Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B1: Quantum Devices Based on Semiconductor Nanowires
Chair: Sankar Das Sarma, University of MarylandRoom: Ballroom A1
Session B1: Quantum Devices Based on Semiconductor Nanowires
Sponsoring Units: DCMPChair: Sankar Das Sarma, University of Maryland
Room: Ballroom A1
Monday, March 21, 2011 11:15AM - 11:51AM |
B1.00001: Cooper-pair splitter: towards an efficient source of spin-entangled EPR pairs Invited Speaker: In quantum mechanics the properties of two and more particles can be \textit{entangled}. In basic science pairs of entangled particles, so called Einstein-Podolsky-Rosen (EPR) pairs, play a special role as toy objects for fundamental studies. They provide such things as ``spooky interaction at distance,'' but they also enable secure encoding and teleportation and are thus important for applications in quantum information technology. Whereas EPR pairs of photons can be generated by parametric down conversion (PDC) in a crystal, a similar source for EPR pairs of electrons does not exists yet. In several theory papers, it has been suggested to use a superconductor for this purpose. The superconducting ground state is formed by a condensate of Cooper-pairs which are electron pairs in a spin-singlet state. Since there are many Cooper pairs in a metallic superconductor like Al, the main tasks are to extract Cooper pairs one by one and to split them into different arms. A controlled and efficient splitting is possible if one makes use of Coulomb interaction [1]. This has recently be demonstrated by two groups [2-4] using hybrid quantum-dot devices with both superconducting and normal metal contacts. In the present talk, I will discuss the Cooper-pair splitter results from the Basel-Budapest-Copenhagen team [3] and compare with the other experiments. As an outlook we discuss approaches that aim at entanglement detection. The Cooper pair splitter holds great promises because very large splitting efficiencies approaching 100{\%} and large pair current rates appear feasible. This work has been done by L. Hofstetter, S. Csonka, A. Geresdi, M. Aagesen, J. Nygard and C. Sch\"{o}nenberger \\[4pt] [1] P. Recher, E. V. Sukhorukov, and D. and Loss, Phys. Rev. B \textbf{63}, 165314 (2001). \\[0pt] [2] C. Strunk, \textit{Towards entangled electrons}, Nature Nanotechnology \textbf{5}, 11-12 (2009). \\[0pt] [3] L. Hofstetter, S. Csonka, J. Nygard, and C. Sch\"{o}nenberger, \textit{Cooper pair splitter realized in a two-quantum-dot Y-junction}, Nature \textbf{460}, 906 (2009). \\[0pt] [4] L.G. Herrmann, F. Portier, P. Roche, A. Levy Yeyati, T. Kontos, and C. Strunk, \textit{Carbon Nanotubes as Cooper Pair Beam Splitters}, Phys. Rev. Lett.\textbf{ 104}, 026801 (2010). [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B1.00002: Spin-orbit qubit in a semiconductor nanowire Invited Speaker: Spin-orbit interaction in InAs nanowires is so strong that spin and motion cannot be separated. The eigenstates of a single electron confined to a quantum dot become a spin-orbital doublet. We perform coherent manipulation of spin-orbit states of a single electron, thereby demonstrating a spin-orbit qubit. Fast and universal qubit control is achieved using gigahertz electric fields, which couple to the orbital part of the wavefunction. Qubits in adjacent quantum dots are addressed separately due to a gate-tunable difference in g-factors. Dephasing due to interaction with nuclear spins is studied in a Ramsey experiment. Coherence is extended using Hahn echo as well as Carr-Purcell-Meiboom-Gill dynamical decoupling pulse sequences. The next step is the demonstration of entanglement between neighbor qubits which can be achieved using exchange interaction. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B1.00003: Prospecting for elusive Majorana particles in nanowires Invited Speaker: We propose and analyze theoretically an experimental setup for detecting the elusive Majorana particle in quasi-one dimensional semiconductor-superconductor heterostructures. The experimental system consists of a quasi one-dimensional semiconductor nanowire with strong spin-orbit Rashba interaction proximity coupled with an s-wave superconductor. Under appropriate conditions, such system can realize a non-trivial topological state supporting Majorana zero energy modes localized at the ends of the wire. These emerging Majorana quasiparticles, i.e. particles that are at the same time their own antiparticles, are effectively fractionalized objects (anyons) obeying non-Abelian statistics. We discuss several experiments for detecting Majorana fermions in nanowires. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:39PM |
B1.00004: Spin States, Spin Correlations, Supercurrent, and Multiple Andreev Reflections in InSb Nanowire Quantum Devices Invited Speaker: Bulk InSb is one of the most promising materials for applications in spintronics and quantum information processing, due to the fact that it has the highest electron mobility $\mu _{e}$ = 77000 cm2/Vs, the smallest electron effective mass m$_{e}$* = 0.015 me, and the largest electron magnetic moment $\vert $g*$\vert $ = 51 among all III-V semiconductors. Here, we report on realization and electrical measurements of InSb quantum dots and superconductor/InSb/superconductor hybrid quantum devices. The devices are made on a SiO2-capped Si substrate from InSb segments of InAs/InSb heterostructured nanowires grown by metal-organic vapor phase epitaxy. Spin states, effective g-factors, and spin-orbit interaction energy are measured for the fabricated InSb nanowire quantum dots [1]. We have also studied strong correlation phenomena and observed a new spin-correlation-induced phenomenon in the devices, namely the conductance blockade at the degeneracy of two orbital states with the same spin [2]. We attribute this conductance blockade to the effect of electron interference between two equivalent, strongly correlated, many-body states in the quantum dots. In superconductor/InSb nanowire/superconductor hybrid devices, we have observed supercurrent and multiple Andreev reflections, and have found that the fluctuations in the supercurrent are correlated to the conductance fluctuations of the corresponding InSb nanowires in the normal state. We have also observed multiple Andreev reflections and interplay between the Kondo correlation and proximity effect in the Coulomb blockade regime. \\[4pt] [1] H. A. Nilsson et al., Nano Lett. 9, 3151-3156 (2009). \\[0pt] [2] H. A. Nilsson et al., Phys. Rev. Lett. 104, 186804 (2010). [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 2:15PM |
B1.00005: Measurement of Spin Relaxation in SiGe nanowire quantum dots Invited Speaker: This abstract not available. [Preview Abstract] |
Session B2: Many-Body Effects for the Excited States of Graphene
Sponsoring Units: DCMPChair: Allan MacDonald, University of Texas at Austin
Room: Ballroom A2
Monday, March 21, 2011 11:15AM - 11:51AM |
B2.00001: Optical properties of single- and few-layer graphene: the role of interlayer and many-body interactions Invited Speaker: Graphene, a single layer of carbon atoms, has attracted much attention in the past few years because of its unique 2D structure and linear dispersion relation near the K-point of the Brillouin zone. Optical spectroscopy provides a powerful tool for probing the electronic structure and interactions in graphene. In this talk we will discuss two types of interactions that affect the optical response -- those arising from interlayer coupling of electrons and those arising from many-body effects. The possibility of altering the low-energy band structure of graphene through the interlayer interactions in few-layer graphene (FLG) was recognized theoretically several years ago and was demonstrated experimentally recently by infrared absorption spectroscopy. Two distinct classes of IR absorption spectra for crystalline samples of the same number of layers, but different stacking order, were also observed. These findings demonstrate the pronounced effect of interlayer interaction and stacking order on the electronic structure of FLG. Furthermore, significant many-body effects are revealed in the optical conductivity spectra. These were manifested as excitonic modifications to optical absorption near the saddle- point singularities. The strong electron-hole interactions produce an asymmetric resonance, significantly red-shifted from the value predicted by ab-initio GW calculations for the band- to-band transitions. Our experiment also showed a weak dependence of the excitonic resonance in FLG on layer thickness. This result reflects the effective cancellation of the increasingly screened repulsive electron-electron and attractive electron-hole interactions. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B2.00002: Band structures of expitaxial graphene: the role of many-body interactions Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B2.00003: Ultrafast Carrier Dynamics in Graphene and Few Layer Graphite Invited Speaker: Graphene and its multilayer counterparts provide unique opportunities to study how the ultrafast carrier dynamics of layered systems evolve with layer number. We have carried out systematic investigations [1] of layered graphitic materials, from graphene to bulk graphite, exfoliated on to a Si/Silicon oxide substrate. The samples are excited using 150 fs, 800 nm pulses at room temperature and the time resolved reflectivity and transmission is probed using 150 fs, 1300 nm pulses. The response is governed by two times constants, one near 250 fs and the other near 3 ps, but both vary with the number of layers. The time constant are related to carrier cooling kinetics, interband transitions and hot phonon effects. The change in the first time constant with layer number is discussed in terms of alterations to the band structure with increasing number of layers over a few layers while the changes in the longer time constant over 10's of layers is related to substrate coupling effects. The results are compared with results from related experiments [2,3] using multilayer graphene, epitaxially grown on SiC, and also from results from experiments [4] using freestanding, thin graphite layers. *Work carried out with R.W. Newson and J.J. Dean. \\[4pt] [1] R.W. Newson, J. Dean, B. Schmidt and H.M. van Driel, Op. Ex. 17, 2326-33 (2009). \\[0pt] [2] D. Sun et al., Phys. Rev. Lett. 101, 157402 (2008). \\[0pt] [3] J.M. Dawlaty et al., Appl. Phys. Lett. 92, 043116 (2008). \\[0pt] [4] M. Breusing, C. Ropers and T. Elsaesser, Phys. Rev. Lett. 102, 086809 (2009). [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:39PM |
B2.00004: The Ordering and Electronic Structure of Multilayer Epitaxial Graphene on SiC Invited Speaker: The structural definition of graphene as a single sheet of hexagonal carbon limits how we view this material. It is the electronic properties of a single isolated graphene sheet that actually defines and motivates current graphene research. Remarkably, the best example of the idealized band structure of graphene comes does not come from a single graphene layer but from multilayer films grown on SiC. Multilayer epitaxial graphene (MEG) not only shows all the 2D properties expected for an isolated graphene sheet, but it the scalability to large scale integrated carbon circuits. I will show that the reason for this remarkable property, i.e. that a multilayer graphene films behaving like a single graphene sheet, is due to MEG's unique stacking. MEG films have a quasi-ordered rotational stacking that breaks the Bernal stacking symmetry associated with graphite. Angle resolved photoemission spectroscopy (ARPES) data demonstrates that the bands are linear at the K-point of these films. We can also show that the rotated stacking is highly ordered and that less than 20\% of the graphene sheets in the film are Bernal stacked. I will also show that ARPES measurements on MEG films demonstrate serious inadequacies with both tight binding and ab initio formalisms. In particular the data shows no reductions in the Fermi velocity or the formation of Van Hove singularity that have been consistently predicted for this material. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 2:15PM |
B2.00005: Raman spectroscopy of pristine, defected and strained graphene Invited Speaker: Raman spectroscopy is the most common and informative characterization technique in graphene science and technology. It is used to determine the number of layers, doping, strain, defects, functional groups, quality and type of edges [1-15]. I will discuss the historical development of the identification of the main Raman bands in graphene, focussing on the 2D'' peak around 2450cm$^{-1}$, and its deep-UV Raman spectrum. I will then discuss the effects of defects, uniaxial and biaxial strain on the Raman spectrum. Combining strain and Raman measurements one can derive the constitutive relation for graphene, and gain insights in the resonant Raman process. The results on graphene are the basis to explain and unify analogous measurements on graphite, carbon fibres and carbon nanotubes reported over the past 30 years. \\[4pt] [1] A. C. Ferrari et al. Phys. Rev. Lett. 97, 187401 (2006).\\[0pt] [2] C. Casiraghi et al. Nano. Lett. 7, 2711 (2007).\\[0pt] [3] C. Casiraghi et al. Appl. Phys. Lett. 91, 233108 (2007).\\[0pt] [4] S. Pisana et al. Nat. Mater. 6, 198 (2007).\\[0pt] [5] S. Piscanec et al. Phys. Rev. Lett. 93, 185503 (2004).\\[0pt] [6] C. Casiraghi, et al. Nano Lett. 9, 1433 (2009).\\[0pt] [7] A. C. Ferrari, Solid State Comm. 143, 47 (2007).\\[0pt] [8] A. Das et al. Nature Nano. 3, 210 (2008).\\[0pt] [9] A. Das et al. Phys. Rev. B 79, 155417 (2009).\\[0pt] [10] T. M. G. Mohiuddin et al. Phys. Rev. B 79, 205433 (2009).\\[0pt] [11] J. Yan et al. Phys. Rev. Lett. 98, 166802 (2007).\\[0pt] [12] D Graf et al. Nano Lett. 7, 238 (2007).\\[0pt] [13] A. C Ferrari et al. Phys. Rev. B 61, 14095 (2000); 64, 075414 (2001).\\[0pt] [14] D. M. Basko et al. Phys Rev B 80, 165413 (2009).\\[0pt] [15] F. Schedin et al. ACS Nano 4, 5617 (2010) [Preview Abstract] |
Session B3: The History of Superconductivity from its Discovery by Kammerlingh Onnes in 1911
Sponsoring Units: FHPChair: Martin Blume, Brookhaven National Laboratory, APS, Retired
Room: Ballroom A3
Monday, March 21, 2011 11:15AM - 11:51AM |
B3.00001: Heike Kamerlingh Onnes and the Road to Superconductivity Invited Speaker: The discovery of superconductivity on 8 April 1911 came as a big surprise. It was stumbled upon in the Leiden cryogenic laboratory of Heike Kamerlingh Onnes in a moment of serendipity. Three years before, the liquefaction of helium on the other hand had been the culmination of a long battle with nature. It was a meticulously prepared operation, ``big science'' in its first appearance. Until recently, careless notebook entries by Kamerlingh Onnes and his terrible handwriting had hindered a complete view to the road to superconductivity. Even a date of the fascinating discovery was lacking. How did the discovery fit into the Leiden research program? What about the research effort Kamerlingh Onnes had to put in to be sure he had found superconductivity rather than a short-circuit? What about superfluidity? Once the right interpretation of the notebooks is clear, the real story can be told. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B3.00002: From the Meissner Effect to the Isotope Effect: Precursors to the Microscopic Theory of Superconductivity Invited Speaker: After the discovery by Kamerlingh Onnes in 1911 of the low temperature disappearance of resistance in mercury to a state of perfect conductivity, there was a long period of more than two decades before there was a major experimental advance. In 1933, Meissner and Ochsenfeld discovered that a superconductor is not only a perfect conductor but in addition it is a perfect diamagnet. In 1935 F. and H. London presented a phenomenological understanding of the electromagnetic properties of the superconducting state, which included the London penetration depth for applied magnetic fields and later introduced the concept of a ``stiffness'' of the superconducting wave function. In 1950, Ginzburg and Landau developed a phenomenological theory for the superconducting state using general thermodynamic arguments.In the same year, Maxwell, and Serin et.al discovered the Isotope Effect which indicated that the electron-phonon~interaction would play an important role in the theory of superconductivity. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B3.00003: BCS: 50 Years Invited Speaker: The road to and from BCS: This talk was presented before an audience at Brown University on December 10th, 2010. The recording of the talk will be shown at the session in Dallas, as Professor Cooper will not be able to be present. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:39PM |
B3.00004: Giaever, Nb3Sn, and Josephson Invited Speaker: The late 1950s and 1960s were times of remarkable progress in both the understanding and utilization of superconductivity. The majority of today's applications can be traced to key scientific advances made during a period of less than a decade, and the majority of those advances were made in the industrial research laboratories of the United States. As examples, in this talk I will mention the measurement of the specific heat and the critical current of Nb3Sn in high magnetic fields, the discovery by Giaever of tunneling between metal films and his direct observation of the superconducting energy gap, the understanding of strong coupling superconductors, leading to the development of tunneling spectroscopy of the electron-phonon pairing interaction, and the prediction by Josephson of pair tunneling. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 2:15PM |
B3.00005: The arrival of high temperature superconductors Invited Speaker: The attainment of high temperature superconductivity has been considered a major advancement of modern science. It was the seminal discovery of the first cuprate high temperature superconductor, the Ba-doped La$_{2}$CuO$_{4}$, with a $T_{c}$ of 35 K in 1986 by Alex M\"{u}ller and George Bednorz of IBM Zurich Lab,\footnote{J. G. Bednorz and K. A. M\"{u}ller, Z. Phys. B 64, 189 (1986).} who were awarded the Nobel Prize in 1987, that ushered in the era of cuprate high temperature superconductivity. It was the first liquid nitrogen high temperature superconductor, YBa$_{2}$Cu$_{3}$O$_{7}$ with a $T_{c}$ of 93 K discovered in 1987 by Paul C. W. Chu, Maw-Kuen Wu and colleagues in the respective groups at the University of Houston and the University of Alabama at Huntsville\footnote{M. K. Wu et al., Phys. Rev. Lett. 58, 908 (1987).} that heralded the new era of high temperature superconductivity, drastically changing the psyche of superconductivity research and bringing superconductivity applications a giant step closer to reality. In the ensuing years, many high temperature superconductors have been found, leading to the current record $T_{c}$ of 134 K which was observed by A. Schilling et al.\footnote{A. Schilling et al., Nature 363, 56 (1993).} of ETH in 1993 in HgBa$_{2}$Ca$_{2}$Cu$_{3}$O$_{9-\delta}$ at ambient and later raised to 164 K under 30 GPa by L. Gao et al.\footnote{L. Gao et al., Phys. Rev. B 50, 4260(R) (1994).} In the present talk, I shall briefly recall a few events leading to and during the arrival of high temperature superconductivity. The prospects for future superconductors with higher $T_{c}$ will also be discussed. [Preview Abstract] |
Session B4: Hybrid Nanomaterials Assembly
Sponsoring Units: DPOLYChair: Richard Vaia, Air Force Research Laboratory
Room: Ballroom A4
Monday, March 21, 2011 11:15AM - 11:51AM |
B4.00001: ``Hairy'' Nanoparticles in Block Copolymers and Homopolymers: Modeling using Hybrid Self-Consistent Field Theory Invited Speaker: Today, dispersed nanoparticles play important role in various applications (toughened plastics, healthcare, personal care, etc.) Mesoscale simulations and theory are important in understanding what governs the morphology of nanoparticles under various conditions. In particular, for nanoparticle/block copolymer mixtures, two popular simulation methods are Self-Consistent Field/Density Functional Theory (SCF-DFT) (Thompson, Ginzburg, Matsen, and Balazs, Science 292, 2469 [2001]), and Hybrid Self-Consistent Field Theory (HSCFT) (Sides et al., Phys Rev Lett 96, 250601 [2006]). The two methods are shown to be very similar in their assumptions and end-results; the choice of the method to be used can depend on the specific problem. Here, we use modified HSCFT to explicitly account for the complicated role of short-chain ligands grafted onto nanoparticles to promote dispersion. In particular, we discuss the phase diagrams of such ``hairy'' nanoparticles in diblock copolymers as function of diblock composition, nanoparticle volume fraction, and ligand length. Depending on the particle size and ligand coverage, particles could segregate into favorable domain, stay close to the interface, or phase-separate from the block copolymer altogether. We also consider the dispersion of ``hairy'' nanoparticles in a homopolymer and analyze the morphologies of particle clusters as function of ligand length. The results could have interesting implications for the design of new nanocomposite materials. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B4.00002: Direct hierarchical assemblies of nanoparticles in thin films Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B4.00003: Aqueous foams stabilised solely by nanoparticles Invited Speaker: Particles are being increasingly used to stabilise foams and emulsions, the corresponding emulsions being known as ``Pickering'' emulsions. One of the peculiarities of these systems is the absence of Ostwald ripening: since the bubbles or drops do not grow (coalescence seems also suppressed) both foams and emulsions are stable over extremely long periods of time (months). These features make particles very interesting surface active agents as compared to standard surfactants or polymers/proteins. The origin of the suppression of ripening can be traced to the unusual behaviour of the interfacial layers made by these particles. The layers are solid-like and the usual characterisation methods (surface tension, surface rheology) are not straightforward to use. In this presentation, we will illustrate these difficulties with experiments made with partially hydrophobic silica nanoparticles. We will also discuss the relevance of foam characterisations methods such as multiple light scattering and X-ray tomography. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:39PM |
B4.00004: Control of Nanoparticle Organization in Thin Homopolymer Films Invited Speaker: The morphological structure of mixtures of homopolymers with chain-grafted nanoparticles is determined by competing interactions between the nanoparticle cores, the free host chains and the grafted chains. In the bulk, when the nanoparticle grafting density is low, the phase behavior is largely determined by a competition between attractive nanoparticle core-nanoparticle core interactions, mediated by the chains grafted to the surface. At high grafting densities, the entropic brush layer/free host chain interactions are dominant, leading to miscibility or to microscopic/macroscopic phase separation. Thin film mixtures are thermodynamically less stable than their bulk analogs due to the preferential attraction of grafted nanoparticles to the external interfaces. The preferential attraction of the nanoparticles to the interfaces is driven by factors that include: entropic gains of the grafted nanoparticles and linear host chains; van der Waals interactions between the nanoparticles and the interfaces. If the grafted chains and host chains are of dissimilar chemical structure, then the nanoparticles exhibit a tendency to segregate to the free surface, provided its grafts possess a lower surface energy than the host chains. Consequences of these interactions on the overall nanoparticle organization in thin homopolymer films will be discussed. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 2:15PM |
B4.00005: Nanoparticle Self-Assembly in a Polymer Matrix and Its Impact on Phase Separation Invited Speaker: The ubiquitous clustering of nanoparticles (NPs) in solutions and polymer melts depends sensitively on the strength and directionality of the effective NP-NP interactions, as well as on the molecular geometry and interactions of the dispersing fluid. Surface functionalization apparently can also lead to emergent anisotropic interactions that can influence NP dispersion. Since NP clustering can strongly influence the properties of polymer nanocomposites and NP solutions, we investigate the reversible self-assembly of model NPs into clusters under equilibrium conditions through a combination of simulation and analytic methods. First, we performed molecular dynamics simulations of polyhedral NPs in a coarse-grained dense bead--spring polymer melt and find a transition from a dispersed to clustered NP state, consistent with the thermodynamic models of equilibrium particle association such as equilibrium polymerization. We also describe the competition between self-assembly and phase separation in an analytic lattice model of a mixture of polymers and NPs. We then focus on the particularly interesting situation where the associating ``monomeric'' NP species form high molecular mass dynamic polymeric clusters and where the assembly process then transforms the phase boundary from a form typical of a polymer solution to one that more resembles a polymer blend with increasing association near the critical point for phase separation. The model calculations elucidate basic physical principles governing the coupling of self-assembly and phase behavior in these complex mixtures. [Preview Abstract] |
Session B5: Mentoring Undergraduate Research
Sponsoring Units: DCMP FEdChair: Theodore Holdap, American Physical Society
Room: Ballroom C1
Monday, March 21, 2011 11:15AM - 11:51AM |
B5.00001: Using seminar-based instruction to convey contemporary research to undergraduates Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B5.00002: Flocking and self-defense: experiments and simulations of avian mobbing Invited Speaker: We have performed motion capture studies in the field of avian mobbing, in which flocks of prey birds harass predatory birds. Our empirical studies cover both field observations of mobbing occurring in mid-air, where both predator and prey are in flight, and an experimental system using actual prey birds and simulated predator ``perch and wait'' strategies. To model our results and establish the effectiveness of mobbing flight paths at minimizing risk of capture while optimizing predator harassment, we have performed computer simulations using the actual measured trajectories of mobbing prey birds combined with model predator trajectories. To accurately simulate predator motion, we also measured raptor acceleration and flight dynamics, well as prey-pursuit strategies. These experiments and theoretical studies were all performed with undergraduate research assistants in a liberal arts college setting. This work illustrates how biological physics provides undergraduate research projects well-suited to the abilities of physics majors with interdisciplinary science interests and diverse backgrounds. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B5.00003: Involving undergraduates in interdisciplinary research: The physics of biomineralization Invited Speaker: Biominerals include mollusk shells, the skeletons of sea urchins, corals, mammals, etc. Their formation mechanisms fascinate physicists, materials scientists, and chemists because they result in materials more robust than their components, with exquisitely intricate nano-structures, fill space more than synthetic nanoparticles, and directly control phase transitions. Because of the fundamental nature of research on the physical aspects of biominerals, their formation mechanisms, the potential for future bio-inspired materials synthesis, and the aesthetic beauty of biomineral structures, students of all ages are interested in biomineralization. While describing the involvement of undergraduates in this research, my talk will address two key questions: Q: How do biominerals achieve the beautiful morphologies we observe? A: By forming through amorphous precursor phases, with morphology and phase transitions directly under biological control [1, 2]. Q: How do organisms order their biominerals to be single-crystalline? A: By controlling crystal growth at the nanoscale, not atom by atom [3, 4]. \\[4pt] [1] Y Politi et al PNAS 105, 17362 (2008). \\[0pt] [2] AV Radha et al PNAS 107, 16438 (2010). \\[0pt] [3] RA Metzler et al PRL 98, 268102 (2007). \\[0pt] [4] PUPA Gilbert et al JACS 130, 17519 (2008). [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:39PM |
B5.00004: Processing and Characterization of New Materials at Pomona College with External Collaborations Invited Speaker: My research program focuses on the evolution of novel lithographic, growth, and characterization processes for use with thin films for microelectronics and photovoltaic technologies. We have established facilities at Pomona College for wet chemistry, spin coating, thermal evaporation,~micro-contact printing, ultra violet ozone cleaning, oxygen plasma cleaning, Au/Pd sputter coating, critical point drying, optical microscopy, optical lithography, ellipsometry, spectral reflectance, electrical conductivity, current-voltage characterization, atomic force microscopy, scanning tunneling microscopy, electron microscopy, electron beam lithography, and energy dispersive x-ray spectroscopy. Active collaborations with researchers at Cornell University and at Ris{\o} National Laboratory for Sustainable Energy (in Denmark) keep the research program vibrant and relevant. Since 2001, I have been an active member of the Cornell Center for Nanoscale Systems. Recent research and publications have focused on carbon nanotubes, graphene sheets, and organic photovoltaics. Pomona College students have played significant roles in all these projects, as well as in the development of our facilities. Connections to a wide range of researchers are invaluable not only for scientific discussions, but provide many opportunities for summer REU internships for my research students. This provides valuable training, access to facilities, and seeds future collaborations. Collaborations at Cornell span 15 years including two sabbatical years and regular summer visits to work at sites such as the Cornell Nanofabrication Facility, the Cornell Center for Materials Research, and the Cornell Center for Nanoscale Systems. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 2:15PM |
B5.00005: Mentoring undergraduates for experimental research in physic Invited Speaker: This abstract not available. [Preview Abstract] |
Session B6: Few-body Aspects of Cold Atomic Gases
Sponsoring Units: DAMOPChair: Robin Cote, University of Connecticut
Room: Ballroom C2
Monday, March 21, 2011 11:15AM - 11:51AM |
B6.00001: Interacting and Rotating Gases of a Few Trapped Atoms Invited Speaker: I will discuss attempts to generate motionally entangled states in small clusters of repulsively interacting Bosonic atoms at nonzero angular momentum in two-dimensional harmonic traps.\footnote{Gemelke, N., Sarajlic, E., Chu, S., arXiv:1007.2677} By constructing an ``array of rotating buckets'' from an optical lattice of spinning and precisely controlled on-site potentials, small clusters of interacting atoms can be adiabatically transferred from uncorrelated states at zero angular momentum through a tabulated sequence of ground state level crossings with increasing atomic correlation and total angular momentum. Results will be shown probing these states with both time-of- flight techniques and by directly interrogating atomic correlation via photo-association to excited molecules. Comparison will be made to numeric models with no free parameters. I will discuss extension of these results to future experiments using Feshbach-resonant interactions, and the use of dynamically modulated lattice potentials to generate effective gauge fields. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B6.00002: Deterministic preparation and control of a tunable few-fermion system Invited Speaker: Systems consisting of only few interacting fermions play a fundamental role in nature with atoms and atomic nuclei being the most prominent examples. In our experiments with ultracold atoms we have recently been able to prepare and control few-atom quantum states consisting of 1-10 fermions. We prepare such a system using ultracold $^6$Lithium atoms in an optical dipole trap in which the interparticle interaction can be tuned over a wide range using a Feshbach resonance. By spilling all atoms occupying higher energy quantum states we can deterministically prepare samples from 1-10 particles in the ground state with fidelities exceeding 90\%. In my talk I will present our first experiments controlling the interaction between particles in the ground state of the trap. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B6.00003: Four-body Efimov effect Invited Speaker: The few-body problem with resonant two-body $s$-wave interaction (that is with an infinite scattering length) can now be studied experimentally with ultracold atomic gases. In particular, the three-body Efimov phenomenon (Efimov, 1971), consisting in the existence of an infinite number of trimer states with an asymptotically geometric spectrum in the vicinity of a zero energy accumulation point, has now obtained experimental evidence. On the contrary, the four-body Efimov effect has remained elusive, both theoretically and experimentally. Strictly speaking, for same spin state bosons, as pointed out by Amado and Greenwood (1973), it is {\sl a priori} excluded by the existence of the three-body Efimov effect: A tetramer state with an energy arbitrarily close to zero has eventually an energy larger than an Efimov trimer state and may decay into this trimer plus a free atom. We have found a system where a four-body Efimov effect takes place: It is made of three same spin state fermions of mass $M$ interacting only with a lighter particle of mass $m$. The mass ratio $\alpha=M/m$ is used as a control knob: This system experiences a three-body Efimov effect if and only if $\alpha>\alpha_c(2;1)\simeq 13.607$ (Efimov, 1973; Petrov, 2003). Using a combination of symmetry arguments and a numerical solution of an integral equation,we show that Efimov tetramers exist over the interval of mass ratio $\alpha_c(3;1) < \alpha < \alpha_c(2;1)$, with $\alpha_c(3;1)\simeq 13.384$. The four-body Efimov exponent $|s|$ is also calculated as a function of $\alpha$ over that interval, and the experimental feasibility is discussed. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:39PM |
B6.00004: Interferometry with ultra-cold few-atom states Invited Speaker: I will explain some of our recent modeling of experiments that loaded an atomic Bose-Einstein condensate into a three-dimensional optical lattice. In an optical lattice, a periodic trap for atoms, the condensate can be divided into millions of independent atomic coherent states. These states are superpositions of different atom number and the analogue of coherent states of light or photons. As in the case of coherent laser light these atomic states can be made to interfere. In fact, the time-evolution of the states leads to collapse and revivals in interference patterns observed in the atomic momentum distribution. I show that long-period revivals are associated to effective three-body interactions that are due to virtual excitations to higher vibrational states within a site of an optical lattice. This work has been published as P. R. Johnson, E. Tiesinga, J. V. Porto, and C. J. Williams, New Journal of Physics {\bf 11}, 093022 (2009). [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 2:15PM |
B6.00005: Virial Expansion for a Strongly Correlated Fermi Gas Invited Speaker: Few-body physics can give considerable insight into the challenging many-body problem. A concrete example is the exact Tan relations [1] linking the ``hard'' (few-body) physics at short distance, large-momentum and high frequency to the ``soft'' physics of the equation of state via a contact parameter. This has been demonstrated clearly using the operator product expansion (OPE) method [2] which separates in a natural way few-body from many-body physics. In this talk, we present another example: the quantum virial expansion that bridges few-body and many-body physics. At large temperatures, the properties of a strongly correlated Fermi gas, either static or dynamic, can be expanded in terms of virial coefficients or expansion functions, calculable from the few-fermion solutions [3]. For the equation of state in the resonant unitarity limit [3], we obtain for the first time an accurate third order virial coefficient. This has been experimentally verified in a measurement at ENS (Paris) [4]. For the single-particle spectral function [5], we demonstrate that an expansion up to second order is able to explain the main features of momentum-resolved RF spectroscopy in a resonantly interacting Fermi gas, as recently reported by JILA [6]. We also obtain a virial expansion of the dynamic structure function, as measured at Swinburne University (Melbourne), and check that the second order expansion functions give the correct OPE coefficients in the limit of large momentum and frequency. The important feature of this expansion is the existence of a small parameter, the fugacity, even for strong interactions. In the future, we anticipate that higher-order virial expansions of dynamic properties such as the single-particle spectral function may provide useful insights into clarifying the debate on the pseudo-gap issue in resonantly interacting Fermi gases. \\[4pt] [1] S. Tan, \textit{Ann. Phys}. \textbf{323}, 2952 (2008); \textbf{323}, 2971 (2008).\\[0pt] [2] E. Braaten, and L. Platter, \textit{Phys. Rev. Lett}. \textbf{100}, 205301 (2008).\\[0pt] [3] X.-J. Liu, H. Hu, and P. D. Drummond, \textit{Phys. Rev. Lett}. \textbf{102}, 160401 (2009).\\[0pt] [4] S. Nascimb\`{e}ne et al. \textit{Nature} \textbf{463}, 1067 (2010).\\[0pt] [5] H. Hu, X.-J. Liu, and P. D. Drummond, \textit{Phys. Rev. Lett}. \textbf{104}, 240407 (2010).\\[0pt] [6] J. T. Stewart, J. P. Gaebler, and D. S. Jin, \textit{Nature} \textbf{454}, 744 (2008). [Preview Abstract] |
Session B7: Superconductivity in Accelerators
Sponsoring Units: DPBChair: Soren Prestemon, Lawrence Berkeley National Laboratory
Room: Ballroom C3
Monday, March 21, 2011 11:15AM - 11:51AM |
B7.00001: Superconducting Accelerator Structures: An Historical Overview Invited Speaker: In 1961 I began doing active research on RF superconducting cavities at the High Energy Physics Laboratory (HEPL) at Stanford University. At that time there were already nascent research programs exploring superconducting cavities at four other laboratories around the world, including the one at the Stanford physics department. However, all attempts to produce a substantial accelerating field in a superconducting cavity had failed. Since a cavity that is capable of acceleration always has a surface electric field, I decided that my first research effort would be to build and test a cavity with only a magnetic field at the surface. The frequency would need to be 2856 MHz, that of the electron linac at HEPL, so that available instrumentation could be used. In order to have only a magnetic field at the surface, the cavity would have to operate in the so-called TE mode. But there was a problem: at 2856 MHz such a cavity would be considerably larger than the single-cell accelerating mode cavities previously built at the Stanford physics department. In collaboration with the low temperature physics group in the Stanford physics department, a larger electroplating facility was built that was capable of handling the cylindrical cavity body and two end plates. The initial measurements gave stunning results: a Q factor of about 10$^{8}$ at 4\r{ }K for a lead-plated cavity was obtained, and there was no degradation in Q up to a surface magnetic field of about 10 mT, (limited by the oscillator power). The results were published in 1963. Experimentation on superconducting accelerator cavities increased rapidly in the decade or so following this initial success. Successful niobium TM-mode (accelerating mode) cavities were built with Q's of about 10$^{11}$. Within a few years the multipactor problem in accelerating cavities was solved by changing the shape of the outer boundary. The initial impetus for superconducting accelerator research at Stanford was to design and build a long pulse superconducting linac with an energy of about one GeV. Such a linac has still not been realized, but in the years from 1970 to 1990 there have been successful applications of RF superconducting structures to storage rings, rf separators, drive linacs for FEL's, and heavy ion accelerators. The evolution superconducting structures and their applications, as outlined above, will be discussed in more detail in my talk [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B7.00002: Superconductors for superconducting magnets Invited Speaker: Even in 1913 Kamerlingh Onnes envisioned the use of superconductors to create powerful magnetic fields well beyond the capability provided by cooling normal metals with liquid helium. Only some ``bad places'' in his Hg and Pb wires seemed to impede his first attempts at this dream, one that he imagined would be resolved in a few weeks of effort. In fact, of course, resolution required another 50 years and development of both a true understanding of the difference between type I and type II superconductors and the discovery of compounds such as Nb$_{3}$Sn that could remain superconducting to fields as high as 30 T. And then indeed, starting in the 1960s, Onnes's dreams were comfortably surpassed. In the last 45 years virtually all superconducting magnets have been made from just two Nb-base materials, Nb-Ti and Nb$_{3}$Sn. Now it seems that a new generation of magnets based on cuprate high temperature superconductors with fields well above 30 T are possible using Bi-Sr-Ca-Cu-O and the RE-Ba-Cu-O compounds. We hope that a first demonstration of this possibility will be an all-superconducting 32 T magnet with RE-Ba-Cu-O insert that we are building for NHMFL users. The magnet application potential of this new generation of superconducting conductors will be discussed. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B7.00003: State of the art superconducting magnet development Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:39PM |
B7.00004: RF Superconductivity: the ultimate limit Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 2:15PM |
B7.00005: Cryogenic Systems: Recent Trends and New Directions Invited Speaker: The production of reliable cryogenic temperatures is vital for the use of superconductivity in accelerators. Cryogenics is found in the accelerating structures and magnets of the accelerator as well as in the magnets and calorimeters of the detectors in the experimental areas. In the century since the discovery of superconductivity, cryogenic systems have gone from small laboratory devices to very large industrial scale systems involving multiple refrigeration plants, containing over 100 tonnes of liquid helium. These systems, while specialized, represent a mature, well understood technology. This paper will survey the current status of cryogenic systems in accelerators and describe recent trends including: the large scale use of He II (superfluid helium) and the development of higher reliability and higher efficiency systems. It will also discuss future directions including the increased use of HiTc current leads, possible applications for small cryocoolers and the potential impact of the world helium supply on accelerator cryogenics. [Preview Abstract] |
Session B8: Critical Materials for Global Science and Technology
Sponsoring Units: FIPChair: William Barletta, U.S. Particle Accelerator School, MIT and UCLA
Room: Ballroom C4
Monday, March 21, 2011 11:15AM - 11:51AM |
B8.00001: Unobtainium? Critical Elements for New Energy Technologies Invited Speaker: I will report on a recently completed study jointly sponsored by the APS Panel on Public Affairs (POPA) and the Material Research Society (MRS). The twin pressures of increasing demand for energy and increasing concern about anthropogenic climate change have stimulated research into new sources of energy and novel ways to harvest, transmit, store, transform or conserve it. At the same time, advances in physics, chemistry, and material science have enabled researchers to identify chemical elements with properties that can be finely tuned to their specific needs and to employ them in new energy-related technologies. Elements like dysprosium, gallium, germanium, indium, lanthanum, neodymium, rhenium, or tellurium, which were once laboratory curiosities, now figure centrally when novel energy systems are discussed. Many of these elements are not at present mined, refined, or traded in large quantities. However new technologies can only impact our energy needs if they can be scaled from laboratory, to demonstration, to massive implementation. As a result, some previously unfamiliar elements will be needed in great quantities. We refer to these elements as energy-critical elements (ECEs). Although the technologies in which they are employed and their abundance in the Earth's crust vary greatly, ECEs have many features in common. The purpose of the POPA/MRS study was to evaluate constraints on availability of energy-critical elements and to make recommendations that can help avoid these obstructions. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B8.00002: Using Earth Abundant Minerals for New Research Pathways in Solar PV and Battery Storage Invited Speaker: Dr. Wadia will provide an in-depth look at his research to discover and develop a new material system toward a more expansive solar photovoltaic future, covering topics of: materials selection based on abundance, new synthetic pathways for scaleable materials, and results of working solar cell devices he has fabricated with these principles in mind. He will also discuss the results of his latest paper exploring the resource constraints on electrochemical storage for both transportation and grid scale applications. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B8.00003: Essentials for Successful and Widespread LED Lighting Adoption Invited Speaker: Solid-state lighting (SSL), with light-emitting diodes (LEDs) as the light source, is a growing and essential field, particularly in regard to the heightened need for global energy efficiency. In recent years, SSL has experienced remarkable advances in efficiency, light output magnitude and quality. Thus such diverse applications as signage, message centers, displays, and special lighting are now adopting LEDs, taking 2010's market to {\$}9.1 billion - 68{\%} growth from the previous year! While this is promising, future growth in both display and lighting applications will rely upon unveiling deeper understanding and key innovations in LED lighting science and technologies. In this presentation, some LED lighting fundamentals, engineering challenges and novel solutions will be discussed to address reduction in efficiency (a.k.a. droop) at high currents, and to obtain uniform light distribution for overcoming LEDs' directional nature. The droop phenomenon has been a subject of much controversy in the industry and despite several studies and claims, a widely-accepted explanation still lacks because of counter arguments and experiments. Recently several research studies have identified that the droop behavior in nitride-based LEDs beyond certain current density ranges can only be comprehensively explained if the current leaking beyond the LED active region is included. Although such studies have identified a few useful current leakage mechanisms outside the active region, no one has included current leakage, due to non-ideal, 3-D device structures that create undesirable current distribution inside and outside the active region. This talk will address achieving desirable current distributions from optimized 3-D device structures that should reduce current leakage and hence the droop behavior. In addition to novel LED design solutions for droop reduction and uniform light distribution, the talk will address cost and yield concerns as they pertain to core material scarcity. Such solutions are expected to make LED lights more energy efficient, pleasant in appearance, longer-lasting, affordable, and thus suitable for green living. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:39PM |
B8.00004: Niobium and its Impact on Superconducting Radiofrequency Technology Invited Speaker: The Continuous Electron Beam Accelerator Facility at Jefferson Lab in Newport News, Virginia, was the first large-scale deployment of superconducting radiofrequency (RF) technology, and was optimized for nuclear/high energy physics research. The success of this technology led to the creation of a Free Electron Laser (FEL) facility at Jefferson Lab that has achieved world-record power, and is leading the way towards the next generation of FELs based on superconducting energy-recovery linacs. Superconducting RF technology has been adopted for other large accelerators (LEP, SNS) and is proposed for many of the future large accelerator facilities (ESS, FRIB, ILC, Project X, etc.). All these applications rely on the superconducting properties of niobium. While the performance of niobium accelerating cavities has been excellent, there are still improvements being developed, in particular, the new ingot niobium technology. This talk will provide a review of superconducting RF technology and the impact it has had, and will continue to have, on the accelerator field. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 2:15PM |
B8.00005: Isotopes for Research - Can We Continue to Depend on Them? Invited Speaker: The use of isotopes as tracers has been critical to many scientific endeavors. The discovery of radioactivity was quickly followed by experiments in which radioisotopes were being used to trace the behavior of those elements in biologic systems including humans. More recently, the ability to separate and enrich stable isotopes has facilitated their use as probes in a variety of applications using magnetic resonance, mass spectrometry and post facto neutron activation to determine their distribution, concentration and chemical form. Recently, shortages of both stable enriched and radioisotopes have had an impact on endeavors on which they play an important role. [Preview Abstract] |
Session B9: Nanofluidics
Sponsoring Units: DFDChair: Daniel Ou-Yang, Lehigh University
Room: D220
Monday, March 21, 2011 11:15AM - 11:27AM |
B9.00001: Message in a bottle: the statistical behavior of nanoparticles in optical confinement H. Daniel Ou-Yang, Joseph Junio, Liangcheng Zhou In an aqueous medium, container surfaces can significantly alter the behavior of suspended nanoparticles. We propose a method to investigate nanoparticle behavior in a boundary-free environment by transiently trapping them with a focused laser beam. While optical confined, as in an optical bottle, these particles are affected by both particle-light and particle-particle interactions. Time-averaged fluorescence imaging produces results in 3D mapping of the nanoparticle concentration in the bottle. We report how we analyze the messages in the bottle, i.e. the statistical behavior of these particles, by using the 3D distributions obtained under both controlled optical and interparticle forces. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B9.00002: Low-frequency dielectric response of a single particle in aqueous suspensions Jingyu Wang, H. Daniel Ou-Yang $\alpha -$relaxation, the counterion diffusion in the electric double layer, has been used to described the anomalous low frequency dielectric dispersion of aqueous suspensions of colloidal particles. A microscopic theory describing this relaxation process proposed by Schwarz, however, has not been investigated systematically. We propose to use a single particle dielectrophoresis (DEP) force spectroscopy to study the relaxation mechanism as a function of particles size, temperature and solvent viscosity. Specifically, we measure the dependence of the DEP crossover frequency force and compare results with predictions by Schwarz. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B9.00003: Investigation of MEMS force sensors for nano-scale water measurements Soyoung Kwon, Wonho Jhe, Corey Stambaugh Nanoscale water formed by capillary condensation has typically been studied by means of an atomic force microscope (AFM). While this approach can provide details about the dynamic visco-elastic properties, it is limited in the type of information that can be measured. Here we propose replacing the fixed sample surface generally used in AFM systems with movable micro-mechanical force sensors (MEMS) fabricated specifically for tapping mode or shear mode. By incorporating a MEMS device we can directly measure the adhesion force, pull-in distance and capillary force of nano confined water while the AFM collects information pertaining to the dynamic visco-elastic properties. In this talk, we will characterize the force measurement in the system and discuss the behavior of the device in the presence of nano-scale water. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B9.00004: Investigation of the Static and Dynamic Mechanical Properties of Nano-scale Water Corey Stambaugh, Soyoung Kwon, Wonho Jhe The behavior of liquids on the nano-scale has become an area of interest as new fabrication techniques have allowed for increasingly smaller structures to be made. While much work has been done on the interactions forces at liquid and solid interfaces, questions still remain regarding the behavior of nano-scale liquids. By incorporating a micro-electromechanical force sensor (MEMS) into the quartz tuning fork based atomic force microscope (QTF-AFM) probe setup we are able to both manipulate and measure nano-scale water, which in turn provides information beyond the standard AFM approach. Here we look at both the static and dynamic mechanical properties of water formed between the tip of a (QTF-AFM) probe and the polysilicon surface of a MEMS device. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B9.00005: Thermophoretic stretching of DNA in polymer nanochannels Jonas Pedersen, Lasse Thamdrup, Henrik Flyvbjerg, Anders Kristensen We demonstrate that thermophoretic forces generated by light-induced local heating can enhance the extension of genomic-length DNA confined in a polymer nanochannel. By temperature control on the micron-scale, bacteriophage T4 DNA is locally stretched to 80{\%} of its contour length, although the cross-section of the nanochannel is as large as 250x250nm$^2$. A coarse-grained model of the forces at play captures the DNA-molecule's response to thermophoretic forces with accuracy and precision, and allows for fitting the density profile of the stretched DNA with only a single fit-parameter. The forces involved are relatively strong, because they add up along the molecule. They are measured by using the molecule as an entropic spring balance. Pending a calculation of these forces, this experiment might discriminate between the competing theories for thermophoretic forces. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B9.00006: Poisson or not Poisson: Probability distribution of colloidal nanoparticles in an optical trap Yi Hu, Xuanhong Cheng, H. Daniel Ou-Yang In a colloidal suspension of nanoparticles, the presence of an optical trap can exponentially enhance the probability of finding the particles in the vicinity of the trap. Intriguing questions arise regarding whether the probably distribution of particle number in the trap follows Poisson approximation, and if so, what is the upper limit of the trapping energy at which Poisson is followed. To answer these questions, we conduct experiments to determine directly the variance and the mean particle number in the trap at different trapping energies and compare with the predictions of the probability theory. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B9.00007: Charging Dynamics of Sub-nanometer Pores Ying Liu, Guang Feng, Jingsong Huang, Bobby Sumpter, Vincent Meunier, Rui Qiao Electrodes featuring sub-nanometer pores can potentially improve the energy density of supercapacitors significantly. However, ions entering such narrow pores often need to pay an energy penalty because part of their salvation shell must be removed. This can potentially limit the charging kinetics of such nanopores. In this work, we investigate the charging dynamics of sub-nanometer pores connected with an electrolyte bath. We quantify the energy barrier for ions to enter 0.82-nm wide slit pores and determine the time constant for charging of the pores using Molecular Dynamics simulations. Strong concentration polarization is found during the charging process and the charging kinetics is much slower than that predicted using the classical equivalent circuit model. The results are rationalized using a modified Poisson-Nernst-Planck model. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B9.00008: Statics and Dynamics of Stretched Single DNA Molecules Tug-of-War at Micro-Nanofluidic Interfaces JiaWei Yeh, Alessandro Taloni, Yeng-Long Chen, Chia-Fu Chou Understanding single molecule dynamics at micro-nanoscale interfaces has implications to polymer transport in biological processes, device design for single molecule analysis and biotechnological applications. We report our study on single DNA molecules straddling across a nanoslit, bridging two micro-nanofluidic interfaces, for both its tug-of-war behavior and confinement-induced entropic recoiling at varying length and height (h: 30$\sim $100 nm) of a nanoslit. From a modified worm-like chain model in the tug-of-war scenario and the scaling analysis in the entropic recoiling process, we demonstrate the entropic recoiling force is essentially constant, given the degree of confinement, irrespective of the DNA length inside the nanoslit and the slit length. The scaling exponents for the entropic force will also be discussed. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B9.00009: Long time dynamics of single linear and circular ds-DNA confined in sub-100nm nanoslits Po-Keng Lin, Jen-Fang Chang, I. Stachiv, Chia-Fu Chou, Y.- L. Chen We investigate the role of topological constraints on DNA dynamics in very strong confinement to study the dynamics of nuclear chromosome and DNA viral packaging. Experiments and simulations were carried out to investigate the equilibrium shape and dynamics of the single linear and circular $\lambda $-DNA confined in a silicon/glass nanoslit. We measured the chain extension $r$, shape asphericity $A$, extensional (\textit{$\tau $}$_{\vert \vert}$) and rotational relaxation time\textit{ $\tau $}$_{r}$, and examined the dependence on chain topology as functions of the slit height $h $(20 $\sim $ 780 nm) and the solvent ionic strength $I$ (0.8 $\sim $250 mM). We observed that the shape asphericity increases as $h$ and$ I$ decrease as the chain shape becomes anisotropic. Moreover, in sub-Kuhn length confinement, the DNA relaxation time increases with decreasing $h$ in a smooth and broad transition. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B9.00010: Heat-Driven Release of a Drug Molecule From Carbon Nanotubes Vitaly Chaban, Oleg Prezhdo Hydrophobicity and ability to absorb light that penetrates through living tissues make carbon nanotubes (CNTs) promising intracellular drug delivery agents. Following insertion of a drug molecule into a CNT, the latter is delivered into a tissue, is heated by near infrared radiation, and releases the drug. In order to assess the feasibility of this scheme, we investigate the rates of energy transfer between CNT, water and the drug molecule, and study the temperature and concentration dependence of the diffusion coefficient of the drug molecule inside CNTs. We use ciprofloxacin (CIP) as a sample drug: direct penetration of CIP through cell membranes is problematic due to its high polarity. The simulations show that a heated CNT rapidly deposits its energy to CIP and water. All estimated timescales for the vibrational energy exchange between CNT, CIP and water are less than 10 ps at 298 K. As the system temperature grows from 278 K to 363 K, the diffusion coefficient of the confined CIP increases 5-7 times, depending on CIP concentration. The diffusion coefficient slightly drops with increasing CIP concentration. This effect is more pronounced at higher temperatures. The simulations support the idea that optical heating of CNTs can assist in releasing encapsulated drugs. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B9.00011: Characterization of Nanostructured Silicon Membranes for Control of Molecular Transport Bernadeta Srijanto, Scott Retterer, Jason Fowlkes , Mitchel Doktycz Fabrication of nanoporous membranes for selective transport of molecular species requires precise engineering at the nanoscale. The membrane permeability can be tuned by controlling the physical structure and the surface chemistry of the pores. We use a combination of electron-beam and optical lithography, along with cryogenic deep reactive ion etching, to fabricate silicon membranes that are physically robust and have uniform pore sizes. Pore sizes are further reduced using plasma enhanced chemical vapor deposition and atomic layer deposition of silicon dioxide onto the membrane surfaces. Integrating nanoporous membranes within a microfluidic network provides a platform for tailoring molecular exchange between microchannels, independent of hydrodynamic effects. In enzymatic reactions, for example, tuning the pores size will allow smaller enzymatic substrates to traverse the membrane at controlled rates while larger enzymes remain spatially separated. Our results from membrane cross-sectioning using focused ion beam milling show that pore sizes can be controlled at dimensions below 10nm. Functional characterization was performed by quantitative fluorescence microscopy to observe the selective transport of molecular species of different sizes. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B9.00012: Thermal resistance of thin water films during phase-change Nitin Shukla, Nenad Miljkovic, Ryan Enright, Evelyn N. Wang The thermal resistance of a thin water film during phase-change processes is of interest for fundamental studies and of importance for various engineering systems. In particular, as the thickness of the water film approaches the nanoscale, the thermal resistance across the liquid-vapor interface can contribute significantly to the overall heat transport. In this work, we experimentally investigate the thermal resistance of thin water films during phase change on metallic substrates using transient thermoreflectance (TTR) spectroscopy. This technique offers a novel method to examine heat transport in evaporating liquid films less than a 100 nm in thickness. The understanding gained from this work will aid in the design of high performance phase-change based micro/nanoscale devices. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B9.00013: Effect of hydrogen bond cooperativity on the behavior of water Kevin Stokely Four scenarios have been proposed for the low--temperature phase behavior of liquid water, each predicting different thermodynamics. The physical mechanism which leads to each is debated. Moreover, it is still unclear which of the scenarios best describes water, as there is no definitive experimental test. Here we address both open issues within the framework of a microscopic cell model by performing a study combining mean field calculations and Monte Carlo simulations. We show that a common physical mechanism underlies each of the four scenarios, and that two key physical quantities determine which of the four scenarios describes water: (i) the strength of the directional component of the hydrogen bond and (ii) the strength of the cooperative component of the hydrogen bond. The four scenarios may be mapped in the space of these two quantities. We argue that our conclusions are model-independent. Using estimates from experimental data for H bond properties the model predicts that the low-temperature phase diagram of water exhibits a liquid--liquid critical point at positive pressure. [Preview Abstract] |
Session B10: SPS Undergraduate Research I
Sponsoring Units: SPSChair: Gary White, American Institute of Physics
Room: D221
Monday, March 21, 2011 11:15AM - 11:27AM |
B10.00001: Which String Breaks? Revisited Christopher Frye Many have seen the common introductory physics demonstration in which a heavy ball hangs from a string, with another identical string hanging freely from the ball. When the instructor pulls the bottom string slowly, the top string breaks. However, when the instructor pulls the bottom string very rapidly, the bottom string breaks. This simple experiment is used to demonstrate inertia and Newton's laws. In The Physics Teacher of November 1996, there is an article in which the authors create a model of this problem in an attempt to explain the outcomes quantitatively. However, their analysis gave strange results. Using an improved model, I will show that the results of this demonstration can be obtained using only simple calculations. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B10.00002: Synchronization effects in chaotic oscillators with spatially dependent frequency mismatch Philip Javernick, Trinanjan Datta We investigate the phenomena of synchronization for two coupled chaotic oscillators with a frequency mismatch which is explicitly spatially dependent. We compute the frequency synchronization plot in the parameter space of coupling strength and frequency mismatch of the chaotic system. We find regimes where the system is frequency locked corresponding to a synchronous state and regimes of non-synchronous state. In the non-synchronous state the frequencies are either zero individually (quenched oscillations) or the difference between them is non-zero. We also find that the region with oscillation quenching is reduced compared to the case when the frequency mismatch is a constant. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B10.00003: Rolling vs. Sliding: The inclusion of non-conservative work in the classic comparison Benjamin Lee, Justin Mueller, Terry Buehler, Alex Chediak If a rolling and sliding object, each of the same material, were to race down the same incline plane, which would win? Last year, we presented a theoretical model with confirming experimental data which showed that the winning object depends on the angle, the effective coefficient of friction, C, and kinetic coefficients of friction: If C $< \quad \mu _{k\_block}$, the rolling object is faster, but if C $> \quad \mu _{k\_block}$, the sliding object is faster. Though the materials were the same, we previously reported that the $\mu _{s\_sphere }$was apparently not equal to $\mu _{s\_block}$. This year, we are directly determining the coefficients of friction using a force sensor, seeking to resolve this apparent discrepancy. We plan to report more accurate values of $\mu _{s\_sphere }$and $\mu _{s\_block}$ and, if they are found to be different, explain why. Steps will be taken to improve track uniformity. We will more precisely determine the transition angle, where the block becomes faster than the sphere, by taking data at smaller angular increments. In addition, we will incorporate results for rolling \textit{with} slipping, as it is expected that as slipping increases, so will linear velocity, as less energy is lost to rotational kinetic energy. Beyond this, we hope to extend the model to different geometries (with different moments of inertia). [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B10.00004: Is Ball Milling An Innovative Technique For the Production of Zn From ZnO? Jeff McLeod, Ali Bakhshai The process of mechanical alloying using ball milling transfers mechanical energy to reactants in powder form, causing the particle size of the reactant powders to be reduced until defects in the lattice structure of the reactants are created. For reactions of sufficient exothermicity, this facilitates a complete mechanochemical reaction through self-heat propagating synthesis (SHS). The oxide reduction reaction of ZnO with Al, which yields pure Zn as a product, cannot be induced using ball milling alone because of its low exothermicity. This study used a systematic combination of ball milling and annealing in order to induce the reaction. Parameters tested were milling time, annealing time, and annealing temperature with the purpose of establishing the importance of each of these variables in inducing a complete reaction in the sample. The completeness of the reaction was determined using XRD analysis and inspection with an optical microscope. Results confirmed that neither ball milling nor heat treatment could induce the reaction individually; only ball milling followed by annealing could cause the reaction to take place. This study suggests that using ball milling in conjunction with heat treatment can produce Zn from ZnO in a less costly, more efficient, and less wasteful manner than traditional methods. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B10.00005: Women in Physics: an Analysis of the Gender Gap Lillie Ghobrial, Michael Evans, Gregory Maslak, Mark Stewart, Anna Bontorno, Brittany Barrett, Nicole Scott, Carolina Ilie It is not a surprise that the number of women in physics is not impressive, and the reasons are diverse and well-known [1]. We conducted several surveys at SUNY Oswego regarding the gender gap. We examined the source of the problem and we developed possible solutions. We propose herein various strategies for short-term and long-term improvement of female representation in Physics. This insight will hopefully benefit other physics departments in which women are underrepresented. \\[4pt] [1] Rachel Ivie and Katie Stowe. June 2000. Women in Physics, 2000, AIP Publication Number R-430. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B10.00006: STAIRSTEP -- a research-oriented program for undergraduate students at Lamar University Cristian Bahrim The relative low number of undergraduate STEM students in many science disciplines, and in particular in physics, represents a major concern for our faculty and the administration at Lamar University. Therefore, a collaborative effort between several science programs, including computer science, chemistry, geology, mathematics and physics was set up with the goal of increasing the number of science majors and to minimize the retention rate. Lamar's Student Advancing through Involvement in Research Student Talent Expansion Program (STAIRSTEP) is a NSF-DUE sponsored program designed to motivate STEM students to graduate with a science degree from one of these five disciplines by involving them in state-of-the-art research projects and various outreach activities organized on-campus or in road shows at the secondary and high schools [1]. The physics program offers hands-on experience in optics, such as computer-based experiments for studying the diffraction and interference of light incident on nettings or electronic wave packets incident on crystals, with applications in optical imaging, electron microscopy, and crystallography. The impact of the various activities done in STAIRSTEP on our Physics Program will be discussed. [1] Doerschuk P, Bahrim C, Daniel J, Kruger J, Mann J, and Martin Ch, \textit{39th ASEE/IEEE Frontiers in Education Conference,}\textbf{ }San Antonio 2009, M3F-1-2. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B10.00007: Modeling Surface Acoustic Waves on Liquid Loaded Surfaces Michael Mitchell, Matthew Kwan, Madeleine Msall Ultrasound excitation of crystals creates acoustic waves that propagate on the surface. The wave velocities vary with directions based on the properties of the crystal. Experiments typically use ultrasound transducers submerged in water. The water loading on the surface creates a perpendicular stress. This alters the boundary conditions of the surface waves, changing their propagation. We model the phase and group velocities of Rayleigh surface waves on water loaded Si (100) and CaWO$_{4}$. The addition of water loaded boundary conditions improves the match between model and experimental data. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B10.00008: Characterization of a MEMS Actuator through Simulation Erik Garcell, Miguel Gonzalez, Byoung Hee Moon, Pradeep Bhupathi, Pan Zheng, George Ling, Yoonseok Lee, Ho Bun Chan Simulations of a laterally shifting micro-electro-mechanical-system (MEMS) were performed to characterize the device for use in liquid $^{3}$He experimentation. Using the multiphysics software COMSOL, we were able to identify the relevant electrostatic and mechanical properties of our device, as well as its various vibrational modes. When actuated with a DC voltage, simulations demonstrated comparatively large out-of-plane displacements, which are in agreement with optical measurements taken from the actual device. New simulations were performed to test the effectiveness of possible efforts to dampen this displacement. Using the data collected from these simulations, future generations of the MEMS will be designed and improved for use in liquid $^{3}$He experiments. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B10.00009: Scanning Tunneling Microscopy of Manganites Icon Mazzaccari, Hyoungjeen Jeen, Amlan Biswas We have built a scanning tunneling microscope (STM) which employs a mechanical coarse approach mechanism. We have tested the mechanical and electronic components of the system and calibrated the piezoelectric scanning mechanism by imaging highly ordered pyrolytic graphite (HOPG) at room temperature. Atomic resolution HOPG images were obtained when the STM was placed inside a vibration isolated liquid helium dewar. We have also scanned single crystals and thin-films of hole-doped manganese oxides (manganites) and obtained images on the scale of about 100 nm to about 10 nm. After obtaining satisfactory images at room temperature, we will cool the apparatus first down to liquid nitrogen temperature (77 K) and then down to liquid helium temperature (4.2 K) to investigate micrometer and nanometer scale phase separation in manganites. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B10.00010: Magnetic field dependence on neutrino-induced electron-positron creation rates Hannah McWilliams The study of neutrino processes in magnetic fields are immensely important for astrophysical phenomena where neutrino interactions are the dominant mode of energy loss and large fields exist. In this talk I will present a phenomenological relationship for the production rate of one such process, the creation of electron-positron pairs $\nu \to \nu \ e \ \bar{e}$, as a function of the magnetic field. I will show that above the critical magnetic field strength and at large neutrino energies there exists a power law dependence on the magnetic field. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B10.00011: Emergent magnetic monopoles and their dynamics in artificial spin ice Yichen Shen, Olga Petrova, Paula Mellado, Oleg Tchernyshyov Electrically charged particles such as electrons are common in our world. In contrast, no elementary particles with a net \textit{magnetic }charge have ever been observed. After a recent discovery that magnetic monopoles can emerge in a system of magnetic dipoles [1], much attention has been paid to the behavior of magnetic monopoles in artificial spin ice, arrays of nano-scale magnetic islands or wires that mimic the behavior of geometrically frustrated materials [2]. We have developed a theoretical model of magnetization dynamics in artificial spin ice under the action of an external magnetic field [3]. Magnetization reversal is mediated by the creation, propagation and absorption of domain walls carrying two units of magnetic. Domain walls are emitted from lattice junctions when the local field becomes large enough to overcome the Coulomb attraction between the magnetic charges of the domain wall and the junction. This interaction is also responsible for a positive feedback that triggers magnetic avalanches observed experimentally in artificial spin ice. \\[4pt] [1] C. Castelnovo, R. Moessner, and S. L. Sondhi, Nature \textbf{451}, 42 (2008). \\[0pt] [2] O. Tchernyshyov, Nat. Phys. \textbf{6}, 323 (2010). \\[0pt] [3] P. Mellado, O. Petrova, Y. Shen, and O. Tchernyshyov, Phys. Rev. Lett. \textbf{105}, 187206 (2010). [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B10.00012: Antihydrogen Production in a Paul Trap Guy Geyer, Reinhold Blumel We investigate the dynamics of anti-hydrogen production within a Paul trap through computational means with the intent to develop a strategy for confining the anti-atom for further experimentation. We obtained first preliminary results on the transient production of anti-hydrogen. We present these results, discuss the experimental implementation of our system, and suggest ways to lengthen the lifetime of anti-hydrogen in the trap. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B10.00013: Effects of interactions on interference pattern formed after release and expansion of two identical Bose-Einstein condensates Catherine Lee, Courtney Lannert We numerically simulate the expansion and interference of two adjacent, identical Bose-Einstein condensates initially trapped by harmonic potentials. We use explicit finite-difference methods to solve the Gross-Pitaevskii equation and time-evolve the condensates. We repeat the simulation, varying the interaction strength of the condensates, and analyze how the interactions affect the time-evolution of the interference pattern. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B10.00014: Search for an Entanglement Measure for N-Qubit States via Phase Symmetry Joshua Geller While quantitative measures of entanglement exist for two-qubit systems, there are no equivalent measures for larger systems. Phase patterns within multi-qubit density matrices could yield clues to constructing quantitative measures for these larger systems. One such pattern within the N-qubit density matrix is observed by reordering the matrix according to the types of coherence terms in the first row and first column so the number of phases in each element increases from left to right in the first row, and from top to bottom in the first column. The resultant matrix contains blocks on its diagonal with elements having only bipartite entanglement. All remaining diagonal elements are part of GHZ-type states in this configuration. A benefit to this matrix structuring is the ability to apply concurrence, a measure of two-qubit entanglement, to the sub-matrix blocks formed on the diagonal. Exploring the meaning of these concurrences with regard to the entanglement of the whole system of N-qubits represented by the full density matrix is a possible next step toward finding a measure of N-qubit entanglement. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B10.00015: Plasmon Enhancement of Organic Solar Cells using Embossed Gratings David Shope, Jennifer Steele Organic photovoltaic cells (OPV) are attractive because of their low cost and easy fabrication. However, because the diffusion length of excitons in most organic photovoltaic material is about 100 nm, the overall thickness and therefore the optical absorbance of the device is limited, reducing the overall efficiency. Surface plasmon excitations have been studied as a possible mechanism to increase the absorption of light in solar cell active layers because of their ability to manipulate and enhance local electromagnetic fields. This work focuses on using metal gratings as one electrode. Gratings support a broad range of surface plasmons that can be tuned by changing the incident angle of light, making them ideal to isolate the contribution of surface plasmons to increases in the quantum efficiency of solar cells. OPV cells are made using a conjugated polymer and fullerene-based active layer with either an aluminum or silver bottom electrode patterned with a grating through microcontact printing. By measuring the efficiency of the solar cells as a function of both incident angle and wavelength, we can match increases in efficiency with specific surface plasmon modes. [Preview Abstract] |
Session B11: Compound and Oxide Semiconductors
Sponsoring Units: FIAPChair: Nelson Tansu, Lehigh University
Room: D222
Monday, March 21, 2011 11:15AM - 11:27AM |
B11.00001: Terahertz radiation mechanism of native $n$-Type InN with different carrier concentrations Kuang-I Lin, Jung-Tse Tsai, Jenn-Shyong Hwang, Hon-Way Lin, Shangjr Gwo, Meng-Chu Chen InN has received considerable attention due to its lower effective mass, higher mobility, and higher velocity saturation compared with GaN or AlN. The fundamental band gap of InN has recently been reevaluated to be around 0.6--0.7 eV, being therefore a promising candidate for terahertz (THz) applications. In this study, the polarity and mechanism of THz radiation from native $n$-type InN excited by femtosecond optical pulses are investigated. The optical properties, electron concentrations, and crystalline quality are characterized by photoluminescence and Raman scattering spectra. The electron concentrations are estimated to be between 0.35$\times $10$^{19}$ and 3.87$\times $10$^{19}$ cm$^{-3}$. The intensity ratio of the $A_{1}$(LO) to $E_{2}$(high) mode increases with increasing electron concentration. The polarity of THz radiation field from the samples with higher electron concentrations is opposite to that from $p$-InAs, indicating that the dominant radiation mechanism is the drift current. However, the samples with lower electron concentrations show the same polarity as $p$-InAs. Under this condition, the radiation mechanism is dominated by the photo-Dember effect. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B11.00002: Self-confined GaN hetero-phased quantum wells Yu-Chi Hsu, Ikai Lo, Chia-Ho Hsieh, Wen-Yuan Pang, Mitch M.C. Chou, Yen-Liang Chen, Cheng-Hung Shih, Ying-Chieh Wang Wurtzite/zinc-blende/wurtzite GaN hetero-phased quantum wells (QWs) were grown by plasma-assisted molecular beam epitaxy. A self-assembling mechanism was used to simulate the hetero-phased QW, in which a wurtzite/zinc-blende phase transition was created by rotating the threefold symmetric N-Ga vertical bond of wurtzite 60$^{\circ}$. From the cathodoluminescence measurement, we observed an additional peak (energy $\sim $3.2eV) associated with GaN zinc-blende phase. From the transmission electron microscopy images and selective area electron diffraction patterns, we confirmed the formation of hetero-phased quantum wells with a transition of wurtzite/zinc-blende GaN [1]. \\[4pt] [1] I. Lo, Y.-C. Hsu, C.-H. Hsieh, W.-Y. Pang, M. M.C. Chou, Y.-L. Chen, C.-H. Shih, and Y.-C. Wang, Appl. Phys. Lett. \underline {\textbf{96}}, 222105 (2010). [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B11.00003: Growths of InGaN Quantum Wells on GaN Micropyramids Renbo Song, Le Zhao, Guangyu Liu, Jing Zhang, Nelson Tansu Selective area epitaxy (SAE) of InGaN quantum wells on GaN micropyramids were grown by using metalorganic chemical vapor deposition (MOCVD). The pattern prepared for the SAE was fabricated by the deposition of 300 nm SiO$_{2}$ film on n-type GaN substrate by using plasma enhanced chemical vapor deposition (PECVD) and followed by photolithography. The grown micropyramid structures were characterized by scanning electron microscope (SEM) and photoluminescence (PL). Uniformly-distributed defect-free GaN micropyramids were observed by SEM. The growths of InGaN quantum wells were performed on the GaN micropyramids, and broadband luminescence were observed from the PL measurements applicable for white light-emitting diodes applications. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B11.00004: Charge Carriers in Boron Nitride Nanotubes Appear to confirm Quantum Capillary Action Richard Kriske A recent article in Physics Today confirmed a previous Theory put forward by this author in that electrons and perhaps other fundamental particles should find the easiest transmission path to be in the center of nanotubes not on the walls. Of course this is somewhat astonishing in that the center of the tube has nothing in it. This author had previously suggested that this might ultimately give a quantum mechanical explaination for capillary action. In any case a model could now be confirmed that this author previously put forward to show that in many cases the flow of particles should occur in the centers of nanotubes not on the walls and this would allow the exploration of the use of nanotubes as reaction devices, and transport devices with a general theory to show how molecules as well as elementary particles could be transported down the centers of these tubes and reacted in the tubes or in chambers attached to the tubes. This would also explain some vexing biological problems involving tubes and capillary action. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B11.00005: Transport Electron Scattering by Structural Defects in InSb Quantum Wells T.D. Mishima, M.B. Santos Among all the binary III-V semiconductors, InSb has the highest electron mobility and the narrowest band-gap. Field effect transistors and magnetic-field sensors are examples of devices based on InSb quantum wells (QWs) that exploit these material properties. In this study, we have investigated electron scattering due to two dominant structural defects, micro-twins (MTs) and threading dislocations (TDs), in InSb QWs grown on GaAs (001) substrates via AlInSb buffer layers. A linear regression analysis shows that the room-temperature electron mobility in InSb QWs has a strong correlation with the densities of both MTs and TDs, with an $R^{2}$ value of 0.9791. The MT-originated energy barrier and reflection coefficient for electron conduction in InSb QWs are estimated to be 0.09 eV and 0.33, respectively. The TD-limited electron mobility in InSb QWs is explained by electric charge with a density of 1.3 $\times$10$^{-10}$ C/m along a TD line. In addition to further discussion of the data, we will show the derivations of some key equations used for the mathematical analyses. This work was supported by the NSF under Grants Nos. DMR-0520550 and DMR- 0808086. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B11.00006: Strain, Confinement and Density Dependence of the Effective Mass of Holes in InSb Quantum Wells Chomani Gaspe, M. Edirisooriya, T.D. Mishima, R.E. Doezema, M.B. Santos, L.C. Tung, Y.J. Wang The valence band structure in a III-V quantum well (QW) is complicated by the presence of two highly non-parabolic bands. The lower (higher) energy band has a hole mass that is lighter (heavier) for motion in the plane of the QW. The energy separation between the two bands increases with increasing biaxial compressive strain and decreasing well width. An expected anticrossing between the two bands can add significantly to their non-parabolicity. We report an experimental study of the effective mass of 2D holes in a series of remotely doped InSb QWs under biaxial compressive strain. Only the lower energy band is occupied at low tempearture. Cyclotron resonance measurements at 4.2K show that the hole effective mass increases with increasing hole density from 0.045m$_{e}$ at 2.1$\times $10$^{11}$ cm$^{-2}$ to 0.083m$_{e}$ at 5.1$\times $0$^{11}$ cm$^{-2}$. The smallest effective mass of 0.017m$_{e}$ was observed in the QW with the largest compressive strain (1.06{\%}) and narrowest well width (7nm). This work was supported by the NSF Grants Nos. DMR-0520550 and DMR-088086. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B11.00007: Carrier and Spin Dynamics in Narrow Gap Parabolic Quantum Well Structures M. Bhowmick, T. Merritt, G.A. Khodaparast, T.D. Mishima, M.B. Santos, D. Saha, G.D. Sanders, C.J. Stanton Heterostructures with parabolic confinement potentials are important systems to study for many reasons. In a perfect Parabolic Quantum Well (PQW), the subbands are equally spaced and electron-electron interactions are virtually non-existent, allowing coupling of long-wavelength radiation only to the center-of-mass coordinate of the electron system. Narrow band PQW systems are well suited for THz devices because by careful design, one can tune the transition frequency, temperature stability, and narrow-band emission. In our studies, the parabolic confinement was created by an effective parabolic Al compositional gradient inside each well. We studied carrier/spin dynamics in an InSb/$Al_{x}In_{1-x}Sb$ multiple- PQW structure using several time resolved differential transmission schemes in the mid-infrared. Our results demonstrate the unique and complex dynamics in InSb heterostructures that can be important for electronic and optoelectronic devices. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B11.00008: Electrical characterization of GeSn grown on Si using ultra high vacuum chemical vapor deposition method Mo Ahoujja, S. Elhamri, J. Kouvetakis, J. Tolle, Mee Yi Ryu, Y.K. Yeo Recently, there has been considerable interest in growing Ge$_{1-x}$Sn$_{x}$ alloys on Si with x$<$ 0.2 for the purpose of developing optoelectronic devices that can be integrated with Si-based electronic technology. Here we report Hall coefficient and resistivity measurements as a function of temperature from thin epitaxial layers of GeSn grown on Si substrates using ultra high vacuum chemical vapor deposition. The Hall measurements show that GeSn samples with Sn concentrations of 1.5 and 2 {\%} are of high quality. The hole concentration for the boron doped Ge$_{0.98}$Sn$_{0.02}$ sample at room temperature is 7.1x10$^{18}$ cm$^{-3}$ while that of the as-grown undoped sample is 9.8x10$^{16}$ cm$^{-3}$. The measured hole mobility for Ge$_{0.98}$Sn$_{0.02}$ alloys with carrier concentrations greater than 10$^{18}$ cm$^{-3}$ are found to be comparable to those found in Ge samples with similar doping concentrations. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B11.00009: An extensive study to observe the effects of thermal annealing and ion fluences in the ion beam synthesis of $\beta $-SiC P.R. Poudel, B. Rout, D.R. Diercks, F.D. McDaniel, J.A. Paramo, Y.M. Strzhemechny A systematic study of the formation of $\beta $-SiC structures by low energy carbon ion (C$^{-})$ implantation into Si followed by high temperature thermal annealing will be presented. The effects of thermal annealing in the formation of $\beta $-SiC structures has been studied. It is observed that the thermal annealing of 1100\r{ }C for 1 hr is required to observe the formation of $\beta $-SiC. The quantitative analysis in the formation of $\beta $-SiC nanostructures has been performed by the implantation of various carbon ion fluences in the range of 1$\times $10$^{17}$ - 8$\times $10$^{17}$ atoms /cm$^{2}$ at an ion energy of 65 keV into Si. It is observed that the average size of $\beta $-SiC crystals decreases whereas the amount of $\beta $-SiC increases monotonically with ion fluence up to a fluence of 5 $\times $ 10$^{17}$ atoms/cm$^{2}$ and appears to saturate for a higher fluence of 8 $\times $ 10$^{17}$ atoms/cm $^{2}$ when the samples were annealed at 1100\r{ }C for 1 hr. The stability of graphitic C-C bonds at 1100\r{ }C limits the growth of SiC precipitates in the sample implanted at a fluence of 8$\times $10$^{17}$ atoms /cm$^{2}$ which results in the saturation behavior of SiC formation in the present study as predicted by various characterization techniques such as FTIR, Raman, XRD, XPS and Transmission electron microscopy. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B11.00010: Complimentary Ferromagnetic Mechanisms in Mn doped ZnO Thin films deposited using Pulsed Laser Ablation Devajyoti Mukherjee, Tara Dhakal, Hariharan Srikanth, Pritish Mukherjee, Sarath Witanachchi We show evidence through experiments and analysis that the ferromagnetism (FM) in 2{\%} Mn doped ZnO (ZMO) thin films is a combination of two complementary mechanisms - the bound magnetic polaron (BMP) percolation at low temperatures and the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction at higher temperatures. Pulsed laser deposition was used to grow ZMO thin films on c-cut sapphire substrates. Films were deposited at various temperatures and background oxygen pressures to study the effect of growth parameters on the FM. While no impurity-phase contributions were detected, a strong correlation between effective carrier densities and FM was established. FM in amorphous films with high defect densities were described by the BMP model whereas that in highly conducting films was consistent with the RKKY mechanism. Detailed characterization of the structural, electrical and magnetic properties of the as-deposited ZMO films will be presented. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B11.00011: Excitation of atomic zinc during excimer laser ablation of zinc oxide at 193 nm Enamul Khan, Stephen Langford, Thomas Dickinson Atomic excitations during UV laser ablation usually involve collisions with energetic electrons. When zinc oxide is ablated with a 193 nm excimer laser, we observe light emission at pulse energies ---well below the threshold for normal electron heating processes. At pulse energies near the threshold for visible light emission, the source is localized and moves away from the surface at a nearly constant velocity. Time-resolved quadrupole mass spectrometry confirms the presence of zinc atoms with velocities consistent with this motion. We propose that these excited zinc atoms are generated by two-photon excitation into the autoionizing 3d$^{10}$4p ($^{2}$P\r{ }$_{3/2})$ 5s~ $^{2}$[3/2]\r{ } state of atomic zinc at 103 001~cm$^{-1}$. The broad ``window resonance'' associated with this state in single-photon absorption is associated with a \textit{drop} in absorption, because the main decay channel (ionization) is hindered by destructive interference effects. We propose that radiative decay, which is otherwise a minor decay channel, produces bound excited states that subsequently decay to yield the observed light. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B11.00012: Role of ligands on the photoluminescence of colloidal CdSe quantum dots and enhancement of photoconductivity of ZnO nanowires by quantum dots Syamanta Kumar Goswami, Tae Soo Kim, Byoung Woo Lee, Eunsoon Oh, Ch. Kiran Kumar, Eui Tae Kim Colloidal core-shell CdSe/ZnS quantum dots (QDs) encapsulated by trioctylphosphine oxide ligand were synthesized via pyrolysis. Then the TOPO ligands by 3-mercaptopropionic acid ligands were replaced under Ar environment. ZnO nanowires were fabricated by sonochemical method on pre-patterned alumina electrodes. With increasing temperature above 220 K, the PL lifetime was found to be increased in case of the TOPO capped QDs, whereas for the MPA capped QDs, the lifetime was short and almost independent of temperature. The conductivity of ZnO nanorods was increased after the deposition of the QDs, which was further enhanced by the exposure of light. This increase in the conductivity with and without light can be explained by the photo-carrier transport and surface modification effect, respectively. The photo-generated electrons in the CdSe QDs will tend to move toward the ZnO nanowires, resulting in the enhancement of photo-conductivity in the ZnO nanowires. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B11.00013: Preparation and characterization of nanostructured ZIO thin films Vipin Kumar Jain, Praveen Kumar, Y.K. Vijay ZnO--In2O3 system has attracted much attention because of chemical and thermal stability in addition to properties comparable to those of ITO. In the present work Zinc indium oxide (ZIO) thin films were deposited on glass substrate with varying concentration (ZnO: In2O3 - 100:0, 90:10, 70:30 and 50:50 wt {\%}) at room temperature by flash evaporation technique. These deposited ZIO films were annealed in vacuum to study the thermal stability and to see the effects on the structural, chemical and electrical properties. Each film has been characterized ex-situ by XRD, XPS, XRF, AFM, SEM, optical band gap and Hall measurements. Results show the properties of the ZIO films strongly depend on the In2O3 concentration and also influenced by the post annealing of these films. XPS core level spectra of Zn(2p), O(1s) and In(3d) have been deconvoluted into their Gaussian components, while valence band spectra shows the change in electronic structures of the films. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B11.00014: M-plane ZnO grown on m-plane sapphire by radio-frequency magnetron sputtering Bi-Hsuan Lin, Wei-Rein Liu, Shao-Ting Hsu, Chin-Chia Kuo, Song Yang, Chia-Hung Hsu, Wen-Feng Hsieh High quality m-plane orientated ZnO films with in-plane epitaxial relationship of (0002)$_{ZnO}\vert \vert $(11-20)$_{sapphire}$ and (11-20)$_{ZnO}\vert \vert $(0006)$_{sapphire}$ have been successfully grown on m-plane sapphire by using radio-frequency magnetron sputtering. The introduction of a nanometer thick low temperature grown ZnO buffer layer effectively eliminated other undesirable orientations. The significant anisotropy of the strain field breaks the hexagonal symmetry and leads to the different physical properties from that of c-plane oriented ZnO films. The structural properties, including crystalline quality, strain state, and defect structures, of the m-plane ZnO layers are thoroughly examined by synchrotron x-ray scattering, transmission electron microscopy and atomic force microscopy. The optical properties are investigated by temperature, polarization as well as power dependent photoluminescence, and polarization dependent Raman spectroscopy. The correlation between the structural and optical properties will be discussed. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B11.00015: Phase diagram, structure, and electronic properties of (Ga$_{1-x}$Zn$_x$)(N$_{1-x}$O$_x$) solid solution Li Li, Philip B. Allen We studied (Ga$_{1-x}$Zn$_x$)(N$_{1-x}$O$_x$) solid solution by Density Functional Theory (DFT). To conduct thermodynamic simulations, we built a database of structures and constructed a Cluster Expansion(CE). The subsequent Monte Carlo simulation gives a calculated phase diagram with a wide miscibility gap and an ordered $x$=0.5 compound. The disordered phase displays strong short range ordering (SRO) at experimental temperatures. We then used snapshots from MC to investigate structural and electronic properties by DFT on large supercells. Consistent with previous theoretical and experimental findings, lattice parameters appear to deviate from Vegard's law with small upward bowing. Bond lengths depend strongly on local environment, with a variation much larger than the difference of bond length between ZnO and GaN. The downward band gap bowing deviates from parabolic by having more rapid onset of bowing at low and high concentrations. Our results show that SRO influences both the structural and electronic properties. [Preview Abstract] |
Session B12: Focus Session: Dopants and Defects in Semiconductors: Compound Semiconductors I
Sponsoring Units: DMPChair: Su-Huai Wei, National Renewable Energy Laboratory
Room: D223/224
Monday, March 21, 2011 11:15AM - 11:27AM |
B12.00001: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 11:27AM - 11:39AM |
B12.00002: Optimized basis sets for coarse-grained electronic structure calculations of point defects Bj\"orn Lange, Christoph Freysoldt, J\"org Neugebauer Density-functional theory is a powerful tool to study the properties of point defects in the supercell approach. Yet, the size limitations make a description of the extended tails of defect states, especially for shallow defects, cumbersome. Atomic orbital basis sets are the method of choice to coarse-grain electronic structure calculations, but are in general not flexible enough for describing the unusual bonding situations, which occur in point defects. We employ a newly developed method that, based on a variational principle, allows to generate small atomic basis sets which optimally mimic the Kohn-Sham wavefunctions with a plane-wave basis set. We show that these basis sets accurately reproduce the underlying plane-wave calculation. We analyze how the atomic orbitals close to the defect are modified in comparison to their bulk counterparts. We are able to extend basis sets generated from small supercells and to reproduce the bandstructure of larger cells. Using this approach we construct and solve a reliable sparse model Hamiltonian for a shallow defect test system containing $10^3 .. 10^4$ atoms. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B12.00003: Strain-Enhanced Doping in Semiconductors: Effects of Dopant Size and Charge State Junyi Zhu, Feng Liu, Gerald Stringfellow, Su-Huai Wei When a semiconductor host is doped by a foreign element, it is inevitable that a volume change will occur in the doped system. This volume change depends on both the size and charge state difference between the dopant and the host element. Unlike the ``common expectation'' that if the host is deformed to the same size as the dopant, then the formation energy of the dopant would reach a minimum, our first-principles calculations discovered that when an external hydrostatic strain is applied, the change of the impurity formation energy is monotonic: it decreases if the external hydrostatic strain is applied in the same direction as the volume change. This effect also exists when a biaxial strain is applied. A simple strain model is proposed to explain this unusual behavior, and we suggest that strain could be used to significantly improve the doping solubility in semiconductor systems. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B12.00004: Defects and Carrier Compensation in CdTe Invited Speaker: CdTe is a very useful semiconductor material for its radiation detection and thin-film solar cell applications. Good carrier mobility and lifetime are needed for CdTe since efficient carrier collection is essential for the success of both applications. On the other hand, high resistivity is required for radiation detection for suppressing dark current and device noise. This is in contrast to CdTe-based solar cells, in which low resistivity is desired. In this talk, I will discuss the properties of native defects and impurities in CdTe with emphasis on carrier compensation and its implications in radiation detection and solar cell applications. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B12.00005: n-type doping in $Cu_{2}O$ by halogen impurities: a first-principles study Qiong Bai, Meng Tao, Qiming Zhang The present work focuses on first-principles calculations on n- type doping by F, Cl, and Br impurities in $Cu_{2}O$ under solution-grown environments. From the formation energy point of view, the substitution of oxygen in $Cu_{2}O$ is favored over the interstitial sites. The electronic structures after doping are carefully studied. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B12.00006: Diffusion of ion implanted indium in ZnO crystals Faisal Yaqoob, Mengbing Huang, David Look We report on diffusion behaviors for ion implanted indium atoms in ZnO crystals. A c-plane ZnO crystal was implanted with In ions for four different energies (40, 100, 200, and 350 keV, respectively) and doses (8.0$\times $10$^{13}$, 1.2$\times $10$^{14}$, 1.6$\times $10$^{14}$ and 6.5$\times $10$^{14}$ /cm$^{2}$, respectively), resulting in a uniform concentration profile of In from surface to the depth $\sim $ 150 nm. The samples were annealed for 30 minutes at temperatures between 700-1000 \r{ }C with an argon or oxygen gas flow. The distributions of In atoms, either aligned or nonaligned along the crystalline directions, were measured by Rutherford backscattering combined with ion channeling. The diffusivities for nonaligned (interstitial) and aligned In atoms varied with annealing temperature via the Arrhenius relationship. The diffusion activation energies (E$_{a})$ for aligned In atoms were lower than those for interstitial In atoms, e.g., for annealing in an Ar gas, E$_{a} \quad \sim $ 0.61 eV for $<$1010$>$ aligned In atoms and E$_{a} \quad \sim $ 1.1 eV for interstitial In atoms between $<$1010$>$ atomic rows. Furthermore, the diffusion activation energies were affected by the gas species used during annealing, e.g., for annealing in an O$_{2}$ gas, E$_{a} \quad \sim $ 0.39 eV for $<$1010$>$ In atoms and E$_{a} \quad \sim $ 0.79 eV for interstitial In atoms between $<$1010$>$ atomic rows. These experimental results will be compared with first-principle calculations for In diffusion in ZnO crystals. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B12.00007: Surface effects in Co-doped ZnO nanocrystals Aline L. Schoenhalz, Gustavo M. Dalpian Semiconducting nanostructures have received high attention by scientific community due to their unusual properties and wide rage of possible applications. In this scale, the understanding of the surface effects of the material is fundamental to explain its properties. By using the Density Functional Theory within the Local Density Approximation, we report on the effects of the surface on the magnetic properties of Co-doped ZnO nanocrystals. For bulk ZnO, it is well known that the most stable magnetic interaction between Co impurities is antiferromagnetic. This is also the case for saturated nanostructures, where surface effects are not taken into account. However, when surface effects are considered, the interaction between transition metal impurities becomes ferromagnetic. We will discuss the interaction between surface and impurity states, comparing our results to experimental findings. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B12.00008: Vacancy-assisted migration of group-III impurities in ZnO Daniel Steiauf, John L. Lyons, Anderson Janotti, Chris G. Van de Walle Zinc oxide is a wide-band-gap material used as transparent conductor. As grown it often shows n-type conductivity, probably due to impurity contamination. High electron concentrations can be achieved by intentional doping with group-III elements, a process that usually involves annealing. It is thus important to understand the diffusion properties of the dopants. We perform first-principles calculations for the vacancy-assisted migration process of Al, Ga and In in ZnO, using both standard density functionals and hybrid functionals to correct the underestimated band gap. Indium induces the largest distortions in the lattice and has the highest formation energy. Its migration barrier to a neighboring Zn vacancy is the lowest. Al shows the highest barrier and thus has the best thermal stability. From the calculated migration barriers and formation energies, we determine diffusion activation energies and estimate annealing temperatures. The results are compared with recent experiments. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B12.00009: Improved LDA+U model for band gap corrected ZnO defect calculations Adisak Boonchun, Walter Lambrecht The local density approximation (LDA) is known to fail dramatically for point defects in ZnO. In the case of the oxygen vacancy, the one electron level of the 1+ charge state lies above the conduction band and leads to improper filling of the levels. Different points of view on how to implement a-posterior gap corrections still leave large uncertainty on the position of the defect levels. For the Zn-vacancy, LDA leads incorrectly to a delocalized wave function of the hole on all four neighbors. Our approach is to apply LDA+U corrections to various orbitals, O-sp and Zn-spd. The U-parameters which lead to orbital shifts $V_i=U_i(1/2-n_i)$ are adjusted to quasiparticle self-consistent GW (QSGW) calculations of the band structure, including the shifts of the band structure relative to the LDA one on an absolute scale. With this improved LDA+U model, good agreement is obtained for the minimum gap, the conduction band mass and the valence and conduction band shifts separately. The structural properties of ZnO also remain intact. When applied to the oxygen vacancy, we find the 2+/0 transition level in good agreement with recent hybrid functional calculations. Applications of the same LDA+U model to the Zn-vacancy are in progress and show that localization of the wave function on two oxygen neighbors is obtained. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B12.00010: Hybrid density functional study of gallium in ZnO Denis Demchenko The properties of interstitial and substitutional gallium impurities as well as their complexes in wurtzite ZnO are modeled using hybrid density functional theory. This approach reproduces experimental band gap and avoids any artificial gap corrections necessary when using LDA/GGA/LDA+U methods. We find that the lattice relaxations at the LDA/GGA level can also introduce large errors to the defect formation energies (up to 1.5 eV). The error is particularly large in cases where shallow occupied defect levels are formed and LDA can produce incorrect charges. The correct defect structure therefore should be obtained by relaxation using hybrid DFT method. We find both interstitial and substitutional Ga forming donor-like defect states, with substitutional Ga being energetically favorable. Acceptor-like Zn vacancy has high formation energy in the isolated state but exhibits strong preference to form defect complexes with both substitutional and interstitial gallium. (Ga$_i$-V$_{Zn}$) complex acts as a shallow donor, while (Ga$_{Zn}-V_{Zn}$) is a deep acceptor. The low formation energies of both complexes suggests an appreciable degree of self-compensation. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B12.00011: Study of defects in TlBr, InI as potential semiconductor radiation detectors Koushik Biswas, Mao-Hua Du Group III-halides such as TlBr and InI are receiving considerable attention for application in room temperature radiation detector devices. It is however, essential that these detector materials have favorable defect properties which enable good carrier transport when operating under an external bias voltage. We have studied the properties of native defects of InI and Tlbr and several important results emerge: (1) Schottky defects are the dominant low-energy defects in both materials that can potentially pin the Fermi level close to midgap, leading to high resistivity; (2) native defects in TlBr are benign in terms of electron trapping. However, anion-vacancy in InI induces a deep electron trap similar to the $F$-centers in alkali halides. This can reduce electron mobility-lifetime product in InI; (3) low diffusion barriers of vacancies and ionic conductivity could be responsible for the observed polarization phenomenon in both materials at room temperature. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B12.00012: Information-based screens for deep traps in semiconducting materials Kim Ferris, Kunal Shah, Dumont Jones The key to a successful materials search is the ability to suggest promising materials and a priori eliminate unfruitful inquiry. For semiconducting radiation detection materials, performance is characterized by several key properties; band gap, density, electron mobility, and carrier lifetime. The material's proclivity to form defects is critical, as even simple antisite and vacancy defects can be sufficiently deep to affect effective carrier lifetime and mobility. We have developed a new model for defect formation proclivity, leveraging prior defect models (van Vechten and Feichter) and our information-based work. Our approach is based upon classification of materials chemistry and properties consistent with high concentrations of particular defects (e.g. antisites and vacancies). One issue is that nearly any charged local defect can potentially form a deep trap, so the screen must cover different defect types. Second, the screening model for new materials cannot rely on generally unknown factors such as 3D crystal geometry. The resulting model is intended to provide design guidance on expected defect behavior for candidate detection materials for which there is little or no prior information. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B12.00013: Ab-Initio analysis of TlBr: limiting the ionic current without degrading the electronic one Cedric Rocha Leao, Vincenzo Lordi Although TlBr in principle presents all the theoretical requirements for making high resolution room temperature radiation detectors, practical applications of TlBr have proven to be nonviable due to the polarization that is observed in the crystal after relatively short periods of operation. This polarization, that is believed to be caused by accumulation of oppositely charged ionic species at the ends of the crystal, results in an electric field that opposes that of the applied bias, counter-acting its effect. In this work, we use state of the art quantum modeling to benchmark the theoretical limits for the performance of TlBr as a radiation detector, showing that the best experimental reports demonstrate near-ideal electronic characteristics. We then propose a model to inhibit the detrimental ionic current in the material without impacting the excellent properties of the electronic current. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Session B13: Focus Session: Polymer Colloids: Structure, Function, and Dynamics II
Sponsoring Units: DFDChair: Eric R. Dufresne, Yale University
Room: D225/226
Monday, March 21, 2011 11:15AM - 11:27AM |
B13.00001: Predicting long-time Brownian dynamics of ultrasoft colloid suspensions from thermodynamics Mark Pond, Jeffrey Errington, Thomas Truskett Suspensions of ultrasoft colloids, such as Gaussian-core particles and Hertzian spheres, have received significant research interest due to their reentrant melting behavior and dynamic anomalies. Many of the previous dynamic studies of these systems have focused on molecular dynamics simulations, which by their nature ignores the solvent medium. We have conducted Brownian dynamics simulations of these ultrasoft colloid suspensions to show their long-time dynamic behavior near the reentrant melting transition. In addition, we have developed a novel method for quantitatively and qualitatively predicting the long-time Brownian dynamics of ultrasoft colloidal suspensions from their thermodynamic properties. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B13.00002: Development of surfaces repelling negatively buoyant solid particles Carina Semmler, Alexander Alexeev Using a hybrid computational method that integrates the lattice Boltzmann model for fluid dynamics and the lattice spring model for solids, we examine the motion of negatively buoyant solid microparticles in shear flow near a solid wall decorated with regularly distributed rigid posts. The posts are arranged in a square pattern and tilted relative to the flow direction. We show that when rigid posts are tilted against flow, secondary flows emerge that prevent the deposition of suspended particles on the solid surface. We probe the effect of post geometry on the development of secondary flows and identify the optimal post architecture in terms of the mass of levitated solid particles. Our results are useful for designing anti-fouling surfaces that repel colloidal particles carried by fluid. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B13.00003: Microscopic structure of confined colloidal suspensions under shear Xinliang Xu, Stuart Rice, Aaron Dinner, Xiang Cheng, Itai Cohen We report a study of driven colloidal suspensions by Stokesian dynamics simulation. The suspension is confined by two parallel plates, and is being driven far away from equilibrium by shearing induced by translation of the parallel plates. The separation of the plates is varied so the suspensions form either a single layer or two layers. Both the structure of the non-equilibrium steady state and the dynamics of the relaxation of the non-equilibrium state back to the equilibrium are examined, at a wide range of shearing strengths (the non-dimensional ratio quantifying the driven motion relative to the Brownian motion of the colloidal particles, the Peclet number is tuned from 0.1 to 100) and packing fractions. We observe string-like structures at low packing fractions and shear-induced crystallization at high fractions. A mechanism is proposed for how hydrodynamic interactions give rise to these structures. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B13.00004: Evaporation of Lennard-Jones Fluids Shengfeng Cheng, Jeremy Lechman, Steven Plimpton, Gary Grest Solvent evaporation is a process frequently used to disperse particles in a bulk material or at a substrate. The local order and packing of particles can be controlled by controlling the evaporation rate. The first step to fully understand this complicated process is to understand the evaporation process of pure liquid at the microscopic scale. We have carried out large scale molecular dynamics simulations to study the evaporation of Lennard-Jones (LJ) fluids composed of monomers, dimers, or trimers. For LJ monomers in contact with a vacuum, the evaporation rate is found to be very high with significant evaporative cooling and an accompanying density gradient in the liquid domain near the liquid/vapor interface. Increasing the chain length to just dimers significantly reduces the evaporation rate. The velocity distributions of evaporated monomers are measured and compared to a kinetic theory and their dependence on the evaporation conditions is discussed. For nanoparticle suspensions, the nanoparticles order at the surface, which causes the evaporation to significantly slow down. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B13.00005: Rotational and Translational Diffusion of PMMA Colloidal Clusters Hyun Joo Park, Mark T. Elsesser, Kazem V. Edmond, David J. Pine Colloidal clusters, 3-7 $\mu $m in size, are a good model system for various 2D and 3D structures depending on the aggregation number, $N$. We measure the translational and rotational diffusion of individual dyed PMMA clusters of dimers and trimers using high speed confocal scanning microscopy and particle tracking. We report measurements of the rotational and translational diffusion coefficients~(and their ratios) as a function of volume fraction. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B13.00006: A diversity of binary colloidal crystals using DNA-directed interactions John Crocker, Marie Ung, W. Ben Rogers, Raynaldo Scarlett, Talid Sinno DNA is the premier tool for directing the controlled self-assembly of nanoscopic and microscopic objects. The interactions between microspheres due to the hybridization of DNA strands grafted to their surface have been measured and can be modeled in detail, using well-known polymer physics and DNA thermodynamics. Knowledge of the potential, in turn, enables the exploration of the complex phase diagram and self-assembly kinetics in simulation. In experiment, at high densities of long grafted DNA strands, and temperatures where the binding is reversible, these system readily form colloidal crystals having a diverse range of symmetries. For interactions that favor alloying between two same-sized colloidal species, our experimental observations compare favorably to a simulation framework that predicts the equilibrium phase behavior, crystal growth kinetics and solid-solid transitions. We will discuss the crystallography of the novel alloy structures formed and address how particle size and heterogeneity affect nucleation and growth rates. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B13.00007: Correlating Structural and Spectral Fluctuations in a Lasing Colloidal Suspension Jason W. Merrill, Hui Cao, Eric R. Dufresne When multiply scattering media with optical gain are optically pumped above a critical threshold, they emit coherent radiation in many spectral lines. This phenomenon is known as random lasing. The wavelengths of these spectral lines depend sensitively on the spatial distribution of scatterers, but this relationship has only just begun to be explored. We study the time and frequency domain statistics of random laser spectra emitted from dense colloidal suspensions doped with laser dye with an eye toward using this information as a probe of the underlying colloid dynamics. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B13.00008: Smart colloidosomes with tunable permeability and a dissolution trigger Adriana San Miguel, Jan Scrimgeour, Jennifer Curtis, Sven Behrens Self-assembly of colloidal particles in the liquid interface of double emulsion droplets can be used to fabricate ``colloidosome'' microcapsules, which have great potential as vehicles for the controlled delivery of drugs or other cargoes. Here we present a novel class of aqueous core colloidosomes that combine the benefit of low capsule permeability (good cargo retention) with the option of a stimulus-triggered fast release in a target environment. Complete or partial dissolution of the capsule walls in response to a mild pH change is achieved in each case through the use of responsive particles made from polymers with pH-switchable solubility. We demonstrate three methods of controlling the capsule permeability prior to release while maintaining the intended response to the release trigger. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B13.00009: A theoretical study of colloidal forces near an amphiphilic polymer brush Jianzhong Wu Polymer-based ``non-stick'' coatings are promising as the next generation of effective, environmentally-friendly marine antifouling systems that minimize nonspecific adsorption of extracellular polymeric substances (EPS). However, design and development of such systems are impeded by the poor knowledge of polymer-mediated interactions of biomacromolecules with the protected substrate. In this work, a polymer density functional theory (DFT) is used to predict the potential of mean force between spherical biomacromolecules and amphiphilic copolymer brushes within a coarse-grained model that captures essential nonspecific interactions such as the molecular excluded volume effects and the hydrophobic energies. The relevance of theoretical results for practical control of the EPS adsorption is discussed in terms of the efficiency of different brush configurations to prevent biofouling. It is shown that the most effective antifouling surface may be accomplished by using amphiphilic brushes with a long hydrophilic backbone and a hydrophobic end at moderate grafting density. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B13.00010: Phase transition of colloidal particles on curved surfaces Guangnan Meng, Jayson Paulose, David Nelson, Vinothan Manoharan Defects and disclinations have to appear in crystalline domains on a curved surface with non-zero Gaussian curvature. These geometrical frustrations can qualitatively change the physics of phase transition. We encapsulate micron sized polystyrene (PS) colloidal particles within emulsion droplets and use nanometer sized polyNIPAM hydrogel particles to introduce depletion attraction between PS particle and interface, as well as between PS particles. We use this experimental model system and confocal microscopy to study phase transitions on curved surfaces. We will present both experimental phenomena and theoretical analysis. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B13.00011: Dielectric effects in self-assembly of binary colloid mixtures Erik Luijten, Kipton Barros Colloidal self-assembly is often controlled by electrostatic interactions. The solvent and colloids typically have different dielectric constants, thereby inducing polarization charge at the colloid surfaces. A shortcoming of previous simulations of charged colloids with implicit solvent is the neglect of the effective many-body interactions resulting from such dielectric effects. We study colloidal self-assembly using a method that properly accounts for polarization charge. In simulations of weakly charged colloids with large size asymmetry, we find that dielectric effects modify the pair correlation function in a nontrivial way and at low temperatures alter the observed crystal phase. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B13.00012: Formation of three-dimensional colloidal nanoparticle supercrystals and probing the formation kinetics Irving Herman, Chenguang Lu, Austin Akey A multiple solvent system consisting of colloidal nanoparticles in several solvents of gradually decreasing vapor pressures was investigated in the self assembly of hundred-layer thick colloidal nanoparticle superlattices in lithographically defined capillaries. Such a solvent system allows a very slow and tunable drying rate of solvents, which, together with the microfluidic flow into the capillaries, leads to the controllable formation of large, single crystalline 3D nanoparticle supercrystals. The underlying mechanism of superlattice formation was investigated via the drying rates for nanoparticle assembly for solvent systems of specific compositions. This technique generates single-crystalline 3D supercrystals of $\sim $micrometer size at spatially controlled locations, and large chunks (up to 40 $\mu $m by 40 $\mu $m by 5 $\mu $m) of single crystalline supercrystals on a flat Si substrate. The ordered nature of the structures formed was probed by high-resolution SEM and small angle x-ray scattering. In-situ x-ray scattering reveals the formation kinetics of the transition of nanoparticle assemblies from amorphous to ordered. This technique is versatile and has been applied to various types and sizes of colloidal nanocrystals, including those composed of CdSe, Au, PbS and Fe$_{3}$O$_{4}$. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B13.00013: Nanoparticle-induced self-assembly of functionalized tetrapods Daniel W. Sinkovits, Erik Luijten Recent advances in synthesis have made it possible to create monodisperse particles with well-defined shapes. In particular, tetrapods have been fabricated in a wide range of well-controlled dimensions and have been functionalized in several different ways. We present Monte Carlo simulations of the self-assembly of functionalized tetrapods. We consider how the addition of charged spherical nanoparticles provides another means to control the self-assembled structure. In addition, we report the results of simulations of planar tripods confined to two dimensions and demonstrate that highly regular structures can be achieved without functionalization, through nanoparticle-mediated depletion interactions. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B13.00014: A ``diffusing diffusivity'' model of ``anomalous yet Brownian'' diffusion of colloidal particles Mykyta V. Chubynsky, Gary W. Slater ``Anomalous yet Brownian'' diffusion of colloidal beads, with a mean-square displacement (MSD) exactly linear in time (as in simple Fickian diffusion) but an exponential (rather than Gaussian) displacement distribution (DD) at short times for large displacements, has been reported recently by Granick's group [1] in several systems. We argue that a strictly linear MSD with a non-Gaussian DD is a universal feature of systems with ``diffusivity memory'' (a particle diffusing faster is likely to keep diffusing faster for some time), but without ``direction memory'' (a jump in a particular direction does not change the probability of subsequent jumps in that direction). We consider a series of toy models reproducing this behavior in which a particle undergoes regular diffusion, but its diffusivity itself performs a (perhaps biased) random walk. The DD is strictly exponential at short times when the diffusivity distribution itself is exponential, but an exponential remains a good fit for a variety of diffusivity distributions. \\[4pt] [1] Wang et al., PNAS 106 (2009) 15160. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B13.00015: Reentrant and Isostructural Transitions in the Cluster-Crystal Forming GEM-4 Kai Zhang, Patrick Charbonneau, Bianca Mladek Systems governed by soft, bounded, purely repulsive interactions show two possible equilibrium behaviors under compression: reentrant melting, as in the Gaussian core model (GCM), or clustering, as in the penetrable sphere model (PSM). The generalized exponential model of power 4 (GEM-4), which is the intermedia of the GCM and PSM with a simple isotropic pair interaction $u(r)\sim e^{-r^4}$, is thought to belong to the second family and was indeed found to form clusters at sufficiently high densities at high temperatures. Here, we present the low-temperature behavior of GEM-4 through Monte Carlo simulations using a specially developed free energy integration scheme. We find the phase behavior to be hybrid between the GCM and the PSM limits, showing a surprisingly rich phase behavior in spite of the simplicity of the interaction form. For instance, S- shaped doubly reentrant phase sequences and evidence of a cascade of critical isostructural transitions between crystals of different average lattice site occupancy are observed. The possible annihilation of lattice sites and accompanying clustering moreover leads to an unusual softening upon compression, which suggest that these materials may have interesting mechanical properties. We discuss possible experimental realizations and challenges of this class of materials. [Preview Abstract] |
Session B14: Applications of Statistical and Nonlinear Physics to Social Systems and GSNP Student Speaker Award Talks
Sponsoring Units: GSNPChair: Victor Yakovenko, University of Maryland
Room: D227
Monday, March 21, 2011 11:15AM - 11:27AM |
B14.00001: Characterization of stock market regimes by data compression Eugenio E. Vogel, Gonzalo Saravia It has been shown that data compression can characterize magnetic phases (Physica A 388 (2009) 4075). In the introduction of this presentation we briefly review this result. We then go onto introducing a new data compressor (wlzip) developed by us to optimize recognition of meaningful patterns in the compressing procedure, yielding sharp transition curves at the magnetic critical temperatures. The advantages of the new compressor, such as better definition and tuning capabilities are presented. The rest of the talk consists of applying wlzip to the Chilean stock market along several months during 2010. The accumulated daily data allow to recognizing days with different types of activity. Moreover, the data recorded every minute allow to analyzing the ``present'' status of the stock market by applying wlzip to the data of the last hour or couple of hours. Possible extensions of the application of this technique to other fields are discussed. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B14.00002: Relativistic statistical arbitrage Alexander Wissner-Gross, Cameron Freer Recent advances in high-frequency financial trading have made light propagation delays between geographically separated exchanges relevant. Here we show that there exist optimal locations from which to coordinate the statistical arbitrage of pairs of spacelike separated securities, and calculate a representative map of such locations on Earth. Furthermore, trading local securities along chains of such intermediate locations results in a novel econophysical effect, in which the relativistic propagation of tradable information is effectively slowed or stopped by arbitrage. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B14.00003: The Impact of Competing Time Delays in Stochastic Coordination Problems G. Korniss, D. Hunt, B.K. Szymanski Coordinating, distributing, and balancing resources in coupled systems is a complex task as these operations are very sensitive to time delays. Delays are present in most real communication and information systems, including info-social and neuro-biological networks, and can be attributed to both non-zero transmission times between different units of the system and to non-zero times it takes to process the information and execute the desired action at the individual units. Here, we investigate the importance and impact of these two types of delays in a simple coordination (synchronization) problem in a noisy environment. We establish the scaling theory for the phase boundary of synchronization and for the steady-state fluctuations in the synchronizable regime\footnote{D. Hunt, G. Korniss, B.K. Szymanski, e-print arXiv:1011.2957 (2010).}. Further, we provide the asymptotic behavior near the boundary of the synchronizable regime. Our results also imply the potential for optimization and trade-offs in stochastic synchronization and coordination problems with time delays. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B14.00004: Statistical regularities in the rank-citation profile of individual scientists Alexander Petersen, H. Eugene Stanley, Sauro Succi Citation counts and paper tallies are ubiquitous in the achievement ratings of individual scientists. As a result, there have been many recent studies which propose measures for scientific impact (e.g. the $h$-index) and the distribution of impact measures among scientists. However, being just a single number, the $h$-index cannot account for the full impact information contained in an author's set of publications. Alternative ``single-number'' indices are also frequently proposed, but they too suffer from the shortfalls of not being comprehensive. In this talk I will discuss an alternative approach, which is to analyze the fundamental properties of the {\it entire} rank-citation profile (from which all single-value indices are derived). Using the complete publication careers of $200$ highly-cited physicists and $100$ Assistant professors, I will demonstrate remarkable statistical regularity in the functional form of the rank-citation profile $c_{i}(r)$ for each physicist $i=1...300$. We find that $c_{i}(r)$ can be approximated by a discrete generalized beta distribution over the entire range of ranks $r$, which allows for the characterization and comparison of $c_{i}(r)$ using a common framework. Since two scientists can have equivalent $h_{i}$ values while having different $c_{i}(r)$, our results demonstrate the utility of a scaling parameter, $\beta_{i}$, in conjunction with $h_{i}$, to quantify a scientist's publication impact. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B14.00005: Modeling the decline of religion Richard Wiener, Haley Yaple, Daniel Abrams People claiming no religious affiliation constitute the fastest growing ``religious'' minority in many countries throughout the world.\footnote{Zuckerman, P. ``Atheism: Contemporary rates and patterns,'' in Cambridge Companion to Atheism, University of Cambridge Press, 2007.} Here we use a minimal model of competition between social groups\footnote{Abrams, D. M. and Strogatz, S.H. Modelling the dynamics of language death. Nature 424(6951), 900 (2003).} to explain historical data on the growth of religious non-affiliation in 85 regions around the world. We also describe numerical experiments that support the validity of the model. According to the model, for societies in which the perceived utility of not adhering is greater than the utility of adhering, religion will be driven toward extinction. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B14.00006: Spontaneous Time Symmetry Breaking in System with Mixed Strategy Nash Equilibrium: Evidences in Experimental Economics Data Zhijian Wang, Bin Xu In social science, laboratory experiment with human subjects' interaction is a standard test-bed for studying social processes in micro level. Usually, as in physics, the processes near equilibrium are suggested as stochastic processes with time-reversal symmetry (TRS). To the best of our knowledge, near equilibrium, the breaking time symmetry, as well as the existence of robust time anti-symmetry processes, has not been reported clearly in experimental economics till now. By employing Markov transition method to analysis the data from human subject 2x2 Games with wide parameters and mixed Nash equilibrium, we study the time symmetry of the social interaction process near Nash equilibrium. We find that, the time symmetry is broken, and there exists a robust time anti-symmetry processes. We also report the weight of the time anti-symmetry processes in the total processes of each the games. Evidences in laboratory marketing experiments, at the same time, are provided as one-dimension cases. In these cases, time anti-symmetry cycles can also be captured. The proposition of time anti-symmetry processes is small, but the cycles are distinguishable. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B14.00007: The Sign Effect in Emerging Markets: the Inherent Instability of Bad News Joel Tenenbaum, Boris Podobnik, Davor Horvatic, Slavica Bajic, Beco Pehlivanovic, H. Eugene Stanley In developed economy market indices, the sign of a term in a series influences the volatility in an asymmetric fashion --- bad news results in larger subsequent fluctuations while good news results in smaller fluctuations. We study this phenomenon of volatility asymmetry using a stochastic process, exploring whether this asymmetry manifests in emerging markets, and if so, how such asymmetry changes over time as economies develop, mature, and react to crises such as the present one. We find that while both developed and emerging markets show distinctive behavior with respect to volatility asymmetry during times of economic tumult, they do so in ways that could be viewed either as universal or qualitatively different, posing interesting questions for further research. B. Podobnik et al., Phys. Rev. E \textbf{80}, 015101(R) (2009). J. Tenenbaum et al., Phys. Rev. E \textbf{82}, 046104 (2010). [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B14.00008: Quantized expected returns in terms of dividend yield at the money Lamine Dieng We use the Bachelier (additive model) and the Black-Scholes (multiplicative model) as our models for the stock price movement for an investor who has entered into an America call option contract. We assume the investor to pay certain dividend yield on the expected rate of returns from buying stocks. In this work, we also assume the stock price to be initially in the out of the money state and eventually will move up through at the money state to the deep in the money state where the expected future payoffs and returns are positive for the stock holder. We call a singularity point at the money because the expected payoff vanishes at this point. Then, using martingale, supermartingale and Markov theories we obtain the Bachelier-type of the Black-Scholes and the Black-Scholes equations which we hedge in the limit where the change of the expected payoff of the call option is extremely small. Hence, by comparison we obtain the time-independent Schroedinger equation in Quantum Mechanics. We solve completely the time independent Schroedinger equation for both models to obtain the expected rate of returns and the expected payoffs for the stock holder at the money. We find the expected rate of returns to be quantized in terms of the dividend yield. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B14.00009: Competition in social systems: three and a half models Daniel Abrams, Haley Yaple, Richard Wiener When groups compete for members, the resulting dynamics of human social activity may be understandable with simple mathematical models. Here, we use techniques from dynamical systems and perturbation theory to analyze a theoretical framework for the growth and decline of competing social groups in three limits. We apply our analysis to an international data set tracking the growth of religious nonaffiliation, and find that data suggest a particular case of our general growth law, leading to clear predictions about possible future trends in society. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B14.00010: Extraordinary Elasticity of the Distorted Kagome Lattice Anton Souslov, Kai Sun, Xiaoming Mao, Tom Lubensky J. C. Maxwell discovered that a system of particles in $d$-dimensions will be marginally rigid, or \emph{isostatic}, if each particle interacts on average with $2 d$ of its neighbors. Isostatic models have been used to describe such diverse soft phenomena as the jamming transition and the elasticity in networks of semi-flexible polymer gels. We develop models based on the isostatic kagome lattice, which has a subextensive number of floppy phonon modes. We show that these can be extended into soft deformations by changing the particle configurations while keeping the bond lengths fixed. Thus, we create families of novel isostatic lattices, which exhibit highly tunable elastic properties as a consequence of isotropic linear elasticity with a zero bulk modulus. They have a negative Poisson ratio, or auxetic (anti-rubber) behavior. Further, we find no bulk soft phonons at large length scales due to conformal symmetry. We discuss the intimate relationship between various symmetries and soft response in these models as well as the relation of these models to other marginally rigid systems. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B14.00011: Theory of cooperation in a micro-organismal snow-drift game Zhenyu Wang, Nigel Goldenfeld We present a mean field model for the phase diagram of a community of micro-organisms, interacting through their metabolism so that they are, in effect, engaging in a cooperative social game. We show that as a function of the concentration of the nutrients glucose and histidine, the community undergoes a phase transition separating a state in which one strain is dominant to a state which is characterized by coexisting populations. Our results are in good agreement with recent experimental results, correctly predicting quantitative trends and the phase diagram. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B14.00012: Intrinsic noise in stochastic models of gene expression with molecular memory and bursting Tao Jia, Rahul V. Kulkarni Regulation of intrinsic noise in gene expression is essential for many cellular functions. Correspondingly, there is considerable interest in understanding how different molecular mechanisms of gene expression impact variations in protein levels across a population of cells. In this work, we analyze a stochastic model of bursty gene expression which considers general waiting-time distributions governing arrival and decay of proteins. By mapping the system to models analyzed in queueing theory, we derive analytical expressions for the noise in steady-state protein distributions. The derived results extend previous work by including the effects of arbitrary probability distributions representing the effects of molecular memory and bursting. The analytical expressions obtained provide insight into the role of transcriptional, post-transcriptional and post-translational mechanisms in controlling the noise in gene expression. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B14.00013: Search for Euler Singularity using Vortex Filaments Sahand Hormoz, Michael Brenner A promising mechanism for generating a finite-time singularity in the incompressible Euler equations is stretching of vortex filaments. An exhaustive search of all possible initial conditions involving filaments, however, is not practically feasible. In this talk, I will show that two interacting vortex filaments can not generate a singularity for any initial conditions, by analyzing the asymptotic self-similar limit of their collapse. Essentially, our approach entails a separation of the dynamics of the filament shape, from the shrinking of its core. We solve for the dynamics using a self-similar ansatz and show that the core does not shrink fast enough for a self-consistent collapse. The similarity solution allows for many different collapse geometries, consistent with the tireless effort in the past of investigating new initial conditions. Potential for a singularity at higher number of filaments is also discussed. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B14.00014: Vibrofluidized melting of geometrically cohesive granular media Nick Gravish, Geoffrey Russell, Scott V. Franklin, David Hu, Daniel I. Goldman Dry granular media composed of particles of special shapes (e.g. long rods or c-shaped particles) can display cohesive effects through particle geometry alone. We study the solid to gas transition in piles of c-shaped particles under vertical vibration as we vary acceleration and frequency. A cylindrical solid of particles is formed with wall angles near $90^\circ$ and is placed on a solid surface. For fixed frequency as acceleration increases, the pile undergoes two transitions. The first is from the solid-like state to a liquid-like state in which the wall angles relax but the mobile particles remain spatially localized. The second is from the liquid-like state to the gaseous state in which particles become separated (not entangled). Using video and accelerometer measurements, we record the temporal evolution of the spatial density and pile-plate collisional impulse. A critical energy scale, set by the particle geometry and gravitational potential energy, governs the liquid-gas transition. [Preview Abstract] |
Session B15: Focus Session: Spins in Semiconductors - Spin Currents I
Sponsoring Units: DMP GMAG FIAPChair: David Awschalom, University of California, Santa Barbara
Room: D171
Monday, March 21, 2011 11:15AM - 11:51AM |
B15.00001: Optical effects of spin currents in semiconductors Invited Speaker: BANG-FEN ZHU, Department of Physics and Institute of Advanced Study, Tsinghua University, REN-BAO LIU, Department of Physics, The Chinese University of Hong Kong -- We predict the linear and second-order nonlinear optical effects of spin currents in semiconductors, based on systematic symmetry analysis and microscopic calculations with realistic models [1, 2]. By an analogue to the Ampere effect and Oersted effect, we conceived and verified that a spin current can be coupled to a ``photon spin curren'' carried by a polarized light beam, which causes sizeable Faraday rotation without involving net magnetization. Furthermore, a spin current can have a strong second-order nonlinear optical effect with unique polarization-dependence due to the special symmetry properties of the spin current. In particular, for a longitudinal spin current, in which the spins point parallel or anti-parallel to the current direction is a chiral quantity, a chiral sum-frequency effect will be induced. The second-order optical effects of spin currents have been experimentally verified immediately after the theoretical prediction [3]. These discoveries represent new phenomena in magneto-optics, with potential spin-photonic applications. They bring new opportunities to research of spintronics and may also facilitate research of topological insulators where the edge states form pure spin currents. References: \\[4pt] [1] J. Wang, B. F. Zhu, and R. B. Liu, \textit{Phys. Rev. Lett.} 100, 086603 (2008); see also Erratum, \textit{ibid }101, 069902 (2008) \\[0pt] [2] J. Wang, B. F. Zhu, and R. B. Liu, \textit{Phys. Rev. Lett.} 104, 256601 (2008). \\[0pt] [3] L. K. Werake and H. Zhao, \textit{Nature Physics} 6, 875 (2010). [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B15.00002: Magnetoelectric Photocurrent Generated by Direct Interband Transitions in InGaAs/InAlAs Two-Dimensional Electron Gas Junfeng Dai, Hai-Zhou Lu, Chunlei Yang, Shun-Qing Shen, Fu-Chun Zhang, Xiaodong Cui We report the observation of magnetoelectric photocurrent generated via direct interband transitions in an InGaAs/InAlAs two-dimensional electron gas by a linearly polarized incident light. The electric current is proportional to the in-plane magnetic field, which unbalances the velocities of the photoexcited carriers with opposite spins and consequently generates the electric current from a hidden spin photocurrent. The spin photocurrent can be evaluated from the measured electric current, and the conversion coefficient of spin photocurrent to electric current is self-consistently estimated to be 10$^{-3}$--10$^{-2}$ per Tesla. The observed light-polarization dependence of the electric current is well explained by a theoretical model which reveals the wave vector angle dependence of the photoexcited carrier density. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B15.00003: Anisotropic conductivity caused by spin-orbit interactions David H. Berman, Michael E. Flatt{\'e} Free propagation in a two-dimensional electron gas with both Rashba and Dresselhaus spin-orbit coupling shows strong anisotropy depending on the ratio of the coupling strength to the Fermi energy and on the ratio of the strengths of the Rashba and Dresselhaus interactions [1]. This spin-orbit induced anisotropy appears also in the local density of states near impurities. In addition the non-local conductivity, $\sigma_{i,j}({\bf r}, {\bf r'})$, computed in the absence of impurities is anisotropic. This is in contrast to the macroscopic conductivity in the presence of impurities which shows no anisotropy when only ladder diagrams are considered [2]. In all these instances, the degree of anisotropy can be controlled by application of electric fields perpendicular to the 2DEG. \\[4pt] [1] D. H. Berman and M. E. Flatt{\'e}, PRL {\bf 105}, 157202 (2010).\\[0pt] [2] O. Chalaev and D.Loss, Phys. Rev. B {\bf 71}, 245318 (2005). [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B15.00004: Drift and diffusion of spin and charge density waves in a two-dimensional electron gas Luyi Yang, J.D. Koralek, J. Orenstein, D.R. Tibbetts, J.L. Reno, M.P. Lilly We use transient grating spectroscopy (TGS) to study the persistent spin helix (PSH) state and electron-hole density wave (EHDW) in a 2D electron gas in the presence of an in-plane electric field parallel to the wavevector of the PSH or EHDW. By directly measuring the phase, we can measure the PSH and EHDW displacement with 10 nm spatial and sub-picosecond time resolution. We obtain both the spin diffusion and mobility and ambipolar diffusion and mobility from the TGS measurements of PSH and EHDW, respectively. The spin transresistivity extracted from the spin diffusion is in excellent agreement with the RPA theory of spin Coulomb drag (SCD). The spin mobility data indicate that SCD may also play a role in the spin wave drifting process. From the ambipolar diffusion and mobility, we obtain the transresistivity of electrons and holes in the same layer, which is much stronger than is typically seen in the conventional Coulomb drag experiments on coupled quantum wells. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B15.00005: Radial spin helix in two-dimensional electron systems with Rashba spin-orbit coupling Valeriy Slipko, Yuriy Pershin We suggest a new long-lived spin-polarization structure, a radial spin helix [1], and study its relaxation dynamics. For this purpose, starting with a system of equations for spin-polarization density, we find its general solution in the axially symmetric case. It is demonstrated that the radial spin helix of a certain period relaxes slower than homogeneous spin polarization and plain spin helix [2]. Importantly, the spin polarization at the center of the radial spin helix stays almost unchanged at short times. At longer times, when the initial nonexponential relaxation region ends, the relaxation of the radial spin helix occurs with the same time constant as that describing the relaxation of the plain spin helix. Experimentally, such a structure can be created using spin injection or extraction in a system with cylindrical electrodes or, possibly, by a modified spin gratings technique. \\[4pt] [1] Y. V. Pershin and V. A. Slipko, Phys. Rev. B 82, 125325 (2010).\\[0pt] [2] Y. V. Pershin, Phys. Rev. B 71, 155317 (2005). [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B15.00006: Anholonomic spin manipulation in drift transport in semiconductors Ben J. Moehlmann, Michael E. Flatt\'e We find that the electronic spin rotation induced by drift transport around a closed path in a wide variety of nonmagnetic semiconductors at zero magnetic field depends solely on the physical path taken. Physical paths that produce any possible spin rotation due to transport around a closed path are constructed for electrons experiencing strain or electric fields in (001), (110), or (111)-grown zincblende semiconductor quantum wells. Spin decoherence due to travel along the path is negligible compared to the background spin decoherence rate. The small size of the designed paths ($< 100$~nm scale in GaAs) may lead to applications in nanoscale spintronic circuits. This work was supported by an ONR MURI.\footnote{B. J. Moehlmann and M. E. Flatt\'e, arXiv:1007.0909} [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B15.00007: Mapping Spin-Orbit Splitting in Strained (In,Ga)As Epilayers B.M. Norman, C.J. Trowbridge, V. Sih, J. Stephens, A.C. Gossard, D.D. Awschalom Time-resolved and spatially resolved Faraday rotation spectroscopy is used to measure the magnitude and direction of the momentum-dependent spin splitting in strained InGaAs epilayers. The epilayers are lattice-matched to the GaAs substrate and designed to reduce inhomogeneous effects related to strain relaxation. Measurements of momentum-dependent spin splitting as a function of electron spin drift velocity along [100], [010], [110] and [1$\overline{1}$0] directions enable separation of isotropic and anisotropic effective magnetic fields that arise from uniaxial and biaxial strain along $\langle$110$\rangle$. Such electrically induced effective magnetic fields can be used for spin generation and manipulation in spintronics devices. We find that anisotropic and isotropic strain-induced effective magnetic fields are comparable in magnitude.~\footnote{B. M. Norman, C. J. Trowbridge, J. Stevens, A. C. Gossard, D. D. Awschalom, and V. Sih, Phys. Rev. B. {\bf 82}, 081304(R) (2010).} [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B15.00008: Spin-orbit interaction from low-symmetry localized defects in semiconductors Oleg Chalaev, G. Vignale, Michael Flatt\'e The presence of low-symmetry impurities or defect complexes in the zinc-blende direct-gap semiconductors (e.g. interstitials, Jahn-Teller distortions) results in a novel spin-orbit term in the effective Hamiltonian for the conduction band. The new spin-orbit interaction is proportional to the matrix element of the defect potential between the conduction and the valence bands. Because this interaction arises already in the first order of the expansion of the effective Hamiltonian in powers of $\Delta/E_g\ll1$ (where~$\Delta$ is the valence band spin-orbit splitting, and $E_g$ is the band gap), its contribution to the spin relaxation time may exceed that of previously studied contributions, such as the Rashba term, even for moderate concentrations of low-symmetry impurities. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B15.00009: Modeling of diffusion of injected electron spins in spin-orbit coupled microchannels Liviu P. Z\^arbo, Jairo Sinova, I. Knezevic, J. Wunderlich, T. Jungwirth Understanding of the collective electron spin dynamics under the influence of spin-orbit fields is a key requirement in the quest for all-electrical semiconductor spintronic devices. We investigate the spin dynamics of an ensemble of spin polarized electrons injected in the diffusive microchannel of a model device with linear Rashba and Dresselhaus spin-orbit coupling. Using a spin-dependent ensemble Monte Carlo method, we analyze the steady state spin density patterns dependence on channel dimension and orientation, spin-orbit coupling strengths and external magnetic fields. We show that in the persistent spin helix regime, the spin density patterns depend only on the system geometry and channel orientation. Magnetic fields of order of tesla are required to affect spin dynamics in the persistent spin helix regime. Our simulation results [PRB 82, 205320 (2010)] have been used to help understand the spin diffusion in the channel of the recently demonstrated spin Hall effect transistor [arXiv:1008.2844]. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B15.00010: Unitarity of scattering and edge spin accumulation in a ballistic and quasiballistic regimes Alexander Khaetskii, Eugene Sukhorukov We consider a 2D ballistic structure with spin-orbit-related splitting of the electron spectrum. We calculated the edge spin density which appears in the presence of a charge current through the structure. Combined effect of the boundary scattering and spin precession leads to oscillations of the edge polarization. The problem is solved with the use of the method of scattering states. We clarified the important role of the unitarity of scattering for the problem of edge spin accumulation. For Rashba Hamiltonian, which is linear in momentum, and in the case of a straight boundary it leads to exact cancellation of long-wave oscillations of the spin density with a period order of spin precession length. However, this appears to be rather exceptional case. In general, the smooth spin oscillations recover, as it happens, e.g., for the wiggly boundary. For qubic Hamiltonian (2D holes) the unitarity scattering conditions are different, as a result, even in the case of a straight boundary the cancellation of the smooth oscillations in spin density does not occur. Similar problem is considered for the case when the sample size is large compared to the mean free path which in its turn is much larger than the spin precession length. For example, for the cubic Hamiltonian the ``edge'' contribution to the spin density can be larger than the ``bulk'' one which appears as a result of the spin flux from the bulk. This demands the reinterpretation of the experimental results [1]. [1]. J. Wunderlich et al., PRL 94, 047204 (2005). [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B15.00011: Visualizing zitterbewegung in spin-orbit coupled semiconductor quantum wires Markku Jaaskelainen, Ulrich Zulicke We study a spin-orbit coupled parabolic quantum wire in the ballistic regime and develop a method for visualizing zitterbewegung in phase space. We introduce a Husimi distribution on the transverse coordinate and calculate the distribution of spin by a decomposition into Bloch vector components. The Husimi distribution corresponds to the simultaneous, unsharp measurement of the transverse position and velocity in accordance with the uncertainty principle. In phase space, the distribution exhibits a combination of spin precession and coherent oscillation along the longitudinal coordinate, i.e. zitterbewegung. This behavior closely matches the semiclassical dynamics for small values of the spin-orbit coupling. For increasing spin-orbit coupling strength, the oscillation amplitude initially increases, whereas for very large values the oscillation amplitude is quenched. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B15.00012: Theory of carrier dynamics in InSb parabolic quantum wells D. Saha, G.D. Sanders, C.J. Stanton, M. Bhowmick, T. Merritt, G.A. Khodaparast, T.D. Mishima, M.B. Santos InSb, with the narrowest gap among the III-V compound semiconductors, shows considerable promise as a quantum well material because its small conduction-band mass gives it a large room temperature electron mobility, and its large g-factor makes it attractive for spintronic devices. We present experiments and theoretical calculations for carrier dynamics in a strained 50-nm thick InSb/AlInSb parabolic quantum well. Our calculations are based on the 8-band Pidgeon-Brown model generalized to include the effects of the parabolic confinement potential as well as pseudomorphic strain. Optical properties are calculated within the golden rule approximation and compared with experiments. We model one and two color, time-resolved pump-probe differential transmission and reflectivity experiments. The change in the infrared probe pulse as a function of delay time provides information on carrier and spin relaxation dynamics. Both interband and intra-band dynamics are studied. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B15.00013: Imaging spin transport in a semiconductor with an iron-filled carbon nanotube Andrew Berger, Vidya Bhallamudi, Dominic Labanowski, Palash Banerjee, Camelia Marginean, Denis Pelekhov, David Stroud, P. Chris Hammel, Kathy McCreary, Roland Kawakami, Franziska Wolny, Thomas Muehl There has been much recent progress in the field of spintronic device fabrication, creating a need for characterization tools. We are developing a low-temperature scanned probe microscope with the ability to position, with high precision, a magnetized iron-filled carbon nanotube above a spin-injected semiconductor device [1]. The inhomogeneous field of this unique magnetic probe will be experienced by spins in the sample. We have developed a technique for simulating the effects of such an inhomogeneous field [2]. Crucially, we find that our scanned probe technique can create highly localized spin density features on a length scale comparable to the nanotube diameter. This will allow for spatial mapping of the spin density with high resolution -- a capability not possible in current electrical detection schemes. Such experiments may provide information about interface effects, scattering, and material properties which influence spin behavior. \\[4pt] [1] F. Wolny, et al. J. Appl. Phys. 104, 064908 (2008)\\[0pt] [2] V. Bhallamudi, et al. arXiv:1010.3747v1 [cond-mat.mes-hall] [Preview Abstract] |
Session B16: Focus Session: Magnetic Nanostructures II
Sponsoring Units: DMP GMAGChair: Stephane Mangin, Nancy-Universite
Room: D173
Monday, March 21, 2011 11:15AM - 11:27AM |
B16.00001: Electronic structure and electron spectroscopy of magnetic iron oxide nanoparticles J. Gazquez, J. Salafranca, M. Varela, S. Pennycook, S.T. Pantelides, P. Morales, N. Perez, A. Labarta, X. Batlle Magnetic iron oxide nanoparticles are good candidates for biomedical applications due to their low toxicity and easy functionalization. We synthesized magnetite (Fe3O4) nanoparticles by a high temperature decomposition method. They present some very desirable properties for applications: very high saturation magnetization, and excellent degree of crystallinity. Transmission electron microscopy images, and electron energy loss spectroscopy with atomic resolution allow a composition map that shows small variations in relative composition between the core and the surface, and subtle changes in the absorption spectra. Our density functional (DFT) calculations address different factors contributing to the magnetic properties. Changes in the electronic structure correlate with different features in the experimental absorption spectra, yielding a better understanding of the magnetic order. We study the role of structural defects, the organic surfactant, stoichiometry and the nominal oxidation state of iron, and their effect in determining the equilibrium magnetic state. This work is supported by DOE Materials Sciences and Engineering Division and the European Research Council Starting Investigator Award. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B16.00002: Magnetic properties of Fe/Fe$_{3}$O$_{4}$ core/shell nanostructure Vivek Singh, Mohindar Seehra, S. Bali, E. Eyring, N. Shah, F. Huggins, G. Huffman Magnetic properties of a core/shell nanostructure with spherical core of Fe/FeB and a shell of Fe$_{3}$O$_{4}$/$\gamma $-Fe$_{2}$O$_{3}$ are reported employing magnetometry, electron magnetic resonance (EMR) and M\"{o}ssbauer spectroscopy. This nanostructure was produced by reducing FeCl$_{3}$$\cdot$6H$_{2}$O with NaBH$_{4}$. Combining the results from XRD, TEM and M\"{o}ssbauer spectroscopy showed the nanostructure to consist of a core of diameter D$\simeq $20 nm containing both $\alpha $-Fe with D$\simeq $7 nm and amorphous Fe-B alloy and a shell of thickness 5 nm containing Fe$_{3}$O$_{4}$/$\gamma $-Fe$_{2}$O$_{3}$. Measurements of the magnetization M vs. temperature (2 K to 370 K) and in H upto 65 kOe show a blocking temperature T$_{B}\simeq $30 K associated with the oxide shell and ferromagnetism upto 370 K with nearly temperature-independent saturation M$_{S}\simeq $70 emu/g and coercivity H$_{C}\simeq $100 Oe. In EMR studies at 9.28 GHz, two lines are observed: a narrower line with linewidth $\Delta $H$\simeq $600 Oe and g$\simeq $2 and a broader line with $\Delta $H$\simeq $4200 Oe and g$\simeq $2.2. These parameters of the narrower line combined with its disappearance below 50 K suggests its origin to be the oxide shell whereas the broader line is due to Fe/FeB core. Research supported by U. S. Dept. of Energy, Contract {\#}DE-FC26-05NT42456. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B16.00003: Fe and Fe oxide nanoparticles ensembles with macroscopic anisotropy Miguel Angel Garcia We report here the fabrication Fe and Fe oxide nanoparticles over glass substrates exhibiting macroscopic anisotropy. Fe thin films were deposited onto glassy substrates by thermal evaporation and were subsequently annealed in air and argon atmosphere. The difference of thermal expansion coefficient between the substrate and the metallic film induces stresses in the substrate-metal interface leading to hole nucleation, growing and percolation, and finally to the formation of a metallic a nanoparticles layer. Anisotropic nanoparticles can be obtained by applying mechanical stress during the thin the film deposition or the annealing process. The applied stress induce anisotropy axis for the NPs shape that lead to the formation of elongated nanoparticles with macroscopic texture. Anisotropy can be increased by applying a magnetic field during thermal annealing. We analyze here the magnetization and anisotropy of individual nanoparticles and nanoparticles interactions and their relationship with the processing parameters. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B16.00004: Magnetic Core-Shell Morphology of Structurally Uniform Magnetite Nanoparticles Invited Speaker: Magnetic nanoscale structures are intriguing, in part, because of the exotic properties that emerge compared with bulk. The reduction of magnetic moment per atom in magnetite with decreasing nanoparticle size, for example, has been hypothesized to originate from surface disordering to anisotropy-induced radial canting, which are difficult to distinguish using conventional magnetometry. Small-angle neutron scattering (SANS) is ideal for probing structure, both chemical and magnetic, from nm to microns across an ensemble of particles. Adding polarization analysis (PASANS) of the neutron spin orientation before and after interaction with the scattering particles allows the magnetic structure to be separated into its vector components. Application of this novel technique to 9 nm magnetite nanoparticles closed-packed into face-centered crystallites with order of a micron revealed that at nominal saturation the missing magnetic moments unexpectedly interacted to form well-ordered shells 1.0 to 1.5 nm thick canted perpendicular to their ferrimagnetic cores between 160 to 320 K [1]. These shells additionally displayed intra-particle ``cross-talk'', selecting a common orientation over clusters of tens of nanoparticles. However, the shells disappeared when the external field was removed and interparticle magnetic interactions were negligible (300 K), confirming their magnetic origin. This work has been carried out in collaboration with Ryan Booth, Julie Borchers, Wangchun Chen, Liv Dedon, Thomas Gentile, Charles Hogg, Yumi Ijiri, Mark Laver, Sara Majetich, James Rhyne, and Shannon Watson.\\[4pt] [1] K.L. Krycka \textit{et al}., Phys. Rev. Lett. 104, 207203 (2010) [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B16.00005: Magnetic Characterization of Ferrite Nanoparticles Matthew Bryan, Paul Sokol, Greg Gumina, Lyudmila Bronstein, Bogdan Dragnea Magnetic nanoparticles (NPs) of different compositions (FeO/Fe$_{3}$O$_{4}$, g-Fe$_{2}$O$_{3}$, FePt, and CoFe$_{2}$O$_{4})$ have been synthesized using high temperature organometallic routes described elsewhere. NPs (16.6 nm in diameter) of a mixed FeO/Fe$_{3}$O$_{4}$ (wuestite/magnetite) composition were prepared by thermal decomposition or iron oleate in the presence of oleic acid as a surfactant in dodocane at 370C in argon atmosphere. After the thermal treatment of the reaction solution at 200 C under air for 2 hours these NPs are transformed into maghemite (g-Fe$_{2}$O$_{3})$, the magnetization of which is significantly enhanced. NPs of CoFe$_{2}$O$_{4}$ (8 nm) have been prepared by simultaneous decomposition of Co(II) and Fe(III) acetylacetonates in the presence of oleic acid and oleylamine. The X-ray diffraction profile of these NPs is characteristic of cobalt ferrite. Alternatively, alloyed 1.8 nm FePt NPs prepared by simultaneous decomposition of Fe and Pt acetylacetonates in the reductive environment demonstrate a completely disordered structure, which is reflected in their magnetic properties. SQUID magnetometry was used to measure the magnetization of NPs at high and low temperatures. Zero-field cooling and field-cooling measurements were taken to demonstrate superparamagnetic behavior and an associated blocking temperature. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B16.00006: Magnetically Tunable Polymer Nanocomposites for RF and Microwave Device Applications K. Stojak, S. Pal, H. Srikanth, C. Morales, J. Dewdney, J. Wang, T. Weller There has been much interest in polymer nanocomposites (PNC) recently due to potential applications for EMI shielding, tunable electromagnetic devices and flexible electronics. We report synthesis, structural, magnetic and RF characterization on PNCs ranging from 20-80 wt-{\%} loadings of Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ nanoparticles ($\sim $8nm) in a thermosetting resin from the Rogers Corporation. Nanoparticles were synthesized by thermal decomposition and characterized by XRD and TEM. Magnetic properties were studied using a Quantum Design PPMS. PNCs displayed characteristic features of superparamagnetism at room temperature and blocking at low temperature. Microwave transmission/reflection studies were done using a microstrip resonator. Strong tunability in the microwave absorption was observed. We extend our study to include nanoparticle-filled multi-walled carbon nanotubes synthesized by CVD. These high-aspect ratio magnetic nanostructures, with tunable anisotropy, are of particular interest in enhancing magnetic and microwave responses in existing PNCs. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B16.00007: Field dependence of T{\_}B in NiO and (Ni, Zn)O Nanoclusters Yung Huh, M. Peck, R. Skomski, R. Zhang, P. Kharel, M. Allison, D. Sellmyer, M. Langell Size dependence of magnetic properties of rocksalt NiO and Zn substituted NiO nanoparticles are investigated. Nanoparticle diameters are determined from 8 to 30 nm by XRD and AFM. Uncompensated spins at the nanoparticle surface contribute to superparametism at low temperatures and their blocking temperatures increase with stronger applied field. The field induced spin canting of the antiferromagnetic sublattices is a bulk effect and studied by the substitution of Zn with transition metal. Nanoparticles start exhibiting bulk magnetic behavior with size greater than 18 nm. Magnetization rotation of uncompensated spins under the magnetic field is mainly due to nanoscale size effect. The anisotropy of the nanoparticle is about four times larger than that of the bulk NiO. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B16.00008: Interacting Superparamagnetic Brownian Particles in an Array of 2D Asymmetric Magnetic Traps Gregory Vieira, Aaron Chen, R. Sooryakumar We report on the ordering and fluctuation of multiple superparamagnetic particles confined by a thin liquid layer in a two-dimensional array of asymmetric magnetic trapping potentials. The repulsive dipolar interaction between magnetic particles and their confinement by the trapping potential cause the particles to form a cluster with characteristic inter-particle spacing within each trapping site, while the particles undergo thermal fluctuations. Applying an external magnetic field offers a convenient way to control the strength of the dipolar interactions and change the trapping potential landscape. Results on (a) Brownian motion of individual particles in the cluster, (b) re-distribution of particles into new clusters driven by a change in the external field, and (c) hopping of particles between clusters under fluid flow will be presented. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B16.00009: Realization of a Bowl-like Potential and Its Confinement of Magnetic Microspheres Aaron Chen, Thomas Henighan, Gregory Vieira, Ratnasingham Sooryakumar Field-induced self-assembly of fluid-borne superparamagnetic microspheres not only has its importance in nanotechnology, but it also serves as a model for studies of phase transitions at the nano- to micro-meter scale. In this report, we experimentally demonstrate and theoretically account for the dynamics and structural order of a two-dimensional cluster of microspheres in the presence of a bowl-like potential. The potential is derived from magnetic patterns imprinted on the surface together with externally applied magnetic fields. Due to competition between the repulsive dipolar interaction amongst the microspheres and the confining force provided by the bowl-like potential, a cluster of microspheres with characteristic inter-sphere spacing is stabilized within the potential. The role of external magnetic fields which provide a convenient means to tune the strength of the dipolar interactions, and thereby control the relative importance of the two competing interactions, will be presented. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B16.00010: Tailored Magnetostructural Transitions Radhika Barua, Felix Jimenez-Villacorta, Donald Heiman, Laura H. Lewis Dominance of the surface atoms over the bulk atoms in nanoscaled magnetostructural systems may alter the ground state of the system and thereby change the transition character. Creation of a nanostructured magnetostuctural system was carried out via rapid solidification of (FeRh)$_{5}$Cu$_{95}$ to precipitate nanoscaled isolated FeRh precipitates in a Cu matrix upon annealing. Bulk FeRh has an abrupt antiferromagnetic - ferromagnetic transition around T =370 K. X-ray diffraction performed on the quenched (FeRh)$_{5}$Cu$_{95}$ alloy indicates only the presence of Cu of slightly expanded lattice parameter a=3.62 {\AA}, with ferromagnetism confirmed at room temperature by SQUID magnetometry. Vacuum annealing at 200 $^{\circ}$C causes a secondary phase to appear with an abrupt magnetic phase transition at T$_{t}$ = 130 K. Details of the magnetic behavior of this nanostuctured phase will be discussed. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B16.00011: Synthesis of Ferrimagnetic Fe$_{3}$Se$_{4}$ Nanostructures with Giant Coercivity Hongwang Zhang, Gen Long, Da Li, Hao Zeng In this study, we present the synthesis of Fe$_{3}$Se$_{4}$ nanostructures by a one-pot high temperature organic solution-phase method. The size of these nanostructures can be tuned from 50 to 500 nm and their shapes can be varied from nanosheets and nano-cactus to faceted nanoparticles by changing the precursors and reaction conditions. These nanostructures exhibit hard magnetic properties, with giant coercivity values reaching 40 kOe at 10 K, and 4 kOe at room temperature. The estimated lower bound of the magnetocrystalline anisotropy constant is 6$\times $10$^{6}$ erg/cm$^{3}$, comparable to that of hcp Cobalt. The large coercivity/anisotropy is rare for compounds without noble metal or rare-earth elements. If Fe$_{3}$Se$_{4}$ based phases can be doped to enhance their Curie temperature and magnetization, they can be a low cost, non-toxic alternative to noble metal or rare earth based advanced magnets. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B16.00012: Synthesis and Magnetic Properties of FePt@MnO Nano-\textit{hetero}-Particles Thomas Schladt, Tanja Graf, Oskar Koehler, Kerstin Schneider, Wolfgang Tremel Monodisperse FePt@MnO nano-\textit{hetero}-particles with different sizes and morphologies were prepared by a seed-mediated nucleation and growth technique. Both, size and morphology of the individual domains could be controlled by adjustment of the synthetic parameters. As a consequence, different particle constructs, including dimers, dumbbells and flowers, could be obtained by changing the polarity of the solvent. The FePt@MnO nano-\textit{hetero}-particles were thoroughly characterized by (HR-)TEM- and XRD analysis and SQUID magnetometry. Due to a sufficient lattice match, the MnO NPs preferentially grow on the (111) surfaces of the \textit{fcc}-FePt seeds. Furthermore, the surface spins of the antiferromagnetic MnO domains pin the magnetic moments of the ferromagnetic FePt NPs which leads to an exchanged biased magnetic hysteresis. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B16.00013: Electron-electron correlations and magnetic properties of small FePt clusters Alamgir Kabir, Neha Nayyar, Volodymyr Turkowski, Talat S. Rahman We have applied the DFT+U and the Nanoscale Dynamical Mean-Field Theory (NDMFT) [1] approaches to study the magnetic properties of small FePt clusters. The role of correlation effects in determining the geometry and the magnetic properties of the clusters as a function of chemical composition and the effective local Coulomb repulsion energy U is examined. We find that the magnetization to decrease with increasing number of Pt atoms. Interestingly, contrary to the bulk case, Pt clusters have nonzero magnetization. The magnetic properties are found to be very sensitive to the value of U, as a result of the dependence of the single particle energy levels on this parameter. Dynamical correlation effects, which are taken into account in the DFT+DMFT approach and which lead to an increase of the average in time double occupancy of the d-orbitals, results in a significant decrease of magnetization as compared to the results from the DFT+U case. \\[4pt] [1] V. Turkowski et al, J. Phys.: Condens. Matt. 22, 462202 (2010) [Preview Abstract] |
Session B17: Focus Session: Bulk Properties of Complex Oxides - Manganites II
Sponsoring Units: GMAG DMPChair: Gang Cao, University of Kentucky
Room: D174
Monday, March 21, 2011 11:15AM - 11:51AM |
B17.00001: Dynamics of bi-stripes and a colossal metal-insulator transition in the bi-layer manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ $(x\sim0.59)$ Invited Speaker: Electronic phases with stripe patterns have been intensively investigated for their vital roles in novel properties of correlated electronic materials. How these real-space patterns affect the conductivity and other properties of materials (which are usually described in momentum space) is one of the major challenges of modern condensed matter physics. By studying the electronic structure of La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ $(x\sim0.59)$ and in combination with earlier scattering measurements, we demonstrate the variation of electronic properties accompanying the melting of so-called bi-stripes in this material. The static bi-stripes can strongly localize the electrons in the insulating phase above $T_c\sim160$K, while mobile electrons grow up and coexist with a significant portion of localized electrons when the static bi-stripes melt below $T_c$. The presence of localized electrons below $T_c$ suggests that the melting bi-stripes exist as a fluctuating counterpart. From static to melting, the bi-stripes lead to a ``colossal'' metal-insulator transition in this material. Work was done in collaboration with Q. Wang, A. V. Fedorov, H. Zheng, J. F. Mitchell, D. S. Dessau. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B17.00002: Probing multiple magnetic transitions and phase coexistence in mixed phase manganites M.H. Phan, N.S. Bingham, H. Srikanth, C.L. Zhang, S.W. Cheong La$_{5/8-y}$Pr$_{y}$Ca$_{3/8}$MnO$_{3 }$(LPCMO) manganites exhibit a complex phase diagram due to coexisting and competing magnetic and electronic phases. A complete understanding of the origin of phase coexistence and separation in this system has remained elusive. To resolve this, it is essential to employ experimental methods that allow detailed investigations of the temperature and magnetic field response of the different phases. In this study we introduce magnetocaloric effect (MCE) and radio-frequency transverse susceptibility (TS) experiments as being ideally suited for this purpose. While MCE is generally considered in the community as an ``applied'' measurement tool to characterize magnetic refrigerant materials, we demonstrate that it is actually a very useful probe of magnetic transitions and ground state magnetic properties in LPCMO. TS experiments probe a phase conversion between the charge-ordered and ferromagnetic phases and magnetic field-induced kinetic arrest. Our studies provide an important understanding of the phase coexistence and separation in mixed phase systems like LPCMO. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B17.00003: Soft x-ray investigation on the spin and orbital states of La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$ K.-T. Ko, H. Jang, J.-H. Park, B.-G. Park, J.-Y. Kim, Sung Baek Kim, S-W. Cheong The spin and orbital states of La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$ was investigated by using the x-ray absorption spectroscopy (XAS) and the soft x-ray resonant scattering (SXRS) at Mn $L_ {2,3}$-edge. The field induced spin reorientation transition was observed by SXRS. The polarization dependent analysis revealed that the AFM spin axis changes from out-of-plane to in-plane axis. Additionally, the orbital states were determined from the polarization dependent XAS and CI model calculation, where the orbital state were changed by cooling temperature and external magnetic field. Here, the orbital states of low temperature ferromagnetic and field induced ferromagnetic are identical. Finally, we discuss the magnetoelastic coupling including spin and orbital structure of La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B17.00004: Optical Investigation on Collective Dynamics of Charge-Orbital Density Wave in Layered Manganites Jun Fujioka, Yoshiaki Ida, Youtarou Takahashi, Noriaki Kida, Ryo Shimano, Yoshinori Tokura We have investigated the broad band optical spectra on the layered manganites $R_{1-x}$Sr$_{1+x}$MnO$_4$ (R=Nd and La) to reveal the collective charge/orbital density wave dynamics by means of the terahertz time domain spectroscopy [1]. The collective charge/orbital density wave excitation is observed around 9 meV in the charge/orbital stripe phase, when the nominal $e_g$-electron filling (1-$x$) of Mn-ion is less than around 1/3. By contrast, such a collective mode almost vanishes at $x$=1/2, which is explained in terms of the enhanced Jahn-Teller interaction cooperative with the electron correlation effect. \\[4pt] [1] J. Fujioka {\it et al.}, Phys. Rev. B. {\bf 82}, 140409(R) (2010). [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B17.00005: Bilayer manganites: polarons in the midst of a metallic breakdown Mark Golden, Freek Massee, Sanne de Jong, Yingkai Huang, Andrew Boothroyd, D. Prabhakaran, Rolf Follath, Andrei Varykhalov, Luc Patthey, Ming Shi, Jeroen Goedkoop The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistive (CMR) transition is still a flagship issue in emergent correlated matter research. By combining new photoemission and tunneling data, we show that in the bilayer ($N$=2) manganite La$_{\mathrm{2-2x}}$Sr$_{\mathrm{1+2x}}$Mn$_2$O$_7$ the lattice/spin/orbital polaronic degrees of freedom win out, all across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong interactions result in vanishing coherent quasi--particle spectral weight at the Fermi level for all locations in $k$--space. The incoherence of the charge carriers offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopic data. The stacking number $N$ is the key factor for true metallic behavior, as an intergrowth-driven breakdown of the polaronic domination to give a robust metal possessing a traditional Fermi surface is seen in the bilayer system. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B17.00006: Synthesis, Structure, and Physical Properties of Ba$_{2}$Mn$_{2}$Sb$_{2}$O Single Crystals Jianneng Li, S. Stadler, A. Karki, Y. Xiong, R. Jin We have grown high-quality single crystals of Ba$_{2}$Mn$_{2}$Sb$_{2}$O, which possesses the hexagonal structure as determined by X-ray powder diffraction technique. The magnetic susceptibility ($\chi )$ is isotropic above T$_{N }\sim $ 60 K, initially increasing with increasing temperature (T). After reaching the maximum at T$_{MAX} \quad \sim $ 150 K, $\chi $ decreases with increasing T and can be described by Curie-Weiss law with negative Curie-Weiss temperature. Below T$_{N}$, magnetic anisotropy is observed: $\chi _{ab}$ decreases sharply but $\chi _{c}$ increases with decreasing T, suggesting an antiferromagnetic type ordering at T$_{N}$. Interestingly, the temperature dependence of electrical resistivity along both \textit{ab} plane and $c$ direction changes from exponential dependence above T$_{MAX}$ to logarithmic dependence below T$_{MAX}$, reflecting strong Kondo effect. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B17.00007: Evaluating Born and local effective charges in nanoscale MnO Q.-C. Sun, X.S. Xu, S.N. Baker, A.D. Christianson, J.L. Musfeldt Phonons are exquisitely sensitive to finite length scale effects in complex materials because they are intimately connected to charge, polarizability, and structure, and a quantitative analysis of their behavior can reveal microscopic aspects of chemical bonding. To investigate these effects in a model correlated oxide, we measured the infrared vibrational properties of 8 nm particles of MnO, compared the results with the analogous bulk material, and quantified the phonon confinement with a calculation of Born effective charge. Our analysis reveals that the Born effective charge decreases by $\sim$20\% compared to the bulk material. This finding is important for understanding finite length scale effects in this simple binary oxide and the more complicated functional oxides that emanate from this parent compound. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B17.00008: Collective Phase Mode and the Role of lattice distortions at T$_{N}\sim $T$_{C}$ in XMn$_{2}$O$_{5}$ (X= Bi, Pr, Sm, Gd, Tb) N.E. Massa, A.F. Garc\'Ia Flores, E. Granado, G.F.L. Fabbris, G. de M. Azevedo, L. del Campo, D. De Sousa Meneses, P. Echegut, M.J. Mart\'Inez-Lope, J.A. Alonso We report on detailed temperature dependent infrared reflectivity, Raman, local structure, and X-ray diffraction measurements of multiferroic XMn$_{2}$O$_{5}$ (X= Bi, Pr, Sm, Gd, Tb). While for BiMn$_2$O$_5$ there are weak but distinct spectroscopic changes that together with high resolution diffraction patterns suggest a lattice role at T$_{N}\sim $T$_{C}$, for the rare earth (R) replaced infrared spectra have as a main feature a broad band at meV energies in addition to progressive rotation of Mn-O polyhedra. That band, independent of the R$^{3+}$ ion size and common to all, suggests hopping of carriers through fluctuations at a local scale. It partially condenses below 40 K, i.e., the collective electronic behavior changes from delocalized to one partially localized. We assimilate that condensate to a CDW-like phase mode. It might indicate induced orbital correlation of charge transfers between Mn sites.Frequency Raman phonon shifts are observed at T$\sim $60 K, due to spin-phonon coupling, and at T$_{N}\sim $T$_{C}$. Below T$_{N}\sim $T$_{C}$ there is no a Raman soft mode that might be associated to a CDW amplitude mode. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B17.00009: Korringa-like relaxation in $A$-site ordered manganites S. Schaile, H.-A. Krug von Nidda, A. Loidl, J. Deisenhofer, T. Nakajima, Y. Ueda Half doped manganites exhibiting charge- and orbital ordering transitions are paradigm materials for studying colossal magnetoresistance, the existence of ferroelectricity or Zener polaron-type features. We report on high-temperature electron spin resonance studies of $A$-site ordered and disordered $A$BaMn$_2$O$_6$ $A=$Y,Sm,La. We find a Korringa-like spin-relaxation above the charge-ordering transition and extending up to 1000~K in the $A$-site ordered antiferromagnetic systems $A=$Y,Sm, a unique feature for a truly metallic state not having been reported in manganites before. In agreement with the ESR intensity this finding suggests that the ESR signal stems from Mn$^{4+}$ core spins which relax via the quasi-delocalized $e_g$ electrons. In contrast in the disordered AFM and the ferromagnetic samples no Korringa relaxation is observed. Hence, the conductivity does not significantly influence the spin relaxation in those compounds. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B17.00010: Magnetic order in La$_{1-x}$Ca$_{x}$MnO$_{3}$ nanopowder Chetan Dhital, Clarina Dela Cruz, Kefong Wang, Jun-Ming Liu, Zhifeng Ren, Stephen Wilson Here we present neutron diffraction studies exploring the spin behavior in nanocrystalline La$_{1-x}$Ca$_{x}$MnO$_{3}$ (LCMO). Confinement effects and the influence of phase separation have long been key issues within the underlying electronic behavior of the manganites. The coexistence of competing electronic phases has been reported across length scales exceeding 100nm in bulk manganites in proximity of the first-order metal-to-insulator phase transition in their phase diagrams. When the grain size of manganite crystals approaches the size of intrinsically phase-separated domains, new magnetic phases can be stabilized and the resulting electronic behavior dramatically altered. In this talk, we present results from our recent investigations of the magnetism in LCMO samples whose grain-size has been reduced to the nanometer scale. Newly stabilized static spin order and its relevance to phase separation in this manganite system will be discussed. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B17.00011: Density functional investigation of the spin frustration and the field-driven long-range ordering in the honeycomb lattice system Bi$_{3}$Mn$_{4}$O$_{12}$(NO$_{3})$ Jia Liu, Won-joon Son, Mike Whangbo Bi$_{3}$Mn$_{4}$O$_{12}$(NO$_{3})$, consisting of the honeycomb lattices of Mn$^{4+}$ (d$^{3})$ ions, has dominant antiferromagnetic interactions ($\theta _{CW}=-$257 K) but its spins do not order down to 0.4 K. However, applied magnetic fields induce a long-range magnetic order, which is believed to arise from the spin canting due to the Dzyaloshinskii-Moriya interaction. To explain these observations, we examined the spin exchanges between the Mn$^{4+}$ ions (J$_{1}$, J$_{2}$, J$_{c})$ by DFT+U calculations and the preferred orientation of their spins by DFT+U+SOC calculations. The spin frustration is reproduced by U close to zero with J$_{2}$/J$_{1}$ $\approx $ 1/2. The cause for the field-induced long-range magnetic ordering was explored on the basis of DFT+U+SOC calculations. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B17.00012: New magnetic structures in novel and conventional manganites Aziz Daoud-aladine, Juan Rodr\'Iguez-Carvajal, Cristian Perca, Loreynne Pinsard-Gaudart The determination of the magnetic structures of manganites has always been at the root of their fundamental understanding [1]. We studied the magnetic structures of half-doped charge ordered manganites that are either show the prototype [1] CE-type magnetic structure (Pr$_{1/2}$Ca$_{1/2}$MnO$_{3}$), or variants of this order (YBaMn$_2$O$_{6}$ [2] and Pr$_{0.6}$Ca$_{0.4}$MnO$_{3} $) with neutron diffraction. The study of Pr$_{1/2}$Ca$_{1/2}$MnO$_{3}$ (ILL, France) is the first ever done on a single crystal and it essentially confirms the pioneering picture [1], whereas the NPD studies of YBaMn$_2$O$_{6}$ [2] (PSI, Switzerland) and Pr$_{0.6}$Ca$_{0.4}$MnO$_{3}$ [3] (ISIS, UK), give two unprecedented results. The YBaMn$_2$O$_{6}$ magnetic structure corroborates the hotly debated ordering of Zener Polarons [4], and high resolution NPD data evidence a new spin reorientation transition around T$\sim$20K far below its TN$\sim170K$ in Pr$_ {0.6}$Ca$_{0.4}$MnO$_{3}$ [3] that has so far only been vaguely observed. We will discuss the consequences that these results have on the still hotly debated understanding of the connection between charge/orbital and spin orderings in the manganites. [1] Wollan, E.O. and Koehler, W.C. Rev. 100, 545 (1955) [2] A. Daoud-Aladine et al., Phys. Rev. Lett.: 101 166404 (2008) [3] A. Daoud-Aladine et al., unpublished [4] M. Coey Nature 430, 155-157 (8 July 2004) [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B17.00013: ABSTRACT WITHDRAWN |
Session B18: Focus Session: Low D/Frustrated Magnetism - Quantum Magnetism
Sponsoring Units: GMAG DMPChair: Adrian del Maestro, John Hopkins University
Room: D172
Monday, March 21, 2011 11:15AM - 11:27AM |
B18.00001: Abelian and Non-Abelian Height Models R. Zach Lamberty, Stefanos Papanikolaou, Chris Henley We present Monte Carlo simulations on a new class of lattice models in which the degrees of freedom are elements of an abelian or non-abelian finite group $G$, placed on directed edges of a two-dimensional lattice. The group product around any plaquette is constrained to be the group identity, as in a discrete gauge model, but in contrast a ``height model" only allows a certain subset of group elements to appear on edges. These models often realize a classical form of topological order, in that the ensemble breaks up into sectors labeled by loop products (group products taken around topologically non-trivial loops). Our implementation uses a non-local Monte Carlo update, whereby a pair of topological defects is created and later recombined after one diffuses; this allows the simulation to visit different topological sector. We measured two quantities as diagnostics of topological order (i) The relative probabilities of different sectors, which were found to converge to unity with increasing system size $L$. (ii) The probability distribution of the separation $R$ of a defect pair, which should approach a constant (be deconfined). Both results show exponential decay as a function of $L$ or $R$, as expected for a liquid-like phase having only topological order. As a check, we measured the same two quantities in a model equivalent to the 6-vertex model, known to be a critical state, and confirmed the algebraic decay in that case. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B18.00002: Application of DFT+U for calculating magnetic parameters for manganese based molecular magnets Shruba Gangopadhyay, Artem Masunov Single-molecule magnets are promising materials for molecular spintronics and quantum computing applications. Two methods feasible to predict the exchange coupling parameters of molecular magnets, broken symmetry Density Functional Theory and DFT with empirical Hubbard U parameter (DFT+U). In this contribution we apply DFT+U to study magnetic coupling for two Mn12-based molecular magnetic wheel using Vanderbilt Ultrasoft Pseudopotential plane wave DFT method implemented in Quantum ESPRESSO. Unlike most previous studies, we adjust U parameters for both metal and ligand atoms using five dineuclear organometallics as the benchmarks. Our study finds antiparallel spin alignment of the weakly interacting fragments of Mn$_{12}$, while the magnetic coupling inside the fragments are much stronger, both are in agreement with experimental observations. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B18.00003: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 11:51AM - 12:03PM |
B18.00004: DMRG Study of Anisotropic Triangular Heisenberg Lattice Andreas Weichselbaum, Steven R. White The anisotropic antiferromagnetic two-dimensional triangular Heisenberg lattice for spin $1/2$ describes certain classes of transition-metal oxides (TMOs) and chalcogenides (TMCs) supported by experimental data. The understanding of the ground state properties of this frustrated system from a theoretical point of view, however, has remained an extraordinary challenge. In the model under consideration, quasi-one-dimensional Heisenberg chains of uniform intrachain coupling strength $J$ interact with their neighboring chains via the triangular interchain coupling $J'$. By varying the anisotropy ratio $j=J'/J$ from $j=0$ (decoupled Heisenberg chains) to $j=1$ (uniform triangular lattice with finite Neel order like local magnetization), it was pointed out [1,2] that spin liquid properties up to remarkably high values of $j$ of about 0.85 exist. We present in detail our results on the incommensurable correlations using DMRG with special care given to finite size effects. We argue that incommensurable correlations persist throughout the entire range of $j\in[0,1]$.\\ $[1]$ S. Yunoki et al., PRB 74, 014408 (2006).\\ $[2]$ D. Heidarian et al., PRB 80, 012404 (2009). [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B18.00005: Interacting antikinks on a diamondback ladder I Mayra Tovar, Kirill Shtengel Recently introduced ``antikinks'' are spin 1/2 excitations of the Heisenberg antiferromagnet on a sawtooth lattice. The idea is that they mimic spinons of the kagome antiferromagnet. Antikinks are triangles of spins which are not in their ground state. Treating antikinks as free non- interacting particles (a good approximation for the sawtooth chain), their energy was found to be substantially reduced by delocalization. We study antikinks on a ``diamondback'' ladder in which all spins are shared between two triangles. Consequently, in a uniform case the concentration of antikinks becomes 1/4 and they strongly interact, making such a model a much better approximation for the kagome case. We treat these effects perturbatively by allowing different Heisenberg couplings on the up- and downward oriented triangles, the two limiting cases being the sawtooth and uniform diamondback ladder. We find a non-monotonic, power-law decay of induced interactions between the antikinks with their separation. The consequences of these interactions will be discussed in this talk. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B18.00006: Interacting antikinks on a diamondback ladder II Kirill Shtengel, Mayra Tovar Recently introduced ``antikinks'' are spin 1/2 excitations of the Heisenberg antiferromagnet on a sawtooth lattice [1]. The idea is that they mimic spinons of the kagome antiferromagnet. Antikinks are triangles of spins which are not in their ground state. Treating antikinks as free non-interacting particles (a good approximation for the sawtooth chain), their energy was found to be substantially reduced by delocalization [1]. We study antikinks on a ``diamondback'' ladder in which all spins are shared between two triangles. Consequently, in a uniform case the concentration of antikinks becomes 1/4 and they strongly interact, making such a model a much better approximation for the kagome case. We treat these effects perturbatively by allowing different Heisenberg couplings on the up- and downward oriented triangles, the two limiting cases being the sawtooth and uniform diamondback ladder. We find a non-monotonic, power-law decay of induced interactions between the antikinks with their separation. The consequences of these interactions will be discussed in this talk. \\[4pt] [1] Z. Hao and O. Tchernyshyov, Phys. Rev. Lett. \textbf{103}, 187203 (2009) [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B18.00007: Partial Kondo screening in geometrically frustrated Kondo lattice systems Yukitoshi Motome, Kyoya Nakamikawa, Youhei Yamaji, Masafumi Udagawa One of the most important concepts in Kondo lattice systems is competition between the Kondo coupling and the RKKY interaction. The competition leads to a quantum critical point between a magnetically-ordered state and a Fermi liquid state, and furthermore, it is the origin of novel phenomena around the quantum critical point, such as a non-Fermi liquid behavior and a superconductivity. To explore a new quantum phase resulting from the competition, we investigate the ground state of geometrically-frustrated Kondo lattice systems by employing a high-precision variational Monte Carlo simulation. We find that a partially-ordered state, in which a magnetic order and a Kondo spin singlet coexists, emerges between a magnetically-ordered state stabilized by the RKKY interaction and a Kondo spin liquid state stabilized by the Kondo coupling. We clarified that this new quantum phase is stabilized by quantum fluctuations as well as magnetic anisotropy, and that it is accompanied by a charge disproportionation. Ref. Y. Motome {\it et al.}, Phys. Rev. Lett. {\bf 105}, 036403 (2010). [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B18.00008: Study of SU(N) magnets on the cubic lattice Hao Song, Michael Hermele We consider a class of $SU(N)$ magnets that have the same spin on every lattice site, which is obtained as the completely antisymmetric tensor product of $m < N$ fundamental representations. These models, which can be realized in ultracold gases of alkaline earth atoms in optical lattice potentials, have the remarkable property that more than two spins must be combined to form a singlet. A recent study of this model on the square lattice in the large-$N$ limit found a chiral spin liquid ground state with topological order. Inspired by this result, we have studied the three-dimensional version of this model, solving it on the cubic lattice in the large-$N$ limit, which addresses the competition among a variety of non-magnetic states, including some with exotic order. We present results on the phase diagram as the fraction $m/N$ is varied. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B18.00009: Trial wave function for the quantum Ising model at zero temperature Julio F. Fern\'andez A trial wavefunction for the ground state of the transverse field Ising model is proposed. It is a product of pair wavefunctions, which is exact for up to three spins, and is amenable to Monte Carlo calculations. We study the phase transition that occurs at zero temperature as the transverse field varies. Results for the Ising ferromagnet and some spin-glass models will be given. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B18.00010: Thermodynamics of deconfined bosonic spinons in two dimensions Valeri Kotov, Anders Sandvik, Oleg Sushkov We consider the quantum phase transition between a Neel antiferromagnet and a valence-bond solid (VBS) in a two-dimensional system of $S=1/2$ spins. Assuming that the excitations of the critical ground state are linearly dispersing deconfined spinons obeying Bose statistics, we derive expressions for the specific heat and the magnetic susceptibility at low temperature $T$. Comparing with quantum Monte Carlo results for the J-Q model, which is a candidate for a deconfined Neel--VBS transition, we find excellent agreement, including a logarithmic correction in the susceptibility. In our treatment, this is a direct consequence of a confinement length scale $\Lambda \propto \xi^{1+a} \propto 1/T^{1+a}$, where $\xi$ is the correlation length and $a>0$ (with $a\approx 0.2$ in the model). \newline Reference: A. W. Sandvik, V. N. Kotov, and O. P. Sushkov, arXiv:1010.2522 (2010). [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B18.00011: Properties of Resonating-Valence-Bond Spin Liquids and Critical Dimer Models Ying Tang, Anders W. Sandvik, Christopher L. Henley We use Monte Carlo simulations to study properties of resonating-valence-bond (RVB) spin liquid states for $s=1/2$ spins on 2D square lattices. It is well known that the spin-spin correlations decay exponentially in these states, but we find that the four-spin (valence-bond-solid, VBS, type) correlations are critical [1]. We compare various properties of the RVB with those of the classical dimer model (CDM), i.e., the exact ground state wavefunction of the critical Rokhsar-Kivelson quantum dimer model. It is well known that the CDM maps to a height model with a gradient-squared elasticity governed by a stiffness constant $K$. We show that also the RVB has such an effective classical field theory description, namely its (i) four-spin (dimer) correlations (ii) probabilities of different winding number sectors, and (iii) separation of monomer defect pairs, are all consistent with the same value of $K$ (which is higher than in the CDM, i.e., the RVB is closer to an ordered VBS state). In addition to the short-bond RVB we also consider systems with longer bonds, and again find consistency with the height-model description. We discuss implications of the critical fluctuations of the RVB states. \\[4pt] [1] Y. Tang, A. W. Sandvik, and C. L. Henley, arXiv:1010.6146. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B18.00012: Quantum spin liquid in two-dimensional Kagome lattice spin-1/2 XY model with 4-site ring exchange Long Dang, Roger Melko We have studied the 2D Kagome lattice spin-1/2 XY model with 4-site exchange. The ground state properties are investigated within the framework of the Stochastic Series Expansion quantum Monte Carlo (QMC) technique. We have found a featureless insulating phase in the regime of large 4-site exchange interaction. This novel phase is a potential candidate for a the $Z_2$ quantum spin liquid phase proposed by Balents, Girvin and Fisher [Phys. Rev. B, ${\bf 65}$, 224412 (2002)] in a related model. Our efforts to characterize this phase using large-scale QMC simulations are also discussed. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B18.00013: Linear independence of nearest neighbor valence bond states on several 2D lattices Julia Wildeboer, Alexander Seidel We show for several two-dimensional lattices that the spin-$1/2$ nearest neighbor valence bond states are linearly independent. To do so, we utilize and further develop a method recently introduced [1] for the kagome lattice. This method relies on the identification of an appropriate cell for the respective lattice, for which a certain local linear independence property can be demonstrated. Whenever this can be achieved, linear independence follows for arbitrarily large lattices that can be covered by such cells, for open or periodic boundary conditions. We report that this method is applicable to a number of 2D lattices including the kagome, honeycomb, square, pentagonal I and II, and the star lattice. Applications of general linear independence properties, such as the derivation of effective quantum dimer models, are discussed. Furthermore, motivated by a spin-$1/2$ Hamiltonian on the kagome lattice that has Anderson's resonating-valence-bond (RVB) spin liquid wave function(s) as ground state(s) [1], we mention possibilities to study the properties of this RVB wave function for the kagome and other frustrated lattices using Monte Carlo techniques. $[$1$]$ A. Seidel, Phys. Rev. B \textbf{80,} 165131 (2009). [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B18.00014: Symmetry Fractionalization in Two Dimensions Hong Yao, Liang Fu, Xiao-Liang Qi Topologically ordered states are often characterized by topological properties, such as braiding statistics and fusion rules, of their excitations. However, excitations also carry symmetry quantum number, namely a representation of a symmetry group, when a topologically ordered state respects the symmetry. If an excitation's symmetry quantum number cannot be obtained from a finite integer number of fundamental constituents of the system, we propose to call such phenomena ``symmetry fractionalization.'' We introduce a solvable SO(3) spin-rotational and time reversal invariant spin-1 model on the honeycomb and decorated honeycomb lattices. We show that the ground state is the equal-weight superposition of all valence loops, which we call ``resonating valence loop'' (RVL) state and which is a quantum spin liquid respecting all the symmetries of the model. Ends of broken loops are excitations with spin-1/2, which are deconfined spinons. Since spin-1/2 cannot be obtained from an integer numbers of spin-1, the system exhibits symmetry fractionalization (specifically the ``SO(3) symmetry fractionalization''). Moreover, for time-reversal $T$, a spinon has $T^{2}$ = -1, while integer spins have $T^{2}$ = +1. Consequently, the system also has ``time-reversal fractionalization.'' [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B18.00015: Defect-driven phase transitions out of the Coulomb phase Hyejin Ju, Simon Trebst, Christopher Henley Lattice models constrained by a local ``conservation law,'' such as close-packed dimer models on 3D bipartite lattices, exhibit an emergent ``Coulomb phase'' with characteristic power-law correlations. We have studied, by Monte Carlo simulations, phase transitions out of the Coulomb phase induced by introducing a finite fugacity of defect excitations in dimer models. We report two cases. (1) In the simple cubic dimer covering, we admit non- bipartite dimers (connecting sites in the same sublattice), which appear as effective charges with Coulomb-like interactions. Non- bipartite defects induce a transition immediately out of the Coulomb phase, exponentially damping the critical correlations via Debye screening. We characterize this transition by extracting the screening length from our numerical calculation of the dimer structure factor.(2) In the diamond lattice, we initially restrict the dimers to a 2D layer forming a (bipartite) honeycomb lattice, and then admit interlayer dimers. These bipartite dimers appear as dipoles and do not destroy the Coulomb phase, but induce an immediate transition from a 2D to 3D Coulomb phase. [Preview Abstract] |
Session B19: Focus Session: Spin Transport & Magnetization Dynamics in Metals II
Sponsoring Units: GMAG DMPChair: Hendrik Ohldag, SLAC National Accelerator Laboratory
Room: D170
Monday, March 21, 2011 11:15AM - 11:51AM |
B19.00001: Effects of disorder on magnetic vortex dynamics Invited Speaker: Experimental measurements of domain wall propagation are typically interpreted by comparison to reduced models that ignore both the effects of disorder and the internal dynamics of the domain wall structure. Using micromagnetic simulations, first we study vortex wall propagation in magnetic nanowires induced by fields or currents in the presence of disorder. We show that the disorder leads to increases and decreases in the domain wall velocity depending on the conditions. These results can be understood in terms of an effective damping that increases as disorder increases. As a domain wall moves through disorder, internal degrees of freedom get excited, increasing the energy dissipation rate [1]. Next we study the effect of disorder on vortex gyration in a magnetic disc. A vortex gyrating in a magnetic disc has two regimes of motion in the presence of disorder. At large gyration amplitudes, the vortex core moves quasi-freely through the disorder potential. As the amplitude decreases, the core can become pinned at a particular point in the potential and precess with a significantly increased frequency. In the pinned regime, the amplitude of the gyration decreases more rapidly than it does at larger precession amplitudes in the quasi-free regime. In part, this decreased decay time is due to an increase in the effective damping constant and in part due to geometric distortion of the vortex. A simple model with a single pinning potential illustrates these two contributions [2]. \\[4pt] [1] Hongki Min, Robert D. McMichael, Michael J. Donahue, Jacques Miltat, and M. D. Stiles, Phys. Rev. Lett. {\bf 104}, 217201 (2010).\\[0pt] [2] Hongki Min, Robert D. McMichael, Jacques Miltat, and M. D. Stiles (unpublished). [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B19.00002: Spin-torque-driven excitations in magnetic thin films C. Wang, H. Seinige, T. Staudacher, M. Tsoi Spin transfer torque (STT) refers to a novel method to control and manipulate magnetic moments using an electrical current. For the past decade it has proven to be a fascinating domain of research with a number of manifestations in various systems interesting both from fundamental science's point of view as well as for technological applications. In ferromagnetic/nonmagnetic (F/N) multilayers a dc electrical current can switch and/or drive its constituent F parts into high-frequency precession which is of interest for microwave and magnetic recording technologies. Interestingly, application of high-frequency currents can also drive the multilayer, e.g., into ferromagnetic resonance (STT-FMR) precession. In our experiments we use point contacts to inject high microwave currents into a variety of magnetic thin films including NiFe/Cu/NiFe/IrMn and NiFe/Cu/Co spin valves, and single ferromagnetic (NiFe or Co) films. The resulting magnetodynamics are detected electrically when a small rectified dc voltage appears across the contact at resonance. We find that in addition to a standard FMR, the microwave currents can excite other resonance modes in our point contacts. We study the behavior of the excitations as a function of applied magnetic field, dc bias current, and microwave frequency. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B19.00003: Minimization of Ohmic losses for domain wall motion in ferromagnetic nanowires Artem Abanov, Oleg Tretiakov, Yang Liu We study current-induced domain-wall motion in a narrow ferromagnetic wire. We propose a way to move domain walls with a resonant time-dependent current which dramatically decreases the Ohmic losses in the wire and allows driving of the domain wall with higher speed without burning the wire. For any domain wall velocity we find the time-dependence of the current needed to minimize the Ohmic losses. Below a critical domain-wall velocity specified by the parameters of the wire the minimal Ohmic losses are achieved by dc current. Furthermore, we identify the wire parameters for which the losses reduction from its dc value is the most dramatic. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B19.00004: Equilibration in All-Perpendicular Spin Valves Subject to Short Current Pulses * Daniel Bedau, Huanlong Liu, Jonathan Sun, Jordan Katine, Eric Fullerton, Stephane Mangin, Andrew Kent Our recent experiments have shown that all-perpendicular spin valves can be switched by short current pulses ($<$5 ns) [1]. In this limit we found that the switching probability only depends on the spin-angular momentum in the pulse [1]. However, such studies do not directly resolve the magnetization dynamics and relaxation. To study equilibration of spin valves driven out of equilibrium by short current pulse we have developed a pump-probe method, capable of 50 ps resolution. A probe pulse, a variable delay after the pump pulse, is used to determine the magnetization relaxation rate. When the delay between the pump and probe pulses is less than 1 ns the net switching probability differs from that at longer delays. An analysis of this difference shows that the free layer angular-momentum decays exponentially with time after the pump pulse. From these studies we obtain a lifetime, which we use to estimate the free layer damping. [1] Bedau et. al. Appl. Phys. Lett. 96, 022514 (2010) {\&} ArXiv:1009.5240 *supported by: USARO Grant No. W911NF0710643 [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B19.00005: Enhanced magnetization drift velocity and current polarization in (CoFe)$_{1-x}$Ge$_x$ alloys Robert McMichael, Meng Zhu, Brian Soe, Matt Carey, Stefan Maat, Jeff Childress We present measurements of current spin polarization and magnetization drift velocity in (CoFe)$_{1-x}$Ge$_x$ alloys $(x \leq 0 \leq 0.3)$, using a spin wave Doppler technique where spin wave transmission is measured between fixed-wavevector antennas coupled to current-carrying wires [1,2]. In a current density $J$, the transmission resonance frequency is shifted by $\Delta f = kv/2\pi$, where $v=Jg\mu_{\rm B}P/(2eM_{\rm s})$ is a magnetization drift velocity. Measurement of $\Delta f$ allows calculation of $v$ and current spin polarization $P$. With increasing Ge concentration, $v$ increases dramatically from (3.1 $\pm$ 0.2) m/s for CoFe to (8.2 $\pm$ 0.6) m/s for (CoFe)$_{0.7}$Ge$_{0.3}$ ($J= 10^{11}$ A/m$^2$). We attribute this increase in drift velocity primarily to decreased magnetization. The current polarization increases from 0.84 $\pm$ 0.04 for CoFe and reaches a maximum of 0.95 $\pm$ 0.05 at approximately 25\% Ge.\\[4pt] [1] V. Vlaminck and M. Bailleul, Science, 322, 410 (2008)\\[0pt] [2] M. Zhu, C. L. Dennis and R. D. McMichael, Phys. Rev. B. 81, 140407R (2010). [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B19.00006: Planar spin-transfer device with dynamical polarizer and analizer Yaroslaw Bazaliy, Anton Kravchenko The behavior of the planar spin-transfer devices with monodomain magnetic layers can be described by the macrospin Landau-Lifshitz-Gilbert (LLG) equation with spin-transfer terms. The LLG description of a device with two layers is simplified after applying the overdamped, large easy-plane anisotropy approximation. A decrease of the magnetic layer thickness asymmetry creates a transition from the conventional polarizer-analizer (``fixed layer -- free layer'') operation regime to the regime of the nearly identical magnets. Here electric current leads to a ``Slonczewski windmill'' dynamic state, rather than producing the magnetic switching. The ``windmill'' precession state of a device with two free layers was investigated by numerical solution of the LLG equation. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B19.00007: Optimal field sweep rate in magnetic switching of a single-domain particle Shu Yan, Yaroslaw Bazaliy, Andrzej Stankiewicz The speed of magnetic switching is an important parameter of memory cells. We consider a magnetic moment with an easy axis anisotropy switched by an external field applied at a small angle to the axis. By solving the Landau-Lifshitz-Gilbert(LLG) equation numerically, it is found that the switching time of the magnet is not monotonically increasing with the field sweep rate of the applied field. The dependence has a minimum, i.e., there exists an optimal field sweep time. Analytic approximations are derived for the dependence of the switching time on the field sweep rate and for the value of the optimal field sweep time. Our results have important implications for the optimization of magnetic memory devices. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B19.00008: Ballistic (precessional) contribution to the conventional magnetic switching Andrzej Stankiewicz, Ya. B. Bazaliy, Shu Yan We consider a magnetic moment with an easy axis anisotropy energy, switched by an external field applied along the axis. Additional small constant bias field is applied perpendicular to the axis. It is found that the magnet's switching time is a non-monotonic function of the rate at which the field is swept from ``up'' to ``down''. Switching time exhibits a minimum at a particular optimal sweep time. This unusual behavior is explained by the admixture of a ballistic (precessional) rotation of the moment caused by the perpendicular bias field in the presence of a variable switching field. Analytic approximations are derived for the dependence of the switching time on the field sweep rate and for the value of the optimal field sweep time. The existence of the optimal field sweep time has important implications for the optimization of magnetic memory devices. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B19.00009: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 1:27PM - 1:39PM |
B19.00010: AC Magnetic Susceptibility and $\mu $SR Study of Spin Dynamics and the Onset of Magnetic Correlations in LiHo$_{x}$Y$_{1-x}$F$_{4}$ R.C. Johnson, B.Z. Malkin, A. Lascialfari, A. Amato, C. Baines, J.S. Lord, S.R. Giblin, B. Barbara, M.J. Graf The onset of correlation effects in the magnetic Ho$^{3+}$-subsystem in LiHo$_{x}$Y$_{1-x}$F$_{4}$ single crystals is studied by comparing measurements and simulations of the field and frequency dependent magnetic AC susceptibility at 1.8 K and field and temperature dependent muon depolarization rates ($\mu $SR) for the concentrations x=0.0017, 0.0085, 0.041 and 0.0855. Specific features in the field and frequency dependence of in-phase and out-of-phase susceptibilities, in particular, broadening of peaks (dips) in $\chi $' ($\chi $'') that indicate enhanced relaxation processes at field induced avoided level crossings, can be associated with x-dependent changes of cross relaxation rates and the phonon bottleneck effect in the spin-lattice relaxation. The observed peak in the measured temperature dependent muon relaxation rate appears to be related to a maximum at the frequency 60 cm$^{-1}$ in the acoustic phonon density of states. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B19.00011: Temperature dependence of the effective exchange and biquadratic coupling in ferromagnets: Calculation in the disordered local moment method Paul Larson, Kirill Belashchenko We have implemented the disordered local moment (CPA-DLM) method within the tight-binding linear muffin-tin orbital (LMTO) basis. This implementation self-consistently determines the angular distribution function of the generalized Heisenberg model and the angular-dependent local potentials in the symmetry-broken state; the CPA procedure involves numerical integration over the polar angle of the spin. We present benchmark calculations for several materials including Fe, Co, FePd, FePt, and CoPt. We further extract the temperature dependence of the effective exchange and biquadratic coupling parameters from the angular dependence of the single-site grand potential. We find that the effective exchange parameter in Fe is almost temperature-independent, while the biquadratic interaction is entirely negligible at all temperatures. In FePd the effective exchange varies noticeably as a function of temperature, while the biquadratic coupling is somewhat more pronounced but still relatively small. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B19.00012: Effect of occupation numbers on exchange coupling in low dimensional magnetic nanostructures D. Terrade, H. X. Yang, A. Kalitsov, L. Nistor, M. Chshiev, B. Dieny Interlayer exchange coupling (IEC) has been of great interest for spintronic community and has been shown directly related to equilibrium spin current (ESC). Here we present a study of the influence of the electronic occupation numbers on the angular dependence of the IEC in magnetic layered nanostructures with finite thickness ferromagnetic (FM) layers. The calculations were performed within the tight-binding model using the nonequilibrium Green function technique both within perturbation theory and exact diagonalization approaches. We found that the period of IEC oscillations as a function of FM layer thickness has nonmonotonic variation with electronic states occupation numbers (Fermi level position). In the limit of 2-site model it is found that perturbation theory fails to describe correctly exchange coupling angular dependence always giving sinusoidal behavior for the ESC while the exact solution alternates between sinusoidal and strongly nonsinusoidal behavior as a number of electrons in the system is varied. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B19.00013: Spin and phase coherence times in lithographically defined bismuth wires Martin Rudolph, J.J. Heremans We performed low temperature magnetotransport measurements on lithographically defined semimetal thin film bismuth wires and used the weak-antilocalization effect to determine spin and phase coherence times. Purpose-made Bi mesoscopic structures have not been extensively studied, yet are of interest due to the strong spin-orbit coupling in the material and its surface states. The spin and phase coherence times in mesoscopic Bi wires are here studied as function of temperature and wire width. The phase coherence time saturates at temperatures below 2 K, and appears limited by electron-phonon interactions above 2 K. The spin coherence time shows a dependence on width unexpected in Bi thin films. The spin coherence time increases as the width is reduced, similar to the dependence observed in wires fabricated on spin-orbit coupled semiconductor two-dimensional systems. The similarity may be an indication that the weak-antilocalization signature is dominated by two-dimensional strongly spin-orbit coupled Bi surface states (DOE DE-FG02-08ER46532). [Preview Abstract] |
Session B20: Focus Session: Physics of Energy Storage Materials II -- Anodes and Capacitors
Sponsoring Units: FIAP/DMP GERA/DCOMPChair: Gholam?Abbas Nazri, General Motors Company
Room: D168
Monday, March 21, 2011 11:15AM - 11:27AM |
B20.00001: Anode-electrolyte double-layer of Li-ion batteries: Structure and Li-ion intercalation David O. Wipf, Ibrahim Abou Hamad, Per Arne Rikvold, Mark A. Novotny The electrochemical double-layer structure plays an important role in Li-ion intercalation during charging of Li-ion batteries with a graphite anode. In our recent Molecular Dynamics studies of a proposed accelerated charging method [I. Abou Hamad~\textit{et al.}, Phys. Chem. Chem. Phys. {\bf 12}, 2740-2743 (2010)], we notice that ethylene carbonate and propylene carbonate molecules of the electrolyte assemble themselves in a preferred orientation at the electrode-electrolyte interface. On the other hand, they are randomly oriented in the bulk electrolyte. We show that the structure of the double layer is affected by the intercalating Li-ion: while the dipole moments of double-layer molecules far from the intercalating Li-ion point toward the graphite sheets of the anode, they point away from the intercalation site close to the intercalating Li-ion. This observation should contribute to a better understanding of the intercalation process. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B20.00002: High Throughput Computational Discovery of Intermetallic Anodes for Li Batteries Scott Kirklin, Chris Wolverton We have developed a framework to perform high-throughput computational screening of intermetallic compounds as candidates for Li battery anodes. We have used our method to calculate, from density functional theory (DFT), more than 5000 anode lithiation reactions, based on more than 100 intermetallic compounds. We have specifically focused on the 3d-transition metal silicides, nidtrides and phosphides. Given the set of DFT total energies for all compounds, the reaction path upon lithiation is predicted using the recently-developed grand canonical linear programming (GCLP) method. The anode performance is then characterized by the cell potential vs lithium metal, energy density and volume expansion. The accuracy of this approach is first validated for pure silicon, and then extended to binary intermetallic compounds. Based on the results of these calculations, future experimental study can be guided toward systems with promising thermodynamic properties. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B20.00003: Li and Si diffusion in Si anodes in Li-ion batteries: An \emph{ab-initio} molecular-dynamics-based study Priya Johari, Vivek B. Shenoy Several studies have been carried out in the past few years to understand the dynamics of Li diffusion in Si anodes of Li-ion batteries, however, most of these studies are restricted to the diffusion of a single Li atom in crystalline Si. While, it is well known that crystalline Si becomes amorphous on lithiation, this phenomenon has not been considered in previous computational work. Here, we report the results of molecular dynamics simulations that were carried out to study the diffusion of Li atoms in crystalline as well as amorphous Si for the LiSi phase. We have also analysed the dynamics of the Si atoms during lithiation to understand its role in stress generation/relaxation. We find that Li diffuses faster in amorphous Si as compared to crystalline Si, while the diffusivity of Si is around two orders of magnitude lesser than Li. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B20.00004: Li-ion Battery Electrode Materials Design from First-Principles Calculations Invited Speaker: First-principles calculations can provide a powerful tool for investigating and optimizing electrode materials. While the strength of computations lies in the ability to control what is being calculated, the challenge is to ensure that the calculation is relevant for the physical processes that dominate the performance of the material. We will discuss this balance and show examples of how computations can aid in the design of current Li-ion rechargeable battery electrode materials by identifying and understanding the performance bottlenecks on the atomistic level. As the most commonly used anode in today's Li-ion batteries, graphite shows poor rate capability at lower temperatures, leading to over-potential and Li plating. Using first-principles calculations, coupled with a cluster expansion of Li interactions and kinetic Monte Carlo we were able to show that \textit{intrinsic} Li diffusion in graphite can be very fast, providing guidance towards designing higher-rate carbonaceous anode materials. On the cathode side, we have studied the layered Li(Ni$_{1/3}$,Mn$_{1/3}$,Co$_{1/3})$O$_{2}$ material, which is an interesting candidate if Co is partially substituted by the cheaper Al. Li migration in this material is influenced by several factors such as Li slab space, cation ordering and interlayer mixing. We present ab initio calculations of Li diffusivity as a function of Al content and slab spacing in the layered material, which elucidates the intrinsic rate performance effect of the Al substitution in the bulk material. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B20.00005: Crystalline-amorphous interfaces in Li-ion batteries Maria K. Chan, Jeffrey Greeley Amorphous and crystalline materials are associated with fast ionic transport and long term structural stability, respectively, both desirable properties in lithium ion battery materials. It is therefore no surprise that amorphous-crystalline interfaces are ubiquitous in Li-ion batteries. Using first principles density functional theory (DFT), and primarily Si as an example, we study models of crystalline-amorphous interfaces in Li-ion batteries. We will discuss the structure of such an interface and its energetic and mechanical effects on lithium insertion, as well as the kinetics of Li ion transport near and across the interface. The mechanism of solid state amorphization will also be discussed. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B20.00006: Li Diffusion Characteristics and Energetics in TiO$_{2}$ H. Yildirim, Subramanian Sankaranarayanan, Jeff Greeley We present the results of density functional theory-based calculations for the activation energies of Li diffusion in TiO$_{2}$ crystalline and amorphous structures. Additionally, molecular dynamics simulations using shell potential models are used to investigate the Li ion diffusion mechanisms for various titania morphology. The diffusion pathways and the corresponding energetics for each diffusion mechanism are further probed using the DFT-based Nudged Elastic Band Method. We will report the calculated diffusion energetics (MD and DFT) for each (Li-TiO$_{2})$ system and compare the atomic scale Li transport characteristics on crystalline and amorphous TiO$_{2}$ structures. We also discuss the effect of Li concentration on the diffusion energetics. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B20.00007: Composite MnO2-Carbon Electrodes for High Energy Density Storage Brad Corso, Israel Perez, Philip Collins The development of batteries with ever higher power densities is challenged by fundamental materials limitations that might be solved with hybrid combinations of materials. For example, metal oxides with high lithium ion capacities lack the conductivity to be good battery anodes, but composites that add graphitic carbon can achieve both capacity and conductivity. In this case, fast interfacial electron transfer between the materials is critical to achieving high performance. Here, we describe an electrochemical synthesis that achieves precise, conformal MnO2 films on graphitic surfaces. Furthermore, by using single-walled nanotubes as the carbon support, we can control defect densities with single defect resolution. Charge-discharge cycling of these electrodes, combined with control over point defects, directly distinguishes the enhanced charge transfer of defects and illuminates the structure-function relationship in interfacial electron transfer.~This research is supported by the NEES Energy Frontier Research Center of the U.S. DOE Office of Basic Energy Sciences ({\#}DESC0001160). [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B20.00008: Charge-Driven Structural Transformation and Valence Versatility of Boron Sheets in Magnesium Borides Yufeng Zhao, Chunmei Ban, Qiang Xu, Suhuai Wei, Anne C. Dillon We show here that boron sheets exhibit highly versatile valence and the layered boron materials may hold the promise for a high energy-density magnesium-ion battery. Practically, boron is superior to previously known multi-valence materials, especially transition metal compounds, which are heavy, expensive, and often not benign. Based on Density Functional Theory simulations, we have predicted a series of stable magnesium borides MgBx with a broad range of stoichiometries, 2 $<$ x $\le $ 16, by removing magnesium atoms from MgB2. The layered boron structures are preserved through an in-plane topological transformation between the hexagonal lattice domains and triangular domains. The process can be reversibly switched as the charge transfer changes with Mg insertion/extraction. The mechanism of such a charge-driven transformation originates from the versatile valence state of boron in its planar form. The discovery of these new physical phenomena suggests the design of a high-capacity magnesium-boron battery. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B20.00009: Nanostructured Mg Thin Film Electrodes for Mg-Air Batteries Taha Demirkan, Wisam Khudhayer, Fatih Cansizoglu, Tansel Karabacak Over the past decades, primary Mg-air batteries got the attention of several researchers due to their low cost, non-toxicity, and theoretically expected high terminal voltage and high specific capacity values. However, corrosion and formation of a passivation layer around the electrode have been among the major challenges resulting in low columbic efficiencies compared to theoretically expected values. In this study, we utilized a glancing angle deposition (GLAD) method for fabricating nanostructured Mg thin film electrodes with unique physical properties to overcome these problems. Electrodes were prepared using a thermal evaporation GLAD system. Magnesium coatings in various forms ranging from conventional dense thin films to highly porous nanoblades were prepared through the control of deposition angle from normal to oblique angles, respectively. We show that the properties of Mg-air batteries can be significantly improved using nanostructured Mg thin film electrodes and lead to enhanced terminal voltage and specific capacity values. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B20.00010: 1-pyrenecarboxylic acid Functionalization of Graphene: Effect on Capacitive Energy Storage Sujoy Ghosh, Rakesh Shah, Xiaohong An, Dinesh Rawat, Swastik Kar, Saikat Talapatra We will present a comparison of Electrolytic Double Layer Capacitance (EDLC) performance of membrane electrodes fabricated using pure and 1-pyrenecarboxylic acid (PCA)-functionalized graphene flakes. A significant increase in specific capacitance as well as energy and power density values in PCA graphene electrodes indicates that surface functionalization (that affects the hydrophilicity) of graphene-based materials is crucial for improving capacitive energy storage ability of these materials. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B20.00011: Electrochemical Double Layer Capacitors Using Few Layers of Graphene Grown on Nickel Foil. R. Shah, U. Philipose, J.M. Perez, S. Talapatra We report on the properties of Electrochemical Double Layer Capacitors (EDLCs) fabricated using few layers of graphene synthesized on Nickel (Ni) foil by Chemical Vapor Deposition (CVD). The graphene films were characterized by Raman spectroscopy and showed that the film comprised more than one layers of graphene. The capacitive behavior of the fabricated EDLCs was examined using cyclic voltammetry, constant current charge/discharge, and impedance spectroscopy. These measurements show that the charge storage phenomenon is non-Faradic in nature. The capacitance of graphene on Ni electrodes was then compared to blank Ni foil electrodes and it was found that the capacitance of graphene on Ni foil is substantially higher than the blank Ni foil electrode. These results show that the few layers of graphene film grown on Ni foil could be promising material to function as electrodes for electrochemical energy storage device applications. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B20.00012: First-principles study of the structure of RuO$_{2} \cdot x$H$_{2}$O Fei Zhou, Yongduo Liu, Mark Asta, Vidvuds Ozolins Hydrous ruthenia, RuO$_{2} \cdot x$H$_{2}$O, is a high-performance electrode material for electrochemical supercapacitors. Two different structural models of hydrous ruthenia had been proposed. In one of them, hydrogen is incorporated in metal vacancies inside the oxide host ( (``bulk model''), while in the other model structural water associated with Ru-O occupies the region between rutile nanograins (``core + grain-boundary model''). We present a theoretical examination of the validity of the bulk model by optimizing hydrogen positions within RuO$_{2}$ with proton-compensated Ru vacancies using a combination of a systematic search algorithm based on electrostatics, database searching and density-functional theory calculations. We find that all the considered bulk model structures are unstable by $\sim 0.3 - 0.4$ eV per H$_{2}$O molecule with respect to phase separation into anhydrous RuO$_{2}$ and water. Structures with hydroxyl groups or aggregate H$_{2}$O are significantly lower in energy (though still unstable with respect to phase separation), demonstrating that the water prefers to agglomerate outside RuO$_{2}$. Our results strongly disfavor the bulk model with hydrogen inside RuO$_{2}$ and support the core+grain-boundary model of hydrous ruthenia. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B20.00013: Ab initio study of the charge storage mechanism of ruthenium dioxide as an electrochemical ultracapacitor Yongduo Liu, Fei Zhou, Vidvuds Ozolins The charge storage mechanisms of ruthenium dioxide were investigated by first principles calculations. Both H-injected bulk and H-adsorbed RuO2(110) surface have been studied in order to obtain a whole picture of the discharging process of ruthenium dioxide as a supercapacitor. We have predicted the crystal structure of ruthenium-oxy-hydroxide (ROOH). By ab-inito voltage calculations, we also found that the RuO2(110) surface is completely hydrated before the usual voltammagram measurements, which suggests that the redox reactions happen in deep layers and should be diffusion dominated processes. [Preview Abstract] |
Session B21: General Theory
Sponsoring Units: DCOMPChair: David Singh, Oak Ridge National Laboratory
Room: D161
Monday, March 21, 2011 11:15AM - 11:27AM |
B21.00001: Optimization of Elastic Constant Values in Non-cubic Crystals using Computational Image Matching Madeleine Msall, Timothy Head Point excitation in ultrasound or heat pulse experiments excites non-equilibrium phonons that carry energy along the group velocity direction. Phonon images map the sharp boundaries between high and low flux regions, called caustics, which are directly related to folds in the acoustic wave surface. Computational simulations show that caustic positions are extremely sensitive to the values of the elastic constants. We explore methods of determining the elastic constants using image matching techniques. Given the dependence of single image features on a constellation of constants, there are many local minima encountered in the search. This talk will present quantifiable criteria for image matching in this context and discuss potential heuristic or stochastic methods to deal with the problem of local minima. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B21.00002: Entropic sampling without windows Ronald Dickman, Ant\^onio Cunha-Netto We describe an entropic sampling method that permits estimation of the number of configurations over the full range of energies, with dividing the latter into subsets or ``windows.'' Our method involves progressive refinement of an initial approximation for the density of states, using a set of random walks that span the energy range. Applied to the two-dimensional Ising model the method yields the critical temperature to an accuracy of about 0.01\%, and critical exponents to 0.5\% or better. Predictions for system sizes $L=10$ - 160, for the temperature of the specific heat maximum, and the specific heat at the critical temperature, are in very good agreement with exact results. The antiferromagnetic transition is well represented. Excellent results are also obtained for the three-dimensional Ising model (simple cubic lattice) and the lattice gas with nearest-neighbor exclusion. We observe that attempts to restrict the sampling to a subset of the full energy range lead to distortions in the density of states, even if the restriction is imposed in a smooth manner, rather than with a sharp barrier. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B21.00003: Nearly exact calculations of small atomic and molecular systems using explicilty correlated gaussians Sergiy Bubin, Kalman Varga, Ludwik Adamowicz We demonstrate how very precise (virtually exact) solutions of various quantum mechanical problems can be obtained using the variational method with explicitly correlated Gaussian basis functions (ECGFs). As examples we consider several benchmark systems, such as few-electron atoms and molecules, as well as Coulomb systems containing exotic particles. We also discuss the evaluation of relativistic corrections in the framework of ECGFs. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B21.00004: Metallic Phase of Water Ice Predicted at Megabar Pressures Burkhard Militzer, Hugh Wilson We predict water ice to attain two new crystal structures with Pbca and Cmcm symmetry at 7.6 and 15.5 Mbar, respectively [Phys. Rev. Lett. 105 (2010) 195701]. With density functional calculations, we analyze the structural and electronic properties of these phases at zero temperature. The Pbca phase, like the known high-pressure ice phases VII, VIII, X and Pbcm, is insulating and consists of two interpenetrating hydrogen bonded networks, but the Cmcm phase is metallic and consists of corrugated sheets of H and O atoms. The H atoms are squeezed into octahedral positions between next-nearest O atoms while they occupy tetrahedral positions between nearest O atoms in lower-pressure phases. Our predictions may be testable with ramp compression experiments that can reach megabar pressures at lower temperatures than conventional shock wave experiments. The predicted insulator-to-metal transition would lead to an increase in reflectivity that can be measured with spectroscopic techniques. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B21.00005: Supersymmetric Quantum Mechanics in Multiple Dimensions Applied to Variational Monte Carlo - A Proof of Principle Study Thomas Markovich, Kaushik Maji, Eric Bittner, Don Kouri We present a new approach to variational monte carlo using our N-Dimensional generalization of Supersymmetric Quantum Mechanics. We do this by introducing a {\em vector} superpotential in an orthogonal hyperspace. In the case of $N$ distinguishable particles in three dimensions this results in a vector superpotential with $3N$ orthogonal components. The original scalar Schr\"odinger operator can be factored into vector ``charge'' operators: $\vec Q_{1}$ and $\vec Q_{1}^{\dagger}$. Using these operators, we can write the original (scalar) Hamiltonian as $H_{1} = \vec Q_{1}^{\dagger}\cdot \vec Q_{1} + E_{0}^{(1)}$. The second sector Hamiltonian is a tensor given by $H_{2} = \vec Q_{1}\vec Q_{1}^{\dagger} + E_{0}^{(1)}$ and is isospectral with $H_{1}$. The vector ground state of sector two, $\vec\psi_{0}^{(2)}$, can be used with the charge operator $\vec Q_{1}^{\dagger}$ to obtain the excited state wave functions of the first sector. We demonstrate the approach with examples of a pair of separable 1D harmonic oscillators and the example of a non-separable 2D anharmonic oscillator (or equivalently a pair of coupled 1D oscillators). [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B21.00006: Electromagnetic and gravitational signatures of black hole and neutron star mergers Steven Liebling Astrophysical binary systems composed of some combination of compact objects (black holes(BH) and neutron stars(NS)) are extremely interesting dynamical systems. Such systems are generally extremely good radiators of gravitational waves, and, in at least some cases, they should be excellent electromagnetic sources. As such, they hold great promise for concurrent detection from both recently completed gravitational wave observatories and from conventional telescopes. I describe recent results achieved with a fully relativistic adaptive code for the merger of BH-BH, BH-NS, and NS-NS systems with magnetic fields. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B21.00007: Quantification of Partially Ordered Sets with Application to Special Relativity Newshaw Bahreyni, Kevin H. Knuth A partially ordered set is a set of elements ordered by a binary ordering relation. We have shown that a subset of a partially ordered set can be quantified by projecting elements onto a pair of chains where the elements of each chain are quantified by real numbers. This results in a quantification based on pairs of real numbers (pair). Intervals, defined by pairs of elements, can be quantified similarly. A pair can be decomposed into a sum of a symmetric pair and an antisymmetric pair and mapped to a unique scalar which results in the Minkowskian form. Changing the basis of quantification from one pair of chains to another, under special conditions, leads to the generalized Lorentz transformation for pairs. We apply these results to a causally-ordered set of events by identifying a chain of events with an observer equipped with a clock in an inertial frame. We obtain the Minkowski metric of flat space-time as well as Lorentz transformations, which results in there being a maximum invariant speed. We find that the mathematics of special relativity arises from quantifying causal relationships among events, and requires neither the principle of relativity nor the fact that the speed of light is constant. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B21.00008: Lunar Orbit Anomaly and GM=tc$^3$ Cosmology Louise Riofrio Studies of the Moon at Johnson Space Center have confirmed a large anomaly in lunar orbital distance, with possible applications to Relativity. Our Lunar Laser Ranging Experiment has reported the Moon's semimajor axis increasing at 3.82 $\pm$ .07 cm/yr, anomalously high. If the Moon were gaining angular momentum at this rate, it would have coincided with Earth less than 2 Gyr ago. The Mansfield sediment (Bills, Ray 2000) measures lunar recession at 2.9 $\pm$ 0.6 cm/yr. Additional observations independently measure a recession rate of 2.82 $\pm$ .08 cm/yr. LLRE differs from independent experiments by 10 sigma. A cosmology where speed of light c is related to time t by GM=tc$^3$ has been suggested to predict the redshifts of Type Ia supernovae, and a 4.507034\% proportion of baryonic matter (Riofrio 2004). If c were changing in the amount predicted, lunar orbital distance would appear to increase by an additional 0.935 cm/yr. An anomaly in the lunar orbit may be precisely accounted for, shedding light on puzzles of ``dark energy.'' In Planck units this may be summarised as M=R=t. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B21.00009: On a Broken Formal Symmetry between Kinetic and Gravitational Energy Armin Nikkhah Shirazi Historically, the discovery of symmetries has played an important role in the progress of our fundamental understanding of nature. This paper will demonstrate that there exists in Newtonian theory in a spherical gravitational field a formal symmetry between the kinetic (KE) and gravitational potential energy (GPE) of a test mass. Put differently, there exists a way of expressing GPE such that the form of the mathematical expression remains invariant under an interchange of KE and GPE. When extended to relativity by a suitable assumption, it leads to a framework that bridges the general relativistic and Newtonian conceptions of gravitational energy, even though the symmetry is broken except in the infinitesimal limit. Recognizing this symmetry at infinitesimal scales makes it possible to write a relativistic equation of an individual graviton, the properties of which under under one interpretation may be unexpected. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B21.00010: The Relativistic Quantized Force: Newton's Second Law, Inertial and Gravitational; Generalization of Schwarzschild Metric for Strong and Weak Gravitational Field Azzam AlMosallami In this paper we derived the relativistic Quantized force, where the force given as a function of frequency[1]. Where, in this paper we defined the relativistic momentum as a function of frequency equivalent to the energy held by a body, and time, and then the quantized force is given as the first derivative of the momentum with respect to time. Subsequently we introduce in section one Newton's second law as it is relativistic quantized, and in section two we introduce the relativistic quantized inertial force, and then the relativistic quantized gravitational force, and the quantized gravitational time dilation. At the end we shall generalize the Schwartzschild metric to describe the weak and strong gravitational field. [Preview Abstract] |
Session B22: Correlated Electrons Including "115" Materials
Sponsoring Units: DCMPChair: Johnpierre Paglione, University of Maryland
Room: D163
Monday, March 21, 2011 11:15AM - 11:27AM |
B22.00001: Quantum critical point in UCo$_{1-x}$Fe$_{x}$Ge Noravee Kanchanavatee, Kevin Huang, James Hamlin, Ryan Baumbach, Diego Zocco, M. Brian Maple We have carried out a comprehensive study of the UCo$_{1-x}$Fe$_{x}$Ge series across the entire range of compositions 0 $\le $ x $\le $ 1, and report the results of x-ray diffraction, magnetization, specific heat, and electrical resistivity measurements to uncover the magnetic and superconducting phase diagram. Substitution of Fe into UCoGe initially results in an increase in the Curie temperature and a rapid destruction of the superconducting state. Near x = 0.2, the ferromagnetic transition is suppressed to zero temperature at an apparent quantum critical point, and the temperature dependences of the electrical resistivity and specific heat suggest non-Fermi liquid behavior. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B22.00002: Non Fermi liquid properties of Ni-V close to the ferromagnetic quantum critical point Almut Schroeder, Sara Ubaid-Kassis, Brendan Wyatt, Thomas Vojta Resistivity ($\rho )$ and magnetization ($M)$ data of the d-metal alloy Ni$_{1-x}$V$_{x}$ are presented in the vicinity of the critical vanadium concentration $x_{c}\approx $11{\%} where the onset of long-range ferromagnetic (FM) order is suppressed to zero temperature. Above x$_{c}$ the temperature ($T)$ dependence of the magnetic susceptibility is best described by simple nonuniversal power laws (e.g. \textit{M/H(T,H$\to $0) $\sim $T}$^{\alpha -1}$ ). Also the resistivity displays power laws (\textit{$\Delta \rho \sim $T}$^{n})$. Both exponents $\alpha $(x) and n(x) vary with x displaying signatures of a disordered quantum phase transition in a metal very different than of a clean 3D FM. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B22.00003: Non-Fermi Liquid behavior in Itinerant Ferromagnet, HfMnGa$_2$ Carlos Marques, Yuri Janssen, Moo Sung Kim, Liusuo Wu, Meigan Aronson Single crystals of HfMnGa$_2$, space group Pnma, were grown using a Ga self flux technique. A sharp peak in the AC susceptibility $\chi_{AC}$ shows a phase transition at $T_C\approx26 K$, followed by Curie-Weiss behavior at higher temperatures. Arrott plot analysis confirms this transition is ferromagnetic with a spontaneous moment of $\mu_0\cong0.3 \mu_B/ $Mn. HfMnGa$_2$ has a coercive field of $\sim$ 0.1 T as well as a large magnetic anisotropy that restricts the moments to point in the [010] direction. Both a large Rhodes-Wohlfarth parameter $\mu_{\mbox{fluct}}/\mu_0\cong3.6$ and low T$_C$ suggest HfMnGa$_2$ is comparable to other itinerant ferromagnets MnSi, ZrZn$_2$ and Ni$_3$Al. Resistivity $\rho(T)$ shows HfMnGa$_2$ to be metallic with $\rho(T)-\rho_0$ having a $T^{5/3}$ dependance in the ordered state. This non-Fermi liquid relationship was also observed in Ni$_3$Al and ZrZn$_2$ over a more limited range of temperatures. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B22.00004: Electronic, magnetic and structural properties of Cr$_{1-x}$V$_x$N Camilo Quintela, Francisco Rivadulla, Jose Rivas We report a systematic study on the electronic, magnetic and structural properties of stoichiometric and hole-doped CrN and present the magnetic and electronic phase diagram for the Cr$_{1-x}$V$_{x}$N series. Stoichiometric CrN is a narrow gap, correlation-induced, semiconductor that orders antiferromagnetically below 286 K. The changes in the chemical bond associated to the magnetic order result in a non-activated behavior of the resistivity in the antiferromagnetic state, showing some similarities with other materials proposed to be itinerant-AF, like CaCrO$_{3}$. Doping this state with holes drives the system towards itinerant electron behavior through a series of inhomogeneous magnetic/electronic states. Given the chemical and structural simplicity of this system, it could provide an interesting place to study the evolution from an antiferromagnet with a non-thermally activated charge transport to a paramagnetic metal in a non-oxide material. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B22.00005: Magnetic Correlations in Yb$_{4}$LiGe$_{4}$: A $\mu $SR and $^{7}$Li NMR Study S. Disseler, M.J. Graf, P. Carretta, S. Peter, N. Svensson, S.R. Giblin, A. Amato, C. Baines We present results from zero and longitudinal $\mu $SR and $^{7}$Li NMR together along with magnetization and dynamic susceptibility on the multivalent, intermetallic compound Yb$_{4}$LiGe$_{4}$. A magnetic transition at 1.4K in dynamic susceptibility is observed, corresponding to a rapid increase in the quasi-static relaxation component of the ZF MuSR spectra. The strong magnetic field dependence exhibited in these measurements demonstrates a non-trivial criticality, and suggests a close relation to the quantum critical phenomenon observed in other Yb and Ce based systems. Based on support from NMR spectra, we discuss the potential of competing low temperature phases, and the importance of the spin fluctuations in describing the observed phenomena. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B22.00006: Low-temperature specific heat of Nd$_{1-x}$Ca$_x$B$_6$ single crystals Jolanta Stankiewicz, Marco Evangelisti, Zachary Fisk We measured the heat capacity on random alloys of Nd$_{1-x }$Ca$_x$B$_6$ ($x < $ 0.4) in the 0.4 to 40 K temperature range. We calculated the lattice contribution to the specific heat, arising from the Debye-type phonons of the boron framework and Einstein-type oscillators of the cation sublattice. To this end, we used data obtained for the heat capacity of a LaB$_6$ single crystal which we measured in the same temperature range. Subtracting lattice and Schottky-type contributions from the measured heat capacity, we find that the electronic portion, linear in temperature, decreases sharply upon doping with Ca, most likely owed to changes in the Fermi surface. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B22.00007: High-pressure resistivity of CeCoIn$_5$ at low temperatures using four-point probe technique and finite element analysis Nathaniel Brady, Georgiy Tsoi, Tesfaye Gebre, Yogesh Vohra, David Hilton We performed high-pressure electronic characterization of the heavy fermion superconductor, CeCoIn$_5$. Using a designer diamond anvil four-point probe system, we measured the temperature-dependent resistivity up to 25 GPa and found evidence for a decrease in the effective mass at high pressures. We determined the resistivity using Van Der Pauws method and a finite element analysis approach. Room temperature resistivity with increasing pressure was also measured and a maximum in resistivity was observed near $\sim$8 GPa. These data suggest the existence of a pressure-dependent modification of the 4$f$ hybridization at the highest pressures [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B22.00008: Exploring the antiferromagnetic superconducting phase in CeCoIn$_5$ Elizabeth Blackburn, Ted Forgan, Pinaki Das, Morten Ring Eskildsen, Mark Laver, Christof Niedermayer, Jonathan White, Cedomir Petrovic CeCoIn$_5$ is a heavy fermion type-II superconductor showing clear signs of Pauli-limited superconductivity. CeCoIn$_5$ is also very close to a magnetically ordered ground state; this can be achieved by, for instance, doping with Cd. A variety of measurements give evidence for a transition at high magnetic fields inside the superconducting state, when the field is applied either parallel to or perpendicular to the $c$ axis. In the latter case, antiferromagnetic order develops on the high-field side of the transition, with a magnetic wavevector of ($q$ $q$ 0.5), where $q$ = 0.44 reciprocal lattice units [1]. We show that this order remains as the field is rotated out of the basal plane, but that the associated moment eventually disappears above 17$^{\circ}$, indicating that anomalies seen with the field parallel to the $c$ axis are not related to this magnetic order [2]. Our measurements emphasise the fragility of this magnetic order.\\[4pt] [1] M. Kenzelmann et al., Science 321, 1652 (2008).\\[0pt] [2] E. Blackburn et al., Phys. Rev. Lett. 105, 187001 (2010). [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B22.00009: Electronic inhomogeneity in heavy Fermions Zachary Fisk, Eric Bauer, Yi-feng Yang, Cigdem Capan, Filip Ronning, Joe Thompson, Roman Movshovich, Andrea Bianchi, Ricardo Urbano, Hironori Sakai The experimentally determined superconducting condensation energy of La-doped CeCoIn$_{5}$ is interpreted to show that the superconducting fraction decreases linearly with La concentration, consistent with the measured residual normal fraction seen in the electronic specific gamma remaining in the superconducting state as T $\to $ 0K. The In NQR data is also discussed. Our result points to an electronically inhomogeneous state that appears to be common to doped heavy Fermion materials near a quantum critical point. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B22.00010: Cooperative Intermediate Valence and Anomalous Stability of the Kondo Lattice in Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$$^{\ast}$ R.E. Baumbach, L. Shu, M. Janoschek, E. Gonzales, K. Huang, T.A. Sayles, J.J. Hamlin, D.A. Zocco, C.A. McElroy, M.B. Maple, J. Paglione, P.-C. Ho, J.R. O'Brien We have investigated the chemical substitution series Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ ($0\leq x \leq1.0$) by means of X-ray diffraction, energy dispersive X-ray, specific heat, electrical resistivity, and magnetic susceptibility measurements. As Yb is substituted for Ce, the lattice constants remain roughly constant up to $x=0.775$, contrary to Vegard's law, after which phase separation is observed for $0.8 \leq x <1$. The superconducting transition temperature shows only a weak linear suppression with increasing $x$, while the coherence temperature remains constant up to $x = 0.775$. We also observe non-Fermi-liquid behavior for $0 \leq x \leq 0.775$ which is sensitive to the exact value of $x$, although there is no indication for a quantum critical point in the $T-x$ phase diagram. These results suggest that the Ce and Yb ions adopt cooperative intermediate valence states which preserve the Kondo lattice behavior and SC, while the associated valence fluctuations lead to strong modifications in the non-Fermi-liquid behavior as a function of $x$. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B22.00011: Scanning tunneling microscopy and spectroscopy measurements of heavy fermion compound CeCoIn$_{5}$ Eduardo da Silva Neto, Pegor Aynajian, Colin Parker, Paul Tobash, Eric Bauer, Ali Yazdani The heavy fermion compound CeCoIn$_{5}$ has a rich electronic phase diagram as a function of doping, pressure, and magnetic field. The interaction between Ce's f-electrons and the conduction bands is expected to form Kondo screening of the spins starting at relatively high temperatures. Below 2.3 K CeCoIn$_{5}$ is known to exhibit an unconventional superconducting ground state. We present scanning tunneling microscopy and spectroscopy (STM/S) as a function of temperature on CeCoIn$_{5}$. The in-situ cleaved samples show three different layer terminations. Acquiring structural information from STM topographies we identify the chemical character of each layer. STS measurements, on all surfaces, performed over a wide range of temperature show the rapid development of an energy gap in the tunneling density of states near the onset of coherence ($\sim $ 40 K). The origin of the observed energy gap and its relation to heavy band hybridization is addressed. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B22.00012: Electron-spectroscopy of the heavy fermion alloy Ce$_{1-x}$Yb$_x$CoIn$_5$ L. Dudy, J.W. Allen, J. Denlinger, L. Shu, M. Janoschek, R.E. Baumbach, M.B. Maple Ce$_{1-x}$Yb$_x$CoIn$_5$ (YbCe115) is a new 115 alloy series with long range magnetic order suppressed in the whole substitution range. Measurements reveal a rich phase diagram in which the Kondo lattice is robust against Yb substitution, superconductivity persists to high values of x, and the non-Fermi-liquid behavior is enhanced by Yb substitution [1].We have characterized the electronic structure of this new alloy by x-ray and angle resolved photoemission spectroscopy (XPS and ARPES) performed at the Advanced Light Source (ALS). Yb 4f XPS spectra vary with increasing x from dominantly Yb$^{3+}$ to a mix of Yb$^{3+}$ and Yb$^{2+}$, in agreement with inferences from the magnetic susceptibility [1]. We will present ARPES data to show the x-dependence of the Fermi surface and discuss the relation of the electronic structure to the transport properties. The effect of surface contributions to the XPS and ARPES data will be assessed. \\[4pt] [1] Lei Shu et al, to be published [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B22.00013: The Fermi surface of CePt$_{2}$In$_{7}$: a two-dimensional analog of CeIn$_{3}$ Moaz Altarawneh, Neil Harrison, Ross McDonald, Fedor Balakirev, Charles Mielke, Paul Tobash, Jian-Xin Zhu, Joe Thompson, Filip Ronning, Eric Bauer We report magnetic quantum oscillations in magnetic fields extending to $\sim$60 T in single crystals of the body-centered tetragonal antiferromagnetic CePt$_{2}$In$_{7}$ recently discovered to exhibit pressure- induced superconductivity at $T_{c}$ = 2.1 K. Despite two-dimensionality of its Fermi surface, the microscopic electronic properties of layered CePt$_{2}$In$_{7}$ are revealed to be more similar to cubic CeIn$_{3}$ than layered CeRhIn$_{5}$. A significant field-induced change in the Fermi surface occurs at $H_{m}$ around 45 T in both CePt$_{2}$In$_{7}$ and CeIn$_{3}$, below which it is broken into small pockets with field-dependent effective masses -signaling 4f-electron involvement in the Fermi surface for $H < H_{m}$. Our findings suggest that CePt$_{2}$In$_{7}$ and CeIn$_{3}$ differ solely by the dimensionality of their Ce sublattices, thus realizing an ideal pair of compounds for investigating the effect of dimensionality on boosting superconductivity. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B22.00014: Tuning the Quantum Critical Points in CeCoIn$_{5}$ and CeRhIn$_{5}$ Through Pt Doping: Synthesis, Single Crystal and Physical Property Studies Paul Tobash, Krzysztof Gofryk, Filip Ronning, Joe Thompson, Stanislav Stoyko, Arthur Mar, Eric Bauer The well known CeCoIn$_{5}$ and CeRhIn$_{5}$ compounds provide an excellent opportunity for understanding the physics of heavy fermion superconductivity as well as quantum criticality in correlated electron intermetallics. Besides using hydrostatic pressure as a means to tune the physics of these materials, chemical doping has also proved essential for moving through the superconducting/magnetic boundary in these materials. We extend chemical substitution in CeCoIn$_{5}$ and CeRhIn$_{5}$ to another transition metal, Pt, and report on the synthesis, structure, and physical properties of single crystals of CePt$_{x}$Co$_{1-x}$In$_{5}$ and CePt$_{x}$Rh$_{1-x}$In$_{5}$. Single crystal X-ray diffraction confirmed the tetragonal structure of both systems which crystallize with the $P$4/\textit{mmm} space group and are derivatives of the parent compounds CeCoIn$_{5}$ and CeRhIn$_{5}$, respectively. We report the physical property measurements, which include magnetic susceptibility, heat capacity, and electrical resistivity. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B22.00015: Quantum criticality and Fermi surface topology in CeRhIn$_5$ H.Q. Yuan, L. Jiao, J.L. Zhang, Y. Kohama, M. Jaime, J. Singleton, E.D. Bauer, Han-Oh Lee, T. Park, J.D. Thompson In the heavy fermion compound CeRhIn$_5$, superconductivity occurs upon partially suppressing the antiferromagnetic state via applying pressure. At the quantum critical point ($p_c$=2.3GPa), observations of a Fermi surface change from a small Fermi volume to a large one [1] seems to favor the scenario of local quantum criticality [2]. In this talk, we will present the first experimental evidence of a magnetic field induced quantum phase transition in CeRhIn$_5$ by measuring the ac specific heat and the de Hass van Alphen effect using the facilities of pulsed magnetic field at Los Alamos. The antiferromagnetic transition of CeRhIn$_5$ is eventually suppressed at a critical field of $H_c\simeq 50$ T. A dramatic change of the Fermi surface is found close to $H_c$, but still on the antiferromagnetic side. At sufficiently low temperatures, the Fermi surface in the antiferromagnetic state undergo a topological change from a small Fermi volume to a large one with increasing magnetic field, the latter being kept unchanged in the paramagnetic state at fields above $H_c$. These findings seem to support a different scenario for the field induced quantum phase transition in CeRhIn$_5$, e.g., the spin-density-wave-type quantum criticality. [1] H. Shishido et. al., J. Phys. Soc. Jpn. 74, 1103 (2005). [2] Q. Si et al., Nature 413, 804 (2001). [Preview Abstract] |
Session B23: Focus Session: Iron Based Superconductors -- Electronic Structure, Theory and Spectroscopy
Sponsoring Units: DCOMP DMPChair: Bruce Harmon, Iowa State University
Room: D165
Monday, March 21, 2011 11:15AM - 11:27AM |
B23.00001: Point contact spectroscopy (PCS) on the Fe122 pnictides and Fe11 chalcogenides H.Z. Arham, C.R. Hunt, W.K. Park, L.H. Greene, J. Gillett, S. Sebastian, Z.J. Xu, J.S. Wen, Z.W. Lin, Q. Li, G. Gu, A. Thaler, S.L. Budko, P.C. Canfield We present PCS results on Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and Fe$_{1+y}$Te. The superconducting (S) crystals (x=0.08) show multigap like Andreev peaks. The non-S crystals (x=0.015, y=0.03) also show a conductance enhancement with split peaks at low temperatures (T). This conductance enhancement does not match with the bulk antiferromagnetic (AFM) transition T and survives up to 90 K for y=0.03 (T$_{N}\sim $69 K) and 130 K for x=0.015 (T$_{N}\sim $115 K). For the S samples in the coexisting regime (x=0.05 {\&} 0.055), in addition to the Andreev peaks below T$_{C}$, a zero bias conductance enhancement develops and survives for $\sim $5K above T$_{C}$. PCS detects conductance changes due to quasiparticles scattering off charge or spin ordering. These conductance enhancements may arise from orbital ordering as detected by photoemission spectroscopy$^{1}$ and AFM ordering (Q-scattering), respectively.$^{2}$ $^{1}$Yi et.al, arXiv:1011.0050. $^{2}$Bobkova et.al, PRL 94, 037005 (2005). UIUC work supported by NSF-DMR-0706013, U.S. DOE Award No.DE-AC02-98CH10886, BNL work by DOE Award No.DE-AC0298CH10886, Cambridge work by EPSRC, Trinity College, the Royal Society, the Commonwealth Trust. Ames Lab operated by ISU under DOE Contract No.DE-AC02-07CH11358. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B23.00002: Point Contact Andreev Reflection Studies on Iron Pnictide Superconductors Xiaohang Zhang, S.R. Saha, N.P. Butch, K. Kirshenbaum, J. Paglione, R.L. Greene, I. Takeuchi, Y.-S. Oh, Y. Liu, L.Q. Yan, K.-H. Kim We have systematically investigated the temperature, doping and the directional dependence of the gap structure for various types of single crystal iron pnictide superconductors by point contact Andreev reflection spectroscopy. Our studies were performed on highly transparent junctions evidenced by sharp and dramatic conductance enhancements at low temperatures. For the 122 family, despite some small features occasionally observed on the spectroscopy curves which may originate from the multiband superconductivity, a more conclusive characteristic of our obtained spectra is the presence of one predominant superconducting gap. By applying the BTK model, we find that the determined gap size scales well with the transition temperature, resulting in the 2$\Delta $/$k_{B}T_{C}$ value of $\sim $ 3.1 for both potassium doped and cobalt doped single crystals. Directional studies suggest that this gap is highly isotropic. Results on chalcogenide and nickel doped 122 superconductors will also be discussed. [X. H. Zhang et al., Phys. Rev. B 81, 024518 (2010)]. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B23.00003: Point contact spectroscopy of Co-doped pnictide superconductors John Timmerwilke, Alessandra Gallastegui, J.S. Kim, G.R. Stewart, Amlan Biswas, N.H. Sung, M.S. Kim, B.K. Cho Iron pnictides are an interesting material for superconductivity research, since they provide a contrasting system for high temperature superconductivity to the cuprates. Point-contact spectroscopy is a unique method which has been used for investigating the density of states of cuprate superconductors and can help shed light on the pnictide superconductors as well. Point-contact measurements have an advantage over tunneling spectroscopy since they are less sensitive to sample surface quality and the dimensionless barrier parameter Z, can be varied with contact pressure. We have performed point contact measurements using an apparatus with a capacitive displacement sensor which enables us to systematically vary Z. We will present point contact spectra obtained on single crystal Ba(Fe$_{0.926}$Co$_{0.074)2}$As$_{2}$ samples and other cobalt doped iron pnictide samples at various temperatures, magnetic fields, and Z. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B23.00004: First-principles studies for understanding diverse high-T$_{c}$ Invited Speaker: In this talk, I survey results and insights gained from first-principles calculations on materials that exhibit superconducting behavior at temperatures higher than those characteristic of conventional BCS superconductors. These range from highly correlated cuprate Mott insulators as represented by the bismuth-strontium-calcium-copper-oxides (BSCCOs) to border-line itinerant-Mott systems such as the recently discovered 1111 and 122 pnictides. ultimate goal of our studies is to correlate T$_{c}$ with specific material composition using detailed first-principles calculations in conjunction with many-body physics techniques via the critical step of constructing real-materials model Hamiltonians. By manipulating impurity doping, which plays a crucial role in the phase diagrams of high T$_{c}$ materials, we hope to find guidance for designing candidate systems with T$_{c}$ higher than ones currently known. BSCCO material, density functional calculations using a good generalized-gradient approximation (GGA) yield structural information that is correlated to the experimentally observed (STM) super-modulation and impurity peak in the high energy regime ($\sim $1 eV), even though the Kohn-Sham bands from such functionals fail to have a band gap. For FeAs-based high-T$_{c}$ systems, DFT band-structure calculations provide a very good starting point for constructing model Hamiltonians for studies of spin fluctuation and electron pairing mechanisms. Fermi sheets that have been constructed using Wannier transformed Kohn-Sham states have provided critical information for understanding this family of superconducting materials. Analysis of the details of magnetic ordering, density of states, and 2D vs. 3D features in both the 1111 and 122 materials have been valuable in understanding sometimes perplexing experimental findings. Effects of Co impurities have been studied and fully analyzed as well., I will discuss persistent challenges related to calculations on the structure of the non-magnetic state Ba$_{1}$Fe$_{2}$As$_{2}$ system. Both further examination of the underlying physics and development of new approximate functionals are needed. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B23.00005: Point-Contact Andreev Reflection Spectroscopy of Iron-Based Superconductors Yi-Tang Yen, J.Y.T. Wei, S.R. Saha, T. Drye, K. Kirshenbaum, J. Paglione, Rongwei Hu*, C. Petrovic, K.W. Yeh, M.K. Wu We carry out point-contact Andreev reflection spectroscopic studies at cryogenic temperatures on single crystals of Fe-chalcogenide and Fe-pnictide superconductors, including FeS$_{1-x}$Te$_{x}$, FeSe$_{1-x}$Te$_{x}$, SrFe$_{1.84}$Pt$_{0.16}$As$_{2}$ and BaFe$_{1.9}$Pt$_{0.1}$As$_{2}$. Our data are analyzed using current theoretical models involving multiband superconductivity. Further interpretations will be made in comparison with scanning tunneling spectroscopy measurements on Fe-chalcogenides and NbSe$_{2}$. *Present address: Ames Laboratory, Iowa State University [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B23.00006: Observation of multiple superconducting gaps in Fe$_{1+y}$Se$_x$Te$_{1-x}$ through Andreev reflection Debtanu De, Carlos Diaz-Pinto, Zheng Wu, Pei-Herng Hor, Haibing Peng Iron-based superconductors have been under intensive study because of the high transition temperature and the intriguing physical mechanisms involving the superconductivity and magnetic orders. Theoretical studies on the role of spin fluctuation suggest unconventional S wave pairing and multiple superconducting (SC) gaps due to the five disjoint Fermi surfaces. However, this multiple SC-gap scenario has yet to be confirmed in experiments. Here we report the experimental observation of five SC gaps in Fe$_{1+y}$Se$_x$Te$_{1-x}$ from Andreev reflection spectra, along with negative differential conductance dips due to the pair breaking related to the largest SC gap. The evolution of the multiple SC gaps is further investigated as a function of both temperature and magnetic field. For the largest SC gap, the Andreev reflection signal persists above bulk Tc, suggesting the existence of phase incoherent Cooper pairs. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B23.00007: Scanning Tunneling Microscopy/Spectroscopy study on Optimally Potassium Doped Single Crystal BaFe$_{2}$As$_{2}$ Jihua Ma, Ang Li, Chenglin Zhang, Pengcheng Dai, Shuheng Pan The iron pnictide parent compound material can be brought into superconducting state by chemical doping. It is worthwhile to study and compare the hole- and electron-doped iron pnictides. Among the well-known family of AEFe$_{2}$As$_{2}$ (AE=Ca, Sr, Ba), the scanning tunneling microscopy/spectroscopy study on hole-doped samples is insufficient. In this talk we will present high resolution STM/STS results on (001) surface of the optimally doped single crystal Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ (Tc$\sim $37K). With the data we will discuss the spatial variation of the superconducting energy gap. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B23.00008: STM Studies on the Surface Structure of Ba122 iron pnictides cleaved at Low Temperature Ang Li, Jihua Ma, A. Sefat, M. McGuire, B. Sales, D. Mandrus, R. Jin, Chenglin Zhang, Pengcheng Dai, Shuheng Pan We have performed scanning tunneling microscopy/spectroscopy (STM/STS) studies on electron- and hole-doped BaFe$_{2}$As$_{2}$ iron pnictides (Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2})$. Two types of surface topography are always found when the samples are cleaved at low temperature. One has a square-like structure and the other shows a dimerization into rows. Details of these two surfaces, particularly at their boundaries, will be utilized to argue about the surface termination. We also show the impacts of these two surface structures and some topographic features on the tunneling spectrum. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B23.00009: STM Studies of the Lattice Distortion at the Surface of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ Shuheng Pan, Ang Li, Jihua Ma, A. Sefat, M. McGuire, B. Sales, D. Mandrus, R. Jin, E. Plummer At low temperatures the bulk of the iron pnictides such as Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ goes from an orthorhombic structure phase to the tetragonal phase with doping. This phase transition is also accompanied by a magnetic phase transition. These phenomena have been discussed in the context of the mechanism of superconductivity in the iron pnictides. With careful examination of our low temperature STM topographic images on the single crystals of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$, we found that$_{ }$the lattice distortions of the two observed surface structures, namely ``Root 2'' and ``1$\times $2'', evolve with doping concentration x across the superconducting dome. Starting from parent compound, the orthorhombic ``Root 2'' structure evolves towards rhombic, while the ``1$\times $2'' structure evolves from rhombic to orthorhombic. We will discuss the implications of such doping dependent lattice distortion. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B23.00010: Observation of multiple gaps and vortex bound states in Ba0.6K0.4Fe2As2 by Scanning Tunneling Microscopy/Spectroscopy Lei Shan, Yong-Lei Wang, Bing Shen, Bin Zeng, Jing Gong, Yan Huang, Huan Yang, Cong Ren, Hai-Hu Wen, Ang Li, Shuheng Pan, Da Wang, Qiang-Hua Wang We report on low-temperature scanning tunneling microscopy/spectroscopy studies of the electronic structure in single crystalline Ba0.6K0.4Fe2As2. Multiple superconducting gaps were observed in the density of states (DOS) and the sizes of the two dominant gaps are 7.6 meV and 3.3 meV, respectively. The flat bottom of the DOS spectra near zero bias indicates the nodeless feature of the gaps, while the global fitting to the spectra requires definitely the anisotropy. By applying magnetic fields, we observed ordered vortices with Andreev bound states in vortex cores. The bound states and their spatial evolution can be qualitatively explained by our numerical calculations when considering the multiband s-wave superconductivity. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B23.00011: Quasiparticle Interference in Iron-Based Superconductors Johannes Knolle, Alireza Akbari, Ilya Eremin, Roderich Moessner The phase diagram of iron-based superconductors exhibits an anti ferromagnetic phase at low doping, an unconventional superconducting phase at larger carrier concentration, and possibly a coexistence regime of both orders in between. What are the signatures of the different orders in the electronic spectrum and how can phase sensitive measurements distinguish between different order parameter symmetries? To address these questions we systematically calculate quasiparticle interference (QPI) signatures for the relevant candidate phases of iron-based superconductors. Experimentally, QPI can be probed through spectroscopic imaging-scanning tunneling microscopy (SI-STM) thanks to impurities unavoidably present in the sample. We show that in the anti ferromagnetic phase the rotational symmetry of the electronic structure is broken, signatures of which are also seen in the coexistence regime with both superconducting and magnetic order. In the superconducting regime the different scattering behavior for magnetic and non-magnetic impurities allows us to verify the $s\pm$ symmetry of the order parameter. Furthermore, we discuss the effect of possible gap minima or nodes. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B23.00012: Cryogenic Scanning Tunneling Spectroscopy of Superconducting Iron Chalcogenide Single Crystals J.Y.T. Wei, Igor Fridman, Kuo-Wei Yeh, Maw-Kuen Wu, Rongwei Hu, C. Petrovic We report scanning tunneling spectroscopy measurements on the iron-based superconductors of the ``11'' family including Fe$_{1-y}$Te$_{1-x}$Se$_{x}$ and Fe$_{1-y}$Te$_{1-x}$S$_{x}$. Conductance spectra and atomically-resolved images are obtained on single crystals down to 300 mK. A gap-like structure is observed, showing an asymmetric spectral background, non-trivial spatial variation and temperature dependence. We discuss our data in terms of possible gap anisotropy and doping inhomogeneities, and in relation to other recent spectroscopic measurements on iron-based superconductors. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B23.00013: Surface lattice dynamics of the 122-type iron pnictides Jing Teng, Chen Chen, Guorong Li, Amar Karki, Jiandi Zhang, Rongying Jin, Ward Plummer We present a systematic High Resolution Electron Energy Loss Spectra (HREELS) study of the surfaces lattice dynamics of the cleaved single crystals of the parent compounds AFe$_{2}$As$_{2}$ (A=Ba, Ca) as a function of temperature and sample treatment. The different phonon signatures on the competing surface reconstructions 1$\times $2 phase and ($\surd $2x$\surd $2)R45$^{^{\circ}}$ phase are studied. For Ba there are two optical phonon modes are observed at 18 and 29 meV, which can be identified as the $A_{1g}$ and $B_{1g}$ vibrations of the As and Fe atoms, respectively. A detailed discussion is given in terms of the interplay between the spin and lattice in this novel system [Preview Abstract] |
Session B24: Focus Session: Multiscale Modeling - Methodology and applications
Sponsoring Units: DCOMP DMPChair: Jincheng Du, University of North Texas
Room: D167
Monday, March 21, 2011 11:15AM - 11:51AM |
B24.00001: Recent Advances in Accelerated Molecular Dynamics Methods Invited Speaker: Many important materials processes take place on time scales that vastly exceed the nanoseconds accessible to molecular dynamics simulation. Typically, this long-time dynamical evolution is characterized by a succession of thermally activated infrequent events involving defects in the material. Over the last 14 years, we have been developing a new class of methods, accelerated molecular dynamics, in which the known characteristics of infrequent-event systems are exploited to make reactive events take place more frequently, in a dynamically correct way. For certain processes, this approach has been remarkably successful, offering a view of complex dynamical evolution on time scales of microseconds, milliseconds, and sometimes beyond. Examples include metallic surface diffusion and growth, radiation damage annealing, and dynamics of nanotubes and nanoscale clusters. After an introduction to these methods, I will present some recent advances and results, and then describe the major ongoing challenges and our current thinking on how to overcome them. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B24.00002: Multiscale (atomistic to mesoscopic) modeling of carbon nanotube materials Invited Speaker: A principal challenge in the development of computational models for investigation of collective dynamic phenomena in multi- component molecular systems or nanocomposites is presented by the gap between the atomistic description of the elementary structural units and the effective material behavior and properties. We approach this challenge through the development of computational models for dynamic simulations at intermediate (mesoscopic) length and time scales. An example of a mesoscopic model that is being currently designed in our group for carbon nanotube (CNT)-based materials and nanocomposites will be discussed in the presentation. The mesoscopic dynamic model for CNT materials is based on a coarse-grained representation of individual CNTs as chains of stretchable cylindrical segments [1] and a computationally- efficient ``tubular potential'' method describing the van der Waals interactions among the CNT segments [2]. Mesoscopic descriptions of CNT buckling and fracture are developed based on the results of atomistic simulations and incorporated into the model. Mesoscopic simulations performed for a system composed of randomly distributed and oriented CNTs predict a spontaneous self-assembly of CNTs into a continuous network of bundles with partial hexagonal ordering of CNTs within the bundles [2]. The structures produced in the simulations are similar to the structures of CNT films and mats observed in experiments. The first results illustrating the applications of the model for investigation of the response of CNT materials to dynamic mechanical loading, analysis of the structural dependence of the thermal transport properties [3] and gas permeability in CNT films will be briefly discussed in the presentation. Challenges and possible future directions in the development of a realistic mesoscopic description of nanocomposite materials will be outlined.\\[4pt] [1] L.V. Zhigilei, C. Wei, D. Srivastava, Phys. Rev. B 71, 165417, 2005.\newline [2] A.N. Volkov, L.V. Zhigilei, J. Phys. Chem. C 114, 5513, 2010; ACS Nano 4, 6187, 2010.\newline [3] A.N. Volkov, L.V. Zhigilei, Phys. Rev. Lett. 104, 215902, 2010. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B24.00003: Toward Distinct Element Method Simulations of Carbon Nanotube Systems Evgeniya Akatyeva, Tyler Anderson, Ilia Nikiforov, David Potyondy, Roberto Ballarini, Traian Dumitrica We propose distinct element method modeling of carbon nanotube systems. The atomic-level description of an individual nanotube is coarse-grained into a chain of spherical elements that interact by parallel bonds located at their contacts. The spherical elements can lump multiple translational unit cells of the carbon nanotube and have both translational and rotational degrees of freedom. The discrete long ranged interaction between nanotubes is included in a van der Waals contact of nonmechanical nature that acts simultaneously with the parallel bonds. The created mesoscopic model is put into service by simulating a realistic carbon nanotube ring. The ring morphology arises from the energy balance stored in both parallel and van der Waals bonds. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B24.00004: Coarse-Grained Monte Carlo Simulations of Continuous Systems Xiao Liu, Warren Seider, Talid Sinno Various types of Monte Carlo simulations are used extensively to simulate an enormous range of material properties. Restricting particle positions to fixed lattice sites can substantially increase the computational efficiency of a simulation, and this benefit increases as the lattice becomes coarser. However, the confinement of particle positions to a rigid lattice necessarily reduces the available configurational degrees of freedom in a system and this constraint can become very important at elevated temperatures. In this presentation, we discuss a new framework for performing Metropolis Monte Carlo and kinetic Monte Carlo (KMC) simulations of continuous systems on coarse, rigid lattices, while preserving the phase-space contributions of the missing degrees-of-freedom. The approach relies on the pre-computation of coarse-grained interaction potentials using equilibrium sampling of small systems. The coarse-grained simulation methodologies are shown to reproduce both equilibrium (e.g. phase diagram), and non-equilibrium (e.g. aggregation dynamics) features in the corresponding fully resolved systems. In the latter case, the coarse potential is used to compute rates for moves in a coarse-grained KMC system. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B24.00005: Quantum-mechanical and QM/MM simulations of proton dissociation free energies in solution Noam Bernstein, Csilla Varnai, Monika Fuxreiter, G\'abor Cs\'anyi Chemical reactions often occur in the presence of a solvent, in particular water for biological systems. To describe such processes a quantum mechanical (QM) description of the reaction site is needed, combined with a large number of solvent molecules that affect the reaction via their electrostatic fields and free energy effects of their long-range structure. We have simulated the dissociation of a proton from the side chain of a tyrosine molecule, as a realistic model system. We compute a free energy difference, using umbrella integration, from the average restraint force as a function of O$^-$-H$^+$ distance as the proton is transferred from the side-chain to nearby water molecules to form H$_3$O$^+$. We use a combination of periodic QM calculations using DFT and force-mixing QM/MM simulations implemented in QUIP and CP2K. The pure QM calculations are used for reference values and for determining appropriate restraint conditions for the free energy calculations. The force-mixing QM/MM method, which gives accurate forces throughout the system, is used to evaluate free energies for comparison with experiment. We extrapolate the free energy for the initial transfer of the proton to the bulk solvated proton regime by analytically computing electrostatic and entropy contributions. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B24.00006: Understanding Vibrational Spectra of Silicon Nanocrystals Dundar Yilmaz, Cem Sevik, Ceyhun Bulutay, Tahir Cagin After the discovery of light emission from porous Si, nanostructured Si became a promising material for opto-electronic applications. For two decades lots of both experimental and theoretical works done in order to understand mechanisms behind the interaction of light with low dimensional forms of Si. In this work we employed MD simulation technique. The simulation details are similar to our earlier work except we used Large Scale Atomistic Molecular Modeling Package Software (LAMMPS) with ReaxFF package as an integrator. We used constant pressure constant temperature (NPT) ensemble with a simulation box size around 4.2 nm. We inserted silicon nanocrystals into amorphous silicon dioxide matrix with diameter ranging from 2 nm to 3.2 nm using a scheme defined in our previous work7. We also simulated free standing hydrogen passivated nanocrystals with same diameters to compare effects of oxide matrix on the nanocrystals. The effect of strain on vibrational spectra of Silicon Nanocrystals is studied as a function of nanocrystal diameter using reactive molecular dynamics simulations technique for both embedded and hydrogen passivated nanocrystals. With use of refined parameters our calculations reproduce the redshift of the Raman active transverse optical peak of Si-Si vibrations with decreasing the nanocrystal size. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B24.00007: \textit{Ab initio} study of the \textit{thermodynamic properties and the} phonon calculations of Zircon and Reidite Mrunalkumar Chaudhari, Jincheng Du Zircon and Reidite are the polymorphs of Zirconium Silicate which find its importance geologically, because of its natural hosting to various radioactive elements in the crust of the earth. High permittivity also makes it a promising material for the gate dielectric material in metal-oxide semiconductors. Knowledge of the thermodynamic properties and the phonon based calculations is very critical to understand the high temperature and high pressure properties in order to consider its application as an effective natural storage for the radioactive wastes. These properties are thoroughly studied both computationally and experimentally for zircon, while significantly less attention was paid to reidite in the literature. The thermodynamic properties and phonon calculations of Zircon and Reidite were studied using ab initio based periodic density-functional theory (DFT) based calculations using the generalized gradient approximation (GGA). Various properties such as free energy, internal energy, entropy, heat capacity and thermal displacement as a function of temperature is calculated using the PHONON software. Various phonon based density of states and dispersion curves are calculated and compared with the experimental data. No first principles based computational results were reported up to now. Calculated bulk properties agree very well with the experimental data in the literature. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B24.00008: One-dimensional model of interacting-step fluctuations on vicinal surfaces: Analytical formulas and kinetic Monte-Carlo simulations Paul Patrone, T.L. Einstein, Dionisios Margetis We study a 1+1D, stochastic, Burton-Cabrera-Frank (BCF) model of interacting steps fluctuating on a vicinal crystal. The step energy accounts for entropic and nearest-neighbor elastic-dipole interactions. Our goal is to formulate and validate a self-consistent mean-field (MF) formalism to approximately solve the system of coupled, nonlinear stochastic differential equations (SDEs) governing fluctuations in surface motion. We derive formulas for the time-dependent terrace width distribution (TWD) and its steady-state limit. By comparison with kinetic Monte-Carlo simulations, we show that our MF formalism improves upon models in which step interactions are linearized. We also indicate how fitting parameters of our steady state MF TWD may be used to determine the mass transport regime and step interaction energy of certain experimental systems.\footnote{P. Patrone, T. L. Einstein, D. Margetis, Phys. Rev. E, in press.} [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B24.00009: Real-time visualization of excited-state dynamics in molecular chains Yonghui Li, Carsten Ullrich Time-dependent density-functional theory allows one to calculate excitation energies and the associated transition densities in principle exactly. The transition density matrix (TDM) provides additional information on electron-hole localization and coherence of a specific excitation. We have extended the TDM concept into the real-time domain in order to visualize the excited-state dynamics in conjugated molecules. Our computational scheme is based on solving the time-dependent Kohn-Sham equations with the OCTOPUS code and then calculating the time-dependent Kohn-Sham TDM using a spatial partitioning scheme. The method is applied to show in real time how locally created electron-hole pairs spread out over neighboring conjugated molecular chains. The coupling mechanism, electron-hole coherence, and the possibility of charge separation are discussed. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B24.00010: Thermal conductivity of bulk crystals from first-principles lattice dynamics Keivan Esfarjani, Junichiro Shiomi, Gang Chen Based on first-principles density-functional calculations, we have developed and tested a force field for Silicon, which can be used for Molecular dynamics simulations and the calculation of its thermal properties. This force field uses the exact Taylor expansion of the total energy about the equilibrium positions up to 4th order. In this sense, it becomes systematically exact for small enough displacements, and can reproduce the thermodynamic properties of Si with high fidelity. Having the harmonic force constants, one can easily calculate the phonon spectrum of this system. The cubic force constants, on the other hand, will allow us to compute phonon lifetimes and scattering rates. Results on equilibrium Green-Kubo molecular dynamics simulations of thermal conductivity as well as an alternative calculation of the latter based on the relaxation-time approximation will be reported. The accuracy and ease of computation of the lattice thermal conductivity using these methods will be compared. Results on other non-trivial materials such as Heuslers will also be presented. This approach paves the way for the construction of accurate bulk interatomic potentials and force constants database, from which lattice dynamics and thermal properties can be calculated and used in larger scale simulation methods such as Monte Carlo. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B24.00011: Optimizing laser pulses for controlled excitation of materials and molecules Roland Allen This talk extends the ideas of recent papers, including [1] Zhou et al., Phys. Rev. B 82, 075433 (2010); [2] Lin et al., J. Phys. Cond. Mat. 21, 485503 (2009); and [3] Allen, Phys. Rev. B 78, 064305 (2008). There are three basic points: (1) A combination of analytical models and density-functional-based simulations provides guidance for tailoring laser pulses to achieve optimum vibrational and electronic excitation. In [1] it was found that the maximum relative response of a specific vibrational mode with period T is achieved when the FWHM duration of a pulse is equal to 0.42 T, and later work by Jiang et al. provided a similar criterion for the duration and delay times in a series of pulses. (2) It is possible for microscopic (density-functional-based) simulations to provide input for larger-scale simulations, in the form of stresses etc. (as demonstrated in [2]) and excitation-dependent interatomic potentials. (3) It is possible to extend current techniques for simulations of the coupled dynamics of electrons, nuclei, and the radiation field in highly-excited materials, using for example nonequilbrium Green's functions. [Preview Abstract] |
Session B25: Superconductivity: Spin properties, Structure and Dynamics
Sponsoring Units: DCMPChair: Young Lee, Massachusetts Institute of Technology
Room: D166
Monday, March 21, 2011 11:15AM - 11:27AM |
B25.00001: Structure, spin-stripe order, and superconductivity in La$_{1.905}$Ba$_{0.095}$CuO$_4$ with and without 1\% Zn substitution of Cu Jinsheng Wen, Z. Xu, G. Xu, Q. Jie, M. Hucker, A. Zheludev, W. Tian, B. Winn, J. Zarestky, D. Singh, T. Hong, Q. Li, G. Gu, J. Tranquada We have performed susceptibility, thermal transport, and neutron scattering measurements to study the effect of Zn and magnetic field on the structure, spin-stripe order and superconductivity, and the interplay between them in La$_{1.905}$Ba$_{0.095}$CuO$_4$ with and without 1\% Zn. It is shown that the bulk superconductivity is depressed by either the Zn doping or the magnetic field, spin stripe order is enhanced, and the structure is unaffected. For a range of magnetic field, the spin stripe order appears to stabilize a quasi-two-dimensional vortex glass phase. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B25.00002: Electron-Spin Excitation Coupling in an Electron Doped Copper Oxide Superconductor Pr0.88LaCe0.12CuO4-$\delta$ Jun Zhao, Francis C. Niestemski, Shankar Kunwar, Shiliang Li, Paul Steffens, Arno Hiess, Hye Jung Kang, Stephen D. Wilson, Ziqiang Wang, Pengcheng Dai, Vidya Madhavan We use polarized and unpolarized inelastic neutron scattering to study the magnetic excitations of the electron doped copper oxide superconductors Pr0.88LaCe0.12CuO4-$\delta $ (PLCCO, Tc =21 K, 24 K) over a wide energy range. We found the energy dependence of the imaginary part of the dynamic susceptibility displays two distinct energy scales in both samples. Interestingly, the STS measurements on the same samples reveal two modes that evolve with $T_{c}$ in a similar manner as neutron modes. A comparison of the spatial and temperature dependence of the neutron and STS modes suggests that the low energy mode is associated with antiferromagnetism while the high energy mode is connected with superconductivity. These results suggest that spin excitations are the mediating glue for the electron pairing in PLCCO. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B25.00003: Fluctuating stripes at the onset of the pseudogap in the high-Tc superconductor Bi2Sr2CaCu2O8+x Colin Parker, Pegor Aynajian, Eduardo H. da Silva Neto, Aakash Pushp, Shimpei Ono, Jinsheng Wen, Zhijun Xu, Genda Gu, Ali Yazdani A long standing question in high-$T_c$ cuprates has been the interplay between pseudogap, which is generic to all hole-doped cuprates, and stripes, whose static form occurs in only one family of cuprates over a narrow range of the phase diagram. I will present new data [1] on the spatial reorganization of electronic states at the onset of the pseudogap state ($T*$) in the high-temperature superconductor $\textrm{Bi}_2\textrm{Sr}_2\textrm{CaCu}_2\textrm{O}_8+x$ taken with a scanning tunneling microscope (STM). The onset of the pseudogap phase coincides with the appearance of electronic patterns whose doping and energy dependence has the predicted characteristics of fluctuating stripes. While demonstrating that the fluctuating stripes emerge with the onset of the pseudogap state and occur over a large part of the cuprate phase diagram, these experiments indicate that they are a consequence of pseudogap behavior rather than its cause.\\[4pt] [1] C. V. Parker, et al, Nature 2010 (in press) doi:10.1038/nature09597 [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B25.00004: Manipulating stripes in La$_{2-x}$Ba$_{x}$CuO$_{4}$ in extreme environments Markus Huecker, Genda Gu, Zhijun Xu, Jinsheng Wen, John M. Tranquada, Martin von Zimmermann Competing magnetic and electronic interactions in the copper-oxide high temperature superconductors often result in nanoscale inhomogeneity of the charge and spin density. Such observations motivated a proposal that dynamic electronic inhomogeneities are intrinsic to the copper-oxide planes, and can result in electronic states that break their four-fold symmetry. We have performed high-energy single-crystal X-ray diffraction in high magnetic fields and at high pressure to show that the charge and spin stripe phase in La$_{2-x}$Ba$_{x}$CuO$_{4}$ constitutes such a state. In particular, our results provide strong evidence that charge stripe correlations in the cuprates are electronically driven and are enhanced within vortices. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B25.00005: Hidden itinerant-spin extreme in heavily-overdoped LSCO revealed by dilute Fe doping: A combined neutron scattering and ARPES study Ruihua He, M. Fujita, M. Enoki, M. Hashimoto, S. Iikubo, S.-K. Mo, H. Yao, T. Adachi, Y. Koike, Z. Hussain, Z.-X. Shen, K. Yamada Fluctuations of the localized spins on Cu and the itinerant spins of doped holes have been theoretically conceived to be both essential for high-Tc superconductivity. While the former clearly leads to an antiferromagnetic order in the undoped Mott phase (the localized-spin extreme), it has remained open whether the latter has an inherent tendency towards the formation of some magnetic order at very high dopings where it becomes dominant (the itinerant-spin extreme). By perturbing the non-magnetically-ordered heavily-overdoped LSCO with 1\% Fe doping, we found by elastic neutron scattering an incommensurate magnetic order induced below 20 K, which cannot be ascribed to the localized spins on Cu or doped Fe. ARPES study of the itinerant doped holes suggests that this order is driven by a strong Fermi surface nesting, which is inherent in the pristine LSCO but has so far eluded a clean revelation. Our finding presents the first experimental example of the long-sought itinerant-spin extreme for cuprates and supports its important fluctuations that should be considered along with its localized counterparts for HTSC at intermediate dopings. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B25.00006: Two distinct electronic sites in the Cu-O plane of the (La, Sr)CuO(4) pseudogap state Robert Smith, Philip Kuhns, Arneil Reyes, Gregory Boebinger At intense magnetic fields (30 T) $^{17}$O NMR exhibits two distinct signatures for planar oxygen sites instead of the singular site expected from the identical lattice symmetry at oxygen sites in the copper-oxygen plane for underdoped, orthorhombic La$_{2- x}$Sr$_{x}$CuO$_{4}$. Analysis of Knight shift, linewidth, quadrupolar splitting and spectral asymmetry indicates that roughly 75$\%$ of the planar oxygens evidence antiferromagnetically- correlated nearest neighbor Cu moments at temperatures below $\sim$30 K, consistent with previous reports. A second planar oxygen site first observed in this study shows that there are mobile holes on roughly 25$\%$ of the planar oxygen sites that (a) suppress magnetism for all T$<$300K and (b) show a Knight shift that drops to zero below $\sim$60 K, evidencing pair formation at a temperature well above the superconducting transition temperature ($\sim$4 K at 30 T) and more than twice the superconducting transition temperature at zero magnetic field. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B25.00007: Oxygen staging in phase separated La2-xSrxCuO4+y Hashini Mohottala, Linda Udby, Samuel Emery, B.O. Wells, J. I. Budnick, Christof Niedermayer, Kim Lefmann, N.H. Anderson, F.C. Chou We studied oxygen staging in a series of La2-xSrxCuO4 (LSCO) samples using neutron scattering. The samples were oxidized using electrochemistry. Electronic phase separation was previously reported in the oxygen rich LSCO system with two stable phases identified as optimally doped superconducting phase and a magnetic phase (1/8th like) with the same ordering temperatures at 40 K [1]. Our present studies show staging in this system. Although staging was observed and extensively studied in the samples with no Sr [2], it has not been reported or systematically studied in the systems with both Sr and oxygen. We do find staging in the oxygenated LSCO system, but the staging peaks evolve as Sr concentration increases and excess oxygen concentration decreases. [1] H Mohottala et al. Nature Materials \textbf{5}, 377 - 382 (2006), [2] B.O. Wells et al., Z. Phys. B 100, 535 (1996). [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B25.00008: Tunable High Q Superconducting Microwave Resonator Z. Kim, C.P. Vlahacos, J.E. Hoffman, J.A. Grover, B.K. Cooper, J.R. Anderson, A.J. Dragt, C.J. Lobb, L.A. Orozco, S.L. Rolston, J.M. Taylor, F.C. Wellstood We have developed a frequency tuning system for a ``lumped-element'' thin-film superconducting Nb microwave resonator on sapphire intended for coupling to cold trapped $^{87}$Rb atoms. $^{87}$Rb has hyperfine ground states, $|^{5}S_{1/2},F=1\rangle$ and $|^{5}S_{1/2},F=2\rangle$, which are separated by about 6.83 GHz and available as a two-level system for a qubit. The resonator consists of a meandering inductor and an interdigitated capacitor coupled to a transmission line. At \emph{T}=12 mK and on resonance at 6.863 GHz, the loaded quality factor is 40,000. We employ an Al pin as a frequency tuner by placing it above the inductor using a piezo stage so that one can effectively change the inductance of the resonator. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B25.00009: Nanomechanical Torque Magnetometry of Individual Mesoscopic Superconductors Abdul Suhel, Doug Vick, Mark R. Freeman, John P. Davis We have developed an extremely sensitive form of torque magnetometry based on thin nanomechanical resonators [1]. Previously, we applied this technique to study single magnetic vortices in nanomagnetic samples [2]. One advantage of this technique is that it has good time resolution, and each measurement can be performed quickly enough to gather significant statistics on such events [2]. We are now applying this technique to study single mesoscopic superconducting samples. We intend to measure the magnetic moment associated with superconducting vortices, as well as other magnetic effects that occur in mesoscopic superconductors [3,4]. We will discuss our progress towards this goal. [1] J.P. Davis, et al. Appl. Phys. Lett. \textbf{96}, 072513 (2010). [2] J.P. Davis, et al. New Journal of Physics \textbf{12}, 093033 (2010). [3] A.K. Geim, et al. Nature \textbf{396}, 144 (1998). [4] A.K. Geim, et al. Nature \textbf{407}, 55 (2000). [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B25.00010: Strong disorders in cuprate superconductors in d-density wave state Hong-Yi Chen, Chung-Pin Chou The local density of states on strong disordered d-wave superconductor in d-density wave state is studied. Recently, E.W. Hudson et al [Nature Physics 4, 108 (2008) ], reported a method to investigate the pseudogap. We explore the selfconsistent Bogoliubov-de Genns' equations with strong disorders. The quantum interference leading to definitive quasipartical spectra has also been considered. Without d- density wave state, the numerical results are in satisfactory agreement with the observations from STM experiments. With d- densitywave state, a new result can be used to determine the properties of pseudogap. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B25.00011: Resonant Inelastic X-Ray Studies on the Cu-L edge in 1-Dimensional Cuprate Chains James Lee, M. Yi, W.S. Lee, K. Zhou, C. Monney, S. Johnston, J. van den Brink, T. Schmitt, L. Patthey, T.P. Devereaux, K. Kudo, Y. Koike, Z.X. Shen Resonant Inelastic X-ray Scattering (RIXS) is a photon-in, photon-out spectroscopy technique with the capability of seeing many-body interactions in great detail. The recent achievement of sub-eV resolution in RIXS has opened up a new avenue for experiments to study these effects quantitatively. Here we present high-resolution RIXS data at the Cu L-edge on the quasi-one-dimensional edge-sharing chain compound, Ca$_{2+x}$Y$_{2-x}$Cu$_{5}$O$_{10}$, which is the only known dopable quasi-1D chain compound. Charge excitations corresponding to doped holes can be clearly resolved when the photon energy is tuned to the resonance at the hole band. In addition, we find that the d-d excitations appear to disperse with incident photon energy and momentum, and have a nontrivial intensity modulation. Effects of hole doping on these excitations will be discussed. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B25.00012: High-resolution RIXS measurement at O K-edge on the edge-shared chain cuprates, Ca$_{2+x}$Y$_{2-x}$Cu$_{5}$O$_{10}$ W.S. Lee, J. Lee, M. Yi, K. Zhou, S. Johnston, T. Schmitt, Jeroen van den Brink, T.P. Devereaux, K. Kudo, Y. Koike, L. Patthey, Z.X. Shen Quasi one dimensional copper oxides have been model systems in the field of correlated electron physics, because of rich phenomena exhibited in a relatively simple geometry. Its magnetic ground states, fluctuations, and excitations have been investigated extensively by theorists and experimentalists. Among the known quasi 1-D spin-chain compounds, Ca$_{2+x}$Y$_{2-x}$Cu$_{5}$O$_{10}$ is the only compound that can be hole-doped in a wide doping range, providing a unique opportunity to study the dynamics of hole in the quasi-1D environment. Here, we report ultrahigh resolution resonant inelastic soft x-ray scattering experiment at the O K-edge. With an energy resolution of $\sim $50 meV, we resolved rich charge excitations in the sub-eV range that has not been observed in the previous RIXS measurement on the same materials. In particular, we have resolved clear multi-phonon excitations near the elastic peak, suggesting a strong electron-phonon coupling in this quasi-1D system. Doping dependence of these excitations will also be demonstrated. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B25.00013: Suppression of superconductivity in the fully frustrated Josephson junction array with site dilution Bruna de Oliveira, Tommaso Roscilde, Stephan Haas We study the effects of geometric randomness on the ordered phases and phase transitions of frustrated classical Josephson junction (JJ) arrays. In particular, we consider a square lattice array with maximal frustration (one half flux quantum through each plaquette) and with site dilution. The homogeneous model is known to feature two phase transitions: an Ising transition for the ordering of vortices into a crystalline state, and a Kosterlitz-Thouless transition for the appearance of superconductivity. A detailed Monte Carlo study shows a strong effect of disorder on the separation of the ordering temperatures. In particular, superconductivity is completely suppressed well before one reaches the percolation threshold of the lattice. We hence suggest that diluted JJ arrays with magnetic frustration are strong candidates for the experimental observation of a vortex crystal in the normal state. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B25.00014: Dynamical Spectral weight transfer in the cuprates is described by the Hubbard model Philip Phillips, Mark Jarrell Recently, Peets and colleagues [1] measured the x-ray intensity at the oxygen K-edge in overdoped La$_{2-x}$Sr$_x$CuO$_{4\pm\delta}$ (LSCO) and Tl$_2$Ba$_2$CuO$_{6+\delta}$. They concluded that, unlike the underdoped samples of LSCO and YBa$_2$Cu$_3$O$_x$ in which the integrated intensity increases at least linearly with doping, it saturates abruptly for a hole count exceeding $x_c\approx 0.23$. They interpreted the saturation as a breakdown of the 1-band Hubbard model in the cuprates. We analyse all the available data and show that they are completely described by the 1-band Hubbard model. The purported saturation is shown to occur at the doping level at which the dynamical contribution to the spectral weight turns off. \\[4pt] [1] D. C. Peets, Phys. Rev. Lett. {\bf 103}, 087402 (2009). [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B25.00015: Superstructure and Magnetism in Na0.825CoO2 - A NMR study P.Y. Chu, B.-L. Young, J.Y. Juang, G.J. Shu, M.-W. Chu, F.C. Chou We report our NMR study of the Na ordering and magnetism in the single crystal of Na0.825CoO2, as follows. 1. The atomic order of Na is observed from the well- resolved 23Na NMR peaks, which suggests 6 Na inequivalent sites. The 59Co NMR also suggests 4 distinct Co sites due to Na ordering. 2. The magnetic field-induced transition from antiferromagnet to ferromagnet is verified by our 23Na spectra at different temperatures and fields. A slow spin dynamic of glassy-like behavior is observed near the AFM and FM boundary. 3. We observed the NMR frequency shift anomaly near 50K, the same temperature where the susceptibility shows the curvature. The anomaly suggests the onset of the magnetic correlation in prior to magnetic ordering. [Preview Abstract] |
Session B26: Focus Session: Iron Based Superconductors -- Growth
Sponsoring Units: DMP DCOMPChair: Leonardo Degiorgi, ETH Zurich
Room: D162/164
Monday, March 21, 2011 11:15AM - 11:27AM |
B26.00001: Influences of material processing on the microstructure and intergranular current of polycrystalline Sm1111 iron-pnictides A. Yamamoto, J. Jiang, F. Kametani, A. Polyanskii, J. Weiss, E. Hellstrom, D. Larbalestier, A. Martinelli, A. Palenzona, M. Tropeano, M. Putti We have prepared polycrystalline Sm1111 bulk samples by sintering and hot isostatic pressing (HIP) and studied the influence of processing on the microstructure and intergranular current. After sintering and HIPping, samples are denser with much less impurity SmOF and grain boundary wetting FeAs. They then show significantly larger hysteresis loops than as- prepared samples. But Tc of HIPed samples is lower than samples with optimal doping, indicating loss of fluorine during later processing. Nevertheless, even after the fluorine loss and its associated carrier density reduction produced by the HIP treatment, both Magneto-Optical and remanent magnetization analyses showed that the intergranular current was enhanced. We conclude that the denser microstructure and smaller impurity phase content of HIPped samples enhances their connected fraction, even as the doping state of the sample is degraded by F loss. Our study emphasizes that current transport in polycrystalline pnictides is a complex balance involving the need to control carrier density, especially at grain boundaries, while densifying the microstructure and avoiding normal-state grain boundary wetting phases like FeAs. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B26.00002: Synthesis and superconducting properties of FeTe1-xSe Single Crystals under high magnetic fields Tesfaye Gebre, G. Li, J. Whalen, B. Conner, M. Kostov, T. Siegrist, L. Balicas Single crystals of superconductor FeTe$_{1-x}$Se$_{x}$ (0.1 $\le $ x $\le $ 0.5) were synthesized using optical floating zone, Bridgeman technique, and solid stat reaction. The samples were synthesized under various temperature gradients and cooling rates. Crystals were characterized via EDX, X-ray scattering, magnetization and transport measurements. Upper critical fields Hc$_{2 }$as estimated through the Werthamer-Hohenberg-Helfand (WHH) formalism indicate that these materials strongly surpass the weak coupling Pauli limiting field indicating that the shape of their phase diagram under field is essentially controlled by the Pauli effect. Annealing, leads to a metallic temperature dependence of the resistivity, and to sharper superconducting transitions. Despite the relatively small increase in single crystallinity, as quantified by single crystal x-ray diffraction measurements, we observe a different phase diagram under high magnetic fields when compared to non-annealed samples. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B26.00003: Single crystal growth and superconducting properties of LiFeAs Bumsung Lee, Seunghyun Khim, Jae Wook Kim, Eun Sang Choi, Jung Soo Kim, G. R. Stewart, Kee Hoon Kim LiFeAs, a representative compound for `111' system of Fe-based superconductors, has a tetragonal Cu$_{2}$Sb-type structure and possesses a single Fe-As tetrahedral layer sandwiched by the double Li layers. This structural characteristic provides a unique opportunity to realize homogeneous Li terminating surface upon cleaving, and this possibility makes LiFeAs attractive for investigating the intrinsic properties of Fe-based superconductors with various spectroscopic tools. In this sense, growing a large LiFeAs single crystal is a necessary step for studying intrinsic properties of the Fe-based superconductors. We report the successful growth of high quality LiFeAs single crystal by Sn-flux. Electrical resistivity studies reveal the superconducting onset temperature is 18.2 K with a transition width less than 1.1 K and RRR is about 24. Bulk superconductivity is supported by perfect shielding in the magnetic susceptibility and a clear jump in the specific heat. Upper critical field slopes of d$H_{c2}^{c}$/d$T\approx $-1.39 and d$H_{c2}^{ab}$/d$T\approx $-2.99 T/K near $T_{c}$ predict $H_{c2}^{c}$(0)$\approx $17.2 and $H_{c2}^{ab}$(0)$\approx $36.9 T, and it points to a modest superconducting anisotropy about 2.3 near $T_{c}$. This observed modest anisotropy is a bit smaller than the band calculation results based on the GGA approximation but larger than the prediction considered effects of electron correlation through the dynamic mean field theory. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B26.00004: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 12:03PM - 12:15PM |
B26.00005: Superconductivity and wire fabrication of FeSe family Toshinori Ozaki, Keita Deguchi, Yoshikazu Mizuguchi, Hiroaki Kumakura, Yoshihiko Takano The 11 family is an fascinating iron-based superconducting system for not only elucidation of superconducting mechanism but also technological applications because of the simplest crystal structures, the less toxic and high critical field. Recently, we found that the superconductivity appears in the specimen immersed in alcoholic beverages. Focused on the pressure dependence of Se height from Fe layer in FeSe, we found that the Tc is correlated to Se height. Moreover, the anion height dependence of $T_{c}$ for all FeAs-based superconductor obeyed a universal curve with a peak around 1.38 {\AA}. We succeeded in observing the transport $J_{c}$ in the single- and 7-core wires of FeTe$_{x}$Se$_{1-x}$ superconductor using an in-situ powder-in-tube (PIT) method. The $J_{c}$ values in single- and 7-core wire are as high as 159 A/cm$^{2}$ and 100 A/cm$^{2}$ at 4.2 K, respectively. It is considered that the optimization of the composition, together with the improvement of the grain boundary in FeTe$_{x}$Se$_{1-x}$ superconducting wires, will lead to higher $J_{c}$. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B26.00006: Synthesis of Large Single Crystals of LaMnPO Greg Smith, Jack Simonson, Carlos Marques, Victor Leyva, Meigan Aronson The compound LaMnPO is isostructural with LaFeAsO, a recently discovered high-temperature superconductor, but optical spectroscopy and transport measurements of this compound have been heretofore limited by small crystal size. Accordingly, crystal syntheses from Sn, Pb and molten salt fluxes (including NaCl/KCl, LiCl/NaCl, KCl, CaCl$_2$, BaCl$_2$/CaCl$_2$ and KCl/CaCl$_2$) were investigated. Fluorine doping was explored; concentrations less than 30 at.\% (nominal) had no effect on crystal size; concentrations greater than 40 at.\% (nominal) did not yield crystals. Once growth parameters were optimized, the crystals grew in a flat rectangular shape with black luster; their composition was verified with powder and single crystal x-ray diffraction. Successful growths yielded crystals with dimensions up to 3.2 mm by 1 mm by 10 $\mu$m, a significant improvement upon previously reported growths in the literature. These large crystals enabled our group to perform a wide range of experiments that were previously restricted to polycrystalline materials. It may be feasible to extend these methods to the synthesis of similar compounds. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B26.00007: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 12:39PM - 12:51PM |
B26.00008: Faceted nanocrystalline growth of FeTe on SrTiO$_{3}$(001) Yi Li, Zhaoliang Liao, Lina Chen, Jianneng Li, Amar Karki, Rongying Jin, Ward Plummer, Jiandi Zhang The new class of iron based superconductors has ignited the materials community. Of all of these new superconductors, the Fe chalcogenide system is probably the simplest at least in structure. One of most interesting questions is how the properties of these systems change as the structure is manipulated. We have studied ultra thin films of FeTe grown on a SrTiO$_{3}$(001) surface by using Pulsed Laser Deposition (PLD) and characterized with surface techniques. We observe the formation of faceted nanocrystalline islands at the surface of thin films. The structure of the islanded surface, including the faceting angle of nanostructures, has been studied by Low Energy Electron Diffraction (LEED) as well as scanning tunneling microscope (STM). The correlation of such a 3D-type growth with growth conditions and substrate-induced strain in the ultra-thin films are discussed. *Supported by NSF-DMR1005562 [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B26.00009: Superconducting Iron-Chalcogenide Thin Films and Coated Conductors Qiang Li, Weidong Si Superconducting iron-chalcogenide superconducting films have been grown on a variety of substrates, including single crystalline and metallic substrates. In this presentation, we will report transport and structural properties of these films. The superconducting transition temperature of these films is significantly higher than that of the corresponding bulk materials. Analytical electron microcopy analysis indicates substantial structural difference between thee films and single crystals at atomistic level. Detailed measurement of the angular dependence of upper critical field and critical current density demonstrated that iron chalcogenide superconductors have lower anisotropy factor in comparison to the high Tc cuprates, that presents the unique opportunity for this class of materials in high field application. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B26.00010: Superconducting properties of FeSe$_{0.5}$Te$_{0.5}$ and FeTe:O$_x$ thin films Weidong Si, Ayan Bhattacharya, Su Jung Han, Ivo Dimitrov, Lijun Wu, Qiang Li High quality superconducting thin films of FeSe$_{0.5}$Te$_{0.5} $ and FeTe:O$_x$ have been grown by pulsed laser deposition on various substrates including coated conductor. Thin films of FeSe$_{0.5}$Te$_{0.5}$ have a higher superonducting transition temperature Tc (onset start around 20K and zero resistance about 16~17K) than that of bulk. High resolution transmission electron microscopy has identified a microstructure only in thin films, which may be associated with the higher Tc. Magneto-transport measurement has been carried out to check the angular dependence of upper critical field Hc2 and the critical current density of the films with the direction of magnetic field. Both have shown a weak anisotropy. These films have a special high dHc2/dT at Tc, especially for FeTe:O$_x$ film. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B26.00011: Maze-like surface reconstruction on pure SrFe$_2$As$_2$ observed by STM/STS Michael Dreyer, Mark Gubrud, Hui Wang, Shanta Saha, Nick Butch, Kevin Kirshenbaum, Johnpierre Paglione We measured undoped SrFe$_2$As$_2$ samples using a low temperature scanning tunneling microscope. Similar samples showed superconductivity in up to 15\% of its volume in measurements of the diamagnetic screening although being undoped as confirmed by energy dispersive x-ray spectroscopy. The samples where cleaved at room temperature at a pressure $P<10^{-8}$ mbar before introducing them into the low temperature scanning tunneling microscope (LTSTM) operating at a temperature of $T=4.2$ K. Beside the usual striped surface reconstruction and disordered regions we observed areas showing a maze like reconstruction. Atomically resolved images strongly suggest that the maze structure is formed by adatoms and thus, by inference, is the striped phase. Scanning tunneling spectroscopy (STS) measurements showed a superconducting gap on both the striped and maze phase of the sample, similar to the spectra on a cobalt doped sample. The results will be discussed in detail. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B26.00012: Cleavage behavior and surface states in iron pnictides Klaus Koepernik, Alexander Lankau, Helmut Eschrig, Jeroen van den Brink, Sergey Borisenko, Erik van Heumen, Mark S. Golden We present a density functional study of the surface electronic structure and the cleavage behavior of LiFeAs and Co-doped BaFe$_{2}$As$_{2}$. The results are discussed together with angle resolved photo emission (ARPES) and low energy electron diffraction (LEED) data. The two systems behave rather differently and we conclude that LiFeAs will be the ideal system for surface sensitive probes among the iron pnictide familly. [Preview Abstract] |
Session B27: Focus Session: Superconducting Qubits - Measurement
Sponsoring Units: GQIChair: Matthias Steffen, IBM Research
Room: C155
Monday, March 21, 2011 11:15AM - 11:27AM |
B27.00001: Quantum Noise in a Chirped Superconducting Nonlinear Resonator Kater Murch, R. Vijay, Ido Barth, Lazar Friedland, Irfan Siddiqi A nonlinear Josephson junction oscillator driven near resonance can exhibit bistability, forming the basis for sensitive, digital quantum state readout. We consider the case of a high-Q resonator embedded with a Josephson junction excited with a chirped frequency signal. For sufficient drive amplitude, the resonator phase locks with the drive signal and enters the high amplitude oscillation state, a phenomenon known as autoresonance. The probability of capture in a given chirped pulse depends on the initial phase difference between the drive signal and of the fluctuation induced oscillations of the resonator. We find that the width of this threshold is in agreement with recent theoretical predictions and is set by zero-point fluctuations of the resonator. Autoresonant capture forms the basis for fast readout of a superconducting qubit coupled to a high-Q resonator. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B27.00002: Measurement backaction and the quantum Zeno effect in a superconducting qubit Daniel H. Slichter, R. Vijay, Irfan Siddiqi Strong measurement of a quantum system can inhibit quantum state evolution, a phenomenon known as the quantum Zeno effect. If the measurement is not perfectly quantum non-demolition, it can also cause spurious transitions between states. We study these effects in a transmon qubit dispersively coupled to a superconducting microwave readout cavity. We use a fast, ultralow-noise parametric amplifier to amplify the microwave photons used to probe the qubit state, enabling continuous high-fidelity monitoring of the qubit. This arrangement allows us to observe quantum jumps between the qubit states in real time. We examine the dependence of the jump times on measurement strength and the qubit excitation protocol. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B27.00003: Investigation of the measurement dynamics of a flux qubit inductively coupled to a readout dc-SQUID Peter Groszkowski, Jay Gambetta, Frank Wilhelm In this paper we investigate the measurement dynamics of a flux qubit inductively coupled to a capacitively shunted, readout dc-SQUID. We study how the measurement induced dephasing and relaxation rates scale as a function of the qubit operation point and measurement strength. We find analytical solutions when the measurement is quantum-non-demolition (QND) and provide a numerical investigation for non-QND operation. This is of importance as the measurement of the flux qubit when operated at its sweet spot is inherently non-QND. We conclude this with a discussion of the measurement efficiency and signal-to-noise ratio. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B27.00004: Optimization of SQUID-based microwave parametric amplifiers for qubit readout Chris Macklin, R. Vijay, E. Levenson-Falk, D. Slichter, Z. Minev, I. Siddiqi We present recent experimental and theoretical results on the optimization of SQUID-based parametric microwave amplifiers for ultra low noise readout of superconducting and spin-based qubits. The devices consist of an unshunted two-junction SQUID in parallel with an on-chip capacitor, forming a non-linear microwave resonator. The SQUID is operated in a non-linear regime below the critical current, thus producing no local dissipation. These amplifiers have gain exceeding 20 dB, 10 MHz of broadly tunable bandwidth, and quantum-limited noise performance. We present measurements on amplifiers with tunnel type and weak link Josephson junctions. We discuss the use of array structures to optimize dynamic range as well as a resonant flux-coupled input capable of operation in a transmission configuration and potentially suitable for on-chip integration. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B27.00005: Fluxonium qubit readout with the Josephson parametric converter M. Hatridge, B. Abdo, A. Kamal, N. Masluk, F. Schackert, M.H. Devoret Rapid, single shot quantum-non demolition readout is a prerequisite for proposed active quantum feedback and error correction experiments in superconducting qubit systems. The fluxonium qubit, an artificial atom comprised of a Josephson junction array inductively shunting a Cooper-pair box, is a non- Purcell limited system with excellent coherence times, making it a natural candidate for such experiments. The largest obstacle towards achieving single shot fluxonium readout is the severe signal-to-noise ratio degradation of the qubit readout by the microwave frequency amplification chain. This degradation can be minimized through the addition of a quantum- limited pre-amplifier to the chain. We have designed and constructed such an amplifier, the Josephson Parametric Converter (JPC), which achieves nearly quantum limited amplification with a bandwidth and dynamic range suitable for readout of our current fluxonium design, and are currently integrating the JPC and fluxonium. We will discuss experimental requirements of the combined JPC and fluxonium system and anticipated improvements in measurement fidelity and speed. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B27.00006: Dispersive Readout of a Superconducting Flux Qubit Using a Microstrip SQUID Amplifier J.E. Johnson, E.M. Hoskinson, C. Macklin, I. Siddiqi, John Clarke Dispersive techniques for the readout of superconducting qubits offer the possibility of high repetition-rate, quantum non-demolition measurement by avoiding dissipation close to the qubit. To achieve dispersive readout, we couple our three-junction aluminum flux qubit inductively to a 1-2 GHz non-linear oscillator formed by a capacitively shunted DC SQUID. The frequency of this resonator is modulated by the state of the qubit via the flux-dependent inductance of the SQUID. Readout is performed by probing the resonator in the linear (weak drive) regime with a microwave tone and monitoring the phase of the reflected signal. A microstrip SQUID amplifier (MSA) is used to increase the sensitivity of the measurement over that of a HEMT (high electron mobility transistor) amplifier. We report measurements of the performance of our amplification chain. Increased fidelity and reduced measurement backaction resulting from the implementation of the MSA will also be discussed. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B27.00007: SQUID-tunable microwave lumped-element oscillators and distributed resonators P. Bhupathi, M.P. DeFeo, M. Ware, J.D. Strand, B.L.T. Plourde We have fabricated lumped-element microwave oscillators and coplanar waveguide resonators consisting of a dc SQUID using submicron Al-AlOx-Al junctions with resonance frequencies in the range of several GHz. The SQUID oscillators consist of a dc SQUID shunted with a capacitor formed from superconducting layers. The CPW resonators are formed from Nb $\lambda $/2 coplanar transmission lines with a center conductor interrupted by an Al dc SQUID at the current anti-node of the fundamental mode. The resonance frequency can be varied by tuning the Josephson inductance of the SQUID with on-chip flux and bias-current lines. We discuss applications employing these devices, including a new readout scheme for superconducting flux qubits and for the detection of microwave cavity photons. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B27.00008: Multiplexing Readout of a Qubit Array via a Single Transmission Line Markus Jerger, Stefano Poletto, Alexander Lukashenko, Alexey V. Ustinov, Pascal Macha, Uwe H\"ubner, Evgeni Il'ichev A resonant circuit coupled to a qubit displays a shift of its resonance frequency depending on the quantum state of the qubit. The qubit state can be thus measured by probing the resonator near its resonance frequency. By coupling every qubit to its individual resonator of distinct frequency, one can read out the state of an array of many qubits through a single microwave line coupled to all resonators. Moreover, this readout can be performed simultaneously by using a multi-tone microwave pulse with frequency-division multiplexing. We will present measurements on an ensemble of 7 superconducting flux qubits located on one chip and each coupled to an individual transmission-line resonator. We performed spectroscopy of all qubits and determined their parameters in a single measurement run. Our latest experiments on simultaneous preparation and readout of the 7-qubit array will be presented. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B27.00009: Multiplexed dispersive readout of superconducting phase qubits Yu Chen, Rami Barends, Radoslaw Bialczak, Julian Kelly, Micheal Lenander, Erik Lucero, Matteo Mariantoni, Matthew Neeley, Aaron O'Connell, Peter O'Malley, Daniel Sank, Amit Vainsencher, Haohua Wang, Martin Weides, James Wenner, Theodore White, Yi Yin, Jian Zhao, Andrew Cleland, John Martinis A dispersive readout scheme is being developed for superconducting phase qubits. By inductively coupling to a LC resonator, the measured state of the qubit (left or right side of the potential well) can be read out as a shift of the resonance frequency. Compared to our current SQUID readout, this method eliminates the generation of quasiparticles, increases the reliability by reducing the junction count per qubit from 4 to 1, and reduces the chip wire count since the readout can be frequency multiplexed. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B27.00010: Non-linear dispersive interaction in superconducting circuit QED Yi Yin, Haohua Wang, Matteo Mariantoni, Radoslaw C. Bialczak, Mike Lenander, Eric Lucero, Matthew Neeley, Aaron O'Connell, Daniel Sank, Jim Wenner, Tsuyoshi Yamamoto, Andrew Cleland, John Martinis In circuit quantum electrodynamics, the strong coupling between superconducting qubits and a coplanar waveguide resonator (CPW) has been utilized to study the light-atom interaction. When the qubit is detuned far away from the resonator in frequency, linear dispersive interaction has been used for the readout of qubit states by measuring the pulling frequency of the resonator. Alternatively, we investigate dispersive interaction in a broader regime by measuring the accumulated dynamic phase with Wigner tomography. In the quasi-adiabatic process of tuning the qubit frequency, the dynamic phase measurement can be pushed to the case of zero detuning with up to the five-photon Fock state in the CPW resonator. The exotic non-linear behaviors of the qubit on resonator cat state and coherent state have been revealed, strongly depending on the strength of dispersive interaction. Our experimental data are consistent with the numerical calculation using the Jaynes-Cumming model. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B27.00011: Circuit QED with a Nonlinear Resonator: ac-Stark Shift and Dephasing Florian R. Ong, M. Boissonneault, F. Mallet, A. Palacios-Laloy, A. Dewes, A.C. Doherty, A. Blais, P. Bertet, D. Vion, D. Esteve Coupling a superconducting qubit to a superconducting resonator enables to investigate the interaction between light and matter with a unique flexibility of design, and allows to reach coupling regimes hardly accessible otherwise [Wallraff Nature 2004]. In this talk, we discuss the ac-Stark shift and the measurement induced dephasing of a qubit embedded in a \emph{nonlinear} resonator, an architecture that has demonstrated high fidelity single-shot qubit state readout [Mallet Nat. Phys. 2009]. In our experiment, a transmon qubit [Koch PRA 2007] is capacitively coupled to a coplanar waveguide resonator incorporating a Josephson junction that provides a Kerr nonlinearity. We have measured the qubit spectrum while pumping the nonlinear resonator with a microwave tone. Measurements of the qubit frequency shift provide a sensitive probe of the intracavity field, yielding a precise characterization of the resonator nonlinearity. The qubit linewidth has a complex dependence on the pump frequency and amplitude, which is correlated with the gain of the nonlinear resonator operated as a small-signal amplifier. The corresponding dephasing rate is found to be close to the quantum limit for most pump parameters. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B27.00012: Improved Superconducting Qubit Readout by Qubit-Induced Nonlinearities in the Straddling Regime Maxime Boissonneault, J.M. Gambetta, J. Bourassa, A. Blais In dispersive readout schemes, qubit-induced nonlinearities have typically limited the measurement fidelities by reducing the signal-to-noise ratio (SNR) when the measurement power is increased [1]. However, it has been recently shown that these nonlinearities, together with the many-level system (MLS) nature of superconducting qubits, can be used to improve qubit readout in some regimes [2]. Moreover, for the transmon qubit [3], it has been shown that when the resonator's frequency sits between two of the MLS' transition frequencies -- the so-called straddling regime -- contributions of higher levels add constructively to improve the SNR [4]. In this talk, we explore the advantages of using both the qubit-induced nonlinearties and the straddling regime for qubit readout.\\[4pt] [1] Boissonneault et al, PRA 77, 060305(R) (2007)\\[0pt] [2] Reed et al, PRL 105, 173601 (2010), Bishop et al, PRL 105, 100505 (2010), Boissonneault et al, PRL 105, 100504 (2010)\\[0pt] [3] Koch et al, PRA 76, 042319 (2007)\\[0pt] [4] Srinivasan et al, V26.00006, 2010 March Meeting. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B27.00013: Purcell Protection and Cycling Transition Measurement with a Superconducting V-system Anthony Hoffman, Srikanth Srinivasan, Jay Gambetta, Andrew Houck We perform time-domain experiments on a superconducting qubit with a V-level energy structure coupled to a superconducting, coplanar waveguide resonator. Quantum interference and the V-level energy scheme allow independent control of the qubit energy and dipole via two on-chip fast flux bias lines [1]. The tunable dipole is predicted to protect the qubit from cavity-induced spontaneous emission. We probe this ``Purcell protection'' by measuring the qubit lifetime at constant cavity-qubit detuning for a range of coupling strengths. We also show how the coupled cavity-qubit energy spectrum allows for a cycling-type measurement that is predicted to improve the signal to noise ratio of qubit state readout by as much as an order of magnitude.\\[4pt] [1] J.M. Gambetta et al., arXiv:1009.4470v1 [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B27.00014: Quantum State Tomography of a Cooper-pair Box Sergey Novikov, V. Zaretskey, B. Suri, Z. Kim, B.S. Palmer, F.C. Wellstood A 4-8 GHz microwave pulse shaping system with 3 ns Gaussian pulse rise time, arbitrary pulse envelope and phase control has been implemented. The system utilizes a two-channel 1 GSa/s DAC board\footnote{Designed by J. Martinis at UCSB and fabricated by HSCC.} to supply control voltages to an IQ mixer. The signals to the mixer have been optimized to obtain an on-off ratio of $>85\mbox{ dB}$ and phase deviations $<5\%$. The setup has been used to manipulate an $Al/AlO_{x}/Al$ Cooper-pair box (CPB) qubit coupled to a lumped-element microwave resonator ($f_0 = 5.446 \mbox{ GHz}$). The CPB has a charging energy $E_C/h = 6.25\mbox{ GHz}$ and a maximum $E_J/h = 19\mbox{ GHz}$ which was decreased to an effective $E_J/h = 6.1\mbox{ GHz}$ by an external magnetic field. By measuring the microwave transmission at $f_0$ in a pulsed-probe scheme, we perform a dispersive readout of the qubit. We present tomography data on the $\vert g \rangle$, $\vert e \rangle$, $(\vert g \rangle + \vert e \rangle)/\sqrt{2}$ and $(\vert g \rangle + i \vert e \rangle)/\sqrt{2}$ states. We find good agreement with theory, confirming that we have achieved the desired microwave pulse control. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B27.00015: A phase qubit coupled to an RF-SQUID resonator Jed Whittaker, Shane Allman, Katarina Cicak, Francois Nguyen, Adam Sirois, John Teufel, Eva Zakka-Bajjani, Raymond Simmonds We have coupled a tunable cavity (an RF-SQUID resonator) to a phase qubit. The resonator can be used both for state transfer experiments as well as a measurement/readout device for the qubit. Specifically, it can be used in three different ways to help interrogate the state of the qubit. First, changes in the resonator frequency can be monitored in order to read out the qubit state after a conventional fast measure pulse is applied to the qubit bias flux. Second, we can perform a linear dispersive measurement of the qubit state using the coupled interaction between the qubit and the resonator. Here, the resonator will have a qubit-state dependent frequency shift. Finally, we can exploit the nonlinearity of the resonator by driving it into the bifurcated regime and performing a single- shot measurement of the state of the qubit. I will discuss the design, fabrication, and operation of this system. [Preview Abstract] |
Session B28: Focus Session: Carbon Nanotubes and Related Materials: Growth, Sorting and Properties
Sponsoring Units: DMPChair: Ray Baughman, University of Texas at Dallas
Room: C156
Monday, March 21, 2011 11:15AM - 11:51AM |
B28.00001: Fundamentals and applications of monodisperse carbon-based nanomaterials Invited Speaker: Carbon-based nanomaterials have attracted significant attention due to their potential to enable and/or improve applications such as transistors, transparent conductors, solar cells, batteries, water purification systems, infrastructure materials, drug delivery, and biosensors. This talk will delineate chemical strategies for tuning and enhancing the properties of these promising nanomaterials. For example, we have developed and commercialized a scalable technique for sorting single-walled carbon nanotubes (SWCNTs) by their physical and electronic structure using density gradient ultracentrifugation (DGU). The resulting monodisperse SWCNTs possess unprecedented uniformity in their electronic and optical properties, which enables the fabrication of high performance thin film field-effect transistors, optoelectronic devices, and transparent conductors. The DGU technique also enables multi-walled carbon nanotubes to be sorted by the number of walls, and solution phase graphene to be sorted by thickness, thus expanding the suite of monodisperse carbon-based nanomaterials. By recently extending our DGU efforts to SWCNTs and graphene dispersed in biocompatible polymers (e.g., DNA, poloxamers, etc.), new opportunities have emerged in biomedical applications. Ultimately, the ability to control structure and surface chemistry with sub-nanometer precision enables optimized properties for a diverse range of technologies that employ carbon-based nanomaterials. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B28.00002: Electronic separation of dispersed carbon nanotubes in solution by Lorentz forces Charishma Subbaiah, Joshua Wood, Joseph Lyding Use of single-walled carbon nanotubes (SWNTs) in industry compatible device applications requires top-down control of SWNT electronic type. Therefore, we develop a technique for SWNT electronic separation, increasing the relative distribution of metallic SWNTs in solution by a magnetically induced Lorentz force. We take solutions of SWNTs in n-methylpyrrolidone and sonicate them, making a disperse solution on which we apply a non-uniform voltage waveform. This waveform generates a magnetic field that couples more strongly with metallic SWNTs than semiconducting SWNTs, due to a higher metallic SWNT magnetic moment, separating the tubes by Lorentz force. By conducting SWNT spectrophotometric measurements in the UV-vis-IR region, we assess the separation effectiveness. From the extracted supernatant solution, we observe a multi-fold absorbance enhancement in the metallic SWNT transition regions [1]. Additionally, the small full-width at half maximum in the absorbance peaks suggests that we are selecting a small number of metallic chiralities in our separation.\\[4pt] [1] Ausman et al., J. Phys. Chem. B. 104, 8911 (2000). [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B28.00003: Horizontally Aligned Carbon Nanotube Growth: Defects and Film Density W.D. Tennyson, D. Shi, E.S. Sanchez, J.C. Keay, M.B. Johnson, D.E. Resasco Horizontally-aligned single-walled carbon nanotubes (SWNTs) were grown on ST-cut quartz by chemical vapor deposition (CVD). The 0.2-0.3 nm thick thermally evaporated Fe catalyst was patterned using standard liftoff processes both parallel and perpendicular to the $<2\bar{1}\bar{1}0>$ quartz surface (the SWNT alignment axis). Enhanced SWNT film density and improved film uniformity were observed by atomic force microscopy (AFM) and scanning electron microscopy (SEM) when water was included with the carbon feed source (ethanol). For SWNT films without water, the SWNT linear density within 1 $\mu$m of the catalyst edge was 8 SWNT/$\mu$m and down to 2 SWNT/$\mu$m at 10 $\mu$m from the edge. However, films grown with water exhibited similar linear densities both near and far from the catalyst edge, 6 SWNT/$\mu$m. AFM observations suggest that tube-tube interactions during growth contribute to a reduced the linear SWNT density. Aligned SWNTs were observed to terminate when they intersected a non-aligned SWNT. Water-assisted growth reduced the concentration of unaligned SWNTs near the catalyst edge, resulting in a higher fraction of nanotubes extending from the catalyst. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:51PM |
B28.00004: Utilizing real time transmission electron microscopy to understand the mechanisms of nanotube nucleation, growth and growth termination Invited Speaker: In order for carbon nanotubes to find widespread application, we must have a deeper understanding of the mechanisms by which they nucleate, growth and cease growth, in an effort to fully control the resulting structures. Here we will describe how we can exploit the unique capabilities of in-situ environmental cell transmission electron microscopy to observe multiple aspects of these processes. With this approach we can directly visualize how the catalysts that mediate nanotube growth respond to various changes in the growth environment, and correlate these changes with the resulting nanotube structures. In the first part of the presentation, we will investigate how dynamic changes in the catalyst morphology are correlated with the termination of growth in vertically aligned SWNT arrays. In particular, we have investigate how the processes of catalyst coarsening, Ostwald ripening and diffusion into the catalyst support can lead to growth termination, and we will describe how changes in the growth feedstock - in particular the incorporation of controlled amounts of water vapor - can alter the catalyst evolution. In the second portion of the presentation, we will describe how altering other aspects of the growth feedstock - in this case the carrier gas, in combination with the water vapor content - can not only affect catalyst morphological evolution, but can also significantly bias the chiral distribution of the resulting nanotubes. We will correlate the changes in growth ambient with a faceting / defacting transition, as well as a resulting change in the rate of Ostwald ripening. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B28.00005: In situ diagnostics of the pulsed growth of graphene and carbon nanotubes David Geohegan, Alex Puretzky, Jason Readle, Christopher Rouleau, Murari Regmi, Gyula Eres, Gerd Duscher, Mina Yoon Non-equilibrium, pulsed gas delivery and pulsed heating synthesis approaches are used to explore and compare the kinetics and mechanisms of carbon nanotube and graphene growth on metal thin-films. Time-resolved, in situ optical reflectivity of growing nanotubes and graphene reveal the growth kinetics resulting from well-controlled, pulsed fluxes of acetylene by chemical vapor deposition. Alternatively, pulsed laser heating of substrates is used to provide well-defined transient growth temperature profiles for growth by chemical vapor deposition. Pulsed gas fluxes are shown to control the density and diameter of nanotubes in vertically-aligned nanotube arrays with nanoparticles of different size repeatedly nucleating, growing, and terminating growth in accordance with an empirical growth model. The pulsed processing approach is used to grow vertically aligned nanotube arrays with variable density. Research sponsored by the Materials Science and Engineering Division, Basic Energy Sciences, U.S. Department of Energy. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, U.S. Department of Energy. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B28.00006: Growth of Ultra-High Density Vertically-Aligned Carbon Nanotube Forests John Robertson, Santiago Esconjauregui, Martin Fouquet, Bernhard Bayer, Stephan Hofmann We present a general catalyst design method to synthesise ultra-high density, aligned forests of carbon nanotubes by cyclic deposition and annealing of catalyst thin-films. This leads to nanotube forests with an area density of at least 10$^{13}$ cm$^{-2}$, over one order of magnitude higher than existing values (Hata 2004, Zhong 2006), and close to the limit of a fully dense forest. The technique consists of cycles of ultra-thin metal film deposition, annealing, and immobilisation. The nanotubes are then grown as normally by Chemical Vapor Deposition. These ultra-dense forests are needed to use carbon nanotubes as vias and interconnects in integrated circuits and as thermal interface materials. Further density increase to 10$^{14}$ cm$^{-2}$ by reducing nanotube diameter is possible. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:51PM |
B28.00007: Carbon Nanotubes with Temperature Invariant Viscoelasticity from -196$^{\circ}$C to 1000$^{\circ}$C Invited Speaker: Viscoelasticity describes the ability of a material to possess both elasticity and viscosity. Viscoelastic materials, such as rubbers, possess a limited operational temperature range, (e.g., for silicone rubber: -55 to 300$^{\circ}$C) above which the material breaks down and below which the material undergoes a glass transition and hardens. This is because molecular motion that is the origin of viscoelasticity is a thermally activated process. We created a viscoelastic material composed from a random network of long interconnected carbon nanotubes that exhibited an operational temperature range from -196$^{\circ}$C to 1000$^{\circ}$C [1]. The viscoelastic properties (storage modulus, loss modulus, and damping ratio) measured by DMA in N$_{2}$ ambient were nearly constant over an exceptionally wide temperature range (-140$^{\circ}$C$\sim $600$^{\circ}$C). As exemplified by the vibration isolator demonstration, the CNT material showed viscoelasticity beyond the DMA limitation at -190$^{\circ}$C (immersed in liquid nitrogen) and at $>$900$^{\circ}$C (exposed to butane torch). And we implemented impact tests at -196$^{\circ}$C, 25$^{\circ}$C and 1000$^{\circ}$C using a steel ball and analyzed the ball tracks. The ball tracks were identical for all cases as observed by SEM and 3-D mapping that suggested unvarying viscoelastic properties across this 1200$^{\circ}$C temperature range. We interpret that the thermal stability stems from energy dissipation through the zipping and unzipping of carbon nanotubes at contacts. Quantitatively, the viscoelastic properties by DMA showed that the CNT material possessed similar stiffness (storage modulus 1MPa), higher dissipation ability (loss modulus (0.3MPa) and damping ratio (0.3) than silicone rubber at room temperature. Further DMA characterization from -140$^{\circ}$C to 600$^{\circ}$C demonstrated temperature invariant frequency stability (0.1-100Hz), the same level of reversible deformation (critical strain 5{\%}) and fatigue resistance (1,000,000 cycles, 100Hz). \\[4pt] [1] Xu, M.; Futaba, D. N.; Yamada, T.; Yumura, M.; Hata, K. \textit{Science} (Accepted) [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B28.00008: Understanding Chiral-Selective Growth of Carbon Nanotubes: In-Situ Raman Studies of Individual Single Walled Carbon Nanotube Growth Rahul Rao, David Liptak, Tonya Cherukuri, Daylond Hooper, Boris Yakobson, Benji Maruyama In-situ Raman scattering has been used to obtain growth kinetics of individual single-walled carbon nanotubes (SWNTs) using a custom designed cold-wall chemical vapor deposition (CVD) chamber coupled to a Raman spectrometer. Raman spectra are collected during SWNT growth and plots of the G band area versus time are fitted to self-exhausting exponential curves, from which we obtain SWNT growth rates and catalyst lifetimes (time constant). Chiral index assignments are made for several individual SWNTs via analysis of the radial breathing modes. The growth rate of the SWNTs is shown to be proportional to the chiral angle. In addition we find a positive correlation between SWNT length (obtained from SEM analysis) and the growth rate. This confirms the model put forth by Ding \textit{et al}. [1] which links SWNT growth rate to the chiral angle. A growth model based on our results illuminates an as-yet unexplained distribution in the chiral yield of typical CVD-grown nanotubes as being driven by chiral-selective growth kinetics. [1] Ding, F; Haturyunyan, A; Yakobson, B, I; Dislocation theory of chirality-controlled nanotube growth, Proc. Natl. Acad. Sci., 106, 2506, 2009 [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B28.00009: Dislocation Dynamics in Multishell Carbon Nano-Onions Traian Dumitrica, Evgeniya Akatyeva, Jianyu Huang Graphite has long served as a model material to understand dislocations. An early work on natural graphite provided factual evidence for the existence of screw dislocations. Recently, synthetic carbon nanostructures began to be explored in order to understand dislocations at the nanoscale. Here we study the 1/2$<$0001$>$ edge dislocation in nested multishell carbon onions [1]. We report in situ electron microscopy observations of dislocation dissociation and annihilation processes in individual nanometer-sized carbon onions. Essential for these processes is the counterintuitive motion of the 1/2$<$0001$>$ edge from the outer surface to the inner region, which cross-links or unlinks a large number of shells. The correlation with atomistic simulations and analysis of the energy, which separates the strain and edge components, indicates that this inward glide originates in the reduction of edge with each inwards glide step, an effect specific to the spherical topology. \\[4pt] [1] E. Akatyeva, J. Y. Huang and T. Dumitrica, Phys. Rev. Lett. 105, 106102 (2010). [Preview Abstract] |
Session B29: Advances in Ion Trap Quantum Computation
Sponsoring Units: GQIChair: Jungsang Kim, Duke University
Room: C148
Monday, March 21, 2011 11:15AM - 11:51AM |
B29.00001: Trapped ion arrays for quantum simulation Invited Speaker: Trapped ions have been used to demonstrate a broad range of quantum information processes with high fidelity\footnote{D. Leibfried, D. J. Wineland, R. B. Blakestad, J. J. Bollinger, J. Britton, J. Chiaverini, R. J. Epstein, W. M. Itano, J. D. Jost, E. Knill, C. Langer, R. Ozeri, R. Reichle, S. Seidelin, N. Shiga, and J. H. Wesenberg, Hyperfine Interactions \textbf{174}, 1 - 7 (2007). Proc. 4th Int. Conf. Trapped Charged Particles and Fundamental Physics (TCP 2006), Parksville, Canada 3-8 Sept. 2006.} and are an obvious choice for quantum simulations. Several quantum simulations have already been demonstrated with ions.\footnote{K. Kim, M.-S. Chang, S. Korenblit, R. Islam, E. E. Edwards, J. K. Freericks, G.-D. Lin, L.-M. Duan, and C. Monroe, \textit{Nature} \textbf{465}, 590 (2010).}$^,$\footnote{E. E. Edwards, S. Korenblit, K. Kim, R. Islam, M.-S. Chang, J. K. Freericks, G.-D. Lin, L.-M. Duan, and C. Monroe, Phys. Rev. \textbf{B 82}, 060412 (2010).} The present goal is to simulate quantum systems that cannot be achieved with classical computation using more than 20 ions. It is challenging to assemble more than 20 ions in suitable arrays for quantum simulation of arbitrary model systems. Present ion trap based quantum simulations with up to 20 ions are now in progress. This talk describes ion trap micro-fabrication techniques and designs that have the potential to increase the number of coupled ions to the range between 50 and 100 ions. High precision ion traps are fabricated using silicon VLSI techniques on silicon wafers with aluminum electrodes.\footnote{D.R. Leibrandt, J. Labaziewicz, R.J. Clark, I.L. Chuang, R.J. Epstein, C. Ospelkaus, J.H. Wesenberg, J.H. Bollinger, D. Leibfried, D. Wineland, D. Stick, J. Sterk, C. Monroe, C.-S. Pai, Y. Low, R. Frahm, and R.E. Slusher, Quant. Inf. Comp. \textbf{9}, 901 (2009)} At the Georgia Tech Research Institute we are designing, fabricating and testing ion trap arrays that will contain and accurately control at least 50 ions in linear chains of equally spaced ions. Large numbers of equally spaced ions have recently been shown\footnote{G.-D. Lin, S.-L. Zhu, R. Islam, K. Kim, M.-S. Chang, S. Korenblit, C. Monroe, and L.-M. Duan, Europhys. Lett. \textbf{86}, 60004 (2009).} to be stable in anharmonic trap potentials that are easily obtained in the micro-fabricated traps. The limits on quantum simulation accuracy due to errors in the ion trap parameters will be discussed. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B29.00002: Laser-induced charging of microfabricated ion traps Guang Hao Low, Shannon X. Wang, Nathan Lachenmyer, Yufei Ge, Peter Herskind, Isaac L. Chuang Microfabricated ion traps are promising candidates for realizing large-scale quantum computers, but small trap sizes leads to increased sensitivity of the trapped ions to surface effects, including localized charging of the trap electrodes. Laser-induced charging on microfabricated ion traps is studied by monitoring the ion micromotion over a period of up to 20 minutes that a laser is incident on the trap. The ion is trapped 100~$\mu$m above the metal surface and the trap is operated at 6K. The lasers used are at 405, 460, and 674 nm, which are relevant atomic transitions in Sr+ ions, and the typical intensity at the trap is 10$^{35}$ photons/sec. The ion's micromotion signal is related to the number of charges created on the trap. A wavelength and material dependence of the charging behavior is observed: lasers at lower wavelengths cause more charging, and aluminum exhibits more charging than copper or gold. We describe the charging dynamic based on a rate equation approach. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B29.00003: Superconducting microfabricated ion traps Shannon X. Wang, Yufei Ge, Jaroslaw Labaziewicz, Eric Dauler, Karl Berggren, Isaac L. Chuang We fabricate superconducting ion traps with niobium and niobium nitride and trap single $^{88}$Sr ions at cryogenic temperatures. The superconducting transition is verified and characterized by measuring the resistance and critical current using a 4-wire measurement on the trap structure, and observing change in the rf reflection. The lowest observed heating rate is 2.1(3) quanta/sec at 800~kHz at 6~K and shows no significant change across the superconducting transition, suggesting that anomalous heating is primarily caused by noise sources on the surface. This demonstration of superconducting ion traps opens up possibilities for integrating trapped ions and molecular ions with superconducting devices. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B29.00004: Microfabricated surface trap for scalable ion-photon interfaces Peter Herskind, Shannon Wang, Molu Shi, Yufei Ge, Marko Cetina, Isaac Chuang The combination of high-finesse optical mirrors and ion traps is attractive for quantum light-matter interfaces, which represents an enabling resource for large-scale quantum information processing. We report on a scalable approach to ion-photon interfaces based on a surface electrode ion trap microfabricated on top of a highly reflective mirror. An aperture in the central electrode, directly below the ion, allows the mirror to interact with the ion. The integration of such mirrors is scalable as several mirror apertures may be added with no additional overhead for fabrication. Furthermore, the design provides a path for reaching the strong coupling regime of Cavity QED, where an ion-cavity system can be realized by adding a small concave mirror above the trap mirror. The quality of the mirror is not significantly compromised in the course of fabrication and we have measured an increase in losses for light at 422~nm at the level of 100~ppm. The functionality of the mirror has also been verified by light collection from, and imaging of, the ion $169\pm 4~\mu$m above the mirror. Despite its proximity, we find that the presence of the mirror does not perturb the trap. Trapping is stable with laser cooled ion lifetimes of several hours and we observe only minimal sensitivity to laser-induced charging. Furthermore, through operation of the trap in a cryostat at 15~K the heating rate of the ion is a the level of only 0.1~quanta/ms. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B29.00005: Ion crystal transducer for strong coupling between single ions and single photons Lucas Lamata, David Leibrandt, Isaac Chuang, Ignacio Cirac, Mikhail Lukin, Vladan Vuletic, Susanne Yelin A quantum interface between single photons and single ions in an ion crystal is proposed. The coupling between single photon and single particle is collectively enhanced via a collective internal ion state and a phonon state. Applications for this scheme include single-photon generation, a memory for a quantum repeater, and a deterministic photon-photon or photon-ion entangler. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B29.00006: Temperature driven structural phase transition for trapped ions Zhe-Xuan Gong, Guin-Dar Lin, Lu-Ming Duan A Wigner crystal formed with trapped ion can undergo structural phase transition, which is determined only by the mechanical conditions on a classical level. Instead of this classical result, we show that through consideration of quantum and thermal fluctuation, a structural phase transition can be solely driven by change of the system's temperature. We determine a finite-temperature phase diagram for trapped ions using the renormalization group method and the path integral formalism, and propose an experimental scheme to observe the predicted temperature-driven structural phase transition, which is well within the reach of the current ion trap technology. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B29.00007: Differential Stark shift measurement of clock states of Yb+ using an optical frequency comb Qudsia Quraishi*, David Hayes, David Hucul, Dzmitry Matsukevich, Shantanu Debnath, Susan Clark, Chris Monroe Quantum information processing with trapped ions has traditionally involved state preparation, manipulation (eg. quantum gates) and detection using CW lasers. Quantum gates implemented with ions typically involve optical Raman transitions between two atomic levels. An optical frequency comb, emitted by a pulsed laser, is an excellent tool for bridging atomic frequency differences. Previously, we demonstrated quantum gates and separately, ultrafast spin manipulation, using pulsed lasers [1,2]. Unlike the CW case, employing pulsed lasers has the marked advantage of both low spontaneous emission and low AC Stark shifts, because the high powers available from pulsed lasers allow for larger detunings from optical resonance. Here, we show both experimentally and theoretically the scaling of the differential Stark shift with detuning (6 THz to 20 THz) of the Raman fields, achieving values of 10$^{-3}$ of the Rabi frequency. [1] D. Hayes, et al., Phys. Rev. Lett. 104, 140501 (2010) [2] W. C. Campbell, et al., Phys. Rev. Lett. 105, 090502 (2010).~ *Currently NRC postdoc with SEDD, ARL, Adelphi, MD. Support: DARPA OLE under ARO contract, IARPA under ARO contract, NSF PIF Program, NSF PFC at JQI and *IC Postdoc administered by the NGA. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B29.00008: ``Tack'' ion trap for efficient photon collection. Chen-Kuan Chou, Gang Shu, Nathan Kurz, Thomas Noel, John Wright, Boris Blinov Trapped, laser-cooled atoms and ions produce intense fluorescence of the order 10$^{7}$ -- 10$^{8}$ photons per second. Detection of this fluorescence enables the efficient measurement of the quantum state of qubit based on the trapped atoms. Thus, it is desirable to collect a large fraction of the (isotropically emitted) photons to make the detection faster and more reliable. Additionally, efficient fluorescence collection can improve the speed and fidelity of remote ion entanglement and quantum gates. Refractive and reflective optics, as well as optical cavities, and, recently, bare multimode optical fibers have all been used to collect the trapped ion fluorescence with up to 10{\%} efficiency. Here we show a novel ion trap design that incorporates a high numerical aperture metallic spherical mirror as the integral part of the trap itself (the RF electrode) which enables up to 35{\%} solid angle collection of trapped ion fluorescence. The movable central needle-shaped electrode of this ``tack'' trap allows precise placement of the ion at the focus of the spherical mirror. We also study the properties of the images formed by the spherical mirror and comment on possible methods for aberration correction. Owning to the simplicity of its design, this trap structure can be adapted for mircofabrication and integration into more complex trap architectures. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B29.00009: Towards laser cooling of a LC-resonator via trapped ions Soenke Moeller, Nikos Daniilidis, Boyan Tabakov, Aaron Bradley, Hartmut Haeffner We will discuss our experimental progress towards coupling strings of trapped ions to an LC-resonator. The goal of our experiments is to cool the resonant mode of a superconducting high-quality resonant circuit to ultra-low temperatures. By continuously laser cooling a crystal of ions coupled to the circuit, energy is removed from the resonator. For quality factors on the order of 10$^5$, the time-scale of the environment-to-mode coupling, i.e. the time for the resonant mode of the LC-resonator to thermally equilibrate, can be on the order of a second. Thus, engineering an ion-resonator coupling of 10$\sim $kHz results in a reduction of the electronic temperature by four orders of magnitude as compared to the ambient temperature of the resonator. The expected temperatures below 1mK are extremely low approaching even the vibrational ground state of the oscillator mode, enabling novel quantum electronics applications in the solid state. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B29.00010: Micro-Fabricated Surface Electrode Y-Junction Ion Traps David Moehring, Matthew Blain, Robert Cook, Kevin Fortier, Raymond Haltli, Clark Highstrete, Daniel Stick, Chris Tigges We will present results of the design, operation, and performance of two different Y-Junction surface ion micro-traps fabricated at Sandia. Recent progress in the testing of the micro-traps will be highlighted, including the successful shuttling of single and multiple ions, ion-chain splitting and recombination, and the validation of simulations with experiments. 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. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B29.00011: Emergent effective spin models in ion-trap-based quantum simulators Cheng-Ching Joseph Wang We show how effective spin models emerge from the interaction of laser light with the ions in a linear Paul trap. It has been shown that quantum Ising models can be studied by adiabatic state evolution in a transverse magnetic field which is ramped from large to small values. The standard proof involves and adiabatic elimination of the phonons in the Lamb-Dicke regime. We discuss here that such an elimination can be problematic due to the inherent entanglement between the spins and the phonons. If the magnetic field is ramped sufficiently fast, one can show that all quantum state probabilities measured along the Ising field axis are independent of the phonons. But if the field is ramped more slowly, then the phonons and spins become entangled. Nevertheless, the main effects are to change the spin entanglement of the quantum states rather than the probabilities of the different states in the wavefunction. We present numerical evidence to illustrate these points. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B29.00012: Individual addressing of trapped ions using a MEMS beam steering system Taehyun Kim, Caleb Knoernschild, Emily Mount, Stephen Crain, Rachel Noek, Daniel Gaultney, Peter Maunz, Jungsang Kim Implementation of single-qubit and two-qubit quantum gates in a long linear chain of trapped ions generally requires the manipulation of qubits stored in individual ions using a set of laser beams. Individual addressing has been demonstrated with acousto-optic and electro-optic deflectors, by using the Zeeman shift due to a magnetic field gradient, and by separating the ions. Microelectromechanical system (MEMS) technology offers an alternative approach using micromirrors to focus laser beams on individual ions. Advantages of this approach are its broadband optical performance and scalability to more beams and multiple dimensions. We report progress towards integrating a MEMS beam steering system with an Yb ion trap experiment. The MEMS system will direct an ultraviolet beam with waist of $\sim $1.5$\mu$m at the ions across a 20$\mu$m range. For a designed ion separation of 4um this allows addressing up to 5 ions. The far-detuned laser will induce an AC Stark shift on a single ion in the chain, and the induced phase shift can be measured by Ramsey spectroscopy. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B29.00013: Scalable micro-scale optics for planar ion traps True Merrill, Harley Hayden, Chien-Shing Pai, Rachel Noek, Jungsang Kim, Curtis Volin Efficient collection of fluorescence from atomic ions is required for fast high-fidelity measurement in ion trap quantum information processing. Conventional multi-element lens stacks can achieve photon collection efficiencies as high as 5\%, however these systems typically have restricted field-of-view and are not generally scalable to image large arrays of ions. We report the development and fabrication of planar traps with integrated micro-scale spherical mirrors with an expected 15\% collection efficiency. The mirror shape is controlled with a combination of silicon wet-processing and polishing techniques while maintaining a surface roughness below $\sigma_{RMS} < 10$ nm. The design allows for multiple integrated mirrors in a single chip allowing for the simultaneous measurement of many ions over a 10 mm object space. [Preview Abstract] |
Session B30: Graphene: Transport and Correlations
Sponsoring Units: DCMPChair: Vladimir Falko, Lancaster University, UK
Room: C147/154
Monday, March 21, 2011 11:15AM - 11:27AM |
B30.00001: Magnetoconductance Oscillations in High-Mobility Suspended Bilayer and Trilayer Graphene Wenzhong Bao, Zeng Zhao, Hang Zhang, Gang Liu, Philip Kratz, Lei Jing, Jairo Velasc, Dmitry Smirnov, Chun Ning Lau We observed pronounced magnetoconductance oscillations on suspended bilayer and trilayer graphene devices with mobilities up to 270,000 cm$^{2}$/Vs. For bilayer devices, we observe conductance minima at all integer filling factors n between 0 and -8, as well as a small plateau at n=1/3. For trilayer devices, we observe features at n=-1, -2, -3 and -4, and at n$\sim $0.5 that persist to 4.5K at B=8T. All of these features persist for all accessible values of Vg and B, and could suggest the onset of symmetry breaking of the first few Landau (LL) levels and fractional quantum Hall states. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B30.00002: Novel Excitonic Effects in Graphene and Bilayer Graphene Li Yang Through first-principles calculations with many-body effects included, we have revealed unique excitonic effects in the high-frequency regime (10 $\sim $ 20 eV) of optical spectra of graphene and bilayer graphene (BLG). Despite their different symmetries, the parallel $\sigma $ and $\pi $* bands result in enhanced excitonic effects in such two-dimensional semimetals; one narrow resonant exciton is discovered to form an isolated peak below the prominent absorption continuum with a surprisingly large binding energy, 270 meV in graphene and 80 meV in BLG. Moreover, because of its extremely weak resonant character, this exciton exhibits a bound electron-hole wave function. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B30.00003: Metal-insulator transitions in graphene Mario Amado, Enrique Diez, Francesco Rossella, Vittorio Bellani, David Lopez-Romero, Duncan Maude We investigate the metal-insulator quantum phase transitions that appear in the quantum Hall effect, namely the plateau-insulator and plateau-plateau transitions We have performed magnetotransport experiments with the magnetic field as the driving parameter in the temperature range from 4K up to 230K and magnetic fields up to 28T. The analysis of the temperature dependence of the Hall and longitudinal resistivity reveals the non-universality of the critical exponent for the metal-insulator transition when varying the density of carriers. We also find relevant discrepancies with recent works concerning the value of the critical exponent of the plateau-plateau transition. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B30.00004: Conductivity of Coulomb interacting massless Dirac particles in graphene Vladimir Juricic, Oskar Vafek, Igor Herbut The ac conductivity of the Coulomb interacting Dirac fermions in graphene is considered in the collisionless regime using a variant of the dimensional regularization with the spatial dimension $D=2-\epsilon$ for $\epsilon>0$. We show that this regularization procedure preserves the Ward-Takahashi identity associated with the charge conservation [1], and as such it can serve as a consistent regularization of the entire interacting field theory. As a consequence of the explicitly preserved $U(1)$ gauge symmetry, the dimensional regularization yields the same result for the Coulomb correction to the conductivity when calculated using the current-current and the density-density correlators, which is, nevertheless, different than the ones previously reported in the literature. References: [1] V. Juricic, O. Vafek, and I. F. Herbut, ArXiv:1009.3269 (Phys. Rev. B, in press). [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B30.00005: Plasma Excitations of Dressed Dirac Electrons in Graphene Oleksiy Roslyak, Danhong Huang, Andrii Iurov, Godfrey Gumbs The dispersion relation for the collective plasma excitations of optically dressed Dirac electrons in single and double graphene layers is calculated in the random-phase approximation. The presence of circularly polarized light gives rise to an energy gap $\epsilon_g$ between the conductance and valence bands. The value of $\epsilon_g$ may be adjusted by varying the frequency and intensity of the light which could be sizable compared to that which is generated by spin-orbit coupling or sub-lattice symmetry breaking. We present numerical results for the dispersion relation for plasma excitations for various energy gaps and separation between graphene layers. The induced $\epsilon_g$ opens up a gap in the particle-hole continuum thus allowing plasmon excitations of short wave-length. An optical and acoustic phonon-like modes are obtained in the double layer configuration. Those are very sensative to the induced energy gap and symmtry breaking between the layers. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B30.00006: Variational approach to the excitonic phase transition in graphene Fernando Sols, Javier Sabio, Francisco Guinea We analyze the Coulomb interacting problem in undoped graphene layers by using an excitonic variational ansatz. By minimizing the energy, we derive a gap equation which reproduces and extends known results. We show that a full treatment of the exchange term, which includes the renormalization of the Fermi velocity, tends to suppress the phase transition by increasing the critical coupling at which the excitonic instability takes place. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B30.00007: First-principles study of polarization in graphene Priyamvada Jadaun, YuGui Yao, Leonard F. Register, Qian Niu, Sanjay Banerjee The emergence of polarization in monolayer graphene is investigated using first-principles calculations. We try to understand electronic polarization calculated using Berry phase technique as well as ionic polarization when in-plane symmetry is broken within the graphene lattice. The effect of underlying substrate as well as stress on this symmetry breaking is also explored. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B30.00008: Graphene K and K' States at the Dirac Point Lawrence Snyder, Christopher Wells The graphene band structure states at the K and K' points and the Fermi level, the Dirac point, computed when a (1x1) unit cell is employed, fall at the gamma point when a (3x3) unit cell is employed. These states at the gamma point of the Brillouin zone for the (3x3) unit cell have a zero phase factor and are conveniently represented as molecular orbitals of pi electrons. These states are illustrated and discussed. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B30.00009: Edges states and anomalous Aharonov-Bohm-type oscillation in anti-dot lattice graphenes formed by nanoporous alumina template mask J. Haruyama, T. Shimizu, J. Nakamura, T. Matsui, H. Fukuyama Edge states of graphene with a zigzag structure theoretically have extremely high electronic density of states (EDOS), electron localization, and polarized spin transport as well [1]. However, few studies have reported on the experimental observation of edge states and related quantum phenomena. Here, we report on the nonlithographic and low-damage fabrication of honeycomb-like nanopore arrays (anti-dot lattice) on thin multilayered graphenes utilizing nanoporous alumina template masks [2]. We confirm the presence of high EDOS at the edges of the nanopores using STM observation. We find periodic magnetoresistance oscillations with two different periods over a wide magnetic field range (anomalous Aharonov-Bohm-type effect [3]) (e.g., high fields at where the diameter of cyclotron-motion electrons is smaller than diameter of the nanopore). These findings clearly suggest the presence of localized electrons and edge states at the nanopore edges of graphene. \\[4pt] [1] K. Nakada, G. Dresselhaus, M. S. Dresselhaus et al., Phys. Rev. B 54, 17954 (1996). \\[0pt] [2]T. Shimizu, J. Haruyama et al., To be published on Phys. Rev. Lett. \\[0pt] [3] D. Weiss, K.von Klitzing et al., Phys. Rev. Lett. 70, 4118 (1993). [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B30.00010: Snake orbits in graphene underneath an array of Ni$_{0.80}$Fe$_{0.20}$ nano-dots Adam Neal, Jiangjiang Gu, Tony Low, Peide Ye The existence of snake orbits in 2DEG formed at AlGaAs/GaAs heterojunction is theoretically predicted and experimentally demonstrated by creating a spatially inhomogeous magnetic field [1]. Due to its ambipolar nature, graphene opens up new possibilities to investigate snake orbits and other exotic phenomena by simply creating a p-n junction in a homogenous magnetic field. We have fabricated periodic arrays of Ni$_{0.80}$Fe$_{0.20}$ nano-dots on graphene with the dot diameter of 80 nm or 150 nm and the period of 160 nm or 300 nm, respectively. A quasi-periodic magneto-resistance oscillation is observed in the low-temperature magneto-transport measurement. We ascribe it to Aharonov-Bohm oscillations induced by snake orbits of carriers underneath the nano-dots. Due to the high work-function of Ni$_{0.80}$Fe$_{0.20}$, it is possible to generate local circular n-p and p--p junctions underneath the nanodots, which form the snake orbits of carriers in an external applied magnetic field. Dependence of these oscillations on temperature and carrier density and simulation work on snake orbits will be presented.\\[4pt] [1] J.E. Muller, Phys. Rev. Lett. 68, 385 (1992); P.D. Ye et al., Phys. Rev. Lett. 74, 3013 (1995). [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B30.00011: Metal Electrode Effect on Electronic Transport through Graphene Cheng Gong, Weichao Wang, Geunsik Lee, Bin Shan, Kyeongjae Cho Metal-graphene contact is one of key issues in graphene-based device applications. In this work, electronic transport through metal/graphene/metal end-contact structures with zigzag interface is investigated by first-principles non-equilibrium Green's function method. Double-dips transmission characteristics in Palladium/Graphene/Palladium are observed with a common positive dip and varied negative dips for graphene of different lengths. Transmission through the structure is suppressed by mode mismatch among different carbon localities perturbed by interface hybridization, yet intensities of the suppression at two dips are featured by distinctive channel potential profiles. Finite transmissions at Fermi level are attributed to both evanescent and propagating modes. This study benefits the understanding of the origins of contact resistance at metal/graphene interfaces. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B30.00012: Casimir Interaction Between Graphene and Planar Systems David Drosdoff, Lilia Woods Casimir forces become increasingly important as systems become miniaturized. Such fluctuation forces are studied between graphene, a potential future substitute for silicon based electronics, and other materials. Because graphene is one atomic layer thick, the Casimir force is relatively weak. Yet its singular electronic properties give rise to an attraction between graphene layers in the order of a factor of $\alpha\approx 1/137$ times smaller than the interaction between two ideal metal plates. For the case of the interaction between graphene and metamaterials, a strong reduction in the Casimir attraction or even repulsion may be found if the metamaterial is mostly magnetic in nature. Metamaterials with strong magnetic responses in the optical range may soon be possible as the rapid development of metamaterials continue. Other graphene configurations with metals and metamaterials are also studied. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B30.00013: Hot electron dynamics and Schwinger mechanism in graphene Meng-Chieh Ling, J\"org Schmalian We investigate the nonlinear dc conductivity of graphene by explicitly solving the Boltzmann equation with relaxation and particle-hole pair production contributions and obtain the non- equilibrium electronic distribution function. First, by considering isotropic elastic electron-phonon scattering, we show that, in the limit of weak external electric field one recovers Ohm's law, while above a threshold field $E=(k_B T)/(ev_F \tau)$ the dc conductivity varies as the inverse of the external electric field. In particular, we obtain an explicit form for the scaling of the conductivity with respect to $E/T$. We then investigate how this result is affected by the Schwinger mechanism, which leads to particle-hole creation and, consequently, to interband transitions. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B30.00014: Band structures of bilayer graphene superlattices Si Wu, Matthew Killi, Arun Paramekanti We have studied the electronic band structures of bilayer graphene (BLG) superlattices. In BLG, there are two distinct types of superlattice modulations - chemical potential modulations and electric field induced gap modulations. We have solved energy bands for one- and two-dimensional superlattices for both kinds of modulations. We found, in particular, for a 2D superlattice with gap modulation, that the energy gap is one order smaller than that in a uniform electric field. The problem of a single charged impurity in gated BLG is also studied. Implications of our results on transport experiments are discussed. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B30.00015: Giant inelastic tunneling in epitaxial grapheme mediated by localized states Kees Flipse, Kevin Ruit van de, Jiri Cervenka Local electronic structures of nanometer-sized patches of epitaxial grapheme and its interface layer with SiC(0001) have been studied by atomically resolved scanning tunneling microscopy and spectroscopy. Localized states belonging to the interface layer of the graphene/SiC system show to have essential influence on the electronic structure of grapheme. Giant enhancement of inelastic tunneling, reaching 50{\%} of the total tunneling current, has been observed at the localized states on a nanometer-sized graphene monolayer surrounded by defects. [Preview Abstract] |
Session B31: Focus Session: van der Waals Bonding in Advanced Materials: Applications to Systems and Behaviors
Sponsoring Units: DMPChair: Yves Chabal, University of Texas at Dallas
Room: C145
Monday, March 21, 2011 11:15AM - 11:51AM |
B31.00001: Physisorbed molecules: How their frictional and diffusive properties impact lubricity Invited Speaker: Friction and its consequences are of great concern from both a national security and quality-of-life point of view, and the economic impact of energy efficiency, wear, and manufacturing cannot be underestimated [1]. Lubrication schemes for many macroscopic applications have been solved, but an era of science and engineering is emerging where control of mechanical and electrical systems at the atomic level will be required [2]. A fundamental understanding of the dissipative and frictional properties of weakly adsorbed films, which are ubiquitous in these systems, is key to a vast range of emerging applications. This talk will begin with a discussion of how diffusive and frictional properties of adsorbed atoms and molecules governed by van der Waals interactions can be measured experimentally [3]. Selected example of how atomic scale mobility in physisorbed materials, even at very low coverage, can directly impact friction, tribological performance and/or device viability will then be presented, for systems spanning Micro- and Nano- ElectroMechanical Systems to avalanches in granular materials [4,5]. \\[4pt] [1] ``Surface science and the atomic-scale origins of friction: what once was old is new again'', J. Krim, Surface Science \textbf{500} (1-3): 741-758, (2002)\\[0pt] [2] ``QCM tribology studies of thin adsorbed films'', J. Krim, Nano Today 2 (5): 38-43, (2007)\\[0pt] [3] ``Sliding friction measurements of molecularly thin ethanol and pentanol films: How friction and spreading impact lubricity'', B.P. Miller and J. Krim, J. Low. Temp. Phys., \textbf{157,} Special issue on Wetting, Spreading, and Filling, p 252 (2009)\\[0pt] [4] ``Friction, Force Chains and Falling Fruit '', J. Krim and R.P. Behringer, Physics Today, \textbf{62, }pp. 66-67 (Sept. 2009)\\[0pt] [5] ``Atomic-scale lubrication at ultra-low vapor coverages'', D.A. Hook, B.P. Miller, B.M. Vlastakis, M.T. Dugger and J. Krim, submitted [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B31.00002: Edge-dependent Static Friction of Adsorbed van der Waals Islands Nicola Varini, Furio Ercolessi, Ugo Tartaglino, Andrea Vanossi, Erio Tosatti Rare gas islands adsorbed through van der Waals forces on metal surfaces do not slide freely, but exhibit static friction in QCM experiments. Static friction appears, unexpectedly, even for incommensurate and defect-free crystal surfaces, where sliding should be frictionless. Via atomistic simulations of Kr islands on Au(111), we show that the island edges may be the ultimate culprits. Adsorbate sliding requires the flow of solitons - tiny density and corrugation modulations with the beat periodicity between the two periodicities. For an island, we find an edge-originated energy barrier that blocks the soliton flow, keeping the island pinned. As the static friction force is reached, the barrier vanishes at one point on the edge, and new solitons enter the island, which becomes depinned. Unsurprisingly, we find that low surface corrugation and high temperature facilitate this edge depinning. However, the island's thermal expansion is large and leads to changeable commensurability upon heating, which gives rise to the possibility of re-entrant static friction. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B31.00003: Why viscosities of Ne and Kr monolayers are so different on the Pb(111) surface Ruqian Wu, Yanning Zhang, V. Bortolani Adsorption and segregation of Ne and Kr monolayers on the Pb(111) surface are examined through density functional calculations to understand the puzzling experimental observations of different tribological properties of these two rare gases. Theoretical results reveal weak but non-negligible interaction between rare gas and Pb(111), manifested as charge polarization and orbital intermixing. Because of its large atomic size, orbital polarizability and wave function extension, Kr binds with Pb(111) more strongly than does Ne. The activation energy of Kr segregation from the ground state hcp site to the metastable fcc site is 3.8 meV, substantially larger than that of Ne, 2.1 meV. This explains the drastic difference between the viscosities of Ne and Kr over Pb(111), observed at low temperatures using a quartz-crystal microbalance technique. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B31.00004: Tribological Characterization of Nanoclustered Lead Films Keeley M. Stevens, Jacqueline Krim For thin films of Pb on Ti, a system which does not wet, it is known that when studying coverages below the percolation transition measurement of surface friction via a sliding gas monolayer is an effective probe of electronic structure for the isolated lead nanoclusters.\footnote{Highland, M.\ et al.\, in preparation.} This technique is capable of studying superconductors as they pass through the transition temperature. Motivated by on-going reports of quantum size effects in thin lead films grown on Si(111)\footnote{Ozer, M.\ et al.\ {\it J.\ Low Temp Phys.} {\bf 2009}, 157: 221-251.} and Cu(001),\footnote{Li, W.\ et al.\ {\it Phys.Rev.B} {\bf 1993}, 48, 11: 8336-8344.} we examine the issue of nitrogen adsorption\footnote{Krim, J.and Widom, A.\ {\it Phys.\ Rev.\ B} {\bf 1988}, 38: 12184-12189.} onto such nanostructured films. Funding provided by NSF DMR. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B31.00005: van der Waals bonded materials: vanadium pentoxide bulk structure Elisa Londero, Elsebeth Schroeder In this work we present a computational study of the layered oxide structure of vanadium pentoxide (V$_2$O$_5$) using the vdW-DF functional (M. Dion et al., Phys.Rev.Lett. \textbf{92}, 246401 (2004); T. Thonhauser et al., Phys. Rev. B \textbf{76}, 125112 (2007); K. Lee et al., Phys. Rev. B \textbf{82}, 081101 (2010)) which has proven to be able to capture the essential van der Waals interactions across matter separated by charge voids. We show that these forces play a substantial role for the description of the lattice constants and cohesion of this compound. In addition we document and handle a sensitivity to numerical noise in the evaluation of some exchange versions used with nonlocal correlation. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B31.00006: Stacking and Registry Effects in Layered Materials: The Case of Hexagonal Boron Nitride Leeor Kronik, Noa Marom, Jonathan Bernstein, Jonathan Garel, Alexandre Tkatchenko, Ernesto Joselevich, Oded Hod The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to anchor the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which band gap modulations of ~0.6 eV are obtained. We propose a simple geometric model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes. Reference: Marom et al., Phys. Rev. Lett. 105, 046801 (2010). [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B31.00007: van der Waals binding and band structure effects in graphene overlayers and graphane multilayers Per Hyldgaard, Jochen Rohrer We study graphene formation (by selective Si evaporation) and adhesion on SiC surfaces as well as stacking and binding of graphane multilayers [1] using a number of versions of the van der Waals Density Functional (vdW-DF) method [2] and plane-wave density functional theory calculations. For the graphene/SiC systems and for the graphane multilayers we document that the bonding is entirely dominated by van der Waals (vdW) forces. At the same time we find that dispersive forces acting on the layers produce significant modifications in the graphene and graphane band structure. We interpret the changes and discuss a competition between wave function hybridization and interaction with the charge enhancement (between the layers) that results from density overlap. \\[4pt] [1] J. Rohrer and P. Hyldgaard, http://arxiv.org/abs/1010.2925\\[0pt] [2] Dion et al, PRL 92, 246401 (2004); V.R.Cooper, PRB 81, 161104(R) (2010), K. Lee et al PRB 82, 081101(R) (2010). [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B31.00008: Noble gas adsorption on carbon nanotubes: insight from a van der Waals density functional study De-Li Chen, Wissam Al-Saidi, Karl Johnson Adsorption of nobel gases (Ar, Kr, Xe) on metallic and semiconducting carbon nanotubes (CNTs) is investigated using the van der Waals density functional (vdW-DF) developed by the Lundqvist and Langreth groups. Standard local and semi-local density functional methods do not describe nonlocal dispersive forces and fail in these systems. We found that the noble gases are underbound or even unbound with the generalized gradient approximation, while the bonding distance is underestimated at the local density approximation level of theory. In contrast, the vdW-DF approach gives considerable improvement in the description of the adsorption energies. We found no difference in the adsorption between the metallic and semiconducting nanotubes, indicating that the adsorption energies for rare gases on carbon nanotube are not strongly influenced by differences in the electronic structure of the nanotubes. The adsorption energies predicted from classical potentials are smaller than those from vdW-DF calculations by about 10-35\%. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B31.00009: Water droplet distributions in pure and functionalized single walled carbon nanotubes Callen Johnson, Supriyo Ghosh, Xenia Tombokan, Parameswar Hari In this study we investigated water droplet distribution in (1-2 nm diameter and 30 $\mu $s long) single walled carbon nanotubes (SWCNT) using time domain nuclear magnetic resonance (NMR). Annealing SWCNTs at 400\r{ }C resulted in a carbon nanotube with closed ends. We attached various amounts of water on the annealed SWCNT samples and measured the NMR spin-spin relaxation (T$_{2}$ ) distribution profile. The T$_{2}$ distributions were analyzed using the inverse Laplace transform to estimate the amount of water attached to the SWCNT. We performed NMR measurements on water distributions in pure CNT and functionalized CNT with OH and COOH radicals. The T$_{2}$ distribution curves for pure and functionalized SWCNTs show significant difference in water attachment. We also studied water distribution profile with the SWCNTs annealed at 800\r{ }C. Annealing at 800\r{ }C opens the ends of the SWCNTs. T$_{2}$ distribution curves at 400\r{ }C and 800\r{ }C will be compared to obtain the amount of free water attached on the outer and inner surface of pure and functionalized SWCNTs. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B31.00010: How do hybrid functionals, dispersion interactions and quantum nuclei affect the structure of liquid water? Zhaofeng Li, Robert A. DiStasio Jr., Roberto Car, Xifan Wu We report {\it ab-initio} molecular dynamics simulations of liquid water at STP and at the volume corresponding to experimental equilibrium density. These simulations are based on the hybrid functional PBE0 for the electrons and include approximate dispersion interactions according to Ref \footnote{A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. {\bf 102}, 073005 (2009).}. Nuclear quantum corrections were included as estimated by Ref \footnote{J. Morrone and R. Car, Phys. Rev. Lett. {\bf 101 }, 017801 (2008).}. We find that all of these components are important to significantly improve the agreement of the simulated structure with recent experimental analyses based on neutron and X-ray diffraction\footnote{ A. Soper and C. Benmore, Phys. Rev. Lett. {\bf 101 }, 065502(2008).} and on NMR experiments. \footnote{K. Modig, B. Pfrommer, B Halle, Phys. Rev. Lett. {\bf 90 }, 075502 (2003).} [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B31.00011: Density, structure and dynamics of water: the effect of Van der Waals interactions Marivi Fernandez-Serra, Jue Wang, Guillermo Roman, Emilio Artacho, Jose Soler We present a DFT AIMD study of liquid water using several GGA functionals as well as the van der Waals density functional (vdW-DF) of Dion et al. [PRL 92, 246401(2004)]. As expected, we find that the density of water is grossly underestimated by GGA functionals. When a vdW-DF is used, the density improves drastically and the experimental diffusivity is reproduced without the need of thermal corrections. We analyze the origin of the density differences between all the functionals. We show that the vdW-DF increases the population of non-H-bonded interstitial sites, at distances between the first and second coordination shells. However, it excessively weakens the H-bond network, collapsing the second coordination shell. This structural problem is partially associated to the choice of GGA exchange in the vdW-DF. We show that a different choice for the exchange functional is enough to achieve an overall improvement both in structure and diffusivity. Jue Wang et al. J. Chem. Phys, 133, (2010). [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B31.00012: Van der Waals Density Functional Simulations of Liquid Water Jun Wu, Cui Zhang, Giulia Galli, Francois Gygi We compare two versions of van der Waals density functionals (DRSLL [1], LMKLL [2]) in electronic structure computations of weakly bonded systems. The functionals are implemented in the Qbox code [3] and are verified by reproducing published binding energies and equilibrium separations of several weakly bonded dimers. Vibrational frequencies of the water monomer and dimer computed using the above van der Waals functionals are not improved compared to PBE results. We present results of molecular dynamics simulations of liquid water using the DRSLL and LMKLL functionals and compare radial distribution functions with corresponding results obtained with GGA functionals.\\[4pt] [1] M. Dion et al. Phys. Rev. Lett. 92, 246401 (2004).\\[0pt] [2] K. Lee et al. Phys. Rev. B 82, 081101 (2010).\\[0pt] [3] http://eslab.ucdavis.edu/software/qbox [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B31.00013: Ice under pressure: the role of van der Waals forces in hydrogen bonding Eamonn Murray, Giulia Galli We will discuss the evolution of the role of van der Waals interactions in hydrogen bonding in high pressure phases of ice. Here, we compare first principles results of the structural and electronic properties of ice using several different approaches to the calculation of exchange and correlation energies. These include the non-local vdW density functional of Dion \textit{et al}\footnote{M. Dion, H. Rydberg, E. Schr\"oder and D. Langreth, Phys. Rev. Lett. \textbf{92}, 246401 (2004)}, the revised vdW density functional of Lee \textit{et al}\footnote{K. Lee, \'E. D. Murray, L. Kong, B. I. Lundqvist and D. C. Langreth, Phys. Rev. B \textbf{82}, 081101 (2010)} and the EXX/RPA approach based on an eigenvalue representation of the dielectric matrix\footnote{D. Lu, Y. Li, D. Rocca and G. Galli, Phys. Rev. Lett. \textbf{102}, 206411 (2009)} along with the semilocal functional PBE and hybrid functional PBE0. [Preview Abstract] |
Session B32: Focus Session: Optical Properties of Nanostructures and Metamaterials II
Sponsoring Units: DMPChair: Tom Driscoll, University of California, San Diego
Room: C144
Monday, March 21, 2011 11:15AM - 11:27AM |
B32.00001: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 11:27AM - 11:39AM |
B32.00002: Optical Properties of Metal-Dielectric-Metal (MDM) Nanoantennas Bhuwan Joshi, Xuejin Wen, Kai Sun, Wu Lu, Qi-Huo Wei We present a new design of plasmonic nanoantennas and study their optical properties. The nanoantennas consist of two metal blocks (cuboids or cylinders) stacked vertically with a dielectric spacer. The results from numerical simulations show that such plasmonic nanoantennas exhibit various cavity resonance modes which produce sharp peaks in the near field spectra and leave dips in the far field scattering spectra. Nanofabrication and characterization of these nanoantennas will also be presented in the talk. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B32.00003: Designing broadband plasmonic nanoantennas for ultrasensing Zhenhuan Yi, Kai Wang, Dmitri V. Voronine, Andrew Traverso, Alexei Sokolov Various designs of broadband plasmonic nanoantennas made of gold and silver nanospheres are considered and optimized for ultrasensitive spectroscopic applications. The simulated nanostructures show a broadband optical response which may be tuned by varying the size, position and composition of nanospheres. Near-field enhancement in nanoantenna hot spots is analyzed and compared with previous literature results in the case of a fractal plasmonic nanolens. Broadband plasmonic nanoantennas may allow detecting ultrasmall concentrations of toxic materials and may be used for decoding DNA and for ultrafast nanophotonics applications. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B32.00004: Optical nanoantennas: controlled emission of single photon sources Invited Speaker: Nanoscale quantum emitters are key elements in quantum optics and sensing. However, efficient optical excitation and detection of such emitters involves large solid angles, due to their omnidirectional interaction with freely propagating light and due to limits of diffraction. Optical nanoantennas offer both nanoscale localization and efficient interaction. Here we focus on the control of the interaction of single photon emitters (molecules, quantum dots) with radiation through metal nanorod antennas. First a novel analytical model is presented, which shows the continuous evolution of the properties of optical antennas as they become increasingly bound, i.e. plasmonic. The model accurately describes the complete emission process, the radiative decay rate, quantum efficiency, and angular emission, moreover gives a quantitative description of the gradual emergence of sub-radiant, super-radiant, and dark modes. Next we investigate experimentally the coupling of a single quantum dot to a nanorod of increasing length. The angular luminescence of the quantum dot is detected through increasingly higher order antenna modes. Simultaneously the emission is strongly polarized and enhanced. Direct confrontation with theory allows to determine the coupling efficiency of the quantum dot to the antenna. Finally, we present unidirectional emission of a single emitter by coupling to a nanofabricated Yagi-Uda antenna. A quantum dot is placed in the near field of the antenna so that it drives the resonant feed element of the antenna. The resulting quantum-dot luminescence is strongly polarized and highly directed into a narrow forward angular cone. The directionality of the quantum dot can be controlled by tuning the antenna dimensions. Thus our results show the potential of optical antennas to communicate energy to, from, and between nano-emitters. \\[4pt] A.G.Curto et al., Science 329, 930 (2010) [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B32.00005: Plasmon-mediated polarization-tuneable enhancement of optical absorption in a polymer film Edward J. Osley, Paul G. Thompson, Claudiu G. Biris, Nicolae C. Panoiu, Paul A. Warburton We have fabricated and characterized arrays of nanoscale apertures displaying polarization-tuneable localized surface plasmon (LSP) resonances in the infrared. Arrays of asymmetric cruciform apertures were milled in a gold film using a focused ion beam and subsequently coated with Poly(methyl methacrylate) (PMMA). The aperture geometry is designed so that for a certain polarization state of the incident wave the LSP resonance occurs at the same wavelength as the C=O bond absorption peak in PMMA. The nanostructured film results in an order of magnitude increase in the absorption in PMMA by comparison with a continuous film. By changing the in-plane electric-field polarization of the incident light the LSP resonance shifts away from the PMMA absorption peak, allowing us to quantify the role of plasmonic field-focussing on infrared optical absorption in the polymer film. Numerical simulations show that the increased optical absorption is due to the field enhancement both inside the apertures as well as in their close proximity. We will discuss how this technique may be applied to studies of plasmon-mediated field focussing in other materials including photovoltaic materials. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B32.00006: Plasmonic-molecular resonance coupling Jianfang Wang Localized plamsons have been widely used to enhance optical signals. The plasmon enhancement requires optically active species to be close to the metal surface. The presence of active species can affect the plasmon resonances. Understanding the plasmon-molecule interactions is of importance for both enhancing optical signals and developing plasmon shift-based sensors. We have studied the resonance coupling between Au nanocrystals (NCs) and dyes. The coupling strength can be tuned by varying NC plasmon wavelength. The maximum plasmon shift reaches above 120 nm, which is about 10 times larger than that caused by the local index increase. The plasmon shift decays rapidly as the dye-NC spacing is increased. In addition, the coupling strength is strongly dependent on the molecular properties but independent on the NC shape and size. We have further measured the resonance coupling on single Au NCs. The resonance coupling reveals a unique three-band structure. These single-particle studies will greatly help in understanding the fundamental aspects of the resonance hybridization and designing various plasmon-enhanced spectroscopies. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B32.00007: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 1:03PM - 1:39PM |
B32.00008: Advanced Material Models for Nano-Plasmonic Systems via Discontinuous Galerkin Methods Invited Speaker: Nano-Plasmonic systems offer a tremendous potential for the controlled delivery and extraction of electromagnetic energy to and from tiny objects such as molecules and quantum dots in their immediate vicinity. In view of the increasing sophistication of fabrication and spectroscopic characterization, quantitative computational approaches face challenges that go well beyond the usual description of metals as linear dispersive materials. These challenges include the development of material models that describe the (potentially) strongly nonlocal and nonlinear optical response of such metallic nano-structures themselves as well as the strongly modified light-matter interaction that is mediated by them. This talk reports on the progress of applying the Discontinuous-Galerkin Time-Domain (DGTD) method to the quantitative analysis of nano-plasmonic systems using advanced material models. This includes the efficient modeling of complex geometric features via curvilinear elements, the improvement of the time-stepping scheme via tailored low-storage Runge-Kutta schemes, and the incorporation of optically anisotropic media. In addition, this talk reports on recent results regarding the development and application of advanced material models that are based on a hydrodynamic description of the metal's conduction electrons. By coupling the Maxwell equations to this treatment of the free electrons as a plasma in a confined geometry one is able to capture nonlocal and nonlinear effects and to analyze their consequences. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B32.00009: Suppression of Landau Damping in Metal Nanostructures via Quantum Size Effect Xiaoguang Li, Di Xiao, Zhenyu Zhang Using the matrix random phase approximation, we study the tunability of localized surface plasmon resonance in small metal nanostructures, where the Landau damping is the dominant dissipation channel and the intrinsic limit to plasmonics technique. We find that the linewidth of plasmon can be effectively suppressed due to the quantization of electron-hole pair energy in various highly confined geometries, where the strength of Landau damping oscillates as the scale of system. Moreover, beyond a classical surface scattering picture, the oscillatory effect can be illustrated with an electron-hole pair description, which can be used to understand many other properties of plasmon. Our results show the possibility to control the Landau damping and therefore should be able to stimulate more efforts on future plasmonics of small nanostructures. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B32.00010: Excitation of electromagnetic modes in spheres Raul Garc\'Ia-Llamas, Luis Ramirez-Rodriguez A study of the excitation of whispering gallery modes (WGM) in non-dispersive dielectric micro sphere is presented. The Near-Field intensity behavior of the nano-jets, sub-wavelength volume regions, is presented as a function of the radii of the sphere in resonant and off-resonant conditions. A similar study is presented for nano metallic sphere. [Preview Abstract] |
Session B33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Electronic Conduction and Defects
Sponsoring Units: DMP DCOMPChair: Ichiro Takeuchi, University of Maryland
Room: C143/149
Monday, March 21, 2011 11:15AM - 11:27AM |
B33.00001: Conduction at different ferroelectric domain walls in BiFeO$_{3}$ Saeedeh Farokhipoor, Christophe Daumont, Beatriz Noheda BiFeO$_{3}$ (BFO) is, at room temperature, a rhombohedrally distorted, ferroelectric perovskite. There are eight possible polarization directions (or domains) and three different types of domain walls, namely, 180\r{ } (ferroelectric) and 109\r{ } and 71\r{ } (ferroelectric and ferroelastic) domain walls. Recent works have shown that the domain walls of BFO can display functionalities different from those of the domains, generating photocurrents [1], inducing exchange bias [2] and displaying conductivity at room temperature [3]. Conduction has been reported at 180\r{ } and 109\r{ } domain walls[3] and it was proposed that the reduction of the band gap, associated with the suppression of ferroelectric distortions, at domain walls was responsible for the observed conduction[3]. It is, however, not yet clear if and how other (extrinsic) mechanisms affect the conductivity at the walls. In order to help clarifying the origin of domain wall conductivity, we have performed temperature, thickness and orientation dependent local conductivity measurements in BFO thin films. The results will be discussed in this presentation. \\[4pt] [1] S.Y. Yang, Nature Nanotech. 5, 143 (2010); [2] L.W. Martin et al. Nano Letters 8, 2050 (2008);[3] J. Seidel et al., Nature Mat. 8, 229 (2009). [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B33.00002: Electronic Transport properties of ultra-thin BiFeO$_{3}$ Dipanjan Mazumdar, Oleg Mryasov, Vilas Shelke, Stephen Jesse, Arthur Baddorf, Sergei Kalinin, Arunava Gupta We have investigated the electronic transport properties of rhombohedral (R) and the nearly-tetragonal (T) phase of BiFeO$_{3}$ using beyond density functional techniques, and combined with nanoscale I-V transport measurements. Using Quasi-particle GW approximation, we show the R and T phase to have significantly different electronic structures. We find that the T phase has significantly lower effective mass at the conduction band edge compared to the R phase leading to a lower effective barrier height for tunnel electrons (0.38 eV vs 3.6 eV). We therefore anticipate that tunnel devices with T phase BFO to have significantly lower resistances. Local transport measurements performed on ultra-thin BFO R phase are consistent with this inference. Tunneling measurements on the tetragonal phase films are also presented. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B33.00003: Oxygen vacancies in lanthanum aluminate (LaAlO$_3$) Joshua Sayre, Nicola Spaldin Oxygen vacancies can affect the properties of an oxide in various manners such as increasing its ion or electronic conductivity, changing its lattice constant or causing dielectric breakdown. The aim of this research is to investigate structural changes and consequent changes in properties caused by oxygen vacancies in a model complex oxide, lanthanum aluminate, LaAlO$_3$. We use density functional theory with the generalized gradiant approximation (GGA) and within the VASP package to calculate the structure and properties of representative oxygen vacancy profiles. We find that the presence of oxygen vacancies modifies the pattern of rotations of the oxygen octahedra. We discuss the implications of our results for understanding the correlation between epitaxial strain in oxide thin films and intrinsic defect profiles. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B33.00004: Defect-Induced Electronic Structures and Formation Energies of Vacancy Complexes in SrTiO$_{3}$ Jiyeon Kim, Choong H. Kim, Rokyeon Kim, Jaejun Yu Recently defect induced ferroelectricity in SrTiO$_{3}$ has been reported at room temperature. Strontium-oxygen vacancies were suggested as a possible source of electric polarization regarding to the existence of mid-gap states. To understand the detailed electronic structures induced by defects and their formation energies, we carried out density-functional-theory calculations for various defects such as Sr, Ti, O, Sr-O, Sr-O- O vacancies. We employed the LDA+$U$ method as implemented in the VASP code to describe the d-orbital occupation at the Ti- site due to the presence of oxygen vacancy. A complex of Sr-O-O vacancies is found to contribute to the localized electronic states in the band gap and its formation energy is small enough to form easily under the poor oxygen limit. We conclude that the vacancy-complex defects play a crucial role in determination of the physical properties of SrTiO$_{3}$ thin films. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B33.00005: Quasiparticle electronic structure calculations of F centers in SrTiO$_{3}$ perovskite Chandrima Mitra, Alex Demkov Among the broad class of oxides, ABO$_{3}$ perovskites have attracted a lot of attention in the recent past due to its beneficial material properties. SrTiO$_{3}$ is one such example of this class of compounds. It shows a wide range of properties from being ferroelectric to exhibiting superconducting properties in doped SrTiO$_{3}$. The anomalous dielectric properties in this material make it a potential candidate for technological applications. However, being a semicovalent oxide, the complexities in its electronic structure have hindered a proper characterization of the system. For instance, the intrinsic excitonic luminescence, in this system, is not well understood and there is no general agreement as to whether it is caused by defects or due to self trapped excitons. This calls for an accurate theoretical description of the electronic levels as well as the various defect states in this material. In this work we present results for quasiparticle GW calculations of pure as well as \textit{defective} SrTiO$_{3}$ containing oxygen vacancies which form F centers in these compounds. From a quasiparticle description of the system excitonic properties of SrTiO$_{3}$ will be examined. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B33.00006: Charge transition levels of oxygen vacancies in monoclinic hafnia Manish Jain, James R. Chelikowsky, Steven G. Louie We perform electronic structure calculations on oxygen vacancies in hafnia using a combined density functional theory (DFT) and GW formalism. This formalism corrects for the error in calculating formation energy and charge transition levels using standard DFT. While the formalism is, in principle, exact; in previous calculations of this kind, one makes several approximations to make the calculation tractable. We assess the impact of these approximations on the charge transition levels of the oxygen vacancy in hafnia. In particular, we examine the assumption that the quasiparticle wavefunctions are the same as DFT wavefunctions for the defect states. We show that this assumption can lead to erroneous results in this system and present the charge transition levels without making use of this assumption. We also explore the possibility that these defects are negative U centers. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B33.00007: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 12:39PM - 12:51PM |
B33.00008: Disproportionation and comproportionation reactions of resistive switching in polycrystalline NiOx films Forest S.-S. Chien, Yi-Ta Wu, Guan-Long Lai, Y.H. Lai The NiO$_{x}$ thin film exhibit excellent bistable unipolar resistive switching, which has strong potential in nanoscale nonvolatile-memory applications. The underlying mechanism of NiOx resistive switching is still in debate. We studied the chemical bonding states of Ni 2p and O 1s at high/low resistance spots by focused X-ray photoemission spectroscopy. The disproportionation and comproportionation reactions of 3NiO $\leftrightarrow $ Ni + Ni$_{2}$O$_{3}$, accounts for the resistive switching of NiO$_{x}$. The calculated Gibbs energy of the reaction indicates the reversibility of the reaction thermochemically. The dynamic breathing of the filaments with switching was observed by conducting atomic force microscopy. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B33.00009: The effect of oxygen migration for random resistance access memory in oxide-based devices Zhaoliang Liao, Peng Gao, Yang Meng, Hongwu Zhao, Xuedong Bai, Dongmin Chen The observed electric field induced resistance switching in metal oxide thin films is generally thought to arise from the creation or annihilation of oxygen defects. By depositing different kinds of metal electrodes on Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ and CeO$_{2}$ thin films to construct sandwiched devices, we found that the devices can be categorized into two groups with different switching behaviors, depending on the Gibbs free energy of oxidation of the top electrodes with respect to that of underneath metal oxide\textit{. In-situ} TEM measurements show a structure change with an applied electric field. Our analysis indicates that the structure change is related to the oxygen migration driven by external electric field. Therefore, it suggests that not only the oxygen defects but also their migration play important roles in the functionality of oxide-based devices [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B33.00010: Conduction and Loss Mechanisms in Flexible Oxide-Based Memristors J.L. Tedesco, N. Gergel-Hackett, L. Stephey, A.A. Herzing, M. Hernandez-Mora, C.A. Hacker, J. Obrzut, L.J. Richter, C.A. Richter In order to study the conduction and loss mechanisms behind their operation, flexible sol-gel based memristors were fabricated with differing oxide film thicknesses and device sizes. XPS, TEM, EELS, and VASE measurements indicated the oxide was amorphous TiO$_{2}$, with a significant fraction of organic material. Analysis of the bias and sweep rate dependence of the devices suggested the switching mechanism was induced by charge flow in the memristor and not by the electric field. Further analysis of the I-V curves indicated that once the memristors were switched into the high-current ``ON'' state, conduction through them generally became ohmic. Once such memristors were cut to yield two smaller devices, there was typically only one device that remained ohmic, indicating that localized conduction pathways caused switching in the flexible memristors. There was a shift in the capacitance-frequency and conductance-frequency measurements following switches between the ``ON'' and ``OFF'' states of the devices, indicating that an additional dielectric loss mechanism was present in these films that was not present in ordinary TiO$_{2}$ films. This loss mechanism is attributed to dipoles in the organic constituents of the films that are by-products of the sol-gel process. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B33.00011: Capacitive network near the metal insulator transition in Vanadium Dioxide J.G. Ramirez, E.J. Patino, R. Schmidt, A. Sharoni, M.E. Gomez, I.K. Schuller Recent infrared spectroscopy and transport measurements in nano-scaled junction of VO2 have revealed the existence of phase separation into metallic and insulating phases. Here we present Impedance spectroscopy measurements performed in high quality Vanadium dioxide (VO$_2$) thin films for the first time. This technique allows distinguishing between the resistive and capacitive response of the VO$_2$ films and provides the dielectric properties across the metal-insulator transition (MIT). The film capacitance exhibits an unusual increase close to the MIT which implies the formation of a capacitor network produced by the nanoscale phase separation of metallic and insulating phases. This work has been supported by AFOSR, COLCIENCIAS, CENM and Ramon y Cajal Fellowship. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B33.00012: Substitution site for Zn in LiNbO$_3$ from detailed EXAFS analysis Frank Bridges, Brad Car, Jaime Castillo, Michael Kozina, Scott Medling We report detailed EXAFS studies of Zn doped LiNbO$_3$, at the Zn and Nb K-edges, as a function of dopant and as a function of temperature. For this material there exist several models concerning the substition site(s) for Zn. Our data are only consistent with Zn substitution on the Li site. Any substitution on the Nb site is very small. Further as the Zn concentration changes from 5-9\% the EXAFS r-space function for the Zn K-edge changes very little, a slight amplitude reduction consistent with increased local disorder for increasing Zn concentration. Our detailed analysis shows that the nearest O neighbors to Zn are slightly pulled inward while the nearst metal atoms - Nb - are pushed away. We cannot tell if there are vacancies on the Li sites because Li is a very weak backscatterer, and the amplitude of the rather long Zn-Li peak is very low. We discuss and compare our results with previous proposed models and with recent calculations for other defects that suggest that many +2 dopants substitute at the Li site. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B33.00013: Structure and Magnetic Properties of Electron Doped YMnO$_{3}$ Tian Yu, Peng Gao, Trevor Tyson Combined local and long range structural measurements were conducted on the electron doped ferroelectric Y$_{1-x}$Zr$_{x}$MnO$_{3}$ system. Doping by Zr is found to maintain the hexagonal structure for a large range of x-values. The location of Zr in the lattice is identified and changes in structure with doping are followed. These details of the local structure are examined by x-ray diffraction and x-ray absorption spectroscopy and compared with detailed magnetic studies to correlate the impact of electron doping and atomic structure on the magnetic order in these systems. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B33.00014: First-principles calculations of Ti and O NMR chemical shift tensors in ferroelectric perovskites Daniel Pechkis, Eric Walter, Henry Krakauer Complementary chemical shift calculations were carried out with embedded clusters, using quantum chemistry methods, and with periodic boundary conditions, using the GIPAW approach\footnote{C.~\ J.~\ Pickard and F.~\ Mauri Phys. Rev. B {\bf 63}, 245101 (2001)} within the Quantum Espresso package.\footnote{P. Giannozzi et al., Journal of Physics: Condensed Matter {\bf 21}, 395502 (2009)} Compared to oxygen chemical shifts, $\hat \delta$(O),\footnote{D.~\ L.~\ Pechkis, E.~\ J.~\ Walter, and H.~\ Krakauer. J.~\ Chem. Phys. {\bf 131}, 184511 (2009)} cluster calculations for $\hat \delta$(Ti) were found to be more sensitive to size effects, termination, and choice of gaussian-type atomic basis set, while GIPAW results were found to be more sensitive to the pseudopotential construction. The two approaches complemented each other in optimizing these factors. We show that the two approaches yield comparable chemical shifts for suitably converged simulations, and results are compared with available experimental measurements. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B33.00015: Electrical Breakdown in Lightning Arrestor Connector (LAC) Devices Harold P. Hjalmarson, Kenneth Kambour, Andrew C. Pineda Lightning arrestor connector (LAC) devices protect electronic devices by providing a conductive path to ground for electrical power surges caused by lightning. Such devices consist of an insulating material between electrodes. This insulation region is composed of an air gap and a high permittivity dielectric. In this presentation, the physics of the phenomena active in the early stages of the flow of transient electrical current will be described. The conditions that lead to thermal breakdown of the dielectric will also be discussed.--Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Session B34: Focus Session: Interfaces in Complex Oxides - Spectroscopy and Growth
Sponsoring Units: DMP GMAGChair: John Hill, Brookhaven National Laboratory
Room: C141
Monday, March 21, 2011 11:15AM - 11:27AM |
B34.00001: Strain-modulated asymmetric orbital--lattice interactions in correlated oxide heterostructures J. Chakhalian, J. Rondinelli , Jian Liu, B. Gray, M. Kareev, E.J. Moon, J. Cohn, M. Varela, S.G. Altendorf, F. Strigari, B. Dabrowski, L.H. Tjeng, P.J. Ryan, J.W. Freeland Artificial structuring of quasi-two dimensional correlated electron thin films and heterointerfaces offers an arena to discover innovative functionalities by harnessing electronic and orbital degrees of freedom. To harness this potential understanding of how structurally linked correlated electronic responses are modified through epitaxial constraints at the substrate--film hetero-interface is clearly required. We use a suite of advanced experimental probes along with ab-initio calculations to show how compressive and tensile bi-axial strain lead to unusual asymmetrical orbital responses. Microscopic studies based on resonant X-ray spectroscopies reveal that the asymmetry leads to a new ground state with a ligand hole density and chemical bond covalency that is modulated by the sign of the epitaxial constraint at the interface. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B34.00002: Structural effects on the electronic properties of epitaxially strained RNiO$_{3}$ thin films I.C. Tung, Jian Liu, B. Gray, J. Chakhalian, J. Rondinelli, P. Ryan, J.W. Kim, M.J. Bedzyk, J.W. Freeland Since the metal--insulator (MI) transition is a hallmark of strongly correlated materials, understanding the behavior of the MI transition of RNiO$_{3}$ (R=rare earth) thin films subjected to confinement, lattice misfit and broken symmetry at the interface in the ultra-thin limit is fundamentally and technologically important [1]. Here we present a study of the effect of the lattice symmetry with epitaxial strain in thin films of LaNiO$_{3}$ and NdNiO$_{3}$ grown on SrTiO$_{3}$(001) substrates by pulsed laser deposition. A combination of x-ray diffraction, soft x-ray absorption spectroscopy, and temperature-dependent resistivity has been applied to elucidate structural and electronic properties of the samples. Work at the Advanced Photon Source, Argonne is supported by the U.S. Department of Energy, Office of Science under Contract No. DE-AC02-06CH11357.\\[4pt] [1] Jian. Liu et al., Appl. Phys. Lett. 96, 233110 (2010). [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B34.00003: Orbital engineering near La$_{2}$NiO$_{4}$-La$_{2}$CuO$_{4}$ superlattice interfaces S. Smadici, J.C.T. Lee, J. Morales, P. Abbamonte, G. Logvenov, A. Gozar, I. Bozovic Orbital states of transition metal oxides present the opportunity of adjusting material properties to a specific purpose (orbital engineering). A comparison of the resonant soft x-ray reflectivity of La$_{2}$NiO$_{4}$-La$_{2}$CuO$_{4}$ superlattices at Ni L and Cu L edges shows different spatial distributions of the occupation of Ni d$_{x}$2$_{-y}$2 and d$_{3z}$2$_{-r}$2 orbitals in the LNO layers. This modulation of the Ni valence is possible through a pronounced modulation of the density of oxygen interstitial dopants within the structure which does not follow exactly the structure itself. This is the first observation of orbital engineering in a 214 oxide. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B34.00004: Orbital Control in single unit cell LaNiO3/LaAlO3 superlattices J.W. Freeland, J. Liu, B. Gray, M. Kareev, J.W. Kim, P.J. Ryan, R. Pentcheva, J. Chakhalian Oxide heterostructures built from strongly correlated electron materials offers unique opportunity to generate new ground-states by altering the balance of competing energies in the system. In pursuit of rational control of orbital polarization, we present a combined experimental and theoretical study of single unit cell LaNiO$_3$/LaAlO$_3$ superlattices[1]. Polarized x-ray absorption spectra show a distinct asymmetry in the orbital response under tensile vs. compressive strain. A splitting of orbital energies $\sim$100 meV with octahedral distortions is found for the case of compressive strain which is much smaller than the $3d$ bandwidth. In sharp contrast, for tensile strain, no splitting is found although a strong orbital polarization is still present. Density functional theory calculations of the electronic properties reveal that the asymmetry results from a combination of strain effects and altered covalency in the bonding across the interfacial apical oxygen to the Al site, leading to the opening of a pseudogap in the heterostructure for tensile strain. Work at Argonne, including the Advanced Photon, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [1] J.W. Freeland et. al. arXiv:1008.5618 [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B34.00005: Realistic DMFT calculations for nickelate superlattices M.J. Han, Xin Wang, Chris A. Marianetti, Andrew J. Millis We present phase diagram, photo-emission and RIXS (resonant inelastic X-ray scattering) spectra, orbital polarization, and Fermi surface plots for LaNiO3/LaXO3 superlattice (X=Al, Ga,...) obtained from DMFT (dynamical mean-field theory) calculation based on a realistic multi-band tight-binding model derived from DFT (density functional theory) calculations and in particular including oxygen orbitals. Our results indicate that heterostructuring is unlikely to produce one band model physics and point toward a new view of metal-insulator transition of this system. This work is supported by ARO via grant No. W911NF0910345-56032PH. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B34.00006: Conductivity enhancement of ultrathin LaNiO$_{3}$ films in superlattices Junwoo Son, James M. Lebeau, S. James Allen, Susanne Stemmer The transport properties of heterostructures with Mott materials, such as LaNiO$_{3}$, have been predicted to exhibit unusual phenomena not present in the bulk. Prior studies have shown that ultrathin LaNiO$_{3}$ films exhibit strongly localized behavior, whereas thicker films remain metallic. Here, we report on epitaxial [SrTiO$_{3}$(3 u.c.)/LaNiO$_{3}$(4 u.c.)]$_{n}$ superlattices on (001) (LaAlO$_{3}$)$_{0.3}$(Sr$_{2}$AlTaO$_{6}$)$_{0.7}$ (LSAT) substrates (u.c. = unit cell). X-ray diffraction and Z-contrast imaging confirm sharp interfaces. The sheet resistance of the superlattices is explored as a function of temperature and number of bilayers. All superlattices with more than 2 layers were metallic whereas 4 u.c. LaNiO$_{3}$ films and a single 4 u.c. LaNiO$_{3}$/3 u.c. SrTiO$_{3}$ bilayer were both insulating. The sheet resistance of superlattices decreases with n. Possible models for the electrical characteristics will be discussed. The first model attempts to describe the sheet resistance with conduction through parallel-connected LaNiO$_{3}$ layers and conductive interfacial layers. The second model is based on coupling of layers, each of which is near the percolation threshold for a metal-insulator transition, and explains the difference in conductivity of single layers and superlattices without invoking interfacial layers. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B34.00007: Electric-Field Control of the Metal-Insulator Transition in Nickelate Thin Films Raoul Scherwitzl, Pavlo Zubko, Ignacio Gutierrez-Lezama, Shimpei Ono, Alberto Morpurgo, Gustau Catalan, Jean-Marc Triscone The rare-earth perovskite nickelates (RNiO$_3$) are a fascinating family of compounds displaying a sharp temperature-driven metal-insulator (MI) transition with resistance changes of several orders of magnitude. From a fundamental point of view, these materials present an ideal system to study MI transitions since, in contrast to most oxides, a complete evolution from itinerant to localized behavior can be achieved without doping. From a technological point of view, the nickelates are just as exciting, as the large changes and thermal hysteresis in resistance may find uses in various electronic applications, particularly if the MI transition could be tuned using an electric field. We discuss the electric field control of the MI transition in NdNiO$_3$. The electric double layer technique was used in order to obtain very large charge carrier density modulations (exceeding 10$^{15}$ cm$^{-2}$), enabling us to reversibly tune the transition temperature by more than 50 K and to achieve electro-conductivities as high as 60000\% \lbrack 1\rbrack.\\[4pt] \lbrack 1\rbrack \, R. Scherwitzl \emph{et al.}, Adv. Mater., doi:~10.1002/adma.201003241 (2010) [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B34.00008: Interface Structures in Ferromagnetic LaMnO$_{3}$-SrMnO$_{3}$ Superlattices Amish Shah, Quentin Ramasse, Steven May, Anand Bhattacharya, Xiaofang Zhai, James Eckstein, Jian-Min Zuo, John Spence We have investigated the interfaces of LaMnO3$_{2n}$-SrMnO3$_{n}$ (LMO/SMO) superlattices. Charge density calculations have predicted a leakage of Mn e$_{g}$ electrons from LMO into SMO.\footnote{C. Aruta et al., Phys. Rev. B 80 (2009).} For n=1, these electrons are expected to be distributed throughout all films in the superlattice, while for n $>$ 3, the electrons are expected to be localized within a few layers near the interfaces. Using aberration corrected STEM coupled with EELS, we probed a LMO$_{11.8}$-SMO$_{4.4}$ superlattice at high spatial resolution to examine interfacial states. We find that the LMO on SMO interface is structurally sharper than SMO on LMO interfaces. Extra interfacial states above the Fermi level are localized to 1 unit cell of the sharp LMO/SMO interface while the states are weak or absent at the rougher SMO/LMO interfaces. The same interfaces that have extra states have an enhanced ferromagnetic moment at low temperatures.\footnote{S. May et al., Phys. Rev. B 77 (2008).} [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B34.00009: Induced Ti magnetism at titanate / manganite interfaces J. Garcia-Barriocanal, F.Y. Bruno, A. Rivera-Calzada, C. Leon, J. Santamaria, J.C. Cezar, P. Thakur, N.B. Brookes, J.W. Taylor, J.A. Duffy, S.B. Dugdale, C. Utfeld, S.R. Giblin, T. Nakamura, K. Kodama, S. Okamoto We show evidence of induced magnetism resulting from the electronic (charge) or orbital reconstruction occurring at the interface. We show a novel form of Ti magnetism at the interface between SrTiO$_{3}$ (STO) and LaMnO$_{3}$ (LMO) [1] as evidenced by evidenced by a strong XMCD signals at Ti and Mn edges. The magnetic alignment (ferromagnetic or antiferromagnetic) of Ti and Mn moments can be tuned by structural parameters. \\[4pt] [1] J. Garcia-Barriocanal et al. Nature Comm. \textbf{1}:82 doi: 10.1038/ncomms1080 (2010) [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:39PM |
B34.00010: Orbital Reflectometry and the Electronic Structure of Oxide Interfaces Invited Speaker: The occupation of $d$-orbitals has a key influence on the physical properties of transition-metal (TM) oxides.\footnote{Tokura, Y. $\&$ Nagaosa, N. Science {\bf 288}, 462-468 (2000).} Due to the strong hybridization with neighboring oxygen ions, the electronic structure is very sensitive to changes in the TM-oxygen bond distances induced by strain and/or by the chemical bonding to other ions with different electronic configuration. Both effects might be important in oxide heterostructures,\footnote{Han, M. J., Marianetti, C. A. $\&$ Millis, A. J. Phys.\ Rev.\ B {\bf 82}, 134408 (2010).} but thus far it has been difficult to probe atomic-scale modulations of the orbital occupation in a quantitative manner.\footnote{Chakhalian, J. \textit{et al.} Science {\bf 318}, 1114-1117 (2007).} We present results from polarized soft x-ray resonant reflectivity, which demonstrate that it is possible to derive quantitative, spatially resolved orbital polarization profiles. We show that this method is sensitive enough to resolve differences of $\sim 3\%$ in the occupation of Ni $e_g$ orbitals in adjacent atomic layers of a LaNiO$_3$-LaAlO$_3$ superlattice, and the experimental findings are in good agreement with electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up new perspectives for orbital physics in oxide heterostructures. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B34.00011: Source oxidation problem in oxide-MBE environment and its solution Namrata Bansal, Yong-Seung Kim, Seongshik Oh Maintaining stable fluxes for multiple source elements is a challenging task when the source materials have significantly different oxygen affinities in a complex-oxide MBE environment. Although this problem has been known to the complex oxide MBE community since the late 1980s, a detailed study and solution is still lacking. Here, using Sr as a test source, because of its easy oxidation and popularity in complex oxides, we investigated the source-oxidation problem in a number of different conditions. We found that the source oxidation was less for higher flux rates, unmelted source shape, and extended port geometry. The extended port geometry was also found to eliminate the flux transient, usually observed in a standard port, after opening the source shutter. Furthermore, a crucible aperture insert scheme was found to be very effective in suppressing the source oxidation. In this scheme, a disk-shaped aperture was mounted inside the crucible and we found that it blocks most of the oxygen species coming to the source. However, the depth of the aperture disk was critical for its performance. We will discuss how these configurations suppress source oxidation and lead to significantly enhanced stability of Sr-flux in harsh oxidation conditions. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B34.00012: Growth of SrTiO3(110) film with oxide molecule beam epitaxy Jiandong Guo, Zhiming Wang, Fang Yang, Jiagui Feng, Fengmiao Li In the past decade, a tremendous amount of evidence has shown that thin films, superlattices and heterointerfaces of oxides display a rich diversity of glamorous properties that is related, but not identical to that in the bulk. To understand the underlying physical mechanism, it is essential to construct the oxide heterostructures under control with atomic precision. We have studied the SrTiO$_{3}$(110) surface that bears intrinsic instability of reconstruction in addition to the broken symmetry due to the surface polarity, which provides us an additional degree to tune the properties of the epitaxial material by manipulating the termination layer of the substrate. Beyond the termination, we are able to tune the stability of a series of surface reconstructions and realize the reversible phase transitions between them. By applying the knowledge to the homoepitaxy, we develop an easy method to coordinate the metal evaporation sources with required flux rate ratio precisely during the oxide MBE growth. We further simplify the growth by controlling the shutter of the Sr source. The atomically well defined grown surface is characterized by scanning tunneling microscopy. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B34.00013: Oxygen Doping Study of Cuprate/Manganite Thin-Film Heterostructures Hao Zhang, J.Y.T. Wei, Wen He Gong, Gianluigi A. Botton Recent studies of thin-film heterostructures comprising superconducting cuprates and ferromagnetic manganites have revealed a range of novel physical phenomena. These phenomena are believed to involve complex interfacial interactions between competing order parameters [1], and appear to be highly sensitive to carrier doping [2]. To further examine these phenomena, we carry out a systematic oxygen-doping study of cuprate/manganite multilayer thin films, grown epitaxially by pulsed laser-ablated deposition. Our samples are characterized by electrical transport and magnetization measurements, as well as x-ray diffraction and several microscopy probes including SEM and TEM. We also make cation substitution in the cuprate layer, in order to study the effects of carrier doping across the interface.\\[4pt] [1] For example, see J. Hoppler \textit{et al.}, Nature Materials \textbf{8}, 315 (2009).\\[0pt] [2] V. Pe\~{n}a \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 177005 (2006). [Preview Abstract] |
Session B35: Topological Insulators: Theory I
Sponsoring Units: DCMPChair: Kai Sun, University of Maryland
Room: C140
Monday, March 21, 2011 11:15AM - 11:27AM |
B35.00001: Classification of Gapped Topological Phases in 1D Interacting System Xie Chen, Zheng-Cheng Gu, Xiao-Gang Wen Topological phases exist in quantum many-body systems beyond the usual symmetry breaking understanding of phase and phase transition. While a full classification of topological insulators and superconductors has been given for non-interacting fermions, the question of what phases exist for strongly interacting systems and how to identify them seems hard. Here we give a full classification of 1D gapped phases with possible topological and symmetry breaking order in both spin and fermion systems, based on the local unitary equivalence relation between short-range correlated matrix product states, which represent well the class of 1D gapped ground states. We find that in certain symmetry classes, the classification result for non- interacting systems is changed when strong interaction is allowed. Understanding about 1D system also allows us to obtain some simple results for topological phases in higher dimensions when certain symmetries are present. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B35.00002: Classification of topological insulators and superconductors using KR theory Abhishek Roy Kitaev's periodic table\footnote{A. Kitaev, Proceedings of the L.D.Landau Memorial Conference Advances in Theoretical Physics, June 22-26, 2008} provided a classification of topological insulators and superconductors. The classes are indexed by dimension and symmetries. We present a method of arriving at the table using KR theory. This gives a unified and systematic method of constructing model Hamiltonians for each class, as well as those for boundaries and defects. We motivate the formulae for the topological invariants and explain the diagonal periodicity in the invariants. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B35.00003: The classification of topological insulators and superconductors Ching-Kai Chiu, Michael Stone, Taylor Hughes We use the process of band crossings during quantum phase transitions to explain the periodic table of topological insulators and superconductors. This is achieved by showing how irreducible representations of the real and complex Clifford algebras are related to the 10 Altland-Zirnbauer symmetry classes of Hamiltonian matrices which are associated with time reversal, particle-hole, and chiral symmetries. The representation theory not only reveals why a unique topological invariant ($0, Z_2, Z$) exists for each specific symmetry class and dimension, but also shows the interplay between quantum phase transitions, topologically protected boundary modes, and topological invariants. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B35.00004: Chern Numbers Hiding in Time of Flight Images Indubala Satija, Erhai Zhao, Parag Ghosh, Noah Bray-Ali Since the experimental realization of synthetic magnetic fields in neural ultracold atoms, transport measurement such as quantized Hall conductivity remains an open challenge. Here we propose a novel and feasible scheme to measure the topological invariants, namely the chern numbers, in the time of flight images. We study both the commensurate and the incommensurate flux, with the later being the main focus here. The central concept underlying our proposal is the mapping between the chern numbers and the size of the dimerized states that emerge when the two-dimensional hopping is tuned to the highly anisotropic limit. In a uncoupled double quantum Hall system exhibiting time reversal invariance, only odd-sized dimer correlation functions are non-zero and hence encode quantized spin current. Finally, we illustrate that inspite of highly fragmented spectrum, a finite set of chern numbers are meaningful. Our results are supported by direct numerical computation of transverse conductivity. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B35.00005: Topological Nature of Phonon Hall Effect Lifa Zhang, Jie Ren, Jian-Sheng Wang, Baowen Li We provide a topological understanding on phonon Hall effect in dielectrics with Raman spin-phonon coupling. A general expression for phonon Hall conductivity is obtained in terms of the Berry curvature of band structures. We find a nonmonotonic behavior of phonon Hall conductivity as a function of magnetic field. Moreover, we observe a phase transition in phonon Hall effect, which corresponds to the sudden change of band topology, characterized by the altering of integer Chern numbers. This can be explained by touching and splitting of phonon bands. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B35.00006: Effects of Metallic Contacts on Topological Insulator Surface States Jimmy Hutasoit, Tudor Stanescu We study the effect of the coupling between a time-reversal invariant topological insulator and a metal. The coupling of the surface states to the metal is studied both numerically, using a tight-binding model, and analytically within a two-dimensional effective theory. The original surface state described by a massless Dirac fermion acquires a non-trivial spectral profile upon interaction with the metal. This results in the broadening of the surface modes and a shift in the position of the $\Gamma$- point, which may sink into the valence band at strong coupling. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B35.00007: Spin active scattering at the interface between a metal and a topological insulator Erhai Zhao, Chun Zhang, Mahmoud Lababidi We present theoretical results for the spin-active scattering and local spectrum at the interface between a metal and a three-dimensional topological band insulator. We show that there exists a critical incident angle at which complete (100\%) spin flip reflection occurs and the spin rotation angle jumps by $\pi$. We discuss the origin of this phenomena, and systematically study the dependence of spin-flip and spin-conserving scattering amplitudes on the interface transparency and metal Fermi surface parameters. The interface spectrum contains a well-defined Dirac cone in the tunneling limit, and smoothly evolves into a continuum of metal induced gap states for good contacts. We also investigate the complex band structure of Bi$_2$Se$_3$. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B35.00008: Floquet Topological Insulator in Semiconductor Quantum Wells Netanel Lindner, Gil Refael, Victor Galitski Topological phase transitions between a conventional insulator and a state of matter with topological properties have been proposed and observed in mercury telluride - cadmium telluride quantum wells. We show that a topological state can be induced in such a device, initially in the trivial phase, by irradiation with microwave frequencies, without closing the gap and crossing the phase transition. We show that the quasi-energy spectrum exhibits a single pair of helical edge states. The velocity of the edge states can be tuned by adjusting the intensity of the microwave radiation. We discuss the necessary experimental parameters for our proposal. This proposal provides an example and a proof of principle of a new non-equilibrium topological state, Floquet topological insulator, introduced in this paper. arXiv:1008.1792 [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B35.00009: A study of localized states in topological insulators Kun Woo Kim, Tamar Pereg-Barnea, Gil Refael Perturbative and Semiclasscial approaches are employed to find the localized state of the toplogical insulators both on sample edges and defects in the bulk. The models used are massive Dirac with either one or two valleys. The topology is provided by the mass term which has either a momentum dependence or a different sign on the two Dirac points. A semiclasscial Hamiltonian is deduced by following a certain classical path and the Hamilton-Jacobi equation determines the dynamics. Our semiclassical results reproduce the lattice model's chiral edge modes and allow us to investigate impurity bound states. These bound states also appear in a T-matrix calculation. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B35.00010: Zero modes in the bulk of the Topological Insulators induced by disorder or dislocations David Schmeltzer The enigma of the finite conductivity which comes from the bulk of the Topological Insulators (T.I.) is solved by showing that domain walls in the T.I. bind protected zero modes. We consider two scenarios: a)-Dislocations: -We solve the massive Dirac equation (which corresponds to the T.I. in four and two dimensions) in a curved space generated by the coordinates transformation induced by a single dislocation or a single disclination. We examine the condition for the protected zero modes caused by Torsion and Curvature. b) Disorder:-We use the Keldish formalism to study the effect of disorder and interaction of the T.I. We identify an effective Non-Linear Sigma model with a Maxweell and Chern-Simon term which correspond to the different phases: regular metal, regular insulator, topological insulator and protected metals. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B35.00011: Zero energy modes in heterostructures Tudor Petrescu, Stephan Rachel, Karyn Le Hur Zero energy gapless modes have been realized in 1-dimensional domain walls of 2-dimensional systems. In the case of single- or bi-layer graphene, such a quantum wire can be realized by inverting the sign of the gap across a one dimensional interface, without time-reversal symmetry breaking. With the experimental realization of artificial graphene, previously unrealistic additional terms in the Hamiltonian such as staggered potential or artificial gauge fields can be exploited towards the same goal. We classify these terms and study the interplay of disorder effects and boundary conditions. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B35.00012: Protected Entanglement Spectrum in Disordered Topological Insulators Emil Prodan, Taylor Hughes, Andrei Bernevig The topological insulating phase is robust against disorder. However, the phase diagram of a topological insulator, more precisely the boundary between the trivial and topological phases, can be strongly reshaped by the disorder. It is therefore important to devise methods that can efficiently map the extent of the topological phase in the presence of disorder. This talk will describe two such methods and presents several applications. First, it is shown that, in the topological phase, the entanglement spectrum remains extended while in the trivial phase it becomes localized, in the presence of disorder. The localized/delocalized character of the entanglement spectrum has a clear signature in the level statistics, which can be used to efficiently map the boundary between topological and trivial phase. The second method is based on efficient real space calculations of the bulk invariants that do not involve twisted boundary conditions. In fact, it is shown that both methods involve only data encoded in the ground states of the systems. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B35.00013: Theory of Inversion Symmetric Topological Insulators Taylor Hughes, Emil Prodan, B. Andrei Bernevig We analyze translationally-invariant insulators with inversion symmetry that fall outside the current established classification of topological insulators. These insulators exhibit no edge or surface modes in the energy spectrum and hence they are not edge metals when the Fermi level is in the bulk gap. However, they do exhibit protected modes in the entanglement spectrum localized on the cut between two entangled regions. There is a direct connection between the inversion eigenvalues of the Hamiltonian band structure and the mid-gap states in the entanglement spectrum. We also analyze the linear response of these insulators and provide examples of when the inversion eigenvalues determine a non-trivial charge polarization, a quantum Hall effect, an anisotropic 3D quantum Hall effect, or a magneto-electric polarization. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B35.00014: Topological Properties of Insulators with Inversion Symmetry Ari Turner, Yi Zhang, Roger Mong, Ashvin Vishwanath There are many phases of insulators with inversion symmetry (with no other symmetry required). In particular, certain inversion parities cannot change unless there is a phase transition. I will show how to use these parities to classify phases of topological insulators and explain which combinations of these parities have physical consequences (e.g. for the magnetoelectric effect). Many of these results can be derived by pictorial arguments using the entanglement spectrum. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B35.00015: Computing topological invariants without inversion symmetry Alexey Soluyanov, David Vanderbilt We consider the problem of calculating the weak and strong topological indices in noncentrosymmetric time-reversal ($T$) invariant insulators. In 2D we use a gauge corresponding to hybrid Wannier functions that are maximally localized in one dimension. Although this gauge is not smoothly defined on the two-torus,\footnote{A. A. Soluyanov and D. Vanderbilt, arXiv:1009.1415} it respects the $T$ symmetry of the system and allows for a definition of the ${Z}_2$ invariant in terms of time-reversal polarization.\footnote{L. Fu and C. L. Kane, Phys. Rev. B {\bf 74}, 195312 (2006)} In 3D we apply the 2D approach to $T$-invariant planes. We illustrate the method with first-principles calculations on GeTe and HgTe under $[100]$ and $[111]$ strain. Our approach is different from the one suggested previously by Fukui and Hatsugai\footnote{T. Fukui and Y. Hatsugai, J. Phys. Soc. Jpn. {\bf 76}, 053702 (2007)} and should be easier to implement in {\it ab initio} code packages. Time permitting, we will also discuss methods for decomposing the band space into $T$-paired Chern subspaces, and for carrying out a general construction of a Wannier representation for ${Z}_2$ insulators. [Preview Abstract] |
Session B36: Photovoltaics: Novel Approaches and System Issues
Sponsoring Units: GERAChair: Fatima toor, National Renewable Energy Laboratory
Room: C142
Monday, March 21, 2011 11:15AM - 11:27AM |
B36.00001: Surpassing the classical light-trapping limit in thin film solar cells Jeremy Munday, Dennis Callahan, Harry Atwater We describe a methodology for designing thin film solar cells that have light-trapping intensity and absorption enhancements that exceed the classical, ergodic light-trapping limit. From thermodynamic arguments, Yablonovitch and Cody determined the maximum absorption enhancement in the ray optics limit for a bulk material to be $4n^2$, where n is the index of refraction of the absorbing layer. Stuart and Hall expanded this approach to study a simple waveguide structure; however, for the waveguide structures they considered, the maximum absorption enhancement was $<4n^2$. Using a combination of analytical and numerical methods, we describe why these structures do not surpass the ergodic limit and show how to design structures that can. We present here a physical interpretation in terms of the waveguide dispersion relations and optical density of states. We further describe the necessary criteria for surpassing the classical limit and provide examples of waveguide structures with absorption enhancements in excess of $4n^2$. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B36.00002: Resonance shifting: A simple, all-optical method for circumventing the reabsorption problem in luminescent concentrators Noel Giebink, Gary Wiederrecht, Michael Wasielewski Luminescent concentrators (LSCs) were developed over three decades ago as a simple route to obtain high concentration ratio for photovoltaic cells without tracking the sun. In principle, high concentration ratios $>$100 are possible for commonly used chromophores. In practice, however, there is typically an overlap between the chromophore absorption and emission spectra that, although small, ultimately leads to unacceptable reabsorption losses, limiting the concentration ratio to $\sim $10 and hence the utility of LSCs to date. We introduce a simple, all-optical means of avoiding reabsorption loss by ``resonance shifting'' from a bilayer cavity that consists of an absorber/emitter waveguide lying upon a low refractive index layer supported by a transparent substrate. Emission is evanescently coupled into the substrate at sharply defined angles and hence, by varying the cavity thickness over the device area, the original absorption resonance can be avoided at each bounce, allowing for extremely low propagation loss to the substrate edges and hence an increase in the optical concentration ratio. We validate this concept for absorber/emitter layers composed of both a typical luminescent polymer and inorganic semiconductor nanocrystals, demonstrating near-lossless propagation in each case. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B36.00003: Computational Design of All-Carbon Photovoltaics Marco Bernardi, Jeffrey C. Grossman We employ ab-initio computational approaches to study interfaces between different carbon nanomaterials (graphene nanoribbons, carbon nanotubes, graphene fragments) with different structures and surface chemistries. The presence of suitable type-II band alignment at these interfaces and significant light-absorption in the visible and infrared make all-carbon heterojunctions appealing as the active material in next-generation flexible photovoltaic devices, particularly given their greatly enhanced stability compared with polymer-based cells. Results for a wide range of carbon nanomaterials interfaces will be presented, and we will discuss possible applications of such a technology, extending the analysis to the thin-film device scale. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B36.00004: Continuum and KMC simulations of realistic bulk heterostructure solar cell photovoltaic devices Kanokkorn Pimcharoen, Daniel Olds, Phillip Duxbury Design of novel solar cell architectures is significantly assisted by reliable continuum device models, and computational methods capable of solving these models in one, two and three dimensions. We are developing computational methods for these models and are validating them using Kinetic Monte Carlo simulations in the same morphologies. We present simulations using idealized morphologies to test approximations in the continuum models, and we present results for bulk heterostructure morphologies deduced by refining digital nanostructures to experimental neutron relectometry and small angle scattering data. In particular we discuss the ability of one dimensional device models to capture the physics of photovoltaic response of realistic bulk heterostructures. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B36.00005: Computational Materials Design for High Efficiency Photovoltaic Solar Cells and Transparent Conducting Sulfides Hiroshi Katayama-Yoshida, Yoshimasa Tani, Kazunori Sato Based on the first-principles electronic structure calculations we propose computational materials design for high efficiency and low price (In free) solar cell materials based on CuInSe2. Firstly, to avoid the use of In, we try to substitute In by Zn and Sn, or by Ga. The electronic structure calculations are performed by using the KKR-CPA method. To calculate band gap energy correctly, we use self interaction corrections proposed by Filippetti et al. It is found that the direct band gap does not collapse and there appears no deep impurity state in the gap, thus it should be possible to avoid In without any deterioration of photovoltaic effect. From the calculations of mixing energy, we predict that the present system favors the spinodal decomposition and we can expect the formation of nano-wire by two dimensional spinodal nano-decomposition. When the nano-wires are formed, we can expect Type 2 band alignment between host material and the nano-wires. Due to this band alignment, efficient electron hole separation is expected leading to highly efficient photovoltaic effect. As an extension of the present design, we also propose a new class of n-type and p-type transparent conducting sulfides with the negative activation energy for the application of high-efficiency photovoltaic solar-cells. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B36.00006: Efficient Black Silicon Solar Cells with Multi-Scale Surface Texture Fatima Toor, William Nemeth, Matthew Page, Qi Wang, Howard Branz, Hao-Chih Yuan A nanostructured, density-graded surface layer can replace conventional quarter-wavelength coatings as the anti-reflection layer in photovoltaics. If the layer is comprised of structures smaller than the wavelength of the incident light and the density is graded across more than about half the wavelength of the light, reflection is strongly suppressed (H. M. Branz et al., APL \textbf{94} 2009). We developed an inexpensive liquid etch technique for silicon to produce ``black Si'' based upon this physics. However, the problem of high carrier recombination within this nanostructured layer must be overcome to improve beyond the present best solar cell with its confirmed 16.8{\%} black silicon sunlight-to-electricity conversion efficiency (H-C. Yuan et al., APL \textbf{95} 2009). In this work, we combine the black Si layer with conventional KOH-etched pyramidal surface texture (Y. Xiu et al., Langmuir \textbf{24 }2008) at micron-scale. Pyramids contribute anti-reflection based on geometric optics. Combining the pyramids with nanostructures only 100 nm deep provides reflectivity below 2{\%} across a wavelength range from 350 -- 1000 nm. To-date, we have obtained a solar cell efficiency of 17{\%} with a V$_{oc}$ of 613 mV, J$_{sc}$ of 35 mA/cm$^{2}$ and fill-factor of 78{\%}. These cells have improved blue response compared to the best planar black Si cells. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B36.00007: Scalability of Nanocoax PV Architecture Michael J. Naughton, Zhifeng Ren, Kris Kempa The radial junction nanocoax-based nanowire solar architecture offers the prospect of high conversion efficiency with thin film PV, due to enhanced light trapping and ultrathin absorbers. We critique the potential applicability of this structure for various PV media. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B36.00008: Physical effects of ultrathin photovoltaic junctions T. Kirkpatrick, K. Kempa, M.J. Naughton Hot carrier photovoltaic cells have potential to increase conversion efficiency beyond the Shockley-Queisser limit. In addition to implementing selective energy filters into the device in order to extract the hot carriers at elevated energies beyond the band edges, a possible requirement, of particular importance for non-crystalline material, is that the device also be constructed ultrathin in order to extract the hot carriers as usable energy on time scales of less than one picosecond, after which thermalisation sets in. Ultrathin amorphous silicon p-i-n junctions have been shown to extract hot carriers as usable energy at fixed short circuit current density for p- and n- region thicknesses of 5 nm, and i-layer thickness less than 50 nm [Appl. Phys. Lett. \textbf{95}, 233121 (2009)]. Physical effects on device performance in ultrathin cells, such as optical absorption, scattering, band structure, and transport are discussed. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B36.00009: Amorphous Silicon-Carbon Nanostructure Solar Cells Maria Schriver, Will Regan, Matthias Loster, Alex Zettl Taking advantage of the ability to fabricate large area graphene and carbon nanotube networks (buckypaper), we produce Schottky junction solar cells using undoped hydrogenated amorphous silicon thin films and nanostructured carbon films. These films are useful as solar cell materials due their combination of optical transparency and conductance. In our cells, they behave both as a transparent conductor and as an active charge separating layer. We demonstrate a reliable photovoltaic effect in these devices with a high open circuit voltage of 390mV in buckypaper devices. We investigate the unique interface properties which result in an unusual J-V curve shape and optimize fabrication processes for improved solar conversion efficiency. These devices hold promise as a scalable solar cell made from earth abundant materials and without toxic and expensive doping processes. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B36.00010: Prediction of enhanced photovoltaic performance of amorphous silicon solar cells with filled nanopores Jeffrey Grossman, Joo-Hyoung Lee We propose a novel hybrid structure for improving the efficiency of thin-film amorphous silicon solar cells. Using {\it ab initio} calculations, we demonstrate that nanoporous, amorphous silicon (pa-Si), when filled with polythiophene (PT) inside the pores, forms a staggered gap (type II) heterojunction at the interfaces, where both the highest occupied and the lowest unoccupied molecular orbitals of PT are positioned in energy higher than those of pa-Si. Furthermore, we find that while the absorption coefficient ($\alpha$) of pa-Si is significantly reduced from that of bulk amorphous Si (a-Si), inclusion of PT recovers $\alpha$ to the values of a-Si and even higher at thicknesses of $\sim 1$$\mu$m. These results suggest that such a hybrid material, which from a manufacturing standpoint may be substantially easier to scale up than nanowire-based approaches, could greatly enhance the hole mobility in the active layer, which is one of the main reasons for poor efficiency in a-Si solar cells. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B36.00011: Optimizing materials for photon-enhanced thermionic emission Jared Schwede, Daniel Riley, Igor Bargatin, Samuel Rosenthal, Roger Howe, Nicholas Melosh, Zhi-Xun Shen We recently described a novel process for solar energy harvesting called photon-enhanced thermionic emission (PETE) based on a semiconductor cathode and a low-workfunction anode separated by a vacuum gap. Previous work explored the limiting theoretical efficiency of a PETE device, which was shown to exceed the Shockley-Queisser limit on single-junction photovoltaic cells, and described experiments that showed strong evidence the PETE effect. In this presentation, I will describe challenges for making the PETE process efficient, some of which were encountered in these proof-of-concept measurements. I will also describe experimental paths to overcoming these challenges and improving efficiency. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B36.00012: Optimization and Characterization of Nanostructured Surfaces for Photon-Enhanced Thermionic Emission and Photoemission cathodes Daniel Riley, Vijay Narasimhan, Joel Jean, Igor Bargatin, Jared Schwede, Zhi-Xun Shen, Roger Howe, Nick Melosh In the cathode of an energy converter based on photon-enhanced thermionic emission (PETE) photoexcited carriers may need to encounter the emissive surface numerous times before having sufficient thermal energy to escape into vacuum and therefore should be confined close to the surface. However, in a traditional planar geometry, a thin cathode results in incomplete light absorption. ~Nanostructuring has the potential to increase light capture and boost emission by decoupling the lengths associated with photon absorption and electron emission. Nanostructures may complicate the properties of the emissive surface; therefore, the effect of nanostructuring on emission efficiency needs to be studied. {\_}We have recently reported preliminary theoretical results from a suite of simulation tools to capture the full photoemission process: photon absorption, carrier transport within the active material, and electron ballistics following emission. In this work we use the simulation suite to optimize nanostructures for applications including PETE-based solar energy converters, photodetectors and electron sources. The samples are then characterized, and the emission efficiency measured in an ultra-high vacuum test chamber under application-centric conditions. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B36.00013: Design and characterization of transparent thin film nanostructure device Uday Trivedi, Utpal Joshi Indium tin oxide (ITO) is one of the most widely used transparent conducting oxides (TCO) because of its electrical conductivity and optical transparency.We have grown ``all oxide'' transparent $p-n$ junction thin film nanostructure device by using chemical solution deposition and e-beam evaporation onto SiO$_{2}$ substrate. The oxide $p-n$ junction was characterized by GIXRD, AFM, UV-Vis. spectroscopy and I-V measurements. Combined GIXRD and AFM confirm phase pure, mono-disperse 30 nm NiO and ITO nanocrystallites. More than 70{\%} optical transparency is achieved across 160 nm thick $p-n$ junction. The forward bias current is greater than the reverse bias current by approximately a factor of 10$^{4}$ in the measured voltage s weeping range. A small leakage current as low as 12 nA was observed at a reverse bias of --5 V. Previously, Tonooka and co-authors [3] reported the average turn on voltage of their n-ZnO / p-Cu-Al-O diode $\sim $ 0.5 V, which is higher than our $p$-NiO/$n$-ITO diode. This is mainly because of the large variations in the carrier concentrations as well as larger lattice mismatch between the oxides forming the $p-n$ junction. The observed optical and electrical properties of oxide transparent diode are attributed to the heteroepitaxial nature and carrier diffusion at the junction interface. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B36.00014: Interface properties of chalcopyrite heterocontacts Christian Pettenkofer, Andreas Hofmann, Eike Janocha, Carsten Lehmann Interface properties of heterocontacts determine the device performance of thin film solar cells. We investigated well defined chacopyrite interfaces and heterocontacts of MBE grown samples by electron spectroscopy to obtain informations on the morphology and electronic properties of the contact phases. In particular CuInSe2 and CuInS2 (001) and (112) surfaces were grown by MBE and studied with respect to contact formation to ZnO, ZnS and ZnSe. Due to Cu back diffusion into the bulk even for stoichiometric samples Cu poor interfaces were observed giving rise to interdiffused Zn3In2X6 (X=S,Se) layers in the contact plane. Band alignments obtained fort the prepared heterocontacts will be compared to models given by M\"{o}nch and Wei et al. The influence of contact preparation on the properties of the interface will be discussed in detail. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B36.00015: Effects of Back Contact Materials on Substrate Configuration CdTe Solar Cells Nathan G.F. Reaver, Kristopher Wieland, Alvin D. Compaan Substrate configuration CdTe photovoltaics has the potential to provide both a reduction in the production costs and improved power to mass ratio. In this study the effect of copper placement in the cells, sequence of CdCl$_{2}$ treatment, and the effect of back contact material on cell performance was examined. Cells were deposited on a Mo coated conductive substrate, on stainless steel or on TCO coated glass, using RF magnetron sputtering. Three different back contacts were used, copper-gold as used in superstrate configuration cells, Sb$_{2}$Te$_{3}$, and ZnTe:N. Cells were measured using a solar simulator at one sun to obtain current density vs. voltage curves and cell efficiencies. The structure that gave the best performance was stainless steel/Mo/Sb$_{2}$Te$_{3}$/CdTe/CdS/ZnO/ZnO:Al, with the best cell having an efficiency of 5.34{\%}. [Preview Abstract] |
Session B37: Focus Session: Graphene Growth, Characterization, and Devices: Devices and Contacts
Sponsoring Units: DMPChair: Thomas Seyller, University of Erlanger-Nuernberg
Room: C146
Monday, March 21, 2011 11:15AM - 11:27AM |
B37.00001: BN / Graphene / BN RF Transistors Han Wang, Thiti Taychatanapat, Allen Hsu, Pablo Jarillo-Herrero, Tomas Palacios In this work we demonstrate the first BN/graphene/BN transistor for high frequency RF applications. This sandwich structure allows a significant improvement in the mobility of graphene, which reaches more than 18,000 cm$^{2}$/Vs at room temperature. Graphene field effect transistors (GFETs) have been fabricated with L$_{DS}$= 800 nm and L$_{G}$=300 nm. The minimum conduction point of these devices is very close to zero, a result of the negligible substrate doping to the graphene. A current density in excess of 1 A/mm and DC transconductance above 200 mS/mm are achieved for both electron and hole conductions. RF characterization is performed for the first time on this device structure and initial results show a current-gain cut-off frequency $f_{T}$=10 GHz. These experimental results have been combined with simulations of the small-signal model to study the scaling potential of these GFETs for high frequency applications. The impact of the access resistances (R$_{s}$, R$_{d})$, the capacitances (C$_{gs}$, C$_{gd}$, C$_{ds})$, and the transconductance (g$_{m}$) on the frequency performance of the GFETs has also been studied. Finally, the fabricated devices have been compared to GFETs fabricated with SiO$_{2}$ substrate and Al$_{2}$O$_{3}$ gate dielectrics. The improved performance obtained by the BN/graphene/BN structure is very promising to enable the next generation of high frequency RF electronics. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B37.00002: Bilayer graphene $p-i-n$ tunnel junction controlled by modulated top gate Hisao Miyazaki, Song-Lin Li, Kazuhito Tsukagoshi, Akinobu Kanda Ambipolar nature of graphene enables us to set charge polarity for electric transport to be $p$-type or $n$-type. We fabricated a bilayer graphene (BLG) with spatially modulated $p$-type and $n$-type regions. The spatial modulation was introduced by a pair of gate electrodes; a uniform back (substrate) gate and a top gate with stepwise geometry. The gate electric field between the top and back gate also induces band gap in the BLG. As a result, an insulating region is inserted between the $p$- and $n$-regions, realizing a $p-i-n$ junction. The current through the junction showed nonlinearity as a function of the source-drain bias. We identified that the origin of nonlinearity is the tunnel current between the $p$- and $n$-regions. The nonlinearity reflects the density of states singularity at the edge of the conduction and the valence band in BLG with the band gap. This observation appends another evidence for electric-field-induced band gap in BLG. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B37.00003: Rectification at the graphene and multi-layer-graphene / semiconductor interface from room temperature up to 900K Sefaattin Tongay, Todd Schumann, Arthur F. Hebard We report on the formation of Schottky diodes on GaN and SiC using a graphite/graphene electrode as a semimetal contact to the semiconductor. The GaN (SiC) /graphene Schottky barriers display rectifying behavior over a wide temperature range with ideality constant close to unity, implying thermionic emission is the dominant transport across the interface. The diodes display larger breakdown voltages (more than 20V) compared to conventional metal junctions (5V). Advantageously, graphite/graphene is stable up to high temperatures and does not diffuse into the semiconductor. We find that these diodes are stable and rectifying up to 900K and are superior to typical metal Schottky diodes reported for the same semiconductors. High temperature measurements are interesting since graphite semimetal contact starts behaving as Boltzmann gas at temperatures well above Fermi energy (T$>>$280K). Our results imply that graphene based junctions fabricated on conventional semiconductors are good candidates for both high and low temperature devices. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B37.00004: Complementary-like semiconducting graphene logic inverters Song-Lin Li, Hisao Miyazaki, Kazuhito Tsukagoshi, Akinobu Kanda The application of graphene as a post-silicon channel material is an interesting but challenging topic due to its metallic nature and low switching ratio. It is expected that the condition would change if a sizeable band gap is introduced. Here we report the electrical characteristics of the first semiconducting graphene-based logic inverters. Free of doping, the \textit{p}- and \textit{n}- branches in the bipolar graphene transistors are delicately used as the complementary components required in logic devices. Within perpendicular electric fields, large transport band gap ($>100$\,meV) and high switching ratio ($\sim200$ at 77\,K) are obtained in bilayer graphene channels. Besides, a simple and high capacitive-efficiency top gate with natural alumina dielectric ($\sim0.9$\,\textrm{$\mu$}F/cm$^2$) is adopted and the operating bias is lowered within 2\,V. For the first time, $>1$ voltage gain are extracted from graphene inverters. Voltage gain up to 8 and 2 are achieved at liquid-nitrogen and room temperatures, respectively. Importantly, a match between input and output voltage levels is realized, indicating the potential for direct cascading between multiple devices for future large-scale integration. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B37.00005: Ferroelectric gating of CVD graphene devices Guangxin Ni, Yi Zheng, Kui Yao, Barbaros \"Ozyilmaz The recent availability of large area graphene has opened up new possibility in graphene research. We will first discuss experiments, where graphene on the ferroelectric substrate~PZT allows the fabrication graphene field effect transistors (GFETs) and graphene memory within $\pm $1 V operating voltage with maximum doping exceeding 10$^{13}$ cm$^{-2}$. Ferroelectric substrates may also be of importance for large scale applications. Graphene's exceptional optical and mechanical properties make it suitable also for transparent conductors (TCs). While chemical doping has been proven to be an efficient approach to achieving ultra-low sheet resistance, some challenges remain. Here we propose an alternative way to obtain low sheet resistance of graphene using ferroelectric gating. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B37.00006: Electrical noise in graphene FETs Nan Sun, Kristof Tahy, Gerald Arnold, Debdeep Jena, Huili Xing, Steven Ruggiero We report on the low-frequency electrical noise measured in graphene FETs. Samples were created by e-beam lithography using both exfoliated graphene and epitaxial graphene films on SiC. The observed 1/f noise varies as a function of gate bias, where the noise amplitude follows Hooge's empirical relation ($S_V \sim 1/N$), and the noise spectrum deviates from 1/f behavior at low carrier densities. We discuss this behavior in the context of a model including random telegraph noise generated by slow traps. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B37.00007: Development of epitaxial graphene based electronics Invited Speaker: Epitaxial graphene (EG) has demonstrated a great potential for novel electronic devices [1]. In micron-sized structures graphene is essentially a gapless semimetal, consequently reasonable on-to-off rations can be achieved, but digital electronics is precluded. There are essentially two methods to introduce a bandgap in graphene. One is to make very small structures [1], and the other is to chemically modify the graphene itself [2]. Electron beam lithography is not commercially viable and the graphene is severely degraded by this method. Graphene's conductivity depends on the doping density. For interconnects, reliable methods need to be developed to highly dope graphene without deteriorating the mobility. Furthermore, metallic interconnects are required for all but the simplest structures and they need to be incorporated without defeating graphene's favorable properties. Finally, in the more distant future, EG device architectures that rely on wave properties of the electrons that go beyond diffusive electronics are envisioned [1]. These will require interconnected, nanoscopic graphene structures. An overview and perspective of these issues will be given. I will present new directions, involving multilayer epitaxial graphene, interconnect schemes, non-conventional patterning methods (templated graphene growth [3] and related methods), as well as methods to chemically modify and dope EG.\\[4pt] [1] Berger et al. ``Ultrathin Epitaxial Graphite: 2D Electron Gas Properties and a Route toward Graphene-based Nanoelectronics'', J. Phys. Chem. B 108, 2004,19912 (2004); W.A.de Heer http://smartech.gatech.edu/handle/1853/31270 \\[0pt] [2] E. Bekyarova, et al, JACS 131, 1336 (2009).\\[0pt] [3] M. Sprinkle, et al., ``Epitaxial graphene: Templated graphene growth'' Nature NanoTechnology 5, 727, (2010)] [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B37.00008: Metal selection criteria for enhancing electrical conductance of metal-graphene junctions Marcelo Kuroda, J. Tersoff, Dennis Newns, Glenn Martyna We study from first principles the electrical conductance of a junction formed by graphitic films in between metal electrodes. We find that for some metals the junction conductance decays exponentially with the number of graphene layers (thickness of the film) while for others it saturates. These different behaviors are attributed to the presence/absence of Fermi-level states in the metal electrode that couple to those of the graphitic thin film. We also find that the bonding between the metal and graphene atoms at the interface has a significant contribution which is dominant for sufficiently thin films. The study may be proven useful for the design and optimization of epitaxially grown electrical contacts. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B37.00009: Influence of Metal Contact on the Operation and Scalability of Graphene Field-Effect-Transistors Pei Zhao, Qin Zhang, Debdeep Jena, Siyuranga O. Koswatta We explore the effects of metal contacts on the operation and scalability of 2D Graphene Field-Effect-Transistors (GFETs) using detailed numerical device simulations based on the non-equilibrium Green's function formalism at the ballistic limit. Our treatment of metal/graphene (M/G) contacts captures: (1) the doping effect due to the shift of the Fermi level in graphene contacts, (2) the density-of-states (DOS) broadening effect inside the graphene contacts, and (3) the Metal-Induced-States inside the graphene channel. Our results confirm the asymmetric transfer characteristics in GFETs due to the doping effect by metal contacts. Furthermore, the DOS broadening effect will increases the on-current at higher M/G coupling strengths. Finally, with scaling of the channel length, influence on the minimum current in the off-state is also discussed. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B37.00010: Flexible and Transparent Field Emission Devices based on Graphene-Nanowire Hybrid Structures Muhammad Arif, Kwang Heo, Byung Yang Lee, David H. Seo, Sunae Seo, Jikang Jian, Seunghun Hong Recent developments in wafer scale synthesis and transfer of graphene have made it possible to fabricate electrodes for versatile flexible devices. However, a flexible and transparent graphene-based field emission device has not been explored yet. Herein, we report the fabrication of flexible and transparent field emission devices based on graphene-nanowire hybrid structures. In this work, we successfully grew vertically-aligned Au nanowires on graphene surface using an electrochemical method and utilized it as a cathode. We also utilized a graphene electrode for an anode resulting in a transparent and flexible field emission device. Our field emission devices can be bent down to 22 mm radius of curvature without any significant change in its field emission currents. This flexible and transparent field emission device based on graphene-nanowire hybrid structures will utilized for various applications such as field emission displays, x-ray tubes, and pressure sensors. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B37.00011: CVD grown graphene field-effect device arrays with water top gate Bei Wang, B. Koger, J. Zhu We synthesize single-layer graphene sheets by chemical vapor deposition (CVD) on copper foil. Large sheets are transferred to Si/SiO$_{2}$ wafers using poly(methyl methacrylate) (PMMA). Raman spectroscopy of transferred graphene shows the signatures of high-quality graphene with a very small D band. Graphene field-effect device arrays are fabricated using conventional photolithography. A thin SiO$_{2}$ film is deposited on top of the finished devices as the last step. We employ two methods of field effect gating. Gate sweeps of the SiO$_{2}$ back gate show large initial hole doping. When a droplet of water is deposited on the device and used as a top gate, the majority of devices show a Dirac point of $\sim$0.3 V and bipolar behavior. The water top gate injects charges much more efficiently than the 290nm SiO$_{2}$ back gate. The mobility of the devices is estimated to be a few thousand cm$^{2}$/Vs. We discuss transport properties and potential applications of these device arrays. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B37.00012: Graphene p-n Junctions via Molecular Functionalization Ren-Jye Shiue, Hung-Chieh Cheng, Chia-Chang Tsai, Yit-Tsong Chen, Wei-Hua Wang An essential challenge in graphene-based electronics is to engineer the carrier type and density and still preserve the transport properties of graphene. We report an experimental investigation of graphene $p-n$ junctions via molecular functionalization. By developing a generic scheme for the chemical functionalization, we have shown that an effective and uniform chemical doping of graphene can be achieved by non-covalent modification of the molecules. The effectiveness and uniformity of the modification is systematically confirmed by optical microscopy, surface potential measurement, and Raman spectroscopic imaging. Furthermore, the chemical doping by molecules is utilized to fabricate the graphene $p-n$ junctions. The transport characteristics of the graphene $p-n$ junctions are investigated by transport and magnetotransport measurements. The signatures of the graphene $p-n$ junctions are presented with high carrier mobility, energy splitting of Dirac points, and non-conventional quantum Hall effect. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B37.00013: Device fabrication progress on epitaxial graphene on SiC Yike Hu, Zelei Guo, Rui Dong, Claire Berger, Walt deHeer Epitaxial graphene on SiC has been demonstrated to be a viable route toward electronic device fabrication. While a top gate is required to locally change doping density and carrier type, specifically for field effect transistors graphene devices, back gating is relevant to globally change carrier and to address the graphene layer at the SiC-graphene interface. Here we report result on back-gating and top-gating epitaxial graphene grown on SiC by the confinement controlled sublimation method. Post-patterning treatments of graphene devices are also discussed. [Preview Abstract] |
Session B38: Earle K. Plyler Prize Session I: Spectroscopy
Sponsoring Units: DCPChair: Vladimir Chernyak, Wayne State University
Room: A130/131
Monday, March 21, 2011 11:15AM - 11:51AM |
B38.00001: Earle K. Plyler Prize for Molecular Spectroscopy Talk: Coherent Ultrafast Multidimensional Spectroscopy of Molecules; From NMR to X-rays Invited Speaker: Multidimensional spectroscopic techniques which originated with NMR in the 1970s have been extended over the past 15 years to the optical regime. NMR spectroscopists have developed methods for the design of pulse sequences that resolve otherwise congested spectra, enhance selected spectral features and reveal desired dynamical events. The major experimental and computational advances required for extending these ideas to study electronic and vibrational motions on the femtosecond timescale will be surveyed. The response of complex molecules and semiconductor nanostructures to sequences of optical pulses provides snapshots of their structure and dynamical processes. Two-dimensional correlation plots of the signals show characteristic cross-peak patterns which carry information about hydrogen bonding, secondary structure fluctuations of proteins and amyloid fibrils, and coherent and incoherent energy and charge transfer in photosynthetic complexes. Double quantum coherence signals that are induced by correlations among electrons or excitons allow the visualization of correlated wavefunctions. Future extensions to the attosecond regime using xray pulses will be discussed. Since core excitations are highly localized at selected atoms, such signals can monitor the motions of valence electron wavepackets in real space with atomic spatial resolution. Common principles underlying coherent spectroscopy techniques for spins, valence electrons, and core electronic excitations, spanning frequencies from radiowaves, infrared, ultraviolet all the way to hard X-rays will be discussed. \\[4pt] [1] ``Coherent Multidimensional Optical Probes for Electronic Correlations and Exciton Dynamics; from NMR to X-rays'', S. Mukamel, D. Abramavicius, L. Yang, W.Zhuang, I.V. Schweigert and D. Voronine. Acct.Chem.Res. Acct.Chem.Res. 42, 553-562 (2009). \\[0pt] [2] ``Coherent Multidimensional Optical Spectroscopy Excitons in Molecular Aggregates; Quasiparticle vs. Supermolecule Perspectives'', D. Abramavicius, B. Palmieri, D. Voronine, F. Sanda and S. Mukamel, Chem. Rev. 109, 2350-2408 (2009). [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B38.00002: Photoelectron spectroscopy of solvated electrons in liquid tetrahydrofuran and methanol microjets Alexander Shreve Solvated electrons are an important species in radiation chemistry, biology, and other areas. As the simplest quantum solute, solvated electrons are a critical benchmark to test our understanding of solvation in general. Furthermore, when formed in cells, they are highly reactive and may lead to irreversible damage. It is, therefore, important to understand the energetics associated with electron solvation. To this end, we have undertaken a series of studies directly probe electron vertical binding energies (VBEs) in solvents introduced to vacuum through liquid microjets. Solvated electrons are generated following the excitation of the charge-transfer-to-solvent (CTTS) precursor state of iodide from a millimolar concentration salt included in the solution, detached to vacuum, and measured with our field-free time-of-flight spectrometer. Here we present preliminary results of the measurement of the VBE of electrons solvated in bulk tetrahydrofuran and methanol. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B38.00003: The Solvated Electron in Acetonitrile Stephanie Doan, Arthur Bragg, Benjamin Schwartz The nature of solvated electrons in liquid acetonitrile is of great interest as it appears that excess electrons in this solvent are stabilized in two forms, a dipole-bound (DB) electron (i.e. a typical solvated electron) and a valence-bound electron (VB) electron (e.g. a solvated CH3CN dimer anion). Previous work has suggested that these two species are in equilibrium and can interconvert. We performed 3-pulse transient hole-burning experiments aimed at better understanding the nature of the VB and DB electrons. We found that photoexcitation of VB electrons produces an increased population of DB electrons, but that exciting DB electrons does not produce VB electrons. This suggests a significant asymmetry in the solvent motions that accompany photoexcitation of the electron: it is easier for a DB electron to relax back into the solvent location from which it came than for the local solvation structure to change enough to create a VB electron, whereas excitation of a VB electron disrupts the local solvent structure to the point where the excited electron can relax into the bulk solvent rather than back to the molecules on which it initially resided. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B38.00004: Dynamics of electron solvation in I\={ }(CH$_{3}$OH)$_{n}$ clusters (4 $\le \quad n \quad \le $ 11) Ryan Young The dynamics of electron solvation following excitation of the charge-transfer-to-solvent (CTTS) precursor state in iodide-doped methanol clusters, I\={ }(CH$_{3}$OH)$_{n=4-11}$ are studied with time-resolved photoelectron imaging (TRPEI). This excitation produces a I$^{\ldots }$(CH$_{3}$OH)$_{n}$\={ } cluster that is unstable with respect to electron autodetachment, and whose autodetachment lifetime increases monotonically from $\sim $800 fs to 85 ps as $n$ increases from 4-11. The vertical detachment energy (VDE) and width of the excited state feature in the photoelectron spectrum show complex time dependences during the lifetime of this state. The VDE decreases over the first 100-400 fs, then rises exponentially to a maximum with a $\sim $ 1ps time constant, decreasing by as much as 180 meV with timescales from 4-10 ps. The early dynamics are assigned to electron transfer from the iodide to a localized portion of the methanol cluster, while the longer-time changes in VDE are attributed to solvent reordering, possibly in conjunction with ejection of neutral iodine from the cluster. Changes in the observed width of the spectrum largely follow those of the VDEs; the dynamics of both are attributed to the major rearrangement of the solvent cluster during relaxation. The relaxation dynamics are interpreted as a reorientation of at least one methanol molecule and the disruption and formation of the solvent network in order to accommodate the excess charge. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B38.00005: Vibronic Enhancement of Exciton Sizes and Energy Transport in Photosynthetic Complexes Andrew Moran, Jordan Womick, Brantley West, Stephen Miller This talk investigates the impact of vibronic couplings on the electronic structures and relaxation mechanisms of two cyanobacterial light harvesting proteins, allophycocyanin (APC) and c-phycocyanin (CPC). Both APC and CPC possess three pairs of pigments (i.e., dimers), which undergo electronic relaxation on the sub-picosecond time scale. Electronic relaxation is approximately 10 times faster in APC than in CPC despite the nearly identical structures of their pigment dimers. Femtosecond laser spectroscopies conducted in conjunction with a Frenkel exciton model find that photo-induced electronic relaxation in these two proteins is understood on the same footing only when the vibronic couplings in high-frequency modes are properly taken into account. In addition to incorporating high-frequency intramolecular modes in the spectral density, we simulate electronic relaxation dynamics using a model in which the excitons delocalize in a vibronic basis. General implications of the present findings for energy transport in artificial systems (e.g., crystalline organic semiconductors) are discussed. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 1:15PM |
B38.00006: Nonlinear Coherent Optical Imaging for Biomedicine: The Quest for Ultimate Sensitivity Invited Speaker: Recent advances in nonlinear coherent optical imaging, particularly stimulated Raman scattering microscopy, have allowed highly sensitive label-free imaging of living cells and organisms based on molecular spectroscopy. Using the ultimate sensitivity of nonlinear optical microscopy, the detection of a single-molecule absorption signal at room temperature has been achieved. These unprecedented sensitivities offer exciting possibilities for biomedicine. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B38.00007: Optical Control of Conjugated Oligomer Planarity Sergei Tretiak, Jenny Clark, Guglielmo Lanzani Using a sequential photo-excitation mechanism we observe the ultrafast conformational planarization of a large fluorene oligomer at $\sim $ 60fs timescale. Novel non-adiabatic excited state molecular dynamics (NA-ESMD) framework incorporating quantum transitions has been used to rationalize this phenomenon. Simulation show the ultrafast relaxation of the photoexcited wavepacket toward the lowest electronic excited state along the torsional coordinate. The process effectively `locks' the oligomer into a planar state within 100~fs, with excess energy being dissipated into other vibrational modes. Ultrafast control of molecular conformation, as demonstrated here, could have impacts for molecular conformational switches for memory or molecular electronics. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B38.00008: Using 2D Fourier-transform spectroscopy to separate homogeneous and inhomogeneous line widths of heavy- and light-hole excitons in weakly disordered semiconductor quantum wells Steven Cundiff, Alan Bristow, Tianhao Zhang, Mark Siemens, Richard Mirin Optical two-dimensional Fourier-transform spectroscopy is used to study the heavy- and light-hole excitonic resonances in GaAs quantum wells with weak structural disorder. Homogeneous and inhomogeneous broadening contribute differently to the two-dimensional resonance line shapes, allowing separation of homogeneous and inhomogeneous line widths. The heavy-hole exciton exhibits more inhomogeneous than homogeneous broadening, whereas the light-hole exciton shows the opposite. This situation arises from the interplay between the length scale of the disorder and the exciton Bohr radius, which affects the exciton localization and scattering. Utilizing this separation of line widths, excitation-density-dependent measurements reveal that many-body interactions alter the homogeneous dephasing, while disorder-induced dephasing is unchanged. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B38.00009: Oxygen atom roaming and multiple dissociation pathways of NO$_{3}$ Michael Grubb, Michelle Warter, Kurt Johnson, Simon North The role of nitrate radical (NO$_{3})$ photolysis in atmospheric has long been known, but mysteries remain regarding the mechanism of the dissociation. In particular, the NO + O$_{2}$ channel has proven to be a challenge both theoretically and experimentally. High resolution velocity map ion imaging studies reveal that there are two distinct mechanisms to form the NO + O$_{2}$ products. Additionally, the dominant of these mechanisms appears to be the non-traditional state ``roaming'' mechanism recently identified in formaldehyde dissociation. The roaming mechanism involves large amplitude motion associated with a frustrated radical dissociation before roaming oxygen atom abstraction to form O$_{2}$. The identification of roaming in the NO$_{3}$ reaction may imply the widespread importance of this type of mechanism in atmospheric chemistry. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B38.00010: First Principle Simulations of the Infrared Spectrum of Liquid Water using Hybrid Density Functionals Cui Zhang, Davide Donadio, Francois Gygi, Giulia Galli We report on calculations of the infrared spectrum (IR) of liquid water carried out using first principle molecular dynamics and the hybrid functional PBE0. We find results in much better agreement with experiment than those obtained using semi-local, gradient corrected exchange correlation functionals. In particular the description of the IR stretching band is greatly improved and in good accord with recent measurements. When adopting the PBE0 functional, substantial improvement is also found in the description of the structural properties of the liquid, consistent with a smaller average number of hydrogen bonds, and a reduced molecular dipole moment, as revealed by our analysis of maximally localized Wannier functions. Finally the average electronic gap of the liquid is increased by 60\% with respect to PBE, when computed at the PBE0 level of theory, and is in fair agreement with experiment. Work supported by NSF/OCI-0749217. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B38.00011: Nonequilibrium Mixed Quantum-Classical simulations of Hydrogen-bond Structure and Dynamics in Methanol-d Carbon tetrachloride liquid mixtures and its spectroscopic signature Kijeong Kwac, Eitan Geva Liquid mixtures of methanol-d and carbon tetrachloride provide attractive model systems for investigating hydrogen-bond structure and dynamics. The hydrogen-bonded methanol oligomers in these mixtures give rise to a very broad hydroxyl stretch IR band ($\sim$150 cm$^{-1}$). We have employed mixed quantum-classical molecular dynamics simulations to study the nature of hydrogen- bond structure and dynamics in this system and its spectroscopic signature. In our simulations, the hydroxyl stretch mode is treated quantum mechanically. We have found that the absorption spectrum is highly sensitive to the type of force fields used. Obtaining absorption spectra consistent with experiment required the use of corrected polarizabile force fields and a dipole damping scheme. We have established mapping relationships between the electric field along the hydroxyl bond and the hydrogen-stretch frequency and bond length thereby reducing the computational cost dramatically to simulate the complex nonequilibrium dynamics underlying pump-probe spectra. [Preview Abstract] |
Session B39: Focus Session: Single Molecule Biophysics II: Novel Single Molecule Approaches to Biology
Sponsoring Units: DBP DPOLY DCPChair: Keir Neuman, National Institutes of Health
Room: A124/127
Monday, March 21, 2011 11:15AM - 11:51AM |
B39.00001: Single-image molecular analysis for accelerated fluorescence imaging Invited Speaker: We have developed a new single-molecule fluorescence imaging analysis method, SIMA, to improve the temporal resolution of single-molecule localization and tracking studies to millisecond timescales without compromising the nanometer range spatial resolution [1,2]. In this method, the width of the fluorescence intensity profile of a static or mobile molecule, imaged using submillisecond to milliseconds exposure time, is used for localization and dynamics analysis. We apply this method to three single-molecule studies: (1) subdiffraction molecular separation measurements, (2) axial localization precision measurements, and (3) protein diffusion coefficient measurements in free solution. Applications of SIMA in flagella IFT particle analysis, localizations of UgtP (a cell division regulator protein) in live cells, and diffusion coefficient measurement of LacI in vitro and in vivo will be discussed. \\[4pt] [1] Shawn DeCenzo, Michael C. DeSantis, and Y. M. Wang, ``Single-image separation measurements of two unresolved fluorophores,'' Optics Express, 18, 16628-16639, (2010)\\[0pt] [2] M. DeSantis, S. DeCenzo, J. L. Li, and Y.M. Wang, ``Precision analysis for standard deviation measurements of single fluorescent molecule images,'' Optics Express, 18, 6563-6576, (2010) [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B39.00002: Super-resolution imaging of multiple fluorescent proteins with highly overlapping emission spectra in living cells Mudalige Gunewardene, Fedor Subach, Travis Gould, Gregory Penoncello, Manasa Gudheti, Vladislav Verkhusha, Samuel Hess Diffraction limits resolution in far field microscopy. Single molecule localization based superresolution imaging has surpassed such limitations and is rapidly gaining popularity, yet limited availability of cell-compatible photoactivatable fluorescent probes with distinct emission spectra have impeded simultaneous visualization of multiple molecular species in living cells. We introduce PAmKate, a monomeric far-red photoactivatable fluorescent protein (PAFP), which has facilitated simultaneous imaging of three PAFPs in biological samples with fluorescence photoactivation localization microscopy (FPALM). Successful probe identification was achieved by measuring the fluorescence emission intensity in two distinct spectral channels spanning approximately 100 nm of the visible spectrum. Raft-, non-raft- and cytoskeleton- associated proteins were simultaneously imaged in both live and fixed fibroblasts co-expressing Dendra2-hemagglutinin, PAmKate-transferrin receptor and PAmCherry1-$\beta $-actin chimeras, revealing evidence for specific interactions between membrane proteins and membrane-associated actin structures. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B39.00003: Single-image diffusion coefficient measurements of proteins in free solution Shannon Kian Zareh, Michael DeSantis, Jonathan Kessler, Yan Mei Wang Diffusion coefficient measurement of biomolecules is important for particle size determination, reaction rate characterization, and molecular dynamics investigation. Here we present a simple and fast method for determining diffusion coefficient of nanometer- and sub-nanometer-sized fluorophores, such as GFP, in free solution by analyzing their single fluorescence images with sub-millisecond exposure times. In this method, the standard deviation (SD) of a diffusing molecule's intensity profile is used to determine its diffusion coefficient. Our SD vs. diffusion coefficient expression is consistent with our simulation and experimental measurement results, rendering this sub-millisecond-long method to be an improvement of at least 100-fold in temporal resolution over current diffusion coefficient measurement methods, such as single-particle-tracking and FCS. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B39.00004: Quantifying kinetics and dynamics of DNA repair proteins using Raster-scan Image Correlation Spectroscopy Salim Abdisalaam, Milan Poudel, David Chen, George Alexandrakis DNA double strand breaks are potentially dangerous lesions as their incomplete repair may lead to carcinogenesis. In this study the confocal Raster scan Image Correlation Spectroscopy technique is used to study kinetics and dynamics of double stand break repair proteins after $\gamma $-irradiation of mammalian cells. Diffusion and binding constants were obtained by fitting with different physical models. Results were compared to ones obtained by creating high density DNA damage with a laser and subsequently performing Fluorescence Recovery after Photobleaching over the damage area. This work presents similarities and differences in double strand break repair response between $\gamma $-irradiation versus laser damage. This is of importance to answering the question of whether the popular use of laser induced DNA damage is a sufficient surrogate for predicting the radiation treatment response of cancer cells. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B39.00005: Overview of single-molecule methods including high-force, force-fluorescence, and dual-trap studies for probing molecular and cellular machinery Invited Speaker: High force optical trapping, including double trap geometry and simultaneous visualization with single molecule fluorescence imaging enables a wide range of measurement capabilities applicable for probing molecular and cellular machinery. A series of single molecule measurement methods will be presented. Force-fluorescence microscopy enables visualizing amyloid fibers while physically probing their structures including direct unfolding and rupture of fibers with a high force optical trap. Force spectroscopy is employed to probe the strength of single peptide aptamer bonds. A dual-trap geometry allows for direct tracking of unfolding and translocation machinery of the biological motor ClpXP. Force fluorescence microscopy directly visualizes T-cell activation. Automation and flexibility in our instruments coupled with advances in physical assay design strategies are leveraged to access a broad set of molecular and cellular measurement targets. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B39.00006: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 1:15PM - 1:27PM |
B39.00007: 3D single molecule tracking in thick cellular specimens using multifocal plane microscopy Sripad Ram, E. Sally Ward, Raimund J. Ober One of the major challenges in single molecule microscopy concerns 3D tracking of single molecules in cellular specimens. This has been a major impediment to study many fundamental cellular processes, such as protein transport across thick cellular specimens (e.g. a cell-monolayer). Here we show that multifocal plane microscopy (MUM), an imaging modality developed by our group, provides the much needed solution to this longstanding problem. While MUM was previously used for 3D single molecule tracking at shallow depths ($\sim $ 1 micron) in live-cells [1], the question arises if MUM can also live up to the significant challenge of tracking single molecules in thick samples. Here by substantially expanding the capabilities of MUM, we demonstrate 3D tracking of quantum-dot labeled molecules in a $\sim $10 micron thick cell monolayer. In this way we have reconstructed the complete 3D intracellular trafficking itinerary of single molecules at high spatial and temporal precision in a thick cell-sample. \\[4pt] [1] Biophys J., 2008, 95:6025-6043. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B39.00008: Single Molecule Analysis of Serotonin Transporter Regulation Using Quantum Dots Jerry Chang, Ian Tomlinson, Michael Warnement, Alessandro Ustione, Ana Carneiro, David Piston, Randy Blakely, Sandra Rosenthal For the first time, we implement a novel, single molecule approach to define the localization and mobility of the brain's major target of widely prescribed antidepressant medications, the serotonin transporter (SERT). SERT labeled with single quantum dot (Qdot) revealed unsuspected features of transporter mobility with cholesterol-enriched membrane microdomains (often referred to as ``lipid rafts'') and cytoskeleton network linked to transporter activation. We document two pools of surface SERT proteins defined by their lateral mobility, one that exhibits relatively free diffusion in the plasma membrane and a second that displays significantly restricted mobility and localizes to cholesterol-enriched microdomains. Diffusion model prediction and instantaneous velocity analysis indicated that stimuli that act through p38 MAPK-dependent signaling pathways to activate SERT trigger rapid SERT movements within membrane microdomains. Cytoskeleton disruption showed that SERT lateral mobility behaves a membrane raft-constrained, cytoskeleton-associated manner. Our results identify an unsuspected aspect of neurotransmitter transporter regulation that we propose reflects the dissociation of inhibitory, SERT-associated cytoskeletal anchors. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B39.00009: Casein Kinase 2 Reverses Tail-Independent Inhibition of Kinesin-1 Jing Xu, Zhanyong Shu, Preetha Anand, Babu Reddy, Silvia Cermelli, Thomas Whisenant, Stephen King, Lee Bardwell, Lan Huang, Steven Gross Kinesin-1 is a plus-end microtubule-based molecular motor, and defects in kinesin transport are linked to diseases including neurodegeneration. Kinesin can auto-inhibit via a direct head-tail interaction, but is believed to be active otherwise. In contrast, this study uncovers a fast but reversible inhibition distinct from the canonical auto-inhibition pathway. The majority of the initially active kinesin (full-length or tail-less) loses its ability to bind/interact with microtubule, and Casein Kinase 2 (CK2) reverses this inactivation (up to 4-fold) without altering kinesin's single motor properties. Motor phosphorylation is not required for this CK2 -mediated kinesin activation. In cultured mammalian cells, knockdown of CK2 level, but not kinase activity, was sufficient to decrease the force required to stall lipid droplet transport, consistent with a reduction in the number of active motors. We propose that CK2 forms a positive regulating complex with the motor. This study provides the first direct evidence of a protein kinase positively regulating kinesin-transport, and uncovers a pathway whereby inactive cargo-bound kinesin can be activated. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B39.00010: Asymmetric Friction and Directed Movement of Brownian Motors Oleg Andreev, Vladislav Markin It is assumed that a Brownian motor is a system that can rectify thermal fluctuations into directed movement. The intriguing question is how this is achieved: what is the mechanism for transferring random pulses from the environment into directed movement. A number of models have been proposed, which, in general, assume the existence of an ``asymmetric flashing potential'' that makes the motor's diffusion predominately in one direction. In this work, we introduce a model of Brownian motors based on asymmetric friction rather than on asymmetric flashing potential. We show that asymmetric friction can break the symmetry of a molecule's ``random walk'' by changing the step size depending on direction. Our model assumes the presence of a symmetrical Brownian force (Gaussian function, average force is 0), an isotropic viscous force, which is proportional to the velocity value but opposite in direction, and an asymmetric friction force, whose value depends on the direction. We present a mathematical model that explains the directed movement for several Brownian motor types. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B39.00011: GSK-3 regulates transport of kinesin-1 driven cargos \textit{in vivo} Christina Leidel, Carole Weaver, Lukasz Szpankowski, Lawrence S.B. Goldstein, George T. Shubeita The Glycogen Synthase Kinase 3 (GSK-3) has been linked to many aspects of the development of Alzheimer's disease and was proposed to play a role in the transport of the Amyloid Precursor Protein (APP) by kinesin-1 motors. Using \textit{Drosophila }embryos and larvae with altered GSK-3 expression, we characterize motor transport of cargos including APP and lipid droplets using DIC microscopy, high-resolution video tracking, fluorescence, and \textit{in vivo} stall force measurements with optical tweezers. By comparing cargo velocities and run lengths we find that GSK-3 is a required negative regulator of \textit{in vivo }transport. Stall force measurements on lipid droplets reveal that enhanced transport under conditions of reduced GSK-3 is a result of a larger number of active motors hauling the cargo. Our findings have implications on the use of GSK-3 inhibitors in treatment of Alzheimer's disease. [Preview Abstract] |
Session B40: Lipid Bilayers and Biological Membranes: Dynamics and Thermodynamics
Sponsoring Units: DBPChair: Mark Henle, Harvard University
Room: A122/123
Monday, March 21, 2011 11:15AM - 11:27AM |
B40.00001: Correlating Anomalous Diffusion with Membrane Obstacle Structure Using Single Molecule Tracking and AFM Michael Skaug, Marjorie Longo, Roland Faller Anomalous diffusion has been observed abundantly in the plasma membrane, but the underlying mechanisms are still unclear. In general, it has not been possible to directly image the obstacles to diffusion in membranes, so the dynamics of diffusing particles are used to deduce the obstacle characteristics. We present a supported lipid bilayer system in which we characterized the anomalous diffusion of lipid molecules using single molecule tracking, while at the same time imaging the obstacles to diffusion with atomic force microscopy. To explain our experimental results, we performed lattice Monte Carlo simulations of tracer diffusion in the presence of the experimentally determined obstacle configurations. We correlate the observed anomalous diffusion with obstacle area fraction, fractal dimension and correlation length. We further discuss our results in the context of confinement models and the generating stochastic process. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B40.00002: Tracking single Kv2.1 channels in live cells reveals anomalous subdiffusion and ergodicity breaking Aubrey Weigel, Blair Simon, Michael Tamkun, Diego Krapf The dynamic organization of the plasma membrane is responsible for essential cellular processes, such as receptor trafficking and signaling. By studying the dynamics of transmembrane proteins a greater understanding of these processes as a whole can be achieved. It is broadly observed that the diffusion pattern of membrane protein displays anomalous subdiffusion. However, the mechanisms responsible for this behavior are not yet established. We explore the dynamics of the voltage gated potassium channel Kv2.1 by using single-particle tracking. We analyze Kv2.1 channel trajectories in terms of the time and ensemble distributions of square displacements. Our results reveal that all Kv2.1 channels experience anomalous subdiffusion and we observe that the Kv2.1 diffusion pattern is non-ergodic. We further investigated the role of the actin cytoskeleton in these channel dynamics by applying actin depolymerizing drugs. It is seen that with the breakdown of the actin cytoskeleton the Kv2.1 channel trajectories recover ergodicity. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B40.00003: Studies of molecular diffusion in single-supported bilayer lipid membranes at high hydration by quasielastic neutron scattering M. Bai, A. Miskowiec, S.-K. Wang, H. Taub, F.Y. Hansen, T. Jenkins, M. Tyagi, D.A. Neumann, S.O. Diallo, E. Mamontov, K.W. Herwig Bilayer lipid membranes supported on a solid surface are attractive model systems for understanding the structure and dynamics of more complex biological membranes that form the outer boundary of living cells. We have recently obtained quasielastic neutron spectra from single-supported bilayer lipid membranes using the backscattering spectrometer BASIS at the Spallation Neutron Source. Protonated DMPC membranes were deposited onto SiO$_{2}$-coated Si(100) substrates and characterized by AFM. Analysis of their neutron spectra shows evidence of a relatively broad Lorentzian component that we associate with bulk-like water above a freezing temperature of $\sim $267 K. At lower temperatures, the spectra differ qualitatively from that of bulk supercooled water, a behavior that we attribute to water bound to the membrane. We also find evidence of a narrow Lorentzian component that we tentatively identify with a slower motion (time scale $\sim $1 ns) associated with conformational changes of the alkyl tails of the lipid molecules. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B40.00004: Studies of molecular diffusion in single-supported bilayer lipid membranes at low hydration by quasielastic neutron scattering A. Miskowiec, M. Bai, M. Lever, H. Taub, F.Y. Hansen, T. Jenkins, M. Tyagi, D.A. Neumann, S.O. Diallo, E. Mamontov, K.W. Herwig We have extended our investigation of the quasielastic neutron scattering from single-supported bilayer lipid membranes to a sample of lower hydration using the backscattering spectrometer BASIS at the SNS of ORNL. To focus on the diffusive motion of the water, tail-deuterated DMPC membranes were deposited onto SiO$_{2}$-coated Si(100) substrates and characterized by AFM. Compared to a sample of higher hydration, the dryer sample does not have a step-like freezing transition at $\sim $267 K and shows less intensity at higher temperatures of a broad Lorentzian component representing bulk-like water. However, the broad component of the ``wet'' and ``dry'' samples behaves similarly at lower temperatures. The dryer sample also shows evidence of a narrow Lorentzian component that has a different temperature dependence than that attributed to conformational changes of the alkyl tails of the lipid molecules in the wet sample. We tentatively identify this slower diffusive motion (time scale $\sim $1 ns) with water more tightly bound to the membrane. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B40.00005: Diffusion in Single Supported Lipid Bilayers C.L. Armstrong, M. Trapp, M.C. Rheinst\"{a}dter Despite their potential relevance for the development of functionalized surfaces and biosensors, the study of single supported membranes using neutron scattering has been limited by the challenge of obtaining relevant dynamic information from a sample with minimal material. Using state of the art neutron instrumentation we have, for the first time, modeled lipid diffusion in single supported lipid bilayers.\footnote{C.L. Armstrong, M.D. Kaye, M. Zamponi, E. Mamontov, M. Tyagi, T. Jenkins and M.C. Rheinst\"{a}dter, Soft Matter Communication, 2010, Advance Article, DOI: 10.1039/C0SM00637H } While we find that the diffusion coefficient for the single bilayer system is comparable to a multi-lamellar lipid system, the molecular mechanism for lipid motion in the single bilayer is a continuous diffusion process with no sign of the flow-like ballistic motion reported in the stacked membrane system. In the future, these membranes will be used to hold and align proteins, mimicking physiological conditions enabling the study of protein structure, function and interactions in relevant and highly topical membrane/protein systems with minimal sample material. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B40.00006: Membrane stress relaxation by transbilayer cholesterol exchange Mark L. Henle, L. Mahadevan Fusion and fission events in the cell membrane play a crucial role in many biological processes, yet the mechanism for inducing the membrane bending deformations required for such events remains poorly understood. In particular, standard membrane elastic models predict a problematically high energy barrier for the strongly curved ``neck'' region formed during fusion and fission. These models assume that the exchange of lipids between membrane leaflets is negligible. While this is valid for phospholipids, other amphiphilic molecules such as cholesterol undergo rapid flip-flop between leaflets. Such exchange can relax bending stresses in the membrane: By flipping from the compressed to the expanded leaflet, cholesterol can reduce the energy required to bend the membrane. In this talk, we present a coarse-grained energetic model (derived from a simple microscopic description of the membrane) for a two-component lipid bilayer that contains a lipid species that can undergo rapid transbilayer exchange. Using this model, we show that lipid flip-flop dramatically reduces the energetic barriers encountered during membrane fusion and fission events and also plays an important role in determining the deformations induced by external forces such as osmotic pressure. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B40.00007: Ethanol enhances collective dynamics of lipid membranes Martin Kaye, Maikel Rheinstadter Lipid bilayers have long been considered simple homogeneous passive barriers. However, there is a growing consensus that bilayer composition and properties impact their role in membrane function. One molecule which participates in lipid bilayers is ethanol. Ethanol is principally known to increase membrane permeability, serving as a model drug enhancer. While bilayer permeability was thought to depend solely on structural properties such as the area per lipid, this may be supported by thermal fluctuations in the bilayer core. Thermal motion results in the formation of small voids in the hydrocarbon chains, which may play a role in the transport small molecules through the membrane core. In both inelastic neutron scattering experiments and molecular dynamics simulations we find evidence for a new low-energy dynamic mode in the fluid phase of DMPC bilayers immersed in a 5{\%} water/ethanol solution [1]. The molecular motion associated with this phonon corresponds to coherent displacements of the carbon atoms in the lipid tails both in, and partially normal to, the plane of the membrane. This finding supports the possibility of a fluctuation supported trans-membrane transport process in lipid bilayers. \\[4pt] [1] ``Ethanol enhances collective dynamics of lipid membranes'', M. D. Kaye, M. Tarek, K. Schmalzl, M. C. Rheinst\"{a}dter, submitted to Physical Review Letters [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B40.00008: Vesicle Shape Transformations Driven by Active and Spontaneous Lipid Flip-flop Thomas Powers, Elnaz Baum-Snow The lipid composition of cell membranes is created and maintained in part by flippases, enzymes that translocate lipid molecules from one layer of the bilayer membrane to the other. We study how lipid translocation can affect membrane shape, using a cylindrical vesicle as a simple model system. For a short pulse of flippase activity, in which a fraction of lipids are flipped from one layer to the other, we calculate the fraction of flipped lipids that makes the cylinder unstable to a periodic modulation in its radius, as well as the growth rate of perturbations of different wavenumber. We also study the cases of continuous flippase activity and spontaneous flip-flop. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B40.00009: \textit{In vitro} approach to the mechanics of lipid membrane area regulation: vesicle absorption and tube formation Margarita Staykova, Douglas Holmes, Clarke Read, Howard A. Stone We have designed an experimental approach that allows us to study the response of supported lipid bilayers to cycles of biaxial expansion and compression. We observed that the bilayer effectively adjusts its area during dilatational or compressive strains in order to reduce its tension. For example, if there is a sufficient lipid reservoir in the form of attached vesicles, then a lipid bilayer may accommodate strains tens of times larger than the critical strain for rupture by expanding its area. Additionally, upon compression the bilayer reduces its area by expelling lipid tubes out of its plane. These observations offer new insights into how cells regulate their surface area in response to various mechanical stimuli, i.e. during physiological volume changes, locomotion, cyclic expansion and compression of the uro- and the alveolar- epithelium, etc. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B40.00010: Structural Phase Diagram for Multi-lamellar Tubular Deformations of Lipid Mesophases Lobat Tayebi, Atul Parikh Stable multi-lamellar cylindrical tubules protrude readily from concentrated mass of amphiphilic molecules in response to a variety of external stresses. Using energetic considerations, we have developed an phase diagram, predicting various types of morphologies of equilibrium multilamellar tubular deformations that stabilize for a broad range of their bending rigidity and surface tension values. Tubular morphologies are described in terms of core radius(rc) and number of lamellae(N). Results of the calculations reveal that emergent tubular morphologies can be classified into three major classes: (1) thin tethers (small rc and low N); (2) solid tubes (high N); and (3) hollow tubes (large rc and and low N). Experimental validation of these predictions is obtained in experiments involving hydration of dry stack lipids Here, tubular deformations, referred to as myelin figures, of all predicted morphologies form in separate populations. Furthermore, the phase diagram also sheds light on a long-standing question of the determinants of the thickness of such myelin figures. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B40.00011: Biomembranes that respond to specific triggers by phase separating Matthew Leroux, Matthew Frantes, Vernita Gordon Lipid membranes are widely used as models for the cell membrane and for applications such as encapsulation, delivery, and controlled release. We have recently found that when membranes adhere nonspecifically, the adhesion site favors the nucleation and growth of more-ordered lipid phases. The physics behind this, which works by suppressing membrane fluctuations, should be applicable to specifically-adhering membranes as well. This will allow better experimental models for cell adhesion, which is mediated by transmembrane proteins and associated with lipid heterogeneities, and also indicates a new category of pathways for making `smart,' responsive materials out of lipid membranes. We are transforming our previous, non-specifically adhering systems into membranes that specifically adhere to a surface via binder molecules. We will determine the thresholds for forming ordered phases as a function of binder stiffness, length, and density, compatibility of the binder structure with the molecular packing of lipids in these phases, and membrane properties such as bending modulus and proximity to a phase transition. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B40.00012: Lipid domains in supported SM-Chol membranes measured by GISANS Mikhail Zhernenkov, Manish Dubey, Boris Toperverg, Jaroslaw Majewski, Michael Fitzsimmons Cell membranes are known to contain regions (called lipid domains, or rafts) described as sphingolipid-cholesterol assemblies which also may contain a subset of membrane proteins. Currently, the main point of discussion is the methodology to study lipid domains and their sizes. We report on Grazing Incidence Small Angle Neutron Scattering (GISANS) measurements of lipid domains in supported sphingomyelin(SM)-cholesterol(Chol) bilayers in a fully aqueous environment. The model bilayers SM:Chol(2:1), SM:Chol(1:2), and a pure SM were deposited using Langmuir-Blodgett/Langmuir-Schaefer technique at a surface pressure of 10 mN/m and measured at 25 \r{ }C. First measurements revealed short range inhomogeneities of the order of 100 {\AA} in both binary systems. The control measurement of a pure SM bilayer exhibited nearly no GISANS indicating an absence of lipid domains in the SM bilayer. This observation is consistent with the notion that a single component system studied below the liquid-gel transition temperature will not produce lipid domains. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B40.00013: Structure of the Stern layer in Phospholipid Systems Sweta Vangaveti, Alex Travesset The structure of the Stern layer in Phospholipid Systems results from a subtle competition of salt concentration, ionic valence, specific ionic-phospolipid interactions and pH. It becomes very challenging to develop a rigorous theory that encompasses all these effects, yet its understanding is extremely relevant for both model and biological systems, as the structure of the Stern layer determines the interactions of phospholipids with proteins or electrostatic phase separation (rafts). In this talk we will present our theoretical model for the Stern Layer and discuss how all these effects are included. Particularly emphasis is made to Phosphoinositides and Phosphatidic acid. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B40.00014: Modeling Signal Transduction and Lipid Rafts in Immune Cells Ashok Prasad Experimental evidence increasingly suggests that lipid rafts are nanometer sized cholesterol dependent dynamic assemblies enriched in sphingolipids and associated proteins. Lipid rafts are dynamic structures that break-up and reform on a relatively short time-scale, and are believed to facilitate the interactions of raft-associated proteins. The role of these rafts in signaling has been controversial, partly due to controversies regarding the existence and nature of the rafts themselves. Experimental evidence has indicated that in several cell types, especially T cells, rafts do influence signal transduction and T cell activation. Given the emerging consensus on the biophysical character of lipid rafts, the question can be asked as to what roles they possibly play in signal transduction. Here we carry out simulations of minimal models of the signal transduction network that regulates Src-family kinase dynamics in T cells and other cell types. By separately treating raft-based biochemical interactions, we find that rafts can indeed putatively play an important role in signal transduction, and in particular may affect the sensitivity of signal transduction. This illuminates possible functional consequences of membrane heterogeneities on signal transduction and points towards mechanisms for spatial control of signaling by cells. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B40.00015: Calcium-mediated rigidity in PIP2 lipid domains Wouter G. Ellenbroek, Andrea J. Liu In lipid mixtures containing the highly negatively charged lipid PIP2 (a crucial component in cell membrane mechanics) multivalent ions such as calcium can drive the formation of PIP2-rich domains by mediating attractions between the lipids. Although the existence of ion-mediated attractions is well known in macromolecular systems, their form is poorly understood because they result from strong correlations between the charged molecules and ions. Within a numerical model of a lipid monolayer, we analyze the mechanics of PIP2-rich domains. We show that they are liquid-like at moderate values of the PIP2-charge but rigid at higher PIP2-charge. We use a recently introduced method to extract the effective pair interaction between the charged lipids in the many-body system, in which the calcium ions and remaining lipids are integrated out. [Preview Abstract] |
Session B41: Focus Session: Supramolecular Self-Assembly--Controlling Network and Gel Formation I
Sponsoring Units: DPOLY DBPChair: Aline Miller, University of Manchester
Room: A115/117
Monday, March 21, 2011 11:15AM - 11:27AM |
B41.00001: Bio-mimetic metal-ligand crosslinks yield self-healing polymer networks with near-covalent elastic moduli Niels Holten-Andersen, Matthew Harrington, Henrik Birkedal, Bruce Lee, Phillip Messersmith, Herbert Waite, Ka Yee Lee Growing evidence supports a load-bearing role for metal-polymer interactions in biological protein networks. In particular, the strength of the coordinate bonds in metal-ligand coordination complexes combined with their capacity to reform after breaking has been proposed as a source of the high toughness and potential self-healing in certain natural materials. Some of the highest stabilities among metal-ligand coordination complexes are found between Fe3+ and catechol ligands at alkaline pH where the tris-catecholato-Fe3+ stoichiometry prevails, yet the effect of such crosslinks on material properties has not been fully characterized due to the low solubility of Fe3+ at high pH. Inspired by the pH jump experienced by marine biomaterials during secretion, we have developed a simple method to control catechol-Fe3+ inter-polymer crosslinking via pH. The resulting gels display elastic moduli (G') that approach covalently crosslinked gels as well as self-healing properties. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B41.00002: Controlling Mechanical Properties of Bis-leucine Oxalyl Amide Gels William Chang, Daniel Carvajal, Kenneth Shull is-leucine oxalyl amide is a low molecular weight gelator capable of gelling polar and organic solvents. A fundamental understanding of self-assembled systems can lead to new methods in drug delivery and the design of new soft material systems. An important feature of self-assembled systems are the intermolecular forces between solvent and gelator molecule; by changing the environment the gel is in, the mechanical properties also change. In this project two variables were considered: the degree of neutralization present for the gelator molecule from neutral to completely ionized, and the concentration of the gelator molecule, from 1 weight percent to 8 weight percent in 1-butanol. Mechanical properties were studied using displacement controlled indentation techniques and temperature sweep rheometry. It has been found that properties such as the storage modulus, gelation temperature and maximum stress allowed increase with bis-leucine oxalyl amide concentration. The results from this study establish a 3-d contour map between the gelator concentration, the gelator degree of ionization and mechanical properties such as storage modulus and maximum stress allowed. The intermolecular forces between the bis-leucine low molecular weight gelator and 1-butanol govern the mechanical properties of the gel system, and understanding these interactions will be key to rationally designed self-assembled systems. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B41.00003: 4D Structural Dynamics of Sheared Collagen Networks Richard Arevalo, Daniel Blair, Jeffrey Urbach Soft biopolymer networks undergo substantial bulk stiffening when subject to shear strain. This nonlinear rheological signature has been observed for a wide range of semiflexible and stiff biopolymers, but the underlying geometric fiber rearrangements have not been measured and the resulting stress propagation through the network has not been experimentally assessed. We apply steady shear strains to collagen gels adhered to a thin elastic polyacrylamide gel substrate embedded with fluorescent displacement markers, while simultaneously imaging the three-dimensional network with a coupled confocal-rheometer. We observe dramatic network realignment towards the shear gradient driven by the nonaffine stretching, buckling, and rotation of constituent fibers and simultaneously measure stress inhomogeneities at the collagen-polyacrylamide interface. These observations elucidate the physical mechanisms governing strain-stiffening and our recent observation of the system-size dependence of this effect. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B41.00004: Molecular Dynamics Modeling of Actin Network Formation Ronald Pandolfi, Peter Becich, Lam Nguyen, Linda Hirst Actin filaments are ubiquitous and critical in cellular functions. The polymer protein F-actin is a semi-flexible filament that forms networks in the presence of binding proteins (i.e. $\alpha$-actinin, filamin, fascin). Molecular dynamics modeling and simulation of the formation of these networks has revealed the dependence of network structure on the ratio of G-actin monomers to cross-linkers, cross-linker shape, and filament length. In this study we focus on the effects of filament length on the assembled system. Comparative experimental work informs the accuracy of the modeled systems. Fourier analysis of the simulated networks allows quantitative characterization of the network structure. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B41.00005: Polyelectrolyte-Surfactant Complexes: A New Class of Organogelators Kevin Cavicchi, Yuqing Liu, Gustavo Guzman Polyelectrolyte-surfactant complexes (PE-SURFs) are a class of polymers generated by neutralizing a polyelectrolyte with an oppositely charged surfactant. It has been found that PE-SURFs composed of polystyrene sulfonate and long chain alkyl dimethyl amines act as good organogelators for a range of hydrophobic, organic solvents. Thermo-reversible organogels are formed by heating and cooling PE-SURF/solvent solutions. The gel transition temperature is influenced by the degree of polymerization, the length of the alkyl side-chain, the solubility parameter of the solvent, and the concentration of the gelator. Freeze-drying and scanning electron microscopy characterization of the resultant xerogels shows the formation of rod- and plate-like network morphologies depending on the system parameters. This behavior is consistent with gelation driven by the self-assembly of the amphiphilic PE-SURFs into micellar networks. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B41.00006: Self-Assembly of DNA--Block Copolymer Micelles Wei Qu, Xuan Jiang, Hai-Quan Mao, Erik Luijten Cationic--hydrophilic block copolymers have been developed as a potential carrier for use in gene delivery, displaying good transfection efficiency and biocompatibility. The cationic blocks effectively condense the DNA into a core surrounded by a protective and stabilizing corona formed by the hydrophilic blocks [Jiang \emph{et al.}, J. Control.\ Release \textbf{122} 297--304 (2007)]. Although the DNA condensation induced by the cationic blocks can be understood from energetic considerations, the formation of micelles with distinct morphologies is more complicated, as it involves several competing interactions. We employ computer simulations to model this interplay of driving forces. By correlating our simulation results with experimental observations, we provide an understanding of the self-assembly process and determine the key structural and experimental parameters that influencing the morphology of the DNA--block copolymer micelles. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B41.00007: LL37-DNA complexes and auto-immune diseases Fan Jin, Lori K. Sanders, Wujing Xian, Michel Gilliet, Gerard C. L. Wong LL37 is an alpha-helical host defense peptide in humans. Recent work has shown that Toll-like receptor-9 (TLR9), an intracellular receptor in plasmacytoid dendritic cells (pDCs) of the immune system that normally responds to pathogen nucleic acids, can be pathologically triggered by self DNA in the form of DNA-LL37 complexes. Synchrotron small-angle x-ray scattering (SAXS) measurements reveal an unanticipated form of self-assembly between DNA and this positively charged macroion. We examine the generality of this with other macroions, and propose a new geometric criterion for immune cell activation. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B41.00008: Stiffness of DNA nanotubes: insights for the design of dsDNA materials Paul Weitekamp, Daniel Schiffels, Alex Iteen, Deborah Fygenson DNA is increasingly used as a material in the design and construction of elaborate structures with nanoscale precision and functionalities. Whether self-assembled from tiles of short, synthetic oligomers or woven from purified genomic strands, most DNA nanostructures are based on parallel arrays of double-stranded DNA (dsDNA) held together by Holliday junction-like cross-links. There is considerable evidence that the double-helices thus intertwined are largely B-form in structure, but the mechanical integrity of the resulting nanostructures has gone largely unexplored. Here we present a systematic study of the stiffness of DNA nanotubes varying parameters such as helix number, cross-link density and strand complexity. We find stiffness is a useful reporter of structural quality for nanotubes and extract design principles for optimizing mechanical integrity of dsDNA materials. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:27PM |
B41.00009: Peptide assemblies: from cell scaffolds to immune adjuvants Invited Speaker: This talk will discuss two interrelated aspects of peptide self-assemblies in biological applications: their use as matrices for regenerative medicine, and their use as chemically defined adjuvants for directing immune responses against engineered antigens. In the first half of the presentation, the design of peptide self-assemblies as analogues for the extracellular matrix will be described, with a focus on self-assemblies displaying multiple different cell-binding peptides. We conducted multi-factorial investigations of peptide co-assemblies containing several different ligand-bearing peptides using statistical ``design of experiments'' (DoE). Using the DoE techniques of factorial experimentation and response surface modeling, we systematically explored how precise combinations of ligand-bearing peptides modulated endothelial cell growth, in the process finding interactions between ligands not previously appreciated. By investigating immune responses against the materials intended for tissue engineering applications, we discovered that the basic self-assembling peptides were minimally immunogenic or non-immunogenic, even when delivered in strong adjuvants. -But when they were appended to an appropriately restricted epitope peptide, these materials raised strong and persistent antibody responses. These responses were dependent on covalent conjugation between the epitope and self-assembling domains of the peptides, were mediated by T cells, and could be directed towards both peptide epitopes and conjugated protein antigens. In addition to their demonstrated utility as scaffolds for regenerative medicine, peptide self-assemblies may also be useful as chemically defined adjuvants for vaccines and immunotherapies. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B41.00010: Multiscale peptide self-assembly Justin Barone, Devin Ridgley Here, we demonstrate a hierarchical peptide self-assembly process from the nanometer to the micrometer scale. The process begins by mixing a short hydrophobic peptide and a longer $\alpha$-helix peptide. Cross-$\beta$ nanostructures spontaneously form that then aggregate into nanometer fibrils and then micron-sized fibers. FT-IR and Raman spectroscopy show unraveling of $\alpha$-helices and packing of aliphatic side groups as the major events leading to $\beta$-sheet and large fiber formation. A thermodynamic model is presented that uses conformational change and hydrogen bond formation to describe free energy change. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B41.00011: Intermolecular Hydrogen Bonding in Peptide and Modified Jeffamine Organogels Daniel Savin, Adam Richardson In these studies, we present two systems whereby supramolecular assembly results in rigid organogels. First, a series of AB diblock copolymers consisting of poly(Lysine(Z)) (P(Lys(Z)) blocks were synthesized and found to form stable, rigid organogels in THF (ca. 1 - 1.5 wt.\% solutions) and chloroform at room temperature. In these systems, the protecting group on the P(Lys) side-chains remains intact and gel formation results from the assembly of the solventphobic P(Lys(Z)) chains through intermolecular beta-sheet formation. The non-peptide block was found to have an effect on organogel properties due to interfacial frustration, which disrupts H-bonding. Second, Jeffamine polymers were modified in a facile way to incorporate intermolecular H-bonding groups to yield networks able to gel various solvents as well as mineral and canola oil. We present the physical and rheological properties of the organogels produced. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B41.00012: Designing responsive peptide hydrogels using peptide-responsive polymer conjugates Alberto Saiani, Anton Maslovskis, Aline Miller Self-assembly represents a simple and efficient route to the construction of large, complex structures. Peptide self-assembly in particular offers the possibility to design new functional bio-materials that find application in drug delivery and tissue engineering. The $\beta $-sheet motif is are of particular interest as short peptides can be designed to form $\beta $-sheet rich fibres that entangle and consequently form hydrogels. These hydrogels can be functionalised using specific biological signals and can also be made responsive through the use of enzymatic catalysis and/or conjugation with responsive polymers. In this presentation we will focus on the design of the latter using peptide-responsive polymer conjugates. The main objective is to create hydrogels possessing an internal transition resulting from the conjugation with the responsive polymer in the gel state that can be used as a trigger for example the release of a drug. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B41.00013: Stimuli-Responsive Peptide-based Triblock and Star Copolymers Jacob Ray, Sandeep Naik, Ashley Johnson, Jack Ly, Daniel Savin Stimuli-responsive copolymers demonstrate diverse aggregation behavior in aqueous solution. In general, the molecular architecture and the balance of hydrophilic and hydrophobic volumes influence morphology. This study involves polypeptide-based ABA linear triblock and AB2 star copolymer (which structurally resemble phospholipids) amphiphiles. Model systems for this study are poly(L-lysine)-b-poly(propylene oxide)-b-poly(L-lysine) (KPK) triblocks and poly(L-glutamate) (PE) based star copolymers. Extensive studies with KPK systems have resulted in morphological transitions by modifying pH, and we hypothesize that a change in individual chain conformation is the driving force for these transitions. Preliminary results for PE-based star copolymers with various hydrophobic moieties suggest polymersome (vesicle) formation. Light scattering (dynamic and static) and TEM were used to determine aggregate size and morphology as a function of pH; furthermore, circular dichroism (CD) spectroscopy was used to measure helix-to-coil transitions of the polypeptide blocks. [Preview Abstract] |
Session B42: Polyelectrolytes, Conformations, Assembly, and Dynamics
Sponsoring Units: DPOLYChair: Venkat Ganesan, University of Texas at Austin
Room: A302/303
Monday, March 21, 2011 11:15AM - 11:27AM |
B42.00001: Scaling behavior of single chain dimension of polystyrene sulfonate Qingbo Yang, Jiang Zhao Scaling behavior of single chain of polystyrene sulfonate (PSS-) has been studied by fluorescence correlation spectroscopy. The scaling power index of the hydrodynamic radius of the PSS- single chain in aqueous solutions was found to depend on the salt condition in the solution and a systematic investigation on salt concentration and salt valency has been conducted. The results clearly demonstrate the change in conformation of PSS- chain due to its interaction with the counterions. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B42.00002: Bridging induced by multivalent counterions in polyelectrolyte brush Nicolas Laugel, Robert Farina, Philip Pincus, Matthew Tirrell When the counterions of a polyelectrolyte brush are multivalent, significant changes in its behavior are observed. One example is its shrinkage at values of ionic strengths where osmotic pressure would be expected to keep chains extended. This effect could be explained through the existence of a bridging phenomenon, with the multivalency of each counterion enabling attractive interactions with more than one polymeric charge at a time. Here we present a variational and phenomenological free energy model in a description of the charged brush as an homogeneous layer subject to classical energy contributions related to both chains and counterions. Two mean-field order parameters are introduced to describe counterion condensation and bridging. The model predicts an abrupt collapse of the brush height upon very fine changes in values of the parameters value which is reminiscent of the experimentally observed behavior. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B42.00003: Collapse of single polyelectrolytes in a.c.\,electric fields Chunda Zhou, Robert Riehn Experimental and theoretical studies of single polyelectrolyte molecules under alternating electric fields have concluded that stretching is the near-universal response. We confined fluorescently stained $\lambda$-DNA (48.5 kbp, $\approx$16 $\mu$m contour length) in TBE buffer solution in 500 nm x 10 $\mu$m microchannels and applied alternating electric fields ranging from 0 kV/cm to about 2 kV/cm. We observed that DNA molecules collapsed under these conditions, in contrast to the literature reports. We observed single molecules with a fluorescence microscope, and analyzed the radius of gyration of each molecule in each frame. The threshold of the electric field at which DNA molecules start to collapse depends on both the concentration of TBE buffer solutions and the frequencies of the alternating electric fields. In particular, the critical electric field for collapse increases as the frequency increases. In our experiment, DNA molecules were suspended in 0.25x TBE, 0.5x TBE, 1x TBE and 2x TBE buffer solution, and the frequency was 100 Hz, 200 Hz, 300 Hz, 450 Hz, 675 Hz or 800 Hz. The critical electric field ranged from 0.5 kV/cm to 1.5 kV/cm. We believe the phenomenon is due to aggregation of density fluctuations within the polymer coil, which is not described in traditional homogeneous coil models. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B42.00004: Conformation of Randomly Sulfonated Pentablock Ionomers in Dilute Solution: Molecular Dynamic Simulation Study Dipak Aryal, Dvora Perahia, Gary S. Grest As part of our efforts to define the factors that control the structure and dynamics of structures ionic polymers, the conformation of a pentablock copolymer that consists of randomly sulfonated polystyrene, an ionomeric block, bound to poly-ethylene-r-propylene end caped by poly-t-butylstyrene has been studied in dilute solutions using molecular dynamic simulations. Multi-block copolymers offer a means to tailor several properties into one molecule, taking advantage of their rich phase diagram together with unique properties of specific blocks. We varied the solvent quality for the different blocks and followed the changes in conformation. The spatial configuration of the pentablock as well as the dynamics of the polymer was studied. We find that, independent on the solvent, the higher the sulfonation level, the lower R$_{g}$. The static and dynamic structure factors were calculated and compared in an implicit poor solvent, water and a common solvent. These data are compared with results obtained from neutron scattering. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B42.00005: Polyelectrolyte Dendrimer Conformations from Mean Field Theory Thomas Lewis, Venkat Ganesan The unique architecture of dendrimers has led to research in a wide array of applications including drug delivery. It is widely accepted that non-charged dendrimers exhibit a dense-core radial density profile in order to balance entropic and excluded volume forces. The use of polyelectrolyte dendrimers in drug delivery has been suggested as a way to attain internal cavities within the dendrimer, which can be tuned by varying salt concentration and pH of the solution. In order to gain insight into the equilibrium behavior of both annealed and quenched polyelectrolyte dendrimers, we have developed and numerically solved a Self-Consistent Field Theory approach for charged dendrimer molecules in an implicit solvent. We then use this method to examine the effects of pH, salt concentration, and generation number upon the conformations of these molecules. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B42.00006: Controlling the swelling and wettability of weak polyelectrolyte brushes Richard Gurtowski, Benxin Jing, Elaine Zhu Weak polyelectrolytes (PE) of tunable ionization shows great potential as ``smart'' polymer materials for diverse applications from drug delivery to energy storage. However, the conformational dynamics of surfaced-tethered weak PE chains remain inadequately understood due to the complexity of their dynamic charge states in response to solvation and surface immobilization conditions. In this work, we investigate the wetting and swelling characteristics of poly(2-vinyl pyridine) (P2VP) brushes grafted to a gold substrate by AFM and water contact angle measurements. We observe the collapse of P2VP brushes, accompanied with increased surface hydrophobicity, as increasing solution pH across a critical transition pH, which is considerably lower than the pKa of free P2VP chains in bulk solution. Surprisingly, the broadness of the transition pH range shows a strong dependence with brush thickness, but not grafting density, suggesting a distribution of chain ionization along grafted P2VP brushes. We further manipulate P2VP brush structures by applying ac-electric fields across the brushes to make tunable and switchable polymer surfaces. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B42.00007: Light- and pH Switchable Supramolecular Nanoparticles through Electrostatic Self-Assembly Franziska Groehn, Immanuel Willerich Supramolecular structures that can respond to external triggers are of high interest for example for nanotechnology or drug delivery. Recently we have introduced an approach to electrostatic self-assembly for the formation of supramolecular particles in solution: polyelectrolytes and multivalent stiff organic counterions build well-defined and stable nano-objects. In addition to electrostatics, secondary interactions between counterions such as pi-pi stacking directs the association. Aggregates with narrow size distribution and varying shape such as spheres, cylinders, vesicles and networks result. PH-responsive assemblies can be repeatedly switched ``on'' and ``off'' through pH. Furthermore, light is an elegant, non-invasive stimulus offering possibilities for new functional nanostructures. By electrostatic self-assembly, supramolecular particles can be built the size of which can be triggered by light. For example, assemblies of dendrimer macroions and divalent azobenzene counterions can respond to light with a size increase from 30 nm to 165 nm radius. Detailed characterization by static and dynamic light scattering, AFM, SANS and zeta-potential measurements as well as thermodynamic studies yield insight into driving forces and structural control in the self-assembly process. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B42.00008: Tunable morphologies from charged diblock copolymers Monojoy Goswami, Bobby Sumpter Molecular Dynamics (MD) simulations are carried out to understand the physical aspects of different bulk morphologies formed in charged diblock copolymers. It has been seen that the bulk morphologies formed by charged block copolymers, 75 vol \% fluorinated polyisoprene (FPI) - 25 vol\% sulfonated polystyrene (PSS) with 50\% sulfonation are substantially different from their diblock counterparts. In this study we show how the bulk morphologies change from the uncharged diblock counterparts and also how morphology can be tuned with volume fraction of the charged block and with a change in dielectric constant. A physical understanding based on the underlying strong electrostatic interactions between the charged block and counterions is obtained. The 75/25 diblock shows hexagonal morphologies with the minority blocks (PSS) forming the continuous phase due to charge percolation and the FPI blocks arranged in hexagonal cylinders. Some long-range order can be sustained even by changing the dielectric of the medium. Diverse and atypical morphologies are readily accessible by simply changing the number distribution of the charges on PSS block. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B42.00009: Finite size effects in polyelectrolyte adsorption: A simulation study Maria Sammalkorpi, Paul R. Van Tassel In recent experiments, we have uncovered conditions where polyelectrolyte adsorption to a conducting surface may become continuous in the sense of scaling linearly with time over hours [1]. This discovery of continuous layer growth offers an enticing possibility of nanoscale thin film growth in a single step process, but also brings forth questions of the underlying mechanisms. Here, we present a molecular Monte Carlo simulation study aimed at understanding mechanistically the continuous adsorption process and, more broadly, polyelectrolyte adsorption in general. Our system consists of two parallel polymer chains composed of charged tangent spheres above a surface of variable dielectric discontinuity between the substrate and the solution, and spherical counterions and salt ions. We find that counter ion correlations act to enable the formation of stable polymer-polymer binding and aggregation. We discuss the sensitivity of the attractive regime to a Coulombic coupling parameter and to finite ion size, and the implications of finite size effects and charge distribution both in the polyelectrolytes and in the ions, and implications to experimental observations. [1] A. P. Ngankam and P. R. Van Tassel, Proc. Nac. Acad. Sci. 104, 1140-1145 (2007); C. Olsen and P. R. Van Tassel, J. Colloid and Interface Science 329, 222-227 (2009). [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B42.00010: Thermal Properties of Linearly and Exponentially Growing Layer-by-Layer Assemblies Ajay Vidyasagar, Jodie Lutkenhaus Polyelectrolyte multilayer thin films have received significant attention for assembling various nanostructured coatings, but their thermochemical properties are challenging to measure. Here, we present results regarding the thermochemical properties of two different ``model'' layer-by-layer (LbL) assemblies. The LbL process involves alternate deposition of positively and negatively charged polymers resulting in interpenetrating networks of layers with fine structural control. Films may grow linearly or exponentially, and each type of growth is expected to give varied internal structure. Poly(allylamine hydrochloride)/poly(styrene sulfonate) (PAH/PSS) multilayers assembled without (or with) added salt are selected as the linear (or exponential) ``models.'' Other systems explored include hydrogen bonding and PAH/poly(acrylic acid) multilayers. In general, linear growth takes place due to charge overcompensation leading to thinner films than exponential growth, where interdiffusion of polyelectrolytes is a major driving force forming much thicker films. Calorimetry and ellipsometry were used to determine glass transition and crosslinking temperatures. A standing hypothesis is that linear (or exponential) growth is observed for glassy (or rubbery) multilayers. The aim of this work is to understand the origin of linear versus exponential growth in polyelectrolytes with respect to their thermal properties. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B42.00011: Polyelectrolyte gel dynamics during volume phase transitions Mithun Mitra, Jing Hua, Murugappan Muthukumar We will address the dynamics of the elastic modes of a polyelectrolyte gel near the first-order volume phase transition. The role of the neutralizing plasma on the modulus of the polyelectrolyte gel will be discussed. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B42.00012: Field theoretic simulations of the interfacial properties of complex coacervates Robert Riggleman, Glenn Fredrickson Many biological processes and emerging technologies, such as wet adhesives and biosensors, rely on the association between oppositely charged polyelectrolytes. Such association is driven not only by the electrostatic interactions between the polyelectrolytes, but there is also a substantial entropy gain associated with counterion release upon complexation. In some cases, the association between oppositely charged polymers can lead to a solid precipitate while others can result in a fluid phase rich in polyelectrolytes (coacervate phase) coexisting with a polyelectrolyte-dilute solvent phase. For many of the applications seeking to exploit coacervation, characterization of the interface between the solvent phase and the coacervate is of paramount importance. In this talk, we will present the results of field-theoretic simulations for a coarse-grained polyelectrolyte model that exhibits complex coacervation. Our simulations sample the fully-fluctuating fields in three-dimensions and provide a detailed characterization of the interface between the solvent and the coacervate phase for symmetric polyelectrolytes (where both the polycations and the polyanions carry identical charge densities) as a function of salt concentration and strength of the electrostatic fields. Finally, we characterize the interfacial properties for a select set of asymmetric conditions. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B42.00013: Electrohydrodynamics of polyelectrolytes using Lattice-Boltzmann simulations without electrostatics Owen A. Hickey, James L. Harden, Christian Holm, Gary W. Slater In computer simulations of polyelectrolyte electrophoresis, the effects of long-ranged hydrodynamics are often ignored due to the high computational cost. However, the hydrodynamic interactions often play a key role in the physics and can lead to some surprising phenomena. We present hybrid Molecular Dynamics simulation methods to study the electrohydrodynamics of polyelectrolytes using a Lattice-Boltzmann (LB) fluid. By applying a local slip between the monomer beads and the LB fluid we are able to reproduce realistic dynamics for free solution electrophoresis as well as the correct stall force for a polyelectrolyte subject to an electric field. Simulations also demonstrate how a net-neutral object, such as a block polyelectrolyte, can have a non-zero net force due to hydrodynamic interactions and that the force can even be perpendicular to the applied electric field. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B42.00014: Thermophoresis of a polyelectrolyte Jennifer Kreft Pearce, Audrey Hammack, Andrew Laster, James Lee, Seth Norman Thermophoresis, the migration of a species due to a temperature gradient, has been shown to be a possible mechanism for manipulating molecules in microfluidic devices. The mechanism governing thermophoresis is complex making a molecule's Soret coefficient (S$_{T})$ and its dependence on different physical factors hard to predict. We experimentally investigate thermophoresis of a polyelectrolyte. For sufficiently high average temperatures, two forms of the molecule are present. We measure the Soret coefficient of both and find that one has positive S$_{T}$ and the other negative. We also investigate the dependence of S$_{T}$ on co-dissolved ionic species, specifically NaOH and NaCl. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B42.00015: Charge transport in confined ionic liquids Joshua Sangoro, Ciprian Iacob, Wycliffe Kipnusu, Friedrich Kremer Charge transport and glassy dynamics in neat and polymerized ionic liquids confined in nanoporous silica are investigated in a wide frequency and temperature ranges by a combination of Broadband Dielectric Spectroscopy and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). By applying the Einstein-Smoluchowski relations to the dielectric spectra, diffusion coefficients are obtained in quantitative agreement with independent PFG NMR. The impact of geometrical confinement as well as the pore wall-ionic liquid interactions on the overall ionic mobility is explored for diverse categories of ionic liquids. The results are discussed within the framework of dynamic glass transition assisted charge transport in ionic liquids. [Preview Abstract] |
Session B43: Focus Session: Polymers for Energy Storage and Conversion -- Nanoscale Structure in Polymer-based Photovoltaics
Sponsoring Units: DMP DPOLY GERAChair: Dean Delongchamp, National Institute of Standards and Technology
Room: A306/307
Monday, March 21, 2011 11:15AM - 11:51AM |
B43.00001: Association of P3HT and PCBM in solution Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B43.00002: Organic solar cells: a theoretical study of the effects of polymer side-chains Nicolas Berube, Helene Antaya, Michel Cote Organic photovoltaic cells received a great interest in the last few years as they offer an environmentally clean and low-cost solution to the world's rising energy needs. One of the main problems limiting the efficiency of an organic solar cell device is the strong binding energy of the excitons, typically situated about a few hundreds of meV, which is ten to one hundred times more than in inorganic devices. Another limiting factor can be the misalignment of the the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital) energy level of the different components of the solar cell. In this presentation, we will discuss how different modifications on organic polymers' side-chains can affect and improve their electronic properties. Our calculations, based on density-functional theory using the B3LYP functional, indicate a HOMO and LUMO lowering of more than 1 eV in various organic polymers like poly-isothianaphtene (pITN) and poly-thienothiophene (pTT), and various side-chains like aldehyde-based ones. Preliminary calculations on oligothiophenes also show a lowering of the exciton binding energy. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B43.00003: Perylene Diimide Based ``Nanofabric'' Thin Films for Organic Photovoltaic Cells Austin Carter, June Hyoung Park, Yong Min, Arthur Epstein We report progress in using a perylene diimide (PDI) nanofabric as an effective electron accepting nanostructure for organic photovoltaics (OPV). A key challenge in OPV continues to be the recovery of electrons after charge separation due to the relatively poor mobility of C60 and related materials. A series of PDI compounds and complexes have been synthesized and used to fabricate nanofibers and thin films using solution and vacuum deposition techniques. Overlaping PDI-based nanofibers form a fast electron-transporting ``nanofabric'' that has been characterized (AFM, PL, UV-vis, etc.) and can be blended with electron donating materials. A solution-processible OPV configuration containing a nanofabric heterojunction (FHJ) of poly(3-hexylthiophene) and the PDI nanofabric was investigated. We observed a significant improvement in power-conversion efficiency due in part to expansion of the interfacial area and the presence of high mobility electron pathways to the LiF/Al electrode. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B43.00004: Influence of Annealing and Blending of Photoactive Polymers on Their Crystalline Structure Matthias A. Ruderer, Stefan M. Prams, Monika Rawolle, Qi Zhong, Peter Mueller-Buschbaum, Jan Perlich, Stephan V. Roth Thin photoactive polymer films of poly(3-octylthiophene-2,5-diyl) (P3OT) and poly(2,5-di(hexyloxy)cyanoterephthalylidene) (CN-PPV) are investigated. With X-ray reflectivity measurements, linear concentration-thickness dependence is found for both. Grazing incidence wide-angle X-ray scattering (GIWAXS) is used to probe the crystallinity of thin films and to determine characteristic length scales of the crystalline structure. Moreover, the orientation of the crystalline parts regarding the substrate of both the homopolymer and the blended films is probed with GIWAXS. Temperature annealing is found to improve the crystallization for both homopolymers. In addition, reorientation of the predominant crystalline structures takes place. Blending both polymers reduces or even suppresses the crystallization during spin coating as well as temperature annealing. Absorption measurements complement the structural investigations [1]. \\[4pt] [1] M.A. Ruderer et al. J. Phys. Chem. B (2010), doi:10.1021/jp106972s. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B43.00005: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 12:39PM - 1:15PM |
B43.00006: Bio-Inspired electro-photonic structure for organic and dye sensitized solar cells Invited Speaker: A major challenge in solar cell technology dwells in achieving an efficient absorption of photons with an effective carrier extraction. In all cases, light absorption considerations call for thicker modules while carrier transport would benefit from thinner ones. This dichotomy is a fundamental problem limiting the efficiencies of most photovoltaics. One pathway to overcome this problem is to decouple light absorption from carrier collection. We present solutions to this problem applying bio-inspired nanostructures to two different types of systems: organic photovoltaic (OPV) and dye sensitized solar cells (DSSC). For OPV devices based on poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM), we describe a 2-D photonic crystal geometry that enhances the absorption of polymer-fullerene photonic cells $\sim $ 20{\%} relative to conventional planar cells. Remarkably, the photonic crystal cell offers the possibility to increase photocurrents by improvements in optical absorption and carrier extraction simultaneously, and particularly through the excitation of photonic resonant modes near the band edge of organic PV materials. We also present an optical method to extract charge transport lengths from device photoactive layers. For DSSCs we introduce a new structural motif for the photoanode in which the traditional random nanoparticle oxide network is replaced by vertically aligned bundles of oxide nanocrystals. We have used a pulsed laser deposition system to ablate titanium oxide targets to obtain the porous and vertically aligned structures for enhanced photoelectrochemical performance. Absorption studies show that in optimized structures for titanium oxide, there is a 1.4 times enhancement of surface area compared to the best sol-gel films, Incident-Photon-Conversion-Efficiency values are better than 3 times thicker sol-gel films, and $\sim $ 92{\%} Absorbed-Photon-Conversion-Efficiency values have been observed when sensitizing with the N3 dye (Ru(dcbpyH)2(NCS)2). The direct pathways provided by the vertical structures appear to indeed provide for enhanced collection efficiency for carriers generated throughout the device. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B43.00007: The Poly(3-hexylthiophene) / ZnO (10-10) interface: structure and energetics Jie Jiang, Sohrab Ismail-Beigi The poly(3-hexylthiophene) (P3HT) polymer on ZnO system is of significant interest for hybrid nanoscale solar energy research and applications. Using density functional theory and periodic supercells, we study the P3HT/ZnO interface where sulfur atoms on the P3HT side chains are used to anchor the polymer onto the ZnO (10-10) surface. We discuss the structure and energetics of the binding modes for low and high polymer coverage. We then apply the Frenkel-Kontorova model to study the likely polymer structures in practice (e.g. dislocation formation to release strain energy). We end with a discussion of the band energy alignment across the interface. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B43.00008: Photovoltaic Device Performance Enhancement by Interfacial Decoration of Bulk-Heterojunctions with Semiconducting Nanocrystals Theodore J. Kramer, Ioannis Kymissis, Irving P. Herman We have developed a facile method for decorating the donor-acceptor interface of organic bulk-heterojunctions (BHJs) with semiconducting nanocrystals (NCs). Using nano-scale phase separation of a poly(3-hexylthiophene)/polystyrene copolymer blend, followed by selective removal of the polystyrene, we are able to expose a nano-scale network of poly(3-hexylthiophene) [P3HT] fibers. These fibers are subsequently decorated with cadmium selenide (CdSe) NCs prior to back filling the structure with thermally evaporated C$_{60}$. Optical characterization techniques have confirmed that NCs located at the donor-acceptor interface show enhanced charge transfer to the surrounding medium compared to NCs randomly dispersed in similar BHJs. Photovoltaic (PV) devices made using this technique show improved external efficiencies compared to similar planar PV structures. This technique provides an elegant mechanism for improving the performance of organic BHJs by tailoring their spectral absorption using semiconducting NCs. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B43.00009: Semiconducting Nanocomposites via Directly Grafting Conjugated Polymer onto Quantum Rods Lei Zhao, Xinchang Pang, Zhiqun Lin Nanocomposites of poly(3-hexylthiophene) (P3HT)-cadmium selenide (CdSe) nanorod (NR) were synthesized by directly grafting P3HT onto bromobenzylphosphonic acid (BBPA) functionalized CdSe NR, dispensing with the need for ligand exchange chemistry. The grafting was accomplished by Heck coupling as well as a newly developed catalyst-free click reaction. The resulting P3HT-CdSe NR nanocomposites possess a well-defined interface, thereby significantly promoting the dispersion of CdSe within the P3HT matrix and facilitating the electronic interaction between them. The success of grafting was confirmed by the NMR and DLS, and the occurrence of charge transfer at P3HT/CdSe NR interface was demonstrated by the UV-vis absorption and photoluminescence (PL) measurements as well as the time-resolved PL study. Similar grafting density was yielded using these two methods. The nanocomposites prepared by the catalyst-free click reaction was found to exhibit a faster charge transfer. To the best of our knowledge, this is the first study of grafting conjugated copolymer directly onto the elongated semiconductor nanomaterials. As such, it provides insight into rational design and fabrication of organic-inorganic nanohybrid solar cells with improved power conversion efficiency. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B43.00010: Directed assembly of core-shell hybrid nanomaterials for polymer photovoltaics Shanju Zhang, Candice Pelligra, Lisa Pfefferle, Chinedum Osuji The creation of large-area aligned nanohybrid films/arrays remains a challenge in the fabrication of ordered heterojunction photovoltaics. We demonstrate a bottom-up approach based on the directed assembly of lyotropic inorganic-organic core-shell nanohybrids. Semiconductor nanowires are prepared by solvothermal synthesis. Diameter and length of the nanowires are controlled by various reaction parameters. Core-shell nanohybrids are prepared by grafting conjugated polymers onto the nanowires. Effect of the nanowire diameter on the polymer coating is demonstrated. We show that high aspect ratio nanohybrids spontaneously form nematic phases in liquid media. These systems show isotropic, bi-phasic and nematic phases on increasing concentration in reasonable agreement with Onsager's theory for rigid rods. Suspensions are readily processed to produce films with large-area monodomains. With a decrease of nanowire diameter, the polymers in the nanohybrids tend to form ordered crystalline layers, in which the conjugated backbone is aligned along the nanowire long axis. The corresponding optoelectronic properties are discussed. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B43.00011: Interconnected and nano-perforated lamellar sheets of metal oxides produced using novel block copolymer templates Paul Zavala-Rivera, Kevin Channon, Vincent Nyugen, Easan Sivaniah, Nataraj Sanna Kotrappanavar, S.A. Al-Muhtaseb Recently, our group has investigated the development of a novel bicontinuous nanostructure using block copolymers. This has lead to the creation of various bicontinuous, mesoporous, and interconnected metal oxides sheets. The high surface area produced by the 3D nanostructure has shown a considerable improvement in efficiency in the method of preparation. The main transformations from polymer scaffold to inorganic matrices produced by our group include the use of oxide of titanium, and a number of other sol-gel transformations. The application of these nanostructures is shown in the development of photovoltaic devices. We highlight future applications in electronic, memory, energy storage and production devices. [Preview Abstract] |
Session B44: Physics of Copolymers I
Sponsoring Units: DPOLYChair: Darrin Pochan, University of Delaware
Room: A309
Monday, March 21, 2011 11:15AM - 11:51AM |
B44.00001: Controlling Cellular Endocytosis at the Nanoscale Invited Speaker: One of the most challenging aspects of drug delivery is the intra-cellular delivery of active agents. Several drugs and especially nucleic acids all need to be delivered within the cell interior to exert their therapeutic action. Small hydrophobic molecules can permeate cell membranes with relative ease, but hydrophilic molecules and especially large macromolecules such as proteins and nucleic acids require a vector to assist their transport across the cell membrane. This must be designed so as to ensure intracellular delivery without compromising cell viability. We have recently achieved this by using pH-sensitive poly(2-(methacryloyloxy)ethyl-phosphorylcholine)- co -poly(2-(diisopropylamino)ethyl methacrylate) (PMPC-PDPA) and poly(ethylene oxide)-co- poly(2-(diisopropylamino)ethyl methacrylate) (PEO-PDPA) diblock copolymers that self-assemble to form vesicles in aqueous solution. These vesicles combine a non-fouling PMPC or PEO block with a pH-sensitive PDPA block and have the ability to encapsulate both hydrophobic molecules within the vesicular membrane and hydrophilic molecules within their aqueous cores. The pH sensitive nature of the PDPA blocks make the diblock copolymers forming stable vesicles at physiological pH but that rapid dissociation of these vesicles occurs between pH 5 and pH 6 to form molecularly dissolved copolymer chains (unimers). We used these vesicles to encapsulate small and large macromolecules and these were successfully delivered intracellularly including nucleic acid, drugs, quantum dots, and antibodies. Dynamic light scattering, zeta potential measurements, and transmission electron microscopy were used to study and optimise the encapsulation processes. Confocal laser scanning microscopy, fluorescence flow cytometry and lysates analysis were used to quantify cellular uptake and to study the kinetics of this process in vitro and in vivo. We show the effective cytosolic delivery of nucleic acids, proteins, hydrophobic molecules, amphiphilic molecules, and hydrophilic molecules without affecting the viability of cells or even triggering inflammatory pathways. Finally we show how size, surface chemistry and surface topology of the vesicles affect their interaction with the cell membrane and hence their cellular uptake. \\[4pt] References:\\[0pt] C. Lo Presti, M. Massignani, T. Smart, H. Lomas, and G. Battaglia \textit{J. Mater. Chem}. (2009) 19, 3576-3590 H. Lomas, I. Canton, S. MacNeil, J. Du, S.P. Armes, A.J. Ryan, A.L. Lewis and G. Battaglia \textit{Adv. Mater}. (2007). 19, 4238-4243 \\[0pt] M. Massignani, I. Canton, N. Patikarnmonthon, N. J. Warren, S. P. Armes, A. L. Lewis and G. Battaglia, \textit{Nature Prec.}, 2010, http://hdl.handle.net/10101/npre.2010.4427.1 M. Massignani, C. LoPresti, A. Blanazs, J. Madsen, S. P. Armes, A. L. Lewis and G. Battaglia \textit{Small}, 2009, 5, 2424-2432. \\[0pt] M. Massignani, T. Sun, A. Blanazs, V. Hearnden, I. Canton, P. Desphande, S. Armes, S. MacNeil, A. Lewis and G. Battaglia \textit{PLoS One}, 2010, 5, e10459. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B44.00002: Microphase Separation in Block-Random Copolymers of Styrene and Hydrogenated Isoprene Bryan S. Beckingham, Richard A. Register The capacity to synthesize block-random copolymers, which are block copolymers with one or more random copolymer blocks, allows for continuous tuning of the interblock segregation strength, $\chi $, through the composition of the random copolymer. The ability to tune $\chi $ effectively decouples the block copolymer molecular weight from its order-disorder transition temperature. By lithium-initiated anionic polymerization with added triethylamine, we synthesize near-monodisperse and near-symmetric block-random copolymers of styrene and isoprene: PI-PSrI (50{\%} wt. styrene). In comparison to PS-PI diblock copolymers, the number of unfavorable segmental contacts in the disordered state is decreased and hence the effective interblock $\chi $ is reduced. Isoprene-hydrogenated derivatives of these block-random copolymers exhibit microphase separation into well-ordered lamellae and display sharp thermally-induced order-disorder transitions via small-angle x-ray scattering. The observed reduction in $\chi $, as gauged by the molecular weight required to achieve a desired T$_{ODT}$, matches well with the mean field prediction. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B44.00003: Micellization kinetics of diblock copolymers in a homopolymer matrix: A self-consistent field study Raghuram Thiagarajan, David Morse Self-consistent field theory is used to calculate free energy barriers and reaction rates for the spontaneous association and dissociation of micelles formed of block copolymers in a homopolymer matrix. The barriers are prohibitively large for copolymers of typical molecular weights when the unimer (free surfactant) concentration is near the equilibrium critical micelle concentration. As a result, polymeric micelles normally cannot reach true thermodynamic equilibrium. The rates of association and dissociation are, however, sensitive to unimer concentration, making it possible to form or destroy micelles at observable rates in sufficiently highly supersaturated or subsaturated solutions, respectively, even when both reactions are suppressed near the equilibrium CMC. The barrier to disassociation is particularly sensitive to unimer concentration, and vanishes when the unimer concentration is only slightly (e.g., tens of percent) below the equilibrium CMC. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B44.00004: Manipulating the structural conformation of block copolymer micelles using co-solvent mixtures Elizabeth G. Kelley, Thomas P. Smart, Millicent O. Sullivan, Thomas H. Epps, III The internal structure of poly(butadiene-$b$-ethylene oxide) (PB-PEO) block copolymer micelles was manipulated through the use of co-solvent mixtures. In aqueous solutions, the PB-PEO block copolymers self-assembled into monodisperse, spherical micelles with well-defined PB cores surrounded by PEO coronas. The addition of tetrahydrofuran (THF) to the micelle solution improved the solvent quality for the PB block and resulted in the swelling of the micelle cores. The average micelle size decreased with increasing THF content as shown by dynamic light scattering, small angle X-ray scattering, and cryogenic transmission electron microscopy. The distribution of PB in the micelles was studied using nuclear magnetic resonance spectroscopy and small angle neutron scattering. Finally, the spherical micelles disassembled in co-solvent mixtures containing greater than 72 vol{\%} THF. This result was consistent with phase behavior studies of PB homopolymer, which indicated that PB is soluble in water-THF mixtures up to a similar THF solution composition. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B44.00005: Molecular Exchange in Ordered Diblock Copolymer Micelles Soo-Hyung Choi, Timothy Lodge, Frank Bates Previously, molecular exchange between spherical micelles in dilute solution (1 vol{\%} polymer) was investigated using time-resolved small-angle neutron scattering (TR-SANS). As the concentration of spherical micelles formed by the diblock copolymers increases, the micelles begin to overlap and eventually pack onto body-centered cubic (BCC) lattice. In this study, concentrated, ordered micelles (15 vol{\%} polymers) prepared by dispersing isotopically labeled poly(styrene-$b$-ethylene-\textit{alt}-propylene) in an isotopic squalane mixture was investigated to understand the micellar concentration dependence of the molecular exchange. Perfectly random mixing of isotopically labeled micelles on the BCC lattice was confirmed by SANS patterns where the interparticle contribution vanishes, resulting in an intensity that directly relates to the exchange kinetics. The measured molecular exchange process for the concentrated, ordered system is qualitatively consistent with the previous observations, but the rate is more than an order of magnitude slower than that for the dilute, disordered system. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B44.00006: Kinetics of Pressure Jump for Block Copolymer Phase Transition in Selective Solvent Yongsheng Liu, Rama Bansil, Milos Steinhart Synchrotron based time-resolved small angle x-ray scattering (SAXS) was used to study the kinetics of the order-disorder transition (ODT) in a 30{\%} (w/v) solution of a diblock copolymer of poly(styrene -- isoprene) (SI 18-12) in diethyl phthalate, a selective solvent for the PS block using pressure jump methods. Time resolved pressure jump SAXS experiments were done to study the kinetics of disorder to BCC phase transition and the reverse transition. The results show that the ODT temperature increases at about 20C/kbar with pressure. Analysis of Percus-Yevik model following pressure jumps and pressure ramps will be presented. The analysis shows that core radius of micelles are independent of pressure, but hard sphere radius increases with pressure. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B44.00007: Transition Behavior of Hydrogen Bonding mediated Block Copolymer complex Sudhakar Naidu, Hyungju Ahn, Hoyeon Lee, Du Yeol Ryu We have investigated transition behavior for block copolymer (BCP) complexes composed of a lamella-forming polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) and phenyl acetamide derivatives. Influence of small molecules on transition temperatures such as order-to-disorder transitions (ODT) were analyzed by by in-situ small angle x-ray scattering (SAXS) and depolarized light scattering (DPLS). The importance of the availability for H-bonding mediation to control over transition behavior for BCP mixtures with the functional molecules was shown by changing the annealing temperatures. Non-covalent interactions between the nitrogen units of P2VP block and small molecules enhances nonfavorable segmental interactions between two block components, leading to a significant increase in d-spacing for BCP mixtures. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B44.00008: Phase Behavior of Binary Mixtures of Block Copolymers having Hydrogen Bonding Sung Hyun Han, Jin Kon Kim The phase behavior of binary mixtures of high molecular weight polystyrene-\textit{block}-poly(2-vinyl pyridine) (PS-$b$-P2VP) and low molecular weight polystyrene-\textit{block}-poly(4-hydroxystyrene) (PS-$b$-PHS) copolymers was investigated by using small angle X-ray scattering and transmission electron microscopy. Both block copolymers exhibited lamellar microdomains. When the weight fraction of PS-$b$-PHS in the blend was less than 0.1, lamellar microdomains are maintained. However, with increasing amount of PS-$b$-PHS, the microdomains are transformed to hexagonally-packed cylindrical microdomains, and body-centered cubic spherical microdomains. This is attributed to the hydrogen bonding between P2VP and PHS blocks. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B44.00009: Highly Localized Optically Induced Melting Transitions in Block Copolymers Azar Alizadeh, Eugene Boden, Xialei Shi, Victor Ostroverkhov, Daniel Brunnelle, Vicki Watkins, Charles Kerbage, Matthew Misner, Brian Lawrence Semi-crystalline block copolymers are well known to exhibit confined crystallization and/or melting phase transitions in sub-50 nm domains. Confined crystallization within these nano-domains is favored under the following conditions: 1) the crystallizable block forms discrete spherical or cylindrical domains; 2) the glass transition temperature of the matrix block is above the crystallization and melting temperatures of the crystallizable minority block; and 3) the block constituents form a strongly segregating system, such that the phase separation dominates the crystallization process. Here we report on optically induced highly localized crystalline-to-amorphous phase transitions in a composite medium comprised of a semi-crystalline block copolymer and a heat generating dye. We use an optical probe-pump and a Bragg reflective grating to both induce and detect the optically induced phase transitions in these block copolymers. We show that extremely fast and localized melting in these block copolymers can be achieved by exposing the samples to very short (5-20 ns) pulses of light. This study provides a new insight on the timescale of melting transitions in polymeric materials. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B44.00010: Self-Assembly of Lamellar Microphases in Linear Gradient Copolymer Melts Nicholas B. Tito, Scott T. Milner, Jane E. G. Lipson The ability to create `designer copolymers' with tunable properties by tailoring their monomer composition has garnered recent interest in their molecular self-assembly. Here we investigate lamellar microphases in linear gradient binary copolymer melts using a variety of techniques, including solutions of self-consistent field equations, scaling theory, and analysis of the strong-segregation limit. The Flory scaling theory predicts the scaling of the equilibrium lamellar width $L_{eq}$ as a function of comonomer incompatibility as characterized by \textit{$\chi $}. From the strongly segregated limit there are conformational fluctuations, and it is the tradeoff between the entropic effect of these relative to repulsive comonomer interactions that determines $L_{eq}$. We discover that $L_{eq}$ /$R_{g} \quad \sim $ (\textit{$\chi $N})$^{1/6}$; remarkably, this is the same result as for symmetric diblock copolymers, although for quite different physical reasons. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B44.00011: Kinetics and Dynamics of HEX to Gyroid Transition of a Diblock Copolymer in Selective Solvent Julian Spring, Yongsheng Liu, Rama Bansil Synchrotron based time-resolved small angle x-ray scattering (SAXS) was used to study the kinetics of the formation of a gyroid phase in solutions of a poly (styrene -isoprene) diblock copolymer in dimethyl phthalate, a selective solvent for the polystyrene block. From temperature ramp measurements on an 80{\%} (w/v) sample, a hexagonally-packed cylinders (HEX) phase was identifed below 95 C,while a gyroid formed above 95C. The kinetics of the transitions from HEX to gyroid was examined using temperature jump and ramp experiments over the temperature range of 50-150C. In addition, x-ray photon correlation spectroscopy was used to study the dynamics of the HEX and Gyroid phases, as well as the transition regime. Analysis of the time evolution of the Bragg peaks to follow the kinetics of the transition between these phases will be presented, in addition to analysis of the dynamics of this sample throughout the phase space under investigation. The formation of the Gyroid structure was also modeled using Molecular Dynamics (MD) simulations, and the results of these simulations will also be presented. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B44.00012: Dynamics of Cloud Point Transitions in Dilute Solutions of Gradient Copolymers with Prescribed Gradient Strengths Keith Gallow, Yueh-Lin Loo We have investigated dilute solutions of gradient copolymers comprising hydroxyethyl methacrylate and dimethylaminoethyl methacrylate with different gradient strengths undergoing their cloud point transitions. The gradient strength defines the maximum difference in instantaneous compositions along the polymer backbone. Isothermal dynamic light scattering tracks the fractions of unimers and aggregates with which the half times characterizing this transition can be quantified. We find the temperature dependence of this transition to depend on gradient strength, ranging from -2.22 decades/$^{\circ}$C for a random copolymer to -0.75 decades/$^{\circ}$C for a gradient copolymer of comparable molecular weight and overall composition but a gradient strength of 0.52. The progressively shallower temperature dependence of this transition with increasing gradient strength suggests of a nucleation and growth mechanism of aggregate formation. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B44.00013: Effect of Macromolecular Architecture on the Morphology of Polystyrene/Polyisoprene Block Copolymers Caleb Dyer, Paraskevi Driva, Scott Sides, Bobby Sumpter, Jimmy Mays, Mark Dadmun, Feng Zuo, Frank Bates The molecular architecture of branched block copolymers has been shown to dramatically effect morphological behavior. A study of four polystyrene/polyisoprene block copolymers with varying architecture (branched PSPI$_{2}$, PS$_{2}$PI, PS$_{2}$PI$_{2}$, and linear PSPI), and constant composition and molecular weight is presented. The morphologies of each sample were determined using SCFT simulations and, experimentally using SAXS and TEM. The PS$_{2}$PI$_{2 }$miktoarm star exhibits the same morphology as the linear$_{ }$diblock but with a reduction in the domain size. The PS$_{2}$PI and PSPI$_{2}$ copolymers demonstrated different morphologies from the diblock copolymer, a result of the architectural asymmetry. The results were then compared to Milner's theoretical predictions and found to be in good agreement. These results, therefore, provide detailed insight into the effect of copolymer architecture on the morphological behavior of block copolymers. [Preview Abstract] |
Session B45: Optomechanics at the Quantum Limit
Sponsoring Units: DAMOP GQIChair: Brian DeMarco, University of Illinois at Urbana-Champaign
Room: A310
Monday, March 21, 2011 11:15AM - 11:27AM |
B45.00001: Sideband cooling micromechanical motion to the quantum ground state John Teufel, Tobias Donner, Dale Li, Konrad Lehnert, Raymond Simmonds Accessing the full quantum nature of a macroscopic mechanical oscillator first requires elimination of its classical, thermal motion. The flourishing field of cavity opto- and electromechanics provides a nearly ideal architecture for both preparation and detection of mechanical motion at the quantum level. We realize such a system by coupling the motion of an aluminum membrane to the resonance frequency of a superconducting, microwave circuit. By exciting the microwave circuit below its resonance frequency, we damp and cool the membrane motion with radiation pressure forces, analogous to laser cooling of trapped ions. The microwave excitation serves not only to cool, but also to monitor the displacement of the drum. A nearly shot-noise limited, microwave Josephson parametric amplifier is used to detect the mechanical sidebands of this microwave excitation and quantify the thermal motion of the oscillator as it is cooled with radiation pressure forces to its quantum ground state. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B45.00002: Cavity Cooling of A Mechanical Resonator in Amorphous Systems Lin Tian The quantum backaction force generated by a cavity coupled with a mechanical resonator can be exploited to achieve sideband cooling of the mechanical mode. By applying a red-detuned driving, the quantum ground state of the mechanical mode can be reached in the resolved-sideband regime, which has recently be demonstrated in experiments. However, in many of these materials, surface defects or adsorbates can couple with the mechanical mode and impair the cavity cooling. These defects can be treated as quantum two-level system (TLS). The mechanical vibration changes the local strain tensor and generates coupling with the TLS via the deformation potential. In this work, we study the cavity cooling of the mechanical mode in the presence of a TLS. By applying the adiabatic elimination technique widely used in quantum optics, we derive the cooling master equation for the resonator-TLS system in the eigenbasis of this system. Our results show that the stationary phonon number depends non- monotonically on the energy of the TLS. We also show that the cooling depends strongly on the decoherence rate of the TLS. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B45.00003: Quantum Interactions of a Torsional Nanomechanical Resonator with a Single Spin Brian D'Urso, Shonali Dhingra While the motions of macroscopic objects may ultimately be governed by quantum mechanics, the distinctive features of quantum mechanics can be hidden by thermal excitations and coupling to the environment. We present a system consisting of a torsional nanomechanical resonator with quantum behavior introduced to the system by coupling the resonator with a single spin through a uniform external magnetic field. The spin originates from a nitrogen vacancy (NV) center in a diamond nanocrystal which is positioned on the resonator. The quadratic coupling is maximized by utilizing a low moment of inertia resonator and an avoided level crossing. This coupling results in quantum non-demolition (QND) measurements of the resonator and spin states, enabling a bridge between the quantum and classical worlds. Furthermore, it provides a high-fidelity readout of the NV center spin and a potential means of observing the discrete states of the resonator. We will describe the potential for these measurements and report on the experimental progress made towards observing this coupling in the torsional resonator-NV system. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B45.00004: Characterization of an oscillator's mechanical impedance using photon pressure Paul Wilkinson, Gordon Shaw, Jon Pratt In recent years, there has been much progress in coupling optical cavities to mechanical oscillators, especially in the pursuit of the quantum ground state of a macroscopic oscillator. Photon pressure due to reflection is of particular interest, and such experiments must be carefully designed to minimize competing contributions. Typically, such unwanted contributions are estimated or modeled. We describe an experimental approach to place an upper bound on unwanted contributions. A fiber coupled superluminous light emitting diode is modulated at an optical power of 6.5 mW rms, driving a highly reflective cantilever at a displacement of over 10 nm rms at resonance (Q=4900) in vacuum (10-5 Torr). The optomechanical transfer function is measured and fit to a simple harmonic oscillator model. The stiffness of the oscillator determined from the fit (k=16.6 +/- 1.3 N/m) is found to be in good agreement with that obtained by calibration against our SI-traceable nanoindenter (k=17.4 +/- 0.5 N/m). We characterize the modal stiffness, mass, and dissipation of the first two eigenmodes of our oscillator with SI traceability. The quantitative agreement in our experiment indicates that our oscillator is actuated by photon pressure, and that all other contributions to the force must sum to less than 11{\%}. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B45.00005: Multi-stability in an optomechanical system with two-component Bose-Einstein condensate Ying Dong, Jinwu Ye, Han Pu We investigate a system consisting of a two-component Bose-Einstein condensate interacting dispersively with a Fabry-Perot optical cavity where the two components of the condensate are resonantly coupled to each other by another classical field. The key feature of this system is that the atomic motional degrees of freedom and the internal pseudo-spin degrees of freedom are coupled to the cavity field simultaneously, hence an effective spin-orbital coupling within the condensate is induced by the cavity. The interplay among the atomic center-of-mass motion, the atomic collective spin and the cavity field leads to a strong nonlinearity, resulting in multi-stable behavior in both matter wave and light wave at the few-photon level. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B45.00006: Optomechanical down-conversion Simon Groeblacher, Sebastian Hofer, Witlef Wieczorek, Michael Vanner, Klemens Hammerer, Markus Aspelmeyer One of the central interactions in quantum optics is two-mode squeezing, also known as down-conversion. It has been used in a multitude of pioneering experiments to demonstrate non-classical states of light and it is at the heart of generating quantum entanglement in optical fields. Here we demonstrate first experimental results towards the optomechanical analogue, in which an optical and a mechanical mode interact via a two-mode squeezing operation. In addition, we make use of the fact that large optomechanical coupling strengths provide access to an interaction regime beyond the rotating wave approximation. This allows for simultaneous cooling of the mechanical mode, which will eventually enable the preparation of pure initial mechanical states and is hence an important precondition to achieve the envisioned optomechanical entanglement. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B45.00007: Photothermally induced dynamics in partially coated loaded microcantilevers Shomeek Mukhopadhyay, Umar Mohideen Cooling of microcantilevers in a Fabry-Perot cavity either by radiation pressure or using the photothermal effect has attracted significant attention lately. We present ongoing experimental results on partially coated microcantilevers which are either loaded ( gold sphere) or have a coating only at the tip. In particular, we will compare the results with that of recent work on fully coated cantilevers. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B45.00008: Micro-optomechanical trampoline resonators Brian Pepper, Dustin Kleckner, Petro Sonin, Evan Jeffrey, Dirk Bouwmeester Recently, micro-optomechanical devices have been proposed for implementation of experiments ranging from non-demolition measurements of phonon number to creation of macroscopic quantum superpositions. All have strenuous requirements on optical finesse, mechanical quality factor, and temperature. We present a set of devices composed of dielectric mirrors on Si$_{3}$N$_{4}$ trampoline resonators. We describe the fabrication process and present data on finesse and quality factor. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B45.00009: Proposal for detecting measurement-induced entanglement between remote mechanical oscillators Kjetil Borkje, Andreas Nunnenkamp, Steven M. Girvin In optomechanical systems where an optical cavity mode interacts with a mechanical oscillator, the light leaking out of the cavity has sidebands at the mechanical frequency. The photon statistics of these sidebands contain information about the mechanical oscillator. We consider driving two similar optical cavities, containing one mechanical system each, in such a way that the mechanical oscillators are laser cooled close to the ground state. When the output fields of the two cavities are made indistinguishable by combining them on a beamsplitter, the detection of sideband photons can lead to measurement-induced entanglement between the two non-interacting mechanical oscillators. We show how this short-lived entanglement between remote mechanical oscillators can be verified through measurements of higher-order coherences of the optical output field. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B45.00010: Investigation of radiation pressure shot-noise in a microwave circuit optomechanical system Jennifer Harlow, John Teufel, Raymond Simmonds, Konrad Lehnert We examine the possibility of measuring the radiation pressure shot-noise of microwave light. When the motion of a nanomechanical oscillator is coupled to the microwave energy stored in a resonant circuit, the oscillator experiences a radiation pressure force. That force must have a random component associated with the quantum nature of the microwave field, a mechanical manifestation of the microwave photon. The variance of this random component increases with increasing circuit excitation power. Until recently, reaching powers where radiation pressure shot-noise would dominate over other random forces was unfeasible due to relatively weak optomechanical coupling and technical power limitations of microwave circuits. However, the recent advent of a mechanical oscillator coupled strongly to a microwave circuit [1] will enable exploration of this regime. We discuss the most favorable circuit parameters and measurement strategy for studying radiation pressure shot-noise. \\[4pt] [1] J. D. Teufel, et al, Circuit cavity electromechanics in the strong coupling regime, arXiv:1011.3067v1. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B45.00011: Levitated Quantum Nano-Magneto-Mechanical Systems Mauro Cirio, Jason Twamley, Gavin K. Brennen, Gerard J. Milburn Quantum nanomechanical sysems have attracted much attention as they provide new macroscopic platforms for the study of quantum mechanics but may also have applications in ultra-sensitive sensing, high precision measurements and in quantum computing. \noindent In this work we study the control and cooling of a quantum nanomechanical system which is magnetically levitated via the Meissner effect. Supercurrents in nano-sized superconducting loops give rise to a motional restoring force (trap), when placed in an highly inhomogenous magnetic field and can yield complete trapping of all translational and rotational motions of the levitated nano-object with motional oscillation frequencies $\nu\sim 10-100$MHz. As the supercurrents experience little damping this system will possess unprecendented motional quality factors, with $Q_{motion}\sim 10^9-10^{13}$, and motional superposition states may remain coherent for days. We describe how to execute sideband cooling through inductive coupling to a nearby flux qubit, cooling the mechanical motion close to the ground state. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B45.00012: Measurement of Casimir force with transparent conducting oxides Alexandr Banishev, Chia-Cheng Chang, Umar Mohideen The Casimir force plays an important role in micro- and nano electro mechanical systems (MEMS and NEMS) fabrication, because it can easily exceed the electrostatic forces used for actuating the systems at small electrode separation distances. The reduction of the Casimir force in devices is a complicated problem that needs to be scientifically investigated to open opportunities for the full exploitation of MEMS and NEMS technology. One of the ways to tune the Casimir force is to properly choose the materials of which the interacting surfaces are made. According to the Lifshitz theory, the interaction between two objects depends on their dielectric permittivity. In that case the transparent dielectrics attract less than reflective materials. This can be used to decrease the Casimir force when the design requires a smaller short range interaction. To achieve low Casimir forces and avoid uncontrolled electrostatic forces as present in dielectrics, transparent but conductive materials can be used. An ideal choice is conductive Indium Oxide such as very low doped Indium Tin Oxide (ITO). In this report we present the results of the Casimir force using transparent electrodes such as Indium Tin Oxide coated SiO$_{2}$ plate. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B45.00013: Precision measurements of the Casimir force at Low temperatures Rodrigo Castillo-Garza, Umar Mohideen We will present research involving the precision measurement of the Casimir force at low temperatures. The role of material losses in this force and its incorporation into the Lifshitz theory remains unresolved. The Casimir force results from the modification of the zero point photon spectrum due to the presence of boundaries. The problem arises when the Casimir force is calculated at non zero temperature, where the role of thermal photons have to be included to that of the zero point photons. We plan to address this problem by measuring the Casimir force for different materials as a function of the temperature. Currently we are involved in making precision measurements of the Casmir force at 6K, 77K, and 300K with a micro cantilever based system that we have designed and built at UC-Riverside. The high sensitivity of this instrument will provide us with the resolution to advance our understanding of the interactions of both virtual photons and real photons when confined to a semi-infinite cavity made out of real metals. The constructed apparatus will also provide a deeper understanding of the role vacuum fluctuations play when the cavity constituents are made of a combination of dielectric, superconductor, and metal surfaces. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B45.00014: Strong interactions of single atoms and photons near a dielectric boundary N.P. Stern, D.J. Alton, T. Aoki, H. Lee, E. Ostby, K.J. Vahala, H.J. Kimble Quantum control of strong interactions between a single atom and photon has been achieved within the setting of cavity quantum electrodynamics (cQED). To move beyond proof-of-principle experiments involving one or two conventional optical cavities to more complex scalable systems that employ $N > 1$ microscopic resonators requires localization of atoms on distance scales $\sim 100$ nm from a resonator's surface where an atom can be strongly coupled to a single intracavity photon while at the same time experiencing significant radiative interactions with the dielectric boundaries of the resonator. As an initial step into this new regime of cQED, we use real-time detection and high-bandwidth feedback to select and monitor motion of single Cesium atoms through the evanescent field of a microtoroid\footnote{D. J. Alton, \textit{et al,} \textit{Nature Physics} (2010); available as arXiv:1011.0740.}. Direct temporal and spectral measurements coupled with simulations reveal both the significant role of Casimir-Polder attraction and the manifestly quantum nature of the atom-cavity dynamics, here in a regime of strong coupling, setting the stage for trapping atoms near micro- and nano-scopic optical resonators. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B45.00015: Distinct single photons strongly interacting at a single atom in a waveguide Pavel Kolchin, Rupert F. Oulton, Xiang Zhang We propose a waveguide QED system where two distinct single photons can interact strongly. The system consists of a single ladder-type three level atom coupled to a waveguide. We show that the nonlinear interaction can be tremendously enhanced by the strong coupling of the cascade atomic transitions to the waveguide mode simultaneously. As a result, a control photon tuned to the upper transition induces a $\pi$ phase shift and tunneling of a probe photon tuned to the otherwise reflective lower transition. Waveguide QED schemes could be an alternative to high quality cavities or dense atomic ensembles in quantum information processing. [Preview Abstract] |
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