Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U1: RNA Folding at the Crossroads Between Molecular Biology and Statistical Physics
Sponsoring Units: DCMPChair: Robijn Bruinsma, University of California, Los Angeles
Room: Colorado Convention Center Four Seasons 2-3
Thursday, March 8, 2007 8:00AM - 8:36AM |
U1.00001: BREAK
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Thursday, March 8, 2007 8:36AM - 9:12AM |
U1.00002: Aggregation and folding phase transitions of RNA molecules Invited Speaker: RNA is a biomolecule that is involved in nearly all aspects of cellular functions. In order to perform many of these functions, RNA molecules have to fold into specific secondary structures. This folding is driven by the tendency of the bases to form Watson-Crick base pairs. Beyond the biological importance of RNA, the relatively simple rules for structure formation of RNA make it a very interesting system from the statistical physics point of view. We will present examples of phase transitions in RNA secondary structure formation that are amenable to analytical descriptions. A special focus will be on aggregation between several RNA molecules which is important for some regulatory circuits based on RNA structure, triplet repeat diseases like Huntington's, and as a model for prion diseases. We show that depending on the relative strength of the intramolecular and the intermolecular base pairing, RNA molecules undergo a transition into an aggregated phase and quantitatively characterize this transition. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:48AM |
U1.00003: Ground state and glass transition of the RNA secondary structure Invited Speaker: RNA molecules form a sequence-specific self-pairing pattern at low temperatures. Understanding the relevant energy scales that govern sequence specificity is important for thermal and mutational stability studies of functional RNAs. This problem has been analyzed using a random pairing energy model as well as a random sequence model that includes a base stacking energy in favor of helix propagation [1]. The free energy cost for separating a chain into two equal halves offers a quantitative measure of sequence specific pairing [2]. In the low temperature glass phase, this quantity is shown to grow quadratically with the logarithm of the chain length, but it switches to a linear behavior of entropic origin in the high temperature molten phase. Numerical studies of the melting transition suggest similarities to the thermal depinning of a two-dimensional elastic manifold in a disordered medium, though details of the analogy need to be further explored. For designed sequences, however, a power-law distribution of pairing energies on a coarse-grained level may be more appropriate. Extreme value statistics arguments then predict a power-law growth of the free energy cost to break a chain, in agreement with numerical simulations. Interestingly, the distribution of pairing distances in the ground state secondary structure follows a remarkable power-law with an exponent 4/3, independent of specific assumptions for the base pairing energies. \newline \newline [1] Sheng Hui and Lei-Han Tang, Eur. Phys. J. B \textbf{53}, 77 (2006). \newline [2] R. Bundschuh and T. Hwa, Phys. Rev. Lett. \textbf{83}, 1479 (1999); Phys. Rev. E \textbf{65}, 031903 (2002). [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:24AM |
U1.00004: Glassiness in RNA folding Invited Speaker: We study secondary structures of random RNA molecules by means of a renormalized field theory based on an expansion in the sequence disorder. We show that there is a continuous phase transition from a molten phase at higher temperatures to a low-temperature glass phase. Based on an exact inequality, we argue that RNA conformations in the glass phase are similar to those at the transition. \\ $[1]$ M. Laessig and K.J. Wiese, The freezing of random RNA, Phys. Rev. Lett. 96 (2006), 228101. \\ $[2]$ F. David and K.J. Wiese, Systematic field theory of the RNA glass transition, q-bio.BM/0607044 (2006); accepted for publication in Phys. Rev. Lett. [Preview Abstract] |
Session U2: Quantum Cryptography and Quantum Communication I
Sponsoring Units: GQI DCMPChair: Paul Kwiat, University of Illinois at Urbana-Champaign
Room: Colorado Convention Center Four Seasons 4
Thursday, March 8, 2007 8:00AM - 8:36AM |
U2.00001: Entanglement-Based Quantum Cryptography and Quantum Communication Invited Speaker: Quantum entanglement, to Erwin Schroedinger the essential feature of quantum mechanics, has become a central resource in various quantum communication protocols including quantum cryptography and quantum teleportation. From a fundamental point of view what is exploited in these experiments is the very fact which led Schroedinger to his statement namely that in entangled states joint properties of the entangled systems may be well defined while the individual subsystems may carry no information at all. In entanglement-based quantum cryptography it leads to the most elegant possible solution of the classic key distribution problem. It implies that the key comes into existence at spatially distant location at the same time and does not need to be transported. A number recent developments include for example highly efficient, robust and stable sources of entangled photons with a broad bandwidth of desired features. Also, entanglement-based quantum cryptography is successfully joining other methods in the work towards demonstrating quantum key distribution networks. Along that line recently decoy-state quantum cryptography over a distance of 144 km between two Canary Islands was demonstrated successfully. Such experiments also open up the possibility of quantum communication on a really large scale using LEO satellites. Another important possible future branch of quantum communication involves quantum repeaters in order to cover larger distances with entangled states. Recently the connection of two fully independent lasers in an entanglement swapping experiment did demonstrate that the timing control of such systems on a femtosecond time scale is possible. A related development includes recent demonstrations of all-optical one-way quantum computation schemes with the extremely short cycle time of only 100 nanoseconds. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U2.00002: Beller Lectureship Recipient Talk: Applications of Quantum Teleportation Invited Speaker: Quantum teleportation, a way to transfer the state of a quantum system from one location to another, is central to quantum communication and plays an important role in a number of quantum computation protocols. Previous experimental demonstrations have been implemented with single photonic or ionic qubits. Very recently long-distance teleportation and open-destination teleportation have also been realized. Until now, previous experiments have only been able to teleport single qubits. However, since teleportation of single qubits is insufficient for a large-scale realization of quantum communication and computation, teleportation of a composite system containing two or more qubits has been seen as a long-standing goal in quantum information science. In my talk, I shall present the first experimental realization of quantum teleportation of a two-qubit composite system. In the experiment, we develop and exploit a six-photon interferometer to teleport an arbitrary polarization state of two photons. Not only does our six-photon interferometer provide an important step towards teleportation of a complex system, it will also enable future experimental investigations on a number of fundamental quantum communication and computation protocols such as multi-stage realization of quantum-relay, fault-tolerant quantum computation, universal quantum error-correction and one-way quantum computation. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:48AM |
U2.00003: Twenty two years of quantum key distribution Invited Speaker: Following their 1984 invention of quantum key distribution (QKD), Bennett and Brassard and colleagues performed a proof-of-principle QKD transmission over a 32-cm air path in 1991. This seminal experiment led other researchers to explore implementations of QKD in optical fibers and over line-of-sight outdoor atmospheric paths (``free-space''), resulting in dramatic increases in range, secret bit rate, security and availability. These advances have led to, and been enabled by, improvements in sources, single-photon detectors and the deeper understanding of QKD security with practical sources and detectors in the presence of transmission loss and channel noise. Today, QKD has been implemented with unconditional security over ranges greater than 100km, over multi-kilometer distances in high background environments in both fiber and free-space, and at high (GHz) clock rates over shorter distances. In my talk I will review the key enabling advances underlying these developments of experimental optical fiber and free-space QKD over the past 16 years, describe the present status of the field, and compare and contrast different approaches to implementing security against photon number splitting attacks. I will describe some recent results from QKD in dedicated (``dark'') optical fiber using ultra-high efficiency, low-noise transition edge sensor (TES) photo-detectors, achieving ultra-long transmission distances, and unconditional security over 107km through the use of a decoy-state protocol. I will also describe progress in making QKD compatible with all-optical fiber networks, including the co-existence of QKD signals with conventional optical data on the same fiber. I will conclude my talk with a survey of the prospects for QKD transmission distances exceeding 200km, which will include a comparison of the various single-photon detector technologies now becoming available for quantum communications. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:24AM |
U2.00004: Security of Quantum Key Distribution Invited Speaker: Quantum Key Distribution (QKD) is the most advanced application of Quantum Information Science. It allows extending secret keys over some distances in such a way that the security of the resulting key material can be guaranteed by the laws of quantum mechanics. In contrast to presently used encryption techniques, the security of QKD can be proven in terms of information-theoretic measures. The resulting key can then be used for many tasks, including exchanging secret messages. QKD has been developed in the language of abstract two-level systems, the qubits. They cannot be easily implemented in optical signals. It took some time to bring the protocols and theory of QKD to the point where they fit to the realities of fiber-optical or free-space applications, including lossy channels. Today, QKD schemes can be implemented reliably using standard off-the-shelf components. Information theoretic security is a theoretical concept. Naturally, it is impossible to demonstrate directly that a given experimental set-up indeed creates a secret key. What one can do is to show that the experiment can give data within a certain parameters regime, such as error rate and loss rate, for which a security proof exists. I will discuss what parameter regime gives provable secure key and which parameter regime cannot lead to secret key. It is desirable to prove `unconditional security,' as it is termed in the world of classical cryptography: no assumption is made about the attacks of an eavesdropper on the quantum channel. However, one has to assume that the signal structure and the measurement device are correctly described by the adopted model and that no eavesdropper can intrude the sender or receiver unit. In this talk I will briefly introduce the concept of QKD and optical implementations. Especially I will discuss security aspects of modern approaches of QKD schemes that allow us to increase the covered distance and the achievable rate. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 11:00AM |
U2.00005: Towards a practical quantum repeater Invited Speaker: Quantum mechanics provides a mechanism for absolutely secure communication between remote parties. For distances greater than 100 kilometers direct quantum communication via optical fiber is not viable, due to fiber losses, and intermediate storage of the quantum information along the transmission channel is necessary. This lead to the concept of the quantum repeater, proposed in 1998 by Briegel, Duer, Cirac, and Zoller. In 2001, Duan, Lukin, Cirac, and Zoller have proposed to use atomic ensembles as the basic memory elements for the quantum repeater. We will outline our program on the use of atomic ensembles as an interface for quantum information transfer and the prospects for long distance quantum networks. [Preview Abstract] |
Session U3: Quantum Chaos in Condensed Matter Physics
Sponsoring Units: DCMPChair: Wentao Lu, Northeastern University
Room: Colorado Convention Center Korbel 2A-3A
Thursday, March 8, 2007 8:00AM - 8:36AM |
U3.00001: Single-Channel Scattering from Disordered Samples: a sensitive probe of the eigenfunctions behavior Invited Speaker: The goal of the talk is to demonstrate that that statistics of waves reflected from a disordered sample via a single open channel can serve as a sensitive probe of the eigenfunctions behaviour inside the sample in all regimes: localized, extended, and critical (multifractal). In particular, it allows one to understand the anomalous scaling exponents governing the multifractal behavior of the moments of the Wigner time delay at the point of the Anderson localization transition. The method also reveals some nontrivial exact symmetry relations which must be satisfied by the anomalous exponents and multifractality spectra. These predictions were recently verified in accurate numerical simulations. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U3.00002: Scattering fidelity in random matrix elastodynamics, the effect of temperature on diffuse ultrasound Invited Speaker: Temperature variations in a high Q elastic body provide access to a slowly and reversibly tuned wave chaotic random ultrasound ``Hamiltonian.'' This allows benchtop measurements of scattering fidelity in quantum chaotic, and other, systems. To a first approximation, temperature changes merely rescale time, as the wave speeds and specimen size change. But inasmuch as the shear and longitudinal wave speeds change by different amounts, the wave fields are distorted as well. The degree of distortion is a measure of how rapidly the shear and longitudinal waves mix. We show how that distortion varies with temperature, with the age of a transient waveform, with frequency, and with specimen size and geometry. Measured scattering fidelities are found to be in accord with predictions from random matrix theory for both irregular and regular bodies, up to a scaling parameter that is related to the rate of mixing of the rays. That rate is very different depending on the regularity of the specimen. Fidelity is greater in ray-chaotic bodies than in regular bodies. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:48AM |
U3.00003: Analog Experiments on Quantum Chaotic Scattering and Transport Invited Speaker: The transport properties of mesoscopic and nanoscopic materials are dominated by quantum interference effects. Nevertheless it is challenging to delineate these effects through conventional transport experiments on real materials. Complications arise from finite temperatures (thermal smearing, inelastic scattering), and the excitation of two-level systems that can cause the electrons to ``decohere'' and drop out of the quantum-coherent transport process. We approach this problem from the perspective of nonlinear dynamics and utilize a unique experimental technique that directly simulates the quantum scattering properties of complicated (ray-chaotic) systems. A microwave cavity is used to simulate solutions to the time-independent Schr\"{o}dinger equation for a two-dimensional ray-chaotic infinite square-well potential. The classically chaotic ray trajectories within a suitably shaped microwave cavity play a role analogous to that of the chaotic dynamics of noninteracting electron transport through a ballistic quantum dot in the absence of thermal fluctuations. In wave chaotic scattering, statistical fluctuations of the scattering matrix $S$ and the impedance (`reaction') matrix $Z$ depend both on universal properties and on nonuniversal details of how the scatterer is coupled to external channels. We remove the non-universal effects of the coupling from the experimental $S $data using the radiation impedance obtained directly from the experiments, thus eliminating one of the most significant complications in conventional transport measurements. The Landauer-B\"{u}ttiker formalism is applied to obtain the conductance of a corresponding mesoscopic quantum-dot device. We find good agreement for the probability density functions of the experimentally derived surrogate conductance, as well as its mean and variance, with the theoretical predictions based on random matrix theory [1]. We also observe a linear relation between the quantum dephasing parameter and the cavity ohmic loss parameter. The results apply to scattering measurements on any wave chaotic system. We also discuss future directions for this work. \newline \underline {[1]} S. Hemmady, \textit{et al.},(http://dx.doi.org/10.1103/PhysRevB.74.195326) Phys. Rev. B 74, 195326 (2006). [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:24AM |
U3.00004: Quantum Networks as Models of Mesoscopic Systems Invited Speaker: We review our work on quantum networks. These are one-dimensional systems consisting of vertices connected by bonds that have incommensurate lengths L. Particles with a fixed wave-number k can propagate freely on the bonds and scatter at the vertices. Combining the free propagation and the vertex scattering we have ended up with a quantum ``evolution'' operator on the graph. The corresponding classical dynamics was defined as follows: we have constructed a Liouville description by considering the evolution of a phase-space density over the space of directed bonds. The classical evolution operator consists of transition probabilities between connected bonds taken from the corresponding quantum evolution operator. Due to the multiple connectivity (stretching) and the compactness of the system (folding), the classical dynamics is chaotic. This analogy enables us to study the connection between statistical properties of eigenvalues and eigenfunctions and the classical dynamics. Finally, connecting them with leads to infinity we have also shown that quantum networks are excellent paradigms for the study of mesoscopic transport. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 11:00AM |
U3.00005: The classical limit of quantum transport Invited Speaker: Weak localization and conductance fluctuations are manifestations of quantum interference on transport. These quantum effects take a finite time, the Ehrenfest time, to appear. We present a semiclassical calculation of the Ehrenfest time dependence of weak localization and conductance fluctuations for ballistic quantum dots. Weak localization is found to be suppressed when the Ehrenfest time is larger than the typical dwell time in the dot. In contrast, the conductance fluctuations are found to be Ehrenfest time independent. The calculated Ehrenfest time dependences are consistent with numerical results. [Preview Abstract] |
Session U4: Interfaces between Synthetic and Biological Polymers
Sponsoring Units: DPOLY DBPChair: Christine Ortiz, Massachusetts Institute of Technology
Room: Colorado Convention Center Korbel 2B-3B
Thursday, March 8, 2007 8:00AM - 8:36AM |
U4.00001: Design Rules for Thermally Responsive Polymer Brushes Invited Speaker: Thermally responsive polymers such as poly(N-isopropylacrylamide) (PNIPAM) are extensively used to thermally tune the interfacial properties of thin polymer films. Above a lower critical solution temperature (LCST) of 32C, PNIPAM becomes insoluble in water and the chains collapse. Below the LCST the polymer chains are swollen. Yet such dramatic changes are not observed in all cases. The molecular weight and grafting density may also influence the phase behavior. This talk describes the systematic investigation of the thermally driven collapse of end-grafted PNIPAM as a function of the chain grafting density, molecular weight, and temperature. The polymer was grafted from the surface of an alkanethiol monolayer on gold containing a brominated alkanethiol initiator. The chains were synthesized by Atom Transfer Radical Polymerization (ATRP). Extensive chain collapse occurred at the highest grafting density and molecular weight, but the change in the film thickness decreased with decreasing density and molecular weight. The LCST was unchanged within 1C for all films. The force profiles measured between the PNIPAM brushes and a second surface at T below the LCST further suggest a one-dimensional phase separation within the polymer brush. These findings are compared with theoretical models of water-soluble polymers. We further discuss design criteria that impact the ability to thermally tune the interfacial properties of grafted PNIPAM films. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U4.00002: Studying Polymer Transport on Soft and Hard Surfaces Invited Speaker: We have employed experiments and simulations to understand the factors controlling the transport of polymers on surfaces. From an experimental viewpoint we have focused on the transport of DNA (single stranded) on lipid bilayers. We show that this behavior is slaved to the mobility of the lipids. More surprisingly, it appears that the transport of molecules adsorbed on surfaces follows the same dependence on lipid mobility as for molecules incorporated into the lipid layer. The ability to control this surface diffusion through the introduction of posts or varying the strength of adsorption (by the use of an AC field normal to the surfaces) will also be studied. Theoretically we have used molecular dynamics simulations of a polymer chain of length N dissolved in explicit solvent and adsorbed as a pancake at the solid-liquid interface to discriminate between respective influences on surface diffusion of hydrodynamics and adsorption energetics. Only for analytically-smooth surfaces do we observe a strong influence of hydrodynamics; the polymer lateral diffusion constant, D, scales as $D \propto 1/N^{3/4}$, more weakly than for implicit solvent. For atomistic surface corrugation with uniform surface chemical makeup, $D \propto 1/N$ instead. This suggests that while we can understand the results for diffusion on lipid surfaces, more recent experimental observations of stronger N dependence for diffusion on hard solid surfaces originate not in hydrodynamic interactions but in spatially patchy energetic interactions. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:48AM |
U4.00003: Design of dendrimer-based drug delivery nanodevices with enhanced therapeutic efficacies Invited Speaker: Dendrimers and hyperbranched polymers possess highly branched architectures, with a large number of controllable, tailorable, `peripheral' functionalities. Since the surface chemistry of these materials can be modified with relative ease, these materials have tremendous potential in targeted drug delivery. They have significant potential compared to liposomes and nanoparticles, because of the reduced macrophage update, increased cellular transport, and the ability to modulate the local environment through functional groups. We are developing nanodevices based on dendritic systems for drug delivery, that contain a high drug payload, ligands, and imaging agents, resulting in `smart' drug delivery devices that can target, deliver, and signal. In collaboration with the Children's Hospital of Michigan, Karmanos Cancer Institute, and College of Pharmacy, we are testing the \textit{in vitro} and \textit{in vivo} response of these nanodevices, by adapting the chemistry for specific clinical applications such as asthma and cancer. These materials are characterized by UV/Vis spectroscopy, flow cytometry, fluorescence/confocal microscopy, and appropriate animal models. Our results suggest that: (1) We can prepare drug-dendrimer conjugates with drug payloads of greater than 50{\%}, for a variety of drugs; (2) The dendritic polymers are capable of transporting and delivering drugs into cells faster than free drugs, with superior therapeutic efficiency. This can be modulated by the surface functionality of the dendrimer; (3) For chemotherapy drugs, the conjugates are a factor of 6-20 times more effective even in drug-resistant cell lines; (4) For corticosteroidal drugs, the dendritic polymers provide higher drug residence times in the lung, allowing for passive targeting. The ability of the drug-dendrimer-ligand conjugates to target specific asthma and cancer cells is currently being explored using \textit{in vitro} and \textit{in vivo} animal models. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:24AM |
U4.00004: Ligand-receptor binding in the presence of polymeric spacers Invited Speaker: Ligand-receptor binding is of fundamental importance in many biological processes. Examples include cell-cell adhesion and cell-surface interactions among others. In several biomimetic materials as well as in some biological systems the ligand is attached to the surface by a spacer. In this talk we address the role that spacers play in ligand-receptor binding. More specifically, we present a series of theoretical studies in which we systematic study the role of polymeric spacers on the efficiency of ligand-receptor binding. The systems of interest correspond to the ligand chemically bound at the free end of polymers tethered to the surface, while the receptor is part of proteins free to move in the solution. Our theoretical approach is based on a molecular theory that has been shown to predict thermodynamic and structural information for tethered polymer layers in excellent agreement with experimental observations. We have generalized the theory to include the equilibrium between the bound and unbound species. We find that the presence of spacers increases the amount of binding as compared to the case in which the ligands are directly on the surface. The maximal binding is obtained at a relatively low surface coverage of spacer and it increases as the spacer chain length increases. The maximal binding is found to correspond to the cases in which the bound proteins can accommodate at different distances from the surface while bound to the ligand. We will show how the binding depends upon the size of the protein, the free energy of binding of the bare ligand-receptor pair, the polymer surface coverage and molecular weight. The predictions of the theory will be compared with recent experimental observations on the interactions between protein coated surfaces and surfaces with ligands at the end of polyethylene oxide spacers. Finally, we will show the use of mixed tethered layers to optimize ligand- receptor binding and at the same time to minimize non-specific adsorption of proteins. Throughout the presentation the interplay between different interactions in determining the binding will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 11:00AM |
U4.00005: Using Liquid Crystallinity to Design Interfaces between Synthetic and Biological Materials. Invited Speaker: This presentation will discuss the spontaneous assembly of amphiphiles and biological macromolecules at interfaces between thermotropic liquid crystalline phases and aqueous phases. This assembly process gives rise to patterned orientations of the liquid crystals that reflect the spatial and temporal organization of the amphiphiles and macromolecules. Strong and weak specific binding events involving proteins at these interfaces drive the reorganization of phospholipids and trigger orientational transitions in the liquid crystals. Because these interfaces are fluid, processes involving the lateral organization of proteins (e.g., formation of protein and phospholipid-rich domains) are also readily imaged via the orientational response of the liquid crystal, as are stereospecific enzymatic events. These results suggest new principles for designing interfaces between synthetic and biological polymers. [Preview Abstract] |
Session U5: Pharmaceutical Materials Science
Sponsoring Units: FIAPChair: Peter Stephens, State University of New York at Stony Brook
Room: Colorado Convention Center Korbel 1A-1B
Thursday, March 8, 2007 8:00AM - 8:36AM |
U5.00001: Solid forms of pharmaceuticals and thermodynamic stability Invited Speaker: |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U5.00002: Water and stability of pharmaceutical solids Invited Speaker: Solid pharmaceuticals are multi-component systems consisting of an active pharmaceutical ingredient (API) and inactive ingredients (excipients). Excipients may include inorganic salts (e.g., NaCl), carbohydrates (e.g., lactose), and polymers, to name a few, whereas APIs range from relatively simple molecules (e.g., aspirin) to proteins and olygonucleotides. Pharmaceutical solids could exist either as single-phase or heterophase systems. They also may have different extent of order, such as highly ordered crystalline phases, amorphous solids that are thermodynamically unstable but might be kinetically stable under the time frame of observation, and crystalline mesophases including liquid crystals. With all this diversity, there are common features for such systems, and two of them will be discussed in the presentation. (i) Requirements for chemical stability of pharmaceuticals are very strict. A very limited (e.g., less than 0.1{\%}) extent of conversion is allowed in these materials over the shelf life, i.e., during several years of storage at ambient and (sometimes) not fully controlled (e.g., a medicine cabinet in one's bathroom) conditions. (ii) All pharmaceutical solids contain some water, although its amount and physical state are highly variable and may change during manufacturing and shelf life. There are many challenging questions and issues associated with the ``Water and stability of pharmaceutical solids'' subject; some of them will be considered in the presentation: (i) What are the features of chemical reactivity of crystalline vs disordered systems? (ii) What is the role of water in solid state chemical reactivity of amorphous solids, e.g., water as plasticizer vs reactant vs reaction media? (iii) How homogeneous are pharmaceutical amorphous solid solutions, e.g., carbohydrate-water systems? (iv) What is the optimal water content? With water being the most common destabilizing factor, is ``the drier - the better'' always the case? [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:48AM |
U5.00003: Physical and Chemical Aspects of Pharmaceutical Solids: Fundamentals of Polymorphs, Hydrates and Solvates Invited Speaker: Crystal polymorphs are solid phases of a given compound resulting from the possibility of at least two different arrangements of the molecules of that compound in the solid state. Solvates form when the solvent is incorporated in the crystal structure of a compound; hydrates form when water is the solvent of crystallization. The potential effects of crystal polymorphism and hydration on the quality and performance of drug products is widely recognized by the pharmaceutical industry. Investigations of crystal polymorphism and hydration are usually conducted early in drug development to optimize the physical properties of a pharmaceutical solid. Although the thermodynamically most stable crystal form is generally selected for commercial development to mitigate the risk of undesired phase transformations, form selection oftentimes involves a compromise among different physical properties of various drug crystal forms. Controlling polymorph (or hydrate) appearance must be accomplished through careful evaluation of both thermodynamic (tendency toward the formation of more stable crystal forms) and kinetic parameters (which lead to the formation of metastable forms) in the crystallization process. In this presentation, fundamental aspects of polymorphs and solvates (hydrates) will be explored. Particular attention will be given to the structure and stability relationships between polymorphs and hydrates, kinetic vs. thermodynamic transitions, and the impact of polymorphism and hydration on the chemical and physical stability of an active pharmaceutical ingredient. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:24AM |
U5.00004: Stability in Glassy Pharmaceuticals: The Role of Glass Dynamics Invited Speaker: Many pharmaceutical products, particularly freeze dried therapeutic proteins, are often produced in the glassy state. Stability or resistance to degradation is often a serious problem, and we find stability differences between formulations of similar composition often differ by an order of magnitude or more. We seek a better understanding of those factors that determine physical and chemical stability so that more stable products may be efficiently developed. Our hypothesis is that differences in dynamics in the glassy state are at least partially responsible for formulation specific stability behavior of materials stored well below their T$_{g}$'s. Various measures of dynamics or ``mobility'' are compared with stability data obtained by chemical assay (HPLC) of samples stored at various temperatures and for various times. While the different measures of mobility are often well correlated, there exist several examples where a trend in dynamics with some variable depends greatly on the measure of dynamics being used for analysis. Experimental stability data suggest stability and calorimetric relaxation dynamics, an indicator of mobility on a large length scale and long time scale, or ``global mobility'', are well coupled in many cases. We review data for physical stability in small molecule amorphous systems, chemical stability in cephalosporin antibiotics, dimer formation in small molecule systems, and both chemical decomposition and aggregation in proteins. We also find evidence that stability may be improved by annealing, presumably as a result of the decrease in free volume (as determined by high precision density measurements) and the corresponding decrease in ``global'' mobility as determined by TAM experiments. We conclude that glass dynamics is an important factor in determining stability, both chemical and physical, of small molecules and proteins in the amorphous solid state. However, the correlations are far from perfect, and it appears that due recognition of ``Fast Dynamics'' may be critical for our understanding, particularly for stability well below T$_{g}$. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 11:00AM |
U5.00005: Cohesion, Cracking, Dilation, and Flow -- Rheological Behavior of Cohesive Pharmaceutical Powders Invited Speaker: Cohesive powders can be loosely defined as systems where the attractive forced between particles exceed the average particle weight. Cohesive powder flow is interesting from a wide range of reasons. Their main characteristic, intermittence, is evidenced both in the interruption of flow out of hoppers (a mundane issue causing great annoyance to industrial practitioners) and in the sudden avalanching of snow and dirt that has terrified and terrified mankind since the dawn of time. At the present time, our ability to predict either of these phenomena (and many more involving cohesive powders) is very limited, primarily due to an incomplete understanding of their constitutive behavior. To wit, consider just a simple fact: \textbf{\textit{a flowing powder never has constant density}}. Equations describing the relationship between velocity, shear, stress, and density are rudimentary at best. Computational and experimental approaches for characterizing flow behavior are in their infancy. In this talk, I will describe some recent progress achieved at Rutgers by our group. New instruments have been developed to determine simultaneously powder density and cohesive flow effects. Extensive measurements have been carried out focusing on pharmaceutical blends. These results have been used to fine-tune computational models that accurately predict dilation, flow in drums, and flow in hoppers. Impact of these observations for pharmaceutical manufacturing applications will be discussed in some detail. [Preview Abstract] |
Session U6: Recent Progress in Computational Approaches for Rare Events
Sponsoring Units: DCOMP DCPChair: David Chandler, University of California, Berkeley
Room: Colorado Convention Center 207
Thursday, March 8, 2007 8:00AM - 8:36AM |
U6.00001: Transition Path Sampling and Rare Events in Complex Fluids Invited Speaker: The transition path sampling technique has matured to a universal tool for the study of rare events in the decade since its development. In this presentation I will give a brief overview of current progress. As an example I will discuss an application to micelle formation, fission and fusion in surfactant solutions. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U6.00002: Phase transformations and transition path sampling methods Invited Speaker: Under pressure, CdSe-nanocrystals transform from the four-coordinated wurtzite structure to the six-coordinated rocksalt structure. The transformation is a rare event that is strongly influenced by the size and the shape of the crystallite. Using this example, we will discuss how the transition path sampling methodology provides a framework for the study of the mechanism and the kinetics of phase transformations. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:48AM |
U6.00003: Computational techniques for the description of rare events Invited Speaker: |
Thursday, March 8, 2007 9:48AM - 10:24AM |
U6.00004: Theory and modeling of rare events Invited Speaker: |
Thursday, March 8, 2007 10:24AM - 11:00AM |
U6.00005: Metadynamics for the description of rare events Invited Speaker: |
Session U7: Teaching Nonequilibrium Statistical Mechanics for the Needs of 21st Century Physicists
Sponsoring Units: GSNP FEdChair: M. Cristina Marchetti, Syracuse University
Room: Colorado Convention Center Korbel 4A-4B
Thursday, March 8, 2007 8:00AM - 8:36AM |
U7.00001: Nonequilibrium Statistical Mechanics for Today's Graduate Students Invited Speaker: The notion of nonequilibrium statistical mechanics as a tool for analysis of many body systems is emphasized to support its value to graduate students in general. Its evolution across a wide range of fields within and beyond physics over the past half century is recalled. Much of the earlier focus on detailed and systematic applications to simple systems has been replaced by more qualitative analysis of complex systems. A case is made for the utility of a carefully composed course in nonequilibrium statistical mechanics as a means of thinking about problems in condensed matter physics, materials sciences, chemical physics, and particle physics. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U7.00002: Entropy, Order Parameters, and Complexity: Incorporating the last 50 years into the statistical mechanics curriculum Invited Speaker: The purview of statistical mechanics has grown rapidly in the past decades, with nonequilibrium extensions and applications to dynamical systems, molecular biology and bioinformatics, complex systems and networks, digital communication and information theory, and econophysics and other social sciences. It is our responsibility to join these new insights to the old wisdom in the field, and to distill the key ideas for the next generation. We should include (a) Shannon entropy, data compression, and reversible computation, (b) chaotic motion, ergodicity and the KAM theorem, and renormalization-group treatments of the onset of chaos, (c) molecular motors and hidden Markov models for analyzing genomic data. We should make statistical mechanics useful and comprehensible to those outside of physics, eschewing applications (Clausius-Clapeyron equations, $c_{p}$ vs. $c_{v})$ and methods (quantum mechanics) accessible and interesting only to condensed-matter physicists and physical chemists. See \textit{Entropy, Order Parameters, and Complexity} (http://www.physics.cornell.edu/sethna/StatMech/), OUP, 2006. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:48AM |
U7.00003: Teaching at the edge of knowledge: Non-equilibrium statistical physics Invited Speaker: As physicists become increasingly interested in biological problems, we frequently find ourselves confronted with complex open systems, involving many interacting constituents and characterized by non-vanishing fluxes of mass or energy. Faced with the task of predicting macroscopic behaviors from microscopic information for these non-equilibrium systems, the familiar Gibbs-Boltzmann framework fails. The development of a comprehensive theoretical characterization of non-equilibrium behavior is one of the key challenges of modern condensed matter physics. In its absence, several approaches have been developed, from master equations to thermostatted molecular dynamics, which provide key insights into the rich and often surprising phenomenology of systems far from equilibrium. \newline In my talk, I will address some of these methods, selecting those that are most relevant for a broad range of interdisciplinary problems from biology to traffic, finance, and sociology. The ``portability'' of these methods makes them valuable for graduate students from a variety of disciplines. To illustrate how different methods can complement each other when probing a problem from, e.g., the life sciences, I will discuss some recent attempts at modeling translation, i.e., the process by which the genetic information encoded on an mRNA is translated into the corresponding protein. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:24AM |
U7.00004: Non-equilibrium statistical mechanics in the context of biological physics Invited Speaker: Living systems are inherently out of equilibrium and operate in a fluctuating environment. The current challenges and interests in quantitative biology thus provide a great opportunity to introduce and develop methods from non-equilibrium statistical physics. For example, Master equations for evolution of probability find applications in mutating sequences, molecular motors, and signaling networks. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 11:00AM |
U7.00005: The Onsager Matrix Reloaded: Teaching Nonequilibrium Statistical Mechanics via Modern Applications Invited Speaker: Traditional texts and courses on nonequilibrium statistical mechanics (NESM) focus on fundamentals and outmoded examples that do not reflect the explosion of applications and developments from recent decades. With the burgeoning interest in multidisciplinary approaches to key problems in science and technology, in which physics and quantitative methods play a central role, NESM emerges as one of the unifying and pivotal techniques that physicists have to offer. I outline an approach to teaching NESM that emphasizes less formal theory and more modern applications, centered around developments in (e.g.) pattern formation, materials science, biology, fluid dynamics and atmospheric science. [Preview Abstract] |
Session U8: Focus Session: Superconductivity and Magnetism
Sponsoring Units: DMPChair: John Wei, University of Toronto
Room: Colorado Convention Center Korbel 1C
Thursday, March 8, 2007 8:00AM - 8:36AM |
U8.00001: Triplet superconductivity and the long-range proximity effect in superconductor/half-metallic ferromagnet systems Invited Speaker: |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U8.00002: Odd-frequency pairing in superconducting heterostructures . A.A. Golubov, Y. Tanaka, T. Yokoyama, Y. Asano We present a general theory of the proximity effect in junctions between unconventional superconductors and diffusive normal metals (DN) or ferromagnets (DF). We consider all possible symmetry classes in a superconductor allowed by the Pauli principle: even-frequency spin-singlet even-parity state, even-frequency spin-triplet odd-parity state, odd-frequency spin-triplet even-parity state and odd-frequency spin-singlet odd-parity state. For each of the above states, symmetry and spectral properties of the induced pair amplitude in the DN (DF) are determined. The cases of junctions with spin-singlet s- and d-wave superconductors and spin-triplet p-wave superconductors are adressed in detail. We discuss the interplay between the proximity effect and midgap Andreev bound states arising at interfaces in unconventional (d- or p-wave) junctions. The most striking property is the odd-frequency symmetry of the pairing amplitude induced in DN (DF) in contacts with p-wave superconductors. This leads to zero-energy singularity in the density of states and to anomalous screening of an external magnetic field. Peculiarities of Josephson effect in d- or p-wave junctions are discussed. Experiments are suggested to detect an order parameter symmetry using heterostructures with unconventional superconductors. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U8.00003: Phase Diagram and Thermodynamic Properties of Layered Ferromagnet-Superconductor Nanostructures Paul H. Barsic, Oriol T. Valls We present results obtained by exact, fully self-consistent numerical solution of the microscopic Bogoliubov-DeGennes equations for Superconductor/Ferromagnet (S/F) nanostructures in the clean limit. We discuss the thermodynamics of SFS trilayers, including the complete phase diagram over experimentally relevant parameter range. Both the first order $0$ to $\pi$ transition and the second order transition to the normal state at $T_c$ are included. We find excellent agreement between theoretical and experimental results for $T_c$. Other relevant quantities such as DOS, magnetization, and conductivity will also be discussed. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U8.00004: Transition Temperature Shifts in Asymmetric FSF Multilayers Paul Cadden-Zimansky, J. Samuel Jiang, John Pearson, Yaroslaw Bazaliy, Sam Bader Ferromagnet-Superconductor-Ferromagnet multilayers, where the superconductor thickness is on the order of the coherence length, exhibit shifts in the superconducting $T_c$ when the relative magnetization of the ferromagnetic layers is changed. However, experimental work has produced shifts orders of magnitude smaller than theoretically predicted. We investigate this discrepancy by examining multilayers where the thickness of the two ferromagnetic layers is varied. Our work indicates that differences between the two superconductor-ferromagnet interfaces may play a role in explaining the discrepancy, and demonstrates how the use of hard vs. soft ferromagnets can effect these devices. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U8.00005: Conductance spectra of ferromagnetic superconductors Jacob Linder, Martin Gronsleth, Asle Sudbo Recent findings of superconductors that simultaneously exhibit multiple spontaneously broken symmetries, such as ferromagnetic order or lack of an inversion center and even combinations of such broken symmetries, have led to much theoretical and experimental research. Ferromagnetic superconductors represent a marriage of two physical phenomena that conventionally have been considered virtually incompatible. We here study quantum transport in a junction consisting of a ferromagnetic metal and a non-unitary p-wave ferromagnetic superconductor. Considering several different possible pairing symmetries, our results show how the magnitude of the superconducting gaps may be inferred from the conductance spectra, in addition to their relative orientation in momentum-space. Also, we investigate how the strength of the magnetic exchange energies on both sides of the junction affect these spectra. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U8.00006: Derivation of the Ginzburg-Landau equations of a ferromagnetic $p$-wave superconductor Eskil Dahl, Asle Sudbo We derive a Ginzburg-Landau free energy for a $p$-wave ferromagnetic superconductor. The starting point is a microscopic Hamiltonian including a spin generalised BCS term and a Heisenberg exchange term. We find that coexistence of magnetisation and superconductivity depends on the sign of the energy-gradient of the DOS at Fermi level. We also compute the tunnelling contribution to the Ginzburg-Landau free energy, and find expressions for the spin-currents and Josephson currents across a tunnelling junction separating two ferromagnetic $p$-wave superconductors. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U8.00007: Nearly ferromagnetic superconductors Dietrich Belitz, Qi Li, Ted Kirkpatrick The coexistence of ferromagnetism and superconductivity has received substantial attention over the years [1]. Here we report on a theory for the electromagnetic properties of superconductors in the paramagnetic phase near a ferromagnetic instability [2]. Using a generalized Ginzburg-Landau theory, we have found that the magnetic flux expulsion capability of the superconductor gets {\it stronger} as the normal-state magnetic susceptibility increases. The temperature dependencies of the London penetration depth, the critical fields, and the critical current are all strongly affected by ferromagnetic fluctuations. For the critical current we find a temperature exponent $\alpha \approx 2$ over an appreciable temperature range. The extent to which proximity to magnetic criticality may be a viable explanation for recent observations in MgCNi microfibers, which find $\alpha \approx 2$ [3], is discussed. \medskip\par\noindent [1] E.I Blount and C.M. Varma, Phys. Rev. Lett. {\bf 42}, 1079 (1979); D.E. Moncton et al., Phys. Rev. Lett. {\bf 45}, 2060 (1980); J.W. Lynn et al., Phys. Rev. Lett. {\bf 46}, 368 (1981). \par\noindent [2] Qi Li et al., Phys. Rev. B {\bf 74}, 134505 (2006). \par\noindent [3] A.P. Young et al., Phys. Rev. B {\bf 70}, 064508 (2004). [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U8.00008: Anisotropic Pauli depairing effects and field-induced nodal excitations in superconductors without inversion symmetry Satoshi Fujimoto Superconductors without inversion symmetry has been currently attracting much interest. Here, we investigate theoretically a novel effect in the vortex state realized when the magnitude of the spin-orbit splitting due to the inversion symmetry breaking (ISB) is of the same order as the superconducting gap. Such a situation with the small ISB may be relevant to a superconductor without inversion symmetry Y$_2$C$_3$, which was recently discovered by Akimitsu et al. We show that in this case the Pauli depairing effect is anisotropic in the momentum space, and thus induces nodal excitations even for s-wave superconductors. We calculate the density of states and the specific heat coefficient on the basis of the Eilenberger's quasiclassical method, and compare the results with experiments for Y$_2$C$_3$. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U8.00009: Superconducting properties and the Fermi surface in noncentrosymmetric CeRhSi$_3$ T. Terashima, T. Yamaguchi, T. Matsumoto, S. Uji, N. Kimura, T. Komatsubara, H. Aoki, H. Harima CeRhSi$_3$ is a recently-discovered noncentrosymmetric superconductor [Kimura \textit{et al}., PRL \textbf{95}, 247004 (2005)]. At ambient pressure $P$, it orders antiferromagnetically below $T_N$ = 1.6~K. $T_N$ decreases with $P$ above $\sim$8 kbar, and disappears somewhere above 20 kbar. Superconductivity is observed above $\sim$12 kbar. We have performed measurements of ac susceptibility and the de Haas-van Alphen effect (dHvA) with the field in the $c$ direction up to $P$ = 29.5 kbar. Remarkably high upper critical fields $B_{c2}$ are observed: e.g., $B_{c2}$ = 17.5 T at 0.46 K for $P$ =29.5 kbar, where the superconducting transition temperature is only 1.1 K. The Fermi surface continuously evolves from $P$ = 0 to 29.5 kbar, and the effective masses decrease with $P$. We argue that these are consistent with theoretical scenarios ascribing antiferromagnetism to spin-density-wave formation. Analyses of dHvA oscillations in the mixed state seem to suggest an anisotropic superconducting energy gap. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U8.00010: Surface states in non-centrosymmetric superconductors Anton Vorontsov, Ilya Vekhter, Matthias Eschrig Since the discovery of superconductivity in CePt$_3$Si, there is a strong interest in superconducting materials without center of inversion. Lack of inversion symmetry results in strong spin-orbit interactions that lifts the spin degeneracy and leads to the mixing of `singlet' and `triplet' pairing channels. We consider surface bound states of a superconductor with bulk spin-orbit interactions described by a Rashba term, $\alpha (\hat{\bf z} \times {\bf k}) \cdot \mbox{\boldmath{$\sigma$}}$. We find that the scattering of the quasiparticle off the interface strongly mixes the two bands of opposite helicity. We analyze the properties of the surface states that appear for different surface orientations, investigate their spin structure and their possible signatures in tunneling experiments. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U8.00011: Magnetic penetration depth in noncentrosymmetric Re$_3$W Yuri Zuev, Valentina Kuznetsova, James Thompson, David Christen The magnetic penetration depth is one of the most fundamental characteristics of a superconductor. We report measurements of temperature dependence of the penetration depth $\lambda$ in Re$_3$W- a superconductor without inversion symmetry. The penetration depth was extracted from dc magnetic susceptibility, measured on aligned quenched powder in epoxy using a SQUID magnetometer. At present, based on the low-temperature behavior of the superfluid density 1/$\lambda^2$, we see no evidence of unconventional behavior, i.e we see a fully-gapped state. Higher resolution data at low temperatures are needed to decide the case. ORNL is managed by UT-Battelle, LLC for USDOE under contract DE-AC05-00OR22725 [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U8.00012: Helical and stripe vortex phases in non-centrosymmetric superconductors Zhichao Zheng, Daniel Agterberg, Raminder Kaur When magnetic fields are applied to non-centrosymmetric superconductors, helical or stripe vortex phases are formed. We develop a quasiclassical microscopic theory for these phases. We will study the resulting phase diagrams and physical properties of these phases as a function of the relative density of states of the two spin-split bands and with varying Zeeman-field strength. We apply these results to CePt3Si, CeRhSi3, CeIrSi3, KOs2O6, Li2Pt3B and Li2Pd3B. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U8.00013: Superconducting properties of noncentrosymmetric Mg$_{10}$Ir$_{19}$B$_{16}$ Tomasz Klimczuk, Robert J. Cava, Joe D. Thompson The magnetic, electrical transport and specific heat properties of the new ternary boride superconductor Mg$_{10}$Ir$_{19}$B$_{16}$ (T$_{C}$ = 5K) have been investigated. The polycrystalline Mg$_{10}$Ir$_{19}$B$_{16}$ sample was synthesized by reaction of Mg and Ir metal powders with amorphous boron powder. The material has a noncentrosymmetric crystal structure (I-43m) and therefore the superconducting properties are a subject of great interest. [Preview Abstract] |
Session U9: Superconductivity: Squid Instruments
Sponsoring Units: DMPChair: Mike Osofsky, Naval Research Laboratory
Room: Colorado Convention Center Korbel 1D
Thursday, March 8, 2007 8:00AM - 8:12AM |
U9.00001: Closing the Quantum Metrological Triangle M. Paalanen, A. Kemppinen, A. Manninen, A. Satrapinski, J. Hassel, P. Helisto, A.O. Niskanen, Heikki Seppa, P. Hakonen, Mikko Mottonen, Jukka Pekola, Juha Vartiainen Quantum Metrological Triangle is made out of Josephson voltage standard, Quantum Hall resistance standard and an accurate current pump. Closing the Triangle consists of applying Ohm's law with great accuracy on the three devices, based on fundamental physical phenomena and quantities, such as Planck's constant and electron charge. The first two devices are already accepted international metrological standards. Recently we have made progress in developing the missing components for the closing experiments, i.e. a differential low-noise amplifier for comparing small currents and a current pump. Both of these new devices are based on superconducting single electronics. We will describe our plans for closing the Triangle along with the expected uncertainties and also report on the progress in developing the low noise current amplifier and the current pump. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U9.00002: A low-field SQUID MRI system: design, performance and characterization. Byeong Ho Eom, Konstantin Penanen, Inseob Hahn A low-field MRI system with SQUID detector and gradiometer sense coil readout is being developed for clinical imaging. The system is compact and low-weight and operates in minimally shielded environment. An in-vivo image of human hand with sensitive field of view of 3cm, in-plane resolution of 1mm and 4mm slice width can be acquired in $\sim $5 minutes. We discuss the system design, optimization, noise characteristics~and imaging performance of the~apparatus.~ [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U9.00003: Optimization of sensing coil geometry for low-field SQUID MRI and MEG. Konstantin Penanen, Byeong Ho Eom, Inseob Hahn The performance of low-field MRI and MEG systems critically depends on the noise-limited sensitivity of the SQUID input subsystem. Optimization tradeoffs between the sensing coil field sensitivity, extraneous field rejection and self-inductance, as well as geometrical and fabrication constraints need to be considered. In addition to coil geometry optimizations, it is beneficial to optimize the shape of the wire loops the coil is wound with. We present the results of such optimization study in configurations typical for SQUID MEG and MRI sensing coils. A significant improvement over the circular cross-section wire can be achieved when flat washer wire is used.~ [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U9.00004: Noise Temperature of the Microstrip SQUID Amplifier with Cooling Fins Darin Kinion, John Clarke Microwave amplifiers, based on washer-type niobium dc SQUIDs with integrated input coils that act as a microstrip resonator operate at frequencies between 50 MHz and 2 GHz, and in principle are capable of reaching the Standard Quantum Limit (SQL) for linear amplifiers. In practice, heating of the shunt resistors may increase their Johnson noise thereby limiting the ultimate noise temperature before the SQL is reached. To reduce this noise contribution, we measure the noise temperature by cooling the devices to 25-500 mK in a dilution refrigerator, and by attaching large-area cooling fins to the shunts to minimize hot-electron effects. We have previously measured the noise temperature as a function of both frequency and physical temperature. At 800 MHz, the minimum noise temperature was 47 $\pm$ 4 mK (within 20\% of the SQL) at a physical temperature of 90 mK, with no further improvement upon cooling. This limiting behavior could be an indication of either the SQL or of hot- electron effects in the shunts. We will present results of separate flux-noise measurements at 140 kHz where similar limiting behavior below 100 mK should be caused only by hot- electron effects, thus resolving the ambiguity. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U9.00005: Cavity Josephson Bifurcation Amplifier: an improved microwave readout for a superconducting qubit Michael Metcalfe, Etienne Boaknin, Vladimir Manucharyan, rajamani Vijayaraghavan, Luigi Frunzio, Michel Devoret Dispersive readouts for superconducting qubits have the advantage of speed and minimal invasiveness. We have developed a new type of dispersive bifurcating amplifier, which consists of a Josephson junction imbedded in a microwave on-chip resonator. In contrast with the Josephson bifurcation amplifier [1,2,3], which has an on-chip capacitor shunting a junction, the resonator is based on a simple coplanar waveguide imposing a pre-determined frequency and whose other RF characteristics like the quality factor are easily controlled and optimized. Furthermore, readout frequencies ranging from 1 to 10GHz have been realized. Under proper microwave irradiation conditions, the resonator has two metastable states. Which state is adopted by the resonator depends on the state of a qubit coupled to the readout junction. We present the characterization of the Cavity Bifurcation Amplifier and demonstrate its application as a readout for the Quantronium qubit. \newline [1] I. Siddiqi et al. Phys. Rev. Lett 93, 207002 (2004) \newline [2] I. Siddiqi et al. Phys. Rev. Lett. 94, 027005 (2005) \newline [3] I. Siddiqi et al. Phys. Rev. B. 73, 0054510 (2006) [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U9.00006: Symmetry of Charge and Thermal Transport in Andreev Interferometers Jeff Weiss, Philippe Jacquod Motivated by recent experiments of Chandrasekhar et al. [Phys. Rev. Lett. 81, 437 (1998); Phys. Rev. B 72, 020502(R) (2005)], we investigate the symmetry of charge and thermal transport through magnetic-field threaded Andreev interferometers. We construct a scattering theory that accounts for multi-terminal geometries with no specific spatial symmetry. We use random matrix theory to calculate both the electrical and thermal four-terminal resistances and the thermopower, focusing on their magnetic flux dependence. Our results on the parity and amplitude of the magnetic field dependence is connected to the experimental results. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U9.00007: Few quasiparticle dynamics with a single-Cooper-pair transistor. Nadia Court, Andrew Ferguson, Robert Clark We employ the single-quasiparticle sensitivity of the single-Cooper-pair transistor (SCPT) to directly measure the quasiparticle population of a small superconducting volume. The SCPT is measured in a radio-frequency configuration allowing $>$ MHz bandwidth and is connected to a quasiparticle trap formed using aluminium bandstructure engineering. Quasiparticles in the trap region tunnel into the SCPT, this interrupts the coherent Cooper-pair transport and is detected by changes in the reflected radio-frequency signal. The quasiparticle population is monitored both in the steady state and under non-equilibrium conditions of quasiparticle injection. In particular, we study the non-equilibrium regime where the quasiparticle recombination time is accessed via the response of the SCPT to pulsed injection. We discuss the application of the SCPT as an ultra sensitive sub-mm photon detector and also as a tool to investigate the statistical mechanics of a few quasiparticle gas. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U9.00008: High Frequency Flux Sampling SQUID Microscope C.P. Vlahacos, J. Matthews, S.P. Kwon, F.C. Wellstood One important application of scanning SQUID microscopes is to fault detection in integrated circuits and multi-chip modules. However, the present generation of computer processors operate at over 1 GHz, well above the bandwidth of the present generation of SQUID microscopes. We have overcome the bandwidth limitations of traditional scanning SQUID microscopes by removing the main bandwidth limiter -- the conventional flux-locked loop electronics -- and using instead a pulsed sampling technique with a hysteretic dc SQUID. We present time-varying magnetic field images obtained with the 4.2 K cryocooled microscope with a time-resolution below 1 nanosecond, and discuss the advantages and limitations of this method. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U9.00009: Fast NDE of Superconducting Magnet Wires using a Flow-Through SQUID Microscope with Coaxial Current Injection J. Matthews, F.C. Wellstood, H. Weinstock We have developed a cryocooled high-Tc SQUID microscope for fast non-destructive evaluation (NDE) of long wires, designed for detecting defects in superconducting magnet wire. A feedthrough mechanism pulls the wire at speeds of up to 20 cm/s through a thin mylar tube that separates the room temperature wire from the SQUID. In order to null the magnetic field from bulk current flow the current return path is coaxial with the wire. We present results on test wires and samples of NbTi superconducting wire. By comparison with analytical and numerical models, we extract information from the data, such as defect size and location, and also outline a method for fast automated detection of defects in long wires. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U9.00010: A three-junction single electron transistor as a diffusive, high-speed thermometer: Experiment and simulation Loren Swenson, D.K. Wood, A.N. Cleland Nanoscale calorimetry promises unprecedented sensitivity and temporal resolution for energy measurements in mesoscopic systems. As the size scale and temperature of a calorimeter are reduced, the heat capacity and equilibration time decrease rapidly. Achieving the lowest temperatures at the smallest size scales therefore requires thermometric sensors that can be operated with large measurement bandwidths ($>$ 1 MHz), at low temperatures ($<$ 1 K), and that minimally perturb the calorimeter itself. We have fabricated a unique, three junction single-electron transistor that allows diffusive thermometric sensing of a nanoscale calorimeter, with minimal power dissipation in the calorimeter volume. In this talk, we will describe the experiment, and our development of a Monte Carlo method to simulate the experimental device. Design optimization, sensitivity and practical implementation considerations will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U9.00011: Ultra-Sensitive Hot-Electron Detectors of IR/sub-mm Radiation Wei Jian, David Olaya, Sergey Pereverzev, Michael Gershenson, Boris Karasik, Andrew Sergeev We have developed a superconducting nanostructure that enables an ultra-sensitive detection of far infrared (FIR) and sub-mm radiation. The nanostructure consists of a Ti nanobridge with a volume of $\sim $3$\times $10$^{-3}$~$\mu $m$^{3}$ flanked with Nb current leads. The electrons in the nanostructure are thermally isolated from the heat bath due to Andreev reflection from superconducting Nb leads and a weak electron-phonon coupling at mK temperatures. Being driven into the resistive state by the temperature and/or magnetic field, this transition-edge sensor is very sensitive to electron overheating. According to our measurements of the thermal conductance between the electrons in the nanobridge and the heat bath, the expected noise equivalent power and the response time of the detector at T=0.1~K are $\sim $10$^{-20}$~W/Hz$^{1/2}$ and $\sim $1 ms, respectively. Alternatively, this nanostructure with a heat capacity C $\sim $ 10$^{-19}$J/K at T=0.1\textsc{~K} can be used for the calorimetry of photons and phonons with an energy resolution $\sim $~10$^{-21}$~J. This resolution is sufficiently high for the detector to operate in a regime of THz photon counting. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U9.00012: Microwave Techniques for SQUID Multiplexing John Mates, Kent Irwin, Leila Vale, Gene Hilton, Konrad Lehnert, Manuel Castellanos-Beltran A single Superconducting Quantum Interference Device, or SQUID, is a particularly good low-noise, low impedance amplifier. However, many applications for which SQUID's are well-suited, ranging from astronomical spectroscopy to nuclear non-proliferation verification, require a large array of amplifiers. Large arrays require multiplexing techniques. Our group at NIST has fabricated several multiplexed SQUID arrays, including 1,280 pixel arrays, that use a time-division multiplexing technique. I will report on existing SQUID multiplexing techniques and the development of a SQUID multiplexer operating at microwave frequencies. This new technique uses non- hysteretic, non-dissipative rf-SQUID's to tune microwave resonators, so that, with high enough Q's, potentially tens of thousands of SQUID's could be read out on one coaxial line. I will also report on our initial experimental work, in which we have demonstrated Q's of around 100,000. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U9.00013: Superconducting Tunnel Junctions as Submillimeter Direct Detectors John Teufel, Minghao Shen, Luigi Frunzio, Daniel Prober, Robert Schoelkopf We are developing superconducting tunnel junctions (STJ) as direct detectors for submillimeter astronomy. Photons with energy greater than the superconducting gap of the aluminum absorber break Cooper pairs and generate excess quasiparticles that are then measured as a tunneling current through the STJ. In order to monitor the response of the detector with large readout bandwidth and maximal sensitivity, we have implemented a novel readout which monitors the impedance of the detector via radio frequency (RF) reflectometry. For calibration of the detector, we have also developed a gold microbridge as an on-chip, submillimeter photon source. When biased, high frequency noise from the microbridge couples via a transmission line to the detector. This allows for a calibrated photon source with near unity coupling, fast chopping, and calculable response. We present results in which we measured the ``optical'' properties of the STJ using the microbridge source and RF-STJ readout at 300 mK, demonstrating good responsivity, high sensitivity, and fast response times. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U9.00014: Single Crystal Superconducting Bolometric Photon Detector Kevin Inderhees, Paul Welander, James Eckstein We have studied bolometric photon detectors made from 10 nm thick single crystal niobium films grown by MBE. The films are atomically flat, have transition temperatures above 7.5 K and residual resistivity ratios $>$10, and can be patterned with uniform cross section, since they are not granular. The critical current density at 4.2 K is greater than 2x10$^{7}$ A/cm$^{2}$. The films are patterned into links between 300 and 500 nm wide. As a function of bias current, the detectivity is sharply peaked near the critical current, which appears to increase the sensitivity. The detected voltage signal has been studied at low frequency so far ($\sim $1MHz), and is linearly dependent on the photon flux. The fact that relatively wide links provide good detectivity is due to the uniform superconducting transport properties of the single crystal niobium. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U9.00015: Ultra-high resolution alpha particle spectroscopy using cryogenic microcalorimeters R. D. Horansky, J. N. Ullom, J. B. Beall, W. B. Doriese, W. D. Duncan, L. Ferreira, G. C. Hilton, K. D. Irwin, C. D. Reintsema, L. Vale, M. W. Rabin, A.S. Hoover, S. P. Lamont, C. R. Rudy We have built a novel alpha particle detector using a transition edge sensor (TES) thermometer attached to a bulk superconducting absorber and have measured $<$ 2.5~keV resolution at 5.3~MeV . We have used the microcalorimeter to measure a mixture of Pu isotopes and are able to individually resolve peaks corresponding to $^{239}$Pu and $^{240}$Pu which are separated by 12.9~keV. The $^{240}$Pu/$^{239}$Pu activity ratio is a key parameter for determining whether a plutonium sample is weapons or reactor grade. Conventional silicon based detectors achieve resolutions no better than 10~keV, severely limiting current actinide assays. In addition, the microcalorimeter alpha detector has shown significantly better resolution for gamma rays. The origin of the resolution difference for gamma and alpha interactions may be due to lattice damage and is currently under study. [Preview Abstract] |
Session U10: Charge Density Waves and 1D Systems
Sponsoring Units: DCMPChair: Weiqiang Yu, University of Maryland
Room: Colorado Convention Center Korbel 1E
Thursday, March 8, 2007 8:00AM - 8:12AM |
U10.00001: A Kadanoff-Wilson renormalization group analysis of half-filled one-dimensional quantum electron-phonon models Hassan Bakrim, Claude Bourbonnais We study the zero temperature phase diagrams of the half-filled one-dimensional Su- Schrieffer-Heeger (SSH) and molecular crystal (CM) models using the Kadanoff-Wilson renormalization group approach. At the one-loop level, the full frequency dependence of the phonon induced electron-electron coupling constants is taken into account in the vertex corrections and the quantum interference between the Cooper and Peierls diffusion channels. This enters as a key ingredient for the description of the quantum to classical transition for the Peierls instability. Our results confirm that finite phonon frequency introduces quantum fluctuations that depress the Peierls gap $\Delta$ compared to the classical - mean field - limit $\Delta_0$. It is found that in the spinless fermion case, the Peierls gap vanishes at the threshold $\omega_D\sim \pi \Delta_0$, whereas for fermions with spins, the gap remains in the quantum spin- charge separated regime. We extend our study to the XY spin-Peierls chain and confirm the DMRG result about the existence of a power law relation between the critical spin-phonon coupling $\alpha_c$ and frequency at the quantum-classical boundary, namely $\alpha_c\sim \omega_D^{0.7}$. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U10.00002: Small Bipolarons in the anisotropic 2-dimensional Holstein-Hubbard model. Jun Zhou, Jerome Dorignac, David Campbell We will investigate the bipolaron states in the anisotropic two-dimensional Holstein-Hubbard model. The interplay between attractive electron-phonon coupling g and the repulsive electron-electron interaction v will generate many different ground states. The anisotropic electron hopping in two dimensions also plays a role in affecting the bipolaron state. The bipolaron could be located on a single site or be two polarons separated by several sites or a quadrisinglet state which is the superposition of 4 electronic singlets with a common central site. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U10.00003: Fermi surface pockets and Luttinger sum rule in low-dimensional systems Christophe Berthod, Thierry Giamarchi, Silke Biermann, Antoine Georges We investigate the Mott transition in a quasi-one dimensional system of weakly coupled interacting fermionic chains. Within a generalization of dynamic mean field theory, we study by quantum Monte Carlo the evolution of the electron self-energy with increasing inter-chain coupling. Our approach is able to capture the closing of the Mott gap at some critical coupling, and is thus ideally suited to examine the characteristics of the Mott transition in this and similar systems. We find that the transition proceeds through an intermediate phase where the Fermi surface is broken into electron and hole pockets. Although these pockets can be very small, we show that the Luttinger sum rule is obeyed throughout the phase diagram. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U10.00004: Does nesting really cause Charge Density Waves? Michelle Johannes, Igor Mazin The concept of a CDW induced by Fermi-surface nesting originated from the Peierls idea of electronic instabilities in purely 1D metals and is now often applied to charge ordering in real, low-dimensional materials. The idea is that if Fermi surface contours coincide when shifted along the observed CDW wave vector, then the CDW is considered to be nesting-derived. We argue that only a tiny fraction, if any, of the observed charge ordering phase transitions are true analogues of the Peierls instability, {\it i.e.} that (a) there is substantial nesting of the FS, as quantified by a peak in Im $\chi_0({\bf q})$ at the CDW wave vector; (b) this peak translates (as in the 1D case) into a peak in Re$ \chi_0({\bf q})$ at the same wave vector; (c) a divergence in the full electronic susceptibility causes the electronic subsystem to be unstable {\it without} ion shifts; and (d) {\it all} phonons are softened at the CDW vector. Using prototypical CDW materials NbSe$_2$, NbSe$_3$, and CeTe$_3$, we show that these conditions are hardly ever fulfilled, and that the CDW phases are actually structural phase transitions, driven by ionic rather than electronic instabilities. We also show mathematically that the original Peierls construction is so fragile as to be unlikely to apply to any real material. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U10.00005: The pseudogap phase in (TaSe$_4$)$_2$I Andras Vanyolos, Balazs Dora, Attila Virosztek We have developed the mean-field theory of coexisting charge-density wave (CDW) and unconventional charge-density wave (UCDW). The double phase transition manifests itself in the thermodynamic quantities and in the magnetic response, such as spin susceptibility and nuclear spin-lattice relaxation rate. Our theory qualitatively applies to the quasi-one dimensional CDW material (TaSe$_4$)$_2$I. This material exhibits peculiar properties: above the CDW transition temperature $T_c$, thermal fluctuations were found to die out rapidly, but robust pseudogap behavior is still detected. Namely, the experimental findings include: (i) sharp increase of the static spin susceptibility above $T_c$, (ii) smooth increase of the spin-lattice relaxation rate above $T_c$, (iii) as opposed to conventional CDW, no sharp feature in the spin-lattice relaxation rate below $T_c$. We have found that our coexisting CDW+UCDW model qualitatively describes these observed properties of (TaSe$_4$)$_2$I. Direct calculations for the magnetic response are shown to evidence the agreement. We also argue, that the fluctuations around $T_c$ are suppressed due to the presence of the ``hidden'' UCDW phase, which partially gaps the Fermi surface, and causes non-Fermi-liquid (pseudogap) behavior. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U10.00006: Charge density wave formation and physical properties of $R_{2}$Te$_{5}$ ($R$=Nd, Gd) compounds K.Y. Shin, N. Ru., M.F. Toney, I.R. Fisher $R_{2}$Te$_{5}$ ($R$=Nd, Gd) has a layered tetragonal structure based on alternating single and double square planar Te sheets separated by corrugated $R$Te layers, and has a quasi-two dimensional electronic structure. The material shares important physical properties with other two single and double Te layer variants, $R$Te$_{2}$ and $R$Te$_{3}$ (R=La~Yb), including a charge density wave (CDW) instability. Using a binary self-flux method, we have grown high-quality single crystals of Gd$_{2}$Te$_{5}$ and Nd$_{2}$Te$_{5}$ compounds and have characterized their structural, thermodynamic and transport properties. Our measurements reveal, for the first time, the charge density wave in this material. We will discuss the properties and origin of the CDW, and the relation to the better known $R$Te$_{2}$ and $R$Te$_{3}$ compounds. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U10.00007: Effect of Chemical Pressure on the Charge Density Wave Transition in Rare-earth Tritellurides $R$Te$_3$ N. Ru, G. Y. Margulis, K. Y. Shin, M. F. Toney, I. R. Fisher The charge density wave transition is investigated in the bi-layer family of rare earth tritelluride $R$Te$_3$ compounds ($R$ = Sm, Gd, Tb, Dy, Ho, Er, Tm) via high resolution x-ray diffraction and electrical resistivity. The transition temperature $T_{CDW}$ increases monotonically with increasing lattice parameter by an extraordinarily large amount, from 244(3) K for TmTe$_3$ to 416(3) K for SmTe$_3$. It is suggested that this behavior, and the observation of a secondary transition for the heaviest members of the series, is intimately linked to the effect of chemical pressure on the degree of bilayer splitting of the Fermi surface. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U10.00008: High-resolution STM imaging and spectroscopy of Cu$_{x}$TiSe$_{2}$ Dale Kitchen, Kenjiro K. Gomes, Abhay Pasupathy, Aakash Pushp, Pedram Roushan, Emilia Morosan, Robert J. Cava, Ali Yazdani The discovery of superconductivity in Cu-doped TiSe$_{2}$ has created a new opportunity to study the competition between charge density wave (CDW) formation and superconductivity in layered chalcogenides [1]. Using a cryogenic scanning tunneling microscope (STM), we have obtained atomic resolution images of \textit{in situ} cleaved Cu$_{x}$TiSe$_{2}$ and perform spatially resolved mapping of the electronic states of this compound. The STM images measured on samples at low Cu doping, reveal the atomic lattice, the CDW organization, and show local signatures consistent with individual Cu-dopants. Imaging and spectroscopy are used to identify how the Cu-dopants alter the local electronic structure of this material and destroy the CDW organization, which eventually gives way to the rise of superconductivity. [1] E. Morosan \textit{et al}., \textit{Nature Physics} \textbf{2}, 544 (2006). [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U10.00009: STM Studies of the CDW System TbTe$_3$ Alan Fang, Zhanybek Alpichshev, Nancy Ru, Ian Fisher, Aharon Kapitulnik We present STM data on the Charge Density Wave (CDW) in the Rare Earth Tri-Telluride TbTe$_3$. Topography scans as large as 250$\times$250 $\AA^2$ were taken with voltage bias as high as 0.8 Volt. Fourier analysis shows an incommensurate unidirectional modulation with wave-vector q $\approx$ 0.71 a*. The topographic scans at different bias voltages are used to highlight the difference in structure of the CDW and lattice period-doubling effects, either from the Te-Te dimerization, or from the Te-Tb layer directly below the surface. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U10.00010: APRES study of a complex charge density wave material Gd$_{2}$Te$_{5}$ Ruihua He, Kyungyun Shin, Hong Yao, Jude Laverock, Stephen Dugdale, Nancy Ru, Donghui Lu, Worawat Meevasana, Steve Kivelson, Ian Fisher, Zhi-Xun Shen By using angle-resolved photoemission spectroscopy based on synchrotron radiation as well as monochromatic He-I UV, we have investigated for the first time a complex charge density wave (CDW) material, Gd$_{2}$Te$_{5}$, of the rare earth (R) telluride family R$_{2}$Te$_{5}$, with a hybrid crystalline structure of its two well-studied relatives, RTe$_{2}$ and RTe$_{3}$. Based on a tight-binding model, combining with the LDA calculation, we analyze the experimental Fermi surface, energy band dispersions and their temperature dependence in detail, which provides valuable insights into its complex CDW phase revealed recently by the TEM and XRD measurements. The nature of different CDW wave vectors involved and the roles of different interlayer split bands in the CDW formation are discussed. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U10.00011: Electronic Structure and CDW Physics in LaTe$_{2}$ using ARPES Daniel Garcia, Gey-Hong Gweon, Shuyun Zhou, Jeff Graf, Chris Jozwiak, Myung-Hwa Jung, Yong Seung Kwon, Alessandra Lanzara We report a direct study of the band structure and charge density wave (CDW) formation in LaTe$_{2}$, by using high-resolution angle-resolved photoemission spectroscopy (ARPES). The nature of the CDW formation, the momentum dependence of the CDW gap and the role of dimensionality in the tellurides will be presented. Finally the transition from a stripe to a checkerboard phase and its layer dependence will be addressed. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U10.00012: Doping (x)- and pressure-dependence of the CDW state in Cu$_{x}$TiSe$_{2}$ using inelastic light scattering Harini Barath, Minjung Kim, S.L. Cooper, Emilia Morosan, R.J. Cava TiSe$_{2}$ has long been known to form a rather simple commensurate charge-density-wave (CDW) below T$_{CDW }\sim $ 200K. Interest in this material has grown recently, however, with the discovery that Cu intercalation between the TiSe2 layers [1] suppresses the CDW transition, and at intermediate compositions (x $\ge $ 0.04 in Cu$_{x}$TiSe$_{2})$, gives rise to a superconducting state. Consequently, Cu$_{x}$TiSe$_{2}$ is a particularly interesting system in which to investigate the competition between CDW and superconducting correlations. In this talk, we discuss our inelastic light scattering studies of the effects of chemical tuning on the CDW state in Cu$_{x}$TiSe$_{2}$, which we compare to pressure-dependent studies of the CDW state in TiSe$_{2}$. By monitoring both the CDW amplitude modes and phonons with chemical substitution and pressure, we are able to sensitively study the different routes to CDW melting in this interesting system. [1]. E. Morosan et al., \textit{Nature Physics }\textbf{2, }544 (2006). [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U10.00013: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U10.00014: CDW dynamics in NbSe$_3$ probed by $^{93}$Nb NMR S. Suh, P. Monceau, W. G. Clark, R. E. Thorne, S. E. Brown Using $^{93}$Nb NMR spin echoes applied on a single crystal of NbSe$_3$, we have probed electric-field induced displacements of the CDW forming below T$_2$=144K. In our experiments, good S/N was achieved by aligning the chain axis of a single large crystal (cross-section O(500$\mu m^2$) with the coil symmetry axis. Evidence for CDW motion throughout the sample for $E>E_T$ was observed in motional narrowing experiments. For $E\le 0.9E_T$, we find a wide distribution of displacements less than one CDW wavelength, for both unipolar and bipolar electric field pulse excitations. At $E\sim 0.9E_T$, the mean displacement is approximately 6-7 degrees, and the width of the distribution is about twice the mean displacement. We discuss the results in the context of the Fukuyama-Lee phase Hamiltonian, and describe the constraints imposed by these experiments on the proposal that CDW depinning is an example of a dynamic critical phenomenon. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U10.00015: Transmission Electron Microscopy of Charge Density Wave Transitions of rare-earth transition-metal silicide R$_{5}$Ir$_{4}$Si$_{10}$ (R=Dy, Ho). C.H. Chen, C.M. Tseng, H.D. Yang The metallic ternary rare-earth transition-metal silicides of R$_{5}$Ir$_{4}$Si$_{10}$ where R=(Dy, Ho) exhibit charge density wave (CDW) transitions despite its seemingly three-dimensional crystal structure. In this talk we present the observation of the CDW phase transitions in this class of materials by electron diffraction and electron microscopy. These compounds exhibit incommensurate to commensurate phase transitions as temperature decreases. The modulation wave vector is found to be along the c-axis of the crystal with a modulation periodicity approximately four times of the unit cell. Real space imaging of CDW domains and/or domain walls using the dark-field technique in transmission electron microscopy will also be presented. [Preview Abstract] |
Session U12: Focus Session: Diluted Magnetic Semiconductors II
Sponsoring Units: GMAG DMP FIAPChair: George Kioseoglou, Naval Research Laboratory
Room: Colorado Convention Center Korbel 3C
Thursday, March 8, 2007 8:00AM - 8:36AM |
U12.00001: High Curie temperature ferromagnetism in self-organized GeMn nanocolumns. Invited Speaker: We have recently evidenced high Curie temperature (T$_{C}>$400 K) Mn-rich nano-columns self-assembled in a diluted Ge$_{0.94}$Mn$_{0.06}$ film. Their composition is close to Ge$_{2}$Mn as given by nanoscale chemical analysis (Electron Energy Loss Spectroscopy). Their average diameter, height and spacing are 3 nm, 80 nm and 10 nm respectively. Their volume fraction in the GeMn film is almost 16 {\%}. From Transmission Electron Microscopy (TEM) cross sections we could clearly evidence that nano-columns are crossing the whole GeMn film. SQUID measurements reveal a very high Curie temperature ($>$ 400 K) and Zero Field Cooled-Field Cooled (ZFC-FC) data rule out the presence of superparamagnetic nanoparticles. Growth is performed by simultaneously evaporating Ge and Mn atoms from standard effusion cells on Ge(001) single crystal substrates. The growth temperature is varied from 80 to 200\r{ }C. In the whole temperature range, Mn-rich nano-columns are clearly observed by TEM. However magnetic properties depend on the growth temperature and high T$_{C }$columns are only obtain in a very narrow temperature range around 130\r{ }C. Magnetotransport measurements have been performed with magnetic fields applied perpendicular to the film plane. A large positive magnetoresistance (up to 7000 {\%} at 30 K) in contradiction with the negative MR in granular systems or other ferromagnetic semiconductors is measured. Taking MR effects into account we have evidenced a large Anomalous Hall Effect (AHE) up to room temperature despite of the low volume fraction of nano-columns in GeMn films. The presence of AHE proves that holes are spin-polarized by crossing nano-columns. In this presentation, we will discuss the kinetic mechanisms leading to the columns formation, the composition and crystal structure of the columns as well as their magnetic properties as a function of the growth temperature and Mn concentration. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U12.00002: Dopant segregation and giant magnetoresistance in manganese-doped germanium A.P. Li, C. Zeng, K. van Benthem, M.F. Chisholm, J. Shen, S.V.S. Nageswara Rao, S.K. Dixit, L.C. Feldman, A.G. Petukhov, M. Foygel, H.H. Weitering Dopant segregation in Mn$_{x}$Ge$_{1-x}$ dilute magnetic semiconductor leads to a remarkable self-assembly of Mn-rich nanocolumns, embedded in a fully compensated Ge matrix. Samples grown at 80 \r{ }C display a giant positive magnetoresistance that correlates directly with the distribution of magnetic impurities. Annealing at 200 \r{ }C increases Mn substitution in the host matrix above the threshold for the insulator-metal transition, while maintaining the columnar morphology, and results in global ferromagnetism with conventional negative magnetoresistance. The qualitative features of magnetism and transport in this nanophase material are thus extremely sensitive to the precise location and distribution of the magnetic dopants. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U12.00003: Ferromagnetism in Mn doped Ge thin films Jiani Yu, Jiwei Lu, Kevin West, Li He, Robert Hull, Stu Wolf Exploring ferromagnetism in Group IV semiconductors is of great interest due to their potential application to spintronics. In this presentation, we discuss the ferromagnetism induced in thin Ge films by Mn$^{+}$ ion implantation as well as the correlation between their magnetism and their transport properties. The as-received Germanium on insulator (GOI) wafer consists of 200nm of (100) oriented Ge on 400nm of oxide both on a Si wafer. Mn ions were implanted at 300 $^{o}$C into the Ge layer at 200 KeV. The ferromagnetism has been observed in Ge with a range of Mn concentration from 0.5 to 2 atom {\%}. The sample with 2 {\%} Mn doping has a Curie temperature near 300K and has a moment of $\sim $ 0.7 $\mu _{B}$/Mn at 10 K. Transmission electron microscopy (TEM) reveals the formation of second phase clusters of which are probably responsible for the majority of the magnetism in this sample. In contrast, the 0.5 {\%} and 1 {\%} Mn as implanted Ge thin films behave like diluted ferromagnetic semiconductors, both have Curie temperatures is around 100 $\sim $150 K and the 0.5{\%} sample doesn't show TEM evidence of a second phase. Our data indicates that the transport properties of Mn doped Ge correlates with the magnetism. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U12.00004: Effects of complementary doping of transition metals into Ge epitaxial films Brian Collins, Liang He, Frank Tsui, Yuncheng Zhong, Stefan Vogt, Yong Chu We report structural and magnetic properties of epitaxial films of Co and Mn co-doped Ge, grown by combinatorial molecular beam epitaxy on Ge (001) substrates. A ternary epitaxial phase diagram has been determined for total doping concentrations up to 30 at. \%, where regions of coherent epitaxy and associated strain states, and regions of rough disordered growth and the nature of the disorders have been examined and identified. In the phase diagram, there are two adjacent regions in composition, one Co-rich and another Mn-rich, where pseudomorphic epitaxial growth can be achieved at combined doping concentrations as high as 17 at. \%. These values are significantly higher than those from using either of the dopants individually. The lattice constants of the Co-rich films obey the Vegard's law, i.e. a linear dependence on the concentration, while the Mn-rich counterparts do not. This finding indicates that two transition metal dopants can compensate for the internal stress caused by the individual dopants in the host lattice. Our results also show that the presence of a second dopant can significantly reduce the tendency for phase separation and disorder, especially when Mn is the primary dopant. A ternary magnetic phase diagram has been determined using the magnetooptic Kerr effect, within which there exit high quality epitaxial films of magnetic semiconductors. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U12.00005: Dopant-assisted Concentration Enhancement of Substitutional Mn in Si and Ge Wenguang Zhu, Zhenyu Zhang, Efthimios Kaxiras Incorporation of Mn atoms as magnetic impurities in bulk Si and Ge is of great importance for integrating magnetism with existing device technology. Here, we study the influence of p- and n-type electronic dopants on Mn incorporation in bulk Si and Ge, using first-principles calculations within density functional theory. We find that in bulk Si, the site preference of a single Mn atom is changed from interstitial to substitutional in the presence of a neighboring n-type dopant (P, As, Sb). In bulk Ge, a Mn atom is more easily incorporated into the lattice when an n-type dopant is present in its immediate neighborhood, forming a stable Mn/dopant dimer with both impurities at substitutional sites. A detailed analysis of magnetic exchange interactions between such dimers reveals that magnetic properties are not degraded when Mn atoms coexist with n-type dopants. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U12.00006: Interstitial Mn in Si: half-metallic heterostructures studied by density-functional theory Peter Kratzer, Hua Wu, Matthias Scheffler Adding magnetic functionality to the most common semiconductor, Si, is in its infancy. So far, research on Mn-doped Si has concentrated on substitutional Mn (Mn$_{sub})$ as done for Mn-doped GaAs and Ge, although Mn$_{sub}$ impurities in Si are energetically less stable than interstitial Mn (Mn$_{int})$. In this work, we investigate the role of Mn$_{int}$ impurities for ferromagnetism in Si, and propose a novel type of heterostructures with Mn$_{int} \quad \delta $-doping. Using density-functional theory within the generalized gradient approximation, we show that Si-based heterostructures with 1/4 layer $\delta $-doping of Mn$_{int}$ are half-metallic. For Mn$_{int}$ concentrations of 1/2 or 1 layer, the $\delta $-doped heterostructures still display a high spin-polarization of conduction electrons, about 85{\%} and 60{\%}, respectively. The proposed heterostructures are more stable than previously assumed $\delta $-layers of Mn$_{sub}$. Contrary to wide-spread belief, the present study demonstrates that interstitial Mn can be utilized to tune the magnetic properties of Si, and thus provides a new clue for Si-based spintronics materials. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U12.00007: Ge-Based Diluted Magnetic Semiconductor films on Si Zuoming Zhao, Xiaoyu Zhou, Kang L. Wang Ge-based diluted magnetic semiconductor (DMS) films with 4{\%} manganese (Mn) are grown on Si (001) substrates using molecular beam epitaxy (MBE). Surface morphology is measured by atomic force microscopy (AFM). For a 24-nm thick film, surface roughness around is around 1nm. Structure properties of the film are characterized by X-ray diffraction (XRD) and single crystal film quality by the results which only (004) Ge peak is observed. Magnetic properties are measured by a superconducting quantum interference device (SQUID). Clear hysteresis is observed at room-temperature. The results indicate that high quality Ge-based DMS can be grown on Si with good crystal quality and magnetic properties. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U12.00008: Local moment and interaction of manganese moment in amorphous silicon thin films Li Zeng, Frances Hellman, Robert Culbertson, Rafiqul Islam, David Smith, Erik Helgren Highly homogeneous Mn doped amorphous silicon ($a-$Mn$_{x}$Si, x = 1$\sim $18 at.{\%}) samples are prepared by e-beam co-evaporation to explore the local Mn moment in $a-$Mn$_{x}$Si. The relationship between the microstructure and the magnetic properties has been investigated. HR-XTEM shows that Mn atoms are uniformly distributed inside the amorphous silicon matrix, greatly exceeding the Mn solubility in crystalline silicon. Rutherford backscattering (RBS) is used to verify the film composition and the oxygen impurity. The effective moment of Mn has a nontrivial Mn composition dependence. The smaller the Mn concentration, the larger the Mn effective moment is found. The saturation moment of the sample is small, indicating a small Mn local moment. Possible explanations of the small effective moment and its concentration dependence are discussed, including direct antiferromagnetic interaction, s-d interaction. Magnetotransport is also studied for this system and magnetoresistance is found very small comparing to other magnetically doped amorphous semiconductor, such as $a-$Gd$_{x}$Si$_{1-x}$. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U12.00009: Spin waves in Mn-doped Si: exchange interactions from first-principles calculations G. Rao and J. E. Raynolds College of Nanoscale Science and Engineering, University at Albany, State University of New York Gayathri Rao, James Raynolds There has been considerable interest in magnetic semiconductors in recent years for potential applications in the field of spintronics.The present work was motivated by recent experimental achievement of above-room- temperature magnetism in Mn doped silicon (Phys.Rev.B \textbf{71}, 033302 (2005)).We present the results of Density Functional calculations that have been carried out to determine the strength of the distance-dependant exchange interaction in Mn-doped Si.The exchange interaction determines the energy difference between ferro-magnetic and anti-ferromagnetic spin configurations and as such it provides for a prediction of spin-wave velocities.Such spin waves, if they exist, are of interest in that they may provide means for transmitting spin-based information. Comparison of the relative energy differences between ferromagnetic and anti-ferromagnetic configurations for a series of Mn locations yielded the distance dependant exchange interaction J(R).Interestingly we find that the exchange interaction is negative (anti-ferromagnetic) for short and long distances and is positive (ferromagnetic) for intermediate distances.This talk will present these findings along with estimates of spin-wave velocities,densities of states,band structure and spin-density distributions. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U12.00010: Controlling magnetic, magnetotransport and optical properties of Al codoped Zn-Co-O thin films Plamen Stamenov, M. Venkatesan, L. Dorneles, J.M.D. Coey Thin films of 5{\%} Co doped ZnO were grown on C- and R-cut sapphire substrates by pulsed-laser deposition, with and without Al codoping ($x$=0--1{\%} Al). Al-doped films retain significant magnetization while exhibiting degenerate semiconductor behaviour. Magnetoresistance of these novel Co-doped ZnO semiconductor films is found to be highly dependent on Al doping and is vanishingly small at ($x>$0.2 {\%}) Large ($\sim $20 {\%} at 2 K) in-plane anisotropy of the magnetoresistance is observed, resembling an AMR effect, which is attributed to Fermi surface anisotropy and most of it has no ``ferromagnetic'' origin. The field dependence of the magnetoresistance can be explained in terms of two-band model and ionised impurity scattering. Hall-effect data indicates completely degenerate electron gas at ($x \quad >$ 0.5 {\%}). High resolution x-ray scattering and magnetisation data on samples with ($x$=0) reviles the presence of Co metal clusters ($\sim $8nm in size) that account for much or all the ferromagnetic magnetisation and exhibit temperature activated decrease of the coercive field. A huge band gap shift is observed with Al doping as a result of \textit{Burstein-Moss} effect. In view of the vanishing magnetoresistive effects at room temperature, it is clear that these Co doped ZnO samples are not dilute magnetic semiconductors. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U12.00011: Electrical transport and magnetic properties of sputtered Co-doped indium-tin oxide films Jolanta Stankiewicz, Francisco Villuendas We report results of electrical resistivity, Hall effect and magnetization measurements in Co-doped indium-tin oxide films, in a temperature range from 5 to 400 K and in magnetic fields of up to 5 T. The films were grown on fused quartz substrates, by magnetron sputtering. ITO (In$_2$O$_3$ with 10 wt \% Sn) homogeneous films doped with less than 20 at.\% of Co seem to show intrinsic FM behavior. Magnetic hysteresis loops with coercive fields of up to 100 Oe at room temperature, as well as a ferromagnetic contribution in the difference between field-cooled and zero field-cooled magnetization, are observed in these films. We find that post-growth treatment strongly affects the electrical and magnetic properties of our films. This allows us to control the electron concentration of the films by varying the temperature and/or changing the ambient gas in the annealing process. A clear correlation between the values of the magnetic moment and of the electron concentration found for the ITO films doped with 10 at.\% of Co seems to follow the predictions for a bound magnetic polaron percolation model. This suggests a carrier-mediated ferromagnetic mechanism. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U12.00012: Investigation of Mn incorporation on GaN(0001) by spin-polarized STM Yun Qi, M. Weinert, L. Li We investigate the Mn substitution of Ga on GaN(0001) by spin-polarized scanning tunneling microscopy (SP-STM) using a Fe coated W tip. The GaN films are grown by plasma-assisted MBE on 6H-SiC(0001), with a metallic pseudo-1x1 (denoted ``1x1'') surface, consists of 2.3 ML Ga on top of the Ga-terminated GaN. Mn deposition on this surface results in the formation of domains of 5x5 and $5\sqrt 3 \times 5\sqrt 3 $ structures. First principles calculations show that Mn substitution of Ga leads to virtual bound states with bandwidth of $\sim $1.5 eV, indicating significant Mn-Ga interactions. We propose that Mn substitution of Ga freezes the Ga motion in the adlayer of the ``1x1'', forcing the extra Ga atoms of the top layer to ``pop up'' and reside at the T$_{4}$ sites, forming the (5x5) reconstruction similar to the DAS structure observed on Si(111) surface. With the Fe/W tip, the regions that contain the incorporated Mn would give rise to the extra bright features that form the $5\sqrt 3 \times 5\sqrt 3 $ structure, due to the higher spin DOS at Mn sites. Implications of these results for understanding the magnetic coupling between Mn atoms in GaN will be discussed at the meeting. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U12.00013: Electrical manipulation of non-volatile spin cell based on diluted magnetic semiconductor quantum dots Ki Wook Kim Electrical manipulation of a memory cell based on a semiconductor nanostructure consisting of a diluted magnetic semiconductor quantum dot (QD) and a reservoir of itinerant holes separated by an energy barrier is investigated theoretically. The operating principle takes advantage of the paramagnetic-ferromagnetic phase transition mediated by the itinerant holes in the diluted magnetic semiconductor QD that can lead to electrically controlled Write/Erase operations. Non-volatility can be achieved when the structure is properly designed to reach a thermodynamic equilibrium at both the PM and FM configurations (i.e., bistability). Assuming a parabolic confining potential in the QD, the performance characteristics of the proposed nanostructure are analyzed including the scalability and the lifetime. An advantage of this memory concept is the extremely small dissipative energy for Write/Erase functions due to the open circuit nature of the process. A readout scheme enabling electrical detection with the repetition rate up to the 10 - 100 MHz range is also explored by utilizing only two contacts. [Preview Abstract] |
Session U13: Focus Session: Maganites, Titanates, & Vanadates
Sponsoring Units: DMP GMAGChair: Yoichi Horibe, Rutgers University
Room: Colorado Convention Center Korbel 4C
Thursday, March 8, 2007 8:00AM - 8:36AM |
U13.00001: Combining DFT and Many-body Methods to Understand Correlated Materials Invited Speaker: Electronic and magnetic properties of strongly-correlated systems are typically controlled by a limited number of electronic states, located near the Fermi level and well isolated from the rest of the spectrum. This opens a formal way for combining the first-principles methods of electronic structure calculations, based on the density-functional theory (DFT), with model many- body methods, formulated in a restricted Hilbert space of states near the Fermi level. The core of this project is the construction of ``\textit{ab initio} model Hamiltonians'', which would incorporate the physics of on-site Coulomb correlations and provide a transparent physical picture for the low-energy properties of strongly- correlated systems. First, I will describe a systematic procedure for constructing such an effective Hubbard-type model, which consists of three major steps, starting from the electronic structure in the local- density approximation.$^1$ (i) Construction of the kinetic-energy part using an exact version of the downfolding method;$^{1,2}$ (ii) Construction of the Wannier functions; (iii) Calculation of screened Coulomb interactions using a hybrid approach, which combines the random phase approximation with the constraint DFT.$^{1,3}$ Then, I will illustrate abilities of this method for resolving a number of controversial issues, related with the interplay of the experimental lattice distortion and magnetic properties of four narrow $t_{2g}$ band perovskite oxides (YTiO$_3$, LaTiO$_3$, YVO$_3$, and LaVO$_3$), for which the obtained Hamiltonian was solved using a number of techniques, including the Hartree-Fock (HF) approximation,$^4$ the second-order perturbation theory and the $t$-matrix approach for the correlation energy,$^ {4,5}$ and a variational superexchange theory, which takes into account the multiplet structure of the atomic states.$^4$ I will argue that the crystal distortion imposes a severe constraint on the form of the possible orbital states, which favors the formation of experimental magnetic structures in YTiO$_3$, YVO$_3$, and LaVO$_3$, even at the level of HF approximation. The correlation effects systematically improve the agreement with the experimental data and additionally stabilize the experimentally observed G- and C-type antiferromagnetic states in YVO$_3$ and LaVO$_3$. The role of relativistic spin-orbit interaction will be also discussed. \newline $^1$ I.~V.~Solovyev, Phys.~Rev.~B~\textbf{73}, 155117 (2006). \newline $^2$ I.~V.~Solovyev, Z.~V.~Pchelkina, and V.~I.~Anisimov, cond- mat/0608528. \newline $^3$ I.~V.~Solovyev and M.~Imada, Phys.~Rev.~B~\textbf{71}, 045103 (2005). \newline $^4$ I.~V.~Solovyev, Phys.~Rev.~B~\textbf{74}, 054412 (2006). \newline $^5$ I.~V.~Solovyev, cond-mat/0608625. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U13.00002: Additional evidence for complex 2-site polarons in CMR manganites. Frank Bridges, Geza Kurczveil, Lisa Downward, John J. Neumeier Recently we have proposed a complex 2-site polaron model (which we call a dimeron) that exists for temperatures near and above the ferromagnetic transition temperature, T$_c$ [1]. The dimeron has a hole delocalized over two Mn sites (i.e. a hole and an electron share the two Mn sites) and the two Mn sites have a reduced distortion compared to the remaining Jahn-Teller distorted electron sites. Magnetic clusters just above T$_c$ are likely clusters of these dimeron quasiparticles. The average valance of the two Mn sites in the dimeron is 3.5 and the spin is 7/2. We show that the Mn K-absorption edge is much better described as a sum of a 3.5 valence edge (fraction 2x) plus a 3 valance edge (fraction 1-2x), compared to earlier simulations using x CaMnO$_3$ plus 1-x LaMnO$_3$. We also show that fitting the Mn-O peak to a sum of two experimental Mn-O standards leads to a similar result as in the earlier study - a fraction 2x of lower distorted Mn sites (dimerons) and a fraction 1-2x of more distorted sites with 1 e$_g$ electron. Both support the proposed complex - 2-site polaron model.\newline Supported under NSF grant DMR0301971.\newline [1] L. Downward et. al., Phys Rev Lett 95, 106401 (2005). [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U13.00003: Magnetic mapping of phase separated manganite films Casey Israel, Diana Sanchez, Takeshi Kasama, Rafal Dunin-Borkowski, Neil Mathur, Tien-Ming Chuang, Alex de Lozanne We summarize the recent results of efforts to image the different phases in phase-separated epitaxial thin manganite films. We present images acquired by three techniques, electron holography in a transmission electron microscope, magnetic force microscopy (MFM), and conducting atomic force microscopy (CAFM), all of which are applied to La$_{1-x}$Ca$_{x}$MnO$_{3}$ (LCMO) films grown on NdGaO$_{3}$ (001) substrates. Electron holography images of a focused ion beam milled LCMO ($x=0.3$) film (uniformly ferromagnetic at low temperatures) demonstrate the feasibility of using this method to distinguish local phases with different magnetic properties and illustrate the dangers of ion implantation during focused ion beam processing. MFM and CAFM scans of an as-deposited LCMO ($x=0.40$) film (phase-separated at low temperatures) indicate the coexistence of ferromagnetic (metallic) and nonferromagnetic (insulating) regions characterized by length scales below roughly 100 nm. It appears that the ferromagnetic regions preferentially form conducting pathways aligned with the film's easy-axis for magnetization. This seems intrinsic, in that there are no topographical features linked with this phase anisotropy. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U13.00004: Observation of Nanoscale Electronic Phase Separation and Charge Density Wave in Manganites Jeehoon Kim, Junwei Huang, Alex de Lozanne, J. S. Zhou, J. B. Goodenough Scanning tunneling microscopy (STM) is used to image the surface topography and local density of states (LDOS) in the bilayer colossal magnetoresistance (CMR) material LSMO below the Curie temperature. While our STM is capable of atomic resolution, on this surface the smallest features are randomly distributed islands with a lateral size of $\sim $1 nm. We obtained conductance maps to investigate the local electronic structure associated with these islands. The 2-D cross-correlation map between the gap map and the topography suggests a random distribution of three different domains which are Mn$^{3+ }$rich, Mn$^{4+}$ rich or mixed valence Mn$^{3/4}$. The spectroscopic data show a large gap around 600 meV. We also observed some modulations in the conductance map. The Fourier transform of the conductance map shows two major modulations with 1.6 nm and 60 nm wavelength along the crystal axis. The 1.6 nm wavelength modulation can be explained by charge density wave (CDW) model resulting from a Fermi surface instability. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U13.00005: Susceptibility imaging of a weakly disordered manganite in high magnetic field Weida Wu, Casey Israel, Alex de Lozanne, Namjung Hur, Soonyong Park, Sang-Wook Cheong We report variable temperature magnetic force microscopy (VT-MFM) studies of (La,Pr)$_{0.625}$Ca$_{0.375}$MnO$_3$ in a 1 T magnetic field with temperature ranging from 10 K to 250 K. {\it In-situ} resistance measurements confirmed the bulk phase transitions between paramagnetic, anti-ferromagnetic charge ordering and ferromagnetic (FM) phases. Above the FM transition ($\sim$80K), MFM images show $\mu$m-scale magnetic contrast which originates from the local susceptibility variations of the different phases.\footnote{C. Israel, W. Wu and A. de Lozanne, APL, {\bf 89}, 032502 (2006).} By comparing the MFM images with polarized optical microscopy images taken at room temperature, it becomes apparent that the magnetic phase inhomogeneity observed at elevated temperature is linked to local strain variations at the surface of the sample. The high field susceptibility imaging technique could be extended to the study of other magnetic phase separated systems. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U13.00006: Evidences for Metal-Insulator Phase Coexistence Below Tc in Self-doped Manganite Films Peng Gao, Trevor A. Tyson, Michael Deleon, Catherine Dubourdieu, Zhenxian Liu We present evidence for the existence of a significant insulating phase component more that 100 K below the magnetic ordering temperature in the self-doped system La$_{0.80}$MnO3. Reflectivity measurements were made over the range 100 to 8000 cm-1. Using the Drude-Lorenz Model, we convert reflectivity spectra into optical conductivity spectra. The insulating phase was evident as sharp resonances corresponding to atomic vibrational modes which disappeared deep in the metallic phase at low temperature. A significant component of the insulating phase exists down to 200 K below Tc. Between 200 K and 150 K, the metallic phase becomes the dominant phase. Comparisons are made with the La0.7Ca0.3MnO3 samples measured under the same conditions. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U13.00007: Phase-segregated Glass Formation Linked to Long-Range Strain using Resonant Ultrasound Spectroscopy Peter Sharma, S. El-Khatib, I. Mihut, J. B. Betts, A. Migliori, S. B. Kim, S. Guha, S-W. Cheong We have observed a very large damping of ultrasonic waves in the magnetically/electronically/structurally phase segregated state of a CMR manganite that suddenly disappears upon the formation of a glassy state. A subtle stiffening of the shear modulus accompanies the putative glass transition. Our observations most explicitly link strain to the proposed formation of an unusual glass state composed of coexisting, macroscopic structural domains found in this material. These results may implicate strain as the determining factor in the formation of non-equilibrium mixed-phase states in other systems that display widely hysteretic first order magneto-structural transitions. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U13.00008: Raman scattering studies of field- and temperature-dependent melting of charge order in La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ Minjung Kim, H. Barath, S.L. Cooper, M. Rubhausen, S.W. Cheong The La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ system provides an interesting opportunity to study the effects of chemical disorder---introduced by replacing La$^{3+}$ with Pr$^{3+}$ having a smaller ionic radius---on the charge-ordered (CO) state and the complex field- and temperature-dependent phase behavior observed in the manganese perovskite La$_{1-x}$Ca$_{x}$MnO$_{3}$. We report field- and temperature-dependent Raman studies of La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$, in which we are able to carefully study the effects of disorder on both field-induced melting of CO and on the field-induced evolution of novel structural and magnetic phases in the (La,Pr,Ca)MnO$_{3}$ system. Among other results we will discuss, La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ exhibits a quantum melting transition from a CO state to a ferromagnetic metal phase at fields less than 7 T, which is a much lower value than that observed in La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ ($\sim $20T). [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U13.00009: Optical study of Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin films on SrTiO$_{3}$ Seung Yup Jang, M.W. Kim, K.W. Kim, T.W. Noh, N. Nakagawa, H.Y. Hwang Doped rare earth manganites show interesting phase diagrams depending on strong coupling among spin, charge, and orbital degrees of freedom. Especially, Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ (NSMO) undergoes two phase transitions of para-magnetic insulating state to ferro-magnetic metallic state (FMM) and FMM to charge/orbital ordered insulating state as temperature decreases. When NSMO is grown as a thin film on SrTiO$_{3}$(STO) (001), it has no phase transition. Recently, however, thin films grown on STO (110) substrates were found to retain the phase transitions of bulk NSMO. This implies that the charge/orbital state could be modulated by controlling lattice strain. We have investigated the optical conductivity spectra of NSMO epitaxial thin films grown on STO substrates. Optical spectra of NSMO grown on STO (110) show drastic change, depending on temperature and polarization. Based on the careful comparison between the films grown on differently oriented STO substrates, we demonstrate that the substrates strain can cause crucial effect on the ground state of NSMO to result in a novel insulating state which is rarely known. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U13.00010: Phase diagram for Bi1-xCaxMnO3 (x $<$ 0.5) Yuhai Qin, Trevor Tyson, Sang-Wook Cheong, Xiaonong Xu The multiferroic BiMnO3 system, in which ferroelectronic and ferromagnetic orders can coexist, has attracted much research work in the past years for its potential technological applications. For the more general system Bi1-xCaxMnO3, the phase diagram for the Ca rich region (x $>$ 0.4) has been established [1]. In order to understand the multiferroic behavior near the x=0 system, the hole-doped region (0$<$x$<$0.5) was investigated. We have completed the magnetic, transport, and structural phase diagram of Bi1-xCaxMnO3, by performing detailed structural (XRD and XAFS), magnetization (ZFC/FC) and electrical measurements on Bi1-xCaxMnO3 (0$<$x$<$0.5), showing the transition form the highly distorted monoclinic phase to the orthorhombic phase. This work is supported by NSF DMR-0512196. \newline \newline [1] H. Woo, T. A. Tyson, M. Croft, S. W. Cheong, and J. C. Woicik, Physical Review B: Condensed Matter and Materials Physics 63, 134412/1 (2001). [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U13.00011: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U13.00012: Investigation of the local Mn-O distortions in the concentration-induced Metal/Insulator transition materials, La$_{1-x}$ Ca$_{x}$MnO$_{3}$ (x= 0.16-0.22) using EXAFS. Yu Jiang, Lisa Downward, Frank Bridges, John Neumeier The pseudo-cubic manganites La$_{1-x}$ Ca$_{x}$MnO$_{3}$ (LCMO) are CMR (Colossal Magnetoresistance) materials for x between approximately 0.2 and 0.5. They have a temperature driven Metal/Insulator (MI) transition at the ferromagnetic transition temperature T$_{c}$ which is caused by Jahn-Teller-like distortions which form around Mn sites as T increases through T$_{c}$. Near x=0.2 the MI is driven by concentration. To investigate the concentration and temperature dependent local distortions for low doped powder samples (LCMO x = 16{\%}, 18{\%}, 20{\%}, 22{\%}), we carried out EXAFS (Extended X-ray Absorption Fine Structure) experiment at SSRL (Stanford Synchrotron Radiation Laboratory). The results of our preliminary analysis show the following: (1) a sharp, step increase in $\sigma ^{2}$ near T$_{c}$ ($\sigma $ is the width of the Mn-O pair distribution function (PDF)), (2) the magnitude of the step decreases rapidly with decreasing x, (3) the value of $\sigma ^{2}$ at low T is close to the value for zero-point motion for x = 0.22, but increases substantially for lower x, indicating that in the magnetized state a large J-T distortions remain at low T for low x. Also the magnetization at low T is small for lower x, indicating that not all of the sample is ferromagnetic. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U13.00013: Temperature- and field-dependent far-infrared studies of (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ films Naveen Margankunte, Tara. P. Dhakal, Alexandre Zimmers, D.J. Arenas, Y.J. Wang, Amlan Biswas, D.B. Tanner We report far-infrared temperature (300 K-20 K) and magnetic field (0-18 T) dependent reflectance and transmittance studies of 1000 A-thick hole-doped LPCMO thin films. Temperature-dependent measurements show evidence of phase coexistence in the sense that the film still shows insulating behavior at very low temperatures. Magnetic field studies at 4.2 K observe an insulator to metal transition, seen as large changes in reflectance and transmittance. The spectra are analyzed by using standard multilayer film-fitting procedures to extract the optical conductivity and other optical constants. The results are discussed in the context of effective medium approximations. [Preview Abstract] |
Session U14: Focus Session: Magnetization Dynamics and Half Metals
Sponsoring Units: GMAG DMP FIAPChair: John Freeland, Argonne National Laboratory
Room: Colorado Convention Center Korbel 4D
Thursday, March 8, 2007 8:00AM - 8:12AM |
U14.00001: Theoretical limit of the minimal magnetization switching field and the shape of a field pulse for minimal reversal time of Stoner particles X.R. Wang, Z.Z. Sun The theoretical limit of the minimal magnetization switching field and the optimal field pulse design for uniaxial Stoner particles are investigated. Two results are obtained. One is the existence of a theoretical limit of the smallest magnetic field out of all possible designs. It is shown that the limit is proportional to the damping constant in the weak damping regime and approaches the Stoner-Wohlfarth (SW) limit at large damping. For a realistic damping constant, this limit is more than ten times smaller than that of so-called precessional magnetization reversal under a non-collinear static field, showing a big room for possible improvement in current available strategies. The other is on the optimal field pulse design: If the magnitude of a magnetic field does not change, but its direction can vary during a reversal process, there is an optimal design that gives the shortest switching time. The switching time depends on the field magnitude, damping constant and magnetic anisotropy. The two results can be used to evaluate various magnetization reversal strategies. Reference: Z.Z. Sun, and X.R. Wang, Phys. Rev. B 73, 092416 (2006); 74, 132401 (2006); Phys. Rev. Lett. 97, 077205 (2006). [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U14.00002: Coupled Precession Modes in Indirect Exchanged-Coupled Thin Films Steven Michalski, Jian Zhou, Bob Buckley, Ralph Skomski, Roger Kirby Static and dynamic magnetic properties of exchange-coupled magnetic layers have been investigated by magneto-optical measurements. Our samples are [Pt/Co] multilayers with perpendicular magnetic anisotropy (PMA) exchange-coupled to a Co (Ni) layer with in-plane magnetic anisotropy by a variable thickness intervening Pt layer. The magnetic properties of such systems are controllable by tuning the exchange strength and PMA. To measure the magnetization precession, we use a femtosecond laser in a pump-probe experiment with direct optical excitation and preliminary measurements using a magnetic field pulse excitation. Both the strength and the angle of an external applied magnetic field were varied and for many samples, two modes with two distinct precession frequencies were observed, with frequencies that depend on the strength and the angle of the applied field. Our results are interpreted by a LLG-based model which predicts two modes whose behaviors depend on the strength and sign of the exchange coupling. This model is in a good qualitative agreement with our data and allows us to estimate the magnitude of the exchange coupling between the two layers. This work is supported by NSF-MRSEC, NCMN, and the W. M. Keck Foundation. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U14.00003: Frequency-dependent magnetization response of CoFe thin film Mingqiang Bao, Alexander Khitun, Jooyoung Lee, Kang L. Wang, Ajey P. Jacob Frequency-dependent magnetization dynamics in CoFe thin films is investigated. Magnetization has been generated and detected by a pair of micrometer-wide antennas. The experimental data have been mapped showing the frequency-dependence of both the amplitude and the phase under external magnetic field. The response on a continuous single frequency excitation shows complex behaviors including those of harmonic resonances and half-frequency resonances. Furthermore, the device time-domain response of any input voltage/current pulse under any external magnetic field (below 350 Oe) can be reconstructed from our measured frequency-domain maps through the Fourier transform, and is validated with our time-domain measurement data. \newline \newline *The authors would like to thank Dr. S. Wang and Dr DW Lee for growing CoFe thin-films. The work was supported in part by Western Institute of Nanoelectronics (WIN) and the Microelectronics Advanced Research Corporation (MARCO) Focus Center on Functional Engineered Nano Architectonics (FENA). [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U14.00004: Vortex Dynamics Imaged by Time-Resolved Scanning Transmission X-Ray Microscopy. Invited Speaker: Substantial progress in the understanding of magnetic vortex dynamics has been achieved by implementing 100 ps time-resolved magnetic imaging techniques into a Scanning Transmission X-Ray Microscope (ALS, Berkeley, BL 11.0.2) with a lateral resolution of 20-40 nm. Gyrotropic vortex motion excited by in-plane alternating magnetic fields was studied in micron-sized ferromagnetic vortex structures. \newline The vortex core, extending over a range of about 20 nanometers in the center of a vortex structure, plays a key role in vortex dynamics. We have been able to perform time-resolved imaging of the out-of-plane magnetization distribution of the excited vortex core. In addition, we have discovered new switching schemes to change the direction of the vortex core polarization in micron-sized ferromagnetic vortex structures, either (i) by altering the amplitude of an alternating magnetic field at a frequency close to the eigenfrequency of the gyrotropic vortex motion or (ii) by applying a short burst (e.g., one single period) of ac magnetic fields [1]. \newline Magnetic vortex cores have already been discussed as candidates for magnetic data storage, but for the switching of their polarization large magnetic fields in the order of half a Tesla were required so far. The vortex core reversal schemes presented here need significantly lower magnetic fields (down to a few mT). The switching mechanism as reproduced by micromagnetic simulations [1] involves: (i) creation of a vortex--antivortex pair, both with opposite polarisation and (ii) annihilation of the antivortex with the original vortex. At the end a vortex with opposite polarization remains. \newline \newline [1] B.Van Waeyenberge, A.Puzic, H.Stoll et al., \textit{Magnetic vortex core reversal by excitation with short bursts of an alternating field}, Nature, Nov.23 (2006) [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U14.00005: Highly-coherent magnetic vortex oscillations driven by a dc spin-polarized current V.S. Pribiag, G.D. Fuchs, P.M. Braganca, O. Ozatay, J.C. Sankey, D.C. Ralph, R.A. Buhrman, I.N. Krivorotov While it has been demonstrated that dc spin-polarized currents can drive microwave spin-wave oscillations in magnetic multilayers via the spin-transfer torque (STT) effect [1], little is known about persistent STT-driven oscillations in strongly non-uniform systems. We report the use of STT to excite steady-state gigahertz-frequency oscillations of a magnetic vortex. We use an elliptical Py-Cu-Py nanopillar spin-valve structure in which one of the Py layers is sufficiently thick that its magnetization assumes a vortex configuration. The oscillations, which can be obtained in essentially zero applied field, are highly coherent, with full-widths at half maximum of less than 300 kHz at room temperature being obtained under certain bias conditions. We will discuss the observed sensitivity of the oscillation line-width to magnetic defects. We will also present measurements of the temperature-dependence of the oscillations, which we are pursuing to obtain a more complete understanding of how magnetic imperfections and thermal fluctuations determine the performance of this new type of nanomagnetic STT oscillator. We will also discuss the use of STT-driven ferromagnetic resonance to examine the various magnetic modes that can be present in these nanoscale vortex structures. [1] S. I. Kiselev \textit{et al.}, \textit{Nature} (London) \textbf{425}, 380 (2003). [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U14.00006: Driven dynamic mode-splitting of magnetic vortices Kristen Buchanan, Marcos Grimsditch, Frank Fradin, Sam Bader, Val Novosad It has been established theoretically and experimentally that a magnetic vortex in restricted geometry possesses a translational excitation that corresponds to circular motion of the vortex core at a characteristic frequency. Here we explore the effect of increased driving-field amplitude on this dynamic mode using a microwave reflection technique. We find a new effect - the vortex translational eigenmode splits into two peaks. The splitting in frequency is $>$25\% for driving magnetic fields $<$25 Oe for micron-sized permalloy ellipses that are 40-nm thick. Splitting effects were detected for driving fields as low as 3 Oe in circular dots. Micromagnetic modeling suggests this effect could be indicative of nonlinear fold-over but further theoretical work will be required to develop a full understanding of these observations. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U14.00007: Electronic and magnetic properties of the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrTiO$_{3}$ interface from first principles calculations Hand Zenia, Gillian Gehring, Walter Temmerman We present results of first principles calculations of the electronic and magnetic properties of the La$_{0.7}$Sr$_{0.3}$MnO$_3$/SrTiO$_{3}$ interface. We are interested in the changes with respect to the bulk as concerns ferromagnetism and half-metallicity. The bulk calculations give a nearly half-metallic ground state for the manganite. In transport the system is predicted to be totally half-metallic. This latter property is preserved at the interface only if the magnetic coupling between the interface region and the bulk is ferromagnetic. We have looked at the two possible interface terminations between La$_{0.7}$Sr$_{0.3}$MnO$_3$ and SrTiO$_{3}$ and found that one of the interfaces preserves the bulk properties of La$_{0.7}$Sr$_{0.3}$MnO$_3$ whereas the other type of termination suppresses them. Reducing the number of holes at the interface by introducing LaMnO$_{3}$ restores ferromagnetism. Hence the possibility of engineering interfaces to improve the Tunneling Magneto-Resistance yield in the La$_{0.7}$Sr$_{0.3}$MnO$_3$/SrTiO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_3$ tunnel junctions. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U14.00008: Heusler Alloys for CPP-GMR C. Culbert, M. Williams, M. Chshiev, P. LeClair, W. H. Butler Half-Metallic full Heusler alloys of composition X$_{2}$YZ and structure type L2$_{1}$ have aroused interest because of their potential application in CPP-GMR spin valves for readers in hard disk drives. The X and Y are typically transition metals and the Z is a non-transition metal element. The structure of these alloys can be viewed as a variant of bcc in which (100) atomic layers of X alternate with layers of YZ. The alloys Co$_{2}$MnSi and Co$_{2}$MnGe have received particular attention because of their high T$_{C}$ which exceeds 900K. We have performed first-principles calculations using the VASP code in GGA to investigate the properties of these materials. We have found them to be half-metals in bulk in agreement with previous work. We obtained minority gaps at the Fermi energy of 0.36 and 0.51eV for Co$_{2}$MnGe and Co$_{2}$MnSi, respectively. We also investigated multilayers consisting of Heusler and various possible spacer materials. Interestingly, we found that for one or two atomic layers of Cr alternating with Co2MnGe along (100), the system remained half-metallic. Cr can actually be used in this way to increase the minority gap. We found that Co$_{2}$MnGe slabs were typically not half metallic, but slabs terminated in a pure Mn layer retained a minority gap. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U14.00009: Spin Polarization and Spin Transport in Co-based Heusler alloys Muhammad M. Faiz, Raghava P. Panguluri, Sabine Wurmehl, Claudia Felser, Boris Nadgorny The Co-based Heusler alloys are of special interest for possible spintronic applications due to their high Curie temperatures and high magnetic moment per unit cell. Co$_{2}$FeSi is especially promising as a candidate half-metal as it has a Curie temperature of approximately 1100K and the integer magnetic moment of 6$\mu _{B}$ per unit cell [Ref. S. Wurmehl et al]$^{1}$. The samples have been prepared by arc melting of stochiometric quantities of pure metals in argon atmosphere followed by annealing in sealed quartz tube\textbf{s} at 1300K. Here, we report comparative spin polarization, $P$, in Co$_{2}$FeSi and Co$_{2}$MnFeSi using Point Contact Andreev Reflection Spectroscopy (PCAR). We have also studied spin transport in Heusler/Au bilayers. Variable thickness Au films were deposited on top of the Heusler samples and the PCAR technique was then used to probe $P $on the Au side. We will give the estimates of the spin diffusion length in Au based on these measurements and compare the results with conventional ferromagnetic spin injectors. 1. S. Wurmehl et al., J. Appl. Phys. 99, 08J103 (2006). [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U14.00010: A magneto-optical study of Heusler alloy Co$_{2}$MnGe films grown on Ge(111) substrates Pranaba Muduli, Wil Rice, Brian Collins, Liang He, Frank Tsui We report an extensive study of magneto-optic properties and magnetic anisotropy of ternary alloys of Co, Mn and Ge over a wide range of composition near the Heusler alloy compound Co$_{2}$MnGe grown on a single Ge(111) substrate by combinatorial molecular-beam epitaxy. The scanning magneto-optic Kerr effect (MOKE) study reveals a range of room temperature ferromagnetic phases near the Heusler alloy composition. A ridge of higher MOKE intensity near the Co/Mn atomic ratio of two is found, which appears to correlate with the structural perfection of the epitaxial film. The MOKE hysteresis loops in this composition range exhibit asymmetries that are associated with the presence of a large second-order magneto-optic effect. The magnetic anisotropy in the same composition range can be described by a combination of a six-fold and a uniaxial term. A single domain model in conjunction with the second-order MOKE is used to explain the magnetic anisotropy in these alloys. The model reveals a systematic change of magnetic anisotropy and the second-order MOKE coefficients as a function of composition and structural and chemical ordering of the films. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U14.00011: The electronic band structure of CoS$_{2}$ Ning Wu, Yaroslav Losovyj, David Wisbey, Kirill Belashchenko, Wai-Ning Mei, Michael Manno, Lan Wang, Chris Leighton, Peter Dowben We have identified a strongly dispersing band of CoS$_{2}$(100), with both sulfur and cobalt weight, along the $\Gamma $-X direction of the bulk Brillouin zone, from photon energy dependent angle resolved photoemission studies. From the critical points of the experimental band structure, the inner potential is estimated at about 4 to 5 eV, consistent with LEED I(V) analysis. The small inner potential indicates that CoS$_{2}$ has a narrow band width, consistent with the theoretical expectations. The clearly favored structural model from the LEED I(V) analysis is sulfur with cobalt terminated surface. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U14.00012: Spin polarization of the ferromagnetic semimetal EuB$_{6}$ Xiaohang Zhang, Stephan von Molnar, Peng Xiong, Zach Fisk Much progress has been made recently in the understanding of the electronic properties of EuB$_{6}$. However, the details of the electronic structure remain controversial. Several band structure calculations have produced different degrees of conduction-valence band overlap for the two spin subbands. In particular, some calculations predict a half-metallic band structure, ie, 100{\%} spin polarization at the Fermi level$^{1}$. We have performed direct measurements of the spin polarization of EuB$_{6}$ crystals using Andreev reflection spectroscopy. Planar junctions of EuB$_{6}$/Pb were fabricated on crystals grown with an Al flux method. The conductance spectra were measured using phase-sensitive detection at several temperatures below T$_{C}$ of Pb. The spectra are well-described by the spin-polarized BTK model. More than seven junctions were measured and a spin polarization of 55 $\pm $ 10{\%} is obtained. Our results indicate that in ferromagnetic EuB$_{6}$ the electrons and holes at the Fermi level are not fully spin-polarized. This work was supported by a FSU Research Foundation PEG grant and NSF grant under DMR-0503360. $^{1}$J. Kunes and W.E. Pickett, PRB 69, 165111 (2004); M. Kreissle and W. Nolting, PRB 72, 245117 (2005). [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U14.00013: Sharp switching of the magnetization in Fe$_{1/4}$TaS$_{2}$ Emilia Morosan, Henny Zandbergen, Lu Li, Minhyea Lee, Joseph Checkelsky, Michael Heinrich, Theo Siegrist, N. Phuan Ong, Robert Cava Anisotropic magneto-transport measurements are reported on Fe$_{1/4}$TaS$_{2}$ single crystals grown by vapor transport. Both the magnetization and resistivity are extremely anisotropic, with the magnetic moments aligned parallel to the c crystallographic direction. Fe$_{1/4}$TaS$_{2}$ orders ferromagnetically below T$_{C}$ = 160 K and displays very sharp hysteresis loops in the ordered state for H$\vert \vert $c. The corresponding magnetoresistance is negative, and it qualitatively reproduces the features observed in the M(H) data, by showing a sharp drop around the critical field H$_{s}$ for the moment reversal. For field applied within the ab plane, the magnetization remains small and linear in field up to 5 T, and the magnetoresistance is positive and quadratic in field, with no visible hysteresis. The squareness of the H$\vert \vert $c M(H) loops and the high critical field for the magnetization switch (H$_{s }$= 3.7 T at T = 2 K) allow us to classify Fe$_{1/4}$TaS$_{2}$ as a strong ferromagnet. [Preview Abstract] |
Session U15: Focus Session: 1D Quantum Magnetism
Sponsoring Units: GMAGChair: Steve Nagler, Oak Ridge National Laboratory
Room: Colorado Convention Center Korbel 4E
Thursday, March 8, 2007 8:00AM - 8:12AM |
U15.00001: Dynamical Structure Factors of quasi-one-dimensional antiferromagnets Rob Hagemans, Jean-S\'ebastien Caux, Jean Michel Maillet For a long time it has been impossible to accurately calculate the dynamical structure factors (spin-spin correlators as a function of momentum and energy) of quasi-one-dimensional antiferromagnets. For integrable Heisenberg chains, the recently developed ABACUS method (a first-principles computational approach based on the Bethe Ansatz) now yields highly accurate (over 99\% of the sum rule) results for the DSF for finite chains, allowing for a very precise description of neutron-scattering data over the full momentum and energy range. We show remarkable agreement between results obtained with ABACUS and experiment. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U15.00002: Dynamical spin structure factor for the anisotropic spin-1/2 chain Rodrigo G. Pereira, Jesko Sirker, Jean-Sebastien Caux, Rob Hagemans, Jean Michel Maillet, Steve White, Ian Affleck The longitudinal spin structure factor for the XXZ-chain at small wave-vector $q$ is obtained using Bethe Ansatz, field theory methods and the Density Matrix Renormalization Group. It consists of a peak with peculiar, non-Lorentzian shape and a high-frequency tail. We show that the width of the peak is proportional to $q^2$ for finite magnetic field compared to $q^3$ for zero field. For the tail we derive an analytic formula without any adjustable parameters and demonstrate that the integrability of the model directly affects the lineshape. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U15.00003: Bound magnon states in the frustrated ferromagnetic 1D chain Lars Kecke, Tsutomu Momoi, Akira Furusaki We studied a one-dimensional Heisenberg chain with ferromagnetic nearest-neighbour $J_1$ and antiferromagnetic next-nearest-neighbour interactions $J_2$ in a magnetic field. Starting from the fully polarized high-field state, we calculated the saturation field and the dispersions of the lowest lying n-magnon excitations, and examined the possibility of a multi-magnon ground state near the ferromagnetic limit. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U15.00004: High Frequency Electron Paramagnetic Resonance studies of NiCl$_{2}$-4SC(NH$_{2})_{2}$ Sung Su Kim, S. Hill, P. Sengupta, V.S. Zapf, R. Mcdonald, M. Jaime, C.D. Batista, S. Tozer, A. Paduan-Filho High-frequency (50 to 311~GHz) Electron Paramagnetic Resonance (HFEPR) measurements have been performed on a single-crystal sample of the easy-plane $S$~=~1 linear-chain Heisenberg antiferromagnet NiCl$_{2}${\-}4SC(NH$_{2})_{2}$ (DTN). DTN has received much interest due to the possibility that it undergoes a Bose-Einstein condensation of the $S^{z}$~=~1 excitations (magnons) in the vicinity of the field tuned crossing of the $S^{z}$~= 0 and 1 states. Low-temperature frequency-dependent studies provide a direct measure of the zero-field spin gap separating the $S^{z}$ = 0 and 1 states. Lower frequency (92~GHz), temperature dependent studies with $B$//$c$ reveal two resonances either side of the $S^{z}$ = 0 and 1 crossing. Strong shifts and changes in the lineshapes of these resonances as the temperature is raised from 2~to ~15~K can be understood in terms of the gradual population of multi-magnon states. The trends observed from HFEPR experiments can be reproduced via numerical simulations using accepted values for the intra-chain Heisenberg interaction $J$ and the local anisotropy parameter $D$. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U15.00005: Magnetic Excitations in the Spin-1 Anisotropic Heisenberg Antiferromagnetic Chain System NiCl$_2$-4SC(NH$_2$)$_2$ Sergei Zvyagin Electron spin resonance studies of magnetic excitations in NiCl$_2$-4SC(NH$_2$)$_2$ (DTN, a quantum $S=1$ chain system with strong easy-pane anisotropy and a new candidate for the Bose-Einstein condensation of the spin degrees of freedom) in fields up to 25 T are presented. Based on analysis of the frequency-field dependence of single-magnon mode in the high- field spin-polarized phase and previous experimental results [Phys. Rev. Lett. 96, 07724 (2006)], a revised set of spin- Hamiltonian parameters is obtained. Our results yield $D=8.9$ K, $J_c=2.2$ K, and $J_{a,b}=0.18$ K for the anisotropy, intra- and inter-chain exchange interactions, respectively. These values are used to calculate the AFM-phase boundary, low- temperature magnetization and the frequency-field dependence of two-magnon bound-state excitations predicted by theory and observed in DTN for the first time. Excellent quantitative agreement with experimental data is obtained. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U15.00006: Far-infrared study of gapped spin excitations in the chains of Sr$_{14}$Cu$_{24}$O$_{41}$ Dan Huvonen, U. Nagel, T. Room, P. Haas, B. Gorshunov, M. Dressel, Y.-J. Wang, J. Akimitsu, T. Sasaki, T. Nagata We studied using far-infrared spectroscopy, magnetic field and temperature dependence of the spin gap modes in the chains of Sr$_{14}$Cu$_{24}$O$_{41}$. Two triplet modes T$_{1}$ and T$_{2}$ were found in the center of the Brillouin zone at E$_{1}$=77.8 cm$^{-1 }$(9.65 meV) and E$_{2}$=87.7 cm$^{-1 }$(10.86 meV) in zero magnetic field. Both excitations are electric dipole active modes. T$_{1}$ mode is excited when the light E-vector is along the $b$ crystallographic axis and T$_{2}$ is excited when the light E-vector is along the $a$-axis, both perpendicular to the chain direction. The selection rules of the transitions are compatible with dynamic Dzyaloshinskii-Moria interaction mechanism. Up to the field of 18T the electron g-factors of two modes are similar, g$_{1c}$=2.049 and g$_{2c}$=2.055 with magnetic field applied along the chains. Linewidth of both modes is 1 cm$^{-1}$ (0.12 meV) at 4K and increases with T. The temperature dependence of the mode energies is in agreement with the inelastic neutron scattering (INS) results from other groups. However the T$_{1}$ mode has not been observed by INS. The zone structure model of magnetic excitations in the chains is not complete and must include a triplet mode at 9.65 meV in the center of the magnetic Brillouin zone. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U15.00007: Quasi one-dimensional magnetism driven by unusual orbital ordering in CuSb$_2$O$_6$ Helge Rosner, Deepa Kasinathan, Klaus Koepernik Cuprate compounds, including the family of high-Tc-superconductors, exhibit a large variety of exotic physical properties. This variety is determined by the interplay of different interactions; mainly covalency, ligand-fields and strong correlation effects. A nearly universal component of cuprate systems is a strongly elongated CuO$_6$-octahedron wherein the exotic behaviour finds its origin in the deceivingly simple planar Cu-O orbital lying in its basal plane. In this well established standard scenario, the involvement of the apical out-of-plane orbitals is not settled completely. The surprisingly regular CuO$_6$-octahedra of CuSb$_2$O$_6$ offer a unique opportunity to elucidate this scenario due to the changed balance of interactions in this system. We present an electronic structure study resulting in an hitherto unobserved ground state originating from a competition of in- and out-of-plane orbitals. Our results show that strong Coulomb correlation drives a surprising and unique orbital ordering. This, gives rise to an unusual and strongly one-dimensional magnetic ordering that is unlike any ordering observed in conventional low-dimensional cuprates. Our results provide a unique and natural interpretation of recent neutron data. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U15.00008: Interplay of Dzyaloshinskii-Moriya and magnetic field in a one dimensional spin-$\frac{1}{2}$ Heisenberg antiferromagnetic chain Jianmin Sun, Suhas Gangadharaiah, Oleg Starykh We study a one dimensional spin-1/2 Heisenberg antiferromagnetic chain in the presence of a magnetic field and a Dzyaloshinskii-Moriya (DM) interaction term. The case of a staggered DM term and a magnetic field perpendicular to it has been considered previously in Ref.[1]. It was shown that in both weak and strong magnetic field, DM term is a strongly relevant operator leading to the opening of a gap. The spin ordering takes place along a direction perpendicular to both the DM vector and the magnetic field. We consider the case of a uniform DM term, perpendicular to the magnetic field. In the limit of a strong magnetic field, the uniform DM term generates a marginally relevant operator that opens up a gap which scales as a power law with the DM term. The spin ordering takes place along the DM vector. In the limit of weak magnetic field, in comparison with the DM term, no relevant terms are generated, and the spin chain remains gapless. \newline [1] I. Affleck and M. Oshikawa, Phys. Rev. B 60, 1038 (1999). [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U15.00009: Oxygen hole-doping effects on magnetic properties of the spin-chain system Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10-\delta}$ Keeseong Park, Theodore Cackowski, John Markert The magnetic properties of the low-dimensional spin-chain system Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10-\delta}$ were studied as a function of oxygen content. The temperature dependence of the magnetic moment and specific heat were measured for a series of samples with different oxygen contents, prepared by solid state reaction under various oxygen pressures up to 225 atm and characterized by x-ray diffraction and iodometric titration. At fixed oxygen pressure, oxygen deficiency increases with Ca doping. For example, when annealed at 1 atm O$_2$ pressure, the $x=0$ sample was multiphased due to excess oxygen, whereas the samples with $x>1.2$ were also multiphased due to oxygen deficiency. With decreasing oxygen deficiency, the antiferromagnetic transition temperature decreases for $x=0.50$, $x=0.75$, $x=0.90$, and $x=1.00$ doped samples. In particular, for the fully oxygenated $x=1.00$ sample the transition is completely suppressed, which is contrary to the single crystal result recently published by K. Kudo $et$ $al$.\footnote{K. Kudo, S. Kurogi, and Y.Koike, Physical Review B \textbf{71}, 104413 (2005)}, where long range order disappeared at $x\approx1.4$ for the apparently oxygen-deficient crystals. A new magnetic phase diagram is proposed to include both Ca doping and oxygen deficiency. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U15.00010: Anomalous Curie response of an impurity in a quantum critical spin-$1/2$ Heisenberg antiferromagnet Kaj H{\"o}glund, Anders Sandvik There is a disagreement concerning the low-temperature ($T$)
magnetic susceptibility $\chi ^z_{{\rm imp}}\sim {\mathcal C}/T$
of a spin-$S$ impurity in a nearly quantum critical
antiferromagnetic host. Field-theoretical work~[1] predicted an
anomalous Curie constant $S^2/3<{\mathcal C} |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U15.00011: Thermal response functions for 1D strongly correlated electron systems Michael R. Peterson, Jan O. Haerter, B. Sriram Shastry Thermal response functions of strongly correlated electron systems are of appreciable interest to the physics community from both a theoretical and technological point of view. Here we focus on one-dimensional models, namely the Hubbard and infinitely correlated $t$-$J$ models. Using exact diagonalization on finite sized systems we calculate the dynamical thermal response functions as functions of temperature, i.e., the electrical conductivity, the Peltier coefficient, and the thermal conductivity via the Kubo formulas. This in turn allows us to calculate the thermopower (Seebeck coefficient), Lorentz number, and the dimensionless figure of merit. By considering a geometrically frustrated system (inclusion of second neighbor hops into the Hubbard model) the thermopower is shown to be enhanced at intermediate temperatures. We also benchmark the finite temperature Lancos method. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U15.00012: Quantum Monte Carlo study of a spin-Peierls model in a magnetic field Jeongpil Song, R.T. Clay We present results of a quantum Monte Carlo study of a quasi one-dimensional XY spin model coupled to quantum phonons. We compare different updating techniques for the Stochastic Series Expansion method and present autocorrelation time data. We are able to reduce autocorrelation times by using loop update techniques for both spin and phonon degrees of freedom. We determine the critical phonon coupling for the spin-Peierls state, and discuss the dependence on the phonon frequency, magnetic field, and inter-chain coupling. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U15.00013: Magnetic field dependence of spin-forbidden electronic excitations reflects the Haldane or paramagnetic ground state V.C. Long, J.R. Montague, A.C. Kozen, X. Wei, B.R. Landry, K.R. Pearson, M.M. Turnbull, C.P. Landee We compare the zero-field and magnetic field-dependent optical spectra of the Haldane chain compound NENB (Ni[en]$_2$NO$_2$BF$_4 $; en = C$_2$N$_2$H$_{8}$) and the paramagnetic compound, Ni(en) $_3$(ClO$_4$)$_2$,H$_2$O. Due to similar electronic coordination of Ni$^{2+}$, the two materials show similar zero-field $d-d$ electronic transitions, including a spin-forbidden (SF) transition at 1.58 eV, overlapping a broad spin-allowed band at 1.45 eV. The relatively greater intensity of the SF band in the Haldane compound suggests activation by a spin exchange mechanism, whereas a spin-orbit coupling origin is likely in the paramagnet. A second narrower SF spin flip transition appears in NENB at 1.66 eV. In both compounds, the SF excitations are sensitive to applied field $H$. In NENB, the SF intensity is suppressed by $H$, consistent with behavior of spin exchange-activated bands. In Ni(en)$_3$(ClO$_4$)$-2$,H$_2 $O, the SF field sensitivity appears to combine an energy shift and intensity decrease. Details of the $H$ dependence reflect the magnetic ground state of the material: the field sensitivity commences only above $H_C$ $\approx$ 10 T, in the Haldane compound, whereas the field-induced modifications begin immediately at $H$ = 0 T in the paramagnet. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U15.00014: Spin waves in LiCu$_{2}$O$_{2}$ and NaCu$_{2}$O$_{2}$: far-infrared study in magnetic field. T. Room, D. Huvonen, U. Nagel, Y.J. Choi, C.L. Zhang, S. Park, S.-W. Cheong, M. Mostovoy Frustrated magnetic interactions influence the ground state properties of spin systems. They may drive spin chain systems into a helicoidally ordered phase. Excitations in the helicoidally ordered phase are gapless spin waves. Anisotropic interactions create a finite gap at zero wave vector, \textbf{\textit{q}} = 0. LiCu$_{2}$O$_{2}$ and NaCu$_{2}$O$_{2}$ are quasi-1D systems with a weak interaction between Cu$^{2+}$ spin chains. Both compounds enter spin spiral state at low temperature (LiCu$_{2}$O$_{2}$ at 24K and NaCu$_{2}$O$_{2}$ at 12K). Here we report results of far-infrared (FIR) study where the absorption of light from 3 to 100cm$^{-1}$ by spin waves in LiCu$_{2}$O$_{2}$ and NaCu$_{2}$O$_{2}$ is measured. Light is absorbed by magnetic excitations at \textbf{\textit{q}} = 0 and at wave vectors that are multiples of the spiral wave vector, $\pm n$\textbf{\textit{Q}}. In magnets with a noncollinear spin ordering, such as a spin spiral, both components of light, magnetic and electric, may induce optical transitions. Measurements were performed from 3K to 30K and in fields up to 12T. To elucidate the selection rules, polarized light and different sample geometries were used. The magnetic field dependence of line positions and intensities is analyzed within a continuum model of a spiral magnet. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U15.00015: Magnetic anisotropy of doped (Fe, Mn, Cr) NbSe$_{2}$ A. F. Isakovic We investigated the behavior of magnetic dopants (Mn, Fe, Cr) in NbSe$_{2}$ at relatively large concentrations of approximately 1{\%}. Magnetic measurements show a pronounced magnetic anisotropy of these interstitial alloys, as evidenced in qualitatively different magnetization reversal dynamics for the magnetic fields applied in-plane and out-of-plane of these quasi two-dimensional samples. Element-specific characterization indicates that magnetic dopants are distributed approximately evenly throughout samples, thus ruling out clustering as a primary mechanism behind the observed anisotropy. For an arbitrary orientation of the applied field with respect to the sample plane, an interesting reorientation-like behavior is observed at magnetic fields below the value needed for the magnetization saturation. The details of this reorientation depend on the angle between the applied field and the sample plane. This behavior is temperature dependant, and was observed below the Peierls transition (T$_{P}$ = 33 K), but above the superconducting transition T$_{C}$. We discuss the nature of these measurements in the light of the proposed models of doped NbSe$_{2}$. This research was supported by the NSF grant (DMR 04-05500). I acknowledge Prof. R. E. Thorne for making samples available, and CCMR staff for assistance. [Preview Abstract] |
Session U16: Correlated Electrons: Heavy Fermions, Kondo Lattice, and f-electron Systems
Sponsoring Units: GMAGChair: Wei Bao, Los Alamos National Laboratory
Room: Colorado Convention Center Korbel 4F
Thursday, March 8, 2007 8:00AM - 8:12AM |
U16.00001: Ultrast study of Kondo-lattice coupling in the d-electron Kenneth Burch, Diyar Talbayev, B.C. Sales, D. Mandrus, A.J. Taylor, R.D. Averitt We report our recent ultrafast optical measurements of the Zintl compound Yb$_{14}$MnSb$_{11}$, which was recently determined to be the first ferromagnetic Kondo lattice in the underscreened limit. The response at low temperatures to has two components, which we attribute to the underscreened nature of this Kondo system. In addition we measure the generation of coherent acoustic and optical phonons, that allow us to demonstrate a clear connection between the Kondo effect and lattice vibrations. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U16.00002: Resonant Ultrasound Studies of Ferromagnetic Yb$_{14}$MnSb$_{11}$ Sriparna Bhattacharya, Veerle Keppens, Ivan A. Sergienko, Brian Sales, David Mandrus The compound Yb$_{14}$MnSb$_{11}$ is believed to be a rare example of an under-screened Kondo lattice. In this material the Yb ions are divalent and hence nonmagnetic. The magnetism comes from the Mn 3d electrons and the antiferromagnetic coupling of these electrons to holes in the Sb 5p bands. The antiferromagnetic coupling between a local magnetic moment and extended Bloch states can result in ferromagnetism due to the RKKY interaction, but can also give rise to Kondo physics. Here we report measurements of the elastic moduli (obtained using resonant ultrasound spectroscopy) as a function of temperature (2-300K) and magnetic field (0-2 T) for Yb$_{14}$MnSb$_{11}$. An unusual lattice stiffening is observed below Tc. Attempts to model this behavior using Landau theory will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U16.00003: Valence state and magnetism of Yb$_{7}$Co$_{4}$InGe$_{12}$. Zsolt Rak, Maria Chondroudi, S.D. Mahanti, M.G. Kanatzidis Ytterbium (Yb) compounds exhibit unusual physical properties due to the Yb f-electrons, which play an active role in bonding, giving rise to intermediate valence, heavy-fermion or Kondo behavior. Many physical characteristics of the Yb systems are related to the fact that Yb can have two valence states: nonmagnetic divalent Yb$^{2+}$ (f$^{14})$ and magnetic trivalent Yb$^{3+}$ (f$^{13})$. We have synthesized a new Yb containing quaternary Yb$_{7}$Co$_{4}$InGe$_{12}$. XPS and magnetic susceptibility measurements indicate that all Yb are all trivalent. To understand the Yb valency in this compound, we have carried out \textit{ab initio }electronic structure calculations within density functional theory using FP-LAPW method. The electronic structure is obtained using LSDA with on-site Coulomb correlation potential (LSDA+U) included for both 4f electrons of Yb and 3d electrons of Co. As a one ``f-hole'' analogue of many Ce compounds$^{1, 2}$, we find that all the Yb atoms are trivalent, in agreement with XPS and magnetic susceptibility measurements. $^{1}$A. I. Liechtenstein, V. P. Antropov, and B. N Harmon, Phys. Rev. B 49, 10770 (1994). $^{2}$E. Bauer, Adv. Phys. 40, 417 (1991). [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U16.00004: Crystal structure and magnetic behavior in CeAu2Si2 and CeAu4Si2 Andriy M. Palasyuk, John D. Corbett, Athena S. Sefat, Paul C. Canfield Interest in Ce-based compounds of systems like CeCu2Si2, CePd2Si2 and CeRu2Si2 has attracted much interest due to magnetic ordering, heavy Fermion behavior and superconductivity. This work is a comparative study of crystal structure and properties of CeAuxSi2 flux-grown crystals with x=2, 4. For CeAuxSi2 system, we have studied structure and anisotropic field- and temperature-dependent magnetization M(H, T). The single-crystal x-ray data indicate that CeAu4Si2 has CeRe4Si2-type structure and is orthorhombic (Cmmm) and CeAu2Si2 is tetragonal (I4/mmm). Although there is an extra layer of Au atoms in the CeAu4Si2 structure the magnetic ordering temperatures of CeAu2Si2and CeAu4Si2 are remarkably similar. In this work we will examine and discuss the similarities and differences between the thermodynamic, transport and structural properties of these related materials. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U16.00005: Interplay of Magnetism and Superconductivity in CeCu$_{2}$Si$_{2}$ and RNi$_{2}$B$_{2}$C Probed by Neutron Scattering Michael Loewenhaupt, Oliver Stockert, Andreas Kreyssig We will discuss results of neutron scattering experiments performed on two systems that were investigated in close collaboration with other groups: the heavy fermion superconductor CeCu$_{2}$Si$_{2}$ and the borocarbides RNi$_{2}$B$_{2}$C (with R = rare earth). In both systems SC and magnetic ordering is observed. Due to minor changes in stoichiometry single crystals of A-, A/S and S-type CeCu$_{2}$Si$_{2}$ can be grown that show only long range magnetic order (LRO), both SC and LRO, and only SC. However, LRO is suppressed as soon as SC sets in (phase separation in A/S-type crystals). In S-type crystals only short range correlations are observed that develop an energy gap in the magnetic excitation spectrum below $T_{C}$ . In the borocarbides both LRO and SC coexist (e.g. for HoNi$_{2}$B$_{2}$C: $T_{C}$ = 8 K and $T_{N}$ = 6 K). However, there is no change in the diffuse neutron scattering (being a signature of the susceptibility of the conduction electrons) for temperatures above and below $T_{C}$ (but still above$ T_{N})$. The origin of SC in the borocarbides is manifested in a strong phonon softening intimately connected with the development of the SC gap. Conclusion: Magnetism and superconductivity in CeCu$_{2}$Si$_{2}$ is (phase) separated in real space while in the borocarbides both phenomena may be understood as being decoupled in momentum space. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U16.00006: Muon spin resonance study on UCu$_{1.5}$Sn$_{2}$ S. El-Khatib, A. Llobet, G. Kalvius, D. Noakes, C. Stronach, E. Ansaldo, M. Torikachvili, H. Nakotte We report on muon spin relaxation measurements results on UCu$_{1.5}$Sn$_{2}$, which crystallizes in the CaBe$_{2}$Ge$_{2}$-type structure. Our analysis is consistent with collinear antiferromagnetic order that occurs below 108 K, in agreement with previous reports. The Brillouin-like behavior of the temperature dependence of the magnetic response is consistent with localized 5$f$ moments. In the paramagnetic regime a fast and a slowly relaxing signal is seen, the former reflecting disturbed local magnetic surroundings caused by the defect structure. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U16.00007: Characteristic energy scales for field-tuned quantum criticality of CeCoIn$_5$ M.A. Tanatar, J. Paglione, Louis Taillefer, C. Petrovic The Phase diagram of field-tuned quantum criticality in CeCoIn$_5$ was characterized by comparative electrical and thermal conductivity measurements. Main two findings for $J \parallel a$ are: (1) finite characteristic temperature of spin fluctuations, $T_{SF}$, terminates linear in $T$ resistivity observed at high temperatures; (2) the Wiedemann-Franz law is obeyed in $T \to 0$ limit, so that the integrity of quasiparticles is preserved, even though the standard $T^2$ Fermi-liquid temperature dependence of resistivity fails. A comparison with inter-plane transport revealed pronounced anisotropy of the phenomenology. 1. J. Paglione, et al. PRL 97, 106606 (2006). [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U16.00008: Angular dependent magnetorestriction study of CeCoIn$_{5}$ Catalin Martin, Victor. F. Correa, Kenneth M. Purcell, Timothy P. Murphy, Eric C. Palm, Stanley W. Tozer, George M. Schmiedeshoff, J. C. Cooley Using a capacitive dilatometer designed to operate on the rotating stage of a top loading dilution refrigerator probe, we measured the magnetorestriction $\left( L(H)-L(0)\right)/L(0)$, of the Heavy Fermion Superconductor CeCoIn$_5$, along all three crystallographic direction, and with the magnetic field applied parallel both to the superconducting \textit{ab}-planes and to the \textit{c}-axis. When $B||\textit{ab}$, the \textit{c}-axis magnetorestriction features, besides a sharp jump at the upper critical field, a continuous, second-order like, transition at a lower field, which is consistent with previous observations from specific heat measurements. Our results are consistent with a field induced modulation of the superconducting order parameter (FFLO state), rather than with a field induced magnetic ordering. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U16.00009: Metamagnetism in CeIrIn5: magnetoresistance and dHvA study C. Capan, R.G. Goodrich, J.F. DiTusa, L. Balicas, T.P. Murphy, E.C. Palm, S.W. Tozer, R. Movshovich, E.D. Bauer, M.F. Hundley, J.L. Sarrao, J.D. Thompson, D. Hall Quantum phase transitions correspond to a continuous ground state transformation at T=0 driven by quantum fluctuations. A Fermi Surface change might be expected at a quantum critical point. The metamagnetic transition, corresponding to a non-linear increase in magnetization of a paramagnet, has been focus of attention since strong deviations from Fermi Liquid theory reported in Sr$_{3}$Ru$_{2}$O$_{7}$ have raised the possibility of a metamagnetic quantum critical end-point. CeIrIn$_{5}$, a heavy fermion compound with a recently discovered metamagnetic transition at high fields, offers yet another playground for such investigations. We report a study of magnetoresistance and de-Haas-van-Alphen effect (dHvA) in CeIrIn5 for magnetic fields up to 45T and in the temperature range 0.03K-1K. We found that the metamagnetic transition is marked by a concomitant drop in the resistivity and in the amplitude of dHvA oscillations as the magnetic field is increased, while the Fermi Surface remains intact. Possible scenarios will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U16.00010: Photoemission in 5f systems - duality and final state effects Tomasz Durakiewicz, John Joyce, Clifford Olson, Eric Bauer, Elzbieta Guziewicz, Martin T. Butterfield, Kevin Graham, John Sarrao, Joe D. Thompson In recent years, the concept of 5f electron duality started to play important role in our understaning of the electronic structure of actinides. With respect to the photoemission spectra, duality may be seen through the existence of two distinct regions in the valence band. First region, usually in the form of one narrow peak, is located near the Fermi level and represents the itinerant part of the spectral function. Second region, comprising a broad peak of major spectral weight is located a few eV below the Fermi level and corresponds to the more localized part of 5f response. The role of 5f duality in establishing electronic structure will be discussed. Aspects of the relevant final-state effects will also be shown. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U16.00011: Probing the Localization Boundary in 5f electron Systems J.J. Joyce, T. Durakiewicz, K.S. Graham, J.M. Wills, E.D. Bauer, J.L. Sarrao, D.P. Moore, L.A. Morales, C.G. Olson, E. Guziewicz, M.T. Butterfield The role of the 5f electrons in the bonding and hybridization for the light actinides is examined. Through use of various types of photoelectron spectroscopy including angle-resolved and resonance techniques, one may define the boundaries for periodic versus central potentials experienced by the outer electrons in actinide materials including UO$_{2}$, PuTe and Pu metal. A comparison with various computational approaches sheds light on the limits of actinide separation in the lattice and bonding to ligand components before one converges on magnetic configurations for materials. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U16.00012: ARPES Study on Ca$_{1.8}$Sr$_{0.2}$RuO$_{4}$ Madhab Neupane, P. Richard, Y. Xu, A.K.P. Shekharan, Z. Wang, H. Ding, R. Jin, D. Mandrus, T. Valla, P.D. Johnson The phase diagram of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ shows a rich variety of physical phenomena covering the unconventional superconductor Sr$_{2}$RuO$_{4}$ to antiferromagnetic Mott-insulator Ca$_{2}$RuO$_{4}$. In particular, the x = 0.2 compound is at the boundary between a magnetic metal and a canted antiferromagnetic insulator. It has been argued that the surprisingly large effective mass observed at low temperature in this compound may be due to the Kondo-like coupling between the localized and itinerant Ru 4d valence electrons which coexist in this system. We will report ARPES results on this material, which may shed light to the unusual heavy-fermion behavior in this d-electron system. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U16.00013: Application of Projective QMC to LDA+DMFT calculations for LiV$_2$O$_4$ Ryotaro Arita, Karsten Held, Alexey Lukoyanov, Vladimir Anisimov By means of the LDA+DMFT method (the local density approximation combined with the dynamical mean field theory), we investigate the electronic structure of LiV$_2$O$_4$, for which various heavy-fermion-like behaviors have been observed experimentally. To obtain the spectral function at $T \rightarrow 0$, we employ the projective quantum Monte Carlo method as the solver of the effective impurity problem in DMFT. We show that a sharp peak appears just above the Fermi level at $T \rightarrow 0$, which is consistent with the recent photoemission experiment by Shimoyamada {\it et al.} [Phys. Rev. Lett. {\bf 96} 026403 (2006)] [Preview Abstract] |
Session U17: Polymer Surfaces
Sponsoring Units: DPOLYChair: Vivek Prabhu, National Institute of Standards and Technology
Room: Colorado Convention Center 102
Thursday, March 8, 2007 8:00AM - 8:12AM |
U17.00001: Forces between polyelectrolyte brushes in various ionic environments Matthew Tirrell End-tethered polyelectrolyte layers (``brushes'') shrink monotonically in response to addition of mono-valent salt, which also produces corresponding monotonic changes in the range of the repulsive normal forces exerted by such brushes. High swelling and very low frictional forces have been reported under low salt concentrations. A new pattern of behavior is demonstrated here via surface force measurement on polyelectrolyte brushes in the presence of multi-valent ionic interactions, introduced via tri-valent aluminum cations (Al$^{3+})$ or aggregates of cationic surfactants. Very low concentrations of added Al$^{3+ }$or surfactant produce much stronger shrinkage of the brush than does mono-valent salt. Normal forces become strongly attractive under these circumstances. Multi-valent interactions enable tuning of polyelectrolyte brush structure and properties over a wide range, from compact, stiff and sticky to swollen, soft and repulsive. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U17.00002: Evolution of Polymer Brush Dynamics by X-ray Photon Correlation Spectroscopy. Pinar Akcora, Suresh Narayanan, Pappannan Thiyagarajan, Linda Schadler, Sanat Kumar We studied the dynamics of PS grafted silica particles dispersed in PS homopolymers of two molecular weights (44k and 150k) through x-ray photon correlation spectroscopy measurements. The particle motions were related to the traditional aging process and also to the wetting/dewetting behavior of polymer brushes controlled by changing the molecular weight of the matrix PS homopolymer. By increasing the particle concentration, relaxations slowed down in both wet and dry brushes. At 5{\%} particle concentration, relaxations were independent of the matrix molecular weight. We observed that particle dynamics represented the features of structural relaxation in wet brush case which was not related directly to the aging. Two modes of relaxations were seen at early times (in the first 2 hours) for 5{\%} particle loading. Slow relaxations were seen at low q's where tau dependency of q was 0.8. Particles that were arranged at closer distances (below average interparticle spacing) showed diffusive motions. In the dry brush, particle motions were found to be ballistic similarly observed in the pure brush case. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U17.00003: Nanoparticle decoration overlayer for producing a surface enhanced Raman scattering spectrum of a pre-existing polymer surface. Bettina Roan, Thomas Furtak A new method was used for obtaining a surface enhanced Raman scattering (SERS) spectrum of a polymer surface. SERS reactive silver nanoparticles were deposited directly onto the surface of the polymer. This differs from conventional methods of producing SERS-active samples where the analyte polymer is deposited onto a SERS reactive metal nanoparticle substrate, an approach that can only give a SERS spectrum of the buried metal-polymer interface. The new technique makes it possible to study the outermost, pre-existing surface of the polymer. We obtained a SERS spectrum of the external surface of poly(methyl methacrylate) (PMMA) using this method. The vacuum evaporation parameters of the silver nanoparticles were adjusted to optimize the SERS intensity of PMMA. We demonstrate the utility of this technique by measurements on surface-modified polymers. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U17.00004: Dewetting of a Polymer Melt on a Chemically Identical Brush Ophelia Tsui, Xueyun Zhang The use of an end-grafted polymer or oligomer layer has been a popular means of engineering surfaces with designable wetting properties. We investigated the factors governing the equilibrium states of the autophobic polystyrene/polystyrene brush/SiO$_{2}$/Si model system, namely the ratio of the molecular weight of the homopolymer to that of the brush, $r$, and the grafting density of the brush, $\sigma $. We found that the wettability of the polymer melt film on the brush -- as measured by the residual thickness, $d$, of the final dewetted film -- decreased monotonically with increasing $r$ or $\sigma $. The experimentally determined form of the $d(r$,$\sigma )$ curves and the polymer-brush interfacial tensions can be described reasonably well by the self-consistent field theory. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U17.00005: Probing Molecular Mobility at the PNIPAM Brush Surface Jiang Zhao, Wei Wang, Shengqin Wang Diffusion of single polyelectrolyte molecules, poly 2- vinylpyrindine (P2VP), on N-isopropylacrylamide (PNIPAM) brush was studied by single molecule fluorescence techniques. The dependence of the surface diffusivity on varying temperature and brush thickness was studied. We observed a strong dependence of the surface diffusivity on the brush thickness while a minor dependence on the temperature. We attribute these effects to the surface mobility of the adsorption sites of the P2VP to the PNIPAM brush surface. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U17.00006: Influence of entropic configurational effects on the surface tension of symmetric star polymers Zhenyu Qian, Venkatachala Minnikanti, Lynden Archer, Bryan Sauer Surface tension of symmetric 4 and 11 arm polystyrenes have been measured for a series of different molecular weights. We observe that entropic effects contribute significantly to the variation of surface tension as a function of molecular weight. Minnikanti and Archer have developed an expression that relates the surface tension of a symmetric star polymer with its molecular weight. To determine the dependence of surface tension on the reciprocal of molecular weight, the number of arms and dimensionless attraction of the ends and branched point towards the surface play a significant role. We estimated the entropic attraction of the ends and joints from independent self consistent field simulations of a polymer on a lattice. The predicted variations in surface tensions due to entropic reasons were found to constitute a large part of the experimentally observed surface tension variation with molecular weights for 4 and 11 arm stars. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U17.00007: Contact Properties of Surface Modified Elastomeric Membranes and the Recognition of Specific Interactions David A. Brass, Kenneth R. Shull Specific interactions (i.e. Biotin/Avidin, etc.) between modified brushes on both a thin elastomeric membrane and a gold coated quartz crystal surface are investigated. To quantify the effect of different end groups, the interactions between opposing unfunctionalized brushes or between an unfunctionalized brush and a substrate were first analyzed. The quartz crystal resonator provides a measured distance between the membrane and quartz surfaces. The surface of the quartz is modified with poly(ethylene glycol) (PEG) molecules with thiol end groups that bond to the gold electrode surface. The membrane surface is modified through the use of Langmuir layers of amphiphilic molecules, producing PEG brushes that extend into the aqueous environment. Self consistent field theory and models of the acoustic wave propagation were used to establish the sensitivity of the technique to the specific interactions arriving from the end groups. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U17.00008: Photoresponsive Polymer Surfaces Spiros H. Anastasiadis, M.I. Lygeraki, K. Lakiotaki, M. Varda, A. Athanassiou, M. Farsari, C. Fotakis Photochromic spiropyran molecules are utilized as additives for the development of polymer surfaces whose wetting characteristics can reversibly respond to irradiation with laser beams of properly chosen photon energy. The hydrophilicity is enhanced upon UV laser irradiation since the embedded non-polar spiropyran molecules convert to their polar merocyanine isomers, which is reversed upon green laser irradiation. Micropatterning of the photochromic-polymer films using soft lithography or photo-polymerization techniques affects their wettability towards a more hydrophobic or more hydrophilic behavior depending on the dimensions of the patterned features and on the hydrophilicity-hydrophobicity of the flat surface. The light-induced wettability variations of the structured surfaces are enhanced by up to a factor of three as compared to those on the flat surfaces. This enhancement is attributed to the photoinduced reversible volume changes to the imprinted gratings, which additionally contribute to the wettability changes due to the light-induced photochromic interconversions. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U17.00009: Learning from the Venus Flytrap: A Biomimetic Responsive Interface Douglas P. Holmes, Alfred J. Crosby The ability to have controllable and fast property changes to a material surface over large length scales is desirable for a variety of functions including chemical sensors, antimicrobial devices, ``smart'' adhesives, and drug delivery coatings. Recently, many strategies, including shape memory and environment selectivity, have been developed for synthesizing responsive surfaces, but the response times are often too slow or non-sensitive. Here, we present a new strategy for responsive surfaces or interfaces that builds upon the response mechanism of the Venus flytrap, which exhibits one of the most rapid movements in the plant kingdom. This rapid, controllable movement is largely attributed to the geometry of the leaflets, which can undergo a snap-through elastic instability upon development of a critical pressure. Learning from nature, we have made a responsive interface with controlled, elastic instabilities capable of large geometric changes across very short timescales. We present the fabrication of a responsive surface of microlenses with a controlled, predictable geometry that undergoes a rapid snap-through transition when triggered by various stimuli. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U17.00010: ``Smart'' Surfaces of Diblock Copolymer Brushes Dong Meng, Qiang Wang We use a self-consistent field (SCF) theory to study the morphological response of diblock copolymers grafted to a planar substrate. One of the two blocks can carry weakly dissociating charges (e.g., poly(acrylic acid)), making the surface responsive to the solution pH, ionic strength and applied electric field, in addition to the solvent selectivity. Three-dimensional, parallel SCF calculations are performed in real space with high accuracy to identify the thermodynamically stable morphology under a given set of controlling parameters. The internal structure and surface switchability of the copolymer brushes are studied in detail as a function of chain length, volume fraction of the two blocks and chain-grafting density. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U17.00011: Polymers containing azobenzene as photo-mechanical materials. Christopher Barrett Polymers containing Azobenzene have received much interest as photo-reversible materials for a variety of optical and photonic applications. Most recently however, Azo Polymers have also been shown to respond physically and mechanically to light, to act as all-optical patterning materials, and photo-mechanical devices. In particular, a photo-induced pressure in soft amorphous thin films of azo polymers can lead to the facile inscription of efficient surface relief gratings (SRGs) upon irradiation with an interference pattern. Irradiation with CW light is also shown to lead to a reversible photo-expansion of these films, allowing the materials to function as photo-mechanical switches or light-actuators. New azo polymers to optimize this effect with be presented, and some simple macroscopic devices will be demonstrated that take mechanical advantage of this effect for larger scale motion driven by light. The mechanism for this effect will be discussed from studies using ellipsometry, surface plasmon resonance spectroscopy, and neutron reflectometry. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U17.00012: Light-directed Control of Macromolecule Organization on a Surface Gregory Carroll, Jeffrey Koberstein, Nicholas Turro This report describes a versatile method to photo-generate and control self-organized polymer patterns on a surface within a larger pattern. Thin polymer films are cross-linked by irradiation with UV light. Crosslinked thin polymer films resist dewetting when heated above the glass transition temperature. Combining pattern formation via instability with pattern formation via photolithography allows the dewetting patterns to be localized to specific areas of a surface, resulting in a self-organized pattern within a light-directed pattern. By confining the uncrosslinked polymer to an area that approaches the size of the equilibrium dewetting morphology, new mesoscopic features result. For thicker films, the polymer organizes into ribbons at the interface between crosslinked and uncrosslinked polymer. When the width of the uncrosslinked area is large enough, droplets form between the ribbons. As the width gets larger, droplet organization evolves from incomplete to complete polygons. In addition, the structure of the dewetting morphologies changes as the thickness of the film changes. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U17.00013: High-speed, sub-15 nm feature size thermochemical nanolithography Elisa Riedo, Robert Szoszkiewicz, Takashi Okada, Simon Jones, Tai-De Li, William King, Seth Marder The past decade has witnessed an explosion of techniques used to pattern materials on the nano and submicrometer scale, driven by a diversity of applications, such as molecular electronics, data storage, optoelectronics, displays, and all forms of sensors. However, there are many challenges to conventional techniques as they are approaching their fundamental size limit. Here we report a nanolithography technique that allows simultaneous direct control of the local chemistry and topography of thin polymer films. Specifically, a heated atomic force microscope tip can write sub-15 nanometer hydrophilic features over a hydrophobic polymer at the rate of 1.4 millimeters per second. This method is simple, direct, extremely rapid, achievable in a range of environments, and easily adaptable to other materials systems. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U17.00014: In-Plane Ordering in Diblock Copolymer Brushes. Bulent Akgun, Gokce Ugur, William J. Brittain, Mark D. Foster, Xuefa Li, Jin Wang Internal and surface structures of polystyrene-b-polyacrylate and polyacrylate-b-polystyrene diblock copolymer brushes have been studied using grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM). Each asymmetric, as-deposited diblock brush that contains a poly(methyl acrylate) (PMA) block shows an in-plane structure with a spacing comparable to the PMA layer thickness. The correlation length of the in-plane ordering is about the nearest neighbor distance and grows with annealing at 180$^{\circ}$ C. After a brush is treated with a solvent selective for the bottom block, Bragg rods appear in the GISAXS pattern. The lateral spacing corresponding to the Bragg rods is on the order of the brush total thickness. This lateral correlation is also detected by power spectral density analysis of AFM images of the samples' surfaces. The Bragg rods disappear upon heating to 80$^{\circ}$ C. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U17.00015: ABSTRACT WITHDRAWN |
Session U18: Spectroscopy and Dynamics of Single Molecules and Nanoparticles
Sponsoring Units: DCPChair: David Nesbitt, University of Colorado
Room: Colorado Convention Center 103
Thursday, March 8, 2007 8:00AM - 8:12AM |
U18.00001: Single-Molecule Spectroscopic Investigations of RNA Structural Dynamics Julie L. Fiore, David J. Nesbitt To function properly, catalytic RNAs (ribozymes) fold into specific three-dimensional shapes stabilized by multiple tertiary interactions. However, only limited information is available on the contributions of individual tertiary contacts to RNA conformational dynamics. The Tetrahymena ribozymes's P4--P6 domain forms a hinged, ``candy-cane'' structure with parallel helices clamped by two motifs, the GAAA tetraloop-tetraloop receptor and adenosine (A)-rich bulge--P4 helix interactions. Previously, we characterized RNA folding due to a tetraloop-receptor interaction. In this study, we employ time-resolved single-molecule FRET methods to probe A-rich bulge induced structural dynamics. Specifically, fluorescently labeled RNA constructs excited by a pulsed 532 nm laser are detected in the confocal region of an inverted microscope, with each photon sorted by arrival time, color and polarization. We resolve the kinetic dependence of A-rich bulge-P4 helix docking/undocking on cationic environment (e.g. Na$^{+}$ and Mg$^{2+}$ concentration.) At saturating [Mg$^{2+}$], the docked structure appears only weakly stabilized, while only 50{\%} of the molecules exhibit efficient folding. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U18.00002: Addressed Grids for Single-Nanoparticle Studies W.D. Tennyson, C.E. Allen, D.S. Hartnett, M.E. Curtis, A.R. Dedigama, D.J. Wasielewski, M.D. McCutchen, D.H. Dahanayaka, M.B. Johnson, L.A. Bumm We have developed a grid structure with a simple and robust address system to assist in locating and relocating individual substrate-supported nanoparticles. We demonstrate application of our addressed grids for facile characterization of the SAME nanoparticles in multiple instruments. Our grids can be prepared on a variety of substrates using lift-off photolithography. We will show addressed grids of Cr/Au on silicon, fused silica, and ITO coated glass as well as application to multiple measurements of the same nanoparticles by scanning electron microscopy, optical microscopy, atomic force microscopy, and single nanoparticle spectroscopy. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U18.00003: Effect of Environment on Single-Photochromic-Molecule Satoshi Yokojima, Yasutaka Fujiu, Masanori Tachikawa, Jun-Wei Shen, Qi Gao, Paul Tchoupe, Takao Kobayashi, Akinori Murakami, Mitsuru Yoneyama, Katsuya Kanda, Shinichiro Nakamura, Toshikazu Ebisuzaki, Tuyoshi Fukaminato, Masahiro Irie Recent experimental results on the photochromic reactions of diarylethene derivatives at the single-molecule level by using a fluorescence technique [T.~Fukaminato, T.~Sasaki, T.~Kawai, N.~Tamai, and M.~Irie, J.~Am.~Chem.~Soc. {\bf 126} (2004) 14843; M.~Irie, T.~Fukaminato, T.~Sasaki, N.~Tamai, and T.~Kawai, Nature {\bf 420} (2002) 759.] is analyzed by the quantum chemical calculations and the molecular dynamics calculations. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U18.00004: Single-molecule fluorescence quenching near small nanoparticles V. N. Pustovit, T. V. Shahbazyan We study theoretically radiative and nonradiative decay of a single molecule near small gold nanoparticle. The local field enhancement leads to an increased radiative decay rate while the energy transfer from molecule to optically-inactive electronic states in nanoparticle results in a decrease in fluorescence quantum efficiency for small molecule-nanoparticle distances. We performed a DFT-TDLDA calculation of both the enhancement and the quenching for small nanometer-sized gold nanoparticles. We found that in a close proximity to the surface, the non-radiative decay rate is dominated by generation of electron-hole pairs out of the Fermi sea resulting in a significantly lower quantum efficiency as compared to that obtained from electromagnetic calculations. For large distances, the efficiency is maximal for molecule polarized normal to the surface, whereas for small distances it is maximal for parallel orientation. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U18.00005: Atomic-Scale Coupling of Photons to Single-Molecule Junctions Shiwei Wu, Naoki Ogawa, Wilson Ho The study of optical phenomena at the atomic scale is expected to provide new understanding of molecules and their chemical dynamics. The combination of lasers with a low temperature scanning tunneling microscope (STM) has led to the observation of photo-induced resonant tunneling with sub-molecular spatial resolution for single molecules adsorbed on a solid surface [\textit{Science} \textbf{312}, 1362, (2006)]. Furthermore, the irradiation of femtosecond laser pulses into this single-molecule junction defined by STM results in non-linear coupling by two-photon excitation, rather than the single-photon coupling in the case of continuous wave (CW) lasers. These experiments lead to new opportunities by tapping into the unique properties of lasers and the STM. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U18.00006: Energy Spectra of Individual Gold Monolayer Protected Clusters Measured by Single Electron Tunneling Force Microscopy Ning Zheng, Jon Johnson, Gangli Wang, Clayton Williams Monolayer Protected Clusters (MPCs) exhibit strong quantum confinement effects and size dependent electronic, optical and chemical properties. The energy levels of individual gold MPCs (Au$_{38}$ {\&} Au$_{140})$ have been directly measured by Single Electron Tunneling Force Microscopy at room temperature in UHV.$^{[1,2]}$ Single electrons, tunneling between a probe tip and individual gold MPCs are detected using a novel surface potential measurement technique.$^{[2]}$ Tunneling events to and from the MPCs are recorded as a function of the applied bias voltage. A clear electronic spectrum is obtained, showing a HOMO-LUMO gap for Au$_{38}$ but not for Au$_{140}$. For both MPCs the single electron charging energy is measured. Spectral differences from particle to particle are observed. The energy spectra obtained by this method are directly compared with existing electrochemical data,$^{[3]}$ showing good agreement. The methodology will be described and the measured electronic spectra for Au$_{38}$ and Au$_{140}$ will be presented and discussed. 1. E. Bussmann, D. J. Kim, and C.C. Williams, \textbf{\textit{Appl. Phys. Lett. 85, 2538 (2004)}} 2. E. Bussmann, N. Zheng, and C. C. Williams, \textbf{\textit{Nano Lett.; 2006; 6(11)}} 3. Lee, D el at, \textbf{\textit{J. Am. Chem. Soc. 2004, 126, 6193}} [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U18.00007: On the reactivity of neutral metal oxide clusters in the gas phase: Detection through 118 nm single photon ionization Sheng-Gui He, Yan Xie, Elliot Bernstein Single photon ionization by a vacuum ultra-violet (VUV, 118 nm) laser is successfully employed for the study of reactions of \textit{neutral} metal oxide clusters (Ti$_{m}$O$_{n}$, Fe$_{m}$O$_{n}$, Co$_{m}$O$_{n})$ with various simple molecules (CO, NO, SO$_{2}$, H$_{2}$O) in the gas phase. Neutral clusters are generated by reaction of laser ablation generated metal plasma with O$_{2}$ in a supersonic expansion. Clusters are reacted with reactant gases in a flow tube reactor. Detection of neutral clusters and products is through ionization with 118 nm laser radiation and time of flight mass spectroscopy. Rich neutral cluster chemistry is observed: (1) Ti$_{m}$O$_{2m}$ and Ti$_{m}$O$_{2m+1}$ absorb one or more H$_{2}$O molecules for $m \quad \ge $ 2 and $m \ge $ 1, respectively; (2) FeO$_{2}$, FeO$_{3}$, and possibly FeO are reactive with CO while Fe$_{2}$O$_{4}$ and Fe$_{2}$O$_{5}$ are less reactive; (3) Fe$_{2}$O$_{5}$ is reactive with NO and SO$_{2}$, but FeO$_{2}$ is much less reactive with them; and (4) small Co$_{m}$O$_{n}$ clusters ($m \quad \le $ 4 and $n \quad \le $ 6) are more reactive than large clusters (6 $\le \quad m \quad \le $ 12 and 8 $\le \quad n \le $ 17) with CO, and among these small clusters, Co$_{3}$O$_{4}$ is particularly reactive. A detailed quantum chemistry study of Fe$_{m}$O$_{n}$ reactions with CO is in progress. Preliminary calculations indicate that reactions of FeO and FeO$_{2}$ with CO to produce CO$_{2}$ are overall barrierless, in agreement with the experimental observations. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U18.00008: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U18.00009: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U18.00010: Structures and energetics of hydrocarbon molecules in the full hydrogen chemical potential range Yongxin Yao, Tzu-Liang Chan, Cai-Zhuang Wang, Kai-Ming Ho Hydrocarbons, especially polycyclic aromatic hydrocarbons (PAH), attract much interest as candidates for the unidentified infrared bands (UIRs), UV extinction curve and diffuse interstellar bands in the interstellar medium. While many experiments and quantum chemical calculations of the infrared spectra on human-selected hydrocarbons have been done, it remains an open question. A key reason may be the complicated phase space of hydrocarbons originated from the enormous bonding ability of carbon. Here we present a series of unbiased global search for hydrocarbon molecules (CnHm, 1$<$n$<$50) in the full hydrogen chemical potential range. The search is based on Genetic Algorithm (GA) combined with Brenner's hydrocarbon empirical potential and our new tight-binding potential. The structures and energies from GA are checked at both DFT and MP2 level. Such a mini database is reported, which is supposed to give more helpful information on hydrocarbon-related researches. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U18.00011: Structural evolution of Au nanoclusters: From planar to cage to tubular motifs Bokwon Yoon, Xiaopeng Xing, Uzi Landman, Joel H. Parks The evolution of structural motifs of gold cluster anions, Au$_{n}^{-}$, in the size range $n=11-24$ has been determined through a comparison of electron diffraction data with density functional calculations. The results provide clear evidence for a transformation from planar to three-dimensional structures in the range $n=12-14$, the development of cage structures for $n=16$ and 17, the appearance of a tetrahedral structure at $n=20$, and the emergence of a highly symmetric tubular structure for $n=24$. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U18.00012: Bulk Quantities of Noble Gas Nanoclusters with Five-Fold Symmetry Stabilized in Superfulid Helium V. Kiryukhin, E.P. Bernard, V.V. Khmelenko, R.E. Boltnev, D.M. Lee, N.V. Krainyukova Bulk quantities (volume$\sim$cm$^3$, atomic density$\sim$10$^ {19}$-10$^{20}$ cm$^{-3}$) of noble gas nanoclusters (size$\sim$5-6 nm) were produced in superfluid helium by injection technique. X-ray diffraction measurements show that the samples consist of weakly interacting nanoclusters with five-fold symmetry axes, such as icosahedra and decahedra. These results open new opportunities for fundamental research of nanoclusters of noble gases and other materials in well-controlled environments using experimental techniques requiring bulk samples. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U18.00013: Support-induced Catalytic Activity of Gold Nanocluster Chun Zhang, Bokwon Yoon, Uzi Landman The catalytic activity of gold nanoclusters supported on metal-oxide surfaces is a topic of active research efforts. Recently, a dimensionality crossover of gold clusters, adsorbed on a metal-supported thin film of MgO(100), has been predicted$^{1}$. We present here a first- principles study of the catalytic activity of a planer Au$_{20}$ cluster on two- layer MgO(100) film supported by a Mo surface. Both Langmuir-Hinshelwood (LH) and Eley--Rideal (ER) mechanisms of CO oxidation are investigated. The barrier of the LH mechanism is found to be 0.15 eV. For the ER mechanism, the barrier depends on the direction of approach of the CO molecule to the preadsorbed oxygen molecule, varying between a vanishing barrier height and 0.2 eV. Charge transfer from the Mo surface to the cluster supported on the thin MgO(100) film plays a key role in the catalyzed CO oxidation process. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U18.00014: The Electronic Structure of Diamondoids: When does a Molecule Become a Nanoparticle? Trevor Willey, Christoph Bostedt, T. Moller, J. E. Dahl, S. G. Liu, R. M. K. Carlson, T. van Buuren, R. W. Meulenberg, E. Nelson, L. J. Terminello Methane and diamond are common materials with well-known, but extremely different properties. Fundamental, compelling questions then arise: with sp$^{3}$ bound carbon molecules/clusters, at what sizes do diamond-like properties emerge? Pure, defect-free, perfectly hydrogen-terminated diamondoids, from $\sim $0.5 nm to $\sim $1 nm, bridge the gap between molecules and nanoparticles. Furthermore, experiments in the gas phase eliminate particle-particle interaction and ensure measurement of pure, pristine, and undamaged molecules. X-ray absorption probes the unoccupied electronic states; the carbon K-edge reveals rich electronic structure in the series methane, cyclohexane, adamantane, diamantane, through to hexamantane. Diamondoids show the emergence of a diamond-like band structure. Using x-ray absorption and soft x-ray emission, we will compare our experimental LUMO/conduction band and HOMO/valence band positions with predicted changes in HOMO-LUMO gap from several calculations. [Preview Abstract] |
Session U19: Focus Session: Ultrafast Dynamics using X-rays and Electrons I
Sponsoring Units: DCPChair: Dwayne Miller, University of Toronto
Room: Colorado Convention Center 104
Thursday, March 8, 2007 8:00AM - 8:36AM |
U19.00001: Ultrafast X-ray Science at SLAC and LCLS Invited Speaker: Hard x-rays (E greater than 1 keV) can probe the structure of matter on the length scale of a chemical bond. Ultrafast lasers (t less than 1 ps) can capture the quantum dynamics of single vibration in a crystal lattice or in a molecule, and they have also been used to view the transient molecular-scale transformations of chemical reactions. Until recently, only laser-induced plasma radiation was capable of capturing these ultrafast dynamics and also viewing them on the scale of a single chemical bond. The recent Sub-Picosecond Pulse Source experiment at SLAC was the first instrument based on synchrotron radiation from an undulator that could do both. During its two-year run, its 8 keV, 80 fs x-ray pulses were the brightest ultrafast x-rays in the world. This is just the beginning. The planned X-ray free electron laser at SLAC (LCLS) will generate focused x-ray fields as strong as atomic binding fields, comparable to today' highest intensity lasers. These new tools are creating some special opportunities for new science, and also some challenges. I will discuss these, and present recent progress in ultrafast x-ray sources and science. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U19.00002: Ultrafast lattice dynamics in laser-excited solids probed with femtosecond X-ray diffraction Invited Speaker: Ultrafast pulsed excitation of solids provides a unique way of depositing energy into materials and to create states of strong electronic excitation and high temperature and pressure. With the initial deposition of energy a complex chain of elementary physical processes is triggered which can lead to structural changes on very rapid time-scales, and often along unusual, non-equilibrium pathways. Due to the unique combination of \textit{atomic-scale} spatial and temporal resolution, the recent progress in the development of ultrafast X-ray sources has provided new opportunities for studying such processes. This talk will discuss our recent work on ultrafast time-resolved X-ray diffraction using laser-driven as well as accelerator-based femtosecond X-ray sources. Examples include the non-thermal melting transition in semiconductors, the direct observation of large-amplitude coherent optical phonons, and studies of the energy relaxation in optically excited solids through measurements of the transient \textit{Debye-Waller} effect. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U19.00003: Non-Thermal Liquid Formation Dynamics Studied with Ultrafast Diffuse X-Ray Scattering Kelly Gaffney, Christian Blome, Simon Engemann, David Fritz, Patrick Hillyard, Jorgen Larsson, Aaron Lindenberg, Matthieu Nicoul, David Reis, Klaus Sokolowski-Tinten, Jerry Hastings The ultrafast melting dynamics of a laser excited semiconductor crystal have been studied with femtosecond x-ray scattering. We have used diffuse x-ray scattering to determine that a liquid structure appears within 2 ps of laser excitation. This structure preserves the density of the crystal, and can be well fit with a hard sphere structure factor, unlike equilibrium liquid InSb. At a delay time of 100 ps, an under-dense liquid structure forms with large amplitude scattering at intermediate momentum transfer. A concurrent rise in small angle scattering intensity suggests that voids form in this under-dense liquid. Cooling and contraction leads to the formation of a dense liquid structure on the ns time scale distinct from that of the equilibrium liquid InSb. The equilibrium liquid structure does not appear until delay times of 20 ns and longer. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U19.00004: Laser-induced phonon-phonon interaction in bismuth Martin Garcia, Eeuwe Zijlstra We demonstrate that the coupling between laser-induced coherent phonons in bismuth leads to the appearance of mixing signals in the isotropic reflectivity. As a consequence modes that cannot usually be detected by means of the isotropic reflectivity show up. We further demonstrate that this interaction is strongly dependent on the laser fluence and is for that reason only observable when sufficiently intense laser pulses are used. In addition, we demonstrate that the coupling between phonons of the same symmetry leads to the appearance of higher harmonics in the isotropic reflectivity. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U19.00005: Magnetic field effects on ultrafast lattice compression dynamics of Si(111) crystal when excited by linearly-polarized femtosecond laser pulses Koji Hatanaka, Hideho Odaka, Kimitoshi Ono, Hiroshi Fukumura Time-resolved X-ray diffraction measurements of Si (111) single crystal are performed when excited by linearly-polarized femtosecond laser pulses (780 nm, 260 fs, negatively-chirped, 1 kHz) under a magnetic field (0.47 T). Laser fluence on the sample surface is 40 mJ/cm$^{2}$, which is enough lower than the ablation threshold at 200 mJ/cm$^{2}$. Probing X-ray pulses of iron characteristic X-ray lines at 0.193604 and 0.193998 nm are generated by focusing femtosecond laser pulses onto audio-cassette tapes in air. Linearly-polarized femtosecond laser pulse irradiation onto Si(111) crystal surface induces transient lattice compression in the picosecond time range, which is confirmed by transient angle shift of X-ray diffraction to higher angles. Little difference of compression dynamics is observed when the laser polarization is changed from p to s-pol. without a magnetic field. On the other hand, under a magnetic field, the lattice compression dynamics changes when the laser is p-polarized which is vertical to the magnetic field vector. These results may be assigned to photo-carrier formation and energy-band distortion. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U19.00006: X-ray studies of acoustic vibrations from semiconductor superlattices. Mariano Trigo, Yu-Miin Sheu, David Reis, Roberto Merlin, Matthew Reason, Rachel Goldman We present ultrafast X-ray studies of acoustic phonons transmitted from a GaAs/AlAs superlattice. An ultrafast laser pulse impulsively excites coherent acoustic waves in the superlattice which subsequently transmit into the GaAs substrate. A short x-ray pulse can be used to probe the wave packet traveling in the bulk material, without the need of a transducer such as a second SL by detecting sidebands of Bragg diffraction. Unlike optical probes, the short wavelength of the x-rays allows momentum resolved detection over a wide range of wavevectors. This method should in principle be able to detect the whole spectrum of the generated excitations. Furthermore, the coherent part of the excitation is followed by a much slower thermal diffusion which, as we will show, can also be studied by time resolved x-ray scattering. [1] R. Merlin, Solid State Comm. 102, 207 (1997). [2] D. A. Reis et al., Phys. Rev. Lett. 86, 3072 (2001) [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U19.00007: Ultrafast X-Ray Diffraction Study of Potential Energy Surface Evolution in InSb Under Intense Laser Excitation Patrick Hillyard, Kelly Gaffney, Aaron Lindenberg, Simon Engemann, David Reis, Aniruddha Deb, Drew Meyer, Jerome Hastings Ultrafast time-resolved x-ray diffraction has been used to directly monitor atomic disordering in InSb as a function of carrier density. The carrier dependent curvature of the potential energy surface has been determined from the time evolution of the atomic structure. Three regimes have been identified. At low carrier densities, atomic disordering occurs via a thermal mechanism with an exponential time constant determined by the electron-phonon coupling constant. Upon increasing excited carriers to roughly 5{\%} of the valence band electron population, a sharp transition is observed and the predominant disordering mechanism is inertial motion on a softened potential energy surface with a Gaussian time constant of $\sim $400 fs. For a carrier density above $\sim $20{\%}, accelerated atomic motion on an inverted potential energy surface is observed. This inverted regime was previously predicted by theory but had been unobserved until now. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U19.00008: Transient Lattice Deformation in Laser-Irradiated Semiconductor Studied by Picosecond Time-Resolved X-ray Diffraction Kazutaka Nakamura, Hiroaki KIshimura, Yoichiro Hironaka, Ken-ichi Kondo, Toshiyuki Atoh The transient lattice behavior of Si(111) single crystal under 300-ps laser irradiation has been studied by using picosecond time-resolved X-ray diffraction. When the laser is irradiated, the rocking curves of the laser-irradiated Si(111) have a higher-angle-shifted component due to lattice compression by laser ablation. The maximum lattice strain is estimated at 5.6 \%, which is larger than the Hugoniot elastic limit for Si (111). After 1000 ps, a broadening of the main peak is observed. In addition, the rocking curve of the recovered sample is clearly broader than that of a pristine sample. Reciprocal space mapping for the recovered sample shows that the lattice spacing of the recovered sample does not change from that of the pristine sample, whereas lattice planes are misoriented. The results of the time-resolved measurement and the assessment of the recovered sample indicate that mosaic blocks with inclined orientations are induced by laer-driven elastic compression and the subsequent pressure release within 1000 ps, rather than plastic deformation. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U19.00009: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U19.00010: Ultrafast Photoinduced Solid State Phase Transitions Probed by Femtosecond Electron Diffraction R. Ernstorfer, M. Harb, C.T. Hebeisen, T. Dartigalongue, R.E. Jordan, G. Sciaini, R.J.D. Miller Femtosecond Electron Diffraction harbors great potential for providing atomic resolution of structural changes as they occur, essentially watching atoms move in real time. It combines temporal resolution on the hundreds of femtoseconds scale -- a time scale typically only accessible by time-resolved optical spectroscopy -- with real-space structural information on the atomic scale. We applied this technique to study the structural response of thin free-standing metal and semiconductor [1] films upon ultrafast electronic photo-excitation within a wide range of excitation levels. These studies distinguish the different mechanisms, thermal vs. non-thermal, of energy transfer from electronic to vibrational degrees of freedom resulting in different melting mechanisms for both classes of materials. In addition, we discuss a technique we recently established to determine the duration of the electron pulses by using the ponderomotive force of an intense femtosecond laser pulse to sequentially scatter parts of the electron pulse and found the electron pulse duration to be about 400 fs [2]. [1] M. Harb \textit{et.al.}, \textit{J. Phys. Chem. B}, in print. [2] C.T. Hebeisen \textit{et al.}, \textit{Opt. Lett} \textbf{31}$,$ 3517 (2006). [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U19.00011: Ultrafast Time-resolved Electron Diffraction with Megavolt Electron Beams Jerome Hastings, Fedor Rudakov, David Dowell, John Schmerge, Stephen Gierman, Peter Weber An rf photocathode electron gun is used as an electron source for ultrafast time-resolved pump-probe electron diffraction. We observed single-shot diffraction patterns from a 160 nm Al foil using the 5.4 MeV electron beam from the Gun Test Facility at the Stanford Linear Accelerator. Excellent agreement with simulations suggests that single-shot diffraction experiments with a time resolution approaching 100 fs are possible. Details of the measurements and applications will be discussed [Preview Abstract] |
Session U20: History of Physics; General Physics
Sponsoring Units: FHPChair: Catherine Westfall, Argonne National Laboratory
Room: Colorado Convention Center 105
Thursday, March 8, 2007 8:00AM - 8:24AM |
U20.00001: Compartmentalization of Science, Power and Social Responsibility as exemplified in the life of J. Robert Oppenheimer. Willem Van De Merwe, Todd Ream Many biographies of J. Robert Oppenheimer have recently been published; each emphasizing some different aspects of his life. Physicists can learn much about physics in the early 1900s and about the practice of physics in society from these biographies. Oppenheimer, the ``father of the atomic bomb,'' seems to have struggled early in life with finding a framework for understanding himself and for finding guidance for making responsible decisions. In this paper, we will briefly consider his upbringing in the Ethical Cultural School, his studies in physics in Europe, passion for poetry, including the influence of the Bhagavad-Gita, and his initial sympathizing with left-wing political groups. In this context, we will consider whether a quality liberal arts education might help physics students formulate their framework to guide them throughout the course of their career in science. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:48AM |
U20.00002: The English Revision of The Blegdamsvej Faust Karen Keck At the 1932 meeting of quantum physicists at Niels Bohr's Copenhagen Institute, participants staged an updated version of Goethe's Faust with Pauli tempting Ehrenfest to accept a chargeless, massless particle, then called the neutron. The most widely read translation of the anonymous Faust: Eine Historie appears in George Gamow's Thirty Years that Shook Physics; his second wife, Barbara, translated the text. Her work masterfully communicates the parallels between Goethe's original and the anonymous parody, but it also rearranges and adds to the parody to strengthen those similarities and to reflect George Gamow's views. The changes emphasize the international and cooperative aspects of physics. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:12AM |
U20.00003: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 9:12AM - 9:36AM |
U20.00004: A 18$^{th}$ century thermometer recipe: The begin of experimental physics courses in Guadalajara, M\'{e}xico? Durruty Jesus de Alba Martinez As a part of the Special Funds Collection of the Jalisco's State Public Library ``Juan Jos\'{e} Arreola'' is a physics course manuscript attributed to Francisco Javier Clavigero s.j. (1731-1787), teacher at the Jesuit \textit{Colegio de Santo Tom\'{a}s} (a college-level institution in Guadalajara before the university opening), inside of the vellum bounded volume is a unbounded folio containing instructions on how to build a thermometer. In this work are discussed some evidences of the belonging of such folio to the manuscript in spite of their differences (it is written in Spanish not in Latin as the whole), we also describe the process to construct the thermometer and how could be the experimental part of the physics course. Also is briefly exposed the importance of the educational role of Clavigero as a builder of the concept of \textit{mexicanity}. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 10:00AM |
U20.00005: Historical perspectives on respiratory fluid dynamics and flow phenomena deep in the lung Josue Sznitman, Akira Tsuda Next year marks 30 years since the first review on pulmonary fluid dynamics was published (TJ Pedley, \textit{Ann Rev Fluid Mech}, 1977). Since the early publications on flow resistance in airways (F Rohrer, Pflugers Arch, 1915), much research has been conducted to deepen our understanding on the role of flow convection in the lung. While many investigations have been aimed at elucidating the nature of airflow in the upper (nose, larynx) and conducting airways (trachea down to the 15$^{th}$ bifurcation generation of the airway tree), comparatively little effort has dealt with airflow in the deeper regions of the lung, characterized by 300 million pulmonary alveoli providing gas exchange with blood. For very long, it has been argued that airflow velocities in the alveolar region are negligible due to a large increase in the total cross-sectional area at that level. This is still reflected today in medical teaching. However, in the last 20 years, new theories have tackled the experimental observation of convective mixing of inhaled particles deep in the lung. These theories suggest that convective airflow in the alveolar region is indeed relevant. In particular, alveolar flows are much more complex than previously thought and may exhibit properties of chaotic flows. Such discoveries have led to a small revolution in our common understanding of respiratory flows deep in the lung. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U20.00006: Sustainability and the Use of Non Renewable Resource, Australia as an Example Albert A. Bartlett A government minister in Australia writes that Australian coal will last ``110 years at present rates of production.'' (1) But production is growing 5\% per year, so the life expectancy will be less than 110 y. Assume production follows a Gaussian Hubbert curve. We can construct a family of curves for the future path of P(t) vs. t, each of which is consistent with the 110 y. This envelope of this family of curves divides the graph of P(t) vs. t into allowed and forbidden areas. The curve with the current value of dP/dt is then the most probable future path of P(t). The curve reaches a maximum and then rapidly declines to zero. Australia's growing population and these Hubbert curves combine to indicate frightening non-sustainability. Sustainability requires a curve of P(t) that declines exponentially with k = (1/110) per y.(2) \newline (1) Ian Macfarlane, World Energy, V.8, 112-117, 2005 \newline (2) A.A. Bartlett, Am.J.Phys., V.54, 398-402, 1986 [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U20.00007: Dynamics of the 1,n compound pendulum John Starrett, John Korbin We analyze the motion of the $1,n$ compound pendulum, that is, a pendulum system with one upper and $n$ lower pendula. In contrast to the more well known $1,1$ pendulum (the double pendulum), the $1,n$ pendulum exhibits an exchange of energy between the lower pendula, which can lead to bursts of over-the-top motion for one or more of the lower pendula as their energy is suddenly pumped up from a lower energy state. The $1,n$ systems can exhibit chaotic dynamics, but as $n \rightarrow \infty$, the motion of the upper pendulum approaches zero and the lower pendula become independent of each other, and the system ceases to be chaotic. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U20.00008: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U20.00009: myADS-arXiv: A fully customized, open access virtual journal Michael Kurtz, Guenther Eichhorn, Edwin Henneken, Alberto Accomazzi, Carolyn Grant, Donna Thompson, Elizabeth Bohlen, Stephen Murray {\bf myADS-arXiv} is a collaboration of the Astrophysics Data System group at the Smithsonian Astrophysical Observatroy and the arXiv group at Cornell University. The {\bf myADS-arXiv} service provides a listing of those articles in physics or astronomy which have been posted in the last week which are of most interest to you. In essence {\bf myADS-arXiv} is a free, weekly, fully customized (for each individual user) open access virtual journal which covers the most important, most recent papers in physics and astronomy. {\bf myADS-arXiv} follows the same format as the existing {\bf myADS-Astronomy} and {\bf myADS-Physics} notification services. These services provide access to the most interesting (to you) recent journal articles in physics and astronomy. These are done in collaboration with nearly every publisher of physics or astronomy journals, including APS, AIP, AAS, RAS, IoP, EDP, World Scientific, Wiley, Springer, and Elsevier. [Preview Abstract] |
Session U21: Focus Session: General Theory: Density Functional Theory and Beyond
Sponsoring Units: DCOMPChair: Eric Shirley, National Institute of Standards and Technology
Room: Colorado Convention Center 106
Thursday, March 8, 2007 8:00AM - 8:36AM |
U21.00001: Berry phase effects on electronic properties Invited Speaker: Geometric phase has found wide applications in physics, and has emerged as a unifying concept in describing and predicting electronic properties in solids. In this talk, I will review recent advances on Berry phase effects in insulators, metals, and semiconductors, with topics covering dielectric, magnetic, and transport properties. I will also discuss non-abealian generalization of the geometric phase concept and applications in spin transport. \newline \newline [1]. D. Xiao, J. Shi, and Q. Niu, Phys. Rev. Lett. \textbf{95}, 137204 (2005); \newline [2]. D. Xiao, Y. Yao, Z. Fang, and Q. Niu, Phys. Rev. Lett. \textbf{97}, 026603 (2006).; \newline [3]. D. Culcer, Y.G. Yao, and Q. Niu, Phys. Rev. B \textbf{72}, 085110 (2005). [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U21.00002: Local and non-local vertex corrections in GW for extended and localized systems Martin Stankovski, Andrew Morris, Benjamin Robinson, Rex Godby, Kris Delaney, Patrick Rinke, Ulf von Barth, Carl-Olof Almbladh, Pablo Garc\'Ia-Gonz\'alez A non-local operator like the self-energy can be consistently calculated through many-body perturbation theory for systems of interacting electrons. This is usually done within the framework of Hedin's $GW$ approximation. If the initial Green's function is obtained within a local approximation like DFT-LDA, there is in principle a local vertex given by the static exchange-correlation kernel in the first iteration (Del Sole \emph{et al.} PRB {\bf 49}, 8024 (1994)). We present total energies and bandwidths for jellium and equivalent quantities for He, Be and Ne. We show that a local vertex implemented in both the screened interaction and the self-energy leads to unphysical results. A local vertex in the screened interaction only provides results on par with or slightly better than standard one-shot $G_ {0}W_{0}$. Finally, we obtain significant improvements by introducing non-local vertex corrections derived from non-local starting aproximations for the self-energy. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U21.00003: GW Calculations Starting from Generalized Kohn-Sham Schemes Frank Fuchs, J{\"u}rgen Furth{\"u}ller, Friedhelm Bechstedt, Maxim Shishkin, Georg Kresse The GW approximation of Hedin is arguably the most successful approach for the calculation of quasi-particle (QP) energies. Its accuracy has been proven for a variety of systems. Usually, the QP eigenvalues are calculated in a perturbative approach, starting from solutions of the Kohn-Sham equations with an exchange-correlation (XC) potential in local density or generalized gradient approximation (LDA/GGA). However, this standard approach fails for a number of systems such as InN with shallow 'semi-core' electrons and a 'negative gap' in LDA/GGA. Here we present $G_0W_0$ calculations for Si, ZnO and InN which start from solutions of the generalized Kohn-Sham (gKS) equations for the screened exchange (sX), HSE03, PBE0 and HF model of exchange and correlation. The calculations are performed in the all-electron wavefunction framework of the PAW method. Starting from a gKS solution is found to yield a positive gap and $d$-band positions close to the experimental values for all the functionals investigated here. Furthermore, with exception of HF the resulting gaps are almost the same for all the gKS functionals choosen as starting point. The results are analysed with respect to the wave-function character and localization. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U21.00004: The GW space-time formalism at finite temperatures Matthieu Verstraete, Christoph Freysoldt, Patrick Rinke, Rex Godby We present the generalization of the space-time formulation of the GW approximation in many-body perturbation theory. The main changes are introduced in the treatment of the imaginary time and frequency dependency of the polarizability, screening, and self-energy. The discrete Matsubara frequencies and finite imaginary time intervals for integration are taken into account. Efficient fitting and interpolation schemes are developed to avoid a large increase in the grid sizes when going to metallic and finite-T systems. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U21.00005: $GW$ study of f-shell systems Mark van Schilfgaarde, Athanasios N. Chantis, Takao Kotani, Robert C. Albers We have applied the recently developed Quasiparticle Self-Consistent GW (QS$GW$) method to rare-earth metals, rare-earth compounds, and actinide metals. Comparison with the available experimental data shows that this method can successfully describe the electronic structure of these materials within a unified ab-initio theoretical framework. Through this approach we can examine the approximations that enter into the LDA+U method and show ways of improving its implementation. Based on the QS$GW$ bands, a discussion of the role of electronic correlations on the band structure of uranium and plutonium will be presented. Finally, we uncover a completely new effect: a first-order metal-insulator transition in GdN, which is driven by dielectric function changes. This result may explain the ambiguities in the experimental data obtained for this material. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U21.00006: Exact-exchange based quasiparticle energy calculations applied to (transition) metal nitrides: ScN, InN and more P. Rinke, M. Scheffler, A. Qteish, J. Neugebauer The transition metal nitride ScN is emerging as a versatile material for promising technological applications, e.g. in spintronics and optoelectronics. Like for InN the electronic band structure of ScN -- a key quantity for devices -- has been difficult to access experimentally (due to growth related problems) and theoretically (due to strong self-interaction effects in the local-density approximation (LDA) to density functional theory (DFT)). Here we show that removing the self-interaction by applying DFT in the exact-exchange optimized effective potential approach (OEPx) correctly predicts ScN and InN to be semiconductors and not (semi)metals as found in the LDA. The OEPx ground state then provides a suitable starting point for quasiparticle energy calculations in the $G_0W_0$ approximation. Our OEPx+$G_0W_0$ gap supports recent experimental observations [1] that ScN has a much lower indirect band gap than previously thought [2]. We further show how a meaningful comparison to LDA based $G_0W_0$ calculations can be constructed that allows us to make contact with self-consistent $GW$ calculations.\\ $[$1$]$ A. A. Al-Brithen {\it et al.}, Phys.\ Rev.\ B {\bf 70}, 045303 (2004)\\ $[$2$]$ P. Rinke {\it et al.}, cond-mat/0611435 [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U21.00007: Electron Correlation Effects And The Electronic Structures of Perovskite Ruthenates SrTi1-xRuxO3 Pao-An Lin, T.Y. Chang, Horng-Tay Jeng, Chen-Shiung Hsue The behavior of the electronic structures of SrTi1-xRuxO3 crystals as the value of x is varied, were studied by ab initio LDA band structure calculation. The roles of on-site Coulomb interaction U were included by carrying out LDA+U calculations. It is found that electron correlation effects play an important role in the electronic structures involving the Ru4d orbitals in the Perovskite ternary Ruthenates SrTi1-xRuxO3. The on-site Coulomb interaction U is needed to describe correctly the metal- insulator transition. Comparing with LSDA calculation (including GGA(generalized gradient correction),the calculated spectrum from LDA+U band structure calculation are in much better agreement with published experimental results such as photoemission (PES) and oxygen 1s X-ray absorption (XAS) spectroscopy for clean surface. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U21.00008: \textit{Ab initio} calculations of intrinsic and extrinsic losses in x-ray spectra J. Kas, M. Prange, J. J. Rehr, L. W. Campbell, J. A. Soininen Typical calculations of x-ray absorption spectra (XAS) include inelastic losses within the quasi-particle approximation, which neglects satellite structure in the spectral function. While satellite effects are relatively small in the EXAFS region, they can be quite large for near edge spectra. Here we present an efficient \textit{ab initio} approach for calculating inelastic losses due to both intrinsic and extrinsic many body interactions, as well as interference between them. The method begins with a real-space multiple-scattering calculation of the dielectric function $\epsilon(\omega)$,\footnote{J.J. Rehr et al., cond-mat/0601241 (2006)} which is then fit to a many-pole model with approximately 10$^2$ poles. This yields a many-pole approximation to GW self-energies, as well as a many-pole spectral function, and estimates for the many-body amplitude factor in XAS. Results for the self-energy agree well with other calculations.\footnote{J.A. Soininen et al., J. Phys. Condens. Matter. 15 (2003)} The approach also gives improved agreement for core-level XAS, especially in the near edge region. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U21.00009: Reciprocal space calculations of EELS and XAS spectra without the supercell Kevin Jorissen, John Rehr Traditionally, one has two ways of calculating \textit{ab initio} x-ray and electron absorption spectra. Band structure calculations can treat periodic materials such as crystals, however, when a core hole is introduced and treated with a supercell approximation, unphysical interactions are often introduced. Real space calculations on the other hand, e.g. based on Green's function theory, can treat the core hole as an impurity, but have the disadvantage of finite cluster size effects. We present a hybrid method in which the perfect crystal is treated in reciprocal space, avoiding cluster size effects, and the inclusion of the core hole is done afterwards in real space, thus avoiding supercell effects. This strategy is implemented into the ab initio Green's function code FEFF8.6, which has recently been adapted for relativistic EELS-spectra. The method is illustrated, for example for the C K-edge of diamond. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U21.00010: Real space, real time approach for linear and non-linear optical response of nano-scale molecules Yoshinari Takimoto, Fernando Vila, John Rehr We present a real time, time-dependent density-functional theory approach for the calculation of the frequency-dependent optical responses, which is based on the approach of Tsolakidis et al. [1] This approach is extended for the calculation of non-linear response. Tensor components of linear polarizabilities and first order hyper-polarizabilities are extracted by fitting the net time dependent polarization with different electric field strengths. This real-space, real-time method is computationally efficient and generic in that it requires no symmetry assumptions. Results are presented for several organic molecules, e.g., FTC and PNA, including both linear and nonlinear response. [1] A. Tsolakidis, D. Sanchez-Portal and R.M. Martin, Phys. Rev. B \textbf{66}, 235416 (2002). [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U21.00011: Assigning Spectral Features in Excitonic Spectra Eric L. Shirley Considerable progress has been made over the last several years in calculations of optical absorption spectra from first principles. This builds on the foundation of accurate electronic band structures, including many-body corrections to band energies, and realistic solution of the couple equation of motion for an electron-hole pair in the excited state. Omitting the electron-hole interaction (or excitonic effects) can severely hamper the accuracy of a spectrum. In narrow-gap semiconductors (Si, Ge, GaAs, etc.), this is a reasonably tolerable situation, in the sense that assigning spectral features can be (and has been) used to establish energies of interband transitions at critical points. In wider gap systems, such as LiF or MgO, the qualitative change in spectral features resulting from electron-hole interactions might call into question assignment of features to interband transitions. This talk presents an attempt to carry out such assignments, nonetheless. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U21.00012: A generalized Sham-Schl\"uter equation: the link between nonlocality and frequency dependence Matteo Gatti, Valerio Olevano, Lucia Reining, Ilya V. Tokatly We present an in principle exact approach to construct effective potentials and kernels for the calculation of electronic spectra. In particular, the potential that yields the spectral function needed to describe photoemission turns out to be dynamical but local and real. Using explicit examples we illustrate how the nonlocality of the physical self-energy is converted into a frequency dependence of the effective potential. We also show that our approach leads to a very short derivation of a kernel that gives a very good description of absorption or energy-loss spectra of a wide range of materials. [Preview Abstract] |
Session U22: Focus Session: Friction
Sponsoring Units: GSNP DMPChair: Elisa Riedo, Georgia Institute of Technology
Room: Colorado Convention Center 108
Thursday, March 8, 2007 8:00AM - 8:36AM |
U22.00001: Fundamental aspects of energy dissipation in friction Invited Speaker: Energy dissipation in friction is mediated by excitation of elementary processes including surface phonons and electronic excitations. These excitations couple through anharmonic interactions or by Frank-Condon nuclear motions to bulk substrate phonons, which ultimately appear as heat. This gives rise to numerous phenomena including friction anisotropy, velocity dependence, and dissipative surface charge motion. Friction anisotropy can appear when phonon modes with specific polarizations are forbidden in particular crystal directions. Electronic excitations have been discussed and investigated but never clearly and definitely identified as primary mechanisms in contact friction. I will discuss these topics using recent experimental results in my laboratory including the large friction anisotropy of Al-Ni-Co decagonal quasicrystals, the role of hydrogen bonding networks in determining the velocity dependence of friction and finally the control of friction by changing the carrier concentration near the surface of p and n semiconductors. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U22.00002: A nanotribology study of self-mated \textit{vs.} unmated interfaces and local packing density effects for octadecyltrichlorosilane monolayers and silicon Erin Flater, W. Robert Ashurst, Robert Carpick We use atomic force microscopy (AFM) to determine the frictional properties of nanoscale single asperity contacts involving octadecyltrichlorosilane (OTS) monolayers and silicon. Quantitative AFM measurements are performed using both uncoated and OTS-coated silicon AFM tips and surfaces. Friction is reduced by the presence of the OTS coating, and the overall shape of the friction \textit{vs. }load plot strikingly depends on whether or not the substrate is coated with OTS, regardless of tip material. Uncoated substrates exhibit the common sublinear dependence, while coated substrates exhibit an unusual superlinear dependence. These results can be explained qualitatively by invoking molecular plowing as a significant contribution the frictional behavior of OTS. Direct \textit{in-situ }comparison of two intrinsic OTS structural phases of otherwise identical molecules on the substrate show that the lower packing density phase exhibits higher friction, decisively observed here in single, uninterrupted images on the same monolayer for the first time. The lateral stiffness of the two OTS structural phases are indistinguishable, which implies that the packing density directly affects the interface's intrinsic resistance to shear as opposed to simply modifying the stiffness of the monolayer. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U22.00003: Dynamical noise and avalanches in quasi-static plastic flow of amorphous solids Ana\"el Lema\^{\i}tre, Christiane Caroli We build a mean-field model of plasticity of amorphous solids, based on the dynamics of an ensemble shear transformation zones, interacting via intrinsic dynamical noise generated by the zone flips themselves. We compare the quasi-static, steady-state properties for two types of noise spectrum: (G) Gaussian; (E) broad distribution derived from quadrupolar elastic interactions. We find that the plastic flow proceeds via avalanches whose scaling properties with system size are highly sensitive to noise tails. Comparison with available data suggests that non-affine strain fields might be of paramount importance in the small systems accessible to molecular simulations. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U22.00004: Dynamics of Phononic Dissipation at the Atomic Scale Haldun Sevincli, Soma Mukhopadhay, R. Tugrul Senger, Salim Ciraci Dynamics of dissipation of a local phonon distribution to the bulk is a key issue in boundary lubrication and friction between sliding surfaces. We consider a highly excited molecule which interacts weakly with the substrate surface. We study different types of coupling and substrates having different types of dimensionality and phonon densities of states. We propose three different methods to solve the dynamics of the combined system, namely the equation of mation technique, Fano-Anderson method and the Green's function method. Using this theoretical framework we present an analysis of transient properties of energy dissipation via phonon discharge at the microscopic level. The methods allow the theoretical calculations to be extended to include any density of states for the substrate including experimental ones and any type of molecule that represent the lubricant or the asperity. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U22.00005: Friction and Viscous Forces in Sub-Nanometer Water Films Tai-De Li, Elisa Riedo Water under nano-confinement is ubiquitous, with examples including clay swelling, aquaporines, ion channels, and water menisci in micro-electrical-mechanical-systems. However, the structural and rheological characteristics of nano-confined pure and ionized water continue to be the subject of discussion and debate. Here, we report an experiment in which an atomic force microscope tip approaches a flat solid surface in purified water, while small lateral oscillations are applied to the tip. Direct measurements of the lateral forces encountered by a nano-size tip approaching a solid surface in purified water are reported for tip-surface distances, 0$\pm $0.03 nm $<$ d $<$ 2 nm. We find that, for hydrophilic surfaces, the dynamic viscosity is measured to grow up orders of magnitude in respect to bulk water, whereas no significant increase in the viscosity has been detected when the confining solid surface is hydrophobic. The origin of the observed different behavior is discussed. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U22.00006: Friction Reduction Using Self-Assembled Hydrogels Michael J. Mackel, Julia A. Kornfield Friction of agarose-based hydrogels against bare glass is examined as a function of added linear polyelectrolyte using a stress rheometer to measure the angular velocity of a clean glass plate against the hydrogel surface as a function of applied torque and normal force. Incorporating linear dextran sulfate into 2 weight percent agarose hydrogel reduces friction on the hydrogel surface. The reduction of friction is a nonmonotonic function of dextran sulfate concentration: a 2 percent doping of dextran sulfate shows the minimum friction. Lubricity enhancement on the agarose doped with 2 percent dextran sulfate occurs at all normal forces examined (0.5, 1, 1.5, and 2 N) and is more pronounced at larger angular velocities. Rheological studies of agarose hydrogels doped with dextran sulfate suggest that the dextran sulfate does not interfere with the porous structure of the hydrogel when present in concentrations of 2 weight percent or less. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U22.00007: High velocity sliding at a compressed Al(111)/Al(100) interface J. E. Hammerberg, R. Ravelo, T.C. Germann, B.L. Holian We discuss high velocity sliding at a compressed Al(111)/Al(100) interface sliding in the $1\overline{1}0$ direction at a pressure of 15 GPa. Three temperatures are considered, T=232, 464 and 696 K. System sizes are $1.4 10^{6}$ atoms .We find that for velocities above a critical velocity, $v_{c}$, the frictional force scales as $(v/v_{c})^{-\beta}$ with $\beta\approx 3/4$. We discuss the temperature and size dependence of $v_{c}$. We find that below $v_{c}$ the frictional force is an increasing function of velocity with an initial linear dependence. Above $v_{c}$ there is a regime of interfacial instability characterized by a (100) transformation front moving into the (111) material. This is followed by a fluid regime for which a Couette flow profile develops at the interface, the thickness of which grows with increasing velocity. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U22.00008: The Dynamics of Precursors to Frictional Sliding Jay Fineberg, Shmuel Rubinstein, Gil Cohen The dynamics of frictional motion are governed by the nature of the interface separating two sliding materials. We report that the spatial profile of the contact-area along an interface is a dynamic quantity which evolves via a discrete sequence of rapid crack-like precursors to overall motion. These precursors, which are generated at stress levels much lower than the critical stress for sliding, significantly modify the initially uniform contact area profile. Thus, when overall sliding finally occurs, the contact area is highly non-uniform in space. These results suggest a fundamentally new view of the processes leading to frictional motion with ramifications to earthquake dynamics and material failure. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U22.00009: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U22.00010: Molecular Dynamics Simulation of Frictional Melting Shigenobu Hirose Frictional melting produces lubricant at the sliding plane and changes the physics of dynamical sliding, which may play a key role on coseismic slipping. In this paper, molecular dynamics simulation is used to study the basic physics of fritional melting. Here, friction between a Lenard-Johns material and a rigid material is considered for simplicity. When the sliding velocity is low enough, there is no melting and the friction coefficient almost does not depend on the sliding velocity. On the other hand, when the sliding velocity is so high that frictional melting occurs, the friction coefficient decreases due to the melting lubricant. A preliminary result shows that the friction coefficient is roughly power-law of the sliding velocity. A discussion will also be given on the themodynamic balance between the frictional heating, cooling by latent heat, and conduction cooling. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U22.00011: Molecular Dynamics Simulations of Nanotribology with Accurate Probe Tip Models Michael Chandross, Christian Lorenz, Gary Grest Results for extensive dynamical nanotribological simulations of amorphous silica tips in contact with alkylsilane self-assembled monolayers (SAMs) will be presented. The radius of curvature of the tips match experimental dimensions. Comparison with contact mechanics models indicate that the standard JKR and DMT models do not give the correct dependence of contact area on applied force. The dependence of the tribological response on the chain length of the SAM has been determined. For short chains and for long chains at low loads the SAM presents a disordered sliding surface to the tip and the chain length is irrelevant. This result is in agreement with our previous simulations for SAMs in contact with a flat surface. For longer chains at higher loads the tip penetrates the monolayer and the friction is dominated by a plowing mechanism. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U22.00012: Molecular dynamics studies of friction between bare and oxidized silicon surfaces Woo Kyun Kim, Michael Falk Using molecular dynamics simulation we examine the friction between a bare silicon tip and a silicon surface under perfect vacuum conditions. The simulations utilize a Stillinger-Weber model for the Si-Si interactions. In the case of bare silicon (100) the proper surface reconstructed is verified. Silicon-silicon sliding leads to high friction and significant wear due to the strong adhesive force between tip and surface. Repeated adhesion and shearing produces a stick-slip motion. The quantity of material lost during sliding depends on the relative orientation of the dimer rows between the reconstructed surfaces of the tip and substrate. Little dependence on the temperature or the normal force is observed in this case. The frictional force does not change significantly even when an upward normal force is applied to the tip force, although the quantity of lost material depends on the magnitude of upward normal force. The geometry and elasticity dependence of the stick-slip motion has also been analyzed. We have also begun investigations of a similar geometry in which the silicon is coated with a thin oxide layer. A charge transfer potential having 3-body terms as well as pair-wise interactions is being used to model the amorphous silica interactions. These simulation results will be compared to the recent AFM experiments by Schirmeisen et al. measuring the frictional forces between an oxidized silicon tip and substrate. [Preview Abstract] |
Session U23: Low-D Metals and Responses
Sponsoring Units: DCMPChair: Rongying Jin, Oak Ridge National Laboratory
Room: Colorado Convention Center 110
Thursday, March 8, 2007 8:00AM - 8:12AM |
U23.00001: Internal friction of a one micron thick silver film between 1 mK and 1 K Andrew Fefferman, R. O. Pohl, J. M. Parpia Metal films are useful for thermalization and electrostatic actuation of both macroscopic and nanoscale oscillators at low temperatures. However, the effect of the metal film on the dynamics of the oscillator substrate is not always known since previous measurements of the internal friction $Q^{-1}$ of metal films extend down down to a few hundred mK. Measurements of X. Liu $\it{et~al}$ [Phys. Rev. B $\bf{59}$, 11767 (1999)] showed that between 0.5 and 1 K many micron-thick metal films exhibit a $Q^{-1}$ plateau at the level of several $10^{-4}$, but the existence of the plateau below 500 mK was unknown. We have measured $Q^{-1}$ of a one micron thick silver film between 1 mK and 1 K using a single crystal silicon double paddle oscillator substrate vibrating at 5.5 kHz. $Q^{-1}$ of the substrate was measured prior to deposition of the film so that $Q^{-1}$ of the film alone could be extracted from measurements of the composite oscillator. $Q^{-1}$ of the silver film was nearly constant at $4 \times 10^{-5}$ between 1 and 10 mK and increased to $10^{-4}$ as temperature increased from 10 mK to 1 K. These data will be valuable for future experiments on metal coated silicon or silicon nitride resonators at mK temperatures. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U23.00002: Thermal Expansion and Specific Heat of some BeCu Alloys J.C. Cooley, J.C. Lashley, T.J. Tucker, W.L. Hults, S.J. Tracy, G.M. Schmiedeshoff We have measured the specific heat and thermal expansion of some polycrystalline BeCu alloys with an atomic concentration of Cu up to 3{\%}. We will present and discuss our measurements in the context of a Gruneisen analysis. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U23.00003: Optical properties of Al at shock-melt conditions Lorin X. Benedict, John E. Klepeis We report calculations of the linear optical absorption spectra of aluminum at pressures of roughly 1 Mbar and temperatures of around 5000 K. Both the hot fcc solid and the liquid have been studied at those conditions, and we conclude that it should be possible to distinguish solid from liquid when Al has been shocked. This suggests that in situ measurements of optical constants may serve as a diagnostic for melting in dynamic high pressure experiments. Calculations were performed using a combination of ab initio and semi-empirical electronic structure schemes, together with a combination of molecular dynamics schemes to generate snapshots of hot solid and liquid ionic configurations. Lower densities and temperatures were considered as well, showing favorable comparison with ambient pressure high-T optical measurements on Al. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U23.00004: Ultrafast Extreme Ultraviolet Holography: Dynamic Measurement of Surface Deformation Mark Siemens, Oren Cohen, Qing Li, Margaret Murnane, Henry Kapteyn, Ra'anan Tobey, Keith Nelson We demonstrate femtosecond time-resolved dynamic holography using coherent extreme ultraviolet (EUV) light generated by high harmonic upconversion of a femtosecond laser. We use a novel excitation geometry in which a pump laser excites a narrow line on the sample, and a much larger EUV beam probes the perturbed sample. The unperturbed portion of the sample reflects the EUV beam to serve as the reference beam, while the pumped region diffracts the EUV probe to from the object beam. The interference of the two beams forms a dynamic hologram that changes as the surface relaxes. By varying the pump-probe delay time, we observe laser-induced surface displacement and subsequent acoustic oscillations in thin metal films. EUV probing in this manner has sub-picometer sensitivity to vertical surface deformation, and is largely free of ambiguities associated with electronic and photoelastic effects that complicate other photoacoustic schemes. In the future, we will extend phase-sensitive detection to study other transient dynamics, such as thermal transport in nanostructures. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U23.00005: Resonant mapping of image states on metal surfaces using tunable femtosecond light Kevin Knox, Mehmet Yilmaz, Nader Zaki, Jerry Dadap, Richard Osgood, Peter Johnson We report resonant band mapping using angle-resolved two-photon photoemission measurements of image states on Cu(111) surfaces using a tunable ultrafast femtosecond optical parametric amplifier source. An optical parametrically amplified visible beam is frequency doubled to obtain a tunable fs UV source with photon energies in the 3.6 to 5 eV range. Unoccupied image states are populated by resonant excitation from the occupied sp-like surface state of Cu(111). The image state electrons are then probed by absorption of a second photon of the same energy. Since the surface and image states have different effective masses, resonant excitation occurs at different parallel momenta for each photon energy. By tuning the photon energy we are able to resonantly map both the surface and image state spectra. Our fs laser provides high signal to noise ratio and ultrafast time resolution and the resonant mapping scheme allows for precise measurement of the dispersion and reference planes (the dispersion minimum) of the occupied and excited bands. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U23.00006: Scattering of an Incident Beam by a Magnetic Structure using FDTD. Miguel A. Alvarez-Cabanillas The electromagnetic scattering by a magnetic structure is modeled using finite difference time domain (FDTD). The electromagnetic wave with normal incidence to the magnetic structure has a plane wave front. The incident electric field is chosen perpendicular to the magnetization in the magnetic structure. The electric field is rotated by the magneto-optical Kerr effect (MOKE) and then reflected. This phenomenon is simulated by building the algorithm for FDTD from the Maxwell's equations, using a Transversal Magnetic distribution of the fields in the numerical mesh. The MOKE is introduced in the dielectric constant of the magnetic material. The space of simulation is surrounded by an absorbing boundary condition (ABC). The Perfect Match Layer (PML) was chosen as an ABC with ten layers, enough to reduce the reflected wave. The same size of cells were used in the while mesh. The size of the cells in the space of simulation and the time step were selected in agreement to reduce the numerical dispersion and avoid numerical instability. The algorithm simulates the correct rotation of the electric field as was predicted by MOKE. The numerical results of the FDTD were compared with the analytic solution in order to verify the algorithm and validate the numerical results. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U23.00007: Measurement of the resonance shift in the radar backscattering cross section of thick stainless steel fibers at 35 GHz Sharhabeel Alyones, Charles Bruce Measurements of the radar backscattering cross section of stainless steel fibers with low length-to-diameter ratio (thick fibers) had been done at 35 GHz. The intention was to confirm the resonance shift in length predicted by a numerical solution of the general problem of electromagnetic scattering and absorption by finite conducting wires [1]. The numerical methods solves the generalized form of the Pocklington equation, which is valid for both thin and thick fibers. Single particle radar backscattering measurement system was used and the resonance shift had been confirmed for four sets of aspect ratios. The position of the first resonance is shifted to shorter lengths in comparison with the previous analytical solution of the problem by P. Watermann and J. Pedersen [2]. \newline [1] Sharhabeel Alyones, Charles W. Bruce, and Andrei Buin$, `` $Numerical methods for solving the problem of electromagnetic scattering by a finite thin conducting wire$'', $ accepted for publication in\textit{ IEEE. Trans. Antennas and Propag.} \newline [2] P. C. Waterman, ``Scattering, absorption and extinction by thin fibers,'' Accepted for publication in \textit{J. Opt. Soc. A.} [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U23.00008: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U23.00009: Plasmon Vibrational Delocalization in 1D Disordered Wigner Lattices Shimul Akhanjee, Joseph Rudnick We explore various aspects of classical 1D Wigner solids in the presence of strong disorder at $T=0$. Two different realizations of electrostatic randomness are studied: a system of particles with spatially random charge strengths and a system of like charges interacting with an external random potential. In the random potential system we have discovered a novel type of vibrational delocalization transition of the plasma oscillations. Finite size scaling studies of the localization length and inverse participation ratio reveal an Anderson transition from extended to localized eigenmodes at larger eigenfrequencies. Other properties of the eigenmodes are also discussed in the context of this criticality. Additionally, for both models the probability density of particle spacings is examined analytically through the use of probability convolutions within a weak disorder approximation and compared to numerically relaxed ensembles. We find that the statistical configuration of the charges is sensitive to the to the specific type of quenched random distribution. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U23.00010: Evidence of Electron-Plasmon Coupling in Single Crystal Bismuth Riccardo Tediosi, N. Peter Armitage, Enrico Giannini, Dirk van der Marel We present a detailed optical study via the extended-Drude model analysis of single crystal bismuth using infrared reflectivity and visible-light ellipsometry. The extremely narrow Drude peak and the small value of the screened plasma frequency are consistent with the small carriers density typical for this semimetallic system. The temperature dependence of the optical properties is dominated on one side by the progressive narrowing of the free electron Drude peak and on the other side by the progressive appearance of an absorption peak in the region between the itra-band and inter-band contributions with a consequent change in the frequency dependent scattering rate $\tau^{-1}(\omega)$. We observed that the inflection point $\omega_{\tau}(T)$ corresponding to the increase of $\tau^{-1}(\omega, T)$ closely follows the change of the plasma frequency with temperature according to the relation $\omega_{\tau}(T) \simeq \omega_p(T)$. This aspect suggests a possible interaction between free electrons and collective modes as already theoretically demonstarted in earlier works. In this scenario we calculated the scattering rate contribution for electron-plasmon interaction starting from the plasmon dispersion relation observing an astonishing good agreement between experimental result and theoretical expectation. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U23.00011: Electromagnetic Contribution to Enhanced Raman Scattering from a Metal Nanoshell Dimer Ke Zhao, Zhenyu Zhang We present a theoretical framework for calculating the electromagnetic contribution to enhanced Raman scattering from a metal nanoshell dimer, using time dependent local density approximation within density functional theory, and going beyond existing dipole excitation treatments. When applied numerically to silver and gold nanoshell dimers, we obtain results that can be compared with those derived from classical electrodynamics, which in turn allows to test the validity of using local dielectric functions to describe nanoshell dimers. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U23.00012: Convective Atom Transport as a Modifier of Near-Surface Alloy Composition Yong W. Kim Thermophysical properties of metallic alloys are manifestly the features of a given material specimen, and, as such, they are dependent on their elemental composition. Some properties are measured at surfaces and others are measured through the bulk as a whole. Complications arise when the elemental composition becomes position dependent within a material specimen. Such occurrences turn out to be common and have been demonstrated by simultaneous measurements of thermal diffusivity and elemental composition by time-resolved spectroscopy of laser-produced plasma (LPP) plume emissions. To further understand the cause, we have investigated the evolution of near-surface composition of Wood's alloy (composed of 50 W{\%} bismuth, 25W{\%} lead, 12.5 W{\%} tin and 12.5W{\%} cadmium) as a model system under the influence of thermal cycling with, and without, temperature gradient over the specimen. Surface composition modification has been found to take place by accumulation of irregularly spaced gray patches of inhomogeneous composition on the surface in the presence of temperature gradient. Surface position and depth-resolved determination of elemental composition by LPP spectroscopy has revealed fully 3-D composition structures of the patches. Candidate mechanisms will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U23.00013: Effects of Zr Doping on the Oxidation of Low-index Crystal Surfaces of Single Crystal beta-Nickel Aluminum Serif Uran, Marcos Grimsditch, Boyd Veal, Paul Paulikas Addition of small amount ($\sim $0.1 atm {\%}) of a reactive element (e.g., Y, Zr, Hf) to substrate alloy prior to oxidation is known to improve the oxidation properties (i.e., adherence) of these alloys. This phenomenon is known as the reactive element effect. The purpose of this investigation is to determine the role of reactive element doping during oxidation of a single crystal. It can be argued that absence of grain boundaries in the underlying metal might change or inhibit the improved scale adherence normally produced by doping with a reactive element. By comparing the effects of reactive element doping on the oxidation of different crystallographic faces, we expect to improve our understanding of this still very poorly understood phenomenon. In this study, we have measured scale thickness, composition and residual stress as a function of oxidation temperature for the three principal low-index surfaces (001), (1-10) and (111) of zirconium doped specimen. Systematic differences are observed among different surfaces and the results are compared to those of the undoped crystal. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U23.00014: Electronic structure of ultrathin films of Co on Cu(775) stepped surfaces using high-resolution photoemission spectroscopy Shancai Wang, Jerry Dadap, Mehmet Yilmaz, Kevin Knox, Nader Zaki, Richard Osgood, Tonica Valla, Peter Johnson We perform high-resolution photoemission spectroscopy, using the U13UB UV line at the NSLS, to study the electronic structure of bare and low-coverage Co on Cu(775) stepped surfaces. Despite the relatively wide terrace widths, the bare surface shows clear evidence of umklapps due to the step edges; this behavior is sharpened in the presence of very low Co coverage due to step pinning. We also measure the dispersion for electron emission along and perpendicular to the steps and obtain an exchange splitting energy for the lower Co d-bands at 16 and 25 eV photon energies. The splitting energy reaches a value of as low as $\sim $0.4 eV, which is considerably smaller than that obtained for both Co and Co/Cu(111) surfaces. In addition, the typical widths of the spin states are larger than those obtained for the other surfaces, indicating the increased scattering channels arising from the strong influence of the steps. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U23.00015: Oxidation of Pt (100) surface: Ab initio studies Eunja Kim, David Stucke, Tao Pang We have performed density-functional calculations to investigate the oxidation process of Pt (100) surface. We carefully examine the previously proposed models and propose a new dissociative model of oxygen molecules on the Pt (100) in this study. Our findings also indicate that one monolayer of oxygen atoms can be covered on Pt (100) surface with 1.09 eV/O$_2$. The role played by oxygen and temperature in the degradation of catalyst will be further discussed in details. [Preview Abstract] |
Session U24: Phase Transitions in Polymeric Systems I
Sponsoring Units: DPOLYChair: Alamgir Karim, National Institute of Standards and Technology
Room: Colorado Convention Center 201
Thursday, March 8, 2007 8:00AM - 8:36AM |
U24.00001: Influence of Phase Separation and Shear on the Crystallization of Polyolefin Blends Invited Speaker: The correlation between liquid-liquid phase separation (LLPS) and crystallization at several compositions in statistical copolymer blends of poly (ethylene-co-hexene) (PEH) and poly (ethylene-co-butene) (PEB) has been examined by optical microscopy (OM), atomic force microscopy (AFM) and differential scanning calorimetry (DSC). The overwhelming change in the crystallization kinetics due to the density fluctuation caused by the spontaneous spinodal LLPS is observed. This coupling mechanism suggests a new mechanism in the nucleation-crystallization process. All evidences are pointing to a cross-over mechanism from the spinodal fluctuations (of liquid-liquid phase separation) to the nucleation and than crystallization. Also, the shear dependence and mechanism of the Shih-kebab formation in the isotactic polypropylene (iPP) and isotactic polypropylene/polyethylene-co-octene blends have been studied. The network strands deformation and the primary nucleation mechanism have been studied by the time resolved small angle light scattering and AFM. New mechanism has been proposed. The above studies are aimed to understand the enhanced primary nucleation mechanisms in crystallizable polymers and polymer blends which have not been emphasized in most of the traditional nucleation and crystallization research in polymers. The detailed experimental evidences and proposed physical model will be presented. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U24.00002: Formation of micelles in homopolymer-copolymer mixtures Marcus M{\"u}ller, Anna Cavallo, Kurt Binder Using Monte Carlo (MC) simulations of the bond fluctuation model and self-consistent field (SCF) calculations, we study the formation of micelles in a mixture of homopolymers and asymmetric AB-diblock copolymers with composition, $f_{A}=1/8$. We work in the semi--grandcanonical ensemble, {\it i.e.}, we fix the monomer density and incompatibility, $\chi N\simeq 100$, and control the composition of the mixture via the exchange chemical potential, $\delta\mu$ between the copolymer and homopolymer solvent. The MC simulation comprises moves that allow homopolymers to mutate into AB-diblock copolymers and {\it vice versa}. These moves are very efficient in equilibrating the configurations. We accurately locate the critical micelle concentration, study the micellar size distribution and characterize the shape of the micelles by the tensor of gyration and radial density profiles. The simulation results are {\it quantitatively} compared to predictions of the SCF theory in the grandcanonical ensemble without adjustable parameter. Only in the limit of high molecular weight the simulation results gradually approach the theoretical predictions. The structure and phase behavior of mixed micelles is investigated by SCF calculations. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U24.00003: Enhancing the segregation strength of amphiphilic block copolymer melts using selectively associating homopolymers: Well ordered systems from inexpensive components. James Watkins, Vijay Tirumala, Alvin Romang, Eric Lin Amphiphilic block copolymers based on poly (oxyethylene-oxypropylene-oxyethylene) are commercially available (Pluronic{\texttrademark}, BASF), inexpensive and used in a variety of solution-based applications. But their use as thin film templates in applications is limited by their low molecular mass, presence of impurities, and weak segregation. Here, we show that the segregation strength of such copolymer melts can be dramatically enhanced by blending with a homopolymer that selectively associates with one of the segments via hydrogen bonding. Examples include poly (acrylic acid), poly (4-vinyl phenol) and poly (styrene sulfonate). SANS measurements indicate that the order-disorder transition temperature of a deuterated copolymer analogous to Pluronic F68 increases by at least 180 $^{o}$C when blended with poly (acrylic acid) suggesting a significantly higher effective interaction parameter. This approach has many implications including the preparation of highly ordered templates from inexpensive components. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U24.00004: Hexagonal Phases in Rod Coil Block Copolymers Rachel Segalman, Bradley Olsen The thermodynamics of rod containing block copolymers is distinct from classical block copolymers due to the conformational asymmetry between the blocks and liquid crystalline interactions between the rods. Understanding the self-assembly of rod containing block copolymers is potentially useful for organic electronics and biopolymers. In the weak segregation limit, a model rod-coil block copolymer shows only lamellar ordered structures, with nematic and isotropic phases observed on heating above the order-disorder transition. As both the strength of segregation and the asymmetry between the rod and coil are increased, new hexagonal phases are observed. Self-assembly of these hexagonal phases requires both high asymmetry in volume fraction of the blocks and high asymmetry between the interfacial area occupied by coil and rod such that there is a strong driving force to break lamellae. Heating of hexagonal polymers with a comparatively low coil fraction can result in an order-order transition to the lamellar phase. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U24.00005: Morphological Characteristics and Phase Behavior of Nanoparticle-Modified Block Copolymers Michelle Bowman, Michael Bockstaller, Kim Rasmussen, Jon Samseth, Steven Smith, Russell Thompson, Richard Spontak Block copolymers exhibit a wealth of nanoscale morphologies that continue to find use in a diverse variety of emergent (nano)technologies. While numerous studies have explored the effects of molecular confinement on such copolymers, few have examined the use of such objects to modify the morphological characteristics and phase behavior of microphase-ordered block copolymers. In this work, a poly(styrene-$b$-methyl methacrylate) (SM) diblock copolymer has been modified with surface-functionalized fumed silica (FS) and colloidal silica (CS). Dynamic rheological measurements have been conducted on the neat and nanoparticle-modified copolymer to generate a quantitative comparison with SM/FS and SM/CS nanocomposites. Transmission electron microscopy (TEM) and self-consistent field theory (SCFT) calculations have also been performed to further elucidate results obtained via dynamic rheology by establishing the morphological characteristics of the copolymer and the dispersion of the functionalized nanoparticles within the resultant nanocomposites. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U24.00006: Structure and Phase Transition in Sulfonated Block Copolymer Moon Jeong Park, Nitash Balsara Ion conducting polymers containing sulfonic acid are of interest for the fuel cell applications. This study is based on sulfonated poly(styrene-b-methylbutylene) block copolymers. Small-angle X-ray scattering experiment was performed on the dry state and these materials show remarkably rich sequence of phase transitions, i.e., lamellae, gyroid, hexagonally perforated lamellae, hexagonal cylinder, and disorder, depending on temperature, molecular weight, and fraction of sulfonated monomers. Small-angle Neutron scattering measurements under controlled temperature and humidity provided us powerful insight into the structure of hydrated materials. We will present data on the relationship between water uptake and morphology. In future work we plan to measure the proton conductivity of these materials. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U24.00007: Symmetry Breaking in Block Copolymer Thin Films Eric Cochran, Gila Stein, Kirill Katsov, Ed Kramer, Glenn Fredrickson This contribution is concerned with the packing of spherical domain block copolymer mesophases in the thin film geometry as a function of the number of layers $n$. In a single layer, $n = 1$, the spheres pack on a hexagonal lattice; in the bulk, $n = \infty$, the $bcc$ ($Im\overline{3}m$) packing is preferred. These symmetries minimize packing frustration in 2- and 3- dimensions, respectively. We find that intermediately segregated films of sphere-forming poly(styrene-\emph{b}-vinylpyridine) pack on a hexagonal lattice ($P6_3/mmc$) for $n$ = 1--3, and then abrupty transform to a $Fmmm$ orthorhombic packing for $n \ge 4$. Beyond the $P6_3/mmc$-$Fmmm$ transition, the unit cell deforms to asymptotically approach the bulk $bcc$ packing. These results are interpreted in terms of the competition between the excess surface energy imposed by the interfaces and the free energy of the bulk system. We construct a simple theory, parameterized using high-resolution SCFT calculations, that successfully accounts for the experimentally observed symmetry breaking. From these calculations we conclude that character of the transition from thin-film to bulk behavior may be either continuous or discontinuous, depending on the degree of the block copolymer segregation. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U24.00008: Dewetting and Phase Separation in Thin Film Polymer Blends Nigel Clarke We present a dynamic theory for simultaneous phase separation and dewetting in ultra thin-film binary mixtures with free surfaces. The model assumes that phase separation only occurs in the plane of the film, thus reducing a three dimensional problem to two dimensions. If the height of the film is orders of magnitude less than the typical lateral dimensions of phase separation and dewetting then such an approximation is highly desirable from a computational viewpoint. We show some preliminary computational results, illustrating how coupling between the two processes can significantly affect pattern formation. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U24.00009: Shear-Induced Phase Transitions in Ternary Polymer Blends Venkat Ganesan, Bharad Narayanan We present a theoretical study of flow-induced phase transitions in the microemulsion phases of ternary polymer blends. The results match qualitatively with recent experimental observations, and suggests that flow transforms the microemulsion phases into distinct lamellar phases at strong shears followed by a macrophase separation of the homopolymers at even stronger shear flows. These transitions are also accompanied by a strong shear-thinning behavior in the rheological response. The results suggest significant differences between ternary polymeric systems and oil-water- surfactant systems, which we rationalize from a molecular viewpoint. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U24.00010: Formation of a superlattice in mixtures of block copolymer micelles Sayeed Abbas, Timothy P. Lodge The formation of a superlattice was observed in a binary solution mixture of diblock copolymer micelles. The larger micelles formed from polystyrene-$b$-polyisoprene (SI) diblocks had an overall radius of 29 nm, and the smaller micelles formed from polystyrene-$b$-polydimethylsiloxane (SD) diblocks had an overall radius of 16 nm. The radius ratio of the micelles was therefore 0.55. By indexing the peaks observed in the scattering pattern from the sample, the structure was assigned to the $Fm\bar {3}c$ space group. The structure is more commonly known in the literature as the binary AB$_{13}$ superlattice. The same lattice was again observed in a similar SI + SD system where the hydrodynamic radii of the bigger and the smaller micelles were 38 and 22 nm, respectively. In this system the size ratio of the micelles was 0.57. Similar to sterically stabilized colloidal hard spheres, the micelles were uncharged and hence the formation of the superlattice was driven solely by free volume entropy. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U24.00011: Phase transitions in block copolymers induced by external fields Marco Pinna, Andrei Zvelindovsky By means of computer simulation we investigate structures formed by polymers consisting of several chemically different blocks, block copolymers. Aim of this contribution is to describe phase transitions between different structures induced by applied electric field and shear flow. We study behaviour of bicontinuous gyroid structure under electric field and compare it with the transition under shear flow. The transition demonstrates non-trivial topological transformations under various fields. We also study spherical, cylindrical, and lamellae phases. Depending on temperature the sphere morphology shows different behaviour under applied shear flow or electric field. We describe kinetics of sphere-to-cylinder transition. We found that about the critical value of electric field the spherical phase transforms into cylinders. While applying shear flow we find sphere-to-cylinders coexistence at one temperature and arrays of hexagonal ordered spheres at another temperature. The results are compared with the experiments and suggest ways of block copolymer structure tailoring. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U24.00012: Phase behavior and morphology of high hard block content polyurethanes. Alberto Saiani, Julia S. Higgins We have investigated the phase behavior and morphology of a set of high hard block content polyurethanes: 50 to 100{\%} (wt.) hard segments. Using mainly calorimetry, scattering and microscopy techniques we were able to elucidate the origins of all the thermal events observed through differential scanning calorimetry. Correlating our thermodynamic work with our structural work we were able to propose a new morphological model of the structure and the phase behavior of high hard block content polyurethanes. We have shown that above 65{\%} hard segment content the melt-quenched samples present a two-phase morphology one pure hard segment phase co-existing with a mixed phase with the same hard segment content of 65{\%} for all samples. When annealed at high temperature the mixed phase undergoes phase separation resulting in the same phase-separated mesophase. This model allowed us to account for our small angle X-ray experiments that showed a single scattering peak at the same position for all the samples. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U24.00013: Monte Carlo simulation of self-assembled polymer chains with inter-chain attractions Xinjiang L{\"u}, James Kindt We have used Monte Carlo simulations to study the phase behavior of self-assembled semiflexible chain polymers with inter-chain attractions. Approximate phase diagrams are obtained with varied physical parameters, such as the chain flexibility, intra- and inter-chain bonding energies. The attraction induced phase separation results in an equilibrium between a bundle and isotropic short chains. The chain length distribution of the phase separated system, as well as the bundle's shape and aspect ratio are presented and discussed. Simulation results are analyzed and compared with related experimental and theoretical work. We also present some other interesting results of observed toroids and branched bundles. [Preview Abstract] |
Session U25: Organic Field-Effect Transistors
Sponsoring Units: DPOLY DMPChair: Lynn Loo, University of Texas at Austin
Room: Colorado Convention Center 203
Thursday, March 8, 2007 8:00AM - 8:12AM |
U25.00001: Intrinsic transport anisotropy in single-crystal FETs on new rubrene derivatives A.F. Stassen, W. Kalb, S. Haas, U. Berens, H.J. Kirner, B. Batlogg For the charge transport in a field-effect transistor (FETs), both the spectral density of gap states and the intrinsic mobility play a critical role. The latter is closely related to the arrangement of the molecules in the solid. We present measurements on FETs fabricated on single crystals of new rubrene derivatives. One of them crystallizes in two polymorphic forms: One polymorph shows a very high field-effect mobility ($>$10 cm$^2$/Vs) and a transport anisotropy which can be directly related to the crystal packing. In the second polymorph, the same molecules are arranged in a different structure with minimal $\pi$-overlap. No charge transport could be induced, highlighting the crucial role played by the wave-function overlap associated with the packing. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U25.00002: RC Transmission Line Characterization of Organic Thin Film Transistors Daniel Lenski, Adrian Southard, Michael S. Fuhrer The transport properties of organic semiconductors are typically measured in a field-effect transistor geometry with DC gate and drain bias. We have developed a novel method of characterizing organic thin films using a 2- or 3-contact transmission line configuration, in which an AC voltage is applied to the thin film and the phase and magnitude of the current are measured. This method can determine both sheet resistance and contact resistance. Additionally, this method may reveal inherent frequency-dependent transport properties of organic semiconductors, which may be significant for future applications of these materials. We present the results of transmission line measurements of pentacene thin films, and compare them to transport data obtained from FET characterization. This work has been supported by the Laboratory for Physical Sciences. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U25.00003: Eliminating gate bias stress effects in organic field-effect transistors Wolfgang L. Kalb, Thomas Mathis, Simon Haas, Arno F. Stassen, Bertram Batlogg We report on organic field-effect transistors with unprecedented resistance against gate bias stress. The single crystal and thin-film transistors combine small molecule organic semiconductors and an organic gate dielectric with a remarkable electrical breakdown strength. The single crystal devices have no current hysteresis. Extended gate bias stress leads to almost unmeasurable changes in the transfer characteristics: the induced interface state density is of order 10$^{9}$/cm$^{2}$. In contrast, stress-induced trap densities of order 10$^{12}$/cm$^{2 }$ have been identified previously in devices with SiO$_{2}$ or OTS-treated SiO$_{2}$ gate dielectrics. Therefore, adverse gate bias stress effects are not generic to oligocene organic semiconductors, and there is no conceptual limitation for the stability of organic-based transistors in contrast to hydrogenated amorphous silicon. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U25.00004: Field dependent hole transport mobility studies on a select group of conjugated polymers N. C. Heston, B. Wilson, E. M. Galand, D. B. Tanner, J. R. Reynolds The thin film hole transport properties of a set of cyanovinylene-linked 3,4-propylenedioxythiophene (ProDOT)/dialkoxyphenylene polymers, and regioregular poly(3-hexyl-thiophene), were measured by fabricating hole dominated devices and measuring the room-temperature I-V characteristics. The data were fitted to a model of field-dependent space-charge-limited current. By carrying out both the fabrication and the measurements inside an inert atmosphere we attained significant improvements to the reproducibility of our results. We also found that exposing the devices to heating increased the field dependent hole mobilities of the polymers with values ranging from 5.2 x 10$^{-7}$ (cm$^{2}$/Vs) to 3.1 x 10$^{-6}$ (cm$^{2}$/Vs). [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U25.00005: Electric Field Induced Conductivity of Disorder Driven Anderson Insulator Vladimir Prigodin, Arthur Epstein The effect of an electric field on the Anderson localization is considered. In the 3d case the field mixing the localized and extended states leads to delocalization. In the 2d case, the localization exponentially weakens with increasing kinetic energy so that in principle any field leads also to total delocalization. The field induced IMT occurs in the 1d case [1]. However, the delocalization effect of field on the Anderson insulator hardly is observable because it is masked by phonon assisted hopping. In the localized phase for each localized state along the field there are remote empty states whose energies are lower and electrons hop to those states by emission of phonons. Therefore the electric field induces the crossover in temperature dependence of hopping conductivity from activation (variable range hopping conductivity) to emission (metallic like conductivity) regime. The results have implications for the recent studies of field effects in polymer based transistors [2]. \begin{enumerate} \item V.N. Prigodin, Sov. Phys. JETP\textbf{ 52}, 1185 (1980 \item F.C. Hsu, \textit{et al}., Phys. Rev. B, in press (2006); A.S. Dhoot \textit{et al.}, Phys. Rev. Lett. \textbf{96, }46403(2006). \end{enumerate} [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U25.00006: Device Model for Organic Semiconductor Light-Emitting Field-Effect Transistors Darryl Smith, P. Paul Ruden Recent experiments demonstrate ambipolar channel conduction and light generation in polymer field effect transistors (FETs).$^{1,2}$ In the ambipolar mode of operation, the gate potential lies between those of the source and drain contacts, hence electrons are injected from one of these contacts and holes from the other. The carriers recombine in channel regions where both types of carriers are present, and the location of the resulting light emission is controlled by the voltages applied to the terminals. We describe a device model for ambipolar organic FETs based on the gradual channel approximation. Trapping of injected carriers in localized states within the polymer energy gap is shown to be important. A non-linear differential equation for the channel potential is derived and solved numerically. Carrier density and recombination profiles are determined. The calculations are in good agreement with experimental data$^{1,2}$. \newline \newline 1) J.S. Swensen, C. Soci, and A.J. Heeger, Appl. Phys. Lett. 87, 253511 (2005). \newline 2) J. Zaumseil, R.H. Friend, and H. Sirringhaus, Nature Materials 5, 69 (2006). [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U25.00007: Electrostatic Injection of Very Large 2D Charge Carrier Densities to Obtain Metallic Conductivities in Organic Semiconductors Matthew Panzer, C. Daniel Frisbie The traditional choice of SiO$_{2}$ for the gate dielectric material in organic field-effect transistors (OFETs) limits the amount of charge that one can induce via the field effect due to its relatively weak dielectric strength. In fact, the maximum 2D charge density achievable (near SiO$_{2}$ breakdown, typically $>$100 V applied) is only $\sim $10$^{13}$ charges/cm$^{2}$, while the 2D molecular packing density of many common organic semiconductors is on the order of 5 $\times $ 10$^{14}$ molecules/cm$^{2}$. In order to achieve a higher fraction of charged semiconductor molecules in the 2D OFET channel, a dielectric layer with a higher capacitance is required. We have used a solid polymer electrolyte as an OFET dielectric in order to obtain 2D charge densities exceeding 10$^{14}$ charges/cm2 at operating voltages under 3 V in a variety of organic semiconductors. We have observed metallic conductivity values ($\sim $1000 S/cm) and nearly temperature-independent resistance ratios in poly(3-hexylthiophene) films using a polymer electrolyte-gated OFET. In addition, conductivity maxima at carrier densities approaching 1 charge/molecule were observed in oligomeric, polymeric, and single-crystal organic semiconductors alike. This phenomenon may be caused by carrier correlations or a complete emptying of the semiconductor transport band at very high charge densities. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U25.00008: Charge mobility of discotic mesophases of hexabenzocoronene derivatives: a multiscale quantum/classical study of the effects of side chain substitution Denis Andrienko, Valentina Marcon, Kurt Kremer, James Kirkpatrick, Jenny Nelson Discotic liquid crystals form columnar phases, where the molecules stack on top of each other and the columns arrange in a regular lattice. The self-organization into stacks results in the one-dimensional charge transport along the columns. Using atomistic molecular dynamics (MD) simulations we study the solid and liquid crystalline columnar discotic phases formed by the alkyl-substituted hexabenzocoronene mesogens. Correlations between the molecular structure, packing, and dynamical properties of these materials are established. Combining Kinetic Monte Carlo and MD trajectories a correlation between the material morphology and charge mobility is then obtained. We are able to reproduce the trends and magnitudes of mobilities as measured by pulse-radiolysis time-resolved microwave conductivity technique. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U25.00009: The electronic structure and charge carrier dynamics in organic molecular crystals Na Sai, Zhiqiang Li, Vitaly Podzorov, Michael Martin, Michael Gershenson, Dimitri Basov, Massimiliano Di Ventra Organic molecular crystal semiconductors have been receiving intense attention recently. The much higher carrier mobility and intrinsic physical properties uncovered in these materials offer many fundamental advantages over their polycrystalline counterpart. Combined with infrared absorption studies of the rubrene based field effect transistor, we study the intrinsic anisotropy in the basal a-b plane of the rubrene crystal. We report the structure and electron structure of the rubrene crystals using density-functional theory. We find fairly light effective masses of the order of the free electron mass, in agreement with those extracted from infrared measurements. In light of these results, we discuss possible mechanisms of charge transport in these crystals. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U25.00010: Analysis of the Injection Efficiency Saturation in Polyfluorene Copolymers David Dunlap, Tianjian Lu, Hon Hang Fong, George Malliaras Recent experiments of the transport of holes across a 1 - 5 $\mu$m layers of the copolymer poly [9,9-dioctylfluorenyl-2,7-diyl)- co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)] (TFB) reveal an injection efficiency that saturates at high voltages, which is indicative of space-charge-limited current. The injection efficiency is on the order of 10$^{-3},$ three orders of magnitude smaller than what would be expected if the current followed the Mott-Gurney law. It is difficult to explain this behavior as arising from trapping phenomena, for time of flight transients show clear plateaus, and the extracted mobility is only weakly dependent on temperature[1]. We propose that the behavior may be accounted for by an interface dipole at the metal/organic injecting contact which is dependent on the applied voltage. Quantitative support to for this mechanism is obtained through numerical simulations of the convection-diffusion equation. \newline [1] H. H. Fong, A. Papadimitratos, and G. G. Malliaras, Appl. Phys. Lett. 89 (2006) 172116 [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U25.00011: High-resolution electrical characterization of polyaniline/p-type organic semiconductor interfaces in thin-film transistors Kwang Seok Lee, Timothy J. Smith, Chris Zangmeister, Joung Eun Yoo, Keith J. Stevenson, Yueh-Lin (Lynn) Loo While the work functions of polyaniline (PANI) and gold are known to be similar (ca. 5eV), we found the electrical properties of PANI/ and gold/p-type organic semiconductor interfaces in dihexylthiophene (DHT-Ant) and pentacene thin-film transistors (TFTs) to be significantly different. Specifically, the current-voltage (IV) characteristics of DHT-Ant TFTs with PANI electrodes show current crowding in the saturation regime. Such current crowding is absent in DHT-Ant TFTs with gold electrodes. Surface potential measurements reveal reduced potential drops at the gold/DHT-Ant interface, relative to the PANI/DHT-Ant interface suggesting limited hole injection into DHT-Ant from PANI. In contrast, pentacene TFTs with PANI electrodes show near-ideal IV characteristics and negligible surface potential drops at the electrode/channel interface, whereas pentacene TFTs with gold electrodes exhibit significant potential drops at the electrode/channel. Hole injection thus appears to be more efficient from PANI than gold in pentacene TFTs, which can be directly correlated with the similar pentacene grain size and the identical fused ring orientation across the PANI/channel interface. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U25.00012: Polymer LED interfaces studied with resonant soft x-ray reflectivity. Cheng Wang, B. Watts, T. Araki, H. Ade, A. Hexemer, A. Garcia, T.-Q. Nguyen, G.C. Bazan, K.E. Sohn, E.J. Kramer Multilayered polymer structures produced by alternate spin casting from polar and non-polar solvents are promising for polymer light emitting diodes (PLEDs). The structure of the interface between the active layers most likely significantly affects the device efficiency, yet little is known about the interfacial structure and how it affects properties of such devices. Recently, it has been shown that Resonant Soft X-ray Reflectivity (RSoXR) is an excellent tool to study polymeric thin films without the need for deuteration. RSoXR can enhance the sensitivity to a particular interface by using specific photon energies. We have used RSoXR and characterized the interfacial width of bilayers of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly[9,9-bis(6'-N,N,N,-trimethylammoniumhexyl)fluorene-co-alt-1,4 phenylene bromide] (PFNBr), materials relevant to PLED devices. MEH-PPV is a neutral conjugated polymer spun-cast from toluene (non-polar solvent) and PFNBr is a charged conjugated polymer (conjugated polyelectrolyte) spun-cast from methanol (polar solvent). Bilayers nominally 20 nm/80 nm PFNBr/MEH-PPV and 80 nm/20 nm MEH-PPV/PFNBr were investigated and their interfacial widths were determined to be 1.3 and 1.4 nm, respectively. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U25.00013: Enhanced transport in metallic particle modulated organic field-effect transistors Yu Chen, Masaya Nishioka, Allen Goldman, Yu Xia, Daniel Frisbie Modulated organic field-effect transistors (FET) of rubrene were made by laminating organic single crystals on the top of two-dimension quantum dots arrays. By introducing the single crystal under-layer dots arrays, we were bale to effectively separate organic crystals into mesoscopic-sized grains and reduced the density of trapping sites. Comparing with the conventional organic FETs, these structures show an enhanced mobility at low temperature. This was exhibited as a sharp increase in motilities when temperature decreased below a characteristic temperature. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U25.00014: Viewing density of states of the contact in organic thin-film transistors Takeo Minari, Kazuhito Tsukagoshi, Tetsuhiko Miyadera, Hiromi Ito, Yoshinobu Aoyagi The large contact resistance is one of the most critical issues in the research and development of organic thin-film transistors (OTFTs). In this study, we developed a method for evaluating the injection process of OTFTs through the effective use of the gate voltage dependence as energetic spectroscopy of charge injection. In this spectroscopy, quasi-Fermi level tuning in contact by scanning the gate voltage provides essential clues to the determination of the energy state of the contact. Based on the concept that the contact resistance is composed of the resistance of a barrier at metal/organic interface and bulk resistance of organic film itself, we attempted to reduce the interface barrier height by inserting a thin doping layer of charge-transfer molecule (CTM) into the metal/organic interface. A suppression of the interfacial barrier height unveiled energetic distribution of the density of states (DOS) localized at the contact. We also found that the bulk transport clearly obeys the Meyer-Neldel rule, according to which the exponential density of states near the band edge limits the charge injection. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U25.00015: ABSTRACT WITHDRAWN |
Session U26: Focus Session: Charge Transport in Nanostructures I
Sponsoring Units: DCPChair: Troy van Voorhis, Massachusetts Institute of Technology
Room: Colorado Convention Center 205
Thursday, March 8, 2007 8:00AM - 8:36AM |
U26.00001: Transport in nanoscale systems: hydrodynamics, turbulence, and local electron heating Invited Speaker: Transport in nanoscale systems is usually described as an open-boundary scattering problem. This picture, however, says nothing about the dynamical onset of steady states, their microscopic nature, or their dependence on initial conditions [1]. In order to address these issues, I will first describe the dynamical many-particle state via an effective quantum hydrodynamic theory [2]. This approach allows us to predict a series of novel phenomena like turbulence of the electron liquid [2], local electron heating in nanostructures [3], and the effect of electron viscosity on resistance [4]. I will provide both analytical results and numerical examples of first-principles electron dynamics in nanostructures using the above approach. I will also discuss possible experimental tests of our predictions. Work supported in part by NSF and DOE. \newline \newline [1] N. Bushong, N. Sai and M. Di Ventra, ``Approach to steady-state transport in nanoscale systems'' Nano Letters, 5 2569 (2005); M. Di Ventra and T.N. Todorov, ``Transport in nanoscale systems: the microcanonical versus grand-canonical picture,'' J. Phys. Cond. Matt. 16, 8025 (2004). \newline [2] R. D'Agosta and M. Di Ventra, ``Hydrodynamic approach to transport and turbulence in nanoscale conductors,'' cond-mat/05123326; J. Phys. Cond. Matt., in press. \newline [3] R. D'Agosta, N. Sai and M. Di Ventra, ``Local electron heating in nanoscale conductors,'' cond-mat/0605312; Nano Letters, in press. \newline [4] N. Sai, M. Zwolak, G. Vignale and M. Di Ventra, ``Dynamical corrections to the DFT-LDA electron conductance in nanoscale systems,'' Phys. Rev. Lett. 94, 186810 (2005). [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U26.00002: Molecular conductance simulations with a `hybrid' DFT-NEGF approach Alexander Prociuk, Barry Dunietz A time propagated DFT-NEGF methodology for describing molecular conductance through extended metal-molecule-metal systems is developed. This innovative method calculates transient currents in the presence of time dependent perturbations applied to the molecular junction. Steady state currents can be calculated in the presence of finite temporal perturbations. The electronic density, represented by the lesser GF, is recast into a form that expresses the temporal propagation of the energy spectrum. The effects of the potential biased metal electrodes are expressed with energy dependent `self-energy' terms. This results in a manageable and compact expression for the electron density. This density can be propagated by a specialized scheme that elucidates the transport properties of the system. Propagation, in the absence of an applied temporal perturbation, reduces to an alternative and novel NEGF transport methodology. In addition, applied perturbations can be propagated fully or approximated to any order in time dependent perturbation theory. Calculations are performed for various DFT functionals with a LANL2DZ ECP basis set. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U26.00003: Local electron heating in nanoscopic conductors Roberto D'Agosta, Na Sai, Massimiliano Di Ventra The electron current density in nanoscale junctions is typically several orders of magnitude larger than the corresponding one in bulk electrodes. Consequently, the electron-electron scattering rate increases substantially in the junction. This leads to local electron heating of the underlying Fermi sea [1] in analogy to the local ionic heating that is due to the increased electron-phonon scattering rates [2]. By using a novel hydrodynamic formulation of transport [3], we predict the bias dependence of local electron heating in quasi-ballistic nanoscale conductors [1], its effect on ionic heating [1], and the consequent observable changes in the inelastic conductance [4]. \noindent [1] R. D'Agosta, N. Sai and M. Di Ventra, accepted in Nano Letters (2006). \noindent [2] Y.-C. Chen, M. Zwolak, and M. Di~Ventra, Nano Lett. {\bf 3}, 1961 (2003); Nano Lett. {\bf 4}, 1709 (2004); Nano Lett. {\bf 5}, 621 (2005). M. J.~Montgomery, T. N. Todorov, and A. P. Sutton, J. Phys. Cond. Matt. {\bf 14}, 5377 (2002). \noindent [3] R. D'Agosta and M. Di Ventra, J. Phys. Cond. Matt. in press. \noindent [4] R. D'Agosta and M. Di Ventra, in preparation. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U26.00004: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U26.00005: Electroluminescence from individual CdS/CdSe nanowires contacted by poly(3-hexylthiophene) Kristin Maher, Lian Ouyang, Dong Yu, Yong-Joo Doh, Chun Yu, Hongkun Park Nanocrystal-polymer composites have been shown to exhibit interesting optoelectronic properties. However, only bulk properties of these materials have been measured, and the single nanocrystal-polymer interface is poorly understood. We report the fabrication and characterization of light-emitting transistors incorporating individual CdS/CdSe heterostructure nanowires and a thin film of poly(3-hexylthiophene) (P3HT). The nanowire is contacted at one end by a metal electrode and at the other by a thin film of P3HT, a $p$-type conducting polymer. The devices show rectifying current-voltage behavior and light emission can be observed at forward bias. The peak wavelength and the full width at half-maximum of the electroluminescence were 1.68 eV and 0.08 eV, respectively. The mechanism for light emission will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U26.00006: Photoconductivity and multiple exciton generation in arrays of coupled semiconductor nanoparticles Matthew Beard, Joseph Luther, Kelly Knutsen, Qing Song, Randy Ellingson, Arthur Nozik Three dimensional arrays of semiconductor nanocrystals (NCs) in p-i-n structures are a novel approach to solar energy conversion that offers the potential to control the microscopic charge generation, separation, and transport so as to maximize solar energy conversion efficiencies. A necessary characteristic of the NC arrays is that they exhibit very high conductivity for electrons and holes; this requires strong inter-NC electronic coupling and the subsequent formation of extended electronic states. Many factors, such as inter-NC spacing, site energy dispersion, NC size and shape, cross linking, and Coulomb charging determine the inter-NC coupling. In addition, efficient carrier transport in NC solids requires minimization of carrier loss processes such as surface trapping. All of these factors are highly interdependent. Time-resolved THz spectroscopy (TRTS) is a powerful experimental tool that measures both photoconductivity, in a non-contact fashion, and carrier dynamics simultaneously, with sub-picosecond temporal resolution. We report TRTS for a series of chemically treated PbSe NCs where the inter-NC separation has been varied in a systematic manner. We also report multiple exciton generation (MEG) QYs within the coupled arrays. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U26.00007: Photoconductivity of Nanotapes Self-assembled from meso-Tri(4-sulfonatophenyl)monophenylporphine A. L. Yeats, B. Massare, W. F. Smith, A. D. Schwab, J. C. de Paula, D. E. Johnston, A. T. Johnson We have shown$^{1}$ that meso-Tetra(4-sulfonatophenyl)porphine (TPPS$_{4})$ forms well-defined nanorods in acidic solution. Experiments on related molecules provide insight into the mechanisms for self-assembly and photoconduction. Meso-Tri(4-sulfonatophenyl)monophenylporphine (TPPS$_{3})$ has one fewer sulfonate group than TPPS$_{4}$, but is otherwise identical. Previous work$^{2}$ has shown that, when deposited onto substrates by rotary evaporation, it forms folded nano-ribbons. We have found that, when deposited via immersion and spin-drying, it forms tape-like aggregates of two distinct heights. The larger width of these nanotapes (compared to TPPS$_{4}$ nanorods) is expected from the smaller in-solution charge of the monomer. The TPPS$_{3}$ aggregates exhibit photoconductive properties very similar to those of TPPS$_{4}$. The aggregates ordinarily form in solution, but can also be surface-catalyzed. $^{1}$ A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). $^{2}$ J. Crusats \textit{et al.,} Chem Commun. 1588 (2003). [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U26.00008: Inelastic electron tunneling -induced light emission from a metal - quantum dot - metal double barrier tunnel junction Antti Makinen, Alan Wan, James Long A double barrier tunnel junction formed by a scanning tunneling microscope (STM) tip together with the substrate-tethered quantum dots provides a flexible architecture to explore the characteristics of envisaged nanoparticle-based optoelectronic devices. STM-induced light emission (STM-LE) measurements of ligand-capped CdSe/ZnS quantum dots, tethered to a gold substrate, reveal a light generation mechanism, which is driven by inelastic electron tunneling (IET) at a threshold voltage. The threshold voltage for STM-LE is found to depend on the optical gap of the quantum dots, stipulating a simple energy conservation rule for light emission through IET. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U26.00009: Break-down of the density-of-states description of scanning tunneling spectroscopy in supported metal clusters Martin Garcia, Mario De Menech, Ulf Saalmann Low-temperature scanning tunneling spectroscopy allows to probe the electronic properties of clusters at surfaces with unprecedented accuracy. By means of quantum transport theory, using realistic tunneling tips, we obtain differential conductance curves which considerably deviate from the cluster's density of states. Our study explains the remarkably small number of peaks in the conductance spectra observed in recent experiments. We demonstrate that the unambiguous characterization of the states on the supported clusters can be achieved with energy-resolved images, which we are able to construct with a complete simulation of the experimental imaging procedure. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:48AM |
U26.00010: Measurement of the Conductance of Single Conjugated Molecules Invited Speaker: Electrical conduction through molecules depends critically on the delocalization of the molecular orbitals, and their weight on the metallic contacts. Thiolated conjugated organic molecules are therefore often considered as good candidates for molecular conductors. In such molecules the orbitals are delocalized throughout the molecular backbone, with substantial weight on the sulfur-metal bonds. However, their relatively small size, typically 1 nm, calls for innovative approaches to realize a functioning single molecule device. In this paper we report a new approach for contacting a single molecule and use it to study the effect of localizing groups within a conjugated molecule on the electrical conduction. Our method is based on synthesizing a dimer structure, consisting of two gold colloids connected by a di-thiolated short organic molecule, and electrostatically trapping it between two metal electrodes. We study the electrical conduction through three short organic molecules: A fully conjugated molecule, 4,4'-biphenyldithiol (BPD), 4,4'-biphenyletherdithiol (BPED) in which the conjugation is broken at the center by an oxygen atom, and 1,4-benzenedimethanethiol (BDMT), where the conjugation is broken near the contacts by a methylene group. We find that the oxygen in the BPED and the methylene groups in the BDMT suppress the electrical conduction relative to the BPD. [Preview Abstract] |
Session U27: Focus Session: Computational Nanoscience VII-Reactions on Surfaces and Nanostructures
Sponsoring Units: DMP DCOMPChair: Hai-ping Cheng, University of Florida
Room: Colorado Convention Center 301
Thursday, March 8, 2007 8:00AM - 8:12AM |
U27.00001: Effects of Rh coordination and surface strain on NO dissociation Stefano de Gironcoli, Pushpa Raghani, Prasenjit Ghosh, Shobhana Narasimhan The Rh(100) surface is known to be a good catalyst for the reduction of NO. We try to understand the effect of Rh coordination and in plane strain on NO adsorption and its dissociation. To distinguish between the strain effects and charge transfer we look at the adsorption and dissociation of NO on Rh(100) and stretched Rh(100), in addition to NO adsorption on 1 ML of Rh/MgO at MgO and Rh lattice constants. As expected, we find that the adsorption energy of NO increases with reduction in effective coordination of Rh, i.e., while going from Rh(100), to stretched Rh(100), to 1 ML Rh/MgO, to 0.25 ML of Rh/MgO. In the case of NO on Rh(100) and on 1 ML Rh/MgO at Rh lattice constant, we find that NO adsorbs vertically at the bridge site between two Rh atoms; whereas, in the case of stretched Rh(100) and 1 ML of Rh/MgO at MgO lattice constant, NO adsorbs horizontally at the hollow site. Hence, as the effective Rh coordination is changed, both adsorption geometries and adsorption energies change in a systematic way. This has an influence on the energetic barriers for the rate-limiting step in the reduciton of NO on these surfaces. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U27.00002: Bridging the materials gap in catalytic kinetics via first principles coarse-grained kinetic Monte Carlo simulations Altaf Karim, Dion Vlachos We describe a first principles coarse-grained kinetic Monte Carlo technique enabling us to simulate catalysis under different technologically relevant conditions. By implementing coarse-grained KMC, various simulations are performed efficiently at much larger time and length scales. Using this approach, we studied catalytic oxidation of CO on different metal surfaces. Especially, we tried to understand the role of defects in catalytic kinetics such as steps, kinks, and multiple facets. These studies led us to some interesting results bridging the material gap in catalysis. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U27.00003: Formation of Pt nano-islands on Ru(0001) surface: insights from ab initio calculations . Talat S. Rahman, Marisol Alcantara Ortigoza, Sergey Stolbov As reported [1], Ru nanoparticles with sub-monolayer of Pt are much more efficient catalysts for hydrogen oxidation than anodes used in standard fuel cells. Since this effect apparently depends on the size of Pt islands, we have performed density functional theory based calculations of energetics of Pt islands of varying size on the Ru(0001) surface. We find the formation energy of the island per atom to decrease monotonically from -5.1 eV to -6.1 eV, as the number of Pt atoms in the 2D island increases from one atom to one monolayer. Propensity of the systems to increase the number of interatomic bonds thus overpowers the effect of the stress caused by the 2.6{\%} misfit of Ru and Pd interatomic bonds. As a result, Pt atoms prefer to coalesce on Ru(0001) increasing the cluster size up to one monolayer, in agreement with experimental data. This raises the question whether the key role of nano-sized Ru clusters [1] is to prevent coalescence of Pt islands by restricting their diffusion through facet edges. To address this issue we consider the effect of surface steps on Ru(0001) on the Pt island formation. 1. S. R. Brankovic, et al. , Electrochem. Sol.-St. Lett. \textbf{4} (12) A217-A220 (2001). [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U27.00004: First principles studies of CO adsorption and diffusion over Pt nano-islands on Ru(0001) surface Sergey Stolbov, Marisol Ancantara Ortigoza, Talat S. Rahman Proton exchange fuel cells are promising tools for hydrogen economy. However, CO, present in hydrogen, blocks active Pt sites of anode that poisons its reactivity. As reported [1], small coverage of Pt on Ru nanoparticles is much less sensitive to CO than commercial catalysts. To understand this effect, we have performed density functional theory based calculations of the energetics of adsorption and diffusion of CO on the 7Pt-atom islands and on the Ru(0001) substrate. We find that CO adsorption energy increases as it moves from the center of the Pt island to its edge and further onto substrate. CO thus tends to move from the Pt island to the Ru substrate. Diffusion barriers are found to be lower than 0.3 eV suggesting this process to be fast. This finding suggests that this hydrogen oxidation catalyst is CO tolerant because of the propensity of CO to move from active Pt island site to the Ru substrate. We present the rationale for this effect using insights from detailed electronic structure calculations. [1] S. R. Brankovic, et al., Electrochem. Sol.-St. Lett. \textbf{4} (12) A217-A220 (2001). [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U27.00005: Mechanism of the Energy Barrier Formation during Dissociation of Hydrogen Molecule on Mg(0001) Shigeyuki Takagi, Kazuo Tsumuraya There has been no clear explanation on the mechanism of the energy barrier for the dissociation of the hydrogen molecules on the metal surfaces. When the hydrogen molecule approaches the surface of metals, the molecule dissociates and forms a new bond with the atoms on the surfaces. During the process the energy barrier appears on the Au or Cu or Mg surface, although no barrier appears on the Pt or Ni surface. Although the stability of hydrogen atoms has been clarified by use of the density of stare,[1] there has been no clear explanation on the mechanism of the barrier formation. In the present paper we use a density functional calculation method to evaluate the charges that belong to the hydrogen atoms during the dissociation process of the hydrogen molecule on the Mg(0001) using Bader analysis. During the barrier formation, the charge transfers from the Mg substrate to the dissociating hydrogen atoms. We will discuss how this energy barrier can be explained with the sum of the independent systems of the electron donated Mg surface and the electron received hydrogen molecule. [1] B.Hammer and J.K.N{\o }rskov, Nature 376, 238(1995). [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U27.00006: Theoretical study of tunneling spectra of tetramantane on Au(111) surface Emmanouil Kioupakis, Y. Wang, R. Yamachika, X. Lu, M. F. Crommie, Steven G. Louie Diamondoids are a class of carbon-based molecules in which the carbon atoms are structured in a diamond-like fashion with all dangling bonds saturated with hydrogen atoms. Recently, scanning tunneling microscopy experiments were carried out to study the tunneling spectra of tetramantane on Au (111) surface. The elastic tunneling images have a LUMO-like character in a broad energy range around the Fermi level, while the inelastic signal shows spatial localization. We use ab-initio density functional theory calculations to study the molecule-surface system and discuss the observed elastic and inelastic tunneling spectra. This work was supported by National Science Foundation Grant No. DMR04-39768, by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and by an NSF Graduate Research Fellowship. Computational resources have been provided by NERSC and NPACI. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U27.00007: Motion of clusters on complex surfaces Sabri Alkis, Jeffrey Krause, Hai-Ping Cheng Polymer and organic molecule assemblies have been investigated intensely in the past decade, due to their vast range of applications in nano-molecular electronics and as bio-sensors. In particular, self-assembled monolyers (SAMs) of alkanethiol on the Au(111) surface are used widely in surface studies because they are simple structurally, stable thermodynamically and have well-defined order. In this project, inspired by recent experiments, we use classical molecular dynamics simulations to study motions of Ag$_{n}$ clusters with various sizes on the alkanthiol SAMs. We report detailed results on dynamics, diffusion, and sintering processes of these nano-clusters. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U27.00008: Effects of water on the reactivity and stability of SiO2 nanostructure. Yao He, Chao Cao, Hai-Ping Cheng To investigate effects of water on the reactivity and stability of SiO2 nanostructure, we have performed first-principles molecular dynamics simulations of SiO2 nano-chain and nano-rod. The SiO2 nanostructures, which have stimulated many current research endeavors, can react with water strongly under internal or external stress. In our study, water monolayer films that cover the entire system are used to study the collective motion of protons. Structure, charge separation, stress dependent bond breaking and formation, and proton conduction are discussed based on results obtained at room temperature. Finally, we extend our effort to carbon nanotubes. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U27.00009: Enantiospecific adsorption of chiral molecules on chiral Au clusters Ignacio L. Garzon, Xochitl Lopez-Lozano, Luis A. Perez Enantioselectivity in gold clusters is investigated by studying the adsorption of a chiral amino acid (cysteine) on a chiral Au$_{55}$ cluster using density functional calculations. The highest adsorption energies were found when the amino and thiolate functional groups of cysteine are bonded to the lowest coordinated edges of the chiral cluster. Enantiospecific adsorption is primarily obtained from the different bond location and strength, at the cluster edge, of the carboxyl groups forming the left- and right-handed enantiomers. These results provide theoretical support to convey enantioselectivity in asymmetric nanocatalysts using chiral gold clusters. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U27.00010: Spin-dependent electronic structure of transition-metal atomic chains adsorbed on single-wall carbon nanotubes Engin Durgun, Salim Ciraci We present a systematic study of the electronic and magnetic properties of transition-metal (TM) atomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs). We examined the effect of the TM coverage and geometry on the binding energy and the spin polarization at the Fermi level. All those adsorbed chains studied have ferromagnetic ground state, but only their specific types and geometries demonstrated high spin polarization near the Fermi level. Their magnetic moment and binding energy in the ground state display interesting variation with the number of d electrons of the TM atom. Spin-dependent electronic structure becomes discretized when TM atoms are adsorbed on finite segments of SWNTs. Once coupled with nonmagnetic metal electrodes, these magnetic needles or nanomagnets can perform as spin-dependent resonant tunneling devices. Our study is performed by using first-principles pseudopotential plane wave method within spin-polarized density functional method. \textbf{Reference:} E. Durgun and S. Ciraci Phys. Rev B 74, 125404 (2006). [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U27.00011: \textit{Ab inition }study of adenine and thymine adsorption on carbon nanotubes. Yaroslav Shtogun, Lilia Woods, Galyna Dovbeshko The adsorption properties of the DNA bases, adenine and thymine, on the surface of single walled metallic and semiconducting carbon nanotubes are calculated from first principle density functional theory calculations using the VASP code (Vienna Ab initio Simulation Package). Calculations for the adsorption of the charged adenine and thymine molecules are also presented. The equilibrium positions and their energies are determined. The adsorption process is analyzed in terms of the electronic structure of the various configurations. The effects of charging of the DNA bases in the adsorption on the nanotubes are also determined. The results for the density of states, band structure and charge transfer analysis are used to establish a model of the interaction of these biological molecules with carbon nanotubes. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U27.00012: First-Principle Calculation on Hydrogen Dissociation on Pd-doped CNT Chao Cao, Alexandre Kemper, Yao He, Hai-Ping Cheng We have calculated hydrogen dissociation on Pd-doped (5,5) CNT using density functional theory (DFT), and examined its influence on the conductance of CNT using a non-equilibrium green's function (NEGF) method. Our results show that the hydrogen dissociation is barrier-less on edge of Pd cluster, while on the top of Pd cluster the molecule would not automatically dissociate. Calculations also show that a dense doping with Pd cluster would modify the band structure of CNT substantially such that the doped tube becomes a semi-metal. The dissociation of hydrogen molecule will further change it into a semiconductor. Our NEGF calculations confirmed the band structure calculation, and suggested that Pd-doped CNT could be used as a hydrogen sensor device by measuring the conductance change of the device induced by hydrogen dissociation. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U27.00013: Dynamics and shape fluctuations of nanosized water pools in reverse micelles Janamejaya Chowdhary, Branka Ladanyi Reverse micelles are surfactant assemblies containing nanosized water reservoirs which can serve as confined media for studying chemical reactions as well as for nanoparticle synthesis. Molecular dynamics simulations are performed for reverse micelles formed by the surfactant CTAB (Cetyl Trimethyl Ammonium Bromide) in cyclohexane with the cosurfactant pentanol. We present results for the dynamics of confined water and for shape fluctuations of the reverse micelles which can be probed experimentally using terahertz time-domain spectroscopy. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U27.00014: First-Principles Studies of sila-Diamondoids Steven L. Richardson, Rajendra R. Zope, Tunna Baruah, Mark R. Pederson While there has been recent progress in the isolation of carbon-based diamondoids from petroleum oil by the Chevron-Texaco group, there is still no experimental evidence for the existence of the silicon analogue of adamantane, sila-adamantane (Si$_{10}$H$_{16}$). As adamantane forms the central building block for diamondoids, we speculate that sila-adamantane could serve as the template for a novel class of materials known as sila-diamondoids. We predict that because sila-diamondoids are nanostructures derived from bulk crystalline silicon they will have important applications in molecular electronics and nanotechnology. Recently, Fischer, Baumgartner, and Marschner (Science {\bf 310,} 825 (2005)) have made an important contribution to the field of sila-diamondoids with the synthesis of a four-fold silylated molecule, C$_{24}$H$_{72}$Si$_{14}$. In this work, we show that density-functional theory is capable of calculating the structural, electronic, and vibrational properties of C$_{24}$H$_{72}$Si$_{14}$ that compare very well with the recent experimental data. [Preview Abstract] |
Session U28: Focus Session: Graphene IV
Sponsoring Units: DMPChair: Barbaros Qezyilmaz, Columbia University
Room: Colorado Convention Center 302
Thursday, March 8, 2007 8:00AM - 8:12AM |
U28.00001: The advent, and physical properties, of new classes of graphene-based materials Rodney S. Ruoff, Inhwa Jung, Supinda Watcharotone, Geoffrey H. B. Dommett, Eric Zimney, Sasha Stankovich, Richard Piner, Sungjin Park, Dmitriy A. Dikin We have developed new materials based on chemically modified graphene (CMG) sheets. By working with aqueous colloidal suspensions of `graphene oxide' sheets (graphene sheets that are surface functionalized with hydroxyl, epoxide, carboxylate, and possibly other oxygen-containing functionalities), and at times by doing further chemistry to create a portfolio of CMG sheets, it has been possible to study (i) individual CMG sheets deposited on substrates designed for optical characterization of them (ii) electrical properties of individual CMG sheets (iii) fabrication {\&} properties of ceramic composites with embedded CMG sheets (iv) fabrication of novel ``paper-like'' materials (analogy: bucky paper) comprised of aligned CMG sheets and having thicknesses from less than 1 up to 20 micrometers. Here, we provide a broad overview of this work, and also new directions of research. Support from NASA ({\#} NCC-1-02037) through theURETI on Bio-inspired Materials, the Naval Research Laboratory ({\#}N00173-04-2-C003) and the NSF (CMS-0510212), is appreciated. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U28.00002: The Structure of Suspended Graphene Jannik Meyer, Andre Geim, Mikhail Katsnelson, Kostya Novoselov, Tim Booth, Siegmar Roth The recent discovery of graphene has sparked significant interest, which has so far been focused on the peculiar electronic structure of this material, in which charge carriers mimic massless relativistic particles. However, the structure of graphene is also puzzling. On one hand, graphene appears to be a strictly 2D material and exhibits such a high crystal quality that electrons can travel submicron distances without scattering. On the other hand, perfect 2D crystals cannot exist in the free state, according to both theory and experiment. This is often reconciled by the fact that all graphene structures studied so far were an integral part of larger 3D structures, either supported by a bulk substrate or embedded in a 3D matrix. We describe individual graphene sheets freely suspended on a microfabricated scaffold. These membranes are only one atom thick and still display a long-range crystalline order. However, our studies by transmission electron microscopy have revealed that suspended graphene sheets are not perfectly flat but exhibit intrinsic microscopic roughening such that the surface normal varies by several degrees and out-of-plane deformations reach 1 nm. The atomically-thin single-crystal membranes offer an ample scope for fundamental research and new technologies whereas the observed corrugations in the third dimension may shed light on subtle reasons behind the stability of 2D crystals. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U28.00003: Electrostatic Deposition of Graphene Anton Sidorov, Gamini Sumanasekera, Mehdi Yazdanpanah, Romaneh Jalilian, P. Ouseph, Robert Cohn Loose graphene sheets, one to a few atomic layers thick are often observed on freshly cleaved HOPG surfaces. A simple and reliable technique using electrostatic attraction is demonstrated to transfer these graphene sheets to a selected substrate. Sheets from one to 22 layers thick have been transferred by this method. One sheet after initial deposition is measured by atomic force microscopy to be only atomic layer thick ($\sim $ 0.35 nm). A few weeks later, this height is seen to increase to $\sim $ 0.8 nm. Raman spectroscopy of a single layer sheet shows the emergence of an intense D band which dramatically decreases as the number of layers in the sheet increase. The intense D band in monolayer graphene is attributed to the graphene conforming to the roughness of the substrate. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U28.00004: Characterization and Patterning of Ultrathin Epitaxial Graphene Grown on 4H-SiC Xuebin Li, Zhimin Song, Michael Sprinkle, Xiaosong Wu, Claire Berger, Walter de Heer Ultrathin graphite films are grown on the C face (000-1) of insulating single crystal 4H-SiC substrates by high temperature thermal decomposition of SiC. The films are characterized extensively. Atomic force microscopy images show extended atomically flat micron size terraces. Magneto-transport measurements indicate that transport of the films is dominated by the interface graphene layer which is electron-doped due to the built-in electric field at the interface. The films can be patterned with conventional lithography techniques and ribbons with widths less than 10nm can be produced. We present experimental results on several patterned gated structures. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U28.00005: ABSTRACT HAS BEEN MOVED TO P27.00015 |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U28.00006: Structural and mechanical properties of `graphene oxide'-based paper Dmitriy A. Dikin, Eric J. Zimney, Sasha Stankovich, Richard D. Piner, Geoffrey H. B. Dommett, Rodney S. Ruoff Free standing membranes (GO paper) were produced by exfoliation of graphite oxide in water to individual `graphene oxide' sheets (as a colloidal suspension) followed by their re-assembly by vacuum filtration. Study of the structure and morphology of the GO paper revealed that it is composed of highly packed and ordered layers of graphene oxide sheets separated by water molecules. Measurements of the mechanical response under tensile load revealed elastic deformation for small strain, followed by plastic deformation again for a relatively small region of strain, and then fracture without pullout of individual sheets or multi-layer stacks. GO paper possesses high modulus values of about 40 GPa and strength values around 130 MPa; each much higher than modulus or strength values for Bucky-paper or Grafoil. The experimental results support the conclusions of very effective load distribution and good binding between the GO sheets in which the self-adjusted amount of interlayer water plays a central role. After the deoxygenation of GO sheets the fabricated paper becomes a tunable semiconductor. Support from NASA (Award {\#} NCC-1-02037) through the University Research, Engineering, and Technology Institute on Bio-inspired Materials is appreciated. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U28.00007: Generation of Carbon Scrolls from Graphene films Humberto Gutierrez, Awnish Gupta, Qiujie Lu, Vincent Crespi, Peter Eklund Using a chemical process to delaminate graphene from HOPG, we are able to produce suspended graphene and n-graphene layer films (i.e., $n$GL\textbf{, }n=integer) in various organic solvents. The $n$GLs have lateral dimensions of several microns. We observe that in a matter of a few hours, the $n$GLs ``roll up'' on themselves to form scrolls. Here we present results of a study which investigates the role of the solvent in determining the characteristic time to ``roll up'' the $n$GL. Raman scattering, AFM and TEM is used to characterize the scrolls. A model will be presented to explain why the scrolling occurs. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U28.00008: Mechanism of growth of a graphitic edge in a Carbon Monoxide atmosphere Sujata Paul, Erik E. Santiso, Marco B. Nardelli The interaction of CO with zigzag and armchair graphite edge has been studied using density functional theory. Our results suggest that the growth of a zig-zag graphene edge in a CO atmosphere may happen through a multiple steps. A possible growth mechanism will start through the formation of pentagon with the adsorption of CO on the edge. The cleaning of oxygen atoms from the edge could happen through the desorption of O$_{2}$ or desorption of CO$_{2. }$ Further chemisorption of CO will cover the edge with the formation of 5-7-5 structures. Such adsorptions of CO will be followed by desorption of O$_{2}$ or desorption of CO$_{2 }$ and finally through the rearrangement of the 5-7-5 structures, the zig-zag edge would be restored. On the contrary the growth of armchair graphite edge have only two steps. The formation of hexagonal structures upon adsorption of CO and desorption of top oxygen atoms as CO$_{2}$ or O$_{2}$ . Our results of energy calculations suggest that the growth in the direction of a armchair wall is more favorable. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U28.00009: Boron doped graphene nanoribbons Thiago Martins, Hiroki Miwa, Antonio J.R. da Silva, A. Fazzio We will present a detailed study of the electronic, magnetic and transport properties of boron doped graphene nanoribbons, for various widths. The electronic structures and the equilibrium geometries were obtained through ab initio total energy DFT calculations. The transport properties were investigated using nonequilibrium Green's functions. Our results reveal that the substitutional boron atoms occupy the edge sites of nanoribbons, quenching the local ferromagnetism along the nanorribon edges. In addition, the presence of edge boron atoms break the symmetry between spin up and spin down transmittance channels. Those results suggest that, through a suitable doping process, it is possible to tailor the electronic current along the graphene nanoribbon. We thank FAPESP, CNPq and CENAPAD-SP. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U28.00010: Thermal-closing of holes put in single-graphene sheets of carbon nanotubes depending on its curvatures. Masako Yudasaka, Jin Miyawaki, Ryota Yuge, Takasumi Kawai, Jing Fan, Sumio Iijima Holes put in the walls of single-wall carbon nanotubes by oxidation are believed to be closed by heat treatment. We investigated this in detail using single-wall carbon nanohorn (SWNH), a type of single-wall carbon nanotubes. SWNHs are suitable to study this because they have high purities (95{\%}, no metal) and closed structure in the as-grown state, and the holes are easily opened by oxidation. Even numbers and sizes of holes are controllable. The nitrogen adsorption quantities measured at 77K clarified that the holes opened at the tips of tubes were closed easily by the heat treatment at 1473K in Ar, but those in the sidewalls were not, suggesting that the closing easiness depended on the tube curvatures. This was confirmed by the computer simulation. The hole closing kinetics of the tip holes was further investigated by changing the heat-treatment duration, as a result, two types of holes were found: one closed in a couple of minutes and the other in 30 minutes by the heat-treatment a 1473K, which may correspond to the variation of hole-sizes at the tips. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U28.00011: Fabrication of graphene nanogaps by electrical breakdown Brian Standley, Emma Schmidgall, Marc Bockrath We have fabricated {\it n}-graphene nanogaps which may be useful as an alternative to the metallic contacts used in current single molecule transistors. The nanogaps are formed by electrical breakdown of two-terminal {\it n}-graphene devices. We have characterized the gaps by atomic force microscopy and electrical transport measurements, both of which suggest that the gaps are narrow enough to capture a single molecule. The {\it n}-graphene contacts' two dimensional nature is expected to improve gate control by reducing charge screening. Additionally, the contacts' atomic flatness may allow in situ scanning tunneling microscopy imaging of the transistor molecule. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U28.00012: Structural Differences Between Graphite Grown on Si- and C- Terminated Polar Faces of 4H-SiC Joanna Hass, Rui Feng, Xuebin Li, Michael Sprinkle, Claire Berger, Edward Conrad In the last two years the transport properties of 2D graphene grown on SiC have shown that electron coherence lengths can exceed many microns. It is now critical to understand the source of these unique transport properties and explain their dependence on which polar face they are grown. We will present surface X-ray diffraction data that highlights the structural differences between graphite grown on C-terminated and Si-terminated 4H-SiC. We will show that the C-terminated graphite grows in domains more than an order of magnitude larger than the Si-terminated graphite.[1] Strain, islanding and complex rotational phases in the graphene will be presented. More importantly, X-ray reflectivity measurements reveal a tightly bound initial graphene layer, with a second graphene layer at an interlayer spacing significantly larger than in the bulk. The implications of this ``buffer'' layer will be discussed in terms of recent band structure calculations[2] and a possible explanation for transport seemingly being confined to a single graphene layer. [1] J. Hass, et al., App. Phys. Lett. \textbf{89}, 143106 (2006). [2] F. Varchon, et al., (to be published). [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U28.00013: Graphane: a two-dimensional hydrocarbon Jorge Sofo, Ajay Chaudhari, Greg Barber We predict the stability of a new\textit{ extended two-dimensional hydrocarbon} on the basis of first-principles total energy calculations. The compound that we call graphane is a fully saturated hydrocarbon derived from a single graphene sheet with formula CH. All of the carbon atoms are in sp$^{3}$ hybridization forming a hexagonal network and the hydrogen atoms are bonded to carbon on both sides of the plane in an alternating manner. Graphane is predicted to be stable with a binding energy comparable to other hydrocarbons such as benzene, cyclohexane, and polyethylene. We discuss possible routes for synthesizing graphane and potential applications as a hydrogen storage material and in two dimensional electronics. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U28.00014: Morphology and crack toughness behaviour of PP-MWNT nanocomposites R. Weidisch, M. Ganss, B. K. Satapathy, P. Poetschke, D. Jehnichen, A. Janke Morphology and crack toughness of PP-MWNT nanocomposites have been studied by AFM-WAXD and essential work of fracture approach respectively. A ductile-to-semiductile transition in the crack resistance behaviour of PP-MWNT nanocomposites and its interrelation to the structural attributes studied by SEM and DSC has been discussed. A maximum in the non-essential work of fracture was observed at 0.5 wt.-{\%} MWNT demonstrating enhanced toughness compared to pure PP, followed by a sharp decline as the MWNT content was increased to 1.5 wt.-{\%} reveals a ductile-to-semiductile transition. Fracture kinetics studies presents a qualitative picture of the nature of such a transition in terms of (a) switch over from non-steady (in pure PP) to steady state crack-tip-opening-displacement rate (in nanocomposites) and (b) ductile-to-semiductile transition being largely due to delayed-yielding in the nanocomposites . The time-resolved analysis of strain field offering insight into the crack propagation kinetics has revealed that such a transition is caused by rapid development of critical local stresses causing a shift of crack initiation to shorter time, resulting in a semi-ductile fracture of nanocomposites containing 1.5 wt.{\%} MWNT. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U28.00015: Electron fractionalization in two-dimensional graphene-like structures Chang-Yu Hou, Claudio Chamon, Christopher Mudry Electron fractionalization is intimately related to topology. In one-dimensional systems, fractionally charged states exist at domain walls between degenerate vaccua. In two-dimensional systems, fractionalization exists in quantum Hall fluids, where time-reversal symmetry is broken by a large external magnetic field. Recently, there has been a tremendous effort in the search for examples of fractionalization in two-dimensional systems with time-reversal symmetry. In this paper, we show that fractionally charged topological excitations exist on graphene-like structures, where quasiparticles are described by two flavors of Dirac fermions and time-reversal symmetry is respected. The topological zero-modes are mathematically similar to fractional vortices in $p$-wave superconductors. They correspond to a twist in the phase in the mass of the Dirac fermions, akin to cosmic strings in particle physics. [Preview Abstract] |
Session U29: Suspensions and Fluid Dynamics
Sponsoring Units: DFDChair: Jerzy Blawzdziewicz, Yale University
Room: Colorado Convention Center 303
Thursday, March 8, 2007 8:00AM - 8:12AM |
U29.00001: Wall-induced Particle migration in Dilute Suspensions of Spheres in Creeping Flow Jerzy Blawzdziewicz, Mauricio Zurita-Gotor, Eligiusz Wajnryb The effects of confinement on the dynamics of binary encounters between spherical particles in shear flow are studied for a system bounded by a single planar wall or two parallel planar walls under creeping flow conditions. We show that wall proximity gives rise to a new class of binary trajectories resulting in cross-streamline migration of particles. In contrast, in unbounded space spherical particles on open trajectories return to their original streamlines after a binary encounter is completed (with no non-hydrodynamic forces present). The physical origin of the new trajectories is explained in terms of counter-rotation of particle pairs that is driven by the dynamic pressure distribution. The new type of trajectories constitutes the dominant cross-streamline migration mechanism in dilute wall-bounded suspensions. We show that this mechanism is responsible for the unusually large self-diffusivity observed in experiments by Zarraga and Leighton (2002). The effect of the new migration behavior in dilute suspensions is illustrated using a Boltzmann--Monte Carlo simulation technique. We show that apart from the enhanced self-diffusivity, the walls may also cause formation of a layered suspension microstructure in the low-concentration regime. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U29.00002: Effect of surface roughness on rate-dependent slip in simple fluids. Nikolai Priezjev The influence of molecular-scale surface roughness on the slip length in a flow of simple fluids is investigated using molecular dynamics simulations. The parabolic fit of the steady state velocity profiles induced by a constant force is used to define the value of interfacial shear rate. At weak wall-fluid interactions, the slip length increases non-linearly with the shear rate provided that the liquid/solid interface forms incommensurable structures. A gradual transition to the linear rate-dependence is observed upon increasing the wall-fluid interaction. Thermal surface roughness is found to affect the slip behavior significantly: for soft walls the slip length weakly depends on the shear rate. With increasing elastic stiffness of the wall, the linear rate-dependence of the slip length is restored again. Periodically and randomly corrugated surfaces strongly suppress both the magnitude and slope of the rate-dependence of the slip length even for weak wall-fluid interactions. A relation to recent slip flow experiments is discussed. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U29.00003: Rheology of Deformable Particle Suspensions by Dissipative Particle Dynamics Anuj Chaudhri, Jennifer R. Lukes Understanding the behavior of colloidal suspensions, emulsions, and other complex fluids under shear flow is important in liquid crystal switching, lab-on-chip processing of biological fluids, self-assembly of polymer structures, and other areas of soft matter physics. Various analytical and computational approaches, including Brownian dynamics, dissipative particle dynamics, and Stokesian dynamics, have been applied to study the rheology of \textit{rigid} particle suspensions. Still lacking are methods capable of treating suspensions containing \textit{deformable} particles such as blood cells or macromolecules. Here we present a new, dissipative particle dynamics-based computational method with this capability. This method is used to calculate the shear rate dependence of viscosity for suspensions of deformable particles with varying stiffnesses. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U29.00004: Reducing Viscosity of Liquid Suspensions by pulsed electric or magnetic field R. Tao Viscosity of liquid suspensions is of great importance. Controlling the viscosity is vital in science and engineering. In electrorheological (ER) or magentorheological (MR) fluids, electric or magnetic field is used to increase the viscosity. However, in most cases we need to lower the viscosity. For example, reducing blood's viscosity improves circulation and prevents cardiovascular events. Lowering the viscosity of crude oil is the key to transporting offshore oil via undersea pipelines. Unfortunately, to date there are no effective methods for reducing the viscosity except by changing the temperature. In case that changing temperature is not an option, such as in the above examples, reducing the viscosity becomes formidable. Here we present a theory and experimental results showing that application of a suitable electric or magnetic field pulse can significantly reduce the viscosity of liquid suspensions for several hours with no change of temperature. The field induces dipolar interactions between the suspended particles and forces them to aggregate into large particles. The aggregation changes the rheological properties of the fluids and reduces the effective viscosity. Positive experimental results with MR fluids and crude oil indicate that this method, developed from the basic mechanism of viscosity, is universal and powerful for all liquid suspensions with broad applications. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U29.00005: Microbubbling viscous liquids and suspensions Ketan Pancholi, Mohan Edirisinghe Using a T-junction together with a cross flow technique, we have carried out a detailed study on the formation of near-monodisperse microbubbles in liquids with viscosities in the range of 5-950 mPa s. The data collected were analysed in the context of the classical momentum equation for viscous liquid flow to propose an analytical equation correlating dimensionless viscosity ratio ($\mu _{l}$/$\mu _{g})$ to the ratio of liquid pressure to gas pressure (P$_{l}$/P$_{g})$ required to generate bubbles. This equation is useful in predicting P$_{l}$/P$_{g}$ for microbubbling a liquid having a known viscosity. Our experimental results show that in the liquids investigated, the ratio of P$_{l}$/P$_{g}$, which is a function of dynamic equilibrium of pressure of liquid and gas at the T-junction, is decreasing proportional to dimensionless viscosity ratio. We calculated radial pressure for a given liquid pressure (P$_{l})$ to establish that for liquid viscosities $\ge 48.5$mPa s the radial velocity of liquid, which is responsible for imposing radial pressure on the gas-jet, dominates the mechanism of microbubble pinch-off. In contrast, in the low viscosity regime ($\le 48.5$mPa s), deceleration of the gas stream from the initial velocity is largely the cause of pinch-off of microbubbles. We made ceramic liquid foams using the technique. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U29.00006: Non-Newtonian Impact Denis Bartolo, Gregoire Narcy, Daniel Bonn Spray deposition is widely used in industry (spray painting, pesticide spraying...), but is often inefficient due to an unfavourable wetting interaction of the liquid with the surface. Non-Newtonian polymer effects have been suggested to improve the deposition efficiency, but so far the mechanism has remained elusive and controversial. Here we provide the detailed and quantitative mechanism of the action of the polymers, opening the way to use the non-Newtonian properties to control deposition. We study the impact and subsequent retraction of aqueous drops onto a hydrophobic surface for which rebound of the droplets limits deposition. Adding very small amounts of large molecular weight, flexible polymers dramatically slows down the retraction, inhibiting rebound. We show that the polymers generate strong normal stress effects near the moving contact line of the drop; these can be measured in conventional rheology and can be used to quantitatively account for the slowing down of the retraction. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U29.00007: Spectra of single bubble sonoluminescence from noble gas mixtures Mogens Levinsen In single bubble sonoluminescence a gas bubble trapped by a resonant sound-field emits pulses of light in synchrony with the exciting field. The exact nature of the light emitting processes is, however, not known, and the extent to which internal compressional waves or even shock-waves in the gas affect these processes is still an open question. Simulations suggest that most likely such waves would lead to segregation of species which presumably would have consequences for the intensity and spectrum of the light emitted. We have measured the spectra from single sonoluminescing bubbles seeded with various mixtures of noble gasses. The results are discussed in the light of theoretical expectations. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U29.00008: Steady-state structure formation of two-phase flow in porous media. Thomas Ramstad, Alex Hansen Transport of fluids in porous media is highly complex and creates remarkable patterns. We study these structures and the physics behind them in numerical models based on real porous sediments. These are embedded in a steady-state environment so that they represent selections of a larger, global system. As the saturation of the phases are changed within our models , we see a process towards creation of fluid clusters that eventually span the whole system and have a distribution that approaches a power law behavior. The critical saturation where this phase transition takes place, is dependent of the ratio between viscous and capillary forces inside the pores. We study these scaling properties and the physics that leads to the cluster behavior. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U29.00009: Numerical simulation of flow past an oscillating cylinder beneath a free surface Serpil Kocabiyik, Oleg Gubanov, Larisa Mironova A computational study of laminar flow of a viscous incompresible fluid past an oscillating cylinder close to a free surface is performed. The integral form of unsteady two dimensional Navier- Stokes equations is only discretized in the fluid flow region using fixed Eulerian staggered grid. Well-posed boundary conditions are used at the inflow and outflow boundaries. The no- slip boundary conditions are prescribed at the solid boundary. At the free surface boundary conditions are described by neglecting the motion of ambient air. The volume of fluid method is used to track a moving free surface interface. A piecewise- linear interface reconstruction algorithm is used at each time step for determining the position of both the free surface and fluid-body interfaces. The reconstructed free surface is then advected using computed local velocity field based on a geometrical area-preserving volume of fluid advection algorithm. The numerical simulations are conducted at a fixed Reynolds number, $R=200$, and at displacement amplitude-to-cylinder diameter ratios of $A=0.25$ and $A=0.5$ when submergence depth- to-cylinder diameter ratio is $1.25$. Previously computed and observed flow fields around submerged cylinders are compared to current numerical results and good agreement is found. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U29.00010: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U29.00011: Non-Newtonian behavior of complex plasma fluids Alexei Ivlev, Victor Steinberg, Roman Kompaneets, Gregor Morfill One of the remarkable aspects of complex plasmas is that although they are intrinsically multiphase systems, the rate of momentum exchange through collisions between the microparticles (grains) can exceed the coupling to the background neutral gas significantly. Therefore complex plasma fluids can act as an essentially single-fluid system. Numerical simulations predict that the shear viscosity of complex plasmas should have strong non-monotonous dependence on the kinetic temperature of grains. We proposed a self-consistent model which allows us to obtain explicit dependence of the viscosity on the velocity shear rate, with well-pronounced shear-thinning and thickening effects. Under certain condition, the stress vs. strain rate dependence becomes N-shaped, suggesting formation of shear bands. We performed a series of experiments in a planar or cylindrical shear flow geometry, similar to the Couette and Poiseuille flows. This allowed us to retrieve the viscosity of complex plasmas, which turned out to be in fairly good agreement with the theory. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U29.00012: Magnetic-Force Enhanced Temperature Gradient Jonathan Fraine, Weili Luo The temperature gradient was established in a quasi-one dimensional magnetic fluid by controlling the initial heating and cooling rates. Measurements were done to monitor temperature gradient verses time before and after the cooling and heating were stopped in both zero and applied magnetic field. We found that the magnetic field can enhance the temperature gradient across the sample. The theoretical calculation shows that the effect of field on the temperature gradient is attributed to the magnetic body force that depends on the gradient of the susceptibility. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U29.00013: Dynamics of the Shock Waves Generated by High-Speed Liquid Jets Kyoung-su Im, Seong-Kyun Cheong, Jin Wang, Ming-Chia Lai Ultra fast x-radiography and a multiphase numerical simulation were used to reveal complete dynamical characteristics of the shock waves generated by supersonic liquid jets. Unlike the conventional shock waves by a rigid body compression, this shock waves generated by highly transient liquid jets are characterized by an immediately expansion after short compression caused by the liquid deformation due to aerodynamic drag on the jet front. A transition mechanism from the transonic to the supersonic has been clearly analyzed. With the quantitative analysis and the numerical simulation, the dynamic behavior induced by the compression and decompression in ambient gas in the vicinity of the shock front has been examined, and also we demonstrated the dependence of the shock characteristics on spray angles. Under specific injection condition, we provided the detailed internal structures and interacting mechanisms between the ambient gas and liquid spray jet by simultaneously simulating the fluid parameters such as gas velocities, density contours, and liquid sprays. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U29.00014: Optical tweezer based study of the motion of a sphere in an oscillatory boundary layer Shankar Ghosh, Prerna Sharma, Shobo Bhattacharya Drag forces on a single polystyrene sphere in the vicinity of an oscillatory plate have been measured using an optical tweezer. The phase of the sphere is found to be a sensitive probe of the dynamics. The evolution of the phase from an inertia-coupled regime to a velocity-coupled regime is explored. The frequency dependence of the response is found to be characteristic of a damped oscillator with an effective inertia which is orders of magnitude greater than that of the bare sphere. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U29.00015: Dynamics of electrorhoelogical(ER) fluids Jianwei Zhang, Chun Liu, Ping Sheng Electrorhoelogical (ER) fluids are a class of colloids whose rheological characteristics can be controlled by applying an external electric field. Most applications of ER fluid are determined by its dynamic properties, reflecting the competition between the kinetic energy and internal energies. The relevant physics of dynamic processes is very different from that in static situations. We derive the fully coupled hydrodynamic system modeling the ER fluid dynamics using the energetic variational approach. The interaction between particles is treated as dipole-dipole in character, with a repulsive core. The solid particles and carrier fluid are treated as a two-component incompressible material. The induced electrical polarization and local fields are obtained self- consistently. The forces on the particles and the fluid are derived from the coupling between the transport of the particles and the induced stress. The total force on the moving boundary in stationary state is calculated via total dissipation inside the ER fluid. [Preview Abstract] |
Session U30: Focus Session: Fluid Dynamics of Animal Motion
Sponsoring Units: DFDChair: Meredith Betterton, University of Colorado
Room: Colorado Convention Center 304
Thursday, March 8, 2007 8:00AM - 8:36AM |
U30.00001: Modeling Bodies Locomoting through Fluids Invited Speaker: Locomotion of an organism through a fluid is one of the most fascinating fluid-structure interactions. How an organism accomplishes this feat depends on many things, such as whether the fluid is inertial (i.e., big bodies, high Reynolds number), overdamped(small bodies, low Reynolds number), or somewhere in between. The presence of boundaries, or of other moving bodies in the fluid, or non-Newtonian behavior of the fluid, makes the problem richer. I will not discuss the biology of locomotion per se, but rather focus on what mathematical models and simulations of prototype physical systems reveal of the core physical interactions that underlie locomotion. This includes how bodies can locomote by taking advantage of symmetry breaking instabilities in fluidic response, the instability and persistence of orientational order in active suspensions, and the effect of visco-elasticity at low Reynolds number. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U30.00002: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U30.00003: Swimming in a viscoelastic fluid Eric Lauga The fluid mechanics of swimming microorganisms was pioneered by G.I. Taylor more than fifty years ago, and is one of the most mature branch of biophysics. Most previous studies have assumed the fluid to be Newtonian. However, a variety of biologically relevant situations involve non-Newtonian fluids, including sperm motion in cervical mucus as well as ciliary transport of mucus in the lungs. In this talk, we present simple models of swimming in viscoelastic fluids and discuss the impact of elastic stresses on swimming kinematics and energetics. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U30.00004: A Model for the Viscous Synchronization of Bacterial Flagella Qian Bian, Leila Setayeshgar, Thomas R. Powers, Kenneth Breuer Many flagellated bacteria propel themselves by rotating several helical flagella. The motors that rotate these filaments operate in a constant torque mode, and can alternate between counter-clockwise and clockwise motion. Although they reverse direction independently and randomly, the filaments are observed to coordinate and form a bundle during the run phase of the cell motion. We bring both experimental and theoretical tools to study a model problem which considers rotating paddles rather than helical filaments. The paddles are simpler both to construct and to model, and exhibit stronger viscous interactions than thin helices. Experimentally, we find that the paddles coordinate in about 15 rotations, and stay in synchronized motion with a phase difference of $\pi/2$, although this phase difference increases if there is a torque mismatch between the two motors. The synchronization is observed to persist indefinitely. However, as the paddle separation increases, the synchronization is weaker, and can exhibit instabilities. Theoretical models based on the long-range hydrodynamic interaction of Stokes flow are compared with the experimental results. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U30.00005: The mechanics of slithering David Hu, John Bush, Michael Shelley Snakes propel themselves over land using a variety of techniques, including a unidirectional accordion-like mode, lateral sinuous slithering and sidewinding. We explore these friction-based propulsion mechanisms through a combined experimental and theoretical investigation. Particular attention is given to classifying the gaits of snakes according to Froude number and the relative magnitudes of the frictional forces in the tangential and normal directions. While the term ``gait'' is usually used to describe a sequence of foot movements, here it refers to a sequence of undulations made by the limbless snake. In a simple mass-spring model, we prescribe the muscle activity of the snake and then calculate its motion as required by the torque and force balances on its body. A key feature of our model is that it allows us to rationalize the mode of locomotion of the snake on the basis of propulsive efficiency. [Preview Abstract] |
Session U31: Supersolids Theory
Sponsoring Units: DCMPChair: Stratos Manousakis, Florida State University
Room: Colorado Convention Center 401
Thursday, March 8, 2007 8:00AM - 8:12AM |
U31.00001: Thermodynamic instability of a vacancy gas in solid Helium Lode Pollet, Massimo Boninsegni, Anatoly Kuklov, Nikolay Prokof'ev, Boris Svistunov, Matthias Troyer The supersolid phase of matter, characterized by non-dissipative flow in a crystal, has been elusive for some 35 years. The recent discovery of a non-classical moment of inertia in solid $^4$He by Kim and Chan has provided the first piece of experimental evidence, although its interpretation in terms of supersolidity of the ideal crystal phase remains controversial. Using Quantum Monte Carlo methods, we investigate the long-standing question of vacany-induced superflow. We find that a uniform gas of vacancies is thermodynamically unstable against separation into two phases, an insulating, vacancy-free crystal and a liquid. We investigate the thermodynamics of other defects, such as edge dislocations. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U31.00002: Superfluidity of Grain Boundaries in Solid $^4$He Nikolay Prokofiev, Lode Pollet, Massimo Boninsegni, Anatoly Kuklov, Boris Svistunov, Matthias Troyer Superfluid grain boundaries (GB) were proposed as a plausible scenario of the effect discovered by Kim and Chan, and have now been experimentally observed (at the melting point) by Sasaki, Ishiguro, Caupin, Maris, and Balibar [Science {\bf 313}, 1098 (2006)]. We report results of large-scale quantum Monte Carlo simulations (up to $\approx 14000$ atoms) of GB in $^4$He crystals at the melting pressure, as well as deep inside the solid phase. We find that generically GB are superfluid, with transition temperatures about $\sim$ 1 K--0.5 K (we estimate the maximum $T_c$ at about $1.50(5)$~K). At the melting point, the density of particles in the GB is slightly higher than that of a crystal and the GB width is about 3 interatomic spacings. We also observe insulating grain boundaries which typically occur at special orientations of the two crystallites. By simulating GB in direct contact with the superfluid liquid and observing that the system of two liquid-solid and one solid-solid interfaces is mechanically stable we prove that the phenomenon of GB-superfluidity is not related to the close vicinity of the melting line. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U31.00003: Supersolid is Dirty Jiansheng Wu, Philip Phillips A microscopic model for the supersolid phase in $^4$He is given. On the grain boundary, the motion of atoms is well described by a disordered Bose-Hubbard model. We argue that the clean system is a commensurate Mott insulator but in the presence of disorder, a supersolid state obtains. At work is the disorder-induced closing of the Mott gap. We find that the transition temperature to the supersolid state is an increasing function of disorder as is seen experimentally. In addition, we are able to explain the saturation of the superfluid fraction below a characteristic temperature and the pressure dependence of the superfluid fraction. Finally, we also find that a glassy Bose metal phase (BM) is possible and possesses a period shift, though it lacks superflow. This latter observation is useful in explaining the period shift without superflow in hydrogen. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U31.00004: Defects and Impurities in Solid $^4$He Keola Wierschem, Efstratios Manousakis We perform path integral Monte Carlo studies of defects and impurities in solid $^4$He near the low temperature melting transition. The worm algorithm, recently developed for continuum systems, is used to study off-diagonal properties such as the one-body density matrix (OBDM). While this quantity approaches zero exponentially with increasing particle displacement for the ``pure'' solid, interstitial defects and $^3$He impurities appear to enhance and/or stabilize the OBDM at long distances. Thus, imperfections in solid helium may lead to the formation of a condensate. These calculations are repeated for two-dimensional solid helium, and compared with results from lattice boson models. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U31.00005: Understanding supersolids Miklos Gulacsi, Andre Stoffel We model the newly discovered supersolid phase of ${}^4$He by a hard-core bosonic quantum lattice model in 3 dimension including nearest and next-nearest neighbor interactions. As hard-core Boson exhibit the same algebra as spin-1/2 operators there exists a one-to-one correspondence to the anisotropic Heisenberg model in an external field. To solve this Heisenberg model we used the Tyablikov Green's function technique and in order to obtain a closed set of equations we used a cumulant decoupling scheme. The obtained Green's functions have been used to study the properties of the system. Here, we are particularly interested in the normal-solid (NS) and supersolid (SS) phases as well as the corresponding phase transition. It was long proposed that vacancies and defects may play a crucial role in the formation of the supersolid phase. Hence we studied the incommensurability which is a measure of the net fraction of vacancies. For the NS phase we re-obtained the well-known thermal activation theory. However, the incommensurability in the SS displays a rather different behavior, which also suggests that the NS to SS transition is a commensurate-incommensurate transition. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U31.00006: Field Induced Supersolid Phase in Spin-One Heisenberg Models Pinaki Sengupta, Cristian Batista We use quantum Monte Carlo methods to demonstrate that the quantum phase diagram of the S=1 Heisenberg model with uniaxial anisotropy contains an extended supersolid phase. We also show that this Hamiltonian is a particular case of a more general and ubiquitous model that describes the low energy spectrum of a class of {\it isotropic} and {\it frustrated} spin systems. This crucial result provides the required guidance for finding experimental realizations of a spin supersolid state. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U31.00007: A striped supersolid phase in bosons on the triangular lattice Roger Melko Using large-scale quantum Monte Carlo simulations, we explore the ground-state phase diagram of bosons hopping on a triangular lattice with nearest (V) and next-nearest (V') neighbor repulsive interactions. In the limit where V=0 but V' is large, we find an example of an unusual striped supersolid state that is stable at 1/2-filling. We discuss the peculiar properties of this phase, as well as the phase transitions out of it into the neighboring superfluid and Mott phases. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U31.00008: Theory of Small Para-Hydrogen Clusters: Magic Numbers and Superfluid Sizes Saad Khairallah, Mikhail Sevryuk, David Ceperley, Peter Toennies We apply the Path Integral Monte Carlo method to study the low temperature structures and superfluidity of para\-hydrogen clusters with up to 40 molecules. We find an enhanced stability at certain `` magic '' cluster sizes and sharp jumps in superfluidity for specific cluster sizes ($N>26$). Superfluidity is largely localized on the surface and coexists with clusters with solid- like core. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U31.00009: Computer simulations of helium-solvated ions: solid-like versus liquid-like defect structures Stefano Paolini, Francesco Ancilotto, Flavio Toigo The local order around several alkali (Li$^+$ and Na$^+$) and alkali-earth (Be$^+$, Mg$^+$ and Ca$^+$) ions in $^4$He clusters has been studied using ground-state path integral Monte Carlo simulations. We apply a criterion based on multipole dynamical correlations to discriminate between solid-like versus liquid-like behavior of the He solvent surrounding the impurity-ion. In agreement with existing experimental measurements in bulk helium, our findings suggest that Be$^+$ produces a solid-(``snowball'')-like structure, similarly to alkali ions and in contrast to heavier alkali-earth ones, for which a liquid-like environment is predicted. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U31.00010: The BCS-BEC crossover in density in GaAs heterostructures for bilayers of electrons and holes with mismatched densities David Neilson, Pierbiagio Pieri, Giancarlo Strinati We have investigated excitonic superfluidity in electron-hole bilayers in GaAs at low temperatures. We analyze the crossover from the BCS limit of overlapping pairs (high carrier density) to the BEC limit of non-overlapping tightly-bound pairs (low carrier density) by independently varying the densities of the electrons and holes. The different electron and hole effective masses in GaAs causes the phase diagram to depend strongly on the direction of the density imbalance. We can identify the crossover region between the BCS and BEC regimes in the phase diagram, and we find the richest variety of phases in this crossover region. We propose detection of a jump in the electron and hole chemical potentials across zero-density imbalance as the criterion for the occurrence of superfluidity. We make a comment on the relation of our results to density and mass imbalances in ultracold Fermi atoms. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U31.00011: ABSTRACT HAS BEEN MOVED TO S31.00014 |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U31.00012: Possible role of $^3He$ impurities in solid $^4He$ Efstratios Manousakis We use a quantum lattice gas model to describe the essential aspects of the motion $^4He$ atoms and of a $^3He$ impurity in solid $^4He$. We find that $^3He$ impurities promote $^4He$ atoms to interstitial sites and this can turn the bosonic quantum crystal into a metastable supersolid. It is suggested that $^3He$ impurity atoms, which produce the interstitial $^4He$ atoms, might have been reabsorbed by pure $^4He$ solid formed during the first stage of a multi-stage solid $^4He$ nucleation process. While we use the ``spin''-wave approximation and low dimensional lattices to illustrate some of the ideas, we argue that the conclusions drawn from these studies may be valid for the real system. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U31.00013: Magnetothermoelectric response at a superfluid--Mott insulator transition Miraculous Bhaseen, Andrew Green, Shivaji Sondhi We present recent results on the finite temperature magnetothermoelectric response in the vicinity of a superfluid--Mott insulator quantum phase transition [cond-mat/0610687]. We focus on the particle-hole symmetric transitions of the Bose--Hubbard model, and combine Lorentz invariance arguments with entropy drift and quantum Boltzmann calculations. Depending on the ratio of the applied fields, the model displays distinct regimes of behavior. We discuss how a non-vanishing thermoelectric tensor and a finite thermal conductivity are supported in this quantum critical regime. [Preview Abstract] |
Session U32: Focus Session: Novel Phases in Quantum Gases
Sponsoring Units: DAMOPChair: Charles Clark, National Institute of Standards and Technology
Room: Colorado Convention Center 402
Thursday, March 8, 2007 8:00AM - 8:12AM |
U32.00001: Cold Atoms on Frustrating Lattices Dagim Tilahun, Allan MacDonald Ultracold atoms in optical lattices undergo a quantum phase transition from a superfluid to a Mott insulator as the lattice potential depth is increased. We present a theory of the ground state and the elementary excitations of cold atoms in which the potential $\Sigma_i$ which induces coherence between different number states on a given site is elevated from a variational parameter to a quantum degree of freedom. In this approach mean-field theory is equivalent to minimizing the energy with respect to the $\Sigma_i$. The theory is applied to the Boson Hubbard model of optical lattice systems, to frustrated lattice models for rotating atoms, and to inhomogenous systems with a harmonic trapping potential superimposed on the lattice potential. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U32.00002: Atomtronics: Ultracold atom analogs of electronic circuits and devices Ronald Pepino, Brian Seaman, Murray Holland Atomtronics focuses on creating an analogy between electronic devices and circuits with ultracold atoms. Such an analogy can come from the Mott-insulator characteristic of ultracold gases trapped in optical lattices. The highly tunable parameters of optical lattices allow one to construct and precisely manipulate them. This lets one to create conditions that cause atoms in lattices to exhibit the same behavior as electrons moving through solid state media. We present our model and show how the atomtronic diode and the field effect transistor can be realized. These fundamental components can lead to the construction of other atomtronic devices such as the bipolar junction transistor and possibly amplifiers and switches. Besides the similarities to condensed matter systems, there are also differences that can be explored: atomtronic current carriers can be either bosons or fermions having spin not equal to 1/2. Also, there are no thermal fluctuations or phonon modes associated with the lattice itself. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U32.00003: Reversible quantum phase dispersion in two-component quantum gases A. Widera, S. Trotzky, P. Cheinet, S. F\"olling, F. Gerbier, I. Bloch Controlling fundamental interactions on an atomic scale has offered the unique possibility to engineer strongly correlated quantum states in ultracold atomic samples during recent years. In particular, controlling the interactions of an ensemble of particles implies the possibility of pushing into the intriguing regime of coherent many-body physics. Here we report on the controlled manipulation of a quantum many-body state in a 2D-array of mesoscopic spinor gases. Starting from a coherent spin-state, controllable interatomic interactions close to a Feshbach resonance are used to induce a dynamics which changes the distribution of intrinsic spin fluctuations. The resulting phase dispersion is detected by monitoring the decay of coherence through Ramsey spectroscopy. We demonstrate the coherent nature of this interaction effect by time-reversal of the dynamics, observing a substantial revival of coherence in the system. These results have implications not only on our understanding of decoherence in ultracold atomic systems but also point towards the possibility of dynamically creating correlated spin states or even maximally entangled mesoscopic Schr\"odinger cats. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U32.00004: Finite quantal systems -- from semiconductor quantum dots to cold atoms in traps Invited Speaker: Many-body systems that are set rotating may form vortices, characterized by rotating motion around a central cavity. This is familiar to us from every-day life: you can observe vortices while stirring your coffee, or watching a hurricane. In quantum physics, vortices are known to occur in superconducting films and rotating bosonic He-4 or fermionic He-3 liquids, and recently became a hot topic in the research on cold atoms in traps. Here we show that the rotation of trapped particles with a repulsive interaction may lead to vortex formation regardless of whether the particles are bosons or fermions. The exact many-particle wave function provides evidence that the mechanism is very similar in both cases. We discuss the close relation between rotating BECs and quantum dots at strong magnetic fields. The vortices can stick to particles to form composite particles, but also occur without association to any particular particle. In quantum dots we find off-electron vortices that are localized, giving rise to charge deficiency or holes in the density, with rotating currents around them. The vortex formation is observable in the energetics of the system. ``Giant vortices'' may form in anharmonic potentials. Here, the vortices accumulate at the trap center, leading to large cores in the electron and current densities. Turning from single traps to periodic lattices, we comment upon the analogies between optical lattices with cold fermionic atoms, and regular arrays of few-electron quantum dots. Trapping a few (N $<$ 12) fermions in each of the single minima of the lattice, we find that the shell structure in the quantum wells determines the magnetism, leading to a systematic sequence of non-magnetic, ferromagnetic and antiferromagnetic states. \newline M. Toreblad \textit{et al}., Phys. Rev. Lett. 93, 090407 (2004); \newline H. Saarikoski, \textit{et al}., Phys. Rev. Lett. 93, 116802 (2004) , Phys. Rev. B 71, 035421 (2005); \newline M. Manninen, \textit{et al.}, Phys. Rev. Lett. 94, 106405 (2005); \newline E. R\"{a}s\"{a}nen, \textit{et al., }Phys. Rev. B 73, 235324 (2006); \newline M. Koskinen, \textit{et al.}, Phys. Rev. Lett. 90, 066802 (2003) ; \newline K. Karkkainen, \textit{et al.,} to be published (2006) [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U32.00005: Thermal Fluctuations of Vortex Matter in Trapped Bose-Einstein Condensates Steinar Kragset, Egor Babaev, Asle Sudbo We perform Monte Carlo studies of vortices in three dimensions in a cylindrical confinement, with uniform and nonuniform density. The former is relevant to rotating 4He, the latter is relevant to a rotating trapped Bose--Einstein condensate. In the former case we find dominant angular thermal vortex fluctuations close to the cylinder wall. For the latter case, a novel effect is that at low temperatures the vortex solid close to the center of the trap crosses directly over to a tension-less vortex tangle near the edge of the trap. At higher temperatures an intermediate tensionful vortex liquid located between the vortex solid and the vortex tangle, may exist. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U32.00006: Radial and angular rotons in trapped dipolar gases Shai Ronen, Daniele Bortolotti, John Bohn We study Bose-Einstein condensates with purely dipolar interactions in oblate (pancake) traps. We find that the condensate always becomes unstable to collapse when the number of particles is sufficiently large. We analyze the instability, and find that it is the trapped-gas analogue of the ``roton- maxon'' instability previously reported for a gas that is unconfined in two dimensions. In addition, we find that under certain circumstances, the condensate wave function attains a biconcave shape (like a red-blood cell), with its maximum density away from the center of the gas. These biconcave condensates become unstable due to azimuthal excitation - an angular roton. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U32.00007: Cold atoms in time dependent optical lattices I. B. Spielman, B. Brown, P. Lee, N. Lundblad, J. V. Porto, W. D. Phillips Cold atoms in optical lattices provide new avenues for studying iconic condensed matter problems. Using an initially Bose condensed sample of $^87$Rb atoms, we first implement the Bose-Hubbard model (the intensity of the static lattice potential determine the constants in the Bose-Hubbard model). This ``native'' Hamiltonian, with only on-site interactions, exhibits just two phases of matter: insulator and superfluid. Additional phases, such as a supersolid and density wave, are expected when nearest-neighbor interactions are added. Here we show preliminary results where we extend the ``native'' Bose-Hubbard Hamiltonian by rapidly varying the lattice potential. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U32.00008: Topological defects and the 2D superfluid transition in $S=1$ spinor condensates Subroto Mukerjee, Cenke Xu, Joel Moore Condensates of non-zero spin have recently attracted a lot of interest both theoretically and experimetally. The spin degree of freedom can give rise to interesting magnetically ordered phases. This talk will focus on condensates of Spin-1 atoms ($^{23}$Na, $^{87}$Rb). These will be shown to have intresting ground state manifolds and topological defects. The toplogical defects play an important role in the superfluid transition in two dimensions. The low temperature phase of $^{23}$Na will be shown to be a spin disordered nematic superfluid of boson pairs. The superfluid transition is of the Kosterlitz-Thouless type but mediated by half vortices. Extensions of these ideas to higher spin systems will be discussed. Journal Ref: Phys. Rev. Lett. 97, 120406 (2006) [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U32.00009: Kosterlitz-Thouless physics in a one-dimensional optical lattice Anibal Iucci, Miguel A. Cazalilla, Thierry Giamarchi We study a system of quasi two-dimensional Bose gases formed in the nodes of a one-dimensional optical lattice potential. We focus on the effect of the tunneling of the atoms between adjacent planes on the Kosterlitz-Thouless crossover recently observed in the experiments of the Paris group [Z. Hadzibabic \emph{et al}., Nature (London) \textbf{441}, 1118 (2006)]. We compute the contrast of the interference pattern between two condensates, finding a behavior different from the one observed in the Kosterlitz-Thouless crossover. Finally, we consider the stack of a large number of pancakes. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U32.00010: Superfluid, Supersolid, and Phase Separation in Strongly Correlated Square Lattice Bosons Liang Ren, Ziqiang Wang The Bose-Hubbard model and its mapping onto the quantum spin-1/2 XXZ model have played an important role in the understanding of the possible phases of strongly correlated lattice bosons. We present here a different mapping to the quantum spin-1/2 model in the hard-core limit and a mean field solution that accounts for both the direct and the exchange correlations on equal footing. We discuss the possible phase structure as a function of boson density, involving superfluid, supersolid, Neel solid, and phase separation, and make comparisons to the available quantum Monte Carlo simulations. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U32.00011: Exotic Superconducting Phases of Ultracold Atom Mixtures on Triangular Lattices Shan-Wen Tsai, Ludwig Mathey, Antonio H. Castro Neto We study two-dimensional Bose-Fermi mixtures of ultracold atoms on a triangular optical lattice, in the limit when the velocity of bosonic condensate fluctuations is much larger than the Fermi velocity$^1$. Interactions, lattice geometry and frustration lead to a rich phase diagram in this system. Using functional renormalization group techniques we show that this phase diagram contains exotic superconducting and spin-density wave phases. For spin-1/2 fermions on an isotropic lattice we find a competition of $s$-, $p$-, extended $d$-, and $f$-wave symmetry, as well as antiferromagnetic order. For an anisotropic lattice, we further find an extended $p$-wave phase. A Bose-Fermi mixture with spinless fermions on an isotropic lattice shows a competition between $p$- and $f$-wave symmetry.\\ $^1$ L. Mathey, S.-W. Tsai, A.H. Castro Neto, cond-mat/0609212 [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U32.00012: Temperature scale and Adiabatic Processes of Bosons in Optical Lattices Qi Zhou, Tin-Lun Ho We show that as the optical lattice is ramped up adiabatically in a Bose gas, the temperature first decreases in the superfluid regime due to kinetic effects, but eventually increases in the Mott regime due to interaction effects. We also show that in the Mott regime, the density profile of superfluid between Mott steps can be used as a temperature scale. [Preview Abstract] |
Session U33: Quantum Algorithms, Simulation, and Error Correction
Sponsoring Units: GQIChair: Ian Durham, Saint Anselm College
Room: Colorado Convention Center 403
Thursday, March 8, 2007 8:00AM - 8:12AM |
U33.00001: Quantum Simulations of Classical Systems Rolando Somma, Gerardo Ortiz, Cristian Batista, Emanuel Knill Understanding the properties of classical systems on a lattice using numerical methods is, in general, a very hard problem. In this talk I will focus then on the quantum simulations of classical systems. That is, if we had a quantum computer, which properties could be obtained more efficiently on it than on a conventional one?. For this purpose, I will introduce a classical-to-quantum mapping that will allow us to understand classical and quantum annealing procedures as two independent paths on Quantum-Hamiltonian space. I present then the corresponding quantum algorithms to simulate classical systems and give convergence rates, determined by the adiabatic theorem, to assure successful simulation. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U33.00002: Simple proof of equivalence between adiabatic quantum computation and the circuit model Ari Mizel, Daniel Lidar, Morgan Mitchell We prove the equivalence between adiabatic quantum computation and quantum computation in the standard circuit model. An explicit adiabatic computation procedure is given that generates a ground state from which the answer can be extracted. The amount of time needed is evaluated by computing the gap. We show that the procedure is computationally efficient. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U33.00003: Quantum cellular automata and quantum simulation Peter Love Quantum information theory can address ground state properties through DMRG-like classical methods, and through proposals for the use of phase estimation-based quantum algorithms. However, the dynamics of quantum systems present greater challenges to both classical and quantum computational methods. Quantum cellular automata provide a simple arena in which to address questions about quantum dynamics. Prior work has yielded decision procedures to determine when the local rule leads to globally unitary dynamics. I will describe a particular class of automata, unitary by construction, and discuss their relevance to the quantum computational complexity of quantum dynamics. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U33.00004: A Two-dimensional Lattice Ion Trap for Quantum Simulation Tongyan Lin, Kenneth Brown, Xie Chen, Grace Cheung, Christopher Pearson, Isaac Chuang Quantum simulations offer the possibility of answering quantum spin system dynamics questions which may otherwise require unrealistic classical resources to solve. Such simulations may be implemented using well-controlled systems of effective spins, including, as we demonstrate, two-dimensional lattices of locally interacting ions. We present experimental results from a model ion lattice system, realized as a surface electrode rf trap with a square lattice geometry. Using 440 nm diameter charged microspheres, we loaded a 30 $\times$ 36 lattice with a spacing of 1.67 mm. When the trap is driven at 2 kHz and 375 volts, we observe isolated ion secular frequencies of 170 Hz perpendicular to the trap, and Coulomb repulsion between ions at different lattice sites consistent with numerical modeling. These results, when scaled to single-atom ion charge-to-mass ratios, and linewidths achievable using standard microlithography, are promising for quantum simulations with planar ion trap lattices. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U33.00005: Making Moderately Large Ancillae Bryan Eastin Block coding has long been hailed as a practical method of decreasing the effective error probability. Compared with concatenated coding, large block codes require fewer encoding qubits to achieve the same logical error rate. The size of the quantum code employed, however, is not the only factor in determining the total number of qubits required. Another important aspect is the efficiency of procedures for constructing ancillary logical states used for computation and error correction. In this regard, block codes are at a disadvantage because there is no known method of efficiently preparing logical ancillae for block codes that does not rely on concatenation. For practical applications, however, it is sufficient to be capable of preparing logical ancillae for codes with large finite block size. This presentation describes a method of making moderately large ancillae with benign error properties. The form of the errors will be discussed as well as the method's resource scaling and the range of parameters for which it is effective. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U33.00006: Quantum Error Correction Beyond Completely Positive Maps Daniel Lidar, Alireza Shabani We present a generalized theory of quantum error correction (QEC) that applies to any linear map, in particular maps that are not completely positive (CP). This theory of ``linear quantum error correction'' is applicable in cases where the standard and restrictive assumption of a factorized initial system-bath state does not apply. For linear maps that preserve positivity and/or Hermiticity, we find that standard QEC based on CP recovery maps still applies. Other linear maps generally require non-CP recovery operations. We illustrate our findings with examples of QEC for non-CP maps. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U33.00007: Adiabatic Quantum Computation and the Theory of Quantum Phase Transitions William Kaminsky, Seth Lloyd We present a general approach to determining the asymptotic scaling of adiabatic quantum computational resources (space, time, energy, and precision) on random instances of NP-complete graph theory problems. By utilizing the isomorphisms between certain NP-complete graph theory problems and certain frustrated spin models, we demonstrate that the asymptotic scaling of the minimum spectral gap that determines the asymptotic running time of adiabatic algorithms is itself determined by the presence and character of quantum phase transitions in these frustrated models. Most notably, we draw the conclusion that adiabatic quantum computers based on quantum Ising models are much less likely to be efficient than those based on quantum rotor or Heisenberg models. We then exhibit practical rotor and Heisenberg model based architectures using Josephson junction and quantum dot circuits. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U33.00008: Simulating the time-dependent Schr\"odinger equation with a quantum lattice-gas algorithm Zachary Prezkuta, Mark Coffey Quantum computing algorithms promise remarkable improvements in speed or memory for certain applications. Currently, the Type II (or hybrid) quantum computer is the most feasible to build. This consists of a large number of small Type I (pure) quantum computers that compute with quantum logic, but communicate with nearest neighbors in a classical way. The arrangement thus formed is suitable for computations that execute a quantum lattice gas algorithm (QLGA). We report QLGA simulations for both the linear and nonlinear time-dependent Schr\"odinger equation. These evidence the stable, efficient, and at least second order convergent properties of the algorithm. The simulation capability provides a computational tool for applications in nonlinear optics, superconducting and superfluid materials, Bose-Einstein condensates, and elsewhere. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U33.00009: Implement quantum search and quantum counting algorithms by geometric quantum computation Qinghua Zhang, Guilu Long, Zidan Wang We derive the matching condition in the SO(3) picture of the generalized Grover's quantum search algorithm. We also give a more concise formula for evaluating the number of the iterations needed in the searching. This will help us a lot in the operation and get the target with certainty easily. We propose a new adiabatic Abelian geometric quantum computation strategy to implement quantum search and quantum counting algorithms based on the non-degenerate energy eigenstates in superconducting phase-qubit systems. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U33.00010: Multi-dimensional linear diffusion for image enhancement on a type-II quantum computer. Gabriel Colburn, Mark Coffey We describe how multi-dimensional linear diffusion with application to image processing could be carried out on a hybrid classical-quantum computer. By using a hybrid approach, a variety of applications are possible that purely quantum computers are not suited for. Although the speed-up over classical computers is not as high as the latter approach, physical realization may be much sooner. One particular hybrid technique utilizes a lattice of quantum computers with relatively few quantum bits per node. Each node is connected by classical communication channels to its nearest neighbors. Such an architecture is referred to as a Type-II quantum computer. This class of architecture is able to efficiently simulate a variety of partial differential equations and is the platform for which our quantum-lattice-gas-based algorithms have been designed. We present the effective finite difference approximations that yield multi-dimensional linear diffusion, and representative simulation results. Additionally we demonstrate an extension to constrained linear diffusion that provides for nonuniform image smoothing. These methods are particularly relevant to image enhancement tasks. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U33.00011: Quantum Algorithm for Partial Search Vladimir Korepin Searching and sorting is used as subroutines in many important algorithms. Grover discovered a quantum algorithm that searches faster than any classical algorithm. If we want less information we can do it faster. Partial search used to find an approximate location of the target item. An example: the exact location of the target item is given by a sequence of many bits, but we want to find only some of them. A partial search considers the following problem: a database is separated into several blocks. We want to find a block with the target item, not the target item itself. Efficiency of search algorithms is measured by number of queries to the oracle [total number of iterations]. Partial search can use the same hardware as the full search. Essential references follow:=$>$ 1) K. Grover, Quantum Mechanics helps in searching for a needle in a haystack, Phys. Rev. Letters, 78(2), 325, 1997. 2) L. K. Grover and J.Radhakrishnan, quant-ph/0407122, 3) V. E. Korepin and L. K. Grover, e-print quant-ph/0504157; Quantum Information Processing, vol. 5, issue 1, page 3, 2006, 4) V. E. Korepin, Journal of Physics A: Math. Gen. vol 38, pages L731-L738, 2005 see also quant-ph/0503238 5) V.E. Korepin, J. Liao, quant-ph/0510179 6) B.-S. Choi, T. A. Walker, S. L. Braunstein, e-print quant-ph/0603136 7) B-S Choi, V. E Korepin, quant-ph/0608106 8) V. E. Korepin, B. C. Vallilo, quant-ph/0609205 [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U33.00012: An Index Theorem for the Majorana Zero Modes in Chiral P-Wave Superconductors Sumanta Tewari, Sankar Das Sarma, Dung-Hai Lee We show that the Majorana fermion zero modes in the cores of odd winding number vortices of a 2D $p_x+ip_y$-paired superconductor is due to an index theorem. This theorem is analogous to that proven by Jackiw and Rebbi for the existence of localized Dirac fermion zero modes on the mass domain walls of a 1D Dirac theory. The important difference is that, in our case, the theorem is proven for a two component fermion field theory where the first and second components are related by parity reversal and hermitian conjugation. The vortices with Majorana zero modes can be used, in principle, to build a topological quantum computer. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U33.00013: Breakdown of a topological phase: Quantum phase transition in a loop gas model with tension Simon Trebst, Philipp Werner, Matthias Troyer, Kirill Shtengel, Chetan Nayak We discuss the stability of topological order against local perturbations by considering the effect of a magnetic field on a spin model -- the toric code -- which is in a topological phase. The model can be mapped onto a quantum loop gas where the perturbation introduces a bare loop tension. When the loop tension is small, the topological order survives. When it is large, it drives a continuous quantum phase transition into a magnetic state. The transition can be understood as the condensation of `magnetic' vortices, leading to confinement of the elementary `charge' excitations. We also show how the topological order breaks down when the system is coupled to an Ohmic heat bath and discuss our results in the context of quantum computation applications. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U33.00014: D-Colexes: Topological Computation and Brane-Net Condensates Hector Bombin, Miguel A. Martin-Delgado Topological quantum order provides a way to reliably store quantum information. These systems show an energy gap and a topology dependent ground state degeneracy. Then one can use this degenerate subspace to protect quantum information from local interactions with the environment. Topological error correction codes are an interesting constructive approach towards topologically ordered systems. However, when one faces the issue of performing computations, not every topological code is suitable. We introduce a new family of topological codes that have such computational capabilities. They are constructed from certain $D$-dimensional lattices that we call $D$-colexes. For $D=2$, they allow to perform any operation in the Clifford group without individually addressing the physical qubits that form the quantum memories. The point is that Clifford group operations are enough for many tasks in quantum information such as quantum distillation. In the $D=3$ case even universal quantum computation is possible. From the perspective of topological order, the resulting systems give rise to interesting new topological orders: brane-net condensates. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U33.00015: Quantum search algorithm with many bosons in optical lattices David Feder One approach to implementing quantum algorithms is the quantum walk (QW). In the continuous-time formulation, the QW is equivalent to the time-evolution of a quantum state under the influence of a discrete-space Hamiltonian. Building on a duality between many boson systems and weighted graphs, I will discuss how one can implement a QW search algorithm with ultracold bosons confined in optical lattices. By applying a specified external potential (using an external laser), the atoms in a shallow lattice will evolve from the uniformly populated ground state to all occupying one particular site. The results indicate that quantum algorithms exhibiting polynomial speed-up should be feasible with current experimental technology. [Preview Abstract] |
Session U34: Focus Session: Non-equilibrium Fluctuations in Biomolecules
Sponsoring Units: DBPChair: Ioan Kosztin, University of Missouri-St. Louis
Room: Colorado Convention Center 404
Thursday, March 8, 2007 8:00AM - 8:36AM |
U34.00001: Fluctuations in Proteins Invited Speaker: Proteins are the machines of life. In order to perform their functions, they must move continuously. The motions correspond to equilibrium fluctuations and to non-equilibrium relaxations. At least three different fluctuation processes occur: $\alpha$- and $\beta$-fluctuations and processes that occur even below one Kelvin. The $\alpha$-fluctuations can be approximated by the Vogel-Tammann-Fulcher relation, while the $\beta$-fluctuations appear to follow a conventional Arrhenius law (but may in some cases be better characterized by a Ferry law). Both are usually nonexponential in time. These phenomena are similar in proteins and glasses, but there is a fundamental difference between fluctuations in glasses and proteins: In glasses, they are independent of the environment, in proteins the $\alpha$-fluctuations are slaved to the $\alpha$-fluctuations in the solvent surrounding the protein; they follow their rate coefficients but they are entropically slowed. The studies of the protein motions are actually still in their infancy, but we can expect that future work will not only help understanding protein functions, but will also feed back to the physics of glasses. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U34.00002: A Tunable Chemical Pattern Filter Constructed by Networks of Reaction Compartments and Tubes Ludvig Lizana, Zoran Konkoli, Owe Orwar We study numerically the filtering capabilities of nanoscale networks built up of containers and tubes hosting chemical reactions. Spatio-temporal patterns of substrate molecules are injected into the network. The substrate propagates by diffusion and reacts with enzymes distributed in the network prior to the injections. The dimensions of the network are tailored in a way that the transport and reaction rates are comparable in size, a situation in which the overall behavior is highly influenced by the geometry and topology of the network. This property is crucial for the functionality of the pattern filter developed in here. It is demonstrated that input patterns can be classified in a crude way using a simple setup (two micrometer-sized containers joined together by a nanotube) and that the classification can be tuned by changing the geometry of the network (the length of the tube connecting the two containers). The filter device we investigate can also be viewed as a primitive chemistry-based computational element since the information encoded in the input patterns is processed using chemical reactions. In particular it is argued that the filter can be used as a frequency sensor. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U34.00003: Effect of Orientation in Translocation of Polymers through Nanopores Stanislav Kotsev, Anatoly Kolomeisky The motion of a polymer with inhomogeneous structure through a nanopore is discussed theoretically. Specifically, we consider the translocation of polymer consisting of one double-stranded and one single-stranded blocks. Since only the single-stranded chain can pass through the nanopore, the double-stranded segment has to unzip before translocating. Utilizing a simple analytical model, translocation times are calculated explicitly for the different polymer orientations - when the single-stranded block enters the pore first and when the double-stranded one enters first. Their dependence on external fields, energy of interaction in the double-stranded segment, total size of the polymer, and the fraction of double-stranded to single-stranded blocks lengths is analyzed. It is found that the order of entrance into the pore has a significant effect on the translocation dynamics. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U34.00004: Assisted DNA hairpin retraction from nanopores Meni Wanunu, Buddhapriya Chakrabarti, Jerome Mathe, David R. Nelson, Amit Meller We present results from recent experimental and theoretical investigations of DNA hairpin retraction from an $\alpha$-hemolysin nanopore in the presence of an assisting voltage. By mapping the translocation process to that of biased diffusion of a Brownian particle we compute the probability of the polymer to stay in the pore as a function of time. Using this model we back out the diffusion constant and the drift velocity of the polymer as a function of the assisting voltage. While the drift-diffusion model gives good agreement with experiments at low voltages it fails for high assisting voltages. We discuss possible reasons for this along with the implications of our work. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U34.00005: Pharmaceuticals in nanopores - A strategy to manipulate the phase behavior M. Beiner, G.T. Rengarajan, S. Pankaj, D. Enke The manipulation of the crystalline state of substances existing in different polymorphic forms is an important issue in many fields of application. In case of pharmaceuticals the stabilization of unstable forms is interesting since solubility and bioavailability are improved. We will show in this presentation that it is possible to manipulate the crystallization behavior of pharmaceuticals and to stabilize unstable crystalline forms by confining the substance in pores with diameters in the range 20-400 nanometers. \footnote{G.T. Rengarajan et al. $J. Am. Chem. Soc.$, to be published.} The crystallization behavior of a pharmaceutical model system in two different types of nanostructured inorganic host systems is studied by DSC and x-ray scattering. The results clearly show that the most unstable crystalline form of this pharmaceutical melts and is stable for long times under confinement which was never observed for bulk samples. This allows to extract the thermodynamic parameters of this crystalline form which have not been reported so far and shows that this is an interesting field of application for nanostructured host-guest systems. The influences of pore geometry and surface interaction are studied and possible explanations for the differences between the crystallization behavior in the bulk and under confinement are discussed. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U34.00006: Thermal fluctuation spectroscopy in histone and nucleosomes during denaturation Arup Raychaudhuri, K.S. Nagapriya Thermal stability of biomolecules is an important issue. We have studied thermal denaturation of histone and nucleosome using precision thermal fluctuation spectroscopy (TFS) . - a problem that we believe has not been studied experimentally before. TFS uses a very sensitive noise calorimeter which can detect thermal fluctuations of micro Kelvin at around room temperature. We find that the thermal denaturation of histones (in particular H1) as well as that of the nucleosome are associated with large fluctuations, which are few orders higher than those away from the denaturation temperature. It involves large energy exchange which can be few tens of kBT0 (T0=300K). It appears that the denaturation occurs in three distinct steps – 1. breaking of bonds leading to the cooling jumps, 2. the change in its secondary, tertiary structure leading to slow dynamics and 3. formation of bonds as it is unfolding and in the newly folded high temperature phase which accounts for the heating jumps. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U34.00007: Development of High-Resolution Magnetic Tweezers for Single-Molecule Measurements Kipom Kim, Omar A. Saleh Magnetic tweezers can sense single-molecule DNA-protein interactions through optical tracking of the motion of a colloidal particle. This is typically done by relating changes in the colloid's diffraction pattern to its position. While diffraction-tracking is relatively simple to implement, it is intrinsically limited in its resolution. To improve this, we have developed a tracking technique based on Reflection Interference Contrast Microscopy (RICM). RICM relies on interference between light reflected from the colloid and a glass surface. To optimize the interference pattern, the reflecting surfaces of the colloid and the glass substrate were coated with gold and dielectric thin-films, respectively. To maintain the focal position of objective against the defocusing due to a thermal drift, the objective was automatically focused on the glass/water interface using feedback control with a piezo-driven actuator. We evaluated the system's performance by measuring fundamental physical properties of the DNA. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U34.00008: Single-biomolecule circuits with carbon nanotube wiring John Coroneus, Brett R. Goldsmith, Vaikunth Khalap, Alexander Kane, Gregory A. Weiss, Philip G. Collins Because of their size and chemistry, carbon nanotubes offer a unique opportunity to couple solid-state electronics with individual proteins or other biomolecules. This talk will describe our success covalently attaching single proteins to functioning, nanotube-based electronic devices. Because the nanotubes are sensitive, one-dimensional conductors, their electrical properties are greatly altered by this attachment, even when only one or two proteins are bound. The single-molecule circuits which result allow the dynamics of molecules to be directly observed without ensemble averaging. This work is partly supported by NSF grant EF-0404057. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U34.00009: Probing DNA-Protein Interactions on Surfaces Using Spectral Self-interference Fluorescence Microscopy Mehmet Dogan, Peter Droge, Anna K. Swan, Selim Unlu, Bennett B. Goldberg We are probing the interactions between double-stranded DNA and integration host factor (IHF) proteins [1] on surfaces using Spectral Self-interference Fluorescence Microscopy (SSFM) [2].The probing technique utilizes the spectral fringes produced by interference of direct and reflected emission from fluorescent molecules. The modified spectrum provides a unique signature of the axial position of the fluorophores. Using the SSFM technique, we probe the average location of the fluorescent markers attached to the DNA molecules to study the conformational changes in double-stranded DNA tethered to SiO$_{2}$ surfaces. In the presence of IHF, a DNA bending protein, we observe reduction in the vertical position of fluorescent molecules suggesting the formation of IHF-DNA complex and IHF-induced DNA bending. We also discuss the results with different IHF strains and different binding conditions. [1] Q. Bao et. al., Gene, Vol.343 pp.99-106 (2004) [2] L.A. Moiseev et. al., Journal of Applied Physics, Vol.96, pp. 5311-5315 (2004) [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U34.00010: Dynamics of Single Actin Filaments and Bundles in Flow Dagmar Steinhauser, Sarah Koester, Heather M. Evans, Holger Stark, Thomas Pfohl Actin filaments, aside from their biological renown as providing the `skeleton' of cells, also proffer an ideal platform from which to study -- more generally -- the properties of semi-flexible polymers. Microfluidic devices made using soft-lithography are easily adapted in dimension and geometry to create well-defined flow environments. Actin filaments are visualized in continuous flow in a microfluidic channel by stroboscopic laser light illumination. A detailed analysis of filament orientation, center-of-mass distribution, and thermal fluctuations as a function of flow rate and channel geometry is reported. In addition, the non-equilibrium bundling behavior of actin in the presence of actin-binding proteins or multivalent ions is studied in microchannel devices using FRET microscopy. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U34.00011: Segregation of molecules in self-spreading lipid bilayer at ultra-small metal nano-gaps arrayed on solid surface Kei Murakoshi, Hideki Nabika, Masahiro Oowada Diffusion of target molecules incorporated in the self-spreading lipid bilayer was controlled by the introduction of periodic array of metallic architecture on solid surface. Retardation of the progress of target molecules became significant when the size of gap between small metal architectures was less than a few hundred nm. The self-spreading dynamics of the lipid bilayer depending on the size of the small gap were analyzed semi-quantitatively. Estimated change in the driving force of the spreading layer suggests that highly localized compression of the spreading layer causes selective segregation of molecules. Surface-modified metal nano-architectures were also used to tune the selectivity of the molecules. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U34.00012: Calibration of Micromachined Force Sensors by Gravitational Force on Precision Microspheres Steven J. Koch, Gayle E. Thayer, Alex D. Corwin, Maarten P. de Boer To complement the existing tools for applying and measuring piconewton-level forces on biomolecules (e.g. optical tweezers, magnetic tweezers, AFM), we are developing a compliant micromachined spring for simple and direct measurements in an aqueous environment. Accurate calibration of the spring constant is crucial and we will present a gravitational method that uses NIST-traceable size standard microspheres. The method is applicable to calibration of other soft cantilevers of both in-plane and out-of-plane varieties. We affixed two microspheres to the force sensor and measured a deflection per bead of 196 nm $\pm $ 6{\%}. Using a weight of 150 pN $\pm $ 4.8{\%} per microsphere, we obtained a spring constant of 0.76 pN / nm $\pm $ 8{\%}. The method proved simpler and more reliable when compared to two other methods: high resolution SEM and thermal equipartition. The versatility of surface micromachining should enable use of the spring in new platforms for biophysical force measurement, for example on-chip load cells for dynamic DNA stretching. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U34.00013: Multimode Analysis of SHG Signal from Complex Biological Systems: Parameterization of Features Using Nearest-Neighbor Analysis and Wavelet Transforms Clayton Bratton, Karen Reiser, Andre Knoesen, Diego Yankelevich, Israel Rocha-Mendoza, Mingshi Wang We have developed a novel computational approach for quantifying structural disorder in biomolecular lattices with nonlinear susceptibility based on analysis of polarization-modulated second harmonic signal. Transient, regional disorder at the level of molecular organization is identified using a novel signal processing algorithm sufficiently compact for near real-time analysis with a desktop computer. Global disorder within the biostructure is assessed using a two-dimensional wavelet transform of the magnitude and phase of the second harmonic signal. Selection of coefficients and the specific wavelet family~is based on topological considerations. Experimental results suggest our signal processing method represents a robust, scaleable tool that allows us to detect both regional and global alterations in signal characteristics of biostructures with a high degree of discrimination. [Preview Abstract] |
Session U35: Focus Session: Cytoskeletal Dynamics and Cell Migration II
Sponsoring Units: DBP GSNP DPOLYChair: Arpita Upadhyaya, University of Maryland
Room: Colorado Convention Center 405
Thursday, March 8, 2007 8:00AM - 8:36AM |
U35.00001: Modelling cell motility and pathways that signal to the actin cytoskeleton Invited Speaker: Gradient sensing, polarization, and motility of rapidly moving cells such as neutrophils involves the actin cytoskeleton, and regulatory modules such as membrane bound phosphoinositides (PIs), kinases/phosphatases, and proteins of the Rho family (Rho GTPases). I describe recent work in my group in which we have modeled components of these modules, their interconversions, interactions, and action in the context of protrusive cell motility. By connecting three modules, we find that Rho GTPases work as a spatial switch, and that PIs filter noise, and define the front vs. back. Relatively fast PI diffusion also leads to selection of a unique pattern of Rho distribution from a collection of possible patterns. We use the model to explore the importance of specific hypothesized interactions, to explore mutant phenotypes, and to study the role of actin polymerization in the maintenance of the PI asymmetry. Collaborators on this work include A.T. Dawes, A. Jilkine, and A.F.M. Maree. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U35.00002: Migration of a Model Lamellipodium by Actin Polymerization: A Molecular Dynamics Simulation Approach Junhwan Jeon, Peter Cummings We performed molecular dynamics simulation of a model lamellipodium with growing F-actin filaments in order to study the effect of stiffness of the F-actin filament, the G-actin monomer concentration, and the number of polymerization sites on lamellipodium motion. The lamellipodium is modeled as a two-end capped cylinder formed by triangulated particles on its surface. It is assumed that F-actin filaments are firmly attached to a lamellipodium surface where polymerization sites are located and actin polymerization takes place by connecting a G-actin monomer to a polymerization site and the first monomer of a growing F-actin filament. It is found that there is an optimal number of polymerization sites for rapid lamellipodium motion. This appearance of the maximum speed is related to the competition between the number of polymerization sites and the number of available G-actin monomers, and the degree of pulling and holding the lamellipodium surface by non-polymerized actin filaments. The model lamellipodium speed distribution is found to be Maxwellian for particles with random motion in two dimensions and is in agreement with experiment. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U35.00003: Intracellular dynamics during directional sensing of chemotactic cells Gabriel Amselem, Eberhard Bodenschatz, Carsten Beta We use an experimental approach based on the photo-chemical release of signaling molecules in microfluidic environments to expose chemotactic cells to well controlled chemoattractant stimuli. We apply this technique to study intracellular translocation of fluorescently labeled PH-domain proteins in the social ameba \textit{Dictyostelium discoideum. }Single chemotactic \textit{Dictyostelium }cells are exposed to localized, well defined gradients in the chemoattractant cAMP and their translocation response is quantified as a function of the external gradient. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U35.00004: Searching strategies in Dictyostelium Liang Li, Edward Cox Levy walks are known to be the best strategy for optimizing non-destructive search times, while an intermittent two-state searching process optimizes the destructive case. Here we ask about hunting strategy in Dictyostelium amoebae when they cannot know where their food is. We show that correlated random walks with two typical correlation time scales bias their search, improving the search outcome. Further analysis indicates that cell trajectories consist of runs and turns. Strikingly, amoebae remember the last turn, and have a strong turning preference away from the last turn. Autocorrelation analysis of turn sequences indicates that this tendency does not persist beyond the nth+1 turn. Computer simulations reveal that this bias contributes to the longer of the two correlation times. The search rules are essentially the same when cells are continuously stimulated by cAMP, with different persistence times and lengths. Interestingly, new pseudopods form in an orientation opposite to the following turn. One of the correlation timescales is approximately 30 seconds in all cases, thus indicating a short-lived cellular process, while the other is 9 to 15 minutes suggesting a process sensitive to external signals, perhaps pseudopod extensions during turning. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U35.00005: Perturbing Streaming in Dictyostelium Discoideum Colin McCann, Paul Kriebel, Carole Parent, Erin Rericha, Wolfgang Losert Upon starvation the social amoebae \textit{Dictyostelium discoideum} aggregate to form multicellular organisms. During the transition from single cells to full aggregates, cells relay the chemotactic signal, align in a head-to-tail fashion, and follow each other in streams. To gain more insight into streaming behavior we investigated its robustness by perturbing the strength of the relayed chemoattractant. We measured the effects of plating the cells at varying densities, placing them in excess extracellular fluid thereby diluting cell-cell signals, or directly mixing up the local external fluid using ultrasound-induced bubble-driven flow. We compared wild type cells to cells devoid of signal relay and measured how streaming affects cell speed, directionality, and extent of directed migration. Results will be discussed and a model describing our findings will be presented. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U35.00006: Traction cytometry applied to chemotacting \textit{Dictyostelium discoideum} Alberto Aliseda, Baldomero Alonso-Latorre, Juan Carlos del Alamo, Javier Rodriguez-Rodriguez, Rudolph Meili, Richard Firtel, Juan C. Lasheras The motion of \textit{Dictyostelium discoideum} cells moving on a elastic substrate has been studied. Joint analysis of time-lapse DIC movies of the cells and UV fluorescence from the beads embedded in the substrate, allows for identification of characteristic time scales of the motion and the quantitative description of the crawling cycle. From the measured displacements of the beads, forces can be computed by analytically solving the elasto-static equation in a finite thickness slab. We found that the finite thickness of the substrate and the distance of the beads to its surface have a substantial effect and that the previous traction cytometry techniques based on the Boussinesq solution effectively low-pass-filtered the force field, reducing the spatial resolution and damping the range of the measured forces by as much as 50{\%}. The improved spatial resolution of this method enables us to determine the spatial extent of the regions where the cells apply force on the substrate and, consequently, the magnitude of the elastic energy spent in its deformation. The measured forces, as well as the elastic energy communicated by the cell to the substrate, will be correlated to the different stages of the crawling cycle for various cell strains. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U35.00007: Stem cell cytoskeleton is slaved to active motors Florian Rehfeldt, Andre Brown, Adam Engler, Dennis Discher Cells feel their physical microenvironment through their adhesion and respond to it in various ways. Indeed, matrix elasticity can even guide the differentiation of human adult mesenchymal stem cells (MSCs) [Engler et al. \textit{Cell} 2006]. Sparse cultures of MSCs on elastic collagen--coated substrates that are respectively soft, stiff, or extremely stiff were shown to induce neurogenesis, myogenesis, and osteogenesis. Lineage commitment was evaluated by morphological analysis, protein expression profiles, and transcription microarrays. Differentiation could be completely blocked with a specific non-muscle myosin II (NMM II) inhibitor, suggesting that contractile motor activity is essential for the cells to sense matrix elasticity. Current studies by AFM and near-field fluorescence imaging show that NMM II inhibition in stem cells on rigid glass surfaces promotes actin-rich dendritic outgrowth resembling neurite extension. Dynamic cell studies have been conducted to elucidate the complex molecular interplay of the contractile apparatus in response to selected physical and biochemical stimuli. Additional insight is being gained by using AFM to investigate the local elasticity of the cell's cytoskeletal force sensing machinery. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U35.00008: The Collective Contractile Dynamics of Confluent Epithelial Cells is Highly Coherent Thomas Angelini, Manuel Marquez, David Weitz We have studied the collective contractile dynamics of confluent Cos-7 epithelial cells in several contexts. We patterned cells in single file lines on confined PDMS 'rubber bands', and quantified substrate deformation by tracking embedded fluorescent particles over the course of approximately 10 hours. Deformations confined to one dimension, well over ten microns in magnitude, correlated over distances exceeding the millimeter scale, were observed. On unpatterened PDMS, collective substrate deformations in two dimensions were over ten times smaller, and exhibited a propagating mechanical excitation. Three dimensional matrix deformation was studied by embedding cells at high density in 1mg/ml collagen. Since collective network deformations are difficult to quantify in the microscope, a dynamic small angle light scattering technique was adapted. With this technique, we have spectrally characterized the three dimensional mechanical network deformations, and observed collective behavior similar to the measurements on compliant surfaces. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U35.00009: Membrane fluctuations driven by actin and myosin: waves and quantized division. Nir Gov, Roie Shlomovitz We present a model which couples the membrane with the protrusive forces of actin polymerization and contractile forces of molecular motors, such as myosin. The actin polymerization at the membrane is activated by freely diffusing membrane proteins, that may have a distinct spontaneous curvature. Molecular motors are recruited to the polymerizing actin filaments, from a constant reservoir, and produce a contractile force. All the forces and variables are treated in the linear limit, which allows us to derive analytic solutions. Our results show that for convex membrane proteins the myosin activity gives rise to propagating membrane waves similar to those observed on different cells. For concave membrane proteins the myosin activity gives rise to an unstable contraction, which yields a length-quantization of the mitosis process. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U35.00010: Simulation of Actin-Polymerization-Mediated Propulsion Kun-Chun Lee, Andrea Liu An important component of the cellular cytoskeleton is F-actin, a biopolymer whose self-assembly is key to the process of cell crawling. The polymerization and branching of F-actin near the cell membrane is known to drive cell crawling, but the precise mechanism by which these processes lead to the generation of a mechanical force is still controversial. We have constructed a Brownian dynamics simulation of F-actin polymerizing near a surface, which includes all known important processes, including polymerization, depolymerization, branching, severing and capping. Using this model, we are able to simulate the cell movement. We measure the speed as function of concentration of different proteins involved in the process. We find the speed to be non-monotonic, consistent with experimental results [Louis et al. Nature \textbf{401} 613 (1999)]. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U35.00011: Symmetry breaking in actin gels - Implications for cellular motility Karin John, Philippe Peyla, Chaouqi Misbah The physical origin of cell motility is not fully understood. Recently minimal model systems have shown, that polymerizing actin itself can produce a motile force, without the help of motor proteins. Pathogens like Shigella or Listeria use actin to propel themselves forward in their host cell. The same process can be mimicked with polystyrene beads covered with the activating protein ActA, which reside in a solution containing actin monomers. ActA induces the growth of an actin gel at the bead surface. Initially the gel grows symmetrically around the bead until a critical size is reached. Subsequently one observes a symmetry breaking and the gel starts to grow asymmetrically around the bead developing a tail of actin at one side. This symmetry breaking is accompanied by a directed movement of the bead, with the actin tail trailing behind the bead. Force generation relies on the combination of two properties: growth and elasticity of the actin gel. We study this phenomenon theoretically within the framework of a linear elasticity theory and linear flux-force relationships for the evolution of an elastic gel around a hard sphere. Conditions for a parity symmetry breaking are identified analytically and illustrated numerically with the help of a phasefield model. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U35.00012: A kinematic description of the trajectories of Listeria monocytogenes propelled by actin comet tails Dhananjay Tambe, Vivek Shenoy The bacterial pathogen Listeria monocytogenes propels itself in the cytoplasm of the infected cells by forming a filamentous comet tail assembled by the polymerization of the cytoskeletal protein, actin. While a great deal is known about the molecular processes that lead to actin based movement, most macroscale aspects of motion, including the nature of the trajectories traced out by the motile bacteria are not well understood. Listeria moving between a glass-slide and cover slip in a Xenopus frog egg extract motility assay is observed to display a number of geometrically fascinating trajectories including sine curves, serpentine shapes, circles, and a variety of spirals. We have developed a dynamic model that provides a unified description of these seemingly unrelated trajectories. A key ingredient of the model is a torque (not included in any microscopic models to date) that arises from the rotation of the propulsive force about the body-axis of the bacterium. The trajectories of bacteria executing both steady and saltatory motion are found to be in excellent agreement with the predictions of our dynamic model. When the constraints that lead to planar motion are removed, our model predicts motion along regular helical trajectories, observed in recent experiments. We discover from the analysis of the trajectories of spherical beads that the comet tail revolves around the bead. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U35.00013: Steady-state configurations and dynamics of the MreB helix within bacteria Andrew Rutenberg, Jun Allard We present a quantitative model of the actin-like MreB cytoskeleton that is present in many prokaryotes. Individual MreB polymers are bundled into a supra-molecular array to make up helical cables. The cell wall imposes constraint forces through a global elasticity model. With variational techniques and stochastic simulations we obtain relationships between observable quantities such as the pitch of the helix, the total abundance of MreB molecules, and the thickness of the MreB cables. We address changes expected with slow cell growth, as well as turnover dynamics that are relevant to FRAP studies. We also address polarized macromolecular trafficking along the MreB cables without motor proteins. [Preview Abstract] |
Session U38: Focus Session: Advances in Scanned Probe Microscopy III: High Frequency and Probe Characterization
Sponsoring Units: GIMSChair: Eric Hudson, Massachusetts Institute of Technology
Room: Colorado Convention Center 501
Thursday, March 8, 2007 8:00AM - 8:12AM |
U38.00001: Radio-frequency Scanning Tunneling Microscopy (rf-STM) Utku Kemiktarak, Tchefor Ndukum, Keith Schwab, Kamil Ekinci Scanning Tunneling Microscopy (STM) possesses unprecedented spatial resolution, extending down to sub-Angstroms. There is a serious obstacle, however, in front of realizing the full potential of the STM technique: insufficient temporal resolution. Electronic bandwidths in typical STM experiments extend only up to $\sim $10 kHz. Here, a novel approach to attain time resolution in STM is demonstrated. The impedance of the tip-sample tunnel junction in the M$\Omega $ range is matched close to 50 $\Omega $ by using a LC tank circuit, which allows for bandwidths up to 20 MHz. To demonstrate broadband rf-STM operation, several experiments have been performed. First, radio-frequency reflectometry has been used to image surfaces, resulting in spatial resolution comparable to that available in conventional STM. Second, shot noise in the tunnel current has been measured at high frequencies. Third, high frequency displacements of surfaces have been detected using the STM tip as a nanomechanical displacement detector. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U38.00002: Very high frequency (VHF) self-sensing nanoscale cantilevers and their mass sensing applications in ambient conditions. Mo Li, Hong Tang, Michael Roukes We report the development a new class of self-sensing, nanometer-scale cantilevers with fundamental-mode mechanical resonances up to very high frequencies (VHF). The sensors employ integrated piezoresistive displacement transducers; we show that, at the nanoscale, these are optimally realized using thin, metallic-density films. Our approach enables detection of VHF cantilever vibrations, to date as high as 127 MHz, at the thermomechanical noise limit. Displacement sensitivity of 39 fm/Hz$^{1/2}$ and extremely low 1/f noise are attained. Our smallest devices have lateral dimensions approaching the mean free path at atmospheric pressure; hence their high quality factors are preserved in ambient. Measurements of molecular chemisorption onto polymer-coated nanocantilevers yield unprecedented mass resolution below 1 attogram (10$^{-18}$ g) level at atmospheric pressure and room temperature. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U38.00003: High Frequency Cantilevers for Magnetic Resonance Force Microscopy Christian Degen, Martino Poggio, Ben Chui, John Mamin, Dan Rugar We are exploring the possibility of using high frequency cantilevers for detection of magnetic resonance spin signals, possibly at the Larmor frequency of nuclear spins. For this purpose we have fabricated smaller, 20 micron long cantilevers that resonate at frequencies near 1 MHz. Operating at 4K, these levers can have surprisingly high Q values, over 300,000, and can achieve force noise levels in the few attonewton range, despite their rather high stiffness of about 0.1 Newton per meter. We discuss some experimental challenges that will be increasingly important for future generations of cantilevers with even smaller dimensions. Finally we look ahead into what we might expect when such high sensitivity, nanomechanical resonators become tightly coupled to small ensembles of nuclear spins. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U38.00004: Construction of a Low Temperature Capable Frequency-Modulation Magnetic Resonance Force Microscope SangGap Lee, Sun Ho Won, Seung-Bo Saun, Soonchil Lee We constructed a low temperature capable frequency-modulation magnetic resonance force microscope (FM-MRFM) and applied to detecting electron spin resonance signals from tiny DPPH particles with ease in control and thereby improved signal-to-noise ratio. The electronics was composed of a spin-polarization modulator, a cantilever-oscillation feedback gain controller, and a phase-locked loop (PLL) FM demodulator. The controller kept the cantilever oscillating at its resonance frequency by positively feeding back to a piezo-actuator the gain-controlled drive signal phase shifted by 90 degrees with respect to the cantilever oscillation in the PLL circuit. The modulator generated waveforms to modulate the strength or frequency of radio-frequency field in phase with the drive signal. The whole setup enabled to sense as readily as spins resonate by tracking the cantilever resonance frequency without breakdown in fastidious conditions. We will discuss the details and features of our microscope and furthermore ongoing MRFM results. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U38.00005: MRFM System Design for the Study of Organic Materials Doran Smith, David Kim We will present an overview of our program to develop an MRFM system specialized for the study of organic materials at 4 K. The system uses the SPAM geometry and the CERMIT protocol and is predicted to be capable of imaging organic materials in 3D. The MRFM probe head design will be overviewed and progress toward system completion will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U38.00006: Progress on Magnetic Resonance Force Microscopy Detection of Statistical Polarization of Electron Spins K.C. Fong, I.H. Lee, P. Banerjee, Yu. Obukhov, D. Pelekhov, P.C. Hammel Here we report our experimental progress on detecting statistical polarization of electron spins. In the condition of low external magnetic field and high temperature, polarizati on due to Boltzman factor could be small, i.e. $\mu B/k_BT \ll 1 $. The $\sqrt{N}$ statistical polarization can dominate the Boltzmann polariation when the spins ensemble is suffi ciently small. With its unprecedented force sensitivity, Magnetic Resonance Force Microscopy (MRFM) has demonstrated the capability to observe this self-polarizing nature of spins via the i-OSCAR detection protocol\footnote {H.J. Mamin, R. Budakian, B.W. Chui and D. Rugar, Phys. Rev. Lett. \textbf{91}, 207604 (2003)}. Our efforts to use MRFM to detect this statistical polarization will be presented. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U38.00007: Cross Polarization Imaging with Magnetic Resonance Force Microscopy Kai W. Eberhardt, Qiong Lin, Andreas Hunkeler, Urban Meier, Beat H. Meier Cross Polarization (CP) is a standard method in Nuclear Magnetic Resonance Spectroscopy (NMR) for signal enhancement of nuclei with a low gyromagnetic ratio and was recently applied in MRFM [1]. We demonstrate two techniques based on CP. In the first we perform frequency-sweept CP to enhance the polarization of low-$\gamma$ S-spins. In the second method the S spins are used as a polarization sink for the high-$\gamma$ I-spins coupled to them. The I-spins can be completely depolarized by adding phase-jumps to the frequency-sweep of the S-spin channel, allowing that their presence is detected indirectly via the high-$\gamma$, often more abundant I-spins with improved SNR. 1D images with $\mu$m-resolution of a $KPF_{6}$ - $CaF_{2}$ sample are presented for both techniques. In the example the depolarization scheme allowed for an order of magnitude signal-to-noise ratio enhancement over direct detection. [1] Q. Lin et al., Phys. Rev. Lett. 2006, 96, 137604. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U38.00008: Near-field scanning microwave microscope with separated excitation and sensing probes Keji Lai, M.B. Ji, N. Leindecker, M. Kelly, Z.X. Shen We present here the design and experimental results of a near- field scanning microwave microscope (NSMM), currently working at a frequency of 1GHz. The coplanar waveguides were patterned onto the silicon nitride cantilever interchangeable with AFM tips, which are robust for high speed scanning. Our microscope is unique in that the sensing probe is separated from the excitation electrode to significantly suppress the common mode signal. The reflected signal, at the same time, can be used for the feedback of height control in a non-contact mode. In the contact mode which we are currently using, the contrast comes from both the sample topography and the difference of the complex dielectric constant. Our NSMM shows the ability to achieve high resolution microwave images on nano-particles, nano-wires, and biological samples with mostly topographical contrast, as well as buried structures with mostly electrical contrast. Numerical analysis of the tip-sample interaction was also performed and will be discussed in detail. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U38.00009: Ferromagnetic resonance force microscopy of a permalloy film E. Nazaretski, I. Martin, J.D. Thompson, R. Movshovich, D.V. Pelekhov, P.C. Hammel, P. Wigen, M. Zalalutdinov, T. Mewes, J. Baldwin, B. Houston We describe Ferromagnetic Resonance Force Microscopy (FMRFM) experiments performed on a 50 nm thick permalloy film. We have studied the evolution of the FMRFM force spectra as a function of temperature. The temperature-dependent studies show a decrease of the ferromagnetic resonance field with increasing temperature which we attribute to the temperature-dependent changes of the saturation magnetization. The experiments demonstrate the potential of FMRFM to study temperature dependent ferromagnetic resonance phenomena. We analyzed the FMRFM force spectra evolution as a function of the probe-film distance and performed numerical simulations of the intensity of the FMRFM probe-sample interaction force. Excellent agreement between the experimental data and the simulation results provides the new insight into the mechanism of the FMR mode excitation in an FMRFM experiment. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U38.00010: Magnetic particle imaging with a cantilever torque magnetometer John Moreland, Jason Eckstein, Yushun Lin, Sy-Hwang Liou, Steven Ruggiero We have demonstrated magnetic particle imaging$^{1 }$with a cantilever torque magnetometer. Imaging is based on measuring the harmonic content of the magnetic moment of a particle driven to saturation by an applied ac magnetic field while adjusting the zero point of the field gradient with a slowly sweeping dc magnetic field. Large field gradients ($>$ 100 T/m) necessary for high resolution imaging can be generated by opposing electromagnets with ferrite cores and thus there is the potential for submicrometer image resolution. Results on an array of 50 $\mu $m Permalloy dots patterned on microcantilevers will be reported. $^{1}$B. Gleich and J. Weizenecker, Nature 435, 1214 (2005). [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U38.00011: Resonant Measurement of Coupling Forces Between Two Microcantilevers Onur Basarir, Kamil L. Ekinci Here we studied the nature of nonlinear coupling forces between two microcantilevers. We employed a resonant measurement technique similar to that used in non-contact atomic force microscopy (NC-AFM). A stiff cantilever, which was driven at its resonance at a constant amplitude, was brought to the close vicinity of a second cantilever excited by thermal noise. A spectral analysis of the displacement signal of the driven cantilever revealed the effects of the coupling forces at the sum and difference frequencies of the resonances of two microcantilevers. From this, the resonance frequency and the quality factor of the thermally excited cantilever were extracted. As the nominal distance between the two cantilevers was reduced, we observed an increase in the dissipation as well as a shift in the resonance frequencies. We shall discuss how these observations may lead to a better understanding of the coupling forces. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U38.00012: Functional Probes for Scanning Probe Microscopy Kotone Akiyama, Toyoaki Eguchi, Toshu An, Yasunori Fujikawa, Yukio Hasegawa, Toshio Sakurai For superior performance of scanning probe microscopy, we are working to fabricate functional probes. For Kelvin probe force microscopy, we fabricated a metal-tip cantilever by attaching a thin metal wire to a regular Si cantilever and milling it by focused ion beam (FIB)$^{1}$. By using the W tip with a curvature radius of 3.5 nm, we obtained the potential profile of Ge/Si(105) surface in atomic resolution with the energy resolution better than 3 meV$^{2}$. For synchrotron-radiation-light-irradiated scanning tunneling microscopy which aims at atomically resolved elemental analysis, we fabricated a glass-coated W tip using FIB$^{3}$. It is found that the glass coating blocks the unwanted secondary electrons, which come from large area of the sample, by a factor of 40 with respect to the case no coating. Using the tip to detect the electrons emitted just below the tip, we obtained element specific images with a spatial resolution better than 20 nm under the photo irradiation whose energy is just above the adsorption edge of the element$^{4}$. 1 K. Akiyama \textit{et al.}, RSI \textbf{76}, 033705 (2005) 2 T. Eguchi, K. Akiyama \textit{et al.}, PRL \textbf{93}, 266102 (2004) 3 K. Akiyama \textit{et al.}, RSI \textbf{76}, 083711 (2005) 4 T. Eguchi, K. Akiyama\textit{ et al.}, APL, in press [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U38.00013: Determination of the cross-sectional area of the indenter in nano indentation tests Weidian Shen, Jeremy McMinis, Rene Crombez, Eva Montalvo To measure the hardness and elastic modulus of a composite system in the nano scale the precise determination of the cross- section area of the indenter tip at different heights is a key. A method of using scanning probing microscopy to image the tip, and then using its analysis software, Histogram program and Bearing curve, combined with the information of the area of each pixel, to determine the cross section area at different heights is introduced in this presentation. Compared with other techniques, it is simple, straightforward, and readily provides a precise relationship between the cross-sectional area and the height. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U38.00014: In-situ observation of the transformation process into the nano-contact of single shell carbon fullerene using TEM-STM Makoto Yoshida, Yoshihiko Kurui, Yoshifumi Oshima, Kunio Takayanagi In this study, we proposed the simple fabrication method of the single shell carbon fullerene (SSF) from an amorphous carbon between two gold electrodes by applying the bias voltage. The STM system which was combined with TEM system was employed as applying the bias voltage and observing the fabrication process in high vacuum condition (about 10$^{-6}$ [Pa]) at room temperature. As the applied bias voltage was increased, the transformation into the SSF via the glassy carbon was caused. It was found that transformation into the SSF occurred above 0.6V. In this method, we obtained the SSF which was range from C$_{60}$ to C$_{620}$. This proposal method is very easier than the previous techniques for obtaining the SSF between two metallic electrodes, for example the combination of the synthesis and evaporation of the SSF, and very useful for researching the electrical conductance property such as the structure effect, the effect of the connection way between both metallic electrodes. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U38.00015: Quantitative imaging of lateral stiffness using sub-{\AA}ngstrom oscillation amplitude nc-AFM Mehrdad Atabak, Sevil Ozer, H. Ozgur Ozer, Ahmet Oral The specially designed and constructed a nc-AFM, capable of measuring lateral stiffness simultaneously with tunneling current will be discussed. In our technique a sensitive fiber interferometer is aligned at the side of a home--made tungsten cantilever with typical stiffness of about 150 N/m. To improve the sensitivity, a RF circuit is designed to inject RF current into the laser diode. The frequency and the amplitude of the RF current can be adjusted to optimize the noise reduction. Using this technique a noise level of $\sim $1$\times $10$^{-4 }${\AA}/$\sqrt {Hz} $ is obtained. The cantilever is dithered in lateral direction respect to the sample with sub-{\AA}ngstrom oscillation amplitudes (A$_{0}$ =0.25 {\AA}) at a frequency, well below the resonance frequency and the changes in lateral oscillation amplitudes. The amplitude at the tip, which is altered by the tip-surface, is detected from the interferometer output using a lock-in amplifier. We present the performance of our microscope and lateral stiffness images as a function of tunnel current (relative tip-sample distance) on HOPG surface. [Preview Abstract] |
Session U39: Focus Session: Emerging Research Devices and Materials for the Microelectronics Industry II
Sponsoring Units: FIAPChair: Victoria Soghomonian, Virginia Polytechnic Institute and State University
Room: Colorado Convention Center 502
Thursday, March 8, 2007 8:00AM - 8:12AM |
U39.00001: Transfer printed organic thin-film transistors using the semiconductors P3HT or pentacene and a polymer dielectric. Adrian Southard, Dan Hines, Elba Gomar-Nadal, Ellen Williams, Michael Fuhrer The assembly via the transfer printing process of all components for organic electronic devices onto a plastic substrate has been demonstrated. Both poly(3-hexylthiophene) (P3HT) and pentacene (Pn) have been used as active semiconducting films for such devices without exposing the films to detrimental chemical processes. Transfer printing relies on the difference in adhesion of two substrates towards the material being transferred. Here we use the transfer printing process to fabricate organic thin-film transistors (OTFT) with a range of channel lengths. These devices are used to characterize the contact resistance for both the P3HT and Pn OTFTs. The field-effect mobility of the P3HT devices is measured to be in the range of 0.02 to 0.035 cm$^{2}$/(Vs) which is comparable to the best values reported in the literature and is an order of magnitude higher than the control devices of the unprinted P3HT film as deposited onto a SiO$_{2}$ dielectric layer. These devices are demonstrated using polystyrene (PS), poly(4-vinylphenol) (PVP) and poly methylmethacrylate (PMMA) as dielectric materials and polyethylene terephthalate (PET) as the substrate material. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U39.00002: First-principles calculations of mobilities in MOSFETs George Hadjisavvas, Leonidas Tsetseris, Matthew Evans, Sokrates Pantelides Nano-scale MOSFETs demonstrate interesting electron transport behavior. Straining the silicon lattice results in significant increases in carrier mobility up to 100{\%}. Transport properties are known to depend also on the presence of interface traps. Due to their significance, a large number of studies have obtained mobilities, but in an empirical and semi-classical fashion, whereas, in nano-devices quantum mechanical effects and atomic-scale structural details are the key factors of mobility calculations. Here we use a recently developed method[1] for first-principles calculations of mobilites within DFT to probe the effect of strain and interface point defects (e.g., dangling bonds) on mobilities in double gate ultra-thin SOI (UTSOI) MOSFETs. The transport properties are described in a fully self-consistent quantum mechanical fashion and mobilities are calculated within the Born approximation. The results show that biaxial tensile strain is shown to significantly increase carrier mobility in UTSOI devices by suppressing the effective scattering from atomic-scale interface inhomogeneities; the effect of dangling bonds on mobility in a UTSOI channel is weaker than in conventional MOSFETs because the carrier density peaks at the center of the channel. This work was supported in part by NSF Grant ECS-0524655 and by AFOSR Grant 4224224232. [1] M.H. Evans et al., Phys. Rev. Lett. \textbf{95, }106802 (2005). [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U39.00003: Nano-circuit design in the nonohmic regime Vijay Arora Ohm's law on the basis of which all electronic circuits are designed and their performance evaluated is not holding its linearity as nanoelectronic devices are scaled down. In a macro-device of twentieth century (typical size L = 1 cm), the critical voltage for triggering nonohmic behavior, $V_c =\frac{V_t }{\ell _o }L$, is 2.6 kV for the room-temperature thermal voltage $V_t =\frac{k_B T}{q}$ = 26 mV and a typical ohmic mean free path = 0.1 $\mu $m. For the integrated circuits, a 5-V logic voltage was within the ohmic regime. For the twenty-first century nano-circuits, a typical device size, L, is below 0.1 $\mu $m, resulting in a critical voltage of 0.26 V. Even for a low logic voltage of 1 V, Ohm's law is not valid. A review of the physical processes behind this breakdown of Ohm's law is reviewed. The paper will show how familiar voltage and current division laws and the transient effects transform in the nano-regime where Ohm's law is not valid. In a circuit environment, the smaller-length resistor is much more resistive as applied voltage is increased. Similarly, RC time constant will be larger than its expected ohmic value. Power consumption in the nano-circuit tends to be linear function of the applied voltage in direct contrast to quadratic dependence for the macro circuits. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U39.00004: Emission from Vertical-Cavity Surface-Emitting Lasers after Femtosecond Pulse Injection Botao Zhang, Albert Heberle Vertical-cavity surface-emitting lasers (VCSEL's) are important devices for optical communication and sensing. Many applications require single-mode operation, which can be achieved by suppressing multiple lateral modes with emission apertures of 10 micrometers or less and by restricting emission to one polarization by reduction of symmetry. Such single-mode lasers still can produce multi-mode emission when subject to high pump currents or high-frequency modulation. Here we will discuss the emission dynamics of single-mode VCSEL's after resonant optical injection of femtosecond pulses from a mode-locked Ti:sapphire laser. The VCSEL emission is time and polarization resolved by cross correlation on a nonlinear optical crystal. This all-optical technique gives access to the VCSEL dynamics without limitation from electronics. Our measurements show dynamics in the 10 GHz range stemming from ordinary and from polarization relaxation oscillations. Interference beats above 100 GHz show the importance of dynamic multi-mode behavior. The decay of these beats gives direct information on the roundtrip gain of the dynamically excited modes. Polarization resolved measurements show the feasibility of polarization switching on a subpicosecond time scale. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U39.00005: Photon storage with ultrafast switching in coupled quantum wells Alexander Winbow, Aaron Hammack, Leonid Butov, Arthur Gossard Photon storage with ultrafast switching was implemented with indirect excitons in coupled quantum wells. The storage and release of photons was controlled by the gate voltage: a pulse of the gate voltage increased the exciton lifetime by orders of magnitude, resulting in storage of the absorbed photons in the form of indirect excitons; the pulse termination led to the emission of the stored photons. The storage time reached microseconds. The write and readout times were subnanosecond, and faster by an order of magnitude than the previously established record for optoelectronic photon storage devices. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U39.00006: High-Power, Single-Mode, Distributed-Feedback Interband Cascade Lasers for the Midwave-Infrared M. Kim, C.S. Kim, W.W. Bewley, C.L. Canedy, J.A. Nolde, J.R. Lindle, I. Vurgaftman, J.R. Meyer Narrow-ridge distributed-feedback interband cascade lasers with HR/AR coating have been fabricated using e-beam lithography followed by Ge lift-off. The quarter wavelength phase shift region has also been introduced near the HR coating facet to robust single mode operation with better wavelength control. Furthermore, amorphous Si layers have been deposited both sides of ridges to keep the single mode for wider ridge width ($\sim $13 $\mu $m) by suppress the additional transverse modes. A single-mode output has been obtained at $T$ = 130 K and $\lambda \approx $ 3.313 $\mu$m. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:48AM |
U39.00007: The Role of Physics in New Information Processing Technologies Invited Speaker: As semiconductor technology moves ever closer to the ultimate physical limits for scaling of devices that utilize electrons as information bearing particles, many new opportunities for research in the physical sciences are emerging. In order to achieve the limits for electron transport scaling, many new materials and processing problems must be overcome including the invention of both low and high dielectric-constant material technologies and new ideas for interdevice communication. If we look beyond the limits of scaling electron devices, many more challenging research opportunities exist in the areas of physics of information carriers and physics of communication. On the physics of information carriers, it may be that for devices with critical dimensions less than one nanometer, an information bearing particle much more massive than the electron would be desirable. As another example, we have observed that for spin-based systems, high magnetic fields are needed for reliable spin manipulation. Breakthroughs in the physics of spin manipulation are needed. For example is it possible to obtain suitable materials that exhibit a high g-factor to enable controllable devices that operate with reasonable energies and magnetic fields. The option operation in a non-equilibrium thermal environment also needs to be considered. On the physics of communication, for branched communication between nanodevices, it may be that we need to supplant metallic or nanotube interconnects with systems that utilize the wave nature of quantum particles to minimize energy dissipation. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U39.00008: Modeling of resonant terahertz detector with two-dimensional electron gas and lateral Schottky junction Nizami Vagidov, Akira Satou, Victor Ryzhii, Michael Shur, Vladimir Mitin We developed a device model for a resonant detector of terahertz (THz) radiation based on a heterostructure with an ungated two-dimensional electron channel and with a lateral Schottky junction at one of the channel edges (recently proposed and assessed by us using an analytical model). The resonant operation of the detector is associated with the excitation of electron plasma oscillations in the channel with the channel serving as a resonant cavity for the plasma waves. The rectification of the signals is due to nonlinearity of the lateral Schottky junction current-voltage characteristics. The model comprises a kinetic equation governing the electron transport in the device channel and a two-dimensional Poisson equation for the self-consistent electric potential. A novel approach, accounts for the electron-electron collisions and other scattering mechanisms, is used for numerical solution of the equations of the model. Using the developed model, we studied the resonant excitation of plasma oscillations by incoming terahertz signals and calculated the detector responsivity at different levels of excitation of the plasma oscillations. The THz detector performance is compared with that of Schottky diodes without the plasma resonant cavity. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U39.00009: Si single electron tunneling transistor with tunable barriers Emmanouel Hourdakis, Neil Zimmerman, Stuart Martin, Akira Fujiwara, Hiroshi Inokawa, Kenji Yamazaki, Hideo Namatsu, Yashuo Takahashi, Yukinori Ono We demonstrate the operation of single-electron tunneling (SET) transistor using electrostatically induced barriers. The barriers are formed on a Silicon nanowire using metal-oxide-metal field effect transistor (MOSFET) gates. This allows us to control the conductance of the tunnel barriers by more than 3 orders of magnitude. It also allows for various configurations of charge islands (changing the gate used for the tunnel barrier changes the shape and size of the island ). Below 4 K, the Coulomb blockade oscillations obtained are highly periodic. We also report the excellent reproducibility of the value of gate capacitances between different devices (variation of about 1 aF). The excellent controllability in these devices increases their potential in a number of applications, like metrological current standard or multivalued memory. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U39.00010: Extraordinary Electroconductance in In-GaAs hybrid thin film structures Yun Wang, A.K.M. Newaz, K.A. Wieland, S.A. Solin Following the demonstration of extraordinary electroconductance (EEC) in metal-semiconductor hybrids (MSHs) in macroscopic structures, we have developed a new design for a microscopic thin film EEC sensor, which is a van der Pauw plate structure consisting of a heavily doped n-type GaAs epi layer (500nm) with metallic shunt (50nm) on top. EEC arises from the current redistribution between the shunt and GaAs when an external E field lowers the interfacial Schottky barrier height (SBH). By defining the EEC effect to be the percentage change in sample conductance with and without the E field, we have obtained a 20\% change in the macroscopic sample in a field of 12kV/cm at 300K. We also compared the response of a sample with a Schottky barrier to an unshunted sample and to a shunted sample with an Ohmic interface. We propose that by applying a new surface treatment to the GaAs mesa, the surface state density can be remarkably reduced, so that the SBH is controlled by judicious choice of the metal. This allows more electron transport over the barrier and results in a geometrically enhanced conductance change. By varying the geometry of the structure and the material of the shunt, we can optimize the design of the EEC sensor. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U39.00011: Investigating the resonant response of a field-effect transistor subjected to an ac signal Manvir Kushwaha A theoretical investigation is made of the response of a field-effect transistor (FET) to an incoming electromagnetic radiation in the presence of a perpendicular magnetic field within the framework of hydrodynamics. The treatment is valid for a nondegenerate electron gas in which the mean free path for electron-electron scattering $\lambda_{ee}$ is much smaller than the device length $L$ and than the mean free path due to collisions with impurities and/or phonons $\lambda_{coll}$. These requirements, written as $\lambda_{ee} \ll L \ll \lambda_{coll}$, are fulfilled for magnetic fields weak enough to prevent Landau quantization. It is our general observation that the shorter device lengths, weaker magnetic fields, and lower temperatures (or higher electron mobility) are most favorable to achieve a greater resonant response of the device to an ac signal. Such resonant response makes FET a promising device for new types of sources, detectors, mixers, and multipliers in the GHz and THz frequency range. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U39.00012: Resistance Switching Behavior in Epitaxial NiO Films Seung Ran Lee, Jino Lee, Kookrin Char Recently NiO has attracted great attention due to its potential applications for nonvolatile ReRAM devices. However, the mechanism of resistance switching has not been clearly elucidated and it still remains controversial. To understand the phenomena in resistance switching, epitaxial film can serve as a good model system. Epitaixial films show atomically flat surface as well as good crystallization with much fewer defects which have been considered to be responsible for resistance switching behavior in polycrystalline NiO (poly-NiO). For these purposes we have grown epitaxial NiO (epi-NiO) films on SRO films prepared on (100) STO by pulsed laser deposition systems. XRD pattern shows clear (200) NiO peaks, which means our films are well crystallized with minimal defects. We also analyzed AFM and TEM images of SRO/NiO/Pt, which show clean and atomically flat interface between each layer. I-V characteristics of epi-NiO show an asymmetric shape and bipolar switching behavior. No abrupt current increase at ambient voltage was found, which is considered to be a necessary process for resistance switching in poly-NiO. These results suggest that unidentified defect states and/or interfaces play an important role in resistance switching phenomena. In order to understand the role of defects and interfaces further, our efforts to control the density of defect states and correlate them with I-V characteristics will be presented. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U39.00013: Nanowire Non-volatile Memory with Silicon Nitride Charge Trapping Layer Qiliang Li, Xiaoxiao Zhu, D.E. Ioannou, J.S. Suehle, C.A. Richter We present the fabrication and characterization of Si nanowire field effect transistors with silicon nitride as a charge trapping layer for non-volatile memory application. The Si nanowires were grown by chemical vapor deposition on defined location on a 60 nm Si nitride which was deposited on a 20 nm thermal grown oxide (blocking oxide). The source/drain electrodes were formed by using photolithographic alignment and metal lift-off processes. The nanowires were then covered with sputtering oxide at room temperature to be isolated from the external environment. We have observed a large threshold voltage shift window (8 V) at 10 V write/erase voltage and non-volatile on/off current states, which is attributed to the small radius ($\sim $ 10 nm) and intrinsic doping of the Si nanowire. The dynamics of the nanowire/nitride charge exchange, and its effect on threshold voltage and memory retention have been studied. [Preview Abstract] |
Session U40: Semiconductors: Defects and Impurities
Sponsoring Units: FIAPChair: Chris G. Van de Walle, University of California, Santa Barbara
Room: Colorado Convention Center 503
Thursday, March 8, 2007 8:00AM - 8:12AM |
U40.00001: Substitutional hydrogen as a cause of the unintentional n-type conductivity in ZnO. Anderson Janotti, Chris G. Van de Walle ZnO is a wide-band-gap semiconductor that is very promising for optoelectronic device applications. Serious challenges remain, however, and controlling the electrical conductivity is the most fundamental among these. As-grown ZnO is n-type and the cause of this n-type conductivity is still widely debated. Oxygen vacancies have long been invoked as the source of the n-type conductivity. This assignment is based on the assumption that oxygen vacancies are shallow donors, and on the fact that the conductivity changes inversely with the oxygen partial pressure. Contrary to the conventional wisdom, we have recently established that oxygen vacancies are not shallow but deep donors and have high formation energies in n-type ZnO. Therefore, they cannot cause conductivity. Based on first-principles calculations we show that SUBSTITUTIONAL hydrogen is a likely cause of the unintentional conductivity in ZnO. Hydrogen on an oxygen site forms a multicenter bond. Substitutional hydrogen is a shallow donor, and has a migration barrier of 2.5 eV, explaining recent observations of a hydrogen-related donor that is stable up to 500 $^{o}$C. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U40.00002: Accuracy of defect formation energies from first-principles calculations: A study of vacancies in ZnO Yiyang Sun Formation energy of point defects determines the concentration of the defects at thermal equilibrium. Accurately calculated formation energies can be used to pinpoint the type of the dominant defects at specific experimental conditions. First-principles calculations based on the density-functional theory and the supercell modeling of the point defects are the state-of-the-art approach to obtaining the formation energies. In this study, taking the O and Zn vacancies in ZnO as examples, we investigate the effects of supercell size on the accuracy of calculated formation energies. It is found that the 5x5x3 supercell is sufficient to reach the accuracy of about 0.06 eV in the formation energies, which means that the defect concentration can be evaluated with an accuracy in the magnitude of 10, considering k$_{B}$T is about 0.026 eV at room temperature. For charged defects, the shift of valence band maximum (VBM) is an additional factor which affects the accuracy of calculated formation energies. It is found that the 5x5x3 supercell is able to obtain the VBM shift at an accuracy of about 0.02 eV. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U40.00003: The shallow donor wavefunction in Si: Corrections to the KL effective mass theory (EMT) Theodore Castner The ENDOR data of Hale \& Mieher$^{1}$ (HM) provides detailed information on $\psi^{*}$$\psi$(R$_{nnm}$) at nearly 25 lattice sites for P, As, \& Sb. Ivey \& Mieher$^{2}$(IM) have given the most comprehensive calculation of $\psi$(r)= $\Sigma$A(k)u$_{k}$(r)e$^{ik.r}$ featuring a complex A(k) (and u$_{k}$(r)) and higher conduction bands. IM could identify most of the sites and reduce the rms error between theory values and experimental results from 60\% to 11\%. However, the IM results are poor for the (1,1,1) site [shell E] and don't provide clear evidence for subsidiary minima$^{3}$ (L$_{1}$,$\Gamma$$_{2}$$^ {'}$) from their region IV in the BZ. A reliable calculation of matrix elements $<$L$_{1}$[U(r)]$\Delta$$_{1,min}$$>$ is difficult because of the complicated core potential in the central cell. Using the equidistant matched lattice pair data [(3,3,3) \& (1,1,5); shells C and Q] provides a good estimate of the \% admixture from the L$_{1}$ minima, somewhat smaller than in [3]. The IM ImA(k) and the L$_{1}$ minima both provide corrections to the uniaxial strain i$_{d}$ parameter$^ {4}$. A data analysis for the odd lattice sites improves the agreement between theory and experiment. Important remaining theoretical issues will be discussed and new ENDOR experiments will be proposed. The corrections to EMT are important, but are smaller than implied in IM. 1) E.B. Hale \& R.M. Mieher, Phys.Rev.184, 739, 751 (1969). 2) J.L. Ivey \& R.M. Mieher, Phys.Rev.B11, 822 (1975). 3) T.G. Castner, Phys.Rev.B2, 4911 (1970). 4) E.B. Hale \& T.G. Castner, Phys.Rev.B1, 4763 (1970). [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U40.00004: Vacancy--interstitial interactions in crystalline Silicon Matthew J. Beck, L. Tsetseris, S.T. Pantelides Extensive experimental and theoretical investigations of fundamental defects in Si have led to the conclusion that both interstitials and vacancies diffuse athermally according to a carrier recombination-enhanced Bourgoin-Corbett mechanism. It is therefore widely accepted that Si vacancy–-interstitial pairs, or Frenkel pairs (FPs), either rapidly recombine or dissociate, even at cryogenic temperatures. This has recently been challenged by X-ray scattering experiments that are interpreted to show FPs in Si persisting to $> 100$ K (Partyka, P., et al., \emph{Phys. Rev. B} v. 64 art. no. 235207). Here we report first-principles density functional theory calculations of the properties of FPs in Si. A novel electronic interaction involving charge transfer from the interstitial to the vacancy in a FP is described. In a bound FP, this interaction suppresses the athermal diffusion mechanisms of both the interstitial and vacancy. This reconciles the existing experimental results by establishing that there are thermal barriers to the recombination or dissociation of bound FPs, but not affecting the previously described athermal diffusion of isolated interstitials and vacancies. This work supported by the AFOSR-MURI program. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U40.00005: First-Principles Study of Er Location in Er-Si Systems with Oxygen Co-Dopants R.H. Pink, Junho Jeong, Dip N. Mahato, M.B. Huang, T.P. Das, R.H. Scheicher, Sitaram Byahut Using the Hartree-Fock cluster procedure, we are investigating possible models for the Er$^{3+}$ ion in silicon with oxygen co-dopants[1]. We are examining first the Hi (hexagonal interstitial) site with six O atoms in the intrabond regions of the six Si-Si bonds for this center[2], allowing for relaxation in positions of the O and Si atoms. The aim of this study is to see if the presence of the O atoms is indeed able to change the situation of a maximum in the potential surface for Er$^{3+}$ found from our recent investigations of the Er-Si system without co-dopants to a minimum in the co-doped Oxygen system. Results for the Er$^{3+}$ potential curve for the Hi center and the geometries of the Si and O atoms will be presented. [1] F. d'Acapito, S. Mobilio, S. Scalese, A. Terrasi, G. Franz\'{o} and F. Priolo, Phys. Rev. B 69, 153310 (2004) and references therein. [2] J.D. Carey, J. Phys. Condens. Matter 14, 8537 (2002) [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U40.00006: \textit{Ab-initio }Study of the Diffusion Mechanisms of Gallium in a Silicon Matrix Kevin Levasseur-Smith, Normand Mousseau We present the results of a study into the diffusion mechanisms of Ga defects in crystalline Si. The dominant neutral configurations for single and multi-atom defects are established by \textit{ab-initio} calculations using the density functional theory in the LDA approximation, with a LCAO basis as implemented in the SIESTA package. We find formation energies of 0.7 eV and 2.9 eV, respectively, for the substitutional and tetrahedral interstitial defects, while the diatomic substitutional-tetrahedral complex has a formation energy of 2.2 eV. Subsequent calculations using this same DFT package in conjunction with the activation relaxation technique (ART nouveau) allow us to determine possible diffusion pathways as well as their corresponding saddle points and energy barriers. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U40.00007: Surface effects in Si interstitial formation energies Ann E. Mattsson, Ryan R. Wixom, Rickard Armiento We are calculating Si self-interstitial formation energies using Density Functional Theory and several different exchange-correlation energy functionals. We show that the difference in results obtained with the LDA, PBE, PW91, and AM05 [1] functionals can be explained by the functionals' different surface intrinsic errors. We explain why surface effects are important for formation energies of interstitials in semi-conductors. Surface effects have previously been studied for metal vacancy formation energies. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] R. Armiento and A. E. Mattsson, Phys. Rev. B {\bf 72}, 085108 (2005). [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U40.00008: Confining P Diffusion in Si by an As-doped Barrier Layer Lugang Bai, Guang-Hong Lu, Feng Liu, Qi Wang, Hamza Yilmaz We investigate the effect of As-doping on P diffusion in Si, using first-principles total-energy calculations. The formation of As-vacancy complex is found to be energetically favorable, which indicates the consumption of vacancy by As to prohibit vacancy-mediated P diffusion. In the vicinity of As-vacancy complex, the diffusion barrier of P via exchanging with Si vacancy is considerably increased, which further decreases the P mobility. These results qualitatively explain the experimental observations and provide useful direction for designing As-doped diffusion barriers for confining P diffusion in Si wafer processing and in MOSFET device fabrication. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U40.00009: Genuine impurity states vs. perturbed host states in a supercell approach Yong Zhang, A. Mascarenhas, L.-W. Wang The spatial localization is the most basic feature of an impurity state(IS) in a semiconductor. However, in the often used supercell approach, this feature might not be practically reliable, especially when the IS's are in resonance with the host state. We have developed a systematic approach for identifying the genuine IS's amongst the large number of states given by the supercell calculation[1], with the help of a charge-patching method that allows us to perform the electronic structure calculation with an accuracy near that of a self-consistent DFT and the ability handling $>$10K atoms[2]. We have applied this approach for two prototype systems: (1) isoelectronic ``acceptor'' in GaP:N, for which we have shown the non-existence of any excited state of N impurity in the conduction band within at least a few hundred meV proximity of the N bound state[1], in contrast to the decades old speculation and recent claim of the existence of such excited states; (2) isoelectronic ``donor'' in GaAs:Bi, for which we have found a resonant IS generated by Bi within the valence band (VB) and the topmost VB state is that of GaAs perturbed by Bi[3], which reveals a strong enhancement in the spin-orbit splitting (confirmed experimentally[4]) and resolves the speculations whether or not Bi could have a bound state in GaAs. [1] Zhang et al.,PRB\textbf{74},R41201 (06). [2] Wang, PRL \textbf{88},256402 (02). [3] Zhang et al., PRB\textbf{71},155201 (2005). [4] Fluegel et al., PRL\textbf{97},67205 (06). DOE/BES [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U40.00010: Origins of transparent conductivity in SnO$_{2}$ Abhishek Singh, Anderson Janotti, Chris Van de Walle, Matthias Scheffler SnO$_{2}$ belongs to a small class of materials that can conduct electricity while remaining transparent to visible light. Along with ITO it is widely used for contacts in flat-panel displays, solar cells, light emitting diodes and other optoelectronic applications. The origin of the observed unintentional $n$-type conductivity in SnO$_{2}$ is generally attributed to oxygen vacancies, $V_{\rm O}$. Using density functional calculations along with the LDA+$U$ method we show that $V_{\rm O}$ is not a shallow donor, but has a deep $\epsilon$(2+/0) level at $\sim$2.0 eV below the conduction band. The Sn interstitial is a shallow donor; however, its formation energy is very high making it very unlikely to be incorporated. Instead of point defects we propose that impurities, and in particular hydrogen, are responsible for the observed conductivity. We find that the hydrogen interstitial H$_{\rm i}$ has low formation energy and acts as a shallow donor in SnO$_{2}$. However, its migration barriers are small and therefore it is not stable at elevated temperatures. {\it Substitutional} hydrogen (H$_{\rm O}$), also acts as a shallow donor, and is more stable. Unlike H$_{\rm i}$ the formation energy of H$_{\rm O}$ depends on the abundance of oxygen and hence explains the experimentally observed dependence of conductivity on oxygen partial pressure in SnO$_{2}$. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U40.00011: Localized Vibrational Modes of $\mathrm{O_{Te}}$ and ($\mathrm{O_{Te}-V_{Cd}}$) Centers in CdTe: Fundamentals and Second Harmonics$^{*}$ Gang Chen, I. Miotkowski, S. Rodriguez, A. K. Ramdas In CdTe, grown with excess Cd vacancies ($\mathrm{V_{Cd}}$), oxygen replacing Te ($\mathrm{O_{Te}}$) displays a pair of fundamental localized vibrational modes (LVMs), $\nu_{1} = 1096.78$ cm$^{-1}$ and $\nu_{2} = 1108.35$ cm$^{-1}$. They are ascribed to the non-degenerate $\Gamma_{1}$ ($\nu_{1}$) and the doubly degenerate $\Gamma_{3}$ ($\nu_{2}$) LVMs of ($\mathrm{O_{Te}-V_{Cd}}$) centers with nearest neighbor Cd missing, having $C_{3v}$ symmetry and $\hat{\bf{c}}$ axis along $\langle 111\rangle$. In CdTe grown with conditions suppressing $\mathrm{V_{Cd}}$, $\mathrm{O_{Te}}$ occurs with all the four Cd nearest neighbors, and exhibits a triply degenerate $\Gamma_{5}$ LVM at $\nu_{0} = 349.79$ cm$^{-1}$ of $T_{d}$ symmetry.[1] The harmonics of ($\mathrm{O_{Te}-V_{Cd}}$), i.e., of $\nu_{1}$ and $\nu_{2}$ occur at $\nu_{4} = 2198.66$ cm$^{-1}$ and $\nu_{5} = 2210.5$ cm$^{-1}$. The temperature dependence of both ($\nu_{1}$, $\nu_{2}$) and ($\nu_{4}$, $\nu_{5}$) pairs display a remarkable behavior: $\nu_{1}$ and $\nu_{2}$ approach each other and coalesce at $T^{*} \sim 300$ K, as do $\nu_{4}$ and $\nu_{5}$; beyond $T^{*}$ they behave as a triply degenerate $\nu_{0}^{*}$ and $\nu_{s}^{*}$, respectively. The relative intensity of $\nu_{2}$ : $\nu_{1}$ approaches $2$ as $T \rightarrow T^{*}$ while that of $\nu_{5}$ : $\nu_{4}$ approaches $1/2$. These features find a convincing explanation on the basis of the dynamic switching of the ($\mathrm{O_{Te}-V_{Cd}}$) dangling bond among the four $\langle 111\rangle$ axes and, for $T \geq T^{*}$, these centers \textquotedblleft acquire\textquotedblright ~$T_{d}$ symmetry. With its $T_{d}$ symmetry, $\mathrm{O_{Te}}$ displays a single second harmonic $\nu_{s}$ at $695.72$ cm$^{-1}$. [1] Chen \textit{et al.}, Phys. Rev. Lett., \textbf{96}, 035508 (2006). $^{*}$Work supported by NSF (DMR 0405082) [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U40.00012: Theoretical study of the deep defect states in PbTe thin films Khang Hoang, S.D. Mahanti, P. Jena The nature of deep defect states (DDS) in bulk PbTe has been studied recently using density functional theory and a supercell model [1]. It was found that substitution of Pb by the trivalent impurities Ga, In, and Tl gave rise to hyperdeep defect states (HDS) below the valence band (VB) and DDS near the band gap region. Here we discuss how these states are affected in a (100) PbTe film using a supercell slab model. The HDS and DDS are preserved in the film geometry. As one goes from the bulk-like layers to subsurface and surface layers, the HDS tends to move closer to the bottom of the VB and becomes narrower; the DDS also gets modified. We also find that the defect formation energy E$_{f}$ as a function of the distance from the surface shows interesting features: all three impurities have lowest E$_{f}$ in the first layer but E$_{f}$ increases monotonically in the case of Ga, whereas there is a potential barrier in the second layer and a shallow potential ``valley'' between the second and the bulk-like layers in the case of In and Tl. This suggests that Ga impurities will be annealed out whereas the other two can be trapped in the subsurface region. [1] Salameh Ahmad, Khang Hoang, and S. D. Mahanti, Phys. Rev. Lett. 96, 056403 (2006). [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U40.00013: \textit{Ab Initio }Study of Electronic Structure of Defects in SnTe and GeTe. Salameh Ahmad, S.D. Mahanti \textit{Ab} \textit{initio} electronic structure calculations have been carried out within density functional theory (DFT) in SnTe and GeTe, two well-known narrow band-gap semiconductors, to understand the nature of deep defect states (DDS) introduced by Cd and In impurities substituting for Sn/Ge. These results are compared with similar studies in PbTe$^{1}$. The calculations have been carried out using a 64 atom super-cell model containing one defect. The density of states near the top of the valence band (VB) and the bottom of the conduction band (CB) get significantly modified by the defects as found in PbTe. The DDS associated with Indium impurity near the top of the VB is resonant in SnTe and lies in the gap in GeTe; its energy increasing in the order Sn-Pb-Ge. Cadmium on the other hand gives resonance (GeTe) and bound states (on SnTe) near the bottom of the CB, the energy of the DDS increasing from Sn-Pb-Ge. The positions of these DDS can have significant impact on thermoelectric and other transport properties of these semiconductors. 1. Salameh Ahmad, Hoang Khang, and S.D. Mahanti, Phys. Rev. Lett. 96, 056403 (2006); Salameh Ahmad et. al. Phys Rev. B74, 155205 (2006). [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U40.00014: Interfacial segregation and electrodiffusion of dopants in AlN/GaN superlattices P. Boguslawski, J. Bernholc, N. Gonzalez Szwacki Properties of semiconductors and their devices entirely rely on accurate control of doping and stoichiometry, determined in turn by solubility of impurities and the presence of native defects. In fact, it has been experimentally shown that thermally activated segregation of impurities through heterointerfaces leads to concentration differences that reach two orders of magnitude. We develop a first-principles theory of interfacial segregation in heterostructures and apply it to hydrogen and typical dopants and defects in GaN/AlN superlattices [1]. We show that the equilibrium concentrations of a dopant at two sides of an interface may differ by up to a few orders of magnitude depending on its identity and charge state, and are not directly given by dopant solubilities of bulk constituents. In addition, the presence of strong electric fields in GaN/AlN systems induces field-driven electromigration and accumulation of hydrogen at the appropriate interfaces. [1] P. Boguslawski, J. Bernholc, and N. Gonzalez Szwacki, Phys. Rev. Lett. 96, 185501 (2006). [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U40.00015: Effect of Native Defects in InN: Metallic bonding and N$_2$ Formation Xiangmei Duan, Catherine Stampfl Despite intense investigations, development of indium nitride technology remains at the stage of seeking to improve the growth techniques and fabrication of device-quality material. Doping and impurity control is essential for the advancement of InN-based electronic and optoelectronic devices, yet even the role of native point defects in InN, and their effect on the physical and electronic properties, is still lacking [1]. We have thoroughly investigated the distribution and electronic properties of native point defects in wurtzite InN through first-principles density- functional theory calculations. We find that both the nitrogen and indium vacancies have a tendency to form ``clusters'' in the neutral and negative charge states. For nitrogen vacancies, this results in local indium- rich regions with metallic-like bonds, while we find molecular nitrogen formation occurs either by the clustering of indium vacancies, or interstitial nitrogen. The formation energies of the indium vacancy clusters are however rather high, but that of interstial nitrogen in the 3+ charge state has a low formation energy under N-rich conditions. Our results help to explain a number of hitherto puzzling experimental observations[1-3]. [1] T. Shubina et al. Phys. Stat. Sol. a \textbf{202}, 377 (2005). [2] Y. Davydov et al. Phys. Stat. Sol. B \textbf{230}, R4(2002); \textbf{240}, 425 (2003). [3] H. Timmers et al. J. Cryst. Growth \textbf{288}, 236 (2006). [Preview Abstract] |
Session U42: Nitrides, Carbides, Oxides: Surfaces and Properties
Sponsoring Units: DCMPChair: Phil Cohen, University of Minnesota
Room: Colorado Convention Center 505
Thursday, March 8, 2007 8:00AM - 8:12AM |
U42.00001: Electronic structure of nitride surfaces Chris G. Van de Walle, David Segev, Anderson Janotti Knowledge of surface reconstructions and the corresponding surface electronic structure is important to control growth, since Fermi-level pinning can affect defect creation and incorporation. In addition, surface states can play an important role in devices, for instance in high-electron mobility transistors where the surface acts as a source of electrons for the channel. In the case of InN a very high, and thus far unexplained, electron accumulation has been observed on all polar surfaces. We have addressed these issues by performing a systematic computational study of reconstructed GaN and InN surfaces in various orientations, including (11-20) ($a$ plane) and (10-10) ($m$ plane), as well as the polar (0001) (+$c)$ and (000-1) (-$c)$ planes. The calculations are based on density-functional theory, combined with an extensively tested approach for correcting the band-gap error through use of modified pseudopotentials. For GaN we identify the microscopic origins of the experimentally observed Fermi-level pinning. For InN we find that on polar surfaces occupied surface states occur above the conduction-band minimum, thus explaining the observed electron accumulation. We predict an absence of electron accumulation on \textit{nonpolar} surfaces grown at moderate In/N ratios. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U42.00002: Electronic structures and work functions of InN(0001) films Jung-Hwan Song, Arthur J. Freeman InN films have attracted great attention with its recently discovered band gap, 0.7eV, and evidence for p-type doping\footnote{R. E. Jones, et al, Phys. Rev. Lett, {\bf 96}, 125505 (2006)}. We have studied theoretically the electronic structures, surfaces, and work functions of InN films using the highly precise FLAPW method\footnote{Wimmer, Krakauer, Weinert, Freeman, Phys. Rev. B, {\bf 24}, 864 (1981)}. The passivation with pseudo-hydrogens\footnote{K. Shiraishi, J. Phys. Soc. Jpn, {\bf 59}, 3455 (1990)} has also been applied to the surfaces of InN(0001) films for comparison with the electronic structure of the ideal InN(0001) films. We compare the work functions of InN films with other wurtzite materials such as ZnO, GaN, and AlN, which we have also calculated. We discuss the mechanism of the structural transition\footnote{C. L. Freeman, et al, Phys. Rev. Lett, {\bf 96}, 066102 (2006)} with layer thickness for the very thin InN(0001) films, for which we have found that the ideal InN(0001) films of the wurtzite structure, up to 4 bilayers, optimize to the graphitic- like structure. We then discuss the relationship between the dipoles and the surfaces (work functions) of the InN(0001) films, and the possibilities of their p-type doping. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U42.00003: HREELS, AES, and LEED of InN(000-1): Surface structure and electron accumulation Rudra Bhatta, Brian Thoms, Mustafa Alevli, Nikolaus Dietz InN layers grown by high pressure chemical vapor deposition (HPCVD) have been studied using several surface sensitive techniques. Following argon sputtering and atomic hydrogen cleaning (AHC), Auger electron spectroscopy showed that surface contaminants had been removed and a 1x1 hexagonal low energy electron diffraction pattern demonstrated that the InN surface was well ordered. HREEL spectra of the atomic hydrogen cleaned layer show a Fuchs-Kliewer surface phonon at 560 cm$^{-1}$ and adsorbate loss peaks at 3260 and 870 cm$^{-1}$ assigned to N-H stretching and bending vibrations, respectively. These assignments are confirmed by isotopic shifts using deuterium. No surface In-H vibrations are observed indicating N-H termination of the surface and the film is N-polar. HREEL spectra also showed a broad loss features due to conduction band plasmon excitations. The plasmon excitation shifted to higher energy as the incident electron energy (and therefore the penetration depth) was decreased. This shift indicates that the surface has a higher plasma frequency than bulk of the InN layer, which in turn indicates the presence of a surface electron accumulation layer. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U42.00004: In-situ X-ray Studies of MOCVD Growth of InN Fan Jiang, A. Munkholm, C.D. Dimitropoulos, R.-V. Wang, S.K. Streiffer, G.B. Stephenson, P.H. Fuoss, K. Latifi, Carol Thompson One of the fundamental issues in the continued development of III-nitride semiconductor alloys is to understand incorporation of indium. Our approach is to use real-time x-ray scattering and fluorescence as \textit{in situ} probes during growth by MOCVD. We observe the equilibrium condensation boundaries for elemental In and InN as a function of temperature and trimethylindium supply, which allow us to determine the effective activities of In and N at the sample surface. We find that the partial pressures of both hydrogen and ammonia in the ambient have strong effects on the activities. We also observe strong effects of the substrate on condensation, including an oscillatory regime, indicating that surface reactions are important. Work supported by the U.S. Dept. of Energy contract DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U42.00005: Atomic and Electronic Structures of Oxygen on the $\beta-$Si$_3$N$_4$ $(10\overline{1}0)$ Surface Weronika Walkosz, Juan C. Idrobo, Serdar Ogut The desirable mechanical and physical properties of Si$_3$N$_4$ ceramics in high temperature applications are hindered by their intrinsic brittleness. Doping Si$_3$N$_4$ with rare-earth oxides has long been known to overcome this limitation creating a tougher material. Precise information about the microscopic origin of this empirical observation has, however, been lacking for many years. In this study, we present {\em ab initio} calculations for the structural stability of $\beta-$Si$_3$N$_4$ $(10\overline{1}0)$ surfaces in the presence of different oxygen concentrations. Two different $(10 \overline{1}0)$ surface terminations, the ``open ring" and the ``half surface",\footnote{J. C. Idrobo {\em et al}., Phys. Rev. B {\bf 72}, 241301(R) (2005).} are investigated in detail using an asymmetric slab. We find that the Si-O bond plays the most important role in the structural stability and passivation of the surface. The theoretical results are analyzed in connection with recent electron microscopy studies on the interface.\footnote{A. Ziegler {\em et al.}, Science {\bf 306}, 1768 (2004); N. Shibata {\em et al.}, Nature {\bf 428}, 730 (2004); G. B. Winkelman {\em et al.}, Phil. Mag. Lett. {\bf 84}, 755 (2004).} [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U42.00006: Growth and Structure of ZrSiN Thin Films Robert Lad, Xuefei Zhang A series of Zr$_{1-x}$Si$_{x}$N thin films were grown on r-plane sapphire substrates using rf magnetron co-sputtering of Zr and Si targets in a N$_{2}$/Ar plasma. The films were grown at 200$^{o}$C and also post-deposition annealed to 900$^{o}$C in vacuum. Pure ZrN grows with high quality (100) epitaxy on r-sapphire as demonstrated by x-ray diffraction reflectivity and pole figure analysis. When small amounts of Si are added into the lattice, the films become strained as evidenced by a continual increase in the lattice parameter (up to a 6{\%} for x=0.12) and become polycrystalline. Higher amounts of Si cause the structure to become amorphous and the films become much rougher. X-ray photoelectron spectroscopy measurements show large shape changes in the N and Zr core levels as the alloy composition changes, whereas the Si peaks exhibit negligible change. UV-visible optical absorption measurements show a direct correlation between the location of the absorption edge and Zr-Si ratio. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U42.00007: Passivation of 4H-SiC Silicon surface G. Pennington, C. Ashman, A. Lelis, N. Goldsman The material properties of the silicon carbide (SiC) 4H polytype are ideally suited for use in metal-oxide-semiconductor field-effect transistors (MOSFETs) operating under high temperature, high power conditions. Currently, the development of lateral SiC MOSFETs is hindered by excessively small field-effect mobilities that are typically measured in these devices. The cause for such small mobilities is believed to be directly related to the very large density of traps measured at the 4H-SiC/SiO2 interface. Recently, oxidation processing in the presence of nitrogen or in the presence of metals, has been shown to improve the mobility of 4H-SiC MOSFETs. However, there is no clear consensus on the physical mechanisms involved in improvement of the 4H-SiC/SiO2 interface. We use \textit{/ab-initio/} density functional theory to study passivation of the 4H-SiC Si surface by nitrogen, oxygen, aluminum, and sodium. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U42.00008: A DFT study of low index polar surfaces: the case of SiC and ZnO Giuseppe Brandino, Giancarlo Cicero, Alessandra Catellani With the advent of nanostructured devices, it appears evident that the role of surface and interface effects may prevail on bulk properties and determine the physical characteristics of the material at the nanoscale: in particular, the understanding of the electronic properties of nanosized structures demands for a proper accurate treatment. Here we report on first principles density functional calculations of the structural and electronic properties of the so called ``non-polar'' low index surfaces of hexagonal silicon carbide (SiC) and zinc oxide (ZnO), specifically focussing on surface polarity. We first provide an accurate analysis of the macroscopic polarization field as a function of the hexagonality and ionicity of the compound, and than describe in details the properties of the relaxed surfaces. Our predictions nicely compare with recent experimental data on similar SiC and ZnO surfaces: in particular we obtain good agreement between the perpendicular surface dipole component and the experimental work functions values. Moreover, for the first time, we highlight the presence of a strong in-plane dipole component related to dangling bond rearrangement at the surface, which opposes the bulk spontaneous polarization. The decaying behaviour of this dipole inside the slab shows that the presence of surfaces deeply change the polar properties of structures of few nanometers size, while bulk polarization is recovered for thicker systems. Given the importance of surface dipoles in adsorption and functionalization processes, we finally analyze the local surface electric field by employing a simple polar model molecule (HF) as a probe: we describe the interaction in terms of the potential energy surface (PES) experienced by the molecule. This allows us to give a complete description of the surface polarity and to draw conclusions on the most likely adsorption sites of charged and polar species. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U42.00009: Role of neutral impurity scattering in the analysis of Hall data from ZnO Xiaocheng Yang, Chunchuan Xu, Nancy Giles Zinc oxide is a wide band-gap semiconductor with bright UV emission. To determine donor and acceptor concentrations affecting electrical properties in n-type ZnO crystals, the relaxation time approximation has been used to analyze mobility ($\mu )$ and carrier concentration data measured from 80 to 400 K. Five scattering mechanisms are included: polar-optical-phonon, piezoelectric potential, deformation potential, ionized impurity, and neutral impurity (NI) scattering. The NI scattering is often ignored but plays an important role in limiting the total $\mu $. By including NI scattering, the experimental deformation potential E$_{1}$ = 3.8 eV can be used. Temperature dependences of the intrinsic Hall r factor and intrinsic $\mu $ are determined. At 300 K, ``pure'' ZnO has an electron $\mu $ of about 210 cm$^{2}$/Vs. Analysis of Hall data from commercial hydrothermally and CVT-grown n-type ZnO crystals is presented. Donor and acceptor concentrations from Hall data are compared with those estimated using infrared absorption and EPR data. Intrinsic hole mobility in p-type ZnO is also discussed. This work was supported by NSF Grant No. DMR-0508140. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U42.00010: Direct observation of zinc vacancies and oxygen vacancies in an electron-irradiated ZnO crystal S. M. Evans, N. C. Giles, L. E. Halliburton, L. A. Kappers Electron paramagnetic resonance (EPR) has been used to monitor zinc vacancies and oxygen vacancies in a ZnO crystal irradiated near room temperature with 1.5 MeV electrons. Out-of-phase detection at 30 K greatly enhances the EPR signals from these vacancies. After electron irradiation, but before illumination, Fe$^{3+}$ ions and nonaxial singly ionized zinc vacancies are observed. Illumination with 325 nm light at low temperature produces spectra from singly ionized oxygen vacancies, neutral zinc vacancies, and axial zinc vacancies. The light also produces spectra from zinc vacancies with an adjacent hydrogen (an OH$^{-}$ ion). The response of the irradiated crystal to illumination wavelengths out to 750 nm is described. Wavelengths shorter than 600 nm convert the Fe$^{3+}$ ions to Fe$^{2+}$ ions and convert the neutral oxygen vacancies to singly ionized oxygen vacancies. Warming above 130 K in the dark reverses the effect of the illuminations. This work was supported by NSF Grant DMR-0508140. One of the authors (SME) acknowledges support from the West Virginia STEM Fellowship Program. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U42.00011: Tunable UV-Luminescent MgZnO Nanoalloys John L. Morrison, Jesse Huso, Heather Hoeck, Erin Casy, James Mitchell, Russell Geisthardtand, Leah Bergman Mg(x)Zn(1-x)O alloys are promising wide-bandgap semiconductors for optoelectronic applications, and also of considerable interest from a fundamental viewpoint. The environmentally friendly chemical composition and the deep excitonic level $\sim $ 60 and 90 meV of ZnO and MgO respectively make it an excellent candidate for high-efficiency next generation ultraviolet light sources. These optical alloys may enable the tuning of the bandgap and the luminescence at the range of $\sim $ 3.0 for ZnO of the wurtzite structure up to $\sim $ 7 eV for the MgO of the rocksalt structure. We will present studies on the photoluminescence and Raman properties of Mg(x)Zn(1-x)O nanocrystallites of average size $\sim $ 30 nm that were synthesized via the thermal decomposition method. For the studied composition range of 0-26{\%} Mg, the room temperature UV-PL was found to be tuned by $\sim $ 0.3 eV towards the UV-spectral range. For that composition range the first-order LO Raman mode was found to exhibit a significant blueshift of $\sim $ 33 cm$^{-1}$ indicating that a good solid solution was achieved at the nanoscale. At higher composition ranges a PL blue shift of at least 1.3 eV was achieved. Issues such as excitonic emissions, alloy spectral broadening, and phonon symmetry will be presented [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U42.00012: Stability, Electronic and Optical Properties of In$_2$O$_3$(ZnO)$_n$ Alloys Juarez L. F. Da Silva, Su-Huai Wei (In$_2$O$_3$)-(ZnO) heterostructural alloy has recently become a promising transparent conducting oxides (TCO) because it possesses combined physical properties such as excellent optical transmission, high electrical conductivity, chemical and thermal stability, and good film smoothness. However, the origin of these superior properties is not well understood. In this work, using first-principles methods, we have investigated the structural stability, electronic, and optical properties of In$_2$O$_3$(ZnO)$_n$ $(n = 1 - 5)$. In$_2$O$_3 $(ZnO)$_n$ forms layered hexagonal or rhombohedral In$_2$O$_3$(ZnO)$_n$ superlattices, which are isostructural with LuFeO$_3$(ZnO)$_n$. The deformed In$_2$O$_3$ layer is highly strained and plays an important role in determining the stability of the system. The calculated band structures show that these alloys has a direct band gap at the $\Gamma$-point and typical features as other known n-type TCOs. The calculated small effective mass for these materials is consistent with the high electron mobility for these system. The optical properties of these alloys are calculated and compared with that of In$_2$O$_3$ and ZnO. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U42.00013: Carrier Dynamics and Emission Efficiency in Sulfur-doped ZnO Powders John V. Foreman, Henry O. Everitt, Jie Liu In previous work [\textit{Nano Lett. }\textbf{6}, 1126 (2006)] it was found that sulfur-doping ZnO micro- and nanostructures dramatically enhanced the broadband, visible wavelength defect emission centered at $\sim $2.5 eV (500 nm), while quenching the band edge ultraviolet emission. The effects of sulfur-doping on carrier dynamics and integrated emission efficiency are further characterized here by studying the time-resolved photoluminescence of band edge and defect emitters as a function of sulfur doping concentration, temperature, and excitation intensity in ZnO powders. The dynamics can be understood in terms of a rate equation model which describes energy transfer between band edge and radiative defect levels, as well as nonradiative centers. The potential application of these materials for efficient visible wavelength phosphors will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U42.00014: Origin of high-density two-dimensional electron gas in ZnO/ZnMgO heterostructures. Shigehiko Sasa, Tetsuya Tamaki, Kazuyuki Hashimoto, Kazuya Fujimoto, Kazuto Koike, Mitsuaki Yano, Masataka Inoue We performed a self-consistent calculation of electronic states in ZnO/ZnMgO multiple quantum wells (MQWs). In ZnO/ZnMgO MQWs, the charges induced by spontaneous and/or piezoelectric polarizations at the heterointerfaces play an important role in determining the optical properties. By comparing the optical transition energies between the calculations and experiments, the polarization charge density was determined. In addition, the band bending effects caused by ionized impurities in the structure were found to be crucial for wider ZnO well thicknesses. Therefore, the electronic states in ZnO/ZnMgO MQWs was calculated by changing the thickness of the ZnO layer, Lw (1-8 nm), the sheet polarization charge,$\sigma $, and donor concentrations in the ZnO, Nw, and in the ZnMgO, Nb. We also calculated the two-dimensional electron gas (2DEG) concentration in a thick ZnO layer grown on a ZnO (5 nm)/ZnMgO (5 nm) MQW buffer layer by using the same parameters in order to validate the calculation. The 2DEG concentration was successfully explained by the calculation. The calculation indicates that the 2DEG concentration is mainly determined by the donor concentration in the ZnMgO barrier layer. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U42.00015: Anisotropic thermal expansion in wurtzite materials: an ab-initio calculation for ZnO A.H. Romero, J. Serrano, I. Margiolaki, M. Cardona Many optoelectronic devices are based on wurtzite materials, e.g., ZnO, GaN, and SiC. Contrary to cubic structures, scarce experimental data have been reported on the linear thermal expansion coefficients of anisotropic materials. To our knowledge, no first principles calculations have been reported for the anisotropic thermal expansion coefficients. We report here two different approaches for first principles calculations of these coefficients based on the lattice dynamics obtained in the quasiharmonic approximation from the \textit{ab initio} electronic band structure. The first method relies on thermodynamic relations for the entropy and the phonon density of states. The second approach requires the explicit calculation of Gr\"{u}neisen parameters in the irreducible Brillouin zone. The two methods are applied to wurtzite ZnO and the obtained expansion coefficients are in excellent agreement with those derived from x-ray diffraction data taken with synchrotron radiation at the beamline ID31 of the ESRF. The calculations also provide the so-called zero-point contribution to the lattice parameters, which is also anisotropic and of interest in the analysis of the temperature dependence of electronic gaps. [Preview Abstract] |
Session U43: Optical Properties of Quantum Well Systems
Sponsoring Units: DCMPChair: Sergio Ulloa, Ohio University
Room: Colorado Convention Center 506
Thursday, March 8, 2007 8:00AM - 8:12AM |
U43.00001: $\Gamma$-phonons in ZnSe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ and ZnTe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ hybrid materials Ivan Naumov, Huaxiang Fu Artificially synthesized organic/inorganic hybrid materials with semiconductor component are of growing interest due to potential applications in flexible microelectronics. Among them, hybrid ZnSe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ and ZnTe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ are two examples capable of tuning electronic and optical properties on a wide range. Engineering of these materials is difficult without deeper understanding of their fundamental physical characteristics, including electronic band structure and phonon spectra. Recent experimental investigations [1] performed on $\beta$--ZnTe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ samples and showed multiple sharp phonon modes in frequencies very different from the LO($\Gamma$) phonon of the binary semiconductor ZnTe. Motivated by these results, we have performed density-functional calculations of the $\Gamma$ phonon modes for both $\alpha$-- and $\beta$--phases of ZnSe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ and ZnTe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$. We found, in particular, that light hydrogen atoms not only define the high frequency motion (1500--3000 cm$^{-1}$), but also dominate in some low frequency modes in the region of 100--150 cm$^{-1}$ where they move mostly perpendicular to the superlattice stacking direction. Based on symmetry of the materials, the Raman modes at the $\Gamma$ point are identified. \newline [1] X. Huang, J. Li, Y. Zhang, and A. Mascarenhas, J. Am. Chem. Soc. {\bf 125}, 7049 (2003). [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U43.00002: The Magnetic-Field Dependent Phonon-Like FIR Absorption in Modulation Doped CdMnTe/CdMgTe quantum Well Structures. Li-Chun Tung, Grzegorz Karczewski, Yong-Jie Wang Spin-dependent phonon had been reported in several magnetic semiconductors. This unique phonon mode is resulted from an ion-position dependent spin Hamiltonian. This mode can be either infrared or Raman active. However, such modes were never observed in the diluted magnetic semiconductors. We have investigated a series of CdMnTe/CdMgTe QW samples with Mn concentration ranged from 0 to 3.9{\%} up to 33T and found an IR absorption near 125cm$^{-1}$. The intensity of this mode increases rapidly with increasing magnetic field, while the energy increases only several wavenumbers. This behavior resembles that of the spin-dependent phonon mode. The unique behavior of the spin-dependent phonon mode was induced by a magnetic transition with decreasing temperatures. In the current studies, however, similar behavior is induced by applying magnetic field. Besides 125cm$^{-1}$ mode, we have also observed two new IR absorptions in addition to the electron cyclotron resonance (CR). The intriguing disappearance of CR at high magnetic field and the stimulating behavior of these two new modes will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U43.00003: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U43.00004: Investigation of the influence of the magnetic field of different orientations on excitonic states in structures with shallow quantum wells Yuri Khavin, Mikhail Skorikov, Nikolai Sibeldin, Vitaliy Tsvetkov Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of GaAs/Al$_{0.5}$Ga$_{0.95}$As structures with two tunneling-coupled quantum wells (QWs) 3 and 4 nm wide and structures with tunneling-isolated QWs of the same width were investigated at liquid helium temperatures in parallel and perpendicular magnetic fields up to 14.6T. It was shown that the parallel magnetic field strongly suppresses tunneling coupling in the structure with coupled QWs, and its PLE spectrum in a strong field becomes similar to the PLE spectrum of the structure with isolated QWs. At the same time, excitonic components of the PL spectrum do not undergo such substantial changes. It was also shown that both parallel and perpendicular field substantially affect the intensity of the trion PL line in both structures, but the influence of the parallel field is much stronger than it is for the perpendicular field. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U43.00005: Photoluminescence Studies of Type-I and Type-II In$_{0.27}$Ga$_{0.73}$Sb/In$_{x}$Al$_{1-x}$As$_{y}$Sb$_{1-y}$ Multiple Quantum Well Heterostructures Grown by MBE E.R. Glaser, R. Magno, B.V. Shanabrook, J.G. Tischler Heterojunction bipolar transistors (HBTs) with lattice constant near 6.2{\AA} using the InAs/AlSb/GaSb family of semiconductors are of interest based on their promise for high-speed operation with low power dissipation. A unique aspect of these materials is the ability to engineer the bandgap energies and the conduction band offsets at the emitter/base and base/collector heterointerfaces by varying the In/Al and Al/Sb ratios. In this work low-temperature PL was performed on a set of In$_{0.27}$Ga$_{0.73}$Sb/In$_{x}$Al$_{1-x}$As$_{y}$Sb$_{1-y}$ MQW heterostructures to provide a measure of the conduction band offsets ($\Delta _{CB})$ that are a critical design parameter for the HBTs. Excitation power studies revealed evidence for strong recombination at 0.56 eV within the InGaSb layers of the MQW structure with x,y = 0.52,0.25 and, thus, confirmed the type-I band alignment. In contrast, weaker PL bands at energies close to 0.4 eV and that exhibited strong shifts with increasing excitation power density were found from the nominally type-II MQW samples with x,y=0.67,0.39 and 0.69,0.41. Neglecting small corrections ($\sim $15 meV) due to the electron and hole confinement energies, we estimate $\Delta _{CB}$ of $\sim $ 120-150 meV in these Type-II structures. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U43.00006: Temperature and Pump Pulse Dependence of Superfluoresence from InGaAs/GaAs Multiple Quantum Well in High Magnetic Fields Xiaoming Wang, Young-Dahl Jho, Jinho Lee, David Reitze, Junichiro Kono, Alexey Belyanin, Vitaly Kocharovsky, Glenn Solomon, Xing Wei, Stephen McGill Using intense near-IR ultrashort pulse laser excitation, we investigate the characteristics of cooperative emission (superfluoresence) from dense electron hole magneto-plasmas in InGaAs/GaAs MQW in high magnetic fields as a function of temperature and excitation pulsewidth. We find strong narrow line emissions from 0-0 and 1-1 Landau levels (LLs), with thresholds depending on magnetic field and temperature. Varying the excitation pulsewidth (180 fs -- 60 ps) and fluence (0.1 -- 1 mJ/cm$^{2})$, we observe qualitative changes in the emission strengths from different LLs. The strong emissions from 0-0 and 1-1 LL excited with short and long pulses are obviously different. Mechanisms of the temperature and excitation pulse width effect on the strong emission are presented. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U43.00007: Interface roughness scattering in p-Si/SiGe asymmetric quantum wells Marco Califano, N.Q. Vinh, P.J. Phillips, Z. Ikonic, R.W. Kelsall, P. Harrison, C.R. Pidgeon, B.N. Murdin, D.J. Paul, P. Townsend, J. Zhang, I.M. Ross, A.G. Cullis Of paramount importance in the design of a quantum cascade laser is the ability to engineer carrier lifetimes. These can be strongly influenced by the quality of the interfaces: fluctuations in the well width result in local fluctuations of the carriers' confining potential, which act as a scattering potential. We report the direct determination of non-radiative lifetimes in Si/SiGe asymmetric quantum wells designed to access spatially indirect interwell transitions between heavy-hole states, at photon energies below the optical phonon energy. We show both experimentally and theoretically that, for the interface quality currently achievable experimentally interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recent data for III-V multiple quantum wells leads us to the conclusion that the dominant role of interface roughness scattering at low temperature found here is a general feature of a wide range of semiconductor heterostructures not limited to IV-IV materials. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U43.00008: Localized charged magnetoexcitons in 2D systems Diana Cosma, Alexander Todd, Alexander Dzyubenko, Andrey Sivachenko We performed a detailed theoretical study of localization of spin-singlet $X_s ^-$ and spin-triplet $X_t ^-$ negatively charged excitons on isolated charged donors $D^+$ located at various distances $L$ from the heteroboundary of a Quantum Well (QW). Our results show that the parent bright singlet state $X_s ^-$ remains always bound. In contrast, the dark $X_{td} ^-$ and bright $X_{tb} ^-$ triplet states survive only for sufficiently large distances $L$ to the donor ion $D^+$. In the presence of the $D^+$ the dark triplet acquires finite oscillator strengths. We also found several new bound $X^-$ states, some of which have surprisingly large oscillator strengths. We showed that shake-up processes are strictly prohibited in magneto-photoluminescence of free charged excitons and only become allowed in the presence of a $D^+$ or other symmetry-breaking mechanisms. Our results show that the main magneto-PL peaks of free and donor bound charged excitons may exhibit very similar features while the shake-up processes in PL are symmetry-breaking signatures. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U43.00009: Effect of disorder on the lifetime of interface polaritons Igor Smolyarenko, Celestino Creatore, Alexei Ivanov We study the effects of weak disorder on the liftime of interface polaritons in quantum wells (QWs) associated with in- plane free QW excitons. In ideal quantum wells, the interface light is evanescent, i.e., it is localized in the z direction (QW growth direction) and is thus invisible at macroscopic distances from the QD plane. However, randomness in the structure of QWs leads to finite radiative lifetimes of the interface light modes which makes them ``visible'' in time- resolved light scattering experiments. We study the distribution of the delay times for scattering of bulk photons off the two- dimensional interface layer at finite incidence angle. While the bulk of the distribution corresponds to the usual threshold effect, the tail of the distribution at long delay times is governed by the quasi-resonant long-lived interface light modes which are essentially the disorder-degraded remnants of the evanescent waves in ideal QWs. These modes are analogous in some ways to the anomalously localised states in two-dimensional disordered electronic systems. Time-resolved spectroscopy of weakly disordered QWs is thus shown to be an effective tool for probing the ``hidden'' optics of the interface polaritons. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U43.00010: Generic Theory of Surface Plasmon Polaritons at Active or Passive Metal-Dielectric Interfaces Dentcho Genov, Muralidhar Ambati, Xiang Zhang In this work we propose a simple analytical approach to study the excitation of surface plasmon polaritons (SPP) at the interfaces between metals and passive or active media. Explicit relationships are derived for both SP dispersion and propagation length. The analytical theory matches the exact numerical calculations for a wide range of excitation frequencies and metal slab thicknesses. Scaling relationships for the critical gain required to achieve infinite propagation length are derived for the symmetric and antisymmetric SPP modes. A specific multiple quantum well (MQW) system is identified as an effective media for prospective experimental studies of SP amplification and enhanced propagation. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U43.00011: Nonequilibrium Green's function modelling of quantum well solar cells Urs Aeberhard, Rudolf H. Morf We present a microscopic model of the photocurrent in $GaAs-AlGaAs$ quantum well solar cells (QWSC), based on the nonequilibrium Green's function formalism (NEGF) for a multiband tight binding Hamiltonian. The quantum kinetic equations of motion are selfconsistently coupled to Poisson's equation. Relaxation and broadening mechanisms are considered by the inclusion of acoustic and optical electron-phonon interaction in a self consistent Born approximation of the scattering selfenergies. The model is applied to different multi-quantum-well sytems to investigate the role of device geometry and coupling between the wells. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U43.00012: Optical and Electronic Properties of Electrochemically Active Perylene Tetracarboxylic Diimide Molecules Nazanin Davani, ken Shimizu, Michael Preiner, Nicholas Melosh Perylene tetracarboxylic diimide (PTCDI) molecules form a unique class of n-type semiconductors with high thermal and photo stability. Understanding the electronic properties of these molecules in nanoscale systems may lead to novel applications in various molecular electronics devices. Using optical spectroscopy, we analyzed the optical and electronic properties of electrochemically active TE-PTCDI molecules self -assembled on gold electrodes. Surface Plasmon Resonance Spectroscopy is used to detect the refractive index of the monolayer. Cyclic voltammetry and SPR measurements are performed simultaneously to probe changes in reflectivity as the molecule undergoes redox reactions. In addition, the TE-PTCDI molecule is used in fabricating metal-insulating monolayer-metal devices using Polymer Assisted Lift Off (PALO) technique. The influence of the top metal contact on molecular properties will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U43.00013: Photoconductivity of Yttrium Neodymium Gadolinium Oxalate Crystals Soosy Kuryan, Rosalin Abraham, Jayakumari Isac Crystals are pillars of modern technology..Yttrium Neodymium Gadolinium oxalate (YNdGaOx) crystals were grown by gel method by the diffusion of Yttrium Chloride, Neodymium Chloride, and Gadolinium Chloride into the set gel containing Oxalic acid. Silica gel method is capable of yielding crystals of high optical perfection and wide morphology. The growing crystals are held in the gel medium in a strain free manner and at the same time nucleation and super saturation are well controlled. Photo conductivity studies of these crystals revealed negative photoconductivity nature. The photocurrent is found to be less than the dark current at every applied electric field. Rare Earth compounds are known for their interesting electric, magnetic and luminescent properties. Recent investigations on the fluorescence of some rare earth oxalates suggest their potentiality for their optical applications. Rare Earth oxalates evoked greater attention because of their ionic conduction. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U43.00014: Plasmon dispersion and Coulomb drag in low-density electron bi-layers S. M. Badalyan, C. S. Kim, G. Vignale, G. Senatore We investigate the effect of exchange and correlation (xc) on the plasmon spectrum and the Coulomb drag between spatially separated low-density two-dimensional electron layers. We adopt a new approach, which employs dynamic xc kernels in the calculation of the bi-layer plasmon spectra and of the plasmon-mediated drag, and static many-body local field factors in the calculation of the particle-hole contribution to the drag. We observe that both optical and acoustical plasmon modes are strongly affected by xc corrections and shift in opposite directions with decreasing density. This is in stark contrast with the tendency observed within the random phase approximation (RPA). We find that the introduction of xc corrections results in a significant enhancement of the transresistivity and qualitative changes in its temperature dependence. In particular, the large high-temperature plasmon peak that is present in the RPA is found to disappear when the xc corrections are included. Our numerical results are in good agreement with the results of recent experiments by M. Kellogg {\it et al.}, Solid State Commun. \textbf{123}, 515 (2002). [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U43.00015: Surface Plasmon Polaritons on Concentric Cylindrical Shells Jakub Pritz, Lilia Woods A theoretical investigation of electromagnetic plasmon modes for multilayered concentric cylindrical shells in the absence of an applied magnetic field is presented. The plasmon excitations in such multiple interface configurations are characterized by the electromagnetic fields. To obtain the dispersion relation for the modes, the Maxwell equations with appropriate boundary conditions are solved numerically. We study the conditions for existence of radiative and nonradiative modes. The effects of varying the different shells radii, their thicknesses and dielectric response functions are also investigated. The physical importance of the different plasmon modes is also discussed. [Preview Abstract] |
Session U44: Focus Session: Optical Properties of Metallic Nanostructures: Theory
Sponsoring Units: DMPChair: Alexander Govorov, Ohio University
Room: Colorado Convention Center 507
Thursday, March 8, 2007 8:00AM - 8:36AM |
U44.00001: Theory and modeling of light interactions with metallic nanostructures Invited Speaker: Metallic nanostructures such as systems containing metal nanoparticles or metal films with nanoscale diameter holes or other nanostructured features are intriguing systems. Surface plasmons, special electronic excitations near the metallic surfaces, can then be excited with visible light. In addition to interest in their fundamental behavior and interactions, surface plasmons are useful in a variety of practical areas, including chemical and biological sensing and optoelectronics. Surface plasmons can be intense and localized, and correctly describing their behavior in complex systems can require numerically rigorous modeling techniques. This talk presents a discussion of the results of rigorous electrodynamics modeling using the finite-difference time-domain (FDTD) method. Such calculations may be used to validate ideas and concepts based on approximate models. Detailed inspection and analysis of the results can also lead to the development of new physical pictures . In particular, FDTD calculations are used to show (i) how it is possible to increase the propagation lengths surface plasmon polaritons with the use of appropriate dielectric underlayers, (ii) how to efficiently bend light in a subwavelength region, and, (iii) how nanoholes and wells in metal films can exhibit complex transmission spectra of relevance to sensing. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U44.00002: General Properties of Local Plasmons in Metal Nanostructures Feng Wang, Ron Shen Local plasmon resonance in metal nanostructures offers the potential to concentrate electro-magnetic energies at nanoscale. Different designs of nanostructures have been proposed to achieve this goal. Here we investigate the general behavior of local plasmon resonances independent of specific structures. We study the local plasmon under quasi-static approximation given that nanostructure dimension is much smaller than optical wavelength.[1] We show that the plasmon resonance frequency depends on the fraction of plasmon energy residing in the metal through the real dielectric function of the metal. Further, at a given resonant frequency, the Q-factor of the resonance is determined only by the complex dielectric function of the metal material and does not depend on the nanostructure form or the dielectric environment. We will also discuss the effect of optical gain on the Q-factor of plasmon resonance. \newline [1] F. Wang and Y.R. Shen, Phys. Rev. Lett. \textbf{97}, 206806 (2006) [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U44.00003: Plasmonic properties of Nanorod Dimers Britain Willingham, Daniel Brandl, Peter Nordlander Using the plasmon hybridization and the FDTD methods, we investigate the plasmonic properties of nanorod dimers as a function of inter-particle separation and relative nanorod orientation. We show that the plasmonic structure of the dimer consists of bonding and anti-bonding combinations of individual nanorod plasmons localized on each particle. For short dimer separations, the plasmons consist of strongly hybridized individual nanorod plasmons of all multipolar orders. The bonding dipolar dimer plasmon displays a strong red shift with decreasing dimer separation and provides large electric field enhancements across the dimer junction. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U44.00004: Plasmon Hybridization in finite and periodic structures Daniel Brandl, Peter Nordlander We have extended the plasmon hybridization method[1] to periodic structures of metallic nanoparticles. The approach allows for simple and intuitive calculations of the plasmonic band structure of periodic chains or arrays of nanoparticles. The method allows for the inclusion of arbitrarily high multipolar interactions between the individual nanoparticles and interaction distances beyond nearest neighbor couplings. We also present an investigation of how the plasmonic structure of a finite chain approaches that of an infinite periodic structure. The plasmonic structure of a nanostructure array is shown to consist of bands made up of hybridized plasmons of the individual nanoparticles. [1] E. Prodan, C. Radloff, N.J. Halas, and P. Nordlander, Science 302(2003) 419; E. Prodan and P. Nordlander, J. Chem. Phys.120(2004) 5444 [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U44.00005: Laser Induced Forces Between Metallic Nanospheres; The Role of Collective Plasmon Resonances Ping Chu, Douglas Mills We explore the theory of laser induced attractive forces between conducting nanospheres. Emphasis is placed on the influence of collective mode resonances on this force. As two spheres approach each other, the dipole active plasmon resonances drop in frequency and can pass through the laser frequency. This produces a dramatic enhancement of the force. We present explicit calculation for Ag nanospheres in solution. We compare the amplitude of the laser induced attractive force with the van der Waals forece. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U44.00006: Plasmons in Metallic Nanoparticles: Effect of Nanoparticle Shape Garnett Bryant, Javier Aizpurua, Javier Garcia de Abajo Plasmonic oscillations of valence electrons determine the optical response of metallic nanoparticles. The energy and strength of these surface oscillations are a function of the size and shape of the nanoparticles. With the use of the boundary element method, we solve Maxwell's equations to calculate light scattering and surface modes in Au nanorods that are commonly used in field enhanced nanoantennas and scanned probe microscopies and spectroscopies. We calculate the near field and far field response of the nanorods to show how the shape of the nanorod determines its optical response. Although it is often assumed that the plasmon wavelength scales with the nanorod aspect ratio, we find that scaling with the aspect ratio does not apply. For small rod radii, the plasmon response blueshifts with increasing radii, as would be expected for scaling with the aspect ratio. However, the plasmon response still depends on both the rod length and radius and does not scale with the aspect ratio. For larger radii, the plasmon response redshifts with increasing radii, in contradiction to scaling with aspect ratio. We discuss the mechanisms that determine the shape dependence in these two regimes. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U44.00007: FDTD calculations of the optical properties of nanostars F. Hao, C. Nehl, J. Hafner, P. Nordlander Using the Finite-Difference Time-Domain (FDTD) method, we calculate the near- and far-field properties of a gold nanostar. The nanostar is modeled as a solid core with protruding tips of prolate spheroidal shape. The shape of this nanostar agrees qualitatively with the shape inferred from an SEM picture. The calculated extinction spectra agree very well with the experimentally observed scattering spectra for different polarization angles of incident light. We show that the plasmon resonances of the nanostar can be viewed as resulting from hybridization of short wavelength primitive plasmons associated with the core and long wavelengths plasmons associated with the individual tips. Due to the asymmetric orientation of the tips, several nanostars plasmons can be observed for an arbitrary polarization of the incident light. The intensity of these plasmons resonances vary with polarization angle. The plasmon hybridization results in bonding and antibonding nanostar plasmons. The bonding plasmons are primarily composed of primitive tip plasmons but with a small but finite admixture of the core plasmons. The admixture of the core plasmon dramatically increases the cross section for excitation of the bonding plasmons and result in enormous local electric field enhancements compared to those for individual tips. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U44.00008: Optical Properties of Semiconducting and Metallic Nanoparticle Structures by TDDFT Emily Townsend, Garnett Bryant Superstructures of semiconducting and metallic nanoparticles display substantially novel properties compared to homogeneous materials or single nanoparticles due to the coupling of elementary excitations between different nanoparticles, i.e. the confined plasmons in the metallic nanoparticles and excitons in semiconductor quantum dots. We use time-dependent density functional theory (TDDFT) to examine the optical response of such structures. This method allows a quantitative, fully quantum mechanical treatment of the electronic response of both the semiconducting and metallic components. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U44.00009: Optical Properties of 2D hexagonal arrays of gold nanoshells Fei Le, Hui Wang, Naomi Halas, Peter Nordlander Using periodic boundary conditions, we employ the Finite Difference Time Domain method to calculate the optical properties of a two dimensional close-packed array of gold nanoshells for different polarizations under normal incidence. The calculated extinction spectrum agrees very well with experimental data. We show that compared with an individual nanoshell or a nanoshell trimer, the nanoshell array shows a significantly red shifted dipolar resonance while the quadrupolar peak remains at almost the same wavelength for all structures. The local field enhancement of the nanoshell array is a factor of 10 higher than that of an individual nanoshell. The calculated Surface Enhanced Raman Spectroscopy (SERS) efficiency of the close-packed array is around three orders of magnitude higher than that for an individual nanoshell. The largest efficiencies occur for incident wavelengths around six microns in the infrared. The 2D hexagonal array of gold nanoshells is therefore highly suitable as a substrate for both SERS and Surface Enhanced Infrared Absorption (SEIRA) applications. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U44.00010: Coherent acoustic vibrations of metal nanoshells A.S. Kirakosyan, T.V. Shahbazyan, C. Guillon, P. Langot, N. Del Fatti, F. Vallee, T. Cardinal, M. Treguer We study vibrational modes of gold nanoshells grown on dielectric core by means of time-resolved pump-probe spectroscopy. The fundamental breathing mode launched by a femtosecond pump pulse manifests itself in a pronounced time-domain modulation of the differential transmission probed at the frequency of the nanoshell surface plasmon resonance. The modulation amplitude is significantly stronger while the period is longer than in a gold nanoparticle of the same overall size. A theoretical model describing breathing mode frequency and damping for a nanoshell in a medium is developed. A distinct acoustical signature of nanoshells provides a new and efficient method for identifying these versatile nanostructures and for studying their mechanical and structural properties. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U44.00011: Surface plasmon polaritons in co-metal nanostructures. Krzysztof Kempa Co-metal structures, such as a strip-line or coaxial cable, are well-known from radio engineering. They are capable of subwavelength guiding of TEM modes, and therefore their visible range analogs are of great interest. At these very high frequencies, however, the propagating modes acquire a plasmon polariton character. I study in detail these plasmon polaritons in co-metal structures, and show that for properly chosen materials and geometry, these modes reduce to the conventional, radio TEM modes. I show, how a metamedium made of an array of such co-metal nanostructures, can simulate negative refraction, suerlensisng and cloaking. [Preview Abstract] |
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