Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W1: Superconducting Qubits: Advances in Single-Shot QND Readout
Sponsoring Units: DCMPChair: John Martinis, University of California, Santa Barbara
Room: Ballroom A1
Thursday, March 24, 2011 11:15AM - 11:51AM |
W1.00001: DC-SQUID Quantum Non-Demolition Readout of Superconducting Flux Qubits Invited Speaker: Extracting state information from a quantum system is a central theme in quantum mechanics. As the process of state extraction by a detector implies system-detector entanglement, reverse action from the detector onto the quantum object can not be avoided. Consequently, detectors that minimise this back action are crucial. For superconducting flux qubits [1] commonly a DC-SQUID detector is used, either in an AC dispersive scheme or in a switching mode. The latter can be by AC bifurcation or by direct DC switching. The DC approach combines simplicity in use with complexity in dynamical behaviour. This complexity results from the fast Josephson phase dynamics and the significant generation of quasi-particles in the dissipative detector ON-state. This gave rise to the long-standing belief that it can not act as a ``good'' detector. This includes it to fail as a Quantum Non-Demolition (QND) detector, i.e. the preservation of the state of the quantum object after a state readout. In a recent experiment for relatively weak qubit-SQUID interaction strength [2] we investigated the detection properties of such a DC-switching SQUID, finding a remarkably good QND fidelity. This was achieved by shunting the SQUID by a low-value resistor, thus strongly suppressing the generation of quasi-particles. Also the detector ON-time was minimised to a few tens of ns using a nearby cryogenic amplifier. The QND-ness was obtained from measuring the correlation between two successive readouts, and found to reach 75{\%} QND fidelity. The weak qubit-detector interaction leads to a limited readout contrast. We will discuss the results as well as its consequences, including the potential for combining high contrast and good QND fidelity.\\[4pt] [1] J.E. Mooij et. al., Science \underline {\textbf{285}}, 1036 (1999) \\[0pt] [2] T. Picot et. al., Phys. Rev. Lett. \underline {\textbf{105}}, 040506 (2010) [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W1.00002: Novel approaches to high fidelity qubit state measurement in circuit quantum electrodynamics Invited Speaker: Qubit state measurement (`readout') in solid state systems is an open problem, which is currently the subject of intensive experimental and theoretical research. Achieving high fidelity in a single-shot measurement is an interesting quantum control problem, as well as an important component for the successful implementation of quantum information protocols. For superconducting qubits we can distinguish between linear dispersive and nonlinear methods, the latter relying on the bistability of a nonlinear resonator. In the context of circuit quantum electrodynamics, the transmon qubit is strongly coupled to a linear resonator and described by a generalized Jaynes-Cummings model (JCM) with external drive and dissipation. Recent novel approaches to achieve high-fidelity readout in the dispersive regime rely on the intrinsic nonlinearity of the JCM and its ultimate linearity in the high excitation regime. In the degenerate regime we rely on the photon blockade and precise transient dynamics of the system. This regime presents a theoretical challenge and the driven damped JCM model exhibits a dynamical phase transition. Another proposed approach extends the Josephson Bifurcation Amplifier and employs the dynamical effects of frequency chirping of the drive on the coupled qubit-resonator system. We will discuss the physics of these different regimes and describe the readout schemes which have been demonstrated by recent experiments and quantum simulations, as well as the role of quantum fluctuations and optimal control. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W1.00003: Observation of quantum jumps in a superconducting quantum bit Invited Speaker: Superconducting qubit technology has made great advances since the first demonstration of coherent oscillations more than 10 years ago. Coherence times have improved by several orders of magnitude and significant progress has been made in qubit state readout fidelity. However, a fast, high-fidelity, quantum non-demolition measurement scheme which is essential to implement quantum error correction has so far been missing. We demonstrate such a scheme for the first time where we continuously measure the state of a superconducting quantum bit using a fast, ultralow-noise parametric amplifier. This arrangement allows us to observe quantum jumps between the qubit states in real time. The key development enabling this experiment is the use of a low quality factor (Q), nonlinear resonator to implement a phase-sensitive parametric amplifier operating near the quantum limit. The nonlinear resonator was constructed using a two junction SQUID shunted with an on-chip capacitor. The SQUID allowed us to tune the operating band of the amplifier and the low Q provided us with a bandwidth greater than 10 MHz, sufficient to observe jumps in the qubit state in real time. I will briefly describe the operation of the parametric amplifier and discuss how it was used to measure the state of a transmon qubit in the circuit QED architecture. I will discuss measurement fidelity and the statistics of the quantum jumps. I will conclude by discussing the implications of this development for quantum information processing and further improvements to the measurement technique. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:39PM |
W1.00004: Probing the quantum fluctuations of a nonlinear resonator with a superconducting qubit Invited Speaker: Coupling a superconducting quantum bit to a superconducting resonator offers the opportunity to investigate the interaction between light and an atom in regimes hardly accessible otherwise [1]. Making the resonator nonlinear has enabled important recent progress in the readout of qubits. Indeed, when pumped by a microwave field of well-chosen amplitude and frequency, nonlinear resonators (NRs) provide parametric amplification close to the quantum limit. In other drive conditions, the intra-cavity field can take two stable values between which the resonator can switch stochastically. Both regimes have been shown to yield a high-fidelity qubit readout [2,3]. Qubits can also be used to obtain interesting insight into the physics of NRs. In this work we use a transmon qubit [4] coupled to such a pumped NR as a probe of its quantum fluctuations. The qubit-NR coupling is manifested by the appearance around the qubit spectral line of two sidebands that we interpret as processes in which the driven resonator fluctuations are effectively cooled down or heated with the assistance of the qubit. The ratio of the sidebands amplitudes gives thus a direct experimental access to the pumped NR effective temperature which is found to be in quantitative agreement with the theory, bringing a clear support to the quantum description of a driven nonlinear resonator [5]. \\[4pt] [1] A. Wallraf, D. I. Schuster, A. Blais, L. Frunzio, R.-S. Huang, J. Majer, S. Kumar, S. M. Girvin and R. J. Schoelkopf, Nature 431 162 (2004). \\[0pt] [2] R. Vijay, D.H. Slichter, I. Siddiqi, arxiv:cond-mat/1009.2969. \\[0pt] [3] F. Mallet, F.R. Ong, A. Palacios-Laloy, F. NGuyen, P. Bertet, D. Vion, and D. Esteve, Nature Physics 5, 791-795 (2009). \\[0pt] [4] J. Koch, T.M. Yu, J.M. Gambetta, A.A. Houck, D.I Schuster, J. Majer, A. Blais, M.H. Devoret, S.M. Girvin, and R.J. Schoelkopf, Phys. Rev. A 76, 042319 (2007). \\[0pt] [5] M. Marthaler and M.I. Dykman, PRA 73, 042108 (2006). [Preview Abstract] |
Session W2: CVD Graphene: Synthesis, Properties and Applications
Sponsoring Units: DCMPChair: Millie Dresselhaus, Massachusetts Institute of Technology
Room: Ballroom A2
Thursday, March 24, 2011 11:15AM - 11:51AM |
W2.00001: Chemical Vapor Deposition of Large-size Monolayer Graphene and Properties Invited Speaker: Graphene is of interest in part due to its electronic and thermal transport, mechanical properties including high stiffness and the possibility of high strength and toughness, high specific surface area, and that it can act as an atom thick layer, barrier, or membrane. Our top-down micromechanical exfoliation approaches conceived of in 1998 [1, 2] yielded multilayer graphene. Two main areas of our research are: (i) CVD growth of large area graphene films on metal substrates, characterization and properties of such films, and (ii) The generation, study, and use of colloids containing graphene-based platelets. We present our work on CVD growth of graphene on metal substrates, including the first achievement of large area growth of monolayer graphene [3], studies on understanding growth [related references: 3-6]. Properties such as TCE [7], thermal conductivity [8], and mechanical properties [related reference: 9], will be presented. An excellent review of graphene is [10]. A history of experimental work on graphene (from its discovery in 1969 until now) will be available on our web site: http://bucky-central.me.utexas.edu/ prior to the meeting. Ruoff group publications: http://bucky-central.me.utexas.edu/publications.htm.\\[4pt] [1] Tailoring graphite with the goal of achieving single sheets, Nanotechnology 10, 269-272 (1999).\\[0pt] [2] APL 75, 193-195 (1999).\\[0pt] [3] Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324, 1312-1314 (2009).\\[0pt] [4] Evolution of Graphene Growth on Ni and Cu by Carbon Isotope Labeling, Nano Letters 9, 4268 (2009).\\[0pt] [5] Synthesis, Characterization, and Properties of Large-Area Graphene Films, ECS Transactions 19, 41-52 (2009).\\[0pt] [6] Graphene Films with Large Domain Size by a Two-Step Chemical Vapor Deposition Process, Nano Letters, (2010).\\[0pt] [7] Transfer of large-area graphene films for high-performance transparent conductive electrodes, Nano Letters, 9, 4359-4363 (2009).\\[0pt] [8] Thermal Transport in Suspended and Supported Monolayer Graphene Grown by Chemical Vapor Deposition, Nano Letters, 10, 1645-1651 (2010).\\[0pt] [9] Mechanical Properties of Monolayer Graphene Oxide, ACS Nano, (2010).\\[0pt] [10] Graphene and Graphene Oxide: Synthesis, Properties, and Applications, Advanced Materials, 22, 3906-3924 (2010). [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W2.00002: Roll-to-roll production of 30-inch graphene films for transparent electrodes Invited Speaker: The outstanding electrical1, mechanical and chemical properties of graphene make it attractive for applications in flexible electronics. However, efforts to make transparent conducting films from graphene have been hampered by the lack of efficient methods for the synthesis, transfer and doping of graphene at the scale and quality required for applications. Here, we report the roll-to-roll production and wet-chemical doping of predominantly monolayer 30-inch graphene films grown by chemical vapour deposition onto flexible copper substrates. The films have sheet resistances as low as $\sim $125 Ohm/sq with 97.4{\%} optical transmittance, and exhibit the half-integer quantum Hall effect, indicating their high quality. We further use layer-by-layer stacking to fabricate a doped four-layer film and measure its sheet resistance at values as low as $\sim $30 Ohm/sq at $\sim $90{\%} transparency, which is superior to commercial transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W2.00003: Structural and electronic properties of graphene grown by chemical vapor deposition (CVD) Invited Speaker: Graphene grown by chemical vapor deposition (CVD) has brought many exciting opportunities for both fundamental studies and practical applications. In this talk, I will present our studies of the structural and electronic properties of graphene synthesized by ambient CVD based growth on polycrystalline Ni and Cu foils. Our earlier work on graphene layers and large scale graphitic thin films grown on Ni and transferred to insulators [1,2] show that such films can have excellent electronic properties, despite their structural non-uniformity. We also characterized the wrinkles in such films, yielding insights on their growth and buckling processes [3]. On Cu foils, we have synthesized wafer-scale graphene films consisting of predominantly monolayer graphene [4]. We have studied the electronic transport properties [4], including field effect, ``half-integer'' quantum Hall effect (electronic hall-mark of graphene) and weak localization (probing carrier scattering) in such synthetic graphene transferred to SiO2/Si substrates and characterized its structural properties by Raman mapping, transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). We have also studied thermal transport in CVD graphene using both electrical and Raman measurements [5]. Finally, one of the outstanding issues in large scale CVD graphene, which can be monolayer but generally polycrystalline, is the role of grain boundaries. I will present our recent studies of single crystal graphene grains (hexagonally-shaped with edges macroscopically aligned close to zigzag directions) grown on Cu, and how individual grain boundaries affect the electronic transport properties [6]. Work in collaboration with Q. Yu, H. Cao, L. Jauregui, J. Tian, N. Guisinger, R. Colby and E.A.Stach. \\[4pt] [1] Q. Yu et al., Appl. Phys. Lett. 93, 113103 (2008);\\[0pt] [2] H. Cao et al., J. Appl. Phys. 107, 044310 (2010);\\[0pt] [3] R. Colby et al., Diamond Relat. Mater. 19, 143 (2010)\\[0pt] [4] H. Cao et al. Appl. Phys. Lett. 96, 122106 (2010)\\[0pt] [5] L. Jauregui et al. ECS Trans. 28 (5), 73 (2010)\\[0pt] [6] Q.Yu and L.A. Jauregui et al., arXiv:1011 (2010) [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:39PM |
W2.00004: Atomistic view in the initial stages of growth of epitaxial graphene on metal substrates Invited Speaker: For both fundamental studies and potential development of graphene electronics, it is pressing to search for reliable methods for mass production of quality graphene. Epitaxial growth of graphene on catalytic metal substrates combined with post-growth transfer has become a promising route towards this goal [1,2]. However, to better control the quality and yield of graphene, a comprehensive understanding of the growth kinetics is essential. In particular, how the carbon atoms adsorbed on the metal surface (or dissolved into the metal) meet to nucleate into stable carbon islands will greatly influence both the growth rate and quality of larger carbon entities such as graphene sheets. In this talk, we first show that the delicate competition between carbon-carbon bonding and carbon-metal bonding dictates the initial nucleation sites of graphene on metal surfaces [3]. These results are discussed in connection with the experimental findings that on Ir(111) and Ru(0001) substrates graphene nucleates from the step edges [4,5]. We also predict that on Cu(111) nucleation should take place everywhere on a terrace [3]. Next we study larger carbon clusters on Cu(111) and explicitly compare the stability of linear and compact structures. We find that the linear carbon ``nanoarches'' are more stable than compact islands consisting of up to 13 carbon atoms, and these nanoarched structures may serve as the missing bridge between carbon dimers and larger graphene nanodomes. Based on these improved understanding of the atomistic rate processes involved, we propose a few kinetic pathways that may lead to better growth control of bilayer graphene and graphene nanoribbons as elemental building blocks for developing graphene electronics. \\[4pt] [1] Q. K. Yu, et al., Appl. Phys. Lett. 93, 113103 (2008).\\[0pt] [2] X. S. Li, et al., Science 324, 1312 (2009).\\[0pt] [3] H. Chen, W. G. Zhu, and Z. Y. Zhang, Phys. Rev. Lett. 104, 186101 (2010); R. van Wesep et al., to be published.\\[0pt] [4] E. Loginova et al., New J. Phys. 10, 093-026 (2008).\\[0pt] [5] J. Coraux et al., New J. Phys. 11, 023-006 (2009). [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 2:15PM |
W2.00005: Graphene synthesis, characterization, and processing: an atomic-scale investigation Invited Speaker: Graphene is nature's ideal two-dimensional conductor that is comprised of a single sheet of hexagonally packed carbon atoms. Since the first electrical measurements made on graphene, researchers have been trying to exploit the unique properties of this material for a variety of applications that span numerous scientific and engineering disciplines. In order fully realize the potential of graphene, large scale synthesis of high quality graphene and the ability to control the electronic properties of this material on a nanometer length-scale remain key challenges. This talk will focus on atomic-scale characterization of graphene synthesis on various materials (SiC, Cu(111), Cu foil, etc) via scanning tunneling microscopy. These fundamental studies explore growth dynamics, film quality, and the role of defects. The chemical modification of graphene following exposure to atomic hydrogen will be discussed, while additional emphasis will be made on graphene's unique structural (not electronic) properties. [Preview Abstract] |
Session W3: Advances in ZnO Physics and Applications
Sponsoring Units: DCMP DMPChair: Scott Chambers, Pacific Northwest National Laboratory
Room: Ballroom A3
Thursday, March 24, 2011 11:15AM - 11:51AM |
W3.00001: Hybrid functional studies of defects and impurities in ZnO Invited Speaker: Zinc oxide is regarded as a highly promising material for light-emitting diodes and lasers. Its features include a direct band gap of 3.4 eV, a large exciton binding energy of 60 meV, and the availability of high-quality single-crystal substrates. Despite the rapid development, fundamental issues regarding $p$-type doping remain unresolved. The most significant barrier to realizing ZnO-based optoelectronic devices is the difficulty in producing reliable and reproducible $p$-type material. Among the possible acceptor impurities, N has been considered the most promising because it has an atomic size close to that of O. In addition, N has been conclusively shown to act as a shallow acceptor in other II-V semiconductors, such as ZnSe. In spite of many published reports on $p$-type conductivity in N-doped ZnO, reproducibility and stability are still major issues, and devices based on $p-n$ homojunctions have remained elusive. In this work, we study the properties of the nitrogen acceptor using advanced density functional techniques. Our first principles calculations are based on hybrid functionals, which include a portion of exact exchange and correct the band gap of semiconductors, allowing us to accurately predict defect and impurity transition levels. Contrary to the conventional wisdom, we find the N acceptor has an exceedingly high ionization energy of 1.3 eV above the valence band, meaning that N cannot lead to hole conductivity in ZnO [1]. We have also analyzed the optical transitions (absorption and luminescence) and charge distribution associated with the N impurity, which offer characteristic signatures that can be compared to experimental results. \\[4pt] [1] J. L. Lyons, A. Janotti, and C. G. Van de Walle, APL \textbf{95}, 252105 (2009). [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W3.00002: Limits of Conductivity in ZnO Thin Films: Experiment and Theory Invited Speaker: Transparent conductive oxides (TCOs) have major (multi-{\$}B) roles in applications such as flat-panel displays, solar cells, and architectural glass. The present workhorse TCO is indium-tin-oxide (ITO), but the recent huge demand for ITO has made In very expensive; moreover, it is toxic. The most commonly suggested replacement for ITO is ZnO, doped with Al, Ga, or In, and indeed the ISI lists 628 papers on Group-III-doped ZnO in 2009. However, to our knowledge, none of these papers has included calculations of donor N$_{D}$ and acceptor N$_{A}$ concentrations, the fundamental components of conductivity in semiconductors. We have developed a simple model for the calculation of N$_{D}$ and N$_{A}$ from temperature-dependent measurements of carrier concentration n, mobility $\mu $, and film thickness d. With the inclusion of phonon scattering in the model, excellent fits of n and $\mu $ are obtained from 15 -- 300 K. Experimentally, we have shown that highly conductive ZnO films can be grown by pulsed laser deposition in a pure Ar ambient, rather than the usual O$_{2}$. In a 278-$\mu $m-thick film, we have achieved a room-temperature resistivity $\rho $ = 1.96 x 10$^{-4}$ $\Omega $-cm, carrier concentration n = 1.14 x 10$^{21}$ cm$^{-3}$, and mobility $\mu $ = 28.0 cm$^{2}$/V-s. From our model, we calculate N$_{D}$ = 1.60 x 10$^{21}$ and N$_{A}$ = 4.95 x 10$^{20}$ cm$^{-3}$; however, the model also predicts that a significant reduction of N$_{A}$ would give $\mu $ = 42.5 cm$^{2}$/V-s and $\rho $ = 7.01 x 10$^{-5} \quad \Omega $-cm, a world record. Such a reduction in N$_{A}$ may be possible by in-diffusion of Zn after growth, since there is evidence that one of the major acceptor species in these films is the Zn-vacancy/Ga$_{Zn}$ complex. We can also decrease the resistivity by annealing in forming gas, and have recently attained $\rho $ = 1.46 x 10$^{-4} \quad \Omega $-cm, n = 1.01 x 10$^{21}$ cm$^{-3}$, and $\mu $ = 42.2 cm$^{2}$/V-s, giving N$_{D}$ = 1.13 x 10$^{21}$ and N$_{A}$ = 1.09 x 10$^{20}$ cm$^{-3}$. In very thin films, quantum effects must be considered. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W3.00003: The Electronic Properties of Native Point Defects at ZnO Surfaces and Interfaces Invited Speaker: Despite nearly sixty years of research, several fundamental issues surrounding ZnO remain unresolved. Among the key roadblocks to ZnO optoelectronics have been the difficulty of p-type doping and the role of compensating native defects. Oxygen vacancies (V$_{O})$, Zn interstitials (Zn$_{I})$, and residual impurities such as H, Al, Ga, and In are reported to be donors in ZnO, while Zn vacancies (V$_{Zn})$ are considered to be acceptors. Electrically active complexes of V$_{O}$, Zn$_{I}$, and V$_{Zn}$ can also exist. Although their impact on free carrier compensation and recombination is recognized, the physical nature of the donors and acceptors dominating carrier densities in ZnO and their effects on carrier injection at contacts is unresolved. The impact of these electronic states on ZnO carriers at the nanoscale is only now being explored. We can now address these issues using a combination of depth-resolved and scanned probe techniques. Taken together, we clearly identify the optical transitions and energies of V$_{Zn}$ and V$_{Zn}$ clusters, effects of annealing on their spatial distributions in ion-implanted ZnO, and how V$_{Zn}$ and V$_{Zn}$ clusters modify the near- and sub-surface carrier densities. Indeed, these native point defects can directly impact the activation of extrinsic dopants. We have now discovered that nanostructures form spontaneously on ZnO polar surfaces and create sub-surface V$_{Zn}$ locally because of Zn diffusion that feeds the nanostructure growth. Overall, this work reveals the interplay between ZnO electronic defects, polarity, and surface nanostructure. \\[4pt] [1] Y. Dong, F. Tuomisto, B. G. Svensson, A. Yu. Kuznetsov, and L. J. Brillson, ``Vacancy defect and defect cluster energetics in ion-implanted ZnO,'' Phys. Rev. B \textbf{81}, 081201(R) (2010).\\[0pt] [2] D. Doutt, H. L. Mosbacker, G. Cantwell,J. Zhang, J. J. Song, and L. J. Brillson, ``Impact of near-surface defects and morphology on ZnO luminescence,'' Appl. Phys. Lett. \textbf{94}, 042111 (2009). [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:39PM |
W3.00004: Realization of high performance random laser diodes Invited Speaker: For the past four decades, extensive studies have been concentrated on the understanding of the physics of random lasing phenomena in scattering media with optical gain. Although lasing modes can be excited from the mirrorless scattering media, the characteristics of high scattering loss, multiple-direction emission, as well as multiple-mode oscillation prohibited them to be used as practical laser cavities. Furthermore, due to the difficulty of achieving high optical gain under electrical excitation, electrical excitation of random lasing action was seldom reported. Hence, mirrorless random cavities have never been used to realize lasers for practical applications -- CD, DVD, pico-projector, etc. Nowadays, studies of random lasing are still limited to the scientific research. Recently, the difficulty of achieving `battery driven' random laser diodes has been overcome by using nano-structured ZnO as the random medium and the careful design of heterojunctions. This lead to the first demonstration of room-temperature electrically pumped random lasing action under continuity wave and pulsed operation. In this presentation, we proposed to realize an array of quasi-one dimensional ZnO random laser diodes. We can show that if the laser array can be manipulated in a way such that every individual random laser can be coupled laterally to and locked with a particular phase relationship to its adjacent neighbor, the laser array can obtain coherent addition of random modes. Hence, output power can be multiplied and one lasing mode will only be supported due to the repulsion characteristics of random modes. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 2:15PM |
W3.00005: Nanogenerators and Piezotronics Invited Speaker: Developing wireless nanodevices and nanosystems is of critical importance for sensing, medical science, environmental/infrastructure monitoring, defense technology and even personal electronics. It is highly desirable for wireless devices to be self-powered without using battery. This is a new initiative in today's energy research for mico/nano-systems in searching for sustainable self-sufficient power sources [1]. We have invented an innovative approach for converting nano-scale mechanical energy into electric energy by piezoelectric zinc oxide nanowire arrays [2]. As today, a gentle straining can output 1-3 V from an integrated nanogenerator, using which a self-powered nanosensor has been demonstrated. A commercial LED has been lid up [3-5]. Due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential\textit{ (piezopotential)} is created in the crystal by applying a stress. The effect of piezopotential to the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. Electronics fabricated by using inner-crystal piezopotential as a ``gate'' voltage to tune/control the charge transport behavior is named \textit{piezotronics [6,7].Piezo-phototronic effect} is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of electro-optical processes by strain induced piezopotential [8]. \\[4pt] [1] Z.L. Wang, \textit{Scientific American}, 298 (2008) 82-87; \\[0pt] [2] Z.L. Wang and J.H. Song, \textit{Science}, 312 (2006) 242-246. \\[0pt] [3] R.S. Yang, Y. Qin, L.M. Dai and Z.L. Wang, \textit{Nature Nanotechnology}, 4 (2009) 34-39. \\[0pt] [4] S. Xu, Y. Qin, C. Xu, Y.G. Wei, R.S. Yang, Z.L. Wang, \textit{Nature Nanotechnology}, 5 (2010) 366. \\[0pt] [5] G. Zhu, R.S. Yang, S.H. Wang, and Z.L. Wang , Nano Letters, 10 (2010) 3151. \\[0pt] [6] Z.L. Wang, \textit{Adv. Mater}., 19 (2007) 889-992. \\[0pt] [7] W.Z. Wu, Y.G. Wei and Zhong Lin Wang , Adv. Materials, DOI: adma.201001925. \\[0pt] [8] Y.F. Hu, Y.L. Chang, P. Fei, R.L. Snyder and Z.L. Wang, \textit{ACS Nano}, $4$ (2010) 1234--1240. \\[0pt] [9] Research supported by DARPA, DOE, NSF, Airforce, NIH, Samsung. For details: http://www.nanoscience.gatech.edu/zlwang/. [Preview Abstract] |
Session W4: Glassy Dynamics and Jamming
Sponsoring Units: DCMP GSNPChair: Arjun Yodh, University of Pennsylvania
Room: Ballroom A4
Thursday, March 24, 2011 11:15AM - 11:51AM |
W4.00001: Growing length-scales at the glass and jamming transition Invited Speaker: Growing timescales are usually associated to growing length-scales. The glass transition is no exception. Recent experimental, numerical and theoretical results unveiled that the slowing down of the dynamics is accompanied by growing dynamic and static correlation lengths.\\ The aim of this talk is to present an overview of what has been recently understood about growing length-scales at the glass transition and their role in glassy dynamics. I will also discuss the main open questions and the predictions of different theoretical approaches. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W4.00002: Structural rearrangements that govern flow in colloidal glasses Invited Speaker: We use colloidal glasses to obtain insight into the flow of amorphous materials. In three dimensions and real time, we track the individual colloidal particles in a flowing glass, and we visualize structural rearrangements that occur during flow. The individual particle trajectories are used to identify regions of non-affine deformation, in which shear concentrates. Under slow shear, we observe thermally activated `shear transformation zones' embedded in an otherwise elastic amorphous material. Connections between these zones result in flow, which is homogeneous on macroscopic length scales. We calculate correlation functions for the fluctuations of non-affine displacements, and find a remarkable scaling, indicating that the flow of glasses is highly correlated in space. By reconstructing the entire three-dimensional strain distribution, we demonstrate that these system-spanning correlations arise from the elastic interactions between shear transformations. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W4.00003: Order in amorphous solids Invited Speaker: A solid is a system which has density modulations that are not erased by thermal motion, even on long timescales. The typical example is a crystal. Particles with soft, inter-penetrable cores may form such a solid. The remarkable fact that one can build a hard building with soft bricks can only be explained by the cooperation between an infinite number of particles. If soft particles may form a true, glassy solid phase, then we are forced to accept that an infinite coherence length must also exist for them. And yet, when we look at glass configurations, they appear definitely disordered, liquid-like. This is the mystery of glasses, which motivates the quest for a hidden order. I shall describe a coherence length that is accessible experimentally, and should diverge in an ideal glass state. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:39PM |
W4.00004: Low frequency vibrational modes and particle rearrangements in colloidal glasses Invited Speaker: We investigate the correlation between low frequency vibrational modes and fragile regions in two dimensional binary colloidal glasses, consisting of thermosensitive microgel particles. The sample packing fraction is tuned by small changes in temperature. The particles remain in their equilibrium positions in jammed states, and rearrangements are observed during temperature changes when packing fraction is changing. Using the particle displacement covariance matrix, we extract the intrinsic vibrational modes of the ``shadow'' colloidal network (i.e., with same geometric configuration and interactions but absent damping). Spatial correlations are observed between low frequency quasi-localized modes and rearranging clusters. The low frequency modes are found to contribute much more to particle rearrangement than high frequency modes. The number of rearranging clusters, as well as the size of particle rearrangements, increases as the system approaches jamming transition. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 2:15PM |
W4.00005: Space-time phase transitions in models of glasses Invited Speaker: Glass forming systems have a much richer dynamical phase structure than their thermodynamics would suggest. I will show how to explore this by means of large-deviation methods. In particular, I will demonstrate the existence of space-time phase transitions in kinetically constrained models of glasses. Similar space-time transitions seem to be present in atomistic models of supercooled liquids. In contrast to equilibrium phase transitions, which occur in configuration space, these transitions occurs in trajectory space, and are controlled by variables that drive the system out of equilibrium. Glass formers appear to live at, or close to, first-order coexistence between two distinct dynamical phases: an active and equilibrium phase, and an inactive and non-equilibrium one. This space-time coexistence helps explain observed fluctuation effects such as dynamic heterogeneity and transport decoupling. The connection of the glass transition to a true order-disorder dynamical transition offers the possibility of a unified picture of glassy phenomena. [Preview Abstract] |
Session W5: Educating Physicists for Industrial Careers
Sponsoring Units: FIAP FEdChair: Mary Lanzerotti, Pacific Lutheran University
Room: Ballroom C1
Thursday, March 24, 2011 11:15AM - 11:51AM |
W5.00001: SPIN-UP and Preparing Undergraduate Physics Majors for Careers in Industry Invited Speaker: Seven years ago, the Strategic Programs for Innovations in Undergraduate Physics (SPIN-UP) Report produced by the National Task Force on Undergraduate Physics identified several key characteristics of thriving undergraduate physics departments including steps these departments had taken to prepare students better for careers in industry. Today statistical data from AIP shows that almost 40{\%} of students graduating with a degree in physics seek employment as soon as they graduate. Successful undergraduate physics programs have taken steps to adapt their rigorous physics programs to ensure that graduating seniors have the skills they need to enter the industrial workplace as well as to go on to graduate school in physics. Typical strategies noted during a series of SPIN-UP workshops funded by a grant from NSF to APS, AAPT, and AIP include flexible curricula, early introduction of undergraduates to research techniques, revised laboratory experiences that provide students with skills they need to move directly into jobs, and increased emphasis on ``soft'' skills such as communication and team work. Despite significant success, undergraduate programs face continuing challenges in preparing students to work in industry, most significantly the fact that there is no job called ``physicist'' at the undergraduate level. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W5.00002: From Grad School to the Glass Industry: One Perspective Invited Speaker: This presentation will reflect one physicist's thoughts regarding a research and development career in the glass industry. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W5.00003: Applying Physics: Opportunities in Semiconductor Technology Companies Invited Speaker: While many physicists practice in university settings, physics skills can also be applied outside the traditional academic track. ~Identifying these opportunities requires a clear understanding of how your physics training can be used in an industrial setting, understanding what challenges technology companies face, and identifying how your problem solving skills can be broadly applied in technology companies. ~In this talk I will highlight the common features of such companies, discuss what specific skills are useful for an industrial physicist, and explain roles (possibly unfamiliar) that may be available to you. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:39PM |
W5.00004: How a Physicist Can Add Value In the Oil and Gas Industry Invited Speaker: The talk will focus on some specific examples of innovative and fit-for-purpose physics applied to solve real-world oil and gas exploration and production problems. In addition, links will be made to some of the skills and areas of practical experience acquired in physics education and research that can prove invaluable for success in such an industrial setting with a rather distinct and unique culture and a highly-collaborative working style. The oil and gas industry is one of the largest and most geographically and organizationally diverse areas of business activity on earth; and as a `mature industry,' it is also characterized by a bewildering mix of technologies dating from the 19th century to the 21st. Oil well construction represents one of the largest volume markets for steel tubulars, Portland cement, and high-quality sand. On the other hand, 3D seismic data processing, shaped-charge perforating, and nuclear well logging have consistently driven forward the state of the art in their respective areas of applied science, as much or more so than defense or other industries. Moreover, a surprising number of physicists have made their careers in the oil industry. To be successful at introducing new technology requires understanding which problems most need to be solved. The most exotic or improbable technologies can take off in this industry if they honestly offer the best solution to a real problem that is costing millions of dollars in risk or inefficiency. On the other hand, any cheaper or simpler solution that performs as well would prevail, no matter how inelegant! The speaker started out in atomic spectroscopy (Harvard), post-doc'ed in laser cooling and trapping of ions for high-accuracy time and frequency metrology (NIST), and then jumped directly into Drilling Engineering with Schlumberger Corp. in Houston. Since then, his career has moved through applied electromagnetics, geological imaging, nuclear magnetic resonance logging, some R and D portfolio management, and more recently, management of applied physics research for evaluating reservoir rocks and fluids and enhancing the productivity of reservoirs. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 2:15PM |
W5.00005: Internet Courses for Career Path Redirections Invited Speaker: The Internet provides a cost-efficient means to reach out to larger audiences, not only to graduate students soon to enter the work force, but also to practicing scientists and engineers who wish to update their own knowledge base and perhaps consider new career directions. In this presentation we describe experience with several graduate courses at the University of California, Berkeley, that were broadcast live over the internet and posted first as Google videos and later at www.youtube.com. Full graduate class lectures, typically 28 per class, together with all presentation materials available for download, plus homeworks (and solutions upon request) are found at www.coe.berkeley.edu/AST/srms and www.coe.berkeley.edu/AST/sxr2009. The video lectures are also available at www.youtube.com by clicking on ``videos'' and then searching ``david attwood''. Based on e-mail queries and personal feedback it is clear that the lectures are widely viewed, both as training lectures in industry and as classes at various universities worldwide. [Preview Abstract] |
Session W6: Alkaline Earth Atoms and SU(N) Magnetism
Sponsoring Units: DAMOPChair: Andrew Daley, University of Pittsburgh
Room: Ballroom C2
Thursday, March 24, 2011 11:15AM - 11:51AM |
W6.00001: Two-orbital SU(N) magnetism with ultracold alkaline-earth atoms Invited Speaker: Fermionic alkaline-earth atoms have unique properties that make them attractive candidates for the realization of atomic clocks and degenerate quantum gases. At the same time, they are attracting considerable theoretical attention in the context of quantum information processing. In this talk, we demonstrate that when such atoms are loaded in optical lattices, they can be used as quantum simulators of unique many-body phenomena. In particular, we show that the decoupling of the nuclear spin from the electronic angular momentum can be used to implement many-body systems with an unprecedented degree of symmetry, characterized by the SU(N) group with N as large as 10. Moreover, the interplay of the nuclear spin with the electronic degree of freedom provided by a stable optically excited state should enable the study of physics governed by the spin-orbital interaction. Such systems may provide valuable insights into the physics of strongly correlated transition-metal oxides, heavy-fermion materials and spin-liquid phases. [Reference: Nature Phys. 6, 289 (2010)] [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W6.00002: Double-degenerate Bose-Fermi mixture of strontium Invited Speaker: We report on the attainment of a double-degenerate Bose-Fermi mixture of strontium. A sample of fermionic $^{87}$Sr atoms is spin-polarized and sympathetically cooled by interisotope collisions with the bosonic isotope $^{84}$Sr. A degeneracy with $T/T_F=0.30(5)$ is reached for a $^{87}$Sr Fermi sea of $2\times 10^4$ atoms together with an almost pure $^{84}$Sr BEC of $10^5$ atoms. The rich electronic structure and the large nuclear spin of $^{87}$Sr make it a promising candidate for quantum simulation of SU($N$) magnetism and quantum information processing. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W6.00003: Degenerate Fermi Gas of Strontium-87 Invited Speaker: Degenerate Fermi gases of alkaline earth metal atoms such as strontium and ytterbium open new possibilities in the study of many-body physics because of the existence of isotopes with large nuclear spin I (e.g. I=9/2 in strontium-87). With the closed-shell electronic ground state in these atoms, the nuclear spin is decoupled from other degrees of freedom. Interactions between atoms are spin-independent, leading to a large SU(N=2I+1) symmetry of the Hamiltonian. This results in a large degeneracy of the ground state, which has been predicted to result in novel spin liquid and valence bond states. Strong attractive interactions would favor formation of N-particle singlets, in analogy to the formation of baryons in quantum chromodynamics. (For a short overview, see C. Wu, \textit{Physics} \textbf{3}, 92 (2010).) We will describe the experimental realization of a degenerate Fermi gas of strontium-87 and characterization of an optical Feshbach resonance in this system that would be needed to manipulate the atom-atom interactions. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:39PM |
W6.00004: Realization of an SU(6) invariant Fermi system Invited Speaker: We report the realization of a novel Fermi system with an enlarged spin symmetry of SU(6) in a cold atomic gas of ytterbium $^{173}$Yb with nuclear spin I=5/2, which will open up a new opportunity for exotic many-body physics . While the achievement of quantum degeneracy of $^{173}$Yb with 6 spin components was already reported three years ago, an important technique of the separate imaging of the nuclear spin components was not developed. Recently we have made this possible by exploiting an optical Stern-Gerlach effect using a spatially inhomogeneous laser beam. The metallic state to Mott insulator transition for SU(6) Fermi gas is also investigated by loading $^{173}$Yb atoms into a 3D optical lattice. We find some results suggesting the formation of SU(6) Mott state at low lattice temperatures expected for SU(N) systems. The similar adiabatic cooling effect is also observed in the Bose-Fermi mixture of spinless boson of $^{174}$Yb and the SU(6) Fermi system of $^{173}$Yb. In addition, an all-optical sympathetic evaporative cooling method is applied to the two fermionic isotopes of ytterbium $^{171}$Yb with the nuclear spin I=1/2 and $^{173}$Yb, and we successfully cool the mixture below the Fermi temperatures. The same scattering lengths for different spin components make this mixture featured with the novel SU(2) x SU(6) symmetry. The mixture is loaded into a 3D optical lattice to implement the SU(2) x SU(6) Hubbard model. In particular, we find interaction-induced suppression of Bloch oscillations for the mixture in the 3D lattice. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 2:15PM |
W6.00005: Exotic magnetism and new states of matter with alkaline earth atoms Invited Speaker: A crucial basic property of antiferromagnetic insulators with SU(2) symmetry is that adjacent spins can (and tend to) combine to form singlets, or valence bonds. The classical analog of this fact is that adjacent spins prefer to be antiparallel. These two facts underly much of our thinking about ground states of quantum antiferromagnets. Ultracold alkaline earth atoms can be used to realize magnetic insulators with SU(N) symmetry, where a minimum of N spins is required to form a singlet, and where N can be as large as 10. These systems belong to a largely unexplored class of quantum magnets. In this talk, I will discuss some of the remarkable new states of matter that are strong candidates to arise in these systems, including chiral spin liquids with fractional and non-Abelian statistics. I will also briefly discuss the issue of temperature, and point out an advantage of high-spin alkaline earths as compared to spin-1/2 magnetic systems. [Preview Abstract] |
Session W8: Bose-Einstein Condensation of Magnons and Related Phenomena
Sponsoring Units: GMAG DAMOPChair: Andrey Zheludev, Paul Scherrer Institute
Room: Ballroom C4
Thursday, March 24, 2011 11:15AM - 11:51AM |
W8.00001: Spin Dimers: from BEC to Luttinger liquids Invited Speaker: Localized spin systems, and in particular dimer systems, provide a fantastic laboratory to study the interplay between quantum effects and the interaction between excitations. Magnetic field and temperature allow an excellent control on the density of excitations and various very efficient probes such as neutrons and NMR are available. They can thus be used as ``quantum simulators'' to tackle with great success questions that one would normally search in itinerant interacting quantum systems. In particular they have provided excellent realizations of Bose-Einstein condensates [1,2]. This allowed not only to probe the properties of interacting bosons in a variety of dimensions but also to study in a controlled way additional effects such as disorder. If the dimensionality is reduced they also allow to test in a quantitative way Luttinger liquid physics [3,4,5]. I will discuss these various cases, and show that we have now good theoretical tools [6] to make quantitative comparisons with the experiments. Finally, how to go from this low dimensional case where the spins behave essentially as fermions, to the higher dimensional case where they behave as (essentially free) bosons, is a very challenging, and experimentally relevant issue.\\[4pt] [1] T. Giamarchi and A. Tsvelik, Phys. Rev. B {\bf 59} 11398 (1999).\\[0pt] [2] T. Giamarchi, C. R\"uegg and O. Tchernyshyov, Nat. Phys. {\bf 4} 198 (2008).\\[0pt] [3] M. Klanjsek et al., Phys. Rev. Lett. {\bf 101} 137207 (2008).\\[0pt] [4] C. R\"uegg et al., Phys. Rev. Lett. {\bf 101} 247202 (2008).\\[0pt] [5] B. Thielemann et al., Phys. Rev. B {\bf 79} 020408(R) (2009).\\[0pt] [6] P. Bouillot et al., arXiv:1009.0840 (2010). [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W8.00002: Luttinger--liquid and BEC physics in spin ladders Invited Speaker: Spin ladder materials serve as model systems in which the fundamental phases, exotic order, and elementary excitations of low-dimensional quantum magnets can be studied experimentally and compared quantitatively to predictions by theory. We have utilised the optimal energy scale of the exchange interactions and excellent low dimensionality of the metal--organic spin ladder material (C$_{5}$H$_{12}$N)$_{2}$CuBr$_{4}$ to study spin Luttinger--liquid (LL) and magnon Bose--Einstein Condensate (BEC) physics realized at low temperatures and in high magnetic fields in this magnetic insulator. Furthermore, the inherent chemical flexibility and the structural tunability of such metal--organic compounds enable studies of the effects of bond randomness and of non--magnetic and magnetic dopants on the spin LL and magnon BEC. Bose glass phases form and the localized impurities dominate the physics near the intrinsic quantum critical points of the ladder. Measurements of the elementary excitations, phase diagrams, and thermodynamic and magnetic properties of the LL and BEC have also been extended recently to BiCu$_{2}$PO$_{6}$ in which these phenomena are combined intriguingly with frustration of the magnetic exchange interactions. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W8.00003: Bose-Einstein Condensation and Asymmetry induced by Quantum Fluctuations in NiCl$_{2}$-4SC(NH$_{2})_{2}$ Invited Speaker: I will review Bose-Einstein condensation (BEC) in quantum magnets, in particular the compound NiCl$_{2}$-4SC(NH$_{2})_{2}$. This compound exhibits field-induced XY antiferromagnetism of the S = 1 Ni system for magnetic fields along the tetragonal c-axis between H$_{c1}$ = 2.1 and H$_{c2}$ = 12.6 T, and the axial symmetry of the spin environment allows us to understand the quantum phase transitions at H$_{c1}$ and H$_{c2}$ in terms of BEC of the spin system. Here the tuning parameter for the BEC transition is the magnetic field and not the temperature. It turns out that mass of the bosons that condense can be strongly suppressed by quantum fluctuations, resulting in a remarkable asymmetry between the properties at H$_{c1}$ and H$_{c2}$. Here I will present magnetization, thermal conductivity and specific heat data to probe BEC and in particular the effect of quantum fluctuations on the boson mass. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:39PM |
W8.00004: Spin Superfluidity and Magnone BEC in He-3 Invited Speaker: The spin superfluidity -- superfluidity in the magnetic subsystem of a condensed matter -- is manifested as the spontaneous phase-coherent precession of spins first discovered in 1984 in $^3$He-B. This superfluid current of spins -- spin supercurrent -- is one more representative of superfluid currents known or discussed in other systems, such as the superfluid current of mass and atoms in superfluid $^4$He; superfluid current of electric charge in superconductors; superfluid current of hypercharge in Standard Model of particle physics; superfluid baryonic current and current of chiral charge in quark matter; etc. Spin superfluidity can be described in terms of the Bose condensation of spin waves -- magnons. We discuss different states of magnon superfluidity with different types of spin-orbit coupling: in bulk $^3$He-B; magnetically traped ``$Q$-balls'' at very low temperatures; in $^3$He-A and $^3$He-B immerged in deformed aerogel; etc. Some effects in normal $^3$He can also be treated as a magnetic BEC of fermi liquid. A very similar phenomena can be observed also in a magnetic systems with dinamical frequensy shift, like $MnC0_3$. We will discuss the main experimental signatures of magnons superfluidity: (i) spin supercurrent, which transports the magnetization on a macroscopic distance more than 1 cm long; (ii) spin current Josephson effect which shows interference between two condensates; (iii) spin current vortex -- a topological defect which is an analog of a quantized vortex in superfluids, of an Abrikosov vortex in superconductors, and cosmic strings in relativistic theories; (iv) Goldstone modes related to the broken $U(1)$ symmetry -- phonons in the spin-superfluid magnon gas; etc. For recent review see Yu. M. Bunkov and G. E. Volovik J. Phys. Cond. Matter. {\bf 22}, 164210 (2010) [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 2:15PM |
W8.00005: Bose-Einstein condensation of magnons at room temperature Invited Speaker: This abstract not available. [Preview Abstract] |
Session W9: Self Assembly II followed by Vesicles and Micelles II
Sponsoring Units: DFDChair: Remi Dreyfus, CNRS
Room: D220
Thursday, March 24, 2011 11:15AM - 11:27AM |
W9.00001: Self-assembly of two-dimensional systems with off-center core-corona architecture Daniel Salgado, Carlos Mendoza Physical systems with core-corona architecture, such as dendritic polymers or hyper-branched star polymers which are characterized by two repulsive length scales, related to the hard and soft repulsions, respectively, show the spontaneous formation of stripe phases. Here we study, by using Monte Carlo simulations, how robust is the stripe formation process upon a shift in the center of the core with respect to the corona in a two-dimensional system of colloidal particles. We find that for sufficiently large shifts, the strip phases are replaced by a sort of plastic (or glassy) colloidal crystal consisting of a regular lattice of coronas inside of which disordered aggregates of cores coexist. The model investigated in this work could be useful for the design of colloidal plastic crystals. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W9.00002: Limit of validity of Ostwald's rule of stages in a model of solution crystallization Lester Hedges, Stephen Whitelam Many systems take ``nonclassical'' crystallization pathways, forming ordered solids via intermediates that do not share the architecture of the stable material. We possess only rules-of-thumb to explain such dynamics. Chief among them is Ostwald's rule of stages, which states that the phase that first emerges is the one closest in free energy to the parent phase. Although widely applicable, the rule breaks down in many experiments and computer simulations. It is therefore clear that the rule is without firm theoretical foundation, but it is not clear when it should apply. To this end we test Ostwald's rule of stages in a lattice model of solution crystallization. We find that rule holds in certain regions of parameter space and breaks down in others. We argue that its breakdown can be predicted using simple arguments. In addition, we find that crystallization pathways depend qualitatively on both the thermodynamic landscape prescribed by inter-particle interactions and on the relative rates of particle rotations and translations. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W9.00003: Mesophase behavior and rheology of polyhedral particles Umang Agarwal, Fernando Escobedo Translational and orientational excluded volume fields can guide assembly of particles with anisotropic shape to diverse morphologies. A roadmap elucidating correlations between phase behavior and particle shape may help devising efficient strategies for self-assembly of desired nanocrystal superlattices. To explore these complex correlations we performed detailed Monte Carlo simulations of six convex multi-faceted shapes belonging to the diverse class of space-filling polyhedrons. Simulations predict formation of various novel liquid-crystalline and plastic-crystalline phases at intermediate volume fractions. By correlating these findings with particle anisotropy and order of rotational symmetry, simple guidelines for predicting phase behavior of polyhedral particles are proposed. Moreover, detailed analysis of the structures of mesophases reveals importance of dynamical order in defining these phases and preliminary information about kinetics of these transitions is also obtained. Finally, to elucidate the effect of particle shape anisotropy on rheology, preliminary results will be reported from non equilibrium molecular dynamics simulations of the isotropic and cubatic(LC) phase of cuboidal particles. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W9.00004: Triblock Janus Spheres Qian Chen, Sung Chul Bae, Steve Granick We show that spheres that attract one another on two polar regions but repel at the middle band (``triblock Janus'') assemble into nontrivial reticulated networks. We have constructed such spheres and have visualized their aqueous assembly dynamics on the single-particle level. The building blocks are simple micron-sized colloidal spheres whose interactions (electrostatic repulsion in the middle, hydrophobic attraction at the poles) are likewise simple, but their self-assembly into this open structure contrasts with previously-known close-packed periodic arrangements of spheres. This strategy of ``convergent'' self-assembly from facilely fabricated colloidal building blocks encodes the target supracolloidal architecture not in localized attractive spots but rather in large redundantly attractive regions of the building blocks. The idea extends to designing other supracolloidal networks. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W9.00005: Novel structure formation of dipolar Janus particles (JP) in electrolytes: A molecular dynamic (MD) simulation study Mahdy Malekzadeh, Bamin Khomami There have been tremendous number of experimental studies and number of simulations in recent years trying to elucidate the underlying principals which determine structure formation of colloidal systems of JP. However most of simulations utilize relatively simple models and lack inclusion of long range columbic interactions. In this work MD simulations have been performed to understand effects of surface charge density and volume fraction (0.01-0.17) on structure formation and radial pair distribution function (RDF) of JPs of 6 nm in diameter with opposite charges on each hemisphere. Inclusion of long range columbic interaction via Ewald summation leads to formation of novel structures such as rings, chains and layered large spheres (about hundreds of nanometers) in accord to experimental observations. Moreover based on possibility of defect formation during synthesis, defects were introduced into each JP by slightly altering the uniform charge distribution on each hemisphere. Our results show in presence of small amount of defects ($<$10{\%}) no significant changes occur in RDF, however increasing defect sites up to around 20{\%} will significantly changes structure formation and combination of aforementioned structures concur to SFM and SEM images. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W9.00006: Glassy Dynamics in the Rotator Phase of Two-Dimensional Janus Crystals Jing Yan, Shan Jiang, Jonathan Whitmer, Stephen Anthony, Erik Luijten, Steve Granick Janus particles, spheres with two different sides, represent the simplest building blocks whose interparticle interaction is orientation dependent. When confined on regular lattices, they epitomize basic physical problems from the arrangement of spins in magnetic materials, to rotating molecules in plastic crystals. Here we study both in experiment and in simulation, the heterogeneous dynamics in a two-dimensional crystal of amphiphilic Janus spheres. Single particle tracking reveals that orientation along can generate phenomenology resembling conventional translational supecooled liquids and glasses. Characteristic cage break events, which requires anti- correlated rotation of particles sitting on neighboring lattices, were indentified and characterized in detail. Recent experiments aiming at selectively perturbing the system using external field, such as magnetic field, will also be discussed. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W9.00007: Chiral Control of Interfacial Tension Mark Zakhary, Thomas Gibaud, Edward Barry, Robert Meyer, Zvonimir Dogic The interfacial tension between molecular species in self-assembling systems plays a crucial role in determining the physical properties of the mesoscopic assemblages. The predominant method for controlling interfacial tension is the addition of surfactant molecules, which preferentially adsorb onto the interface and modify the interactions between the two phases. In this talk, using a model colloidal membrane composed of chiral, rod-like \textit{fd }viruses, I will present a new method for controlling interfacial tension which does not require additional surfactant components, but instead utilizes the intrinsic chirality of the constituent rods. I will demonstrate that chirality can be used to continuously tune the interfacial tension of a membrane and to drive a dramatic phase transition from two-dimensional membranes to one-dimensional twisted ribbons. Using a wide variety of microscopic techniques, this transition is characterized over all relevant length-scales, ranging from nanometers to microns. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W9.00008: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W9.00009: Dynamics of a compound vesicle in shear flow Shravan Veerapaneni, Yuan-Nan Young, Petia Vlahovska, Jerzy Blawzdziewicz The dynamics of compound vesicle (a lipid bilayer membrane enclosing a fluid with a suspended particle) in shear flow is investigated using both numerical simulations and theoretical analysis. We find that the non-linear coupling (via hydrodynamic interaction) between the inclusion motion and the confining membrane deformation gives rise to new features in the vesicle dynamics. Transition from tank--treading to tumbling can occur even in the absence of any viscosity mismatch. An initially non-concentric inclusion induces transient vesicle waltzing. A swinging-like vesicle motion is observed if the enclosed particle is an ellipsoid. The rheology of a suspension of compound vesicles is also strongly affected by the inclusion confinement. Our results highlight the complex effects of internal cellular structures on cell dynamics in external flow. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W9.00010: Exploring Structure, Shape, and Dynamics of Elastin-like Polypeptide Nanoparticles Kiril A. Streletzky, Kaitlin Vandemark, Ali Ghoorchian, Nolan Holland Environmentally responsive nanoparticles synthesized from elastin-like polypeptides (ELP) present a promising system for applications as biosensors, drug delivery vehicles, and viscosity modifiers. These nanoparticles undergo a transition from a soluble state at room temperature to micellar aggregates above the transition. The size, shape, and dynamics of micelles above the transition as well as effects of the solvent salt concentration and pH on the transition are important to understand from a fundamental science point of view as well as for potential applications. The system has been characterized with high resolution multiangle Dynamic and Static Light Scattering Spectroscopies. It was confirmed that the system undergoes a transition from mixture of ELP extended trimers and their non-spherical formations to a solution of micelles. It was discovered that micellar size and structure are very sensitive to solution's pH. The micelles were generally found to exhibit properties of the hyperbranched spheres below pH of 10 and above pH of 10.3 with their shape becoming significantly elongated in the pH window of 10 to 10.3. It was also found that the size of micelles strongly depends on salt concentration displaying at least two size regimes (20-45nm at 0-20mM and 100-150nm at 25-40mM) with different salt concentration dependences. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W9.00011: The Lipid domain Phase diagram in a Dipalmitoyl-PC/Docosahaexnoic Acid-PE/Cholesterol System Chai Lor, Linda Hirst Lipid domains in bilayer membrane and polyunsaturated fatty acids (PUFAs) are thought to play an important role in cellular activities. In particular, lipids containing docosahaexnoic acid are an interesting class of PUFAs due to their health benefits. In this project, we perform oxidation measurements of DHA-PE to determine the rate of oxidation in combination with antioxidants. A ternary diagram of DPPC/DHA-PE/cholesterol is mapped out to identify phase separation phenomena using atomic force microscope (AFM). Fluorescence microscopy is also used to image lipid domains in a flat bilayer with fluorescent labels. As expected, we observe the phase, shape, and size of lipid domains changes with varying composition. Moreover, we find that the roughness of the domains changes possibly due to overpacking of cholesterol in domains. This model study provides further understanding of the role of cholesterol in the bilayer membrane leading towards a better understanding of cell membranes. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W9.00012: Interfacial Microrheology with a Magnetic Needle Viscometer and Two-Particle Correlated Motion James Sebel, Kenneth W. Desmond, Eric R. Weeks We measure the viscoelastic moduli of thin films using two different methods. First, we use a magnetic needle viscometer. Our apparatus employs Helmholtz coils to control the position and orientation of the needle in the film. By driving the needle we can produce a response in the film which allows us to probe the bulk viscoelastic properties of the film. Second, we use two particle microrheology to probe the local properties of the film. Tracking the correlated motion between two particles as they undergo Brownian motion probes the local viscoelastic properties of any heterogeneous domains. Examining the correlations between pairs of particles with large separations helps us infer information about the bulk properties. Coupling this technique with the magnetic needle viscometer provides information on the effect local viscoelastic properties have on the bulk properties. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W9.00013: Ion-Specific Induced Charges at Aqueous Soft Interfaces Wenjie Wang, Alex Travesset, David Vaknin Surface-sensitive X-ray scattering and spectroscopic techniques are employed to monitor ion binding specifically to Langmuir monolayers of densely packed carboxyl or phosphate groups. By systematically varying pH of Fe$^{3+}$, Fe$^{2+}$ and La$^{3+}$ solutions, we show that the critical surface pressure at the tilted (L2) to untilted (LS) transition is ionic specific and pH dependent. While the maximum density of surface bound La$^{3+}$ per carboxylic group is $\sim$ 0.3, the amount necessary to neutralize the fully charged surface, for Fe$^{3+}$ it is nearly 0.6. Furthermore, the binding of Fe$^{3+}$ is accompanied with a significant accumulation of Cl$^-$ co-ions implying interfacial charge inversion. Similar experiments with charged phosphate groups at the interface show that the bindings of Fe$^{2+}$ and La$^{3+}$ are electrostatically driven. Our results have implications on biomineralization processes and ionic functions at cell membranes. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W9.00014: Structural characterization of a multiple stacked supported bilayer system Curt Decaro, Justin Berry, Laurence Lurio, Yicong Ma, Gang Chen, Sunil Sinha, Lobat Tayebi, Atul Parikh Supported Lipid Bilayers are a popular model system for cell membranes since their defined orientation allow characterization with probes such as AFM, x-ray and neutron scattering. A significant concern, however, is that strong interactions with the substrate can suppress dynamics within the bilayer. One method that has been successful at overcoming this limitation is to cushion the supported bilayer on a softer material. In the present work, we have stacked up to five successive bilayers of DPPE on top of each other, in effect using the lower bilayers as cushions. X-ray reflectivity shows that each stack preserves the orientation of the first, and that each bilayer exhibits full coverage of the one below. The roughness of each bilayer is found to increases with distance from the substrate as would be expected if thermal fluctuations are increasing with distance from the substrate. We also find that upon heating from the gel to the fluid state that an unbinding transition is observed. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W9.00015: Shape and Size of highly concentrated micelles in CTAB/NaSal solutions by small angle neutron scattering (SANS) Hu Cao, Helmult Kaiser, Narayan Das, Paul Sokol, Joseph Gladden Highly concentrated micelles CTAB/NaSal with a fixed salt/surfactant ratio of 0.6 have been studied by small angle neutron scattering (SANS) as a function of temperature and concentrations. A modeling analysis with a combination of ellipsoid, Gaussian size distribution and Hard Sphere Model (HSM) on SANS data suggests that these micelle solutions have an ellipsoidal structure, which is independent on the concentrations and temperature. However, the micelle size decreases monotonically as increasing the temperature or concentration. Besides, it was found that the number density of particles increases as increasing the temperature, while the total volume keeps unchanged. These observations indicate that large micelles at low temperature begin to break to form small ones as increasing the temperature and these broken surfactant molecules aggregate again under the effect of strongly binding counterions to form more micelles. [Preview Abstract] |
Session W10: Adsorption, Wetting, and Complex Interfaces
Sponsoring Units: DCMPChair: Milton Cole, Penn State University
Room: D221
Thursday, March 24, 2011 11:15AM - 11:27AM |
W10.00001: Surface Tension Anisotropy of Lennard-Jones Systems Emre Esenturk Anisotropy in the surface tension plays a significant role in the evolution of interfaces and in determining the equilibrium shapes of materials (dendritic growth, motion of grain boundaries). We present a discrete version of integral Phase Field Model with non-local potential for the crystal-melt interfaces of Lennard-Jones Systems. The model provides a methodology to understand the process of transfer of microscopic anisotropy to macroscopic scale. We calculate the surface tension and the anisotropy of the crystal-melt interface in the [100] and [110] directions and compare our results with recent simulations. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W10.00002: Adsorption of Methane and Propane on a LJ Wall and on Molybdenum Surface: A Simulation Study Ali Abu Nada, Gary Leuty, Mesfin Tsige Atomic-scale MD simulations were used to study multilayer adsorption as a function of temperature for two different alkanes (CH$_{4}$ and C$_{3}$H$_{8})$ on a fixed Lennard-Jones wall and on the (001) surface of molybdenum. In sets of simulations on the molybdenum surface, the substrate atoms were made to interact via a Lennard-Jones potential in one study and via an embedded-atom model (EAM) potential in the next. The results show that CH$_{4}$ and C$_{3}$H$_{8}$ on the flat wall possess a highly ordered packing arrangement exhibiting a higher degree of order than films adsorbed on the molybdenum substrate. Additionally, the number of ordered layers seen in the case of adsorption of CH$_{4}$ was noted to be greater than the number of ordered layers in the case of adsorption of C$_{3}$H$_{8}$. In each case, the first layer appears frozen, implying there is no translational motion for the molecules in this layer, and we can confidently surmise that the first adsorbed layer exists in the solid phase far above the bulk melting temperature. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W10.00003: Evaporation kinetics of CO$_{2}$ laser heated fused silica Selim Elhadj, M.J. Matthews, S.T. Yang, D. Cooke, J.S. Stolken, M.D. Feit Laser-based machining strategies of optical surfaces remain mostly empirical, yet, systematic and controlled studies that relate gas chemistry and surface temperature to evaporation kinetics are limited, especially at extreme temperatures ($>$2800K) reached during laser irradiation. We present experimental results of CO$_{2}$ laser heating of silica in oxidizing and non-oxidizing environments, along with analysis of surface shape from which a near-equilibrium evaporation model is derived. Based on this model, temperature dependent enthalpies of evaporation are determined and compared to published results. This model reproduces experimental laser-etch rates, while still accounting for laser, mass transport, and gas chemistry parameters. Although heat and mass transport processes are complex and tightly coupled, general conditions for which such an approach can be used to guide laser-based evaporation will be presented. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W10.00004: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W10.00005: Simulation study on structure of water in aqueous solutions confined between graphene electrodes under very high applied electric field Gary Leuty, Mesfin Tsige, Saikat Talapatra Arising from questions regarding electric double-layer capacitors utilizing graphene electrodes and aqueous electrolyte (KOH solution), atomistic MD simulations of electrolyte confined between graphene electrodes were performed to understand the behavior of electrolyte as a function of electric field strength and solution concentration, from pure water to 6M KOH. It was noted that the strength of the electric field had a demonstrable effect on the structure of pure water between the electrodes (as has previously been seen in highly confined multilayer water systems), creating regularly spaced channels and densely packed sheets of highly ordered molecules. We also saw a clear effect due to the presence of electrolyte ions and their separation from the water due to the action of the field; different field strengths appear to greatly alter the distribution of ions, which in turn affects the structure and ordering of the water. Time dependence in the strength of the electric field was also studied to determine what effect, if any, it has on induced structure. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W10.00006: Thermodynamics and kinetics of wetting transition of an oily fluid on surfaces with nanoscale roughness: A molecular dynamics study Elizabeth Savoy, Fernando Escobedo Surface wettability has garnered significant interest in recent years, as design and manufacture of nanoscale features allows fabrication of highly non-wetting surfaces. Such behavior is more difficult to achieve for low surface-tension fluids such as oils, and requires novel approaches. One approach is to create roughness features that provide an energy barrier to the fluid's transition from the composite to fully wetted state. We use molecular dynamics of small droplets in combination with various simulation techniques, such as umbrella sampling and forward flux sampling, to probe the energy landscape associated with the wetting transition and compute transition rates and their dependence on key topological parameters such as feature height. We find that the drop does not transition with a flat liquid-vapor interface when it penetrates and wets the subsurface features (as is often assumed in continuum treatments) and that the hysteresis in the wetting and dewetting transitions is associated with differences in the evolution of that interface. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W10.00007: Understanding and designing of steam-phobic surfaces Ila Badge, Sunny Sethi, Ali Dhinojwala The wetting behavior of a surface under steam condensation depends on its intrinsic wettability and micron or nanoscale surface roughness. A typical superhydrophobic surface may not be suitable as a steam-phobic surface due to nucleation and growth of water inside the valleys and thus, failure to form air-liquid- solid composite interface. Here, we present the results of steam condensation on chemically modified nano-structured carbon nanotube carpets. The combination of surface chemistry and surface roughness provides a mechanism to retain superhydrophobicity of the nanotube surfaces under steam condensation. Ability of withstand steam temperature and pressure also implies improved hydrostatic stability of the surface. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W10.00008: Effects of host relaxation on gas uptake in porous media Milton Cole, Annie Grosman, Susana Hernandez, Angela Lueking We have recently predicted [1] an \textit{imbibition transition} as a prototype of a general phenomenon-substrate relaxation due to adsorption. That transition is exemplified by a graphene sheet's lifting off of a surface in order to intercalate gas. Other relevant phenomena include the expansion of nanotube bundles or MOFs to accommodate imbibed gases. In our new work, we first analyze the relaxation problem, in general, and then address infinite cylindrical and slit pore geometries, for which simplifications occur because there is just one finite dimension. Research supported by DOE. \\[4pt] [1] K. E. Noa, A. D. Lueking and M. W. Cole, \textit{Imbibition transition: gas intercalation between graphene and silica}, submitted to J. Low Temp. Phys. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W10.00009: Surface metal-oxygen bond length on hydrated rutile(110) and cassiterite(110) surface - A measure of the local environment Nitin Kumar, Paul Kent, Andrei Bandura, David Wesolowski, James Kubicki, Jorge Sofo We study the dynamics of water on the surface of rutile (110) and cassiterite (110) using ab-initio molecular dynamics simulation. The water molecule covalently attach with the fivefold coordinated metal atoms on the surface. It can remain in a molecular form or it can dissociate to form hydroxyls on the surface. The distance between the metal and the oxygen depends on the protonation state of the latter. Moreover, we find that the local environment is not only limited to the number of covalently bonded hydrogen but it also depends on number of hydrogen bonds and the species participating in it. In general, the metal oxygen distance shows much larger fluctuations in rutile compared with cassiterite. The half width half maximum (HWHM) of the metal oxygen distance histogram, for the terminal oxygen, is 0.27 Angstrom for rutile and 0.16 Angstrom for cassiterite. Also, for bridging oxygen HWHM is 0.18 and 0.12 Angstrom for rutile and cassiterite, respectively. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W10.00010: Water-Thin-Film Adsorption on Alpha-Quartz (0001) Surface Yun-Wen Chen, Yan Wang, Hai-Ping Cheng We investigated thin water films adsorbed on quartz (0001) surfaces using first-principles density functional theory calculations. Interfacial structure and energetics were studied through a layer-by-layer deposition. From monolayer to multilayer, the low energy state configurations and adsorption sites show a transition due to formation of a highly stable bilayer membranelike structure. The water adsorption energy on a quartz surface coated by this membrane is of typical hydrogen bond strength for both dry and fully hydroxylated surfaces. The interactions between the surface and the water films are short-ranged due to shielding of the bilayer. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W10.00011: Mn Adsorption on MgO/Ag(100): DFT and DFT+U Calculations Hossein Hashemi, Barbara Jones The adsorption properties of a Mn adatom on MgO ultrathin films deposited on a Ag(100) substrate are determined from first principles DFT calculations and compared with the corresponding adsorption characteristics from DFT+U calculations. First, we investigate the properties of a pure Ag (100) surface and a MgO/Ag(100) system. The structural relaxation, work function and surface energy for the Ag(100) surface, as a function of thickness dependence, is discussed. Next, we discuss the addition of a Mn adatom. Regular (U=0) DFT calculations show that the most stable site for Mn adsorption on MgO/Ag(100) is the bridge site, followed closely by the Oxygen site, and a very unlikely position, the Mg site. We have also investigated the role of strong electron correlations in the substrate on the chemisorption properties of a Mn adatom. DFT+U calculations predict the Oxygen site to be the most stable site, instead of the bridge site, in contrast to U=0 DFT calculations. The energies, geometry, and magnetic properties of the Mn adatom are all influenced by adding a Coulomb energy. Altogether our results show that the on-site Coulomb repulsion in the Mn d band plays an important role in the description of adsorption on MgO/Ag(100). [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W10.00012: Stabilizing Graphitic Thin Films of Wurtzite Materials by Epitaxial Strain Dangxin Wu, Feng Liu Recent theoretical and experimental work showed that (0001) ultrathin films of wurtzite materials transform into a stable graphite-like structure if their thickness is reduced to only a few atomic layers. Using first-principles calculations of both freestanding and substrate-supported thin films, we predict that the thickness range of stable graphitic films can be greatly extended by epitaxial tensile strain but reduced by compressive strain. The band gap of the resulting graphitic films can be tuned by strain and film thickness either above or below that of the bulk wurtzite phase. Our prediction suggests a plausible physical mechanism to be explored by future experiments for strain engineering of graphitic films from wurtzite materials with a wide range of potential applications. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W10.00013: Simulating phase formation during exothermic reactions in Al/Ni and Al/Zr multilayered foils Rong-Guang Xu, Michael L. Falk, Hong-Wei Sheng Reactive multilayered foils are composed of thousands of alternating micro- to nano- scale layers of elements which have a large negative enthalpy of mixing. When a small pulse of energy (such as an electric spark or a thermal pulse) is provided, highly exothermic, self-propagating chemical reactions can be triggered. Both theoretical models and experimental data indicate that even a relatively small amount of premixing can have a dramatic effect on the heats and velocities of the propagating reaction front. We have implemented molecular dynamics simulation to study the phase transformation sequence during multilayered reactions and to elucidate how premixing can affect the sequence of phase formation during such reactions. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W10.00014: Chemical Reactivity at the Ti/CuO Interface A. Chourasia, J. Edmondson The chemical reactivity between titanium and copper oxide at the Ti/CuO interface has been investigated using x-ray photoelectron spectroscopy. About 15 nm thick copper film was deposited on silicon substrates by the e-beam method. Such samples were oxidized in an oxygen environment in a quartz tube furnace at 400\r{ }C. The formation of CuO was checked by the XPS spectral data. Thin films of titanium were then deposited on these CuO samples. The titanium 2p, oxygen 1s and copper 2p regions were investigated by XPS. The magnesium anode (energy = 1253.6 eV) has been used for this purpose. The spectral data show chemical reactivity at the Ti/CuO interface. The samples were annealed afterwards in air at 400\r{ }C. The spectral data were recorded at different take-off angles. Comparison of the data with the pre-annealed samples shows diffusion of Cu through the titanium overlayer alongwith the formation of CuO. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W10.00015: Composite TiO2-Carbon nano films with enhanced photocatalytic activity Dinko Chakarov, Raja Sellappan Composite TiO2-carbon thin films prepared by physical vapor deposition techniques on fused silica substrates show enhanced photocatalytic activity, as compared to pure TiO2 films of similar thickness, towards decomposition of methanol to CO2 and water. Raman and XRD measurements confirm that annealed TiO2 films exhibit anatase structure while the carbon layer becomes graphitic. Characteristic for the composite films is an enhanced optical absorption in the visible range. The presence of the carbon film causes a shift of the TiO2 absorption edge and modifies its grain size to be smaller. The observed enhancement is attributed to synergy effects at the carbon-TiO2 interface, resulting in smaller crystallite size and anisotropic charge carrier transport, which in turn reduces their recombination probability. [Preview Abstract] |
Session W11: Electronic Structure: Thermodynamics and Optical Properties
Sponsoring Units: FIAPRoom: D222
Thursday, March 24, 2011 11:15AM - 11:27AM |
W11.00001: Comparison of Experimental and Theoretical Vibrational Spectra for Pentacene Derivatives Gregory Maslak, Mark Stewart, Lillie Ghobrial, Wes Laurion, Jinyue Jiang, Li Tan, Carolina C. Ilie The practical use of pentacene in the area of organic field-effect transistors is limited by its sensitivity to oxygen and poor solubility in organic solvents. To overcome these disadvantages, new organic semiconductors as 2,3,9,10-tetrakis(3,5-di-t-butylphenylethynyl)-6,13- bis (trimethylsilylethynyl) pentacene are synthesized. The new pentacene derivatives may be useful for electronic devices such as organic field-effect transistors or organic light-emitting diodes. Here we compare the calculated vibrational spectra to the experimental data in order to characterize the new derivative. The methods and similarities between the theoretical calculations and the experimental data are discussed. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W11.00002: Optical characterization of p-doped InP epitaxial layers in mid and far infrared region R.C. Jayasinghe, Y.F. Lao, A.G.U. Perera, M. Hammar, C.F. Cao, H. Wu The optical properties of p-doped Indium Phosphide (InP) epitaxial thin films with 1, 3, and 24 $\times $ 10$^{18}$ cm$^{-3}$ carrier concentrations were investigated by infrared reflection, transmission, and absorption measurements in 5 - 40 $\mu $m wavelength range. The absorption spectra were modeled by complex dielectric function using the classical Lorentz--Drude model. The phonon absorption in InP was modeled using eight Lorentzian oscillators. This method gives a straightforward approach for modeling the experimental absorption spectra when compared to the two-phonon absorption spectroscopy technique. The calculated spectra are in a good agreement with experimental spectra. The effects of doping on fitting parameters are also investigated. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W11.00003: Photoluminescence intensity oscillations with magnetic field in InGaAs quantum wells Lars Schweidenback, Andreas Russ, Tariq Ali, Joseph Murphy, Alexander Cartwright, Athos Petrou, Alexander Govorov, Connie Li, Aubrey Hanbicki, Berend Jonker, George Kioseoglou We have observed magnetic field oscillations in the photoluminescence (PL) intensity from InGaAs quantum wells (QWs) with indium compositions of 5{\%} and 15{\%} with laser excitation close to the bandgap for temperatures $<$ 20 K. For all samples the intensity maxima occur at 2.2 and 4.5 tesla when the magnetic field is applied perpendicular to the QW plane. Experiments in which the sample normal (z-axis) is tilted with respect to the applied magnetic field $B$ show that the PL intensity maxima positions depend on the magnetic field component $B_{z}$. Time-resolved PL comparison with GaAs QWs yields much longer recombination times for the InGaAs QWs. Furthermore, cross sectional scanning tunneling microscopy studies indicate the formation of Indium rich InGaAs clusters in the InGaAs QWs. We interpret the observed oscillations in terms of the Aharonov-Bohm effect and quasi-indirect excitons with ring-like trajectories of carriers. The oscillation period corresponds to orbits with radius equal to 24 nm. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W11.00004: Optical two-dimensional Fourier transform spectroscopy of single GaAs quantum wells Yuri D. Glinka, Zheng Sun, Xiaoqin Li, Allan Bracker Optical two-dimensional Fourier transform spectroscopy is applied to study the coherent coupling between light-hole and heavy-hole excitons in single GaAs quantum wells instead of those consisted of ten or four periods of GaAs separated by Al$_{0.3}$Ga$_{0.7}$As barriers measured previously. The effect of the confinement energy as well as Coulomb and disorder correlation lengths on coherent coupling dynamics is discussed. The financial support from ARO, NSF, and Welch foundation is gratefully acknowledged. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W11.00005: Origin of the Terahertz Absorption Peak in Single-Walled Carbon Nanotubes Qi Zhang, Lei Ren, H. Gojuki, E.H. Haroz, T. Arikawa, J. Kono, C.L. Pint, R.H. Hauge, A.K. Wojcik, A.A. Belyanin Single-walled carbon nanotubes (SWNTs) are promising for high-frequency electronics and terahertz (THz) applications, as well as for fundamental studies of finite-frequency dynamics of one-dimensional electronics. Previous studies of dynamic conductivities of various types of SWNTs have revealed a pronounced and broad absorption peak around 4 THz, whose origin has been a matter of controversy. Both the effects of curvature-induced band gaps and plasmonic absorption due to finite lengths have been proposed to be important, but a consensus has not emerged. We have studied 4THz peak in highly aligned and length-controlled SWNT films and metallically-enriched SWNT films through FTIR and THz time-domain spectroscopy. We provide evidence that this peak is observable only when the THz polarization is parallel to the nanotubes and only in metallic tubes. We will discuss the origin of this absorption peak in light of these new findings. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W11.00006: UV light emission from ZnO nanostructures in SiO$_{2}$ synthesized by ion implantation and thermal annealing Bimal Pandey, Akhilesh Singh, Prakash Poudel, Arup Neogi, Duncan Weathers Zinc Oxide (ZnO) nanostructures were synthesized by the implantation of low energy (35 keV) ZnO molecular ions into thermally grown SiO$_{2}$ at a fluence of 5 $\times $ 10$^{16}$ ions/cm$^{2}$. Implanted samples were annealed in an oxygen environment to allow the growth of ZnO precipitates. X-ray photoelectron spectroscopy (XPS), Fourier transform spectroscopy (FTIR) and energy dispersive x-ray spectroscopy (EDS), confirm the formation of ZnO. High resolution transmission electron microscopy (HRTEM) shows the formation of nanostructures having diameters ranging from 2 nm to 5 nm in the SiO$_{2}$. Photoluminescence (PL) measurements show excitonic and band-edge emission in the ultraviolet region at temperatures ranging from 4 K - 300 K. Time-resolved PL measurements performed at 4K showed an electron-hole recombination lifetime on the order of a few hundred picoseconds. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W11.00007: Ab initio calculation of indirect light absorption by free carriers in transparent conducting oxides Hartwin Peelaers, Emmanouil Kioupakis, Chris G. Van de Walle Phonon-assisted absorption of light by free carriers is an important optical process in many materials and a challenging problem for computational condensed-matter physics. As transparent conducting oxides play an important role as contacts to light emitters and photovoltaic devices, it is important to consider not only the optical absorption across the band gap, but also the absorption by free carriers due to indirect processes mediated by phonons and defects. Here we calculate these losses using a full ab initio methodology as opposed to a phenomenological Drude model containing empirical parameters. These calculations involve the electron-phonon coupling matrix elements, which are dominated by the longitudinal optical phonon modes. We also compare with the Fr\"ohlich model, which describes the electron-phonon matrix elements in the long-wavelength limit. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W11.00008: Optical absorption of light carrying orbital angular momentum by semiconductors: free-particle quantum kinetics P.I. Tamborenea, G.F. Quinteiro We develop a free-carrier theory of the optical absorption of light carrying orbital angular momentum (twisted light) by bulk and quasi-two-dimensional semiconductors. We obtain the optical transition matrix elements for Bessel-mode twisted light and use them to calculate the wave function of photo-excited electrons to first-order in the vector potential of the laser [1]. We then pose the problem of the quantum kinetics of interband transitions in terms of the Heisenberg equations of motion of the electron populations, and interband and intraband coherences [2]. We solve the equations of motion in the low-excitation regime, and obtain analytical expressions for the coherences and populations; with these, we calculate the orbital angular momentum transferred from the light to the electrons and the paramagnetic and diamagnetic electric current densities. \\[4pt] [1] G.\ F.\ Quinteiro and P.\ I.\ Tamborenea, EPL {\bf 85}, 47001 (2009).\\[0pt] [2] G.\ F.\ Quinteiro and P.\ I.\ Tamborenea, Phys.\ Rev.\ B {\bf 82}, 125207 (2010). [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W11.00009: Optical harmonics generation in semiconductor quantum dots: A tunable terahertz source Yan Xie, Weidong Chu, Suqing Duan, Wei Zhang The high-order harmonic generation (HHG) study have been extended to semiconductor quantum dots (QDs) and coupled QDs (CQDs), the so-called ``artificial atoms and molecules.'' One motivation of the study of the HHG in QDs is to find an efficient way of terahertz wave generation due to their controllable energy spectra and wave functions. With the help of Floquet theory, we show that the HHG in quantum dot structures can be changed from only odd orders to both odd and even orders by controlling the coupling parameters. The selection rules of the odd-even HHG in a noninversion-symmetric multilevel system are determined by the parity of emitted photon numbers during allowable virtual steps. On the other hand, by mapping the optical problem to a transport problem, we find that the terahertz generation efficiency is determined by the bandwidth of the quasienergy spectrum. Our studies are useful for engineering tunable terahertz sources based on semiconductor quantum dots. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W11.00010: Ultrafast Quantum Control in Semiconductor Nanostructures using Twisted Light Guillermo Quinteiro, Pablo Tamborenea, Jamal Berakdar We investigate possible uses of twisted light (TL) ---or light carrying orbital angular momentum (OAM)--- as a tool to control semiconductor-based nanostructures. Two systems are considered, namely quantum dots (QD) and quantum rings (QR). For both structures we employ a simplified two-band model in the effective-mass approximation, having a conduction and a heavy-hole valence bands. In the case of disk-shaped QDs, we predict that the TL would allow to induce optical transitions which are normally regarded to be forbidden. The OAM $l$ and other parameters of the TL beam can be used to precisely control the final state of the electron. In the case of QRs, we study induced electric currents. We analyze the evolution of the system in terms of Heisenberg equations of motion. We find an analytical solution that resembles the standard Optical Bloch Equations. Using this solution, we find the evolution of the $z$-component of OAM and the electric current circulating the ring. Our results indicate that the electric current could be as large as $\mu$A, in the time-scale of pico-seconds. For an appropriate radius of the ring, the photo-induced magnetic field would be large enough to switch within picoseconds the magnetic moment of particles placed within the ring. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W11.00011: Far-infrared absorption of PbSe nanorods Byung-Ryool Hyun, Adam Bartnik, Weon-kyu Koh, Nikolay Agladze, Jun Yang, Al Sievers, Christopher Murray, Frank Wise The far-infrared absorption spectra of PbSe nanodots and nanorods are measured as a function of aspect ratio, and show the expected splitting of the single Frohlich sphere mode in nanocrystals into two modes parallel and perpendicular to the nanorod axis. We analyze this splitting by modeling the dependence of the nanocrystal's local field factor on its shape. Excellent agreements is found with the features measured in experiment. We predict that this shape-dependent local field factor will cause a two-order of magnitude increase of the third-order susceptibility of long nanorods in the near-infrared. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W11.00012: Bright spot pattern generation in GaAs/AlGaAs multiple quantum wells Angelo Mascarenhas, Brian Fluegel, D.W. Snoke Exciton photoluminescence pattern generation is investigated in multiple quantum wells. High-contrast outer rings and localized bright spots are generated using efficient field-assisted upconversion of laser light whose photon energy lies below the energy of the luminescing quantum well transition. Time-resolved images of the bright spot reveal that the ring transients are driven by carrier diffusion both from the laser excitation spot as well as from the bright spot. These dynamics are not explained simply by two-dimensional rate equations for generation and diffusion. The behavior must be understood as a result of three-dimensional transport in the vertically extended samples. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W11.00013: Nonlinear optical properties of polychlorotriphenylmethyl radicals: a computational study Claudia Cardoso, Bruce Forbes Milne, Fernando Nogueira The special interest in molecular design for the development of novel second-order nonlinear optical (SONLO) materials is driven by their potential applications in new optoelectronic technologies. Various strategies are currently employed to enhance the molecular SONLO activity, and studies revealed that species having open-shell electronic states exhibit larger $\beta$ values than analogous closed-shell systems. Their open-shell electronic structure leads to accessible low-lying charge transfer electronic states which enhance the $\beta$ values with respect to their closed-shell counterparts. We present the results of a Density Functional Theory calculations of a series of substituted polychlorinated triphenylmethyl radicals. This family of radicals has been measured by Ratera et al using Hyper-Rayleigh Scattering and showed to have enhanced $\beta$ values. The present study compares results obtained within LDA and several hybrid functionals, namely long-range corrected functionals. The effect of solvents was also considered through the use of the polarizable continuum model. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W11.00014: Binding energies of indirect excitons in double quantum well systems Alex Rossokhaty, Stefan Schmult, Werner Dietsche, Klaus von Klitzing, Igor Kukushkin A prerequisite towards Bose-Einstein condensation is a cold and dense system of bosons. Indirect excitons in double GaAs/AlGaAs quantum wells (DQWs) are believed to be suitable candidates. Indirect excitons are formed in asymmetric DQW structures by mass filtering, a method which does not require external electric fields. The exciton density and the electron-hole balance can be tuned optically. Binding energies are measured by a resonant microwave absorption technique. Our results show that screening of the indirect excitons becomes already relevant at densities as low as ~5$\times$10$^{9}$cm$^{-2}$ and results in their destruction. [Preview Abstract] |
Session W12: Electronic Transport in Novel Materials and Nanostructures
Sponsoring Units: FIAPRoom: D223/224
Thursday, March 24, 2011 11:15AM - 11:27AM |
W12.00001: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W12.00002: Random Telegraph Noise in Silicon Nanowire BioFETs Nitin Rajan, David Routenberg, Jin Chen, Mark Reed Noise spectroscopy is important for nanostructures because it represents a highly sensitive and non-destructive means of studying surface states/defects. In this study we characterize the low frequency noise of top-down fabricated silicon nanowire FETs with exposed channels at low temperature. For some devices, we observe a change in the noise spectra as temperature is lowered, from 1/f to Lorentzian. This indicates the presence of random telegraph signals (RTS) due to an interface trap which we confirm from the time-domain measurements. By making measurements at different temperatures, we can probe into the dynamic properties of the trap. In this way, the activation energies for the emission and capture of electrons are determined. The nature and position of the trap is deduced from the gate voltage dependence of the emission and capture time constants. We also observe an increase in the relative RTS noise amplitude as temperature is decreased and report on very large ($>$100{\%}) relative noise amplitudes for measurements carried out at low temperature. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W12.00003: Distinct microwave photoresistivity peak in a high-mobility quantum Hall system A.T. Hatke, M.A. Zudov, L.N. Pfeiffer, K.W. West We report on a distinct resistivity peak in a microwave-irradiated high-mobility two-dimensional electron system at low temperatures. This peak appears in the regime of well separated Landau levels near the second harmonic of the cyclotron resonance and is in addition to microwave-induced resistance oscillations. This talk will focus on the generic characteristics of this peak, such as its dependence on microwave power and temperature. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W12.00004: Theory of nonlinear transport in separated Landau levels of two-dimensional electron systems M. Khodas, H.-S. Chiang, A.T. Hatke, M.A. Zudov, L.N. Pfeiffer, K.W. West Recent experiments have shown that the differential magnetoresistivity of a high mobility two-dimensional electron system (2DES) is strongly suppressed under applied dc bias. This phenomenon is most pronounced when the Landau level width becomes smaller than the cyclotron energy. Using the quantum kinetics approach we calculate the characteristic current responsible for the suppression and compare the results to the experimental data obtained on a high mobility 2DES at low temperatures. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W12.00005: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W12.00006: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W12.00007: Terahertz Spectroscopy of Semiconductor Materials and Nanostructures William Baughman, Shawn David Wilbert, Lee Butler, Nick Harris, Gang Shen, Nabil Dawahre, Joseph Brewer, Patrick Kung, Seongsin Margaret Kim Terahertz (THz) time-domain spectroscopy is an attractive method to obtain the electronic transport properties in a variety of semiconductor materials and nanostructures. Unlike traditional techniques, THz spectroscopy does not require the realization of electrical contacts or even direct contact to the material probed. Here, we report the use of THz time-domain spectroscopy to determine the dielectric constant of a variety of semiconductor materials in the THz spectral range, and extract the refractive index, absorption coefficient and electrical conductivity. We also present a comparison of the results obtained from other techniques, including four-point probe resistivity measurements. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W12.00008: Statistics of excitations in the electron glass model Matteo Palassini We study the statistics of elementary excitations in the classical electron glass model of localized electrons interacting via the unscreened Coulomb interaction in the presence of disorder. We reconsider the long-standing puzzle of the exponential suppression of the single-particle density of states near the Fermi level, by measuring accurately the density of states of charged and electron-hole pair excitations via finite temperature Monte Carlo simulation and zero-temperature relaxation. We also investigate the statistics of large charge rearrangements after a perturbation of the system, which may shed some light on the slow relaxation and glassy phenomena recently observed in a variety of Anderson insulators. In collaboration with Martin Goethe. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W12.00009: Kondo Effect in a mesoscopic system Seungjoo Nah, Michael Pustilnik At low temperatures, transport and thermodynamic properties of Coulomb blockade systems are characterized by the energy scale $T_K$ (the Kondo temperature). We show that the Kondo temperature is subject to strong mesoscopic fluctuations. In a quantum dot system with many single-particle energy levels, the Kondo temperature acquires a log-normal distribution. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W12.00010: Two-Dimensional Electrostatic Lattices for Excitons Mikas Remeika, Leonid Butov We report on a method for the realization of two-dimensional electrostatic lattices for excitons in quantum well structures. The lattice structure is set by an electrode pattern and the amplitude of the lattice potential is controlled by applied voltages. We demonstrate square, hexagonal, and honeycomb lattices created by this method. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W12.00011: Non-Markovian effects in the quantum noise of interacting nanostructures Clive Emary, David Marcos, Ramon Aguado, Tobias Brandes We present a theory of finite-frequency noise in non-equilibrium conductors, and in particular, interacting nanostructures. We employ a quantum master equation approach and treat correlations between the system and the reservoirs in a nonMarkovian fashion. These correlations are pivotal in properly describing current fluctuations in situations where the measuring frequency is larger than both the applied voltage and the temperature. We explicitly show the importance of nonMarkovian effects in different contexts, including the finite-frequency current noise through a double quantum dot charge qubit and the short-time counting statistics of quantum dots. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W12.00012: Nonlinear Insulator in Complex Oxides Zhiqi Liu, T. Venkatesan, A. Ariando For complex oxides, the very concept of an insulator must be re-examined as they behave differently from conventional insulators such as SiO$_{2}$ due to the presence of multiple defect levels within bandgap. As the semiconductor industry is moving to such oxides for high-$k$ materials, we need to truly understand the insulating properties of them under various electrical excitations. We report a class of material which we coin as nonlinear insulators that exhibit reversible electric-field-induced metal-insulator transitions (MIT). We show this behaviour for an insulating LaAlO$_{3 }$thin film with a large bandgap of $\sim $5.6 eV in a metal/LaAlO$_{3}$/Nb-SrTiO$_{3}$ heterostructure. The reversible MIT is attributed to the formation of a quasi-conduction band (QCB) in the defect states of LaAlO$_{3}$ that forms a continuum state with the conduction band of the Nb-SrTiO$_{3}$. An opposing voltage is required to deplete the charges from the QCB. The implications of these nonlinear insulators are far-reaching. For example, the use of multi-component oxides as insulators in devices (e.g., high-$k$ dielectrics in silicon CMOS devices) must be exercised with caution. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W12.00013: Tailoring the Crystal Structure of Individual Silicon Nanowires by Polarized Laser Annealing Chia-Chi Chang, Haitian Chen, Chun-Chung Chen, Chongwu Zhou, Stephen Cronin We study the effect of polarized laser annealing on the crystalline structure of individual amorphous and nano-crystalline silicon nanowires (Si NWs) using Raman spectroscopy. The crystalline fraction of annealed NWs increases dramatically from 0 to 0.93 with increasing incident laser power. We observe Raman line shape narrowing and frequency hardening upon laser annealing due to the increase in crystal grain size. The Raman anti-Stokes:Stokes intensity ratio is used to determine the local heating temperature caused by the intense focused laser spot, which shows a strong polarization dependence on both single crystal bulk Si and nano-crystalline Si NWs. This method provides a new approach to control the crystal structure rather than by simply adjusting the laser power. Furthermore, strain induced linewidth broadening and frequency softening was also observed in bent nano-crystalline Si NWs, and the deformation stress can be released via laser annealing. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W12.00014: A novel method for measuring electrical conductance in thin solid films that is insensitive to contact effects Tamar Mentzel, Moungi Bawendi, Marc Kastner The synthesis of novel materials has been a primary driver in the emerging fields of organic-based electronics and nanoelectronics. One major obstacle to the research and development of novel materials is the ability to electrically characterize the material without introducing a significant contact resistance or damaging the material. In organic materials, the contact material has been found to alter the morphology of and to penetrate into the organic molecules, to form Schottky barriers, and to be thermally and mechanically unstable. Throughout nanoelectronics, unstable contacts and large contact resistances arise because of the reduced contact area as devices shrink in size. I will present a novel method for measuring electrical conductance in thin solid films that is insensitive to contact effects. In place of standard current measurements, a nanoscale metal-oxide-semiconductor field-effect transistor (MOSFET) is used to sense charge diffusion in a thin film of amorphous germanium. The contact resistance between the amorphous germanium and a pair of gold electrodes can be modulated \textit{in situ} without affecting the conductance measurement. Moreover, our technique enables the measurement of conductance as low as 10$^{-19}$ S with application of only 1 V to the film. This method can be used to electrically characterize any thin film which is sensitive to contact effects or where the resistance is too high to measure with conventional methods. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W12.00015: Spectrometry of electron pumping by surface acoustic waves M.J. Benesh, M. Kataoka, C.J.B. Ford, C.H.W. Barnes, J.P. Griffiths, G.A.C. Jones, I. Farrer, D.A. Ritchie Surface acoustic waves (SAWs) generate an electrostatic potential wave when applied to a GaAs/AlGaAs heterostructure. Electrons may be captured in a SAW minimum, creating a dynamic quantum dot (QD). SAW-defined QDs may be useful for certain quantum computing schemes, since, for example, they provide reliable single-electron transport and reduce the need for fast gate switching. Surface gates above a 2D electron gas (2DEG) are used to define a quasi-1D channel (Q1DC) at a potential far above the Fermi level. A SAW pulse captures electrons from the 2DEG and pumps a number of them controllably through the Q1DC. As a SAW minimum rises up the potential slope at the channel entrance, the QD is squeezed and some electrons are ejected back into the 2DEG with energies above the Fermi level. In our experiment, we probe the range of energies at which the electrons are emitted using a narrow potential barrier as an energy spectrometer. We can also measure electrons that have been pumped through the channel. We compare these results with a model for the SAW capture/pumping process. [Preview Abstract] |
Session W13: Applications of Statistical and Nonlinear Physics in the Life Sciences
Sponsoring Units: GSNP DBPChair: Uwe Tauber, Virginia Polytechnic Institute and State University
Room: D225/226
Thursday, March 24, 2011 11:15AM - 11:27AM |
W13.00001: Mean-field theory of four species in cyclic competition C.H. Durney, S.O. Case, M. Pleimling, R.K.P. Zia We consider a simple model of cyclic competition of $M$ species: When a pair of individuals from species $k$ and $k+1$ interact, the latter transforms into the former. Even with no spatial structure, such systems often display interesting and counterintuitive behavior. With possible applications in both biological systems (e.g., Min proteins, E. Coli, lizards) and game theory (e.g., rock-paper-scissors), the $M=3$ case has attracted considerable recent attention. We study a $M=4$ system (with no spatial structure) and find major differences, e.g., (1) the presence of macroscopically many absorbing states, (2) coexistence of species, and (3) violation of the ``law'' of survival of the weakest - a central theme in the $M=3$ case. Like the game of Bridge, the system typically ends with ``partner pairs.'' After describing the full stochastic model and its master equation, we present the mean-field approximation. Several exact, analytic predictions will be shown. Their limitations and implications for the stochastic system will also be discussed. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W13.00002: Stochastic evolution of four species in cyclic competition: exact and simulation results S.O. Case, C.H. Durney, M. Pleimling, R.K.P. Zia We study a stochastic system with $N$ individuals, consisting of four species competing cyclically: $A+B \longrightarrow A+A$, $\cdots$, $D+A \longrightarrow D+D$. Randomly choosing a pair and letting them react, $N$ is conserved but the fractions of each species evolve non-trivially. At late times, the system ends in a static, absorbing state $-$ typically, coexisting species $AC$ or $BD$. The master equation is shown and solved exactly for $N=4$, providing a little insight into the problem. For large $N$, we rely on simulations by Monte Carlo techniques (with a faster dynamics where a reaction occurs at every step). Generally, the results are in good agreement with predictions from mean field theory, after appropriate rescaling of Monte Carlo time. The theory fails, however, to describe extinction or predict their probabilities. Nevertheless, it can hint at many remarkable behavior associated with extinction, which we discover when studying systems with extremely disparate rates. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W13.00003: Random inheritance in a stochastic Lotka-Volterra model Ulrich Dobramysl, Gabriel Martinez, Uwe C. T\"auber We introduce a stochastic two-species Lotka-Volterra predator-prey model that includes random inheritance features. Specifically, each individual particle takes on a predation rate value which is determined when the particle is created and is dependent on the particle's parent. Thus we arrive at a simple model for evolution due to selection pressure. We employ Monte Carlo simulations to study the time evolution of the predation rate distribution as a function of the prescribed variability. We find that this model yields a steady state with optimized rates for both predator and prey species. Contrary to, e.g., gene expression models, the rates do not experience fixation at extreme values. An approximate description of the resulting data is achieved by means of an effective master equation approach for the predation rate distribution. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W13.00004: Time-dependent mechanical response of a network model for the cytoskeleton Nasrin Afzal, Michel Pleimling Motivated by a series of experiments that study the response of the cytoskeleton in living cells to time-dependent mechanical forces, we investigate through Monte Carlo simulations a three-dimensional network subjected to time-dependent perturbations. After having prepared the system in a relaxed state, time-dependent shear or stress is applied and the response is monitored. We discuss the possible implications of our results for the time-dependent mechanical response of the cytoskeleton. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W13.00005: Hopf Bifurcations in a Model for Circadian Rhythms in Arabidopsis Thaliana Orrin Shindell, Randall Tagg Arabidopsis thaliana is a plant used for many fundamental studies, including circadian rhythms. Numerically integrating the 7-equation kinetic model of Locke et al. [J. Theor. Bio. 234 (2005) 383], we have mapped regions of parameter space where circadian expression of key mRNA and proteins undergoes limit cycle oscillation. We seek to relate this to the work of Fukuda et al. [Phys. Rev. Lett. 99 (2007) 098102], where a coupled system of cells individually described by Stuart-Landau equations is used phenomenologically to describe experimentally observed spatio-temporal patterns in the plant leaves. To that end we have done a weakly nonlinear analysis of the system of kinetic equations. We also comment on possible experimental directions to further connect the kinetic models to dynamics in this multi-cellular system. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W13.00006: Translation with secondary structure: Dynamic blockages in totally asymmetric simple exclusion process Leah Shaw The totally asymmetric simple exclusion process (TASEP) is often used as a model for protein synthesis, with the lattice and particles representing the mRNA and ribosomes, respectively. Here we model the effect of secondary structure (folding) of the mRNA by introducing a dynamic blockage region in the lattice. If the region is unoccupied by particles, the blockage can close and prevent upstream particles from moving into it, representing the folding of that section of mRNA. Reopening of the blockage, allowing particles to pass, represents unfolding. We study the effects of the blockage size, closing/opening probabilities, and TASEP parameters on the particle current and blockage switching rates. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W13.00007: Rescue Interventions in Biological and Physical Networks Sean Cornelius Gene knockout experiments on single cells have established that expression of most genes is not needed for optimal growth. Yet, environmental and genetic perturbations to these organisms are known to be accompanied by the transient activation of a large number of latent metabolic pathways, suggesting that the temporarily activated reactions increase growth in the presence of perturbations. We have tested this hypothesis computationally and found, surprisingly, that the availability of latent pathways tends in fact to inhibit growth after genetic perturbations. This adverse effect indicates that latent pathway activation is derivative of a suboptimal response and that consequently, growth can actually be improved by removing these pathways from the network. In this talk, I will relate this counterintuitive effect to very recent research showing that a loss in network performance inflicted by an external perturbation can be mitigated by the application of additional perturbations. The challenge is to identify such ``rescues'' under constraints that limit the type of perturbations that can be made. I will present an approach to identify such eligible rescues for general networks modeled as dynamical systems, and present computational examples for biological and physical networks. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W13.00008: Feedback control for stabilizing chaotic spiral waves during cardiac ventricular fibrillation Ilija Uzelac, John Wikswo, Richard Gray The cardiac arrhythmias that lead to ventricular fibrillation (VF) arise from electrical spiral waves (SW) rotating within the heart with a characteristic period $\tau$. A single drifting SW can degenerate into a chaotic system of multiple SWs and VF. Hence early SW detection and termination is crucial to prevent VF. Time-delayed feedback control (TDFC) is well known approach for stabilizing unstable periodic orbits embedded in chaotic attractors. We hypothesize that cardiac SWs can be stabilized by TDFC with a time-delay of $\tau$. Implementing this approach will require precise, closed-loop control of the charge delivered to the heart during the defibrillation process. To do this, we have developed a 2 kW arbitrary-waveform voltage-to-current converter (V2CC) with a 1 kHz bandwidth that can deliver up to 5 A at 400 V for 500 ms, and a photodiode system for recording in real time an optical electrocardiogram, OECG(t). The feedback signal driving the V2CC will be the time-difference (OECG(t) - OECG(t-T), where we hypothesize that T is $\tau$, the period of the SW. This may dramatically decrease defibrillation voltages by using a defibrillation waveform customized to the VF event, unlike commercial capacitor defibrillators. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W13.00009: Experimental and theoretical evidence for fluctuation driven activations in an excitable chemical system Harold Hastings, Sabrina Sobel, Richard Field, Scott Minchenberg, Nicole Spinelli, Keith Zauderer An excitable medium is a system in which small perturbations die out, but sufficiently large perturbations generate large ``excitations.'' Biological examples include neurons and the heart; the latter supports waves of excitation normally generated by the sinus node, but occasionally generated by other mechanisms. The ferroin-catalyzed Belousov-Zhabotinsky reaction is the prototype chemical excitable medium. We present experimental and theoretical evidence for that random fluctuations can generate excitations in the Belousov-Zhabothinsky reaction. Although the heart is significantly different, there are some scaling analogies. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W13.00010: The statistical physics of decision-making in insect colonies Patrick M. Hogan, Thomas Schlegel, Nigel R. Franks, James A.R. Marshall We apply the stochastic methods of statistical physics to analyse collective-decision making in social insect colonies, allowing us to derive the colony-level behaviour from an individual-level model. This contrasts with the traditional approach where a differential equation model, with or without arbitrary noise terms, is assumed. Social insect colonies vary in size from on the order 100 to 10,000,000 individuals, and such a statistical physics approach allows us explicitly to derive equations for both the average behaviour and the noise in the system, across this entire scale. We develop such a framework by building upon an existing stochastic model of opinion formation to model the decision-making processes in emigrating ant colonies. This new model is both driven by and evaluated against results from experiments with rock ants. This allows us to elucidate rigorously the role played by the individual-level phenomena of direct switching in the colony-level decision-making process, which optimality theory has predicted to be of crucial importance, and which we compare with our experimental results. This illustrates the power of the stochastic methods of statistical physics for understanding social insect colonies as complex systems. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W13.00011: Thermodynamic efficiency out of equilibrium David Sivak, Gavin Crooks Molecular-scale machines typically operate far from thermodynamic equilibrium, limiting the applicability of equilibrium statistical mechanics to understand their efficiency. Thermodynamic length analysis relates a non-equilibrium property (dissipation) to equilibrium properties (equilibrium fluctuations and their relaxation time). Herein we demonstrate that the thermodynamic length framework follows directly from the assumptions of linear response theory. Uniting these two frameworks provides thermodynamic length analysis a firmer statistical mechanical grounding, and equips linear response theory with a metric structure to facilitate the prediction and discovery of optimal (minimum dissipation) paths in complicated free energy landscapes. To explore the applicability of this theoretical framework, we examine its accuracy for simple bistable systems, parametrized to model single-molecule force-extension experiments. Through analytic derivation of the equilibrium fluctuations and numerical calculation of the dissipation and relaxation time, we verify that thermodynamic length analysis (though derived in a near-equilibrium limit) provides a strikingly good approximation even far from equilibrium, and thus provides a useful framework for understanding molecular motor efficiency. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W13.00012: A simple model for the transmission of malaria Adriana Dickman We study a simple lattice model describing the transmission of malaria. The transmission of the disease to humans occurs through contact with an infected mosquito, while a healthy mosquito can become infected through contact with an infected human. Recovered individuals are susceptible to re-infection. The mosquitoes diffuse through the lattice, spreading the disease. We show preliminary results for the model obtained via site approximation (mean-field theory). [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W13.00013: Genearlized force-extension relation for DNA confined in sub-100nm nanoslits Yeng-Long Chen, Po-Keng Lin, Chia-Fu Chou We generalize the force-extension relation of DNA molecules confined in persistence length scale nanoslits. In strong confinement with slit geometry, the segmental correlation length of DNA molecules have two components -- in the confined and unconfined dimensions. In the confined dimension, the segmental correlation length is controlled by the slit height. In the unconfined dimension, the segmental correlation length increases as the slit height decreases. We characterize this effect, and generalize how this affects the entropic elasticity of confined DNA molecules. In addition, we investigate the structure of dense strongly confined semi-flexible polymers. [Preview Abstract] |
Session W14: Focus Session: Extreme Mechanics: Elasticity and Deformation III
Sponsoring Units: GSNPChair: Pedro Reis, Massachusetts Institute of Technology
Room: D227
Thursday, March 24, 2011 11:15AM - 11:27AM |
W14.00001: A general theory of mechanical instabilities in soft solids Evan Hohlfeld, L. Mahadevan Some instabilities in soft solids, e.g. buckling and wrinkling, can be detected in linearized analysis. Surprisingly, linearly stable configurations can still have nonlinear instabilities with strictly zero energy barrier. Two examples are cavitation (formation of voids) and sulcification (formation of sharply creased free surface folds), wherein singularities nucleate and grow when a critical strain is achieved. Here we present the first general theory of stability in nonlinearly elastic materials. The theory predicts when singularities spontaneously form, irrespective of linearized analysis, and how these can be controlled with geometry. Such ``hidden'' instabilities arise from the scale-free geometric and constitutive nonlinearities common in soft materials, and can be understood as scale symmetry breaking processes in simple cases. More deeply, even buckling and wrinkling can be traced back to scale-free linear instabilities (loss of ellipticity at an interface) as was first explained by M. A. Biot. We illustrate the theory with simulations and experiments on sulcification. Time allowing we will also discuss fracture and delamination. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W14.00002: Elastocapillary imbibition Camille Duprat, Jeffrey Aristoff, Howard Stone The deformation of elastic structures under capillary forces (elastocapillarity), and their interaction with fluid flow (elastohyrodynamics), are relevant to many biological, geophysical and engineering processes. Here, we present the dynamics of surface-tension-driven flow into a gap between flexible boundaries (i.e. elastocapillary imbibition). We examine two model systems of elastocapillary imbibition, with and without gravitational effects, using a combination of experiment, theory, and numerical simulation. We identify the characteristic length and time scales, and demonstrate how the prescence of flexible boundaries leads to a departure from classical imbibition. The time to reach equilibrium (if one exists) is determined, and a criterion for the coalescence of the boundaries is established. Good agreement between experiment and theory is obtained. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W14.00003: Buckling of swelling gels under constraints Howon Lee, Jiaping Zhang, Yonghao An, Hanqing Jiang, Nicholas Fang Buckling is a traditional topic in mechanics and has been thought to be well studied for the last hundred years. Recently, buckling has drawn new attention in a different perspective; a novel scheme for pattern transformation. Here we present an experimental study on buckling using swelling of gels under constraints. Under critical conditions combined with proper mechanical constraints, non-homogenous stress develops as gel swells, which gives rise to buckling instability. We developed a fabrication technique to make a 3D cylinder-shaped microgel, the bottom end of which is tightly fixed on a rigid substrate to impose constraints. Equilibrium swelling study of such gel structure allowed us to determine a critical geometrical condition for buckling. Furthermore, exploiting slow gel swelling process, we recorded time evolution of buckling as gel swells to study post-buckling morphologies. Numerical simulation also showed close relationship between geometric parameters and resulting buckling pattern. We believe our study on buckling of swelling gels will not only help us better understand the mechanics of soft materials, but it will also contribute to increasing the breadth of possible application of soft materials in many emerging fields such as photonic crystals. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W14.00004: Optimal control of growing sheets Gareth Jones, L. Mahadevan There has been much recent interest in plates and sheets that have the ability to actively swell, grow and bend. In this presentation an inhomogeneously growing plate is modeled by prescribing the in-plane growth strain and the active change-of-curvature function. The plate will then change shape to accommodate the induced strains. For applications of this phenomenon, an important problem is how best to choose these functions in order for the plate to deform to a given target shape. In seeking an answer to this question, we have developed a computational approach, where the growth strains will be found as solutions to a numerical optimization procedure. Example results will be presented which will provide some insight into the mechanical behavior of growing thin structures. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W14.00005: Decoupling thermal, chemical, and mechanical strain components in thin films Meredith Silberstein, Ethan Crumlin, Yang Shao-Horn, Mary Boyce Many electrochemical systems have performance which is affected by internal strains due to thermal and/or chemical stimuli. The bi-material curvature method is a means to quantify these thermal and chemical strains and their coupling with mechanical stress. In this method, a thin layer of the material of interest is deposited on a substrate of intermediate thickness. The composite assumes a curvature that depends on the mismatch strains between the substrate and film. The Stoney formula provides an explicit expression for the film stress as a function of the elastic substrate properties, film and substrate thickness, and curvature. Here we study two distinct materials systems: Nafion used as the polymer electrolyte in low temperature fuel cells, and epitaxial perovskite thin films used as a catalyst for the oxygen reduction reaction in solid oxide fuel cells. The thermal, chemical, and mechanical strains are quantitatively determined as functions of temperature and atmospheric conditions by monitoring the curvature evolution with changes in these parameters. The extent of coupling of the thermal and chemical strains with mechanical stress is evaluated by conducting the experiment at multiple substrate thicknesses. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W14.00006: Plane deformations generating a prescribed finite rotation field Gregory Rizza, Janet Blume Compatibility conditions for various strain measures are well known in both small and finite strain kinematics. For many problems, such conditions enable boundary value problems to be formulated using strains, stresses, or a generating potential function, as the fundamental dependent variable(s). These methods are effective, as most strain fields fully determine the generating deformations up to an arbitrary rigid deformation. Our research is concerned with the compatibility issue for the rotation field. Although it is not a direct measure of the distortion in a deformation, the rotation associated with a deformation and its variation from point to point within a body turns out to carry quite a bit of information about the actual deformation. For the case of plane deformation, we have been able to show that any suitably smooth plane proper orthogonal tensor field may serve as a finite rotation tensor for a generating deformation. We have developed several examples demonstrating this relationship between material deformation and rotation fields. Our results demonstrate in the case of plane deformation, any skew-symmetric two-dimensional tensor field can serve as a plane rotation field. The relation between the position-dependence of a rotation field and generating deformation information has implications in both mechanical twinning and shear banding. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W14.00007: Mechanics of Suture Joints Yaning Li, Juha Song, Christine Ortiz, Mary Boyce Biological sutures are joints which connect two stiff skeletal or skeletal-like components. These joints possess a wavy geometry with a thin organic layer providing adhesion. Examples of biological sutures include mammalian skulls, the pelvic assembly of the armored fish Gasterosteus aculeatus (the three-spined stickleback), and the suture joints in the shell of the red-eared slider turtle. Biological sutures allow for movement and compliance, control stress concentrations, transmit loads, reduce fatigue stress and absorb energy. In this investigation, the mechanics of the role of suture geometry in providing a naturally optimized joint is explored. In particular, analytical and numerical micromechanical models of the suture joint are constructed. The anisotropic mechanical stiffness and strength are studied as a function of suture wavelength, amplitude and the material properties of the skeletal and organic components, revealing key insights into the optimized nature of these ubiquitous natural joints. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W14.00008: Mechanical and thermal stability of adhesive membranes with nonzero bending rigidity Tuomas Tallinen, Jan Astrom, Pekka Kekalainen, Jussi Timonen Membranes at a microscopic scale are affected by thermal fluctuations and self-adhesion due to Van der Waals forces. Methods to prepare membranes of even molecular scale, e.g. graphene, have been recently developed, and the question of their mechanical and thermal stability is of crucial importance. To this end we modeled microscopic membranes with a short-range attractive interaction and applied Langevin dynamics. Their behavior was also analyzed under external loading. Even though these membranes folded during isotropic compression as a result of energy minimization, the process at high confinement did not differ much from crumpling of macroscopic thin sheets. The main difference appeared when the external load was released. In such cases, for membranes of sufficiently large size $L$, folded or scrolled conformations emerged. At high enough temperature $T$ entropic effects made such conformations unfavorable, however. Possible conformations of free-standing membranes (``phase diagrams'') were determined in the $TL$-plane. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W14.00009: Universal Shapes formed by Interacting Cracks Melissa Fender, Frederic Lechenault, Karen Daniels Brittle failure through multiple cracks occurs in a wide variety of contexts, from microscopic failures in dental enamel and cleaved silicon to geological faults and planetary ice crusts. In each of these situations, with complicated curvature and stress geometries, pairwise interactions between approaching cracks nonetheless produce characteristically curved fracture paths known in the geologic literature as en passant cracks. While the fragmentation of solids via many interacting cracks has seen wide investigation, less attention has been paid to the details of individual crack-crack interactions. We investigate the origins of this widely observed crack pattern using a rectangular elastic plate which is notched on each long side and then subjected to quasistatic uniaxial strain from the short side. The two cracks propagate along approximately straight paths until the pass each other, after which they curve and release a lenticular fragment. We find that, for materials with diverse mechanical properties, the shape of this fragment has an aspect ratio of 2:1, with the length scale set by the initial cracks offset $s$ and the time scale set by the ratio of $s$ to the pulling velocity. The cracks have a universal square root shape, which we understand by using a simple geometric model and the crack-crack interaction. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W14.00010: Pericyte Actomyosin-Mediated Contraction at the Cell-Material Interface can Modulate the Microvascular Niche Adam Zeiger, Maciej Kotecki, John Maloney, Ira Herman, Krystyn Van Vliet Here we employ the experimental finding that pericytes can wrinkle a freestanding, underlying membrane via actin-mediated contraction. Pericytes were cultured on deformable silicone substrata. Local stiffness of subcellular domains was investigated by using AFM-enabled nanoindentation. Substratum contraction was quantified by normalized change in wrinkle contour lengths, and a model was used to relate local strain energies to pericyte contractile forces. The nature of pericyte-generated wrinkling and contractile protein-generated force transduction was further explored by the addition of pharmacological cytoskeletal inhibitors that affected contractile forces and the effective elastic moduli of pericyte domains. Actin-mediated forces are sufficient for pericytes to exert an average contraction of 38{\%} on the substrata employed in these in vitro studies. Pericyte generated contractile forces thus serve as a direct mechanical stimulus to adjacent vascular endothelial cells, potentially altering the effective mechanical stiffness of nonlinear-elastic extracellular matrices, to modulate pericyte-endothelial cell interactions that directly influences physiologic angiogenesis. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W14.00011: Extraordinary Elasticity of the Distorted Kagome Lattice Anton Souslov, Kai Sun, Xiaoming Mao, Tom Lubensky J. C. Maxwell discovered that a system of particles in $d$-dimensions will be marginally rigid, or \emph{isostatic}, if each particle interacts on average with $2 d$ of its neighbors. Isostatic models have been used to describe such diverse soft phenomena as the jamming transition and the elasticity in networks of semi-flexible polymer gels. We develop models based on the isostatic kagome lattice, which has a subextensive number of floppy phonon modes. We show that these can be extended into soft deformations by changing the particle configurations while keeping the bond lengths fixed. Thus, we create families of novel isostatic lattices, which exhibit highly tunable elastic properties as a consequence of isotropic linear elasticity with a zero bulk modulus. They have a negative Poisson ratio, or auxetic (anti-rubber) behavior. Further, we find no bulk soft phonons at large length scales due to conformal symmetry. We discuss the intimate relationship between various symmetries and soft response in these models as well as the relation of these models to other marginally rigid systems. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W14.00012: Suppression of Viscous Fingers in Miscible Hele-Shaw Flow Radha Ramachandran, Justin Burton, Sidney Nagel The flow of two immiscible fluids between closely-spaced parallel plates can be highly unstable and produce a series of complex fingering patterns when the less viscous injected fluid invades the more viscous one. Air displacing granular material in such a Hele-Shaw geometry shows similar patterns with sharp features consistent with the granular/air surface tension being virtually zero [1]. Here we investigate the flow of two $\textit{miscible}$ fluids in a radial Hele-Shaw cell, with an inner liquid displacing an outer one of higher viscosity. We use two glycerol- water mixtures so that the viscosity can be tuned by varying the glycerol concentration. We vary the plate spacing and flow rate as well as the fluid viscosites. The non-equilibrium interfacial tension between these two miscible fluids is expected to be nearly zero. However, extrapolating to zero surface tension in the linear theory for Hele-Shaw flow does not describe our results. Specifically, flow becomes \textit{stable} even when the inner liquid has a much lower viscosity than the outer one. At higher velocity, it is possible to see small amplitude fingering patterns develop. \\[4pt] [1] X. Cheng, L. Xu, A. Patterson, H. M. Jaeger and S. R. Nagel \textit{Nature Physics} 4, 234 (2008). [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W14.00013: Patterns on thin sheets: buckling, wrinkling, crumpling, folding Benny Davidovitch Recent experiments on thin sheets under various geometric confinements and distributions of exerted forces found a multitude of pattern types. I will discuss the possibility of classifying this diverse phenomenology by generalizing concepts of primary and secondary instabilities, and basic types of symmetry breaking. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W14.00014: Mechanics and chemical thermodynamics of a temperature-sensitive hydrogel Shengqiang Cai, Zhigang Suo A temperature-sensitive hydrogel is a network of polymers containing monomers, whose interaction with water molecules can be tuned dramatically by changing temperature. In most cases, the swelling ratio of a temperature-sensitive hydrogel changes discontinuously upon heating above or cooling below a critical temperature, which is called volume phase transition. Interestingly, the coexistence of swollen phases and shrunk phases are frequently observed in the experiments for temperature-sensitive hydrogels and additionally, people have also discovered that a uniaxial force can induce phase transition in a temperature-sensitive gel bar .In order to understand these phenomena, we studied the mechanics and chemical thermodynamics of a temperature-sensitive hydrogel bar, by using the free-energy landscape of a bar made from PNIPAM gel. Following Gibbs, we plot the phase diagram of a temperature-sensitive hydrogel bar under uniaxial force. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W14.00015: Mechanics of Curved Folds Marcelo A. Dias, Christian D. Santangelo Despite an almost two thousand year history, origami, the art of folding paper, remains a challenge both artistically and scientifically. Traditionally, origami is practiced by folding along straight creases. A whole new set of shapes can be explored, however, if, instead of straight creases, one folds along arbitrary curves. We present a mechanical model for curved fold origami in which the energy of a plastically-deformed crease is balanced by the bending energy of developable regions on either side of the crease. Though geometry requires that a sheet buckle when folded along a closed curve, its shape depends on the elasticity of the sheet. [Preview Abstract] |
Session W15: Focus Session: Spins in Semiconductors - Spin-Orbit Effects and Confinement
Sponsoring Units: DMP GMAG FIAPChair: Giovanni Vignale, University of Missouri
Room: D171
Thursday, March 24, 2011 11:15AM - 11:51AM |
W15.00001: Chern Number Spins of Mn Acceptor Magnets in GaAs Invited Speaker: State-of-the-art STM techniques have made it possible to substitute transition metal impurities for individual atoms in semiconductor crystals and have provided detailed information on the nature of the bound acceptor or donor states. Individual coupled acceptor (or donor)-impurity centers represent a new class of nanomagnets which we refer to as \textit{acceptor or donor magnets}. Here we determine the effective total spin $J $of local moments formed from acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern numbers. When individual Mn atoms are close to the sample surface, the total spin changes from $J=1$ to $J =2,$ due to quenching of the acceptor orbital moment. For Mn pairs in bulk, the total $J $depends on pair orientation in the GaAs lattice and on the separation between the Mn atoms. We point out that Berry curvature variation as a function of local moment orientation can profoundly influence the quantum-spin dynamics of these magnetic entities. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W15.00002: Effect of magnetic field on the local density of states of Mn acceptor magnets in GaAs M.R. K. Mahani, C.M. Canali, A.H. MacDonald Advances in atomic manipulation, real-space imagining and spectroscopic power of STM techniques have recently made it possible to investigate the local electronic properties of a few substitutional Mn impurities inserted in the GaAs surfaces [1]. Theoretical work [2] predicts that the local density of states in the vicinity of the Mn impurities should depend strongly on the direction of the Mn magnetic moment. In contrast, recent STM experiments [3] from several groups find a negligible dependence of the tunneling LDOS on the magnetic field direction for applied fields up to 7 T. Based on tight- binding calculations we interpret these findings by arguing that large LDOS signals require large angle moment rotations, and that the strength of the magnetic field used in present experiments is not strong enough to substantially modify the magnetic anisotropy landscape of Mn impurities near the GaAs surface.\\[4pt] [1] D. Kitchen et al., Nature, 442, 436 (2006); J. K. Garleff et al., Phys. Rev. B 82, 035303 (2010).\\[0pt] [2] T. O. Strandberg, C. M. Canali, and A. H. MacDonald, Phys. Rev. B 80, 024425 (2009). [3] P. M. Koenraad, Private Communication. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W15.00003: Effects of Anisotropy in Magnetic Quantum Dots Rafal Oszwaldowski, Igor Zutic, Andre Petukhov Magnetic ordering in semiconductor Quantum Dots (QDs) doped with Mn is mediated by the confined carriers (typically holes), which interact with Mn through exchange interaction. The ordering can be affected by the QD flat shape [1], and, by the resultant anisotropic g-factor of holes [2]. A reduction of the in-plane symmetry of the QD by an external potential may influence the magnetic alignment as well [3]. We study the magnetic ordering at different degrees of anisotropy. The ordering arises in absence of external magnetic field, e.g., through formation of magnetic polarons. A typical number of Mn in a QD is large, so we replace their spins by classical magnetic moments. We emphasize the limit of full isotropy (electrons) and extreme anisotropy (holes in a flat QD). Supported by DOE-BST, ONR, AFOSR, and NSF-ECCS CAREER. \\[4pt] [1] I. R. Sellers, R. Oszwaldowski, et al., Phys. Rev. B \textbf{82}, 195320 (2010)\\[0pt] [2] P. Dorozhkin, et al., Phys. Rev. B \textbf{68}, 195313 (2003).\\[0pt] [3] R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. \textbf{101}, 207202 (2008) [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W15.00004: Incorporation of Mn into Ge Quantum Dots: Growth Strategies to Control Structure and Magnetism Christopher Nolph, Petra Reinke Manganese doped, magnetic germanium quantum dots are important building blocks for the future of spintronic devices. Our goal is to understand and control how the manipulation of the Mn-environment within the Si(100)-Ge wetting layer-Ge QD systems influence the magnetic properties. We investigate several pathways for Mn-doping of Ge QDs which suppress detrimental germanide formation. The first pathway uses a surface-driven approach: Mn is deposited on the Ge QD surface, forms well-defined clusters on the QD and dissolve during annealing. The second pathway uses co-deposition of Ge and Mn (i) throughout the entire QD growth process, and (ii) only during the formation of the wetting layer. The highest concentration of Mn is about 20{\%}, and the Ge QD growth is only marginally perturbed, albeit germanides begin to form. All processes are observed with scanning tunneling microscopy, which yields morphological and electronic structure information of the reaction sequence. A comprehensive model of all processes will be presented. Preliminary magnetism results, obtained with a vibrating sample magnetometer, indicate a ferromagnetic material with a Curie temperature up to 100K. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W15.00005: Control and properties of magnetic nanostructures in nitride semiconductors Invited Speaker: We review [1] studies of MOVPE (Ga,Fe)N [2-7] and (Ga,Mn)N [8,9], combining magnetic [2-4,6,8,9], magnetooptical [3], and XANES [4,5,8] investigations with a comprehensive structural and chemical characterization by SIMS, TEM, EDS [2,4,6,8,9], synchrotron-XRD [4,6,8], EXAFS [5,8], and PEEM [7]. We show that the Fe ions aggregate into Fe$_{x}$N nanocrystals either by crystallographic or by chemical phase separation, controlled by the growth conditions and by co-doping. Depending on the degree of nitridation, these nanocrystals are either ferromagnetic or antiferromagnetic. \\[4pt] [1] A. Bonanni and T. Dietl, Chem. Soc. Rev. 39, 528 (2010);\\[0pt] [2] A. Bonanni et al., Phys. Rev. B 75, 125210 (2007);\\[0pt] [3] W. Pacuski et al., Phys. Rev. Lett. 100, 037204 (2008);\\[0pt] [4] A. Bonanni et al., Phys. Rev. Lett. 101, 135502 (2008);\\[0pt] [5] M. Rovezzi et al., Phys. Rev. B 79, 195209 (2009);\\[0pt] [6] A. Navarro-Quezada et al., Phys. Rev. B 81, 205206 (2010);\\[0pt] [7] I. Kowalik et al., arXiv:1011.0847;\\[0pt] [8] W. Stefanowicz et al., Phys. Rev. B 81, 235210 (2010);\\[0pt] [9] A. Bonanni et al. arXiv~: 1008.2083. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W15.00006: From simplicity to complexity: Can transcendental equation and transfer matrix enlighten us about the nature of Rashba physics? Chih-Piao Chuu, Roland Winkler, Qian Niu We present an analytic model of Rashba spin-splitting of conduction electrons in asymmetric quantum wells based on transcendental equation and transfer matrix approaches. The sources of asymmetries of quantum wells, such as interface discontinuity, conduction and valence band profiles, external field or presence of a potential gradient, related to Rashba spin-splitting will be discussed. We will use type I and type II semiconductor heterostructure quantum wells for demonstration. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W15.00007: Signatures of the crystal symmetry after Dyakonov-Perel spin relaxation of photoexcited hot electrons in semiconductor heterostructures Lan Qing, Hanan Dery We reveal unique manifestations of the intimate relation between the crystal structure of zincblende semiconductors and their spin-orbit coupling. We show that reflection of photoexcited hot electrons is capable of tipping the direction of the optically injected net spin vector away from the propagation axis of the exciting circularly polarized beam. The effect is robust even in case of complete electron reflection from a non magnetic target (e.g., GaAs/AlAs). The tipping angle of the net spin vector after spin relaxation is determined by the effective Dyakonov-Perel magnetic field as well as by the momentum alignment and spin-momentum correlation of the initial photoexcited electron population. All of these crystal-structure dependent mechanisms contribute due to the reflection plane induced symmetry breaking. We perform Monte Carlo simulations to calculate the tipping angle and also provide qualitative derivations of the effect. Results are shown for non-magnetic semiconductor heterostructures and for hybrid semiconductor/ferromagnet systems. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W15.00008: The spin dynamics in the strong spin-orbit coupling regime-A collective Rabi oscillation Xin Liu, Xiong-Jun Liu, Jairo Sinova We study the spin evolution in a high-mobility two dimensional electron gas (2DEG) with generic spin-orbit interactions(SOI). A fully understanding of the D'yakonov-Perel's(DP) mechanism is presented by using the microscopic linear response theory from the diffusive to the ballistic regime. We derive a set of spin dynamical equations which capture the characters of the purely exponential and damped oscillatory spin evolution modes in different spin-orbit coupling(SOC) regimes. It is shown that the oscillatory spin dynamics appear when the electron life time is larger than the half of the spin precession time due to the SOI. The Rabi oscillation between two spin bands is the physical origination of the damped oscillatory modes. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W15.00009: Rasbha Spin-Orbit Interaction in Digital Alloys Joseph Pingenot, Kieran Mullen The Rashba spin-orbit interaction couples the electron spatial wavefunction to its spin through breaking the inversion symmetry of a structure. From work by Lange[1], the Rashba spin-orbit effect can be divided into a structural component, originating primarily in the valence band offsets within the nanostructure, and an electric field component, originating primarily in the internal self-consistent electric field and in an applied electric field. A common growth technique, digital alloying, breaks the well into discrete steps and then varies the material composition for each step by constructing a well within the step such that the average material across the step corresponds to the desired percentage, e.g. a Ga$_{0.5}$In$_{0.5}$As well would be pure GaAs through half the step, followed by pure InAs for the rest of the step. With digital alloying, the electron wavefunction is approximately the same as is obtained by using a real alloy. The Rashba spin-orbit couplings, however, are considerably smaller for the digital alloy than the real alloy. This comes about because the digital alloy has a series of positive and negative interfaces at each step, whereas the real alloy has a series of identical steps. We calculate the Rashba coefficient for a variety of realistic well geometries, for both digitial and continuous alloying, and discuss how the consequences for experiment. [1] Jens Lange, {\it Quantentransport in Halbleiter-Heterostrukturen} (1996). [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W15.00010: Superconductivity in the repulsive Rashba model Luyang Wang, Oskar Vafek We study the superconducting instability of a two dimensional Rashba spin-orbit coupled system with a weak repulsive interaction by a two step renormalization group (RG) method introduced by Raghu et.al. (PRB 81, 224505 (2010)). We present the superconducting transition temperature Tc in terms of the correlation functions of the non-interacting system. The RG flows in the Cooper channel break down below some scale, which we identify with Tc and verify that the Tc is independent of the intermediate cutoff. Finally, we present results of Tc as a function of spin-orbit coupling strength. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W15.00011: Kohn-Luttinger superconductivity of two-dimensional electrons in the presence of Rashba spin-orbit coupling Ali Ashrafi, Dmitrii Maslov We consider a two-dimensional(2D) system of fermions with weak short-range repulsive interac- tion in the presence of Rashba Spin-Orbit coupling(SOC). We show that although Kohn-Luttinger instability in 2D in the absence of SOC occurs only to third order in the interaction, it occurs to second order in the presence of SOC. The critical temperature of the p-wave transition is calculated. [Preview Abstract] |
Session W16: Focus Session: Bulk Properties of Complex Oxides - Other
Sponsoring Units: DMP GMAGChair: Rongying Jin, Louisiana State University
Room: D173
Thursday, March 24, 2011 11:15AM - 11:27AM |
W16.00001: Magnetic Force Microcopy of the Magnetostructural Phases of Mn3O4 Xu Wang, Minjung Kim, Raffi Budakian, S. L. Cooper In this talk, we report temperature- and field- dependent magnetic force microscopy (MFM) studies of Mn3O4. The spinel Mn3O4 has novel phase structure at low temperatures due to a three-way competition between spin-orbital coupling, geometric frustration and external magnetic field. This competition could lead to complex magnetic pattern formations. A particularly interesting phase exists where the crystal lattice undergoes a transition to resolve the spin frustration due to internal and external magnetic fields. In this phase the spins were expected to be in a disordered state, but local magnetic ordering has not been explored in previous studies. We present results from our investigation of Mn3O4 phase space using a variable temeprature MFM. Measurements were made at temperatures between 4K and 75K and in magnetic fields up to 5 tesla. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W16.00002: Magnetic excitations in the geometric frustrated multiferroic CuCrO$_2$ Matthias Frontzek, Georg Ehlers, Andrey Podlesnyak, Masaaki Matsuda, Andrew Christianson, Randy Fishman, Jason Haraldsen, Sergei Barilo The delafossite CuCrO$_2$, crystallizing in the rhombohedral $R\overline{3}M$ space group, is an interesting case of a multiferroic compound due to its apparent strong coupling of spin and charge. In contrast to other multiferroic compounds CuCrO$_2$ shows a spontaneous electric polarization upon antiferromagnetic ordering at \emph{T}$_{\textrm{N}}\approx$~24~K without an accompanying structural phase transition. Further, CuCrO$_2$ is a rare example where the magnetoelectric properties are tunable by both an electric and a magnetic field. In our contribution we present inelastic neutron scattering experiments on CuCrO$_2$ single crystals. The measured magnetic excitation spectra have been modeled by Monte-Carlo spin wave calculations and allowed the determination of the relevant exchange interaction and anisotropy terms. We will present evidence for a weak ferromagnetic Cr-Cr-interlayer exchange interaction and show that this interaction is relevant for the multiferroic properties of CuCrO$_2$. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W16.00003: Charge dynamics in frustrated charge ordered system with strong electron correlation Makoto Naka, Sumio Ishihara Electronic ferroelectricity is known as phenomena where the electric polarization is caused by the electronic charge order without inversion symmetry. This is seen in some transition metal oxides, e.g. LuFe$_{2}$O$_{4}$, and charge transfer organic salts. It is suggested from the theoretical works [1,2] that large charge fluctuation and frustration are responsible for the electric polarization. This charge fluctuation is expected to govern dynamical properties and external field effects. Actually, the relaxer like large dielectric fluctuation in some electronic ferroelectricity is observed. Motivated by these experimental and theoretical results, we study charge dynamics in charge ordered fermion system on the layered triangular lattice. We adopt the V-t model where the inter-site electron transfers and the inter-site Coulomb interactions are taken into account. We analyze this model by utilizing the exact diagonalization method and focus on effects of frustration in the charge dynamics. In the 3-fold charge ordered phase associated with the electric polarization, the optical conductivity shows multiple-peak structure in a wide energy range. In the non-polar 2-fold charge ordered phase, a precursor of the 3fold charge order appears in the low energy charge fluctuation around the phase boundary. [1] A. Nagano et al. Phys. Rev. Lett. \textbf{99} 217202. [2] M. Naka et al. Phys. Rev. B. \textbf{77} 224441. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W16.00004: Photo induced spin-state change in itinerant correlated electron systems Sumio Ishihara, Yu Kanamori, Hiroaki Matsueda Recently developed ultrafast optical techniques open up a new frontier for research of the phase transition. This is the so-called photo-induced phase transition (PIPT). The spin-state transition between different magnitudes of the spin-angular moment is one of the targets. In particular, the photo-induced spin-state transition is seen in the cobalt oxides with a perovskite structure. The ultrafast optical measurements in the low-temperature low-spin insulators show transient metallic spectra which are completely different from the spectra in the high-temperature high-spin phase. We study theoretically the photo-induced spin-state change in itinerant correlated electron systems. We derive the model Hamiltonians where numbers of photo-excited electron-hole pair are fixed. A bound state of the photo-doped hole and the high-spin state are created inside of the low-spin sites. This bound state brings about a characteristic peak structure in the optical pump-probe spectra which are completely different from the spectra in thermally excited states. The present theory provides a possible scenario in recently observed photo-induced hidden state in perovskite cobaltites. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W16.00005: Rapidly fluctuating orbital occupancy above the orbital ordering transition in spin-gap compounds Francisco Rivadulla, Beatriz Rivas-Murias, Haidong Zhou, Jose Rivas Low-dimensionality spin systems develop a spin-dimer phase within a molecular orbital that competes with long-range antiferromagnetism below TS. Very often, preferential orbital occupancy and ordering are the actual driving force for dimerization, as in the orbitally-driven spin-Peierls (MgTi2O4, CuIr2S4, La4Ru2O10, NaTiSi2O6, etc.). Through a microscopic analysis of the thermal conductivity in La4Ru2O10, we show that the orbital occupancy fluctuates rapidly above TS, resulting in an orbital-liquid state. Strong orbital-lattice coupling introduces dynamic bond-length fluctuations that scatter the phonons to produce a glass-like thermal conductivity above TS. This phonon-glass to phonon-crystal transition occurs in other spin-dimer systems, like NaTiSi2O6, pointing to a general phenomenon. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W16.00006: Influence of particle size on the magnetic and magnetocaloric properties of nanocrystalline La$_{2/8}$Pr$_{3/8}$Ca$_{3/8}$MnO$_{3}$ P. Lampen, N.S. Bingham, M.H. Phan, H. Srikanth, T.H. Hoang, H.D. Chinh Bulk manganites La$_{5/8-y}$Pr$_{y}$Ca$_{3/8}$MnO$_{3 }$(LPCMO) exhibit a complex phase diagram due to coexisting and competing charge-ordered (CO) and ferromagnetic (FM) phases. Of particular interest is the CO phase that is unstable under various perturbations, such as carrier doping, strain, magnetic and electric field. We report systematic studies of the influence of particle size on the magnetic and magnetocaloric properties of nanocrystalline LPCMO (y=3/8) synthesized by sol-gel method. The nanocrystalline samples with mean particle sizes of 30 nm, 150 nm, and 250 nm were structurally characterized by XRD, SEM, and TEM. Magnetic and magnetocaloric measurements were conducted using a Quantum Design PPMS. We find that the 150 nm and 250 nm samples exhibit features similar to their bulk counterpart. However, the case is very different for the 30 nm sample where only a paramagnetic to ferromagnetic transition occurs. Size reduction has been found to suppress the CO phase, decrease the magnetization, and strongly modify the magnetocaloric effect in LPCMO. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W16.00007: Studies on manganese substituted cobalt ferrite prepared by autocombustion route Y. Kolekar, R. Kambale, R. Gupta, P. Kahol, K. Ghosh Compositions of Co1.2-xMnxFe1.8O4 (0 = x = 0.4) were synthesized by autocombustion route keeping oxidizer to fuel ratio at 1. Structural and compositional characterizations of all the samples were performed by XRD, SEM and EDS. Magnetization measurements showed that the Ms increases form 106.5 emu/g for x = 0.0 to 138.5 emu/g for x = 0.2 and then decreases from x = 0.3 (124.71 emu/g for x = 0.3 and 97.78 emu/g for x =0.4), whereas the coercivity (Hc) decreases with manganese (Mn) substitution, except for x = 0.3. Room temperature dielectric properties such as relative dielectric permittivity (er), dielectric loss and ac conductivity, were studied as a function of frequency in the range from 20 Hz to 1 MHz. These studies indicates that the relative dielectric permittivity increasing (from er = 600 for x =0.0 to er = 2400 for x = 0.4) with the increase of Mn content in cobalt ferrite and also all samples show the usual dielectric dispersion which may be due to the Maxwell-Wagner-type of interfacial polarization. Dr. Y. D. Kolekar gratefully acknowledges the award of BOYSCAST fellowship by Department of Science and Technology, India. *On leave from Department of Physics, University of Pune, Pune- 411 007, India. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W16.00008: Single magneto-chiral domain observed in langasite Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ by non-resonant magnetic X-ray scattering Laurent Chapon, Alessandro Bombardi, Federica Fabrizi, Chris Stock, Des McMorrow, Paolo Radaelli, Sang-Wook Cheong The helical magnetic ground state of the chiral langasite compound Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ has been investigated using a left-handed single crystal using non resonant x-ray magnetic scattering. This technique, when combined with circularly polarized x-ray and a full polarization analysis of the scattered beam, is sensitive to the chirality of the spiral order previously reported in this compound and it allows an unique determination of the chirality of the magnetic ground state. A topographic map of the sample surface shows that the crystal is made of a single magneto-chiral domain. Azimuthal scans revealed that the long range magnetic order with wave-vector k=(0,0,1/7) is characterized by an elliptical modulation rather than a circular one, as initially reported. We also discuss the possible spin-driven ferroelectric state in this compound. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W16.00009: Simple point-ion electrostatic model explains the cation ordering in A2BO4 spinel oxides Vladan Stevanovic, Mayeul d'Avezac, Alex Zunger The A2BO4 spinel oxides are distinguished by having either a normal or an inverse distribution of the A, B cations over the octahedrally and tetrahedrally coordinated sites. While normal spinel represents a single structure (A-octahedral, B-tetrahedral) the inverse spinel is similar to a 50-50 alloy with octahedral sublattice occupied randomly by A and B. We show that a simple point-ion electrostatic (PIE) model parameterized by the oxygen displacement parameter u and by the relative formal cation valencies ZA vs ZB provides a simple rule: if ZA$>$ZB the structure is normal for u$>$0.2592 and inverse for u$<$0.2578, while if ZA$<$ZB the structure is normal for u$<$0.2550 and inverse for u$>$0.2578. This rule is illustrated for the known spinel oxides, proving to be 98 \% successful (PRL 105, 075501). Moreover, in inverse spinels the PIE model also explains the origin of the experimentally observed ordered phase that emerges from the random alloy at low T. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W16.00010: Anionogenic Magnetism: Mixed Alkali -- Alkaline Earth Metal Oxides Shivakumara Giriyapura, Syarif Riyadi, Baomin Zhang, Robert A. de Groot, Thomas T.M. Palstra, Graeme R. Blake The scientific and technological potential of materials in which magnetism arises from p-electrons is little explored. Accordingly, we have synthesized the solid solution Ba$_{1-x}$K$_{x}$(O$_{2}^{2-})_{1-x}$(O$_{2}^{-})_{x}$ which contains a nominal mixture of magnetic superoxide and non magnetic peroxide anions. Magnetization measurements reveal short range antiferromagnetic ordering below $\sim $65 K for all compositions, with an increasing tendency toward ferromagnetic interactions at lower temperatures, characterized by the opening of magnetic hysteresis loops in applied fields.Field induced reorientation of the dioxygen dumbbells probably occurs, altering the magnetic exchange interactions between nearest neighbor anions. [Preview Abstract] |
Session W17: Focus Session: Magnetic Oxide Thin Films - Cobaltate and Ferrous Oxide Thin Films
Sponsoring Units: GMAG DMPChair: Chris Leighton, University of Minnesota
Room: D174
Thursday, March 24, 2011 11:15AM - 11:51AM |
W17.00001: Fist-principles design of magnetic oxides Invited Speaker: First-principles design of magnetic oxides is one of most ambitious challenges in modern computational physics. The electronic and magnetic properties of these materials are substantially affected by strongly localized electrons, complex crystalline structures, and mixed valencies of magnetic ions. In my talk I shall present an ab-initio Green function method within density functional theory which provides an accurate description of the electronic structure of these materials. In particular, the treatment of strongly localized electrons is improved considerably by applying a self-interaction correction (SIC), thereby removing the spurious interaction of an electron with itself. By localizing a particular electronic configuration using the SIC, we simulate various valencies of magnetic ions. A mixed-valency state can then be efficiently treated within the coherent potential approximation which is implemented in our multiple-scattering Green function code that can be used as well for simulations of any kind of substitutional disorder. In my talk I shall demonstrate the power of our approach on complex magnetic oxidic surfaces and interfaces. In particular, I shall discuss effective exchange interactions in systems with mixed valency and the influence of structural imperfections, such as defects and relaxations, on the electronic and magnetic properties of these materials. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W17.00002: Cobalt spin state and hyperfine interaction in ferromagnetic insulating LaCoO$_3$ thin films Renata Wentzcovitch, Han Hsu, Peter Blaha, Chris Leighton At low temperatures, bulk LaCoO$_3$ is a non-metallic diamagnet. In contrast, thin-film LaCoO$_3$ experiencing a tensile epitaxial strain is a ferromagnetic insulator in the same temperature range. It is difficult to properly describe this phenomenon with density functional theory (DFT) calculations, even with the Hubbard $U$ correction. Previous calculations have found ferromagnetic conducting thin-film LaCoO$_3$ with all Co ions in the intermediate-spin (IS) state, incompatible with experimental data. In this work, using the DFT+$U$ approach, we show that a strained LaCoO$_3$ thin film can be stabilized in a ferromagnetic insulating state. We also predict the electric field gradient (EFG) at the Co nucleus in the magnetic thin film, which can be helpful in identifying Co spin states via nuclear magnetic resonance (NMR) spectroscopy. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W17.00003: First-principles study of strain-induced ferromagnetism in LaCoO$_{3}$ Hosung Seo, Alexander Demkov We study theoretically the effect of biaxial strain on magnetic properties of LaCoO$_{3}$ (LCO) using density functional theory combined with the Hubbard U method. LCO is normally a non-magnetic insulator with trivalent cobalt ions in low-spin state (t$_{2g}^{6})$. Owing to close interplay between orbital, spin, and lattice degrees of freedom, it shows rich magnetic behavior such as temperature-induced spin state transition. Recently, the ferromagnetic tensile-strained LCO films have been reported. The underlying physics of the ferromagnetic state is, however, unclear. Using a large tetragonal cell we calculate full structural response of the system to applied strain for non-magnetic and magnetic solutions. We show that beyond tensile strain of 3.8{\%} the ferromagnetic solution with Co ions in intermediate-spin state (t$_{2g}^{5}$ e$_{g}^{1})$ is stabilized accompanied by partial untilting of CoO$_{6}$ octahedral network. We also perform the calculation for compressive-strained structures and the difference between these and the tensile strained structures will be presented. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W17.00004: Spin-polarized scanning tunneling spectroscopic (SP-STS) studies of the intrinsic electronic heterogeneity in ferromagnetic (FM) cobaltites and manganites Jing Shi, Li Zhang, Wei-Hsun Lin, Hao Chu, Cameron Hughes, Nai-Chang Yeh The perovskite manganites Ln$_{1-x}$A$_{x}$MnO$_{3}$ and cobaltites Ln$_{1-x}$A$_{x}$CoO$_{3}$ (Ln: trivalent rare earth ions; A: divalent alkaline earth ions) exhibit interesting magnetotransport properties in their FM state: the former show colossal magnetoresistance (CMR) and the latter display giant anomalous Hall effect (AHE), where the AHE coefficient peaks near the Curie temperature ($T_{C})$. These novel phenomena are associated with the intrinsic electronic heterogeneity resulting from strongly correlated multi-valence multi-spin electronic configurations. We perform SP-STS studies on epitaxial films of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ and La$_{1-x}$A$_{x}$CoO$_{3}$ (A = Sr, Ca; 0.4 $\le $ x $\le $ 0.6). In the manganites electronic heterogeneity on the scale of $\sim $10$^{2}$ nm is found to develop below $T_{C}$ and diminish with increasing magnetic field. A surface FM insulating phase is manifested by the spin filtering effect and is attributed to the MnO$_{2}$ surface layer. Similar studies are conducted on the cobaltites to reveal possible correlation between magnetic clustering effects and the Berry phase. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W17.00005: Epitaxial growth and characterization of strained LaCoO$_{3}$ on Si (100) Agham Posadas, Morgann Berg, Hosung Seo, David Smith, Alexander Kirk, Dmitry Zhernokletov, Robert Wallace, Alex de Lozanne, Alexander Demkov LaCoO$_{3}$ is a correlated oxide that normally has a diamagnetic ground state in bulk. The material undergoes a spin-state transition and becomes paramagnetic at higher temperatures. In this work, we report the epitaxial growth of strained LaCoO$_{3}$ on silicon via a fully-relaxed SrTiO$_{3}$ buffer layer using molecular beam epitaxy (MBE). We confirm that the strained LaCoO$_{3}$ becomes ferromagnetic with a Curie temperature of ~85 K, similar to recent reports for films grown by pulsed laser deposition. We will discuss the issues related to the MBE growth of LaCoO$_{3}$ and show results of x-ray diffraction, x-ray photoelectron spectroscopy, transmission electron microscopy, and SQUID magnetometry measurements on LaCoO$_{3}$ films grown on silicon. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W17.00006: Control of the Octahedral Tilts in Lanthanum Cobaltite and the Impact on Magnetic Properties Michael Biegalski, Haile Ambaye, Valeria Lauter, Hans Christen Strain can be accommodated in two ways in perovskite materials, either via the extension of bond lengths or the rotation of the relatively rigid BO$_{6}$ octahedra. To explore the effects of octahedral tilts on magnitism, we have used epitaxy to control the octahedral tilting in the unit cell of La$_{0.5}$Sr$_{0.5}$CoO$_{3}$ by growth on cubic La$_{0.3}$Sr$_{0.7}$Al$_{0.65}$Ta$_{0.35}$O$_{3}$ (LSAT) and orthorhombic NdGaO$_{3}$. From X-ray diffraction, the films grown on LSAT were shown to have a cubic structure whereas the films grown on NdGaO$_{3}$ show an orthorhombic distortion. Due to the large paramagnetic response of the NdGaO$_{3}$ polarized neutron reflectometery was used to probe the magnetic structure of the films. The polarized neutron reflectometry demonstrates that changes in the crystal structure due to the epitaxially imposed symmetries alter the magnetism in these materials. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W17.00007: Magnetic Phase Separation in Oxygen Doped SrCoO$_{3-y}$ F.J. Rueckert, C.K. Xie, Y.F. Nie, B.O. Wells, J.I. Budnick, W.A. Hines, B. Dabrowski SrCoO$_{3-y}$ forms the perovskite structure with oxygen vacancies and is ferromagnetic for y$<$0.25. We have performed a study on polycrystalline samples, controlling the oxidation state using electrochemistry. Under these conditions we have found that magnetically the system segregates into separate, stable phases that correspond to SrCoO$_{2.75}$(T$_{C}$ = 165 K), SrCoO$_{2.875 }$(T$_{C}$ = 220 K), and SrCoO$_{3}$ (T$_{C}$ = 280 K), with two phase behavior for intermediate oxygen concentrations. Surprisingly, these same samples show only a single structural phase that evolves smoothly. We have recently learned to grow high quality epitaxial films of SrCoO$_{y}$, allowing for more typical single crystal diffraction experiments. Our initial results indicate that magnetic phase separation is suppressed in the films. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W17.00008: Examining the Magnetic Properties of LaCoO$_3$ Thin Films Using Magnetic Force Microscopy Morgann Berg, Agham Posadas, Alex de Lozanne, Alexander Demkov In contrast to the non-magnetic ground state of bulk LaCoO$_3$ (LCO) at low temperatures, ferromagnetism has been observed in elastically strained thin film specimens. The origins of ferromagnetism in strained LCO thin films have been obscured by conflicting experimental results. Pulsed laser deposition (PLD) is the current standard of preparation techniques used to grow thin films of LCO, but results from thin film LCO samples prepared by PLD have been questioned on the basis of chemical inhomogeneity and film defects. Using magnetic force microscopy, we investigate the microscale magnetic properties of strained thin films of LCO prepared by molecular beam epitaxy and deposited on lanthanum aluminate and strontium titanate substrates. We observe these properties across a temperature range surrounding the Curie temperature (T$_c \sim$ 80K) and compare our results to global magnetic characteristics of these films as measured by a SQUID magnetometer. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W17.00009: Magnetic resonance at the Verwey transition in epitaxial $Fe_3O_4$ M. Pechan, B. Kaster, J. Dou, P. Jayathilaka, C. Bauer, C. Miller $Fe_3O_4$ is of interest due to its potential applications in the field of spintronics. Previous studies on magnetite films and $Fe_3O_4$/Cr/NiFe spin valves indicate the presence of a uniaxial anisotropy when the magnetite is grown in a magnetic field. Two 170 nm thick films of $Fe_3O_4$ were reactively sputtered simultaneously onto (100) MgO substrates without and with an applied field (100 Oe). Epitaxy was confirmed by in-plane x-ray diffraction and optimal oxygen stoichiometry is confirmed by the 119 K Verwey transition temperature ($T_V$). Ferromagnetic resonance (FMR) measurements at 35 GHz in the plane of the film revealed four-fold anisotropy confirming high quality (100) epitaxy, with an additional uniaxial contribution present in the field-grown sample. Temperature dependent FMR on the sample grown without field clearly reflects $T_V$ in the linewidth and in-plane and out-of-plane anisotropies. The in-plane uniaxial anisotropy for the sample grown with field exhibits an even stronger temperature response at $T_V$. Detailed discussions of thermal variations of magnetization, anisotropy and relaxation processes will be presented. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W17.00010: LuFe2O4 nanostructures on MgO(111) substrate Xiaoshan Xu, Wenbin Wang, Gai Zheng, Paul Snijder, Thomaz Ward, Jian Shen LuFe$_2$O$_4$ nanostructures have been deposited on MgO(111) substrate using pulsed laser deposition. Substrate temperature and gas pressure are found to be very critical to form the LuFe$_2$O$_4$ phase. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) show very well crystalized morphology with triangular symmetry. The common orientations of the nanosctructurals are consistent with epitaxial growth. X-ray diffraction data show that (001) face of LuFe$_2$O$_4$ is parrallel to the substrate face MgO(111). [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W17.00011: Edge-Imposed Domain Ordering in Antiferromagnetic LaFeO3 Nanostructures J.K. Grepstad, E. Folven, T. Tybell, A. Scholl, A. Young, S.T. Retterer, Y. Takamura The antiferromagnetic (AFM) domain structure of submicron-sized LaFeO$_{3}$ nanostructures was imaged with photoemission electron microscopy in combination with x-ray magnetic linear dichroism. These nanostructures were defined in epitaxial LaFeO$_{3}$ thin films using e-beam lithography and Ar$^{+}$ ion implantation to locally destroy the magnetic order in the surrounding matrix. Extended domains were found to form along the perimeter of rectangular-shaped islands, when their edges were aligned with the in-plane ${<}$100${>}$ axes of the cubic SrTiO$_{3}$ substrate. The AFM spin axis of these domains was confined to lie within the film plane, aligned with the edges of the nanostructures. This domain configuration predominated for nanoislands scaled down to 500x500 nm$^{2}$. However, no edge-imposed domain ordering was observed for rectangular islands rotated by 45$^{\circ}$ with respect to the in-plane crystalline axes, suggesting a magnetocrystalline origin of the extended edge-bound AFM domains. These findings may prove important to spintronic devices relying on exchange-biased nanostructures, where domain engineering in antiferromagnets remains relatively unexplored and has the potential to provide new device opportunities. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W17.00012: Fe$_3$O$_4$/ZnO: a high-quality magnetic oxide-semiconductor heterostructure Andreas Mueller, Markus Paul, Dominik Kufer, Sebastian Brueck, Eberhard Goering, Martin Kamp, Jo Verbeeck, He Tian, Michael Sing, Ralph Claessen Magnetite (Fe$_3$O$_4$) is ranked among the most promising materials to use as a spin injector into a semiconducting host. We demonstrate epitaxial growth of Fe$_3$O$_4$ films on ZnO which presents a further step towards incorporation of magnetic materials into semiconductor technology. X-ray spectroscopy results evidence that the iron-oxide is phase-pure and nearly stoichiometric magnetite. Diffraction measurements indicate highly oriented epitaxy and almost complete structural relaxation. The microstructure consists of domains separated by anti-phase boundaries or twin boundaries as a result of island-like growth. The magnetic behavior shows a rather slow approach to saturation at high fields in comparison with bulk crystals, which is likely due to antiferromagnetic coupling at the anti-phase boundaries. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W17.00013: The control of Morin transition temperature on hematite $\alpha$-Fe$_2$O$_3$(0001) thin film SeongHun Park, J.-H. Park, B.-G. Park, J.-Y. Kim The Morin transition of $\alpha$-Fe$_2$O$_3$(0001) thin film was investigated by using soft x-ray absorption spectroscopy (XAS).The epitaxial thin films were grown by cycles of evaporation and post-oxydation method on Al$_2$O$_3$(0001) substrate. The x-ray diffraction (XRD) revealed that the strain is changed with variation of the thickness and the buffer layer. The Morin temperature were measured by x-ray magnetic linear dichroism (XMLD).Interestingly, the Morin transition temperature increased up to room temperature in hematite thin film. In addition, the Co metal overlayer suppressed the Morin transition temperature. Finally, we discuss the magnetic anisotropy including the strain and the interlayer exchange interaction. [Preview Abstract] |
Session W18: Many Body
Sponsoring Units: DCOMPChair: Munehisa Matsumoto, University of California, Davis
Room: D172
Thursday, March 24, 2011 11:15AM - 11:27AM |
W18.00001: Exact asymptotes of static and dynamic correlation functions of the 1D Bose gas Aditya Shashi, Leonid Glazman, Jean-Sebastien Caux, Adilet Imambekov Recent experiments with ultracold atomic gases have provided realizations of one-dimensional systems of bosons with contact interactions, described by the Lieb-Liniger model. These experiments have revived interest in the correlation functions for this model. Since a fully analytical calculation of the correlation functions is still lacking, our results [1] represent a significant step forward. We have combined field theoretical approaches with an analysis of the finite size scaling of exact form factors of the Lieb-Liniger model to analytically calculate ``non-universal'' prefactors in the long-distance behavior of correlation functions as well as prefactors of singularities in dynamic response functions such as the density structure factor and spectral function. We have also proved the existence of singularities within a continuum spectrum. \\[4pt] [1] A. Shashi, L. I. Glazman, J-S. Caux and A. Imambekov, arXiv:1010.2268. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W18.00002: Correlation functions of nonlinear Luttinger liquids as Fredholm determinants Yinbin Ma, Adilet Imambekov One dimensional quantum liquids are often described within an effective linear hydrodynamic approach known as Luttinger liquid theory. As the principal simplification, a generic spectrum of the constituent particles is replaced by a linear one, which leads to a linear hydrodynamic theory. It has been shown recently [1] that the nonlinearity of the generic spectrum leads to a significant modification of the dynamic response functions. Their description can be achieved within the universal framework of nonlinear Luttinger liquid theory. We show here that correlation functions within such approach can be expressed as Fredholm determinants, and evaluate them in low energy regions for arbitrary interaction strength and small temperatures. \\[4pt] [1] ``Universal theory of nonlinear Luttinger liquids,'' A. Imambekov and L.I. Glazman, Science 323, 228 (2009) [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W18.00003: Ab initio Low-Dimensional Physics Opened Up by Constrained RPA for Energy and Space: Applications to LaFeAsO and $\kappa $-(BEDT-TTF)$_{2}$X Kazuma Nakamura, Yoshihide Yoshimoto, Yoshiro Nohara, Masatoshi Imada Studies on outstanding electron correlation effects such as non-Fermi liquid behavior and unconventional superconductivity discovered in systems with low-dimensional anisotropy have continuously been at a front of condensed matter physics. Analyses for 1D or 2D simplified models have played a primary role in understanding essence of correlation effects, but to a large extent, the studies rely on ad hoc adjustable parameters as in the Hubbard models. We develop a new ab initio downfolding scheme for deriving effective low-energy models with low spatial dimensions [1]. The scheme is based on constrained random-phase-approximations by imposing constraints not only in ``energy'' but also in ``space''. We show real applications for 2D-layered superconductors of LaFeAsO and $\kappa $-(BEDT-TTF)$_{2}$X. The derived interactions in the effective models become short ranged essentially within up to next-nearest neighbors and thus justify multiband 2D Hubbard models as effective models for these materials from first principles. This work is supported from MEXT Japan under grant numbers 22740215 and 22104010.\\[0pt] [1] K. Nakamura, Y. Yoshimoto, Y. Nohara, and M. Imada, arXiv:1007.4429. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W18.00004: Beyond Landau Adiabacity: Weak Interaction Quenches in a Fermi Gas Stefan Kehrein, Michael Moeckel The crossover from Landau's Fermi-liquid paradigm with adiabatic switching on of the interaction to a sudden interaction quench is investigated [Phys. Rev. Lett. 100, 175702 (2008); Ann. Phys. 324, 2146 (2009); New J. Phys. 12, 055016 (2010)]. The real time dynamics for weak interactions is calculated in a systematic expansion and one finds three clearly separated time regimes: (i) An initial buildup of correlations where the quasiparticles are formed. (ii) An intermediate quasi-steady prethermalized regime resembling a zero temperature Fermi liquid with a nonequilibrium quasiparticle distribution function. (iii) The long-time limit described by a quantum Boltzmann equation leading to thermalization of the momentum distribution function. This thermalization behavior is contrasted with interaction quenches in 1d models. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W18.00005: Dynamical Freezing of Response in Driven Many-Body Quantum System Arnab Das We show, that the response of a periodically driven quantum many- body system may freeze drastically, when driven with certain combinations of driving parameters (amplitude and frequency). We demonstrate this effect with analytical results for a broad class of integrable quantum spin models (illustrated particularly for one-dimensional Transverse Ising Model)and direct numerical integration data for large system-size. We show that the immunity of the freezing behavior to external noise can be controlled arbitrarily by controlling the strength of the local (on-site) field. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W18.00006: Bethe-Salpeter Equation calculations of transition metal L$_{2,3}$ edge x-ray spectra including multiplet effects J. Vinson, E.L. Shirley, J.J. Rehr Calculations of x-ray spectra at transition metal L$_{2,3}$ edges often present theoretical difficulties due to strong core-hole multiplet effects. Here we discuss an {\it ab initio} method for treating these effects following the Bethe-Salpeter Equation (BSE) approach of Shirley.\footnote{E. L. Shirley, J. Electron Spectrosc. Relat. Phenom. {\bf 144}, 1187 (2005).} The method builds in spin-orbit interactions, intra-atomic Coulomb integrals, core-hole screening, and band-structure effects, and thus accounts for multiplet effects without the need for phenomenological ligand-field parameters. The method has been implemented in the core-level BSE code OCEAN,\footnote{J. Vinson, E. L. Shirley, J. J. Rehr, and J. J. Kas, arXiv:1010.0025} which uses as input wavefunctions from planewave, pseudopotential DFT calculations and PAW transition matrix elements. Examples are presented for several transition metal systems and compared with experiment. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W18.00007: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W18.00008: Real Space Green's Function Calculations of RIXS J.J. Kas, J.J. Rehr, J.A. Soininen We present an {\it ab initio} theory of resonant inelastic x-ray scattering (RIXS) based on the real-space multiple scattering Green's function (RSGF) formalism and a quasi-boson model Hamiltonian. It is shown that the RIXS spectrum is quasi-local in nature, depending primarily on the Green's function close to the absorbing site. Based on several assumptions, we derive an approximation to the RIXS spectrum in terms of a convolution of the x-ray absorption and x-ray emission spectra. In addition, quasi-particle self-energy and other many-body effects are calculated using a many-pole model dielectric function, and included via a convolution of the RIXS spectrum with an energy dependent spectral function. Core hole effects are also investigated. The method is implemented in an extension of the RSMS code FEFF90\footnote{J.J. Rehr et al., Comptes Rendus Phys. \textbf{10}, 548 (2009)} and illustrated with several examples. Results are found to be in qualitative agreement with experiment. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W18.00009: Finite-Temperature Constraint-Pairing Mean Field Theory Jianmin Tao, Gustavo Scuseria Recently Tsuchimochi and Scuseria have developed a constrained-pairing mean-field theory (CPMFT), based on the Hartree-Fock-Bogoliubov theory. Application to molecular systems shows that CPMFT can accurately describe the binding energy curve in the dissociation of molecules, where electron correlation is strong. However, because CPMFT is a zero-temperature theory, it is not suitable for the description of high-temperature superconductivity and normal state of high-temperature superconductors. Here a finite-temperature generalization is formulated for the thermodynamic state of quantum many-body systems. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W18.00010: Fermionic duality approach to strongly-interacting lattice models Joe Mitchell, Tigran Sedrakyan, Victor Galitski We derive an exact Grassmann path-integral representation for strongly interacting fermion systems that is dual to the conventional Hubbard-Stratonovich approach. In contrast to the latter, we decouple the interaction Hamiltonian by introducing additional (dual) fermionic fields. This fermionic decoupling naturally forms in particle-hole channels and leads to alternative order parameters given in terms of the dual fields. The new formalism is tested by calculating the partition function in a simple solvable model with four-fermion interaction and is argued to be very effective for strong or repulsive interactions. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W18.00011: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W18.00012: Breakdown of the coherent state path integral: two simple examples Justin Wilson, Victor Galitski We show how the time-continuous coherent state path integral breaks down for both the single-site Bose-Hubbard model and the spin path integral. Specifically, when the Hamiltonian is quadratic in a generator of the algebra used to construct coherent states, the path integral fails to produce correct results following from an operator approach. As suggested by previous authors, we note that the problems do not arise in the time-discretized version of the path integral. Further, a na\"ive use of the semiclassics agrees with our conclusions. [Preview Abstract] |
Session W19: Focus Session: Novel Magnetic Devices
Sponsoring Units: GMAG DMPChair: Sujoy Roy, Lawrence Berkeley National Laboratory
Room: D170
Thursday, March 24, 2011 11:15AM - 11:27AM |
W19.00001: Novel Spintronic Device-Terahertz Magnon-Photon Laser Boris Tankhilevich A novel spintronic -based method of generating THz radiation is proposed. The method is based on pumping of non-equilibrium electrons into the upper (spin-down) sub-band of spin-polarized half-metallic ferromagnets or ferromagnetic semiconductors, which makes it possible to build tunable, narrow-band, high-power THz sources. Non-equilibrium electrons pumped into the spin-down subband rapidly emit non-equilibrium magnons with THz frequency, pass into highly excited states of the spin-up subband, and fall into the ground state due to interaction with the equilibrium spin-up electrons or by emitting optical phonons. The mechanism of magnon generation is similar to a three-level conventional laser, and at a critical pumping intensity, which depends on the magnon damping, magnon lasing begins. In this regime the number of excited magnons increases exponentially with time. Merging of two THz magnons with frequency f generates a THz photon with frequency 2f. Thus, a magnon laser becomes a THz photon laser. The proposed one-stage device is capable of generating THz power being of orders of milliwatt and is tunable by tuning the magnetic field and/or the bias. The device has nano-dimensions and can be mass produced on a large scale. Recently THz radiation by spin-polarized current in a ferromagnetic structure was observed. However, the material used in this experiment has not met the conditions for magnon lasing. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W19.00002: Micro-Structured Ferromagnetic Tubes for Spin Wave Excitation Alexander Kozhanov, Daniel Ouellette, Mark Rodwell, Dok Won Lee, Shan X. Wang, S. James Allen Small scale magnetostatic spin wave devices are potentially important for on-chip filters for communication systems and spin wave logic devices. Low efficient coupling the electronic signals into the spin waves as well as coupling-out makes it difficult to build logical circuits especially when structures are scaled down to nanometer sizes. In this work we study the effect of external biasing magnetic field on the propagation of backward volume magnetostatic spin waves (BVMSW) in ferromagnetic CoTaZr stripe with micron sized ferromagnetic tubes fabricated at the ends. Spin waves are excited by shorted coplanar waveguides signal line of which is placed inside the tubes. Ferromagnetic tubes placed at the ends of the stripe form closed magnetic circuit that traps the RF magnetic field produced by the coupling loop. Transmission S-parameters of fabricated structures were measured using a vector network analyzer in the frequency range (0.5-20) GHz and biasing magnetic fields (0-1000) Oe. Experimental data is analyzed with use of theoretical model for BVMSW in ferromagnetic stripe. This work is supported by the Nanoelectronics Research Initiative (NRI) - Western Institute of Nanoelectronics (WIN). [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W19.00003: Ferromagnetic STM tip operating as a Spin-diode Poliana H. Penteado, Fabricio M. Souza, Ant\^onio C. Seridonio, Renato M. Coutinho, Edson Vernek, J. Carlos Egues We study spin-dependent transport in a system composed of a ferromagnetic STM tip coupled to an adsorbed atom (adatom) and to a host metallic (non-magnetic) surface. Electrons can tunnel directly from the tip to the surface or through the adatom. Our calculation is based on the nonequilibrium Green functions technique (Keldysh formalism). We self-consistently calculate the adatom spin occupation and its magnetization as a function of the tip position. We find that the adatom becomes magnetized when the tip approaches it; this magnetization switches sign as the voltage changes from forward to reverse bias. We also calculate the spin-resolved currents. If the tip is near the adatom, we obtain the spin-diode effect [PRB \textbf{75}, 165303 (2007)] - i. e., unpolarized current for positive bias and polarized current for reverse bias - when the adatom is singly occupied. We also observe Friedel oscillations in the current as the tip-adatom distance increases [F. M. Souza, P. H. Penteado, et al. - to be submitted]. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W19.00004: Spin voltage generation across rare earth spin filter barriers Guoxing Miao, Joonyeon Chang, Jagadeesh Moodera When a metal is in close contact with a rare-earth based magnetic compound, strong exchange interaction exists between the localized 4f electrons and the free moving conduction electrons. One important consequence is that the spin degeneracy among the conduction electrons is lifted, showing up as an effective Zeeman splitting higher than tens of Tesla in low dimensional systems such as graphene and other 2DEG. We perform our work using a vertical transport geometry, which consists of double spin filtering barriers based on a ferromagnetic Eu chalcogenide - EuS. A thin Al metallic layer is sandwiched in the middle and its conduction electrons thus experience the strong spin splitting, which is subsequently detected via the spin filtering effect. A spontaneous spin dependent voltage appears across such a device, and its polarity is directly determined by the EuS/Al interface. The voltage level difference between the spin-parallel and -antiparallel configurations is as large as a few mV. Such spin splitting also induces a clear universal behavior in the observed TMR bias dependence. Such spin voltage effect offers a possibility of directly converting magnetic exchange energy into electrical power. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W19.00005: Generation of spin currents due to mechanical rotation Mamoru Matsuo, Jun'ichi Ieda, Eiji Saitoh, Sadamichi Maekawa In the frontier of spintronics, much attention is paid on the control and generation of spin currents. Due to the exciting progress of nanomechatrononics, the importance of mechanical manipulation of electron spin will increase. We discuss theoretically effects of mechanical rotation on spin currents using generally covariant Dirac equation with gauge fields in the non-relativistic limit. We derive semi-classical equations of motion for a wavepacket of electrons in two dimentional planes subject to the spin-orbit interaction argumented by a mechanical rotation. We show that a circular spin current is created by the mechanical rotation with a magnetic field. The magnitude of the spin current becomes $10^8 \mbox{A/m}^2$ in Pt with the magnetic field $\approx 1$T and the rotational velocity $\approx 1$kHz. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W19.00006: Magnetic Field Effects on Mechanical Cantilevers with Deposited Thin Film Micromagnets Rosa Elia C\'{a}rdenas, Francisco M\'{a}rquez, John T. Markert We report on the techniques used to deposit magnetic material onto mechanical cantilevers. The deposition of the magnetic material, permalloy, onto the cantilevers was achieved by using a precise masking technique before mounting the cantilevers inside an electron beam evaporation chamber. This method resulted in a mechanical cantilever with a deposited micromagnet on its tip. A typical size of the resulting micromagnet is 200 nm thick by 20 microns wide by 10 microns in height. Using a laser interferometer, the driven response of the cantilevers with the deposited micromagnets are currently being studied in vacuum as a function of the external magnetic field. We will analyze the magnetic-field-dependent changes in the resonant frequencies and the quality factors of the cantilevers to determine the micromagnet net moment and anisotropy constants. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W19.00007: Torque magnetometry of permalloy-coated microcantilevers using higher order vibrational modes Joseph Losby, Jacob A.J. Burgess, Douglas Vick, John P. Davis, Wayne K. Hiebert, Mark R. Freeman There has been an accumulation of recent interest in the development of magnetometry techniques facilitating the use of nano- and micro-resonators. A finite element model describing the interaction of a magnetic cantilever driven at its fundamental resonance frequency by an external field is described and illustrated for the simple case of a straight domain wall propagating across the cantilever during magnetization reversal. The experimental results are compared to the finite element mechanical transformation of Landau-Lifshits-Gilbert based micromagnetic simulations. This idea is then extended to higher order (flexural and torsional) modes, with the intent of moving towards increased sensitivity and functionalization of magnetometers for the observation of quasi-static magnetization processes. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W19.00008: Forces due to Patterned Magnetic Traps within Microfluidic Channels M. Howdyshell, G. Vieira, A. Chen, M. Simon, M. Poirier, R. Sooryakumar An array of microscopic ferromagnetic disks patterned onto a silicon surface has been previously utilized to trap and transport magnetic microspheres as well as magnetically labeled biological cells across the surface. The transport is activated through programmable weak external magnetic fields that do not damage the cells and enable remote control on the magnitude and direction of the fields. In this talk we present results in which the array of magnetic bits is imprinted within microfluidic channels where now competing hydrodynamic drag forces come into play. The trapping forces on individual microspheres are directly determined from the flow rates required to overcome the local magnetic forces. These findings are compared to results derived from micromagnetic simulations of the magnetic profile of individual disks. The fluid flow within the channel is also used to stretch DNA molecules tethered between two microparticles. With one of the ends trapped on a magnetic disk, the extension is controlled by the fluid flow rate. Comparisons to DNA stretching achieved with conventional magnetic tweezers reported in the literature serve as an additional calibration of the measured forces. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W19.00009: Giant Magnetioimpedance in Co-Based Amorphous Ribbons Coated in Magnetic Nanoparticles for Biosensing Applications N. Laurita, A. Chaturvedi, K. Stojak, S. Chandra, M.H. Phan, H. Srikanth Giant magnetoimpedance (GMI) is a large change in the ac impedance of a ferromagnetic conductor subject to a dc magnetic field. It forms the basis for developing highly sensitive magnetic sensors. We report studies aimed at developing GMI as a magnetic biosensing technique. We have investigated the GMI effect and its field sensitivity in Co-based amorphous alloys with and without coated magnetic nanoparticles. Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ nanoparticles (mean size, 5-10 nm) were patterned onto the ribbon surfaces and the number of particle layers was varied from 10 to 80. The influences of particles size, concentration, and layer thickness on the GMI and field sensitivity have been investigated systematically. The coating of the nanoparticles has been shown to enhance the GMI and field sensitivity, both of which increase with increase of particle concentration and layer thickness. Overall, our studies demonstrate the possibility of using GMI as a magnetic biosensor with high sensitivity for applications in biomolecular detection. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W19.00010: Correlation between magnetic softness, sample surface and magnetoimpedance in Co$_{69}$Fe$_{4.5}$\textit{X}$_{1.5}$Si$_{10}$B$_{15}$ (\textit{X} = Ni, Al, Cr) amorphous ribbons A. Chaturvedi, T. Dhakal, S. Witanachchi, M.H. Phan, H. Srikanth, A.T. Le In this work we have studied the giant magnetoimpedance (GMI) effect and its field sensitivity ($\eta )$ in Co$_{69}$Fe$_{4.5}$X$_{1.5}$Si$_{10}$B$_{15}$ (X = Ni, Al, Cr) amorphous ribbons in the frequency (f) range of 0.1 to 10 MHz. We find that at f $<$ 5 MHz, the GMI effect and $\eta $ reach the largest values for the Al-containing sample and the smallest values for the Ni-containing sample, while an opposite trend is observed at f $>$ 5 MHz. Magnetization and atomic force microscopy (AFM) experiments reveal that the largest values of the low-frequency GMI effect and $\eta $ for the Al-containing sample result from the largest value of magnetic permeability, while the largest values of the high-frequency GMI effect and $\eta $ for the Ni-containing sample are attributed to the smallest surface roughness of this sample. These results point to the importance of the sample surface in determining high-frequency GMI behavior. A correlation between the sample surface and high-frequency GMI is established in the investigated ribbons. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W19.00011: Modulating the Magnetic Field to Improve Magnetic Sensors Alan Edelstein, Jonathan Petrie, Jonathan Fine, Greg Fischer, James Burnette, Gopal Srinivasan, Sanjay Mandal The sensitivity of most magnetic sensors is affected by 1/$f$ noise. Modulating the magnetic field to be detected by magnetic sensors can improve their performance by minimizing the effect of this 1/$f$ noise and, in some cases, also have them operate in a narrow frequency band where they have higher sensitivity. We present approaches for modulating the field. One approach is the MEMS flux concentrator can be used with small magnetic sensors and another, based on using a rotating disc containing flux concentrators that can be used with large magnetic sensors, such as magnetoelectric sensors, that have an increased sensitivity at their mechanical resonance frequency. Sidebands observed around the modulation frequency demonstrate the applicability of these approaches. The MEMS flux concentrator has improved the signal to noise ratio in the power spectrum by a factor of 15. The sensors have the potential to achieve sensitivities of a few pT/Hz$^{1/2}$ at 1 Hz. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W19.00012: Ultra high sensitivity, room temperature magneto-optic field sensor made of ferromagnetic bismuth rare-earth iron garnet thick films Dong Ho Wu, Anthony Garzarella, Vince Fratello The ferrimagnetic bismuth rare-earth iron garnet (BiGdLu)$_{3}$(FeGa)$_{5}$O$_{12}$ thick film has a specific Faraday rotation $\theta _{S}$ of 0.09 \r{ }/mm at 1550 nm and excellent transparency at infrared wavelengths. Using the thick film we recently have demonstrated a magneto-optic (MO) field sensor with a sensitivity of about 10$^{-14}$T/Hz$^{1/2}$, comparable with SQUID. The sensor is made of all dielectric materials including the bismuth rare-earth iron garnet and optical fibers, and is operated at room temperature without any cooling requirement. The MO field sensor is capable to measure a magnetic field over a very large dynamic range (from a very weak field to a very high magnetic field exceeding several hundred Tesla) and over a very wide frequency range, which may be from DC to a few hundred GHz. However, presently, our MO sensor's frequency range is limited from DC to 2 GHz. We think that this limited frequency range is due to the presence of magnetic domains in the bismuth rare-earth iron garnet film. In this presentation we will report our experimental results obtained from this MO field sensor as well as the effect of magnetic domains. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W19.00013: Spin-Flipping at Sputtered Co(90)Fe(10)/Cu Interfaces Hoang Yen Thi Nguyen, Rakhi Acharyya, William P. Pratt Jr., Jack Bass Knowledge of the spin-flipping probability, P$_{F/N}$ = 1 -- exp[-$\delta $(F/N)], at ferromagnetic/non-magnetic (F/N) interfaces in the Current-Perpendicular-to-Plane (CPP) geometry is minimal [1,2]. We use a new technique [2] to find $\delta $(CoFe/Cu) at 4.2K, where CoFe = Co(90)Fe(10). With thin (3 nm) CoFe layers, the spin-diffusion length of CoFe $\sim $ 12 nm doesn't mask spin-flipping due to $\delta $. Our most important samples, sensitive to $\delta $(CoFe/Cu), have the form FeMn/Py/Cu/X/Cu/Py/FeMn. Here Py = Ni(80)Fe(20), the antiferromagnet FeMn pins the two Py layers to flip at a much higher field than X = [CoFe(3)/Cu(1.4)]$_{n}$CoFe(3), and 1.4 nm of Cu couples the CoFe layers ferromagnetically so X reverses as a unit. We measure, versus the number of repeats $n$, the change in specific resistance, A$\Delta $R = AR(AP) -- AR(P), between states where the X moment is anti-parallel (AP) or parallel (P) to the pinned Py moments. CPP current flows through area A. Our resulting best estimate is $\delta $(CoFe/Cu) $\approx $ 0.2. [1] J. Bass and W.P.Pratt Jr., J. Phys. Cond. Matt. \textbf{19}, 183201 (2007). [2] B. Dassonneville et al., Appl. Phys. Lett. \textbf{96}, 022509 (2010). [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W19.00014: Magnetic Properties of Niobium-Permalloy hybrid system Jiyeong Gu, Jill Pestana, David Christiansen Ferromagnet/superconductor hybrid system has been studied intensively due to the proximity effect between ferromagnetism and superconductivity. Not only the superconducting property changes because of the different magnetic configuration, the presence of the superconductivity can also often drastically change the magnetic properties of the ferromagnets. In our current work we focused on the investigation of the magnetic property change of the Nb-Py (Permalloy; NiFe) hybrid system through the superconducting transition. Nb-Py bilayer and trilayers were fabricated using Multi-target sputtering system. Magnetization was measured by Physical Property Measurement System using the Vibrating Sample Magnetometer (VSM) or Alternating Current Measurement System (ACMS) options. In addition to the magnetometry measurement, we also measured magneto optical Kerr effect. Magnetization changed when the system goes through the superconducting transition; however the result varied depending on many parameters, such as magnetic measurement history, cooling or warming rate, and measurement method. Magnetic responses obtained from different methods would be compared and discussed. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W19.00015: Localized magnetism on the surface of niobium: experiments and theory Thomas Proslier, John Zasadzinski, Gianluigi Ciovati, Mike Pellin The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact spectroscopy (PCT), SQUID magnetometry and Electron paramagnetic resonance (EPR). All niobium (Nb) samples displayed a small impurity contribution to the magnetic susceptibility at low temperatures which exhibited Curie-Weiss behavior, indicative of weakly coupled localized paramagnetic moments. By examining Nb samples with widely varying surface-to-volume ratios it was found that the impurity contribution is correlated with surface area. Tunneling measurements which use the native oxide layers as barriers exhibit a zero-bias conductance peak which splits in a magnetic field $>$ 4T, consistent with the Appelbaum model for spin flip tunneling. Viewed together the experiments strongly suggest that the native oxides of Nb are intrinsically defective, and consistently exhibit localized paramagnetic moments caused by oxygen vacancies in Nb$_{2}$O$_{5}$. The computation of the surface impedance (R$_{S})$ in presence of magnetic impurities in the Shiba approximation reveals the saturation at low temperature of Rs, suggesting that magnetic impurities are responsible for the so-called residual resistance. [Preview Abstract] |
Session W20: Focus Session: Thermoelectric Materials: Clathrates and Oxides
Sponsoring Units: DMP GERA FIAPChair: Donald Morelli, Michigan State University
Room: D168
Thursday, March 24, 2011 11:15AM - 11:27AM |
W20.00001: The Atomic Structure of Ga and Ge in Ba-Ga-(Si,Ge) Clathrate A.N. Mansour, J.B. Martin, W. Wong-Ng, G.S. Nolas Compression studies on Sr$_{8}$Ga$_{16}$Ge$_{30}$ type I clathrate revealed a 3 fold increase in ``ZT'' [J. F. Meng et al., J. Appl. Phys., 89, 1730 (2001)]. Substitution of Si for Ge in Ba-Ga-Ge clathrate could mimic the effect of bulk compression, and subsequently enhance ``ZT''. Recent studies on Si substituted Ba-Ga-Ge clathrate have shown a decrease in the lattice constant and an increase in the power factor with Si substitution. However, the effects of Si on the electronic and local atomic structures of Ga and Ge have not been investigated in detail. We have used XAS to characterize the electronic and local atomic structures of Ga and Ge for a number of samples with varying degree of Si substitution. Analysis of Ga and Ge K-edge XANES spectra revealed that the partial density of $p$-states was modified for both Ga and Ge with Si substitution with the changes being more pronounced in the case of Ga. Comparisons of Fourier transforms of EXAFS spectra revealed that the local structure of Ga is significantly changed with Si substitution while changes in the local structure of Ge with Si substitution are moderate. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W20.00002: Examining Lattice Disorder in Type-I Clathrate Ba$_{8}$Ga$_{16}$Sn$_{30}$ using EXAFS Scott Medling, Michael Kozina, Frank Bridges, Koichiro Suekuni, Toshiro Takabatake Semiconducting type I clathrates, such as Ba$_{8}$Ga$_{16}$Ge$_{30}$ (BaGaGe) and Ba$_{8}$Ga$_{16}$Sn$_{30}$ (BaGaSn), have a cage-like crystal structure with ``rattler" atoms (Ba) located near the center of cages (Ga-Ge/Sn). Such compounds have a low thermal conductivity which is attributed mainly to vibrations of the ``rattler" atoms inside the cages which strongly scatter phonons. BaGaSn has a surprising lower thermal conductivity than BaGaGe. To better understand why, we studied samples of BaGaSn using Extended X-ray Absorption Fine Structure (EXAFS). The analysis shows that the average Ga-Sn distance is shorter and the average Sn-Sn distance is longer than the distances found from diffraction; also, the Ba-Ga and Ba-Sn distances have greatly increased disorder. This suggests that the cage-like structure is severely distorted, in contrast to BaGaGe; such a large distortion will strongly scatter phonons, decreasing the thermal conductivity, but unfortunately also will reduce the electrical conductivity. We compare our results for BaGaSn with earlier results for BaGaGe and discuss them in light of recent transport measurements. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W20.00003: Structural, Electronic and Vibrational Properties of Na$_{x}$Si$_{136 }$(0 $<$ x $<$ 24) Clathrates Craig Higgins, Emmanuel Nenghabi, Charles Myles, Koushik Biswas, Matt Beekman, George Nolas CRAIG HIGGINS, EMMANUEL NENGHABI$^{\dag }$, CHARLES W. MYLES, Texas Tech U.; KOUSHIK BISWAS, Oak Ridge National Lab; MATT BEEKMAN, U. of Oregon; GEORGE S. NOLAS, U. of South Florida - Na$_{x}$Si$_{136}$ is a Type II clathrate with important thermoelectric properties. It's face-centered cubic lattice contains polyhedral ``cages'' of silicon atoms with Na atom ``guests'' in the cages. This material is very interesting because powder X-ray diffraction experiments$^{1 }$for differing Na content x have shown that, for increasing x in the range 0 $<$ x $<$ 8, lattice contraction occurs. After all Si$_{28}$ cages in the unit cell are filled (x = 8) and x is increased further, causing a filling of the Si$_{20}$ cages, a contrasting lattice expansion results. Using the local density approximation, we have calculated the x dependences of the structural, electronic and vibrational properties of Na$_{x}$Si$_{136}$. Results are presented for the x dependences of the lattice constant, electronic bands, and vibrational modes. Our results for the x dependence of the lattice constant are in agreement with our X-ray data$^{1}$. $^{\dag }$Deceased. $^{1}$M. Beekman, E.N. Nenghabi, K. Biswas, C.W. Myles, M. Baitinger, Y. Grin, G.S. Nolas, Inorg. Chem. 49, 5338--5340 (2010). [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W20.00004: NMR relaxation and phonon rattling in type-I Ba$_{8}$Ga$_{16}$Sn$_{30}$ clathrates Xiang Zheng, Sergio Y. Rodriguez, Joseph H. Ross, Jr. The atomic motion of guest atoms inside clathrate cages has been considered as one of the important reasons for the observed glasslike thermal behavior. $^{69}$Ga and $^{71}$Ga Nuclear Magnetic Resonance (NMR) studies of type-I Ba$_{8}$Ga$_{16}$Sn$_{30}$ clathrates show a clear low temperature spin-lattice relaxation peak attributed to the influence of Ba rattling dynamics on the framework-atom resonance. Analysis indicates that the quadrupolar relaxation is the leading contribution. The data are analyzed using a two-phonon Raman process, according to a recent theory involving localized one dimensional anharmonic oscillators. Excellent agreement is obtained using this model, with the parameters corresponding to a double well with very large anharmonicity. We have extended the theory to include a two dimensional anharmonic well, with similar parameters providing the best fit to the data. We also examine the Einstein type peak observed in heat capacity using this model, and compare to previous reported results obtained using different models for the anharmonic oscillator. This work is supported by Robert A. Welch Foundation (A-1526). [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W20.00005: Zintl stabilization and site preference in Ba-Cu-Ge clathrates Sergio Y. Rodriguez, Xiang Zheng, Laziz Saribaev, Joseph H. Ross, Jr. Sn, Ge and Si clathrates have cage-like structures, and many exhibit enhanced thermoelectric performance. To understand Cu substitution and Zintl stabilization in Ba$_8$Cu$_x$Ge$_{46-x}$ type-I clathrates with $4 \leq x \leq 6$, we performed NMR measurements coupled with first principles calculations. The $^{63}$Cu NMR resonance exhibits a lineshape characteristic of Cu occupation of the high-symmetry 6$c$ site. All electron computational results showed that the lowest energy configuration is the one with all the Cu atoms located in the 6$c$ site, in agreement with NMR and crystallographic analysis. From bandstructure calculations we find that the preferred structure is a semiconductor, consistent with the observed Zintl stabilization in this material. A direct band gap of about 0.27 eV is found within the Generalized Gradient Approximation formalism. The preferred compositions follow quite closely the valence-counting scheme. From an Atoms In Molecules analysis it is seen that the Ba charge is less than 2$^+$, even though the simple electron-counting argument works well. The framework is seen to be composed of a $sp^3$ bonded network, with strongly polar bonds for Cu. Ba atoms in the large cages are least strongly bond in the plane of the hexagonal faces, corresponding to ease of rattling as seen for other clathrates. This work is supported by Robert A. Welch Foundation (Grant A-1526). [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W20.00006: Thermoelectric properties of low-dimensional clathrates from first principles Deepa Kasinathan, Helge Rosner Type-I inorganic clathrates are host-guest structures with the guest atoms trapped in the framework of the host structure. From a thermoelectric point of view, they are interesting because they are semiconductors with adjustable bandgaps. Investigations in the past decade have shown that type-I clathrates $X_{8}$Ga$_{16}$Ge$_{30}$($X$ = Ba, Sr, Eu) may have the unusual property of ``phonon glass-electron crystal'' for good thermoelectric materials. Among the known clathrates, Ba$_{8}$Ga$_{16}$Ge$_{30}$ has the highest figure of merit (ZT~1). To enable a more widespread usage of thermoelectric technology power generation and heating/cooling applications, ZT of at least 2-3 is required. Two different research approaches have been proposed for developing next generation thermoelectric materials: one investigating new families of advanced bulk materials, and the other studying low-dimensional materials. In our work, we concentrate on understanding the thermoelectric properties of the nanostructured Ba-based clathrates. We use semi-classical Boltzmann transport equations to calculate the various thermoelectric properties as a function of reduced dimensions. We observe that there exists a delicate balance between the electrical conductivity and the electronic part of the thermal conductivity in reduced dimensions. Insights from these results can directly be used to control particle size in nanostructuring experiments. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W20.00007: Nanostructured Oxides and Sulfides for Thermoelectrics Invited Speaker: Thermoelectric power generation can be applied to various heat sources, both waste heat and renewable energy, to harvest electricity. Even though each heat source is of a small scale, it would lead to a great deal of energy saving if they are combined and collected, and it would greatly contribute to reducing carbon dioxide emission. We have been engaged in developing novel thermoelectric materials to be used for energy saving and environmental protection and are currently developing nanostructured ceramics for thermoelectric conversion. We have demonstrated a quantum confinement effect giving rise to two dimensional electron gas (2DEG) in a 2D superlattice, STO/STO:Nb (STO: strontium titanate), which could generate giant thermopower while keeping high electrical conductivity. One unit-cell thick Nb-doped well layer was estimated to show ZT=2.4 at 300K. Then, a ``synergistic nanostructuring'' concept incorporating 2DEG grain boundaries as well as nanosizing of grains has been applied to our STO material and 3D superlattice ceramics was designed and proposed. It was verified by numerical simulation that this 3D superlattice ceramics should be capable of showing ZT=1.0 at 300K which is comparable to or even higher than that of conventional bismuth telluride-based thermoelectrics. We have recently proposed titanium disulfide-based misfit-layered compounds as novel TE materials. Insertion of misfit-layers into the van der Waals gaps in layer-structured titanium disulfide thus forming a natural superlattice gives rise to internal nanointerfaces and dramatically reduces its lattice thermal conductivity. ZT value reaches 0.37 at 673 K even without optimization of electronic properties. Our challenge to further increase ZT by controlling their electronic system and superlattice structures will be presented. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W20.00008: Electronic Structure Determination of the Thermoelectric CuRh$_{1-x}$Mg$_{x}$O$_{2}$ using Soft X-Ray Spectroscopies Eric Martin, Paolo Vilmercati, Christine Cheney, Takao Sasagawa, Norman Mannella Magnesium-doped rhodium oxides with formula unit CuRh$_{1-x}$Mg$_{x}$O$_{2}$ and delafossite-type structure exhibit a high thermoelectric figure of merit at elevated temperatures. The electronic structure of CuRh$_{1-x}$Mg$_{x}$O$_{2}$ has been studied with x-ray emission spectroscopy (XES), x-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). The data reveal that the states at the Fermi level are Rh-derived. Measurements carried out by changing the orientation of the linear photon polarization further indicate that the Rh states have a more localized character along the c-axis, consistent with the layered crystal structure. Given the similarity of the electronic configurations of Co and Rh, these data provide solid experimental evidence that the orbital degrees of freedom of the d$^{6 }$ionic configuration of the states rooted in transport are key for explaining the thermoelectric properties of oxide materials. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W20.00009: Thermal Conductivity of Aluminum Oxide from First Principles Moses Ntam, Jianjun Dong, Bin Xu Alumina (Al$_{2}$O$_{3})$ is a well-known ceramic material. First-principles study of lattice thermal conductivity can assist our understanding in extreme conditions that are difficult to achieve experimentally, as well as analyze the fundamental difference between other materials. We combine density functional theory and the Peierls--Boltzmann transport theory to predict the temperature and pressure dependencies of lattice thermal conductivity of the corundum phase. We use a real space super cell method to extract second force constants and third order lattice anharmonicity tensors. These are then used to directly evaluate the phonon scattering rates due to lattice anharmonicity. Our preliminary results show that at a density of 4.23 g/cm$^{3}$ Al$_{2}$O$_{3}$ has thermal conductivities of 14.8Wm$^{-1}$K$^{-1}$ at 300K and 5.31Wm$^{-1}$K$^{-1}$ at 1000K. Moreover, we calculated the thermodynamic properties such as thermal expansion coefficient, bulk modulus and heat capacity, which are in excellent agreement with available measurements and previous theoretical calculations. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W20.00010: Thermoelectric properties of Ca$_{3}$Co$_{4}$O$_{9}$ thin film Robert Klie, Qiao Qiao, Ahmet Gulec, Tadas Paulauskas, Stanislaw Kolesnik, Bogdan Dabrowski, Cihat Boyraz, Mehmet Ozdemir, Dipanjan Mazumdar, Arun Gupta Thermoelectric oxides have attracted increasing attention due to their high thermal power and temperature stability. In particular, Ca$_{3}$Co$_{4}$O$_{9 }$(CCO), a misfit layered structure consisting of single layer hole-doped CoO$_{2}$ sandwiched between insulating Ca$_{2}$CoO$_{3}$ rocksalt layers, exhibits a high Seebeck coefficient at 1000 K. It was suggested that the Seebeck-coefficient can be further increased by growing doped thin films with controlled defects structures. This study combines pulsed layer deposition thin film synthesis of pristine CCO on several oxide substrates, as well as CCO thin films doped with Ti, Bi or La, with aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) to examine the effects of interfacial strain and doping on the atomic and electronic structures of CCO. The thermoelectric properties will be measured and correlated to the local changes in the atomic and electronic structures. We will further evaluate the role of CoO$_{2}$ stacking faults, as well as film thickness on the thermoelectric properties of CCO. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W20.00011: Improvement of the thermoelectric properties of substituted SrTiO$_{3}$ by synthesis conditions S. Kolesnik, S. Boona, B. Dabrowski, K. Swierczek, K. Wojciechowski We have studied thermoelectric properties of polycrystalline Sr$_{1-x}$La$_{x}$TiO$_{3}$ and SrTi$_{1-x}$Nb$_{x}$O$_{3}$ (x$<$=0.2) synthesized by a solid state synthesis method in a H$_{2}$/Ar atmosphere. The incorporation of La and Nb into the crystal structure was confirmed by x-ray diffraction and energy dispersive x-ray spectroscopy. By increasing the synthesis temperature (up to $\sim $1570$^{o}$C) and decreasing the partial pressure of oxygen, we were able to optimize the thermoelectric properties of the studied materials. The determined values of the thermoelectric figure of merit ZT$\sim $0.1 at 400 K and $\sim $0.3 at 800 K are comparable to those of single crystals of La- and Nb- substituted compounds. Our results show that the synthesis conditions play a crucial role in tailoring of the thermoelectric properties of substituted strontium titanates. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W20.00012: Thermoelectric Properties of a-InGaZnOx D.S. Williams, Stephan Piotrowski, B.E. White Primarily known as an active layer in thin film transistors, the electrical and thermal properties of a-InGaZnOx indicate promise as a thermoelectric material. In contrast to most phonon-blocking, electron-transmitting thermoelectric materials, a-InGaZnOx is a structurally amorphous material that maintains relatively high electron mobility (10-50 cm2/V-s) and optical transparency. Here we report on the electrical conductivity, thermal conductivity, and Seebeck coefficient of this material as a function of charge carrier concentration. Carrier concentration is modulated through thin film annealing in a reducing ambient. Room temperature thermal conductivity is found to be 0.35 W/m-K with a Seebeck coefficient of approximately 200$\mu $V/K. These data suggest room temperature thermoelectric figures of merit in the range of 0.1-0.3 are achievable with these materials, offering the possibility of transparent thermoelectric energy generation. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W20.00013: ABSTRACT WITHDRAWN |
Session W21: Novel Imaging Techniques and Calorimetry
Sponsoring Units: GIMSChair: Wilhelmus Geerts, Texas State University
Room: D161
Thursday, March 24, 2011 11:15AM - 11:27AM |
W21.00001: Particle distribution and dynamics in a complex fluid suspension studied by an image-analysis light-scattering technique Saad Algarni, H. Kashuri, Germano Iannacchione A relatively unique approach is described to analyze the scattered laser light from a complex fluid suspension for both static and dynamic behavior. Recent development of speckle analysis using CCD recorded direct imaging of the scattered coherent light has opened many new avenues for the application of static and dynamic light scattering experiments. The straightforward nature of this approach is somewhat offset by the constraints of the CCD chip size and placement to probe wide (or narrow) ranges of the wave vector. An alternative, and greatly simplified variation of this technique, is to convert the scattered light into diffuse scattering using a translucent screen placed at a desired location down beam then imaging the resulting pattern on the screen. A thru-beam stop and axis scales can be easily placed on the screen and recorded to improve the image quality and later analysis. One of many possible applications is the study of the particle (7nm diam aerosil SiO2 spheres) distribution and dynamics due to Brownian motion as well as sedimentation in a complex fluid (glycerol). [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W21.00002: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W21.00003: 3D Optical Field Mapping of a Focused Cylindrical Vector Beam Using Rayleigh Nanoparticles Liangcheng Zhou, Qiwen Zhan, H. Daniel Ou-Yang We report a novel method of mapping the optical field distribution of a focused cylindrical vector beam (CVB) using optically trapped Rayleigh nanoparticles. By using an ensemble method to measure the potential energy of nanoparticles in a CVB trap, optical trapping energy as low as 0.05 k$_{B}$T was measured. We demonstrated that the absolute intensity of a highly localized optical field is measured \textit{in situ} using low concentration of polystyrene nanoparticles sized at 48 nm acting as optical nanoprobes. Their collective behavior in the focal volume gives very accurate reading of the optical field distribution, which shows excellent consistency with numerical simulations. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W21.00004: High Resolution Micro-Optical Wall Shear Stress Sensor Based on Whispering Gallery Modes of Dielectric Microspheres Ulas Ayaz, Tindaro Ioppolo, Volkan Otugen We report the performance of a photonic wall shear stress sensor based on Whispering gallery mode (WGM) shifts of dielectric microsphere resonators. In particular, issues related to the sensitivity, resolution, frequency response and cross-axis sensitivity of the sensor are investigated experimentally. The sensor used in this prototype is a dielectric hollow microsphere made of Polydimethylsyloxane (PDMS). The wall shear stress acting on a sensing element of 125 $\mu $m diameter, is transmitted mechanically to the microsphere and the transmitted stress leads to shifts in the WGMs of the microsphere. By monitoring these WGM shifts, the magnitude as well as the direction of the wall shear stress are measured. Measurement resolutions better than 1 mPa have been observed. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W21.00005: An effect of probe current on ADF image intensity of Si crystal Suhyun Kim, Yoshifumi Oshima, Yasumasa Tanishiro, Kunio Takayanagi Annular dark field (ADF) scanning transmission electron microscope has been used to identify elements in the crystal. To analyze ADF image intensity more quantitatively, simulation of ADF image is required. However, the simulation has been known to overestimate the intensity because source size used in the simulation is assumed to be a point. Therefore, finite effective source size has been taken into an account by convolving Gaussian function to simulation. And, the Gaussian convolution has been usefully used to solve the mismatch in intensity, providing us a way of quantitative analysis for ADF image. Here, we quantitatively estimated an effective size of the cold field emission source. We obtained different Gaussian convolution size for ADF image acquired with various probe current by comparing ADF image contrast between experiment and simulation. As a result, we found that the effective source size which is needed for explaining contrast of ADF image decreased with decreasing probe current. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W21.00006: Fast and Computationally Efficient Boundary Detection Technique for Medical Images Arpita Das, Partha Goswami, Susanta Sen Detection of edge is a fundamental procedure of image processing. Many edge detection algorithms have been developed based on computation of the intensity gradient. In medical images, boundaries of the objects are vague for gradual change of intensities. Therefore need exists to develop a computationally efficient and accurate edge detection approach. We have presented such algorithm using modified global threshold technique. In our work, the boundaries are highlighted from the background by selecting a threshold (\textbf{T}) that separates object and background. In the image, where object to background or vice-verse transition occurs, pixel intensity either rises greater or equal to \textbf{T} (background to object transition) or falls less than \textbf{T} (object to background). We have marked these transition regions as object boundary and enhanced the corresponding intensity. The value of \textbf{T} may be specified heuristically or by following specific algorithm. Conventional global threshold algorithm computes the value of \textbf{T} automatically. But this approach is not computationally efficient and required a large memory. In this study, we have proposed a parameter for which computation of \textbf{T} is very easy and fast. We have also proved that a fixed size memory [256$\times $4Byte] is enough to compute this algorithm. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W21.00007: Integrated instrument for holographic optical trapping and multicolor holographic video microscopy Bhaskar Jyoti Krishnatreya, David G. Grier We designed and constructed an integrated holographic materials characterization and processing workstation that combines dynamical holographic optical trapping with multicolor holographic video microscopy. Unlike previously described systems, which are based on conventional light microscopes, our holographic workstation features a rigid, compact, adaptable, and modular design that can be replicated easily using standard off-the-shelf optical components. We demonstrate enhanced efficiency in micro-manipulation of colloidal materials using our instrument. By illuminating these colloidal particles with multiple laser wavelengths concurrently and analyzing the multicolor holograms independently for each color, we can acquire complementary information about the particles' size, position, and composition, and also gain insights into their material-dependent properties. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W21.00008: Graded Index Lens Design for Aqueous Applications Theodore Martin, Michael Nicholas, Gregory Orris, Liang-Wu Cai, Daniel Torrent, Jose Sanchez-Dehesa A graded refractive index (GRIN) provides a means of controlling wave propagation within the bulk of a material without relying on curved interfaces between dissimilar materials. We report the design and experimental testing of a GRIN metamaterial that behaves as a lens for acoustic waves in water. The graded index is achieved using a regular array of cylindrical scatterers with an anisotropic distribution of sizes. The metamaterial lens operates at sonic frequencies and has flat interfaces. We demonstrate that this metamaterial design focuses acoustic signals in the same manner as an ideal optical lens. By comparing with calculations using multiple scattering theory and finite difference time domain methods we show that the elastic properties of the scatterers are important for achieving an accurate model of the transmitted signal. We consider perturbations of the metamaterial design and their impact on the transmission. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W21.00009: WASTED at work: the Webcam Alpha Spectrometer Experiment Demonstrator Arthur Pallone, Nicole Newton Ion beam analysis (IBA) methods are commonly used to determine the composition or other properties of samples. The scanning of micrometer diameter and smaller ion beams across sample surfaces produces spatial distribution maps of those properties for the samples. Electron microscope providers offer radiation-hardened CMOS camera options to directly image sample areas as a complement to scanning the electron beam. A modified webcam operated in the radiographic mode of transmission ion microscopy (TIM), with alpha particles in place of electrons, has been shown to be an effective low-cost alternative to that camera for TIM. IBA under ambient pressure is still not commonly practiced. Even less common is the use of a radioactive source of ions, such as Po-210, in IBA. The synthesis of these three ideas -- the direct imaging by a modified webcam of Po-210 alpha particles that first pass through a sample under ambient pressure -- is explored with the Webcam Alpha Spectrometer Experiment Demonstrator (WASTED). A description of the experiment, first results, conclusions and future work will be presented. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W21.00010: Fourier phase contrast microscopy technique for real time imaging of phase and fluorescence features simultaneously Chandra Yellleswarapu, Alexey Veraksa, Bhargab Das, Devulapalli Rao Understanding of biological cell response is facilitated by microscopy techniques, but has been limited by our ability to image cell structure and function at the same time. Current procedure is to obtain separate images, such as phase and fluorescence features, using different imaging techniques one after the other and digitally register the resulting images together. Mostly this procedure requires switching between the light sources and the associated optical paths, making it difficult to image biological events at short time scales. Recently we developed a novel optical Fourier phase contrast microscopy technique for real time display of phase and fluorescence features of biological specimens at the same time. We were able to image the brightfield+fluorescence, phase+fluorescence, and edge enhanced+fluorescence features of the \textit{Drosophila} embryo at once without the need for digital image registration and fusion. This comprehensive microscope has the capability of simultaneously providing both structural and functional information. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W21.00011: Atomic Resolution imaging with non-contact Atomic Force Microscope (nc-AFM) in a closed liquid cell Umit Celik, Demet Catcat, H. Ozgur Ozer, Ahmet Oral We have designed a non-contact Atomic Force Microscope, which can achieve true atomic resolution in a closed liquid cell, which does not suffer from evaporation of fluids during imaging. We have designed a closed liquid cell, where we can flow the fluid using a syringe or a peristaltic pump. The AFM cantilever holder was designed to eliminate acoustical resonances. We can obtain resonance curves without spurious acoustic peaks in liquid using a piezoactuator. We have used an RF modulated 635 nm low noise diode laser. RF modulation is effective to reduce the optical feedback noise and the optical interference noise. Deflection noise density of designed system is $\sim $20 fm/$\surd $Hz in air and $\sim $25 fm/$\surd $Hz in water. The observed frequency noise at the PLL output was $\sim $1Hz$_{pp}$ in water using a 300kHz, 32N/m cantilever oscillated at 0.9nm amplitude with Q$\sim $11. Force sensitivity of our system is demonstrated by imaging cleaved mica surface in water environment with atomic resolution. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W21.00012: Scanning Tunneling Microscope Study of Atomic Steps in Gold Films on Muscovite Mica Olesya Sitnikova, Ramesh Mani We are carrying Scanning Tunneling Microscope (STM) studies of gold on Muscovite mica to observe the atomic step migration and test the functionality of our STM. The experiment was performed by using a RHK ultra-high-vacuum STM. The gold sample was prepared in low pressure vacuum chamber, and the films were grown at constant rate of 0.1 nm/sec to the desired thickness. After deposition, the films were thermally annealed to produce flat terraces which are being studied. The migration of atomic steps over time is being observed and the results are presented. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W21.00013: X-Ray Fluorescence Imaging of Ancient Artifacts Robert Thorne, Ethan Geil, Kathryn Hudson, Charles Crowther Many archaeological artifacts feature inscribed and/or painted text or figures which, through erosion and aging, have become difficult or impossible to read with conventional methods. Often, however, the pigments in paints contain metallic elements, and traces may remain even after visible markings are gone. A promising non-destructive technique for revealing these remnants is X-ray fluorescence (XRF) imaging, in which a tightly focused beam of monochromatic synchrotron radiation is raster scanned across a sample. At each pixel, an energy-dispersive detector records a fluorescence spectrum, which is then analyzed to determine element concentrations. In this way, a map of various elements is made across a region of interest. We have succesfully XRF imaged ancient Greek, Roman, and Mayan artifacts, and in many cases, the element maps have revealed significant new information, including previously invisible painted lines and traces of iron from tools used to carve stone tablets. X-ray imaging can be used to determine an object's provenance, including the region where it was produced and whether it is authentic or a copy. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W21.00014: Measuring the Imaginary Part of the Permittivity Using Calorimetry Hektor Kashuri, Krishna Sigdel, Klaida Kashuri, Germano S. Iannacchione Modulated or AC calorimetry is a well established technique for measuring the temperature dependence of the heat capacity of many complex fluids. Employing a dielectric or RF heating method, the heat capacity, thermal conductivity, and the dielectric properties of the sample are all probed simultaneously. Combining the results obtained by this technique for the liquid crystal 4-n-pentyl-4-cyanophenyl (5CB) with those obtained by our novel AC calorimetric technique employing RF (dielectric) heating, we have been able to directly measure the temperature dependence of the imaginary part of the permittivity of this liquid crystal. Measurements were performed over a temperature range from 303 to 313 K, spanning the nematic to isotropic phase transition, as well as radio frequencies from 10 to 30 MHz [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W21.00015: Calorimetry of epitaxial thin films Frances Hellman, David Cooke, James Groves, Bruce Clemens Thin film growth allows for the manipulation of material on the nanoscale, allowing for the creation of metastable phases not seen in the bulk. Heat capacity provides a direct way of measuring thermodynamic properties of these new materials, but traditional bulk calorimetric techniques are inappropriate for such a small amount of material. Micro- and nanocalorimetry techniques exist for the measurements of thin films but rely on an amorphous membrane platform, limiting the types of films which can be measured. In this work, ion-beam-assisted deposition is used to provide a biaxially-oriented MgO template on a suspended membrane microcalorimeter. Synchrotron X-ray diffraction was used to successfully assess the biaxial order of the MgO template. X-ray diffraction was also used to prove the high level of epitaxy of a film grown onto this MgO template. The contribution of the MgO layer to the technique will be discussed. An Fe$_{.49}$Rh$_{.51}$ film grown epitaxially onto the device was measured, comparing favorably to literature data on bulk crystals. This shows the viability of the MgO microcalorimeter as a way of measuring the thermodynamic properties of epitaxial thin films. [Preview Abstract] |
Session W22: Magnetic Phase Transitions II
Sponsoring Units: DCMPChair: Thomas Vojta, Missouri University of Science and Technology
Room: D163
Thursday, March 24, 2011 11:15AM - 11:27AM |
W22.00001: Bound states and E8 symmetry effects in perturbed quantum Ising chains Jonas Kjall, Frank Pollmann, Joel Moore In a recent experiment on $\mathrm{CoNb}_2\mathrm{O}_6$, Coldea et al. found for the first time experimental evidence of the exceptional Lie algebra $E_8$. The emergence of this symmetry was theoretically predicted long ago for the transverse quantum Ising chain in the presence of a weak longitudinal field. We consider an accurate microscopic model of $\mathrm{CoNb}_2\mathrm{O}_6$ incorporating additional couplings and calculate numerically the dynamical structure function using a recently developed matrix-product-state method. We compare the signatures of this model to those found in the transverse Ising chain in a longitudinal field and to experimental data, with focus on how far the effects of integrability extends and how robust they are to the additional interactions. The excitation spectra show bound states characteristic of the weakly broken $E_8$ symmetry and a bound state continuum carrying spectral weight comparable to the higher bound states. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W22.00002: Comparison between two simple models for the magnetoelectric interaction in multiferroics G.E. Barberis, C.J. Calderon Filho We developed numerical calculations to simulate the magnetoelectric coupling in multiferroic compounds, using the Monte Carlo technique. Two simple models were used to simulate the compounds. In the first one, the magnetic ions are represented by a spin 1/2 2D\ Ising lattice of ions, and the electric lattice by classical moments, coupled one to one with the magnetic moments. The coupling between both lattices allows to the leading lattice, that is, the magnetic one, to change the orientation of the electrical dipoles in one direction perpendicular to the magnetic dipoles. This direction was chosen to accomplish the symmetry requirements of the magnetoelectric effect. In the second case, the magnetic lattice is also a 2D Ising lattice, but the electric momenta are in a lattice that also behaves as an Ising lattice, perpendicular to the magnetic moments. In this case, the one-to-one coupling of the electric and magnetic momenta is represented by a two-valued energy parameter, allowing the possibility of independent transition temperatures for both lattices. Both models contain three independent parameters. We studied the physical properties obtained with both models, as functions of the ratio of the three parameters. The results in both cases allowed us to compare changes in the physics of the models, and with the physics of compounds measured experimentally. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W22.00003: The itinerant ferromagnetic phase of the Hubbard model Giuseppe Carleo, Saverio Moroni, Federico Becca, Stefano Baroni Using a newly developed quantum Monte Carlo technique, we provide strong evidence for the stability of a saturated ferromagnetic phase in the high-density regime of the two-dimensional infinite-U Hubbard model. By decreasing the electron density, a discontinuous transition to a paramagnetic phase is observed, accompanied by a divergence of the susceptibility on the paramagnetic side. This behavior, resulting from a high degeneracy among different spin sectors, is consistent with an infinite-order phase transition. The remarkable stability of itinerant ferromagnetism renews the hope to describe this phenomenon within a purely kinetic mechanism and will facilitate the validation of experimental quantum simulators with cold atoms loaded in optical lattices. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W22.00004: Two species Bosonic Hubbard model in a two-dimensional optical lattice Kalani Hettiarachchilage, Valy Rousseau, Juana Moreno, Mark Jarrell We study a two-component hardcore bosonic Hubbard model in a two- dimensional optical lattice by performing Quantum Monte Carlo (QMC) simulations. Our model contains a repulsive interspecies interaction between the two species of bosons and a hopping term between nearest neighbors. The phase diagram shows magnetic orderings, insulatings and superfluid phases as a function of doping for balanced populations. We predict the appearance of a first order phase transition from an antiferromagnetic phase to a superfluid phase near half filling. A phase transition from superfluid to an exotic phase occurs away from half filling at very low temperature. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W22.00005: An Anomalously Elastic, Intermediate Phase in Randomly Layered Planar Magnets, Superfluids, and Superconductors Thomas Vojta, Paul Goldbart, Priyanka Mohan, Rajesh Narayanan, John Toner We show that layered quenched randomness in planar magnets leads to an unusual intermediate phase between the conventional ferromagnetic low-temperature and paramagnetic high-temperature phases. In this intermediate phase, which is part of the Griffiths region, the spin-wave stiffness perpendicular to the random layers displays anomalous scaling behavior, with a continuously variable anomalous exponent, while the magnetization and the stiffness parallel to the layers both remain finite. Analogous results hold for superfluids and superconductors. We study the two phase transitions into the anomalous elastic phase, and we discuss the universality of these results, and implications of finite sample size as well as possible experiments. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W22.00006: Quantum fidelity in the thermodynamic limit Marek Rams, Bogdan Damski A quantum phase transition happens when dramatic changes in the ground state properties of a quantum system can be induced by a tiny variation of an external parameter (e.g., a magnetic field in spin systems). Quantum fidelity -- the overlap between two ground states calculated at slightly different values of the external parameter -- provides the most basic probe into the dramatic change of the wave-function. In this talk I will discuss quantum fidelity focusing on thermodynamic regime. I will present novel analytical results for quantum fidelity of the Ising chain, a paradigmatic model of quantum phase transitions, and discuss a theory extending these findings to systems characterized by other universality classes. In particular, I will show how quantum fidelity approaches a non-analytic limit, quantify how the Anderson catastrophe takes place in quantum critical systems, and discuss scaling properties of quantum fidelity when it cannot be approximated by the popular fidelity susceptibility approach. This approach provides a promising way of characterizing quantum phase transition in strongly correlated systems. The work is summarized in M.M. Rams, B. Damski, arXiv:1010:1048 [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W22.00007: Finite-Temperature Fidelity Susceptibility for One-Dimensional Quantum Systems Jesko Sirker We calculate the fidelity susceptibility $\chi_f$ for the Luttinger model and show that there is a universal contribution linear in temperature $T$ (or inverse length $1/L$) by using conformal field theory. Furthermore, we develop an algorithm - based on a lattice path integral approach - to calculate the fidelity $F(T)$ in the thermodynamic limit for one-dimensional quantum systems. We check the Luttinger model predictions by calculating $\chi_f(T)$ analytically for free spinless fermions and numerically for the $XXZ$ chain. Finally, we study $\chi_f$ at the two phase transitions in this model.\\*[0.2cm] J. Sirker, PRL {\bf 105}, 117203 (2010) [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W22.00008: Thermodynamics of itinerant metamagnetic transitions Andrew Berridge Itinerant metamagnetic transitions may be driven by features in the electronic density of states. These features produce signatures in the entropy and specific heat near to the transition. We study these signatures for a variety of different cases, identifying the key features which differ from naive expectations, such as enhanced critical fields and `non-Fermi liquid' temperature dependencies above the transition. We will consider the generic case of a logarithmically divergent density of states, as caused by a van Hove singularity in 2D. We also study a specific model for the bandstructure of Sr$_3$Ru$_2$O$_7$, a material with a well-studied metamagnetic transition and quantum critical endpoint. We consider how far the behaviour of the system can be explained by the density of states rather than quantum fluctuations, and the distinctive features of this mechanism. The most intriguing feature of Sr$_3$Ru$_2$O$_7$ is an unusual phase with a higher entropy than its surroundings, we consider how this may arise in the context of a density of states picture and find that we can reproduce the observed thermodynamic behaviour and first-order phase transitions. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W22.00009: Low-energy behavior of the generalized Golden chain at an integrable point Paata Kakashvili, Eddy Ardonne Recently, properties of collective states of interacting non-Abelian anyons have attracted a considerable attention. In particular, it has been shown to that the generalization of the Golden chain, a chain of interacting Fibonacci anyons, has a rich phase diagram with various critical and gapped phases. In additions, several integrable points have also been studied. We identify a new integrable point in the parameter space of the model and diagonalize the Hamiltonian exactly using the Bethe Ansatz method. To describe the corresponding low-energy conformal field theory, we perform the finite-size analysis to calculate the central charge and critical exponents. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W22.00010: Tensor renormalization group: Local magnetizations, correlation functions, and phase diagrams of systems with quenched randomness Can G\"uven, Michael Hinczewski, A. Nihat Berker The tensor renormalization-group method, developed by Levin and Nave, brings systematic improvability to the position-space renormalization-group method and yields essentially exact results for phase diagrams and entire thermodynamic functions. The method, previously used on systems with no quenched randomness, is extended in this study to systems with quenched randomness [1]. Local magnetizations and correlation functions as a function of spin separation are calculated as tensor products subject to renormalization-group transformation. Phase diagrams are extracted from the long-distance behavior of the correlation functions. The approach is illustrated with the quenched bond-diluted Ising model on the triangular lattice. An accurate phase diagram is obtained in temperature and bond-dilution probability for the entire temperature range down to the percolation threshold at zero temperature.\\[4pt] [1] C. G\"uven, M. Hinczewski, and A.N. Berker, Phys. Rev. E 82, 051110 (2010). [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W22.00011: Nature of Fermi Systems near l=0 Pomeranchuk Instability: A Tractable Crossing Symmetric Equation Approach Kelly Reidy, Khandker Quader, Kevin Bedell In Fermi liquids, a Pomeranchuk instability occurs when one of the Landau parameters $F^{a,s}_{\ell} \rightarrow -(2\ell+1)$. The Pomeranchuk instabilities at $F^{a,s}_0 = -1$ are related to respectively to a ferromagnetic transition ($a$), and to a density wave or charge instability resulting in phase separation ($s$). We use the tractable crossing symmetric equations (TSCE) method to explore the nature of quantum fluctuations, excitations and pairing in a 3D Fermi system, around these points. We obtain interesting limiting results at zero and finite momentum (q), and in the limits of large and small coupling strengths. We develop methods to deal with a set of finite-q singularities in the competing quantum fluctuation terms contained in TSCE; these may have physical significance. Using graphical and numerical methods to solve coupled non-linear integral equations that arise in the TSCE scheme, we obtain results for the behavior of spin and density excitations, and pairing properties around the instability points. Our results may have relevance to ferromagnetic superconductors. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W22.00012: Microscopic model for the Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ Ruddlesden-Popper series of materials Jean-Michel Carter, Hae-Young Kee The Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ family of materials displays an insulator to metal transition as the number of layers (n) increases. The presence of large spin-orbit coupling is believed to be a significant ingredient for the novel J$_{eff}= 1/2$ state found in Sr$_{2}$IrO$_{4}$. We offer a microscopic tight-binding Hamiltonian with spin-orbit coupling and Hubbard interactions, and compare our results with experimentally observed phases. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W22.00013: Protecting clean critical points by local disorder correlations J.A. Hoyos, Nicolas Laflorencie, Andr\'e Vieira, Thomas Vojta We show that a broad class of quantum critical points can be stable against locally correlated disorder even if they are unstable against uncorrelated disorder. Although this result seemingly contradicts the Harris criterion, it follows naturally from the absence of a random-mass term in the associated order-parameter field theory. We illustrate the general concept with explicit calculations for quantum spin-chain models. Instead of the infinite-randomness physics induced by uncorrelated disorder, we find that weak locally correlated disorder is irrelevant. For larger disorder, we find a line of critical points with unusual properties such as an increase of the entanglement entropy with the disorder strength. We also propose experimental realizations in the context of quantum magnetism and cold-atom physics. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W22.00014: Quantum antiferromagnet on a Bethe lattice at percolation I. Low-energy states, DMRG, and diagnostics Hitesh Changlani, Shivam Ghosh, C.L. Henley We investigate ground and excited state properties of randomly diluted spin-1/2, exchange-coupled Heisenberg antiferromagnets on the Bethe lattice with coordination 3. In the case of square lattice percolation clusters, previous Quantum Monte Carlo (QMC) calculations [1] found that the singlet-triplet gaps scaled ``anomalously,'' being much smaller than the $1/N$ scaling expected from the tower of ``quantum rotor'' states (due to $E=M^2/2N\chi$). The low energies were attributed to the interaction of distant ``dangling spins,'' forced by the local imbalance of even and odd sites. In the present study we further study this effect on the Bethe lattice, using Exact Diagonalization and density-matrix RG. (DMRG applies naturally since the Bethe lattice lacks loops). We introduce inter-site correlation and susceptibility matrices as diagnostics to identify the spatial locations of the low-energy degrees of freedom, and to understand interactions between them. These matrices have been computed within the harmonic spin-wave theory, in order to highlight the deviations seen in the spin-1/2 system. In addition to the above, we propose a simple effective Hamiltonian which explains the magnitude of the singlet-triplet gap. \\[4pt] [1] L. Wang and A. Sandvik, Phys. Rev. B 81, 054417 (2010). [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W22.00015: Quantum antiferromagnet on a Bethe lattice at percolation II.Effective Hamiltonian for dangling spins Shivam Ghosh, Hitesh Changlani, Sumiran Pujari, C.L. Henley The lowest energy excitations of spin 1/2 Heisenberg antiferromagnets on percolation clusters (about the Neel ordered state) were believed to be ``quantum rotor states'' scaling with cluster size as 1/N, until Wang and Sandvik [Wang et al, Phys. Rev. B 81, 054417 (2010)] discovered a class of states in the diluted square lattice that had even lower energies and had a different finite size scaling of the gap exponent. They conjectured these anomalous states were due to local even/odd sublattice imbalances, leading to emergent local moments called ``dangling spins'' that interact over large distances, mediated through intervening spins. We have pursued this question on the z=3 Bethe lattice at the percolation threshold. Exact diagonalization shows, forevery cluster, a split-off group of low-energy states having the same quantum numbers as can be made using the dangling spins. We identify these with the Wang-Sandvik anomalous states and model their energies using an effective pair Hamiltonian coupling the ``dangling spins.'' The couplings are a function of separation and geometry; the parameters are solved by fitting to a database of different clusters.The separation dependence of these interactions can be related to the gap scaling with N. We will also compare the effective Hamiltonian predictions to the intersite susceptibility matrix of each cluster. [Preview Abstract] |
Session W23: Superconductivity: Mesoscopic and Nanometer Scale Phenomena
Sponsoring Units: DCMPChair: Myron Salamon, University of Texas at Dallas
Room: D165
Thursday, March 24, 2011 11:15AM - 11:27AM |
W23.00001: Fluxoid Quantization in Superconducting Al Nano-Rings Stephen Snyder, Allen Goldman The Little-Parks experiment on superconducting cylinders is an important demonstration of fluxoid quantization in superconductors. The transition temperature oscillations in magnetic field have a period of $h/2e$ for the micro cylinders in their studies, which was further evidence for Cooper paring at the time {[}W. A. Little, R. D. Parks, PRL 1964, 9, 9{]}. However recent theoretical works have suggested that in superconducting loops smaller than the coherence length this period changes from $h/2e$ to $h/e$, for details see {[}F. Loder, et al. PRB, 2008, 78, 174526{]} and references therein. We present experimental work in an effort to achieve this limit with Al nano-rings prepared by electron beam lithography. The rings presented here are smaller than others reported in the literature by as much as a factor of two or three {[}H. Wang, et al. PRB, 2007, 75, 064509{]}. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W23.00002: Destructive regime in Al loops prepared by e-beam lithography Neal Staley, Ying Liu For doubly connected superconductors, cylinders, loops, or rings, the fundamental fluxoid quantization leads to oscillations in the superconducting transition temperature (T$_c$) with the applied flux. This is known as the Little-Parks effect. For sufficiently small loops, with a circumference smaller than $\pi$$\xi$(0), where $\xi$(0) is the zero temperature coherence length, superconductivity is completely destroyed near the half-flux quanta. This ``destructive regime" emerges because of the competition between the kinetic energy carried by the supercurrent, and the condensation energy of the system. Theoretically, it has been shown that adding a tail to the loop can increase the condensation energy and possibly eliminate the destructive regime. We present electrical transport measurements on Al loops defined by e-beam lithography with a size comparable to $\xi$(0). The loops were varied to have different condensation energies by adding explicit tails, and by lengthening and shortening the measurement leads. We observed strongly enhanced Little-Parks oscillations due to the reduction of the sample size, and the transition to the destructive regime when the size of sample was further reduced. These experimental results will be examined in the context of the competition between the kinetic and condensed energies. Work supported by NSF. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W23.00003: Superconducting nanowires as nonlinear inductive elements for qubits Jaseung Ku, Vladimir Manucharyan, Alexey Bezryadin We report microwave transmission measurements of superconducting Fabry-Perot resonators, having a superconducting nanowire placed at a supercurrent antinode. As the plasma oscillation is excited, the supercurrent is forced to flow through the nanowire. The microwave transmission of the resonator-nanowire device shows a nonlinear resonance behavior, significantly dependent on the amplitude of the supercurrent oscillation. We show that such amplitude-dependent response is due to the nonlinearity of the current-phase relationship of the nanowire. The results are explained within a nonlinear oscillator model of the Duffing oscillator, in which the nanowire acts as a purely inductive element, in the limit of low temperatures and low amplitudes. The low-quality factor sample exhibits a ``crater'' at the resonance peak at higher driving power, which is due to dissipation. We observe a hysteretic bifurcation behavior of the transmission response to frequency sweep in a sample with a higher quality factor. The Duffing model is used to explain the Duffing bistability diagram. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W23.00004: Electrical transport properties of topological insulator Bi2Te3 nanowires contacted with superconducting electrodes Mingliang Tian, Jian Wang, Meenakshi Singh, Moses Chan Single-crystal Bi2Te3 nanowires were fabricated by template-assisted electrochemical deposition. The electrical transport properties of the nanowires in the temperature range 1.8-300 K were investigated by connecting nonsuperconducting or superconducting electrodes. When the wire was connected to focused-ions beam deposited W-electrodes, a series of exotic quasi-periodic oscillations were found and the amplitude of the oscillations was unusually enhanced near 3.5 K below the Tc, 4.5 K, of W-electrodes. When the wire was connected to nonsuperconducting Pt electrodes, the wire showed positive magnetoresistance accompanied with random fluctuations. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W23.00005: Observation of Little-Parks Oscillations of the Kinetic Inductance at Low Temperatures Using a GHz Resonator with Two Parallel Superconducting Nanowires Andrey Belkin, Matthew Brenner, Thomas Aref, Jaseung Ku, Alexey Bezryadin Little-Parks (LP) effect manifests the phenomenon of the fluxoid quantization in doubly connected superconductors. Usually it is observed at high temperatures, i.e. slightly below the critical temperature (Tc). We demonstrated that a thin-film Fabry-Perot superconducting resonator with a pair of nanowires inserted at the point of supercurrent antinode can be used to reveal LP effect even at temperatures much lower than Tc. As magnetic field (H) is applied, the Meissner current develops, changing the kinetic inductance of the wires and, correspondingly, the resonance frequency of the resonator and its transmission S21 measured at the fixed frequency. The periodicity of the LP effect is revealed as a periodic set of distorted parabolas S21(H) corresponding to the states with different vorticities. The transition from one state to another corresponds to a Little's phase slip. We suggest a theoretical explanation to the shape of the observed parabolas. We also report a statistical analysis of the jumps between the parabolas. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W23.00006: Spin Resonance and dc Current Generation in a Quantum Wire Peng Zhou, Artem Abanov, Wayne Saslow, Valery Pokrovsky We show that in a quantum wire the spin-orbit interaction leads to a narrow spin resonance at low temperatures, even in the absence of an external magnetic field. A relatively weak dc magnetic field of a definite direction strongly increases the resonance absorption. Linearly polarized resonance radiation produces dynamic magnetization as well as electric and spin currents. The effect strongly depends on the external magnetic field. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W23.00007: Large oscillations of the magnetoresistance in nano-patterned high-temperature superconducting films Ilya Sochnikov, Avner Shaulov, Yosef Yeshurun, Gennady Logvenov, Ivan Bozovic Measurements on nano-scale structures made of high-temperature superconductors are expected to shed light on the origin of superconductivity in these materials. The size of loops made of these compounds was so far limited to the submicron scale. We report the results of measurements on loops of La$_{1.84}$Sr$_{0.16}$CuO$_{4}$, with dimensions down to tens of nanometers. We observe oscillations in the loops resistance as a function of the magnetic flux through the loops. The oscillations have a period of $h/2e$ and their amplitude is much larger than the amplitude of resistance oscillations expected from the Little-Parks effect [1-2]. Unlike the Little-Parks oscillations, caused by periodic changes in the superconducting transition temperature, the oscillations we observe are caused by periodic changes in the interaction between thermally-excited moving vortices and the oscillating persistent current induced in the loops. Despite the enhanced amplitude of these oscillations, we have not detected oscillations with a period of $h/e$, as recently predicted for nanoscale loops of superconductors with d-wave symmetry, or with a period of $h/4e$, as predicted for superconductors that exhibit stripes. [1] I. Sochnikov \textit{et al.}, Nature Nano. \textbf{5}, 516 (2010). [2] I. Sochnikov \textit{et al}., PRB\textbf{ 82,} 094513 (2010). [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W23.00008: Penetration Depth of Superconductors with Random Superfluid Density Thomas Lippman, Kathryn Moler Electronic inhomogeneity is inevitable in doped superconductors, due simply to the random nature of the doping process. We argue that when the superconducting coherence length is not much larger than the lattice scale, this creates spatial randomness in the superconducting properties. In particular, we expect the superfluid density to be stochastic, which modifies the measured diamagnetic response. We approximate the London equation with random superfluid density as a modified equation for the disorder averaged field response, and speculate on implications for the interpretation of measurements of the penetration depth. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W23.00009: MBE Growth for Qubit Devices Anthony Megrant, Alex Kozhanov, Ludwig Feigl, Martin Weides, Jian Zhao, Yi Yin, James Wenner, Andrew Cleland, Chris Palmstrom, John Martinis A major component of the UCSB research program is to find and understand proper growth conditions and procedures for epitaxial Re and sapphire thin films. Separately, we are taking what we have learned from this research and applying it to develop growth-optimized procedures in our dedicated MBE chamber for qubit devices. I will report on important parameters that need to be adjusted to obtain optimum growth, some of which include: temperature, deposition rate and surface preparation. Measurement tools such as in-situ RHEED and ex-situ XRD and AFM are used to characterize the quality of the films, as well as the fabrication of resonators to measure the quality factor. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W23.00010: Single Electron Turnstiles: Improving Performance for a Quantum Metrological Current Standard Thomas Aref, Ville Maisi, Olli-Pentti Saira, Antti Kemppinen, Jukka Pekola A top priority in metrology is to develop measurement standards that are based on fundamental physical constants. Although such standards exist for resistance and voltage (the quantum Hall effect and the Josephson effect respectively), no such standard exists for the unit of current, the ampere, at the present time. The best candidate for a quantum metrological standard for current is a single electron turnstile. Such turnstiles allow electrons through one at a time. When operated at a specific frequency, they produce a proportional current traceable to the single electron charge. Our turnstiles are based on a normal metal copper island contacted through insulating barriers by superconducting aluminum electrodes. By engineering improvements such as on-chip filtering and increased charging energy, we have improved our turnstiles to suppress both first order leakage and second order Andreev currents thus approaching the required accuracy for a new quantum metrological current standard. This current standard would allow the closing of the quantum metrological triangle created by voltage, resistance and current. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W23.00011: Tunneling spectroscopy of the Andreev Bound States in a Carbon Nanotube Jean-Damien Pillet, Charis Quay, Cristina Bena, Alfredo Levy Yeyati, Philippe Joyez Carbon nanotubes are not intrinsically superconducting but they can carry a supercurrent when connected to superconducting electrodes. This supercurrent is mainly transmitted by discrete entangled (electron-hole) states confined to the nanotube, called Andreev Bound States. These states are a key concept in mesoscopic superconductivity as they provide a universal description of Josephson-like effects in quantum-coherent nanostructures (e.g. molecules, nanowires, magnetic or normal metallic layers) connected to superconducting leads. We report here the first tunneling spectroscopy of individually resolved ABS, in a nanotube-superconductor device. Analyzing the evolution of the ABS spectrum with a gate voltage, we show that the ABS arise from the discrete electronic levels of the molecule and that they reveal detailed information about the energies of these levels, their relative spin orientation and the coupling to the leads. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W23.00012: Quantum impurity model for microwave photons Moshe Goldstein, Michel Devoret, Manuel Houzet, Leonid Glazman We consider propagation of microwave photons along an array of superconducting grains with a set of weakly-coupled grains at its center. Quantum fluctuations of charge on the weakly- coupled grains make the process of ``photon splitting'' effective. In such a process, an incoming photon may be split into a number of photons of lower energy. The minimal number of photons created in such process depends on the symmetry properties of the corresponding quantum impurity model. As an example, we consider a specific circuit allowing quantum fluctuations between two charge configurations of two weakly- coupled grains, thus mimicking the behavior of an anisotropic Kondo impurity. Both ferromagnetic and antiferromagnetic Kondo regimes may be reached this way. We relate the rate of conversion of the incoming photons into the lower-energy ones to the dynamic spin susceptibility of the Kondo model. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W23.00013: Elliptic billiard is a nontrivial integrable system Tao Ma, Rostislav Serota We investigate the semiclassical energy spectrum of quantum elliptic billiards. The nearest neighbor spacing distribution, level number variance and spectral rigidity support the notion that the elliptic billiard is a generic integrable system. A classical simulation shows that all periodic orbits, except two, are not isolated. From Fourier analysis of the spectrum, all peaks correspond to periodic orbits. The two isolated periodic orbits have relatively small contribution to the fluctuations of the level density as compared to non-isolated periodic orbits. We argue that elliptic billiard is a nontrivial classically integrable system that enables us to gain new insights into their properties. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W23.00014: Measurement of the statistical properties of the persistent current in normal metal rings Manuel Castellanos Beltran, Will Shanks, Dustin Ngo, Ania Bleszynski-Jayich, Jack Harris A striking manifestation of quantum mechanics at the mesoscopic scale is the existence of an equilibrium persistent current in normal metal rings threaded by a magnetic flux. A theory of non-interacting diffusive electrons predicts that the amplitude of these currents is a stochastic function of the disorder profile of the specific ring. Thus the persistent current is different from sample to sample, with a Gaussian distribution. Due to the difficulty of measuring these currents, experiments to determine the form of the persistent current distribution had not yet been performed. However, our group recently developed a technique for measuring persistent currents in normal metal rings with high SNR, low measurement back-action, excellent background rejection, and over a large range of magnetic fields. We have measured a total of roughly 100 independent realizations of persistent current amplitudes in single rings. Within the statistical limits of our data, we corroborate that the first five cumulants are consistent with a Gaussian distribution. As a further test of the higher-order statistical properties of the persistent current, we also show that the quadrature amplitudes of the current's Aharonov-Bohm oscillations are uncorrelated. [Preview Abstract] |
Session W24: Density Functional Theory I
Sponsoring Units: DCOMPChair: Alexey Zayak, Lawrence Berkeley National Laboratory
Room: D167
Thursday, March 24, 2011 11:15AM - 11:27AM |
W24.00001: A conventional, massively parallel eigensolver for electronic structure theory V. Blum, M. Scheffler, R. Johanni, H. Lederer, Th. Auckenthaler, Th. Huckle, H.-J. Bungartz, L. Kr\"amer, P. Willems, B. Lang, V. Havu We demonstrate a robust large-scale, massively parallel conventional eigensolver for first-principles theory of molecules and materials. Despite much research into $O(N)$ methods, standard approaches (Kohn-Sham or Hartree-Fock theory and excited-state formalisms) must still rely on conventional but robust $O(N^3)$ solvers for many system classes, most notably metals. Our eigensolver overcomes especially parallel scalability limitations, where standard implementations of certain steps (reduction to tridiagonal form, solution of reduced tridiagonal eigenproblem) can be a serious bottleneck already for a few hundred CPUs. We demonstrate scalable implementations of these and all other steps of the full generalized eigenvalue problem. Our largest example is a production run with 1046 Pt (heavy-metal) atoms [1] with converged all-electron accuracy in the numeric atom-centered orbital code FHI-aims,[2] but the implementation is generic and should easily be portable to other codes. [1] P. Havu \emph{et al.}, Phys. Rev. B \textbf{82}, 161418 (2010). [2] V. Blum \emph{et al.}, Comp. Phys. Comm. \textbf{180}, 2175 (2009). [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W24.00002: GPAW on Blue Gene/P Nichols Romero, Jussi Enkovaara, Marcin Dulak, Christian Glinsvad, Ask Larsen, Jens Mortensen, Sameer Shende, Vitali Morozov, Jeffrey Greeley Density function theory (DFT) is the most widely employed electronic structure method due to its favorable scaling with system size and accuracy for a broad range of molecular and condensed-phase systems. The advent of massively parallel supercomputers have enhanced the scientific community's ability to study larger system sizes. Ground state DFT calculations of systems with $O(10^3)$ valence electrons can be routinely performed on present-day supercomputers. The performance of these massively parallel DFT codes at the scale of 1 - 10K execution threads are not well understood; even experienced DFT users are unaware of Amdahl's Law and the non-trivial scaling bottlenecks that are present in standard $O(N^3)$ DFT algorithms. The GPAW code was ported an optimized for the Blue Gene/P. We present our algorithmic parallelization strategy and interpret the results for a number of benchmark tests cases. Lastly, I will describe opportunities for computer allocations at the Argonne Leadership Computing Facility. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W24.00003: Application of partition density-functional theory to model systems Larry Boyer, Michael Mehl Elliott et al.\footnote{P. Elliott, K. Burke. M. H. Cohen and A. Wasserman, Phys. Rev. A {\bf 82}, 024501 (2010)} have introduced a method called partition density-functional theory (PDFT) for expressing the Kohn-Sham charge density as a sum of overlapping fragment densities, which promises accuracy and efficiency along with a framework for developing and testing useful approximations for kinetic-energy functionals, T[n]. They illustrate their method using results obtained for non-interacting electrons in a one-dimensional model potential. Following their approach, we apply PDFT to similar models which examine its usefulness in developing approximations for T. We also discuss how PDFT compares with the self-consistent atomic deformation\footnote{L. L. Boyer, H. T. Stokes, M. M. Ossowski and M. J. Mehl, Phys. Rev. B {\bf 78}, 045121 (2008)} method. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W24.00004: Simple Impurity Embedded in a Spherical Jellium: Approximations of Density Functional Theory compared to Quantum Monte Carlo Benchmarks Michal Bajdich, Jeognim Kim, Paul R.C. Kent, Fernando A. Reboredo We study the electronic structure of a simple Gaussian impurity embedded in a spherical jellium in order to mimic the localization effects present in $d$- and $f$-electron compounds. We use quantum Monte Carlo benchmarks to validate approximations of density functional theory (DFT), such as local density approximation (LDA) and generalized gradient approximation (GGA) as well as the Hartree--Fock (HF) method. We identify distinct transitions between delocalized and localized states in the phase space of realistic densities ($1 \le r_s\le 5$) and moderate depths of the Gaussian impurity. We also extend the previous fixed-node diffusion Monte Carlo calculations of impurity-free jellium spheres and extract very accurate jellium surface exchange-correlation energies. Computer resources supported by DOE, Office of Science under contract DE-AC05-00OR22725 (NCCS). Research sponsored by DOE, BES, Materials Sciences and Engineering Division (FAR) and LDRD program (MB) and DOE SUF, CNMS (PRCK). [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W24.00005: Periodic Density Functional Theory Solver using Multiresolution Analysis with MADNESS Robert Harrison, William Thornton We describe the first implementation of the all-electron Kohn-Sham density functional periodic solver (DFT) using multi-wavelets and fast integral equations using MADNESS (multiresolution adaptive numerical environment for scientific simulation; http://code.google.com/p/m-a-d-n-e-s-s). The multiresolution nature of a multi-wavelet basis allows for fast computation with guaranteed precision. By reformulating the Kohn-Sham eigenvalue equation into the Lippmann-Schwinger equation, we can avoid using the derivative operator which allows better control of overall precision for the all-electron problem. Other highlights include the development of periodic integral operators with low-rank separation, an adaptable model potential for nuclear potential, and an implementation for Hartree Fock exchange. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W24.00006: A Robust Spectrum Slicing Method Applied to the Kohn-Sham Equation for the Liquid/Solid Silicon Interface Grady Schofield, James Chelikowsky A difficult aspect of solving the Kohn Sham equation is the super-linear scaling of eigensolvers with the number of valence orbitals desired. We present a robust spectrum slicing technique that calculates the valence orbitals in a divide and conquer fashion through the use of smooth Chebyshev-Jackson filters. This algorithm allows for a ``parallel'' implementation of the eigensolver. Our calculations are done in the real-space density functional framework implemented in the program PARSEC. We apply this method to examine the liquid-solid silicon interface. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W24.00007: Beyond the LDA in density functional theory: empirical Laplacian-based models for the exchange-correlation energy Antonio C. Cancio, Christopher E. Wagner We report recent work in developing a GGA-level density functional theory using primarily the Laplacian of the density $\nabla^2 n$ as an input beyond the LDA. Our starting point and motivation is a model fit to the exchange-correlation energy density of the valence shell of the Si crystal and other systems, as calculated by quantum Monte Carlo simulations, which show a strong, roughly linear dependence of this quantity on the Laplacian. The model respects the Lieb-Oxford bound for large positive Laplacian but suffers from a pole at negative values. A better treatment of $\nabla^2 n$ in this limit can be used to construct an all-electron extension of our model, and as an added benefit, avoid the singularity in the Kohn-Sham potential that gradient-based models suffer due to the cusp in electron density at the nucleus. Using an expansion in $1/\nabla^2 n$ we fit exchange energy densities in the cusp region accurately; obtaining reasonable potentials is a harder task but made easier by keeping the gradient of the density. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W24.00008: Better GGA and meta-GGA Functionals: VT{84}, meta-VMT, meta-VT{84} Alberto Vela, J. Martin del Campo, J.L. Gazquez, S.B. Trickey The goal of fast DFT calculations on large families of highly complicated systems (e.g.\ large clusters, biomolecules) implicitly conflicts with the heavy emphasis of recent years on inclusion of exact exchange. In response we have worked on improving non-empirical GGA X functionals. Here we report extension of our VMT GGA functional (J.\ Chem.\ Phys.\ \textbf{130} 244103 (2009)) to satisfy a relevant asymptotic constraint, yielding the VT\{84\} X functional. With the PBE C functional, VT\{84\} gives about 10\% improvement over VMT in energetics on the G3 223 molecule set. At the meta-GGA level of complexity, we have both meta-VMT and meta-\{84\}. The former is about 10\% better on the G3 set than the TPSS meta-GGA, while meta-VT\{84\} gives roughly 10\% further improvement over meta-VMT. Details of these assessments, including improvements in chemical shifts, will be presented. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W24.00009: All-Electron and Pseudopotential Orbital-Free Density Functional Calculations Valentin Karasiev, T. Sjostrom, S.B. Trickey Generalized gradient approximation (GGA) and modified-conjoint GGA kinetic energy functionals, proposed recently, have been implemented in an all-electron diatomic molecule code and in a periodic boundary condition code which uses local pseudopotentials. Self-consistent OF-DFT calculations confirm earlier non-self-consistent results. The GGA KE functionals give qualitatively incorrect total energy surfaces in the attractive region (isolated molecule) and the equilibrium crystalline cell volume is strongly expanded. In contrast, the mcGGA functional predicts a qualitatively correct energy surface for isolated systems, and the equilibrium geometry for pseudopotential calculations is in agreement with the Kohn-Sham results. We show the closeness in behavior between GGA-based functionals and simpler approximations defined by mixing of the Thomas-Fermi and the von Weizs\"acker KE functionals. Effects of the pseudopotential in OF-DFT calculations also are discussed. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W24.00010: Contributions to the Non-interacting Free Energy Density Functional S.B. Trickey, James Dufty, T. Sjostrom Phenomenological models for the T=0 non-interacting kinetic energy density functional often use a linear combination of the von Weizs\"acker (vW) and Thomas-Fermi (TF) functionals. A more systemmatic approach, for any temperature follows from extracting the vW functional from the exact free energy density functional, and analyzing the remainder in a local density approximation. We show that the vW functional is a lower bound for the free energy functional, extending a well-known T=0 result and indicating its priority in the decomposition. The exact remainder involves gradients of the off-diagonal one-body Fermi density matrix, for which a local density approximation is ambiguous. We discuss the extent to which a TF contribution can be extracted. Extension of the original vW phenomenological approach gives complementary insight. Modeling the orbitals as modulated plane waves, with coefficients identified in terms of the density and its gradients leads to vW and TF functionals plus higher-order gradient and temperature corrections. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W24.00011: Finite-size correction in many-body electronic structure calculations of spin polarized systems Fengjie Ma, Shiwei Zhang, Henry Krakauer We extend the post-processing finite-size (FS) correction method, developed by Kwee, Zhang, and Krakauer\footnote{H.~Kwee, S.~Zhang, and H.~Krakauer, Phys. Rev. Lett. {\bf 100}, 126404 (2008)}, to spin polarized systems. The method estimates the FS effects in many-body (MB) electronic structure calculations by a modified density functional theory (DFT) calculation, without having to repeat expensive MB simulations. We construct a unified FS DFT exchange-correlation functional for spin unpolarized and fully spin polarized systems, and then interpolate the results to arbitrary spin polarizations using the formula of Perdew and Wang\footnote{J.~P.~Perdew and Y.~Wang, Phys. Rev. B {\bf 45}, 13244 (1992)} or that of Perdew and Zunger.\footnote{J.~P.~Perdew and A.~Zunger, Phys. Rev. B {\bf 23}, 5048 (1981)} The application of this FS correction method to several typical magnetic systems with varying supercell sizes demonstrates that it consistently removes most of the FS errors, leading to rapid convergence of the MB results to the infinite size limit. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W24.00012: Generalization of the Hohenberg-Kohn theorem to the case of the presence of a magnetic field Viraht Sahni, Xiaoyin Pan We generalize the HK theorem for the nondegenerate ground state of electrons in an external electrostatic field ${\bf{E}}({\bf{r}}) = -$ {\boldmath $\nabla$} $v ({\bf{r}})$ to the presence of an additional external magnetostatic field ${\bf{B}} ({\bf{r}}) =$ {\boldmath $\nabla$} $\times {\bf{A}} ({\bf{r}})$. We prove that the nondegenerate ground state wave function $\Psi$ is a functional of the ground state density $\rho ({\bf{r}})$, the physical current density ${\bf{j}} ({\bf{r}})$, and a gauge function $\alpha ({\bf{R}})$, with ${\bf{R}} = \{{\bf{r}} \}$. In other words, the basic variables, viz. those that uniquely determine the external potentials $ \{v ({\bf{r}}), {\bf{A}} ({\bf{r}}) \}$, are $\{ \rho ({\bf{r}}), {\bf{j}} ({\bf{r}}) \}$. As the choice of $\alpha ({\bf{R}})$ is arbitrary, it is possible to construct a $\{ \rho ({\bf{r}}), {\bf{j}} ({\bf{r}}) \}$ functional theory, as well as the corresponding Kohn-Sham and quantal density functional theories. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W24.00013: Quantal density functional theory (QDFT) in the presence of a magnetic field Xiaoyin Pan, Tao Yang, Viraht Sahni We present the QDFT of electrons in an external electrostatic ${\bf{E}}({\bf{r}}) = -$ {\boldmath $\nabla$} $v({\bf{r}})$ and magnetostatic ${\bf{B}}({\bf{r}}) =$ {\boldmath $\nabla$} $\times {\bf{A}} ({\bf{r}})$ field. This is the mapping from the interacting system of electrons to one of noninteracting fermions with the same density $\rho ({\bf{r}})$ and physical current density ${\bf{j}} ({\bf{r}})$. The mapping, based on the `quantal Newtonian' first law, is in terms of `classical' fields and quantal sources, the fields being separately representative of electron correlations due to the Pauli exclusion principle and Coulomb repulsion, and correlation-kinetic and correlation-magnetic effects. The theory is valid for ground and excited states. It is explicated by application to a ground state of the exactly solvable Hooke's atom in the presence of a magnetic field. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W24.00014: Quantum Continuum Mechanics for Many-Electron Systems in a Strong Magnetic Field Stefano Pittalis, I.V. Tokatly, G. Vignale A quantum continuum mechanics approach for the determination of the excitation energies of many-electron systems in strong magnetic field is introduced by means of linear response theory (LRT) and time-dependent deformation-functional theory (TD-DefT). In the high-frequency (anti-adiabatic) limit the collective modes of the system appear as the small oscillations of an elastic body in the presence of non-inertial forces reminiscent of the Coriolis and centrifugal forces. Interestingly, the complexity of the problem does not increase significantly with the particle number and only ground state properties are needed as an input. Further results, together with elementary and illustrative examples, may be presented as well. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W24.00015: First principles finite temperature magnetism of defects in Fe using Wang-Landau method Aurelian Rusanu, D.M. Nicholson, Kh. Odbadrakh, Gregory Brown, Markus Eisenbach Magnetic structure of materials with defects presents a strong dependence on local atomic arrangements. This dependence affects mechanical, magneto-caloric, and magnetization properties. Insights into thermodynamic and magnetic fluctuations at defects in Fe are obtained from first principle analysis by deploying the first principle local self consistent multiple scattering method(LSMS) and Wang-Landau statistical method. The computation of thermodynamic properties requires the sampling of a large number of configurations. To reduce the computational effort a Heisenberg model will be used to speed the configuration sampling procedures. The approach will be demonstrated for Fe systems and will address the magnetic structure of defects. [Preview Abstract] |
Session W25: Novel Superconductors I
Sponsoring Units: DCMPChair: Taner Yildirim, NIST/University of Pennsylvania
Room: D166
Thursday, March 24, 2011 11:15AM - 11:27AM |
W25.00001: Parallels and contrasts in the pairing mechanism between cuprate and organic superconductors Thomas Maier The organic superconductors share many characteristics with the cuprates, such as the existence of antiferromagnetism, unconventional superconductivity and Mott insulating behavior. In addition, despite their complexity, their physics can be approximated by a single-band Hubbard model on a two-dimensional lattice. Here, we will present dynamic cluster quantum Monte Carlo simulations of a half-filled ``dimer'' Hubbard model on an anisotropic triangular lattice, which find a transition from an antiferromagnetic phase to a d-wave superconducting phase with increasing frustration. In particular, we will discuss commonalities and differences in the nature of the superconducting behavior between the dimer Hubbard model of the organic compounds and the standard Hubbard model of the cuprate materials. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W25.00002: Can simple hydrocarbon molecular solids superconduct? A theoretical investigation of superconductivity in K$_{3}$Picene Xuhui Luo, Serdar Ogut, Taner Yildirim Unlike many well established high temperature (T$_{c})$ superconductors such as cuprates, M$_{3}$C$_{60}$, MgB$_{2}$ and iron-pnictides, the possibility of superconductivity in molecular hydrocarbon solids remains a controversial issue. This topic became active again by a recent study reporting superconductivity up to T$_{c} \sim $ 17 K in potassium doped Picene [1], a wide-bandgap semiconducting solid hydrocarbon. However, there is no theoretical study about possible mechanism of superconductivity. In this talk, we present a detailed first-principles study of the electron-phonon (el-ph) coupling in doped organic molecular solids. Due to large system size, the calculation of el-ph coupling using the standard linear response theory is not feasible. Hence, we have developed a finite-displacement method where both the phonon energies and el-ph coupling can be easily calculated for large systems. We have tested our code for well-established superconductors such as K$_{3}$C$_{60}$ and MgB$_{2}$. As a comparison, we have also studied the el-ph coupling in alkali-doped Pentacene, a similar well-studied hydrocarbon in which no superconductivity has been observed. We discuss the effect of charge transfer as well as pressure on T$_{c }$for solid Picene and make predictions for future possible experiments. \\[0pt] [1] Mitsuhashi et al. Nature, 464, p. 76 (March, 2010). [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W25.00003: Electronic structure and magnetic properties of the hydrocarbon K$_{3}$picene superconductor near the metal-insulator transition Minjae Kim, B.I. Min, Geunsik Lee, Hee Jae Kwon, Y.M. Rhee, Ji Hoon Shim Superconductivity has recently been observed in K-doped picene, K$_{3}$picene, which is a first organic superconductor in the hydrocarbon system with high transition temperature Tc=18K [1]. We have investigated the electronic structures and magnetic properties of K$_{3}$picene by density-functional theory. We have shown that the metal-insulator transition (MIT) is driven in K$_{3}$picene by 5{\%} volume enhancement with a formation of local magnetic moment. Active bands for superconductivity near the Fermi level (E$_{F}$ ) are found to have hybridized character of LUMO and LUMO+1 picene molecular orbitals. Fermi surfaces of K$_{3}$picene manifest neither prominent nesting feature nor marked two-dimensional behavior. By estimating the ratio of the Coulomb interaction $U $and the band width $W $of the active bands near E$_{F}$ , $U/W$, we have demonstrated that K$_{3}$picene is located in the vicinity of the Mott transition. \\[4pt] [1] R. Mitsuhashi \textit{et. al}. Nature. 464, 76 (2010) [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W25.00004: Slow-Motion $^{1}H$ NMR Study of $\kappa$-(ET)$_{2}$Cu[N(CN)$_{2}$]Br J.C. Gezo, Tak-Kei Lui, R.W. Giannetta, C.P. Slichter, J.A. Schleuter The recent discovery of an anomalous Nernst signal in the pseudogap phase of organic superconductor $\kappa$-(ET)$_{2}$Cu[N(CN)$_{2}$]Br suggests the presence of magnetic flux vortices above T$_{c}$.\footnote{M. S. Nam et al, Nature 449, 584-587 (2007)} We report spin-locked and line-narrowed proton NMR data on the pseudogap phase of this material. These experiments are sensitive to magnetic fluctuations, and probe far slower timescales ($10^{-6}$-$10^{-3}$s) than the previously explored NMR parameter space. Other experiments\footnote{C. H. Recchia et al, Phys. Rev. Lett. 78, 3543-3546 (1997)} have suggested that vortices leave an NMR fingerprint at these low frequencies. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W25.00005: NMR Study of $\kappa$-$\mathrm{(ET)_{2}Cu[N(CN)_{2}]Br}$ Tak-Kei Lui, J.C. Gezo, R.W. Giannetta, C.P. Slichter, J.A. Schlueter We report measurements of the $\mathrm{^{13}C}$ spin-lattice relaxation rate, $(T_{1}T)^{-1}$ of $\kappa$-$\mathrm{(ET)_{2}Cu[N(CN)_{2}]Br}$, an organic superconductor with $T_{C}=11.9\,\mathrm{K}$. The shape of the $(T_{1}T)^{-1}$versus $T$ graph suggests the existence of a finite spin singlet-triplet gap\footnote{C. P. Slichter, Experimental Evidence for Spin Fluctuations in High Temperature Superconductors, in \emph{Strongly Correlated Electronic Materials}: A Los Alamos Symposium, pp. 427-479, ed. K.S. Bedell \emph{et al}., Addison-Wesley (1994)}. Knight shifts were measured to search for a two-component electronic system. We also report measurements using an ``S-shape'' pulse sequence\footnote{L. R. Becerra, C. A. Klug, C. P. Slichter, and J. H. Sinfelt, J. Phys. Chem. \textbf{97}, 12014 (1993)} that inverts half of the absorption line, permitting one to study slow motions and spin density waves. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W25.00006: 13C NMR measurements of Zeeman limited superconductivity in $\kappa $-(BEDT-TTF)2Cu(NCS)2 Jeffrey Wright, James Brugger, John Schlueter, Reizo Kato, Stuart Brown The class of superconductors formulated by the BEDT-TTF molecule offers a unique ability to study superconductivity which is Zeeman limited, utilizing their qausi-2D layered structure. The critical field H$_{c2}$, which usually limits superconductivity due to orbital screening currents, can be suppressed by aligning the applied magnetic field parallel to the conducting layers. In this orientation, the field penetrates in the form of Josephson vortices, and the dominant effect of the magnetic field is from the Zeeman interaction with quasiparticles. We present 13C NMR measurements of the spin lattice relaxation rate as a function of applied field on a single crystal of $\kappa $-(BEDT-TTF)2Cu(NCS)2 after aligning the field in this orientation. A quadratic dependence is observed: R(B) $\sim $ B$^{2}$, which gives clear evidence of k-space nodes in the superconducting gap. Extending these measurements to field strengths near and above the Pauli limit, we observe a phase transition within the superconducting state at B=21.5T, and we comment on the compatibility of these results with the sought after FFLO state. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W25.00007: Upper Critical Field in the Molecular Organic Superconductor (DMET)$_{2}$I$_{3}$ Pashupati Dhakal, Harukazu Yoshino, Jeong Il Oh, Koichi Kikuchi, Michael J. Naughton We report the temperature dependence of the upper critical magnetic field in the quasi-one-dimensional molecular organic superconductor (DMET)$_{2}$I$_{3}$, for magnetic field applied along the intrachain, interchain, and interplane directions. The upper critical field tends to saturation at low temperature for field in all directions and does not exceed the Pauli paramagnetic limit. Thus the superconductivity appears to be conventional spin singlet, in contrast to the status of the isostructural Bechgaard salts. Furthermore, we will discuss a magnetic field-induced dimensional crossover effect in the normal metallic state leading to a magnetoresistance minimum for the field parallel to the most conducting plane. This result will be useful to understand the nature of superconductivity in the molecular organic conductors. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W25.00008: London penetration depth of $\lambda$-(BETS)$_2$GaCl$_4$ superconductor Jason Murphy, H. Kim, K. Cho, M.A. Tantar, A. Kobayashi, H. Kobayashi, R. Prozorov After many years of study, it is still unclear whether the superconductivity of organic charge transfer salts is conventional or not. In particular, symmetry of the superconducting gap is an important unresolved question. The gap anisotropy and nodal structure can be probed via precision measurements of London penetration depth, $\lambda(T)$, for which tunnel-diode resonator has been developed over past 20 years. Here we report $\lambda(T)$ in $\lambda$-(BETS)$_2$GaCl$_4$ crystals ($T_c \sim 5$~K ) measured down to 400 mK. We find that different samples exhibit different widths of superconducting transition, $\Delta T_c$. It is known for organic superconductors that cooling rate from room temperature is important for the properties probed at the low temperatures. We discuss observed variation of $\Delta T_c$ with the cooling rate as well as sample size and aspect ratio. Results are compared with thermal conductivity measurements [1]. \\[4pt] [1] M. A. Tanatar, T. Ishiguro, H. Tanaka, H. Kobayashi, Phys.Rev. B {\bf 66}, 134503 (2002). [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W25.00009: Thermodynamic study of the superconducting gap structure of (TMTSF)$_2$ClO$_4$ Shingo Yonezawa, Yoshiteru Maeno, Klaus Bechgaard, Denis Jerome We have studied the superconducting (SC) gap structure of the quasi-one-dimensional molecular conductor (TMTSF)$_2$ClO$_4$ based on our high-resolution heat capacity measurement. We developed a new calorimeter, which allowed us field-angle resolved calorimetry using one single crystal weighing as small as 76~$\mu$g. From our results, we investigate the SC gap structure in the $k$-space as well as in the spin space. Comparison between the SC phase diagram deduced from the present results and those obtained from previous resistivity measurements are also discussed. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W25.00010: Superconducting energy gap features of MgB$_{2}$ thin films on different substrates and orientations Wenqing Dai, Qi Li, Ke Chen, Xiaoxing Xi We report a detailed study of tunneling spectra of MgB$_{2}$/I/Pb planar junctions with MgB$_{2}$ films on various substrates and of various doping levels. Planar trilayer junctions were fabricated using MgB$_{2}$ films with native oxide barrier grown by the Hybrid Physical-Chemical Vapor Deposition technique. Both $\pi $ and $\sigma $ bands contribute to the tunneling spectra of tilted-axis films on MgO (211) substrate and mainly $\pi $ band was observed on c-axis MgB$_{2}$ films on SiC (0001), MgO (111) and c-sapphire substrates. We observed $\sigma $ gap value of $\sim $7.9 meV in MgB$_{2}$ films on SiC substrates which display higher T$_{c}$ due to the lattice strain. This is larger than 7.4 meV on unstrained substrates. However, the $\pi $ gap value of all samples is $\sim $2.3 meV. We concluded that the strain in MgB$_{2}$ films on SiC substrates mainly affects the $\sigma $ band of MgB$_{2}$. In addition, small amount of nitrogen gas was added during film growth to introduce more scattering in MgB$_{2}$ films. We systematically studied the change of two gaps with nitrogen doping from the tunneling spectra of MgB2/I/Pb junctions. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W25.00011: Measurements of Multiple Gap Substructure in MgB$_2$ Below 1 Kelvin Steven Carabello, Joseph Lambert, Roberto Ramos The two superconducting energy gaps of magnesium diboride (MgB$_2$) are well established. First-principles calculations have also predicted substructure within the sigma- and pi-band gaps. However, due to anticipated intraband impurity scattering, there is controversy as to whether these finer structures can be observed in real samples. Prior experimental evidence above 1 Kelvin has provided evidence supportive of these features. We have performed tunneling spectroscopy experiments on MgB$_2$/insulator/Pb Josephson junctions on SiC substrates, at temperatures as low as 20 mK. By measuring differential conductance at low temperatures, and by using extremely clean MgB$_2$ thin films, we have resolved features within the energy gaps to under 1 meV. We report results of these experiments, which are in remarkable agreement with theoretical predictions for this substructure. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W25.00012: Time Dependent Transport Properties of Superconducting MgB$_{2}$ Murat Olutas, Atilgan Altinkok, Atilla Kilic, Kivilcim Kilic The time dependent transport properties of polycrystalline superconducting MgB$_{2}$ sample was investigated in details by means of current-voltage (I -- V curves) measurements with different current sweep rates (dI/dt) and transport relaxation (V -- t curves) measurements. The corresponding measurements were carried out as functions of temperature (T), transport current (I) and external magnetic field (H). Upon cycling of the transport current, it was observed that the hysteresis effects in I -- V curves are negligible. Further, for slow or fast sweep rates (dI/dt) of the transport current, the measured voltage dissipation at a given transport current is approximately stable and does not change much with time. Analysis of the I -- V curves shows that the pinning potential is also practically independent of dI/dt. The time evolution of sample voltage reveals that the increase in sample voltage is quite a sharp and non-linear. The V -- t curves were interpreted in terms of re-organization of flux lines and suppression of the effective superconducting order parameter along the sample as the time progresses. The quenched state created in V -- t curves was correlated to the supercooling of flux lines. Finally, the experimental results of polycrystalline MgB$_{2}$ sample were compared to previous similar studies on superconducting Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-x}$ and Bi$_{1.7}$Pb$_{0.3}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{x}$ polycrystalline samples. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W25.00013: Measurement of Hc1 in MgB2 thin films and multilayer structures by a microwave absorption technique Chenggang Zhuang, Ke Chen, Som Tyagi, Xiaoxing Xi For superconducting RF applications, Gurevich suggested a route to enhance the vortex penetration field, Hc1, and thermal breakdown field by a multilayer structure consisting of alternating insulator and superconductor layers with thicknesses smaller than the penetration depth. We have measured Hc1 of MgB2 thin films and multilayer structures by measuring the microwave absorption of the sample at 9.3 GHz in a TE102 rectangular cavity under an applied magnetic field. The magnetic fluxon penetration into the sample as the applied field is increased to greater than Hc1 leads to an increase in the microwave absorption. Preliminary results indicate an enhancement of Hc1 in the MgB2 thin films from the bulk value, consistent with Gurevich's thickness effect model, which is very promising for RF applications of MgB2. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W25.00014: Magnetic force microscopy study of the penetration depth in MgB$_{2}$ thin films Jeehoon Kim, Leonardo Civale, Evgueni Nazaretski, Nestor Haberkorn, Josh Thibodaux, Ilya Vekhter, Brian Moeckly, Joe D. Thompson, Roman Movshovich We performed magnetic force microscopy (MFM) investigations of superconducting vortices in thin films of MgB$_{2}$. Our MFM instrument has the capability of scanning multiple samples. Prior to imaging vortices in MgB$_{2}$, vortex imaging in a Nb thin film was performed to characterize the cantilever's tip. This procedure allows extraction of the penetration depth with only one fitting parameter. Images of two MgB$_{2}$ films with different thickness were taken as a function of temperature, together with periodic checks of the condition of the magnetic tip via imaging of vortices in the Nb reference sample, during a single cooldown. The temperature dependent penetration depth determined by MFM will be compared to that obtained via SQUID magnetometry on the same samples. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W25.00015: Metastable Vortex Lattice Phases in MgB$_2$ C. Rastovski, K. Schlesinger, P. Das, M.R. Eskildsen, L. DeBeer-Schmitt, N. Zhigadlo, J. Karpinski We present small-angle neutron scattering (SANS) studies of vortex lattice (VL) metastability in MgB$_2$. Three different VL phases are observed, all of which have a triangular symmetry. At low fields the VL is aligned with the crystalline $a$-axis. At intermediate fields the VL rotates away from the $a$-axis, leading to two degenerate domain orientations. Once the rotation reaches $30^{\circ}$ a single domain, high field VL is reformed, now aligned along the $a^*$-axis. Metastable configurations are obtained when crossing the equilibrium VL transition lines by cooling or heating in a constant field. At any given field and temperature the equilibrium VL can be obtained by inducing vortex motion. We have explored the details of how the metastable VL transitions to the ground state, and established that the equilibrium VL phase propagate from the edge of the sample towards the center as the magnetic field is reduced. We have successfully prepared samples with a coexistence of metastable and ground state VL domains, and explored how large field changes are necessary to completely suppress the metastable VL phase. The SANS results are compared to measurements of the critical current obtained from magnetization measurements. This work was supported by DOE BES award no. DE-SC0005051. [Preview Abstract] |
Session W26: Focus Session: Iron Based Superconductors -- Tuning Magnetism and Superconductivity
Sponsoring Units: DMP DCOMPChair: David Mandrus, University of Tennessee and Oak Ridge National Laboratory
Room: D162/164
Thursday, March 24, 2011 11:15AM - 11:27AM |
W26.00001: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W26.00002: Superconductivity in (Sr,Ba)Fe2As2 single crystals by Pt substitution Tyler Drye, Shanta Saha, Kevin Kirshenbaum, Nicholas Butch, Johnpierre Paglione, Peter Zavalij Iron-based superconducting materials with the ThCr2Si2 tetragonal crystal structure appear to show a maximum superconducting transition temperature of Tc $\sim $ 20-25 K when transition metals (e.g., Co, Ni, Ru, Rh, Pd, or Ir) are substituted for Fe, effectively doping d-electrons and suppressing the antiferromagnetic order of the parent compounds. However, this trend is known to be broken in the case of SrFe2-xNixAs2 and SrFe2-xPdxAs2, which both have lower optimal Tc values near 10 K. We will present our recent work on Pt substitution in single crystalline BaFe2As2 and SrFe2As2, which induces a maximum Tc of 23 K and 17 K, respectively. The relation between Pt substitution in these systems and the related cases of isoelectronic Ni and Pd substitution will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W26.00003: Tetragonal lattice collapse in SrFe$_2$As$_2$ - a combined experimental and theoretical study Helge Rosner, Deepa Kasinathan, Miriam Schmitt, Alim Ormeci, Katrin Meier, Ulrich Schwarz, Michael Hanfland, Klaus Koepernik, Yuri Grin, Andreas Leithe-Jasper In a joint experimental and theoretical study we investigate the crystal structure of the Fe pnictide compounds SrFe$_2$As$_2$ under applied hydrostatic pressure. Applying high pressure X-ray diffractyion, for a critical pressure of about 10 GPa we observe a sudden collapse of the tetragonal $c$ axis, accompagnied by a small expansion of the basal plane. This results in a drastic reduction of the $c/a$ ratio and a significant decrease of the unit cell volume. This tetragonal collapse is well described by DFT band structure calculations and can be assigne d to the formation of an additional As-As bond along the tetragonal $c$ axis. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W26.00004: Structural tuning of superconductivity and magnetism in intermetallic iron-pnictide materials Invited Speaker: The relationship between superconductivity, magnetism and crystallographic structure remains as one of the intriguing properties of the new family of iron-based superconducting materials. A well established requirement for high-temperature superconductivity in these systems is a substructure of iron ions tetrahedrally coordinated with either pnictogen or chalcogen anions stacked together to form a layered material, suggesting that both tetrahedral geometry and quasi-two-dimensionality are key ingredients. Through an investigation of solid solutions of (Ba,Sr,Ca)Fe$_2$As$_2$ series of parent compounds, we present a study of the importance of internal tetrahedral structure in stabilizing both magnetic and superconducting ground states in these materials, revealing an intimate relationship between the energy scale that dictates magnetic order and the internal structure of the FeAs$_4$ tetrahedra even far above the magnetic ordering temperature. In addition, interlayer coupling is investigated by exploiting the ``collapse'' of the tetragonal unit cell of CaFe$_2$As$_2$ under pressure, where interlayer pnictogen-pnictigen bonding changes dramatically. We investigate the effect of this collapse on superconductivity via chemical substitution, demonstrating an intriguing interplay of structure, magnetic and superconducting properties. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W26.00005: Magnetic ordering in EuCo$_2$As$_2$ Balazs Sipos, Athena S. Sefat, Brian C. Sales We have synthesized and studied EuCo$_2$As$_2$ single crystals by resistivity, magnetoresistance, and susceptibility measurements. We found antiferromagnetic (AFM) ordering of the Eu spins at $T_N = 50$ K. Upon applying a magnetic field $H \| ab$ at $T = 2$ K this phase exhibits a metamagnetic (MM) transition at $H_{MM} = 3.5$ T. In case of $H \| c$ the magnetisation increases linearly up to 7 T. The same AFM to MM transition was found at 0.5 T in EuFe$_2$As$_2$ where it was found to be due to the reorientation of the Eu spin. We found that replacing Fe with Co strengthens the coupling between the Eu moments resulting in a higher $T_N$ and H$_{MM}$. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W26.00006: High Magnetic Field Studies of Pressure-induced Superconductor EuFe$_2$As$_2$ Nobuyuki Kurita, Motoi Kimata, Kota Kodama, Atsushi Harada, Hiroyuki Suzuki, Takehiko Matsumoto, Shinya Uji, Taichi Terashima, Keizo Murata We have performed resistivity and susceptibility measurements of the pressure-induced superconductor EuFe$_2$As$_2$ under high magnetic fields up to 27 T. The upper critical field $B_{c2}$ and its pressure evolution up to 3.2 GPa were determined in a wide temperature range down to 1.6 K. At 2.5 GPa, nearly the optimal pressure with $T_c$=30 K, $B_{c2}$(0) obtained by the onset of resistive transitions are 25 T and 22 T for $B\parallel ab$ and $B\parallel c$, respectively, which are appreciably smaller than those for other Fe-based superconductors with similar $T_c$. The small $B_{c2}$(0) values and the peculiar $B_ {c2}$($T$) curves in EuFe$_2$As$_2$ can be explained by a multiple pair-breaking model including the exchange field due to the magnetic Eu$^{2+}$ moments. We will also present the results of quantum oscillations observed above the optimal pressure. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W26.00007: Pressure shift of the superconducting T$_{c}$ of (Pr$_{1-x }$Sr$_{x})$FeAsO and Sm(O$_{1-x}$F$_{x})$FeAs Kalyan Sasmal, G. Mu, H.-H. Wen, B. Lorenz, Ching-Wu Chu Pressure plays important role in discovery and unraveling physics of novel superconductors.High Tc iron-based layered compounds can be obtained by hole/electron-doping.To determine if a symmetry between electron and hole-doping exists,we investigated pressure-induced shift in T$_{c}$ by carryingout resistivity measurements under hydrostatic pressure on hole-doped Pr$_{1-x }$Sr$_{x}$FeAsO up to 1.8GPa using piston-cylinder clamp cell device.The coexistence of superconductivity {\&} spin-density wave behavior were observed and pressure effects on both being investigated. Four probe resistance measurements show T$_{c}$ increases (+dT$_{c}$/dP) with pressure for under-doped Pr$_{1-x}$Sr$_{x}$FeAsO similar to high-T$_{c}$ cuprates. High pressure can compress crystalline structure of material and force its layers to be closer,which might increase material's T$_{c}$ by improving pressure-induced charge transfer between (Fe$_{2}$As$_{2})$ and (Pr/Sr)O layers. The pressure effect on T$_{c}$ of Pr$_{1-x }$Sr$_{x}$FeAsO is being compared with that of electron doped Sm(O$_{1-x}$F$_{x})$FeAs. The results suggest a symmetry appear to exist between electron and hole-doping Fe-pnictide superconductors. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W26.00008: Pressure induced superconductivity in LaFeAsO: the role of anionic height and magnetic ordering Ravhi Kumar, James Hamlin, Yuming Xiao, Stanislav Sinogeikin, Paul Chow, Brian Maple, Yusheng Zhao, Andrew Cornelius We have investigated the pressure effect on the crystal structure and magnetic ordering of LaFeAsO at low emperature ($\sim $18K) using high pressure powder x-ray diffraction (HPXRD) and nuclear forward scattering (NFS) to pressures up to 40 GPa. We demonstrate a continuous suppression of the long range antiferromagnetic ordering in this compound under pressure. Furthermore we show here a direct correlation between the pressure induced changes in the anionic height parameter to the transition temperature (\textit{Tc}) and is first observed in the 1111 class of iron arsenide compounds under pressure. Our findings suggest that pressure induced suppression of magnetic ordering and the anionic height variation both play important roles in the origin of pressure induced superconductivity in LaFeAsO. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W26.00009: The Pressure Effects on SmFeAsO$_{0.85}$ and PrFe$_{0.925}$Co$_{0.075}$AsO Superconductors X.L. Dong, W. Lu, J. Yang, W. Yi, Z.C. Li, C. Zhang, Z.A. Ren, G.C. Che, L.L. Sun, F. Zhou, X.J. Zhou, Z.X. Zhao We have measured magnetic susceptibility of iron pnictide superconductors SmFeAsO$_{0.85}$ and PrFe$_{0.925}$Co$_{0.075}$AsO under hydrostatic pressure up to 1.15 GPa. Our results indicate that the pressure dependences of T$_{C}$ and superfluid density in both systems are positively correlated which suggests that these quaternary iron-based superconductors are not conventional BCS ones. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W26.00010: Valence change of europium and its relation to superconductivity in EuFe$_2$As$_{1.4}$P$_{0.6}$ and compressed EuFe$_2$As$_2$ Liling Sun, Jing Guo, Genfu Chen, Xianhui Chen, Xiaoli Dong, Wei Lu, Chao Zhang, Zheng Jiang, Yang Zuo, Shuo Zhang, Yuying Huang, Qi Wu, Xi Dai, Yuanchun Li, Jing Liu, Zhongxian Zhao Superconductivity can be realized in Eu-containing pnictides by applying chemical (internal) and physical (external) pressure, the intrinsic physical mechanism of which attracts much attention in the studies of pnictide superconductors. Here we present the experimental evidence for the pressure-induced valence change of europium in EuFe$_{2}$As$_{1.4}$P$_{0.6}$ exposed to ambient pressure and EuFe$_{2}$As$_{2}$ to high pressure by x-ray absorption measurements on L$_{3}$-Eu edge. We found that the absorption spectrum of EuFe$_{2}$As$_{1.4}$P$_{0.6}$ showed a clear spectra weight transfer from divalent to trivalent state. Furthermore, a similar behavior of valence transition as in EuFe$_{2}$As$_{1.4}$P$_{0.6}$ was also observed in EuFe$_{2}$As$_{2}$ when pressure was applied. This is the first to report the observation of valence change in pnictide superconductors and the analysis of its influence on superconductivity in EuFe$_{2}$As$_{1.4}$P$_{0.6}$ and compressed EuFe$_{2}$As$_{2}$. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W26.00011: Pressure effects on strained FeSe$_{0.5}$Te$_{0.5}$ thin films Melissa Gooch, Bernd Lorenz, SunXiang Huang, Chia-Ling Chien, Paul Chu FeSe is the simplest structure in the family of the iron pnictides, with a reported superconducting transition of 8K for the $\alpha$-PbO- type structure. With the substitution of Te for Se, FeSe$_{0.5}$Te$_{0.5}$, was found to have an increased superconducting transition up to 15.2 K. To investigate the strain effect on the superconducting properties of the FeSe$_{0.5}$Te$_{0.5}$, thin films were grown under different conditions. The Tc and the normal state properties show a correlation to the induced strain of the system. The application of external pressure resulted in an increase of T$_c$, but at different rates depending on the pre-existent strain in the system. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W26.00012: Enhanced SDW transition temperature under hydrostatic pressure in Fe1.02Se0.10Te0.90 Naoyuki Katayama, Kazuyuki Matsubayashi, Juscelino Leao, Sungdae Ji, Sung Chang, Yoshiya Uwatoko, Taku Sato, Seunghun Lee We will present the spin glass (SG) to spin density wave (SDW) transition in Fe1.02Se0.10Te0.90 under hydrostatic pressure. At ambient pressure, the present compound shows SG transition, characterized by a broad weak magnetic peak. By applying hydrostatic pressure, SG develops to SDW at Pc $\sim $ 0.5 GPa and the sharp strong magnetic Bragg peak appears. The SDW transition temperature is increased up to $\sim $ 250 K at 8 GPa, which is sharply contrast to the results of pressure experiments in LaFeAsO families and BaFe2As2 families. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W26.00013: Competition between magnetic order and superconductivity in Fe oxy-pnictides Samuele Sanna, P. Carretta, G. Prando, A. Rigamonti, R. De Renzi, T. Shiroka, G. Lamura, M. Putti, A. Martinelli, R. Cimberle, M. Tropeano, C. Ferdeghini, A. Palenzona We have microscopically investigated the interplay between
magnetism (M) and superconductivity (SC) of the RFeAsO (1111)
oxy-pnictide for R=La, Sm and Ce as a function of F doping,
isoelectronic Fe/Ru substitution and external pressure. In
contrast to earlier data, our results suggest a unique
behaviour in different 1111 families at the M-SC crossover,
showing a sharp crossover between the two types of order as a
function of F doping [1-3]. In the optimally e$^-$-doped SmFe$_
{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ compound, magnetic order
appears in the FeAs layers for $0.1 |
Session W27: Focus Session: Semiconductor Qubits- Optical Control, Donors, and Hybrid Systems
Sponsoring Units: GQIChair: Jake Taylor, University of Maryland
Room: C155
Thursday, March 24, 2011 11:15AM - 11:27AM |
W27.00001: Ultrafast optical entanglement control between two quantum dot spins Sam Carter, Danny Kim, Alex Greilich, Allan Bracker, Daniel Gammon A single electron spin in an InAs quantum dot is very attractive as a qubit since this system is potentially scalable and allows complete quantum control on an ultrafast timescale using optical pulses. While great progress has been achieved with single spin qubits, it is essential for quantum information applications to move toward entangled multi-qubit systems. Two-qubit systems have been studied in electrostatically-defined quantum dots, but their optical functionality remains unexplored. Here we demonstrate ultrafast optical control of two interacting qubits consisting of two electron spins in separate InAs dots. We initialize the system into a spin singlet state using a cw laser. We then manipulate the entangled state of the two spins with single qubit gates (acting only on one spin) by using pulses faster than the exchange interaction. This allows us to generate all four Bell states. Two-qubit gates are obtained either by the natural exchange precession or by using a longer laser pulse that induces a phase shift in the precession. The two-qubit exchange rate (30 GHz) here gives SWAP gate times of 16 ps, the fastest of any candidate for quantum information processing. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W27.00002: Complete ultrafast optical coherent control and spin echo of single InAs quantum dot spins Kristiaan De Greve, Peter McMahon, David Press, Thaddeus Ladd, Christian Schneider, Dirk Bisping, Martin Kamp, Lukas Worschech, Sven Hoefling, Alfred Forchel, Yoshihisa Yamamoto We report on recent progress on the complete ultrafast optical coherent control of individual InAs quantum dot spin qubits. We demonstrate Rabi-oscillations and Ramsey-fringes, and implement a spin echo to overcome time-averaged dephasing.\footnote{D. Press, K. De Greve, P. McMahon \textit{et al.}, Nat. Phot. \textbf{4}, 367 (2010)} We probe the hyperfine interaction of a single spin using optical pulse control. Interesting non-Markovian dynamics could be observed in the single electron spin free-induction decay, resulting from feedback between the strong electron spin Overhauser shift and spin dependent nuclear relaxation.\footnote{T. D. Ladd, D. Press, K. De Greve \textit{et al.}, Phys. Rev. Lett. 105, 107401 (2010)} [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W27.00003: Control of exciton relaxation channels in quantum dot molecules Kushal C. Wijesundara, Juan E. Rolon, Sergio E. Ulloa, Eric A. Stinaff, Allan Bracker, Dan Gammon We observe modulations in radiative lifetimes and intensities of the spatially indirect exciton as the InAs/GaAs coupled quantum dot system is tuned between molecular and atomic like states. With standard time-resolved single photon counting techniques the measured lifetimes were found to vary between 0.3 and 2.0 ns which resulted in modulations of the observed photoluminescence intensity of the indirect exciton. These modulations can be attributed to phonon mediated relaxations and carrier tunneling processes in good agreement with the modeled results. We clearly see the structure of the acoustic phonon distribution as shown in recent theoretical predictions. Tuning the relative energy levels in coupled quantum dots results in controllable modulation of exciton relaxation channels that may provide new directions in engineering decoherence in these systems. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W27.00004: Charge dynamics and phonon induced oscillatory relaxation rates of indirect excitons in quantum dot molecules J.E. Rolon, K.C. Wijesundara, E.A. Stinaff, S.E. Ulloa Optoelectronic control of quantum dots is a thriving area of research with impact on fundamental physics and quantum information devices. Time-resolved photoluminescence experiments, carried out in charge tunable coupled quantum dots, have demonstrated non-monotonic behavior of neutral indirect exciton lifetimes over a wide range of applied electric fields [1]. We present a model for neutral indirect exciton lifetimes in electric field tunable quantum dot molecules. Our model includes field-dependent oscillatory phonon-induced relaxation rates [2], carrier tunneling rates, and carrier relaxation into nearby charged exciton states. To this end we have used a multi-excitonic Hamiltonian, and calculated the exciton population dynamics using a master equation with electric field dependent rates. We find that lifetime suppression is dominated by scattering with LA phonons at low fields, and that the maximum lifetime gives information on the effective dimensions of the molecule. In contrast, at high fields the lifetime suppression is dominated by the interplay of carrier population exchange with nearby charged excitons. This prompts for ways of controlling exciton lifetimes and possible decoherence in quantum dots. [1] K. C. Wijesundara et al., (unpublished), [2] J. I. Climente et al., Phys. Rev. B 74, 035313 (2006). [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W27.00005: Ultralong Coherence of Phosphorus Donors in High-Purity $^{28}$Si Silicon S.A. Lyon, A.M. Tyryshkin, S. Tojo, K.M. Itoh, J.J.L. Morton, T. Schenkel, M.L.W. Thewalt, H. Riemann, N.V. Abrosimov, P. Becker, H.-J. Pohl We report on electron spin coherence measurements for phosphorus donors in high purity, highly-enriched $^{28}$Si, with residual $^{29}$Si of less than 50 ppm. At this low $^{29}$Si density, spectral diffusion processes by nuclear spin flip-flops are suppressed, and therefore other relaxation processes become prominent. By examining a series of $^{28}$Si crystals with a donor concentration of 1$\times $10$^{14}$ to 3$\times $10$^{15}$/cm$^{3}$, we identified three decoherence mechanisms, all related to dipole interactions between donors: (1) instantaneous diffusion, caused by flips of donor spins induced by the applied microwave pulses; (2) spectral diffusion caused by T$_{1}$-induced flips of neighboring donors; (3) spectral diffusion caused by donor spin flip-flops. We demonstrate how all three mechanisms can be suppressed, leading to measured coherence times extrapolating to T$_{2}\sim $10 sec. The work was funded by DOE and LPS. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W27.00006: Electrical Control of the high spin system Mn2+ in ZnO Richard George, John Morton, Arzhang Ardavan, James Edwards We examine the high spin impurity Mn$^{2+}$ in single crystal ZnO (S=5/2, I=5/2), and report a strong linear coupling (K = 52.3 rad/V/m) of the manganese electrical and magnetic moments that preserves quantum coherence. We combine pulsed EPR and electric field techniques to manipulate the Mn states and study electron spin lifetimes, finding $T_{2e}$ and $T_{1e}$ times of 0.8ms and 100ms at 2K in the natural material. We investigate the `forbidden' transitions that become allowed in the low symmetry environment and use these to manipulate the nuclear spin state on a sub-microsecond timescale that is inaccessible via ENDOR and RF techniques. Finally, we explore the existence of subspaces that are robust against strain-induced decoherence and the application of this material as an entanglement-based field sensor. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W27.00007: Electron spin coherence and electron nuclear double resonance of Bi donors in natural Si John Morton, Stephanie Simmons, Richard George, Wayne Witzel, H. Riemann, Nikolai Abrosimov, N. Notzel, Mike Thewalt We have shown that the electron spin coherence times of Si:Bi donors in natural silicon are limited by the same mechanism of spectral diffusion as seen in Si:P, though the smaller Bohr radius of the Bi donor leads to $\sim30\%$ longer T$_2$ times (up to 0.8~ms). We have mapped out the 36 ENDOR transitions observable at X-band arising from the $I=9/2$ nuclear spin of $^{209}$Bi, going up to 1.3~GHz. We also demonstrate the transfer of electron spin coherence to and from the $^{209}$Bi nuclear spin with a fidelity of $\sim63\%$. Using pulsed ESR at W-band (100~GHz), we observe optically-induced dynamic nuclear polarisation, consistent with the mechanism of exciton capture proposed in by T. Sekiguchi \emph{et al.}. Finally, we explore the zero-field splitting of 7.5~GHz in this system, within the context of coupling to superconducting resonators. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W27.00008: Neutral donors interacting with a two-dimensional electron gas measured by electrically detected magnetic resonance up to 94GHz C.C. Lo, J. Bokor, V. Lang, R.E. George, J.J.L. Morton, A.M. Tyryshkin, S.A. Lyon, T. Schenkel Electrically detected magnetic resonance of a silicon field-effect transistor with channel-implanted donors is measured in a W-band ($94\:$GHz, $3.36\:$T) resonant microwave cavity. It is found that the two-dimensional electron gas (2DEG) resonance signal intensity increases by two orders of magnitude compared with conventional low-field X-band ($9.7\:$GHz, $0.35\:$T) measurements. On the other hand, the neutral donor resonance signals increase by over one order of magnitude. We interpret the results in terms of direct spin-dependent scattering and a polarization transfer from the donors to the 2DEG spin system. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W27.00009: Electrical Manipulation of Spin Qubits in Li-doped Si Andre Petukhov, Luke Pendo, Erin Handberg, Vadim Smelyanskiy We propose a complete quantum computing scheme based on Li donors in Si under external biaxial stress. The qubits are encoded on the ground state Zeeman doublets and coupled via long-range spin-spin interaction mediated by acoustic phonons. This interaction is unique for Li donors in Si due to their inverted electronic structure. Our scheme takes advantage of the fact that the energy level spacing in $1s$ Li-donor manifold is comparable with the magnitude of the spin-orbit interaction. As a result the Li spin qubits can be placed 100 nm apart and manipulated by a combination of external electric field and microwave field impulses. We present a specially-designed sequence of the electric field impulses which allows for a typical time of a two-qubit gate $\sim$~1~$\mu$s and a quality factor $\sim 10^{-6}$. These estimates are derived from detailed microscopic calculations of the quadratic Stark effect and electron-phonon decoherence times. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W27.00010: Entanglement in a Solid State Spin Ensemble Stephanie Simmons, Richard Brown, Helge Riemann, Nikolai Abrosimov, Peter Becker, Hans-Joachim Pohl, Mike Thewalt, Kohei Itoh, John Morton Entanglement is a both a fascinating phenomenon and a critical ingredient in most emerging quantum technologies. Spin ensembles manipulated using magnetic resonance have demonstrated the most advanced quantum algorithms to date, however these studies contain no entanglement and hence constitute classical simulations of quantum algorithms. Here we report the on-demand generation of entanglement between an ensemble of electron and nuclear spins in isotopically engineered phosphorus-doped silicon. High field/low temperature electron spin resonance (3.4~T, 2.9~K) was used in conjunction with a hyperpolarisation sequence to reduce the spin entropy to a level sufficient to form an inseparable state. The generated entanglement was confirmed by measuring the state's density matrix which displayed a fidelity of 98\% compared to the ideal state at this field and temperature. The entanglement operation was performed simultaneously, with high fidelity, to $10^{10}$ spin pairs, and represents an essential requirement of a silicon- based quantum information processor. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W27.00011: Optimized Electron-spin-cavity coupling in a double quantum dot Xuedong Hu, Yu-xi Liu, Franco Nori We search for the optimal regime to couple an electron spin in a semiconductor double quantum dot to a superconducting stripline resonator via the electrically driven spin resonance technique. In particular, we calculate the spin relaxation rate in the regime when spin-photon coupling is strong, so that we can identify system parameters that allow the electron spin to reach the strong coupling limit. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W27.00012: Near Field Photon Emission and Revival in Quantum Dot Qubits S. Tafur, M.N. Leuenberger Modeling the spontaneous emission of photons coupled to the electronic states of quantum dots is important for understanding quantum interactions and entanglement in condensed matter as applied to proposed solid-state quantum computers, quantum networks, single photon emitters, and single photon detectors. A quantum dot initially in an excited state can be experimentally observed to decay to its ground state and the observed homodyne tomography of the emitted photon can yield information about the qubit state of the emitter. Though the characteristic lifetime of photon emission is traditionally modeled via the Weisskopf-Wigner approximation, we seek to model the fully quantized spontaneous emission, including near field effects, of a photon from the excited state of a quantum dot beyond the Markovian limit. We further investigate subsequent interactions between the emitted photon and adjacent quantum dots in an effort to describe multipartite entanglement. We propose the use of discretized central-difference approximations of space and time partial derivatives, similar to finite-difference time domain models, to describe single photon states via single photon operators. Additionally, within the future scope of this model, we seek results in the Purcell and Rabi regimes for spontaneous emission events from quantum dots embedded in micro-cavities. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W27.00013: A Theoretical Model of Single Photon Source at Room Temperature Ahmed Elhalawany, Michael Leuenberger In this work we present a theoretical model for an electrically injected single photon source at room temperature. The source is made of three regions. The region containing the source is n-doped, the middle region is an intrinsic semiconductor heterostructure. The region containing the drain is p-doped. The configuration of the intrinsic region is designed to trap a single pair of electron and hole; this is due to Pauli Exclusion Principle and Coulomb blockage. This is achieved by applying a reverse voltage to neutralize the intrinsic electric field between the n- and p-doped regions. Based on the calculated tunneling time of the electron/hole, the reverse voltage will be switched o. For the kinetics at the room temperature operation is calculated by means of the Master equations. For this we use an effective Hamiltonian in the tight-binding approximation. The results show that a single electron and a single hole are trapped simultaneously for an adequate period of time until they recombine. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W27.00014: Phonons and solid-state qubits for quantum technology \"{O}.O. Soykal, Rusko Ruskov, Charles Tahan Phonons in the context of quantum information processing are traditionally negatives. They induce relaxation or decoherence of or between qubit states. Learning to control phonons for positive purposes, both as supporting technology for quantum information processing, and for other quantum devices is of great possible interest. Already, acoustic waves are used as a supporting technology in microelectronics and optoelectronics (e.g. their slow speed can be useful in certain contexts). Here we consider some methods for making phonons useful and describe the physics of such systems in several potential solid-state systems including silicon. Our results may also be of interest to the optomechanics community. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W27.00015: Towards electrons floating over diamond M.P. Ray, J.W. Baldwin, M.K. Zalalutdinov, J.L. Shaw, J.E. Butler, B.B. Pate, T.I. Feygelson The opportunities for development of a 2D electron system of image potential surface electrons over negative electron affinity diamond are examined. Image potential surface electron states, located spatially outside the solid, are well established on a variety of surfaces (metals, semiconductors and dielectrics). In particular, laterally confined electrons above liquid helium have been demonstrated and proposed for advanced computing applications [1,2]. Unlike the surface of liquid helium, the electron affinity of the diamond surface can be varied [3], providing the ability to lithographically pattern surface electron `pools' and `wires'. We present candidate structures for lateral charge control that make use of buried and surface features patterned in and on diamond. Electronic properties and spectroscopy of electrons over diamond in our fabricated structures are discussed. \\[4pt] [1] S. A. Lyon, Phys. Rev. A \textbf{74}, 052338 (2006).\\[0pt][2] P. M. Platzman and M. I. Dykman, Science \textbf{284}, 1967(1999).\\[0pt][3] J. Ristein, Surf. Sci. \textbf{600}, 3677 (2006). [Preview Abstract] |
Session W28: Graphene: Nanoribbons and Electronic Transport
Sponsoring Units: DCMPChair: Nancy Sandler, Ohio University
Room: C156
Thursday, March 24, 2011 11:15AM - 11:27AM |
W28.00001: Low Bias Negative Differential Resistance in Graphene Nanoribbon Superlattices Gerson J. Ferreira, Michael N. Leuenberger, Daniel Loss, J. Carlos Egues We theoretically investigate negative differential resistance (NDR) for ballistic transport in semiconducting armchair graphene nanoribbon superlattices at low bias voltages $V_{SD}$. We combine the modulated graphene-Dirac hamiltonian with the Landauer formalism to calculate the current $I_{SD}$ through the system. This description is expected to be valid at low biases and for narrow samples. We find three distinct transport regimes in which NDR occurs: (i) a ``classical regime'' in which the transport across the crossings of barrier and valley bandgaps is suppressed; (ii) a quantum regime dominated by superlattice miniband conduction, with current suppression arising from the misalignment of miniband states with increasing $V_{SD}$; (iii) a Wannier-Stark ladder regime with current peaks occurring at the crossings of Wannier-Stark rungs from distinct ladders. We emphasize that all the above mechanisms show NDR at voltages lower than 500 mV. Interestingly, within the miniband transport regime the NDR occurs at biases as low as 10 mV, i.e., comparable to the miniband width. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W28.00002: A finite difference method for transport of massless Dirac fermions: The case of graphene nanoribbons Caio Lewenkopf, Alexis Hernandez We develop a new finite difference scheme to numerically compute the scattering matrix of two-dimensional massless Dirac fermions propagating in a ribbon geometry. The method is nonlocal, avoids the fermion doubling problem, and is suitable for introducing different kinds of boundary conditions. To illustrate its utility we compute the Landauer conductance of a monolayer graphene sheets with zig-zag boundary conditions in presence of a perpendicular magnetic field. The method is particularly useful in the study of long range disorder effects (much larger than the lattice spacing) in large graphene strips. In passing, we also show how the method works in the description of electronic transport at the surface of three-dimensional topological insulators. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W28.00003: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W28.00004: Graphene edge from A to Z Yuanyue Liu, Somnath Bhowmick, Boris I. Yakobson We introduce an energy decomposition anzats, which leads to an analytical expression for the edge energy G(X) of arbitrary direction-cut angle X in two dimensional materials [1]. We further show that thermodynamic conditions at the edge simply add a ``chemical phase shift'' C, G(X) = cos(X + C), making the favorable shapes controllable, according to the Wulff construction. Direct atomistic computations and analysis for graphene, as well as 2D boron nitride (h-BN), and zinic oxide (ZnO) support the universal nature of the relationship.\\[4pt] [1] Y. Liu, A. Dobrinsky, and B.I. Yakobson, Phys. Rev. Lett., in press (Dec 10 2010 issue). [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W28.00005: Boron nitride nanoribbons become metallic Alejandro Lopez-Bezanilla, Jingsong Huang, Humberto Terrones, Bobby Sumpter Boron nitride (BN) sheets can be grown on nickel substrates, similar to graphene, and BN domains coexist with graphene. The synthesis of zig-zag BN nanoribbons (zBNNRs) brings interesting possibilities regarding edge chemistry: since both boron and nitrogen atoms are exposed on each edge the functionality of the nanostructure is enriched. We report first principles calculations on the electronic properties of zBNNR nanoribbons with several types of functionalization. Sulfur and oxygen edge doping and topological one-dimensional defects are studied and the possibility of having half metallicity is also analysed. Sulfur and oxygen edge passivation converts zBNNRs into a metallic material which offers several possibilities for new applications in electronics, molecular sensing and spintronics. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W28.00006: Graphene valley pseudospin filter using an extended line defect Daniel Gunlycke, Carter White Although graphene exhibits excellent electron and thermal transport properties, it does not have an intrinsic band gap, required to use graphene as a replacement material for silicon and other semiconductors in conventional electronics. The band structure of graphene with its two cones near the Fermi level, however, offers opportunities to develop non-traditional applications. One such avenue is to exploit the valley degeneracy in graphene to develop valleytronics. A central component in valleytronics is the valley filter, just as the spin filter is central in spintronics. Herein, we present a two-dimensional valley filter based on scattering of electrons and holes off a recently observed extended line defect [Nat. Nanotech. \textbf{5}, 326 (2010)] within graphene. The transmission probability depends strongly on the valley pseudospin and the angle of incidence of the incident quasiparticles. Quasiparticles arriving at the line defect at a high angle of incidence lead to a valley polarization of the transmitted beam that is near 100 percent. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W28.00007: Zero Landau level in folded graphene nanoribbons Elsa Prada, Pablo San-Jose, Luis Brey Graphene nanoribbons can be folded into a double layer system keeping the two layers decoupled. In the Quantum Hall regime folds behave as a new type of Hall bar edge. We show that the symmetry properties of the zero Landau level in metallic nanoribbons dictate that the zero energy edge states traversing a fold are perfectly transmitted onto the opposite layer. This result is valid irrespective of fold geometry, magnetic field strength and crystallographic orientation of the nanoribbon. Backscattering suppression on the N=0 Hall plateau is ultimately due to the orthogonality of forward and backward channels, much like in the Klein paradox. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W28.00008: Effective time-reversal symmetry breaking in the spin relaxation in a graphene quantum dot Philipp Struck, Guido Burkard We study the relaxation of a single electron spin in a circular gate-tunable quantum dot in gapped graphene [1]. Direct coupling of the spin to out-of-plane phonons via the intrinsic spin-orbit coupling leads to a lowered relaxation time $T_1$ at intermediate B-fields. At low fields, $T_1$ increases as $\propto B^{-2}$ due to the suppression of the phonon density of states at long wavelengths in a finite system. We also find that Rashba spin-orbit induced admixture of opposite spin states in combination with the emission of in-plane phonons provides various further relaxation channels via deformation potential and bond-length change. In the absence of valley mixing, spin relaxation takes place within each valley separately and thus time-reversal symmetry is effectively broken, thus inhibiting the van Vleck cancellation at $B=0$ known from GaAs quantum dots. Both the absence of the van Vleck cancellation as well as the out-of-plane phonons lead to a behavior of the spin relaxation rate at low magnetic and intermediate fields which is markedly different from the known results for GaAs. At high fields there is a crossover to $T_1\propto B^{-2}$ or $\propto B^{-4}$.\\[4pt] [1] P. R. Struck and G. Burkard, Phys.\ Rev.\ B \textbf{82}, 125401 (2010). [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W28.00009: Sensory Organ Like Response of Zigzag Edge Graphene Nanoribbons Vijay Shenoy, Somnath Bhowmick Using a continuum Dirac theory, we study the density and spin response of zigzag edge terminated graphene ribbons subjected to edge potentials and Zeeman fields. Our analytical calculations of the density and spin responses of the closed system (fixed particle number) to the static edge fields, show a highly nonlinear Weber-Fechner type behavior where the response depends logarithmically on the edge potential. The dependence of the response on the size of the system (e.g.~width of a nanoribbon) is also uncovered. Zigzag edge graphene nanoribbons, therefore, provide a realization of response of organs such as the eye and ear that obey Weber-Fechner law. We validate our analytical results with tight binding calculations. These results are crucial in understanding important effects of electron-electron interactions in graphene nanoribbons such as edge magnetism etc., and also suggest possibilities for device applications of graphene nanoribbons. \\ Reference: Somnath Bhowmick and Vijay B. Shenoy, {\it Physical Review B}, {\bf 82}. 155448 (2010) [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W28.00010: Transport Through Graphene Surface States Douglas Mason, Eric Heller Of particular interest to models of transport through graphene has been the theoretical prediction of long-lived surface states on zigzag cuts. These states may have a strong influence on transport through finite graphene structures since, unlike infinite nanoribbon surface states, they can absorb and emit electrons traveling through the bulk of the structure. We will be presenting a novel approach to these surface states and postulate on their role in recent transport calculations. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W28.00011: Quantum charge pumping in graphene nanoribbons Tejinder Kaur, Liliana Arrachea, Nancy Sandler The mechanism to generate DC currents in open-quantum systems by applying local de-phased time-dependent potentials is known as charge pumping. For graphene ribbons, pumping techniques provide an alternative route for current production that overcomes the role of contacts. We have analyzed the properties of zero-bias current through graphene nanoribbons using a tight-binding Hamiltonian description and the Keldysh formalism, which provides the proper description for these systems in the quantum non-equilibrium regime. After reviewing results for quantum pumping in a one-dimensional chain attached to two reservoirs, with two local single-harmonic potentials oscillating in time, we will introduce results for finite-width ribbons of square and graphene lattices. A discussion on the differences in transmission functions and DC currents between these two cases will be presented and the role of edge termination in graphene ribbons will be addressed. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W28.00012: Electronic properties of graphene nanoflakes: energy gap engineering Carlos Ramos, Eduardo Cifuentes, Romeo de Coss, Edgar Martinez Graphene nanostructures show an energy gap resulting of the finite size, and are of current interest because of the potential applications in electronic devices. Thus, we discuss some recent progress in the synthesis of graphene nanoflakes obtained from the reaction of polyaromatic hydrocarbons. We are presenting ab-initio results for the electronic properties of graphene nanoflakes with a hexagonal-zigzag (HZ) structure and different effective radius (R). The calculations were performed using the Density Functional Theory as implemented in the pseudopotential-LCAO method. We find that the, Kohn-Sham gap decreases with size as R$^{-1}$, while the quasi-particle energy gap follow the R$^{-0.8}$ scaling rule. A formula to evaluate the energy gap of a HZ graphene nanoflake of arbitrary size is provided. This research was supported by Conacyt-M\'exico under Grant No. 83604. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W28.00013: Electric and Magnetic Field Induced Insulating States in High Quality Bilayer Graphene pnp Junctions Lei Jing, Jairo Velasco, Philip Kratz, Gang Liu, Wenzhong Bao, Marc Bockrat, Chun Ning Lau Band gap opening in bilayer graphene has generated significant interest in both technological application and fundamental research. By applying external electric and magnetic field, we observe an insulating state in our dual-gated bilayer graphene device. In addition, we also observe Quantum Hall plateaus with fractional values of $e^{2}/h$ at large magnetic field, which arises from edge state equilibration at the interface of differentially doped regions, in agreement with theoretical predictions. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W28.00014: Electron-Hole Asymmetry and Electron-Electron Interaction in Bilayer Graphene K. Zou, X. Hong, J. Zhu We report precision measurements of the effective mass $m$* in high-quality bilayer graphene samples using the temperature dependence of Shubnikov-de Haas oscillation. In the measured density range of 0.7x10$^{12}$/cm$^{2}$ $<$ $n$ $<$ 4.1x10$^{12} $/cm$^ {2}$, both the hole mass $m$*$_{\mathrm{h}}$ and the electron mass $m$*$_ {\mathrm{e}}$ increase with increasing $n$, with $m$*$_{\mathrm {h}}$ being roughly 20-30$\%$ larger than $m$*$_{\mathrm{e}}$ at the same density. We compare our results to tight-binding calculations and provide an accurate determination of several hopping parameters. The measured $m$* is substantially suppressed compared to non- interacting values, demonstrating the importance of electron- electron interaction in bilayer graphene. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W28.00015: Magneto-transport study of band structure of tri-layer graphene Ryuta Yagi, Seiya Fukada, Midor Shimomura We have studied magneto-transport of high-quality tri-layer graphene samples. It has been observed Shubnikov-de Haas oscillations with two different frequencies that corresponded to two bands in tri-layer graphene. Detailed analysis of gate voltage dependence of the frequencies showed that sum of the carrier density for each band gave approximately the nominal carrier density tuned by the gate voltage. From temperature dependence of magnitude of the oscillation we have estimated cyclotron masses. [Preview Abstract] |
Session W29: Symmetric Discrete Structures for Finite Dimensional Quantum Systems
Sponsoring Units: GQIChair: Christopher Fuchs, Perimeter Institute for Theoretical Physics
Room: C148
Thursday, March 24, 2011 11:15AM - 11:51AM |
W29.00001: Pairwise complementary observables and their mutually unbiased bases Invited Speaker: Pairs of complementary observables (PCO) characterize all quantum degrees of freedom and are central to a technical formulation of Bohr's principle of complementarity. A defining property of such pairs are their mutually unbiased bases (MUB) of eigenstates. MUB have found many applications for tasks in quantum information processing. Maximal sets of PCO and MUB are known, by explicit construction, for degrees of freedom that live in finite-dimensional Hilbert space whose dimension is a power of a prime; continuous sets of MUB are also known for most continuous degrees of freedom. I will review the situation and mention a couple of open problems. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W29.00002: Quantum States as Probabilities from Symmetric Informationally Complete Measurements Invited Speaker: If you pick $d^2$ symmetrically spread vectors in a $d$-dimensional Hilbert space, you get a symmetric informationally complete set of quantum states (or SIC for short). SICs have applications within quantum information science, such as to quantum state tomography and quantum cryptography, and are also of interest for foundational studies of quantum mechanics. In this talk I will review the representation of quantum states as probability distributions over the outcomes of a SIC measurement. Not all probability distributions correspond to quantum states, thus quantum state space is a restricted subset of all potentially available probabilities. We will explore how this restriction can be characterized. A recent publication (Fuchs and Schack, arXiv:0906.2187) advocates the SIC-representation and suggests that the Born rule rewritten in this language can be taken as a postulate for quantum mechanics. This motivates the introduction of so-called maximally consistent sets (Appleby, Ericsson, and Fuchs, arXiv:0910.2750); one such set is quantum state space. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W29.00003: The Lie Algebraic Significance of Symmetric Informationally Complete Measurements Invited Speaker: Examples of symmetric informationally complete positive operator valued measures (SIC-POVMs) have been constructed in every dimension less than 68. However, it remains an open question whether they exist in all finite dimensions. A SIC-POVM is usually thought of as a highly symmetric structure in quantum state space. However, its elements can equally well be regarded as a basis for the Lie algebra gl(d,C). We examine the resulting structure constants, which are calculated from the traces of the triple products of the SIC-POVM elements and which, it turns out, characterize the SIC-POVM up to unitary equivalence. We show that the structure constants have numerous remarkable properties. In particular we show that the existence of a SIC-POVM in dimension d is equivalent to the existence of a certain structure in the adjoint representation of gl(d,C). We hope that transforming the problem in this way, from a question about quantum state space to a question about Lie algebras, may help to make the existence problem tractable. This is joint work with M. Appleby and C. Fuchs. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:39PM |
W29.00004: Experimental access to higher-dimensional discrete quantum systems, towards realizing SIC-POVM and MUB measurements, using integrated optics Invited Speaker: The aim of our work is to access and explore higher-dimensional photonic quantum systems. In terms of stability and complexity, normal bulk-optic setups greatly limit the capabilities of reaching higher-dimensional systems. However, the recent development in integrated photonic circuits has opened new possibilities [1]. Our approach is to use integrated photonic circuits on-chip, as well as in fiber, to reach photonic states of higher dimension. We are working toward a fully integrated realization of a multiport [2], a device which can apply any unitary transformation based on tunable internal parameters. Our first step is to realize a multiport in dimension four, implementing any unitary transformation on Qubits, Qutrits and Ququarts. Furthermore, we have built an integrated source using purely in-fiber components for creating higher-dimensional entangled photons. The combination of this source with the multiport yields a very general system applicable to a variety of experiments in higher dimensional Hilbert spaces. It is possible to realize different experimental setups by setting the device for different incoming entangled states, and subsequently applying unitary transformations. For example, this opens the possibility to observe new types of higher-order perfect correlations [3], or to realize full SIC-POVM measurements in higher dimensions.\\[4pt] [1] J.L.O'Brien, G.J.Pryde, A.G.White, T.C.Ralph and D.Branning, Nature Vol.426, pp264-267 (2003) \newline [2] M.Reck and A.Zeilinger, PRL Vol.73, No.1 (1994) \newline [3] M.Zukowski, A.Zeilinger and M.A.Horne, PRA Vol.55, No.1 (1997) [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 2:15PM |
W29.00005: Isotropic States in Discrete Phase Space Invited Speaker: An energy eigenstate of a harmonic oscillator is isotropic in phase space, in the sense that the state looks the same along any ray emanating from the origin. It is possible to extend this notion of ``isotropic'' to quantum systems with finite-dimensional state spaces---the rays are then rays in discrete phase space. In this talk I present examples of discrete isotropic states and discuss their properties. One can show that every isotropic state minimizes a specific information-theoretic measure of uncertainty with respect to a complete set of mutually unbiased bases. Numerical results on a certain class of isotropic state vectors suggest that their components, in any of those same mutually unbiased bases, exhibit a semicircular distribution when the dimension of the state space is large. [Preview Abstract] |
Session W30: Nanowires and Nanotubes: Optical Properties and Spectroscopy
Sponsoring Units: DCMPChair: Alec Talin, National Institute of Standards and Technology
Room: C147/154
Thursday, March 24, 2011 11:15AM - 11:27AM |
W30.00001: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W30.00002: Low temperature electron transport spectroscopy of mechanically templated carbon nanotube quantum dots Saiful Khondaker, Paul Stokes We report on the low temperature electronic transport measurements of mechanically templated carbon nanotube quantum dots (QDs). The devices were fabricated by precise dielectrophoretic placement of single walled carbon nanotubes (SWNTs) between 1 $\mu $m spaced Pd electrodes and over a local Al/Al$_{2}$O$_{3}$ bottom gate. The local gate defines the quantum dot due to the bending of SWNT at the edges of the gate, as well as controls its operation [1]. We performed detailed transport spectroscopy measurements of the templated SWNT QDs to determine how the tunnel barriers evolve with both the global back gate and local gate voltage. We will present models to explain the evolution of these devices as a function of local gate and back gate voltage. This study may allow for future size tunabiliy of SWNT QDs by further control over the tunnel barrier transparency and source, drain, and gate capacitance to fabricate room temperature single electron transistors. \\[4pt] [1] Paul Stokes and Saiful I. Khondaker, Appl. Phys. Lett. \textbf{92}, 262107 (2008). [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W30.00003: Tunneling spectroscopy using carbon nanotubes quantum dots Yanjing Li, Nadya Mason Tunneling spectroscopy is an important measurement technique, encompassing, for example, planar tunneling, scanning tunneling microscopy and superconducting tunnel probes. Here, we demonstrate that carbon nanotube quantum dots (QDs) can be used as tunneling probes. The sharp features in the density of states of the QDs in the Coulomb blockade regime can map out the density of states and the energy distribution function of the system to be studied. We present preliminary data showing tunneling from a carbon nanotube quantum dot into mesoscopic metal wires that have been driven out of equilibrium by a bias voltage. Previous measurements of these systems using superconducting probes [1-2] showed that the electron energy distribution functions and electron interactions can be determined. With the present measurements, the use of a QD instead of a superconductor allows us to probe at significantly higher temperatures and biases. \\[4pt] [1] H. Pothier, S. Gueron, Norman O. Birge, D. Esteve, and M. H. Devoret, Phys Rev Lett 79, 3490 (1997) \\[0pt] [2] Yung-fu Chen, Travis Dirks, Gassem Al-Zoubi, Norman O. Birge, and Nadya Mason, Phys Rev Lett 102, 036804 (2009) [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W30.00004: Photoconductivity Spectroscopy of Colloidal Lead Selenide Nanowires Rion Graham, Ka Leung, Dong Yu PbSe, with a large Bohr exciton radius of 46 nm, is ideally suited to study strong quantum confinement effects. Photoconductivity spectroscopy of single PbSe nanowire devices can remove the inhomogeneous broadening of ensemble measurements and allows for extraction of fine electronic structures in two dimensionally confined semiconductors. We have grown PbSe nanowires via a colloidal, oriented attachment mechanism, with diameters down to 6nm. Using a tunable wavelength laser, we have measured photoconductivity of single nanowire field effect transistors (FETs) as a function of excitation energy, temperature, and nanowire diameters. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W30.00005: The optical properties of SnO$_{2}$ nano-wires by THz time-domain spectroscopy Dongwook Lee, Xingquan Zou, Chuanwei Cheng, Saritha K. Nair, Hongjin Fan, Elbert E.M. Chia As feature seizes of devices are reduced below a hundred nanometers and chip frequencies reaches high GHz, a convenient method of characterizing thin films in the GHz to THz frequency range has been required. THz time-domain spectroscopy provides a non-destructive, non-contact, and high-sensitive tool to characterize thin films as well as nano-materials. The optical properties of SnO$_{2}$ nano-wires, which were grown on z-cut quartz, have been investigated by THz time-domain spectroscopy. The real and imaginary parts of the complex refractive index and optical conductivity are measured at THz frequency. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W30.00006: Direct mapping of the exciton-polariton dispersion in tree-like ZnO micro-structures Xiangshun Lu, Huajun Zhou, Z. Ryan Tian, Min Xiao We report a direct observation of branches of the exciton polaritons in tree-like ZnO micro-structures using second-harmonic generation (SHG) spectroscopy. Within the tunable range of a mode-locked Ti:Sapphire laser with pulse width of 100 femto-second, we are able to tune the second harmonic energy of the incident laser across the A, B and C excitons of ZnO. Under the resonant enhancement of the strong coupling between photons and excitons, we obtain for the first time the direct mapping of the branches of exciton-polariton dispersion at the room as well as liquid nitrogen temperatures. We also observed strong modulation of the polariton spectra by the whispering-gallery modes (WGMs) formed inside the tree-like micro-structure. The disappearance of the SHG in tree-like ZnO near/above the energy of the excitonic band gap indicates the much higher efficiency of energy conversion, comparing to the simple ZnO rod, and reveal the practical and important applications of the tree-like micro-structures in solar cell. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W30.00007: Optical anisotropy of transparent multiwalled carbon nanotube sheets Julia Bykova, Yakov Lesnichii, Aleksey Arsenin, Dmitry Fedyanin, Jonathon Smith, William Holmes, Anvar Zakhidov Replace this text with your abstract body. The oriented carbon nanotube (CNT) sheets absorb polarized light anizotropically depending on its relative orientation to the incident light. Oriented aerogel of multi-walled carbon nanotubes (MWNT) created by dry-drawing of spinnable CNT forests has been shown to be a relatively good polarizer [1] even at high temperatures [2]. In this presentation it is shown how the qualitative factor of polarization (QFP) depends on the CVD synthesis conditions and CNT sheets process. The optimized QFP of MWNT sheet is found to be 2-3 times higher than reported before for CNT-based polarizers. Systematic characterization by UV-Vis spectroscopy, elipsometry combined with SEM showed how the polarization properties can be improved by annealing, stretching and electrical field, which enhance the anisotropy of MWCNT sheets. This work is supported by AFOSR grant FA 9550-09-10384 and AFRL/Rice grant via CONTACT consortium of Texas. [1] M. Zhang et al., Science, 309 (2005) 1215 [2] A. Aliev at al. Phys Let A, 372 (2008) 4938 [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W30.00008: Synthesis and Optical Properties of Free Standing Titania Nanotubes Arrays for Photovoltaic and Photocatalytic Applications Dalmau Reig-i-Plessis, Mohamed Abd Elmoula, Eugen Panaitescu, Donald Heiman, Latika Menon Titanium oxide nanostructures, nanotubes arrays in particular, are key components for several emerging technologies, notably dye sensitized solar cells and supported gold nanoparticle catalysts. Free standing nanotube ordered arrays were produced by anodization of titanium foil in non-aqueous solutions. For optical measurement purposes the arrays were flaked off the titanium substrate. Flakes as large as several square centimeters, with a thickness of 30-50 $\mu $m were obtained, and coated either with N719 dye, or with gold nanoparticles (2-7nm). Both annealed (crystalline) and non-annealed (amorphous) samples were characterized by means of SEM and TEM imaging coupled with EDS and XRD spectroscopy. Subsequent optical measurements on gold decorated nanotubes offered insight on the gold-titania interaction, while measurements on both naked and dye coated tubes provided information on diffraction and thin film effects. These effects caused a significant wavelength dependent difference in the reflection spectrum depending on whether illumination was incident on the open or closed side of the tubes. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W30.00009: Electro-Optic Effects in Colloidal Dispersion of Metal Nano-Rods with Nematic Ordering Oleg D. Lavrentovich, Andrii B. Golovin, Jie Xiang, Heung-Shik Park, Luana Tortora, Yuriy A. Nastishin, Sergij V. Shiyanovskii In modern transformation optics, one explores metamaterials with properties that vary from point to point in space and time, suitable for applications in devices such as an ``optical cloak'' [1] and an ``optical black hole'' [2]. We propose an approach to construct spatially varying and switchable metamaterials that are based on colloidal dispersions of gold (Au) nano-rods (NRs) in dielectric fluids [3], in which dielectrophoretic forces, originating in the electric field gradients, create spatially varying configurations of aligned NRs. We quantify the electric field induced orientational order and concentration distribution of Au NRs and the describe the ensuing optical effects. We demonstrate that the gradient electric-field induces a nematic birefringent phase in the toluene dispersions of AuNR and determine how the refractive indices change in space. [1] W. Cai, U.K. Chettiar, A.V. Kildishev, V.M. Shalaev, Nature Phot. \textbf{1}, 224 (2007); [2] E.E. Narimanov, A.V. Kildishev, Appl. Phys. Lett. \textbf{94}, 041106 (2009); [3] A.B. Golovin, O.D. Lavrentovich, Appl. Phys. Lett. \textbf{95}, 254104 (2009). [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W30.00010: Measurement of carrier lifetimes in silicon vapor-liquid-solid wires Brian Bryce, Mark Reuter, Brent Wacaser , Sandip Tiwari Minority carrier lifetimes are critically important to many semiconductor devices. For example, optimal photovoltaic design is almost completely dependent on knowledge of carrier lifetimes. We have extended traditional microwave photoconductance methods for use on aggregated films of nanowires. Using these methods we have measured the carrier lifetimes of both gold and aluminum catalyzed silicon vapor-liquid-solid wires in the 100-800 nm range. This approach allows for rapid characterization of wire quality prior to device design and fabrication. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W30.00011: Exciton dynamics for single walled carbon nanotubes in the presence of a single ion Benjamin Tayo, Slava Rotkin We study the dynamics of excitons in single walled carbon nanotubes in the presence of a single ion placed on the surface of the tube. The scattering process is described in three main stages. First, we solve the Schr\"{o}dinger equation in the tight binding approximation to calculate the quasiparticle wave functions and energies. Second, we use quasiparticle wave functions and energies in the Bethe-Salpeter equation to calculate exciton binding energies and wave functions. Finally, we use the exciton energies and wave functions to investigate the process of exciton-ion scattering. We model the potential of the single ion by that of a point charge. Our studies show exciton trapping in the presence of the ion. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W30.00012: The emissivity of an incandescent carbon nanotube B.C. Regan, Scott Singer, Matthew Mecklenburg, Edward White A classical thermal emitter has physical dimensions large compared to the wavelength $\lambda$ of the emitted light, and radiates power in proportion to its surface area. To explore the non-classical limit, we build tiny incandescent lamps with individual multi-walled carbon nanotubes as their filaments. We image a filament with atomic resolution in a transmission electron microscope, determining its length $L$ and radius $r$. Separately we apply Joule heating to reach temperatures $\sim 2000$~K, where the nanotube radiates in the visible ($L \sim \lambda \gg r$), and collect the light with an optical microscope. Comparing the filament's brightness with its tiny physical dimensions, we find that a single carbon nanotube is surprisingly black. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W30.00013: Polarized Light Emission from a Single Hot Carbon Nanotube S.B. Singer, Matthew Mecklenburg, Edward White, B.C. Regan We fabricate nanoscale lamps, incandescent in the visible, which have a filament consisting of a single multiwall carbon nanotube. The radius $r$ of the nanotube is much smaller than the wavelength $\lambda$ of the emitted light, making it a very unusual thermal emitter. Transmission electron microscopy is used to determine the nanotube's axis as well as the parameters of the tube's geometry. We image both light polarizations on a CCD camera simultaneously and observe a degree of polarization between 70\% and 85\% along the tube's axis at visible wavelengths--highly polarized, yet less so than is expected for a conducting antenna. Furthermore, the polarization's variation with wavelength trends opposite to that predicted by classical models and analogy with graphene. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W30.00014: Photoconductivity of complexes of chrysotile with tetra(4-sulfonatophenyl) porphyrins created via ionic self-assembly Wesley Chu, Walter Smith, Ye Lu, A.T. Johnson, Giovanna De Luca, Luigi Scolaro Tetrakis(4-sulfonatophenyl) porphine (TPPS4) molecules form complexes with chrysotile nanotubes in a chloroform-methanol solvent; the self-assembly is driven by Coulombic attractions. The UV/vis absorption spectrum indicates J-aggregation. In AFM, the complexes appear as straight, long tubes when deposited onto oxidized silicon substrates. Preliminary experiments conducted in a dry nitrogen atmosphere (0.2{\%} oxygen concentration) show that the aggregates are photoconductive. When illuminated for long periods at 428 nm, the photoconductivity grows slowly, and there is also a growth of persistent photoconductivity. These observations are similar to those for nanorods self-assembled from TPPS4 (without chrysotile),\footnote{C. K. Riley et al., \textit{J. Phys. Chem. C }\textbf{2010}, 114, 19227--19233.} though the current levels are much lower in the chrysotile complexes. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W30.00015: A confinement phase in carbon nanotubes as captured by the extended massive Schwinger model Takashi Oka, Hideo Aoki We propose that the strong-coupling excitons in carbon nanotubes with electric fluxes confined in one dimension can be a condensed-matter candidate for a confinement phenomenon. Namely, we show that the system, with the Coulomb interaction proportional to $\vert x\vert $, is in a confinement phase with many properties similar to QCD in 4D. Low-energy physics is described by the massive Schwinger model with multi-species fermions labelled by the band and valley indices. We propose two means to detect this. One is an optical measurement of the exciton spectrum, where the confinement phase should be hallmarked by an absence of continuous component in the exciton spectrum. The spectrum is actually calculated with the 't Hooft-Berknoff equation utilizing the light-front field theory, where the Gell-Mann-Oakes-Renner relation is shown to be satisfied by dark excitons. The second way is through the nonlinear transport, which is shown to be related to Coleman's ``half-asymptotic" state. (arXiv:1007.5393) [Preview Abstract] |
Session W31: Energy Production & Distribution, Nuclear, Hydrogen, Bio and Infrastructure
Sponsoring Units: GERAChair: Judy Pang, Oak Ridge National Laboratory
Room: C145
Thursday, March 24, 2011 11:15AM - 11:27AM |
W31.00001: Phonon dynamics of UO$_{2}$ at high temperature J.W.L. Pang, A. Chernatynskiy, B.C. Larson, S.R. Phillpot, W.J.L. Buyers Inelastic neutron scattering and numerical simulations are being used to investigate the fundamental aspects of phonons and thermal transport in UO2 as part of a DOE-EFRC ``Center for Materials Science of Nuclear Fuel'' project. Understanding thermal transport associated with nuclear fuel environments requires a correct accounting for a wide range of phonon scattering processes, including anharmonic phonon-phonon, phonon-fission product, and phonon-defect cluster. Reactor and spallation neutron measurements of phonon dispersion, phonon linewidths and density of states in UO$_{2}$ at room and high temperature are in progress for direct comparison with atomic potential lattice-dynamics simulations of phonon dispersion, phonon group velocity, phonon linewidth, and phonon density of states. Direct comparisons between experimental measurements and numerical simulations in UO$_{2}$ as a function of temperature will be presented. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W31.00002: An \textit{Ab Initio} Study of $\alpha $ --Pu Sarah C. Hernandez, Asok K. Ray Hybrid density functionals, which replace a fraction of density functional theory exchange with exact Hartree-Fock exchange has been used to study the electronic, geometric, and magnetic properties of $\alpha $ -- Pu. Different fractions of Hartree-Fock exchange have been used and the computations have been performed using the all-electron \underline {f}ull-\underline {p}otential \underline {l}inearized \underline {a}ugmented \underline {p}lane \underline {w}ave plus \underline {l}ocal \underline {o}rbitals basis method. Pu has been studied at the non-magnetic, ferromagnetic and anti-ferromagnetic configurations with spin-orbit coupling, orbital polarization, and \textit{full} geometry optimizations. The variations of the optimized lattice constants, magnetic moments, bulk moduli, density of states, and the degree of $5f $electron localization with the amount of Hartree -- Fock exchange will be reported. Results will be compared with those of $\delta $ -- Pu for which the performance of the hybrid functionals did \textit{not} seem superior to that of the pure density functionals.\footnote{R. Atta-Fynn and A. K. Ray, Europhys. Lett. 85, 27008 (2009).} [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W31.00003: Electron Correlation and Tranport Properties in Nuclear Fuel Materials Quan Yin, Kristjan Haule, Gabriel Kotliar, Sergey Savrasov, Warren Pickett Using first principle LDA+DMFT method, we conduct a systematic study on the correlated electronic structures and transport properties of select actinide carbides, nitrides, and oxides, many of which are nuclear fuel materials. Our results capture the metal--insulator Mott transition within the studied systems, and the appearance of the Zhang-Rice state in uranium dioxide. More importantly, by understanding the physics underlying their transport properties, we suggest ways to improve the efficiency of currently used fuels. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W31.00004: Reaction Rate Measurement at the Californium User Facility (CUF) for unfolding the neutron spectrum Mohammad Hannan, Ruben Ortega Neutron Activation Analysis was used to determine Reaction Rate measurement of several activation detectors at the ORNL Californium User Facility (CUF). The irradiations were performed with 34 mg Cf$^{252}$ neutron source strength.. Ten source capsules $>$ 34 mg were positioned concentrically around a sample cavity. We have determined absolute activity per atom of 9 detectors: Au$^{197}$ (n,$\gamma )$ Au$^{198}$, Al$^{27}$(n,$\alpha )$Na$^{24}$, Al$^{27}$(n,p) Mg$^{27}$, Fe$^{56}$(n,p) Mn$^{5}$, Fe$^{54}$(n,p) Mn$^{54}$, In$^{115}$ (n,$\gamma )$In$^{116}$, Ti$^{46}$(n,p)Sc$^{46}$, Ni$^{60}$ (n,p) Co$^{60}$, Fe$^{58}$(n,$\gamma )$ Fe$^{59}$. The errors are within 1.5-8{\%} except Ni$^{60}$ and Fe$^{58}$ have errors of 46{\%} and 32 {\%}. These high errors may be attributed to the counting statistics. These reaction rate values will be used to unfold the neutron spectrum of the CUF using the MAXED 2000, a computer code for the de convolution of multi sphere neutron spectrometer data and the results are discussed. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W31.00005: A first-principles investigation of III-V semiconductor-water interfaces for solar hydrogen production Brandon Wood, Tadashi Ogitsu, Eric Schwegler Photoelectrochemical devices promise sustainable hydrogen production using sunlight and water. Currently, the highest efficiency devices use III-V semiconductor photoelectrodes; however, stability of these materials under operating conditions remains an issue. In an effort to understand the chemical properties of the electrode-water interface, we have performed first-principles molecular dynamics simulations on model III-V surfaces in realistic aqueous environments. The structure, stability, and chemical activity of these surfaces are investigated, with the aim of understanding the reactive states precursory to photoexcitation and hydrogen evolution. Our results show that surface oxide nucleation is key to facilitating surface reactivity, and that the surface oxygen bonding arrangement is important for determining of the available pathways for water dissociation and corrosion. This points to the importance of III-V surface oxides as intermediates in the water-dissociation component of hydrogen evolution. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W31.00006: Hydrogen Generation and Photoelectrochemical Effect of InGaN alloys Krishna Aryal, Bed Pantha, Rajendra Dahal, Jing Li, Jingyu Lin, Hongxing Jiang Generation of hydrogen gas, a clean source of energy with the highest conversion efficiency, via water splitting, using renewable resources has attracted tremendous research work in recent years. For producing hydrogen gas, a promising method using semiconductor materials is direct photoelectrolysis by solar water splitting. In$_{x}$Ga$_{1-x}$N alloys grown by metal organic chemical vapor deposition (MOCVD) are very promising candidates for water splitting because of their direct band gap which can be tuned to the entire solar spectrum through band gap engineering. It was found that n-GaN has a higher photocurrent density (J$_{ph})$ at zero bias, while an InGaN alloy provides much higher hydrogen generation rate (R$_{H})$ with a small external bias. R$_{H}$ of about 0.024 mL/min.cm$^{2}$ was obtained using an In$_{0.18}$Ga$_{0.82}$N as working electrode. The characteristics of time dependent J$_{ph}$ for a prolonged period of time (up to 7 days) showed higher chemical stability of the InGaN electrodes in aqueous solution of HBr. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W31.00007: Sieving hydrogen based on its high compressibility Hangyan Chen, Deyan Sun, Xingao Gong, Zhifeng Liu Based on carbon nanotube intramolecular junction and a C60, a molecular sieve for hydrogen is presented. The small interspace between C60 and junction provides a size changeable channel for the permselectivity of hydrogen while blocking Ne and Ar. The sieving mechanism is due to the high compressibility of hydrogen. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W31.00008: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W31.00009: Combining micro-structures and micro-algae to increase lipid production for bio-fuel Saurabh Vyawahare, Emilly Zhu, Troy Mestler, Andr\'e Est\'evez-Torres, Robert Austin 3rd generation bio-fuels like lipid producing micro-algae are a promising source of energy that could replace our dependence on petroleum. However, until there are improvements in algae oil yields, and a reduction in the energy needed for processing, algae bio-fuels are not economically competitive with petroleum. Here, we describe our work combining micro-fabricated devices with micro-algae \textit{Neochloris oleoabundans}, a species first isolated on the sand dunes of Saudi Arabia. Inserting micro-algae of varying fitness into a landscape of micro-habitats allows us to evolve and select them based on a variety of conditions like specific gravity, starvation response and Nile Red fluorescence (which is a marker for lipid production). Hence, we can both estimate the production of lipids and generate conditions that allow the creation and isolation of algae which produce higher amounts of lipids, while discarding the rest. Finally, we can use micro-fabricated structures and flocculation to de-water these high lipid producing algae, reducing the need for expensive centrifugation and filtration. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W31.00010: Structure and diffusion of furans and other cellulose-derived compounds in solvents via MD simulation Brooks Rabideau, Ahmed Ismail There is now a large push towards the development of energy sources that are both environmentally friendly and sustainable; with the conversion of cellulose derived from biomass into biofuels being one promising route. In this conversion, a variety of intermediary compounds have been identified, which appear critical to successful expansion of the process to an industrial scale. Here we examine the structure and diffusion of these furans and acids derived from cellulose within ionic liquids via molecular dynamic simulation. Ionic liquids have shown the ability to dissolve cellulose with certain `green' benefits over existing, conventional solvents. Specifically, we study the solvation properties of these chemicals by examining the pair correlation functions of solute with solvent, and by exploring the agglomeration and separation of these chemicals from the solvent as well as the hydrogen bonding between species. Additionally, we determine the diffusion constant of these compounds in ionic liquid and aqueous solvents. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W31.00011: Molecular-dynamics study of proton transport near an ionomer-electrode interface Philip Taylor, Elshad Allahyarov Coarse-grained molecular-dynamics simulations have been used to study the behavior of an ionomer electrolyte in response to an induced current. We observe the changes in the distribution of charge concentration and local electrostatic field in the region near an electrode in contact with a Nafion-like ionomer. We have also analyzed how the morphology of the sulfonate clusters and the transport of water molecules depends on the current strength. In this study we insert protons at the electrode interface of the material and remove them at a plane some distance into the material. When a steady state is achieved we note the new charge distribution and average voltage difference between the faces of the simulation cell. We also note the change in distribution of water molecules within the material in response to the induced current of protons. We compare these results with those predicted by one-dimensional theoretical models. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W31.00012: Origin of colossal ionic conductivity in YSZ/STO multilayers Timothy Pennycook, Mark Oxley, Matthew Beck, Javier Garcia-Barriocanal, Flavio Bruno, Carlos Leon, Jacobo Santamaria, Maria Varela, Stephen Pennycook, Sokrates Pantelides A colossal eight order of magnitude increase in the ionic conductivity of yttria- stabilized zirconia (YSZ) near room temperature was recently reported in YSZ/strontium titanate (STO) epitaxial heterostructures [1]. We present density functional theory results that explain the enhancement in terms of strain- and interface-induced disorder of the YSZ O-sublattice [2]. We further present experimental confirmation of O disorder using a combination of scanning transmission electron microscopy and electron energy loss spectroscopy. The O K-edge fine structure shows blurred-out features indicative of disorder [3]. Atomic-resolution elemental mapping clearly resolves the O sublattice in the STO but is blurred out in the YSZ, indicating disorder. This work is supported by DOE grant DE-FG02-09ER46554 and DOE Materials Sciences and Engineering Div. 1 J. Garcia-Barriocanal et al. Science 321, 676 (2008); 2 T.J. Pennycook et al., Phys. Rev. Lett. 104, 115901 (2010); 3 T.J. Pennycook et al., European Phys. J. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W31.00013: Microscopic Understanding of Reactivity of Clinkers for Green Cement Engin Durgun, Hegoi Manzano, Roland J. M. Pellenq, Jeffrey C. Grossman Cement is the cause of up to 10 percent of global CO2 emissions, and yet, while it is one of the most common materials in use, we have remarkably little understanding of its microscopic properties. Toward this end, we use quantum mechanical simulations to examine the electronic properties and structure of cement crystals and to understand the surface reactivity of various clinker phases. Using these results, our aim is to clarify the mechanisms of cement dissolution, which is the initial stage of hydration and also one of the key processes that leads to the need for high temperature/energy manufacturing. As a first step we modeled the crystal structure of two major clinker phases, alite and belite and analyzed both electronic and mechanical properties. Next, we cleaved the clinker crystal in the simulation along different symmetry directions in order to obtain a prediction of the most stable surfaces. Dissolution occurs at the surface so accurate determination of the surface pattern is crucial. Using the computed surface energies, we can predict the full structure of the clinker nanocluster. This allows us to examine the interaction of water molecules with different nanocluster phases, in order to shed light on the dissolution mechanism and use this new understanding to predict possible novel routes for modifying and controlling the dissolution reactions. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W31.00014: Comparison of Solar and Wind Power Output and Correlation with Real-Time Pricing Kathryn E. Hoepfl, Alvin D. Compaan, Andrew Solocha This study presents a method that can be used to determine the least volatile power output of a wind and solar hybrid energy system in which wind and solar systems have the same peak power. Hourly data for wind and PV systems in Northwest Ohio are used to show that a combination of both types of sustainable energy sources produces a more stable power output and would be more valuable to the grid than either individually. This method could be used to determine the ideal ratio in any part of the country and should help convince electric utility companies to bring more renewable generation online. This study also looks at real-time market pricing and how each system (solar, wind, and hybrid) correlates with 2009 hourly pricing from the Midwest Interconnect. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W31.00015: Monte Carlo optimization of a matrix-based power-grid islanding algorithm Ibrahim Abou Hamad, Per Arne Rikvold, Svetlana V. Poroseva Spectral matrix methods are widely used for intelligent intentional islanding of power grids, the purposeful partitioning of a utility system to limit cascading disturbances. However, these methods may produce unbalanced islands of generators and loads when applied recursively. While some of the resulting islands have surplus generating capacity, others are deficient. We here implement a Monte Carlo simulated-annealing optimization procedure to load-balance the islands and increase their internal connectivity or modularity. After a matrix-based initial agglomeration of nearby loads and generators, Monte Carlo is used to redistribute loads among neighboring islands. The resulting network of islands is treated as a new network with the first-generation islands as the new nodes (``supergenerators'' and ``superloads''), and the same agglomeration and MC procedures are iteratively applied. We show here that combining matrix-based agglomeration and Monte Carlo methods results in well balanced, internally connected islands. [Preview Abstract] |
Session W32: Focus Session: Frontiers in Computational Thermodynamics of Materials I
Sponsoring Units: FIAP DCOMPChair: Gus Hart, Brigham Young University
Room: C144
Thursday, March 24, 2011 11:15AM - 11:51AM |
W32.00001: Modeling the interactions of adsorbates with each other and with metal surfaces Invited Speaker: The interactions of molecules with metallic surfaces are fundamental to the ability of metals to catalyze reactions. One often thinks of a metal like platinum as the catalyst, but under reaction conditions the reactivity of the metal surfaces is modified by the molecules that adsorb on them. We have used quantum chemical calculations in conjunction with cluster expansions to probe the adsorption behavior of atomic adsorbates such as C, N, O, and S on late transition metal surfaces such as Rh, Ir, Pd, Pt, Cu, Ag, and Au(111). There are remarkable similarities in the adsorption behavior of these adsorbates that can be interpreted in terms of a simple adsorbate-induced surface electronic structure modification mechanism that is common to all the adsorbates and surfaces. The variations between the adsorbates and metals are readily explained in terms of the size of the metal and adsorbate orbitals and the geometry dependent overlap of these orbitals. We have constructed a new Solid State Table of these orbital radii from the quantum chemical calculations that can be used in conjunction with a simple model to rapidly estimate the electronic structure of metal and alloy surfaces with adsorbates on them. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W32.00002: An Automatic Symmetry-Leveraging Approach for Solving Incomplete Many-Atom Crystal Structures Bryce Meredig, Chris Wolverton We present a new first principles-based method, called a symmetry-leveraging genetic algorithm (SLGA), for fully and automatically solving large crystal structures when experimental diffraction studies do not identify all internal atomic positions. Such incomplete structural refinements may occur when crystals contain light atoms or when the characterization is performed under extreme conditions such as high pressure. We apply our method to solve the crystal structure of the promising hydrogen storage candidate magnesium imide (MgNH), which has remained a mystery for over 40 years. We also confirm \emph{via} a fully automated procedure a recent specialized ``by hand" prediction for the high-pressure phase of ammonia borane, NH$_3$BH$_3$. The MgNH prediction, which involves 36 atoms and a notoriously complex configuration space, to the best of our knowledge represents the largest-ever crystal structure solution derived from first-principles calculations without making simplifying assumptions about atom connectivity. The 32-atom NH$_3$BH$_3$ prediction is nearly as demanding. Our approach, which takes full advantage of existing experimental information to solve for structural unknowns, has great potential for completing thousands of partially determined crystal structures. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W32.00003: Finite Temperature Lattice Vibrations and the Magnetic Structure of Fe and Ni G. Malcolm Stocks, Yang Wang, Roger Stoller, Aurelian Rusanu, Markus Eisenbach, Donald Nicholson, German Samolyuk Modern \textit{ab initio} theories of the magnetic phase transition (Curie Temperature, T$_{C})$ of Fe and Ni based on the Disordered Local Moment (DLM) type models generally rely on (constrained) density functional theory calculations performed at 0K and assume that the atoms occupy their equilibrium lattice sites. Here we point out that finite temperature lattice vibrations can result in large fluctuations in the local moments associated with individual site beyond those already accounted for in these approaches. These conclusions are based on large cell ($\sim $10$^{4}$ -- atoms) \textit{ab initio} calculations of the magnetic state of Fe and Ni based on the O[N] Locally Self-consistent Multiple Scattering (LSMS) method. Atom positions are obtained from freezes of individual time steps of molecular dynamics simulations based on classical interaction potentials. Calculations are performed for a range of temperatures up and beyond T$_{C}$ that illustrate the extent of the moment fluctuations. We discuss the consequences of these findings for the adequacy of existing theories T$_{C}$. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W32.00004: Combined Experimental and Theoretical Studies of Core-Shell Nanostructures in Al-Sc-Li Alloys Colin Ophus, Abhay Gautam, Emmanuelle Marquis, Velimir Radmilovich, Ulrich Dahmen, Mark Asta We have used two aging treatments of Al-Li-Sc alloys to create highly monodisperse, coherent L$1_2$ structure core-shell precipitates. We perform detailed analyses of the compositional distributions in the precipitate structures with electron microscopy and atom probe tomography. By combining this information with first principles calculations and Monte Carlo simulations based on the cluster expansion formalism, we compute bulk and interfacial thermodynamic properties relevant to precipitate formation. We specifically focus on understanding how the presence of Li modifies the nucleation rate relative to that of pure Al3Sc precipitates. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W32.00005: New structures in Pd-rich ordered alloys Jacqueline Corbitt, Gus Hart An intriguing intermetallic structure with 8:1 stoichiometry was discovered in the 1950s in the Pt-Ti system. Since then, a handful of other Pt/Pd/Ni binary systems have been observed to exhibit this curious structure (e.g., Pt$_8$Zr, Pd$_8$Mo, Ni$_8$Nb, etc). This ordered structure can significantly increase the hardness of an alloy by forming precipitates. Recent calculations and experiments suggest that the 8:1 structure may form in about 20 previously unsuspected Pt/Pd binary systems. Using first-principles calculations, cluster expansion, Monte Carlo modeling, we have explored possible precipitate hardening (via the 8:1 structure) in Pd-Nb, Pd-Mg and Pd-Cu. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W32.00006: Pseudo-Random Number Generation for Brownian Dynamics and Dissipative Particle Dynamics Simulations on GPU Devices Carolyn Phillips, Joshua Anderson, Sharon Glotzer Brownian Dynamics (BD) and Dissipative Particle Dynamics (DPD) are implicit solvent methods commonly used in models of soft matter and biomolecular systems. The interaction of the numerous solvent particles with larger particles is coarse-grained as a Langevin thermostat is applied to individual particles or to particle pairs. The Langevin thermostat requires a pseudo-random number generator (PRNG) to generate the stochastic force applied to each particle or pair of neighboring particles during each time step. In a GPU parallel computing environment, small batches of random numbers must be generated over thousands of threads and millions of kernel calls. We introduce a PRNG scheme, in which a micro-stream of pseudorandom numbers is generated in each thread and kernel call. These high quality, statistically robust micro-streams are more computationally efficient than other PRNG schemes in memory-bound kernels, and uniquely enable the DPD simulation method . This scheme has been implemented in HOOMD-blue, a GPU-accelerated open-source general purpose molecular dynamics simulation package. By enabling BD and DPD to be performed in HOOMD-blue, a broad range of mesoscale coarse-grained simulations can now be accelerated in a massively parallel architecture. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W32.00007: Efficient ab initio molecular dynamics using exact reweighting Vidvuds Ozolins, Mark Asta Density-functional theory (DFT) based {\it ab initio\/} molecular dynamics (AIMD) is a promising method for calculating high-temperature thermodynamic properties of solids and liquids. Nevertheless, computational expense associated with AIMD simulations has prevented general adoption of these methods. We show that substantial savings of computational effort can be realized by using less expensive (and less accurate) Hamiltonians to generate long MD trajectories and by recalculating statistically independent snapshots with high-accuracy DFT methods. A formally exact reweighting formula, based on the Jarzynski switching approach, is used to obtain thermal averages and thermodynamic properties in the high-accuracy DFT ensemble. If under-converged AIMD simulations with low energy cutoffs and coarse k-point meshes are used to generate trajectories, this approach can lead to savings of CPU time of a factor of 10 to 100, depending on the relevant correlations times. We also present extensions of the reweighting method to calculate impurity free energies and free energy barriers for interstitial diffusion. Robust methods for estimation of statistical errors based on random subsampling and variance extrapolation are discussed. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W32.00008: Ab Initio Simulations of Hydrogen in Crystalline and Amorphous Metal Membranes William Huhn, Mike Widom Solid metallic membranes are used to separate hydrogen from other gases for clean energy applications. In order to create cheaper, more effective membranes for hydrogen separation, it is desirable to model hydrogen transport through the membrane. Amorphous metal membranes in particular have potential for this type of application due to low expense and high theoretical hydrogen capacity. We computationally model hydrogen absorption and transport through materials in order to find materials that can be used to construct effective membranes for hydrogen capture. In this talk, we will obtain hydrogen binding sites and diffusion barriers in order to model the hydrogen diffusion through various nickel-based amorphous alloys and compare them to associated crystalline structures as well as elemental palladium, which is favored for this application despite its high expense. Ab initio methods (specifically the Vienna Ab Initio Simulation Package, VASP) are used to develop the hydrogen binding energy spectrum, from which thermodynamic models can be constructed. Kinetic Monte Carlo methods are used to model the hydrogen transport through the bulk, from which we can obtain the permeability. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W32.00009: The enigmatic Ag-Pt phase diagram and yet another derivative structure algorithm Gus L.W. Hart, Lance J. Nelson, Rodney W. Forcade The Ag-Pt phase diagram as published in the most recent phase diagram compilations (Massalski, Pauling File) is entirely speculative below 1000$^{\circ}$ C. The phase diagrams and our calculations both suggest a stable L1$_1$ phase at 50 at.-\% Pt. However, an experimental study published after the compilations supports a significantly different phase diagram. In this new phase diagram, the only stable phases at low temperatures are the elemental fcc Ag and Pt phases and one ordered phase at the unusual concentration of 53 $\pm 0.5$ at.-\% Pt. The experimental study shows that the homogeneity range for the ordered phase is narrow (almost like a line compound), and its X-ray data suggests that the unit cell of this phase contains 32 atoms with a stoichiometry of 15:17. We developed a new derivative structure enumeration algorithm specifically designed for large unit cells with known concentrations. This is necessary because our old algorithm enumerated all concentrations and was therefore limited to smaller unit cells. We have explored, via first-principles, the structural details of this enigmatic phase in the Ag-Pt phase diagram. I will discuss our first-principles results for Ag-Pt, and I will discuss the how the new algorithm is useful for large unit cells when partial structural information is known. [Preview Abstract] |
Session W33: Insulators and Dielectrics I
Sponsoring Units: DCMPChair: William Parker, Argonne National Laboratory
Room: C143/149
Thursday, March 24, 2011 11:15AM - 11:27AM |
W33.00001: An \textit{Ab Initio }Study of PuO$_{2\pm 0.25}$, UO$_{2\pm 0.25}$, and U$_{0.5}$Pu$_{0.5}$O$_{2\pm 0.25}$ Li Ma, Asok Ray Hybrid density functional theory has been used to systematically study the electronic, geometric, and magnetic properties of strongly correlated materials PuO$_{2\pm x}$, UO$_{2\pm x}$, and U$_{0.5}$Pu$_{0.5}$O$_{2\pm x}$ with x=0.25. The calculations have been performed using the all-electron \underline {f}ull-\underline {p}otential \underline {l}inearized \underline {a}ugmented \underline {p}lane \underline {w}ave plus \underline {l}ocal \underline {o}rbitals basis (FP-L/APW+lo) method. Each compound has been studied at the ferromagnetic (FM) and anti-ferromagnetic (AFM) configurations with and without spin-orbit coupling (SOC) and \textit{full} geometry optimizations. The optimized lattice constants, bulk moduli, and band gaps will be reported. Total energy calculations indicate that the ground states are AFM for all compounds studied here and the band gaps are typically higher than 1.0 eV, and characteristic of semiconductors. The total energy is lowered significantly and the band gaps increase with the inclusion of SOC. The chemical bonds between the actinide metals and oxygen atoms are primarily ionic in character. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W33.00002: Characterization of point defects in UO2 by positron annihilation spectroscopy: a first-principles study Marc Torrent, Gerald Jomard abstract- Positron Annihilation Spectroscopy is a powerful experimental tool for probing defects in crystalline materials. The correct identification of defects with PAS requires the knowledge of accurate positron lifetimes for the various kinds of defects. That can be provided by numerical calculations in the framework of the Two-Component Density-Functional Theory. This method accurately treat on the same footing, the electrons and positron densities as well as the atomic structure. We have implemented this formalism within the Projector Augmented-Wave method in the ABINIT code, optimizing the electrons and positron densities self-consistently and calculating positron-induced forces accurately. That allows to properly determining the relaxed geometries of defects that trapped positron. We have applied the TC-DFT to various point defects in UO2. The use of the PAW method allows considering large super cells to simulate point defects, we have typically used cells that contain 32 UO2 unit formulas. We use the LDA+U framework in order to treat strong electronic correlations. This work is a first attempt to help for the interpretation of PAS experiments on UO2 but it seems really promising.- [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W33.00003: Effect of Dielectric Materials on the Topological Insulator Bi$_{2}$Se$_{3}$ Surface States Jiwon Chang, Leonard Register, Sanjay Banerjee, Bhagawan Sahu We study the effects of crystalline dielectric materials on the electronic surface states of a strong topological band insulator (TI) Bi$_{2}$Se$_{3}$ using a density functional based electronic structure method [1]. We will discuss the sensitivity of Dirac point degeneracy and linear band dispersion of the TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and the TI. Both passivated and non-passivated substrate surfaces will be considered. Two representative dielectrics SiO$_{2}$ and boron nitride will be chosen to understand the physics of interplay of interface potential, linear band dispersion and the chemical environments of the TI surface states. Our findings have implications in interpreting experiments and designing novel nanoelectronics device concepts based on TIs. \\[4pt] [1] ``Intrinsic and extrinsic perturbations on the surface states of topological insulator Bi$_{2}$Se$_{3}$,'' J. Chang, P. Jadaun, L. F. Register, S. K. Banerjee and B. Sahu (In preparation) [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W33.00004: First-Principles Investigation of Low Energy E' Center Precursors in Amorphous Silica Nathan Anderson, Ravi Vedula, Peter Schultz, Renee Van Ginhoven, Alejandro Strachan We show that oxygen vacancies are not necessary for the formation of E' centers in amorphous SiO2 and that a single O-deficiency can lead to two charge traps. Employing molecular dynamics with a reactive potential and density functional theory we generate an ensemble of stoichiometric and oxygen-deficient amorphous SiO2 atomic structures and identify low-energy network defects. Three-coordinated Si atoms appear in several low-energy defects both in stoichiometric and O-deficient samples where, in addition to the neutral oxygen vacancy, they appear as isolated defects. Various charge transition levels for each defect are also presented. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W33.00005: Structure and energetics of ferroelectric domain walls in LiNbO$_{3}$ from atomic level simulations Donghwa Lee, Haixuan Xu, Volkmar Dierolf, Venkatraman Gopalan, Simon Phillpot Due to its unique ferroelectric and nonlinear optical properties, LiNbO$_{3}$ has a wide range of applications in optoelectronics and nonlinear optics. These unique properties of LiNbO$_{3}$ are, however, quite sensitive to point defects and ferroelectric domain walls. Therefore, detailed characterization of the ferroelectric domain walls and their interaction with the defects at atomistic scale is highly important The local structure and energetics of the ferroelectric domain walls in LiNbO$_{3}$ are examined using density functional theory (DFT) and atomistic-level simulation methods. The energetics and electric dipoles associated with the pertinent point defects and domain walls in LiNbO$_{3}$ are investigated in detail. In particular, the variation of polarization due to 180\r{ } ferroelectric domain walls, the coupling of defect-domain wall interactions and their effects on domain wall motion are discussed [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W33.00006: Study of Defects That Trap Excitons in Yttrium Aluminum Garnets Doped With Rare-Earth Elements Farida Selim, Chris Varnery, Gary Collins, David Mckay, Sherif Reda Excitons play a fundamental role in transporting energy in photonic materials. Understanding and controlling excitons dynamics through their interactions with activating impurities and lattice defects is key to improving scintillation and optical properties. Singles crystals of yttrium aluminum garnet (YAG) crystals doped with rare-earths were studied by positron annihilation, thermolunuinescence and optical spectroscopy. Evidence of defect complexes was found in the YAG structure. Positron lifetime measurements were performed to characterize those defects. Effects of dopants on the optical properties and lattice defects were investigated. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W33.00007: Charge-Flow Regulation at Material Interfaces in Atomistic Models Steven Valone An important class of materials problems of great interest consists of composites of metals and metal oxides. At sharp metal/metal oxide interfaces, the oxygen concentration is varying radically over short distances. For this reason, at the atomistic level, variable-charge atomistic models are required that control charge flow at these interfaces. Charge flow is controlled through chemical potential equalization among its constituents. Existing models of chemical potential, such as the Iczkowski-Margrave (IM) model, are linear in the charge as is appropriate for metals. Here a new, ``fragment'' model Hamiltonian is constructed at the atomistic level commensurate with the IM model, as opposed to the one-electron model Hamiltonians that underlie tight-binding and related methods. Essential properties of the fragment model Hamiltonian include an alternative charge dependence compared to the IM model, charge-flow regulation, absent in the IM model, preservation of a sense of covalent-ionic balance, and capture of important theoretical limits. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W33.00008: Infrared laser ablation of polystyrene from microseconds to picoseconds Richard Haglund, Sergey Avanesyan, Kenneth Schriver, Malte Duering, Barry Luther-Davies, Hee Park, Singaravelu Senthilraja, Michael Klopf, Michael Kelley We describe experiments on both resonant and non-resonant infrared pulsed laser of polystyrene across time scales varying from microseconds to picoseconds for the purpose of determining the ways in which the rate of energy deposition changes the response of both the ablated material and the residual substrate. RIR-PLD has been shown to be a relatively low-temperature process leading to evaporation and deposition of intact molecules. We compare the characteristics of ablation craters and ablation plumes deposited by Nd:YAG and Er:YAG lasers, picosecond and nanosecond optical parametric oscillators, and two different infrared free-electron lasers with differing pulse profiles. The films were characterized by profilometry, digital optical microscopy, scanning electron microscopy, and Fourier-transform infrared spectroscopy. Based on the experiments and computational modeling, we discuss the constraints on laser parameters that produce non-destructive ablation by resonant infrared excitation. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W33.00009: The effect of substrate temperature on the structure and morphology of titanium nitride compounds grown by DC magnetron sputtering Mohammad R. Hantehzadeh, Reza Bavadi The TiN thin films were deposited on p-type silicon (100) substrates using reactive planar DC magnetron sputtering system. The target was 99.99{\%} pure Ti. The reactive sputter gas was a mixture of Ar (99.999{\%}) and N$_{2}$ (99.999{\%}) with the ratio Ar (97{\%}) and N$_{2}$(3{\%}) by volume. Structural characterization of the coating was done using X-ray diffraction (XRD). The surface roughness of the coating was determined using an Atomic Force Microscope (AFM). The reflectivity of thin films was investigated by a spectrophotometer system. The X-ray diffraction measurements showed that by increasing the substrate temperature during the growth, change in crystalline structure will occur. The crystallite size of the films determined by Scherer's equation, and the crystallite size measured by AFM also increased by increasing the substrate growth temperature. The surface reflectivity measurements indicate that by increasing the substrate growth temperature, the optical properties of the films changes. The change in optical properties and crystalline structure of the films indicate that substrate growth temperature plays an important role in structure and morphology of the grown layers. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W33.00010: Growth of Erbium doped Yttrium oxide thin films by atomic layer deposition Nicholas Becker, Thomas Proslier, J. Klug, J. Elam, T. Sanamyan, M. Dubinckii, J. Girolami, M. Pellin Er-doped Yttrium oxide (Er$^{3+}$:Y$_2$O$_3$) has gained recent attention for its possible use in optoelectronic devices. Here we report the use of atomic layer deposition (ALD) to synthesize thin films of Yttrium oxide with various doping levels of Erbium ions (Er$^{3+}$) using different chemistries. ALD uses self-limiting surface reactions to deposit highly conformal thin films over large areas and substrates of arbitrary shape. This allows for the uniform layered doping of Yttrium oxide with Er$^{3+}$. The spatial separation of the Erbium ions can be controlled using organometallic precursors with varying ligand sizes. The doping concentration (volume ratio of Er$^{3+}$ sites to Y$^{3+}$ sites) is controlled by the ratio of the precursor pulses. We comprehensively studied ALD-grown films of Er$^{3+}$:Y$_2$O$_3$ obtained from the Erbium precursors Er(MCp)$_3$ and Er(BA)$_3$ and the Yttrium precursors Y(MCp)$_3$ and Y(Cp)$_3$ using either water or hydrogen peroxide as an oxygen source. Detailed description of the studied optical and spectroscopic properties, stoichiometry, and physical characteristics of these films will be presented. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W33.00011: Atomic Layer Deposition of AlOx for Metal-AlOx-Metal Trilayers and Resonators A. Kozen, L. Henn-Lecordier, X. Chen, M. Schroeder, C. Musgrave, G. Rubloff The dielectric layer in conventional Josephson junction qubits is fabricated by thermal oxidation of aluminum. These dielectrics suffer from high loss tangents thought to be due to defect-related quantum two level systems. Our collaborators have identified the -OH rotor associated with hydroxyl species in the AlOx as a prime defect candidate. We demonstrate the fabrication of the AlOx dielectric in MIM structures using atomic layer deposition (ALD) from trimethylaluminum and both H2O and D2O precursors. ALD enables precise control of film growth at the atomic scale, while comparison of D2O vs H2O as the oxidation precursor should reveal the role of this defect in the loss tangent. We have developed the D2O based ALD process, observed similar kinetics and properties for D2O and H2O based ALD for AlOx, and characterized the materials by SIMS, XPS, and electrical measurements of MIM capacitor structures. Correlation between room temperature electrical measurements and low temperature resonator performance will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W33.00012: Improvements of Defect Analysis by Space-Charge Wave Spectroscopy Kay-Michael Voit, Burkhard Hilling, Heinz-J\"urgen Schmidt, Mirco Imlau We report on our recent advancements in space-charge wave spectroscopy, which can be used to investigate defect structures in classical high-resistive semiconductors and insulators. It permits to estimate the effective trap concentrations as well as the effective donor density $N_{eff}$ and the product $\mu\tau$ of electron mobility and life-time in the conduction band. We present a novel method of space-charge wave excitation, using a superposition of a running and a static sinusoidal illumination pattern. Thus we acquire -- in contrast to the former oscillating pattern -- a distinct direction of movement. The proposed new technique can be regarded as an effective amelioration, as the theoretical analysis is no longer limited by numerous presumptions like low modulation depth or small oscillation amplitudes. It not only overcomes these limits of the experimental configuration improving accuracy of SCW spectroscopy, but it also provides additional information, such as the sign of the charge carriers. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W33.00013: First-principles studies of Ce and Eu doped inorganic scintillator gamma ray detectors Andrew Canning, Anurag Chaudhry, Rostyslav Boutchko, Stephen Derenzo We have performed DFT based band structure calculations for new Ce and Eu doped wide band gap inorganic materials to determine their potential as candidates for gamma ray scintillator detectors. These calculations are based on determining the 4f ground state level of the Ce and Eu relative to the valence band of the host as well as the position of the Ce and Eu 5d excited state relative to the conduction band of the host. Host hole and electron traps as well as STEs (self trapped excitons) can also limit the transfer of energy from the host to the Ce or Eu site and therefore limit the light output. We also present calculations for host hole traps and STEs to compare the energies to the Ce and Eu excited states. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W33.00014: Novel Approaches in Energy Conversion by Molecular Charge Transfer from Diamond Surfaces Franz A.M. Koeck, Jeff Sharp, Robert J. Nemanich Vacuum thermionic energy conversion is based on electron transfer from a hot emitter across a vacuum gap to the collector. Our approach for an efficient emitter utilizes nanostructured, negative electron affinity doped diamond films. With a low effective work function of less than 1.3 eV thermionic emission commences at 260 C and observes the law of Richardson -- Dushman with a significant emission current $>$ 5 mA at 500 C. Pairing this emitter with a similar collector results in a potential across the gap and introduction of an ohmic load establishes a current indicative of energy conversion. Utilizing ionization processes of gaseous species at the emitter surface can enhance inter-electrode charge transfer and increase output power. In the ionization process an electron is trapped in an occupied molecular orbital establishing a negative ion state. The electron affinity and negative ion binding energy determines stability of the transient negative species, and we present results for H2 and CH4. As these species are introduced in the inter-electrode gap an increase in output power is observed with a concurrent shift of maximum output power to lower load resistance. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W33.00015: Diamond Based Electron Emitters for Photon Enhanced Thermionic Energy Conversion Tianyin Sun, Franz A. Koeck, Robert J. Nemanich Energy conversion cells utilize either direct photon illumination or indirect thermal energy for electron excitation. Nitrogen-doped, hydrogen terminated nanocrystalline diamond films display a negative electron affinity and have shown low temperature thermionic emission which can be employed for energy conversion in a vacuum thermionic emission cell. However, the low work function of such films suggests that the current could be enhanced through visible light illumination to induce photoelectron emission. We present measurements of the spectrum of emitted electrons from N-doped diamond films for light illumination between 600 and 340nm, while the film is heated from ambient to 500C. Features due to thermionic and photo-emission are identified, and a complex interaction is observed between the two processes at various temperatures and illumination wavelengths. The results indicate the potential application of diamond emitters as combined thermal and photon energy converters, and we present a new approach to enhance the performance of diamond-film energy converters. [Preview Abstract] |
Session W34: Focus Session: Optical Properties of Nanocrystals
Sponsoring Units: DMPChair: Alexander Govorov, Ohio University
Room: C141
Thursday, March 24, 2011 11:15AM - 11:27AM |
W34.00001: Evolution of CdSe/ZnS Quantum Dot Ensembles Under Prolonged Excitation Georgiy Shcherbatyuk, Richard Inman, Sayantani Ghosh We study the spectral evolution of self-assembled ensembles of CdSe/ZnS core/shell quantum dots (QDs) under photo-excitation in ambient conditions. We use spatially-resolved photoluminescence (PL) scanning microscopy in conjunction with spectrally-resolved time-resolved spectroscopy to measure variations in spectral intensity, emission wavelength and recombination lifetimes. Our results indicate that the spectral intensity of the ensemble undergoes both photo-induced brightening and darkening, and these rates have a complicated dependence on the concentration of the QD samples. They initially decrease with decreasing concentration but are greatly enhanced in the dilute limit. The photo-exposure also causes a rapid spectral red-shift followed by a slow blue-shift. The recombination lifetimes increase with the red-shift for all concentrations but do not correlate to the blue-shift in a straightforward manner in different samples. We conclude the possible explanation of this behavior is the inter-play of photo-induced surface trap discharging and preferential photo-oxidation of the smaller QDs in the ensemble. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W34.00002: Photoluminescence spectroscopy and lifetime measurements from self assembled semiconducting quantum dot- metal nanoparticle hybrid arrays M. Haridas, J.K. Basu We demonstrate how the emission properties of a hybrid array consisting of semiconducting quantum dot (QD) and metal nanoparticles (NP) can be controlled by varying the density and distance between QD and NP independently. Our hybrid system consists of chemically synthesized cadmium selenide quantum dots (CdSe QDs) and polymer capped gold nanoparticles (Au NP) embedded in a block copolymer matrix having the topology of cylinders oriented perpendicular to the substrate. We have prepared hybrid arrays with two different densities of CdSe QDs ($\rho _{QD})$ each having same Au NP densities ($\rho _{Au})$. The photoluminescence measurements (PL) from such hybrid system shows enhancement in emission with increase in $\rho _{Au}$, compared to the CdSe QD film and the enhancement factor is lower for hybrid films with high $\rho _{QD}$. The lifetime measurement shows double exponent PL decay with systematic reduction in exciton lifetime for hybrid arrays with respect to $\rho _{Au}$. The film with high $\rho _{QD}$ shows larger reduction in lifetime. Similarly, the amplitudes of the two relaxations switch over with increase in $\rho _{Au}$. It is clear that the shorter time becomes the dominant relaxation mode with increasing $\rho _{Au.}$ Observed phenomena have been explained in terms of exciton plasmon interaction$_{.}$ [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W34.00003: Manipulating coupling between a single semiconductor quantum dot and single gold nanoparticle Farbod Shafiei, Daniel Ratchford, Suenne Kim, Xiaoqin Li, Stephen Gray We report the manipulation of coupling in a simple model system, a single semiconductor quantum dot (QD) near a single metallic nanoparticle, and study the resulting changes in QD photoluminescence (PL) dynamics. We used atomic force microscopy nanomanipulation to controllably push a Au NP proximal to a CdSe/ZnS QD. We observed gradual and reversible changes in the QD PL lifetime and blinking dynamics. In some cases, the total lifetime reduced from 30 ns to well below 1 ns. This decrease is accompanied by the disappearance of blinking behavior as the nonradiative energy transfer from QD to the Au NP becomes the dominant decay channel. In comparison to previous studies, our experiments report changes in the PL dynamics of the same QD, therefore, eliminating the ambiguity of variable properties of individual QDs. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W34.00004: Carbon Nanotube CdSe Nanoparticle Hybrid Materials: Synthesis and Optical Properties Austin Akey, Chenguang Lu, Irving P. Herman Carbon nanotubes present remarkable opportunities as a base for construction of advanced nanomaterials with uniques properties. We report novel heterostructures composed of single-walled carbon nanotubes and monodisperse cadmium selenide nanoparticles (3.5 to 6 nm in diameter). The optical properties of the hybrid material differ significantly from those of the unbound nanoparticles and nanotubes, pointing to the existence of strong electronic/optical interaction effects between the two. Specific differences in absorption/emission behavior and the photoluminescence Stokes shift in the nanoparticles will be presented, along with experiments exploring the underlying mechanisms of this interaction. We believe this system exhibits ``hot'' excitation-transfer behavior from the nanoparticles to the nanotubes, which makes it of great interest for photovoltaic applications. This work is primarily supported by the Nanoscale Science and Engineering Center at Columbia University, which is supported by the Nanoscale Science and Engineering Initiative of the NSF under Award Number CHE-0641523. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W34.00005: Enhanced Optical Absorption of Glancing Angle Deposited Semiconducting Nanostructures for Photovoltaic Solar Cell Applications Hilal Is, Mehmet Cansizoglu, Miria Finckenor, Tansel Karabacak Semiconducting nanostructures with controlled geometries can provide enhanced optical absorption and effective collection of photo-charges for high efficiency photovoltaic solar cells and photoconductive devices. Glancing angle deposition (GLAD) provides a unique capability of producing nanostructured arrays of various materials with controlled shapes, size, and separation. In this study, as a model system, we fabricated arrays of semiconducting indium sulfide nanostructures by GLAD in the shapes of springs, screws, rods, and zigzags. We show that GLAD nanostructures have significantly lower reflectance and higher optical absorption compared to conventional flat thin films. In addition, we observed a superior photoconductivity (PC) response of about 80{\%} for nanorod array samples, which is believed to be mainly due to their high optical absorption. On the other hand, PC response was less than 1{\%} for conventional thin films of indium sulfide. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W34.00006: In-Situ Studies of Photoluminescence Quenching in Single Crystal Quantum Dot Sensitized Solar Cells Douglas Shepherd, Yong-Qi Liang, Justin Sambur, Bruce Parkinson, Martin Gelfand, Alan Van Orden Single crystal quantum dot sensitized solar cells (QDSSC) are a promising photovoltaic system in which collection of multiple charge carriers per photon has recently been reported.\footnote{Justin B. Sambur, Thomas Novet, B.A. Parkinson, \textit{Science} \textbf{330} (6000) 63-66} Utilizing time-correlated single photon counting we have studied both the fluorescence intensity and fluorescence decay time from CdSe quantum dots coupled to both single crystal TiO$_{2}$ and ZnO substrates through short and long chain ligands. We find that for all configurations the fluorescence decay time is quenched compared to unbound quantum dots in solution, while the photovoltaic properties of the system strongly depend on the chain-length of the ligand. These results suggest there exist interactions between either the individual quantum dots or the quantum dots and substrate that may compete with the charge injection process in QDSSCs. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W34.00007: Particle Morphology and Interfacial Energy Transfer in CdSe/CdS Nanocrystals Nicholas Borys, Manfred Walter, Jing Huang, Dmitri Talapin, John Lupton CdSe/CdS core-shell nanocrystal heterostructures are unique systems to study nanoscale energy migration. We perform single-particle excitation spectroscopy at low temperatures by monitoring both the luminescence intensity and energy of the core as a function of optical excitation energy in three different heterostructure shapes: spherical particles, rods, and tetrapods. In the tetrapod and rod structures, the shapes of the PLE spectra fall into one of two classes while the spherical particles all exhibit one universal form, which we attribute to the general shape and quantum confinement symmetry of the CdS shell. We confirm this assignment by correlated single particle SEM and PLE measurements. By resolving the core emission energy as a function of excitation energy, we identify two distinct exciton species in the tetrapods indicating the presence of a barrier that prevents charge transfer across the heterostructure interface [1]. \\[4pt] [1] Borys et al., Science (in press) [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W34.00008: Copper-doped core-shell ZnSe/CdSe nanocrystals with efficient and widely tunable photoluminescence Ranjani Viswanatha, Sergio Brovelli, Victor I. Klimov We report synthesis and spectroscopic studies of Cu-doped ZnSe/CdSe nanocrystals (NCs) with a wide range of shell thicknesses. Incorporation of copper ions into the NCs introduces an atomic-like state within the NC band gap. This results in a three-level system in which emission occurs due to the transition coupling the NC lowest-energy conduction-band level to the localized hole state associated with the Cu ions. Cu is incorporated into the NC core while the shell remains nominally undoped, which allows us to manipulate the shell thickness (and thus emission color) without affecting the overall level of NC doping. We demonstrate a wide-range spectral tunability of photoluminescence (PL) (from 3.1 eV to 1.25 eV), and ``giant'' Stokes shifts ($\sim $ 0.8 eV), which reduce emission losses due to reabsorption. We show that hole trapping at the Cu sites occurs primarily from the top of the valence band (i.e., no hot-hole transfer processes are detected) and that the main nonradiative mechanism is electron trapping at surface defects, which can be suppressed at cryogenic temperatures, resulting in PL quantum yields of $\sim $40{\%}. High intrinsic emission efficiencies, wide-spectral tunability, and a large Stokes shift make these novel NCs attractive candidates for radiation detection, light-emitting diodes, and lasers. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W34.00009: Breakdown of Volume Scaling in Auger Recombination in CdSe/CdS Heteronanocrystals: The Role of the Core-Shell Interface Sergio Brovelli, Florencio Garc\'Ia-Santamar\'Ia, Ranjani Viswanatha, Han Htoon, Scott Crooker, Victor I. Klimov Spatial confinement of electronic excitations in semiconductor nanocrystals (NCs) results in a significant enhancement in nonradiative Auger recombination (AR) of multiexcitons, which is detrimental to promising NC lasing applications.~In standard NCs, AR times scale linearly with NC volume. We investigate multiexciton dynamics in NCs composed of CdSe cores and CdS shells of tunable thickness. Thicker shells dramatically reduce AR, particularly during initial shell growth, which cannot be explained by traditional volume scaling alone. Rather, low-temperature fluorescence-line-narrowing studies strongly suggest that suppressed AR derives primarily from the formation of an alloy layer at the CdSe/CdS interface, and a corresponding ``smoothing'' of the confinement potential (CP). These findings support the recent theory, which predicts that the change from abrupt to smoothly-varying CPs reduces the high-spatial-frequency Fourier components of the exciton wave function, thereby minimizing overlap with the high-energy states involved in nonradiative Auger decay. Our results highlight the importance of NC interfacial structure in the AR process in zero-dimensional NCs and provide general guidelines for the design of new nanostructures with suppressed AR for future lasing applications. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W34.00010: Complete suppression of Blinking and convergence to a single emissive state in giant nanocrystal quantum dots Anton Malko, Siddharth Sampat, Young-Shin Park, Javier Vela, Youngfen Chen, Jennifer Hollingsworth, Victor Klimov, Han Htoon We report a systematic study of photoluminescence (PL) emission intensities and lifetimes of individual core-shell CdSe/CdS ``giant'' nanocrystal quantum dots (gNQDs) as a function of the shell thickness. We observed a complete suppression of blinking for gNQDs overcoated with more than 16 monolayers (ML) of CdS shell. An analysis of the photon emission statistics reveals a highly super-Poissonian distribution for thin shell (4-12 ML) gNQDs and near perfect Poissonian distribution for non-blinking, thick-shell gNQDs. Measurements of PL decay rates as a function of PL intensity show that while PL lifetimes vary continuously with PL intensity for thin-shell gNQDs, only one PL decay constant is observed for the thicker shell gNQDs. This result clearly indicates that while the thin-shell gNQDs possess continuous distribution of emission states, PL of the non-blinking, thick-shell gNQDs originates from a single emissive state. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W34.00011: A comparative study of carrier multiplication in PbS and PbSe nanocrystals John Stewart, Lazaro Padilha, Doh Lee, Bishnu Khanal, Jeffrey Pietryga, Victor Klimov In this talk I will present our recent studies of carrier multiplication (CM) in PbS and PbSe nanocrystals (NCs). CM is a process in which absorption of a single photon produces multiple excitons. In our experiments, we evaluate its efficiency based on the amplitude of the Auger decay signature of multiexcitons in carrier dynamics recorded using both transient absorption and time-resolved photoluminescence. In the case of PbSe NCs, we have measured the CM quantum efficiencies and Auger lifetimes for a large range of NC sizes, including very large particles with a band gap around 0.5 eV. Using excitation at 3.1eV we observe that the quantum efficiency is low for small NCs and increases monotonically toward the bulk value for larger dots. Despite many apparent similarities in the band structure of PbSe and PbS NCs, our preliminary studies of PbS nanoparticles hint at a quite different spectral behavior of quantum efficiencies compared to PbSe NCs. In particular, while the quantum efficiencies are similar for smaller NCs, we see discrepancies for the larger NCs for which the energy gap approaches the bulk limit. We discuss these discrepancies in the context of our comparative studies of intraband relaxation and Auger recombination in these two types of the NCs. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W34.00012: Carrier recombination pathways in Copper Indium Sulfide (CIS) nanocrystals Anshu Pandey, Liang Li, Jeffrey M. Pietryga, Victor I. Klimov Ternary and Quaternary compounds are rapidly gaining interest because of potential applications in areas such as thin-film photovoltaics and light-emitting diodes. We will discuss carrier dynamics in Copper Indium Sulfide (CIS) nanocrystals (NCs), one of the better studied members of the ternary-quaternary family. While as-prepared CIS NCs exhibit photoluminescence (PL) quantum yields less than 10{\%}, overgrowth with a few monolayers of CdS or ZnS increases PL quantum efficiency to more than 80{\%}. We investigated the reasons for this dramatic improvement in efficiency through time-resolved PL and transient absorption measurements. Our results suggest that while electrons in CIS NCs remain delocalized, the holes are rapidly localized due to trapping at defects. PL emission arises through the radiative recombination of a delocalized electron with a hole at the interior defect site (radiative decay center). We also observe surface defects that serve primarily as centers for nonradiative recombination. Overcoating of CIS NCs with CdS or ZnS eliminates surface traps and results in a long single-exponential PL decay that appears to be unique among visible-emitting NCs. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W34.00013: Nonradiative Energy Transfer in Assemblies of Nanostructures with Mixed Dimensionality Pedro Ludwig Hernandez-Martinez, Hilmi Volkan Demir We study the exciton-exciton interaction and nonradiative energy transfer in nanostructure assemblies with mixed dimensionality. We investigate possible combinations in terms of dimensionality for these nanostructures, and analyze the resulting nonradiative energy transfer rates as a function of dimensionality. Depending on the direction of the energy transfer, arrangements of such nanostructures have potential applications in both photovoltaics [1] and light generation [2]. \\[4pt] [1] J. Sambur, et al., ``Multiple Exciton Collection in a Sensitized Photovoltaic System'', Science 330, 63 (2010). \\[0pt] [2] R. Yan, et al., ``Nanowire Photonics,'' Nature Photonics, 3, 569-576 (2009). [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W34.00014: Charge and Shape Effects on the Carrier Dynamics of 25 Atom Au Nanoclusters Matthew Sfeir, Huifeng Qian, Rongchao Jin We study a series of semiconducting gold clusters that exhibit strong quantum confinement effects on their optical properties. In contrast to larger metallic nanoparticles, the surface plasmon resonance disappears a large optical gap ($>$ 1.3 eV) is formed. Recent synthetic advances have permitted the study of truly monodisperse clusters with precise control on the atomic scale. Using femtosecond and nanosecond transient absorption spectroscopy, we have investigated the excited state relaxation dynamics of spherical and prolate 25 atom isomers. We have determined that these particles exhibit long excited state lifetimes and that the carrier dynamics that are strongly influenced by the charge state and the physical arrangement of the atoms. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W34.00015: Transparent Conducting Metallic Film for Applications in Photovoltaics and Optoelectronic Devices Trilochan Paudel, Piotr Patoka, Wen-Chen Chen, Michael Giersig, Willie Padilla, Zhifeng Ren, Kris Kempa Oxides such as Indium-Tin Oxide (ITO) known as transparent conducting oxide (TCO) have been used for a long time in most of the thin film solar cell fabrications. It simultaneously works as an electrically conducting and optically transparent electrode. However, the electrical conductivity and optical transparency are not good enough. Here, we discuss our experimental and simulated results on nanostructured metallic films as a possible alternative replacement to TCO. This structure may possibly outperform the conventional ITO for applications in photovoltaics and optoelectronic devices such as LEDs. [Preview Abstract] |
Session W35: Topological Insulators: Superconductivity
Sponsoring Units: DCMPChair: Taylor Hughes, University of Illinois
Room: C140
Thursday, March 24, 2011 11:15AM - 11:27AM |
W35.00001: New forms of superconductivity and magnetism in a doped topological insulator L. Andrew Wray, Suyang Xu, Hsin Lin, M. Zahid Hasan Topological insulators achieve a phase of matter characterized by the quantum topology of electron kinetics rather than by broken symmetries. The topological insulator state gives rise to spin-helical surface states that dramatically alter the surface physics and allow new phenomena in the presence of purturbations such as superconductivity and magnetism. We have used angle resolved photoemission spectroscopy to map electron dynamics at the surface of a topological insulator in the presence of magnetic surface ions and with doping compositions that superconduct. Our measurements establish that bulk Cu-doped Cu$_{0.12}$Bi$_2$Se$_3$ realizes a new form of superconductivity (Wray et al, Nat. Phys. 6, 855 (2010)) and is likely to host localized non-Abelian Majorana fermions on the crystal surface. We observe that surface-deposited ions lead to the formation of new topoligically-derived surface Dirac bands, and our data suggest that magnetic moments of deposited Fe undergo a phase transition to align along the out-of-plane axis (arXiv:1009.6216). [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W35.00002: Coupling superconductors to topological insulators Menno Veldhorst, Marcel Hoek, Marieke Snelder, Xiaolin Wang, Hans Hilgenkamp, Alexander Brinkman The recent discovery of the topological insulators sparked an enormous attention. The experimental investigation of topological insulators with surface sensitive spectroscopic techniques evidently exposed the helical Dirac fermions living at the surface of a bulk insulator. In transport experiments, intrinsic bulk conduction challenges the observation of the topological surface states. We have observed electrical transport through the surface states of Bi$_{2}$Te3 by utilizing the electrical field effect and by utilizing intrinsically transparant interfaces to metallic electrodes. Of particular interest is coupling topogical insulators to superconductors, where at the interface the elusive Majorana fermion is predicted to recide. We fabricated superconductor -- topological insulator -- superconductor Nb-Bi$_{2}$Te$_{3}$-Nb junctions and observed a supercurrent over a length scale of more than a micrometer. Shapiro steps appear under microwave irradiation. The supercurrent is found to be surprisingly robust in large magnetic fields. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W35.00003: Stability of surface Majorana modes in superconducting doped topological insulators Ashvin Vishwanath, Pavan Hosur, Pouyan Ghaemi, Roger Mong In recent years, several model condensed matter systems have been predicted to harbor Majorana fermion zero modes. One such system is the surface of a strong topological insulator with proximity-induced superconductivity. A vortex in this surface superconductor was shown to host a topologically protected Majorana mode. Since then, bulk superconductivity has been induced in several strong topological insulators via doping or application of pressure. Here, we address the question of whether a vortex in these superconductors will trap Majorana zero modes at the surface. Viewed as a 1D system, the vortex can be characterized by a $Z_2$ topological invariant which denotes the presence or absence of a Majorana mode at its end. For weak pairing, we find that, the transition point between the two topological phases is determined by a Fermi surface property in the normal state. Hence, the phase transition can be achieved by simply varying the Fermi level. At the transition, the vortex supports gapless Majorana excitations along its length. Using this criterion, we discuss whether surface Majorana modes exist in the experimentally established superconductors Cu$_x$Bi$_2$Se$_3$, p-doped TlBiTe$_2$ and Pd$_x$Bi$_2$Te$_3$. Interestingly, the Fermi surface criterion also allows superconducting vortices in systems with non-topological band structures to be associated with surface Majorana modes. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W35.00004: Surface Spectral Function in the Superconducting State of a Topological Insulator Ting-kuo Lee, Lei Hao We discuss the surface spectral function of superconductors realized from a topological insulator, such as the copper-intercalated Bi$_{2}$Se$_{3}$. These functions are calculated by projecting bulk states to the surface for two different models proposed previously for the topological insulator. Dependence of the surface spectra on the symmetry of the bulk pairing order parameter will be discussed with particular emphasis on the odd-parity pairing. Exotic spectra like an Andreev bound state connected to the topological surface states will be presented. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W35.00005: Superconductivity in Topological Parent Compound Induced via Pressure C.Q. Jin, J.L. Zhang, S.J. Zhang, H.M. Gong, W. Zhang, P.P. Kong, J. Zhu, R.Z. Yu, L.X. Yang, S.M. Feng, Q.Q. Liu, X.C. Wang, R.C. Yu, W.G. Yang, L. Wang, S.C. Zhang, X. Dai, Z. Fang We report successful observation of pressure induced superconductivity in topological compound of Bi$_{2}$Te$_{3}$ single crystal induced via pressure [1]. The combined high pressure structure investigations with first-principles calculations indicated that the superconductivity occurs at the ambient phase of topologically nontrivial. The results suggest topological superconductivity can be realized in the parent state of Bi$_{2}$Te$_{3}$ topological material. Ref: [1] J. L. Zhang et al., ``Pressure induced superconductivity in the parent compound of Bi$_{2}$Te$_{3}$'' (submitted).. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W35.00006: Detecting Majorana Bound States Colin Benjamin, Jiannis Pachos We propose a set of interferometric methods on how to detect Majorana bound states induced by a topological insulator. The existence of these states can be easily determined by the conductance oscillations as function of magnetic flux and/or electric voltage. We study the system in the presence and absence of Majorana bound states and observe strikingly different behaviors. Importantly, we show that the presence of coupled Majorana bound states can induce a persistent current in absence of any external magnetic field. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W35.00007: Chain of Majorana States from Superconducting Dirac Fermions at a Magnetic DomainWall Titus Neupert, Shigeki Onoda, Akira Furusaki We study theoretically a strongly type-II s-wave superconducting state of two-dimensional Dirac fermions in proximity to a ferromagnet having in-plane magnetization. It is shown that a magnetic domain wall can host a chain of equally spaced vortices in the superconducting order parameter, each of which binds a Majorana-fermion state. The overlap integral of neighboring Majorana states is sensitive to the position of the chemical potential of the Dirac fermions. Thermal transport and scanning tunneling microscopy experiments to probe the Majorana fermions are discussed. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W35.00008: Global Properties of 3D Topological Insulator Surface/Superconductor Junctions Matthew Gilbert, Taylor Hughes, Ching-Kai Chiu, Andrei Bernevig The ability to precisely find and manipulate non-Abelian anyons\footnote{A. Kitaev, \textit{Ann. Phys (N.Y.)} \textbf{303}, 2 (2003).} has long been sought after as a potential means for the realization of robust quantum information processing. The simplest of these particles, Majorana fermions, have been predicted to exist in a new class of materials commonly referred to as topological insulators\footnote{B. A. Bernevig and S. C. Zhang, \textit{Phys. Rev. Lett.} \textbf{96}, 106802 (2006).} when they are coupled with s-wave superconducting contacts.\footnote{L. Fu and C. L. Kane, \textit{Phys. Rev. Lett.} \textbf{100}, 096407 (2008).} This proposal is the focus of intense experimental research whose aim is to prove the existence of Majorana fermions trapped at the surface of topological insulators paired with superconductors. We will present our initial theoretical investigations into the surface properties of 3D topological insulator/superconductor junctions. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W35.00009: Order parameter and triplet correlations near a superconductor-topological insulator interface Mahmoud Lababidi, Erhai Zhao At the interface between a 3D topological band insulator and an s- wave superconductor forms a remarkable 2D superconductor that can host Majorana fermions at vortex cores. Going beyond the original work of Fu and Kane [Phys. Rev. Lett. 100, 096407 (2008)], we present a microscopic theory for the proximity effect near the TI-S interface. We compute the superconducting order parameter as a function of the distance away from the interface by self-consistently solving the Bogoliubov-de Gennes equation. We discuss the suppression of the order parameter by the topological insulator. Moreover, we show that triplet superconducting correlations of $p_x\pm ip_y$ orbital symmetry are induced near the interface by the spin-orbit coupling inside the topological insulator. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W35.00010: Superconductivity and Majorana fermion creation at the quantum spin Hall insulator edge Annica M. Black-Schaffer We focus on properties related to Majorana fermion creation in a self-consistent study of a microscopic interface between a quantum spin Hall insulator (QSHI) and a superconductor (SC). For a spin-singlet $s$-wave SC we show that large odd-in-momentum, or $p$-wave, order parameters exist for all doping levels of the QSHI and that they can be related to different spinless Cooper pair amplitudes. This result demonstrates that it is natural that the Majorana mode at a SC-ferromagnet (SF) interface survives even at zero doping. Despite the induced $p$-wave order parameters, the induced superconducting gap in the QSHI always retains its $s$-wave character, thus validating the commonly used effective model for superconductivity in a topological insulator. We also self-consistently model an SFS Josephson junction along the QSHI edge and report on Majorana mode occurrence in the junction, the current-phase relation, and novel odd-frequency spin-triplet correlations. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W35.00011: Measurements of Electrical Noise at the Interface between Bi2Se3 and a Superconductor J.S. Lee, D.M. Zhang, A.R. Richardella, Nitin Samarth The narrow band gap semiconductor Bi$_2$Se$_3$ is a promising candidate for exploring exotic quantum states that might arise at the interface between topological insulators and superconductors (Phys. Rev. Lett. {\bf 100}, 096407 (2008)). Motivated by proposed approaches for detecting such states (Phys. Rev. B {\bf 79}, R161408 (2009)), we have embarked on measurements of electrical noise in mesoscopic Bi$_2$Se$_3$ devices with superconducting electrodes. Present measurements focus on CVD-grown Bi$_2$Se$_3$ nanoribbons which show proximity-induced superconductivity below $\sim 5$ K when contacted by (dirty) W electrodes. The measurements are carried out using a balanced bridge technique over a temperature range of $0.5 \rm{K} \leq T \leq 40$ K and in magnetic fields up to 80 kOe. We observe 1/f noise over a wide range of temperature and discuss the variation in noise spectral density across the normal-superconductor transition as well as its dependence upon excitation current and magnetic field. We will also describe extensions of these studies to mesoscopic devices lithographically patterned from thin films of Bi$_2$Se$_3$ grown by molecular beam epitaxy. Supported by NSF and ONR. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W35.00012: Study of the interface of supercondutor and topological insulator Dong Qian, Xiaomei Wang, Chunlei Gao, Canhua Liu, Jinfeng Jia State-of-art Molecular beam epitaxy (MBE) has been carried out to grow high quality topological insulator (TI) films on some BCS s-wave superconductor (SC) substrates. The growth dynamics and the electronics structures of the SC/TI interface were studied using high energy reflected electron diffraction and ultralow-temperature scanning tunneling microscopy (STM). Electronic structure, superconducting gap, vortex dynamics would be reported in this presentation. The superconducting state of the topological insulator due to approximate effect and the formation of Majorana Fermion would be discussed. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W35.00013: Self-consistent study at the superconductor-3D topological insulator interface Xiaoting Zhou, Chen Fang, Jiangping Hu We perform a theoretical study of the interface between a 3D topological insulator (TI) and an s-wave conventional superconductor (both in normal and superconducting state) using the BdG self-consistent formulation. The robustness of the TI surface state when in contact with a metal is discussed and the topological superconductivity on the interface induced by the proximity effect is studied in detail. We find that in general a mixed singlet and triplet pairing can be observed on the interface as the spin-orbit coupling breaks the SU(2) symmetry in TI. The Majorana edge states of such a mixed pairing 2D topological superconductor are also studied. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W35.00014: Proximity induced superconductivity in Bi$_2$Se$_3$ nanoribbons D.M. Zhang, J. Wang, J.S. Lee, H.R. Gutierrez, M.H.W. Chan, N. Samarth Proposals for possible realizations of Majorana fermions in condensed matter provide a strong motivation for interfacing superconductors with topological insulators (PRL {\bf 100}, 096407 (2008)). We describe experiments that accomplish an important first step in this context: the realization of proximity-induced superconductivity in a candidate topological insulator. We have measured the bias-dependent differential conductance in Bi$_2$Se$_3$ nanoribbons contacted with superconducting electrodes over a temperature range 0.5 K $\leq T \leq$ 6 K in magnetic fields up to 8 T. We observe signatures of both proximity-induced superconductivity and incoherent multiple Andreev reflections in these mesoscale devices. In addition, we find periodic magneto- resistance oscillations for magnetic field perpendicular to both the nanoribbon axis and the superconducting contacts. The temperature- and field-dependence of the magneto-resistance oscillation amplitude and period are suggestive of dissipative vortex dynamics in the vicinity of the contacts. Supported by NSF-MRSEC, NSF-NNIN and ONR. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W35.00015: Majorana Fermions in a Spin Ladder System Wade DeGottardi, Smitha Vishveshwara, Ditpiman Sen We consider a two-legged spin chain version of Kitaev's honeycomb model. Like its parent, this model supports $Z_2$ vortices at every plaquette. The topological phase of the system is sensitive to, {\it inter alia}, the spatial pattern of these vortices. The topological phases are gapped in the bulk but possess isolated zero energy Majorana modes at each end. The existence of such Majorana modes can be inferred from a new topological invariant. We show that there is an intimate relation between the existence of Majorana modes and the spontaneous breaking of the global $Z_2$ symmetry. [Preview Abstract] |
Session W36: Focus Session: Graphene Structure, Dopants, and Defects: Graphene Oxide and Fluoride
Sponsoring Units: DMPChair: Masahiro Ishigami, University of Central Florida
Room: C142
Thursday, March 24, 2011 11:15AM - 11:51AM |
W36.00001: Properties of Dilute Fluorinated Graphene Invited Speaker: I will discuss our recent studies on dilute fluorinated graphene (DFG). Fluorine adatoms are covalently added to a graphene sheet using a controlled and reversible approach to create a dilute coverage on the order of 10$^{12}$/cm$^2$, as determined by scanning tunneling microscopy studies. These adatoms are atomically sharp defects, interact strongly with the electronic states of graphene and drastically modify the transport properties of pristine graphene. This unusual 2D system exhibits several remarkable properties. Mid-gap state scattering dominates conduction at high temperature, the magnitude of which is determined by the adatom density and is correlated with Raman spectra. The temperature-dependent conductivity of the DFG sample follows weak localization at high carrier density and variable-range hopping at low carrier density. The transition is strongly correlated with the fluorine adatom density. In the variable-range hopping regime, DFG samples exhibit very large, negative magnetoresistance, which shows unusual staircase-like field dependence at low temperature. In the weak localization regime, we observe anomalous phase breaking behavior. I will discuss possible origins of these observations in the context of magnetism and localization. (In collaboration with S.-H. Cheng, C. Herding, and J. Zhu.) [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W36.00002: Electronic and structural properties of fluorinated graphene Chad E. Junkermeier, Stefan C. Badescu, Thomas L. Reinecke Experiments have shown that the electronic structure of graphene can be tailored from that of a semimetal to that of a wide bandgap semiconductor through adsorption of fluorene. This makes fluorinated graphene C$_{x}$F (x$\ge $1) attractive for electronics applications. Here we present first-principle calculations that reveal the dependence of C$_{x}$F electronic structure on the degree of fluorination in the range 1 $\le $ x $\le $ 8. We present a systematic analysis of bandgap opening and p-doping, as well as of adsorption energies, lattice constants, bulk modulus and surface corrugation for single-face and two-face functionalization. We rationalize these with a band-interpolation scheme in terms of localized orbitals that clarify the C-C, C-F and F-F bonding. We discuss the relevance of tunable Young modulus for nanomechanical resonators. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W36.00003: Patterning nanoroads and quantum dots on fluorinated graphene Morgana Ribas, Abhishek Singh, Pavel Sorokin, Boris Yakobson Graphene nanoroads [1] and quantum dots [2] patterned on functionalized graphene combine metallic and semiconducting properties on a same mechanically intact sheet. Using density functional methods we investigate different stoichiometric phases of fluorinated graphene and find that the complete ``2D-teflon'' CF phase is thermodynamically more stable. The formation of fluorinated graphene favors the nucleation of aromatic ``magic'' clusters, but unlike hydrogenated graphene [3] it does not have a nucleation barrier. The CF is an insulator and turns out to be a perfect matrix-host for patterning nanoroads and quantum dots of pristine graphene. Depending upon the edge orientation and width the electronic and magnetic properties of the nanoroads can be tuned. The HOMO-LUMO energy gaps are size dependent and show a typical confinement of Dirac fermions. Furthermore, we study the effect of different coverage of F on graphene (CF and C$_{4}$F) on the band gaps, and show their suitability to host quantum dots of graphene with unique electronic properties. References: [1] Singh, A. K.; Yakobson, B. I., Nano Lett. 2009, 9 (4), 1540.[2] Singh, A. K.; Penev, E. S.; Yakobson, B. I., ACS Nano 2010, 4 (6), 3510. [3] Lin, Y.; Ding, F.; Yakobson, B. I., Phys. Rev. B 2008, 78 (4), 041402. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W36.00004: Anomalous Phase Breaking in Dilute Fluorinated Graphene Xia Hong, Shih-Ho Cheng, Jun Zhu Quantum interference induced weak localization and phase breaking measurements are sensitive tools to probe the existence of magnetic impurities in mesoscopic systems. In this work, we study the low-field magnetoresistance of dilute fluorinated graphene (DFG), with a fluorine adatom density of $\sim$~10$^{12}$/cm$^2$. In the DFG samples, the phase breaking time $\tau_\phi$ follows $T^{-1}$ at high temperature and saturates at $T$ $\sim$ 10 K. The former is consistent with electron-electron interaction. The latter cannot be accounted for by conventional theories based on sample size and charge inhomogeneity. We show the dependence of the saturated $\tau_\phi$ on the carrier density and fluorine coverage and discuss the effects of spin-flip scattering and localization in phase breaking. Our observations point to the presence of adatom induced local magnetic moments in dilute fluorinated graphene. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W36.00005: Functionalization of exfoliated graphene by grafting aryl groups Hang Zhang, Elena Bekyarova, Zeng Zhao, Wenzhong Bao, Jhao-Wun Huang, Mikhail Itkis, Sandip Niyogi, Xiaoliu Chi, Huazhou Wei, Fenglin Wang, Robert Haddon, Chun Ning Lau We studied the transport properties of aryl functionalized exfoliated graphene. For the functionalization the graphene devices were immersed in a solution of diazonium salt. The attachment of aryl free radical to the basal carbon atoms changes the hybridization of the graphitic atoms from sp2 to sp3 thereby~modifying the lattice's electronic structure. We observe that this opens a Coulomb gap at low temperatures and transport measurements indicate a variable range hopping mechanism. In suspended graphene, which allows for functionalization on both sides, we observe a large transport gap. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W36.00006: Field emission from atomically thin edges of reduced graphene oxide Hisato Yamaguchi, Katsuhisa Murakami, Goki Eda, Takeshi Fujita, Julien Boisse, Pengfei Guan, Fujio Wakaya, Kyeongjae Cho, Yves Chabal, Mingwei Chen, Mikio Takai, Manish Chhowalla Point sources show the best electron emission properties due to local field enhancement at the tip. A drawback of tip emitters is that they must be positioned sufficiently apart to achieve field enhancement, limiting the number of emission sites and therefore the overall current. In contrast, we report ultra-low threshold voltage emission of multiple electron beams from atomically thin edges of individual reduced graphene oxide (rGO) sheets. The emission sites observed by field emission (FEM) and field ion (FIM) microscopies are atomically spaced along the edge. FEM measurements indicate evidence for interference, suggesting that the emitted electron beams are coherent. Based on our spectroscopy, high-resolution transmission electron microscopy and theory results, field emission is attributed to the aggregation of oxygen groups in the form of cyclic edge ethers. Such closely spaced electron beams from rGO offer prospects for novel applications and understanding the physics of linear electron sources. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W36.00007: Electronic structure of graphite oxide Hae Kyung Jeong, Cheolsoo Yang, Bong Soo Kim, Ki-jeong Kim We have investigated the electronic structure of graphite oxide by photoelectron spectroscopy at the Pohang Accelerator Laboratory, Korea. The typical sp$^{2}$ hybridization states found in graphite were also seen in graphite oxide. However, the $\pi $ state disappeared near the Fermi level because of bonding between the $\pi $ and oxygen-related states originating from graphite oxide, indicating electron transfer from graphite to oxygen and resulting in a downward shift of the highest occupied molecular orbital (HOMO) state to higher binding energies. The band gap opening increased to about 1.8 eV, and additional oxygen-related peaks were observed at 8.5 and 27 eV.\\[4pt] This research was supported by the Basic Science Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2010-0004592), and partly by the MEST (2009-0087138). Experiments at the PLS were supported in part by POSTECH and MEST. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W36.00008: Computational Studies for Reduced Graphene Oxide in Alcohol- and Hydrogen-Rich Environments Ramin Abolfath, Cheng Gong, Muge Acik, Yves Chabal, Kyeongjae Cho We employ \textit{ab-initio} molecular dynamic simulations to analyze the chemical reaction mechanisms for the oxygen removal process of graphene oxide upon annealing in the presence of water molecules and compare various thermal pathways in alcohol- and hydrogen-rich environments. Our first principles calculation shows damage-repair mechanisms of sp$^{2}$-C bonds in the etch holes of reduced graphene oxide and formation of dangling and/or sp$^{3}$-C bonds. The initial oxygen-abstraction results in the propagation of broken bonds and multi-site sp$^{2}$-C bond damage driven by the cascade of chemical reactions. The interplay between the environmentally induced damages and self-repair mechanisms of sp$^{2}$-C bonds determines the quality of the sheets after chemical treatments in alcohols or with hydrogen-rich environment. Water molecules form C=O and C-H bonds in the etch holes. We show that the alcohol- and hydrogen-rich environment provide an efficient transformation of C=O to the C-O bonds, and the removal of oxygen that is rarely observed with alcohol-rich environment alone. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W36.00009: Electron Phonon Coupling Mechanism in Thermally Reduced Graphene Muge Acik, Geunsik Lee, Cecilia Mattevi, Manish Chhowalla, Kyeongjae Cho, Yves J. Chabal Infrared absorption of atomic and molecular vibrations in solids can be affected by electronic contributions through non-adiabatic interactions, such as the Fano effect. Typically, the IR absorption lineshapes are modified or IR forbidden modes are detectable as a modulation of the electronic absorption. In contrast to such known phenomena, we report here the observation of a giant IR absorption band in reduced graphene oxide (GO), arising from the coupling of electronic states to the asymmetric stretch mode of a yet unreported structure [1], consisting of oxygen atoms aggregated at edges of defects. DFT calculations show that free electrons are induced by the displacement of the oxygen atoms, leading to a strong IR absorption that is in-phase with the phonon mode. This new phenomenon is only possible when all other oxygen-containing chemical species including hydroxyl, carboxyl, epoxide and ketonic functional groups are removed from the region adjacent to the edges, i.e. clean graphene patches are present. *The authors acknowledge funding from the NRI SWAN program and Texas Instruments. [1] Acik, M.; Lee, G.; Mattevi, C.; Chhowalla, M.; Cho, K.; Chabal, Y. J. \textit{Nature Materials}. \textbf{9,} 840-845 (2010) [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W36.00010: Grain-boundary unzipping by oxidation in polycrystalline graphene Simone Alexandre, Aline Lucio, Ricardo Nunes The need for large-scale production of graphene will inevitably lead to synthesis of the polycrystalline material [1,2]. Understanding the chemical, mechanical, and electronic properties of grain boundaries in graphene polycrystals will be crucial for the development of graphene-based electronics. Oxidation of this material has been suggested to lead to graphene ribbons, by the oxygen-driven unzipping mechanism [3]. A cooperative-strain mechanism, based on the formation of epoxy groups along lines of parallel bonds in the hexagons of graphene's honeycomb lattice, was proposed to explain the unzipping effect in bulk graphene [3] In this work we employ \textit{ab initio} calculations to study the oxidation of polycrystalline graphene by chemisorption of oxygen at the grain boundaries. Our results indicate that oxygen tends to segregate at the boundaries, and that the unzipping mechanism is also operative along the grain boundaries, despite the lack of the parallel bonds due to the presence of fivefold and sevenfold carbon rings along the boundary core. \\[4pt] [1] J. Cervenka et al., PRB 79, 195429 (2009). \\[0pt] [2] J. da Silva-Ara\'{u}jo and R. W. Nunes, PRB 81, 073408 (2010). \\[0pt] [3] J-L. Li \textit{et al.}, PRL 96, 176101 (2006). [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W36.00011: Influence of Gate Voltage on the diffusion of Oxygen on Graphene Alejandro Suarez, Ljubisa Radovic, Ezra Bar-Ziv, Jorge Sofo We calculate the surface diffusion of Oxygen on Graphene using Density Functional Theory. We find the activation energy for diffusion to be 0.71 eV. Charging the graphene plane causes the diffusion barrier to change substantially. Electron doping graphene lowers the diffusion barrier, resulting in activation energies as low as 0.15 eV for a carrier concentration of 7.6x10$^{13}$ cm$^{-2}$. This barrier reduction yields diffusion coefficients reaching over nine orders of magnitude lower than that of diffusion on neutral graphene. After study of the change in charge density distribution and local density of states, this effect is explained by a mixture of bond weakening under the equilibrium state and bond strengthening during the transition state. With this large fluctuation in diffusivity, patterning of oxidized regions in graphene may be achieved through variation of the gate voltage. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W36.00012: Coulomb blockade and hopping conduction in graphene quantum dots array Daeha Joung, Lei Zhai, Saiful Khondaker We show from the low temperature electron transport measurements that the transport properties of chemically reduced graphene oxide (RGO) sheets can be explained as a sequential tunneling of charges through a two dimensional polydispersed array of graphene quantum dots (GQD), where graphene domains act like QDs while oxidized domains behave like tunnel barriers between QDs. As the temperature is decreased to lower than 15 K, a complete suppression of current ($I)$ below a threshold voltage ($V_{t})$ was observed due to Coulomb blockade (CB) of charges through GQD array. For $V> \quad V_{t}$, the current follows a scaling behavior, $I \quad \propto [(V-V_{t})/V_{t}]^{\alpha }$ with $\alpha \quad \sim $ 2.8, implying a quasi 2D GQD array. Temperature dependent current -- gate voltage ($I-V_{g})$ curves show reproducible Coulomb oscillations due to a single electron tunneling through GQD array that washes out between 70 and 120 K corresponding to charging energies of 6.2 $\sim $ 10 meV giving estimated GQD sizes of 5 - 8 nm. Temperature dependent resistance data show Efros-Shklovskii variable range hopping (ES VRH) arising from CB, structural and size induced disorder. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W36.00013: Ink jet printed graphene oxide (GO) coplanar waveguide (CPW) structures for measurement of microwave propagation in GO Kate Duncan, Edwin Barry, Mark Griep, Johhny Daniel, Derek Morris, Shashi Karna Chemically reduced graphene~(CGR) has been successfully inkjet printed using a commercially available printer. The CGR with sheet sizes below 200 nm were dispersed in a mixture of water and ethanol. Coplanar waveguide (CPW) structures were deposited on CGR and plastic substrates, scattering (S) parameters were measured in order to extract material parameter for incorporation into simulation tools. Measurements and modeling of microwave propagation in graphene shall be presented. [Preview Abstract] |
Session W37: Focus Session: Graphene: Growth, Characterization, and Devices: Quantum Hall Effect
Sponsoring Units: DMPChair: Aaron Bostwick, Lawrence Berkeley National Laboratory
Room: C146
Thursday, March 24, 2011 11:15AM - 11:27AM |
W37.00001: A study of the quantum Hall effect in CVD graphene synthesized on Cu Tian Shen, Wei Wu, Helin Cao, Yong Chen, David Newell, Curt Richter, Qingkai Yu Graphene films grown by Chemical Vapor Deposition (CVD) have provided a viable way to large area, low cost graphene-based electronics. Graphene CVD grown on Cu was transferred to SiO2/Si substrates and Hall-bar devices with sizes as large as 150 $\mu $m were fabricated, exhibiting carrier mobility of $\sim $3000cm$^{2}$/Vs. At low temperatures, the half integer quantum-Hall effect (QHE) and Shubnikov-de Haas (SdH) oscillations confirmed the single layer quality of the transferred graphene films. Systematic measurements in the quantum Hall region such as the DC/AC current dependence, temperature dependence, and AC frequency dependence were carried out, and their impact on the breakdown of the QHE was investigated. From weak localization peak fitting, the phase coherence length of the CVD graphene is determined to be $\sim $3$\mu $m at 1.6K. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W37.00002: Strain-induced pseudo-magnetic fields and charging effects on CVD-grown graphene R.T.-P. Wu, M.-L. Teague, N.-C. Yeh, S. Yeom, B.L. Standley, D.A. Boyd, M.W. Bockrath Atomically resolved imaging and spectroscopic characteristics of chemical vapor deposition (CVD) grown graphene on Cu are studied using scanning tunneling microscopy and spectroscopy. CVD-grown graphene remaining on Cu exhibits large ripples and appears strongly strained. Different regions show different lattice structures and electronic density of states (DOS). Ridges appear along the boundaries of different lattice structures, which reveal excess charging effects. The large, non-uniform strain induces pseudo-magnetic fields up to $\sim $50 Tesla, yielding integer and fractional quantum Hall effects (IQHE and FQHE) as quantized conductance peaks in the DOS. For CVD-grown graphene transferred from Cu to SiO$_{2}$, the average strain and the resulting charging effects and pseudo-magnetic fields are much reduced. Fourier transformation of the local DOS of strained samples as well as data on the effects of real magnetic fields versus pseudo-magnetic fields will be presented. This work was jointly supported by NSF and NRI. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W37.00003: Transport phenomena of suspended graphene in the quantum Hall regime Hsin-Ying Chiu, Ching-Tzu Chen, David DiVincenzo, Phaedon Avouris Towards the goal of making qubits in graphene, it is important to develop graphene quantum point contact for quantum-gate operations. Klein tunneling effect implies that confining charge carriers in graphene using external potential is challenging. Hence, we resort to electric-field controlled band gap opening for charge confinement. We have successfully fabricated high quality suspended few-layer graphene devices with local electrostatic gates. In this talk, we present our observation of voltage-controlled band gap opening and anomalous quantum Hall features upon applying vertical electrical filed, revealing rich physics of symmetry breaking in the few-layer graphene system. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W37.00004: Measurement of the 1/3 Fractional Quantum Hall Effect Energy Gap in Multi-terminal Suspended Graphene Devices Fereshte Ghahari Kermani, Yue Zhao, Paul Cadden-Zimansky, Kirill Bolotin, Philip Kim In recent investigations of transport properties in two-terminal high mobility suspended graphene devices, a quantized conductance corresponding to the 1/3 FQHE state has been observed. However, due to the inherent mixing between longitudinal and transverse resistivities in this two-terminal measurement, quantitative characterization of the observed FQHE states such as the FQHE energy gap is difficult. In this talk, we present the measurement of multi-terminal IQHE and FQHE states in ultraclean suspended graphene samples in low density regime. The energy gap of the 1/3 FQHE, measured by its temperature-dependent activation, is found to be much larger than the corresponding state found in the 2DEGs of high-quality GaAs heterostructures, indicating that stronger e-e interactions are present in graphene. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W37.00005: Symmetry breaking of zero energy landau level in monolayer graphene Yue Zhao, Paul Cadden-Zimansky, Fereshte Ghahari, Philip Kim We experimentally study the nature of the symmetry breaking of the zero energy landau level (LL) in monolayer graphene using Corbino geometry and Hall bar geometry devices. At high magnetic fields, in the absence of the edge state channel connection in Corbino devices, we observe a gap opening in $\nu=0$ QH state whose gap is independent of in-plane magnetic field. In Hall-bar geometry devices where edge state connection is allowed, we observe similar QH Insulator behavior independent of the in-plane magnetic field, indicating that the observed insulating behavior at the charge neutrality point of monolayer graphene at high magnetic field is originated from the degeneracy lifting of the zero LL via the valley pseudospin polarization rather spin polarization. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W37.00006: Fractional quantum Hall effect in graphene on boron nitride Cory Dean, Andrea Young, Paul Cadden-Zimansky, Lei Wang, Hechen Ren, Kenji Watanabe, Takashi Taniguchi, Philip Kim, James Hone, Ken Shepard Graphene is a remarkable 2D material exhibiting many unique and surprising many-body effects resulting from strong electron interactions. A continuing challenge remains the fabrication of ultra-high mobility devices that allow the intrinsic character of graphene to be fully explored. In my talk I will discuss our recent advancements in fabricating very-high quality graphene devices on boron nitride. Magnetoresistance measurements under very large applied fields will be presented including our recent observation of the fractional quantum Hall effect in multi-terminal devices over a broad range of carrier densities. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W37.00007: Transport on gated C-face epitaxial graphene Invited Speaker: We present transport and electronic properties on single layer and multilayered epitaxial graphene layers grown on 4H-SiC-(000-1) (C-face) by the Confinement Controlled Sublimation method [1]. Single layers present all the characteristics of isolated graphene layers. In particular quantum Hall effect plateaus develop at half-integer values, concomitant with vanishing longitudinal resistivity. High mobility up to $\mu $=14,000 cm$^{2}$/V.s at 300 K is achieved despite contamination and substrate steps. Multilayered epitaxial graphene (MEG) on the C-face consists of non-graphitic rotationally stacked graphene layers, exhibiting the band structure of a single graphene layer [2]. Transport in MEG presents also graphene characteristics. In some cases transport anomalies are observed indicating a much richer picture. \\[4pt] [1] R. Ming et al. Materials Science and Engineering -- Reports (submitted) \\[0pt] [2] M. Sprinkle et al, Phys. Rev. Lett. 103, 226803 (2009). [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W37.00008: Quantum Hall Effect of Hybrid Monolayer-bilayer Graphene Structures: Observation of Broken electron-hole Symmetry Yong P. Chen, Jifa Tian, Isaac Childres, Helin Cao Quantum Hall Effect (QHE) in both monolayer (1L) and bilayer (2L) graphene has been well studied in the past few years. Little attention has been paid to the magneto-transport across the 1L $\sim $2L graphene interface. Here, we present the magnetotransport measurements of several exfoliated graphene quasi-Hall bar devices which consist of partly 1L and partly 2L graphene. We focused on the Hall resistance (R$_{xy})$ across the interface between 1L and 2L graphene when the carrier types and densities are changed using a back gate voltage. We observed that when the carrier type is p type (hole), R$_{xy}$ typically shows QHE of bilayer graphene with filling factor of 4N, N being integers. When the carrier type is changed into n type (electron), the corresponding R$_{xy}$ typically shows QHE of single layer graphene with filling factor of 4(N+1/2), N being integers. We discuss possible explanations for the observed broken electron-hole symmetry in such hybrid structures. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W37.00009: Multi-terminal transport property on hybrid structure of monolayer and bilayer graphene Yongjin Jiang, Jiangping Hu We study the transport properties of Hybrid structures formed by monolayer and bilayer graphene. The energy spectrum and edge states of various junctions in strong magnetic field are analyzed. The transport properties are affected by the mixing effect of interfaces and the scattering between edge states, and display many novel features in multi-terminal measurements, Our results explains recent experimental results on such hybrid structure systems. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W37.00010: Electric field induced transition between spin to valley polarized $\nu $=0 quantum Hall state in dual-gated graphene bilayers Kayoung Lee, Seyoung Kim, Babak Fallahazad, Emanuel Tutuc Graphene bilayers in Bernal stacking exhibit a transverse electric field dependent energy gap, thanks to the on-site electron energy asymmetry between the two layers. In a perpendicular magnetic field, the applied transverse electric field ($E)$ will induce a quantum Hall state (QHS) at the charge neutrality point (filling factor $\nu $=0) marked by a insulating behavior of the longitudinal resistance ($\rho _{xx})$, and a plateau in the Hall conductivity. Using dual-gated graphene bilayers, we investigate here the $E$-field dependence of the $\nu $=0 QHS in high perpendicular magnetic fields ($B)$, up to 30T. The temperature dependence of $\rho _{xx}$ measured at $\nu $=0 shows an insulating behavior, which is strongest in the vicinity of $E$=0 as well as at large $E$-fields. At a fixed $B$-field, as a function of the applied $E$-field the $\nu $=0 QHS undergoes a transition, marked by a $\rho _{xx }$minimum, as well as a temperature independent $\rho _{xx}$ at a finite $E$-field value. This observation can be explained by a transition from a spin polarized $\nu $=0 QHS at small $E$-fields, to a valley (layer) polarized $\nu $=0 QHS at large $E$-fields. The $E$-field value at which the transition occurs follows a linear dependence on the applied perpendicular magnetic field, with a slope of $\sim $18 mV/nm$\cdot $T. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W37.00011: Giant Spin-Hall Effect and Nonlocal Transport in Graphene Dmitry Abanin, K.S. Novoselov, A.K. Geim, L.S. Levitov Graphene provides a unique opportunity to explore quantum-relativistic phenomena in a condensed matter laboratory. Interesting phenomena associated with the parity anomaly, including quantum Hall effect in the absence of magnetic field and quantum spin-Hall effect in quantizing magnetic fields, have been theoretically proposed, but could not be observed so far largely due to disorder and density inhonogeneity. We show that weak magnetic field induces large bulk non-quantized spin-Hall effect in graphene. The effect occurs due to Zeeman spin splitting which generates the imbalance of the Hall resistivities of the two spin species. The spin-Hall effect is robust in the presence of disorder and interactions. It will manifest itself in large nonlocal transport mediated by long-lived spin currents, as well as in spin injection and spin accumulation experiments. The effect peaks at the Dirac point, and can serve as a hallmark of the relativistic character of carriers in graphene and other Dirac materials. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W37.00012: Quantum Hall effect and Landau level crossing in trilayer graphene Thiti Taychatanapat, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero We report the experimental observation of quantum Hall effect in Bernal stacked trilayer graphene (TLG) on hexagonal boron nitride substrate. The mobility of our TLG reaches 110,000~cm$^2 $/V$\cdot$s allowing us to observe the Shubnikov-de Haas oscillation at a magnetic field as low as $300$~mT and broken- symmetry states at high magnetic field. In addition, the unique band structure of Bernal stacked TLG which consists of monolayer-like and bilayer-like subbands at low energy allows us to observe the Landau level crossing between these two subbands. The positions of these crossings in magnetic field and filling factors enable us to estimate relevant Slonczewski-Weiss-McClure parameters. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W37.00013: Suspended graphene electromechanics in quantum Hall regime Vibhor Singh, Ganesh Subramanian, Bushra Irfan, Hari Solanki, Mandar Deshmukh There has been a keen interest in the NEMS community in probing the coupling between charge and mechanical degrees in NEMS resonators. These experiments show that electron transport and mechanical motion of the resonator influence each other. Motivated by this, we have probed the electron transport in the ultra clean graphene devices in quantum Hall regime at low temperature while it is mechanically perturbed. There can be several mechanisms that can lead to the resistance change due to mechanical vibrations like, by strain due to deformation of the flake, redistribution of the carrier density etc, electron scattering within the flake due to changes in the energy landscape. In our study we find that upon mechanical vibrations the resistance of the device changes. We try to understand these changes caused by the non-linear dependence of resistance on carrier density. [Preview Abstract] |
Session W38: Focus Session: Organic Electronics and Photonics -- Charge transport
Sponsoring Units: DMP DPOLYChair: Almantas Pivrikas, Johannes Kepler University Linz
Room: A130/131
Thursday, March 24, 2011 11:15AM - 11:27AM |
W38.00001: Charge Transport in Functionalized Fluorinated Pentacenes Oana Jurchescu, Daniel David, Claire McLellan, Balaji Purushothaman, Shubin Liu, Veaceslav Coropceanu, John Anthony, Laurie McNeil We report on charge transport in fluorinated functionalized pentacenes and discuss the effect of trialkylsilyl and the number of fluorine atoms. We show that modifications in the chemical composition influence the molecular packing, crystal formation and electrical properties, allowing us to measure mobilities from 10$^{-5}$ to 1.7 cm$^{2}$/Vs. The mobilities correlate with the packing, demonstrating that tuning the solid-state order to induce pi-stacking improves electrical properties. By combining Raman measurements with theoretical calculations predicting the vibrational spectrum, we explore the vibrational modes of the crystals, providing information about the intermolecular coupling and electron-phonon interactions governing charge transport. We calculate the intermolecular electronic couplings and band structures by using density functional theory, and study the effect of fluorination and trialkylsilyl substitution on crystal packing and the electronic properties. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W38.00002: Semicrystalline high performance poly (thienothiophene) thin films: crystallites and defects Chenchen Wang, Javier Dacu\~na, Bj\"orn Br\"auer, Dan Daranciang, Alberto Salleo Effects of liquid crystalline temperature annealing and surface treatment on Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (PBTTT) thin films were studied. Time resolved terahertz spectroscopy (TRTS), which measures the local carrier mobility, suggests that the mobility improvement of annealed PBTTT on octadecyltrichlorosilane (OTS), compared with as cast film, is mainly due to superior local carrier mobility. Scanning transmission x-ray microscopy (STXM), which measures the in-plane molecular orientation with 30nm spatial resolution, shows similar domain size of annealed films on both OTS and $SiO_{2}$, and implies that the higher mobility of film on OTS cannot be accounted for by domain size. These results are also supported by the mobility edge model, which extracts trap density and quasi-free carrier mobility in crystallites from transistor characterization. Annealing film on OTS improves mobility in crystallites, but has little effects of reducing trap density. The modeling shows great elimination of interface trap density of film on OTS, compared with film on $SiO_{2}$, which might be the main reason for its higher mobility. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W38.00003: First-principles study of polythiophene and polyselenophene crystals for organic electronics Takao Tsumuraya, Jung-Hwan Song, A.J. Freeman Semiconducting polymers, like regioregular poly(3-hexylthiophene)/poly(3-hexylselenophene) (rr-P3HT/rr-P3HS) are currently the most widely studied materials in a variety of applications for polymer based bulk-heterojunction (BHJ) solar cells and organic field-effect transistors (OFET). [1,2] For both applications, the performance of devices has been attributed to thin film structures of rr-P3HT/rr-P3HS on substrates. To understand their mechanisms, the crystal structure has been extensively investigated by using various experimental techniques. However the crystal structure has yet to be unambiguously characterized. Here, we proposed several possible structures and investigated their stabilities from first-principles density functional calculations based on the all-electron FLAPW method. [3] We found that two base-centered monoclinic structures belonging to space group $A$2 are in the degenerate lowest energy structures. The electronic and transport properties are also discussed. Lastly, we report on the differences in electronic and crystal structure between rr-P3HT and rr-P3HS.\\[0pt] [1] G. Li $et$ $al$., Nature Mater. \textbf{4}, 864 (2005).\\[0pt] [2] H. Sirringhaus $et$ $al$., Nature \textbf{401}, 685 (1999).\\[0pt] [3] E. Wimmer $et$ $al$., Phys. Rev. B \textbf{24}, 864 (1981). [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W38.00004: Analysis of Metallic Conduction at the Interface of TTF and TCNQ Crystals Viktor Atalla, Mina Yoon, Matthias Scheffler Organic materials are promising candidates for a next generation of electronic devices, since they offer a variety of new intriguing electronic phenomena while being environmentally friendly, low cost, and mechanically flexible. Here we study the donor/acceptor interface of TTF and TCNQ organic molecular crystals which was found to exhibit metallic conduction whereas the individual crystals are large band-gap semiconductors. Using density functional theory (DFT) we first compare the performance of different exchange-correlation (XC) functionals for TTF and TCNQ dimers. All employed XC functionals consistently give electron transfer from TTF to TCNQ and the van der Waals (vdW) corrected molecular binding distances are within $\approx 0.1$~{\AA} of the MP2 value, indicating that within DFT the system can be qualitatively described by semilocal functionals. We construct interfaces between the two types of crystals and calculate their electronic structures. On a PBE + vdW level we find indications for metallic conduction at the interface, due to metallic bands that are exclusively induced from the interface layers of TTF and TCNQ molecules. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W38.00005: Charge Trapping in Organic Thin-Film Transistors Claire McLellan, Jack Owen, Marsha Grimminger, John Anthony, Oana Jurchescu Charge trapping in the bulk of the organic semiconductors and at interface with the gate dielectric and/or contacts is one of the determining factors governing charge transport in organic thin-film transistors (OTFTs). We explore the current-voltage characteristics in different charge density regimes and extract the field-effect mobility. The dependence of mobility on gate/drain voltage give us valuable insight into the mechanism of charge transport and the relevance of trapping states. We perform measurements on devices fabricated using different methods, such as spin-coating, drop-casting or spray-coating, on a silicon gate electrode, silicon dioxide gate dielectric, and gold source and drain contacts. We demonstrate that the performance of OTFTs is strongly dependent on processing details. We show that even when using the same processing method, we are able to systematically tune the charge trapping states by chemically modifying the contact and dielectric surfaces with self-assembly monolayers. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W38.00006: Organic electrical double layer transistors gated with ionic liquids Wei Xie, C. Daniel Frisbie Transport in organic semiconductors gated with several types of ionic liquids has been systematically studied at charge densities larger than 10$^{13}$ cm$^{-2}$. We observe a pronounced maximum in channel conductance for both p-type and n-type organic single crystals which is attributed to carrier localization at the semiconductor-electrolyte interface. Carrier mobility, as well as charge density and dielectric capacitance are determined through displacement current measurement and capacitance-voltage measurement. By using a larger-sized and spherical anion, tris(pentafluoroethyl)trifluorophosphate (FAP), effective carrier mobility in rubrene can be enhanced substantially up to 3.2 cm$^{2}$V$^{-1}$s$^{-1}$. Efforts have been made to maximize the charge density in rubrene single crystals, and at low temperature when higher gate bias can be applied, charge density can more than double the amount of that at room temperature, reaching 8*10$^{13}$ cm$^{-2}$ holes (0.4 holes per rubrene molecule). [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W38.00007: Hopping transport in electrolyte-gated P3HT organic field effect transistors Shun Wang, Mingjing Ha, Michael Manno, C. Daniel Frisbie, C. Leighton Using ion-gel-gated poly (3-hexylthiophene) (P3HT), we successfully fabricated p-type organic field effect transistors (OFET) with on/off ratios of 10$^{5}$ and mobility of the order of 1 cm$^{2}$/V s at room temperature. We studied charge transport in the electrochemically doped P3HT as a function of gate voltage, temperature, magnetic field, film thickness, and roughness. Carrier concentrations were obtained from both gate charging current and Hall effect measurements. The resistance vs. temperature (down to 5K for large gate voltages) characteristics indicate 2D hopping transport. Large positive magnetoresistance at temperatures lower than 50 K was observed, but with anomalously low anisotropy due to high roughness. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W38.00008: Charge transport and velocity distribution in ambipolar organic thin film Transistors based on a diketopyrrolopyrrole-benzothiadiazole copolymer Tae-jun Ha, Prashant Sonar, Samarendra Pratap Singh, Ananth Dodabalapur There have been reports of charge transport mechanisms in organic thin film transistors (OTFTs) focusing on steady-state characteristics but these measurements provide limited information. Time-resolved measurements can provide additional information in understanding transport mechanisms but existing reports have focused on unipolar organic characteristics. No previous reports on ambipolar organic devices have involved entire velocity distribution and charge transport mechanisms. Recently, we have fabricated ambipolar OTFTs based on a diketopyrrolopyrrole-benzothiadiazole copolymer (PDPP-TBT) with a field-effect mobility of more than 0.2 cm$^2$ V$^-1$ s$^-1$. Velocity distributions are measured by performing specialized dynamic measurements while keeping the RC-time constant of the measurement circuit small. This yields a distribution in arrival times of charge carriers from source to drain which can be converted to velocity distributions. We will also describe dynamic transport measurements on high-k-dielectric PDPP-TBT OTFTs. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W38.00009: Organic field effect transistor fabricated by directly grown poly (3 hexylthiophene) crystalline nanowires on solution processed carbon nanotube aligned array electrodes Biddut Sarker, Jianhua Liu, Lei Zhai, Saiful Khondaker We demonstrate convenient and highly reproducible approach to fabricate organic field effect transistors (OFETs) using the direct growth of crystalline P3HT nanowires on aligned array SWNT interdigitated electrodes. Compared to the OFETs with metal electrodes, the OFETs with SWNT electrodes show high mobility and high current on-off ratio with a maximum of 0.13 cm$^{2}$/Vs and 3.1$\times $10$^{5}$, respectively. The improved device characteristics are also demonstrated by the absence of short channel effect which is dominant in gold electrode OFETs. Such remarkable improvement of the device performance as high mobility, high current on-off ratio, absence of short channel effect and better charge carrier injection can be attributed to the improved contact via strong \textit{$\pi -\pi $} interaction SWNT electrodes with the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one dimensional crystalline structure. . [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W38.00010: Interfacial Width Measurements of Dielectric/P(NDI2OD-T2) Using Resonant Soft X-ray Reflectivity Hongping Yan, Ziran Gu, Eliot Gann, Brian Collins, Sufal Swaraj, Cheng Wang, Torben Schuettfort, Chris McNeill, Harald Ade Interfaces between a conjugated polymer and a dielectric play a critical role in organic thin-film transistors, yet it's difficult to measure. Resonant Soft X-ray Reflectivity (R-SoXR) is a unique and relatively simple method to investigate such interfaces. By tuning the soft X-ray energies, we are able to selectively and quantitatively characterize the interfacial width and thicknesses of the films. In an effort to relate performance to interface structure, we have used R-SoXR to investigate polystyrene (PS) or poly(methyl methacrylate) (PMMA) as the top layer and poly{\{}[N,N9-bis(2-octyldodecyl)-naphtha-lene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5, 59-(2,29-bithiophene){\}} (P(NDI2OD-T2)) as bottom layer supported on a Si substrate. We found that the device with PS as dielectric has a higher threshold voltage, which correlates to the increased interfacial trapping due to increased interfacial roughness. The extension of R-SoXR to the energy of fluorine K absorption edge is also demonstrated. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W38.00011: Temperature Induced Structure Evolution of Regioregular Poly (3-hexylthiophene) in Dilute Solution and its Influence on Thin Film Morphology Charles Han, Ye Huang, He Cheng The structure evolution of regioregular Poly (3-hexylthiophene) (P3HT) in THF dilute solution, and its influence on thin film morphology were studied. A thermal treatment at high temperature effectively re-disperses P3HT micro-sized aggregates, and introduces two modes in DLS measurements. The structures of these 2 modes are identified, and the two structures of P3HT in dilute solution can greatly influence the morphology on subsequent thin films produced. Since the P3HT structures are carried into the film morphology, proper understanding and control of the structures in solution are important and can enhance electronic and opto-physical properties of the final devices. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W38.00012: Effect of Cooling Rate on Microstructure and Charge Transport in Semiconducting Polymer Thin Films Evan Kang, Eunseong Kim Thermal annealing of polymer thin films often enhances charge carrier mobility which can be attributed to self-healing of the film morphology. We have investigated the effect of cooling rate following the annealing treatment on the thin film microstructure and the charge transport properties using a high performance semiconducting polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). The cooling rate plays a key role in determining the microstructure and performance of polymer thin films. Differential scanning calorimeter measurement shows that fast cooling suppresses the crystallization process. The microstructure of thin films is investigated by using 2D X-ray diffraction and atomic force microscopy. Slow cooling results in well-connected large domains with enhanced three dimensional ordering whereas fast cooling leads to misalignment of small domains with relatively rough surface. Transport characteristics at various temperatures show increase in the charge carrier mobility and decrease in the activation energy when the cooling rate is slowed. This change in the mobility and activation energy becomes saturated with cooling rate below 15 \r{ }C/min. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W38.00013: Exponential behavior of the Ohmic transport in organic films Corneliu Colesniuc, Rudro Biswas, Samuel Hevia, Alexander Balatsky, Ivan Schuller An exponential dependence of conductance on thickness and temperature was found in the low voltage, Ohmic regime of copper (CuPc) and cobalt (CoPc) phthalocyanine, sandwiched between palladium and gold electrodes, unlike ever claimed in organic materials. A comparison with conventional models fails to explain all the data with a single set of parameters. On the other hand, a model which incorporates tunneling between localized states with thermally-induced overlap agrees with the data, and decouples the contributions to conductance from the electrode-film interface and the bulk of the film. Work supported by AFOSR, DOE and the UCOP program on carbon nanostructures. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W38.00014: Atomistic Molecular Dynamics Simulation of the Surface Properties of P3HT Films Yeneneh Yimer, Sima Mofakham, Ali Dhinojwala, Mesfin Tsige In recent years P3HT has attracted much interest mainly because of its potential applications in solar cells, light emitting diodes and field effect transistors. The performance of these devices is strongly dependent on the structural packing, morphology and interfacial properties of the P3HT. In order to improve the devices efficiency, understanding the structural and dynamical properties of P3HT at the atomic level is important. Most studies on P3HT have mainly focused on understanding its bulk properties. However, the orientation of P3HT chains at the polymer/air interface has not been well investigated. Using molecular dynamics simulations we have studied the interfacial properties of free-standing P3HT films. The simulation results show that at the air/polymer interface the alkane side groups of the P3HT chains orient mainly to the interface in qualitatively good agreement with SFG experimental results. The surface tension of P3HT in its melt state shows strong dependence on temperature and chain length and is directly related to the roughness of the P3HT surface. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W38.00015: Switching or triggering by light organic materials in the 100 nm size range Vina Faramarzi, Jean Francois Dayen, Bernard Doudin We investigate optoelectronic fabrication and characterization of organic electronics devices in the 100 nm range. This intermediate size has advantages in simplicity of device fabrication and robustness of observed properties. For this aim high aspect ratio lateral electrodes separated by a sub 100nm gap were produced by means of simple optical lithography techniques [1]. The electrical measurements set-up is integrated with an inverted optical microscope, allowing simultaneous optical and electrical measurements followed by temperature and magnetic field studies. We demonstrate that electrical contacts are suitable for a wide range of current measurements going from 10$^{-13}$ to 10$^{-2}$ A. This versatility makes the nanotrench design compatible for studying a broad variety of nanoparticles and molecular systems. Electrical transport properties of different devices are presented, e.g molecular switches, Iron based spin-transition nanoparticles, Conductive molecular chains and 2D nanoparticle networks. The promising reproducible results reveal novel intrinsic transport properties and confirm the high interest and reliability of this approach for further studies in the field of molecular electronics and spin dependent transport in molecular structures. \\[0pt] [1] J-F Dayen, V Faramarzi et al, \textit{Nanotechnology} \textbf{21}(33), 335303 (2010) [Preview Abstract] |
Session W39: Experimental Techniques in Biophysics
Sponsoring Units: DBPChair: Mark Reeves, George Washington University
Room: A124/127
Thursday, March 24, 2011 11:15AM - 11:27AM |
W39.00001: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W39.00002: Spatially resolved absorption spectroscopy of bio - assemblies on a micron scale Silki Arora, Jennifer Mauser, Debopam Chakrabarti, Alfons Schulte We have developed a novel approach to measure optical absorption spectra with spatial resolution at the micron scale. The setup employs a confocal microscope with a broadband white light excitation beam in transmission geometry. An aperture controls the amount of illuminating light and localizes the area of excitation. The setup is employed to measure the absorption spectrum of single red blood cells ($\sim $ 7 microns diameter) under solution conditions. The spatial resolution in the lateral direction is found to be better than three microns. Through measurements of the transmitted intensity in met- myoglobin and calcein dye nanoliter solutions at fixed path lengths, we establish that the absorbance varies linearly with concentration over the range from 0.1 to 2 mM. Our instrument enables measurements of spatial variations in the optical density of small samples and may find application in monitoring biological assemblies at the single cell level. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W39.00003: Stochastic super resolution imaging by diffusive probes Thomas Dorn, John Ewalt, Francisco Marquez, George Shubeita Optical microscopy is a powerful tool for the imaging of cells and bio-materials, however the resolution is limited by diffraction and thus objects closer than a few hundred nanometers cannot be individually resolved. We report a novel stochastic super-resolution technique which relies on diffusing probes in which the resolution is determined by probe size and Forster radius of energy transfer. By recording a time-series of images similar to other super-resolution techniques, the centers of bright spots can be determined with sub-pixel accuracy by fitting to the point spread function. The centroids can then be used to reconstruct a super-resolution image. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W39.00004: Near-field Approaches to Subcellular Tissue Abalation Deepa Raghu, Joan Hoffmann, Benjamin Gamari, Andrew Gomella, Mark Reeves We report on the development of a near-field approach to MALDI (Matrix-assisted laser desorption and Ionization). In this technique analytes embedded in an energy- absorbing matrix are ablated from the surface of a sample. In the infrared region, the matrix can be water by exciting the 3-micron vibrational mode of the water molecule. We use a 3-micron wavelength lasers, coupled with a near-field scanning microscope to ablate material from cells of different membrane stiffness. We have been able to reproducibly ablate features as small as 1 micron in diameter in cell and have characterized the power-dependence of the ablation process. We will review our findings and describe demonstrations of tissue modification by this approach at length scales smaller than a single cell. This approach has the potential to allow the identification and mapping of proteins expressed in intact cells and tissues, which is of great interest as protein expression connects genomic information with the functioning of an organism. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W39.00005: Rotational Diffusion of Plasmon-Resonant Gold Nanorods for Depth-Resolved Microrheology Using Optical Coherence Tomography Amy Oldenburg, Raghav Chhetri, Krystian Kozek, Aaron Johnston-Peck, Joseph Tracy The ability to perform microrheology in optically thick samples would enable analysis of bulk tissues. Optical coherence tomography (OCT) provides imaging several mean free scattering path lengths into tissue. In this study we report the use of plasmon-resonant gold nanorods as microrheological sensors in OCT. Nanorods exhibit a longitudinal mode that is excited when they are oriented parallel to the polarization of the incident light, which is favorable for passive microrheology using polarized light to monitor their rotational diffusion. We demonstrate measurements of the rotational diffusion of unconfined, colloidal gold nanorods using polarization-sensitive OCT, and validate the Stokes-Einstein relationship for the nanorods in simple fluids of varying viscosity. We then show that OCT provides depth-resolved imaging of fluid viscosity through measurements of the rotational diffusion rate of the nanorods. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W39.00006: Monte-Carlo Study of Binding Kinetics in Surface Plasmon Resonance Systems Matthew Raum, Uwe T\"auber, Kimberly Forsten-Williams Surface plasmon resonance (SPR) has become a standard tool for studying ligand-receptor binding reactions in real-time. Ideally the data obtained with this technique allows measurement of kinetic reaction rates (rather than merely the equilibrium constant for the reaction). In typical experimental configurations one species is immobilized near the active surface while its binding partner is initially suspended in solution, flowing across the active surface. It is generally appreciated that reaction rates observed in SPR experiments are affected by mass transport if the time scales for reaction and transport in the system are comparable. The issue of ensuing effective reaction rates has been treated through different approaches in the literature. The goal of this research is a quantitative study of how faithfully intrinsic binding rates can be measured in SPR devices. We employ a lattice Monte Carlo method to simulate SPR experiments in order to test the efficacy of common SPR analytical techniques. We point out where existing analytical techniques succeed or fail in measuring binding and dissociation rates, and investigate the influence of secondary parameters in the system (such as the flow rate) on experimental data. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W39.00007: Fluorescence Correlation Spectroscopy of Tryptophan-containing Proteins in Sugar Solutions using Two Photon Excitation David Sidebottom, Nathan Holman, Yuli Wang, Michael Nichols Simple sugars are often incorporated in cryopreserving media to aid in the preservation of biomaterials and functional proteins. However, the mechanism by which sugars provide protection is still openly debated. As part of a project to investigate the behavior of proteins in sugar solutions, we are developing Fluorescence Correlation Spectroscopy (FCS), using a novel two photon excitation at 532 nm, as a selective probe of protein dynamics for tryptophan-containing proteins. Our goal is to monitor possible alterations in the protein's hydrodynamic radius caused by preferential binding of sugars to its surface. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W39.00008: Continuous Nano-Particle Transport in a Standing Wave Optical Line Trap Vassili Demergis, Ernst-Ludwig Florin Since the introduction of the single beam optical trap (SBT) by Ashkin et. al. in 1986, trapping and manipulation of micron-sized particles by optical forces has become instrumental in many areas of research. However, controlled transport of large numbers of particles is difficult using a SBT. Here we introduce a technique for controlled transport that we call an Optical Capillary (OC), named for its ability to strongly confine and continuously transport nanometer-sized particles. The OC, generated by an optical standing wave pattern, is especially strong along the optical axis due to the compensation of the axial scattering force. We utilize the lateral scattering forces to control the transport of particles along a line perpendicular to the optical axis. The measured velocity profiles of single particles in the OC agree with our model predictions. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W39.00009: Laser- based Insect Tracker (LIT) Leonardo Mesquita, Shiva Sinha, Rob de Ruyter Van Steveninck Insects are excellent model systems for studying learning and behavior, and the potential for genetic manipulation makes the fruitfly especially attractive. Many aspects of fruitfly behavior have been studied through video based tracking methods. However, to our knowledge no current system incorporates signals for behavioral conditioning in freely moving flies. We introduce a non-video based method that enables tracking of single insects over large volumes ($>$8000cm3) at high spatial ($<$1mm) and temporal ($<$1ms) resolution for extended periods ($>$1 hour). The system uses a set of moveable mirrors that steer a tracking laser beam. Tracking is based on feedback from a four-quadrant sensor, sampling the beam after it bounces back from a retro reflector. Through the same mirrors we couple a high speed camera for flight dynamics analysis and an IR laser for aversive heat conditioning. Such heat shocks, combined with visual stimuli projected on a screen surrounding the flight arena, enable studies of learning and memory. By sampling the long term statistics of behavior, the system augments quantitative studies of behavioral phenotypes. Preliminary results of such studies will be presented. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W39.00010: Power dependent study of kinetics of TRF2 recruitment in cells due to DNA damage caused by ultrafast near-IR Laser Manas Bhalerao, Nazmul Huda, David Gilley, Samarendra Mohanty Ultrafast laser microbeam is finding widespread applications in eliciting highly localized damage to cellular components allowing study of in-situ repair machinery. While the high peak power density that exists in ultrafast laser can cause various types of DNA damage including double strand breaks (DSB), tuning the power of these laser microbeams may cause specific type of DNA damage. Here, we report wavelength and dose dependent parametric study of kinetics of TRF2 recruitment in cells due to DNA damage caused by ultrafast near-IR Laser. A tunable Ti: Sapphire laser beam was coupled via laser port of an inverted microscope. Spot and line laser micro-irradiation pattern in nuclear sites of HT1080 cells expressing YFP-tagged TRF2 was achieved by piezo-scanning mechanism. The recruitment of TRF2-YFP was found to depend highly on the peak irradiance of the near-IR laser microbeam, the required threshold irradiance being much higher than that observed for DSB. Further, recruitment kinetics revealed that the time constant for TRF2 recruitment depends on the laser irradiance parameters. The time required for TRF2 recruitment was found to decrease with increased peak irradiance. We will present these results and also elucidate on physical mechanism of DNA damage caused by ultrafast laser microbeam. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W39.00011: Background Elimination and Noise Reduction by Mechanical Modulation Raman Spectroscopy Kathleen Hinko, Chieze Ibeneche, Andrea Keidel, Tobias Bartsch, Ernst-Ludwig Florin Raman spectroscopy is widely used by biophysicists for the molecular identification of cellular substructures. However, there are high levels of background and noise associated with Raman spectra from other molecules in the microscopic detection volume. We present two methods of mechanical modulation for background subtraction and noise reduction in a Raman microscope: (1) a three-axis stage modulation for fixed objects and (2) a separate optical trap modulation for objects in solution. With our technique, we completely eliminate the background in our spectra and improve the signal-to-noise ratio by two orders of magnitude. We applied this technique to lipid vesicles and fission yeast cells in solution. Additionally, we obtained mechanical modulation Raman spectra of fission yeast in three dimensions and observed spatial differences in the molecular composition for different metabolic states of a single yeast cell. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W39.00012: Ultraweak bioluminescence dynamics and singlet oxygen correlations during injury repair in sweet potato Marius Hossu, Lun Ma, Wei Chen Ultraweak bioluminescence at the level of hundreds of photons per second per square centimeter after cutting injury of sweet potato was investigated. A small emission peak immediate after cutting and a later and higher peak were observed. Selective singlet oxygen inhibitors and sensors have been use to study the contribution of singlet oxygen during the curing process, demonstrating increased presence of singlet oxygen during and after the late bioemission peak. It was confirmed that singlet oxygen has direct contribution to ultraweak bioluminescence but also induces the formation of other exited luminescent species that are responsible for the recorded bioluminescence. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W39.00013: Obtaining optical properties using Representative Layer Theory Neema Razavi, Brain Yust, Dhiraj Sardar Reliable and minimally invasive methods for diagnosis of toxicity and onset of disease are important for advances in clinical practices. This is commonly achieved through the optical properties, such as a change in the absorption or scattering strength of the diseased tissue. Thus, being able to quantitatively characterize these changes is important to advancements in medical diagnostic methods. By adapting the Representative Layer Theory to the integrating sphere technique, very thin biological samples may be optically characterized, yielding a quick and easy method for monitoring optical changes as a function of disease progression. Samples, consisting of cells, dyes, and nanoparticles of known concentrations were optically characterized at multiple wavelengths. Optical properties obtained by the Representative Layer Theory are compared to those obtained through other methods, such as Kubelka-Munk and Inverse Adding Doubling which are known to have sample thickness limitations. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W39.00014: An \textit{in vitro} approach to understanding intracellular motor-based cargo transport Rafael Longoria Casasa, Hayley Manning, George Shubeita Microtubule-based molecular motors are responsible for the long range transport of intracellular cargoes. Most cargoes move bidirectionally yet reach their destination in the cell. The mechanism by which the seemingly random bidirectional motion of cargoes is regulated by the cell to produce directed transport remains unclear. Two distinct models have been proposed: coordination via a tug-of-war, the dynamics of which depend only on the properties of the motors; and coordination via non-motor proteins. However, no direct evidence for either one has been found yet. We present an experimental method that can address the different predictions of these models. We reconstitute \textit{in vitro} transport of endogenous motor-driven lipid droplets purified from \textit{Drosophila} embryos. Global transport dynamics are observed under varied medium conditions by DIC microscopy. Combined with stall force measurements using an optical trap, these investigations relate the global dynamics to local changes in force production of the motors which give us a direct handle to differentiate between the different models of transport. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W39.00015: Nonlinear optical microscopy in biology: Combining second-harmonic generation and two-photon fluorescence imaging Koen Clays Optical microscopy has been since long a truly enabling visualization technique in the biological and biomedical sciences. Linear optical microscopy relies on simple linear optical effects. Nonlinear optical microscopy relies on the nonlinear optical properties of endogenous or exogenous chromophores to produce a better image. Two-photon fluorescence (TPF), a third-order nonlinear optical effect and observed at the focal spot only due to the quadratic intensity dependence, results in inherently higher resolution than possible for one-photon fluorescence, observed over the complete Rayleigh range. Second-harmonic generation (SHG) is a second-order nonlinear optical effect only observed for non-centrosymmetric arrangements of non-centrosymmetric chromophores. While this does put a restriction on the chromophores that can be used, it also results in structural information about symmetry when used in combination with TPF. TPF, being a third-order nonlinear process, is not restricted by any symmetry consideration. We will review the molecular design criteria for exogenous probes for combined SHG and TPF nonlinear microscopy, provide examples of optimized chromophores and show microscopy images demonstrating the use of such chromophores in nonlinear microscopy. [Preview Abstract] |
Session W40: Polymer Blends
Sponsoring Units: DPOLYChair: Mesfin Tsige, University of Akron
Room: A122/123
Thursday, March 24, 2011 11:15AM - 11:27AM |
W40.00001: Entanglement Dynamics in Miscible Polyisoprene / Poly(p-{\it tert}-butyl styrene) Blends Hiroshi Watanabe Viscoelastic and dielectric behavior was examined for well entangled, miscible blends of high-$M$ {\it cis}-polyisoprene (PI) and poly(p-{\it tert}-butyl styrene) (PtBS). The dielectric data of the blends, reflecting the global motion of the PI chains having the type-A dipoles, indicated that PI and PtBS were the fast and slow components therein. At high temperatures $T$, the blends exhibited two-step entanglement plateau. The high frequency ($\omega$) plateau height was well described by a simple mixing rule of the entanglement length based on the number fraction of the Kuhn segments. At low $T$, the blend exhibited the Rouse-like power-law behavior of storage and loss moduli, $G' = G'' \sim \omega^{0.5}$, in the range of $\omega$ where the high-$\omega$ plateau was supposed to emerge. This lack of the high-$\omega$ plateau was attributed to retardation of the Rouse equilibration of the PI chain over the entanglement length due to the hindrance from the slow PtBS chains: The PI and PtBS chains were equilibrated cooperatively, and the retardation due to PtBS shortened the plateau for PI to a width not resolved experimentally. A simple model for this cooperative equilibration formulated on the basis of the dielectric data described the viscoelastic data surprisingly well. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W40.00002: Local Relaxation Behavior and Dynamic Fragility in Hydrogen Bonded Polymer Blends James Runt, Kevin Masser, Hanqing Zhao, Paul Painter The dynamics of intermolecularly hydrogen bonded polymer blends of poly(p-(hexafluoro-2-hydroxyl-2-propyl)styrene) with poly(vinyl acetate), poly(ethylene[30]-co-vinyl acetate[70]) and poly(ethylene[55]-co-vinyl acetate[45]) are investigated by broadband dielectric relaxation spectroscopy and Fourier transform infrared spectroscopy. Each blend component exhibits a glassy state (beta) relaxation, and these relaxations are affected by the formation of intermolecular associations. The glassy state behavior of the blends can be modeled using the Painter-Coleman association model. All blends exhibit a single Tg and a single dielectric segmental (alpha) relaxation, indicative of strong segmental-level coupling. The fragility of the glass-formers depends on the volume fraction of intermolecularly associated segments, and the association model predicts which compositions have the highest fragilities. A relaxation related to the breaking and reforming of hydrogen bonds is observed at temperatures above the alpha process, and its temperature dependence varies systematically with ethylene content. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W40.00003: Partial Miscibility in Copolymer Blends Elizabeth Clark, Jane Lipson Copolymers can be used to affect the miscibility of otherwise immiscible polymer blends by acting as compatibilizers. To better understand the energetics of these types of systems, we use a simple lattice model to study phase separation in binary copolymer/homopolymer blends. We focus on a copolymer that contains both A and B type monomers and a homopolymer that contains purely A type monomer. An example of a system that we are investigating is polyethylene mixed with either random or alternating poly(ethylene-co-propylene). The sequence effect on miscibility as the copolymer microstructure is varied from random to alternating is investigated as well. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W40.00004: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W40.00005: Properties of polystyrene/poly(dimethyl siloxane) blends partially compatibilized with star polymers containing a gamma-cyclodextrin core and polystyrene arms C. Maurice Balik, Brad J. Busche, Alan E. Tonelli Cyclodextrins (CDs) are cyclic starch molecules having a hollow central cavity which can be threaded by a polymer to form an inclusion compound. This characteristic is exploited in a new type of compatibilizer: a star polymer with a gamma-CD (g-CD) core and polystyrene (PS) arms (CD-star). Spun-cast thin films of PS containing up 20 weight percent poly(dimethyl siloxane) (PDMS) are compatibilized by CD-star. The mechanism of compatibilization involves threading of the CD core by PDMS and solubilization of the resulting slip-ring graft copolymer via the PS star arms. Thin spun-cast films of these blends exhibit a a nanoscale level of mixing and remain well-mixed after annealing at 125 C for three days. In contrast, thicker solution-cast films of these blends exhibit larger-scale phase separation since the film solidification process occurs over a period of days rather than seconds. This allows some of the PDMS to de-thread from the CD-star and phase separate. However, DSC, DMA and PDMS leaching data show that PS and PDMS remain partially compatibilized in the solution-cast films. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W40.00006: Crystallization in the Binary Blends of Crystalline-Amorphous Diblock Copolymers Bearing Chemically Different Crystalline Block Che-Yi Chu, Hsin-Lung Chen, Bhanu Nandan, Ming-Siao Hsiao The crystallization behavior of a series of lamellae-forming blends of a shorter PS-$b$-PEO (SEO) and a longer PS-$b$-PLLA (SLLA) has been studied. In SLLA-rich blends, the junction point constraint coupled with the poor chain mobility at low $T_{c}$ ($\le $45 $^{\circ}$C) hampered the formal crystallization of PLLA. In this case, a local demixing between a fraction of PEO and PLLA chains took place, yielding the PLLA crystalline domains in which the PLLA crystalline stems were intervened by the PEO chains. This crystalline species gave rise to a relatively broad peak at 2$\theta $= 15.92$^{\circ}$ in the WAXS profile and displayed a much lower melting point of ca. 100 $^{\circ}$C compared to that of the typical $\alpha $-form crystal of PLLA. It was suggested that the inserted PEO chains served as the molecular defects which induced an expansion of the $a$-axis and $b$-axis of the $\alpha$-form PLLA unit cell and lowered the crystal melting point due to introduction of defect free energy. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W40.00007: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W40.00008: Effect of Organoclays on Immiscible Polymer Blends Mai Ha, Ramanan Krishnamoorti The effect of adding organoclays on the phase behavior, rheological properties and bulk mechanical properties of immiscible polymer blends of polystyrene (PS) and poly(methyl methacrylate) (PMMA) is investigated. Traditional organoclays, prepared using alkyl ammonium chains, display a preference to segregate to the PS phase for high PS volume fraction blends where the PS forms the continuous matrix. On the other hand, for blends with low PS volume fractions, the organoclay segregates to the interface between the PS and PMMA domains and leads to a decrease in the domain size that does not change much with organoclay concentration variations from 0.1 to 2 wt {\%}. Linear dynamic rheological data of these samples show significant increase in the low-frequency modulus of the blends with added organoclay. A thermodynamic model for estimating the interfacial modulus is proposed and the results agree well with the interfacial modulus calculated by Palierne's emulsion model. The toughness of the blends increases at low concentrations of added organoclays with the optimal improvements observed for less than 0.5 wt {\%} added organoclay. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W40.00009: Characterization of the Early Stages of Phase Separation in PS/PVME Blends Using Fluorescence Annika Kriisa, Sung Park, Connie Roth Controlling the early stages of phase separation in polymer blends provides a potentially easy route towards obtaining interconnected nanostructured domains. We present results of thermally induced phase separation in polystyrene (PS) / poly(vinylmethylether) (PVME) blends using different fluorophores covalently attached to the PS component. Fluorescence identifies the phase separation temperature Tc at earlier stages then the more traditional method of light scattering. At Tc, a large increase in fluorescence intensity is observed due to a strong reduction in the fluorescence quenching caused by the intimate presence of the more polar PVME component. We discuss the spectral red shifts of pyrene associated with the dissolution of the weak hydrogen bonding in this blend and the change in polarity of the local environment during phase separation. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W40.00010: Non-Isothermal Crystallization of PET/PLA Blends Huipeng Chen, Marek Pyda, Peggy Cebe Binary blends of poly(ethylene terephthalate) with poly(lactic acid), PET/PLA, were studied by differential scanning calorimetry. The solution cast blends were miscible in the melt over the entire composition range. We report the non-isothermal crystallization of: a.) PET, with and without presence of PLA crystals, and b.) PLA, with and without presence of PET crystals. PET can crystallize in all blends, regardless of whether PLA is amorphous or crystalline, and crystallinity of PET decreases as PLA content increases. PLA crystallization is strongly affected by the mobility of the PET. When PET is wholly amorphous, PLA can crystallize weakly even in 70/30 blends. When PET is crystalline, PLA cannot crystallize when its own content is below 0.90. The different behaviors may be related to the tendency of each polymer to form constrained chains, i.e., to form rigid amorphous fraction, RAF. PET is capable of forming a large amount of RAF, whereas relatively smaller amount of RAF forms in PLA. Like the crystals, rigid amorphous fraction of one component may inhibit growth of crystals of the other blend partner. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W40.00011: Critical Micelle Concentrations for Different Micelle Shape in Diblock Copolymer/Homopolymer Blends Jiajia Zhou, An-Chang Shi Diblock copolymers (AB) blended with homopolymers (A) may self-assemble into lamellar, cylindrical and spherical micelles. The critical micelle concentrations for different micelle shape are determined using self-consistent field theory. The effect of varying copolymer block asymmetry, homopolymer molecular weight and monomer-monomer interaction are considered. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W40.00012: Ordered Materials via Additive Driven Assembly and Reaction using Surfactant-Based Templates Michael R. Beaulieu, Vikram K. Daga, Alan J. Lesser, James J. Watkins We recently reported (1) the ordering behavior of Pluronic surfactant melts through the addition of aromatic additives with hydrogen bond donating groups, which exhibit selective interactions with the polyethylene oxide (PEO) block. The ordered blends had domain sizes ranging from 12 to 16 nm at additive loadings up to 80{\%}.The goal of this work is to utilize condensation chemistries based on the functionality of similar additives, to yield ordered composite materials that could be used for applications involving membranes or dielectric materials. The structure of the blends and composites are determined by small angle x-ray scattering, which indicates that the ordered structure is preserved following reaction of the additives. Differential scanning calorimetry indicates that an increase in additive loading causes a decrease in the melting temperature and enthalpy of melting of the PEO, which demonstrates that the interaction between the PEO segments and the additive is strong. (1) Daga, V.K., Watkins, J. J. Macromolecules, ASAP. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W40.00013: Interfacial Structure and Dynamics of the liquid/liquid interface between Polydimethylsiloxane and Polystyrene Mesfin Tsige Many important phenomena in biology, chemistry and in various fields involve processes that occur at the interface between two immiscible liquids. A molecular level understanding of such interfaces is crucial for insight into the complex dynamics that are observed at such interfaces. In this study, atomistic molecular dynamics simulations were performed to study the structural and dynamical properties of the liquid/liquid interface between two immiscible polymers, polydimethylsiloxane (PDMS) and polystyrene (PS). A series of simulations is carried out to examine the temperature and molecular weight dependence of the orientation of molecules at and away from the interface, intermolecular correlation at the interface, interfacial tension and interfacial thickness. The results from these detailed simulations will be presented. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W40.00014: Effect of linear contaminants on the dynamics and rheology of ring polymer melts Jonathan Halverson, Gary Grest, Kurt Kremer Understanding the behavior of ring polymer melts remains a challenge. Early experimental efforts to characterize the rheological behavior of pure ring polymer melts have led to controversial results most likely because the samples were contaminated with linear chains. Recent studies found that stress relaxation follows a simple power law with no sign of a plateau. To further investigate these systems we have conducted molecular dynamics simulations for a semiflexible bead-spring model for chain lengths up to 14 entanglement lengths. The structure, dynamics and rheology of these systems are investigated for different concentrations of linear chains. We find that the viscosity of a ring melt increases dramatically when trace quantities of linear contaminants are present. The rings are found to swell slightly and diffuse more slowly with increasing linear concentration while the linear chains mostly behave as if in a pure linear melt. We use the concept of threading, analogous to thread passing through the eye of a needle, to explain the response of the ring melts to linear contaminants. [Preview Abstract] |
Session W41: Focus Session: Electronic Structure and Applications to Energy Conversion I
Sponsoring Units: DCPChair: Sergei Tretiak, Los Alamos National Laboratory
Room: A115/117
Thursday, March 24, 2011 11:15AM - 11:51AM |
W41.00001: Tuning the electronic structure of II-VI semiconductors and nanostructures for energy applications Invited Speaker: Using first-principles calculations within density functional theory (DFT), we study the impacts of quantum confinement, strain, and surface ligand passivation on the electronic structure of typical II-VI wurtzite semiconductors and nanostructures. In CdSe/CdTe core/shell nanowires, large anisotropic strains develop due to the large lattice mismatch. These strains result in significant reductions in band gap in the CdSe core with increasing CdTe shell thickness, by amounts comparable to that expected from reduced quantum confinement [1]. The response of band gaps of wurtzite compounds to anisotropic strain is further shown to be large and highly non-linear, and system-dependent [2]. In addition, we also explore the effects of chemisorbed ligand on the electronic structure of CdSe surfaces. Substantial shifts in band edge energies are predicted due to the induced dipole at the CdSe-ligand interface and the intrinsic dipole of the ligand [3]. Our studies suggest well-defined routes to control both the band gaps and band edge energies of nanomaterials for light-harvesting applications. \\[4pt] [1] S. Yang, D. Prendergast, and J. B. Neaton, Nano Lett. 10, 3156 (2010).\\[0pt] [2] S. Yang, D. Prendergast, and J. B. Neaton, Appl. Phys. Lett., in press (2011).\\[0pt] [3] S. Yang, D. Prendergast, and J. B. Neaton, in preparation (2011). [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W41.00002: TR-2PPE Studies of Ultrafast Charge Separation at Organic Photovoltaic Interfaces S.W. Robey, G.J. Dutton, W. Jin, J.E. Reutt-Robey Dissociation of excitons in organic photovoltaic (OPV) devices occurs exclusively at interfaces between donor and acceptor molecular materials. To help understand critical charge separation processes, we have performed time-resolved two-photon photoemission (TR-2PPE) studies of sub-picosecond exciton dynamics at well-characterized organic donor-acceptor interfaces. Interfaces between phthalocyanines and C$_{60}$ were engineered using organic MBE and characterized using STM, STS, and UPS. Ultrafast TR-2PPE measurements were performed on CuPc$\backslash $C$_{60}$ structures by pumping the lowest optical $\pi \to \pi $* transitions (Q-band) to generate CuPc singlet (S$_{1})$ excitons and probing this population with a time-delayed UV pulse. For thick films, CuPc S$_{1}$ decay is dominated by vibrational relaxation (several 100's femtoseconds) and singlet-to-triplet conversion ($\sim $ 1 picosecond). Directly at the interface, however, charge transfer to C$_{60}$ dominates decay of S$_{1}$ exciton, ($\sim $ 100 femtoseconds) . We also find evidence for important recombination routes from the charge separated state back to lower-lying CuPc T$_{1}$ triplet excitons. To test the impact of intersystem crossing to triplet levels, we have performed analogous investigations for H$_{2}$Pc$\backslash $C$_{60}$ interfaces. Results for this interface will be compared and contrasted with the CuPc$\backslash $C$_{60}$ case. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W41.00003: Near- and Far-Field Effects on Excited States at Organic Semiconductor and Metal Interfaces Oliver Monti, Mary Steele, Nahid Ilyas, Leah Kelly We present an investigation of the evolution of excited states at the interface of the dipolar organic semiconductor vanadyl naphthalocyanine on highly oriented pyrolytic graphite. Using two-photon photoemission we observe several excited states at sub-monolayer to few-monolayer coverages. Excited states of this organic semiconductor are progressively stabilized with coverage, an effect that is somewhat mirrored in the image state manifold as well. These findings can be understood in the context of a simple electrostatic model that considers how molecular levels and vacuum level are influenced differentially by the the strong electrostatic fields present at the interface with dipolar molecules: While the vacuum level rises continuously with coverage, the molecular states are significantly depolarized as a function of electric fields in the near-field regime. This indicates that the interfacial excited state electronic structure is strongly sensitive to long-range intermolecular interactions mediated by the surface, with direct implications for energy level alignment and charge transfer dynamics at the interface. Interfacial electrostatic fields may therefore be used to manipulate in a concrete fashion interfacial charge transfer processes such as photoinduced interfacial electron transfer. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:51PM |
W41.00004: Time-domain ab initio studies of excitation dynamics in semiconductor quantum dots Invited Speaker: Solar energy applications require understanding of dynamical response of novel materials on nanometer scale. Our state-of-the-art non-adiabatic molecular dynamics techniques, implemented within time-dependent density functional theory, allow us to model such response at the atomistic level and in real time. The talk will focus on single and multiple exciton generation, relaxation, annihilation and dephasing in semiconductor quantum dots.\\[4pt] References:\\[0pt] [1] O. V. Prezhdo, ``Multiple excitons and electron-phonon bottleneck in semiconductor quantum dots: Insights from ab initio studies'', \textit{Chem. Phys. Lett. -- Frontier Article}, \textbf{460}, 1 (2008) \\[0pt] [2] O. V. Prezhdo ``Photoinduced dynamics in semiconductor quantum-dots: insights from time-domain ab initio studies'', \textit{Acc. Chem. Res.}, \textbf{42}, 2005 (2009) \\[0pt] [3] A. B. Madrid, H.-D. Kim, O. V. Prezhdo, ``Phonon-induced dephasing of excitons in silicon quantum dots: multiple exciton generation, fission and luminescence'', \textit{ACS-Nano}, \textbf{3}, 2487 (2009) \\[0pt] [4] C. M. Isborn, O. V. Prezhdo, ``Quantum dot charging quenches multiple exciton generation: first-principles calculations on small PbSe clusters'', \textit{J. Phys. Chem. C,} \textbf{113}, 12617 (2009) \\[0pt] [5] S. V. Kilina, D. S. Kilin, O. V. Prezhdo, ``Breaking the phonon bottleneck in PbSe and CdSe quantum dots: time-domain density functional theory of charge carrier relaxation'', \textit{ACS-Nano}, \textbf{3}, 93 (2009). \\[0pt] [6] S. A. Fischer, A. B. Madrid, C. M. Isborn, O. V. Prezhdo, ``Multiple exciton generation in small Si clusters: A high-level, ab initio study'', \textit{J. Phys. Chem. Lett.,} \textbf{1}, 232 (2010). [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W41.00005: Hole Localization in Molecular Crystals From Hybrid Density Functional Theory Na Sai, Paul F. Barbara, Kevin Leung Charge trapping in organic solids and interfaces plays an important role in organic photovoltaic efficiencies. Experimental confirmation of intrinsic charge trapping at the atomic scale and the tools to directly probe the trap energy landscape, however, remain lacking. We use first principles computational methods to examine hole trapping in organic molecular crystals. We present a computational scheme based on the tuning of the fraction of exact exchange in hybrid density functional theory to eliminate the many-electron self-interaction error [1]. With small organic molecules, we show that this scheme gives accurate descriptions of ionization and dimer dissociation. We demonstrate that the excess hole in perfect molecular crystals can form self-trapped hole polarons. The predicted absolute ionization potentials of both localized and delocalized holes are consistent with experimental values.\\[4pt] [1] N. Sai, P. Barbara, and K. Leung (submitted). [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W41.00006: Linear Dichroism and Photoluminescence Microscopy Imaging of Grain Boundaries in Crystalline Metal-Free Phthalocyanine Thin Films Zhenwen Pan, Cody Lamarche, Ishviene Cour, Naveen Rawat, Lane Manning, Randall Headrick, Madalina Furis We employed a combination of linear dichroism and photoluminescence microscopy with spatial resolution of 5$\mu $m to study the excitonic properties of solution-processed metal-free phthalocyanine (H2Pc) crystalline thin films with millimeter-sized grains. We observe a highly-localized, sharp, monomer-like emission at the high angle grain boundaries, in contrast to samples with more uniform grain orientation where no such feature has been observed. The energy difference between the grain boundary luminescence and the HOMO-LUMO singlet exciton recombination of the crystalline H2Pc is measured to be 160meV. Our systematic survey of grain boundaries indicates this localized state is never present at low angle boundaries where the $\pi $-orbital overlap between adjacent grains is significant. It supports recent results which associated a decrease in carrier mobility with the presence of large angle boundaries in similar crystalline pentacene films. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:51PM |
W41.00007: Theories and applications for characterizing electronic coupling factors Invited Speaker: The transport of charges and excitation energy are two processes of fundamental importance in many biological and material systems. One of the fundamental parameters in the transport rates is the electronic coupling, which is essentially an off-diagonal Hamiltonian matrix element between the initial and final diabatic states. We have developed ways to define the diabatic states and calculate the coupling factors, including those for electron transfer (ET) and excitation energy transfer (EET). The fundamental method development and applications will be discussed. For characterizing TEET, the Fragment Spin Difference (FSD) was developed and it can be to calculate the TEET coupling over a general class of systems. TEET in bacterial light-harvesting complex LH2 and the peridinin chlorophyll-a protein (PCP) of dinoflagellates were calculated and analyzed. Our results are in good agreement with experimental results and it offers limits to the photoprotection models. Therefore, with the FSD scheme, it is possible to quantify and analyze the electronic couplings in TEET processes in large systems, and to derive insights and limits of theoretical models. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W41.00008: Ab initio calculation of optical spectra of solvated molecules: GW+BSE method for liquid environments Jeehye Lee, Tomas Arias Electronic excitations for solvated systems have drawn a great interest in the energy community because they provide a possibility to engineer photoexcitation processes. Time-dependent DFT (TDDFT) and the hybrid QM/MM approach successfully calculate the solvent shift in excitation energies (solvatochromic shift) for confined systems, but are well known to work best for small systems and Frenkel excitons. Here we present a new modification of the GW and Bethe-Salpeter equation (GW+BSE) methods which allows treatment of solvated systems beyond the TDDFT level by including the frequency-dependent polarizability of the solvent at the diagrammatic level. In this initial work, we present the solvatochromic and ionization potential shifts for a series of molecules in aqueous solution. [Preview Abstract] |
Session W42: Focus Session: Polymer Brushes
Sponsoring Units: DPOLYChair: You Yeon Won, Purdue University
Room: A302/303
Thursday, March 24, 2011 11:15AM - 11:51AM |
W42.00001: Responsive Grafted Polymer Layers: the role of pH, temperature and surface geometry Invited Speaker: The competition between chemical equilibrium, e.g. protonation, and physical interactions determines the molecular organization and functionality in biological and synthetic systems. Charge regulation by displacement of acid-base equilibrium induced by changes in the local environment provides for a feedback mechanism that controls the balance between electrostatic, van der Waals, steric interactions and molecular organization. What are the mechanisms that determine the interplay between all these different factors? In this talk I will describe a molecular theory to address this question. In particular, the theory will be used to study the structural and thermodynamic properties of end-grafted polyacids with hydrophobic backbones. The molecular theory explicitly includes the size, shape, conformations, charge and charge distribution of all the molecular species in the system and incorporates excluded volume, van der Walls and electrostatic interactions coupled with acid-base equilibrium. On planar surfaces, the theory predicts the formation of surface micelles with morphologies that depend upon the bulk pH, solution ionic strength, temperature and surface coverage. The self assembled aggregates, present domains of varying local pH that is very different from that of the bulk solution. We show that a qualitatively new form of local organization arises that is only found when there is explicit coupling between charge regulation and physical interactions. The different morphologies can be manipulated by changing the bulk solution conditions and they provide for local domains with controlled charge and pH with large gradients with a characteristic size of a few nanometers. Following this we will discuss how ion conductivity in nanopores functionalized with polybases changes as a function of solution pH. The predictions of the theory are in quantitative agreement with experiments and they provide a physical explanation of the interplay between molecular organization and charge in the nanopore. The last part of the talk will be devoted to a new system in which we show how to use the theory under non-equilibrium conditions to study the flux of ions in finite nanopores with grafted polybases (or polyacids) in the presence of external potentials. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W42.00002: Polyelectrolyte brushes in mixed ionic medium studied via intermolecular forces Robert Farina, Nicolas Laugel, Philip Pincus, Matthew Tirrell The vast uses and applications of polyelectrolyte brushes make them an attractive field of research especially with the growing interest in responsive materials. Polymers which respond via changes in temperature, pH, and ionic strength are increasingly being used for applications in drug delivery, chemical gating, etc. When polyelectrolyte brushes are found in either nature (e.g., surfaces of cartilage and mammalian lung interiors) or commercially (e.g., skin care products, shampoo, and surfaces of medical devices) they are always surrounded by mixed ionic medium. This makes the study of these brushes in varying ionic environments extremely relevant for both current and future potential applications. The polyelectrolyte brushes in this work are diblock co-polymers of poly-styrene sulfonate (N=420) and poly-t-butyl styrene (N=20) which tethers to a hydrophobic surface allowing for a purely thermodynamic study of the polyelectrolyte chains. Intermolecular forces between two brushes are measured using the SFA. As multi-valent concentrations are increased, the brushes collapse internally and form strong adhesion between one another after contact (properties not seen in a purely mono-valent environment). [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W42.00003: Microsecond MD Simulations of Nano-patterned Polymer Brushes on Self-Assembled Monolayers Creighton Buie, Liming Qiu, Soyeun Park, Mark Vaughn, Kwan Cheng Nano-patterned polymer brushes end-grafted onto a self-assembled monolayer interface have unique properties and application potential. However, the molecular-level interactions of these brushes with the substrate interface and the solvent are still not clear. Using a coarse-grained MD simulation approach we investigate the structure and dynamics of brushes of monomer length ranging from 25 to 75 units and an implant density $\sigma$ of 0.2 to 1.0 nm$^{-2}$ that were end-grafted onto a $5\times 5$ nm$^2$ well in a self-assembled hexadecane monolayer. The behavior of each polymer-monolayer-water complex was simulated from 3 to 12 $\mu$s.The excess width and the extended height of the polymers, the nanosecond-resolved conformational transition kinetics from a compact helical to a random coil-like structure, and the time-averaged monomer density maps were determined. The scaling behavior of these brushes differs from that of previous thermodynamic and computational studies of homogeneous brushes and nanopatterned stripes. Here, we find a weaker dependence of brush height on implant density, $\sigma^{0.29}$ rather than $\sigma^{1/3}$ and near linear scaling of the excess width on $\sigma$ rather than the $\sigma^{1/2}$. Our structural dynamics data and molecular templates are useful for future experimental and computational investigations of nano-patterned polymer brushes at the nanoscopic length and mesoscopic time scales. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W42.00004: Nano-pillars created from the surface-grafted crosslinked polymer chains Soyeun Park, Dipika Patel, Fernando Monjaraz, Wolfgang Frey Nano-patterned polymer chains with the controlled mechanical properties are widely applicable to biological and chemical studies. We synthesized linear and crosslinked polymer chains grafted onto micro/nano-patterned substrates by developing a series of unique bottom-up fabrication steps based on the iniferter-driven quasi-living polymerization. We incorporated conventional photolithography and nanosphere lithography. The AFM study provides insight into the influence of the addition of the crosslinker on the configurative, kinetic, mechanical, and wetting properties of polymer chains grafted onto micro/nano-patterns. We found that the addition of crosslinker successfully converts the mushroom-like configuration of nanopatterned polymer chains into the well-standing brush like polymer chains, i.e, soft nano-pillars. By analyzing the AFM force-distance curves obtained in the two-dimensional array and lateral force images, we found that the shear moduli of the obtained soft nano-pillar can be adjusted by varying the concentration of crosslinkers. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W42.00005: Polymer brushes: Tools for surface design Invited Speaker: Polymers brushes are ideal materials for interfacing with biological systems as they share many of the same molecular components and properties. Polymer brushes provide remarkable screening power in shielding a substrate from the environment through both steric and charge interactions. However, the majority of biomolecular species will still non-specifically bind to polymer brush surfaces unless some care is given to molecular design. Several polymer brush systems are described to control interaction of biomacromolecules and cells by design of specific and non-specific interactions in polymer brush architectures. ``Grown from'' and block copolymer brushes are described, both of which provide excellent substrates for study of brush surfaces. Examples of polymer brushes used for sensor creation and for investigation of cellular interaction are given. Brushes used in non-fouling coatings tailored for marine applications and in which amphiphilic structures play an important role are also described. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W42.00006: Preparation, Patterning, and Electrical Characterization of Conjugated Polymer Brushes Jose Alonzo, Jihua Chen, Onome Swader, Nikolay Lavrik, Mark Dadmun, S. Michael Kilbey II Intimate contact at donor-acceptor interfaces and electrode-film interfaces is considered important for optoelectronic devices. This presentation will describe the formation and characterization of novel conjugated polymer brushes based on end-functionalized poly(3-hexylthiophene) and poly(para-phenylene) (PPP). In each case, end-functionalized polymers were synthesized and grafted to silicon substrates, with changes in film preparation method and polymer molecular weight used to manipulate the grafting density of the interfacial layers. Highly tunable PPP brushes having thickness ranging from 4 to 108~nm were obtained by in situ aromatization of poly(cyclohexadiene) brushes. Exceptionally smooth brush-modified interfaces were prepared, and neutron reflectometry, ellipsometry, AFM, and transmission electron microscopy were used to characterize layer structure and chain density. This presentation will also describe efforts to measure properties of these nanostructured layers using DC conductivity and AC electrical impedance measurements, as well as micro and nano-patterning of conjugated polymer brushes in the context of nanocircuit or organic solar cell applications. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W42.00007: Quantifying Fluctuations/Correlations in Polymer Brushes Qiang Wang, Xinghua Zhang, Pengfei Zhang, Baohui Li Fast lattice Monte Carlo (FLMC) simulations with multiple occupancy of lattice sites and Kronecker $\delta$-function interactions give orders of magnitude faster/better sampling of the configurational space of multi-chain systems than conventional lattice MC simulations with self- and mutual- avoiding walks and nearest-neighbor interactions.\footnote{Q. Wang, \textbf{Soft Matter, 5}, 4564 (2009).} Using FLMC simulations with Wang-Landau -- Transition-Matrix sampling, we have studied polymer brushes in both an implicit and explicit solvent. The various quantities obtained from simulations (including the internal energy, Helmholtz free energy, constant-volume heat capacity, segmental distribution, and chain sizes) are compared with predictions from the corresponding lattice self-consistent field theory and Gaussian fluctuation theory that are based on the same Hamiltonian as in FLMC simulations (thus without any parameter-fitting) to unambiguously and quantitatively reveal the effects of system fluctuations and correlations neglected or treated only approximately in the theories. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W42.00008: Structures of One and Two Polymer Mushrooms Delian Yang, Qiang Wang A polymer mushroom here is referred to as a group of chains end- grafted at the same point on a flat and impenetrable substrate. Using lattice self-consistent field (LSCF) calculations with the Kronecker $\delta$-function interactions (instead of the commonly used nearest-neighbor interactions), we have studied the structures of one and two polymer mushrooms in an explicit solvent as a function of the polymer volume fraction in the system, solvent quality characterized by the Flory-Huggins $\chi$ parameter, and distance between the two mushrooms. Since LSCF results are exact only in the limit of number of chains $n \to \infty$, we also use fast lattice Monte Carlo (FLMC) simulations\footnote{Q. Wang, \textbf{Soft Matter, 5}, 4564 (2009).} with the same Hamiltonian as in LSCF theory to examine how this limit is approached with increasing $n$. Direct comparisons between LSCF and FLMC results without any parameter-fitting quantify the fluctuation/correlation effects neglected in LSCF theory. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W42.00009: Tethered chains in good and poor solvent - effects of lateral confinement on adsorption and chain collapse Jutta Luettmer-Strathmann, Ryan M. Van Horn The grafting density of a polymer brush affects the response of the system to changes in solvent quality and surface interactions. In this work, we focus on low (mushroom) and intermediate (semi-dilute regime) grafting densities and model a polymer chain in a brush as a single tethered chain subject to an applied force field of cylindrical symmetry that pulls the chain segments toward an axis through the tethering point and normal to the surface. The polymer chain is represented by a bond-fluctuation model with extended range attractive bead-bead interactions and variable bead surface interactions. Monte Carlo simulations with a Wang-Landau type algorithm are performed to determine the density of states in the state space of monomer- monomer contacts, monomer-surface contacts, and lateral chain extension. We present results for the effect of lateral confinement on conformational transitions such as chain adsorption in good and poor solvent. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W42.00010: Semiflexible Polymer Brushes Shiliyang Xu, J.M. Schwarz Non-crossing flexible polymer brush configurations with $N$ polymers correspond to $N$ vicious random walkers, i.e. the system stalls when any two random walkers meet. We study a system of $N$ vicious accelerating walkers with the velocity undergoing Gaussian fluctations, as opposed to the position, to model semiflexibility. We numerically compute the survival probability exponent, $\alpha$, for this system, which characterizes the probability for any two semiflexible polymers in the brush not to cross. The data suggest that $\alpha=\frac{1}{8}N(N-1)$ . We also numerically study $N$ vicious Levy flights and find, for example, for $N=3$ and a Levy index $\gamma=1$ that $\alpha=1.26\pm0.01$. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W42.00011: Interfacial properties of statistical copolymer brushes David Trombly, Victor Pryamitsyn, Venkat Ganesan The interfacial properties of statistical copolymers have important ramifications for the design of patterned thin films with preferred morphologies. In order to explore these properties, we study the interfacial properties of random copolymer brushes in contact with a thin film composed of a homopolymer of one of the blocks. We calculate the interfacial widths and interfacial energies between them as a function of different parameters. We find that the interfacial widths decrease (signifying expulsion of the free chains from the brush) with increasing free chain length, grafting density, and Flory interaction parameter $\chi N$ as well as with decreasing grafted chain length. The interfacial energies show inverse trends to the interfacial widths, except that results for varying grafting density depend on how chemically similar the brush is to the film. We also compare the interfacial efficacies for different types of randomness and find that, except for the case of very blocky chains, blockiness has only little effect on the properties of the interface. We discuss our findings in terms of the design of neutral surfaces and show that our results are consistent with comparable experimental results. [Preview Abstract] |
Session W43: Morphology and Transport in Charged Polymers, Block Copolymers, Membranes, and Films
Sponsoring Units: DPOLYChair: LaShanda Korley, Case Western Reserve University
Room: A306/307
Thursday, March 24, 2011 11:15AM - 11:27AM |
W43.00001: Characterization of a Model Polyelectrolyte Membrane Using a Semi-crystalline Block Copolymer Keith Beers, Xin Wang, Nitash Balsara The microstructured block copolymer sulfonated polystyrene-block-polyethylene is studied as model system for use as a proton exchange membrane in a fuel cell. Self-assembly of this system creates proton conducting hydrophilic channels in the form of sulfonated polystyrene domains, while the polyethylene domains create a hydrophobic matrix to provide mechanical stability. This system serves as a powerful model system since the effects of domain size, morphology and crystallinity on water uptake and proton conductivity can be investigated. Similar systems have shown the ability of small hydrophilic channels to prevent drying at high temperatures in humid air, but have focused on amorphous hydrophobic blocks. The morphology, water uptake, and proton conductivity of this semi-crystalline model system will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W43.00002: Structure-Property Relationships in Sulfonated Pentablock Copolymers Jae-Hong Choi, Carl Willis, Karen I. Winey Membranes of pentablock copolymers consisting of poly(\textit{tert}-butyl styrene) (TBS), hydrogenated polyisoprene (HI), and partially sulfonated poly(styrene-\textit{ran}-styrene sulfonate) (SS) were studied using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The TBS-HI-SS-HI-TBS pentablock copolymer in solution forms spherical micelles with a core of SS and a corona of solvated HI and TBS. The spherical micelles in solution compact as the solvent evaporates and some of SS cores merge to form interconnected SS microdomains without substantially changing their shape. The number of connections increases with the volume fraction of the SS block, which increases with sulfonation level. The structure does not have long-range order, because strong ionic interactions prevent extensive rearrangement. The morphologies of the sulfonated pentablock copolymers will be correlated with their transport properties. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W43.00003: Interfacial Stability of Solid Block Copolymer Electrolytes for Rechargeable Lithium Metal Batteries Greg Stone, Scott Mullin, Nitash Balsara Solid electrolytes that can resist dendrite growth from the lithium surface and adhere to the electrode surface are needed for the development of rechargeable batteries with lithium metal anodes. We show that self-assembled block copolymer electrolytes are inherently more stable against lithium metal anodes than homogeneous homopolymers. This is due to an unusual combination of solid-like properties in the bulk to resist dendrite growth, arising from a randomly oriented granular structure, and the liquid-like properties of perpendicularly oriented lamellae that are formed at the lithium-electrolyte interface providing adhesion to the electrode. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W43.00004: Local dielectric constant and its effects on the microphase separation in charged-neutral diblock copolymer melts Rajeev Kumar, Scott Sides, Bobby Sumpter Using block copolymers as mesoscale templates has potential applications for improved photovoltaic devices and fuel-cells. Charged species in these polyelectrolytic copolymers play a vital role in determining the details of the nanoscale morphologies formed when these systems phase segregate. We have carried out a quantitative analysis of the local dielectric constant for charged-neutral diblock copolymer melts using field-theoretic simulations based on the self-consistent field theory (SCFT). Quantitative expression for the local dielectric constant in terms of the local electric field will be presented along with its effects on the microphase separation in these systems. Using large-scale SCFT simulations, we will explore the effects of different experimental parameters on the morphology diagram. These parameters include the chain length, temperature, degree of ionization and length fraction of the charged block. Also, the effect of added salt on the disorder-order transition temperature and the domain spacings of the ordered morphologies along with the distribution of small ions (counterions and co-ions) will be presented. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W43.00005: Magnetically aligned ion-transport polymer membranes Pawel Majewski, Manesh Gopinadhan, Chinedum Osuji We present the use of magnetic fields to direct the self assembly and impose long-range order in amphiphilic block-copolymers which can be utilized as solid electrolytes for ion-transport membranes or nanomaterials synthesis templates. Our approach allows us to produce highly aligned hexagonally packed cylindrical or lamellar polymer microdomains over macroscopic areas. We systematically explore the influence of several parameters; the strength of magnetic field used for alignment, lithium ion content and temperature on the conductivity of such membranes. A surprising order of magnitude increase in conductivity is found in films aligned in the conduction direction relative to the non-aligned case. The conductivity of field aligned samples shows a non-monotonic dependence on temperature, with a distinct decrease on heating in the proximity of the order-disorder transition of the system before increasing again at elevated temperatures in homogenous melt state. The data suggest that domain-confined ion transport in hexagonally packed cylindrical systems differs greatly in anisotropy compared to lamellar systems. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W43.00006: The effect of salt-doping on the lamellar phase of AB diblock copolymers Issei Nakamura, Zhen-Gang Wang We study the effect of adding salts on the lamellar phase of AB diblock copolymers by means of the self-consistent field theory. We consider a model in which the A and B blocks have different dielectric constants. We include the Born energy to account for the preference of salt ions to be solvated by higher dielectric polymer. We first show that the effective $\chi$ parameter can be increased upon addition of salt, depending on the size of salt particles, with an accompanied increase in the domain spacing of the lamellar phase. The salt ions tend to be localized in the microphase where the higher dielectric components are dominant. The effect of the incompressibility on the distribution of salt ions is also studied. Moreover, we include a binding interaction between one of the blocks and one of the salt ions (e.g., cations), and study the effects of such strong binding on the distribution of the counterions (anions). [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W43.00007: Impact of morphology on conductivity of lamellar block copolymer electrolytes for battery applications Venkat Ganesan, Victor Pryamitsyn We use bond fluctuation model based Monte Carlo simulations to study the correlations between structure and the conductivity of the lamella phase of block copolymer electrolytes. We investigate the effects of degree of segregation, polymer molecular weights and the alignment of the lamellae upon the conductivity of the block copolymer lamella. Our results indicate different influences of the preceding factors upon the conductivities parallel and perpendicular to the lamellae. These results are rationalized in terms of the distributions of the ions and the overall inhomogeneous dynamics of the polymer molecules. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W43.00008: Dynamics of water in sulfonated poly(phenylene) membranes Naresh Osti, Thusitha Etampawala, Umesh Shrestha, Dvora Perahia, Christopher Cornelius The dynamics of water in networks formed by highly rigid ionic polymers, sulfonated poly(phenylene) as observed by quasi elastic neutron scattering (QENS) is presented. These rigid ionic polymers have potential as effective ion exchange membranes with impact on a large number of applications from water purification to clean energy, where its rigidity distinguishes it from other ionic polymers. Its transport characteristics are affected by its rigidness as well as by direct interactions with the solvent. Our QENS studies as a function of sulfonation levels, temperature and solvent content have shown that on the time scale of the measurement, the polymers are rigid. While macroscopically all samples swell, and transport water, the water molecules appear locally rather confined. Water however remind non-frozen to subzero temperatures. The results will be discussed in view of theoretical models including continues diffusion and hopping of solvent molecules. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W43.00009: Morphology and Transport Properties of Phosphonium-containing Styrenic Ionomers with Random Charge Placement Rick Beyer, Kristoffer Stokes, Joshua Orlicki, Yuesheng Ye, Yossef Elabd Alkaline fuel cell (AFC) technology is currently of interest for portable power supplies due in part to the use of less expensive non-noble metals (nickel, iron, cobalt) as the catalyst material. Wide-spread use of the AFC has been prevented by the use of aqueous KOH as the liquid electrolyte, easily poisoned by CO$_{2}$. Development of an semipermeable polymeric alkali anion exchange membrane (AEM) would significantly improve the usefulness of AFCs. We have synthesized a series of random copolymers of styrene and p-vinylbenzyl-trimethylphosphonium chloride, via RAFT polymerization. Detailed $^{1}$H-NMR analysis of the polymerization conditions allowed us to refine our approach and generate materials with random monomer addition. $^{1}$H-NMR was also used to quantify ion contents, which range from 15 mol{\%} to 100 mol{\%}. In this presentation, we will review the synthesis and characterization of these novel cationomers, and then report on their anion transport characteristics and morphological behavior as characterized via SAXS and TEM. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W43.00010: Morphology of precise acid copolymers neutralized with monovalent cations Michelle Seitz, Kathleen Opper, Kenneth Wagener, Karen Winey Poly(ethylene-co-acrylic acid) copolymers with precisely spaced acid groups along the strictly linear chain backbone form the basis of a new family of ionomers with unprecedented molecular uniformity. These copolymers were neutralized with monovalent cations (Li, Na, and Cs) and their morphologies were studied using X-ray scattering. In order to more fully understand the ionic aggregation in these systems, both the low and high angle features are considered. At low angle a sharp ionomer peak arises from interaggregate interference and shifts with acid spacing. At high angle, the amorphous halo from the average backbone separation is observed. For materials with an acid group on every 9th carbon, additional high angle scattering is observed which may be related to either the internal aggregate structure or isolated ion pairs. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W43.00011: Cubic Ordering of Aggregates in Precise Phosphonic Acid Copolymers Francisco Buitrago, Kathleen Opper, Kenneth Wagener, Karen Winey Polyethylene-acid copolymers were synthesized by acyclic diene metathesis (ADMET) chemistry. The result is a series of strictly linear, high molecular weight polyethylenes with pendent acid groups separated by a precisely controlled number of methylene units. Previous studies have been focused on acrylic acid copolymers and ionomers. Here, we focus on phosphonic acid pendent groups in single and geminal architectures. The morphology of these materials has been studied by X-ray scattering at 25 and 150\r{ }C, along with transmission electron microscopy. For a geminal acid copolymer with low acid content, the precise molecular structure produces thermally persistent acid aggregates on a cubic lattice. This is the first report of cubic aggregate packing in polyethylene-acid copolymers. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W43.00012: The Effect of Sulfonation and Neutralization on the Dynamics of Zn Neutralized Sulfonated Polystyrene Ionomers Alicia Castagna, Wenqin Wang, Karen I. Winey, James Runt The effect of sulfonation and neutralization levels on structure and dynamics of Zn neutralized sulfonated polystyrene (SPS) ionomers were investigated using scanning transmission electron microscopy (STEM), X-ray scattering, and dielectric relaxation spectroscopy. STEM and X-ray scattering revealed the presence of spherical aggregates 2 nm in diameter. Successful fitting of the scattering data to the Kinning-Thomas modified hard sphere model revealed that aggregate size is independent of degree of sulfonation and neutralization level, and that aggregate composition becomes increasingly ionic with increasing neutralization. Two segmental relaxations were identified in dielectric loss spectra corresponding to cooperative motion of chain segments in the unrestricted matrix and motions of chain segments restricted by aggregates. A Maxwell-Wagner-Sillars interfacial polarization process was revealed, with relaxation times that were in good agreement with predictions from a simple model of dispersed ionic spheres. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W43.00013: Proton Transport in Nanostructured Block Copolymer/Ionic Liquid Membranes Megan Hoarfrost, Madhu Tyagi, Jeffrey Reimer, Rachel Segalman Nanostructured block copolymer/ionic liquid mixtures are of interest for creating membranes having high proton conductivity coupled with high thermal stability. In these mixtures, it is anticipated that nanoconfinement to block copolymer domains will affect ionic liquid proton transport properties. Using pulsed-field gradient NMR and quasi-elastic neutron scattering, this relationship has been investigated for mixtures of poly(styrene-b- 2-vinylpyridine) (S2VP) with ionic liquids composed of imidazole and bis(trifluoromethane)sulfonimide (HTFSI), where the ionic liquids selectively reside in the P2VP domains of the block copolymer. Proton mobility is highest in the neat ionic liquids when there is an excess of imidazole compared to HTFSI due to proton hopping between hydrogen-bonded imidazoles. As predicted, the amount of proton hopping can be tuned by nanoconfinement, as evidenced by the finding that a lamellar mixture of an imidazole- excess ionic liquid with S2VP has greater proton mobility than a corresponding disordered mixture of the ionic liquid with P2VP homopolymer. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W43.00014: Simultaneous Electronic and Ionic Charge Transport in Poly(3-hexylthiophene)-b-Poly(ethylene oxide) Shrayesh Patel, Anna Javier, Nitash Balsara Block copolymers can self-assemble to distinct channels, which allows for simultaneous transport of electronic and ionic charge carriers. A potential polymer system is Poly(3-hexylthiophene)-b-Poly(ethylene oxide) (P3HT-b-PEO). P3HT serves as the electronic conducting channel while the PEO serves as the ionic conducting channel. Both conductive blocks are doped to induce simultaneous electronic and ionic conduction. The PEO phase is doped with LiTFSI while the P3HT is doped with F$_{4}$TCNQ, which generates hole carriers. In addition, we take into account the case where no electronic dopant is added to P3HT phase. The charge transport properties of the material are analyzed via ac impedance spectroscopy and dc polarization techniques. These experiments provide decoupled electronic and ionic transport in P3HT-b-PEO. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W43.00015: Confinement Effects on Watery Domains in Hydrated Block Copolymer Electrolyte Membranes Moon Jeong Park, Sung Yeon Kim, Joomi Yeo The morphology of a series of diblock copolymers comprising randomly sulfonated polystyrene (PSS) and polymethylbutylene (PMB) blocks equilibrated with humid air was determined by in- situ small angle neutron scattering (SANS). In-situ SANS data were collected over a wide angular range permitting the determination of the superstructure of the hydrophilic PSS-rich and hydrophobic PMB-rich domains and the substructure within the hydrophilic PSS-rich domains. When the characteristic length of the superstructure is larger than 10 nm, the hydrophilic PSS domains are heterogeneous with periodically arranged watery domains. The scattering signature of the watery domains is very similar to the well-established ``ionomer peak.'' This peak vanishes when the neutron scattering length density of the water (H2O/D2O mixture) is matched to that of the PSS block. The spacing between watery domains depends only on sulfonation level of the PSS block. When the characteristic length of the superstructure is less than 10 nm, the watery substructure disappears and homogeneous hydrated PSS-rich domains are obtained. [Preview Abstract] |
Session W44: Focus Session: Dynamics of Polymers-Phenomena due to Confinement - Diffusion, Particles, & Pores
Sponsoring Units: DPOLYChair: Karin Jacobs, Saarland University, Germany
Room: A309
Thursday, March 24, 2011 11:15AM - 11:27AM |
W44.00001: Single-molecule measurements of adsorbed polymer Changqian Yu, Juan Guan, Sung Chul Bae, Steve Granick Single-molecule tracking is used to study the surface mobility of PEG (polyethylene glycol) chains adsorbed to the solid-liquid interface from dilute aqueous solution. The end-labeled chains are visualized by objective-based total internal reflection fluorescence microscopy (TIRFM) and their trajectories are analyzed after cleaning the images with denoising algorithms. Surface mobility, which in this system depends on pH, is decomposed into one family of chains which remains adsorbed over the observation time window, and another family that appears to translate from point to point by hopping. This we quantify with nm-level resolution. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W44.00002: Segmental motion in polystyrene thin film: a single molecule fluorescence study Zhongli Zheng, Jiang Zhao Single molecule fluorescence de-focus microscopy is used to study the segmental motion by observing the rotational motion of single fluorophores chemically attached to polystyrene chain ends. The collective nature of the rotational motion was noticed: a sudden change of the fraction of rotating fluorophores was discovered at a temperature 60 degree below the glass transition temperature of polystyrene. The dependence of the critical temperature on film thickness and surface chemistry was investigated and the results show that the effect of confinement, surface interaction and free surface. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W44.00003: Order of Magnitude Decrease in Dye Diffusion in Nanoconfined Polymer Films: Fluorescence Nonradiative Energy Transfer Technique Hui Deng, Manish Mundra, John Torkelson A fluorescence nonradiative energy transfer/multilayer film technique was used to determine the diffusion coefficient of the dyes decacyclene and Disperse Red 1 in supported polystyrene (PS) films as a function of film thickness. Previous studies on the glass transition temperature (Tg) of PS show a decrease in Tg as films are nanoconfined. This Tg-reduction is due to the enhanced role of the polymer/air interface which results in a region of increased polymer mobility as thickness is reduced. However, dye diffusion coefficients decrease upon film nanoconfinement, with the onset of diffusion coefficient reduction appearing at film thicknesses much thicker than the onset of Tg-confinement effects. These results can be explained by the fact that Tg reflects the longer time side of the polymer relaxation time distribution while dye diffusion reflects the shorter time side of the relaxation distribution. We hypothesize that confinement suppresses the shorter time side of the relaxation distribution which results in the observed decrease in diffusion coefficients. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W44.00004: Non-classical diffusion of PDMS confined in a surface forces apparatus Subhalakshmi Kumar, Changqian Yu, Sung Chul Bae, Steve Granick We present FRAP measurements inside a surface forces apparatus. Polydimethylsiloxane (PDMS), well above its glass transition, was confined into molecularly-thin films between atomically smooth mica sheets. Translational diffusion was measured using fluorescence recovery after photobleaching (FRAP) as the polymer film thickness was changed from tens of Rg to 3 Rg . The FRAP recovery curves of confined films are distinctly non-classical. Huge heterogeneity is suggested by stretched exponential behavior in which the power of time varies smoothly from $\beta $=1 (thick films) to $\beta $=0.3 (confined films) with a sharp transition between these limits. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W44.00005: Probing In-Plane Diffusion of Nano-Confined Polymers in Ultrathin Films Joshua Katzenstein, Justin Chandler, Haley Hocker, Christopher Ellison In-plane (parallel to the substrate) polymer diffusion at and near interfaces has significant implications for polymeric surfactants used in tertiary oil recovery, exfoliation of clay sheets in polymer nano-composites, and several other high technology applications. Here, we report a study on the in-plane diffusion of whole polymer chains confined between interfaces using fluorescence recovery after photobleaching. Adapted from quantitative biology, FRAP provides a platform to independently study the effect of temperature, molecular weight, and film thickness on in-plane diffusion of polymers confined between interfaces. Fluorescently labeled polymers were synthesized, spin coated onto quartz substrates and the self-diffusion coefficient was measured by irreversibly photobleaching fluorophores in a pre-defined pattern and monitoring recovery of fluorescence over time. Preliminary results indicate that for thick films the diffusion coefficient is consistent with bulk values. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W44.00006: Screening Effect of Supercritical Carbon Dioxide on Polymer/Substrate Interactions Peter Gin, Naisheng Jiang, Maya Endoh, Bulent Akgun, Sushil Satija, Tad Koga The kinetics and thermodynamic properties of polymer melts near interfaces and in confined geometries can vary significantly from their bulk counterparts. This behavior can be attributed to the presence of an immobile layer at the polymer/substrate interface, which has been reported to hinder the mobility of polymer chains in thin films even at a large length scale. Here, we investigate the use of supercritical carbon dioxide (scCO$_{2})$ as a medium to screen the polymer/substrate interactions and enhance chain mobility in polymer thin films. In-situ neutron reflectivity was utilized to measure the interdiffusion of deuterated polystyrene (d-PS) into various matrices of hydrogenated PS (h-PS) with thicknesses ranging from 0.5 Rg to 5 Rg. We found that at the unique T (36\r{ }C) and P (8.2 MPa) conditions, where the anomalous adsorption of CO$_{2}$ molecules in polymer thin films occurs, the diffusion constants remained unchanged regardless of bottom layer thickness, while no diffusion occurred below 1Rg at high temperature (170\r{ }C). [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W44.00007: The Glass Transition at Silica/PMMA Nanocomposite Interfaces Rahmi Ozisik, Katelyn Parker, Ryan T. Schneider, Richard W. Siegel, Juan Carlos Cabanelas, Berna Serrano, Claire Antonelli, Juan Baselga Local glass transition temperatures (Tg) have been measured in the interfaces of solution blended silica/poly(methyl methacrylate) (PMMA) nanocomposites using florescence spectroscopy and compared with Tg measured by differential scanning calorimetry (DSC). It was found that the two types of measurements yielded significantly different information. Combinations of silanes and poly(propylene glycol)- based molecular spacers bound to fluorophores were covalently linked to the surface of the nanoparticles, allowing for variation of the fluorophore response with respect to the distance from the nanofiller surface. Increases in the bulk Tg from the neat PMMA value were found upon the addition of nanofillers, but were independent of the nanofiller concentration when the filler concentration was above 2\% by weight. Furthermore, as the size of the grafted molecular spacer was increased, Tg values were found to decrease and approach Tg of the neat PMMA. Owing to variable conformations of the spacers, an effective distribution of fluorophore-silica distances exists, which influences the fluorophores' response to the transition. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W44.00008: Structural Relaxation of 3-Dimensionally Confined Polymer Glasses: Isobaric versus Isochoric Glass Formation Yunlong Guo, Chuan Zhang, Rodney Priestley We have measured the glassy-state structural relaxation of aqueous suspended polystyrene (PS) nanoparticles and the corresponding silica-capped PS nanoparticles via modulated differential scanning calorimetry. Suspended and capped-PS nanoparticles undergo glass formation and subsequent physical aging under isobaric and isochoric conditions, respectively. To account for glass transition temperature (Tg) changes with confinement, physical aging measurements were performed at a constant value of Tg minus Ta, where Ta is the aging temperature. With deceasing diameter, aqueous suspended PS nanoparticles exhibited enhanced physical aging rates in comparison to bulk PS. At all values of Tg minus Ta investigated, capped-PS nanoparticles aged at reduced rates compared to the corresponding aqueous suspended PS nanoparticles. Due to differences in paths to glass formation, suspended and capped-PS nanoparticles aged to different apparent equilibrium states. We captured the physical aging behavior of all nanoparticles via the Tool, Narayanaswamy, and Moynihan (TNM) model of structural relaxation. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W44.00009: Molecular Dynamics Study of Single Conjugated Polymers Confined to Nanoparticles Sabina Maskey, Flint Pierce, Dvora Perahia, Gary Grest Optically active polymers confined into nanoparticles are highly fluorescent and have potential applications in intracellular fluorescence imaging, bio-sensors and other optoelectronic devices. Internal conformation and dynamics of the polymers determines their optical properties. Using molecular dynamics (MD) simulations, we have explored the structure and dynamics of nanoparticles formed by conjugated polymers in a collapsed conformation, which is not the most stable conformation of the polymer. Nanoparticles were formed in a collapsed conformation and followed as the function of time in both poor and good solvents. We found that these nanoparticles are stable and remain collapsed in a poor solvent but rapidly expands and unraveled in a good solvent. The lengths of the side chains affect the internal packing of the side chains which in turn affect the size of the nanoparticles. S(q,t) was measured to characterize the internal dynamics of the collapsed nanoparticles. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W44.00010: The Glass Transition Temperature of Polymer Nanoparticles under Soft and Hard Confinement Chuan Zhang, Yunlong Guo, Rodney Priestley When confined to the nanoscale, the glass transition temperature (Tg) of polymers can deviate substantially from the bulk, i.e., the Tg-confinement effect. Due to ease of processing, most studies have focused on the size-dependent Tg of thin films, while few have extended investigations to other geometries. As polymers confined in higher geometrical dimensions become the enabling material in technologies ranging from drug delivery to plastic electronics, a greater understanding of size effects on Tg is warranted. Here, we investigate the effect of soft and hard three-dimensional confinement on the Tg of polymer nanoparticles. Via modulated differential scanning calorimetry, we show that Tg decreases with size for bare polymer nanoparticles, i.e., the case of soft confinement while Tg is invariant with size for silica-capped polymer nanoparticles, i.e., the case of hard confinement. These results suggest that the free surface is a key factor in Tg reductions of three-dimensionally confined polymer. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W44.00011: Statics and dynamics of confined DNA in a nanopit array Alexander Klotz, Walter Reisner Polymers have been proposed as tools for self-assembly in nanotechnology. There is interest in controlling the movement and conformation of the polymers by modifying the free energy landscape of their environment. It is necessary to understand the free energy and equilibrium behavior of a polymer in a nanoscale environment in order to control its dynamics. In these experiments, DNA molecules are placed in slits on the order of 100 nanometers. The slits are embedded with a lattice of square pits that act as entropic traps for which it is energetically favorable for the DNA to occupy. Based on the geometric properties of the lattice, the molecule in equilibrium will occupy a discrete number of pits. The dynamics of the system can be understood in terms the number of occupied pits. A partition function based on these states can be used to make testable predictions. Measurement of the static conformations of DNA in these pits, as well as the diffusion of the molecule throughout the lattice, as a function of geometric parameters can be used to test models of polymer free energy. Measurements show that the mean occupancy state scales as expected with various pit parameters. Early diffusion results indicate that the diffusion of DNA can be fine tuned by modifying the topography. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W44.00012: Modeling of Free Radical Polymerization of Methyl Methacrylate (MMA) in Nanoporous Confinement Fatema Begum, Sindee Simon Nanoconfinement of methyl methacrylate free radical polymerization is known to impact the molecular weight and molecular weight distribution of the polymer produced, and the results in the literature generally indicate an increase in molecular weight and a concommitant decrease in polydispersity index. In the present work, the mathematical model described by Verros et al. (2005) for free radical bulk polymerization of methyl methacrylate is extended to account for polymerization in nanopores. The model of Verros et al. (2005) incorporates diffusion effects and is capable of describing the conversion and the number- and weight-average molecular weights of the resulting poly(methyl methacrylate) as a function of polymerization time and process conditions. The model is extended by incorporating the effect of nanoconfinement on diffusivity using the scaling reported in the literature. The results indicate that nanoconfinement will lead to higher molecular weights, lower polydispersity, and the gel effect occurs earlier. The results are compared to experimental work and implications discussed. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W44.00013: Effect of Nanopore Confinement on the Polymerization Rate of Linear Polymers Po-Han Lin, Rajesh Khare Confinement to a nanopore has a significant impact on the thermal properties as well as the rate of chemical reactions such as polymerization as compared to these processes in the bulk. In this work, we have studied the effect of nanopore confinement on the rate of free radical polymerization by using molecular simulations. In order to capture the physics of this process, we have implemented a coarse-grained model to carry out reactive molecular dynamics simulations. Our simulation method considers the three stages of polymerization process: initiation, propagation and termination. Our simulation results will be used to compare the polymerization rate in the confinement with that in the bulk. The results will be explained by focusing on the dynamics of the reacting species in the confinement. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W44.00014: Theoretical Study of Tethered Polymers inside a Cylindrical Tube Tongchuan Suo, Mark Whitmore We present a numerical self-consistent mean-field theoretical (SCMFT) study of polymer chains tethered to the inside walls of cylindrical tubes. We consider cases ranging from relatively thin to relatively thick tubes, from low to high tethering densities, and in various solvents. Our focus is on the polymer concentration profiles and the chain end distributions, in particular the concentrations and chain overlap at the tube centers. We show that these quantities depend primarily on only two parameters, and that this dependence becomes exact in the limit of low polymer concentration. We find that there can be significant polymer interpenetration at the tube centers even in cases where the tube radius is greater than the polymer $R_{\rm g}$, and this can be tuned by changing the solvent quality and/or tethering density. [Preview Abstract] |
Session W45: Atom-Light Interactions: Experiment and Theory
Sponsoring Units: DAMOPChair: Charles Clark, National Institute of Standards and Technology
Room: A310
Thursday, March 24, 2011 11:15AM - 11:27AM |
W45.00001: Lattice-induced nonadiabatic frequency shifts in optical lattice clocks Kyle Beloy We consider the frequency shift in optical lattice clocks which arises from the coupling of the electronic motion to the atomic motion within the lattice. For the simplest of three-dimensional lattice geometries this coupling is shown to affect only clocks based on blue-detuned lattices. We have estimated the size of this shift for the prospective strontium lattice clock operating at the 390-nm blue-detuned magic wavelength. The resulting fractional frequency shift is found to be on the order of $10^{-18}$ and is largely overshadowed by the electric quadrupole shift. For lattice clocks based on more complex geometries or other atomic systems, this shift could potentially be a limiting factor in clock accuracy. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W45.00002: Magic wavelengths for optically trapped atoms Bindiya Arora, Marianna Safronova, Charles Clark The ability to trap neutral atoms inside high-Q cavities in the strong coupling regime is of particular importance for quantum computation and communication schemes, where it is essential to precisely localize and control neutral atoms with minimum decoherence. In a far-detuned optical dipole trap, the potentials experienced by an atom in its ground and excited states may be of opposite sign affecting the fidelity of experiments in which excited states are temporarily occupied. ``Magic wavelengths'' are those for which such potentials are equal. Single-laser schemes offer few cases in which magic wavelengths exist for Rb [Arora et al. PRA 76, 052509 (2007).] Here we explore bichromatic schemes for state-insensitive optical trapping of the Rb. We describe the use of trapping and control lasers to minimize the variance of the potential experienced by a trapped Rb atom in ground and excited states. We have also identified wavelengths $\lambda_{\rm{zero}}$ where the ground state frequency-dependent polarizabilities in alkali-metal atoms are zero. These are relevant for cooling and trapping experiments involving mixtures such as Rb/Yb, where the vanishing lattice potential for Rb facilitates interesting applications. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W45.00003: Laser frequency stabilization for narrow linewidth cooling of $^6$Li atoms Adam Reed, Kevin Jourde, Pedro Duarte, Randall Hulet Laser cooling to micro-Kelvin temperatures requires a laser with active frequency stabilization. The linewidth $\Gamma$ of an atomic transition sets a lower bound on the Doppler cooling temperature $k_B T_D = \hbar \Gamma/2$. The $2s-2p$ transition in $^6$Li has a lower bound temperature of $T_D \approx 140 \, \mu$K. In contrast, the $2s-3p$ transition has a narrower linewidth and thus provides a lower temperature limit of $T_D \approx 20 \, \mu$K. We present a method for stabilizing a laser to an atomic line in a vapor cell using modulation transfer spectroscopy and a home-built lock-in amplifier. Our results demonstrate successful locking of a 323 nm laser to the $2s-3p$ transition. The stabilized laser provides a second stage of magneto-optical trapping that results in an order of magnitude increase in the phase space density before evaporating to degeneracy in an optical dipole trap. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W45.00004: Demonstration of a $^{6}$Li magneto-optical trap using the $2S_{1/2}\rightarrow 3P_{3/2}$ transition R. Hart, P.M. Duarte, T.L. Yang, J.M. Hitchcock, T.A. Corcovilos, R.G. Hulet We demonstrate narrow linewidth laser cooling on the $2S_{1/2}\rightarrow 3P_{3/2}$ transition of $^{6}$Li at 323 nm. Typically, magneto-optical traps (MOTs) of alkali atoms cool on the D2 transition. The linewidth of this transition determines the Doppler limit of cooling which in the case of $^{6}$Li is 140 $\mu$K, given a 5.9 MHz transition linewidth. Due to a lack of resolved hyperfine structure that prohibits polarization gradient cooling, typical Li MOTs reach minimum temperatures near 300 $\mu$K. Cooling on the $2S_{1/2}\rightarrow 3P_{3/2}$ transition, however, allows for a Doppler limit of 20 $\mu$K since the transition linewidth is only 790 kHz. We have implemented this cooling scheme and demonstrate $^{6}$Li MOT temperatures of 65 $\mu$K. With the increased phase space density from this MOT, initial loading of the gas to an optical trap is substantially enhanced. We present our results on the characteristics of the narrow linewidth MOT and our results on the benefits of using this cooling scheme in the preparation of a degenerate gas of fermions. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W45.00005: Pulsed Counter-Rotating Source of Slow, Cold Molecules Igor Lyuksyutov, Les Sheffield, Mark Hickey, Vitaliy Krasovitskiy, Daya Rathnayaka, Dudley Herschbach We describe the performance of a new design of the counter-rotating source (CRS) of slow molecules introduced originally by Gupta and Herschbach. The CRS produces a supersonic expansion from a nozzle near the tip of a hollow rotor spun at high speed contrary to the exit beam velocity. Thereby the lab velocity can be markedly reduced. Introducing a pulsed feeding system, cryo-cooling, and shutter system has eliminated the main problem of the original CRS apparatus, in which continuous gas flow imposed high background pressure. The new version provides intense pulses, typically of duration 0.1 ms with lab speeds as low as 40 m/s and longitudinal temperature as low as 0.5 K. This device can, in principle, decelerate (or accelerate) any molecule available as a gas; we report experiments producing slow beams of krypton, oxygen, ammonia, and nitrogen dioxide. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W45.00006: Nanofriction in Cold Ion Traps Andrea Vanossi, Andrea Benassi, Erio Tosatti Sliding friction between crystal lattices and the physics of cold ion traps are so far non-overlapping fields. Two sliding lattices may either stick and show static friction or slip with dynamic friction; cold ions are known to form static chains, helices, or clusters, depending on trapping conditions. Based on simulations, we show that much could be learnt about friction by sliding (e.g., via an electric field) the trapped ion chains over a periodic corrugated potential. Unlike infinite chains where, according to theory, the classic Aubry transition to free sliding may take place, static pinning always shows up in trapped chains. Nonetheless we find that a properly defined static friction still vanishes Aubry-like at a symmetric-asymmetric structural transition, ubiquitous for decreasing corrugation in both straight and zig-zag trapped chains. Dynamic friction can also be addressed by ringdown oscillations of the ion trap. Long theorized static and dynamic one dimensional friction phenomena could thus become exquisitely accessible in future cold ion tribology. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W45.00007: Pattern formation with trapped ions Tony Lee, Michael Cross We propose an experiment to study collective behavior in a nonlinear medium of trapped ions. Using laser cooling and heating and an anharmonic trap potential, one can turn an ion into a nonlinear van der Pol-Duffing oscillator. A chain of ions interacting electrostatically has stable plane waves for all parameters. The system also behaves like an excitable medium, since a sufficiently large perturbation generates a travelling pulse. Small chains exhibit multistability and limit cycles. We account for noise from spontaneous emission in the amplitude equation and find that the patterns are observable for realistic experimental parameters. The tunability of ion traps makes them an exciting setting to study nonequilibrium statistical physics. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W45.00008: Spontaneous emission modification via quantum interference with energy shifts remained Shuai Yang, Shi-Yao Zhu, M. Suhail Zubairy Quantum interference in spontaneous emission from a four-level atom is investigated with the counter rotating terms and the energy shifts included. The atom has two upper levels coupled to a common lower level by the same vacuum modes and is driven by a coherent field to an auxiliary level. The effect of the counter rotating terms in coupling through the vacuum modes is taken into account by a unitary transformation method. We show how the quantum interference due to the energy shifts effects the spontaneous emission spectrum. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W45.00009: Optical chirality and superchiral fields Yiqiao Tang A chiral object is any material body whose mirror image may not be superimposed on the original. Electromagnetic (EM) fields may be chiral too, with circularly polarized light as the paradigmatic example. We propose a measure of the local chirality of EM fields, which we call optical chirality. Optical chirality determines the degree of chiral asymmetry in the interaction of light with small molecules. We predict the existence of superchiral forms of light which show larger bias for exciting a single chiral enantiomer, in some regions of space, than does circularly polarized plane waves. We performed a conceptually simple experiment to probe the interaction of superchiral light with a chiral biperylene derivative. We selected this molecule for its strong intrinsic optical activity and fluorescence in the visible. The regions of enhanced chiral selectivity are too thin to detect by absorption, so we used fluorescence instead. We demonstrated experimentally a 12-fold enhancement in the chiral selectivity of superchiral fields for these chiral compounds. The demonstrated chiral enhancement is not a fundamental limit. Larger enhancement may be obtained at the expense of lower overall excitation rate. These results establish that optical chirality is a fundamental property of the electromagnetic field, with possible applications ranging from plasmonic sensors to absolute asymmetric synthesis. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W45.00010: Observation of a Red-Blue Detuning Asymmetry in Matter-Wave Superradiance Lu Deng, Edward W. Hagley, Qiang Cao, Xiaorui Wang, Xinyu Luo, Ruquan Wang, Marvin G. Payne, Fan Yang, Xiaoji Zhou, Xuzong Chen, Mingsheng Zhan We report the first experimental observation of strong suppression of matter-wave superradiance using blue-detuned pump light and demonstrate a pump-laser detuning asymmetry in the collective atomic recoil motion. In contrast to all previous theoretical frameworks, which predict that the process should be symmetric with respect to the sign of the detuning of the pump laser from the one-photon resonance, we find that for condensates the symmetry is broken. With high condensate densities and red-detuned pump light the distinctive multi-order, matter-wave scattering pattern is clearly visible, whereas with blue-detuned pump light superradiance is strongly suppressed. However, in the limit of a dilute atomic gas symmetry is restored. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W45.00011: Large and rapidly-responding Kerr nonlinear phase shift using a four-level~N-scheme with an active Raman gain core Ke Li, Lu Deng We report rapidly-responding Kerr nonlinear phase shift using a long pencil-shaped cold $^{85}$Rb atom samples. Our system is a four-level~N-scheme based on both the D1 and D2 transition lines of Rb atom. The nonlinear phase shift and nonlinear optical index of the cold medium are studied using an interferometric method for various pump and phase-inducing laser intensities. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W45.00012: Experimental Observation of Carrier-Envelope Phase Effects in Multi-Cycle Pulses Pankaj Jha, Yuri Rostovtsev, Hebin Li, Vladimir Sautenkov, Marlan Scully Using intense RF pulses interacting with the magnetic Zeeman sub-levels of Rubidium (Rb) atoms, we have experimentally and theoretically shown the CEP effects in the population transfer between two bound atomic states interacting with pulses consisting of many cycles (up to 15 cycles) of the field. It opens several exciting applications and interesting possibilities that can be easily transfer to optical range and enhance current and create new set of tools to control CEP of laser pulses. These tools allow researchers to improve laser systems that generate laser pulses with better reproducibility and accuracy and better controlled. Also the tools provide an additional handle to control the process of collisions, and the current approach of extending the duration of the pulses with measurable or controllable CEP allows researchers to extend the coherent control to a new level where they are able to study molecular collisions or electron collisions in nano-structures under the action of strong electromagnetic fields with known CEP. In particularly, the obtained results can be applied to control of chemical reactions. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W45.00013: Counter-Propagating Coherent Stimulated Raman Spectroscopy for Remote Sensing in Air Luqi Yuan, Andrew Traverso, Dmitri Voronine, Pankaj Jha, Kai Wang, Alexei Sokolov, Marlan Scully We analyze phase-matching conditions in various four-wave mixing schemes for coherent nonlinear optical spectroscopy in the counter-propagating beam configuration. Coherent stimulated Raman spectroscopy satisfies the conditions and gives a signal containing specific molecular spectroscopic information. A counter-propagating broadband and a narrowband pulses are used to measure the Raman spectrum with a single shot. In addition, the nonresonant background due to the nondegenerate four-wave mixing is suppressed. Using this technique we develop a new scheme for standoff spectroscopy in atmosphere by using nitrogen molecules in air as a gain medium for remote lasing. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W45.00014: Efficient Backward Emission from Optically Pumped Air Andrew Traverso, Rodrigo Sanchez-Gonzalez, Michael Grubb, Dmitri Voronine, Kai Wang, Luqi Yuan, Alexei Zheltikov, Arthur Dogariu, James Michael, Richard Miles, Vladimir Sautenkov, Alexei Sokolov, Simon North, Marlan Scully We demonstrate the generation of backwards emitted coherent light in atmosphere via optical pumping. The backwards emitted light is narrow band centered at 845 nm and is generated from the dissociation of molecular oxygen and then subsequent two photon excitation of these newly dissociated oxygen atoms. Both the dissociation and excitation of oxygen are driven by a single 226 nm $\sim $10 nanosecond pulsed pump beam. The produced 845 nm light is a pulse approximately 10 nanosecond in duration and not only exhibits threshold characteristics, but is also nearly diffraction-limited. This optically-pumped mirror-less light source which propagates back towards the pump source presents a unique opportunity to develop new techniques for remote sensing. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W45.00015: Quantum lithography beyond the diffraction limit via Rabi-oscillations Zeyang Liao, Mohammad Al-Amri, M. Suhail Zubairy We propose a quantum optical method to do the sub-wavelength lithography. Our method is similar to the traditional lithography but adding a critical step before dissociating the chemical bound of the photoresist. The subwavelength pattern is achieved by inducing the multi-Rabi-oscillation between the two atomic levels. The proposed method does not require multiphoton absorption and the entanglement of photons. This method is expected to be realizable using current technology. [Preview Abstract] |
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