivity we study the interface between a high-temperature superconductor (PrY)Ba2Cu3O7 and CMR manganite La2/3Ca1/3MnO3 by resonant x-ray spectroscopy, magneto-optics and neutron reflectivity. The resulting data provide a hint of orbital changes and strong modification of magnetic structure in the heterojuction. $^{1}$J. Chakhalian et al, Science, v. 318, 1155 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H30.00004: Dynamics of Proximate Order Parameters Measured by the Time-Resolved Magneto-Optical Kerr Effect in SrRuO$_{3}$ /YBa$_{2}$Cu$_{3}$O$_{7-x }$Heterostructures C.L.S. Kantner, M.C. Langner, S.P. Crane, L.W. Martin, Y.-H. Chu, P. Yu, R. Ramesh, J.W. Orenstein The interaction between ferromagnetic and superconducting complex oxides in a heterostructure is a subject of great interest. The recent observation of ferromagnetic resonance in SrRuO$_{3}$ by the time-resolved magneto-optical Kerr effect (TRMOKE) presents a new method for insight into such a system. TRMOKE has been used to compare the temperature dependence of magnetization dynamics in SrRuO$_{3}$ grown on insulating substrates and ferromagnetic SrRuO$_{3}$/superconducting YBa$_{2}$Cu$_{3}$O$_{7-x }$heterostructures. The substantial differences between SRO grown on an insulating substrate compared to YBCO as well as the effects seen upon passing through the YBCO transition temperature are reported. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H30.00005: Competition between high Tc superconductivity and ferromagnetism in oxide multilayers Invited Speaker: Artificial multilayers offer unique opportunities for combining materials with antagonistic orders such as superconductivity and ferromagnetism. It was previously shown for multilayers of classical superconductors and ferromagnets that fascinating new quantum states can be realized by tuning the layer thicknesses or the interface properties. In my talk, I will show that multilayers of cuprate high T$_{c}$ superconductors (HTSC) and oxide-based ferromagnets provide an equally fascinating playground for studying the competition between the superconducting and ferromagnetic orders under the condition that both opponents are of comparable strength. I will present experimental results from neutron reflectometry and low energy muon spin rotation ($\mu $SR) measurements on thin film superlattices that were grown by pulsed laser deposition (PLD). These measurements establish that the interaction between superconductivity and ferromagnetism is surprisingly strong and gives rise to a number of unexpected and very unusual phenomena. In particular, our data provide evidence that a giant superconductivity-induced modulation of the vertical profile of the ferromagnetic magnetization takes place in some of these superlattices. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H30.00006: Enhanced Superconductivity in Superlattices of High-$T_c$ Cuprates Satoshi Okamoto, Thomas Maier We investigate the electronic properties of multilayers of strongly correlated models for cuprate superconductors using cluster dynamical mean-field techniques. We focus on combinations of underdoped and overdoped layers and find that the superconducting order parameter in the overdoped layers is enhanced by the proximity effect of the strong pairing scale originating from the underdoped layers. The enhanced order parameter can even exceed the maximum value in uniform systems. This behavior is well reproduced in slave-boson mean-field calculations which also find higher transition temperatures than in the uniform system. This work was supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. A portion of this research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H30.00007: Anomalous Expansion of the Cu-Apical O Distance in Superconducting Cuprate Oxide Bilayers Hua Zhou, Yizhak Yacoby, Ronald Pindak, Vladimir Butko, Gennady Logvenov, Ivan Bozovic Interfaces between complex oxides have received considerable attention due to the observation of fascinating quasi two- dimensional phenomena such as a high-mobility electron gas, interfacial ferromagnetism, and, recently, the observation of interfacial high-temperature superconductivity in epitaxially grown bilayers of metallic La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ and insulating La$_{2}$CuO$_{4}$ on LaSrAlO$_{4}$ substrates$^{1}$. To help understand the mechanism underlying the observed interfacial superconductivity, we directly measured the 3D electron density of this epitaxial bilayer system with sub- atomic resolution using the Coherent Bragg Rod Analysis phase retrieval method$^{2}$. The Cu to apical O distance, which is believed to be a critical parameter controlling T$_{C}$, was found to expand dramatically from the substrate/film interface towards the surface. The correlation between structural features and interface transport properties will be discussed. $^{1}$A. Gozar et al., Nature, 455, 782(2008). $^{2}$Y. Yacoby et al., Phys. Rev. B, 77, 195426(2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H30.00008: Andreev Nanoprobe of Half-Metallic Oxides Using Superconducting Cuprate Tips C.S. Turel, T.L. Wu, J.Y.T. Wei, I.J. Guilaran, P. Xiong Andreev reflection has been extensively used to determine the spin polarization of various ferromagnetic materials, with conventional $s$-wave superconductors as a counterelectrode. In this work, we use the high-$T_{c}$ superconductor YBa$_{2} $Cu$_{3}$O$_{7-\delta }$ (YBCO) as point-contact tips to probe the half-metallic ferromagnets CrO$_{2}$ and La$_{0.66}$Ca$_ {0.33}$MnO$_{3}$ (LCMO) in thin-film form. High-impedance point-contact junctions are made and their differential conductance spectra are measured at 4.2K. Strong suppression of the $d$-wave Andreev reflection characteristics is observed, in contrast to spectra taken on Au films, indicating strong spin polarization in the CrO$_{2}$ and LCMO films. Our point contacts are estimated to range between several nanometers to tens of nanometers in size, attesting to their ballistic and microscopic nature. These results demonstrate the feasibility of using superconducting cuprate tips as spin-sensitive nanoprobes of itinerant ferromagnets. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H30.00009: Magnetic and structural properties of half-metallic Sr$_{2}$FeMoO$_{6}$ epitaxial films fabricated by ultra-high vacuum sputtering Adam Hauser, R.A. Ricciardo, A. Genc, R.E. Williams, P.M. Woodward, H.L. Fraser, F.Y. Yang Sr$_{2}$FeMoO$_{6}$, a double-perovskite half-metallic ferromagnet, has attracted much attention because of its high Tc of 420 K. However, the fabrication of Sr$_{2}$FeMoO$_{6}$ epitaxial films has been challenging due to impurity phases and disorder. Using ultrahigh vacuum off-axis RF sputtering with precisely controlled low-concentration H$_{2}$ in Ar, we have fabricated phase-pure Sr$_{2}$FeMoO$_{6}$ epitaxial films on SrTiO$_{3}$ (001) and (111) substrates. X-ray diffraction confirms pure phase with double perovskite ordering. The phase purity and magnetic moments are highly sensitive to the H$_{2}$ partial pressure. The optimal range for the H$_{2}$ concentration is 0.4{\%} to 0.6{\%} in Ar with 70 mTorr total pressure. The saturation magnetization of the Sr$_{2}$FeMoO$_{6}$ films grown in this range is 1.5 $\mu _{B}$ per formula unit at 5 K, which is a strong magnetization considering the epitaxial strain. Aberration-corrected HAADF TEM images reveal atomically sharp interface between Sr$_{2}$FeMoO$_{6}$ and SrTiO$_{3}$. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H30.00010: Atomic structure of the polar Fe$_2$O$_3$(0001)/MgO(111) interface K. Pande, M. Gajdardziska-Josifovska, M. Weinert We present a first-principles investigation of the stability and structural properties of layer-by-layer growth of thin films of Fe$_2$O$_3$(0001) (hematite) on polar MgO(111). The interface is ``oxide-like'', atomically abrupt, and stabilized by significant structural relaxations. The electronic and magnetic properties are found to vary as a function of hematite film thickness. In contrast to the insulating and antiferromagnetic nature of bulk hematite, the heterointerface is half-metallic and ferromagnetic. Drastic structural rearrangements of the Fe$_2$O$_3$ overlayer are observed at a critical thickness of three Fe bilayers, resulting in an effective expulsion of oxygen from the hematite film. To clarify the effect of the MgO(111) substrate polarity on the nature and growth of the Fe$_2$O$_3$ films, comparisons will be made to unsupported hematite slabs and to Fe$_2$O$_3$/Ti(0001) interfaces. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H30.00011: Investigation of Electrically Driven Phase Transition in Magnetite Thin Films Alexandra A. Fursina, R.G. Sumesh Sofin, Igor V. Shvets, Douglas Natelson Magnetite, Fe$_{3}$O$_{4}$, is an example of strongly electronically correlated system. It undergoes so called Verwey transition at T$_{V}\sim $122 K accompanied both by structural distortion and drastic decrease in electrical conductivity, i.e. metal-insulator transition. Recently, we discovered a new electrically driven phase transition in magnetite nanoparticles and thin films. We observed that a low-temperature (T below T$_{V})$ insulating state is broken upon applying an electric field, resulting in a sharp transition to the state with much higher conductivity. We report on further electrical characterization of this newly discovered state. There is a question whether this state is the same as high-temperature phase above T$_{V}$ or this is a new state of magnetite. In standard two-terminal measurement dominant contribution of contact resistance impedes intrinsic electrical properties. Thus, four-terminal configuration is necessary. Electrical and magnetoresistance properties are measured in challenging four-terminal geometry at nanoscale. [Preview Abstract] |
Session H31: Focus Session: Vanadate Thin Films
Sponsoring Units: DMP GMAGChair: Dmitri Basov, University of California San Diego
Room: 335
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H31.00001: Magneto-transport studies in thin film vanadium dioxide across the metal-insulator transition D. Ruzmetov, V. Narayanamurti, S. Ramanathan, D. Heiman, B.B. Claflin Temperature dependent magneto-transport measurements in magnetic fields of up to 12 Tesla were performed on thin film vanadium dioxide (VO$_{2})$ across the metal-insulator transition (MIT). The Hall carrier density increases by 4 orders of magnitude at the MIT and accounts almost entirely for the resistance change. The Hall mobility varies little across the MIT and remains low. Electrons are unambiguously found to be the major carriers on both sides of the MIT. The Hall coefficient temperature dependence in the semiconducting phase can be well fitted within a single band model. Position of the Fermi level in the band gap is estimated from the fit and an unusually large effective electron mass is found. The positive magnetoresistance at room temperature is measured to be 0.09{\%} in a 12T field. The results will be compared with single crystal data and discussed in detail. The studies are of relevance towards understanding mechanisms governing phase transition in complex oxide thin films. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H31.00002: Mott metal-insulator transition-induced electrical oscillation in VO$_{2}$ Hyun-tak Kim, Yong Wook Lee, Bong-jun Kim, Sun Jin Yun, Sungyoul Choi, Byung-gyu Chae Since Mott predicted the abrupt first-order metal-insulator transition (MIT) in 1949, one of the most important issues in contemporary solid-state physics has been to experimentally prove Mott's MIT in a strongly correlated system with electron-electron interaction. The MIT has many practical applications and is believed to facilitate the understanding of physical phenomena, such as high-$T_{c}$ superconductivity, colossal magnetoresistance, etc. In particular, in order to reveal the mechanism of the Mott MIT, many physicists have paid attention to a representative paramagnetic insulator, VO$_{2}$(4$d^{1})$, with an abrupt resistance change near 68$^{\circ}$C. The key issue is whether VO$_{2}$ is a Mott insulator, in which the abrupt MIT is not caused by a structural phase transition (SPT), or a Peierls insulator undergoing the SPT near $T_{SPT }\approx$ 68$^{\circ}$C; this question can be answered when a monoclinic and correlated metal (MCM) phase different from a normal metal is observed. Here we show an MCM phase, high frequency electrical oscillations in the MCM phase of VO$_{2}$. The oscillation possibly is generated from a temporal capacitor, which is comprised of both temporary dielectric components, arising from inhomogeneity in a VO$_{2}$ film, and MCM phases acting like electrodes. This work concluded that the electrical oscillation is a characteristic of the Mott MIT. (Ref: Applied Physics Letters 92 (2008) 162903). [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H31.00003: De-coupling Electrical and Thermal Effects in Triggering Metal-Insulator Transition in $VO_{2}$ Thin Film Devices Gokul Gopalakrishnan, Shriram Ramanathan Vanadium dioxide ($VO_{2}$) has been shown to undergo an abrupt electronic phase transition near $70^{\circ}C$ from a semiconductor to a metal, with an increase in dc conductivity of over three orders of magnitude, making it an interesting candidate for advanced electronics as well as for fundamental research into understanding the Mott transition. Recent experiments strongly suggest that this transition can be manifested independent of a structural phase transition in the system at a similar temperature, and that it can be triggered by the application of a critical field across the $VO_{2}$ thin film. To address the important question of thermal effects due to the applied field, we report the results of electro-thermal simulations on a number of common and promising device geometries showing the extent of heating caused by the leakage current in the ``off'' state of the $VO_{2}$ device. The simulation results are compared with experimentally observed device characteristics. Valuable insights into the nature of the metal-insulator transition can be obtained from such simulations and will be discussed in the presentation. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H31.00004: Properties of Complex Oxides at the Nanoscale: First Order Phase Transitions through Avalanches. Invited Speaker: Properties of complex oxides have been studied for decades, including many systems which exhibit a phase transition, among which are high temperature superconductors, multiferroics, and metal insulator (M-I) transition materials. We have studied a member of the later group, vanadium oxide (VO$_{2})$, which in spite of its long history, keeps surprising researchers today. We find that the M-I transition of nano-scaled VO$_{2}$ devices is drastically different from the smooth transition curves generally reported. The temperature driven M-I transition occurs through a series of resistance jumps ranging over two decades in magnitude, indicating that the transition between the two phases of the system happens by resistive avalanches. The avalanche magnitude follows statistically a power law similar to that observed in many other physical systems, such as Barkhausen noise in ferromagnets or sand avalanches in sand piles, and non-physical systems such as connectivity of the internet. We discuss the effects on the distribution of avalanches due to the: device dimensions, percolative nature of the measurement and interactions between the different phases within the phase transition. We present additional evidence for the importance of interactions in macroscopic FORC measurements, and their role in the opening of the hysteresis in VO$_{2}$ M-I transitions. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H31.00005: FORC Analysis of the thermal hysteresis at the Metal Insulator Transition in VO$_2$ Juan Ramirez, Amos Sharoni, Jonathan Dubi, Maria E. Gomez, Ivan K. Schuller We use the first order reversal curve method (FORC) in order to obtain a quantitative analysis of the temperature-driven metal-insulator transition hysteresis in VO$_2$ thin films. By studying the hysteresis properties of resistance vs. temperature we were able to obtain information regarding inter-domain interactions. An unexpected tail like feature in the contour plot of the FORC distribution indicates the existence of irreversible regions outside of the hysteresis loop. This irreversibility may arise from metallic domains present at temperatures below the closing of the hysteresis, which interact with the surrounding medium and change the reversal path relative to one coming from a \emph{fully} insulating state. We develop a model where the driving force which opens hysteresis in VO$_2$ are inter-domain interactions. This model is intrinsically different from the Preisach model that is usually used to describe hysteresis, since it identifies a microscopic origin of the hysteresis, and provides physical parameters to characterize it. Work Supported by the US Department of Energy, AFOSR and the Colombian agencies COLCIENCIAS and the Excellence Center for Novel Materials, CENM. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H31.00006: Electric field effects on phase transition and electronic transport mechanisms in vanadium oxide thin films Changhyun Ko, Shriram Ramanathan Metal-insulator transitions (MIT) in complex oxide thin films are of great interest from both scientific and application perspectives. Vanadium oxide is a model system to exploit strongly correlated electronic phenomena such as Mott transition. Furthermore wide attention for application has been attracted by its transition functionality that can be tuned in terms of various external parameters: temperature, electric field, photoexcitation, and stress. The tunable conduction states allow creating novel devices whose functions are controlled by multiple parameters. We report on recent observations of electric-field assisted phase transition in vanadium oxide thin films. The conduction mechanisms were analyzed using both in-plane and out-of-plane modes and matched well. In the insulator phase, Poole-Frenkel emission was suggested to govern non-ohmic behavior at high field regime, while under low field application, ohmic conduction with activation energy of $\sim $0.24 eV was observed. Activation energy of ohmic conduction in the metallic phase was $\sim $0.08 eV. These preliminary results are encouraging towards exploring correlated oxides for computing device technologies. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H31.00007: Conductivity Dynamics in the Correlated Metallic State of V$_{2}$O$_{3}$ M. Liu, B. Pardo, M.M. Qazilbash, S.J. Yun, B.G. Chae, B.J. Kim, H.T. Kim, D.N. Basov, R.D. Averitt V$_{2}$O$_{3}$ is a strongly correlated electron material that undergoes a transition from antiferromagnetic insulator at low temperatures to a strongly correlated metal above $\sim $140K. We report on time resolved spectroscopic studies of V$_{2}$O$_{3}$ thin films where we have observed coherent oscillations in the far-infrared conductivity following excitation with a 35-fs optical pulse. The resultant $\sim $100 ps conductivity oscillations result from the optically induced generation of strain which modulates the orbital overlap and hence the conductivity thus revealing a strong coupling of carriers to the lattice in the metallic state. This contrasts with other vanadates such as VO$_{2}$ where this effect is not observed. We will discuss the potential of V$_{2}$O$_{3}$ as a candidate material for investigating photoinduced phase transitions. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H31.00008: Subdomain studies of the metal-insulator transition in VO$_{2}$ nanobeams Jiang Wei, Jae Hyung Park, Jacob Beedle, Zenghui Wang, Wei Chen, Geeta Yadav, David Cobden In many correlated materials, domain structure causes the bulk properties to differ from those on the sub-domain level. In addition, near first-order phase transitions it leads to transition broadening, hysteresis, and sample degradation. Studies of nanoscale crystals enable investigations of the domain-free homogeneous material. We demonstrate this by working with nanobeams of vanadium dioxide, thereby discovering or clarifying multiple aspects of its famous metal-insulator transition at 67$^{\circ}$C. Amongst them are that the transition to the metal occurs at a constant value of the resistivity of the insulating phase; large supercooling of the homogeneous metallic phase is possible; and the activation energy in the insulating phase is consistent with the optical gap, in contrast with earlier reports on bulk samples. The nanobeams also enable new classes of experiments, including investigating a single metal-insulator interphase wall, employing nanomechanical effects to determine the equilibrium transition temperature, and investigating the dynamics of a phase transition in quasi-one-dimensional geometry. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H31.00009: Infrared nano-imaging of metallic and insulating domains in single crystalline vanadium dioxide nanowires Andrew Jones, Jiang Wei, David Cobden, Markus Raschke Correlated electron systems are often associated with heterogeneous electronic and structural phase transitions with ordering and domain formation on nanometer length scales. Vanadium Dioxide (VO$_{2}$) has long been a material of research focus due to behavior associated with its temperature induced metal-insulator transition (MIT) occurring around 340K. The underlying mechanism of this transition is thought to be the result of a complex interplay between the lattice and electronic degrees of freedom as the material passes through the MIT, whose origin is as of yet poorly understood. We study the nanometer scale formation of insulating and metallic domains of single crystal VO$_{2}$ nanowires bonded to silica substrate using infrared scattering-scanning near field optical microscopy (s-SNOM). Imaging contrast is obtained due to the distinct optical dielectric properties of the respective metallic and insulating phases. A hierarchy of domain sizes is observed, suggesting two distinct insulating phases in addition to the metallic phase as the material moves through the MIT. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H31.00010: Mapping the spatial scale of domain switching in heteroepitaxial vanadium dioxide thin films and nanoparticles Joyeeta Nag, Richard Haglund Vanadium dioxide is a strongly correlated electron system exhibiting a hysteretic semiconductor-to-metal transition around 67C, accompanied by a structural change from monoclinic to tetragonal and huge changes in its electrical conductivity and near-infrared transmission. As interest grows in very thin films and nanoparticles of vanadium dioxide, the spatial scale and domain structure of the metal-insulator transition are critical issues. To elucidate these questions, thin films and nanoparticles of vanadium dioxide were grown on R-, C- and A-cut sapphire substrates, and the substrate-dependent epitaxial growth habits and in-and-out-of-plane orientations were characterized by near-infrared transmission, X-ray diffraction, scanning and transmission electron microscopy. Temperature variable XRD scans at intervals of one degree were performed from the onset to the completion of the transition hysteresis to map out the percentage of coexisting domains of monoclinic and tetragonal phases in one such 100nm thick epitaxial film. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H31.00011: Novel devices in VO$_{2}$ nanobeams Jiang Wei, Jae Hyung Park, Jacob Beedle, Geeta Yadav, Zenghui Wang, David Cobden, Marco Rolandi Working with nanobeams and sheets of vanadium dioxide attached to a rigid substrate, one can avoid the difficulties associated with multiple domains and sample degradation that occur in the bulk material due to the first-order metal insulator transition. Taking advantage of this, we illustrate the possible uses of single-crystal nanobeams and sheets for making new kinds of nanoelectronic devices and switches. For switching, one can exploit the instabilities due to coupling between the metal-insulator transition and mechanical buckling or the supercooling of the metallic phase. For nanoelectronic devices we explore the patterning of a conducting layer on the surface by controlled reduction and oxidation using conducting atomic force microscopy. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H31.00012: Self-Limiting Growth of Magnetic Nanoparticles in a Glassy Matrix Sergio Picozzi, Mark Laurenzi III, Ian Pegg Nanoparticles of magnetite can be grown by heat treatment of suitable glass compositions slightly above the glass transition temperature. We have investigated the transformation kinetics and magnetic properties, including the size dependence of the Verwey transition, in such systems. The initially rapid growth is quickly arrested leading to tight size distributions that become essentially independent of time. The mean size (a few nm) is dependent on the glass composition and temperature. In this paper, we investigate a simple model in which the self-limiting nature of this process is ascribed to the experimentally observed strong dependence of the glass transition temperature of the matrix on the concentration of one of the diffusing species, which in turn gives rise to a concentration-dependent diffusivity. In addition, the relationship between the equilibrium concentration of the diffusing species and the curvature of the particle-matrix interface (the Gibbs-Thomson effect) is shown to play a prominent role. The model reproduces the essential features of the transformation kinetics, predicting an initial power law growth that becomes nearly logarithmic at long times, and identifies the key physical parameters that determine the self-limited particle size. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H31.00013: Unusual electronic states in TiO$_2$/VO$_2$ (001) multilayers. Victor Pardo, Warren Pickett Abrupt interfaces between oxides display a wealth of unexpected behavior, and the interface between a band insulator and a Mott insulator is expected to display extra richness. Several multilayered TiO$_2$/VO$_2$ structures have been studied by ab initio density functional theory techniques, including the thin VO$_2$ regime corresponding to the quantum confined Mott insulator. VO$_2$ undergoes a metal-insulator transition near room temperature, but when deposited in thin films of thickness smaller than 5 nm, the metal-insulator transition disappears. Our calculations (using the correlated LDA+U method with modest values of U and J) show that the electronic character (metallic versus insulating) changes with the number of VO$_2$ layers embedded within insulating TiO$_2$ layers: metallic for five VO$_2$ layers, semimetallic and half-metallic for three layers, and insulating for a single VO$_2$ layer. These trends, and the peculiar nature of the three VO$_2$ layer case, will be discussed in some detail. [Preview Abstract] |
Session H32: Focus Session: Theory and Simulation of Spin-Dependent Effects and Properties I: (First-Principles Calculations)
Sponsoring Units: GMAG DCOMP DMPChair: Andre Petukhov, South Dakota School of Mines
Room: 336
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H32.00001: Longitudinal spin fluctuations in itinerant ferromagnets Invited Speaker: Finite-temperature properties of magnetic materials strongly depend on their magnetic configuration given by relative orientation of local magnetic moments. In itinerant ferromagnets, an additional degree of freedom becomes important: the value of local magnetic moment, which is sensitive to the magnetic state and the local chemical environment of atoms as well as the thermal electronic excitations of different type, leading to the so-called longitudinal spin fluctuations (LSF). LSF play an important role in itinerant magnetic systems at high temperatures. In particular, they are responsible for the existence of finite local magnetic moments on atoms in the paramagnetic state, which affect different physical properties. At the same time, an ab initio based description of the LSF is problematic. The LSF are ignored in the local density and related approximations of density functional theory, which for instance leads into a substantial underestimation or complete disappearance of the local magnetic moments on atoms in the itinerant ferromagnets in the corresponding calculations at high temperature in the paramagnetic state. Although the LSF can be included in more elaborate schemes, such as the dynamical mean-field theory, the application of such techniques to real systems is too cumbersome in most cases. In this work a generalized form of classical magnetic Hamiltonian is suggested, which includes both transverse and longitudinal spin fluctuations on equal footing. Parameters of the Hamiltonian can be determined in the first-principles calculations, within the local spin density approximation. The method is applied to the calculations of high-temperature magnetic properties of Fe, Co and Ni, including the Curie temperature. The effect of the LSF in the high-temperature paramagnetic state on chemical interactions and other physical properties is demonstrated for several alloys, including fcc Fe-Cr-Ni alloys, which is the basis of austenitic stainless steels. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H32.00002: Spin excitations in solids from first principles Arno Schindlmayr, Ersoy Sasioglu, Manfred Niesert, Christoph Friedrich, Stefan Bl\"ugel The long-range order of the electron spins in magnetic solids gives rise to additional excitation modes that preserve the charge density but change the total spin of the electron system. While Stoner excitations, which correspond to spin-flip transitions between the majority and minority channels, can be described within a single-particle picture, spin waves are collective modes that result from the spin-dependent exchange interaction between the electrons. Here we discuss different approaches that we have explored for material-specific spin-wave calculations from first principles. All of these methods focus on the non-local and dynamic transverse spin susceptibility, whose spectral function can be directly related to experimental spectroscopies, but employ either time-dependent density-functional theory or many-body perturbation theory to treat exchange and correlation. In the latter case, maximally localized Wannier orbitals are used to efficiently obtain the electron-hole vertex of the multiple-scattering $T$ matrix, which is constructed with full frequency and wave-vector dependence. The implementation uses the full-potential linearized augmented-plane-wave (FLAPW) method. For ferromagnetic transition metals like Fe, Co or Ni our results are in good agreement with experimental data and reproduce all important spectral features. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H32.00003: Transverse spin susceptibility in Ni, Fe and Co Liqin Ke, Takao Kotani, Mark van Schilfgaarde, Vladimir Antropov We calculate the full transverse spin susceptibility $\chi^{\pm}(\bf q, \omega)$ in the time-dependent local density approximation(TDLDA) for elemental Ni, Fe and Co. We extract the Heisenberg exchange parameters from both the energy-dependent $\chi^{\pm}$ and the static one. The results are compared with those given by a method assuming the rigid rotation of the magnetic moments at each site(ref.kotani2008). We observe some differences between these two methods, especially around the Brillouin zone boundaries. We also calculate $\chi^{\pm}$ starting from the non-interacting Hamiltonian generated from the quasiparticle self-consistent GW(QSGW) approximation (ref.Kotani2007). We analyze how the QSGW potential alter the LDA results. \\ Ref.\\ T.Kotani and M van Schilfgaarde, J. PHYS. C 20, 295214 (2008) T.Kotani, M. van Schilfgaarde, S. V. Faleev Phys. Rev. B 76, 165106 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H32.00004: Ab-initio electronic structure calculations of periodic systems in the presence of arbitrary magnetic fields Alfredo A. Correa, Eunseok Lee, Wei Cai, Giulia Galli Ab initio electronic structure calculations in the presence of magnetic fields have been mainly performed for isolated systems, or, in the case of periodic systems, by adopting perturbative approaches. Building on a recent formulation of electronic structure calculations in the presence of magnetic fields [1,2], we will discuss calculations for periodic systems under arbitrary conditions, which include arbitrary (finite) magnetic field, arbitrary periodic cell shapes, and magnetic field spatial variations. Preliminary results based on a planewave numerical approach and local approximations to Density Functional Theory will be presented.[1] W.Cai, G.Galli, Phys. Rev. Lett. 92, 186402 (2004).[2] E. Lee, W. Cai, G. Galli, J. Comput. Phys. 226, 1310 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H32.00005: Real space first-principles derived semiempirical pseudopotentials applied to tunneling magnetoresistance Kirk Bevan, Tony Low, Hong Guo We present a real space density functional theory (DFT) localized basis set semi- empirical pseudopotential (SEP) approach. The method is applied to iron and magnesium oxide, where bulk SEP and local spin density approximation (LSDA) band structure calculations are shown to agree within approximately 0.1 eV. Subsequently we investigate the qualitative transferability of bulk derived SEPs to Fe/MgO/Fe tunnel junctions. We find that the SEP method is particularly well suited to address the tight binding transferability problem because the transferability error at the interface can be characterized not only in orbital space (via the interface local density of states) but also in real space (via the system potential). To achieve a quantitative parameterization, we introduce the notion of ghost semi-empirical pseudopotentials extracted from the first-principles calculated Fe/MgO bonding interface. In general the results underscore the need for separate tight binding interface and bulk parameter sets when modeling conduction through thin heterojunctions on the nanoscale. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H32.00006: Rational Design of Half-Metallic Alloys William Butler, Claudia Mewes, Chunsheng Liu, Mairbek Chshiev A half-metal is a material that is a metal for one spin-channel and an insulator or semiconductor in the other. Half-metals are potentially important for spintronic applications such as magnetic sensors for hard drives and magnetic random access memory. We show using very simple ideas that it is possible to rationally design a class of magnetic alloys by placing a gap at the center of one of the d-bands and placing the Fermi energy in this gap. We will present design rules that can be used to make an infinite number of half-metallic heterostructures. We will also show how the half-metallic feature may be maintained at surfaces and interfaces. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H32.00007: Density-functional theory study of the Co$_2$MnSi/MgO interface Bj\"orn H\"ulsen, Peter Kratzer, Matthias Scheffler Magnetic memory devices that exploit the tunneling magneto-resistance (TMR) effect depend crucially on the spin polarization of the electrode materials. Using ferromagnetic half-metals, such as the full Heusler alloy Co$_2$MnSi, perfect electrodes with 100\% spin polarization could possibly be realized, at least at zero temperature. Here, we use density functional theory (DFT) calculations to model an epitaxially grown Co$_2$MnSi/MgO(001) interface in a prospective TMR device. The stability, the electronic and magnetic properties of different terminations of Co$_2$MnSi (stoichiometric Co- and MnSi- and non-stoichiometric Mn- and Si- planes) and different registry with respect to the insulating barrier (Mg-top, O-top, bridge and hollow site) are investigated. We find that the electronic and magnetic properties (including the existence of the spin gap) depend strongly on the termination. The formation energy of the various interfaces is presented in form of a phase diagram. Both the interface Co/O (Co at O top site) with a high spin polarization of $P = 70$\%, and the interface MnSi/O with only small $P$ form part of the thermodynamically accessible region. The MnMn/O interface preserves the half-metallicity of the bulk, but is found to be only metastable. Interface band structures are presented, and magnetic moments are compared to experimental data. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H32.00008: Magnetoelectric polarizability and axion electrodynamics in crystalline insulators Andrew Essin, Joel Moore, David Vanderbilt Spin-orbit coupling can lead in two- and three-dimensional solids to time-reversal-invariant insulating phases that are ``topological'' in the same sense as the integer quantum Hall effect and similarly have protected edge or surface states. The three-dimensional topological insulator is known to have unusual magnetoelectric properties referred to as ``axion electrodynamics'': it supports an electromagnetic coupling $\Delta{\cal L}_{EM} = (\theta e^2 / 2 \pi h) {\bf E} \cdot {\bf B}$ with $\theta=\pi$, giving a half-integer surface Hall conductivity $\sigma_{xy}=(n + 1/2) e^2 / h$. An approach to $\theta$ in any three-dimensional crystal is developed based on the Berry-phase theory of polarization: $\theta e^2/ 2 \pi h$ is the bulk orbital magnetoelectric polarizability (the polarization induced by an applied magnetic field). We compute the orbital magnetoelectric polarizability for a simple model and show that it predicts the fractional part of surface $\sigma_{xy}$, computed using a slab geometry. Although $\theta$ is not quantized once time-reversal and inversion symmetries are broken, it remains a bulk quantity for the same reasons as ordinary polarization. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H32.00009: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H32.00010: Flexomagnetic effect in Mn-based antiperovskites Renat Sabirianov, Pavel Lukashev We report our theoretical results on the induced magnetization appearance in antiferromagnetic antiperovskites, such as Mn$_{3}$GaN, due to the gradient of applied external strain (flexomagnetic effect). We model the external flexure by forming a 40-atom Mn$_{24}$Ga$_{8}$N$_{8}$ supercell with 4 domains under external strain gradient. This structure shows a net magnetization which increases parabolically up to 0.03$\mu _{B}$ (per Mn atom) in the (0,-1,1) direction reflecting non-linear contribution to local piezomagnetric effect in the considered range of up to 0.005{\%} external strain gradient. The calculated flexomagnetic effect is found to be relatively small with induced magnetic moment order of magnitude smaller than that of piezomagnetic contribution. The flexomagnetic effect can be especially important in the nanostructures, where the stress gradients are usually large due to the surface tension. All calculations were performed using the projector augmented wave method. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H32.00011: Layered antiferromagnetism with high Neel temperature in the intermetallic compound Mn$_{2}$Au Sergii Khmelevskyi, Peter Mohn On the basis of earlier experimental studies the intermetallic compound Mn$_{2}$Au has been characterized as a non-magnetically ordered material. Here we report the results of first-principles calculations based on Local Spin-Density Approximation which describe Mn$_{2}$Au to have a narrow band antiferromagnetic ground state with rigid local moments on the Mn sites. Calculations of the inter-atomic exchange constants based on the magnetic force theorem and a Monte-Carlo modeling of the resulting Heisenberg-like Hamiltonian predict a very high Neel-temperature of $\sim $1580K. This temperature is considerably higher than for the other known high-temperature antiferromagnetic L1$_{0}$-type Mn based binary alloys, which are widely used in magnetic storage applications. The source of the difficulties in determining magnetic order from the earlier experiments is discussed. The observed meta-magnetic like behavior and a susceptibility anomaly at low temperatures are linked to the frustrated magnetism on Mn anti-site impurities. We believe that the high temperature antiferromagnetism of Mn$_{2}$Au may have quite an impact in technology. In particular, it can be considered as a candidate for the application as a ``pinning'' layer in GMR devices. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H32.00012: 3d impurities in wide gap oxides---magnetism and carrier doping. Hannes Raebiger, Stephan Lany, Alex Zunger 3d transition metal impurities in wide-gap oxides exist in multiple charged configurations[1], and may function as (i) donor/acceptor defects to modify carrier concentrations, (ii) magnetic elements to induce collective magnetism, and (iii) shift the host band edges. While previous investigations have addressed some of these phenomenologies separately, we link them together, and present the chemical trends for electronic properties, carrier doping, and magnetism along the series of 3d1...3d8 impurities in the paradigmatic wide-gap oxide hosts ZnO and In2O3. For these general trends we find that, in In2O3 most 3d impurities are amphoteric and exhibit deep transitions, whereas in ZnO the early 3d impurities (Sc-V) have shallow donor transitions, and only the late 3d's (Co, Ni, Cu) have acceptor transitions inside the band gap. Ferromagnetic interactions emerge upon the partial filling of 3d levels resonant inside the conduction band, an in general require electron doping from additional sources. [1] H. Raebiger, S. Lany, and A. Zunger, Nature {\bf 453}, 763 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H32.00013: Multifarious-magnetism in copper oxide nanostructures from first-principles X.-Y. Cui, A. Soon, B. Delley, S.-H. Wei, C. Stampfl Driven by the ever-increasing demand for novel spin-dependent advanced materials, investigation of nanoscale magnetic materials is currently actively pursued. With the latest developments focusing more on magnetic semiconducting oxides, materials based on cuprous oxide, Cu$_2$O, are of high interest as potential $p$-type semiconducting candidates. Thus developing an understanding of how intrinsic defects influence both its electronic and magnetic properties is important. We perform density-functional theory calculations [1] and analyze both the electronic and magnetic properties of native defects in both bulk Cu$_2$O and its surfaces, as well as their respective formation/surface energies under different growth conditions. We find that under oxygen-lean conditions, the experimentally observed ferromagnetic behaviour [2] could originate from copper vacancies on Cu$_2$O(111) while under oxygen-rich conditions, low energy bulk oxygen interstitials might explain the ferromagnetic moment found in the same material. This suggests that the origin of observed magnetism in sub-stoichiometric copper oxide nanoparticles could be multifarious, highlighting the complimentary role of bulk and surface native magnetic defects.\\[0pt] [1] A. Soon \textit{et al}. submitted\\[0pt] [2] A. Ye. Yermakov\textit{et al.} J. Magn. Magn. Mater. \textbf{310}, 2102 (2007) [Preview Abstract] |
Session H33: Focus Session: Iron Pnictides and Other Novel Superconductors V: Tunneling, PCAR, Josephson
Sponsoring Units: DCMPChair: Laura Greene, University of Illinois
Room: 403
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H33.00001: Observation of the Josephson effect in Pb/Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ single crystal junctions Xiaohang Zhang, Richard Greene, Ichiro Takeuchi, Yoon Seok Oh, Yong Liu, Liqin Yan, Kee Hoon Kim We have fabricated Josephson junctions using single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ and Pb (or PbIn) as the counter electrode in two geometries. The $c$-plane single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ were synthesized by the Sn-flux method with the nominal composition of $x$ = 0.4. In one junction geometry, Ag (30 nm) and PbIn (200 nm) were evaporated on the surface of the crystals. In the other geometry, a Pb point contact was used. Both geometry junctions show resistively shunted junction $I-V$ curves below the $T_{C}$ of the counter electrode. Microwave induced steps were observed in the $I-V$ curves, and the critical currents are completely suppressible with applied magnetic field in a manner consistent with a small junction limit. $I_{C}R_{N}$ products of up to 0.3 mV have been observed in these junctions at 4.2 K. The observation of Josephson coupling along the $c$-axis between an iron pnictide superconductor and a conventional superconductor suggests the existence of a non-d-wave superconducting order parameter in iron pnictide superconductors. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H33.00002: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H33.00003: Multigap superconductivity in pnictides Peter Samuely, Pavol Szabo, Zuzana Pribulova, Gabriel Pristas, Paul Canfield, Sergey Bud'ko Point-contact Andreev reflection studies of the superconducting energy gap on the NdFeAsOF, (Ba,K)Fe$_{2}$As$_{2}$ and Ba(Fe,Co)$_{2}$As$_{2}$ crystals will be presented. The analysis of the data points to a two-gap superconductivity in these materials. Possible existence of the pseudogap in the non-superconducting density of states related to the static or dynamic magnetic order will be discussed as well. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H33.00004: Coexistence of two order parameters and a pseudogap in the iron-based superconductors Invited Speaker: The number, the symmetry and the amplitude of the order
parameters (OPs) in the Fe-As superconductors are still open
issues, as well as the origin of the electron pairing. To
address these issues, we performed point-contact Andreev-
reflection measurements in SmFeAsO$_{0.8}$F$_{0.2}$ ($T_c^{on}=53
$ K) and LaFeAsO$_{0.9}$F$_{0.1}$ ($T_c^{on}=27$ K)
polycrystals. In both cases, the low-temperature conductance
curves clearly indicate the presence of two OPs in the
superconducting state. No zero-bias peaks were observed, which
-- considering the non-directional current injection -- clearly
rules out the $d$-wave symmetry. If a superconducting character
is supposed for both the OPs, their amplitudes can be extracted
from a generalized two-band BTK fit (with two s-wave gaps, as in
MgB$_2$) of the normalized conductance curves. The fit is indeed
very good and gives OP amplitudes, $\Delta_1$ and $\Delta_2$,
that lie slightly below and well above the BCS value,
respectively. In Sm-1111, their low-temperature values are
$\Delta_1(0)=6.15\pm0.50 $ meV and $\Delta_2(0)=18\pm3$
meV, which give gap ratios ($2 \Delta/k_BT_c$) of about 2.7 and
8.0. Both $\Delta_1$ and $\Delta_2$ show a BCS-like temperature
dependence and close at the bulk $T_c$. In La-1111 we obtained
point contacts with different local $T_c$ (from 27.3 to 31.0 K)
in crystallites with slightly different doping. Here $\Delta_1$
shows a non-BCS temperature dependence with a high-
temperature ``tail,'' while $\Delta_2$ seems to close at $T |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H33.00005: Point-contact Andreev reflection tunneling spectroscopic (PCARTS) study on Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ Xin Lu, W. K. Park, L. H. Greene, H. Q. Yuan, G. F. Chen, G. L. Luo, N. L. Wang, A. Sefat, M. A. McGuire, R. Jin, B. C. Sales, D. Mandrus PCARTS is applied to investigate the gap structure in the newly-discovered iron pnictide superconductors Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. Double peaks due to Andreev reflection with strongly-sloping background are frequently observed in the conductance curves G (V) for the Au-Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ point-contact junctions on the freshly-cleaved surface along the c-axis. . If normalized by the background baseline and analyzed by Blonder-Tinkham-Klapwijk model, the data show a gap size $\sim $ 4 meV with 2$\Delta $/k $_{B}$T$_{C }\sim $ 2.6. However, it is observed that, for the Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$, G(V) curves evolve from V-shape to zero-bias conductance peak with increasing tip pressure, where the tip is likely to penetrate through surface layer. The existence of surface oxide layer is confirmed by comparative XPS characterization on freshly-cleaved and uncleaved surfaces. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H33.00006: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H33.00007: Probing the Spin-Density-Wave Transition in SmOFeAs using Point-Contact Spectroscopy T. Y. Chen, S.X. Huang, R. H. Liu, X. H. Chen, C. L. Chien In spring 2008, a new family of superconductors with the general composition of SmFeAs(O$_{1-x}$F$_{x})$ has been discovered. The metallic parent compound SmOFeAs has a spin-density-wave (SDW) transition near 150 K with hysteretic temperature dependence in magnetic structure, crystalline structure, and resistance. Doping the parent compound with fluorine results in the suppression of the SDW transition and the emergence of superconductivity with transition temperatures up to 55 K. Some theories suggest that the SDW transition may be intimately related to the superconductivity. In this work, we use point-contact spectroscopy to investigate the SDW transition in the parent SmOFeAs compound. Instead of varying temperature, under a high bias voltage a small region underneath a point contact can be heated up through the SDW transition, resulting in differential conductance peaks. Similar to the temperature dependence of the resistance and structure, the SDW transition is hysteretic with bias voltage at the transition. We further show that this feature in differential conductance, which may be easily mistaken as a pseudo gap of a superconductor, is actually a characteristic of the ballistic heating effect. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H33.00008: Theory of Tunneling Spectroscopy in LaFeAsO$_{1-x}$F$_{x}$ Seiichiro Onari, Yukio Tanaka Recent discovery of superconductivity in the iron based LaFeAsO$_{1-x}$F$_{x}$ with Tc = 26K has stimulated great interests as a new class of non-cuprate compound. We calculate the surface density of state of LaFeAsO$_{1-x}$F$_{x}$ superconductor. The gap function is obtained microscopically by solving the Eliashberg equation in a 5-band Hubbard model with the random phase approximation (RPA). Green's function of the surface state is obtained by inserting infinite potential barriers using Matsumoto and Shiba method. Although the gap function has a sign change between Fermi surfaces, we cannot find mid gap Andreev bound state in [100] and [110]-oriented interface. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H33.00009: Novel properties in Josephson junctions involving $s_{x^2 y^2}$-pairing state in Iron-Pnictides Wei-Feng Tsai, Dao-Xin Yao, JiangPing Hu, B. Andrei Bernevig We present theoretical results of Andreev bound states in superconductor-normal metal (or insulator)-iron-pnictide junctions. Within the two-orbital exchange coupling model [1], the presence of non-trivial in-gap states, which uniquely appear in the $s$-wave $\cos k_x\cos k_y$ ($s_{x^2 y^2}$) pairing state, can be taken as a sharply distinct feature in contrast to other singlet pairing states. In addition, a proposed novel trilayer $\pi$-junction involving $s_{x^2 y^2}$ superconductivity is also discussed as a new possible signature of such unconventional pairing symmetry.\\[3pt] [1] K. Seo, B. A. Bernevig, and J. Hu, Phys. Rev. Lett. 101, 206404 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H33.00010: Possible phase-sensitive test of pairing symmetry in superconducting pnictides David Parker, Igor Mazin The discovery of the new class of pnictide superconductors has engendered a controversy about their pairing symmetry, with proposals ranging from an extended s-wave or ``s$_{\pm}$'' symmetry to nodal or nodeless d-wave symmetry to still more exotic order parameters such as p-wave. Although there is evidence that a fully gapped state may exist in the pnictides, the symmetry of this state remains indeterminate. Building on the earlier, similar work performed for the cuprates, we propose here a phase-sensitive Josephson interferometry experiment that may allow resolution of the issue, taking into consideration novel features such as the local orbital character (from DFT calculations), and employing a particular potential barrier to restrict the tunneling orientations to favorable directions. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H33.00011: Andreev Bound states as a phase sensitive probe of the pairing symmetry of the FeAs superconductors Pouyan Ghaemi, Fa Wang, Ashvin Vishwanath A leading contender for the pairing symmetry in the Fe-pnictide high temperature superconductors is extended s-wave $s_-$, a nodeless state in which the pairing changes sign between Fermi surfaces. Verifying such a pairing symmetry requires a special probe that is sensitive to both phase and magnitude of the order parameter. We show that the sign structure of $s_-$ pairing leads to Andreev bound states at the edge. In the clean limit they only occur when the edge is along the Fe-Fe bond, but not for a diagonal edge. In contrast to d-wave Andreev bound states, they are not at zero energy and, in general, do not produce a zero bias tunneling peak. Consequences for tunneling measurements are derived, within a simplified two band model and also for a more realistic five band model. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H33.00012: Examining SrFe$_{2}$As$_{2}$ with a Low Temperature Scanning Tunneling Microscope Francis Niestemski, J. Gillett , Suchitra Sebastian, Vidya Madhavan The new pnictide superconductors have generated huge excitement. These materials are the first to add some chemical diversity to the limited high-Tc list previously exclusive to cuprates. We examine the pnictide material parent compound SrFe$_{2}$As$_{2}$ with a low-temperature ultra-high vacuum scanning tunneling microscope (STM) at 4 K. We find multiple types of topography and spectroscopy with low energy features. We relate our data to results from ARPES and other experiments. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H33.00013: Electronic Structure on (001) Surface of Co-doped BaFe$_{2}$As$_{2}$ Studied with Scanning Tunneling Spectroscopy A. Li, D. R. Jayasundara, Y. Xuan, J. P. O'Neal, Y. Chen, W. Kim, C. S. Ting, S. H. Pan, R. Jin, E. W. Plummer, R. Jin, A. S. Sefat, M. A. McGuire, B. C. Sales, D. Mandrus Co-doping makes the pnictide compound BaFe$_{2}$As$_{2}$ superconducting. We cleave the single crystals of this compound in UHV and study their surfaces with a low temperature STM. In this talk, we present the scanning tunneling spectrums obtained on the (001) surface of the optimally doped BaFe$_{2}$As$_{2}$ single crystals (Tc = 23K) and compare these spectrums with the ones obtained on the surface of the parent compound. We have found that the major feature of the spectrums on the superconducting compounds is the opening of a superconducting gap of about 6 meV for the optimally doped one. We have also observed other detailed spectrum features. We will discuss the relation between spectrum features and the local environment and also present some theoretical fit to the superconducting energy gap spectrum. [Preview Abstract] |
Session H34: Superconductivity: Transport Properties
Sponsoring Units: DCMPChair: J.R. Thompson, University of Tennessee
Room: 404
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H34.00001: Resistivity and superfluid density measurements on under- and over-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ films. Thomas Lemberger, Iulian Hetel, A. Tsukada, Michio Naito We have measured the resistivities and superfluid densities (or, penetration depths, $\lambda )$ of a series of LSCO films with a wide range of Sr concentrations. Films are grown by MBE on LaSrAlO$_{3}$ substrates under nominally identical conditions. Due to substrate mismatch, films are under compression. Resistivities decrease smoothly as Sr concentration increases, and resistive transitions are sharp. T$_{c}$ has a maximum at x = 0.15, while superfluid density 1/$\lambda ^{2}$(0) has a maximum at x $\approx $ 0.18. Interesting features in the T-dependence of 1/$\lambda ^{2}$ will be discussed. Absolute values of resistivity and superfluid density in these films indicates quality comparable to bulk materials. Falloff of superfluid density with overdoping, together with a smooth decrease in resistivity, is consistent with an interpretation in terms of a mesoscopically inhomogeneous superconducting state. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H34.00002: Strong Reduction of T$_{c }$Suppression by Magnetic Field in YBa$_{2}$Cu$_{3}$O$_{7+x}$ Films with Dispersed Nanoparticles E. Cimpoiasu, J. D. Feldmann, C. V. Varanasi, T. J. Haugan, P. N. Barnes, G. A. Levin Improvements in the critical current density $J_{c}$ in applied magnetic fields are of great importance for applications of the YBa$_{2}$Cu$_{3}$O$_{7-x}$ coated conductors. Nanosize inclusions have shown to be effective in increasing $J_{c}$, but the precise physical mechanisms of their action remains elusive. A broader range of experiments is needed in order to elucidate the physics of this phenomenon. Here we discuss the magnetic field H- and temperature T-dependence of the resistivity of thin films in the normal state and near T$_{c}$. Pure YBCO films will be compared with those that contain either dispersed Y$_{2}$O$_{3 }$nanoparticles or BaSnO$_{3}$ nanorods. The resistance of highly c-axis oriented YBCO films was measured by the Montgomery method in the range 20 K $<$ T $<$ 300 K and in fields up to 9 T. The films with inclusions show a much sharper and less broadened in-field transition (smaller T$_{c}$ suppression by field) than pure YBCO. This correlates well with increased $J_{c}$ measured by conventional methods and indicates increased pinning strength at all temperatures. In order to further identify the signatures of the nano-inclusions, the samples were annealed in air at 420 deg C. The changes induced by the annealing will be discussed. \textit{This work was partially supported by AFOSR and the AFRL Propulsion Directorate. } [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H34.00003: Quantum Oscillations and Hall Resistivity in YBCO and Tl-2201: Exploring the Fermi Surface of the Cuprates Brad Ramshaw The field of high temperature superconductivity has enjoyed something of a Renaissance in the past two years with the discovery of quantum oscillations in Shubnikov de Haas and de Haas-van Alphen measurements preformed on YBCO and Tl-2201. DC transport measurements around the one eighth hole doping region of high Tc phase diagram have shown a temperature and doping dependence to the Hall coefficient. In this doping region the Hall coefficient changes sign from positive to negative as temperature goes to zero, indicating a competition of mobilities between holes and electrons. In high magnetic fields these measurements also exhibit oscillations in one over the field strength, indicating the existence of small pockets of Fermi surface. This information, coupled with the Hall data, gives rise to the interpretation that the large hole like Fermi surface found in the overdoped region on Tl-2201 reconstructs into smaller electron like Fermi pockets on the underdoped side. Combined with other theoretical and experimental techniques, these experiments are allowing us to develop an understanding of the electronic structure of the cuprates across the entire phase diagram. This understanding is crucial to uncovering the underlying mechanism that gives rise to high temperature superconductivity. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H34.00004: Longitudinal and Transverse Transport properties of disordered graphene Vincent Ugarte, Vivek Aji We present results of calculations of the properties of thermal and electrical transport coefficients of disordered graphene in a weak magnetic field. In particular, we are interested in the effect of vanishing density of states and unitary scattering on transport coefficients near the Dirac point. The effect of impurity states is included within a self-consistent t-matrix approximation. We find a peak in both the Nernst coefficient and thermopower around the chemical potential which grows as one approaches the Dirac point. We compare our results to recent experimental data and discuss the importance of unitary scattering in various limits of temperature and chemical potential. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H34.00005: Linear-T resistivity and change in Fermi surface at the pseudogap critical point of a high-T$_c$ superconductor Ramzy Daou, Nicolas Doiron-Leyraud, David LeBoeuf, Shiyan Li, Francis Laliberte, Olivier Cyr-Choiniere, Y.J. Jo, Luis Balicas, J.-Q. Yan, J.-S. Zhou, John Goodenough, Louis Taillefer A fundamental question of high-temperature superconductors is the nature of the pseudogap phase which lies between the Mott insulator at zero doping and the Fermi liquid at high doping p. Here we report on the behaviour of charge carriers near the zero-temperature onset of that phase, namely at the critical doping p* where the pseudogap temperature $T^*$ goes to zero, accessed by investigating a material in which superconductivity can be fully suppressed by a steady magnetic field. Just below $p^*$, the normal-state resistivity and Hall coefficient of La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_{4}$ are found to rise simultaneously as the temperature drops below $T^*$, revealing a change in the Fermi surface with a large associated drop in conductivity. At $p^*$, the resistivity shows a linear temperature dependence as $T \rightarrow 0$, a typical signature of a quantum critical point. These findings impose new constraints on the mechanisms responsible for inelastic scattering and Fermi-surface transformation in theories of the pseudogap phase. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H34.00006: Transport Properties in Electron-Doped La$_{2-x}$Ce$_{x}$CuO$_{4}$ Thin Films Kui Jin, Xiaohang Zhang, Paul Bach, Richard Greene The electron-doped high-Tc cuprate La$_{2-x}$Ce$_{x}$CuO$_{4}$ (LCCO) is quite different from other members, such as Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ (PCCO) and Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ (NCCO). One distinct difference is that the optimal Ce doping in LCCO is $\sim $0.10, compared to Ce $\sim $0.15 in PCCO and NCCO. Here, we will present a detailed and systematic study of the magnetic field and temperature dependence of the transport properties of LCCO, including the low-temperature Hall effect and in-plane angular magnetoresistance. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H34.00007: Evidence for individual quantum phase-slip events in homogeneous superconducting nanowires Mitrabhanu Sahu, Myung-Ho Bae, Andrey Rogachev, David Pekker, Nayana Shah, Tzu-Chieh Wei, Paul Goldbart, Alexey Bezryadin We report strong evidence for individual quantum tunneling events undergone by the superconducting order-parameter field in homogeneous Mo$_{79}$Ge$_{21}$ nanowires. We obtain this via measurements of the distribution of switching currents, whose width exhibits a rather counter-intuitive, monotonic increase with decreasing temperature. We outline a stochastic model of phase-slip kinetics, which relates the basic phase- slip rates to switching rates. Comparison with this model indicates that the phase predominantly slips via thermal activation at high temperatures, but at sufficiently low temperatures switching is caused by individual topological tunneling events of the order-parameter field, i.e., quantum phase slips (QPS). Importantly, measurements on several wires show that quantum fluctuations tend to dominate over thermal fluctuations at larger temperatures in wires having larger critical currents. This fact supports the view that the anomalously high switching-rates observed at low temperatures are indeed due to QPS, and are not consequences of extraneous noise or inhomogeneity of the wire. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H34.00008: Collapse of peak effect by alternating current and its frequency dependence in MgCNi3 single crystal Dong-jin Jang, Hyun-Sook Lee, H-G Lee, M-H Cho, Sung-Ik Lee The peak effect, which appears as sharp rise in critical current near superconductor-normal transition of a superconductor, is first order phase transition. However, if vortices happen to move across sample by direct current (DC), sharp transition nature becomes blunted by edge contamination as intensively studied in NbSe$_{2}$. This edge contamination has been shown to be removed by alternating current (AC) or by using edgeless Corbino geometry. Among few superconducting materials showing peak effect, MgCNi$_{3}$ exhibits fairly sharp peak effect even in DC strip geometry. And remarkably, critical current measured by using AC is greatly suppressed as frequency of AC increases. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H34.00009: Analytical Procedure for Measuring Electrical Resistivity of Anisotropic Materials C.A.M. dos Santos, B.S. de Lima, C.Y. Shigue, A. de Campos, M.S. da Luz, A.T. Rice, B.D. White, J.J. Neumeier The Montgomery method is used to determine the resistivity tensor of anisotropic materials [1] such as high-T$_{C}$ and FeAs superconductors, 2-layer Mn oxides, organic conductors, and quasi-1D conductors. It uses the Wasscher transformation [2], which calculates an isotropic equivalent sample of the anisotropic sample. This is a timing-consuming task because it is a numerical method based upon graphical analyses obtained from calculations by Logan, Rice, and Wick [3]. In this work we report a simplification of the Montgomery method. Analytical equations are derived and applied to several isotropic and anisotropic samples (Cu, Al, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$, Graphite, SrNbO$_{x}$, $\gamma $-Mo$_{4}$O$_{11})$. Comparisons with results obtained by using the standard four-probe method demonstrate the quality and simplicity of the procedure, which can easily be extended to data acquisition systems. This material is based upon work supported by FAPESP (grant No. 07-04572-8), NSF (grants Nos. DMR-0504769 and 0552458), and CNPq (grant Nos. 301334/2007-2 and 201439/2007-7). [1] H. C. Montgomery, J. Appl. Phys. \textbf{42}, 2971 (1971). [2] J. D. Wasscher, Philips Res. Repts. \textbf{16}, 301 (1961). [3] B. F. Logan, S. O. Rice, and R. F. Wick, J. Appl. Phys. \textbf{42}, 2975 (1971). [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H34.00010: Doping dependence of the dynamic and static critical exponents in Pr$_{2-x}$Ce$_x$CuO$_4$ M.C. Sullivan, R. Isaacs, J.B. Olson, J. Sousa, M. Salvaggio, R.L. Greene Scaling analysis of voltage vs.\ current isotherms is a favorite tool to study the normal-superconducting phase transition in cuprate superconductors. This measurement has never been performed on the electron-doped cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$, due in part to difficulties which result from finite-thickness effects, even in thick ($d \approx 3000$\AA) films.\footnote{ Phys. Rev. B \textbf{69}, 214524 (2004)} If finite-thickness effects are taken into consideration, we can find the critical isotherm and the dynamic critical exponent, and we can use small magnetic fields to find the static critical exponent. Similar measurements have been made on the more familiar hole-doped cuprates such as YBa$_2$Cu$_3$O$_{7-\delta}$.\footnote{ arXiv:0803.0969} We present our results of the dynamic critical scaling exponent $z$ and static critical exponent $\nu$ in our Pr$_{2-x}$Ce$_x$CuO$_4$ films as a function of doping. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H34.00011: Experimental study of superconductivity in single crystal few-layer NbSe$_2$ and the effect of high electric fields Neal Staley, Linjun Li, Zhuan Xu, Ying Liu There have been many studies on superconducting properties in two dimensional films. However, a detailed study of superconducting properties in the two-dimensional limit when crystallinity is still retained, which will allow the probing of band dependent superconductivity in 2D, has not been performed. Due to concerns over defects in ultra thin films deposited in the usual methods, we use the methods developed in preparing micromechanically exfoliated graphene devices. In these samples the band structure is present while maintaining extremely low defect density. Inspired by this simple process that created single crystal single sheet graphite we fabricated ultra thin single crystalline NbSe$_2$ flakes ranging from single to many sheets as estimated using an optical technique correlated to AFM and Raman spectroscopy measurements. Transport and planar tunnel junction devices were fabricated using standard ebeam lithography techniques. We will also study the behavior of of these devices in high electric fields. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H34.00012: Size effects in the nonlinear resistance in a virtual Berezinskii-Kosterlitz-Thouless state of superconducting films Alex Gurevich, Valerii Vinokur We show that the size effects strongly affect the nonlinear electric field-current ($E-I$) relation of superconducting films. We calculate $E(J)$ due to thermally-activated hopping of single vortices driven by current $I$ across the film in a magnetic field $H$, taking into account interaction of free vortices with their antivortex images and peaks in the Meissner currents at the film edges. Unbinding of virtual vortex-antivortex pairs not only mimics the transport uniform BKT behavior, it can dominate the observed $E(J)$ and result in the field-dependent ohmic resistance at small $I$. We show that $E(I)$ can be tuned by changing the film geometry and propose experimental tests of this theory. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H34.00013: Geometric Effects on the Tunneling Apparent Barrier Height Aran Garcia-Lekue, Thomas Frederiksen, Andres Arnau An experimental quantity which may help understanding the mechanism of electron tunneling, in STM experiments or across broken nanojunctions for example, is the apparent barrier height. In order to extract information from this experimental observable one can consider a simple one-dimensional tunneling model, where the apparent barrier height is the rate of change of the logarithm of the conductance with the tip-apex separation or vacuum gap. Theoretically, a faithful analysis of the apparent barrier height requires a precise description of the tunneling conductance in the vacuum region. However, most of the conductance calculations are performed using atom centered localized basis sets, which cannot adequately describe the tunneling current crossing the vacuum gap and can therefore lead to erroneous results. In this work, we present tunneling conductance calculations obtained using the transport calculation method introduced in Ref.\,[1]. Since this method employs a plane-wave basis set, it provides accurate results for the electron tunneling across the vacuum gap and, consequently, for the apparent barrier height. Here, we report results for broken Au nanojunctions with different geometries, which allows us to thoroughly investigate geometric effects on the apparent barrier height. 1. A. Garcia-Lekue and L.W. Wang, Phys. Rev. B. {\bf 74}, 245404 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H34.00014: Thermopower across the pseudogap critical point of La(1.6-x)Nd(0.4)Sr(x)CuO(4): Evidence for a quantum critical point in a hole-doped high-Tc superconductor Olivier Cyr-Choiniere, Ramzy Daou, Francis Lalibert\'e, David LeBoeuf, Nicolas Doiron-Leyraud, Jiaqiang Yan, Jianshi Zhou, John B. Goodenough, Louis Taillefer The thermopower S of the high-Tc superconductor La(1.6-x)Nd(0.4)Sr(x)CuO(4) was measured as a function of temperature T near its pseudogap critical point, the critical hole doping p* where the pseudogap temperature T* goes to zero. Just above p*, S/T varies as ln(1/T) over a decade of temperature. Below p*, S/T undergoes a large increase below T*. As with the temperature dependence of the resistivity, which is linear just above p* and undergoes a large upturn below T*, these are typical signatures of a quantum phase transition. This suggests that p* is a quantum critical point below which some order sets in, causing a reconstruction of the Fermi surface, whose fluctuations are presumably responsible for the linear-T resistivity and logarithmic thermopower. We discuss the possibility that this order is the ``stripe'' order known to exist in this material. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H34.00015: Low-Temperature Thermal Conductivity in a $d$-Wave Superconductor with Coexisting Checkerboard Charge Order Philip Schiff, Adam Durst Given the experimental evidence of charge order in the underdoped cuprate superconductors, we consider the effect of coexisting checkerboard charge order on low-temperature thermal transport in a d-wave superconductor. We compute the quasiparticle excitation spectrum in the presence of both order parameters and perform a diagrammatic Kubo formula calculation of the zero-temperature thermal conductivity tensor as a function of the magnitude and wave vector of the charge order. Results depend on disorder, indicating that, in the presence of charge order, zero-temperature thermal transport is no longer universal. [Preview Abstract] |
Session H35: Focus Session: Iron Pnictides and Other Novel Superconductors VI: Model Hamiltonians
Sponsoring Units: DMPChair: Zlatko Tesanovic, Johns Hopkins
Room: 405
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H35.00001: Effects of Magnetic Order and Pairing on the Fermi Surface of the Pnictides A. Moreo, M. Daghofer, R. Yu, J. A. Riera, E. R. Dagotto Based on numerical and mean-field calculations performed on models for the FeAs planes of the newly discovered Fe-based superconductors, we present results for the expected shape of the Fermi surface both in the undoped and the doped regime. In the undoped case, numerical studies, Lanczos and VCA, are performed for a two-orbital model, while a mean-field formalism allows us to study a more realistic 4 orbital case. A pocket structure is obtained for the intermediate Hubbard coupling regime of parameters in which the system is magnetically ordered but still metallic [1]. We construct a mean-field model for light electronic doping based on the pairing operator that is found by the unbiased Lanczos calculations in the two orbital model, i.e. a spin singlet, orbital even, operator transforming according to the $B_{2g}$ representation of the group $D_{4h}$.[2] We present the resulting nodal structure [3] and discuss comparisons with ARPES results.\\[0.0001em] [1] R. Yu et al., submitted. [2] M. Daghofer et al., arXiv:0805.0148, to appear in PRL. [3] A. Moreo et al., submitted. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H35.00002: Nematic spin order and spin-lattice coupling in Fe-based Superconductors Jiangping Hu, Chen Fang, Wei-Feng Tsai, Hong Yao, Steve Kivelson We show that the structure transitions observed in Fe-based superconductors are magnetically driven. A quantum Heisenberg model ($J_1-J_2-J_z$) exhibits a sequence of two phase transitions: from a high temperature symmetric phase to a narrow region of intermediate ``nematic'' phase, and then to a low temperature spin ordered phase when $J_z$ is small. Identifying phases by their broken symmetries, these phases correspond precisely to the sequence of structural (tetragonal to monoclinic) and magnetic transitions that have been recently revealed in neutron scattering studies of 1111 series of Fe- based superconductors. The structural transition can thus be identified with the existence of incipient (``fluctuating'') magnetic order. We also discuss the effect of spin-lattice coupling on the phase diagram of the model. \\[3pt] Reference: Chen Fang, Hong Yao, Wei-Feng Tsai, JiangPing Hu and Steven A. Kivelson, Phys. Rev. B 77 224509 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H35.00003: Modeling of Fe pnictides: the Magnetic Order and Pairing Channels M. Daghofer, A. Moreo, J. A. Riera, E. Arrigoni, D. J. Scalapino, E. R. Dagotto We use numerical methods - exact diagonalization and the variational cluster approach - to study a two-orbital model for Fe-pnictide superconductors, including onsite Coulomb interaction $U$ and Hund's rule coupling $J$ [1]. Robust next-nearest neighbor hoppings stabilize the spin ``striped'' AF order for undoped clusters, in agreement with neutron scattering data. The ordered magnetic moment depends on $U$ and $J$, and we find a bad metal with small ordered moment at intermediate $U$, as observed experimentally. By adding two electrons to the undoped cluster, we identify three different pairings channels: An inter-orbital triplet at small $U$, which transforms as the $A_{2g}$ representation of the $D_{\rm 4h}$ group, an inter-orbital singlet transforming as $B_{2g}$ at the most realistic intermediate $U$, and an intra-orbital $A_{1g}$ singlet at large $U$. We compare the results to a three-orbital model including the $xy$ orbital in addition to the $xz$ ans $yz$ orbitals. [1] M. Daghofer et al., arXiv:0805.0148, to appear in PRL. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H35.00004: Existence of a metallic magnetically ordered state at intermediate Hubbard couplings in multi-orbital models for undoped iron pnictides Rong Yu, Kien Trinh, Adriana Moreo, Maria Daghofer, Jose Riera, Stephan Haas, Elbio Dagotto We present the results of a mean-field study for models that describe undoped iron pnictides. A realistic four-orbital model including iron $d_{xz}$, $d_{yz}$, $d_{xy}$, and $d_{x^2-y^2}$ orbitals is mainly discussed. Results for a two-orbital model with $d_{xz}$ and $d_{yz}$ orbitals are also shown. In both models, we report the existence of a novel intermediate coupling regime where the system is metallic and exhibits a striped spin order. Several properties of this state are discussed. By performing a mean-field study of other models for iron pnictides, we argue that such a metallic striped ordered phase is a general feature of the theoretical models describing iron pnictides.[1] \\[3pt] [1] R. Yu et al., submitted for publication. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H35.00005: Low Ordered Magnetic Moment in Fe-As High-T$_{c}$ Superconductors by Violation of Hund's Rule Jose Rodriguez, Edward Rezayi We study by exact diagonalization the J$_{0}$-J$_{1}$-J$_{2}$ model over the square lattice that Si and Abrahams introduced recently to describe magnetism in the newly discovered iron-arsenic class of high-T$_{c}$ superconductors. The case of maximum frustration between the nearest-neighbor and the next-nearest-neighbor Heisenberg exchange terms, J$_{2}=\vert $J$_{1}\vert $/2, over a 4 by 4 square lattice with periodic boundary conditions is focused on. Each site hosts two Fe orbitals. Hidden long-range antiferromagnetic order can appear in the absence of Hund's rule coupling, J$_{0}$ = 0. It shows no net ordered magnetic moment. The ordered collinear/SDW moment steadily increases from zero as Hund's rule coupling turns on: J$_{0} \quad <$ 0. This result compares well with recent determinations of a low ordered magnetic moment for the insulating parent compounds of Fe-As based high-T$_{c}$ superconductors by elastic neutron scattering. Our numerical results are also consistent with a quantum phase transition at intermediate Hund's rule coupling, J$_{0}$ = - 2$\vert $ J$_{1}\vert $, that separates the latter hidden-order state from a more familiar frustrated magnetic state that obeys Hund's rule. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H35.00006: Magnetic Excitations in the Iron-based Superconductors Daoxin Yao, JiangPing Hu, Erica W. Carlson We calculate the expected inelastic neutron scattering response based on the spin-orderings found in the iron-based superconductors, using spin-wave theory. For the two-sublattice collinear antiferromagnet, we consider two types of superexchange couplings between Fe atoms: nearest-neighbor coupling J$_1$ and next-nearest-neighbor coupling J$_2$. We show how to distinguish experimentally between ferromagnetic and antiferromagnetic J$_1$. We show the existence of saddlepoints near ($\pi$, $\pi/2$) and ($0$, $\pi/2$), which are expected to give rise to extra scattering intensity. We find that the sublattice magnetization can be reduced by the zero-point motion of spin waves, although not enough to account for the small moments observed in experiment. By comparison with experimental results on SrFe$_2$As$_2$, we estimate that the effective magnetic interlayer coupling is rather large, about $1/8$ the value of the in-plane couplings. References: 1) Phys. Rev. Lett. 101, 167203 (2008); 2) Phys. Rev. B 78, 052507 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H35.00007: Iron pnictides as a model system for heavy fermion behavior: influence of conduction-electron magnetic ordering on Kondo effect Jianhui Dai, Qimiao Si, Jian-Xin Zhu The rare-earth iron pnictides exhibit a number of magnetic ground states besides the unconventional superconductivity. With CeOFeP [1] and CeOFeAs [2] in mind as prototypes, we derive an extended Anderson lattice model which incorporates the hybridizations of the pnictogen 4p (or 3p)- orbitals with both the iron 3d-orbitals and rare earth 4f-orbitals[3]. We show a new type of Kondo lattice physics: Kondo screening of the f-moments are suppressed by the antiferromagnetic ordering of the d-electrons. Inside the d- electron AF state (as in CeOFeAs), the f-moments are dominantly coupled by superexchange with competing components. The resulting magnetic frustration in general favors a helical order. The regime where d-electrons are paramagnetic (including CeOFeP) features the usual RKKY vs. Kondo competition. The implications of our results for heavy fermion physics in general are discussed.\\[3pt] [1] E.M. Br\"uning et al., PRL101, 117206(2008)\\[0pt] [2] G. F. Chen et al., PRL100, 247002(2008). J. Zhao et al., Nat. Mater.(2008)\\[0pt] [3] J. Dai, Q. Si {\&} J-X Zhu, to be published. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H35.00008: Tight-binding Hamiltonian for LaFeAsO Dimitrios Papaconstantopoulos, Lane Nixon, Michael Mehl There have been several first-principles calculations reported recently for the superconducting pnictinide LaFeAsO and related compounds. In addition, tight-binding(TB) Hamiltonians for these systems have been constructed with varying degrees of success. In this work we have used the NRL-TB method to fit our LAPW results to a TB basis with the aim of reproducing the band structure very accurately. We have included the s and d orbitals of Fe, the s and p orbitals of As, and the p orbitals of O. We present a study of these TB results in terms of the effect of each of the above orbitals on how accurately the first-principles band structure can be reproduced. Finally, we assess the feasibility of carrying out many-body theory with a Hamiltonian that may contain more than just the d-Fe orbitals. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H35.00009: Competing magnetism and superconductivity in two-band metals Anton Vorontsov, Maxim Vavilov, Andrey Chubukov Recently discovered FeAs-based superconductors have a distinct multiple band structure - the hallmark feature of these materials. We consider a simple two-band model for these metals, with one electronic and one hole bands. Within this model, we treat on equal footing magnetic spin density wave (SDW) and superconducting (SC) orders. We find that at low doping, magnetism wins, but at higher dopings superconducting instability comes first. We discuss the type of a transition between the two states, incommensurate SDW order at finite dopings, and co-existence of SDW and SC orders at $T=0$ and finite temperatures. Our results reasonably well explain the phase diagram of LaO$_{1-x}$F$_{x}$FeAs compounds. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H35.00010: Superconductivity in the multiband matrix t-J1-J2 model and its implications for the iron pnictides Qimiao Si, Pallab Goswami, Predrag Nikolic, Elihu Abrahams We describe the iron pnictides in terms of an incipient Mott picture. We use local moments with frustrating J1-J2 interactions to model the incoherent electronic excitations, and couple them to the coherent electronic carriers. The resulting multiband matrix t-J1-J2 model is analyzed in terms of a slave boson theory, leading to a superconducting phase diagram as a function of doping and J2/J1 ratio. The different pairing symmetries reflect a competition between the strong coupling effects of the J1-J2 interactions, and the kinematic effects associated with the multiple sheets of Fermi surfaces. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H35.00011: Electronic states and material dependences of Fe-based superconductors Michiyasu Mori, Takami Tohyama, Navid Afzal Shooshtary, Sadamichi Maekawa In this study, we will discuss the electronic states of Fe- based superconductors and its material dependences, in particular, by taking account of the bond angle dependences. First, we calculate the crystal field splitting ($\Delta$) of Fe 3d orbital coordinated by four As's. Next, the hopping integrals (t) are estimated by using the Slater and Koster fs method. Note that these parameters, t and $\Delta$ change with $\alpha$. Finally, we can obtain the dispersion relation as a function of $\alpha$. It is found that the spectral weights near the Fermi energy are dominated by yz, zx and x2-y2 orbitals. The yz and zx orbitals are higher in energy around the regular tetragonal geometry, in which $\alpha$ is almost 109$^{\circ}$. On the other hand, those two orbitals become lower in energy for the larger value of $\alpha$. Such an orbital crossing is crucial for the electronic states. The ground state phase diagram is obtained by the Hartree-Fock calculation of multi-band Hubbard model. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H35.00012: Minimal two-band model for the superconducting iron oxypnictides Chao-Xing Liu, Srinivas Raghu, Xiao-Liang Qi, Douglas Scalapino, Shoucheng Zhang Following the discovery of the Fe-pnictide superconductors, LDA band structure calculations showed that the dominant contributions to the spectral weight near the Fermi energy came from the Fe 3d orbitals. The Fermi surface is characterized by two hole surfaces around the $\Gamma$ point and two electron surfaces around the M point of the 2 Fe/cell Brillouin zone. Here, we describe a 2-band model that reproduces the topology of the LDA Fermi surface and exhibits both ferromagnetic and $q=(\pi,0)$ spin density wave (SDW) fluctuations. We argue that this minimal model contains the essential low energy physics of these materials. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H35.00013: Spin-orbital frustrations and anomalous metallic state in iron-pnictide superconductors Frank Kruger, Sanjeev Kumar, Jan Zaanen, Jeroen van den Brink We develop an understanding of the anomalous metal state of the parent compounds of recently discovered iron based superconductors starting from a strong coupling viewpoint. On the basis of an intermediate-spin ($S=1$) state for the Fe$^{2+}$ ions, we derive a Kugel-Khomskii spin-orbital Hamiltonian for the active $t_{2g}$ orbitals. It turns out to be a highly complex model with frustrated spin and orbital interactions. We compute the classical phase diagrams and provide an understanding for the stability of the various phases by investigating the spin-only and orbital-only limits of the full Hamiltonian. The experimentally observed spin-stripe state is found to be stable over a wide regime of physical parameters and can be accompanied by three different types of orbital orders. Of these the orbital-ferro and orbital-stripe orders are particularly interesting since they break the in-plane lattice symmetry; a robust feature of the undoped compounds. We also compute the magnetic excitation spectra for the $S=1$ Heisenberg model, treating orbital correlations as static. The stable orbital-stripe state provides an explanation for the observed strong reduction of magnetic moment. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H35.00014: Theory for the Magnetic Ordering in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ Chen Fang, B. Andrei Bernevig, Jiangping Hu We develop a local spin model to explain the rich magnetic structures in the iron-based superconductors $Fe_{1+y}Te_{1-x}Se_x$. We show that our model exhibits both commensurate antiferromagnetic and incommensurate magnetic order along the crystal a-axis, the transition between which can be obtained by increasing the concentration of the excess $Fe$ to a critical value. The incommensurate modulation vector is also shown to be $Fe^2$ concentration dependent. Experimentally measurable spin-wave features and Fermi surface properties are calculated and compared to those of other Fe-based superconductors. Our model also suggests the existence of a large quantum critical region due to strong spin frustration upon increasing $Se$ concentration. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H35.00015: Global phase diagram and the spin structure in the Fe-based superconductors Kangjun Seo, Chen Fang, Jiangping Hu, B. Andrei Bernevig We study the global phase diagram and calculate the spin susceptivility for different states based on a two-orbital $J_1 - J_2$ model for Fe-based superconductors. Unique features associated with the unconventional $s_{x^2 y^2}\sim \cos (k_x) \cos (k_y)$ wave pairing symmetry are identified. [Preview Abstract] |
Session H36: Carbon Nanotubes: Electrothermal Transport and Raman Spectroscopy
Sponsoring Units: DCMPChair: Stephen Cronin, University of Southern California
Room: 408
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H36.00001: Restricted Wiedemann-Franz law in 1D conductors Marcelo Kuroda, Jean-Pierre Leburton We show that under external electric fields or thermal gradients, carrier distributions in one-dimensional (1D) conductors with linear $E$-$k$ dispersion have different temperatures for forward and backward (branch) carrier populations, as a consequence of self-consistent carrier-heat transport. We derive the moment equations of the Boltzmann transport equation, in the presence of elastic scattering, for which: (a) The Wiedemann-Franz law is restricted to each branch with its specific temperature; (b) thermoelectric power vanishes due to electron-hole symmetry. The model depicts different regimes such as ballistic and diffusive and shows excellent agreement with diffusive carrier transport in 1D conductors. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H36.00002: Observation of Joule heating in multi-walled carbon nanotubes by electron thermal Microscopy Kamal Baloch, Todd Brintlinger, Norvik Voskanian, John Cumings We report Joule heating in multi-walled carbon nanotubes under voltage bias by using an electron thermal imaging technique [1]. Briefly, the temperature profile is obtained by observing the solid to liquid phase transitions of indium islands sub-100nm in diameter, thermally deposited on the back side of an electron transparent dielectric membrane. The high spatial-resolution maps thus obtained demonstrate that in the high-voltage-bias regime the thermal dissipation occurs not at the electrode contacts but along the entire length of the nanotube as predicted in [2]. The low temperatures involved ($<$200~C) extend previous results [3] into a new temperature regime. The implications of these results when combined with other observations in the literature will be discussed. \\[4pt] [1] Brintlinger et al, Nano Lett., \textbf{8}, 582 (2008)\\[0pt] [2] Yao et al, Phys. Rev. Lett., \textbf{84}, 2941 (2000)\\[0pt] [3] Begtrup et al, Phys. Rev. Lett. \textbf{99}, 155901 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H36.00003: Theory of phonon thermal conductivity of single-walled carbon nanotubes L. Lindsay, D. A. Broido, N. Mingo We have developed a Boltzmann transport approach to calculate the intrinsic lattice thermal conductivity of single-walled carbon nanotubes. Harmonic and anharmonic interatomic force constants are obtained from Brenner [1] and Tersoff [2] empirical potentials, while the full phonon Boltzmann equation is solved using an iterative scheme [3]. We employ symmetry based selection rules for anharmonic phonon-phonon scattering, and we include higher order anharmonicity in our examination of the phonon-phonon scattering for the acoustic modes. We compare our results to those obtained from other recent theoretical calculations [3, 4]. [1] D. Brenner et al., J. Phys. Condens. Matter 14, 783 (2002). [2] J. Tersoff, Phys. Rev. B 39, 5566 (1989). [3] N. Mingo and D. A. Broido, Nano Letters 5, 1221 (2005). [4] D. Donadio and G. Galli, Phys. Rev. Lett. 99, 25502 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H36.00004: Thermal Transport Through Carbon Nanotube Junctions and Carbon Nanotube Nanopapers Charles A. Barr, Alper Buldum Carbon nanotubes have demonstrated exceptional thermal transport properties that show promise in a wide range of applications. Nanotube nanocomposites and nanopapers have great potential as electronic thermal management materials. Here we present our theoretical investigations on thermal properties of nanotube junctions and nanopapers. Equilibrium and non-equilibrium molecular dynamics simulations are performed on non-bonded and bonded (fused) nanotube junctions and extended two dimensional structures (papers) containing these junctions. The investigation includes the effects of chirality, the off set in atomic register and the angle between the axes of the nanotubes on the thermal conductivity. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H36.00005: Imaging single carbon nanotubes with thermal radiation Yuwei Fan, Scott Singer, Raymond Bergstrom, B.C. Regan We have constructed tiny light bulbs, visible to the naked eye, using individual carbon nanotubes as filaments. A nanotube is suspended over a hole in a solid silicon substrate, and is heated to incandescence with electrical current. Diffraction-limited optical microscopy identifies the nanotube position and orientation, and allows direct comparison with high-resolution transmission electron micrographs of the same nanotube. Our current progress toward quantitative pyrometry will be described. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H36.00006: Thermal conductance and bolometric response of individual single-walled carbon nanotubes Daniel Santavicca, Joel Chudow, Anthony Annunziata, Luigi Frunzio, Daniel Prober, Meninder Purewal, Philip Kim We describe low temperature electrothermal characterizations of individual single-walled carbon nanotubes on insulating substrates. The increase in differential resistance with increasing dc bias current is attributed to Joule heating. This is confirmed by Johnson noise thermometry, and thus the resistance can be used as a direct probe of the average electron temperature. These measurements enable us to determine the nanotube thermal conductance. We also measure the rf heterodyne response and find that the data agree well with a linear response bolometric model using our experimental value for the thermal conductance. This is the first demonstration of bolometric detection in an individual nanotube. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H36.00007: Investigation of Optical Absorption and Thermal Transport in Suspended Carbon Nanotube Bundles I-Kai Hsu, Adam Bushmaker, Mehmet Akyol, Stephen Cronin, Michael Pettes, Li Shi The optical absorption in suspended carbon nanotube (CNT) bundles is measured using Raman spectroscopy and two platinum resistance thermometers (PRTs), located at both ends of the suspended CNTs. The power absorbed from an incident focused laser is determined from the thermal power flowing through both ends of the CNT, detected by resistance changes in the PRTs. The results show 0.03 to 0.44{\%} absorption of a focused 532nm laser with a 0.4$\mu $m diameter spot size incident on CNT bundles with diameters and lengths varying from 7.1-8.2nm and 11.7-14.3$\mu $m, respectively. The thermal conductance of the suspended CNT bundles can also be obtained by measuring the temperature difference between the incident laser spot and both ends of the suspended CNT. Here, temperatures in the center of the nanotube are extracted from the temperature-induced downshifts of the $G$ band Raman mode. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H36.00008: Terahertz Bolometric Detection in an Individual Single-Walled Carbon Nanotube Joel Chudow, Daniel Santavicca, Anthony Annunziata, Luigi Frunzio, Daniel Prober, Charles Schmuttenmaer, Philip Kim We describe measurements of terahertz detection in individual single-walled carbon nanotubes. The terahertz power dissipated in the antenna-coupled nanotube is determined from the induced temperature change via the nanotube's temperature-dependent resistance. This is the first demonstration of terahertz bolometric detection in an individual nanotube. This experimental technique is being developed to study high-frequency charge excitations in the nanotube, which are predicted to display Luttinger-liquid behavior due to the lack of screening in one dimension. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H36.00009: Quantum-dot thermometry applied to the study of electron-phonon interaction in nanowires Eric Hoffmann, Jason Matthews, Henrik Nilsson, Lars Samuelson, Heiner Linke The thermal properties of mesoscopic devices are greatly influenced by quantum and finite-size effects. For example, the influence of electron-phonon coupling on heat flow through nanowires is different than in bulk materials and has not been studied in detail. One challenging aspect of performing thermal experiments with a mesoscopic device is the application and quantification of a temperature difference across a sub-micron distance. The recently introduced quantum-dot thermometry[1,2] uses a quantum dot to measure the electronic temperature difference across the dot's dimension. We present here experimental results demonstrating quantum-dot thermometry using a quantum dot embedded in an InAs nanowire. In addition, we show result which suggest that quantum-dot thermometry can be used to measure the strength of electron-phonon interaction in a one-dimensional nanowire. 1. Hoffmann, E.A. \textit{et al.}, \textit{Quantum-dot thermometry}, Appl. Phys. Lett. \textbf{91}(25), 252114 (2007). 2. Hoffmann, E.A. \textit{et al.}, \textit{Measuring temperature gradients over nanometer length scales}, Submitted to Nano Letters (2008). [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H36.00010: Numerical study of heat flow and electron-phonon coupling in nanowires Jason Matthews, Eric Hoffmann, Henrik Nilsson, Lars Samuelson, Heiner Linke The strength of electron-phonon (e-ph) interaction in one-dimensional systems is an important mechanism that controls heat flow generated by Joule heating, e.g. in nanowires. Here we use finite element modeling to study the effects of e-ph interactions on the electron temperature profile within a heterostructure nanowire. In recent experiments, we have measured the electron temperatures in the vicinity of a double-barrier quantum dot embedded in a nanowire. We find a significant electron temperature rise in the non-heated (drain) end of the nanowire near the dot. Such a temperature rise is unexpected due to electrons seeing the dot as both electrically and thermally insulating. It is suspected that this temperature rise is due to heat bypassing the quantum dot via phonons, which in turn heat electrons in the nanowire drain by means of e-ph interaction. Our modeling results are in agreement with measured electronic temperatures, suggesting that these measurements could be used to determine the strength of e-ph interaction. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H36.00011: Nonlinear Optical Properties of Carbon Nanotubes from First Principles Jack Deslippe, David Prendergast, Steven Louie The optical excitation spectra of both semiconducting and metallic single-walled carbon nanotubes (SWNTs) as well as other 1D materials are dominated by exciton states of large binding energy and well defined symmetry in the group of the k-vector along the periodic direction. The optical oscillator strength is transferred almost entirely from the continuum into the excitons and the corresponding exciton-phonon states. Recent experiments have probed the spectral structure of the excited states of various symmetry in SWNTs using nonlinear optics techniques such as ultrafast spectroscopy, multi-photon spectroscopy, and phonon-assisted spectroscopy. We have developed and applied a new method based on the first-principles GW-Bethe Salpeter approach to the study of the nonlinear optical properties of the SWNTs. Supported by NSF Grant No. DMR07-05941, US DOE Contract No. DE- AC02-05CH11231 and DOE CSGF grant DE-FG02-97ER25308 and computational resources from Teragrid and NERSC. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H36.00012: The BO Approximation Breakdown - Raman Spectroscopy of Suspended Single-Walled Carbon Nanotubes under Gate Voltages. Adam W. Bushmaker, Vikram V. Deshpande, Scott Hsieh, Marc W. Bockrath, Stephen B. Cronin Since the creation of the field effect transistor, gate voltage response has been central to solid state devices. Typically, changing the Fermi energy in a metal with a gate voltage does not substantially change any of its properties. We present Raman spectra of pristine, suspended, metallic single walled carbon nanotubes observed under gate voltages, in which the LO mode of the G band downshifts, and then upshifts, giving the predicted ``W'' shaped gate voltage response. The data give interesting insight into the electron-phonon coupling through the Kohn anomalies in carbon nanotubes, and for the first time, confirm the predicted Born-Oppenheimer approximation breakdown in metallic nanotubes. We also report on Raman intensity variations in metallic nanotubes in response to gate voltages. Understanding these effects in pristine systems is crucial for the future development of low-dimensional devices based on metallic nanotubes. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H36.00013: Raman Scattering Study of Carbon Nanotube Serpentines Bei Wang, Awnish Gupta, Peter Eklund, Jun Huang, Wonbong Choi Single-walled carbon nanotubes (SWNTs) were grown on step-edge single crystal quartz using CVD. SEM images were taken using FESEM showing serpentines and loops. Raman spectra were taken on SWNTs using excitation lines 514nm and 647nm with laser spot diameter $\sim $0.8$\mu $m. By tracing the Raman spectrum along a SWNT, we were able to record the change of Raman features with respect to the bending radius R of tube in the plane of the substrate. We found that there is linear upshift of the G-band and R-band with the curvature (1/R) of the bend. We attribute the shift of these frequencies to strain related change of carbon-carbon bond length. In semiconducting SWNTs changes were also observed in the intensities of these bands which we identify with a curvature induced change in the band-gap. This interpretation is also consistent with the results of electronic measurements. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H36.00014: Raman Spectroscopy of isolated double wall carbon nanotubes with different metallic and semiconducting configurations. Federico Villalpando, Daniel Nezich, Yoong Ahm Kim, Daisuke Shimamoto, Hiroyuki Muramatsu , Takuya Hayashi , Jing Kong, Endo Moribu, Mauricio Terrones, Mildred Dresselhaus We have developed an experimental technique to obtain the Raman spectra from individual double wall carbon nanotubes (DWNT). A chemical vapor deposition (CVD) derived sample of DWNTs is dispersed into solution and placed on a Si substrate. The Si substrate contains lithographic markers that allow us to record the exact location of individual and isolated DWNTs and obtain their Raman spectra with various laser energies. The laser energy can be in resonance with the inner and/or the outer layers of the same DWNT. We report on the differences between individual DWNTs with different metallic and semiconducting configurations and compare our results to previous experiments performed on DWNT bundles. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H36.00015: Softening of the Radial Breathing Mode in Metallic Carbon Nanotubes Hootan Farhat, Ken-ichi Sasaki, Martin Kalbac, Mario Hofmann, Riichiro Saito, Mildred S. Dresselhaus, Jing Kong In this work, the Fermi level $(\epsilon_{\rm F})$ dependence of the radial breathing mode (RBM) of metallic single walled carbon nanotubes (M-SWNTs) has been investigated. In situ Raman spectra were obtained from several individual M-SWNTs while varying $\epsilon_{\rm F}$ electrochemically. The RBM frequency of an intrinsic M-SWNT is shown to be downshifted relative to highly doped tubes by $\sim2{\rm cm}^{-1}$. The downshift is greatest for small diameter and small chiral angle nanotubes. Most tubes do not show any change in RBM linewidth. A comparison is drawn between the RBM and the G-band ($A_{\rm LO}$ phonon) with respect to the $\epsilon_{\rm F}$ dependence of their frequencies and linewidths. [Preview Abstract] |
Session H37: Focus Session: Spectroscopic Probes of Biomolecular Structure and Function I
Sponsoring Units: DCPChair: Sunil Saxena, University of Pittsburgh
Room: 409
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H37.00001: Structure and dynamics in B12 enzyme catalysis revealed by electron paramagnetic resonance spectroscopy Invited Speaker: Challenges to the understanding of how protein structure and dynamics contribute to catalysis in enzymes, and the use of time-resolved electron paramagnetic resonance (EPR) spectroscopic techniques to address the challenges, are examined in the context of the coenzyme B12-dependent enzyme, ethanolamine ammonia-lyase (EAL), from \textit{Salmonella typhimurium}. EAL conducts the homolytic cleavage of the coenzyme cobalt-carbon bond, intraprotein radical migration (5-6 {\AA}), and hydrogen atom transfers, which enable the core radical-mediated rearrangement reaction. Thermodynamic and activation parameters are measured in two experimental systems, which were developed to isolate sub-sequences from the multi-step catalytic cycle, as follows: ($1)$ A dimethylsulfoxide (DMSO)/water cryosolvent system is used to prepare the kinetically-arrested enzyme/coenzyme/substrate ternary complex in fluid solution at 230 K.[1] Temperature-step initiated cobalt-carbon bond cleavage and radical pair separation to form the Co(II)-substrate radical pair are monitored by using time-resolved, full-spectrum EPR spectroscopy (234$\le T\le $250 K).[1] ($2)$ The Co(II)-substrate radical pair is cryotrapped in frozen aqueous solution at $T<$150 K, and then promoted to react by a temperature step. The reaction of the substrate radical along the native pathway to form the diamagnetic bound products is monitored by using time-resolved, full-spectrum EPR spectroscopy (187$\le T\le $217 K).[2] High temporal resolution is achieved, because the reactions are dramatically slowed at the low temperatures, relative to the initiation and spectrum acquistion times. The results are combined with high resolution structures of the reactant centers, obtained by pulsed-EPR spectroscopies,[3] and the protein, obtained by structural proteomics[4] and EPR and electron spin echo envelope modulation (ESEEM) in combination with site directed mutagenesis,[5] to approach a molecular level description of protein contributions to catalysis in EAL. \\[4pt] [1] Wang, M. {\&} Warncke, K. \textit{J. Am. Chem. Soc.} \textbf{2008}, $130$, 4846. \\[0pt] [2] Chen, Z. and Warncke, K. \textit{Biophys. J. }\textbf{2008}, $95$ (December) \\[0pt] [3] Canfield, J. M. and Warncke, K. \textit{J. Phys. Chem. B }\textbf{2002}, $106$, 8831. \\[0pt] [4] Sun, L. and Warncke, K. \textit{Proteins} \textbf{2006}, $64$, 308. \\[0pt] [5] Sun, L., Groover, O., Canfield, J. M., and Warncke, K. \textit{Biochemistry} \textbf{2008}, $47$, 5523. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H37.00002: Time-resolved X-ray scattering of proteins in solution: a new method for probing biomolecular structure and function Invited Speaker: X-ray scattering patterns from proteins in solution exhibit a radial intensity distribution that is sensitive to protein size, shape, and structure. When acquired in a time-resolved fashion, these `fingerprints' unveil conformational changes that occur as a protein executes its designed function. We recently developed the infrastructure required to record X-ray scattering snapshots with $\sim $100-ps time resolution on the BioCARS beamline at the Advanced Photon Source in Argonne, IL. This methodology was used to probe structural changes in hemoglobin after photodissociating a bound ligand. Remarkably, the scattering fingerprint exhibited changes at the earliest times resolved, evidently corresponding to small amplitude tertiary structure changes. On longer time scales, the allosteric quaternary structure transition was resolved. These scattering fingerprints provide robust constraints for structural models of intermediates and their dynamics, which are crucial to develop a detailed understanding of biophysical processes. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:48AM |
H37.00003: Ultrafast Photodynamics in Diverse DNA Structures from A-tracts to Z-DNA Invited Speaker: The vulnerability of the genome to UV photodamage has sustained interest in excited electronic states in DNA for over 50 years. Progress in understanding the nature and dynamics of electronic excitations in DNA has accelerated rapidly thanks in part to ultrafast spectroscopy. Most excitations in single DNA bases decay nonradiatively in hundreds of femtoseconds. Surprisingly, much longer-lived excited states are observed in femtosecond pump-probe experiments on single- and double-stranded DNAs. Localized charge transfer states are prominent in runs of adenine bases (A tracts). DNA is polymorphic and can adopt a range of structures beyond the iconic B-form double helix. The effect of helix conformation on excited-state dynamics has been studied in a double-stranded oligonucleotide that can be switched between B- and Z-forms. Experiments on G quadruplex structures and on $i$-motif DNA reveal that these forms have significantly slower relaxation than B-DNA. By altering $\pi -\pi $ stacking and hydrogen bonding, structure profoundly affects the complex photoprocesses observed in DNA. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H37.00004: Charge Transfer States of Aqueous B-DNA at Energies Above the Bright $^1\pi\pi^\ast$ Exciton States Adrian Lange, John Herbert Charge transfer states have been proposed to explain experimentally observed long-lived excited state dynamics in aqueous DNA oligomers\footnote{Crespo-Hern\'andez,~C.~E.;\ \ Cohen,~B.;Kohler,~B. \textit{Nature} \textbf{2005}, \textsl{436}, 1141.}. Due to the large number of atoms, tractably describing such excited states in DNA systems with \textit{ab initio} theory is limited to TD-DFT. However, standard TD-DFT exchange-correlation functionals significantly underestimate CT excitation energies owing to incorrect asymptotic behavior. To circumvent this error, we instead apply recently developed and optimized long-range corrected TD-DFT functionals to better assess the low lying CT and exciton states of DNA oligomers. We show that long-range corrected TD-DFT yields results comparable to correlated wave function models, placing CT states of aqueous B-DNA at energies above the optically bright $^1\pi\pi^\ast$ exciton states, contrary to TD-DFT results which find CT states below the exciton states. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H37.00005: Low frequency dynamics of Cytochrome c Karunakaran Venugopal, Paul Champion Femtosecond coherence spectroscopy is used to investigate the low frequency dynamics of cytochrome c (cyt c). There is good agreement between the higher frequency oscillatory components of the coherence spectra and the low frequency Raman spectra. A mode near $\sim $40 cm$^{-1}$ is a universal feature of heme systems and has been assigned to doming motions that are strongly enhanced upon ligand photolysis [1]. A dominant heme ruffling mode near $\sim $60 cm$^{-1}$ [2] appears in ferric cyt c for excitation in the region 425-432nm, to the red of the Soret maximum (408nm). This, along with a phase jump of $\sim \pi $ in this region, suggests the ruffling mode is coupled to a charge transfer (CT) band underlying the Soret band [3] and that it is a potentially important electron transfer reaction coordinate. [1] F. Gruia, M. Kubo, X. Ye, P. M. Champion, \textit{Biophys. J}., \textbf{2008}, $94$, 2252. [2] M. Kubo, F. Gruia, A. Benabbas, A. Barabanschikov, W. R. Montfort, E. M. Maes, P. M. Champion, \textit{J. Am. Chem. Soc.,} \textbf{2008}, $130$, 9800. [3] K. T. Schomacker, P. M. Champion, \textit{J. Chem. Phys.,} \textbf{1986}, $84$, 5314. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H37.00006: Ultrafast Dynamics of Leu-Enkephalin in Water and Membranes Soohwan Sul, Yuan Feng, Uyen Le, Nien-Hui Ge Ultrafast two-dimensional infrared (2D IR) spectroscopy has been applied to investigate the peptide-membrane interaction and conformational distribution of Leu-enkephalin (Lenk) in bilayer membranes. We compare the results from linear and 2D IR experiments on p-cresol in water, Lenk in water, and Lenk in membranes, focusing on the ring stretching mode of the Tyr side chain. Frequency-frequency correlation functions obtained from a series of waiting-time-dependent 2D IR spectra reveal a fast decaying component with a $\sim $ 1 ps time constant that is common for all three systems. This spectral diffusion component is attributed to hydrogen-bond making-breaking dynamics of the Tyr side chain. Unlike p-cresol in water, both Lenk systems exhibit substantial spectral inhomogeneity that does not decay within the 4 ps window. The observed hydrogen-bond dynamics suggests that the Tyr side chain of Lenk in membranes is located at the water-abundant region at the water-membrane interface. The experimental results are compared with those from MD simulations and DFT calculations. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H37.00007: Flap Conformations in HIV-1 Protease are Altered by Mutations Gail Fanucci, Mandy Blackburn, Angelo Veloro, Luis Galiano, Ding Fangu, Carlos Simmerling HIV-1 protease (PR) is an enzyme that is a major drug target in the treatment of AIDS. Although the structure and function of HIV-1 PR have been studied for over 20 years, questions remain regarding the conformations and dynamics of the \textit{$\beta $}-hairpin turns (flaps) that cover the active site cavity. Distance measurements with pulsed EPR spectroscopy of spin labeled constructs of HIV-1 PR have been used to characterize the flap conformations in the apo and inhibitor bound states. From the most probably distances and the breadth of the distance distribution profiles from analysis of the EPR data, insights regarding the flap conformations and flexibility are gained. The EPR results clearly show how drug pressure selected mutations alter the average conformation of the flaps and the degree of opening of the flaps. Molecular dynamics simulations successfully regenerate the experimentally determined distance distribution profiles, and more importantly, provide structural models for full interpretation of the EPR results. By combining experiment and theory to understand the role that altered flap flexibility/conformations play in the mechanism of drug resistance, key insights are gained toward the rational development of new inhibitors of this important enzyme. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H37.00008: An Investigation of Ionic Binding to Fatty Acid Monolayers by Broad-bandwidth Sum Frequency Generation Vibrational Spectroscopy Cheng Tang, Heather Allen Model study of ionic binding of fatty acid monolayer is a good proxy towards understanding the fundamental chemistry in biological processes. In this study, we used broad-bandwidth sum frequency generation (BBSFG) vibrational spectroscopy to investigate the ionic binding event that leads to deprotonation of the fatty acid head groups. Palmitic acid (C15C00H) exists as monolayer on aqueous surfaces, and on aqueous alkali and alkaline solutions surfaces. Surface vibrational stretching modes of palmitic acid from 1400 cm$^{-1}$ to 3700 cm$^{-1}$ were observed (COO$^{-}$, C=O, C-H, and O-H). Palmitic acid is mostly protonated at the aqueous surface at neutral pH ($\sim $6). However, various degrees of deprotonation are initiated by introduction of different cations in the salt solutions albeit at neutral pH. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H37.00009: Using Rotationally Resolved Electronic Spectroscopy to Probe Chiral Molecules in the Gas Phase Justin Young, Leonardo Alvarez-Valtierra, David Pratt It is well established that biological processes involving chiral molecules can show a preference of one enantiomer relative to the other. Reported here are high resolution spectroscopy experiments that allow one to distinguish one diastereomer from another, and thereby establish if the structural requirments for diastereoism are present. [Preview Abstract] |
Session H38: Focus Session: Theory of Electron Transport Through Molecules I
Sponsoring Units: DCPChair: Matthias Ernzerhof, Université de Montréal
Room: 410
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H38.00001: The Role of Symmetry in Molecular Electronic Conduction Invited Speaker: The Greens Function Density-Functional Tight-Binding (gDFTB) method is applied to determine the role that molecular symmetry in single-molecule conductivity.~ Both coherent elastic electron transport and inelastic electron-tunnelling spectroscopy (IETS) are considered.~ Symmetry becomes manifest in various ways: through the molecular point-group symmetry of the conducting molecule (D$_{2h}$ for chemisorbed benzenedithiol between two gold electrodes), through the conductance point-group symmetry displayed by the gDFTB equations (this embodies junction asymmetry and may be very low and nominally non-existent), and through an approximate molecular-conductance point group (C$_{2v}$ for chemisorbed benzenedithiol).~ Indeed, the conductivities for a range of relevant problems are well approximated using the restriction of molecular-conductance point-group.~ This allows the complex transmission curves calculated by many research groups to be dramatically simplified and partitioned into symmetry-depicted channels.~ Means are introduced that isolate a very small number of component channels describing different aspects of single-molecule conductivity: input junction channels, through-molecule channels, and output-junction channels. For elastic transport, all through-molecule channels are totally symmetric and hence a rigorous selection rule appears that transport is allowed involving only input-junction and output-junction channels of the same symmetry.~ However, for IETS, the through-molecule channels have the symmetry of the scattering molecular vibration and hence the input-junction and output-junction channel symmetries may vary.~ In general, just one channel is expected to dominate the junctions, leading to the IETS propensity rule that totally symmetric transitions are the most intense ones. Simple physical pictures are presented showing the input, vibrational scattering, and output channels for IETS, leading to predictions of how this effect can be controlled chemically. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H38.00002: Molecular and Nano Scale Device-conductance: steady state and dynamical analysis Invited Speaker: A computational approach is used and developed to study electron transport through molecular and nano scale devices. New models and methods are employed to describe the dynamics of electron transport under the influence of time dependent (TD) perturbations. Quantum interferences affecting the TD conductance are analyzed for transient aspects, effects of present bound states and transport under the effect of coherent excitations. I will also discuss our modeling of several recent high-profile experimental studies achieving molecular scale (steady state) conductance which provides intriguing insight at the molecular structural level on the functionality of the conducting devices. The studies involve metal recognition properties of short peptides or fabricated molecular sockets based on surface confined terpyridine ligands. If time permits I will describe the required structural features for a gating field to tune the conductance of a molecular conjugated system. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H38.00003: The spectroscopic dynamics of electron transport through molecular junctions Alexander Prociuk, Barry Dunietz A non-equilibrium Green's-Function (NEGF) model based on time dependent perturbation theory is developed to compute the spectroscopic dynamics of electron transport through molecular junctions under the influence of weak time dependent classical fields. In this model, we use the two time variable nature of the Kadanoff-Baym equations of motion to formulate a mixed time-frequency representation for the electronic density. The resulting highly informative time dependent Wigner distributions are used to shed light on the features of dynamical observables, such as electron current, dipole moment and population. We analyze laser induced coherence and population transfer effects for both Markovian and non-Markovian electrode models. If time permits, the analysis of transient conductance with respect to the system's fundamental parameters will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H38.00004: Model \textit{ab initio} studies of solvation and excess charge localization on conjugated carbon chains Michael Mayo, Yuri Gartstein Using long C$_N$H$_2$ conjugated carbon chains with the polyynic structure as prototypical examples of one-dimensional (1D) semiconductors, we discuss self-localization of excess charge carriers in the presence of the interaction with a surrounding polar solvent. The solvation mechanism of self-trapping is different from the self-localization due to coupling with bond-length modulations of the underlying atomic lattice well-known in conjugated polymers. Model \textit{ab initio} computations are carried out and compared that employ various methods such as hybrid density functionals and Hartree-Fock within the framework of the polarizable continuum model. We demonstrate the possibility of the formation of large 1D electron- and hole-polarons entirely due to solvation, but even larger degrees of charge localization occur when accompanied by atomic displacements. Also discussed are doubly-charged bipolaron states and topological kink-solitons that may be formed in these systems. For a brief report, see M.~L.~Mayo and Yu.~N.~Gartstein, Phys. Rev. B 78, 073402 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H38.00005: Measuring single electron charging energy in self-assembled single nanoparticle devices: Coulomb blockade threshold vs. Arrhenius energy Al-Amin Dhirani, Amir Zabet-Khosousi Single-nanoparticle (NP) devices formed by self-assembling NPs onto alkanedithiol-functionalized break junctions exhibit Coulomb blockade (CB) conductance suppressions at low temperatures. We have studied temperature dependence of conductance inside the CB region and find \textit{multiple }activation energies (\textit{Ea}): A small \textit{Ea }at low temperatures, and a larger \textit{Ea }at high temperatures. The small \textit{Ea }is independent of NP size and is attributed to an energy state located at the metal--molecule contact. The larger \textit{Ea }scales with NP size and is attributed to single electron charging energy of the NPs. Importantly, we observe a significant ($\sim $5--100 fold) discrepancy between values of charging energies obtained from CB voltage thresholds and \textit{Ea}. To account for the discrepancy, we propose a model in which electrons are temporarily localized at the energy states near the metal--molecule interface and lose energy. The proposed model is supported by ultraviolet photoelectron spectroscopy of alkanedithiol monolayers on gold which indicates a presence of energy states close to the Fermi level of gold likely arising from gold--thiolate bonds. A suitably modified Orthodox theory successfully describes our measurements. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H38.00006: Molecular transport in the language of many-body states Michael Galperin Recent advancements in experimental techniques at nanoscale caused a surge in research of transport through molecular junctions. Nonlinearity of current-voltage characteristic at resonance makes this regime particularly important for potential molecular based memory, switchers and logic devices. One of important differences of molecular junctions (compared e.g. to semiconductor QDs) is sensitivity of electronic and vibrational structure of the junction to oxidation/reduction of the molecule. This implies necessity of treating the transport at resonance in the language of molecular states rather than single particle orbitals. The latter are the choice of majority of available ab initio approaches. We consider two possible schemes capable of incorporating isolated molecule (many-body) states as a basis for transport calculations. The schemes utilize Hubbard operators for description of single electron transitions between many-body states and go beyond previously proposed scattering theory and standard quantum master equation approaches. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H38.00007: WKB modeling of single molecular transport and Molecular Nanometrology Vladimir Burtman, Andrei V. Pakoulev Wentzel--Kramers--Brillouin (WKB) approach to model transport mechanism in molecular nanostructures is discusses in content of molecular nanometrology. Two WKB models, direct tunneling (Simmons model) and field emission tunneling (Fowler-Nordhaim tunneling), could be used to model conductivity in single molecular structure at low and elevated biased. Potentially, Simmons model could extract two molecular barriers, one for electrons and one for holes from conductivity spectra. Following this assumption electrical and optical gap-probed molecular nanometrology (GMN) could be developed. The main GMN principle is the small difference between the values of the HOMO-LUMO energy gap detected by electrical and optical measurements. We will compare experimentally derived electrical and optical probed gap and energy offsets between E$_{F}$ and nearest molecular orbital to discus applicability and feasibility of this approach. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H38.00008: Exact real-time dynamics of electron transport in mesoscopic systems Xiao Zheng, Jinshuang Jin, YiJing Yan We present a formally exact and numerically tractable quantum dissipation theory for time-dependent quantum transport in mesoscopic systems. It is formulated in terms of hierarchically coupled equations of motion, which govern the non-Markovian dynamics of an arbitrary fermionic system interacting with grand canonical electron reservoirs, in the presence of arbitrary time-dependent applied voltages [1-2]. We also present numerical results on the real-time dynamics of open quantum dot systems. The linear response admittance is mapped to classical equivalent circuits; while the nonlinear response dynamics is associated with dot-state transitions, such as the dynamic Coulomb blockade effect involved in interacting quantum dots [3-4]. Real-time Kondo phenomena are also demonstrated, with the cotunneling induced Kondo transition distinguished in the transient response current. This work highlights the significance and versatility of quantum dissipation theory for transient dynamics calculations. [1] J. S. Jin, S. Welack, J. Y. Luo, X. Q. Li, P. Cui, R. X. Xu, and Y. J. Yan, J. Chem. Phys. \textbf{126}, 134113 (2007). [2] J. S. Jin, X. Zheng, and Y. J. Yan, J. Chem. Phys. \textbf{128}, 234703 (2008). [3] X. Zheng, J. S. Jin, and Y. J. Yan, J. Chem. Phys. \textbf{129}, 184112 (2008). [4] X. Zheng, J. S. Jin, and Y. J. Yan, New J. Phys. \textbf{10}, 093016 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H38.00009: Theoretical study of electron transport through $\pi $-stacked ethylbenzene lines bonded to a Si surface Manuel Smeu, Robert Wolkow, Hong Guo Recently, experimental techniques were developed for lines of $\pi $-stacked ethylbenzene molecules to self-assemble on an H-terminated Si (100) surface in the laboratory of one of the authors. In this work, we use density functional theory (DFT) combined with the nonequilibrium Green's function formalism (NEGF) to model electron transport through these ethylbenzene lines to determine if they could be used as molecular wires. In our calculations, the molecules are bonded to an H-terminated Si (100) surface and are bridging two Al leads. The transmission spectrum and its associated scattering states are determined by the NEGF-DFT technique. The presence of the Si substrate is found to play an important role for conduction: there is a dominant transmission peak near the Fermi level which is contributed by the Si substrate and not the $\pi $-stacked molecular line. The low-bias resistance is found to increase exponentially with the length of the molecular line, indicating a tunneling behavior in conduction. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H38.00010: Stark Spectroscopy of Conjugated Oligomers and Polymers Important for Organic Devices Alberto Moscatelli, David C. Coppock, Linda A. Peteanu Fluorescent conjugated polymers have attracted a great deal of attention among scientists and engineers for their potential use in opto-electronic devices. One of the points that remain to be fully understood, however, is the undesirable sensitivity of their charge transport efficiency and emission characteristics on variations of the polymer structure and morphology. Using Stark spectroscopy it is possible to measure directly two important photophysical molecular parameters: (i) the change in the dipole moment, which is related to the degree of charge transfer associated with an optical transition; and (ii) the change in polarizability, which is related to the extent of the electronic delocalization. Poly(phenylenevinylene) (PPV), poly(dialkylfluorene) (PDAF) and ladder-type polyphenylenes (LPPP), as well as related oligomers, have been tested using this approach. Comparison of the results from single chains and from aggregates reveal how intermolecular interactions impact charge transfer and electronic delocalization in these technologically-important systems. [Preview Abstract] |
Session H39: Lipid Bilayers: Structure and Function I
Sponsoring Units: DBPChair: Alex Levine, University of California, Los Angeles
Room: 411
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H39.00001: A New Mechanism for Domain Size Selection in Curved Lipid Membranes Fangfu Ye, Jonathan Selinger Lipid membranes, composed of saturated lipids, unsaturated phospholipids and cholesterols, play important roles in maintaining cellular activities. It is now well established that lipid membranes under proper conditions separate into saturated-lipid-enriched liquid-ordered (L$_{o})$ phase regions and unsaturated-lipid-enriched liquid-disordered (L$_{d})$ phase regions, with the L$_{o}$ phase having a larger bending modulus than the L$_{d}$ phase. In this project, we study how the bending modulus difference between L$_{o}$ and L$_{d}$ phases may affect the phase separation behavior of uniformly curved lipid membranes. We predict that, for membranes of a spherical lipid vesicle, when the line tension between the L$_{o}$ phase and L$_{d}$ phase is small the phase separation process is truncated and the underlying curvature leads to formation of stable L$_{o}$-phase domains of finite size. We also compare these predictions with experiments on lipid rafts. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H39.00002: Making Sense of the Polymorphous Shapes of Giant Liposomes Yan Yu, Stephen Anthony, Julie Vroman, Sung Chul Bae, Steve Granick Lipid vesicles, especially giant unilamellar vesicles (GUVs) are often used as simplified models for biological membranes, but their polymorphous panoply of shapes and shape changes is notorious to those who work with them. This affords opportunities to study why phospholipid membranes so often fail to minimize their surface area to adopt spherical shapes. Instabilities can be triggered by the tension caused by optical tweezers, osmotic perturbations, or polymer anchorage. This talk will describe the evolution of GUVs from spherical to pearl-like and to tube-like shapes, and back again reversibly. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H39.00003: Near-Field structural studies of lipid bilayers Merrell Johnson, Ricardo Decca We use a Near-field Scanning Optical Microscope (NSOM) in conjunction with a Photo Elastic Modulator (PEM) to conduct birefringence $(\mathop n\nolimits_e -\mathop n\nolimits_o )$ measurements with a spatial resolution of $\sim $80nm. With our current setup we are able to distinguish changes in retardance $\alpha =\frac{2\pi (\mathop n\nolimits_e -\mathop n\nolimits_o )}{\lambda }$ on the order of $5\times 10^{-3}$ radians. Simultaneously while gathering information about $\alpha $ we extract information about the samples optical orientation$\theta $, reference to the system's axis, with an accuracy of $3.64\times 10^{-3}$radians. We use our system on 1,2-dipalmitoylphosphatidylchorline (DPPC) bilayers, which at room temperature are in the gel state,( i.e.: their acyl chains have a $\sim $32 degree azimulthal tilt with respect to the membranes normal). Modeling the membrane as a uniaxial crystal we are able determine the position of the acyl chains by measuring the birefringence and optical orientation. By controlling the temperature of our sample we hope to better study the structural changes that occur during phase transitions from gel to liquid states. The investigation of other lipid mixtures and the transformations they undergo during different phases will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H39.00004: X-ray insight into cholesterol-phospholipid interactions David Gidalevitz The mechanism of nonideal cholesterol-lipids mixing yet remains controversial. We report on a systematic study of cholesterol-phospholipid interactions in lipid monolayers using Langmuir isotherms, synchrotron X-ray reflectivity (XR), and grazing-incidence X-ray diffraction (GIXD) techniques. Lipid monolayers consisted of cholesterol-DPPC mixtures with cholesterol mole fractions $\chi _{CHOL}$ varying from 0 to 1. GIXD reveals that at both $\chi _{CHOL}$ and $\chi _{DPPC}$ above .85 mixed films exhibit packing order of a prevalent lipid. In between, cholesterol seizes places in DPPC crystalline lattice at the stoichiometry similar as that of the mixture inducing short-range regular-hexagonal packing order with increasing spacing between molecules as a function of cholesterol content. XR shows that cholesterol tends to stay in DPPC acyl chains at low $\chi _{CHOL}$ while gradually descending to a subphase at higher $\chi _{CHOL}$ accompanied by rearrangement of DPPC headgroups. Thus, a desire of highly nonpolar cholesterol to avoid contacts with polar water molecules and/or DPPC headgroups defines a mode of cholesterol-lipid interactions. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H39.00005: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H39.00006: Monte Carlo Simulation of Coexisting Phases in DOPC/DSPC/Cholesterol Ternary Mixtures Rejwan Ali, Jian Dai, Juyang Huang Lipid raft domain has been a topic of current interest in both computational and experimental membrane biophysics. Understanding raft domain will open up path for modeling many cellular phenomena. Extensive studies on model raft consists of DOPC/DSPC/cholesterol ternary system have been reported by many experimental groups. We report Monte Carlo simulation to reconstruct experimental phase diagram. Both pair-wise and multi-body interactions have been used to simulate the phase boundary of liquid-ordered phase and liquid-disordered phase coexistence region. A new algorithm, named the ``Composition Evaluation Method,'' was implemented to determine the compositions of the coexisting phases in simulations. The new method is about 20$\sim $50 times faster in determining phase boundaries, comparing to the traditional free energy calculation. In addition, pair correlation functions were used to map the phase boundaries in the critical region. We found that pair-wise interactions can reproduce the experimental critical point as well as the slope of tie lines, but not the compositions of the coexisting phases. Simulations with multi-body interactions produced a much better fit to the experimental phase diagram. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H39.00007: Structural studies of mixed lipid bilayers on solid substrates using x-ray reflectivity Gang Chen, Mrinmay Mukhopadhyay, Yicong Ma, Sunil Sinha, Zhang Jiang, Curt DeCaro, Justin Berry, Laurence Lurio, Adrian Brozell, Atul Parikh The lipid bilayers of natural membranes generally exist in a fluid state which occurs above the gel to liquid crystalline phase transition temperature. Knowledge of the structure of such bilayers is important for understanding fundamental biological processes mediated by or occurring within membranes. We have performed systematic measurements on bilayers of 1,2-Dipalmitoyl-\textit{sn}-Glycero-3-Phosphoethanolamine (DPPE) and its mixture with 1,2-Dioleoyl-\textit{sn}-Glycero-3-Phosphocholine (DOPC) and cholesterol (CH) on silicon substrates with x-ray reflectivity both below and above their phase transition temperatures. Structural variations as a function of temperature are demonstrated by fitting the reflectivity data with both a model dependent and a model independent routine. Studies of Au nanoparticle labeled DOPC and DOPC + DPPE + CH mixture are also performed and the location of Au nanoparticles in these bilayers is established by analyzing the x-ray reflectivity data. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H39.00008: Cellular adhesion and dynamic membrane tether extraction Sarah Nowak, Tom Chou We consider the energetics and dynamics of pulling a ligand bound to an integral membrane receptor. Deformation of the cell membrane and cytoskeleton is considered as the ligand is pulled. We assume that deformation of the cytoskeleton obeys Hook's law up to a critical force at which the cell membrane locally detaches from the cytoskeleton and a membrane tether forms. Depending on the pulling velocity and force, a membrane tether of varying length may form before the receptor-ligand bond breaks. We study the probability of tether formation and the mean tether length at the moment of ligand detachment as a function of system parameters. This problem is applicable to AFM studies of cellular adhesion molecules, and to the biological problem of leukocyte rolling. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H39.00009: Lipid Gymnastics: Tethers and Fingers in membrane Lobat Tayebi, Gregory Miller, Atul Parikh A significant body of evidence now links local mesoscopic structure (e.g., shape and composition) of the cell membrane with its function; the mechanisms by which cellular membranes adopt the specific shapes remain poorly understood. Among all the different structures adopted by cellular membranes, the tubular shape is one of the most surprising one. While their formation is typically attributed to the reorganization of membrane cytoskeleton, many exceptions exist. We report the instantaneous formation of tubular membrane mesophases following the hydration under specific thermal conditions. The shapes emerge in a bimodal way where we have two distinct diameter ranges for tubes, $\sim $20$\mu $m and $\sim $1$\mu $m, namely fat fingers and narrow tethers. We study the roughening of hydrated drops of 3 lipids in 3 different spontaneous curvatures at various temp. and ionic strength to figure out the dominant effect in selection of tethers and fingers. Dynamics of the tubes are of particular interest where we observe four distinct steps of birth, coiling, uncoiling and retraction with different lifetime on different thermal condition. These dynamics appear to reflect interplay between membrane elasticity, surface adhesion, and thermal or hydrodynamic gradient. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H39.00010: Pseudo-phase Diagram of Cholesterol-Rich Filamentous, Helical Ribbon, and Crystal Microstructures Y.A. Miroshnikova, M. Elsenbeck, Guanqing Ou, Y.V. Zastavker, K. Kashuri, G.S. Iannacchione Optical and calorimetric techniques are employed to study temperature and concentration dependence of three self-assembled microstructure types formed in Chemically Defined Lipid Concentrate (CDLC): filaments, helical ribbons, and crystals. CDLC consists of cholesterol, bilayer-forming amphiphiles, and micelle-forming amphiphiles in water, and is considered to be a model system for cholesterol crystallization in gallbladder bile. Phase contrast and DIC microscopy indicate the presence of all three microstructure types in all samples studied. Optically observed structural evolution indicates that filaments first bend to form helical ribbons followed by clustering and ``straightening'' of these structures into short and increasingly thickening filaments that dissolve with increasing temperature. Complementary calorimetric studies (differential-scanning and modulation) reveal thermal signatures that correspond to this observed structural evolution, which occurs throughout a large region of metastable chemical coexistence. These results suggest that a pseudo-phase diagram for the microstructures formed in CDLC may be developed to explain the observed behavior of the system. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H39.00011: PTEN interaction with tethered bilayer lipid membranes containing PI(4,5)P$_{2}$ R. Moldovan, S. Shenoy, P. Shekhar, A. Kalinowski, A. Gericke, F. Heinrich, M. Loesche Synthetic lipid membrane models are frequently used for the study of biophysical processes at cell membranes. We use a robust membrane model, the tethered bilayer lipid membrane (tBLM), based on a (C14)$_{2}$-(PEO)$_{6}$-thiol anchor, WC14 [1]. Such membranes can be prepared to contain single phospholipids or complex lipid mixtures [2], including functional lipids involved in cell signaling, such as the highly charged phosphatidylinositol phosphates (PIPs). To study the interaction between the tumor suppressor PTEN (phosphatase and tensin homologue deleted on chromosome 10) and model membranes we have incorporated phosphatidylinositol-4,5-bisphosphate (PI(4,5)P$_{2}$) in tBLMs and use fluorescence correlation spectroscopy (FCS), neutron reflectometry (NR) and surface plasmon resonance (SPR) for their characterization. NR shows that tBLMs formed with PI(4,5)P$_{2}$ are complete. FCS of labeled PI(4,5)P$_{2}$ shows that diffusion occurs at the time scale characteristic of membrane-incorporated lipid. Finally, SPR shows specific binding of PTEN to the model membrane thus confirming the incorporation of PI(4,5)P$_{2}$ into the tBLM. [1] McGillivray et al, Biointerphases 2, 21-33 (2007) [2] Heinrich et al, Langmuir, submitted [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H39.00012: Subdiffusion and diffusion of lipid atoms and molecules in phospholipid bilayers Elijah Flenner, Jhuma Das, Maikel Rheinstadter, Ioan Kosztin We examine the dynamics of lipid atoms and molecules using a 0.1 $\mu$s all-atom molecular dynamics simulation of a hydrated diyristoyl-phosphatidycholine (DMPC) lipid bilayer. We identify three well separated time regimes in the mean square displacement, $\langle \delta r^2(t) \rangle$, of the lipid atoms and molecules: (1) a ballistic regime for $t<10$ femptoseconds; (2) a subdiffusive regime where $\langle \delta r^2(t) \rangle \sim t^\beta$ and $\beta < 1$ for times between 10 picoseconds and 10 nanoseconds; and (3) a Fickian diffusion regime where $\langle \delta r^2(t) \rangle \sim t$ for $t > 30$ nanoseconds. We propose a memory function approach for describing the mean square displacement over the whole time range, and find that the lateral self diffusion coefficient found from the memory function approach agrees well with the one determined from the mean square displacement. We use the cumulant expansion of the self-intermediate scattering function to connect the three time scales in the mean square displacement to the interpretation of neutron scattering signals. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H39.00013: Suppression of fluid membrane fluctuations by a periodic pinning potential: Applications to red blood cells. Mark L. Henle, Alex J. Levine The membrane of the red blood cell (RBC) is tethered to a two- dimensional triangular network of semi-flexible elastic spectrin filaments. This network allows the cell to maintain its structural integrity during the large shape deformations that occur as it circulates through the microvasculature. The lipid membrane is anchored to the spectrin filaments at the nodes of the network. Consequently, these attachments impose a two-dimensional periodic pinning potential upon the membrane. In this talk, we investigate the effect of this pinning potential on the thermal bending fluctuations of the membrane. We show that there is an exact mapping of this system onto the classic problem of non-interacting electrons subject to a periodic potential; we exploit this mapping to obtain an exact analytic solution for a defect-free triangular array of harmonic pinning sites. The pinning potential affects both the local and global structure of the bending fluctuations. To investigate the local structure we consider the bending correlations between two nearby points in the membrane, while for the global structure we consider the total area stored in the fluctuations. We also investigate the effective area modulus of the membrane/spectrin composite structure. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H39.00014: Rough-Smooth-Rough Interface transition in a supported lipid bilayer system Piyush Verma, Nick Melosh Dynamic evolution of interfaces with quenched disorder is common in nature including fluid flow in porous media, granular particle flow and bacterial colony growth. These interfaces, which are either modeled using the Quenched-KPZ equation or the Quenched-Edward-Wilkinson (QEW) equation, expand due to a driving force while the edge profile roughens monotonically over time due to a distribution of disordered trapping defects. We studied interface expansion of a supported phospholipid bilayer, which is an ideal two dimensional viscoelastic material. Surprisingly, we observed a unique rough-smooth-rough bilayer interface transition on chromium oxide which has never been reported before. This transition was found to be a result of the viscoelasticity of the lipid bilayer and could be modeled using a modified QEW equation, which includes a spring-like term to account for the bilayer elasticity. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H39.00015: The effect of curvature on the undulation spectrum of Red Blood Cell membranes Tatiana Kuriabova, Mark L. Henle, Alex J. Levine The human red blood cell (RBC) membrane has a composite structure of a fluid lipid bilayer tethered to an elastic 2D spectrin network. The study of the mechanical properties of RBCs is crucial to our understanding of their ability withstand large amplitude deformations during their passage through the microvasculature. The linear mechanical response of this composite membrane can be measured by observing its undulatory dynamics in thermal equilibrium, i.e. microrheology. Previous models of these dynamics postulated an effective surface tension. In this talk, we show that surface tension is not necessary. Rather, the coupling of membrane bending to spectrin network compression by curvature can account for the observed dynamics. We use a simplified theoretical model to describe the undulatory dynamics of RBCs, measured experimentally by the Popescu group.\footnote{G. Popescu et al. ``Imaging red blood cell dynamics by quantitative phase microscopy, Blood Cells, Molecules, and Diseases, (2008), in print''} Analyzing their data using our model, we observe dramatic changes in RBC membrane elasticity associated with cells' morphological transition from discocytes to echinocyte to spherocyte. [Preview Abstract] |
Session H40: Biological Physics I
Sponsoring Units: DBPChair: David Weitz, Harvard University
Room: 412
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H40.00001: Application of EPR studies on Biologically Important Copper Acetyl Acetonate and Copper Tetraphenyl Porphyrin via Bayesian Inference and Density Functional Theory Laxman Mainali, Indra Sahu, Keith Earle Quantitative lineshape analysis can allow one to infer information about spin probe structure and dynamics. Experiments were performed at different frequencies (S, X, K, Q, and W Band) for Copper acetyl acetonate (Cu(acac)$_{2})$ and 5,10,15,20 - Tetraphenyl - 21H,23H -porphine copper(II) (CuTPP) in toluene at different temperatures. In order to obtain unbiased estimates of model parameters within the context of a given model, EPR spectra were analyzed via methods of Bayesian Inference. Four different sets of model parameters used to describe cw EPR spectra for two different probe symmetries (axial and rhombic) were explored using a model for rotational diffusion that was analyzed via Stochastic Liouville Equation. The optimized magnetic and dynamic tensor parameters were inferred from individual and simultaneous multifrequency fits and were compared with the values obtained from density functional theory (DFT). The isotropic g values estimated with PW1PW for Cu(acac)$_{2}$ and CuTPP with the respective basis sets 6-31G and 6-31G(d) agree well with the experimental values, whereas the isotropic A values for Cu(acac)$_{2}$ and CuTPP estimated with Local and gradient corrected functionals PWP and Ahlrichs basis set DZ agree well with the experimental values. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H40.00002: Application of Bayesian Inference and Relativistic Density Functional Theory on EPR Study of Biologically important Transition Metal (Vanadium) Compounds Indra Sahu, Laxman Mainali, Keith Earle Quantitative lineshape analysis based on the stochastic Liouville equation allows one to infer information about spin probe structure and dynamics. The EPR parameters extracted from Experimental spectra can be used to check theoretical calculation based on Relativistic Density Functional Theory. Experiments on Vanadyl acetylacetonate [VO(acac)$_{2}$] and Vanadyl mesotetraphenyl porphine [VO(TPP)] in toluene at different temperatures were performed at five different frequencies(S,X,K,Q {\&} W-band). Spectral Analysis was performed using methods of Bayesian Inference at the various frequencies, both independently and in a simultaneous multifrequency fit. Rotational diffusion parameters were inferred for two symmetries (Axial and Rhombic). The isotropic A value (A$_{iso})$ calculated for VO(acac)$_{2 }$ with unrestricted and hybrid-functional PW1PW and for VO (TPP) with unrestricted and hybrid functional BHLYP and Pople style basis set with polarization function 6-311G(3df,3pd), deviated by 0.95{\%} and 0.23{\%} respectively compared to experimental Aiso values. The isotropic g values (g$_{iso})$ calculated for VO (acac)$_{2}$ and VO(TPP) with unrestricted and hybrid-functional PW1PW and Ahlrichs basis set TZV, deviated by 0.18{\%} and 0.05{\%} respectively compared to experimental g$_{iso}$ values. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H40.00003: Electrically stimulated contractions of Vorticella convallaria Deependra Kantha, David Van Winkle The contraction of \textit{Vorticella convallaria }was triggered by applying a voltage pulse in its host culturing medium. The 50V, 1ms wide pulse was applied across platinum wires separated by 0.7 cm on a microscope slide. The contractions were recorded as cines (image sequences) by a Phantom V5 camera (Vision Research) on a bright field microscope with 20X objective, with the image size of 256 pixels $\times $ 128 pixels at 7352 pictures per second. The starting time of the cines was synchronized with the starting of the electrical pulse. We recorded five contractions of each of 12 organisms. The cines were analyzed to obtain the initiation time, defined as the difference in time between the leading edge of the electrical pulse and the first frame showing zooid movement. From multiple contractions of same organism, we found the initiation time is reproducible. In comparing different organisms, we found the average initiation time of 1.73 ms with a standard deviation of 0.63 ms. This research is supported by the state of Florida (MARTECH) and Research Corporation. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H40.00004: Pressure Effects on the Morphology of Mammalian Cells John Schroeder, Charles R. Keese, Ivar Giaever Living mammalian cells can be perturbed by high pressure in a continuous and controlled manner. The effects can be easily measured and quantified using the well-established method of Electric Cell-substrate Impedance Sensing (ECIS). ECIS is an electrical biosensor that quantitatively monitors behaviours of living cells including spreading and adhesion, micromotion and migration. Here we describe the experimental set-up of the pressure equipment and how ECIS is adapted to data collection under these conditions. Preliminary results suggest that high pressure affects the cell attachment and spreading and causes well-attached cells to round up. Results will be presented on the behavior of monolayers of BSC-1 cells upon application of pressures up to 2 kbars of nitrogen gas.. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H40.00005: The electronic properties of microbial nanowires: An STM investigation Josh Veazey, Becky Steidl, Gemma Reguera, Stuart Tessmer \textit{Geobacter} species of bacteria present the prospect of an interesting physical system through the expression of pili that act as electrically conductive nanowires. These nanowires serve the biological role of transporting metabolically generated electrons outside the cell body to electron acceptors in the organism's native environment. We have performed scanning tunneling microscopy and spectroscopy on \textit{Geobacter sulferreducens} in an effort to elucidate the mechanism of conductivity. Understanding this system may lead to the enhancement in the effectiveness of \textit{Geobacter} species' roles in microbial fuel cells and the bioremediation of hazardous waste, such as uranium and petroleum. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H40.00006: Probing Ion Channel Insertion into a Bilipid Membranes with a Radio Frequency Tank Circuit Hyun Cheol Shin, Eric Stava, Minrui Yu, Hua Qin, Hyun-Seok Kim, Robert Blick We fabricated a radio frequency resonant circuit which can be applied for probing ion channels formed in bilipid membranes. The insertion of ion channels can be probed by monitoring the resonant response of the tank circuit. The circuit itself is realized on a glass chip, which simultaneously uses DC channel recordings (i.e. conventional on-chip patch clamping) and RF detection. The direct current recordings of the ion channels responses allows for the calibration of the radio frequency signal. Such radio frequency recordings of ion channel activity have great potential for high-throughput drug screening. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H40.00007: Highly oil-producing microalgae selected through directed-evolution on a microfludic chip Troy Mestler, Andre Estevez-Torres, Guillaume Lambert, Robert H. Austin Some species of photosynthetic microalgae produce signi?cant amounts of oil which can be easily converted to diesel fuel. However, as it stands today, biodiesel is signi?cantly more expensive than fossil fuels. We wish to improve the oil yield and production rate of a single species of microalgae through directed evolution. We propose to utilize our microfabication technology to create microhabitats to control the nutrient environment of the species, monitor oil production through Raman Spectroscopy, and punish colonies of algae which have low oil yield. We believe this process will produce a mutant species with a high oil yield. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H40.00008: Structural Color of Biological and Biomimetic Amorphous Nanostructures Jason Forster, Heeso Noh, Vinodkumar Saranathan, Hui Cao, Simon Mochrie, Chinedum Osuji, Richard Prum, Eric Dufresne The feathers of many bird species have amorphous nanostructures of beta-keratin and air that produce non-iridescent color. These structural colors are the result of wavelength-selective scattering from nanostructures which have well-defined length scales but no long-range translational order. We quantify the optical properties of feathers with angle-resolved reflectance spectra and compare them to the nanoscale structures observed with X-ray scattering. We are exploiting self-assembly of polymeric and colloidal systems to create biomimetic nanostructures that capture the essential optical properties of bird feathers. By varying the characteristic length scale and index of refraction contrast of these structures we aim to enhance and tune wavelength selectivity. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H40.00009: Directed Fluid Transport and Mixing with Biomimetic Cilia Arrays A.R. Shields, B.A. Evans, B.L. Carstens, M.R. Falvo, S. Washburn, R. Superfine We present results on the long-range, directed fluid transport and fluidic mixing produced by the collective beating of arrays of biomimetic cilia. These artificial cilia are arrays of free-standing nanorods roughly the size of biological cilia, which we fabricate from a polymer-magnetic nanoparticle composite material and actuate with permanent magnets to mimic biological cilia. Biological cilia have evolved to produce microscale fluid transport and are increasingly being recognized as critical components in a wide range of biological systems. However, despite much effort cilia generated fluid flows remain an area of active study. In the last decade, cilia-driven fluid flow in the embryonic node of vertebrates has been implicated as the initial left-right symmetry breaking event in these embryos. With silia we generate directional fluid transport by mimicking the tilted conical beating of these nodal cilia. By seeding fluorescent microparticles into the fluid we have noted the existence of two distinct flow regimes. The fluid flow is directional and coherent above the cilia tips, while between the cilia tips and the floor particle motion is complicated and suggestive of chaotic advection. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H40.00010: Physics of phagocytosis of foreign versus self-tolerance Richard Tsai, Pia Rodriguez, Dennis Discher The first cells to `attack' an implanted or injected foreign material or microbe are phagocytic cells of the innate immune system. These cells actively and rapidly phagocytose foreign cells, surfaces, or particles, but the process that is inefficient when faced with ``self'' cells. We have examined the biochemistry and some of the physics of this decision to eat or not eat. One particular protein on all animal cell membranes, called CD47, seems to engage phagocytic cell couter-receptors, and deactivate the force-generating myosin machinery that otherwise makes phagocytosis efficient. We will map the phagocytic synapse between phagocytes and particles or cells and describe the physicochemical dynamics that mediate this key decision in compatability. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H40.00011: The Dynamics of Foraging Ants G. William Baxter We experimentally study the foraging of small black ants, Formicinae lasius flavus, in order to describe their foraging behavior mathematically. Individual ants are allowed to forage on a two-dimensional surface in the {\it absence} of any food sources. The position of the ant as a function of time is determined using a high-resolution digital camera. Analysis of the average square displacements of many ants suggests that the foraging strategy is a non-reversing random walk. Moreover, the ants do not retrace their steps to return home but instead continue the random walk until it brings them back near their starting point. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H40.00012: Elasticity in Ionically Cross-Linked Neurofilament Networks Norman Yao, Yi-Chia Lin, Chase Broedersz, Karen Kasza, Frederick MacKintosh, David Weitz Neurofilaments are found in abundance in the cytoskeleton of neurons, where they act as an intracellular framework protecting the neuron from external stresses. To elucidate the nature of the mechanical properties that provide this protection, we measure the linear and nonlinear viscoelastic properties of networks of neurofilaments. These networks are soft solids that exhibit dramatic strain stiffening above critical strains of 30-70{\%}. Surprisingly, divalent ions, such as Mg$^{2+}$, Ca$^{2+}$, and Zn$^{2+}$, act as effective cross-linkers for neurofilament networks, controlling their solid-like elastic response. This behavior is comparable to that of actin-binding proteins in reconstituted filamentous actin. We show that the elasticity of neurofilament networks is entropic in origin and is consistent with a model for cross-linked semiflexible networks, which we use to quantify the cross-linking by divalent ions. Ultimately, we are able to extract microstructural network parameters such as the persistence length and the average distance between cross-links directly from bulk rheology. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H40.00013: Exploring Cell-Assisted Cell Growth Elijah Bogart, Sharon Lau, Amrish Deshmukh, Carl Franck The population dynamics of microbial life in sheared liquid suspension affords opportunities to explore the ways in which cells encourage each other to proliferate. Such elegant systems continue to inspire us to develop and test simple theories for cooperative behavior (e.g. Phys. Rev. E v. 77, p. 041905 (2008)) in living matter. We report on new insight afforded by the observation of the amoebae Dictyostelium discoideum of the effect on population growth of the introduction of adhesive contacts of cells with each other as well as solid substrates. Through a hydrodynamic scaling argument we find that that mechanical triggers provided by intercellular collisions are more important than collisions with container walls in encouraging growth. Finally, we confirm the discovery of a strain that lacks growth regulation due to density sensing. This work was supported by the NIH (P01 GM078586). [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H40.00014: Small Molecules Target Carcinogenic Proteins Claudiu Gradinaru An ingenious cellular mechanism of effecting protein localization is prenylation: the covalent attachment of a hydrophobic prenyl group to a protein that facilitates protein association with cell membranes. Fluorescence microscopy was used to investigate whether the oncogenic Stat3 protein can undergo artificial prenylation via high-affinity prenylated small-molecule binding agents and thus be rendered inactive by localization at the plasma membrane instead of nucleus. The measurements were performed on a home-built instrument capable of recording simultaneously several optical parameters (lifetime, polarization, color, etc) and with single-molecule sensitivity. A pH-invariant fluorescein derivative with double moiety was designed to bridge a prenyl group and a small peptide that binds Stat3 with high affinity. Confocal fluorescence images show effective localization of the ligand to the membrane of liposomes. Stat3 predominantly localizes at the membrane only in the presence of the prenylated ligand. Single-molecule FRET (fluorescence resonance energy transfer) between donor-labeled prenylated agents and acceptor-labeled, surface tethered Stat3 protein is used to determine the dynamic heterogeneity of the protein-ligand interaction and follow individual binding-unbinding events in real time. The data indicates that molecules can effect protein localization, validating a therapeutic design that influences protein activity via induced localization. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H40.00015: Direct observation of DNA dynamics toward solid state nanopore studied by fluorescence microscopy Tomoharu Takita, Noriyuki Toyama, Kaya Kobayashi, Toshiyuki Mitsui Translocation of single DNA through a solid state nanopore provides information of the length and the folding configuration of the DNA by sensing the ionic current profile. This sensing method opens the possibility to characterize individual polynucleotide molecule such as DNA and RNA and their interaction with various proteins. The interesting phenomena related to the nanopore based DNA sensing with the translocating ionic current have been reported recently and we also have found unexpected clogging probability of DNA into pore as a function of the biased voltage across the pore membrane. To visualize these phenomena as the dynamics of individual DNA molecule near nanopore, we have used fluorescence microscopy. The acceleration of DNA caused by an attractive force toward nanopore was observed in sequential sets of the microscope images. By applying the langevin equations to follow the dynamical motion of DNA, the electric fields near the nanopore under the various biased voltages and ionic concentrations were estimated in DNA solution. In this presentation, we will report the results of the estimated electric fields near nanopore and discuss the shape of the potential. [Preview Abstract] |
Session H41: CDW, SDW and Charge Order in Organic Conductors
Sponsoring Units: DMP DCMPChair: Claude Bourbonnais, Universite de Sherbrooke
Room: 413
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H41.00001: X-ray Diffraction measurements of the CDW phase in doped TiSe$_{2}$ and NbSe$_{2}$ J.P. Castellan, C.D. Malliakas, M. Lavarone, S. Rosenkranz, R. Osborn, F. Weber Layered transition metal chalcogenides display a phase diagram similar to that of high Tc superconducting materials. In high Tc materials the superconductivity coincides with the suppression of magnetic order. In the case of TiSe$_{2}$ and NbSe$_{2}$ the onset of superconductivity involves a charge density wave (CDW) rather than magnetic order. The phase diagram of doped TiSe$_{2}$ and NbSe$_{2}$ has so far been interpreted as a competition of the CDW order and superconductivity order parameters. Another interpretation is that the softening of the excitons responsible for the CDW order enables the superconductivity. We have measured diffuse scattering associated with the fluctuations in the order parameter of the CDW on either side of the quantum phase transition using X-ray scattering. We will present the behavior of the critical exponents as the quantum phase transition is approached and discuss whether there is a coupling of the CDW and superconducting order parameters. Work supported by US DOE BES-DMS DE-AC02-06CH11357 [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H41.00002: Interplane and Intraplane Coupling in Charge-Density-Wave Phases of $\mbox{1T-TaS}_2 $ and $1\mbox{T-TaSe}_2 $ Yizhi Ge, Amy Liu At low temperatures, the layered transition metal dichalcogenides $\mbox{1T-TaS}_2 $ and $1\mbox{T-TaSe}_2 $ adopt similar charge-density wave structures corresponding to a $\sqrt {13} \times \sqrt {13} $ reconstruction in which the Ta atoms cluster within the triangular layers. Yet $\mbox{TaS}_2 $ also undergoes a metal-insulator transition, while $\mbox{TaSe}_2 $does not. Here we present a density-functional-theory study of the electronic structure of $\mbox{1T-TaS}_2 $ and $1\mbox{T-TaSe}_2 $. The half filled Ta d band at the Fermi level is found to differ significantly in the two materials: in $\mbox{TaSe}_2 $ the band is three dimensional, while in $\mbox{TaS}_2 $ it is highly one dimensional. These results are analyzed using maximally localized Wannier functions. The effects of stacking sequence and the spin-orbit interaction will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H41.00003: A time-resolved observation of collective phenomena in the CDW compound TbTe3 F. Schmitt, P. S. Kirchmann, U. Bovensiepen, R. G. Moore, L. Rettig, M. Krenz, J.-H. Chu, N. Ru, L. Perfetti, D.-H. Lu, M. Wolf, I. Fisher, Z.-X. Shen Strong correlations and collective phenomena in solids are a fascinating and challenging area of physics. Understanding how the interactions between the constituents give rise to collective phenomena like phase transitions and collective modes will greatly enhance our understanding in solid state physics. One such system, the Rare Earth Tritelluride RTe$_3$, is an excellent model system for a systematic study of charge-density wave (CDW) physics. We have performed time-resolved angle-resolved photoelectron spectroscopy (trARPES), a novel form of pump-probe spectroscopy, on TbTe3 and we will present our newest results. We observe two collective modes and what we term the time-dependent melting of the CDW, which we discussed earlier\footnote{F. Schmitt et al., Science \textbf{321}, 1649-1652 (2008)}. Here, we will focus primarily on the detailed properties of the two collective modes. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H41.00004: Voltage Induced Torsional Strain in Tantalum Trisulfide in its CDW State J. Nichols, Jun Zhou, L. Ladino, J.W. Brill In 2007, Pokrovskii et al [Phys. Rev. Lett. 98, 206404 (2007)] reported that crystals of tantalum trisulfide spontaneously twist when a voltage near the CDW depinning threshold is applied. The direction of twist reverses when the voltage is reversed, so that the twist angle describes a characteristic hysteresis loop as a function of applied voltage or current. We have studied this effect by placing the sample inside a resonant RF cavity so that the twisting sample modulates the resonant frequency of the cavity. A magnetic wire is attached to the sample so that magnetic torque can also be applied to the sample. We have reproduced the ``Pokrovskii effect'' and also observed the complement: application of torque induces an additional voltage when the sample is biased above threshold. We have also found that the onset of the torsional strain occurs at a voltage slightly below that at which the shear modulus of the sample softens. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H41.00005: Evidence for coupling between charge-density-wave and phonons in two-dimensional rare-earth tri-tellurides M. Lavagnini, M. Baldini, A. Sacchetti, D. Di Castro, B. Delley, R. Monnier, J.H. Chu, N. Ru, I.R. Fisher, P. Postorino, L. Degiorgi We report on a Raman scattering investigation of the charge-density-wave (CDW), quasi two-dimensional rare-earth tri-tellurides $R$Te$_3$ ($R$= La, Ce, Pr, Nd, Sm, Gd and Dy) at ambient pressure, and of LaTe$_3$ and CeTe$_3$ under externally applied pressure. The observed phonon peaks can be ascribed to the Raman active modes for both the undistorted as well as the distorted lattice in the CDW state by means of a first principles calculation. The latter also predicts the Kohn anomaly in the phonon dispersion, driving the CDW transition. The integrated intensity of the two most prominent modes scales as a characteristic power of the CDW-gap amplitude upon compressing the lattice, which provides clear evidence for the tight coupling between the CDW condensate and the vibrational modes. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H41.00006: Pressure dependence of the single particle excitation in the charge-density-wave CeTe$_3$ system L. Degiorgi, M. Lavagnini, A. Sacchetti, C. Marini, M. Valentini, R. Sopracase, A. Perucchi, P. Postorino, S. Lupi, J.H. Chu, I.R. Fisher We present data on the pressure dependence at 300 K of the optical reflectivity of CeTe$_3$, which undergoes a charge-density-wave (CDW) phase transition well above room temperature. The collected data cover an unprecedented broad spectral range from the infrared up to the ultraviolet, which allows a robust determination of the gap as well as of the fraction of the Fermi surface affected by the formation of the CDW condensate. Upon compressing the lattice there is a progressive closing of the gap inducing a transfer of spectral weight from the gap feature into the Drude component. At frequencies above the CDW gap we also identify a power-law behavior, consistent with findings along the $R$Te$_3$ series (i.e., chemical pressure) and suggestive of a Tomonaga-Luttinger liquid scenario at high energy scales. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H41.00007: Far infrared magnetospectroscopy of quasi-1D and 2-D density wave compounds. A.F. Isakovic, G.L. Carr We report a far infrared reflectance study of quasi-1D blue bronze (K$_{0.3}$MoO$_{3})$ and magnetically doped quasi-2D Mn$_{0.01}$NbSe$_{2}$ over the temperature range from 10K up to 180K and fields up to 10T. For blue bronze, several features in the magnetoreflectance spectra, including amplitudon and phonon-like modes, change substantially (up to 50{\%}) when a magnetic field is applied. For 100K$<$T$<$180K (below the CDW T$_{C}$ of 183K), the spectra change nearly monotonically with T. Changes at lower T involve primarily the amplitudon mode (CDW order parameter), consistent with creep transport of the CDW. The B and T dependent spectra of Mn doped NbSe$_{2}$ reveal the presence of an isosbestic point near 5 meV photon energy and having a shape suggesting a local transfer of oscillator strength. The magnetoreflectance changes abruptly around a temperature of 16K for fields below 6T. Though a Drude-Lorentz fit can be applied to the spectra, a full understanding of charge transport in these materials will require a more detailed model. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H41.00008: Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)$_{2}$PF$_{6}$ salts Mario Poirier, Alexandre Langlois, Claude Bourbonnais, Pascale Foury-Leylekian, Alec Moradpour, Jean-Paul Pouget Using a microwave cavity perturbation technique at 16.5 GHz, we report a temperature and magnetic field study of the complex dielectric function along the stacking $a$-axis for a (TMTTF)$_{2}$PF$_{6}$ single crystal and its deuterated analog (d$_{12})$. For both salts, the charge ordering transition (CO) is characterized by a decrease of the dielectric constant $\varepsilon _{a}$ centered at T$_{CO}$ (65 K and 85 K); concomitantly, the dielectric losses go through a maximum near T$_{CO}$ and decreases rapidly below. The spin-Peierls transition (SP) is rather signalled by a rapid increase of $\varepsilon _{a}$ below T$_{SP}$ (16.5 K and 13 K) accompanied by a small peak in the losses. For the deuterated salt, we have observed important relaxation effects below 40 K that complicate the analysis of the dielectric function in the SP ground state. The temperature dependence of the SP anomalies was analysed in magnetic field values up to 18 Tesla. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H41.00009: NMR spin echo measurements of sliding spin density wave repinning in (TMTSF)$_{2}$PF$_{6}$ W.G. Clark, M.E. Hanson, Ernest W.H. Wong We report the repinning rate of sliding spin density waves (SDW's) in (TMTSF)$_{2}$PF$_{6}$ obtained from proton spin echo measurements. This method provides a local measurement of the square of the magnitude of the SDW velocity averaged over the sample as a function of time after the electric field responsible for the SDW sliding is reduced to below its depinning value. It shows a slow decay of the SDW current which we attribute to the corresponding adjustment of the SDW phase to the pinning centers and relaxation of the strain of the sliding SDW. Above the temperature $T\sim $4~K, a thermally activated behavior for the characteristic decay time is observed whose activation energy of 22$\pm $3 K is close to the single electron excitation gap. This indicates that above 4 K, the repining rate is dominated by thermally excited electrons. Below 4 K, a much weaker $T$-dependence is observed suggesting that tunneling may become the dominant repinning mechanism at lower $T$. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H41.00010: Low-temperature states in quasi-one-dimensional charge transfer salt (TMTTF)$_{2}$SbF$_{6}$ studied by $^{13}$C NMR under hydrostatic high pressures Fumitatsu Iwase, Koichi Sugiura, Kou Furukawa, Toshikazu Nakamura We report $^{13}$C NMR study of a quasi-one-dimensional charge transfer salt, (TMTTF)$_{2}$SbF$_{6}$, under the application of the hydrostatic pressure. The antiferromagnetic phase transition at ambient pressure was confirmed by the line splitting and the divergent increase of the spin-lattice relaxation rate 1/$T_{1}$. Under $\sim $5 kbar, 1/$T_{1}$ deceases abruptly at low temperatures without the enhancement, indicating that the ground state is a spin-gapped phase. However, the decrease of the Knight shift expected for spin-Peierls phase transition has not been observed. We observed additional lines, which split symmetrically, at low temperatures under the pressure of $\sim $17 kbar. The possible reentrant antiferromagnetic phases are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H41.00011: The Theory of the Field-Induced Charge-Density-Wave Phases. Andrei Lebed We study a problem about the appearance of the Field-Induced Charge-Density-Wave (FICDW) phases in a quasi-one-dimensional conductor in high magnetic fields. The calculated ratios for critical magnetic fields of the different FICDW phase transitions are in very good agreement with the existing experimental data in alpha-(ET)2KHg(SCN)4 materials. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H41.00012: Realistic Parameters for the Description of Organic Metals Andreas Dolfen, Erik Koch, Volker Blum, Laura Cano-Cort\'es, Jaime Merino In molecular crystals correlation effects are often significant. For a non-perturbative description of the full Coulomb interaction we have therefore to resort to a model description in terms of generalized Hubbard models. The derivation of parameters for such models is crucial for realistic simulations. While hopping parameters are easily derived from density-functional theory (DFT) the Coulomb parameters pose a significant problem due to screening processes. We decompose their contributions into intra- and inter-molecular parts. The intra-molecularly screened Coulomb parameters are treated within DFT whereas the inter-molecular corrections are evaluated using classical electrostatics with DFT-derived polarizabilities and the distributed-dipole approach in combination with a Ewald summation. Even for simple lattices of polarizable point dipoles we find intriguing screening phenomena. As realistic applications we discuss the one- and two-dimensional organic metals TTF-TCNQ and $\Theta$-(BEDT-TTF)$_2$I$_3$. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H41.00013: Spinodal decomposition of the charge ordering in the $\theta $-based organic salt Majed Abdel-Jawad, Ichiro Terasaki, Hatsumi Mori, Takehiko Mori We have measured with X-ray diffraction, resistivity and thermopower the spinodal decomposition of the long range charge ordering in the organic compounds $\theta $-(BEDT-TTF)$_{2}$RbZn(SCN)$_{4}$ and $\theta $-(BEDT-TTF)$_{2}$RbCo(SCN)$_{4}$ . Details of this spinodal decomposition reveals that the charge ordering growth in these compounds follows at first a two dimensional diffusion controlled mode followed abruptly by a change to a three dimensional diffusion controlled growth. Thermopower reveals that the long ranged charge ordered state in these compounds is characteristic of a system with strong on-site repulsion with narrow bandwidths. In contrasts to this, the thermopower of the short ranged charge ordering is anomalous not only in its value but also in its temperature dependence. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H41.00014: Possible verification of tilted and anisotropic Dirac cone in quasi-two dimensional organic compound $\alpha$-(BEDT-TTF)$_2$ I$_3$ using inter-layer magnetoresistance Takao Morinari, Takahiro Himura, Takami Tohyama It is proposed that a tilted and anisotropic Dirac cone is verified using the interlayer magnetoresistance in the layered Dirac fermion system, which is realized in $\alpha$-(BEDT-TTF)$_2$I$_3$, quasi-two-dimensional organic compound. The formula for the inter-layer magnetoresistance is derived using the analytic Landau level wave functions and assuming local tunneling of electrons. Reflecting the deformation of the Landau level wave function due to tilt and anisotropy of the Dirac cone, the inter-layer magnetoresistance depends on the direction of the magnetic field in the plane. We discuss how to determine the parameters of the tilt and anisotropy experimentally using the theoretical formula. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H41.00015: Interlayer Magnetoresistance of the Layered Organic Conductor (DMET)$_{2}$I$_{3}$ P. Dhakal, J.I. Oh, H. Yoshino, K. Kikchui, M.J. Naughton The interlayer magnetoresistance $\rho _{zz}(\theta $,$\phi )$ of the layered organic conductor (DMET)$_{2}$I$_{3}$ is reported for all possible orientations in a 9T magnetic field at 100 mK, and compared with existing theoretical models. We have also calculated $\rho _{zz}(\theta $,$\phi )$ for such Q1D conductors using semiclassical calculations employing, for the first time$^{1}$, the true triclinic crystal structure. These calculations are in qualitative agreement with our data, especially in the $y-z$ plane, where all previous models fail to reproduce the experimentally observed$^{2}$ Lebed magic angle effect$^{3}$. These results will be useful to understand the relationship between the different kinds of angular effects in this and other families of layered organic conductors. This work was supported by NSF Grant No. DMR 0605339. [1]. H. Yoshino, \textit{et al.}, (unpublished). [2]. S. Uji, \textit{et al.}, Proceeding of Physical Phenomena at High Magnetic Fields III (World Scientific, London, 1998), p 227. [3]. A. G. Lebed, JETP Lett. \textbf{43}, 174 (1986); T. Osada, \textit{et al.}, Phys. Rev. Lett. \textbf{66}, 1525 (1991); M. J. Naughton, \textit{et al.}, Phys. Rev. Lett. \textbf{67}, 3712 (1991). [Preview Abstract] |
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