Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J1: Toward Single Spin Electronics
Sponsoring Units: DCMPChair: Andreas Heinrich, IBM Almaden Research Center
Room: Ballroom A1
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J1.00001: Imaging and Manipulating Single and Interacting Spins on Surfaces: Towards Atomic-Scale Spin Devices Invited Speaker: Spin-Polarized Scanning Tunneling Microscopy (SP-STM) provides new insight into spin structures at a length scale and a sensitivity level which are inaccessible by other magnetic-sensitive measurement techniques [1]. The combination of atomic resolution in direct space, single spin sensitivity, and high energy resolution nowadays offers unique possibilities for probing spin-dependent states and interactions in natural or artificially created nanostructures [2]. The ultimate goal has been the combination of spin-resolved imaging with atomic resolution and magnetometry at the single-atom level in order to probe spin states and magnetic interactions of individual adatoms and nanostructures at solid surfaces quantitatively and in a most direct way. This challenging goal has been achieved by operating a SP-STM system at temperatures below 1 Kelvin and in external magnetic fields up to several Tesla. The new method of single-atom magnetometry with an unprecedented degree of magnetization measurement sensitivity is applicable to metallic [3, 4] as well as to semiconducting [5] and molecular systems [6]. The combination of single-atom manipulation techniques and single-atom magnetometry has recently led to the first demonstration of atomic-scale spin logic devices based solely on spin- rather than charge-transport for realizing computation and information transmission at the atomic level. \\[4pt] [1] R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009).\\[0pt] [2] D. Serrate, et al., Nature Nanotechnology 5, 350 (2010). \\[0pt] [3] F. Meier, et al., Science 320, 82 (2008). \\[0pt] [4] L. Zhou, et al., Nature Physics 6, 187 (2010). \\[0pt] [5] A. A. Khajetoorians, et al., Nature 467, 1084 (2010). \\[0pt] [6] J. Brede, et al., Phys. Rev. Lett. 105, 047204 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J1.00002: All-electric control of single atom spin states Invited Speaker: The quantum state of a single spin is a great candidate for forming a qubit. Spin systems in various forms are considered for the task, ranging from electrons trapped in artificial quantum dots to magnetic dopants in semiconductors and diamond. In this talk I will review recent progress towards controlling the spins of individual atoms on a surface through local access with an STM probe tip: an intriguing approach in view of the possibility to rearrange the atoms at will so as to build multi-atom structures. Magnetic d-metal atoms, separated from a metal substrate by a thin decoupling layer, are studied through inelastic electron tunneling spectroscopy (IETS): a tool by which transition energies of the spin state can be accurately followed. By addressing the atoms with a spin-filtered probe tip, controlled excitations or de-excitations can be made, effectively pumping the spin into a magnetization direction of choice. In a more recent experiment, spin pumping is performed in short pulses, opening up ways to control atomic spins in the time domain. I will discuss avenues to further develop this technique, eventually leading to coherent control of an atomic spin qubit. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J1.00003: Quantum control and nanoscale placement of single spins in diamond Invited Speaker: Diamond is a unique solid state platform for fundamental studies of spintronics and quantum information science that has recently enabled control, readout, and storage of quantum states at the single spin level. Nitrogen-vacancy (NV) center spins can be individually addressed and have remarkably long spin coherence times at room temperature. We show that the spin of single NV centers in both the orbital ground\footnote {G. D. Fuchs, V. V. Dobrovitski, D. M. Toyli, F. J. Heremans, and D. D. Awschalom, \emph{Science} \textbf{326}, 1520 (2009).} and excited state\footnote{G. D. Fuchs, V. V. Dobrovitski, D. M. Toyli, F. J. Heremans, C. D. Weis, T. Schenkel, and D.D. Awschalom, \emph{Nat. Phys.} \textbf{6}, 668 (2010).} can be controlled on sub-nanosecond time scales using intense microwave fields. Moreover, coherent light-matter interactions enable non-destructive spin measurement and localized single spin manipulation with near-resonant light.\footnote{B. B. Buckley, G. D. Fuchs, L. C. Bassett, and D. D. Awschalom,\emph {Science Express} (DOI: 10.1126/science.1196436)} An associated quantum memory is also demonstrated using the intrinsic nuclear spin of nitrogen.\footnote{G. D. Fuchs, G. Burkard, P. Klimov, and D. D. Awschalom, in preparation.} Scaling these findings toward a spin network is a key challenge - to this end we present a simple method for patterning NV center formation on 50 nm length scales.\footnote {D. M. Toyli, C. D. Weis, G. D. Fuchs, T. Schenkel, and D. D. Awschalom,\emph{NanoLett.} \textbf{10}, 3168 (2010).} These results represent progress toward control, coupling, and scaling of single spins for future spin and photon based quantum information processing. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J1.00004: Exploring the quantum frontier of spin dynamics Invited Speaker: Our familiar classical concept of a \textit{spin} is that of a system characterized by the \textit{direction} in which the spin is \textit{pointing.} In this picture, we may think of the dynamics of a spin as the motion of a \textit{classical gyroscope}, wich we can aptly describe the spin dynamics as the motion of a point on a sphere. This classical description of the spin dynamics, formalized in the Landau-Lifshits-Gilbert equation, has proved extremely successful in the field micro- and nanomagnetism. However, as the size of the system is further decreased (e.g., when considering molecular magnets such as the Fe$_{8}$ or Mn$_{12}$ systems, which have a spin $S$=10), \textit{quantum} effects such as tunneling, interference, entanglement, coherence, etc., play an essential role, and one must adopt a fully quantum mechanical description of the spin system. The landscape in which the system evolves is then no longer a mere sphere, but rather it is the projective Hilbert space (wich is the projective complex space $\le $P$^{2S}$ for a spin $S)$, as space of considerably greater richness and complexity than the sphere of classical spin dynamics. A very appealing tool to describe a quantum spin system is Majorana's stellar representation, which is the extension for a spin $S$ of the Bloch sphere description of a spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. I shall discuss how this representation can help us in improving our understanding of fundamental quantum processes and concept such as Landau-Zener transitions, Rabi oscillations, Berry phase, diabolical points and illustrate this on the example of spin dynamics of molecular magnets. [Preview Abstract] |
Session J2: Force Probes of Materials' Structure and Function
Sponsoring Units: DCMPChair: Chris Hammel, Ohio State University
Room: Ballroom A2
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J2.00001: Nanoscale Mechanical Resonators for Probing Physical Phenomena: Fluid Dynamics of High-frequency Flows Invited Speaker: With their miniscule sizes, high frequencies, and small force constants, nanoelectromechanical systems (NEMS) resonators are expected to emerge as tools for sensing a variety of analytes, for probing biological entities, and for measuring molecular-scale forces. Because many of these foreseeable applications are in fluids, it is natural to consider the operation of NEMS resonators in fluids. When immersed in a fluid, however, the NEMS resonator loses most of its vibrational energy to the fluid. In other words, the quality factor (Q) of the resonator decreases significantly. Reductions in Q result in a reduction in the resonator's sensitivity to added mass or force. In order to understand the fluid dynamics of NEMS, we have revisited a well-known fluid dynamics problem: Stokes' second problem of the oscillating plate in a fluid. At the typical frequencies of NEMS resonators, Stokes' second problem needs to be reformulated using a relaxation time approach in order to accurately describe the fluidic effects. Our experiments and theory show that the fluid relaxation time in conjunction with the resonator frequency determines the nature of the flow; linear dimension and geometry appear to have weak effects. Our results support a universality in oscillating flows and suggest a deep connection between simple and complex fluids. With this understanding, we are making progress toward reducing NEMS dissipation in water. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J2.00002: Consistency and discrepancy between single molecule force spectroscopy experiments and theoretical models Invited Speaker: Single molecule force spectroscopy is a well-established tool to study molecular interactions in a wide range of binding affinities on the single-molecule level. Information about the strength of the molecular bond can be quantified in terms of the dissociation rate k$_{off}$, and the reaction length x$_{b}$ (i.e., the distance between potential minimum and maximum along the direction of pulling). The analysis and interpretation of the underlying force-distance curves is still challenging and various models describing the experimental data are under discussion. In this talk, I will present experimental data for a protein-RNA interaction related to posttranscriptional regulation on the single molecule level, and the interaction between DNA bases forming two or three hydrogen bonds. I will use these examples to discuss the advantages and limitations of this technique, and the consistency and discrepancy to theoretical models. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J2.00003: Magnetic force microscopy of superconductors: vortex manipulation and measuring the penetration depth Invited Speaker: We use a low temperature magnetic force microscope (MFM) to image superconductors. The interaction between the magnetic tip and individual vortices allows us to both image vortices and to manipulate them. The manipulation results depend on sample thickness and on the superconducting properties. Here I concentrate on YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) samples and on Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$, an underdoped pnictide. In thin films, if the force exerted by the tip is strong enough to overcome the pinning potential a vortex jumps as a whole to a new pinning site. The behavior in thick YBCO single crystals depends on the doping level. In a slightly overdoped sample vortices stretch rather than jump when we perturb them strongly [1]. The dragging distance in this crystal is anisotropic: it is easier to drag vortices along the Cu-O chains than across them, consistent with the tilt modulus and the pinning potential being weaker along the chains. We also find that when we ``wiggle'' the top of a vortex we can drag it significantly farther than when we do not, giving rise to a striking dynamic anisotropy between the fast and the slow directions of the scan pattern. In an underdoped YBCO single crystal, where superconductivity is so anisotropic that a vortex should be viewed as a stack of two dimensional pancakes, we show that vortices kink rather than tilt when we perturb them [2]. Since the discovery of the pnictides, a new family of high temperature superconductors, we have also been developing ways to determine the absolute value of the magnetic penetration depth, which is notoriously difficult to measure, as well as its dependence on temperature. For that we either use the Meissner repulsion of the magnetic MFM tip from the sample or the magnetic interaction between the tip and the magnetic field from a vortex. The temperature dependence that we find allows us to comment on the symmetry of the order parameter [3]. \\[4pt] Work done in collaboration with Lan Luan and Kathryn A. Moler (Stanford)\\[4pt] [1] O. M. Auslaender et al., Nat. Phys. 5, 35 (2009).\\[0pt] [2] Lan Luan et al., Phys. Rev. B 79, 214530 (2009).\\[0pt] [3] Lan Luan et al., Phys. Rev. B 81, 100501 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J2.00004: Nanoscale Magnetic Resonance Imaging Invited Speaker: Magnetic resonance imaging (MRI), based on the sensitive detection of nuclear spins, enables three dimensional imaging without radiation damage. Conventional MRI techniques achieve spatial resolution that is at best a few micrometers due to sensitivity limitations of conventional inductive detection. The advent of ultrasensitive nanoscale magnetic sensing opens the possibility of extending MRI to the nanometer scale. If this can be pushed far enough, one can envision taking 3D images of individual biomolecules and, perhaps, even solving molecular structures of proteins. In this talk we will discuss issues related to nanoscale magnetic resonance imaging, especially its implementation using magnetic resonance force microscopy (MRFM). We will also consider the future possibility of using NV centers in diamond for detection of nanoMRI. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 2:15PM |
J2.00005: Histone Post-Translation Modifications Influence Chromatin Mechanical Stability Invited Speaker: Histone proteins organize the human genome into chromatin fibers while their post-translation modification (PTM) regulates genome replication, expression and repair. The mechanistic connections between histone PTMs and biological functions remain enigmatic. We find with a combination of magnetic tweezers mechanical measurements and biochemical studies that a number of histone PTMs influence the DNA mismatch repair process by mechanically destabilizing chromatin. The location of the PTM within the chromatin structure appears to determine the mechanism by which it alters the mechanical stability. These findings have direct implications for understanding the repair of the human genome. [Preview Abstract] |
Session J3: The Kavli Foundation Special Symposium: Nobelist Perspectives on 100 Years of Superconductivity
Sponsoring Units: DMP DCMPChair: Warren Pickett, University of California, Davis
Room: Ballroom A3
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J3.00001: Theoretical work on superconductivity up to 1956 Invited Speaker: In this talk I survey some of the attempts to understand superconductivity which preceded the 1957 work of Bardeen, Cooper and Schrieffer, and conclude by asking if there are any lessons we can draw for our current efforts to understand apparently ``non-BCS'' superconductors such as the cuprates and ferropnictides. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J3.00002: The Exceptional Properties of Superconductivity in Cuprates Invited Speaker: Copper oxides are the only materials that have transition temperatures, $T_c$, \textit{above} the boiling point of liquid nitrogen, with a maximum $T_c^m$ of 162 K under pressure. Their structure is layered, with one to several CuO$_2$ planes, and upon hole doping, their transition temperature follows a dome-shaped curve with a maximum at $T_c^m$. In the underdoped regime, i.e., below $T_c^m$, a pseudogap $T$* is found, with $T$* always being larger than $T_c$, a property unique to the copper oxides [1]. In the superconducting state, Cooper pairs (two holes with antiparallel spins) are formed that exhibit coherence lengths on the order of a lattice distance \textit{in the} CuO$_2$ plane and one order of magnitude less perpendicular to it. Their macroscopic wave function is parallel to the CuO$_2$ plane near 100\% $d$ at their surface, but only 75\% $d$ and 25\% $s$ in the bulk, and near 100\% $s$ perpendicular to the plane in YBCO. There are two gaps with the same $T_c$ [2]. As function of doping, the oxygen isotope effect is novel and can be quantitatively accounted for by a two-band vibronic theory [3]. These cuprates are intrinsically heterogeneous in a dynamic way. In terms of quasiparticles, bipolarons are present at low doping, and aggregate upon cooling [1], so that probably ramified clusters and/or stripes are formed, leading over to a more Fermi-liquid-type behavior at large carrier concentrations above $T_c^m$. \newline \newline [1] For an overview, see: K.A.\ M\"uller, J. Phys:\ Condens.Matter \textbf{19}, 251002 (2007). \newline [2] R.\ Khasanov, A.\ Shengelaya \textit{et al.}, Phys.\ Rev.Lett.\ \textbf{98}, 0570007 (2007). \newline [3] H.\ Keller, A.\ Bussmann-Holder, and K.A.\ M\"uller, Materials Today \textbf{11}, 38 (2008). [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J3.00003: Discovery of Superconductive Tunneling Invited Speaker: Some times unlikely events happens: How can a mechanical engineer from Norway end up with a Nobel Prize in Physics? I had the great fortune to receive the prize in Physics for using electron tunneling to measure the energy gap in superconductors. In this talk I will recollect some of the events that led to this discovery and hopefully be able to convey to you some of the fun and excitement of that area. My great fortune was really to be at the right place at the right time, where I had access to outstanding and helpful physicists. If you become real interested, you may look up the talk at the web site http://nobelprize.org/ . [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J3.00004: Superconductivity and particle physics Invited Speaker: I will briefly review and give a modern perspective on some classic applications of the ideas of superconductivity theory to fundamental particle physics: spontaneous chiral symmetry breaking in vacuum QCD, the Higgs mechanism in electroweak theory, and color superconductivity in dense hadronic matter; and also the confinement problem. Then I will discuss some frontier topics that carry the ideas further and in new directions: supersymmetry and the superHiggs mechanism, exotic quantum statistics of superconducting vortices. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 2:15PM |
J3.00005: Superfluidity in an Atomic Gas of Strongly Interacting Fermions Invited Speaker: What is the benefit of realizing superfluidity in a gas a million times more dilute than air? Such systems consist of well-separated atoms which can be observed and manipulated with the control and precision of atomic physics, and which can be treated with first-principles calculations. By implementing scattering resonances, we have realized the strong-coupling limit of the Bardeen Schrieffer-Cooper (BCS) mechanism and observed a normalized transition temperature of 15\% of the Fermi temperature, higher than in any superconductor. By tuning the strength of the interactions, the BEC-BCS crossover is realized. When the population of the two spin states is imbalanced, pairing is frustrated; and superfluidity is quenched at the Chandrasekhar-Clogston limit. These studies illustrate a new approach to condensed-matter physics where many-body Hamiltonians are realized in dilute atomic gases. [Preview Abstract] |
Session J4: Interactions Between Pore Forming Peptides and Membranes
Sponsoring Units: DPOLY DBPChair: Gerard Wong, University of California, Los Angeles
Room: Ballroom A4
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J4.00001: Membrane Disruption Mechanism by Antimicrobial Peptides Invited Speaker: Antimicrobial peptides (AMPs) are a class of small (less than100 residues) host defense peptides that induce selective membrane lytic activity against microbes. To understand the mechanism of membrane disruption by AMPs, we investigated, via atomic force microscopy, topological changes in supported phospholipid bilayers induced by protegrin-1 (PG-1). We have observed that PG-1 induces structural transformations, progressing from fingerlike instabilities at bilayer edges, to the formation of sievelike nanoporous structures and finally to a network of stripelike structures in a zwitterionic dimyristoylphosphatidylcholine (DMPC) model membrane in buffer, with increasing PG-1 concentration. Our results suggest that AMPs act to lower the interfacial energy of the bilayer in a way similar to detergents. By varying the lipid composition, temperature and using AMPs with different secondary structures, we are able to identify factors other than electrostatics that are important for the efficacy of AMPs. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J4.00002: Deconstruction of biophysical function in the HIV fusion peptide Invited Speaker: We have synthesized a library of variants of the 23-residue fusion peptide domain found at the $N$-terminus of gp-41 glycoprotein of HIV. This sequence is critical for viral infectivity and is thought to be central in the membrane fusion of viral envelope with the host endosomal membrane. There has been extensive discussion in the literature regarding the mechanism by which this viral fusion sequence initiates membrane fusion, with importance placed on glycine-content, particular oligomeric states and secondary structure; both helical and sheet structures have been proposed to be the active fusogenic structure. Our library was designed to address the biophysical importance of secondary structure, peptide flexibility, glycine content and location as well as the nature of the membrane anchor. Each member of this library also bears a positively charged hexapeptide at the $C$-terminus for solubility and to facilitate binding to negatively charged membranes. We assayed each peptide for its ability to induce lipid-mixing and lysis in both large and giant unilamellar vesicles, and searched for correlations between aggregated peptides and heightened activity. We find that the information encoded in the viral fusion peptide required for may be greatly simplified: glycine is not required for fusion, aggregation is not correlated with activity, and any peptide within a window of hydrophobicity can be an effective fusion catalyst. Given the wide range of sequences which may be effective in catalyzing vesicle membrane fusion, it appears highly unlikely that a particular stably folded secondary structure is important for fusion. Rather, our data show that many flexible, linear, minimally hydrophobic peptides may achieve the biophysical function of fusion. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J4.00003: Relation between amino acid sequence and peptide-induced membrane curvature Invited Speaker: This abstract not available. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J4.00004: Structural Studies of Biological Solids Using NMR Invited Speaker: High-resolution structure and dynamics of biological molecules are important in understanding their function. While studies have been successful in solving the structures of water-soluble biomolecules, it has been proven difficult to determine the structures of membrane proteins and fibril systems. Recent studies have shown that solid-state NMR is a promising technique and could be highly valuable in studying such non-crystalline and non-soluble biosystems. I will present strategies to study the structures of such challenging systems and also about the applications of solid-state NMR to study the modes of membrane-peptide interactions for a better assessment of the prospects of antimicrobial peptides as substitutes to antibiotics in the control of human disease. Our studies on the mechanism of membrane disruption by LL-37 (a human antimicrobial peptide), analogs of the naturally occurring antimicrobial peptide magainin2 extracted from the skin of the African frog Xenopus Laevis, and pardaxin will be presented. Solid-state NMR experiments were used to determine the secondary structure, dynamics and topology of these peptides in lipid bilayers. Similarities and difference in the cell-lysing mechanism, and their dependence on the membrane composition, of these peptides will be discussed. Atomic-level resolution NMR structures of amyloidogenic proteins revealing the misfolding pathway and early intermediates that play key roles in amyloid toxicity will also be presented. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 2:15PM |
J4.00005: How antimicrobial peptides disrupt lipid bilayers? Invited Speaker: The molecular basis for the activity of cyclic and linear antimicrobial peptides is analysed. We performed multi-scale molecular dynamics simulations and biophysical measurements to probe the interaction of antimicrobial peptides with model membranes. Two linear antimicrobial peptides, magainin and melittin and a cyclic one, BPC194 have been studied. We test different models to determine the generic and specific forces that lead to bilayer disruption. We probe whether interfacial stress or local membrane perturbation is more likely to lead to the porated state. We further analyse the reasons that determine specificity and increase of activity in antimicrobial peptides. The results provide detailed insight in the mode of action of antimicrobial peptides. [Preview Abstract] |
Session J5: Hildred Blewett Scholars and their Research followed by Panel Discussion
Sponsoring Units: CSWPChair: Sherry Yennello, Texas A&M University
Room: Ballroom C1
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J5.00001: M. Hildred Blewett and the Blewett Scholarship Invited Speaker: M. Hildred Blewett became a physicist at a time when few women were physicists. After beginning her career at General Electric, she became a respected accelerator physicist, working at Brookhaven, Argonne, and eventually CERN. Blewett was married for a time to John Blewett, another accelerator physicist, but the couple divorced without children and she never remarried. She felt that her career in physics was hampered by her gender, and when she died in 2004 at the age of 93, she left the bulk of her estate to the American Physical Society, to found a Scholarship for women in physics. Since 2005 the Blewett Scholarship has been awarded to women in physics who are returning to physics after a career break, usually for family reasons. Family/career conflicts are one of the most important reasons why young women in early careers leave physics---a loss for them as well as the physics community, which has invested time and money in their training. The Blewett Scholarship is one way for the physics community, under the leadership of CSWP, to help these young women resume their careers. I will discuss the life and work of Hildred Blewett, the Blewett Scholarship, and its benefits to the physics community. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 12:03PM |
J5.00002: Atomically Thin Graphene Hall Cross Devices as Sensitive Magnetic Field Probes Invited Speaker: Experimentally realized in 2004, graphene has ignited great interest in physics, material science, chemistry and engineering. Graphene is a one-atom thick sheet of carbon atoms arranged in a hexagonal lattice. This elegantly simple material has excellent electronic and mechanical properties, as well as remarkable physics resulting from its relativistic Dirac electrons. Graphene is a highly promising material for many applications, including sensors. One of my research projects has been investigating graphene for use as a nanoscale magnetic sensor. Such sensors could be used to measure small magnetic particles or as a scanning probe to map out magnetic fields. The Hall cross geometry of the devices gives a noninvasive and straightforward magnetic field probe. I fabricated Hall crosses from mechanically exfoliated single-, bi-, and multi-layer graphene with cross junction widths down to a few hundred nanometers. The devices were tested in a small applied field and the noise spectra of the Hall signal as a function of bias current and back gate voltage was measured at room and cryogenic temperatures. The best field sensitivity at room temperature obtained for a 400 nm graphene Hall cross was 15 G/Hz$^{1/2}$ at 1 Hz and 1 G/Hz$^{1/2}$ at 4 kHz, which is on par with similar crosses made from other materials in the literature. In addition, because they are made from graphene, graphene Hall probes have further advantages such as tunability with a gate, being extremely thin and at the surface, and having mechanical stability for ultra-small device fabrication. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:39PM |
J5.00003: When Nano Isn't Small Enough: Lattice Quantum Chromodynamics and My Quest to Understand Particle Physics Invited Speaker: Although I started my research life as a condensed matter theorist (nanotribology), I now work in the field of high-energy physics. I use effective field theories and Lattice Quantum Chromodynamics (Lattice QCD) to calculate strong-interaction effects in Standard Model processes. I have worked on quark and meson mass calculations, and contributed to decay-process calculations used to probe the weak-interation. After setting the stage for this research, I will briefly describe calculations I have worked on over the past few years and give an introduction to my current research project(s). [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 1:15PM |
J5.00004: Lateral Modulation in Antimonide Superlattices Invited Speaker: Lateral modulation is the spontaneous formation of a periodic modulation in structure or alloy composition perpendicular to the growth direction in an epitaxial structure. It has been observed in many epitaxially grown III-V semiconductor alloys, occurring during both the homogenous growth of III-V alloys and in III-V superlattices. In [001] oriented zinc blende structures, lateral modulation typically occurs along one of the [110] directions and is associated with strain and growth kinetics. It begins when strain is relieved through elastic surface undulations, typically in layers much thinner than the critical thickness for dislocation formation. In a superlattice, these undulations can lead to either compositional or purely structural modulation, depending on the relative phase and amplitude of the undulations in the constituent layers. Compositional modulation is by far the most commonly observed form in superlattice structures. We report on the analysis of purely structural lateral modulation in AlSb/AlAs digital superlattices, using x-ray diffraction. This is the first report of lateral modulation in an antimonide superlattice, and a rare observation of purely structural modulation in a superlattice. InAs/Al(In)Sb digital superlattices were also studied, and exhibited no lateral modulation. The composition and strain of the structures and critical thickness for lateral modulation will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 2:15PM |
J5.00005: Panel discussion on the Blewett Scholarship, and how it helps young women achieve career/family balance. Invited Speaker: This panel discussion will be preceded by talks by three women who have had career interruptions due to family reasons and have restarted their research careers with some assistance from a Blewett Scholarship. They will discuss their experience balancing career and family. Questions from the audience will be encouraged. This session should provide inspiration for young women to pursue their passions and motivation for department chairs to be flexible and accommodating of career/family balance. [Preview Abstract] |
Session J6: The Use of GPUs in Computational Physics
Sponsoring Units: DCOMPChair: David Ceperley, University of Illinois at Urbana-Champaign
Room: Ballroom C2
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J6.00001: Fully accelerating quantum Monte Carlo simulations of real materials on GPU clusters Invited Speaker: Quantum Monte Carlo (QMC) has proved to be an invaluable tool for predicting the properties of matter from fundamental principles, combining very high accuracy with extreme parallel scalability. By solving the many-body Schr\"{o}dinger equation through a stochastic projection, it achieves greater accuracy than mean-field methods and better scaling with system size than quantum chemical methods, enabling scientific discovery across a broad spectrum of disciplines. In recent years, graphics processing units (GPUs) have provided a high-performance and low-cost new approach to scientific computing, and GPU-based supercomputers are now among the fastest in the world. The multiple forms of parallelism afforded by QMC algorithms make the method an ideal candidate for acceleration in the many-core paradigm. We present the results of porting the QMCPACK code to run on GPU clusters using the NVIDIA CUDA platform. Using mixed precision on GPUs and MPI for intercommunication, we observe typical full-application speedups of approximately 10x to 15x relative to quad-core CPUs alone, while reproducing the double-precision CPU results within statistical error.\footnote{K. Esler, J. Kim, L. Shulenburger, D. Ceperley, ``Fully accelerating quantum Monte Carlo simulations of real materials on GPU clusters'', Computing in Science and Engineering ({\em preprint}) DOI: 10.1109/MCSE.2010.122 (2010).} We discuss the algorithm modifications necessary to achieve good performance on this heterogeneous architecture and present the results of applying our code to molecules and bulk materials.\footnote{K.P. Esler, R.E. Cohen, B. Militzer, Jeongnim Kim, R.J. Needs, and M.D. Towler, ``Fundamental High-Pressure Calibration from All-Electron Quantum Monte Carlo Calculations'', Phys. Rev. Lett. {\bf 104}, 185702 (2010).} [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J6.00002: The Use of GPUs in Lattice Gauge Theory Invited Speaker: In the past few years, GPUs have been put to use in lattice gauge theory calculations. This talk will consider the successes and challenges of this approach including such issues as how to take advantage of multiple threads, the effort involved in porting, the resulting performance and the challenge of writing multi-GPU code that scales well. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J6.00003: Wavelet-Based DFT calculations on Massively Parallel Hybrid Architectures Invited Speaker: In this contribution, we present an implementation of a full DFT code that can run on massively parallel hybrid CPU-GPU clusters. Our implementation is based on modern GPU architectures which support double-precision floating-point numbers. This DFT code, named BigDFT, is delivered within the GNU-GPL license either in a stand-alone version or integrated in the ABINIT software package. Hybrid BigDFT routines were initially ported with NVidia's CUDA language, and recently more functionalities have been added with new routines writeen within Kronos' OpenCL standard. The formalism of this code is based on Daubechies wavelets, which is a systematic real-space based basis set. As we will see in the presentation, the properties of this basis set are well suited for an extension on a GPU-accelerated environment. In addition to focusing on the implementation of the operators of the BigDFT code, this presentation also relies of the usage of the GPU resources in a complex code with different kinds of operations. A discussion on the interest of present and expected performances of Hybrid architectures computation in the framework of electronic structure calculations is also adressed. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J6.00004: Graphical Processing Units for Quantum Chemistry Invited Speaker: This abstract not available. [Preview Abstract] |
Session J7: The Dynamics of Co-Evolving and Interdependent Networks
Sponsoring Units: GSNPChair: Alessandro Vespignani, Indiana University
Room: Ballroom C3
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J7.00001: Robustness of Interdependent Networks Invited Speaker: In interdependent networks, when nodes in one network fail, they cause dependent nodes in other networks to also fail. This may happen recursively and can lead to a cascade of failures. In fact, a failure of a very small fraction of nodes in one network may lead to the complete fragmentation of a system of many interdependent networks. We will present a framework for understanding the robustness of interacting networks subject to such cascading failures and provide a basic analytic approach that may be useful in future studies. We present exact analytical solutions for the critical fraction of nodes that upon removal will lead to a failure cascade and to a complete fragmentation of two interdependent networks in a first order transition [1]. Surprisingly, analyzing complex systems as a set of interdependent networks may alter a basic assumption that network theory has relied on: while for a single network a broader degree distribution of the network nodes results in the network being more robust to random failures, for interdependent networks, the broader the distribution is, the more vulnerable the networks become to random failure. We also show [2] that reducing the coupling between the networks leads to a change from a first order percolation phase transition to a second order percolation transition at a critical point. These findings pose a significant challenge to the future design of robust networks that need to consider the unique properties of interdependent networks. \\[4pt] [1] S. Buldyrev, R. Parshani, G. Paul, H.E. Stanley, S. Havlin, Nature, 465, 0893 (2010)\\[0pt] [2] R. Parshani, S. Buldyrev, S. Havlin, PRL, 105, 048701 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J7.00002: Interconnected Financial Networks Invited Speaker: It is known that different dynamics on a network are associated to different resilient structures. In particular in the case of financial networks the fragility is associated to the kind of relation between the vertices (financial agents). Since the same agents can be connected on different networks for different financial relations, a study on the global resilience can only be obtained by considering the whole structure. Here we present our activity on this topic. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J7.00003: Emergent phenomena in interacting networks Invited Speaker: This abstract not available. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J7.00004: Dynamical network approach for social interactions Invited Speaker: This abstract not available. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 2:15PM |
J7.00005: The Structure and Dynamics of Economic Complexity Invited Speaker: Can network science help us understand the structure and evolution of the global economy? In this talk I summarize recent research that uses networks and complexity science to describe and explain the evolution of the mix of products that countries, and cities, produce and export. First, I show how to use information on the network connecting industries to locations to measure the complexity of an economy. Using these measures I demonstrate that countries tend to approach a level of income that is dictated by the complexity of their economies. Next, I study the evolution of economic complexity by showing that it is constrained by a coordination problem that countries, and cities, deal with using three different channels: First, they move to products that are close by, in the Product Space, to the products that they already do. Second, they are more likely to develop a product if a geographical neighbor has already developed it. And third, they follow the nestedness of the network connecting industries to locations. Finally, I introduce a simple model to account for the stylized facts uncovered in the previous sections. [Preview Abstract] |
Session J8: Physics Education Research in Upper-division Physics Courses
Sponsoring Units: FEdChair: Paula Heron, University of Washington
Room: Ballroom C4
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J8.00001: Using research to enhance student learning in intermediate mechanics Invited Speaker: For many undergraduate physics majors the sophomore/junior level course in intermediate mechanics represents their first step beyond the introductory sequence. Over the past several years research has shown that intermediate mechanics students often encounter conceptual and reasoning difficulties similar to those that arise at the introductory level. Many difficulties suggest deeply-seated alternate conceptions, while others suggest loosely or spontaneously connected intuitions. Furthermore, students often do not connect the physics to the more sophisticated mathematics they are expected to use. This presentation will highlight results from research conducted at Grand Valley State University, the University of Maine (by co-PI Michael Wittmann) and pilot sites in the \textit{Intermediate Mechanics Tutorials} project. These results, taken from the analysis of pretests (ungraded quizzes), written exams, and classroom observations, will illustrate specific student difficulties as well as examples of guided-inquiry teaching strategies that appear to address these difficulties. (Supported by NSF grants DUE-0441426 and DUE-0442388.) [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J8.00002: A Research-Based Approach to Transforming Upper-Division Electricity \& Magnetism Invited Speaker: We present research on transforming an upper-division undergraduate electricity and magnetism course using principles of active engagement and learning theory. We build on a systematic investigation of student learning difficulties, with the goal of developing useful curricular materials and suggestions for effective teaching practices. We observe students in classroom, help-session, and interview settings, and analyze their written work. To assess student learning, we have developed and validated a conceptual instrument, the CUE (Colorado Upper-division Electrostatics) diagnostic. We collaborate with faculty to establish learning goals, and have constructed a bank of clicker questions, tutorials, homeworks, and classroom activities. We find that students in the transformed courses exhibit improved performance over the traditional course, as assessed by common exam questions and the CUE, but there is still much work to be done. Our work underlines the need for further research on the nature of student learning and appropriate instructional interventions at the upper division. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J8.00003: Investigating student understanding in an upper-division analog electronics course Invited Speaker: The Physics Education Group at the University of Washington has recently begun an in-depth investigation of student understanding of analog electronics. As part of this investigation, we have been examining student learning in an upper-division laboratory course on this subject. In particular, we have administered written questions on fundamental electric circuits concepts (typically covered in introductory physics courses) and on canonical topics in analog electronics (e.g., filters, diodes, transistors, and operational amplifiers). Drawing on the results from such questions, we are investigating the impact of the analog electronics course on student conceptual understanding. Specific examples will be used to illustrate how the findings from this investigation have implications for instruction in both introductory and upper-division courses. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J8.00004: Research on Student Learning of Upper-Level Thermal and Statistical Physics Invited Speaker: Within the last decade, physics education researchers have begun to extend the tools and methods used at the introductory level to conduct systematic investigations of student learning of thermal and statistical physics in the upper division. Most research in thermodynamics has focused on student ideas about the first and second laws and the associated concepts (e.g., work, heat, entropy). Several studies yield insights about broader ideas, such as state functions. Research in statistical physics has focused on the concepts underlying multiplicity and related ideas in probability. Research has identified a number of conceptual difficulties with varied degrees of persistence, some of which are consistent with findings at the introductory level. Some investigations further probe connections between physics and relevant mathematics concepts in these areas, including student interpretation of canonical representations such as pressure-volume (P-V) diagrams. Results from research are guiding the development of curricular materials in order to address several known difficulties. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 2:15PM |
J8.00005: Improving students' understanding of quantum mechanics Invited Speaker: Learning quantum mechanics is especially challenging, in part due to the abstract nature of the subject. We have been conducting investigations of the difficulties that students have in learning quantum mechanics. To help improve student understanding of quantum concepts, we are developing quantum interactive learning tutorials (QuILTs) as well as tools for peer-instruction. The goal of QuILTs and peer-instruction tools is to actively engage students in the learning process and to help them build links between the formalism and the conceptual aspects of quantum physics without compromising the technical content. They focus on helping students integrate qualitative and quantitative understanding, confront and resolve their misconceptions and difficulties, and discriminate between concepts that are often confused. In this talk, I will give examples from my research in physics education of how students' prior knowledge relevant for quantum mechanics can be assessed, and how learning tools can be designed to help students develop a robust knowledge structure and critical thinking skills. [Preview Abstract] |
Session J9: Liquid Crystals: Nematics, Lyotropics and Vesicles
Sponsoring Units: DFDChair: Satyendra Kumar, Kent State University
Room: D220
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J9.00001: Dynamics of chiral liquid-crystal films driven by water transport Jonathan V. Selinger, Lena M. Lopatina In previous experimental and theoretical research, Tabe and Yokoyama investigated Langmuir monolayers of chiral molecules on the surface of water, and found that evaporation of water induces collective precession of the molecular orientation [1]. More recently, they have found a similar effect in freely suspended films of chiral smectic liquid crystals, but with one new feature: the molecular rotation is accompanied by large- scale flow of the molecules, indicating a strong coupling between orientation and flow. To model the coupled rotation and flow driven by water transport, we construct the Lagrangian and Rayleigh dissipation function appropriate for a film in the smectic-A or smectic-C phase, derive the equations of motion, and solve these equations in geometries corresponding to the experiments. In particular, we calculate the flow patterns in terms of the viscosity coefficients of the liquid-crystal films, in order to understand the mechanisms that control this dynamic behavior. The theoretical predictions are compared with experimental results, and with related work on granular materials [2]. \\[4pt] [1] Y. Tabe and H. Yokoyama, Nature Mat. 2, 806 (2003).\\[0pt] [2] J.-C. Tsai, F. Ye, J. Rodriguez, J. P. Gollub, and T. C. Lubensky, Phys. Rev. Lett. 94, 214301 (2005). [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J9.00002: Linear aggregation and liquid-crystalline order: comparison of Monte Carlo simulation and analytic theory Tatiana Kuriabova, M.D. Betterton, Matthew A. Glaser Many soft-matter and biophysical systems are composed of monomers that reversibly assemble into rod-like aggregates. The aggregates can then order into liquid-crystal phases if the density is high enough, and liquid-crystal ordering promotes increased growth of aggregates. Systems that display coupled aggregation and liquid-crystal ordering include wormlike micelles, chromonic liquid crystals, DNA and RNA, and protein polymers and fibrils. Coarse-grained molecular models that capture key features of coupled aggregation and liquid-crystal ordering common to many different systems are lacking; in particular, the roles of monomer aspect ratio and aggregate flexibility are not well understood. We study a system of sticky cylinders that interact primarily by hard-core interactions but can stack and bind end to end. We use Monte Carlo simulations and analytic theory. We present results for several different cylinder aspect ratios and a range of end-to-end binding energies. The phase diagrams are qualitatively similar to those of chromonic liquid crystals, with an isotropic-nematic-columnar triple point. Our analytic theory shows improvement compared to previous theory in quantitatively predicting the I--N transition for relatively stiff aggregates, but requires a better treatment of aggregate flexibility. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J9.00003: Modeling the Kerr effect in polymer-disordered liquid crystals Lena M. Lopatina, Jonathan V. Selinger In the Kerr effect, an electric field applied to an optically isotropic material induces orientational order and hence induces optical birefringence. Recently, many investigators have used the Kerr effect to develop liquid-crystal displays and other electro-optic devices that can operate at high speed and with no need for aligning substrates. This application requires a large and fairly temperature-independent Kerr coefficient. One approach to achieve this goal is by using liquid-crystal blue phases, perhaps with polymer stabilization. As an alternative approach, D.-K. Yang has suggested using a nematic phase within a disordered polymer network. This structure would be disordered and optically isotropic in the absence of a field, but it would develop order and birefringence under an applied field. To assess this approach, we perform Monte Carlo simulations of a nematic liquid crystal in a disordered polymer network, and calculate the response to an applied field. We compare the results with analytic studies of liquid crystals under quenched disorder and with experiments. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J9.00004: A nonlocal model of inhomogeneous nematic liquid crystals Peter Palffy-Muhoray, Xiaoyu Zheng, Roland Ennis The free energy cost of spatial inhomogeneities in nematic liquid crystals is usually described in terms of gradients of the director field or of the order parameter tensor. The origins of such gradient expansions are not clear; they can also lead to ill-posedness of the variational problem of minimizing the free energy. We propose a simple nonlocal form of the single particle potential from which the free energy may be constructed. Our model reduces to the Maier-Saupe form for homogenous systems, but describes inhomogenous systems in general. We demonstrate the validity of the model by using it to describe the electric field induced Freedericksz transition. We discuss the connection between our non-local model and gradient expansions. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J9.00005: Freely Suspended Nematic Films Wilder Iglesias, Jeffrey Choi, Elizabeth K. Mann, Antal Jakli Using one of the most commonly studied synthetic molecule, 4-Cyano-4'-pentylbiphenyl (5CB), we were able to pull freely suspended membranes of different thicknesses into circular frames of up to 20mm diameter. Films pulled this way were distorted using a speaker, while a laser light was shone onto them for studying the far field reflection and learn about resonant frequency modes and subtract valuable information about the viscoelastic terms that hold the membrane stable. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J9.00006: Fluctuation Modes of a Bent-Core Nematic Liquid Crystal Madhabi Majumdar, S. Chakraborty, B. Senyuk, O.D. Lavrentovich, James T. Gleeson, Antal Jakli, Samuel Sprunt We present a dynamic light scattering study of the bent-core nematic liquid crystal compound \textit{DT6Py6E6. }We utilize a ``dark'' scattering geometry, which allows us to search for fluctuation modes that are not purely associated with the uniaxial director. Indeed, we observe two modes (hydrodynamic and non-hydrodynamic) in addition to the expected twist-bend director mode. We present a model for the additional modes based on fluctuations of the biaxial order parameter, which leads to an estimate of 10-100 nm for the correlation length associated with these fluctuations. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J9.00007: Three dielectric constants and orientation order parameters in nematic mesophases Hyung Guen Yoon, Seung Yeon Jeong, Satyendra Kumar, Min Sang Park, Jung Ok Park, M. Srinivasarao, Sung Tae Shin Temperature dependence of the three components $\varepsilon _{1}$, $\varepsilon _{2}$, and $\varepsilon _{3}$ of dielectric constant and orientation order parameters in the nematic phase of mesogens with rod, banana, and zero-order dendritic shape were measured using the in-plane and vertical switching geometries, and micro-Raman technique. Results on the well-known uniaxial (N$_{u})$ nematogens, E7 and 5CB, revealed two components $\varepsilon _{1~}$=~$\varepsilon _{\vert \vert }$ and $\varepsilon _{2~}$=~$\varepsilon _{3~}$=~$\varepsilon _{\bot }$, as expected. The three dielectric constants were different for two azo substituted (A131 and A103) and an oxadiazole based (ODBP-Ph-C12) bent core mesogens, and a Ge core tetrapode. In some cases, two of the components became the same indicating a loss of biaxiality at temperatures coinciding with the previously reported N$_{u}$ to biaxial nematic transition. This interpretation is substantiated by micro-Raman measurements of the uniaxial and biaxial nematic order parameters. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J9.00008: The interplay between fluctuations in physical structure and power consumption in electroconvecting liquid crystals John Cressman, Zrinka Greguric, Tyrus Berry, Timothy Sauer We will present results from experiments performed on the nematic liquid crystal MBBA. We have made simultaneous measurements of the optical patterns formed in the weakly-driven electroconvective state, as well as the electrical power consumed by the sample. By performing a dimensionality reduction on the optical data we identify the dominant modes in the system and go on to elucidate the role of these modes in the measured power fluctuations. We will conclude by discussing these results in the context of the free energy derived by de Gennes for nematic liquid crystals. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J9.00009: Magneto-optical technique for detecting biaxial nematic phase Tanya Ostapenko, J.T. Gleeson, S.N. Sprunt, A. Jakli There have been numerous attempts to find a thermotropic liquid crystal that exhibits a biaxial phase. There have been findings of biaxial order in bent-core nematic liquid crystals; however, there are recent reports that call this into question. One reason for this discrepancy is the difficulty in unambiguously identifying the biaxiality. Based on a previously described electro-optical technique, we have developed a technique that uses magnetic field, thus widening its application to any bent-core nematic material. The field orients the uniaxial director along the optical path length, and we search for birefringence perpendicular to this direction. We expect one of two situations to occur: if the material is uniaxial, the induced phase difference will decrease asymptotically to zero as the field increases. However, if the material is biaxial, the induced phase will extrapolate to a non-zero value. Results on one calamitic liquid crystal show that this method yields the expected result, namely the lack of biaxial nematic phase. We also tested several bent-core nematic liquid crystals and found that none of these materials exhibits a biaxial nematic phase. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J9.00010: Nonlinear electrophoresis in nematics: Flows and effects of salts Israel Lazo-Martinez, Oleg D. Lavrentovich Electrophoresis (EP) in a nematic liquid crystal (LC) is dramatically different from its isotropic counterpart, as the EP velocity has a component that is quadratic in the applied electric field [1]. Unlike the regular EP velocity that is linear in the field, this component does not vanish in an ac field with a zero time average, which makes the LC EP attractive for applications where the steady flows are needed. EP propulsion is caused by distortion of the LC orientation around the particles that break the fore-aft (or left-right) symmetry, leading towards an imbalance of field-induced flows around the particles. We visualize the flows and measure the EP velocity by recording 3D trajectories of passive tracers suspended in the LC under the fluorescent confocal polarizing microscope. We demonstrate that doping the LC with organic salts increases the EP velocities. The work was supported by NSF DMR 0906751. \\[4pt] [1] O. D. Lavrentovich, I. Lazo, O. P. Pishnyak, Nature 467, 947-950 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J9.00011: Anisotropic Stokes Drag and Dynamic Lift on Cylindrical Colloids in a Nematic Liquid Crystal Joel Rovner, Clayton Lapointe, Daniel Reich, Robert Leheny Unlike isotropic fluids, nematic liquid crystals exhibit a complex assortment of hydrodynamic properties that can strongly depend on the director field and local boundary conditions set by inclusions. To understand further these characteristics, measurements were taken of the Stokes drag on magnetic nanowires suspended in nematic 4-cyano-4'-pentylbiphenyl (5CB). Effective drag viscosities for wires moving perpendicular and parallel to the nematic director were measured and were found to differ by factors of approximately 0.88 to 2.4, depending on the wire orientation and surface anchoring. Additionally, a lift force was observed when wires were forced at an oblique angle to the director resulting in motion divergent from the line of force. The lift was greater for wires with homeotropic anchoring and smaller for wires with longitudinal anchoring, suggesting that the lift force can act as a mechanism for sorting colloidal particles according to their surface chemistry. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J9.00012: Morphology and Rheology of the Liquid Crystal-Colloid Composites Lu Zou, Chanjoong Kim Liquid crystal (LC)-colloid composites form aggregates and are arrested in various network structures. We study viscoelastic properties and three-dimensional structure of nematic LC-colloid composites using fluorescence confocal polarized rheoscope and fluorescence microscope. We observe various morphological transformations of the composites when we cool them down below $T_{NI}$ from the high temperature isotropic phase. We find that colloidal particles are self-organized to ferny structures, and that the morphological characteristics of the ferny structures depend on the applied shear rates, the cooling rate, the particle volume fraction and the particle size. This study may offer a new route to form novel colloidal structures using anisotopic fluid, which could not be obtained from isotropic suspensions. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J9.00013: Structural Reorganization of Liquid Crystals Revealled by Fast Scanning Calorimeter Dongshan Zhou, Jing Jiang, Xiaoliang Wang, Gi Xue Liquid crystal glass of 4-Cyano-4'-octylbiphenyl is obtained by rapid cooling with rates over 2000 Kelvin per second (K/s) on the chip calorimeter. The glass can crystallize easily upon heated above its glass transition temperature. Depending on the prior cooling rate and annealing history thereafter, melting-structural reorganization-remelting behavior similar to that of semicrystalline polymer can be observed during subsequent heating. The complex melting behavior is attributed to the transformation of metastable crystal forms formed during annealing or heating induced cold crystallization. Increasing the heating rate ($>$15000 K/s) can suppress the transformation and, additionally, enables us to capture the multiple N-I transition. This implies the coexistence of two different types of nematic states. To avoid above complex structural reorganization, one can anneal the sample at 260K for 2 seconds to get the stable crystal form. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J9.00014: Parity breaking in nematic tactoids of lyotropic chromonic liquid crystals Luana Tortora, Oleg D. Lavrentovich In many colloidal systems, an orientationally ordered nematic phase emerges from the isotropic melt in the form of spindle-like birefringent tactoids. In cases studied so far, the tactoids always reveal a mirror-symmetric non-chiral structure, even when the building units are chiral, as in the case of tobacco mosaic virus [1] and fd virus [2]. We report on parity breaking in the nematic tactoids formed in molecularly non-chiral polymer-crowded solutions of lyotropic chromonic liquid crystals. The effect is manifested by twist of the director and optical activity. Fluorescent confocal polarizing microscopy reveals that the tactoids nucleate at boundaries of cells. We explain the chirality induction by the effect of geometrical anchoring [3] and by increase of the splay elastic constant in condensed nematic regions of crowded solutions.\\[4pt] [1] J. D. Bernal and I. Fankuchen, J. Gen. Physiol. \textbf{25}, 111 (1941);\\[0pt] [2] Z. Dogic, Phys. Rev. Lett. \textbf{91}, 165701 (2003);\\[0pt] [3] O.D. Lavrentovich, Phys. Rev. A15 \textbf{46}, R722 (1992) [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J9.00015: Orientational order and defect structures on curved surfaces Subas Dhakal, Francisco J. Solis, Monica Olvera de la Cruz We study the topological defects in a nematic liquid crystal confined to a surface. Using Monte Carlo simulations, we investigate how the position and number of defects depend on the interaction strength, the shape of the surface and other physical parameters. On a spherical surface, we find that the interaction changes the location of four +1/2 defects initially sitting on a great circle of the sphere to the vertices of a tetrahedron. In deformed spheres, we observe the coalescence of defects into two single +1 defects. [Preview Abstract] |
Session J10: Surface and Interfaces with Metals
Sponsoring Units: DCMPChair: Ted Einstein, University of Maryland
Room: D221
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J10.00001: Ordered and disordered surface alloys in Au-Pt(111) and their effect on oxygen adsorption Wei Chen, C. Wolverton, David Schmidt, William Schneider Bimetallic surface alloys are considered a promising type of catalyst for improved activity and selectivity. Understanding surface structure and its effect on catalytic performances plays a critical role in designing catalysts from surface alloys. We have studied the surface structure and ordering of AuPt(111) using a first-principles cluster expansion based method. Even though the Au-Pt system is phase-separating in the bulk, we find a series of thermodynamically stable, laterally ordered striped structures of AuPt(111) surfaces. The formation of such ordered structures is the result of a competition between the strain relaxations from stripes and the unfavorable Au-Pt bonds at stripe interfaces. We have also investigated the oxygen adsorption on these structures. The oxygen binding energy is found to be highly correlated with the type of nearest neighbor surface atoms of oxygen. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J10.00002: Order-Disorder Transitions for Au/Mo(112) Keisuke Fukutani, Yaroslav Losovyj, Natalia Lozova, Ivan Yakovkin, Ning Wu, Peter Dowben Order-disorder overlayer phase transitions are observed at the surface of Au/Mo(112) for the nominal Au coverages of 1.66 and 1.75 monolayers. These transitions are characterized by the abrupt change in the surface Debye temperature. In the search for the detailed mechanism of this phase transition, we investigated the electron-phonon coupling (EPC), in the vicinity of the Fermi level, for the surface states of Au-covered Mo(112) surface from high- resolution angle resolved photoemission data taken parallel to the surface corrugation (i.e. $<$\underline {11}1$>)$. The changes of the widths of the surface weighted bands, induced by Au layers, are discussed in terms of electron-electron interactions, electron-impurity scattering and electron-phonon coupling. Gold overlayers suppress the mass enhancement of the Mo(112) surface band crossing the Fermi level at 0.54 {\AA}$^{-1}$. The data indicate that significant contributions from impurity and defect scattering must be considered in any serious analysis of the imaginary part of the self energy and that there interface effects can have a profound influence on the imaginary part of the self energy. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J10.00003: Ab initio study of Mg self-diffusion on Mg(0001) terraces and steps Maral Aminpour, Marisol Alcantara Ortigoza, Talat Rahman The high density of states (DOS) at the Fermi level and high density of quasi-free electrons result in a singular behavior for Mg surfaces and thin films. We find, however, that the DOS around the Fermi level, surface energy and cohesive energy converge beyond 15 layers. We also show that the Friedel charge density oscillations of Mg(0001) are more complex than depicted previously by 1D and 2D plots. These oscillations are, in fact, responsible for the stacking fault of Mg adatoms and islets on Mg(0001) and also, indirectly, for the low adatom self-diffusion barrier (20 meV) on Mg(0001), which is in agreement with effective-medium theory calculations. [1] We will compare this barrier with that of Mg adatom on a narrow terrace and across the steps on Mg(0001), as well as with predictions from Kinetic Monte Carlo simulations made to fit the growth mode observed for Mg/Mg-thin-films via scanning tunneling microscopy. \\[4pt] [1] Z.J. Tian, U. Yxklinten, B.I. Lundqvist and K.W. Jacobsen.Surf. Sci.\textbf{258} (1991), p. 427 [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J10.00004: Theoretical aspects of studies of high coverage oxidation of the Cu(100) surface using low energy positrons N.G. Fazleev, W.B. Maddox, J.A. Reed The study of adsorption of oxygen on transition metal surface is important for the understanding of oxidation, heterogeneous catalysis, and metal corrosion. The structures formed on transition metal surfaces vary from simple adlayers of chemisorbed oxygen to more complex structures which results from diffusion of oxygen into the sub-surface regions. In this work we present the results of an ab-initio investigation of positron surface and bulk states and annihilation probabilities of surface-trapped positrons with relevant core electrons at the Cu(100) missing row reconstructed surface under conditions of high oxygen coverage. Calculations are performed for various surface and subsurface oxygen coverages ranging from 0.50 to 1.50 monolayers. Calculations are also performed for the on-surface adsorption of oxygen on the unreconstructed Cu(001) surface for coverages up to one monolayer to use for comparison. Estimates of the positron binding energy, positron work function, and annihilation characteristics reveal their sensitivity to atomic structure of the topmost layers of the surface and charge transfer. Theoretical results are compared with experimental data obtained from studies of oxidation of the Cu(100) surface using positron annihilation induced Auger electron spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J10.00005: Deposition of metal onto a sulfur loaded substrate Daeho Kim, Dezheng Sun, Wenhao Lu, Eric Chu, Jon Wyrick, Zhihai Cheng, Ludwig Bartels A Cu(111) surface can be loaded with sulfur to form a variety of surface patterns. In this work, we study the deposition of copper and molybdenum on a Cu(111) surface and the resultant film morphology as a function of the sulfur pre-loading of the substrate. For copper deposition, we find the formation of adstructures of different geometry depending on the sulfur decoration of the substrate. A 0.143 ML S coverage leads to rectangular structure consisting of 6 lobes while a 0.118 ML S coverage leads to 7$\times $7 structure. Notably, annealing allows the sulfur to float up decorating the newly deposited layer. Deposition of molybdenum shows a similar pattern, with ordered MoS2 forming as a result of annealing. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J10.00006: Oxides on Nanoscale Platinum Surfaces Daniel Hennessy, Vladimir Komanicky, Michael S. Pierce, Kee-Chul Chang, Hoydoo You We demonstrate the existence of oxide layers on nanoscale Pt interfaces annealed in an oxygen environment. The sample is a Pt single crystal cut at the midpoint between the 100 and 111 crystal directions; annealing in Ar produces a smooth surface, while annealing in air produces $\sim $10 nm-sized 100 and 111 facets. Synchrotron x-ray crystal truncation rod (CTR) measurements indicate a bilayer Pt oxide structure on the nanofacets. Fitted Pt occupancies are consistent with a nearest-neighbor avoidance structure of the surface oxygen atoms. Electrochemical cycling of the faceted surface in CO-saturated solution removes the oxide and leaves clean, ordered facets. Pt single crystals of 100 and 111 surface orientations prepared the same way did not support an oxide layer. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J10.00007: The amazing kinetic stability of the high temperature ($\sqrt{3}$ $\times$ 6)rect. striped structure of decanethiol SAMs on Au(111) and other interesting properties. A scanning tunneling microscopy study. Lloyd Bumm, Daminda Dahanayaka, Abhijit Biswas, Ronald Halterman We present an STM study of the properties of the high temperature ($\sqrt{3}$ $\times$ 6)rect. phase of decanethiol SAMs on Au (111). Although this phase is known, it has not been extensively studied. We show a simple reliable way to grow the ($\sqrt{3}$ $\times$ 6) rect. phase and show that its coverage is 75\% of the normal (2 $\sqrt{3}$ $\times$ 3)rect. phase. Although it has lower density compared to the normal alkanethiol SAM structure, it shows a remarkable kinetic stability with respect to uptake of additional alkanethiol molecules and reversion to the normal (2$\sqrt{3}$ $\times$ 3) rect. phase. Other properties of the ($\sqrt{3}$ $\times$ 6)rect. phase will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J10.00008: Epitaxial orientations of para-sexiphenyl platelets grown on alkali halide (001) surfaces Edward Kintzel, Detlef Smilgies Thin film growth of simple aromatic molecules has been researched intensely in recent years in the burgeoning field of organic electronics. Film growth for simple rodlike molecules on the atomically well-defined and nonreactive alkali halide (001) surfaces also constitutes an archetypical model system for the study of molecular epitaxy. We have observed a surprising variety of preferential orientations of para-sexiphenyl platelets on a series of alkali halide surfaces with lattice constants ranging from 4.6 to 6.6 Angstroms. We present a metric that helps to classify the dominant epitaxial orientations and allows us to predict epitaxial orientations on other rocksalt-type substrates, and we identified surface corrugation as the driving force for these preferred relative orientations. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J10.00009: Pentacene thin films on vicinal Ag(111) surfaces Fatih Danisman, Ilker Demiroglu, Ersen Mete, Sinasi Ellialtioglu Here we present a structural study of pentacene thin films on different vicinal Ag(111) surfaces by helium atom diffraction measurements and density functional theory (DFT) calculations. Our helium atom diffraction results suggest a step flow growth mechanism evidenced by initial slow specular reflection intensity decay rate as a function of pentacene deposition time. This is in agreement with our previous helium diffraction results on flat Ag(111) surfaces with a small miscut angle. In parallel with the experimental findings, our DFT calculations predict the step edges as the most stable adsorption site on the surface. Isolated pentacene molecules adsorb on the step edges in a tilted configuration with a binding energy of 0.615 eV. In addition a complete monolayer with tilted pentacene on the step edges is found to be more stable than one with all lying flat molecules. Hence our results suggest, in agreement with prvious predictions, that step edges can trap the pentacene molecules and act as nucleation sites for the growth of ordered thin films with a crystal structure similar to that of bulk pentacene. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J10.00010: Scaled Shell-like Pattern Formation of Selenium-Based Anthracene Derivatives at a Metal Surface Ludwig Bartels, Zhihai Cheng, Jonathan Wyrick, Dezheng Sun, Daeho Kim, Yeming Zhu, Miaomiao Luo, Robert Carp, Michael Marsella We investigated the behavior of selenium-substituted anthracene molecules at a Cu(111) surface. In our previous work, the sulfur and oxygen counterparts of this molecule exhibited controlled diffusion on Cu(111) violating the substrate's symmetry. In contrast Diseleno-ateanthracene shows an isotropic and very high mobility suggesting very non-local substrate interactions. However, we observe pronounced sensitivity of the diffusion to the oscillation of the Cu(111) substrate surface state. In this talk, we will focus on the coverage-dependent pattern formation of this species: at coverages close to 1 ML, two kinds of hexagonal patterns with large unit cells are formed. Both of them show a shell superstructure with an identical central empty hole. The smaller of the features one molecular shell of 6 molecules the larger a double shells of 6 and 12 molecules. Another kind of rectangular pattern is also observed, which could be an intermediate superstructure between the small and large hexagonal patterns. We will compare these patterns to prior work on sulfur and oxygen based molecules. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J10.00011: Stable Carbon Nanoarches in the Nucleation of Graphene on Cu(111) Robert Van Wesep, Hua Chen, Wenguang Zhu, Zhenyu Zhang To fully exploit the device potential of graphene, reliable production of large-area, high-quality samples is required. Epitaxial growth on transition metal surfaces have shown promise in this regard, but further improvement would be facilitated by a more complete understanding of the nanoscale processes involved. Using density functional theory calculations, we have investigated the energetics and kinetics of graphene nucleation and growth on a Cu(111) surface. Our calculations have revealed an energetic preference for the formation of stable 1D carbon nanoarches consisting of 3-13 atoms when compared to 2D compact islands. Our findings may provide the structural link between nucleated carbon dimers [1] and larger carbon nanodomes [2], and may also explain some recent experimental observations. We will also present results on estimating the critical cluster size that marks the transition from nanoarch dominance to island dominance in the growth sequence. \\[4pt] [1] Hua Chen, et al., Phys. Rev. Lett. 104, 186101 (2010). \\[0pt] [2] Paolo Lacovig, et al., Phys. Rev. Lett. 103, 166101 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J10.00012: An \textit{Ab Initio} Study of Bulk $\gamma $-U and the (100) Surface Dayla Morrison, Asok Ray The properties of bcc $\gamma $-U have been studied using the formalisms of the generalized gradient approximation to density functional theory (GGA-DFT) and hybrid density functional theory. The computational formalism is the full potential linearized augmented plane wave method as implemented in the suite of software WIEN2k. Computations have been performed both without and with spin orbit coupling (SOC). Results indicate that GGA-DFT with SOC performs comparatively better in the description of the properties of $\gamma $-U, such as the non-magnetic ground state, lattice constant, and the bulk modulus. The predicted lattice constant and bulk modulus are 3.463 A and 114 GPa, respectively, to be compared with the experimental values of 3.467 A and 113 GPa, respectively. For the (100) surface, the monolayer exhibited significant contraction but the lattice constants tend to converge after 5 layers. Based on the results of the five layers, we predict the surface energy and the work function to be 1.46 J/m$^{2}$ and 3.24eV, respectively. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J10.00013: Electronic Structures of Hydrogen and Oxygen Adsorbed Tungsten (3, 2, 0) and Tungsten (8, 7, 0) Surfaces Zhuo Bao, Aaron Bostwick, Eli Rotenberg, Stephen Kevan The Valence band electronic structues of Hydrogen adsorbed and Oxygen adsorbed Tungsten stepped surfaces, Tungsten (3, 2, 0) and (8, 7, 0) surface are investigated using angular-resolved photoemission techniques and ab-initio electronic structure calculation methods. The band features of surface states at different Hydrogen and Oxygen coverages are experimentally distinguished by using photon-energy scanning method. Quasi-one- dimensional band features are found in the surface states with saturated Oxygen coverages of both stepped surfaces. The effects of adsorbate coverages on dimensionalities of surface electronic states are studied using high-resolution band mapping methods and ab-initio calculation methods. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J10.00014: The suppression surface state near a monostep of Au(111) surface studied by low temperature scanning tunneling microscopy Qing Li, Peter Maksymovych, Sergi Kalinin, Minghu Pan The dynamics of electronic states on metal surfaces is a fundamental probe of electron transport and electronic interactions with practical relevance for nanodevices and reactions. In our study, series of scanning tunneling spectroscopy near a monostep of Au(111) surface are used to investigate the behavior of surface state. We found that the Shockley surface state of Au (111) was suppressed near the step. By carefully analyzing each dI/dV spectroscopy, we determined the lateral tip-step distance dependence of the lifetime of the surface electrons. The lifetime broadening of surface state shows linear decay close to the monostep, possibly due to the electron-electron interactions. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J10.00015: Low Energy Hot Electron Scattering in Nanometer Scale Metal Films Using Ballistic Electron Emission Microscopy John Garramone, Joseph Abel, Vincent LaBella Inelastic and elastic scattering lengths of hot electrons have been measured at low energies ($<$2~eV) in nanometer thick silver films utilizing ballistic electron emission microscopy (BEEM). BEEM is a scanning tunneling microscopy (STM) based technique that is capable of injecting electrons a few eV above the Fermi level and utilizes a third collector contact on the semiconductor of a Schottky diode\footnote{L. D. Bell, et al., Phys. Rev. Lett. 61 2368 (1988)}. Electrons tunnel from the STM tip into the metal base layer and a small fraction of these electrons travel ballistically to the metal/semiconductor interface. Electrons with energy greater than the Schottky barrier height (SBH) are collected as BEEM current. The silver attenuation length is extracted by measuring the BEEM current as a function of the Ag overlayer thickness over a series of samples for both electron and hole injection. The relative contribution of inelastic and elastic scattering is extracted by modeling the change in attenuation length with respect to the tip bias. A drastic increase in the attenuation length is observed as energies approach the SBH, which we attribute to the ballistic nature of the electrons and holes that are collected at these energies. [Preview Abstract] |
Session J11: Fractional Quantum Hall Effect I
Sponsoring Units: FIAPChair: Nikolai Zhitenev, National Institute of Standards and Technology
Room: D222
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J11.00001: The Real-Space Entanglement Spectra of Fractional Quantum Hall States A. Chandran, Parsa Bonderson, Nicolas Regnault, Andrei Bernevig We investigate the entanglement spectra arising from a sharp real-space cut on the topologically ordered fractional quantum hall (FQH) ground states. We find that the counting of the real-space entanglement spectra (the number of edge excitations of the liquid) is identical to the number of bulk quasihole excitations, in accordance with the bulk-boundary correspondence. The spectra of cuts with two edges display the shape and counting of counter-propagating non-interacting modes as well. Initial estimates of the topological entanglement entropy seem to be in agreement with theory. The real-space entanglement spectra also allows us to distinguish between particle-hole conjugate states, providing us with a new probe to interacting edge modes. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J11.00002: Identification of 331 quantum Hall states with Mach-Zehnder interferometry Chenjie Wang, D.E. Feldman It has been shown recently that non-Abelian states and the spin-polarized and unpolarized versions of the Abelian 331 state may have identical signatures in Fabry-P\'{e}rot interferometry in the quantum Hall effect at filling factor 5/2. We calculate the Fano factor for the shot noise in a Mach-Zehnder interferometer in the 331 states and demonstrate that it differs from the Fano factor in the proposed non-Abelian states. The Fano factor depends periodically on the magnetic flux through the interferometer. Its maximal value is $2\times 1.4e$ for the 331 states with a symmetry between two flavors of quasiparticles. In the absence of such symmetry the Fano factor can reach $2\times 2.3e$. On the other hand, for the Pfaffian and anti-Pfaffian states the maximal Fano factor is $2\times 3.2e$. The period of the flux dependence of the Fano factor is one flux quantum. If only quasiparticles of one flavor can tunnel through the interferometer then the period drops to one half of the flux quantum. We also discuss transport signatures of a general Halperin state with the filling factor $2+k/(k+2)$.\\[4pt] [1] Chenjie Wang and D. E. Feldman, Phys. Rev. B {\bf 82}, 165314 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J11.00003: Bulk excitonic currents in a bilayer quantum Hall system and Andreev reflection A.D.K. Finck, J.P. Eisenstein, L.N. Pfeiffer, K.W. West Bilayer 2D electron systems in the quantum Hall regime can support a novel interlayer coherent phase which may be viewed as a Bose condensate of interlayer excitons. While numerous experiments over the past decade have revealed a host of remarkable properties of this strongly correlated quantum fluid, heretofore none have directly demonstrated the transport of excitons across the electrically insulating bulk of the system. We report here just such an observation. Our experimental results show that excitons may be launched into the bulk of the 2D system via a process analogous to Andreev reflection. Excitons are emitted into the bulk of the bilayer when electrons are injected into one 2D layer and withdrawn from the other along a common edge of the system. Similarly, we demonstrate that excitons arriving at the edge of the Hall droplet can drive current through external circuitry connected to contacts along that edge. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J11.00004: Phase diagram of the even-denominator fractional quantum Hall state at $\nu$ = 1/2 in wide quantum wells Javad Shabani, Mansour Shayegan We have studied the fractional quantum Hall (FQH) effect in very high quality two dimensional electrons confined to GaAs single wide quantum wells. In these systems typically two electric subbands are occupied at zero magnetic field and the electron charge distribution in the quantum well is bilayer-like. For a symmetric charge distribution and appropriate electron density, a unique even-denominator FQH state emerges at filling factor $\nu$ = 1/2 which has no counter-part in standard, single-layer systems [1]. We have revisited this problem by studying wide quantum well samples with narrower well widths, $47 \leq w \leq 64$ nm, and hence larger tunneling, $\Delta$ (up to 35 K). The new $\nu$ = 1/2 data in these narrower samples allow us to expand the $d/l_{B}$ vs. $\Delta/(e^{2}/4 \pi \epsilon l_{B})$ phase diagram for the stability of the $\nu$ = 1/2 FQH state (d is the layer distance and $l_{B}$ is the magnetic length). Based on this phase diagram, we find that, it is not clear whether this state has a Pfaffian or a two-component Halperin origin. \\[4pt] [1] Y. W. Suen et al., Phys. Rev. Lett. 72, 3405 (1994). [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J11.00005: When is the fractional quantum Hall effect stable Yang Liu, Javad Shabani, Mansour Shayegan The fractional quantum Hall (FQH) effect, signaled by the vanishing of the longitudinal resistance and the quantization of the Hall resistance, is the hallmark of interacting two-dimensional electrons in a large perpendicular magnetic field. The effect is most prominently observed at low Landau level (LL) filling factors ($\nu$) and is conspicuously absent for $\nu>4$. We examine the stability of the FQH states at high fillings in a 2D electron system in a wide GaAs quantum well which we can tune the Fermi energy ($E_F$) to lie, at a given filling factor, in different LLs of two electric subbands. The data provide direct and definitive evidence that the stability of the FQH states is linked to the LL where $E_F$ resides. We observe FQH states at high filling factors such as 13/3, 14/3, 16/3, and 17/3, but only when $E_F$ lies in the ground state ($N=0$) orbital LLs of either of the two electric subbands, regardless of the underlying, fully occupied levels. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J11.00006: Evolution of odd-denominator fractional quantum Hall states in a two-subband system Mansour Shayegan, Javad Shabani, Yang Liu Our magneto-transport measurements reveal that the sequence of fractional quantum Hall (FQH) states observed in two-subband, wide GaAs quantum wells at high fillings ($\nu >$ 2) are very different from those of a single-subband system. When the Fermi level lies in the lowest Landau level of either of the two subbands the odd-denominator FQH states following the usual, composite fermion filling sequences are observed. These include states at $\nu $ = 7/3, 8/3, 12/5, 13/5, 10/3, 11/3, 17/5, 18/5, and 25/7. The evolution of these states with changing the Zeeman and subband energies is consistent with coincidences of composite fermion Landau levels. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J11.00007: Negative spin wave dispersion for composite fermions U. Wurstbauer, D. Majumder, S. Mandal, I. Dujovne, A. Rigosi, T.D. Rhone, B. Dennis, K. West, L. Pfeiffer, J. Jain, A. Pinczuk The FQHE is a result of strongly interacting electrons that can be understood as QHE of composite fermions. We use inelastic light scattering experiments to study the collective excitations of CF with 2 flux quanta focusing on filling factors $\nu$ = 4/9, 3/7 and 2/5. For these fillings, the lowest collective excitation modes are spin-waves, which display a distinct spectral weight below the bare Zeeman energy indicating a negative dispersion relation. The determined energies for these ``spin-wave roton minima'' are in excellent quantitative agreement with numerical calculations. Using the real experimentally transferred momentum the addressed DOS and hence inelastic light scattering spectra can be modeled. We demonstrate that the observed modes are very similar for positive and negative effective magnetic field at the same CF-filling factor. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J11.00008: Subband Engineering Even-Denominator Quantum Hall States Vito Scarola, Christian May, Michael Peterson, Matthias Troyer Proposed even-denominator fractional quantum Hall effect (FQHE) states suggest the possibility of excitations with non-Abelian braid statistics. Recent experiments on wide square quantum wells observe even-denominator FQHE even under electrostatic tilt. We theoretically analyze these structures and develop a procedure to accurately test proposed quantum Hall wavefunctions. We find that tilted wells favor partial subband polarization to yield Abelian even-denominator states. Our results show that tilting quantum wells effectively engineers different interaction potentials allowing exploration of a wide variety of even-denominator states. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J11.00009: Quasiparticle tunneling amplitute in fractional quantum Hall states Zixiang Hu, Kihoon Lee, Edward H. Rezayi, Xin Wan, Kun Yang We study qp tunneling in the MR state, in which qp of charge e/4 and e/2 may co-exist and both contribute to edge transport. The tunneling amplitude for charge e/2 qp is exponentially smaller than that for e/4 qh, and the ratio between them can be (partially) attributed to their charge difference. The tunneling amplitude shows some scaling behavior which originates from the propagation and tunneling of charged qhs in an effective field analysis. In the ring limit, we conjecture the exact functional form for several cases. The results for Abelian qp tunneling is consistent with the scaling anaysis; this allows for the extraction of conformal dimensions of the qps. We analyze the scaling behavior of both Abelian and non-Abelian qps in the Z$_k$ parafermion states. Interestingly, the non-Abelian qp tunneling amplitudes exhibit nontrivial $k$-dependent corrections to the scaling exponent. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J11.00010: Extracting Excitations From Model State Entanglement Nicolas Regnault, Antoine Sterdyniak, Andrei Bernevig We extend the concept of entanglement spectrum from the geometrical to the particle bipartite partition. We apply this to several Fractional Quantum Hall wavefunctions on both sphere and torus geometries to show that this new type of entanglement spectra completely reveals the physics of bulk quasihole excita tions. While this is easily understood when a local Hamiltonian for the model state exists, we show that the quasiholes wavefunctions are encoded within the model state even when such a Hamiltonian is not known. As a nontrivial example, we look at Jain's composite fermion states and obtain their quasiholes directly from the model state wavefunction. We reach similar conclusions for wavefunctions described by Jack polynomials. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J11.00011: Neutral mode heat transport and fractional quantum Hall shot noise So Takei, Bernd Rosenow We study nonequilibrium edge state transport in the fractional quantum Hall (FQH) regime for states with a counter-propagation neutral mode. Focusing on the filling fraction of 2/3, we consider a setup in which the neutral mode is heated by a hot spot, and where heat transported by the neutral mode causes a temperature difference between the upper and lower edges in a Hall bar. This temperature difference is probed by the excess noise it causes for scattering across a quantum point contact (QPC). We find that the excess noise in the QPC provides evidence for counter-propagating neutral modes, and we calculate its dependence on both the temperature difference between the edges and on source drain bias. We generalize our results to the non-abelian Moore-Read quantum Hall state at filling fraction 5/2. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J11.00012: Anisotropic quantum Hall liquids - a small system Monte Carlo study Orion Ciftja, Brittney Cornelius, Kesha Brown, Emery Taylor While no fractional quantum Hall effect states were expected to stabilize in the second excited Landau level, the discovery of extreme magneto-transport anisotropy around filling factor 9/2 was found quite surprising. A unidirectional charge density wave is a plausible candidate for the anisotropic states as indicated by the earlier theoretical work. An alternative approach that would be consistent with observed experimental facts would view the onset of anisotropy as signature of a phase transition from an isotropic to an anisotropic liquid crystalline phase. In this work we present a small-system Monte Carlo study for anisotropic quantum Hall liquid states observed at filling factor 9/2. The anisotropic electronic liquid phases are described by a broken rotational symmetry wave function and electrons interact with Landau level-projected interaction potentials. Our small-system Monte Carlo study indicates that such an anisotropic liquid crystalline quantum Hall phase with broken rotational symmetry is energetically favored relative to an isotropic liquid one. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J11.00013: Chiral Abelian anyons from interacting non-Abelian vortices Ville Lahtinen, Jiannis Pachos We demonstrate the existence of a new topologically ordered phase in Kitaev's honeycomb lattice model. This new phase appears due to the presence of a tightly packed vortex lattice and it supports chiral Abelian anyons. We characterize the phase by its low-energy behavior that is described by four Fermi points as opposed to two Fermi points in the absence of the vortex lattice. This doubling is shown to be related to an emergent vortex lattice symmetry that arises due to interactions between the anyonic vortices. By mapping the Hamiltonian of the model to a BCS one, we show that the chiral Abelian phase can be understood as two coupled p-wave superconductors, one living on the original honeycomb lattice and the other on the dual lattice that coincides with the vortex lattice. Finally, we identify two physically distinct types of topological phase transitions in the model and show that the Fermi surface evolution associated with them is described by Dirac fermions coupling to chiral gauge fields. The study of the Fermi point transport across the Brillouin zone enables us to obtain analytic results on the extended phase space. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J11.00014: The Weakly Coupled Pfaffian as a Type I Quantum Hall Liquid S.A. Parameswaran, S.A. Kivelson, S.L. Sondhi, B.Z. Spivak The Pfaffian phase of electrons in the proximity of a half-filled Landau level is understood to be a $p+ip$ superconductor of composite fermions. We consider the properties of this paired quantum Hall phase when the pairing scale is small, {\it i.e.} in the weak-coupling, BCS, limit, where the coherence length is much larger than the charge screening length. We find that, as in a Type I superconductor, the vortices attract so that, upon varying the magnetic field from its magic value at $\nu=5/2$, the system exhibits Coulomb frustrated phase separation. We propose that the weakly and strongly coupled Pfaffian states exemplify a general dichotomy between Type I and Type II quantum Hall fluids. [Preview Abstract] |
Session J12: Focus Session: Electricity-to-Light Conversion: Solid State Lighting II
Sponsoring Units: GERA DMPChair: Fred Schubert, Rensselaer Polytechnic Institute
Room: D223/224
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J12.00001: Surface origin of the LO phonon feature in the Raman spectrum of InN Esther Alarcon-Llado, Nate Miller, Marie Mayer, Joel W. Ager Wurtzite InN presents an intrinsic Fermi level pinning above the conduction band edge at the surface. For this reason, a large surface electron accumulation (SEA) occurs in InN. The interaction between the free electrons at the surface and the longitudinal optical (LO) phonon has been addressed by previous studies, but questions still remain. Here, we use the insulating nature of the Helmholtz layer that forms on a surface of an object in contact with an an aqueous electrolyte to apply potentials to n-type (undoped) and p-type (Mg-doped) InN. Applying a potential to the electrolyte changes the electric field and distribution of free carriers near the surface. This enables us to perform electrolyte gated Raman spectroscopy on differently doped InN layers and while simultaneously modulating and probing the SEA region in InN. We find that the intensity of the forbidden LO Raman feature is changed by the external potential, showing that this feature is due, at least in part, to the SEA. This is the first experimental evidence of the relation between the LO feature in InN and free electron accumulation at its surface. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J12.00002: Hydrogen incorporation in high hole density GaN:Mg M.E. Zvanut, Y. Uprety, J. Dashdorj, M. Moseley, W. Alan Doolittle We investigate hydrogen passivation in heavily doped p-type GaN using electron paramagnetic resonance (EPR) spectroscopy. Samples include both conventionally grown GaN (10$^{19}$ cm$^{-3}$ Mg, 10$^{17}$ cm$^{-3}$ holes) and films grown by metal modulation epitaxy (MME), which yielded higher Mg (1-4x10$^{20}$ cm$^{-3})$ and hole (1-40x10$^{18}$ cm$^{-3})$ densities than found in conventionally grown GaN. The Mg acceptor signal is monitored throughout 30 minute annealing steps in N$_{2}$:H$_{2}$ (92{\%}:7{\%})) and subsequently pure N$_{2}$. N$_{2}$:H$_{2}$ heat treatments of the lower hole density films begin to reduce the Mg EPR intensity at 750 $^{o}$C, but quench the signal in high hole density films at 600 $^{o}$C. Revival of the signal by subsequent N$_{2}$ annealing occurs at 800 $^{o}$C for the low hole density material and 600 $^{o}$C in MME GaN. The present work highlights chemical differences between heavily Mg doped and lower doped films; however, it is unclear whether the difference is due to changes in hydrogen-Mg complex formation or hydrogen diffusion. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J12.00003: Enhancing erbium emission by strain engineering in GaN heteroepitaxial layers I-Wen Feng, Jing Li, Ashok Sedhain, Jingyu Lin, Hongxing Jiang, John Zavada Rare earth doped III-nitrides have been intensively studied due to their intra-4$f$ transitions covering the window from visible emissions to infrared wavelengths. Trivalent Erbium (Er$^{3+})$ has driven particular interests since the intra-4$f$ transition from its first excited ($^{4}$I$_{13/2})$ to the ground state ($^{4}$I$_{15/2})$ gives 1.54 $\mu $m emission, which sits in the low optical loss band of silica fibers and potentially affords light emitters and optical amplifiers at optical communication wavelength. Due to the structural and thermal stability of GaN, GaN appears to be the promising candidate as the host semiconductors. We prepared Er doped GaN (GaN:Er) samples by metal organic chemical vapor deposition. GaN:Er epilayers were simultaneously grown on different templates, including GaN/Al$_{2}$O$_{3}$, AlN/Al$_{2}$O$_{3}$, GaN/Si(111), and c-GaN bulk. The effects of stress, caused by the lattice mismatch between GaN:Er epilayers and the substrates, on the intensity of 1.54 $\mu $m emission were probed. The emission intensity at 1.54 $\mu $m increased with greater tensile stress in the c-direction of GaN:Er epilayers. The correlation between stress and 1.54 $\mu $m emission will be presented. The results implied the potential to design efficient photonic devices based on GaN:Er semiconductors. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J12.00004: Effect of electrostatic image charge effect on the photoluminescence in Gallium droplet coated AlGaAs-GaAs Single Quantum Wells Karol Gryczynski, Jie Lin, Tony LLopis, Zhiming Wang, G. Salamo, Arkadii Krokhin, Arup Neogi Gallium (Ga) droplets deposited on the cap layers of AlGaAs-GaAs single quantum wells (QWs) lead to a large blue shift in the observed photoluminescence of quantum wells compared to identical single wells without Ga droplets. Furthermore the intensity of the blue shifted emission peaks is enhanced with respect to the reference QWs. As the emission energies for all depths of QWs used (1.5nm to 10nm) exceed the plasmon resonance energy of the deposited Ga droplets, surface Plasmon polariton interactions cannot account for an increase in exciton recombination energy of about 20meV it is concluded that the blue shift and enhancement from the Ga droplets is not plasmonic in nature. The observed phenomena are described and modeled by applying an additional electrostatic potential to the confined excitons within the QW. An electrostatic attraction between the confined exciton and inhomogeneous nanoscale metal surfaces exert forces on the carriers both parallel and perpendicular to the surface of the well. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J12.00005: Polarization induced doping in graded AlGaN films Morgan Ware, Shibin Li, Vasyl Kunets, Michael Hawkridge, Paul Minor, Jiang Wu, Gregory Salamo The fixed polarization field which is intrinsic to nitride based III-V semiconductors in the wurtzite crystal phase can be manipulated during growth by varying the alloy composition. We report on initial experiments to use the space charge field which results from changing the internal polarization field of graded AlGaN films in a simple p-n junction device. Our devices are fabricated from films which are graded from GaN to AlGaN then reverse graded back to GaN without the intentional addition of impurity dopants. Structural characterization of the films is reported through X-Ray diffraction rocking curves and reciprocal space maps, and the rectifying behavior of the device is demonstrated through temperature dependent I-V measurements. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J12.00006: Electroluminescence from $n-n$ isotype heterojunctions of graded-band-gap ZnMgO:Al and ZnO films Jong-Gul Yoon, Sung Woo Cho, Woo Seok Choi, Dae Yeol Kim, Joonhee Lee, Chang Oh Kim, Hojoon Chang, Heonsu Jeon, Suk-Ho Choi, Tae Won Noh We report room temperature electroluminescence (EL) from $n-n$ isotype heterojunction composed of Al-doped graded-band-gap Zn$_{1-x}$Mg$_{x}$O ($g$-ZnMgO:Al) and ZnO films fabricated on Pt/Ti/SiO$_{2}$/Si substrates. The graded-band-gap of $g$-ZnMgO:Al film was investigated by spectroscopic ellipsometry and found to change contiuously from 3.22 to 3.56 eV. The EL emission spectra covered visible and near infrared regions under unipolar operation condition, with $g$-ZnMgO:Al as positive, at the operation voltages as low as 3-5 V. Impact ionization/excitation process in a narrow region of the graded layer was suggested as a possible origin of the EL. We discussed multistep excitation process mediated by defect-related deep levels and the effect of quasi-electric field in the graded-band-gap layer in conjunction with the apparent upconversion EL in the heterojuncition device. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J12.00007: Efficiency Droop in III-nitride LEDs: a differential carrier lifetime analysis Invited Speaker: GaN-based LEDs suffer from a phenomenon known as efficiency droop, which causes a (non- thermal) roll-over of the IQE at high current density, and whose underlying physical origin is not well understood. Identifying the correct process is of importance, as it dictates which strategies can be employed to quench or mitigate droop. Among the most often cited hypotheses are: localization effects related to InGaN alloy fluctuations, leakage effects, and Auger scattering. In this contribution, we will present recent experimental results which aim at testing these scenarios. We will first show why droop appears to be a bulk-like phenomenon, rather than transport-related. We will present PL measurements to illustrate how droop scales with carrier density, and biased-PL measurements which quantify the magnitude of the leakage current. In a second part, we will present differential carrier lifetime measurements, which aim at characterizing the various recombination processes in InGaN heterostructures. We will review recently published results, which show that droop is caused by the onset of a high-order non-radiative process, and confirm that lifetimes are quantitatively compatible with the hypothesis of Auger scattering. Finally, we will present new lifetime measurements on QW samples with various In contents, and discuss how the variations in droop can be explained by the impact of piezoelectric fields on the carrier lifetime. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J12.00008: MOCVD Growths of Linearly-Shaped Staggered InGaN Quantum Wells Light-Emitting Diodes Hongping Zhao, Jing Zhang, Takahiro Toma, Guangyu Liu, Jonathan Poplawsky, Volkmar Dierolf, Nelson Tansu High-efficiency InGaN-based quantum wells (QWs) light-emitting diodes (LEDs) play an important role in solid state lighting. However, the existence of both spontaneous and piezoelectric polarization fields in III-Nitride semiconductors leads to severe charge separation in InGaN QWs, which significantly reduces the electron-hole wavefucntion overlap ($\Gamma _{e\_hh})$ in InGaN QWs. In this work, the growths of linearly-shaped (LS) staggered InGaN QWs LEDs are investigated. The InGaN QWs with LS staggered In-content profile were grown by metalorganic chemical vapor deposition (MOCVD). The use of LS staggered In-contents in InGaN QWs results in improved electron-hole wavefunction overlap ($\Gamma _{e\_hh})$, in comparison to that of conventional InGaN QW. The power dependent cathodoluminescence (CL) measurement shows 2.5-3.5 times enhancement of CL intensity for LS staggered InGaN QWs as compared to that of the conventional InGaN QWs. Theoretical calculations using self-consistent 6-band \textbf{\textit{k.p}} method were performed for both LS staggered InGaN QWs and conventional InGaN QWs. The experimental measurements show good agreement with the theoretical simulation. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J12.00009: Indirect Auger recombination in nitride light emitters Emmanouil Kioupakis, Patrick Rinke, Kris T. Delaney, Chris G. Van de Walle Nitride-based light emitters suffer from an efficiency loss at high drive currents (droop), which limits their high-power performance. The origin of this efficiency droop is not fully understood, and several loss mechanisms have been suggested as its cause. One such mechanism is Auger recombination, a three-carrier non-radiative recombination process that dominates over the radiative one at high carrier densities. We have employed first-principles computational techniques to show that Auger recombination is strong in nitride materials and therefore a likely cause of the droop in nitride LEDs. The underlying microscopic Auger recombination processes occur in an indirect way, mediated by electron-phonon and alloy scattering. Our work elucidates the origin of the droop and suggests ways to improve the high-power efficiency of nitride LEDs. This work was supported by the Center for Energy Efficient Materials, an Energy Frontier Research Center funded by the U.S. DOE, by the UCSB Solid State Lighting and Energy Center, and by the NSF MRSEC Program. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J12.00010: Micro- and Nanostructural Properties of Wide Bandgap Semiconductors Nabil Dawahre, Gang Shen, Shawn David Wilbert, Nick Harris, William Baughman, Lee Butler, Joseph Brewer, Seongsin Margaret Kim, Patrick Kung Wide bandgap semiconductor materials based on ZnO and GaN have attracted considerable attention in recent years because of the practical applications such as green and blue light emitting and laser diodes, solid-state lighting, photovoltaics, RF and microwave electronics, and gas sensors. However, the resulting device performance strongly depends on the quality, both compositionally and structurally, of the constituent materials. Here we report the combined use of high resolution transmission electron microscopy imaging, micro-Raman and micro-photoluminescence spectroscopy, and local electrode atom probe tomography to understand the micro- and nanostructural properties of materials synthesized by chemical vapor deposition, including defects and impurities. Growth and doping process, sample preparation, and analysis results will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J12.00011: Auger Recombination in Defect-Free III-Nitride Nanowires Meng Zhang, Wei Guo, Pallab Bhattacharya, Junseok Heo, Animesh Banerjee Defect free InGaN nanowires (NWs) and InGaN/GaN dot-in-nanowires (DNWs) were grown on (001) Si by plasma assisted molecular beam epitaxy. The nanowires have a density of $\sim $ 1x10$^{11 }$cm$^{-2}$ and exhibit photoluminescence emission peak at $\lambda \sim $500 nm. The Auger recombination coefficients of these nanowires are determined by excitation power dependent photoluminescence and time-resolved photoluminescence techniques. The measured Auger coefficients are 6.1x10$^{-32}$ cm$^{6}\cdot $s$^{-1}$ and 4.1x10$^{-33}$ cm$^{6}\cdot $s$^{-1}$, in the NW and DNW samples, respectively, which are nearly two orders of magnitude lower than those measured in InGaN/GaN quantum wells and agree very well with theoretical calculations. This suggests that the abnormally high Auger coefficients measured in traditional wide bandgap nitride materials is related to the high density of dislocations. InGaN NW and InGaN/GaN DNW light emitting diodes are demonstrated. The external quantum efficiency does not decrease up to an injection current density of 400A/cm$^{2}$. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J12.00012: Effects of oxygen annealing treatment on formation of ohmic contacts to n-GaN Wenting Hou, Theeradetch Detchprohm, Christian Wetzel Low-resistance ohmic contacts are essential for the fabrication of efficient light emitting diodes (LEDs). A commonly used ohmic contact to n-type GaN is a layer sequence of Ti/Al/Ti/Au, followed by rapid thermal annealing (RTA) in nitrogen ambient at a high temperature. We present an ohmic contact on n-GaN by a surface treatment process of rapid thermal annealing (RTA) in oxygen ambient before the n-metal deposition. As deposited n-contacts are not linear. After RTA in nitrogen ambient, ohmic contact are obtained. The annealed n-contact degrades during the p-metal anneal in oxygen ambient. However, if the sample is annealed in oxygen ambient prior to the metal deposition, the as-deposited n-contact on the treated surface is ohmic, similar to or even better than processes optimized for single-type contacts. This benefit was seen for both, mesa-etched and as-grown n-GaN epi layers. Contacts improvement is also found on unintentionally doped GaN (u-GaN). Oxygen ambient is crucial in the treatment and RTA in nitrogen ambient fail to give ohmic contacts. XPS analysis of the surface shall give us more information on the mechanism of the treatment. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J12.00013: Strong non-plasmonic mechanism of light emission from semiconductor quantum well Arkadii Krokhin, Antony Llopis, Arup Neogi, Sergio Pereira, Ian Watson We report a new mechanism of enhancement of light emission from InGaN/GaN quantum wells (QW). This mechanism is due to electrostatic attraction of the carriers to gold nanoparticles (NP) imbedded within a QW. Metal equally attracts electrons and holes, causing the carriers to concentrate near its surface. Since the probability of e-h recombination is proportional to carrier densities, the QW with NPs generates a stronger emission than the same QW without the NP. We observed roughly a $60\%$ ($80\%$) enhancement with NPs at room temperature (77K). In these nitride heterostructures, dislocations result in hexagonal pits at the surface. Gold NPs were incorporated inside the pits with no effect on the quality. The same nitride material system used to demonstrate {\it plasmonic} enhancement [1]. It has been shown that gold film {\it do not} enhance the emission because of mismatch of surface plasmon energy and the emission energy of the QW. Here we observe the enhancement caused by electrostatic mechanism that does not require energy matching. This mechanism provides another means for enhancing the efficiency of solid- state emitters. This work is supported by the DOE grant \# DE-FG02-06ER46312. \\[4pt] [1] K.Okamoto, \textit{et al.}, \textit{Nature Mat.} \textbf{3}, 601 (2004). [Preview Abstract] |
Session J13: Focus Session: Jamming Theory and Experiment II
Sponsoring Units: GSNPChair: Leonardo Silbert, Southern Illinois University
Room: D225/226
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J13.00001: Jammed particulate systems are inherently nonharmonic Invited Speaker: Normal mode analysis in the harmonic approximation underlies most of solid-state physics and applies well to both ordered and dis- ordered systems. Naturally, researches apply this analysis to jammed particulate systems, such as granular media, colloids, and foams, that interact via one-sided interactions, which are nonzero only when particles overlap. However, we find that systems with one-sided repulsive interactions possess no linear, harmonic response regime for large systems ($N\rightarrow\infty$) at finite pressures $P$, and for all $N$ near jamming onset $P\rightarrow 0$. We perform simulations on 2D frictionless bidisperse mechanically stable disk packings over a range of packing fractions $\Delta \phi = \phi-\phi_J$ above jamming onset $\phi_J$. We apply perturbations with amplitude $\delta$ to the packings along each eigen-direction from the dynamical matrix and determine whether the response of the system evolving at constant energy remains in the original eigenmode of the perturbation. For $\delta > \delta_c$, a single contact breaks and fluctuations abruptly spread to all discrete harmonic modes. As $\delta$ increases further all harmonic modes disappear into a continuous frequency band. We find that $\delta_c \sim \Delta \phi/N$, and thus jammed particulate systems are inherently nonharmonic with no linear vibrational response regime as $N\rightarrow \infty$ over the full range of $\Delta \phi$, and as $\Delta \phi \rightarrow 0$ at any $N$. This breakdown of harmonic behavior dramatically affects all aspects of system response including heat capacity, density of states, elastic moduli, and energy propagation. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J13.00002: Vibrational density of states for granular solids Carl Schreck, Thibault Bertand, Mark Shattuck, Corey O'Hern It was recently shown that granular packings composed of frictionless particles with purely repulsive contact interactions are strongly anharmonic. When perturbed along an eigenmode of the static packing (in the harmonic approximation), energy leaks from the original mode of vibration to a continuum of frequencies even when the system is under significant compression due to the breaking of the weakest contact. In light of this, we perform numerical simulations to measure the displacement matrix averaged over fluctuations and the associated eigenspectrum of weakly vibrated frictionless packings, which possess well-defined equilibrium positions that are different than those of the nearest static packing. We find that there is an increase in the number of low-frequency eigenmodes of the displacement matrix in the harmonic approximation (over the number of low-frequency modes in the static case) and these modes provide a more accurate description of the system dynamics. We also investigate the extent to which these results hold for systems with continuous potentials with repulsive and attractive interactions. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J13.00003: Spatiotemporally resolved granular acoustics Eli Owens, Karen Daniels Acoustic techniques provide a non-invasive method of characterizing granular material properties; however, there are many challenges in formulating accurate models of sound propagation due to the inherently heterogeneous nature of granular materials. In order to quantify acoustic responses in space and time, we perform experiments in a photoelastic granular material in which the internal stress pattern (in the form of force chains) is visible. We utilize two complementary methods, high-speed imaging and piezoelectric transduction, to provide particle-scale measurements of the amplitude of the acoustic wave. We observe that the average wave amplitude is largest within particles experiencing the largest forces. The force-dependence of this amplitude is in qualitative agreement with a simple Hertzian-like model for contact area. In addition, we investigate the power spectrum of the propagating signal using the piezoelectric sensors. For a Gaussian wave packet input, we observe a broad spectrum of transmitted frequencies below the driving frequency, and we quantify the characteristic frequencies and corresponding length scales of our material as the system pressure is varied. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J13.00004: Edge Effects in Jammed Systems Carl Goodrich, Wouter Ellenbroek, Andrea Liu Packings of spheres at zero temperature and shear stress exhibit a
jamming/unjamming transition as a function of density. For spheres
that repel when they overlap and do not otherwise interact, packings
are jammed with a nonzero static shear modulus at high densities. As
density decreases towards the unjamming transition, the number of
interacting neighbors per particle, $z$, decreases towards a critical
value $z_c$, so that at the unjamming transition the system just has
the minimum number of interacting neighbors to be mechanically stable.
In 2005, Wyart, et al. [1] proposed that there is a diverging length
scale, $l^*$, associated with this transition, that can be understood
from a ``cutting argument." Thus, if one cuts a cluster of linear
dimension $L$, the cluster will have zero-frequency vibrational modes
(soft modes) only for $L |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J13.00005: Dislocations jam at any density Georgios Tsekenis, Nigel Goldenfeld, Karin Dahmen Crystalline materials deform in an intermittent way via dislocation-slip avalanches. Below a critical stress, the dislocations are jammed due to long-range interactions and the material exhibits plastic response, while above this critical stress the dislocations are mobile (the unjammed phase) and the material fails. We use dislocation dynamics and scaling arguments to show that the critical stress grows with the square root of the dislocation density. Consequently, dislocations jam at any density, in contrast to granular materials, which only jam below a critical density. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J13.00006: Jammed packings of bumpy spherical particles Dominic Kwok, K. Vijay Kumar, Carl Schreck, Corey O'Hern, Mark Shattuck Static packings of soft frictionless spheres are a simple model to understand the jamming transition in granular media, and have provided great insight. However, friction in granular media plays an important role in determining the structural and mechanical properties of jammed packings. In particular, the number and location of contacts near the Coulomb sliding threshold is strongly correlated with plastic rearrangements. To better understand friction, we numerically generate jammed packings of bumpy spherical particles as a function of the rms roughness of the particles without incorporating {\it ad hoc} single contact frictional forces between particles, {\it i.e.} frictional contacts in the Hertz-Mindlin (HM) model. The frictional interactions in the bumpy particle model emerge in a natural way via the interdigitation of bumps between contacting particles. We calculate the number of contacts, packing fraction, interparticle forces, eigenmodes of the dynamical matrix, and mechanical properties of jammed packings of bumpy particles and compare our results with those obtained using the HM model. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J13.00007: Shock Waves in Jammed Solids Leopoldo Gomez, Ari Turner, Martin van Hecke, Vincenzo Vitelli We study shock propagation in two-dimensional jammed packings of soft repulsive spheres with Herzian contacts. The critical amplitude above which acoustic waves propagate as shocks displays power law scaling with density and vanishes as the jamming point is approached. Thus close to jamming elastic energy is mainly propagated in the form of shock waves. We determine the characteristic speed and attenuation of the resulting shocks as a function of the amplitude of the initial impulse and applied load. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J13.00008: Jamming: A Peek Behind the Curtain Martin van Hecke We will discuss two of the ``dirty little secrets'' concerning the (anomalous) scaling of the elastic moduli near jamming. First, there is no linear response near jamming, and we propose a novel scaling law for the onset of rearrangements as function of number of particles and distance to the jamming point. Second, the elastic moduli obtained from the dynamical matrix have a very broad distribution, and we discuss how to deal with packings with negative elastic moduli. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J13.00009: Local anisotropy in globally isotropic granular packings Kamran Karimi, Craig Maloney We study local stresses and elastic moduli defined at various coarse-graining scales, $R$, and volume fractions, $\phi$, in a two dimensional (2D) mixture of frictionless granular particle packings. We measure the average deviatoric stress normalized by pressure, $\tau/p$, and normalized anisotropic component of the shear modulus, $\delta\mu/\mu$, as a function of $R$. As the packings are prepared isotropically, both $\tau/p$ and $\delta\mu/\mu$ vanish at large $R$. However, in local regions, where single force chains dominate, the response can be quite anisotropic. We show that $\tau/p$ exhibits two power-law regimes in $R$ with a cross-over that is only weakly dependent on $\phi$. In contrast, $\delta\mu/\mu$, behaves like a pure power law up to $R\sim640D$ (where $D$ is the characteristic particle diameter) at all $\phi$. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J13.00010: Thermodynamic stability of dense packings of hard, regular tetrahedra Amir Haji-Akbari, Michael Engel, Sharon C. Glotzer The question of how densely regular tetrahedra can pack in three dimensions has attracted many researchers in recent years. In the first thermodynamic study of dense phases of the hard tetrahedron system, we recently reported the spontaneous formation of a dodecagonal quasicrystal [1]. The (3.4.3$^2$.4) approximant of the quasicrystal, with an 82-particle unit cell, was compressed to a packing fraction of 85.03$\%$ [1]. Very shortly after, a much simpler crystal of tetrahedron dimers [2] with a slightly higher packing fraction of 85.63$\%$ was discovered [3], the current densest packing [3]. Since the dimer crystal packs more densely than the quasicrystal and its approximant, it is thermodynamically favored in the limit of infinite pressure. However, which structure is stable at finite pressures is an open question. Here, we explore the relative thermodynamic stability of these very different ordered phases as a function of packing density. \\[4pt] [1] Haji-Akbari A, Engel M, Keys A S, Zhang X Y, Petschek R, Palffy-Muhoray P, Glotzer S C, Nature 462: 773-777 (2009). \\[0pt] [2] Kallus Y, Elser V, Gravel S, Disc. Comp. Geom 44(2):245-252 (2010). \\[0pt] [3] Chen E R, Engel M, Glotzer S C, Disc. Comp. Geom. 44(2):253-280 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J13.00011: Jamming of frictional tetrahedra Max Neudecker, Stephan Ulrich, Stephan Herminghaus, Matthias Schr\"{o}ter We present experimental results on the packing of polypropylene tetrahedra with 7mm side length. Analysis via X-ray-tomography allows for a detailed analysis of the radial distribution function and the number and type of geometrical contacts. We focus particularly on the dependence of these packing properties on the bulk packing fraction. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J13.00012: Rheology of Minimally Jammed Frictionless Rodpiles James Graham, Scott Franklin The ability of large aspect ratio granular particles to form solid plugs is now well-documented but, apart from a general phenomenological explanation of geometric entanglement, remains unexplained. Recent experiments on the collapse of granular columns~[1] suggest that rods with even moderate aspect ratios can maintain angles of repose of 90$^{\circ}$ or larger, implying that the shear modulus increases continuously with aspect ratio. Our simulations generate minimally jammed packings of frictionless, aspect ratio 1-48 spherocylinders through an energy-minimization process. Once the minimally jammed state is reached, we continue the process to larger packing fractions in order to determine the bulk modulus. Packings are then subjected to infinitesimal strain in order to calculate the shear modulus as a function of particle aspect ratio. Shear simulations can be extended to large strain and used to investigate the long-time reordering of rod-like particles that accompanies macroscopic shear. \\[4pt] [1] M. Trepanier and S. V. Franklin, Phys. Rev. E {\bf 82}, 011308 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J13.00013: Couette Shear for Elliptical Particles Near Jamming Somayeh Farhadi We have performed 2D Couette shear experiments on systems of photoelastic particles. The particles are identical ellipses with aspect ratio 2. We use the photoelastic property of the disks to obtain the forces acting on a particle. We use two cameras to simultaneously image the particle motion and the photoelastic force response. Using ellipses enables us to understand the effect of particle shape asymmetry on the large-scale behavior on the rheological behavior of granular systems near jamming. Of particular interest are the nematic ordering of the ellipses, the formation of shear bands and the nature of force transmission. [Preview Abstract] |
Session J14: Focus Session: Physics of Active Materials
Sponsoring Units: GSNPChair: Aparna Baskaran, Brandeis University
Room: D227
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J14.00001: Active Currents and Stresses on the cell surface: Clustering, Instabilities and Budding Invited Speaker: We study the contractile dynamics of a collection of active polar filaments, such as actin, on a two dimensional substrate, using a continuum hydrodynamic description in the presence of spatiotemporal noise. The steady states, characterized by a variety of phases generically consisting of a transient collection of inward pointing asters. We next study the dynamics of particles advected along these active filaments. This is relevant to the dynamics and organization of a large class of cell surface molecules. We make several predictions regarding the statistics of fluctuations of these passive advective particles which we confirm using fluorescence based experiments. We then show how such active patterning of filaments can give rise to membrane stresses leading to membrane shape deformations. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J14.00002: Pattern formation in Active Polar Fluids Arvind Gopinath, Michael Hagan, Aparna Baskaran Systems such as bacterial suspensions or cytoskeletal filaments and motility assays can be described within the paradigm of active polar fluids. These systems have been shown to exhibit pattern formation raging from asters and vortices to traveling stripes. A coarse-grained description of such a fluid is given by a scalar density field and a vector polarization field. We study such a macroscopic description of the system using weakly nonlinear analysis and numerical simulations to map out the emergent pattern formation as a function of the hydrodynamic parameters in the context of two specific microscopic models - a quasi-2D suspension of cytoskeletal filaments and motor proteins and a system of self propelled hard rods that interact through excluded volume interactions. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J14.00003: Non-Equilibrium Dynamics of the Metaphase Spindle Daniel Needleman, Jan Brugues A wide variety of cellular structures exist in a nonequilibrium steady-state with a constant flux of molecules and energy continuously modifying and maintaining their architecture. Understanding such self-organizing structures is not only crucial for cell biology, but also poses a fundamental challenge for physics, since these systems are materials that behave drastically differently from those that have been traditionally studied in condensed matter physics. Physical theories of active materials have been used to describe the cytoskeleton, but it is still unclear how applicable these theories are to complex biological systems \textit{in vivo}. We are experimentally testing if such phenomenological theories of cytoskeletal behavior can be profitably used to model the metaphase spindle. Our approach is to use polarized light microscopy, spinning disk fluorescence microscopy, single molecule imaging, and magnetic tweezers to quantitatively measure spatial-temporal correlation functions of spontaneous fluctuations in the director, concentration, and internal stress in spindles. We are comparing these measurements with predictions from various continuum theories to determine how best to describe the non-equilibrium dynamics of these structures. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J14.00004: Modeling active materials based on self-oscillating gels Victor V. Yashin, Anna C. Balazs The Belousov-Zhabotinsky (BZ) reaction in solution is a classical example of an active medium that demonstrates various chemical oscillations and waves, which can be observed visually. Grafting a ruthenium metal-ion complex, the catalyst to the BZ reaction, to a chemo-responsive polymer gel creates an active material (BZ gel), which exhibits periodic volumetric changes in the course of the reaction. The redox oscillations of the catalyst affect the polymer-solvent interactions and cause the periodic swelling and de-swelling of the gel, so that chemo- mechanical energy transduction occurs within the material. We consider a model that couples the polymer gel dynamics and the BZ reaction kinetics; the latter is described by the modified Oregonator model. The model equations are solved numerically in 2D. We demonstrate that the dynamical behavior of the BZ gel can be controlled by a heterogeneous distribution of the catalyst and by such structural features as the solvent-filled voids. The dynamics of an active membrane having the self-oscillating pores is considered as an example. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J14.00005: Homeostatic pressure, tumor growth and fingering of epithelial tissues: Some generic physics arguments Invited Speaker: We propose that one aspect of homeostasis is the regulation of tissues to preferred pressures, which can lead to a competition for space of purely mechanical origin and be an underlying mechanism for tumor growth. Surface and bulk contributions to pressure lead to the existence of a critical size that must be overcome by metastases to reach macroscopic sizes. This property qualitatively explains the observed size distributions of metastases, while size-independent growth rates cannot account for clinical and experimental data. It also potentially explains the observed preferential growth of metastases on tissue surfaces and membranes, suggests a mechanism underlying the seed and soil hypothesis introduced by Stephen Paget in 1889, and yields realistic values for metastatic inefficiency [1]. Treating epithelial tissues as viscous fluids with effective cell division, we find a novel hydrodynamic instability that leads to the formation of fingering protrusions of the epithelium into the connective tissue. Arising from a combination of viscous friction effects and proliferation of the epithelial cells, this instability provides physical insight into a potential mechanism by which interfaces between epithelia and stroma undulate, and potentially by which tissue dysplasia leads to cancerous invasion.\\[4pt] [1] M. Basan, T. Risler, J.-F. Joanny, X. Sastre-Garau, and J. Prost, \textit{HFSP Journal}, \textbf{3}, 4, p.265 [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J14.00006: Hydrodynamics of an Active Smectic Tapan Chandra Adhyapak, Sriram Ramaswamy, John Toner We show that self-driven particles, in suspension or on a substrate, can support striped phases with long-range order in three dimensions and quasi-long-range order in two dimensions. This is in contrast to the situation for smectic phases at thermal equilibrium, which have the same spatial symmetry. We analyse the fluctuation properties of stable active smectics as well as the nature of characteristic instabilities that these systems can display. Our results apply to any active system that spontaneously develops layers, including apolar orientable cells, monolayers of rods either fluidized or shaken and, most significantly, the Rayleigh-Benard instability. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J14.00007: Spontaneous Oscillations in Nonlinear Active Solids Shiladitya Banerjee, Tanniemola B. Liverpool, M. Cristina Marchetti We present a generic continuum model of a nonlinear active gel with both passive and active crosslinks. The model is relevant for actin gels with passive elasticity provided by ABPs such as filamin-A or $\alpha$-actinin and dynamic active crosslinkers such as myosin-II. We consider an one dimensional continuum active solid where compressional deformations are coupled to molecular motor dynamics. Three kinds of nonlinearities are incorporated : (a) nonlinear load dependence of unbinding rate of molecular motors, (b) pressure nonlinearities stemming from excluded volume interactions, and (c) nonlinearity due to convection of bound motors along the gel. Unbinding rate nonlinearity stabilizes the oscillatory instabilities predicted by the linear theory and lead to sustained oscillations at intermediate concentrations of ATP. Pressure nonlinearity due to excluded volume interactions stabilizes the contractile instability and leads to a contracted ground state. Our work provides a generic framework for the description of the large scale properties of nonlinear isotropic active solids. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J14.00008: Self-diffusiophoresis in the strongly advecting regime Gareth Alexander, Andrea Liu Certain forms of biological motility, such as actin-based propulsion and chromosomal translocation in certain bacteria, have recently been proposed to have their physical origins in the phenomenon of self-diffusiophoresis. In diffusiophoresis, a particle in a fluid with an inhomogeneous concentration of solute will move along the concentration gradient with a well-defined velocity due to surface interactions with the solute. If the particle has the means of generating the concentration gradient itself--by catalyzing a chemical reaction on one side of its surface, for example--then self-diffusiophoresis serves as a mechanism of self-propulsion. Until now, self-diffusiophoresis has been studied under conditions of rapid diffusion, or small P\'eclet number, where the effects of advection on the solute dispersion can be neglected. However, in the biological examples of interest, the P\'eclet number is high. We present an analysis of the large P\'eclet number limit, where diffusion is slow and advection by the fluid flow is the primary means of solute dispersal. The resulting motion is still described in terms of a slip velocity generated in a thin boundary layer, but with a different origin, arising not from diffusion but from local outward flow to carry away the solute together with fluid continuity. A simple model is developed on this basis, contrasted with the rapid diffusion regime, and applied to provide insight into relevant biological processes. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J14.00009: Phase transitions and solitons in a rule-based model of active particles Thomas Ihle I study the Vicsek model [Phys. Rev. Lett. {\bf 75} (1995) 1226] by means of kinetic theory. In this non-equilibrium model, self-driven particles try to align their travel directions with the average direction of their neighbours. At strong alignment, rotational symmetry is spontaneously broken and a global flocking state forms. The alignment is defined by a stochastic rule, not by a Hamiltonian. The corresponding interactions are non-additive and are typically of genuine multi-body nature. Due to this and the discreteness of the time evolution, the kinetic equations are different from the usual ones found in textbooks. I derive the phase diagram for the flocking transition and show that it agrees very well with simulations at large particle velocities and is qualitatively different from the one of a continuous version of the Vicsek-model. The theory starts with the Liouville equation, the hydrodynamic equations are derived by a Chapman-Enskog expansion. These equations contain more terms than previously postulated; their coefficients are given in terms of microscopic parameters. I show how a large-wavelength instability of the flocking state leads to an inhomogeneous soliton state which is very stable and shows a first-order phase transition to the disordered state. I determine the speed of the solitons, investigate the hysteresis of the transition and estimate the system size beyond which the first order nature of the transition should be visible in computer simulations. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J14.00010: Collective motion of vibrated polar granular disks Olivier Dauchot, Deseigne Julien, Hugues Chate In many interesting situations, the interactions among self-propelled agents lead to the spontaneous emergence of self-organized collective motion. The ubiquity of the phenomenon at all scales raises the question of the existence of some underlying universal mechanisms. Recent numerical and analytical studies have confirmed the existence of a transition from a disordered state at large noise to a state with various collective properties reflecting the local symmetry of the particles and their interactions. Though, there are still very few experimental situations where the onset of collective motion can be attributed to spontaneous symmetry breaking. Here, we report on experiments conducted with both polar self propelled and a-polar Brownian disks and by comparing the dynamics of both systems in the same experimental conditions, we demonstrate without ambiguity that collective motion emerges from the interplay of self-propulsion and hard-core repulsion only [1]. Interestingly the alignment, which has no nematic origin, is effectively induced during the collisions because of the self propulsion. \\[4pt] [1] Phys. Rev. Lett 105 135702 (2010) [Preview Abstract] |
Session J15: Focus Session: Spins in Semiconductors - Spin Currents II
Sponsoring Units: DMP GMAG FIAPChair: Jairo Sinova, Texas A&M University
Room: D171
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J15.00001: Spin-Seebeck effect: Local nature of thermally induced spin currents in GaMnAs Invited Speaker: The spin-Seebeck effect refers to a spatial distribution of spins in a ferromagnetic material induced by a thermal gradient. This macroscopic spatial distribution of spins is several orders of magnitude larger than the spin diffusion length [1]. Here we describe measurements of the spin-Seebeck effect in the ferromagnetic semiconductor, GaMnAs [2]. The thermally induced spatial distribution of spins is inferred from the sign and magnitude of the inverse spin Hall voltage generated from local spin currents in platinum bars that are in electrical contact with the ferromagnetic material. From an experimental point of view, GaMnAs provides unique measurement geometries since the magnetic easy axes can be engineered in different directions and the low Curie temperature makes it convenient to perform spin-Seebeck measurements across the magnetic phase transition. Using different experimental configurations we measure either the isolated spin-Seebeck signal, the planar and transverse Nernst effect, or a combination of the spin-Seebeck and Nernst effects. One of the most intriguing aspects of the spin-Seebeck effect is the observation that the spatial distribution of spins is maintained across electrical breaks revealing that the effect does not arise from a longitudinal spin current of charge carriers. \\[4pt] [1] K.~Uchida, S.~Takahashi, K.~Harii, J.~Ieda, W.~Koshibae, K.~Ando, S.~Maekawa, E.~Saitoh, \textit{Nature} \textbf{455}, 778 (2008). \\[0pt] [2] C.~M. Jaworski, J.~Yang, S.~Mack, D.~D. Awschalom, J.~P. Heremans, R.~C. Myers, \textit{Nature Materials} \textbf{9}, 898 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J15.00002: Theory of the Anomalous Hall Effect in the Insulating Regime Xiong-Jun Liu, Xin Liu, Jairo Sinova The anomalous Hall effect (AHE) has been an enigmatic problem that has resisted theoretical and experimental assault for almost one century. The AHE in the metallic regime has been separated into different contributions, i.e. skew scattering side jump and intrinsic contribution. However, the recent experiments on AHE in the insulating regime discover a qualitatively different behavior described by a scaling relation which is different from that in the metallic regime. The new finding cannot be explained by available microscopic theories of metals based on impurity scattering. Here we present a theory to study the anomalous Hall conductivity (AHC) in this regime With this theory we calculate the lower and upper limits for the AHC by taking simple assumptions of the impurity distributions. Our results are quantitatively in agreement with the experimental discoveries, and thus provides the understanding of the AHE in the insulating regime. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J15.00003: Strain manipulation of anomalous Hall response in GaMnAs micromechanical buckled Hall beam structure Chanuk Yang, Hyung Kook Choi, Tai Hoon Kim, Yun Daniel Park The non-magnetic manipulation of magnetic properties of diluted magnetic semiconductors (DMS) has recently received much attention, such as magnetization by electric field [1], and magnetic anisotropy by strain engineering [2]. Especially, in GaMnAs, the spin-orbit interactions (SOIs) are highly strain-dependent and the applied magnetic field plays a crucial role in determining magnetic anisotropies (MA), anisotropic magnetoresistances (AMR), and the intrinsic anomalous Hall effect (AHE) [3]. Here, we present AHE of local-strain-induced GaMnAs micro-Hallbeam by fabricating mechanical suspended structure. We observe a suppression of the AHE that varies symmetrically about the centre of the buckled beam due to strain-related SOIs. \\[4pt] [1] H. Ohno et al., Nature 408, 944 (2000); Y.D. Park et al. Science 295, 651 (2002) \\[0pt] [2] T. Dietl et al. PRB 63, 195205 (2001); M. Glunk et al., PRB 79,195206 (2009); J. Wenisch et al., PRL 99, 077201 (2007); A. W. Rushforth et al., PRB 78, 085314 (2008) \\[0pt] [3] Nagaosa, N. et al., Reviews of Modern Physics 82 (2), 1539 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J15.00004: The spin-Seebeck effect in a GaMnAs/MnAs bilayer Christopher Jaworski, Jing Yang, Shawn Mack, David Awschalom, Joseph Heremans, Roberto Myers The spin-Seebeck effect, recently discovered in ferromagnetic metals such as permalloy, semiconductors such as GaMnAs and insulators such as YIG, consists of a thermally generated spin redistribution. This effect is measured by detecting an inverse spin Hall voltage that varies spatially across a sample due to the thermally generated local spin currents. Here, we describe measurements of the spin-Seebeck effect in metallic ferromagnetic MnAs thin films grown on GaMnAs. The difference in H$_{c}$ and T$_{c}$ of each layer allows independent measurement of spin-Seebeck signals arising from MnAs from that of GaMnAs. We discuss the effect of the exchange bias between these layers on the spin-Seebeck effect above and below the magnetic phase transition. Work support in parts by NSF, NSF-CBET-0754023, ONR, and DMR-0820414. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J15.00005: Magnetoresistance in Lateral GaMnAs devices with Nano-constrictions Bhim Paudel, Grant Riley, Leonidas Ocola, Xinyu Liu, Jacek Furdyna, Khalid Eid Mn-doped GaAs (or GaMnAs) offers opportunities to demonstrate both new device concepts with added functionality and new phenomena in condensed matter physics, since it is both a ferromagnet and a semiconductor. We will present our recent results on fabricating and characterizing GaMnAs-based nano- devices. The resistance of these deep-nanoscale devices can be manipulated either by varying the applied voltage or via an external magnetic field. The nano-devices were prepared using electron-beam lithography and wet chemical etching. The magnetoresistance of the devices was as high as 50{\%} at 4.2k and the behavior was different from previous results reported in literature. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J15.00006: A grate field-dependent change of magnetic damping in Fe/(Ga,Mn)As Satoi Kobayashi, Keita Suda, Hiro Munekata Reported here is the field dependence of the magnetic damping in the photo-induced precession of magnetization in three different samples, a simple (Ga,Mn)As, two hybrids Pt/(Ga,Mn)As and Fe/(Ga,Mn)As. The Mn content $x $is $x$ = 0.045 for all cases. In (Ga,Mn)As, the precession frequency \textit{$\omega $} increases and the precession lifetime\textit{ $\tau $} decreases with increasing a lateral, external field applied along the easy axis, whereas the \textit{$\omega \tau $} product is hardly changed. Similar trend is observed in Pt/(Ga,Mn)As, except that the \textit{$\omega \tau $} product is smaller than that of (Ga,Mn)As. An inverse value of the \textit{$\omega \tau $} product, so called the Gilbert damping constant $\alpha $, is $\alpha $ = 0.1 and 0.15, respectively, for (Ga,Mn)As and Pt/(Ga,Mn)As. The enhanced magnetic damping in Pt/(Ga,Mn)As can be understood qualitatively in terms of the spin pumping. In Fe/(Ga,Mn)As, the \textit{$\omega \tau $} product around the zero field is even smaller than that of Pt/(Ga,Mn)As, being indicative of a larger damping ($\alpha $ $\sim $ 0.26), whereas the \textit{$\omega \tau $} product increases steeply with an external field. At around 400 Oe and higher, the \textit{$\omega \tau $} product saturates at the value comparable to that of a simple (Ga,Mn)As. Taking magnetization data into account, a great field-dependent change in damping could be attributed to the spin-wave excitation at the Fe/(Ga,Mn)As interface caused by non-parallel magnetization configuration between Fe and (Ga,Mn)As in microscopic scale. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J15.00007: Interfacial Spin Filtering at Copper/GaMnAs Contacts Khalid Eid, Bhim Paudel, Grant Riley, Xinyu Liu, Jacek Furdyna We determine the spin injection efficiency using a single ferromagnetic film without the need for a spin-detection layer. This is accomplished by studying the temperature dependence of the specific contact resistance (AR$_{C})$ of a copper/GaMnAs contact using a circular transmission line method. AR$_{C}$ is as low as 5x10$^{-8} \quad \Omega $cm$^{2}$, and decreases slowly with decreasing temperature T. However, as T approaches Curie temperature T$_{C}$, AR$_{C}$ abruptly jumps to about double its initial value. We suggest that this behavior arises from the suppression of one of the two spin conduction channels, which results in substantial spin polarization. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J15.00008: Spin-dependent Transport in GaAs/MnAs Core/shell Nanowires J. Liang, J. Wang, N.S. Dellas, B.J. Cooley, S.E. Mohney, R. Engel-Herbert, M.H.W. Chan, N. Samarth The integration of a metallic ferromagnet (FM) with a semiconductor (S) in axially- and radially modulated nanowires (NWs) has the potential to open up new opportunities in nanospintronics. We describe a comprehensive study of the structure, magnetism and electrical transport in hybrid core/shell S(GaAs)/FM(MnAs) NWs synthesized by molecular beam epitaxy. This is an unusual system where the competition between magnetocrystalline and shape anisotropies in the FM shell creates a magnetic ordering regime which is distinct from conventional FM metal NWs. We report four probe measurements of the temperature dependence of conductivity and the magnetoresistance (MR) in single NWs over a temperature range 0.5 K - 300 K and in magnetic fields ranging up to 80 kOe. Assuming that electrical transport is dominated by the metallic shell, we use the measured anisotropic MR in conjunction with micromagnetic simulations to gain insight into the magnetization reversal process of the FM shell. We also discuss the possible origins of a striking negative linear MR at high field which becomes more pronounced with increasing temperature. Supported by NSF-MRSEC and ONR. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J15.00009: Ohmic spin injection from a half-metal at finite temperatures: Is the conductivity mismatch problem relevant? James Glasbrenner, Aleksander Wysocki, Kirill Belashchenko Spin injection from a normal ferromagnet into a semiconductor requires a highly-resistive tunnel or Schottky barrier at the interface to overcome the conductivity mismatch problem. This barrier limits the current that can be achieved in a device. A half-metallic ferromagnet used as a spin injector obviously overcomes this problem at zero temperature, but the situation at finite temperatures is nontrivial. We argue that the two-current model is inapplicable to half-metals, and that Ohmic (barrierless) spin injection from a half-metal is possible even at finite temperatures. This conclusion is reached using an intuitive model which sums up multiple scatterings at the interface. To complement this model, we calculate the spin injection efficiency for a half-metallic electrode using a single-band tight-binding model with explicit statistical averaging over thermal spin fluctuations. The results are contrasted with the case of a normal ferromagnet. We also consider a practically interesting case of a CrAs electrode within the tight-binding LMTO method. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J15.00010: MnAs/Al(AsSb)/InAs Heterostructure-Based Spin LEDs H. Zhang, E.D. Fraser, S. Hegde, J. Kwon, J.B. Hatch, H. Luo, G.P. Lindberg, B.A. Weinstein, B.D. McCombe MnAs is a promising spin alignment material for spin-injection into InAs-based structures due to its well studied structural and magnetic properties. A well known difficulty of spin injection from a ferromagnetic metal spin aligner into a semiconductor is the so-called conductivity mismatch, which has been overcome via tunnel barrier contacts.\footnote{B.T. Jonker \textit{et al.}, PRB \textbf{62}, 8180 (2000).}$^,$\footnote{X. Jiang \textit{et al.}, PRL \textbf{94}, 056601 (2005).} Lattice matching the tunnel barrier to the active region is important because a highly strained interface and resulting defects can reduce spin polarization of the injected carriers. We report development of a spin-LED structure with a lattice matched AlAs$_{0.16}$Sb$_{0.84}$ tunneling barrier between the MnAs spin aligner and an InAs quantum well. The composition was characterized through XRD and Raman spectroscopy. Sample growth, characterization, LED fabrication and optical polarization studies of electroluminescence in the 3 micron spectral region will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J15.00011: Efficient injection of spin-polarized electrons from MnAs contacts into GaAs quantum well LEDs Everett Fraser, Shridhar Hegde, Lars Schweidenback, Andreas Russ, Athos Petrou, Hong Luo, George Kioseoglou Recent studies of ferromagnetic MnAs have revealed a wide range of properties desirable for spintronic applications. In this work, ferromagnetic MnAs contacts have been used to inject spin polarized electrons into AlGaAs(n)/GaAs(i)/AlGaAs(p) light emitting diodes. The band-edge electroluminescence emitted from these devices has a saturation circular polarization of 26\% at 7K. The circular polarization was found to track the out of plane magnetization of MnAs, confirming spin injection. Using optical pumping measurements, the corresponding electron spin polarization was determined to be 52\%. Emission persists up to room temperature, with a saturation circular polarization of 6\%. The improved performance over similar structures is attributed to the use of MnAs/AlGaAs Schottky barrier tunneling and minimal interdiffusion of Mn ions near the materials interface. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J15.00012: Oscillatory spin polarization and magneto-optical Kerr effect in Fe$_{3}$O$_{4}$ thin films on GaAs(001) Yan Li, Wei Han, A.G. Swartz, K. Pi, J.J.I. Wong, S. Mack, D.D. Awschalom, R.K. Kawakami Magnetite is an attractive material for spin injection and detection, because the theory predicts completely negative spin polarization at the Fermi level at room temperature. We fabricated high quality Fe$_{3}$O$_{4}$ films on GaAs (001) by molecular beam epitaxy. The Fermi level spin polarization of the Fe$_{3}$O$_{4}$ film was probed using the ultrafast optical measurement of ferromagnetic proximity polarization (FPP). The systematic thickness dependence of FPP and MOKE were measured on wedged Fe$_{3}$O$_{4}$ films on GaAs(001), and similar oscillatory and sign reversing behaviors were observed even though the two measurements rely on different mechanisms (spin dependent electron reflection for FPP, and optical transitions for MOKE). Quantum confinement of the t$_{2g}$ states near the Fermi level provides an explanation for the similar thickness dependences of the FPP and MOKE oscillations. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J15.00013: TMR study of GaMnAs/AlGaAs:Be/GaMnAs trilayers Joseph Hagmann, Xinyu Liu, Malgorzata Dobrowolska, Jacek Furdyna, Taehee Yoo, Sungwon Khym, Sanghoon Lee GaMnAs/GaAs:Be/GaMnAs trilayers have recently demonstrated antiferromagnetic (AFM) coupling between the two ferromagnetic (FM) layers, mediated by holes in the spacer layer. In this work, GaMnAs/Al$_{x}$Ga$_{1-x}$:Be/GaMnAs trilayer samples with varying Al concentrations were fabricated into magnetic tunnel junction (MTJ) devices with range of pillar diameters to measure tunneling magnetoresistance (TMR) under various conditions. SQUID measurements were use to measure the magnetization of the samples, including switching fields for parallel and antiparallel magnetization alignments of the FM layers. TMR was observed in the sample with Al$_{0.22}$Ga$_{0.78}$As:Be spacer, but was massively suppressed in the samples with lower Al content. The presence of holes in the spacer layer is shown to suppress TMR. This illustrates the difference in conditions for TMR and for AFM interlayer coupling. [Preview Abstract] |
Session J16: Focus Session: Magnetic Nanostructures III
Sponsoring Units: DMP GMAGChair: Valentyn Novosad, Argonne National Laboratory
Room: D173
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J16.00001: Studies of isolated and interacting ferromagnetic gapped nanorings Jie Li, Sheng Zhang, Jason Bartell, Chris Grigas, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have used micromagnetic simulation and magnetic force microscopy (MFM) to study isolated and interacting permalloy nanorings that are lithographically fabricated with gaps that prevent a rotationally symmetric magnetic state. The gapped nanorings have inner and outer radii of 200 and 300 nm respectively, and the gap has a subtended width of $\sim $20 degrees. The nanorings generate a strong magnetic field only in the gap, and thus the magnetization states of gapped nanorings are much more accessible to MFM imaging than complete rings. We have investigated the properties of these gapped nanorings, including the anisotropy in their coercive field and the relative alignment of the magnetic polarization in coupled pairs. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J16.00002: Broad-Band FMR of Patterned Square Arrays of Square Permalloy Antidots Vinayak Bhat, Joseph Sklenar, John Ketterson, Lance DeLong We have used electron beam lithography to pattern 25-nm-thick Permalloy films with square arrays of square antidots of size D = 300, 400, 500 and 700 nm and same lattice constant d = 1000 nm, using a lift-off technique. Broadband FMR\footnote{C. C. Tsai et al., Rev. Sci. Instrum \textbf{80}, 023904 (2009)} was used to observe localized modes\footnote{C. T. Yu, M. J. Pechan, and G. J. Mankey, Appl. Phys. Lett. \textbf{83}, 3948 (2003)}$^,$\footnote{Minghui Yu et al., J. Appl. Phys \textbf{101}, 09F501 (2007)} showing four-fold rotational symmetry for in-plane DC magnetic field. We have studied FMR spectra spanning the ferromagnetic hysteresis regime around 250 MHz, up to the saturation regime ending near 14 GHz, and observe the appearance and disappearance of various FMR modes, especially at frequencies below 7 GHz. We have observed history-dependent modes below 3 GHz that may be associated with domain walls. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J16.00003: Broadband Magnetic Resonance Measurements on Periodic Patterned Disc and Hole Arrays J. Sklenar, V.S. Bhat, L. Delong, V. Metlushko, C.C. Tsai, O. Chernyashevskyy, K. Rivkin, J.B. Ketterson We have made broadband ferromagnetic resonance measurements on patterned permalloy arrays consisting of circular dots (discs) and both square and circular anti-dots (holes). We employ a transmission meander line approach as opposed to a resonant cavity technique, and cover the frequency range 10MHz to 20GHz. Experiments are performed at a fixed frequency by sweeping the field (through positive and negative values) and at a fixed field while varying the frequency; both magnetic field and frequency modulation are employed to suppress noise and background effects. Experiments on hole arrays show two dominant resonances which from their symmetry appear to be standing spin waves centered at the X- points of the square Brillouin zone. Low field measurements on disc arrays where the field is swept over varying ranges in the region where the sample is hysteretic while tracking the history dependent disappearance and reappearance of the uniform FMR mode, allows a determination of the phase boundaries separating the single and double vortex states, and are in agreement with simulations by Rivkin. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J16.00004: Exciton-Mn ion interaction in CdTe quantum dots Anna Trojnar, Marek Korkusinski, Eugene Kadantsev, Pawel Hawrylak We develop a microscopic theory of optical properties of quantum dots containing a single magnetic ion [1,2] which includes electron-hole correlations, short range exchange of Mn ion with electron and with heavy hole, long range electron-hole exchange, quantum dot anisotropy and external strong magnetic field. A new quantum interference (QInt) effect between electron-hole Coulomb scattering and scattering by Mn ion is obtained. Special role is played here by configurations with electron and hole on the p-shell and degenerate with it configurations with electron (hole) on s-shell and hole(electron) on a d-shell. QInt is shown to significantly reduce exciton-Mn coupling. The signature of this QInt effect in emission and absorption spectrum is discussed. The effect of strong magnetic field on the characteristic emission spectrum is discussed and the limitations of the spin model are established.\\[4pt] [1] S.-J.Cheng et al, Eur. Phys. Lett. 81, 37005 (2008)\\[0pt] [2] L.Besombes, Phys. Rev. Lett. 93, 207403 (2004). [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J16.00005: Antiferromagnetic coupling in cobalt atomic clusters on (110) surface of tungsten Renat F. Sabirianov, Pavel Lukashev, Axel Enders We report results of the first principles calculations on the structural and magnetic properties of cobalt atomic clusters on (110) surface of tungsten. We found that for certain geometry these clusters can exhibit antiferromagnetic order. The result is unexpected, as in the bulk as well as in the thin films and free standing clusters Co always exhibits ferromagnetic structure. We compare results for Co with the ones for the analogues Fe atomic clusters. We found that Fe clusters deposited on (110) surface of tungsten tend to couple ferromagnetically similar to bcc Fe in considered geometries. In our calculations we analyzed different configurations of atomic islands, in particular N=3, 4, 5, 6, 8, 12, where N is the number of atoms in the cluster. We perform full structural and magnetic relaxation, and we show that depending on the geometry and number of cobalt atoms in the cluster, the system can be non-magnetic (N=4, 6, 8), ferromagnetic (N=3, 5) and antiferromagnetic or ferrimagnetic (N=4, 12). We present phenomenological model to explain this intriguing magnetic properties of Co atomic islands on (110) surface of tungsten. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J16.00006: Non-liftoff block copolymer nanolithography of magnetic nanodot arrays A. Baruth, M.D. Rodwogin, A. Shankar, M.A. Torija, M.J. Erickson, M.A. Hillmyer, C. Leighton Nanolithographic techniques based on self-assembled block copolymer templates offer exceptional potential for fabrication of large-area nanostructure arrays from a wide variety of functional materials. Despite significant progress with control of the template ordering, and development of pattern transfer schemes, significant issues exist with common techniques such as lift-off and etching. Here, we demonstrate successful execution of a nanolithographic process based on climate-controlled solvent annealing of easily degradable cylinder-forming poly(styrene-$b$-lactide) block copolymer films that avoids both lift-off and the most challenging aspects of etching. Essentially, we use an overfill/planarize/etch-back ``Damascene-type'' process, exploiting the large Ar ion beam etch rate contrast between polystyrene and typical metals. The process is demonstrated via formation of a large-area array of 12 nm thick, 25 $\pm $ 3 nm diameter Ni$_{80}$Fe$_{20}$ nanodots ($\sim $0.4 x 10$^{12}$ dots/in$^{2})$ with hexagonally-close-packed local order. Extensive microscopy, magnetometry, and electrical measurements provide detailed characterization of the pattern formation and fidelity. We argue that this generic approach can be applied to a wide variety of materials and is scalable to even smaller feature sizes. Funded by NSF MRSEC. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J16.00007: Investigating the origin of the magnetic switching field distribution in bit patterned media Olav Hellwig, Bastian Pfau, Christian Guenther, Stefan Eisebitt, Thomas Hauet, Elizabeth Dobisz, Xiaoyu Xu, Yaney Deborah Bit patterned media (BPM) is a promising approach for extending densities in magnetic data storage. One critical challenge for BPM is a tight magnetic switching field distribution (SFD), i.e. the bit-to-bit variations in reversal field. The SFD has three components: the dipolar interactions between neighboring islands within the array, pattern non-uniformities, such as variations in island size, position or shape and the so-called intrinsic SFD of each individual island, which is due to variations in the intrinsic magnetic material properties. We use soft X-ray spectro-holography and high resolution transmission electron microscopy (TEM) to study the origin of the magnetic SFD in BPM. For this we fabricated pattern arrays with 80 nm islands by e-beam lithography and integrated these into a SiN membrane design suitable for x-ray transmission studies. After identifying individual easy and hard to switch islands in the tails of the SFD we performed plane-view TEM analysis of these islands and correlate their magnetic with their structural properties, such as misaligned grains or irregular island shapes. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J16.00008: Control of Magnetic States of Cobalt Nanorings by an External Azimuthal Field Nihar Pradhan, Tianyu Yang, Abbey Licht, Yihan Li, Mark Tuominen, Katherine Aidala Ferromagnetic nanorings attract interest due to their potential application in high density data storage and Magnetoresistive Random Access Memory (MRAM) devices. These nanorings show multidomain stable states that need to be well controlled by external in-plane or circular magnetic fields. This talk presents a new method to generate circular magnetic fields to control the magnetic states in different geometries of Cobalt nanoring structures, of varying diameter, width and thickness. A solid platinum AFM tip was used to pass current through a single nanoring, generating a circular magnetic field. In applying this field we were able to change the state of the individual ring without affecting the states of other neighboring rings. The evolution of the magnetic states of individual symmetric and asymmetric Cobalt nanorings with applied azimuthal field will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J16.00009: Rotationally induced magnetic chirality in clusters of single-domain permalloy islands and gapped nanorings Sheng Zhang, Jie Li, Jason Bartell, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied magnetic moment configurations of clusters of single-domain ferromagnetic islands in different geometries.\footnote{J. Li , S. Zhang, J. Bartell, C. Nisoli, X. Ke, Paul E. Lammert, Vincent H. Crespi, and P. Schiffer, Physical Review B \textbf{82}, 134407 (2010).} The magnetic moments of these clusters are imaged by MFM after rotational demagnetization, following our previous protocols.\footnote{R. F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B. J. Cooley, M. S. Lund, N. Samarth, C. Leighton, V. H. Crespi, P. Schiffer, Nature \textbf{439}, 303 (2006).} We observed that two types of the clusters showed a significant imbalance of their two-fold degenerate ground states after demagnetization, and this inequality is correlated to the rotational direction of the demagnetization. A similar imbalance was also found in nano-scale rings with a small gap: the chirality of their magnetic state can be precisely controlled by the rotational direction during demagnetization. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J16.00010: Chirality control and vortex manipulation in asymmetric Co dots Invited Speaker: Magnetic vortices in sub-micron sized dots have gained considerable interests in recent years due to their fascinating physics and potential applications in information storage, spin-torque oscillators, and magnetic memory and logic devices. Reproducible control of the vortex chirality is of critical importance for these studies. Here we report on two distinctly different chirality control mechanisms in asymmetric Co dots. Arrays of Co dots were fabricated using electron beam lithography and the circular symmetry was broken by introducing a flat edge. Below a critical diameter and/or thickness, chirality control is achieved by the nucleation of a single vortex within each dot, as conventionally observed. The vortex can be manipulated to annihilate at particular sites under different field orientations and cycle sequences. Interestingly, above these critical dimensions a new chirality control mechanism is realized by the nucleation and subsequent coalescence of double vortices, resulting in a single vortex at remanence with the \textit{opposite} chirality as found in smaller dots. Magneto-optical Kerr effect and magnetic force microscopy measurements confirm this new process. Micromagnetic simulations not only reproduce the experimentally observed behavior, but also elucidate the delicate interplay between exchange, demagnetization, and Zeeman energies and the role of fractional vortices bound to the dot edge. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J16.00011: Manipulation of Magnetization States of Permalloy Nanorings by an External Azimuthal Field Tianyu Yang, Nihar Pradhan, Abby Goldman, Moureen Kemei, Abbey Licht, Yihan Li, Mark Tuominen, Katherine Aidala This experimental research investigates a new method of manipulating the magnetic states of ferromagnetic nanorings using a circular magnetic field directed along the ring circumference. This type of azimuthal field can naturally select a vortex magnetization of desired chirality. The understanding of the magnetization switching behavior in an azimuthal field could lead to new designs of practical magnetic data storage devices. Symmetric and asymmetric nanorings made of permalloy are fabricated by a standard technique using electron-beam lithography and e-beam evaporation. Azimuthal fields are generated by passing current through an atomic force microscope tip, which is positioned at the center of the ring. The magnetic field direction and magnitude are controlled by the current. We demonstrate control over switching from an onion state to a vortex state, and also between two vortex states, using magnetic force microscopy to image the resulting magnetic states. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J16.00012: Magnetization reversal of surface and subsurface Co/Pt multilayers in a porous matrix B.J. Kirby, M.T. Rahman, R.K. Dumas, J.E. Davies, Kai Liu, C. Lai Deposition of magnetic multilayers onto porous host matrices has been studied as a simple and cost-effective method for fabrication of nano-patterned magnetic arrays [1]. For such structures, the magnetic reversal properties of the surface multilayer are twofold dependent on the size and depth of the host pores. First, the pore size determines the lateral size of the surface multilayer with respect to that of a single domain. Second, the pore size determines the amount and location of magnetic material within the pore - material that can exchange couple to the surface multilayer. To study these effects, we have used polarized neutron reflectometry to measure the structural and field-dependent magnetic depth profiles of a series of Co/Pt multilayers deposited on nanoporous alumina (diameter: 13, 20, or 28 nm). Despite the film porosity, we observe robust spin-dependent reflectivities, providing strong sensitivity to interfaces throughout the structure. The determined nuclear profiles show impressive agreement with cross-sectional transmission electron microscopy, and the magnetic profiles feature clearly distinct surface and subsurface magnetizations. The surface magnetization reversal and the role of exchange coupling will be discussed. [1] M. T. Rahman, et al., APL. 94, 042507 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J16.00013: Hysteresis-Loop Overskewing Ralph Skomski, T.A. George, D.J. Sellmyer The performance of permanent magnets is largely determined by the magnetostatic energy stored in free space (energy product), which depends on both materials properties and magnet geometry. The latter usually differs from laboratory shapes such as spherical samples, and demagnetizing-field corrections must be applied to compare different geometries. However, in nanostructures, especially in thin films, the macroscopic demagnetizing factors \emph{D} predicted from Maxwell's equations [1] yield unphysical overskewed hysteresis loops [2]. The overskewing is probably a nanoscale effect, but its origin has remained controversial. Our explanation is that nanoscale magnetization processes violate a main condition for the applicability of macroscopy demagnetizing factors, namely the uniform character of the magnetization. In bulk magnets, the magnetization inhomogeneities effectively average to zero, but this is no longer the case if any of the dimension of the magnet becomes small. We explicitly consider granular thin films, where we find a real-structure dependent reduction \emph{D}, as contrasted to the sometimes assumed infinite slope \emph{M}(\emph {H}) at coercivity. --- This research is supported by BREM (RS), ARPA-E, DOE (DJS), and NCMN. --- \textbf{References:} [1] J. A. Osborn, Phys. Rev. \textbf {67}, 351 (1945); [2] R. Skomski, J.- P. Liu, and D. J. Sellmyer, Phys. Rev. B \textbf{60}, 7359 (1999). [Preview Abstract] |
Session J17: Focus Session: Bulk Properties of Complex Oxides - Ferrites + Vanadates
Sponsoring Units: DMP GMAGChair: Jaime Fernandez-Baca, Oak Ridge National Laboratory
Room: D174
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J17.00001: New structural phase transition in Bi$_{2}$(Fe$_{4-x}$Mn$_{x}$)O$_{10-x}$ complex oxides and it implications in the mullite family of materials Patricia Kalita, Andrew Cornelius, Stanislav Sinogeikin, Kristina Lipinska, Oliver Hemmers, Michael Lufaso, Zachary Kann, Hartmut Schneider Complex oxides with the mullite crystal structure belong to the most important phase in both traditional (porcelains and alumino silicate refractories) and advanced ceramics (heat exchangers, shock resistant composites, optical devices). New complex oxides in the mullite family Bi$_{2}$(Fe$_{4-x}$Mn$_{x})$O$_{10-x}$ were synthesized and characterized. Using synchrotron x-ray diffraction we demonstrate a new structural phase transition in Bi$_{2}$(Fe$_{4-x}$Mn$_{x})$O$_{10-x}$ induced by pressure. We contrast it with the structural stability for mullite senso stricto Al$_{4+2x}$Si$_{2-2x}$O$_{10-x}$ where we did not observe any phase transition. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J17.00002: The role of charge and orbital order for the Verwey transition in Fe$_3$O$_4$ S. de Jong, R. Kukreja, M. Hossain, C. Back, A. Scherz, D. Zhu, W. Schlotter, J. Turner, W. Lee, Y. Chuang, R. Moore, O. Krupin, M. Trigo, L. Patthey, H. D\"urr, N. Pontius, T. Kachel, A. F\"ohlisch, M. Beye, F. Sorgenfrei, W. Wurth, C. Chang, M. D\"ohler, C. Trabant, C. Sch\"ussler-Langeheine Magnetite, Fe$_3$O$_4$, displays a strong decrease in resistivity upon heating above $T_C=123$~K: the Verwey transition. Since long it has been proposed that charge and orbital order (CO/OO), via Fe$^{3+}$ and Fe$^{2+}$ charge disproportionation, play a crucial role. However, the mechanism behind the Verwey transition to date remains unclear. Using pump-probe O K-edge resonant soft X-ray scattering at the new LCLS SXR beamline, we have studied the role of CO/OO for the Verwey transition on ultra-fast time-scales. We focus on the structurally forbidden (00 $^1/_2$) peak. Upon excitation, the charge gap of 200~meV is quenched on resolution limited time-scales, $<250$~fs, while we still observe a residual CO/OO signal. This may indicate the existence of a new transient state of matter, displaying charge and orbital order in coexistence with metallic behavior. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J17.00003: Pseudogap Phase of Magnetite Yohanes Pramudya, Hanna Terletska, Efstratios Manousakis, Vladimir Dobrosavljevic Despite extensive experimental and theoretical work, the description of the electrical transport mechanism in magnetite (Fe$_{3}$O$_{4})$ is still an unresolved issue [1]. This unusual resistivity behavior close to the Verwey transition in magnetite has long been a matter of controversy. In our study, we focus on the temperature regime above Verwey transition and far below the magnetic phase transition, where a nearly charge ordering state (due to the long-range Coulomb frustration) is expected. Here, we expect similar behavior to what has been discussed in a nearly frozen Coulomb liquid [2] with the existence of a pseudogap phase. Following this line of thought, we use extended dynamical mean field theory (EDMFT) and Monte Carlo simulation to study the simplest spinless model describing this system. Our studies do capture the main transport trends in this temperature regime with a typical pseudogap--like behavior. \\[4pt] [1] N. F. Mott, ``Metal-Insulator Transitions'', Taylor{\&}Francis (1990). \\[0pt] [2] S. Pankov and V. Dobrosavljevic, Phys. Rev. Lett. \textbf{94}, 046402 (2005). [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J17.00004: Synchrotron x-ray single-crystal structure analysis of a spinel oxide FeV$_{2}$O$_{4}$ with spin and orbital degrees of freedom Yoichi Nii, Hajime Sagayama, Taka-hisa Arima, Riu Sakai, Shinobu Aoyagi, Eiji Nishibori, Hiroshi Sawa, Kunihisa Sugimoto, Hiroyuki Ohsumi, Masaki Takata It has been reported that FeV$_{2}$O$_{4}$, which has orbital and spin degrees of freedom both in tetrahedral Fe$^{2+}$(d$^{6})$ sites and octahedral V$^{3+}$(d$^{2})$ sites, exhibits successive structural phase transitions, accompanying a ferrimagnetic transition. The origin of the phase transitions is supposed to be a cooperation and/or competition between the orbital and spin degrees of freedom both in Fe$^{2+}$ and V$^{3+}$. By a synchrotron x-ray single-crystal structure analysis, we determined the space group and atomic coordinate of each phase (cubic- HT-tetra.- HT-ortho.- LT-tetra.). The results suggest that the HT-tetra. (a$>$c) and HT-ortho. phases should be ascribed to the FeO$_{4}$ local compression, whereas VO$_{6}$ elongation should be responsible for the LT-tetra. (c$>$a) phase. We also discuss the orbital ordering (OO) pattern assuming strong electron-lattice coupling. A conceivable OO pattern of V$^{3+}$ at the LT-tetra. (c$>$a) is \textit{ferroic} one with \textit{yz} and \textit{zx} orbitals occupied, which is unique among spinel-type vanadates. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J17.00005: Single-ion Anisotropy, Dzyaloshinskii-Moriya Interaction and Negative Magnetoresistance of the Spin-1/2 Pyrochlores R2V2O7 Hongjun Xiang, Erjun Kan, M.-H. Whangbo, C. Lee, Su-Huai Wei, X.G. Gong The electronic and magnetic properties of spin-1/2 pyrochlores R$_2$V$_2$O$_7$ were investigated on the basis of density-functional calculations. Contrary to the common belief, the spin-1/2 $V^{4+}$ ions are found to have a substantial easy-axis single-ion anisotropy. The $|D/J|$ ratio deduced from the magnon quantum Hall effect of Lu$_2$V$_2$O$_7$, where $J$ is the nearest-neighbor spin exchange and $D$ is the Dzyaloshinskii-Moriya parameter, is much greater than the value estimated from our calculations (i.e., 0.32 vs. 0.05). We show that this discrepancy is due to the neglect of the single-ion anisotropy of the V$^{4+}$ ions, and the negative magnetoresistance observed for R$_2$V$_2$O$_7$ arises from a new mechanism. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J17.00006: Stability of the Ni$_{3}$V$_{2}$O$_{8}$ phase diagram on substitution with magnetic and non-magnetic transition metal ions Akila Kumarasiri, Ambesh Dixit, Gavin Lawes There is considerable interest in understanding the materials properties underlying the development of simultaneous magnetic and ferroelectric order in multiferroics. Ni$_{3}$V$_{2}$O$_{8}$ develops strongly coupled ferroelectric and antiferromagnetic order simultaneously at low temperatures and has a rich magnetic phase diagram due to competing magnetic interactions. We investigated how the magnetic phases of Ni$_{3}$V$_{2}$O$_{8}$ were affected by systematic doping by transition metal ions. For these studies, polycrystalline Ni$_{3}$V$_{2}$O$_{8}$ samples substituted by various concentrations of transition metal ions M (M = Zn, Cu, Co, Mn, Fe) were prepared. Heat capacity, magnetization, dielectric, AC susceptibility, and pyrocurrent measurements were used to track the change in phase transition temperatures. On doping with spin-0 Zn, the system behaves as expected for site dilution consistent with 2-D spins, where the phase transition temperatures are suppressed linearly to lower temperatures. The modifications to the phase diagram for magnetic dopants (Co, Cu, Mn and Fe) show more variation, but the multiferroic phase transition appears to persist over a range of concentrations. This suggests that the specific spin structure in Ni$_{3}$V$_{2}$O$_{8}$ responsible for the development of ferroelectric order is relatively robust against perturbations produced by both magnetic and non-magnetic dopants. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J17.00007: Orbitally driven trimerization in LiVO$_2$ and LiVS$_2$: a ``partial Mott transition" Hua Wu, D.I. Khomskii Layered triangular-lattice transition-metal compounds often display interesting magnetic and electronic properties. Here we studied the formation of the trimerized spin-singlet state of the V$^{3+}$ ($S$=1) in vanadates LiVO$_2$ and LiVS$_2$ and their electronic structure with a special orbital order, using constrained LSDA+$U$ calculations combined with lattice optimization. The obtained results show that the trimerization distortion in LiVO$_2$ increases as the effective $U$ decreases, and the calculated distortion of $\sim$0.3 \AA~ at the small $U$=1 eV agrees well with the experiments, indicating that LiVO$_2$ is close to a metal-insulator transition. The corresponding distortion in LiVS$_2$ is even stronger, being $\sim$0.4 \AA~ at the $U$=1 eV, which is due to enhanced electron delocalization via increased V-S covalency, in spite of a lattice expansion. This agrees with the experimental finding that LiVS$_2$ has a metal-insulator transition. The calculated energy gain associated with the trimerization well accounts for the observed structural phase transition temperature in LiVO$_2$ and LiVS$_2$. We conclude that the trimerization in LiVO$_2$ and LiVS$_2$ is due to a partial delocalization of the orbitally ordered electrons---a ``partial Mott transition,'' occurring not in the whole system but in small clusters (here in trimers). This situation is contrasted with that in NaVO$_2$, which is further away from the localized-itinerant crossover and thus remains insulating with different orbital ordering. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J17.00008: Simultaneous electrical transport and Raman spectroscopic measurements on individual nanowires of $W_{x}V_{1-x}O_{2}$ Tai-Lung Wu, Luisa Whittaker, C.J. Patridge, S. Banerjee, G. Sambandamurthy Vanadium oxide is a well-know material to study the metal-insulator transition (MIT) in correlated electron systems. Upon heating to about 340 K, $VO_{2}$ undergoes orders of magnitude drop in resistance from an insulating phase (I) to a metallic phase (M) and accompanies a lattice structural phase transition from a low-temperature monoclinical phase (M1) to a high-temperature tetragonal phase (R). We present results from combined electrical transport and Raman spectroscopic measurements to discern the effects of doping in controllably tuning the MIT in individual nanowires of single crystal $W_{x}V_{1-x}O_{2}$. The MIT temperature ($T_{c}$) in our $W_{x}V_{1-x}O_{2}$ nanowires can be tuned through a wide range from 280 to 330 K by controlling the dopant concentration. The M-I transition can also driven electrically in these nanowires. Our simultaneous measurement of electrical transport and Raman spectroscopic measurement help us understand the role of structural transition in affecting the macroscopic electrical transition in individual wires. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J17.00009: Strain- and temperature-induced phase transitions in single crystalline VO$_2$ Joanna Atkin, Emily Chavez, Samuel Berweger, Jinbo Cao, Wen Fan, Junqiao Wu, Markus Raschke The metal-insulator transition (MIT) of VO$_2$ exhibits a rich phase behavior involving two monoclinic (M1, M2), triclinic, and tetragonal phases that can form a complex domain structure and accompany the electronic transition. The interplay between these structural variants arises from strain due to differing lattice constants, temperature-dependent phase stability, and possible external strain from the substrate; the coupling between these effects renders a systematic study of the phase behavior difficult. We report on phase mapping of the structural changes through independent control of temperature and uniaxial strain in individual single-crystal nanorods, using Raman spectroscopy and near-field imaging. This allows us to investigate the transformation between the various insulating phases, elucidating the nature of the triclinic phase as a continuously distorted variant of the M1 monoclinic phase, intermediate in the first-order transformation into the monoclinic M2 phase. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J17.00010: Discrepancy of structural and electronic transitions in the vicinity of the Metal-Insulator-transition in V$_{2}$O$_{3}$ Hyun-Tak Kim, Jun-Hwan Shin, Jung-Young Choi, Bong-Jun Kim Vanadium sesquioxide (V$_{2}$O$_{3})$, representative of strongly correlated electronic system, has been known as undergoing the MIT (Metal-Insulator-Transition) which is between rhombohedral paramagnetic metallic phase and monoclinic antiferromagnetic insulating phase near the transition temperature, (T$_{c})$ \textbf{$\approx $}150 K. In order to reveal a relation between electronic and structural atomic transition, we has measured the temperature dependence of DC conductivity and structural crystallographic characterization with various temperatures from 90 K to 300 K by using low-temperature X-Ray diffraction (LTXRD). The obtained results show a discrepancy of structural and electronic transitions. This discrepancy can be explained by forming of the metallic puddles whose the size and number increased by nucleation and percolation[1,2] during the electronic transition progress from 120 K to 180 K. The puddles have an insulating monoclinic structure before the structural phase transition at $\sim $185 K. These metallic puddles are induced by the MIT not related to the SPT (structure phase transition). (1. M. M. Qazilbash et al., Science 318, 1750 (2007); 2. B. J. Kim et al., Phys. Rev. B 77, 235401(2008)) [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J17.00011: Interplay of chemical pressure and spin degrees of freedom on the magnetic properties of the $A$Ag$_{2}M$[VO$_{4}$]$_{2}$ type of compounds Angela M\"{o}ller, Ngozi Amuneke, Phillip Daniel, Dana Gheorghe, Bernd Lorenz A series of layered compounds of the $A$Ag$_{2}M$[VO$_{4}$]$_{2}$ type of structures, featuring the magnetic ions on a triangular lattice, have been synthesized by solid state reactions. Studies on the interplay of i) the chemical pressure induced by the differences in cation sizes ($A^{II}$ = Sr, Ba) and ii) the spin system ($M^{II}$ = Mn, Co, Ni, Cu) provide further insights into the structure-properties relationships which have been investigated by specific heat and magnetization measurements. Furthermore, spectroscopic methods have been employed to evaluate vibrational and electronic structural aspects in detail. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J17.00012: Trimer Formation and Metal-Insulator Transition in Triangular-Lattice Systems LiV{\it{X}}$_2$ ({\it{X}}=O,Se,S) Junki Yoshitake, Yukitoshi Motome Geometrically-frustrated systems sometimes lift their degeneracy by spontaneous formation of multisite clusters via coupling to other degrees of freedom. A fascinating example is found in a triangular-lattice system LiVO$_2$, which exhibits a three-site trimer formation. The origin was argued to be an orbital ordering under strong electronic correlation [1], however, recent experiments on a series of LiV{\it{X}}$_2$ ({\it{X}}=O,Se,S) suggest that the system is intermediately correlated and rather close to a metal-insulator transition [2]. In this contribution,we revisit this problem within a multiorbital Hubbard model in a wide range of Coulomb interaction by strong-coupling perturbation and Hartree-Fock approximation. We find a new trimer state under substantial trigonal crystal-field splitting; it is located in the vicinity of a metal-insulator transition and not adiabatically connected to the trimer state previously proposed. We discuss the origin of this new trimerization and the relation to experimental results.\\[4pt] [1] H. F. Pen {\it et al}., Phys. Rev. Lett. {\bf 78}, 1323 (1997).\\[0pt] [2] N. Katayama {\it et al}., Phys. Rev. Lett. {\bf 103}, 146405 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J17.00013: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J17.00014: Electronic Excitations in Vanadates William Thornton, Anton Kozhevnikov, Adolfo Eguiluz Vanadates represent an electronic analogue to the cuprates, which have one hole in the 3d shell. There are many realizations of the vanadates, as the solid-state chemistry of the V-O system allows various coordination numbers. Here we probe the electronic structure of vanadates by studying neutral electronic excitations computed within a time-dependent density functional theory framework. We evaluate the dynamical electronic response in both a Bloch basis and a Wannier basis, for both large momentum transfers, and in the optical limit. We compare our results with available experimental data, and assess the importance of many-body (excitonic) interactions. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J17.00015: A microscopic view on the Mott transition in chromium-doped V$_2$O$_3$ G. Sangiovanni, S. Lupi, L. Baldassarre, D. Nicoletti, M. Marsi, P. Hansmann, N. Parragh, A. Toschi, T. Saha-Dasgupta, O.-K. Andersen, K. Held V$_2$O$_3$ is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal-to-insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator. This or related MITs have a high technological potential, among others, for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V$_2$O$_3$ with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure, that promotes a genuine Mott transition to an eventually homogeneous metallic state. \\[4pt] Nature Communications {\bf 1}, 105 (2010) doi: 10.1038/ncomms1109 [Preview Abstract] |
Session J18: Focus Session: Low D/Frustrated Magnetism - Molecular Magnets I
Sponsoring Units: GMAG DMPChair: Vivien Zapf, Los Alamos National Laboratory
Room: D172
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J18.00001: Magnetic field-induced XY-AFM in 2D Heisenberg Antiferromagnet [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})$ Yoshimitsu Kohama, Marcelo Jaime, Jamie Manson Specific heat ($C_{p})$ and magnetic susceptibility (\textit{$\chi $}) measurements were performed on the two-dimensional spin-1/2 Heisenberg antiferromagnet [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})_{2}$ in DC and pulsed magnetic fields up to H = 15 T and 40 T, respectively [1]. We observe no long-range magnetic order down to 500 mK in zero applied magnetic field, suggesting that [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})_{2}$ is close to an ideal 2D AFM and instead undergoes a Berezinskii-Kosterlitz-Thouless (BKT) transition. However, the application of a finite magnetic field induces a clear anomaly in $C_{p}$, although not in \textit{$\chi $}. This behavior is known to be a remarkable signature of magnetic field induced XY-AFM [2]. In addition, $C_{p}$(H) measurements in pulsed fields, performed down to T = 1.5 K and up to H = 40 T, were used to map out the asymmetric Field-Temperature phase diagram which provides additional support for an ideal realization of field-induced XY-AFM in [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})_{2}$.\\[0pt] [1] Y. Kohama et al., \textit{Rev. Sci. Instrum.} \textbf{81}, 104902 (2010).\\[0pt] [2] A. Cuccoli et al., \textit{Phys. Rev. }\textbf{B68}, 060402(R) (2003). [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J18.00002: Pressure-induced Jahn-Teller axis switching in Cu(pyz)F$_2$(H$_2$O)$_2$? J.L. Musfeldt, Z. Liu, S. Li, J. Kang, C. Lee, P. Jena, J.L. Manson, J.A. Schlueter, G.L. Carr, M.-H. Whangbo We employed infrared spectroscopy along with complementary lattice dynamics and spin density calculations to investigate local structure and magnetism through the series of pressure-driven transitions in Cu(pyz)F$_2$(H$_2$O)$_2$. Rather than frequency shifts that dovetail with the recently proposed pressure-induced Jahn-Teller switching model, we overall mode hardening, particularly in the Cu--OH$_2$ bending mode. We combine these findings with a reanalysis of the crystal structure to reveal the series of pressure-induced transitions as a combination of $a$-axis rotation, $c$-directed compression that acts to weaken O-H...F hydrogen bonds, and pyrazine ring buckling. The magnetic dimensionality crossover can be understood in terms of changes in magnetic orbital overlap. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J18.00003: Lost and found: The missing diabolical points in the Fe$_8$ molecular magnet Feifei Li, Anupam Garg The tunneling spectrum of the single-molecule-magnet Fe$_8$ is known to have diabolical points (DP's). For magnetic fields along the hard axis, there are four such points for tunneling between the ground pair of levels, whereas the simplest model including only second-order anisotropy would predict ten DP's. The difference is due to a very weak fourth-order anisotropy, which in a semiclassical picture generates instantons with endpoint discontinuities, one of which dominates for large enough fields, and having no interfering partner, causes six of the underlying DP's to go away. However, as shown by Bruno, the six missing DP's do not truly disappear, but merely move off the hard axis into the hard-medium plane. In this talk, we report on a numerical search for these ``missing" DP's. This search is nontrivial because the energy surface is like a smooth golf course, on which the DP's are extremely localized and deep holes. We therefore locate the DP's by following the lines of the Berry curvature which have monopole singulairities at the DP's. This exercise is performed for tunneling between excited pairs of levels also. An experimental observation of the rediscovered DP's would be an important test of the underlying spin Hamiltonian for Fe$_8$. (Submitted to the arxiv: Nov.~18, 2010.) [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J18.00004: Magnetic Superatoms J. Ulises Reveles, Victor M. Medel, A.C. Reber, S.N. Khanna, V. Chauhan, P. Sen The electronic states in metal clusters are grouped in shells much in the same way as in atoms. Filling of the electronic shells leads to stable species called magic numbers. This has led to the preposition that selected stable metal clusters can mimic chemical properties of atoms on the periodic table and can be classified as superatoms. Here, we propose an extension of the superatom concept to magnetic species by invoking systems that hybridize localized and delocalized electronic states. Through first principles studies focusing on the electronic structure and magnetic moment, we show that TMMg$_{n}$ (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) clusters exhibit a new class of magnetic superatoms stabilized by magnetic supershells. The talk will include possible applications of the new building blocks. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J18.00005: Tunnel-diode Resonator Spectroscopy of Quantum Levels in Cr$_{12}$Ln$_{4}$ (Ln=Y,Eu,Gd,Tb,Dy,Ho,Yb) Magnetic Molecules Steven Yeninas, Marshall Luban, Ruslan Prozorov, William A. Coniglio, Charles C. Agosta, Larry Engelhardt, Grigore A. Timco, Richard E.P. Winnpenny The differential magnetic susceptibility for a series of Cr$_{12}$Ln$_{4}$ (Ln=Y,Eu,Gd,Tb,Dy,Ho,Yb) magnetic molecules was measured in static (up to 16 T) and pulsed (up to 45 T) magnetic fields using a rf tunnel-diode resonator (TDR). At low temperatures, the behavior of these finite spin systems is governed by discrete energy spectra of the individual molecules. In magnetic field, low-energy quantum levels Zeeman-split, crossing at field values where magnetization exhibits a step corresponding to switching between different spin states. In high fields, we detect multiple level crossings which allow for a detailed mapping of the energy diagram. We then perform quantum Monte Carlo (QMC) using a Heisenberg Hamiltonian with three adjustable exchange constants whose values are chosen so as to optimize agreement with the experimental energy spectrum. The variations in results for the studied molecules are correlated to the magnetic properties of the lanthanide ions. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J18.00006: Spin-electric coupling in Cu$_3$, V$_{15}$ and other frustrated molecular magnet rings: a first principle study M.F. Islam, J.F. Nossa, C.M. Canali, M.R. Pederson Frustrated triangular single-molecule magnets (SMMs) without inversion symmetry, such as Cu$_3$ and V$_{15}$, are characterized by a doubly degenerate S=1/2 ground-state (GS) with opposite chirality. Recently it has been proposed theoretically [1] and verified by ab-initio calculations [2] that an external electric field can couple these two chiral spin states, even in the absence of spin-orbit interaction (SOI). The efficiency of these coupling depends on the electric dipole moment between chiral states. In this talk we report on first-principle calculations of the coupling strength for the triangular SMMs Cu$_3$ and V$_{15}$. The spin-electric coupling is found to be considerably stronger in V$_{15}$ than in Cu$_3$. We discuss the mechanism leading to an enhanced spin-electric coupling, which can be used as a convenient guide to synthesize SMMs that can respond more efficiently to an external electric field. \\[4pt] [1] M. Trif et.al. Phys. Rev. B 82, 045429 (2010). Mircea Trif et.al. Phys. Rev. Lett. 101, 217201 (2008) \\[0pt] [2] M.F. Islam et.al. Phys. Rev. B 82, 155446 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J18.00007: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J18.00008: Magnetic anisotropy and high-spin effects in single-molecule transistors Alexander Zyazin, Johan van den Berg, Edgar Osorio, Nikos Konstantinidis, Martin Leijnse, Falk May, Walter Hofstetter, Chiara Danieli, Andrea Cornia, Maarten Wegewijs, Herre van der Zant Fabrication of single-molecule transistors where electron transport occurs through an individual molecule has become possible due to the recent progress in molecular electronics. Three-terminal configuration allows charging molecules and performing transport spectroscopy in multiple redox states. Single-molecule magnets combining large spin with uniaxial anisotropy are of special interest as appealing candidates for high density memory applications and quantum information processing. We study single-molecule magnets Fe$_4$. Three-terminal junctions are fabricated using electromigration of gold nanowires followed by a self-breaking. High-spin Kondo effect and inelastic cotunneling excitations show up in transport measurements. Several excitations feature the energy close to the energy of zero-field splitting (ZFS) of a ground spin multiplet in bulk. This splitting is caused by the anisotropy and is a hallmark of single-molecule magnets. We observe nonlinear Zeeman effect due to a misalignment of an anisotropy axis and a magnetic field direction. The ZFS energy is increased in oxidized and reduced states of the molecule indicating enhancement of the anisotropy in these states. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J18.00009: Magneto-optical spectroscopic studies of solid and solution-phase tetra-phenyl porphyrin Jacob Wahlen-Strothman, Zhen Wen Pan, Lane Manning, Madalina Furis, Kelvin Chu Tetraphenylporphyrin (TPP) is a synthetic heterocyclic compound that serves as a model system for heme proteins and cytochromes. TPP can accomodate a metal ion in the center; D-shell ion porphyrin complexes with a crystalline solid phase are of interest for magnetic studies because of the possibility of macroscopic long range magnetic order of the ion spins. We have investigated the 5K magnetic properties of poly-crystalline thin films of the heme protoporphyrin IX analogue tetra-phenyl porphyrin, complexed with Zn and Mn, deposited through a capillary pen technique that produces 100um to 1 mm sized grains. Our novel experimental setup measures the UV/VIS, linear dichroism and magnetic circular dichorism simultaneously, incorporates a photoelastic modulator and a microscopy superconducting magnet for high-field (5T) measurements. We present solution and crystalline data on metal-complexed TPP; data are analyzed in terms of A and B-type MCD using a perimeter model. We find good agreement with previous solution data, and novel crystalline phase spectra that are correlated to the long range ordering. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J18.00010: Chiral molecule for spin filtering purposes: the study of L- and D-Alanine Esmeralda Yitamben, Richard Rosenberg, Nathan Guisinger The field of molecular electronics has attracted scientists by the great opportunities and versatility it offers as a replacement for standard semiconductor electronics with organic materials, thus bringing down the cost, and opening endless possibilities for chemical synthesis, and scientific breakthrough. Of particular interest is the use of chiral molecules, such as alanine, for spin filtering studies in hope of creating highly spin-polarized charge carriers for spintronics applications. Preliminary studies of both L- and D-alanine on Cu(111) were conducted using scanning tunneling microscopy and spectroscopy, revealing the formation of a 2-dimensional phase at low coverage, a hexagonal ``flower'' pattern at intermediate coverage, and a chain and ring superstructures at high coverage. A model is proposed to explain the surface chemistry and bonding of the molecules on the metallic surface. Current studies of L- and D-alanine on Fe/W show promises in the intermediate coverage regime. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J18.00011: DFT and STM studies of magnetism in single Co(TCNE) complexes on an ultrathin insulating film M. Badal, T. Choi, D. Stroud, J.A. Gupta We present results from large-scale ab initio DFT calculations for geometry and electronic structure of Co(TCNE) complexes on a c($2\times 2)$Cu$_2$N substrate. The long-term aim is to study charge and spin transport in molecular systems. The work is done in concert with STM experiments. In particular, we perform calculations to help explain STM observations indicating that the electronic and magnetic properties of Co(TCNE) complexes vary with apparent molecular orientation. Using plane wave DFT with a GGA functional and spin polarization, we perform geometry optimization to identify the most commonly seen orientations of Co(TCNE). We further study the resulting electronic structure, using calculated LDOS and simulated STM images, to compare with observations. To study the strong in-plane magnetic anisotropy suggestd by spin-flip spectroscopy, we do noncollinear magnetic calculations on the relaxed structure, including spin-orbit coupling effects. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J18.00012: Non-equilibrium transport through a single molecular Kondo impurity in a scanning tunneling microscope junction Ungdon Ham, Wilson Ho An unpaired spin from a single electron trapped in a molecular orbital in a double barrier scanning tunneling microscope (STM) junction at sub-Kelvin temperature and high magnetic field showed a non-equilibrium transport through a Kondo impurity. Hysteresis and switching in a conductance allows the spin and charge state of the molecule in the junction to be controlled. Mechanically tuning the coupling of the single spin to STM tip showed a gradual change from lowest order spin-flip inelastic tunneling spectroscopy (IETS) to the Kondo resonance. Using the imaging capability of STM, we observed clear sub-molecular node structures of the spin-flip IETS and the Kondo resonance. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J18.00013: Electronic transport through single-molecule magnets by scanning tunneling spectroscopy Simranjeet Singh, Jyoti Katoch, Taketo Taguchi, George Christou, Masa Ishigami, Enrique del Barco Atomic structure of molecules and electrodes are expected to sensitively influence the properties of molecular spintronics devices. We have studied the transport properties of individual Mn$_{4}$ single-molecule magnets bound to a surface using atomic force and scanning tunneling microscopy at cryogenic temperatures Unlike previous scanning probe microscopy experiments, we are able to continuously tune the density of states of individual molecules using novel device geometries in-situ We will discuss transport properties of single-molecule magnets as a function of their atomic structure, coupling to electrodes and the Fermi levels. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J18.00014: Magnetic excitations from an $S$=1/2 antiferromagnetic tetramer system Cu$_2$PO$_4$OH M. Matsuda, D.L. Abernathy, K. Totsuka, A.A. Belik Cu$_2$PO$_4$OH is a candidate material for the $S$=1/2 diamond-shaped antiferromagnetic tetramer system.\footnote{A. A. Belik $et$ $al.$, Inorg. Chem. 46, 8684 (2007).} The magnetic susceptibility shows a spin-gap behavior and the exchange interaction $J$ was estimated to be 138 K. Since there have not been so many experimental studies in the spin tetramer systems, it is important to clarify the magnetism in this compound. We have performed inelastic neutron scattering experiments on a powder sample of Cu$_2$PO$_4$OH on a chopper neutron spectrometer ARCS installed at SNS at ORNL in order to study the magnetic excitations from the tetramer spin system. We have clearly observed two magnetic excitations at $\sim$12 and $\sim$20 meV, whose widths in energy are broader than the instrumental resolution. It was found that the energy levels cannot be explained with the simple antiferromagnetic tetramer model with only nearest-neighbor interaction. We will discuss the results including further-neighbor interactions. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J18.00015: High-field EPR study of a ReCl$_{4}$(CN)$_{2}$ molecular magnet building block Junjie Liu, T. David Harris, Jeffrey Long, Stephen Hill Slow magnetic relaxation has been observed in the single-chain magnet (DMF)$_{4}$MReCl$_{4}$(CN)$_{2}$ (M = Mn, Fe, Co, Ni) [D. Harris \textit{et al}., J. Am. Chem. Soc. \textbf{132}, 3980 (2010)]. The ReCl$_{4}$(CN)$_{2}$ \textbf{(1)} molecule has been synthesized in which the local environment of the Re$^{IV}$ ion is same as in the single-chain magnet. Electron Paramagnetic Resonance (EPR) measurements have been performed on single crystal of complex \textbf{1} to determine the magnetic anisotropy of the Re$^{IV}$ ions. Both intra and inter Kramer's doublet transitions are observed in high-field (up to 36T) EPR experiments. The data indicate a significant axial anisotropy of the easy-plane type ($D>$ 0), with sizeable rhombic $E$ term. In light of these findings, we are developing a theoretical model to account for the slow relaxation in the single-chain magnet. [Preview Abstract] |
Session J19: Focus Session: Spin Transport & Magnetization Dynamics in Metals IV
Sponsoring Units: GMAG DMP FIAPChair: Peter Fischer, Lawrence Berkeley National Laboratory
Room: D170
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J19.00001: Vortex-Core Reversal Dynamics: Towards Vortex Random Access Memory Invited Speaker: An energy-efficient, ultrahigh-density, ultrafast, and nonvolatile solid-state universal memory is a long-held dream in the field of information-storage technology. The magnetic random access memory (MRAM) along with a spin-transfer-torque switching mechanism is a strong candidate-means of realizing that dream, given its nonvolatility, infinite endurance, and fast random access. Magnetic vortices in patterned soft magnetic dots promise ground-breaking applications in information-storage devices, owing to the very stable twofold ground states of either their upward or downward core magnetization orientation and plausible core switching by in-plane alternating magnetic fields or spin-polarized currents. However, two technologically most important but very challenging issues --- low-power recording and reliable selection of each memory cell with already existing cross-point architectures --- have not yet been resolved for the basic operations in information storage, that is, writing (recording) and readout. Here, we experimentally demonstrate a magnetic vortex random access memory (VRAM) in the basic cross-point architecture. This unique VRAM offers reliable cell selection and low-power-consumption control of switching of out-of-plane core magnetizations using specially designed rotating magnetic fields generated by two orthogonal and unipolar Gaussian-pulse currents along with optimized pulse width and time delay. Our achievement of a new device based on a new material, that is, a medium composed of patterned vortex-state disks, together with the new physics on ultrafast vortex-core switching dynamics, can stimulate further fruitful research on MRAMs that are based on vortex-state dot arrays. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J19.00002: Dynamics of coupled vortices in spin-valve nanostructures Paolo Bortolotti, N. Locatelli, V. Cros, J. Grollier, V.V. Naletov, G. de Loubens, C. Ulysse, G. Faini, O. Klein, A. Fert Recently, vortex dynamics driven by spin-transfer torque have been considered for new generation of nano-oscillators and memory devices. In this work we study the coupled vortex dynamics in FeNi(15nm)/Cu(10nm)/FeNi(4nm) samples where one single vortex state is favoured in both magnetic layers. Our experimental data are in good agreement with the corresponding simulations obtained through a 3D spin diffusion approach. Each vortex is characterized by a given chirality and polarity controllable separately by varying the external field (both in-plane and out-of-plane) and by applying a DC current perpendicular to the sample plane. The system modes are detected by static magneto-transport and microwave emissions analysis. In particular, it can be shown that vortex dynamics with large power appear only for configuration characterized by vortex cores pointing in opposite directions. The coupling of those two vortices allows to reach very narrow peak linewidth (down to 50 kHz), two order of magnitude smaller than in the uncoupled case. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J19.00003: Statistical Behavior of Formation Process of Magnetic Vortex State in Permalloy Nanodisks Mi-Young Im, Peter Fischer, Yamada Keisuke, Shinya Kasai Magnetic vortices in magnetic nanodots, which are characterized by an in-plane (chirality) and an out-plane (polarity) magnetizations, have been intensively attracted because of their high potential for technological application to data storage scheme and their scientific interest for an understanding of fundamental physics in magnetic nanostructures. Complete understanding of the formation process of vortex state in magnetic vortex systems is very important issue in both technical and scientific points of view. In our work, we have statistically investigated the formation process of vortex state in permalloy (Ni$_{80}$Fe$_{20})$ nanodisks through the direct observation of vortex structure utilizing a magnetic transmission soft X-ray microscopy (MTXM) with a high spatial resolution down to 20 nm. We found a particular selectivity between the circulation sense of chirality and orientation sense of polarity for each other in the formation process of vortex state. Dzyaloshinskii-Moriya interaction inevitably generated in magnetic nanodisks is mainly responsible for the experimentally witnessed selectivity between chirality and polarity. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J19.00004: Imaging Magnetic Normal Modes Driven by Spin Transfer Torque in Magnetic Nanopillars using Soft X-ray Microscopy Yong-Tao Cui, Lin Xue, Peter Fischer, Mi-Young Im, R.A. Buhrman, D.C. Ralph Motivated by the desire to understand the spatial structure of the high-frequency dynamical magnetic modes that can be excited by spin transfer from spin-polarized currents, we report measurements using X-ray microscopy to image magnetic normal modes in nanopillar devices resonantly excited by spin torque from a microwave frequency current. The frequency of the microwave current is phase locked to the incident X-ray pulses. We achieve 70 ps time resolution and 25 nm spatial resolution, enabling us to study the spatial configuration of the magnetization throughout the cycle of resonant magnetization dynamics. We will discuss the initial results of our measurements and comparisons with micromagnetic simulations. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J19.00005: Voltage-controlled Spin Wave Logic Device: Ring Interferometer Tianyu Liu, Giovanni Vignale Spin wave logic circuitry transmits information by propagating spin waves along magnetically insulating wave guides. This is less power-consuming than ordinary circuits and is expected to work at THz frequency and room temperature. Logical operations are performed by modulating the interference of spin waves through a phase shifter. A great deal of effort has been devoted to the problem of controlling the phase of spin waves by means of a spatially varying magnetic field, either extrinsic or intrinsic (i.e., by passing the spin wave through a non-uniform magnetic texture). Here we introduce a new approach, which exploits the response of spin waves to an external {\it electric field} via the spin-orbit coupling of this electric field to the electrons that mediate the magnetic interaction. Based on the Heisenberg Hamiltonian modified by spin-orbit coupling, we show how a ring interferometer made of a magnetic insulator (e.g, YIG) can be used to implement NOT logic (and other logic functions) under voltage control. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J19.00006: Phenomenology of Current-Induced Dynamics in Antiferromagnets Kjetil M.D. Hals, Yaroslav Tserkovnyak, Arne Brataas In antiferromagnets, an electric current can induce a torque on the staggered magnetization. We derive a novel phenomenology of current-induced dynamics in antiferromagnets. The theory includes effects of damping, external magnetic fields, and both adiabatic and non-adiabatic current-induced torques. We apply our theory to an antiferromagnetic domain wall system, and find an analytic solution for the domain wall motion in the low current density regime. In this regime, the domain wall velocity is proportional to the ratio between the non-adiabatic torque and the damping coefficient. In addition, the domain wall develops a net magnetic moment. This opens the route to an alternative way to observe current-induced effects in antiferromagnets. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J19.00007: Temperature dependence of domain wall dynamics in Permalloy nanowires Jusang Yang, James L. Erskine Current-driven [1] and current-assisted field-driven [2] domain wall dynamics in ferromagnetic nanowires have the thermal effects resulting from Joule heating, which make difficult to separate the spin-torque effects on domain wall displacements. To understand the thermal effects on domain wall dynamics, temperature dependence of field-driven domain wall velocity was studied using high-bandwidth scanning Kerr polarimetry. Domain wall velocity curves of 20 nm thick Permalloy nanowires with various widths (from 400 nm to 1000 nm) were measured with increasing temperature from 300 K to 400 K. Walker critical fields decreased with increasing temperature, which can be attributed to thermal excitations, and temperature-induced stochastic dynamics mode changes were observed. The results will be discussed in relation to internal domain wall structures.\\[4pt] [1] M. Klaui et al., Phys. Rev. Lett. 95, 026601 (2005).\\[0pt] [2] G.S.D. Beach et al., Phys. Rev. Lett. 97, 057203 (2006). [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J19.00008: Voltage induced by domain wall motion in a ferromagnetic nanowire Yang Liu, Oleg Tretiakov, Artem Abanov We study current-induced domain-wall motion in a narrow ferromagnetic wire. This motion is described by effective equations of motion which depend only on four parameters. These parameters are set by the magnetic Hamiltonian and the shape of the wire. We propose a new way to measure these parameters by measuring time dependent voltage generated by the domain wall motion. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J19.00009: Discrete positioning of domain walls due to localized pinning sites in current driven motion along nanowires Xin Jiang, Luc Thomas, Rai Moriya, Stuart Parkin Current driven domain wall motion is studied in spin-valve nanowires. The position of the domain wall after nanosecond long driving current pulses is determined with an accuracy of better than 50 nm by measuring the resistance of the nanowire. Although the domain wall displacement scales linearly with the current pulse length, its final position is discretized. This is attributed to relaxation of the domain wall into local pinning potential minima along the nanowire after the current pulse is turned off. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J19.00010: Direct imaging of domain wall pinning in artificially created asymmetric potentials D.E. Read, L. O'Brien, S. Ladak, K. Zeissler, T. Tyliszczak, A.-V. Jausovec, H.T. Zeng, E.R. Lewis, J. Sampaio, A. Fernandez-Pacheco, D. Petit, R.P. Cowburn, W.R. Branford Domain walls (DWs) in ferromagnetic nanowires are ideal candidates for a wide variety of technological applications including high density data storage devices. To realise functional DW devices the processes associated with DW pinning must be understood and controlled. Magnetostatic pinning of DWs has already been observed experimentally using spatially resolved MOKE measurements. Using high resolution scanning transmission x-ray microscopy (STXM) we were able to directly image DWs in NiFe nanowires pinned in asymmetric potentials which were created by nano-patterning additional ferromagnetic wires perpendicular to the DW conduit. Tailoring the pinning potential in this way allows us to probe the rigidity of the DW in the region of the pinning site, increasing our understanding of DW deformation in magnetostatic traps and paving the way for future commercial applications. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J19.00011: Spin-torque-driven ballistic switching with $<$ 50ps pulses OukJae Lee, Dan Ralph, Robert Buhrman Spin-torque-driven ballistic switching is a fast, energy-efficient, non-thermal operation in which the magnetization of a nanomagnet rotates from one equilibrium state to the other without any preceding small-angle precession. This reversal scheme can be implemented with a non-collinear structure in which the magnetic free layer is located between an out-of-plane spin polarizer and an in-plane polarizer. Both for achieving better fundamental understanding of magnetic dynamics and for realizing technological advances, it is desirable to demonstrate experimentally that the free layer can be reliably reversed with a current pulse as short as possible. Moreover it is necessary to achieve an asymmetrical response as the function of both the initial state and the pulse current polarity in order to obtain the desired final state with a simple unipolar pulse. We will discuss experimental results that show that the interval of pulse widths giving reliable switching is strongly dependent on the initial magnetic state and on the current. We will also discuss strategies to further improve ballistic switching operations. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J19.00012: First-principles calculation of the photon-shortage mystery in femtosecond magnetism Guoping Zhang, Mingsu Si, Yihua Bai, T.F. George Laser-induced femtosecond magnetism needs photons to influence the magnetization in a sample, but there is a debate on whether the photon-shortage really exists [1]. Here we directly compute the number of photons used in ferromagnetic nickel, and we find that for nearly all the experiments, there are enough photons. The key is that one has to compute this number correctly using the surface instead of volume as a parameter [1,2]. Then we use the first-principles method to compute the magnetization for a fixed number of photons. Our results show that the number of photons is not a decisive factor, since for a fixed number, the laser amplitude and pulse duration can be changed systematically. We suggest that it is more appropriate to use the laser amplitude and pulse duration as two decisive parameters to characterize the role of photons, instead of the photon number. [1] M. S. Si and G. P. Zhang, J. Phys.: Cond. Matt. {\bf 22}, 076005 (2010). [2] G. P. Zhang, W. H\"{u}bner, G. Lefkidis, Y. H. Bai, and T. F. George, Nature Phys. {\bf 5}, 499 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J19.00013: \emph{Ab initio} investigation of ultrafast spin-manipulation: $\Lambda$ processes in charged two-magnetic-center nanostructures with bridging atoms Chun Li, Wei Jin, Georgios Lefkidis, Wolfgang H\"ubner We present a fully {\it ab initio} investigation of ultrafast laser-induced magnetic switching mechanisms in charged two-magnetic-center nanostructures via $\Lambda$ processes [1,2]. In order to improve the spin transferability between the magnetic centers and fulfill the energy-difference requirements for the $\Lambda$ processes [3], a small number of nonmagnetic bridging atoms (O and Mg) is used to connect the magnetic centers. These bridging atoms influence the overlap between the magnetic centers. It is shown that both bridging atoms can redistribute the spin density on the structure by changing either the local spin density or even the total spin localization. Especially, the spin-transfer scenario achieved in [Fe-O(Mg)-Co]$^+$ confirms that using bridging atoms can significantly enhance the spin transferability between the magnetic centers. \\[0pt] [1] C. Li, T. Hartenstein, G. Lefkidis {\it et al}, PRB {\bf 79}, 180413(R) (2009).\\[0pt] [2] T. Hartenstein, C. Li, G. Lefkidis {\it et al}, JPD {\bf 41}, 164006 (2008).\\[0pt] [3] G. Lefkidis, G. P. Zhang, and W. H\"{u}bner, PRL {\bf 103}, 217401 (2009). [Preview Abstract] |
Session J20: Optoelectronic Devices & Applications
Sponsoring Units: FIAPChair: Patrick Folkes, Army Research Laboratory
Room: D168
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J20.00001: Scanning Ladar: Spatial Imaging Performance Through Turbulence Mazen Nairat, David Volez, Srinivasu Pudi The performance of scanning laser radar is studied for generating two-dimension spatial images at long ranges. Performance is described in terms of the Modulus Transfer Function (MTF). A simple analytic expression for the MTF associated with wave front tilt caused by propagation through atmospheric turbulence is explicitly derived. The derivation includes consideration of the influence of the Fresnel length. A physical optics simulation is employed to demonstrate the applicability of the MTF approach. The results are compatible with theoretical expressions that describe the image. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J20.00002: Stimulated Terahertz Smith-Purcell Radiation in Planar Gunn Diodes Alexey Belyanin, Don D. Smith We propose a room-temperature semiconductor source of coherent narrowband Smith-Purcell radiation (SPR) in the spectral range of 0.1-1.2 THz. Spontaneous SPR in semiconductors has been observed at low temperature with very low power. Practical vacuum SPR devices utilize a pre-bunched electron beam to achieve the stimulated mode of operation. However, electron bunches quickly dissipate in semiconductors. We propose to utilize the Gunn instability to form stable charge bunches (Gunn domains) that enable semiconductor sources of stimulated SPR. The device is a planar Gunn diode with a thin dielectric spacer layer and metallic grating deposited on the drift region. The SPR frequency is determined by the domain velocity and the grating period. In contrast to conventional Gunn diodes, the frequency is not limited by the transit time. Our calculations show that technologically relevant power density levels (1-100 nW per micrometer of device width) may be achieved by this method. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J20.00003: Correlated photon fluctuations at the onset of first-order optical coherence Patrick Folkes We report the observation of correlated photon fluctuations over a narrow range of current at threshold of an interband cascade laser using single-detector photon noise measurements. The correlated photon noise is manifested by large fluctuations in the low-frequency photon noise spectral density at certain discrete frequencies which are sensitive to the laser gain characteristics. We observe the concurrent emergence and growth of the lasing mode over the same current range indicating that the correlated photon noise provides evidence of the occurrence of a change in the photon fluctuation statistics and the onset of first-order coherence in the laser emission. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J20.00004: Self Generation of Chaos From Electrical Solitons Ozgur Yildirim, Donhee Ham The nonlinear transmission line (NLTL) is a structure that can generate electrical solitons of subpicosecond duration. As an autonomous soliton generator utilizing the NLTL, thus far, only periodic electrical soliton oscillators have been reported. These circuits self generate a periodic train of solitons, where an amplifier with a saturable absorber prevents the generation of multiple solitons and hence their nonlinear collisions. However, if the amplifier encourages the generation of multiple solitons and their collisions, the system can attain chaos, because the position of the soliton modulates after each collision, disrupting the periodicity. In this work, for the first time, we experimentally demonstrate such a chaotic system. Our circuit self generates an aperiodic signal, which has a continuous spectral distribution. We confirm its chaotic behavior by calculating the largest Lyapunov exponent, and show that the dimensionality of the generated chaos is high (d$>$3) by performing a false-nearest-neighbors analysis. Moreover, we explicitly measure the route from periodic soliton oscillation to chaotic oscillation via decreasing the time constant of the saturable absorber, showing the effect of soliton collisions on period-doubling bifurcations and finally the creation of chaos. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J20.00005: Harmonic Bloch and dipole oscillations and their transition in elliptical optical waveguide arrays Yun San Chan, Ming Jie Zheng, Kin Wah Yu We have studied harmonic oscillations in an elliptical optical waveguide array in which the couplings between neighboring waveguides are varied in accord with a Kac matrix so that the propagation constant eigenvalues can take equally spaced values. As a result, the long-living optical Bloch oscillation (BO) and dipole oscillation (DO) are obtained. Moreover, when a linear gradient in the propagation constant is applied, we achieve a switching from DO to BO and vice versa by ramping up or down the gradient profile [1]. The various optical oscillations as well as their switching are investigated by field evolution analysis and confirmed by Hamiltonian optics. The equally spaced eigenvalues in the propagation constant allow viable applications in transmitting images, switching and routing of optical signals. \\[4pt] [1]. M. J. Zheng, Y. S. Chan and K. W. Yu, J. Opt. Soc. Am. B 27, 1299 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J20.00006: Bloch-dipole-Zener Oscillations in Binary Parabolic Optical Waveguide Arrays Ming Jie Zheng, Yun San Chan, Kin Wah Yu We have studied the propagation and Zener tunneling of light in the binary parabolic optical waveguide arrays (BPOWA) consisting of two evanescently coupled dissimilar optical waveguides. BPOWA attains two minibands with a gap at the zone edge due to Bragg reflections. Various superposition of optical oscillations and Zener tunneling are identified for different parameters on the phase diagram. In particular, both Bloch-Zener oscillation [1] and Bloch-dipole-Zener oscillation are obtained in the BPOWA by the field-evolution analysis. The research results may have potential applications in optical splitting and waveguiding devices and shed light on the coherent phenomena in lattice structures [2]. \\[4pt] [1] F. Dreisow, A. Szameit, M. Heinrich, T. Pertsch, S. Nolte, A. Tunnermann, and S. Longhi, ``Bloch-Zener Oscillations in Binary Superlattices,'' Phys. Rev. Lett. 102, 076802 (2009). \\[0pt] [2] M. J. Zheng, G. Wang, and K. W. Yu, ``Tunable Hybridization at Mid Zone and Anomalous Bloch-Zener Oscillations in Optical Waveguide Ladders,'' Opt. Lett. (in press). [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J20.00007: Cavity phase matching for a high efficient sheet optical parametric oscillator Z.D. Xie, X.J. Lv, S.N. Zhu Cavity phase matching (CPM) was first proposed in the early days in nonlinear optics by J. A. Armstrong, N. Bloembergen, J. Ducuing and P. S. Pershan in theory in 1962 as one of the three protocols to realize phase matching in nonlinear medium. The other two protocols have been developed into the well-known quasi-phase matching (QPM) technique. CPM has equivalent capability to compensate for the phase mismatching as QPM in principle and people has been attempting to achieve CPM in several kinds of semi-conductor materials. However, there is no convincing experimental realization up to date. In the work, we manufactured, for the first time, a CPM optical parametric oscillator (OPO) which consisted of a 217$\mu$m thick KTP crystal sheet whose two surfaces were optically coated for the resonance recirculation of signal and idler. The sheet OPO could emitted the near-frequency-degenerate signal and idler beams in near-infrared region with a quasi-continuously tunable frequency difference ranging from 0.35 to 26.1\textit{THz}. This mini-device showed the high slope efficiency up to 22{\%}, and as well the unique spectral and spatial features, like single-longitudinal-mode, single-spatial-mode, narrow linewidth, etc. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J20.00008: Variation of Losses with Detuning in Bragg Gratings Sergiy Mokhov, Derrek Drachenberg, George Venus, Boris Zeldovich, Leonid Glebov The optical losses due to small scattering and absorption in Bragg gratings are proportional to the loss coefficient, and also depend on the integral of optical power over grating length. At different resonant conditions this integral of stored optical power inside a grating differs from the product of power by length for transmitted beams far from resonance. We have found an analytical expression for the relative value of this integral in the case of a uniform grating. If it equals unity for a beam propagating out of resonance, then for a grating in Bragg resonance with reflectance of 99{\%} it equals 0.332 due to exponential decay of reflected and transmitted power inside the grating and grows up to 2.027 near the first zero of the reflection spectrum due to increased resonant capacity of the grating similar to a Fabry-Perot resonator. Also, we have found analytically that in the case of spatially modulated grating losses with Bragg period the odd-functional term will be present in the expression for relative losses in addition to the term for averaged losses. We have measured this variation of losses in volume Bragg grating at a small incidence angle, and in this case can resolve the incident and reflected beams and precisely measure the power balance in the experimental setup, and the results show good agreement with theory. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J20.00009: Temperature dependence of polaritons in ZnO based hybrid micro-cavity Ryoko Shimada, Sankar Davuluri, Hadis Markoc, Arup Neogi We have studied the temperature dependence of cavity polaritons in bulk ZnO-based hybrid microcavities. The bulk ZnO-based micro cavity is formed by 36 pairs of AlGaN/(Al)GaN distributed Bragg reflector at the bottom of (lambda)/4 thick ZnO cavity and eight pairs of SiO2/SiN DBR as top mirror. Shift in exciton resonances with temperature resulted in shift in the energy levels of upper and lower polariton modes. The magnitude of observed energy shifts in polariton modes is dependent on the angle at which photoluminescence is collected. It can be possible to obtain either a upper or lower polariton mode that is stable over a long range of temperature by selectively collecting the polaritons modes emitted at a particular angle. The temperature dependent carrier dynamics of the upper or lower polariton mode has been studied by time resolved spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J20.00010: Study of the Coherent Phonon-Polariton effect on the Terahertz pulse generation in $<$110$>$ ZnTe crystal Chien-Ming Tu, Jeng-Chung Chen, Cheng-Chung Chi We report a study of the wave form and spectrum of the THz radiation generated by illuminating $<$110$>$ ZnTe crystal with femto-second optical pulses of 750 nm in wavelength. The co-linearly measured wave form consists of a main W-shaped THz pulse and a trailing quasi monochromatic damped oscillation (QMDO) with a duration of several tens of pico-seconds. In Fourier-transformed spectrum of the measured THz waveform, there are two peaks, one centered at 0.6 THz and the other one at 2.7 THz, which correspond to the main THz pulse and the QMDO respectively. Our calculation of the THz pulse generated by the optical pulse indicates that the QMDO is caused by the phase matching of the optical pulse and the coherent phonon-polariton in ZnTe. We observe that, by increasing the optical pulse width, the duration of the trailing QMDO shrinks in time domain, and the amplitude of the phase-matching component also reduces, both of which are consistent with our calculations. There remain some subtle differences between the experimental results and the theoretical calculations, the origin of which will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J20.00011: Accurate Formulation and Numerical Calculation of Faraday, Magnetic Circular Dichroism (MCD) and Kerr Effect of Light in Magnetized Cubic Crystal Jin T. Wang, Sean Hall, Yi Zhen, Dong-Sheng Guo Faraday, magnetic circular dichroism and Kerr effects are three important magneto-optic effects. They are significant in fundamental sciences and applications. Presently, scientists in this field believed that Faraday and Kerr effects are caused by the difference in real parts of the refractive indices of the magnetic crystal for left-and right-circularly polarized light and the magnetic circular dichroism is caused by the difference in the imaginary parts of the refractive index (absorption) of the magnetic crystal for left-and right-circularly polarized light. However, the derived equations for these effects are approximated only. In our paper we obtained accurate formulations for these effects and found that there are mistakes in the present conclusions with respect to the above mentioned these effects. The precise equations, conclusions from our derivation and the results of numerical calculation are presented. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J20.00012: Quantum cascade laser with low threshold and high characteristic temperature T$_{0}>$ 300K at $\sim $ 14 $\mu $m Xue Huang, William Charles, Claire Gmachl High-performance quantum cascade (QC) lasers with wavelength in 4 $\sim $ 12 $\mu $m range are widely used in trace gas sensing. However, lack of high performance for longer wavelength in the 12 $\sim $ 16 $\mu $m range, where exist the strongest absorption lines of BTEX (benzene, toluene, ethylbenzene, and xylenes) and Uranium Hexafluoride, prohibits QC laser applications in sensing these important gases. The QC laser emitting at $\sim $ 14 $\mu $m we investigate here is based on a diagonal-transition design. The depletion of the lower laser state is achieved by a one-phonon-continuum scheme instead of the widely used ``continuum'' lower mini-bands in existing long-wavelength lasers. This scheme reduces LO scattering from the upper laser state, the leakage from the injector and thermal back-filling to the lower laser state, thus attaining population inversion efficiently. The laser shows low threshold (J$_{th}$ =2.4 kA/cm$^{2}$ for a 1.97-mm-long laser at room temperature), and a high characteristic temperature T$_{0}$ =309K fitted from $\mbox{J}_{\mbox{th}} \mbox{(T) = J}_0 e^{\mbox{T/T}_0 }$ , which is comparable with the record highest characteristic temperature. The peak power is 1.4W at 80K and 0.25W at 300K. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J20.00013: The effect of energy density on laser flyer velocity H.R. Brierley, T.A. Vine Laser driven flyers are produced on the interaction of a laser pulse with a thin film of metal. When an Nd:YAG laser is focussed through a fused silica substrate onto a 5 micron layer of aluminium, a fraction of the metal is ablated. This causes the remaining aluminium to be punched from the film and launched as a discrete flyer. By varying the energy of the incident laser pulse, the velocity of the resulting flyer is changed. The Nd:YAG laser beam was spatially filtered to remove higher order modes. This improved the beam quality and reduced the focal spot diameter. The resulting higher energy densities led to faster flyer velocities for a given pulse energy. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J20.00014: Spatially-sculpted aberrated optical tweezers for delivery of nanoparticles onto cells Shivaranjani Shivalingaiah, Suyash Chhajed, Samarendra Mohanty Nanoparticles (NP) are emerging as photochemical and photothermal agents for delivery of drugs and heat onto the targeted cells. Here, we report spatially-sculpting of transverse potential landscape by introducing aberration in the optical tweezers beam for delivery of therapeutic NP on to the prostate cancer PC3 cells. A tunable Ti-Sapphire laser beam was focused to a diffraction limited spot by use of a high numerical aperture microscope objective for optical trapping. A cylindrical lens was used to create the beam profile astigmatic, which led to spatially extended potential landscape. In order to facilitate transport of NP, Comatic potential was created by tilting of the astigmatic beam with respect to the optic axis. NPs were attracted towards the potential minima, transported along the major axis of the elliptic spot and ejected out along the direction having lower stiffness. The Carbon NPs as well as Poly Lactic-\textit{co}-Glycolic Acid NPs were efficiently transported and concentrated near the PC3 cells\textit{ in-vitro}. The direction and the speed of transport of nano-particles could be reversed by change in tilt direction and angle. Further, by utilizing the scattering force with the asymmetric gradient force, three-dimensional transport of nanoparticles was achieved. The effect of laser beam power and size / refractive index of the nano-particles on the speed of transport will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J20.00015: Mapping force of interaction between PLGA nanoparticle with cell membrane using optical tweezers Suyash Chhajed, Ling Gu, Homa Homayoni, Kytai Nguyen, Samarendra Mohanty Drug delivery using magnetic (Fe$_{3}$O$_{4})$ Poly Lactic-\textit{co}-Glycolic Acid (PLGA) nanoparticles is finding increasing usage in therapeutic applications due to its biodegradability, biocompatibility and targeted localization. Since optical tweezers allow non-contact, highly sensitive force measurement, we utilized optical tweezers for studying interaction forces between the Fe$_{3}$O$_{4}$-PLGA nanoparticles with prostate cancer PC3 cells. Presence of Fe$_{3}$O$_{4}$ within the PLGA shell allowed efficient trapping of these nanoparticles in near-IR optical tweezers. The conglomerated PLGA nanoparticles could be dispersed by use of the optical tweezers. Calibration of trapping stiffness as a function of laser beam power was carried out using equipartition theorem method, where the mean square displacement was measured with high precision using time-lapse fluorescence imaging of the nanoparticles. After the trapped PLGA nanoparticle was brought in close vicinity of the PC3 cell membrane, displacement of the nanoparticle from trap center was measured as a function of time. In short time scale ($<$ 30sec), while the force of interaction was within 0.2 pN, the force increased beyond 1pN at longer time scales ($\sim $ 10 min). We will present the results of the time-varying force of interactions between PLGA nanoparticles with PC3 cells using optical tweezers. [Preview Abstract] |
Session J21: Focus Session: Imaging and Modifying Materials at the Limits of Space and Time Resolution I
Sponsoring Units: DMP GIMS DCPChair: Arnaud Delcorte, Universite Catholique de Louvain, Belgium
Room: D161
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J21.00001: Transformation of carbon nanoparticles under laser microirradiation Ninad Ingle, Vijayalakshmi Varadarajan, Ali Koymen, Samarendra Mohanty Functional, mechanical, electrical and thermal properties of carbon nanoparticles (CNP) have been shown to change significantly with change in its shape and structure. Here, we show that shape of the CNPs can be transformed by exposure to tightly focused near-infrared Ti: Sapphire laser beam. The CNPs were prepared using electric plasma discharge generated in an ultrasonic cavitation field of liquid benzene. High resolution TEM image showed nanoparticles with average radius of $\sim $5nm with crystalline structure. A Nanonics Multiview Atomic Force Microscopy (AFM) was integrated on the laser micro-irradiation system to reveal the shape transformation of the CNPs before and after laser irradiation. Since near-IR laser irradiation can lead to significant heat generation in CNP in absence of aqueous solution (sink), the system is far from thermal equilibrium and can curve or bend graphitic layers by introducing topological defects. The photothermally-induced shape transformation can occur below laser power required for complete melting of CNP since surface melting can suffice the observed shape transformation. The results show significant reduction in the volume of irradiated CNP-clusters, which was attributed coalescing of melted CNPs. Raman spectroscopic measurements are being carried out to evaluate possibility of ultrastructural changes. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J21.00002: The effect of Au condensation in laser desorption/ionization of organic materials Aneesh Prabhakaran, Arnaud Delcorte Matrix-assisted desorption/ionization (MALDI) mass spectrometry, where the analyte is mixed in a low molecular weight matrix, often constitutes a limitation for the analysis and imaging of real world samples. Herein, we investigate the influence of a thin layer of gold (1-15nm) deposited on the surface of different organic materials, in the laser ablation using 355nm wavelength light. We see a significant effect of the condensed metal nanoparticles in the laser ablation process. Compared to pristine samples, the metallized samples show a significant intensity of characteristic fragments as well as metal cationized molecules. Relatively soft desorption/ionization is indicated by the observation of characteristic molecular ions of the different analytes. The observed effects can be explained by the increased laser absorption by the gold nanoparticles in this wavelength range and the increased ionization by the gold. Hence the metallization improves the surface characterization using lasers and also proves to be a novel technique for chemical imaging of organic surfaces. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J21.00003: Atomic-level simulations of structural transformations in layered Au-Cu and Ag-Cu metal targets irradiated by a femtosecond laser pulse Chengping Wu, Derek Thomas, Zhibin Lin, Leonid Zhigilei The structural transformations in Ag/Au film - Cu substrate systems irradiated by femtosecond laser pulses are investigated in simulations performed with a model that couples the molecular dynamics method with a continuum-level description of the laser excitation and subsequent relaxation of the excited electrons. The higher strength of the electron-phonon coupling in Cu compared to Ag and Au results in a preferential sub-surface heating and melting of the Cu substrate. The melting is followed by rapid cooling and resolidification. In the case of Cu-Ag system, the rapid resolidification results in a complex structure of the interfacial region, where the lattice-mismatched interface is separated from the Ag-Cu mixing region by an intermediate pseudomorphic bcc Cu layer that grows epitaxially on the (001) face of the fcc Ag film during the final stage of the resolidification process. The new lattice-mismatched interface consists of a periodic array of stacking fault pyramids outlined by stair-rod partial dislocations. The intermediate bcc layer and the stacking fault pyramid structure of the mismatched interface present a barrier for dislocation propagation, resulting in the effective hardening of the layered structure treated by laser irradiation. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J21.00004: Optical Antennas for Enhanced Light Absorption and Emission Invited Speaker: Lukas Novotny The absence of optical antennas in technological applications is primarily associated with their small scale. Antennas have characteristic dimensions on the order of a wavelength, demanding fabrication accuracies better than 10nm for the optical frequency regime. The advent of nanoscience and nanotechnology provides access to this length scale but material challenges associated with optical antennas remain. For example, the penetration of radiation into metals can no longer be neglected. The electromagnetic response is then dictated by collective electron oscillations (plasmons) characteristic of a strongly coupled plasma. These collective excitations make a direct downscaling of traditional antenna designs impossible and demand the careful study of plasmon resonances in metal nanostructures. The introduction of the antenna concept into the optical, infrared and terahertz frequency regime holds promise for a wide range of novel technological applications. Optical antennas can be employed to enhance the efficiency of photovoltaics, to release energy from nanoscale light-emitting devices, and to boost the efficiency of photochemical or photophysical detectors. In this presentation, I will outline the physical properties of optical antennas, review relevant history and recent work. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J21.00005: In Situ 3D Coherent X-ray Diffraction Imaging of Shock Experiments: Possible? John Barber In traditional coherent X-ray diffraction imaging (CXDI), a 2D or quasi-2D object is illuminated by a beam of coherent X-rays to produce a diffraction pattern, which is then manipulated via a process known as iterative phase retrieval to reconstruct an image of the original 2D sample. Recently, there have been dramatic advances in methods for performing fully 3D CXDI of a sample from a single diffraction pattern [Raines et al, Nature 463 214-7 (2010)], and these methods have been used to image samples tens of microns in size using soft X-rays. In this work, I explore the theoretical possibility of applying 3D CXDI techniques to the in situ imaging of the interaction between a shock front and a polycrystal, a far more stringent problem. A delicate trade-off is required between photon energy, spot size, imaging resolution, and the dimensions of the experimental setup. In this talk, I will outline the experimental and computational requirements for performing such an experiment, and I will present images and movies from simulations of one such hypothetical experiment, including both the time-resolved X-ray diffraction patterns and the time-resolved sample imagery. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J21.00006: Lattice dynamics of laser excited self-assembly gold nanocrystals by time resolved X-ray diffraction Kouhei Ichiyanagi, Hiroshi Sekiguchi, Shunsuke Nozawa, Tokushi Sato, Shin-ichi Adachi, Yuji C. Sasaki The self-assembled gold nanoparticle has attracted considerable interest from researchers as the new nanodevices and bio-sensors. Functional groups such as thiols and amines have assembled on the gold nanoparticles in solution. For using the functional optical nanomaterial, it is necessary to reveal the mechanism of interaction between the laser and the functional nanomaterial. In the present work, we observed the effect of photo-excited process of self-assembled gold nanocrystal in ethanol solution using picosecond time-resolved X-ray diffraction. Gold nanocrystals deposited on the NaCl (100) substrate. After isolation of gold nanocrystals from the substrate, these nanocrystals were assembled with 10-Carboxydecyl disulfide molecules in ethanol. The nanocrystals size was the diameter of about 60 -- 120 nm. The X-ray energy, pulse width and repetition rate for probing the gold nanocrystals were 15 keV, 100 ps and 945 Hz, respectively. The excitation wavelength and the pulse width were 400 nm and 150 fs. The detailed results of the lattice dynamics inside gold nanocrystals will be presented in the presentation. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J21.00007: Ultrafast Laser Matter Interaction and Pump-probe Imaging of Transient Electric Fields Jian-Min Zuo, Hyuk Park Ultrafast electron diffraction and microscopy use pulsed laser as pump to initiate dynamic processes in solids. Under irradiation of pulsed laser beam of picoseconds or less, electrons inside a solid can be heated to high temperatures for a short period of time (several picoseconds). A part of hot electrons can be emitted from the surface in a similar way of thermionic emission. The emitted electrons, travel at speeds, produce transient electric fields (TEFs) together with the positively charged surface [1]. However, the effect of photoemitted electrons and their electric fields on ultrafast electron diffraction and microscopy has been a subject of debate [2]. Here we report direct measurement of TEFs using time-resolved electron beam imaging techniques based on the pump-probe approach. Results obtained from Pt thin films, Cu and ZnO nanowires will be shown. We demonstrate that TEFs produced by ultrafast laser irradiation can lead to large beam deflections that depend on the electron beam distance to sample surface, laser fluence and laser wavelength. The work shows that there is clearly a critical need for better understanding of TEFs in the field of ultrafast electron microscopy. The work is supported by DOE DEFG02-01ER4592, DEFG02-91-ER45439 and DOE DEFG02-07ER46453. [1] H. Park and J. M. Zuo, Applied Physics Letters 94, 251103 (2009). [2] H. Park and J. M. Zuo, Physical Review Letters 105, 059603 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J21.00008: Femtosecond Nanocrystallography with X-ray Free-Electron Lasers Invited Speaker: The ultrafast pulses from X-ray free-electron lasers have opened up a new form of protein nanocrystallography. The X-ray pulses are of high enough intensity and of sufficiently short duration that individual single-shot diffraction patterns can be obtained from a sample before significant damage occurs. This ``diffraction before destruction'' method may enable the determination of structures of proteins that cannot be grown into large enough crystals or are too radiation sensitive for high- resolution crystallography. Ultrafast pump-probe studies of photoinduced dynamics can also be studied. We have carried out experiments in coherent diffraction from protein nanocrystals, including Photosystem I membrane protein, at the Linac Coherent Light Source (LCLS) at SLAC. The crystals are filtered to sizes less than 2 micron, and are delivered to the pulsed X-ray beam in a continuously flowing liquid jet. Millions of diffraction patterns were recorded at the LCLS repetition rate of 60 Hz. Tens of thousands of the single-shot diffraction patterns have been indexed, and combined into a single crystal diffraction pattern, which can be phased for structure determination and analysed for the effects of pulse duration and fluence. Experimental data collection was carried out as part of a large collaboration involving CFEL DESY, Arizona State University, Max Planck Institute for Medical Research, University of Uppsala, SLAC, LBNL, LLNL, using the CAMP apparatus which was designed and built by the Max Planck Advanced Study Group at CFEL. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J21.00009: Plans for an Upgrade of the Advanced Photon Source Dennis Mills We are presently developing plans for an upgrade of the Advanced Photon Source facility. Science has formally issued Critical Decision 0 and approved the Mission Need Statement in April of 2010, authorizing the APS to develop a conceptual design for the APS Upgrade (APS-U) project. The proposed upgrade will include enhancements to the accelerator, beamlines, and facility infrastructure. The high brilliance x-ray beams at high photon energy (e.g. $>$ 25 keV) provided by the APS Upgrade will have strong impact on research in energy, the environment, new or improved materials, and biological studies. High-energy x-rays can penetrate into a wide range of realistic and/or extreme environments and allow imaging of structures and processes in unprecedented detail on picosecond time scales and nanometer length scales. The presentation will include some of the essential goals of the APS-U and proposed strategies to attain those goals. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J21.00010: The Jefferson Lab VUV-FEL at 10 eV and above Gwyn Williams We will present details of the vacuum ultraviolet performance of the Jefferson Lab Free Electron Laser. The JLab FEL is oscillator-based [1] and uses a superconducting energy recovered linac for CW RF operation at up to 75 MHz. Lasing at a fundamental wavelength of 372 nm, the third harmonic is at 124 nm, corresponding to a photon energy of 10 eV. The energy per pulse in the fundamental is 20 microJoules, which at 9 MHz yields an average power of 180 Watts. The pulses have a FWHM of order 300 fs, which essentially determines the optical bandwidth. The third harmonic, which is a 0.1 - 1\% fraction of this, is considerably brighter than any other source in the region. Further, being an FEL, there is a wide range of tunability in the 1 eV to 15 eV range. Additional reach is possible with increased electron beam energy, and some options will be discussed in the talk.\\[4pt] [1] S. Benson et al. Nucl. Instrum. Methods A582, 14-17 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J21.00011: Ultrafast optical fiber microbeam for in-depth fabrication, trapping and fluorescence excitation Mervyn Pinto, Yogeshwar Mishra, Ninad Ingle, Samarendra Mohanty Micro-focused laser beam is finding widespread application in two-photon polymerization (TPP), microsurgery, two-photon fluorescence microscopy and optical trapping of microscale objects. However, limited by short working distance of the microscope objective, it is essential to develop fiber based laser microbeam for in-depth applications. While fiber-optic two-photon fluorescence excitation (TPE) has been explored in past for endoscopic imaging, only recently we demonstrated optical trapping and microsurgery using single fiber optical microbeam. Here, we present use of ultrafast laser coupled to microfabricated single mode optical fiber for in-depth fabrication of microstructures by TPP as well as TPE and manipulation of microscopic objects by fiber optical microbeam tweezers. The microfabrication of fiber optic axicon tip was optimized so as to perform all the four functions, namely fabrication, excitation, manipulation and collection of fluorescence from the trapped object. Owing to the propagation distance of Bessel-like beam emerging from the axicon-fiber tip, relatively longer streak of fluorescence was observed along the microsphere length. Stable trapping of the fluorescent objects was observed due to reduced scattering force as compared to axial gradient force. These results using multifunctional optical fiber will be presented. [Preview Abstract] |
Session J22: Theory of Non Fermi Liquids
Sponsoring Units: DCMPChair: Subir Sachdev, Harvard University
Room: D163
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J22.00001: Resistivity of a non-Galilean Fermi liquid near Pomeranchuk Quantum Criticality Dmitrii Maslov, Vladimir Yudson, Andrey Chubukov We analyze the effect of the electron-electron interaction on the resistivity of a metal near a Pomeranchuk quantum critical point (QCP). We show that Umklapp processes are not effective near a QCP, and one must consider the interplay between interaction and disorder. By power counting, the correction to the residual resistivity at low $T$ scales as $AT^{(D+2)/3}$ at QCP ($T^{4/3}$ in 2D). We show, however, that that $A=0$ for a simply connected and convex Fermi surface in 2D due to hidden integrability of the electron motion. We argue that $A >0$ in a two-band ($s-d$) model with light and heavy carriers, and propose this model as an explanation for the observed $T^{(D+2)/3}$ behavior. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J22.00002: Transport Properties near Quantum Critical Point in 2D Hubbard Model Kuang-Shing Chen, Sandeep Pathak, Shuxiang Yang, Shi-Quan Su, Dimitris Galanakis, Karlis Mikelsons, Juana Moreno, Mark Jarrell We obtain high quality estimates of the self energy $\Sigma(K,\omega)$ by {\em{direct}} analytic continuation of $\Sigma(K,i\omega_n)$ obtained from Continuous-Time Quantum Monte Carlo. We use these results to investigate the transport properties near the quantum critical point found in the 2D Hubbard model at finite doping. Resistivity, thermal conductivity, Wiedemann-Franz Law, and thermopower are examined in the Fermi liquid, Marginal Fermi liquid (MFL), and pseudo-gap regions. $\Sigma''(k,\omega)$ with $k$ along the nodal direction displays temperature-dependent scaling similar to that seen in the experiment. A next-nearest neighbor hopping $t'<0$ increases the doping region where MFL character is found. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J22.00003: Quantum Phase Transition in Interacting Quantum Wires Mehul Dixit, Tobias Meng, Markus Garst, Achim Rosch, Julia Meyer We consider the quantum phase transition of interacting electrons in a quantum wire from a one-dimensional (1D) to a quasi-1D state as a function of an external gate voltage. At weak interactions, a Lifshitz transition occurs when electrons start filling the second subband of transverse quantization. The physics in the vicinity of the transition is characterized by pronounced correlations as interactions in the second subband are effectively strong due to the diverging density of states close to the band bottom. Inter-subband interactions lead to the formation of polarons, but the nature of the transition is unchanged, i.e., one finds a Lifshitz transition of impenetrable polarons. By contrast, strongly interacting electrons form a (quasi-)1D Wigner crystal, and the transition corresponds to the linear crystal splitting into a zigzag crystal. This Ising transition in the charge sector is decoupled from the spin excitations in the system. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J22.00004: Conductance of Tomonaga-Luttinger liquid wires and junctions with resistances Diptiman Sen, Abhiram Soori We study the effect that resistive regions have on the conductance of a quantum wire with interacting electrons which is connected to Fermi liquid leads. Using the bosonization formalism and a Rayleigh dissipation function to model the power dissipation, we use Green's function techniques to derive the DC conductance. The resistive regions are generally found to lead to incoherent transport. For a single wire, we find that the resistance adds in series to the contact resistance of $e^2/h$ for spinless electrons, and the total resistance is independent of the Luttinger parameter $K_W$ of the wire. We numerically solve the bosonic equations to illustrate what happens when a charge density pulse is incident on the wire; the results depend on the parameters of the resistive and interaction regions in interesting ways. For a junction of Tomonaga-Luttinger liquid wires, we use a dissipationless current splitting matrix to model the junction. For a three-wire junction, there are two families of such matrices; we find that the conductance matrix depends on $K_W$ for one family but is independent of $K_W$ for the other family. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J22.00005: Holographic metals and fractionalized Fermi liquids Subir Sachdev I show that there is a close correspondence between the physical properties of holographic metals near charged black holes in anti-de Sitter (AdS) space, and the fractionalized Fermi liquid phase of the lattice Anderson model. The latter phase has a ``small'' Fermi surface of conduction electrons, along with a spin liquid of local moments. This correspondence implies that certain mean-field gapless spin liquids are states of matter at non-zero density which realize the near-horizon, AdS$_2 \times$R$^2$ physics of Reissner-Nordstrom black holes. I will also go beyond this mean-field theory, and discuss connections between gauge theories of fractionalized Fermi liquids and holographic theories. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J22.00006: Dynamically Generated Gap from Holography: Mottness from a Black Hole Ka-Wai Lo, Mohammad Edalati, Robert Leigh, Philip Phillips In the fermionic sector of top-down approaches to holographic systems, one generically finds that the fermions are coupled to gravity and gauge fields in a variety of ways, beyond minimal coupling. In this paper, we take one such interaction -- a Pauli, or magnetic dipole, interaction -- and study its effects on fermion correlators. We find that this interaction modifies the fermion spectral density in a remarkable way. As we change the strength of the interaction, we find that spectral weight is transferred between bands, and beyond a critical value, a hard gap emerges in the fermion density of states. A possible interpretation of this bulk interaction then is that it drives the dynamical formation of a (Mott) gap, in the absence of any symmetry breaking. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J22.00007: Quantum phase transitions in the pseudogap Anderson Holstein model Mengxing Cheng, Kevin Ingersent We study a pseudogap Anderson-Holstein model of a magnetic impurity level that (1) hybridizes with a conduction band whose density of states vanishes in power-law fashion at the Fermi energy, and (2) couples, via its charge, to a nondispersive bosonic mode (e.g., an optical phonon). The model exhibits quantum phase transitions (QPTs) of different types depending on the strength $\lambda$ of the impurity-boson coupling. For small $\lambda$, the suppression of the density of states near the Fermi energy leads to QPTs between strong-coupling (Kondo) and local-moment phases. A sufficiently large $\lambda$, however, transforms the bare Coulomb repulsion between a pair of electrons in the impurity level into an effective attraction, leading to QPTs between strong-coupling (charge-Kondo) and local-charge phases. Critical exponents characterizing the response to a local magnetic field (for small $\lambda$) or electric potential (for large $\lambda$) suggest that the QPTs belong to the same universality class as the QPT of the previously studied pseudogap Anderson model. One specific case of the pseudogap Anderson-Holstein model may be realized in a double-quantum-dot device, where the QPTs manifest themselves in the finite- temperature linear electrical conductance. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J22.00008: Statistical fluxes and the sodium cobaltate Curie-Weiss metal Kai Wu, Zheng-Yu Weng, Jan Zaanen A central pursuit in the study of quantum matter is whether non Fermi liquid states exist, as invoked in trying to explain e.g. high-$T_{c}$ superconductivity. A quite different context is the search for thermodynamic materials in energy applications, which require at the same time a very large thermopower and a low resistivity. Here we predict a new state of matter that descends from a strongly interacting microscopy described by a t-J model on a triangular lattice. Due to the altered role of quantum statistics the spins are ``localized'' in statistical Landau orbits, while the charge carriers form a Bose metal that feels the spins through random gauge fields. In contrast to the Fermi-liquid state, this state naturally exhibits a Curie-Weiss susceptibility, large thermopower, and linear-temperature resistivity, explaining the physics of \textrm{Na}$_{x}$\textrm {CoO}$_{2}$ at $x>0.5.$ A ``smoking gun'' prediction for neutron scattering is presented. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J22.00009: Spin-incoherent behavior in the ground state of strongly correlated systems Gregory Fiete, Adrian Feiguin It is commonly believed that strongly interacting one-dimensional Fermi systems with gapless excitations are effectively described by Luttinger liquid theory. However, when the temperature of the system is high compared to the spin energy, but small compared to the charge energy, the system becomes ``spin-incoherent.'' We present numerical evidence showing that the one-dimensional ``t-J-Kondo'' lattice, consisting of a t-J chain interacting with localized spins, displays all the characteristic signatures of spin-incoherent physics, but in the ground state. We argue that similar physics may be present in a wide range of strongly interacting systems. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J22.00010: DMRG study of the Phase Diagram of the Infinite U Hubbard Model Li Liu, Hong Yao, Erez Berg, Steve White, Steve Kivelson Despite decades of discussion, the phase diagram of the paradigmatic Hubbard model in the strong coupling limit remains uncertain. Here, we study Hubbard ladders with infinite on site repulsion and electron density ranging from n=0 to n=1 per site. DMRG calculations shows that the phase diagrams of two, three and four-leg laddders share the following similarities: as a function of decreasing n a fully polarized (half metallic ferromagnetic phase is followed by a partially polarized ferromagnetic metallic state, and finally by a paramagnetic (unpolarized) phase for n less than a critical value of roughly n $\sim $ 0.5, but which differs somewhat depending on the number of legs. Unexpectedly, the ferromagnetic metal phase is reentrant in the sense that it is interrupted at a special commensurate density (n=0.75 for the two-leg and 4-leg ladders and n=0.8 for the three leg) by an incompressible commensurate density wave phase with zero net ferromagnetic moment. All results appear to extrapolate smoothly to the limit of infinite ladder length. We conclude with some speculations about the phase diagram of the 2D infinite U Hubbard model. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J22.00011: Mobile impurities in ferromagnetic liquids Adrian Kantian, Ulrich Schollwoeck, Thierry Giamarchi Recent work has shown that mobile impurities in one dimensional interacting systems may exhibit behaviour that differs strongly from that predicted by standard Tomonaga-Luttinger liquid theory, with the appearance of power-law divergences in the spectral function signifying sublinear diffusion of the impurity. Using time-dependent matrix product states, we investigate a range of cases of mobile impurities in systems beyond the analytically accessible examples to assess the existence of a new universality class of low-energy physics in one-dimensional systems. \newline\newline Correspondence: Adrian.Kantian@unige.ch [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J22.00012: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J22.00013: Nonlinear Collective Field Theory for models with inverse square interaction and exchange Fabio Franchini, Manas Kulkarni, Alexander Abanov We present fully nonlinear dynamics [1] in inverse square models such as spin-Calogero model and Haldane-Shastry model. Hydrodynamic equations of motion are written for these models in the regime where gradient corrections to the exact hydrodynamic formulation of the theory may be neglected. We then show how this collective field theory allows to calculate correlation functions [2] that cannot be considered with conventional bosonization. We will also present the case of including external harmonic confinement [3] and show that the Calogero family is strikingly similar to models with delta (short-ranged) interaction and can be used as a toy model for cold atom experiments. Including harmonic trap usually ends up destroying integrability. However, Calogero family is special in this regard and the system remains integrable. In addition, we will present results of collective field theory which include gradient corrections thereby enabling us to go beyond gradient catastrophe.\\[4pt] [1] M. Kulkarni, F. Franchini, A. G. Abanov, Phys. Rev. B 80, 165105 (2009)\\[0pt] [2] F. Franchini, M. Kulkarni, Nucl. Phys. B, 825, 320 (2010)\\[0pt] [3] M. Kulkarni, A. G. Abanov, arXiv:1006.0966 [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J22.00014: Renyi entropy of gapless spin liquids Tarun Grover, Yi Zhang, Ashvin Vishwanath Spin liquids are exotic quantum states that do not break any symmetry. Though much is known about gapped spin-liquids, critical spin-liquids with strongly interacting gapless excitations in two and three spatial dimensions are less understood. Candidate ground state wave-functions for such states however can be constructed using the Gutzwiller projection method. We use bipartite entanglement entropy, in particular the Renyi entropy $S_2$ to investigate the quantum structure of these wave-functions. Using the Variational Monte-Carlo technique, we calculate the Renyi entropy of a critical spin liquid - the projected Fermi sea state on the triangular lattice. We find a violation of the boundary law, with $S_2$ enhanced by a logarithmic factor, an unusual result for a bosonic wave-function reflecting the presence of emergent spinons that form a Fermi surface. The Renyi entropy for algebraic spin liquids is found to obey the area law, consistent with the presence of emergent Dirac fermions in the system. Projection is found to completely alter the entanglement properties of nested Fermi surface states. These results show that the Renyi entropy calculations could serve as a diagnostic for gapless fractionalized phases. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J22.00015: Power-Law Behavior of Bond Energy Correlators in a Kitaev-type Model with a Parton Fermi Surface Hsin-Hua Lai, Olexei I. Motrunich We study bond energy correlation functions in an exactly solvable quantum spin model of Kitaev type on the kagome lattice with stable Fermi surface of partons proposed recently by Chua {~\textit{et~al.}}, Ref. [arXiv:1010.1035]. Even though any spin correlations are ultra-short ranged, we find that the bond energy correlations have power law behavior with a $1/r^{3}$ envelope and oscillations at incommensurate wavevectors. We determine the corresponding singular surfaces in momentum space, which provide a gauge-invariant characterization of this gapless spin liquid. [Preview Abstract] |
Session J23: Superconductivity: Fluctuation Phenomena
Sponsoring Units: DCMPChair: Michelle Johannes, Naval Research Laboratory
Room: D165
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J23.00001: Systematic determination of the superconducting fluctuation regime in the cuprates with torque magnetometry Guichuan Yu, Yuan Li, Ruihua He, Xudong Zhao, Martin Greven Among the most important temperature scales in the high-$T_{c }$cuprates are those associated with the normal state pseudogap and the superconducting (SC) gap. Early Nernst effect and torque magnetometry measurements appeared to suggest an onset of SC fluctuations well above $T_{c}$ in La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO). Recently, stripe correlations were found to contribute to the observed large Nernst signal in LSCO, leading to a reinterpretation of the earlier Nernst data. We report a systematically torque magnetometry study of the SC fluctuation regime in the simple model compound HgBa$_{2}$CuO$_{4+\delta }$, which has the highest $T_{c}$ (97 K) among all single-layer cuprates, as well as initial results for LSCO. We demonstrate that the SC fluctuation regime is narrow and that it closely tracks T$_{c}$, which implies that the higher temperature scale observed in LSCO does indeed not result from SC fluctuations. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J23.00002: Superconducting fluctuation regime in HgBa$_{2}$CuO4$_{+\delta }$ revealed by microwave measurements Neven Barisic, Mihael Grbic, Antonije Dulcic, Yuan Li, Xudong Zhao, Guichuan Yu, Martin Greven, Miroslav Pozek There have been many attempts to measure the fluctuations preceding the superconducting long-range order in the cuprates with various experimental techniques, yet the onset temperature of the fluctuation regime has proven difficult to determine. We used a novel approach to microwave conductivity measurements in order to elucidate the phase diagram of the cuprates. Initial measurements were performed on the single-layer material HgBa$_{2}$CuO$_{4+\delta }$. From c-axis data for a sample close to optimal doping, we clearly discern the opening of the pseudogap at T*=185 K, the appearance of the superconducting fluctuations at the much lower temperature T'=105 K, and the transition to the superconducting state at T$_{c}$=94.3 K. Our result implies that the superconducting fluctuations extend only to about 10 K above T$_{c}$. Using the same approach, a narrow fluctuation regime is also found in other cuprates. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J23.00003: Superconducting phase diagram and fluctuations in SmFeAsO$_{0.85}$F$_{0.15}$ single crystals U. Welp, C. Chaparro, W.-K. Kwok, A. Rydh, N.D. Zhigadlo, J. Karpinski, S. Weyeneth We use micro-calorimetry to investigate the anisotropic phase diagram and effects of superconducting fluctuations in sub-micro-gram single crystals of SmFeAsO$_{0.85}$F$_{0.15}$. Our measurements reveal that SmFeAsO$_{0.85}$F$_{0.15}$ is characterized by a large anisotropy of $\Gamma \sim $8 and a short in-plane Ginzburg-Landau coherence length of $\xi _{ab}$(0) $\sim $ 1.3 nm. These materials parameters promote strong superconducting fluctuations which are seen in the zero-field specific heat as clear upwards curvature in C/T at temperatures below T$_{c}$ = 49.5 K and long tails above T$_{c,}$. The resulting anomaly is cusp-shaped with height of $\Delta $C/T$_{c}$ = 24 mJ/moleK$^{2}$, which can be fitted with 3D-Gaussian fluctuations. The transition shows pronounced broadening in magnetic fields applied along the c-axis. The field evolution in fields higher than 3 T is well described in the frame of 3D lowest-Landau-level scaling of fluctuations using an upper critical field slope of -4.4 T/K. We will compare these characteristics to the behavior of other members of the FeAs-family. This work was supported by DOE-BES under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J23.00004: A dynamical study of phase fluctuations and their critical slowing down in amorphous superconducting films Wei Liu, Minsoo Kim, Ganapathy Sambandamurthy , Peter Armitage We report a comprehensive study of the complex AC conductance of amorphous superconducting InO$_x$ thin films. Using a novel broadband microwave ``Corbino'' spectrometer we measure the explicit frequency dependency of the complex conductance and the phase stiffness over a range from 0.21 GHz to 15 GHz at temperatures down to 350 mK. Dynamic AC measurements are sensitive to the temporal correlations of the superconducting order parameter in the fluctuation range above $T_c$. Among other aspects, we explicitly demonstrate the critical slowing down of the characteristic fluctuation rate on the approach to the superconducting state and show that its behavior is consistent with vortex-like phase fluctuations and a phase ordering scenario of the transition. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J23.00005: Superconducting fluctuations in high-Tc cuprate superconductors Brigitte Leridon Experimental results on electrical transport in various high-Tc cuprate superconductors are shown (namely in YBa$_{2}$Cu$_{3}$O$_{6+x}$, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$, La$_{1-x}$Sr$_{x}$CuO$_{4})$. The part of the conductivity associated to superconducting fluctuations in the vicinity of the transition -- also named paraconductivity - is extracted using different techniques for evaluating the normal state conductivity. Whenever possible, the conductivity measured under high pulsed field (50T) is used for the analysis. The results are compared to other experimental probes such as the Nernst effect, and are discussed from one compound to the other. It is shown that some straightforward conclusions can be drawn from relatively simple observations. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J23.00006: Pairing associated with a single quantum critical energy in superconducting electron-doped cuprates Kui Jin, Nicholas Butch, Kevin Kirshenbaum, Paul Bach, Johnpierre Paglione, Richard Greene Though a comprehensive study of magnetotransport on electron-doped La$_{2-x}$Ce$_{x}$CuO$_{4}$ thin films, we show that an envelope of spin fluctuations yielding non Fermi liquid behavior ($\rho =\rho _{0}$ +AT) surrounds the superconducting dome in the overdoped region (x =0.15 to 0.21). This behavior survives to zero temperature over a range of fields exceeding the upper critical field. For example, the resistivity of x = 0.15 is linear in temperature over three decades down to 20 mK at 7.5 T. We demonstrate that all of the relevant energy scales in this system: those determining superconducting pairing, spin correlations, and the Fermi liquid metallic state, emanate from one common critical point at the end of the superconducting dome. These observations suggest that the superconductivity pairing is associated with spin fluctuations and with a single quantum critical energy in electron-doped cuprates. This work was partially supported by NSF-DMR 0653535. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J23.00007: Shot noise measurements in mesoscopic N-S-N structures Martin Stehno, D.J. Van Harlingen Nonlocal subgap transport in mesoscopic superconductor-hybrid devices has received attention as a possible route towards creating and detecting entangled electron pairs in solid state devices. We study local and nonlocal transport in multi-terminal Cu/Al structures with transparent interfaces and separation between contacts comparable to the coherence length in the superconductor. The current shot noise in the two branches of the device is measured simultaneously and compared to the shot noise in a single contact. We discuss cross-correlations expected from Crossed Andreev Reflection and Elastic Co-tunneling processes, non-equilibrium transport in the superconductor, and device heating. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J23.00008: Transport noise crossovers in disordered electron nematics Wan-Ju Li, Benjamin Phillabaum, Erica Carlson, Karin Dahmen Recently, low-frequency transport noise in underdoped YBCO was shown to exhibit an enhancement below 250K, consistent with fluctuations associated with a symmetry-breaking collective electronic state [1]. We discuss these results in relation to crossovers associated with the development of local electronic nematic order. Using a mapping of disordered electron nematics to random anisotropic resistor networks, we predict the thermal evolution of the noise power in transport associated with the crossover to local electron nematic behavior. \\[0pt] [1] Caplan et al., Phys. Rev. Lett. {\bf 104}, 177001 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J23.00009: Vortex Noise in Thin Nb Films on a Triangular Anti-Dot Lattice Tanner Schulz, Liwen Tan, Beth Stadler, E. Dan Dahlberg Thin Nb films are deposited on a periodic, triangular, anti-dot lattice with a lattice constant of 100nm. The lattice serves as pinning sites where superconducting vortices are trapped. The vortex density is set by an external field. At a vortex density commensurate with the pinning lattice transport measurements show an increase in current density. Interstitial vortex pinning produces similar current features at integer multiples of the matching field. We examine the voltage noise spectra as the applied field and DC current bias are varied. Noise signals appear above a field dependent threshold current and show minima at the matching fields. The noise is due to vortex motion in a pinning potential that varies with vortex density and driving forces. We use our results to study vortex motion and compare our signals to existing vortex noise models. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J23.00010: Inelastic scattering effects in current noise for a one dimensional Landauer system Manohar Kumar, Zheng P. Baardman, Roel H.M. Smit, Jan M. van Ruitenbeek Generally, current shot noise is measured at low bias currents, and it reflects the transmission probability of the electrons. Here we present the first measurement at bias currents above the phonon energy of the system, $i.c.$ a chain of Au atoms. The onset of phonon emission processes is signaled by an abrupt jump in differential conductance which results from the change in the transmission probability of the electrons due to phonon excitation. One should expect a sign of this change to be visible in shot noise. Indeed, a distinct signature in the current shot noise signal is observed due to inelastic scattering as a linear deviation from the Levitov- Lesovik classical shot noise. Surprisingly, we have observed that the deviation of noise from the classical noise at the phonon frequency is either positive or negative, depending on whether the transmission is above or below 0.96G0. These observations agree with recent predictions of a sign change in the phonon-induced correction to the noise [1-3], but the point of cross-over is higher than predicted. References: \textit{1. Federica Haupt et.al., PRL 103, 136601 (2009)} \textit{2. R. Avriller et. al., PRB 80 041309 (2009)} \textit{3. T.L.Schmidt et al., PRB 80 041307(R), (2009)} [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J23.00011: AC conductivity across the disorder driven superconductor insulator transition Yen Lee Loh, Karim Bouadim, Nandini Trivedi, Mohit Randeria The superconductor-insulator transition (SIT) is defined, at the most fundamental level, in terms of electromagnetic response. The Mattis-Bardeen theory for conventional superconductors becomes inadequate near the disorder-tuned SIT, where phase fluctuations become important. We present AC conductivity results obtained using determinant quantum Monte Carlo simulations, which include both quantum and thermal phase fluctuations. We find unexpected low-energy weight in the AC conductivity especially near the SIT, and we identify possible sources of this weight. We comment on implications for experiments [1,2]. \\[4pt] [1] R. Vald\'es Aguilar et al., Phys. Rev. B 82, 180514 (2010)\\[0pt] [2] I. Hetel et al., Nature Physics 3, 700-702 (2007) [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J23.00012: Spectroscopic probes of the disorder-driven superconductor-insulator transition Mohit Randeria, Karim Bouadim, Yen-Lee Loh, Nandini Trivedi In spite of decades of research, the mechanism of the disorder- driven superconductor- insulator transition (SIT) and the nature of the insulator are not understood. We use quantum Monte Carlo simulations that treat, on an equal footing, inhomogeneous amplitude variations and phase fluctuations, a major advance over previous theories. The energy gap in the density of states survives across the transition, but coherence peaks exist only in the superconductor. A characteristic pseudogap persists above the critical disorder and critical temperature, in contrast to conventional theories. Surprisingly, the insulator has a two-particle gap scale that vanishes at the SIT, despite a robust single-particle gap. Our predictions are testable with scanning probe experiments. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J23.00013: Fast vortices in the cuprates? A vortex plasma model analysis of the THz conductivity and diamagnetism in $La_{2-x} Sr_x CuO_4 $ Lucas Bilbro, Rolando Valdes Aguilar, Gennady Logvenov, Oshri Pelleg, Ivan Bozovic, N.P. Armitage We present measurements of the fluctuation superconductivity in an underdoped thin film of$La_{1.905} Sr_{0.095} CuO_4 $ using time-domain THz spectroscopy. We compare our results with the measurements of diamagnetism in a similarly doped crystal of $La_{2-x} Sr_x CuO_4 $. Through a vortex-plasma model, we show that if the fluctuation diamagnetism originates in vortices, then we necessarily obtain an anomalously large vortex diffusion constant, more than 100 times larger than estimates from the Bardeen-Stephen model. [Preview Abstract] |
Session J24: Focus Session: Multiscale Modeling: Structural Materials
Sponsoring Units: DCOMP DMPChair: Zhiqiang Wang, University of North Texas
Room: D167
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J24.00001: Origin of Plasticity Length-Scale Effects in Fracture and Deformation Invited Speaker: Engineering design of essentially all metallic components used in structural applications relies heavily on the framework of continuum plasticity. However, many experiments now show that the plastic flow stress in metals increases in micron-scale material volumes, i.e. ``smaller is stronger". The failure of conventional plasticity is particularly manifest at a crack tip, where infinite toughness can be predicted. Phenomenological strain-gradient plasticity models and discrete-dislocation models have emerged to handle size effects but there is no clear physical identification of material length scales controlling size-dependence, in spite of wide speculation. Here, we use a new discrete-dislocation/cohesive-zone model to unambiguously demonstrate that the spacing between obstacles to dislocation motion is one dominant material length scale controlling the fracture toughness of plastically deforming metals. With this insight, we propose a new ``stress gradient plasticity" concept based on the behavior of dislocations in a ``pile-up" at an obstacle under a stress gradient, which (i) rationalizes our fracture results and (ii) predicts size-effects under other loading conditions (bending, torsion, indentation). Quantitative agreement between theory and experiments is then demonstrated in several cases. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J24.00002: Chemistry and Deformation: First Principles Studies of Local Plasticity Invited Speaker: In order to understand the how chemistry influences deformation, an adequate description of the strain field near the center of dislocations (i.e. the core) is required. Continuum level descriptions of deformation ignore this short-range coupling between dislocations and the local atomic lattice. Interactions at this scale are non-linear and can strongly influence plastic deformation in fcc and bcc metals. Here, density functional theory is used in conjunction with a flexible boundary condition method to calculate the equilibrium dislocation core structure in a variety of bcc and fcc metals. The problem is divided into two parts: a solution for the nonlinear dislocation-core region and a solution for the long-range elastic response. Solving these individual problems is straightforward and by iteratively coupling the two solutions we can efficiently solve for the strain field in all space. Chemical effects, in the form of local solute-dislocation interactions, can also be calculated using this method. Derived solute-dislocation interactions are used to inform new models of solution hardening (and softening) in bcc Mo-X (X=Re, Pt) and fcc Al-X (X=Mg, Cr, Si, Cu) alloys. Currently, solute dislocation interactions are being assessed in bcc Fe-H alloys using this first principles technique. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J24.00003: Ab-initio Concurrent Multiscale Method to Address Defects in Metals Georg Schusteritsch, Thomas K\"uhne, Efthimios Kaxiras We present a concurrent multiscale method for metallic systems based on coupling a region calculated using Kohn-Sham Density-Functional-Theory (KS-DFT) to a macroscopic region employing the Embedded Atom Method (EAM). By construction, our method is particularly well suited for treating defects such as grain boundaries (GBs), dislocations and chemical impurities, where quantum mechanical interactions in a small region near the defect may affect the mechanical properties at the macroscopic scale. Results for two metals, Nickel and Copper, are presented in the context of chemical embrittlement. We study the effects of impurities near GBs and investigate the surrounding strain field. This gives us insights into the role defects play in the underlying physical mechanism of chemical embrittlement. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J24.00004: Multiscale Modeling of Irradiation Induced Hardening in Ferritic-Martensitic Steels Hussein Zbib, Ioannis Mastorakos, Mohammad Khaleel, Xin Sun The development of structural materials for use in new generation nuclear reactors depends critically on predicting and understanding the underlying physical mechanisms responsible for microstructural evolution along with corresponding dimensional instabilities and mechanical property changes. As the phenomena involved are very complex and span in several length scales, a multiscale approach is necessary in order to fully understand the degradation of materials in irradiated environments. The purpose of this work is to study the mechanical behaviour of Fe systems (namely a-Fe, Fe-Cr and Fe-Ni) under irradiation using both Molecular Dynamics (MD) and Dislocation Dynamics (DD) simulations. Critical information is passed from the atomistic (MD) to the microscopic scale (DD) in order to study the degradation of the material under examination. In particular, information pertaining to the dislocation-defects (particularly voids, helium bubbles and prismatic loops) interaction is obtained from MD simulations. This information is used in large scale DD simulations to analyze systems with high dislocation and defect densities, predicting the dependence of strength and ductility on defect density. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J24.00005: A QCDFT Study of Hydrogen embrittlement at Crack Tip Qing Peng Study of hydrogen embrittlement is of great importance due to widespread availability of hydrogen in all environmentally influenced cracking phenomena. We used QCDFT: Density functional theory based Quasi-continuum method to study the system where hydrogen atoms are presented on crack tip surface in single aluminum crystal under mode-I loading. We found that the presence of 0.1{\%} hydrogen atoms increases the energy for nucleation of dislocations and enhance the brittlement of aluminum by 5{\%}. The presence of hydrogen atoms also makes the geometry of crack tip to be sharp. The bonding and electronic charge transfer between hydrogen atoms and aluminum atoms were studied and the mechanism of hydride-induced embrittlement will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J24.00006: Models of defects at bi-material interfaces Kipton Barros, Turab Lookman Multi-phase composite materials with a high density of bi-material interfaces can exhibit striking strength and robustness in extreme conditions such as shock and radiation damage. Laminar composites of Ag-Cu, Cu-Nb, and Ag-Fe with submicron to nano-scale layer thicknesses have recently been fabricated, but theoretical models of such systems are lacking. The plastic deformation behavior of nano-scale composites is dominated by defects, such as dislocations and twins, that are controlled by the interfaces. We investigate the phenomenology of defect dynamics at bi-material interfaces using Landau theory based models that span atomic and mesoscales. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J24.00007: First Principles Simulations of Beta to Omega Transformation in the Titanium-Molybdenum System Arun Devaraj, Niraj Gupta, Soumyu Nag, Hamesh Fraser, Raj Banerjee, Srinivasan Srivilliputhur The omega phase precipitation in beta titanium (Ti) alloys influence the beta to alpha phase transformation, and ultimately the mechanical properties of these alloys. Molybdenum (Mo) and other alloying additions affect both the relative phase stability and the energy barrier of the transformation. In this work we perform first principle calculations using Nudeged Elastic Band Method(NEB) implemented in Vienna Ab initio Simulation Package (VASP) to determine the minimum energy path, and thereby the energy barrier in beta Ti-Mo alloys with up to 20wt.{\%} Mo. We report the energetics of beta to omega transformation path, proposed by De. Fontaine et al (Acta Metallurgica, vol. 19, p 1153 (1971)). The atomic configurations along the minimum energy transformational path will be compared with our 3D atom probe tomography and probe corrected high-resolution scanning transmission electron microscopy results. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J24.00008: Shock-induced Spallation Phenomena in Copper-Niobium Nanolayered Composites Niraj Gupta, Alexander Stukowski, Michael Baskes, Srinivasan Srivilliputhur Shock-induced spallation phenomena in Copper-Niobium nanolayered composites conforming to a Kurdjumov-Sach's orientation relation were simulated using molecular dynamics to determine both spallation strength and the nature of void formation. The target structures consisted of varying numbers of alternating copper and niobium layers with thicknesses varying from 1 nm to 22 nm. Flyer velocities ranged from 3.5 to 11.5 A/ps, corresponding to an approximate strain rate of 10$^{9}$ s$^{-1}$. Spallation occurs in the vicinity of the Cu-Nb interface, and always in the copper layer. The proposed factors contributing to spallation will be discussed, as well as what effect the layer morphology has on the strength of the target. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J24.00009: A First-Principles Study of Structure and Stability of Nickel Carbides Josh Gibson, Jamal Uddin, Nelli Bodiford, Thomas Cundari, Angela Wilson Computational studies of nickel carbides, particularly Ni2C, are scarce. A systematic density functional theory study is reported for Ni2C, along with NiC and Ni3C, to understand the stability and electronic structure of nickel carbides of varying stoichiometry. A comprehensive study was executed that involved 28 trial structures of varying space group symmetry for Ni2C. An analysis of the electronic structure, geometry and thermodynamics of Ni2C is performed, and compared with that for Ni3C and NiC as well as several defect structures of varying composition. It is found that the most stable ground state arrangement of Ni2C exists within a simple orthorhombic lattice and that it has metallic character. The calculated formation energies (kcal/mol) of NiC, Ni2C, and Ni3C are 48.6, 7.9 and 6.4, respectively. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J24.00010: Atomistic Simulations of Deformation of Nanoscale FCC Materials Shivraj Karewar, Niraj Gupta, Alex Stukowski, Michael Baskes, Srinivasan Srivilliputhur We compare the deformation behavior of gold single crystal nanospheres with $\sim$6-30 nm diameters with gold spherical shells of varying inner to outer diameter ratios. Gold nanospheres are modeled with an EAM potential and the indenter is described by a repulsive potential. Yield strength dependence on sample size, geometry and temperature was studied in these nanospheres. The deformation mechanism is aided by the continuous displacement burst accompanying dislocation escape from the nanospheres. Based on this, a dislocation starvation mechanism has been discussed. Extended dislocations are found to be the prominent defect type in both solid and hollow nanospheres. Flow stresses are lower in hollow nanoshells. Low flow stresses are accounted for the presence of additional surface area for dislocation nucleation and emergence at the inner surface of the hollow shell. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J24.00011: First-principles investigation of migration barriers in bulk (Cr,Co)-doped Ni$_{3}$Al ($\gamma )$ and Ni$_{3}$Al/Ni ($\gamma $'/$\gamma )$ interface Priya Gopal, Srinivasan Srivilliputhur Ni-based super-alloys possess desirable high-temperature properties including ductility, fracture toughness as well as resistance to creep and oxidation mainly due to the precipitation of ordered Ni$_{3}$Al $\gamma $' precipitates within a $\gamma $ (Ni,Al) matrix. Various studies have shown that the mechanical properties can be improved by adding substitutional elements. It is thus very important to understand the electronic structure and diffusion kinetics of the substitutional elements and the role each one has on the overall microstructure. $\backslash $pardthis work we present our results on the systematic study of the energetics and migration barriers of Cr and Co in bulk Ni$_{3}$Al and Ni$_{3}$Al/Ni ($\gamma $'/$\gamma )$ interface. We did simulations of migration of vacancy and substitutional element in a complete set of migration paths and evaluated the barrier energies in both bulk Ni$_{3}$Al and Ni$_{3}$Al/Ni ($\gamma $'/$\gamma )$ interface using density functional theory methods. We will briefly discuss our results on the effect of migration barriers on the partitioning behavior of Cr and Co between the and ' phases in Ni-based super-alloy. [Preview Abstract] |
Session J26: Focus Session: Iron Based Superconductors -- Vortices & High Fields
Sponsoring Units: DMP DCOMPChair: Stephen Wilson, Boston College
Room: D162/164
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J26.00001: Anomalous High-Field Meissner Effect in Pnictide Superconductors Ruslan Prozorov, Makariy A. Tanatar, Sergey L. Bud'ko, Paul C. Canfield, Bing Shen, Peng Cheng, Hai-Hu Wen The Meissner effect has been studied in Ba(Fe$_{0.926}$Co$_ {0.074}$)$_2$As$_2$ and Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystals and compared to well known, type-II superconductors LuNi$_2$B$_2$C and V$_3$Si. Whereas flux penetration is mostly determined by the bulk pinning (and, perhaps, surface barrier) resulting in a large negative magnetization, the flux expulsion upon cooling in a magnetic field is very small, which could also be due to pinning and/or surface barrier effects. However, in stark contrast with the expected behavior, the amount of the expelled flux increases almost linearly with the applied magnetic field, at least up to our maximum field of 5.5 T, which far exceeds the upper limit for the surface barrier. One interpretation of the observed behavior is that there is a field-driven suppression of magnetic pair-breaking. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J26.00002: Flux Creep associated with Strong Pinning in Isovalently Doped Iron-Based Superconductors M. Konczykowski, Sultan Demirdis, C.J. van der Beek, R. Prozorov, M. Tanatar, P.C. Canfield, S. Kasahara, T. Shibauchi, Yuji Matsuda Strong pinning in Iron-Based Superconductors leads to the ubiquitous central peak of the irreversible magnetization. Notably, isovalently doped materials such as BaFe$_2$(As$_{1-x}$P$_x$)$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ offer a paradigm for the study of strong pinning because it is the only contribution to the critical current density $j_c$. We have studied flux creep rates as function of field and temperature in the low- and high field regimes in which $j_c$ is limited by the line tension of a single pinned vortex, and by vortex interactions, respectively. For $T < \frac{1}{2}T_c$, screening currents $j$ are of the order of $10^9$ Am$^{-2}$, in spite of a creep rate $d \ln j / d \ln t \sim 0.02$. Creep is initially Anderson Kim-like, \em i.e. \rm, creep barriers $U$ depend on $j$ as $U \propto (1 - j / j_c )$ over an order of magnitude in $j$, before crossing over to a nonlinear behavior. $j_c$ is easily extracted from the high-current, short-time part of the magnetic relaxation. The results cast doubt on the range of applicability of the often-used ''interpolation formula'' $j \propto [1 + (k_BT/U_c) \ln( t + t_0 / \tau )]^{-1/\mu}$ for weak collective pinning. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J26.00003: Strong Pinning and Nonlinear Creep Barriers in Iron-Pnictide Superconductors S. Demirdis, M. Konczykowski, C.J. van der Beek, R. Prozorov, S. Kasahara, T. Shibauchi, Yuji Matsuda The irreversible magnetization of Iron-Based Superconductors is characterized by the presence of an ubiquitous peak of the critical current density $j_c$, centered around zero field. Closer examination shows that the field-dependence of $j_c$ corresponds, in all cases, to a low-field plateau, followed by a power-law decrease, $j_c \propto B^{-\alpha}$ (with $\alpha \sim \frac{5}{8}$) above a cross-over field $B^{*}$. This strongly suggests that vortex pinning at low magnetic field is due to strong pinning by nanometer-scale defects. In isovalently doped materials such as BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, strong pinning is the only contribution to the critical current. The analysis of $j_c$ allows one to extract, without a priori assumptions, the elementary pinning force and the defect density. In BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, the latter quantity is in qualitative agreement with that obtained by H. Shishido \em et al.\rm [Phys. Rev. Lett. {\bf 104}, 057008 (2010)]. The temperature dependence of the screening current above $B^{*}$ is strongly affected by flux creep. The current decays as $j \sim [ (k_BT/U) \ln( t + t_0 / \tau )]^{-1/\mu}$, with $\mu \sim 1.6$, showing that nonlinear creep barriers are not an exclusive feature of weak collective pinning. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J26.00004: Model of vortex states in hole-doped iron-pnictide superconductors Yi Gao, Huai-Xiang Huang, Chun Chen, C.S. Ting, Wu-Pei Su Based on a phenomenological model with competing spin-density- wave (SDW) and extended $s-$wave superconductivity, the vortex states in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J26.00005: Angular dependence of the critical current and vortex phase diagram in Co-doped BaFe2As2 films with strong pinning B. Maiorov, T. Katase, H. Hiramatsu, H. Hosono, L. Civale Studying the angular dependence of the critical current density (Jc) as a function of temperature in superconductors with complex pinning landscapes is very important both from the technical and fundamental points of view. The low anisotropy found in the Ba122 family tightened with strong naturally grown pinning make Ba122 films very attractive. It is also interesting to understand the different factors affecting vortex pinning in different regimes of temperature (T) and magnetic field (H). We present results on iron-arsenide superconducting films with varied pinning landscapes composed of columnar defects and nanoparticles. We analyze different field and angular regimes. We find that the naturally grown correlated defects found in Co-doped BaFe2As2 films are effective up to very high fields (m0H=15T) affecting a very wide region o the angular phase diagram. We also investigate the effects of film's thickness and the addition of defects produced by irradiation [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J26.00006: Comparative study of flux pinning characteristics of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x)_2$ single crystals Noriko Chikumoto, Wataru Hirata, Shigeki Miyasaka, Setsuko Tajima, Keiichi Tanabe We have studied the magnetization behavior of iron-pnictide superconductor, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (Co-doped Fe122) with various Co doping and BaFe$_2$(As$_{0.65}$P$_{0.35})_2$ (P-doped Fe122) single crystals. All of the Co-doped Fe122 crystals showed a very pronounced ``peak effect'' in all the temperature range, irrespective of doping state. It is important to mention that a similar peak effect was previously reported for REBa$_2$Cu$_3$O$_y$. In order to get further insight into the pinning mechanism of the present system, we analyzed the pinning force density $F_p=J_cB$. A good scaling of the $F_p$ versus the reduced field, $b=B/B_{irr}$, was established for all the Co-doped Fe122 crystals and the scaling curves were well fitted with the function given by $F_p/F_{p,max} =Ab^p(1-b)^q$, where $A$ is a numerical parameter, $p$ and $q$ are describing the actual pinning mechanism. It was found that $p$ value monotonically increases with x, while $q$ value decreases with x. On the other hand, P-doped Fe122 did not show ``peak effect''. We will discuss about the possible pinning mechanism causing the peak effect. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J26.00007: Dynamics of Vortices in Heavy-ion Irradiated Co-doped BaFe$_2$As$_2$ Tsuyoshi Tamegai, Toshihiro Taen, Hidenori Yagyuda, Tomotaka Taniguchi, Shyam Mohan, Yasuyuki Nakajima, Satoru Okayasu, Masato Sasase, Hisashi Kitamura, Takeshi Murakami, Tadashi Kambara, Yasuyuki Kanai Effects of heavy-ion irradiation on the critical current density, $J_c$, and vortex dynamics are investigated in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ irradiated by heavy- ions of various kinds. Irradiations induce a large enhancement of $J_c$ in the case of 200 MeV Au creating nearly continuous columnar tracks. On the other hand, in the case of 800 MeV Xe irradiation, despite the enhancement of $J_c$, clear columnar defects are not observed. In the case of 2.6 GeV U irradiation, new types of structure appears in the $M$-$H$ loop at high matching field. We also discuss the behavior of vortex dynamics in the irradiated Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J26.00008: High critical current density in BaAs$_{2}$(Fe,Co)$_{2}$ thin films up to 35 T J. Jiang, C. Tarantini, J.D. Weiss, J. Jaroszynsky, E.E. Hellstrom, D.C. Larbalestier, S. Lee, C.W. Bark, H.W. Jang, C.M. Folkman, S.H. Baek, J.W. Park, C.B. Eom, Y. Zhang, C.T. Nelson, X.Q. Pan In the Co-doped BaFe$_{2}$As$_{2}$ thin films we intensively investigated field and angular dependences of J$_{c}$ down 4.2 K in high field. We found a strong correlated c-axis pinning and Jc for field along the c-axis exceeds J$_{c}$ for H//ab plane up to $\sim $20T, inverting the expectation of the Hc2 anisotropy. As a consequence the angular dependence is very weak and J$_{c}$ is still over 10$^{5}$ A/cm$^{2}$ at 20T. Moreover the maximum pinning force F$_{p}$(4.2K) reaches 35-40 GN/m$^{3}$ at 15-20T depending on the field configuration, indicative of strong high-field vortex pinning. High resolution transmission electron microscopy reveals that the strong vortex pinning is due to a high density of non-superconducting Ba-Fe-O nanocolumnar defects whose diameter is $\sim $2$\xi $, perfect conditions for a strong pinning. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J26.00009: Critical current in disordered iron-pnictide superconducting wires Dushko Kuzmanovski, Maxim Vavilov, Anton Vorontsov We evaluate the critical current in narrow wires of disordered iron-based pnictide superconductors. We present the Eilenberger and Usadel equations for a two-band model of a pnictide superconductor which take into account both inter and intra-band scattering events. The intra-band scattering events are responsible for the momentum relaxation of charged excitations, but do not suppress the homogeneous superconducting state. On the contrary, the inter-band scattering acts as the depairing mechanism. We apply the Usadel equation to analyze the dependence of the critical current on the strength of disorder in narrow iron-pnictide wires. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J26.00010: Upper critical field measured up to 85T in different iron-based compounds C. Tarantini, J. Jaroszynsky, A. Gurevich, D.C. Larbalestier, F. Balakirev, H.H. Wen, E. Bellingeri, I. Pallecchi, C. Ferdeghini We report magneto-transport measurements of H$_{c2}$(T) at very high dc and pulsed magnetic field up to 85T on different families of iron-based single crystals and thin film superconductors with different doping levels and stress. Some of these materials show high H$_{c2}$ extrapolating to $\sim $100T and extremely high slopes up to 20 T/K for H//c and over 200 T/K for H//ab, indicating significant Pauli pair breaking and a possibility of the Fulde-Ferrel-Larkin-Ovchinnikov state. The superconducting transitions remain sharp also at the highest field showing an irreversibility field close to H$_{c2}$. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J26.00011: Flux Pinning and Quasi-particle Scattering in Charge- Doped Iron-Based Superconductors Kees van der Beek, S. Demirdis, M. Konczykowski, S. Kasahara, T. Terashima, R. Okazaki, T. Shibauchi, Yuji Matsuda Whereas isovalently doped iron-based superconductors, such as BaFe$_2$(As$_{1-x}$P$_x$)$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ show only strong, ''individual-defect'' vortex pinning due to nanometer-sized defects, charge-doped iron-pnictide superconductors show a low-field, field-independent contribution to the critical current density $j_c$ that is well described by the collective pinning theory. Quantitative analysis of the magnitude, temperature, and field-dependence of $j_c$ in the PrFeAsO$_{1-y}$ compound shows that the behavior of $j_c$ can be fully explained, if one assumes the oxygen vacancies in this material to be responsible for quasi-particle scattering in the vortex cores. Analysis of $j_c$ of this and other charge-doped compounds such as NdFeAs(O,F), (Ba,K)Fe$_2$As$_2$, and Ba(Fe,Co)$_2$As$_2$ yields estimates for the transport scattering cross-section of the dopant impurities in all these materials. We find scattering to be in the Born limit, with a scattering phase angle $\delta_0$ such that $\sin \delta_0 \sim 0.2 - 0.3$. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J26.00012: Nearly Isotropic Critical Currents in SmFeAs(O,F) in High Magnetic Fields Bertram Batlogg, Philip Moll, Roman Puzniak, Fedor Balakirev, Luis Balicas, Janusz Karpinski, Nikolai Zhigadlo The layered structure of SmFeAs(O,F) naturally raises questions about the electronic anisotropy of this 55K superconductor. To investigate the transport anisotropy, we performed electric 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with sub-$\mu$m$^2$ cross-section, with current along and perpendicular to the FeAs layers. The normal state resistivity is indeed anisotropic ($\rho_c$/$\rho_{ab} \approx$ 2 at RT, $\approx$ 10 at 50K) and consistent with the calculated Fermi velocity anisotropy. In contrast, the dissipation in high fields below $T_c$ is more isotropic. The critical current densities at 4K are nearly isotropic and very high ($>$ 2 10$^6$ A/cm$^2$), and up to 14 T, they are almost independent of the field orientation and strength. These values agree well with magnetization measurements. Additional measurements to much higher fields are presented. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J26.00013: Low-energy muons and polarized neutrons for studying superconductivity Vladimir Kozhevnikov, Kristiaan Temst, Andreas Suter, Timothy Charlton, Helmut Fritzsche, Thomas Prokscha, Elvezio Morenzoni, Margriet Van Bael, Christian Van Haesendonck, Joseph Indekeu The penetration of the magnetic field into superconductors (SC) in the Meissner state is one of the major resources for studying SC. The merit of this resource will be greatly enhanced by quantitative measurements of the field distribution over the penetration layer, which, in particular, may lead to new insights for unconventional SC. We will report on measurements of the magnetic field profile in In and Sn using low-energy muons (LE-mSR) and polarized neutrons (PNR). The results solidly demonstrate nonexponential decay of the magnetic induction in accord with the Pippard and BCS theories. However, in contrast to In, results on which were reported last year, the data for Sn show significant differences in the values of the London penetration depth and the Pippard coherent length with those known from the literature. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J26.00014: Incoherent Interplane Response of FeTe$_{0.55}$Se$_{0.45}$ S.J. Moon, C.C. Homes, A. Akrap, Z.J. Xu, J.S. Wen, Z.W. Lin, Q. Li, G.D. Gu, D.N. Basov We investigated the interplane $c$ axis electronic response of iron-chalcogenide superconductor FeTe$_{0.55}$Se$_{0.45}$ using infrared spectroscopy. We found that the normal-state $c$ axis electronic response of FeTe$_{0.55}$Se$_{0.45}$ is incoherent. The $c$ axis optical conductivity does not display well-defined Drude response and it becomes further suppressed with decreasing temperature. This normal-state $c$ axis optical response is remarkably similar to that of the mildly underdoped cuprates but is in sharp contrast to the coherent $c$ axis response of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. From the analyses on the electronic anisotropy of various layered superconductors, we found a close correlation between the degree of the coherence in the $c$ axis transport and the strength of the dissipation in the \textit{ab} plane response. [Preview Abstract] |
Session J27: Focus Session: Quantum Optics with Superconducting Circuits II
Sponsoring Units: GQIChair: David Schuster, University of Chicago
Room: C155
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J27.00001: Tomography and Correlation Function Measurements of Itinerant Microwave Photons Invited Speaker: At optical frequencies the radiation produced by a source, such as a laser, a black body or a single-photon emitter, is frequently characterized by analysing the temporal correlations of emitted photons using single-photon counters. At microwave frequencies, however, there are no efficient single-photon counters yet. Instead, well-developed linear amplifiers allow for efficient measurement of the amplitude of an electromagnetic field. Here, we demonstrate first- and second-order correlation function measurements of a pulsed microwave-frequency single-photon source integrated on the same chip with a 50/50 beam splitter followed by linear amplifiers and quadrature amplitude detectors [1]. We clearly observe single-photon coherence in first-order and photon antibunching in second-order correlation function measurements of the propagating fields [2]. We also present first measurements in which we reconstruct the Wigner function of itinerant single photon Fock states and their superposition with the vacuum. To perform these measurements we have developed efficient methods to separate the detected single photon signal from the noise added by the amplifier by analyzing the moments of the measured amplitude distribution up to 4th order. The techniques and methods demonstrated in this work may find application in quantum optics and quantum information processing experiments at microwave frequencies.\\[4pt] [1] M.~P.~da~Silva, D.~Bozyigit, A.~Wallraff, and A.~Blais, Phys. Rev. A 82, 043804 (2010)\\[0pt] [2] D.~Bozyigit, C.~Lang, L.~Steffen, J.~M.~Fink, C.~Eichler, M.~Baur, R.~Bianchetti, P.~J.~Leek, S.~Filipp, M.~P.~da~Silva, A.~Blais, and A.~Wallraff, Nat. Phys. in print (2010), also arXiv:1002.3738 [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J27.00002: An integrated circuit for generating distributable and unconditional entanglement at microwave frequencies Hsiang-Sheng Ku, Francois Mallet, William F. Kindel, Konrad W. Lehnert, Kent D. Irwin, Gene C. Hilton, Leila R. Vale, Emanuel Knill, Scott C. Glancy Entanglement, the unique feature of quantum mechanics, is the central resource of quantum information. In the strategy of continuous-variables quantum information processing, unconditional and distributable entanglement can be obtained by combining two squeezed states on a balanced beam splitter. Our group has recently demonstrated the generation of squeezed microwave states using a Josephson Parametric Amplifier [1] and implemented on-chip balanced beam splitters [2]. This talk will present a device that combines all these components on a single chip. The design requirements for such an ``on-chip entangler'' of the electromagnetic field modes will be discussed. \\[4pt] [1] M. A. Castellanos-Beltran et al, Nature Physics, 4, 929 (2008). \\[0pt] [2] Hsiang-Sheng Ku et al, arXiv:1010.3232v1 [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J27.00003: Measuring on-chip distributable and unconditional entanglement at microwave frequencies Francois Mallet, Hsiang-Sheng Ku, Will Kindel, Scott Glancy, Emanuel Knill, Kent D. Irwin, Gene C. Hilton, Leila R. Vale, Konrad W. Lehnert A squeezed mode of the light field exhibits reduced fluctuations, below the vacuum level, along one of its quadratures and conversely amplified fluctuations along the conjugate quadrature. In that sense, it is the electromagnetic analog of the particle states used by Einstein-Podolsky-Rosen to derive their famous paradox. Indeed, by combining two such squeezed modes on a balanced beam splitter, entanglement can be generated, in an unconditional and distributable way. Such experiments have been performed for some years at optical frequencies. This talk will present an experimental attempt to generate and characterize entanglement with squeezed light at microwaves frequencies, using superconducting electrical circuits. We will discuss the achieved degree of entanglement from the perspective of implementing quantum teleportation protocols at microwave frequencies. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J27.00004: Observation of photon blockade in circuit QED using second-order correlation function measurements C. Lang, D. Bozyigit, C. Eichler, L. Steffen, J.M. Fink, A.A. Abdumalikov Jr., M. Baur, S. Filipp, A. Wallraff Circuit quantum electrodynamics (QED) provides an attractive platform to effectively study photon-photon interactions mediated by their strong and resonant coupling to a superconducting qubit embedded into a transmission line resonator. Driving the coupled system with a coherent microwave frequency tone the anharmonicity of the Jaynes-Cummings ladder blocks the transmission of more than a single photon through the resonator at a time. Using on-chip microwave beam splitters, linear amplifiers, and quadrature amplitude detectors we observe fluorescence and Rayleigh scattering in Mollow-triplet-like spectra. We investigate the phenomenon of photon blockade in second-order correlation function measurements which show antibunching and signatures of Rabi oscillations induced by the continuous drive coupling the ground and first excited states of the Jaynes-Cummings ladder. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J27.00005: Generation and reconstruction of two mode squeezed states in the microwave domain Christopher Eichler, Deniz Bozyigit, Christian Lang, Matthias Baur, Lars Steffen, Johannes Fink, Stefan Filipp, Andreas Wallraff Squeezing between two radiation field modes at optical frequencies has already been used to realize various quantum information processing tasks such as teleportation and quantum key distribution. Here we present measurements at microwave frequencies in which we generate and reconstruct a two mode squeezed state in a circuit QED setup. We prepare the desired state with a Josephson parametric amplifier and detect all four quadrature components simultaneously in a two channel heterodyne setup using amplitude detectors. Recording two dimensional phase space histograms for all possible pairs of quadratures allows for the reconstruction of the full covariance matrix and the four dimensional Wigner function of the squeezed state which shows strong correlations between the quadrature noise in the two modes. Combining parametric amplifier devices in networks with beamsplitters and superconducting qubits could allow for future linear optics quantum computation with propagating microwave photons. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J27.00006: Using Superconducting Qubit Circuits to Engineer Exotic Lattice Systems Dimitris Tsomokos, Sahel Ashhab, Franco Nori We propose an architecture based on superconducting qubits and resonators for the implementation of a variety of exotic lattice systems, such as spin and Hubbard models in higher or fractal dimensions and higher-genus topologies. Spin systems are realized naturally using qubits, while superconducting resonators can be used for the realization of Bose-Hubbard models. Fundamental requirements for these designs, such as controllable interactions between arbitrary qubit pairs, have recently been implemented in the laboratory, rendering our proposals feasible with current technology. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J27.00007: Microwave Cavity Lattices for Simulating Condensed Matter Systems Devin Underwood, Arthur Safira, Srikanth Srinivasan, Anthony Hoffman, Jens Koch, Andrew Houck Recently, quantum phase transitions of light have been the focus of much theoretical attention. One possible experimental realization relies upon the circuit quantum electrodynamics architecture (cQED); however, in order for this to be successful, coupled arrays of superconducting resonators must first be realized with low disorder. Here we fabricate and characterize an array with low disorder consisting of 12 niobium resonators on a sapphire substrate in a honeycomb pattern with the photonic lattice sites forming a Kagome star. The structure is characterized by measuring transmission through different input-output port pairs and by varying the hopping rate between resonators. A family of resonant peaks corresponding to the various modes of the coupled array is identifiable and agrees well with both a tight-binding Hamiltonian and simulations from a commercial microwave software package. These experiments are an important step in realizing strongly correlated interactions in cQED. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J27.00008: Synthetic gauge fields in Jaynes-Cummings-Hubbard ring lattices Andreas Nunnenkamp, Jens Koch, Steven Girvin Recently there has been much interest in many-body physics with photons in circuit-QED arrays. Here we explore the physics of a Jaynes-Cummings-Hubbard ring lattice subject to a synthetic gauge field, i.e.~where the hopping terms carry a complex phase factor due to Josephson couplers between the resonators. There are critical phase twists at which the single-particle spectrum is degenerate so that even weak interactions can give rise to strong correlations. We compare to ultracold bosons in rotating ring lattices and study the out-of-equilibrium physics as relevant for current experiments. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J27.00009: Multi-Resonator Circuit QED Part I: The Photon Shell Matteo Mariantoni, H. Wang, Radoslaw C. Bialczak, M. Lenander, Erik Lucero, M. Neeley, A.D. O'Connell, D. Sank, M. Weides, J. Wenner, T. Yamamoto, Y. Yin, J. Zhao, John M. Martinis, A.N. Cleland The generation and control of quantum states of light constitute fundamental tasks in cavity quantum electrodynamics (QED). The superconducting realization of cavity QED, circuit QED, enables on-chip microwave photonics, where superconducting qubits control and measure individual photon states. A long-standing issue in cavity QED is the coherent transfer of photons between two or more resonators. Here, we use circuit QED to implement a three-resonator architecture on a single chip, where the resonators are interconnected by two superconducting phase qubits. We use this circuit to shuffle one- and two-photon Fock states between the three resonators, and demonstrate qubit-mediated vacuum Rabi swaps between two resonators. This illustrates the potential for using multi-resonator circuits as photon quantum registries and for creating multipartite entanglement between delocalized bosonic modes. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J27.00010: Multi-resonator circuit QED - Part 2: Generation and detection of NOON states Frank Wilhelm, Seth Merkel NOON states, states between two modes of light of the form $|N,0\rangle+e^{i\phi}|0,N\rangle$ allow for super-resolution interformetry. We show how NOON states can be efficiently produced in circuit quntum electrodynamics using superconducting phase qubits and resonators. We propose a protocol where only one interaction between the two modes is required, creating all the necessary entanglement at the start of the procedure. This protocol makes active use of the first three states of the phase qubits. Additionally, we show how to efficiently verify the success of such an experiment, even for large NOON states, using randomly sampled measurements and semidefinite programming technique. This is more efficient that the full tomography implemented to-date, allowing to reliably verify higher NOON-states. Based on New J. Phys. {\bf 12}, 093036 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J27.00011: Multi-Resonator Circuit QED Part III: Two-Resonator Entanglement Haohua Wang, Matteo Mariantoni, Radoslaw C. Bialczak, M. Lenander, Erik Lucero, M. Neeley, A.D. O'Connell, D. Sank, M. Weides, J. Wenner, Y. Yin, J. Zhao, John M. Martinis, A.N. Cleland, T. Yamamoto Quantum entanglement, a defining feature of quantum mechanics, has been demonstrated in a variety of nonlinear spin-like systems. Quantum entanglement in linear systems has proven significantly more challenging, as the intrinsic energy level degeneracy associated with linearity makes quantum control more difficult. Here we demonstrate the quantum entanglement of photon states in two independent linear microwave resonators utilizing two superconducting phase qubits coupled through a band-pass resonator. After entangling two qubits into a Bell state, we demonstrate the controlled sequential photon amplification and transferring procedures, creating N quanta excitations distributed in two resonators. We completely characterize the two-resonator states with bipartite Wigner tomography and prove the existence of entanglement. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J27.00012: Observation of Collective Strong Coupling between a Superconducting Resonator and Bismuth Dopants in Silicon Natania Antler, R. Vijay, Christoph Weis, Eli Levenson-Falk, Thomas Schenkel, Irfan Siddiqi All electrical readout and control of spin systems with superconducting circuitry is an attractive route for implementing hybrid quantum information processing. Isolated spins, in general, have much longer coherence times than present day superconducting qubits, and thus could be utilized as memory elements. Species with a zero-field splitting (ZFS), such as bismuth doped silicon or NV centers in diamond, are particularly attractive as the absence of a strong magnetic bias field facilitates compatibility with low loss superconducting circuitry. We present results on the interaction of a tunable superconducting resonator and an ensemble of Bi spins implanted in an epitaxial layer of 28Si. As the resonator tunes through the ZFS, we observe an avoided crossing indicative of collective strong coupling. We discuss coherence properties as a function of spin density as well as progress on the detection of a small number of spins using a dispersive nanoSQUID magnetometer. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J27.00013: Qubit-oscillator systems in the ultrastrong-coupling regime and their potential for preparing nonclassical states Franco Nori, Sahel Ashhab We consider a system composed of a two-level system (i.e. a qubit) and a harmonic oscillator in the ultrastrong-coupling regime, where the coupling strength is comparable to the qubit and oscillator energy scales. We explore the possibility of preparing nonclassical states in this system, especially in the ground state of the combined system. The nonclassical states that we consider include squeezed states, Schrodinger-cat states and entangled states. We also analyze the nature of the change in the ground state as the coupling strength is increased, going from a separable ground state in the absence of coupling to a highly entangled ground state in the case of very strong coupling. Reference: S. Ashhab and F. Nori, Phys. Rev. A 81, 042311 (2010). [Preview Abstract] |
Session J28: Focus Session: Computational Materials Design - Data-Driven
Sponsoring Units: DCOMP DMPChair: Long-Qing Chen, Penn State University
Room: C156
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J28.00001: Distributed Strategies for Materials Development Invited Speaker: Through appropriate combinations of ``ab-initio,'' ``data-mining-high-throughput,'' ``cluster expansion,'' ``vibrational,'' and ``electronic structure'' techniques, we have parameterized the whole set of transition-metal binary intermetallics (435 alloys) and a list of $\sim$10,000 inorganic crystals. The presentation will introduce the method, the tools, the standards, and the approach for automatic discovery of trends in material development. We will analyze rules for miscibility in metallic catalytic materials, electronic structure correlations in scintillators, and high-throughput search of thermoelectric materials and topological insulator through the distributed network of data, accessible to the scientific community. The presentation will also extend the hybrid method to study phenomena at the nanoscale, like size-induced viscosity effects on the catalytic rate, self-consistent variational approaches to the shape of nano-catalysts and size-dependent Wulff plots for tailoring catalysts compositions and size (Sponsors: ONR, NSF, DHS, Teragrid). [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J28.00002: A database survey to search for new candidate {\em p}-type TCOs Giancarlo Trimarchi, K. Poeppelmeier, A. J. Freeman New {\em p}-type TCOs are often produced by reacting the prototype binary {\em p}-type oxides, i.e., Ag$_{2}$O and Cu$_{2}$O, with binary oxides of other transition metals or main-series elements. Yet, so far only a small part of all the multi-cation Cu and, in particular, Ag oxides have been assessed as candidate {\em p}-type TCOs. Furthermore, numerous multi-species Cu and Ag oxide systems are poorly characterized, which leaves ample scope for discovery of yet unknown compounds belonging to them, and, likely, of unsuspected new TCOs, too. Here, we survey a {\em complete} database of known multicomponent Ag and Cu oxides, without restrictions on element composition, to search for new candidate TCOs. We indexed all the compounds in this database by applying selected crystal structure descriptors as structure type, stoichiometry, and coordination environment of the Cu and Ag cations. Chemical insight points to a significant likelihood that 2- and 4-fold coordination of the noble metal cations yield band structure properties suitable for the transparency and hole conductivity needed in TCOs. We scanned the indexed database to find compounds that could match these requirements and identified a set of materials that could be interesting candidate {\em p}-type TCOs. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J28.00003: Ab-Initio Prediction of Phase Diagrams Using a Genetic Algorithm Will Tipton, Richard Hennig The computational design and prediction of materials' properties is a goal on which much progress has been made. However, it is generally necessary to first determine a material's crystal structure, and this remains a difficult problem. Previously, genetic algorithms have been successful in searching for stable crystal structures at particular compositions. However, when approaching a new material system, it is often unknown at which compositions stable structures might form. In order to search all of composition space simultaneously, Trimarchi and Zunger have recently suggested a modification to the traditional GA approach. In this method candidate structures are evaluated according to their formation energies with repect to structure found previously. We have implemented this technique in our genetic algorithm code and are investigating the practical details of its use. We have predicted previously-unknown phases in the Li-Be and elemental Eu systems under high pressure. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J28.00004: The Harvard Clean Energy Project. Large-scale computational screening and design of molecular motifs for organic photovoltaics on the World Community Grid Johannes Hachmann, Roberto Olivares-Amaya, Sule Atahan-Evrenk, Carlos Amador-Bedolla, Alan Aspuru-Guzik Organic solar cells are one of the promising approaches to ubiquitously establishing renewable energy sources; alas the necessary 10\% energy conversion efficiency remains elusive. We present the Harvard Clean Energy Project (CEP, http://cleanenergy.harvard.edu) which is concerned with the screening and design of organic photovoltaics (and organic electronics in general) by means of first-principles computational quantum chemistry. We use modern DFT to assess the quality of candidate structures and systematically improve upon these based on the gathered understanding of structure-property relations. The CEP is a high-throughput investigation which utilizes the massive computational resource of the IBM World Community Grid, which allows us to characterize millions molecules of interest in the course of the next year. We address the combinatorial generation of our molecular library, our database, workflow organization and automation, data calibration and cheminformatics analysis, and the closure of the development cycle provided by our experimental collaborators. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J28.00005: Computational design of new A2BX4 materials Invited Speaker: The A2BX4 family of ternary compounds represents an important class of materials. Members of this group, in addition to being among the earth most abundant materials, also span a significant range of physical properties including ferromagnetism, coexistence of transparency and p-type conductivity, ferroelectricity, etc. Today we know for about 800 A2BX4 compounds that have been characterized experimentally. This is only a portion of nearly 5000 A2BX4 combinations that could be constructed throughout the periodic table. In this talk I will present a systematic theoretical approach, based on ab initio calculations, for predicting new A2BX4 compounds. For a given new A2BX4 combination we find the candidate crystal structures from the classification of the existing A2BX4 in terms of the atomic orbital radii of the constituent A and B atoms (Zhang and Zunger, Adv. Funct. Mat. 20, 1944, 2010). This step is followed by the set of high-throughput ab initio calculations which are used to sort out the ground-state structure and compute the corresponding heat of formation. The stability of a given A2BX4 with respect to decomposition into competing phases is then tested against all possible combinations of known compounds involving the same elements. This is done by comparing the heat of formation of the new ternary and the heats of formation of the competing (existing) binary and ternary compounds. I will also discuss the algorithms for searching the chemical space of ternary compounds in order to find the materials with target properties. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J28.00006: Discovering potentially overlooked Filled Tetrahedral Structure compounds by high-throughput first-principles calculation Xiuwen Zhang, Alex Zunger Filled Tetrahedral Structures (FTS) such as LiZnP are derived from the binary zincblende family by splitting a cation such as Ga in GaP into two lower-valent cations Li+Zn, placing one on the original cation site and the other on one of the empty interstitial sites. Generalizing this process, it is possible to generate a few hundred of ABX compounds. Depending on the position of A, B, and X in the periodic table, the structure of such ABX can deviate from the parent tetrahedral framework. Using high-throughput total-energy calculation in GGA+U we have examined the stable structures and possible metastable structures of a few hundred ABX compounds, establishing the basic regularities relating structure to chemical identity. Their thermodynamical stability has been checked by taking into account the competing binary and ternary phases. We identify dozens of ABX compounds likely to have large band gaps, potentially suitable as solar absorbers and transparent conductors. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J28.00007: GW band gap of Filled Tetrahedral Structuctures: absorbers and topological insulators ? Julien Vidal, Xiuwen Zhang, Jun-Wei Luo, Alex Zunger Filled Tetrahedral Structures (FTS) such as LiZnP are derived from the binary zincblende material such as GaP by splitting a cation such as Ga into two lower-valent cations Li+Zn, placing one on the original cation site and the other on one of the empty interstitial sites. Generalizing this process, it is possible to generate a few hundred of ABX compounds. Their electronic structure has been previously calculated by bandgap-underestimating DFT assuming a zincblende-derived crystal structure. We use instead GW to establish (i) which ABX materials are potentially suitable as absorbers in solar cells and (ii) which of the ABX materials previously proposed as topological insulators based on DFT may not be so in a better approximation such as GW. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J28.00008: Quantum Monte Carlo for Materials Design Tim Mueller, Lucas Wagner, Jeffrey Grossman When designing new materials it is important to have an accurate measure of the material's formation energy to assess thermodynamic stability and chemical activity. Computational materials science holds the potential to accurately predict formation energies, but widely-used methods such as density functional theory often yield large errors when calculating energy differences between compounds with significantly different electronic structures. More accurate quantum chemical methods tend to scale poorly with system size, making it infeasible to apply them to many materials. One exception is quantum Monte Carlo (QMC), which effectively scales linearly or better with system size when calculating formation energy per atom. QMC scales perfectly with the number of processors, making it ideally positioned to take advantage of the rapidly growing core count in central and graphics processing units. It has been shown that quantum Monte Carlo can successfully predict formation energies for some solid state materials, but a broad assessment has been lacking. We have run QMC calculations on a variety of different materials for which high-quality experimental data exists. We present data on the cost and accuracy of QMC, providing insight into the role QMC will play in materials design. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J28.00009: Off-lattice self-learning kinetic Monte Carlo: application to 3D island decay on fcc(100) surface Giridhar Nandipati, Abdelkader Kara, Syed Islamuddin Shah, Talat S. Rahman We report the development of an off-lattice kinetic Monte Carlo (KMC) method with a new three-dimensional (3D) pattern recognition scheme to better identify the local environment and processes involving 3D motion which was not possible in the earlier approach [1]. In the present scheme, to uniquely identify the 3D neighborhood around the central atom or leading atom we split it into 3D rectangular boxes whose dimensions dictate the accuracy with which the motion of the diffusing entity to be accounted. This technique combines the idea of self-learning KMC (SLKMC) [2] method with the new pattern-recognition scheme fitted to an off-lattice model. We present application of this off-lattice SLKMC to 3D island decay on fcc (100) surface and compare the results and computational efficiency to that available in the literature. \\[4pt] [1] A. Kara et al, J. Phys.: Condens. Matter, 21 (2009)\\[0pt] [2] O. Trushin et al, Phys. Rev. B, 72, 115401 (2005) [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J28.00010: Density-functional study of U-Mo alloys Alexander Landa, Per Soderlind, Patrice E.A. Turchi The U-Mo and U-Zr alloys proved to be very promising fuels for advanced fast nuclear reactors. According to numerous experiments, the main advantages of U-Mo fuels over U-Zr fuels lies in a much lower constituent redistribution due to the existence a single $\gamma $-U-Mo phase with body-centered cubic structure over typical fuel operation temperatures. Density-functional theory (EMTO-CPA technique) previously used to describe phase equilibria in U-Zr alloys [A. Landa, P. S\"oderlind, P. E. A. Turchi, Journal of Alloys and Compounds, 478 (2009) 103] is extended to investigate the ground-state properties of U-Mo solid solutions. Calculated heats of formation of bcc U-Zr and U-Mo alloys are compared with CALPHAD assessments. We discuss how the heat of formation in both alloys correlates with the charge transfer between the alloy components, and how the specific behavior of the density of states in the vicinity of the Fermi level promotes the stabilization of the U$_{2}$Mo compound. Our calculations prove that, due to the existence of a single $\gamma $-phase over the typical fuel operation temperatures, $\gamma $-U-Mo alloys should indeed have much lower constituent redistribution than $\gamma $-U-Zr alloys for which binodal decomposition causes a high degree of constituent redistribution. This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J28.00011: Inverse band design of SiGe superlattices with direct band-gaps Mayeul d'Avezac, Jun-Wei Luo, Alex Zunger, Thomas Chanier Integrating optoelectronic functionalities directly into the mature Silicon-Germanium technology base would prove invaluable for many applications. Unfortunately, both Si and Ge display indirect band-gaps unsuitable for optical applications. It was previously shown (Zachai \textit{et al.} PRL \textbf{64} (1990)) that epitaxially grown [(Si)$_n$(Ge)$_m$]$_p$ (i.~e.~ a single repeat unit) grown on Si can form direc-gap heterostructures with weak optical transitions as a result of zone folding and quantum confinement. The much richer space of \emph{multiple-period} superlattices [(Si)$_{n_1}$(Ge)$_{n_2}$(Si)$_{n_3}$(Ge)$_{n_4}$\ldots$Ge_{n_N}$]$_p$ has not been considered. If $M=\sum n_i$ is the total number of monolayers, then there are, roughly, $2^M$ different possible superlattices. To explore this large space, we combine a (i) genetic algorithm for effective configurational search with (ii) empirical pseudopotential designed to accurately reproduce the inter-valley and spin-orbit splittings, as well as hydrostatic and biaxial strains. We will present multiple-period SiGe superlattices with large electric dipole moments and direct gaps at $\Gamma$ yielded by this search. [Preview Abstract] |
Session J29: Focus Session: Quantum Information for Quantum Foundations - Structures in Hilbert Space
Sponsoring Units: GQIChair: Asa Ericsson, Institute Mittag-Leffler
Room: C148
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J29.00001: Proofs of the Kochen-Specker theorem based on two qubits Mordecai Waegell, P.K. Aravind The observables for a pair of qubits yield a system of 60 rays and 105 bases in a complex Hilbert space of four dimensions that contains over a hundred million parity proofs of the Kochen-Specker theorem. An overview of these proofs is given and they are compared with those in other 4-d systems, such as the 600-cell. The significance of the results is discussed. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J29.00002: Proofs of the Kochen-Specker theorem based on the 600-cell P.K. Aravind, Mordecai Waegell, Norman Megill, Mladen Pavicic It is shown that the system of 60 rays and 75 bases derived from the vertices of a 600-cell (a regular polytope in four dimensions) contains over a hundred million parity proofs of the Kochen-Specker theorem. An overview of the proofs is given, some examples of them are presented and the significance of the results is discussed. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J29.00003: MUB Entanglement Patterns by Transformations in Phase Space Jay Lawrence All possible MUB entanglement patterns for systems of N prime-state particles are obtained from standard ones by unitary transformations in the Hilbert space, thus preserving the relationships between the generalized Pauli operators, the phase point operators, and the MUB projectors. The transformations are described geometrically in discrete phase space. Illustrative examples show the invariance of the total entanglement content and the connection of entanglement with Galois fields. Different field representations for the same dimension may produce inequivalent MUB sets. This work provides alternative constructions and generalizes previous work on qubit systems [1,2]. \\[4pt] [1] J L Romero, G Bjork A B Klimov, and L L Sanchez-Soto, Phys. Rev. A {\bf 72}, 062310 (2005).\\[0pt] [2] A B Klimov, J L Romero, G Bjork, and L L Sanchez-Soto, Ann. Phys. {\bf 324}, 53 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J29.00004: Entanglement in Mutually Unbiased Bases Marcin Wiesniak, Tomasz Paterek, Anton Zeilinger Higher-dimensional Hilbert spaces are still not fully explored. One issue concerns mutually unbiased bases (MUBs). For primes [1] and their powers (e.g. [2]), full sets of MUBs are known. The question of existence of all MUBs in composite dimensions is still open. We show that for all full sets of MUBs of a given dimension a certain entanglement measure of the bases is constant. This fact could be an argument either for or against the existence of full sets of MUBs in some dimensions and tells us that almost all MUBs are maximally entangled for high-dimensional composite systems, whereas this is not the case for prime dimensions. We present a new construction of MUBs in squared prime dimensions. We use only one entangling operation, which simplifies possible experiments. The construction gives only product states and maximally entangled states. \\[4pt] [1] I. D. Ivanovi\'c, J. Phys. A 14, 3241 (1981). \\[0pt] [2] W. K. Wootters and B. D. Fields, Ann. Phys. (N.Y.) 191, 363 (1989). [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J29.00005: Qutrits under a microscope Gelo Noel Tabia Gleason's theorem states that the set of quantum states is complete, in the sense that density operators specify the unique probability measure definable on the lattice of Hilbert space of projection operators according to the Born Rule. Particularly, Gleason showed that the theorem holds in all finite dimensions if and only if it holds in dimension 3. This suggests that the essential features defining the probability structure of quantum theory can already be found in 3-dimensional quantum systems. Hence, we establish key geometric properties of qutrit state space as they are expressed in terms of symmetric, informationally-complete (SIC) measurements. We provide a variety of important results, which include an elegant formula for describing pure qutrits, affine plane symmetries and the Hesse configuration in qutrit SICs derived from algebraic structure constants for $\mathrm{GL}(3,C)$, and a comparison of the SIC and generalized Bloch representations by analyzing plane cross-sections of qutrit state space. In addition, we present a new way of implementing SIC-POVMs using multi-port devices built from waveguide-based micro-optics, in particular, by proposing experimental circuits for qubits and qutrits. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J29.00006: A Linear Dependency Structure Arising from Weyl-Heisenberg Symmetry Hoan Bui Dang, Marcus Appleby, Ingemar Bengtsson, Kate Blanchfield, Asa Ericsson, Christopher Fuchs, Matthew Graydon, Gelo Tabia The Weyl-Heisenberg (WH) group was used by Hermann Weyl to construct finite-dimensional quantum mechanics in the earliest days of the theory and, through its ubiquitous use in quantum information theory, is even more important today. While investigating properties of symmetric informationally-complete (SIC) measurements, we found a linear dependency structure in a class of Weyl-Heisenberg covariant sets when certain conditions on the dimensionality of the Hilbert space are met. This result reveals more structure in WH symmetry than previously noted and helps us gain a better understanding of quantum state space. For example in the Quantum Bayesian framework of Fuchs and collaborators, the number of zeros of a quantum state in a SIC representation is directly related to this linear dependency. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J29.00007: Regrouping phenomena of SIC POVMs covariant with respect to the Heisenberg--Weyl group Huangjun Zhu Symmetric informationally complete positive operator valued measures (SIC POVMs) covariant with respect to the Heisenberg--Weyl (HW) group form disjoint orbits under the action of the normalizer of the HW group---the (extended) Clifford group. Additional SIC POVMs can be obtained by a suitable regrouping of the fiducial vectors on certain orbits, for example, in Hilbert spaces of dimension three, four, eight and twelve. To understand these SIC POVM regrouping phenomena, we need to go beyond the Clifford group and consider a larger group, in particular the normalizer of the Clifford group. We prove that, when the dimension of the Hilbert space is not a multiple of four, the HW group is a characteristic subgroup of the Clifford group, and the normalizer of the Clifford group is itself; when the dimension is a multiple of four, there are exactly two normal subgroups in the Clifford group that are isomorphic to the HW group, which are conjugated to each other in the normalizer of the Clifford group. Based on this observation, we provide a unified framework for understanding the regrouping phenomena mentioned above and those potential candidates. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J29.00008: Quantum Computational Geodesic Derivative Howard Brandt In recent developments in the differential geometry of quantum computation, the quantum evolution is described in terms of the special unitary group of n-qubit unitary operators with unit determinant. The group manifold is taken to be Riemannian. In the present work the geodesic derivative is clarified. This is applicable to investigations of conjugate points and the global characteristics of geodesic paths in the group manifold, and the determination of optimal quantum circuits for carrying out a quantum computation. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J29.00009: Affine Maps of the Polarization Vector for Quantum Systems of Arbitrary Dimension Mark Byrd, C. Allen Bishop, Yong-Cheng Ou The operator-sum decomposition (OS) of a mapping from one density matrix to another has many applications in quantum information science. To this mapping there corresponds an affine map which provides a geometric description of the density matrix in terms of the polarization vector representation. This has been thoroughly explored for qubits since the components of the polarization vector are measurable quantities (corresponding to expectation values of Hermitian operators) and also because it enables the description of map domains geometrically. Here we extend the OS-affine map correspondence to qudits, briefly discuss general properties of the map, the form for particular important cases, and provide several explicit results for qutrit maps. We use the affine map and a singular-value-like decomposition, to find positivity constraints that provide a symmetry for small polarization vector magnitudes (states which are closer to the maximally mixed state) which is broken as the polarization vector increases in magnitude (a state becomes more pure). The dependence of this symmetry on the magnitude of the polarization vector implies the polar decomposition of the map can not be used as it can for the qubit case. However, it still leads us to a connection between positivity and purity for general d-state systems. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J29.00010: Pseudo-unitary freedom in the operator-sum representation Yong Cheng Ou, Mark S. Byrd A general dynamical map can be written in an operator-sum representation (OSR) by using a spectral decomposition, which needs not be completely positive. The OSR is not unique; there is freedom to choose a different set of operators in the OSR, yet still obtain the same map. We will show that, whereas the freedom for completely positive maps is unitary, the freedom for not completely positive maps is pseudo-unitary. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J29.00011: Long-range spin-coupled interactions: a \emph{Gedankenexperiment} on the nature of spin Ian Durham What is intrinsic spin? It is at the heart of the quantum information revolution and yet it defies many of the efforts to better understand it, even to the point of pushing particle physics beyond the Standard Model. Long assumed to require the relativistic theory of Dirac, in 1967 L\'{e}vy-Lablond demonstrated that this was not the case: it is not necessarily a relativistic effect. In this article, we apply the L\'{e}vy-Lablond model to a simple \emph{Gedankenexperiment} that suggests the existence of a quasi-fundamental long-range spin-coupled interaction. Calculations of the eigenfunctions of a test particle and the coupling constant of the force gives insight into the behavior of the potential that gives rise to this interaction. For large separation distances the potential looks like a simple potential well while for very small separation distances it exhibits a more complex nature. This, in turn, sheds additional light on the nature of intrinsic spin and a suggests a path for future research. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J29.00012: Quantum simulation of time-dependent Hamiltonians and the convenient illusion of Hilbert space Rolando Somma, David Poulin, Angie Qarry, Frank Verstraete We consider the manifold of all quantum many-body states that can be generated by arbitrary time-dependent local Hamiltonians in a time that scales polynomially in the system size, and show that it occupies an exponentially small volume in Hilbert space. This implies that the overwhelming majority of states in Hilbert space are not physical as they can only be produced after an exponentially long time. We establish this fact by making use of a time-dependent generalization of the Suzuki-Trotter expansion, followed by a counting argument. This also demonstrates that a computational model based on arbitrarily rapidly changing Hamiltonians is no more powerful than the standard quantum circuit model. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J29.00013: Mathematical Constraint on Realistic Theories James Franson We consider realistic theories in which some physical property f(r,t) is assumed to exist regardless of whether or not we measure it. It is shown that the value of f(r,t) at position r and time t is completely determined by its value at all other locations r$'$ and earlier times t$' <$ t provided that f(r,t) has continuous second partial derivatives [1]. Mathematical functions of this kind are sufficiently general to describe many situations of physical interest. These results are based on a mathematical identity that is similar in some respects to Cauchy's integral theorem and it can be viewed as a generalization of Green's third identity. The physical implications of weak determinism of this kind will be discussed and it will be contrasted with the properties of quantum systems. \\[4pt] [1] J.D. Franson, arXiv: 1007.1941. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J29.00014: Construction of optimal witness for unknown two-qubit entanglement S.-S.B. Lee, H.S. Park, H. Kim, S.-K. Choi, H.-S. Sim Whether entanglement in a state can be detected, distilled, and quantified without full state reconstruction is a fundamental open problem. We demonstrate a new scheme encompassing these three tasks for arbitrary two-qubit entanglement, by constructing the optimal entanglement witness for polarization-entangled mixed-state photon pairs without full state reconstruction. With better efficiency than quantum state tomography, the entanglement is maximally distilled by newly developed tunable polarization filters, and quantified by the expectation value of the witness, which equals the concurrence. This scheme is extendible to multiqubit Greenberger-Horne-Zeilinger entanglement. This work is to appear in Physical Review Letters. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J29.00015: ABSTRACT WITHDRAWN |
Session J30: Nanowires & Nanotubes: Electronic Properties
Sponsoring Units: DCMPChair: Mark Reed, Yale University
Room: C147/154
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J30.00001: Interplay between structural and electronic properties of bundled Mo$_6$S$_{9-x}$I$_x$ nanowires: an {\em ab initio} study Seoung-Hun Kang, Young-Kyun Kwon, David Tomanek We use first principles density functional theory to investigate the structural, electronic and magnetic properties of isolated and bundled Mo$_6$S$_{9-x}$I$_x$ nanowires with $x=3,4.5,$ and $6$. The skeleton of these nanowires consists of linear arrays of Mo$_6$ octahedra decorated with S and I atoms that are connected by flexible S$_3$ linkages. Due to the bi-stability of each sulfur linkage, free-standing and bundled nanowires are capable of stretching or compressing axially at almost no energy cost, giving rise to many structural minima. We explore the structural stability, elastic behavior and electronic structure at all these minima for different compositions. We find that axial strain and inter-wire interaction in bundles modify significantly the electronic structure. Most intriguing changes occur in nanowires with $x=4.5$ and $6$, which change from metal to semiconductor or undergo a magnetic transition upon axially stretching or compressing the nanowires or upon changing the inter-wire separation. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J30.00002: A Hybrid Density Functional Study of Capped Silicon Carbide Nanotubes Kapil Adhikari, Asok Ray A systematic study of fullerene hemisphere capped finite SiC nanotubes of type 1 using cluster approximation is presented. Nanotubes (3,3) and (5,0) are capped by C$_{20}$-fullerene hemisphere(C$_{10})$ and (5,5) and (9,0) are capped by C$_{60}$-fullerene hemisphere (C$_{30})$. Geometries of the tubes have been spin optimized using the functional B3LYP, 3-21G* basis set and the GAUSSIAN 03 software. The study indicates that fullerene capping of a SiC nanotube changes the electronic and geometric structure properties of SiC nanotubes. For example, the binding energy per atom for infinite nanotube (5,5) is 4.993eV whereas the same nanotube with C- and Si-caps has the binding energy per atom of 5.989eV and 4.812eV, respectively. C-capped nanotubes are energetically more preferable compared to Si-capped. The HOMO-LUMO gaps of the capped nanotubes are significantly lower compared to those of infinite nanotubes. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J30.00003: Low-energy spectral weights of the 1D Hubbard chain Stefan Soeffing, Imke Schneider, Alexander Struck, Sebastian Eggert We investigate the low-energy spectral weights of the 1D Hubbard chain by means of Density Matrix Renormalization Group (DMRG) calculations in comparison with Bosonization results. We identify the bosonic excitations of the underlying Luttinger liquid and analyze their evolution upon increasing the interaction strength in terms of their density of states (DOS). Comparing analytical and numerical results we point out the competition of spin/charge degrees of freedom vs. non-interacting spin up and down particles, which here become important due to the lattice nature of the model and higher order operators. Furthermore, we discuss the spatially resolved (local) DOS that can be calculated analytically by a recursive formula vs. numerically using DMRG. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J30.00004: Implications of time-reversal symmetry for band structure of single-wall carbon nanotubes Serguei Goupalov When electron states in carbon nanotubes are characterized by two-dimensional wave vectors with the components $K_1$ and $K_2$ along the nanotube circumference and cylindrical axis, respectively, then two such vectors symmetric about a ${\bf M}$-point in the reciprocal space of graphene are shown to be related by the time-reversal operation. To each nanotube there correspond five relevant ${\bf M}$-points with the following co\"ordinates: $K_1^{(1)}={\cal N}/2R$, $K_2^{(1)}=0$; $K_1^{(2)}={\cal M}/2R$, $K_2^{(2)}=-\pi/T$; $K_1^{(3)}=(2 \, {\cal N} -{\cal M})/2R$, $K_2^{(3)}=\pi/T$; $K_1^{(4)}=({\cal M} +{\cal N})/2R$, $K_2^{(4)}=-\pi/T$, and $K_1^{(5)}=({\cal N} -{\cal M})/2R$, $K_2^{(5)}=\pi/T$, where ${\cal N}$ and ${\cal M}$ are the integers relating the chiral, ${\bf C}_h$, symmetry, ${\bf R}$, and translational, ${\bf T}$, vectors of the nanotube by ${\cal N} \, {\bf R}={\bf C}_h + {\cal M} \, {\bf T}$, $T=|{\bf T}|$, and $R$ is the nanotube radius. We show that the states at the edges of the one-dimensional Brillouin zone which are symmetric about the ${\bf M}$-points with $K_2=\pm \pi/T$ are degenerate due to the time-reversal symmetry. Explicit expressions are obtained for the co\"ordinates of the ${\bf K}$-points in the reciprocal space of graphene relevant to a given nanotube. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J30.00005: Ab initio simulations of the eletronic and transport properties of nanotube bundles used as gas sensors Alexandre Rocha, Rodrigo Amorim, Adalberto Fazzio, Ant\^onio J.R. da Silva Carbon nanotubes (CNT) have exceptional mechanical and - particularly - electronic properties that make this material of great potential interest for applications in different areas of materials science. One of the possibilities which raises the highest hopes is the area of nanotube-based gas sensors. From the fabrication point of view, one is probably going to use bundles of CNTs instead of a single tube. In this work we initially use density functional theory (DFT) calculations to determine the electronic structure properties of different molecules interstitially positioned between the nanotubes in a bundle. From the most stable structures we couple the DFT calculations to a recursive Green's function method to simulate. The electronic transport properties of a disordered nanotube bundle containing a large number of molecules randomly distributed along the different tubes forming the ropes. This way one is able to simulate a realistic sensor based on three-dimensional nanotube bundles taking into consideration the effects of disorder. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J30.00006: Theoretical studies of the electronic and transportation properties of Gd disilicide nanowires on Si(001) Wenjie Ouyang, Yanning Zhang, Shengyong Qin, Anping Li, Ruqian Wu The scanning tunneling microscopy data demonstrate the successful growth of isolated GdSi$_{2}$ nanowires and wire bundles on Si (100) surface and the nano transport measurement shows the isolated nanowires exhibit a metal-insulator transition (MIT) upon cooling while the wire bundles maintain a metallic state. We investigate the structural and electronic properties of isolated GdSi$_{2}$ nanowires and wire bundles surface through extensive density functional calculations. A 8aSi-wide supercell was used to mimic the environment of a single nanowire, and a 5aSi-wide supercell was used for wire bundles. Interestingly, we found that the bundle structures frustrate the Perils-type structural transition that occurs easily in single nanowires. This can be regarded as the reason for the observed MIT. We also explored the effect of Si adatoms on top of wires and wire bundles. The electrical transport behaviors of GdSi$_{2}$ nanowires are further explained using the calculated local electronic density of states and band structures. The special magnetic ordering and its effect on other properties of nanowires will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J30.00007: Structural and Electronic properties of a bismuth nanowire encapsulated inside a boron nitride nanotube Chi-Hsuan Lee, Chih-Kai Yang The structural and electronic properties of a bismuth nanowire (BiNW) encapulated inside the boron nitride nanotube (BNNT) are investigated by first principles calculation. The results show that they depend both on the configuration of BiNW and the diameter of the BNNT. The interaction between the two constituents induces hybridization of energy bands from each subsystem, causing unexpected variation of dispersion and splitting of energy bands near the Fermi level. The role of spin-orbit interaction is especially decisive in the later outcome. It enhances the stability of the hybrid structure and produces more band-edge states. These results should be observable with the tool of scanning tunneling spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J30.00008: Jastrow-Correlated Wavefunctions for Flat-Band Lattices Hao Wang, V.W. Scarola The electronic band structure of many compounds, e.g., carbon-based nanostructures, can exhibit essentially no dispersion. Models of electrons in such flat-band lattices define non-perturbative strongly correlated problems by default. We construct a set of Jastrow-correlated ansatz wavefunctions to capture the low energy physics of interacting particles in flat bands. We test the ansatz in an example honeycomb ribbon. The model Hamiltonian is projected on a flat band of the ribbon, thus containing only the Coulomb interaction term. The properties of the ground states are studied using numerical diagonalization. We find that the ansatz wavefunction accurately captures the ground state in a transition from a crystal to a uniform quantum liquid. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J30.00009: Electronic structures of potassium-doped C$_{60}$ encapsulated in BN nanotubes Takashi Koretsune, Susumu Saito, Jesse Noffsinger, Marvin L. Cohen Boron-nitride nanotubes have large band gap independent of chirality and are promising candidates for nanostructure control. Here, we investigate the electronic structure of potassium-doped C$_{60}$ encapsulated in boron-nitride nanotubes using first-principles methods based on the density functional theory. We demonstrate that the material is one-dimensional metal where conducting electrons are only in the C$_{60}$ chain. Interestingly, the material can have a large Fermi-level density of states, which indicates the possibility of various phase transitions including superconductivity as in the case of fcc K$_3$C$_{60}$. We therefore discuss the electron-phonon couplings as well as the pressure dependence of the electronic structures of this material. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J30.00010: Effect of atomic defects and interwire coupling on the electronic properties of one-dimensional Gd silicide nanowires Shengyong Qin, Tae-Hwan Kim, Arthur P. Baddorf, An-Ping Li, Hanno H. Weitering, Chih-Kang Shih, Wenjie Ouyang, Yanning Zhang, Ruqian Wu Metallic nanowires have attracted great interest for understanding the electronic interactions and conductivity in one dimension. Electron transport is often dictated by quantum instabilities and strong localization at low temperature. Well-ordered and uniformly oriented GdSi2 nanowires are self-assembled on Si(100) in the form of either isolated nanowires or wire bundles with atomic interwire spacing. The effects of interwire coupling and atomic defects in these quasi-one-dimensional systems are studied by correlating the 4-probe STM electrical transport with STM local density of states of individual nanowires. While the isolated nanowires exhibit a metal-insulator transition associated with atomic defects, the wire bundles remain metallic at low temperature which we believe the interwire coupling suppress the lattice disorder and stabilize a robust metallic conductance. This research at ORNL's CNMS was sponsored by the Scientific User Facilities Division, Office of BES, U.S. DOE. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J30.00011: Negative curvature energy in magnesium-boride nanotubes Hui Tang, Sohrab Ismail-Beigi Mg-boride nano-materials have attracted much attention due to constant quest for novel superconducting materials on nanoscale. A recent experiment on Mg borides nanostructures has hinted at a possible superconducting temperature as high as 80K. More generally, studying the physics of pure and metal-doped boron nanosystems enhances understanding of novel properties that emerge in reduced dimensions. Here, based on first principles calculations, we describe an unusual nanoscale curvature effect in Mg-boride nanotubes and discuss its origin. We show that a number of 2D Mg-boride sheets prefer to spontaneously curve themselves into small diameter nanotubes and thus have negative curvature energies. This is rather unique when compared to other nanotubular materials: usually, curving the parent 2D sheet to create a nanotube imposes an energy cost. We explain the reason for the negative curvature energy by analyzing the charge state of the Mg atoms, its relation to the type of boron sublattice present in the nanostructure, and its consequences for the Mg-Mg interactions and hence the energetics. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J30.00012: Quantum Size Effect and Electronic Stability of Freestanding Metal Atom Wires Haiping Lan, Ping Cui, Jun-Hyung Cho, Qian Niu, Jinlong Yang, Zhenyu Zhang Using DFT calculations, we present a thorough study of the quantum size effects on the stability of freestanding metal atom wires. Our systems include Na, Ag, Au, In, Ga and Pb atom wires, i.e. s, sd, and sp electron prototypes. We found that the total energy always oscillates with the wire length, which clearly indicates the existence of preferred lengths. Increasing the length, the s-system exhibits even-odd oscillations following a 1/x decay law in the stability, which can be attributed to electron band filling and quantum confinement along the wire. The sd-system exhibits a similar oscillation pattern, even in the presence of sd hybridization. In sp-system, the energy oscillations are beyond the simple even-odd nature, likely due to unpaired p orbitals and the corresponding nontrival band filling. Our findings clearly demonstrate that electronic contribution is quite critical to the stability of freestanding wires, and this stability may be important even when wires are deposited on substrates or strained. This study sheds light on the underlying formation mechanism of metal atom wires. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J30.00013: Gate controlled donor activation in silicon nanowires Adam Gali, Binghai Yan, Thomas Frauenheim Due to the proximity to an embedding medium with low dielectric constant (e.g., oxides), semiconductor nanowires have higher impurity ionization energy than their bulk counterparts, resulting lower free carrier density. Using ab initio calculations within density functional theory, we propose a way to reduce the ionization energy in nanowires by fabricating a special cross section with appropriate engineering of doping and an applied gate voltage. We demonstrate on a phosphorus-doped silicon nanowire that the ionization energy can be effectively tuned and the impurity backscattering can also be reduced. For instance, the free carrier density may increase by 40{\%} in a silicon nanowire with 15 nm diameter and special cross section without special engineering of doping. Our proposal has profound implications to fabricate nanowire devices with high carrier density. Our proposed Si NW device realizes a fine manipulation of the interaction between electron and nuclear spins by using an external electric field which is a fundamental step to a silicon-based nuclear spin quantum computer. Moreover, with a negative voltage the ionization energy of P-donors can be increased even in larger silicon nanowires which opens up the possibility to manipulate the donor electron spin at room temperature [1]. [1] B. Yan, Th. Frauenheim and A. Gali, \textit{Nano Lett.}, 2010, 10, 3791 [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J30.00014: Non-Collinear Ferromagnetic Luttinger Liquids Nicholas Sedlmayr, Sebastian Eggert, Jesko Sirker In the now classic Tomonaga-Luttinger model the presence of the electron-electron interaction in one dimension is shown to profoundly change the properties of the system. We consider here the magnetic and electronic properties of a \emph{ferromagnetic} Luttinger liquid when it has a region of non-collinearity present, i.e. a domain wall. Spin-charge separation does not survive in this system, and the absence of both spin-charge separation and coherent spin-charge excitations has consequences for the spin-transfer-torque effects which cause domain wall motion. Furthermore the presence of the domain wall introduces a spin dependent scatterer into the problem, which will alter both the transport, and the static electronic, properties of the system. Finally we show how the magnetization dynamics of the domain wall will be modified for a Luttinger liquid. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J30.00015: How to Extract Luttinger Liquid Velocity from Carbon Nanotubes Darryl H. Ngai, Chang-Yu Hou, Eun-Ah Kim We propose direct detection of Luttinger Liquid velocity of the charge collective mode in carbon nanotubes using optical conductivity and Coulomb blockade effect. We note that detection of such fractionalized excitation needs to exploit the energy or frequency scale tied to the finite length of the nanotube. This is why previous experimental attempts have been unsuccessful.\footnote[1]{Z. Zhong {\it{et al.}}, Nature Nanotechnology {\bf{3}}, 201 (2008)} We will discuss features in the optical conductivity sensitive to the velocity of the collective mode which would be observable in the high temperature limit. In the low temperature limit, spacing between the Coulomb blockade peaks in the conductance as a function of gate voltage will be a sensitive probe. [Preview Abstract] |
Session J31: Focus Session: Materials at High Pressure II: Elements
Sponsoring Units: DMP GSCCM DCOMPChair: Viktor Struzhkin, Carnegie Institution
Room: C145
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J31.00001: Multi-Scale Shock Compression Simulations of Metals and Metallic Phase Transitions Nir Goldman, Larry Fried We present a straightforward method for efficient molecular dynamics (MD) simulation of shock compression of materials that experience thermal electronic excitations at high pressure and temperature. Previous studies have shown that exclusion of the electronic temperature at extreme conditions can result in incorrect computation of dynamic and equation of state properties. The Multi-Scale Shock Technique (MSST) is a simulation methodology based on the Navier--Stokes equations for compressible flow that enables MD simulation of a shock wave with relatively small computational cost. We extend MSST to allow for changes in the electronic entropy during shock compression while conserving Hugoniot conditions. This allows for simulation of significantly higher shock velocities than previously possibly with MSST. We have used our simulation methodology in density functional tight binding simulations of shock compressed silicon. We observe that at high shock velocities inclusion of a non-zero electron temperature results in lower computed shock Hugoniot temperatures and pressures. Our methodology is well suited for shock compression simulations of any material that experiences changes in its electronic entropy under extreme thermodynamic conditions. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J31.00002: Large-Scale Molecular Dynamics Simulations of Shock-Induced Plasticity in Tantalum Single Crystals R. Ravelo, Qi An, T.C. Germann, B.L. Holian We report on large-scale non-equilibrium molecular dynamics (NEMD) simulations of shock wave compression in Ta single crystals. The atomic interactions are modeled via a recently developed and optimized embedded-atom method (EAM) potential for Ta, which reproduces the equation of state up to 200 GPa. We examined the elastic-plastic transition and shock wave structure for wave propagation along the low index directions: (100), (110) and (111). Shock waves along (100) and (111) exhibit an elastic precursor followed by a plastic wave for particle velocities below 1.1 km/s for (100) and 1.4 km/s for (111). The nature of the plastic deformation along (110) is dominated by twinning for pressures above 40 GPa. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J31.00003: Transport of particulate matter from a shocked interface W.T. Buttler, J.E. Hammerberg, D. Oro, C. Morris, F. Mariam, C. Rousculp We have performed a series of shock experiments to measure the evolution and transport of micron and sub-micron Tungsten particles from a 40 micron thick layer deposited on an Aluminum substrate. Densities and velocity distributions were measured using proton radiography at the Los Alamos Neutron Science Center for vacuum conditions and with contained Argon and Xenon gas atmospheres at initial pressures of 9.5 bar and room temperature. A common shock drive resulted in free surface velocities of 1.25 km/s. An analysis of the time dependence of Lithium Niobate piezo-electric pin pressure profiles is given in terms of solutions to the particulate drag equations and the evolution equation for the particulate distribution function. The spatial and temporal fore-shortening in the shocked gas can be accounted for using reasonable values for the compressed gas shear viscosities and the vacuum distributions. The detailed form of the pin pressure data for Xenon indicates particulate breakup in the hot compressed gas. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J31.00004: Thermal Emission Determination of Argon under Extreme Pressure and Temperature D. Allen Dalton, Michael Wong, Alexander F. Goncharov, Julius Ojwang, Viktor V. Struzhkin, Zuzana Konopkova, Peter Lazor Argon is a common pressure-transmitting medium in diamond anvil cell (DAC) experiments, and is often used as thermal insulation in the laser heated DAC. A more thorough understanding of the thermal properties of argon under extreme conditions is essential for measuring thermal properties of materials under similar conditions. A transient heating technique was applied to a symmetric DAC up to 50 GPa and 2500 K. A 1 $\mu $m thick iridium foil positioned within a recessed gasket hole filled with argon served as a laser absorber to pump thermal energy into the sample. Pump pulses of 6 $\mu $s temporal width were provided from an electronically modulated Yb-based fiber laser. We determined the temperature of the coupler with 500 ns time resolution by applying a Planckian fit to the thermal emission spectrum. Finite element calculations were also used to simulate thermal diffusion in the DAC cavity. The experimental results show slightly larger thermal conductivity with theory, but the results converge in the limit of high temperature. This work is supported by NSF EAR 1015239, NSF-REU, Carnegie Institution of Washington, and DOE-NNSA (CDAC). [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J31.00005: Thermal diffusivity of metals at high pressure Bruce Baer, William Evans Very few measurements of thermal diffusivity have been taken at high pressure. This is especially true of metals above 2 GPa. In earlier experiments, the Angstrom method has been employed for these types of measurements. However, this method is limited for high pressure because it requires a relatively large sample. We will discuss the use of sinusoidally modulated laser heating to measure thermal diffusivity in the diamond-anvil-cell. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J31.00006: Pressure-Induced Structure Transitions in Europium Metal to 92 GPa W. Bi, J. Schilling, Y. Meng, R. Kumar, A. Cornelius, Y. Zhang, C. Chen, R. Hennig Motivated by the recent discovery of pressure-induced superconductivity in Eu for pressures above 80 GPa [1], we have carried out high pressure angle-dispersive synchrotron x-ray diffraction measurements on Eu metal in a diamond anvil cell to 92 GPa. Our experiments confirm the bcc-to-hcp transition at 12 GPa reported in previous studies and identify two further phase transitions. The predictions of two independent density functional theory calculations are compared to the experimental results.\\[4pt] [1] M. Debessai, T. Matsuoka, J. J. Hamlin, J. S. Schilling, and K. Shimizu, Phys. Rev. Lett. \textbf{102}, 197002 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J31.00007: Electronic structure and dynamics of elements at high pressures Invited Speaker: Electronic structure and dynamics information of materials under high pressure has been very scarce due to the experimental difficulties. The standard electronic probes using electron energy loss spectroscopy (EELS) is limited to vacuum pressures. The optical probes that can reach high-pressure samples through the diamond windows, on the other hand, are limited by the energy accessibility ($<$ 5 eV) and near-zero momentum transfer, $q=(4\pi /\lambda _0 )\sin \theta $. These problems can be overcome by the newly advanced, two-photon, inelastic, xray, scattering (IXS) spectroscopy which uses high energy xrays ($\sim $ 10$^{4}$ eV) to provide the atomic-level momentum transfer and to enter (with energy $E)$ and exit (with energy $E_0 )$ the pressure vessel. The electronic spectra are revealed by analyzing the xray energy loss between the two photons, $\hbar \omega =E-E_0 $. Using IXS facilities at third-generation synchrotron source, we studied electronic structure and dynamics of two elements at high pressures in a diamond-anvil cell: i.e., He, the widest-gap insulator, and Na, the archetypal free-electron metal. At 11.9-17.9 GPa in a single crystal $^{4}$He, we observed rich electron excitation spectra, including a cut-off edge above 23 eV, a sharp exciton peak showing linear volume dependence, and a series of excitations and continuum at 26 to 45 eV. We determined electronic dispersion along the 100 direction over two Brillouin zones, and provided a quantitative picture of the helium exciton beyond the simplified Wannier-Frenkel description. At 1.6-4.39 GPa in a polycrystalline Na sample, we observed the sharp plasmon peak at low $q$ and its dispersion beyond the critical $q_{c}$. The plasmon shifts to higher energy under compression and drastic reduction of $r_{s}$. \textit{Ab-initio} theoretical calculations are conducted for interpretation of the experimental results. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J31.00008: Diffusion Monte Carlo calculations of Xenon melting under pressure L. Shulenburger, T.R. Mattsson The slope of the melting temperature as a function of pressure yields, via the Clausius-Clapeyron equation, important information regarding the changes in density, energy, and entropy. It is therefore crucial to resolve the long-standing differences in melt lines under pressure between Diamond Anvil Cell data (low/flat melt line) and other methods, including density functional theory (DFT) simulations$^1$ (high/steep melt line). The disagreement for Ta was recently resolved$^{2}$ and although a similar situation exists in the literature on Xe,$^{3}$ the resolution may be quite different. For example, DFT with its lack of van der Waals forces is a prima facie less credible simulation method for Xe, although excellent agreement has been obtained between calculations of the Hugoniot of Xe and experiments.$^4$ We investigate whether this theoretical shortcoming is significant for the melting transition by applying diffusion Monte Carlo. The energy differences obtained in this way are compared to the DFT results in order to address any systematic errors that may be present near the melting transition. $^1$ Taioli et al. PRB {\bf 75}, 214103 (2007); $^2$ Dewaele et al. PRL {\bf 104}, 255701 (2010); $^3$ Belonoshko el al. PRB {\bf 74}, 054114 (2006); $^4$ Root et al. PRL {\bf 105}, 085501 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J31.00009: Lattice dynamics at ultra-high pressures using high-resolution inelastic x-ray scattering Daniel Farber While our understanding of many physical properties is enhanced by the large body of neutron, the restrictions on sample size imposed by the technique relegated the achievable information to low or at most, moderate pressures ($\sim$10 GPa) and to materials readily available in reasonable large quantities. The advent of third generation synchrotron sources and the construction of beamlines dedicated for inelastic x-ray scattering experiments (IXS), these limitations have to a great degree been overcome. Over the past few years our group has focused a large experimental and theoretical effort on quantifying the vibrational energies in metals at high-pressures and high-temperatures. Most recently, we have determined the phonon dispersions across the isostructural gamma- to alpha-cerium transition. Our new data place important thermodynamical and theoretical constraints on the underlying physics of this important transition. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J31.00010: High-pressure phases of calcium Amanuel Teweldeberhan, Jonathan Dobois, Stanimir Bonev The high-pressure phases of calcium have been investigated using a combination of density functional theory and diffusion quantum Monte Carlo calculations. Finite-temperature Gibbs free energies of several competing structures are computed at pressures near 50 GPa. The discrepancy between theory and experiment both at low and room temperature is resolved with input from diffusion quantum Monte Carlo. Furthermore, diffusion quantum Monte Carlo calculations are performed on 0 K crystalline structures up to 150 GPa. The resulting structures differ from those obtained with density functional theory. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J31.00011: Lifshitz transition in \textit{cI}16 Li at high pressures: Unfolding first-principles Fermi surfaces Chia-Hui Lin, Tom Berlijn, Wei Ku The Fermi surface topology of \textit{cI}16 Li is investigated using the recently developed first-principles band structure unfolding method [1]. The resulting unfolded Fermi surfaces display a clear Lifshitz transition at 47 GPa, explaining the anomalous change of superconducting transition temperature [2]. The unfolded Fermi surfaces also reveals a more complete picture of the driving force of the \textit{cI}16 phase starting at 39 GPa [3]. In addition to the previously proposed ``nesting'' effect [3] along [1$\frac{1}{2}\frac{1}{2}$], both [100] and [$\frac{1}{2}\frac{1}{2}$0] wavevectors are found to contribute significantly to the structural instability as well, due to their large phase space, a more effective effect in 3D. We expect a wide range of applications of this Fermi surface unfolding method to the study of high pressure electronic structure.\\[4pt] [1] Wei Ku et al, Phys. Rev. Lett. {\bf 104}, 216401 (2010)\\[0pt] [2] S. Deemyad and J. S. Schilling, Phys. Rev. Lett. {\bf 91}, 167001 (2003)\\[0pt] [3] M. Hanfland et al, Nature {\bf 408}, 174 (1998) [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J31.00012: On the role of quantum ion dynamics for the anomalous melting of lithium Sabri Elatresh, Stanimir Bonev Lithium has attracted a lot of interest in relation to a number of counterintuitive electronic and structural changes that it exhibits under pressure. One of the most remarkable properties of dense lithium is its anomalous melting. This behavior was first predicted theoretically based on first-principles molecular dynamics (FPMD) simulations, which treated the ions classically [1]. The lowest melting temperature was determined to be about 275~K at 65~GPa. Recent experiments measured a melting temperature about 100~K lower at the same pressure. In this talk, we will present FPMD calculations of solid and liquid lithium free energies up to 100 GPa that take into account ion quantum dynamics. We examine the significance of the quantum effects for the finite-temperature phase boundaries of lithium and, in particular, its melting curve. \\[4pt] [1] I. Tamblyn, J-Y. Raty, and S. A. Bonev, Phys. Rev. Lett. 101, 075703 (2008).\\[0pt] [2] E. Gregoryanz et al, Nature, in press. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J31.00013: Graphite under high pressure Yuejian Wang As one of the longest-known forms of carbon, graphite has been extensively studied for several decades. However, its phase diagram under high pressures is still poorly understood. Here we use both in-situ high-pressure Raman spectroscopy and synchrotron x-ray diffraction, collected on both compression and decompression, to elucidate the high-pressure behavior of highly-ordered pyrolitic graphite (HOPG) at room temperature. The Raman spectra show that G band (1580 cm$^{-1}$ at ambient pressure) of HOPG shifts to higher frequency with increased pressure, which has been attributed to pressure-induced in-plane lattice contraction. Above 19 GPa the broadening of this Raman peak indicates a reordering of the atomic structure, and is consistent with synchrotron x-ray diffraction measurements that also show a slight change in symmetry. [Preview Abstract] |
Session J32: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures: Nanowires
Sponsoring Units: DMPChair: Leigh Smith, University of Cincinnati
Room: C144
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J32.00001: Finite Size Effects in Electrical Transport in Nanowires and Nanowire-like Devices. Invited Speaker: Semiconductor nanowires posses many unique qualities including high crystallinity, simple growth techniques compatible with variety of low cost substrates, increased ability to accommodate strain, and nanoscale dimensions not easily accessible by `top-down' lithographic means. These and other interesting characteristics have made nanowires an attractive topic for fundamental research, as well as for potential applications in nanoelectronics, photonics, sensors, and more recently energy conversion and storage. Most of these applications rely on charge transport, which can be profoundly affected by the high aspect ratio, high surface to volume ratio and nanoscale diameter typical of semiconductor nanowires. Properly identifying the factors that influence electrical transport characteristics is important for device design but also because extraction of material parameters such as the mobility relies on analysis with specific models. In my talk I will discuss several specific examples where nanoscale dimensions and geometry profoundly affect transport and device characteristics, including GaN and GaN/III-N core/shell nanowires; Si radial pn-junction photovoltaics; and all-nanowire Li-ion batteries.. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J32.00002: Photocurrent Spectroscopy of single ZB, WZ InP Nanowire devices K. Pemasiri, S. Perera, A. Wade, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish Photocurrent spectroscopy was performed on single InP nanowire devices having either zinc-blende (ZB) or wurtzite (WZ) crystal structures at 300~K and 10~K. Photolithography was used to fabricate Ohmic Ti/Al metal contact pads separated by 5 $\mu $m. Using a monochromatic white light set up or a tunable (1.30 to 1.55 eV) CW laser, the photocurrent is measured as a function of bias voltage and excitation energy. At room temperature, the lowest energy band of In WZ (1.408~eV) is found to be 70 meV above the ZB band gap (1.338~eV), consistent with previous photoluminescence measurements. At low temperatures (10 K), the ZB device shows strong evidence for a broadened excitonic resonance peak at 1.432 eV and the WZ device shows three excitonic peaks at 1.504 eV, 1.56 eV, and 1.65 eV corresponding to the A,B and C valence band energies, respectively, which coincide with recent photoluminescence excitation measurements. Support for this work was provided by the NSF ({\#}0701703, {\#}0806700 and {\#}0806572) and the Australian Research Council. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J32.00003: Photoresponse of solution-synthesized PbSe nanowire devices Todd Brintlinger, Edward E. Foos, Joseph G. Tischler, Janice E. Boercker, Thomas J. Zega, Rhonda M. Stroud, Steve C. Erwin PbSe nanowires are of interest for photovoltaic applications due to their variable band gap in the near infrared and potential for efficient multiexciton generation. Contributing to this effort, we present our ongoing studies of the photoresponse of PbSe nanowire devices. These materials are synthesized in solution by reaction of Pb and Se precursors in the presence of stabilizing organic ligands, and possess 10-20 nm diameters with lengths $>$1 $\mu $m. The nanowires are then dispersed on transparent silicon nitride membranes, allowing for both transmission electron microscopy and optical spectroscopy of individual devices. Devices show up to two-fold increases in current under illumination, with current density in the $\sim $1x10$^{9}$ A/m$^{2}$ range. Electrical transport and characterization of nanowires will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J32.00004: Energy and Spatially Resolved Measurements of Plasmonically Enhanced Photocurrent in a Single Si Nanowire with Au Nanoparticles Jerome Hyun, Lincoln Lauhon Hybrid assemblies of nanowires and metallic particles have attracted great interest because of their potential as light harvesting systems. Optoelectronic measurements of the most basic light absorbing unit in such systems, consisting of a single nanowire and plasmonic particles, would provide further guidance for performance optimization schemes. Here, we present spatially and energy resolved photocurrent measurements across the visible spectrum on a Si nanowire device with Au nanoparticles using a confocal scanning microscope and a tunable wavelength laser source. A 50 nm diameter nanowire was used due to its monotonic optical response in the wavelength range of interest, and 50 nm size Au nanoparticles were selected in order to neglect the effects of Mie scattering. The photocurrent is shown to depend on the azimuthal location of the nanoparticles on the nanowire. Nanoparticles resting on the substrate adjacent to the nanowire can significantly modify the absorption with a strong polarization-dependent plasmonic response while nanoparticles resting directly in the line of sight between the nanowire and light source show minimal contribution to the photocurrent. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J32.00005: Charge injection and transport in nanowires Invited Speaker: Semiconductor nanowires show promise in electronic, optoelectronic, and sensing devices. To realize this promise, a fundamental understanding of charge injection and electronic transport in these novel nanomaterials is necessary. In this presentation, I will discuss recent work that couples experiment and theory to address this topic. For example, in GaN and InAs nanowires, we achieve efficient charge injection and find that space-charge-limited currents are unusually strong. In contrast, charge transport across individual Au-nanoparticle/Ge-nanowire interfaces is injection-limited, and surprisingly, the conductance increases with decreasing nanowire diameter due to a dominance of electron-hole recombination. Furthermore, we find that transport in GaAs nanowires is governed by charge traps, which can be activated to reveal the nature of the charge injection at the contacts. More generally, our results indicate that a broad range of electronic transport regimes can be observed in semiconducting nanowires depending on the particular material system and growth process. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J32.00006: Photocurrent spectroscopy of CdS nanosheets P. Kumar, A. Wade, H.E. Jackson, L.M. Smith, J. Yarrison Rice, Y.-J. Choi, J.-G. Park We study the photocurrent from photoexcited charge carriers in CdS nanosheet (NS) structures. Metal-semiconductor-metal nanodevices are made with both Schottky and Ohmic contacts using photolithography followed by Ti/Al (20nm/200nm) metal evaporation and lift-off. Ohmic contacts are formed by Ar ion bombardment before the metal deposition to create donor sulfur vacancies which increases the electron concentration. Photocurrent spectra using a white light source filtered by a monochrometer show excitonic resonances at low temperatures corresponding to each of the A, B, and C hole bands. The photocurrent increases linearly with power for above gap excitation, and nonlinearly (quadratic) with laser power for below gap excitation, consistent with two-photon absorption with a nonlinear coefficient of $\beta $ = 2 cm/GW. A wavelength dependence of the photocurrent with sub-band gap excitation to find the resonances and hence band structure is in progress. We acknowledge the financial support of the National Science Foundation through grants DMR-0806700, 0806572 and ECCS-0701703, and the KIST institutional research program 2E21060R. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J32.00007: Electronic Devices with Dichalcogenide Nanolayers Andras Kis, Branimir Radisavljevic, Mohamed Benameur, Jacopo Brivio We have exfoliated single, two-dimensional layers 6.5 Angstrom thick from a number of dichalcogenide materials such as MoS2, using the micromechanical cleavage technique commonly used for the production of graphene. Optical microscopy together with AFM was used to characterize the nanolayers and establish optimal conditions for rapid identification of monolayers using optical methods. Our nanolayers are mechanically and chemically stable under ambient conditions. We have electrically contacted nanolayers using electron-beam lithography and fabricated field-effect transistors. Electrical transport measurements show that our devices have high on/off ratios and high mobilities. Our results indicate that two-dimensional dichalcogenide nanolayers could be interesting building blocks for nanoelectronic applications. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J32.00008: Electric Field Dependent Photocurrent Decay Length in Single Lead Sulfide Nanowire Field Effect Transistors Dong Yu, Rion Graham, Chris Miller, Eunsoon Oh We determined the minority carrier diffusion length to be $\sim $1 $\mu $m in single PbS nanowire field effect transistors by scanning photocurrent microscopy. PbS nanowires grown by the vapor-liquid-solid method were p-type with hole mobilities up to 49 cm$^{2}$/Vs. We measured a photo-response time faster than 14 $\mu $s with near-unity charge separation efficiency at the contacts. For the first time, we also observed a field dependent photocurrent decay length, indicating a drift dominant carrier transport at high bias. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J32.00009: Exciton Diffusion Measurements in III/V Nanowires using Spatially and Time Resolved Photoluminescence M.A. Fickenscher, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, J.H. Kang, Q. Gao, H.H. Tan, C. Jagadish We present an optical investigation of transport in GaAs/AlGaAs core shell nanowires utilizing low temperature spatial and time resolved photoluminescence (PL). We use a solid immersion lens (SIL) to achieve a laser spot size and image resolution of 600 nm. With the laser spot fixed on the nanowire, the image of the wire is scanned across the entrance slit of the spectrometer taking time-decays at each point. Thus, we measure the spatial profiles of the exciton distribution in the wire as a function of time. We then extract the diffusion constant from the width squared of each spatial distribution as a function of time. The measured exciton diffusion constants are of the order of 100 cm$^{2}$s$^{-1}$, equivalent to a mobility of 100,000 cm$^{2}$ V$^{-1}$s$^{-1}$ by using the Einstein relation. These values are comparable to the best hole mobilities seen in modulation doped two dimensional GaAs/AlGaAs heterostructures. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J32.00010: Electron transport in coupled InGaAs quantum wires Vasyl Kunets, Sergey Prosandeev, Sabina Koukourinkova, Vitaliy Dorogan, Yuriy Mazur, Marcio Teodoro, Morgan Ware, Mourad Benamara, Peter Lytvyn, Gregory Salamo Remotely doped InGaAs/GaAs heterostructures were grown by molecular beam epitaxy on the (311)A plane of GaAs. Applying strain driven epitaxy on the (311)A GaAs surface, two-dimensional quantum wells (QW) and quasi-one-dimensional quantum wires (QWr) were formed by varying InGaAs coverage between 6 and 11 monolayers. Polarization dependent photoluminescence and electrical conductivity experiments revealed a remarkable anisotropy in the QWr samples, which was insignificant in the QWs, the dimensionality of which was confirmed by atomic force and cross-sectional transmission electron microscopies. The resulting complex behavior of the electric current anisotropy as function of InGaAs coverage, doping and temperature is explained through a multi-band conductivity model, which is supported by magneto-transport measurements at low and high magnetic fields along with the Hall effect theory in anisotropic media with multi-band conduction. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J32.00011: Probing charge density wave transition at the nanoscale in NbSe$_{2 }$using NEMS resonators Shamashis Sengupta, Hari Solanki, Vibhor Singh, Sajal Dhara, Mandar Deshmukh We study nanoelectromechanical (NEMS) resonators fabricated from suspended flakes of NbSe$_{2}$ (thickness $\sim $ 30--50 nm) to probe charge density wave (CDW) physics at nanoscale. Variation in elastic and electronic properties accompanying the CDW phase transition (around 30 K) are investigated simultaneously using the devices as self-sensing heterodyne mixers. Elastic modulus is observed to change by 10{\%}, an amount significantly larger than what had been reported earlier in the case of bulk crystals. We also study the modulation of conductance by electric field effect, and examine its relation to the order parameter and the CDW energy gap at the Fermi surface. [Preview Abstract] |
Session J33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Multiferroics & Magnetoelectrics II
Sponsoring Units: DMP DCOMPChair: Craig Fennie, Cornell University
Room: C143/149
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J33.00001: Correlation between magnetocapacitance effect and polarization flop direction in a slanted magnetic field in Tb$_{1-x}$Dy$_{x}$MnO$_{3}$ Nobuyuki Abe, Hajime Sagayama, Hiroshi Umetsu, Taka-hisa Arima, Kouji Taniguchi Recent extensive studies show that the cycloidal spin system can possess electric polarization through the spin-orbit coupling. The magnetoelectric coupling in multiferroics is enhanced by the clamping of helimagnetic and ferroelectric domain walls. For example, DyMnO$_{3}$ shows a gigantic magnetocapacitance effect caused by the microscopic motion of multiferroic domain walls at a magnetic field induced \textbf{\textit{P}}-flop transition. In contrast, the enhancement of capacitance at the \textbf{\textit{P}}-flop transition is much smaller in TbMnO$_{3}$. Here, we show the systematic control of magnetocapacitance effect in helimagetic ferroelectric Tb$_{1-x}$Dy$_{x}$MnO$_{3}$ as a function of the composition ratio $x$ and the intensity of the applied magnetic field. It has been also found that the rotation direction of \textbf{\textit{P}} in a slanted magnetic field changes with $x$ and $H$. The crossover between small and large enhancement in magnetocapacitance corresponds to the switch in the \textbf{\textit{P}}-flop direction. The correlation can be explained by assuming the mobility of domain wall would be dominated by the thickness of domain walls in a helical magnet. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J33.00002: Giant Magnetoelectric Effect in Antiferromagnetic BaMnO$_{3-\delta}$ and Its Derivatives M. Ge, O.B. Korneta, T.F. Qi, S. Parkin, L.E. DeLong, G. Cao, P. Schlottmann Hexagonal perovskite 15R-BaMnO$_{2.99}$ with a ratio of cubic to hexagonal layers of 1/5 in the unit cell is an antiferromagnetic insulator that orders at a N\'eel temperature T$_{N }$= 220 K. Here we report structural, magnetic, dielectric and thermal properties of single crystal BaMnO$_{2.99}$ and its derivatives BaMn$_{0.97}$Li$_{0.03}$O$_{3}$ and Ba$_{0.97}$K$_{0.03}$MnO$_{3}$. The central findings of this work are: \textbf{(1)} these materials possess a usually large, high-temperature magnetoelectric effect that amplifies the dielectric constant by more than an order of magnitude near their respective N\'eel temperature; \textbf{(2)} Li and K doping can readily vary the ratio of cubic to hexagonal layers and cause drastic changes in dielectric and magnetic properties. These findings provide a new paradigm for developing \textit{novel}, \textit{high-temperature magnetoelectric} \textit{materials} that may eventually contribute to technology. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J33.00003: Electric-field control of magnetic orderings in the tetragonal BiFeO$_{3}$ Chun-Gang Duan We present a systematic first-principles study on the magnetic properties of the tetragonal BiFeO$_{3}$ (P4mm) with various in-plane lattice constants. The Heisenberg model is applied to study the behaviors of exchange constants (J$_{1a}$, J$_{1c}$, J$_{2a}$, J$_{2c})$ under the influence of the in-plane strain. We find that in certain region of the in-plane lattice constant, switching the direction of polarization from out of plane to in-plane by electric field could result in transition of magnetic orderings, e.g., from G-type to C-type antiferromagnetic states in tetragonal BiFeO$_{3}$. This may open a new avenue to controlling magnetoresistance using electric field. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J33.00004: Ferroelectric polarization in the magnetic world Invited Speaker: Switchable spontaneous polarization in ferroelectrics is produced by a structural distortion of a high-symmetry reference phase which lowers the symmetry to a polar space group. Under certain conditions, this structural distortion and symmetry breaking can also induce ferromagnetism and other changes, such as a metal-insulator transition, allowing the possibility of electric and magnetic field control. In this talk, I will present first-principles illustrations of specific materials realizations of the rich variety of this behavior in magnetic perovskite oxides, identified using a database of first-principles calculations of the full phonon dispersions of a range of magnetic perovskites, including the d3 compounds SrMnO3and SrCaO3, the d5 compounds BiFeO3, and the series SrMO3 (M= V, Cr, Mn, Fe, Co). First, I will discuss an epitaxial-strain-induced multiferroic phase produced by large spin-phonon coupling in SrMnO3 [1]. Then, I will turn to colossal magnetoresistance based on a ferromagnetic- metal/antiferromagnetic-ferroelectric phase boundary with epitaxial strain in SrCoO3, which exhibits typical ferromagnetic metallic character in room-temperature but with a large spin-phonon coupling by which antiferromagnetic ordering favors a polar distortion. Lastly, I will discuss the identification of perovskite superlattice systems in which the symmetry lowering produced layer-by-layer ordering produces a phase with ferroelectrically-induced weak ferromagnetism. I will present first-principles calculations demonstrating these behaviors in BaMnO3/SrMnO3 superlattices and other systems which could provide robust experimental realizations. \\[4pt] [1] J. H. Lee and K. M. Rabe, ``Epitaxial-strain-induced multiferroicity in SrMnO3 from first principles,'' Phys. Rev. Lett. 104, 207204 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J33.00005: Magnetic and multiferroic phases of single-crystalline Mn$_{1-x}$Co$_{x}$WO$_{4}$ K.-C. Liang, R.P. Chaudhury, Y.-Q. Wang, Y.Y. Sun, B. Lorenz, F. Ye, J.A. Fernandez-Baca, H.A. Mook, C.W. Chu The recent interest of MnWO$_{4}$ system is due to the strong correlation between the long-wavelength magnetic structure and the ferroelectric polarization. To understand the effects of Co substitutions on magnetic and multiferroic phases of MnWO$_{4}$, we studied the magnetic and dielectric properties of the single-crystalline Mn$_{1-x}$Co$_{x}$WO$_{4}$ compounds. At lower Co substitution, the commensurate (CM) AF1 phase was found suppressed but could be restored in external magnetic fields along b axis. We also observed the ferroelectric polarization along b axis suppressed by a b-axis magnetic field. On the other hand, the higher Co substitution such as 15{\%} showed more complex magnetic phases, which warrants future investigation. With Neutron scattering data, more detailed magnetic orders of the various phases would be revealed, and the relationship between magnetic phases and ferroelectric polarization will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J33.00006: Magnetoelectricity in BiFeO$_3$ films - first-principles-based computations and phenomenology Sergey Prosandeev, Igor Kornev, Laurent Bellaiche A first-principles-based effective Hamiltonian is used to compute linear and quadratic magnetoelectric (ME) coefficients in epitaxial (001) BiFeO$_3$ thin films. Its predictions are analyzed within a phenomenological model that provides analytical expressions of the ME coefficients in terms of polarization, as well as, dielectric and magnetic susceptibilities. Main discoveries are: (i) the quadratic ME coefficient is dramatically enhanced by increasing the magnitude of the compressive strain within the Cc phase, as similar to the previously reported enhancement of the linear ME coefficient in these films; (ii) the enhancements of the linear and quadratic ME coefficients have the same macroscopic origin, namely an increase in the dielectric permittivity; and (iii) the relative contribution of {\it two} different free-energy terms on the total linear ME coefficient is extracted from the simulations. The analytical expressions also help in understanding other ME effects. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J33.00007: First Principles Study on Magnetoelectric Effects in Ba$_2$CoGe$_2$O$_7$ Kunihiko Yamauchi, Silvia Picozzi Magnetoelectric (ME) effects, or magnetically-induced ferroelectricity, are attracting large interests due to promising applications for novel type of devices. While the microscopic origin of ME effects is mostly classified as relate to spin current and/or exchange striction, a novel mechanism originating in spin- dependent $p$-$d$ hybridization has been proposed and well explained ME effects observed in antiferromagnetic Ba$_2$CoGe$_2$O$_7$. In this study, we theoretically confirmed the magnetically induced electric polarization, whose size is dependent on the direction of Co spins. By means of both Landau theory and density functional calculations, the behavior of the experimentally observed polarization was well reproduced. Microscopically, we suggest single-site spin orbit coupling to slightly change the Co-$d$ orbital shape upon changing the direction of Co spins, resulting in ``asymmetric" $p$-$d$ hybridization and consequent change in the electric polarization. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J33.00008: Magnetoelectric resonance in S=3/2 two-dimensional antiferromagnet Ba$_{2}$CoGe$_{2}$O$_{7}$ Shin Miyahara, Nobuo Furukawa We have investigated dynamical magnetoelectric effects in S = 3/2 two-dimensional antiferromagnet Ba$_{2}$CoGe$_{2}$O$_{7}$. It is a quasi two-dimensional antiferromagnet. Below T$_{N}$ = 6.7 K, the Co magnetic moments show an antiferromagnetic ordering [1]. Recently, the material is paid attention to due to the magnetic field induced ferroelectric polarization [2,3]. Such multiferroics behaviors can be explained well by considering a spin-dependent metal-ligand hybridization mechanism on a Heisenberg model with strong uniaxial anisotropy term [3,4]. Through the spin dependent polarization, electric component of light can modulate the spin structures. As a result, electric component of light can excite magnetic excitations. We can clarify selection rules and resonance peak positions in absorption. When both magnetic and electric components induce the same magnetic excitation, cross correlated term, where electric component induced excitation is deduced to the ground state by magnetic component and vice versa, is realized. The effects of such cross correlation can be observed directly as linear directional dichroism in absorption process.[1] A. Zheludev \textit{et al.}, Phys. Rev. B \textbf{68 }024428 (2003). [2] H.T. Yi \textit{et al}., Appl. Phys. Lett. \textbf{92 }212904 (2008). [3] H. Murakawa et al., Phys. Rev. Lett. \textbf{105} 137202 (2010). [4] T. Arima, J. Phys. Soc. Jpn. \textbf{76} 073702 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J33.00009: Investigation of Multiferroic properties of the Co doping SrBi$_{2}$Nb$_{2}$O$_{9}$ William Perez, Nora Ortega, Ashok Kumar, Ram Katiyar Multiferroics (MF) are novel class of next generation multifunctional materials, however there are very few single-phase MF materials existing in nature. Thin films (TF) of (Sr$_{x}$Co$_{1-x})$Bi$_{2}$Nb$_{2}$O$_{9}$ (SCBN) with x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3 with thickness $\sim $400 nm were fabricated from individual SCBN targets on Pt/Ti/SiO$_{2}$/Si substrate by pulsed laser deposition technique. The x-ray diffraction studies revealed orthorhombic structure of SCBN for up to 20{\%} Co doped TF without any phase segregation, the splitting in (200) peak was observed above 25{\%} of Co doping. The room temperature (RT) Raman spectra of SCBN TF showed SrBi$_{2}$Nb$_{2}$O$_{9 }$ peaks in all compositions, however additional modes appeared in the 600-800 cm$^{-1 }$frequency region. The dielectric constant of all SCBN films showed linear frequency dispersion above 1 kHz, and their values are in the range of 400 to 650 at 1 kHz. An increase in tangent loss from $\sim $ 0.040 to 0.135 at 1 kHz was observed with increase in Co concentration. All SCBN TF show well defined hysteresis loop with remanent polarization of about 16 $\mu $C/cm$^{2}$. However, less saturation in the ferroelectric loop was observed with increase of Co content. Both magnetic and magneto-electric behavior of TF along with the ferroelectric properties will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J33.00010: Large crystal-symmetry-induced magnetoelectric coupling in the quadruple perovskite BiMn$_3$Mn$_4$O$_{12}$ A. Gauzzi, G. Rousse, F. Mezzadri, G. Calestani, G. Andre, F. Bouree, M. Calicchio, E. Gilioli, R. Cabassi, F. Bolzoni, A. Prodi, P. Bordet, M. Marezio The remarkable properties of manganese oxides $A$MnO$_3$ with perovskite structure, such as the colossal magnetoresistance and the multiferroicity, arise from peculiar charge, spin and orbital orderings of the Mn \textit{e$_g$} electrons driven by cooperative Jahn-Teller distortions of the MnO$_6$ octahedra. Mastering these orderings remains a challenge owing to local structural distortions and electronic inhomogeneities enhanced by chemical substitutions and oxygen defects. We show that these difficulties are absent in \textit{quadruple} perovskites $A$Mn$_3$Mn$_4$O$_{12}$. These compounds share with \textit{simple} perovskites $A$MnO$_3$ a similar pseudo-cubic network of corner-sharing MnO$_6$ octahedra and similar electronic properties associated with the Mn$^{3+}$ and Mn$^{4+}$ ions. However, they display smaller distortions thanks to the higher crystal symmetry an no defects. For $A$=Bi, by means of neutron powder diffraction we give direct crystallographic evidence of a large coupling between the electric dipole of the 6s lone pair of Bi$^{3+}$ and the magnetic structure of the Mn$^{3+}$ ions, which provides a hint for enhancing the magnetoelectric coupling in proper ferroelectrics in view of multiferroic applications. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J33.00011: Micorscopic theory of temperature-dependent magnetoelectric effect in Cr$_{2}$O$_{3}$ Andrea Scaramucci, Maxim Mostovoy, Nicola A. Spaldin, Kris T. Delaney We study the temperature dependence of the magnetoelectric effect in Cr$_{2}$O$_{3}$ by considering the coupling between electric polarization and spins induced by Heisenberg exchange interactions. The form of the coupling is obtained by symmetry analysis and its strength is calculated by {\it ab initio} methods. Using Monte Carlo simulations, we evaluate the temperature dependence of the largest component of the magnetoelectric susceptibility. The quantitative agreement of our results with experimental measurements shows that the dominant contribution to the linear magnetoelectric effect originates from nonrelativistic exchange interactions and spin fluctuations. The approach used can be applied to study other magnetoelectrics with collinear spin ordering and opens a new route for the design of materials with large magnetoelectric effect at high temperatures. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J33.00012: Pressure- and field-dependent Raman studies of magnetodielectric behavior in Mn$_{3}$O$_{4}$ M. Kim, S. Yuan, S.L. Cooper We present simultaneous pressure- and field-dependent Raman scattering studies of the magnetodielectric spinel, Mn$_{3}$O$_{4}$, which allow us to investigate both the microscopic origins and pressure dependence of magnetodielectric phenomena in this material. We identify the specific phonon modes responsible for the dramatic magnetodielectric behavior observed in this material, and show that these modes provide quantitative information regarding magnetodielectric behavior in Mn$_{3}$O$_{4}$ via the Lyddane-Sachs-Teller (LST) relationship. We also show that pressure can induce monoclinic distortions accompanied with magnetic ordering in Mn$_{3}$O$_{4}$. Finally, by exploring the field-dependent phonon spectrum at different pressures, we are able to map out the pressure-field structural phase diagram of Mn$_{3}$O$_{4}$ and explore the pressure-dependence of magnetodielectric behavior in this material. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J33.00013: Infrared optical properties of multiferroic FeTe$_{2}$O$_{5}$Br single crystal Kevin H. Miller, C. Martin, X. Xi, H. Berger, G.L. Carr, D.B. Tanner Reflection as a function of temperature has been measured on a single crystal of the anisotropic multiferroic FeTe$_{2}$O$_{5}$Br utilizing light spanning the far infrared to the visible portions of the electromagnetic spectrum. The complex dielectric function and optical properties along all three principal dielectric axes were obtained via Kramers-Kronig analysis and by fits to a Drude-Lorentz model. Transmission in the terahertz region as a function of temperature and magnetic field is also described, with particular focus on temperatures around the $\sim $10 K transition to the multiferroic state. [Preview Abstract] |
Session J34: Focus Session: Interfaces in Complex Oxides - Microscopy and Local Structure
Sponsoring Units: DMPChair: Suzanne Stemmer, University of California, Santa Barbara
Room: C141
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J34.00001: Unexpected ordering at the atomic scale interface of SrRuO$_{3}$ and BaTiO$_{3}$ Invited Speaker: Atomically engineered oxide multilayers and superlattices display unique properties responsive to the electronic and atomic structures of the interfaces. Interfaces can exhibit not only two-dimensional functionality, but have the power to dictate the properties of thin films. A clear example is found in ferroelectric thin films, where the domain size, orientation, and transport properties are controlled by top and buried interfaces. We have explored a prototypical ferroelectric - bottom electrode interface by characterizing BaTiO$_{3}$ grown on SrRuO$_{3}$. Films were grown on SrTiO$_{3}$ substrates by pulsed laser deposition, monitored by high-pressure reflection high-energy diffraction, exhibited high crystalline quality in electron diffraction and cross-sectional transmission electron microscopy (STEM), and were flat according to atomic force microscopy. Despite multiple indicators commonly accepted to confirm a sharp interface, atomically the structure is more complex. When grown in a typical oxygen pressure, at or below 10 mTorr, in situ scanning tunneling microscopy (STM) revealed the SrRuO$_{3}$ surface is littered with oxygen vacancies. For growth or post-annealing above $\sim $100 mtorr, vacancies were removed, but STM and low energy electron diffraction (LEED) disclosed a surface reconstruction consisting of parallel rows with periodicity doubled in one direction. Density function theory (DFT) suggests these rows are chains of Sr and O raised by excess oxygen. Both LEED and cross-sectional STEM revealed that this reconstruction persists at the buried interface and modifies the structure of subsequent BaTiO$_{3}$ layers. By four layers, the BaTiO$_{3}$ surface returns to a bulk-like structure with upward polar distortion. This study emphasizes the importance of atomic scale structural studies of what may otherwise appear as sharp interfaces. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J34.00002: Highly confined 2D electron gas in SrTiO$_{3}$/SrRuO$_{3}$ superlattices Marcos Ver\'Issimo-Alves, Daniel Bilc, Pablo Garc\'Ia-Fern\'andez, Philippe Ghosez, Javier Junquera In recent years, experimental advances in the creation of atomically smooth interfaces between complex oxides have made possible the observation of unusual electronic phenomena such as the formation of 2D electron gases confined to the interface between insulating compounds. In this work, we explore an alternative for creating a 2D electron gas in oxide interfaces by means of a (SrTiO$_3$)$_5$/(SrRuO$_3$)$_1$ superlattice. LSDA+U and hybrid functional B1-WC DFT calculations with full relaxation of atomic geometry show the presence of a 2D electron gas, and the system can be described with $U=4$ eV. Electronic correlations thus become important with quantum confinement: the system is a minority spin half metal with a magnetic moment of $\mu=4 \mu_B$. Degenerate Ru 4d$_{(xz,yz)}$ states are the main responsible for the DOS at the Fermi level, whose shape strongly resembles that of 1D free-electron gases. Our results suggest that the material could find uses in spintronic devices. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J34.00003: Ferroelectric thickness effects on LaSrMnO3/PbZrTiO3 Heterostructures Jinling Zhou, Evan Wolf, Charles Frye, Disheng Chen, Srinivas Polisetty, Mikel Holcomb Magnetoelectric (ME) coupling is the coupling of magnetic and electric properties within a material. It allows the possibility of dual control of the material through the manipulation of either electric or magnetic fields and therefore could potentially revolutionize the current technology. However, little is known about the factors that influence the strength of this magnetoelectric coupling. In the presented research, ferromagnetic LSMO and ferroelectric PZT are constructed as wedged adjacent layers for the purpose of studying the coupling effects and physical properties in each layer and the resulting interface. X-ray absorption spectroscopy (XAS) and photoemission electron microscopy (PEEM) are used as the major techniques to map out magnetism, ferroelectricity, and the interfacial coupling. The XAS spectra illustrate a strong effect on the magnetic properties depending on ferroelectric thickness. PEEM images display the magnetic and ferroelectric domains in each material layer, allowing further insight into why the coupling depends on layer thickness. This research will aid the understanding of coupling in not only magnetoelectric heterostructures, but also in other similar complex oxide systems. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J34.00004: SrTiO$_3$ on silicon: interface chemistry, polarization pinning, and electronic states Sohrab Ismail-Beigi, Alexie Kolpak, Fred Walker, Jim Reiner, Charles Ahn We use SrTiO$_3$/Si as a model system to examine the effects of interface atomic structure and composition on the functional properties of epitaxial oxide films on silicon. Using first-principles computations, we show that intrinsic chemo-mechanical boundary conditions at the interface fix a single polarization direction in the SrTiO3 thin film independent of the interface composition, inhibiting ferroelectric functionality. In contrast, the transport properties of the interface are quite sensitive to the interface composition, which can be used to tune the interface from an insulator to an interfacial 2D electron gas. We describe the origins of both functionalities and discuss their applicability to the general class of epitaxial oxides on semiconductors. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J34.00005: First Principles Study of Resonant Phonon Coupling across the LSMO / STO Interface Kevin F. Garrity, Yaron Segal, Carlos A.F. Vaz, Jason D. Hoffman, Fred J. Walker, Charles A. Ahn, Sohrab Ismail-Beigi Epitaxial interfaces permit dynamical modification of the properties of a thin film via coupling to the substrate. In particular, the coupling of phonons between two materials allows one to manipulate the atomic structure and vibrational modes near an interface. We use first principles density functional theory (DFT) to study the octahedral oxygen rotations at and across an interface between La$_x$Sr$_{(1-x)}$MnO$_3$ (LSMO) and SrTiO$_3$. By performing finite temperature Monte Carlo sampling on a classical potential built to reproduce our DFT energetics, we demonstrate that as the SrTiO$_3$ is driven through the phase transition where its octahedral rotations become frozen in place, phonons from the SrTiO$_3$ couple into the interfacial LSMO. These couplings can then modify the LSMO transport properties, as observed in our experiments. The decay length of the phonon coupling into the LSMO agrees with our experimental determinations on this system. We demonstrate that the observed changes in resistance are not due to static changes in the LSMO structure, confirming the phonon coupling. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J34.00006: Coupling of strain and magnetism in LPCMO films Surendra Singh, M. Fitzsimmons, Hyoungjeen Jeen, Amlan Biswas Complex oxides show extraordinary structural, magnetic and magneto-transport properties and these properties are closely coupled with atomic structure and strain. The temperature- magnetic field phase diagram and transport studies of La$_{0.27}$Pr$_{0.40}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) films suggest the existence of two phases at low temperatures, i.e. ferromagnetic metallic (FMM) phase and charge order insulating (COI) phase. We report the magneto transport properties of LPCMO films on application of external strain using mechanical jig. The study shows the shift in metal to insulator transition (MIT) temperature on application of external strain. To understand the effect of strain as well as kinematics of formation of FMM phase from COI phase of LPCMO films and vice versa, we have performed detailed in situ transport and specular polarized neutron reflectivity measurements across the MIT as functions of temperature, magnetic field and applied strain. The study reveals a variation of magnetic scattering length density along the depth of the film, which may be attributed to chemical inhomogeneity of the film as a function of depth. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J34.00007: Coupling between oxygen octahedron rotations, Jahn-Teller distortion, magnetic ordering and epitaxial strain in LaMnO3 from first principles Jun Hee Lee, Karin M. Rabe, Kris Delaney, Eric Bousquet, Nicola Spaldin LaMnO3 is known to have rich physics due to coupling among orbital ordering, Jahn-Teller distortions and magnetism. However, less attention has been paid to the role of oxygen octahedron rotations and epitaxial strain. In this talk, we show from first principles calculations that oxygen octahedron rotations induce weak ferromagnetism in \textit{Pbnm} LaMnO3 and that octahedral rotations are cooperatively coupled to the Jahn-Teller distortion.. Furthermore, we predict that compressive epitaxial strain drives bulk $A$-type antiferromagnetic Pbnm insulating phase to a ferromagnetic metallic phase. At the phase boundary between the ferromagnetic-metallic and antiferromagnetic-insulating phases, colossal magneto-resistance is expected. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J34.00008: Local Octahedral Distortions and Magnetic Properties Controlled by Substrate Symmetry at Perovskite Oxide Interfaces Jun He, Albina Borisevich, Sergei Kalinin, Stephen Pennycook, Sokrates Pantelides We have investigated the oxygen octahedral distortions and local magnetism at the interfaces of magnetic perovskite oxide heterostructures using first principles calculations. The studied prototype oxide heterostructures include La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, SrRuO$_{3}$, and BiFeO$_{3}$. The results show that the symmetry mismatch at interfaces between two perovskite oxides imposes an interfacial layer with distortion modes that do not exist in either bulk material, creating new interface properties by symmetry alone. The thickness of such interface layer depends on the resistance of the octahedra to deformation. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J34.00009: Atomic Resolution and First Principles Study of the Structure and Bonding at SrTiO$_{3}$/GaAs Hetero-interfaces Qiao Qiao, Robert Klie, Serdar Ogut Ultrathin metal-oxide films on polar semiconductor surfaces have received much attention in recent years due to occurrence of novel functional properties, including ferroelectricity, superconductivity and the presence of an interfacial 2-dimensional electron gas. In this study, we examine the atomic and electronic structures of epitaxial ultrathin SrTiO$_{3}$ (100) films on GaAs (001) using aberration corrected atomic-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) in combination with first principles calculations to develop a fundamental understanding of the interfacial structure-property relationships. Using atomic-column resolved EELS, we show that Ti diffuses into the first few monolayers of GaAs depending on the thin film growth condition. The effects of Ti diffusion into subsurface GaAs (001) with (4x2)-$\beta $2 surface reconstructions will be investigated via first principles calculations. We will also discuss the results for the formation energies of Ti-related impurity defects in the bulk and surface regions of GaAs to help in the interpretation of electron microscopy experiments. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J34.00010: Accessing hidden isosymmetric phase transitions in perovskite thin films James Rondinelli, Sinisa Coh Isosymmetric phase transitions (IPT), which show no change in occupied Wyckoff positions or crystallographic space group, are exceedingly rare in crystalline matter because most condensed systems respond to external stimuli by undergoing ``conventional'' symmetry-lowering displacive, martensitic or reconstructive transitions. In this work, we use first-principles density functional calculations to identify an elusive IPT in orthorhombic $AB$O$_3$ perovskite oxides with tendency towards rhombohedral symmetry. Using perovskite LaGaO$_3$ as our prototypical system, we show that the latent isosymmetric phase transition, which manifests as an abrupt change in the octahedral rotation axis, is accessible only with an external elastic constraint---bi-axial strain. We show the transition originates from a soft phonon that describes the geometric connectivity and relative phase of the GaO$_6$ polyhedra. By connecting the origin of IPT to a chemical and structural incompatibility between the lattice and the elastic constraints, we describe how subtle changes in bulk orthorhombic and monoclinic symmetries are critical to the complete engineering of structure-correlated electronic properties in thin films. Because bi-axial strain is the critical parameter controlling the IPT, we suggest heteroepitaxial synthesis of IPT materials is a plausible route to realize high-$\kappa$ dielectric actuators with variable band gaps and dielectric anisotropies. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J34.00011: Electronic properties of ultrathin GdTiO3 thin films and GdTiO3/SrTiO3 interfaces Pouya Moetakef, Bharat Jalan, Jack Zhang, S. James Allen, Susanne Stemmer Interfaces between Mott insulators, such as the rare earth titanates, and band insulators, such as SrTiO3, have recently attracted much attention. We report on the transport properties of epitaxial rare earth titanate thin films, GdTiO3, grown by molecular beam epitaxy (MBE) and those of heterostructures with SrTiO3 and GdTiO3. Growth of GdTiO3 was performed by shuttered growth of alternating titanium tetra isopropoxide (TTIP) and Gd fluxes, in the absence of any additional oxygen. We show that to stabilize the GdTiO3 perovskite phase, SrTiO3 buffer layers are needed for growth on perovskite substrates, such as LSAT ((LaAlO3)0.3(Sr2AlTaO6)0.7). The contribution of n-type SrTiO3 buffer layers and that of the SrTiO3/GdTiO3 interfaces to the transport properties are determined by measurements of the electrical resistance and Hall coefficient as a function of temperature and magnetic field. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J34.00012: Atomically-resolved mapping of polarization and electric fields across ferroelectric-oxide interfaces by Z-contrast imaging Albina Borisevich, Hye Jung Chang, Sergei Kalinin, Anna Morozovska, Ying-Hao Chu, Pu Yu, Ramamoorthy Ramesh, Stephen Pennycook Polarization, electric field, charge and potential across ferroelectric-oxide interfaces are obtained from direct atomic position mapping by aberration corrected scanning transmission electron microscopy combined with Ginsburg-Landau-Devonshire theory. We compare two antiparallel polarization orientations, which allows separation of the polarization and intrinsic interface charge contributions. Using the Born effective charges, the complete interface electrostatics is obtained in real space, providing an alternative method to holography. The results provide new microscopic insight into the thermodynamics of polarization distribution at the atomic level. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J34.00013: Inherited (In)stabilities in Transition Metal Superlattices Sven Rudin Many transition metals exhibit a solid phase with a body-centered cubic (bcc) crystal structure. For some elements, e.g., tungsten (W), bcc is the only solid phase; for others, e.g., titanium (Ti), the bcc phase only appears at high temperatures. Titanium's high-temperature bcc phase exhibits soft phonon modes. These reflect the atomic movements upon transformation into the low-temperature phases. One such mode shows atomic displacements that also appear in the top few layers of tungsten's surface reconstruction. Superlattices constructed from alternating nanometer-thick layers of W and Ti would allow the two displacement patterns to interact. The work presented here uses density functional theory calculations to predict how the structure and mechanical response of such superlattices depends on the choice of transition metal elements and the layer thicknesses. [Preview Abstract] |
Session J35: Topological Insulators: Optics
Sponsoring Units: DCMPChair: Carlo Piermarocchi, Michigan State University
Room: C140
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J35.00001: Far-infrared magneto-spectroscopy study of ultrathin Bi$^{2}$Se$^{3}$ layers in high magnetic field Wenlong Yu, Li-Chun Tung, Xunchi Chen, Dmitry Smirnov, Ireneusz Miotkowski, Helin Cao, Yong P. Chen, Zhigang Jiang We present a far-infrared (FIR) magneto-spectroscopy study of thin Bi2Se3 layers. Transmittance and reflectance measurements are performed in the Faraday geometry at 4.2 K and in a magnetic field up to 17.5 T. The thin samples (much less than 1 micron) are stabilized on the Scotch tape, which enable us to obtain reliable FIR transmission signals. A pronounced electron-phonon coupling, i.e., the coupling of a FIR phonon of Bi2Se3 with the continuum free-carrier spectrum, is observed and strongly enhanced by the applied magnetic field. The phonon lineshape is asymmetric and can be fit by a Fano formula. After examining the fitting parameters, we find no systematic broadening of the lineshape in our measurements. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J35.00002: Terahertz and magneto-optical studies of HgTe films Jason Hancock, Georgy Astakhov, Christophe Bruene, Dirk van der Marel, Laurens Molenkamp We present a frequency-domain infrared, time-domain terahertz, and magneto- optical study of thin films of high-mobility HgTe. The conductivity at low frequency and a wide range of temperature is described, and clear cyclotron effects are observed at low temperature and frequency. The context of these findings to the possible 3D topological phase will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J35.00003: Magneto-Optics in the search for the topological insulating state Alexander Schafgans, Andrew LaForge, A. Taskin, Yoichi Ando, Dimitri Basov We present infrared spectroscopic data for the thermoelectric material Bi-Sb at the insulating composition, taken in magnetic field applied both in the Voight and Faraday geometries, in order to gain insight into the recently discovered topological insulating state. We observe a metallic plasma edge that displays significant blue shift at high temperatures. The form of the plasma edge shows broadening suggestive of sample inhomogeneities. The plasma edge is remarkably sensitive to the applied magnetic field, resulting in a complicated spectral lineshape. We compare these results with magneto-optics of the topological insulator Bi$_{2}$Se$_{3}$.\footnote{A. D. LaForge, \textit{et. al.}, Phys. Rev. B 81, 125120 (2010).} [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J35.00004: Magneto-optical spectra of topological insulators Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Se$_3$ in magnetic fields up to 18 Tesla M.S. Wolf, S.V. Dordevic, N. Stojilovic, Hechang Lei, C. Petrovic, L.C. Tung Topological insulators are a novel class of materials that behave as insulators in the bulk, but have conducting states on the surface. Studies of their behavior in magnetic field is an important avenue towards understanding their complex properties. We will report the results of our magneto-optical measurements of topological insulators Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Se$_3$ in magnetic fields up to 18 Tesla. In all three compounds we detect magnetic-field induced changes in optical properties, which are most pronounced around the plasma edge. The induced changes are much bigger in Bi$_2$Se$_3$ than in Bi$_2$Te$_3$ and Sb$_2$Te$_3$. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J35.00005: Optical properties of novel topological insulators Andrei Sushkov, G.S. Jenkins, H.D. Drew, N.P. Butch, J. Paglione, K.R. Choi, S.-W. Cheong The recent discovery of topological insulators (TI) with the spin-polarized Dirac-like electrons on their surfaces have attracted a lot of attention and theoretical and experimental research efforts. One of the fascinating properties of TIs is the presence of the linear magneto-electric coupling which is responsible for coupled charge-spin modes in dynamical response. We will report our results on the search for such modes together with the conventional optical properties of materials predicted to be TI such as 5d oxide Na$_{2}$IrO$_{3}$, SmB$_{6}$, and Bi$_{2}$Se$_{3}$ and other materials. Measurement results include optical reflectance and/or transmission in the frequency range 5 - 50,000 cm$^{-1}$ and Faraday or Kerr rotation and circular dichroism at selected THz frequencies. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J35.00006: Terahertz Kerr and Reflectivity Measurements on the Topological Insulator Bi$_2$Se$_3$ Gregory S. Jenkins, A.B. Sushkov, D.C. Schmadel, N.P. Butch, P. Syers, J. Paglione, M.-H. Kim, H.D. Drew, J.G. Analytis, I.R. Fisher We report the first terahertz Kerr measurements on bulk crystals of the topological insulator Bi$_2$Se$_3$ with and without Fe doping at 4\,K and magnetic fields up to 8\,T. Transport evidence and characterization of the surface states will be presented. By employing a gate that creates a small depletion layer, the optical signals from the surface state carriers are modulated with no contribution arising from the bulk carriers. The real and imaginary parts of the Kerr angle yeild the transport scattering rate, spectral weight, and mass of the surface state carriers. FTIR and magneto-optical measurements characterize the bulk carriers. Comparisons with ARPES and other transport measurements will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J35.00007: Magneto-optical and Magneto-electric Effects of Topological Insulators in Quantizing Magnetic Fields Wang-Kong Tse, A.H. MacDonald Topological insulators show novel magneto-electric effect when the surface Dirac cone dispersion is gapped by a weak Zeeman field. In this talk, we present our study of the magneto-optical and magneto-electric effects of a thin-film topological insulator in the presence of a strong quantizing magnetic field. We find that low-frequency magneto-optical properties depend only on the sum of top and bottom surface Dirac-cone filling factors, whereas the magneto-electric response depends only on the difference. The Faraday rotation is quantized in integer multiples of the fine structure constant $\alpha$ and the Kerr effect exhibits a full-quarter rotation. Strongly enhanced cyclotron-resonance features appear at higher frequencies that are sensitive to the filling factors of both surfaces. When the product of the bulk conductivity and the film thickness in $e^2/h$ units is small compared to $\alpha$, magneto-optical properties are only weakly dependent on accidental doping in the interior of the film. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J35.00008: Photo-Induced Kerr Rotation in the Bulk and Surface of a Topological Insulator Fahad Mahmood, David Hsieh, James McIver, Dillon Gardner, Young Lee, Nuh Gedik We report ultrafast bulk- and surface-sensitive optical pump-probe spectroscopy from Bi$_{2}$Se$_{3}$ (111). Using second harmonic generation, we demonstrate that the bulk and surface electronic contributions can be separated and exhibit qualitatively different relaxation dynamics. Ultrafast surface-sensitive optical measurements reveal a large photo-induced Kerr rotation from the surface that is dependent on the helicity of the circularly polarized excitation pulse. We will discuss the microscopic origin of this observation and how it relates to the strong spin-charge coupling on the surface of a topological insulator.~ [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J35.00009: Infrared studies of topological insulators Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Se$_3$ S.V. Dordevic, M.S. Wolf, N. Stojilovic, Hechang Lei, C. Petrovic In this study we have used infrared spectroscopy to probe the electrodynamic response of topological insulators Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Se$_3$. Infrared spectra are collected over a broad frequency and temperature range. The results reveal similar spectra in all three compounds, with well defined plasma edge located in the far-infrared part of the spectrum. However there are some important differences in the temperature evolution of the spectra. Namely, as temperature decreases the plasma edge shifts to lower frequencies in Bi$_2$Se$_3$, whereas in Bi$_2$Te$_3$ and Sb$_2$Te$_3$ it shifts to higher frequencies. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J35.00010: Surface optical study of topological insulator Bi2Se3 David Hsieh, James McIver, Darius Torchinsky, Dillon Gardner, Young Lee, Nuh Gedik We report the observation of optical surface second harmonic generation from the (111) surface of Bi2Se3 using ultrafast laser pulses. We demonstrate that second harmonic generation is sensitive to both the surface crystal structure as well as the surface carrier density, which we tune through surface molecular doping. Protected nodes in the second harmonic circular dichroism spectroscopy provide a method to study time-reversal symmetry breaking effects from a single surface in a contact free manner. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J35.00011: Ultrafast carrier and phonon dynamics in Bi$_{2}$Se$_{3}$ crystals Xunchi Chen, Jingbo Qi, Wenlong Yu, Paul Cadden-Zimansky, Dmitry Smirnov, Norman Tolk, Ireneusz Miotkowski, Helin Cao, Yong P. Chen, Yizheng Wu, Shan Qiao, Zhigang Jiang Ultrafast time-resolved differential reflectivity of Bi$_{2}$Se$_{3}$ crystals is studied using optical pump-probe spectroscopy. Three distinct relaxation processes are found to contribute to the initial transient reflectivity changes: a sub-ps and a few-ps electron-phonon relaxation process due to different phonon modes and a defect-induced charge trapping process. After the crystal is exposed to air, the relative strength of these processes is altered and becomes strongly dependent on the excitation photon energy. We argue that the observed behavior is likely due to the presence of Se vacancies. Further, weaker charge trapping process and vanishing air doping effect are observed in magnetically doped samples, supportive of our argument. Part of this work is published on Appl. Phys. Lett. 97, 182102(2010). [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J35.00012: Photo-induced currents in the topological insulator Bi2Se3 James McIver, David Hsieh, Hadar Steinberg, Pablo Jarillo-Herrero, Nuh Gedik We report the observation of photo-induced currents in micro-devices built from exfoliated single crystals of Bi2Se3. Our experiments are performed using ultrashort laser pulses at 800 nm and in the absence of an applied bias. We find that the induced currents scale quadratically with laser field strength, confirming their second order nature. We will present the temperature dependence of these second order currents and discuss their microscopic origin. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J35.00013: Circular photogalvanic effect on topological insulator surfaces: Berry curvature-dependent response Pavan Hosur Strong spin-orbit coupling is commonly exploited for generating electric currents using circularly-polarized light. We study, theoretically, the direct current generated by circularly-polarized light on the surface of a topological insulator, focusing on the part that reverses on switching the light-helicity. Interestingly, the dominant current, due to an interband transition, is controlled by the Berry curvature of the surface bands. This extends the connection between photocurrents and Berry curvature beyond the quasiclassical approximation where it has been shown to hold. Explicit expressions are derived for the (111) surface of the topological insulator Bi$_2$Se$_3$ where we find significant helicity-dependent photocurrents when the rotational symmetry of the surface is broken by an in-plane magnetic field or a strain. Moreover, the dominant current grows linearly with time until a scattering occurs, which provides a means for determining the scattering time. DC spin density is generated on the surface as well, and is also dominated by a linear-in-time, Berry curvature-dependent contribution. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J35.00014: Raman studies of irradiation-induced defects in thin flakes of Bi2Se3 and related materials Isaac Childres, Jifa Tian, Ireneusz Miotkowski, Yong P. Chen We report a Raman spectroscopy study of exfoliated Bi2Se3 flakes of various thicknesses after exposure to irradiation by lasers, electron-beam and oxygen plasma. We observe little effect of irradiations on Raman spectra of thicker ($>$50 nm) Bi2Se3 flakes, which exhibit characteristic Raman peaks at $\sim $130 cm-1 and $\sim $170 cm-1 similar to bulk Bi2Se3. However, spectra from irradiated thinner ($<$20 nm) flakes show the appearance of an extra Raman peak ($\sim $250 cm-1) and attenuation and broadening of the peaks at $\sim $130 cm-1 and $\sim $170 cm-1. This additional peak is not seen in flakes exposed to electron-beam irradiation and lower-power lasers. We interpret the new peak in the Raman spectra as due to irradiation-induced disorder. We also performed similar Raman studies on Bi2Te3, Sb2Se3 and Sb2Te3. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J35.00015: Tuning optical and electrical transport properties of Bi2Se3 with Ca Zhiyong Wang, Tao Lin, Peng Wei, Xinfei Liu, Randy Dumas, Kai Liu, Jing Shi We have systematically tuned the carrier type and density in Bi$_{2}$Se$_{3}$ single crystals by introducing a calcium dopant. By controlling Ca-concentration x in Ca$_{x}$Bi$_{2-x}$Se$_{3}$, a minimum carrier density of $\sim $ $1 \times 10^{17}$ cm$^{-3}$ is achieved in both n- and p-type materials. The Fourier transform infrared (FTIR) measurements were carried out in samples with different doping levels to obtain the inter-band transition energy, sample thickness, and the plasma frequency. The band gap and reduced effective mass of carriers were determined from the relation between the inter-band transition energy and carrier density. The undoped samples show a high electron density ($\sim $ $5 \times 10^{18}$ cm$^{-3})$ and the electrical resistivity shows a typical metallic behavior. At high magnetic fields (up to 14 T), the undoped samples show the Shubnikov-de Haas oscillations. Near the compensation point or x=1.2{\%}, the electrical resistivity shows an insulating behavior with a low temperature saturation. This work was supported in part by DOE and NSF. [Preview Abstract] |
Session J36: Focus Session: Graphene Growth, Characterization and Devices: SiC and Metal Substrates
Sponsoring Units: DMPChair: Kurt Gaskill, Naval Research Laboratory
Room: C142
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J36.00001: SiC structuring and step bunching for C-face epitaxial graphene growth John Hankinson, Yike Hu, Ming Ruan, Baiqian Zhang, Claire Berger, Walt de Heer Recent research at Georgia Tech has focused on understanding and improving the epitaxial graphene growth process. Electronic experiments have demonstrated the excellent properties that high quality epitaxial graphene can posses when grown by the confinement controlled sublimation (CCS) method in an induction furnace [1]. Here we focus on the mechanisms at work in the early stages of graphitization. Experimental observations of C-face epitaxial graphene growth have revealed that when step-pinning defects are present they seem to act as preferential graphene nucleation sites. In addition we have observed preferential graphene growth on silicon carbide sidewalls and mesas. Ongoing work seeks to take advantage of the correlation between silicon carbide structure and graphene growth by pre-patterning the SiC substrate in order to better control the graphene grown on it. With CCS growth we have created flat graphene regions extending over tens of microns with RMS roughness below 2.5 angstroms. Growth results and electronic measurements on graphene grown on structured SiC mesas will be presented. \\[4pt] [1] R. Ming et al. Materials Science and Engineering -- Reports (submitted) [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J36.00002: Novel epitaxy of graphene using substrate microfabrication Hirokazu Fukidome, Masato Kotsugi, Takuo Ohkouchi, Toyohiko Kinoshita, Thomas Seyller, Karsten Horn, Yusuke Kawai, Maki Suemitsu, Yoshio Watanabe Epitaxy of graphene on SiC is promising for device applications owing to the capability to produce large-area film. For the further applications toward integrated devices, the microscopic thickness variation of graphene should be minimized because the thickness of graphene critically determines the electronic properties, such as carrier mobility and bandgap. One of the effective solutions is the epitaxy on microfabricated substrates to spatially control surface reactions involved in the epitaxy. The controllability of the epitaxy using substrate microfabrication has been already proven for the homoepitaxy on microfabricated Si substrates. We therefore study heteroepitaxy of graphene on microfabricated 6H-SiC(0001) substrates as a model system to produce epitaxial graphene without thickness variation. It has been in fact demonstrated by using photoemission and low energy electron microscopies that the epitaxial graphene exhibits no thickness variation when the size of microfabrication pattern is small (below 10 micrometer). Further the shape of the microfabrication pattern is also influential to the microscopic variation of the graphene. The controlled epitaxy of graphene by substrate microfabrication is thus demonstrated to be vital for future integrated graphene devices. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J36.00003: Selective Epitaxial Graphene Growth on SiC via AlN Capping Farhana Zaman, Miguel Rubio-Roy, Michael Moseley, Jonathan Lowder, William Doolittle, Claire Berger, Rui Dong, James Meindl, Walt de Heer Electronic-quality graphene is epitaxially grown by graphitization of carbon-face silicon carbide (SiC) by the sublimation of silicon atoms from selected regions uncapped by aluminum nitride (AlN). AlN (deposited by molecular beam epitaxy) withstands high graphitization temperatures of 1420$^{o}$C, hence acting as an effective capping layer preventing the growth of graphene under it. The AlN is patterned and etched to open up windows onto the SiC surface for subsequent graphitization. Such selective epitaxial growth leads to the formation of high-quality graphene in desired patterns without the need for etching and lithographic patterning of graphene itself. No detrimental contact of the graphene with external chemicals occurs throughout the fabrication-process. The impact of process-conditions on the mobility of graphene is investigated. Graphene hall-bars were fabricated and characterized by scanning Raman spectroscopy, ellipsometry, and transport measurements. This controlled growth of graphene in selected regions represents a viable approach to fabrication of high-mobility graphene as the channel material for fast-switching field-effect transistors. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J36.00004: Multi-Layer Epitaxial Graphene Formed from Poly-Crystalline Silicon Carbide Grown on C-Plane Sapphire Timothy McArdle, Jack Chu, Yu Zhu, Zihong Liu, Mahadevaiyer Krishnan, Chris Breslin, Christos Dimitrakopoulos, Robert Wisnieff, Alfred Grill Growth of epitaxial graphene on substrates as large as eight inches in diameter is of great interest for integration with current CMOS technology. We use ultra-high vacuum chemical vapor deposition to grow poly-crystalline silicon carbide (SiC) on c-plane sapphire wafers, which are then annealed at high temperature in vacuum to create multi-layer epitaxial graphene films. Despite the roughness and small domain size of the poly-crystalline SiC films, a thick, conformal layer of graphene is formed. Reducing the surface roughness by chemical-mechanical polishing the SiC surface prior to the anneal results in a dramatic reduction of the Raman defect band observed in the final graphene film. Additionally, the graphene formed on polished SiC demonstrates significantly more ordered layer-by-layer graphene growth and increased carrier mobility for the same carrier density as the unpolished samples. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J36.00005: Theory of the Growth of Epitaxial Graphene on Silicon Carbide Fan Ming, Andrew Zangwill We present a one-dimensional kinetic Monte Carlo model for the growth of epitaxial graphene on 6H-SiC. The model parameters are effective energy barriers for the nucleation and subsequent propagation of graphene at step edges. For growth on vicinal substrates with half-unit-cell height steps, we predict first and second layer graphene coverages and the distribution of first-layer graphene strip widths as a function of total coverage, vicinal angle, and the model parameters. Comparing our results to experiment will provide the first quantitative insights into the kinetics of growth for this unusual epitaxial system. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J36.00006: A density-functional theory study for the mobility of carbon atoms on 6H SiC(0001) Christian Ratsch Graphene is a very promising material for many microelectronic applications because of its unique electronic properties. Among the several proposed routes to fabricate (single) layers of graphene, the growth of epitaxial graphene on 4H and 6H SiC(0001) appears to be particularly promising. The 6H SiC(0001) surface has 3 different polytypes. In this talk, results from density-functional theory calculations will be presented for the potential energy surfaces and different diffusion rates of C atoms on these different polytype surfaces. Both, the Si or C terminated surfaces will be investigated. Results for the adsorption of single and multiple graphene layers will also be presented. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J36.00007: Epitaxial graphene on SiC(0001) Invited Speaker: Epitaxial graphene on SiC is considered to open a route towards graphene based electronics such as, e.g., high frequency transistors. Recently considerable progress has been made in the growth of epitaxial graphene on SiC. On the Si-face of SiC, where the growth is slower as compared to the C-face, monolayers can be grown reliably. However, several open questions remain. Transport studies as well es photoelectron spectroscopy has shown that the pristine layers on SiC(0001) are heavily electron doped ($n=1\times10^{13}$~cm$^{-1}$). This results in rather low electron mobilities of the order of 2000~cm$^2$/Vs at 25~K. In addition, the carrier mobility shows a strong temperature dependence so that it drops to around 1000~cm$^2$/Vs at 300~K. In my presentation I will first show how chemical gating of graphene by deposition of F4TCNQ affects the carrier mobility. Hall effect measurements on samples close to charge neutrality show a carrier mobility of 29,000~cm$^2$/Vs at 25~K. Then I will discuss measurements demonstrating inertial-ballistic transport in nanoscale cross junctions fabricated from epitaxial graphene on SiC(0001). Finally, I will review recent results obtained by hydrogenation of the interface between graphene and SiC(0001). The latter process leads to a decoupling of the bufferlayer which is converted into quasi-freestanding graphene (QFMLG). The electronic, structural, and transport properties of QFMLG will be discussed in detail. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J36.00008: Structural and Electronic Properties of Graphene on Cu(111) and SiC(0001) Li Gao, Paolo Sessi, Jongweon Cho, Jeffrey R. Guest, Nathan P. Guisinger Graphene has shown attractive physical properties and is a promising new material. The structural and electronic properties of graphene on Cu(111) and SiC(0001) have been investigated by scanning tunneling microscopy and spectroscopy and Raman spectroscopy. The growth of graphene on these two substrates was achieved by thermal decomposition of ethylene on Cu(111) and thermal decomposition of SiC(0001) surface, respectively, in an ultra high vacuum chamber. On Cu(111), the nucleation of monolayer islands and two predominant domain orientations have been observed, which leads to the formation of numerous domain boundaries with increasing coverage [1]. Raman spectroscopy verifies the single layer thickness and shows the defect-induced bands for graphene on Cu(111). On SiC(0001), the electronic structure of the first two carbon layers on top of the $6\sqrt 3$ surface reconstruction has been studied by scanning tunneling spectroscopy. \\[4pt] [1] L. Gao, J. R. Guest, and N. P. Guisinger, Nano Lett. 10, 3512 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J36.00009: Wafer scale synthesis of bilayer graphene film Kyunghoon Lee, Seunghyun Lee, Zhaohui Zhong The discovery of electric field induced bandgap opening in bilayer graphene paves the way for making semiconducting graphene without aggressive size scaling, or using expensive substrates. Despite intensive research, synthesizing homogeneous bilayer graphene in large size has proven extremely challenging, and the size of bilayer graphene was limited to micrometer scale by exfoliation Here we demonstrate homogeneous bilayer graphene films over at least square inch area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates. Bilayer coverage of over 99{\%} is confirmed by spatially resolved Raman spectroscopy. The result is further supported by electrical transport measurements on bilayer graphene transistors with dual-gate configuration, where field induced bandgap opening is observed in 98{\%} of the devices. The size of our bilayer graphene film is only limited by the synthesis apparatus and can be readily scaled up, thus enabling wafer scale graphene electronics and photonics. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J36.00010: Visualizing graphene grown by chemical vapor deposition on metal substrates at the atomic scale Liuyan Zhao, Kwang Rim, Christopher Gutierrez, Rui He, Keunsoo Kim, Hui Zhou, Tony Heinz, Philip Kim, Aron Pinczuk, George Flynn, Abhay Pasupathy We present an atomic-scale scanning tunneling microscopy (STM) study of large-area graphene films grown by chemical vapor deposition (CVD) on metal substrates. We will first describe experiments where pristine graphene is grown in UHV conditions on single crystal Cu(111) and Cu(100) surfaces. We will compare this with graphene grown on copper foils and thin films in a typical low-pressure tube furnace. We will describe the effect of substrate quality and orientation on the quality and electronic structure of the graphene film produced. Finally, we will describe experiments where the graphene film is doped by nitrogen during growth. We will describe the bonding environment and the local electronic structure caused by the incorporation of nitrogen atoms into the graphene lattice. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J36.00011: Graphene Growth and Defects on Ni(111) Matthias Batzill, Jayeeta Lahiri Using scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES) we have investigated the growth of graphene on Ni(111) surfaces by carbon segregation from the bulk. We reveal two distinct growth modes for graphene growth. Between 480 and 650 C graphene forms on clean Ni(111) and below 480 C graphene grows by an in-plane conversion of a surface carbide phase. This is the first time that graphene formation is observed by transformation of a surface carbide. STM indicates that a lattice-matched, one-dimensional in-plane domain boundary between graphene and the carbide forms and graphene grows by replacing Ni-atoms with carbon at this interface. In addition to the growth of graphene we will also briefly discuss atomic-scale defects that can be synthesized in Ni-supported graphene. In particular we emphasize the formation of an extended line-defect with metallic properties [1]. \\[4pt] [1] J. Lahiri, Y. Lin, P. Bozkurt, I.I. Oleynik, M. Batzill Nature Nanotechnol. 5, 326 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J36.00012: Atomic Structures and Electronic Scattering of Graphene Edges Jifa Tian, Helin Cao, Jongweon Cho, Li Gao, Jeffrey R. Guest, Nathan P. Guisinger, Wei Wu, Qingkai Yu, Yong P. Chen The success of growing monolayer gaphene on Cu foils has stimulated intense interests to study its structural and electronic properties at the atomic scale. Here we present a scanning tunneling microscopy (STM) investigation on single crystalline graphene islands synthesized on polycrystalline Cu foils by chemical vapor deposition (CVD). Our studies reveal that most of the graphene edges are macroscopically parallel to the zigzag directions with microscopic roughness. The observed rough edges follow the zigzag directions at atomic scale and make many 120-degree turns. Strong electron scattering was observed from a rarely-occurring armchair-oriented edge, and there is little such scattering observed from zigzag-oriented edges. In addition, we also observed nearly periodic parallel lines attributed to the surface dislocations of the Cu underneath graphene. [Preview Abstract] |
Session J37: Focus Session: Graphene Growth, Characterization, and Devices: Surface Studies
Sponsoring Units: DMPChair: Phil First, Georgia Institute of Technology
Room: C146
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J37.00001: Scanning Tunneling Microscopy of Graphene on a Boron Nitride Substrate Yang Wang, Regis Decker, Victor Brar, William Regan, Hsin-Zon Tsai, Qiong Wu, Alex Zettl, Michael Crommie Graphene placed on a boron nitride (BN) substrate has been shown to result in increased mobility and improved Quantum Hall measurements.\footnote{Dean, CR \textit{et al.} Boron nitride substrates for high-quality graphene electronics. \textit{Nature Nanotechnology} \textbf{5}, 722-726 (2010)} It is therefore of great interest to understand how BN substrates differently influence graphene compared to more standard SiO$_{2}$ substrates. I will present new scanning tunneling microscopy measurements of graphene placed on a BN substrate. Differences in the local behavior of graphene on a BN substrate versus a SiO$_{2}$ substrate will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J37.00002: STM study of graphene on boron nitride Jiamin Xue, Danny Bulmash, Javier Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, Pablo Jarillo-Herrero, B.J. LeRoy We have performed low-temperature STM topographic and spectroscopic measurements of graphene on h-BN. We found that the topographic variations are reduced as compared to graphene on SiO2. We also performed scanning tunneling spectroscopy measurements to study the spatial variation of the Dirac point. We will present our latest results on the topographic and spectroscopic features for graphene on h-BN and compare them with similar measurements for graphene on SiO2. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J37.00003: Atomic-Scale Topographic and Electronic Structure of Graphene Films on Ultraflat Insulating Materials Christopher Gutierrez, Liuyan Zhao, Fereshte Ghahari, Cory Dean, Kwang Rim, James Hone, George Flynn, Philip Kim, Abhay Pasupathy Graphene, a unique two-dimensional material, has attracted much attention for its exotic electronic properties. But owing to its nature as a single monolayer, many of these interesting properties depend heavily on the substrate on which the graphene rests. Scanning tunneling microscope (STM) experiments offer the unique ability to investigate the effect of the substrate on the surface roughness (via topography maps) as well as the local electronic properties (via spectroscopy maps) of graphene. In this talk we will present such experimental results of graphene on atomically flat insulating substrates such as mica and boron nitride, as well as suspended graphene sheets. We will describe experiments performed both on exfoliated graphene flakes as well as large-area graphene films grown by chemical vapor deposition (CVD). [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J37.00004: Structural and electronic properties of epitaxial graphene on SiC(0001): Growth, transfer doping and atomic intercalation Invited Speaker: Epitaxial graphene on SiC(0001) promises a scalable graphene technology. Growth methods and experimental techniques for layer counting will be reviewed. The graphene layers are n-doped due to the influence of a covalently bonded carbon interface layer. This doping level can be precisely tailored and completely neutralized by functionalizing the graphene surfaces with electronegative molecules. In particular the Fermi level can be shifted into the band gap of bilayer graphene. The influence of the interface can be completely eliminated by hydrogen intercalation. Hydrogen migrates under the interface layer, passivates the underlying SiC layer and decouples the graphene from the substrate. The interface layer alone transforms into a quasi-free standing monolayer and epitaxial monolayer graphene turns into a decoupled bilayer. By intercalation of Germanium the graphene layers can also be decoupled. In this process both p- and n-doping can be obtained, depending on the amount of Ge intercalated. Both phases can be prepared simultaneously on the surface, so that lateral p-n junctions can be envisioned. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J37.00005: Scanning Tunneling Spectroscopy of Suspended Graphene in the Quantum Hall Regime Nikolai N. Klimov, Suyong Jung, Gregory M. Rutter, Nikolai B. Zhitenev, David B. Newell, Joseph A. Stroscio The discovery of graphene, a unique two-dimensional electron system with extraordinary physical properties, has ignited tremendous research activity in both science and technology. Graphene interactions with a substrate such as, for example, SiO$_{2}$/Si are known to strongly limit the electrical performance of graphene devices. Suspended graphene devices, where interaction with substrates can be strongly reduced, have been studied by macroscopic transport measurements and shown to have a 10-fold increase in mobility. However, a detailed investigation on a microscopic scale is still missing. In this talk we present a scanning probe microscopy (SPM) study of a free-standing graphene membrane. The device was fabricated from a graphene flake exfoliated over an array of 1$\mu $m holes etched in SiO$_{2}$/Si substrate. Electronic spectra of both suspended and supported regions of single-layer graphene can be probed using SPM in a perpendicular magnetic field and in varying back gate voltages applied to the Si substrate. The significant differences found in electronic spectra of suspended and non-suspended graphene will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J37.00006: Quantized Landau level spectrum and its density dependence in graphene supported by SiO$_{2}$ Adina Luican, Guohong Li, Eva Andrei Scanning tunneling microscopy and spectroscopy in magnetic field was used to study Landau quantization in graphene and its dependence on charge carrier density. Measurements were carried out on exfoliated graphene samples deposited on a chlorinated thermal SiO$_{2}$ which allowed observing the Landau level sequence characteristic of single layer graphene while tuning the carrier density through the Si back-gate. Upon changing the carrier density we find abrupt jumps in the Fermi level after each Landau level is filled. Moreover, at low doping levels a marked increase in the Fermi velocity is observed which is consistent with the logarithmic divergence expected due to the onset of many body effects close to the Dirac point. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J37.00007: The effect of the tip in scanning tunneling spectroscopy of graphene Landau levels Kevin Kubista, David Miller, Ming Ruan, Walt de Heer, Phillip First, Gregory Rutter, Joseph Stroscio Landau Level (LL) spectroscopy measurements were performed on multilayer epitaxial graphene using 4 K scanning tunneling spectroscopy in magnetic fields up to 8 T. Fits of the LL energies to the form expected for graphene show a slight difference in the Fermi velocity of hole and electron states. We show that this may be a consequence of the work function difference between graphene and the tip material (iridium). Data sets consisting of LL energies versus magnetic field are used to fit a model tip potential. The calculated spectrum of tip-perturbed LLs reveals the possible source of some ``extra'' peaks. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J37.00008: Scanning tunneling spectroscopy of adsorbates and vacancies on graphene Jyoti Katoch, Masa Ishigami Adsorbates and vacancies sensitively influence transport properties of graphene. We have investigated the impact of adsorbates, such as atomic hydrogen and potassium, and vacancy defects on electronic properties of graphene at atomic scale using scanning tunneling microscopy and spectroscopy Our results will be discussed in comparison with previous transport measurements to understand the effect of extrinsic disorder on transport properties of graphene. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J37.00009: A Scanning Tunneling Microscopy and Spectroscopy Study of K-doped Graphene Jeonghoon Ha, Hongwoo Baek, Beomyong Hwang, Minjun Lee, Jungseok Chae, Young Kuk Understanding the role of impurity scattering is crucial in explaining the carrier transport phenomena in a graphene device. Although unique two-dimensional Dirac fermion behavior have been confirmed by many transport experiments, direct observation of the local electronic structure around impurities may provide detailed picture of carrier scattering. In this study the local electronic structure of potassium deposited graphene film were studied using scanning tunneling microscopy (STM) and spectroscopy (STS). Chemical vapor deposition (CVD) graphene were transferred on a SiO$_{2}$ substrate after confirming the thickness and flatness by Raman spectroscopy and atomic force microscopy. STM images show relatively long-range screening around K impurities and the spatially resolved STS revealed unique electronic structure within the screening range. It was found that the screen range varies with the applied back gate bias, suggesting carrier density dependence. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J37.00010: Scanning Tunneling Microscopy Study of Fluorinated Graphene on Copper Scott Schmucker, Joshua Wood, Rick Haasch, Joseph Lyding We probe by ultrahigh vacuum scanning tunneling microscopy (UHV-STM) the structural and electronic properties of monolayer fluorinated graphene (C$_{x}$F, x$\approx $4) synthesized by chemical vapor deposition on copper substrates and fluorinated by xenon difluoride gas [1]. The chemical composition and structure of the resulting film is probed by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning tunneling spectroscopy (STS). In contrast to metallic graphene, this material exhibits a large ($>$3 eV) band gap with a muted gap state corresponding to a copper surface state near -0.4 eV. We further investigate by STM alignment between the fluorographenic surface layer and copper substrate, uniformity of fluorination, and stability of fluorinated graphene under electron bombardment and thermal annealing.\\[4pt] [1] J.T. Robinson, et al., Nano Lett. 10, 3001-2005 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J37.00011: Scanning tunneling spectroscopy of chemical vapor deposition grown graphene Daniel Cormode, Collin Reynolds, Brian LeRoy The electronic properties of CVD grown graphene were investigated by scanning tunneling microscopy. Mono and multi layered samples were prepared by growth on copper and transferred to 300 nm SiO$_2$ substrates. Raman spectroscopy mapping was used to determine the thickness of the samples as well as characterize regions of higher disorder as evidenced by an increased D peak. The samples were then measured in ultra high vacuum by scanning tunneling spectroscopy at 5 K. The type and density of defects measured with the STM were compared with measured D peak intensity. We have examined the correlation between changes in the local density of states and disorder in monolayer graphene. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J37.00012: Directed Assembly of Iron Phthalocyanine and Pentacene Molecules on a Graphene Monolayer Grown on Ru(0001) Shixuan Du, H.G. Zhang, J.T. Sun, L.Z. Zhang, Q. Liu, J.H. Mao, Y. Pan, M. Gao, H.T. Zou, H.M. Guo, W.A. Hofer, H.-J. Gao Monolayer graphene was successfully fabricated on 4$d $transition metal surfaces. The resulting ordered Moir\'e pattern was found to be an ideal template for the formation of ordered nanoclusters and molecules. Using scanning tunneling microscopy we show the selective adsorption process and assembly of iron phthalocyanine and pentacene molecules with different structural symmetries on a graphene monolayer, epitaxially grown on Ru(0001). The combination of first principles calculations and experimental measurements suggests that the lateral dipole field is the main driving mechanism for assembling molecules into ordered arrays. These findings should be important for achieving a large scale well-defined molecule-graphene interface. And such a detailed understanding of the molecular assembly will be essential in the actual fabrication process. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J37.00013: Ab initio scanning tunneling spectroscopy simulation of graphene with metal adatoms: weak and strong coupling regimes Gunn Kim, Jae-Hyeon Parq, Jaejun Yu, Young-Kyun Kwon Metal atoms on graphene, when ionized, can act as a point-charge impurity to probe a charge response of graphene with the Dirac cone band structure. To understand charge and spin polarization in graphene, we present scanning tunneling spectroscopy STS simulations based on density-functional theory calculations. We find that a Cs atom on graphene is fully ionized with a significant band-bending feature in the STS whereas the charge and magnetic states of Ba and La atoms on graphene appear to be complicated due to orbital hybridization and Coulomb interaction. By applying external electric field, we observe changes in charge donations and spin magnetic moments of the metal adsorbates on graphene. [Preview Abstract] |
Session J38: Focus Session: Ultrafast Dynamics and Imaging II
Sponsoring Units: DCPChair: Markus Guehr, Stanford University
Room: A130/131
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J38.00001: Probing Ultrafast Solution-Phase Chemistry in the X-ray Water Window and beyond via Soft X-ray Spectroscopy Invited Speaker: A prerequisite for a microscopic understanding of chemical reactions is knowledge of the ultrafast interplay of valence charge distributions, spin states, and nuclear degrees of freedom. These degrees of freedom are often intricately coupled, leading to very complex dynamics. Femtosecond core-level spectroscopy is very well suited to study such dynamics via x-ray absorption near-edge structure (for information on changes in valence charge distribution / spin-state) and via the extended x-ray absorption fine structure (for information in nuclear arrangements) due to the very localized nature of the initial states of well-defined symmetry and the high chemical specificity of core-level excitations. We have employed femtosecond core-level spectroscopy to study metal-ligand interactions in solvated transition metal complexes as an important class of model systems to demonstrate the feasibility and merit of ultrafast solution-phase soft X-ray spectroscopy.\footnote{N. Huse, T. K. Kim, L. Jamula, J. K. McCusker, F. M. F. de Groot, R. W. Schoenlein, {\it J. Am. Chem. Soc.}, {\bf 132}, 6809.}$^,$\footnote{H. Wen, N. Huse, R. W. Schoenlein, A. M. Lindenberg, {\it J. Chem. Phys.}, {\bf 131}, 234505.} Laser-induced charge transfer reactions on sub-100 fs time scales trigger structural dynamics in first-row transition-metal complexes that display multiple spin-state changes within 300 fs upon photo-excitation. The combined analysis of vibrational, optical, and core-level spectroscopy reveals a complex interplay of nuclear, electronic, and spin degrees of freedom in these systems that leads to detailed insights into the underlying reaction mechanisms. These are prototypical in nature for a variety of organometallic systems. The chemical specificity of core-level spectroscopy is exploited by probing metal-centered transitions to elucidate the ``metallic view.'' We have very recently succeeded in also following the ``ligand view'' via soft X-ray spectroscopy in the X-ray water window. The later experiment has far-reaching consequences as it demonstrates the feasibility of studying ultrafast processes and short-lived species of solvated organic compounds via Nitrogen K-edge spectroscopy to deliver a detailed picture of the evolving valence charge density in chemical reactions. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J38.00002: Ultrafast Dynamics in Helium Nanodroplets Probed by Femtosecond Time-Resolved EUV Photoelectron and Ion Imaging Oleg Kornilov, Oliver B\"unermann, Ali Eftekhari-Bafrooei, Stephen R. Leone, Daniel M. Neumark, Oliver Gessner Femtosecond time-resolved EUV photoelectron and ion imaging are employed to study the relaxation dynamics of electronically excited helium nanodroplets. Excitation into a broad droplet absorption band ($\sim $23.8 eV) is followed by ionization with a delayed IR pulse. The transient photoelectron spectra and angular distributions indicate that electronically excited helium atoms are predominantly emitted in either an aligned 1s4p Rydberg state within less than $\sim $100~fs or in a non-aligned 1s3d state within $\sim $200~fs. The transient ion imaging results suggest that different Rydberg atoms are emitted with significantly different kinetic energy distributions that closely resemble Maxwell-Boltzmann distributions with temperatures of 2700~K (1s3d) and 490~K (1s4p). The results are interpreted in terms of a dynamic model that is based on the local density dependent blue shift of atomic Rydberg states in the droplet environment. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:39PM |
J38.00003: ``Molecular spectrometers'' in the condensed phase: local THz-FIR response from femtosecond fluorescence Invited Speaker: We examine dye molecules whose color depends on the polarity of the environment. Following fast optical excitation, their fluorescence band typically red-shifts by 0.5 eV on femtosecond to nanosecond time scales. This ``dynamic Stokes shift'' reflects the joint molecular and environmental reorganisation of the system. Solvation dynamics has been studied for decades in the hope that the dynamics of the environment itself can be extracted. We contribute with two research lines: (1) development of rigid polar solvation probes whose vibrational response is removed from that of water, for example, and (2) fluorescence techniques which measure the dynamic Stokes shifts more precisely. Two results will be shown. The frequency-dependent permittivity $\varepsilon (\omega )$ of water surrounding N-Methyl-6-Quinolone is extracted up to about 100 cm$^{-1}$ from the time-resolved fluorescence shift R(t). The key consists in an analytical connection $\varepsilon (\omega ) \quad \to $ R(t) which is needed for data fitting. Measurements with the cryoprotectant disaccharide trehalose in water serve to establish the method. Its unique feature is locality, $i.e.$ the possibility to measure $\varepsilon (\omega)$ around a supramolecular structure with a covalently connected or embedded probe. THz vibrational activity of a biopolymer is thus measured locally, on the effective length scale for polar solvation, with an embedded molecular probe. For this purpose 2-hydroxy-7-nitro-fluorene was linked into a 13mer duplex opposite an abasic site. The NMR solution structure shows that the fluorene moiety occupies a well-defined position in place of a base-pair. The dynamic Stokes shifts for solution in H$_{2}$O and D$_{2}$O are quantified. Their difference is much larger than expected for free water, suggesting that only bound water is observed. A weak 26 cm$^{-1}$ spectral oscillation of the emission band is observed which is not present when the probe is free in solution, and is therefore caused by the supramolecular structure (DNA and hydration water). [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J38.00004: Tunable acoustic terahertz generation in InGaN quantum wells effected by metal nanocrystals Meg Mahat, Antonio Liopis, Sergio Periera, Ian Watson, Tae Choi, Arkadii Krokhin, Arup Neogi The strained semiconductor multiple quantum wells have the capability to generate acoustic terahertz emission via coherent acoustic phonon oscillations. The frequency of the THz emission is usually limited by the periodicity of the quantum wells or superlattice structures. We propose a novel technique to modify the frequency and amplitude of THz oscillations by the inclusion of the metal nanocrystals (NCs) within InGaN/GaN multiple quantum wells via the self-assembled inverted hexagonal pits. Time resolved differential transmission measurements demonstrate a four-five folds decrease in the THz frequency under band edge excitation conditions. A theoretical model predicts a strong dependence of the amplitude and period of the oscillations on the radius of the metal NCs. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:27PM |
J38.00005: ``Making the Molecular Movie'': First Frames Invited Speaker: Femtosecond Electron Diffraction has enabled atomic resolution to structural changes as they occur, essentially watching atoms move in real time--directly observe transition states. This experiment has been referred to as ``making the molecular movie'' and has been previously discussed in the context of a gedanken experiment. With the recent development of femtosecond electron pulses with sufficient number density to execute single shot structure determinations, this experiment has been finally realized. A new concept in electron pulse generation was developed based on a solution to the N-body electron propagation problem involving up to 10,000 interacting electrons that has led to a new generation of extremely bright electron pulsed sources that minimizes space charge broadening effects. Previously thought intractable problems of determining t=0 and fully characterizing electron pulses on the femtosecond time scale have now been solved through the use of the laser pondermotive potential to provide a time dependent scattering source. Synchronization of electron probe and laser excitation pulses is now possible with an accuracy of 10 femtoseconds to follow even the fastest nuclear motions. The camera for the ``molecular movie'' is well in hand based on high bunch charge electron sources. Several movies depicting atomic motions during passage through structural transitions will be shown. Atomic level views of the simplest possible structural transition, melting, will be presented for a number of systems in which both thermal and purely electronically driven atomic displacements can be correlated to the degree of directional bonding. Optical manipulation of charge distributions and effects on interatomic forces/bonding can be directly observed through the ensuing atomic motions. New phenomena involving strongly correlated electron systems will be presented in which an exceptionally cooperative phase transitions has been observed. The primitive origin of molecular cooperativity has also been discovered in recent studies of molecular crystals. These new developments will be discussed in the context of developing the necessary technology to directly observe the structure-function correlation in biomolecules--the fundamental molecular basis of biological systems. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J38.00006: Two-dimensional dynamical reconstruction of the valence exciton in LiF Xiaoqian M. Chen, Yu Gan, Peter Abbamonte, Chen-Lin Yeh, Diego M. Casa, Wei Ku The structure and dynamics of excitons are interesting because excitons are model many-body excitations with technological relevance, e.g. to the behavior of photocells. In a previous study, we used inelastic X-ray scattering, together with inversion techniques, to reconstruct one-dimensional projection images of exciton propagation in LiF in real space and time, and showed that the exciton in LiF is of the Frenkel type. Here we generalize our previous work to a two dimensional plane in LiF. Our new images of exciton propagation show intricate shape changes arising from scattering off of the crystal lattice. Our results are compared to model Wannier function calculations for a more detailed test of the Frenkel model. [Preview Abstract] |
Session J39: Physics of Proteins III: Folding, Structure and Stability
Sponsoring Units: DBPChair: Bernard Gerstman, Florida International University
Room: A124/127
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J39.00001: The Speed Limit of Protein Folding: Alpha-Helix Initiation Modeled and Observed Milo Lin, Omar Mohammed, Ahmed Zewail As a primary event of protein folding, alpha-helix initiation is the starting point of macromolecular complexity. In this work, an analytic coarse-grained model which predicts the initiation rate as a function of temperature, is presented. Helix initiation was measured via ultrafast temperature-jump fluorescence refolding experiments on two penta-peptides, and the measured rates agreed well with those of the model. In addition, the temporal separation of rate-limiting diffusion from fast annealing stipulated by the model was confirmed via ensemble-converging all-atom molecular dynamics simulations, which reproduced both the diffusion and the picosecond annealing processes and rates observed experimentally. Some of these results were published in: Mohammed OF, Jas GS, Lin MM, Ma H, Zewail AH (2009) Primary peptide folding dynamics observed with ultrafast temperature jump. Angew Chem 48: 5628-5632. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J39.00002: Protein folding microenvironments within the cytoplasm of living cells Minghao Guo, Apratim Dhar, Martin Gruebele The protein folding kinetics in a living cell strongly depends on the local environment. Viscosity of cytoplasm and crowding by macromolecules modulate stability, folding rates and folding mechanism in the folding progresses. We use Fast Relaxation Imaging (FReI) to map out the stability and folding kinetics of a FRET-labeled phosphoglycerate kinase (PGK) in the cytoplasm of individual eukaryotic cells with 500 nm spatial resolution. It shows that this modulation results in large variation of folding mechanism compared to in vitro experiment. We have developed the folding-diffusion model of protein folding in cell with hetergenous microenvironment, which includes spatial hetergenous of folding rates of the multiple-state folding of PGK and diffusion between pixels. It is shown that diffusion contributes little to the large variation of folding kinetics, which can only result from the change of folding mechanism due to microenvironment. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J39.00003: First step in folding of nonconstitutive membrane proteins: spontaneous insertion of a polypeptide into a lipid bilayer and formation of helical structure Yana Reshetnyak, Alexander Karabadzhak, Dhammika Weerakkody, Donald Engelman, Vladislav Markin, Oleg Andreev There are two questions we would like to address: 1) what is the molecular mechanism of a polypeptide insertion into a lipid bilayer and formation of transmembrane helix? 2) Are there any transient changes of a lipid bilayer in process of a polypeptide insertion and folding? As a convenient system we are studying pHLIP (pH (Low) Insertion Peptide) insertion into a membrane and folding, which is modulated by pH. The insertion of pHLIP occurs with rapid (0.1 sec) interfacial helix formation followed by a much slower (100 sec) insertion pathway to form a transmembrane helix. The reverse process of unfolding and peptide exit from the bilayer core proceeds much faster than folding/insertion and through different intermediate states. Our kinetic studies with pHLIP variants indicate that insertion can occur 100 times faster and with less number of intermediate states. To study changes, which might occur with a lipid bilayer in a process of peptide insertion and folding, we employed stopped-flow SAXS. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J39.00004: Protein barrel fluctuations and the barrel permeability: A comparison between Green and Red Fluorescent proteins Chola Regmi, Prem Chapagain, Bernard Gerstman As compared to the Green Fluorescent Proteins (GFP), the monomeric variants of the Red Fluorescent Proteins (RFP), also known as mFruits, are substantially less photostable, possibly due to the barrel permeability for molecular oxygen into the protein barrel. We performed molecular dynamics simulations to compare the protein barrel fluctuations of the GFP as well as a monomeric variant of the RFP. We also performed implicit ligand sampling for uncovering the pathways for molecular oxygen entry into the barrels. We found that, as compared to the GFP barrel, the RFP barrel has significantly larger structural fluctuations and these large barrel fluctuations lead to clear pathways through which molecular oxygen or other ions can enter the barrel more easily. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J39.00005: Heterogeneous helical propensity and its effects on dimerization and the stability of a model protein dimer Yuba Bhandari, Prem Chapagain, Bernard Gerstman We will present the results of Monte Carlo simulations of the dimerization and unfolding of a helical protein dimer. Using a three dimensional lattice model, we investigate the role of including sections of amino acids with strong alpha-helix propensity at different locations along the helices on the dimerization kinetics and the dimer stability. Specifically, we focus on the rate limiting steps in both folding and unfolding processes. We find that these processes can be optimized by tuning the ease of access through diffusion to the metastable intermediate state and its stability. The kinetics and thermodynamical stability is tuned by a combination of the locations of the amino acids with the high helical propensity and the salt bridges. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J39.00006: Protein-like folding and free energy landscape of a homopolymer chain Mark Taylor, Wolfgang Paul, Kurt Binder Many small proteins fold via a first-order ``all-or-none'' transition directly from an expanded coil to a compact native state. We have recently reported an analogous direct coil-to-crystallite transition for a flexible homopolymer [1]. Wang-Landau sampling was used to construct the 1D density of states for square-well chains up to length 256 and a microcanonical analysis shows that for short-range interactions the usual polymer collapse transition is preempted by a direct freezing transition. A 2D configurational probability landscape, built via multi-canonical sampling, reveals a dominant folding pathway and an inherent configurational barrier to folding. Despite the non-unique homopolymer ground state, the thermodynamics of this direct freezing transition are identical to those of two-state protein folding. Homopolymer folding proceeds over a free energy barrier via a transition state folding nucleus, displays a protein-like Chevron plot, and satisfies the van't Hoff two-state criterion.\\[4pt][1] Phys. Rev. E 79, 050801(R) (2009); J. Chem. Phys. 131, 114907 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J39.00007: Unfolding Kinetics of Egg Protein Dipti Sharma This study explores denaturing kinetics of egg white using high resolution calorimetric technique. Fresh egg was scanned fro heating and cooling to see the thermodynamics 10$^{\circ}$ C to 100$^{\circ}$ C at different heating ramp rates varying from 1 to 20$^{\circ}$ C/min. An endothermic peak was found on heating scan showing denaturing of protein which was found absent at the cooling indicating the absence of any residue after heating. The denature peak shifted towards higher temperature as ramp rate increases following Arrhenius behavior and shows an activated denaturing kinetics of the egg protein. This peak was also compared with the water to avoid water effects. Behavior of denaturing peak can be explained in terms of Arrhenius theory. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J39.00008: Probing the dynamics of biomolecules in liquid water by terahertz spectroscopy Nguyen Vinh, Jim Allen, Kevin Plaxco Decades of molecular dynamics and normal mode calculations suggest that proteins are rife with collective vibrational modes with ps to ns time constants. Given that proteins are ``decorated'' with charged groups, these motions should lead to oscillating dipoles that, in turn, will lead to strong gigahertz to terahertz absorption. Investigation of these harmonic motions by absorption spectroscopy, however, is extremely challenging due to the strong absorption of water. In response, we have developed a sensitive Vector Network Analyzer based spectrometer that operates from 65 to 700 GHz and can measure both the absorbance and refractive index of protein solutions. In order to extract the complex dielectric response of the protein in solution we employ an effective medium approximation for the mixture of the protein and aqueous buffer. The extracted dielectric response suggests that each protein molecule is surrounded by a tightly held layer of 164 +/- 5 water molecules that behave as if they are an integral part of the protein. The size of this hydration shell and the dielectric response of the solvated protein are all independent of protein concentration. Our measured dielectric response, however, does not agree with published computation models of the protein: the measurements indicate a low frequency cutoff in the density of modes of $\sim$250 GHz. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J39.00009: MD-simulations of Beta-Amyloid Protein Insertion Efficiency and Kinetics into Neuronal Membrane Mimics Liming Qiu, Creighton Buie, Mark Vaughn, Kwan Cheng Early interaction events of beta-amyloid (A$\beta )$ peptides with the neuronal membranes play a key role in the pathogenesis of Alzheimer's disease. We have used all-atom MD simulations to study the protein insertion efficiency and kinetics of monomeric A$\beta _{40}$ and A$\beta _{42}$ into phosphatidylcholine lipid bilayers (PC) with and without 40 mole{\%} cholesterol (CHOL) that mimic the cholesterol-enriched and depleted lipid nanodomains of the neuronal plasma membranes. Independent replicates of 200-ns simulations of each protein pre-inserted in the upper lipid layer were generated. In PC bilayers, only 25{\%} of A$\beta _{40}$ and 50{\%} of A$\beta _{42}$ in the replicates showed complete insertion into the lower lipid layer, whereas the percentages increased to 50{\%} and 100{\%}, respectively, in PC/CHOL bilayers, providing evidence that cholesterol improves the protein insertion efficiency into the bilayers. The rate of protein insertion was proportional to the hydrophobic, transmembrane helix length of the inserted peptide and depended on the cholesterol content. We propose that the lysine snorkeling and C-terminus anchoring of A$\beta $ to the PC headgroups at the upper and lower lipid/water interfaces represent the dual-transmembrane stabilization mechanisms of A$\beta $ in the neuronal membrane domains. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J39.00010: Characterization of Polyethylene Glycol Modified Hemoglobins Gil Salazar, James Barr, Wayne Morgan, Li Ma Polyethylene glycol modified hemoglobins (PEGHbs) was characterized by liquid chromatography and fluorescence methods. We prepared four samples of two different molecular weight PEG, 5KDa and 20KDa, modified bovine and human hemoglobin. We studied the oxygen affinities, stabilities, and peroxidase activities of PEGHbs. We have related oxygen affinities with different degrees of modifications. The data showed that the modification on the beta subunits was less stable than that of the alpha subunits on the human Hb based samples especially. We also compared peroxidase activities among different modified PEGHbs. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J39.00011: Multi-scale structure of a protein (histone H3.1) via a knowledge-based potential Ras Pandey, Barry Farmer A coarse-grained computer simulation model is used to investigate the multi-scale structures of a histone H3.1, a protein with 136 residues in an effective solvent medium. The protein chain consisting of residues (nodes) tethered together by fluctuating bonds on a cubic lattice where empty lattice sites constitute the effective solvent matrix. Each residue interacts with surrounding solvent sites and other residues via Lennard-Jones (LJ) potential. A knowledge-based interaction matrix is used for the residue-residue interaction coefficient of the LJ potential. Interaction between the residue and solvent sites, a measure of the solvent quality, is varied. Each residue executes its stochastic motion with the Metropolis algorithm. We examine a number of local and global physical quantities some of which include mobility and energy profiles of each residue and their local structural histogram, radius of gyration ($R_{g})$, radial distribution function, and structure factor of the protein for a range of the solvent interactions. Variation of $R_{g}$ with the solvent quality of solvent exhibits a maximum. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J39.00012: Calculating the shift in pKa of the position 66 for an Staphylococcal nuclease mutant with the Replica Exchange Free Energy Perturbation method Danial Sabri Dashti, Adrian Roitberg The Experimental pKa value of Glutamate66 in a hyperstable mutant of Staph Nuclease, which has been measured by Moreno et al., shows a large shift of around 5 pKa units with respect to a glutamate in solution. In order to reproduce the large experimental shift by single structure continuum solvent computational methods, it is required that the dielectric constant of the interior of the protein be set to around ten in the simulations. The physical reason behind this is not understood as of yet and hypotheses have been produced by the Moreno group regarding solvent penetration, protein reorganization etc. We tried to resolve this inconsistency between experimental and continuum methods by introducing a four-state thermodynamic cycle that has couples conformational states with protonation state of the side chain of E66. We propose that what the experimental methods, (which are mostly sensitive to configurational changes) are measuring is actually the equilibrium constant between the two configurational states rather than between the two protonation states. In this regard we applied our recently developed Replica Exchange method Free Energy Perturbation (REFEP) in implicit solvent to calculate the pKa value of E66 for each of the configurational states as well as the mixed configuration, and our results are in almost perfect agreement with the experiments of Moreno. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J39.00013: Brownian dynamics simulations of amelogenin microribbons formation Wei Li, Anthony Perez Lopez, Ya Liu, Amit Chakrabarti, James Gunton Recent advances in chemical particle synthesis have emphasized the fundamental role of surface colloidal heterogeneities and their detailed chemical composition, which is particularly significant for an important subclass of colloidal systems, namely, proteins. Recently, the process of self-assembly of amelogenin monomers with a hydrophobic/hydrophilic bipolar nature into ordered ribbon structures has been studied experimentally. In this work, we study this dynamical process by means of a Brownian dynamic simulation of a simple model which represents the bipolar character of the globular amelogenin molecule and the hydrophilic C-terminal tail. We monitor the kinetics of self-assembly through a study of the structure factor. We also calculate the phase diagram of the model using Gibbs ensemble Monte Carlo simulation and thermodynamic perturbation theory. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J39.00014: Stability of proteins inside a hydrophobic cavity Mithun Radhakrishna, Sumit Sharma, Sanat K. Kumar Previous studies have shown that enclosing a protein in an athermal cavity stabilizes the protein against reversible unfolding by virtue of eliminating many open chain conformations. Examples of such confined spaces include pores in chromatographic columns, Anfinsen's cage in Chaperonins, interiors of Ribosomes or regions of steric occlusion inside cells. However, the situation is more complex inside a hydrophobic cavity. The protein has a tendency to adsorb on the surface of the hydrophobic cavity, but at the same time it loses conformational entropy because of confinement. We study this system using a simple Hydrophobic Polar (HP) lattice protein model. Canonical Monte Carlo (MC) simulations at different temperatures and surface hydrophobicity show that proteins are stabilized at low and moderate hydrophobicity upon adsorption. The range of surface hydrophobicity over which a protein is stable increases with a decrease in radius of the cavity. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J39.00015: Wang-Landau sampling of protein adsorption using the HP model Ying-Wai Li, D.P. Landau, T. Wuest We have applied Wang-Landau sampling\footnote{F. Wang and D. P. Landau, Phy. Rev. Lett. \textbf{86}, 2050 (2001).} with appropriate trial moves\footnote{T. W\"{u}st and D. P. Landau, Phy. Rev. Lett. \textbf{102}, 178101 (2009).} to investigate the thermodynamics and structural properties of lattice hydrophobic-polar heteropolymers (commonly known as the HP protein model) interacting with an attractive substrate. We estimate the density of states of the system, from which the partition function and all thermodynamic quantities, e.g. specific heat, radius of gyration, end-to-end distance and surface contacts, can be calculated. ``Transitions'' between ``phases'' are then identified based on a comprehensive analysis of these observables. Generally speaking, three transition processes are observed: adsorption-desorption, collapse (formation of hydrophobic core), and ``flattening'' of adsorbed structures. These have been confirmed by ``snapshots'' of typical states of the system. Depending on the surface attractive strength, these transitions take place in different order upon cooling, giving rise to different thermodynamic behaviors. Such dependence of folding hierarchy on the surface attraction is found to be universal for different HP sequences. [Preview Abstract] |
Session J40: Theory of Clusters and Nanoscale Systems
Sponsoring Units: DCPChair: Shiv Khanna, Virginia Commonwealth University
Room: A122/123
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J40.00001: Nanomaterials synthesized by electrochemical discharges: qualitative and quantitative performance Anis Allagui, Rolf Wuthrich During the electrochemical discharges in aqueous solutions, the electronic avalanches induce several reactions of dimerization and recombination of the hydrated electrons and H* and OH* radicals, generated by the radiation of water molecules. With the introduction of metallic ions M$^{z+}$, the successful manufacture of nanoparticles is controlled by the continuous competition of reduction of M$^{z+}$, by the powerful reducing agents e$^{-}_{H}$ and H* to lower levels of valency, and the back reaction of oxidation by OH*. With the assumption that the concentration of metal ions is high enough when compared to those of species e$^{-}_{H}$, H* and OH*, the differential yield G between the formation and consumption of M$^{z+}$ in a given finite volume around the electron-emitting electrode is modeled by homogeneous kinetics. It is found G to be proportional to the concentration of metal ions, the speed and penetration depth of the electrons, and the ratio of rate constants of reactions of nucleation and polymerization, which are supported by previous contributions on the dynamics and stability of the phenomenon. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J40.00002: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J40.00003: Structural Oscillation in Pd$_{13}$ During Oxidation/Reduction D.R. Roy, J. Ulises Reveles, S. Vincent Ong, S.N. Khanna, A.M. K\"oster, P. Calaminici First principles electronic structure calculations within a gradient corrected density functional formalism have been carried out to investigate the electronic structure and magnetic properties of bare and oxidized Pd$_{13}$ clusters. It is shown that the ground state of neutral Pd$_{13}$ is a bilayer structure that can be regarded as a fragment of the bulk, while a compact icosahedron is higher in energy. The addition of an electron, however, reverses the ordering of structures and Pd$_{13}^{-}$ has an icosahedral ground state. Similar reordering of structure occurs as an O$_{2}$ molecule is added to the neutral cluster. The talk will focus on the oscillations between the two structures during catalysis process. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J40.00004: Computational Studies on the Energy Landscape of Pt-Pd nanoparticles Alvaro Posada-Amarillas, Rafael Pacheco-Contreras, Dora J. Borb\'on-Gonz\'alez, Lauro Oliver Paz-Borb\'on, Roy L. Johnston, J. Christian Sch\"on Bimetallic nanoparticles such as Pt-Pd are currently the subject of intense research mainly due to their important catalytic properties. Clusters structure, composition and degree of mixing or segregation all play important roles in determining their chemical activity. It is presented here an exhaustive study of the structure of Pt-Pd nanoparticles, obtained by a Genetic Algorithm (GA) which incorporates the Gupta potential to mimic interaction for bimetallic atoms. This procedure provided an icosahedral structure as the lowest in energy. The threshold method (TM) is used to analyze the energy landscape of 13-atom Pt@Pd$_{12}$ nanoparticle, as well as the transition probabilities for those structures with pentagonal symmetry found by the TM. Disconnectivity graphs are obtained for both a vast exploration of the potential energy surface (PES) and the exploration around the lowest energy structure. We found low interconversion transition rates for the putative global minimum provided by the GA code, which was confirmed by the TM algorithm. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J40.00005: Negative ions of transition metal-halogen clusters Kalpataru Pradhan, Gennady L. Gutsev, Purusottam Jena A systematic density functional theory based study of the structure and spectroscopic properties of neutral and negatively charged MX$_{n}$ clusters formed by a transition metal atom M (M=Sc, Ti, V) and up to seven halogen atoms X (X=F, Cl, Br) has revealed a number of interesting features: (1) Halogen atoms are bound chemically to Sc, Ti, and V for n $<$ n$_{max}$, where the maximal valence n$_{max}$ equals to 3, 4, and 5 for Sc, Ti, and V, respectively. For n $>$ n$_{max}$, two halogen atoms became dimerized in the neutral species, while dimerization begins at n = 5, 6, and 7 for negatively charged clusters containing Sc, Ti, and V. (2) Magnetic moments of the transition metal atoms depend strongly on the number of halogen atoms in a cluster and the cluster charge. (3) The number of halogen atoms that can be attached to a metal atom exceeds the maximal formal valence of the metal atom. (4) The electron affinities of the neutral clusters abruptly rise at n=n$_{max}$, reaching values as high as 7 eV. The corresponding anions could be used in the synthesis of new salts, once appropriate counterions are identified. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J40.00006: Crystal Field Splitting and Stabilization of CuMgx- Clusters Victor M. Medel, J. Ulises Reveles, A.C. Reber, M.C. Qian, S.N. Khanna The electronic states in clusters group into shells much in the same way as in atoms. Clusters with filled electronic shells exhibit enhanced stability as manifested through observed magic numbers in metal clusters. An important issue is if stable species can be attained at sub-shell fillings. In this work we have carried out first principles electronic structure calculations on CuAl$_{x}^{-}$ and CuMg$_{x}^{-}$ clusters to demonstrate this intriguing effect. It is shown that the ionic cores in the clusters can order to generate internal electric fields that lead to splitting of the supershells, much in the same way as the crystal field splitting of atomic states in crystals. The studies offer a new approach to forming magic species though control of the composition and the arrangement of atoms. The talk will highlight these effects and how they can be seen in experiments. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J40.00007: {\em Ab initio} study of dimer and one-dimensional chain structures of M@Au$_{12}$ (M = W, Mo) clusters Sora Park, Gunn Kim, Young-Kyun Kwon Using density functional theory, we investigate the structural and electronic properties of the dimer and one-dimensional (1D) chain structures composed of metal-encapsulated Au$_{12}$ nanoclusters (M@Au$_{12}$, M = W, Mo) with icosahedral ({\em I} $_{h}$) and cuboctahedral ({\em O}$_{h}$) symmetries. We consider various dimer configurations with different compounds and symmetries to find the most stable dimer structure in each case. We find that during dimerization (either homogeneous or heterogeneous dimer), Au atoms in the one cluster tend to form triangular bonds with counterpart Au atoms in the other. By maximizing the number of Au-Au bonds by dimerization, any cluster is stabilized by about 3 eV. We further find their stable 1D chain structures by considering various 1D chain configurations with different compounds and symmetries. Our results demonstrate that the spin-orbit coupling effects are significant on the electronic and magnetic properties as well as the structural stability due to 5{\em d} electrons in a transition metal atom M of the M@Au$_{12}$ nanocluster. We also present interesting differences in electronic and magnetic properties between {\em I}$_{h}$- and {\em O}$_{h}$-symmetric 1D polymerized M@Au$_{12}$ chain structures. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J40.00008: Interpretation of Cp(*) - protected Aluminum Clusters as Superatom Complexes P. Andre Clayborne, Olga Lopez-Acevedo, Robert Whetten, Henrik Gr\"onbeck, Hannu H\"akkinen Metal clusters stabilized by a surface ligand shell represent an interesting intermediate state of matter between molecular metal-ligand complexes and bulk metal. Such ``metalloid'' particles are characterized by the balance between metal-metal bonds in the core and metal-ligand bonds at the exterior of the cluster. In previous studies, the electronic stability observed for selected ligand-protected aluminum clusters is not fully understood. By density functional theory calculations, we illustrate here that the electronic stability of various experimentally isolated Cp(*) -- protected aluminum clusters can be explained using the electron shell model for the aluminum core, coupled with an ionic Al-Cp(*) interaction at the surface. Thus, one may classify ligand-protected aluminum clusters as ``superatom complexes'' similar to the ligand-protected gold clusters. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J40.00009: Probing the existence of energetically degenerate cluster isomers by chemical tagging Qian Wang, Qiang Sun, Puru Jena Current methods for identifying the ground state geometry of a cluster require synergy between theory and experiment. However, this becomes a difficult problem when the accuracy of the theoretical methods is not sufficient to distinguish between nearly degenerate isomers. Using density functional theory based calculations we show that the near degeneracy between the planar and cage structures can be lifted by tagging these with halogens and superhalogens moieties such as Cl and BO$_{2}$. The energy of the planar Au16- isomer is lowered from 0.15 eV before tagging to 0.51 $\sim $ 0.55 eV after tagging, thus providing a way to probe its coexistence. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J40.00010: Gold clusters at finite temperature: influence of fluxionality on ligand adsorption Luca M. Ghiringhelli, Elizabeth C. Beret, J\"{o}rg Meyer, Matthias Scheffler Metal clusters, in particular in relation with their catalytic properties, have been the object of intensive experimental and theoretical studies, in the recent years. A great deal of effort has been devoted by many theoretical groups to understanding the zero kelvin properties of such clusters. Here, by focusing on small gas phase An$_N$ clusters $(3 \leq N \leq 20)$ and their interaction with CO and O$_2$ as a showcase, we illustrate a methodology for the study of small clusters and their interaction with atoms and molecules at finite temperature. We combine all-electron density functional theory, including scf-density dependent van-der-Waals tail corrections, with finite temperature sampling techniques, like Biased MD and Parallel Tempered MD. We find an unusual flexibility of the clusters, at room and lower temperature. At certain sizes, Au$_N$ clusters at room temperature are liquid droplets. This has an important implication, when accounting for the dynamics of ligand adsorption. One has to consider that the energy released by an exothermic ligand adsorption heats up the newly formed complex, and the equilibration with the environment is much longer than the typical timescale for conformational rearrangement. In this respect, the very concept of a preferred adsorption site in the bare cluster might be meaningless. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J40.00011: Pseudohalogens as Building Blocks of Hyperhalogens: A Case Study with Au(CN)$_{x}$ Complexes Devleena Samanta, Miao Miao Wu, Purusottam Jena Electron affinity (EA) is one of the major factors that govern reactivity. Halogen atoms possess the highest electron affinities among the elements in the periodic table since it takes only one electron to close their shell. Pseudohalogens also require one electron to close their shell and thus mimic the properties of halogens. A typical example is the CN moiety whose electron affinity (3.8 eV) is slightly larger than that of Cl. Using calculations based on density functional theory we show that when a Au atom is surrounded by CN moieties, the electron affinity of Au(CN)$_{x}$ complexes rise above that of CN for x$\ge $2 and reach a value as high as 8.4 eV, thus forming hyperhalogens. Electron affinities also show odd even alternation with the clusters with even x having higher EA values. Equilibrium geometries, electronic structure and spectroscopic properties of these complexes will be presented and results will be compared with available experimental data. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J40.00012: Chirality in Metallic Clusters Ignacio L. Garzon, Luis A. Perez In this work, we present a theoretical study on the structural, vibrational, electronic, and optical properties of chiral bare gold clusters. We consider the case of the Au$_{34}^{-}$ cluster for which extensive experimental studies on its structural and electronic behavior had been published recently. Our results show that the lowest-energy isomers of the Au$_{34}^{-}$ cluster correspond to two chiral structures with C$_{1}$ and C$_{3}$ point symmetry groups, being the C$_{1}$ isomer slightly more stable than the C$_{3}$ one. The calculated structure factors, which have been measured using trapped ion electron diffraction, indicate that these isomers are almost indistinguishable. On the other hand, their electronic DOS show different features around the HOMO-LUMO energy gap, which may be detected through optical spectroscopies. In fact, our calculated absorption and circular dichroism spectra show clear differences in the optical behavior of these chiral clusters. Another important property that distinguishes the C$_{1}$ and C$_{3}$ isomers is the different spatial distribution of the atomic coordination on the cluster surface, which would generate distinct enantiospecific adsorption patterns with chiral molecules. These results confirm the existence of intrinsically chiral bare gold clusters. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J40.00013: Free gold clusters in CO and O$_2$ atmosphere: an ab initio study Elizabeth C. Beret, Luca M. Ghiringhelli, Matthias Scheffler The marked catalytic activity of gold nanoparticles has inspired a large number of scientific contributions~ from different fields. However, many questions still lack a satisfying answer, for example what are the structures and stoichiometries of the gold particles in the presence of the reactive gases, and how do their catalytic properties depend on the particle size [1]. We answer these questions for neutral gold clusters modeled in a gas phase atmosphere containing CO and O$_2$ in variable compositions, and in a temperature range between 100 and 600 K. To this aim, DFT (PBE)--based \textit{ab initio atomistic thermodynamics} technique [2] is applied, including full account of the vibrational contribution to the free energy. As a result, the preferred cluster+adsorbate structures for different environmental conditions are obtained and interpreted as candidate intermediates in the catalytic CO oxidation reaction.\\[4pt] ~[1] R. Meyer, C. Lemire, S. K. Shaikhutdinov and H. J. Freund, \textit{Gold Bull}. \textbf{2004}, \textit{37}, 72--124.\\[0pt] [2] K. Reuter and M. Scheffler, \textit{Phys. Rev. B} \textbf{2001}, \textit{65}, 035406; C. M. Weinert and M. Scheffler, \textit{Mat. Sci. Forum} \textbf{1986}, \textbf{10--12}, 25--30; M. Scheffler and J. Dabrowski, \textit{Phil. Mag. A} \textbf{1988}, \textit{58}, 107--121. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J40.00014: Structure-specific spectroscopy of plasmon-supporting nanoparticles Kenneth Knappenberger Recent advances in the development of sensitive ultrashort laser-based spectroscopic probes to investigate dynamics of high surface-to-volume metal and alloy nanostructures will be discussed. Electronic relaxation and interparticle electromagnetic coupling processes in hollow gold nanospheres (HGNs) and HGN aggregates were studied using femtosecond pump-surface plasmon probe and second harmonic generation spectroscopies, including single-particle measurements. In the case of HGNs, an unexpected, but systematic, blue shift of the spectral position of the surface plasmon resonance was observed upon nanoparticle aggregation. Femtosecond time-resolved measurements, high-resolution TEM, and Finite-Difference Time-Domain calculations demonstrate that this blue shift results from interparticle cavity coupling, an effect not possible for solid nanospheres. The efficiency of this coupling was tailored by controlling HGN aspect ratio over a vast range of sizes (20 nm to 80 nm outer diameters). This effect may be applied to developing more efficient optical and electronic devices, including photovoltaics. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J40.00015: Rare Earth Doped Magnetic Clusters of Gold for Medical Application Brahm Deo Yadav, Vijay Kumar In recent years gold clusters have been studied extensively due to their unusual properties and applications in cancer treatment and catalysis. Small gold clusters having up to 15 atoms are planar as shown in figure 1. Thereafter a transition occurs to 3D structures but the atomic structures continue to have high dispersion. Doping of these clusters could transform them in to new structures and affect the properties. Gold clusters with cage structures such as W@Au12 can be prepared with large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gap by doping with a transition metal atom such as W. By changing the transition metal atom, cage structures of different sizes as well as different HOMO-LUMO gaps can be formed which could be useful in different optical applications. In these structures gold clusters are generally non-magnetic. However, it is also possible to form magnetic clusters of gold such as Gold clusters have been found to be good for cancer treatment. We have performed ab initio calculations on doping of rare earths in small gold clusters to obtain magnetic clusters using projector augmented wave pseudopotential method within generalized gradient approximation for the exchange-correlation energy. Elemental gold clusters having up to 15 atoms are planar and thereafter 3D structures become favorable. We have explored the changes in the growth behavior when a rare earth atom is doped and studied the variation in the magnetic behavior as a function of size. Our results suggest that gold clusters may have twin advantage of treating cancer as well as be helful in magnetic imaging such as by MRI. [Preview Abstract] |
Session J41: Focus Session: Polymers for Energy Storage and Conversion -- Structure in Organic Semiconductor Blends
Sponsoring Units: DMP DPOLY GERAChair: Enrique Gomez, Pennsylvania State University
Room: A115/117
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J41.00001: Organic devices. New perspectives provided from soft x-ray characterization Invited Speaker: Organic semiconductors are continuing to receive significant interest for application in photovoltaic cells, field-effect transistors and light-emitting diodes. Conjugated polymers in particular offer the convenience of solution processibility with the flexibility of materials design afforded by synthetic chemistry. One of the disadvantages of conjugated polymers is the complexity of their film structure that, while key for understanding and optimizing device performance, is difficult to characterize. Here I will present new insights into the structure of films based on conjugated polymers using synchrotron-based soft x-ray techniques. By exploiting molecular resonances near the carbon K-edge, soft x-ray techniques such as x-ray spectromicroscopy and resonant soft x-ray scattering afford enhanced material contrast with high spatial resolution. This enhanced material specificity has been exploited to reveal the complex, hierarchical structure of conjugated polymer blends used in polymer solar cells. Furthermore, we have recently demonstrated a significant degree of miscibility of fullerene derivatives used in high-efficiency polymer/fullerene blends calling into question the assumed paradigm of phase-separated, pure phases. The polarized nature of synchrotron radiation can also be exploited to probe local molecular orientation and order using soft x-rays. This facilitates mapping of domain orientation and molecular order important for understanding charge transport in polycrystalline polymer films used in field-effect transistors. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J41.00002: Systematic Multiscale Modeling of Polymers Roland Faller, David Huang, Beste Bayramoglu, Adam Moule The systematic coarse-graining of heterogeneous soft matter systems is an area of current research. We show how the Iterative Boltzmann Inversion systematically develops models for polymers in different environments. We present the scheme and a few applications. We study polystyrene in various environments and compare the different models from the melt, the solution and polymer brushes to validate accuracy and efficiency. We then apply the technique to a complex system needed as active layer in polymer-based solar cells. Nano-scale morphological information is difficult to obtain experimentally. On the other hand, atomistic computer simulations are only feasible to studying systems not much larger than an exciton diffusion length. Thus, we develop a coarse-grained (CG) simulation model, in which collections of atoms from an atomistic model are mapped onto a smaller number of ``superatoms.'' We study mixtures of poly(3-hexylthiophene) and C$_{60}$. By comparing the results of atomistic and CG simulations, we demonstrate that the model, parametrized at one temperature and two mixture compositions, accurately reproduces the system structure at other points of the phase diagram. We use the CG model to characterize the microstructure as a function of polymer:fullerene mole fraction and polymer chain length for systems approaching the scale of photovoltaic devices. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J41.00003: Exploiting solvent additives to introduce processability to organic solar cells James Rogers, Kristin Schmidt, Michael Toney, Guillermo Bazan, Edward Kramer Solution processable, highly efficient, organic photovoltaics typically consist of a two component donor-acceptor type system composed of a low bandgap conjugated polymer donor blended with a fullerene acceptor. Efficient charge extraction from these blends demands that donor and acceptor components form nanoscale phase separated percolating pathways to their respective electrodes. Although post deposition thermal annealing has been shown to degrade device performance in low bandgap polymer systems, the incorporation of a small concentration of solvent additive (e.g. diiodooctane) into the solution from which a bulk heterojunction solar cell is cast has been shown to nearly double device efficiency without the need for subsequent thermal annealing. In situ grazing incidence wide angle x-ray scattering measurements as a function of time after spin coating suggest that the role of additives is to induce nucleation of crystals of the polymeric component and to facilitate changes in the correlation length (size and/or perfection) of these crystallites during the film drying process. The resulting structural order in additive processed films suggests novel processing routes for existing organic photovoltaics. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J41.00004: Kinetics of Structure Formation in Polymer-Fullerene Solutions for Organic Photovoltaics Margaret Sobkowicz, Ronald Jones, R. Joseph Kline, Dean DeLongchamp Bulk heterojunctions (BHJs) composed of poly(hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are promising active layers for organic photovoltaics. The nanoscale morphology of the BHJ is critical to the performance of solar devices because exciton diffusion, charge separation, and carrier transport require domains that have specific optimal sizes and connectivity. Processing parameters have been shown to influence the morphology and thus device performance. Because P3HT crystallization during film formation is the driving force for phase segregation, casting solution properties are vital to film electronic properties. Small angle neutron scattering (SANS) is an ideal measurement technique to study the blend morphology and phase formation in P3HT:PCBM solutions due to the large difference in neutron scattering length density between P3HT and PCBM, the length scale probed, and the excellent sensitivity of SANS to concentration fluctuations. In this work SANS and solution rheology are developed as characterization tools for organic photovoltaic materials. Scattering data from P3HT:PCBM solutions are correlated to gelation kinetics to develop a picture of the nanoscale organization and the influence of processing on morphology. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J41.00005: Molecular-Scale and Nanoscale Morphology of P3HT:PCBM Bulk Heterojunctions: Energy-Filtered TEM and Low-Dose HREM Lawrence Drummy, Robert Davis, Diana Moore, Michael Durstock, Richard Vaia, Julia Hsu The performance of bulk heterojunction organic photovoltaic devices is critically dependent on the morphology of the active layer. Here we describe the combination of two electron microscopy techniques to quantitatively examine the molecular level structure and mesoscopic domain morphology of the active layer of P3HT:PCBM bulk heterojunction solar cells. Energy-filtered transmission electron microscopy (EFTEM) revealed the nanoscopic, interpenetrating fibrillar structure of the phase separated blend, providing unique assignments of the P3HT-rich and PCBM-rich regions. Low-dose high-resolution electron microscopy (LD-HREM) provided direct images of the P3HT crystals and their orientation within the P3HT-rich domains. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J41.00006: Neutron Scattering Provides a New Model for Optimal Morphologies in Organic Photovoltaics: Rivers and Streams Mark Dadmun, Nathan Henry, Wen Yin, Kai Xiao, John Ankner The current model for the ideal morphology of a conjugated polymer bulk heterojunction organic photovoltaic (OPV) is a phase-separated structure that consists of two pure phases, one an electron donor, the other an acceptor, that form an interpenetrating, bicontinuous, network on the length scale of 10-20 nm. In this talk, neutron scattering experiments that demonstrate that this model is incorrect for the archetypal conjugated polymer bulk heterojunction, poly[3-hexylthiophene] (P3HT) and the fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6]) C$_{61}$ (PCBM) will be presented. These studies show that the miscibility of PCBM in P3HT approaches 20 wt{\%}, a result that is counter to the standard model of efficient organic photovoltaics. The implications of this finding on the ideal morphology of conjugated polymer bulk heterojunctions will be discussed, where these results are interpreted to present a model that agrees with this data, and conforms to structural and functional information in the literature. Furthermore, the thermodynamics of conjugated polymer:fullerene mixtures dominate the formation of this hierarchical morphology and must be more thoroughly understood to rationally design and fabricate optimum morphologies for OPV activity. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J41.00007: Organic Photovoltaic Interfaces: Back Contact Study Brett Guralnick, Michael Mackay, Raul Lobo Charge transfer between the polymer and contact greatly affects organic photovoltaics' (OPV) performance. The processing conditions are key since depositing the contact incorrectly reduces the polymer cell efficiency by up to fifty percent. The back contact, typically aluminum, is thermally evaporated onto the OPV active layer which has long been suspected to be affected by the process. To analyze this, the aluminum layer was dissolved after deposition and the resulting surface was imaged with an atomic force microscope. A fast aluminum deposition rate pitted the polymer surface creating regions of high resistivity thereby reducing cell efficiency. The addition of a LiF blocking layer between the active layer and aluminum was found to eliminate pitting allowing faster deposition. Interestingly, thermally annealing the active layer prior to aluminum deposition was also found to eliminate pitting. Neutron reflectivity experiments were used to determine that the fullerene derivative, used as the electron acceptor in the active layer, migrated to the surface during the annealing step and apparently act as a shielding layer preventing damage. With this knowledge the optimum deposition conditions were determined and has led to the highest efficiencies from OPVs. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J41.00008: Long-range Order in Self-assembled Poly(3-alkylthiophene)-Diblock Copolymers Victor Ho, Bryan Boudouris, Rachel Segalman Poly(3-alkylthiophenes) (P3ATs) are used commonly as active layer components in plastic electronic devices due to their relatively high hole mobilities, low optical band gaps, and their ability to be processed from solution. To date, however, block copolymers containing these molecules as a functional component predominantly have shown nanofibrillar morphologies identical to that of the P3AT homopolymers due to the large thermodynamic driving force for crystallization. We show that by decreasing rod-rod interactions through rational side chain substitution, well-ordered ($e.g.$, lamellar and hexagonally-packed) geometries can be obtained with P3AT-containing diblock copolymers as evidenced by x-ray scattering and electron microscopy. Additionally, we demonstrate that the structural and optoelectronic properties of the P3EHT domains remain in place. The ability to pattern these functional macromolecules on the nanoscale opens many doors for advanced design of organic electronic active layers. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J41.00009: P3HT-based copolymers as interfacial compatibilizers in P3HT/PCBM system S. Michael Kilbey II, Jihua Chen, Xiang Yu, Kai Xiao, Mark Dadmun, Deanna Pickel, Bobby Sumpter To lower the interfacial tension and control the donor-acceptor phase separation in organic photovoltaic devices, a poly(3-hexylthiophene)-\textit{block}-poly(ethylene oxide) (P3HT-$b$-PEO) diblock copolymer compatibilizer was added to a binary blend of regioregular P3HT and the fullerene derivative 6,6-phenyl C$_{61}$ butyric acid methyl ester (PCBM). We systematically examined the ternary phase behavior of spin-coated films of P3HT/ P3HT-$b$-PEO/ PCBM before and after annealing with selected area electron diffraction, grazing-incidence X-ray diffraction, AFM, optical and transmission electron microscopy. Neutron reflectivity experiments were also carried out to study thermodynamic behaviors of P3HT/P3HT-$b$-PEO/PCBM trilayer films. The addition of 5{\%} P3HT-$b$-PEO (block molecular weights of 10kDa and 3kDa, respectively) to a 1:1 P3HT/PCBM blend reduces the size of P3HT-rich domains in P3HT/PCBM films by up to 40{\%} while the $\pi -\pi $ stacking of P3HT (i.e. (020) crystallinity) remains nearly unchanged. In addition we will discuss the effect of compatibilizer type, additive concentration, and thermal annealing conditions on power conversion efficiencies of compatibilized organic photovoltaic cells. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J41.00010: Cooperative Assembly of Hydrogen-bonded Block Copolythiophenes/Fullerene Blends for Photovoltaic Devices James J. Watkins, Ying Lin, Jung Ah Lim, Qingshuo Wei, Alejandro L. Briseno The current work provides a general approach to obtain reliable donor acceptor morphologies by H-bonding cooperative assembly and to achieve efficient photovoltaic devices with enhanced device stability. Herein, we utilize P3HT-based block copolymer (BCP), in which one block is P3HT and the other block is a P3HT derivative containing a poly(ethylene oxide) (PEO) oligomer side chain. This design both enables self-assembly of the devices via microphase segregation into lamellar, cylindrical or spherical morphologies depending on the relative volume fractions of the blocks and provides a means for establishing strong preferential interaction between fullerene derivatives containing hydrogen bond donating groups (such as COOH groups) and the PEO side chain. One advantage of this approach is excellent device stability due to the suppression of macrophase separation resulting from fullerene crystallization under harsh annealing condition. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J41.00011: Enhanced Photovoltaic Performance of All-Conjugated Poly(3-alkylthiophene) Diblock Copolymers Ming He, Wei Han, Jing Ge, Yuliang Yang, Feng Qiu, Zhiqun Lin Control of the ratio of blocks in the all-conjugated poly(3-butylthiophene)-b-poly(3-hexylthiophene) (P3BHT) diblock copolymer provides a simple route to precisely tune the molecular organization and nanoscale morphology in the resulting bulk heterojunction (BHJ) solar cells made of P3BHT/ PC$_{71}$BM. An attractive high PCE of 4.02 {\%} was found in P3BHT21 (i.e., P3BT/P3HT block ratio of 2:1 mol/mol)/PC$_{71}$BM, compared to that of 1.08 {\%} in P3BT and 3.54 {\%} in P3HT homopolymer-based devices. The enhanced performance is attributed to improved phase separation, interpenetrating pathway and the formation of crystalline domain size of 10.4 nm in the active layer; the latter also elucidated the importance of alkyl side-chain lengths in the molecular organization and final film morphology. In the P3BHT21/PC$_{71}$BM blend films, P3BT block facilitated the self-assembly of P3BHT chains into interpenetrating crystalline pathway for efficient charge transport, while P3HT block provided P3BHT chains with necessary flexibility to form improved phase separation at the nanoscale with maximum interfacial areas for charge generation. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J41.00012: Morphological Characterization of Low-Bandgap Crystalline Polymer:PCBM Bulk Heterojunction Solar Cells Haiyun Lu, Thomas Russell Understanding the morphology of polymer-based bulk heterojunction (BHJ) solar cells is key to improving device efficiencies. Blends of a low-bandgap silole-containing conjugated polymer, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b;2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5'-diyl] (PSBTBT) with phenyl-C61-butyric acid methyl ester (PCBM) were investigated using different processing conditions. Scanning force microscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, dynamic secondary ion mass spectrometry and neutron reflectivity studies showed that thermal annealing did not induce obvious changes in the structure of the active layer. Grazing-incidence X-ray diffraction and small-angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and a brief thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains with 10-nm in size, leading to an improvement in photovoltaic performance. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J41.00013: Donor-acceptor conjugated polymers used as electron acceptors in bulk heterojunction photovoltaics Christopher Bailey, Barney Taylor, Jianguo Mei, John Reynolds, John Henderson, Benjamin Leever, Michael Durstock Synthetic control over the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) has been of significant importance in organic photovoltaics due to the nature of charge separation in donor/acceptor blends. One technique for obtaining tunability of the HOMO and LUMO levels with polymer synthesis is to combine electron donating and electron accepting moieties separated by a conjugated linkage unit. This technique has been utilized to produce highly efficient devices reaching power conversion efficiencies above 8{\%} in polymer/fullerene blends. In this work, we report the characterization of poly(2,7{\-}divinylene fluorene{\-}co{\-}benzothiadiazole) (F10DVBT), and performs best as an electron acceptor when mixed with poly(3-hexylthiophene) (P3HT) with an open circuit voltage of 1.2V. A combination of morphological and photo-physical studies highlights interesting properties of this material and its interactions with P3HT. The donor-acceptor conjugated structure of F10DVBT appears to strongly affect the photocurrent of these devices, and may result from the interactions between intramolecular and intermolecular charge transfer processes. [Preview Abstract] |
Session J42: Padden Award Symposium
Sponsoring Units: DPOLYChair: John Torkelson, Northwestern University
Room: A302/303
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J42.00001: Directing crystallization of organic semiconductors around corners in solution-processed thin films Stephanie S. Lee, Samuel Tang, Marsha Loth, John E. Anthony, Detlef-M. Smilgies, Arthur Woll, Yueh-Lin Loo We demonstrate the ability to pre-specify the crystallization direction of triethylsilylethynyl anthradithiophene (TES ADT), an organic semiconductor, in solution-processed thin films. Manipulating the substrate surface energy allows us to control the crystallization rate of TES ADT, which ranges from 9 to 25 $\mu $m/s, during solvent-vapor annealing. Grazing-incidence x-ray diffraction experiments on as-spun TES ADT films indicate that the initial in-plane orientation of TES ADT is influenced by the surface energy of the underlying substrate, likely due to the competition between strong molecule-molecule interactions and its wettability on the substrate. By imposing surface energy specific patterns on the substrate prior to the deposition of TES ADT, we can preferentially direct TES ADT crystallization around bends and sharp corners to form channels with high hole mobility for charge transport. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J42.00002: Solid-State Structure and Crystallization in Double-Crystalline Diblock Copolymers Sheng Li, Sasha Myers, Richard Register Crystalline-crystalline block copolymers, containing two or more chemically distinct crystallizable blocks, can potentially exhibit a rich array of complex solid-state structures. Double-crystalline diblock copolymers of linear polyethylene (LPE) and hydrogenated polynorbornene (hPN) were synthesized, and their crystallization behavior and morphology were examined using two-dimensional simultaneous time-resolved synchrotron small-angle and wide-angle x-ray scattering. Previously, we showed that in symmetric diblock copolymers of hPN and LPE, with molecular weights above 50 kg/mol, the hPN block crystallizes first and sets the solid-state microstructure. In the present work, we extend these studies to lower molecular weights, and more importantly, we examine the structural relationship between the crystals formed by the two blocks under different conditions of confinement. When the diblock molecular weight is reduced to 20 kg/mol, the LPE block crystallizes first, even when LPE is the minority component, and restricts hPN to crystallize between the LPE lamellae. Furthermore, in both the high and low molecular weight diblock copolymers, the second-to-crystallize block always orients its crystals orthogonally to the first-to-crystallize block. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J42.00003: Electric Field Induced Ordering of a Battery Electrolyte Scott Mullin, Nitash Balsara A disordered mixture of a symmetric poly(styrene-\textit{block}-ethylene oxide) (SEO) copolymer and lithium bis(trifluoromethanesulfonimide) (a lithium salt) was placed between two lithium metal electrodes. Application of a 3V potential across the electrodes results in a current density of 15 mA/cm$^{2}$ and order formation as evidenced by the instantaneous development of a sharp small-angle X-ray scattering (SAXS) ring and Bragg spots due to the presence of a few large coherent grains. With time, radial streaks emanated from the ring, leading to a scattering pattern that resembles a sun dial. Our preliminary hypothesis is that these streaks are due to salt concentration gradients that occur when the current is passed. This gradient results in coherent grains within which the domain size changes continuously. To our knowledge, this kind of structure has not been observed previously in block copolymers. The disordered phase is recovered in the bulk when the applied potential is turned off, and the open circuit voltage of the cell and the SAXS invariant relax with similar time constants. This work represents the first step toward designing responsive battery electrolytes wherein structure and ion transport depends on the state-of-charge of the battery. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J42.00004: Gradient Solvent Vapor Annealing of Thin Films Julie Albert, Timothy Bogart, Ronald Lewis, Thomas Epps The development of block copolymer materials for emerging nanotechnologies requires an understanding of how surface energy/chemistry and annealing conditions affect thin film self-assembly. Specifically, in solvent vapor annealing (SVA), the use of solvent mixtures and the manipulation of solvent vapor concentration are promising approaches for obtaining a desired morphology or nanostructure orientation. We designed and fabricated solvent-resistant devices to produce discrete SVA gradients in composition and/or concentration to efficiently explore SVA parameter space. We annealed copolymer films containing poly(styrene), poly(isoprene), and/or poly(methyl methacrylate) blocks, monitored film thicknesses during annealing, and characterized film morphologies with atomic force microscopy. Morphological changes across the gradients such as the transformation from parallel cylinders to spheres with increasing solvent selectivity provided insight into thin film self-assembly, and the gradient device has enabled us to determine transition compositions and/or concentrations. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J42.00005: Injectable solid peptide hydrogel: shear-thinning and instant recovery Congqi Yan, Joel Schneider, Darrin Pochan Peptides were designed to fold into $\beta $-hairpins once exposed to physiological conditions and consequently self-assemble into rigid hydrogel. The network consists of branched and entangled 3nm-wide fibrils. These physical gels shear thin and flow under a proper shear stress but immediately recover back into solids on removal of stress with further rigidity restoring over time. To elucidate mechanisms of these physical properties, gel behavior during and after flow was investigated. Gel stiffness recovered immediately after shear, as well as gel stiffening over time post-recovery, were found dependent on shear rate and shear duration. From scattering measurements during flow, the gel network structure was observed unchanged from the static state at all shear rates investigated. Thus, the peptide gel networks fracture into gel domains ($>$200nm as determined by scattering) during shear thinning/flow but can instantly percolate back into a solid hydrogel after cessation of shear, stiffening further as particle boundaries relax. As these gels are essentially the same solid material, before and after shear, they offer great potential as well-defined, injectable carriers of biomedical therapies where a desired encapsulated therapeutic payload is delivered to an \textit{in vivo} site by simple syringe injection. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J42.00006: Transport properties of mechanically deformed polymer networks Hassan Masoud, Alexander Alexeev We develop a hybrid computational method to probe how the permeation and hindered diffusion change when an isotropic polymer network is deformed by an externally applied force. We use a bond-bending lattice spring model to capture the micromechanics of random networks of interconnected elastic filaments coupled with the dissipative particle dynamics to explicitly model the viscous fluid and diffusive solutes. Our simulations reveal that the network transport properties are defined by the network porosity and by the degree of network anisotropy due to network mechanical deformations. We also show that the internal network structure does not affect the permeation and diffusion of stressed and unstressed networks. Furthermore, our results indicate that permeability of mechanically deformed networks can be predicted based on the alignment of network filaments that is characterized by a second order orientation tensor. Our findings have implications for designing drug delivery agents, tissue engineering, and understanding the function of certain biological systems. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J42.00007: Numerical coarse-graining of polymer field theories Michael Villet, Glenn Fredrickson Field theoretic models of polymers are widely used to investigate polymer self-assembly, but numerical simulations of these models that include full fluctuation physics are computationally demanding and infrequently conducted. To enable efficient multi-scale simulations, we propose the use of systematically coarse-grained field theories that can be simulated on coarse computational lattices while accurately incorporating the effects of important sub-lattice-scale physics. We present a rigorous formalism for generating such coarse-grained theories from data obtained from small-scale fine-grained simulations, and demonstrate our methodology's effectiveness for a representative polymer solution model. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J42.00008: Molecular mobility and cation conduction in sulfonated polyester copolymer ionomers Gregory Tudryn, Daniel King, Michael O'Reilly, Karen Winey, Ralph Colby Poly(ethylene oxide) ionomers are candidate materials for electrolytes in energy storage devices due to the ability of ether oxygen to solvate cations. Copolyester ionomers are synthesized via condensation of sulfonated phthalates with mixtures of PEG and PTMG to make random copolymer ionomers with identical ion content. Variation of the PEG/PTMG composition changes Tg, dielectric constant and ionic aggregation; each with consequences for ion transport. Dielectric spectroscopy is used to determine number density of conducting ions, their mobility and extent of aggregation. Conductivity and mobility show Vogel temperature dependence and increase with PEG content; even though PTMG ionomers have lower Tg. Conducting ion densities show Arrhenius temperature dependence and are nearly identical for polymers containing PEG. SAXS confirms the extent of aggregation and temperature response from dielectric results, and exposes phase separation as PTMG content is increased. The tradeoff between ion-solvation and low Tg in this study provides fundamental understanding of ionic aggregation and ion transport in polymer electrolytes. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J42.00009: Strain Localization and Sliding Friction in Physically Associating Networks Kendra A. Erk, Kenneth R. Shull Experimental evidence, constitutive models, and scaling law arguments are presented for shear-induced strain localization in triblock copolymer gels deformed at reduced rates spanning almost four orders of magnitude. Strain-stiffening behavior proceeded by rapid softening is believed to result from the formation of highly localized regions of deformation in the macromolecular network. This behavior is described by a constitutive model that incorporates the strain energy and relaxation of individual strands in the network. Flow curves predicted from the model are non-monotonic, consistent with the onset of flow instabilities at high shear rates. Connections are established between the stress response of the gel at large strain and traditional sliding friction experiments of gelatin gels on glass. The gel's well-defined network structure and tunable range of relaxation times allow for these gels to be useful model systems for future studies of flow instabilities in physically associating solutions. [Preview Abstract] |
Session J43: Physics of Copolymers II
Sponsoring Units: DPOLYChair: Bradley Olsen, Massachusetts Institute of Technology
Room: A306/307
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J43.00001: Coil-globule Transitions in Model Bioinspired Polymers Hannah Murnen, Adrianne Rosales, Ronald Zuckermann, Rachel Segalman The monomer sequence of a polypeptide chain has a profound effect on the coil to globule transition of the protein. Both theoretical and experimental efforts to probe the effect of monomer sequence have included the use of chemical modifications post chain collapse in a homopolymer/solvent system followed by further analysis of the chain to understand the resulting sequence. Polypeptoids, or N-substituted glycines, are a far more precise sequence specific model system that can be used to test the effect of monomer sequence on the coil to globule transition. In this study, we synthesized 50mer sequences of a blocky protein-like copolymer using 40 monomers of N-(methyl)glycine and 10 of N-(carboxyethyl)glycine). As predicted in theoretical simulations, the protein like copolymer forms a smaller globule than the periodic control sequence. In addition, decreasing the relative hydrophobicity of the two comonomers results in a looser globule size at room temperature. Future work will focus on using polypeptoids to further probe this transition and to gain insight into the fundamental forces at play in polypeptide folding. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J43.00002: Modeling the Heat Capacity of Spider Silk Inspired Di-block Copolymers W. Huang, S. Krishnaji, D. Kaplan, P. Cebe We synthesized and characterized a new family of di-block copolymers based on the amino acid sequences of Nephila clavipes major ampulate dragline spider silk, having the form HABn and HBAn (n=1-6), comprising an alanine-rich hydrophobic block, A, a glycine-rich hydrophilic block, B, and a histidine tag, H. Using temperature modulated differential scanning calorimetry (TMDSC), we captured the effect of bound water acting as a plasticizer for copolymer films which had been cast from water solution and dried. We determined the water content by thermogravimetry and used the weight loss vs. temperature to correct the mass in TMDSC experiments. Our result shows that non-freezing bound water has a strong plasticization effect which lowers the onset of the glass transition by about 10$^{\circ}$C. The reversing heat capacities, Cp(T), for temperatures below and above the glass transition were also characterized by TMDSC. We then calculated the solid state heat capacities of our novel block copolymers below the glass transition (Tg) based on the vibrational motions of the constituent poly(amino acid)s, whose heat capacities are known from the ATHAS Data Bank. Excellent agreement was found between the measured and calculated values of the heat capacity, showing that this model can serve as a standard method to predict the solid state Cp for other biologically inspired block copolymers. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J43.00003: Effect of chain shape and monomer sequence on self-assembly of polypeptoid-polystyrene block copolymers Adrianne Rosales, Hannah Murnen, Ronald Zuckermann, Rachel Segalman Polymer chain shape has profound effects on block copolymer self-assembly. In nature, chain shape is controlled by the monomer sequence of biological polymers, but such precise control is difficult with classical synthetic systems. Polypeptoids, a class of sequence-specific bioinspired polymer, are shown to have a chain shape which can be tuned by the introduction of monomers with bulky, chiral side chains. Here, it is shown that introducing chiral monomers into the peptoid chain increases chain stiffness, as reflected by a 20C increase in the glass transition temperature for a chiral polypeptoid compared to its achiral analog. Incorporation into block copolymers enables systematic study of the effect of chain shape while maintaining similar enthalpic interactions. For two otherwise analogous block copolymers, conformational asymmetry is shown to affect both the self-assembled morphology and its order-disorder transition temperature. The ability to tune polymer properties with this biomimetic system will lend insight to the relationship between monomer sequence and self-assembled nanostructures. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J43.00004: Self-Assembly of Globular Protein-Polymer Diblock Copolymers C.S. Thomas, B.D. Olsen The self-assembly of globular protein-polymer diblock copolymers into nanostructured phases is demonstrated as an elegant and simple method for structural control in biocatalysis or bioelectronics. In order to fundamentally investigate self-assembly in these complex block copolymer systems, a red fluorescent protein was expressed in \emph{E. coli} and site-specifically conjugated to a low polydispersity poly(N-isopropyl acrylamide) (PNIPAM) block using thiol-maleimide coupling to form a well-defined model globular protein-polymer diblock. Functional protein materials are obtained by solvent evaporation and solvent annealing above and below the lower critical solution temperature of PNIPAM in order to access different pathways toward self-assembly. Small angle x-ray scattering and microscopy are used to show that the diblock forms lamellar nanostructures and to explore dependence of nanostructure formation on processing conditions. Circular dichroism and UV-vis show that a large fraction of the protein remains in its folded state after conjugation, and wide angle x-ray scattering demonstrates that diblock copolymer self-assembly changes the protein packing symmetry. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J43.00005: Phase Behavior and Significantly Enhanced Toughness in Polylactide Graft Copolymers Megan Robertson, Grayce Theryo, Feng Jing, Marc Hillmyer Polylactide (PLA), a biodegradable polyester derived from plant sugars, is commercially available and used in a variety of applications ranging from serviceware to resorbable sutures. One limitation to diversifying the applications of the material is its inherent brittleness. Graft copolymers containing PLA arms and a rubbery aliphatic polymer backbone were synthesized by a combination of ring-opening metathesis and ring-opening transesterification polymerizations. The high degree of incompatibility between the arms and backbone resulted in microphase separation of the graft copolymer at increasingly low fractions of the backbone polymer, as evidenced by small-angle x-ray scattering. In graft copolymers with a rubbery content of only 5 wt percent, the tensile strain at break was observed to be as high as twenty times that of neat PLA. Studies are underway to provide insight into the critical polymer molecular parameters for enhanced toughness and the deformation mechanisms. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J43.00006: Thermoresponsive Polymers and Block Copolymers in Ionic Liquids Hau-Nan Lee, Zhifeng Bai, Nakisha Newell, Timothy Lodge We recently discovered that poly(ethylene oxide) (PEO) and poly(n-butyl methacrylate) (PnBMA) exhibit two completely different types of lower critical solution temperature (LCST) phase behavior in certain ionic liquids (ILs). While typical LCST type phase diagrams were shown in PnBMA/IL systems, we observed unusual temperature-composition phase diagrams in the PEO/IL systems, in which the cloud point curves are strongly asymmetric, with the critical composition located at 80 wt {\%} of PEO. In addition, an important feature of these thermosensitive polymer/IL systems is that the LCST can be easily tuned over a wide range by blending different ILs, without changing the chemical structure of the polymers. On the basis of the LCST of PEO and the upper critical solution temperature (UCST) of poly(N-isopropylacrylamide) (PNIPAm) in ILs, we designed a PEO-PNIPAm block copolymer that exhibits interesting doubly thermosensitive self-assembly. The block copolymer forms PNIPAm-core micelles at low temperatures and transforms into PEO-core micelles at high temperatures. The critical micellization temperatures (CMT) of both blocks can be manipulated by adjusting the mixing ratio of ILs. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J43.00007: Effect of homopolymer additives on texture evolution in block copolymer composites Hyung Ju Ryu, Jane Sun, Michael Bockstaller This contribution presents a systematic study of the effect of homopolymer (hP) addition on the texture evolution in block copolymer (BCP) blends. The microstructures of poly(styrene-b-isoprene) based symmetric di-BCP blended with homopolystyrene additives at various filling fraction were analyzed after different thermal annealing time. For the analysis we utilized serial electron imaging in conjunction with image reconstruction {\&} stereological analysis. Particular emphasis was on the elucidation of the evolution of type and frequency of grain boundary (GB) formation as well as average grain size and orientation. Relative GB energies were determined from triple junction analysis. The results demonstrate that the presence of even small amount of hP impurities significantly reduce grain growth and annealing of high energy GB surfaces. This is interpreted as a consequence of selective segregation of the fillers within high energy GB regions and the associated stabilization of the GB surfaces. These results have important implication on the use of BCPs in areas ranging from plastic electronics to tunable photonic crystals. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J43.00008: Influence of Chemical Heterogeneity on the Viscoelastic Properties of Polystyrene-b-Poly(alkyl methacrylate) Baroplastics Asem Abdulahad, Chang Y. Ryu The development of purification and fractionation techniques of block copolymers is important for overcoming the synthetic difficulty of preparing well-defined block copolymers using various living polymerization techniques. A large scale separation technique would lead us to obtaining sufficient amounts of homopolymer-free block copolymers for subsequent physical characterization. This can potentially aid in the elucidation of the role of chemical heterogeneity on the thermodynamic transitions and viscoelastic properties of block copolymer materials. Atom transfer radical polymerization by the activators regenerated by electron transfer method (ARGET-ATRP) was used to prepare a series of polystyrene-b-poly(alkyl methacrylate) copolymers that would inherently consist of homopolymers and a high polydispersity. Leveraging the understanding of polymer adsorption/desorption in solution onto silica and C18-modified silica surfaces during HPLC, we demonstrate how a large scale purification and fraction is achievable using flash chromatography. Finally, the viscoelastic properties of the purified, homopolymer-free block copolymers will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J43.00009: Effects of B Segment Polydispersity on ABA Triblock Copolymer Phase Behavior Mahesh Mahanthappa, Joan Schroeder, Andrew Schmitt, Adam Schmitt, Kyuhyun Im Advanced polymerization techniques enable the synthesis of a variety of polymeric materials with well-defined chain architectures, compositions, and tunable molecular weights and molecular weight distributions. The inherent chain length polydispersity of polymers derived from these syntheses affects their ultimate materials properties and applications. Relying on tandem ring-opening metathesis polymerization with chain transfer (ROMP-CT) and atom transfer radical polymerization (ATRP), we have synthesized a series of poly(styrene-b-1,4-butadiene-b-styrene) triblock copolymers in which the polybutadiene blocks are polydisperse (Mw/Mn = 1.7-2.0) and the polystyrene end blocks are monodisperse (Mw/Mn = 1.05-1.30). We systematically explore the role of block polydispersity, a molecular chain length heterogeneity, on the melt-phase self-assembly behavior of these block copolymers. Using a combination of temperature-dependent X-ray scattering and transmission electron microscopy, we demonstrate that monodispersity is not a necessary condition for molecular self-assembly into well-defined supramolecular morphologies. The origins of these effects are discussed and a preliminary experimental phase portrait for this system is presented. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J43.00010: Elucidation of an Unusual Pull Out Mechanism for the Additive-Driven Assembly of Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide)Tri-block Copolymers Vikram Daga, Hua-Gen Peng, Ying Lin, Wen-Li Wu, Christopher Soles, James Watkins The addition of poly(acrylic acid) (PAA) to disordered poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) tri-block copolymers induces microphase segregation to yield well ordered blends with sub-10 nm domain sizes. Unexpectedly, even with large changes in the chain length (up to 10 times that of tri-block copolymer) and loading of PAA ($>$40{\%}), the domain spacing of the ordered blend remains nearly invariant although order-to-order transitions are still observed. Here we use neutron scattering and selective deuteration of the tri-block copolymer to probe phase segregation and structure in these systems. One interesting observation is the emergence of a pullout mechanism during ordering in which PEO chain segments are drawn across the interface of a mixed PEO-PPO phase to stabilize the addition for PAA and to create a mixed PAA-PEO phase. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J43.00011: Influence of thermal fluctuations on nucleation in a diblock copolymer melt Russell Spencer, Robert Wickham We study the effect of thermal fluctuations on the kinetics of nucleation of lamellar droplets from a metastable cylinder phase in a diblock copolymer melt by simulating the time-dependent Landau-Brazovskii model in three dimensions. We investigate the shift in the location of phase coexistence, due to fluctuations, as well as changes in nucleus shape, critical size and interfacial velocity. We also examine the kinetics of the transition in the spinodal regime. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J43.00012: Disorder-to-Order Transition Induced by Alkyne/Azide Click Chemistry in Diblock Copolymer Thin Films Xinyu Wei, Wei Chen, Joseph Strzalka, Thomas Russell The thin film morphology of binary blends of poly(ethylene oxide)-\textit{block}-poly(n-butyl methacrylate-\textit{random}-propargyl methacrylate) (PEO-$b$-P(nBMA-$r$-PgMA)) diblock copolymer and Rhodamine B azide was investigated. During thermal annealing, the click reaction between the alkyne-bearing diblock copolymer and the azide lead to a significant increase in non-favorable segmental interaction and thus microphase separation of the block copolymer. Different morphologies were realized by controlling block copolymer composition and the mole ratio between the alkyne and azide groups. The effects of film thickness and annealing temperature on microdomain orientation and lateral ordering were also revealed. Our studies suggest a promising approach to fabricate nanostructured materials with long-range lateral ordering. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J43.00013: Correlation in SANS $\chi$ upon heating and pressurization for a diblock copolymer Junhan Cho, Jumi Lee, Du Yeol Ryu The response of phase behavior to pressure for an A-$b$-B diblock copolymer in the disordered state has been studied by small-angle neutron scattering (SANS). Deuterated polystyrene-$b$-poly(n-propyl methacrylate) (dPS-$b$-PPrMA) copolymer, which possesses ordering transition upon heating and baroplasticity (suppressed demixing by pressurization), was taken as our model system. It was shown that effective Flory-Huggins parameter $\chi _F$ from scattering intensity profiles upon heating and pressurization forms a characteristic curve that is a function of pressure increment $\Delta P$ ($\equiv P-P_0$) divided by temperature dependent bulk modulus $B_0 $ at a reference pressure $P_0$. Each isotherm of $\chi _F$ is superposed into the curve by a scale factor $\tau$ determined by $B_0 $. The scattering intensity maxima $I_{max}$, which is governed by $\chi _F$, were also shown to reveal a similar superposition. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J43.00014: Rouse and Entangled Dynamics in Coarse Grain Polymeric Systems Abelardo Ramirez-Hernandez, Darin Pike, Francois Detcheverry, Juan de Pablo The understanding of the kinetics of microphase ordering of block copolymers is important for controlling the morphology of these polymeric materials. Much of our current understanding of the equilibrium morphologies of block copolymers has emerged from studies using Self-Consistent Field Theory (SCFT), in which the effect of non-crossability of chains is not taken into account. In this work, we use a particle-based coarse grain model of block copolymers, and introduced elastic slip-links to model the effect of entanglements on the dynamics of the melts. These effects can be important when the self-assembly occurs in non-equilibrium conditions. We show that our model is able to reproduce both Rouse and Entangled dynamical behavior for a homopolymeric melt. We apply our computational approach to block copolymer systems under equilibrium and non equilibrium (shear flow) conditions. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J43.00015: Facile Synthesis and Characterization of Well-Defined Rod-Coil Block Copolymers Composed of Regioregular Poly(3-hexyl thiophene) Hong Chul Moon, Jin Kon Kim We synthesized rod-coil block copolymers composed of regioregular poly(3-hexyl thiopene) (P3HT) block via anionic coupling reaction. Three different coil blocks (poly(2-vinyl pyridine) (P2VP) and polyisoprene (PI)) and poly(methyl methacrylate) (PMMA)) were selected. For the synthesis of P2VP-$b$-P3HT-$b$-P2VP and P2VP-$b$-P3HT-$b$-P2VP, the chain ends of the P3HT were capped by the aldehyde group. On the other hand, phenyl acrylate (PA)-capped P3HT was prepared for coupling reaction with living PMMA anions. When the excess amount of the used living anions was removed by column chromatography, all of the neat block copolymers showed lower PDI without leaving any homopolymers. We also investigated the optical property and thin film morphology of synthesized various block copolymers. [Preview Abstract] |
Session J44: Focus Session: Kinetic Control of Solution Assemblies
Sponsoring Units: DPOLY DFDChair: Margarita Herrera-Alonso, Johns Hopkins University
Room: A309
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J44.00001: Helical Assembly of Janus Particles Jonathan Whitmer, Qian Chen, Shan Jiang, Sung Chul Bae, Steve Granick, Erik Luijten Amphiphilic Janus particles, which have hydrophobic and hydrophilic hemispheres, assemble into a variety of clusters depending on salt concentration and particle volume fraction. At low salt concentration, repulsion due to surface charges keeps cluster sizes small, whereas at higher salt concentrations beautiful elongated helices of tetrahedra emerge. We demonstrate that the emergence of these helical structures is a nonequilibrium effect, and that kinetic selection drives formation of polytetrahedral shapes relative to polyhedral shapes which are entropically more favorable. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J44.00002: Nucleation of Nanoparticle Superclusters from Solution Siddique J. Khan, C.M. Sorensen, A. Chakrabarti Colloids of surface ligated nanoparticles (NP) often act as solutions with the NP displaying reversible temperature and solvent dependent solubility. In many cases when the nanoparticles are highly uniform, the precipitating solid is a two- or three-dimensional superlattice of the nanoparticles. Thus there is strong analogy to the phase behavior of molecular solutions, and it is reasonable to ask what controls the phase behavior of nanoparticle solutions and what is the nature of nucleation and growth of the insoluble phase? We have recently developed [1] a phenomenological model for the effective interaction potential between two ligated gold nanoparticles. In the current work, we carry out Brownian Dynamics simulations using this NP-NP interaction potential. We will report results from our simulations for both dynamics and shape of pre- nucleating and post-nucleating superclusters. \\[4pt] [1] S.J. Khan, F. Pierce, C.M. Sorensen and A. Chakrabarti, Langmuir, 25, 13861 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J44.00003: Directing the self-assembly of polyhedral silver nanocrystals Michael Gruenwald, Joel Henzie, Asaph Widmer-Cooper, Phillip Geissler, Peidong Yang Self-assembly of nanocrystals with complex shapes requires precise control of nanoscale interactions and driving forces. Here we show with experiment and simulation that large 3D supercrystals with exceptional order can be assembled by tuning the shape and attraction between polyhedral building blocks. When passivated with adsorbing polymer, Ag nano-polyhedra can behave as quasi-hard particles, and assemble into their densest known packings under a simple gravitational driving force. Excess polymer in solution induces depletion attractions that can stabilize less dense, ordered packings. In the case of octahedra, controlling polymer concentration allows us to tune between the well-known Minkowski lattice, and a novel packing with complex helical motifs. Such large-scale ordered arrangements of Ag nanocrystals provide many possibilities for designing scalable 3D plasmonic metamaterials with applications including chemical and biological sensing, nanophotonics and photocatalysis. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:27PM |
J44.00004: Nanoscale systems assembled with DNA: from principles to rational design Invited Speaker: This abstract not available. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J44.00005: A direct comparison of crystallization transitions and glassy dynamics in polymers and colloids Robert S. Hoy, Corey S. O'Hern Using computer simulations, we compare the freezing transitions in systems composed of $N$ spherical particles with and without covalent-bonding constraints. Linear polymer chains with $N-1$ permanent covalent bonds are compared to ``colloidal'' systems with the same nonbonded interactions but no covalent bonds. In the ``sticky hard sphere'' limit, where covalent bonds act as holonomic constraints, the melting temperatures $T_{\rm melt}$ for both polymers and colloids (with the same $N$) are inversely proportional to the number of unconstrained degrees of freedom. We also examine the effect of the thermal quench rate $|\dot{T}|$ on collapse. At $|\dot{T}|$ below a lower ``critical'' value, which decreases rapidly with increasing $N$, polymers and colloids form similar ground states. This critical value is lower for polymers than colloids and has different $N$-dependence. In both cases, the dynamics of the systems slow down near $T_{\rm melt}$ as the system approaches isostaticity. For high $|\dot{T}|$, glassy dynamics produces disordered final states. At intermediate $|\dot{T}|$, we find complex nonmonotonic behavior in $T$, which we relate to the very different rearrangement kinetics of polymeric and colloidal clusters. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J44.00006: Dynamics of Fatty Acid Single Molecule Islands on Metal-exchanged Mica Mourad Chennaoui, Aleks Ponjavic, Janet Wong Under certain conditions, surface-active molecules are known to self-organise into SAMs according to two main driving forces: molecular surface adsorption via diffusive/convective transport, and surface reorganisation and growth. For the latter in-situ methods are required to deconvolute the complex underlying kinetics and dynamics. To this end, a single molecule fluorescence imaging technique is used to observe the dynamics of fatty acid molecules on different metal-exchanged Mica substrates (K, Li, H). It is shown that the molecular surface re-organisation proceeds via an initial islandisation step. These islands spread and grow until forming a stable and organised SAM. Islands formation kinetics/dynamics according to different surface metal types is investigated. Diffusive mechanisms within and between the islands are also explored. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:27PM |
J44.00007: Multicompartment and multigeometry nanostructures with block copolymers and kinetic control Invited Speaker: The combination of charged block copolymer architecture with the kinetic control of solvent processing offers great flexibility for the creation of new assembled morphologies in solution and outstanding ability to control and manipulate those morphologies. When charged, acidic blocks are present, assembled structures are tunable in a well-defined way via co-assembly of organic bases with adjustable chain structure and control of the solution assembly pathway. A rich variety of polymeric micelles have been made such as toroids, disks, and helical cylinders from poly(acrylic acid)-\textit{block}-poly(methyl acrylate)-\textit{block}-polystyrene (PAA-$b$-PMA-$b$-PS) triblock copolymers in THF/water mixtures with multiamines to complex with the PAA. Both the type and amount of multiamine were found to be critical for formation of specific micelles. Kinetic pathways and temporal stabilities of different micelles and nanoscale aggregates have also been studied. Due to low chain exchange dynamics between block copolymeric micelles in solution, global thermodynamic equilibrium is extremely difficult, if not impossible, to achieve. In our block copolymer/THF/water/multiamine quaternary systems, thermodynamics and kinetics of morphological evolution are governed by three important factors, including chain length of hydrophobic blocks, ratio of THF to water, and the interaction of multiamine with hydrophilic PAA block in the corona. Slow kinetics associated with these factors in solution greatly hinders the system from reaching a global equilibrium. However, by taking advantage of slow kinetics behavior of polymeric micelles in solution, one can purposely produce multicompartment micelles and mulitgeometry micelles by now mixing different PAA-containing block copolymers together but forcing them to ultimately reside in the same nanoscale structure through kinetic processing. While kinetically trapped in common nanostructures, local phase separation can occur producing compartments. This compartmentalization can be used within common micelle geometries to make complex spheres and cylinders or can be used to make new nanostructures such as multigeometry aggregates (e.g. hybrid cylinder-sphere aggregates). All is possible through the kinetic control of the assembly process. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J44.00008: Determination of critical micelle concentrations in ionic liquid/block copolymer systems Michelle Mok, Timothy Lodge The micellization of block copolymers in ionic liquids is of great interest, due to their potential as cargo carriers for separations, transfer and extraction applications. In this study, we investigate the critical micelle concentration (cmc) of block copolymers in ionic liquids using fluorescence-based techniques. Specifically, the cmcs of poly(styrene-b-ethylene oxide) and poly(styrene-b-methylmethacrylate) copolymers were determined from the polarity-sensitive emission spectra of pyrene probes. At the onset of micellization, the probes preferentially partition to the non-polar styrene cores, analogous to pyrene-based cmc studies of aqueous micelle systems. The cmcs are explored as a function of copolymer block molecular weight and composition, as well as ionic liquid composition. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J44.00009: Hierarchical Helical-Assembly of Conjugated Poly(3-hexylthiophene)-$b$-poly(3-triethylene glycol-thiophene) Diblock Copolymers Eunji Lee, Brenton Hammer, Todd Emrick, Ryan C. Hayward One-dimensional crystalline fibrillar assemblies of poly(3-hexylthiophene) (P3HT)-based materials hold significant potential for fabrication of low-cost optoelectronic devices. We have studied the crystallization-driven assembly of a series of poly(3-hexylthiophene)-\textit{block}-poly(3-triethylene glycol-thiophene) (P3HT-$b$-P3TEGT) diblock copolymers, which provide a large contrast in solubility due to the presence of non-polar (hexyl) and polar (TEG) side-chains. P3HT-$b$-P3TEGT diblock copolymers were found to form well-defined fibrillar structures in mixed solvents of chloroform and methanol, with lengths could be tuned easily by changing the solvent composition or relative block lengths. For polymers containing relatively short P3TEGT blocks, the resulting fibers show twisted ribbon-like structures. For appropriate block ratios, complexation of the TEG side chains to alkali metal cations drives formation of clearly defined single helical ribbons and superhelical structures. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J44.00010: Molecular simulation study of random block copolymer films prepared through solvent evaporation Dmitry Bedrov, Keith Hambrecht, Grant Smith Molecular simulations have been used to study equilibrium and non-equilibrium morphologies of random block copolymer films prepared through solvent evaporation. The polymer chains are comprised of ``A'' and ``B'' beads connected by FENE springs. Chains comprised of six blocks (ten beads each) and representing all possible combinations of A and B blocks were used to form films with 50/50 A/B fraction. Bead-bead interactions were chosen such that one of the blocks had higher glass transition temperature than the other and that the A and B blocks were incompatible in absence of the solvent. Initially the polymer chains were dissolved in a monomeric solvent at 75/25 solvent/polymer ratio. Then, polymer films were formed through solvent evaporation at various processing parameters. The nanoscale structure and viscoelastic properties of the polymer were investigated as a function of solvent quality, segment incompatibility and rate of evaporation. It was found that when the temperature is below the glass transition temperature of one of homopolymers, the morphology and properties of the film are strongly dependent on evaporation rate. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J44.00011: Blood Clotting-Inspired Control of Single-Chain Molecules in Flows Charles Sing, Alfredo Alexander-Katz Recent experimental evidence has demonstrated a clear link between mechanical stimuli and the activation of von Willebrand Factor (vWF), a protein that plays a critical role in the blood clotting cascade. This protein exhibits counter-intuitive conformational and adsorption responses that suggest novel ways of controlling the single-chain dynamics of polymer chains. Specifically, we are using simulation and theoretical approaches to elucidate the fundamental physics that govern globule-stretch transitions in collapsed polymers due to the effect of fluid flows. We begin to extend this general approach to the case of globule adsorption-desorption transitions in the presence of fluid flows, and demonstrate how kinetic considerations must be taken into account to describe the basic features of these transitions. We expect that these results will both allow the development of novel techniques for single-chain targeting and assembly and offer insight into the physiological behavior of vWF. [Preview Abstract] |
Session J45: Focus Session: Non-equilbrium Physics with Cold Quantum Gases
Sponsoring Units: DAMOPChair: Miguel Angel Garcia March, Colorado School of Mines
Room: A310
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J45.00001: Universal Dynamics Near Quantum Critical Points Invited Speaker: asp@physics.bu.edu [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J45.00002: Dynamics of a finite-rate quantum quench in an ultra-cold atomic BCS superfluid Chih-Chun Chien, Bogdan Damski We study dynamics of an ultra-cold atomic BCS superfluid driven towards the BCS superfluid-Fermi liquid quantum critical point by a gradual decrease of the pairing interaction. We analyze how the BCS superfluid falls out of equilibrium and show that the non-equilibrium gap and Cooper pair size reflect critical properties of the transition. We observe three stages of evolution: adiabatic where the Cooper pair size is inversely proportional to the equilibrium gap, weakly non-equilibrium where it is inversely proportional to the non-equilibrium gap, and strongly non-equilibrium where it decouples from both equilibrium and non-equilibrium gap. These phenomena should stimulate future experimental characterization of non-equilibrium ultra-cold atomic BCS superfluids. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J45.00003: Slow quench dynamics of a trapped one-dimensional Bose gas confined to an optical lattice Jean-Sebastien Bernier, Guillaume Roux, Corinna Kollath We analyze the effect of a linear time-variation of the interaction strength on a trapped one-dimensional Bose gas confined to an optical lattice. The evolution of different observables such as the experimentally accessible onsite particle distribution are studied as a function of the ramp time using time-dependent exact diagonalization and density-matrix renormalization group techniques. We find that the dynamics of a trapped system typically display two regimes: for long ramp times, the dynamics are governed by density redistribution, while at short ramp times, local dynamics dominate as the evolution is identical to that of an homogeneous system. In the homogeneous limit, we show that the energy absorbed scales non-trivially with the ramp time. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J45.00004: Quench dynamics of paired states of fermions in two dimensions with breaking of parity and time-reversal symmetries Noah Bray-Ali Resonantly paired fermions in two spatial dimensions with breaking of parity and time-reversal symmetry are believed to exhibit two topologically distinct phases at low temperature: the weak-pairing (Bardeen-Cooper-Schrieffer or BCS) phase and the strong-pairing (Bose-Einstein condensate or BEC) phase. We examine the dynamic response of each phase to a rapid quench towards and away from the quantum critical regime. The weak-pairing (BCS) phase has a higher residual defect concentration after the quench than the strong-pairing (BEC) phase. We relate this to the presence of a topologically protected, Majorana fermion edge excitation in the weak-pairing phase, and propose quench dynamics as a practical, experimental probe of this excitation in these systems. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J45.00005: Title: Time-Dependent Mean Field Theory for Quench Dynamics in correlated electron systems Marco Schiro', Michele Fabrizio A simple and very flexible variational approach to the out-of-equilibrium quantum dynamics in strongly correlated electron systems is introduced through a time-dependent Gutzwiller wavefunction. As an application, we study the simple case of a sudden change of the interaction in the fermionic Hubbard model and find at the mean field level an extremely rich behaviour. In particular, a dynamical transition between small and large quantum quench regimes is found to occur at half-filling, in accordance with the analysis of Eckstein {\sl et al.}, Phys. Rev. Lett. {\bf 103}, 056403 (2009), obtained by dynamical mean field theory, that turns into a crossover at any finite doping. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J45.00006: Non-equilibrium dynamics and heating of cold atoms in optical lattices Andrew Daley, Hannes Pichler, Peter Zoller We study the dissipative many-body dynamics of cold atoms in optical lattices that is induced by incoherent scattering of light from the lattice lasers. The resulting heating process is intrinsically non-equilibrium, and involves an important interplay between the atomic physics of the spontaneous emission process and the many-body physics of the state present in the system. In particular, we observe important differences for strongly and weakly interacting regimes, as well as a strong dependence on the sign of the laser detuning from the excited atomic state. We compute heating rates and changes to characteristic correlation functions based on a microscopic master equation. In 1D this equation can be propagated exactly by combining time-dependent density matrix renormalization group (t-DMRG) methods with quantum trajectory techniques. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J45.00007: Quantum effects on Fermi-Pasta-Ulam recurrence in ultracold lattice bosons Ippei Danshita, Rafael Hipolito, Vadim Oganesyan, Anatoli Polkovnikov We propose an experimental scheme for studying the Fermi-Pasta-Ulam (FPU) problem in a quantum mechanical regime with use of ultracold one-dimensional Bose gases in an optical lattice. In the classical limit, we identify parameter regions in which FPU recurrence can occur in this system. The strength of quantum fluctuations can be widely controlled by tuning the number of atoms per lattice sites (filling factor). To investigate the effects of quantum fluctuations on the FPU recurrence, we simulate the real time dynamics of the Bose-Hubbard model by means of the exact numerical method of time-evolving block decimation. We show that strong quantum fluctuations cause significant damping of the FPU oscillation. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J45.00008: Dynamic Stimulation of Phase Coherence in Lattice Bosons Andrew Robertson, Victor Galitski, Gil Refael The existence of superfluidity depends on the energy distribution of~excitations in a system. However, the distribution at thermal equilibrium is rarely optimal for the manifestation of long-range phase coherence. We show that by pushing a system of lattice bosons out of equilibrium with periodic driving, it is possible to increase or decrease the phase coherent region in the phase diagram of the Bose-Hubbard model.~We demonstrate this by calculating the non-equilibrium spatial correlation function~using a synthesis of Keldysh and Floquet theories. This work is supported by DARPA-MTO. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J45.00009: Phase Kink Dynamics in fluctuating Bose condensates Amy Cassidy, Ludwig Mathey, Charles Clark We study the dynamics of Bose gases following a phase imprint. Numerical results within truncated Wigner approximation, which includes both quantum and thermal fluctuations, are compared with analytical predictions. In order to emphasize the effects of fluctuations in these approximations, we also compare our results with dynamics governed by the Gross Pitaevskii equation. We study the dynamics of several observables, including the density and single-particle and density-density correlation functions, with particular focus on experimentally relevant quantities. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J45.00010: Exploring topological phases with quantum walks Takuya Kitagawa, Mark Rudner, Erez Berg, Eugene Demler The quantum walk was originally proposed as a quantum mechanical analogue of the classical random walk, and has since become a powerful tool in quantum information science. In this talk, we show that the dynamical protocols called discrete time quantum walks provide a versatile platform for studying topological phases, which are currently the subject of intense theoretical and experimental investigation. In particular, we demonstrate that recent experimental realizations of quantum walks simulate a non-trivial one dimensional topological phase. With simple modifications, the quantum walk can be engineered to realize all of the topological phases which have been classified in one and two dimensions. We further discuss the existence of robust edge modes at phase boundaries, which provide experimental signatures for the non-trivial topological character of the system. Reference: T.Kitagawa, M.Ruder, E.Berg, and E. Demler, Phys. Rev. A 82, 033429 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J45.00011: Universal energy distribution in thermally isolated driven systems Luca D'Alessio, Guy Bunin, Anatoli Polkovnikov, Yariv Kafri The evolution of the energy distribution of a thermally isolated and repeatedly driven system is studied. A general formalism to calculate the width of the energy distribution is derived and the result is compared with the thermal width. This comparison allow us to identify two regimes: quasi-thermal and run-away. In the quasi-thermal regime the width on the energy distribution of the driven system is proportional to the thermal width with a protocol-dependent universal coefficient. In the run-away regime the width of the energy distribution is an universal function of the energy with an exponent different from the thermal case. A simple formulation in terms of entropy production allow us to distinguish these two regimes. Examples and application to both classical and quantum system (mainly cold atoms) are presented. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J45.00012: Ramping through Superfluid-to-Mott transition in the Bose-Hubbard Model Bernhard Wunsch, David Pekker, Takuya Kitagawa, Efstratios Manousakis, Eugene Demler We discuss equilibrium and dynamic properties of cold bosonic atoms in optical lattices which can be described by the Bose-Hubbard Model. Motivated by recent experiments we study local density fluctuations and their correlations both in equilibrium and for a ramp from the superfluid to the Mott regime. We compare mean-field Gutzwiller approach with exact diagonalization studies and analyze the effect of a trapping potential. In order to describe fluctuations and finite temperature we include quadratic fluctuations on top of the mean field. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J45.00013: Dynamics of thermalisation: a Gaussian regime Sam Genway, Andrew Ho, Derek Lee We study numerically the thermalisation and temporal evolution of subsystems in a fermionic Hubbard model prepared far from equilibrium at a definite energy. Taking motivation from cold atoms in optical lattices with single-site addressability, we consider measurements on a two-site subsystem. We ask the question: how do observables on the subsystem thermalise when the total system is in a pure state? Even for very small systems near quantum degeneracy, the subsystem can reach a steady state resembling thermal equilibrium. This occurs for a non-perturbative coupling between the subsystem and the rest of the lattice where relaxation to equilibrium sharply contrasts perturbative results. To examine the extent to which this behaviour is generic for small quantum systems, we also investigate small Bose-Hubbard model systems and fermionic systems with random couplings between the subsystem and the rest of the lattice. [Preview Abstract] |
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