Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F1: Invited Session: Physics from the Laboratory to the Universe: Davisson-Germer/Heineman/Onsager/Lilienfeld Prizes
Sponsoring Units: DCMP GSNPChair: Barbara Jones, International Business Machines
Room: Ballroom I
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F1.00001: Davisson-Germer Prize in Atomic or Surface Physics Lecture: Line 'Em All Up: Macromolecular Assembly at Liquid Interfaces Invited Speaker: Geraldine Richmond Advances in our molecular level understanding of the ubiquitous fluid interface comprised of a hydrophobic fluid medium, and an aqueous solution of soluble ions and solutes has been slow until recently. This more recent upsurge in interest and progress comes from advances in both experimental and computational techniques as well as the increasingly important role that this interface is playing in such areas as green chemistry, nanoparticle synthesis, improved oil and mineral recovery and water purification. The presentation will focus on our most recent efforts in understanding (1) the molecular structure of the interface between two immiscible liquids, (2) the penetration of aqueous phase ions into the interfacial region and their effect on its properties, and (3) the structure and dynamics of the adsorption of surfactants, polymers and nanoparticles at this interface. To gain insights into these processes we use a combination of vibrational sum frequency spectroscopy, surface tension measurements using the pendant drop method, and molecular dynamics simulations. The results demonstrate that weak interactions between interfacial oil and water molecules create an interface that exhibits a high degree of molecular structuring and ordering, and with properties quite different than what is observed at the air-water interface. As a consequence of these interfacial oil-water interactions, the interface provides a unique environment for the adsorption and assembly of ions, polymers and nanoparticles that are drawn to its inner-most regions. Examples of our studies that provide new insights into the unique nature of adsorption, adsorption dynamics and macromolecular assembly at this interface will be provided. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F1.00002: Lars Onsager Prize Lecture: Statistical Dynamics of Disordered Systems Invited Speaker: Daniel S. Fisher The properties of many systems are strongly affected by quenched disorder that arose from their past history but is frozen on the time scales of interest. Although equilibrium phases and phase transitions in disordered materials can be very different from their counterparts in pure systems, the most striking phenomena involve non-equilibrium dynamics. The state of understanding of some of these will be reviewed including approach to equilibrium in spin glasses and the onset of motion in driven systems such as vortices in superconductors or earthquakes on geological faults. The potential for developing understanding of short-term evolutionary dynamics of microbial populations by taking advantage of the randomness of their past histories and the biological complexities will be discussed briefly. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F1.00003: Dannie Heineman Prize for Mathematical Physics Prize Lecture: Correlation Functions in Integrable Models: Ising Model and Monodromy Preserving Deformation Invited Speaker: Tetsuji Miwa Studies on integrable models in statistical mechanics and quantum field theory originated in the works of Bethe on the one-dimensional quantum spin chain and the work of Onsager on the two-dimensional Ising model. I will talk on the discovery in 1977 of the link between quantum field theory in the scaling limit of the two-dimensional Ising model and the theory of monodromy preserving linear ordinary differential equations. This work was the staring point of our journey with Michio Jimbo in integrable models, the journey which finally led us to the exact results on the correlation functions of quantum spin chains in 1992. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F1.00004: Dannie Heineman Prize for Mathematical Physics Prize Lecture: Correlation Functions in Integrable Models II: The Role of Quantum Affine Symmetry Invited Speaker: Michio Jimbo Since the beginning of 1980s, hidden infinite dimensional symmetries have emerged as the origin of integrability: first in soliton theory and then in conformal field theory. Quest for symmetries in quantum integrable models has led to the discovery of quantum groups. On one hand this opened up rapid mathematical developments in representation theory, combinatorics and other fields. On the other hand it has advanced understanding of correlation functions of lattice models, leading to multiple integral formulas in integrable spin chains. We shall review these developments which continue up to the present time. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F1.00005: Julius Edgar Lilienfeld Prize Lecture: Mapping the Universe: Physics Writ Large Invited Speaker: Margaret Geller The age of mapping the universe began in earnest in the late twentieth century. I will describe the enormous strides we have made in mapping the galaxy distribution and in understanding its nature and history. I will show how techniques for measuring the (primarily dark) matter distribuion in massive systems of galaxies are an astrophysical route to tests of fundamental physics. [Preview Abstract] |
Session F2: Invited Session: Low Energy Excitations in Iridates
Sponsoring Units: DCMPChair: Paolo Radaelli, University of Oxford
Room: Ballroom II
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F2.00001: Correlated phases and excitations in the iridates Invited Speaker: Leon Balents The iridium oxides form an intriguing set of materials controlled by a delicate balance of kinetic, spin-orbit, and Coulomb interaction energies. Many possible exotic phases and phenomena have been suggested for them in the literature. I will review the theoretical context for these compounds, emphasizing effects arising from the combination of strong spin-orbit coupling and electron-electron correlations. Finally, I will discuss our group's on-going efforts to understand the excitations and magnetic phases in these materials. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F2.00002: RIXS Studies of Magnetic Excitations in Layered Iridates Invited Speaker: B.J. Kim 5$d$ Transition metal oxides lie at the intersection of strong spin-orbit coupling and electron correlation, and open a new playground for novel electronic phases with unconventional magnetic, superconducting, magneto-electric, and band-topological properties. In particular, a rich variety of magnetic phases are predicted from the magnetic interactions that take various forms ranging from Heisenberg to bond-directional dipolar-like couplings in the strong spin-orbit coupling limit. In this talk, I will review on these novel aspects of magnetism in iridates studied using resonant x-ray scattering techniques. Specifically, following topics will be discussed: (i) Heisenberg-like nature of magnetic coupling in Sr$_{2}$IrO$_{4}$ that sharply contrast with the unusually large spin-wave gap in Sr$_{3}$Ir$_{2}$O$_{7}$, (ii) the origin of strong Ising anisotropy in Sr$_{3}$Ir$_{2}$O$_{7}$, and (iii) the contrasting dynamics of ``spin-orbit exciton'' modes in the Heisenberg and Ising magnets. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F2.00003: Interplay of spin-orbit coupling, correlations, and crystal anisotropy in 5d oxides Invited Speaker: Liviu Hozoi We investigate the correlated $d$-level electronic structure of $5d$ Ir and Os oxide compounds by fully {\it ab initio} quantum-chemical many-body calculations on finite embedded clusters. The wave-function quantum-chemical methods provide a promising alternative to density-functional-based approaches to the electronic structure of solids. The computed $d$-$d$ excitations in square-lattice, honeycomb, pyrochlore, and chain-like iridates compare well with recent RIXS (resonant inelastic x-ray scattering) data. We also perform a detailed analysis of the relativistic spin-orbit wave functions and compute observables such as the $\langle \mathbf{L}\!\cdot\!\mathbf{S} \rangle $ ground-state expectation value of the spin-orbit operator. The latter is in principle accessible from x-ray absorption and provides information on the role of $t_{2g}$--\,$e_g$ couplings in the ground-state wave function and on the strength of non-cubic fields that lift the degeneracy of the $t_{2g}$ levels. As concerns the departure from cubic symmetry, interesting effects are found in $A_2$Ir$_2$O$_7$ pyrochlores, where the highly anisotropic, hexagonal configuration of the adjacent $A$-site ions breaks cubic symmetry even in the absence of O-ligand trigonal distortions and moreover competes with the latter. Our findings open new perspectives in pyrochlore oxides. In 227 iridates, the outcome of this competition is decisive for the actual realization of any type of non-trivial topological ground state. In 227 spin systems with $S\!>\!1/2$, e.g., Cd$_2$Os$_2$O$_7$, this interplay decides the sign of the single-ion anisotropy and the degree of magnetic frustration. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F2.00004: Low energy excitations in iridates studied with Resonant Inelastic X-ray Scattering Invited Speaker: Xuerong Liu In the iridium oxides, the strong spin-orbit coupling (SOC) of the 5d iridium electrons entangles the orbital and spin degrees of freedom, providing opportunities for exotic magnetic states with highly anisotropic exchange interactions. At the same time, the spatially extended 5d electrons are expected to have much stronger hybridization with the oxygen 2p orbitals, comparing with that in 3d transition element compounds. Both factors make crystal symmetry and local environment crucial in determining the electronic and magnetic properties of the iridates. We present here our resonant inelastic X-ray scattering (RIXS) studies of a number of octahedrally coordinated iridates with special structures, exploring these effects. In particular, for the 1-D spin 1/2 chain compound, Sr$_3$CuIrO$_6$, the wavefunction of the hole in the t2g manifold was reconstructed based on the RIXS spectra. Our results show that it is significantly modified from the isotropic shape expected for J$_{\mathrm{eff}} = 1/2$ states in the strong SOC limit, due to the distortion of the oxygen octahedral cage. This distortion is comparable to, or smaller than, that present in most iridates and thus this work emphasizes the importance of local symmetry for the iridate families. Further, the magnetic excitations of this material were also measured. A large gap of $\sim$30 meV, was found, comparable to the magnetic dispersion bandwidth. This is in contrast to the gapless dispersion expected for linear chain with isotropic Heisenberg exchange interaction. We also studied Na$_4$Ir$_3$O$_8$ which has a hyperkagome lattice, and is a candidate quantum spin liquid. Here, a low energy continuum is observed below the d-d excitations. Optical conductivity measurements performed on the same sample and polarization dependence of the RIXS signal suggest that these excitations are magnetic in origin, agreeing with the spin-liquid state prediction. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F2.00005: Spin Dynamics in Na2IrO3 Probed by Inelastic Neutron Scattering: Implications for Kitaev Physics Invited Speaker: Radu Coldea We explore the spin dynamics in the layered antiferromagnet Na$_2$IrO$_3$, a candidate for the Kitaev spin model on the honeycomb lattice [1]. Using powder inelastic neutron scattering with an optimised setup to minimise neutron absorption by Ir we observed evidence for dispersive spin wave excitations of the Ir moments below a zone-boundary energy of 5 meV [2]. Results are compared quantitatively with predictions of a Kitaev-Heisenberg model, as well as a Heisenberg model with further neighbour couplings, both with a magnetic ground state of zig-zag ferromagnetic chains ordered antiferromagnetically. By combining single-crystal xray diffraction and ab initio calculations we propose a revised crystal structure model with significant departures from the ideal case of regular IrO6 octahedra and 90$^{\circ}$ Ir-O-Ir bonds required for large Kitaev exchanges.\\[4pt] [1] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010); arXiv:1209.5100 (2012).\\[0pt] [2] S.K. Choi, R. Coldea, A.N. Kolmogorov, T. Lancaster, I.I. Mazin, S.J. Blundell, P.G. Radaelli, Yogesh Singh, P. Gegenwart, K.R. Choi, S.-W. Cheong, P.J. Baker, C. Stock and J. Taylor, Phys. Rev. Lett. 108, 127204 (2012). [Preview Abstract] |
Session F3: Invited Session: Quantum Computing in AMO
Sponsoring Units: GQI DAMOPChair: Ivan Deutsch, University of New Mexico
Room: Ballroom III
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F3.00001: Quantum Computation with Trapped Rydberg Atoms Invited Speaker: Mark Saffman Highly excited atomic Rydberg states provide strong, long range dipolar interactions which can be used to create entanglement between atoms, between atoms and optical photons, and between atoms and microwave photons. I will review recent progress in this rapidly developing area including optical trapping of Rydberg atoms, experiments with a 2D array of qubits, and progress towards a coherent quantum interface between neutral atom and superconducting qubits. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F3.00002: Quantum computation with atomic ensembles Invited Speaker: Tommaso Calarco |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F3.00003: Hybrid quantum information processing Invited Speaker: Akira Furusawa There are two types of schemes for quantum information processing (QIP). One is based on qubits, and the other is based on continuous variables (CVs), where the computational basis for qubit QIP is \{$|0 \rangle$, $|1 \rangle$\} and that for CV QIP is \{$|x \rangle$\} ($-\infty < x < \infty$). A universal gate set for qubit QIP is \{`bit flip'$\sigma_x$, `phase flip'$\sigma_z$, `Hadamard gate'H, `$\pi/8$ gate', `controlled NOT (CNOT) gate'\}. Similarly, a universal gate set for CV QIP is \{`$x$-displacement'$\hat{D}(x)$, `$p$-displacement'$\hat{D}(ip)$, `Fourier gate'$\hat{F}$, `cubic phase gate'$e^{ik\hat{x}^3}$, `quantum non-demolition (QND) gate'\}. There is one-to-one correspondence between them. CV version of `bit flip'$\sigma_x$ is `$x$-displacement'$\hat{D}(x)$, which changes the value of the computational basis. Similarly, CV version of `phase flip'$\sigma_z$ is `$p$-displacement'$\hat{D}(ip)$, where `phase flip'$\sigma_z$ switches the ``value'' of `conjugate basis' of qubit \{$|+ \rangle,|- \rangle$\} ($|\pm \rangle = (|0 \rangle \pm |1 \rangle)/\sqrt{2}$) and `$p$-displacement'$\hat{D}(ip)$ changes the value of CV conjugate basis \{$|p \rangle$\}. `Hadamard' and `Fourier' gates transform computational bases to respective conjugate bases. CV version of `$\pi/8$ gate' is a `cubic phase gate'$e^{ik\hat{x}^3}$, and CV version of CNOT gate is a QND gate. However, the origin of nonlinearity for QIP is totally different, here the very basic nonlinear operation is calculation of multiplication and of course it is the heart of information processing. The nonlinearity of qubit QIP comes from a CNOT gate, while that of CV QIP comes from a cubic phase gate. Since nonlinear operations are harder to realize compared to linear operations, the most difficult operation for qubit is a CNOT gate, while the counter part, a QND gate, is not so difficult. CNOT and QND gates are both entangling gates, it follows that creating entanglement is easier for CV QIP compared to qubit QIP. Here, creating entanglement is the heart of QIP. So, it is a big advantage of CV QIP. On the other hand, the fidelity of CV QIP is not so high because perfect fidelity needs infinite energy, which comes from the infinite dimensionality of CV QIP. To overcome the difficulty, ``hybrid" approach is proposed. In this approach, qubits are used as inputs for CV QIP. It is possible because qubits can be regarded as a special case of CVs. So, we can circumvent the infinite dimensionality problem of CV QIP by using qubits as the inputs. The basic example is qubit teleportation with a CV teleporter, where the qubit is a so-called ``dual-rail'' qubit with a single photon; $c_0 |1,0 \rangle + c_1 |0,1 \rangle$. We recently succeeded in creating time-bin qubits with single photons, and now we are working on the teleportation experiment with the technology developed for teleportation of highly nonclassical wave packets of light. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F3.00004: Photonic quantum technologies Invited Speaker: Jeremy O'Brien Of the approaches to quantum computing [1], photons are appealing for their low-noise properties and ease of manipulation [2], and relevance to other quantum technologies [3], including communication, metrology [4] and measurement [5]. We report an integrated waveguide approach to photonic quantum circuits for high performance, miniaturization and scalability [6--10]. We address the challenges of scaling up quantum circuits using new insights into how controlled operations can be efficiently realised [11], demonstrating Shor's algorithm with consecutive CNOT gates [12] and the iterative phase estimation algorithm [13]. We have shown how quantum circuits can be reconfigured, using thermo-optic phase shifters to realise a highly reconfigurable quantum circuit [14], and electro-optic phase shifters in lithium niobate to rapidly manipulate the path and polarisation of telecomm wavelength single photons [15]. We have addressed miniaturisation using multimode interference architectures to directly implement NxN Hadamard operations [16], and by using high refractive index contrast materials such as SiO$_{\mathrm{x}}$N$_{\mathrm{y}}$, in which we have implemented quantum walks of correlated photons [17], and Si, in which we have demonstrated generation of orbital angular momentum states of light [18]. We have incorporated microfluidic channels for the delivery of samples to measure the concentration of a blood protein with entangled states of light [19]. We have begun to address the integration of superconducting single photon detectors [20] and diamond [21,22] and non-linear [23,24] single photon sources. Finally, we give an overview of recent work on fundamental aspects of quantum measurement, including a quantum version of Wheeler's delayed choice experiment [25].\\[4pt] [1] TD Ladd, \textit{et al} \textbf{\textit{Nature }}\textbf{464}, 45 (2010) [2] JL O'Brien, \textbf{\textit{Science}}\textbf{ 318}, 1567 (2007) [3] JL O'Brien, A Furusawa, J Vuckovic \textbf{\textit{Nature Photon.}}\textbf{ 3}, 687 (2009 [4] T Nagata, \textit{et al} \textbf{\textit{Science}}\textbf{ 316}, 726 (2007) [5] R Okamoto, \textit{et al} \textbf{\textit{Science}}\textbf{ 323}, 483 (2009) [6] A Politi, \textit{et al} \textbf{\textit{Science }}\textbf{320}, 646 (2008). [7] A Laing, \textit{et al} \textbf{\textit{Appl. Phys. Lett.}}\textbf{ 97}, 211109 (2010) [8] JCF Matthews, \textit{et al} \textbf{\textit{Nature Photon.}}\textbf{ 3}, 346 (2009) [9] A Politi, \textit{et al} \textbf{\textit{Science}}\textbf{ 325}, 1221 (2009) [10] JCF Matthews, \textit{et al} \textbf{\textit{Phys. Rev. Lett.}} \quad \textbf{107}, 163602 (2011) [11] X-Q Zhou, \textit{et al} \textbf{\textit{Nature Comm. }}\textbf{2} 413 2011 [12] E Mart\'{\i}n-L\'{o}pez, \textit{et al} \textbf{\textit{Nature Photon. }}\textbf{6}, 773 (2012) [13] X-Q Zhou, \textit{et al} arXiv:1110.4276 [14] PJ Shadbolt, \textit{et al }\textbf{\textit{Nature Photon.}} \textbf{6}, 45 (2012). [15] D. Bonneau, \textit{et al.} \textbf{\textit{Phys. Rev. Lett.}}, 108, 053601 (2012) [16] A Peruzzo, \textit{et al} \textbf{\textit{Nature Comm.}}\textbf{ 2,} 224 (2011) [17] A Peruzzo, \textit{et al} \textbf{\textit{Science }}\textbf{329}, 1500 (2010) [18] X Cai, \textit{et al }\textbf{\textit{Science}} \quad \textbf{338}, 363 (2012) [19] A Crespi, \textit{et al} \textbf{\textit{Appl. Phys. Lett. }}\textbf{100}, 233704 (2012) [20] CM Natarajan, \textit{et al} \textbf{\textit{Appl. Phys. Lett.}}\textbf{ 96}, 211101 (2010) [21] JP Hadden, \textit{et al }\textbf{\textit{Appl. Phys. Lett.}}\textbf{ 97}, 241901 (2010) [22] L Marseglia, \textit{et al} \textbf{\textit{Appl. Phys. Lett. }}\textbf{98}, 133107 (2011) [23] C. Xiong, \textit{et al.} \textbf{\textit{Appl. Phys. Lett.}}\textbf{ 98}, 051101 (2011) [24] M. Lobino, \textit{et al}, \textbf{\textit{Appl. Phys. Lett. }}\textbf{99}, 081110 (2011) [25] E. Engin, \textit{ et al.} arXiv:1204.4922 [25] A. Peruzzo, \textit{et al} \textbf{\textit{Science}} \textbf{338}, 634 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F3.00005: Quantum information processing with trapped ions Invited Speaker: John Gaebler Trapped ions are one promising architecture for scalable quantum information processing. Ion qubits are held in multizone traps created from segmented arrays of electrodes and transported between trap zones using time varying electric potentials applied to the electrodes. Quantum information is stored in the ions' internal hyperfine states and quantum gates to manipulate the internal states and create entanglement are performed with laser beams and microwaves. Recently we have made progress in speeding up the ion transport and cooling processes that were the limiting tasks for the operation speed in previous experiments. We are also exploring improved two-qubit gates and new methods for creating ion entanglement. [Preview Abstract] |
Session F4: Industrial Physics Forum: Frontiers in Biophysics
Sponsoring Units: FIAPChair: James Hollenhorst, Agilent Technologies
Room: Ballroom IV
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F4.00001: Biophysical Variables Which Are Available from Single-Molecule Optical Studies Invited Speaker: W.E. Moerner Since the first optical detection and spectroscopy of a single molecule in a condensed phase host in 1989, a wealth of new information has been obtained from time-dependent measurements and single-molecule probability distributions. When single-molecule imaging is combined with active control of the emitter concentration, enhanced spatial resolution well beyond the optical diffraction limit can be obtained for a wide array of biophysical structures in cells. Single-molecule emitters also provide precise and accurate 3D position as well as dipole moment orientation when combined with Fourier plane processing. Examples here include the implementation of a double-helix point spread function for 3D position information (Backlund, Lew et al. PNAS (2012)), and the creation of a quadrated pupil response to sense emission dipole orientations (Backer et al. submitted 2012). If high-resolution spatial information is not needed, a machine called the Anti-Brownian ELectrokinetic (ABEL) trap provides real-time suppression of Brownian motion for single molecules in solution for extended analysis of dynamical state changes (Wang et al. Acc. Chem. Res. (2012)). With proper design of reporter fluorophore, individual electron transfer events to a single Cu atom in a redox enzyme may be sensed under turnover conditions (Goldsmith et al. PNAS (2011)). Optical counting of fluorescent ATP nucleotides on a multisubunit enzyme provides measurement of ATP number distributions, which can be used to generate a new window into enzyme cooperativity devoid of ensemble averaging (Jiang et al PNAS (2011)). With advanced control system design of feedback to enable optimal trapping performance, the ABEL trap also allows direct, simultaneous measurement of three variables: brightness, excited state lifetime, and emission spectrum, for objects as small as individual $\sim$1-2 nm sized fluorophores in solution (Wang et al. JPCB (in press 2013)). These examples illustrate some of the wide variety of physical variables which may now be measured for single molecules in a various condensed phase environments ranging from aqueous solutions to living cells. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F4.00002: Accounting for conformational flexibility when targeting proteins Invited Speaker: Sara Nichols Molecular simulation techniques are well-established tools for understanding protein motion and complementing experimental observations. Molecular conformations from such simulations provide insight into receptor flexibility, particularly with respect to the binding of activity-modulating molecules, such as drugs. With the ultimate goal of predicting and designing these favorable interactions, incorporating information about flexibility can enhance structure-based drug design. While modeling all receptor degrees of freedom can be challenging due to conformational space sampling restrictions, advances in computing technology, hybrid and hierarchical protocol, as well as enhanced sampling algorithms are making an impact now, and will continue to do so in the future. An overview of these topics and applications to specific therapeutic targets will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F4.00003: Changing Chasses and Inventing Elements: Developing a Combined Systems Biology and Engineering Approach to Designing Complex Function in Cells Invited Speaker: Adam Arkin To meet the goal of creating reliable, predictable, efficient, and transparent methods to harness cellular capabilities for human benefit, it is necessary both to have standard libraries of elements from which useful pathways can be constructed and an understanding of the how host physiology and the environment impacts the functioning of these heterologous circuits. We show how variations in cellular and environmental context affect the operation of the basic central dogma functions underlying gene expression. Then we describe progress on creating a complete, scalable, and relatively homogeneous and designable sets of part families that can control central dogma function predictably in the face of varying configurations, genetic contexts, and environments. We show the challenges that arise in attempting this in applications such as a tumor destroying bacteria. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F4.00004: Frontiers of Biophysics: Single Molecule and Single Cell Sensing with Nanomechanical Systems Invited Speaker: Michael Roukes Nanoelectromechanical systems (NEMS) resonators can detect inertial mass with exceptional sensitivity. We have used NEMS devices to realize a new method for single-molecule mass spectrometry. In our first-generation approach, mass spectra from several hundred adsorption events were assembled into mass spectra using statistical analysis. Our second-generation approach now enables NEMS-based mass spectrometry (MS) in real time: as each molecule in the sample adsorbs upon the NEMS resonator, its mass and position-of-adsorption are determined by continuously tracking two driven vibrational modes of the device. We demonstrate the potential of this method by analyzing individual IgM antibody complexes and other biological analytes in real-time. NEMS-MS is a unique and promising new form of mass spectrometry: it can resolve neutral species, provides resolving power that increases markedly for very large masses, is readily scalable to millions of channels, and is and producible \textit{en masse} by methods from the semiconductor industry for very-large-scale integration. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F4.00005: Cytometry and Atomic Mass Spectrometry Converge in Single Cell Deep Profiling of the Human Immune System Invited Speaker: Scott Tanner Mass cytometry addresses the challenges of polychromatic flow cytometry by capitalizing on the analytical benefits of atomic mass spectrometry. Fluorescence flow cytometry has helped to define the cell subsets of the immune system. The addition of intracellular staining facilitated examination of signaling networks and, recently stratification of patients correlated with clinical outcome. However, the potential for further advances has been stymied by the physical and spectral limitations of fluorophores. This technical barrier has been broached by replacing fluorophores with heavy metal isotopes, and optical detection with atomic mass spectrometry. Antibodies raised against phenotypic and functional proteins are tagged with polymers that are labeled with the transition metal isotopes. More than 30 lanthanide isotopes, complemented by noble metals, permit the immunological recognition of more than 40 (and conceptually up to 100) proteins in single cells simultaneously. Individual cells are injected at nearly 1 kHz into an Inductively Coupled Plasma where the cells are vaporized, atomized and ionized. The reporting ions within the vaporization cloud of each cell are extracted, separated and counted by a time-of-flight mass spectrometer. The data output is a massively multivariate signature of each cell. Already the technology has offered dramatic new insights into the operation and function of the human hematopoietic hierarchy, shown novel application for the screening and mechanistic understanding of drug candidates, and foresees improved prognostic and diagnostic application in the clinic. We will report on our work, and the work of others, in profiling the signaling and functional responses of the suite of cell populations in human bone marrow, the revealing of unappreciated levels of organization in virus-specific memory T cell compartments, and massively multiplexed single-cell kinase inhibitor profiling. [Preview Abstract] |
Session F5: Focus Session: Computational Discovery and Design of New Two-dimensional Materials beyond Graphene
Sponsoring Units: DMP DCOMPChair: Richard Hennig, Cornell University
Room: 301
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F5.00001: Electronic and magnetic properties of 2D BCN nanostructures Invited Speaker: Hyoungki Park Recent developments of two-dimensional (2D) nanomaterials hold great promises for future electronics, optics and spintronics. Since the isolation and electronic characterization of graphene, other layered 2D crystals also have been synthesized. In particular, carbon can be combined with its neighboring atoms in the periodic table, boron and nitrogen as hexagonal BN (h-BN), to obtain hybrid BCN configurations. These BCN 2D nanostructures show a rich variety of physical properties, distinct from parent materials. Their electronic properties can in principle be tuned by varying the concentration of each of the three elements. We study electronic structures of a variety of 2D BCN nanostructures using hybrid functional HSE in density functional theory (DFT). We show that their electronic properties can be gradually tuned by composition and the atomic configuration of three elements. We demonstrate that the substitution-induced impurity states, associated with carbon atoms, and their interactions dictate the electronic structure and properties of C-doped h-BN. Stacking of localized impurity states in small C clusters embedded in h-BN forms a set of discrete energy levels in the wide gap of h-BN, leading to electronic structures of quantum dots made of carbon nano-domains for applications in optics and opto-electronics. We also show that half-metallic electron transport can be achieved by low concentration substitutional doping of only one sublattice of graphene by nitrogen or boron atoms. The delocalized spin-densities induced by the unpaired electrons at substitutional sites permeate only through the sublattice where the nitrogen (boron) atoms belong. For interacting nitrogen (boron) atoms located along the ``zigzag'' direction and in the same sublattice the ferro-magnetic spin-ordering is energetically favored, and substitution-induced impurity states selectively disturb the spin-polarized $\pi$-orbital of that same sublattice. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F5.00002: Electronic Structures of Single-Layer Boron Pnictides Houlong L. Zhuang, Richard G. Hennig Single layered materials such as graphene and boron nitride promise alternative routes to electronic devices. We use density-functional calculations to identify potential novel 2D materials in the boron pnictide family and determine their stability and electronic properties.\footnote{H. L. Zhuang and R. G. Hennig. Appl. Phys. Lett., \textbf{101}, 153109 (2012)}Hybrid density functional calculations show that BN, BP, BAs and BSb in this family exhibit a direct bandgap of 6.1, 1.4, 1.2 and 0.6 eV, respectively, that originates from the energy difference of the $p_z$ orbitals of the species and is tunable by strain. The bandgap linearly decreases with strain for BN, while it increases non-linearly for BP, BAs, and BSb. The calculated natural band offsets between the various boron pnictides are all of type I. We expect that these results will provide valuable guidance in designing electronic devices based on single-layer boron pnictides. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F5.00003: Impurity induced states in monolayer hexagonal BN Susumu Saito, Yoshitaka Fujimoto, Takashi Koretsune Ever since the experimental production of graphene, it has attracted much attention as a future device material with monoatomic-layer thickness although the material has metallic electronic transport properties. In this respect, a monoatomic layer of hexagonal boron nitride (hBN) can be even more interesting device material to be used in the future since it possesses semiconducting electronic properties with the fundamental energy gap. We study the electronic properties of the hBN monolayer in the framework of the density-functional theory and the many-body theory with Hedin's GW approximation. Both donor and acceptor-type states induced by the substitutional C impurity atom at B and N sites respectively are studied in detail. In addition, we also study the impurity states induced by the substitution of the cluster of atoms in hBN by the graphene flake. These impurity states are found to be generally rather deep, and therefore we discuss the possible methods to change the ionization energies of these impurity-induced states [1]. \\[4pt] [1] Y. Fujimoto, T. Koretsune, and S. Saito, to be published. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F5.00004: Ab initio study of the buckling on silicene and germanene Edgar Martinez-Guerra, Karla Hern\'andez, Eduardo Cifuentes-Quintal, Romeo de Coss Recently, a new graphene-like silicon structure was discovered: silicene. Since its discovery, silicene has been more exciting than graphene because this is a semiconductor and it should be compatible with silicon-based electronic. Silicon and germanium atoms have similar electronic configurations as those of carbon and this the reason that the bandstructure of silicene and germacene exhibits the Dirac cones at K point, with a very similar linear dispersion around it, like in graphene. The disvintage is that sp$^{\mathrm{2}}$ bonded Si is much less stable than for carbon resulting that to be stable in the planar layer their atoms must buckle. In this work, we calculated the sp character on silicene and germacene to correlate its hibridization with the velocity of electrons and holes at Dirac cones. The calculations were performed using the pseudopotential LCAO method with GGA for the exchange-correlation energy functional. The buckling of silicene and germacene layer was 0.50 and 0.69 {\AA}, respectively. In addition, the sp- character of silicene and germacene buckled was 2.33 and 2.64, respectively. Thus, a detailed analysis on the electronic band structure of these system show that as sp character goes from sp2 to sp3 it is correlated with a decrease of velocity of electrons and holes at Dirac cones. This study is primarly important and it could address a new future to modulate carrier velocities on bidimensional systems. This research was supported by Conacyt under Grant No. 133022. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F5.00005: Properties of silicene on graphene Lok Lew Yan Voon, Ruiping Zhou, Yan Zhuang Silicene, the silicon analog of graphene, was first shown by one of the authors in 2007 to have similar properties to graphene. Three groups have reported the fabrication of silicene on metal in Phys. Rev. Lett. in 2012. In this talk, we will present results on the structure and properties of silicene on graphene obtained from ab initio calculations. A new structure of bilayer silicene on graphene is obtained. The band structure reveals a phenomenon of self-doping. Finally, the application of a transverse electric field and I-V characteristics will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F5.00006: Gated Silicene as a tunable source of nearly 100\% spin-polarized electrons Wei-Feng Tsai, Cheng-Yi Huang, Tay-Rong Chang, Hsin Lin, Horng-Tay Jeng, Arun Bansil We demonstrate, via first-principles calculations, that gated silicene with a low-buckled honeycomb structure posseses two gapped Dirac cones with nearly full spin-polarization at the corners of the Brillouin zone. By using this key finding, we further propose a design of a silicene-based spin-filter to switch the output spin current simply by gating without the need to switch magnetic domains. Quantum transport calculations indicate that such designs will be highly efficient (nearly 100\% spin-polarized) and robust against weak disorder and edge imperfections. We also propose a Y-shaped spin/valley separator that produces spin-polarized current at two output terminals with opposite spins. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F5.00007: Density functional investigation of epitaxial silicene on semiconducting substrates G.P. Das, A. Bhattacharya, S. Bhattacharya In spite of the uniqueness of carbon to form pristine fullerene, nanotube and graphene, there have been attempts to replicate these nanostructures with silicon. Most recently, the free-standing quasi-2D honeycomb structure of silicene has been predicted to be stable with linear band dispersion and Dirac cone feature similar to graphene. Epitaxial silicene on Ag(110) and on ZrB$_{2}$(0001) substrates have already been reported [1,2]. We have carried out first principles density functional investigation of the structural and electronic properties of silicene monolayer on various wurzite structured III-V and II-VI semiconducting substrates, with metal terminated (MT) as well as non-metal terminated (NMT) top surface [3]. The binding energies of silicene on MT semiconductors are in the range 0.5 - 0.7 eV/atom and their behavior can be metallic, semi-metallic or even magnetic, depending on the choice of substrates. The silicene overlayer undergoes n-/p-type doping on MT/NMT semiconductor surface, depending upon the direction of the charge transfer. [1] P. Vogt, et al, Phys. Rev. Lett. \textbf{108} (2012) 155501. [2] A. Fleurence et al, Phys. Rev. Lett. \textbf{108} (2012) 245501 [3] A. Bhattacharya et al,, to be published. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F5.00008: Physisorption of nucleobases on silicene and applications for DNA sequencing Rodrigo Amorim, Ralph Scheicher We have used density functional theory including van der Waals corrections combined with the non-equilibrium Green's function (NEGF) method to study the adsorption of individual nucleobases on top of a 2-D allotrope of silicon, known as silicene, which was experimentally discovered to exist in a hexagonal buckled form. Our study focused on the stability, electronic properties and transverse electronic transport, i.e., changes in the transmission and the conductance caused by each base (A, C, G, T) in silicene compared to its pristine form. Intriguingly, despite the weak interaction between nucleobases and silicene, considerable changes in the transmittance at zero bias are predicted by us. This opens up the possibility to utilize silicene as an integrated-circuit biosensor as part of a lab-on-a-chip device. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F5.00009: Normal Compressive Strain Induced Metallic Transition of Semiconducting Bilayer Transition Metal Dichalcogenides Abhishek Singh, Swastibrata Bhattacharyya First principle density functional theory based calculation was carried out to investigate the effect of strain on the band gap of bilayer semiconducting transition metal dichalcogenides (TMDs). The band gap of these materials was observed to decrease smoothly with the application of normal compressive strain. Most importantly, the materials exhibit semiconductor to metal (S-M) transition after a critical pressure (inter-layer distance) is reached. This critical pressure varies with the type and stacking pattern of the material. The S-M transition is attributed to lifting of degeneracy of the bands at the fermi level caused by inter-layer interactions via charge transfer from metal to chalcogens. The GGA result was validated by incorporating the band gap corrections using hybrid functionals and GW method. The tuning of band gap of TMDs by applying normal compressive strain opens a possibility to use these materials in various applications of nanoelectronics such as electromechanical sensors, switches etc. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F5.00010: First-principles study of electric field effect on GaN bi- and trilayers Dongwei Xu, Haiying He, Ravindra Pandey, Shashi P. Karna First-principles calculations based on density functional theory (DFT) are performed to study bilayers and trilayers of GaN. The calculated results suggest that the bi- and trilayer systems both prefer planar graphene-like configurations rather than buckled bulk-like configurations in their ground states. The most stable configurations are predicted to be the so-called AA$'$ stacking for the bilayer and the AA$'$A stacking for the trilayer at the GGA-DFT level of theory. By appling an external perpendicular electric field to the AB-stacked bilayer, its band gap increases monotonically. However, this is not case for the symmetric AA$'$ stacked bilayer, ABA or AA$'$A stacked trilayer where the applied electric field reduces the band gap. Furthermore, a semiconductor-metal transition is predicted for the ABA stacked GaN trilayer at about 0.4 V/ {\AA}. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F5.00011: Interfaces between buckling phases in Silicene Matheus P. Lima, Ant\^onio J.R. da Silva, Adalberto Fazzio Silicene has a honeycomb buckled lattice, with two energetically degenerate geometric phases ($\alpha$ and $\beta$). The $\alpha$ phase has one atom shifted up, and its neighbors shifted down, whereas the $\beta$ phase the shifts are reversed. Some consequences of this buckling pattern are: i) the increase of spin-orbit coupling, thus enhancing the Quantum Spin Hall Effect; ii) potential to tune several properties with the application of an external electric field. Therefore, the understanding of the effects caused by this buckling is crucial to fully explore the potential of this material. In this work we performed simulations based on Density Functional Theory to investigate the co-existence of the $\alpha$ and $\beta$ phases in the same sample. We show that: i) This phase inversion is stable in the zigzag and armchair directions, and can make curves, allowing the formation of islands; ii) The formation energy per unit length is approximately $0.02~eV/$Ang; iii) The modifications caused in the Density of States (DOS) are small, and appear $0.5~eV$ below the Fermi energy. Finely, we show how these linear defect will appear in Scanning Tunneling Microscopy (STM) experiments. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F5.00012: Electron-Phonon Coupling in Two-Dimensional Germanene Ryan Stein, David Schaefer, Jia-An Yan The phonon properties of the two-dimensional honeycomb allotrope of germanium, germanene, were studied by first-principles calculations. We found that the highest optical branches on the phonon dispersions at $\Gamma$ and K symmetry points of the first Brillouin zone exhibit similar behavior as in graphene and graphite, indicating possible Kohn anomalies in germanene. Electron-Phonon coupling for the high symmetric modes will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F5.00013: Optical signatures of valley-spin coupling in graphene-like materials: silicene and germanene E.J. Nicol, C.J. Tabert, L. Stille With the success of graphene and the development of the field of two-dimensional crystals, other graphene-like materials are now of interest, such as, monolayers of silicon (silicene) and germanium (germanene). The interplay of spin orbit coupling, due to the buckled structure of these materials, and a perpendicular electric field is predicted to give rise to a rich variety of phases via an electrically tunable band gap [1,2]. These span a topological or quantum spin Hall insulator, a valley-spin-polarized metal and a band insulator [2]. We have calculated the dynamical conductivity [3] and show that it should reveal signatures of these different phases which would allow for their identification along with the determination of parameters such as the spin orbit energy gap. Furthermore, the effect of spin-valley coupling can be seen in the response to circularly polarized light as a function of frequency. Using right- and left-handed circular polarization it is possible to select a particular combination of spin and valley index. The frequency for this effect can be varied by tuning the band gap.\\[4pt] [1] N.D. Drummond, V. Zolyomi, V.I. Fal'ko, PRB 85, 075423 (2012).\\[0pt] [2] M. Ezawa, New J. Phys. 14, 033003 (2012).\\[0pt] [3] L. Stille, C.J. Tabert, E.J. Nicol, PRB 86, 195405 (2012). [Preview Abstract] |
Session F6: Focus Session: Van der Waals Bonding in Advanced Materials - Functional Materials
Sponsoring Units: DMPChair: Per Hyldgaard, Chalmers University of Technology
Room: 302
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F6.00001: Active Pharmaceutical Ingredients: Prediction of Physical-Chemical Properties from First Principles Loredana Valenzano Polymorphism in active pharmaceutical ingredients (APIs) plays a crucial role both for medical and intellectual property concerns but despite ongoing efforts, experimental and computational investigations of the existence and the physical-chemical properties of the same compound in different forms is still an open question.While comparison between computed and experimental values for properties derived from differences between states is often promising (such as bulk modulus), results are disappointing for absolute values (such as density). Quantum mechanical computational methods describe the systems at 0K, experimentally properties are often evaluated at room temperature. Therefore it is not surprising that results determined from first principles dramatically differ from those obtained experimentally. By applying a quantum mechanical periodic approach that takes into account long range London dispersion forces fitted for solid materials, and by imposing different cell volumes corresponding to different thermodynamic conditions, we show how results from calculations at 0K (structures, vibrational spectra, elastic constants) may be compared to experimental values at higher temperatures, helping to foster a stronger linkage between computational and experimental work on systems such as APIs. Where experimental results are not available, our work represents an innovative approach in addressing the properties of APIs. Our results can also serve as foundation for the developing of new force fields to be adopted within a multi-scale computational approach. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F6.00002: Many-Body van der Waals Effects in Advanced Materials Alexandre Tkatchenko, Anatole von Lilienfeld, Robert A. DiStasio Jr. Van der Waals (vdW) interactions are ubiquitous in molecules and condensed matter. These interactions are inherently quantum mechanical phenomena that arise from concerted correlations between many electrons within a given molecular system. Despite this fact, the vast majority of theoretical calculations include long-range vdW interactions based on a simple effective interatomic pairwise model. We introduce an efficient method that accurately describes the full long-range many-body vdW energy [1,2], and demonstrate that many-body contributions can significantly exceed the highly coveted ``chemical accuracy''. Cases studied include intermolecular binding energies, the conformational hierarchy of DNA structures [2], the geometry and stability of molecular crystals [1], and supramolecular host--guest complexes [3]. Our findings suggest that inclusion of the many-body vdW energy is essential for achieving chemical accuracy and therefore must be accounted for when studying advanced materials. [1] Tkatchenko, DiStasio, Car, Scheffler, PRL (2012), [2] DiStasio, von Lilienfeld, Tkatchenko, PNAS (2012), [3] Tkatchenko, Alfe, Kim, JCTC (2012). [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F6.00003: First-Principles Calculations of the Role of PVP in the Controlled Synthesis of Au Nanostructures Shih-Hsien Liu, Wissam Al-Saidi, Kristen Fichthorn Structure-directing agents such as PVP play an important role in determining the shape of metal nanostructures in solution-phase syntheses. It is usually hypothesized that structure-directing molecules bind more strongly to certain crystal facets, which grow at the expense of facets on which they are less strongly bound. In this study, we use dispersion-corrected density functional theory to resolve the role of PVP in the shape-selective synthesis of Au nanostructures. We calculate binding energies for the 2-pyrrolidone ring of PVP on Au(111), (5 $\times$ 1) Au(100)-hex, and Au(100) slabs in vacuum. The results show that there is no significant difference between the binding of 2-pyrrolidone to Au(111) and Au(100)-hex, while 2-pyrrolidone binds more strongly to Au(111) than to Au(100). We discuss the origins of these trends. Our results are consistent with experiments, in which (111)-faceted Au nanostructures are formed with the assistance of PVP. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F6.00004: van der Waals Density Functional Studies of Gas Binding and Transport in Zeolitic Imidazolate Frameworks Keith Ray, David Olmsted, Ning He, Yao Houndonougbo, Brian Laird, Mark Asta Gas adsorption selectivity and transport barriers in a series of Zeolitic Imidazolate Frameworks (ZIFs) are calculated with the van der Waals density functional [1]. In these microporous materials, promising for natural gas upgrading applications, CO2 molecules are found to preferentially adsorb [2] when compared with CH4 depending on the ZIF chemical functionalization. The role of the interaction between the CO2 quadrupole and the host framework, as well as the significant dispersion contribution to both CO2 and CH4 binding are discussed. Diffusion barriers are calculated with the nudged elastic band method (NEB) and results are found to depend on the inclusion of the van der Waals energy.\\[4pt] [1] M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, B. I. Lundqvist, Phys. Rev. Let. 92, 246401 (2004)\\[0pt] [2] K. G. Ray, D. Olmsted, N. He, Y. Houndonougbo, B.B. Laird, M. Asta, Phys. Rev. B 85, 085410 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F6.00005: Self-assembly of functionalized anthradithiophene on Au(111) Brad Conrad, Shawn Huston, Jiuyang Wang, Marsha Loth, John Anthony, Daniel Dougherty We utilize scanning tunneling microscopy (STM) to characterize the initial growth and crystallization of the high-performance, small organic molecule 2,8-difluoro-5,11-triethylsilylethynyl (diF TESADT) on Au(111). Two ordered structures are observed with diF TESADT backbone planes parallel to the substrate. Submolecular resolution imaging of the first monolayer ordered film regions realizes structures with close approach of fluorine-sulfur and fluorine-fluorine atoms of alternating molecules. These measurements provide evidence for the importance of non-covalent F-S and F-F interactions in driving 2D self-assembly. Scanning Tunneling Spectroscopy indicates a 2.4 eV transport gap which is insensitive to the local domain. Structures and growth are put in context of bulk measurements and device performance measurements. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F6.00006: Using NMR to study small molecule adsorption in metal organic frameworks M.G. Lopez, P. Canepa, T. Thonhauser We calculate the carbon nuclear magnetic resonance (NMR) chemical shift for the CO$_2$ molecule and the hydrogen shift for both H$_2$ and H$_2$O inside the metal organic framework structure Mg-MOF74 using \emph{ab initio} calculations at the density functional theory level\footnote{T. Thonhauser et al., J. Chem. Phys. \textbf{131}, 101101 (2009).}$^,$\footnote{C. J. Pickard and F. Mauri, Phys. Rev. B {\bf 63}, 245101 (2001).} with the van der Waals density functional (vdW-DF).\footnote{M. Dion et al., Phys. Rev. Lett. \textbf{92}, 246401 (2004).} These shifts are obtained while placing the small molecules throughout the structure, including the calculated adsorption site for various loading scenarios. Our binding energy results agree well with previous experiments and calculation, and the NMR calculations show that it is reasonable to expect an experimentally observable change in the chemical shift depending on adsorbant, position, and loading. By providing this mapping of chemical shift to position and loading for these adsorbants, we argue that NMR probes could be used to provide information about the position at which these small molecules bind within the MOF and provide information about the loading of the adsorbed molecule. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F6.00007: Spectroscopic studies of van der Waals bonding and interactions in microporous materials Invited Speaker: Yves Chabal Van der Waals interactions govern the interaction of gas phase molecules in microporous materials. New theoretical approaches, such as DF-vdW methods, have brought great insight into the results of vdW forces on the adsorption and diffusion properties of molecular guests. In this talk, we highlight the role of vibrational spectroscopies (infrared and Raman) in providing information that can directly test such theoretical approaches. Typically, vdW interactions lead to measurable shifts in molecular internal modes, which can be calculated. We also show that vdW interactions often lead to minor structural alteration or reconfiguration of the microporous hosts, which can clearly be observed by IR or Raman spectroscopy. Examples will be taken from molecular hydrogen storage and gas phase separation in Metal Organic Framework materials, which represent a versatile class of porous materials. For example, the origin of interesting ``gate opening'' phenomena in flexible MOFs, leading to highly selective adsorption, will be described. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F6.00008: Van der Waals density functional study of water binding in metal-organic frameworks Kyuho Lee, Berend Smit, Jeffrey B. Neaton Metal-organic frameworks (MOFs) are promising candidate materials for gas storage, gas separation and catalysis. However, MOFs are vulnerable to humid air and effective surface area drops dramatically on an exposure to water [1]. In this theoretical study, we investigate the interaction of single water molecule with MOF-74 on different binding sites by using van der Waals density functionals. We also explore how different type of metal cations affect the interaction.\\[4pt] [1] S. S. Kaye, A. Daily, O. M. Yaghi and J. R. Long, J. Am. Chem. Soc. \textbf{129}, 14176 (2007). [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F6.00009: Understanding CO$_{2}$/N$_{2}$ Selectivity and Binding in MOFs Using Dispersion-Corrected DFT Joshua Howe, Kyuho Lee, Berend Smit, Jeffrey Neaton Metal-organic frameworks (MOFs) are a class of highly ordered, highly customizable nanoporous materials that are attractive for use in energy-relevant gas separations. MOF-253 (AlOH)(bpydc) can be post-synthetically modified by introduction of metal cations and charge-stabilizing anions [1]. Post-synthetically modified MOF-253 samples have been shown to exhibit enhanced CO$_{2}$/N$_{2}$ selectivity over the unmodified framework [1]. Here we focus on the following series of post-synthetic modifications: ~CoCl$_{2}$, CuCl$_{2}$, FeCl$_{2}$, NiCl$_{2}$, PdCl$_{2}$. We use the vdW-DF, vdW-DF2, and DFT-D2 dispersion-corrected density functional theory (DFT) methods to study CO$_{2}$ and N$_{2}$ binding trends in this series of modified frameworks. Particular focus is paid to examining the predictive power of our calculations on both the modified framework and modified bipyridine clusters as a proxy for the full framework. Additionally, we examine the suitability of an approximate Henry coefficient model to predict measured gas selectivity trends [1]. \\[4pt] [1] E. Bloch, et. al, J. Am. Chem. Soc., 132, 14382-14384, 2010. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:48AM |
F6.00010: The Role of Many-Body Dispersion Interactions in Molecular Crystals Invited Speaker: Noa Marom The structure, energetics, and electronic properties of molecular crystals are studied using density functional theory (DFT) with the recently developed many-body dispersion (MBD) method [Tkatchenko et al. Phys. Rev. Lett. 108, 236402 (2012)]. It is shown that accounting for the long-range electrostatic screening in extended systems is essential for obtaining the correct dielectric constants and ensuing optical properties of molecular crystals [Schatschneider et al., arXiv:1211.1683]. Furthermore, accounting for the non-additive many-center dispersion interactions is crucial for obtaining a highly accurate description of the energetics of molecular crystals. This includes lattice energies, sublimation enthalpies [Reilly et al., to be published], and relative stabilities of polymorphs [Marom et al. arXiv: 1210.5636] \\[4pt] In collaboration with Leslie Leiserowitz, Weizmann Institute of Science, Israel; Bohdan Schatschneider, The Pennsylvania State University, Fayette; Robert DiStasio, Princeton University; Anthony Reilly and Guo-Xu Zhang, Fritz Haber Institute of the Max Planck Society, Berlin; James Chelikowsky, The University of Texas at Austin; and Alexandre Tkatchenko, Fritz Haber Institute of the Max Planck Society, Berlin. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F6.00011: Stiffness of Diphenylalanine-Based Molecular Solids from First Principles Calculations Ido Azuri, Oded Hod, Ehud Gazit, Leeor Kronik Diphenylalanine-based peptide nanotubes were found to be unexpectedly stiff, with a Young modulus of 19 GPa. Here, we calculate the Young modulus from first principles, using density functional theory with dispersive corrections. This allows us to show that at least half of the stiffness of the material comes from dispersive interactions and to identify the nature of the interactions that contribute most to the stiffness. This presents a general strategy for the analysis of bioinspired functional materials. [Preview Abstract] |
Tuesday, March 19, 2013 11:00AM - 11:12AM |
F6.00012: Molecular Transport in Metal Organic Framework Ma- terials P. Canepa, N. Nijem, Y.J. Chabal, T. Thonhauser Metal organic frameworks (MOF) materials are a class of porous materials well suited for hydrogen storage and gas separation. While current work on MOFs focuses mostly on the adsorption properties of small molecules, their diffusion is still poorly understood. To elucidate the diffusion process, we study the diffusion of H$_2$, CO$_2$, and H$_2$O in the nano-pores of MOF-74-Mg by combining \emph{ab initio} simulations with infrared (IR) spectroscopy. We present computed adsorption energies and changes in the IR frequencies upon adsorption. We also discuss several diffusion mechanisms and their calculated barriers. We further verify the existence of the debated secondary binding sites for guest molecules and we discuss the role played by H$_2$O. We find that H$_2$O is much more likely to adsorb in the MOF than H$_2$ and CO$_2$, leading to a significant reduction of the adsorption capabilities of the MOF towards these target molecules, and hence resulting in limitations for practical applications. Overall, our computational findings are in very good agreement with experiment and they provide a fundamental understanding of the diffusion processes of small molecules in these nano-porous materials, with implication for the usability of MOFs in gas separation and storage applications. [Preview Abstract] |
Session F7: Focus Session: Graphene Devices IV
Sponsoring Units: DMPChair: Cory Dean, City College of New York
Room: 303
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F7.00001: Multi-state current switching by the interference between standing electronic waves in two misoriented crossed graphene nanoribbons K.M. Masum Habib, Roger Lake In semi-infinite armchair graphene nanoribbon (aGNR), the electronic wavefunctions are standing waves with energy dependent wavelengths. The wavelength of the electrons can be controlled by an external electric field. These standing electronic waves show some unique transport phenomena in crossed graphene nanoribbon (xGNR) consisting of two semi-infinite aGNRs with one placed on top of the other and a relative rotation of 90 degrees in between. At any given energy, the matrix element between a bottom aGNR state and a top aGNR state depends on the phases of the standing waves at that energy. The matrix element and hence the inter-aGNR transmission is strongly suppressed when a zero of the standing wave of either the top or the bottom aGNR falls inside the overlap region. An external bias applied between the aGNRs can control the wavelengths and hence the phases of the standing waves which in turn modulates the inter-aGNR transmission and current. Calculations show that the inter-aGNR current is an oscillatory function of the bias voltage with multiple negative differential resistance (NDR) regions and that the period of the oscillation is controlled by the length of the finite ends of the xGNR. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F7.00002: Vertical Transport through Twisted Graphene/h-BN Heterostructure Xingyuan Pan, Shayan Hematiyan, Jairo Sinova, Marco Polini, Allan MacDonald Graphene and its heterostructues are promising candidates for high-frequency electronics. Vertical heterostructures created by stacking graphene layers and hexagonal boron nitride layers together display orientational disorder, due to rotational stacking faults. In this work we report our theoretical study of vertical charge transport through a rotated graphene/h-BN heterostructure. Our theoretical model combines the microscopic tight-binding method with the Landauer formalism for electrical transport. Electrical conductances are calculated for a variety of system configurations and system sizes. We found that the electrical conductance has a maximum value when the rotation angle is commensurate. Away from commensurate angles transport is suppressed but cannot be completely ignored. We show that the distance dependence of the transfer integrals between two atoms is crucial in modeling the rotation-angle dependence of the vertical transport. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F7.00003: Vertical Transport Properties of Graphene/h-BN Hetrostructures Shayan Hemmatiyan, Xingyuan Pan, Marco Polini, Allan MacDonald, Jairo Sinova We present results of extensive first principles, studying the scaling behaviour of inter-layer tight-binding hopping parameters in vertical graphene/h-BN heterostructures. We focus, in particular, on the dependence of these parameters on orientational disorder and inter-layer distances. We will report relevant inputs for numerical studies of the vertical transport in graphene/h-BN heterostructures. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F7.00004: Superconducting Graphene Nanodevices in Ballistic Transport Regime Yu-An Chen, Joel I-Jan Wang, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero Superconductivity carried by Dirac fermions can be realized through induced superconductivity in grapheme. Observation of novel phenomena anticipated by theories requires graphene devices with low disorder whereas the carrier transport is ballistic. Current fabrication procedures to make graphene devices with low disorder like suspension or ultra-flat substrates all call for certain kinds of annealing to remove organic residues derived from the fabrication process. Applying these methods to superconducting devices can be challenging since the transparency at the graphene/superconductor interface will be destroyed. Here we present a method to do dry transfer of patterned hexagonal Boron Nitride (hBN) flakes onto graphene. The ultra flatness and lack of dangling bond in the boron nitride substrate reduces the disorder in graphene, and the top layer hBN can protect the graphene from contamination in the nanofabrication procedures and yield the geometry desired for different experimental exploration. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F7.00005: Exploring graphene properties in a periodic electrostatic potential Nikolai N. Klimov, David B. Newell Graphene, a unique two-dimensional honeycomb lattice of carbon atoms, exhibits rich new physics and great promise for applications in electronics. It was been predicted that a slowly varying nanoscale external periodic electrostatic potential applied to a graphene modifies its lectronic structure in a very unique way and leads to novel phenomena and possible applications [1-4]. In particular, a one-dimensional electrostatic potential applied to graphene may result in strong anisotropy of the group velocity of Dirac fermions, appearance of new zero-energy states at the Fermi energy, unusual Landau levels and quantum Hall effects. Both the anisotropy of the group velocity and the number of zero-energy modes can be altered by varying parameters of the superlattice potential. Although graphene in periodic potentials has been intensively studied theoretically, a thorough experimental investigation is still missing due to difficulties of fabricating of graphene devices, in which an external periodic potential can be applied with nanoscale periodicity. In this talk we present our results on the fabrication of graphene devices with nanoscale periodic local gates. The devices will be used to investigate graphene electronic properties in a one-dimensional periodic electrostatic potential using both magnetotransport and scanning probe microscopy measurement techniques. [1] C.-H. Park et al., Nat. Phys. 4, 213 (2008); Nano Lett. 8, 2920 (2008); Physica E 43, 651 (2011). [2] M. Barbier et al., PRB 77, 115446 (2008). [3] L. Brey, H.A. Fertig, PRL 103, 046809 (2009). [4] P. Burset et al., PRB 83, 195434 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F7.00006: Effect of non-uniform magnetic field on Dirac fermions in graphene Fen Guan, Naomi Mizuno, Bent Nielsen, Xu Du It has been theoretically proposed that non-uniform magnetic field can trigger bound, quasi-bound and scattering states in graphene, while electrostatic barriers cannot serve this purpose due to Klein tunneling. To observe this tuning effect on the transport properties of graphene experimentally, we need high quality graphene and microscopically inhomogeneous magnetic field. Here we report building of the inhomogeneous magnetic field through magnetic vortices in type II superconductor and study the effect of this magnetic field on the transport properties of the Dirac electrons in graphene. We present the fabrication and measurements of suspended graphene over Nb thin films which generate superconducting vortices. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F7.00007: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F7.00008: Alternative polymer scaffolds for clean transfer of CVD-grown graphene Joshua Wood, Gregory Doidge, Basil Aruin, Hefei Dong, Justin Koepke, Enrique Carrion, Isha Datye, Kamalika Chatterjee, Jeffrey Moore, Eric Pop, Joseph Lyding We investigate and benchmark polymer scaffolds used to support large-area chemical vapor deposition (CVD) grown graphene on Cu during transfer. CVD graphene must be transferred off of Cu to be used in various applications. PMMA transfers introduce hard-to-remove residues, and thermal release tape transfers have removable residue but give holey graphene films. Films transferred by poly(bisphenol A carbonate) (PC) are atomically clean after room-temperature polymer dissolution, and we confirm this by atomic force microscopy, Raman spectroscopy, device transport, and scanning tunneling microscopy. Compared to PC-, PMMA-transferred films have fewer wrinkles but higher RMS roughness. When we use a PC/PMMA bilayer, we find lower graphene wrinkle density but higher RMS roughness from polymer co-mixing. We also transfer graphene with other industrially relevant scaffolds like polylactic acid (PLA) and chemically modified photoresists. PLA-transferred films, after polymer dissolution, have sub-nm RMS roughness, and this improves upon PLA gasification above 180 $^{\circ}$C. Graphene transfer polymers that require low thermal budgets will open possibilities for temperature-sensitive substrates or graphene encapsulation of biological specimens (e.g. viruses, bacteria). [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F7.00009: Effect of sample preparation on charged impurities in graphene substrates K.M. Burson, C.R. Dean, K. Watanabe, T. Taniguchi, J. Hone, P. Kim, W.G. Cullen, M.S. Fuhrer The mobility of graphene as fabricated on SiO$_{2}$ has been found to vary widely depending on sample preparation conditions. Additionally, graphene mobility on SiO$_{2}$ appears to be limited to $\sim$20,000 cm$^{2}$/Vs, likely due to charged impurities in the substrate. Here we present a study of the effect of fabrication procedures on substrate charged impurity density (n$_{imp}$) utilizing ultrahigh-vacuum Kelvin probe force microscopy. We conclude that even minimal SEM exposure, as from e-beam lithography, induces an increased impurity density, while heating reduces the number of charges for sample substrates which already exhibit a higher impurity density. We measure both SiO$_{2}$ and h-BN and find that all n$_{imp}$ values observed for SiO$_{2}$ are higher than those observed for h-BN; this is consistent with the observed improvement in mobility for graphene devices fabricated on h-BN over those fabricated on SiO$_{2}$ substrates. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F7.00010: Graphene / Boron Nitride Heterostructures Invited Speaker: Roman Gorbachev The talk is dedicated to multilayer boron nitride/graphene heterostructures. It will review several aspects of microfabrication of such structures as well as the transport experiments. Graphene placed on boron nitride and exhibiting nanometer-scale moir\'e patterns showing strong anomalies in the density of states which can be associated with new Dirac cones formed high up in graphene's original spectrum. We describe quantum transport in specially aligned graphene-on-hBN devices such that the DoS anomalies reproducibly appear within the Fermi energy range achievable in transport measurements. We report a strongly reconstructed graphene spectrum with new sharp neutrality points and extra sets of Landau levels and quantum Hall states. Different experiments done on multilayer structures containing two interacting graphene layers will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F7.00011: Characterization Of Graphene-Ferroelectric Superlattice Hybrid Devices Mohammed Yusuf, Xu Du, Matthew Dawber Ferroelectric materials possess a spontaneous electrical polarization, which can be controlled by an electric field. A good interface between ferroelectric surface and graphene sheets can introduce a new generation of multifunctional devices, in which the ferroelectric material can be used to control the properties of graphene. In our approach, problems encountered in previous efforts to combine ferroelectric/carbon systems are overcome by the use of artificially layered superlattice materials grown in the form of epitaxial thin films. In these materials the phase transition temperature and dielectric response of the material can be tailored, allowing us to avoid polarization screening by surface absorbates, whilst maintaining an atomically smooth surface and optimal charge doping properties. Using ferroelectric PbTiO$_3$/SrTiO$_3$ superlattices, we have shown ultra-low-voltage operation of graphene field effect devices within $\pm$ 1 V at room temperature. The switching of the graphene field effect transistors is characterized by pronounced resistance hysteresis, suitable for ultra-fast non-volatile electronics. Low temperature characterization confirmed that the coercive field required for the ferroelectric domain switching increases significantly with decreasing temperatures. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F7.00012: Transport properties of graphene devices transferred to STO substrates Raymond Sachs, Patrick Odenthal, Roland Kawakami, Jing Shi The effect of substrate on graphene transport properties can help us understand the scattering mechanisms relevant to its carrier mobility. Single-layer graphene is easily located on the surface of Silicon with 300nm SiO$_{2}$ using optical microscopy. We have developed a technique for wet-etching the SiO$_{2}$, peeling the device with metallic leads from the surface, and transferring it to any substrate. This technique eliminates the need to locate the graphene flake on the target substrate for aligning and patterning. A direct comparison can be made between the transport properties of graphene on SiO$_{2}$ and the target substrate. A device has been transferred to 500um and 200um thick Strontium Titanate (STO) substrates as well as 250nm thick layer of STO that has been grown epitaxially on Nb-doped STO via Pulsed Laser Deposition. The STO layer, with a higher dielectric constant than SiO$_{2}$, has a higher capacitance and produces a more effective graphene FET. A higher mobility is expected for a device on the surface of a material with a higher dielectric constant if charged impurity scattering is a primary limiting factor. The devices transferred to STO display a gate voltage dependent hysteresis in both the longitudinal and Hall resistances. However, the mobility obtained from these measurements remains the same as that of the device on SiO$_{2}$. Possible reasons for the absence of the high dielectric substrate effect on graphene carrier mobility and hysteretic behavior will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F7.00013: Impact of atomic hydrogen on graphene on hexagonal boron nitride Masa Ishigami, Jyoti Katoch We have measured the transport property of graphene on hexagonal boron nitride as a function of density of adsorbed atomic hydrogen. Atomic hydrogen is reversibly chemisorbed and has a large carrier scattering cross section. The impact was previously found to be radically different on graphene on silicon oxide where atomic hydrogen is mostly physisorbed and the saturation coverage of hydrogen was found to correspond to the number of native scatterers. Our results can directly test the theoretical results on the resonant impurities and suggest the nature of the native scatterers in graphene on hexagonal boron nitride. These finding will be outlined in this talk. [Preview Abstract] |
Session F8: Functionalization and Decoration of Graphene
Sponsoring Units: DCMPChair: Jun Zhu, Pennsylvania State University
Room: 307
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F8.00001: Hydroxyl-decorated Graphene Systems: Organic metal-free Ferroelectrics, Multiferroics, and Proton battery Cathode Materials Menghao Wu, J.D. Burton, Evgeny Tsymbal, Xiao Cheng Zeng, Puru Jena Through density-functional-theory calculations we show that hydroxylized graphene systems are ideal candidates for light-weight organic ferroelectric materials with giant polarizations. For example, the polarization of semi-hydroxylized graphane and graphone as well as fully hydroxylized graphane are, respectively, 41.1, 43.7, 67.7 $\mu$C/cm$^{2}$, much higher than any organic ferroelectric materials known to date. In addition, hydroxylized graphone is multiferroic due to the coexistence of ferroeletricity and ferromagnetism. Zigzag graphene nanoribbons decorated by hydroxyl groups also exhibit ferroelectric properties with a large polarization of 27.0 $\mu$C/cm$^{2}$. Moreover, proton vacancies at the end of ribbons can induce large dipole moments that can be reversed by both hopping of protons and rotation of O-H bonds under an electric field. These materials have the potential as high-capacity cathode materials with specific capacity six times larger than lead-acid batteries and five times that of lithium-ion batteries. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F8.00002: Quantum Sticking of Atomic Hydrogen to Graphene Yanting Zhang, Adam Doherty, Andrew Geragotelis, Dennis Clougherty We consider the low-energy behavior of the sticking probability of atomic hydrogen to suspended graphene. For energy transfer through the flexural modes of graphene, we find that the inelastic coupling falls in the subOhmic regime. Thus the effects of low-frequency fluctuations of the graphene sheet are crucially important for quantum sticking. We analytically solve for the low-energy asymptotic behavior of the sticking coefficient using a variational mean-field method [D.P. Clougherty and Y. Zhang, Phys. Rev. Lett. 109, 120401 (2012)]. We find that as a result of strong coupling to the low-frequency flexural modes of graphene, a new scaling law results. For suspended graphene at finite temperature, we find that at a critical incident energy, the sticking probability drops discontinuously; below this critical energy, the sticking probability is suppressed by the orthogonality catastrophe. We compare our nonperturbative variational results to those obtained by using Fermi's golden rule. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F8.00003: Electronic structure of oxygen functionalized graphene nanoribbons Adam Simbeck, Deyang Gu, Neerav Kharche, Saroj Nayak We investigate the electronic and magnetic properties of armchair graphene nanoribbons whose edges are passivated by oxygen. Using a first-principles density functional approach and the many-body GW method we find that oxygen-passivation results in a rich geometrical environment which in turn determines the electronic and magnetic properties of the ribbon. For planar systems we report magnetic ground states whose electronic structure depends upon the magnetic coupling between edges. For non-planar ribbons we report a nonmagnetic ground state with a band gap that decreases as a function of increasing ribbon width. Our results will be discussed in light of previous experimental and computational studies. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F8.00004: Optical properties of functionalized monolayer and bilayer graphene JinLuo Cheng, Cuauht\'emoc Salazar, John E. Sipe We use {\it ab initio} calculations to investigate the structures, band structures, and optical properties of functionalized monolayer and bilayer graphene, where a hydrogen atom is attached to only one carbon atom site periodically every few unit cells. The hydrogen atom distorts the carbon atoms vertically, but the inplane structure is approximately unchanged. The ground state acquires a bandgap due to adsorption depending on the supercell size, and shows magnetic order, which is in agreement with a recent experiment [1]. The calculated optical absorption spectra displays detailed structures at lower photon frequencies than that of the pristine graphene.\\[4pt] J. Hong {\it et~al.}, Sci. Rep. {\bf 2}, 624 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F8.00005: Ferromagnetism in hydrogenated epitaxial graphene on 6H-SiC A.J.M. Giesbers, K. Uhlirova, M. Konecny, J. Aarts, C.F.J. Flipse Graphene remains a material of interest in both fundamental and applied physics due to its unique combination of properties [1] such as its mechanical strength, surface sensitivity, relativistic bandstructure and large spin relaxation length. Functionalizing graphene leads to a whole new range of properties [2] varying from photoluminescence in graphene oxide [3] to ferromagnetism in hydrogenated graphene [4]. Here we will show a detailed investigation of the (ferro-)magnetic properties of hydrogenated epitaxial graphene on SiC (HeG). The magnetization of the of the HeG shows a clear hysteresis loop, which remains visible up to room temperature with a saturation magnetization of 0.5 $\mu_{\mathrm{B}}$/hexagon. The saturation magnetization depends on the hydrogen coverage and shows a strong anisotropy to the sample orientation with respect to the magnetic field. [1] N. M. R. Peres, Rev. Mod. Phys. 82, 2673 (2010) [2] W. Wei and X. Qu, Small 8, 2138 (2012). [3] Z. Luo et al., Appl. Phys. Lett. 94, 111909 (2009). [4] L. Xie et al., Appl. Phys. Lett. 98, 193113 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F8.00006: Electronic structures and magnetism of hydrogenated and fluorinated graphene with vacancies Bi-Ru Wu, Chih-Kai Yang Graphene is a gapless semiconductor. As graphene is covered with one layer of hydrogen or fluorine, it becomes a wide band gap insulator. However, vacancies are easily found during the hydrogenated or fluorinated processes. We investigate the electronic structure and magnetism of the hydrogenated and fluorinated graphene with a variety of configuration of vacancies. We found that a continuous zigzag chain distribution of vacancies will result linear energy dispersion both in the hydrogenated and fluorinated graphene. This finding should be very useful for the design of graphene based electronic devices. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F8.00007: First-principles study of the spin-orbit interaction in graphene induced by hydrogen adatoms Martin Gmitra, Denis Kochan, Jaroslav Fabian We have performed first principles calculations of the spin-orbit coupling effects in hydrogenated graphene structures, for varying hydrogen coverage densities, using the linearized augmented plane wave method as implemented in the FLEUR code. The covalent bonding between the hydrogen and carbon atoms leads to a local structural puckering of graphene sheets, giving rise to an overlap between the Dirac and sigma electrons and a giant enhancement (from roughly 0.01 to 1 meV) of the local spin-orbit interaction. The calculated effects on the band structure and the emerging spin patterns of the electronic states can be well explained by effective Hamiltonian models derived from group theoretical principles. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F8.00008: A Model for the Origin of Spin half Para-magnetism in Fluorinated Graphene Piali Aditya, Alejandro Suarez, Tyler Maunu, Diego B. Carrasco, Jorge Sofo It came as a surprise when the Manchester group reported a paramagnetic response in fluorinated graphene [\textit{Nair et al., Nature Physics 8, 199-202 (2012)}]. The response is characteristic of non-interacting spin 1/2 with a concentration that is almost zero up to 60{\%} fluorination and peaks at 80{\%} fluorination. The density is never larger than a few spins per 1000 carbon atoms. Prior DFT calculations show an absence of magnetism for dilute fluorinated graphene samples [\textit{Sofo et al., Phys. Rev. B Rapid Comm. 83, 081411 (2011)}]. We propose that the magnetic response originates from regions with a small number of non-fluorinated carbon atoms surrounded by fluorinated ones. In support of this model we combine the exact response of the non-fluorinated regions with a stochastic model to account for the fluorination process. Our calculation reproduces the magnetic response of the samples and tracks the origin of this magnetic phenomenon to the grain boundary between fluorinated patches. If our model is correct, the number of spins in this sample is not an intrinsic quantity but is determined by the fluorination process. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F8.00009: Analytic Local and Total Density of States for Hydrogen Adatoms on Graphene Nicholas Pike, David Stroud Spin transport through graphene is strongly influenced by the presence of adatoms with unpaired spins, such as hydrogen adatoms. In this work, we calculate the local density of states (LDOS) for a simple model of hydrogen on graphene using a tight binding model. The model includes nearest neighbor hopping between carbon atoms, the value of the hydrogen energy level, hopping between the carbon and hydrogen atoms, and a Hubbard U-term to account for the on-site Coulomb interaction. When U = 0, we develop an exact analytic equation for the LDOS on the adatom site, and for the total density of states (DOS). When U $\neq 0$, we carry out the same calculation but treat the Hubbard term using mean-field theory. We find that the hydrogen adatom has a net non-integer spin polarization, and that some of the electronic density is transferred from the hydrogen adatom to the graphene host. Possible implications of these results for spin transport through graphene will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F8.00010: Electrical detection of phase changes in adsorbed neutral dipolar molecules on graphene Yilin Wang, Wenzhong Bao, Shudong Xiao, Michael Fuhrer, Janice Reutt-Robey Graphene is a very promising material for sensing application because its transport properties are highly sensitive to adsorbates on its surface. Here, we study the carrier-density-dependent resistance of bilayer graphene to neutral dipolar adsorbates under ultra-high vacuum condition. Halocarbon molecules with known dipole moment are deposited on graphene at $\sim$ 20 K. After deposition of a few monolayers of molecules, the resistance of graphene near the Dirac point is measured as a function of carrier density (tuned by gate voltage) and temperature, from 20 K to room temperature. We observe negligible shifts of the gate voltage of maximum resistance, indicating negligible charge transfer from adsorbate to graphene. In the temperature-dependent-resistance curve, a sharp step-like increase and decrease in resistance occur at $\sim$ 45 K and $\sim$ 65 K, respectively. We relate these abrupt changes in resistance to phase transitions in the adsorbate overlayer. The same molecules adsorbed on graphite are known to exhibit a complex temperature - coverage phase diagram. We will discuss the relationship between graphene resistance and the phases of molecules on graphite. This work was supported by the NSF-MRSEC at the University of Maryland, DMR 0520471 [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F8.00011: Optical properties of hydrogenated graphene from first principles Sebastian Putz, Martin Gmitra, Jaroslav Fabian We investigate the effect of hydrogen coverage on the optical properties of single-side hydrogenated graphene from first principles. To account for different degrees of uniform hydrogen coverage we calculate the complex dielectric function for graphene supercells of various size, each containing a single additional H atom. We use the Linearized Augmented Planewave (LAPW) method, as implemented in WIEN2k, to show that the hydrogen coverage strongly influences the complex dielectric function and thus the optical properties of hydrogenated graphene. The absorption coefficient in the visible range, for example, has different characteristic features depending on the hydrogen coverage. This opens up new possibilities of determining the hydrogen coverage of hydrogenated graphene samples in the experiment by contact-free optical absorption measurements. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F8.00012: A Theoretical Analysis of the Effect of the Hydrogenation of Graphene to Graphane on Its Mechanical Properties Q. Peng, Chao Liang, Wei Ji, Suvranu De We investigated the mechanical properties of graphene and graphane using first-principles calculations based on density-functional theory. A conventional unitcell containing a hexagonal ring made of carbon atoms was chosen to capture the finite wave vector ``soft modes", which affect the the fourth and fifth elastic constants considerably. Graphane has about 2/3 ultimate strengths in all three tested deformation modes -- {\em armchair}, {\em zigzag}, and {\em biaxial}-- compared to graphene. However, graphane has larger ultimate strains in {\em zigzag} deformation, and smaller in {\em armchair} deformation. We obtained the second, third, fourth, and fifth order elastic constants for a rigorous continuum description of the elastic response. Graphane has a relatively low in-plane stiffness of 240 N/m which is about 2/3 of that of graphene, and a very small Poisson ratio of 0.078, 44\% of that of graphene. The pressure dependence of the second order elastic constants were predicted from the third order elastic constants. The Poisson's ratio monotonically decreases with increasing pressure. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F8.00013: Fluorination of CVD graphene: the role of wrinkles, folds, multi-layer islands and grain boundaries Bei Wang, Junjie Wang, J. Zhu Chemical functionalization, such as fluorination, can modify the gapless band structure of graphene and turn it into an insulator. Fluorinated graphene (FG) can potentially be integrated into graphene electronics and serve as ultrathin gate dielectrics or tunnel barriers. Here we present our effort in synthesizing and understanding the properties of FG. Graphene sheets synthesized by chemical vapor deposition (CVD) are fluorinated using CF$_{4}$ plasma under varying conditions. The resulting FG is systematically examined using a wide range of spectroscopic and microscopic tools including XPS, Raman, FTIR, electrical transport and conductive AFM. We obtain high F:C ratio of 0.1-1. Our results show that 1. Morphological features of CVD graphene (wrinkles, folds, multi-layer islands) are less fluorinated and charge transport in FG occurs through the conductive network formed by these features. 2. Lattice defects and grain boundaries play a significant role in the chemical reactivity of CVD graphene. XPS studies indicate the formation and evolution of CF$_{x}$ (x=1,2,3) bonds, as well as oxygen-passivated defect sites in FG. These studies highlight current challenges in realizing electronics-grade FG and point to the possible pathways forward. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F8.00014: Theory of the hydrogen adatoms induced spin-orbit coupling in graphene Denis Kochan, Martin Gmitra, Jaroslav Fabian We have analyzed the first-principles data of the electronic structure of hydrogenation in graphene by means of group theory derived effective Hamiltonians. We propose effective models for semihydrogenated graphene as well as for graphene with a single hydrogen adatom. The chemisorption of hydrogen modifies the structural symmetry of the plane graphene in two essential ways---it breaks the pseudospin (sublattice) symmetry and induces rippling. We show that in addition to the Rashba spin-orbit interaction there emerges another spin-orbit field which is induced by the pseudospin inversion asymmetry due to the adatoms. Our realistic effective Hamiltonians should be useful for spin transport and spin relaxation investigations. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F8.00015: Adsorption Configurations of Carbon Monoxide on Gold Monolayer Supported by Graphene or Hexagonal Boron Nitride Film: A First-Principles Study Lu Wang, Wai-Ning Mei, Jiaxin Zheng, Jing Lu, Peter Dowben Using density functional theory with a semiempirical van der Waals approach proposed by Grimme, the adsorption behavior of carbon monoxide on a gold monolayer supported by graphene or monolayer hexagonal boron nitride has been investigated. Based on the changes in the Dirac cone of graphene and a Bader charge analysis, we observe that the Au(111) monolayer gains a small electron charge from graphene and monolayer $h$-BN. The adsorbed CO molecule adopts similar adsorption configurations on Au(111)/graphene and Au(111)/$h$-BN with Au-C distance 2.17$-$2.50 {\AA} and Au-C-O angle of $123.9^{\circ} - 139.6^{\circ}$. Moreover, we found that for low CO coverages, bonding to the gold surface is surprisingly energy-favorable. Yet the CO adsorption binding energy diminishes at high coverage due to the repulsive van der Waals interactions between CO molecules. [Preview Abstract] |
Session F9: Invited Session: The Impact of Hydraulic Fracturing
Sponsoring Units: FPS GERAChair: Richard Wiener, Research Corporation for Scientific Advancement
Room: 308
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F9.00001: The EPA's Study on the Potential Impacts of Hydraulic Fracturing on Drinking Water Resources Invited Speaker: Susan Sharkey Natural gas plays a key role in our nation's clean energy future. The United States has vast reserves of natural gas that are commercially viable as a result of advances in horizontal drilling and hydraulic fracturing technologies, which enable greater access to gas in rock formations deep underground. These advances have spurred a significant increase in the production of both natural gas and oil across the country. However, as the use of hydraulic fracturing has increased, so have concerns about its potential human health and environmental impacts, especially for drinking water. In response to public concern, the US Congress requested that the US Environmental Protection Agency (EPA) conduct scientific research to examine the relationship between hydraulic fracturing and drinking water resources. In 2011, the EPA began research to assess the potential impacts of hydraulic fracturing on drinking water resources, if any, and to identify the driving factors that may affect the severity and frequency of such impacts. The study is organized around the five stages of the hydraulic fracturing water cycle, from water acquisition through the mixing of chemicals and the injection of fracturing fluid to post-fracturing treatment and/or disposal of wastewater. EPA scientists are using a transdisciplinary research approach involving laboratory studies, computer modeling, toxicity assessments, and case studies to answer research questions associated with each stage of the water cycle. This talk will provide an overview of the EPA's study, including a description of the hydraulic fracturing water cycle and a summary of the ongoing research projects. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F9.00002: Induced Seismicity Potential of Energy Technologies Invited Speaker: Murray Hitzman Earthquakes attributable to human activities--``induced seismic events''--have received heightened public attention in the United States over the past several years. Upon request from the U.S. Congress and the Department of Energy, the National Research Council was asked to assemble a committee of experts to examine the scale, scope, and consequences of seismicity induced during fluid injection and withdrawal associated with geothermal energy development, oil and gas development, and carbon capture and storage (CCS). The committee's report, publicly released in June 2012, indicates that induced seismicity associated with fluid injection or withdrawal is caused in most cases by change in pore fluid pressure and/or change in stress in the subsurface in the presence of faults with specific properties and orientations and a critical state of stress in the rocks. The factor that appears to have the most direct consequence in regard to induced seismicity is the net fluid balance (total balance of fluid introduced into or removed from the subsurface). Energy technology projects that are designed to maintain a balance between the amount of fluid being injected and withdrawn, such as most oil and gas development projects, appear to produce fewer seismic events than projects that do not maintain fluid balance. Major findings from the study include: (1) as presently implemented, the process of hydraulic fracturing for shale gas recovery does not pose a high risk for inducing felt seismic events; (2) injection for disposal of waste water derived from energy technologies does pose some risk for induced seismicity, but very few events have been documented over the past several decades relative to the large number of disposal wells in operation; and (3) CCS, due to the large net volumes of injected fluids suggested for future large-scale carbon storage projects, may have potential for inducing larger seismic events. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F9.00003: Environmental Dimensions of Shale Gas Extraction and Stray Gas Migration Invited Speaker: Robert Jackson Shale gas extraction is growing rapidly in the United States and elsewhere, developed in part through advances in technologies such as horizontal drilling and hydraulic fracturing. Concerns over potential environmental impacts have accompanied the boom in natural gas extraction. For several years we have studied drinking water quality, asking the question, ``Is water quality different for homeowners living near natural gas wells?'' We have sampled shallow groundwater systems of \textgreater\ 300 homeowners, the majority of them in the Marcellus formation of Pennsylvania and New York, for brines, dissolved gases, and other attributes. We have also examined how much methane reaches the atmosphere during the extraction and distribution of natural gas. In a study published in May of 2011 (Osborn et al. 2011, PNAS 108:8172-8176), we found no evidence of increase salt concentrations or fracturing fluids with distance to gas wells for 68 sampled homes. However, dissolved methane concentrations were 17 times higher on average for water wells found within 1km distance of them. A subset of homeowners also had groundwater that indicated the presence of natural hydraulic connections to deeper formations, suggesting specific structural and hydrodynamic regimes where shallow drinking water resources might be at greater risk of contamination with fugitive gases during drilling and hydraulic fracturing of shale gas (Warner et al. 2012, PNAS 109:11961-11966). This presentation will discuss new results from shale gas sampling in 2011 and 2012. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F9.00004: The natural gas revolution -- Scale, cost and uncertainty Invited Speaker: Francis O'Sullivan Over the past decade, the natural gas industry landscape in North America has undergone tremendous change. The focus of exploration and production has shifted from ``conventional'' to ``unconventional'' resources, and in particular to shale formations. The fact that some shale formations contain significant volumes of gas-in-place has been known for as long as gas production has taken place -- these rocks have always been viewed as the source rock for conventional gas resources. What changed over the past decade is that it became possible to recover this gas directly from the source rock at economically attractive production rates. Horizontal drilling and hydraulic fracturing technologies were key to these developments. This presentation will describe how the unlocking of shale gas through horizontal drilling and fracturing has changed perspectives regarding the scale of the overall recoverable natural gas resource in the United States. The potential impact of shale gas on the global gas resource will also be described. The results of volumetric assessments of recoverable shale gas will be presented and the critical issue of uncertainty surrounding these estimates will be highlighted. The economics of shale gas relative to conventional resources in the United States will be described, and this will be compared with the economics of gas elsewhere in the world. In discussing the economics of shale gas, the very important issue of intra and inter-play well-to-well performance variability will be highlighted. The presentation will also describe some of the major environmental concerns that surround that shale gas production. The issue of water intensity in hydraulic fracturing operations will be examined, as will the concerns regarding surface and subsurface water contamination. The debate regarding the GHG footprint of hydraulic fracturing operations will be described and an assessment of ``potential'' and ``actual'' fugitive methane emissions from hydraulic fracturing operations in the major U.S. shale plays will be presented. [Preview Abstract] |
Session F10: Invited Session: Spin-Orbit Transfer Torques in Magnetic Bilayers
Sponsoring Units: GMAGChair: Kyung-Jin Lee, Korea University
Room: 309
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F10.00001: The spin Hall effect in transition metal-ferromagnetic material bilayer devices Invited Speaker: Chi-Feng Pai The strong spin-orbit interaction from certain heavy metal/ferromagnetic material bilayer systems has been shown to be intense enough to drive the magnetization into steady dynamics and/or magnetic switching via spin transfer torque mechanism. The spin Hall effect, which describes the generation of a transverse spin current from a longitudinal charge current, plays an important role in these bilayer devices that typically contain a heavy transition metal underlayer. Here we demonstrate that the spin Hall effect induced spin transfer torque (SHE-STT) from Ta and W based systems can be utilized to control the magnetization direction in magnetic tunnel junctions through a three-terminal device architecture. We also demonstrate DC current induced dynamics in the magnetic layer due to the SHE-STT in these three-terminal devices. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F10.00002: Interfacial current induced torques in Pt|Co|GdOx Invited Speaker: Geoffrey S. D. Beach |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F10.00003: Rashba spin-orbit coupling and orbital chirality in magnetic bilayers Invited Speaker: Hyun-Woo Lee The phenomenon of the Rashba spin-orbit coupling is examined theoretically for an ultrathin magnetic layer in contact with a non-magnetic heavy metal layer. From first-principles calculation, large Rashba parameter of order 1 eV$\cdot$\AA\ is obtained, which is strong enough to generate large spin transfer torque of spin-orbit coupling origin. Large Rashba parameter is attributed to the orbital mixing of 3$d$ magnetic atoms and non-magnetic heavy elements with significant atomic spin-orbit coupling. Interestingly the magnitude and sign of the parameter vary from energy bands to bands, which we attribute to band-specific chiral ordering of orbital angular momentum. Through a simple tight-binding model analysis, we demonstrate that $d$-orbital hybridization allowed by the breaking of structural inversion symmetry generates band-specific chiral ordering of orbital angular momentum, which combines with atomic spin-orbit coupling to give rise to band-specific Rashba parameter. The band-dependence of the Rashba parameter is discussed in connection with recent experiments and we argue that the dependence may be utilized to enhance device application potentials. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F10.00004: Vector measurements of the current induced effective fields in Ta/CoFeB/MgO heterostructures Invited Speaker: Masamitsu Hayashi Ultrathin magnetic heterostructures exhibit a variety of rich physics owing to the strong effects from the interfaces. Power efficient current induced magnetization switching and domain nucleation, fast current driven domain wall motion have been observed in ultrathin Co or CoFeB layer sandwiched between a heavy metal (Pt, Ta) and an oxide. Most of the current (or voltage) induced effects in these systems can be represented by the ``effective magnetic fields'', which illustrate the strength and direction of the torque exerted on the magnetic moments. A comprehensive understanding of the effective fields is key to the development of magnetic nano-devices aimed for memory and logic applications. We have studied the current induced effective field vector in Ta\textbar CoFeB\textbar MgO heterostructure to reveal the underlying physics of the interaction between the magnetic moments and current in such structure. A low current lock-in detection scheme is used to evaluate the effective field vector. The CoFeB layer is perpendicularly magnetized owing to the interface magnetic anisotropy of CoFeB\textbar MgO. We find that the effective field is very sensitive to the thickness of the Ta and CoFeB layers. The effective field even changes its direction when the Ta layer thickness is varied, indicating that there are competing effects that contribute to the effective field generation. We discuss our results in light of the spin Hall effect and an effect due to Rashba-like Hamiltonian. (Acknowledgment: FIRST program) [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F10.00005: Giant spin Hall effect in CuBi alloys Invited Speaker: Yoshichika Otani Spintronic devices manipulating pure spin currents, flows of spin angular momentum without net charge current, should play an important role in low energy consumption electronics for next generation. This explains the current interest for the spin Hall effect (SHE) which provides a purely electrical way to create spin currents without ferromagnets and magnetic fields. In this work, we have studied extrinsic SHEs in Cu-based alloys [1]. Copper itself does not show any SHE, but by adding impurities with strong spin-orbit interactions such as Ir and Bi, the extrinsic SHEs can be generated and one can tune the SH angle which represents the maximum yield of conversion of charge to spin current density. The SH resistance was measured by means of spin absorption method using a lateral spin valve structure with an inserted wire of SHE material [1]. We found that Cu$_{99.5}$Bi$_{0.5}$ exhibited a very large negative SH resistance whereas Pt and a Cu$_{99}$Ir$_{1}$ alloy had positive SH resistances. From nonlocal spin valve measurements with the SHE materials, we can obtain the spin absorption rates as well as the spin diffusion lengths of the SHE materials. The spin Hall angle was determined by fitting experimental data to two theoretical models, i.e., a purely 1D model [2] and a 3D spin transport model based on an extension to 3D of the Valet-Fert formalism [3]. For Pt and CuIr alloys, the spin diffusion lengths are smaller than their thickness (20 nm), and the SH angles obtained from the 1D and 3D analyses are similar to each other (about 2{\%} for both Pt and CuIr). For CuBi alloys, however, the analysis in the 3D model gave much larger SH angle of about - 24{\%} than the 1D of about -12{\%}. More interestingly the fact that Bi impurities generated much larger SH angle than Pt and Ir, was consistent with a recent ab-initio theoretical calculation [4].\\[4pt] [1] Y. Niimi et al., Phys. Rev. Lett. 106 (2011) 126601; Y. Niimi et al., Phys. Rev. Lett. 109 (2012) 156602.\\[0pt] [2] S. Takahashi and S. Maekawa, Phys. Rev. B 67 (2003) 052409.\\[0pt] [3] T. Valet and A. Fert, Phys. Rev. B 48 (1993) 7099.\\[0pt] [4] M. Gradhand et al., Phys. Rev. B 81 (2010) 245109. [Preview Abstract] |
Session F11: Invited Session: Polymer Physics Prize Session
Sponsoring Units: DPOLYChair: John Torkelson, Northwestern University
Room: 310
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F11.00001: Polymer Physics Prize Lecture: Self-assemblies of Giant Molecular Shape Amphiphiles as a New Platform for Engineering Structures with Sub-Nanometer Feature Sizes Invited Speaker: Stephen Z.D. Cheng Utilizing nano-building blocks rather than atoms to construct and engineer new structures is a fresh approach to design and develop functional materials for the purpose of transferring and amplifying microscopic functionality to macroscopic materials' property. As one of the important elements of these nano-building blocks, giant molecular shape amphiphiles (GMSAs) provide a latest platform for generating self-assembled ordered structures at nanometer scale, which are stabilized by collective physical bonds (such as collective hydrogen bonding). In this talk, two topics will be focused on. First, composed of functionalized hydrophilic molecular nanoparticles as the heads with rigid shape and fixed volume, and tethered polymer chains as the tails (such as giant molecular surfactants and lipids and other topologies), these GMSAs of various architectures can self-assemble into highly diversified, thermodynamically stable microstructures at sub-10 nm length scale in the bulk, thin film and solution states. Second, GMSAs could also be constructed solely from nanoparticles interconnected via different numbers of the rigid linkages in specific symmetry, simulating the overall shapes of small molecules but with sizes that are one-order of magnitude larger in length and three-order of magnitude larger in volume. Giant crystal structures can then be obtained from this class of ``giant molecules'' via supramolecular crystallization. These findings are not only scientifically intriguing in understanding the physical principles underlying their self-assembly, but also technologically relevant in industrial applications. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F11.00002: Polymer Spheulites Invited Speaker: Bernard Lotz The growth and/or structural features that determine lamellar shape in polymer spherulites and therefore their structure and properties have been debated for many years. The spectacular twisting of lamellae in optically banded spherulites has been explained by the existence of unbalanced stresses in opposite fold surfaces of the lamellae. This mechanical origin implying the folds explains also the demonstrated absence of correlation between lamellar twist sense and molecular chirality of chiral polymers. Unbalanced surface stresses may also generate spherulites made of scrolled lamellae, with the scroll axis radial. This original morphology was first observed in spherulites of poly(vinylidene fluoride) in its $\gamma $ phase. It arises from a chemical disparity of folds formed on opposite fold surfaces, the volumes of which differ by 10{\AA}*3. Similar chemical disparities have been suggested to explain the formation of highly unusual scrolled single crystals of polyamide 66 obtained from solution under specific annealing and crystallization conditions. Related thermal histories lead to the formation, in the bulk, of unusual optically negative spherulites of polyamide 66 that were first observed in the 1940s. These still poorly understood negative spherulites may well display a similar scrolled lamellar morphology. The analysis of unbalanced surface stresses requires to evaluate the interplay and mutual impact of crystal and fold structures. The stresses associated with different fold structures are locally small perturbations but are cumulative and are exerted on thin, flexible lamellae. The latter non-planar morphologies reveal these stresses and help reach sub-moleculr insights on the fold structures. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F11.00003: Cellulose as Sustainable Materials for Separation Membranes Invited Speaker: Benjamin Chu Polysaccharides, while complex, form one of the most abundant sustainable resources on earth. We want to take advantage of fundamental advances in materials understanding across length and time scales to investigate the interrelationships between structure, morphology, processing, properties, performance, and cost to meet the specific challenges arising from separation membranes for water purification. Non-woven fiber mats have unique properties, such as interconnected pores, a very large surface-to-volume ratio, and a high capacity for surface modifications. The breakthrough concept of combining fibrous mats composed of different fiber diameters for fabricating scaffolds as a unique platform for water purification is presented. Further, we take advantage of recent advances in chemical modifications, structural studies using synchrotron X-rays, and physical scale-up transformations to drastically improve filtration membrane development. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F11.00004: Computational Modeling Studies of Peptides and Proteins on Inorganic Surfaces Invited Speaker: Barry Farmer Biological moieties offer exquisite sensitivity and selectivity in their interactions with small molecules, offering considerable potential in applications as chemical sensors. To detect binding events between the peptide and the intended molecule, a transduction mechanism is needed. This often involves an association of the peptide with an inorganic surface, such as a metal nanoparticle, a carbon nanotube, or graphene. Understanding the nature of the association of the peptide with the surface and its effect on the conformational (and thus, binding) properties of the peptide are key to optimizing the sensing mechanism. We utilized computational approaches ranging from \textit{ab initio} to molecular dynamics to bond-fluctuation Monte Carlo methods to study the adsorption of peptides and proteins on inorganic surfaces to develop an understanding of the role that composition and substrate character plays in the adsorption process, and in turn, the effects on the binding events with the molecules of interest. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F11.00005: Periodic Polymers Invited Speaker: Edwin Thomas Periodic polymers can be made by self assembly, directed self assembly and by photolithography. Such materials provide a versatile platform for 1, 2 and 3D periodic nano-micro scale composites with either dielectric or impedance contrast or both, and these can serve for example, as photonic and or phononic crystals for electromagnetic and elastic waves as well as mechanical frames/trusses. Compared to electromagnetic waves, elastic waves are both less complex (longitudinal modes in fluids) and more complex (longitudinal, transverse in-plane and transverse out-of-plane modes in solids). Engineering of the dispersion relation between wave frequency w and wave vector, k enables the opening of band gaps in the density of modes and detailed shaping of w(k). Band gaps can be opened by Bragg scattering, anti-crossing of bands and discrete shape resonances. Current interest is in our group focuses using design - modeling, fabrication and measurement of polymer-based periodic materials for applications as tunable optics and control of phonon flow. Several examples will be described including the design of structures for multispectral band gaps for elastic waves to alter the phonon density of states, the creation of block polymer and bicontinuous metal-carbon nanoframes for structures that are robust against ballistic projectiles and quasi-crystalline solid/fluid structures that can steer shock waves. [Preview Abstract] |
Session F12: Focus Session: Complex Oxide Interfaces - Polar interfaces II
Sponsoring Units: DMPChair: Jeremy Levy, University of Pittsburgh
Room: 314
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F12.00001: Mott p-n junctions in layered materials Maxime Charlebois, Syed Hassan, Rajesh Karan, David Senechal, A.-M.S. Tremblay Correlated electron heterostructure became a possible alternative when thin film deposition techniques achieved structures with a sharp interface transition [1]. We study here the electronic reconstruction of doped Mott insulator p-n junctions based on a Cluster Dynamical Mean Field Theory (CDMFT) calculation of the Hubbard model in the limit where electrostatic energy dominates over the kinetic energy associated with transport across layers. The grand potential of individual layers is first computed within CDMFT and then the electrostatic potential energy is taken into account in the Hartree approximation. The charge reconstruction in an ensemble of stacked planes of different nature can lead to a distribution of electron charge [2], density of states, and optical properties that are unique to doped-Mott insulators.\\[4pt] [1] J. Mannhart, D. G. Schlom, Science 327, 1607 (2010)\\[0pt] [2] T. Oka, N. Nagaosa, PRL 95, 266403 (2005) [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F12.00002: Strong electron correlation enhancement to capacitance via frustrated phase separation James Freericks, Simon Hale Recent experiments on strongly correlated capacitors from Mannhart's group have shown that the capacitance can be enhanced by about 50\% over the geometric capacitance when the metallic leads are gated to be nearly depleted of electrons. More recently, direct measurements of the electron compressibility in those leads show that they become phase separated in this regime. It has long been known that proximity to phase separation, or equivalently negative electron compressibility, should lead to an enhancement of capacitance. In this work, we show that this phenomenon is quite general. By employing a microscopic model of a strongly correlated capacitor composed of multilayers of electronic leads and a Mott insulating dielectric, we show that by tuning the barrier to lie in the regime where it is phase separated in the bulk, it exhibits a type of frustrated phase separation in the multilayer, which gives rise to an enhancement in the capacitance with capacitance curves versus gate voltage resembling quite close to those of experiment. In the calculations, the enhancement effect is lower (on the order of 10\%), and the mechanism is different, because here the phase separation is in the dielectric instead of the metallic plates. Nevertheless, this behavior seems to be ubiquitous. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F12.00003: Theory of optical response and ellipsometry spectra of LaAlO$_3$/SrTiO$_3$ heterostructures Se Young Park, Andrew Millis We present a theory of the optical and ellipsometric properties of the electron gas at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface. The reflectivity and ellipsometry angles are obtained by calculating the random phase approximation (RPA) dielectric constant including the optical phonon of STO and the charge response of the electron gas. We find a dip in the ellipsometry angle at the plasma edge of STO phonon that is related with in-plane Drude response and a peak in high energy from the plasmon excitation of yz and xz electrons and show how these may be related to subband occupancy and scattering rates. Comparison of the theory to published data indicates that about 80\% of electrons in xy band are inert to optical transition, possibly explaining the discrepancy in charge density between transport measurements and polar catastrophe scenario. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F12.00004: Optical conductivity of SrTiO$_3$ based interfaces Ming Xie, Guru Khalsa, Allan MacDonald Since the discovery of a high mobility two-dimensional electron gas at the interface of LaAlO$_{3}$/SrTiO$_{3}$, there has been a large scientific effort to understand the properties of perovskite interfaces. Naturally, this effort has focused on magneto-transport and photoemission studies. Here we use the Kubo formalism to study the optical conductivity of SrTiO$_3$ based interfaces and discuss its implications on the underlying physical properties of these systems. In particular, the response to light polarized in- and out-of-plane will be contrasted. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:24AM |
F12.00005: Oxygen vacancies and magnetism at titanate interfaces Invited Speaker: Natalia Pavlenko Breaking the translation or inversion symmetry at surfaces and interfaces may lead to the formation of new charge, spin and orbital electronic states which are different than the bulk states. The emergence of these states is particularly relevant for oxides where the balance of competing interactions and the resulting stable electronic phase crucially depend on the local oxidation state near the interface. A prominent example is the interface of LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO), which exhibits a two-dimensional electron liquid state in the structures with LaAlO$_{3}$-layers more than 4uc thick, and undergoes a transition into a superconducting state below 0.2 K. Depending on growth conditions LAO/STO has also been found to display pronounced magnetotransport effects indicating the existence of local moments. Recently, even a coexistence of ferromagnetism and superconductivity has been reported, possibly due to an electronic phase separation within the interface. We analyze the magnetic state at the LAO/STO interface within density functional theory and provide evidence that it is caused by the spin polarization of Ti 3d interface electrons. The magnetic state depends strongly on the oxidation state of the interfaces. We show that oxygen vacancies at titanate interfaces induce a complex multiorbital reconstruction which involves a lowering of the local symmetry and an inversion of t$_{2g}$ and e$_{g}$ orbitals resulting in the occupation of the e$_{g}$ orbitals of Ti atoms neighboring the O vacancy. In contrast to stoichiometric nonmagnetic interfaces of LaAlO$_{3}$ and SrTiO$_{3}$, the vacancy-induced orbital reconstruction at LAO/STO interfaces generates a two-dimensional interface magnetic state not observed in bulk SrTiO$_{3}$. We demonstrate that oxygen vacancies in the TiO$_{2}$ interface layer enhance the tendency for ferromagetism considerably. This allows for the notion that areas with increased density of oxygen vacancies produce ferromagnetic puddles and account for the previous observation of a superparamagnetic behavior in the superconducting state. Using generalized gradient approximation (LSDA) with intra-atomic Coulomb repulsion (GGA$+$U), we find that this magnetic state is common for titanate surfaces and interfaces. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F12.00006: Local Moment Formation and Magnetism at LaAlO$_3$/SrTiO$_3$ Interfaces Onur Erten, Sumilan Banerjee, Mohit Randeria One of the most exciting observations at oxide interfaces relate to the observation of magnetism at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface, since neither material is magnetic in the bulk even with doping. Experiments [1,2] give incontrovertible evidence for local moments at the LAO/STO interface, consistent with an areal density close to 0.5 per interfacial Ti atom. The particular splitting of the $t_{2g}$ orbitals at the interface, leads to a quarter-filled $d_{xy}$ band on the top band. Using a slave-rotor approach for the on-site Coulomb interaction U and Hartree-Fock for nearest neighbor V, we show that local moments form in a checkerboard charge-ordered insulating (COI) state, even for a very modest values of $U$. Phonons further stabilize the COI state as the breathing mode couples cooperatively to the charge order. To understand the magnetic interactions between moments, we examine both the small superexchange and the dominant kinetic exchange mediated by conduction electrons. We show that this leads to a ferromagnetic double exchange model with some very interesting twists arising from the Rashba SOC of the conduction electron due to broken inversion at the interface. Ref: [1] L. Li et al., Nature Phys. 7 762. [2] J. A. Bert et al., Nature Phys. 7 76. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F12.00007: Theory for magnetic exchange and anisotropy at the LaAlO$_3$/SrTiO$_3$ interface Sumilan Banerjee, Onur Erten, Mohit Randeria The LaAlO$_3$/SrTiO$_3$ interface exhibits unusual magnetic properties with a large density of local moments seen by both torque and scanning SQUID experiments. We develop a model where local moments are formed on a 2D checkerboard lattice due to correlation-driven charge ordered insulator. We focus on the double exchange interaction of these moments via conduction electrons with a large Rashba spin-orbit coupling (SOC) due to broken inversion at the interface. We derive an effective Hamiltonian for the local moments that has an unusual double square-root ferromagnetic exchange, previously seen in a different context [1]. Two new features arise from SOC, direction-dependent anisotropic exchanges and Dzyaloshinskii-Moriya type terms, which can be tuned by gating. We show that SOC accounts for the the unusually large easy-plane magnetic anisotropy seen in experiment. We will explore the phase diagram, as a function of the strength of Rashba SOC, to see what unusual magnetic states might be stabilized. We will comment on the possibility of reconciling the apparently different conclusions reached by torque and scanning SQUID measurements regarding the magnetic ordering and ordered moment.\\[4pt] [1] O.~Erten et al., Phys. Rev. Lett. 107, 257201 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F12.00008: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F12.00009: Intrinsic spin Hall effect at oxide interfaces: a simple model Lorien Hayden, Roberto Raimondi, Michael Flatte', Giovanni Vignale An asymmetric triangular potential well provides one of the simplest model for the confinement of mobile electrons at the interface between two insulating oxides, such as LaAlO$_3$ and SrTiO$_3$ (LAO/STO). In this paper we study the intrinsic spin Hall effect due to Rashba coupling in an asymmetric triangular potential well. Besides splitting each subband into two branches of opposite chirality, the spin-orbit interaction causes the wave function in the direction perpendicular to the plane of the quantum well (i.e., the growth direction) to depend on the in plane wave vector $kv$. In contrast to the extreme asymmetric case, i.e., the wedge-shaped quantum well, for which the intrinsic spin Hall effect is known to vanish due to vertex corrections, we find that the asymmetric well supports a non-vanishing intrinsic spin Hall conductivity, proportional to the square of the spin-orbit coupling constant. Its origin lies in the non-vanishing matrix elements of the spin current between subbands corresponding to different states of quantized motion perpendicular to the plane of the well. Vertex corrections are carefully considered, both for the intra-band and the inter-band contributions to the spin Hall conductivity. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F12.00010: Theory of spin-orbit effects in the t$_{2g}$ band of pseudo-cubic perovskites Guru Khalsa, Byounghak Lee, Allan MacDonald Epitaxial interfaces of perovskite systems have recently been the focus of an enormous amount of research due to their novel properties and potential for integration with silicon based technologies. Although the role of spin-orbit effects has been discussed in the literature, a first principles study of their influence on electronic structure has been lacking. We have conducted a study of spin-orbit effects in pseudo-cubic t$_{2g}$ perovskite systems in which inversion symmetry has been broken by the presence of an external electric field. In this talk, we discuss our results and compare with available magneto transport studies on LAO/STO and related systems. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F12.00011: Theory of spin-orbit coupling at LaAlO3/SrTiO3 interfaces and SrTiO3 surfaces Zhicheng Zhong, Anna Toth, Karsten Held A full theoretical understanding of the spin-orbit coupling (SOC) effects at LaAlO$_{3}$/SrTiO$_{3}$ interfaces and SrTiO$_{3}$ surfaces is still needed. We perform first-principles density-functional-theory calculations and derive from these a simple tight-binding Hamiltonian, through a Wannier function projection and group theoretical analysis. We find striking differences to the standard Rashba theory for spin-orbit coupling in semiconductor heterostructures, because the relevant $t_{2g}$ orbitals are very different from nearly free electrons. The key ingredients to the spin splitting are the atomic SOC and the interface asymmetry, which enters via asymmetric $t_{2g}$ orbital lobes. ArXiv:1209.4705 by Zhicheng Zhong, Anna Toth, Karsten Held [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F12.00012: Transport through oxide interfaces - The case of SrTiO$_3$ based hetero-structures M. Ben Shalom, E. Flekser, Y. Dagan, M. Kim, C. Bell, Y. Hikita, H.Y. Hwang Sharp interfaces can host phenomena that are absent in their constituting materials. By depositing a thin layer of LaAlO$_3$ on top of SrTiO$_3$, the interface between these two band-insulator is highly conducting. Conductivity emerges only for TiO$_2$ termination and above a critical LaAlO$_3$ thickness of 4 unit cells, pointing to the importance of the polar structure. The transition, from insulating to high mobility electron gas, can be controlled continuously by gate voltage, thus enabling a careful study of the dependence of system properties on charge density. Carrier-controlled two-dimensional superconductivity, and magnetic hysteresis were observed between the two non-magnetic oxides. We have found anisotropic magnetoresistance (AMR) in our samples, an outcome of magnetic scattering, which affect the transport through the spin orbit (SO) interaction, and coexists with superconductivity. Gate bias enables tuning the SO energy, which dominates the magnetotransport properties. The exceptionally large amplitude and sign of the AMR suggests a Rashba-type SO coupling. The different AMR characteristics for Nb doped SrTiO$_3$, a symmetric non-polar with similar resistivity and carrier density, demonstrates the significant role of interface polarity for its magnetic properties. [Preview Abstract] |
Session F13: Topological Insulators: Theory I
Sponsoring Units: DCMPChair: Pouyan Ghaemi, University of Illinois
Room: 315
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F13.00001: Homological Order in Three and Four dimensions: Wilson Algebra, Entanglement Entropy and Twist Defects Abhishek Roy, Xiao Chen, Jeffrey Teo We investigate homological orders in two, three and four dimensions by studying $Z_k$ toric code models on simplicial, cellular or in general differential complexes. The ground state degeneracy is obtained from Wilson loop and surface operators, and the homological intersection form. We compute these for a series of closed 3 and 4 dimensional manifolds and study the projective representations of mapping class groups (modular transformations). Braiding statistics between point and string excitations in (3+1)-dimensions or between dual string excitations in (4+1)-dimensions are topologically determined by the higher dimensional linking number, and can be understood by an effective topological field theory. An algorithm for calculating entanglemnent entropy of any bipartition of closed manifolds is presented, and its topological signature is completely characterized homologically. Extrinsic twist defects (or disclinations) are studied in 2,3 and 4 dimensions and are shown to carry exotic fusion and braiding properties. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F13.00002: Weyl points and line nodes in gapless gyroid photonic crystals Ling Lu, Liang Fu, John Joannopoulos, Marin Soljacic Weyl points and line nodes are three-dimensional linear point- and line-degeneracies between two bands. In contrast to Dirac points, which are their two-dimensional analogues, Weyl points are stable in the momentum space and the associated surface states are predicted to be topologically non-trivial. However, Weyl points are yet to be discovered in nature. Here, we report photonic crystals, based on the double-gyroid structures, exhibiting frequency-isolated Weyl points with complete phase diagrams by breaking the parity and time-reversal symmetries. The surface states associated with the non-zero Chern numbers are demonstrated. Line nodes are also found in similar geometries; the associated surface states are shown to be at bands. Our results are based on realistic ``numerical experiments'' with true predictive power and should be readily experimentally realizable at both microwave and optical frequencies. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F13.00003: The optical conductivity of quasicrystals: evidence of a Weyl semimetal with 3D Dirac spectrum Thomas Timusk, Jules Carbotte, Christopher Homes, Dimitri Basov, Sergei Sharapov The optical conductivity of quasicrystals is characterized by an absence of the Drude peak and a conductivity that rises linearly over a wide range of frequencies. The absence of the Drude peak has been attributed to a pseudogap at the Fermi surface but a detailed explanation of the linear behavior has not been found. This unusual behavior is seen in all icosahedral quasicrystal families and their periodic approximants. A simple model that assumes that the entire Fermi surface is gapped, with the exception at a finite set of Dirac points, fits the data. There is no evidence of a semiconducting gap in any of the materials suggesting that the massless Dirac spectrum is protected by topology leading to a Weyl semimetal. The model gives rise to a linear conductivity with only one parameter, the Fermi velocity. In accord with this picture decagonal quasicrystals should have a frequency independent conductivity, without a Drude peak. This is in accord with the experimental data as well. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F13.00004: Chiral magnetic effect in Weyl semimentals and insulators Mohammad Vazifeh, Marcel Franz It has been proposed recently, on the basis of field-theoretical considerations, that the effective electromagnetic action of Weyl semimetals (as well as the closely related Weyl insulators) contains an axion term with the $\theta$-angle dependent on time $t$ or spatial position ${\bf r}$. If correct this would lead to a number of novel observable phenomena, such as the chiral magnetic effect, whereby an applied uniform magnetic field induces a dissipationless bulk current ${\bf j}\propto {\bf B}$. In this work we construct a simple lattice model for a Weyl semimental (insulator) and use it to explicitly test for the chiral magnetic effect by means of numerical techniques combined with analytical calculations. We discuss possible ways to engineer a suitable material in layered nanostructures and comment on the physical observability of the effect. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F13.00005: Axion field theory, chiral anomaly and anomalous non-dissipative transport properties of (3+1)-dimensional Weyl semi-metals and superconductors Pallab Goswami, Sumanta Tewari From a direct calculation of the anomalous Hall conductivity and an effective electromagnetic action obtained via Fujikawa's chiral rotation technique, we conclude that an axionic field theory with a non-quantized coefficient describes the electromagnetic response of the (3+1)-dimensional Weyl semi-metal. The coefficient is proportional to the momentum space separation of the Weyl nodes. Akin to the Chern-Simons field theory of quantum Hall effect, the axion field theory violates gauge invariance in the presence of the boundary, which is cured by the chiral anomaly of the surface states via the Callan-Harvey mechanism. A direct linear response calculation also establishes an anomalous thermal Hall effect and a Wiedemann-Franz law. But, thermal Hall conductivity does not directly follow from the well known formula for the (3+1)-dimensional gravitational chiral anomaly. By calculating the gravitational chiral anomaly at finite temperature we show the existence of a new term, which correctly accounts for the thermal Hall effect in (3+1)-dimensional Weyl materials, topological insulators and topological superconductors. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F13.00006: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F13.00007: Friedel oscillations due to Fermi arcs in Weyl semimetals Pavan Hosur Weyl semimetals harbor unusual surface states known as Fermi arcs, which are essentially disjoint segments of a two-dimensional Fermi surface. We describe a prescription for obtaining Fermi arcs of arbitrary shape and connectivity by stacking alternate two-dimensional electron and hole Fermi surfaces and adding suitable interlayer coupling. Using this prescription, we compute the local density of states--a quantity directly relevant to scanning tunneling microscopy--on a Weyl semimetal surface in the presence of a point scatterer and present results for a particular model that is expected to apply to pyrochlore iridate Weyl semimetals. For thin samples, Fermi arcs on opposite surfaces conspire to allow nested backscattering, resulting in strong Friedel oscillations on the surface. These oscillations die out as the sample thickness is increased and Fermi arcs from the opposite surface retreat and weak oscillations, due to scattering between the top surface Fermi arcs alone, survive. The surface spectral function, accessible to photoemission experiments, is also computed. In the thermodynamic limit, this calculation can be done analytically and separate contributions from the Fermi arcs and the bulk states can be seen. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F13.00008: Topological Phases of Point Group Symmetric Weyl Superconductors Vasudha Shivamoggi, Chen Fang, Taylor Hughes, Matthew Gilbert We study superconductivity in a Weyl semimetal with broken time-reversal symmetry and stabilized by a point-group symmetry. The resulting superconducting phase is characterized by topologically protected bulk nodes and surface states with Fermi arcs. We derive a phase diagram of possible superconducting phases which are distinguished by the number of bulk nodes and discuss novel properties of the corresponding surface states. We show how the topological behavior may be understood in terms of the properties of the parent Weyl semimetal at high-symmetry momenta. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F13.00009: Excitonic Phases of Weyl Semi-Metals with Coulomb Interaction Huazhou Wei, Sung-Po Chao, Vivek Aji Weyl semi-metals have an even number of nodes which are perfectly nested in the absence of a chiral chemical potential. For repulsive interactions these are susceptible to excitonic instabilities. The vanishing density of states requires that the coupling be larger than a critical value for the states to be realized. There are eight possible states in the particle-hole channel, only two of which gap out the weyl nodes for long range Coulomb interactions. The lowest energy state is the Charge Density Wave state, which is more stable than the ferromagnetic insulator that arises in the context of short range repulsion. The defects of the state, i.e. dislocations, have been shown in the literature, to carry gapless chiral modes. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F13.00010: Excitonic Phases from Weyl Semi-Metals with short range interaction Sung-Po Chao, Huazhou Wei, Vivek Aji Weyl semimetal, possibly realized in Pyrochlore irridates or supperlatice of 3D topological-normal insulators system, has strong spin orbit interactions leading to effective low energy described by massless linearly dispersing fermions. In the absence of interactions chirality is a conserved quantum number, protecting the semi-metallic physics against perturbations that are translationally invariant. We show that the interplay between short range repulsive interaction and topology yields a novel chiral excitonic insulator. The state is characterized by a complex vectorial order parameter leading to a gapping out of the Weyl nodes. An interesting feature is that it is ferromagnetic, with the phase of the order parameter determining the direction of the induced magnetic moment. The case of Coulomb interaction will be discussed by Huazhou Wei in his report. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F13.00011: Probing the Chiral Anomaly via Nonlocal Transport in Weyl Semimetals Siddharth Parameswaran, Tarun Grover, Ashvin Vishwanath Weyl semimetals are three-dimensional analogs of graphene in which a pair of bands touch at points in momentum space, known as Weyl nodes. Electrons originating from a single Weyl node possess a definite topological charge, the chirality. Consequently, they exhibit the Adler-Jackiw-Bell anomaly, which in this condensed matter realization implies that application of parallel electric ($\mathbf{E}$) and magnetic fields ($\mathbf{B}$) pumps electrons between nodes of opposite chirality at a rate proportional to $\mathbf{E}\cdot\mathbf{B}$. We argue that this pumping is measurable via transport experiments, in the limit of weak internode scattering. Specifically, we show that injecting a current in a Weyl semimetal subject to an $\mathbf{E}\cdot\mathbf{B}$ term leads to nonlocal features in transport. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F13.00012: Effective field theories for topological insulators by functional bosonization Pak On Chan, Taylor L. Hughes, Shinsei Ryu, Eduardo Fradkin Effective field theories that describe the dynamics of electric current for topological insulators in general dimension D = d+1 are discussed using the functional bosonization. For non-interacting topological insulators with a conserved U(1) charge and characterized by an integer topological invariant, we derive the BF-type topological field theories supplemented with the Chern-Simons (when D is odd) or the Axion term (when D is even). For topological insulators characterized by a Z2 topological invariant, their topological field theories are obtained by dimensional reduction. Building on this effective field theory description for non- interacting topological phases, we also discuss, following the spirit of the parton construction of the fractional quantum Hall effect, the putative ``fractional'' topological insulators and their possible effective field theories. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F13.00013: Physics of three dimensional bosonic topological insulators I Ashvin Vishwanath, Todadri Senthil We discuss physical properties of ``integer'' topological phases of bosons in D=3+1 dimensions, protected by internal symmetries like time reversal and/or charge conservation. These phases invoke interactions in a fundamental way but do not possess topological order and are bosonic analogs of free fermion topological insulators and superconductors. Here we develop a field theoretic description of several of these states and show that they possess unusual surface states, which if gapped, must either break the underlying symmetry, or develop topological order. In certain cases the topological phases are characterized by a quantized magneto-electric response $\theta$, which, somewhat surprisingly, is an odd multiple of $2\pi$. A surface theory in which vortices transform under a projective representation of the symmetry group is shown to capture these properties. A bulk field theory of these states is also identified, which furthermore predicts phases characterized by the thermal analog of the magneto-electric effect, that lie beyond the current cohomology description. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F13.00014: Physics of three dimensional bosonic topological insulators II S. Todadri, Ashvin Vishwanath We discuss physical properties of interacting boson/spin analogs of free fermion topological insulators and superconductors. We discuss general constraints on the surface theories of these phases, and their field theoretic descriptions. We illustrate some of the results in the context of quantum paramagnetic phases of spin systems. For the 3d states we describe the 2d surface either spontaneously breaks symmetry or is in a spin liquid phase. In the latter case the symmetry is realized in the surface spin liquid in a way that is forbidden in strictly two dimensional quantum magnets. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F13.00015: Wilson-loop Classification of Inversion-Symmetric Topological Insulators and the Z$_2$ Crystalline Topological Insulator A. Alexandradinata, Xi Dai, B. Andrei Bernevig In the context of translationally-invariant insulators, Wilson loops describe the adiabatic evolution of the ground state around a closed circuit in the Brillouin zone. We propose that the Wilson-loop eigenspectrum provides a complete characterization of (a) the inversion-symmetric topological insulator, and (b) the \textbf{Z}$_2$ crystalline topological insulator: time-reversal symmetric insulators with either C$_4$ or C$_6$ rotational symmetry, but with no spin-orbit coupling. For the 1D inversion-symmetric insulator, we formulate a \textbf{Z} Wilson-loop index, which is identifiable with the number of protected boundary modes in the entanglement spectrum. For the 2D inversion-symmetric insulator, we identify a \textbf{Z} relative-winding number, which is the inversion-analog of the first Chern class (for charge-conserving insulators). For the \textbf{Z}$_2$ crystalline topological insulator, we show how the \textbf{Z}$_2$ invariant can be extracted from the Wilson-loop eigenspectrum; this aids the identification of materials that realize this phase. [Preview Abstract] |
Session F14: Focus Session: Material Properties Important for Spin-torque Dynamics
Sponsoring Units: DMP FIAP GMAGChair: Ting Yong Chen, Arizona State University
Room: 316
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F14.00001: Quantifying spin torque effects using a current-driven magnetic vortex Invited Speaker: Kristen Buchanan Spin transfer torques offer great potential for the development of spin-based devices for processing and storing information but there is still debate surrounding the relative contributions of the adiabatic and non-adiabatic spin torque effects. Magnetic vortices in patterned magnetic films provide a model system that can be used to quantify these effects. Micromagnetic calculations of the current-driven motion of a magnetic vortex in a patterned Permalloy element show that the two spin torque effects have distinguishable influences on the trajectories of the vortex core and, furthermore, that the effect of the current-generated magnetic fields (Oersted) that are often non-negligible when current flows through magnetic nanostructures can also be separated out. An analysis of a series of experimental images of vortex trajectories obtained using a recently developed dynamic Lorentz transmission electron microscopy technique provides a measure of the non-adiabatic spin torque parameter with greatly improved precision [1]. The work described here was carried out in collaboration with Shawn Pollard, L. Huang, Dario Arena, and Yimei Zhu (Brookhaven). \\[4pt] [1] S. Pollard, L. Huang, K. S. Buchanan, D. Arena, and Y. Zhu, {\it Nature Communications} {\bf 3}, 1028, 2012. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F14.00002: Anisotropy of spin relaxation in metals and ultrathin metallic films Nguyen H. Long, Phivos Mavropoulos, Bernd Zimmermann, Swantje Heers, Stefan Blugel, Yuriy Mokrousov We predict a hitherto overlooked anisotropy of the spin relaxation time $T_1$ in non-magnetic metallic systems with respect to the orientation of the spin porlarization $\hat{s}$ of the injected electrons relative to the crystallographic directions. In the Elliott-Yafet mechanism, the spin relaxation time is related to the Elliott-Yafet parameter $b^2$ that quantifies the degree of spin-mixing of Bloch states due to spin-orbit interaction. It can be demonstrated that $b^2$ depends on $\hat{s}$ due to the directional dependence of the spin-orbit matrix-elements between Bloch states comprising directional orbitals. The directional dependence becomes very pronounced in the case of degeneracies or near-degeneracies leading to {\it spin-flip hot spots} or even extended {\it hot areas} on the Fermi surface. The calculated anisotropy can reach values as large as 830\% for hcp Hf or 87\% in W(110) 10-layer-films, as we find from first-principles calculations employing the Kohn-Korringa-Rostoker Green function method. The anisotropy offers interesting new functionalities in spintronics applications such as GMR, spin Hall effect as well as spin dynamics. [1] B. Zimmermann, P. Mavropoulos, S. Heers, N. H. Long, S. Blugel, and Y. Mokrousov, Phys. Rev. Lett., in press (arXiv:1210.1801). [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F14.00003: Extraction of spin-transport parameters from ferromagnetic resonance measurements of spin-pumping in metallic multilayers Carl Boone, Hans Nembach, Justin Shaw, Thomas Silva We use ferromagnetic resonance to measure damping due to spin-pumping in symmetric multilayers of Ta(3)/Ni(x)/Pd(y)/CoFe(2)/Pd(y)/Ni(x)/Ta(3) (thicknesses in nm, 0$\le $x$\le $2nm, 0$\le $y$\le $10nm). The stack's symmetry ensures that spin-pumping on both sides of the ferromagnet is identical and allows us to unambiguously characterize the multilayer spin-transport properties. When x $=$ 0 (no Ni), the Pd-Ta interface is found to be strongly spin-impedant, due to the low spin conductivity of Ta, leading to greatly reduced damping for thin Pd. As the Pd thickness approaches the spin diffusion length, the damping increases. By inserting Ni, a strong spin absorber, at the Pd/Ta interface, the damping for thin Pd is maximized. Varying the thickness of the Ni layer can be used to tune the inter-layer spin current flow in the Pd/Ni/Ta heterostructure. Comparison of the data with the conventional model for diffusive spin-polarized transport in normal metals permits quantitative determination of the spin-diffusion length in the normal metals. The results have implications for the detection of spin-currents in lateral spin valves via the inverse spin Hall effect in high-resistivity materials such as Ta. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F14.00004: Controlling the Gilbert damping using spin pumping and magnetic impurities Tim Verhagen, Holly Tinkey, Jan van Ruitenbeek, Jan Aarts The ability to control the magnetic damping parameter of thin magnetic films is an important issue when designing for example giant magnetoresistance (GMR) devices. A well-known way to influence the damping of the ferromagnetic (F) layer is by using the spin pumping effect in which a spin current is emitted into an adjacent normal (N) layer by bringing the F-layer into ferromagnetic resonance (FMR). As N layer, we used the well studied strongly spin sinking material Pt and the bad spin sink Cu, but also a Cu layer with Co impurities. We find that by adding a small amount of Co impurities, the Cu layer becomes as effective in damping as a Pt layer. In the latter case, the damping is caused by the strong spin orbit coupling. Using magnetic impurities, we rather make use of the inelastic spin scattering. This opens up new ways to control the damping of a ferromagnetic thin layer, for example in current-in-plane (CIP) GMR sensors, where the extra damping can suppress the spin transfer torque which becomes dominant with the further decrease of the size of the sensor. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F14.00005: Gilbert damping parameter characterization in perpendicular magnetized Co$_{2}$FeAl films Yishen Cui, Jiwei Lu, Behrouz Khodadadi, Sebastian Sch\"afer, Tim Mewes, Stuart Wolf Materials with perpendicular magnetic anisotropy(PMA) have gotten extensive recent attention because of their potential application in spintronic devices such as spin transfer torque random access memory (STT-RAM). It was shown that a much lower switching current density(J$_{\mathrm{C}})$ is required to write STT-RAM tunnel junctions with perpendicular magnetic anisotropy ferromagnetic electrodes (p-MTJ). Additionally Heusler alloy Co$_{2}$FeAl is expected to further reduce J$_{\mathrm{C}}$ due to its ultra low Gilbert damping parameter. In our study, Heusler alloy Co$_{2}$FeAl films were prepared using a Biased Target Ion Beam Deposition (BTIBD) technique. We demonstrated a low Gilbert damping parameter achieved in thick B2-Co$_{2}$FeAl films. Besides, we achieved an interfacial PMA in ultra thin Co$_{2}$FeAl films by rapid thermal annealing (RTA) with no external field presented. Annealing conditions were carefully adjusted to maximize the interfacial PMA. However it was noticed that a higher annealing temperature was required for a low damping parameter which to some extent sacrificed the interfacial PMA. We also deposited ultra thin CoFeB films and characterized their damping parameters for comparison. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F14.00006: Bonding, magnetic properties and stability of the half-Heusler alloys LiMnZ (Z=N, P, Si) Liam Damewood, Brian Busemeyer, C.Y. Fong, Michael Shaughnessy We examined the bonding and magnetic properties, as well as the stability, of three magnetic half-Heusler alloys, namely LiMnZ, with Z=N, P or Si, in the three different atomic ordering of the C1$_b$ crystal structure (i.e. $\alpha$, $\beta$, and $\gamma$ phases). Using LiMnP as a prototype, we examined the bonding properties of the three phases and found, at the optimized lattice constant, each phase is a ferromagnetic metal. Assuming a proper matching substrate could be found, we found that $\alpha$-LiMnN, LiMnSi in the $\beta$ and $\gamma$ phases, and LiMnP in the $\alpha$ and $\beta$ phases can be ferromagnetic half-metals at lattice constants larger than the optimized values. Volume stability studies showed that the $\beta$ phase is the most stable for these materials. In our search for more half-metals, we found that $\beta$-Li$_{0.75}$MnSi, $\beta$-Li$_{0.75}$MnP and $\gamma$-Li$_{0.75}$MnN can be half-metals at the respective LiMnSi half-metallic lattice constants. By comparing $\beta$-LiMnP to the metastable zinc blende phase of MnP, the role of Li in the structure, with respect to the elastic stability, electronic properties, and magnetic properties, was studied. Finally we examined the possibility of a compensated half-metallic phase in these materials. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F14.00007: Dirac Half-Metal in a Triangular Ferrimagnet Hiroaki Ishizuka, Yukitoshi Motome Massless Dirac fermions show substantially different nature from ordinary electrons due to the peculiar cone-like dispersion with the point node. While it was originally introduced in the relativistic quantum theory, recent discovery of graphene, a single layer sheet of graphite, has carved out a new direction of their study in condensed matter systems. From the viewpoint of potential application to electronics, it is of great interest to control the electronic spin degree of freedom. However, there is not so much flexibility in graphene, as the relativistic spin-orbit interaction is very weak. Here, we present an alternative idea for realizing massless Dirac fermions in itinerant electrons coupled to a well-known ferrimagnet on a triangular lattice [1]. The Dirac fermions are spin-polarized, and stable in a wide range of the spin-charge coupling including typical values in solids. We demonstrate that, by an unbiased Monte Carlo simulation, such Dirac half-metal with ferrimagnetic order spontaneously emerges in a minimal Kondo-lattice type model. The realization will be beneficial for spintronics as a candidate for spin-current generator.\\[4pt] [1] H. Ishizuka and Y. Motome, preprint (arXiv:1210.6700), PRL in press. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F14.00008: GMAG PhD Dissertation Research Award Talk: Experimental characterization of non-isochronous properties of spin torque nano-oscillators Invited Speaker: Michael Quinsat Spin Torque Oscillators (STO) are nano-sized auto- oscillators whose frequency can be tuned in a wide range. This tuning originates from the nonlinear properties of the underlying magnetization dynamics that is induced by spin transfer torque (STT) in magnetic nanostructures. Being highly tunable in frequency has the inconvenience of being very sensitive to noise. As a result the spectral purity of STOs is below the one required for applications. The magnetization dynamics induced by STT has been described theoretically in the frame of a non-isochronous auto-oscillator model [1]. Within this model the important information on the excitation mode is contained within two phenomenological parameters which are, the amplitude-phase coupling $\nu $, and the amplitude relaxation rate $\Gamma_{\mathrm{p}}$. They completely determine the non-autonomous oscillator response and their experimental determination is thus an important issue. This presentation describes several experimental methods to extract these parameters. The first involves time domain noise spectroscopy which permits the power spectral density of phase and amplitude noise to be extracted [2]. The analysis of such noise in light of theoretical models allows not only direct extraction of the nonlinear parameters, but also to quantify the phase noise - the characteristics important for applications. This is demonstrated for magnetic tunnel junction devices. A second method involves the analysis of higher harmonic linewidths $\Delta $f$_{\mathrm{n}}$ [3], where it is shown that due to the non-isochronous property of STOs, the relationship between $\Delta $f$_{\mathrm{n}}$ and $\Delta $f$_{\mathrm{1}}$ is nontrivial and allows one to extract $\nu $ and $\Gamma_{\mathrm{p}}$. We then apply the information gathered on the autonomous dynamics of STOs to understand their non-autonomous dynamics that are a prerequisite for the use of STOs in more complex devices (field sensors, frequency synthesizers, etc). It is shown experimentally how the nonlinear parameters $\nu $ and $\Gamma_{\mathrm{p}}$ determine this non-autonomous behavior of the STO. \\[4pt] [1] A. Slavinand and V. Tiberkevich, IEEE Trans on Mag 45, 1875 (2009)\\[0pt] [2] M. Quinsat et al. APL 97, 182507 (2010)\\[0pt] [3] M. Quinsat et al. PRB 86, 104418 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F14.00009: Correlation effects in disordered conductors with spin accumulation Alexander Zyuzin We consider the effect of electron-electron interaction on the density of states of disordered thin film paramagnetic conductor in the presence of spin accumulation and magnetic field. We assume a mechanism of electrical spin injection from a ferromagnet into a paramagnet as a particular realization of the spin accumulation. We show that interaction correction to the electron density of states of the paramagnet exhibits singularities at energies corresponding to the difference between chemical potentials of electrons with opposite spins. The correction to the conductivity of the paramagnet in the metallic region as well as in the hopping region in the presence of spin accumulation is calculated. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F14.00010: Spin transport in the Neel and collinear antiferromagnetic phase of the two dimensional spatial and spin anisotropic Heisenberg model on a square lattice Trinanjan Datta, Zewei Chen, Dao-Xin Yao We analyze and compare the effect of spatial and spin anisotropy on spin conductivity in a two dimensional S=1/2 Heisenberg quantum magnet on a square lattice. We explore the model in both the Neel antiferromagnetic (AF) phase and the collinear antiferromagnetic (CAF) phase. We find that in contrast to the effects of spin anisotropy in the Heisenberg model, spatial anisotropy in the AF phase does not suppress the zero temperature regular part of the spin conductivity in the zero frequency limit - rather it enhances it. In the CAF phase (within the non-interacting approximation) the zero frequency spin conductivity has a finite value which is suppressed as the spatial anisotropy parameter is increased. Furthermore, the CAF phase displays a spike in the spin conductivity not seen in the AF phase. We also explore the finite temperature effects on the Drude weight in the AF phase (within the collision less approximation). We find that enhancing spatial anisotropy increases the Drude weight value and increasing spin anisotropy decreases the Drude weight value. Based on these studies we conclude that antiferromagnets with spatial anisotropy are better spin conductors than those with spin anisotropy at both zero and finite temperatures. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F14.00011: Study of the Switching Current Density Reduction in Spin Transfer Torque Magnetic Tunneling Junction by Using Micromagnetic Simulations Chun-Yeol You, Myung-Hwa Jung We investigate the reduction of switching current density of the spin transfer torque magnetic tunneling junction (STT-MTJ) with micromagnetic simulations for the various parameters and structures. We introduce a non-collinear magnetization polarizer layer and find noticeable switching current density reduction. The physical origin of the reduction ascribe to the enhanced coherent spin rotation due to the asymmetry breaking. Furthermore, when we cut the one edge of the MTJ ellipse structure, conspicuous reduction of switching current density is also found. In contrast to the normal MTJ structure where the switching process is accompanied with non-coherent spin dynamics, enhanced coherent spin rotations play an important role in both new structures. In addition, we find that the switching current density is sensitively varied with the exchange stiffness and junction size due to the weakly quantized spin wave vector. Based on our micromagnetic simulations, we open new path to reduce the switching current density with new MTJ structures. [Preview Abstract] |
Session F15: Focus Session: New Frustrated Quantum States: Theory & Materials
Sponsoring Units: GMAG DMPChair: Piers Coleman, Rutgers University
Room: 317
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F15.00001: Lattices of Magnetic Vortices in a Frustrated Mott Insulator Invited Speaker: Cristian Batista Chiral spin textures with different length scales emerge in some itinerant magnets and are attracting increasing interest in the study of magneto-transport and possible applications to magnetic data storage and spin-electronic devices. It is natural to ask if similar topological textures can emerge in Mott insulators and also lead to magneto-electric effects. In this talk I will show that this is indeed possible when the exchange interactions are geometrically frustrated. For this purpose, I will consider a frustrated S$=$1/2 XXZ Hamiltonian that is a low-energy effective model for Ba$_{\mathrm{3}}$Mn2O$_{\mathrm{8}}$, a novel layered spin-dimer compound, comprising magnetic dimers of Mn$^{\mathrm{5+}}$ ions arranged on triangular planes. Successive layers are stacked following an ABC sequence, such that the dimer units on adjacent planes are positioned in the center of the triangular plaquettes of the layers above and below. The effective exchange anisotropy of the low-energy model results from frustration between exchange interactions connecting the same pair of dimmers. The competition between intra and inter-layer exchange interactions leads to a triplon dispersion with six-fold degenerate minima at the incommensurate wave vectors $\pm $Q$_{n}$ (1$\le n\le $3). This degeneracy leads to a very rich quantum phase diagram near the magnetic field induced quantum critical point, that is constructed by adding ladder diagrams and minimizing the resulting energy functional. The phase diagram includes different multi-Q magnetic orderings, which combine up to the six degenerate incommensurate lowest-energy modes $\pm $Q$_{n}$ (1$\le n\le $3). In particular, it includes a six-Q state that consists of a lattice of magnetic vortices and other complex spin textures associated with different multi-Q ordered states. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F15.00002: Unconventional quantum critical points in the generalized quantum dimer models Zi-Xiang Li, Fan Yang, Hong Yao We study a class of generalized quantum dimer models with both NN and NNN dimers on the square lattice, whose exact ground state wave function can be constructed. By varying the weight of NNN dimers and the interactions between dimers, we obtain a rich quantum phase diagram which hosts quantum spin liquid phases and various valence bond solids. We then investigate the quantum critical behavior of the transitions between spin liquids and valence bond solids by analytically studying its effective field theory and numerically doing variational Monte Carlo simulations. We discuss unconventional quantum critical points in this system. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F15.00003: Supersolids and Anomalous Hysteresis in Frustrated Spin-Dimer Systems Daisuke Yamamoto, Ippei Danshita We study the ground-state properties of weakly coupled spin dimers on a triangular lattice. The competition of the two (direct and crossed) interdimer interactions and the geometry of the triangular lattice lead to a strong frustration. By using a large-size cluster mean-field method and the cluster-size scaling, we determine the quantitative magnetic phase diagram of the system under the presence of a magnetic field. The strong intradimer interaction provides a gapped spin-singlet ground state. If the magnetic field exceeds a certain critical value, the system undergoes a phase transition to a magnetically ordered state, which is known as a Bose-Einstein condensation (BEC) of spin-triplet excitations called ``triplons.'' We find that for strong magnetic fields, the magnetization curve shows plateaus at 1/3 and 2/3 of the total magnetization, in which the local singlet and triplet states form a superlattice pattern. This state can be regarded as a solid of triplons. We also find that if increasing (decreasing) the magnetic field from the 1/3 (2/3) plateau, the BEC of triplons occurs on the superlattice background, leading to the transition into ``magnon supersolid'' phase. The region of supersolid phase in the phase diagram is reasonably large compared to the square-lattice case. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F15.00004: Quantum spin-ordered states for the frustrated XY model on the honeycomb lattice Andrea Di Ciolo, Juan Carrasquilla, Federico Becca, Victor Galitski, Marcos Rigol We consider the frustrated XY model on the honeycomb lattice and determine the stability of several classical spin states supplemented with a long-range Jastrow factor that introduces quantum fluctuations. In particular, we focus on the competition between antiferromagnetic, collinear, and generic spiral order upon increase of frustration. Our investigation is based on Variational Monte Carlo calculations. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F15.00005: Evidence of valence bond condensation in the frustrated cluster magnet LiZn$_{2}$Mo$_{3}$O$_{8}$ John Sheckelton, James Neilson, Daniel Soltan, Tyrel McQueen The reduced molybdenum oxide LiZn$_{2}$Mo$_{3}$O$_{8}$ is a Mott insulating material built of two dimensional layers of magnetic Mo$_{3}$O$_{13}$ triangular clusters, arranged on a triangular lattice. Between these magnetic layers are disordered non-magnetic LiZn$_{2}$ layers. The formal oxidation state and calculations show each molybdenum cluster collectively produces a S$=$1/2 moment. The ``triangle of triangles'' arrangement of magnetic clusters gives rise to exciting frustrated magnetic physics while also preventing Jahn-teller instabilities and site disorder seen in single ion frustrated systems. In addition, the structure allows for facile electronic doping of the magnetic layers. Structural and measured physical properties and ongoing research will be discussed. The evidence discussed indicates the formation of an exotic condensed valence bond state, reminiscent of the long-sought resonating valence bond state. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F15.00006: LiZn$_2$Mo$_3$O$_8$: honeycomb spin liquid in a triangular lattice material? Rebecca Flint, Patrick Lee LiZn$_2$Mo$_3$O$_8$ is a S=1/2 triangular lattice material in which two-thirds of the spins vanish at 100K, while the remaining spins remain free down to the lowest temperatures. There is no thermodynamic phase transition, and does not appear to be any magnetic order. The experimental proposal is that the triangular lattice decouples into a honeycomb lattice with free spins in the center of each hexagon, however, it is not immediately clear what favors this decompostion. We argue that a set of alternating octahedral rotations can strengthen the bonds of the honeycomb lattice while weakening those to the central spin. Furthermore, if the honeycomb lattice forms a $Z_2$ spin liquid, as proposed for the $J_1-J_2$ Heisenberg model, instead of a N\'{e}el or valence bond solid state, the central spin can delocalize over the hexagon, further favoring this decomposition, and also stabilizing the spin liquid phase over the N\'{e}el and VBS phases. Experimentally, this proposal can be tested by searching for signatures of the octahedral rotations, which may be short range or dynamic, but should result in a $q=0$ soft phonon mode. The spinon spectrum of the gapped $Z_2$ spin liquid should also have signatures in inelastic neutron scattering. We also discuss possible 3D analogues. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F15.00007: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F15.00008: Improved cluster-effective-field study on frustrated quantum spin systems in 2D Yoshihiko Nonomura Although frustrated quantum spin systems in two dimensions are fascinating playground of novel quantum states, systematic numerical study with the quantum Monte Carlo method is difficult in such systems owing to the negative sign problem. Then, cluster-effective-field approach may be useful as an alternative numerical tool. Crucial points of formulation are to use periodic boundary clusters and to compare two different clusters. As an example, the $J_{1}$-$J_{2}$ model, where $S=1/2$ Heisenberg spins are located on a square lattice with the nearest-neighbor and next-nearest-neighbor antiferromagnetic couplings $J_{1}$ and $J_{2}$. Classical N\'eel or sublattice N\'eel orders become unstable in the vicinity of $J_{2}/J_{1}=0.5$, where novel quantum states are expected to be stable. When the $16$- and $20$-spin clusters are used and the columnar or staggered dimer orders are taken as order parameters, we have coexisting regions of magnetic and dimer orders and first-order phase transitions between the columnar and staggered dimer orders. Further results based on larger clusters and improved formulations including multi-body effective fields are also discussed in the presentation. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F15.00009: Theory of inelastic neutron scattering in a quantum spin nematic Nic Shannon, Andrew Smerald The idea that a quantum magnet could act like a liquid crystal, breaking spin-rotation symmetry without breaking time-reversal symmetry, holds an abiding fascination. However, despite being a viable form of magnetic order, None the less, experimental evidence for ``spin nematic'' states in magnetic insulators remains scarce. And the very fact that spin nematic states do not break time-reversal symmetry renders them ``invisible'' to the most common probes of magnetism --- they do {\it not} exhibit magnetic Bragg peaks, a static splitting of lines in NMR spectra, or oscillations in $\mu$SR. However, as a consequence of breaking spin-rotation symmetry, spin-nematic states {\it do} posses a characteristic spectrum of dispersing excitations which could be observed in inelastic neutron scattering. With this in mind, we develop a symmetry-based description of long-wavelength excitations in a broad class of spin-nematic states, based on an SU(3) generalisation of the quantum non-linear sigma model. We use this field theory to make explicit predictions for inelastic neutron scattering, and argue that the wave-like excitations it predicts could be used to identify the symmetries broken by the unseen spin-nematic order. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F15.00010: Gapless spin liquid phase in the J1-J2 Heisenberg model Wenjun Hu, Federico Becca, Sandro Sorella We study the stability of a Z2 spin liquid in the highly frustrated regime of the J1-J2 Heisenberg model in the square lattice, namely with nearest and next nearest antiferromagnetic superexchange interactions. We use state of the art quantum Monte Carlo methods[S. Sorella {\it et al.}, prl {\bf 88}, 117002 (2002)] and show that, by means of few Lanczos steps acting over our initial wave function, we can achieve basically exact energies for the ground state and the low lying spin excitations, whenever our results can be compared with exact diagonalization reference data. For large clusters we show evidence that our calculations remain very accurate because we can estimate exact eigenvalues by extrapolating our results to the exact zero variance limit. By taking into account these important corrections, our final phase diagram seems to be inconsistent with an opening of a sizable spin gap in the spin liquid region, as recently found by a DMRG study [H.-C. Jiang {\it et al.}, prl {\bf 86}, 024424 (2012)]. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F15.00011: Symmetric and nematic $Z_2$ quantum spin liquids: applications to the $J_1$-$J_2$ Heisenberg model Yifan Jiang, Fan Yang, Hong Yao We classify symmetric and nematic $Z_2$ quantum spin liquid states on the square lattice by analyzing bosonic PSG. We then compute the energies of various symmetric and nematic $Z_2$ spin liquid states for the $J_1$-$J_2$ square Heisenberg model by doing variational Monte Carlo simulations. The connections of our variational Monte Carlo studies with the recent DMRG results on the same model will also be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F15.00012: Deconfined Criticality in a $J-Q$ model on Honeycomb lattice Sumiran Pujari, Fabien Alet, Kedar Damle The Deconfined Criticality scenario\footnote{T. Senthil \emph{at al}, Science 303, 1490 (2004).} describes in the context of quantum magnets a generic non-Landau second-order transition between two orders that break different symmetries - antiferromagnetic order that breaks $SU(2)$ symmetry and Valence bond (VB) order breaking lattice translational symmetry. We investigate this physics in the context of a $J-Q$ model\footnote{A. W. Sandvik, Phys. Rev. Lett. 98, 227202 (2007).} on the honeycomb lattice using both $T=0$ Projector Quantum Monte Carlo (QMC) and finite-$T$ Stochastic Series Expansion QMC techniques. We find evidence for a continuous transition from different measurements including scaling of N\'eel and VB order parameters, Binder ratios of staggered magnetization, stiffness and uniform susceptibility. We have indications that this critical point belongs to the same universality class as the one observed on square lattice $J-Q$ model. Our results also suggest that this critical fixed point controlling deconfined critical behaviour remains essentially unchanged even on the honeycomb lattice which allows three-fold hedgehog defects in the N\'eel order to be present in the continuum description of the critical point. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F15.00013: Monte Carlo studies of spinon deconfinement in two dimensions Ying Tang, Anders Sandvik We have used a recently proposed quantum Monte Carlo algorithm [1] to study spinons (emergent $S=1/2$ excitations) in 2D Resonating-Valence-Bond (RVB) spin liquids and in a $J$-$Q$ model hosting a N\'eel--VBS phase transition at zero temperature. We found that spinons are well defined quasi-particles with finite intrinsic size in the RVB spin liquid. The distance distribution between two spinons show signatures of deconfinement. However, at the N\'eel--VBS transition, we found that the spinon size itself is comparable to the confinement length (the size of the bound state), even showing a shrinkage of the bound state (triplon) relative to the single spinon. Both the spinon size and the confinement length diverge as the critical point is approached. We attempt to extract the corresponding exponent. $[1]$ Y. Tang and A. W. Sandvik, Phys. Rev. Lett. {\bf 107}, 157201 (2011). [Preview Abstract] |
Session F16: Focus Session: High Magnetic Anisotropy Materials
Sponsoring Units: GMAG DMPChair: Randy Dumas, Gothenburg University, Sweden
Room: 318
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F16.00001: Growth, structure and magnetism of self-organized epitaxial nano-alloys on a metallic substrate S. Rousset, N. Moreau, V. Repain, C. Chacon, Y. Girard, J. Klein, J. Lagoute, H. Bulou, F. Scheurer, C. Goyhenex, Ph. Ohresser The CoPt alloy is one of the most studied bimetallic compounds, due to its potential application for magnetic recording. We report here on Co$_{x}$Pt$_{1-x}$ nano-alloys deposited on the well-known Au(111) reconstructed surface since it has been recognized as a powerful substrate in order to investigate the magnetic properties of metallic nano-clusters [1,2]. The growth of Co$_{x}$Pt$_{1-x}$ clusters on the Au(111) surface observed by STM revealed a morphological transition from single layer to bilayer islands with the Co concentration x. Using molecular dynamics calculations, we show that this transition is driven by the local strain due to Co atoms. These results are interpreted by a competition between the interface energy, the mixing energy and the elastic energy. Using X-ray Magnetic Circular Dichroism, we have studied the magnetic properties of these nano-alloys. The out-of-plane anisotropy of pure Co clusters strongly decreases, until it goes in-plane for 40\% of Pt. This spin reorientation transition is interpreted by a phenomenological pair model for magnetic anisotropy. \textbf{References} [1] N. Weiss et al., Phys. Rev. Lett. 95, 157204 (2005) [2] S. Rohart et al., Phys. Rev Lett. 104, 104, 137202 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F16.00002: Tailoring anisotropy in (001) oriented (Fe$_{\mathrm{1-x}}$Cu$_{\mathrm{x}})_{55}$Pt$_{45}$ films Dustin Gilbert, Liang-Wei Wang, Timothy Klemmer, Jan-Ulrich Thiele, Chih-Huang Lai, Kai Liu High anisotropy magnetic materials are central to future spintronic and recording technologies. Binary alloy FePt in its fct-L1$_{0}$ phase offers ideal magnetic properties, but usually requires high temperature annealing. Alloying with Cu has been suggested to lower the annealing temperature. However, it has been difficult to grow oriented films and prior studies have often focused on non-ideal compositions. In this work we investigate (Fe$_{\mathrm{1-x}}$Cu$_{\mathrm{x}})_{55}$Pt$_{45}$ films -- an ideal ratio for the L1$_{0}$ phase. Fully ordered films with a strong (001) texture were grown by an atomic-scale multilayer sputtering method and rapid thermal annealing at 400 $^{\circ}$C. The room-temperature deposition, low annealing temperature, and lack of a seed layer shows the strength of this technique. An increase in the tetragonal lattice distortion and fragmentation of the microstructure [while retaining the (001) texture] were observed with added Cu. Magnetic properties were evaluated and show a strong perpendicular anisotropy. The Cu inclusion is demonstrated to decrease T$_{\mathrm{C}}$ without hefty sacrifices to M$_{\mathrm{S}}$ and K$_{\mathrm{U}}$, making such films ideal for heat-assisted magnetic recording. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F16.00003: Direct Chemical synthesis of L1$_{\mathrm{0}}$ FePt Nanoparticles with High Coercivity Xiaocao Hu, Ryan Gallagher, George Hadjipanayis FePt particles with tetragonal L1$_{\mathrm{0}}$ structure have been of great interest as one of the most promising candidate for ultra-high density recording media. Chemical synthesis is one of the two major methods to fabricate FePt nanoparticles because it can lead to high uniformity and patterned assembly. However, traditional approaches require post annealing above 500${^\circ}$ to transform the FePt nanoparticles from the disordered face-centered cubic (fcc) to the ordered L1$_{\mathrm{0}}$ phase which introduces undesirable agglomeration and sintering. In this study, we have fabricated ordered L1$_{\mathrm{0}}$ FePt nanoparticles using one-step chemical synthesis without post annealing. The traditional synthesis method of reduction of Pt(acac)$_{\mathrm{2}}$ and Fe(CO)$_{\mathrm{5}}$ was used at higher temperatures in the range of 300 to 400${^\circ}$ . Monodispersed Au nanoparticles with average size of 10 nm were used as catalysts. X-ray diffraction (XRD) spectra and selected area electron diffraction (SAED) patterns revealed that the FePt nanoparticles are in L1$_{\mathrm{0}}$ phase. The highest coercivity obtained was 8 kOe at room temperature and 11 kOe at 50 K and is achieved at the reaction temperature of 400${^\circ}$. Transmission electron microscopy (TEM) images showed that FePt nanoparticles are partially agglomerated which needs further improvement. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F16.00004: Structure and Magnetic Properties of Sm-Co Particles Synthesized From Nanostructured Precursor Oxides Brian Kelly, Karl Unruh Sm-Co particles have been synthesized by a calciothermic reduction/diffusion reaction from a Sm-Co-O precursor. The precursor oxide was prepared by an autocombustion reaction and was subsequently milled with CaO, which served as a grain-growth inhibitor and thermal ballast. The effects of varying the amount of Ca metal between 2 and 6 times the amount needed for complete reduction of the available Sm-Co-O precursor, the reaction temperature between 850 and 1000 C, and reaction time between 15 minutes and 24 hours were studied. The structural and magnetic properties of the Sm-Co reaction products were studied by x-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry measurements. The results of these measurements indicated that magnetically hard SmCo$_{\mathrm{5}}$ was preferentially formed in samples synthesized with high Ca amounts and short diffusion times, or low Ca amounts and long diffusion times. Particle sizes were also observed to increase with both increasing diffusion time and increasing Ca amount. The influence of changes in both reaction temperature and size of the CaO additive on the final product were also studied. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F16.00005: Magnon softening in exchange-coupled hard-soft nanocomposites Alexander Belemuk, Siu Tat Chui We study spin excitations of the fully aligned state for three-dimensional nanocomposites of exchange coupled hard (SmFeN) and soft (FeCo) phases. When the amount of soft phase is increased the energy of low-lying spin excitation is considerably softened and contains a contribution proportional to the anisotropy constant of the soft phase. The dipolar interaction further lowers the magnon energy and controls the spin wave gap at ${\bf k}= 0$, which closes when the amount of soft phase exceeds a critical value. With the addition of soft phase or increasing the temperature the system moves to another ground state characterized by a magnetization mismatch between spins of hard and soft phases. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F16.00006: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F16.00007: Atomic structure of high-coercivity cobalt-carbide nanoparticles ensembles D.A. Arena, G. Sterbinsky, P.W. Stephens, K.J. Carroll, H. Yoon, S. Meng, Z. Huba, E.E. Carpenter Permanent magnets are increasingly important in numerous applications, including the quickly expanding area of green technologies ($e.g.$ high efficiency electric car motors and wind turbine power systems). We present studies of novel permanent magnet materials based on cobalt carbide nanoparticles (NPs), where the energy product ($BH_{max}$) exceeds 20 kJ / m$^3$ [1]. The NPs are synthesized via a polyol process, which offers a flexible approach to modify the Co-carbide phase (Co$_2$C and Co$_3$C), and NP morphology, size and size dispersion. The Co$_2$C and Co$_3$C phases have unique magnetic properties, and the combination exhibits the high $BH_{max}$. We present a detailed assessment of the structure of mixtures of Co$_2$C and Co$_3$ NPs, measured by high-resolution, synchrotron based powder x-ray diffraction (p-XRD). Both the Co$_2$C and Co$_3$ phases exhibit an orthorhombic structure (Pnnm and Pnma space groups, respectively). The high-resolution p-XRD facilitates identification of mixed phase samples, enabling detailed comparisons of the atomic structure with the magnetic properties, measured by both lab-based magnetometry and x-ray spectroscopy (soft x-ray XAS \& XMCD). \newline [1] V. G. Harris et al., J. Phys. D: Appl. Phys. 43, 165003 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F16.00008: Magnetic and electronic structure of high-coercivity cobalt-carbide nanoparticles for permanent magnet applications George Sterbinsky, Kyler Carroll, Hyojung Yoon, Shirley Meng, Zachary Huba, Everett Carpenter, Dario Arena Permanent magnets are important in numerous technological applications. However, those with the largest energy product (\textit{BH}$_{max})$ contain rare earth elements, which increase costs and introduce volatility into the supply chain. Recently, rare-earth free Co$_{\mathrm{2}}$C and Co$_{\mathrm{3}}$C nanoparticles (NPs) with large magnetic coercivity and \textit{BH}$_{max}$ have been synthesized using a polyol process [1]. Optimal \textit{BH}$_{max} $is found in a mixture of the two phases. In this system, the nature of the magnetic interparticle interactions and the origins of intrinsic magnetic properties of the Co-carbide phases are not fully understood. We have investigated the origins of the magnetic properties of Co$_{\mathrm{2}}$C and Co$_{\mathrm{3}}$C NPs using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Co $L$-edge and C $K$-edge. From differences in the electronic structures of the two Co-carbide phases, as determined by XAS, the nature of their unique magnetic properties can be deduced. Furthermore, the role of the spin and orbital moments in determining magnetic anisotropy and \textit{BH}$_{max}$ in these materials is examined with XMCD. [1] V. G. Harris et al. J. Phys. D: Appl. Phys. \textbf{43}, 165003 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F16.00009: Brilliant CoC nanomagnets: highly magnetocrystalline anistropy for potential applications Ahmed El-Gendy, Turki Almugaiteeb, Everett Carpenter No doubt that the development of novel materials and their understanding on a smaller size scale is still a challenging issue at the basis of progress in many areas of materials science. This is almost entirely true in the development of new magnetic materials for a various types of vital applications. Recently the focus has moved from the microcrystalline to the nanocrystalline magnetic regime. The most common amorphous and nanocrystalline magnetic materials are classified to be one of either magnetically hard (a quite larger coercivity) or soft (a material with a very smaller coercivity) materials. In the work at hand we are concerning the hard magnetic CoC nanopareticles. This material shows a mono-dispersed, stable against air environment and larger magnetocrystalline anisotropy as well as larger coercivty. In addition, the mono-dispersed and small particle size led to getting a Curie temperature much smaller than the related bulk materials. Based on the relation between the Curie temperature and the number of atoms, the shape of the particles can be determined. Therefore, the CoC nanomagnes with impressive magnetic properties open the root for various essential applications such as permanent magnets, magnetic sensors and contrast agent for MRI diagnostic tools. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F16.00010: Structural stability of HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ magnetic nanoclusters Balamurugan Balasubramanian, Bhaskar Das, Ralph Skomski, David Sellmyer The gas-aggregation-type cluster deposition has emerged as an attractive method to create uniaxially aligned nanoparticle building-blocks of metastable and new permanent-magnet materials such as HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ with appreciable coercivities ($H_{c} \approx $ 5.0 kOe), magnetocrystalline anisotropies ($K_{1} \approx $ 10 Mergs/cm$^{3})$, and magnetic polarization ($J_{s} \approx $ 10 kG) at 300 K. In comparison, bulk HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ alloys form only at ideal stoichiometries and high temperatures above 1000 $^{\circ}$C at thermal equiliburium conditions. We have investigated the structural stability of HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ phases on varying their stoichiometries from ideal values in HfCo$_{7\pm \delta }$ and Zr$_{2}$Co$_{11\pm \delta}$ nanoclusters (0$\le \delta \le $1), respectively and compared these results with the corresponding bulk phase diagrams. This study provides new insights to understand the underlying crystal structure and magnetic properties of the nanoclusters and to explore them for significant applications. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F16.00011: Growth and Properties of Mn$_{\mathrm{x}}$Ga Magnetic Nanostructures Michelle Jamer, Badih A. Assaf, Marius Eich, Jagadeesh S. Moodera, Don Heiman Rare-Earth (RE) magnets are becoming more expensive and less available for current applications in technology. Mn$_{\mathrm{x}}$Ga (x$=$2-3) has previously shown coercivity of \textgreater\ 2.5 T, close to that of RE magnets.\footnote{T.J. Nummy, S.P Bennett, T. Cardinal, and D. Heiman, \textit{Large Coercivity in Nanostructured Rare-earth-free Mn}$_{x}$\textit{Ga Films}, Appl. Phys. Lett. \textbf{99}, 252506 (2011).} In this project, the vapor-liquid-solid (VLS) method was used to grow nanoparticles of Mn$_{\mathrm{x}}$Ga (x$=$1-3) with MBE. The goal was to study the magnetic properties as a function of reduced dimensionality. The samples were prepared by depositing a 3-6 nm layer of Au on Si, GaAs, and glass. It was found that the miscibility of Ga and Au is high, but for Mn and Au it is much lower. Therefore, during the growth process Ga was deposited on the gold catalyst followed by Mn deposition. The samples were then annealed at temperatures 100-500 $^{\circ}$C. Nanostructures, including regions of nanowires, were found using scanning electron microscopy on all samples. The magnetic properties of the nanostructured samples were studied with SQUID magnetometry and found to have a magnetization of 200 emu/cm$^{3}$. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F16.00012: Magnetism of MnGa-based nanostructures Parashu Kharel, Yung Huh, Valloppilly Shah, Ralph Skomski, David Sellmyer Materials with high magnetic anisotropy and Curie temperature well above room temperature have potential for a range of applications including high-density recording, nonvolatile memory and permanent-magnet materials. Mn$_{\mathrm{y}}$Ga (1 $\le $ y $\le $ 2) is one such compounds that can be synthesized in the tetragonal L1$_{0}$ or D0$_{22}$ structures based on the value of y in Mn$_{\mathrm{y}}$Ga. Our experimental investigation of the rapidly quenched nanostructured ribbons shows that the material with y $=$1.2, 1.4 and 1.6 prefers the L1$_{0}$ structure and that with y $=$1.9 and 2.1 prefers D0$_{22}$ structure. We have found a maximum saturation magnetization of 88 emu/g in Mn$_{1.2}$Ga which decreases monotonically to 50 emu/g as y reaches 2.1. Although both the L1$_{0}$ and D0$_{22}$Mn$_{\mathrm{y}}$Ga samples show a high Curie temperature (T$_{\mathrm{c}})$ well above room temperature, the value of T$_{\mathrm{c}}$ decreases almost linearly from 740 K for Mn$_{2.1}$Ga to 550 K for Mn$_{1.2}$Ga. We will also discuss the effect of boron doping on the structural and magnetic properties of this material. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F16.00013: MnBi: a better magnet via computational design Nikolai A. Zarkevich, Lin-Lin Wang, Ichiro Takeuchi, Matthew J. Kramer, Duane D. Johnson Using DFT-based methods, we study the magnetic properties of MnBi in the technologically important low-temperature phase. We identify the origin and behavior of the magnetoanisotropy and magnetism versus structure and doping. We perform high-throughput screening for dopants that improve magnetoanisotropy (larger, $c$-axis only -- no reorientations) and magnetization, and chemical and structural stability. We also assess the best-in-class materials for exchange-spring coupled magnet, without the use of rare-earth elements. Experimental assessment of the predictions is also provided. Work was supported by the U.S. Department of Energy, ARPA-E under REACT (0472-1526), using methods develop under support by the Office of Basic Energy Science, Division of Materials Science and Engineering (DE-FG02-03ER46026 and DE-AC02-07CH11358). Ames Laboratory is operated for the U.S. DOE by Iowa State University under contract DE-AC02-07CH11358. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F16.00014: DFT high-throughput screening of transition metal dopant in MnBi for better magnetic properties Lin-Lin Wang, Nikolai A. Zarkevich, Ichiro Takeuchi, Yiying Ye, Vladimir Antropov, Matthew J. Kramer, Duane D. Johnson To improve the magnetic properties of MnBi, especially magnetic energy product, we use density functional theory (DFT) calculations to study alloying effects on MnBi properties with transition metals (TMs), both as dopant and soft phase in an exchange-spring magnet composite. We have considered various defects in the NiAs-type structure with interstitial and substitutional sites. Via high-throughput screening for dopants from groups 3-16, we have DFT trends in impurity formation energy, magnetization, structural parameters, magnetoanisotropy, etc. Early and late TMs prefer to occupy the Mn sites, while mid-TMs are not stable. Early and late TMs are antiferromagnetically coupled, while mid-TMs are ferromagnetically coupled to MnBi. For 3$d$ mid-TMs, there is no increase in magnetization. However, energetically favorable mid-late-TM substitutes at Mn sites can improve the magnetic anisotropy. To investigate improving the magnetic energy product of a composite alloy system, we also detail the coupling between MnBi thin films with soft magnetic materials having a high magnetization. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F16.00015: First Principles~Studies of the Magnetic Properties of Alnico Permanent Magnet Materials Balazs Ujfalussy, German Samolyuk, Khorgolkhuu Odbadrakh, G. Malcolm Stocks Until the advent of rare earth based magnets Alnico was one of the highest energy product hard magnets available. Recently, interest in this system has been rekindled as system whose properties and utility may be further enhanced but does not contain rare earth elements. Recent experiments on Alnico alloy suggest that there is no sharp interface between the disordered bcc FeCo magnetic phase and the ordered B2 NiAl non-magnetic phase; thereby undermining our understanding of the large coercivity of this material. By utilizing several electronic structure methods we first study the issue of the effect of substitutions of additional elements into B2 NiAl phase. We also calculate the magnetic moment distribution across the interface and examine the magnetic ground state. These calculations suggest that the magnetic structure of the B2-phase as well as the interface in much more complex than previously thought. [Preview Abstract] |
Session F17: Focus Session: Magnetic Spinel and Perovskite Heterostructures
Sponsoring Units: DMP GMAGChair: Anand Bhattacharya, Argonne National Lab
Room: 319
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F17.00001: Growth and Properties of Magnetic Spinel Ferrite Thin Films and Heterostructures Invited Speaker: Arunava Gupta There is considerable interest in the growth of single crystal spinel ferrites films because of their numerous technological applications in areas such as microwave integrated devices, magnetoelectric coupling heterostructures, and potentially as an active barrier material in an emerging class of spintronic devices called spin filters. Unlike perovskites, the study of spinel ferrite films is quite limited in part due to the complex crystal structure with a large unit cell consisting of many interstitial sites and that the transition metal cations can adopt various oxidation states. We have grown high-quality, atomically smooth epitaxial ferrite (NiFe$_{\mathrm{2}}$O$_{\mathrm{4}}$, CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ and LiFe$_{\mathrm{5}}$O$_{\mathrm{8}})$ films using chemical vapor deposition and pulsed laser deposition techniques and carried out detailed studies of their structural, magnetic and optical properties. Of particular interest are systematic studies on the formation of antiphase boundaries in epitaxial NiFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ films grown on different substrates and the accurate determination of the band gap of this material using optical spectroscopy and first principles calculations. Additionally, we have grown ferrite films on piezoelectric substrates and observed large shifts in the ferromagnetic resonance profile due to magnetoelectric coupling resulting from electrostatic field-induced changes in the magnetic anisotropy field. Work done in collaboration with N. Z. Bao, W. H. Butler, R. Datta, B. S. Holinsworth, M. Iliev, S. Kanuri, S. V. Karthik, G. Kim, T. M. Klein, N. Li, M. Liu, P. R. LeClair, J. X. Ma, D. Mazumdar, T. Mewes, D. V. B. Murthy, J. L. Musfeldt, K. R. O'Neal, N. Pachauri, V. M. Petrov, H. Sato, S. Sch\"{a}fer, L. Shen, H. Sims, G. Srinivasan, N. X. Sun, Q. -C. Sun, and Z. Zhou. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F17.00002: Temperature-dependent time-domain THz spectroscopic study of spinel NiCo$_{2}$O$_{4}$ thin films Punam Silwal, Tianqi Shan, Daeho Kim, Diyar Talbayev The unique combination of electrical conductivity, infrared transparency, electro catalytic activity, and ferrimagnetic order makes the spinel NiCo$_{2}$O$_{4}$ an attractive material for various technological applications. Our previous study showed that high quality epitaxial spinel NiCo$_{2}$O$_{4}$ films on MgAl$_{2}$O$_{4}$ (001) substrate exhibit metallic behavior accompanied by ferrimagnetic order. The electrical properties of these films can be tuned from metallic to insulating by changing the growth temperature. The comprehensive understanding of the microscopic details of carrier transport in these films requires the study of frequency-dependent optical properties. Terahertz time-domain spectroscopy (THz TDS) can determine the frequency dependent complex dielectric constant, refractive index, and optical conductivity. We used THz TDS to measure the optical properties of NiCo$_{2}$O$_{4}$ in the 0.2 -- 2.7 THz spectral region. The complex conductivities display a Drude-type frequency response. The extrapolated DC conductivity is consistent with our previous work. The temperature- and growth-condition dependent Drude parameters provide further insight in the metal-insulator transition in these materials. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F17.00003: Theory of the magnetic and metal-insulator transitions in RNiO3 bulk and layered Bayo Lau, Andrew J. Millis A slave rotor-Hartree Fock formalism is presented for studying the properties of the p-d model describing perovskite transition metal oxides, and a flexible and efficient numerical formalism is developed for its solution. The methodology is shown to yield, within an unified formulation, the significant aspects of the rare earth nickelate phase diagram, including the paramagnetic metal state observed for the LaNiO$_3$ and the correct ground-state magnetic order of insulating compounds. It is then used to elucidate ground state changes occurring as morphology is varied from bulk to strained and un-strained thin-film form. For ultrathin films, epitaxial strain and charge-transfer to the apical out-of-plane oxygen sites are shown to have significant impact on the phase diagram. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F17.00004: Perovskite BaCrO$_{3}$: completing a materials system with an anomalous Mott transition Z.H. Zhu, F.J. Rueckert, J.I. Budnick, W.A. Hines, M. Jain, H. Zhang, B.O. Wells Perovskite BaCrO$_{3}$ cannot be stabilized in bulk but we have synthesized this compound as a film. BaCrO$_{3}$ films have a substantially larger lattice constant than other chromates, are insulating, and exhibit weak ferromagnetism likely associated with canted antiferromagnetism. Comparison with the sister compounds CaCrO$_{3}$ and SrCrO$_{3}$ suggests an anomalous Mott transition caused by lattice expansion where magnetism is independent of whether the compound is metallic or insulating. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F17.00005: Above room temperature ferroelectricity and weak ferromagnetism in LaFeO$_3$/LnFeO$_3$ digital superlattices Saurabh Ghosh, Craig J. Fennie We have studied from first principles the structural, ferroelectric, and magnetic properties of the (LaFeO$_3$)$_1$/ (LnFeO$_3$)$_1$ digital superlattices, with Ln = lanthanide (or Y). We show that in this class of artificial materials constructed from Pnma perovskites, which are highly amenable to advanced oxide thin film growth techniques, octahedral rotations induce a spontaneous electrical polarization (consistent with the recently developed rules of Rondinelli and Fennie). Furthermore, this rotation pattern is shown to induce linear magnetoelectricity and weak-ferromagnetism, much like the recently discussed `327' manganite Ruddlesden-Popper. In these ferrite superlattices, however, it is clear that both the ferroelectric and magnetic ordering temperatures should occur above room temperature. Finally we discuss how the `La/Ln' cation radius mismatch controls the magnitudes of the induced polarization and magnetization, as well as the barrier to switch the polarization. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F17.00006: Polarization in epitaxial LaFeO$_{3}$/SrFeO$_{3}$ superlattice thin films Rohan Mishra, Sokrates Pantelides, Young-Min Kim, Albina Borisevich, Stephen Pennycook, Seongkeun Kim, Seohyoung Chang, Anand Bhattacharya, Jeffrey Eastman, Dillon Fong Controlling ferroelectricity in perovskite thin-films requires an understanding of the many factors that are known to affect their polar behavior, such as octahedral rotations, cation ordering, oxygen vacancies, and the surface and interface terminations. Here, we report a study using a combination of aberration-corrected scanning transmission electron microscopy, electron-energy loss spectroscopy (EELS), and density functional calculations (DFT) to demonstrate how these factors work in concert to give rise to polarization in LaFeO$_{3}$/SrFeO$_{3}$ superlattices grown on SrTiO$_{3}$ substrates. Although both LaFeO$_{3}$ and SrFeO$_{3}$ are non-polar in the bulk, microscopy results show Fe-displacements in the superlattices indicating a dipole-like electric field. The magnitude of the observed displacements peaks in the interior of the films and goes to zero towards the substrate and the surface. O K EELS results show variation in intensities within the films, suggesting that oxygen vacancies may play a role. DFT results explaining the origin of the observed polar displacements within the superlattices and the effect of the abovementioned factors will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F17.00007: Tuning the electronic structure of (SrTiO$_{3}$)$_{n}$/(SrFeO$_{3-x}$)$_{m}$ superlattices Robert Berger, Daniel Broberg, Carolina Adamo, Shawn Sallis, Nicholas Quackenbush, Louis Piper, Craig Fennie, Darrell Schlom, Jeffrey Neaton SrTiO$_{3}$ and other $d^{0}$ perovskite-derived compounds are of interest as possible solar water-splitting catalysts, due to their band-edge energies and stability in water. To optimize their ability to absorb and convert solar energy, it is desirable to understand how to tune the electronic structure and band gap of these compounds. One controllable way to experimentally tune the crystal structure, and consequently the electronic structure, of these compounds is through the growth of epitaxially layered superlattices. Past computational work has studied the interleaving of SrTiO$_{3}$ and SrFeO$_{3}$, in which the $d^{4}$ Fe$^{4+}$ atoms result in metallic electronic structure. However, the synthesis of related compounds suggests that oxygen vacancies in these superlattices would likely reduce some or all of the Fe$^{4+}$ to Fe$^{3+}$ ($d^{5}$), which could once again open a tunable band gap. We use density functional theory and beyond to study the energetics of oxygen vacancy patterns in (SrTiO$_{3}$)$_{n}$/(SrFeO$_{3-x}$)$_{m}$, and the possibility of favorably tuning the electronic structure and band gap of these superlattices via changes in layering, oxygen vacancy concentration, and biaxial strain. Our results are thoroughly discussed in the context of recent experiments. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F17.00008: Electronic and Magnetic Reconstruction at Manganite Interfaces Kalpataru Pradhan, Arno P. Kampf We investigate interfaces between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites using a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long range Coulomb interactions. In FM/AFI heterostructures the magnetic and the transport properties critically depend on the thickness of the AFI layers. We focus on superlattices where the constituent parent FM and AFI manganites have the same electron density n. For n=0.6, the induced ferromagnetic moment in the AFI layers sandwiched between FM manganites decreases monotonically with increasing layer width. For n=0.5 instead, the induced ferromagnetic moment varies non-monotonously with the layer width. These differences for n=0.6 and n=0.5 originate from different charge-transfer profiles and magnetic reconstructions at the FM/AFI interfaces. The width of the AFI layers furthermore controls the magnitude of the magnetoresistance and the metal to insulator transition of the FM/AFI heterostructure. These results are discussed in the context of recent experiments on LSMO/PCMO [1] and LCMO/PCMO superlattices [2].\\[4pt] [1] D. Niebieskikwiat {\it et al.}, Phys. Rev. Lett. 99, 247207 (2007).\\[0pt] [2] H. Li {\it et al.} Appl. Phys. Lett. 80, 628 (2002). [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F17.00009: DFT investigation of structural effects on perovskites exhibiting metal-insulator phase transitions Joseph Schick, Lai Jiang, Diomedes Saldana-Greco, Andrew Rappe The rich variety of electronic, magnetic, etc. properties available in perovskite materials are closely linked to octahedral tilting and other details of the arrangements of atoms within these materials. Furthermore, it has been demonstrated that the tilts and structural details are altered by the growth of films of these materials on substrates that provide strain and changed by the creation of new layered or superlattice structures from these materials. We employ density functional methods to investigate the relationship between tilting and charge ordering in a variety of strained-layered perovskite materials. We present these DFT results along with model calculations that aid in interpreting the complex connections between atomic structure and electronic properties, \emph{e.g.}\ structural control of metal-insulator phase transitions. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F17.00010: The effect of interfacial octahedral behavior on magnetic properties in ultrathin manganite films Eun Ju Moon, X.M. Cheng, D.J. Keavney, S.J. May In \textit{AB}O$_{3}$ perovskites, the rotation and distortions of $B$O$_{6}$ octahedra lead to crystal symmetric variants of the basic perovskite structure. The rotation angles play a role in magnetic exchange with previous work demonstrating a clear relationship between bond angles and ordering temperatures. Recent work has shown that heteroepitaxial oxide films can be stabilized with non-equilibrium crystal structures due to structural coupling of octahedral behavior across the substrate/film interface. However, it is not yet apparent how the crystal symmetry across a heteroepitaxial oxide interface contributes to magnetic properties. Here, we report on the effect of crystal symmetry in La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO), a canonical magnetic oxide, grown using molecular beam epitaxy on different symmetric substrates with similar lattice parameters. For this study, we have used x-ray magnetic circular dichroism, transport, and magnetoresistance measurements to explore the magnetic properties of ultrathin LSMO films for a direct comparison of magnetic behavior in isocompositional perovskites with different octahedral behavior. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F17.00011: Electronic and Magnetic Tunability of Sr$_{2}$CrReO$_{6}$ Thin Films by Growth-mediated Oxygen Modulation and Template Variation Jeremy Lucy Highly ordered epitaxial films of ferrimagnetic semiconductor Sr$_{2}$CrReO$_{6}$ have been fabricated by off-axis magnetron sputtering, and characterized as a function of the oxygen partial pressure at optimal growth conditions. In this letter, we report 18,000{\%} modulation in electrical resistivity at T$=$ 7K (60{\%} at room temperature) from a 1{\%} modulation in the oxygen partial pressure during film growth. The growth window was chosen to center around the condition for peak saturation magnetization, which drops due to both increasing and decreasing oxygen growth pressure. The results suggest that n-type doping from oxygen vacancies in the film likely play the dominant role in the electrical properties and modulation of Sr$_{2}$CrReO$_{6}$ thin films. We also explore the effects of substrate templates on the structural, electrical, and magnetic properties of Sr$_{2}$CrReO$_{6}$. Sr$_{2}$CrReO$_{6}$ films fabricated on double perovskite substrates or buffer layers exhibit increased resistivities at low temperatures. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F17.00012: High-resolution terahertz spectroscopy of Sr$_{2}$CrReO$_{6}$ at cryogenic temperatures and high magnetic fields D.R. Daughton, R. Higgins, S. Yano, C.H. Du, A.J. Hauser, R. Adur, J.M. Lucy, H.L. Wang, D.V. Pelekhov, E. Johnston-Halperin, F.Y. Yang, P.C. Hammel Temperature and magnetic field dependent terahertz spectroscopies have proven useful in elucidating the interplay between structure charge, and magnetism in complex oxide systems. To this end, we are developing a turn-key, continuous-wave (CW) terahertz transmission spectrometer operating from 6 K to 300 K and in fields up to 9 T. Fiber-coupled photoconductive switches operate from 200 GHz to 1.8 THz in the cryogenic and high-field sample environment -- eliminating the need to align a THz beam through multiple cryostat windows. In this work we compare CW-THz measurements on epitaxial thins films of Sr$_{2}$CrReO$_{6}$, a double-perovskite ferrimagnet, with conventional THz time-domain spectroscopy. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F17.00013: Indications of spin-polarized transport in thin film double perovskites Sr2FeMoO6 and Ba2FeMoO6 Simon Granville, Ian Farrell, Adam Hyndman, Duncan McCann, Grant Williams Double perovskite oxides Sr2FeMoO6 and Ba2FeMoO6 have attracted attention for spintronic device development due to their predicted highly spin-polarized transport characteristics. However, most published experimental results are from bulk material, not thin films that are more relevant for realistic device development. We will present our results of the growth as well as structural, magnetic and transport properties of thin films of double perovskites Sr2FeMoO6 and Ba2FeMoO6 produced by pulsed laser deposition. We have produced highly crystalline, near-epitaxial thin films of each material. We will survey the magnetic and magnetotransport properties, including the magnetoresistance, planar and anomalous Hall effects, which provide evidence for the presence of spin-polarized charge carriers well above room temperature and the potential for developing high sensitivity magnetic sensors. [Preview Abstract] |
Session F18: Two Dimensional Topological Insulators I
Sponsoring Units: DCMPChair: Yong Chen, Purdue University
Room: 320
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F18.00001: Imaging currents in HgTe quantum wells in the quantum spin Hall regime Katja Nowack, Eric Spanton, Matthias Baenninger, Markus K\"onig, John Kirtley, Beena Kalisky, Christopher Ames, Philipp Leubner, Christoph Br\"une, Hartmut Buhmann, Laurens Molenkamp, David Goldhaber-Gordon, Kathryn Moler Dissipationless edge channels are a key feature of the quantum spin Hall (QSH) state, which was predicted and experimentally demonstrated to exist in HgTe quantum wells. The existence of the edge channels has been inferred from local and non-local transport measurements. Here we image the current in Hall bars made from HgTe quantum wells by probing the magnetic field generated by the current using a scanning superconducting quantum interference device. We observe that the current flows mainly along the edge of the device in the QSH regime, and furthermore that an identifiable edge channel exists even in the presence of disorder and considerable bulk conduction as the device is gated and its temperature is raised. Our results represent a versatile method for the characterization of new quantum spin Hall materials systems. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F18.00002: Electronic properties of HgTe/CdTe heterostructure under perturbations preserving time reversal symmetry Tome Schmidt, Jonas Anversa, Paulo Piquini, Adalberto Fazzio Using first principles calculations, the Dirac cone of HgTe/CdTe heterostructure is identified at the interface, inside the valence band. The spin texture of the 2D Dirac states is totally in-plane for all interface directions, different from the 3D topological insulators, where there is always some out-of-plane spin components. The masless Dirac states are strongly affected by applying positive or negative biaxial pressure. While negative pressure turns the system metallic, suppressing the Dirac states, positive pressure maintains the protected topological states, but dislocates the Dirac cone upward in energy. The protected Dirac states are kept up to a contraction of 3\% in the lattice parameter. Larger compressive pressures leads to suppression of the protected metallic states. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F18.00003: Quantized Conductance in InAs/GaSb Quantum Wells Lingjie Du, Ivan Knez, Rui-Rui Du, Gerald Sullivan We have studied electrical transport in inverted InAs/GaAs quantum wells (QWs) made by molecular beam epitaxy, in which the evidences for helical edge modes were observed in messoscopic samples with either normal or superconductor contacts. Here we report on measurements of QWs that are doped with Si at the InAs/GaSb interface, where Si is a donor in InAs and an acceptor in GaSb. The influences of induced disorder in the quantum Spin Hall effect as well as outside this regime are systematically studied and results will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F18.00004: Scanning SQUID measurements of current flow in InAs/GaSb Quantum Wells Eric Spanton, Lingjie Du, Katja Nowack, Gerry Sullivan, Rui-Rui Du, Kathryn Moler InAs/GaSb quantum wells have been predicted theoretically to exhibit the quantum spin hall phase in the inverted regime. In this phase, spin-polarized helical edge modes are expected to exist. Previous published results on length and width dependence of InAs/GaSb 4-terminal devices suggests these helical edge states coexist with a residual bulk conductivity when the device is tuned into the minigap. We probe the spatial distribution of currents in devices using a scanning SQUID to measure the resulting magnetic fields. Specifically, we find that when the device is tuned into the gap with a front gate, current flows along the edge and coexists with bulk current. We also look at dependence on back gate voltage and temperature dependence. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F18.00005: Quantum Transport near the Charge Neutrality Point in Inverted Type-II InAs/GaSb Field-Effect Transistors W. Pan, J.F. Klem, J.K. Kim, M. Thalakulam, M.J. Cich, S.K. Lyo We present here our recent quantum transport results around the charge neutrality point (CNP) in a type-II InAs/GaSb field-effect transistor. At zero magnetic field, a conductance minimum close to $4e^2/h$ develops at the CNP and it follows semi-logarithmic temperature dependence. In quantized magnetic ($B$) fields and at low temperatures, well developed integer quantum Hall states are observed in the electron as well as hole regimes. Electron transport shows noisy behavior around the CNP at extremely high B fields. When the diagonal conductivity $\sigma_{xx}$ is plotted against the Hall conductivity $\sigma_{xy}$, a conductivity circle law is discovered, suggesting a chaotic quantum transport behavior. 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] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F18.00006: Quantum Anomalous Hall Effect in 2D Organic Topological Insulators Zhengfei Wang, Zheng Liu, Feng Liu Quantum anomalous Hall effect (QAHE) is a fundamental transport phenomenon in the field of condensed-matter physics. Without external magnetic field, spontaneous magnetization combined with spin-orbit coupling give rise to a quantized Hall conductivity. So far, a number of theoretical proposals have been made to realize the QAHE, but all based on inorganic materials. Here, using first-principles calculations, we predict a family of 2D organic topological insulators (OTIs) for realizing the QAHE. Designed by assembling molecular building blocks of triphenyl-transition-metal compounds into a hexagonal lattice, this new classes of organic materials are shown to have a nonzero Chern number and exhibit a gapless chiral edge state within the Dirac gap. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F18.00007: Quantum anomalous Hall effect with in-plane magnetization in HgMnTe Hsiu-Chuan Hsu, Xin Liu, Chao-Xing Liu A quantum anomalous Hall (QAH) insulator carries quantized Hall conductance which is similar to Quantum Hall (QH) effect. However, it originates from the exchange coupling of magnetization instead of Landau levels. It was proposed that QAH effect can be realized in HgTe quantum wells doped with Mn ({\bf Phy. Rev. Lett. 101, 146802 (2008)}) and evidenced by recent experiments. However, Mn is paramagnetic and an external magnetic field, which also leads to Landau levels, is required to obtain Mn polarization. Thus, it is essential to find an experimentally feasible way to distinguish between the two effects. In this study, we propose to distinguish QH effect and QAH effect by inducing the in-plane magnetization of Mn with an in-plane magnetic field. The in-plane magnetic field reduces the QAH effect by tilting the magnetization of Mn into the quantum well plane and reducing the out-of-plane magnetization. In contrast, the in-plane magnetic field has little influence on the conventional QH effect which only depends on the out-of-plane magnetic field. The phase diagram is identified based on the band structure calculation and Landau level calculation with the realistic material parameters of HgMnTe quantum wells, which can serve as the guidance for the future transport experiment. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F18.00008: Engineering quantum anomalous Hall phases with orbital and spin Hongbin Zhang, Frank Freimuth, Gustav Bihlmayer, Stefan Bl\"ugel, Yuriy Mokrousov Combining tight-binding models and first principles calculations, we demonstrate that under external exchange fields, non-zero Chern numbers and nontrivial QAH effect could be induced by on-site spin-orbit coupling (SOC) in buckled honeycomb lattices with $sp$ orbitals. In the Haldane model [1], the occurrence of QAH effect is attributed to complex next nearest neighbor hopping. Detailed analysis of a generic tight-binding models reveals that there exist different mechanisms giving rise to complex hoppings, utilizing both orbital and spin degrees of freedom of electrons on a lattice. Furthermore, it is shown that in Bi/Sb(111) bilayers [2], different topological phases exist as function of the magnitude of SOC and external exchange fields. These phases are characterized using Chern and spin Chern numbers [3] together with transverse charge and spin conductivities. At last, we show that introducing ferromagnetic dopants provides a practical way to induce nontrivial topological phases, whereas the physics is modified due to incompletely filled d states around the Fermi energy.\\[4pt] [1] F.D.M. Haldane, Phys. Rev. Lett. 61, 2015 (1988).\\[0pt] [2] H. Zhang, {\it et al.}, Phys. Rev. B 86, 035104 (2012).\\[0pt] [3] E. Prodan, Phys. Rev. B {\bf 83}, 195119 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F18.00009: Effective field theory of nematic QAH for interacting fermions Yizhi You, Eduardo Fradkin We consider 2D fermionic lattice models with quadratic band touching. By turning on a marginal relevant interaction of this system, the system~condenses into a state that spontaneously breaks time reversal and/or rotation (point-group) symmetry. When both symmetries are broken the state is a nematic quantum anomalous Hall (QAH) phase. We derive an effective field theory which describes the quantum phase transition into this state from a spontaneous QAH state. The effective field theory has the form of Maxwell-Chern-Simons action for the hydrodynamic degrees of freedom of the spontaneous QAH state with a coupling to the nematic order-parameter field that induces a spatial anisotropy. The fluctuations of the nematic field modify the local spatial geometry and couples to the Maxwell term as the spatial components of a metric tensor. We will discuss the behavior at quantum criticality and the relation with recent theories that associate transitions of this type with quantum Lifshitz criticality [1]. We will also discuss extensions of our theory to nematic fractional QAH state. [1] M. Mulligan, C. Nayak, and S. Kachru, Phys. Rev. B 82, 085102 (2010); Phys. Rev. B 84,195124 (2011) This work was supported in part by the NSF grant DMR-1064319 at the University of Illinois. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F18.00010: Symmetry protected Spin Quantum Hall phases Zheng-Xin Liu, Xiao-Gang Wen Symmetry protected topological (SPT) states are short-range entangled states with symmetry. Nontrivial SPT states have symmetry protected gapless edge excitations. Topological insulators are examples of nontrivial SPT phases. We study Bosonic SPT phases protected by $SU(2)$ or $SO(3)$ symmetry in 2D. There are infinite number of such phases, which can be described by $SU(2)/SO(3)$ nonlinear-sigma models with a quantized topological $\theta$-term. At open boundary, the $\theta$-term becomes the Wess-Zumino-Witten term and consequently the boundary excitations are decoupled gapless left movers and right movers. Only the left movers (if $\theta>0$) carry the $SU(2)/SO(3)$ quantum numbers. As a result, the $SU(2)$ SPT phases have a half-integer quantized spin Hall conductance and the $SO(3)$ SPT phases have an even-integer quantized spin Hall conductance. Both the $SU(2)/SO(3)$ SPT phases are symmetric under their $U(1)$ subgroup and can be viewed as $U(1)$ SPT phases with even-integer quantized Hall conductance. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F18.00011: Bloch Model Wavefunctions and Pseudopotentials for All Fractional Chern Insulators Yang-Le Wu, N. Regnault, B. Andrei Bernevig We introduce a Bloch-like basis in a $C$-component lowest Landau level fractional quantum Hall effect (FQH), which entangles the real and internal degrees of freedom and preserves an $N_x\times N_y$ full lattice translational symmetry. We implement the Haldane pseudopotential Hamiltonians in this new basis. Their ground states are the model FQH wavefunctions, and our Bloch basis allows for a mutatis mutandis transcription of these model wave functions to the fractional Chern insulator (FCI) of arbitrary Chern number $C$, obtaining wavefunctions different from all previous proposals. For $C>1$, our wavefunctions are related to color-dependent magnetic-flux inserted versions of Halperin and non-Abelian color-singlet states. We then provide large-size numerical results for both the $C=1$ and $C=3$ cases. This new approach leads to improved overlaps compared to previous proposals. We also discuss the adiabatic continuation from the FCI to the FQH in our Bloch basis, both from the energy and the entanglement spectrum perspectives. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F18.00012: First-principles study on quantum valley Hall effects in silicene Youngkuk Kim, Hosub Jin, Keunsu Choi, Jisoon Ihm Silicene is a two-dimensional honeycomb lattice of silicon atoms, similar to graphene. Based on first-principles calculations, we suggest that silicene is an ideal host for realization of quantum valley Hall effects. We show that the intrinsic buckled structure allows the formation of topological domain walls in silicene under a uniform applied electric field and valley-polarized kink states emerge on the domain walls. Peculiar behaviors of the kink states under various applied electric fields are demonstrated, and simulated scanning tunneling microscopy images are presented to show that they can be used to identify the topological domain walls as well as valley-polarized kink states. Our findings suggest that the one-dimensional domain wall may be used as an electrical wire through which valley-polarized current can flow, and silicene can be used as a valley polarizer. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F18.00013: Quantum Topological Hall Effect in Kagome Ice Yukitoshi Motome, Hiroaki Ishizuka The quantum Hall state was originally discovered in two-dimensional electron systems associated with the formation of quantized Landau levels in external magnetic field. Later, a quantum anomalous Hall effect without Landau levels was proposed, and the idea has been generalized to topological insulators in the presence of the spin-orbit coupling. Besides, a noncoplanar magnetic order was shown to give rise to the quantum anomalous Hall effect through the Berry phase mechanism. Here, we present yet another example of the quantum anomalous Hall state which emerges in the absence of Landau levels, spin-orbit coupling, and magnetic ordering. The new state is realized in itinerant electrons coupled with local spin textures subject to geometrical frustration of lattice structure. Considering the double-exchange model with spin-ice type Ising spins on a kagome lattice, we numerically show that a local spin correlation called kagome-ice opens a charge gap, resulting in quantization of the Hall conductivity in the absence of magnetic ordering. By Monte Carlo simulation, we discuss the stability of the anomalous Hall insulating region in the magnetic phase diagram. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F18.00014: Critical behavior of the transport coefficients at the Chern-to-normal insulator transition Yu Xue, Emil Prodan Using the non-commutative Kubo formula for disordered lattice systems, we mapped the conductivity tensor $\sigma_{xx}$($E_F$,$T$) and $\sigma_{xy}$($E_F$,$T$) as function of Fermi level $E_F$ and temperature $T$, for a model of a Chern insulator in the presence of strong disorder. In line with previous studies, $\sigma_{xy}$ displays a quantized non-trivial value near the half-filling, value that changes rapidly to a trivial value as $E_F$ crosses a critical value $E^c_F$. As expected, the $T$-dependence of $\sigma_{xx}$ display the typical signature of the insulating behavior, except at $E^c_F$. Examining the resistivity tensor $\hat\rho=\hat\sigma^{-1}$, we found that the data looks extremely similar to the experimental data for the plateau-insulator transition in the Integer Quantum Hall Effect: 1)$\rho_{xx}$($E_F$,$T$) vs $E_F$ plots for various temperatures intersect each other at precisely one point; 2) At this $E^c_F$, $\rho_{xx} \approx 1$ and $\sigma_{xy} \approx 0.5$; 3) The plots near $E^c_F$ for different temperatures collapse into one curve when rescaled with an exponent that is consistent with the universally accepted value. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F18.00015: Topological protection of localization against the hybridization Bohm-Jung Yang, Mohammad Saeed Bahramy, Naoto Nagaosa Localization of electronic wave functions is governed by their topological nature as well as the symmetry and dimensionality of the system. Two prominent examples are the presence of the extended states in two-dimensional quantum Hall systems and the absence of localization for the surface states of three dimensional topological insulators. In these cases, the extended states are protected by topological invariants. Here, we show that the two-dimensional quantum Hall system put on the three dimensional trivial insulator manifests a new class of localization phenomena of topological origin, where the two dimensional states remain localized along the surface normal direction in spite of the hybridization with the continuum extended states. The one-dimensional edge channel is also localized along the same direction as long as its energy is within the band gap. This finding demonstrates that the localized states are protected by the topological invariants against the hybridization with the continuum. [Preview Abstract] |
Session F19: Heavy Fermions and Quantum Criticality in 115's
Sponsoring Units: DCMPChair: Johnpierre Paglione, University of Maryland
Room: 321
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F19.00001: The electronic structure of Ce-based 115's Filip Ronning, Yusuke Nomura, Ryotaro Arita, Hiroaki Ikeda, Anton Kozhevnikov, Jianxin Zhu The Ce and Pu-based 115's embody the notion that reduced dimensionality and increased spin fluctuation energy scales are good for unconventional superconductivity. Often these materials are considered to be quasi two-dimensional systems similar in spirit to the high temperature cuprate superconductors. However, in reality the systems are rather three dimensional. Consequently, we construct an accurate down-folded Hamiltonian from ab-initio electronic structure calculations for the Ce-based 115 materials. We subsequently perform a constrained RPA calculation to obtain effective Coulomb parameters as a starting point to further investigate the magnetic, superconducting, and heavy fermion phenomena which these materials possess. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F19.00002: Stability of the Kondo Lattice and Field-tuned Antiferromagnetic Structures in the Ce$_{1-x}$Yb$_x$RhIn$_5$ System Sooyoung Jang, Benjamin White, Marc Janoschek, Brian Maple We have investigated the series Ce$_{1-x}$Yb$_x$RhIn$_5$ (0 $\leq x \leq$ 0.8) by means of x-ray diffraction, energy dispersive x-ray spectroscopy, electrical resistivity ($\rho$), specific heat ($C$), and magnetic susceptibility measurements. The coherence temperature $T$$_{coh}$ inferred from $\rho(T)$ remains nearly constant over a wide range of Yb concentrations 0 $\leq x \leq$ 0.8. Measurements of $C(T)$ were made in various magnetic fields up to 9 tesla on the Ce$_{1-x}$Yb$_x$RhIn$_5$ samples. In CeRhIn$_5$, the peak in $C(T)$ associated with the incommensurate antiferromagnetic (AFM) transition is accompanied by another peak that is associated with the commensurate AFM transition that emerges in an applied magnetic fields. Measurements on Ce$_{1-x}$Yb$_x$RhIn$_5$ samples ($x$ = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8) reveal that the field induced commensurate AFM peak shifts relative to the incommensurate peak with Yb substitution. The results indicate that Yb substitution stabilizes the electronic state and tunes the AFM structures in Ce$_{1-x}$Yb$_x$RhIn$_5$. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F19.00003: London penetration depth in Ce$_{1-x}$Yb$_x$CoIn$_5$ ($0 \le x \le 0.4$) Hyunsoo Kim, Makariy A. Tanatar, Ruslan Prozorov, Benjamin D. White, Ivy K. Lum, M. Brian Maple The London penetration depth was measured in single crystals of superconducting Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ ($0\leq x\leq0.4$) by means of a tunnel diode resonator technique operating at 15 MHz in a dilution refrigerator down to 100 mK. Judging from the suppression of the superconducting transition temperature, the superconductivity in CeCoIn$_{5}$ is relatively robust to Yb - substitution on Ce site unlike the substitution with other rare earth elements. On the other hand, the Yb substitution induces a drastic change of the Fermi surface near $x=0.2$, from which one may expect a significant change in properties of superconducting pairing upon doping. Possible evolution of superconducting order parameter in Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ with increasing $x$ will be discussed based on the results from penetration depth measurements. \\ \\ Work in Ames was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358. Work in San Diego was supported by the Department of Energy Office of Science (Grant DE-FG02-04-ER46105). [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F19.00004: Non-Fermi liquid behavior with and without quantum criticality in Ce$_{\mathrm{1-x}}$Yb$_{\mathrm{x}}$CoIn$_5$ Y.P. Singh, T. Hu, L. Shu, M. Janoschek, M. Dzero, M.B. Maple, C.C. Almasan In a growing number of f-electron systems the non-Fermi liquid (NFL) behavior occurs in the absence of an obvious quantum phase transition (QPT), which takes place at a quantum critical point (QCP). An intriguing candidate is Ce$_{\mathrm{1-x}}$Yb$_{\mathrm{x}}$CoIn$_5$ that exhibits an unconventional T -- x phase diagram without an apparent QCP. Therefore, it is important to elucidate the nature of the NFL behavior and to search for possible QCPs in this system Here we reveal a field induced QCP (H$_{\mathrm{QCP}})$ through normal state magneto-resistivity measurements and find its evolution with x. The full suppression of H$_{\mathrm{QCP}}$ for x \textgreater\ 0.2 has surprisingly little effect on the Kondo lattice coherence. At low Yb concentrations, resistivity consists of two contributions with linear and sub-linear temperature dependences, while at higher concentrations only the sub-linear term is present. These results imply that the NFL behavior could be a new state of matter in its own right rather than a consequence of the underlying QPT. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F19.00005: Superfluid density in heavy fermion superconductor Ce$_{1-x}$Yb$_x$CoIn$_5$ Lei Shu, D.E. MacLaughlin, O.O. Bernal, X.P. Shen, S. Pham, I. Lum, M.B. Maple Recent x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on the superconducting heavy fermion system Ce$_{1-x}$Yb$_x$CoIn$_5$ reveal that the correlated electron state is stabilized throughout the range $0 < x < 0.8$, apparently due to cooperative behavior of Ce and Yb ions involving their unstable valences. Phase separation occurs for $x > 0.8$. Interestingly, the superconducting critical temperature decreases linearly with $x$ from 2.3 K at $x = 0$ towards 0 K at $x = 1$. Transverse-field muon spin rotation experiments have been performed on Ce$_{1-x}$Yb$_x$CoIn$_5$ alloys. Based on these measurements, we report the absolute value of magnetic penetration depth as a function of $x$ and discuss whether $T_c$ is controlled by the superfluid density of superconducting carriers. The results are compared to a recently proposed theory for the superconductivity in Ce$_{1-x}$Yb$_x$CoIn$_5$ [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F19.00006: Anomalous upper critical field in CeCoIn$_5$/YbCoIn$_5$ superlattices with a Rashba-type heavy fermion interface Masaaki Shimozawa, S.K. Goh, Y. Mizukami, H. Shishido, D. Watanabe, S. Yasumoto, M. Yamashita, T. Terashima, Y. Yanase, T. Shibauch, A.I. Buzdin, Y. Matsuda We report the precise angular dependence of the upper critical field ($H_{\mathrm{c2}})$ in the epitaxial superlattices CeCoIn$_5(n)$/YbCoIn$_5$(5), formed by alternating layers of $n$ and 5 unit-cells thick CeCoIn$_5$ with a strong Pauli effect and normal metal YbCoIn$_5$, respectively [1]. For the $n =$ 3 superlattice, $H_{\mathrm{c2}}(\theta )$ changes smoothly as a function of the field angle $\theta $. However, near the superconducting transition temperature,$ H_{\mathrm{c2}}(\theta )$ shows a cusp near the angle parallel to the plane of the superlattice. This cusp behavior disappears for $n =$ 4 and 5. This sudden disappearance suggests the relative dominance of the orbital depairing effect in the $n =$ 3 superlattice, which may be due to the suppression of the Pauli effect in a system with local inversion symmetry breaking [2].\\[4pt] [1] Y. Mizukami \textit{et al.}, Nature Phys. \textbf{7,} 849 (2011).\\[0pt] [2] S. K. Goh \textit{et al.}, Phys. Rev. Lett. \textbf{109,} 157006 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F19.00007: Strong pressure dependence of the magnetic penetration depth in single crystals of the heavy fermion system CeCoIn$_5$ studied by muon spin rotation Ludovic Howald, Alexander Maisuradze, Pierre Dalmas de R\'eotier, Alain Yaouanc, Christopher Baines, Gerard Lapertot, Karine Mony, Jean-pascal Brison, Hugo Keller The pressure dependence ($0-1$ GPa) of the in-plane magnetic penetration depth ($\lambda_a$), the penetration depth anisotropy ($\gamma=\lambda_c/\lambda_a$) and the temperature dependence of $1/\lambda^2_i$ ($i=a,c$) were studied in single crystals of the heavy fermion system CeCoIn$_5$ by means of muon spin rotation. A strong decrease of $\lambda_a$ with pressure was observed, while $\gamma$ and $\lambda_i^2(0)/\lambda_i^2(T)$ are pressure independent. A linear relationship between $1/\lambda_a^2$(270 mK) and Tc was also found. The large decrease of $\lambda_a$ with pressure is the signature of an increase of the number of superconducting quasiparticles by a factor of about 2. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F19.00008: STM Spectroscopic Mapping of Quasiparticle States in the Superconducting State of CeCoIn$_5$ Shashank Misra, Brian Zhou, Eduardo H. da Silva Neto, Pegor Aynajian, Ryan Baumbach, J.D. Thompson, Eric Bauer, Ali Yazdani The heavy fermion compounds provide an interesting playground to study strongly correlated physics, as a variety of unusual low-temperature states emerge in relatively close proximity to one another in their phase diagrams. However, to date, very little spectroscopic information about these low-temperature phases, including unconventional superconductivity, is known. Recently, at comparatively high temperatures, Aynajian and coworkers\footnote{P. Aynajian, {\it et al.}, {\it Nature} {\bf 486}, 201-206 (2012).} used scanning tunneling microscopy (STM) to visualize the formation of heavy quasiparticles in one of the prototype 115 compounds, CeCoIn$\mathrm{_5}$. Here, we use a new home-built STM to extend the spatial mapping of the electronic states of CeCoIn$_5$ down to its superconducting state at mK temperatures. This work was supported by the DOE and NSF. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F19.00009: STM Spectroscopic Mapping of Quasi-Particle States in the Vortex State of CeCoIn$_5$ Brian Zhou, Shashank Misra, Pegor Aynajian, Eduardo da Silva Neto, Ryan Baumbach, J.D. Thompson, Eric Bauer, Ali Yazdani The superconducting properties of the heavy-fermion CeCoIn$_5$ emerge from a remarkable backdrop of strong electron correlation and magnetic criticality. Fittingly, this superconducting phase is itself remarkable, displaying signatures of unconventional pairing with (d-wave) line nodes in the order parameter and a Pauli-limited upper critical field below 700 mK [1]. Through scanning tunneling microscopy at milli-kelvin temperatures, we present, for the first time, atomically-resolved spectroscopy of CeCoIn$_5$ as the application of a magnetic field weakens and eventually destroys superconductivity.\\[4pt][1] J. D. Thompson and Z. Fisk, J. Phys. Soc. Jpn. \textbf{81}, 011002 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F19.00010: Long range order and two-fluid behavior in heavy electron materials Nicholas Curro, Abigail Shockley, Kent Shirer, Adam Dioguardi, Nicholas abRoberts-Warren, John Crocker, Ching Lin, David Nisson The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexists with the local moment component until the material orders at low temperatures. The two-fluid formula captures this behavior in a broad range of materials in the paramagnetic state. In order to investigate two-fluid behavior and the onset and physical origin of different long range ordered ground states in heavy electron materials, we have extended Knight shift measurements to URu$_2$Si$_2$, CeIrIn$_5$ and CeRhIn$_5$. Our results indicate that the ordered state can emerge from either the Kondo liquid or heavy electron component, and imply that the nature of the ground state is strongly coupled with the hybridization in the Kondo lattice. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F19.00011: Study of the Kondo lattice on La doped CeCoIn$_5$ G. Koutroulakis, H. Yasuoka, T. Zhou, S. E. Brown, E. D. Bauer, J. D. Thompson The effect of non-magnetic impurities on the properties of the Kondo lattice was investigated through nuclear magnetic/quadrupolar resonance (NMR/NQR) experiments on Ce$_{1-x}$La$_x$CoIn$_5$. Specifically, comprehensive $^{115}$In, $^{139}$La NQR and NMR measurements were carried out on single crystals of various La concentration levels (x=0, 2, 3, and 5\%) for temperatures 1.5K-80K and applied magnetic field values 0T-7T. Our results indicate that the ramifications of the Kondo-ion substitution extend well-beyond the vicinity of the particular site, readily affecting the heavy-fermion forming hybridization. It is suggested that the spin polarization around La impurities is modulated on a much larger length scale than that of charge oscillations. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F19.00012: High Field Knight Shift studies in CeIrIn5 Abigail Shockley, Nicholas apRoberts-Warren, David Nisson, Phil Kuhns, Arneil Reyes, Peter Klavins, Nicholas Curro All heavy fermion compounds that have been measured with NMR exhibit a Knight shift anomaly, in which the Knight shift does not scale with the bulk susceptibility below a characteristic temperature, T*.~ Typically this temperature corresponds with the Kondo lattice coherence temperature as measured by other probes.~ In order to investigate the microscopic origin of this anomaly, we have conducted high field measurements of the In-115 Knight shift in CeIrIn5 up to 30 T.~ We find that although the onset temperature T* is field independent, the overall low temperature shift below T* is suppressed.~ In the context of the two-fluid model, these results suggest that that the dominant change is in the local moment channel. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F19.00013: Probing the hybridization gap in heavy fermions by temperature dependent ARPES Cris Adriano, Fanny Rodolakis, Priscila Rosa, Francisco Restrepo, Dimitar Tenev, Mucio Continentino, Zachary Fisk, Juan Carlos Campuzano, Pascoal Pagliuso We report temperature dependent angle-resolved photoemission spectroscopy (ARPES) for pure and Cd-doped Ce$_{2}$RhIn$_{8}$ heavy fermion compounds. Our results reveal that for Ce$_{2}$RhIn$_{8}$ at T $=$ 100 K once the $f $-- conduction electrons magnetic scattering becomes larger than the phonon scattering, even states of different parities can hybridize, forming many-body quasiparticles with heavy masses. We further show that at a temperature of 20 K, where the hybridization of conduction electrons and $f $states is stronger, a spectral gap is observable in the ARPES spectra. Interestingly, when replacing In by Cd to tune the local density of conduction electrons states at the Ce$^{3+}$ site, we find a strong reduction of the $f$ - conduction electrons hybridization strength, and the suppression of the hybridization gap at low temperatures. We also observe that the $f $states near the chemical potential hybridize mostly with out-of-plane $p$ states (presumably from In). These findings have important consequences for the understanding of the different antiferromagnetic and exotic superconducting ground states that occur in these families of materials. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F19.00014: Fermi Surface evolution as a function of temperature in heavy fermion Ce$_{2}$RhIn$_{8}$ probed by ARPES Fanny Rodolakis, Cris Adriano, Francisco Restrepo, Dimitar Tenev, Pascoal Pagliuso, Juan Carlos Campuzano The crossover of 4$f$ localized magnetic moments at high temperatures into itinerant states of heavy mass at low temperatures in Cerium-based heavy fermion materials is a fundamental problem in condensed matter physics, involving a temperature-dependent hybridization between the $f$ levels immersed in a sea of conduction electrons (\textit{ce}). Due to the Luttinger theorem, this hybridization leads to a Fermi surface (FS) enlargement at low temperature: as the $f$ electrons become itinerant, their contribution to E$_{\mathrm{F}}$ increases. We have studied the evolution of the heavy fermion FS in Ce2RhIn8 as a function of temperature using angle resolved photoemission. We observed topological changes that emerge at a temperature scale much higher than the onset of the coherence character of the $f$ electrons. This behavior can be related to the evolution of the electrical resistivity as a function of temperature: as typically found for Kondo lattice materials, it first decreases when temperature is lowered, but increases below $\sim $ 150K as the magnetic scattering of the \textit{ce} by the localized $f$ electrons becomes larger than the phonon scattering. It reaches a maximum and then drops when the magnetic scattering becomes coherent for T$^{\ast} \sim $ 5K. This multiple scale behavior of the $f$ electrons is in good agreement with a recent theoretical study performed in the parent compound CeRhIn$_{5}$ [1].\\[4pt] [1] Choi et al, Phys. Rev. Lett. \textbf{108}, 016402. [Preview Abstract] |
Session F20: Focus Session: Mesoscopics - Optics and Plasmonics
Sponsoring Units: DMPRoom: 322
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F20.00001: Electron Microscopy: an Analytical Tool for Solid State Physicists Invited Speaker: Gustaaf Van Tendeloo For too long the electron microscope has been considered as ``a big magnifying glass.'' Modern electron microscopy however has evolved into an analytical technique, able to provide \textit{quantitative} data on structure, composition, chemical bonding and magnetic properties. Using lens corrected instruments it is now possible to determine atom shifts at interfaces with a precision of a few picometer; chemical diffusion at these interfaces can be imaged down to atomic scale. The chemical nature of the surface atoms can be visualized and even the bonding state of the elements (e.g. Mn$^{2+}$ versus Mn$^{3+}$) can be detected on an atomic scale. Electron microscopy is by principle a projection technique, but the final dream is to obtain atomic info of materials in three dimensions. We will show that this is no longer a dream, but that it is possible using advanced microscopy. We will show evidence of determining the valence change Ce4+ versus Ce$^{3+}$ at the surface of a CeO$^2$ nanocrystal; the atomic shifts at the interface between LaAlO$^3$ and SrTiO$^3$ and the 3D relaxation of a Au nanocrystal.\\[4pt] References:\\[0pt] ``2D atomic mapping of oxidation states in transition metal oxides by scanning transmission electron microscopy and electron energy-loss spectroscopy,'' Tan,H., Turner, S., Yucelen, E., Verbeeck, J., Van Tendeloo, G. \textit{Physical Review Letters}, 107, 107602, (2011).\\[0pt] Three-dimensional atomic imaging of crystalline nanoparticles, Van Aert, S., Batenburg, K.J., Rossell, M.D., Erni, R., Van Tendeloo, G. \textit{Nature}, 470, 374 (2011).\\[0pt] ``Advanced Electron Microscopy for Advanced Materials,'' Van Tendeloo, G., Bals, S., Van Aert, S., Verbeeck, J., Van Dyck, D. \textit{Advanced Materials, DOI}: 10.1002/adma.201202107 (2012).\\[0pt] Atomic-scale determination of surface facets in gold nanorods, Goris, B., Bals, S., Van den Broek, W., Carbo-Argibay, E., Gomez-Grana, S., Liz-Marzan, M., Van Tendeloo, G. \textit{Nature Materials},11, 930 ( 2012)\\[0pt] ``Handbook of Nanoscopy'' Eds. G. Van Tendeloo, D. Van Dyck, S.J. Pennycook, Wiley-VCH (2012) [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F20.00002: Spatial mapping of surface plasmons in nanoscale Ag islands on graphite using Scanning Probe Energy Loss Spectroscopy Karl Bauer, Shane Murphy, Lin Tang, Richard Palmer A scanning STM tip operated at high voltage can be used to obtain localized spectroscopic information about surfaces via energy loss measurements [1]. In this technique, known as Scanning Probe Energy Loss Spectroscopy (SPELS), the STM tip is used as a localized source of field-emitted electrons, which, upon backscattering from a surface, are analyzed by an energy-dispersive detector to obtain localized energy loss spectra. Characteristic surface excitations such as plasmons and excitons (as well as secondary electrons) can be probed with a spatial resolution below 50 nm and an energy resolution approaching 0.3 eV [2]. We report the development of a new generation SPELS instrument utilizing a 400-Channel detector, allowing sufficiently fast sampling of the energy loss spectra to allow us to obtain 2D spatially-resolved maps of energy loss features in a reasonable timeframe. We demonstrate the new instrument by mapping plasmons in (thermally evaporated) Ag nano-islands on the surface of graphite and illustrate the various mechanisms give rise to the contrast obtained in the energy-resolved maps. [1] A. Pulisciano, S.J. Park and R. E. Palmer, Appl. Phys. Lett. 93, 213109 (2008). [2] F. Festy and R. E. Palmer, Appl. Phys. Lett. 85, 5034 (2004). [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F20.00003: Ultrafine and Smooth Full Metal Nanostructures for Plasmonics Xinli Zhu, Jaseng Zhang, Jun Xu, Zhimin Liao, Xiaosong Wu, Dapeng Yu Surface plasmon polaritons (SPPs), which are coupled excitations of electrons bound to a metal-dielectric interface, show great potential for application in future nanoscale photonic systems due to the strong field confinement at the nanoscale, intensive local field enhancement, and interplay between strongly localized and propagating SPPs. The fabrication of sufficiently smooth metal surface with nanoscale feature size is crucial for SPPs to have practical applications. A template stripping (ST) method combined with PMMA as a template was successfully developed to create extraordinarily smooth metal nanostructures with a desirable feature size and morphology for plasmonics and metamaterials. The advantages of this method, including the high resolution, precipitous top-to bottom profile with a high aspect ratio, and three-dimensional characteristics, make it very suitable for the fabrication of plasmonic structures. By using this ST method, boxing ring-shaped nanocavities have been fabricated and the confined modes of surface plasmon polaritons in these nanocavities have been investigated and imaged by using cathodoluminescence spectroscopy. The mode of the out-of-plane field components of surface plasmon polaritons dominates the experimental mode patterns, indicating that the electron beam locally excites the out-of-plane field component of surface plasmon polaritons, and quality factors can be directly acquired. Numerous applications, such as plasmonic filter, nanolaser, and efficient light-emitting devices, can be expected to arise from these developments. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F20.00004: Low Loss Plasmonic Oxide Nanocrystals with Controlled Morphology Thomas Gordon, Taejong Paik, Dahlia Klein, Matteo Cargnello, Christopher Murray Localized surface plasmon resonance (LSPR) is a observed in metallic particles and results from the resonant oscillation of free electrons on the particle surface. One can manipulate the resonant frequencies through adjustment of the shape and size of the metal. A series of recent papers report LSPR at NIR and IR frequencies resulting from doped semiconductor nanocrystals. Free carriers in semiconductor particles result from atomic vacancies or through doping with aliovalent cations. While the plasma frequency ($\omega_{p})$ is considered an intrinsic property of metals, through adjustment of dopant concentrations, $\omega_{p}$ can be tuned in plasmonic semiconductors, opening the possibility of producing tunable, low-loss plasmonic nanocrystals to substitute for Au and Ag. We report the size and shape controlled synthesis of plasmonic oxide nanocrystals with highly uniform morphology and shape dependent optical properties. The size, shape, and doping concentration are independently controlled by modifying the synthetic parameters, allowing for precise modulation of optical response. These nanocrystals may be assembled to form superlattices, which function as plasmonic metamaterials, or used as precursors to produce bulk like films with tunable plasma frequencies. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F20.00005: Aluminum Plasmonic Nanoantennas Henry Everitt, Mark Knight, Lifei Liu, Yumin Yang, Lisa Brown, Shaunak Mukherjee, Nicholas King, Peter Nordlander, Naomi Halas We have explored the plasmonic properties of individual Al nanorod antennas fabricated by planar lithography on lightly doped n-type silicon. Energy-resolved cathodoluminescence was used to image the local density of optical states with a spatial resolution of $\sim$ 20 nm and thereby identify the radiative modes of these nanostructures. Al nanoantenna emission exhibited highly tunable plasmonic resonances from the deep UV through the visible region of the spectrum. The dependence of the radiative dipolar and quadrupolar plasmon modes on antenna length and photon energy agreed well with finite difference time domain-based analysis of these nanostructures. The results herald nano-structured aluminum as a practical and highly promising material system for the design and implementation of UV and visible frequency plasmonics, broadening the range of potential applications of plasmonics into areas where complementary metal$-$oxide$-$semiconductor (CMOS) compatibility or low-cost, mass producibility are desired. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F20.00006: Plasmon coupling between distance-controlled gold nanoparticles Holger Lange, Beatriz Hernandez Juarez, Christian Thomsen, Tony F. Heinz For small enough distances of noble metal nanoparticles in a matrix an additional plasmon-coupled mode is known to appear as a collective excitation between the nanoparticles. We show an approach of combining gold nanoparticles that allows to obtain coupled plasmons that can be dynamically changed, allowing systematic studies of the coupling. Poly-(N-isopropylacrylamide) pNIPAM is a polymer that can be used to produce thermo responsive gels, which have a volume phase transition at around $32^\circ$C. A ligand exchange on Au nanoparticles allows the attachment of the nanoparticles to pNIPAM spheres. The combined hybrid Au-pNIPAM system shows a plasmon-coupled mode above the pNIPAM's phase transition, additionally to the well-known shift and broadening of the fundamental plasmon peak. This plasmon mode can be switched on and off and modified simply by changing the temperature. We present discrete dipole approximation (DDA) calculations that characterize this resonance as a quadrupole Au plasmon mode, which results from close-to-contact-particles within the statistically distributed nanoparticles in the pNIPAM matrix. The presented approach is generalizable and allows to investigate the interaction between different kinds of metal nanostructures. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F20.00007: Energy concentration of periodic nanoparticle array using Green function formalism King Chun Lai, Sze Fung Lee, Kin Wah Yu We have studied a periodic array of nanoparticle wires by using the Green function formalism (GFF). When light is incident on the wire, a collective oscillation of the free electrons is excited on the surface of the wires, which is called the coupled surface plasmon. The excitation of coupled surface plasmon can cause an enhancement of the local energy density. By tuning the separation relative to the radius of the wires, an energy concentration can be controlled. When the separation of the wires is small, multipolar effect becomes significant. Dealing with tight-binding model by Park and Stroud (2004) would involve interaction term which appears to be non-existent and the resolution of FDTD is insufficient to resolve the multipole interaction as the multipole field can vary rapidly. We applied GFF to this problem which expresses all interaction in a Greenian within one unit cell. The system was studied under spectral representation and the relation between different resonance modes and the outcoming energy concentration was examined. The energy concentration is largest several hot spots which depend on the incident directions. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F20.00008: Tunable optical excitations in transition-metal doped arrays of noble-metal chains Neha Nayyar, Volodymyr Turkowski, Talat S. Rahman We apply time-dependent density-functional theory to study the absorption spectrum of arrays of nano-scale pure noble and transition metal (TM) chains. We find that as the number of chains in the noble atom array increases the plasmon peak shifts to higher energies and appears in the visible range for an array of three gold chains, each consisting of more than 10 atoms. We also find collective excitations (plasmons) in arrays of TM chains: a behavior distinct from bulk TM systems. Doping noble metal chains with TM atoms leads to additional plasmon peaks close in energy to the main one for the undoped case. We compare the calculated optical absorption spectrum of the doped chains for several different types of TM atoms at different positions in the chains, and provide rationale for the trends. In the multi-chain case, the response is very sensitive to the position of the doped atoms. We argue that the origin of the additional modes is charge oscillations around the impurity atoms. Finally, we analyze the effect of interaction of excitonic modes created in infinite chains with plasmons in neighboring nanochains, including the possibility of resonance excitations and their trapping by the TM impurity atoms. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F20.00009: Hollow Shells Of Dipoles: A Group Theoretical Approach Christopher Devulder, Slava Rotkin We investigate the plasmonic properties of hollow cylindrical lattices whose constituent elements are modeled as point dipoles. The symmetry of the lattice is described within the framework of group theory, which enables us to obtain the eigenmodes and eigenvalues of the entire polarization field by diagonalizing the dipole interaction part of the Hamiltonian. An incoming plane wave electric field that couples resonantly with the dipole lattice is expanded in terms of cylindrical harmonics, allowing us to precisely determine the contribution of various modes in its response function. The latter can then be obtained analytically for an arbitrary plane wave excitation. This work facilitates the study of cylindrical plasmonic shells with various geometry, as in the case of gold nanoparticles surrounding carbon nanotubes. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F20.00010: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F20.00011: Band edge excitons and trions in CdSe/CdS core/shell nanocrystals Andrew Shabaev, Anna Rodina, Alexander Efros We have developed a theory of positevly and negatively charged excitons (trions) in ``giant'' CdSe/CdS core-shell nanocrystals. The theory describes the energy structure of excitons and trions. We present the results of calculations for the fine structure of the the positively charged trion, the binding energy of the negatively charged trion, and the radiative decay time for excitons and trions. The theoretical results are compared with available experimental data. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F20.00012: Controlling orientational order of multivalent prisms in superlattice assemblies Kevin L. Kohlstedt, Monica Olvera de la Cruz, George C. Schatz Multivalent nanostructures are an increasingly important player in the self-assembly of optically responsive superlattices. Understanding the role nanostructure coordination plays in the ordering of superlattice assemblies is crucial for the plasmonic response of the material. We developed a simple design rule for the assembly of multivalent DNA-Au triangular nanoprisms into 1D ordered superlattices based on both the length of the valent DNA and the size of the prism. Using MD simulations, we describe an order parameter that captures the short-range order of the mesoscale assembly controlled by the design rule. The order parameter shows that even short chains of prisms have a high-degree of orientational order when 1D superlattices are formed. Unlike isotropic polyvalent nanostructures, we find the highly oriented prism superlattices lose orientational order in a multistage fashion through loss of coordination during melting. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F20.00013: Quantum Size Effects in $\alpha $-Pu (020) Layers Sarah C. Hernandez, Asok K. Ray, Christopher D. Taylor First principles calculations using the projector-augmented wave method and a plane wave basis set as implemented in the Vienna \textit{Ab Initio} Simulation Package (VASP) have been performed for the $\alpha $-Pu (020) layers. Because of severe demands on computational resources, scalar-relativistic computations were performed at the experimental geometry. The surface was assumed to be anti-ferromagnetic (AFM) since previous theoretical studies indicate the ground state of bulk $\alpha $-Pu to be AFM. Up to ten layers have been considered in this study. Work functions and surface energies appear to converge as the number of layers increase. We predict the work function to be around 3.4eV, with the surface energy being approximately 1.6eV. While no experimental results are available for $\alpha $-Pu, experimental results for $\delta $-Pu indicate a work function of approximately 3.2eV and a surface energy of 2.0eV. We will also present results on the magnetic moments and density of states of the layers. Results will be compared with results using the full-potential linearized-augmented-plane-wave method as implemented in the WIEN2k suite of software. [Preview Abstract] |
Session F21: Focus Session: Multiferroics and Magnetoelectrics
Sponsoring Units: DMPChair: Hans Christen, Oak Ridge National Laboratory
Room: 323
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F21.00001: Magnetoelectricity in Spinel FeCr$_{2}$S$_{4}$ Lin Lin, Dan Liu, Zhengyin Zhao, Jiajia Wen, Zhibo Yan, Shuai Dong, Junming Liu We report on ferroelectricity, magnetic susceptibility, dielectric property, and specific heat capacity of the polycrystalline spinel FeCr$_{2}$S$_{4}$. We provide clear evidence of a ferroelectric transition at $\sim$ 8.5K, which accompanies an orbital-ordering transition and a dielectric anomaly. The polarization increases with decreasing temperature, and reaches 3.6$\mu $C/m$^{\mathrm{2}}$ at 2 K. We also carry out detailed multiferroic measurements, and a remarkable magnetoelectric coupling is observed. A very small magnetic filed $H$ $\sim$ 500Oe enhances the polarization to 8.13.6$\mu $C/m$^{\mathrm{2}}$ at 2 K, with a magnetoelectric coupling coefficient $\alpha \sim$ 120{\%}. However, the polarization rapidly decreases for fields beyond $H \sim$ 1T. The multiferroic behavior of FeCr$_{2}$S$_{\mathrm{4}}$ is proposed to arise from competition between the spin-orbital coupling and the Jahn-Teller effect for the Fe ion.\\[4pt] [1] V. Fritsch, et al, Phys. Rev. Lett. \textbf{92}, 116401 (2004).\\[0pt] [2] R. Fichtl, et al, Phys. Rev. Lett. \textbf{94}, 027601 (2004).\\[0pt] [3] V. Tsurkan et al, Phys. Rev. B \textbf{81}, 184426 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F21.00002: The internal and external magnetoelectric effect in LiFeP$_{2}$O$_{7}$ K.-C. Liang, W. Zhang, B. Lorenz, Y.Y. Sun, P.S. Halasyamani, C.W. Chu We study the internal and external magnetoelectric (ME) effect of the single-crystalline LiFeP$_{2}$O$_{7}$ by magnetic, thermodynamic, and magnetoelectric measurements. The Fe$^{3+}$ spins form a canted antiferromagnetic (AFM) order below $T_{N}$ $\sim$ 27K with weak ferromagnetic components along the $b$-axis. A sharp peak found in the pyroelectric current at 27K indicates the strong internal ME interaction resulting in a sizeable polarization decrease. With external magnetic field applied, the ME polarization shows a combination of a linear and a quadratic field dependence below $T_{N}$, but it only shows the quadratic ME response above then. A large uniaxial magnetoelastic response in the thermal expansion data indicates strong spin-lattice coupling. A qualitative explanation regarding to the ME interaction between electric polarization and magnetic order parameters will be proposed and discussed. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F21.00003: Epitaxial Strain Induced Robust Multiferroicity in BiMnO$_{3}$ Xuezeng Lu, Xingao Gong, Hongjun Xiang By performing first principles calculations, we investigate the effects of the epitaxial strain on the properties of BiMnO$_{\mathrm{3}}$ films grown along the pseudocubic [001] direction. Unlike the ground state with the centrosymmetric $C$2/$c$ space group in bulk, two previously unreported phases, namely, paraelectric \textit{Pnma} and ferroelectric \textit{Cc} phases, are stabilized by epitaxial strain. Several surprising and interesting phenomena are revealed. In particular, we find a metal-insulator transition between the ferromagnetic metallic state and antiferromagnetic insulating ferroelectric state under compressive epitaxial strain. On the other hand, the tensile epitaxial strain stabilizes the ferromagnetic and ferroelectric \textit{Cc} state with the large polarization ($P$ \textgreater 80 $\mu $C/cm$^{2})$ and high Curie temperature (estimated $T_{c}$ $\sim$ 395 K). Moreover, there is a novel intrinsic magnetoelectric coupling in the multiferroic \textit{Cc} state with the easy magnetization axis tunable by the external electric field. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F21.00004: Chemistry and synthesis of new polar perovskites with small tolerance factors Invited Speaker: Alexei Belik ``Usual'' perovskite-type compounds with the general formula ABO3, where A is La-Lu and Y and B is V, Cr, Mn, Fe, Co, Ni, and Cu have been attracting a lot of attention for decades. ``Exotic'' perovskites are also highly interesting because new phenomena may emerge in them. The term ``exotic'' may include compounds with unusual oxidation states, unusual ion distribution, and unusual ions at the A site and B site. Perovskites with A $=$ Sc and In have small tolerance factors, and they can be prepared only at high pressure. We will discuss ``exotic'' perovskites with A $=$ Sc and In. A limited number of compounds can be prepared at 6 GPa. Results on (A1-yMy)MnO3 (A $=$ Sc and In, M $=$ Mn, Mg, Co, and Ni), InCrO3, ScCrO3, InRhO3, ScRhO3, InNi0.5Mn0.5O3, and ScNi0.5Mn0.5O3 will be presented. We will also describe a new class of multiferroic polar materials: In-based perovskites. We show that (In1-yMy)MO3 with y $=$ 0.112-0.176 and M $=$ Fe0.5Mn0.5 is isostructural with BiFeO3 (space group R3c) and has a high ferroelectric Curie temperature; (In1-yMy)MO3 is a canted antiferromagnet with the N\'eel temperature close to RT. Our results give a significant contribution to the development of RT multiferroics and also show new ways for the preparation of perovskite-type materials. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F21.00005: The search for new multiferroic ABF$_4$ fluorides via first-principles structure maps Brian Abbett, Aditi Krishnapriyan, Craig J. Fennie Transition metal ABF$_4$ fluorides are observed in a wide variety of different structure types. One, the BaMnF$_4$ structure, is an interesting family of polar (possibly ferroelectric) materials that display canted-antiferromagnetism, which has been predicted (Ederer and Spaldin) to reverse when the polarization reverses. This strong coupling between magnetism and polarization has motivated us to explore additional ABF$_4$ structure types. In this talk we will discuss our search for new multiferroic ABF$_4$ fluorides by creating structure maps from first principles. As a first step we categorize the ABF$_4$ compounds found in the ICSD. We focus on structures for which the B-site is octahedrally coordinated; these can be fitted into one of four categories: BaMF$_4$, Dion-Jacobson, and the so-called slip (100) or slip (110) structures. These four categories represent high symmetry structures which allow distortions to lower symmetry structures. Note that most of the known multiferroic ABF$_4$ compounds form in the BaMF$_4$ structure. We elucidate a simple descriptor that helps to build the chemical and physical intuition as to why a compound forms in this structure type needed for the rational design of new multiferroic ABF$_4$ fluorides. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F21.00006: Magnetic field enhanced structural instability in EuTiO$_{3}$ Zurab Guguchia, Hugo Keller, J\"{u}rgen K\"{o}hler, Annette Bussmann-Holder EuTiO$_{3}$ undergoes a structural phase transition from cubic to tetragonal at $T_{\rm S}$ = 282 K which is not accompanied by any long range magnetic order. However, it is related to the oxygen ocathedra rotation driven by a zone boundary acoustic mode softening. Here we show that this displacive second order structural phase transition can be shifted to higher temperatures by the application of an external magnetic field (${\Delta}$$T_{\rm S}$ ${\simeq}$ 4 K for $\mu_{0}$$H$ = 9 T). This observed field dependence is in agreement with theoretical predictions based on a coupled spin-anharmonic-phonon interaction model. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F21.00007: Spin-Lattice Coupling and Third Neighbor Magnetic Interactions in EuTiO$_3$ Turan Birol, Craig J. Fennie An ongoing challenge in materials physics is to identify materials that display a strong coupling between the electrical polarization and magnetism. EuTiO$_3$ is one such material that has been of much recent interest. This novel material is antiferromagnetic and paraelectric in bulk but becomes simultaneously ferromagnetic and ferroelectric under biaxial strain due to a rather large spin-lattice (phonon) coupling. In this talk we will present the results of our first-principles study on the effect of ferroelectric distortions and octahedral rotations on the magnetic exchange interactions in EuTiO$_3$. We elucidate the evolution of the octahedral rotation pattern with strain and show how they influence the properties of the multiferroic phase. Going beyond the proposed cation-mediated exchange for EuTiO$_3$, which has been linked to the large spin-lattice coupling in this material, we uncover the importance of third-neighbor magnetic interactions and illustrate how it is responsible for the ``giant'' cross-field magnetoelectric effect recently demonstrated. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F21.00008: Magneto-orbital helices: a novel coupling mechanism between magnetism and ferroelectricity in multiferroic CaMn$_7$O$_{12}$ Paolo G. Radaelli, Natasha Perks, Roger D. Johnson, Christine Martin, Laurent Chapon The trigonal quadruple perovskite CaMn$_7$O$_{12}$ displays one of the largest magnetically induced ferroelectric polarisations measured to date (2870 $\mu$C m$^{-2}$). Ferroelectricity appears below 90 K, together with an incommensurate helical magnetic modulation, and cannot be explained within the framework developed for cycloidal magnets [2]. We report an unprecedented magneto-orbital texture in multiferroic CaMn$_7$O$_{12}$, which is directly connected to ferroelectricity[3]. X-ray and neutron diffraction characterisation of the structural and magnetic modulations in these ``magneto-orbital helices'', and analysis of magnetic exchange shows that orbital order is crucial in stabilising a chiral magnetic structure. Additionally, the presence of a global structural rotation enables the magnetic helicity to couple with the lattice, giving rise to electric polarisation. These novel principles open up the possibility of discovering new multiferroics with even larger polarization and higher transition temperatures. [1] G. Zhang, \textit{et al.}, Phys. Rev. B 84 (2011) 174413. R.D. Johnson \textit{et al.}, Phys. Rev. Lett. 108, 067201 (2012). [2] M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006). [3] N. Perks \textit{et al.}, Nat. Comm., \textit{in press}. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F21.00009: Magneto-Electric Coupling in Single Crystal Cu$_2$OSeO$_3$ Studied by a Novel Electron Spin Resonance Technique Alexander Maisuradze, Alexander Shengelaya, Helmuth Berger, Dejan Djoki\'c, Hugo Keller The magneto-electric (ME) coupling on spin-wave resonances in single-crystal Cu$_2$OSeO$_3$ was studied by a novel technique using electron spin resonance combined with electric field modulation. An external electric field ${\bf E}$ induces a magnetic field component $\mu_0 H^i = \gamma E$ along the applied magnetic field ${\bf H}$ with $\gamma=0.7(1)~ \mu$T/(V/mm) at 10 K. The ME coupling strength $\gamma$ is found to be temperature dependent and highly anisotropic. $\gamma(T)$ nearly follows that of the spin susceptibility $J^M(T)$ and rapidly decreases above the Curie temperature $T_{\rm c}$. The ratio $\gamma/J^M$ monotonically decreases with increasing temperature without an anomaly at $T_{\rm c}$. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F21.00010: Strong Dzyaloshinskii-Moriya Interaction and Origin of Ferroelectricity in Cu$_{2}$OSeO$_{3}$ Ji-Hui Yang, Zheng-Lu Li, Xuezeng Lu, X.G. Gong, Hongjun Xiang, M.-H. Whangbo, Su-Huai Wei In this work, we try to understand the skyrmions recently observed experimentally in Cu2OSeO3 system, as well as its origin of ferroelectricity. Based on the spin Hamiltonian, we developed four-state-energy-mapping method to derive these spin interaction parameters. For this system, we found a very large ratio between the DM term and the symmetric exchange interaction. Besides, the spin arrangements in the ground state are found degenerate and the spin energy is independent of the propagation vector q. Taking these two factors into account, we explained the experimental observation of skyrmions to some extent. Then we built a model to describe the polarization of this system. By the symmetry analysis, the ferroelectricity is supposed to result from the spin single-site term, as is confirmed by direct calculations of our model. Using this model, we analyzed its ferroelectricity dependence of the spin arrangement and find the largest polarization happens when the spins are along \textless 111\textgreater\ direction, in excellent agreement with the experimental results. [Preview Abstract] |
Session F22: Strongly Correlated Electron Theory I
Sponsoring Units: DCMPChair: Steve Hellberg, Naval Research Laboratory
Room: 324
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F22.00001: Spin flip in spin-orbit split quantum wires in magnetic field Oleg A. Tretiakov, K. S. Tikhonov, V. L. Pokrovsky We study spin-flip processes induced by ac electromagnetic field in quantum wires with strong spin-orbit coupling in the presence of an external magnetic field. The dc magnetic field is essential to enable the electric dipolar excitation of the spin-flip processes. We consider the electron spin-flip resonance in the framework of Luttinger liquid theory. The electron-electron interaction is strong in quantum wires and changes the shape of the spin-flip resonance curve at the spin wave frequency and produces an additional cusp at the frequency of collective charge excitation. We discuss how this spin flip is affected by the dissipation processes and the dispersion curvature. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F22.00002: Theoretical study of a one-dimensional chain of alternating spin-1 and electron sites with spin-mediated hopping Wing-Ho Ko, Hong-Chen Jiang, Jeffrey Rau, Leon Balents Motivated by the nickel valance controversy in the perovskite nickelate RNiO$_3$, we consider a one-dimensional chain consisting of alternating spin-1 (``nickel'') and electron (``oxygen'') sites, which in addition to the usual electron hopping and spin-spin interaction between the spin-1 and the electron also contains a spin-1 mediated electron hopping term. Using density-matrix renormalization group (DMRG), we obtain the phase diagram of such model, as well as various correlation functions in each phase. Importantly, for certain range of parameters the model exhibits a quasi-long-range spiral (QS) order. To understand the DMRG results, we construct a mean-field theory based on Schwinger fermion decomposition of the spin-1 spins, from which we argue that the QS phase corresponds to a phase in proximity to the spin Bose metal state proposed by Sheng, Motrunich, and Fisher [Phys. Rev. B, 79, 205112 (2009)]. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F22.00003: Magnetic Phase Transition Induced by the Hubbard and Spin-orbit Interactions in a Nanoribbon Geometry Hyeong Jun Lee, Moo Young Choi, Gun Sang Jeon The local repulsive Coulomb interaction between the electrons tends to cause a Mott transition into a magnetically ordered phase. In a honeycomb lattice, particularly, the magnetic order is known to emerge on the edges of graphene, which is attributed to the electron interactions. Meanwhile, the introduction of the spin-orbit interaction gives rise to metallic boundary states, which is a prominent characteristic of the topologically nontrivial materials. We study the effect of the spin-orbit interaction on the edge states as well as the bulk properties of the electron system on the honeycomb lattice. By employing a Hartree-Fock approximation, we compute the local magnetization in the half-filled nanoribbon system at zero temperature. We pay particular attention to the decaying behavior of the local magnetization from the edge toward the center. It is found that the characteristic length associated with the decay is divergent on the phase boundaries. Such slow decay is found to be algebraic in the thermodynamic limit. We discuss the relation between the bulk phase transitions and the decay of magnetization at the edges. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F22.00004: Nonequilibrium thermal transport and its relation to linear response Christoph Karrasch, Roni Ilan, Joel Moore We study the real-time dynamics of spin chains driven out of thermal equilibrium by an initial temperature gradient $T_L\neq T_R$. We demonstrate that the nonequilibrium energy current saturates fast to a finite value if the linear-response thermal conductivity is infinite, i.e. if the Drude weight $D$ is nonzero. Our data suggests that a nonintegrable dimerized chain might support such dissipationless transport ($D > 0$). We show that the steady-state value of the current for arbitrary $T_L \neq T_R$ is completely determined by the linear conductance. Inhomogeneous systems exhibiting different bulk parameters as well as Luttinger liquid boundary physics induced by single impurities are discussed shortly. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F22.00005: Quantum Monte-Carlo simulation of spin-one antiferromagnets with single-ion anisotropy Yasuyuki Kato, Keola Wierschem, Yusuke Nishida, Cristian Batista, Pinaki Sengupta We study a spin-one Heisenberg model with uniaxial single-ion anisotropy, $D$, and Zeeman coupling to a magnetic field, B, parallel to the symmetry axis. We compute the $(D/J, B/J)$ quantum phase diagram for square and simple cubic lattices by combining analytical and Quantum Monte Carlo approaches, and find a transition between XY-antiferromagnetic and ferronematic phases that spontaneously break the U(1) symmetry of the model. In the language of bosonic gases, this is a transition between a Bose-Einstein condensate (BEC) of single bosons and a BEC of pairs. For the efficient simulation of ferronematic phase, we developed and implemented a new multi-discontinuity algorithm based on the directed-loop algorithm. The ordinary quantum Monte-Carlo methods fall into freezing problems when we apply them to this system at large $D/J$ and finite $B/J\sim 1$. The new method does not suffer from the freezing problems. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F22.00006: Majorana dimerised order in magnetic systems Edmund Bennett We consider the analysis of quantum critical points (QCPs) using a Majorana fermion\footnote{W. Mao, P. Coleman, C. Hooley \& D. Langreth; PRL, 91, 20, p. 2072031-2072034; 2003} representation of spin. Majorana fermions are a useful spin representation as they obey Wick`s theorem and automatically provide the correct $S_{\rm tot.}^{2} = 3/4$ for stationary spin-$1/2$ lattice spins. We consider an Ising model in various dimensions in an applied transverse field, a model which exhibits a QCP and has an exact solution in 1D. In the Majorana fermion representation, the interaction vertex may be decoupled into either a ``Majorana dimerisation (MD)'' decoupling or an Ising magnetic decoupling. A mean-field analysis of the MD decoupling (which involves two Majorana fermions of the same flavour on adjacent lattice sites) suggests an ordered phase in the region above the QCP extant in the model, which extends through to high magnetic fields. Full RPA corrections to this mean-field theory are also presented, which give insight into the stability of this ordered phase to quantum perturbations. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F22.00007: Spin Orbit Magnetism and Unconventional Superconductivity Yi Zhang, Kevin Bedell We find an exotic spin excitation in a magnetically ordered system with spin orbit magnetism in 2D, where the order parameter has a net spin current and no net magnetization. Starting from a Fermi liquid theory, similar to that for a weak ferromagnet, we show that this excitation emerges from an exotic magnetic Fermi liquid (EMFL) state that is protected by a generalized Pomeranchuck condition. We derive the propagating mode using the Landau kinetic equation, and find that the dispersion of the mode has a q$^{\mathrm{1/2}}$ behavior in leading order in 2D. We find an instability toward superconductivity induced by this exotic mode, and a further analysis based on the forward scattering sum rule strongly suggests that this superconductivity has p-wave pairing symmetry. We perform similar studies in the 3D case, with a slightly different magnetic system and find that the mode leads to a Lifshitz-like instability most likely toward an inhomogeneous magnetic state in one of the phases. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F22.00008: Magnetic properties of $d-$atomic systems with unquenched orbital moments Victor Antonov, Liqin Ke, Anton Jesche, Vladimir Antropov Many systems of $d$-atoms with unquenched orbital moments demonstrate unusually large values of atomic magnetic moments, high magnetic anisotropy and small magnetic ordering temperatures. Using electronic structure analysis, we study a mechanism of the formation of strong on-site electronic correlations that lead to a strong orbital polarization, and in turn, generate a highly orbitally polarized hybridization with non-magnetic host atoms. In this case, even a small spin orbital coupling of $3d$-atoms can create a significant effect. We introduce a consistent model of the formation of large orbital moments and magnetic anisotropy both in the metallic and insulating cases, and apply it to several realistic systems. Detailed calculations of magnetic properties, including magneto-optical studies of the Kerr angle rotation, are performed for several nitrometalates of Mn, Fe and Co where a rather large (3-5 degrees) Kerr angle rotation is predicted for the first time. We further discuss the nature of critical temperature in magnetic phase transition in such systems and the opportunity to increase it. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F22.00009: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F22.00010: Fractional Chern insulator on the triangular lattice Stefanos Kourtis, J\"orn Venderbos, Jeroen van den Brink, Maria Daghofer The opportunity for the formation of fractional quantum-Hall (FQH) states in 3-orbital Hubbard and Kondo lattice models on the triangular lattice without an external magnetic field has been recently demonstrated [1,2]. With this as motivation, an effective interacting spinless-fermion model, which is designed to capture the essential relevant physics, has been extensively studied. Its numerically obtained ground states at several fillings exhibit features which suggest that the former are spontaneously occurring FQH states on a lattice, i.e. fractional Chern insulator (FCI) states. The most unequivocal feature of such states is arguably their fractionally quantized Hall conductivity. This effect, as seen in numerical calculations for finite clusters, will be presented and discussed. Having thus identified FCI states, further signatures of their nature are highlighted, e.g. fractional quasihole statistics. \\[4pt] [1] J.W.F. Venderbos, S. Kourtis, J. van den Brink, and M. Daghofer, Phys. Rev. Lett. 108, 126405.\\[0pt] [2] S. Kourtis, J.W.F. Venderbos, and M. Daghofer, arXiv:1208.3481. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F22.00011: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F22.00012: Magnetic phase transitions in the chiral helimagnet Cr$_{1/3}$NbS$_2$ Nirmal Ghimire, Michael McGuire, Brian Sales, Lisa Debeer-Schmitt, Huibo Cao, Bryan Chakoumakos, Adam Aczel, Balazs Sipos, Siwei Tang, Yuen Yiu, Jiaqiang Yan, Stephen Nagler, David Mandrus Cr$_{1/3}$NbS$_{2}$ is a long period chiral helimagnet crystallizing in the noncentrosymmetric, hexagonal~space group P6$_3$22. Helimagnetic ordering along the c-axis is attributed to the competition between the symmetric exchange interaction, favoring parallel moments, and the anti-symmetric Dzyaloshinsky-Moriya interaction, favoring perpendicular moments. Recently, the ground state helical ordering is found to be destabilized by a magnetic field applied perpendicular to c, forming a chiral soliton lattice phase, and, above a critical field, a commensurate ferromagnetic state. Thermal and transport properties also show interesting behaviors in the vicinity of the transition temperature. Here we present magnetic, thermal and transport properties of Cr$_{1/3}$NbS$_{2}$ measured on single crystals, along with recent results from neutron scattering experiments conducted on the four circle single crystal diffractometer and general purpose SANS at the High Flux Isotope Reactor, ORNL. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F22.00013: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F22.00014: Possible non-coplanar spin structure and large Hall effect in Na$_{x}$CoO$_{2}$ J.W. Kim, E.D. Mun, R.D. McDonald, V. Zapf, J.D. Thompson, L. Balicas, I. Marting, D. Argyriou We present magnetotransport studies of Na$_{x}$CoO$_{2}$ ($x=$0.46) and its relation to possible non-coplanar spin texture. This compound exhibits a unique insulating state below temperature ($T)$ of 53 K related to charge-order which is different from other composition with metallic behavior. It also has frustrated local spin texture owing to its hexagonal structure. Previous works report a very large Hall signal for composition $x=$0.5 (M. Foo \textit{et al.}, Phys. Rev. Lett. 92, 247001 (2004)) at low magnetic field ($B)$ and prior high-field studies (L. Balicas \textit{et al.}, Phys. Rev. Lett. 94, 236402 (2005)) have found the existence of a small Fermi surface in the system and a two-fold angular magnetoresistance. Using pulse and hybrid magnets at NHMFL, we mapped out a detailed $T$-$B$ phase diagram up to 65 T which is strong enough to suppress the charge-order. When $B$ is applied along the $c$-axis, the charge-ordered state is suppressed at $B$ $\sim$ 41 T with highly non-monotonic shape in $\rho_{xy}$. We found that this Hall signal reaches a maximum around $T \sim$ 30 K and $B \sim$ 27 T and on further cooling the absolute change of $\rho_{xy}$ decreases significantly. Interestingly, we found no significant changes in field-dependent magnetization which suggests that this behavior does not come from the ordinary anomalous Hall effect. We discuss the origin of this unique Hall signal by existence of a non-coplanar spin structure that may exist in this compound (I. Martin, C. D. Batista, Phys. Rev. Lett. 101, 156402 (2008)). [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F22.00015: Combined Transport, Magnetization and Neutron Studies of Structural and Magnetic Behavior in Ca3Ir4Sn13 Zhensong Ren, Mani Pokharel, Tom Hogan, Athena Sefat, Clarina de la Cruz, Huibo Cao, Bo Li, Cyril Opeil, Stephen Wilson Ca3Ir4Sn13, synthesized by Espinosa and his coworkers almost 30 years ago, was recently suggested to possess an unconventional superconducting ground state in the presence of a background of strong spin fluctuations. This signature for this claim stemmed from charge transport and magnetization anomalies near 45K, yet, later a detailed single crystal XRD investigation revealed that the anomaly is produced by a second order superlattice transition and that this transition can be tuned to zero temperature---suggesting a structural quantum critical point. Here in an attempt to characterize this phase further, we present a picture of the evolution of the structural and magnetic behavior in Ca3Ir4(Sn1-xSbx)13 via a combined transport, magnetization and neutron scattering study. [Preview Abstract] |
Session F23: Focus Session: Dopants and Defects in Semiconductors IV
Sponsoring Units: DMPChair: Beall Fowler, Lehigh University
Room: 325
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F23.00001: The electronic structure of Group V dopants in silicon -- The requirements for a realistic DFT model Veronika Brazdova, David R. Bowler, Andrew J. Fisher Typical concentrations of Group V donors in Si wafers used in experiment are up to 10$^{18}$ cm$^{-3}$. In contrast, the simulation cell in a typical atomistic simulation would contain a few hundred Si atoms and one or two dopants. That is equivalent to concentrations on the order of at least 10$^{20}$ cm$^{-3}$. We investigate the effect of donor concentration on the electronic structure of doped bulk silicon in density functional simulations (DFT) using the linear scaling DFT code Conquest on very large cells, and the cell sizes required to model the metal-semiconductor transition correctly. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F23.00002: Charged defect in GaSb by selective occupation in density functional theory Jianwei Wang, Yong Zhang In a density functional theory (DFT) approach, the transition energy of an acceptor-like defect is typically calculated by the total energy difference between E(N$+$1) and E(N), where N is the total number of the valence electrons of the defected system. Effectively, in this scheme, the hole in the valence band is simulated by a uniform positive background charge or a plane wave. A scheme closer to the reality would be to move one electron from the valence band maximum (VBM) to the defect level, because the VBM state usually is quite different from a plane wave. We apply this selective occupation scheme to a defect problem, an antisite defect of Ga on Sb in GaSb, and compare the results of two schemes with varying supercell size, using a pseudopotential DFT theory. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F23.00003: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F23.00004: Small polaron characteristics of the OH center in TiO$_{2}$ Invited Speaker: W. Beall Fowler Most insulating crystals have nearly-free-electron conduction bands and corresponding conduction properties, with the effective mass increased slightly by large-polaron effects. In TiO$_{2}$, the lowest conduction bands contain considerable admixture of Ti 3d states. In this case the conduction electrons become localized, or self-trapped, into small-polaron states [1], and their conduction properties differ considerably from the usual case. EPR experiments by Halliburton \textit{et al.} [2] have shown that this self-trapping is also present in association with point defects, namely substitutional F and interstitial H (which forms a bond with a lattice O). In each case the spin of the unpaired electron is localized on a nearest neighbor Ti. Infrared absorption experiments as a function of temperature on the OH center by Bekisli \textit{et al.} [3] have resolved apparent inconsistencies in the model used to fit earlier IR data. Through detailed analysis they have interpreted their results in terms of a small polaron model which involves several configurations corresponding to the localization of the OH electron on different Ti sites, each of which yields an IR line of slightly different frequency. These conclusions are supported by theoretical results in the literature and by our calculations using the CRYSTAL06 code [4] with a hybridized DFT Hamiltonian.\\[4pt] [1] A. Yidiz \textit{et al.}, J. Appl. Phys. \textbf{108}, 083701 (2010).\\[0pt] [2] S. Yang and L. E. Halliburton, Phys. Rev. B \textbf{81}, 035204 (2010); A. T. Brant \textit{et al.}, J. Appl. Phys. \textbf{110}, 053714 (2011).\\[0pt] [3] F. Bekisli \textit{et al.}, Phys. Rev. B \textbf{86}, 155208 (2012).\\[0pt] [4] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual} (University of Torino, Torino, 2006). [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F23.00005: H and D centers in In$_{2}$O$_{3}$ studied by IR spectroscopy Weikai Yin, Kirby Smithe, Michael Stavola, W. Beall Fowler, L.A. Boatner Hydrogen has been predicted to be an important source of n-type conductivity in transparent conducting oxides (TCO's) [1]. We have used IR spectroscopy to investigate the properties of H (and D) in single crystals of the prototypical TCO, In$_{2}$O$_{3,}$ and to test the predictions of recent theory [2]. H (or D) introduces several O-H (or O-D) stretching lines and also the broad absorption arising from free carriers. We have used the vibrational properties of H- (and D-) containing centers as a probe of microscopic structure and as a strategy to monitor H-related reactions that occur upon annealing. [1] M. McCluskey \textit{et al.}, J. Mater. Res. \textbf{27}, 2190 (2012) [2] S. Limpijumnong \textit{et al.}, Phys. Rev. B \textbf{80}, 193202 (2009). [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F23.00006: Controlled introduction of defects in GaMnAs and GaBeAs thin films by ion-beam irradiation Marcelo Sant'Anna, Elis Sinnecker, Tatiana Rappoport, Mauricio Pires, Germano Penello, David Souza, Sergio Mello, Joaquim Mendes, Jacek Furdyna, Xinyu Liu The existence of interstitial Mn atoms, and other point defects, significantly modify magnetic and transport properties of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As. This opens a door to manipulate these properties in a controlled way by ion-beam irradiation of thin films. We study how the simultaneous lowering of hole concentration and increasing of disorder, introduced by ion-beam irradiation, affects the magnetization and conductivity of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As samples [1,2]. Highly doped Ga$_{\mathrm{1-x}}$Be$_{\mathrm{x}}$As is a material that can be produced with similar doping levels but that shows no ferromagnetism, acting as an interesting experimental standard for comparison of transport properties of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As. We irradiate Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As and Ga$_{\mathrm{1-x}}$Be$_{\mathrm{x}}$As thin films with 2 MeV oxygen ion beams. Samples were grown by molecular beam epitaxy. Sheet resistance of the thin films was measured in situ in the irradiation chamber as a function of the incident dose.\\[4pt] [1] E. H. C. P. Sinnecker et al., Phys. Rev. B. 81, (2010) 245203.\\[0pt] [2] M. M. Sant'Anna, et al., Meth. in Phys. Res. B. 273 (2012) 72. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F23.00007: The role of d levels of substitutional magnetic impurities at the (110) GaAs surface M.R. Mahani, Anna Pertsova, Fhokrul Islam, C.M. Canali The study of the spin of individual transition-metal dopants in a semiconductor host is an emergent field known as magnetic solotronics, bearing exciting prospects for novel spintronics devices at the atomic scale. Advances in different STM based techniques allowed experimentalists to investigate substitutional dopants at a semiconductor surface with unprecedented accuracy and degree of details [1]. Theoretical studies based both on microscopic tight-binding (TB) models and DFT techniques have contributed in elucidating the experimental findings. In particular, for the case of Mn dopants on the (110) GaAs surface, TB models [2] have provided a quantitative description of the properties of the associated acceptor states. Most of these TB calculations ignore dealing explicitly with the Mn d-levels and treat the associated magnetic moment as a classical vector. However recent STM experiments [3] involving other TM impurities, such as Fe, reveal topographic features that might be related to electronic transitions within the d-level shell of the dopant. In this work we have included explicitly the d levels in the Hamiltonian. The parameters of the model have been extracted from DFT calculations. We have investigated the role that d levels play on the properties of the acceptor states of the doped GaAs(110) surface, and analyzed their implications for STM spectroscopy. [1] Yakunin et al., PRL 92, 216806 (2004), Kitchen et al., Nature 442, 436 (2006). [2] Tang et al., PRL 92, 047201 (2004), Strandberg et al., PRB 80, 024425 (2009). [3] J. Bocquel et al., arXiv:1203.6293v.1. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F23.00008: Optical measurements of trap state density and minority carrier lifetime in GaAs heterostructures grown at varying rates Chelsea Haughn, Kenneth Schmieder, Joshua Zide, Allen Barnett, Chris Ebert, Robert Opila, Matthew Doty Semiconductor growth rates are a critical factor for production costs and can have a significant impact on electrical properties. We use time resolved photoluminescence (TRPL) to characterize the effective lifetime of carriers in gallium arsenide - indium gallium phosphide (GaAs/InGaP) double heterostructures grown at varying rates. We measure the PL decay time as a function of laser fluence and extract an approximate trap state density by fitting this data with the Shockely-Read-Hall model of carrier recombination. Using the approximate trap densities, we then calculate minority carrier lifetimes for a range of doping conditions. The results suggest that the increased density of trap states associated with a two-fold increase in growth rate are less limiting to carrier lifetime than doping at the levels required for devices. The techniques and analysis developed here can be applied for rapid, non-destructive quantification of trap state densities in materials grown under varying conditions. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F23.00009: First-principles study on scattering potentials of defects on Ge(001) surfaces Tomoya Ono As new techniques for the nanoscale manipulation and modification of materials progress, the electron scattering properties of nanostructures are the focus of attention both experimentally and theoretically. The spatial maps of the local density of states obtained by scanning tunneling spectroscopy can give us the images of standing waves, which provide important information about the dispersion relation of the electron scattering process at the potential barrier. I examined the scattering potential of the Ge-Si and Ge-Sn dimers on Ge(001) surfaces using a first-principles calculation. By calculating the scattering wave functions, the standing waves in the spatial map of the local density of states are examined; the waves correspond to the image of the differential conductance obtained by scanning tunneling spectroscopy. The period of the standing wave and its phase shift agree with those obtained by the experiment. I found that the scattering potential acts as a barrier when the electronegativity of the upper atom of the dimer is larger than that of the lower atom, while it becomes a well in the opposite case. The scattering potential is related to the stabilization of the ? bands of the Ge(001) surface due to the difference in electronegativity between Ge and the impurity. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F23.00010: Why Cu diffuses fast in semiconductors? Jie Ma, Su-Huai Wei It is well-known that experimentally Cu diffuses fast in semiconductors and the fast diffusion plays an important role in many applications. However, the theoretical reason for the fast diffusion is still unclear. Using first-principles calculations, we compare the diffusion behavior between Cu and group-IA atoms in CdTe, and find that the fast diffusion of Cu can be explained by the existence of the symmetry-induced strong s-d coupling in the system, which lowers the energy significantly at the site usually consists the barrier for group-IA system. Due to this s-d coupling, the most stable doping site, diffusion pathway, and diffusion energy curve of Cu are different from those of group-IA atoms, and the diffusion barrier for Cu$+$ is usually larger than that for neutral Cu. The mechanism is expected to be general for all tetrahedral semiconductors. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F23.00011: Density functional study of the properties of Tl6SeI4 for radiation detection applications Koushik Biswas, Mao-Hua Du, David Singh The extra compositional freedom available in ternary compounds allow flexibility to tune their electronic and structural properties compared to the binary counterparts. Indeed, the Tl-based ternary semiconductor Tl6SeI4 is a promising candidate for radiation detectors. It has a band gap (1.86 eV) that is intermediate between those of Tl2Se (0.6 eV) and TlI (2.75 eV) and suitable for room temperature detectors. However, the flexibility in ternary semiconductors may come at the expense of more channels for defect formation and more complex defect chemistry, which need to be studied in details. To better understand the properties of Tl6SeI4$_{\mathrm{\thinspace }}$in relation to the radiation detection, we performed first-principles study of electronic structure, phase diagram, and dielectric, optical, and defect properties in Tl$_{\mathrm{6}}$SeI$_{\mathrm{4}}$.[1] We will discuss the properties of defects and their diffusion barriers in the context of resistivity and polarization phenomenon in Tl6SeI4. [1] K. Biswas, M.-H. Du, and D. J. Singh, \quad Phys. Rev. B \quad \textbf{86}, 144108 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F23.00012: Electronic Structure Engineering of Elpasolites for Brighter and Faster Scintillators Mao-Hua Du, Koushik Biswas Utilization of scintillator materials is one of the primary methods for radiation detection. Elpasolites are a large family of quaternary halides that have attracted considerable interest for their potential applications as $\gamma $-ray and neutron scintillators. However, many elpasolite scintillator materials currently under development suffer from low light yield and long scintillation decay time. The low light yield is partially due to a large band gap while the long scintillation decay time is a result of slow carrier transport to Ce dopants, where electrons and holes recombine to emit photons. We suggest that these problems may be mitigated by optimizing the band gap and carrier mobility by selecting constituent elements of proper electronegativity. For example, cations with lower electronegativity may lower the conduction band and increase the conduction band dispersion simultaneously, resulting in higher light yield and faster scintillation. First-principles calculations of electronic structure, small polarons, and Ce dopants in Cs$_{\mathrm{2}}$LiYCl$_{\mathrm{6}}$ and Cs$_{\mathrm{2}}$AgYCl$_{\mathrm{6}}$ compounds show that the strategy of manipulating electronegativity can lead to brighter and faster elpasolite-based scintillators. This work was supported by the U.S. DOE Office of Nonproliferation Research and Development NA22. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F23.00013: Hydrogen configurations at a high-angle grain boundary in yttria-stabilized zirconia Apostolos Marinopoulos Hydrogen is a common impurity in many technologically-relevant semiconductors and oxides. Being mobile and reactive it can form defect complexes with native defects or other extrinsic point defects. Ab initio calculations based on density-functional theory (DFT) have so far been instrumental in elucidating the tendency of hydrogen to form stable complexes with oxygen vacancies and acceptor dopants. The interaction of hydrogen with internal extended defects, such as grain boundaries, needs also to be addressed given the fact that metal oxides are commonly used in polycrystalline or nanocrystalline forms. The present DFT study aims to determine the type of hydrogen configurations that can exist at the core of a high-angle tilt grain boundary in yttria-stabilized zirconia (YSZ). The core is characterized by strong distortions for both anion and cation sublattices and lower ionic density and coordination numbers that lead to larger interstitial spaces at the interface with respect to the bulk. Formation energies and charge transition levels are determined and compared to those in the bulk YSZ where hydrogen was found to incorporate either at hydroxide-bond configurations or at interstitial sites with strong atomic character. [Preview Abstract] |
Session F24: Focus Session: Computational Studies of Interactions between Electromagnetic Fields and Materials I
Sponsoring Units: DCOMPChair: Carsten Ullrich, University of Missouri
Room: 326
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F24.00001: Coherent phonon generation in time-dependent density functional theory Invited Speaker: George Bertsch We apply the time-dependent density functional theory (TDDFT) to the generation of coherent optical phonons in Si and Sb crystals. The computations are carried out by real-time evolution of the orbital wave functions on a coordinate-space mesh. The theory reproduces the main phenomena observed experimentally: dependence on polarization, strong growth at the direct band gap, and the change in phase from below to above the band gap. Comparing with more phenomenological models, we find that the TDDFT supports the impulsively stimulated Raman mechanism at low frequencies and the qualitative aspects of the displacive mechanism at higher frequencies. We also compare with the more detailed model of displacive excitation by Stevens, Kuhl, and Merlin. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F24.00002: Spectral Representation analysis of dielectric screening in solids and molecules Amandeep Kaur, Erik Ylvisaker, Deyu Lu, Tuan Anh Pham, Giulia Galli, Warren Pickett We propose a new approach to identify and rationalize the contribution of core electron polarization to dielectric screening, based on ab initio calculations of the dielectric matrix in its eigenpotential basis. We also present calculations of phonon frequencies, dielectric constants for alkali hydrides and quasiparticle energies of several sp bonded molecules, and we discuss the quantitative effect of including core polarization. We find that inclusion of semi-core (SC) electrons leads to new eigenmodes in the dielectric matrix with respect to those with valence electron only. These eigenmodes are highly localized in real space. Polarization arising from SC orbitals may contribute 4-6\% to the computed dielectric constants in alkali hydrides, and to differences in QP energies of $\sim$100 meV for sp bonded molecules. Our findings illustrate efficient ways of approximating the spectral decomposition of dielectric matrices used, e.g. in many body perturbation theory and dielectric constant calculations, with substantial computational gains for large systems composed of heavy atoms. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F24.00003: Real Time Dynamical Core-hole Effects in X-ray Spectra A.J. Lee, F.D. Vila, J.J. Kas, J.J. Rehr We present an extension of the real-time x-ray spectroscopy code RTXS\footnote{A. J. Lee \textit{et al.}, Phys. Rev. B \textbf{86}, 115107 (2012)} to introduce dynamic effects due to the sudden creation of a core hole in x-ray absorption (XAS) and emission (XES) spectra. RTXS is based on a local, time-correlation function approach using a real-time extension of the SIESTA code with a Crank-Nicolson time-evolution operator, and projector augmented wave (PAW) transition matrix elements. Originally RTXS used a statically screened core hole, an approximation equivalent to the final state rule as in $\Delta$SCF approaches. To introduce dynamic effects, we now start with the system in the ground state, suddenly introduce the core-hole, and then propagate the system in real time, again with the Crank-Nicolson approach. This implementation yields a generally applicable code that builds in full-potential electronic structure and dynamic core-hole screening. Illustrative examples are presented and compared with initial and final state rule approximations. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F24.00004: New method for calculating the optical absorption spectrum for solids using the transcorrelated method Masayuki Ochi, Shinji Tsuneyuki \textit{Ab initio} calculation of an accurate optical absorption spectrum for solids is a challenging problem, and various methods are proposed for this purpose, such as TDDFT using new functionals and GW+BSE. In this study, we propose a new method for calculating the optical spectra, using the transcorrelated (TC) method[1-6], which is one of the promising wave-function theories. In the TC method, the total wave function is approximated as the Jastrow-Slater-type wave function, and the many-body Hamiltonian is similarity-transformed by the Jastrow factor. Then we solve an SCF equation and optimize one-body orbitals in the Slater determinant with a relatively low computational cost.[6] For excited-state calculations, we use configuration interaction singles (CIS) for the TC method, and will present an optical absorption spectrum of LiF calculated with this method. [1] S. F. Boys and N. C. Handy, Proc. R. Soc. London Ser. A 309, 209 (1969). [2] S. Ten-no, Chem. Phys. Lett. 330, 169 (2000). [3] N. Umezawa and S. Tsuneyuki, J. Chem. Phys. 119, 10015 (2003). [4] R. Sakuma and S. Tsuneyuki, J. Phys. Soc. Jpn. 75, 103705 (2006). [5] H. Luo, J. Chem. Phys. 133, 154109 (2010). [6] M. Ochi, K. Sodeyama, R. Sakuma, and S. Tsuneyuki, J. Chem. Phys. 136, 094108 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F24.00005: Computationally efficient dielectric calculations of molecular crystals Kathleen Schwarz, T.A. Arias The dielectric response is a key quantity for electronic materials such as organic semiconductors. Calculations of the dielectric response for molecular crystals are currently either expensive, or rely on extreme simplifications such as multipole expansions. We present an alternate approach using an analogue of the Clausius-Mossotti equation, which constructs the crystal's dielectric response from an eigenvalue decomposition of the molecular dielectric response. This method can be used to examine the effects of defects and surfaces on the dielectric properties of molecular crystals. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F24.00006: Exciton and trions binding energies in single-layer MoS$_2$: applications of the density-matrix time dependent density Alfredo Ram\'Irez-Torres, Volodymyr Turkowski, Talat S. Rahman Exciton and trion binding energies of a single layer of MoS$_2$ are studied using a time- dependent density-functional theory formalism. Kohn-Sham orbitals of the initial state were obtained using ab initio electronic structure calculations based on density functional theory. Several types of exchange-correlation (XC) kernels are implemented in our code to compare their performance. As expected our results depend crucially on the XC kernels used. In particular, the exchange-only adiabatic local density approximation kernel results in the binding energy about 0.1 eV, which is smaller than those obtained using the GW theory approximation ($\sim$ 0.9 eV) [1]. We have generalized the approach on the case of trion excitations, which gives the trion binding energy $\sim$ 0.3eV when one used the LDA approximation. On the other hand, we demonstrate that the results for the experimental binding energies can be reproduced by using phenomenological local and long-range XC kernels. [1] T. Cheiwchanchamnangij and W. R. L. Lambrecht, Phys. Rev. B \textbf{85}, 205302 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F24.00007: Landau-Zener Tunneling in 1-d periodic potential Jiajun Li, Jong Han Landau-Zener tunneling can be used to model the transition between energy bands of a particle in 1-d periodic potential [1-2]. It is pointed out that a specific model could be utilized to explain the transition driven by a uniform external force, between energy bands in a periodic lattice [3]. Here we examine the transition driven by an external force, in a sinusoidal periodic potential, by solving Schr\"odinger equation numerically. As an exact solution, all bands and transitions between them are included. By considering arbitrary crystal potential of any supercell size, we can approximate random potential scattering and examine how random elastic scattering modifies the inter-band transition and eventually the electron transport. Non-exponential decays and other patterns for different ranges of parameters will be presented. We will also make a connection between the numerical results and conventional Landau-Zener transition model, and show how a time-dependent periodic potential will change the nature of transition. Supported by NSF.\\[4pt] [1] Zener, C., 1932, Proc. Roy. Soc. London Ser. A 137, 696.\\[0pt] [2] C. Zener, A theory of the electrical breakdown of solid dielectrics, Proc. Royal Soc. A 145 (1934) 523.\\[0pt] [3] Q. Niu and M. G. Raizen, Phys. Rev. Lett. 80, 3491(1998) [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F24.00008: Efficient Numerical Modeling of Nonequilibrium Fluctuation Phenomena M. T. Homer Reid, Alejandro Rodriguez, Steven Johnson We present efficient numerical methods for computing non-equilibrium Casimir forces and radiative heat transfer between bodies of complex shapes and realistic material properties. Our methods borrow techniques from computational electromagnetism (specifically, surface integral equations and boundary-element methods) to describe fluctuations in \textit{fields} in terms of fluctuating \textit{sources} on the surfaces of material bodies. We obtain concise formulas expressing forces and heat-transfer rates in terms of traces of matrix products, where the elements of the matrices describe the interactions of tangential currents flowing on the surfaces of the interacting material bodies. Using our methods, we obtain new predictions of nonequilibrium phenomena in geometries that would be difficult or impossible to treat using other methods for modeling nonequilibrium fluctuations. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F24.00009: Solution of electric-field-driven tight-binding lattice in contact with fermion reservoirs Jong Han Electrons in tight-binding lattice driven by DC uniform force field dissipate their energy through on-site fermionic thermostats. Due to the translational invariance in the transport direction, the problem can be block-diagonalized. We solve this time-dependent quadratic problem and demonstrate that the problem has an oscillatory steady-state. The steady-state occupation number shows that the Fermi surface disappears for any damping from the thermostats and any finite electric field. Despite the lack of momentum scattering, the conductivity takes the same form as the semi-classical Ohmic expression from the relaxation-time approximation. Despite the similarity of the Ohm's law with the Boltzmann transport, this solution does not support gradual shift of Fermi surface by drift velocity and, therefore, when used for many-body steady-state calculations, may lead to pathological effects. We discuss extensions of this model for more realistic dissipation mechanisms. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F24.00010: Investigation of quantum-confined Stark effect on exciton binding energy and electron-hole recombination in GaAs qdots Christopher Blanton, Arindam Chakraborty We present the field-dependent explicitly correlated full configuration interaction (XCFCI) method for calculating highly accurate electron-hole wavefunction in presence of external electric field. The XCFCI is a variational method which is based on performing FCI calculation using explicitly correlated reference wavefunction. Field-dependent basis functions were used and were constructed using a variational field-dependent coordinate transformation. A discussion between this method and the variational polaron transformation for spin-boson system will be presented. The exciton energy, exciton binding energy (EB), and electron-hole recombination probability (eh-RP) were computed using XCFCI and the analysis of the scaling laws will be presented. One of the key results is that EB and eh-RP follow very different scaling with respect to the field strength. It was found that for a 500 kV/cm change in the field reduces the EB and eh-RP by a factor of 2.6 and 166, respectively. The explicitly correlated term was found to be crucial for accurate computation of eh-RP and was also responsible for improving convergence of the XCFCI energy with respect to basis size. The field dependent basis functions were found to very important and comparison with field independent basis will be presented [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F24.00011: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F24.00012: A Computational Framework for Cavity Mediated Energy Transfer in Nanostructures Andrew Baczewski, Nicholas Miller, Daniel Dault, Carlo Piermarocchi, Balasubramaniam Shanker Cavity mediated energy transfer is vital to numerous technologies, such as systems that harvest/generate light, quantum information, and platforms for studying strongly coupled cavity QED. In these processes, the density of photonic states through which a donor and acceptor complex exchange energy is dramatically modified by a resonant structure such as a photonic crystal or a distributed Bragg reflector. The design and optimization of new systems of this nature is greatly facilitated by the development of high fidelity numerical methods for resolving the fields in structures not amenable to analytical methods. This is increasingly relevant at the nanoscale, wherein optically dense geometric features exist at or below the scale of the free space wavelength. To this end, we have implemented a nodal Discontinuous Galerkin discretization of the curl-curl Maxwell eigenproblem for the resolution of the spectrum of photonic modes in nanostructures. This framework delivers a high accuracy representation of light-matter coupling constants and optical eigenfrequencies that can be fed into quantum mechanical models of energy transfer. Details of our framework, implementation/validation, and applications germane to energy transfer between cavity-confined quantum dots will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F24.00013: Real-Time TDDFT simulation for coherent phonon generation in crystalline solids Yasushi Shinohara, Shunsuke A. Sato, Kazuhiro Yabana, Tomohito Otobe, Jun-Ichi Iwata, George F. Bertsch We have been developing a theoretical framework to describe electron dynamics in a crystalline solid under an ultrashort laser pulse. We rely upon the time-dependent density functional theory, solving the time-dependent Kohn-Sham equation in real-time and real-space. Using our method, it is possible to describe both linear and nonlinear light-matter interactions in a unified way. In my presentation, I will focus on the application to coherent phonon generation, a coherent atomic oscillation over a macroscopic volume. I will show applications to two material, semiconductor Si and semimetal Sb. For Si, we have found that the TDDFT is capable of describe two distinct mechanisms of the coherent phonon generation. When the laser frequency is below the direct bandgap, virtual electronic excitation induces impulsive force to atoms. When the laser frequency is above the gap, real electronic excitation causes the atomic motion. For Sb, we study the frequency dependence of the coherent phonon generation and compare our results with phenomenological theories. [Preview Abstract] |
Session F25: Superconducting Qubits: Read-out, Feedback and Stabilization
Sponsoring Units: GQIChair: Will Oliver, Massachusetts Institute of Technology
Room: 327
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F25.00001: Non-linear processes in thin titanium nitride transmission lines for parametric amplification Michael Vissers, Jiansong Gao, Suptarshi Chaudhuri, Clint Bockstiegel, Martin Sandberg, David P. Pappas Nitride superconductors, such as titanium nitride and niobium titanium nitride, are a non-linear, low dissipation medium at microwave frequencies. The lossless nonlinearity may be probed and utilized. Important applications include generation of higher harmonics , e.g. 3f, and a microwave version of the optical paramagnetic amplifier, i.e. the degenerate-pump case of four-photon mixing (FPM). An amplifier based on these principles should allow for very wide bandwidth, low noise (quantum limited) and high dynamic range devices. These measurements are performed via a single layer, 3 meter long TiN spiral and measured at temperatures below 100 mK. Initial results of the design, fabrication, testing, and impedance optimization of a titanium nitride based parametric amplifier are presented. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F25.00002: Efficiency of a microwave photon detector based on a current-biased Josephson junction Amrit Poudel, Canran Xu, Maxim Vavilov In this talk we discuss the efficiency of a microwave photon detector based on a current-biased Josephson junction driven by a classical microwave source. We consider the evolution of the junction in the presence of the environment and tunneling events to the voltage state. We calculate the switching time distribution to the voltage state and evaluate the efficiency of the photon detector as a function of input power and the junction parameters. We present conditions for the optimal power matching between the detector and the microwave source. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F25.00003: Optimization of single shot readout of a transmon qubit using a SLUG microwave amplifier Yanbing Liu, Srikanth Srinivasan, David Hover, Robert McDermott, Andrew Houck We report on measurement of a superconducting transmon qubit using a number of optimization techniques and a low noise amplifier. ~Optimization is performed over power and frequency, and a genetic algorithm is employed to optimize the readout fidelity as a function of the measurement pulse shape. ~In addition, a superconducting low-inductance undulatory galvanometer (SLUG), a SQUID-based microwave amplifier, is used to reduce system noise. ~The SLUG amplifier has very high dynamic range and low noise over a relatively wide frequency range. ~Both the SLUG amplifier and genetic algorithm lead to improved readout fidelity over analytic pulse shaping and HEMT amplification. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F25.00004: Large gain quantum-limited qubit state measurement using a two mode nonlinear cavity Saeed Khan, Aashish Clerk A single nonlinear cavity dispersively coupled to a qubit functions as a large gain detector near a bifurcation, but also has an unavoidable large backaction that prevents QND measurement at weak couplings~[1]. We show theoretically that a modified setup involving two cavities (one linear, one nonlinear) and a dispersively coupled qubit allows for a far more optimal measurement. In particular, operating near a point of bifurcation, one is able to both achieve a large gain as well as a near quantum-limited backaction. We present analytic results for the gain and noise of this detector and a heuristic understanding of the physics, thus presenting a complete description of this new way of performing weak qubit state measurements. The setup we describe can easily be realised in experiments with superconducting circuits involving Josephson junctions~[2,3].\\[4pt] [1] C. LaFlamme, A.A. Clerk, Phys. Rev. A \textbf{83}, 033803 (2011)\\[0pt] [2] F.R. Ong \emph{et al.}, Phys. Rev. Lett. \textbf{106}, 167002 (2011)\\[0pt] [3] M. Hatridge \emph{et al.}, Phys. Rev. B \textbf{83}, 134501 (2011) [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F25.00005: A $>$10 GHz JPC with Trans-Gain for Qubit Readout K. Sliwa, A. Narla, M. Hatridge, F. Schackert, B. Abdo, S. Shankar, L. Frunzio, M.H. Devoret For multi-qubit circuit QED experiments, it is desirable to work with cavities at frequencies $>$10 GHz to allow for design flexibility. However, performance of following electronics can be best optimized at low frequencies (3-5 GHz). These seemingly contradictory requirements can be naturally reconciled using the Josephson Parametric Converter (JPC). The JPC is a quantum limited amplifier comprised of two non-degenerate resonators coupled via a ring of Josephson junctions. It can bridge frequency ranges separated by more than an octave via its trans-gain, a process in which a signal incident on one port is frequency converted and transmitted with gain on the other port. Here we present data on the trans-gain of a JPC with one resonator at 11.5 GHz and the other at 4.5 GHz which could be used in such a readout scheme without any significant compromise on gain, dynamic range, or bandwidth. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F25.00006: Characterization of a Multi-Layer Parametric Amplifier with On-Chip Bias Line T. White, R. Barends, J. Bochmann, B. Campbell, Y. Chen, B. Chiaro, E. Jeffrey, J. Kelly, M. Mariantoni, A. Megrant, J. Mutus, C. Neill, P. O'Malley, S. Ohya, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, A.N. Cleland, J.M. Martinis Single shot dispersive readout of superconducting qubits requires a near quantum limited microwave amplifier. Based on the parametric amplifier design from UC Berkeley, we have developed a parametric amplifier using the UCSB multilayer fabrication with a single ended input and an on-chip flux bias line. These changes enable us to use a smaller and simpler chip mount with separate signal and flux ports. The high bandwidth of the flux port allows us to flux pump the amplifier and should allow dynamic frequency tuning on ns timescales. Flux pumping also requires fewer components in the measurement line, reducing signal loss. With this design we have achieved parametric amplification using two kinds of input pumping and three kinds of flux pumping; for each mode we have characterized gain bandwidth product, saturation power, and noise temperature. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F25.00007: Increasing dynamic range in microwave parametric amplifiers J. Mutus, R. Barends, J. Bochmann, B. Campbell, Y. Chen, B. Chiaro, E. Jeffrey, J. Kelly, M. Mariantoni, A. Megrant, C. Neill, P. O'Malley, S. Ohya, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. White, A.N. Cleland, J.M. Martinis Parametric amplifiers have long been of interest in quantum information due to their high gain and near quantum limited performance. In collaboration with UC Berkeley, we are improving upon their proven parametric amplifier design, which consists of a lumped element LC resonator, with a SQUID providing a tunable nonlinear inductance. In order to improve the dynamic range of these amplifiers, multiple SQUIDs are used in series in order to distribute the non-linearity across many junctions. We report on the design of a single-ended amplifier using our 7-layer fabrication process, combining photo and electron beam lithography. We explore the experimental optimization of such a design, specifically the impact of adding additional SQUIDs on overall device performance. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F25.00008: Improved JPC performance via a low inductance, lumped element design A. Narla, K. Sliwa, M. Hatridge, S. Shankar, F. Schackert, B. Abdo, L. Frunzio, R.J. Schoelkopf, M.H. Devoret The Josephson Parametric Converter (JPC), a linear, non-degenerate, nearly quantum-limited amplifier, is a promising tool for quantum information applications. We propose a new JPC design characterized by the use of multi-pF parallel-plate capacitors. By decreasing the geometric inductance of the system, higher critical-current Josephson junctions can be used. Both the bandwidth and dynamic range can thus be increased by a factor of two relative to existing microstrip devices. When integrated with a shunted ring of Josephson junctions [1], these devices should also be tunable over more than a GHz. We present simulations of the circuit behavior and preliminary measurements of a proof-of-concept device. \\[4pt] [1] N. Roch et al., Phys. Rev. Lett. 108, 147701, 2012 [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F25.00009: Optimizing bandwidth and dynamic range of lumped Josephson parametric amplifiers A. Eddins, R. Vijay, C. Macklin, Z. Minev, I. Siddiqi Superconducting parametric amplifiers have revolutionized the field of quantum measurement by providing high gain, ultra-low noise amplification. They have been used successfully for high-fidelity qubit state measurements, probing nano-mechanical resonators, quantum feedback, and for microwave quantum optics experiments. Though several designs exist, a simple and robust architecture is the Lumped Josephson Parametric Amplifier (LJPA). This device consists of a capacitively shunted SQUID directly coupled to a transmission line to form a low quality factor (Q) nonlinear resonator. We discuss amplifiers which can be tuned over the full 4-8 GHz band with 20-25 dB of gain and 10 - 50 MHz of signal bandwidth. However, similar to other parametric amplifiers employing a resonant circuit, the LJPA suffers from low dynamic range and has a -1 dB gain compression point of order -130 dBm. We explore new designs comprised of an array of SQUIDs to improve the dynamic range. We will present the results of numerical simulations and preliminary experiments. We will also briefly discuss improvements obtained from different biasing methods and packaging. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F25.00010: Real-time digital processing of qubit readout and feedback control Y. Liu, N. Ofek, K. Geerlings, M. Hatridge, R.J. Schoelkopf, M.H. Devoret Rapid progress in high fidelity readout of superconducting qubits paves the way for measurement-based feedback control of quantum systems and error correction protocols. A traditional data acquisition and processing setup, consisting of separate digitizer card for qubit readout, PC for processing and commercial arbitrary waveform generator (AWG) for qubit control, however, can have latency of at least several milliseconds and cannot meet the timing requirement of quantum feedback experiments. We have implemented an all-in-one system that contains a digitizer, a demodulator, a qubit-state estimator and an AWG on a commercial field-programmable-gate-array (FPGA) board. The FPGA system shows superior performance in terms of throughput, timing stability and on-the-fly programmability compared to traditional technology. Latency of the FPGA system can be on the order of only hundreds of nanoseconds. Results from our project of integrating the real-time processing power of the FPGA with a qubit + amplifier system will be shown. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F25.00011: Stabilizer quantum error correction toolbox for superconducting qubits Simon Nigg, Steven Girvin Rudimentary quantum error correction (QEC) has been achieved in a superconducting qubit circuit [1]. Realization of topological protection and QEC based on stabilizer codes will require protocols for QND measurement of multi-qubit Pauli operators on arbitrary selected subsets of qubits. Initial progress towards this goal has been achieved with four-qubit stabilizer pumping in a trapped ion system [2]. We present a general protocol for stabilizer measurement and pumping in a system of $N$ superconducting qubits. We assume always-on, fixed dispersive couplings $\chi$ to a single mode of a high-$Q$ microwave resonator in the strong-dispersive limit defined by $\chi\gg 1/T_2,\kappa$, where $T_2$ is the qubit coherence time and $\kappa$ is the cavity line width. In this limit, we show how to measure an arbitrary weight $M\leq N$ Pauli operator, by entangling the multi-qubit state with two distinguishable coherent states of the cavity. Together with a fast cavity readout ($T_{\rm meas}\ll 1/\kappa$), which can be achieved by tunable coupling to a low-$Q$ cavity mode, this enables the efficient measurement of mulit-qubit Pauli operators.\newline [1] M. D. Reed et al. Nature 2012, {\bf 482}, 382-385\newline [2] J. T. Barreiro et al. Nature 2011, {\bf 470}, 486 [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F25.00012: Autonomous stabilization of an entangled state of two transmon qubits S. Shankar, Z. Leghtas, M. Hatridge, A. Narla, U. Vool, S.M. Girvin, M. Mirrahimi, M.H. Devoret Recent circuit QED (cQED) experiments on superconducting transmon qubits have shown good progress towards measurement-based quantum feedback, that should allow the stabilization of interesting quantum states, such as an entangled state of two qubits. These experiments crucially depend on fast, high-fidelity, quantum non-demolition qubit readout using superconducting parametric amplifiers as well as high-speed room-temperature electronics. We describe an alternate autonomous-feedback strategy to stabilize two qubits dispersively coupled to a single cavity into an entangled state, while obviating the need for an optimized measurement chain. The system Hamiltonian is designed to be in the strong dispersive cQED regime where the dispersive shifts of the two qubits are tuned to be equal ($\chi/2\pi = 5$~MHz) and larger than the cavity linewidth ($\kappa/2\pi = 1.5$~MHz). By applying continuous microwave drives at the cavity and qubit frequencies, the system is forced into the desired quantum state. The stabilization rate of this scheme is of order $\kappa$ which can be made much faster than all decoherence rates 1/T$_1$, 1/T$_\phi$ that take the system out of the entangled state. We will discuss initial experimental progress towards the goal of autonomous high-fidelity entanglement. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F25.00013: Efficient Experimental Characterization of a Feedback Scheme for Qubit Initialization Yves Salathe, Christopher Eichler, Thomas Karg, Philipp Kurpiers, Christian Lang, Andreas Wallraff Quantum feedback based on high-efficiency projective measurements has a variety of potential applications such as active qubit initialization and quantum teleportation. Here, we experimentally investigate active initialization of a single transmon qubit in circuit quantum electrodynamics using parametric amplification similar to the experiment by Rist\`e \emph{et. al.} [1]. We implement the feedback scheme using field-programmable gate array (FPGA) electronics which conditions a $\pi$-pulse on the outcome of a prior quantum nondemolition measurement. Our processing unit also records multi-dimensional histograms which reveal the correlations between the initial and final state of the feedback process. We use these histograms to characterize the efficiency of our feedback implementation without the necessity of storing all individual single-shot measurement traces. The presented histogram-based measurement technique has potential applications in other experiments which involve feedback such as quantum teleportation. \newline [1] D.~Rist\`e, C.~C. Bultink, K.~W. Lehnert, L.~DiCarlo, arXiv:1207.2944v1. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F25.00014: Superconducting qubit parameter optimization for remote entanglement N. Roch, M.E. Schwartz, C. Macklin, R. Vijay, I. Siddiqi The combination of coherent lifetimes in excess of 100 microseconds and robust operation of low noise parametric amplifiers has enabled experiments in which high fidelity continuous measurement can be performed, opening the door for measurement based quantum feedback. The first experiment realized in this regime aimed at stabilizing a dynamical state of a superconducting qubit using a closed feedback loop [1]. We explore the prospects of extending this unprecedented control to engineered networks comprised of several superconducting qubits and microwave cavities, with the particular goal of stabilizing a central feature of quantum mechanics: the entanglement. We will discuss the optimal choice of hardware---qubit, cavity, and circuitry---as well as measurement protocols for maximizing entanglement. \\[4pt] [1] R. Vijay et al., Nature 490, 77-80 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F25.00015: Progress towards measurement-induced entanglement of remote superconducting qubits M. E. Schwartz, N. Roch, C. Macklin, R. Vijay, I. Siddiqi Generation and distribution of entanglement are critical capabilities for quantum computation and simulation. In superconducting qubits, entanglement can be achieved via direct qubit-qubit coupling on chip. In contrast to this type of local interaction, we present experiments and simulations targeted at generating entanglement between remote (non-coupled) 3D transmons. Entanglement is achieved via joint measurement in a basis that does not project, and thus does not dephase, the odd-parity Bell manifold (\textbar 01\textgreater /\textbar 10\textgreater ). The experiments rely on coherent state detection, rather than photon-counting, and are a step towards deterministic feedback stabilization of remote qubit entanglement. We also model the effects of experimental realities, including excess amplifier noise, cable insertion loss, and finite qubit coherence times. [Preview Abstract] |
Session F26: Quantum Crytography, Quantum Communication, and Quantum Measurement
Sponsoring Units: GQIChair: Graeme Smith, IBM
Room: 328
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F26.00001: Quantum to classical randomness extractors Stephanie Wehner, Mario Berta, Omar Fawzi The goal of randomness extraction is to distill (almost) perfect randomness from a weak source of randomness. When the source yields a classical string X, many extractor constructions are known. Yet, when considering a physical randomness source, X is itself ultimately the result of a measurement on an underlying quantum system. When characterizing the power of a source to supply randomness it is hence a natural question to ask, how much classical randomness we can extract from a quantum system. To tackle this question we here introduce the notion of quantum-to-classical randomness extractors (QC-extractors). We identify an entropic quantity that determines exactly how much randomness can be obtained. Furthermore, we provide constructions of QC-extractors based on measurements in a full set of mutually unbiased bases (MUBs), and certain single qubit measurements. As the first application, we show that any QC-extractor gives rise to entropic uncertainty relations with respect to quantum side information. Such relations were previously only known for two measurements. As the second application, we resolve the central open question in the noisy-storage model [Wehner et al., PRL 100, 220502 (2008)] by linking security to the quantum capacity of the adversary's storage device. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F26.00002: Quantum secret sharing with minimized quantum communication Ben Fortescue, Gilad Gour Standard techniques for sharing a quantum secret among multiple players (such that certain subsets of the players can recover the secret while others are denied all knowledge of the secret) require a large amount of quantum communication to distribute the secret, which is likely to be the most costly resource in any practical scheme. Two known methods for reducing this cost are the use of imperfect ``ramp'' secret sharing (in which security is sacrificed for efficiency) and classical encryption (in which certain elements of the players' shares consist of classical information only). We demonstrate how one may combine these methods to reduce the required quantum communication below what has been previously achieved, in some cases to a provable minimum, without any loss of security. The techniques involved are closely-related to the properties of stabilizer codes, and thus have strong potential for being adapted to a wide range of quantum secret sharing schemes. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F26.00003: Quantum teleportation over 143 kilometres using active feed-forward Xiaosong Ma, Thomas Herbst, Thomas Scheidl, Daqing Wang, Sebastian Kropatschek, William Naylor, Alexandra Mech, Bernhard Wittmann, Johannes Kofler, Elena Anisimova, Vadim Makarov, Thomas Jennewein, Rupert Ursin, Anton Zeilinger Quantum teleportation is a quintessential prerequisite of many quantum information processing protocols. By using quantum teleportation, one can circumvent the no-cloning theorem and faithfully transfer unknown quantum states to a party whose location is even unknown over arbitrary distances. Ever since the first experimental demonstrations of quantum teleportation of independent qubits and of squeezed states, researchers have progressively extended the communication distance in teleportation. Here we report the first long-distance quantum teleportation experiment with active feed-forward in real time. The experiment employed two optical links, quantum and classical, over 143 km free space between the two Canary Islands of La Palma and Tenerife. To achieve this, the experiment had to employ a combination of advanced techniques such as a frequency-uncorrelated polarization-entangled photon pair source, ultra-low-noise single-photon detectors, and entanglement-assisted clock synchronization. The average teleported state fidelity was well beyond the classical limit of 2/3. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F26.00004: Ultrafast quantum communications over long distances using quantum encoding Sreraman Muralidharan, Liang Jiang Quantum repeaters provide a way of enabling long distance quantum communication by establishing entangled qubits between remote locations. The first generation quantum repeater protocols involve time consuming entanglement purification steps that demand a long lived quantum memory and two-way classical communication that makes them slow. This problem can be circumvented by the new generation quantum repeater protocols that use quantum encoding, one-way classical communication and classical error correction techniques. Furthermore, each quantum repeater station only needs short lived quantum memory bits, the number of which has favorable poly-logarithmic scaling with the distance. We investigate the tolerance of these schemes against photon losses and depolarizing errors, and discuss the possibility of realizing these schemes in physical systems with the current state of art. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F26.00005: Generating and verifying entanglement of itinerant microwave modes H.S. Ku, W.F. Kindel, S.C. Glancy, E. Knill, L.R. Vale, G.C. Hilton, K.D. Irwin, K.W. Lehnert Entanglement is a critical resource for quantum information science. In particular, shared entanglement between pairs of electromagnetic fields propagating on two physically separate channels is required for quantum communication protocols with continuous variables. Moreover, the ability to entangle propagating microwave fields provides possible schemes to create quantum communication channels between localized superconducting qubits. In this talk, we will present an experimental demonstration of this type of entanglement. We generate the entangled state by combining a squeezed state and vacuum on a balanced beam splitter. The entanglement is then revealed by strong correlations between the quadrature amplitudes of the two output modes of the beam splitter. Crucial for an eventual teleportation demonstration, the two modes are measured efficiently and with independent choice of measured quadratures. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F26.00006: Microwave Photon Counter Based on Josephson Junctions Guilhem Ribeill, Umeshkumar Patel, Joseph Suttle, Robert McDermott We describe progress in the development of a microwave photon counter based on the current biased Josephson junction; absorption of a single photon causes the junction to switch to the voltage state, producing a large and easily measured classical signal. We discuss a self-resetting bias scheme based on a superconducting kinetic inductor that causes the junction to reset automatically to the active state following photon absorption. We investigate detector quantum efficiency and dark rate, and discuss applications to mesoscopic noise and circuit quantum electrodynamics. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F26.00007: Utilization of an Electron Multiplying CCD camera for applications in quantum information processing Monika Patel, Jian Chen, Jonathan Habif Electron Multiplying Charge-Coupled Device (EMCCD) cameras utilize an on-chip amplification process which boosts low-light signals above the readout noise floor. Although traditionally used for biological imaging, they have recently attracted interest for single-photon counting and entangled state characterization in quantum information processing applications. In addition, they exhibit some photon number-resolving capacity, which is attractive from the point-of-view of several applications in optical continous-variable computing, such as building a cubic phase gate. We characterize the Andor Luca-R EMCCD camera as an affordable tool for applications in optical quantum information. We present measurements of single-photon detection efficiency, dark count probability as well as photon-number resolving capacity and place quantitative bounds on the noise performance and detection efficiency of the EMCCD detector array. We find that the readout noise floor is a Gaussian distribution centered at 500 counts/pixel/frame at high EM gain setting. We also characterize the trade-off between quantum efficiency and detector dark-count probability. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F26.00008: High-efficiency Cooper pair splitting demonstrated by two-particle conductance resonance and positive noise cross-correlation Yuval Ronen, Anindya Das, Moty Heiblum, Diana Mahalu, Andrey Kretinin, Hadas Shtrikman Entanglement is at the heart of the Einstein-Podolsky-Rosen paradox, where the non-locality is a necessary ingredient. Cooper pairs in superconductors can be split adiabatically, thus forming entangled electrons. Here, we fabricate such an electron splitter by contacting an~aluminum superconductor strip at the centre of a suspended~InAs~nanowire. The nanowire is terminated at both ends with two normal metallic drains. Dividing each half of the nanowire by a gate-induced Coulomb blockaded quantum dot strongly impeds the flow of Cooper pairs due to the large charging energy, while still permitting passage of single electrons. We provide conclusive evidence of extremely high efficiency Cooper pair splitting via observing positive two-particle correlations of the conductance and the shot noise of the split electrons in the two opposite drains of the nanowire. Moreover, the actual charge of the injected quasiparticles is verified by shot noise measurements. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F26.00009: An operational approach to indirectly measuring tunneling time Yunjin Choi, Andrew Jordan The tunneling time through an arbitrary one-dimensional barrier is investigated using the dwell time operator approach Since the tunneling time contains a natural post-selection ( we only average over particles that successfully tunnel), the tunneling time can ve related to the weak value of the dwell time operator. We analyze the situation by considering a specific measurement context containing experimentally observable quantities, since measuring the dwell time operator directly is not strictly achievable in the laboratory. Therefore, we reconstruct the average value of the dwell time operator applying the contextual values formalism [J. Dressel and A. N. Jordan, Phys. Rev. A \textbf{85}, 022123 (2012)] for generalized measurements based on the Larmor clock [ M. B\"{u}ttiker, Phys. Rev. B \textbf{27}, 6178 (1983)]. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F26.00010: A Stochastic Path Integral Formulation for Continuous Quantum Measurement Areeya Chantasri, Justin Dressel, Andrew Jordan We consider the continuous quantum measurement of a two-level system, for example, a double-quantum dot weakly measured by a quantum point contact. In a weak measurement regime, the measurement outcome at each time step is non-deterministic and fluctuates around its mean value. While the stochastic master/Schr\"{o}dinger equations are commonly used to study the state of the qubit, we propose an alternative formalism -- the stochastic path integral -- which can compute moments and correlation functions of the measurement outcomes, and the distributions of possible qubit states. By constructing a probability functional of the measurement outcomes in a path integral form, the moments can be computed from perturbative expansions, which can be resumed to exact solutions in some cases. We show that this approach reproduces and extends existing solutions derived using different methods, and introduces a new way to compute conditioned moments and correlation functions. We also show how real-time feedback can be naturally included in this approach. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F26.00011: Weak values are universal in von Neumann measurements Justin Dressel, Andrew Jordan We refute the widely held belief that the quantum weak value necessarily pertains to weak measurements. To accomplish this, we use the transverse position of a free particle as the detector for the conditioned von Neumann measurement of a system observable. For any coupling strength, any initial states, and any choice of conditioning, the averages of the detector position and momentum are completely described by the real parts of three generalized weak values in the joint Hilbert space. Higher-order detector moments also have similar weak value expansions. Using the Wigner distribution of the initial detector state, we find compact expressions for these weak values within the reduced system Hilbert space, demonstrating that the effective preselection for a measured system weak value is decohered by the detector. As an optical application of the approach, we consider an arbitrary Hermite-Gauss mode for a paraxial beam-like detector. For non-Gaussian modes the momentum shift involves the imaginary part of the system weak value plus an additional weak-value-like correction. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F26.00012: Implementing general quantum measurements on linear optical and solid-state qubits Yukihiro Ota, Sahel Ashhab, Franco Nori We show a systematic construction for implementing general measurements on a single qubit, including both strong (or projection) and weak measurements. We mainly focus on linear optical qubits. The present approach is composed of simple and feasible elements, i.e., beam splitters, wave plates, and polarizing beam splitters. We show how the parameters characterizing the measurement operators are controlled by the linear optical elements. We also propose a method for the implementation of general measurements in solid-state qubits. Furthermore, we show an interesting application of the general measurements, i.e., entanglement amplification. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F26.00013: Embedded SIC-POVMs Hoan Bui Dang, Kate Blanchfield, Ingemar Bengtsson, Marcus Appleby Symmetric informationally complete (SIC) sets of quantum states have applications in foundational studies of quantum mechanics, quantum tomography, quantum communication, quantum cryptography, and classical signal processing. However, their existence in every dimension has not been proven, and no general construction has been known. During our study of linear dependencies in Weyl-Heisenberg orbits [1], we discovered 2-dimensional SICs embedded in a 6-dimensional Hilbert space. This offers a robust construction for 2-dimensional SICs, and may potentially impact the SIC existence problem. In this talk, I will explain how this construction works, and present numerical results for some other dimensions. References: [1] Hoan Bui Dang, Kate Blanchfield, Ingemar Bengtsson, D. M. Appleby, ``Linear Dependencies in Weyl-Heisenberg Orbits,'' arXiv:1211.0215. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F26.00014: A multiport scheme for performing SIC-POVMs Gelo Noel Tabia SIC-POVMs comprise a family of generalized quantum measurements known to be optimal for linear quantum tomography, according to fairly standard Hilbert-Schmidt measures of statistical efficiency [1]. Because of the practical significance of state estimation in quantum information processing, it should prove useful to develop experimental methods for implementing SIC-POVMs directly. Based on the idea of Naimark extensions for POVMs, I propose the design for a SIC-POVM experiment using multiport devices with path-encoded qudits and demonstrate how it can be realized with integrated linear optics for qubits and qutrits [2]. References: [1] A. J. Scott, J. Phys. A 39, 13507 (2006). [2] G. N. M. Tabia, arXiv:1207.6035 (2012). [Preview Abstract] |
Session F27: Physical Implementations of Qubits
Sponsoring Units: GQIChair: John Preskill, California Institute of Technology
Room: 329
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F27.00001: Majorana fermions in 1D superconducting wires subject to disorder and other spatial inhomogeneities Wade DeGottardi, Diptiman Sen, Smitha Vishveshwara We present a systematic study of the role that disordered and quasiperiodic potentials play in the topology of 1D p-wave superconducting systems characterized by boundary Majorana modes. We forge a connection between Majorana wave functions and the localization properties of the corresponding normal state system (i.e, one which, though otherwise identical, lacks superconducting order). This enables the leveraging of the vast body of literature on Anderson localization to extensively map the topological phase diagram in superconducting wires. We find that the phase boundary is extremely sensitive to the detailed form of the potential. Our analysis provides a mapping between the limits of weak and strong disorder; in some cases, we are able to find the full phase boundary analytically. A noteworthy discovery is a tell-tale singularity in the phase boundary at the point corresponding to the quantum Ising model, a feature which offers a window into the physics of Majorana fermions. Our results can be directly applied to a spin-1/2 XY chain in a transverse magnetic field which is quasiperiodic or disordered. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F27.00002: Universal transport signatures of Majorana fermions in superconductor-Luttinger liquid junctions Jason Alicea, Lukasz Fidkowski, Netanel Lindner, Roman Lutchyn, Matthew Fisher One of the most promising proposals for engineering Majorana fermions employs a spin-orbit-coupled nanowire proximate to an s-wave superconductor. When only part of the wire's length contacts to the superconductor, the remaining conducting portion serves as a natural lead that can be used to probe these Majorana modes via tunneling. The enhanced role of interactions in 1D dictates that this configuration should be viewed as a superconductor-Luttinger liquid junction. We demonstrate that low-energy transport in such junctions is \emph{universal}, and governed by fixed points describing either perfect normal reflection or perfect Andreev reflection. In addition to capturing (in some instances) the familiar Majorana-mediated zero-bias anomaly in a new framework, we show that interactions yield dramatic consequences in certain regimes. Implications for conductance and local density of states measurements will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F27.00003: Coherent oscillations between single fluxonium qubit and Majorana fermion qubit Chang-Yu Hou, David Pekker, Vladimir Manucharyan, Eugene Demler We propose a hybrid device that couples a Majorana qubit to a superconducting fluxonium qubit. The devices consists of a conventional s-wave superconductor (e.g. Nb) ring interrupted by a narrow gap and a section of topological 1D wire bridging across the gap. Such topological 1D wire can be realized by using a semiconducting nanowire with strong spin orbit scattering (e.g. InSb) subjected by magnetic field. The nanowire hosts a topological qubit formed by four Majorana fermions and acts as a Josephson junction that completes the superconducting ring and makes a fluxonium qubit. As the current-phase relation of the Josephson junction is controlled by the state of the Majorana qubit, the fluxonium and Majorana qubit are naturally coupled. We demonstrate how this coupling can be exploited to construct two qubit operations. Remarkably, quantum information can be transformed between two distinct types of qubits solely using well-controlled operations on the fluxonium qubit. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F27.00004: Majorana Zero Modes in Semiconductor Nanowires in Contact with Higher-Tc Superconductors Younghyun Kim, Jennifer Cano, Chetan Nayak We present the prospects for stabilizing Majorana zero modes in semiconductor nanowires that are proximity-coupled to higher-temperature superconductors. We begin with the case of iron pnictides which, though they are $s$-wave superconductors, are believed to have superconducting gaps that change sign. We then consider the case of cuprate superconductors. We show that a nanowire on a step-like surface, especially in an orthorhombic material such as YBCO, can support Majorana zero modes at an elevated temperature. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F27.00005: Coherent population trapping of hyperfine-coupled single spins in diamond Andrew Golter, Nima Dinyari, Hailin Wang Coherent population trapping (CPT) provides a highly sensitive means for probing the energy level structure of an atomic system. For diamond nitrogen vacancy (NV) centers, this technique offers an alternative to the standard optically-detected magnetic resonance (ODMR) for measuring the hyperfine structure of the electronic ground states. Here, we report an experimental study using CPT to probe the hyperfine splitting of these states as well as the Autler-Townes effect induced by a strong resonant microwave field. This nuclear spin dependent CPT was also employed along with other coherent spin operations for the initialization and manipulation of hyperfine-coupled nuclear spins. In addition, the use of CPT process to incorporate NV centers into a cavity QED system will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F27.00006: Phonon sideband studies of the spin-triplet optical transition in diamond nitrogen-vacancy centers Audrius Alkauskas, David M. Toyli, Bob B. Buckley, David D. Awschalom, Chris G. Van de Walle In the past decade, the nitrogen-vacancy center in diamond has emerged as a promising solid-state system for quantum-information processing, and also for nanoscale magnetic, electric, and thermal sensing. All of these applications are partly enabled because the spin of the center can be measured through photoluminescence. This calls for a deeper understanding of the photoluminescence spectrum, in particular its phonon side-band. In this work we study the coupling of lattice vibrations to the triplet ($^3$E$\rightarrow$$^3$A$_2$) optical transition from first-principles electronic structure calculations. Our formulation includes both quasi-localized and bulk phonons, and leads to an excellent agreement of the calculated and the measured photoluminescence lineshape. This good agreement enables the application of the developed methodology to other defects in semiconductors that are currently being investigated as viable quantum bits. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F27.00007: Measurement-Only Topological Quantum Computation via Tunable Interactions Parsa Bonderson I examine, in general, how tunable interactions may be used to perform anyonic teleportation and generate braiding transformations for non-Abelian anyons. I explain how these methods are encompassed by the ``measurement-only'' approach to topological quantum computation. The physically most relevant example of Ising anyons or Majorana zero-modes is considered in detail, particularly in the context of Majorana nanowires. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F27.00008: Quantum information processing using quasiclassical electromagnetic interactions between qubits and electrical resonators Andrew Kerman Electrical resonators are widely used in quantum information processing with any qubits that are manipulated via electromagnetic interactions. In most cases they are engineered to interact with qubits via real or virtual exchange of (typically microwave) photons, and the resonator must therefore have both a high quality factor and strong quantum fluctuations, corresponding to the strong-coupling limit of cavity QED. Although great strides in the control of quantum information have been made using this so-called ``circuit QED'' architecture, it also comes with some important disadvantages. In this talk, we discuss a new paradigm for coupling qubits electromagnetically via resonators, in which the qubits do not exchange photons with the resonator, but instead exert quasi-classical, effective ``forces" on it. We show how this type of interaction is similar to that induced between the internal state of a trapped atomic ion and its center-of-mass motion by the photon recoil momentum, and that the resulting entangling operations are insensitive both to the state of the resonator and to its quality factor. The methods we describe are applicable to a variety of qubit-resonator systems, including superconducting and semiconducting solid-state qubits, and trapped molecular ions. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F27.00009: Generation and characterization of hypercubic cluster states in the optical frequency comb Moran Chen, Pei Wang, Nicolas Menicucci, Olivier Pfister We report on experimental progress toward generating and characterizing a very large quantum wire cluster state of the continuous electromagnetic variables (``Qmodes'') in the optical frequency comb of an optical parametric oscillator (OPO). We also present a proposal for creating higher-dimensional graph states by entangling the linear cluster states of several OPOs, each OPO adding a dimension to the graph (line, square grid, cube, hypercube). Besides the scalable (in number, size, and dimension) creation of sophisticated quantum graphs over hundreds to thousands of Qmodes, this work also constitutes a considerable experimental simplification of our previous proposal for generating a square-grid cluster state in a single OPO. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F27.00010: Entangled Photon Holes Todd Pittman, Junlin Liang, James Franson Entangled photon hole (EPH) states represent a new form of entanglement that is based on the existence of ``missing pairs'' of photons in two optical modes. In contrast to the more familiar photon pairs entangled in polarization or other variables, the entanglement in EPH states arises from the absence of the photon pairs themselves. We will review recent experimental work on the generation of these states, and theoretical work showing that they can be relatively insensitive to loss and amplification noise in certain situations. We will also report on our recent efforts to generate time-bin EPH states which have different properties than energy-time EPH states. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F27.00011: Information-efficient phase imaging with heralded single photons Reihaneh Shahrokhshahi, Niranjan Sridhar, Olivier Pfister, Saikat Guha, Jonathan Habif, Aaron Miller, Adriana Lita, Brice Calkins, Thomas Gerritts, Antia Lamas-Linares, Sae Woo Nam We report progress toward the experimental realization of information-efficient quantum imaging, here at two bits per photon. A heralded single-photon source ($g^{2}(0)<0.08$) is used as the input to a 4x4 multiport interferometer, compactly implemented using both polarization and spatial degrees of freedom. The interferometer can be used to read out all 4 Hadamard phase codes with a single photon. We investigate the use of cavity-enhanced spontaneous parametric downconversion for the coherent source of heralded photons. The photon-number-resolving ability of high-quantum-efficiency transition edge sensors is used for the heralding and detection. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F27.00012: A Two-Qubit Geometric Phase Gate for Localized Electron Spin Qubits using Cavity Polariton Resonance Shruti Puri, Na Young Kim, Yoshihisa Yamamoto We propose a two-qubit geometric phase gate, in which the interaction between a pair of localized electron spins, is mediated by quantum well microcavity exciton-polaritons. The entanglement between the electrons is a result of their spin dependent Coulomb exchange interaction with the exciton-polaritons. This optical coupling, resembling the electron-electron Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, offers high speed, high fidelity two-qubit gate operation with moderate cavity quality factor Q. The long ranged interaction by microcavity polaritons (order of micrometers) makes this gate suitable for fault tolerant operations. By the use of electrostatic quantum dots, the errors caused by unwanted excitations to charged excitons or trions are eliminated. The errors due to the finite lifetime of the polaritons can be minimized by optimizing the optical pulse parameters (duration and energy). The proposed design maximizes entanglement and ensures scalability. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F27.00013: Quantum Computing using Photons Ahmed Elhalawany, Michael Leuenberger In this work, we propose a theoretical model of two-quantum bit gates for quantum computation using the polarization states of two photons in a microcavity. By letting the two photons interact non-resonantly with four quantum dots inside the cavity, we obtain an effective photon-photon interaction which we exploit for the implementation of an universal XOR gate. The two-photon Hamiltonian is written in terms of the photons' total angular momentum operators and their states are written using the Schwinger representation of the total angular momentum. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F27.00014: Charge pumping by a surface acoustic wave in an undoped quantum well: a potential single-photon source S.K. Son, Y. Chung, J. Pedros, C.J.B. Ford, C.H.W. Barnes, J.P. Griffiths, G.A.C. Jones, I. Farrer, D.A. Ritchie Single-electron transfer between distant quantum dots using the potential minima of surface acoustic waves (SAWs) has been demonstrated recently, with possible applications for quantum computation. We have developed a technique to induce electrons and holes in an undoped GaAs/AlGaAs quantum well in different regions of the same device using gates, and to transport a stream of single electrons or holes along a narrow, empty channel using SAWs. The potential has a steep slope at the edges of the inducing gates, but we have modelled the potential profile of the active region to find designs in which the potential slope is shallow enough to allow the SAW potential to drag electrons out of the induced region, towards the region of holes. Recombination of each electron with one of the holes should produce a photon and we are investigating the use of this device as a single-photon source. If the electrons are spin-polarised then their spins can be detected by measuring the circular polarisation of the photons, and this may be useful for spin readout in a quantum processor, or as part of a quantum repeater in quantum cryptography. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F27.00015: ABSTRACT WITHDRAWN |
Session F28: Physical Approaches to Social Modeling
Sponsoring Units: GSNPChair: Bruno Goncalves, Indiana University
Room: 336
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F28.00001: Online Networks and the Diffusion of Protests Invited Speaker: Yamir Moreno Undoubtedly, online social networks have an enormous impact on opinions and cultural trends. Also, these platforms have revealed as a fundamental organizing mechanism in country-wide social movements. Recent events in the Middle East and North Africa (the wave of protests in the Arab world), across Europe (in the form of anti-cuts demonstrations or riots) and United States (the OWS movement) have generated much discussion on how digital media is connected to the diffusion of protests. In this talk, we investigate the mechanisms driving the emergence, development and stabilization of unrest movements in Spain and the USA by analyzing data from Twitter. Messages related to the protests are analyzed at both static and dynamic levels. We show that the online trace of the protests provides a unique opportunity to tackle central issues like recruitment patterns, information cascades and their spatiotemporal dynamics. Our findings shed light on the connection between online networks and social movements, and offer an empirical test to elusive sociological questions about collective action. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F28.00002: Role of Committed Minorities in Times of Crisis Malgorzata Turalska, Paolo Grigolini, Bruce J. West The surprising social phenomena of the Arab Spring and the Occupy Wall Street movement posit the question of whether the active role of committed groups may produce political changes of significant importance. Under what conditions are the convictions of a minority going to dominate the future direction of a society? We use a Cooperative Decision Making (CDM) model to study the effect of committed minorities on group behavior in time of crisis. The CDM model has been shown to generate consensus through a phase-transition process that at criticality establishes long-range correlations among the individuals within a model society. In a condition of high consensus, the correlation function vanishes, thereby making the network recover the ordinary locality condition. However, this state is not permanent and times of crisis occur when there is an ambiguity concerning a given social issue. The correlation function within the cooperative system becomes similarly extended as it is observed at criticality. This combination of independence (free will) and long-range correlation makes it possible for very small but committed minorities to produce substantial changes in social consensus. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F28.00003: Beating the news using social media: the case study of American Idol Fabio Ciulla, Delia Mocanu, Andrea Baronchelli, Bruno Goncalves, Nicola Perra, Alessandro Vespignani We present a contribution to the debate on the predictability of social events using big data analytics. We focus on the elimination of contestants in the American Idol TV shows as an example of a well defined electoral phenomenon to assess the predictive power of twitter signals. We provide evidence that Twitter activity during the time span defined by the TV show airing and the voting period following it allows the anticipation of the voting outcome. Twitter data have been analyzed to attempt the winner prediction ahead of the airing of the official result. We also show that the fraction of Tweets that contain geolocation information allows us to map the fanbase of each contestant, both within the US and abroad, showing that strong regional polarizations occur. The geolocalized data are crucial for the correct prediction of the final outcome of the show, pointing out the importance of considering information beyond the aggregated twitter signal. Although American Idol voting is just a minimal and simplified version of complex societal phenomena, this work shows that the volume of information available in online systems permits the real time gathering of quantitative indicators that may be able to anticipate the future unfolding of opinion formation events. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F28.00004: Heterogeneity of Human Activity Levels Gives Rise to Power-Law Distribution in Online Social Networks Lev Muchnik, Sen Pei, Lucas Parra, Saulo Reis, Jos\'e Andrade, Jr, Shlomo Havlin, Hernan Makse It is well established that the distribution of social ties (degree) of an individual in a social network follows a power-law. How this heavy-tailed distribution arises in practice, however, has not been conclusively demonstrated. Mechanisms of ``preferential-attachment'' and optimization are often cited as the origin of heavy-tailed degree distributions. Our data indicate that there is a different cause for these phenomena. For different social networks we find an intrinsic relationship degree and activity (number of posts, edits etc): The degree distribution is entirely random except for its mean value which depends deterministically on the volume of the users' activity. This suggests that heavy-tailed degree distribution is a consequence of the intrinsic activity of users. More importantly, human activity deterministically affects the mean success at establishing links in a social network, and the specific degree of a given user is otherwise random following a ``maximum entropy attachment'' model. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F28.00005: Nonlinear Opinion Dynamics on Networks Michael Gabbay, Arindam Das A model which treats group decision making as nonlinear opinion dynamics occurring over a network is presented. The model makes predictions regarding the interaction of network structure and initial disagreement level upon decision outcomes and consensus formation. The model displays bifurcations at high disagreement levels which lead to behaviors that are qualitatively distinct from those at low disagreement. For example, at high disagreement, the model exhibits asymmetric, majority rule outcomes that arise even when the system is symmetric with respect to the distribution of initial opinions and network structure. Analytical approximations for the bifurcation boundaries agree well with numerically-determined boundaries. An ongoing experimental effort involving the use of online discussion groups to test the model predictions is briefly described. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F28.00006: Language Geography from Microblogging Platforms Delia Mocanu, Andrea Baronchelli, Nicola Perra, Bruno Gon\c{c}alves, Alessandro Vespignani Microblogging platforms have now become major open source indicators for complex social interactions. With the advent of smartphones, the everincreasing mobile Internet traffic gives us the unprecedented opportunity to complement studies of complex social phenomena with real-time location information. In this work, we show that the data nowadays accessible allows for detailed studies at different scales, ranging from country-level aggregate analysis to the analysis of linguistic communities withing specific neighborhoods. The high resolution and coverage of this data permits us to investigate such issues as the linguistic homogeneity of different countries, touristic seasonal patterns within countries, and the geographical distribution of different languages in bilingual regions. This work highlights the potentialities of geolocalized studies of open data sources that can provide an extremely detailed picture of the language geography. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F28.00007: The rise and fall of social communities: Cascades of followers triggered by innovators Yanqing Hu, Shlomo Havlin, Hernan Makse New scientific ideas as well as key political messages, consumer products, advertisement strategies and art trends are originally adopted by a small number of pioneers who innovate and develop the ``new ideas''. When these innovators migrate to develop the novel idea, their former social network gradually weakens its grips as followers migrate too. As a result, an internal ``cascade of followers'' starts immediately thereafter speeding up the extinction of the entire original network. A fundamental problem in network theory is to determine the minimum number of pioneers that, upon leaving, will disintegrate their social network. Here, we first employ empirical analyses of collaboration networks of scientists to show that these communities are extremely fragile with regard to the departure of a few pioneers. This process can be mapped out on a percolation model in a correlated graph crucially augmented with outgoing ``influence links''. Analytical solutions predict phase transitions, either abrupt or continuous, where networks are disintegrated through cascades of followers as in the empirical data. The theory provides a framework to predict the vulnerability of a large class of networks containing influence links ranging from social and infrastructure networks to financial systems and markets. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F28.00008: Extraordinary variability and sharp transitions in a maximally frustrated dynamic network Wenjia Liu, Beate Schmittmann, R.K.P. Zia Most previous studies of complex networks have focused on single, static networks. However, in the real world, networks are dynamic and interconnected. Inspired by the presence of extroverts and introverts in the general population, we investigate a highly simplified model of a social network, involving two types of nodes: one preferring the highest degree possible, and one preferring no connections whatsoever. There are only two control parameters in the model: the number of ``introvert'' and ``extrovert'' nodes, $N_I$ and $N_E$. Our key findings are as follows: As a function of $N_I$ and $N_E$, the system exhibits a highly unusual transition, displaying extraordinary fluctuations (as in 2nd order transitions) and discontinuous jumps (characteristic of 1st order transitions). Most remarkably, the system can be described by an Ising-like Hamiltonian with long-range multi-spin interactions and some of its properties can be obtained analytically. This is in stark contrast with other dynamic network models which rely almost exclusively on simulations. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F28.00009: Self-organization of plutonomy in competitive societies Takashi Odagaki, Yuuni Todate, Ryo Fujie The plutonomy, where the top 1 percent of households accounts for more wealth than the bottom 99 percent, is an extreme form of hierarchy. It is an important question to find out what traits of competitive society lead to the extreme form of hierarchy. We investigate conditions for emergence of plutonomy in two model competitive societies. In a model where individuals make random walk and fight when they meet, we show that the order of move plays an important role and the plutonomy emerges when individuals have the same right to move first. For a model society where individuals participate in a competition with equal right, we show that the plutonomy can be self-organized when individuals grouped into several classes compete with those in the same class for a certain period (season) and they are regrouped at the end of every season. We also discuss various features of the emergence of plutonomy. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F28.00010: Popularity-Driven Networking Eli Ben-Naim, Paul Krapivsky We investigate the growth of connectivity in a network. In our model, starting with a set of disjoint nodes, links are added sequentially. Each link connects two nodes, and the connection rate governing this random process is proportional to the degrees of the two nodes. Interestingly, this network exhibits two abrupt transitions, both occurring at finite times. The first is a percolation transition in which a giant component, containing a finite fraction of all nodes, is born. The second is a condensation transition in which the entire system condenses into a single, fully connected, component. We derive the size distribution of connected components as well as the degree distribution, which is purely exponential throughout the evolution. Furthermore, we present a criterion for the emergence of sudden condensation for general homogeneous connection rates. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F28.00011: Threshold model with multiple initiators P. Singh, S. Sreenivasan, B. Szymanski, G. Korniss In social networks, adoption of a new behavior or opinion by an agent strongly depends on its neighborhood. We study the threshold model where every node is in one of two possible states (0 or 1) and a node in state '0' changes to '1' if at least a threshold fraction $\phi$ of its neighbors are already in state '1'[M. Granovetter, AJS, Vol. 83, No. 6]. Initially all nodes are in state '0' except initiators. Previous studies have shown that a small seed of such initiators can give rise to large cascades if $\phi$ is less than some critical $\phi_{c}$. The focus of our work is the effect of the size of the initiator fraction $p$ on the size of the cascade for different threshold values, on empirical networks as well as stylized models of social networks. We observe that global cascades are possible for arbitrary values of $\phi$, if $p$ is sufficiently large. We find that there exists a critical $p_{c}$ (unique for every $\phi$), such that $p\geq p_{c}$ results in global cascades whereas for $p |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F28.00012: Spreading of infectious diseases considering age contact patterns for Latin America Ana Pastore y Piontti, Marcelo F.C. Gomes, Luca Rossi, Alessandro Vespignani The dynamics of infectious diseases strongly depends on the structure of the social contact patterns among individuals. In order to have an accurate estimate of the impact of epidemic outbreaks and which effective control measures to take, we need an appropriate description of these patterns. A simple way to improve the homogeneous mixing assumption is to introduce age contact patterns. Here we follow the approach of Fumanelli et al (PLoS Computational Biology, 8(9):e1002673, 2012) to estimate the age mixing patterns of virtual populations using highly detailed census data for Argentina, Brazil and Mexico. Considering age contact matrices for these countries we study the epidemiological relevant quantities and their relation with the sociodemographic data. Our results show that even for the same country the impact of epidemics outbreaks could be very different when we consider age contact matrices. This results can be explained as a result of a change in the average age of the population in the different regions of the countries. This study also provides the first estimates of contact matrices for Latin American countries. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F28.00013: S-curves and the Mechanism of Propagation in Language Change Richard Blythe, W. Croft Linguists have proposed a wide variety of mechanisms for the propagation of a linguistic innovation through the speech community. The complexity of social systems makes it difficult to evaluate the different mechanisms empirically. We introduce a four-way typology of mechanisms and provide mathematical definitions based on the symmetries that may (or may not) be present between different speakers in the community and/or between different linguistic variants. As in physics, such symmetries impose strong constraints on the patterns of change that may emerge as a result. In particular, we conclude that the widely observed empirical pattern of an S-curve temporal trajectory of change can be captured only by theories that invoke a pre-existing shared preference among speakers for the incoming variant. [Preview Abstract] |
Session F29: Focus Session: Spin Glasses: Advances, Algorithms, and Applications
Sponsoring Units: GSNPChair: Jonathan Machta, University of Massachusetts Amherst
Room: 337
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F29.00001: Is there a de Almeida-Thouless line in finite-dimensional spin glasses? Invited Speaker: Peter Young The question of whether there is a line of transitions in a magnetic field in an Ising spin glass (the de Almeida-Thouless, or AT, line) is important for two reasons: (i) its existence or otherwise is a major difference between the ``droplet'' and ``replica symmetry breaking (RSB)'' pictures of the spin glass state, and (ii) the spin glass in a field is argued to be quite similar to structural glasses, and, in this analogy, the spin glass AT line corresponds to the ``ideal glass'' transition of structural glasses. ``Standard'' finite-size scaling (FSS) methods do not find evidence for an AT line in three- or four-dimensional spin glasses. However, these results have been called into question by Leuzzi et al., Phys. Rev. Lett. 103, 267201 (2009) who perform a ``non-standard'' FSS analysis, in which they state that one should not include fluctuations at $k=0$ since these are argued to have larger corrections to FSS than $k > 0$ fluctuations. Using the ``non-standard'' analysis Leuzzi et al. find an AT line in four dimensions and also in a one-dimensional long-range model which is a proxy for four dimensions. In this talk I will describe results of large-scale Monte Carlo simulations for one-dimensional models which are proxies for three and for four dimensions, analyzed using both the ``standard'' and ``non-standard'' FSS approaches. I will also briefly discuss the merits of these two approaches to FSS. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F29.00002: Low-temperature behavior of the spin overlap distribution in one-dimensional long-range diluted spin glasses Matthew Wittmann, Helmut G. Katzgraber, J. Machta, A. P. Young Computer simulations of the spin glass state find that the \emph{average} order parameter distribution $P(q)$ has a weight in the region of small overlap $q$ which does not appear to decrease with size for the range of sizes that can be studied. This is in agreement with the ``Replica Symmetry Breaking'' (RSB) picture as opposed to the droplet picture which predicts $P(0)=0$ in the thermodynamic limit. Recently, a detailed study [1] has been made of peaks in $P(q)$ for \emph{individual samples} of a three-dimensional spin glass to gain more understanding of the situation. Here we pursue a similar approach but for long-range models in one dimension for which the interactions fall off with a power of the distance. Varying the power is analogous to varying the space dimension of a short-range model, so we can conveniently study models which are proxies for a \emph{range} of space dimensions. We will present results on the nature of the peaks in $P(q)$ for individual samples for several such models, and interpret them in terms of the RSB and droplet pictures. References: [1] Phys. Rev. Lett. 109, 177204 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F29.00003: Numerical evidence against both mean field and droplet scenarios of the Edwards-Anderson model Julio F. Fernandez, Juan J. Alonso From tempered Monte Carlo simulations, we have obtained accurate probability distributions $p(q)$ of the spin-overlap parameter $q$ for finite Edwards-Anderson (EA) and Sherrington-Kirkpatrick (SK) spin-glass systems at low temperatures. Our results for $p(q)$ follow from averages over $10^5$ disordered samples of linear sizes $L=4-8$ and over $15 \; 000$ samples for $L=10$. In both the SK and EA models, at temperatures as low as $0.2T_{sg}$, where $T_{sg}$ is the transition temperature, $p(q)$ varies insignificantly with $L$. This does not fit the trend that the droplet model predicts for large $L$. We have also calculated correlation functions, $F(q_1,q_2)$, from which rms deviations, $\delta p$, over different realizations of quenched disorder, as well as thermal fluctuations, $w$, of $q$ values, follow. Our numerical results for $\delta p$ and $w$ scale as $\sqrt{L}$ and $1/L$, respectively, in the SK model. This fits in well with mean field predictions. On the other hand, our data for $w$ and $\delta p$ vary little, if at all, for the EA model. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F29.00004: Evidence of Non-Mean-Field-Like Low-Temperature Behavior in the Edwards-Anderson Spin-Glass Model Burcu Yucesoy, Helmut G. Katzgraber, Jonathan Machta The three and four-dimensional Edwards-Anderson and mean-field Sherrington-Kirkpatrick Ising spin glasses are studied via large-scale Monte Carlo simulations at low temperatures, deep within the spin-glass phase. Performing a careful statistical analysis of several thousand independent disorder realizations and using an observable that detects peaks in the overlap distribution, we show that the Sherrington-Kirkpatrick and Edwards-Anderson models have a distinctly different low-temperature behavior. The structure of the spin-glass overlap distribution for the Edwards-Anderson model suggests that its low-temperature phase has only a single pair of pure states. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F29.00005: Overlap distributions in two-dimensional spin glasses A. Alan Middleton Numerical results are presented for overlaps of configurations of two-dimensional Ising spin glasses. At low temperatures, the correlation length greatly exceeds the system size, so that spin-spin correlations are relatively long range and domain wall energies exhibit sensitive dependence to temperature,as seen in the low temperature phase of three-dimensional spin glasses. Exact sampling algorithms are used so that there is no doubt of equilibration. High statistics runs are carried out, with tens of thousands of samples of size $L^2=256^2$ simulated. The results of the size-dependent spin overlap distribution $P(q)$ are evaluated using statistics recently developed by Yucesoy, Katzgraber and Machta. The statistics for two-dimensional models at low temperature are found to be quite similar to those of three-dimensional spin glasses at finite temperatures below the spin-glass transition. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F29.00006: Monte Carlo Simulation of three dimensional Edwards Anderson model with multi-spin coding and parallel tempering using MPI and CUDA Sheng Feng, Ye Fang, Ka-Ming Tam, Bhupender Thakur, Zhifeng Yun, Karen Tomko, Juana Moreno, Jagannathan Ramanujam, Mark Jarrell The Edwards Anderson model is a typical example of random frustrated system. It has been a long standing problem in computational physics due to its long relaxation time. Some important properties of the low temperature spin glass phase are still poorly understood after decades of study. The recent advances of GPU computing provide a new opportunity to substantially improve the simulations. We developed an MPI-CUDA hybrid code with multi-spin coding for parallel tempering Monte Carlo simulation of Edwards Anderson model. Since the system size is relatively small, and a large number of parallel replicas and Monte Carlo moves are required, the problem suits well for modern GPUs with CUDA architecture. We use the code to perform an extensive simulation on the three-dimensional Edwards Anderson model with an external field. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F29.00007: Extremal Optimization for Ground States of the Sherrington-Kirkpatrick Spin Glass with Levy Bonds Stefan Boettcher Using the Extremal Optimization heuristic (EO),\footnote{S. Boettcher \& A.G. Percus, {\it PRL} {\bf 86}, 5211 (2001)} ground states of the SK-spin glass are studied with bonds $J$ distributed according to a Levy distribution $P(J)\propto1/|J|^{1+\alpha}$ with $|J|>1$ and $1<\alpha<4.$ The variation of the energy densities with $\alpha$, their finite-size corrections, their fluctuations, and their local field distribution are analyzed and compared with those for the SK model with Gaussian bonds.\footnote{S. Boettcher, {\it Philosophical Magazine} {\bf 92}, 34 (2012)} We find that the energies attain universally the Parisi-energy of the SK when the second moment of $P\left(J\right)$ exists ($\alpha>2$). They compare favorably with recent one-step replica symmetry breaking predictions well below $\alpha=2$. Near $\alpha=2$, the simulations deviate significantly from theoretical expectations. The finite-size corrections exponent $\omega$ decays from the putative SK value $\omega_{SK}=\frac{2}{3}$ already well above $\alpha=2$. The exponent $\rho$ for the scaling of ground state energy fluctuations with system size decays linearly from its SK value for decreasing $\alpha$ and vanishes at $\alpha=1$. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F29.00008: Equilibrium and nonequilibrium properties of Boolean decision problems on scale-free graphs with competing interactions with external biases Zheng Zhu, Juan Carlos Andresen, Katharina Janzen, Helmut G. Katzgraber We study the equilibrium and nonequilibrium properties of Boolean decision problems with competing interactions on scale-free graphs in a magnetic field. Previous studies at zero field have shown a remarkable equilibrium stability of Boolean variables (Ising spins) with competing interactions (spin glasses) on scale-free networks. When the exponent that describes the power-law decay of the connectivity of the network is strictly larger than 3, the system undergoes a spin-glass transition. However, when the exponent is equal to or less than 3, the glass phase is stable for all temperatures. First we perform finite-temperature Monte Carlo simulations in a field to test the robustness of the spin-glass phase and show, in agreement with analytical calculations, that the system exhibits a de Almeida-Thouless line. Furthermore, we study avalanches in the system at zero temperature to see if the system displays self-organized criticality. This would suggest that damage (avalanches) can spread across the whole system with nonzero probability, i.e., that Boolean decision problems on scale-free networks with competing interactions are fragile when not in thermal equilibrium. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F29.00009: Self-organized criticality in glassy spin systems requires long-range interactions Juan Carlos Andresen, Ruben S. Andrist, Helmut G. Katzgraber, Vladimir Dobrosavljevic, Gergerly T. Zimanyi We investigate the conditions required for general spin systems with frustration and disorder to display self-organized criticality, a property which so far has been established in spin models only for the infinite-range Sherringtion-Kirkpatrick Ising spin-glass model [PRL 83, 1034 (1999)]. We study the avalanche and the magnetization jump distribution triggered by an external magnetic field in the short-range Edward-Anderson Ising spin glass for various space dimensions, between 2 and 8. Our numerical results, obtained on systems of unprecedented size, demonstrate that self-organized criticality is recovered only in the strict limit of infinite space dimensions (or equivalently of long-ranged interaction), and is not a generic property of spin-glass models in finite space dimensions. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F29.00010: Minimal spanning trees at the percolation threshold: a numerical calculation Sean Sweeney, A. Alan Middleton Through computer simulations on a hypercubic lattice, we grow minimal spanning trees (MSTs) in up to five dimensions and examine their fractal dimensions. Understanding MSTs is imporant for studying systems with quenched disorder such as spin glasses. We implement a combination of Prim's and Kruskal's algorithms for finding MSTs in order to reduce memory usage and allow for simulation of larger systems than would otherwise be possible. These fractal objects are analyzed in an attempt to numerically verify predictions of the perturbation expansion developed by T.~S.~Jackson and N.~Read for the pathlength fractal dimension $d_{s}$ of MSTs on percolation clusters at criticality [T.~S.~Jackson and N.~Read, Phys.\ Rev.\ E \textbf{81}, 021131 (2010)]. Examining these trees also sparked the development of an analysis technique for dealing with correlated data that could be easily generalized to other systems and should be a robust method for analyzing a wide array of randomly generated fractal structures. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F29.00011: Are the diluted antiferromagnet in a field and the random-field Ising model in the same universality class? Helmut G. Katzgraber, Bjoern Ahrens, Alexander K. Hartmann We perform large-scale Monte Carlo simulations using the Chayes-Machta and parallel-tempering algorithms to study the critical behavior of both the diluted antiferromagnet in a field (30\% dilution) and the random-field Ising model with Gaussian random fields for different field strengths. For small fields, analytical calculations by Cardy [Phys.~Rev.~B 29, 505 (1984)] predict that both models should share the same universality class. However, a detailed finite-size scaling analysis of both the Binder cumulant and the two-point finite-size correlation length suggests that even in the limit of small fields both models are not in the same universality class. Therefore, care should be taken when interpreting (experimental) data for diluted antiferromagnets in a field using the random-field Ising model. Finally, we present approximate analytical expressions based on our numerical data for the phase boundaries of both models. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F29.00012: Universality in the three-dimensional random-field Ising model Victor Martin-Mayor, Nikolaos Fytas We present the results of a large scale numerical simulation of the three-dimensional random-field Ising model at zero temperature. A combination of graph theoretical algorithms with a proper re-weighting scheme allows us to obtain data for systems with linear sizes $L \leq 192$ and extreme ensembles of disorder realizations, up to $5\times 10^{7}$. Three types of field distributions are considered, namely the Gaussian, the Poissonian, and the double Gaussian for two values of its width. In particular, for the double Gaussian case we choose parameters such that the distribution of random fields is bimodal. Our finite-size scaling analysis, based on the quotients method and universal quantities, indicates the existence of a unique random fixed-point. Therefore, the random-field Ising model is ruled by a single universality class, in disagreement with early mean-field theory predictions and the current opinion in the literature. The complete set of critical exponents characterizing this universality class is given, including the correction-to-scaling exponent $\omega$ and the violation of hyper-scaling exponent $\theta$. Finally, discrepancies with previous works are explained in terms of strong scaling corrections. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F29.00013: Competing Antiferromagnetic and Spin-Glass phases in a hollandite structure Yanier Crespo Hernandez, Alexei Andreanov, Nicola Seriani We introduce a simple model to explain recent experimental results on spin freezing in a hollandite-type structure. We argue that geometrical frustration of the lattice with antiferromagnetic (AFM) interactions is responsible for the appearance of a spin-glass phase in presence of disorder. We check our predictions numerically using parallel tempering on a model that considers Ising spins and nearest-neighbor AFM interactions. The proposed model presents a rich phenomenology: in absence of disorder two ground states are possible, depending on the strength of the interactions, namely an AFM or a geometrically frustrated phase. Remarkably for any set of AFM couplings having an AFM ground state in the clean system, there exist a critical value of the disorder for which the ground state is replaced by a spin-glass one while maintaining all couplings AFM. To the best of our knowledge in the literature there is not a model that presents this kind of transition considering just short-range AFM interactions. Therefore we argue that this model would be useful to understand the relation between AFM coupling, disorder and the appearance of spin glasses phase. [Preview Abstract] |
Session F30: Membranes, Micelles, Vesicles, Gels and Complex Fluids
Sponsoring Units: DCMPChair: Elizabeth Mann, Kent State University
Room: 338
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F30.00001: For a Safe Diamide Extraction Process, Elucidated by Atomistic Simulations Baofu Qiao, Ross J. Ellis, Monica Olvera de la Cruz The diamide extraction process has been successfully employed in separating trivalent actinides from used nuclear fuels. The extractant, which is an amphiphilic molecule with a metal-binding polar headgroup and hydrophobic tail, binds the actinides, thus extracting them from the aqueous phase into the oil phase. However, the oil phase will split into two phases, once a critical concentration of actinide is reached. This phase splitting is suspected to have caused the Red Oil events, which can decompose explosively. Therefore, it is extremely important for an extractant to have a high extraction efficiency, on one hand, and resist phase splitting, on the other. In comparison with DMDBTDMA, DMDOHEMA has both higher extraction efficiency and phase stability, which we suspect stem from the supramolecular aggregated structures influenced by the different extractant tails. To test our hypothesis, atomistic molecular dynamics simulations were performed on DMDBTDMA in bulk oil system and DMDOHEMA in bulk oil system. Our preliminary results indicate that DMBTDMA is more disposed toward formation of chain-like aggregates, especially at lower water concentration, in comparison with the branched structures observed in DMDOHEMA. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F30.00002: Stochastic nature of clathrin-coated pit assembly Anand Banerjee, Alexander Berezhkovskii, Ralph Nossal Clathrin-mediated endocytosis is a complex process through which eukaryotic cells internalize various macromolecules (cargo). The process occurs via the formation of invaginations on the cell membrane, called clathrin-coated pits (CCPs). The dynamics of CCP formation shows remarkable variability. After initiation, a fraction of CCPs, called ``productive pits", bind to cargo and then grow and mature into clathrin-coated vesicles (CCVs). In contrast, a large fraction of CCPs, called ``abortive pits", fail to bind to cargo, grow only up to intermediate sizes and then disassemble. There is notable heterogeneity in the lifetimes of both productive and abortive pits. We propose a stochastic model of CCP dynamics to explain these experimental observations. Our model includes a kinetic scheme for CCP assembly and a related functional form for the dependence of free energy of a CCP on its size. Using this model, we calculate the lifetime distribution of abortive pits (via Monte Carlo simulation) and show that the distribution fits experimental data very well. By fitting the data we determine the free energy of CCP formation and show that CCPs without cargo are energetically unstable. We also suggest a mechanism by which cargo binding stabilizes CCPs and facilitates their growth. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F30.00003: Budding transition of a self-avoiding polymer confined by a soft membrane adhering onto a flat wall Yu-Cheng Su, Jeff Z. Y. Chen The Monte Carlo simulation is used to study the structural properties of the system consisting of a self-avoiding polymer chain confined between a fluid membrane and a flat hard surface. As the adhesion between the soft membrane and the hard-wall surface increases, the polymer is subject to a strong confinement; the state containing a pancake-shaped polymer conformation eventually yields to a bud state, through an abrupt, first-order phase transition. We explore the scaling behavior of the physical properties of the system as functions of the polymer's size, the membrane's surface tension, and the adhesion energy, for both pancake and bud states, in terms of Monte Carlo data and analytic scaling theories. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F30.00004: Why square lattices are not seen on curved ionic membranes Creighton Thomas, Monica Olvera de la Cruz Ionic crystalline membranes on curved surfaces are ubiquitous in nature, appearing for example on the membranes of halophilic organisms. Even when these membranes buckle into polyhedra with square or rectangular sides, the crystalline structure is seen to have hexagonal symmetry. Here, we theoretically and numerically investigate the effects of curvature on square lattices. Our model system consists of both positive and negative ions with a 1:1 charge ratio adsorbed onto the surface of a sphere. In flat space, the lowest-energy configuration of this system can be a square lattice. This bipartite arrangement is favored because there are two types of ions. It leads to a fundamentally different defect structure than what has been seen when triangular lattices are favored. We classify these defects and find that curvature disrupts long-range square symmetry in a crystal. Through numerical simulations, we see that small square regions are possible in some cases, but this phase coexists with other structures, limiting the scale of these square-lattice microstructures. Thus, at large length scales, curvature leads to triangular structures. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F30.00005: Amphiphilic lipids in solution: a simulational study of lipid bilayer formation Thomas Vogel, David P. Landau, Lili Gai, Katie A. Maerzke, Christopher R. Iacovella, Clare M. McCabe, Peter T. Cummings Amphiphilic molecules consisting of hydrophilic head and hydrophobic tail groups self-assemble into a wide variety of structures, such as bilayers (membranes), micelles, or vesicles (liposomes) when mixed with a suitable solvent. The understanding of this lipid self-assembly is essential for industrial, biological, or medical applications, but computer simulations are generally challenging due to the complex structure of the energy landscape. We show results for the lipid bilayer formation process obtained by newly developed parallel Wang--Landau Monte Carlo and statistical temperature molecular dynamics simulations. By applying those methods to a generic coarse-grained model for amphiphilic molecules in solution, we were able to obtain the thermodynamical data over the whole relevant temperature and energy range and to unravel the membrane formation process including all structural sub-transitions between different fluid and gel-phase \hbox{bilayers}. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F30.00006: Study of vesicle size distribution dependence on pH value based on nanopore resistive pulse method Yuqing Lin, Yauheni Rudzevich, Adam Wearne, Daniel Lumpkin, Joselyn Morales, Kathleen Nemec, Suren Tatulian, Oleg Lupan, Lee Chow Vesicles are low-micron to sub-micron spheres formed by a lipid bilayer shell and serve as potential vehicles for drug delivery. The size of vesicle is proposed to be one of the instrumental variables affecting delivery efficiency since the size is correlated to factors like circulation and residence time in blood, the rate for cell endocytosis, and efficiency in cell targeting. In this work, we demonstrate accessible and reliable detection and size distribution measurement employing a glass nanopore device based on the resistive pulse method. This novel method enables us to investigate the size distribution dependence of pH difference across the membrane of vesicles with very small sample volume and rapid speed. This provides useful information for optimizing the efficiency of drug delivery in a pH sensitive environment. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F30.00007: Spontaneous Thermoreversible Formation of Cationic Vesicles in a Protic Ionic Liquid Dongcui Li, Carlos Lopez-Barron, Leo DeRita, Madivala Basavaraj, Norman Wagner The search for stable vesicular structures is a long-standing topic of research because of the usefulness of these structures and the scarcity of surfactant systems that spontaneously form vesicles in true thermodynamic equilibrium. We report the first experimental evidence of spontaneous formation of vesicles for a cationic double tail surfactant (didodecyldimethylammonium bromide) in a protic ionic liquid (ethylammonium nitrate) [1-2]. Using small and ultra-small angle neutron scattering, rheology and bright field microscopy, we identify the coexistence of two vesicle containing phases in compositions ranging from 2 to 68 wt \%. A low density highly viscous solution containing giant vesicles and a sponge phase coexists with a dilute high density phase containing large vesicles. Vesicles form spontaneously via different thermodynamic routes, with the same size distribution, which strongly supports that they exist in a true thermodynamic equilibrium. The formation of equilibrium vesicles and the L3 phase is facilitated by ion exchange between the cationic surfactant and the ionic liquid, as well as the strength of the solvophobic effect in the protic ionic liquid.\\[4pt] [1] Lopez-Barron et al., J. Phys. Chem. B 116, 813 (2012).\\[0pt] [2] Lopez-Barron et al., J. Am. Chem. Soc., Accepted. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F30.00008: Phase separation in a DMPC/Dchol mixed Langmuir Film: A combined Brewster Angle, Fluorescence and Light Scattering Microscopy study Pritam Mandal, Fanindra Bhatta, Arne Gericke, Edgar Kooijman, David Allender, Elizabeth Mann Fluorescence microscopy (FM) is one of the most direct imaging techniques for in situ observation of morphology and phase-separation at the macroscopic scale [1] in lipid mono- or bi-layers. However, the presence of fluorescent dye-molecules can affect the system. In Brewster Angle Microscopy (BAM), one can image monomolecular Langmuir films without probes. Here, using a composite set-up of BAM, FM and Light Scattering Microscopy (LSM), we present a comparative study of the three techniques on a binary lipid mixture in the presence of two different probes. In most cases, all three techniques show precisely the same domains. However, depending on conditions, some domain types were more evident in one technique than the others. This established, we can directly test the influence of probe on the domain structure. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F30.00009: Mesoscopic Membrane Morphology Regulated by Molecular Crystallization Cheuk-Yui Leung, Liam Palmer, Bao Fu Qiao, Sumit Kewalramani, Rastko Sknepnek, Christina Newcomb, Megan Greenfield, Graziano Vernizzi, Samuel Stupp, Michael Bedzyk, Monica Olvera de la Cruz A grand challenge in self-assembly of multi-component systems is to control the crystal symmetries and the resulting geometries of co-assembled molecular structures. We generate here crystalline ionic bilayers in a large variety of geometries, which~resemble unusual cellular shell shapes, by mixing $+$3 and -1 ionic amphiphiles. To structurally characterize the co-assembly from the mesoscopic to nanometer scale, we combine electron microscopy with small and wide angle x-ray scattering. We use pH to control the degree of ionization of the amphiphiles and hence their intermolecular electrostatic interactions. At low and high pH, closed faceted vesicles with 2D hexagonal molecular arrangements were observed, while at intermediate pH ribbons with rectangular-C packing of the amphiphiles were observed. Thus pH acts as a switch to control the morphology of the ionic bilayers via transitions in the crystalline lattice. This work promotes the design of nanocontainers for various applications and improves our understanding of the origin of polyhedral shells in nature. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F30.00010: Thermodynamics of protein driven self assembly in membranes Ramakrishnan Natesan, Richard Tourdot, Ryan Bradley, Ravi Radhakrishnan Recent experimental evidences strongly point to the role of proteins and other membrane binding macromolecules in reshaping biological membranes, at length scales of the molecule and the structure enclosed by the membrane. In this work, we investigate the interplay between the membrane curvature induced at the molecular scale, mainly due to peripheral membrane proteins, and the resulting membrane morphologies, of varying complexity, observed at the mesoscale. The biological membrane, in our approach, is represented by a dynamically triangulated surface while the proteins are modeled as curvature fields on the membrane, which can either be isotropic or anisotropic. Thermal undulations in the membrane and cooperativity in the curvature field, due to the stabilization of a nematic phase, drives the membrane into conformations that resembles those in experiments in vivo and vitro. The stability of these structures are examined by two approaches to compute the free energy of the system: (i) Widom insertion technique to compute excess chemical potentials and (ii) thermodynamic integration using the Kirkwood coupling parameter to compute absolute free energies. Building on these methods, we propose a hybrid scheeme that couples both the approaches for computing free energies. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F30.00011: Morphology and Performance of PLLA Based Porous Membranes by Phase Separation Control Qian Xing, Xia Dong, Rongbo Li, Charles C. Han, Dujin Wang Poly (L-lactic acid) (PLLA) porous membranes with different morphologies and properties were prepared through immersion precipitation method. It has been proved that the rate and level of phase separation between PLLA/dioxane solution and coagulation baths were the original drive force for the ultimate structure and corresponding performance of PLLA membranes. The equilibrium thermodynamic phase diagram of PLLA/solvent/nonsolvent and the kinetic diffusion rate between solvent and nonsolvent were systematically investigated. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F30.00012: Correlating bulk properties and nanoscale rearrangement during UV-initiated gelation of hybrid nanoparticle/ block copolymer systems K. Anne Juggernauth, Soenke Seifert, Brian Love We use rheology and Small Angle X-Ray Scattering (SAXS) to investigate UV initiated gel formation in aqueous dispersions of clay nanoparticles in the presence of poly(ethyleneoxide-b-propyleneoxide-b-ethyleneoxide) block copolymer surfactants (Pluronics\textregistered ) and small amounts of a photoacid generator (PAG). This material system demonstrates stable liquid-like behavior in the absence of UV but undergoes bulk gelation upon UV exposure. Rheology was used to monitor the bulk properties of a series of samples undergoing UV exposure and confirm bulk gel formation. We further probe nanoparticle rearrangement using time resolved synchrotron SAXS with simultaneous UV exposure. Time dependent SAXS indicate an absence of long range order and crystallinity while changes in the scattering profile are related to short range interparticle interactions leading to a stable or arrested structure. Finally, we compare the time scales for structural rearrangement of nanoparticles with the bulk gelation behavior. Our results show that the kinetics for local structural changes between particles and bulk gelation from UV exposure are strongly correlated. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F30.00013: Cooperative Processes in Restructuring Gel Networks Jader Colombo, Asaph Widmer-Cooper, Emanuela Del Gado Colloidal gel networks are disordered elastic solids that can form even in extremely dilute particle suspensions. Similarly to other network-forming soft materials, including many with important biological function or technological potential, they can locally restructure via breaking and reforming interparticle bonds. Although controlling the link between local restructuring and mechanical response bears enormous potential for designing smart nanocomposites, there is at present little understanding of how local bond changes affect the dynamics of the gel network and the stress transmission through it. Here, using numerical simulations of a model system and a space-resolved analysis of dynamical heterogeneities, we show that bond breaking has non-local consequences and induces cooperative relaxation further away along the network. This provides explicit microscopic insight into why non-local constitutive relations are required to rationalize the non-trivial mechanical response of colloidal gels. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F30.00014: Elimination of branching in self assembled beta-hairpin based peptide hydrogels Sameer Sathaye, Darrin Pochan Hydrophobic collapse of amphiphilic $\beta $-hairpin peptides (e.g. MAX1 VKVKVKVKV$^{\mathrm{D}}$PPTKVKVKVKV-NH$_{\mathrm{2}})$ into fibrils and their hierarchical assembly into branched, hydrogel networks has been extensively studied. A physically crosslinked hydrogel network is formed due to fibrillar entanglement and branched defects in hydrophobic collapse during fibril formation. Alternating valine residues with side chains of the same size are responsible for the hydrophobic collapse of the molecule into a b-hairpin and fibril nanostructure with branching. In a new sequence LNK1 (LNK1 (Nal)K(Nal)KAKAKV$^{\mathrm{D}}$PPTKAKAK(Nal)K(Nal)-NH$_{\mathrm{2}})$ the non-beta turn valines were replaced with Napthylalanine and alanine amino acid residues, with hydrophobic side chains of larger and smaller volume, respectively, than valine. Thus, formation of a `lock and key' type structure was attempted in the hydrophobic core of the peptide fibrils that would eliminate fibril branching. ~The folding and network formation of LNK1 has been studied by Circular Dichroism spectroscopy (CD), Transmission Electron Microscopy (TEM) and Oscillatory Rheology. Preliminary rheological characterization suggests the elimination of branching in the fibrils and also a possibility that LNK1 networks, unlike MAX1, are just nanofibrillar suspensions rather than permanently physically crosslinked hydrogels. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F30.00015: Microstructure and rheology of a thermoreversible gel under large amplitude oscillatory shear (LAOS) deformation using time-resolved oscillatory rheo-small-angle neutron scattering (tOr-SANS) Jung Min Kim, A. Kate Gurnon, Norman Wagner, Aaron Eberle Large amplitude oscillatory shear (LAOS) rheology is an effective way of studying the nonlinear dynamics of complex fluids. Here, we present a new method for a direct, quantitative study of the microstructure under LAOS deformation in the framework of the alignment factor, \textit{Af.} We use a model thermoreversible adhesive hard-sphere system composed of octadecyl-coated silica particles suspended in $n$-tetradecane. With temperature the particle potential is controlled and the system is shifted from behaving as a near hard-sphere to an adhesive hard-sphere system leading to aggregation and ultimately a dynamical arrest transition to macroscopic gelation. Time-resolved oscillatory rheo-small-angle neutron scattering (tOr-SANS) measurements in the 1-3 plane are performed by stroboscopically probing the structural evolution as a function of time during LAOS. Under strong shear, the 2D scattering pattern of the system in the gelled state exhibits a strong anisotropy commonly known as a ``butterfly'' pattern, which corresponds to the stretching of the microstructure along the flow direction. The first structure-Lissajous plots of this model system are presented in terms of an order parameter and \textit{Af} as a function of instantaneous strain and strain rate. This new analysis demonstrates a novel method for simultaneously measuring the rheology and microstructure during a time-dependent deformation (LAOS). [Preview Abstract] |
Session F31: Focus Session: Nano to Meso-Scale Structure in Ordered Soft Matter: Liquid Crystal Structure, Dynamics and Function II
Sponsoring Units: DPOLYChair: Chinedum Osuji, Yale University
Room: 339
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F31.00001: POLYMER PHYSICS PRIZE BREAK
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Tuesday, March 19, 2013 8:36AM - 8:48AM |
F31.00002: Liquid Crystals of Disks of Controlled Aspect Ratios Zhengdong Cheng, Min Shuai, Andres F. Mejia Nanoparticles with quasi two-dimensional shapes serve as building blocks for discotic colloidal liquid crystals. However, due to difficulty of synthesis and especially shape-tuning of disk-shaped nanoparticles, good model systems for the study of discotic colloidal liquid crystals are hard to found. $\alpha $-zirconium phosphate (ZrP) crystals synthesized through hydrothermal treatment has regular disk shapes and controllable size, thickness, as well as size polydispersity. We experimentally illustrate that aqueous suspensions of these ZrP disks form stable liquid crystal phase easily. By choosing the thickness of the disks, an iridescent liquid crystal phase has been achieved. The critical concentration of the phase transition was found to be dependent on aspect ratios. We will also discuss our recent results on the phase diagram of discotic liquid crystals as a function of aspect ratio and particle concentration using ZrP monolayers and wax disks. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F31.00003: Synthesis and Self-Assembly Behaviors of Polyhedral Oligomeric Silsesquioxane Based Giant Molecular Shape Amphiphiles Kan Yue, Xinfei Yu, Chang Liu, Wen-Bin Zhang, Stephen Cheng Recently, our group has focus on the synthesis and characterization of novel giant molecular shape amphiphiles (GMSAs) based on functionalized molecular nanoparticles (MNPs), such as polyhedral oligomeric silsesquioxane (POSS), tethered with polymeric tails. A general synthetic method via the combination of sequential ?click? reactions has been developed and several model GMSAs with various tail lengths and distinct molecular topologies, which can be referred as the ?giant surfactants?, ?giant lipids?, ?giant gemini surfactants?, and ?giant bolaform surfactants? etc., have been demonstrated. Studies on their self-assembly behaviors in the bulk have revealed the formation of different ordered mesophase structures with feature sizes around 10 nanometers, which have been investigated in detail by small angle X-ray scattering (SAXS) technique and transmission electron microscopy (TEM). These findings have general implications on understanding the underlying principles of self-assembly behaviors of GMSAs, and might have potential applications in nano-patterning technology. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F31.00004: 2D Smectic of a T-shaped Liquid Crystal Mesogen D. Chen, D.A. Coleman, C. Zhu, N. Chattham, X. Cheng, C. Tschierske, J.E. Maclennan, M.A. Glaser, N.A. Clark We report structural studies of a T-shaped mesogenic molecule. Upon cooling from the isotropic, the molecules first form a lamellar phase, with the molecular tails organized into sheets and the head groups isotropic in the plane of the lamellae. On further cooling, the head groups self-assemble into phases with 2D nematic and smectic order. The 2D smectic has only short-range positional correlations, with dislocations in the layering. The development of the 2D smectic reduces the long-range correlations of the fundamental lamellar structure, with the system evolving into a biaxial nematic with the T-shaped molecules ordered in three dimensions but with only short-range correlations. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F31.00005: Hierarchical Structure in Liquid Crystalline Polymers and Block Copolymers Invited Speaker: Rajeswari Kasi We are interested in developing general molecular engineering approaches to liquid crystalline and semicrystalline brush random and block copolymers. These polymers self-assemble into hierarchical supramolecular nano structures with organization over several length scales that allows for evolution for unique property and function. In these polymeric libraries, we exploit liquid crystalline units for its responsive optical and mechanical features and semicrystalline brush units for its mechanical and thermal features. These materials are useful for applications in 1D photonic band gap materials as well as templates for preparation of nanoporous scaffolds. A series of liquid crystalline monomers and semicrystalline brush macromonomers are synthesized and polymerized by ring opening metathesis methods to prepare liquid crystalline random brush copolymers and liquid crystalline block brush copolymers. All these copolymers exhibit atleast two levels of hierarchy: LC mesophase assembly and brush microphase segregation due to incompatibility with the LC phase. We investigate the phase evolution of these materials based on composition, molecular weight and length of the semicrystalline brush and we map out the phase behavior by a variety of techniques including thermal analysis, UV visible analysis, polarized optical microscopy, temperature controlled small angle x-ray, wide-angle x-ray, electron microscopy, dynamic mechanical analysis. In addition to thermal and microstructural analysis, we determine the order-disorder transition of the self-assembled copolymers. In closing, by exploiting molecular architecture and composition to modulate the self-assembly, hierarchical structure at multiple length scales can be obtained and preserved which allows for the creation of unique 1D-photonic band gap materials as well as nanoporous scaffolds. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F31.00006: Triply Periodic Multiply Continuous Lyotropic Liquid Crystals Derived from Gemini Dicarboxylate Surfactants Gregory Sorenson, Mahesh Mahanthappa A delicate balance of non-covalent interactions drives the supramolecular assembly of hydrated small molecule amphiphiles into aqueous lyotropic liquid crystals (LLCs). High symmetry multiply continuous phases, exemplified by the gyroid phase, are particularly desirable for many applications due to their interpenetrating hydrophilic and hydrophobic domains with well-defined chemical functionality decorating the interface between the two domains. However, these high symmetry assemblies are often difficult to obtain due to limited levels of hydration and temperature ranges over which they are accessible. Recent work suggests that small molecule amphiphiles known as ``gemini'' surfactants readily form these lyotropic network phases. Herein we report the lyotropic phase behaviors of a new class of dicarboxylate gemini surfactants that form stable, multiply continuous, high symmetry network structures over broad hydration and temperature ranges. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F31.00007: Alignment and Stiffening of Liquid Crystal Elastomers under Dynamic Compression Aditya Agrawal, Prabir Patra, Pulickel Ajayan, Walter Chapman, Rafael Verduzco Biological tissues have the remarkable ability to remodel and repair in response to disease, injury, and mechanical stresses, a phenomenon known ``functional adaptation'' or ``remodeling''. Herein, we report similar behavior in polydomain liquid crystal elastomers. Liquid crystal elastomers dramatically increase in stiffness by up to 90 {\%} under low-amplitude, repetitive (dynamic) compression. By studying a systematic series of materials, we demonstrate that the stiffness increase is directly influenced by the liquid crystal content of the elastomers, the presence of a nematic liquid crystal phase and the use of a dynamic as opposed to static deformation. Through a combination of rheological measurements, polarizing optical microscopy and 2-D X-ray diffraction, we demonstrate that self-stiffening arises due to rotations of the nematic director in response to dynamic compression, and show that the behavior is consistent with the theory for nematic rubber elasticity. Previous work with liquid crystal elastomers has focused primarily on `soft elastic' deformations at large strains, but our findings indicate rich behavior at previously overlooked low-strain, dynamic deformations. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F31.00008: Large area Magnetic alignment of a Cylindrical liquid crystalline Brush Block Copolymer for Generating Nanoporous Templates Manesh Gopinadhan, Prashant Deshmukh, Pawel Majewski, Rajeswari Kasi, Chinedum Osuji Magnetic fields have been shown to be a facile route to directing the self-assembly of both lamellar and cylinder forming diamagnetic block copolymer nanostructures over macroscopic areas. Here we present magnetic field directed self-assembly of a novel strongly segregated cylindrical block copolymer with polynorbonene backbone bearing a poly(lactic acid) PLA minority cylindrical brush block which is amenable to selective removal by chemical etching while a ciano-biphenyl species forms the liquid crystalline magneto-responsive block. We found that the PLA brush length is critical to obtain hexagonally packed cylindrical domains, while the system was not susceptible to magnetic field alignment due to large separation of LC clearing transition and order-disorder transition temperatures. Surprisingly, doping a small amount of free ciano-biphenyl mesogens induces strong and fast alignment of block copolymer microdomains under 5T magnetic field. Subsequent etching of the PLA block from the aligned material and cross-linking norbonene backbone by thiol-ene chemistry yield highly aligned nanoporous membranes which could potentially serve as templates for the synthesis of nanomaterials. Magnetic field directed self-assembly thus offer a simple route to generate nanoporous templates where porosity and the dimensions can be controlled by the molecular parameters. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F31.00009: Observations of phase behavior of chiral mesogens in diastereomeric domains of bent-core helical nanofilament networks B. Horanyi, D. Chen, E. Korblova, D.M. Walba, J.E. Maclennan, M.A. Glaser, N.A. Clark Blends of NOBOW, a helical nanofilament-forming B4 bent-core liquid crystal with organic guest molecules are completely mixed in the high temperature, isotropic phase. Upon cooling, the B4 filaments nucleate from the isotropic melt and grow into a homochiral dendritic network which acts as a porous medium of large internal area, with the guest material confined to nanoscale interstitial volumes between the twisted filaments. A typical sample is a conglomerate of independently nucleated left- and right-handed B4 domains many tens of $\mu $m across. Polarized optical microscopy reveals that chiral liquid crystal guest materials nanoconfined in the helical nanofilament networks form diastereomeric domains with distinct thermal behavior. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F31.00010: The Role of Confinement on Biologically Derived Liquid Crystals Marguerite Brown, Daniel Blair Suspensions of stabilized, dilute microtubules provide a versatile model system for understanding the structure of confined liquid crystals. Microtubule solutions are easily transported as a simple monomeric fluid that can easily be polymerized into rod-like macromolecules after they are confined within quasi-2D geometries (microfluidics). Using polarization and confocal microscopy, we analyze the structure of liquid crystals in a variety of geometries. We will present results on the role of confinement, boundary conditions and concentration, specifically discussing how each variable alters nematic ordering. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F31.00011: Bio-based liquid crystalline polyesters Carolus Wilsens, Sanjay Rastogi The reported thin-film polymerization has been used as a screening method in order to find bio-based liquid crystalline polyesters with convenient melting temperatures for melt-processing purposes. An in depth study of the structural, morphological and chemical changes occurring during the ongoing polycondensation reactions of these polymers have been performed. Structural and conformational changes during polymerization for different compositions have been followed by time resolved X-ray and Infrared spectroscopy. In this study, bio-based monomers such as vanillic acid and 2,5-furandicarboxylic acid are successfully incorporated in liquid crystalline polyesters and it is shown that bio-based liquid crystalline polymers with high aromatic content and convenient processing temperatures can be synthesized. [Preview Abstract] |
Tuesday, March 19, 2013 11:00AM - 11:12AM |
F31.00012: The liquid-crystal phase transition in suspensions of soft particles Miguel Pelaez-Fernandez, Anton Souslov, L. Andrew Lyon, Paul M. Goldbart, Alberto Fernandez-Nieves We experimentally determine the equation of state of swollen microgel suspensions: $\pi=\pi(\zeta, N_c)$, with $\pi$ the suspension osmotic pressure, $\zeta$ the generalized volume fraction and $N_c$ the number of chains per particle, which determines the microgel stiffness. We find that the melting and freezing lines shift to higher $\zeta$ as the particle becomes softer. Concomitantly, the liquid-crystal coexistence region becomes wider. We suggest that this behavior is due to the internal degrees of freedom of the microgel particles, which increase as the particle becomes softer. In this case, crystallization requires freezing some of these additional degrees of freedom resulting in the observed widening of the coexistence region. Our experiments provide the starting point to understand how the single-particle elasticity affects the phase behavior of colloidal suspensions. [Preview Abstract] |
Session F32: Polymer Nanocomposites II
Sponsoring Units: DPOLYChair: Nigel Clarke, University of Sheffield
Room: 340
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F32.00001: POLYMER PHYSICS PRIZE BREAK
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Tuesday, March 19, 2013 8:36AM - 8:48AM |
F32.00002: Polystyrene Nanocomposites: Shear and Bulk Rheology and PVT Behavior Ran Tao, Sindee Simon One potential strategy to mitigate thermal residual stresses in polymer materials is to reduce the thermal pressure coefficient, a product of the bulk modulus and thermal expansion coefficient. Recent model predictions show that the liquid bulk modulus could decrease by incorporation of well-dispersed spherical nanoparticles into polymer matrix. In this work, the pressure-volume-temperature (PVT) behavior and pressure relaxation response of a 10 wt{\%} silica nanoparticle-filled polystyrene nanocomposite sample are measured using a custom-built pressurizable dilatometer. The glass transition temperature (T$_{g})$ is calculated as a function of pressure from the PVT data, and the PVT data are fitted to the Tait equation. Isothermal pressure relaxation experiments are performed in the vicinity of the pressure-dependent T$_{g}$, from which the time-dependent bulk modulus is calculated. The temperature dependence of the horizontal shift factors is examined and compared to those obtained from the shear response. In addition, the retardation spectra for the bulk and shear responses are compared and the implications are discussed. The results are consistent with literature prediction indicating that bulk modulus will increase in aggregated nanocomposites system. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F32.00003: Simulating Fiber Aggregation in Shear Flow with Dissipative Particle Dynamics Justin Stimatze, David Egolf, Jeffrey Urbach We have developed a mesoscale simulation of fiber aggregation using LAMMPS and its implementation of dissipative particle dynamics. Fiber-fiber interactions are approximated by combinations of standard pairwise forces, allowing exploration of multiple interaction-influenced fiber behaviors such as aggregation and bundling. We determine viscosity, stresses, fluid velocity field, and fiber forces while simulating the evolution of a model fiber system in shear flow. Preliminary simulations supported by AFOSR HPC resources have demonstrated several aggregate types dependent on system parameters. Explorations of fiber interaction mechanisms and parameters may enable greater insight into processes such as nanocomposite material manufacturing and silk fibrillation. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F32.00004: Effects of functional groups and ionization on the structure of alkanethiol coated gold nanoparticles Dan S. Bolintineanu, J. Matthew D. Lane, Gary S. Grest We report fully atomistic molecular dynamics simulations of alkanethiol coated gold nanoparticles solvated in water and decane. The structure of the coatings is analyzed as a function of various functional end groups, including amine and carboxyl groups in different neutralization states. We study the effects of charge in the end groups for two different chain lengths (10 and 18 carbons) and different counterions (mono- and divalent). For the longer alkanes we find significant local phase segregation of chains on the nanoparticle surface, which results in highly asymmetric coating structures. In general, the charged end groups attenuate this effect by enhancing the water solubility of the nanoparticles. Based on the coating structures and density profiles, we can qualitatively infer the overall solubility of the nanoparticles. The asymmetry in the alkanethiol coatings is also likely to have a significant effect on aggregation behavior. More importantly, our simulations suggest the ability to modulate end group charge states (e.g. by changing the pH of the solution) in order to control coating structure, and therefore control solubility and aggregation behavior. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F32.00005: Shear and Extensional Flow-Induced Particle Orientation in Polypropylene/Clay Nanocomposites Wesley Burghardt, Erica McCready Synchrotron-based in situ x-ray scattering is used to monitor the orientation of dispersed particles in molten polypropylene/clay nanocomposite melts during flow. Nanocomposite samples were prepared via twin screw extrusion processing, and the degree of clay exfoliation assessed in terms of the magnitude of the low frequency enhancement in viscoelasticity. In shear flow, an annular cone and plate flow cell is used which allows measurement of the degree and direction of particle orientation in the flow-gradient (1-2) plane. Samples were also studied in extensional flow, using an SER extensional flow fixture installed in a custom-built convection oven that provides x-ray access. In both shear and extensional flow, only a moderate degree of particle orientation is observed. Extensional flow studies are complicated by (i) the tendency of samples to fail at moderate Hency strain, and (ii) a heterogeneous initial distribution of particle orientation in the SER specimens, prepared by compression molding of extruded pellets of the nanocomposite. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F32.00006: Effect of Ionic Groups on the Assembly of Polymer-Grafted Magnetic Nanoparticles Yang Jiao, Pinar Akcora Hydrophobic iron oxide nanoparticles grafted with hydrophobic polymer chains at low grafting density assemble into long strings of nanoparticles. Brush-brush entanglement and the effective dipolar interactions of these elongated clusters drive this aggregation process. In this work, we investigate the influence of ionic attractions on the morphologies of these polymer functionalized nanoparticles at different grafting densities. The effect of sulfonic group locations incorporated into poly(styrene) chains on the aggregation state of nanoparticles will be discussed with small-angle x-ray scattering measurements in solution and melts. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F32.00007: Synthesis and Assembly of Janus Gold Nanorods in Polymer Matrices Robert C. Ferrier, Hyun-Su Lee, Michael J.A. Hore, Matthew Caporizzo, David M. Eckmann, Russell J. Composto Gold nanorods (AuNRs) possess unique optical properties that depend on the local orientation and separation of the individual rods. Previous research focused on assembling AuNRs either end-to-end or side-by-side in solution. Our group has explored the dispersion of polymer grafted AuNRs in polymer matrices. The present work investigates the end-to-end assembly of polymer grafted Janus AuNRs (JNRs) in polymer thin films. JNRs are synthesized by exploiting the anisotropic surface chemistry of CTAB-coated AuNRs. Poly(ethylene oxide) (PEO) brushes are grafted to the side of the AuNR, while leaving the ends unmodified. Using alkane dithiols of different lengths, the JNRs are covalently linked in various solutions and the optical properties are characterized by UV/visible spectroscopy. Linked JNRs are spin-cast in poly(methyl metracrylate) (PMMA) or PEO thin films and characterized via electron microscopy and UV/visible spectroscopy. Using this procedure, linked JNRs can be dispersed in a polymer matrix and linked end-to-end to control the optical properties of coating. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F32.00008: Dispersion of Polymer-Grafted Nanorods in Polymer Films Amalie L. Frischknecht, Michael J. A. Hore, Russell J. Composto Gold nanorods (NRs) exhibit unique optical properties, i.e. their surface plasmon resonances, which can be tuned by the separation between the NRs. One strategy for controlling the assembly of NRs in a polymer film is to coat them with a polymer brush. The resulting dispersion or aggregation of the rods depends on the details of their interactions, which we examine using both theory and experiment. Classical density functional theory (DFT) and self-consistent field theory calculations of the structure of the brush around an isolated NR in a polymer melt predict a gradual transition from a ``wet'' to a ``dry'' brush as the NR radius, the grafting density, and/or the ratio of matrix to brush chain lengths is increased. DFT calculations of the interaction free energy between two NRs find an attractive well at intermediate NR separations. The strength of the attraction increases as the brushes become more dry. Including the van der Waals attractions between the NRs gives an estimate of their total interaction free energy, which can be used to predict when the NRs are dispersed or aggregated. A dispersion map shows good agreement between DFT calculations and experimental observations. Our calculations can be used as a guide to the design rules for tuning NR assembly in polymer films. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F32.00009: Dispersion of small nanoparticles in random copolymer melts Debapriya Banerjee, Kenneth S. Schweizer, Bobby Sumpter, Mark D. Dadmun Microscopic PRISM integral equation theory is applied to study the structure and miscibility of extremely small nanoparticles (e.g., C60 buckyballs) dissolved at low concentrations in a chemically heterogeneous random AB copolymer melt. The effects of polymer stiffness, melt isothermal compressibility, and the strength and spatial range of polymer-particle, polymer-polymer, and filler-filler attractions on the miscibility of the nanoparticles are studied. Complex, subtle and highly nonuniversal behavior is found. Appropriate tuning of the chemical interactions can result in the emergence of an intermediate range of random copolymer compositions where miscibility is maximized and larger than in either homopolymer limit. The physical origin involves a competition between depletion, steric stabilization, and bridging polymer-mediated interactions. When the direct interaction between the small fillers is tuned to model fullerenes, the potentials of mean force exhibit a competition between contact aggregation and bridging, and miscibility is enhanced with decreasing contact aggregation until the onset of bridging. Qualitative comparisons to recent experiments have been performed using attractive interaction strengths motivated by quantum chemical calculations. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F32.00010: Dispersion of Soft Nanoparticles in a Chemically Identical Polymer Matrix Dilru Ratnaweera, D. Baskaran, D. Holley, M. Ruppel, J. Mays, V. Urban, Mark Dadmun The mechanical, thermal and rheological properties of polymers can be improved by embedding organic nanoparticles (NPs). However, controlling the dispersion of NPs is often challenging due to thermodynamic and kinetic incompatibilities between particles and matrices. The current work focuses on the dispersion of chemically identical NPs in a polystyrene matrix. These NPs were made through a micro-emulsion technique using styrene and divinylbenzene (DVB) monomers. Polystyrene nanoparticles with controlled interfacial roughness and targeted styrene densities were achieved by controlling DVB volume fraction during synthesis. The dispersion of these NPs in deuterated polystyrene matrices was followed by Small Angle Neutron Scattering as a function of NP concentration and matrix molecular weight. At low NP concentrations, individual NPs are well distributed in the matrix, while aggregates were formed at higher concentrations in high molecular weight polymer matrices. Shape of the aggregates as well as the minimum concentration of NPs required to form aggregates were affected by the surface roughness and softness of the NPs. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F32.00011: Using Polydispersity in Polymer Grafted Nanoparticles for Tuning Morphology in Polymer Nanocomposites Tyler Martin, Arthi Jayaraman Polymer nanocomposites, consisting of nanoscale additives in a polymer matrix, are used in many applications where high thermal and wear resistance is important e.g. automotive tires. To achieve uniform mechanical and thermal properties of the nanocomposite, the nanoparticles need to be well dispersed in the polymer matrix. One way to control the nanoparticle spatial organization in the polymer matrix is by grafting the nanoparticle surface with polymers that are chemically similar to the matrix polymer and tuning the effective interactions between the particles by simply tuning the grafting density, graft length, matrix length, particle size, filler concentration, and matrix density. In this study, we demonstrate that polydisperse polymer grafts can stabilize dispersions of polymer grafted nanoparticles in a polymer matrix in cases where monodisperse grafts would cause aggregation of particles. The change in the effective inter-particle interactions with increasing polydisersity is because of increased wetting of the grafted polymers by the matrix polymers. The implication that polydispersity can stabilize particle dispersions in matrix shows that it can be used as a design tool to program inter-particle interactions in a polymer matrix. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F32.00012: Synchrotron radiation studies of the evolution dynamics of self-assembled nanoparticle Langmuir films Yeling Dai, Oleg Shpyrko, Binhua Lin, Mati Meron, Kyungil Kim, Brian Leahy Nanoparticle Langmuir films self-assembled on a liquid sub-phase represent a class of systems that is of great interest for studies of phase transitions in quasi-2D systems, chemical self-assembly, surfactant behavior and biologically relevant monolayers and membranes. We utilize Grazing Incidence X-ray Off-Specular (GIXOS) scattering to study elastic properties, structure and surface fluctuating modes of these systems. We present here a comparison between the GIXOS and the X-ray Reflectivity (XR) measurements, where XR is conventionally used to provide structural information of samples along the surface-normal direction. We further present a detailed analysis of GIXOS data from the self-assembled nanoparticle films and describe how we use it to obtain quantitative, Angstrom-resolution details of the electron density profile normal to the surface, complementary to that obtained with XR. Additionally, GIXOS provides us with improved temporal resolution that allows us to directly study the evolution dynamics of self-assembled nanoparticle films in response to lateral compression. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F32.00013: HipGISAXS: A Massively Parallel Code for GISAXS Simulation Slim Chourou, Abhinav Sarje, Xiaoye Li, Elaine Chan, Alexander Hexemer Grazing Incidence Small-Angle Scattering (GISAXS) is a valuable experimental technique in probing nanostructures of relevance to polymer science. New high-performance computing algorithms, codes, and software tools have been implemented to analyze GISAXS images generated at synchrotron light sources. We have developed flexible massively parallel GISAXS simulation software ``HipGISAXS'' based on the Distorted Wave Born Approximation (DWBA). The software computes the diffraction pattern for any given superposition of custom shapes or morphologies in a user-defined region of the reciprocal space for all possible grazing incidence angles and sample rotations. This flexibility allows a straightforward study of a wide variety of possible polymer topologies and assemblies whether embedded in a thin film or a multilayered structure. Hence, this code enables guided investigations of the morphological and dynamical properties of relevance in various applications. The current parallel code is capable of computing GISAXS images for highly complex structures and with high resolutions and attaining speedups of 200x on a single-node GPU compared to the sequential code. Moreover, the multi-GPU (CPU) code achieved additional 900x (4000x) speedup on 930 GPU (6000 CPU) nodes. [Preview Abstract] |
Session F33: Focus Session: Organic Electronics and Photonics - Light Emission and Management
Sponsoring Units: DMPChair: Russell Holmes, University of Minnesota
Room: 341
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F33.00001: POLYMER PHYSICS PRIZE BREAK
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Tuesday, March 19, 2013 8:36AM - 8:48AM |
F33.00002: Pure Bending Loss in Nanowire Waveguides Jaeyeon Pyo, Ji Tae Kim, Jewon Yoo, Jung Ho Je One of the major concerns in designing waveguides is unavoidable bending that causes energy loss due to the distortion of modal field. Bending loss in nanowire waveguides has been studied while including substrate coupling loss. Pure bending loss unaffected by substrate coupling in nanowire waveguides still remains unclear. A challenging task in study of pure bending loss is to introduce bending on nanowire waveguides in the air and to tune the radius of bending. We report the characterization of pure bending loss in nanowire waveguides by bending a vertical freestanding nanowire in the air. Specifically, vertical freestanding active nanowire waveguides of MEH-PPV have been fabricated by our meniscus-guided method. To characterize pure bending loss, desired bending was remotely introduced by applying electrostatic force near the top end of the waveguide. Finite-difference-time-domain simulation was performed to confirm the experimental result. We show that the bending losses, by conventional experimental approaches of nanowires rested on substrates, were strongly overestimated attributed to the coupling of the enhanced evanescent field to the substrate. We suggest that our system could be also utilized for studying various intrinsic properties of nanowire waveguides. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F33.00003: Molecular shear and the induced massive enhancement of conjugated polymer MEH-PPV photoluminescence by solvent-dewetting Chi-Ching Liu, Tsang-Lang Lin, Gunter Reiter, Arnold C.-M. Yang The molecular flows triggered by dewetting above Tg in ultrathin polymer films were shown previously generating huge photoluminescence (PL) enhancements for conjugated polymers contained within. By means of annealing in solvent vapor at room temperature, MEH-PPV molecules dispersed in inert polystyrene (PS) manifested massive PL enhancements up to $\sim$ 10 folds when flowed into tiny droplets and residual layer. The enhancement was independent of MEH-PPV chain length but, in contrast to thermal dewetting, decreasing with MEH-PPV concentration ($c$). In addition, the blue shift accompanying thermal dewetting was also reduced. As annealing continued on, the blue shift reversed, illustrating the increase of conjugation length under stretching. The transient blue shift increased with PS molecular weight, unveiling the alteration of inter-segmental chain entanglements up to this stage of dewetting. Surprisingly, vapor of poorer solvent induced larger PL enhancements with narrower transient blue shifts, revealing that solvent was effective in inducing molecular flows, even when in the plasticizing Feakean precursor, relaxing the residual stresses and simultaneously stretching polymer chains for dramatically enhanced optoelectronic efficiencies. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F33.00004: Multilayer polymer light emitting devices Zachary Barcikowski, Adam Thomas, Marian Tzolov The interplay of device layers and their interfaces is a major area of study in Polymer Light Emitting Devices (PLEDs). Many factors such as the degradation, efficiency, and overall performance depend on how these layers interact with each other. A fundamental understanding of the interfaces of these layers can lend to better performing devices using a multitude of organic polymers deposited in conjunction with each other in several ways. We have studied basic PLED devices in which we vary the emissive layer used, along with final bake temperatures. Devices include a glass substrate with Indium Tin Oxide anode, Aluminum cathode, and Plexcore Hole Injection layer. The active polymer films were spin casted from solution of MEH-PPV and PFO. Single layer and dual layers of several polymers are studied by examining current-voltage characteristics, film densities, impedance measurements, light emission, and efficiency calculations. We have found that not only do dual layers positively alter the performance of the device in the majority of cases, but the solvents in which each layer is originally in when deposited affects the formation of the interface, thereby altering the device mechanisms. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F33.00005: Color change in organic light-emitting diodes using the magnetic field effect Tek Basel, Dali Sun, Bhoj Gautam, Eitan Ehrenfreund, Z. Valy Vardeny The magnetic field effect has been widely used to generate magneto-electroluminescence (MEL) in organic light-emitting diodes (OLEDs). We have used the MEL effect to change the emission color from OLED in which the active layer is composed of a host polymer with fluorescence (FL) emission and a guest, heavy atom-based molecule with phosphorescence (PH) emission. The color change has been studied as a function of the guest/host weight ratio, and the optimal ratio was determined. The underlying mechanism of the magnetic-field induced color change is the difference that exists between the MEL intensity of the FL emission band respect to that of the PH emission band. The MEL difference between the two types of emission bands will be thoroughly discussed within models used to explain the MEL in organic devices. Research sponsored by National Science Foundation-Material Science {\&} Engineering Center (NSF-MRSEC), University of Utah. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 10:00AM |
F33.00006: Highly efficient organic light-emitting diodes by delayed fluorescence Invited Speaker: Chihaya Adachi Although typical organic molecules only contain carbon (C), hydrogen (H), nitrogen (N) and oxygen (O) atoms, the unique bonding of C involving sp3, sp2 and sp hybrid orbitals enables generation of very complicated molecular architectures that have extensive functions in a wide variety of organisms and industrial products. In the last two decades, the allure of the unlimited freedom of design of organic molecules has shifted a significant proportion of electronics research from inorganic into organic materials. In particular, great advances have been achieved in organic light-emitting diodes (OLEDs). First-generation OLEDs containing fluorescent molecules have progressed to second-generation ones using phosphorescent molecules, which is an attractive design for practical electronics. Herein, new organic electroluminescent (EL) molecules lacking precious metals are presented. The energy gap between the singlet (S1) and triplet (T1) excited states is minimized by strategic design, promoting highly efficient spin up-conversion from T1 to S1 states while maintaining a high radiative decay rate of \textgreater 106/s, leading to a high fluorescence efficiency of \textgreater 90{\%}. Using these unique molecules, a very high external EL efficiency of \textgreater 19{\%} is realised, which is comparable to those of high-efficiency phosphorescence-based OLEDs. These molecules harvest both singlet and triplet excitons for light emission through fluorescence decay channels. We call this new luminescence concept ``hyperfluorescence.'' [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F33.00007: Kinetic Monte Carlo simulation of organic devices Alison Walker, Edward Wright At Bath, we have developed a model of organic devices that links morphology (packing arrangements) to device characteristics. The model, based on the dynamical Monte Carlo approach pioneered in surface physics, allows us to include interaction processes between different species on many different timescales. In this talk I will show how we have used this approach to compare organic solar cells of rod, blend and gyroid morphologies and to model the influence of interlayers, layers added to improve efficiency and lifetime, in organic light emitting devices, OLEDs. We have developed the model to allow it to distinguish between triplet and singlet excitons and allows for the interactions of these species (triplet-triplet annihilation, triplet-singlet annihilation, triplet-polaron quenching). I will show our predictions for current-voltage-illumination characteristics (solar cells) and current-voltage-luminance characteristics (OLEDs). I will also show how through prediction of emission zone profiles in an OLED, we can gain insight into what determines changes in OLED efficiency with current and how in the longer term this approach can be used to address degradation. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F33.00008: Carrier Conduction and Light Emission by Modification of Poly(alkylfluorene) Interface under Vacuum Ultraviolet Light Irradiation Yutaka Ohmori, Hirotake Kajii, Daiki Terashima, Yusuke Kusumoto Organic field effect transistors (OFETs) have been extensively studied for flexible electronics. The characteristics of poly(9,9-dioctylfluorenyl-2,7-dyl) (F8) modified by thermal or light are strongly dependent on the carrier transport and optical characteristics. We investigate all solution-processed OFETs with Ag nano-ink as gate electrodes patterned by Vacuum Ultraviolet (VUV) (172 nm). Bi-layer gate insulators of amorphous fluoro-polymer CYTOP (Asahi Glass Corp.) and poly(methylmethacrylate) (PMMA) were used. Top-gate-type OFETs with ITO source/drain electrode utilizing F8 or poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as an active layer were fabricated, and investigated the carrier conduction and emission characteristic. Without VUV irradiation, both OFETs showed the ambipolar and light-emitting characteristics. On the other hand, F8 devices with VUV exhibited only p-type conduction. The quenching centers were generated in F8 layer by VUV irradiation, which are related to the electron trap sites at the interface. OFETs with F8BT showed both p- and n-type conduction even after VUV. F8BT suffers less damage by VUV and maintain light emission. Light emitting transistors were realized utilizing F8BT patterned by VUV irradiation. [Preview Abstract] |
Session F34: Charged Colloids with Short-Range Attractions I
Sponsoring Units: DPOLY DCMP DBIOChair: Yun Liu, University of Delaware/NIST
Room: 342
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F34.00001: POLYMER PHYSICS PRIZE BREAK
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Tuesday, March 19, 2013 8:36AM - 9:12AM |
F34.00002: A colloidal perspective of protein solutions manipulated by multivalent ions: Phase behavior and associated dynamics Invited Speaker: Frank Schreiber After a brief overview of interactions in aqueous protein solutions, we will discuss how ions can be used to manipulate these interactions and the associated phase behavior as well as the diffusion dynamics. We show that multivalent ions do not only influence the ionic strength and the resulting interactions including effective attraction, but lead to qualitatively new effects. Particular attention will be given to the reentrant condensation of proteins (F. Zhang et al, PRL 101 (2008) 148101; F. Zhang et al, Soft Matter 8 (2012) 1313) and its relationship with liquid-liquid phase separation and protein crystallization. In particular, we attempt to rationalize crystallization controlled by trivalent ions and discuss the role of specific ions and their impact on the effective interaction potential. These results are compared to the diffusion dynamics in these systems studied using neutron spectroscopy and light scattering (F. Roosen-Runge et al, PNAS 108 (2011) 11815; Heinen et al, Soft Matter 8 (2012) 1404) and the question of transient clusters is discussed. Finally, we critically discuss to which extent proteins can be described by colloidal concepts. The work was performed in collaboration with F. Zhang, T. Seydel, M. Hennig, F. Roosen-Runge, M. Skoda, R. Jacobs and others. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F34.00003: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F34.00004: Protein clusters in biomembranes Nicolas Destainville We propose that proteins embedded in lipidic bio-membranes can spontaneously self-organize into stable membrane nano-domains (or clusters), due to the competition between short-range attractive and longer-range repulsive forces between proteins, specific to these systems, and propagated by the lipidic membrane. We compare different long-range potentials (including notably three-body terms) and we demonstrate that the existence of cluster phases in this context should be quite generic. Furthermore, a real membrane contains hundreds of different protein species that are far from being randomly distributed in these nano-domains, which is crucial in terms of biological functions. We take this protein diversity into account by modulating protein-protein interactions both at short and longer range. Both theoretical and numerical investigations explain why protein clusters recruit only a few protein species, thus leading to cluster biological specialization. In this respect, we highlight that cluster phases can turn out to be an advantage at the biological level, for example by enhancing the cell response to external stimuli. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F34.00005: Assembly of Spherical Colloids by Short-range Out-of-plane Attraction and Long-range In-plane Repulsion Fuduo Ma, David T. Wu, Ning Wu The electric-field assembly of spherical colloids with isotropic surface properties has been studied in both two- and three- dimensions. Structures, such as FCC, HCP, and BCT crystals were observed. Recently, we have found surprisingly new types of structures within a previously unexplored experimental regime: low frequency regime (100 Hz to 10 kHz) and low salt concentrations (below 10$^{-4}$ M). At low particle concentrations, a family of well-defined clusters, ranging from 3 to 10 was observed. Statistical analysis of the population distribution reveled non-trivial peaks for trimers, tetramers, hexamers, and nonamers. We attribute these new types of non-planar structures to a short-range out-of-plane (the plane refers to the substrate) attraction and a long-range in-plane repulsion. For example, the double layer and in-plane dipolar repulsion could make bottom particles in the clusters separate from each other. While the out-of-plane dipolar attraction and particle-substrate attraction could be responsible for the formation of the clusters, i.e., the top central sphere is associated with the bottom spheres. Phase diagrams from experiments and simulation will be compared. These clusters could be used as building blocks for making photonic crystal, filtration, and plasmonic structures. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F34.00006: Colloidal stability in concentrated electrolyte solutions using large counterions Guillermo Guerrero Garcia, Pedro Gonzalez Mozuelos, Monica Olvera de la Cruz The stability of charged colloids in solution has been widely studied because it has ubiquitous applications in science and engineering. According to the classical DLVO theory, the electrostatic repulsion among charged colloids is significantly screened at high electrolyte concentrations. As a result, highly charged particles are expected to aggregate due to short-range van der Waals attractive interactions. Nevertheless, the classical DLVO theory relies in the linear Poisson-Boltzmann equation, which is usually restricted to low electrolyte concentrations and weakly charged colloids. In this work, we propose a novel mechanism beyond the classical DLVO picture that uses large counterions to prevent highly charged nanoparticles from aggregating in salt solutions with concentrations up to 1 M, in agreement with experimental observations. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F34.00007: Size and interaction-strength effects on the phase behavior of colloidal particle assemblies Ray Sehgal, David Ford, Dimitrios Maroudas We report the findings of a systematic computational study of the inherently complex phase behavior of thermodynamically small assemblies (clusters) of colloidal particles interacting via a potential that includes electrostatic repulsion and depletion-based short-ranged attraction. Using Monte Carlo umbrella sampling with coarse graining in two order parameters and a biasing scheme based on a genetic algorithm, we generate free-energy landscapes (FELs) that can indicate coexistence between fluid-like and crystalline phases. We have used the data mining technique of diffusion mapping to determine the dimensionality of the order-parameter space and assess the suitability of chosen order parameters that represent metrics of cluster density and crystallinity. Evaluation of phase behavior metrics from analysis of the FELs leads to predictions of conditions for formation of stable phases of such small colloidal clusters. A stable crystalline phase emerges as the number of particles in the assembly increases beyond a critical value. We find that the critical cluster size for the onset of crystallization decreases with increasing strength of the interparticle attraction. This FEL analysis also enables a mean-field description of the phase transitions undergone by these assemblies. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F34.00008: Distinguishing cluster phases as a unique scenario of intermediate range order in colloidal suspensions and protein solutions Paul Douglas Godfrin, Ramon Casta\~neda-Priego, Yun Liu, Norman J. Wagner A state of stable clusters is characterized by the reversible aggregation of colloidal particles to a finite, energetically favored size. Clusters can arise from a competition between short range attraction, driving aggregation, and long range repulsion, stabilizing clusters. Recent interest in systems with these interactions has brought attention to the formation of a low-q peak in the structure factor and the proposition that this peak directly indicates cluster formation. To understand the structures that produce a low-q peak, Metropolis Monte Carlo simulations are performed to calculate the partial structure factors by decomposing the system into cluster-cluster, monomer-monomer, and cross-correlations. We find that a low-q peak appears in fluids with strong cluster-cluster correlations but also in systems dominated by monomer-monomer correlations and percolated states. Thus, this low-q peak is more appropriately termed the intermediate range order (IRO) peak. Consequently, an IRO peak does not necessarily signal the existence of a cluster state in solution. Rather, it reflects the presence of a preferred length scale related to the two competing potential features. Determining cluster formation is most accurately accomplished by combining experiment with simulation. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F34.00009: Clustering and state diagram of charged colloids with short-range attraction in shear flows Alessio Zaccone, Massimo Morbidelli Under static conditions, the superposition of short-range (e.g. van der Waals) attraction and electrostatic repulsion gives rise to interesting phases such as equilibrium clusters in globular protein suspensions. What is much less understood is their behavior under external flow, which is important for the physiological aggregation of proteins and for industrial systems as well. I will present theoretical and experimental results showing that clustering of these systems in shear flow is characterized by the crossover from a reaction-limited clustering kinetics at low shear into a convection-dominated aggregation regime at high Peclet numbers. The kinetics may rise by up to many orders of magnitude in the crossover regime. This behavior is due to the singularly-perturbed character of the governing diffusion equation where the shear drift term induces a singularity and a boundary-layer at large interparticle distances. This understanding, together with a theoretical description of cluster breakup, is used to rationalize the peculiar nonequilibrium state diagram (including gelation) of these colloidal suspensions in shear flow with applications ranging from microfluidic self-assembly to proteins. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F34.00010: Percolation and local density fluctuations for a Colloidal System with competing interactions Nestor Valadez-Perez, Yun Liu, Ramon Castaneda-Priego The gelation is believed to result from the particle aggregation in a complex structure. The aggregate span in the entire volume gives it a capability for supporting stresses. Gelled systems possess a high degree of inhomogeneity, while locally the particles and their near neighbors present a defined array as can be seen in their coordination number and bonding angles. Using Monte Carlo simulations, we investigate the structure of a system of hard spheres interacting through a combined potential: a short-ranged Square Well (SW) and a long-ranged repulsive Yukawa potential (RY). We made an exhaustive study for several conditions of temperature (T*) and concentration ($\varphi$) corresponding to different repulsion strengths ($A$). Our results show that the percolation threshold is shifted to lower concentrations when the repulsion is increased, but this shift gradually disappears at low temperature. Besides we also computed the local density through the system; we particularly identified a length scale at which the density fluctuations are attenuated. This length coincides with the intermediated range order recently identified in protein systems. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F34.00011: Drag coefficient of an electrophoretic colloidal particle Kathryn Reddy, Ming-Tzo Wei, Joel A. Cohen, H. Daniel Ou-Yang Electrophoretic mobility is a measure to determine the electric charges on a colloidal particle. Zeta potential, a concept originated by Smoluchowski, has been a standard for quantifying the surface charge density for the electric double-layers that are thin compared to the particle radius. Various models have been suggested to improve Smoluchowski's theory for systems with Debye length not thin compared to the particle radius. Central to the issue is that the fluid flow due to the external field-induced counter-ion motion is unknown. Using optical tweezers to trap a colloidal particle in a low-frequency electric field, we found the drag coefficient of the particle in the field to be non-Stokes. We discuss how the non-Stokes' drag coefficient as a function of salt concentration and particle size may be useful for interpreting different models of Zeta potential. [Preview Abstract] |
Session F35: HTSC: ARPES and TR-ARPES
Sponsoring Units: DMPChair: Lex Kemper, Lawrence Berkeley National Laboratory
Room: 343
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F35.00001: Determinining the Critical Condition for Superconductivity in Bi$_{2}$Sr$_{2}$CaCuO$_{8+\delta }$ T. Reber, S. Parham, Y. Cao, J. Waugh, H. Li, N. Plumb, Q. Wang, G. Gu, Y. Yoshida, Y. Aiura, H. Eisaki, G. Arnold, D. Desssau Using the tomographic density of states (TDoS) ARPES-based technique we present a detailed study of the temperature and doping dependences of the pair-forming, represented by the gap magnitude ($\Delta )$, and pair-breaking, represented by the scattering rate ($\Gamma )$, ~processes in BSCCO.~ We find that $\Delta $ is finite through the superconducting transition, T$_{C}$, and ceases only at the higher temperature T$_{Pair}$, which corresponds with the T$_{Onset}$~from Nernst experiments rather than the T* tied to the antinodal pseudogap.~ Furthermore, we find $\Gamma $ is large and strongly temperature dependent and T$_{C}$~is strongly correlated with the ratio of $\Delta $/$\Gamma $.~ Consequently, the presence of pairs is not sufficient for superconductivity: the pair-breaking processes must also be diminished to the point that pairs have a long enough lifetime to develop long range coherence.~ [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F35.00002: The Effects of Impurities and Disorder on the ARPES lineshapes of Bi2212 Stephen Parham, Theodore Reber, Yue Cao, Justin Waugh, Haoxiang Li, Z. Xu, J. Schneeloch, R.D. Zhong, Genda Gu, Hiroshi Eisaki, Daniel Dessau We present a study of Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCuO$_{\mathrm{8}}$ doped with various magnetic impurities, Fe and Ni. Through the use of our Tomographic Density of States or TDoS technique, we show that these magnetic impurities decrease the lifetime of the Cooper pairs in this material, while leaving the superconducting gap essentially unchanged. These effects are masked using traditional MDC/EDC analyses and thus our results highlight the usefulness of the TDoS technique. Even without impurities, there is gap disorder in these materials that is readily seen in STM experiments. This gap disorder affects the TDoS lineshape, and we show that the disorder predicted from our TDoS technique is consistent with that measured through STM. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F35.00003: Observation of symmetry-distinct states proximate to the Fermi level in a high-Tc cuprate family Rui-Hua He, M. Hashimoto, K. Tanaka, A. Sorini, S.-K. Mo, T. Sasagawa, M. Fujita, T. Adachi, M. Enoki, S. Iikubo, N. Mannella, Hong Yao, M. Yi, W. Meevasana, Y. He, K. Yamada, Y. Koike, T. P. Devereaux, Z. Hussain, Z.-X. Shen Current understanding of cuprate superconductivity is based exclusively on an effective one-band electronic band structure formed by states of in-plane dx2-y2 symmetry. By studying the La-based cuprates with polarization-dependent angle-resolved photoemission spectroscopy, here we uncovered another group of states of distinct c-axis symmetry that coexists with the dx2-y2-symmetry states near the Fermi level and eluded previous detection. As functions of momentum and doping, these new states show overall different dispersion relations yet a qualitatively similar low-energy (pseudo)gapping behavior as the dx2-y2-symmetry states, until they become closely degenerate above $\sim$20{\%} doping. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F35.00004: Fine doping and temperature dependent ARPES study in deeply undersoped LSCO system Yu He, Ruihua He, Makoto Hashimoto, Sung-Kwan Mo, Seiki Komiya, Zhi-Xun Shen Deeply underdoped cuprates are known to be a host system for strong electron-phonon coupling physics. Set in the picture of lightly doped Mott insulator, extremely underdoped cuprates show prevailing evidence of polaron formation, as a natural result of strong bosonic interaction, which have gained support from both optical and transport measurements. Based on K. Shen and O. Roesch's pioneering work, we further studied fine doping and temperature dependence in the low-doping LSCO system, where antiferromagnetism and spin glass phases still persist. In this work, we will discuss the change in Fermi velocity in terms of doping, evolution of nodal gap as function of temperature and the possible contribution from lattice/spin degree of freedom in light of the small polaron's existence. Comparison with similar observations in manganites and iron-chalcogenides will be discussed to further address the ubiquity of the polaron physics in strongly correlated electron systems. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F35.00005: Laser-ARPES studies of dispersion kinks in cuprate phase diagram I.M. Vishik, M. Hashimoto, S. Johnston, W.-S. Lee, F. Schmitt, R.G. Moore, D.H. Lu, T. Sasagawa, S. Uchida, S. Ishida, K. Fujita, M. Ishikado, Y. Yoshida, H. Eisaki, R.-H. He, Z. Hussain, T.P. Devereaux, Z.-X. Shen Angle-resolved photoemission spectroscopy (ARPES) reveals ubiquitous dispersion kinks in cuprates which are manifestations of electron-boson coupling, potentially related to the superconductivity pairing mechanism. We report new temperature-, momentum-, and doping- dependent laser-ARPES measurements of the energy and coupling strength of the ubiquitous kink near 70 meV in Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+\delta }}$ (Bi-2212). The apparent kink energy below T$_{\mathrm{c}}$ is related to the mode energy modulo the maximum of the $d$-wave superconducting gap, and the pseudogap similarly needs to be considered above T$_{\mathrm{c}}$. Following improvements in data quality as well as recent comprehensive gap measurements throughout the phase diagram, we can better assess the phenomenology and origin of dispersion anomalies. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F35.00006: Autocorrelation of ARPES Spectra in the Pseudogap State of the Underdoped Curpates Jonathan Rameau, Hongbo Yang, Cherise Burton, Tim Kidd, Maurice Rice, Peter Johnson It has long been known that the normal state of the underdoped cuprate high $T_{c}$ superconductors is characterized by a pseudogap in the anti-nodal region of the Brillouin zone and a set of disconnected set of excitations at the chemical potential, in the nodal region, known as Fermi arcs. Recently, quantum oscillation and angle resolved photoemission spectroscopy (ARPES) experiments have indicated that these Fermi arcs actually represent one side of a Fermi surface reconstructed into nodal hole pockets. These pockets, as well as a number of consequences of their formation, have been shown to be well described by the phenomenological model of Yang Rice and Zhang (YRZ) for the single particle Green's function. Here, we show how the autocorrelation of ARPES spectra - so-called AC-ARPES -- acquired in the normal pseudogap state of the cuprates may be used to examine this phenomenon. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F35.00007: Examining Spin Fluctuations Pairing Model From Angle Resolved Photoemission Spectra Vivek Mishra, U. Chatterjee, J.C. Campuzano, M.R. Norman The mechanism of superconductivity is a long standing puzzle in the cuprates. Among various proposed models, pairing through the exchange of spin fluctuations is one of the leading candidates. Here we use spectral functions measured from angle resolved photoemission spectroscopy to calculate this pairing interaction within a random phase approximation, and then determine whether for a reasonable choice of the Hubbard parameter `$U$', we obtain a reasonable $T_c$. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F35.00008: Weak doping dependence of nodal transverse velocity in underdoped cuprates: explanation and significant implications Wei Ku Recent high resolution angular resolved photoemission spectroscopy has revealed a surprising nearly doping-independent transverse velocity in the nodal region of underdoped cuprates [1,2], in great contrast to the strong doping dependent superconducting transition temperature. This behavior is qualitatively incompatible with standard Bogoliubov quasi-particle picture currently employed in the field, and implies a fundamentally new regime of physics. Here, we will show analytically that this novel behavior follows naturally the recently derived kinetic nature of the quasi-particle gap in the strong binding limit [3]. This realization further confirms the strong pairing nature of the superconductivity and the novel nature of superconducting gap in underdoped cuprates. This study also suggests the crucial need for future experiments in the overdoped regime. [1] PRL 104, 207002 (2010) [2] PNAS 109, 18332 (2012) [3] PRX 1, 011011 (2011) [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F35.00009: Deduction of the self-energy and bosonic spectrum of Bi2212 from ARPES experiments Jin Mo Bok, Han-Yong Choi, Junfeng He, X.J. Zhou, Chandra M. Varma We analyzed the ARPES intensity of slightly underdoped ($T_{c}$=89K)and overdoped ($T_{c}$=82K) Bi2212 superconductors. The diagonal and off-diagonal self-energy, $\Sigma$ and $\phi$, were extracted by performing MDC (momentum distribution curve) fitting using superconducting Green's function at the tilt angle $\theta$ with respect to the nodal direction. Using the extracted self-energy as input, the Eliashberg function $\alpha^{2}F^{(+)}$ and $\alpha^{2}F^{(-)}$ corresponding to $\Sigma$ and $\phi$ were deduced by inverting the d-wave Eliashberg equation. Our main results are follows: (1) The deduced Eliashberg functions are similar for slightly underdoped and overdoped Bi2212. (2) The Eliashberg function $\alpha^{2}F^{(+)}$ has two peaks at ~15meV and ~50meV. Both peaks were enhanced as the tilt angle increases or temperature decreases. The Eliashberg function $\alpha^{2}F^{(-)}$ has one peak at ~15meV. Its energy scale is almost the same as the energy scale of the low energy peak of $\alpha^{2}F^{(+)}$. Then, we will compare our results with other experiments and modal calculations. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F35.00010: Quantum Monte Carlo simulations of ARPES spectra on correlated materials with electron-phonon coupling Elizabeth Nowadnick, Steven Johnston, Brian Moritz, Richard Scalettar, Thomas Devereaux Results from a variety of experiments have revealed the importance of the electron-phonon interaction in strongly correlated electron materials. In particular, ARPES experiments on the cuprates have observed signatures of polaron formation in the underdoped regime, indicative of strong electron-phonon coupling, as well as ``kinks'' in the dispersion in the doped compounds, which have been interpreted in a phonon picture. In order to study the role of electron-phonon coupling in strongly correlated systems, we simulate the single-band Hubbard-Holstein model using determinant quantum Monte Carlo, an approach that allows the non-perturbative study of strongly interacting systems, and treats the electron-electron and electron-phonon interactions on an equal footing. We present electronic spectral functions, which can be compared to ARPES results, as well as phonon spectral functions, which shed light on phonon renormalizations. In particular, we focus on an antiferromagnetic---charge density wave transition that occurs in the Hubbard-Holstein model at half filling, and present signatures of an emergent intermediate metallic phase that occurs between the two insulating phases. We also discuss the influence of phonons on the electronic dispersion. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F35.00011: Phonon-mode couplings studied by pump-probe photoemission Michael Sentef, Alexander Kemper, Brian Moritz, James Freericks, Zhi-Xun Shen, Thomas Devereaux Motivated by recent pump-probe photoemission experiments on cuprate superconductors, we show how the coupling of electrons to phonon modes at the same time leads to a prominent kink in the equilibrium band dispersion and to a distinct behavior of relaxation time scales in nonequilibrium experiments. Here, using the nonequilibrium solution of a model photoexcited electron-phonon system we show that the return of the electrons to equilibrium is governed by the equilibrium self-energy so that the phonon frequency sets a window for``slow'' versus ``fast'' relaxation. The overall structure of the relaxation spectroscopy in the time domain allows for a reliable and quantitative extraction of the electron-phonon coupling strength. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F35.00012: Probing Many-Body Interactions in High-Tc Superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ using Time- and Angle-Resolved Photoemission Tristan Miller, Wentao Zhang, Christopher Jozwiak, Christopher Smallwood, Hiroshi Eisaki, Dung-hai Lee, Alessandra Lanzara Laser-based time- and angle-resolved photoemission spectroscopy (trARPES) is a technique that uses an initial laser pulse to pump a system, and a second pulse to probe it by photoemission. By using trARPES on the high temperature superconductor, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$, we open a window into its many-body quasiparticle interactions. Here we report on the effect of pumping on the quasiparticle band structure of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. We will discuss the dynamics of this effect, and its relation to the superconducting state. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F35.00013: Gap Dynamics in Bi2212 Studied by Time- and Angle-Resolved Photoemission Christopher Smallwood, Wentao Zhang, Tristan Miller, Chris Jozwiak, Hiroshi Eisaki, Alessandra Lanzara Recent developments in ultrafast spectroscopy have shown that irradiating cuprate superconductors with intense, short pulses of light can induce nonequilibrium dynamics that may hold clues for understanding why the critical temperature (Tc) in these materials exceeds that of almost all other superconductors by an order of magnitude or more. Using a 1.5 eV pump pulse, and 5.9 eV probe, we use time- and angle-resolved photoemission spectroscopy (trARPES) to characterize the non-equilibrium dynamics of the gap and transient quasiparticle population in the cuprate superconductor Bi2212 (optimally doped, Tc=91 K). Correlations between these two quantities reveal clues for the underlying mechanism that drives the formation of the pseudogap and superconducting states in the hole-doped cuprates. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F35.00014: Time-resolved Ultrafast Spectroscopy Experiments on High Temperature Superconductor Bi2Sr2CaCu2O8 Jianqiao Meng, Georgi L. Dakovski, Jian-Xin Zhu, Peter S. Riseborough, Genda Gu, Steve M. Gilbertson, George Rodriguez, Jingbo Qi, Antoinette Taylor, Tomasz Durakiewicz Time-resolved~ultrafast spectroscopy experiments have been carried out on various dopings of high temperature superconductor Bi2Sr2CaCu2O8In this talk, we will report our observation and analysis of ultrafast dynamics in Bi2Sr2CaCu2O8, with special emphasis on the quasiparticle dynamics in the pseudogap and SC gap regimes. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F35.00015: Probing the Nodal Dynamical Electronic States in Bi$_2$Sr$_2$CaCu$_2$O$_8$ by Time- and Angle- Resolved Photoemission Wentao Zhang, Chris Smallwood, Tristan Miller, Chris Jozwiak, Hiroshi Eisaki, Dung-Hai Lee, Alessandra Lanzara Ultra-high resolution time- and angle- resolved photoemission (trARPES) measurements have been carried out on various dopings of Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. In this talk, we will report on the study of the dynamical quasiparticle excitation and recombination of the nodal electronic states in cuprate. The power of trARPES will be discussed in this talk. [Preview Abstract] |
Session F36: Superconductivity: Josephson Effect
Sponsoring Units: DCMPChair: Carmen Almasan, Kent State University
Room: 344
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F36.00001: High Temperature Superconducting Terahertz Emitters with Various Mesa Structures Kaveh Delfanazari, M. Tsujimoto, T. Kashiwagi, H. Asai, T. Kitamura, T. Yamamoto, M. Sawamura, K. Ishida, C. Watanabe, S. Sekimoto, H. Minami, M. Tachiki, T. Hattori, R. A. Klemm, K. Kadowaki In 2007, the first observation of the coherent terahertz (THz) electromagnetic (EM) waves from a mesa structures of intrinsic Josephson junctions (IJJs) in high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi-2212) is reported [1]. The ac-Josephson effect as well as the cavity resonance conditions is considered as the principle mechanism of the THz radiation [1, 2]. In order to understand the cavity effect in THz radiation from IJJ mesas more clearly, we studied mesas with various geometries; various kinds of triangles [3], and pentagonal mesas with various sizes and thicknesses. The focused ion beam (FIB) milling technique is used in all mesa fabrications. In this talk, we discuss our recent progress in THz emission observation in pentagonal mesas.\\[4pt] [1] L. Ozyuzer \textit{et al}., Science \textbf{318} (2007) 1291.\\[0pt] [2] M. Tsujimoto \textit{et al}., Phys. Rev. Lett. \textbf{108}, (2012) 107006.\\[0pt] [3] K. Delfanazari \textit{et al}., Submitted (2012) [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F36.00002: Tunable THz radiation from intrinsic Josephson junctions in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ in a localized phase rotating mode Manabu Tsujimoto, Kaveh Delfanazari, Takeo Kitamura, Masashi Sawamura, Kazuya Ishida, Shunsuke Sekimoto, Chiharu Watanabe, Takashi Yamamoto, Takanari Kashiwagi, Hidetoshi Minami, Kazuo Kadowaki After the first report of intense continuous THz electromagnetic wave radiation from high-$T_{\mathrm{c}}$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ [L. Ozyuzer \textit{et al.}, Science \textbf{318}, 1291 (2007)] with remarkably higher intensity, a great deal of interest has been drawn not only to the physical mechanism of the radiation but also to the possible variety of applications in the vast fields of THz science and technology. Recently, the authors pointed out that the contributions to the output power from the Josephson current source was found to be comparable in magnitude [K. Kadowaki \textit{et al.}, J. Phys. Soc. Jpn. \textbf{79}, 023703 (2010); M. Tsujimoto \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 107006 (2012)]. As R. Kleiner \textit{et al.} observed in 1992 [R. Kleiner \textit{et al.}, Phys. Rev. Lett. \textbf{68}, 2394 (1992)], the intrinsic junctions in the phase rotating mode produce an equal number of $I$-$V$ characteristic branches. Here we show clear evidence that the mesas can emit radiation at many frequencies in various localized phase rotating modes, and that the resulting radiation is tunable over a broad range of frequencies, allowing us to construct a powerful THz source device that could fill the THz gap. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F36.00003: Direct imaging of hot spot in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ mesa terahertz sources Timothy Benseman, Ken Gray, Alexei Koshelev, Wai-Kwong Kwok, Ulrich Welp, Vitalii Vlasko-Vlasov, Kazuo Kadowaki, Hidetoshi Minami Stacks of intrinsic Josephson junctions (IJJs) made from high-temperature superconductors such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) are a promising source of coherent continuous-wave terahertz radiation. When electrical power is applied to these devices, it is thought that hot spots may form due to resistive self-heating, and that these spots may be highly beneficial for the generation of high levels of THz power from Bi-2212 stacks. In order to better understand these hot spots, we have performed a thermal imaging study of BSCCO stacks which generate approximately 50 microwatts of radiation power at 0.59 THz. Utilizing the temperature-dependent 612nm fluorescence line of Eu$^{3+}$, we are able to directly measure the temperature distribution at the top surface of these stacks with a resolution of $+$/- 1K. The images reveal a highly non-uniform temperature distribution in which the temperature in the middle of the stack can exceed the superconducting transition temperature by tens of Kelvin under biasing conditions typical for THz-emission. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F36.00004: Magnetic field effects on THz radiation from Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ mesa structures Takeo Kitamura, Takanari Kashiwagi, Manabu Tsujimoto, Kaveh Delfanazari, Masashi Sawamura, Kazuya Ishida, Shunsuke Sekimoto, Chiharu Watanabe, Takashi Yamamoto, Hidetoshi Minami, Masashi Tachiki, Kazuo Kadowaki In a previous our study, coherent and continuous electromagnetic radiation phenomena in mesa structures of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystal have been investigated precisely in magnetic fields up to only 150 Oe [1]. This experimental result showed that the emission intensity decreases sharply for the field parallel to the $c$-axis, while it decreases gradually as increasing magnetic field for the in-plane field. In order to improve the measurement, we developed a new system with a better angular resolution and much wider magnetic field range up to 6 T, and a mesa having much stronger THz emission power. The mesa structure is also changed to the stand-alone type of mesa, which produces higher power THz radiation with ideal distribution of radiation [2]. In this presentation, the recent detailed results will be shown in magnetic fields both parallel and perpendicular to the \textit{ab}-plane of Bi2212, where the Josephson and pancake vortices are playing an important role for THz radiation.\\[4pt] [1] K. Yamaki \textit{et al}., physica C \textbf{470} (2010) S804.\\[0pt] [2] T. Kashiwagi \textit{et al}., Jpn. J. Appl. Phys. \textbf{51} (2012) 010113. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F36.00005: Simultaneous observation of temperature distribution and THz emitting spectra of Bi2212 THz devices Chiharu Watanabe, Hidetoshi Minami, Takashi Yamamoto, Takanari Kashiwagi, Kazuo Kadowaki When the intrinsic Josephson junctions in high-$T_{\mathrm{c}}$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ are fabricated to a mesa structure and biased by a dc-voltage, it is known to emit coherent, stable and continuous THz electromagnetic waves [1] whose frequency lies between 0.3-1 THz with the line width of 0.5 GHz and the radiation maximum power of 30 $\mu $W. Recently, we have succeeded in measuring the temperature distribution of the mesa directly while the mesa is emitting THz radiation and found an extreme temperature inhomogeneity (hot-spot) in the mesa [2]. By this way, we could determine the temperature of a mesa, as a result we could observe inhomogeneous temperature distribution, and we found that relation between THz emitting characteristics and temperature distribution. The simultaneous observation of the emission frequency by FTIR strongly indicates that the THz emission phenomenon is not influenced by the formation of the hot-spot at all. In this meeting, we would like to discuss the relations between THz emission phenomena hot-spot formation.\\[4pt] [1] L. Ozyuzer, \textit{et al}., Science \textbf{318}, 1291 (2007).\\[0pt] [2] H. Minami, \textit{et} \textit{al}., in preparation. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F36.00006: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F36.00007: Control of Spin-Triplet Josephson Junctions with Perpendicular Anisotropy Eric Gingrich, Simon Diesch, William Pratt, Norman Birge We present recent work on S/F'/F/F''/S Josephson Junctions with the magnetic multilayer Ni/[Co/Ni]$_n$ as the central F layer [1]. This multilayer possesses strong intrinsic perpendicular (out-of-plane) anisotropy at Co and Ni thicknesses of a few monolayers. If a hard ferromagnet is used for F', and a softer ferromagnet is used for F'', both with magnetizations in plane, the direction of the F'' layer's magnetization is predicted to control the state of the junction [2]. We are fabricating such junctions with the goal of controllably switching the junction between the $0$ and $\pi$ states. By integrating these junctions in a SQUID device, a measurement of the $0$and $\pi$ state of the junction can be performed. We will report on our progress.\\ \\ $[$1$]$ E.C. Gingrich, P. Quarterman, Y. Wang, R. Loloee, W.P. Pratt, and N.O. Birge, arXiv:1208.3118v1.\\ $[$2$]$ A.F. Volkov, F.S. Bergeret, and K.B. Efetov, Phys. Rev. Lett. \textbf{90}, 117006 (2003). [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F36.00008: Search for $\pi $/2 state in large-area spin-triplet ferromagnetic Josephson junctions Yixing Wang, Norman O. Birge, William P. Pratt, Jr. The current-phase relationship of ferromagnetic spin-triplet Josephson junctions was predicted to be tuned by the magnetization orientations of different magnetic layers [1]. Given the random domain structure in large-area ferromagnetic junctions, the possibility of a random distribution of 0 or $\pi $ sub-junctions across the area could lead to a global $\pi $/2 junction [2]. Critical current measurements as a function of area provide indirect evidence for $\pi $/2 coupling [3], but they do not provide phase-sensitive information. Unfortunately there are technical difficulties faced by a direct current-phase measurement, especially for large area junctions. We are currently working toward a SQUID-based experiment that should be able to distinguish the $\pi $/2 state from either the 0 or $\pi $ states. In this talk we will report our progress toward this goal. [1] A.F. Volkov, F.S. Bergeret, and K.B. Efetov, Phys. Rev. Lett. 90, 117006 (2003). [2] A. Zyuzin, B. Spivak, Phys. Rev. B 61 5902 (2000). [3] Y Wang, W. P. Pratt Jr., N. O. Birge, Phys. Rev. B 85 214522 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F36.00009: Odd-Frequency Triplet Josephson Current Through an Exchange Spring Andreas Bill, Thomas E. Baker, Adam Richie-Halford, Adam K. Moke The existence of an odd-frequency long range triplet component in the order parameter of a proximity system with singlet superconductors is a recent prediction that has garnered great interest. The experimental fingerprint of this phenomenon is difficult to establish. We investigate a hybrid structure in which the emergence of the long range triplet component may be measured and identified. We consider a superconductor - exchange spring - superconductor Josephson junction as a function of increasing twist of the magnetic domain wall in the exchange spring. We show that as the domain wall is generated the long range triplet component emerges and modifies the current flowing through the Josephson junction. The critical temperature is also affected by the increased twist of the domain wall. The calculations lead us to propose an experiment where the long range triplet component can unequivocally be identified. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F36.00010: Spin-triplet supercurrent in planar geometry ferromagnetic Josephson junctions William M. Martinez, W.P. Pratt, Jr., Norman O. Birge The spin-triplet supercurrent in ferromagnetic Josephson junctions is obtained by surrounding the central ferromagnet with noncollinear ferromagnetic layers, F' [1]. In metallic ferromagnets, the long-range nature of the spin-triplet supercurrent has only been tested to lengths of a few tens of nm [2]. In this work, we are fabricating and measuring S/F'/F/F'/S junctions where the central F layer has a lateral geometry with lengths up to a few hundred nm. We will report on our recent progress. \\[4pt] [1] A.F. Volkov, F.S. Bergeret and K.B. Efetov, Phys. Rev. Lett., \textbf{90}, 117006 (2003).\\[0pt] [2] M.A. Khasawneh, T.S. Khaire, C. Klose, W.P. Pratt, Jr., and N.O. Birge, Supercond. Sci. Technol., \textbf{24}, 024005 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F36.00011: Skewness and Kurtosis of the Switching Current Distribution in Superconductor-Graphene-Superconductor Junctions and Superconductor-Nanowire-Superconductor Devices Andrew Murphy, Thomas Aref, Ulas Coskun, Phillip Weinberg, Alex Levchenko, Victor Vakaryuk, Alexey Bezryadin We study statistical properties of the switching current in superconductor-graphene-superconductor proximity junctions and superconductor-nanowire-superconductor devices. The fluctuations of the switching current are related to Little's phase slips, generated by thermal and quantum fluctuations of the superconducting order parameter. The study focuses on higher moments of the statistical probability distributions of the switching current. Namely we study the skewness, which defines the asymmetry of the distribution, and kurtosis, which is a measure of the ``peakedness.'' The skewness is defined as sk= $m_3/m_2^{3/2}$ where $m_2$ is the second moment of the distribution, called the variance, and $m_3$ is the third moment. Kurtosis is defined as kur= $m_4/m_2^2$, where $m_4$ is the fourth moment of the distribution. It is known that for Gaussian distributions sk=0 and kur=3. On our devices we find, in most cases, sk $\sim$ -1 and kur $\sim$ 5. These results are in agreement with numerical simulations as well as an analytic model. Finally we present preliminary experimental results for a two-nanowire device. We have found that the standard deviation, skewness and kurtosis of the switching current distributions in these devices vary periodically with magnetic field. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F36.00012: Realization of short ballistic vertical graphene Josephson junction Gil-Ho Lee, Hu-Jong Lee We realized short ballistic vertical graphene Josephson junctions (vGJJs), where a monolayer graphene sheet is sandwiched between two superconducting electrodes along the c-axis of graphene. To enhance the transparency between electrodes and graphene layer we thermally deposited aluminum superconducting electrodes on both surfaces of the graphene sheet by using a ``flip-transfer'' scheme instead of transferring graphene onto the bottom electrode. With the highly transparent contacts and atomically short channel length, vGJJ shows a very large value of I$_c$R$_N$ product (2.2$\Delta_{Al}$). This value is in sharp contrast to much suppressed value of $I_cR_N$$ < \Delta_{Al}$, observed in planar graphene Josephson junctions. Surprisingly, $I_c$ decreases superlinearly with increasing temperature ($T$) from 50 mK up to the junction critical temperature, which is a typical character of a short ballistic Josephson junction. To our best knowledge, this feature has long been predicted but never been reported in proximity-coupled Josephson junctions. $I_c$-$T$ curve fits well to the short ballistic Josephson junction model (KO-2)\footnote[1]{K. K. Likharev, Rev. Mod. Phys. \textbf{51}, 101 (1979)} with the transparency of 0.94. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F36.00013: Direct measurements of the current-phase relation in graphene Josephson junctions Christopher English, David Hamilton, Dale Van Harlingen, Nadya Mason The current-phase relation (CPR) of a Josephson junction can provide key information about the microscopic processes and symmetries that control the supercurrent. In this talk, we present CPR measurements on Josephson junctions incorporating single-layer graphene as a weak link between Al superconducting electrodes with spacing \textless 100nm that are in the quasi-ballistic regime. We use a phase-sensitive SQUID technique to determine the supercurrent amplitude and phase as a function of temperature and electrostatic doping (gate voltage). As the critical current is varied, we observe a crossover from forward skewing in the CPR that arises from the low density of discrete electronic states in the junction to backward skewing induced by noise-rounding in the CPR measurement. We compare our results to theoretical models. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F36.00014: Josephson currents in semiconductor nanowire / s-wave superconductor nanostructures Li Mao, Sumanta Tewari, Chuanwei Zhang It has been theoretically predicted that a nanostructure composing of a semiconductor nanowire with strong spin-orbit coupling and an s-wave superconductor can support two Majorana fermions at the ends of the nanowire in the presence of a Zeeman field. Recently, following the theoretical proposals, some preliminary experimental signatures (e.g., zero-bias conductance peak) which may be related to the existence of Majorana fermions have been observed in the charge transport experiments. Here we investigate the Josephson currents with the zero-bias voltage in the topologically trivial region of a superconductor-insulator-superconductor junction in the presence of strong spin-orbit coupling and Zeeman field. This structure may be relevant to the Delft experiment by considering the possible proximity effect of the superconductor lead to the normal part of the nanowire. Our results indicate that the experimentally observed zero-bias conductance peak may not originate from Majorana fermions. [Preview Abstract] |
Session F37: Focus Session: Fe-based Superconductors: Order Parameter Symmetry/Fe(Te,Se) Films
Sponsoring Units: DMP DCOMPChair: Qiang Li, Brookhaven Natl Lab
Room: 345/346
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F37.00001: Symmetry measurements of the order parameter of BaFe$_2$As$_2$ superconductors Juan Atkinson, Dale Van Harlingen, Paul Canfield, Duck Chung Since the discovery of the Fe-pnictide superconductors, extensive efforts have been directed toward understanding the symmetry and mechanism of the superconducting pairing. Extended s-wave models, predominately the s$\pm$ model, are predicted by many theories, but a definitive experimental verification has been elusive. We are using phase-sensitive Josephson interferometry to test for magnitude and phase anisotropy in electron (Co-doped) and hole (K-doped) BaFe$_2$As$_2$ single crystals. In particular, we are looking in the heavily K-doped regime that is predicted to exhibit d-wave symmetry characterized by a sign change in the order parameter. We are also searching for proximity-induced structure in the density-of-states of an s-wave superconductor proximity-coupled to an Fe-pnictide superconductor that is predicted to arise for s$\pm$ pairing (Koshelev, 2012). [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F37.00002: Sign-changing nodal $s$-wave gap in heavily over doped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ evidenced from thermal-transport measurement Daiki Watanabe, Shigeru Kasahara, Takuya Yamashita, Takumi Ota, Takasada Shibauchi, Yuji Matsuda, Minoru Yamashita, Hideto Fukazawa, Taku Saito, Yoh Kohori, Shigeyuki Ishida, Kunihiro Kiho, Chul-Ho Lee, Akira Iyo, Hiroshi Eisaki, Anton Vorontsov The superconducting state of hole-doped Fe-based superconductors, (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$, changes from a fully-gapped state near the optimally doping ($x\sim$0.5) to a nodal one at the end material ($x$= 1)[1,2]. Here we report the results of thermal-transport measurements for heavily overdoped $x$=1, 0.93, 0.88, 0.76 crystals and discuss the doping evolution of the superconducting gap. For $x$=0.88, 0.93 and 1, the $T$-dependence of thermal conductivity in zero field shows a finite $\kappa _0/T$ in the zero-temperature limit. In low magnetic fields, $\kappa/T$ ($T\to 0$ K) increases as $\propto \sqrt{H}$. These results indicate the presence of gap nodes in the gap function. We find that the residual $\kappa _0/T$ exhibits a non-monotonic $x$-dependence, which is inconsistent with $d$-wave symmetry. We show that the observed $x$-dependence can be explained by nodal $s$-wave pairing with sign change between zone centered hole pockets. [1] K. Hashimoto {\it et al.}, Phys. Rev. B {\bf 82}, 014526 (2010). [2] K. Okazaki {\it et al.}, Science {\bf 337}, 1314 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F37.00003: Angle Dependent Specific Heat Study of BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ Liam Malone, Yuta Mizukami, Philip Walmsley, S. Kasahara, T. Shibauchi, Y. Matsuda, Antony Carrington The structure of the superconducting gap of the pncitide superconductors is an unresolved but crucial issue to understanding their mechanism of superconductivity. While some experiments and theories support a fully gapped s+/s- state, several experiments have revealed evidence for nodes in some families of pnictides. Detailed knowledge of the superconducting gap structure and how it varies between different families can be useful in helping to decide between microscopic theories. BaFe$_2$(As$_x$P$_{1-x}$)$_2$ is a pnictide family where penetration depth and thermal conductivity measurements show evidence for nodes [1]. We have measured the specific heat of a single crystal of BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ (T$_c\sim$29\,K) at low fields and as a function of applied field angle. The angle dependence of specific heat at low fields and low temperature is expected to show minima whenever it is along a nodal direction and can be used to differentiate between gap symmetries [2]. Our results show a clear angle dependent component consistent with the presence of nodes and we discuss the implications on the gap structure of BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$.\\[4pt] [1] K. Hashimoto et al, Phys. Rev. B, 81, 220501R,(2010).\\[0pt] [2] A. B. Vorontsov et al, Phys. Rev. Lett. 105 ,187004 (2010) [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F37.00004: Isotropic superconducting gap structure in BaFe1.90Pt0.10As2 from low temperature thermal conductivity Kevin Kirshenbaum, Shanta Saha, Steven Ziemak, Rongwei Hu, Jean-Philippe Reid, Ryan Gordon, Louis Taillefer, Johnpierre Paglione In this study we present measurements of thermal transport down to 50 mK in single crystals of the iron-based superconductor BaFe1.90Pt0.10As2 with Tc $=$ 23 K [1]. Magnetic fields up to 15 T were applied along the c-axis of the crystal as well as along the basal plane direction to probe the anisotropy of the superconducting gap. The lack of any significant residual electronic term in thermal conductivity for all field directions and values confirms the absence of nodes and places limits on the depth of gap minima in this system.\\[4pt] [1] S.R. Saha et al, JPCM 22 072204 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F37.00005: Investigation of Pairing Symmetry in Pt-Substituted BaFe2As2 S. Ziemak, K. Kirshenbaum, S.R. Saha, R. Hu, J. Paglione, J.-Ph. Reid, R. Gordon, L. Taillefer, A. Ignatov, D. Kolchmeyer, G. Blumberg, D. Evtushinsky, S. Thirupathaiah, S.V. Borisenko We present results from several measurements on BaFe1.9Pt0.1As2 single crystals designed to measure the superconducting gap structure. Low temperature thermal conductivity was measured in magnetic fields up to 15 T and will be compared to other materials. Point-contact Andreev reflection spectroscopy measurements were made using the needle-anvil technique and the spectra analyzed using BTK theory. Raman spectroscopy was used to probe excitations in multiple channels. Finally, angle-resolved photoemission spectroscopy was used to provide further information about the band structure and superconducting gap. We will discuss the implications that the combination of these results reveal about the superconducting order parameter in this system. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F37.00006: In-plane transport anisotropy study of oxygen doped iron telluride MBE thin films on LaAlO3 substrate Can Zhang, Huihuo Zheng, Mao Zheng, Brian Mulcahy, Xiaoxiao Wang, Ying Jia, Ulrich Welp, James Eckstein FeTe is well known for its simple crystal structure in the 11 family iron-based high temperature superconductors. We have observed two distinct in-plane transport phenomena in MBE grown FeTe thin films on LaAlO3 substrates. The first one is an unexpected global alignment of the in-plane transport anisotropy. A low temperature resistivity upturn feature has been observed in the neighborhood of the superconducting transition temperature as a function of transport direction. The resistivity upturn feature emerges from 8K to 20K. The second one is the coexistence of superconductivity with the low temperature resistivity upturn. We will report our studies of these distinct transport anisotropy experimental results comparing them with first principle simulations. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F37.00007: Electric transport properties in FeSe0.3Te0.7 / Au c-axis tunneling junctions Yu Tien Shen, You Sheng Li, Cheng Chung Chi Currently favored pairing symmetry in the iron-based superconductor is the nodaless S$+$- wave. Based on this theory, the conductance spectra of the normal metal to superconductor tunneling junctions do not exhibit ZBCP. We report the fabrication and the transport properties of c-axis tunneling junctions formed by FeSe0.3Te0.7 (FeSeTe) and Au. When FeSeTe is in its normal state, the conductance spectrum shows a V-shape background; while when FeSeTe becomes superconducting, the conductance spectrum shows some remarkable features: First, a pronounced ZBCP was obtained as temperature is just below Tc, and when the temperature was further decreased to below 4K, a clear double peak structure appears; Second, there were two dip structures at around 4 and 20mV. We found out that there is a linear dependence of the voltage difference between the double peaks versus applied field. Though the origin of the ZBCP, the double peak structure, and the dip structures were unclear and still under investigation, we believe that they all related to the superconducting gaps due to their dependence on applied magnetic fields and temperature. The existence of the prominent ZBCP is not consistent with the proposed S$+$- wave symmetry of the superconducting gap. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F37.00008: New bi-epitaxial Grain boundary Josephson Junction of FeSe$_{0.3}$Te$_{0.7}$ You-Sheng Li, Y.T. Shen, M.J. Wang, M.K. Wu, C.C. Chi We have successfully fabricated epitaxial FeSe$_{0.3}$Te$_{0.7}$films on MgO substrate with its in-plane crystalline axis either parallel to or rotated 45$^{\circ}$ with respect to the MgO lattice. We use this technique to fabricate the 45$^{\circ}$ grain-boundary Josephson junction. The IV-curve measured at 4.2 K can be fitted with the RSJ model, and the measured I$_{c}$R$_{n}$value is 9.24$\mu$V, which is in general agreement with the values obtained by previous results for Fe-based grainboundary junctions on bi-crystal substrates. We have also measured the dc Josephson current as a function of applied magnetic fields, which shows a clear Fraunhofer-like pattern. Hence we can rule out the possibility of d-wave symmetry in FeSe$_{0.3}$Te$_{0.7}$ superconductor. Upon applying 6 GHz microwave irradiation, the junction IV curve exhibits clear Shapiro steps. Thus we have demonstrated our ability to fabricate high quality grain-boundary Josephson junctions of this new class of material. Further physical properties, such as the noise power spectrum, are currently being investigated. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F37.00009: The tunneling spectra and superconducting gaps observed by using scanning tunneling microscopy near the (100)/(110) grain-boundary of FeSe0.5Te0.5 films Kuang Cheng Lin, You-Sheng Li, Cheng-Chung Chi We have found that, using PLD method, the a- and b-axis of the FeSe$_{0.5}$Te$_{0.5}$ film deposited on pristine MgO substrate are parallel to those of MgO, while these axes of the film grown on MgO substrate treated with Ar-ion milling rotate 45$^{\circ}$ along its c-axis. Here, we prepared such film with two kinds of orientations (0$^{\circ}$ and 45$^{\circ}$ ab plane with respect to the substrate axis) on MgO substrate with the connection between them form a ramp at an angle about 30$^{\circ}$ to the substrate plane. We used STM to study the tunneling spectra of two orientations of c-axis planes and the connection ramp between them. In the planar region with different orientation, we have observed similar tunneling spectra with a superconducting gap about 5 meV. This gap value is consistent with the previous studies of a variety of FeSeTe samples. However, a much larger gap about 18 meV is observed in the ramp region. The only paper we found to have such a large gap in the family of Fe-based superconductors is the one by Xue et.al. They have shown a gap of 20 meV in one unit-cell thick of FeSe on STO substrate. Furthermore, we have also observed a small ZBCP inside the large gap at 4.3K. The ZBCP becomes smaller with increasing temperature and disappears near and above TC. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F37.00010: Transport properties of transition-metal substituted FeTe0.65Se0.35 single crystals Valeriy L. Bezusyy, Dariusz J. Gawryluk, Artur Malinowski, Marek Berkowski, Marta Z. Cieplak We use the ab-plane resistivity and Hall effect measurements to evaluate the influence of substitutions on the superconductivity and normal-state transport in Fe$_{\mathrm{1-y}}$M$_{\mathrm{y}}$Te$_{\mathrm{0.65}}$Se$_{0.35}$ single crystals, where M$=$Co, Ni or Cu. The crystals, with 0 \textless\ y \textless\ 0.11, are grown by Bridgman's method. We find that the Co impurity induces markedly different effects than the other two impurities. Superconducting transition temperature (T$_{\mathrm{c}})$ is suppressed with the rate of about 1.3 K per at.{\%} of Co impurity, while the rate is about 3.5 and 4 times larger in case of Ni and Cu, respectively. The resistivity at the T$_{\mathrm{c}}$ onset remains almost unaffected by Co doping, while it increases substantially for Ni and Cu. The Hall constant (R$_{\mathrm{H}})$ is positive for all samples, indicating that hole carriers dominate the transport. However, while the R$_{\mathrm{H}}$ is gradually suppressed towards zero with increasing Co content suggesting that electron doping occurs, it remains almost unchanged by Ni or Cu doping, suggesting that these impurities are rather of isovalent nature. The implications of these results will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F37.00011: FeSe$_{0.5}$Te$_{0.5}$ thin film Josephson junction on SrTiO$_{3}$ bicrystal substrates Weidong Si, Cheng Zhang, Xiaoya Shi, Qiang Li Josephson junctions were fabricated in the epitaxial FeSe$_{0.5}$Te$_{0.5}$ thin films on [100] tilted SrTiO$_{3}$ bicrystal substrates with a CeO$_{2}$ buffer layer. These junctions with a 24 degree of grain boundary misorientation show a typical resistive-shunt-junction like current-voltage behavior. Critical current densities across the grain boundary in these junctions were observed to be remarkably suppressed and modulated by the magnetic field. Films without the grain boundary show a critical current density much higher than those with the grain boundary. These results indicate a Josephson Effect in those grain boundary junctions. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F37.00012: Planar tunnel junction on oxygen doped iron telluride thin films Mao Zheng, Han Zhao, Can Zhang, Gustaf Olson, Brian Mulcahy, Valentin Stanev, Alexei Koshelev, Laura Greene, James Eckstein Since its discovery, iron based superconductivity has garnered much interest from the research community for its potential in both application and fundamental science. One of the questions awaiting an answer is the pairing symmetry of this new class of superconductors. Recently, Koshelev and Stanev proposed a fingerprint of s$+$- symmetry in the NIS tunneling spectrum where the iron based superconductor is proximity-coupled to a thin s-wave superconductor[1]. We have prepared oxygen doped iron telluride (FeTe:Ox) thin films, along with an in-situ grown tunnel barrier and top electrode by Molecular Beam Epitaxy (MBE). We have fabricated them into planar tunnel junction and will report the temperature dependence of both tunneling and point contact spectra. [1]. A. E. Koshelev and V. Stanev, EPL (Europhysics Letters) \textbf{96} (2), 27014 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F37.00013: Non-Fermi liquid behavior in overdoped iron-pnictide Ba(Fe,Co,Ni)2As2 Alex Hughes, Yasuyuki Nakajima, Kevin Kirshenbaum, Shanta R. Saha, Tyler Drye, Johnpierre Paglione Very low-temperature specific heat was used to study a series of single-crystal iron-based intermetallic compounds with the ThCr2Si2 structure with transition metal substitution used to heavily over-dope the system. This system has been found to exhibit non-Fermi liquid characteristics in transport, magnetic and thermodynamic properties. We will present low-temperature specific heat capacity measurements of this compound in order to elucidate the non-Fermi liquid nature of the ground state and to help elucidate the origin of these properties and their relation to superconductivity. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F37.00014: Observation of a c-axis collapse in superconducting FeTeO$_{\mathrm{x}}$ films below T$_{\mathrm{c}}$ Lahiru Narangammana, Xuerong Liu, Yuefeng Nie, Joseph Budnick, Christof Niedermayer, John Hill, Genda Gu, Barrett Wells We compared the temperature dependent crystal structure of superconducting FeTeO$_{\mathrm{x}}$ films and non superconducting Fe$_{1.02}$Te single crystals. The primary difference between the two is that the superconducting FeTeO$_{\mathrm{x}}$ films show a collapse of the c-axis below the superconducting transition temperature 13K. No such transition occurs in the single crystal. The room temperature structures of the two are similar and both show a tetragonal to monoclinic transition near 60K. Preliminary neutron diffraction studies indicate a suppression in antiferromagnetic order below T$_{\mathrm{c}}$ on superconducting FeTeO$_{\mathrm{x}}$ thin films. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F37.00015: Effect of doping on the specific heat jump in iron-based superconductors Dushko Kuzmanovski, Saurabh Maiti, Maxim Vavilov, Andrey Chubukov, Frederic Hardy In this talk we present a theoretical description of the jump of the specific heat at the transition to a superconducting phase of iron-based pnictides. We discuss both the overdoped regime, when the transition occurs between non-magnetic and superconducting phases, and the underdoped regime, when superconductivity emerges from a pre-emptive SDW phase. Both effects lead to a qualitatively similar phase diagram as a function of doping, but details differ. We presume that doping simultaneously modifies the Fermi surface of pnictides and introduces disorder. By fitting the transition temperatures for the SDW and SC phases, we establish the relative strengths of the the rigid band shift caused by doping and doping-induced disorder. We then evaluate the specific heat jump as a function of doping. Our theory is consistent with measurements made by Karlsruhe group of the specific heat jump in $\mathrm{BaFe}_{2}\mathrm{As}_{2}$ compounds with $\mathrm{K}$- and $\mathrm{Co}$-doping. [Preview Abstract] |
Session F38: Physics Education Programs, Policy and the Media
Sponsoring Units: FEdChair: Aaron Wade, University of West Florida
Room: 347
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F38.00001: Engaging community college students in physics research Megan Valentine, Maria Napoli, Arica Lubin, Liu-Yen Kramer, Ofelia Aguirre, Jens-Uwe Kuhn, Nicholas Arnold Recruiting talent and fostering innovation in STEM (Science, Technology, Engineering and Mathematics) disciplines demands that we attract, educate, and retain a larger and more diverse cohort of students. In this regard, Community Colleges (CC), serving a disproportionate number of underrepresented minority, female and nontraditional students, represent a pool of potential talent that, due to a misguided perception of its students as being less capable, often remains untapped. We will present our strategies to attract and support the academic advancement of CC students in the STEM fields through our NSF-sponsored Research Experience for Undergraduates program entitled Internships in Nanosystems Science Engineering and Technology (INSET). For more than a decade, INSET has offered a physics research projects to CC students. The key components of INSET success are: 1) the involvement of CC faculty with a strong interest in promoting student success in all aspects of program planning and execution; 2) the design of activities that provide the level of support that students might need because of lack of confidence and/or unfamiliarity with a university environment; and 3) setting clear goals and high performance expectations. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F38.00002: Pathways to Excellence Scholarship Program for women in STEM fields Joseph Di Rienzi Notre Dame of Maryland University (NDMU) has an NSF S-STEM grant, Pathways to Excellence, that gives 10 scholarships annually to academically talented women undergraduates with demonstrated financial need who are pursuing degrees in mathematics, physics, computer information systems, or engineering. NDMU has been cited (Whitten, et al. (2007)) as providing a female friendly environment for the study of physics. In this program we are using a tri-part mentoring system involving a faculty member in the student's discipline, a peer mentor from the program and an external alumnae mentor. The program also has a thematic seminar course for the scholars. Each student in the program is tasked to construct a career development plan in assistance with her faculty mentor and set measured annual goals. In addition, all scholarship students are requested to have an experiential experience. As a result, NDMU aims to strengthen its role in increasing the numbers of well-educated and skilled women employees from diverse backgrounds, including mostly first-generation college students, in technical and scientific areas. Early assessment of the success of the program will be presented as well as modifications that resulted from the formative evaluation. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F38.00003: Diversity in Physics: Impact of Using Minimum Acceptable GRE Scores for Graduate Admissions Casey W. Miller About 180 graduate programs in physics are listed in the AIP Graduate Programs book. $\sim$ 96{\%} require the general GRE test; a quarter of these have an explicitly stated minimum score for admission, with the median stated cut-off being 700 (64$^{\mathrm{th}}$ percentile) on GRE Quantitative; $\sim$ 48{\%} require the physics GRE; about half of these have an explicitly stated minimum score for admission, with the median being 600 (32$^{\mathrm{nd}}$ percentile). It does not seem unreasonable to expect students to be among the top test scorers, until you dissect the test results by race and gender. In this talk, I will present data showing that the use of minimum acceptable scores on the GRE exam will have (have had?) a negative impact on diversity in Physics. I will remind the community that this practice is in opposition to ETS's Guide to the Use of Scores. I will make some suggestions for admissions committees, based in part on analyses I have performed. I will then pose challenges related to reducing the influence of GRE scores to the community, ranging from the department and university administration, to ranking bodies and professional societies. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F38.00004: Interdisciplinary Research and Education in STEM in a Discipline Dominated Academic Structure- Research and Education at the Cross Roads Solomon Bililign Major issues in society - developing alternate sources of energy and a sustainable environment, improving health, and minimizing the effects of climate change require a collective effort by different disciplines working in interdisciplinary groups. Many major breakthroughs in science take place at the boundaries or intersections of disciplines. The need to create a new generation of students who combine a rigorous disciplinary depth with the ability to reach out to other disciplines and work in interdisciplinary teams is more urgent. There is a consensus that the current academic administrative structure is the most important barrier to interdisciplinary collaboration; other barriers like poor communication, etc., emanate from it. How can interdisciplinary education and research flourish while maintaining strong backgrounds in the disciplines? How can universities lower or remove barriers to faculty participation in interdisciplinary education and research and create porous, flexible, less redundant environment that facilitates the flow of ideas, people and resources across disciplinary boundaries? Is possible to have disciplines without disciplinary departments? In this short paper, the barriers and the challenges for developing interdisciplinary education and research will be summarized, lessons from some successful attempts and failures will be presented, and some approaches will be recommended for further discussion. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F38.00005: Personifying self in physics problem situations involving forces as a student help strategy A.E. Tabor-Morris How can physics teachers best guide students regarding physics problem situations involving forces? A suggestion is made here to personify oneself as the object in question, that is, to pretend to be the object undergoing forces and then qualify and quantify those forces according to their vectors for the system at hand. This personification is not meant to empower the object to act, just to sense the forces it is experiencing. This strategy may be especially useful to beginning physics learners attacking problems that involve both multiple forces AND multiple objects, since each object acted upon needs to be considered separately, using the idea that one cannot be two places at once. An example of this type of problem expounded on here is Atwood's machine: two weights hung over a pulley with a single rope. Another example given is electromagnetic forces on one charge caused by other charges in the vicinity. Discussion is made on implementation of classroom strategies. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F38.00006: High School Physics Teacher Outreach Programs at California State University Long Beach Chuhee Kwon, Galen Pickett, Laura Henriques One of the goals of the CSULB PhysTEC project has been to establish a physics teaching community that partners CSULB faculty, high school teachers, pre-service teachers, and physics students. In two years, we have created a solid sustainable Physics Teacher Network with local high school teachers. We will discuss the successful outreach programs for high school physics teachers at CSULB and the detailed logistics. Teacher-In-Residence (TIR), high school physics teachers working with the CSULB PhysTEC team, has provided invaluable input for designing and implementing outreach events. The department organizes biannual open house for local high school teachers and their students. The open house event is attended by pre-service teachers, physics undergraduate and graduate students, and faculty. We also host the monthly demo-sharing day that physics teachers bring and share topical demos, which has about 30 - 50 attendees each month. The CSULB PhysTEC project also distributes a monthly newsletter for local physics teachers with upcoming events and information about teaching, and this newsletter is organized and written by TIR. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F38.00007: Using the science of granular materials to engage middle and high school students in the process of scientific enquiry Jennifer Podel, Nalini Easwar, Shubha Tewari, Karl Martini, Kristin Dolcimascolo, Eric Newman We describe outreach efforts that use the science of granular materials to engage middle and high school physics students in local public schools in scientific investigations. In the middle school, the students were provided with a set of questions, and starting materials to set up their experiments. Examples of investigations pursued by the students include looking at the influence of the size and shape of grains on (i) their rate of flow through a hopper and (ii) their tendency to desegregate in a flow. The high school students were introduced to the properties of granular materials via a series of activities that explored the complex behavior of these materials. Following this, groups of students were challenged to pose a question and design an experiment to investigate a particular aspect of the properties of granular materials. Examples of questions that the students chose to investigate include: How does the shape of grains influence how well they stack in a pile? What factors affect the probability of avalanches down an incline? Both sets of students worked in groups over a period of two months to take quantitative data to test their hypotheses. The investigations culminated in a set of presentations by the students to local faculty and students. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F38.00008: Science Days: Graduate Student Run Outreach on a Budget Justin K. Perron, George P. Lindberg We will describe a new and ongoing program at the University at Buffalo (UB) aimed at exposing underrepresented K-12 students to the Science Technology Engineering and Math (STEM) fields. This program has been an entirely graduate student run effort, from idea to inception and finally through implementation. Graduate students, under supervision from faculty members, received a grant from NYSS-APS and matching funds from Physics, Chemistry, and Biology departments at UB. Graduate students set up an outreach program that buses students from inner city Buffalo to UB campus to participate in STEM-based activities. We have held two three hour events so far. Each event involved $\sim$30 students, 99{\%} of which are from underrepresented demographics. Their responses to brief questionnaires showed overwhelming positive views of the event and their genuine interest in science. We will discuss what has made this program a success including what faculty members have done and can do, to support the effort while still leaving it entirely in the graduate students' hands. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F38.00009: Bringing education to your virtual doorstep Vitaliy Kaurov We currently witness significant migration of academic resources towards online CMS, social networking, and high-end computerized education. This happens for traditional academic programs as well as for outreach initiatives. The talk will go over a set of innovative integrated technologies, many of which are free. These were developed by Wolfram Research in order to facilitate and enhance the learning process in mathematical and physical sciences. Topics include: cloud computing with Mathematica Online; natural language programming; interactive educational resources and web publishing at the Wolfram Demonstrations Project [1]; the computational knowledge engine Wolfram Alpha [2]; Computable Document Format (CDF) and self-publishing with interactive e-books; course assistant apps for mobile platforms. We will also discuss outreach programs where such technologies are extensively used, such as the Wolfram Science Summer School [3] and the Mathematica Summer Camp [4].\\[4pt] [1] http://demonstrations.wolfram.com\\[0pt] [2] http://www.wolframalpha.com\\[0pt] [3] http://www.wolframscience.com/summerschool\\[0pt] [4] http://www.mathematica-camp.org [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F38.00010: A Mobile Nanoscience and Electron Microscopy Outreach Program Tonya Coffey, Kyle Kelley We have established a mobile nanoscience laboratory outreach program in Western NC that puts scanning electron microscopy (SEM) directly in the hands of K-12 students and the general public. There has been a recent push to develop new active learning materials to educate students at all levels about nanoscience and nanotechnology. Previous projects, such as Bugscope, nanoManipulator, or SPM Live! allowed remote access to advanced microscopies. However, placing SEM directly in schools has not often been possible because the cost and steep learning curve of these technologies were prohibitive, making this project quite novel. We have developed new learning modules for a microscopy outreach experience with a tabletop SEM (Hitachi TM3000). We present here an overview of our outreach and results of the assessment of our program to date. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F38.00011: The Good, The Bad, and The Ugly: Using Movies to Teach Science Joanne Budzien Can the plane outrun the explosion? Could the heroes escape injury from the bomb by hiding in the bathtub? Are we in danger of being overrun by 50-foot-tall bugs that have been exposed to radiation? Many people in the general public do want to know the science behind much of what they see in the movies and on television. However, those people are unlikely to take a whole class because ``everyone knows'' that science classes are boring and irrelevant. On the other hand, an evening with an hour or so of video clips interspersed with explanations of the science can be a big hit both to raise general science fluency and recruit students into general education science classes. Film-editing technology has advanced to the point that anyone who has a computer and is willing to invest a couple days in learning to use the software can make a clips-with-PowerPoint DVD that can be shown to a local audience for discussion or used in a science class to show the exact scenes to save time. In this presentation, I'll show an example of my work and talk about how you can make your own DVD. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F38.00012: The Physics of Babies Philip Shemella Since the 2011 birth of my daughter I have been a 100\% as a stay-at-home dad and 50\% researcher. My ``Routine Adventures'' in the baby universe are the subject of this fun talk that presents the unique challenges of baby physics. Topics include ``Schroedinger's Baby'' and ``The Entropy of Rice.'' [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F38.00013: An IYPT-based undergraduate physics tournament in China ChuanYong Li, Feng Song, Yubin Liu, Qian Sun International Young Physicists' Tournament (IYPT) is a team-oriented scientific competition of secondary school students. The participants present their solutions to scientific problems they have prepared over several months and discuss their solutions with other teams. It can also be implemented in university level as its physics problems are all open questions and have no standard answers, especially suitable for undergraduates' ability training in China. The annual tournament of physics learning of undergraduates in our school of physics was started in 2008. Each year, there are 15-18 teams, 20 more student volunteers and 30 more faculty jurors involved. The students benefited in different ways. It is project-based, requiring students to solve the problems in a research way. Team work is developed in both experimenting and discussing stages. The knowledge learned in classrooms can be used to solve these practical and life-related problems, raising their interest and initiative in physics learning. Finally, they are building up their skills in scientific presentation and communication. An IYPT-based program called CUPT (China undergraduate physics tournament) was launched in 2010 and annually attracts about 40 universities to attend. It gains its important role in physics education. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F38.00014: Met The Press: What It's LIke to Talk to Reporters about Physics Rebecca Thompson Someone from the Huffington Post just called you because they are doing a story about science and you are a physicist. The problem is that they need you to take time away from your grapheme experiments to talk about the physics of exploding anvils. It's been a long time since you've shot an anvil in the air so you think you might not be right for this. But, as long as you understand general physics and can explain things well, you can be a real asset. This talk will recount first-hand experiences talking to a range of news outlets from the PBS New Hour to Real Simple Magazine about everything from quick-freezing water to pumpkin boats. It will include helpful information about preparing for an interview, learning new physics fast, timelines and follow-up. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F38.00015: Talking to Journalists about Your Research James Riordon Many physicists have the opportunity to speak to members of the media from time to time. A journalist may want to ask about your work, or they may be in search of expert comments on the work of others in your field. I will offer some thoughts on ways to prepare for various types of interviews. I will also suggest some things you should always try to bring up in an interview, and others that you might want to avoid entirely. Finally, I will talk about what you can do when a reporter gets it wrong. [Preview Abstract] |
Session F39: Focus Session: Materials in Extremes: High Pressures
Sponsoring Units: GSCCM DCOMP DMPChair: Evan Reed, Stanford University
Room: 348
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F39.00001: Pressure-induced metallization and phase transitions in GeS$_2$ Ranga Dias, Choong-Shik Yoo We have studied the pressure-induced structural and electronic phase transitions of crystalline GeS$_2$ ($P$2$_{1}/c)$ to 50 GPa, using micro-Raman spectroscopy and electrical resistivity measurements in diamond anvil cells. The result shows a steady decrease in resistivity to that a metal at around 40GPa. The visual appearance of GeS$_2$ supports the insulator-metal transition: initially transparent GeS$_2$ becomes opaque and eventually reflective with increasing pressure. The Raman and X-ray diffraction result indicates that the metallization is preceded by a structural phase transition. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F39.00002: Density Functional Theory Investigation of Sodium Azide at High Pressure Brad Steele, Aaron Landerville, Ivan Oleynik Sodium azide is intriguing because it could potentially be used as a precursor to a high-nitrogen energetic material. Furthermore, recent absorption and Raman spectroscopic results have shown that novel nitrogen structures may indeed be attainable from sodium azide. First-principles density functional theory calculations were performed to characterize possible novel crystalline structures of sodium azide including their atomic structure, vibrational properties, Raman spectra, and equation of state up to 90 GPa. Calculated Raman peaks and intensities show good agreement with experiment. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F39.00003: Simple binary mixtures of hydrogen and ammonia under extreme pressures Gustav Borstad, Choong-Shik Yoo Binary mixtures under pressure are of interest as fundamental systems in physics and chemistry as they allow the effects of the environment on the behavior of different chemical compounds to be examined. Furthermore, mixtures of simple molecular systems are of interest for applications in fuel cells and also to planetary science due to their presence in the interiors of the giant gas planets. In this presentation, Raman data on the ammonia and hydrogen system under pressure will be presented, and the extent and nature of the interactions in this mixture will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F39.00004: First-principles simulations on chemical transformation bonding pathways of compressed graphite Anguang Hu, Fan Zhang Chemical transformation bonding pathways for cubic diamond, hexagonal diamond, and cold compressed carbon has been investigated using first-principles simulations of the enthalpy minimization with various target pressures. The high-pressure bonding pathways of carbon from initial bonding conformations can be divided into three bonding evolution stages, which are defined as the van der Waals bonding destruction, bond breaking and forming reaction, and bonding equilibrium process. The principal stress tensor components were used to characterize the response of C-C bonds in graphite to compressive loading. It was found that the local stress field starts to rapidly increase towards the positive direction at the onset of the van der Waals bonding destruction. Bond breaking and forming reaction then takes place, leading to a cell volume collapse accompanied with a drop in stress components. The three bonding evolution stages demonstrated that the bonding evolution of the system towards chemical transformation under compression can be dictated by the local stress field together with the initial bonding conformation. Thus, the local stress field provides an understanding on how atoms and electrons move during the course of chemical transformation under compression. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F39.00005: Thermo-Physical Properties of Ammonium Azide under High Pressure from First-Principles Aaron Landerville, Brad Steele, Ivan Oleynik Polynitrogen compounds offer tremendous promise for use as insensitive high-explosives or propellants. While the existence of such compounds have been observed in Diamond Anvil Cells (DAC) under high pressure, recovery to ambient pressure and temperature has proven problematic. A current thrust towards the recovery, and ultimate manufacture, of materials rich in polymeric nitrogen has brought renewed attention to various nitrogen-rich compounds, particularly crystalline azides, as possible precursors. We investigate the thermo-physical properties and Raman spectra of one azide candidate -- ammonium azide -- under hydrostatic compression using density functional theory with an empirical van der Waals correction. Additionally, we perform structural minima searches to discern possible polymorphs that may help to elucidate dynamical processes leading to the production of a material rich in polymeric nitrogen, as well as its recovery from DAC. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F39.00006: Theory-driven discovery of an exotic CaB$_{6}$ high-pressure crystal structure phase Aleksey Kolmogorov, Sheena Shah, Elena Margine, Annette Kleppe, Andrew Jephcoat We synthesized and solved an unexpectedly complex crystal structure of CaB6 under high pressures and temperatures [1]. The only known crystal structure in the large family of metal hexaborides, a simple cubic cP7 type, has been shown to transform into a tetragonal tI56 configuration comprised of unfamiliar 24-atom boron units above 30 GPa and remain metastable under ambient pressure. The interpretation of the convoluted x-ray diffraction pattern was accomplished with an ab initio evolutionary search implemented in MAISE [2] which identified the tI56 structure (28 atoms per primitive unit cell) without any parameter input, i.e. truly ``from scratch.'' I will describe the performance of different ground state search techniques in such challenging cases.\\[4pt] [1] A.N. Kolmogorov et al., Phys. Rev. Lett. 109, 075501 (2012)\\[0pt] [2] Module for Ab Initio Structure Evolution, http://maise-guide.org [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F39.00007: Modeling of the amorphous phase of poly-CO I.G. Batyrev We studied theoretically the details of amorphous structure of extended CO solid obtained by isotropic compression of solid CO phases in the range of 3-25 GPa. We performed DFT simulations of 128, 432, and 1024 atom models. Structures of random networks found at zero temperature were used for equilibration at finite temperatures up to 50 ps by employing first principles MD. We found that the polymerization begins at 6 - 8 GPa and a random network of 4-7 atom rings obtained above 15 GPa could exist up to 0.1 -0.25 GPa. We studied pressure induced changes in topological characteristic of the random network based on the rings statistics, radial distribution function and average number of the nearest neighbors (NN). NN found to be 3.2 for C and 1.7 for O for 128 atom system at 15 GPa. We performed vibration analysis of the systems as a function of pressure and calculated in dipole approximation IR intensity with identification of contributions from several main motifs of the amorphous structure. To understand charge distribution and localization and to find the possible ``weakest link'' in the network we calculated electron localization function for the most common fragments of amorphous poly-CO structure. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F39.00008: The role of anharmonicity in the ab-initio phase diagram of calcium Marco Di Gennaro, Srijan Kumar Saha, Matthieu Jean Verstraete In the 32-119 GPa pressure range and at room temperature, a simple cubic phase was reported for calcium in many different experiments. Standard linear response theory, both within density functional perturbation theory and frozen phonon calculations, presents dynamical instabilities for simple cubic in the whole pressure range. Many other possible candidate phases, as well as several possible stabilization mechanisms for simple cubic phase, have been proposed as the result of \emph{ab-initio} predictions but the role of temperature on the relative stability of the different phases has not been investigated systematically. We revisit the stability of three candidate phases of calcium for the intermediate pressure range and for various value of temperatures, taking explicitly into account thermal corrections relative to electronic as well as phononic entropy and anharmonic contributions. This corrects the discrepancies among previous theoretical results and experiments, and presents a different picture of the temperature driven phase transition, which results from dynamical anharmonic stabilization of simple cubic and de-stabilization of the tetragonal phase. Transport quantities are calculated in the stabilized phases, to provide additional points of comparison with experiments. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F39.00009: Origin of Metallization of FeO at High Temperatures and Pressures from First-principles DFT-DMFT Computations R.E. Cohen, Kristjan Haule Experiments and theory show that FeO metallizes at high temperatures ($\sim$2000K) and pressures ($\sim$80 GPa) [1]. Here we use DFT+Dynamical Mean Field Theory (DMFT) with continuous time quantum Monte Carlo (CTQMC) to study the origin of the metallization. We find with increasing pressure in paramagnetic FeO in a cubic lattice a high-spin low-spin transition, with a wide transition region between characterized by intermediate occupancies of the t2g and eg states between. We find that at 300K cubic FeO remains insulating to a factor of two compression (over 600 GPa), except for a small region of high spin metal. However, at high temperatures (e.g. 2000K) a metallic state is found under compression. The metallization occurs from thermal fluctuations among different multiplets representing high- and low-spin states. We are now studying the AFM ground state, the N\'eel transition, and (Mg,Fe)O solid solutions. This work is supposed by NSF.\\[4pt] [1] Ohta, K., Cohen, R. E., Hirose, K., Haule, K., Shimizu, K. \& Ohishi, Y. Experimental and Theoretical Evidence for Pressure-Induced Metallization in FeO with Rocksalt-Type Structure. Phys. Rev. Lett. 108, 026403 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F39.00010: Towards a Predictive First-Principles Description of High Pressure Hydrogen with Density Functional Theory Miguel A. Morales, Jeffrey M. McMahon, Carlo Pierleoni, David M. Ceperley We present a study of the influence of the main approximations employed in first-principles descriptions of high pressure hydrogen with Density Functional Theory. We focus on the importance of nuclear quantum effects (NQE) on equilibrium properties of both liquid and solid molecular hydrogen close to dissociation. We find that NQEs strongly influence intramolecular properties, such as bond stability, and are thus an essential part of the dissociation process. In addition, we show how the combination of both thermal and quantum effects make a drastic change to the predicted optical properties of the molecular solid, demonstrating the very limited value of predictions based on classical ions and static crystals. We also focus on the influence of the chosen exchange--correlation density functional on the predicted properties of hydrogen, including the location of the Liquid-Liquid Phase Transition and the pressure dependence of the band gap in the solid. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F39.00011: Lattice dynamics beyond the harmonic approximation: a compressive sensing approach Fei Zhou, Weston Nielson, Vidvuds Ozolins First-principles modeling of materials in extreme conditions of increasing complexity has had profound impact on revealing and predicting the materials properties and explaining experimental results. Therefore methods and algorithms that can automatically scale to large systems with quantum mechanical accuracy are in dire need. Recently we have shown that a recently developed technique in the field of signal processing, compressed sensing (CS), provides a simple, general, and efficient way of constructing cluster expansion models for alloy systems. Here CS is applied to calculate force constants, including anharmonic effects up to high orders, in solids. CS performs well in extracting accurate lattice dynamics with highly competitive computational costs and reduced human efforts. Compressive sensing for lattice dynamics can be readily applied to much larger systems than ab initio methods can handle and with superior accuracy than classical force fields. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F39.00012: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F39.00013: Neon Hydrate at High Pressure: an in-situ Neutron Diffraction Study Xiaohui Yu Clathrate hydrates are a group of ice-like, crystalline inclusion compounds which form through the combination of water and suitably sized ``guest'' molecules. There are mainly three crystallographic structures of the hydrate clathrate: SI, SII and SH, which are determined by the shape and size of the included gas molecular. However, when the neon gas pressure got increased to 0.48 GPa, we found that the neon gas could be enclathrate in the ice II frameworks which is totally different structure from the traditional cubic clathrate. Through the in-situ neutron diffraction study, the detail structure of Ne hydrate, including the atom positions, can be derived using the Rietveld refinements. The Ne atoms are just in the middle of H$_{2}$O channels and sandwiches by two H$_{2}$O rings The thermal equation of state was calculated and compared with pure ice II. We found that inclusion of Ne atoms could enlarge the ice II H$_{2}$O hexagonal rings, however, shortened the H$_{2}$O channels. Although the Ne atoms crystallized in ice II frameworks, the thermal vibration is significant compared to the host atoms. The distribution of Ne atoms are presented by MD simulations. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F39.00014: Neutron Scattering Study of Hydrogen in Copper Alexander I. Kolesnikov, Vladimir E. Antonov, Garrett E. Granroth, Valery I. Kulakov, Michail A. Kuzovnikov, Ken C. Littrell, Eugene Mamontov Until now, vibrational spectra of hydrogen in the group 1b metals, Cu, Ag and Au, have never been investigated. Meanwhile, these elements are often used in hydrogen containing atmospheres, therefore the properties of hydrogen in these metals are of significant interest. For the present study, Cu-H samples were synthesized by exposing bulk copper to a hydrogen gas at a pressure of 7 GPa and T=900 K and recovering the samples to ambient conditions. The samples were studied by inelastic (INS), quasielastic and small angle neutron scattering. Nearly all hydrogen ($\sim$10 at.\%) contained in the samples proved to be in the form of H2 molecules trapped in large ($>>$100 A) pores/bubbles in the copper matrix. Para $<=>$ ortho transitions in these molecules give intense peaks at +-14.4 and 28.8 meV in the INS spectra. On heating the sample, the molecular hydrogen melts in a temperature interval from 14 to 60 K corresponding to the gradual increase in the H2 pressure in the pores from 5 bar to 3 kbar. A small narrow peak at 73 meV is also observed in the INS spectra. The peak can only be assigned to a local mode of a 0.03 at.\% H impurity in the copper bulk. This is the first observation of H vibrations in a group 1b metal. [Preview Abstract] |
Session F40: Surfaces, Interfaces, and Thin Films: Molecules on Surfaces
Sponsoring Units: DCMPChair: Daniel Dougherty, North Carolina State University
Room: 349
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F40.00001: Molecular Ordering in PCBM Monolayer Films on Ag and Au (111): From $\mu $-aerosol deposited glasses to hcp packing Qian Shao, Levan Tskipuri, Janice Reutt-Robey Functionalized C$_{\mathrm{60}}$ and C$_{\mathrm{70}}$ fullerenes are increasingly employed as active components in organic electronic devices. The structure of the PCBM electrode interface is expected to strongly impact charge transfer processes in photovoltaic devices. Here we report molecularly-detailed studies of PCBM ordering at coinage metal surfaces. We have developed a vacuum-compatible liquid delivery source to generate thin films of C$_{\mathrm{60}}$- and C$_{\mathrm{70}}$-$_{\mathrm{\thinspace }}$PCBM from organic solvents. Structure is tracked from the sub-monolayer to multilayer regime on (111)-oriented Ag and Au surfaces with molecular detail by UHV-STM. Glassy morphologies of as-grown films reflect solvent retention. Upon thermal annealing solvent molecules are released and films evolve into ordered packing arrangements that depend upon the PCBM density in the original films. The hcp monolayer phase of C$_{\mathrm{60}}$-and C$_{\mathrm{70}}$-PCBM are newly produced and characterized, indicating the accessibility of new growth phases by $\mu $-aerosol deposition. Acknowledgement: This work was supported by the NSF-MRSEC at the University of Maryland, DMR 0520471. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F40.00002: Doping of Grain Boundaries in diF TESADT Transistors Cortney Bougher, Shawn M. Huston, Jeremy W. Ward, Abdul Obaid, Marsha A. Loth, John E. Anthony, Oana D. Jurchescu, Brad R. Conrad We utilize Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM) to characterize the dynamics of electronic transport across 2,8-difluoro-5,11-triethylsilylethynyl anthradithiophene (diF TESADT) grain boundaries. We show that the morphology of grain boundaries and the adsorption of atmospheric dopants at these local boundaries have a direct impact on the electrical behavior of diF TESADT in thin film transistor (TFT) devices. Device voltage drops at grain boundaries are characterized as a function of both atmospheric dopants and transition time between dopants. The morphology, including crystallization and packing motifs, of diF TESADT grown on thermally grown SiO$_2$ will be discussed and related to other semiconducting small organic molecules. This work will be put in the context of other, recent advances in small molecule organics. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F40.00003: Scanning Tunneling Microscopy investigation of multilayer diF-TES-ADT on Au(111) Shawn Huston, Jiuyang Wang, Marsha Loth, John Anthony, Brad Conrad, Daniel Dougherty Organic thin film transistors (OTFT) partially composed of solution processed 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene (diF-TES-ADT) have shown high performance with hole mobilities up to 1 cm$^{2}$/(V s). Pretreatment of the gold electrodes results in growth of large diF-TES-ADT crystals extending well out into the channel of the OTFT. Without electrode pretreatment, the crystal sizes are small and possess a non-preferred molecular orientation. We have chosen to investigate the reasons for the reduced crystal size of these films on untreated gold electrodes by studying a model system generated by vapor deposition of multilayers of diF-TES-ADT on Au(111). The initial wetting layer forms a highly ordered film such that the anthradithiophene backbone is oriented parallel to the substrate and the unit cell is 1.49 nm x 1.25 nm with an included angle of 56.8$^{\circ}$. The second layer is poorly ordered with only weak evidence of crystallinity in small regions. Growth beyond the second layer appears essentially bulk-like and crystalline with domain sizes that are potentially limited by the disordered bilayer growth. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F40.00004: STM and optical investigations of molecules on graphene Ozgun Suzer, Joseph Smerdon, Nathan Guisinger, Jeffrey Guest We describe efforts to understand the structural, electronic and optical properties of an archetypal organic molecular building block for graphene-based nano-optical and photovoltaic devices, presenting UHV STM studies of pentacene (Pn) molecules deposited on graphene that was grown epitaxially on SiC(0001). Isolated electronic states are observed and associated molecular orbitals are resolved; also, a large HOMO-LUMO spacing indicates that we are probing a ``transport gap'' in the monolayer Pn. The electronic properties of this system indicate a de-coupling of the molecules from the graphene and underlying substrate, similar to results obtained for the complementary molecular system, C60 on graphene [Cho, et.al. Nano Letters 12, 3018 (2012)], suggesting a path for developing molecular-scale electronic and optically active devices that are not dominated by substrate interactions. We will also discuss our efforts to correlate these studies with the optical properties of the systems using a UHV STM that incorporates confocal optical microscopy and spectroscopy at the tip-sample junction. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F40.00005: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F40.00006: Bonding of anthracene derivatives to a Cu (111) surface: a combined STM and DFT study Jonathan Wyrick, Yeming Zhu, Daniel Salib, Connor Holzke, Zhihai Cheng, Ludwig Bartels We compare and contrast three anthracene derivatives whose 9,10 hydrogens are replaced by the elements O, S, and Se respectively that act as ``feet'' binding the molecules to a Cu (111) substrate. DFT calculations are compared with and shed light on STM data for the three molecules. We analyze the three species in terms of their geometric and electronic structure upon adsorption, taking into account the competing effects that the ``feet'' have with the anthracene moiety in their interactions with the underlying Cu surface. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F40.00007: Substrate Mediated Short-range and Long-range Adsorption Pattern of CO on Ag(110) Wai-Leung Yim Yim, Thorsten Kluener Substrate-mediated intermolecular interactions were proposed in the literature to explain the adsorption of CO on Ag(110) but the underlying mechanism is yet to be known. Here, short-range and long-range relaxation patterns for CO adsorption on Ag(110) surfaces have been investigated. The relaxation mode can be explained by the interaction of heavy electrons on metal substrates in electron momentum space. We identified two relaxation modes for CO on Ag(110). The long-range relaxation involved a (6$\times$6) commensurate phase, while the short-range relaxation involved an alleviation of Fermi surface nesting along the $\langle$1$\bar{1}$0$\rangle$ direction of the Ag(110) substrate. The symmetry broken ground state structure at high CO coverage can be rationalized, and this structure is consistent with the interpretation of available experimental data. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F40.00008: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F40.00009: 5,6,7-trithiapentacene-13-one on vicinal gold (788): a STM study Amanda Larson, Jeremiah van Baren, Jeremy Kintigh, Jun Wang, Glen Miller, Karsten Pohl Scanning tunneling microscopy was used to examine the atomic interface between gold and 5,6,7-trithiapentacene-13-one (TTPO), an electron donor of potential interest for photovoltaic applications. TTPO is a polar species of pentacene with centered oxygen and sulfur bridge substituents. TTPO is a thermally and photo-oxidatively robust molecule with a HOMO-LUMO gap of 1.90 eV that can be thermally evaporated onto an electrode. The vicinal gold (788) surface is a well-studied surface on which pentacene molecules and other pentacene derivatives self assemble in long range order. We examined TTPO on gold to gain a better understanding of the structure of photovoltaic interfaces at the nanoscale. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F40.00010: DFT based modeling of C60/Dichloropentacene on stepped Au surfaces Jun Wang, Jian-Ming Tang, Karsten Pohl The co-assembly of functionalized pentacenes (electron-donor materials) and fullerenes (electron-acceptor materials) on metal substrates provides a model for studying the structural and electronic properties for novel organic photovoltaic (OPV) heterojunctions [1]. Our previous STM experimental results show C$_{60}$ to form molecular chains on an intact single-domain, brick-wall structured 6,13-dichloropentacene (DCP) monolayer adsorbed on stepped Au(788) [2]. Here, we have included a stepped gold substrate in DFT calculations for the geometric and electronic structure of this interacting three-component system. Our calculations show that C$_{60}$ molecular chain prefer to absorb on top of the DCP molecules on the upper step edge. We calculate the dipole moments for various C$_{60}$ configurations. The stepped gold substrate interaction shows a major influence on this unique molecular chain formation. [1] J. Wang, I. Kaur, B. Diaconescu, J.-M. Tang, G. P. Miller, and K. Pohl, ACS Nano 5 (2011) 1792. [2] J. Wang, J.-M. Tang, G. P. Miller, and K. Pohl, in preparation. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F40.00011: First Principles Study of the Electronic Structure of Organic Adsorbates on Cleaved GaP Surfaces Min Yu, Peter Doak, Jeffrey Neaton We report a first principles calculations of structural, electronic, and spectroscopic properties of organic molecules, such as ethylene and benzene, adsorbed on cleaved GaP (110) surface to assess their potential to allow controlled coupling and to modify charge transport between light absorbing semiconductors and catalysts for applications in artificial photosynthesis. We compute adsorbate geometries, binding energetics, surface band structures, constant current scanning tunneling microscopy images, and electronic energy level alignment of organic molecules on GaP surfaces using density functional theory and many-body perturbation theory within the GW approximation. We quantify the impact of coverage, interface dipoles, hybridization, and nonlocal polarization effects on level alignment, and validate our understanding through direct comparison recent measurements. Work supported by JCAP and computational resources provided by NERSC. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F40.00012: Low Temperature STM Study of Single-Molecule Attachment to GaP(110) Aaron Bradley, M.M. Ugeda, M. Yu, K.L. Meaker, J. Neaton, G. Moore, I. Sharp, M.F. Crommie Engineering efficient artificial photosystems for catalytic and photovoltaic (PV) purposes is a major challenge for the development of viable solar fuel generators. One possible route toward this goal is to employ molecular catalysts covalently attached to semiconductor light absorbers through molecular linkages. The effect of such linkage on local electronic structure, however, remains an important question. Scanning tunneling microscopy (STM) is a useful tool for answering this question since it enables characterization of molecular interfaces at the atomic level. Here we describe our progress at measuring the structural and electronic properties of single organic molecules adsorbed to a p-doped GaP(110) surface. Low temperature STM was used to explore the surface chemistry and reactivity of GaP(110) by exposing UHV-cleaved GaP surfaces to sub-monolayer coverages of ethylene (C2H4) and iodobenzene (IC6H6), the latter being a candidate linker for connecting catalysts and PV molecules to semiconducting light absorbers. Our high-resolution STM images in combination with DFT calculations provide guidance for future attachment strategies involving improved molecule/semiconductor interfaces. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F40.00013: Interplay between dynamics of molecule and surface plasmons in scanning tunneling microscope-induced light emission Kuniyuki Miwa, Mamoru Sakaue, Hideaki Kasai Scanning tunneling microscope (STM)-induced light emission spectroscopy of molecules has unique advantage to investigate the luminescence properties of molecules with the atomic-scale spatial resolution. Recently, many attempts have been made to control the molecular luminescence by using the intense electromagnetic field generated by surface plasmons localized near the tip-substrate gap region. In this study, the nonequilibrium Green's function method are utilized to investigate effects of coupling between an exciton composed by electron and hole in the molecule and the surface plasmons on the luminescence properties of the molecule and the surface plasmons. It is found that the luminescence intensities of the molecule are suppressed due to the re-absorption of the surface plasmons by the molecule. Molecular absorption and enhancement by molecular electronic and vibrational modes lead to dip and peak structures in the luminescence spectra of the surface plasmons. Corresponding structures can be seen in a recent experiment. Moreover we found that the re-absorption by the surface plasmons plays important roles in determining the luminescence spectral profiles. We will discuss the detailed mechanisms of variation in these luminescence spectral profiles. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F40.00014: In-situ spectro-microscopy on organic films: Mn-Phthalocyanine on Ag(100) Abdullah Al-Mahboob, Jerzy T. Sadowski, Elio Vescovo Metal phthalocyanines are attracting significant attention, owing to their potential for applications in chemical sensors, solar cells and organic magnets. As the electronic properties of molecular films are determined by their crystallinity and molecular packing, the optimization of film quality is important for improving the performance of organic devices. Here, we present the results of in situ low-energy electron microscopy / photoemission electron microscopy (LEEM/PEEM) studies of incorporation-limited growth [1] of manganese-phthalocyanine (MnPc) on Ag(100) surfaces. MnPc thin films were grown on both, bulk Ag(100) surface and thin Ag(100)/Fe(100) films, where substrate spin-polarized electronic states can be modified through tuning the thickness of the Ag film [2]. We also discuss the electronic structure and magnetic ordering in MnPc thin films, investigated by angle- and spin-resolved photoemission spectroscopy. \\[4pt] [1] Al-Mahboob et al., Phys. Rev. B 82, 235421 (2010).\\[0pt] [2] E. Vescovo et al., Phys. Rev. B 51, 12418 (1995). [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F40.00015: Adsorption structure analysis of co-adsorption dye-sensitized solar cells by the NEXAFS and XPS Mitsunori Honda, Masatoshi Yanagida, Liyuan Han Adsorption structures of N719 dye alone and a N719$+$D131 co-adsorption system on a TiO$_{2}$ electrode were studied with the objective of increasing the efficiencies of dye-sensitized solar cells (DSCs). However, adsorption structure of isothiocyanate (R-N$=$C$=$S) in the alone and co-adsorption system was not completely understood because the surface morphology about nanocrystalline TiO$_{2\, }$is complex. Therefore, we have investigated the adsorption structure on nanocrystalline TiO$_{2}$ surface using the Sulfur K absorption edge (S K-edge) and core level (S 1s) in details by using the near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS), respectively. To consider the co-adsorption effect on DSCs, we analyze the depth profiling by the angle dependent NEXAFS spectroscopy and the chemical state on top of surface by the XPS analysis. As the results, we can determine the electronic structure around S atom in R-N$=$C$=$S in N719 on nanocrystalline TiO$_{2}$ surface. We clarify the adsorption structure of alone and co-adsorption system from the S K-edge NEXAFS and S 1s XPS analysis. I will talk about these results in my presentation. [Preview Abstract] |
Session F41: Rotation, Effective Fields, and Hydrodynamics in Atomic Gases
Sponsoring Units: DAMOPChair: Phil Johnson, American University
Room: 350
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F41.00001: Hydrodynamics and universality in cold atomic gases Alexander Abanov, Manas Kulkarni Recent flurry of experiments on out-of-equilibrium dynamics in cold gases (Bosonic and Fermionic) has raised great interest in understanding collective behaviour of interacting particles. Although the dynamics of interacting gases depends on many details of the system, a great insight can be obtained in a rather universal limit of weak non-linearity, dispersion and dissipation. In this limit, using a reductive perturbation method we map many hydrodynamic models relevant to cold atoms to well known chiral one-dimensional equations such as Korteweg-de Vries (KdV), Burgers, KdV-Burgers, and Benjamin-Ono equations. This mapping [1] of rather complicated hydrodynamic equations to known chiral one-dimensional equations is of great experimental and theoretical interest. For instance, this mapping gives a simple way to make estimates for original hydrodynamic equations and to study phenomena such as shock waves, solitons and the interplay between nonlinearity, dissipation and dispersion. All these phenomena have been observed in experiments and are the hallmarks of nonlinear hydrodynamics.\\[4pt] [1] M. Kulkarni, A. G. Abanov, Phys. Rev. A 86, 033614 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F41.00002: Time-of-flight expansion dynamics of a circulating ring BEC Mark Edwards, Noel Murray, Kevin Wright, Gretchen Campbell, William D. Phillips, Charles W. Clark We have studied the effect of non--zero circulation on the time--of--flight expansion dynamics of a ring--shaped BEC, under conditions matching recent experiments at the Joint Quantum Institute/NIST in Maryland. We modeled the dynamics of the condensate by first solving the time--independent Gross--Pitaevskii equation (GPE) to obtain the initial condensate wavefunction, with the (quantized) circulation set by imprinting an azimuthal phase gradient. This state was then propagated using the time--dependent GPE in real time, with the trapping potential turned off. In the absence of circulation, the BEC expands and closes the central hole in a few milliseconds, eventually resulting in a density profile with a central peak surrounded by a pedestal modulated by weak concentric fringes. When the ring BEC is circulating, the central hole initially decreases in size but never closes due to the phase singularity. In the long--time limit, the size of the central hole scales nearly linearly with the winding number of the circulation state, in good agreement with the NIST experimental results. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F41.00003: Driving phase slips in a neutral-atom analog of an RF SQUID Kevin C. Wright, R.B. Blakestad, J.G. Lee, S.P. Eckel, C.J. Lobb, W.D. Phillips, G.K. Campbell We can deterministically control the quantized circulation state of a toroidal atomic Bose-Einstein condensate by rotating a weak link around the ring above a critical velocity. We vary this critical velocity by controlling the strength of the repulsive optical dipole potential creating the weak link. This system is directly analogous to a superconducting loop in an external magnetic field, where the loop is interrupted by a weak link with a dynamically tunable current-phase relation. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F41.00004: Observation of hysteresis in a superfluid Bose-Einstein condensate with a weak link S. Eckel, J.G. Lee, K.C. Wright, W.D. Phillips, C.J. Lobb, G.K. Campbell Hysteresis is a common feature of superfluid and superconducting systems with Josephson junctions. We have observed hysteresis in the persistent current state of a toroidally-shaped, Bose-Einstein condensate, stirred with a rotating barrier potential. The barrier, which is modeled as a weak link, induces phase slips in the superfluid between well-defined persistent current states. The rotation frequency at which these phase slips occur differ, depending on whether the phase slip results in an increase or decrease of the persistent current. Such behavior in a toroidal BEC is analogous to an RF SQUID, allowing this device to possibly be used as a sensitive rotation sensor. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F41.00005: Stirring a ring Bose-Einstein condensate: vortices and overall circulation Noel Murray, Mark Edwards, Charles W. Clark We have studied the process whereby stirring a superfluid Bose--Einstein condensate confined in a ring-shaped potential leads to an overall circulation. We solved the time-dependent Gross--Pitaevskii equation under conditions chosen to match those of an experiment recently conducted at NIST. Briefly, 500,000 Na atoms where confined at the ring-shaped intersection of a red-detuned horizontal light sheet and a vertically propagating Laguerre--Gauss beam. Stirring was carried via a blue--detuned gaussian beam. We found that, at first, the stirring spawned a number of vortex--antivortex pairs and then stopped. These vortices displayed a complicated dynamical behavior which slowly reduced the number of vortices pairwise via annihilation and singly via diffusion into surface modes of the condensate. At the end of this dynamics, the set of vortices was replaced by an overall circulation of atoms around the ring. We present examples of this behavior, give a simple model of vortex motion and vortex-vortex interaction, and show how the production and annihilation of vortices gets turned into a overall circulation of the ring Bose--Einstein condensate. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F41.00006: Quantum Hall states in rapidly rotating two-component Bose gases Shunsuke Furukawa, Masahito Ueda Ultracold atomic gases under rapid rotation offer interesting analogues of quantum Hall systems with variable statistics and spins of constituent particles. Here we study strongly correlated phases of two-component (or pseudo-spin-$1/2$) Bose gases under rapid rotation by means of exact diagonalization. As the ratio of the inter-component contact interaction $g_{\uparrow\downarrow}$ to the intra-component one $g$ increases, the two components are expected to be entangled to form novel ground states. For $g_{\uparrow\downarrow}=g$, we find the formation of gapped spin-singlet states at the filling factors $\nu=k/3+k/3$ (the $k/3$ filling for each component) with integer $k$. In particular, we present numerical evidences that the gapped state with $k=2$ is well described as a non-Abelian spin-singlet (NASS) state, in which excitations feature non-Abelian statistics. Furthermore, we find the phase transition from the product of composite fermion states to the NASS state by changing the interaction ratio $g_{\uparrow\downarrow}/g$. Reference: Phys. Rev. A 86, 031604(R) (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F41.00007: Vortex formation in a rotating reference frame Michael Ray, Thomas Langin, David Hall We create vortices in a trapped Bose-Einstein condensate by cooling the atomic sample through the phase transition in the presence of a rotating magnetic trapping potential. The thermal cloud remains in quasi-equilibrium during the cooling, ultimately producing condensates in the rotating ground state. We show that the trap rotation frequency at which a vortex first appears agrees closely with theoretical predictions. The number of vortices within the condensate is established by the rotation frequency at the phase transition; once the condensate has started to form, its vortex content is robust against frequency changes. Images of the condensate taken during evaporation suggest that the vortex spatial configuration is similarly determined early on in the growth of the condensate. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F41.00008: Quantum Monte Carlo study of the drag coefficient for two-component BECs Thomas Goldstein, Christopher Varney, Egor Babaev, Nikolay Prokofiev, Boris Svistunov Groundbreaking advances in experimental techniques for ultracold gases have resulted in considerable interest in multi-component systems, which exhibit richer physics than single species systems. Recent theoretical work has established the strong possibility of ``entrainment'' coupling between components in a two-component BEC. In this talk, we present quantum Monte Carlo simulations of the drag coefficient in a two-component Bose-Hubbard model. Next, we utilize Langevin dynamics to determine manifestations of the intercomponent drag in the ground state structure of vortices in multi-component superfluids. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F41.00009: Periodically kicked quantum Hall system of cold atoms Mahmoud Lababidi, Indubala Satija, Erhai Zhao The integer quantum Hall state is characterized by chiral edge modes associated with the topological invariant, the Chern number. We numerically study a non-equilibrium, periodically driven quantum hall system of fermionic atoms in a square optical lattice. We show that periodically modulated tunneling gives rise to new edge states inside the quasi-energy band gaps. We present a phase diagram with a zoo of interesting phases as functions of driving parameters, along with the spectral evolution of the edge states through the topological quantum phase transitions. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F41.00010: Experimental Validation of Interferometry Simulations on an Atom Chip Violeta Prieto, Jason Alexander, Christopher Rowlett, William Golding, Patricia Lee We report on recent experimental results on manipulating cold atoms trapped on a chip for the development of a compact atom interferometer using a double-well potential. The experiment uses $^{87}$Rb atoms magnetically confined in an atomic waveguide produced by wires on the surface of a lithographically patterned chip. The double-well potential is created by dynamically changing the current configuration on our atom chip.~ By dynamically powering traces on the atom chip while simultaneously varying external bias fields, we offer a means to coherently split the atomic cloud.~ We investigate real-time transformations, both adiabatic and non-adiabatic, between different double-well configurations and study their effects on the initially trapped atoms. We examine the coherence properties of the two atomic wavepackets and evaluate their potential use in an atom interferometer. [Preview Abstract] |
Session F42: Focus Session: Multiscale Modeling--Coarse-graining in Space and Time III
Sponsoring Units: DCPChair: William Noid, Pennsylvania State University
Room: Hilton Baltimore Holiday Ballroom 3
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F42.00001: Adaptive Resolution Simulations: Applications and New Developments towards Open Systems MD Invited Speaker: Kurt Kremer The relation between atomistic structure, architecture, molecular weight and material properties is a basic concern of modern soft matter science. A typical additional focus is on surface interface aspects or the relation between structure and function in nanoscopic molecular assemblies. Here computer simulations on different levels of resolution play an increasingly important role. To progress further adaptive schemes are being developed, which allow for a free exchange of particles (atoms, molecules) between the different levels of resolution. The lecture will concentrate on these methods, however will also include first approaches to connect particle based simulations to continuum as well as to include quantum effects. Furthermore the extension to open systems MD as well as new recent methodology advances will be explained. A general review on the first part can be found in M. Praprotnik et al. Ann. Rev. Phys. Chem. 59, 2008 and recent advances in S. Fritsch et al. PRL 108, 170602 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F42.00002: Ions without Charges, Hydrogen-Bonds without Hydrogen: Coarse-Grained Models with Short-Range, Anisotropic Interactions Invited Speaker: Valeria Molinero Water, ions, hydrophilic and hydrophobic moieties are the building blocks of materials and biomolecules. Modeling of the hydrogen-bonded structure of water is particularly challenging for coarse-grained simulations. Nevertheless, reproducing the hydrogen-bonded order of water is necessary not only to reproduce the anomalous thermodynamics, structure and dynamics of liquid water, but also its properties as solvent of ions and hydrophobes, and water-driven interactions. In this talk I will discuss a strategy for the development of coarse-grained models based on short-range anisotropic interactions, and their application for the development of accurate and efficient coarse-grained models of water, solvated ions and DNA, methane and hydrophobic nanoparticles and cavities. These models are 100 to 1000 times computationally more efficient than atomistic models while having quite high fidelity in the description of the structure and -with some caveats- their thermodynamics. I will discuss the level of agreement of the coarse-grained simulations with experimental or atomistic results, and highlight some of their applications. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F42.00003: Solving Multiscale Polymer Field Theory Simulations with Lattice Boltzmann Equation Hsieh Chen, YongJoo Kim, Alfredo Alexander-Katz A new Lattice Boltzmann (LB) approach is introduced to solve for the modified diffusion equations in polymer field theory. This method bridges two desired properties from different numerical techniques, namely: (i) it is robust and stable as the pseudo-spectral method, and (ii) it is flexible and allows for grid refinement and arbitrary boundary conditions. While the LB method is not as accurate as the pseudo-spectral method, full self-consistent field theoretic (SCFT) simulations of block copolymers on graphoepitaxial templates yield indistinguishable results from pseudo-spectral calculations. Furthermore, we were able to achieve speedups of about 100x compared to single CPU core implementations by using graphics processing units (GPUs). We expect this method to be very useful in truly multi-scale studies where small length scale details have to be resolved, such as in strongly segregating block copolymer blends, nanoparticle-polymer interfaces, or polymer wetting phenomena. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F42.00004: Model path-integral dynamics for nonadiabatic reactions in the condensed phase Nandini Ananth, Artur Menzeleev, Thomas Miller We introduce mapping-variable ring polymer molecular dynamics (MV-RPMD), a direct, real-time dynamic technique for the atomistic simulation of nonadiabatic reactions. The dynamics are based on the recently derived exact path-integral Stock-Thoss (PI-ST)representation for the quantum Boltzmann operator that has been previously used to calculate equilibrium properties for N-level systems, and as a way to initialize semiclassical trajectories for the calculation of thermal correlation functions. Both these methods use the Stock-Thoss (ST) mapping protocol to map from a discrete electronic states basis to a continuous Cartesian variables basis, providing the even-handed treatment of electrons and nuclei required to accurately describe their dynamically coupled motions and to describe resonance energy transfer. Like the existing RPMD approach, this method can be used to generate statistically meaningful ensembles of reactive trajectories but, unlike RPMD, it is applicable to photochemical reactions and reactions where proper electronic state quantization is essential. We present the results of simulations using MV-RPMD to calculate correlation functions for a series of model N-level systems over a wide range of nonadiabatic coupling strengths. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F42.00005: Coarse-Grained Simulation of Solvated Cellulose Ib Microfibril Bingxin Fan, Janna Maranas We construct a coarse-grained (CG) model of cellulose microfibrils in water. The force field is derived from atomistic simulation of a 40 glucose-unit-long microfibril by requiring consistency between the chain configuration, intermolecular packing and hydrogen bonding of the two levels of modeling. Intermolecular interactions such as hydrogen bonding are added sequentially until the force field holds the microfibril crystal structure. This stepwise process enables us to evaluate the importance of each potential and provides insight to ordered and disordered regions. We simulate cellulose microfibrils with 100 to 400 residues, comparable to the smallest observed microfibrils. Microfibrils longer than 100nm would form a bending region along their longitudinal direction. Multiple bends are observed in the microfibril containing 400 residues. Although the cause is not clear, the bending regions may provide us insights about the periodicity and the behavior of the disordered regions in the microfibril. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F42.00006: Development and application of coarse-grained models for lipids Invited Speaker: Qiang Cui I'll discuss a number of topics that represent our efforts in developing reliable molecular models for describing chemical and physical processes involving biomembranes. This is an exciting yet challenging research area because of the multiple length and time scales that are present in the relevant problems. Accordingly, we attempt to (1) understand the value and limitation of popular coarse-grained (CG) models for lipid membranes with either a particle or continuum representation; (2) develop new CG models that are appropriate for the particular problem of interest. As specific examples, I'll discuss (1) a comparison of atomistic, MARTINI (a particle based CG model) and continuum descriptions of a membrane fusion pore; (2) the development of a modified MARTINI model (BMW-MARTINI) that features a reliable description of membrane/water interfacial electrostatics and its application to cell-penetration peptides and membrane-bending proteins. Motivated specifically by the recent studies of Wong and co-workers, we compare the self-assembly behaviors of lipids with cationic peptides that include either Arg residues or a combination of Lys and hydrophobic residues; in particular, we attempt to reveal factors that stabilize the cubic ``double diamond" Pn3m phase over the inverted hexagonal H$_{II}$ phase. For example, to explicitly test the importance of the bidentate hydrogen-bonding capability of Arg to the stabilization of negative Gaussian curvature, we also compare results using variants of the BMW-MARTINI model that treat the side chain of Arg with different levels of details. Collectively, the results suggest that both the bidentate feature of Arg and the overall electrostatic properties of cationic peptides are important to the self-assembly behavior of these peptides with lipids. The results are expected to have general implications to the mechanism of peptides and proteins that stimulate pore formation in biomembranes. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F42.00007: Coarse-Grained Molecular Simulation of Lipid Self-Assembly Invited Speaker: Wataru Shinoda The talk will review our recent work on understanding the behavior of lipid self-assembly using a coarse-grained (CG) force field model developed recently [1]. The CG model is designed to reproduce experimental surface/interfacial properties as well as distribution functions from all-atom (AA) molecular dynamics (MD) simulations. A series of MD simulations has elucidated that the CG model reproduces the phase diagram reasonably and produces the membranes with reasonable elastic moduli, surface and line tensions. With a help of technical development of free energy computation, we have evaluated the stability of liposome. [2] A comparison of CG-MD and a simple continuum theory for the free energy barrier to the vesicle-to-bicelle transformation reveals that the internal structural relaxation in the bilayer membrane plays an important role in lowering the free energy barrier in case of a small unilamellar vesicle. [2] The effects of lipid components and additives are also discussed in this talk. Especially the effect of fullerenes on the membrane properties will be discussed in details.[3] The behavior of fullerenes in the bilayer membrane and the resultant membrane properties depend on the size of fullerene and bilayer thickness quite sensitively. To discuss these details, we need a chemically accurate CG model constructed based on extensive AA-MD results.\\[4pt] [1] W. Shinoda, R. DeVane, M. L. Klein, Mol Simul 33, 27 (2007); ibid, Soft Matter 4, 2454 (2008); ibid, J. Phys. Chem. B 114, 6836 (2010); ibid, Soft Matter, 7, 6178 (2011).\\[0pt] [2] W. Shinoda, T. Nakamura, S. O. Nielsen, Soft Matter, 7, 9012(2011).; T. Nakamura, W. Shinoda, T. Ikeshoji, J. Chem. Phys. 135, 094106 (2011).\\[0pt] [3] R. DeVane et al. J. Phys. Chem. B, 114, 6386 (2010); C. Chiu et al. J. Phys. Chem. B 114, 6394 (2010). R. DeVane et al. J. Phys. Chem. B, 114, 16364 (2010); A. Jusufi et al. Soft Matter 7, 1139 (2011); C. Chiu et al. Soft Matter, 8 9610(2012). [Preview Abstract] |
Session F43: Focus Session: Motor dynamics---from Single Molecules to Cells I
Sponsoring Units: DCPChair: Zev Bryant, Stanford University
Room: Hilton Baltimore Holiday Ballroom 2
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F43.00001: Molecular Motors from DNA Invited Speaker: Andrew Turberfield DNA is a wonderful material for nanoscale construction: its self-assembly can be programmed by making use of its information-carrying capability and its hybridization or hydrolysis can be used as to provide energy for synthetic molecular machinery. With DNA it is possible to design and build three-dimensional scaffolds, to attach molecular components to them with sub-nanometre precision--and then to make them move. I shall describe our work on autonomous, biomimetic molecular motors powered by chemical fuels and the use of synthetic molecular machinery to control covalent chemical synthesis. I shall demonstrate bipedal motors whose operation depends on the coordination of the chemomechanical cycles of two separate catalytic centres and burnt bridges motors that can be programmed to navigate networks of tracks. I shall also discuss the use of kinesin motor proteins to power synthetic devices. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F43.00002: Remote control of molecular motors using light-activated gearshifting Invited Speaker: Zev Bryant Engineering molecular motors with dynamically controllable properties will allow selective perturbation of mechanical processes in vivo and provide sophisticated components for directed nanoscale transport in vitro. We previously constructed myosin motors that respond to a change in [Ca++] by reversing their direction of motion along the polarized actin filament [1]. To expand the potential applications of controllable molecular motors, we have now developed myosins that shift gears in response to blue light illumination. Light is a versatile control signal that can be readily modulated in time and space, and is generally orthogonal to cellular signaling. Using structure-guided protein engineering, we have incorporated LOV photoreceptor domains into the lever arms of chimeric myosins, resulting in motors that robustly speed up, slow down, or switch directions upon illumination. These genetically encoded motors should be directly deployable inside living cells. Our successful designs include constructs based on two different myosin classes, and we show that optical velocity control can be implemented in motors that move at microns/sec speeds, enabling practical biological and bioengineering applications.\\[4pt] [1] Chen, L., Nakamura, M., Schindler,T.D., and Bryant Z. (2012). Nat. Nanotechnol. 7, 252-6. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F43.00003: Anomalous dynamics in intracellular transport Invited Speaker: Aaron Dinner This talk will describe quantitative analyses of particle tracking data for systems with cytoskeletally associated molecular motors to better understand the motions contributing to intracellular transport and, more generally, means for characterizing systems far from equilibrium. In particular, we have studied the motions of insulin-containing vesicles (granules) in a pancreatic beta cell line. We find subdiffusive behavior with correlations in both space and time. These data can be modeled by subordinating an ergodic random walk process to a non-ergodic one. We relate the dynamics to the underlying microtubule structure by imaging in the presence of the drug vinblastine. Our results provide a simple physical mechanism for how diverse pools of insulin granules and, in turn, biphasic secretion could arise. Time permitting, these dynamics will be compared with those of actomyosin assemblies. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F43.00004: Morphogenetic Functions of Actomyosin Invited Speaker: Stephan W. Grill Morphogenesis refers to the generation of form in Biology. Much is known about molecular mechanisms of regulation, but little is known about the physical mechanisms by which an unpatterned blob of cells develops into a fully structured and formed organism. The actomyosin cortex is a thin layer underneath the cellular membrane that can self contract, which drives many of the large-scale morphogenetic rearrangements that are observed during development. How this cortex reshapes and deforms, and how such morphogenetic processes couple to regulatory biochemical pathways is largely unknown. I will discuss two emergent physical activities of the actomyosin cytoskeleton, an active contractile tension and an active torque, both of which can serve to drive flows and large-scale chiral rotations of the actomyosin cytoskeleton. Discussing two biological examples, polarization of the Caenorhabditis elegans zygote and epiboly during zebrafish gastrulation, I will illustrate how active tension drive flows, how molecular constituents of the cortex affect flows, and how morphogenetic patterns can be formed by coupling regulatory biochemistry to active cortical mechanics. A particular focus will be the investigation of how active chiral torques drive chiral flow, and the resulting functions of such chiral activities of the actomyosin cytoskeleton for left-right symmetry breaking in development. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F43.00005: Force-balance model of suppression of multipolar division in cancer cells with extra centrosomes Invited Speaker: Jie Zhu Cancer cells often possess extra centrosomes which have the potential to cause cell death due to catastrophic multipolar division. Many cancer cells, however, are able to escape multipolar mitosis by clustering the extra centrosomes to form bipolar spindles. The mechanism of centrosome clustering is therefore of great interest to the development of anti-cancer drugs because the de-clustering of extra centrosomes provides an appealing way to eliminate cancer cells while keeping healthy cells intact. We present a physical model assuming 1) dynamic centrosomal microtubules interact with chromosomes by both pushing on chromosome arms and pulling along kinetochores; 2) these microtubules interact with force generators associated with actin/adhesion structures at the cell boundary; and 3) motors act on anti-parallel microtubules from different centrosomes. We find via computer simulations that chromosomes tend to aggregate near the cell center while centrosomes can be either clustered to form bipolar spindles or scattered to form multipolar spindles, depending on the strengths of relative forces, cell shape and adhesion geometry. The model predictions agree with data from cells plated on adhesive micropatterns and from biochemically or genetically perturbed cells. Furthermore, our model is able to explain various microtubule distributions in interphase cells on patterned substrates. [Preview Abstract] |
Session F44: Focus Session: Stochasticity in Cellular Networks
Sponsoring Units: DBIOChair: Ilya Nemenman, Emory University
Room: Hilton Baltimore Holiday Ballroom 1
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F44.00001: Positive feedback produces broad distributions in maximum activation attained within a narrow time window in stochastic biochemical reactions Jayajit Das Stochastic fluctuations in biochemical reactions can regulate single cell decision processes. Using exact solutions and semi-analytical methods we calculate distributions of the maximum value ($N$) of species concentrations ($P_{max}(N)$) and the time ($t$) taken to reach the maximum value ($P_{max}(t)$) in minimal models of stochastic chemical reactions commonly found in cell signaling systems. We find, the presence of positive feedback interactions make $P_{max}(N)$ more spread out with a higher ``peakedness'' in $P_{max}(t)$. Thus positive feedback interactions may help single cells to respond sensitively to a stimulus when cell decision processes require upregulation of activated forms of key proteins to a threshold number within a time window. Moreover, unlike other models of strongly correlated random variables such as Brownian walks or fluctuating interfaces, the extreme value distributions for the chemical reactions display multiscaling behavior emphasizing the presence of many time scales in cell signaling kinetics. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F44.00002: The effect of extrinsic noise on cellular decision making Elijah Roberts, Michael Assaf, Zaida Luthey-Schulten, Nigel Goldenfeld Many cellular processes are not deterministic, i.e., genetically identical cells can display different phenotypic behavior even in identical environments. Such processes involve cellular decision making, in which individual cells randomly make choices determining their fate. One view is that the stochastic nature of cellular decision making is due to noise present in the biomolecular interaction networks. Most previous work has focused on the role of intrinsic noise of these networks. Yet, especially in the high copy-number regime, extrinsic noise may be much more significant, likely governing the overall dynamics. Here we develop a theoretical framework describing the combined effect of intrinsic and extrinsic noise on the stochastic dynamics of genetic switches responsible for cellular decision making. We do so by devising a semi-classical theory accounting for extrinsic noise as an effective species. Our theory, corroborated by extensive Monte-Carlo simulations, is tested on a simple bistable self-regulating gene model, and is then generalized to gain insight on the behavior of the lac genetic switch under extrinsic noise. We show that extrinsic noise not only significantly lowers the escape time from a phenotypic state, but can fundamentally change the actual escape process. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F44.00003: Temporally Resolved Axonal Growth Rates: A Stochastic Study Daniel Rizzo, Ross Beighley, Matt Wiens, James White, Timothy Atherton, Cristian Staii Description of neuron growth behavior is essential in elucidating the environmental factors that prompt the formation of neural networks. However, the staggering number of physical and chemical guidance cues that influence axonal growth prohibits understanding of growth behavior from a purely mechanistic perspective. Using a phenomenological approach, we record the distribution of growth speeds in neurons at several time points, under well-controlled conditions. Using these distributions in combination with a 1-dimensional Fokker-Planck equation, we solve for the velocity potential of axonal growth for our system as a function of time. In so doing, we aim to resolve time-sensitive growth events that are otherwise overlooked in post-growth studies. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F44.00004: Coarse-graining stochastic biochemical networks Invited Speaker: Ilya Nemenman Biochemical processes typically involve huge numbers of individual reversible or irreversible steps, each with its own dynamical rate constants. Does the structural complexity of these biochemical networks necessarily result in complex dynamics? I will discuss a few examples where simple, nearly universal stochastic dynamical behaviors emerge from this complexity, and sometimes precisely because of this complexity. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F44.00005: The addition of a coarse-grained looping state enhances bistability in a gene expression model of lac Tyler Earnest, Elijah Roberts, Michael Assaf, Karin Dahmen, Zaida Luthey-Schulten Bistability of the \textit{lac} genetic switch in \textit{Escherichia coli} is known to depend on its ability to form DNA loops with the \textit{lac} repressor. Here we present a stochastic gene--mRNA--protein model of the \textit{lac} switch that includes a third transcriptional state describing the DNA loop. We introduce a novel geometric burst extension to the finite state projection method, which allows us to eliminate mRNA as an independent species and rapidly search the parameter space of the model. We evaluate how the addition of the third state changes the model's bistability properties and find a region of parameter space where the system behaves in a way consistent to that seen experimentally for \textit{lac}. Induction in the looping model is preceded by a rare complete dissociation of the loop followed by an immediate burst of mRNA rather than a slower build up of mRNA as in the two-state model. The overall effect of the looped state is to allow for faster switching times while at the same time further separating the uninduced and induced phenotypes from each other. These properties of loop regulatory elements give them intriguing implications for use in synthetic biology. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F44.00006: Large number of receptors may reduce cellular response time variation Xiang Cheng, Lina Merchan, Martin Tchernookov, Ilya Nemenman Cells often have tens of thousands of receptors, even though only a few activated receptors can trigger full cellular responses. Reasons for the overabundance of receptors remain unclear. We suggest that the large number of receptors results in a competition among receptors to be the first to activate the cell. The competition decreases the variability of the time to cellular activation, and hence results in a more synchronous activation of cells. We argue that, in simple models, this variability reduction does not necessarily interfere with the receptor specificity to ligands achieved by the kinetic proofreading mechanism. Thus cells can be activated accurately in time and specifically to certain signals or ligands. We predict the minimum number of receptors needed to reduce the coefficient of variation for the time to activation following binding of a specific ligand. Further, we predict the maximum number of receptors so that the kinetic proofreading mechanism still can improve the specificity of the activation. These predictions fall in line with experimentally reported receptor numbers for multiple systems. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F44.00007: Information flow through calcium binding proteins Ji Hyun Bak, William Bialek Calcium signaling is a ubiquitous mode of biological communication, which regulates a great variety of vital processes in living systems. Such a signal typically begins with an elementary event, in which calcium ions bind to a protein, inducing a change in the protein's structure. Information can only be lost, from what was conveyed through this initial event, as the signal is further transduced through the downstream networks. In the present work we analyze and optimize the information flow in the calcium binding process. We explicitly calculate the mutual information between the calcium concentration and the states of the protein, using a simple model for allosteric regulation in a dimeric protein. The optimal solution depends on the dynamic range of the input as well as on the timescale of signal integration. According to our result, the optimizing strategy involves allowing the calcium-binding protein to be ``activated'' by a partial occupation of its sites, and tuning independently the strengths of cooperative interactions in the binding and unbinding processes. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F44.00008: Noise and fidelity of information transmission through the Tumor Necrosis Factor signaling circuit Invited Speaker: Andre Levchenko Molecular noise restricts the ability of an individual cell to resolve input signals of different strengths and gather information about the external environment. We developed an integrative theoretical and experimental framework, based on the formalism of information theory, to quantitatively predict and measure the amount of information transduced by molecular and cellular networks. Analyzing tumor necrosis factor (TNF) signaling revealed that individual TNF signaling pathways transduce information sufficient for accurate binary decisions, and an upstream bottleneck limits the information gained via multiple integrated pathways. Negative feedback to this bottleneck could both alleviate and enhance its limiting effect, despite decreasing noise. Bottlenecks likewise constrain information attained by networks signaling through multiple genes or cells. We further use this new analysis formalism to ``map'' the noise amplitude across different parts of the network. Finally, we show that the redundancy in signaling due to the existence of parallel pathways is not absolute, and that parallel pathways can transmit different types of information about the input, i.e., the duration vs. amplitude. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F44.00009: Morphogenesis at criticality? Dmitry Krotov, Julien Dubuis, Eric Wieschaus, Thomas Gregor, William Bialek Embryonic development of many multicellular organisms begins with the generation of spatially varying patterns of morphogens that encode the body plan of the future organism. We study the spatial pattern formed by the gap gene proteins in the early fruit fly embryo, which is anchored by ``crossing points'' between expression levels of different genes; these are thought to result from mutual repression. We explore a broad class of models for such interacting genes and show that the parameters implied implied by recent quantitative measurements are non-generic, but rather tuned to certain values, so that the entire gap gene network operates close to the critical surface in its phase diagram. We develop a mean field description of this system as well as derive signatures of critical behavior in the structure of expression noise. One such signature is that fluctuations are dominated by a single ``massless'' mode, so that fluctuations of expression levels of different genes are highly correlated/anticorrelated. We find a surprisingly high degree of anticorrelation in the real experimental data. These results suggest an interesting possibility that the network of genes responsible for development is operating near criticality. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F44.00010: Modeling the Dynamics of Bivalent Histone Modifications in Embryonic Stem Cells Wai Lim Ku, Guo Cheng Yuan, Francesco Sorrentino, Michelle Girvan, Edward Ott Epigenetic modifications to histones may either promote the activation or repression of the transcription of nearby genes. Recent experiments have discovered bivalent domains of nucleosomes in which the domain as a whole contains both active and repressive marks. These domains occur in the promoters of most lineage-control genes in embryonic stem cells. It is generally agreed that bivalent domains play an important role in stem cell differentiation, but the mechanisms remain unclear. Here we propose and study a dynamical model of histone modification which, unlike previous models, captures the general features of the bivalent domains observed in experiments. A key feature of our model is the existence of ``A/R states,'' by which we mean states in which there are a significant number of nucleosomes \textit{each} of which \textit{individually} has both active and repressive marks. We use our model to investigate the formation and decay of A/R states, the localization of A/R nucleosomes, and the effect of DNA replication on the stability of A/R states. The goals of our model are to help understand the underlying principles and mechanisms of bivalent domain dynamics and to suggest directions for future experiments. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F44.00011: Using entropy to cut complex time series David Mertens, Julia Poncela Casasnovas, Bonnie Spring, L.A.N. Amaral Using techniques from statistical physics, physicists have modeled and analyzed human phenomena varying from academic citation rates to disease spreading to vehicular traffic jams. The last decade's explosion of digital information and the growing ubiquity of smartphones has led to a wealth of human self-reported data. This wealth of data comes at a cost, including non-uniform sampling and statistically significant but physically insignificant correlations. In this talk I present our work using entropy to identify stationary sub-sequences of self-reported human weight from a weight management web site. Our entropic approach--inspired by the infomap network community detection algorithm--is far less biased by rare fluctuations than more traditional time series segmentation techniques. [Preview Abstract] |
Session F45: Focus Session: Physics of Proteins I
Sponsoring Units: DBIO DPOLYChair: Aihua Xie, Oklahoma State
Room: Hilton Baltimore Holiday Ballroom 4
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F45.00001: Exploring the landscape for protein folding: from function to molecular machines Invited Speaker: Jose Onuchic Globally the energy landscape of a folding protein resembles a partially rough funnel with reduced energetic frustration. A consequence of minimizing energetic frustration is that the topology of the native fold also plays a major role in the folding mechanism. Some folding motifs are easier to design than others suggesting the possibility that evolution not only selected sequences with sufficiently small energetic frustration but also selected more easily designable native structures. The overall structures of the on-route and off-route (traps) intermediates for the folding of more complex proteins are also strongly influenced by topology. Going beyond folding, the power of reduced models to study the physics of protein assembly, protein binding and recognition, and larger biomolecular machines has also proven impressive. Since energetic frustration is sufficiently small, native structure-based models, which correspond to perfectly unfrustrated energy landscapes, have shown to be a powerful approach to explore larger proteins and protein complexes, not only folding but also function associated to large conformational motions. A discussion of how global motions control the mechanistic processes in the ribosome and molecular motors will be presented. For example, this conceptual framework is allowing us to envisage the dynamics of molecular motors and the ribosome from the structural perspective and it provides the means to make quantitative predictions that can be tested by experiments. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F45.00002: Predicting folding-unfolding transitions in proteins without a priori knowledge of the folded state Osman Okan, Deniz Turgut, Angel Garcia, Rahmi Ozisik The common computational method of studying folding transitions in proteins is to compare simulated conformations against the folded structure, but this method obviously requires the folded structure to be known beforehand. In the current study, we show that the use of bond orientational order parameter (BOOP) Q$_{l}$ [Steinhardt PJ, Nelson DR, Ronchetti M, Phys. Rev. B 1983, 28, 784] is a viable alternative to the commonly adopted root mean squared distance (RMSD) measure in probing conformational transitions. Replica exchange molecular dynamics simulations of the trp-cage protein (with 20 residues) in TIP-3P water were used to compare BOOP against RMSD. The results indicate that the correspondence between BOOP and RMSD time series become stronger with increasing \textit{l}. We finally show that robust linear models that incorporate different Q$_{l}$ can be parameterized from a given replica run and can be used to study other replica trajectories. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F45.00003: Exploring Beta-Amyloid Protein Transmembrane Insertion Behavior and Residue-Specific Lipid Interactions in Lipid Bilayers Using Multiscale MD Simulations Liming Qiu, Mark Vaughn, Kelvin Cheng Beta-amyloid (Abeta) interactions with neurons are linked to Alzheimer's. Using a multiscale MD simulation strategy that combines the high efficiency of phase space sampling of coarse-grained MD (CGD) and the high spatial resolution of Atomistic MD (AMD) simulations, we studied the Abeta insertion dynamics in cholesterol-enriched and -depleted lipid bilayers that mimic the neuronal membranes domains. Forward (AMD-CGD) and reverse (CGD-AMD) mappings were used. At the atomistic level, cholesterol promoted insertion of Abeta with high (folded) or low (unfolded) helical contents of the lipid insertion domain (Lys28-Ala42), and the insertions were stabilized by the Lys28 snorkeling and Ala42-anchoring to the polar lipid groups of the bilayer up to 200ns. After the forward mapping, the folded inserted state switched to a new extended inserted state with the Lys28 descended to the middle of the bilayer while the unfolded inserted state migrated to the membrane surface up to 4000ns. The two new states remained stable for 200ns at the atomistic scale after the reverse mapping. Our results suggested that different Abeta membrane-orientation states separated by free energy barriers can be explored by the multiscale MD more effectively than by Atomistic MD simulations alone. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F45.00004: Combined copper/zinc attachment to prion protein Miroslav Hodak, Jerry Bernholc Misfolding of prion protein (PrP) is responsible for diseases such as ``mad-cow disease'' in cattle and Creutzfeldt-Jacob in humans. Extensive experimental investigation has established that this protein strongly interacts with copper ions, and this ability has been linked to its still unknown function. Attachment of other metal ions (zinc, iron, manganese) have been demonstrated as well, but none of them could outcompete copper. Recent finding, however, indicates that at intermediate concentrations both copper and zinc ions can attach to the PrP at the octarepeat region, which contains high affinity metal binding sites. Based on this evidence, we have performed density functional theory simulations to investigate the combined Cu/Zn attachment. We consider all previously reported binding modes of copper at the octarepeat region and examine a possibility simultaneous Cu/Zn attachment. We find that this can indeed occur for only one of the known binding sites, when copper changes its coordination mode to allow for attachment of zinc ion. The implications of the simultaneous attachment on neural function remain to be explored. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F45.00005: Direct observation of apolipoprotein B refolding at single molecule level by ultra sensitive fluorescence microscopy and solution transmission electron microscopy Chia-Ching Chang, Hsueh-Liang Chu, Hsing-Yuan Lee, Tsai-Mu Cheng, Gong-Shen Chen, Fu-Rong Chen Apolipoprotein (apo) B is the only protein of low-density lipoprotein (LDL). The huge size and extreme hydrophobicity of apoB make examination of its lipidation process an experimental challenge. In this study, we showed that apoB lipidation and its intermediates could be observed at single molecule level by an on-path folding process. When carboxyl-terminal-truncated mutants apoB-29 and apoB-48, representing the amino-terminal 29{\%} and 48{\%}, respectively, of the full-length apoB (apoB-100), were used for comparison, we observed that the refolded apoB-100 resembled both native LDL and VLDL precursors. Thus the process of lipidation recapitulates that of pre-VLDL assembly, \textit{in vitro}. These results suggest that the assembly of mature VLDL requires involvement of factors in addition to apoB-100 and lipids. Using solution transmission electron microscopy (TEM), we were able to detect incorporation of hydrophobic super-paramagnetic iron oxide nanoparticles into apoB-100 particles at the initial, but not final, stage of refolding. The current study thus demonstrates that VLDL assembly can be monitored at single molecule level, too. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F45.00006: The strength of side chain hydrogen bonds in the plasma membrane Kalina Hristova, Sarvenaz Sarabipour There are no direct quantitative measurements of hydrogen bond strengths in membrane proteins residing in their native cellular environment. To address this knowledge gap, here we use fluorescence resonance energy transfer (FRET) to measure the impact of hydrogen bonds on the stability of a membrane protein dimer in vesicles derived from eukaryotic plasma membranes, and we compare these results to previous measurements of hydrogen bond strengths in model lipid bilayers. We demonstrate that FRET measurements of membrane protein interactions in plasma membrane vesicles have the requisite sensitivity to quantify the strength of hydrogen bonds. We find that the hydrogen bond-mediated stabilization in the plasma membrane is small, only -0.7 kcal/mole. It is the same as in model lipid bilayers, despite the different nature and dielectric properties of the two environments. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F45.00007: Molecular Dynamics Simulations of Hydrophobic Residues Diego Caballero, Alice Zhou, Lynne Regan, Corey O'Hern Molecular recognition and protein-protein interactions are involved in important biological processes. However, despite recent improvements in computational methods for protein design, we still lack a predictive understanding of protein structure and interactions. To begin to address these shortcomings, we performed molecular dynamics simulations of hydrophobic residues modeled as hard spheres with stereo-chemical constraints initially at high temperature, and then quenched to low temperature to obtain local energy minima. We find that there is a range of quench rates over which the probabilities of side-chain dihedral angles for hydrophobic residues match the probabilities obtained for known protein structures. In addition, we predict the side-chain dihedral angle propensities in the core region of the proteins T4, ROP, and several mutants. These studies serve as a first step in developing the ability to quantitatively rank the energies of designed protein constructs. The success of these studies suggests that only hard-sphere dynamics with geometrical constraints are needed for accurate protein structure prediction in hydrophobic cavities and binding interfaces. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F45.00008: Solvation Free Energy and Classical Density Functional Theory Eric Mills, Steven Plotkin The cell environment in which proteins fold and function is crowded with biological molecules, at densities of $\sim$300g/L. Treating these molecules explicitly in a MD simulation introduces enormous computational cost, so accurate ways of modelling their contribution to protein behaviour is desirable. I will discuss existing models and propose a new approach, which uses classical density functional theory to calculate the effect of these solutes on protein folding. I will discuss implementing this approach as both an implicit solvent and a post-processing method, and discuss some general conclusions we can derive from it. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F45.00009: Hydrogen Bonding in the Electronic Excited State Guang-Jiu Zhao, Ke-Li Han Here, I will give a talk on our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that intermolecular hydrogen bond in excited state can be greatly strengthened or weakened for many chromophores. We have also clarified that intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced by excited-state hydrogen bonding. References: [1] Guang-Jiu Zhao, and Ke-Li Han, \textit{Hydrogen Bonding in the Electronic Excited State, } \textit{Accounts of Chemical Research} \quad 45, 404--413 \quad (\textbf{2012}). http://pubs.acs.org/doi/pdf/10.1021/ar200135h [2] Book: \textit{Hydrogen Bonding and Transfer in the Excited State}, Editors: Ke-Li Han and Guang-Jiu Zhao, ISBN: 978-0-470-66677-7, \textit{John Wiley {\&} Sons Ltd}, Chichester, UK (\textbf{2011}). http://onlinelibrary.wiley.com/book/10.1002/9780470669143 [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F45.00010: Intermediate Resolution Models and Protein Folding and Allostery Abhijeet Kapoor, Alex Travesset Intermediate Resolution Models (IRM)s model proteins with nearly all atom precision but consider solvent implicitly, and treat electrostatics as short-range interactions. In this talk, we describe a new IRM. We discuss its differences from other existing IRMs and test it again a set of 13 proteins. The model successfully folds 12 of them into its native state, starting from a random configuration. The stability of the native state versus other states with different topologies (arrangement of the secondary structure) is also discussed. Implications for general protein motion are also presented. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F45.00011: Using extremely halophilic bacteria to understand the role of surface charge and surface hydration in protein evolution, folding, and function Wouter Hoff, Ratnakar Deole Halophilic Archaea accumulate molar concentrations of KCl in their cytoplasm as an osmoprotectant, and have evolved highly acidic proteomes that only function at high salinity. We examine osmoprotection in the photosynthetic Proteobacteria Halorhodospira halophila. We find that H. halophila has an acidic proteome and accumulates molar concentrations of KCl when grown in high salt media. Upon growth of H. halophila in low salt media, its cytoplasmic K$+$ content matches that of Escherichia coli, revealing an acidic proteome that can function in the absence of high cytoplasmic salt concentrations. These findings necessitate a reassessment of two central aspects of theories for understanding extreme halophiles. We conclude that proteome acidity is not driven by stabilizing interactions between K$+$ ions and acidic side chains, but by the need for maintaining sufficient solvation and hydration of the protein surface at high salinity through strongly hydrated carboxylates. We propose that obligate protein halophilicity is a non-adaptive property resulting from genetic drift in which constructive neutral evolution progressively incorporates weakly stabilizing K$+$ binding sites on an increasingly acidic protein surface. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F45.00012: Infrared Structural Biology of Proteins: Development of Vibrational Structural Markers for Probing the Structural Dynamics of COO- of Asp/Glu in Proteins Zhouyang Kang, Aihua Xie Asp and Glu often play critical roles in the active sites of proteins. Probing the structural dynamics of functionally important Asp and/or Glu provides crucial information for protein functionality. Time-resolved infrared structural biology offers strong advantages for its high structural sensitivity and broad dynamic range (ps to ks). In order to connect the vibrational frequencies to specific structures of COO- groups, such as the number, type, and geometry of hydrogen bond interactions, we develop two vibrational structural markers (VSM), built on the symmetric and asymmetric COO- stretching frequencies. Extensive quantum physics (density functional theory) based computational studies, combined with 13C isotopic editing of Asp/Glu and experimental FTIR data on Asp/Glu in proteins, are used to establish a unique correlation between the symmetric and asymmetric COO- vibrations with more than 10 types of hydrogen bonding interactions. Development of the COO- VSM markers enhances the power of time-resolved infrared structural biology for the study of functionally important structural dynamics of COO- in proteins, including rhodopsin for biological signaling, bacteriorhodopsin for proton transfer, photosystem II for energy transformation, and HIV protease for enzymatic catalysis. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F45.00013: Controlling allosteric networks in proteins Nikolay Dokholyan We present a novel methodology based on graph theory and discrete molecular dynamics simulations for delineating allosteric pathways in proteins. We use this methodology to uncover the structural mechanisms responsible for coupling of distal sites on proteins and utilize it for allosteric modulation of proteins. We will present examples where inference of allosteric networks and its rewiring allows us to ``rescue'' cystic fibrosis transmembrane conductance regulator (CFTR), a protein associated with fatal genetic disease cystic fibrosis. We also use our methodology to control protein function allosterically. We design a novel protein domain that can be inserted into identified allosteric site of target protein. Using a drug that binds to our domain, we alter the function of the target protein. We successfully tested this methodology \textit{in vitro}, in living cells and in zebrafish. We further demonstrate transferability of our allosteric modulation methodology to other systems and extend it to become ligh-activatable. [Preview Abstract] |
Session F46: SPS Undergraduate IV
Sponsoring Units: SPSChair: Crystal Bailey, American Physical Society
Room: Hilton Baltimore Holiday Ballroom 5
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F46.00001: Density Functional Study of the structural properties in Tamoxifen Romeo de Coss-Martinez, Jorge A. Tapia, Ramiro F. Quijano-Qui\~nones, Gabriel I. Canto Using the density functional theory, we have studied the structural properties of Tamoxifen. The calculations were performed with two methodological approaches, which were implemented in SIESTA and Spartan codes. For SIESTA, we considerate a linear combination of atomic orbitals method, using pseudopotentials and the van der Waals approximation for the exchange-correlation potential. Here we analyzed and compared the atomic structure between our results and other theoretical study. We found differences in the bond lengths between the results, that could be attributed to code approaches in each one. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F46.00002: Cytotoxicity of Gold Nanoparticles with Varying Concentration and Under Low Dose Environmental Radiation on Human Embryonic Kidney 293 Cells (HEK-293) Shalana Crudup, Bruce Braender, Cristina Iftode, Tabbetha Dobbins Nanomaterials are increasingly being used in medicine. Most research surrounding the health and safety effects of nanomaterials examine the cytotoxicity of nanoparticles alone. Few studies, as this one does, examines the combined effects of nanoparticle concentration and radiation exposure on cytotoxicity to human embryonic kidney 293 cells (HEK-293). Nanoparticles injected in the body are supposed to undergo biodegradation once they are done their specified task, however, some do not and accumulate in the cells (particularly at the liver and kidney) and this causes intracellular changes. Examples of intracellular changes are the disruption of organelle integrity or gene alterations. This will cause the cells to die because the cells are very sensitive to changes in their pH. The experiments reported here focus on the cytotoxicity of gold nanoparticles as a function of varying particle concentrations and also with and without exposure to UV radiation. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F46.00003: Migration Modes in Cancer Cell Motility Di Wu, Helim Aranda-Espinoza Cancer cell metastasis is a result of secondary tumor proliferation after single or collective cancer cell migration from a primary tumor. The biophysical mechanisms of cancer cell migration and transmigration through the body vasculature, while investigated, is not extensively quantified. In general, directed cell motility is traditionally viewed as the result of lamellipodia generation through which the cell moves by extending an actin protrusion and adhesion beneath its plasma membrane. However, cancer cells also exhibit motility through blebbing, which involves momentary membrane detachment from the actin cortex, membrane expansion and retraction. While blebbing, cancer cells do not form cell-substrate attachments as with lamellipodia. In vitro studies of single cancer cell migration through microfluidic microchannels of constant or linearly changing widths model in vitro conditions of single cell migration through capillary pores. We study both modes of motility and observe that cancer cell migration using lamellipodia or blebbing depends on channel width. Drug treatments to manipulate the cytoskeleton demonstrate that cancer cell migration changes speed but not the mode of migration. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F46.00004: Developing a Novel, Interdisciplinary Approach to Study Protein Unfolding Ian Bentley, Justin Link The ability of a protein to function is a direct result of its ability to properly obtain its native, folded structure. In order to determine the structural stability of proteins and to gain knowledge of their folding mechanism, we must develop protocols that allow us to monitor the controlled unfolding of proteins. Here, we investigate the stability of cytochrome $c$, a well-studied, model protein, under denaturing conditions using circular dichroism (CD) and fluorescence. Using either a chemical denaturant (Guanidine HCl) or heat, we can cause a protein to gradually unfold. The changes in the fluorescence and CD spectra can provide insight into the stability of proteins by providing us with thermodynamic parameters such as the Gibbs free energy, melting temperature and enthalpy. Research in this lab has been explored with mutant proteins and change in CD signal, however further work must still be done to observe their unfolding monitored by fluorescence. This technique will allow us to determine which regions of native cytochrome $c$ have the greatest impact on the protein folding process. The objective of this session is to present recent work in developing a protocol to observe the unfolding of wild type and mutant proteins with fluorescence. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F46.00005: Optimization of radiation damage to proteins using X-ray nanofocusing optics Selwa Boularaoui, K. Evans-Lutterodt, S. Lee, A.F. Isakovic The need to understand protein structure and perform treatment lead to the use of X-ray and particle-based radiation. Since the use of such radiation has undesirable side effects, mostly through the damage to proteins, it is important to continuously work on decreasing radiation damage. We outline the proposal to use the kinoform refractive optics to focus X-rays on the nanoscale to minimize the radiation damage to protein crystals under study. These optics devices are nanofabricated from low-Z elements (silicon, diamond) and can be used at synchrotron X-ray radiation facilities. We discuss the automated setup that performs nanopositioning of the nanofocusing element, and collects the chemical and structural protein solution under study. We offer simple mathematical models in irradiation and in treatment that help optimize the radiation parameters. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F46.00006: Driving Sodium-Potassium Pumps With An Oscillating Electric Field: Effects On Muscle Recovery In The Human Biceps Brachii Matt Bovyn, Wei Chen, Olivia Lanes, Jason Mast Dr. Chen has developed a technique called synchronization modulation, which uses an oscillating electric field to increase the rate at which the sodium-potassium pumps in the cell membrane work. Because the sodium-potassium pump is integral in the recovery of skeletal muscle fibers after an action potential, we investigated the effects of applying synchronization modulation to muscles which had already undergone fatigue due to repeated action potentials during exercise. Fatigue was induced in human subjects' biceps brachii through isometric contraction. Surface electromyography measurements of fatigue index were used to quantify how the muscle recovered over the minutes following fatigue, both when synchronization modulation was applied and when it was absent. The preliminary results were inconclusive, but it is hoped that in later work it will be shown that applying synchronization modulation is effective in increasing the rate at which the muscle recovers to its initial state. This would demonstrate not only that synchronization modulation can be successfully applied to human muscle, but also that it has many potential applications in sports medicine and novel disease treatments. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F46.00007: Driving Sodium/Potassium Pumps with an Oscillating Electric field: Effects on Muscle Fatigue Olivia Lanes, Matthew Bovyn, Wei Chen Dr. Chen has developed a technique called Synchronization Modulation, which has already been proven to be an effective tool in synchronizing and speeding up the sodium/potassium pumps in cell membranes. When synchronized, it is thought that these pumps are more efficient because they require less ATP. We hypothesized that if this was correct, this technique may be used to reduce muscle fatigue. To test our hypothesis, we had multiple test subjects hold a 15 lb weight for as long as they could while isolating the bicep muscle and applying an oscillating electric field. We compared the EMG data we took during these trials to the control, which was done the same way but without applying the electric field. To compare how fatigued subjects were, we did a Fast Fourier Transform on the first and last 10 seconds of each trial to measure the Fatigue Index. Our preliminary results suggest that the Fatigue Index decreased at a slower rate in the trials where the subject held the weight with Synchronization Modulation. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F46.00008: Multiscale MD Simulations of Folding Dynamics and Mobility of Beta-Amyloid Peptide on Lipid Bilayer Surfaces Scott Van Tilburg, Kelvin Cheng Early interaction events of beta-amyloid peptides with the neuronal membranes play a key role in the pathogenesis of Alzheimer's disease. We have used multiscale Molecular Dynamics (MD) simulations to study the protein folding dynamics and lateral mobility of beta-amyloid protein on the cholesterol-enriched and -depleted lipid nano-domains. Several independent simulation replicates of all-atom and coarse-grained MD simulations of beta-amyloid on different lipid bilayer nano-domains have been generated. Using Define Secondary Structure of Proteins (DSSP) algorithm and mean-square-distance (MSD) analysis, the protein conformation and the lateral diffusion coefficients of protein, as well as the lipid and water, were calculated as a function of simulation time up to 200 nanoseconds for atomistic and 2 microseconds for coarse-grained simulations per replicate in different bilayer systems. Subtle differences in the conformation and mobility of the protein were observed in lipid bilayers with and without cholesterol. The structural dynamics information obtained from this work will provide useful insights into understanding the role of protein/lipid interactions in the membrane-associated aggregation of protein on neuronal membranes. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F46.00009: The structure of immiscible lipid phases as revealed by the Anton special purpose supercomputer Michael Sandar, Edward Lyman We present simulation data for a bilayer composed of a ternary mixture of cholesterol, dioloeoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine. The chosen composition is in the two-phase region and the temperature is in the vicinity of the miscibility transition. Using the Anton special purpose supercomputer to generate continuous trajectories longer than ten microseconds--- which admits complete lipid mixing ---we observe robust liquid-liquid phase coexistence. We characterize the phase separated state by considering the local composition fluctuations. Correlation functions of the position reveal that the structure of the domain is circular on average, but that the boundary is subject to significant fluctuations, as expected in the neighborhood of a critical point. The domain diffuses on a slower timescale than the lipids, but by way of lipid exchange, rather than as a well-defined cluster. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F46.00010: Computational Analysis of ECGs Kevin Waters Electrocardiogram is among the most powerful methods at present to diagnose heart conditions. Here we employed Fourier transform to analyze Electrocardiograms. The goal of the project is to find a way to isolate different wave signals in ways that today's technology is not capable of. Our focus was on building on a code that is capable of filtering out P, QRS, T waves and noise from the ECG, so we created frequency filters that omitted selected amount of data. We first deconstructed and then constructed the ECG this way to find an optimal code assembly for each ECG wave (P-wave, QRS-wave, T-wave). By focusing on one patient, we succeeded to disentangle the complicated ECG signal. We plan to extend this method to more patients. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F46.00011: Nonlinear Dynamical Analysis of Fibrillation John A. Kerin, Justin M. Sporrer, David A. Egolf The development of spatiotemporal chaotic behavior in heart tissue, termed fibrillation, is a devastating, life-threatening condition. The chaotic behavior of electrochemical signals, in the form of spiral waves, causes the muscles of the heart to contract in an incoherent manner, hindering the heart's ability to pump blood. We have applied the mathematical tools of nonlinear dynamics to large-scale simulations of a model of fibrillating heart tissue to uncover the dynamical modes driving this chaos. By studying the evolution of Lyapunov vectors and exponents over short times, we have found that the fibrillating tissue is sensitive to electrical perturbations only in narrow regions immediately in front of the leading edges of spiral waves, especially when these waves collide, break apart, or hit the edges of the tissue sample. Using this knowledge, we have applied small stimuli to areas of varying sensitivity. By studying the evolution of the effects of these perturbations, we have made progress toward controlling the electrochemical patterns associated with heart fibrillation. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F46.00012: Imaging The Genetic Code of a Virus Jenna Graham, Justin Link Atomic Force Microscopy (AFM) has allowed scientists to explore physical characteristics of nano-scale materials. However, the challenges that come with such an investigation are rarely expressed. In this research project a method was developed to image the well-studied DNA of the virus lambda phage. Through testing and integrating several sample preparations described in literature, a quality image of lambda phage DNA can be obtained. In our experiment, we developed a technique using the Veeco Autoprobe CP AFM and mica substrate with an appropriate absorption buffer of HEPES and NiCl$_{\mathrm{2}}$. This presentation will focus on the development of a procedure to image lambda phage DNA at Xavier University. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F46.00013: Single Molecule Study on the Direct Transfer of \textit{E. coli} Single-Stranded Binding protein between Single-Stranded DNA Molecules Teckla Akinyi, I-Ren Lee, Taekjip Ha Single molecule fluorescence resonance energy transfer (smFRET) techniques allow a direct study of the mechanism of the spontaneous transfer of \textit{Escherichia coli} Single-Strand Binding (SSB) protein from single-stranded DNA to a competitor single-stand (ss)DNA. This investigation attempts to understand the kinetics of dissociation and ultimately figure out how long can SSB remain bound to ssDNA in midst of competitor free ssDNA. Application of single molecule techniques as described by Taekjip Ha, (\textit{Ha. Methods 25, 78--86 (2001)}) allow the quantification of the rapid dissociation of SSB from ssDNA as a function of ssDNA length and concentration. We also examined, whether the dissociation occurs with the SSB subunits released simultaneously or consecutively with the possibility of an intermediate state. The variation of dissociation time with DNA length and concentration of the competitive ssDNA demonstrate direct proportionality implying SSB is transferred between ssDNA molecules with a ratio of 1:1, with an abrupt transition from a high FRET state to a low FRET state indicating instantaneous dissociation limited by our time resolution. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F46.00014: Images of Cone Photoreceptors Using Spatially Non-Coherent Light Allison Hartman, Changgeng Liu, Myung Kim In order to get clear images of the photoreceptors in a living human eye, we constructed a collimated beam of light with controllable spatial coherence. In the past, imaging techniques using coherent light have shown interference speckles that are the similar size and shape as photoreceptors; these experiments have been unable to differentiate the speckles and the photoreceptors that are in the retina of the eye. We used MatLab to create a simulation of the optical system using a light source with variable spatial coherence reflecting off of a resolution target and we were able to eliminate the speckle patterns. We then created an experimental setup to verify out simulation. We were able to get clear images of resolution targets and our future work will be to image retina samples using spatially non-coherent light and apply this technique in Digital Holography experiments. [Preview Abstract] |
Session F47: Invited Session: Solid-State Nanopores: Translocation and Applications
Sponsoring Units: DBIO DCMPChair: Gustavo Stolovitzky, IBM Research
Room: Hilton Baltimore Holiday Ballroom 6
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F47.00001: The time distribution of charged biopolymers translocation through voltage-biased solid-state nanopores Invited Speaker: Jiali Li When a charged DNA or protein molecule is passing through a voltage biased solid-state nanopore in an ionic solution, it generates a current blockage signal characterized by its amplitude and time duration (or translocation time). Many parameters such as solution viscosity, applied voltage, the size, conformation, charge, and the charge sequence of the molecule could all contribute to the translocation time and its distribution. By fitting the translocation times to the solution of a Smoluchowski-type equation for 1D-biased diffusion and using the Einstein relation, the viscous drag force on uniformly charged DNA molecules and the uncertainty in determining the DNA chain length due to the contribution of Brownian motion can be evaluated. Furthermore, the time distribution of globular shaped particles and not uniformly charged unfolded protein molecules will also be discussed.\\[4pt] [1] Li, J. and D.S. Talaga, \textit{The distribution of DNA translocation times in solid-state nanopores.} J. Phys. Condens. Matter 2010. \textbf{22}: p. 454129 (8pp).\\[0pt] [2] Ling, D. and X. Ling, \textit{First-passage-time analysis of DNA translocation in solid-state nanopores}, in \textit{APS March Meeting 2012 }2012: Boston, Massachusetts.\\[0pt] [3] Ledden, B., D. Fologea, D.S. Talaga and J. Li, \textit{Sensing Single Protein Molecules with Solid-state Nanopores}, in \textit{Nanopores: Sensing and Fundamental Biological Interactions}, S.M. Iqbal and R. Bashir, Editors. 2011, Springer: New York. p. 129-150. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F47.00002: Controlling DNA Translocation Speed through Solid-State Nanopores by Surface Charge Modulation Invited Speaker: Amit Meller The Nanopore method is an emerging technique, which extends gel-electrophoresis to the single-molecule level and allows the analysis of DNAs, RNAs and DNA-protein complexes. The strength of the technique stems from two fundamental facts: First, nanopores due to their nanoscale size can be used to uncoil biopolymers, such as DNA or RNA and slide them in a single file manner that allows scanning their properties. Consequently, the method can be used to probe short as well as extremely long biopolymers, such as genomic DNA with high efficiency. Second, electrostatic focusing of charged biopolymers into the nanopore overcomes thermally driven diffusion, thus facilitating an extremely efficient end-threading (or capture) of DNA. Thus, nanopores can be used to detect minute DNA copy numbers, circumventing costly molecular amplification such as Polymerase Chain Reaction. A critical factor, which determines the ability of nanopore to distinguish fine properties within biopolymers, such as the location of bound small-molecules, proteins, or even the nucleic acid's sequence, is the speed at which molecules are translocated through the pore. When the translocation speed is too high the electrical noise masks the desired signal, thus limiting the utility of the method. Here I will discuss new experimental results showing that modulating the surface charge inside the pore can effectively reduce the translocation speed through solid-state nanopores fabricated in thin silicon nitride membranes. I will present a simple physical model to account for these results. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F47.00003: Advanced Solid State Nanopores Architectures: From Early Cancer Detection to Nano-electrochemistry Invited Speaker: Rashid Bashir Solid-state nanopores (ssNPs) are potentially low-cost and highly scalable technologies for rapid and reliable se-quencing of the human diploid genome for under {\$}1,000. The ssNPs detect ionic current changes while molecules translocate through the pore. Several key challenges must be overcome in order for ssNPs to become ubiquitous in the fields of medical diagnostics and personalized healthcare. One major challenge is to reduce the speed at which DNA translocates through the nanopore from microseconds to milliseconds per nucleotide, enabling reliable identification of single nucleotides. The other major challenge is to improve the sensitivity of the approach requiring new sensing modalities and novel device architectures. In this paper, we review our recent efforts to (i) develop ssNPs for early cancer detection, (ii) to embed graphene electrodes in dielectric nanolaminates to form 3 and 4 terminal nanopore devices, and (iii) we demonstrate a nanopore based structure consisting of stacked graphene and Al$_{2}$O$_{3}$ dielectric layers to study electrochemical activity at graphene edges. The electrochemical signal corresponding to the atomically thin graphene layer could also provide a pathway to DNA sequencing. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F47.00004: Nonlinear transport of fd virus particles through a solid-state nanopore Invited Speaker: Xinsheng Ling In this talk I will discuss our recent experiments on fd virus particles. The fd particles provide an interesting model system for testing the first-passage time theory of electric-field-driven translocation. We find that the distribution of translocation time can be understood using Schrodinger's first-passage time distribution function. The extracted diffusion constant for fd is significantly larger than the expected value from the Stokes-Einstein relation. We also find that the extracted translocation velocity is a nonlinear function of the electric field. We attribute the large effective diffusion constant to a Taylor dispersion effect in the electroosmotic flow profile in the nanopore and the nonlinear electrophoretic mobility to a Stotz-Wien effect. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F47.00005: Nanopore Graphene-based Electronic Devices Invited Speaker: Marija Drndic Graphene is an exceptional material for high-speed electronics, as well as a revolutionary membrane material due to its strength and atomic thickness. Nanopores in suspended graphene membranes are currently regarded as candidates for ultrafast DNA sequencing. When a single DNA molecule passes through a nanopore, it blocks the field-driven ions passing through the pore and is detected by measuring the ion current reduction. Due to the thin nature of graphene membranes and reduced pore resistance, we observe larger current signals than in the case of traditional solid-state nanopores. Use of graphene as a membrane material opens the door to a new class of nanopore devices in which electronic sensing and control are performed directly at the pore. [Preview Abstract] |
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