Session V1: Iron Pnictides versus Iron Chalcogenides: Magnetism and Pairing Fluctuation

Symmetry and structure of the pairing gap in Fe-based superconductors

I review recent works on the symmetry and structure of superconducting gap in Fe-pnictides and related compounds. I show that the gap very likely has s-wave symmetry, and is either nodal or has nodes along the two electron Fermi surfaces, depending on the parameters. I argue that the nodal gap is most likely outcome in systems with less pronounced tendency towards antiferromagnetism. I compare 4-and 5-pocket models for Fe-pnictides and argue that the parameter range where the gap is nodal is much wider in 4-pocket models. I review recent experiments aimed to understand whether the gap has nodes, e.g., experiments on the variation in the field-induced component of the specific heat C(H) with the direction of the applied field in $FeSe_{0.4}Te_{0.6}$. I show that, for extended s-wave gap, C(H) has $\cos{4 \phi}$ component, where $\phi$ is the angle between H and the direction between hole and electron Fermi surfaces, but only if the gap has no nodes. When the gap has accidental nodes, the $\cos{4\phi}$ variation does not hold. I also plan to discuss the interplay between direct Coulomb interaction at large momentum transfer and spin-fluctuation contribution to the pairing, and the interplay between antiferromagnetism and superconductivity. In particular, I show that in 4-pocket (but not in 5-pocket) model superconductivity becomes the leading instability in some range of parameters even at perfect nesting, i.e, antiferromagnetism is not a pre-condition for superconductivity. This agrees with functional RG studies.   

Anisotropic Superconducting Gap Revealed by Angle Resolved Specific Heat, Point Contact Tunneling and Scanning Tunneling Microscope in Iron Pnictide Superconductors

Angle resolved specific heat was measured in FeSe$_{0.55}$Te$_{0.45}$ single crystals. A four-fold oscillation of C/T, with the minimum locating at the Fe-Fe bond direction, was observed when the sample was rotated at 9 T, which can be understood as due to the gap modulation on the electron pocket within the scheme of S$\pm $ pairing. Accordingly, by measuring the point contact Andreev reflection spectrum on the BaFe$_{2-x}$Ni$_{x}$As$_{2}$ single crystals in wide doping regimes, we found a crossover from nodeless to nodal feature of the superconducting gap. In K-doped BaFe$_{2}$As$_{2}$ single crystals, we performed the low temperature STM measurements and observed a well ordered vortex lattice in local region. In addition, the statistics on over 3000 dI/dV spectra illustrate clear evidence of two gaps with magnitude of 7.6 meV and 3.3 meV, respectively. Detailed fitting to the tunneling spectrum shows an isotropic superconducting gap. Work collaborated with B. Zeng, C. Ren, L. Shan, Y. L. Wang, B. Shen, G. Mu, H. Q. Luo, T. Xiang, H. Yang, I. I. Mazin and P. C. Dai. References: \\[4pt] [1] B. Zeng, et al., arXiv:1007.3597, Nature Communications, 2010, in press.\\[0pt] [2] C. Ren, et al., to be published.\\[0pt] [3] L. Shan, et al., arXiv:1005.4038.   

From magnetism to superconductivity in FeTe$_{1-x}$Se$_{x}$

The iron chalcogenide FeTe$_{1-x}$Se$_{x}$ is structurally the simplest of the Fe-based superconductors and exhibits a Fermi surface similar to iron pnictides. Despite this similarity, the parent compound Fe$_{1+y}$Te orders antiferromagnetically with an in-plane magnetic wave vector ($\pi$,0) with an ordered moment of $\sim$2$\mu_{B}$/Fe, suggestive of a localized rather than itinerant character of the magnetic order. This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector ($\pi$,$\pi$) that likely arises from Fermi Surface nesting. Regardless both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around ($\pi$,$\pi$) as probed by neutron scattering. A central question in this burgeoning field is therefore how ($\pi$,$\pi$) superconductivity emerges from a ($\pi$,0) magnetic instability ? Using neutron scattering we show that incommensurate magnetic excitations around ($\pi$,$\pi$) are found even in the undoped parent compound Fe$_{1+y}$Te. With increasing $x$, the ($\pi$,0)-type magnetic long-range order becomes unstable and correlates with a weak charge carrier localization, while the mode at ($\pi$,$\pi$) becomes dominant for x$>$0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors. This work was carried out in close collaboration with the groups of W.Bao (Renmin), Arno Hies (ILL), Zhiqiang Mao (Tulane), C. Brohom (John Hopkins) and I. Eremin (MPI-Dresden/Bochum).   

The nature of electronic nematic states in iron-pnictides

We show that the electronic nematic states in iron-pnictides is driven by frustrated spin fluctuations. A three dimensional effective spin model is constructed to explain the nematicity. This model explains the relation between the structural and magnetic transitions, and the spin excitations measured in recent neutron scattering experiments. Moreover, the model naturally predicts the separation between the two transitions are controlled by the c-axis magnetic exchange coupling , and the existence of an non-collinear magnetic state before spin-glass state upon replacing irons by nonmagnetic impurities. The experimental evidence supporting the predictions and the relation to orbital ordering and superconductivity will also be addressed.   

Differences in the degree of correlations between Pnictides and Chalcogenides: LaFeAsO vs. FeSe

The discovery of high-temperature superconductivity in iron-based compounds triggered an enormous amount of research in condensed matter physics. A very intriguing property of these new compounds is the rather high flexibility concerning elemental substitutions, leading to several families of superconductors, termed ``1111,'' ``122,'' ``11,'' and so on, depending on their chemical composition. In this talk we will analyse the single-particle properties of prominent iron-based supercondutors using a combination of density-functional theory with the Dynamical Mean-Field Theory, where also the interaction parameters are calculated ab-initio. This approach enables us to understand also these more complex materials at a first-principle level. We will show that there are significant differences in the electronic properties, when going from more weakly correlated members as LaFeAsO, to more correlated ones like FeSe. For reasonable Coulomb parameters, the properties range from Fermi-liquid like to incoherent bad-metal like.   

Session V2: Fermi Surface Reconstruction and Competing Orders in High Tc Cuprates

Broken rotational and translational symmetries in the pseudogap phase of cuprates

A large in-plane anisotropy of the Nernst coefficient in YBCO is found to set in precisely at the pseudogap temperature $T^\star$ throughout the doping phase diagram [1]. This implies that the pseudogap phase is an electronic state that breaks the four-fold rotational symmetry of the copper-oxide planes. At a somewhat lower temperature, of order $T^\star$/2, the positive Hall and Seebeck coefficients of YBCO start dropping, and they reach large negative values at $T = 0$, in the normal state accessed by applying high magnetic fields [2,3]. We interpret this in terms of an electron pocket forming in the Fermi surface of YBCO as a result of a Fermi-surface reconstruction caused by some order which breaks the translational symmetry of the lattice. Because very similar transport anomalies are observed in Eu-LSCO [4], where they coincide with the onset of stripe order, we infer that some form of stripe order is also at play in YBCO, and argue that the pseudogap phase is a precursor region of stripe (or spin-density-wave) fluctuations [5]. \\[4pt] [1] R. Daou {\it et al.}, Nature {\bf 463}, 519 (2010). \\[0pt] [2] J. Chang {\it et al.}, Physical Review Letters {\bf 104}, 057005 (2010). \\[0pt] [3] D. LeBoeuf, arXiv:1009.2078. \\[0pt] [4] O. Cyr-Choini\`ere {\it et al.}, Nature {\bf 458}, 743 (2009). \\[0pt] [5] L. Taillefer, Annual Review of Condensed Matter Physics {\bf 1}, 51 (2010); arXiv:1003.2972.   

Quantum oscillation measurements and their reconciliation with ARPES results in underdoped cuprates

A current conundrum relating to the normal state electronic structure of the underdoped cuprates is the apparent dichotomy between photoemission and quantum oscillations measurements. New quantum oscillation measurements performed over an extended regime on underdoped YBCO are presented that bring results of these two techniques into closer agreement. Further, from the latest quantum oscillation results, we are able to distinguish between various scenarios involving single or multiple carrier types $-$ we show that the experimental findings are only consistent with one of these possibilities. *Work performed in collaboration with N. Harrison, M. Altarawneh, P. A. Goddard, R. Liang, W. N. Hardy, D. A. Bonn, and G. G. Lonzarich   

Magnetic phase diagram and magnetic dynamics of YBa$_{2}$Cu$_{3}$O$_{6+x}$: Implications for the mechanism of high-T$_{c}$ superconductivity

We will present a comprehensive study of the magnetic phase diagram of the high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{6+x}$. Using elastic neutron scattering, we show that the phase diagram includes incommensurate spin density wave and electronic liquid-crystal states, in addition to the well-known antiferromagnetic, d-wave superconducting, and paramagnetic phases [1]. Using a combination of inelastic neutron scattering and resonant inelastic x-ray scattering, we have also arrived at a comprehensive picture of the magnetic excitation spectrum in all of these phases [2]. The dispersion relations and spectral weights of all compounds, including the slightly overdoped superconductor YBa$_{2}$Cu$_{3}$O$_{7}$, are closely similar to those of magnons in undoped, antiferromagnetically ordered YBa$_{2}$Cu$_{3}$O$_{6}$. The results are in excellent agreement with the spin excitations obtained by exact diagonalization of the t-J Hamiltonian on finite-sized clusters. A numerical solution of the Eliashberg equations based on the experimental spin excitation spectrum of YBa$_{2}$Cu$_{3}$O$_{7}$ reproduces its superconducting transition temperature T$_{c}$ within a factor of two, strongly supporting magnetic Cooper pairing models for the cuprates. Neutron scattering data of the buckling phonon in YBa$_{2}$Cu$_{3}$O$_{7}$ suggest that coupling to this phonon does not substantially enhance d-wave pairing [3]. \\[4pt] [1] D. Haug, V. Hinkov, Y. Sidis, P. Bourges, N. B. Christensen, A. Ivanov, T. Keller, C. T. Lin, B. Keimer, New J. Phys. \textbf{12}, 105006 (2010). \\[0pt] [2] M. Le Tacon, G. Ghiringhelli, J. Chaloupka, M. Moretti Sala, V. Hinkov, M. W. Haverkort, M. Minola, M. Bakr, K. J. Zhou, S. Blanco-Canosa, C. Monney, Y. T. Song, G. L. Sun, C. T. Lin, G. M. De Luca, M. Salluzzo, G. Khaliullin, T. Schmitt, L. Braicovich, B. Keimer, to be published. \\[0pt] [3] M. Raichle \textit{et al}., to be published.   

The spin density wave quantum phase transition in two dimensional metals

The mean-field theory for the onset of spin density wave order in a metal describes a quantum phase transition which reconstructs the ``large'' Fermi surface to small Fermi pockets. Fluctuations near this transition are described by a quantum field theory of fermions near the reconstruction points, and the bosonic order parameter. This field theory is shown to be strongly-coupled in two spatial dimensions. We find an instability near the quantum critical point to $d$-wave pairing: this is a ``log-squared'' instability, stronger than the familiar logarithmic BCS instability. A similar instability is also found towards a modulated bond order. We also discuss an alternative route for the onset of spin density wave order, via intermediate non-Fermi liquid phases which have small Fermi pockets but without long-range spin density wave order.\\ ~\\ M. A. Metlitski and S. Sachdev, Physical Review B {\bf 82}, 075128 (2010)\\ S. Sachdev, M. A. Metlitski, Y. Qi, and C. Xu, Physical Review B {\bf 80}, 155129 (2009)   

Quantum Oscillations and High Magnetic Field Heat Capacity in Underdoped YBCO

This abstract not available.   

Session V3: Controlling Quantum Interactions of Single Spins and Photons in Diamond

Spin quantum measurements on diamond defects

Diamond defects allow for precise measurement of single electron and nuclear spin quantum states. The excellent controllability of these spins as well as efficient decoupling from environment make them an ideal playground for engineering complex quantum states and development of elaborate control schemes. The talk will describe how nuclear spin states can be efficiently read-out and used as Qbits in spin clusters. Routs towards the controlled engineering of extended spin arrays as well as coupling to control structures will be discussed.   

Observation of spin-light coherence for single spin measurement and control in diamond

The long spin coherence and optical addressability of nitrogen-vacancy (NV) centers in diamond makes them excellent candidates for studies of quantum information science with potential technological applications. We demonstrate the coherent coupling of light to the electronic spin of a single NV center for both non-destructive, single-spin readout via the Faraday effect and unitary, single-spin control via the optical Stark effect\footnote{B. B. Buckley, G. D. Fuchs, L. C. Bassett, D. D. Awschalom, {\em Science Express} (DOI: 10.1126/science.1196436)}. By monitoring the Faraday effect of laser light focused on a single NV center and detuned from optical resonances, we are able to read out an NV center's spin state without destroying it, in contrast to traditional spin readout techniques which polarize the spin during measurement. In a complimentary way, the spin coherently rotates in response to the light through the optical Stark effect, which we demonstrate as a method of all-optical spin control. These measurements have important consequences for future single-spin quantum non-demolition measurements and spin-photon entanglement schemes in diamond that may be exploited for the development of quantum repeater technologies and photonic coupling of spins over large distances.   

Quantum entanglement between an optical photon and a solid-state spin qubit

Quantum entanglement is among the most fascinating aspects of quantum theory. In this work quantum entanglement between the polarization of a single optical photon and a solid-state spin qubit is realized. The experimental entanglement verification demonstrates that a high degree of control over interactions between a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond and the quantum light field can be achieved. The reported entanglement source can be used in studies of fundamental quantum phenomena and provides a key building block for the solid-state realization of quantum optical networks.\footnote{Quantum entanglement between an optical photon and a solid-state spin qubit, Nature 466, 730-734, (2010)}   

Coupling single electron spins in diamond to integrated optical structures

Diamond is an attractive material for some electronic and photonic applications because of its excellent chemical stability and high thermal conductivity and carrier mobility. Diamond appears to be an excellent material for quantum information processing and magnetic sensing applications as well, with many optically active paramagnetic centers. Of these, the most carefully studied to date has been the nitrogen-vacancy (NV) color center, since it is optically addressable and can exhibit electron spin coherence lifetimes exceeding 1 ms at room temperature. This long-lived coherence is usually attributed to the nuclear spin-zero environment of the diamond lattice, which can be further improved with isotopic purification. These capabilities have recently allowed for some remarkable demonstrations in this system such as controlled coupling between single electronic and nuclear spins. For quantum information technologies, NV centers that are mutually optically coupled could enable long-distance quantum communication through repeaters. However, given the low branching ratio of natural emission from NV centers in bulk diamond into the zero phonon line, coupling these centers to cavities with at least moderately large Purcell factors is a critically important step. In this talk I will describe our recent progress in realizing microcavities with low loss and small mode volume in two hybrid systems: silica microdisks coupled to diamond nanoparticles, and gallium phosphide microdisks coupled to single-crystal diamond. In both cases, we have demonstrated coupling between NV centers and whispering-gallery-type cavity modes. Finally, I will present our most recent progress toward fabricating waveguides and microcavities directly in diamond.   

Quantum control and decoherence of a single spin in diamond

Nitrogen-vacancy (NV) impurity centers in diamond have recently emerged as a unique platform for investigating quantum dynamics and quantum control of single spins in solid-state environments. NV centers demonstrate an unusual combination of spin-dependent optical properties, individual addressability, and long spin coherence times. The NV spin state can be manipulated both optically and magnetically, and very fast quantum control operations can be performed with high fidelity [1,2]. Due to these uniquely favorable properties, quantum dynamics of a single NV spin can be investigated in great detail. I will present the results of our work on decoherence of NV spins by spin baths of atomic nitrogen impurities (P1 centers) and the spins of $^{13}$C nuclei, and discuss different regimes of the decoherence dynamics. We will consider modern dynamical decoupling techniques which aim at preserving coherence of quantum spins, and the experimental implementation of the decoupling protocols, as well as more advanced quantum control of NV spins. Using a variety of analytical and numerical tools, we can characterize and optimize the factors which limit our control over these quantum spin systems [2]. We will also examine how the quantum control approaches can be used to elucidate the quantum dynamics of NV centers and the properties of the spin bath [3]. \\[4pt] [1] G. D. Fuchs, V. V. Dobrovitski, D. M. Toyli, F. J. Heremans, and D. D. Awschalom, Science 326, 1520 (2009).\\[0pt] [2] V. V. Dobrovitski, G. de Lange, D. Riste, and R. Hanson, Phys. Rev. Lett. 105, 077601 (2010).\\[0pt] [3] G. de Lange, Z. H. Wang, D. Riste, V. V. Dobrovitski, and R. Hanson, Science 330, 60 (2010).   

Session V4: Dynamics of Polymers: Phenomena Due to Confinement

Direct measurement of molecular motion in freestanding polystyrene thin films

An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding polystyrene films as thin as 14 nm. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with differing dynamics. While the slow subset shows bulk-like dynamics, the more mobile subset reorients within a few hundred seconds even at Tg- 25 K (Tg is the glass transition temperature of bulk polystyrene). At Tg -5 K, the mobility of these two subsets differs by 4 orders of magnitude. These data are consistent with the presence of a high mobility layer at the film surface whose thickness is independent of polymer molecular weight and total film thickness. The thickness of the mobile surface layer increases with temperature and equals 7 nm at Tg.   

Molecular dynamics at nanometric length-scales

Broadband Dielectric Spectroscopy, Spectroscopic vis-Ellipsometry, X-Ray Reflectometry, Alternating and Differential Scanning Calorimetry are combined to study glassy dynamics and the glass transition in nanometric thin ($\ge $5 nm) layers of polystyrene (PS) having widely varying molecular weights and Polymethylmethacrylate (PMMA) deposited on different substrates. For the dielectric measurements two sample geometries are employed, the common technique using evaporated electrodes and a recently developed approach taking advantage of nanostructures as spacers. \textit{All} applied methods deliver the concurring result that deviations from glassy dynamics and from the glass transition of the bulk never exceed margins of $\pm $3 K \textit{independent} of the layer thickness, the molecular weight of the polymer under study and the underlying substrate. Our findings are discussed in the context of the highly controversial literature and prove that an appropriate sample preparation is of paramount importance in order to avoid artefacts. \\[4pt] [1] Erber et al., \textit{Macromolecules} \textbf{43}, 7729 (2010).\\[0pt] [2] Mapesa et al., \textit{Europ. Phys. J. - Special Topics} \textbf{189}, 173-180 (2010).\\[0pt] [3] Tre{\ss} et al., \textit{Macromolecules} (2010). DOI:10.1021/ma 102031k.   

Gas Permeation in Thin Glassy Polymer Films

The development of asymmetric and composite membranes with very thin dense ``skins'' needed to achieve high gas fluxes enabled the commercial use of membranes for molecular level separations. It has been generally assumed that these thin skins, with thicknesses of the order of 100 nm, have the same permeation characteristics as films with thicknesses of 25 microns or more. Thick films are easily made in the laboratory and have been used extensively for measuring permeation characteristics to evaluate the potential of new polymers for membrane applications. There is now evidence that this assumption can be in very significant error, and use of thick film data to select membrane materials or predict performance should be done with caution. This presentation will summarize our work on preparing films of glassy polymers as thin as 20 nm and characterizing their behavior by gas permeation, ellipsometry and positron annihilation lifetime spectroscopy. Some of the most important polymers used commercially as gas separation membranes, i.e., Matrimid$^{\mbox{{\textregistered}}}$ polyimide, polysulfone (PSF) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), have been made into well-defined thin films in our laboratories by spin casting techniques and their properties studied using the techniques we have developed. These thin films densify (or physically age) much faster than thicker films, and, as result, the permeability decreases, sometimes by several-fold over weeks or months for thin films. This means that the properties of these thin films can be very different from bulk films. The techniques, interpretations and implications of these observations will be discussed. In a broader sense, gas permeation measurements can be a powerful way of developing a better understanding of the effects of polymer chain confinement and/or surface mobility on the behavior of thin films.   

Surface Dynamics in Glass forming Materials

There is mounting evidence that the surface of glassy polymers exhibits enhanced dynamics compared to the bulk material at the same temperature. Using nanoparticle embedding and relaxation of nanodeformations on the surface, we have developed a detailed characterization of the dynamics of glassy polymers (polystyrene (PS), isotactic-poly(methyl methacrylate) ). This includes the effects of temperature, molecular weight and film thickness on the surface dynamics. We have extended the studies to the molecular glass former TNB, which display striking similarities to PS. The results of these studies allow us to begin to develop an understanding of the surface properties of glassy material, and how these properties may lead to observed changes in thin film polymer properties.\\[4pt] Work done in collaboration with Chad Daley and Dongping Qi, University of Waterloo; Zahra Fakhraaim, University of Pennsylvania; and Mark Ilton, University of Waterloo.   

Confinement effects on polymer structure and glassy dynamics

We have performed molecular-dynamics simulations to explore the influence of confinement on the glass-transition temperature $T_{g}$ for supported atactic-polystyrene thin films of different thickness (1 nm $\div $ 10 nm) and different strengths of attraction to the substrate (0.1 kcal/mol $\div $ 3.0 kcal/mol). The films have been equilibrated in a melt at 540 K and further cooled down with a constant cooling velocity of 0.01 K/ps below $T_{g}$ to room temperature, 300 K. Based on the density measurements we have defined three different (substrate, middle and surface) layers for each film. We found that the monomers close to the surface and in the substrate layer are partially oriented, which leads to more effective monomer packing. For the whole film the average $T_{g}$ value remains almost constant for films down to 2 nm thickness, where middle layer vanishes. For the middle layer itself $T_{g}$ does not depend on the total film thickness, while an increase up to 70 K is measured for the substrate layer depending on the strength of attraction to the actual substrate. The surface layer remains liquid-like in the whole temperature range (300 K $\div $ 540 K). We claim that the redistribution of mass in the three film layers may explain the change with film thickness of the average $T_{g}$, if the latter is determined from linear fits of the average glass and melt densities. First results on the shear cycling and the rejuvenation phenomena will be discussed as well.   

Session V5: Physics for Everyone

Biomagnetism and Magnetotaxis in Bacteria: What Bacteria Know About Magnetic Materials and Permanent Magnet Design

Magnetotactic bacteria (mtb) migrate along geomagnetic field lines, i.e., they behave like self-propelled magnetic compass needles. Mtb make single-magnetic-domain crystals of magnetite (Fe$_{3}$O$_{4})$ and greigite (Fe$_{3}$S$_{4})$ in intracellular structures called magnetosomes. The magnetosomes are arranged in linear chains that comprise permanent magnetic dipoles with remanent moments approaching the saturation moment, causing the mtb to be oriented in the geomagnetic field as they swim. This allows them to keep their heading and efficiently migrate to, and remain in, a preferred, microaerobic, aquatic habitat. The mtb have solved the difficult problem of designing a permanent magnet that is sufficiently robust to cause the cell to be oriented in the geomagnetic field at ambient temperature, yet fit inside a micron-sized object, and be assembled \textit{in situ} from potentially toxic materials scavenged from the environment. I will describe some recent advances in mtb genetics that illuminate the process by which they make and arrange their magnetosomes.   

Voodoo Science

A remarkable scientific result that appears to violate natural law may portend a revolutionary advance in human knowledge. It is, however, more likely an experimental screw up. Error is normal; it can be reduced by repeating measurements and better design of controls, but the success and credibility of science is anchored in a culture of openness. Ideas and observations are freely exposed to independent testing and evaluation by others. What emerges is the book of nature. On its pages we find, if not a simple world, at least an orderly world, in which everything from the birth of stars to falling in love is governed by the same natural laws. These laws cannot be circumvented by any amount of piety or cleverness, they can be understood - with the possible exception of String Theory. For those who elect to work outside the scientific community, errors may go unrecognized. We will examine examples of this, including claims of perpetual motion and cancer caused by cell-phone radiation.   

M&Ms Packing and Research with Undergrads

This abstract not available.   

Science and Cooking: Motivating the Study of Freshman Physics

This talk will describe a course offered to Harvard undergraduates as a general education science course, meant to intrduce freshman-level science for non-science majors. The course was a collaboration between world-class chefs and science professors. The chefs introduced concepts of cooking and the professors used these to motivate scientific concepts. The lectures were designed to provide a coherent introduction to freshman physics, primarily through soft matter science. The lectures were supplemented by a lab experiments, designed by a team of very talented graduate students and post docs, that supplemented the science taught in lecture. The course was very successful in motivating non-science students to learn, and even enjoy, basic science concepts.   

Making a frothy shampoo or beer

The terms ``foam'' and ``froth'' refer to a dispersion of gas bubbles in a liquid. Why do certain liquids show a tendency to foam while others do not? For example, bubbles can be produced in pure water by vigorous agitation, but then they rapidly coalesce and disappear. While foams cannot be produced with pure water, foams associated with beer or shampoo can persist for several minutes or even hours. What ingredient(s) in shampoo and beer make their foams stable, and what physical concepts control their stability? In this talk I'll review three basic mechanisms underlying foam stability, and I'll make connection with current research on coarsening by the diffusion of gas from smaller to larger bubbles.   

Session V6: Hybrid Functionals Applied to Solids

The shortcomings and advantages of hybrid functionals in the description of solids

For extended systems, density functional theory currently possesses the optimal balance between computational efficiency and accuracy. Hence its wide spread acceptance and application in quantum chemistry, materials science and computational catalysis is in no way astonishing. However, it is also well known that standard density functionals underestimate the band gap in particular for small gap systems, and resultantly structural properties related to the band gap are difficult to predict. In quantum chemistry this has long be realized, and hybrid functionals mixing Hartree-Fock and local density functionals are generally preferred over purely local functionals. Such functionals usually predict a much larger band gap than purely local density functionals. With the exception of the Crystal code, hybrid functionals were not available in popular periodic codes, and only recently successful implementations and applications using plane wave based codes, such as VASP, were reported. In this talk, I will survey the expertise we have acquired for hybrid functionals for extended systems. For simple prototypical materials, such as metals, semiconductors, and insulators many ground state properties are indeed found to be in much better agreement with experiment than using purely local functionals. This concerns lattice constants and bulk moduli, but phonon dispersion relations are also often significantly improved, in particular, for those semiconductors where DFT predicts no band gap. Some specific applications will be discussed in detail. This includes the description of (small) polarons in semiconductors, the properties of ferroelectric materials (specifically BaTiO$_3$ and SrTiO$_3$), phonon dispersion relations in the group IV elements, and magnetic impurities in semiconductors.   

The effects of long-range exact exchange in hybrid-functional methods

Over the past several years, hybrid density functional theory, which mixes a fraction of exact exchange into conventional semilocal exchange-correlation functionals, has become the dominant method used in molecular electronic structure calculations. While hybrid functionals are responsible for many successes of modern Kohn-Sham theory, they suffer from several drawbacks as well. The rapid decay of semilocal exchange causes errors in describing many phenomena, including charge transfer and Rydberg excitations, polarizabilities of long chains, and several others. Further, different properties seem to require different amounts of exact exchange; calculations near equilibrium, for example, appear to require less exact exchange than do calculations far from equilibrium. Including long-range exact exchange in finite systems improves accuracy in quantities sensitive to the long-range potential and makes it possible to treat systems both near and away from equilibrium on a fairly even footing. This talk discusses the motivations for including long-range exact exchange, as well as some remarkable successes and notable failures caused by doing so. Alternatives which attempt to retain most of the successes while eliminating most of the failures are also discussed.   

Structure of oxygen vacancies and electron localization on CeO$_2$(111)

In the many applications of ceria-based materials in heterogeneous catalysis, the reducibility of ceria is essential to the catalytic function. Thus, having a theoretical approach that is able to describe the changes in the oxidation state of the multivalent cerium atoms appears desirable. The use of density functional theory with hybrid functionals is shown to be adequate [1]. It has been generally accepted that the electrons left behind upon oxygen removal from CeO$_2$ surfaces, driving the Ce$^{4+}$$\rightarrow$Ce$^{3+}$ reduction, localize on cationic sites in next-neighbor distance to the defect. We apply density-functional theory (DFT) with the HSE06 hybrid functional as well as the DFT+U approach and predict that vacancies on CeO$_2$(111) are likely to be bound to Ce$^{4+}$ ions rather than to Ce$^{3+}$ as priorly suggested. This prediction has been recently confirmed by means of STM imaging and spectroscopy [3]. We further find that subsurface vacancies are energetically preferred when compared to surface vacancies by up to 0.5 eV, and thus provide support for the most recent experimental result [4]. Defect-induced lattice relaxations are crucial to the electron localization on more distant cation sites to the defect and to the subsurface preference.\\[4pt] [1] J. L. F. Da Silva, M. V. Ganduglia-Pirovano, J. Sauer, V. Bayer, G. Kresse, Phys. Rev. B 75, 045121 (2007)\\[0pt] [2] M. V. Ganduglia-Pirovano, J. L. F. Da Silva, J. Sauer, Phys. Rev. Lett. 102, 026101 (2009).\\[0pt] [3] J. F. Jerratsch, X. Shao, N. Nilius, H-J Freund, C. Popa, M. V. Ganduglia-Pirovano, J. Sauer, unpublished.\\[0pt] [4] S. Torbr\"ugge et al., Phys. Rev. Lett. 99, 056101 (2007).   

Orbital-dependent functionals in FLAPW: hybrid functionals and optimized effective potentials

Orbital-dependent functionals are a new class of exchange-correlation (xc) functionals for density-functional theory. Hybrid functionals combine a local or semi-local xc functional with a nonlocal orbital-dependent exchange functional and improve the band gaps of semiconductors and insulators as well as the description of localized states. As an alternative to nonlocal hybrid potentials, one can also construct local optimized effective potentials (OEP) from the exact exchange (EXX) functional. So far, most implementations for periodic systems use a pseudopotential plane-wave approach. We present an efficient all-electron, full-potential implementation of the PBE0 [1] and HSE [2] hybrid functionals as well as the OEP-EXX functional [3] within the FLAPW method ({\tt Fleur} code: www.flapw.de). Results for prototype semiconductors and insulators are in very good agreement with other implementations. We will demonstrate the improvement over conventional local or semilocal functionals for oxide materials and focus in particular on systems where standard functionals yield qualitatively wrong results. In particular, we will discuss the geometric and magnetic structures of EuO and GdN. Additionally, we will address the possibility of using the hybrid-functional ground state as starting point for a $GW$ quasiparticle correction [4] and show results for complex perovskite systems. \\[4pt] [1] M. Betzinger, C. Friedrich, and S. Bl\"ugel, Phys. Rev. B 81, 195117 (2010). \\[0pt] [2] M. Schlipf, M. Betzinger, C. Friedrich, M. Lezaic, and S. Bl\"ugel, inpreparation. \\[0pt] [3] M. Betzinger, C. Friedrich, S. Bl\"ugel, and A. G\"orling, to be published in Phys. Rev. B. \\[0pt] [4] C. Friedrich, S. Bl\"ugel, and A. Schindlmayr, Phys. Rev. B 81, 125102 (2010).   

Hybrid functional studies of InGaN alloys and oxides for photochemical watersplitting

Photochemical watersplitting can potentially be a future sustainable energy source, converting sunlight and water into hydrogen. However, in order to have highly efficient devices materials are needed that absorb a large proportion of the solar spectrum while at the same time having valence and conduction bands that straddle the hydrogen and oxygen evolution redox potentials. It is well known that DFT consistently underestimates the band gap (the so-called ``band-gap problem''). As a consequence, the positions of the valence and conduction bands (and hence the band offsets) also suffer from uncertainties. To address these deficiencies of the local density approximation (LDA) and generalized gradient approximation (GGA) we use the HSE exchange correlation functional in order to accurately calculate the electronic band structure [1]. We will discuss bowing effects in InGaN alloys based on accurate calculation of band gaps of InGaN alloys and on an analysis of experimental results using our calculated deformation potentials to disentangle the effect of strain and alloying on the band gap. We will also discuss calculations of the absolute position of the valence band maximum and the conduction band minimum. Including a discussion and comparison with generalized gradient and local density approximations results. Finally we show that HSE may be used to understand the nature of surface defects. \\[4pt] [1] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003)   

Session V7: High Resolution Tunneling Spectroscopy of Dirac Fermions

Strain-Induced Pseudo--Magnetic Fields in Graphene: MegaGauss in Nanobubbles

Recent theoretical proposals suggest that strain can be used to modify graphene electronic states through the creation of a pseudo--magnetic field. This effect is unique to graphene because of its massless Dirac fermion-like band structure and particular lattice symmetry (C3v). Scanning tunneling microscopy shows that graphene grown on a platinum (111) surface forms nanobubbles, which are highly strained due to thermal expansion mismatch between the film and the substrate. We find that scanning tunneling spectroscopy measurements of these nanobubbles exhibit Landau levels that form in the presence of strain-induced pseudo--magnetic fields greater than 300 Tesla. This demonstration of enormous pseudo--magnetic fields opens the door to both the study of charge carriers in previously inaccessible high magnetic field regimes and deliberate mechanical control over electronic structure in graphene or so-called ``strain engineering''. \\[4pt] In collaboration with S. A. Burke$^{\S ,2}$, K. L. Meaker$^{2}$, M. Panlasigui$^{2}$, A. Zettl$^{2,3}$, F. Guinea$^{4}$, A. H. Castro Neto$^{5}$ and M. F. Crommie$^{2,3}$. {\S}. Present address: Department of Physics and Astronomy and Department of Chemistry, University of British Columbia, Vancouver, BC V6T 121, Canada. 2. Department of Physics, University of California, Berkeley, CA 94720, USA. 3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. 4. Instituto de Ciencia de Materiales de Madrid (CSIC), Madrid 28049, Spain. 5. Department of Physics, Boston University, Boston, MA 02215, USA.   

Scanning tunneling microscopy studies of topological insulators grown by molecular beam epitaxy

I will summarize our recent activities of using scanning tunneling microscope (STM) to study topological insulators grown by molecular beam epitaxy (MBE). The Landau quantization in three-dimensional topological insulators was directly observed in the tunneling spectra. In particular, we discovered the zeroth Landau level, which is predicted to give rise to the half-quantized Hall effect for the topological surface states. The existence of the discrete Landau levels and the suppression of Landau levels by surface impurities strongly support the 2D nature of the topological states. In addition, we studied the quantum interference pattern formed by the topological surface states near the step edges and magnetic impurities in Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$. The decay behavior of the standing waves is in good agreement with the Dirac cone structure of the topological surface states. We show that the combination of MBE and high energy resolution scanning tunneling spectroscopy provides a powerful~way to probing the novel physics in the topological insulators.   

Spatially Resolved Spectroscopy of Magnetic States in Epitaxial Graphene

Graphene grown epitaxially on silicon carbide provides a potential avenue toward industrial-scale graphene electronics. A predominant aspect of the multilayer graphene produced on the carbon-terminated ($000\bar{1}$) face of SiC is the rotational stacking faults between graphene layers and their associated moire-pattern superlattice. We use scanning tunneling microscopy (STM) and spectroscopy (STS) in high magnetic fields to obtain detailed information about the massless Dirac fermions that carry charge in graphene. In agreement with prior investigations, we find that for small magnetic fields, the rotational stacking effectively decouples the electronic properties of the top graphene layer from those below. However, in maps of the wavefunction density at magnetic fields above 5 Tesla, we discover atomic-scale features that were not previously known or predicted. A phenomenological theory shows that this high-field symmetry-breaking is a consequence of small cyclotron-orbit wavefunctions, which are sensitive to the local layer stacking internal to the moire superlattice cell.   

Landau-level spectroscopies of a topological insulator

Topological insulators such as Bi$_2$Se$_3$ are characterized by massless Dirac surface state which would give rise to unique quantum phenomena in a magnetic field. Although it was experimentally verified by many ARPES experiments that the surface electrons are indeed massless, there has been a lack of studies exploring their quantum properties due to the inevitable contribution from the bulk electrons in a real material. Using surface-sensitive STM/STS technique, we selectively probed the surface massless electrons in Bi$_2$Se$_3$. Under magnetic field perpendicular to the cleaved surface, a series of Landau levels (LLs) has been observed in the tunneling spectra. Remarkably, there is a field-independent LL at the Dirac point, which is a hallmark of Dirac fermions. We developed a scaling analysis scheme of LLs based on the Bohr-Sommerfeld quantization condition which allowed us to determine the energy-momentum dispersion of the surface state~[1]. Width of the LL peaks in the spectra becomes smaller near the Fermi energy, which may suggest that electron-electron correlation plays a role. In addition to the narrowing of LLs, the spectra near the Fermi energy exhibit complicated fine structures, which may be responsible for the anomalous magneto-fingerprint effect~[2]. This work has been done in collaboration with K. Igarashi, M. Kawamura, H. Takagi and T. Sasagawa.\\[4pt] [1] T. Hanaguri {\it et al.}, Phys. Rev. B {\bf 82}, 081305(R) (2010).\\[0pt] [2] J. G. Checkelsky {\it et al.}, Phys. Rev. Lett. {\bf 103}, 246601 (2009).   

Scanning Tunneling Spectroscopy of a Gated Single- and Bilayer Graphene Devices in the Quantum Hall Regime

We have performed scanning tunneling spectroscopy (STS) measurements on a gated single- and bilayer graphene devices. The combination of STM/STS capability and an electrostatic back gate enables us to investigate the interactions of Dirac particles with local impurities at the atomic scale in zero magnetic field and in the quantum Hall regime while varying the Fermi-energy with respect to a Dirac (charge neutrality) point. In an applied magnetic field, well-resolved Landau levels (LLs) following the Dirac particle scaling are observed in both single- and bilayer graphene. Additionally, in single layer graphene, spatial dispersion of LLs caused by disorder potential lead to formation of graphene quantum dots (QDs). The tunneling spectra measured as a function of gate and sample biases are governed by Coulomb blockade physics. In contrast, no QDs are seen in bilayer devices. Instead, the main feature of the spectra is an energy gap formed around the charge neutrality point. The possible origin of energy gap will be discussed in a context of broken layer symmetry caused by gate electric field and disorder potential variation. Other noticeable features in the tunneling spectra of single- and bilayer graphene such as the formation of electron- and hole-puddles and the Fermi-level pinning effect will be discussed.   

Session V8: Enhancing Graduate Education in Physics: Focus on Skills

Teaching graduate students The Art of Being a Scientist

Graduate education in the classroom traditionally focuses on disciplinary topics, with non-disciplinary skills only marginally discussed, if at all, between graduate student and adviser. Given the wide range of advisers with different types and quality of communication skill (or lack thereof), the professional coaching delivered to students often is restricted to just the technical aspects of research. Yet graduate students have a great need to receive professional training aimed at, among other things, helping their graduate career be more efficient, less frustrating and less needlessly time-consuming. We have addressed this gap in graduate education by developing the one-credit course ``The Art of Being a Scientist.'' This course covers a diverse range of topics of importance to being an effective and creative researcher. Topics covered include the following: What is science? Choosing a research topic, department, and adviser. The adviser and thesis committee. Making a work plan. Setting goals. Ethics of research. Using the scientific literature. Perfecting oral and written communication. Publishing papers and writing proposals. Managing time effectively. Planning a scientific career. Applying for jobs in academia or industry. In evaluations of the course, students invariably comment that they could have avoided significant problems in their graduate study and saved valuable time if they would have taken the course earlier on. This is an indication that the course not only useful for students, but also that it is best taken early in a their graduate career. The material covered in the course is captured in the book ``The Art of Being a Scientist: A Guide for Graduate Students and Their Mentors,'' published by Cambridge University Press; more information can be found at: {\tt www.mines.edu/$\sim$rsnieder/Art\_of\_Science.html} From this website one can download a description of the curriculum used in the class, including homework exercises. Currently we are expanding of professional education by offering more lectures and workshops in order to better prepare graduate students for a career in science.   

Got Skills? On-the-Job Activities of Physicists

It goes almost without saying that physics doctorates do a lot more than just physics research or teaching at their jobs. But what exactly do they do? First, I will share basic data showing where physics doctorates are employed. Then I will present data from two of AIP's surveys about the employment of physicists. The first set of data comes from our survey of physics PhDs one year after doctorate. We will consider how often physics doctorates do a variety of activities on the job, including management, technical writing, teamwork, design and development, programming, and advanced mathematics. The second set of data comes from AIP's new survey of PhDs in physics 10 to 13 years after graduation. Data for many of the same activities will be shown for physics doctorates who have been in the workplace about a decade. Depending on the type of job, most industrially employed physics doctorates do some type of physics at work, but they are also very likely to report managing projects, writing for technical audiences, working on a team, and collaborating with non-physicists, among many other activities. This examination of the types of activities physics doctorates perform in the workplace will provide insight on the non-scientific training that would benefit graduate students the most.   

Communication and Critical Thinking Skills

This talk will discuss how faculty can help graduate students (and even postdocs) improve non-technical professional skills required for success in scientific careers. Examples to be covered will include a) planning and delivering high-quality presentations b) listening critically to others' presentations c) writing grant proposals, cover letters, and CV's d) reviewing manuscripts and responding to referee reports. The faculty member(s) involved must be prepared to project a welcoming attitude, to convey the importance of these skills, and to make a consistent investment of time.   

Tuning Higher Education

In April 2009, the Lumina Foundation launched its Tuning USA project. Faculty teams in selected disciplines from Indiana, Minnesota, and Utah started pilot Tuning programs at their home institutions. Using Europe's Bologna Process as a guide, Utah physicists worked to reach a consensus about the knowledge and skills that should characterize the 2-year, batchelor's, and master's degree levels. I will share my experience as a member of Utah's physics Tuning team, and describe our progress, frustrations, and evolving understanding of the Tuning project's history, methods, and goals.   

Shedding light on molecular dynamics: The role of physicists in the age of biomedical science

Fundamental discoveries of the physics of imaging in the areas of microscopy, MRI, and CCD image sensing have produced innovations throughout the 20th century and continuing into the 21st. Not only have these fundamental discoveries received recognition from the Nobel Foundation in 1953, 1986, 1986, 2003, and 2009, but they have also revolutionized basic interdisciplinary research in areas such as biophysics and biomedical physics to the point at which applied physicists, engineers, and medical clinicians are working together to design experiments and develop tools for use in a broad range of areas including clinical diagnosis and pharmaceutical clinical trials. In this presentation, I will describe several innovative approaches in physics combined with engineering that have revolutionized the frontier in the biomedical sciences. Specifically, I will present examples of basic research as well as design, development, and commercialization of photonics research in the biomedical area within the context of biophotonics and molecular imaging. These examples will include the use of optical, photonics, and imaging techniques to (1) understand and elucidate the fundamental physics and chemistry of biological functions; and (2) understand and describe the critical role of these techniques for disease diagnosis, prognosis, prevention, and treatment with novel noninvasive (or minimally invasive) procedures.   

Session V9: Self Assembly I

Effects of cluster diffusion on the island density and size-distribution in submonolayer island growth

The effects of cluster diffusion on the submonolayer island density and island-size distribution (ISD) $N_s(\theta)$ (where $N_s(\theta)$ is the number of islands of size $s$ at coverage $\theta$) are studied for the case of irreversible submonolayer growth of compact islands on a 2D substrate. In our model, monomers are deposited with deposition rate $F$ while the mobility $D_s$ of an island of size $s$ satisfies $D_s \sim s^{-\mu}$. Results are presented for $\mu = 1/2$ (corresponding to Brownian motion) as well as for higher values of $\mu$. In general, we find that the exponents describing the flux-dependence of the island and monomer densities vary continuously as a function of $\mu$. For $\mu < 1$ we also find that the ISD exhibits power-law behavior up to a cross-over size $S_c$. However, the values of the corresponding exponents are significantly larger than previous theoretical predictions. A generalized scaling form for the ISD for $\mu < 1$ is also proposed which leads to excellent scaling of the entire distribution. In contrast, for $\mu \ge 1$ we find that, due to a competition between size-scales, neither our generalized scaling form nor the standard scaling form $N_s(\theta) = \theta/S^2~f(s/S)$ (where $S$ is the average island-size) lead to scaling of the entire ISD. Instead, the scaled ISD becomes more sharply peaked with increasing $D_1/F$ and coverage. This is in contrast to models with limited cluster mobility for which good scaling occurs over a wide range of coverages and $D_1/F$.   

Rate-equation approach to irreversible island growth with cluster diffusion

A self-consistent rate-equation (RE) approach to irreversible island growth and nucleation is presented which takes into account the effects of cluster mobility. As a first application we consider the irreversible growth of compact islands on a 2D surface in the presence of monomer deposition (with rate $F$) and monomer diffusion (with rate $D_1$) while the mobility of an island of size $s$ is assumed to satisfy $D_s = D_1 s^{-\mu}$ where $\mu \ge 0$. For coverages up to the peak island-density, we find excellent agreement between our RE and simulation results for the dependence of the island-density $N(\theta)$ on coverage $\theta$ for all values of $\mu$ considered, ranging from $\mu = 1/2$ (Brownian motion) to $\mu = \infty$ (immobile clusters). For $\mu \le 2$, excellent agreement is also found between our simulation and RE results for the island-size distribution (ISD), while for higher values of $\mu$ the effects of correlations become important. We also demonstrate that the discrepancies between recent theoretical predictions for the exponents $\tau(\mu)$ and $\zeta(\mu)$ describing the size-dependence of the ISD for $\mu < 1$ and simulations can be explained by the geometry of compact islands. Our self-consistent RE approach may also be generalized to higher dimensions as well as to an arbitrary dependence of the cluster mobility on island-size.   

Kinetics and Thermodynamics of the Association of DNA Coated Colloids

We have investigated the aggregation kinetics and thermodynamics of complementary DNA coated particles as a function of DNA coverage. The streptavidin on our particles can accommodate 69800 biotinalated DNA which has 50 base pair double strands and 11 base sticky ends. For full 100\% coverage, the melting temperature, $T_m$, is 50.3 C. The transition width, $\Delta T$, is 0.8 C, and the characteristic aggregation time, $\tau$, is 4 minutes. For 2.5\% (40 times less) coverage $T_m$ = 22 C, $\Delta T$ = 5 C, and $\tau$ = 11 hours. A simple model which takes into account the number of DNA bonds and the multiplicity of their arrangements accounts for the full time and temperature dependence of the particle aggregation.   

DNA driven 2D Assembly of Nanoparticles on Lipid Surfaces

Use of biomolecular linkers such as DNA due to its sequence-specific hybridization properties provides a versatile platform for assembly of nanoscale components. Here we investigated the DNA-based self-assembly of gold nanoparticles in 2D using lipid layer as fluid substrate. We examined the effect of lipid composition by vary the fraction of cationic and zwitterion lipids on formation of a particle monolayer. Using in-situ X -ray reflectivity we observed adsorption of DNA functionalized nanoparticles on charged lipid surfaces. The surface density of the particle monolayer can be tuned by changing the electrostatic interaction between the particles and the lipid surface. The in-situ measured particle desorption from the lipid surface due to a change of a salt concentration provides quantitative information on particle-surface interactions. The ex-situ studies on samples using XPS under similar conditions support our observations. Our studies explore the possibility to form regulated 2D systems, as well as provide basic understanding of interactions of charge nano-objects with lipids, which is important for the biomedical applications.   

Computational Analysis of DNA-Mediated Crystallization of Binary Colloidal Superlattices

Colloidal self-assembly provides a potential avenue for the design of novel devices with unique optical and structural properties. Colloidal systems also provide useful insights into fundamental mechanisms of phase transitions such as crystal nucleation, growth and melting that are otherwise difficult to probe in atomic systems. A promising approach for realizing highly tunable colloidal assembly is to graft single-stranded DNA oligomer brushes onto the surfaces of particles in order to create attractive interactions between them. Using this approach, micro- and nanoscale particles have now been successfully assembled into several crystalline phases, including ordered, binary superlattice structures. Here, we apply Monte Carlo simulations and free energy calculations to generate a detailed picture for the assembly binary superlattice crystals. The interparticle potential used to perform the calculations was generated specifically for DNA-mediated interactions and verified by measurements. We develop a pseudo-phase diagram for the binary superlattice system which includes both thermodynamic and kinetic influences. The predictions of the pseudo-phase diagram are validated using direct simulations of crystal nucleation. Finally, we discuss recent findings related to diffusionless transformations in growing superlattice crystals that may be important in experiments aimed at growing these structures.   

DNA Linker Mediated Assembly of Gold Nanoparticles Superlattice

A BCC (body-centered-cubic) crystalline phase forms when flexible ssDNA linkers are added to the mixture of two types of dispersed, ssDNAs capped gold nanocolloids which are mutually non-complementary but complementary to the respective ends of the linker DNA. The state diagram of DNA linker mediated nanoparticle assemblies has been experimentally investigated and constructed by using in-situ small angle x-ray scattering. The optically active three-dimensional superlattice containing plasmonic particles and DNA-encoded chromophors were further fabricated using this approach. We investigated structural tunability and corresponding optical response of the multicomponent superlattices.   

Directed Self-Assembly of Colloidal Particles

In nature, simple constituents like atoms, molecules and polymer chains, spontaneously organize into larger, higher order structures. Interactions involved in this self-assembly act on a local level. These facts inspire experimental and theoretical engineering of components able to organize into pre-designed complex systems. We perform numerical simulations of collections of DNA coated colloidal particles. We test different design rules for self-assembly with short-range interactions and explore the stability of equilibrium structures.   

Replication of nanoscale DNA patterns

We present an artificial supramolecular system mimicking self- replication and information transmission strategies in nature, but without the aid of enzymes or equivalent biological mechanisms. Using DNA nanotechnology techniques, we can make DNA tiles with selective interactions based on complementary single-strand connections. A linear tile pattern distinguished by their connector sequences is transmitted to a subsequent generation of copies by connector hybridisation. Longitudinal pattern formation and transverse copy attachment are well separated by different melting temperatures. We have achieved a faithful transmission of the pattern information to the second replication generation. We use AFM imaging to test for pattern fidelity and gel electrophoresis for quantitative yield analysis.   

Designing colloids for alignment

Inducing the spontaneous association of microscopic building blocks into macroscopic structures has been a promising way to create new materials for a variety of useful applications. Such fabrication processes typically require interactions between microscopic building blocks. The interactions that govern the assembly of these microscopic building clocks: electrostatic, magnetic, Van der Waals, depletion, and DNA interactions, are all currently being investigated. For all these cases, the attractive energy between the particles is proportional to the overlapping surface between the colloids. Controlling the positions and orientations of the microscopic building blocks is a critical issue in such processes. To date there has been no efficient or reliable process that enables such spontaneous assembly of building blocks. For the successful alignment of any particles that we desire to self-assemble, a shape with unique physical and mathematical properties must be identified. Under the assumption that energy is reduced in proportion to area overlap, we present a geometrical shape which, when encountering a similar shape from any initial configuration, is forced into a single relative orientation maximizing the overlap. The unique minimum of energy in the energy landscape drives the particles to self-assemble in a controlled orientation.   

Self-assembly of Nanoparticles into Planar Modulated Superstructures

The advance in the synthesis of nanoparticles and colloids opens up the possibility to use them as building blocks for self-assembling novel materials. Ordered structures are especially interesting because they have unique photonic and electronic properties. Among the most complex ordered phases are commensurately and incommensurately modulated crystals. Although frequently found on the atomic scale in the bulk and as ordered structures of noble gases in adsorbed layers, modulated phases have so far not been known to self-assemble with nanoparticles. Here, we use computer simulations to study a two-dimensional system characterized by a simple isotropic interaction that could be realized in future with building blocks on the nanoscale. We find that the particles arrange themselves into planar hexagonal superstructures whose superlattice vector can be tuned reversibly by changing the temperature. Thermodynamic stability is confirmed by calculating the free energy with a combination of thermodynamic integration and the Frenkel-Ladd method. Different contributions to the free energy difference are discussed.   

Programmable, directed assembly of micron-scale components

Self assembly is a nascent paradigm for assembling components in the micron to millimeter size range. Such assemblies are often performed by modifying the surface chemistries of the individual components or by creating flow fields directing them into position. We propose a method of directed assembly using dielectric contrast between the components and a surrounding fluid. A hybrid integrated-circuit / microfluidic device\footnote{Thomas Hunt, David Issadore, Robert Westervelt ``Integrated Circuit/Microfluidic Chip to Programmably Trap and Move Cells and Droplets with Dielectrophoresis'' \textit{Lab on a Chip 8}, 81-87 (2008)} will be used to trap and manipulate pieces into pre-defined patterns. The device contains an array of electrically-chargeable pixels on its surface, with a resolution of 10 $\mu $m.   

Synthesis and Evaporative Self-Assembly of Polystyrene Nanotubes under Confinement

Synthesis and manipulation of anisotropic building blocks into ordered structures has attracted increasing attention in recent years as nanowires and nanotubes (NWs/NTs) show great potential in many emerging technologies such as novel electric devices, optical units and biosensors. Here we use evaporation to align polystryrene NWs/NTs into distinct and interesting patterns. We synthesized polystyrene (PS) NWs/NTs of varied aspect ratio using anodic aluminum oxide (AAO) templates (200 nm pore size) using a melt-wetting technique. The template was removed, and NWs/NTs of controllable length ranging from several hundred nanometers to a few micrometers were released from the bulk PS film under ultrasonication. We further investigate the evaporative self-assembly of the synthesized polystyrene NTs under confined and ``open'' geometries and observe the alignment and assembled structures of the polystyrene NTs using scanning electron microscopy. Confocal laser scanning microscopy was also used to monitor the kinetics of the alignment process during evaporation. Results indicate that many factors (solvent, aspect ratio) contribute to the degree of NW/NT alignment relative to the evaporation front.   

Dynamic self-assembly of chemically-propelled nanoscale building blocks

Self-assembly technique offers spontaneous, massively-parallel structure formation from bottom-up. So far, most research efforts have been focused on static self-assembly that is thermally driven towards a thermodynamic equilibrium. Less attention has been paid to dynamic self-assembly that evolves to a non-equilibrium steady state under a dissipative driving force. This project aims to investigate the non-equilibrium self-assembly behaviors of chemically-propelled nanoscale building blocks via molecular dynamics simulations. We utilize a catalytic building block, that has been shown, when isolated, to exhibit self-motile behavior when immersed in a fuel environment. Upon increasing the number density of the building blocks, interesting collective behaviors emerge due to direct interactions between the building blocks or indirect interactions via the fuel environment. The simulation system is also subjected to an artificial operation of converting products back to fuel molecules. The heat generated by the exothermic chemical reaction will also be removed. In this way, a steady-state, as well as the resulting dynamic self-assembly pattern, can be obtained.   

Dynamic Self-Assembly and Self-Propulsion in Nonequilibrium Magnetic Colloidal Ensembles at a Liquid/Liquid Interface

Ensembles of interacting particles subject to external periodic energy fluxes often develop nontrivial dynamics. Magnetic colloidal particles suspended over an interface of two immiscible liquids and energized by vertical alternating magnetic fields give rise to novel dynamic self-assembled structures (``asters'') which are not accessible at the liquid/air interfaces. Ferromagnetically ordered nickel spherical particles have been used in our experiments. Novel structures are attributed to the interplay between surface waves, generated at the liquid/liquid interface by the collective response of magnetic microparticles to the alternating magnetic field, and hydrodynamic fields induced in the boundary layers of \textit{both} liquids forming the interface. Two types of magnetic order is reported. We show that self-assembled aster structures become distorted in the presence of a small in-plane dc magnetic field and develop self-propulsion. The speed of locomotion can be effectively tuned by the amplitude of the dc field.   

A Tunable Terahertz Detector Based On Self Assembled Plasmonic Structure on a GaAs 2DEG

Recently compact frequency sensitive THz detection has been achieved using gated gratings on 2DEG structure. The method is based on the resonant absorption of the 2D plasmon dependence on system dimension and the tunability of that dimension by depletion gating. Here we attempt to improve detector sensitivity, tunability and remove polarization dependence through the development of a gated grid design. The requirement for imaging applications of device dimensions on the order of $<$ 1 micron over a detector area of 4 mm2, suggest that standard lithographic approaches will be too costly for large scale detector production. Here we realize the gated grid plasmonic structure on 2DEG material by using nanosphere self assembly lithography. This fabrication method has not been widely developed for III-V processing but allows us to achieve large area sensitive detectors with tunability in the 1-4 THz range. In this paper we will discuss the fabrication method and characterization of the devices as a function of gate bias and temperature using FTIR and THz time domain measurements.   

Session V10: Focus Session: Growth, Structure, Dynamics, and Function of Nanostructured Surfaces and Interfaces -- Semiconductors

In situ x-ray scattering investigation of Ag/Si(111)7x7

We have used in situ synchrotron x-ray scattering to investigate the growth of quantum-size-effect (QSE) Ag nanocrystals on Si(111)-7x7. The experiments explore the buried interface and the wetting layer as well as the interlayer spacings and the height distribution of the islands at different coverage and temperatures. The areal density of the wetting layer is found to be 30{\%} of Ag(111) and it is located above the adatom layer at a sharp interface. As the coverage is varied, all Ag layer heights are observed in the height distribution except for 2 atomic layers (measured from the Si surface), which were negligible. The structure of the islands and wetting layer will be discussed in relation to recent work that questions whether Ag/Si(111) is a QSE system. Research funding is supported by NSF DMR-0706278. The experiments were performed at the Advanced Photon Source Sector 6 beam-line at Argonne National Laboratory, which is supported by the US-DOE through Ames Lab under Contract No. W-7405-Eng-82.   

Grazing incidence surface scattering in the Pb/Si(111) system using an Area Detector

Geometrical effects are considered when using an area detector for in situ x-ray grazing incidence scattering studies of the Pb/Si(111) system. Rod-like scattering and 3D-crystallite diffraction can both occur during in situ studies and these require different geometrical considerations. The Pb/Si(111) system conveniently exhibits different surface phases that provide useful examples, including randomly oriented 3D-crystallites on the surface that form powder diffraction rings. The shape of a diffraction ring depends on the position of the detector in real space. For rod scattering, the length of the image on the detector depends on resolution as well as domain size. We will discuss methods for obtaining reciprocal space information from area detector images in surface diffraction. Research funding is supported by NSF DMR-0706278. The experiments were performed at the Advanced Photon Source Sector 6 beam-line at Argonne National Laboratory, which is supported by the US-DOE through Ames Lab under Contract No. W-7405-Eng-82.   

Increased structural ordering of the low temperature wetting layer in the Pb/Si(111)-7x7 system

The Pb/Si(111)-7x7 system exhibits interesting quantum size effects (QSE) for Pb nano-islands, including anomalously fast island coarsening that is facilitated by the wetting layer between the islands. While it is known that the wetting layer has a disordered 8x8 structure, the exact structure of the layer is still an open question. Our in situ x-ray scattering studies show that the wetting layer structure evolves temporally over a remarkably broad range of temperatures due to \textit{two} physically independent mechanisms. The as grown low temperature wetting layer is found to slowly anneal into a \textit{better-ordered 8x8 structure}, which suggests that it is highly dynamic as it attempts to accommodate the large corrugation of the Si(111)7x7 substrate. This increased order has important implications for the fast atom transport between the QSE-islands. Research funding is supported by NSF DMR-0706278 and the Ministry of Knowledge Economy of Korea 2009-F014-01 (CK). The experiments were performed at the Advanced Photon Source Sector 6 beam-line at Argonne National Laboratory, which is supported by the US-DOE through Ames Lab under Contract No. W-7405-Eng-82.   

Quantum Growth of a Metal/Insulator System

Quantum confinement of electrons in thin metal films can lead to novel effects on the growth, structure, stability, and various other physical and chemical properties, as demonstrated by recent work on metal films grown on semiconductor substrates. We report herein the observation of quantum growth behavior in a metal-on-insulator system; the results show substantial differences. Insulating substrates, with their large band gaps, offer minimal electronic coupling at the interface. This decoupling should maximize quantum confinement effects. Indeed, in a study of Pb film growth and thermal processing on sapphire, we have observed robust preferred island height selection over a wide thickness range -- a hallmark of quantum confinement effects -- for processing temperatures up to 250 degrees C. By contrast, room temperature is the limit for Pb films prepared on the semiconducting substrate Si(111). These results provide the first evidence connecting the quantum growth behavior of overlayers with the substrate band gap.   

Electron coherence in Pb/Ag heterostructures epitaxially grown on Si(111)

Along with other metals, Pb and Ag can form globally flat ultra- thin films on the Si(111) surface. Due to electron confinement along the growth direction, such films exhibit distinctive quantum well states (QWS's). Confinement occurs between the vacuum-solid and solid-solid interfaces. It was reported earlier, using angle-resolved photoemission, that quantum confined states existing in Ag thin films can coherently propagate through a Pb overlayer with thickness much thicker than the typical electron mean free path. Here we use scanning tunneling microscopy and spectroscopy to investigate the quantum well states formed in double quantum wells (Pb quantum well and Ag quantum well) formed in Pb/Ag/Si(111) double- heterostructures. Both the growth mechanism and the coherent coupling between the Pb and Ag quantum wells will be reported.   

Surface plasmon excitation in ultrathin Mg films on Si(111)

We investigated the dispersion of the surface plasmon in ultrathin Mg(0001) films, grown on a Si(111)-7$\times $7 surface, as a function of film thickness and parallel momentum ($q_{\vert \vert })$, using angle-resolved high-resolution electron-energy-loss spectroscopy (HREELS). In Mg films thicker than $\sim $ 3 ML, surface plasmon excitations exhibit negative dispersions for small $q_{\vert \vert }$(long wavelength limit). In contrast, the surface plasmons of ultrathin Al(111) films are known to exhibit positive dispersions near $q_{\vert \vert }\sim $ 0. The surface plasmon energies of the Mg films increase as the film thickness decreases. The plasmon line widths reveal similar trends, namely, for a given film thickness the line width decreases initially with increasing $q_{\vert \vert }$ while it increases with film thickness. Possible explanations for the observed thickness dependence of the surface plasmon dispersion and damping will be discussed.   

Plasmon response of a quantum-confined electron gas probed by core-level photoemission

The emergence of the ``bulk'' plasmon in \textit{atomically-smooth} ultrathin Mg(0001) films on Si(111) has been determined using x-ray photoelectron spectroscopy (XPS). Plasmons in this quasi two-dimensional (2D) regime turn out to be excited primarily via the sudden creation of the core hole, as the extrinsic loss channel (which is dominant in bulk XPS spectra) is suppressed by electron confinement. The collective plasmon response of the films is remarkably similar to that of a thin slice of \textit{bulk matter}, subject to quantum-size boundary conditions, in spite of the fact that the one-electron degrees of freedom are quantized. The energy-loss spectra of the thinnest films are characterized by a gradual transfer of spectral-weight from the bulk-like collective modes to the low-energy one-electron excitations, and the plasmon ultimately collapses below six monolayers. Our results represent striking manifestations of the role of electronic confinement on plasmon resonances in precisely-controlled nanostructures.   

Controlled Surface functionalization via self-selective metal adsorption and pattern transformation on vicinal Si(111) surface

We demonstrate a self-selective metal adsorption and pattern transformation process on vicinal Si(111) surfaces. When Au atoms are deposited onto the self-organized periodic Si(111) surface patterns, the Au atoms self-select to adsorb predominantly onto one of the two distinct domains, the Si(111) terrace or the step-bunched facet, at different Au coverage. This leads to a systematic transformation of the surface pattern, whose domain population changes while its periodicity remains intact with the increasing Au coverage. A stress-domain model is used to explain the observed phenomenon. Our findings suggest a unique method for controlled functionalization of surfaces at the nanoscale, as illustrated further by domain- selective self-assembly of uniform CoSi$_2$ nanoclusters on the Au-functionalized vicinal Si(111) surface.   

Epitaxial silicene formed on single-crystalline ZrB$_{2}$ thin films: structure and electronic properties

The experimental realization of extended, two-dimensional sheets of silicene, the silicon counterpart of graphene, has been elusive so far. Here, we demonstrate that such a two-dimensional, epitaxial honeycomb Si layer forms through surface segregation on a metallic zirconium diboride (ZrB$_{2})$ film grown itself epitaxially on Si(111). The honeycomb Si layer uniformly covers the ZrB$_{2}$(0001) surface forming a (2$\times $2) reconstruction. Surface-sensitive core-level photoelectron spectroscopy performed using a photon energy of 130 eV identifies Si atoms in different chemical states that are either in contact with Zr atoms or not, confirming details of the slightly-buckled honeycomb structure obtained through scanning tunneling microscopy. Angle-resolved ultraviolet photoelectron spectra reflect surface electronic states related to the predicted band structure of slightly-buckled, free standing silicene together with those of the uppermost Zr layer.   

Endotaxial Si nanolines in Si(001):H

The study of one dimensional wires is of great interest in the area of low-dimensional physics, and these structures also have potential applications in future nanodevices. A perfectly straight nanoline embedded in a H-terminated silicon surface has been fabricated by a process of hydrogenation of a Bi nanoline surface using an atomic H beam source, and comprises a triangular core of Si embedded in the top five layers of the Si substrate. The defect density of this nanoline is extremely low, and being H- terminated, it is stable in air for limited periods of time. Scanning Tunnelling Microscopy experimental data and Density Functional Theory calculations have been used to determine the atomic structure of this nanoline, so-called the Haiku Stripe, and have revealed that there exists a 1D state localised to the nanoline core, lying just above the conduction band minimum.   

Atomic Layer Epitaxy of Si and Ge on Si(100)-(2x1)

Atomic Layer Epitaxy of Si and Ge on Si(100) surface using disilane (Si$_{2}$H$_{6})$ and digermane (Ge$_{2}$H$_{6})$ as precursors is a critical step for constructing 3-D nano-structures, and is indispensable for Atomically Precise Manufacturing of new devices such as quantum dots. Using IRAS and STM together with DFT calculations, we show that Si$_{2}$H$_{6}$ chemisorbs on clean Si(100)-(2x1) via beta-hydride elimination pathway, involving the intermediate states Si-H and Si-SiH$_{2}$-SiH$_{3}$. Thermal decomposition of the chemisorbed Si$_{2}$H$_{5}$ leads to the formation of Si$_{2}$H$_{2}$ as an added dimer rotated 90 degrees with respect to the initial dimer row. A similar chemisorption pathway is observed for Ge$_{2}$H$_{6}$ on Si(100)x(2x1). The thermal decomposition of Ge$_{2}$H$_{5}$ involves the migration of H from Ge to Si, and Ge ad-dimer formation. Evidence for Ge epitaxial growth on Si(100)x(2x1) using Ge$_{2}$H$_{6}$ will be presented.   

Kinetics-Limited Composition Profile of Semiconductor Alloy Quantum Dots

Semiconductor alloy quantum dots (QDs) with controlled composition profile are promising nanoscale building blocks for modern nanophotonic and nanoelectronic devices. The overall composition profile of such low-dimensional nanostructures is usually far from equilibrium, because bulk diffusion is negligible at typical growth conditions. However, local equilibrium may be established in the surface regions via surface diffusion. Consequently, the kinetic growth mode, which dictates the way of surface mass transport and alloy mixing in the growth fronts, becomes a key factor in determining the kinetics-limited composition profile. In this talk, we report our recent discovery of a striking correlation between the composition profiles of the strained semiconductor alloy QDs and their growth modes, based on atomistic-strain-model Monte Carlo simulations of InGaN (GeSi) QDs. The layer-by-layer growth forms core-shell structures with the core-rich unstrained component; while the faceted growth forms the core-rich strained component. Our findings suggest a promising method for the control of composition profile of semiconductor alloy QDs by selecting the growth mode.   

Coarsening and Saturation of Quantum Dot Evolution during Strained Film Heteroepitaxy

Morphological properties of an epitaxially grown film and the self-organization process of coherent strained islands are analyzed via the development of a continuum elasticity model based on the 2nd order perturbation method. Effects of wetting stress due to film-substrate interactions have been incorporated in the resulting nonlinear dynamic equation governing the film morphological profile. We study the formation and evolution of surface strained islands or quantum dots for different film/substrate misfit strains, via analyzing the time-dependent behavior of the structure factor for surface heights, its various moments, and the surface roughness. Three regimes of island array evolution have been identified, including a film instability regime at early stage, a slow power-law-type coarsening at intermediate time, and the crossover to a saturated state, with detailed behavior dependent on misfit strains but not qualitatively on finite system sizes. The results are compared to previous experimental and theoretical efforts on quantum dots coarsening and saturation.   

Directed Self Assembly and Self-Limiting Growth (SLG) of Mound Formation on Patterned GaAs(001) Surface During MBE Homoepitaxy

We present the results of molecular beam epitaxial growth experiments on nanopit-patterned GaAs(001) surfaces at temperatures near 500$^{\circ}$C. We find that in the initial stage of growth, the pattern directs the spontaneous formation of multilayer islands at 2-fold bridge sites between neighboring nanopits along [110], seemingly due to the presence of an Ehrlich-Schwoebel barrier [1]. However, as growth continues, the height of mounds at 2-fold bridge ``self-limits'': the mounds cease to grow. Beyond this point an initially less favored 4-fold bridge site for mounds dominates and a different pattern of self assembled mounds begins. We propose that a minimum, ``critical terrace size'' at the top of each mound is responsible for the observed self-limiting growth. \\[4pt] [1] T. Tadayyon-Eslami, H.C. Kan, L.C. Calhoun {\&} R.J. Phaneuf, \textit{Phys. Rev. Lett. } \textbf{97}, 126101 (2006)   

Spontaneous Microfaceting and Pyramid Formation during Si(100) Etching

The spontaneous, etching-induced transformation of an initially flat Si(100) surface to a completely nanofaceted morphology consisting of overlapping pyramidal hillocks has been observed using a combination of morphological and spectroscopic probes and modeled using a kinetic Monte Carlo (KMC) simulation of Si(100) etching. The morphological transformation is driven by highly anisotropic chemical reactions that generate self-propagating pyramidal features with near-perfect microfacets. The atomic-scale mechanism of this etching-induced transformation will be discussed. In contrast to the more commonly studied Si(111) surface, the reactivity of the (100) face is dominated by interadsorbate strain.   

Session V11: Electronic Structure: Theory and Spectra II

First principles study of strained Si/Ge core-shell nanowires along [110] direction

First principles density-functional calculations were performed to study the electronic properties of Si/Ge core-shell nanowires along the [110] direction with the diameter of the wires up to 5 nm. It was found the band gap of the core-shell wires is smaller than that of both pure Si and Ge wires, given the same diameter. This reduced band gap is ascribed to the intrinsic strain between Ge and Si layers, which partially counters the quantum confinement effect. External uniaxial strain is further applied to the Si/Ge core-shell nanowires for tuning the band structure. At the $\Gamma $ point, the energy levels of both conduction and valence bands are significantly altered by applied strain, which results in an evident change of the band gap. In contrast, for the K vectors far away from $\Gamma $, the variation of the conduction/valence band with strain is much reduced. In addition, with a sufficient tensile strain ($\sim $1{\%}), the valence band edge shifts away from $\Gamma $, which indicates that the band gap of the Si/Ge core-shell nanowires experiences a transition from direct to indirect.   

Electronic structure of partially hydrogenated graphene superlattices

First-principles calculations based on density functional theory are performed to investigate the electronic structure of graphene-graphane superlattices (GSLs) by varying the widths of both the graphene and graphane regions. For the armchair-type interface between the graphene and graphane strips (AGSLs), the superlattices become semiconducting with a band gap exhibiting a similar dependence on the width of the graphene region as in armchair graphene nanoribbons. In contrast with the nanoribbons, however, the band gap of AGSLs shows both direct and indirect characteristics, depending on the graphane width. On the other hand, GSLs with a zigzag interface (ZGSLs) possess magnetic ground states except for those with a very narrow graphene strip. While an anti-ferromagnetic (AFM) phase is found to be energetically more stable than the ferromagnetic (FM) one, the energy difference between the two phases is so small ($< 10$~meV) that these two phases become nearly degenerate. These findings point toward an alternative route for graphene-based applications without requiring physical cutting as in graphene nanoribbons.   

First-Principle Calculations of The Conductivity of Br-Doped Graphite

First-principles calculations are used to study the enhanced in- plane conductivity that was observed experimentally in Br-doped graphite systems.\footnote{Tongay et al. Phys. Rev. B 81, 115428 (2010)} The band structure near the Fermi surface of the doped systems with different bromine concentrations compared to that of pure graphite, and the charge transfer between carbon and bromine atoms are analyzed to understand the conductivity change in the different directions. In addition, we address the effect of the compression of graphite layers on the stability of the bromine molecule Br$_{2}$ between these layers and thus on the conductivity of the system. Our calculations show that for large separation between doped graphite layers bromine is more stable in the molecular form (Br$_{2}$) and has a negligible effect on in- plane conductivity. However, with increased compression (decreased layer-layer separations) Br$_{2}$ molecule tend to dissociate and exchange charge with the nearby graphite layers causing an increase in hole conductivity.   

Study of Bulk Modulus in Zincblende Nitrogen-doped Gallium Phosphide Alloys Using Density Functional Theory

The prospect of solar energy as a renewable resource is ever-increasing. Density functional theory (DFT) calculations can elicit reliable behavior predictions in energy conversion materials to achieve higher efficiencies. Chemical stability of the photo-catalysts in aqueous solution is of particular interest for its long term performances. The bulk modulus is a mechanical property that is a good indicator of material stability. GaP has a low band gap and is a good candidate for use as a photocatalyst for hydrogen evolution by splitting water. Unfortunately, it is not stable and highly susceptible to corrosion over a very short time period, making it unfeasible for long-term use. GaN has too high of a band gap but a good stability factor. While these materials both possess desirable qualities, they cannot be used solitarily. We will report electronic properties and bulk moduli from the total energy calculations of the zincblende and wurtzite species using DFT-GGA and DFT+U as a function of doping concentration $x$. We will also present the density of states and charge density distribution of the alloy materials to study the localization/delocalization effects of N defects levels and their impact on the alloys' stability.   

Topological Insulators in Ternary Compounds with a Honeycomb Lattice

One of the most exciting subjects in solid state physics is a single layer of graphite which exhibits a variety of unconventional novel properties. The key feature of its electronic structure are linear dispersive bands which cross in a single point at the Fermi energy. This is so-called Dirac cone. The ternary compounds, such as LiAuSe and KHgSb with a honeycomb structure of their Au-Se and Hg-Sb layers feature band inversion very similar to HgTe which is a strong precondition for existence of the topological surface states. These materials exhibit the surface states formed by only a single Dirac cone at the G point together with the small direct band gap opened by a strong spin-orbit coupling (SOC) in the bulk. These materials are centro-symmetric, therefore, it is possible to determine the parity of their wave functions, and hence, their topological character.   

The electronic structures of Cu delafossites nanocrystals for PEC hydrogen production: A density functional theory study

Efficient photo-electrochemical (PEC) splitting of water to hydrogen by sun light usually requires that the semiconductor which will be used as a photoelectrodes will satisfy several electronic criteria. As naturally available semiconductors do not meet all these criteria, a thorough understanding of ``band-engineering'' for mixed alloys both at bulk and nano phases is necessary to successfully design these photoelectrodes. Recently Cu delafossites, Cu$M$O$_{2}$, have received much attentions as photo-catalysts for hydrogen production due to their unique properties such as stability in most aqueous solutions and excellent hole mobility. However, due to their large optical band gap they can absorb sun light only in the ultra-violet region. Hence, it is necessary to tailor their electronic properties to enhance their catalytic activities in the visible light regions. In this presentation density functional study of the Cu-delafossite nanocrystals will be presented. The stability of the nanocrystals will be discussed along with the reactivity of the different crystal faces. It will be shown that O-terminated$ M$-O octahedrons play a major role in the stability of these nanocrystals, which also makes these surfaces less reactive. We will discuss the charge (electron-hole) separation problems in these nanocrystals.   

Lateral Quantum Well States on Pb Films Grown on Cu Step Surface

The highly ordered Pb films were found to grow on Cu step surface as a ``magic'' heteroepitaxial grown model. The lateral quantum well states in these Pb have been investigated by angle-resolved photoemission. Across the step direction, the quantum well state display a dispersive character, with periodicity in reciprocal space defined by the step superlattice geometry. These observations are compared and analyzed with \textit{ab initio} calculations based on the full-potential linearized augmented plane wave method.   

Influence of local environment on the characterization of the p-type TCO in silver vanadates

Cu and Ag oxides are often considered as possible candidates for p-type transparent conducting oxides (TCO's) because the d$^{10}$ valence structure usually gives rise to dispersive d-bands at the valence band maximum. Among them, multi-cation oxides of silver and vanadium show various atomic structures such as the $\alpha-$ and the $\beta-$phase of Ag$_{3}$VO$_{4}$ and KAg$_{11}$(VO$_{4}$)$_{4}$. Hence, these compounds, especially KAg$_{11}$(VO$_{4}$)$_{4}$, offer several local environments at Ag sites and it is interesting to assess how they influence the electronic structure. Based on first-principles density functional theory, we point out a relation between the local environment and d-s orbital mixing at the Ag site. In turn, this mixing determines the orbital composition of the band extrema and band gaps. The influence on band gaps of the substitution of Nb and Ta for V in Ag$_{3}$VO$_{4}$ and of the substitution of alkali metals for K in KAg$_{11}$(VO$_{4}$)$_{4}$ will also be discussed.   

Electronic structures of Tl-based materials for $\gamma$-ray detectors; First-principles study

For Tl-based semiconductors, investigated to find good candidate materials for $\gamma$-ray detectors, we performed ab-initio calculations using the full-potential linearized augmented plane wave (FLAPW) method\footnote{Wimmer, Krakauer,Weinert, Freeman, Phys. Rev. B, {\bf 24}, 864 (1981)} to find their electronic structures and to estimate their physical properties such as band gaps, effective masses, absorption coefficients, dielectric constants, and work functions. Within the LDA scheme, the underestimation of the band gap is well-known and causes serious problems in obtaining optical properties. Therefore, we adopted the screened-exchange LDA (sX-LDA) scheme and acquired correct gap values close to experimental ones. With the sX-LDA, we found that Tl$_6$I$_4$S and Tl$_6$I$_4$Se have direct band gaps of 2.36 and 1.88 eV, respectively, and they exhibit dispersive bands near the band edges. Based on the calculated and experimental results, we discuss the relationship between atom species/crystal structure and electronic characteristics, and suggest several materials for $\gamma$-ray detectors.   

Non-equilibrium Fermi-edge Singularity in Mesoscopic Devices

The non-equilibrium Fermi-edge singularity (NFES) was observed as a non-linearity in the random telegraph signal [1] in mesoscopic devices at low temperatures. Based on a modified NFES theory, we found that when a low-frequency ac signal is applied, the Fumi shift and the electron dephasing contribute in an opposite way to the violation of the detailed balance, which is measured in the logarithmic ratio of tunneling rates. In the case of large electron phase shift and weak dephasing, the usually-ignored Fumi shift plays an important role, and manifests itself as an ``S''- shape curve in the logarithmic ratio of rates. For stronger dephasing, near the transition threshold, the tunneling spectra are dominated by the increased effective temperature because of the bias voltage, while for energies larger than that of the probing signal, the thermal excitation restores the system to pseudo-equilibrium. The overall shape of the logarithmic ratio of rates shows a ``Z''-shape. \\[4pt] [1] D. H. Cobden, and B. A. Muzykantskii, Phys. Rev. Lett. 75, 4274 (1995)   

Dirac Point Degenerate with Massive Bands at a Topological Quantum Critical Point

In the band structure of the Skutterudite, as the Sb sublattice in the unit cell is moved slightly retaining the crystal symmetry, the small gap at the Fermi energy closes due to a band crossing at Gamma. At this critical point a pair of linear (``Dirac'') bands are degenerate with two conventional bands. Because of the crystal symmetry three out of the four bands are degenerate even when one is away from the critical point. Insulators in 3D, as well as in 2D, can be characterized by topological invariants. When inversion symmetry is present (as in the space group 204 of Skutterudite), the Z2 invariant can be obtained from the parities of the occupied states at the invariant momenta, which in the bcc structure consist of Gamma, three H points, and the four P points. Here only the Gamma point requires consideration, since reoccupation occurs only there. The singlet has odd parity at Gamma while for the triplet it is even. As the critical point is crossed, the product of the parities of the occupied bands at Gamma, and hence the Z2 invariant, changes sign due to the reversal of the singlet- triplet position; the signal of a trivial to topological transition.   

Sixteen-band atomic bond-orbital model for zinc-blende structures

We develop a sixteen-band atomic bond-orbital model (16ABOM) which is able to compute the spin splitting induced by bulk inversion asymmetry. This model is derived from the linear combination of atomic orbital (LCAO) scheme such that the characteristics of real atomic orbitals can be preserved for spin-splitting calculations. We derive the Hamiltonian of 16ABOM by performing a similarity transform on the nearest-neighbor LCAO Hamiltonian, followed by taking a second-order Taylor series expansion over $k$-vector at the $\Gamma $ point. The spin-splitting energies in bulk zinc-blendes are calculated using this model, and the results are in good agreement with LCAO and first-principles calculations. In addition, it is found the spin-orbit coupling between anti-bonding and bonding $p$-like states, which can be evaluated directly by this 16ABOM, dominates the magnitude of the spin splitting of the lowest conduction bands in middle-bandgap and wide-bandgap materials.   

Real-space Green's Function Calculations including Hubbard Contributions

Hubbard model contributions are introduced into the real space Green's function formalism in terms of an effective self-energy, based on the LDA+$U$ method of Anisimov et al.\footnote{V. I. Anisimov, F. Aryasetiawan, and A. I. Lichtenstein, J. Phys.: Condens Matter 9, 767 (1997)} The effective self-energy is then applied to localized $d$-states in a material, e.g. at the metal sites of transition metal oxides. The approach is implemented in an extension of the FEFF9 spectroscopy code and leads to an efficient procedure for including strong correlation effects in the electronic structure and x-ray spectra of $d$-electron materials, such as transition metal oxides and high T$_c$ cuprates. Calculations are presented for the angular momentum projected density of states of MnO, NiO and La$_{(2-x)}$Sr$_x$CuO$_4$ and for the K-edge x-ray absorption and emission spectra of the O atoms in these materials, and the results are found to be in reasonable agreement with experiment.   

A Plane-Wave Implementation of Quasiparticle Self-Consistent GW (QSGW)

The use of GW techniques in calculating the quasiparticle properties of certain classes of materials, e.g. complex oxides, is sometimes hindered by the poor mean-field starting point that density functional theory (DFT) within standard Kohn-Sham implementations provides. There has been considerable effort in the community to improve upon the mean-field starting point for a broad range of materials. A recently proposed method, the quasiparticle self-consistent GW (QSGW) method, employs a process in which a mean-field exchange-correlation potential is approximated from and updated self-consistently using the self-energy operator from previous iteration GW calculations. We present an implementation of this method in a plane-wave basis, and discuss its accuracy, computational cost, and physical implications for a variety of semiconducting materials.   

Assessment of dispersion-corrected density functional approaches for extended systems

Standard density functional (DFT) methods are known to fail in describing the long range van der Waals interactions, and currently, there is a great interest in incorporating dispersion corrections in density functionals. Recently, Tkatchenko and Scheffler introduced a new scheme where dispersion corrections are included by a summation of damped interatomic C$_6$/R$^6$ terms. However, contrary to the DFT-D2 approach of Grimme, the C$_6$ coefficients depend on the electron density through a Hirshfeld atom-in-a-molecule decomposition scheme. We have implemented the vdW-TS approach in VASP and applied it to the study of a series of prototype dispersion-dominated systems including layered materials, noble-gas solids and molecular crystals. Full optimization of all degrees of freedom is possible in our implementation because dispersion corrections are computed for the forces acting on the atoms, and also the stresses on the unitcell. Our results show that the vdW-TS method yield good structure, bulk moduli, and cohesive energies of weakly bonded systems in much better agreement with experiment than those obtained with standard DFT approaches.   

Session V12: Transport in 2-D Systems

In-Plane Field Magneto-transport in a Six-fold Degenerate Si-(111) 2DEG

In-plane magneto-transport is an effective tool for measuring sub-band occupancy and differentiating between effects such as the so-called ``reentrant Metal-Insulator Transition'' or a ferromagnetic to paramagnetic phase transition. Using a two-dimensional electron gas (2DEG) on high mobility (up to $100,000$ cm$^2/$Vs) hydrogen terminated Si-(111) surfaces [1], we have studied the magneto-resistance due to in-plane magnetic fields of this six-fold degenerate system. While high perpendicular field (up to 12 T) measurements indicate field-dependent valley splitting, parallel field data helps differentiate this dependence from spin dynamics. The application of an in-plane field polarizes the 2DEG into distinct sub-bands. I will present measurements of both spin and valley sub-band polarization in parallel magnetic fields from samples of various mobility ($10,000 - 100,000$ cm$^2/$Vs) and discuss these results in the context of the broader question of field-dependent valley splitting. In a simple picture of valley splitting on Si-(111) surfaces, one would expect two valley polarization fields in addition to the spin polarization. I will discuss how this interaction-free model fits with the perpendicular field measurements, and what we can learn about the six-fold degenerate system.\\[0pt] [1] R. N. McFarland et al., {\it Phys.~Rev.~B} {\bf 80} 161310R (2009)   

Zero Differential Resistance State in double GaAs quantum wells at high filling factors

Differential resistance $r_{xx}$ of 2D electrons was investigated in double GaAs quantum wells placed in magnetic fields $B<0.5$ (T) at temperatures $T=1.6-4.2$ (K). Electron state with Zero Differential Resistance (ZDR) is found in finite current range at maximums of inter-subband quantum oscillations. The experiment shows that the ZDR state exists at $2R_cE_H/ \hbar \omega_c < 1/2$, where $R_c$ is electron cyclotron radius at Fermi level, $E_H$ is Hall electric field, induced by the $dc$ bias, and $\omega_c$ is cyclotron frequency.\\[4pt] [1] A.A. Bykov, E. G. Mosulev and S. A. Vitkalov, JETP Letters v92, (2010) to be published   

Degenerate versus semi-degenerate transport in a correlated 2D hole system

It has been puzzling that the resistivity of high mobility two-dimensional (2D) carrier systems in semiconductors with low carrier density often exhibits a large increase followed by a decrease when the temperature ($T)$ is raised above a characteristic temperature comparable with the Fermi temperature ($T_{F})$. We find that the metallic 2D hole system (2DHS) in GaAs quantum well (QW) has a linear density ($p)$ dependent conductivity, \textit{$\sigma \approx $e$\mu $}$^{\ast }(p-p_{0})$, in both the degenerate ($T <T_{F})$ is originated from the reduced $p_{0}$, the density of immobile carriers in a two-phase picture. Quantum oscillations in the magneto-resistivity are also found to persist into the semi-degenerate regime in our strongly correlated 2DHS.   

Nonlinear transport in very high Landau levels of a high mobility quantum Hall systems

When a dc current is passed through a high-mobility two-dimensional electron system its differential resistivity exhibits oscillations with the applied magnetic field. The minima of these oscillations can extend all the way to zero leading to states with zero-differential resistance. This talk will discuss our recent experiments studying the evolution of the differential resistivity with temperature and with perpendicular and in-plane magnetic fields.   

Charge carrier velocity distribution in amorphous oxide field-effect transistors

Charge transport in field-effect transistors (FETs) and the underlying physical mechanisms have been the subjects of numerous studies. Many types of transistors have been studied utilizing organic/polymer, amorphous silicon, and thin-film inorganic active layers. Most of these studies involve the evaluation of charge carrier mobility from steady-state characteristics as a function of temperature, electric field, channel dimensions, etc. In this study, we describe a technique to measure the velocity distribution of charge carriers in a thin-film transistor. We use this technique to evaluate velocity distributions in zinc-tin oxide (ZTO) thin-film transistors at various temperatures. In ZTO FETs, we observe multiple distinct transport pathways, each with a distinct activation energy. In contrast, steady state measurements yield a single activation energy. This shows that new insights into charge transport mechanisms and phenomena can be obtained with such time-resolved transport measurements which are not possible with steady-state approaches.   

Electronic Transport Properties of Epitaxial ZnO Films by Electron Dephasing and Mobility Spectrum Analysis

Epitaxial ZnO films were grown by pulsed laser deposition on (111) Si substrates with bixbyite oxide buffers. Carrier transport properties were investigated using Hall measurements (4--300 K) under magnetic fields of 0--10 T, indicating mobility up to 113 cm$^{2}$/Vs. A diffusive Fermi surface (DFS) model incorporating electron dephasing theories was used to fit the abnormally positive magneto-conductivity observed below 150 K. Quantitative mobility spectrum analysis revealed the presence of a hole group at lower mobility accompanying the major electron group. Geometric distribution of the conducting groups was examined by capacitance-frequency experiments, while both temperature-dependent photoluminescence and mobility fitting confirmed a donor binding energy of $\sim$60 meV.   

Response of the Cyclotron Harmonic Spike to an In-Plane Magnetic Field

Microwave-induced resistance oscillations (MIRO) have been commonly observed in high- mobility GaAs/AlGaAs two-dimensional electrons systems (2DES) under microwave irradiations. In ultraclean GaAs/AlGaAs quantum wells (mobility $\sim 3.0\times 10^7cm^2/Vs)$ we have recently observed an extraordinary resistance spike at the second harmonic of cyclotron resonance. In order to elucidate its origin, we have studied the response of microwave photoresistances in a two-axis magnetic field configuration, where the perpendicular (B$_{perp})$ and the in-plane (B$_{//})$ components can be independently applied to the sample. The experiments reveal a distinctive response of the spike to the B$_{//}$ as compared with that of the MIRO. While the major MIRO peaks show an increasing phase-shift towards 0.25 in increasing B$_{//}$, the spike position shows an essentially zero shift. This finding lends additional support for the notion that the spike is a new effect in the microwave-driven 2DES.   

Conductivity kinks in the transport of ultra-dilute GaAs two-dimensional hole systems in zero field

Though Wigner crystal was first observed for electrons on helium in 1979, a Fermi Liquid-to-Wigner crystal transition has never been demonstrated. Important questions on how interaction drives such a transition and the nature of the transition remain unanswered. Apart from the complexity associated with the disorder which competes with or even dominates interaction by rendering the system into an Anderson insulator, an important question is whether there exists intermediate phases that hinder a direct first order transition. We report findings obtained via measuring ultra-high-purity GaAs two-dimensional hole systems with dilute charge concentrations down to 8x10$^{8}$ cm$^{-2}$. For fixed charge densities below 4x10$^{9}$cm$^{-2}$, a conductivity ($\sigma )$ kink is observed while sweeping the temperature across some characteristic value where the derivative d$\sigma $/dT exhibits a discontinuous step. For charge densities above 4x10$^{9}$cm$^{-2}$, the kink evolves into a dip which diminishes for charge densities beyond 7x10$^{9}$cm$^{-2}$. A possible first order phase transition will be discussed.   

The Zero-Resistance States in InN Films

InN is a narrow gap semiconductor which possesses interesting transport properties. Besides the 2D electron accumulation layer on the surface, low temperature zero-resistance states have been observed in InN thin films and attributed to superconductivity. In order to elucidate the origin of superconductivity, we have studied systematically the temperature and magnetic field dependences of resistance in InN films of various thicknesses. High quality InN film samples of thickness between 50 nm and 5 $\mu $m were grown by molecular beam epitaxy on Al$_{2}$O$_{3}$ substrate with a GaN buffer layer. Typically these films have an electron density of 3x10$^{17}$- 6 x10$^{18}$ cm$^{-3}$, and the mobility between 1000-2400 cm$^{2}$/Vs at 300 mK. Zero-resistance states were observed in films of thickness above 1 $\mu $m with the transition temperature of $\sim $ 1K, along with marked nonlinear I-V characteristics suggesting the presence of supercurrent. We observed anisotropic dependences of resistance on in-plane magnetic fields with respect to the direction of applied current. This work in Rice was supported by NSF DMR-0706634 and Welch Foundation C-1682, in Peking University was supported by the NSFC of China (Nos. 60990313, 10774001), and RFDP (No. 20090001120008).   

Electron interaction effects on Aharonov-Bohm resonances in an antidot-based quantum Hall interferometer

We theoretically study the electron interaction effects on Aharonov-Bohm resonances in an antidot-based quantum Hall interferometer in the integer quantum Hall regime. We introduce a general capacitive interaction model for an antidot with multiple bound modes of edge states, and find that the pattern of Aharonov-Bohm resonances is governed by the spectator behavior: The resonances of some modes disappear and instead are replaced by those of the other modes, due to charge relaxation between bound modes in the cotunneling regime. This behavior gives a reasonable understanding on the nontrivial features of previous experimental data, e.g., spectator behavior in an antidot molecule and resonance peaks in a single antidot with two, three, or four modes. References:\\[4pt] [1] W.-R. Lee and H.-S. Sim, Phys. Rev. Lett. 104, 196802 (2010);\\[0pt] [2] W.-R. Lee and H.-S. Sim, arXiv: 1009.1004.   

Effective interlayer charge transfer in an electron bilayer system

An electron bilayer system is realized in a wide GaAs quantum well. The chemical potentials of both layers can be tuned by intrinsic back and top gates. The Landau level spectrum for various charge distributions is probed by photoluminescence (PL), able to discriminate between both layers independently. The PL spectra show unambiguously how the system spontaneously deforms itself in strong magnetic fields as a consequence of energy minimization under Landau quantization and huge SAS energy gaps, reaching up to the cyclotron energy, become visible in the PL spectra [1]. \\[4pt] [1] V.V. Solovyev, S. Schmult, W. Dietsche, I.V. Kukushkin, PRB 80, 241310, 2009.   

Exciton condensates in Pfaffian Quantum Hall states

We apply recently developed (arXiv:1004.3657) conformal field theory techniques to describe exciton condensates in non-abeliann quantum Hall states of the Pfaffian type.   

Spin-full quantum Hall states: squeezing techniques

Despite the high magnetic field does the spin of the electron play an important role in the quantum Hall effect. Various spin (singlet) quantum Hall states have been proposed to explain various observed quantum Hall plateaux. In this talk, we present a method to generate spin-full quantum Hall states, by employing a squeezing procedure, as is used in the polarized state. This squeezing procedure also sheds additional light on the underlying (topological) structure of such states. By using connections between polarized and un-polarized states, one gains insight in the polarized state, as is the case for the Haffnian and Haldane-Rezayi states.   

Spin polarization measurement of the $\nu $ = 5/2 fractional quantum Hall state via NMR

The $\nu $ = 5/2 fractional quantum Hall state has attracted much interest due to its possible non-Abelian statistics, which are expected for the \textit{Pfaffian} state. While the \textit{Pfaffian} model assumes full spin polarization, recent optical experiments suggest a spin-unpolarized ground state at $\nu $ = 5/2 instead [1, 2], and have thus posed a new challenge for understanding the true nature of the 5/2 state. Here, we report a spin polarization measurement of the $\nu $ = 5/2 state using resistively-detected NMR and demonstrate its full polarization. The measurements were performed at $T$ = 10 mK on a gated 30-nm quantum well at 4.4 T. For the resistive read-out of the nuclear resonance frequencies, we used the $\nu $ = 2/3 spin transition by comparing its $R_{xx}$ prior to and after the application of an \textit{rf} pulse at $\nu $ = 5/2. The NMR spectrum at $\nu $ = 5/2 is shifted to lower frequencies from the one at $\nu $ = 2, where the system is unpolarized, indicating a nonzero polarization at $\nu $ = 5/2. Our analysis, which considers the changes of the sub-band wave function under a gate bias, indicates that the polarization at $\nu $ = 5/2 is very close to its maximal value. This, in turn, gives support for the Pfaffian state.\\[0pt] [1] T. D. Rhone, APS March Meeting 2010, Y2.00003 [2], M. Stern \textit{et al}., Phys. Rev. Lett. \textbf{105}, 096801 (2010).   

Observation of a Well-Developed $\nu $ = 5/2 Fractional Quantum Hall State in Low-Mobility Electron Systems

The fractional quantum Hall (FQH) state at filling factor 5/2 is currently one of the hottest issues in FQH physics. This is mainly due to the predicted non-Abelian statistics of its quasiparticles and their possible implementation in quantum computation. However, experimental efforts to explore the 5/2 FQH state are severely hampered by the extremely high sample quality required for its emergence. Indeed, well-developed 5/2 FQH states have been reported only in samples with an ultra high mobility in excess of 1x10$^{7}$ cm$^{2}$/Vs. Here, we report the observation of a fully developed 5/2 FQH state in a GaAs/Al$_{x}$Ga$_{1-x}$As quantum well with a mobility of 4.8x10$^{6}$ cm$^{2}$/Vs, which was established after illumination at low temperatures by a red LED. To clarify the mechanism underlying the emergence of the 5/2 state, we carried out a systematic study on a series of samples with different parameters. Our study unveils that the screening of the remote impurity (RI) potential by the nearby neutral donors in the modulation doping layer plays the essential role. We also find that while the emergence of the 5/2 state is governed by the RI potential, once this state has emerged, the energy gap at 5/2 is still limited by the background impurity potential. Based on the analysis of the transport and quantum lifetimes, the relation between the 5/2 gap and these parameters will be discussed.   

Session V13: Glassy Systems and Jamming II

Phonon Spectra in Disordered Clusters of Colloidal Particles with Attractive Interactions

The influence of size and morphology on the vibrational properties of disordered clusters of colloidal particles with attractive interactions is studied experimentally. Water- lutidine mixtures induce fluid mediated attraction between micron-sized polystyrene particles, leading to the formation of attractive glasses with high local packing fractions. By measuring displacement correlations between particles, we extract the vibrational properties of these disordered clusters. Surprisingly, the spectra and character of vibrational modes did not depend on the number of particles involved. Rather, it depended strongly on the average number of nearest neighbors. An increase in the number of nearest neighbors shifted the phonon spectrum to higher frequencies, independent of the total number of particles in in the cluster. Simulations of structureless random networks of springs support these results, and further suggest that the dependence of phonon spectrum on number of nearest neighbors is a generic property of disordered networks.   

Dynamics of Small-Molecule Glass Formers Confined in Nanopores

We report a comparative neutron scattering study of the molecular mobility and non-exponential relaxation of three structurally similar glass-forming liquids (isopropanol, propylene glycol, and glycerol) in bulk and confined in porous Vycor glass. Confinement reduces molecular mobility in all three liquids, and suppresses crystallization in isopropanol. High-resolution quasi-elastic neutron scattering spectra were fit to Fourier transformed Kohlrausch functions $\exp[-(t/\tau) ^{\beta}]$, describing $\alpha$-relaxation. The stretching parameter $\beta$ is roughly constant with wavevector $Q$ and temperature. Average relaxation times $\langle\tau(Q)\rangle$ are longer at lower temperatures and in confinement. They obey a power law $\langle\tau(Q)\rangle \propto Q^{-\gamma}$, where the exponent $\gamma$ is modified by both temperature and confinement. Comparison of the bulk and confined liquids lends support to the idea that structural and/or dynamical heterogeneity underlies the non-exponential relaxation of glass- formers, as widely hypothesized in the literature.   

Emergence of rigidity at the dynamic glass transition: a replica approach calculation

According to the mean-field replica theory of the glass transition, at the so-called dynamic transition the relaxation stops and the liquid freezes into one of many metastable states. We identify Goldstone modes of the resulting amorphous solid and derive a formal expression for its shear modulus. This expression is complementary to the formula used by Yoshino and Mezard [Phys. Rev. Lett. \textbf{105}, 015504 (2010)]. We combine our formal expression with the recently proposed version of the replica approach [G. Szamel, Europhys. Lett. \textbf{91}, 56004 (2010)] to calculate the shear modulus.   

Experimental study of dynamic rearrangements in repulsive and attractive glasses

The influence of interparticle attraction versus repulsion on heterogeneous glass dynamics is explored with colloidal particles suspended in water-lutidine mixtures. The mixtures permit interparticle potentials to be tuned in situ from short-range repulsive to short-range attractive. Thus, a direct comparison of colloidal glass dynamics in samples composed of the same particles at the same volume fraction is possible. In both types of glasses, dynamics are found to be heterogeneous, and particles rearrange in a cooperative manner. By comparison to repulsive glasses, attractive glasses exhibit dynamics that are heterogeneous over a wider range of time and length scales, and involve more particles. Clusters of rearranging particles form string-like structures in repulsive glasses, and more compact clusters in attractive glasses. The experiments demonstrate explicitly that interparticle interactions affect glass dynamics.   

A family of systematically softened glass-formers

We present a computational study of a family of binary glass-forming mixtures that interact via the generic $U= 4\epsilon[\lambda (\sigma /r)^n-\alpha (\sigma /r)^6]$, where $n = 7, 8, 9, 10, 11, 12$. $\lambda$ and $\alpha$ are chosen such that the location and depth of the potential minimum are constant across all members of the family. We investigate the effects of softening on thermodynamic quantities such as energy and entropy, as well as dynamic properties such as diffusion and scattering. We also investigate the effects of softening on the energy landscape. In spite of the imposed constraint on well depth and location, we find profound effects of softening on all aspects of liquid and glassy behavior. The stability of the glasses is greatly enhanced by softening (soft liquids make hard glasses), and the relaxation rates in the corresponding liquids increase markedly upon softening. We present a comprehensive analysis of kinetic and thermodynamic fragilities in this family of glass-formers.   

Low-Frequency Vibrational Modes and Rearrangements in a Colloidal Glass Subject to Point Expansion

We conduct experiments on two-dimensional packings of colloidal thermosensitive hydrogel particles. The packing fraction of the colloidal suspension is tunable from liquid to deeply jammed by varying the global temperature of the sample. In addition, by tightly focusing an infrared laser on the sample, point expansion of the colloidal glass is induced via thermophoretic forces. We utilize displacement correlation matrix techniques employed in recent papers, and we employ video microscopy to derive the vibrational modes. The response of the sample to induced point expansion is analyzed over a range of packing fractions, with particular focus on the correlation between quasi-localized low-frequency vibrational modes and regions of rearrangements.   

Crystallization of the Lewis-Wahnstr\"{o}m ortho-terphenyl model

Crystallization is observed during long molecular dynamics simulations of bent trimer molecules - one of the standard models in computational studies of viscous supercooled liquids. The crystal was not anticipated, but is surprisingly simple: the three spheres that make up the rigid molecule sit near the sites of a body centered cubic lattice (the trimer bond angle being almost optimal for this structure). Interestingly, the crystal exhibits orientational disorder with molecules aligned randomly along the three Cartesian axis (an example of cubatic orientational order). While crystallization does not disqualify this model for viscous dynamics studies (it may even be valuable that the crystal is known), it illustrates the stubborn ingenuity of molecules to pack in periodic structures and questions our intuition to predict such structures. Finally, this is a rare example of crystallization of a molecular model from melt.   

Vitrification of a monatomic simple liquid in two dimensions

We investigate vitrification and crystallization process of a monatomic system by molecular dynamics simulation, where atoms interact via Lennard-Jones-Gauss potential. We first determine the time-temperature-transformation diagram by observing the crystallization time of the rapidly quenched state from the melt. The crystallization time becomes shortest at a certain temperature T*. The glassy state at low temperatures is shown to be fairly long-lived. In order to examine atomic mechanism of the crystallization, we introduce a modified incoherent intermediate scattering function which measures the structural correlation to a target structure. We show that the crystallization above and below T* take different paths. We also determine the free energy landscape (FEL) and show that the atomic dynamics is consistent with the FEL picture of the glass transition.   

Time reparametrization symmetry in a structural glass model

We explore the existence of time reparametrization symmetry in a particle system with quenched disorder. The system's density fluctuations are described by a stochastic equation (D.~S.~Dean,~J.~Phys.~A:Math. Gen \textbf{29}, L613 (1996)). Using the Renormalization Group (RG) on the Martin-Siggia-Rose generating functional, we analytically probe the long time dynamics by systematically integrating over short time scale fluctuations. We find that the RG flow converges to a fixed point that is invariant under reparametrizations of the time variable.   

Viscosity, Shear Waves and Atomic Level Stress Correlations

The Green-Kubo equation relates the macroscopic stress-stress correlation function to a liquid's viscosity. The concept of the atomic level stresses allows the macroscopic stress-stress correlation function in the equation to be expressed in terms of the space/time correlations between the atomic level stress-stress correlation functions. Molecular dynamics studies show surprisingly long spatial extension of stress-stress correlations and also longitudinal and transverse waves propagating in liquids over ranges exceeding the system size. The results reveal that the range of propagation of shear waves corresponds to the range of distances relevant for viscosity. Thus our results show that viscosity is a fundamentally non-local quantity. We also show that periodic boundary conditions play very non-trivial, previously undiscussed, role in molecular dynamics simulations effectively masking the long range nature of viscosity.   

Study of the de Almeida-Thouless line using power-law diluted one-dimensional Heisenberg spin glasses

In a recent study, we showed that in mean-field theory, there is a de Almeida-Thouless (AT) line, that separates the low-temperature, low-field spin-glass phase from a high-temperature, high-field paramagnetic phase, for arbitrary $m$-component vector spin glasses, provided one applies a magnetic field that is \emph{random in direction.} Building on this piece of work, here, we investigate whether or not there is an AT line beyond mean-field theory for Heisenberg spin glasses by performing Monte Carlo simulations on a power-law diluted one-dimensional Heisenberg spin glass for very large system sizes.   

Renormalization group analysis of the random first order transition

We consider the approach describing glass formation in liquids as a progressive trapping in an exponentially large number of metastable states. To go beyond the mean-field setting, we provide a real-space renormalization group (RG) analysis of the associated replica free-energy functional. The present approximation yields in finite dimensions an ideal glass transition similar to that found in mean field. However, we find that along the RG flow the properties associated with metastable glassy states, such as the configurational entropy, are only defined up to a characteristic length scale that diverges as one approaches the ideal glass transition. The critical exponents characterizing the vicinity of the transition are the usual ones associated with a first-order discontinuity fixed point.   

Influence of pressure on fast relaxation in glass-forming materials

The spectra of GHz-THz dynamics in glass forming materials have two main contributions: the boson peak and the fast relaxation that overlaps with the low-frequency flank of the boson peak. The nature of both contributions remains a subject of active discussions. Applying pressure helps to separate the temperature and volume effects on the fast dynamics. Although the boson peak under pressure was investigated recently by several groups, less attention was devoted to the fast relaxation. In this work we present the study of the fast relaxation measured in some molecular and polymeric glass formers under pressure by light (Raman and Brillouin) scattering. Different experimental conditions were applied: isothermal, isobaric, isokinetic, and isochoric. The results are analyzed within the frames of various theoretical models. In particular, we check in detail the predictions of the soft-potential model of glassy dynamics.   

Session V14: Focus Session: Statistical Mechanics of Complex Networks I

Sensitive Dependence on Network Structure

Much of the recent research in complex networks has been focused on establishing relations between network structure and dynamics and on exploiting these relations to optimize network processes. Using diffusion, consensus, and synchronization dynamics as model processes of broad significance, I will show that optimization can often lead to sensitive dependence of the dynamics on the structure of the network. This sensitivity, which is characterized by cuspy or discontinuous dependence of the fitness function on network structural parameters, is shown to be determined by transitions in the complement graph that are reminiscent of explosive percolation. I will also discuss the prevalence of sensitive dependence. I will argue that this phenomenon is not limited to optimized systems, and may in fact be observed under rather general conditions in systems as diverse as power-grid and laser networks. This phenomenon sets experimental limits but also leads to improved controllability, in which the dynamics can be enhanced by exploiting antagonistic interactions between different fitness-inhibiting network structures.   

Critical percolation phase, geometric phase transitions with continuously varying exponents, and thermal Berezinskii-Kosterlitz-Thouless transition in a scale-free network with short-range and long-range random bonds

Percolation in a scale-free hierarchical network is solved exactly by renormalization-group theory in terms of the different probabilities of short-range and long-range bonds [1]. A phase of critical percolation, with algebraic [Berezinskii-Kosterlitz-Thouless (BKT)] geometric order, occurs in the phase diagram in addition to the ordinary (compact) percolating phase and the nonpercolating phase. The algebraically ordered phase is underpinned by a renormalization-group fixed line along which the flows reverse stability, thus also leading to geometric phase transitions with continuously varying exponents. It is found that no connection exists between, on the one hand, the onset of the geometric BKT behavior and, on the other hand, the onsets of the highly clustered small-world character of the network and of the thermal BKT transition of the Ising model on this network. Nevertheless, both geometric and thermal BKT behaviors have inverted characters, occurring where disorder is expected, namely, at low bond probability and high temperature, respectively. This may be a general property of long-range networks. [1] A.N. Berker, M. Hinczewski, and R.R. Netz, Phys. Rev. E 80, 041118 (2009).   

Renormalization Group Classification of Critical Phenomena in Complex Networks

We discuss critical phenomena for a variety equilibrium statistical models on hierarchical networks with long-range bonds. An exact renormalization group (RG) study reveals that the observed critical behavior, albeit non-universal, can be classified into three generic categories. The non-universality is a direct result of the existence of long-range bonds, while the categories derive from their relative coupling strength. One of these categories is characterized by an infinite-order transition similar in appearance to the Kosterlitz-Thouless type, which has been observed recently in a number of network problems. Our result, if applicable to a wider set of networks, may explain the prevalence of such transitions, and may provide the basis for a generalized RG classification of criticality in complex networks.   

Contact process on static and adaptive preferred degree networks

We consider epidemic spreading on an adaptive network where individuals have a fluctuating number of connections around some preferred degree $\kappa$. Using very simple rules for forming such a network, we find some unusual statistical properties which provide an excellent platform to study adaptive contact processes. For example, by letting $\kappa$ depend on the fraction of infected individuals, we can model behavioral changes in response to how the extent of the epidemic is perceived. Specifically, we explore how various simple feedback mechanisms affect transitions between active and inactive states. In addition, we investigate the effects of two interacting networks, e.g., with a variety of $\kappa$'s and cross links.   

Synchronization with Time Delays in a Noisy Environment

We study the effects of nonzero time delays in stochastic synchronization problems with linear couplings in an arbitrary network. We provide the synchronizability threshold using the known exact threshold value from the theory of differential equations with uniform delays and establish the limit of synchronization efficiency by constructing the scaling theory of the underlying fluctuations \footnote{Hunt, Korniss, Szymanski, Phys. Rev. Lett. {\bf 105}, 068701 (2010)}. Nonzero delays lead to a scaling function for each fluctuation mode that does not vary monotonically as communication is improved (i.e., increasing strength or frequency). The strength and/or frequency of communication can then be tuned in order to subdue the stresses caused by growing the network to larger sizes, the presence of hubs, or lengthening delays. The implications can be counterintuitive: Improving communication is not always beneficial. In fact, making communication worse may salvage an otherwise unsynchronizable network. Insights into these trade-offs allow one to maintain and optimize the synchronization of the networks.   

A simple model for studying interacting networks

Many specific physical networks (e.g., internet, power grid, interstates), have been characterized in considerable detail, but in isolation from each other. Yet, each of these networks supports the functions of the others, and so far, little is known about how their interactions affect their structure and functionality. To address this issue, we consider two coupled model networks. Each network is relatively simple, with a fixed set of nodes, but dynamically generated set of links which has a preferred degree, $\kappa$. In the stationary state, the degree distribution has exponential tails (far from $\kappa$), an attribute which we can explain. Next, we consider two such networks with different $\kappa$'s, reminiscent of two social groups, e.g., extroverts and introverts. Finally, we let these networks interact by establishing a controllable fraction of cross links. The resulting distribution of links, both within and across the two model networks, is investigated and discussed, along with some potential consequences for real networks.   

Controllability of Complex Networks

The ultimate proof of our understanding of natural or technological systems is reflected in our ability to control them. While control theory offers mathematical tools to steer engineered systems towards a desired state, we lack a general framework to control complex self-organized systems, like the regulatory network of a cell or the Internet. Here we develop analytical tools to study the controllability of an arbitrary complex directed network, identifying the set of driver nodes whose time-dependent control can guide the system's dynamics. We apply these tools to real and model networks, finding that sparse inhomogeneous networks, which emerge in many real complex systems, are the most difficult to control. In contrast, dense and homogeneous networks can be controlled via a few driver nodes. Counterintuitively, we find that in both model and real systems the driver nodes tend to avoid the hubs. We show that the robustness of control to link failure is determined by a core percolation problem, helping us understand why many complex systems are relatively insensitive to link deletion. The developed approach offers a framework to address the controllability of an arbitrary network, representing a key step towards the eventual control of complex systems.   

Optimization of flow and cascading effects in weighted complex networks

We investigate the effect of edge weighting scheme $\sim $(k$_{i}$.k$_{j})^{\beta }$ on the optimality of flow efficiency and robustness in complex networks. We achieve this by analyzing a simple distributed flow model: current flow in resistor networks. In this scenario the centrality measure of a node (edge) is given by the current-flow betweenness, that is the amount of current flowing through the node (edge), averaged over all source-target pairs, when unit current enters simultaneously at each node and flows towards a randomly chosen target. The largest loads formed on either the nodes or the edges set the maximum amount of input current for which the network is still congestion free. These two optimal values do not occur for the same value of $\beta $. As congestion may appear on nodes as well as on edges, we also study the cascading behavior of networks, triggered by the removal of one or more entities.   

Winning consensus on social networks

The adoption of a specific behavior (opinion) by a population of individuals is influenced dramatically by the social network through which the individuals interact. Here, we show the conditions under which a randomly distributed sub-population of committed agents -- nodes on the network that consistently profess a unique opinion and are not influenceable to change -- can win over an entire population of individuals initially opposed to that opinion. We model the opinion dynamics by a variant of the Naming Game (Baronchelli et al. (2006)), which effectively captures the persistence of dominant opinions. Given this model, we demonstrate that in the asymptotic network size limit, there exists a critical value p$_{c}$ of the fraction of committed agents, above which the network-state attains consensus, and below which the network-state converges to a non-consensus fixed point. We also discuss finite size corrections to p$_{c}$ and the scaling of consensus times for finite networks.   

Eigenvalue Spectra of Random Geometric Graphs

The spectra of the adjacency matrix and the graph Laplacian of networks are important for characterizing both their structural and dynamical properties. We investigate both spectra of random geometric graphs, which describe networks whose nodes have a random physical location and are connected to other nodes that are within a threshold distance. Random geometric graphs model transportation grids, wireless networks, as well as biological processes. Using numerical and analytical methods we investigate the dependence of the spectra on the connectivity threshold. As a function of the number of nodes we consider cases where the average degree is held constant and where the connectivity threshold is kept at a fixed multiple of the critical radius for which the graph is almost surely connected. We find that there exists an eigenvalue separation phenomenon causing the distribution to change form as the graph moves from well connected to sparsely connected. For example, the Laplacian spectra of well connected graphs exhibit a Gaussian envelope of integer values centered about the mean connectivity and superimposed on a real valued distribution. As connectivity decreases, the distribution shifts and includes an accumulation of eigenvalues near zero.   

Renormalization group fixed point analysis on small-world Hanoi networks

The Hanoi networks (HN) are a class of small-world hierarchical networks with varying degree distributions. Because of their unique self-similar structure, the renormalization group (RG) can be solved exactly on these networks.\footnote{S. Boettcher, C.T. Brunson, http://arxiv.org/abs/1011.1603} The real-space RG framework is used to study the Ising model phase diagrams on HNs and to interpolate between different types of networks using a tunable parameter in the recursion equations. This interpolation between different HNs reveals tunable transitions and critical behavior including the inverted Berezinskii-Kosterlitz-Thouless transition. The fixed point analysis of the RG in HNs explains the behavior of the divergence of the correlation length at critical temperatures as well as other critical phenomena observables.\footnote{See also http://www.physics.emory.edu/faculty/boettcher/.}$^,$\footnote{See also http://www.physics.emory.edu/students/tbrunson/.}   

Quantum Transport through Hanoi Networks

We present a renormalization group (RG) method to calculate the transmission of quantum particles through networks. The RG method is based on finite-dimensional matrix algebra for a tight-binding Hamiltonian [1], not a Green's function method [2]. The RG method is particularly well suited to application to hierarchical lattices. We apply the RG to obtain the quantum transmission $T$ for Hanoi networks [3] HN3 (three bonds per site) and HN5 (on average 5 bonds per site). We give the transmission $T$ as a function of the energy $E$ of the incoming particle and the tight-binding parameters (on-site energy $\epsilon$ and hopping parameters $t$) for both linear and ring geometries. We have obtained $T$ for up to $2^{200}$ sites, and have analyzed the RG equations to obtain asymptotic expressions. We find that the HN3 lattice exhibits band gaps, while no such band gaps exist in linear networks or in HN5.\\[4pt] [1] D. Daboul, I. Chang, and A. Aharony, Eur. Phys. J. B {\bf 16}, 303 (2000).\break [2] S. Datta, {\it Electronic Transport in Mesoscopic Systems} (Cambridge U. Press, Cambridge UK, 1997), and references therein.\break [3] S. Boettcher and B. Goncalves, Europhysics Lett. {\bf 84} 30002 (2008).   

Session V15: Focus Session: Spins in Semiconductors - Spin Currents III

Temperature Dependent Spin Transport in Silicon Controlled by an Electrostatic Gate

Long-distance ($\sim $500$\mu $m) lateral spin polarized electron transport in undoped silicon under the control of an electrostatic gate is studied from 40K to 120K. The temperature dependence of average spin polarization, transport time, and spin dephasing during coherent precession can be largely attributed to reduction of finite spin lifetime at higher temperatures. Measurements on devices with different transport lengths are shown to modify the effect of electrostatic gating.   

Electrical detection of spin accumulation at 500K at FM/SiO2/Si(001) contacts via the Hanle effect

We demonstrate the electrical detection of spin accumulation in Si (doped n-type 3E18 and 3E19/cm3) via injection from a ferromagnetic contact through a SiO2 tunnel barrier formed by plasma oxidation. The injection of spin-polarized carriers produces a net spin accumulation described by the splitting of the spin-dependent electrochemical potential, and is detected as a voltage. The decrease of this voltage with increasing out-of-plane magnetic field due to spin dephasing, i.e., Hanle precession of the electron spin, is observed at temperatures up to 500K. Lorentzian fits to the Hanle curves yield a spin lifetime of 100 and 320ps for the high and lower doped Si. The direct correlation between spin lifetime and carrier concentration in the Si, and that the magnitude of the Hanle signal agrees well with that expected from theory [1], provide clear evidence that the spin accumulation indeed occurs in the Si and not interface states. These results demonstrate that spin accumulation in Si can be a viable basis for spin-based devices. Supported by ONR.   

Quantifying Electron Spin Polarization from Polarized EL in Si spin-LEDs

We analyze the circular polarization (P$_{circ}$) of the electroluminescence (EL) from Si-based spin-LEDs using a recent theory [1] which provides a quantitative relation between the polarization of phonon-assisted optical transitions measured in the EL, and the electron spin polarization electrically injected from Fe/Al2O3 and Fe/SiO2 tunnel barrier contacts [2,3]. EL spectra include features due to transverse acoustic (TA) and transverse optical (TO) phonon-mediated recombination occurring in the p-doped (p$\sim$10$^{19}$cm-3) substrate. P$_{circ}$ of 3.5\% is typical for the TA at 5K, and is systematically higher than that of the TO by a factor $\sim$1.7, consistent with theory. The maximum polarization predicted for the TA is 13\% for recombination of 100\% polarized electrons in p-type Si (10$^{19}$cm-3). Thus the measured P$_{circ}$ 3.5\% corresponds to an electron spin polarization (P$_{spin}$) of 27\% produced by electrical injection from our tunnel barrier contacts. A similar analysis applied to the TO phonon at 80K yields P$_{spin}$ of 25\%. Thus the theory enables quantitative interpretation of optical polarization in indirect gap semiconductors, facilitating future studies of spin injection. [1] P. Li and H. Dery, Phys. Rev. Lett. 105, 037204 (2010). [2] B.T. Jonker, et al., Nature Physics 3, 542 (2007). [3] C.H. Li, et al, Appl. Phys. Lett. 95, 172102 (2009).   

Electrical spin injection to Germanium using a single crystalline Fe/MgO/Ge tunneling junction

Germanium has long been predicted a superior candidate for spintronics with enhanced spin lifetime and transport length due to low spin--orbit interaction and lattice inversion symmetry. One of the critical challenges, however, is to electrically create spin accumulation in otherwise non-magnetic Ge. In this work, we report electrical spin injection to bulk n-type Ge using a single crystalline Fe/MgO/Ge tunneling junction. The spin lifetime and diffusion length are extracted from both 3-terminal Hanle measurement and non-local spin valve measurement. The spin relaxation mechanism in n-type Ge has also been explicitly analyzed from the bias and temperature dependence of the spin relaxation rate.   

Spin accumulation in Fe/MgO/Si heterostructures

We report on spin injection experiments at Fe/MgO/Si interfaces using all electrical injection and detection. MgO is a promising magnetic tunnel junction material, and its incorporation with Si-based spintronics has only recently been reported in degenerately doped Si (n $\sim 10^{20} cm^{-3})$ [1]. We focus here on spin accumulation under the injecting contact for much lower n-doping levels by measuring the Hanle effect in a standard 3-terminal scheme where injection and detection are done using the same contact. The Fe/MgO spin injector was sputter deposited onto various n-doped Si bulk substrates using a variety of different substrate temperatures. The best tunnel barriers were obtained when the MgO was deposited at 70$^{\circ}$C and annealed $in-situ$ before Fe deposition. Fits to Hanle curves using the drift-diffusion model for Si samples with n=$4x10^{18} cm^{-3}$ yield spin lifetimes $\tau_{s}$ = 0.28 ns up to 30 K and a spin diffusion length L$_{s}$=$\sqrt{D\tau_{s}}$ of 0.65 $\mu$m (the diffusion constant D is obtained from the mobility assuming degenerate statistics). We determine the dependence on n, and comment on the potential differences between SOI and bulk Si wafer transport channels. [1] T. Sasaki, et al., Appl. Phys. Exp. 2 (2009).   

Electrical spin injection and detection in Si

We report electrical spin injection from Fe into Si in a Fe/MgO/Si tunnel diode grown by molecular beam epitaxy. Incorporating the spin-degree of freedom into Si adds significant new functionality in a system with established utility. In addition, the use of spin as an intrinsically quantum mechanical degree of freedom may enable more speculative computing paradigms such as spin-based quantum computation. In this work, we investigate spin injection and spin detection and spin-related transport properties in Si. This work also lays the foundation for ongoing studies correlating structural, electronic and magnetic device properties with spin injection efficiency, spin transport mechanism and real-space imaging of spin transport.   

Spin injection into Silicon using Al$_2$O$_3$, SiO$_2$ and MgO tunnel barriers

We recently demonstrated injection of spin-polarized electrons from an Fe film into Si.\footnote{B. T. Jonker et al., Nature Phys. 3, 542 (2007), O.M.J. van 't Erve et al., App. Phys. Lett. 91, 212109, (2007)} The tunnel barrier is the key component in achieving a large spin accumulation in the semiconductor. Here we will compare three different tunnel barriers, Al$_2$O$_3$, SiO$_2$ and MgO, on highly doped Si using three terminal Hanle measurements. Hanle measurements give insight in the spin-accumulation directly underneath the spin injecting contact. We will compare temperature dependence and bias dependence as well as the tunnel barrier properties such as density of interface states based on I-V and C-V measurements. We will compare spin-injection properties, such as spin lifetimes and spin injection efficiency with the oxide/Silicon interface. A spin lifetime of 120ps was obtained for 3e19 n-doped Silicon for both the Al$_2$O$_3$ and SiO$_2$ tunnel barrier, indicating that the spin accumulation occurs in the Si rather than in surface states. Support by ONR.   

Spin injection studies on thin film Fe/MgO/Si tunneling devices

We report progress on the injection of spin polarized electrons into 35 nm thick Si films, using Fe/MgO injector/tunnel barrier structures grown by molecular beam epitaxy on SIMOX silicon-on-insulator substrates. The device requires heavy top-surface n-type doping of the Si film to produce a suitable tunnel barrier, accomplished by diffusion from a spin-on phosphorous-doped glass. Measurements indicate a roughly exponential doping profile with 7E20 per cubic cm at the top surface and a 2 nm decay length. Three terminal measurements showed evidence of spin injection similar to reports of Jansen et al. [1], while injection with a thinner MgO barrier shows more complicated behavior. On-going measurements and modeling will be discussed.\\[4pt] [1] R. Jansen et al.; Nature 462; 491 (2009) Funding for this research was provided by the Center for Emergent Materials at the Ohio State University, an NSF MRSEC (Award Number DMR-0820414).   

Room-temperature magnetocurrent in antiferromagnetically coupled Fe/Si/Fe

Epitaxial Si-based ferromagnet/semiconductor structures demonstrate strong antiferromagnetic coupling (AFC) as well as resonant-type tunneling magnetoresistance, which vanishes at temperatures above T$\sim $50K [1]. Magnetoresistance effects in Fe/Si/Fe close to room temperature (RT) were not established yet. By using the ballistic electron magnetomicroscopy (BEMM) techniques, with its nanometer-scaled locality [2] we observed for the first time a spin-dependent ballistic magnetotransport in AFC structures. We found that the hot-electron collector current with energies above the Fe/GaAsP Schottky barrier reflects magnetization alignment and changes from I$_{cAP}\sim $50fA for antiparallel alignment to I$_{cP}\sim $150fA for the parallel one. Thus, the magnetocurrent [(I$_{cP}$-I$_{cAP})$/ I$_{cAP}$]*100{\%} is near 200 {\%} at RT. The measured BEMM hysteresis loops match nicely with the magnetic MOKE data.\\[0pt] [1]. R.R. Gareev, M.Weides, R. Schreiber, U. Poppe, Appl. Phys. Letts \textbf{88}, 172105 (2006); [2]. E. Heindl, J. Vancea, C.H. Back, Phys. Rev.\textbf{B75}, 073307 (2007).   

Theory of spin-dependent phonon-assisted optical transitions in Si and quantifying spin polarization in Si

The spin polarization of conduction electrons in a direct-gap semiconductor is readily quantified by measuring the circular polarization of the recombination light luminescence. However, in silicon, owing to its indirect band-gap, such a direct connection between spin polarization and luminescence has been conspicuously absent. This missing link is established with a theory that provides concise relations between the degrees of spin polarization and measured circular polarization for each of the dominant phonon-assisted optical transitions [1]. This theory has two important applications. First, it allows one to determine in a parameter-free manner the spin polarization of electrons from the measured circular polarization of the luminescence. Second, it provides a means to extract the spin relaxation time or the spin injection efficiency across ferromagnet/silicon interfaces. In the first part of the talk, by invoking symmetry arguments, I will derive concise optical selection rules for each of the phonon-assisted optical transitions in unstrained bulk silicon. It will be shown that phonon symmetries play a key role in determining the circular polarization degrees of the various phonon-assisted luminescence peaks. In the second part, the optical selection rules will be used to analyze the polarized luminescence spectrum that is calculated by a comprehensive rigid-ion model for doped silicon. The analysis is used to elucidate results of recent spin injection experiments in silicon [2]. The effect of the (weak) spin-orbit coupling in silicon on the luminescence turns out to be unique due to the proximity of the split-off band to the heavy and light hole bands in unstrained bulk silicon (44 meV). This proximity gives rise to a fast reduction in the circular polarization degree of the luminescence in p- type silicon.\\[4pt] [1] P. Li and H. Dery, Phys. Rev. Lett. 105, 037204 (2010).\\[0pt] [2] B. T. Jonker et al., Nature Phys. 3, 542 (2007).   

Tunability of spin lifetimes in strained silicon and germanium

The spin lifetimes due to electron-phonon interactions in silicon and germanium are calculated using a $sp^3$ tight-binding model. Despite of the strong spin-orbit interaction in germanium, the spin lifetime in germanium is only about one order of magnitude shorter than what is in silicon. Near room temperature, the spin-flip scattering is dominated by the inter-valley $f$ processes in silicon and by the inter-valley $X$ processes in germanium. The inter-valley scattering processes can be suppressed by shifting the valley minima with strain. We show that the spin lifetimes can be enhanced by about an order of magnitude in both materials.   

Theory of optical spin orientation in silicon

Despite weak spin-orbit coupling and an indirect band gap, significant optical spin orientation is possible in silicon. We show this by performing full band-structure calculations of the phonon-assisted absorption of circularly polarized light in bulk silicon. At 4~K a maximum spin polarization of 25\% is found at the band edge; at room temperature the polarization is still $15\%$. We present the selection rules and give the contributions from the individual phonon branches, valence bands, and conduction band valleys. Dominant are the TO/LO phonon-assisted transitions from the heavy-hole to the conduction band.   

Spin-Dependent Phonon-Assisted Optical Transitions in Germanium

We study the circular polarization of the photoluminescence due to phonon-assisted indirect optical transitions in Germanium. The band structure is calculated by empirical pseudopotential method with the spin-orbit interaction. Phonon modes are obtained by the adiabatic bond charge model and the L - $\Gamma$ electron-phonon matrix elements are calculated within the rigid-ion approximation. We have used group theory extensively to account for all possible transitions. We quantify the circular polarization of various phonon-assisted optical transitions.   

Session V16: Focus Session: Magnetic Nanostructures, Exchange Coupled System

Mu metal exchage bias

The exchange bias of the soft ferromagnet mu-metal, Ni77Fe14Cu5Mo4, with the metallic antiferromagnet Fe50Mn50 has been studied. Two series of multilayer heterostructures were grown with (111) texture induced by different buffer layer materials: Cu(300 A)/Ni77Fe14Cu5Mo4(200 or 400 A)/Fe50Mn50 (100 A)/Cu(300 A) and Ta(50 A)/Ni77Fe14Cu5Mo4(60--400 A)/Fe50Mn50(150 A); control samples were grown without Fe50Mn50. The samples have a clear unidirectional anisotropy induced by depositing in a magnetic field, the exchange bias magnitude is inversely proportional to the mu-metal thickness, and the interfacial coupling energy of 0.045 erg/cm2 agrees with previous results for FeMn antiferromagnets. While the Cu-buffered samples reveal a significant increase in coercivity and saturation field when exchange biased, the Ta-buffered samples retain the soft magnetic properties of the mu-metal simultaneously with the exchange bias. The ability to preserve soft ferromagnetic behavior in an exchange biased heterostructure may be useful for device and sensing applications.   

Intrinsic Exchange Bias and Origin of Uncompensated Magnetization in FeF$_{2}$

After more than 50 years since the discovery of Exchange Bias, its microscopic mechanism remains unknown. Several experimental findings demonstrate and many models agree that uncompensated magnetization (UM) in the antiferromagnet (AF) plays an important role in exchange bias. However, the origin of the UM is unknown. Magnetometry and polarized neutron reflectometry (PNR) measurements indicate that the UM is present even in the AF-only, (110)-FeF$_{2}$ grown on MgF$_{2}$, samples, and the PNR reveals the spatial distribution of the UM. Exchange bias in the AF-only sample is reported. Coupling of the UM to the bulk antiferromagnetic order parameter is supported by several experimental results, including high value of exchange bias field, its temperature dependence and the absence of training effect. We will discuss the origin of the UM based on general symmetry arguments.   

Tuning the Magnetic Properties in Exchange Coupled FeO/Fe$_{3}$O$_{4 }$Core-Shell Nanoparticles

Chemically synthesized FeO with a native oxide shell has recently received attention due to the large exchange bias effects observed in this system. The magnetic properties reported thus far have been highly dependent upon the aging affects of the system given the vulnerability of FeO to further oxidation resulting in degradation of the exchange bias effects. We report on the magnetic properties of FeO nanoparticles chemically synthesized to form several base diameters (10 nm, 20 nm, and 30 nm) which have each been annealed at various temperatures to obtain a variety of core/shell FeO/Fe$_{3}$O$_{4}$ size ratios. This controlled oxidation method has also given excellent chemical stability to the particles. XRD analysis confirms the existence of polycrystalline phases of FeO and Fe$_{3}$O$_{4}$, and magnetometry experiments reveal the existence of large exchange bias (up to 230 mT) as well as coercivity enhancements (up of 250 mT) which persist up the N\'{e}el temperature (which scales with core size). Other exchange coupling effects such as a large vertical shift in the field cooled hysteresis loops and asymmetric magnetization reversal are observed. Our results further advance the understanding of this exchange coupled system and imply that the properties can be chosen utilizing as-synthesized particle size and annealing temperatures.   

Tuning exchange bias in Ni/FeF$_{2}$ heterostructures using antidot arrays

The transition from positive to negative exchange bias can be systematically tuned with antidot arrays artificially introduced into Ni/FeF$_{2}$ ferromagnetic (FM)/antiferromagnetic (AF) heterostructures. This is a consequence of the energy balance between the Zeeman coupling of the AF spins to the cooling field, and the AF exchange coupling at the FM/AF interface. The nanostructure plays a key role in the formation of pinned uncompensated spins in the AF: the antidot carving produces regions of locally pinned uncompensated spins throughout the antidot faces of the FeF$_{2}$ and these \textit{non} interfacial magnetic moments favor the onset of positive exchange bias at lower cooling fields, by increasing the Zeeman energy of AF domains and favoring the alignment with the latter. Those \textit{non} interfacial AF spins, and the pinned uncompensated interfacial AF spins responsible for the exchange bias (loop shift), align simultaneously with the cooling field since they belong to the same AF domain and become pinned below the N\'{e}el temperature.   

Depth Profiles of Exchange Stiffness and Anisotropy in a Spring Magnet with Intermixed Interfaces

With complementary studies of Polarized Neutron Reflectometry (PNR) and micromagnetic simulations, we determined the depth profiles of the intrinsic magnetic properties in an Fe/Sm-Co spring magnet with intermixed interfaces, including saturated magnetiztion, exchange Stiffness and magnetic anisotropy. We found that intermixed region at the Fe/Sm-Co interface is about 8 nm wide, where the magnetic properties change gradually. We compared the results to a model based on a simple mixture of the Fe phase and the Sm-Co phase, as determined from the chemical depth profile using x-ray and neutron reflectivities. In the intermixed region, the saturation magnetization is slightly lower than the value estimated from the model but the exchange stiffness is higher. The magnetic anisotropy is also lower than the expected value from the model. Therefore the intermixed interface yields superior exchange coupling between the Fe and the Sm-Co layers but at the cost of total magnetization.   

Geometric Structure of Magnetic Domains in CoPd/IrMn Multilayer Films

Using coherent x-ray resonant scattering in a transmission geometry, we collected magnetic scattering signal from CoPd/IrMn exchange biased multilayer films. The incident photon energy was tuned to the Co L3 edge allowing the magnetic domain configuration in Q space to be probed. Rotational autocorrelation functions of the resulting speckle diffraction patterns manifest the local geometric character of domain structure in Q-dependent fashion. These results are compared to microscopic magnetic domain memory probed by cross correlating different patterns. The memory under two different cooling conditions, with saturating field or with zero field was investigated, as well as the dependence of memory on external parameters, such as applied magnetic field and temperature.   

Correlation between bias fields and magnetoresistance in CoPt biased NiFe/Ta/NiFe heterosystems

Exchange coupled magnetic hard layer/soft layer (SL) thin films show SL biasing in close analogy to exchange bias systems with antiferromagnetic pinning. Here we study CoPt(35nm)/NiFe(450nm)/Ta(d)/NiFe(450nm) heterostructures with 0.7 $<$d$<$5nm. We use alternating gradient force magnetometry to measure the overall magnetization reversal and minor loop behavior. Magnetoresistance (MR) is measured by four-point methodology and modeled using magnetization data thus confirming the assumptions of uniform rotation of the top layer and exchange spring behavior of the pinned NiFe layer. In addition, Polarized Neutron Reflectometry (PNR) provides an independent data set for magnetization depth profiles. We compare and contrast results from our magnetometry and MR technique with PNR results. The objective of this comparison is to show that single-component magnetometry in concert with MR and modeling reveals the full vector and depth profile information of the distinct magnetization reversal mechanisms. Financial support by NSF through Career, MRSEC, DOE-OBES   

Growth and magnetism of highly (001)-oriented [Fe/Pt]$_{n}$/Pt films

Highly (001) textured non-epitaxial $L$1$_{0}$ FePt films have been fabricated on SiO$_{2}$ substrates by post-deposition annealing 11 nm magnetron sputtered multilayers of Fe and Pt with an additional overlayer of 1 nm Pt. An identical series of films was made without the thick Pt terminating layer for comparison. All films were post-deposition annealed at 600 \r{ }C for 300 s in a rapid thermal processor and show a high degree of chemical order. The ordered films without a Pt overlayer include a mixture of (001) and randomly oriented grains. In the samples with a Pt overlayer only the (00$l)$ peaks are visible, demonstrating an enormous enhancement in the degree of (001) texture. Structural analysis reveals a decrease in surface roughness from over 2 nm to less than 1 nm, elimination of voided regions, and an increase in average grain size from 50 to 150 nm with the inclusion of a Pt overlayer. Magnetic hysteresis loops show a high squareness ratio for Pt-overlayer samples with coercivities much smaller than their no-overlayer counterparts. The effects of Fe:Pt stoichiometry and bilayer thickness are investigated along with the involved grain-growth process.   

Exchange bias and magnetic anisotropy in ultrathin iron films grown on (001) GaAs

Ultrathin iron films grown by MBE on GaAs substrates were studied by SQUID and by ferromagnetic resonance (FMR). Exchange bias (EB) was observed in this system at temperatures below 20 K, but disappeared at higher temperatures. The angular dependence of asymmetric hysteresis loops of the sample were understood as resulting from the coexistence of the cubic and uniaxial magnetic anisotropy fields of the Fe film and the EB field arising from a yet unidentified inter-layer between Fe and GaAs (possibly Fe2As). Magnetic anisotropy of the Fe films was investigated by FMR in a manner similar to that described by Aktas et al. [1]. By fitting the angular dependence of the FMR field we have obtained magnetic parameters of the sample, which are similar to those reported in Ref. [1]. However, the g- factor obtained from the fitting shows an unexpected anomalous increase in the low temperature range. Since this behavior occurs exactly in the range where EB appears, it is tempting to speculate that these two effects are causally related.\\[4pt] [1] B. Aktas et al., J. Appl. Phys. 102, 013912 (2007).   

Measuring Exchange Bias in Patterned Films using Ferromagnetic Resonance

Exchange bias exploits the exchange interaction at the interface between a~ferromagnet and an adjacent antiferromagnet to create a preferred orientation for the ferromagnet. He-ion bombardment has been used to create stripe-patterned films displaying anti-parallel exchange bias in adjacent stripes. As the width of these stripes approaches micron-size, magnetization reversal within individual stripes can be hindered by dipolar fields from magnetic charges at boundaries, making magnetometry measurements difficult~to interpret. Here we report Ferromagnetic Resonance measurements of the magnitudes of the two opposing exchange bias fields perpendicular to the stripe axis, the dipolar fields experienced by neighboring stripes, and we quantify the effect of ion irradiation on the saturation magnetization of the Ni$_{81}$Fe$_{19}$ films.   

Scanning Probe Ferromagnetic Resonance Imaging of Stripe Patterned Exchange Bias IrMn-NiFe Film Using Nanoscale Confined Modes

We report scanned probe Ferromagnetic Resonance (FMR) imaging of the spatially modulated internal exchange-bias field in the exchange coupled ferromagnet (FM)/antiferromagnet (AF) Ni$_{81}$Fe$_{19}$ /Ir$_{23}$Mn$_{77}$ bilayer material using Magnetic Resonance Force Microscopy (MRFM). The exchange bias is spatially modulated by ion beam irradiation into a periodic stripe pattern having 2 or 20 micron periods. Adjacent stripes have oppositely aligned exchange bias fields. Our new method of FMR imaging employs the locally confined FMR modes created by a strong, non-uniform probe tip field on the out-of-plane saturated Ni$_{81}$Fe$_{19}$ film. We image the spatial variation of the inhomogeneous internal field with spectroscopic precision clearly resolving two exchange bias regions. Analysis of the local magnetic properties and their transition at the boundary of two exchange bias regions will be presented. This work was supported by the U.S. Department of Energy through Grant No. DE-FG02-03ER46054.   

Large exchange bias after zero-field cooling from an unmagnetized state

Exchange bias (EB) is usually observed in systems with interface between different magnetic phases after \textit{field cooling}. Here, we report an unexpected finding that a \textit{large} EB can be realized in Ni-Mn-In bulk alloys after \textit{zero-field cooling from an unmagnetized state}. We propose that the size of superparamagnetic domains in the alloys can grow up under external magnetic fields, which induces a transition from a superspin glass to a superferromagnetic (SFM) state. The SFM unidirectional anisotropy, which is the origin of EB effect, can be created at the \textit{newly} formed SFM-antiferromgantic interface during the initial magnetizing process.   

High energy product of Sm$_{2}$Co$_{7}$/FeCo nanocomposites prepared by severe plastic deformation

Nanocomposite magnets, consisting of exchange coupled hard magnetic and soft magnetic phases exhibit enhanced remanent magnetization and therefore high energy product \textit{(BH)}$_{ max}$. In addition to the high performance, nanocomposites magnets are of commercial interest because the alloys require less expensive rare-earth elements. Here, we report Sm$_{2}$Co$_{7 }$+ x wt {\%} FeCo (x = 0 to 50) nanocomposites prepared by high energy ball-milling and subsequent heat treatments. The evolution of structure and magnetic properties with soft phase fraction was systematically studied. Effect of the soft phase composition Fe$_{100-X}$ Co$_{X}$ ( x = 20, 35 and 50) was also investigated. Microstructural studies by energy filter transmission electron microscopy revealed a homogeneous distribution of -Fe phase in the matrix of hard magnetic Sm-Co phase with grain size less than 20 nm after severe plastic deformation. Enhanced remanence and \textit{(BH)} $_{max}$ (up to 17 MGOe) in the nanocomposites with 40 {\%} of the soft phase are obtained.   

Microstructure refinement in Nd$_{2}$Fe$_{14}$B/(Fe,Co) nanocomposite ribbons produced by melt-spinning in a magnetic field

Nd$_{2}$Fe$_{14}$B/(Fe,Co) ribbons were prepared by melt-spinning in a magnetic field perpendicular or parallel to the wheel surface. The starting alloy Nd$_{15}$Fe$_{77}$B$_{8}$ was mixed with soft magnetic Fe, or Co, or Fe$_{65}$Co$_{35}$. The amount of the soft phases was varied from10 to 40 wt.{\%} of the hard phase. The wheel was reconstructed to provide the surface magnetic fields in the range of 1 to 4 KG perpendicular or parallel to the wheel surface. The obtained results show that grain size in the ribbons was significantly reduced while the texture was enhanced. The mechanism remains to be fully understood, though it may be related to a change of the C-shaped diagram. The observed results suggest that a magnetic field can be used to control and optimize microstructures of nanocomposite ribbons. The effect of field strength and configuration is also discussed in details.   

Structure and magnetism of epitaxial NiMn single layers and Co/NiMn bilayers on Cu$_{3}$Au(100)

The structure of single-crystalline NixMn100-x (NiMn) ultrathin films on Cu3Au(100) and also the magnetic properties of Co films on the NiMn/Cu3Au(100) films have been investigated by multiple techniques. For 10 $\le $ x $\le $ 77, our results revealed good epitaxial, layer-by-layer growth at a substrate temperature of 300 K for all NiMn films with near equiatomic composition. The results indicate a face-centered tetragonal (fct) structure for NiMn, as expected for the L1o phase, and with the c-axis along the film normal. For the Co/NiMn bilayers, MOKE hysteresis loops show a thickness independent coercivity, suggesting no magnetic coupling at the Co/NiMn interface. Although the structural results indicate the formation of ftc NiMn in the equiatomic concentration range, we have no indication of antiferromagnetism for NiMn on Cu3Au(100) at room temperature. This is contrary to the observations for Co/NiMn on Cu(100).   

Session V17: Focus Session: Bulk Properties of Complex Oxides - 5d Oxides

Spin-Orbit Interaction Rediscovered in Transition Metal Oxides

The 5$d$-transition metal oxides are a class of novel materials that exhibit nearly every collective state known for solids. It is commonly expected that iridium oxides should be more metallic and less magnetic than their 3$d $and 4$f$ counterparts due to the extended nature of the $5d $orbitals. In marked contrast, many iridates are magnetic insulators that exhibit a large array of phenomena seldom or never seen in other materials. We review the anomalous physical properties of several iridates and address potential underlying mechanisms, which include strong orbital magnetism, the J$_{eff}$ = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ insulating state, and spin-orbit coupling; the latter strongly competes with other interactions to create an unusual balance between relevant degrees of freedom in this class of materials.   

Resonant inelastic x-ray scattering study of charge ordering in CuIr2S4

We present Ir L$_{3}$-edge resonant inelastic x-ray scattering (RIXS) spectra and resonant x-ray emission spectra (RXES) on the thiospinel, CuIr$_2$S$_4$, which has been attracting much interest due to intriguing metal-insulator transitions. At room temperature CuIr$_2$S$_4$ is metallic, but goes through a metal insulator transition at T$_{\rm{MI}} \sim$ 226 K due to the formation of charge order (CO) of Ir$^{3+}$ and Ir$^{4+}$ together with spin dimerization between Ir$^{4+}$ ions. By exposing the sample to x-ray below T $ = $ 50 K, the crystal symmetry goes from triclinic to tetragonal, accompanied by reduced resistivity. The RIXS signal was dominated by a broad and strong feature around 3 eV, arising from t$_{2g}$ to e$_{g}$ transition, but we were able to observe a clear signature of opening of the insulating gap across the metal-insulator transition. In addition, we also found that this gap is partially filled in the irradiation-induced phase. The emission spectra reveals the existence of an excited Ir-5d \emph{t$_{2g}$} state, which is hidden in the Ir L$_3$-edge XAS of CuIr$_2$S$_4$. The result indicates that the electronic reconstruction that takes place in the irradiation-induced phase comes from the Ir$^{3+}$ while the Ir$^{4+}$ dimers are unchanged.   

Electron-doped Sr$_{2}$IrO$_{4-\delta}$ $(0\leq \delta \leq 0.04)$: Evolution of a disordered J$_{eff}=1/2$ Mott insulator into an exotic metallic state

Stoichiometric Sr$_{2}$IrO$_{4}$ is a ferromagnetic J$_{eff} = 1/2$ Mott insulator driven by strong spin-orbit coupling. Introduction of very dilute oxygen vacancies into single-crystal Sr$_{2}$IrO$_{4-\delta}$ $(\delta < 0.04)$ leads to significant changes in lattice parameters and drives a number of intriguing phenomena such as insulator-to-metal transition at $T_{MI} \approx 105K$, anomalous non-Ohmic behavior and an abrupt current-induced transition in the resistivity. Highly-anisotropic resistivity of the samples continues to decrease by several orders of magnitude below $T_{MI}$ without saturation to a residual limit at the lowest temperature studied $T=1.8K$. The low-temperature metallic state exhibits two distinct regimes (separated at $T\approx 52K$) of switching in the non-linear $I-V$ characteristics. The novel behavior illustrates an exotic ground state and constitutes a new paradigm for device structures.   

Magnetic ordering in Sr$_{2}$IrO$_{4}$ from first principles

Sr$_{2}$IrO$_{4}$ is a layered compound with IrO$_{2}$ planes, separated by two SrO planes. Experimentally Sr$_{2}$IrO$_{4}$ shows weak ferromagnetism. This behavior can be assigned either as band magnetism or canted antiferromagnetic ordering. The latter has been confirmed by Arpes. With DFT calculations (using the WIEN2k package and Quantum Expresso) we show that the antiferromagnetic ordering is more stable than the ferromagnetic one, and due to the Dzyaloshinskii-Moriya rules there is a possibility of canted magnetic ordering.   

Effect of spin-orbit coupling on the band structure, magnetic ground states and low energy excitations of double perovskites

We investigate a model for double perovskites A$_{2}$BB$^{\prime}$O$_{6}$ that describes the coupling of local moments on the B site to itinerant electrons contributed by the B$^\prime$ sites. To model materials like Sr$_2$CrOsO$_6$ we examine the role of spin-orbit coupling on the the B$^\prime$ site, which cannot be ignored because of the large $Z$ of Os. First, we present $T=0$ results for the net moment in the ferrimagnetic state. We show that direct B$^\prime$-B$^\prime$ hopping plays just as important role as the spin orbit coupling in determining the ordered moment. We use our model Hamiltonian approach to discuss the question of metallic versus insulating ground states, by including the effects of Coulomb $U$ on the spin-orbit split electronic structure. Finally, we investigate the low energy excitations of this model to understand the origin of the experimentally observed nonmonotonic behavior of magnetization as a function of temperature.   

Magnetic and structural properties of Sr2CrReO6 epitaxial films fabricated by ultra-high vacuum sputtering

Sr2CrReO6, a double-perovskite half-metallic ferromagnet, has attracted much attention due to its high Tc of 620 K. However, balancing the stoichiometry and ordering of a quaternary oxide is no trivial matter. We have deposited pure-phase Sr2CrReO6 epitaxial films on SrTiO3 substrates by ultrahigh vacuum off-axis magnetron sputtering with precise control of the oxygen partial pressure and in-situ monitoring by high-pressure residual gas analyzer. The films exhibit saturation magnetization at T = 5K approaching 0.9 Bohr magnetons per formula unit and Tc close to 600 K. X-ray diffractometry spectra demonstrate epitaxy and phase purity with a rocking curve FWHM of 0.012 degrees. Laue oscillations give evidence of exceptionally smooth surface and interface as well as precise film thickness determination. Finally, direct observation of the films by HAADF STEM show nearly defect free films with double-perovskite ordering. We will discuss the effects of stoichiometry, growth pressure and oxygen content on sample properties.   

Electron doped CrO$_2$: An unsual example of a charge ordered ferromagnet

Usually metallicity accompanies ferromagnetism. K$_2$Cr$_8$O$_{16}$ is one of the less common examples of magnetic materials, exhibiting ferromagnetism in the insulating state [1]. Analyzing the electronic and magnetic properties within first principle electronic structure calculations, we find [2] that K acts like a donor. The doped electrons associated with the introduction of K in the lattice, induces a charge ordered and insulating ground state and interestingly also introduces a ferromagnetic coupling between the Cr ions. The primary considerations driving the charge ordering are found to be electrostatic ones with the charge being localized on two Cr atoms that minimize the electrostatic energy. The structural distortion that accompanies the ordering, generates a pathway for the electron localized on one site to hop on to the neighboring sites, a process more favorable in the ferromagnetic case, thus, giving rise to a rare example of a charge-order driven ferromagnetic insulator. \\[4pt] [1] Kunihiro Hasegawa {\it et al.}, Phys. Rev. Lett. {\bf 103}, 146403(2009). \\[0pt] [2] Priya Mahadevan, Abhinav Kumar, Debraj Choudhury and D.D. Sarma, Phys. Rev. Lett {\bf 104}, 256401 (2010).   

Ab initio study of the anti-ferromagnetic, non-collinear CuB$_2$O$_4$ Crystal

The spiky features in the crystal absorption spectrum, and the distinct differences in the directional oxygen K-edge absorption spectroscopy of the non-collinear anti-ferromagnetic, incommensurate CuB$_2$O$_4$, had led us to this LDA+U study of the crystal, although in the commensurate phase, due to the instrumental limitation. The calculated band structure matches the spiky features in the absorption spectrum, while the orbital analyzed DOS data explain the differences in the directional oxygen K-edge absorption spectroscopy. The two groups of dispersion-less bands, immediately above the gap, come from different groups of plaquettes, of Cu(A) and Cu(B), and are responsible for the spiky features observed experimentally.   

Doping Rules in A$_{2}$BO$_{4}$ Spinel Oxides

Many of the physical phenomena surrounding Complex Oxide involve the creation and annihilation of charge carriers by cross --substitution of atoms or by the formation of vacancies and interstitials. We have used the machinery of First-Principles defect calculation, developed and tested over the years on semiconductors (where experimental data needed to test DFT corrections is rather clear), applying it to a large number of oxides, initially from the Spinel family. We calculate defect formation energies as a function of temperature and oxygen partial pressure, as well as the concentration of donors and acceptors and the ensuing free carriers. A number of regularities emerge. (i) Oxygen vacancies are not a viable source of electrons and cation vacancies are (usually) not a viable source of holes. (ii) Instead, cation-anti-sites (A-on-B donor and B-on-A acceptors) tend to form in significant numbers and release carriers. (iii) For the group of A3+ and B2+ spinels we find four ``doping classes'' (a) both donor and acceptor are in the gap (Al$_{2}$MgO$_{4})$ (b) Only acceptor is in the gap (Co$_{2}$ZnO$_{4})$ (c) only donor in the gap and (d) none in the gap. Simple regularities can be used as first-order rules to guess electrical behavior from composition. This work was supported through the Center for Inverse Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.   

Magnetoelectricity and magnetostriction due to the rare-earth moment in TmAl$_{3}$(BO$_{3}$)$_{4}$

We investigated the magnetic, magnetostriction and magnetoelectric properties of d-electron free rare-earth aluminum borate TmAl$_3$(BO$_3$)$_4$ between room temperature and 2 K. The magnetoelectric polarization along the `a' and `c' directions reaches up to 300 $\mu$ C/m$^2$ at 70 kOe with the field is applied along the `a' axis. `c' axis magnetic field does not show any significant effect, which correlates with the fact that $\chi_a$ changed very rapidly compared to $\chi_c$. We find that the polarization is proportional to the magnetostriction. The result of this investigation prove the existence of a significant coupling between the rare-earth magnetic moment and the lattice in RAl$_3$(BO$_3$)$_4$ compounds (R=rare earth). This compound shows that the rare-earth moment is sufficient enough to generate a large magnetoelectric effect. This is comparatively a simpler system to study and understand the origin of magnetoelectric effect.   

On the Pairing Instability in Rutile MO$_2$

The metal-dioxide family of compounds such as TiO$_2$, SnO$_2$, VO$_2$, NbO$_2$, and WO$_2$ have been of much recent interest for reasons as diverse as understanding novel correlated electron physics to designing new photo/electro-catalysis. All can be thought of as forming in a prototypical te\-tra\-gonal ``rutile'' structure, yet members with unpaired d$^1$ and d$^2$ electrons undergo a structural phase transition to a monoclinic, ``distorted rutile,'' structure. In some cases this metal pairing transition accompanies a metal to insulator transition, the precise role, however, is still not clear. Here we present a comparative first-principles study of the lattice instabilities in d$^1$ and d$^2$ MO$_2$ rutile, paying particular attention to the d$^2$ system WO$_2$, which remains metallic even in the distorted phase. We calculate the phonon dispersion in the tetragonal prototypical structure. Using unstable high symmetry modes as a guide, we calculate the energy surface around the high-symmetry structure and perform full structural relaxations in the distorted ground states. We elucidate the interplay between the electronic structure and the pairing transition and discuss the possibility of controlling it with strain.   

Relativistic effect determines the oxidation states: a study of Rh and Ir oxides by first principles methods

The relativistic effect becomes significant on determining the structure and properties of $4d$ and $5d$ transition-metal compounds. It is found in some iridates that the relativistic effect, mainly contributed as spin-orbit interactions, can enhance the otherwise weak correlation of $5d$ electrons and cause an unusual Mott transition. Utilizing such effects in creating new phase such as topological insulator has grown into a hot spot in the frontier of functional oxide research. However, the relativistic effects on orbital energies, although important on determining the structure, has not been systematically studied. The general trend of the oxidation states of transition metals in the same group is to decrease with increasing atomic number. However, in contrast to this trend, Ir tends to form IrO$_{2}$ (4+) whereas Rh forms both Rh$_{2}$O$_{3}$ (3+) and RhO$_{2}$. Using relativistic and non-relativistic first principles calculations, we demonstrate that the unusually high oxidation state of Ir and the high stability of IrO$_{2}$ is caused by relativistic effect. Because relativity contracts the $s$ and $p$ orbitals, it repels Ir $5d$ electrons outwards and increases their energies. As a consequence, Ir tends to be oxidized to 4+ state and forms IrO$_{2}$.   

Session V18: Focus Session: Low D/Frustrated Magnetism - Spin Ice, et al.

Magneto-optical Kerr Effect Studies of Artificial Frustrated Magnets

We use the magneto-optical Kerr effect (MOKE) to study the collective magnetic behavior of geometrically frustrated arrays of single-domain ferromagnetic islands. By varying the island spacing, lattice geometry and the orientation relative to the magnetic field, we probe the properties of the arrays via MOKE measurements of the net moment of the arrays. We study the influence of local geometry and frustration on the collective magnetization reversal process, using the switching field as a measure. Further, angle-resolved MOKE measurements probe the influence of individual island shape anisotropy on the collective anisotropy of interacting arrays. Finally, we present preliminary measurements in an oscillating magnetic field. The results are compared to the results of micromagnetic simulation. We thank M. Ericson and C. Leighton for sample preparation. This research was supported by the US Dept. of Energy.   

Measuring Disorder in Artificial Kagome Ice

Artificial spin ice is proving to be a valuable tool in understanding magnetic interactions on the nanoscale. It can directly show the interactions responsible for geometric frustration, and different geometries have been used to model real pyrochlore spin ice compounds and other lattices. The strength in the approach lies in the ability of a synthetic material, fabricated from macroscopic artificial ``atoms,'' to mimic real materials, where atoms are essentially identical with low disorder from lattice site to lattice site. However, in artificial spin ice materials there can be substantial variation among the artificial atoms in relevant quantities such as coercive field, with some systems showing standard deviations as high as 20{\%}. By carefully studying the reversal process of artificial kagome ice along specific crystallographic directions, we can directly measure the distribution of coercivities of the individual nanoscale magnets. ~By using a lattice of connected magnets fabricated from Ni80Fe20, we find that the coercivity distribution can have a deviation of less than 5{\%}. These narrow deviations should allow the observation of behavior that mimics more closely what would be expected in real spin ice materials.   

Dynamics of magnetization in artificial spin ice on kagome

We model magnetization dynamics in artificial spin ice on kagome under an applied magnetic field. Magnetization reversal is mediated by domain walls carrying two units of magnetic charge emitted from and absorbed by lattice junctions and propagating along the wires. The Coulomb interaction between magnetic charges induces avalanches in magnetization reversal. Distributions of avalanche lengths for various angles between the initial magnetization and the applied magnetic field were considered. We used a Gaussian distribution in the magnitude of the links' critical fields to mimic disorder in a real system [1]. An asymmetric distribution of topological defects at a wire junction gives rise to an offset angle $\alpha$ in the reversal field $H(\theta)=H_{c}/\cos{(\theta+\alpha)}$ where $\theta$ is the angle between the link and the applied magnetic field [2]. The model reproduces the salient features of magnetization reversal curves observed experimentally. \\[4pt] [1] Y. Qi, T. Brintlinger, and J. Cumings, Phys. Rev. B \textbf{77,} 094418 (2008). \\[0pt] [2] P. Mellado, O. Petrova, Y. Shen, and O. Tchernyshyov, Phys. Rev. Lett. \textbf{105,} 187206 (2010).   

Disorder and field-induced dynamics in artificial spin ice

Artificial spin ices are athermal systems for which dynamics are induced by a time varying applied field. The field induced dynamics have received a lot of attention, both experimental and theoretical (see, e.g., [1,2]), but these studies have not dealt explicitly with the effects of disorder. We show, through numerical simulations and studies of the phase space of the system, that disorder in fact has a strong and fundamental effect on the field-induced dynamics. This highlights the fact that an understanding of the dynamics of artificial spin ice must take into account both the sequence of applied fields and the spin ice lattice. \\[4pt] [1] X. Ke, J. Li, C. Nisoli, P. E. Lammert, W. McConville, R. F. Wang, V. H. Crespi, and P. Schiffer, Phys. Rev. Lett. 101, 037205 (2008).\\[0pt] [2] Z. Budrikis, P. Politi, and R. L. Stamps, Phys. Rev. Lett. 105, 017201 (2010).   

Control of Ground State Order in Artificial Square Ice

Anisotropy in nanomagnet arrays can be tailored to enforce geometrical frustration, so that analogs of spin-ice materials can be fabricated [1-2]. We have studied artificial square ice, which consists of interlinked vertices of four Ising moments. Previously, energy minimisation via ac demagnetization has received significant attention, however, the long-range ordered ground state (GS) is inaccessible via this method. Furthermore, equilibriation is disallowed in the athermal limit so far explored. We show it is possible to realise GS order in as-prepared arrays, fabricated via electron beam lithography and evaporation, due to early-growth thermalization [3]. Monopole and string-like excitations from the GS are seen to be Boltzmann factor-weighted. Monopole propagation and interactions can be inferred within an energy band structure. Lattice spacing and buffer material allow control of ordering. \\[4pt] [1] Wang et al., Nature (2006), \textbf{439}, 303-306 \\[0pt] [2] Harris et al., PRL (1997), \textbf{79}, 2554-2557 \\[0pt] [3] Morgan et al., Nature Phys. (at press)   

Monopole Dynamics in Spin Ice

The last couple of years have witnessed intense interest in spin ice materials due to the unique nature of its low energy excitations, which take the form of emergent magnetic monopoles. Through combined theoretical and experimental work, it has become increasingly apparent that an effective description of these excitations in terms of free, Coulomb interacting point-like quasiparticles is essential to develop an understanding of the thermodynamic properties of these materials beyond numerical simulations. On the other hand, we are only just beginning to unravel the repercussions of such exotic excitations on the dynamics of spin ice, in relation for instance to how the system relaxes when driven out of equilibrium, or in relation to thermal transport experiments. In this talk we review some of the latest theoretical and experimental results on the out of equilibrium properties of spin ice materials, ranging from thermal and field quenches [Castelnovo, Moessner, \& Sondhi, PRL 104, 107201 (2010) and ongoing work] to thermal runaways in response to a varying magnetic field [Slobinsky \textit{et al.}, arXiv:1010.4143v1]. In particular, we discuss how these phenomena can be understood as consequences of the specific nature of the low energy excitations.   

Magnetic Monopoles in Matter: An Analytic Theory

Transport theory is presented which, starting from the microscopic field equations, incorporates the magnetic monopole physics occurring in materials such as spin ice. Hall effect, Landau levels, and thermopower are calculated for magnetic charges. Magnetic charge currents in elementary lattices are shown to exist even in the absence of geometrical frustration.   

Theoretical and computational models of emergent magnetic monopoles and Dirac strings in kagome spin-ice

Magnetic monopoles and their associated Dirac strings have recently been experimentally observed as emergent quasiparticles in frustrated magnetic spin-ice systems. Detection of reciprocal signatures of monopoles were reported for 3D pyrochlore systems, and subsequently, direct real-space observations of monopoles and their associated Dirac strings were made in 2D artificial kagome lattices. In contrast to conventional domain growth, the magnetization process in these spin ice systems proceeds through nucleation and avalanche-type propagation of overturned dipoles - physical versions of a Dirac string. The 1D nature of these avalanches in a 2D system provides an example of dimensional reduction through frustration. We establish a theoretical model and perform Monte Carlo simulations which faithfully reproduce the observed hysteresis, string-avalanche statistics and monopole densities.   

Hunting a [111] magnetization plateau to test the quantum spin ice model in Tb2Ti2O7

The pyrochlore magnet Tb2Ti2O7 may be described by a quantum spin ice model. This model predicts a magnetization plateau will occur for weak fields applied along the [111] axis at low-temperature. We have carried out muon-spin relaxation measurements to test this hypothesis. Features are observed at 15 and 65mT, agreeing with the predicted boundaries of the magnetization plateau. In the intermediate region the field dependence of the muon relaxation rate suggests a constant distribution of local magnetic fields of 10mT, and a constant fluctuation time of 20ns. ac susceptibility measurements are being carried out to investigate the bulk response on a longer timescale.   

Creation and Measurement of Magnetic Charge Currents in Spin Ice

The recent discovery of magnetic charge in spin ice raises the question of whether long-lived currents of magnetic ``monopoles'' can be created and manipulated by applying magnetic fields. Here we show that they can; by applying a magnetic field pulse to a Dy$_2$Ti$_2$O$_7$ spin ice crystal at 0.36 Kelvin, we create a relaxing magnetic current that lasts for several minutes. We measure the current by means of the electromotive force it induces in a solenoid coupled to a susceptometer and quantitatively describe it using a chemical kinetic model of point-like charges obeying the Onsager-Wien mechanism of carrier dissociation and recombination.   

$d$-wave Metal phase of itinerant electrons with ring exchange on a 2-leg ladder

I will present recent results in the search for theoretical insights into non-Fermi liquid phases in two dimensions. In particular, we propose a novel conducting phase for itinerant electrons on the square lattice with strong $d$-wave correlations (i.e. a ``$d$-wave metal''; see [1] for the bosonic analog of this phase) and a candidate Hamiltonian (hopping plus four-site electron ring exchange), which we examined in search of this phase (at $T=0$) over some portion of the phase diagram. For numerical tractability, we specialize to the 2-leg ladder and study this model using variational Monte Carlo (VMC) and the density matrix renormalization group (DMRG). For the VMC, we construct trial wavefunctions corresponding to a particular slave particle decomposition of the electrons and consistent with the properties of the proposed $d$-wave metal as well as for the ``normal'' Fermi liquid phase to map out a VMC phase diagram for the candidate Hamiltonian. Meanwhile, DMRG is employed as a quasi-exact probe of this Hamiltonian and the successes and failures of the trial wavefunctions, relative to the unbiased DMRG results, will be presented. [1] O. I. Motrunich and M. P. A. Fisher, Phys. Rev. B, {\bf 75}, 235116 (2007).   

Entanglement Hamiltonians for quantum spin chains

We report on our analysis of entanglement phenomena in gapped and gapless quantum spin chains. In particular we discuss criteria of correspondence between entanglement spectra and Hamiltonian spectra with respect to symmetries, spectral gaps, and eigenstate properties. We find that the structure of the entanglement Hamiltonian associated with the ground state is helpful to discover various spectral properties of the full system.   

Session V19: Focus Session: Spin Transport & Magnetization Dynamics in Metals VIII

Ferromagnetic resonance studies of CoFeB-MgO

There has been much interest in ferromagnetic magnetic tunnel junctions (MTJs) as a potential candidate for spin transfer torque memories. Many parameters are important in order to optimize the spin transfer torque effect to minimize the critical switching current density (J$_{c})$ without compromising an energy barrier (E$_{B})$ between stable states. CoFeB/MgO systems have many desirable properties including high spin polarization and, thereby high tunnel magnetoresistance. Recently, Ikeda et al. reported that Fe-rich CoFeB/MgO MTJs can induce perpendicular anisotropy that is strong enough to overcome the in-plane shape anisotropy, demonstrating CoFeB-based perpendicular MTJs [1]. In this work, we have performed FMR studies as a function of alloy composition, layer thickness, pre and post annealing of CoFeB/MgO systems. Coplanar waveguide method VNA FMR experiments were performed [2]. From the FMR resonance frequency and linewidth we were able to extract the Gilbert damping as well as the effective magnetization. Experimental details as well as results will be presented. \\[4pt] [1] S. Ikeda et al, \textit{Nature Materials}~\textbf{9}, 721 - 724 (2010)\\[0pt] [2] J.M. Beaujour et al, Eur. Phys. J. B \textbf{59}, 475--483 (2007)   

Energy distributions of defects causing barrier-resistance noise in CoFeB/MgO/CoFeB tunnel junctions

Magnetic tunnel junction devices, such as field sensors, are well known to exhibit low frequency resistance noise having a 1/$f$ spectrum. This noise has its origins in a combination of electrical and magnetic mechanisms. Previously, we have shown that the resistance noise can be reduced significantly through thermal annealing. Here, we report on the energy distribution of the defects causing tunnel-barrier-resistance noise. The distributions are determined from a Dutta-Horn model for thermally activated charge trapping and detrapping kinetics. We also discuss how the distribution changes as a function of annealing time and its relation to the current-voltage characteristics and the voltage bias dependence of the 1/$f $noise.   

Ultrafast Switching in Magnetic Tunnel Junction based Orthogonal Spin Transfer Devices

Orthogonal spin-transfer magnetic random access memory (OST-MRAM) uses a spin-polarizing layer magnetized perpendicularly to the free layer to achieve large spin-transfer torques and ultrafast energy efficient switching. We have fabricated and studied OST-MRAM devices that incorporate a perpendicularly magnetized polarizer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer. A switching probability of 100{\%} is observed for 500 ps pulses, requiring an energy of 250 fJ. The fast switching process indicates there is no incubation delay of several nanoseconds as observed in conventional collinear magnetized devices. Due to the perpendicular polarizer switching is possible for both pulse polarities. There is also evidence for precessional switching in the non-monotonic dependence of the switching probability versus pulse amplitude. This work was supported by Spin Transfer Technologies.   

The effect of annealing on the spin-transfer torques of MgO MTJ nanopillars

Thermal annealing is essential for enhancing the tunneling magnetoresistane (TMR) of magnetic tunnel junctions, and many studies have focused on the effect of annealing on MTJ chemical, structural, and electrical transport properties. Here, we report the magnetic, electronic properties and the in-plane and field-like spin-transfer torques (STT) in both as-grown and post-annealed FeCoB/MgO/FeCoB MTJs nanopillars. We find that the 350 \r{ }C vacuum annealing breaks the symmetry of the bias dependence of the TMR, conductivity, and switching phase diagram (SPD). Moreover STT-FMR measurements indicate that annealing substantially increases the in-plane torque asymmetry with bias voltage direction, as well as affecting the field-like torque magnitude, with the latter indicating a very significant enhancement of interlayer exchange coupling across the barrier. This STT change is consistent with the change in chemical composition and structural coherency of the MTJ interfaces and electrodes, indicated by XRD and analytical STEM analyzes.   

Bias voltage dependence of the total magnetic field in CoFeB magnetic tunnel junctions

We report experimental thermal-noise spectrum-based ferromagnetic resonance (T-FMR) measurements on CoFeB magnetic tunnel junctions in magnetic fields perpendicular to the film plane. The junctions tested have lateral sizes of $45 \times 80$ nm$^2$. In a simple model a dc junction bias voltage should affect both the slope and the intercept of the T-FMR frequency's dependence on applied magnetic field. The intercept would vary linearly with changes in bias voltage due to an electric field-induced change in uniaxial anisotropy [1]. The slope would have a quadratic dependence on changes in bias voltage based on the existence of a perpendicular spin-torque as discussed by Sankey {\it et al.} [2]. In this experiment we attempt to de-construct the contribution from these two mechanisms. This is done by a careful analysis of the magnetic field dependence of the T-FMR spectra [3]. \\[4pt] [1] Suzuki et al, Appl. Phys. Lett. {\bf 96}, 022506 (2010). \newline [2] Sankey et al, Nature Physics {\bf 4}, 67 (2008). \newline [3] Mascaro et al, Intermag/MMM paper FB-11.   

Confirm existence of 90\r{ }-type coupling in Fe/MgO/Fe junction by investigating magnetic components perpendicular to the plane of incidence

We study 90\r{ }-type interlayer exchange coupling (IEC) in a Fe/MgO/Fe junction by linear magneto-optical Kerr effect (MOKE) in $p_{in}-p_{out}$ configuration, in which only in-plane magnetization perpendicular to the external field $H$is detected. By investigating the switching processes of the ferromagnetic vectors from parallel with- to perpendicular to $H$, we find there is a switching correlation between them: the ferromagnetic vector in the bottom layer always follows the switching direction of that in the top layer. Further analysis shows this kind of switching sequence is the direct consequence of 90\r{ }-type coupling between the two magnetic vectors, i.e., 90\r{ }-type coupling is indeed exists in Fe/MgO/Fe junction.   

Influence of Interface on Conductance in AlO$_{x}$ Based Magnetic Tunnel Junctions

A surprising minimum in the differential conductance at nonzero bias is observed in some magnetic tunnel junctions consisting of CoFe/AlO$_{x}$/CoFe; this pronounced conductance feature occurred for electrons tunneling from the bottom to top electrode. The presence of this conductance feature depends upon the oxidation time for creating the barrier from a thin Al layer; for short and moderate oxidation times the feature was present while for long oxidation times the conductance was found to be both symmetric about zero bias and monotonic with increasing bias voltage. To determine the origin of this feature samples were prepared where the oxidation states of the CoFe on each side of the barrier were studied by X-ray photoelectron spectroscopy: the conductance feature is observed only when the top CoFe layer is partially oxidized and it disappears when the CoFe on both sides of the junction has some oxidation present. More interestingly, the bias voltage of the conductance feature decreases with oxidation time. We attribute the differential conductance feature to the electronic structure and the chemical bonding at the bottom CoFe/AlO$_{x}$ interface.   

Spin transfer torque in magnetic tunnel junctions with a perpendicularly magnetized polarizer

Spin-torque devices containing magnetic layers with perpendicular magnetic anisotropy are of interest for strategies to reduce the switching currents in memory applications. We report spin-torque-driven ferromagnetic resonance (ST-FMR) measurements of the bias-dependent torque in magnetic tunnel junctions containing [Co/Ni]$_{x}$ multilayers possessing perpendicular anisotropy, acting as the polarizer layer providing spin-polarized current. We observe unusual dependence of the bias-dependent torque as a function of the magnetic orientation of the [Co/Ni]$_{x}$ multilayer. We speculate that this sensitivity to the magnetic orientation may originate from changes in the occupation of spin-polarized states at the Co/Ni interfaces associated with the perpendicular magnetic anisotropy.   

Spin transfer induced domain wall motion by perpendicular current injection in MgO-based magnetic tunnel junctions

The spin transfer effect allows to manipulate magnetic domain walls in ferromagnetic wires by current injection. Most experiments use the lateral configuration in which the current is injected directly through the wire where the domain wall (DW) propagates. In this geometry the critical current densities are of the order of 10$^8$ A.cm$^{-2}$. Here we show that by using the current-perpendicular to plane geometry, the current densities can be decreased by two orders of magnitude. Depending of the current sign, the DW propagates in the free layer of a magnetic tunnel junction in both directions, inducing large resistance variations. By investigating the physical origin of DW motion, we find that the field-like torque has a large contribution to the effect, as recently predicted by Khvalkovskiy \textit{et al.} (Phys. Rev. Lett. 2009). This result paves the way towards a new type of domain wall based magnetic memories.   

High Voltage Pulse Measurements of Microwave Emission and Spin-Torque Effects in Magnetic Tunnel Junction

The character and strength of the in-plane and field-like spin transfer torque (STT) components in magnetic tunnel junctions at high bias voltages are crucial to the successful utilization of MTJs in STT MRAM. If the field-like torque (FLT), which is generally found to be symmetric with respect to bias direction for moderate voltages, $< \quad \pm $ 0.5 V, is too large it could result in unreliable switching (back-hopping) for negative bias voltage pulses (anti-parallel to parallel switching). Here we discuss pulse measurements of MgO MTJs at high bias that yield important information about the FLT component in the $\pm $ 0.5 to 1.0 V regime through analysis of both the thermally-excited FMR behavior and spin torque driven oscillations. In the structures studied we find a strong and highly asymmetric voltage-dependent FLT at high bias that under some field and voltage conditions can result in large amplitude, incoherent microwave dynamics that could have a strong effect in enhancing back-hopping. We will analyze possible mechanisms, including junction asymmetries and inelastic tunneling.   

Microwave phase detection with a magnetic tunnel junction

A magnetic tunnel junction (MTJ) can detect microwave magnetic field due to the interplay between the ferromagnetic resonance and tunneling magneto resistance. Based on the fact that the tunneling resistance change is quadratically proportional to the rf magnetic field, we have designed a mixing circuit in which two microwaves interfere, giving rising to a dc voltage containing the phase difference between the two microwaves. With ability to detect microwave intensity and phase, the MTJ-based device may be used for on-chip microwave network analyzer and spectrum analyzer.   

Time-resolved detection of spin-transfer-driven ferromagnetic resonance and spin torque measurement in magnetic tunnel junctions

Several experimental techniques have been introduced in recent years in attempts to measure spin transfer torque in magnetic tunnel junctions (MTJs). The dependence of spin torque on bias is important for understanding fundamental spin physics in magnetic devices and for applications. However, previous techniques have provided only indirect measures of the torque and their results to date for the bias dependence are qualitatively and quantitatively inconsistent. Here we demonstrate that spin torque in MTJs can be measured directly by using time-domain techniques to detect resonant magnetic precession in response to an oscillating spin torque. The technique is accurate in the high-bias regime relevant for applications, and because it detects directly small-angle linear-response magnetic dynamics caused by spin torque it is relatively immune to artifacts affecting competing techniques. At high bias we find that the spin torque vector differs markedly from the simple lowest-order Taylor series approximations commonly assumed.   

Session V20: Focus Session: Physics of Energy Storage Materials V -- Thermal Storage and Conventional Hydrides

Origin of the Diverse Melting Behaviors of Aluminum Nanoclusters with Around 55 Atoms

Microscopic understanding of thermal behaviors of metal nanoparticles is important for nanoscale catalysis and thermal energy storage applications. Using first-principles molecular dynamics simulations, we reveal the microscopic origin of the diverse melting behaviors of Al$_{N}$ clusters with N around 55 [1,2]. The conceptual link between the degree of symmetry (e.g., T$_{d}$, D$_{2d}$ and C$_{s})$ and solidity of atomic clusters is quantitatively demonstrated through the analysis of the configuration entropy. The size-dependent, diverse melting behaviors of Al clusters originate from the reduced symmetry (T$_{d} \quad \to $ D$_{2d} \quad \to $ Cs) with increasing the cluster size. In particular, the sudden drop of the melting temperature and appearance of the dip at N = 56 are due to the T$_{d}$-to-D$_{2d}$ symmetry change, triggered by the surface saturation of the tetrahedral Al$_{55}$ with the T$_{d}$ symmetry.\\[4pt] [1] G. A. Breaux, C. M. Neal, B. Cao, and M. F. Jarrold, Phys. Rev. Lett. \textbf{94}, 173401 (2005).\\[0pt] [2] J. Kang, S.-H. Wei, and Y.-H. Kim, J. Am. Chem. Soc. (in press).   

Azonbenzene-functionalized carbon nantubes as a high energy density solar thermal fuel

Solar thermal fuels, which store energy from the sun in the chemical bonds of molecules, are a fascinating energy storage prospect: in principle they are 100\% renewable, produce no emissions or by-products, are easily transportable in the form of liquids or powders, and can be recharged by the sun without any special equipment. However, adaptation of solar fuels as a viable, low-cost, large-scale means of energy storage will require the discovery of new materials other than the one known case based on Ruthenieum that can perform the process over many cycles with no degradation. Here we discuss a novel approach to the design of solar fuels based on photoswitchable molecules covalently bonded to carbon nanotubes (CNTs). Using density functional theory, we examine the potential for maximizing the energy density via a combination of steric and intermolecular interactions between metastable azobenzene photoisomers and a CNT substrate. In addition, we investigate how a tuning parameter unique to the nanoparticle/molecule geometry --- the packing density of the molecules on the nanotube --- can be varied to produce significant, controlled changes in the transition pathway and barriers via ordered molecule-molecule interactions, potentially leading to new classes of nanoparticle-based solar fuels.   

A high volume, high throughput volumetric sorption analyzer

In this talk we will present an overview of our new Hydrogen Test Fixture (HTF) constructed by the Midwest Research Institute\footnote{http://www.mriresearch.org} for The Alliance for Collaborative Research in Alternative Fuel Technology\footnote{http://all-craft.missouri.edu} to test activated carbon monoliths for hydrogen gas storage. The HTF is an automated, computer-controlled volumetric instrument for rapid screening and manipulation of monoliths under an inert atmosphere (to exclude degradation of carbon from exposure to oxygen). The HTF allows us to measure large quantity (up to 500 g) of sample in a 0.5 l test tank, making our results less sensitive to sample inhomogeneity. The HTF can measure isotherms at pressures ranging from 1 to 300 bar at room temperature. For comparison, other volumetric instruments such as Hiden Isochema's HTP-1 Volumetric Analyser can only measure carbon samples up to 150 mg at pressures up to 200 bar.   

Synthesis of Li$_{2}$MgIr and LiMgIrH$_{6}$: Guidance from DFT

Formation of Li$_{2}$MgIr was suggested by theoretical modeling of Li$_{2}$MgX systems and their hydrides with density functional theory (DFT). Verifying our DFT results, we have synthesized Li$_{2}$MgIr and determined its crystal structure and hydrogen sorption behavior. The phase crystallizes in the cubic $P\bar {4}3m$ space group and is isostructural to the known ternary Li$_{2}$MgSi. Its reaction with hydrogen proceeds according to Li$_{2}$MgIr + $\textstyle{7 \over 2}$H$_{2}\to $ LiMgIrH$_{6}$ + LiH. The hydride LiMgIrH$_{6}$ also features $P\bar {4}3m$ symmetry; its detailed crystal structure is established via a combination of x-ray diffraction and DFT analyses. A metal $\to $ insulator transition accompanies formation of the hydride.   

Thermodynamics of MgH$_{2}$ hydrogen storage materials: nanoparticle size and topological structure effects

Via plane-wave-based Density Functional Theory calculations, we investigate H-desorption from (110) rutile MgH$_{2}$, a surface step, and surfaces of nanoscale Mg$_{30}$XH$_{62}$ clusters having catalytic dopants (X=Mg, Ti, or Fe). All calculated desorption enthalpies are endothermic, in contrast to results in the literature,\footnote{Larsson, P.; Araujo, C. M.; Larsson, J. A.; Jena, P.; Ahuja, R. \textit{P Natl Acad Sci USA} 2008, $105$, 8227} and no particle size effect is found for desorption of H singly, doubly, or triply-bonded to metal atoms, indicating only local bond energy is relevant. In contrast to recent results, we show that exothermic results are not obtained when initial cluster structures are carefully relaxed globally via simulated annealing, in which amorphous structures are found to be favored. A topological feature is identified that offers potential utility for using nanostructured MgH$_{2}$ as a hydrogen-storage solution.   

Hydrogen desorption from MgH$_{2}$(110) surface with transition-metal catalyst: a DFT study of energetics and barriers

Transition-metal (TM) catalysts are widely used in hydrogen-storage materials to increase hydrogen absorption and desorption kinetics. Using density functional theory calculations, we elucidate the catalytic effect of Ti on H-desorption from MgH$_{2}$(110) surface. Kinetic energy barriers of different reaction pathways of hydrogen desorption are calculated via nudged-elastic-band method. We find that Ti dopant is effective in reducing kinetic barriers, in agreement with experimental observations. We also find that magnetic degrees of freedom must be carefully included to describe the change of magnetic states during catalytic-enhanced desorption. As vacancy migration barriers are lower than desorption barrier, bulk diffusion of H inherently feeds into the favorable surface desorption mechanism.   

Tuning the Hydrogen Storage in Magnesium Alloys

We investigate the hydrogen storage properties of promising magnesium alloys. MgH$_{2}$ (7.6 wt {\%} H) would be a very useful storage material if the (de)hydrogenation kinetics can be improved and the desorption temperature is markedly lowered. Using first principles calculations, we show that hydrides of Mg-transition metal (TM) alloys adopt a structure that promotes faster (de)hydrogenation kinetics, as is also observed in experiment [1]. Within the lightweight TMs, the most promising alloying element is titanium. Alloying Mg with Ti alone, however, is not sufficient to decrease the stability of the hydride phases, which is necessary to reduce the hydrogen desorption temperature [2]. We find that adding aluminium or silicon markedly destabilizes Mg-Ti hydrides and stabilizes Mg-Ti alloys. Finally, we show that controlling the structure of Mg-Ti-Al(Si) system by growing it as multilayers, has a beneficial influence on the thermodynamic properties and makes it a stronger candidate for hydrogen storage [3].\\[4pt] Ref: [1] S. Er \textit{et al.}, Phys. Rev. B, \textbf{79}, 024105 (2009). [2] S. Er \textit{et al.}, J. Phys.: Condens. Matter, \textbf{22}, 074208 (2010). [3] S. Er \textit{et al.}, J. Phys. Chem. Lett., \textbf{1}, 1982 (2010).   

Neutron, Thermodynamic, and Modeling Studies of Hydrogen Interaction with MgO(100)

We report our investigations of thermodynamic, neutron diffraction, inelastic and quasielastic neutron scattering (INS and QENS) studies of hydrogen adsorbed onto MgO(100). Guided by our volumetric adsorption measurements, we used INS and QENS to probe the dynamics of the adsorbed H2 molecules. Our structural studies indicate that near monolayer completion the intermolecular distance of the H2 molecules on MgO are $\sim $20{\%} more compressed than the closest packed bulk solid plane. The melting of this compressed solid takes place at temperatures above the bulk triple point, whereas most other 2D films melt at about 70{\%} of the triple point. Using INS, the motion of the adsorbed hydrogen is examined as a function of film thickness. For rotational motions, we use the ortho-to-para transition as a guide and find that the rotational barrier for H2 adsorbed on MgO is shifted to lower energy at low surface coverage. These results are compared with modeling for additional insight into the microscopic processes that underpin the observed behavior. This work partially supported by the U.S. DOE, BES under contract DE-AC05-00OR22725 with ORNL managed and operated by UT-Battelle, LLC, the NSF under grant DMR-0412231 and a grant from the University of TN, JINS.   

Enhanced Hydrogen Storage Properties of Magnesium Nanotrees by Glancing Angle Deposition

Magnesium has a high hydrogen storage capacity of 7.6 wt {\%}. In addition it is one of the most abundant low cost materials in nature. However, absorption/desorption of hydrogen in Mg mainly suffer from slow kinetics. In this study, we investigate the hydrogen storage properties of Mg ``nanotrees with nanoleaves'' fabricated by glancing angle deposition (GLAD) method and compare to those of conventional thin films of Mg. A recently developed quartz crystal microbalance (QCM) gas absorption/desorption technique was used for hydrogen storage measurements on our thin film and nanostructured coatings. Storage experiments were performed at temperatures between 100-300 $^{\circ}$C, and at 30 bars of H$_{2}$ pressure. Our results reveal that Mg nanotrees have significantly faster kinetics and lower absorption temperatures for hydrogen storage compared to Mg thin films. The enhancement in absorption properties is believed to be due to decreased diffusion lengths, favorable crystal orientations for diffusion of hydrogen, and resistance to surface oxidation of Mg nanotrees.   

Neutron spectroscopy of gamma-MgH2

Under ambient conditions, magnesium dihydride exists in two forms, alpha-MgH2 (the most stable modification) and gamma-MgH2 (a less stable modification). The alpha-phase partly transforms to gamma-MgH2 in the course of ball-milling and under high pressure and temperature. Due to the high hydrogen content of 7.6 wt.{\%}, MgH2 has been intensively studied as a prospective material for hydrogen storage. By exposing of alpha-MgH2 to a pressure of 5 GPa and temperature 840 K, we prepared a sample, in which about 60{\%} of the alpha-MgH2 was transformed to gamma-MgH2. We have measured inelastic neutron scattering (INS) spectra of both the high pressure treated MgH2 and starting alpha-MgH2, and extracted the spectrum for gamma-MgH2. The differences between the INS spectra and their agreement with the first-principles calculations for these compounds will be discussed.   

The chemical potential of hydrogen in Mg-films and metal-doped carbon nanostructures

We use first-principles density functional theory to study the binding mechanism of hydrogen to nanoscale systems. We investigate the performance of the exchange-correlation functional in describing the interaction between hydrogen and metal systems and the importance of the vibrational contribution in the formation enthalpy. In ultrathin Mg films the stability of hydrides is much lower than in the corresponding bulk systems and it can be modified by metal alloying. We calculate the chemical potential of hydrogen in Mg films for different dopant species and film thicknesses while including all vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature and pressure, we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption/desorption. The vibrational contribution to the chemical potential of hydrogen becomes more prominent for dihydrogen adsorption to metals, where its significance dramatically changes depending on the binding characteristics. This feature is illustrated by the example of metal-doped nanocarbon systems.   

Computational search for hydrogen storage materials: accuracy and alloys

Metal hydride materials are among the strongest contenders for hydrogen storage, offering good weight and volume density. The main reason that these materials are not used now is that it is very challenging to find a material that is both light enough and has the proper binding to allow for easy absorption/desorption near room temperature. We will evaluate two routes to controlling the binding energy: particle size and alloy composition using the highly accurate quantum Monte Carlo method. We find that traditional methods of calculating the binding energy such as the Wulff construction and density functional theory should be applied with caution, as they can lead to misleading results. We will also report on the prospects for finding a sweet spot of size and alloy composition that has the correct binding energy for hydrogen storage applications.   

Ultra-low diffusion barriers for the ${\rm AlH_3}$-related vacancies in $\gamma$-${\rm NaAlH_4}$

It has been suggested that the diffusion of ${\rm AlH_3}$-related vacancies plays an essential role in the decomposition of ${\rm NaAlH_4}$, a prototypical material for hydrogen storage[1,2]. We find from first-principles calculations that the diffusion barrier for both the neutral ${\rm AlH_3}$ vacancy and the charged ${\rm AlH_4^-}$ vacancy in the newly proposed $\gamma$-phase of ${\rm NaAlH_4}$ [3] is only about 0.1 eV, much lower than the barrier for the diffusion of corresponding vacancies in the conventional $\alpha$-phase 0.5 eV, calculated with the same method. Possible schemes to facilitate the $\alpha\rightarrow\gamma$ phase transformation in order to improve the kinetics of the decomposition reaction of ${\rm NaAlH_4}$ will also be discussed.\\[4pt] [1] H. Gunaydin, K. N. Houk, and V. Ozoli\c{n}\v{s}, Proc Natl Acad Sci USA {\bf 105}, 3673 (2008).\\[0pt] [2] G.~B.~Wilson-Short, A.~Janotti, K.~Hoang, A.~Peles, and C.~G.~Van de Walle, Phys. Rev. B {\bf 80}, 224102 (2009).\\[0pt] [3] B.~Wood and N.~Marzari, Phys. Rev. Lett. {\bf 103}, 185901; {\bf 104}, 019901.   

Defect-mediated Alane formation on Ti-doped Al(111) surfaces: a DFT study

Understanding of Alane (AlH$_{3})$ formation on Al surface remains elusive, including interpreting STM results under various conditions. Using density functional theory calculations, we study Alane formation on close-packed (111) and stepped surfaces with {\{}111{\}} and {\{}100{\}} microfacets of Al, with and without Ti as a catalyst. We find that Ti dopants act as catalyst in the formation of Alane on Al(111) via a vacancy-mediated mechanism. Additionally, we find the Alane formation energy at steps is 40{\%} less than that from the flat surface. We assess the energetics of various surface-defect configurations to understand the concerted roles that Ti dopants, surface vacancies, and step defects play in Alane formation. Work was supported in part by Department of Energy, Office of Basic Energy Science under contract DEFC36-05GO15064 (Sandia Metal-Hydride Center of Excellence), DE-FG02-03ER15476, DE-FG02-03ER46026, and DE-AC02-07CH11358 at the Ames Laboratory operated by Iowa State University.   

Molecular hydrogen interaction with Ti doped Al(111) surfaces

Alanates are promising hydrogen storage materials, but have poor re-hydrogenation kinetics. Decomposition\footnote{\textit{J. Alloys Compd.} 1997, 253, 1.} of NaAlH$_{4}$ can be made reversible at reasonable temperatures and pressures by adding titanium. There is however little understanding of the role of Ti as a catalyst,\footnote{\textit{J Am Chem Soc} 2006, 128, (35), 11404-11415.} and no experimental evidence for H$_{2}$ dissociation on Ti-doped Al surfaces. Using CO as a probe molecule in conjunction with in-situ infrared absorption spectroscopy, we present unambiguous evidence for molecular hydrogen dissociation, chemisorptions and spill over on with Ti doped Al(111) surfaces. The optimum catalytic activity of the Ti-doped Al surface occurs for a Ti coverage of 0.1 monolayer. At high hydrogen coverage, no CO physisorption is observed, indicating that the dissociated hydrogen spill over from the catalytic active Ti site. CO molecules can be chemisorbed at the catalytic sites but do not spill over. These findings provide important information on the nature of the catalyst during the hydrogenation reactions.   

Session V21: Focus Session: Teaching Computational Physics to Classroom and Research Students

Computational (Physics Education) vs. (Computational Physics) Education: Many Body for Anybody

The substantial role of computational approaches to physics research is not currently reflected proportionately in how we prepare future physicists. We can do better in using computation to teach the concepts of physics --Computational (Physics Education) -- and to prepare students to be computational physicists -- (Computational Physics) Education. We have the opportunity to demonstrate that effective use of computing in physics really matters. A computational approach in physics education is essential because quantitative reasoning, computational thinking, and multiscale modeling are the intellectual ``heart and soul'' of 21st Century physics and therefore are the essential skills of the 21st Century physicist. Computing matters because we can apply the power of interactive computing to reach a deeper understanding of physics and the mathematics underlying the theory and their role in understanding the world. We will explore a transformation in physics education, supported by interactive computing resources, promoting a dynamic encounter with the world through guided discovery. We will explore a variety of free and low-cost sources for modeling tools from Shodor and its Computational Science Education Reference Desk, a pathway project of the National Science Digital Library.   

Computing Band Structures in Undergraduate Solid State

Understanding band structures is quite challenging for undergraduate solid state physics students. Calculating the band structures is even more difficult. However, using the techniques developed earlier [1], and which were applied to the simple cubic structures, it is possible to extend them to semiconducting systems in a simple way. The idea is to employ the 8-band model concept of Harrison's Hamiltonian approach [2] to model and parametrize the bands. The method also uses the system's band structure's Green's function and employs the k-space Brillouin-zone ray approach [3] combined with a complex integration method [4] to obtain the density of states. The number of occupied electron states up to a certain energy is obtained using Romberg's method and example results will be shown. [1] Javier Hasbun (J42.00013) http://meetings.aps.org/Meeting/MAR10/Event/119248 [2] S. Froyen, and W. A. Harrison, Phys. Rev. B Vol. 20, 2420 (1979). [3] An-Ban Chen, Phys. Rev. B, Vol. 16, 3291 (1977). [4] Javier Hasbun http://meetings.aps.org/link/BAPS.2009.MAR.L29.12   

Introducing Computational Physics in Introductory Physics using Intentionally Incorrect Simulations

Students in physics courses routinely use and trust computer simulations. Finding errors in intentionally incorrect simulations can help students learn physics, be more skeptical of simulations, and provide an initial introduction to computational physics. This talk will provide examples of electrostatics simulations that students can correct using Easy Java Simulations and are housed in the Open Source Physics Collection on ComPADRE (http://www.compadre.org/osp).   

Computational {\ldots} Physics Education: Letting physics learning drive the computational learning

For several years I have been part of a team researching and rethinking why physicists are more willing to admit the value of computational modeling than to include it in what they teach. We have concluded that undergraduate faculty face characteristic barriers that discourage them from starting to integrate computation into their courses. Computational tools and resources are already developed and freely available for them to use. But there loom ill-defined ``costs'' to their course learning objectives and to them personally as instructors in undertaking this. In an attempt to understand these issues more deeply, I placed myself in the mindset of a relative novice to computational applications. My approach: focus on a physics problem first and then on the computation needed to address it. I asked: could I deepen my understanding of physics while simultaneously mastering new computational skills? My results may aid appreciation of the plight of both a novice professor contemplating the introduction of computation into a course and the students taking it. These may also provide insight into practical ways that computational physics might be integrated into an entire undergraduate curriculum.   

Teaching computational physics: An embarrassment of riches for teaching computational physics

The first decade of the 21st century has provided a wealth of exceptional resources for teaching computational physics, including numerous textbooks, libraries of computer codes (visual as well as numerical), and high-level interfaces for accessing these libraries. We are now faced with the very real challenge of choosing which of these resources to incorporate into the finite number of courses available in a given curriculum. This choice depends on several factors: How much time can be allocated to teaching computational methods and at what stage in the curriculum? What are the goals? (Learning physics better? Being prepared to work in research labs studying large-scale problems?) Are commercial packages an appropriate option for your student population? In recent years one of us (L.E.) has taught three undergraduate computational physics courses per year. The other (A.B.) has taught at various points in the undergraduate spectrum (a seminar for seniors, a computational methods lab for sophomores, a summer research experience for freshmen from underrepresented groups). Thus, while there are no right or wrong answers to these questions, we will present some of the decisions we have made, and will discuss the consequences.   

A course in Computational Physics

This course, taught at UConn, has several objectives: 1) To make the students comfortable in using MATLAB; 2) To reveal the existence of unavoidable inaccuracies due to numerical roundoff errors and algorithm inaccuracies; 3) to introduce modern spectral expansion methods [1], and compare them with conventional finite difference methods. Some of the projects assigned in the course will be described, such as the motion of a falling parachute, and the vibrations of an inhomogeneous vibrating string [2]. \\[4pt] [1] Lloyd N. Trefethen, ``Spectral Methods in MATLAB (SIAM, Philadelphia, PA, 2000)''; John P. Boyd, ``Chebyshev and Fourier Spectral Methods,'' (Dover Publications, Inc. Mineola, New York, Second revised edition, 2001). \newline [2] G. Rawitscher and J. Liss, ``The vibrating inhomogeneous string,'' Am. J. of Phys., to be published; and arXiv:1006,1913v1 [physics.comp-ph]   

Using Python as a first programming environment for computational physics in developing countries

Python unique features such its interpretative, multiplatform and object oriented nature as well as being a free and open source software creates the possibility that any user connected to the internet can download the entire package into any platform, install it and immediately begin to use it. Thus Python is gaining reputation as a preferred environment for introducing students and new beginners to programming. Therefore in Africa, the Python African Tour project has been launched and we are coordinating its use in computational science. We examine here the challenges and prospects of using Python for computational physics (CP) education in developing countries (DC). Then we present our project on using Python to simulate and aid the learning of laboratory experiments illustrated here by modeling of the simple pendulum and also to visualize phenomena in physics illustrated here by demonstrating the wave motion of a particle in a varying potential. This project which is to train both the teachers and our students on CP using Python can easily be adopted in other DC.   

Undergraduate research in numerical relativity: How to put a black hole on a graphics card

Andreas Weyhausen, a diploma student at the Friedrich-Schiller University, ported a standard code for the full 3D stable simulation of black holes to run on a graphics processing unit (GPU), a first in the field. A presentation will be made describing the task he accomplished with key results, including a speed-up comparison to the serial code. This will be placed in context of the course work that prepared him for the project and advising provided by the FSU gravity group prior, during and after the execution leading to his thesis on Numerical Algorithms of General Relativity for Heterogeneous Computing Environments.   

Session V22: Magnetic Phase Transitions I

Pressure-induced antiferromagnetism in pure CeFe2

CeFe2 is a ferromagnet that exhibits antiferromagnetic fluctuations in its ground state at low temperature. We use x-ray diffraction to measure directly the emergence of antiferromagnetic order in pure CeFe2 at high pressure. We present an analysis of both the magnetic and lattice symmetries in the newly discovered high pressure phase, and compare our results to those from doped CeFe2 systems. This comparison provides insights into the roles of pressure and chemical doping in driving the magnetic quantum phase transition.   

Transition-metal dihalide $MX_2$ as magnetoelectric multiferroics

Magnetoelectric properties were investigated for transition-metal dihalide $MX_2$, which turns out to be the first example of non-chalcogen based spiral-spin induced multiferroics. We discovered the emergence of ferroelectric polarization ($P$) in helimagnetic state for several compounds such as CuCl$_2$ with edge-shared $S=1/2$ chain and MnI$_2$ with stacked triangular lattice. In the latter material, in-plane magnetic field ($H$) was found to induce the rearrangement of six possible multiferroic domains. With every $60^{\circ}$-rotation of $H$ around the $c$-axis, discontinuous $120^{\circ}$-flop of $P$-vector is observed as a result of the flop of magnetic modulation vector ($q$). In-plane $H$ also alters the stable direction of $q$-vector from original $q \parallel \langle 1\bar{1}0 \rangle$ to $q \parallel \langle 110 \rangle$ above 3 T, which leads to significant change of $P$-flop patterns under rotating $H$. At the critical field region ($\sim$ 3T), due to the enhanced $q$-flexibility, $P$-vector smoothly rotates clockwisely twice while $H$ rotates counter-clockwisely only once.   

Infrared spectroscopy of phonons and electromagnons in multiferroic TbMnO$_3$

We measured the temperature dependent infrared reflectivity spectra of TbMnO$_3$ with the electric field of light polarized along each of the three crystallographic axes. We analyzed the effect, on the phonon spectra, of the different phase transitions occurring in this material. We show that the antiferromagnetic transition at $T_N$ renormalizes the phonon parameters along the three directions. Our data indicate that the electromagnon, observed along the $a$ direction, has an important contribution in building the dielectric constant. We show that this electromagnon spectral weight comes only from a few phonons which can be clearly identified. We also determined that only one phonon, observed along the $c$-axis, has anomalies at the ferroelectric transition. This phonon is built mostly from Mn vibrations, suggesting that Mn displacements are closely related to the formation of the ferroelectric order.   

Ultrafast pump-probe spectroscopy of multiferroic TbMnO$_{3}$

TbMnO$_{3}$, exhibiting simultaneously both magnetic and ferroelectric phases, is an excellent multiferroic candidate for demonstrating the strong coupling between different degrees of freedom, i.e. spin, orbital and charge order. Previously, ultrafast optical pump-probe spectroscopy has proven to be an ideal technique for unraveling the interplay between different orders in the time domain. In this work, we used this technique to study ultrafast dynamics in multiferroic TbMnO$_{3}$. At low temperatures, we initially observed an extraordinarily slow rising process, with a timescale of tens of picoseconds, followed by another decay process with a relaxation time of hundreds of picoseconds. An analysis of these two processes as a function of temperature reveals the influence of the magnetic and ferroelectric phase transitions on carrier dynamics in TbMnO$_{3}$, which agrees well with other experimental results.   

Photoinduced Femtosecond Formation of Ferromagnetism in a Strongly Correlated Antiferromagentic Manganite

There has been strong current interest to manipulate collective spins and even induce magnetic phase transitions in their highly \textit{non-equilibrium, non-thermal }states at \textit{femtosecond} time scales. Such processes offer opportunities to exceed the upper limit of the magnetic switching speed (0.1-10 GHz) in modern magneto-optical recording industry and magnetic storage/logic devices. One prominent system to explore such femtosecond magnetism is strongly correlated manganites, which are truly ``responsive'' near the phase boundary, exhibiting extreme sensitivity to external stimuli, such as light, electric and magnetic fields. Using ultrafast two-color magnetic circular dichroism spectroscopy, we have observed a substantial photoinduced magnetization enhancement in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ within 180 fs above a threshold pump fluence and at low temperature. Such a photoinduced critical behavior vanishes at elevated temperature. These results clearly show a photoinduced ultrafast antiferromagnetic to ferromagnetic phase transition, demonstrating particularly, that one can reveal a hidden, thermally inaccessible ground state at fs time scales.   

Polaron Glass in La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$

Manganite compounds in the La$_{1-x-y}$Pr$_y$Ca$_x$MnO$_3$ series are known for exhibiting phase separation over a large temperature range. We combined the x-ray photoemission electron microscopy (PEEM) and resonant elastic soft x-ray scattering (REXS) techniques to study the interplay between the ferromagnetic and charge-ordered/antiferromagnetic phases, respectively, in La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$. We found a polaronic glassy state at intermediate temperatures, when the material is dominated by charge- and orbital-order domains. When the sample is cooled below T$_{\mathrm{C}}$, the magnetization increases, accompanied by a relaxation of the lattice deformations that accompany the polaron glass.   

The role of oxygen in the colossal magnetoresistance in manganites

We have used Low temperature Photo-Emission Electron Microscopy (PEEM) measurements on the bi-layered manganite compound La$_ {1.8}$Sr$_{1.2}$Mn$_2$O$_7$ to explore the origin of the colossal magnetoresistance (CMR) effect. It is generally agreed that CMR cannot be explained by double exchange only, and that other interactions mediated by oxygen atoms such as polarons must be important. We have imaged the magnetic domain structure by x-ray magnetic circular dichroism PEEM spectro-microscopy at both the O and Mn sites. By probing the ferromagnetic domain formation below T$_c$, we find that the insulator-to-metal transition is mediated by magnetic interactions involving a strong magnetic moment on oxygen. The spatially resolved oxygen K-edge XMCD signal reveals the role of the in-plane and out-of- plane e$_g$ orbitals in the magnetic transitions and thereby sheds light on the very origin of CMR.   

Momentum-Space Dichotomy in the Metal-Insulator Transition in doped EuO

EuO possesses a wide variety of remarkable properties, most which can be accessed only upon carrier doping. In addition to its large ferromagnetic moment (S = 7/2), doped EuO exhibits a metal-insulator transition with a change in resistivity of over $10^{13}$ and highly spin polarized carriers. Furthermore, the ferromagnetic Curie temperature can be enhanced from 69 K in undoped EuO to over 200 K in carrier doped EuO. We present angle-resolved photoemission studies of Eu$_{1-x}$Gd$_x$O thin films which elucidate the electronic structure and mechanism of the metal-insulator transition. Our ARPES studies verify that the exchange coupling between the Eu 4f moments and the delocalized Eu 5d states pushes the bottom of the majority-spin conduction band through $E_F$ below $T_C$. We also reveal a surprising dichotomy between the delocalized carriers at the Brillouin zone boundary below $T_C$, and localized carriers around the zone center above $T_C$ which are responsible for the respective low-temperature ferromagnetic metallic and high-temperature paramagnetic semiconducting behaviors observed in transport measurements.   

Anharmonic Lattice Effect in the Giant Negative Thermal Expansion Antiperovskite Cu$_{1-x}$Sn$_{x}$NMn$_{3}$

The antiperovskite ANMn$_{3}$ (A: transitional metals and semiconducting elements) often shows a large, discontinuous volume contraction at the magnetic transition associated with a large negative thermal expansion (NTE). The NTE property was initially observed in Cu$_{1-x}$Ge$_{x}$NMn$_{3}$ where Ge-doping broadens the discontinuous volume contraction. It was recently reported that although the average symmetry is cubic in the solid solutions, locally, the symmetry is tetragonal with the I4/mcm symmetry of the end member GeNMn$_{3}$. We investigated the local structure of a ``Ge''-free NTE system, the Cu$_{1-x}$Sn$_{x}$NMn$_{3}$ with $x$= 0.1 and 0.5. On average, both compounds are cubic at all temperatures. At base temperature, the local structure is cubic as well. As the temperature rises however, local lattice distortions evidenced by the splitting of the Mn-Cu and Mn-Sn bonds are observed. This local distortion can be described by the I4/mcm symmetry but this symmetry is different from the P4/mmm symmetry of SnNMn$_{3}$. The splitting of the Mn-Cu and Mn-Sn bond gives rise to a local lattice anharmonicity that may in turn be significant in the NTE behavior in this class of compounds.   

Magneto-elastic coupling in molecule-based materials: [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] and Mn[N(CN)$_2$]$_2$

We measured the infrared vibrational response of two prototypical molecule-based magnets, [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] and Mn[N(CN)$_2$]$_2$. We find that both temperature and magnetic field driven transitions impact spin-lattice interactions in these materials. For instance, through the N\'eel transition, Cr--CN stretching and bending modes as well as Ru-O stretching mode in [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] display sudden frequency shifts and a strong hysteresis that reveal local structure changes around Cr and Ru centers in response to magnetic ordering. On the other hand, the dicyanamide ligands in Mn[N(CN)$_2$]$_2$ display pronounced sensitivity to the 30 T magnetic quantum critical transition, in line with our calculations that point toward the importance of N--C--N and C--N--C angles for the mediation of Mn$\cdots$Mn spin exchange interactions.   

Neutron diffraction study of the magnetic order in magnetic coordination polymer CuF$_{2}$(H$_{2}$O)$_{2}$(pyz) (pyz=pyrazine)

The new linear chain coordination polymer CuF$_{2}$(H$_{2}$O)$_{2}$(pyz)(pyz=pyrazine) provides an interesting example for the study of low dimensional physics. CuF$_{2}$(H$_{2}$O)$_{2}$(pyz) has a monoclinic structure where the Cu$^{2+}$ ions form a 2 dimensional (2D) square lattice in the bc-plane. Short range magnetic order appears below 10 K followed by a transition to long range antiferromagnetic order with T$_{N} \quad \sim $ 2.6 K. To further understand the magnetic behavior of CuF$_{2}$(H$_{2}$O)(pyz), we have performed neutron diffraction experiments on deuterated single crystals CuF$_{2}$(D$_{2}$O)$_{2}$(d$_{4}$-pyz). Below 2.6 K we observe magnetic Bragg peaks which are consistent with the propagation vector (1/2 1 0). Refinement of the data shows that the magnetic moment lies in the ac-plane. Fitting the temperature dependence of the magnetic order parameter to a power-law form in the reduced temperature range of 1-T/T$_{N}$ = 0.01-0.4 yields a critical exponent, $\beta $, of 0.25$\pm $0.01. This result is consistent with the expectation for a 2D XY model where $\beta $=0.23 [1]. \\[4pt] [1] S. T. Bramwell and P. C. W. Holdsworth, J. Phys.: Condens. Matter. \textbf{5} L53-L59 (1993).   

Valence instability of Eu in EuPd$_3$B$_x$ ($ 0 < x < 0.53$)

In a joint theoretical and experimental study large series of intermetallic compounds EuPd$_3$B$_x$ and GdPd$_3$B$_x$ are characterized by X-ray diffraction, metallography, EPMA and chemical anlysis assessing the range of formation up to $x < 0.53$ and $x < 0.42$, respectively. Density functional based electronic structure calculations predict a valence change in EuPd$_3$B$_x$ above $x = 0.19(0.02)$ from a non-magnetic Eu$^{3+}$ state into a magnetic Eu$^{2+}$ state which is reflected in a discontinuity of the lattice parameter. Consistent with the calculations X-ray diffraction data show a kink in the lattice parameter at $x = 0.22(0.02)$. X-ray absorption spectroscopy measurements assign this kink to a transition into a heterogeneous mixed valence state of Eu. The influence of external pressure on the valence instability will be discussed.   

Efficient First-principles Wang-Landau Calculations

The Wang-Landau (WL) method of finding the density of states g(E) contributing to the partition function Z(kT) is useful for determining thermodynamic properties from first-principles energy calculations for magnetic systems. Since DFT calculations require significant computer resources, it is important to make the convergence of the WL method to a self-consistent g(E) as efficient as possible. We present approaches for making accurate initial estimates of g(E) based on similar Hamiltonians or estimates of g(E) for a different number of atoms. These approaches can include workstation-based calculations using classical Heisenberg Hamiltonians based on exchange parameters calculated from initial data from DFT WL calculations. In addition, we annunciate serveral insights we have gained into the convergence of the WL method. For instance, the minimum curvature of the calculated g(E) is limited by the update parameter and the maximum energy step of the Markov chain. This material is based upon work supported as part of the Center for Defect Physics, an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences. This research used resources of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which is supported by the Office of Science of the Department of Energy.   

First principles calculations of magnetic properties of Fe and $\textrm{Fe}_3\textrm{C}$ at finite temperature

We demonstrate a method to investigate finite temperature magnetism from first principles that harnesses massively parallel computers to obtain the free energy, specific heat, magnetization, susceptibility, and other quantities as function of temperature by combining classical Wang-Landau Monte-Carlo calculations with a first principles electronic structure code that allows the energy calculation of constrained magnetic states. Here we will present our calculations of finite temperature properties such as specific heat, magnetization and susceptibility of Fe and $\textrm{Fe}_3\textrm{C}$ using this approach where we find the Curie temperatures to be in good agreement with experiment at 980K and 425K respectively. This work was conducted at Oak Ridge National Laboratory (ORNL), which is managed by UT-Battelle for the U.S. Department of Energy (US DOE) under contract DE-AC05-00OR22725 and sponsored in parts by the Center for Nanophase Material Sciences, Scientific User Facilities Division, the Center for Defect Physics, an Energy Frontier Research Center funded by the US DOE Office of Basic Energy Sciences and by the US DOE Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program. This research used resources of the Oak Ridge Leadership Computing Facility at ORNL, which is supported by the US DOE, Office of Science.   

Fist Principles Approach to the Magneto Caloric Effect: Application to Ni$_2$MnGa

The magneto-caloric effect (MCE) has potential application in heating and cooling technologies. In this work, we present calculated magnetic structure of a candidate MCE material, Ni$_2$MnGa. The magnetic configurations of a 144 atom supercell is first explored using first-principle, the results are then used to fit exchange parameters of a Heisenberg Hamiltonian. The Wang-Landau method is used to calculate the magnetic density of states of the Heisenberg Hamiltonian. Based on this classical estimate, the magnetic density of states is calculated using the Wang Landau method with energies obtained from the first principles method. The Currie temperature and other thermodynamic properties are calculated using the density of states. The relationships between the density of magnetic states and the field induced adiabatic temperature change and isothermal entropy change are discussed. This work was sponsored by the Laboratory Directed Research and Development Program (ORNL), by the Mathematical, Information, and Computational Sciences Division; Office of Advanced Scientific Computing Research (US DOE), and by the Materials Sciences and Engineering Division; Office of Basic Energy Sciences (US DOE).   

Session V23: Superconductivity: Josephson Effects II

Lorentzian crater in superconducting microwave resonators with inserted nanowires

We report on observations of nonequilibrium pulsing states in microwave (i.e., GHz) coplanar waveguide(CPW) resonators consisting of superconducting MoGe strips interrupted by a trench and connected by one or more suspended superconducting nanowires. The Lorentzian resonance peak shows a ``crater'' when driven past the critical current of the nanowire, leading to a ``pulsing'' state. In the pulsing state, the supercurrent grows until it reaches the critical current, at which point all stored energy quickly dissipates through Joule heating. We develop a phenomenological model of resonator-nanowire systems, which explains the experimental data quantitatively. For the case of resonators comprising two parallel nanowires and subject to an external magnetic field, we find field-driven oscillations of the onset power for crater formation, as well as the occurrence of a new state, in which the periodic pulsing effect is such that only the weaker wire participates in the dissipation process.   

Characterizing Chiral Domains in Sr2RuO4 via Nanoscale Josephson Junctions

There is substantial evidence that the ruthenate superconductor Sr$_{2}$RuO$_{4}$ has a chiral order parameter of the form p$_{x}\pm $ip$_{y}$ and forms chiral domains. In order to verify this picture and determine the size of the domains, we have fabricated Josephson junctions on the order of and smaller than the domain width of $\sim $1$\mu $m implied by Josephson interferometer experiments. Using Focused Ion Beam milling, we have patterned Sr$_{2}$RuO$_{4}$-Cu-PbIn junctions ranging in size from 0.5$\mu $m x 0.5$\mu $m to 4$\mu $m x 4$\mu $m on the edge face of a Sr$_{2}$RuO$_{4}$ crystal. By measuring the magnetic field modulation of the Josephson critical current, we can probe the phase anisotropy across the junction and determine the size and dynamics of chiral domains. Our data is consistent with the predicted domain width but also exhibits signatures that suggest the formation of chiral domain structure along the c-axis. The theoretical model of domain wall movement proposed by Bouhon and Sigrist is supported by our data.   

Josephson effect and Andreev scattering in a three-terminal quantum dot in the Kondo regime

We study low-temperature transport through a single quantum dot (QD) connected to three terminals, consisting of two superconducting (SC) leads and one additional normal lead. This system shows interesting behavior caused by an interplay between Josephson, Andreev and Kondo physics. The low-energy excitations of this system can be described by a local Fermi liquid theory for the renormalized Bogoliubov particles. We calculate the renormalized parameters using the Wilson numerical renormalization group approach. The Kondo temperature and the residual interaction between the renormalized Bogoliubov particles depend sensitively on the Josephson phase $\phi$ at the crossover region between the local Cooper paring and the Kondo singlet. This crossover reflects the quantum phase transition between non-magnetic singlet and magnetic doublet states, which takes place in the case where the addition normal lead is disconnected. We will also discuss the results for the Josephson current and the DC conductance due to the Andreev reflection.   

Dissipation of mobile vortices in self-dual Josephson junction arrays

Mutual U(1) $\times $ U(1) Chern-Simons Landau-Ginzburg theory appears as an effective field theory in self-dual Josephson junction arrays. In this theory two complex fields associated with disordering electric and magnetic charges are minimally coupled to two gauge fields related to the currents of Cooper pairs and vortices. The condensation of disorder fields is employed to explore the various phases (superconducting, insulating, and metallic) of the model. In this work we investigate the interplay between the dissipation of mobile vortices and the condensation of magnetic and charge excitations. We evaluate the electromagnetic response functions of the system, and we analyze the longitudinal and the Hall conductivities as a function of the strength of dissipation.   

Measurement of a weak-link Josephson junction in a p-wave superconducting ring

We report the fabrication of a weak-link Josephson junction in a micron-size Sr$_{2}$RuO$_{4}$ ring by focused ion beam milling, and the measurement of the current-phase relation (CPR) using cantilever torque magnetometry. In the presence of a magnetic field applied perpendicular to the crystal $c-$axis, a second harmonic term in the CPR appears which may be related to the underlying spin texture of the spin-triplet condensate. The observed CPR is similar to that previously reported for weak-link junctions in $^{3}$He-B. By including the contribution from both the charge and the spin current into the Gibbs-free energy, we can accurately model the observed CPR of the Sr$_{2}$RuO$_{4}$ weak-link junction.   

Periodic critical current pattern in the superconductor-graphene-superconductor junction induced by the current in one of the leads

We have formed superconducting metal contacts to graphene, resulting in supercurrent through graphene visible up to several degrees Kelvin. In our geometry, graphene bridges a gap between two closely spaced superconducting wires. We have found that passing a current along the length of one of the wires periodically modulates the magnitude of the supercurrent through graphene. We discuss the origins of the observed interference patterns   

Intrinsic Josephson Junctions with Intermediate Damping

In cuprate superconductors, adjacent cuprate double-planes are intrinsically Josephson-coupled. For bias currents perpendicular to the planes, the current-voltage characteristics correspond to those of an array of underdamped Josephson junctions. We will discuss our experiments on sub-micron Tl-2212 intrinsic Josephson junctions (IJJs). The dynamics of the IJJs at the plasma frequency are moderately damped (Q $\approx$ 8). This results in a number of counter-intuitive observations, including both a suppression of the effect of thermal fluctuations and a shift of the skewness of the switching current distributions from negative to positive as the temperature is increased. Simulations confirm that these phenomena result from repeated phase slips as the IJJ switches from the zero-voltage to the running state. We further show that increased dissipation counter-intuitively increases the maximum supercurrent in the intermediate damping regime (PRL vol. 103, art. no. 217002). We discuss the role of environmental dissipation on the dynamics and describe experiments with on-chip lumped-element passive components in order control the environment seen by the IJJs.   

Intrinsic Josephson effect in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ after doping by current injection

By current injection we can change the properties of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals electronically in a wide range. In this work the properties of the same sample were changed multiple times in very small steps in order to investigate the doping process by current injection in greater detail. By measuring the IV characteristic of the intrinsic Josephson junctions as well as doping current and doping voltage simultaneously, the change of superconducting properties is monitored. Macroscopic quantum tunneling experiments in intrinsic Josephson junctions were performed. An exponential increase of the critical current density with hole concentration was observed. Simultaneously, the capacitance of the intrinsic Josephson junctions increases with the doping level by a factor of 5. We will discuss possible reasons for these results.   

Statistics of voltage fluctuations in resistively shunted Josephson junctions

The intrinsic nonlinearity of Josephson junctions converts Gaussian current noise in the input into non-Gaussian voltage noise in the output. For a resistively shunted Josephson junction with white input noise we determine numerically exactly the properties of the few lowest cumulants of the voltage fluctuations, and we derive analytical expressions for these cumulants in several important limits. The statistics of the voltage fluctuations is found to be Gaussian at bias currents well above the Josephson critical current, but Poissonian at currents below the critical value. In the transition region close to the critical current the higher-order cumulants oscillate and the voltage noise is strongly non-Gaussian. For coloured input noise we determine the third cumulant of the voltage.   

Josephson effect in S/F/S junctions: spin bandwidth asymmetry vs. Stoner exchange

We analyze the dc Josephson effect in a ballistic S/F/S junction in the quasiclassical Andreev approximation. We consider the possibility of ferromagnetism originating from a mass renormalization of carriers of opposite spin, i.e. a spin bandwidth asymmetry (SBAF). We provide a general formula for Andreev levels which is valid for arbitrary interface transparency, exchange interaction, and bandwidth asymmetry. We analyze the current-phase relation, the critical current, and the free energy in the short junction regime, showing that a larger number of $0-\pi$ transitions is expected when the ferromagnetism is driven by SBAF compared to Stoner magnetism. We compare the phase diagrams of two identical junctions differing only in the mechanism by which the mid layer becomes magnetic, pointing out that the phase difference across the junction in the ground state need not be the same, even for equal polarizations.   

Fluxon dynamics in two-band superconductor-based long Josephson junctions

We investigate the phase dynamics of a long Josephson junction (LJJ) with two-band superconductors such as $MgB_2$ and iron pnictides. Due to two condensates in each superconductor layer, the phase dynamics of a two-band LJJ, described by the perturbed sine-Grodon equation, becomes more complex than that for the usual LJJ with one-band superconductors. This complexity arises from the presence of inter-band Josephson current that yields soliton-like excitation. This excitation represents a large stable variation of the phase difference of the two condensates. Accounting for the soliton- like excitation, we find that the fluxon dynamics in the LJJ with two-band superconductors is influenced by the modulation of Josephson current. The Josephson current modulation yields radiation emission by the moving fluxon. Also, we discuss the effects of this current modulation on the current- voltage characteristics of the LJJ.   

Enhancement of macroscopic quantum tunneling in Josephson junctions with multigap superconductors through zero-point fluctuations of Josephson-Leggett mode

We theoretically study macroscopic quantum tunneling (MQT) in a hetero Josephson junction formed by a conventional single-gap superconductor and a multigap one such as $\mbox{MgB}_{2}$ and iron-based superconductors. In such a Josephson junction, multiple phase differences are defined and MQT dynamics are extended on a space expanded by the multiple phases. We clarify the quantum dynamics of the multiple phase differences and construct a theory for the MQT in Josephson junctions with multiple gaps. The dynamics of the phase differences are strongly affected by Josephson-Leggett mode, i.e., the fluctuation mode between the multiple phase differences and the escape rate characterizing MQT dynamics is calculated based on the effective action renormalized by the Josephson-Leggett mode. We show that the escape rate is drastically enhanced when the frequency of the Josephson-Leggett mode is less than the Josephson-plasma frequency.   

Parametric Amplification in Discrete Josephson Transmission Line

A Series-connected discrete Josephson transmission line (DJTL) which is periodically loaded by open stubs is studied to investigate various aspects of traveling-wave parametric amplification. The dispersion analysis is made to ensure the existence of three non-degenerate time-harmonic waves interacting with each other through the phase matching condition which is imposed by the cubic nonlinearity associated with each junction. Having weak nonlinearity and slow varying assumptions, we exploit the perturbation theory with the multiple scale technique to derive the three coupled nonlinear partial differential equations to describe their spatial and temporal amplitude variations in this parametric interaction. Cases of perfect phase-matching and slight mismatching are addressed in this work. The numerical analysis based on the spectral method in space and finite difference in time domain are used to monitor the unilateral gain, stability and bandwidth of the proposed structure. This structure can be used as a mesoscopic platform to study the creation of squeezed states of the microwave radiation. These properties make this structure desirable for applications ranging from superconducting optoelectronics to dispersive readout of superconducting qubits where high sensitivity, fast speed and low-noise operation is required.   

Session V24: History of Physics and International Programs

Castles in the Air: The Einstein-De Sitter Debate, 1916-1918

The Einstein De Sitter debate marked the birth of modern cosmology and the infamous cosmological constant. For Einstein, the controversy was essentially a philosophical one. Einstein's insistence on a static Universe and Mach's Principle guided him in the construction of his own cosmological model, and compelled him to criticize De Sitter's. For De Sitter, the debate began as idle conjecture. Before long, however, he began to wonder if the ``spacious castles'' he and Einstein had constructed might actually represent physical reality. We plan to write a volume that reproduces the documents relevant to the debate. Our commentary will retrace and explain the arguments of the historical players, complete with calculations. For the first time readers will be able to follow the arguments of Einstein and De Sitter in a detailed exploration of the first two relativistic cosmological models. Readers will see how Einstein's flawed criticisms of De Sitter were supported by Herman Weyl, and finally how Felix Klein settled the whole matter with a coordinate transformation.   

Ettore Majorana, Ugo Fano and Autoionization

In his brief career, Ettore Majorana posed some questions that remain of compelling interest today, such as the nature of the neutrino. Less remembered now is his virtuosity as an atomic theorist. His first published paper (1928) dealt successfully with complex atomic structures like those of Gd and U, using Fermi's statistical model which was only a few months old at the time. In the early 1930s he solved two outstanding problems of atomic spectroscopy, correctly interpreting them as involving multiply-excited discrete states of atoms that were embedded in single-electron continuua, and thus first identifying the phenomenon of autoionization in atomic spectra.$^{1}$ His unpublished notebooks$^{2}$ show that he grasped this phenomenon at a level of detail comparable to that of modern theory, which derives from the independent work of Ugo Fano.$^{3}$ We review Majorana's work on this subject and show how it still guides present understanding. $^{1}$E. Arimondo, C. W. Clark and W. C. Martin, \textit{Rev. Mod. Phys.} \underline {\textbf{82}}\underline {, 1947} (2010) $^{2}$E. Di Grezia and S. Esposito, \textit{Found. Phys.} \underline {\textbf{38}}\underline {, 228} (2008) $^{3}$U. Fano, Nuovo Cimento \textbf{12}, 154 (1935); \textit{Phys. Rev. }\textbf{124}, 1866 (1961); \textit{J. Res. Natl. Inst. Stand. Technol.} \underline {\textbf{110}}\underline {, 583} (2005)   

The Golden Age of Radio: Solid State's Debt to the Rad Lab

While MIT's Radiation Laboratory is rightly celebrated for its contributions to World War II radar research, its legacy extended beyond the war. The Rad Lab provided a model for interdisciplinary collaboration that continued to influence research at MIT in the post-war decades. The Rad Lab's institutional legacy--MIT's interdepartmental laboratories--drove the Institute's postwar research agenda. This talk examines how solid state physics research at MIT was shaped by a laboratory structure that encouraged cross-disciplinary collaboration. As the sub-discipline of solid state physics emerged through the late-1940s and 1950s, MIT was unique among universities in its laboratory structure, made possible by a large degree of government and military funding. Nonetheless, the manner in which MIT research groups from physics, chemistry, engineering, and metallurgy interfaced through the medium of solid state physics exemplified how the discipline of solid state physics came to be structured in the rest of the country. Through examining the Rad Lab's institutional legacy, I argue that World War II radar research, by establishing precedent for a particular mode of interdisciplinary collaboration, shaped the future structure of solid state research in the United States.   

Willie Hobbs Moore (1934-1994): The First Female African American Physicist

We discuss the life and career of Willie Hobbs Moore, the first African American woman to receive a doctorate degree in physics. This achievement occurred in June 1972 at the University of Michigan, Ann Arbor, MI. Her dissertation, directed by the renowned spectroscopist Samuel Krimm, was on the subject of ``A Vibrational Analysis of Secondary Chlorides," and focused on a theoretical analysis of the secondary chlorides for polyvinal-chlorine polymers. From 1972--1977, she, Krimm, and collaborators published more than thirty papers on this and related research issues. In addition to an overview of her family background, her careers as a research physicist and scientist working in various industrial laboratories, we discuss the obstacles and successes she encountered at various stages of her life.   

Encounters with John Van Vleck: Elevating the Self-Esteem of an Experimentalist

As a Harvard freshman in 1960, John H. Van Vleck was assigned to guide me through physics course selections. As a Harvard first year graduate student in 1964, I took his course on Group Theory. He was an esteemed theoretician, and I was an experimentalist. Nevertheless, I liked him, and he both educated me and gave me good advice. Later, I learned that Van Vleck was responsible for bringing Nicolaas Bloembergen, my Ph. D. advisor, back to Harvard from Holland, with a faculty appointment in the Division of Engineering and Applied Physics. (Bloembergen had done his Ph.D. thesis with Ed Purcell, making NMR into a science, and Purcell was one of the best teachers I encountered in 9 years at Harvard.) Stepping forward in time to May, 1969, just after defending my doctoral thesis, I ran into Van Vleck in the hallway. I told him I had earned my Ph.D., and he asked me what I had done for my thesis. Feeling defensive while talking to a great theoretician, I started out cautiously to say that I had built a carbon dioxide laser. He immediately started praising me as the best of the breed, someone who could build things and actually make them work. The praise continued, he told some stories (which I will share), and I left the building on ``Cloud 9.'' It was a really good day.   

US-Finland Planning Visit: Cooperative Research and Education Activities in Integrated Access Networks

This planning grant visit sponsored by the NSF Office of International Science and Engineering occurred from October 3-10, 2010. The Dean of the School of Computer, Mathematical and Natural Sciences from Morgan State University (MSU), the PI and a faculty member from engineering at MSU along with a faculty member from the University of Arizona and two advanced level graduate students from the NSF-funded Center for Integrated Access Networks participated in this visit. The topic of novel low dimensional nano-materials was determined to be one possible area for future collaboration. As a result of this visit, a Materials World Network proposal has been submitted to the NSF involving MSU and CIAN in the US and Aalto University in Finland. A companion proposal on novel low dimensional nano- materials has also been submitted to the Academy of Finland. Another anticipated outcome of this collaboration of MSU with Aalto University and CIAN expands the outreach and diversity component to MSU, an institution serving largely an underrepresented minority student.   

Session V25: Superconductivity: HTSC Theory, Mostly Nematics and Inhomogeneous Systems

The role of nematic fluctuations in the thermal melting of pair-density-wave phases in superconductors

We study properties of phase transitions of the superconductor liquid crystal phases, and analyze the competition between the recently proposed Pair Density Wave (PDW) and nematic $4e$ superconductor (4eSC). Nematic fluctuations enhance the 4eSC and suppress the PDW phase. For a system decoupled from a lattice, the PDW state exists only at $T=0$ and the low temperature phase is a nematic $4e$SC with short ranged PDW order.   

Nematicity in 3-band Hubbard model of cuprate superconductors

The recent discovery of intra-unit-cell nematicity in STM studies of cuprate superconductors [1] underscores the importance of the role played by oxygen orbitals in CuO2 plane. Motivated by this observation we study 3-band Hubbard model using exact diagonalization. In particular, we investigate the effects various interaction parameters (Ud, Up, Vpd, Vpp) have on nematicity. Interestingly, we find that Ud, the on-site repulsion at copper sites, enhances nematicity in the strongly coupled regime.\\[4pt] [1] Lawler, M. J. \textit{et al}. Intra-unit-cell electronic nematicity of the high-Tc copper-oxide pseudogap states. \textit{Nature} \textbf{466}, 347 (2010).   

Mean-Field Nematic Phase Diagram for the Three-Band Hubbard Model

We map out the phase diagram of the three-band Hubbard model of a CuO$_2$ plane for nematic order in the parameter space of various on-site and nearest-neighbor interactions. For this, we define an intra-unit cell nematic order parameter in terms of a charge imbalance between the two oxygen sites in the unit cell and employ a self-consistent mean-field analysis. This study is motivated by recent STM experiments on high-T$_c$ cuprate superconductors pointing towards intra-unit cell nematicity.   

Dynamical electronic nematicity from Mott physics

We study the two-dimensional Hubbard model with small band anisotropy using dynamical-mean-field theory for clusters. We found that very large transport anisotropies can be induced by very small band anisotropy as in many strongly correlated materials. This happens when the interaction is large enough to yield a Mott transition. The maximum effect on conductivity anisotropy occurs in the underdoped regime as observed in high temperature superconductors. The anisotropy decreases at large frequency and is not associated with static stripe order. Thus we call the phenomenon ``dynamical electronic nematicity''. This work was supported by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. DOE (S.O.), NSERC (Canada) and the Tier I Canada Research Chair Program (A.- M.T.), with part of the computational resources by RQCHP and Compute Canada.   

Quantum Nematic Physics in the Hubbard Model of Cuprate Superconductors

Recent experiments have provided strong evidence that quantum electronic nematic order plays an important role in characterizing the pseudogap region of the cuprate superconductors. Starting from the generic Hubbard model of the cuprates, we introduce a small anisotropy in the hopping integral to model a small orthorhombic distortion. We perform a dynamic cluster quantum Monte Carlo approximation of this model, in order to study the effects of this anisotropy on various properties. In particular, we investigate the effects on superconductivity and pseudogap behavior, as well as the competition between different effects.   

On the origin of quantum criticality found at finite doping in 2D Hubbard model

To better understand the excitations responsible for quantum criticality (QC) found at finite doping in the 2D Hubbard model, we analyze the vertices for different scattering channels obtained from the Dynamical Cluster Continuous-Time Quantum Monte Carlo simulation. By decomposing these vertices using the parquet equations we find that both superconductivity and the charge instabilities responsible for the QC come from the crossed spin channel contribution, and thus are driven by the spin-fluctuations. On contrast, the spin instability comes from the fully irreducible spin vertex contribution.   

Superconductivity from purely repulsive interactions in the 2D electron gas

We present a well-controlled perturbative renormalization group (RG) treatment of superconductivity from short-ranged repulsive interactions in a variety of model two dimensional electronic systems. Our analysis applies in the limit where the repulsive interactions between the electrons are small compared to their kinetic energy.   

Effect of competing orders on superconductivity in the Hubbard model

We study the superconducting transition in the repulsive Hubbard model incorporating different competing orders at the mean field level. To the model on the square lattice with nearest and next-nearest neighbor hopping amplitudes, we add appropriate modulations of the hopping terms or onsite energies, such that they produce the desired broken symmetry. We then study the superconducting instability in the (theoretically tractable) limit in which the onsite repulsion U is small compared to the bandwidth. We obtain the pairing symmetry and strength as a function of the magnitude of the order parameter. Specific cases of broken symmetry states we study include antiferromagnetism, ferromagnetism, nematic order, d-density wave, and orbital loop order.   

$d$-wave Cooper pairing in multiband models for the high-$T_c$ cuprates

We investigate possible reasons for the significant differences of $T_c$s in high-$T_c$ cuprate compounds, based on renormalization group treatments of downfolded models for the electronic structure. Generally, cuprates with a square like Fermi surface exhibit lower critical temperatures than materials with a more rounded Fermi surface, which contradicts with many theoretical studies of the one band Hubbard model. To resolve this contradiction we study multiband models which in addition to the $d_{x2-y2}$ orbital contain $4s$ and $d_{z2}$ orbitals using different approximation levels of the functional renormalization group technique. Our results suggest that the observed material trend can be explained in parts by the influence of orbital mixing, which can dominate over the effect of the Fermi surface shape.   

Inhomogeneity and d-wave superconductivity in the Hubbard model

Whether or not inhomogeneity plays a significant role in determining the superconducting properties of the cuprate high-Tc superconductors remains an open issue, in spite of extensive theoretical and experimental focus. To this end, we study d-wave superconductivity in the checkerboard Hubbard model on a square lattice. We employ the Cellular Dynamical Mean Field theory method with an exact diagonalization solver at zero temperature. The d-wave order parameter is computed for various inhomogeneity levels over the entire doping range of interest. We find a monotonic decrease in the maximum amplitude of the superconducting order parameter with inhomogeneity. However, the order parameter increases with inhomogeneity in a small doping interval lying in the extreme overdoped regime. For any doping, an inhomogeneity-induced change in the height of the lowest energy peak in the antiferromagnetic spin susceptibility correlates with the change in amplitude of the order parameter.   

Disorder effects on pair binding in the checkerboard Hubbard model

The checkerboard Hubbard model is an inhomogeneous fermionic Hubbard model in which hopping on plaquettes takes a different value to hopping between plaquettes. The pair binding energy in the clean checkerboard Hubbard model, interpreted as a tendency towards superconducting order, is positive over a wide part of the zero temperature phase diagram. We perform exact diagonalization studies of the checkerboard Hubbard model with on-site disorder. For systems up to twelve sites, we study the distribution of pair binding energies that results from the introduction of potential disorder and find that weak disorder enhances the region of the phase diagram over which there is a non-zero probability of pair binding, without greatly changing the average pair binding energy. We also study how stronger disorder destroys pair binding.   

Theoretical investigation of superconductivity and antiferromagnetism in trilayer cuprate superconductors

Recent ARPES experiment on the optimally doped trilayer cuprate superconductors Bi2223 has revealed a layer variation of both doping density and d-wave gap. In particular, the two outer layers are overdoped with a gap which is larger than the gap for optimally doped single layer cuprates while the inner layer is underdoped with an even larger gap. Here we propose a minimal model composed of three layer t-J model, single particle interlayer tunneling as well as Cooper pair tunneling terms. By using renormalized mean field method, both the superconducting (SC) and antiferromagnetic (AFM) properties are theoretically investigated. Both tunneling effects may influence the phase configurations of both d-wave SC and AFM order parameters on each layer which plays a crucial role in determining the electronic structures of trilayer cuprates. In particular, the inphase state for both SC and AFM phases is found to be relevant to the Bi2223 trilayer system. In such a state, the superconducting order parameter of inner plane will be further enhanced due to the constructive proximity effect from the two outer planes and the hole density of inner plane will be much suppressed. Furthermore we predict that the appearance of interlayer ferromagnetic correlations for such system which could be tested by future NMR experiments. **This work is in collaboration with Chun Chen and A. Fujimori.   

Confinement-deconfinement interplay in quantum phases of doped Mott insulators

It is generally accepted that doped Mott insulators can be well described by the t-J model. In the latter, the electron fractionalization is dictated by the phase string effect. We found that in underdoped regime, the antiferromagnetic and superconducting phases are dual: in the former, holons are confined while spinons are deconfined, and vice versa in the latter. These two phases are separated by a novel phase, the so- called Bose-insulating phase, where both holons and spinons are deconfined. A pair of Wilson loops was found to constitute a complete set of order parameters determining this zero- temperature phase diagram. The quantum transitions between these phases are suggested to be of non-Landau-Ginzburg-Wilson type.   

FLEX calculation of the pairing state symmetry and quasiparticle excitations for SrRu$_2$O$_4$

We calculate the superconducting phase diagram for a two-dimensional, three-band tight-binding model of SrRu$_2$O$_4$ using the fluctuation exchange approximation (FLEX). Electron interactions are modeled by an atomically-local interaction with intra-band, inter-band and exchange terms and an atomically-local spin-orbit interaction is included as well. Preliminary results suggest that FLEX produces a singlet-pairing state with d$_{x^2 - y^2}$ orbital symmetry, a result that is not in agreement with many experimental results. However, we do find that the values for the interaction strengths that are required to generate $T_c \simeq 1.5\,K$ lead to normal state quasiparticle line widths that are in reasonable agreement with experimental results.   

Diagrammatic Quantum Monte Carlo Solution of Two dimensional Cooperon-Fermion model

We investigate the two-dimensional Cooperon-fermion model in the strong coupling limit with continuous-time diagrammatic determinant quantum monte carlo (DDQMC). We obtained the same Kosterlitz-Thouless transition temperature $T_{c}$ for the fermion's off-diagonal long range order $\chi_{OD}$ ({\boldmath $k$}=0,$\omega=0)$ and cooperon's Greens function $G^{b}$({\boldmath $k$}=0,$\omega=0)$ as expected. The renormalized cooperon's band (band gap and mass) is examined carefully. The delocalization of the cooperons enhances the diamagnetism. When applied to study the diamagnetism of pseudogap state in high-T$_{c}$ cuprate, the results we obtained is in good agreement with recent torque magnetization measurements.   

Session V26: Focus Session: Iron Based Superconductors -- Doping Studies

Isotope substitution induced lattice expansion in the iron based superconductors

In the iron based superconductors there are indications of a wide range of exponents for the iron isotope effect on Tc in different families of these materials. In this work we explore to what extent this spread of exponents may be a result of different isotope induced lattice expansions in combination with an extreme sensitivity of Tc to the interatomic distances. We estimate the magnitude of anharmonicity in the dominant Fe-As(Se) bond based on a model fit of the temperature dependence of Raman phonon frequencies. Using a variational approach to treat the effect of the anharmonic potential we calculate the magnitude and sign of the expansion of lattice parameters and interatomic distances due to Fe isotope substitution in several different iron-based superconductors.   

Doping evolution of the electronic structure in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ as revealed by polarization dependent ARPES and soft X-ray absorption

Here we present a study based on ARPES and X-ray absorption spectroscopies in order to unveil the electronic structure evolution upon Co-doped in BaFe2As2 high Tc superconductors, for the doping levels x=(0,6,8,12,22){\%}. This study focuses on two points: i) the effective role of Co at different doping levels; ii) the shift upon doping of the band structure. X-ray absorption experiments carried out at the Co L23 edge highlight the chemical state of cobalt at the various doping levels, thus unveiling its role as charge donor. The orbital selectivity of polarization dependent ARPES is used to show the filling evolution of each band. The spectra have been collected in different geometries along the $\Gamma $X and $\Gamma $M high symmetry crystallographic directions and at two different photon energies. The experimental results show a general rearrangement of the charge within the various orbitals upon doping, with a non rigid band shift.   

Doping dependence of the optical spectra in Fe$_{1.02}$Te$_{1-x}$Se$_{x}$

We measured the optical reflectivity and extracted the optical conductivity over a broad spectral range of the Fe-chalcogenide Fe$_{1.02}$Te$_{1-x}$Se$_{x}$, at various doping levels, from $x$=0.12 to $x$=0.45. Except for the highest concentration of Se ($x$=0.45), optical properties show unusual frequency and temperature dependence. The zero frequency Drude peak is absent in optical conductivity, a plasma edge cannot be unambiguously defined and both reflectivity and optical conductivity have non-monothonic temperature dependence. For the superconducting samples, we found a weak enhancement of the reflectivity in the far-infrared region upon cooling below Tc and its possible association with the superconducting gap will be discussed.   

Doping-Induced Evolution of Superconducting Order Parameter in Ba(Fe1-xNix)2As2 Single Crystals

We report a systematic investigation on the $c$-axis point- contact Andreev reflection (PCAR) in BaFe$_{2-x}$Ni$_x$As$_2$ superconducting single crystals with the Ni concentrations from underdoped to overdoped regions (0.075 $\leq x\leq 0.15$). At low temperatures, an in-gap peak at low-bias voltage is observed in PCAR for overdoped samples, in contrast to the case of underdoped junctions, in which an in-gap plateau is observed. The spectra are fitted using a generalized Blonder- Tinkham-Klapwijk (BTK) formalism with two gaps: one isotropic and another angle dependent. The second gap, resulted from the fitting, shows a clear crossover from a nodeless in the underdoped side to a nodal feature in the overdoped region. This intriguing observation provides strong evidence of the doping induced evolution of the superconducting order parameter when the inter-pocket and intra-pocket scattering are tuned through doping, as expected in the $s_{\pm}$ scenario.   

First-principles study of light-element doping effects on iron-based superconductors

Since the discovery of the iron-based superconductor, LaFeAsO$_{1-x}$F$_x$ whose Tc reached 26K, various types of iron-based superconductors have been fabricated to attain higher $T_c$. Recently, it is reported that Tc of an iron-based superconductor LaFeAsO$_{1-y}$ is enhanced to 35K by doping hydrogen. This result implies that atoms of light elements penetrate into the crystal of iron-based superconductors and transform their structures into more useful ones for superconductivity. In this talk, we investigate how the light elements are doped in the iron-based superconductors by using the first-principles density functional theory. Furthermore, we evaluate the effects of doping on the crystal structures and electronic states and explore the origin of the $T_c$ enhancements.   

Electron spin resonance in iron pnictides

We report on electron spin resonance studies in Eu based 122-superconductors where the Eu$^{2+}$ ions serve as a probe of the normal and superconducting state. In polycrystalline Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ the spin-lattice relaxation rate $1/T_1^{\rm ESR}$ obtained from the ESR linewidth exhibits a Korringa-like linear increase with increasing temperature above $T_{\rm c}$ evidencing a normal Fermi-liquid behavior. Below $T_{\rm c}$ the spin lattice relaxation rate $1/T_1^{\rm ESR}$ follows a $T^{1.5}$-behavior without any appearance of a coherence peak. In superconducting EuFe$_2$As$_{1.8}$P$_{0.2}$ single crystals we find a similar Korringa slope in the normal state and observe anisotropic spectra for measuring with the external field parallel and perpendicular to the $c$-axis. In addition, we will discuss the ESR properties of selected systems from the 1111 and 11 families.   

Effect of Isoelectronic Doping at the As Site in Iron-based Superconducting Systems

FeAs-based superconductivity can be induced with various doping strategies---either electron doping, hole doping or isoelectronic doping. In this talk, we will focus on the unique isoelectronic doping at the As site. By using different dopants such as P [1,2] or Sb [3], positive or negative chemical pressure can be generated onto the FeAs layers. The positive chemical pressure suppresses/destroys the spin-density-wave (SDW) ordering, and then superconductivity emerges around a quantum critical point. In contrast, the negative pressure tends to recover the suppressed/hidden SDW ordering. The isoelectronic doping also influences the electronic and magnetic state of 4f electrons in the rare-earth atomic layers and 3d electrons of the Fe planes, especially in the case of proximity between 4f and 3d energy levels. This was manifested by the observation of local-moment ferromagnetism of 4f electrons in EuFe$_{2}$(As$_{1-x}$P$_{x})_{2}$, CeFeAs$_{1-x}$P$_{x}$O and CeFeAs$_{1-x}$P$_{x}$O$_{0.95}$F$_{0.05}$ [4] systems. Our results demonstrate the intriguing interplay/competition of intersite RKKY coupling among 4f-moments, intrasite Kondo interaction between 4f- and 3d- electrons, and \textbf{k}-space Cooper pairing of 3d electrons. This work was done in collaboration with Zhu-An Xu and Jian-Hui Dai, and was supported by National Basic Research Program of China (Grant Nos. 2007CB925001 and 2010CB923003). \\[4pt] [1] Zhi Ren et al., \textit{Phys. Rev. Lett}. 102, 137002 (2009). \\[0pt] [2] C. Wang et al., \textit{EPL }86, 47002 (2009)~; S. Jiang et al., \textit{JOP-CM }21, 382203 (2009)~. \\[0pt] [3] C. Wang et al., \textit{Science China G }53, 1225 (2010). \\[0pt] [4] Y. K. Luo et al., \textit{Phys. Rev. B} 81, 134422 (2010)~; Y. K. Luo et al., to be published.   

Suppression of critical temperature in proton irradiated Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

The study on the superconducting gap structure of iorn-pnictide superconductor is one of the most important issues to uncover the pairing mechanism of high temperature superconductivity. To elucidate the superconducting gap structure, a detailed study on the effect of defects is very crucial because the pair-breaking effects due to scattering centers are phase-sensitive. We report the suppression of $T_{c}$ due to the pair- breaking effect introduced by 3 MeV proton irradiation in Ba(Fe$_{1- x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals at under-, optimal-, and over-doping levels. We find that $T_{c}$ decreases and residual resistivity increases monotonically with increasing the proton dose. We also find no resistive upturn at low temperatures, which suggests that the proton irradiation provides nonmagnetic scattering centers. The critical scattering rate for all samples estimated by three different ways is much higher than that expected in $s_{\pm}$-pairing scenario based on inter-band scattering due to antiferro-magnetic spin fluctuations.   

Nanometer Scale Phase Separation and Chemical Inhomogeneity in the Iron Chalcogenide Superconductor Fe$_{1+y}$Te$_{x}$Se$_{1-x}$

We report direct evidences of phase separation and chemical inhomogeneity in Fe$_{1+y}$Te$_{x}$Se$_{1-x}$ single crystals from scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). In STEM, images recorded using an annular dark field (ADF) detector show characteristic nanometer scale patterns of phase separation from the Z dependent contrast. The separation was observed in both non-superconducting samples with excess iron as well as superconducting samples. Using the line scan EELS technique, we determined $\sim $20{\%}, or less, fluctuation in Te concentration from the local average compositions by integrating the intensity of the Te-M$_{4,5}$ edge. The energy-loss near-edge structure (ELNES) of the Fe-L$_{2,3}$ edge changes as the composition varies, especially the L$_{3}$ and L$_{2}$ ratio, which is sensitive to the d-state occupancy of the Fe atom. The results suggest a miscibility gap in the Fe$_{1+y}$Te$_{x}$Se$_{1-x}$ system and changes in the d-electron states at the nanometer scale from the separated phases.   

Local topography and spectroscopy of the doped chalcogenide FeTe$_{x}$Se$_{1-x}$

The atomically resolved structural and electronic properties of FeTe and FeTe$_{0.55}$Se$_{0.45}$ have been investigated using scanning tunneling microscopy/spectroscopy. The STM topography of the doped sample clearly distinguishes two types of atoms. Statistically the topography shows the correct concentration of the two species and reveals that they are not randomly distributed but prefer to congregate with tens of identical atoms in nanometer scale. Consequently, in contrast to extremely flat surface of parent compound FeTe, FeTe$_{0.55}$Se$_{0.45}$ shows structural phase separation globally breaking the pure \textit{p4/nmm} symmetry. Surprisingly the local density of states on tellurium and selenium atoms in FeTe$_{0.55}$Se$_{0.45}$ are identical, but not the same as that in pure FeTe. This indicates an itinerant (rather than localized) electronic character in this doped system. This behavior is opposite to phase separation in many other doped materials.   

Revealing the electronic structure of the iron pnictides with electron energy-loss spectroscopy

We report electron energy-loss spectroscopy (EELS) studies of the parent compounds (LnFeAsO, Ln=La, Ce, Pr, Nd, Sm, Gd) using scanning transmission electron microscopy. We find that all the studied LnFeAsO present a Fe L-edge fine structure closer to that of metallic iron than iron oxides. We observe a direct correlation between the Fe valence state (obtained from EELS) and $T_C$, i.e. the smaller the calculated Fe valence state, the larger is the $T_C$ for that compound. We also find an anomalous crystallographic orientation-dependence of the Ln $M_{45}$ edge fine structure. In particular, we find difference in the apparent crystal field splitting of Ce and Gd f- bands when the spectra are collected parallel and perpendicular to the c-axis. This research was partially supported by NSF Grant No. DMR-0938330 (JCI, WZ), by ORNL's Shared Research Equipment (SHaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy (JCI) and the Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy (MC, ASS, MAM, BCS \& SJP), DOE grant DE- F002-09ER46554 (MP, STP), and by the McMinn Endowment (STP) at Vanderbilt University.   

Doping dependent vortex pinning in single crystal BaFe$_{2}$(As$_{1-x}$P$_{x})_{2 }$

We report on magnetization measurements on doped single crystals of superconducting BaFe$_{2}$(As$_{1-x}$P$_{x})_{2 }$. For optimum doped crystals, we observe a second magnetization peak effect (fish tail). With further doping of phosphur for arsenic, the fish tail effect evolves into a peak effect close to H$_{c2}$, similar to that found in conventional type II superconductors. In heavily overdoped crystals, the magnetization loop is mostly reversible and no peak effect is observed. The evolution of the peak effect with doping is attributed to the reduction in defects as the crystal's purity is increased, going from optimum doping to over-doping. Possible pinning mechanism for the peak effect will be discussed within the framework of recent heat capacity and resistivity measurements.   

Session V27: Focus Session: Semiconductor Qubits- Dynamic Decoupling, Dephasing, and Relaxation

Increasing Quantum Dot Electron Spin Coherence with Persistent Spin Narrowing

We demonstrate reproducible initialization of the Overhauser field in a single InAs self-assembled quantum dot using the hole assisted nuclear feedback mechanism. This fixes the mean the Overhauser field to a value determined by two pump lasers and dramatically reduces the statistical broadening of the electron spin resonance arising from averaging over the nuclear spin ensemble, (1/T2*). By initializing for tens of milliseconds, the prepared Overhauser field distribution lasts for well over a second even in the presence of a fluctuating electron spin. Furthermore, we find a mechanism which will initialize the nuclear spins using only a single laser, and that this mechanism involves the evolution of the nuclear spins ``in the dark'', that is, absent any optical field. This new method is directly compatible with the CW readout technique used in recent time-domain spin manipulation experiments.   

Effects of Multi-pulse Dynamical Decoupling Schemes on Dephasing in a GaAs Spin Qubit

Coherence time ($T_2$) of a singlet-triplet qubit in a GaAs double quantum dot is studied as a function of the number of $\pi$-pulses in a Carr-Purcell-Meiborn-Gill (CPMG) dynamical decoupling sequence. In this system, the dominant forms of dephasing are expected to be hyperfine coupling to the nuclei and electrical noise. For $n_\pi$ ranging from 2 to 32, we find a power law dependence of $T_2$ with the number of pulses, $T_2 \propto n_\pi^\beta$, where $n_\pi$ is the number of pulses and $\beta \sim 0.7$ is a fit parameter.   

The central spin problem: electron spin qubit evolution due to coherently evolving nuclear spins

In recent years, electron spin qubits in solid state quantum dots have emerged as promising candidates for the implementation of quantum information processing. We study the dephasing of two electron spins in a double quantum dot system due to the evolution of the underlying nuclear spins, as was measured in a recent spin echo experiments. We develop a semi-classical model for such a system, by treating the Overhauser field induced by the nuclear spins as a classical time-dependent vector. Comparing the outcome of this model to experimental echo signal of the electron qubit allows us to identify MNR-like signatures from the nuclear spin evolution, such as spin diffusion, coherent nuclear Larmor precession and the spread of the Larmor frequencies by various mechanisms.   

Generating Entanglement and Squeezed States of Nuclear Spins in Quantum Dots

Entanglement generation and detection are two of the most sought-after goals in the field of quantum control. Besides offering a means to probe some of the most peculiar and fundamental aspects of quantum mechanics, entanglement in many-body systems can be used as a tool to reduce fluctuations below the standard quantum limit. For spins, or spin-like systems, such a reduction of fluctuations can be realized with so-called squeezed states [1]. Here we present a scheme for achieving coherent spin squeezing of nuclear spin states in single electron quantum dots [2]. This work represents a major shift from earlier studies, which have explored classical ``narrowing'' of the nuclear polarization distribution through feedback involving stochastic spin flips. In contrast, we use the nuclear-polarization-dependence of the electron spin resonance (ESR) line to provide a non-linearity which generates a non-trivial, area-preserving, ``twisting'' dynamics which squeezes and stretches the nuclear spin Wigner distribution without the need for nuclear spin flips. \\[4pt] [1] M. Kitagawa, M. Ueda, Phys. Rev. A 47, 5138 (1993). \\[0pt] [2] M. S. Rudner, L. V. M. Vandersypen, V. Vuletic, L. S. Levitov, to be published.   

Dephasing of two-spin qubits due to their charge and nuclear environments

We consider dephasing of qubits encoded in the singlet and unpolarized triplet states of pairs of spins localized in biased double quantum dots. The charge environment is modeled by both two-center charge traps in the insulator (where electrons tunnel between the two centers), and single charge traps located near the gate electrodes and QPCs (where electrons charge and empty the trap). The couplings of these trapped charges to the qubits are calculated by considering their charge distributions within a multipole expansion. It is demonstrated that the summation over these random telegraph processes in mesoscopic devices results in non-Markovian and non-Gaussian noise. For the nuclear environment we consider hyperfine-induced electron-spin dephasing in a nuclear spin bath with narrowed distribution. Nuclear state preparation using dynamical polarization cycles was experimentally achieved recently, and it is also essential to enable $X$-rotations for two-spin qubits. Our analysis is performed for both free induction and echo signals. The scaling of these dephasing mechanisms with the number of qubits is also discussed.   

Noisy (spin) neighbors of a solid state (spin) qubit

Powerful computational methods have been developed in recent years for understanding decoherence induced by environmental spins. Specifically, the cluster correlation expansion [Phys. Rev. B 78, 085315 (2008)] and adaptations [Phys. Rev. Lett. 105, 187602 (2010)] provide successive approximations that approach the solution to the full quantum mechanical problem for small and large spin baths with good efficiency. We present our findings from these computations. These have implications for solid state spin qubit fabrication and materials choices. In silicon where nuclear spins may be eliminated through isotopic enrichment, we consider other sources of bath spins in the bulk and near interfaces. We also investigate the conditions under which we may abstract out an approximate noise model that is independent of operations applied to the qubit.   

Competing effects of hyperfine and spin-orbit interactions in two-electron spin qubits

We analyze the dynamics of a double quantum dot system with two electrons in a uniform magnetic field, taking into account the hyperfine interaction as well as the interdot tunneling-induced Rashba spin-orbit coupling. The former mixes the singlet and triplet (1,1) states while the latter accounts for mixing triplet states and the doubly occupied (0,2) singlet. We focus on the effects on experimental results in GaAs dots [1], involving the generation and control of a nuclear field gradient, necessary for full quantum control of this electron spin qubit. Using a complete description of the quantum states involved in the dynamics and numerical solution of the time-dependent Schr\"{o}dinger equation, we study different pumping processes used to polarize (and read) the nuclear system, creating a large inhomogeneous nuclear field. We evaluate the fidelity of gate operations involving the two-electron qubit in the presence of competing spin-flip interactions as well as the implementation of these operations in quantum computation with characteristic experimental dot systems. \\[4pt] [1] S. Foletti et al., Nature Physics 5, 903 (2009)   

Two-eletron spin relaxation in double quantum dots and P donors

We study singlet-triplet relaxation of two electrons confined in a double quantum dot or bound to P donors in Silicon. Hyperfine interaction of the electrons with the host/phosphorus nuclei, in combination with the electron-phonon interaction, leads to relaxation of the triplet states. We calculate the triplet relaxation rates in the presence of an applied magnetic field. This relaxation mechanism affects, for example, the resonance peaks in current Electron Spin Resonance (ESR) experiments on P-dimers. Moreover, the estimated time scales for the spin decay put an upper bound on the gate pulses needed to perform fault-tolerant two-qubit operations in spin-based quantum computers. We have found the optimal regimes, which mitigate this relaxation mechanism, yet permit sufficiently fast two-qubit operations.   

Spin-orbit induced two-electron spin relaxation in double quantum dots

We study the spin decay of two electrons confined in a double quantum dots via the spin-orbit interaction and acoustic phonons. We have obtained a generic form for the spin Hamiltonian for two electrons confined in (elliptic) harmonic potentials in doubles dots and in the presence of an arbitrary applied magnetic field. Our focus is on the interdot bias regime where singlet-triplet splitting is small, in contrast to the spin-blockade regime. Our results clarify the spin-orbit mediated two-spin relaxation in lateral/nanowire quantum dots, particularly when the confining potentials are different in each dot.   

Impurity effects on coupled quantum dot spin qubits in semiconductors

Localized electron spins confined in semiconductor quantum dots are being studied by many groups as possible elementary qubits for solid-state quantum computation. We theoretically consider the effects of having unintentional charged impurities in laterally coupled two-dimensional double (GaAs) quantum dot systems, where each dot contains one or two electrons and a single charged impurity in the presence of an external magnetic field. We calculate the effect of the impurity on the 2-electron energy spectrum of each individual dot as well as on the spectrum of the coupled-double-dot 2-electron system. We find that the singlet-triplet exchange splitting between the two lowest energy states, both for the individual dots and the coupled dot system, depends sensitively on the location of the impurity and its coupling strength (i.e. the effective charge). We comment on the impurity effect in spin qubit operations in the double dot system based on our numerical results. This work is supported by LPS-CMTC and CNAM.   

Theory of anisotropic exchange in laterally coupled quantum dots

We consider an interacting pair of quantum dot electron spin qubits (a two electron double quantum dot). In this setup, two-qubit operations are generated by the (isotropic) exchange interaction, which results from the tunable inter-dot coupling. In the presence of spin-orbit interactions, additional effective inter-qubit coupling arises, termed anisotropic exchange. We show that in GaAs, where spin-orbit interactions are weak, the magnitude of the anisotropic exchange is proportional to the external magnetic field and therefore directly controllable, boosting prospects for spin-based quantum computing. We show how the form of anisotropic exchange follows from its spin-orbit origin and that its magnitude can be traced down to dipole moment matrix elements. Based on this findings, we propose an effective spin Hamiltonian suitable for practical modeling of two-electron spin dynamics. We prove the effective Hamiltonian quantitative accuracy confronting it with a microscopic numerical model.\\[4pt] [1] F. Baruffa, P. Stano, J. Fabian, Phys. Rev. Lett. 104, 126401 (2010)\\[0pt] [2] F. Baruffa, P. Stano, J. Fabian, Phys. Rev. B 82, 045311 (2010)   

Many-body singlets of nuclear spins

We show that dynamic spin polarization by collective raising and lowering operators can drive a spin ensemble from arbitrary initial state to many-body singlets, the zero-collective-spin states with large scale entanglement. For an ensemble of $N$ arbitrary spins, both the variance of the collective spin and the number of unentangled spins can be reduced to $O(1)$ (versus the typical value of $O(N)$), and many-body singlets can be occupied with a population of $\sim 20 \%$ independent of the ensemble size. We implement this approach in a mesoscopic ensemble of nuclear spins through dynamic nuclear spin polarization by an electron. The result is of two-fold significance for spin quantum technology: (1) a resource of entanglement for nuclear spin based quantum information processing; (2) a cleaner surrounding and less quantum noise for the electron spin as the environmental spin moments are effectively annihilated.   

Atomistic theory of spin relaxation in self-assembled (In, Ga)As/GaAs quantum dots at zero magnetic field

We investigated the spin-flip time (T$_{1}$) of electrons and holes mediated by acoustic phonons in self-assembled In(Ga)As/GaAs quantum dots at zero magnetic field, using an atomistic pseudopotential method. At low magnetic field, the first-order process is suppressed, and the second-order process becomes dominant. We find that the spin-phonon-interaction induced spin relaxation time is 40 - 80 s for electrons, and 1 -20 ms for holes at 4.2 K. The calculated hole-spin relaxation times are in good agreement with recent experiments, which suggests that the two-phonon process is the main relaxation mechanism for hole-spin relaxation in the self-assembled quantum dots at zero field. We further clarify the structural and alloy composition effects on the spin relaxation in the quantum dots.   

Optically controlled electron-nuclear spin dynamics in a quantum dot

In recent years, a large number of experiments involving coherent and incoherent control of electron spins in quantum dots have revealed the important role of the nuclear spins of the host material. Experiments with optical controls, both pulsed and continuous wave, have shown that the feedback of the nuclear spins on the electron spin strongly affects the electron spin response. However, a microscopic theory of this mechanism is not available at present. We introduce a formalism that allows us to investigate this system without invoking any phenomenological spin-flip rates for the nuclei. We derive the electron-nuclear dynamics under the influence of external periodic pulsed control to second order in the electron-nuclear hyperfine coupling. Our formalism should have wide applications in both coherently and incoherently driven electron spins interacting with a nuclear spin bath, including self-assembled and gated quantum dots.   

Session V28: Focus Session: Carbon Nanotubes and Related Materials: Devices III

High Frequency Rectification by Carbon Nanotube Schottky Diodes

Carbon nanotubes (CNTs) display many properties that make them appealing for RF electronics, including room-temperature mean- free paths approaching 1 $\mu$m and a carrier mobility of 10$^5$ cm$^2$ / Vs. Further, small junction capacitances on the order of 10 aF promise cut-off frequencies approaching 1 THz, but high impedances make microwave measurements of individual CNTs challenging. We have fabricated single and few-tube CNT Schottky diodes on high-frequency compatible substrates and measured their ac rectification as a function of dc bias, ac power and frequency, up to 40 GHz. The bias dependence of the cut-off frequency is used to extract the effective junction capacitance for diodes of various channel lengths. This capacitance is found to have a weak dependence on applied bias and a strong relation to channel length. Electrostatic simulations corroborate that stray capacitance from the 1D channel to the metal electrode dominates over the effect of carrier depletion near the junction. The results demonstrate that chottky rectification is a viable method of probing transport in high-impedance semiconducting nanostructures.   

Logarithmic time response of carbon nanotube field-effect transistors

When observing the source-drain current of a carbon nanotube field effect transistor (FET) held at constant bias, several different processes can produce a time response of the current, including fluctuations due to current noise, heating of contacts, and releasing of trapped charges in the gate dielectric. This third phenomenon is investigated by pulsing voltages on the gate and observing the source-drain current over time. Instead of a typical exponential decay with one or a few time constants, the current was observed to decrease linearly with the logarithm of time, possibly indicating exponential decay with multiple time constants. This trend was seen to continue for over 20 hours, which spans five orders of magnitude in time with respect to the measurement resolution. This trend of scaling logarithmically with time is also seen with the rate of current change with respect to a gate voltage pulse width. This behavior has been investigated in various FET geometries and different materials, all with comparable results. These measurements may be a new way to investigate and characterize the hysteresis in carbon nanotube FETs and the materials used in their fabrication.   

Mirage Effect From Thermally-Modulated Transparent Carbon Nanotube Sheet

The single beam mirage effect, also known as photothermal deflection, is studied using a free-standing, highly-aligned carbon nanotube sheet as a heat source whose temperature can be modulated over a wide frequency range. The extremely low thermal capacitance and high heat transfer ability of these transparent forest-drawn carbon nanotube sheets enables high frequency modulation of sheet temperature over an enormous temperature range, thereby providing a sharp, rapidly changing gradient of refractive index in surrounding liquid or gas. The advantages of temperature modulation using carbon nanotube sheets are multiple: in inert gases the temperature can reach $>$2500 K; the obtained frequency range for photothermal modulation is $\sim $100 kHz in gases and over 100 Hz in high refractive index liquids; and the heat source is transparent for optical and acoustical waves. The remarkable light deflection in gases and liquids suggests possible application of carbon nanotube sheets for large laser projectors and cloaking systems.   

Application of Carbon Nanotube Assemblies for Sound Generation and Heat Dissipation

Nanotech approaches were explored for the efficient transformation of an electrical signal into sound, heat, cooling action, and mechanical strain. The studies are based on the aligned arrays of multi-walled carbon nanotubes (MWNT forests) that can be grown on various substrates using a conventional CVD technique. They form a three-dimensional conductive network that possesses uncommon electrical, thermal, acoustic and mechanical properties. When heated with an alternating current or a near-IR laser modulated in 0.01--20 kHz range, the nanotube forests produce loud, audible sound. High generated sound pressure and broad frequency response (beyond 20 kHz) show that the forests act as efficient thermo-acoustic (TA) transducers. They can generate intense third and fourth TA harmonics that reveal peculiar interference-like patterns from ac-dc voltage scans. A strong dependence of the patterns on forest height can be used for characterization of carbon nanotube assemblies and for evaluation of properties of thermal interfaces. Because of good coupling with surrounding air, the forests provide excellent dissipation of heat produced by IC chips. Thermoacoustic converters based on forests can be used for thermo- and photo-acoustic sound generation, amplification and noise cancellation.   

Thermoelectric Transport Through Arrays Of Carbon Nanotube Junctions

The work addresses the voltage-controlled thermal flow and electric current through the carbon nanotube (CNT) junction arrays. The CNT thermoelectric generation (TEG) promises a high efficiency for thermal and electric energy conversion in a variety of applications. [1] The energy generation had been studied using advanced methods of the condensed matter physics and nanotechnology. We will outline our experimental findings based on CNTs - TEG devices. We will report on our results that involve TEG-CNTs devices in array and /or single CNTs junctions geometries. We will describe fabrications protocols for preferential CVD growth of CNTs and nanoscale precision patterning of the catalyst on predefined device architectures. Electronic transport and optical properties of the CNTs-TEG nanostructures will also be discussed. I.K. and S.S. acknowledge support from the U.S. Army CECOM Acquisition Center {\#}W909MY-10-C-0032. I.K. acknowledge collaboration with NanoInk Inc.   

Carbon nanotube quantum dot in a dissipative environment

We study conductance through a resonant level in a single-walled carbon nanotube quantum dot embedded in a dissipative environment. The dissipation is provided by environmental modes in the nanotube leads and the strength of the dissipation is experimentally controlled in several samples. At base temperature, dissipation suppresses the resonant tunneling peak height while maintaining resonant level width. We also observe a regime where the height and the width of a conductance peak demonstrate qualitatively different energy scaling.   

Energy Dissipation and Transport in Carbon Nanotube Devices

Power consumption is a significant challenge in electronics, often limiting the performance of integrated circuits from mobile devices to massive data centers. Carbon nanotubes have emerged as potentially energy-efficient future devices and interconnects, with both large mobility and thermal conductivity. This talk will focus on understanding and controlling energy dissipation [1-3] and transport [4-6] in carbon nanotubes, with applications to low-energy devices, interconnects, heat sinks, and memory elements [7]. Experiments have been used to gain new insight into the fundamental behavior of such devices, and to better inform practical device models. The results suggest much room for energy optimization in nanoelectronics through the design of geometry, interfaces, and materials. \\[4pt] [1]. E. Pop, ``Energy Dissipation and Transport in Nanoscale Devices,'' Nano Research 3, 147 (2010). \\[0pt] [2]. Z.-Y. Ong and E. Pop, ``Molecular Dynamics Simulation of Interfacial Thermal Resistance between Single-Wall Carbon Nanotubes and SiO2,'' Phys. Rev. B 81, 155408 (2010). \\[0pt] [3]. A. Liao, R. Alizadegan, Z.-Y. Ong, S. Dutta, F. Xiong, K. J. Hsia, E. Pop, ``Thermal Dissipation and Variability in Electrical Breakdown of Carbon Nanotube Devices,'' Phys. Rev. B 82, 205406 (2010). \\[0pt] [4]. A. Liao, Y. Zhao, E. Pop, ``Avalanche-Induced Current Enhancement in Semiconducting Single-Walled Carbon Nanotubes,'' Phys. Rev. Lett. 101, 256804 (2008). \\[0pt] [5]. Y. Zhao, A. Liao, E. Pop, ``Multiband Mobility in Semiconducting Carbon Nanotubes,'' IEEE Elec. Dev. Lett. 30, 1078 (2009). \\[0pt] [6]. D. Estrada, A. Liao, S. Dutta, E. Pop, ``Reduction of Hysteresis for Carbon Nanotube Mobility Measurements Using Pulsed Characterization,'' Nanotechnology 21, 085702 (2010). \\[0pt] [7]. F. Xiong, A. Liao, E. Pop, ``Inducing Chalcogenide Phase Change with Ultra-Narrow Carbon Nanotube Heaters,'' Appl. Phys. Lett. 95, 243103 (2009).   

Proximal heating by a current-carrying nanotube

The 1D nature of carbon nanotubes makes them an excellent candidate for thermal management and thermal logic devices. Using an established thermal measurement technique based on the melting of indium islands [1], we have studied the thermal characteristics of Joule-heated MWNTs. Our experimental observations contradict prevailing theoretical models for heat dissipation in CNT. Despite the high thermal contact resistance between the CNT and the substrate we observe that a current-carrying nanotube dissipates power readily into the substrate, suggesting an alternate mode of heat transport based on scattering of hot electrons in the CNT from the substrate phonons. Experimental results, simulations, and a review of the experimental technique will be presented in this talk. \\[4pt] [1] T. Brintlinger, et al., Nano Lett. \textbf{8}, 582 (2008).   

Correlated breakdown of carbon nanotubes in an ultra-high density aligned array

Many proposed applications of single walled carbon nanotubes (SWNTs) require a massively parallel array and selective removal of metallic pathways from the array via electrical breakdown. Since experimental and theoretical studies of individual SWNTs demonstrate that the breakdown is due to Joule heating which occurs at defect sites, a straightforward extrapolation to an array would suggest that the breakdown would occur at random point inside each SWNT. Here we demonstrate that in a densely packed aligned array of SWNTs containing up to 30 SWNT/$\mu $m, the breakdown of one of the SWNTs leads to a highly correlated breakdown of neighboring SWNTs, thereby producing a ``nano fissure'' shaped pattern. We show theoretically that the correlated breakdown is due to the electrostatic field of broken nanotubes that produces locally inhomogeneous current distributions in the neighboring nanotubes triggering their breakdowns in the vicinity of the broken nanotubes. Our results suggest that the densely aligned array works like a correlated solid and have strong implications in the future development of fault-tolerant nano-electronic circuits based on SWNT array.   

Thermionic Emission Properties of Surface modified conical carbon nano tubes (CCNT)

We have studied field emission and thermionic emission properties of surface modified arrays of CCNTs. The CCNTs with narrow tip radii (about 10 nm) were synthesized using microwave plasma assisted chemical vapor deposition on platinum wire and planar graphite foils. They show enhanced field emission properties with geometrical enhancement factor as high as about 7000 and turn-on electric field as low as approximately 0.7 V/$\mu $m. The thermionic emission characteristics show work function of aproximately 4.2 eV which is considerably lower than that of aligned MWNT (4.8 eV). The reduced work function value was further confirmed using ultraviolet photoemission spectroscopy (UPS). The surface modified CCNT arrays were also studied and shown to exhibit poorer emission properties compared to pristine CCNTs. We have further coated CCNTs with diamond nanocrystals and doping of the nanocrystals is underway.   

Highly Efficient Field Emission from Carbon Nanotube-Nanohorn Hybrids Prepared by Chemical Vapor Deposition

It is reported that the carbon nanotube (CNT) is one of the best cold cathode emitters for field emission display (FED) and field emission lamp (FEL) due to their large aspect ratio, high mechanical strength, and high electrical conductivity. For the manufacture of highly efficient field emission (FE) devices, we synthesized single-wall carbon nanotube (SWNT) on catalyst-supported single-wall carbon nanohorn (SWNH). We incorporated Fe acetate into SWNHs, heat-treated them, and obtained Fe oxide nano-particles attached to the tips of SWNHs (Fe@NHox). Using Fe@NHox as the catalyst, SWNTs were grown by ethanol-CVD technique (NTNH). In the obtained NTNH, the SWNTs diameters were 1--1.7 nm and the bundle diameters became almost uniform, $i.e.$, less than 10 nm, since the SWNTs were separated by SWNH aggregates. We also confirmed that a large-area FE device with NTNH cathodes made by screen printing was highly and homogeneously bright, suggesting the success of the hybrid strategy.   

Enhanced electron field emission from simultaneously purified and nitrogen incorporated CNTs via tip opening by Novel \textit{in-situ} nitrogen ECR plasma

We report a novel single step process by means of \textit{in-situ} nitrogen ECR plasma treatment with very low power and treatment time for the simultaneous metal catalyst (iron) removal via tip opening and nitrogen functionalization/incorporation of the vertically aligned multiwalled carbon nanotubes synthesised using a microwave plasma enhanced chemical vapour deposition. Microscopic (SEM) and spectroscopic (NEXAFS, XPS and Raman) studies reveal negligible remaining Fe content (0{\%}) and limited/no damage structure and alignment of the nanotubes. The incorporation of nitrogen was elucidated by the N-k edge NEXAFS spectra, where the sharp $\pi $* peak splits into three distinct peaks at energies 399, 399.5 and 401.1 eV. Increase in the at. {\%} conc. of N 1s from 0.7 to 6.9 {\%} and the disappearance of the peak at 780 eV by XPS and Raman corroborate the inclusion of nitrogen in the CNTs and the complete removal of iron metal catalyst. Metal catalyst removal and nitrogen addition by N-ECR plasma leads to enhanced field emission with very low turn on and threshold fields of 0.52 V/$\mu $m and 0.76 V/$\mu $m as compared to the recent studies of other nitrogen doped nanomaterials by plasma treatments.   

Spin-polarized field emission from nanotubes

Time-dependent density functional theory has been used to calculate the spin-polarized field emission from carbon nanotubes with and without Fe adsorbates (atoms and clusters). Using our previously-developed approach [1], the electronic wave function was propagated in real time. Complex absorbing potentials have been employed to avoid artificial reflections from the boundaries and to allow long time simulations. It was observed that various adsorbates cause the separation of density into spin-polarized regions. The calculations predict that carbon nanotubes with various adsorbates can be used as spin-polarized current sources. \\[4pt] [1] J. A. Driscoll and K. Varga, Phys. Rev. B 80, 245431 (2009).   

Session V29: Quantum Control and Measurement

Robust high-fidelity universal quantum gates

We show how a robust high-fidelity universal set of quantum gates can be produced using a single form of non-adiabatic rapid passage whose parameters are optimized to enhance gate fidelity and robustness. All gates in the universal set are found to: (i) operate with fidelities in the range 0.999 --- 0.99999, and (ii) use control parameters requiring no more than 14-bit precision. Such precision is within the reach of commercially available arbitrary waveform generators, suggesting the feasibility of an experimental study of this approach to high-fidelity quantum control.   

Exact, Floquet-based, Single Qubit Control

Single-qubit gate design using oscillatory controls is related to the Rabi problem of rotating a spin. In the classical solution one drives the spin with an oscillatory electromagnetic field orthogonal to a background field. Here, we introduce a new, general method for constructing continuous, oscillatory quantum controls based on Floquet's theorem. We then derive a family of exact, analytical solutions to the generalized Rabi problem of completely controlling a single-qubit in a fixed background field.   

Protection of quantum systems by nested Uhrig dynamical decoupling

Based on a theorem we establish on dynamical decoupling of time-dependent systems, we present a scheme of nested Uhrig dynamical decoupling (NUDD) to protect multi-qubit systems in generic quantum baths to arbitrary decoupling orders. This scheme uses only single-qubit operations. Higher order decoupling is achieved at the cost of a polynomial increase in pulse number. For general multi-level systems, this scheme protects a set of unitary Hermitian system operators which mutually either commute or anti-commute, and hence all operators in the Lie algebra generated from this set of operators, generating an effective symmetry group for the system up to a given order of precision. We also show how to implement NUDD with pulses of finite amplitude, up to an error in the second order of the pulse durations.   

Quantum noise of an electromagnetically controlled two level system

A coherent control of a spin is limited by both the decoherence due to coupling with the environment and noise coming from the quantized control. A quantum noise study is particularly important in fault tolerant quantum computation where a very high fidelity is demanded. Here, we present a time evolution study of a two level system interacting with a laser pulse and the electromagnetic vacuum based on the multimode Jaynes- Cummings model. We develop a diagrammatic formalism in which one can easily identify the coherent Rabi oscillation of the TLS and its relaxation from corresponding diagrams. In the small time limit ($t\ll T_1$), where the noise level is small but still an issue to fault tolerant quantum computing, this method gives a quantitative evaluation of the quantum noise of the TLS under an optical control with an arbitrary pulse shape. Furthermore, this approach can be naturally extended from the Markovian to the non-Markovian regime, resulting in dynamics different from that obtained in the optical Bloch analysis. All these calculations are done without any stochastic assumption.   

Optimal control of an ensemble of atoms in an optical lattice

Controlling quantum systems in a manner that is robust to experimental errors and inhomogeneities is vital for practical realization of quantum gates. We demonstrate numerically the control of motional degrees of freedom of an ensemble of neutral atoms in an optical lattice of shallow trapping potential. Taking into account the range of quasi-momenta across different Brillouin zones results in an ensemble whose members effectively have inhomogeneous control fields as well as spectrally distinct control Hamiltonians. We present a modified optimal control technique that yields high fidelity control pulses, irrespective of quasi-momentum, with average fidelities above 90\%. The resultant controls show a broadband spectrum with gate times in the order of several Rabi oscillations to optimize gates with up to 75\% dispersion in the energies from the band structure.   

High Fidelity State Transfer Over an Unmodulated Linear $XY$ Spin Chain

We provide a class of initial encodings that can be sent with a high fidelity over an unmodulated, linear, $XY$ spin chain. As an example, an average fidelity of 96\% can be obtained using an 11-spin encoding to transmit a state over a chain containing 10,000 spins. An analysis of the magnetic-field dependence is given, and conditions for field optimization are provided.   

Geometric Phase Gates via Adiabatic Control Using Electron Spin Resonance

High fidelity operations are essential elements of quantum information processing. In contrast with the dynamic pulses that are routinely used in electron spin resonance for spin qubit manipulation, geometric phase gates achieved via adiabatic control are less sensitive to certain kinds of noise and field inhomogeneities. Here, we employ theoretical and numerical tools to show that these geometric operations can be realized in electron spin systems with greater fidelities than composite dynamic pulses for large inhomogeneities in the microwave field. We further show that the adiabatic geometric phase is robust against fast fluctuations in the static magnetic field. Finally, we investigate adiabatic geometric phase operations experimentally, showing that we are able to apply such robust phase gates to the electron spin on the microseconds timescale.   

Nanoscale control of individual proximal NV spins via a scanning magnetic field-gradient

Nanoscale ensembles of nitrogen-vacancy (NV) spins have been proposed for implementing quantum information protocols as well as performing sensitive nanoscale magnetometry. However, it has proven experimentally difficult to control individual NV spins without affecting the state of other, proximal spins, as spins are read-out optically and are often collectively driven by applied radio-frequency fields. We demonstrate that single-spin control in NV-spin ensembles can be achieved via a scanning magnetic field-gradient, which locally splits the electron spin resonances of proximal NVs. With this method, we achieve 9 nm spatial resolutions in imaging, characterization, and simultaneous manipulation of individual NVs, roughly two orders of magnitude better than the optical diffraction limit. We discuss applications of this individual control such as generating entangled spin-states and performing sensitive magnetometry.   

Nested Uhrig Dynamical Decoupling with Non-uniform Error Suppression

Here the performance of Nested Uhrig Dynamical Decoupling (NUDD) for qubit systems is analyzed when error suppression is non-uniform. The error suppression provided by NUDD is controlled by the sequence order of each nested sequence. The properties of the error suppression are characterized with respect to varying sequence order to verify the expected error suppression scaling of UDD, order $N+1$ error suppression with respect to the total time of evolution for an $N$th order sequence. The system operators present in the system-environment evolution are isolated and used to quantify the order of error suppression associated with each system error operator. Using this as a measurement, error suppression is examined with respect to the strength of system-enviroment interaction, as well as the pure bath strength. In the case of single-qubit NUDD, known as Quadratic Dynamical Decoupling (QDD), the results show that the error suppression provided by the inner sequence scales exactly with that of UDD, while the outer sequence dynamics leads to error suppression greater than or equal to that expected from UDD. These results can be extended to multi-qubit systems where the error suppression scaling for the inner sequence applied to each qubit follows that of UDD and the outer sequence applied to each qubit gives an error suppression greater than or equal to $N+1$.   

Pulsed Quantum Optomechanics

By combining quantum optics with mechanical resonators an avenue is opened to extend investigations of quantum behavior into unprecendented mass regimes. The field resulting from this combination - ``cavity quantum optomechanics'' -- is receiving a surge of interest for its potential to contribute to quantum measurement and control, studies of decoherence and non-classical state preparation of macroscopic objects. However, quantum state preparation and especially quantum state reconstruction of mechanical oscillators is currently a significant challenge. We are pursuing a scheme that employs short optical pulses to realize quantum state tomography, squeezing via measurement and state purifcation of a mechanical resonator. The pulsed scheme has considerable resilience to initial thermal occupation, provides a promising means to explore the quantum nature of massive oscillators and can be applied to other systems such as trapped ions. Our theoretical proposal and experimental results will be discussed.   

Quantum measurement with Mach-Zehnder Interferometer

We use an electronic Mach-Zehnder Interferometer (MZI) as a measurement device. We perform a measurement on a system by coupling with MZI using a phase shift induced by Clulombic coupling. By reading current and noise cross-correlations, strange conditioned averages can be constructed using the contextual values technique.   

Continuous phase amplification with a Sagnac interferometer

We describe a weak value inspired phase amplification technique in a Sagnac interferometer. We monitor the relative phase between two paths of a slightly misaligned interferometer by measuring the average position of a split-Gaussian mode in the dark port. Although we monitor only the dark port, we show that the signal varies linearly with phase and that we can obtain similar sensitivity to balanced homodyne detection. We derive the source of the amplification both with classical wave optics and as an inverse weak value.   

Conservation of Vacuum in an Interferometer

Source efficiency and photon loss are major problems in optical metrology and quantum information. To understand how to address loss for these applications, it is vital to know how the loss behaves under linear optical (LO) processing including conditional measurements. We have developed a theory for the behavior of loss under LO processing, resolving many long-standing questions from previous work [1,2]. In particular, we have shown that, provided the efficiency of the sources is appropriately quantified, the efficiency of the state in any single mode cannot be increased beyond that of the highest-efficiency mode available at the input [1]. It is also not possible to increase efficiency in a catalytic way, using some high-efficiency modes to increase the efficiency of other modes [2]. The results provide a powerful unifying framework for quantifying efficiency by the incoherent vacuum contribution to optical states, even when entangled over multiple modes. The amount of vacuum is invariant under interferometers, and can only be increased by measurement. \\[4pt] [1] D.\ W.\ Berry and A.\ I.\ Lvovsky, Phys.\ Rev.Lett.\textbf{105}, 203601 (2010).\\[0pt] [2] D.\ W.\ Berry and A.\ I.\ Lvovsky, e-print:1010.6302 (2010).   

Dynamics of entanglement in two-dimensional spin system

We consider the time evolution of entanglement in a finite two dimensional transverse Ising model. The model consists of a set of 7 localized spin-$\frac{1}{2}$ particles in a two dimensional triangular lattice coupled through exchange interaction $J$ in presence of an external time dependent magnetic field $h(t)$. The magnetic field is presented in various function forms. We find that the magnetic field with sudden change does not provide a way to control or tuning the entanglement. While for the smoothly changing field, when its the character frequency is small, entanglement tends to follow the change of external magnetic field; when it gets larger, entanglement gradually loses pace with the field. It is also shown that the mixing of even a few excited states by small thermal fluctuation is devastating to the entanglement of the ground state.   

Probing Majorana edge states with a qubit

A pair of counter-propagating Majorana edge modes can be described by an Ising conformal field theory. These modes appear in a chiral p-wave superconductor or in some superconducting system belonging to the same universality class. We show how a superconducting flux qubit attached to a such system couples to the two chiral edge modes via the disorder field of the Ising model. Thus, measuring the back-action of the edge states on the qubit allows to probe the properties of Majorana edge modes.   

Session V30: Nanowires and Nanotubes: Thermal and Mechanical Properties

Thermal boundary resistance between carbon nanotubes in nanocomposites with Monte Carlo simulations

Enhancing the thermal conductivity of composites by incorporating carbon nanotubes (CNTs) has been an area of vigorous research recently. Measurements of the effective thermal conductivity (keff) for CNT-polystyrene composites at high CNT \%wt found that the ratio (keff/kpolymer) at high concentration of CNTs is not as good as that at low CNT concentration [1]. It appears that the CNT dispersion pattern becomes worse, resulting in the formation of CNT bundles. In this work, we apply Monte Carlo simulations to investigate the keff at different weight fractions taking into account the bundle size and orientation, as well as the thermal boundary resistance. By validating with the experiment data, we found that the phonon transmission probability at the interface decreases by temperature. In addition, the poor enhancement of keff at high CNT concentration is because of the CNT-CNT contact resistance and because of the bundle geometry itself, which is equivalent to the presence of one low aspect ratio nanotube. References [1] Peters J. E.; Papavassiliou D.V; Grady B. P., Macromolecules 2008, 41, 7274-7277.   

Strain Engineering of the thermal conductance in Si nanowires

Silicon nanowires (SiNWs) are promising semiconductor structures spanning a wide range of applications from CMOS devices to thermoelectric modules. Recently, SiNWs have shown tremendous potential as good thermoelectric materials with ZT coefficients larger than 1 [1]. This figure of merit can be further improved by tuning the thermal conductivity of the SiNWs. Here, we show that strain provides a natural way of tuning the thermal conductance of ultra-scaled SiNWs. We utilize a modified Valence Force Field (MVFF) model [2] to calculate the phonon dispersion in these SiNWs under strain and extract their thermal conductance using Landauer's approach. Our investigation shows that uniaxial tensile and hydrostatic compressive help reduce the thermal conductance of SiNWs. For example, a 3nm X 3nm, $<$100$>$ oriented nanowire undergoing a 2{\%} uniaxial tensile strain exhibits a thermal conductance reduced up to 2.6{\%}, whereas a compressive hydrostatic strain of 2{\%} gives a reduction of around 8{\%}. Thus, strain engineering can prove beneficial in tuning the thermal conductance in Si nanowires and offers an efficient way to further improve the ZT figure of merit. Finanacial support from MSD, SRC, MIND and NSF, computational support from nanoHUB.org under NCN. Refs: [1] A. I. Hochbaum et al., 'Enhanced thermoelectric performance of rough silicon nanowires', Nature 451, no. 7175, pp. 163, 2008. [2] A Paul, M Luisier and G Klimeck, 'Modified valence force field approach for phonon dispersion: from zinc-blende bulk to nanowires.', arXiv:1009.6188v2[cond-mat.mes-hall].   

Thermal Conductivity of Metallic Nanowires near Room Temperature

In metallic structures with nanoscale dimension both electrical and thermal conductivities are significantly different from their bulk counterparts. The total thermal conductivity is generally the sum of the electronic and phonon contributions. We examine electron and phonon heat transport in metals, in the temperature range near to or above the Debye temperature, where it is generally assumed that phonon component is negligible for metals, an assumption that has not been subjected to rigorous experimental verification, particularly at the nanoscale, due to difficulties in direct measurement of thermal conductivity. Experimental evidence suggests that the Wiedemann-Franz (W-F) law breaks down at the nanoscale. The neglected phonon component is one factor that has been cited as contributing to the apparent discrepancy in W-F. Another factor is inelastic electron-phonon scattering that influences transport due to a temperature gradient, but not due to an electric field. We report experimental results for Al nanowires and develop a model based on the Boltzmann transport equation for size dependence of electrical and thermal conductivity in nanowires. The model is validated with available data reporting direct measurements of thermal conductivity of nanowires, ribbons, and thin films. The W-F law and Lorenz factor are examined and a modified version of W-F is presented, corrected for these two factors and valid from macro- to nanoscale provided characteristic sizes exceed the phonon mean free path.   

Diffusion-induced dephasing and bistability of nanoresonators

We study dephasing of an underdamped harmonic oscillator due to frequency fluctuations. The spectrum of the response to an external field is sensitive to the nature of the fluctuations. For nanomechanical resonators, if the dephasing is due to diffusion of adsorbed particles along the resonator, the spectrum varies from a single Lorentzian peak to two closely spaced peaks depending on the parameters. If the dephasing depends on the oscillator state, the oscillator can exhibit bistability of forced vibrations. We study this bistability for a nanomechanical resonator with diffusing adsorbed particles. The vibrations affect the particles by driving them toward the antinodes of the vibrational mode. Unexpectedly, even though diffusion is a random process, the bistability arises if it is sufficiently strong, i.e., fast compared to the vibrations decay time. For fast diffusion, we find the bistable response in the mean-field approximation. We also study, analytically and numerically, the rate of fluctuation-induced switching between the coexisting stable states.   

Electrical Detection of Resonance of ZnO nanowires using Harmonic Detection of Radiation

ZnO nanowires exhibit semiconducting and piezoelectric properties making them a technologically promising material. We have measured the mechanical resonance of cantilevered ZnO nanowires using the Harmonic Detection of Resonance (HDR) method.\footnote{J. Gaillard, M.J. Skove, R. Ciocan and A.M. Rao, Rev. Sci. Instrum. 77, 073907(2006)} The resonance is induced by an oscillating electric field and detected by second harmonic electric response of the ZnO nanocantilever. The diameter of the nanowires used was about 200 nm and length varied from 60 to 300 $\mu $m. Other mechanical properties of the cantilever, such as Young's modulus, are calculated from the observed resonance frequency.   

Semiconducting nanowire electromechanics in the Coulomb blockade regime

We fabricate and study Indium Arsenide (InAs) nanowire electromechanical resonators, in field effect transistor (FET) geometry, which allows us to tune the carrier density and tension in the wire at electromechanical resonance by tuning the dc gate voltage. At temperatures below 5K, quality factor (Q) of these resonators is $\sim$10000, two orders of magnitude larger than at room temperature, and the dynamic range reduces by an order of magnitude at low temperatures. Further in Coulomb blockade regime (charging energy $\sim$10 meV), using rectification technique, we have observed the modification in Coulomb diamond structure at the resonance frequency of the wire. Near the electromechanical resonance frequency, Coulomb peaks become broader symmetrically (independent of dc gate voltage and frequency sweep direction) and right at the resonance frequency their intensity is significantly reduced. This indicates a strong coupling between electron transport and mechanical vibration of the nanowire.   

Capacitive Spring Softening in Single-Walled Carbon Nanotube Nanoelectromechanical Resonators

Due to their low mass density and high Young's modulus, single-walled carbon nanotubes (SWNTs) offer great promise as nanoelectromechanical (NEM) resonators with applications in ultrasmall mass and force sensing. Nanotube resonators can be actuated and detected simultaneously through electrostatic gate coupling. This gate induced frequency tuning of NEM resonators is known to be governed by two mechanisms: the elastic hardening effect and the capacitive softening effect. Although elastic hardening effect has been widely reported in SWNT resonators, the field-induced capacitive spring softening has rarely been observed. Here we report the capacitive spring softening effect observed in SWNT resonators. The nanotube resonators adopt dual-gate configuration with both bottom-gate and side-gate capable of tuning the resonance frequency through capacitive coupling. Interestingly, downward resonance frequency shifting is observed with increasing side-gate voltage, which can be attributed to the capacitive softening of spring constant. Furthermore, in-plane vibrational modes exhibit much stronger spring softening effect than out-of-plan modes. Our dual-gate design should enable the differentiation between these two types of vibrational modes, and open up new possibility for nonlinear operation of nanotube resonators. Other nonlinear effects in SWNT resonators will also be discussed.   

Vibrational Behaviour of Metal Nanowires under Tensile stress

We present results of calculations on vibrational density of states (VDOS) of a thin Cu nanowire with $<$100$>$ axial orientation and discuss on the effect of axial strain. The calculations are performed using real space Green's function method with the force constant matrix extracted from the interaction potentials based on the embedded atom method. It is shown that the characteristics of the VDOS of a strain-free nanowire are quite distinctive compared to that of a bulk atom. Among the striking features of this type nanowire is the existence of high frequency modes above the top of the bulk spectrum. From an examination of VDOS of local atoms it is seen that the corner and core atoms are the primary moderators for the anomalous increase in low frequency and high frequency modes, respectively. We, additionally, find that while the high frequency band above the top of the bulk phonon shifts to even higher frequencies, the characteristics at low frequencies remains almost the same upon stretching the nanowire along the axial direction.   

Understanding and Controlling Intrinsic Dissipation in Driven Single Walled Carbon Nanotube Resonators

A ``Phonostat'' algorithm that can regulate total energy in a given internal degree of freedom within a molecular dynamics (MD) simulation is presented. The algorithm computes modal energies at every MD timestep, controls energy in a chosen vibrational mode with an external driving force and an internal damping. Using a test case of driven damped anharmonic oscillator, two different approaches of force correction are presented and various parameters that control the phonostat algorithm are analyzed. This algorithm is then employed to drive a chosen vibrational mode in carbon nanotube resonator to understand intrinsic dissipation under continuous driving, simultaneously computing its quality factor to mimic experimental conditions. The \textit{gateway} modes that couple the driven mode to the thermal background are identified. Regulating these gateway modes hold the key to control intrinsic dissipation and improve quality factor for mass sensing application.   

Anomalous Electromechanical Resonance Behavior of Single-walled Carbon Nanotubes under High Bias Voltages

By monitoring the nanoelectromechanical response of suspended individual carbon nanotubes (CNT), we observe the onset of optical phonon (OP) emission in these CNTs under high bias voltages. An abrupt upshift in the mechanical resonance frequency is observed at high voltage biases. The underlying cause of this behavior is the sudden increase in the lattice temperature of the CNT that causes contraction of the lattice due to the negative thermal expansion coefficient. This, in turn, results in increased tension in the suspended nanotube and an upshift in the mechanical resonance frequency. The sudden increase in temperature is explained by the OP emission in CNT. This effect is also observed in the Raman spectra of CNTs as a sudden downshift in the G band OP frequencies at high bias voltages.   

Transport in partially equilibrated inhomogeneous quantum wires

We study transport properties of weakly interacting one-dimensional electron systems including on an equal footing thermal equilibration due to three-particle collisions and the effects of large-scale inhomogeneities. We show that equilibration in an inhomogeneous quantum wire is characterized by the competition of interaction processes which reduce the electrons total momentum and such which change the number of right- and left-moving electrons. We find that the combined effect of interactions and inhomogeneities can dramatically increase the resistance of the wire. In addition, we find that the interactions strongly affect the thermoelectric properties of inhomogeneous wires and calculate their thermal conductance, thermopower, and Peltier coefficient.   

Energy loss of the electron system in individual single-walled carbon nanotubes

We characterize the energy loss of the non-equilibrium electron system in individual metallic single-walled carbon nanotubes at low temperature. Using Johnson noise thermometry, we demonstrate that, for a nanotube with ohmic contacts, the dc resistance at finite bias current directly reflects the average electron temperature. This enables a straightforward determination of the thermal conductance associated with cooling of the nanotube electron system. In analyzing the temperature- and length-dependence of the thermal conductance, we consider contributions from acoustic phonon emission, optical phonon emission, and hot electron outdiffusion [1]. In the same sample, we also characterize the radio frequency heterodyne response. Distinct responses are seen from bolometric detection and from the electrical nonlinearity due to non-ohmic contacts. \\[4pt] [1] D.F. Santavicca, J.D. Chudow, D.E. Prober, M.S. Purewal, and P. Kim, Nano Lett. 10, 4538 (2010).   

Heat pumping in nanomechanical systems

We propose using phonon pumping mechanism to transfer heat from a cold to a hot body. The mechanism is based on inducing a traveling modulation of the acoustic phonon velocity along the medium connecting the two bodies. This phonon pumping can cool nanomechanical systems without the need for active feedback. We have derived an estimate of the lowest achievable temperature. We have also analyzed this mechanism in the framework of simple one-dimensional microscopic models, which can be exactly solved with non-equilibrium Green function techniques.   

Phonon spectrum in a CdSe nanowire

It is important to calculate the phonon spectrum of realistic nanowires, e.g. to understand its thermo conductivity or to calculate the electron-phonon interaction. In this talk, we will present results of phonon spectrum calculation using valence force field (VFF) method. An important issue is to construct the VFF to describe the surface atomic displacement. We have developed a general VFF formalism to fit our VFF result with the density functional theory (DFT) calculated surface atom displacement energies. In particular, the (10-10) CdSe surface is modelled with Cd-Se dimerization. We will discuss the quality of such VFF model. The phonon spectrum of the nanowire will be presented, and its implication on the phonon transport and electron-phonon coupling will also be discussed.   

Curvature-induced Effects on the Phonon Modes in Sub-nanometer Diameter Single-walled Carbon Nanotubes

Sub-nanometer diameter single-walled carbon nanotubes (sub-nm SWNTs) are of great interest for fundamental studies due to the effect of large curvature on their properties. We have recently synthesized high quality, narrow diameter distribution sub-SWNTs using CoMn catalysts supported on MCM-41 silica templates in a thermal chemical vapor deposition process [1]. The high curvature in the sub-nm SWNTs leads an unusual S-like dispersion of the G-band frequency due to the strong electron--phonon coupling. In addition, we observe diameter-selective intermediate frequency modes (IFMs) that are as intense as the low frequency radial breathing modes (RBMs). The effect of large curvature in the sub-nm SWNTs is also evident in the lower phonon dispersion of the double resonant Raman features compared to SWNTs with larger diameters. The origin of previously unidentified IFM features (600-1100 cm$^{-1})$ and the dispersion of high frequency phonons (1650 -- 2300 cm$^{-1})$ will be discussed.\\[0pt] [1] C. Z. Loebick \textit{etal.}, \textit{J. Am. Chem. Soc.},132, 11125 (2010)   

Session V31: Energy Production: Combustion, Heat Engines, Solar Thermal and Thermoelectrics

Electricity from Coal Combustion: Improving the hydrophobicity of oxidized coals

To reduce pollution and improve efficiency, undesirable mineral impurities in coals are usually removed in coal preparation plants prior to combustion first by crushing and grinding coals followed by gravity separation using surfactant aided water flotation. However certain coals in the US are not amendable to this process because of their poor flotation characteristics resulting in a major loss of an energy resource. This problem has been linked to surface oxidation of mined coals which make these coals hydrophilic. In this project, we are investigating the surface and water flotation properties of the eight Argonne Premium (AP) coals using x-ray diffraction, IR spectroscopy and zeta potential measurements. The role of the surface functional groups, (phenolic -OH and carboxylic -COOH), produced as a result of chemisorptions of O$_{2}$ on coals in determining their flotation behavior is being explored. The isoelectric point (IEP) in zeta potential measurements of good vs. poor floaters is being examined in order to improved the hydrophobicity of poor floating coals (e.g. Illinois {\#}6). Results from XRD and IR will be presented along with recent findings from zeta potential measurements, and use of additives to improve hydrophobicity. Supported by USDOE/CAST, Contract {\#}DE-FC26-05NT42457.   

Electrorheology for Efficient Energy Production and Conservation

At present, most of our energy comes from liquid fuels. The viscosity plays a very important role in liquid fuel production and conservation. For example, reducing the viscosity of crude oil is the key for oil extraction and its transportation from off-shore via deep water pipelines. Currently, the dominant method to reduce viscosity is to raise oil's temperature, which does not only require much energy, but also impacts the environment. Recently, based on the basic physics of viscosity, we proposed a new theory and developed a new technology, utilizing electrorheology to reduce the viscosity of liquid fuels. The method is energy-efficient, and the results are significant. When this technology is applied to crude oil, the suspended nanoscale paraffin particle, asphalt particles, and other particles are aggregated into micrometer-size streamline aggregates, leading to significant viscosity reduction. When the temperature is below 0$^{\circ}$C and the water content inside the oil becomes ice, the viscosity reduction can be as high as 75{\%}. Our recent neutron scattering experiment has verified the physical mechanism of viscosity reduction. In comparison with heating, to reach the same level of viscosity reduction, this technology requires less than 1{\%} of the energy needed for heating. Moreover, this technology only takes several seconds to complete the viscosity reduction, while heating takes at least several minutes to complete.   

Small angle neutron (SANS) and X-ray scattering (SAXS) investigation of microstructure and porosity with fractal properties of coal, shale, and sandstone from Indiana

We have applied SAXS, SANS and adsorption isotherms to study the porosity, pore structure and interaction of confined fluids in the various Indiana rock samples. This study included a bituminous coal, a sandstone, and a grey shale from formations investigated as possible targets for CO2 sequestration. SAXS and SANS are demonstrated quantitative information about the microstructure and pore morphology of the coals and other rocks at length scale (1 nm to 0.3 micron) as well as the fractal nature of pore matrix interfaces. The different scattering cross sections of X-rays and neutrons provide information on the distribution of pore sizes in organic and inorganic components. Neutrons are relatively sensitive to the presence of either hydrocarbons or water in the pores, and always give a smaller Porod exponent that that for X-ray. Construction of LENS was supported by the NSF, the 21$^{st}$ Century Science and Technology fund of Indiana, and the DOD. LENS operation is supported by Indiana University.   

Optimizing the performance of a heat engine: A simulation study

We performed a simulation study of~a simple heat engine as it undergoes Carnot-type cyclic motion in a finite time over a wide range of piston speeds. There exists a specific piston speed at which the power delivered by the engine is maximum ($P_{max})$ and its corresponding efficiency is slightly larger than \textit{half~}of the Carnot efficiency (1/2 $\eta _{c})$. An optimization criterion leads to a trade-off between high power and high efficiency with respective values of 4/5 $P_{max}$ and 3/4 $\eta _{c}$. In addition, we found the time taken at the optimized state to be twice the time taken when operating at maximum power.   

Analysis of Binary Cycle Efficiency Using Redlich-Kwong Equation of State

Coal, natural gas and nuclear power plants operate using various forms of Rankine cycle. We present an efficiency maximization strategy of binary cycle, which has two Rankine cycles in tandem, using Redlich-Kwong equation of state for wide ranging working fluids: alkali metals, mercury, water, and ammonia. Binary cycle efficiency can approach the Carnot efficiency at a cost. The mercury/ammonia working fluid combination yields the highest efficiency for typical binary cycle conditions. We discuss practical implications given that mercury and ammonia create safety concerns, especially on finding other fluids having similar efficiency based on our simulations.   

Carbon dioxide adsorption on H$_{2}$O$_{2}$ treated single-walled carbon nanohorns

Carbon nanohorns are closed single-wall structures with a hollow interior. Unlike SWNTs, which assemble into cylindrical bundles, nanohorns form spherical aggregates. In our experiments we used dahlia-like carbon nanohorn aggregates. Our sample underwent treatment with H$_{2}$O$_{2}$ which opened access to the interior spaces of the individual nanohorns. We measured carbon dioxide adsorption at several temperatures between 167 and 195 K. We calculated the isosteric heat as a function of loading, and the binding energy values for CO$_{2}$ on the nanohorn aggregates from the isotherm data. Results on the H$_{2}$O$_{2}$-treated nanohorns will be compared with those obtained on other carbon substrates. We have also determined detailed equilibration profiles for CO$_{2}$ adsorption on the nanohorn aggregates; these results will also be presented.   

GOFs and ZIFs: Experimental Results and Analysis of Carbon Dioxide Sorption

In recent years, growing concerns about global warming and the environment have spurred an accelerated development of materials technology for carbon dioxide (CO2) capture and storage. Two recent categories of materials being investigated for their CO2 storage capabilities are graphene oxide frameworks (GOFs) [1] and zeolitic imidazolate frameworks (ZIFs). We have synthesized graphene-oxide-frameworks (GOFs) by linking the OH groups on graphene oxide with benzene-boronic acids. Our initial GOF materials exhibit isosteric heats at low coverage of 32 kJ/mol for CO2. The nitrogen BET surface area of these initial materials is around 500 m2/g. Also, ZIFs are particularly useful for CO2 capture and storage due to high selectivities, CO2 uptakes and sample robustness. Neutron scattering and spectroscopic results of GOFs and select ZIFs with in-situ gas sorption will be presented. Neutrons are able to determine locations and strengths of binding sites. We will present detailed isotherms of carbon dioxide, methane and nitrogen at different temperatures of these interesting GOF and ZIF materials. \\[0pt] [1] J. W. Burress et al., Angewandte Chemie International Edition 49, 8902 (2010).   

Eutectics and Phase Diagrams of Molten Salts from Molecular Dynamics simulations

The use of alkali nitrate salt mixtures as heat transfer fluids in solar thermal power plants is limited by their relatively high melting point. Certain compositions of quaternary and higher dimensional mixtures of alkali and alkaline earth nitrates and nitrites have low melting points. However, the high dimensionality of the search space makes it difficult to find lowest melting compositions. Molecular simulations offer an efficient way to screen for promising mixtures. A molecular dynamics scheme general enough to identify eutectics of any HTF candidate mixture will be presented. The eutectic mixture and temperature are located as the tangent point between free energies of mixing for the liquid and a linear plane connecting the pure solid-liquid free energy differences. The free energy of mixing of the liquid phase is obtained using thermodynamic integration over ``alchemical'' transmutations sampled with molecular dynamics, in which particle identities are swapped gradually. Numerical results for binary and ternary mixtures of alkali nitrates agree well with experimental measurements.   

Energy Harvesting with Stochastic, Subharmonic and Ultraharmonic Vibrations

Non-linear bi-stable systems have been shown to provide improved efficiency for harvesting energy from random and broad band vibration sources. This paper explores the distinct frequency response in the broadened spectrum of a particular non-linear energy harvester, a piezoelectric cantilever with magnetic coupling. The cantilever response evolves dynamically with frequency around the main cantilever resonance. Both stochastic and multi-frequency vibration responses are observed, and account for some of the improved efficiency. In addition, sub-harmonics and ultra-harmonics of the main resonance, along with various combinations of these appear. Taken together, the sub-harmonic and ultra-harmonic response produces an average of four-fold increase in voltage production. For energy harvesting purposes, the mixtures of the stochastic and various harmonic features together with the un-damped resonant response enhances the performance well beyond that of a standard energy harvester. An analytical model of the bi-stable dynamics produces results consistent with those observed experimentally.   

Enhanced thermoelectric properties of n-type filled skutterudite Yb0.35Co4Sb12 by substitution on both the Co and Sb sites

A dimensionless thermoelectric figure of merit (ZT) of about 1.2 was reported in Yb0.35Co4Sb12 at 550$^{\circ}$C by ball milling and hot pressing. Through alloying on both the Co and Sb sites, we expect to achieve lower thermal conductivity while maintaining the power factor. The composition tuning is aimed for reducing the electrical conductivity and increasing the Seebeck coefficient, which will lead to a lower thermal conductivity, and ultimately higher ZT. In this report, we present the thermoelectric properties of skutterudites Yb0.35FexCo4-2xNixSb12 and Yb0.35Co4Sb12-yMy (M=Si, Ge, Sn, B, Al, Ga, In, etc.).   

Optimum Working Fluid Selection For Rankine Cycle Using Redlich-Kwong Equation of State

Efficiency of Rankine cycle as a function of working fluid molecule is modeled using Redlich-Kwong equation of state. We have evaluated 12 molecules, ranging from water to ethylene glycol, and have parameterized their individual performance on several material parameters, including heat capacity and compressibility. This research aims to understand at the molecular level what drives some molecules to perform better at certain temperature and pressure range of the Rankine cycle. Immediate applications we are interested in are geothermal power, solar thermal energy conversion and waste heat recovery.   

Thermoelectric properties of correlated materials

The discovery of large Seebeck coefficients in transition metal compounds such as FeSi, FeSb2, or the iron pnictides, has stirred renewed interest in the potential merits of electronic correlation effects for thermoelectric properties. The notorious sensitivity in this class of materials to small changes in composition (doping, chemical pressure) and external stimuli (temperature, pressure), makes a reliable and, possibly, predictive description cumbersome, while at the same time providing an arena of possibilities in the search for high performance thermoelectrics. Based on state-of-the-art electronic structure methods (density functional theory with the dynamical mean field theory) we here compute the thermoelectric response for several of the above mentioned exemplary materials from first principles. With the ultimate goal to understand the origin of a large thermoelectricity in these systems, we discuss various many-body renormalizations, and identify correlation controlled ingredients that are pivotal for thermopower enhancements.   

Electron correlation effect on temperature and magnetic-field dependences of thermopower

We theoretically investigate the temperature {\it T} and the magnetic field dependences of thermopower. To focus on the strong electron correlation, the Hubbard model is solved in the dynamical mean field theory with the non-crossing approximation impurity solver. The thermopower shows a non-monotonic behavior as a function of {\it T} and asymptotes to the high-{\it T} values given by the Heikes formula, depending on the ratio of Coulomb repulsion and {\it T}. The large response to the magnetic-field, which is observed in the cobalt oxides [1], can be associated with the sharp quasiparticle peak intrinsic to the strongly correlated electron system. We discuss the effect of orbital degeneracy, which is another key factor to enhance the thermopower in the correlated system [2].\\[4pt] [1] Y. Wang {\it et al.}, Nature {\bf 423}, 425 (2003).\\[0pt] [2] W. Koshibae {\it et al.}, Phys. Rev. B {\bf 62}, 6869 (2000).   

Unusual Transport and Strongly Anisotropic Thermopower in PtCoO$_{2}$ and PdCoO$_{2}$

Thermoelectrics provide a technology for producing electrical energy from solar and other heat sources. Thermoelectric performance requires materials with high thermopower, normally found in doped semiconductors, where the thermopower is generally nearly isotropic. We discovered using first principles calculations and Boltzmann transport theory that two oxides, PtCoO$_{2}$ and PdCoO$_{2}$, which are not semiconductors, but rather good metals, have exceptionally large thermopowers in one direction, and moreover that the thermopower in these materials is highly anisotropic. This places these compounds in a highly unusual transport regime. Besides providing a new direction for thermoelectric materials research, they may be very useful in probing the fundamental limits of conventional transport theory for metals.   

Analysis of Wind Forces on Roof-Top Solar Panel

Structural loads on solar panels include forces due to high wind, gravity, thermal expansion, and earthquakes. International Building Code (IBC) and the American Society of Civil Engineers are two commonly used approaches in solar industries to address wind loads. Minimum Design Loads for Buildings and Other Structures (ASCE 7-02) can be used to calculate wind uplift loads on roof-mounted solar panels. The present study is primarily focused on 2D and 3D modeling with steady, and turbulent flow over an inclined solar panel on the flat based roof to predict the wind forces for designing wind management system. For the numerical simulation, 3-D incompressible flow with the standard k-$\varepsilon $ was adopted and commercial CFD software ANSYS FLUENT was used. Results were then validated with wind tunnel experiments with a good agreement. Solar panels with various aspect ratios for various high wind speeds and angle of attacks were modeled and simulated in order to predict the wind loads in various scenarios. The present study concluded to reduce the strong wind uplift by designing a guide plate or a deflector before the panel.   

Session V32: Photonics: Metamaterials, Nanotechnology and Sensors

Topological photonic systems: from integer to fractional quantum Hall states

Topological properties of systems lead to remarkable robustness against disorder. The hallmark of such behavior is the quantized quantum Hall effect, where the electronic transport in two-dimensional systems is protected against scattering from impurities and the quantized Hall conductance is the manifestation of a topological invariance. Here we suggest an analogous approach to quantum Hall physics to create robust photonic devices. Specifically, we show how quantum Hall and quantum spin Hall Hamiltonians can be implemented with linear optics using coupled resonator optical waveguides (CROW) in two dimensions. Key features of quantum Hall systems could be observed via reflection spectroscopy, including the characteristic Hofstadter ``butterfly'' and edge state transport. Furthermore, the addition of an optical non- linearity to our proposed system leads to the possibility of implementing a fractional quantum Hall state of photons, where phenomenon such as non-abelian statistics may be observable.   

Analytical and numerical analysis of a generic cloaking system

We present a technique to realize a multi-shell generic cloaking system. By considering specific geometrical and material properties for the shells around the object, we were able to achieve a transparency conditions independent of object's optical properties in quasi-static regime. A complete suppression of dipolar scattering is demonstrated for an arbitrary object enclosed in such a system. We propose \textit{tunable-low loss} realistic shell designs based on composite media and the effect of dispersion on the overall scattering cross-section is evaluated. Full wave analytical and numerical simulations based on the transparency conditions obtained in the quasi-static limit are performed. It is shown that strong reduction of the scattering by a factor of up to $10^3$ can be achieved across the entire optical spectrum.   

Optical spin-orbit coupling and Darwin terms in epsilon-near-zero materials

In optical cavities formed from spatially inhomogeneous epsilon-near-zero (ENZ) metamaterials, optical spin-orbit coupling can be made nearly isospectral to relativistic electron spin-orbit coupling in atoms; the only difference is that the 3x3 classical spin-orbit operator shifts transverse fields by a different integer amount than the quantum operator. When the electric field is rescaled to account for unequal dispersive energy density in the electric and magnetic field quadratures, Maxwell's equations give a Darwin term with the same form as in quantum mechanical systems. These classical/QM similarities, combined with a pronounced importance of the Kerr nonlinearity, make ENZ materials ideal for coaxing electron-like behavior from light.   

Design of Optical Microcavities for Coupling to Nitrogen-Vacancy Centers in Diamond

Nitrogen-Vacancy (NV) centers in diamond have emerged as promising candidates for solid state qubits. When placed in a confined optical field, such as exists in an optical microcavity, the properties of single quantum emitters can be drastically modified. In the weak coupling regime, the rate of spontaneous light emission from the quantum emitter can be enhanced via the Purcell effect. We demonstrate deterministic coupling between single NV centers and photonic crystal microcavities in Gallium Phosphide (GaP). Designs of novel optical cavities for coupling to NV centers in diamond will also be discussed.   

Radiation Tuning of Optical Nanoantennas for Design of Nanofilter Elements

We experimentally and numerically explore the radiation characteristics of optical nanoantennas. These nanoantennas are dipole antennas with dimensions on the order of several tens of nanometers that are fabricated to form lumped capacitance and Inductance through the use of sandwich structures made of dielectric and plasmonic-material layers. We then investigate tuning the response of these optical nanoantennas by varying the material and/or thickness of the dielectric layer.~ After each experiment, experimental results are compared with numerical simulations to verify the validity of the results. We then exploit these characteristics in building a ``lumped'' nanofiltering device, and thereby extending the concept of antennas and circuit elements such as filters from the microwave regime to the visible regime.   

Confined Three-Dimensional Plasmon Modes inside a Ring-Shaped Nanocavity on a Silver Film Imaged by Cathodoluminescence Microscopy

The confined modes of surface plasmon polaritons in boxing ring-shaped nanocavities have been investigated and imaged by using cathodoluminescence spectroscopy. The mode of the out-of-plane field components of surface plasmon polaritons dominates the experimental mode patterns, indicating that the electron beam locally excites the out-of-plane field component of surface plasmon polaritons. Quality factors can be directly acquired from the spectra induced by the ultrasmooth surface of the cavity and the high reflectivity of the silver reflectors. Because of its three-dimensional confined characteristics and the omnidirectional reflectors, the nanocavity exhibits a small modal volume, small total volume, rich resonant modes, and flexibility in mode control.   

Novel meta-surfaces for wave manipulation

Meta-materials are man-made electromagnetic (EM) materials composed by subwavelength local resonance structures of electric and/or magnetic type, and thus possess arbitrary values of permittivity and permeability dictated by such resonance structures. Many novel EM properties, such as the negative refraction, the superlensing effect, and even the invisibility cloaking were predicted or discovered based on meta-materials. By carefully designing metamaterials with appropriate EM wave properties, one can employ metamaterials to efficiently manipulate various properties of EM waves, including the wave propagation, polarization, and so on. Here, we present our latest theoretical and experimental efforts in designing novel meta-surfaces (ultra-thin metamaterials) with anomalous EM wave properties to allow efficiently manipulating wave propagation directions. Furthermore, our system can also convert propagating wave to surface plasmon polariton. Microwave experiments are performed on realistic structures to successfully realize the theoretical predictions, and the obtained results are in agreements with FDTD simulations.   

Percolation and polaritonic effects in periodic planar nanostructures evolving from holes to islands

We study interaction of the electromagnetic radiation with a series of thin film periodic nanostructures evolving from holes to islands. We show, through model calculations, simulations and experiments, that the responses of these structures evolve accordingly, with two topologically distinct spectral types for holes and islands. We find also, that the response at the transitional pattern is singular. We show that the corresponding effective dielectric function follows the critical behavior predicted by the percolation theory, and thus the hole-to-island structural evolution in this series is a topological analog of the percolation problem, with the percolation threshold at the transitional pattern.   

An XPS study of gas phase interaction with Au nanoparticles coated TiO$_{2}$ nanosprings

The interaction of CO and O$_{2 }$on the surface of the Au nanoparticles (NPs) supported on TiO$_{2 }$(Au/TiO$_{2})$ nanosprings (NS) by x-ray photoelectron spectroscopy will be discussed. The Au NPs were coated onto the TiO$_{2 }$NS by plasma enhanced chemical vapor deposition, where the average particle size is 7-8 nm. The gas interactions with the Au NPs is evaluated by examining binding energy shifts of the Au 4f, C 1s, Ti 2p and O 1s electron core level states. For both of the gases, all of the core levels shifted to higher binding energy. Temperature dependent desorption, or the lack thereof, as determined by XPS analysis, indicates that the gas-substrate interaction is chemisorption, as opposed to physisorption. A detailed discussion on the mechanism of adsorption, as well as the roles of the Au NP and the TiO$_{2}$ substrate, will be presented.   

Paper based Flexible and Conformal SERS Substrate for Rapid Trace Detection on Real-world Surfaces

One of the important but often overlooked considerations in the design of surface enhanced Raman scattering (SERS) substrates for trace detection is the efficiency of sample collection. Conventional designs based on rigid substrates such as silicon, alumina, and glass resist conformal contact with the surface under investigation, making the sample collection inefficient. We demonstrate a novel SERS substrate based on common filter paper adsorbed with gold nanorods, which allows conformal contact with real-world surfaces, thus dramatically enhancing the sample collection efficiency compared to conventional rigid substrates. We demonstrate the detection of trace amounts of analyte (140 pg spread over 4 cm$^{2})$ by simply swabbing the surface under investigation with the novel SERS substrate. The hierarchical fibrous structure of paper serves as a 3D vasculature for easy uptake and transport of the analytes to the electromagnetic \textit{hot spots} in the paper. Simple yet highly efficient and cost effective SERS substrate demonstrated here brings SERS based trace detection closer to real-world applications.   

Sensitive detection of nitro aromatic explosives using novel polythiophene nanoparticles

Fluorescent polythiophene nanoparticles were fabricated by surfactant assisted mini emulsion technique. The size distribution of the synthesized nanoparticles was characterized using dynamic light scattering (DLS) and scanning electron microscopy (SEM). The synthesized nanoparticles were also characterized using UV-Vis and fluorescence spectroscopy. Strong two-photon induced fluorescence was observed from these nanoparticles using 800 nm pulses from a femto second laser. The fluorescence response of these nanoparticles to nitro-aromatic explosives 2,4-dinitrotoluene and 2,4,6-trinitrotoluene in solution was investigated at different concentrations of the analytes. Strong fluorescence quenching was observed using both one photon and two-photon excitation source. The Stern Volmer constant is also higher.   

A novel nanostructure for ultrasensitive volatile organic compound sensing

We have developed an arrayed nanocoaxial structure for the ultrasensitive sensing detection and identification of volatile organic compounds (VOC) by dielectric impedance spectroscopy. VOC molecules are absorbed into porous dielectric material in the annulus between nanoscale coax electrodes. A theoretical expression for the basic adsorption mechanism agrees with the experimental results. Detection sensitivities at parts-per-billion levels were demonstrated for a variety of VOCs. A limit-of-detection of ethanol reached $\sim $100 parts-per-trillion, following a Freundlich power-law isotherm across four decades of ethanol concentration. A linear dependence on VOC dielectric constant was observed. Dielectric impedance nanospectroscopy was also performed by scanning frequency from 10 mHz to 1 MHz, with distinctive spectra of different VOCs discovered. These were utilized to conduct colorimetric identification of VOCs. The results suggest our novel nanocoaxial sensor can be used as a sensitive, broadband, and multimodal sensing platform for chemical detection.   

Session V33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Vortices and Novel Mechanisms

Effect of $A$-site size difference on polar behavior in $M$BiScNbO$_6$ ($M$=Na, K, and Rb) perovskite: Density functional calculations

We investigated the effect of $A$-site size difference in the double perovskites BiScO$_3$-$M$NbO$_3$ ($M$=Na, K, and Rb) using first-principles calculations. The materials studied have increasing ionic radii at the $A$-site ($r_{\rm{Na}^+}<$ $r_{\rm{K}^+}<$ $r_{\rm{Rb}^+}$) but are otherwise chemically similar. We find that the polarization of these materials is 70-90 $\mu$C/cm$^2$ along the rhombohedral direction, which increases as the $A$-site size difference becomes larger. The main contribution to the high polarization comes from large off-centerings of Bi ions, which are strongly enhanced by the suppression of octahedral tilts as the $M$-ion size increases. A high Born effective charge of Nb also contributes to the polarization and this contribution is also enhanced by increasing the $M$-ion size. This work was supported by ONR and DOE, BES, Materials Sciences and Engineering.   

First Principles Study of Flexoelectricity

Flexoelectricity is the linear response of polarization to a strain gradient. Because strain gradients break inversion symmetry, flexoelectricity allows for charge to be extracted from deformations even in materials that are not piezoelectric. The flexoelectric effect is negligible on conventional length scales, but it becomes very strong at the nanoscale where large strain gradients can significantly affect the functional properties of dielectric thin films and superlattices. We present first-principles calculations of flexoelectric effects in nonpiezoelectric materials by introducing the strain gradient artificially in a slab geometry and obtain the flexoelectric coefficients. Furthermore, we model the results in terms of quantities, such as dynamical charges and higher\footnote{R. Resta, Phys. Rev. Lett. {\bf 105}, 127601 (2010).} multipole moments that can be computed in the bulk, bringing us closer to a full theory of flexoelectricity.   

Coexistence of ferroelectricity and octahedral rotations in ABX$_3$ perovskites

Nearly all cubic perovskite materials are unstable to energy-lowering structural distortions. The most intensively studied distortions are those that induce ferroelectricity and tilts or rotations of the anion octahedra. The phonon dispersion curves of many perovskites contain both types of instability, although competition between the different types of distortions usually leads to ground-state structures in which one type of instability has been eliminated. Hence, whereas there are many perovskites that are \emph{either} ferroelectric or have rotated octahedra, there are very few perovskites that are \emph{both} ferroelectric and have rotated octahedra. We use a combination of Density Functional Theory, group theoretical techniques and crystal chemistry arguments to study the competition between ferroelectric and octahedral rotation distortions in a family of ferroelectric perovskite fluorides and oxides. By considering both ``long-range'' distortions (phonons) and the local bonding environment of each ion, we are able to build up a picture of which factors favor one type of distortion over the other.   

Vortex Domains in Ferroelectric Nano-Structures

Recently the study of submicron-diameter ferroelectric disks and squares and rectangles fabricated from films of ca. 100-300 nm thick have revealed usual domain patterns, qualitatively different from the stripe domains commonly studied in macroscopic specimens in the past. These include doughnut-shaped domains, four-fold vertex closure domains, and fractal domains. The static configurations offer a variety of puzzles, and the structures differ from those in magnetic vortex domains, presumably due to the much larger anisotropy in ferroelectrics, which generally prohibits true vortex configurations with polarization forced out of plane. The dynamics also differ qualitatively from early studies: For decades ferroelectrics were thought to be highly Ising-like, but recent data and theoretical simulations favor Bloch walls and more Heisenberg-like kinetics. This talk will include data from Alina Schilling and Marty Gregg in Belfast, Marin Alexe in Halle, and modeling from Hlinka and Marton in Prague and Bellaiche and Prosandeev in Arkansas.   

Multiferroic Vortices and Graph Theory

Hexagonal REMnO$_{3}$ (RE= rare earths) with RE=Ho-Lu, Y, and Sc, is an improper ferroelectric where the size mismatch between RE and Mn induces a trimerization-type structural phase transition, and this structural transition leads to three structural domains, each of which can support two directions of ferroelectric polarization. We reported that domains in h-REMnO$_{3}$ meet in cloverleaf arrangements that cycle through all six domain configurations, Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the cycle of domain configurations is reversed. Vortices and antivortices are topological defects: even in a strong electric field they won't annihilate. Recently we have found intriguing, but seemingly irregular configurations of a zoo of topological vortices and antivortices in h-REMnO$_{3}$. These configurations can be neatly analyzed in terms of graph theory.   

Hidden Roto Symmetries in Crystals and Handed Structures

Symmetry is a powerful framework to perceive and predict the physical world. The structure of materials is described by a combination of rotations, rotation-inversions and translational symmetries. By recognizing the reversal of static structural rotations between clockwise and counterclockwise directions as a distinct symmetry operation, here we show that there are many more structural symmetries than are currently recognized in right- or left-handed handed helices, spirals, and in antidistorted structures composed equally of rotations of both handedness. For example, though a helix or spiral cannot possess conventional mirror or inversion symmetries, they can possess them in combination with the rotation reversal symmetry. Similarly, we show that many antidistorted perovskites possess twice the number of symmetry elements as conventionally identified. These new symmetries, referred to as ``roto'' symmetries, predict new forms for roto properties that relate to static rotations, such as rotoelectricity, piezorotation, and rotomagnetism. They also enable a symmetry-based search for new phenomena, such as multiferroicity involving a coupling of spins, electric polarization and static rotations. This work is relevant to structure-property relationships in all material structures with static rotations.   

Spontaneous Vortex Nanodomain Arrays at Ferroelectric Heterointerfaces

The polarization of BiFeO$_{3}$ subjected to different electrical boundary conditions by hetero-interfaces is imaged with atomic resolution using a Cs-corrected transmission electron microscope. Unusual nanodomains are seen and their role in providing polarization closure is understood through phase-field simulations. Hetero-interfaces are key to the performance of ferroelectric devices and this first observation of vortex arrays at ferroelectric hetero-interfaces reveals properties unlike the surrounding film including mixed Ising-N\'{e}el domain walls, which will affect switching behavior, and a drastic increase of in-plane polarization. Imaging this magnetic analogous effect at ferroelectric hetero-interfaces provides the ability to see device-relevant interface issues.   

Helical strain patterns and charge ordering in YbFe$_2$O$_4$

The $R$Fe$_2$O$_4$ compounds exhibit simultaneously charge ordering (CO) of the Fe$^{2+}$ and Fe$^{3+}$ ions,\footnote{Ikeda \textit{et al}, Nature \textbf{436} 1136 (2005)} together with magnetic ordering of the Fe spins\footnote{Christianson \textit{et al}, PRL \textbf{100} 107601 (2008)} and possible multiferroic behavior.\footnote{S-W Cheong \textit{et al}, Nat. Mater. \textbf{6} 13 (2007)} Synchrotron data collected below the 3D CO transition show intensity concentrated around peaks separated by $\sim$1/3 $c$* but slightly displaced in the $(a^*, b^*)$ plane. Calculations modelling an oxygen displacement pattern are in excellent agreement with the data, suggesting an incommensurate charge ordering of the Fe ions close to the commensurate $\sqrt{3} \times \sqrt{3}$ structure, associated with a helical strain pattern. At high temperature, ordering wavevectors corresponding to many different displacement patterns are simultaneously populated by the system, leading to diffuse but highly structured features in reciprocal space.   

Three-dimensional distribution of ferroelectric vortices in multiferroic hexagonal YMnO$_3$

Multiferroics are a rich source for ``unusual'' forms of ferroelectric order. The spontaneous polarizations is induced by magnetism, charge order, geometric effects, etc., and may lead to novel domain states and functionalities. Recently it was shown that ferroelectric domains in hexagonal multiferroic YMnO$_3$ form vortex-like structures around the direction of polarization [1]. It was assumed that the sixfold character of the domain vortices reflects the uniaxial hexagonal structure. Here we show by piezoresponse force microscopy that high densities of sixfold vortices are also present {\it perpendicular} to the direction of the spontaneous polarization in spite of the merely twofold rotation-inversion symmetry in this direction [2]. We present a simple geometric explanation for this unexpected result and discuss the principal difference between the present case and vortex formation in discommensurate systems.\\[4pt] [1] T. Choi et al., Nature Mater. {\bf 9}, 253 (2010)\\[0pt] [2] T. Jungk et al., Appl. Phys. Lett. {\bf 97}, 012904 (2010)   

Cloverleaf domain patterns in multiferroic RMnO$_{3}$ (R = Ho, Er, and Lu)

Hexagonal RMnO$_{3}$ (R=rare earths) exhibits a unique improper ferroelectricity induced by structural trimerization. Intriguing domain pattern associated with ferroelectricity and trimerization, so-called ``cloverleaf'' domain pattern, has been reported in YMnO$_{3}$ [1] In this talk, we will report the domain structures in a series of RMnO$_{3}$ with different rare earth elements, obtained from the results of our transmission electron microscopy. Characteristic cloverleaf domain patterns are clearly observed in RMnO$_{3}$ (R = Ho, Er, and Lu). The results imply that the cloverleaf domain pattern is a common domain feature in the hexagonal manganites. \\[4pt] [1] T. Choi et al., Nature Materials 9, 818 (2010)   

Session V35: Topological Insulators: ARPES & STM

Local probing of Quantum Spin Hall edge states

Since their recent experimental discovery, topological insulators have attracted a lot of interest. The two- dimensional manifestation of a topological insulator, the Quantum Spin Hall (QSH) state, is characterized by counter- propagating edge states with opposite spin-polarization, while the bulk is insulating. We use Scanning Gate Microscopy to demonstrate the edge state nature of transport in the QSH state. Utilizing the high spatial resolution of this technique, we gain insight into the spatial properties of the edge states. Furthermore, the experiments can yield information regarding the sensitivity of the QSH edge states to local perturbations, which can be useful for future applications.   

Visualizing Strong Scattering of Topological Surface States from Magnetic Impurities in Bi$_2$Te$_3$

Bi$_2$Te$_3$ is a topological insulator with a single Dirac cone in the band structure of its helical surface states. The associated spin texture protected by time reversal symmetry (TRS) is thought to suppress scattering off non-magnetic defects. We tested this using scanning tunneling microscopy and spectroscopy. At high energies, far above the Dirac point, backscattering off non-magnetic defects, such as step-edges, is facilitated by quasi-nesting conditions brought about by the hexagonal warped surface band. At lower energies at which the surface dispersion is linear backscattering is highly suppressed by the helical spin texture protected by TRS. In contrast, in Mn-doped Bi$_2$Te$_3$ the measured quasi- particle interference pattern shows the onset of strong scattering both in the warped region as well as in the conic one. The scattering processes involved are affected both by the spin texture as well as by the geometry of the scattering potential. Furthermore, close to the Dirac point the increased scattering in Mn-doped Bi$_2$Te$_3$ seems to promote localization of the surface states.   

Magnetic Versus Non-magnetic Scattering of Topological Surface States in Bi$_{2}$Te$_{3}$

Due to their novel spin texture, the surface states of topological insulators are predicted to be impervious to backscattering from non-magnetic disorder. For impurities which break time-reversal symmetry, however, such backscattering is not forbidden by the topological character of the states. Here we use scanning tunneling microscopy to study scattering from impurities in doped Bi$_{2}$Te$_{3}$. In Mn-doped Bi$_{2}$Te$_{3}$, we have observed an interference pattern from the surface states throughout a broad range of energies, even in the region of linear dispersion near the Dirac point. We contrast these findings of the scattering of topological surface states from magnetic defects with similar measurements on Ca-doped Bi2Te3 using spectroscopic mapping. We will use the results of these experiments to probe whether the presence of magnetic impurities gives rise to backscattering in topological surface states.   

Scanning tunneling spectroscopic (STS) studies of MBE-grown topological insulators of Bi$_{2}$Se$_{3}$ epitaxial films on Si(111)

We report STS studies of MBE-grown Bi$_{2}$Se$_{3}$ epitaxial films on Si(111) with varying thicknesses. The films were atomically flat on the scale of hundreds of nanometers, with occasional atomic steps of one c-axis lattice constant. In the case of thick Bi$_{2}$Se$_{3}$ films, the tunneling spectra were consistent with those found in single crystalline Bi$_{2}$Se$_{3}$, except that the Dirac point ($E_{Dirac}=-$50 $\sim $ -100 meV) of the MBE-film is generally much closer to the Fermi level ($E$ = 0), in contrast to the large downshift of $E_{Dirac}$ (= -400 $\sim $ -200 meV) commonly found in single crystalline bulk grown Bi$_{2}$Se$_{3}$. The STS spectra of the thinner films deviate from those of the thicker samples, probably the result of strain. Fourier transformed (FT) STS data as a function of energy reveals several quasiparticle scattering interference wave-vectors that are consistent with the topologically protected surface states with chiral spin texture, although the overall FT-STS maps are simpler than those reported on the Bi$_{0.92}$Sb$_{0.08}$ (111) surface due to simpler electronic band-structures of Bi$_{2}$Se$_{3}$. The effect of time reversal symmetry breaking on the FT-STS will be investigated by either magnetic doping or application of magnetic fields. This work was supported by a grant from FENA of FCRP and DARPA.   

Electron interference in the 3D topological insulator Bi$_{2}$Se$_{3}$ probed by scanning tunneling microscope

Three-dimensional topological insulators (TIs) have aroused great attention to the new state of quantum matter originating from the surface state that forms a massless Dirac cone. Among the recently discovered TIs, Bi$_{2}$Se$_{3}$ is regarded as the most promising candidate [1]. However, recent magnetotransport measurements showed that the bulk conductance dominates even in low carrier samples [2], which raises the question of possible scattering channels responsible for the reduced surface mobility. Band structure calculations predict the Dirac point of the surface state to be located close to the bulk valence band maximum [1]. In order to clarify the surface state scattering feature, we have performed differential tunneling conductance mapping for the surface of Bi$_{2}$Se$_{3}$. The fast Fourier transformation image shows an electron interference pattern near the Dirac node, which provides the evidence of near-surface scattering of the spin polarized surface electrons at the Dirac point in Bi$_{2}$Se$_{3}$ into the spin-degenerate bulk continuum states. \\[4pt] [1] Y. Xia et al., Nat. Phys. \textbf{5}, 398 (2009). \\[0pt] [2] N. P. Butch, Phys. Rev. B \textbf{81}, 241301(R) (2010).   

Power laws and STM image of standing wave of the topological surface states

We have theoretically and experimentally studied the quasiparticle interference pattern caused by scattering off the step edges of topological surface states in Bi$_{2}$Te$_{3}$ and Bi$_{2}$Se$_{3}$. We propose a general formalism to identify the power law that governs the decaying spatial oscillations of standing wave of the quasiparticle. With strong hexagonal warping of the surface states in Bi$_{2}$Te$_{3}$, the standing wave will have different decay index as the Fermi energy varies; while in Bi$_{2}$Se$_{3}$, the standing wave has only a single decay index due to weak warping effect. Using a scanning tunneling microscope, we directly observe the standing waves in the local density of states on both surfaces, which together with the analysis of such oscillations at different voltage confirms our theoretical predictions. We further show that, the characteristic scattering wavevectors of the standing wave of surface states caused by scattering off the nonmagnetic impurity in both Bi$_{2}$Te$_{3}$ and Bi$_{2}$Se$_{3 }$can also be well explained by this general formalism.   

Multifunctional electronic structure in a topological insulator class

The discovery of topological properties in three dimensional bulk solids have opened up many new research avenues in condensed matter physics. Only a very few compounds have been identified to be topological insulators to this date. However, none of them is proven to be suitable for the majority of experimental configurations including giant magnetoelectric and anomalous optical rotation, unusual exciton condensation, or the neutral half-fermions and interface superconductivity. In fact the realization of even any one of these proposals requires a number of multiply-connected topological compounds with modulated surface band dispersions and naturally tuned in-gap Fermi level, as well as spin variations in the presence of long life-time of the surface states. Here, using conventional and spin-sensitive probes, we report the discovery of several classes of positive band-gap high figure of merit topological insulators with critically important functional properties such as high degree of bulk resistivity and insulation, electronic structure with both in-gap Dirac point and Fermi level crossing, long surface state life-times, as well as chirality inversion through the Dirac node. The unprecedented combinations of electronic, spin, life-time and resistive bulk transport featured by the topological insulators uncovered here not only provide a new platform for research on topological quantum phenomena but also pave the way for functional devices.   

Observation of novel interference patterns in BixFe1-xTe3 by Fourier transform scanning tunneling spectroscopy (FT-STS)

We utilize Fourier transform scanning tunneling spectroscopy (FT-STS) to probe the surface of the magnetically doped TI, Bi2-xFexTe3. Our measurements show the appearance of a hitherto unobserved channel of electronic backscattering along the surface q-vector. By referencing the FT-STS with angle-resolved photoemission spectroscopy (ARPES) data, we formulate a simple model showing that these new vectors are fully consistent with spin-flip scattering. Our combined data therefore present compelling evidence for the first momentum resolved measurement of enhanced backscattering due to magnetic impurities in a prototypical TI.   

Local interaction of magnetic impurities and topological surface states

Topological insulators have garnered much attention as a vehicle to explore exotic Dirac physics through the projection of unpaired Dirac cones into conducting surface states wrapping a spin-orbit twisted bulk band structure. We use an ultrahigh-vacuum low-temperature scanning tunneling microscope (STM) to gain access to and manipulate the chiral Dirac particles present on the Sb(111) surface. Understanding the interplay between local spins and Dirac fermions represents a key foundation to the development of new spintronic applications. Magnetic moments break time-reversal symmetry and provide an additional local quantum degree of freedom to engineer topological states. By dosing magnetic impurities of varying concentration and species, we show how STM can atomically manipulate individual magnetic adatoms on topological surfaces, and in the process gain insight into the physical bonding arrangement of magnetic impurities on top of and embedded inside the host crystal lattice. Using scanning tunneling spectroscopy, we map in real and momentum space how local spins interact with the chiral surface Dirac carriers.   

Visualizing spin-vortex evolution of a topological insulator

Charge carriers on the surface of a topological insulator are predicted to form a spin-vortex in momentum space with the direction of spin rotation determined by whether the carriers are electron-like or hole-like. We show that the angular momentum of photon is extremely sensitive to the spin of carriers by performing time-of-flight based angle-resolved-photoelectron spectroscopy (TOF-ARPES) with photons of different helicity. We demonstrate the first reciprocal space volumetric mapping of the vectorial spin-texture of the surface states of $Bi_2 Se_3$ and directly observe spin-vortex evolution from electron-like to hole-like states and the departure from perpendicular momentum-spin locking.   

Spin and angular reoslved photoemission studies of Bi2Se3

Topological insulators (TL) have attracted much attention because of their exotic properties. Bi$_2$Se$_3$ is a model TL with a relative large bulk gap and a simple surface state structure. By depositing various non-magnetic and magnetic impurities on the surface, we were able to fill the topological surface state and higher lying Rashba splitting surface states. The spin texture of the surface electronic structure was determined in spin resolved photoemission measurement.   

Surface states never die by surface impurities in topological insulators

The metallic surface states in topological insulators are one of the most distinguished features among the characters of this newly discovered quantum state of matter. These states, if properly exploited, may open a new era in spintronics and quantum computing. However, full characterization and understanding of the surface states toward these goals are still far from satisfactory. Here, we focus on the robustness of the metallic surface states in a topological insulator Bi$_{0.9}$Sb$_{0.1}$, and demonstrate their durability over magnetic/non-magnetic surface impurities by measuring the scattering rates of the quasiparticles via angle-resolved photoemission spectroscopy.   

Experimental Realization of Three-dimensional Topological Insulator in Ternary Chalcogenides

Three-dimensional topological insulators (TIs) featured with spin-helical massless surface state have attracted a great attention. Up to now, the experimentally confirmed topological insulators are limited to some binary compounds, such as Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$ and so on. Recently, several ternary chalcogenides have been proposed as a new family of TI. In contrast to the layered binary chalcogenides, in ternary chalcogenides with a more substantial three dimensional character, the surface state depends on the topmost layer because the broken bonds at the surface may give rise also to trivial surface state. Therefore, the experimental realization of non-trivial surface state in TI has been strongly required. In this work, we have performed an angle resolved photoemission spectroscopy by using synchrotron radiation to prove the surface state in the ternary compounds. Especially, for one of the candidate materials, TlBiSe$_{2}$, two important aspects have been revealed: (i) The Dirac cone is more ideal than that of Bi$_{2}$Se$_{3}$. (ii) There are no bulk continuum states that energetically overlap with the Dirac point. This means that the scattering channel from the topological surface state to the bulk continuum is strongly suppressed in TlBiSe$_{2}$.   

STM/STS studies of the surface of Bi2Se3

Building upon previous work,\footnote{Urazhdin S. et al. Physical Review B 69, 085313 (2004); Physical Review B 66, 161306(R) (2002).} we apply scanning tunneling microscopy/spectroscopy to characterize the surface of the topological insulator Bi2Se3. We see clover-like defect states in the topographic scans and a residual image that appears in conductance scans, which we attribute to Bi substitutions in Se lattice sites. Spectroscopy reveals features in the density of states consistent with the topological surface state, with the defect states appearing as an additional enhancement. We will discuss the interaction of the topological surface state with the defect states.   

Scattering on Magnetic and Non-Magnetic Impurities on a Surface of a Topological Insulator

Dirac-like surface states on surfaces of topological insulators have a chiral spin structure that supresses back-scattering and protects the coherence of these states in the presence of potential scatterers. In contrast, magnetic scatterers are expected to open the back- scattering channel via the spin-flip processes and to degrade the state's coherence. We present angle-resolved photoemission spectroscopy studies of the electronic structure and the scattering rates upon adsorption of various magnetic and non-magnetic impurities on the surface of Bi$_2$Se$_3$, a model topological insulator. We uncovered an unusual insensitivity of the topological surface state to both non-magnetic and magnetic impurities. The electrons donated by the impurities fill the topological surface state and pairs of higher lying spin-orbit split surface bands, preserving the non-trivial spin texture of the surface.   

Session V36: Graphene: Optical Properties I

Imaging stacking order in few-layer graphene

Few-layer graphene (FLG) has been predicted to exist in various crystallographic stacking sequences, which can strongly influence the material's electronic properties We demonstrate an accurate and efficient method of characterizing stacking order in FLG using the distinctive features of the Raman 2D-mode. Raman mapping allows us to visualize directly the spatial distribution of Bernal (ABA) and rhombohedral (ABC) stacking in tri- and tetra-layer graphene. We find that $\sim $15{\%} of exfoliated graphene tri- and tetra-layers is comprised of micron-sized domains with rhombohedral stacking, rather than the Bernal stacking. These domains are stable and remain unchanged for annealing to temperatures exceeding 800 $^{\circ}$C.   

Electric-field induced changes in the band structure of trilayer graphene: The effect of crystallographic stacking order

We have studied by means of infrared spectroscopy the influence of a strong perpendicular electric-field on the band structure of graphene trilayers with two different types of crystallographic stacking: ABA (Bernal) and ABC (rhombohedral) stacking. The symmetries of the two crystallographic structures are different, the former having mirror symmetry and the latter inversion symmetry. Distinct infrared response was observed when breaking their respective symmetries by the application of the electric field. We observed an electrically tunable band gap of over 100 meV in ABC trilayers, while no band gap was found for ABA trilayers. Our results will be compared to the induction of a band gap in AB bilayer graphene [K. F. Mak \textit{et al}, PRL \textbf{102}, 256405 (2009); Y. Zhang \textit{et al}, Nature \textbf{459}, 820 (2009)]   

Photoluminescence in highly doped graphene

Pristine graphene is a zero-bandgap semiconductor. Usually no photoluminescence can be observed from such zero-bandgap material upon laser excitation. In highly doped graphene, however, we observed a strong broadband photoluminescence. We will discuss the mechanism of this photoluminescence in graphene, which arises from new recombination pathways enabled by strong electrical doping. We will also describe the polarization dependence of this newly observed photoluminescence.   

Response of graphene to intense optical irradiation

We investigate the modification of graphene under intense ultrashort laser irradiation. Our observations indicate that the graphene structure is very resilient and exhibits a high damage threshold, which is promising for high order non-linear applications. In the case of epitaxially grown samples, we find that single-shot damage threshold is 5x10$^{10 }$Wcm$^{-2}$ for 50 fs pulse duration. Raman and optical microscopy measurements of irradiated samples show that the carbon lattice completely disappears from the region where the laser intensity exceeds the threshold without leaving any visual or spectroscopic signature. Below the threshold, single-shot irradiation does not exhibit a significant defect formation. However, repeated laser irradiation below the threshold leads to formation of defects. The mechanisms underlying the defect formation and lattice reduction will be discussed.   

Mid Infrared Near Field Study of Monolayer Graphene

We have performed near-field spectroscopic studies of both monolayer suspended graphene (SG) and graphene on SiO$_{2}$/Si substrate (GOS) using scattering-type scanning near-field optical microscope (s-SNOM). Our data show that SG produces reliable near-field signal in mid-infrared frequencies. Images taken with high spatial resolution ($\sim $20nm) show nanoscopic features such as ripples and electronic inhomogeneities. The SiO$_{2}$/Si substrate contributes a phonon resonance in the near-field signal around 1130 cm$^{-1}$. This resonance is remarkably strengthened and broadened by just a single layer of graphene in the case of GOS. By probing the resonance spectrum we find over 400{\%} contrast in near field signal between GOS and the bare substrate. The detailed analysis of the contrast suggests that GOS is slightly doped. This study therefore provides much needed insight into the thickness resolution of the s-SNOM technique, proving it can be sensitive to just a single layer of atoms, and advances the fundamental understanding of graphene-light interactions by probing in the near-field regime.   

Terahertz and Infrared Conductivity of Large-Area Graphene

Graphene is predicted to offer new opportunities for terahertz (THz) science and technology. Its zero-gap linear band dispersion is expected to lead to exotic nonlinear electromagnetic properties, which can be probed through frequency-dependent conductivity measurements. Here, we use THz time-domain spectroscopy and Fourier-transform infrared spectroscopy to investigate carrier dynamics in large-area graphene grown by chemical vapor deposition. We studied both nitrogen-doped and nominally-undoped graphene; the latter had accidental doping presumably through air and acid exposure. Absorption increased with the number of graphene layers and was larger in the nominally-undoped samples especially in the 0.2-2.2 THz range. For the highest-mobility samples, we observed Drude-like frequency dependence in the THz range. Further measurements in a wider spectral range are in progress to understand the differences between these samples and the interplay between intra-band and inter-band dynamics.   

Carrier Cooling in Graphene Measured by THz Time-Domain Spectroscopy

We present results on the ultrafast relaxation dynamics of photoexcited electrons and holes in graphene using optical-pump terahertz-probe spectroscopy. Measurements done at different temperatures show that the measured differential transmission as a function of the probe delay decays on time scales that become very long at low temperatures with decay times exceeding $\sim$150 ps at temperatures lower than $\sim$50K. We interpret these transients as carrier cooling due to a combination of electron-optical phonon and electron-acoustic phonon scattering. When the carrier temperature goes below $\sim$250 K, optical-phonon scattering ceases to effectively cool the carriers given the large optical phonon energies in graphene. Since acoustic phonon scattering is not efficient in removing the heat from the carriers, the carrier distribution cools very slowly. Our data is in agreement with the theoretical predictions [1].\\[4pt] [1] Phys. Rev. B 79, 235406 (2009) and Phys. Rev. Lett., 102, 206410 (2009).   

Ultrafast electron dynamics in freely suspended graphene

The optical conductivity of free-standing graphene under the non-equilibrium conditions was investigated by femtosecond pump-probe spectroscopy. The conductivity transient exhibited a strong dependence on pump fluence, with a crossover from enhanced to reduced absorbance occurring with increasing pump fluence. The observed phenomena can be understood by taking into account both the induced intra- and inter-band optical response. Intra-band transitions dominate the transient at low pump fluence (and electronic temperature) and inter-band transitions dominate at high pump fluence (and electronic temperature). Analysis within a model incorporating these two responses allows us to infer the variation of carrier scattering rate with electronic temperature. The temporal evolution of the conductivity transient is controlled by the anharmonic decay of the optical phonons; a lifetime of $\sim$1.4 ps was inferred for intrinsic, suspended graphene.   

Ultrafast dynamics of highly-excited Dirac fermions in monolayer graphene

One of the striking optical properties of single-layer graphene is the universal absorbance in the near-infrared-to-visible spectral range due to the Dirac spectrum of the low-energy electronic structure. High-fluence laser pump can produce superdense Dirac-fermionic excitations at the order of 10 femtoseconds so to reach the non-linear saturation of absorption. We construct a simple model for the transient state of the photo-excited graphene to explore the non-linear saturation of photoexcitations and the transport property of carries. The comparison of our model calculations with the experimental results shows good agreements.   

Ultrafast relativistic response of Photo-excited Carriers in Graphene

Understanding the ultrafast non-equilibrium dynamics of photocarriers in graphene's unique relativistic band structure is important for the development of such high-speed, graphene-based photonic devices and also from a fundamental point of view. Here, we directly demonstrate the relativistic nature of a non-equilibrium gas of electrons and holes photogenerated in a graphene monolayer as early as 100 femtoseconds (fs) after photoexcitation. We photoexcited carriers in graphene and then measured the time-resolved, pump-induced change in reflection at various visible probe photon energies. We observe a nonlinear scaling in the Drude-like optical conductivity of the photocarriers with respect to their density, in striking contrast to the linear scaling expected from conventional materials with parabolic dispersion relations.   

Carrier Dynamics in Colloidal Graphene Quantum Dots

We describe carrier dynamics for single and multiple excitons in colloidal graphene quantum dots (GQDs). Strong confinement and corresponding size-tunable electronic structure make GQDs potentially useful sensitizers in photovoltaic devices. We have studied the optical response of GQDs consisting of 132 and 168 sp$^{2}$ hybridized carbon atoms dissolved in toluene with HOMO-LUMO transitions of 1.4-1.6 eV. From measurements of ultrafast ($\sim$100 fs) transient absorption over nanosecond timescales, we extract the single-photon absorption cross-section and observe carrier-induced Stark shifts of the order of 0.1 eV indicating strong carrier-carrier interactions, as expected for the relatively weak screening of a two-dimensional nanostructure. Multiexcitons are observed to decay nonradiatively on $\sim$1 to 20 ps timescales, while single excitons display dynamics on multiple timescales due to carrier cooling, singlet-to-triplet intersystem crossing, and, on nanosecond to microsecond timescales, radiative recombination.   

Session V37: Focus Session: Graphene Growth, Characterization, and Devices: Transport

Electronic Transport Properties of Graphene on Aluminum Nitride

We have fabricated graphene field-effect transistors on aluminum nitride (AlN) gate dielectric over silicon back gates. AlN thin films are prepared on Si by pulsed laser deposition, and exfoliated graphene on SiO2 is transferred onto the AlN/Si surface by using thermal tape as a transfer medium. After transfer, Raman spectra and AFM measurement have been performed to confirm the quality of graphene on AlN. Electron transport measurements will be reported.   

Exploring Transport Effects in Nanoscale Graphene Devices

Graphene, the single- to few-atomic layers cousin to graphite, has become a very interesting topic of research owing to its unique mechanical, optical, thermal and electrical properties. Many of the properties of graphene can be traced to its structural uniformity, allowing both electrons and holes to travel long distances (up to several microns) before scattering. However, studying graphene on the micron level can mask its true nanoscale behavior. Using very short length scales allows for the investigation of the behavior of charge impurities, contact effects and ballistic transport. In this work, we fabricate sub-30 nanometer suspended graphene 3-terminal devices on gold and platinum electrodes. We present data from electrical measurements on charge impurities that are apparent at this length scale and the effect of electrode work function on contact resistance. We compare this to mechanically exfoliated graphene on a silicon/SiO2 substrate with gold electrodes.   

Electrical transport study of suspended graphene nanoribbons near the Dirac point

We have fabricated graphene nanoribbon Field-effect transistors from high-quality graphene nanoribbons produced by sonicating multiwall carbon nanotubes in an organic solvent. To minimize the influence of the underlying substrate, individual nanoribbons in the devices were suspended by removing the underneath silicon oxide using a wet etching method. Subsequently, in situ current annealing was carried out in high vacuum to further reduce the impurities adsorbed to the ribbon surfaces. The electrical transport properties of the devices were measured for a wide range of temperatures, revealing a range of unusual phenomena pertinent to the competing effects of improved overall charge homogeneity and reduced charge puddle sizes when the graphene nanoribbons are tuned close to the Dirac point. The electrical transport results on suspended graphene nanoribbon with varying disorder will be presented and discussed.   

Effects of disorder on the transport properties of chemically derived graphene

Transport properties of chemically derived graphene (CDG) are strongly influenced by the concentration of defects that are introduced during synthesis. We present a comprehensive transport study on a range of CDG films with varying degrees of disorder. The electric properties of CDG were found to be tunable over several orders of magnitude via controlled oxidation and reduction. The structural properties of CDG were monitored by analyzing the defect-related features in the Raman spectra and correlated with transport. The temperature dependence of the resistivity of these samples indicate that the conduction mechanism evolves from tunneling to hopping for strongly disordered samples and to activated transport for weakly disordered samples. Strong disorder causes localization of carriers and field-dependent modulation of hopping conduction. We discuss the temperature- and gate-bias-dependence of the resistivity of weakly disordered samples in terms of scattering dominated by midgap states, as is the case in ion irradiated graphene [2]. \\[4pt] [1] G. Eda et al., J. Phys. Chem. C, 113, 15768 (2009). \\[0pt] [2] J.-H. Chen et al. Phys. Rev. Lett. 102, 236805 (2009)   

Influence of Strain on Quantum Transport in Graphene

Strain is unavoidable in graphene, either suspended or supported on a dielectric substrate such as SiO2. It is therefore crucial to identify the role of strain on transport properties in graphene. Experimentally, it is shown that local strain in graphene on a SiO2 substrate can modify graphene's conductance near the Fermi energy. The modification is attributed to the coupling of strain and phonon-mediated inelastic tunneling effects. However, conductance on the dielectric substrate is not ballistic and isolating the influence of strain is a difficult task. In this study, strain effects on ballistic conductance, an experimentally attainable transport property for suspended graphene, is studied using a combination of density functional theory and the Landauer-Buttiker formalism. It is found that, unlike in a CNT, regardless of the applied strain graphene's conductance at the Fermi energy is 0.21G0. Furthermore, for conducting electrons with energies higher or lower than the Fermi energy of the system, tensile hydrostatic strain is found to increase conductance but compressive hydrostatic strain decreases conductance. For an 8{\%} compressive hydrostatic strain, conductance increases by as large as 30{\%}. Surprisingly, for uni-axial strain, if the energy of the conducting electrons is higher than the Fermi energy, conductance remains approximately unchanged, whereas conductance by electrons less than the Fermi energy decreases (increases) with compressive (tensile) strain along the transport direction.   

Thermal and Electronic Transport Properties of Graphene Nanoribbons with Defects

The interplay between graphene nanoribbon (GNR) structure and conductivity, both thermal and electrical, is probed with molecular dynamics and tight binding models. A variety of randomly oriented defects, vacancies and Stone-Wales, as well as edge terminations, zig-zag, armchair, and roughened, are studied in experimental sized systems ($>$100 nm long and $>$15 nm wide). It is found that GNR thermal conductivity responds similarly to edge roughness and moderate defect concentrations (0.0023) with a drastic reduction (81\%) in lattice thermal conductivity, compared to pristine GNR value. Conversely, the presence of randomly oriented defects completely erodes the ballistic nature of the electrons, reducing conductance by two orders of magnitude, while edge roughened structures leave the electrical conductance intact.   

First principles study of transport properties of pristine and passivated bilayer graphene nanoribbons

Transport properties of pristine and hydrogen passivated bilayers of zigzag-edged graphene nanoribbons (ZGNRs) coupled with gold electrodes are investigated using first-principles methods based on density-functional theory. The calculated ground state of the passivated bilayer 6-ZGNRs is non-magnetic and the antiferromagnetic coupling is energetically preferred for the pristine counterpart. The results of the bias and spin-dependent electron transmission and current calculated using the nonequilibrium Green's function formalism will be presented. The role of interlayer interaction in determining the I-V characteristics of bilayer graphene nanoribbons will also be discussed.   

1/f noise as a probe to investigate the band structure of graphene

The flicker noise or low frequency resistance fluctuations in graphene depend explicitly on its ability to screen external potential fluctuations and more sensitive compared to the conventional time average transport. Here we show that the flicker noise is a powerful probe to the band structure of graphene that vary differently with the carrier density for the linear and parabolic bands. We have used different types of graphene field effect devices in our experiments which include exfoliated single and multilayer graphene on oxide substrate, freely suspended single layer graphene, and chemical vapor deposition (CVD)-grown graphene on SiO$_{2. }$We find this difference to be robust against disorder or existence of a substrate. Also, an analytical model has been developed to understand the mechanism of graphene field effect transistors. Our results reveal the microscopic mechanism of noise in Graphene Field Effect Transistors (GraFET), and outline a simple portable method to separate the single from multi layered graphene devices. References A. N. Pal and A Ghosh, Phys Rev. Lett~102, 126805 (2009). A. N. Pal and A.~Ghosh, Appl. Phys. Lett$.,$ 95, 082105 (2009). A. N. Pal, A. A. Bol, and A. Ghosh, Appl. Phys. Lett$.$ 97, 133504 (2010). A. N. Pal et al., arXiv: 1009.5832v2.   

Charge Trapping and Transport in Epitaxial Graphene

A thorough characterization of the electronic transport behavior of charge carriers in graphene that is epitaxially grown on the silicon face of 6H(0001) SiC is presented. A nonlinear temperature dependence of the carrier density is observed, and is attributed to the presence of charge traps in the material. Observation of this trapping effect has been previously unidentified, and gives critical information about the material properties of epitaxially grown graphene. The nature of the electrostatic screening associated with these traps is evaluated using zero screening, full screening, and RPA screening approximations, and it is found that the zero screening approximation best describes the measurements. Electrostatic homogeneity of this material allows for exceptionally low carrier densities to be attained, where the carrier mobility sharply increases. The entire mobility profile can be phenomenologically simulated assuming Coulomb and short-range scattering as the dominant scattering mechanisms at low temperatures. Based on this result, the temperature independent residual impurity concentration of this material can be directly extrapolated.   

Transport Through Andreev Bound States in a Graphene Quantum Dot

We have performed transport measurements on a graphene-insulator-superconductor junction, and report the direct observation of sharp, gate-tunable Andreev bound states (ABS) in a graphene quantum dot (QD)[1]. The quantum dot is formed underneath the superconducting lead by local gating due to a work-function mismatch. We show that the ABS form when the discrete QD levels are proximity coupled to the superconducting contact. We find subgap resonant features which are remarkably narrow, can be tuned to zero energy by gating, and show a striking pattern as a function of applied bias and gate voltage. \\[4pt] [1] T. Dirks et al.,arXiv:1005.2749 (2010)   

Transport Properties of Graphene on Strontium Titanate

We have investigated transport properties of mechanically exfoliated graphene on strontium titanate(STO) in ultra high vacuum at cryogenic temperatures. Field- and temperature-dependent dielectric constant of STO is used to measure the impact of substrate-induced screening on transport properties of graphene.   

Session V38: Focus Session: The Physics of Evolution II

Natural Selection in Large Populations

I will discuss theoretical and experimental approaches to the evolutionary dynamics and population genetics of natural selection in large populations. In these populations, many mutations are often present simultaneously, and because recombination is limited, selection cannot act on them all independently. Rather, it can only affect whole combinations of mutations linked together on the same chromosome. Methods common in theoretical population genetics have been of limited utility in analyzing this coupling between the fates of different mutations. In the past few years it has become increasingly clear that this is a crucial gap in our understanding, as sequence data has begun to show that selection appears to act pervasively on many linked sites in a wide range of populations, including viruses, microbes, \textit{Drosophila}, and humans. I will describe approaches that combine analytical tools drawn from statistical physics and dynamical systems with traditional methods in theoretical population genetics to address this problem, and describe how experiments in budding yeast can help us directly observe these evolutionary dynamics.   

Understanding Biological Fitness From First Principles

This abstract not available.   

Geometry Genetics and Evolution

Darwin argued that highly perfected organs such as the vertebrate eye could evolve by a series of small changes, each of which conferred a selective advantage. In the context of gene networks, this idea can be recast into a predictive algorithm, namely find networks that can be built by incremental adaptation (gradient search) to perform some task. It embodies a ``kinetic'' view of evolution where a solution that is quick to evolve is preferred over a global optimum. Examples of biochemical kinetic networks were evolved for temporal adaptation, temperature compensated entrainable clocks, explore-exploit trade off in signal discrimination, will be presented as well as networks that model the spatially periodic somites (vertebrae) and HOX gene expression in the vertebrate embryo. These models appear complex by the criterion of 19th century applied mathematics since there is no separation of time or spatial scales, yet they are all derivable by gradient optimization of simple functions (several in the Pareto evolution) often based on the Shannon entropy of the time or spatial response. Joint work with P. Francois, Physics Dept. McGill University.   

TBD

This abstract not available.   

Dynamical Mueller's Ratchet: Population Size Dependence of Evolutionary Paths in Bacteria

Experimental evolution has recently enabled the complete quantitative description of small-dimensional fitness landscapes. Quasispecies theory allows the mathematical modeling of evolution on such a landscape. Typically, analytic solutions for these models are only exactly solvable for the case of an infinite population. Here we use a functional integral representation of population dynamics and solve it using the Schwinger Boson method. This allows us to compute the first-order correction to the average fitness for finite populations. We will use these results to explain the experimental observations of dynamics of evolution in finite populations.   

At the crossroads of biophysics and evolution: protein robustness and evolvability

Proteins consist of only 20 different amino acids with modest chemical reactivity, but perform a breathtaking range of functions. How do proteins achieve such functional versatility? Novel insights are emerging from research at the interface of protein biophysics and molecular evolution. Proteins are robustness against point mutations: most mutations do not abolish function. How can such robustness be reconciled with the effective evolution of protein function? We examine these issues using photoactive yellow protein (PYP), a prototype of the PAS domain superfamily. High-throughput biophysical measurements of active site properties, functional kinetics, stability, and production level on libraries of PYP mutants reveal that almost all mutants retain photocycle activity, but that the majority of substitutions significantly alter functional properties. Thus, PYP combines robustness with evolvability. The data also reveal the mysterious role of the conserved residues that define protein superfamilies: most PAS-conserved residues are required for maintaining protein production. Asn43, the most conserved residue in PAS domains, regulates PYP signaling kinetics. This residue is often substituted by Ser, Asp, and Thr in PAS domains while retaining two side chain hydrogen bonds. Thus, not residue identity at position 43 but the pattern of side chain hydrogen bonds is conserved.   

Deciphering evolutionary instructions for specifying protein fold and function

Classical studies show that proteins have evolved to fold into functional native states that are, at best, only marginally stable through weak non-covalent interactions encoded by their primary sequences. How such fold and functional information is stored in a single amino acid sequence remains elusive. Using the statistical analysis of covariation between pairs of amino acids at all positions in a protein, here we identify groups of a few key physically-interconnected residues, which we term sectors. What information about the fold and function is captured by sectors? Using simulated-annealing Monte Carlo, we introduce variation in the sequence of a single member of the PDZ family in a manner that either preserves or disrupts sector correlations. Experimentally we show that function is specifically retained in designed proteins that obey sector correlations, and strikingly, even in the absence of a native state. Thus, we suggest that native-state stability is not a fundamental requirement for function, and is encoded in the sequence in an idiosyncratic manner in the PDZ family.   

Session V39: Cellular Biomechanics

Three-dimensional traction force distribution in migrating amoeboid cells

We have studied the 3D traction forces exerted by migrating \textit{Dictyostelium} cells moving over flat elastic substrates. For that purpose, we have developed a method to calculate both vertical and tangential cell traction forces from measurements of 3D substrate deformation, based on the solution of the elastostatic equation for a linearly elastic medium. 3D substrate deformation is measured by applying correlation techniques to a volume of substrate containing fluorescent markers. We have performed experiments for wild-type (WT) and mutant cell lines with crosslinking defects to study how cytoskeletal organization affects the overall distribution of traction forces. We find that cells push the substrate downwards near their center and pull upwards at their periphery with forces of comparable magnitude. Our initial findings show that the effect of the crosslinking mutations on the tangential forces do not necessarily predict the effect on the vertical forces. For instance, myosin II-null cells show a significant reduction of the front-back organization of the tangential traction forces, while the distribution of vertical forces basically remains unaffected.   

The Physics Of Cell Crawling In Elastic Media

Understanding the motion of cells through deformable media, such as the extra-cellular matrix (ECM), is important for understanding many biological processes, such as cancer metastasis, wound healing, and organismal development. We propose a model to understand the cells' movements through ECM, described by an elastic medium. The deformations and the stress tensor are then calculated for different values of Young's modulus and Poisson's ratio. The results are then compared to the values measured experimentally.   

How Deep Cells Feel

Lacking eyes to see and ears to hear, cells can still sense their microenvironment by physically touching and deforming, thus sensing not only their immediate surroundings but also feeling beyond the cell-matrix interface. To elucidate how deeply cells feel we cultured mesenchymal stem cells on gels-made microfilms with controlled elasticity (E) and thickness (h). After 36hrs in culture cells spread area was smaller on thick and on soft than on thin and on stiff films, respectively, and correlated with nuclei morphology. Transition in spread area was obtained at $<$5 microns gel thickness. Transcription levels of Lamin-A predominantly decreased with E and in a similar fashion to Lamin-A expression levels increased with h. RNA levels of histones and of chromatin-remodeling enzymes were similar for stiff gels and for soft but thin films but suppression of cell contractility resulted in transcriptional profiles that were uncorrelated with matrix-emerging cues. We conclude that cells actively sense up to 20 microns into soft, adipose-like matrix. Cellular response to E and h includes cytoskeletal reorganization, NE remodeling with evidence of coupling between matrix-emerging signals and regulation of gene expression   

Delineating cell-matrix interaction at high resolution

It is increasingly evident that mechanic cues affect a wide variety of cells and can sometimes override biochemical cues to control cell division, cell death and even specify stem cell differentiation lineage. To understand how cells interact physically with their surrounding matrix, it is imperative to investigate the spatiotemporal distribution of forces and molecular players as cells undergo contractile activity. We examine human mesenchymal stem cell contractility at high temporal and spatial resolution on soft and hard substrates.   

How substrate rigidity regulates the cellular motility

Mechanical stiffness of bio-adhesive substrates has been recognized as a major regulator of cell motility. We present a simple physical model to study the crawling locomotion of a contractile cell on a soft elastic substrate. The mechanism of rigidity sensing is accounted for using Schwarz's two spring model (Schwarz et al. (2006) BioSystems 83, 225-232). The predicted dependency between the speed of motility and substrate stiffness is qualitatively consistent with experimental observations. The model demonstrates that the rigidity dependent motility of cells is rooted in the regulation of actomyosin contractile forces by substrate deformation at each anchorage point. On stiffer substrates, the traction forces required for cell translocation acquire larger magnitude but show weaker asymmetry which leads to slower cell motility. On very soft substrates, the model predicts a biphasic relationship between the substrate rigidity and the speed of locomotion, over a narrow stiffness range, which has been observed experimentally for some cell types.   

Local nano-mechanical properties in cancer metastasis

We investigated whether the local nano-mechanical properties of cells can represent metastatic potential using the Atomic Force Microscope. As models, we used the lowly (LNCaP) and highly (CL1) metastatic prostate cancer cells. By varying the applied forces, we determined the heterogeneity in the local elastic properties of cells in the vertical direction. We also obtained the 2D array of the force-distance curves over the entire region of cells to investigate the lateral heterogeneity of local elastic moduli. By analyzing the force-distance curves using the Hertz and the advanced models, we delineated the 2D maps of elastic moduli and adhesiveness of cells. We found that the CL1 is more heterogeneous in the local elastic moduli compared to LNCaP. We also found that the CL1 adheres much better on the substrates than the LNCaP. The enhanced adhesion generates the tensional force and thus results in higher elastic moduli. We conclude that there is an optimal range of elastic moduli to make cells actively elicit the directional movements, leading to the enhance metastasis. We will discuss our results correlated with our intercellular calcium transit.   

Cell Shape Dynamics: From Waves to Migration

We analyzed the dynamic shape of migrating Dictyostelium discoideum cells. We found that regions of high boundary curvature propagate from the front to the back of cells in an organized fashion. These waves of high curvature are stabilized by surface contact, and so, at the sides of cells, are stationary relative to the surface. The initiation of curvature waves, though, which usually occurs at the front of cells, is associated with protrusive motion. The protrusion location shifts rapidly in a ballistic manner at speeds nearly double that of cellular migration. To examine curvature waves in the absence of surface contact, we guided cells to extend over the edge of micro-cliffs. The curvature wave speed of cells extended over a cliff was triple the wave speed of cells migrating on a surface, which is consistent with the higher wave speeds observed near the non-adherent leading edge of cells.   

Self-organized cell motility

Cell migration plays a key role in a wide range of biological phenomena, such as morphogenesis, chemotaxis, and wound healing. Cell locomotion relies on the cytoskeleton, a meshwork of filamentous proteins, intrinsically out of thermodynamic equilibrium and cross-linked by molecular motors, proteins that turn chemical energy into mechanical work. In the course of locomotion, cells remain polarized, i.e. they retain a single direction of motion in the absence of external cues. Traditionally, polarization has been attributed to intracellular signaling. However, recent experiments show that polarization may be a consequence of self-organized cytoskeletal dynamics. Our aim is to elucidate the mechanisms by which persistent unidirectional locomotion may arise through simple mechanical interactions of the cytoskeletal proteins. To this end, we develop a simple physical description of cytoskeletal dynamics. We find that the proposed description accounts for a range of phenomena associated with cell motility, including spontaneous polarization, persistent unidirectional motion, and the co-existence of motile and non-motile states.   

Differentiation and Behavior of Dental Pulp Stem Cells in Hydrogel Scaffolds of Various Stiffnesses

Dental Pulp Stem Cells (DPSCs) are known to differentiate in bone, dentine, or nerve tissue through different environment signals. This work investigates whether differentiation could occur in the absence of chemical induction and through mechanical stimuli only. For this study, we chose enzymatically cross-linked gelatin hydrogels as our substrates. Rheological studies carried out by oscillatory shear rheometry indicated that the modulus of the hardest hydrogel was of the order of 8kPa where as the medium and the softest hydrogel had modulus of the order of 1kPa and 100Pa respectively. DPSC were then plated on all three substrates and cultured with and without dexamethasone induction media. After 21 days of incubation, SEM analysis indicated that the cells cultured in the induction media produced biomineralized deposits on hard, medium as well as soft hydrogels. On the other hand, the cells cultured without the induction media also produced large amounts of biomineralized deposits.The modulus of the cells was also measured using AFM. En mass cell migration was also studied to determine the average velocity of migration of DPSCs. We also investigated whether stem cells that are induced to differentiate by their scaffold environment would continue to differentiate and biomineralize when removed from the inducing scaffold.   

Model of myosin recruitment to the cell equator for cytokinesis: feedback mechanisms and dynamical regimes

The formation and constriction of the contractile ring during cytokinesis, the final step of cell division, depends on the recruitment of motor protein myosin to the cell's equatorial region. During animal cell cytokinesis, cortical myosin filaments (MF) disassemble at the flanking regions and concentrate in the equator [1]. This recruitment depends on myosin motor activity and the Rho proteins that regulate MF assembly and disassembly. Central spindle and astral microtubules help establish a spatial pattern of differential Rho activity [2]. We propose a reaction-diffusion model for the dynamics of MF recruitment to the equatorial region. In the model, the central spindle and mechanical stress [3] promote self-reinforcing MF assembly. Negative feedback is introduced by MF-induced recruitment of inhibitor myosin phosphatase. Our model yields various dynamical regimes and explains both the recruitment of MF to the cleavage furrow and the observed damped MF oscillations in the flanking regions [1], as well as steady MF assembly [4]. Space and time parameters of MF oscillations are calculated. We predict oscillatory relaxation of cortical MF upon removal of locally-applied external stress. [1] Zhou {\&} Wang, Mol. Biol. Cell \textbf{19}:318 (2008); [2] Murthy {\&} Wadsworth, J. Cell Sci. \textbf{121}:2350 (2008); [3] Ren et al., Curr. Biol. \textbf{19}:1421 (2009); [4] Vale et al., J. Cell Biol. \textbf{186}:727 (2009)   

Biomechanics and dynamics of red blood cells probed by optical tweezers and digital holographic microscopy

Red blood cells (RBC), with their unique viscoelastic properties, can undergo large deformations during interaction with fluid flow and migration through narrow capillaries. Both local and overall viscoelastic property is important for cellular function and change in these properties indicate diseased condition. Though biomechanics of the cells have been studied using variety of physical techniques (AFM, optically-trapped anchoring beads and microcapilary aspiration) in force regime $>$ 10pN, little is studied at low force regime $<$1pN. Such perturbations are not only hard to exercise on the cell membrane, but quantification of such deformations becomes extremely difficult. By application of low power optical tweezers directly on cell membrane, we could locally perturb discotic RBC along the axial direction, which was monitored dynamically by digital holographic microscopy-a real time, wide-field imaging method having nm axial resolution. The viscoelastic property of the RBC at low force regime was found to be significantly different from that of high-force regime. The results were found to be in good agreement with the simulation results obtained using finite element model of the axially-stretched RBC. The simulations and results of viscoelestic measurements will be presented.   

Effect of Surface Adhesion on Individual and Collective Migration

Cell-surface adhesion plays a critical role in amoeboid cell motion by supplying the traction allowing a cell to move itself forward.~ The amoeba Dictyostelium discoideum, a model system for individual and collective cell migration, naturally exhibits both cell-substrate and cell-cell adhesion during the aggregation process.~ We used both high- and low-magnification time-lapse microscopy to investigate the individual and collective migration of D. discoideum on substrates of varying adhesiveness, as well as on interfaces between surfaces.~ We find that surface adhesion can affect both individual cell migration as well as the behavior of cell groups.~ At the population scale, non-ideal surfaces slow down the initiation of aggregation and change the aggregation dynamics. At the scale of single cells, we measure both adhesion ability as well as the area of contact between cells and surface for individual cells and cells that are part of groups.~ We find that comparable forces are needed to pull cells off all surfaces, indicating that surface adhesion is actively regulated by migrating cells.   

Importance of spectrin network reorganization in computer simulations of RBC shapes

The shape of red blood cells (RBCs) has been the subject of intensive investigations in both experiments and theoretical models. Various computational models for RBCs have also been developed. However, a rigorous quantitative comparison of the observed shapes is still lacking. We have developed a flexible model that allows to study the influence of the various contributions to the membrane stress and their relevance for RBC shape. Our model reveals that a pure curvature model does not fully explain the experimentally observed discocyte shapes. We demonstrate that the in-plane stresses of the spectrin network have a crucial effect on the cell shapes and their transitions, and that the dynamic relaxation of the stresses due to spectrin reorganization is important. We present an extended model that incorporates the effects of dynamic spectrin remodeling and study their role on the dynamics of RBC shapes.   

The contribution of cytoskeleton networks to stretch is strain dependent

The interaction between the cytoskeleton filaments in a cell provides it with mechanical stability and enables it to remodel its shape. The rheological response of cells has been characterized either as viscoelastic or soft-glassy which neglects the molecular origin of cell response. In this work, by using a large amount of cells ($>$10,000 in total) exceeding previous statistics by a decade, we link observed cell response to its molecular origin by showing that actin and microtubule networks maintain the mechanical integrity of cells in a strain dependent manner. While the actin network solely regulated cell deformation at small strain, the microtubule network was responsible for cell relaxation. At large strain, actin and microtubule networks dominated cell response with microtubules having a bipolar effect on cells upon stabilization. This effect could explain the relapse of some cancer after chemotherapy treatment using Taxol thus providing a bridge between soft condense matter physics and systems biology.   

Magnetic Carbon nanoparticles enabled efficient photothermal alteration of mammalian cells

While cw near-infrared (NIR) laser beams have been finding widespread application in photothermal therapy of cancer and pulsed NIR laser microbeams are recently being used for optoporation of exogeneous impermeable materials into cells. Since, carbon nanomaterials are very good in photothermal conversion, we utilized carbon nanoparticles (CNP) doped with Fe, so that they can be localized in a defined area by two fold selectivity, (i) external magnetic field for retention of the CNP in targeted area and (ii) surface functionalization for binding the targeted cells. Here, we report efficient photothermal therapy as well as poration of cells using magnetic CNPs with very low power continuous wave laser beam. Localization of CNPs on cell membrane under application of magnetic field was confirmed by scanning electron microscopy. At different power levels, cells could be damaged or microinjected with fluorescence protein-encoding plasmids or impermeable dyes. Monte Carlo simulation showed that the dose of NIR laser beam is sufficient to elicit response for magnetic CNP based photothermal treatment at significant depth. The results of our study suggest that magnetic CNP based photothermal alteration is a viable approach to remotely guide treatments offering high efficiency with significantly reduced cytotoxicity.   

Session V40: Thesis Award Session: Nucleic Acids -- Structure, Function, and the Genome

2010 Award for Outstanding Doctoral Thesis Research in Biological Physics Talk: How the Genome Folds

I describe Hi-C, a novel technology for probing the three-dimensional architecture of whole genomes by coupling proximity-based ligation with massively parallel sequencing. Working with collaborators at the Broad Institute and UMass Medical School, we used Hi-C to construct spatial proximity maps of the human genome at a resolution of 1Mb. These maps confirm the presence of chromosome territories and the spatial proximity of small, gene-rich chromosomes. We identified an additional level of genome organization that is characterized by the spatial segregation of open and closed chromatin to form two genome-wide compartments. At the megabase scale, the chromatin conformation is consistent with a fractal globule, a knot-free conformation that enables maximally dense packing while preserving the ability to easily fold and unfold any genomic locus. The fractal globule is distinct from the more commonly used globular equilibrium model. Our results demonstrate the power of Hi-C to map the dynamic conformations of whole genomes.   

Recognition Tunneling towards Next Generation Single Molecule DNA Sequencing

A novel approach has been developed to trap and sequence DNA within a molecular junction formed by a pair of functionalized Au electrodes so individual DNA nucleotides could be recognized on small pieces of DNA with single base resolution. The cost of labeling reagents is totally eliminated since different nucleotides are recognized through their intrinsic physical properties. Unexpectedly long residence time of DNA (on the order of a second) in the molecular gap is observed which indicates that a pN force is required to achieve the sequencing speed as fast as 10 bases/Sec. Providing such ionic driving force, a nanopore device incorporated with recognition tunneling reader will provide a revolutionary way for fast, accurate and economic next generation DNA sequencing.   

Electronic Signatures of all Four DNA Nucleosides in a Tunneling Gap

New approaches to DNA sequencing are required to reduce costs and increase the availability of personalized genomics. Using Scanning Tunneling Microscope as a tool, we report measurements of the current signals generated as free nucleosides diffuse into a tunnel junction in which both electrodes are functionalized with a reagent that presents a hydrogen bond donor and acceptor to the nucleosides. This functionalization serves to both limit the range of molecular orientations in the tunnel gap and reduce the contact resistance, increasing the selectivity of the tunneling signal, so that a direct readout may be possible with a few repeated reads.   

Gel electrophoresis of partially denatured DNA: split-ends, bubbles, and squids

Gel electrophoresis separates partially denatured DNA fragments based on chemical sequence. Upon an increase in temperature, AT-rich regions melt into two strands which is thought to be the main contributor to the rapid reduction of the fragment's mobility. The reduction in mobility is often predicted from the average number of denatured bases regardless of their positions. We re-visit the theoretical basis of this approach and determine that the analysis only holds for denatured domains that occur at the ends. Langevin Dynamics simulations are used to study the effect that the placement of the melted regions has on the mobility by discriminating between denatured domains which occur in the middle of the fragment (bubbles) and at the ends (split-ends). It is found that the split-ends dominate the blocking mechanism. In addition, we find a novel conformation (the ``squid'') which seems to be responsible for the blocking at high fields.   

Temperature dependence of two distinct DNA overstretching transitions

Recent experiments show that two distinct transitions are involved in the DNA overstretching that occurs at around 65 pN: a strand-unpeeling transition leading to strand separation from free DNA ends or nicks, and a B to S transition leading to an overstretched double-stranded DNA called ``S-DNA.'' Here we show that the two transitions have distinct temperature dependence: in the strand-unpeeling transition, the transition force decreases when the temperature increases; while in the B to S transition, the temperature dependence of the transition force is opposite. Our results are in agreement with the notion that the two transitions involve distinct types of double-helix reorganization.   

An Analytic Theory for Single Molecule Manipulation of DNA

We introduce a minimal, analytically solvable model for thermomechanical behavior of DNA under tension and torque, and predict critical temperature for denaturation at unwinding and overwinding, phase diagrams for stable b--DNA, and supercoling-elongation curves as functions of applied torque, tension and temperature. Our results are in agreement with experimental data from experiments in single molecule manipulation. We also propose simple thermodynamical formulae for temperature, tension, torque, and supercoiling at criticality.   

Multimerization of DNA Origami Structures in Two Dimensions

DNA nanotechnology, here in particular DNA origami, is based on self-assembly and can be used to construct arbitrary three-dimensional structures with nanometer precision. The dimensions of such DNA origami structures are typically on the order of a hundred nanometers or smaller. To achieve large-scale two-dimensional lattices that could be employed as scaffolds for crystalline arrangement of biomolecules and proteins, individual DNA origami tiles need to be assembled hierarchically. We work on the multimerization of DNA origami structures into extended one- and two- dimensional lattices that can cover areas of several square micrometers. This is achieved by complementary single stranded DNAs (sticky ends) at specific positions on the DNA origami objects that we intend to grow into periodic structures. We study the effect of varying multimerization conditions such as annealing temperatures, length of sticky ends and salt concentration on the quality and size of the resulting lattice.   

Fluctuation Pressure Assisted Ejection of DNA From Bacteriophage

The role of thermal pressure fluctuations excited within tightly packaged DNA while it is ejected from protein capsid shells is discussed in a model calculation. At equilibrium before ejection we assume the DNA is folded many times into a bundle of parallel segments that forms an equilibrium conformation at minimum free energy, which presses tightly against capsid walls. Using a canonical ensemble at temperature T we calculate internal pressure fluctuations against a slowly moving or static capsid mantle for an elastic continuum model of the folded DNA bundle. It is found that fluctuating pressures on the capsid from thermal excitation of longitudinal acoustic vibrations in the bundle whose wavelengths are exceeded by the bend persistence length may have root-mean-square values that are several tens of atmospheres for typically small phage dimensions. Comparisons are given with measured data on three mutants of lambda phage with different base pair lengths and total genome ejection pressures.   

Effects of sequence on DNA wrapping around histones

A central question in biophysics is whether the sequence of a DNA strand affects its mechanical properties. In epigenetics, these are thought to influence nucleosome positioning and gene expression. Theoretical and experimental attempts to answer this question have been hindered by an inability to directly resolve DNA structure and dynamics at the base-pair level. In our previous studies we used a detailed model of DNA to measure the effects of sequence on the stability of naked DNA under bending. Sequence was shown to influence DNA's ability to form kinks, which arise when certain motifs slide past others to form non-native contacts. Here, we have now included histone-DNA interactions to see if the results obtained for naked DNA are transferable to the problem of nucleosome positioning. Different DNA sequences interacting with the histone protein complex are studied, and their equilibrium and mechanical properties are compared among themselves and with the naked case.   

A quantitative model of nucleosome dynamics

The expression, replication and repair of eukaryotic genomes require the fundamental organizing unit of chromatin, the nucleosome, to be unwrapped and/or disassembled. We have developed a quantitative model of nucleosome dynamics which provides a fundamental understanding of these DNA processes. We calibrated this model using results from high precision single molecule nucleosome unzipping experiments, and then tested its predictions for experiments in which nucleosomes are disassembled by the DNA mismatch recognition complex hMSH2-hMSH6. We found that this calibrated model quantitatively describes hMSH2-hMSH6 induced disassembly rates of nucleosomes with two separate DNA sequences and four distinct histone modification states. In addition, this model provides mechanistic insight into nucleosome disassembly by hMSH2-hMSH6 and the influence of histone modifications on this disassembly reaction. This model's precise agreement with current experiments suggests that it can be applied more generally to provide important mechanistic understanding of the numerous nucleosome alterations that occur during DNA processing.   

Fitness and structure landscapes for pre-miRNA processing

The processing from pre-miRNA to mature miRNA in plants involves a mechanism, which depends on an extended stem in the secondary structure of the pre-miRNA. Here, we show how natural selection acts on this secondary structure to produce evolutionary conservation of the processing mechanism together with modularity of the pre-miRNA molecules, making this molecular function independent of others. Our main results are: 1. Selection on miRNA processing can be described by a fitness landscape which depends directly on the secondary structure of the pre-miRNA. 2. This fitness landscape predicts the divergence of the phenotype between orthologous pre-miRNA molecules from different species. 3. Actual pre-miRNA structures are modular: their phenotype is significantly less affected by deleterious mutations in the remainder of the molecule than for random RNA molecules.   

DNA-damage by free radicals in solution

We employ a molecular simulation based on GROMACS-CPMD QM/MM method to study the initial damage to a fragment of DNA-molecule in the solution by ionizing radiation. We illustrate that the diatomic OH-radicals that are primary product of megavoltage ionizing radiation in water-based systems form a network of hydrogen bonds with the nearby water molecules. Our molecular simulation illustrates that the Hydrogen bonds strongly alter the relative orientation of the OH-radicals and DNA molecule. This results to an angular anisotropy in the chemical pathway and a lower efficiency in the hydrogen abstraction mechanisms than previously anticipated for identical system in the vacuum. We illustrate that the thermal fluctuations of the water molecules in the solution strongly compete with the H-abstraction that shows more energitically favorable in solution than in vacuum. As a result the chemical reaction takes place with slower rate in solution than in vacuum.   

Transport properties of nucleotides in a graphene nanogap for DNA sequencing

The application of graphene nanogaps for DNA sequencing has been proposed [H. W. Ch. Postma, Nano Lett. 10, 420 (2010)]. We used density functional theory and the non-equilibrium Green's function method to study the electron transport properties of nucleotides located inside a graphene nanogap. Our setup considered different positions and orientations of the bases with respect to the graphene electrodes, and we analyzed how the transmission spectra depend on such shifts and rotations. Even when taking into account current changes due to base fluctuations, we find that each nucleotide possesses a different characteristic current magnitude, owing to its distinctive electronic properties. Based on our results, it thus seems that the electrical readout from a graphene nanogap could in principle be sufficiently sensitive to distinguish between the four nucleotides, and thus achieve the goal of rapid and economical whole-genome sequencing.   

Session V41: Focus Session: The Role of Water in Energy Production and Utilization II

The structure of water/hydroxyl phases at metal interfaces

On all but the least reactive metals, the first contact layer with water is a mixture of water and hydroxyl, often formed by spontaneous dissociation [1]. Understanding the composition and stability of these layers is a key step in describing both the wetting and the redox behavior of the surface. Here we discuss the wetting of Cu(110) and the formation of mixed water-hydroxyl layers by reaction with adsorbed O. This surface does not conform to the traditional hexagonal symmetry associated with an ice Ih, and the hydrogen bonding structure must accommodate to the surface symmetry. A number of unusual structures are seen, including 1D chains of interlocking pentagons [2], an intact 2D network with a (7x8) unit cell at higher coverage [3] and several partially dissociated structures, including both 1D and 2D phases [4]. The composition of these structures and hydrogen bonding arrangements will be discussed, highlighting the way changing the composition and relative metal-adsorbate and adsorbate-adsorbate interactions drives the structural rearrangement of these phases. \\[4pt] [1] A. Hodgson and S. Haq, Surf. Sci. Rep., 64(9), 381 (2009). \\[0pt] [2] J. Carrasco et al. Nat. Mat., 8, 427 (2009). \\[0pt] [3] T. Schiros et al., Chem. Phys. Lett., 429, 415 (2006). \\[0pt] [4] M. Forster et al., submitted.   

Quantum nature of the proton in water-hydroxyl overlayers on metal surfaces

Using \textit{ab initio} path integral molecular dynamics we show that water-hydroxyl overlayers on transition metal surfaces exhibit surprisingly pronounced quantum nuclear effects. The metal substrates serve to reduce the classical proton transfer barriers within the overlayers and, in analogy to ice under high pressure, to shorten the corresponding intermolecular hydrogen bonds. Depending on the substrate and the intermolecular separations it imposes, the traditional distinction between covalent and hydrogen bonds is lost partially (e.g. on Pt(111) and Ru(0001)) or almost entirely (e.g. on Ni(111)). We suggest that these systems provide an excellent platform on which to systematically explore the magnitude of quantum nuclear effects in hydrogen bonds.   

Water adsorption on oxygen covered Ru(0001) surfaces

We present a combined scanning tunneling microscopy (STM) and density functional theory (DFT) study of the adsorption of water on a Ru(0001) surface covered with half monolayer of oxygen. The adsorption of water causes a shift of half of the oxygen atoms in the O(2x1) structure from hcp sites to fcc sites, creating a honeycomb structure where water molecules bind strongly to the exposed Ru atoms [1]. The energy cost of reconstructing the oxygen overlayer is more than compensated by the larger adsorption energy of water on the newly exposed Ru atoms. STM images reveal a (4x2) superstructure due to alternating orientations of the water molecules. Heating to 185 K results in the complete desorption of the water layer, leaving behind the oxygen honeycomb structure, which is metastable relative to the original (2x1). This stable structure is not recovered until after heating to temperatures close to 260K. \\[4pt] [1] S. Maier et al. Phys. Rev. B 82, 075421 (2010).   

Water monolayers on metals -- a new framework

The first wetting layer on a solid embodies the boundary condition for water transport along its surface, is the template for ice nucleation, and governs aqueous surface chemistry. Today's talk is focused on the wetting of close-packed, precious metal surfaces, which are both relatively easily prepared, and susceptible to study by a host of surface science techniques. For decades, wetting layers on such surfaces have been thought to be ``ice-like'' -- strained into registry with the metal lattice, but otherwise like the layers that stack to form the naturally occurring crystal, ice Ih. Interpretations of STM images of periodic wetting layers on Pt(111) [1], of TPD [2], and of IR absorption spectra [3] contradict the ``ice-like'' picture, but submit to a common, physics-based and DFT-supported interpretation. It is that several ice-like hexagonal rings of H$_{2}$O molecules, per repeated cell, are replaced by pentagons and heptagons, allowing a compact subset of H$_{2}$O's, with planes parallel to the metal surface, to approach the metal exceptionally closely and to anchor the wetting layer strongly to it. This motif, amounting to formation of energetically favorable di-interstitial ``defects,'' appears to be general; similar molecular arrangements account for what we know experimentally (and, largely, could not previously explain) of water bonding to Ni, Ru, and Pd close-packed surfaces. \\[4pt] [1] S. Nie, P. J. Feibelman, N.C. Bartelt, and K. Th\"{u}rmer, Phys. Rev. Lett. ~\textbf{105}, 026102(2010). \\[0pt] [2] P. J. Feibelman, N.C. Bartelt, S. Nie, and K. Th\"{u}rmer. ~J. Chem. Phys. \textbf{133}, 154703(2010). \\[0pt] [3] P. J. Feibelman, G. A. Kimmel, B. D. Kay, N. Petrik, R. S. Smith and T. Zubkov, unpublished.   

First-principles simulation of water on graphene using different levels of theory

We show results from molecular-dynamics simulations of water confined between flat graphene sheets using different levels of theory. DFT simulations using PBE exchange-correlation show strong layering near the graphene sheet, with the hydrogens closer to the graphene surface. Correlations die off to bulk values after $\sim $10{\AA} from the surface. Inclusion of an empirical Grimme-type van der Waals potential has a small effect on the interfacial C-H distance but a seemingly large effect from the second coordination shell onwards from the surface. Existing reactive force-fields for water, e.g. ReaxFF, do not capture the structure of water on graphene accurately and require refitting to more closely reproduce the DFT results. Molecular dynamics results with available self-consistent vDW-DFx kernels and contrasts with existing classical water models will also be presented.   

How Water Meets Graphene

The interactions of electrolyte fluids with solids control many complex interfacial processes encountered in electrochemical energy storage systems. In this talk, we will demonstrate how to develop a fundamental atomic-scale understanding of interfacial structures at the water-graphene interface, a model fluid-solid interface combination. We have performed systematic measurements of high resolution X-ray reflectivity from epitaxial graphene films in contact with electrolytes including deionized water and aqueous salt solutions. The electron density profiles and structural models from the fully analyzed data reveal the intrinsic interfacial structures. It is noted that the interfacial water structure above the first graphene layer exhibits remarkable differences with those of subsequent graphene layers. The latter one, resembling water on freestanding graphene, is well predicted by parallel computational simulations. Moreover, the pH of aqueous solutions was found to have a subtle influence on the interfacial water structure above the first graphene layer. This may well be an indication of the interfacial structural distortions that might exist in this layer, and which may play an important role in controlling the chemical activity of monolayer epitaxial graphene.   

The Dynamics of Supercooled Water

We present an overview of recent experiments performed on transport properties of water in the deeply supercooled region, a temperature region of fundamental importance in the science of water. We report data of nuclear magnetic resonance, quasi-elastic neutron scattering, Fourier-transform infrared spectroscopy, and Raman spectroscopy, studying water confined in nano-meter-scale environments (nano-tubes and the protein hydration water) and in bulk solutions. When contained within small pores, water does not crystallise, and can be supercooled well below its homogeneous nucleation temperature Th. On this basis it is possible to carry out a careful analysis of the well known thermodynamical anomalies of water. Studying the temperature and pressure dependencies of water dynamics, we show that the liquid-liquid phase transition (LLPT) hypothesis represents a reliable model for describing liquid water. In this model, water in the liquid state is a mixture of two different local structures, characterised by different densities, namely the low density liquid (LDL) and the high-density liquid (HDL). The LLPT line should terminate at a special transition point: a low-T liquid-liquid critical point. In particular We discuss the following experimental findings on liquid water: (i) a crossover from non-Arrhenius behaviour at high T to Arrhenius behaviour at low T in transport parameters; (ii) a breakdown of the Stokes-Einstein relation; (iii) the existence of a Widom line, which is the locus of points corresponding to maximum correlation length in the p-T phase diagram and which ends in the liquid-liquid critical point; (iv) the direct observation of the LDL phase; (v) a minimum in the density at approximately 70K below the temperature of the density maximum. In our opinion these results represent the experimental proofs of the validity of the LLPT hypothesis.   

Structural, Dynamic, and Spectroscopic Properties of the SCC-DFTB Water Model

The interactions of water and many interfaces are not understood at a mechanistic level. The accuracy of simulations of the system are limited by the accuracy of the water model used. Classical models such as SPC/E use empirically derived parameters to match their behavior to desired bulk water properties, but cannot participate in reactions that require the making or breaking of bonds. Ab initio quantum mechanical methods such as Car-Parrinello (CP) do allow water to dissociate, but are computational intractable for large systems. A potential middle ground is the self-consistent charge density-functional tight-binding method (SCC-DFTB), which has a smaller associated computational cost, and therefore can access larger systems than CP, while still allowing for the making and breaking of bonds. The DFTB+ implementation of SCC-DFTB allows for 2nd and 3rd order expansions of the density fluctuations in the energy and, in the 3rd order expansion, an optional damping correction factor. For each of these models we compare the structural, dynamic, and spectroscopic properties of bulk SCC-DFTB water to classical SPC/E and experimental results.   

Interfacial water on TiO$_2$ anatase (101) and (001) surfaces: First-principles study with TiO$_2$ slabs dipped in bulk water

Density functional molecular dynamics simulations using supercells with ``bulk'' water between the TiO$_2$ anatase (101) and (001) surfaces were carried out to elucidate the behaviour of water molecules and hydrogen bond networks on the interfaces of representative photocatalysts. It is demonstrated that the adsorption manners (molecular or dissociative) of water molecules on the vacuum surfaces still hold in the presence of ``bulk'' water on the interfaces. We also showed explicit atomistic structures of strong and weak hydrogen bonds on the TiO$_2$/water interfaces, which had been proposed experimentally so far. We then suggested a two-layer model for the interfacial water on both surfaces investigated. Our results also give insights into the H$_2$O or OH adsorption coverage on the interfaces and their hydrophobicity- hydrophilicity, which is important to understand the photocatalytic reaction mechanisms microscopically.   

Session V42: Focus Session: Supramolecular Self-Assembly--Controlling Network and Gel Formation II

Control of semi-flexible polymer networks by architecture and dynamic cross-linking

The rigidity of elastic networks depends sensitively on their internal connectivity and the nature of interactions between constituents. Particles interacting via central forces become rigid above the isostatic connectivity threshold first identified by Maxwell. Stiff or semi-flexible polymers, such as those that form the cellular cytoskeleton, develop a finite network shear modulus $G$ at a lower threshold, although the degree to which the mechanics of such networks are governed by filament bending resistance remains a subject of considerable debate. Such networks also exhibit rich viscoelastic properties. For cytoskeletal networks, there is increasing evidence that the network response is governed by the compliance and dynamics of the cross-links, many of which are transient in nature. Here we study the effects of both local network architecture and dynamic cross-linking in disordered fibrous networks. Surprisingly, the network mechanics in both 2D and 3D are still governed by the central-force isostatic point, which acts as a zero-temperature critical point. Near this point, we find divergent strain fluctuations and an associated diverging length-scale, as well as an anomalous elastic regime that exhibits fractional power-law dependence of $G$ on both fiber bending stiffness and stretch modulus. Furthermore, dynamic cross-linking gives rise to a broad, power-law viscoelastic regime at low frequency \textit{$\omega $} in which $G\sim $\textit{$\omega $}$^{1/2}$.   

Macro- and microphase separation in multifunctional supramolecular polymer networks

We develop a field-based model for a binary melt of multifunctional polymers that can reversibly bond to form copolymer networks. The mean-field phase separation behavior of several model networks with heterogeneous bonding is calculated via the random phase approximation (RPA). The extent of bonding between polymers is controlled by specified bond energies. The phase boundary calculated via RPA is the stability limit of the homogeneous disordered phase to coexisting homogeneous macrophases, for low bond strengths, and to microphases, for high bond strengths. An isotropic Lifshitz point separates these two regions along the spindodal boundary. It is demonstrated that higher functionality and higher bond strength suppresses macrophase separation due to greater connectivity between unlike species. Gelation first occurs at a bond strength higher than the Lifshitz point for tri- or higher functional polymer components.   

Equilibrium and nonequilibrium gelation in TPR protein/linker mixtures

Using simulations we model gelation in two-component systems consisting of tetratricopeptide repeat (TPR) proteins and peptide cross linkers. These have recently been shown [1] to form strong, mechanically stable gels with remarkable shape recovery - but only within narrow parameter regimes. Within our minimal, coarse grained model, we elucidate the effects of the packing fraction $\phi$, temperature $T$ and concentration ratio $r$ of TPR and cross linkers on the gel transition. Two gelation mechanisms are identified. At low $\phi$ and $T$, nonequilibrium microphase-separated gels may be formed by rapid temperature quenches. At higher $\phi$ and $T$, homogeneous equilibrium gelation occurs. At low $r$, gelation is suppressed due to depletion of linkers, while at high $r$ gelation is suppressed due to the ``coating'' of proteins by linkers. The gel transition line in the $(r,T)$ plane has an unusual, asymmetric form. We also briefly compare these results to those for a more realistic ``patchy'' model which incorporates the directional TPR-linker binding present in the experimental systems.\\[4pt] [1] T.\ Z.\ Grove \textit{et. al.}, JACS, \textbf{132}, 14024 (2010).   

Effects of inclusions on the dynamics of viscoelastic media

The modification of material properties due to the presence of inclusions in solutions and elastic composites are well known, modifying properties such as viscosity and elastic moduli. We calculate the mobility of such an inclusion in a viscoelastic medium as well as effective dynamic material properties due to the presence of such inclusions at various frequencies.   

Conformation and mechanical properties of diblock fibers

We analyze the conformations of closed diblock fibers comprised of different bending rigidities and spontaneous curvatures. In each fiber, one block is a bare polymer while the other is an adsorbed protein-filament complex. The length fraction of each component and the total fiber length is controlled by tunable chemical potentials. We analytically calculate the shape of these two-component polymers for all values of the material parameters and chemical potentials. Our results yield: a complete analytical description of all possible two-component polymer conformations in two and three dimensions, a phase portrait detailing the parameter spaces in which these shapes occur, and the identification of spontaneous transitions between shapes driven by environmental changes.   

The Role of Multiple, Reformable Parallel Bonds on the Self-healing Behavior of Dual Crosslinked Nanogel Materials

Using computational modeling, we design novel self-healing materials composed of nanoscopic polymer gel particles, or nanogels. The particles are interconnected via both labile bonds (e.g., disulfide bonds) and stronger, less reactive bonds (e.g, C-C bonds) and therefore the nanogels form a ``dual crosslinked'' network. The stable bonds provide a rigid backbone while the labile bonds allow the material to undergo a dynamic reconfiguration in response to stress. We adapt the Hierarchical Bell Model (HBM) to describe the labile bonding interactions. The HBM effectively allows us to model cases where the ligands on neighboring nanogels interact through multiple sites. We show that the introduction of a small number of labile bonds that lie in parallel significantly increases the strength of the material relative to samples crosslinked solely by the stable bonds. We also isolate an optimal range of labile interconnections that provide high-strength, tough materials that are capable of self-repair.   

Forming Reversible Gels with Triblock Polyelectrolytes: a Field-theoretic Study

Recently, two research groups have formed reversible gels using triblock polyelectrolytes (Lemmers et al. 2010; Hunt et al., in preparation). This gel formation is driven by a phenomenon called complex coacervation, in which two oppositely charged homopolymers in solution phase separate into a polymer rich phase, known as a coacervate, and a solution phase. If instead, the polymers are triblocks with a neutral midblock and charged end blocks, under appropriate conditions they will microphase separate into micelles with cores of coacervated charged groups and coronas of neutral midblocks. These neutral midblocks act as bridges between the micelles, thereby creating a gel. One of the advantages of forming gels in this way is that the coacervate domains, and thus the gel, can be easily tuned by varying parameters such as pH, salt concentration and temperature. In order to understand the microstructures and solution sensitivity of these reversible gels, we have numerically simulated field-theoretic models of triblock polyelectrolyte mixtures in an implicit solvent. Because coacervation is driven by charge correlations, the usual mean-field assumption fails, and it is necessary to study the model beyond the level of SCFT.   

Supramolecular Arrest and Activation for the Network Formation of Acid Catalyzed Epoxy Polymerization

Epoxy resins are limited currently as they must be externally activated by radiation in combination with a photo acid generator or implemented using the more ubiquitous two-component approach. Two component systems begin to react upon mixing are while UV cured epoxies are limited to situations where light can reach the monomer. We propose a novel one-component system that is externally activated with the application of heat. A considerable room temperature lifetime is attributed to the systems ability to sequester super acids through a system of hydrogen bonding coordination. The model system utilizes an alkyl glycidal ether which is made universal by the addition of crown ethers to non-ether epoxy monomers. Our supramolecular-based approach to retard and trigger the epoxy polymerization is likely to enable more widespread applications in microelectronic packaging.   

Polymer Phononic Meta-material Networks

Phononic Meta-materials (PMM) offer unique opportunities for molding the flow of phonons through artificial structuring of the material at relevant length scales; however, most structures rely on combining mechanically ``stiff'' and ``soft'' materials to create the desired phononic properties, usually focusing on resonances to stop phonon flow. Such an approach suffers from lack of scalability, placing fabrication and material compatibility constraints on technological realization. Here, we show that these constraints are unnecessary and that phonon propagation behavior relies on the fundamental requirements of avoided crossings in the frequency dispersion relations. In particular, we demonstrate 1) polymer/air PMMs possessing i) multiple complete spectral gaps (MCSG), ii) negative index bands, iii) both a complete sub-wavelength transverse gap and Bragg-type longitudinal gap and 2) waveguides of pma2 \textit{Frieze} group symmetry that possess MCGS; we verify their dispersion relation using Brillouin light scattering. This opens up the ability to develop novel integrated low-cost all-polymer phononic platforms for information processing via mesoscale polarization manipulation, filtering and superlensing.   

Formation of Anisotropic Block Copolymer Gels

Anisotropic, fibrillar gels are important in a variety of processes. Biomineralization is one example, where the mineralization process often occurs within a matrix of collagen or chitin fibers that trap the mineral precursors and direct the mineralization process. We wish to replicate this type of behavior within block copolymer gels. Particularly, we are interested in employing gels composed of cylindrical micelles, which are anisotropic and closely mimic biological fibers. Micelle geometry is controlled in our system by manipulating the ratio of molecular weights of the two blocks and by controlling the detailed thermal processing history of the copolymer solutions. Small-Angle X-ray Scattering and Dynamic Light Scattering are used to determine the temperature dependence of the gel formation process. Initial experiments are based on a thermally-reversible alcohol-soluble system, that can be subsequently converted to a water soluble system by hydrolysis of a poly(t-butyl methacrylate) block to a poly (methacrylic acid) block.   

Microstructural Organization of Elastomeric Polyurethanes with Siloxane-Containing Soft Segments

In the present study, we investigate the microstructure of two series of segmented polyurethanes (PUs) containing siloxane-based soft segments and the same hard segments, the latter synthesized from diphenylmethane diisocyanate and butanediol. The first series is synthesized using a hydroxy-terminated polydimethylsiloxane macrodiol and varying hard segment contents. The second series are derived from an oligomeric diol containing both siloxane and aliphatic carbonate species. Hard domain morphologies were characterized using tapping mode atomic force microscopy and quantitative analysis of hard/soft segment demixing was conducted using small-angle X-ray scattering. The phase transitions of all materials were investigated using DSC and dynamic mechanical analysis, and hydrogen bonding by FTIR spectroscopy.   

Viscoelastic Properties of Photo-crosslinked Shape Memory Elastomers

Lightly crosslinked polymer networks containing self-complementary hydrogen-bonding side-groups (e.g. ureidopyrimidinones, UPy) have been shown to exhibit unique shape-memory properties. Our synthetic approach, involving photo-crosslinking, enables both the crosslink density and UPy-content to be systematically varied. To better understand how hydrogen bond dynamics impact viscoelastic properties, dynamic mechanical analysis was performed on a series of photo-crosslinked elastomers. The presence of UPy side-groups imparts a broad dynamic transition over a frequency range that depends on the UPy content. The UPy side-group dynamics result in a high level of mechanical damping, and they enable damping characteristics to be tailored. Time temperature superposition (TTS) analysis was performed, and resulting shift factors show Arrhenius behavior. Activation energies were calculated, and elastomers with higher UPy content exhibit higher activation energies.   

Origins and stability of the polydomain regime in isotropic-genesis nematic elastomers

We address the physical properties of nematic elastomers that have been randomly crosslinked in the high-temperature isotropic state. We do this by constructing a replica Landau theory in terms of a coupled pair of order-parameter fields: one for vulcanization, the other for nematic order. We focus on the correlations of the trapped-in nematic fluctuations as a diagnostic of the structure of the elastomer, determining them for a range of temperatures and disorder strengths. Our analysis shows that, in fewer than four spatial dimensions, the quenched randomness associated with the crosslinking prevents the emergence of long-range order, either of the mondomain nematic or of the spatially modulated type. It also shows that, for sufficiently strong disorder and low enough temperatures, the system exhibits unusual short-range oscillatory structure in the local nematic alignment.   

Session V43: Focus Session: Translocation Through Nanopores II

The Statistics of DNA Capture and Re-Capture by Solid-State Nanopores

We studied repeated electrophoretic translocations of the same DNA molecule through $\sim $10 nm nanopores using the voltage-reversal re-capture technique. Correlations were observed in the folding conformations of molecules re-captured within the Zimm relaxation time of the polymer. A trend was observed, whereby more compact conformations of DNA evolved over time. Consecutive event charge deficit measurements were narrowly distributed about a well defined mean, suggesting that the analysis of multiple translocations through a pore can be used to improve estimates of the length of long polymers.   

DNA translocation through a solid-state nanopore coated with a self-assembled monolayer

The translocation of DNA through a solid-state nanopore can be dramatically affected by surface properties of a pore, such as charge density, roughness and hydrophobicity, since the pore surface serves as a boundary for the hydrodynamic flow accompanying with DNA motion. Recent experiment demonstrated the coating of a self-assembled monolayer (SAM) on the surface of a nanopore, allowing an active control on the surface property. Using all-atom molecular dynamics simulation, we investigated the tribological effect on DNA translocation through a solid-state nanopore coated with a SAM. When DNA is confined to the center of a pore, i.e. no direct interaction between DNA and pore surface, charge density and roughness of a pore surface can affect electroosmotic and hydrodynamic flows inside a nanopore, respectively. When allowing direct interaction between DNA and a SAM, adhesive interaction via hydrogen bonds can substantially increase friction force on DNA during translocation but repulsive interaction permits a fast translocation of DNA. We found two types of motion of DNA, stick-slip and steady-sliding, that are qualitatively explained using a Langevin-like model.   

Nanopore DNA translocation studies of tri-oligomer DNA with two hybridization segments

We have earlier detected 12-base hybridizations in trimer DNA complexes formed by three single-stranded DNA oligomers hybridized at their ends sequentially, using nanopores of $\sim $ 10 nm diameter [1]. These complexes are connected to a polystyrene bead at one end to slow down their translocation. Here, we report translocation experiments at different voltages with nanopores $\sim $ 5 nm diameter. The measured time lapses between the passage of consecutive double-strand DNA segments in a trimer complex allow us to study the translocation dynamics. The measured mean-first-passage time between two consecutive hybridization segments is found to be consistent with theoretical estimates based on the Fokker-Planck equation.\\[4pt] [1] V.S.K.Balagurusamy, P.Weinger and X.S.Ling, \textit{Nanotechnology} \underline {21}, 335102 (2010).   

Dehydration and Ionic Conductance Quantization in Synthetic Nanopores

Synthetic nanopores and nanochannels create new opportunities - beyond biological ion channels - to study ionic transport at the nanoscale. One process that occurs at this scale is dehydration. Ions in water do not move freely, but are instead surrounded by tightly bound water molecules held by the charge-dipole interaction. These water molecules are organized into hydration layers. For the ion to move through a nanopore of sufficiently small radius, these hydration layers must be shed as there is not enough space within the pore to accommodate them. We use molecular dynamics simulations to develop a model of dehydration based on the energy cost associated with removing water molecules. We predict that the ionic current would show sudden drops as the pore radius is reduced due to the exclusion of the hydration layers. We also examine the effect of both the sign and magnitude of the ion charge, demonstrating that divalent ions will more clearly exhibit the effect of dehydration on the ionic current.\\[4pt] [1] M. Zwolak, J. Wilson, and M. Di Ventra, J. Phys.: Condens. Matter 22, 454126 (2010); See also M. Zwolak and M. Di Ventra, Phys. Rev. Lett. 103, 128102 (2009)   

Nanowire-nanopore transistor sensor for DNA detection during translocation

Nanopore sequencing, as a promising low cost, high throughput sequencing technique, has been proposed more than a decade ago. Due to the incompatibility between small ionic current signal and fast translocation speed and the technical difficulties on large scale integration of nanopore for direct ionic current sequencing, alternative methods rely on integrated DNA sensors have been proposed, such as using capacitive coupling or tunnelling current etc. But none of them have been experimentally demonstrated yet. Here we show that for the first time an amplified sensor signal has been experimentally recorded from a nanowire-nanopore field effect transistor sensor during DNA translocation. Independent multi-channel recording was also demonstrated for the first time. Our results suggest that the signal is from highly localized potential change caused by DNA translocation in none-balanced buffer condition. Given this method may produce larger signal for smaller nanopores, we hope our experiment can be a starting point for a new generation of nanopore sequencing devices with larger signal, higher bandwidth and large-scale multiplexing capability and finally realize the ultimate goal of low cost high throughput sequencing.   

How to improve the sensitivity in transverse electronic measurements of DNA for nucleobase distinction?

In an attempt to realize third-generation whole-genome sequencing technologies, nanopores have been at the center of the research focus. Key issues with this approach involve how to slow down the translocation speed of DNA and how to achieve single-base resolution. We have previously proposed [arXiv:0708.4011; J. Phys. Chem. C 112, 3456 (2008)] the use of functionalized nanopore-embedded gold electrodes to address both these issues. More recently, we demonstrated [Appl. Phys. Lett. 97, 043701 (2010)] through molecular dynamics and electron transport simulations that the transverse differential conductance of a translocating DNA may allow for distinction between the four bases and can withstand electrical noise caused by DNA structure fluctuations. Our findings demonstrate several advantages of the transverse conductance approach, which may lead to realistic applications in rapid genome sequencing.   

Novel effects of chain flexibility, external force, and background stochasticity on polymer translocation

The polymer translocation through membranes and the polymer crossing over activation barriers in general, are ubiquitous in cell biology and biotechnological applications. Because they are interconnected flexible systems, polymers in translocation incur entropic barriers but can thermally surmount them with unusual sensitivity to background biases. In the presence of non-equilibrium noises characteristic of living environments, the translocation can speed up much when resonant activation occurs. As a related issue, I will also discuss the problem of polymer surmounting a potential barrier, where the chain flexibility enhances the crossing. Furthermore, when the chain flexibility leads to conformational changes, the crossing rate can be even more dramatically increased. This conformational flexibility and variability enhance the stochastic resonance, where the chain crossing dynamics at an optimal temperature and chain length is maximally coherent and resonant to a minute periodic force. Utilizing the self-organizing behaviors mentioned above, we may learn about bio-molecular machinery of living as well as clever means of manipulating it. \\[4pt] [1] W. Sung and P. J. Park, Phys. Rev. Lett. \textbf{77}, 783 (1996) \\[0pt] [2] J. J. Kasianowicz, E. Branton and D. W. Deamer, Proc. Natl. Acad. Sci. USA \textbf{89},13370(1996) \\[0pt] [3] P. J. Park and W. Sung, J. Chem. Phys. \textbf{111}, 5239 (1999) \\[0pt] [4] M. Asfaw and W. Sung, Euro. Phys. Lett. \textbf{90}, 30008 (2010)   

Simulations of Single DNA Nucleotide Transport Through Nanoslits

Transport of single molecules in nano-scale geometries might be used to identify them via their flight times. The motion of nucleotides in aqueous NaCl solution flowing through atomically smooth nanoslits composed of disordered carbon atoms was studied using nonequilibrium molecular dynamics simulations. The fluid was driven by gravity-like forces or the nucleotide was moved electrophoretically. Velocities were on the order of 1 m/s or 3 m/s, respectively. The relatively hydrophobic base parts of the nucleotides adsorbed to the walls multiple times while moving along the slit. The bases tended to adsorb/desorb with the sugar end of the base contacting the surface last/first. The distance required for separation of the flight time distributions (required channel length) was 8.8 $\mu $m for the gravity case. In the electrophoretic case with this surface, the nucleotides moved nearly as fast while adsorbed as while desorbed which made the separation more difficult than in the gravity case.   

Heat Treatment to Shrink Solid-State Nanopores

Solid-state nanopores have a promising application in the area of selective sensing of DNA. Therefore, it is imperative to have a simple and repeatable method for nano-fabrication of pores. This paper focuses on solid-state nanopore fabrication in a silicon-dioxide membrane with heat treatment. A 375 $\mu $m thick pre-oxidized silicon wafer with approximately 1 $\mu $m oxide is used. Photolithography followed by BHF etching, with well-cured photo-resist covering the back-side to preserve its oxide layer, was performed on the wafer in order to open square windows in the front-side oxide layer. Using the front-side oxide layer as a mask and the back-side oxide layer as an etch-stop, the silicon substrate underwent anisotropic etching to create SiO$_{2}$ membranes. The wafer was then cut into small squares approximately 1 cm on a side with each containing one membrane. A focused ion beam was used to open an initial pore in each membrane. Finally, a method for causing SiO$_{2}$ membranes to diffuse was used to shrink the pores to the desired diameter.   

The Nanofluidic Field-Effect in Electrically Actuated Nanopores

We employed high-resolution milling techniques to create solid-state nanopores with integrated electrodes for exerting field-effect control over the transport of ions and single DNA molecules in solution. An embedded, annular gate electrode was used to voltage gate the ionic conductance through a nanopore. An absence of leakage currents confirms the electrostatic origin of this effect. The measurements also reveal strong dependencies on the pH and on the ionic strength of the fluid. These results reflect the crucial difference between the modulation of charge at a solid-liquid interface where surface chemistry plays an important role, versus at a chemically inert semiconductor interface. An electrochemical model of electro-fluidic gating that captures the gate-field-induced shift in the chemical equilibrium of the ionizable surface groups describes our measurements quantitatively. We seek to electrostatically control the translocation of DNA through such gated nanopores, and thereby mimic the single-molecule regulatory capabilities of biological transmembrane channels.   

FIB direct fabrication of sub-10 nm synthetic nanopores

Nanopores in thin solid state membranes are used as single molecule electronic detectors or sensors. The membrane acts as a dividing wall in an electrolytic cell and draws charged molecules attracted by an electric field through the pore. Among the very few patterning techniques applicable to nanopores, one promising approach is to use a FIB system, which can produce small holes directly at specified locations with customized organization and shape into dielectric membranes. We detail an innovative FIB-based approach and the methodologies developed for sub-10 nm nanopore realization. Our method allows direct fabrication of nanometer-sized pores in relatively large quantities with excellent reproducibility. This approach offers the possibility to further process or to functionalize each pore on the same scale keeping the required nm-scaled positioning and patterning accuracies, for i.e. adding detection marks or local membrane thinning at nanopore site. Then we describe solutions for conditioning surface properties and for integrating such single nanopore devices for translocation experiments. Results involving DNA, proteins, polymers, colloids are presented.   

Incremental Mean First Passage Analysis of Unbiased Polymer Translocation

To provide a measure of how translocation progresses, we have recently developed a method of mapping the process as a series of mean first passage processes of increasing displacement. Starting with a simplified, ``quasi-static'' model of translocation, exact numerical and analytic calculations using this Incremental Mean First Passage Time (IMFPT) approach yield insight into the robustness of the scaling of the translocation time \textit{$\tau $} with polymer length $N$ given by \textit{$\tau $}$\sim N^{2}$ as predicted in early theoretical studies of translocation. This approach reveals fundamental differences in the dynamics between absorbing and reflective boundary conditions when only one monomer is in the pore - both experimentally relevant scenarios. IMFPT is also applied to Langevin Dynamics simulations of a full polymer to test the impact of including features neglected in the simplified model. While the scaling for much of the process is now $\tau \sim N^{2.2}$ due to internal degrees of freedom, the exponent as measured by only the net translocation time is shown to depend greatly on the details of the simulation setup as a result of non-equilibrium effects.   

Nanoscale fluid transportation through individual carbon nanotubes

There are great interest in both simulation and experiment of fluid flow on the nanoscale. Carbon nanotubes, with their extremely small inner diameter (usually below 2 nm) and atomic smooth inner surface, are ideal materials for studying nanoconfinement and ion and molecule nanoscale translocation. The excellent electrical properties of CNTs can also be integrated to achieve nanoelectrofluidic device. This presentation describes our recent progress in studying fluid transport through individual carbon nanotubes, including simultaneously ionic and electronic measurements during water, ion and molecule translocation.   

Session V44: Focus Session: Organic Electronics and Photonics -- Small molecule semiconductors and molecular electronics

The Electronic Structure of a Local Charge-Transfer-Induced Spin Transition Molecular Adsorbate

The spin crossover phenomena has been identified in the [Fe(H$_{2}$B(pz)$_{2})$~bpy] where pz=(1-pyrazolyl)borate [Fe(H$_{2}$B(pz)$_{2})$~bpy], and but there is currently a lack of knowledge of the physical nature of this phenomena and the electronic structure of this organometallic compound has not been well characterized. We have investigated the interface electronic characteristics of molecular thin films of the metal-organic [Fe(H$_{2}$B(pz)$_{2})$~bpy] by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission (IPES). X-ray absorption spectroscopy (XAS) and Infrared spectroscopy (IR spectroscopy) were also used to study [Fe(H$_{2}$B(pz)$_{2})$~bpy]. The IPES results coincide with XAS, and the model calculations. The molecular vibrational modes have been identified from a comparison of the IR spectroscopy with model calculations.   

Raman scattering studies of organic semiconducting charge-transfer compounds

Organic semiconductors offer the possibility of devices with greater mechanical flexibility and lower production costs compared to existing materials. Reports of carrier mobilities in monomolecular organic semiconductors in the 10-50 cm$^{2}$/V-s range and success in fabricating electronic devices from organic materials has increased the interest in their properties for electronic applications. However, the range of properties displayed by the monomolecular crystals is rather narrow. Charge-transfer compounds composed of two different organic molecules in which one acts as a donor and the other as an acceptor may represent the next generation of organic semiconductors. Control of their properties by modification of the molecules or changes in stoichiometry and crystalline structure makes them particularly attractive for a wide range of applications provided that the relationship between the structure and constituents of the compounds and their physical properties can be elucidated. Raman scattering studies of single crystals of two representative charge-transfer compounds, perylene-TCNQ and anthracene-TCNQ, will be presented. Theoretical calculations suggest that these materials have the potential for ambipolar charge transport, and so intermolecular interactions in these compounds are of particular interest.   

Observation of optically forbidden states in PC$_{60}$BM due to interfacial distortion

PCBM is a fullerene derivative used extensively in organic solar cells. PC$_{60}$BM shows strong absorbance at wavelengths below 400 nm. A series of sub-gap transitions exist, but are symmetry forbidden in C$_{60}$, and only weakly observed in the PC$_{60}$BM absorbance. Recent theoretical calculations predict that the symmetry rules for C$_{60}$ can be lifted by the proximity of a metallic substrate due to perturbation of the electronic spatial distribution. Here we describe capacitive photocurrent measurements of PC$_{60}$BM in which the optically forbidden features are strongly observed. In agreement with the theoretical predictions, this is thought to be due to the influence of a high conductivity ITO layer in contact with the PC$_{60}$BM. The influence of the ITO is tested by introducing a thin insulator (Al$_{2}$O$_{3})$ of varying thickness between the PC$_{60}$BM and the ITO. The photocurrent due to the symmetry forbidden states drops strongly compared to the above gap photocurrent with increasing separation. Implications of these results on the polythiophene/fullerene blends will also be discussed. DOE-3048103802-08-073, NSF- DMR-0906961   

Free exciton emission and vibrations in pentacene monolayers

Pentacene is a benchmark organic semiconductor material because of its potential applications in electronic and optoelectronic devices. Recently we demonstrated that optical and vibrational characterizations of pentacene films can be carried out down to the sub-monolayer limit. These milestones were achieved in highly uniform pentacene films that were grown on a compliant polymeric substrate. Films with thickness ranging from sub- monolayer to tens of monolayers were studied at low temperatures. The intensity of the free exciton (FE) luminescence band increases quadratically with the number of layers N when N is small. This quadratic dependence is explained as arising from the linear dependence of the intensity of absorption and the probability of emission on the number of layers N. Large enhancements of Raman scattering intensities at the FE resonance enable the first observations of low-lying lattice modes in the monolayers. The measured low- lying modes (in the 20 to 100cm$^{-1}$ range) display characteristic changes when going from a single monolayer to two layers. The Raman intensities by high frequency intra-molecular vibrations display resonance enhancement double-peaks when incident or scattered photon energies overlap the FE optical emission. The double resonances are about the same strength which suggests that Franck-Condon overlap integrals for the respective vibronic transitions have the same magnitude. The interference between scattering amplitudes in the Raman resonance reveals quantum coherence of the symmetry-split states (Davydov doublet) of the lowest intrinsic singlet exciton. These results demonstrate novel venues for ultra-thin film characterization and studies of fundamental physics in organic semiconductor structures.   

The interaction of charge carriers with lattice phonons in oligoacene crystals

We use density functional theory calculations to investigate the non-local electron-phonon interactions between charge carriers and lattice phonons (i.e., the modulation of transfer integrals by vibrations) in oligoacene crystals as a function of molecular size from naphthalene through pentacene. The results point to a significant coupling to both translational and librational intermolecular phonon modes as well as to intra-molecular vibrational modes. The impact of the interplay among these mechanisms on charge transport is investigated by treating the lattice dynamics classically. The impact of quadratic electron-phonon interaction on charge transport is studied as well.   

Spectroscopy of organic semiconductors from first principles

Advances in organic optoelectronic materials rely on an accurate understanding their spectroscopy, motivating the development of predictive theoretical methods that accurately describe the excited states of organic semiconductors. In this work, we use density functional theory and many-body perturbation theory (GW/BSE) to compute the electronic and optical properties of two well-studied organic semiconductors, pentacene and PTCDA. We carefully compare our calculations of the bulk density of states with available photoemission spectra, accounting for the role of finite temperature and surface effects in experiment, and examining the influence of our main approximations -- e.g. the GW starting point and the application of the generalized plasmon-pole model -- on the predicted electronic structure. Moreover, our predictions for the nature of the exciton and its binding energy are discussed and compared against optical absorption data. We acknowledge DOE, NSF, and BASF for financial support and NERSC for computational resources.   

Angle resolved photoemission study of rubrene single crystal

We report the direct experimental observation of the band structure of a bulk organic single crystal. The electronic structure of rubrene single crystal grown by physical vapor transport method was studied with angle-resolved photoemission spectroscopy. Highly reproducible dispersive features were observed with nice symmetry about the Brillouin zone center and boundaries, representing the band structure measured for a bulk organic single crystal. The high quality of the surface was confirmed with scanning tunneling microscopy. The energy dispersion of the highest occupied molecular orbitals derived bands showed strong anisotropic behavior in the a-b plane of the unit cell. The measured band structure, however, differs unexpectedly from theoretical calculations in terms of the amount of the dispersion and the separation of the bands.   

Charge localization and inhibition of self-assembly in tetraphenyl porphyrin on Cu(111)

A study of the nature of the electronic structure and inter-molecular interaction of the adsorbed tetraphenyl porphyrin (H$_{2}$TPP)/Cu(111) system using scanning tunneling spectroscopy (STS) and inverse photoemission spectroscopy (IPES) is presented. By studying STS and IPES spectra as a function of increasing coverage, significant upshifts in the local shockley surface state near the adsorbate as a well interfacial HOMO-LUMO gap state are observed in monolayer-thick films. This, combined with observations of changes in the local workfunction and distortions of the Cu(111) surface within 1 {\AA}~of the molecules, indicates strong molecule-interface electronic interaction and stronger bonding. Such strong electron transfers and resulting charge dipoles are the origin of observed inter-molecular Coulomb repulsion, thereby preventing self-assembly of first-monolayer H$_{2}$TPP/Cu(111) systems, while allowing for self-assembly of second-monolayer and higher, where no such surface states are observed.   

Scanning Tunneling Spectroscopy measurements of the Electronic Structure of C60 films on the Cu(100) surface

Successful implementation of organometallic electronic and photoelectronic device architectures requires understanding and engineering of molecule-electrode interfaces. Here we investigate the electronic structure of a monolayer film of C60 on a Cu(100) surface with low temperature (5 K) scanning tunneling microscopy (STM) and spectroscopy. C60 adopts four unique orientations on the Cu(100) surface, and shifts in the molecular orbital resonances for the four geometries indicate different degrees of electronic molecule-surface hybridization. At higher bias, Stark-shifted image state resonances are shown to spatially vary across the molecular film. Modulation of the image state energies are attributed to shifts in the interfacial dipole that derive from the interplay of interfacial charge transfer, surface reconstruction, and orientational ordering of the molecular film. These observations suggest the need for nanoscale interface characterization for optimizing the performance of molecular electronic devices.   

Highly Conducting Contacts for Single Molecule Transport Measured by STM-Break Junction

We present a novel method to directly link single alkane chains to gold electrodes using trimethyl tin (SnMe$_{3})$ linkers. We characterize electron transport through single molecule junctions using the STM-based break-junction technique, where a gold point contact is repeatedly formed and broken in a solution of the SnMe$_{3}$-alkanes while conductance is measured.~Based on analysis of more than 10,000 individual junctions, we find that we create single molecule junctions which are $\sim $100 times more conducting than those with alkanes terminated with any other linker previously studied. The contact resistance, determined by extrapolating to zero carbons, is 4k$\Omega $, two orders of magnitude lower than analogous values found using amine linkers. Strong evidence supports the hypothesis that \textit{in situ} cleaving of the SnMe$_{3}$ end groups facilitates the formation of a direct bond between the carbon backbone and gold leads, thereby enhancing conductance.~We corroborate this result by comparing the conductance of junctions formed from SnMe3- and Ph$_{3}$PAu-terminated benzenes.   

34 nm Charge Transport through DNA

Long-range charge transport through DNA has broad-reaching implications due to its inherent biological recognition capabilities and unmatched capacity to be patterned into precise, nanoscale shapes. We have observed charge transport through 34 nm DNA monolayers (100 base pairs) using DNA-mediated electrochemistry. Cyclic voltammetry of multiplexed gold electrodes modified with 100mer DNAs reveal sizable peaks from distally-bound Nile Blue redox probes for well matched duplexes but highly attenuated redox peaks from 100mer monolayers containing a single base pair mismatch, demonstrating that the charge transfer is DNA-mediated. The 100mers on the gold surface are efficiently cleaved by the restriction enzyme RsaI. The 100mers in the DNA film thus adopt conformations that are readily accessible to protein binding and restriction. The ability to assemble well-characterized DNA films with these 100mers permits the demonstration of charge transport over distances surpassing most reports of molecular wires.   

Electrical properties of metal-molecule-silicon structures with varying molecular backbones, dipoles, and atomic tethers

We present the results of an extensive experimental investigation of metal-monolayer-silicon junctions. By varying the molecular dipole, the molecular backbone, the Si-molecule linkage, and the Si-doping, we indentified critical features that determine the electrical transport and injection properties of the junctions. Two basic structures were used. One is an enclosed planar structure in which an organic monolayer is directly assembled on silicon and contacted with evaporated silver. The other was made via Flip Chip Lamination, a novel approach that relies on the formation of monolayers on a gold surface first, which enables the study of a wider range of molecular layers on silicon of very high-quality. Two charge transport regimes dominate: (1) a Schottky barrier limited regime where the molecular dipole results in silicon band bending at the junction interface, and (2) a tunneling regime where the molecular dipole creates a small local electric field that screens the electrical transport. Transition Voltage spectroscopy was used to identify electrical differences between $\pi$-conjugated and alkyl backbones attributed to the extended $\pi$-delocalization and variations due to the chemical nature of Si-atom linkage.   

Engineering controlled Au/GaAs junctions with partial molecular monolayers

Advances in molecular electronics offer the possibility to use molecular-based components to enhance integrated circuits and other electronic devices. Therefore, the studies of the electronic transport properties of junctions containing molecular layers are of great interest. The local hot-electron transmittance at buried metal-dicarboxylic acid-semiconductor [1] interfaces was directly investigated with nanometer spatial resolution and meV- level energy resolution using BEEM by spatially mapping hot-electrons that were injected into the top metal thin film and passed through the diode [2]. That study found that the dominant electronic transport mechanism for some dicarboxylic ligands was though pinholes rather than direct tunneling through the molecular film, and that the effective Schottky barrier height (SBH) at the pinholes was increased by a negative electric dipole moment in the surrounding molecular film [2]. We present the results of finite element electrostatic calculations of Au/discontinuous-molecular film/GaAs structures with both positive and negative dipole films, and show that the expected decrease (or increase) of the effective SBH is consistent with BEEM measurements of these types of samples. Work supported by NSF Grant No. DMR-0805237. [1] H. Haick et al., Adv. Mater. 16, 2145 (2004). [2] H. Haick, et. al., Phys. Stat. Sol. (A) 2031, 3438 (2006).   

Session V45: Rotation and Artificial Gauge Fields: Vortices and Quantum Hall Physics

Acoustic Hawking radiation in dynamically expanding Bose-Einstein condensate

Black holes are not just an absorver but emit radiation. Verification of this phenomenon, called Hawking radiation, for real black holes is almost hopeless because the characteristic temperature is much lower than the cosmic microwave background radiation. There are attempts to verify Hawking radiation physics using acoustic black holes. In this paper, I will present a numerical simulation result for a dynamically expanding Bose-Einstein condensate of cold atoms. The result shows that the radiation spectrum obeys the Planck distribution function with the temperature on the order of 0.1nK and the particle creation occurs near the horizon. I will discuss the result from the view point of the phase coherence of Bose-Einstein condensate.   

Interferometry with Synthetic Gauge Fields

We propose a compact atom interferometry scheme for measuring weak, time-dependent accelerations. Our proposal uses an ensemble of dilute trapped bosons with two internal states that couple to a synthetic gauge field with opposite charges. The trapped gauge field couples spin to momentum to allow time dependent accelerations to be continuously imparted on the internal states. We generalize this system to reduce noise and estimate the sensitivity of such a system to be $S\sim 10^{-7} \frac{\textrm{m} / \textrm{s}^2}{\sqrt{\textrm{Hz}}}$.   

Pulling Fluxes Away from Particles in Quantum Hall States of Atomic Gases

Quantum Hall states are often described as states with magnetic fluxes attached to the particles. In the case of rapidly rotating atomic gases, we show that by deforming the trapping potential of a rotating cluster, one can in fact pull the fluxes away from the particles in their quantum Hall state, as a consequence of the balance between rotational energy and interaction energy. This phenomenon can be revealed clearly from the density profile of the clusters after releasing the atoms from the trap, as well as from photoassociation experiments which measure the short range correlations.   

Lattice Quantum Hall Effect

We study the groundstate of a two-dimensional system of interacting ultra-cold atoms (bosons and fermions), trapped in the periodic potential of an optical lattice, under the influence of a strong synthetic magnetic field. In the absence of inter-particle interactions, the energy spectrum is depicted by the Hofstadter butterfly --- a fractal structure seemingly very different from the Landau levels in the continuum. However, when the number of flux quanta per lattice cell is close to a rational fraction, the energy splittings in the Hofstadter butterfly resemble Landau levels. Inspired by this similarity we establish a mapping between the wavefunctions of the non-interacting system in the lattice near rational fractions and the corresponding wavefunctions in the continuum. Using these single-particle wavefunctions we calculate pseudopotential coefficients for the interacting system. These effective interaction potentials can then be used to construct trial wavefunctions for the groundstate of the interacting system on a lattice. For the case of bosons with contact interaction, in addition to the interaction obtained by Palmer et al. [1], we find anomalous terms in the pseudopotential coefficients resembling the umklapp process.\\[0pt] [1] R. N. Palmer, A. Klein and D. Jaksch, Phys. Rev. A 78, 013609 (2008).   

Fractionalization via $Z_{2}$ Gauge Fields at a Cold Atom Quantum Hall Transition

We study a single species of fermionic atoms in an ``effective'' magnetic field at total filling factor $\nu_{f}=1$, interacting through a p-wave Feshbach resonance, and show that the system undergoes a quantum phase transition from a $\nu_{f} =1 $ fermionic integer Quantum Hall state to $\nu_{b} =1/4 $ bosonic fractional Hall state as a function of detuning. The transition is in the $(2+1)$D-Ising universality class. We formulate a dual theory in terms of quasiparticles interacting with a $Z_{2}$ gauge field, and show that charge fractionalization follows from this topological quantum phase transition. The resultant effective theory contains the lattice $Z_{2}$ gauge theory action along with a ``Hopf'' term which encodes the quasiparticle statistics. The transition occurs in the $Z_{2}$ sector and is a confinement-deconfinement transition for the quasiparticles.   

Bogoliubov theory of interacting bosons on a lattice in a synthetic magnetic field

We present a theoretical study of the effect of a magnetic field on a bosonic superfluid in a tight-binding lattice, motivated by advances in the synthesis of gauge potentials for ultracold atoms. An analysis based on the magnetic symmetry group shows that the superfluid has simultaneous spatial order, and that this depends on commensuration between the magnetic field and lattice. We predict clear signatures of many-body effects in time-of-flight images, and use a Bogoliubov expansion to calculate quasiparticle spectra that may be measured using Bragg spectroscopy. This work has been supported by JQI-NSF-PFC, ARO-DARPA-OLE, and Atomtronics-ARO-MURI.   

How does a synthetic non-Abelian gauge field influence the bound states of two spin-$1/2$ fermions?

We study the bound states of two spin-$1/2$ fermions interacting via a contact attraction (characterized by the scattering length) in the singlet channel in $3D$ space in presence of a uniform non-Abelian gauge field. The configuration of the gauge field that generates a Rashba type spin-orbit interaction is described by three coupling parameters $(\lambda_x, \lambda_y, \lambda_z)$. For a generic gauge field configuration, the critical scattering length required for the formation of a bound state is {\em negative}, i.e., shifts to the ``BCS side'' of the resonance. Interestingly, we find that there are special high-symmetry configurations (e.g., $\lambda_x = \lambda_y = \lambda_z$) for which there is a two body bound state for {\em any} scattering length however small and negative. Our results show that the BCS-BEC crossover is drastically affected by the presence of a non-Abelian gauge field. We discuss possible experimental signatures of our findings both at high and low temperatures.   

Dynamic vortex unbinding following a quantum quench in bosonic mixtures

We study the many-body dynamics of a mixture of two hyperfine states of bosonic atoms in 2D, following a pi/2-pulse. Using both a numerical implementation of the Truncated Wigner approximation and an analytical approach, we find that a dynamic phase transition can be triggered, in which the system relaxes from a superfluid to a disordered state via vortex unbinding. This process can be dynamically suppressed, which creates a long-lived metastable supercritical state. We discuss the realization and detection of these effects.   

Macroscopic superposition states of cold bosons in an asymmetric double well with Orbital Degrees of freedom

We study the dynamics of ultracold bosons in three-dimensional double wells when they are allowed either to condense in single-particle ground states or to occupy excited states. On the one hand, the introduction of second level single-particle states opens a range of new dynamical regimes. On the other, since the second level eigenstates can carry angular momentum, NOON-like macroscopic superposition (MS) states of atoms with non-zero angular momentum can be obtained. This leads to the study of the dynamics of atoms carrying vorticity while tunneling between wells. We obtain new tunneling processes, like vortex hopping and vortex-antivortex pair superposition along with the sloshing of atoms between both wells. The resulting vortex MS states are much more robust against decoherence than the usual NOON states, as all atoms in the vortex core region must be resolved, not just a single atom.   

Optical Lattice Hamiltonians for Relativistic Quantum Electrodynamics

We show how interpenetrating optical lattices containing Bose-Fermi mixtures can be constructed to emulate the thermodynamics of 2+1d quantum electrodynamics (QED3). We present a model of neutral atoms on planar lattices whose low energy effective action reduces to that of photons coupled to Dirac fermions. We overview the properties of QED3 and discuss how two of its most interesting features, chiral symmetry breaking and Chern-Simons physics, could be observed experimentally in our cold atom system.   

Vortex Dynamics and Hall Conductivity of Hard Core Bosons

Magneto-transport of hard core bosons (HCB) is studied using an XXZ quantum spin model representation, appropriately gauged on the torus to allow for an external magnetic field. We find strong lattice effects near half filling. An effective quantum mechanical description of the vortex degrees of freedom is derived. Using semiclassical and numerical analysis we compute the vortex hopping energy, which at half filling is close to magnitude of the boson hopping energy. The critical quantum melting density of the vortex lattice is estimated at 6.5x10-5 vortices per unit cell. The Hall conductance is computed from the Chern numbers of the low energy eigenstates. At zero temperature, it reverses sign abruptly at half filling. At precisely half filling, all eigenstates are doubly degenerate for any odd number of flux quanta. We prove the exact degeneracies on the torus by constructing an SU(2) algebra of point-group symmetries, associated with the center of vorticity. This result is interpreted as if each vortex carries an internal spin-half degree of freedom ('vspin'), which can manifest itself as a charge density modulation in its core. Our findings suggest interesting experimental implications for vortex motion of cold atoms in optical lattices, and magnet-transport of short coherence length superconductors.   

Birefringent break up of Dirac fermions in a square optical lattice

We introduce a model of spinless fermions on a square lattice in a spatially periodic magnetic field. This model has Dirac points in its spectrum when there is an average flux of half a flux quantum per plaquette. The dispersion in the vicinity of these Dirac points has the unusual feature that the double degeneracy of Dirac cones is broken. This corresponds to a situation in which the low energy excitations have two different ``speeds of light.'' This is a consequence of broken chiral symmetry in the model, which occurs in the kinetic energy term, and hence leaves the spectrum gapless in the vicinity of the Dirac points. This chiral symmetry breaking is fundamentally different from spontaneous chiral symmetry breaking that leads to mass generation in field theoretic models. We investigate the effects of several perturbations on the spectrum such as staggered potentials, nearest neighbor interactions, and domain wall topological defects. We provide a physical setting in which this model might be realized, namely for fermions in an optical lattice in an artificial magnetic field.   

Generation and Characterization of Free Electron Vortices

Free electron vortex beams -- composed of electron wavefunctions imprinted with a helical phase -- are remarkable for their unique topology, quantized orbital angular momentum, and magnetic moment. We recently produced free electron vortex beams in a transmission electron microscope (TEM) using nanofabricated diffraction holograms. We used this technique to generate well-defined free electron vortices in various orbital states, demonstrating beams with up to 100 $\hbar $ of orbital angular momentum per electron.The helical phase of the electrons was measured directly using interferometric techniques. The orbital magnetic moment of the electron vortex scales with the topological charge of the vortex, and leads to interesting behavior in magnetic fields. As one example of several immediate applications for the electron vortex beam, we discuss how these beams can provide elementally sensitive magnetic imaging capabilities in a TEM by using the transfer of quantized orbital angular momentum to induce preferred atomic excitations in a sample.   

Vortex structures in ultra-rapidly rotating two-component Bose-Einstein condensates

We investigate the vortex structures in rotating two-component Bose-Einstein condensates with a rotating frequency larger than the harmonic trapping frequency. Representative cases for the three phases, miscible, symmetric phase-separated, and asymmetric phase-separated, are studied. It is shown that the three different phases are manifested in the vortex structures to which at the cannular region around the center, vortices of each component form an annular structure and interlace with those of the other component. To determine the vortex structure in an authentic equilibrium state, the result obtained via imaginary-time propagating method is used as the initial state of the stochastic Gross-Pitaevskii equation and one keeps it propagating until the density profile saturates.