Session N1: Topological Phases and Quantum Computing

Protected qubits and quantum computation using Josephson junctions

Several schemes of topological protection have been proposed, in which qubits are realized as degenerate ground states of quantum many-body systems so that all likely perturbations are exponentially suppressed. In the realm of Josephson junction physics, this approach was pioneered by Doucot, Vidal, Ioffe, and Feigelman in 2002. I will report a variation of their scheme that offers greater robustness and flexibility. Its key element is a ``quantum transformer'', a superconducting current mirror operated in the quantum regime. This is a four-terminal device whose energy depends only on $\phi_1-\phi_2+\phi_3-\phi_4$, with exponentially small ``error terms'' like $\cos(\phi_1-\phi_4)$. The qubit is implemented by connecting terminal $1$ with $3$ and $2$ with $4$. I will describe a realization of the basic element, qubit measurements and unitary gates, and also discuss some parameter tradeoffs.   

Quantum critical phases in two dimensions: U(1) spin liquids

Usually, we expect that stable phases of matter can be described in terms of quasiparticle excitations that interact only weakly at low energies. However, it is now clear that certain quantum spin liquids dramatically violate this expectation, but can nonetheless exist as stable zero-temperature phases in two-dimensional systems. These are the critical or algebraic spin liquids, which have no broken- symmetry ordering, but support gapless spin-carrying excitations. These states are promising candidates for the longstanding goal of the unambiguous experimental detection of a quantum spin liquid state; they have been suggested to play a role in certain strongly correlated materials, and they possess a variety of striking, and measurable, properties. I will discuss recent work on the simplest algebraic spin liquids. These are a type of two-dimensional U(1) spin liquid, and can be described at low energies by gapless Dirac fermions (spinons) coupled to a compact U(1) gauge field (photon). I will outline an argument that establishes the stability of these states in a large-N limit, and thus resolved a longstanding controversy. Next, I will discuss some of the remarkable properties of these states, and conclude with a discussion of open issues.   

Non-Abelian topological phases

I will discuss the role of topology in the storage and processing quantum information. Chern-Simons theories in their chiral and doubled versions will be discussed. The Fractional quantum hall effect is the leading example of topological phases and may be home to striking nonabelian examples such as the $\nu=5/2$ state. These examples will be considered within a larger mathematical framework.   

Proposed experiments to probe the non-abelian $\nu=5/2$ quantum Hall state

We propose several experiments to test the non-abelian nature of quasi-particles in the fractional quantum Hall state of $\nu=5/2$. One set of experiments studies interference contribution to back-scattering of current, and is a simplified version of an experiment suggested recently by Das Sarma et al. A second set looks at thermodynamic properties of a closed system. A third set looks at electronic transport in an array of immobile quesi-particles. The first two sets are only weakly sensitive to disorder-induced distribution of localized quasi-particles.   

Realizing non-Abelian statistics in time-reversal invariant systems

Motivated by the search for a quantum computer robust against errors, much theoretical effort has been devoted to finding systems with quasiparticles obeying non-abelian statistics. I discuss a general method of constucting quantum loop gases with such behavior, focusing in particular on the simplest time-reversal-invariant model (P. Fendley and E. Fradkin, Phys. Rev. B 72 (2005) 024412 [cond-mat/0502071]). The quasiparticles of this model are called ``Fibonacci anyons'', and their braiding is related to SO(3) Chern-Simons theory. I also discuss the quantum critical point governing the transition from a topological phase to a conventionally-ordered phase.   

Session N2: Nanoscale Crystals

LeRoy Apker Award (2005): Tunable Nonlocal Spin Control in a Coupled Quantum Dot System

The effective interaction between magnetic impurities in metals that can lead to various magnetic ground states often competes with a tendency for electrons near impurities to screen the local moment (known as the Kondo effect). The simplest system exhibiting the richness of this competition, the two-impurity Kondo system, was realized experimentally in the form of two quantum dots coupled through an open conducting region. We demonstrate nonlocal spin control by suppressing and splitting Kondo resonances in one quantum dot by changing the electron number and coupling of the other dot. The results suggest an approach to nonlocal spin control that may be relevant to quantum information processing.   

Electrical Transport Through a Single Semiconductor Nanocrystal Tetrapod

Semiconductor nanocrystal tetrapods represent a unique complex nanostructure of interest for multiterminal electrical and electromechanical studies. We demonstrate by single electron transport measurements the electronic coupling between the nanotetrapod core quantum dot and the four arm quantum rods. Either ionic or covalent bonding-type of coupling can exist when the interaction between quantum dot at the junction and arm rods is weak or strong. In addition, we demonstrate a new integrated single electron transistor scheme enabled by the unique coupled nanotetrapod systems: one arm can be used as a sensitive arm-gate to control the electrical transport through the whole system. The work here reveals that nanotetrapods and other branched colloidal nanocrystals represent a new class of chemically controlled ``artificial molecules'' of coupled quantum dots.   

Controlled assembly and electronics in semiconductor nanocrystal-based devices

I will discuss the assembly of semiconductor nanocrystals (CdSe and PbSe) into electronic devices and the basic mechanisms of charge transport in nanocrystal arrays [1-4]. Spherical CdSe nanocrystals show robust memory effects that can be exploited for memory applications [1]. Nanocrystal memory can be erased electrically or optically and is rewritable. In PbSe nanocrystal arrays, as the interdot coupling is increased, the system evolves from an insulating regime dominated by Coulomb blockade to a semiconducting regime, where hopping conduction is the dominant transport mechanism [2]. Two-dimensional CdSe nanorod arrays show striking and anomalous transport properties, including strong and reproducible non-linearities and current oscillations with dc-voltage [4]. I will also discuss imaging of the charge transport in nanocrystal-based electronic devices. Nanocrystal arrays were investigated using electrostatic force microscopy (EFM) and transmission electron microscopy (TEM) [3]. Changes in lattice and transport properties upon annealing in vacuum were revealed. Local charge transport was directly imaged by EFM and correlated to nanopatterns observed with TEM. This work shows how charge transport in complex nanocrystal networks can be identified with nm resolution [3]. This work was supported by the ONR grant N000140410489, the NSF grants DMR-0449553 and MRSEC DMR00-79909, and the ACS PRF grant 41256-G10. \newline \newline References:\newline 1) Fischbein M. D. and Drndic M., ``CdSe nanocrystal quantum-dot memory,'' Applied Physics Letters, 86 (19), 193106, 2005.\newline 2) H. E. Romero and Drndic M., ``Coulomb blockade and hopping conduction in PbSe quantum dots,'' Physical Review Letters 95, 156801, 2005.\newline 3) Hu Z., Fischbein M. D. and Drndic M., ``Local charge transport in two-dimensional PbSe nanocrystal arrays studied by electrostatic force microscopy",'' Nano Letters 5 (7), 1463, 2005.\newline 4) Romero H.E., Calusine G. and Drndic M., ``Current oscillations, switching and hysteresis in CdSe nanorod superlattices,'' Physical Review B 72 (23), 2005.   

Localized charge of single CdSe quantum rods and the role of lattice imperfections

The local electronic structure of colloidal semiconductor nanoparticles is of significant fundamental and technical interest. Electrostatic force microscopy was used to determine that single CdSe quantum rods (QRs) have a permanent polarization surface-charge density, an unexpected observation for supposedly well-shaped, neutral dielectric particles. To investigate the source of the surface charge, we performed electron nanodiffraction studies with a scanning transmission electron microscope (STEM). Electron nanodiffraction patterns suggest that rotations exist between various ``sections'' of individual QRs, and that the rotation axes may form substantial angles with the c-axis. Thus, the surface charge results from the slight angle between the QR sides and the direction of internal electric polarization. Despite the large dipole moment expected for CdSe QRs, none was observed. The unavoidable presence of permanently charged surfaces on CdSe QRs has the potential to impede the development of novel devices incorporating these materials.   

Session N3: Insulating Cobaltates: Mottness on a Triangular Lattice

Neutron scattering study of novel magnetic order in Na$_{0.5}$CoO$_{2}$.

The layered sodium cobaltates, Na$_{x}$CoO$_{2}$, have attracted much recent attention, due to their unusual thermodynamic properties, as well as the recent discovery of superconductivity in the hydrated composition.~ These strongly correlated systems exhibit a rich electronic phase diagram as a function of sodium doping, $x$. A particularly intriguing insulating phase is realized at $x$=1/2, featuring a long range sodium order, a metal-insulator phase transition at 51 K, and a magnetic ordering transition at 88 K.~ We present polarized and unpolarized neutron scattering measurements of the magnetic order in single crystals of Na$_{0.5}$CoO$_{2}$. Our data indicate that below T$_{N}$ = 88 K the spins form a novel antiferromagnetic pattern within the CoO2 planes, consisting of alternating rows of ordered and non-ordered Co ions. The domains of magnetic order are closely coupled to the domains of Na ion order, consistent with such a two-fold symmetric spin arrangement. Magnetoresistance and anisotropic susceptibility measurements further support this model for the electronic ground state.   

Quasiparticles, Fermi surface topology and Phase transitions in Na$_x$CoO$_2$

Recently discovered triangular cobaltate class is a novel realization of doped Mott insulators on a triangular spin- lattice. This system exhibits superconductivity, spin-density- waves, charge-order, metal-insulator phase transitions and colossal thermopower as well as Mott and Band insulation. We employ state-of-the-art ARPES to uncover the nature of electron motion in the cobaltates over the phase diagram. Quasiparticle dynamics (Fermi velocity, bandwidth, FS topology, correlation parameters, quasiparticle coherence) we extract from the data provides valueable insights into the novel phases of matter realized on this first realization of a triangular lattice Mott system. Low-T metal-insulator (order-disorder) phase transition will be discussed in this presentation.   

Sodium Ion Ordering in double-layered and triple-layered Na$_{x}$CoO$_{2}$

The layered sodium cobalt oxide Na$_{x}$CoO$_{2}$ is studied by electron diffraction for a wide range of sodium contents, 0.15$<$x$<$0.75. This range in compositions is obtained by removal of Na by various methods for the starting materials Na$_{0.7}$CoO$_{2}$, and Na$_{1.0}$CoO$_{2 }$ The structure of Na$_{x}$CoO$_{2}$ is based the stacking of triangular O-Co-O layers with Na planes. The Co atoms are in edge-sharing CoO$_{6}$ octahedra. For the starting compound Na$_{0.7}$CoO$_{2}$, the Na$^{+}$ ions are in a trigonal prismatic coordination whereas for Na$_{1.0}$CoO$_{2 }$ the Na$^{+}$ coordination is octahedral. Prismatic coordination occurs when the close packed oxygen planes directly adjacent to the Na plane have the same projection into the basal plane (A-Na-A), whereas octahedral coordination of Na occurs when the directly adjacent oxygen planes have different projections (A-Na-B) into the basal plane. Due to this difference in stacking the a axis is about 1.08 nm and 1.65 nm for Na$_{0.7}$CoO$_{2}$ and Na$_{1.0}$CoO$_{2 }$respectively. For Na$_{0.7}$CoO$_{2}$ as well as Na$_{1.0}$CoO$_{2 }$a series of superstructures are observed, which can be explained with ordered Na ion-Na vacancy superlattices. The structural principle for some of the observed ordering schemes, particularly near x=0.5, is, surprisingly, the presence of lines of Na ions and vacancies rather than simply maximized Na-Na separations. With Na$_{0.7}$CoO$_{2}$ as starting material, the most strongly developed superlattice is found for the composition Na$_{0.5}$CoO$_{2}$. With Na$_{1,0}$CoO$_{2}$ as starting material, the most strongly developed superlattice is found for the compositions Na$_{0.75}$CoO$_{2 }$and Na$_{0.5}$CoO$_{2}$. In particular the superstructure Na$_{0.75}$CoO$_{2 }$of is very complicated. \newline \newline In collaboration with M.L. Foo, Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USA; Q. Xu and V. Kumar, National Centre for HREM, Department of Nanoscience, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands ; L. Viciu, Department of Chemistry and Princeton Materials Institute, Princeton University; Q. Huang, NIST Center for Neutron Research, NIST, Gaithersburg, MD 20899; and R.J. Cava, Department of Chemistry and Princeton Materials Institute, Princeton University.   

Magnetic-Field-Induced Suppression of the Charge Ordered State in Na$_{0.5}$CoO$_{2}$ and the observation of Shubnikov--de Haas Oscillations in Na$_{x}$CoO$_{2}$

We performed electrical transport measurements at low temperatures and high magnetic fields in Na$_{x}$CoO$_{2}$ single crystals for both $x$ = 0.5 and $x$ = 0.3. For $x$ = 0.5 Shubnikov de Haas oscillations corresponding to only 1{\%} of the area of the orthorhombic Brillouin zone (BZ) were clearly observed, indicating that most of the original Fermi surface vanishes at the charge ordering (CO) transition. While in-plane magnetic fields were found to strongly suppress the charge ordered state observed for x = 0.5 via a field-induced strongly hysteretic transition. When the external fields are rotated within the conducting planes, we observe angular magnetoresistance oscillations whose periodicity changes from two-to six-fold at the transition suggesting the reconstruction of the Fermi surface of this material. These facts indicate that the charge order is a delicate one, more akin to a charge-density-wave, and consistent with the small gap observed in the optical conductivity. While for $x$ = 0.3 we clearly observe quantum oscillatory phenomena for two frequencies $f_{1} \cong $ 480 and $f_{2}$ $\cong $ 800 T corresponding respectively to only 0.8 and 1.36{\%} of the first Brillouin zone (FBZ), with very weak indications of possible additional frequencies. These values contrast markedly with what is predicted by band structure calculations for $x$ = 0.3, i.e., 2.26{\%} and 22.3{\%} of the FBZ for the pockets resulting from the $e_{g}$$'$ and the $a_{1g}$ bands, respectively. We speculate that the Na superstructures seen for both concentrationsm re-define the Brillouin zone and thus the geometry of the Fermi surface explaining perhaps such discrepancies.   

$^{17}$O and $^{59}$Co NMR Studies of Strongly Correlated Electrons in Na$_{x}$CoO$_{2}$

The anomalous electronic properties of triangular-lattice system Na$_{x}$CoO$_{2}$ has been attracting strong interest over the last several years since the discovery of superconductivity in hydrated Na$_{1/3}$CoO$_{2}\cdot _{4/3}$[H$_{2}$O]. The electronic phase diagram of these materials is quite rich, as the physical properties depend very strongly on Na concentration. Here we report our $^{17}$O and $^{59}$Co NMR studies of the local electronic properties and low-frequency spin dynamics in these materials for a variety of Na concentrations [1,2]. \newline \newline [1] F.L. Ning, T. Imai, B.W. Statt, and F.C. Chou, PRL \underline {93} (2004) 237201.\newline [2] F.L. Ning and T. Imai, PRL \underline {94} (2005) 227004.   

Session N4: Polymer Crystallization

A new approach to study of the onsets of tethered chain overcrowding and highly stretched brush regime utilizing crystalline-amorphous diblock copolymers

Two series of diblock copolymers, PEO-$b$-PS and PLLA-$b$-PS, were used as templates to generate tethered PS blocks on the single crystal surfaces. Controlled and tunable reduced tethering density, $\tilde {\sigma }$, defined by \textit{$\sigma \quad \pi $} $R_{g}^{2}$ (where \textit{$\sigma $ }is the tethered chain density), could be achieved in a broad range (up to 24) by changing the molecular weights (MW's) of the crystalline and amorphous blocks and by varying the crystallization temperature ($T_{x})$ of different PEO-$b$-PS and PLLA-$b$-PS solutions. The $\tilde {\sigma }$ of the tethered PS chains on the crystal surface increased with increasing $T_{x}$ because at a fixed MW of the PEO or PLLA block, an increase in the lamellar thickness ($d_{CRYST})$ was evidence of a decrease in the number of folds. When we plotted the relationships between 1/$d_{CRYST}$ and $T_{x}$ for these two series of diblock copolymers, sudden and discontinuous changes of the slopes in some of these were observed at $\tilde {\sigma }$ = 3.7 ($\tilde {\sigma }$*). This was as a result of the drastic interaction change of the neighboring PS tethered chains. An average reduced surface free energy of the tethered PS chains (\textit{$\Gamma $}$^{PS})$ was used as a parameter to characterize the PS tethered chain interactions. The relationship between \textit{$\Gamma $}$^{PS}$ and $\tilde {\sigma }$ showed a discontinuous transition at $\tilde {\sigma }$*. This could be identified as the onset of the tethered PS chain overcrowding in solution. This transition indicates that the extra entropic surface free energy created by the repulsion of tethered PS chains started to affect the nucleation barrier of the PEO or PLLA block crystallization. Based on the scaling laws, the onset of highly stretched brush regime could be identified at $\tilde {\sigma }$ = 14.3 ($\tilde {\sigma }$**). In the \textit{$\Gamma $}$^{PS}$ versus $\tilde {\sigma }$ plot, the transition appears to be continuous. Thus, a crossover regime in the tethered PS chains exists between $\tilde {\sigma }*$ = 3.7 and $\tilde {\sigma }$*$*$ = 14.3. It is defined as the regime where the interaction of the tethered PS chains undergo changes from being non-interacting towards penetration to, finally, chain stretching normal to the surface.   

Flow-Induced Crystallization Precursor Structure in Entangled Polymer Melt.

Flow-induced crystallization has long been an important subject in polymer processing. Varying processing conditions can produce different morphologies, which lead to different properties. Recent studies indicated that the final morphology is in fact dictated by the initial formation of crystallization precursor structures (i.e., shish kebabs) under flow. In this talk, factors that affect the shish-kebab formation in entangled polymer melts are systematically reviewed, including the concept of coil-stretch transition, chain dynamics, critical orientation molecular weight, phase transition during shish and kebab formations. In particular, recent experimental results from in-situ rheo-X-ray studies and ex-situ microscopic examinations have been presented to illustrate several new findings of flow-induced shish-kebab structures in polymer melts. (1) The shish entity consists of stretched chains (or chain segments) that can be in the amorphous, mesomorphic or crystalline state. (2) The kebab entity mainly arises from the crystallization of coiled chains (or chain segments), which seems to follow a diffusion-control growth process. (3) A shish-kebab structure with multiple shish was seen in the ultra-high molecular weight polyethylene (UHMWPE) precursor. Based on the above results and recent simulation work from other laboratories, a modified molecular mechanism for the shish-kebab formation in entangled melt is presented.   

Curved faces in polymer crystals with asymmetrically spreading growth patches

Polymer crystals often have curved faces. Understanding such morphology is of major interest since it allows distinction between fundamentally different theories of polymer crystallization. E.g. Sadler's ``roughness-pinning'' theory assumes that the curvature is a result of roughening transition on lateral faces. It has since been shown by Mansfield that the curvature can be explained quantitatively, essentially within the Lauritzen-Hoffman nucleation theory. However, the step propagation rates $v$ implied in their treatment are substantially lower than predicted by the LH theory. The retardation appears to be due to the ``self-poisoning'' or ``pinning'' effect of incorrect chain attachment, effectively demonstrated by the extreme cases of growth rate minima in long-chain monodisperse n-alkanes. Recently crystals of poly(vinylidene fluoride) and alkanes C$_{162}$H$_{326}$ and C$_{198}$H$_{398}$ have been found with habits that can be best described as bounded by curved {\{}110{\}} faces. The interesting feature is the asymmetry of the curvature: while the faces are curved at one end, they are straight at the other. We carried out mathematical analysis of the curvature, generalizing the Mansfield model. We suggest that such asymmetric curvature arises from the propagation rates to the left, $v_{l}$, and to the right, $v_{r}$, being different because of the lack of mirror bisecting planes such as (110). By solving appropriate equations with moving boundaries, we obtained the shape of the growth front $y(x,t)$. Calculated crystal habits gave excellent fits to the observed growth shapes of $a$-axis lenticular crystals of long alkanes and PVDF, as well as of single crystals of PEO. This explains some hitherto poorly understood morphologies and, in principle, allows independent measurements of step initiation and propagation rates in all polymers.   

New Paradigms for Polymer Crystallization

The ordering process of topologically connected chains into a crystalline state is distinctly different from that of low molar mass substances. One of the key differences arises fundamentally from entropic barriers due to substantial reduction in configurational entropy of the system during the ordering process. We have derived a new theoretical model with the following essential features: (1) For a single lamella, the free energy landscape exhibits many metastable states (separated by free energy barriers), and a globally stable state. Among the metastable states, even the first viable state with its free energy just below that of the melt is long-lived, due to the barrier for thickening. The thickness of this long-lived metastable state increases with temperature. However, if enough time is granted for this metastable state to evolve, then the equilibrium thickness would be reached for each temperature. The equilibrium thickness decreases with temperature, until the approach of the equilibrium melting temperature. The equilibrium melting temperature does not correspond to that of extended chain dimensions. (2) The lateral growth faces a free energy barrier, due to temporal crowding of entangled chains at the growth front. A general formula is derived for the growth kinetics of the growth front, providing a crossover description for crystallization of low and very high molar mass polymer chains. The predictions are compared with available experimental data.   

Growth and form of spherulites: A phase field study.

Polycrystalline patterns termed spherulites are present in a broad variety of systems including metal alloys, polymers, minerals, and have biological relevance as well (see e.g. semi-crystalline amyloid spherulites and spherultic kidney stones). The fact that similar polycrystalline patterns are observed in systems of very different nature suggests that a minimal model based on coarse-grained fields, which neglects the details of molecular interactions, might be appropriate. Although such a field-theoretic approach disregards most of the molecular scale details of formation, some features such as crystal symmetries can be incorporated via the anisotropies of the model parameters. The rationale for developing such coarse-grained models is the current inability of fully molecular models to address the formation of large scale morphologies. A phase field theory of polycrystalline growth, we developed recently, is applied for describing spherulitic solidification in two and three dimensions. Our model consists of several mechanisms for nucleating new grains at the perimeter of the crystallites, including homogeneous (trapping of orientational disorder and branching in certain crystallographic directions) and heterogeneous (particle-induced nucleation) processes. It will be shown that the diversity of spherulitic growth morphologies arises from a competition between the ordering effect of discrete local crystallographic symmetries and the randomization of the local crystallographic orientation that accompanies crystal grain nucleation at the growth front. This randomization in the crystal orientation accounts for the isotropy of spherulitic growth at large length-scales and long times. We find the entire range of observed spherulite morphologies can be reproduced by this generalized phase field model of polycrystalline growth.   

Session N5: Pake and AIP Industrial Physics Prizes

The Future of Research in Industry

Since 1990 the environment for and execution of industrial research has changed profoundly. See, e.g., R. Buderi, Engines of Tomorrow (Simon and Shuster, New York, 2000); H. W. Chesbrough, Open Innovation (Harvard Business School Press, Boston, 2003); C. B. Duke, Creating Economic Value from Research Knowledge (The Industrial Physicist, Aug-Sept. 2004, pp. 29-31). According to Thomas L. Friedman (``The World is Flat,'' Farrar, Straus and Giroux, New York, 2005) a new global communications-collaboration platform has ``flattened'' the world. National alarms have been raised about the US capability to compete in this changed environment. See, e.g., ``America's Tech Might Slipping?,'' Business Week, March 14, 2004; ``Globalization and Engineering,'' The Bridge, National Academy of Engineering, Fall 2005; ``Rising Above the Gathering Storm,'' National Academy of Sciences, 2005. In this presentation I indicate why firms perform research and how they generate economic value from it. Then I discuss the profound changes in the environment for these activities since 1990. This leads to a consideration of how firms are modifying their Research and Development activities to deal with this situation. I close by noting implications of these developments on the role of physics and the careers of physical scientists in the 21st century.   

NIST Role in Advancing Innovation

According to the National Innovation Initiative, a report of the Council on Competitiveness, innovation will be the single most important factor in determining America's success through the 21$^{st}$ century. NIST mission is to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology -- in ways that enhance economic security and improve the quality of life for all Americans. NIST innovations in measurement science and technology often become the basis for new industrial capabilities. Several examples of such developments will be discussed, including the development of techniques for manipulation and measurement of biomolecules which may become the building blocks for molecular electronics; expansion of the frontiers of quantum theory to develop the field of quantum computing and communication; development of atomic scale measurement capabilities for future nano- and molecular scale electronic devices; development of a lab-on-a-chip that can detect within seconds trace amounts of toxic chemicals in water, or can be used for rapid DNA analysis; and standards to facilitate supply chain interoperability.   

Liquid Crystals: From Discovery to Products

Liquid crystals, constituting a new phase of matter, were discovered in 1888. They remained a scientific curiousity until the late 1960s, when liquid crystal displays were invented by Heilmeier at RCA and Fergason at Kent State University. Today, LCDs dominate the flat panel display industry, with production primarily in the Far East. In this talk, I will briefly review the history of liquid crystals and LC devices, discuss emerging LC technologies and speculate on their commercial potential. I will outline new directions in liquid crystal research and describe some of the remarkable new products that may result. I will conclude by considering the connection between support for basic and applied research and successful product commercialization.   

Benchmarking Competitiveness: Is America's Technological Hegemony Waning?

For more than half a century, by almost every standard, the United States has been the world's leader in scientific discovery, innovation and technological competitiveness. To a large degree, that dominant position stemmed from the circumstances our nation inherited at the conclusion of the World War Two: we were, in effect, the only major nation left standing that did not have to repair serious war damage. And we found ourselves with an extraordinary science and technology base that we had developed for military purposes. We had the laboratories -- industrial, academic and government -- as well as the scientific and engineering personnel -- many of them immigrants who had escaped from war-time Europe. What remained was to convert the wartime machinery into peacetime uses. We adopted private and public policies that accomplished the transition remarkably well, and we have prospered ever since. Our higher education system, our protection of intellectual property rights, our venture capital system, our entrepreneurial culture and our willingness to commit government funds for the support of science and engineering have been key components to our success. But recent competitiveness benchmarks suggest that our dominance is waning rapidly, in part because other nations have begun to emulate our successful model, in part because globalization has ``flattened'' the world and in part because we have been reluctant to pursue the public policies that are necessary to ensure our leadership. We will examine these benchmarks and explore the policy changes that are needed to keep our nation's science and technology enterprise vibrant and our economic growth on an upward trajectory.   

MRI from 400 gauss to 1.5 tesla and beyond

Magnetic Resonance Imaging (MRI) is arguably the most novel and important medical imaging modality since the advent of the X-ray. MRI grew out of the long development of atomic spectroscopy, atomic and molecular beam resonance and, finally, nuclear magnetic resonance (NMR) in condensed matter. The operation and economics of MRI systems depend on the performance of magnets, pulsed magnetic field gradient windings and rf (radiofrequency) coils. Physics and physicists have made critical contributions to these technologies. Superconducting magnets have come to be the magnet of choice. Magnetic gradient windings present theoretical electromagnetic and practical challenges. The need for rf antennas that resonate at high frequencies while surrounding sizable spatial regions inspired large coils producing uniform rf magnetic fields while minimizing electric field interactions with the imaging subject. This development enabled MRI at high magnetic fields. Additionally it is possible to use arrays of small rf coils to obtain MRI images with the high signal-to-noise ratio of a small surface coil and the field of view of a large coil. We recently investigated the intense acoustic noise (110 dB or more) produced in MRI scanners. Surprisingly, eddy currents induced in the magnet cryostat inner bore make a major contribution to this noise. Calculations indicate that a thin layer of Cu on the outside of the gradient assembly could substantially decrease eddy currents and help reduce noise. GE R{\&}D work was focused on the science underlying MRI, MRI technology and the MRI product. Corporate management sometimes discourages technical publication related to evolving products because it might help rivals. Our practice of extensive publication and participation in open scientific exchange---after filing appropriate patent applications---served as quality control for company science and technology. GE conference presentations and journal publications helped establish technical leadership and determine which ideas were most important. GE scientists built reputations leading to leadership prominent within the MRI technical community. Openness underpinned a highly effective development process that enabled GE to pull ahead of competitors.   

Session N6: Fermi Superfluid Gases : Non-equal Spin Polarization (FFLO State) and p-Wave pairing

Phase Diagram of Cold Polarized Fermi Gas

We propose the phase diagram of cold polarized atomic Fermi gas with zero-range interaction. We identify four main phases in the plane of density and polarization: the superfluid phase, the normal phase, the gapless superfluid phase, and the modulated phase. We argue that there exist a Lifshitz point at the junction of the normal, the gapless superfluid and the modulated phases, and a splitting point where the superfluid, the gapless superfluid and the modulated phases meet. We show that the physics near the splitting point is universal and derive an effective field theory describing it. We also show that subregions with one and two Fermi surfaces exist within the normal and the gapless superfluid phases.   

Superfluid stability in polarized Fermi atomic gases

For a two-species atomic Fermi gas with equal populations, it is now widely accepted that a smooth BCS-BEC cross-over occurs when the Feshbach resonance is crossed. However, the situation is very different if the populations of the two species are different. In particular, the uniform state is stable only either (a) for sufficiently negative detuning, where the system is a gapless mixture of Bose condensed pairs and unpaired normal Fermions, or (b) for sufficiently positive detuning, where Fermions are unpaired and the system is in the normal state. No uniform state is stable in between. Phase transition(s) must therefore occur when the resonance is crossed. We discuss the theoretical phase diagram of this system in this talk.   

Pairing and Phase Separation in a Polarized Fermi Gas

BCS pairing can only occur when the Fermi energies of the individual particles are equal. There has been great interest, however, in the consequences of mismatched Fermi energies that may arise in several important situations, including magnetized superconductors or cold dense quark matter at the core of neutron stars. Pairing is qualitatively altered by the Fermi energy mismatch, and there has been considerable speculation regarding the nature and relative stability of various proposed exotic phases. We have created a two-component gas of $^6$Li atoms in which the relative Fermi energies are altered by changing the relative numbers of each component \footnote{G.B. Partridge \textit{et al.}, cond-mat/0511752.}. The BEC/BCS crossover with tunable interactions is realized via a Feshbach resonance. Above a critical number polarization, which depends on the interaction energy, the gas separates into a superfluid paired core surrounded by a shell of normal unpaired atoms. Below the critical polarization the gas exists in a paired state with asymmetric Fermi surfaces. The critical polarization is largest in the BEC regime, and becomes small in the BCS regime. We also measure the universal interaction parameter $\beta$ for a strongly interacting Fermi gas to be -0.54 (5), in good agreement with recent Monte-Carlo calculations.   

Fermi superfluids with p-wave pairing near a Feshbach resonance

Fermi superfluids with s-wave pairing near an s-wave Feshbach resonance are being extensively studied both theoretically and experimentally. Recently, Feshbach resonances in the p-wave channel have been observed in both $^{40}$K and $^6$Li, raising the possibility that fermionic superfluids with p-wave pairing could be attained at low temperatures. Since the pairing wavefunction in this case breaks rotational symmetry, the superfluid properties of this system in the BEC-BCS crossover will be much different from the s-wave case. In particular, I will discuss the symmetry of the ground state as well as the experimental signatures of these novel superfluids as a function of the parameters defining the resonance. Other new results on resonance physics will also be reported.   

Evolution from BCS to BEC Superfluidity in Dilute Fermi Gases

I will review briefly some old results [1,2] of the evolution from BCS to BEC superfluidity in dilute Fermi gases, including critical temperature, order parameter amplitude, chemical potential and time dependent Ginzburg-Landau theory for the s-wave channel in three dimensions. Following this discussion, I will present new results for the BCS to BEC evolution of Fermi gases in the p-wave channel [3]. I will make comparisons between s-wave and p-wave superfluidity and point out the main differences between the two cases. Lastly, I will discuss supefluidity of s-wave and p-wave Fermi gases in a restricted two-dimensional geometry (one dimensional optical lattice), where a Berezinkii-Kosterlitz-Thouless-type transition is proposed as the system evolves from the weak to the strong attraction limit. In this case, I will show that spontaneous vortex-antivortex pairs form and that they can condense into a vortex-antivortex lattice at lower temperatures [4]. [1] C. A. R. Sa de Melo, M. Randeria, and J. R. Engelbrecht, PRL 71, 3202 (1993). [2] J. R. Engelbrecht, M. Randeria, and C. A. R. Sa de Melo, PRB 55, 15153 (1997). [3] M. Iskin, and C. A. R. Sa de Melo, cond-mat/0510300 (2005). [4] S. S. Botelho, and C. A. R. Sa de Melo, cond-mat/0509387 (2005).   

Session N7: Recent Advances in the Computation of Optical and Transport Properties of Nanostructures

Optical Properties of Nano-Crystallites

The optical properties of nanostructured materials are interesting due to the tunability of the electronic structure, of the lifetimes, and of the excitation spectra. This calls for precise knowledge of the physical effects which create the desired properties. Thereby it is of utmost importance to settle the question as to how many-body effects have to be incorporated in the description of the excitation aspects inherent in any optical process. \\ Static DFT-LDA \textit{ab initio} calculations have now become possible for systems of about 1000 atoms for the ground state. Time-dependent DFT (TDDFT) can in principle describe excitations as exhibited in optical spectra. However, approximations for the exchange and correlation contributions that are valid in a wide range of situations and efficient enough to be applied to large nanostructures are still to be found. GW calculations deal only with charged (electron addition and removal) excitations. The solution of the Bethe-Salpeter equation (BSE) gives good answers for neutral excitations like absorption but is numerically heavy and so far tractable for rather small systems only.\\ In my talk I will briefly review the state of the calculation of optical properties. Using bulk semiconductors and Ge, Si, and alloy nanocrystals as illustrations, I will then discuss the following points: \begin{itemize} \item Manifestation of confinement effects in various spectra; \item Importance of surface effects; \item Interplay between many-body effects and confinement and surface effects; \item Importance of short- and long-range contributions. How are they adequately described? (Important, e.g., for embedded nanostructures.) \end{itemize} These questions will be discussed in view of the optical properties, but also for loss spectra and photo-emission. Methods used are DFT-LDA, TDDFT in various approximations, GW, and BSE.   

Atomistic Pseudopotential Calculations of the Electronic and Optical Properties of Self-Assembled Quantum Dots

The optical spectrum and the charging energies of semiconductor quantum dots have been recently measured with high accuracy. Both of these experimental techniques probe many-body states that are not directly described by independent particle theories such as the density functional theory. On the other hand, quasi- particle theories that can in principle address the problem, such as GW, are computationally too demanding for the study of nanostructures (as opposed to clusters) where many thousands of atoms are involved. One way to approach this problem is to use the effective mass approximation or the k.p method and choose a confinement potential that reproduces a few known experimental facts (e.g. the splitting between confined levels). These methods can provide a good initial guess but were shown to be too crude to enable a quantitative comparision with recent experiments. We therefore adopt a bottom-up atomistic approach where instead of starting from a simplified approach, such as effective mass, and progressively increase the complexity by adding parameters, we start from the accurate atomistic description (LDA or GW) and work ourselves up using a few well controlled approximations.\\ I will first present the method, namely (i) the scheme that is used to derive the empirical pseudopotentials including the piezoelectric effect, (ii) the choices that have to be made for the basis used to expand the wave functions, (iii) the inclusion of corelations through Bethe-Salpeter-like treatment. I will then present recent applications of the theory to calculate the fine-structure [1] of excitons and charged excitons, the charging spectra of holes [2] and the degree or entanglement stored in a quantum dot molecule [3].\newline \newline [1] G. Bester, S.V. Nair, A. Zunger, prb {\bf 67}, 161306 (2003). \newline [2] L. He, G. Bester, A. Zunger, PRL (in press). \newline [3] G. Bester, J. Shumway, A. Zunger, PRL {\bf 93}, 047401 (2004)   

Density Functional Theory of the electrical conductivity of molecular devices

The theoretical modeling of electrical transport through nanoscale devices is a very challenging task: On one hand, the conduction properties of a molecular junction depend crucially on details of the chemical bonding, particularly at the interface. Such properties are routinely studied using methods based on density-functional theory (DFT). On the other hand, ground-state theories like DFT cannot be directly applied to systems with a finite current, because such devices are out of equilibrium. One possibility to overcome this problem is to study electron transport in the time domain. In the spirit of what is done in semiclassical Boltzmann approaches, one considers the system subject to both an external electrical field and to dissipation due to inelastic scattering. The combined influence of the external driving force and dissipation leads to a steady state with finite current. In this presentation I will first show how time-dependent DFT can be formally extended to dissipative systems, described by a Liouville master equation for the reduced density matrix. In a second step this formalism is then applied to calculate the current-voltage characteristics of molecular junctions, like e.g. carbon nanotubes suspended between metallic contacts.   

Self-interaction errors in density functional calculations of electronic transport

All density functional (DFT) calculations of single-molecule transport to date have used continuous exchange-correlation approximations, such as the local density approximation (LDA) or the generalized gradient approximation (GGA). These usually provide a good description of metallic systems, but fail in predicting the correct I-V curve for molecules weakly coupled to the current/voltage probes. Most of the problem can be attributed to the lack of the derivative discontinuity of the DFT potential in local approximations. These in fact continuously connect the orbital levels for different integer occupations, leading to qualitative errors such as the erroneous prediction of the dissociation of heteronuclear molecules into fractionally charged ions In this talk I will first describe the typical errors arising from neglecting the derivative discontinuity in transport calculations [1], namely the erroneous prediction of metallic transport for insulating molecules. Then I will present a simple and computationally undemanding atomic self-interaction correction scheme for transport. This preserves the computational and conceptual simplicity of standard LDA, and nevertheless re-introduces part of the derivative discontinuity. The method is implemented in our quantum transport code Smeagol [2] (www. smeagol.tcd.ie) and several examples will be given. \newline \newline [1] C.Toher, A.Filippetti, S.Sanvito, and K.Burke, Phys. Rev. Lett. 95, 146402, (2005). \newline [2] A.R.Rocha, V.M.Garcia Suarez, S.W.Bailey, C.J.Lambert, J.Ferrer, and S.Sanvito, cond-mat/0510083   

Electron-vibration interaction in molecular electronics and GW approximation for the e-e interaction in transport theory

The field of molecular electronics has seen a tremendous expansion in recent years, thanks to the realization of ingenious experimental setups and the fundamental achievement of reproducible results and behaviours. Significant progresses have also been made from a theoretical point of view, although the agreement with experiments is still not satisfactory. The challenges for a complete understanding of transport in such systems are still considerable. Inelastic electron tunnelling spectroscopy is becoming very popular in the field thanks to its powerful capability of probing molecular vibrational properties and could provide in the future a valuable characterization tool if correctly related to theoretical calculations. We simulate IETS spectra of various molecules between metal contacts and show the importance of such simulation for the interpretation of the experiments. Particular attention is devoted to the evaluation of Joule heating and thermal dissipation. The problem is tackled within the formalism of NEGF by the calculation of appropriate electron-phonon self-energies. The electron-phonon coupling is derived from the DFTB Hamiltonian. The Power dissipated is calculated from the virtual contact current originated from phonon emission and absorption processes. Preliminary results of thermal dissipations of molecules coupled to Au and Si substrates will be shown. As well known, all DFT methods tend to underestimate the electronic band-gap of semiconducting and insulating materials. In particular the band-gap of conjugated organic molecules is usually underestimated by few electronvolts. However, band-gap corrections are crucial for quantitatively correct calculations of the tunneling current through organic molecules. We show a novel implementation of the \textit{GW} correction applied to our DFTB method and show its applications to molecular systems sandwitched in-between electrodes to obtain a first-principle correction of the $e-e$ interaction energy. The resulting self-energy is used to improve the system \textit{GF} and to obtain a correction of the tunneling current. We also apply the \textit{GW} correction in the context of the computation of the complex band-structures of polymers such as poly-acetylene or poly-phenylene and show how the energy gap and decay lengths of the evanescent states should be corrected by quasi-particle effects.   

Session N8: Granular Flows

Assessing a Continuum Description of Wide Shear Zones in Slow Granular Flow by Discrete Element Simulations

While the rheology of rapid granular flows is becoming well established, slow, dense flows are not well characterized in part because the strain localization (i.e., shear bands) they often exhibit is not easily amenable to continuum descriptions. Recently, a novel experimental system (split-bottom Couette Cell) was developed with promising potential to give new insight into these flows due to its wide, smooth shear zones (Fenistein et al. PRL \textbf{92}, 94301). Subsequent experimental and numerical studies have lead to a good understanding of the nature of the flow in this device, which has lead Depken et al. (cond-mat/0510524) to propose a set of testable constitutive relations between the internal stresses and flow field. In particular, they suggest that the bulk, effective friction coefficient between sliding layers of particles is not constant, but has a subtle dependence on the orientation of the layers with respect to the bulk force. Here we present large-scale Discrete Element Simulations to analyze the bulk flow in both circular, above and below the critical height, and linear, where no critical height for slip at the base is found, split-bottom geometries. We check the proposed form of the stress tensor and assess the validity of the claim that the effective friction coefficient depends on the shape of the shear zone with respect to gravity.   

Dynamical heterogeneities in dense granular flow: timescales and large-scale particle rearrangements

Recent interest in understanding the dynamical arrest leading to a fluid $\rightarrow$ solid transition in both thermal and athermal systems has led to questions about the nature of these jamming transitions (PRL {\bf 86}, 111 (2001), Nature {\bf 411}, 772 (2001)). It is believed that these jamming transitions are dependent on the influence of extended structures on the dynamics of the system (Science {\bf 287}, 627 (2000)). Simulations of steady-state gravity-driven flows of inelastically colliding hard disks show the formation of large-scale linear chains of particles with a high collision frequency even at flow velocities well above the jamming transition (EPL {\bf 66}, 277 (2004)). These chains can be shown to carry much of the collisional stress in the system due to a dynamical correlation that develops between the momentum transfer and time between collisions in these ``frequently-colliding'' particles. While measurements of slowly decaying stress correlations yield an average lifetime for these structures which scales inversely with the flow velocity (cond-mat/0505496), distributions of time scales associated with the stress chains may provide more information about their effect on the dynamics of the flowing granular medium. These distributions may be obtained by considering time scales related to large-scale rearrangements of neighbouring particles in analogy with measurements done on supercooled fluids.   

Dense granular flows down an inclined plane

Granular flow on a rough inclined plane is an important model system in which to study the basic rules of the dynamics of granular materials. Despite intensive study, many features of such flows are still incompletely understood. For uniformly flowing layers at relatively shallow inclination, we consider experimentally the the basic flow rheology of the granular media and propose new scalings to collapse our data for glass beads and rough sand as a function of inclination angle and particle diameter. At steep inclinations above some angle $\theta_s$ ($\tan\theta_s/\tan\theta_r \approx 1.3-1.5$, where $\theta_r$ stands for the angle of repose) for flowing grains, numerics and theory predict that the surface roughness is inadequate to dissipate energy gained in the gravitational field, and the flow should continue to accelerate. We report on our experimental results on the properties of granular flows on a steeply inclined plane and define the domains of steady flows. We also discuss the instabilities of such flows leading to spatial patterns.   

Transverse Instability of Avalanches in Granular Flows down Incline

Avalanche experiments on an erodible substrate are treated in the framework of ``partial fluidization'' model of dense granular flows. The model identifies a family of propagating soliton-like avalanches with shape and velocity controlled by the inclination angle and the depth of substrate. At high inclination angles the solitons display a transverse instability, followed by coarsening and fingering similar to recent experimental observation. A primary cause for the transverse instability is directly related to the dependence of soliton velocity on the granular mass trapped in the avalanche.   

Swirling in a Vibrated Monolayer of Rods

We report observations of the spatiotemporal behaviour of a vertically vibrated horizontal monolayer of copper rods (aspect ratio $\approx $ 5) etched to a rolling-pin-like shape. The spatial organization of the rods resembles a highly-defected nematic state with large, coherently moving swirls. We measure spatiotemporal correlations of the single-particle and collective velocities, and study the structure and dynamics of the system as a function of density and vibration amplitude. We analyze the observed patterns in the light of theories\footnote{ J. Toner, Y. Tu and S. Ramaswamy, Ann. Phys. 318 (2005) 170.} of orientational ordering, dynamics, and topological defects in systems of driven particles. We make comparisons to related but different experiments\footnote{ D.L. Blair, T. Neicu, and A. Kudrolli, Phys. Rev. E 67, 031303 (2003).}, as well as to our earlier measurements\footnote{ V. Narayan, N. Menon and S. Ramaswamy, J. Stat. Mech. (2005) in press; cond-mat/0510082.} on similar particles with higher aspect ratio.   

Simple Power Law for Transport Ratio with Bimodal Distribution of Coarse Sediment

Using a discrete particle model, we have simulated sheet flow transport of coarse bimodal sediment distributions in the bottom boundary layer over a range of oscillatory waves and steady currents. The ratio of large grain to small grain diameter was varied as 5:4, 3:2, and 2:1. For each bimodal distribution, the mass ratio $M_{L}/M_{S}$ ($M_{L}$ and $M_{S}$ are the masses of large and small grains respectively -- the total mass was fixed for all runs) was varied from 1/9 up to 9/1. We find that, independent of wave and current forcing for the range of conditions considered, the ratio of large to small grain time-average transport rate obeys the power law $Q_{L}/Q_{S}=C(M_{L}/M_{S})^{k}$, where $Q_{L}$ and $Q_{S}$ are the time-average transport rates of the large grains and small grains respectively and $C$ and $k$ are regression constants. A linear regression in log space (including 81 different simulations per diameter ratio) suggests that \textit{k$\approx $D}$_{L}/D_{S}$ with R$^{2}>$0.9. The robust nature of the results suggests that the new power law may have a broad range of applications for shear flows of bimodal granular mixtures.   

Large scale surface flow generation in driven suspensions of magnetic microparticles.

Nontrivially ordered dynamic self-assembled snake-like structures are formed in an ensemble of magnetic microparticles suspended over a fluid surface and energized by an external alternating magnetic field. These self-assembled multi-segment structures emerge as a result of the collective interaction between the particles oscillations induced by an external magnetic field and the standing waves on the surface of fluid. Surprising large-scale vortex flows are generated by these snake-like structures. The flows can be as fast as 2 cm/sec and strongly depend on the driving magnetic field parameters. We report on systematical experimental study of the vortex flow properties and generation mechanisms.   

Green-Kubo expressions for transport coefficients of a granular fluid.

A formal derivation of linear hydrodynamics for a granular fluid is given. The linear response to small spatial perturbations of the homogeneous state is studied in detail using methods of nonequilibrium statistical mechanics. A transport matrix for macroscopic excitations in the fluid is defined in terms of the response functions. An expansion in the wavevector to second order allows identification of all phenomenological susceptibilities and transport coefficients through Navier - Stokes order [1]. The transport coefficients in this representation are the generalization of Helfand and Green-Kubo relations to granular fluids. The analysis applies to a wide range of collision rules. Several differences from the corresponding expressions in the elastic limit are noted. Then, the particular case of inelastic hard spheres is considered and some approximate analytical evaluations illustrated. [1] A preliminary report of these results can be found in J. W. Dufty, A. Baskaran and J. J. Brey cond-matt/0507609   

Dynamics angle and surface flow properties of wet and cohesive granular matter

We will discuss an experimental study of the flow of grains mixed with a small amount of liquid using a horizontally rotated drum apparatus, extending on our previous work on the maximum angle of stability of wet granular materials [1]. We focus on the continuous avalanching regime observed at high rotation rates, and examine the shape of the granular surface and depth of flow with imaging techniques as a function of amount, viscosity and surface tension of the liquid. Glass beads with 1mm diameter, and a drum with a diameter 295 mm and a width of 145mm is used to minimize the effect of the boundary. We find that the shape of the surface may be approximated by two linear segments in the upper and lower halves. The slope of the upper segment corresponding to the dynamical angle of repose $\theta_d$ is observed to initially increase with rotation rate and volume fraction of liquid as expected, while the lower segment has an approximately constant slope. Interestingly, $\theta_d$ is observed to peak before decreasing to an approximately constant value as the volume fraction is increased. The rate of increase of $\theta_d$ is observed to decrease with rotation rate and viscosity. The role of the time scale over which wet grains shear past each other and the time over which grains actually come into contact due to lubrication forces on the observed change in scaling will be discussed.\newline [1]: S. Nowak, A. Samadani, and A. Kudrolli, Nature Physics {\bf 1}, 50 (2005).   

Characterizing the banded state of granular material in a rotating drum.

Why do particles of different size segregate axially in a horizontal rotating tumbler? We aim to understand the microscopic mechanisms for axial segregation through direct measurements of the motion of individual particles. Imaging the surface of the flowing layer, we extract flow angles, velocities, drift and diffusion for different particle types and mixtures of particles. Surprisingly, the direction of surface drift and steepest flow angle do not coincide and that surface drift cannot explain the axial segregation in our mixtures. On the other hand, particles in small particle bands flow significantly faster then particles in large particle bands, and this can be observed before visible band formation. We discuss the possible role of velocity differences in the axial segregation process. We characterize the fluidity of the flowing layer from its response to gentle sideways forcing.   

Mechanisms in size segregation of binary granular mixtures

Shaking of a mixture of large and small particles can lead to segregation. One distinguishes between the Brazil-nut effect (large particles go to the top) and its opposite, the reverse Brazil-nut effect. In this talk, experiments of vertically shaken binary mixtures are presented. Using image analysis, the number of large particles visible at the top and bottom of the granulate are counted to determine the state of segregation. By complementing these results with molecular dynamics simulations, we are able to identify different segregation mechanisms discussed in recent theoretical approaches: a geometrical mechanism called void filling, transport of particles in sidewall-driven convection rolls, and thermal diffusion, a mechanism predicted by kinetic theory.   

Upward penetration through a granular medium

We measure the force needed to push a flat plunger upwards through a granular medium. The plunger begins flush with the base of the grains' container, and we focus upon the force necessary to initiate motion. The data show that this break-out force increases monotonically with plunger diameter and pile height as expected. In contrast to previous measurements of the force needed for vertical penetration from above and of the horizontal drag force, this break-out force has a strong dependence on bead diameter. Research supported by NASA grant NAG3-2384 and the NSF REU program.   

Correlation in granular shear flows

We investigate the effects of long-range correlation in simulations of sheared granular materials and develop theories to model force propagation in the dense regime. Measurements of spatial force correlations determine the size of force networks that emerge as the density is increased. The magnitude of the correlation length separates the dilute regime, where kinetic theory holds, from a dense regime where its assumptions break down. In the dense regime we introduce theories that successfully predict constitutive relations for the stress tensor, using geometrical properties of the force networks. Additionally, we observe that the behavior of the contact force distribution at small forces is highly dependent on the size of the force networks.   

Shear reversal in granular flows

The reversal of the shear direction in flow of monodisperse and bidisperse granular matter in a shear cell of Taylor-Couette type is characterized experimentally. By changing the boundary conditions we tune the location and width of the shear band in steady state flow. When the shear direction is reversed, the system compacts over a characteristic length of half a particle diameter, and shear forces reach a steady state over a chacteristic length of 1-3 particle diameter. A linear strain is found at the onset of shear reversal before a steady state shear band develops. We associate this extra strain during shear reversal with the displacement needed to jam particles in regions away from the shearband. We find that the strain decreases with increasing particle size for a fixed system size. We also find radial components in average particle velocities at the top surface, suggesting a convection current in the bulk.   

Flux from a vibrated granular medium

We have studied vertically vibrated granular media by measuring the flux through a hole in the container's bottom surface. We find that when fully fluidized, the flux is controlled by the peak velocity of the vibration, $v_{max}$, i.e., the flux is nearly independent of the frequency and acceleration amplitude for a given value of $v_{max}$. The flux decreases with increasing peak velocity and then becomes constant for the largest values of $v_{max}$. We demonstrate that the data at low peak velocity can be quantitatively described by a hydrodynamic model. By contrast, the nearly constant flux at larger peak velocity signals a crossover to a state in which the granular density near the bottom is insensitive to the energy input to the system. This research was supported by the NASA through grant NAG3-2384 and the NSF REU program through grant DMR 0305238.   

Session N10: Focus Session: Frontiers in Computational Chemical Physics III

Development of effective models for vectorial biological processes

Progress towards understanding vectorial processes in biological systems using theoretical and computational approaches will be discussed. The specific problems of interest include mechanochemical coupling in the molecular motor myosin and vectorial proton pumping in cytochrome c oxidase. Several related methodological developments will be discussed, which include boundary potential for QM/MM simulations, effective QM methods for treating long-range proton transfers and phosphate chemistry, and coarse-grained models for describing large-scale motions in biomolecules. Quantitative benchmark calculations of these methods have been carried out using both model and realistic biological systems. The application of these methods to myosin and cytochrome c oxidase will be briefly presented.   

Hierarchical Coarse-Grained Models for Polymer Simulations

Structural and thermodynamic properties of industrial and bioengineering materials can be better investigated using atomistic simulations. However, current large-scale atomistic simulations remain computationally demanding. It is thus desirable to seek alternatives to perform efficient and informative mesoscopic simulations. We have developed a coarse-grained intermolecular force (CGIF) model for polymer nanostructures and nanocomposites. This model can effectively capture the stereochemical response to anisotropic long-range interactions. The coarse-graining procedure forms the basis to perform a hierarchy of simulations starting with the quantum-chemistry calculations to coarse-grained molecular dynamics toward continuum modeling. We have applied this procedure to several cases from alkane to benzene to fullerene. For liquid methane, molecular dynamics simulations using the CGIF model reproduce the structural properties calculated using the atomistic force field. The coarse-grained energetics of benzene clusters has well reproduced the results using electronic structure calculations. The subtle anisotropy in the interaction potential of fullerene dimer is also well represented by the CGIF model and is consistent with that calculated using the Brenner force field.   

Dynamic Phase Transitions in Coupled Motor Proteins

The effect of interactions on dynamics of coupled motor proteins is investigated theoretically. A simple stochastic discrete model, that allows to calculate explicitly the dynamic properties of the system, is developed. It is shown that there are two dynamic regimes, depending on the interaction between the particles. For strong interactions the motor proteins move as one tight cluster, while for weak interactions there is no correlation in the motion of the proteins, and the particle separation increases steadily with time. The boundary between two dynamic phases is specified by a critical interaction that has a non-zero value only for the coupling of the asymmetric motor proteins, and it depends on the temperature and the transitions rates. At the critical interaction there is a change in a slope for the mean velocities and a discontinuity in the dispersions of the motor proteins as a function of the interaction energy.   

Reduced-Dimensional Models for Chemical Dynamics in Complex Environments

Nonstationary Langevin models have been developed that are capable of capturing feedback between complex environments and the underlying molecular constructs which in turn collectively comprise the environment. Although initial justifications for this formalism were heuristic and phenomenological, in recent work we have shown that in some cases it arises as the projection of a simple model of a chemical system bilinearly coupled to a harmonic bath with a time-dependent coupling. Moreover, the stochastic model can be used to surmise the diffusion of a tagged particle in a colloidal suspension which swells or shrinks with time. Alternatively, a liquid crystal, modelled as a colloidal suspension of orientable bodies, can also exhibit driven (time-dependent) behavior by way of the rotation of a magnetic field. Once again, the diffusion of a tagged particle under such time-dependence, can be surmised by the stochastic model. Thus these models allow for a substantial reduction of the dimensionality of a complex environment while retaining its multiple-time-scale features.   

Structure of Alzheimer's 10-35 $\beta$ peptide from replica-exchange molecular dynamics simulations in explicit water

We report a replica-exchange molecular dynamics study of the 10-35 fragment of Alzheimer's disease amyloid $\beta$ peptide, A$\beta$10-35, in aqueous solution. This fragment was previously seen~[J. Str. Biol. 130 (2000) 130] to possess all the most important amyloidogenic properties characteristic of full-length A$\beta$ peptides. Our simulations attempted to fold A$\beta$10-35 from first principles. The peptide was modeled using all-atom OPLS/AA force field in conjunction with the TIP3P explicit solvent model. A total of 72 replicas were considered and simulated over 40 ns of total time, including 5 ns of initial equilibration. We find that A$\beta$10-35 does not possess any unique folded state, a 3D structure of predominant population, under normal temperature and pressure. Rather, this peptide exists as a mixture of collapsed globular states that remain in rapid dynamic equilibrium with each other. This conformational ensemble is seen to be dominated by random coil and bend structures with insignificant presence of $\alpha$-helical or $\beta$-sheet structure. We find that, overall, the 3D structure of A$\beta$10-35 is shaped by salt bridges formed between oppositely charged residues.Of all possible salt bridges, K28-D23 was seen to have the highest formation probability, totaling more than 60\% of the time.   

Orbital Energetics and Molecular Recognition

We present data demonstrating that orbital eigenenergy fluctuation recorded in the course of ab initio molecular dynamics calculations contains information relevant in determining molecular behavior and recognition. A simple scheme is presented that maps these data to molecular descriptors. Using computational drug design as the context, these descriptors are compared with previous electronic eigenvalue descriptor methods with encouraging results. Finally we discuss further methods of mapping electronic structure based molecular dynamics trajectories to Quantitative Structure Activity Relationships (QSAR).   

Unrestricted Hartree-Fock Investigation of the Electron Distribution on the Heme System in Azidohemoglobin-$^{57m}$Fe and $^{14}$N Hyperfine Interactions.

We have a program of investigations in progress on the electronic structure of azidohemoglobin by the first-principles Unrestricted Hartree-Fock procedure to understand the substantial amount of magnetic (g-tensor), magnetic hyperfine, and nuclear quadrupole interaction, data available [1] from electron paramagnetic resonance, Mosbauer and electron-nuclear double resonance measurements. Earlier semi-empirical Self-Consistent Charge Extended Huckel investigations have provided semiquantitative results [2] with different degrees of agreement for the available properties and suggested the need for more accurate and quantitative investigations. Results of our investigations will be presented for the $^{57m}$Fe and $^{14}$N nuclear quadrupole and magnetic hyperfine interaction properties and compared with experimental data. *Also UCF Orlando [1] See Refs. 2-4 listed in Ref.[2]. [2] Santosh K. Mishra, J.N. Roy, K.C. Mishra and T.P. Das, Theo. Chim. Acta 75, 195(1989).   

Polymer dynamics and the folding rates of fast folding proteins

In recent years, minimal models of fast folding proteins has enabled considerable agreement between computation, theory, and experiment. The assumptions associated with most simple models of fast folding proteins (Go-models) give rather robust results in terms of coarse grained description of the transition state ensemble. One aspect of the folding mechanism that has received less attention is describing the conformational dynamics responsible for the folding rate prefactor, $k_0$. Here, we consider the distribution of prefactors of fast folding proteins: does local dynamics influence $k_0$, or can one reasonably expect that $k_0$ is essentially the same regardless of contact order or mean structure of the transition state ensemble. We address this question by considering the folding routes of a wide variety of fast folding proteins using a polymer based model in which structural ensembles are parameterized by the degree of localization about the native structure.   

A United Atom Model for Simulation of DNA from Angstroms to Microns in Length

For several years, single molecule pulling experiments have given insights into the stability of DNA. Many descriptions of DNA, from atomistic to continuum, have proven successful at reproducing observed behavior.~ We have found, however, that there is no suitable model for several problems of interest, including viral packaging of DNA and microarray interactions, where the size of the molecules prohibits atomistic representations, but continuum and linear bead-spring models do not contain the required molecular level of detail.~ Emerging technologies require that mesoscopic models of DNA be developed, capable of describing length scales in the 5 to 500 nm range. One of the main challenges is to preserve a coupling between the phenomena seen at longer length scales (e. g.~ a persistence length of 50 nm) while incorporating the features needed for smaller scales (e. g. charge effects, geometry, and base specificity).~ We have developed a coarse grain description of DNA which reduces the complexity of a nucleotide to three interaction sites.~ The model is capable of describing sequence information, bubble formation, and salt effects in simulations of DNA up to a few microns in length.~ The predictions are in remarkable, quantitative agreement with experiment, and shed light into the coupling of multiple length scales and interactions to yield unique behaviors and functions.   

Diffusion Monte Carlo applied to weak interactions - hydrogen bonding and aromatic stacking in (bio-)molecular model systems

Dispersion (Van der Waals) forces are important in many molecular phenomena such as self-assembly of molecular crystals or peptide folding. Calculating this nonlocal correlation effect requires accurate electronic structure methods. Usual density-functional theory with generalized gradient functionals (GGA-DFT) fails unless empirical corrections are added that still need extensive validation. Quantum chemical methods like MP2 and coupled cluster are more accurate, yet limited to rather small systems by their unfavorable computational scaling. Diffusion Monte Carlo (DMC) can provide accurate molecular total energies and remains feasible also for larger systems. Here we apply the fixed-node DMC method to (bio-)molecular model systems where dispersion forces are significant: (dimethyl-) formamide and benzene dimers, and adenine-thymine DNA base pairs. Our DMC binding energies agree well with data from coupled cluster (CCSD(T)), in particular for stacked geometries where GGA-DFT fails qualitatively and MP2 predicts too strong binding.   

Computational Methods for Enhanced Conformational Kinetics

We present and analyze two general method to calculate time-correlation functions from molecular dynamics on scaled potentials or from molecular dynamics with artificial momenta distributions. They are useful for complex systems whose simulations are affected by broken ergodicity. Depending on the value of the scaling factor or of the details of the momentum distributions, correlation functions can be accurately calculated for times that can be orders of magnitude longer than those accessible to current molecular dynamics simulations. We show that the exact value of the correlation functions of the original system can be obtained, in principle, using an action-reweighting scheme based on a stochastic path-integral formalism. Tests on model systems and peptides are exemplified. We also show that free energy profiles using Jarzynski's identity can be more effectively calculated within this scheme.   

Session N11: Focus Session: Aerosols, Clusters, Droplets: Physics and Chemistry of Nanoobjects I: Helium Nanodroplets I

Imaging the Photodynamics of Doped Helium Droplets

During the last decade helium nanodroplets have been established as an ideal spectroscopic matrix. Helium droplets are also thought to be ideal low temperature nanoreactors because of their ability to stabilize weakly bound species. As the focus is nowadays shifting to the study of chemical reactions in liquid helium droplets, question related to the energy relaxation and solvation dynamics become more and more prominent. To address these questions experiments have been performed in with species with a well defined kinetic energy distribution have been created via the photodissociation molecules residing inside helium droplets. The velocity distributions of the photofragments that have escaped from the droplets have been determined using ion imaging techniques. The analysis of speed distributions as function of droplet size and precursor has enabled to obtain a consistent picture of the mechanisms underlying the translational motion of these non-thermal species through this quantum liquid. Additional information on the solvation dynamics could be obtained by using non-resonant ionization techniques in these experiments. More recently the translational dynamics of quasi-free electrons in helium droplets has been investigated by means of photoelectron spectroscopy. The results on these experiments indicate that the relaxation of the electrons is governed by the same mechanism responsible for the kinetic energy relaxation of non-thermal neutral molecules.   

Photoionization and photoelectron spectroscopy of doped helium nanodroplets

Photoionization and photoelectron spectra for helium nanodroplets doped with rare gas atoms and SF$_{6}$ will be reported. The experiments were conducted using tunable synchrotron radiation at the Advanced Light Source in the photon energy range of 14-26 eV. Time-of-flight mass spectra will be presented, along with photoion and photoelectron images. The results will be compared to previous electron impact ionization data.   

Photoinitiated processes in and on liquid helium

Photoinitiated processes that involve molecules in He$_{n}$ droplets are examined. Excitation to states that contain Rydberg character results in repulsion between the electronically excited embedded molecules and the surrounding helium. Even after the helium has, moved further from the molecular core, the situation is unstable in the sense that the electronically excited species prefer the surface. The timescale for transport to the surface is $<$ 10 \textit{ns} duration of the laser pulse. The resulting surface-bound species can be ionized, yielding small clusters of the form He$_{m}$NO$^{+}$, where $\langle {\rm g} \quad \rangle $ is of order 10,000. The possibility of observing high Rydberg states in which the electron is outside the helium droplet will be discussed. A vastly different case of photoexcitation occurs when the excited potential is coupled to a lower one via conical intersection.   

Wave packet propagation of alkali dimers attached to helium nanodroplets

Real-time spectroscopy of alkali dimers attached to helium nanodroplets has been studied by femtosecond pump-probe spectroscopy. Wave packet propagation in different electronic states of Na$_2$ and K$_2$ molecules was investigated. The perturbation of the helium environment allows in particular to observe electronic ground state vibrational motion. Furthermore, for the first time wave packets in alkali dimer triplet states are observed. Finally, the slight change of the vibrational structure when desorbing from the helium droplet can be utilized to determine desorption times upon laser excitation.   

Metal clusters in helium droplets: Fulfilling the promise

In 2001 we demonstrated that superfluid helium droplets, coupled to high-resolution infrared spectroscopy, could be used to investigate the intermolecular interactions and structures of metal cluster-adsorbate systems. The HCN-Mg$_{n}$ (n = 1-6) clusters investigated provided several interesting surprises and taught us many valuable lessons but nevertheless remained a somewhat uninteresting system from the point of view of catalysis and reactivity. Recently, we have overcome some significant experimental challenges and are finally beginning to fulfill the promise of using superfluid helium droplet spectroscopy for the investigation of more ``chemically interesting'' systems. In this talk we will present the infrared spectra of a single HCN molecule bound to copper and silver clusters. From these spectra we were able to obtain information about the adsorbate-metal cluster interactions, as well as obtaining direct structural information through high-resolution spectra.   

Adaptive Clustering of Adatoms Around Ionic Dopants in He Droplets: Quantum Calculations

The structuring and collocation of individual He atoms as quantum objects around simple atomic and molecular impurities has been the subject of a great number of studies, both experimentally and from the theoretical viewpoint [1,2] since the advent of droplets experiments, where such nanoscopic containers have been exploited to provide a sort of nanocryostat for the analysis of the dopant's spectroscopic behavior [3]. We have carried out computations of potential fields within small clusters which contain a variety of ionic dopants using post-Hartree-Fock, ab initio methods and have further endeavoured to extract from them the corresponding classical and quantum structuring of such impurities within clusters of variable size. For the latter enquiry we have employed both classical optimization methods and Quantum Diffusion Monte Carlo analysis. Results for both atomic (Li$^{+})$ and molecular (LiH$^{+}$, OH$^{+}$, OH$^{-})$ ionic dopants will be presented at the meeting. \newline \newline [1] J.P. Toennies and A.F. Vilesov, \textit{Angewandte Chemie} \textbf{43}, 2622 (2004). \newline [2] e.g. see: F. Paesani, A. Viel, F.A. Gianturco and K. Whaley, \textit{Phys. Rev. Lett.} \textbf{90}, 073401 (2003). \newline [3] J.P. Toennies and A.F. Vilesov, {\{}$\backslash $it Ann. Rev. Phys. Chem.{\}} {\{}$\backslash $bf 49{\}}, 1 (1998).   

Electron-impact ionization mass-spectrometry of molecules and clusters in a pulsed helium droplet source

A pulsed helium droplet source has been developed and characterized. The nozzle geometry was found to be critical in allowing controlled tuning of helium nanodroplet size by variation of the stagnation pressure and temperature. The average droplet size scales according to a simple {\{}$p$,$T${\}} scaling law, placing pulsed helium nanodroplet sources on a par with cw sources for the first time. Using this pulsed source, the ability of helium nanodroplets to impede ion fragmentation in electron impact mass spectrometry has been explored. A number of haloalkanes and C$_{1}$--C$_{6}$ alcohols were selected as the target species. The presence of helium alters the fragmentation patterns when compared with the gas phase, with some ion product channels being more strongly affected than others. Parent ion intensities are also enhanced by the helium for alcohols, but only for the two cyclic alcohols studied, cyclopentanol and cyclohexanol, is this effect large enough to transform the parent ion from a minor product (in the gas phase) into the most abundant ion in the helium droplet experiments. The results obtained are difficult to explain solely by rapid cooling of the excited parent ions by the surrounding superfluid helium, although this undoubtedly takes place. A second factor also seems to be involved, a cage effect which favors hydrogen atom loss over other fragmentation channels.   

Electron and Ion Emission from Clusters exposed to Strong Laser Fields

When clusters interact with intense optical laser pulses energetic and highly charged atomic fragment ions e.g. are generated$^1$. In contrast to atoms the efficiency of the process could be enhanced by choosing a pair of optical delayed pulses instead of a single but more intense femtosecond pulse$^2$. In metals the stronger charging of the clusters can qualitatively be explained by a plasmon enhanced ionization process. We extended our studies and have made a compared analysis of the emission of highly charged ions and energetic electrons the interaction dynamics of intense femtosecond laser fields with nanometer-sized silver clusters. Using a pair of laser pulses with variable optical delay the time-dependent cluster response is resolved. A dramatic increase both in the atomic charge state of the ions and the maximum electron kinetic energy is observed for a certain delay of the pulses. Corresponding Vlasov calculations on a metal cluster model system indicate that enhanced cluster ionization as well as the generation of fast electrons coincide with resonant plasmon excitation.$^3$ \begin{enumerate} \item L.~K{\"o}ller, M.~Schumacher, J.~K{\"o}hn, S.~Teuber, J.~Tiggesb{\"a}umker, and K.-H. Meiwes-Broer, Phys. Rev. Lett. {\bf 82}, 3783 (1999). \item T.~D{\"o}ppner, Th. Fennel, Th. Diederich, J.~Tiggesb {\"a}umker, and K.-H. Meiwes-Broer, Phys. Rev. Lett. {\bf 94}, 013401 (2005). \item Th.~Fennel, G.F. Bertsch, and K.-H. Meiwes-Broer, Eur. Phys. J. D {\bf 29}, 367 (2004). \end{enumerate}   

Energy and angular momentum densities of states of ripplons on the surfaces of helium nanodroplets and bubbles

We present an analytical evaluation of the statistical densities of states of surface excitations (“ripplons”) of (1) isolated liquid-drop helium nanoclusters and (2) large multielectron bubbles in bulk liquid helium [1]. For the former case, the calculation of the energy density of states, $\rho (E)$, can be accurately performed in a microcanonical ensemble formalism [2] and yields an expression which is extremely close both to the exact numerical calculation and to its fitted form [3]. For case (2) the canonical ensemble formulation is appropriate. For both systems, the calculation is then extended to yield the energy- and angular-momentum- resolved density of states $\rho (E,L)$ (c.f. [3]); in other words, the ripplon moment of inertia is described. \newline \newline [1] J.Tempere, I.F.Silvera, J.T.Devreese, Phys.Rev.Lett. {\bf 87}, 275301 (2001). \newline [2] J.U.Andersen, E.Bonderup, K.Hansen, J.Chem.Phys. {\bf 114}, 6518 (2001). \newline [3] K.K.Lehmann, J.Chem.Phys. {\bf 119}, 3336 (2003).   

Excited states of He$_{N}$H$^{- }$Clusters

We use correlation function quantum Monte Carlo (CFQMC) method to compute the excited states of the weakly bonded Helium clusters with the H$^{-}$ impurity (He$_{N}$ H$^{-}$, N=1,{\ldots},5). The methodology was tested through comparison with previously published results for the ground state of the system with N=1-11. Our test basis set consists of a standard pair-product ground state multiplied by a polynomial. Our tests for HeH$^{-}$ and He$_{2}$H$^{-}$ demonstrated very good agreement with previously published discrete variable representation (DVR) results. We believe the lowest excited states of the larger clusters to be of similar quality and they can reveal important properties of these weakly bound systems, mainly on the effect of the impurity on the cluster and vice versa.   

Microwave Spectroscopy in Helium Nanodroplets.

We have implemented a microwave resonator, i.e. a Fabry-Perot cavity, into a helium nanodroplet instrument. The cavity consists of two spherical aluminum mirrors with radius of curvature of 13 cm and diameter of 10 cm. The cavity is mounted in a coaxial fashion into the instrument to maximize the interaction length between radiation and doped helium droplets. The helium droplet beam enters and exits the cavity through a hole in each of the mirrors. One of the mirrors can be adjusted to tune the cavity into resonance. The output of a cw microwave synthesizer can be amplified by a traveling wave tube amplifier to powers of about 25 Watt and is coupled into the cavity through a simple wire hook antenna. Detection is accomplished using the depletion technique. We have measured the spectrum of the J=2-1 transition of carbonylsulfide demonstrating the sensitivity of this method. Power saturation was observed and will be analyzed as will be the observed line width of the transition.   

Session N12: Focus Session: Alloy and Interface Composition

Boron nitride nanostructures: complete layers and nanomeshes

A highly ordered self-assembled nanomesh grows on a hot (1000 K) Rh(111) surface during 40 L (1 Langmuir=10$^{-6}$ $torr \times s$) of borazine (HBNH)$_{3}$ exposure [1]. Hexagonal boron nitride (\textit{h}--BN) units aggregate to form this double-layer network of 3 nm periodicity and 2 nm hole size. The two layers are offset so that nearly the entire underlying metal surface is covered. This system can be used as a template for supramolecular structures, as demonstrated with C$_{60}$ molecules, or for any purpose where a nanopatterned surface that is stable at high temperatures (1000 K) is needed. One of the driving forces for its formation is the large lattice mismatch of –6.9 \% between the \textit{h}--BN film and the Rh substrate. The growth of similar nanomeshes on different substrates is investigated, with the purpose to control hole size and shape. It is found that not only the lattice mismatch and the symmetry of the underlying metal play an important role but also the bonding between the nitrogen atoms and the substrate. In fact nanomeshes can be grown on Ru(0001) and on Ir(111) thin films but it does not form on Pd(111) nor on Pd(110). [1] M. Corso et al. Science, 303 (2004) 217.   

Measurements of Molecular Dynamics in Atomically Engineered Molecular Nanostructures

The quantum yield for exciting the motion of a single atom within a molecular nanostructure was measured with atomic spatial resolution. The molecular nanostructures consisted of a series of CoCu$_{n}$ and CoCu$_{n}$Co linear molecules fabricated on a Cu(111) surface. The Co atoms at the end of the molecules were induced to switch between two lattice positions using electron excitation in a scanning tunneling microscope (STM). The electron excitation and quantum yield were found to be spatially localized on an atomic scale. Above an electron energy threshold, the Co atom motion resulted from a predominantly single electron process. By systematically varying the molecular structure, atom motion within the molecule was shown to be dependent on molecular length and composition.   

Formation and Vibrational Entropy-Driven Disordering of Mo(100) and W(100) Surface Alloys

Atoms that are deposited on a surface of a dissimilar material may either remain on the surface or they may become incorporated in a surface or bulk alloy. Although the energetic differences between alloy and overlayer structures at T = 0 can now be understood from first principles in many systems, the entropic contribution to the system free energy, which governs the equilibrium structure, is less well understood. The formation and stability of Cu, Ag and Au-induced c(2x2) alloys at the Mo(100) and W(100) surfaces have been investigated with low energy electron microscopy and diffraction. The dependence of the c(2x2) diffraction intensity upon metal deposition flux reveals that alloy formation is governed by atomistic processes that are analogous to those that dictate overlayer island nucleation. An order-disorder transition is also observed that converts the surface from ordered alloy to disordered overlayer structure. Combined with knowledge of energetics that is obtained from first principles calculations, a comparison of disordering temperatures for alloys of the different metal species and substrates provides information on the decisive contribution of vibrational entropy to the system free energy. Effective Debye temperatures for metal adatoms are determined that are substantially lower than bulk values, but exhibit the expected mass and bond strength dependence.$^{ }$Vibrational entropy may also play a role in the stability of alloys at other surfaces.   

Measuring 3D Alloy Composition Profiles at Surfaces

A key challenge in thin-film growth is controlling structure and composition. Of particular importance is understanding how and why atomic-scale heterogeneity develops during growth. We have used low-energy electron microscopy (LEEM) to measure how the three-dimensional composition of an alloy film evolves with time at the nanometer length scale. By quantitatively analyzing the reflected electron intensity in LEEM, we determine the alloy composition and structure, layer by layer near a surface, with 9 nm lateral spatial resolution. As an example, we show that heterogeneity during the growth of Pd on Cu(001) arises naturally from a generic step-overgrowth mechanism that is likely to be relevant in many growth systems. This work was performed in collaboration with Jiebing Sun (UNH), Karsten Pohl (UNH), and Gary Kellogg (Sandia Labs).   

Threading Dislocation Pair Annihilation as a Mechanism for the Growth of Ordered 2D Nanocluster Arrays

The bottom-up approach of growing nanostructured ordered arrays of clusters on the misfit dislocation networks of strained metallic thin films requires a detailed understanding of the nucleation and film-adsorbate interaction processes. In the case of S adsorption on submonolayer Ag / Ru(0001), the Ag's short herring bone rectangular misfit dislocation unit cell of 54{\AA}x40{\AA} (19x16 Ag atoms) reconstructs into a well ordered triangular array of S filled vacancy islands 50{\AA} apart. Atomically resolved VT-STM measurements show that the S cluster growth mechanism involves a local restructuring of the misfit dislocation network of Ag with the final structure free of threading dislocations. The new symmetry and morphology of the composed S/Ag films is obtained via a threading dislocation annihilation mechanism where adjacent and opposite pairs of threading dislocations are replaced by the S filled Ag vacancy islands. The local character of the annihilation process is shown by the conservation of the unit cell size area of 21.5nm$^{2}$.   

Highly-spatial resolved surface structure and composition by LEEM image intensity analysis

Controlling the local structure and composition of a surface alloy is of great importance in thin film technologies. ~However, measuring the alloy's heterogeneity is very difficult, because existing experimental techniques either assume lateral homogeneity or have limited subsurface or chemical sensitivity. ~In this work~we have analyzed the electron diffraction intensity vs. incident energy curves of the (00) beam acquired from low-energy electron microscopy (LEEM) images. ~In contrast to conventional LEED-IV we are able to extract structural and local composition in the surface region with a lateral resolution of 8 nm. Two challenges in applying multiple electron scattering calculations to the analysis of LEEM data are the low and limited electron energy range (10 to 100 eV), which we address by a careful choice of the energy-dependent real and imaginary part of the optical potential. Our analysis of the LEEM IV curves for the clean Cu(001) and Pd/Cu(001) surfaces gives excellent agreement between experimental and best-fit data and good agreement with previous structural investigations. Our new analysis technique is capable of determining surface structure and composition with high accuracy.   

Structure of an ultra-thin Ag film on the Al(100) surface

The surface structure for one monolayer of Ag deposited on the Al(100) surface at room temperature has been studied using low energy electron diffraction (LEED), ion scattering spectroscopy (ISS) and Rutherford back-scattering spectroscopy (RBS). The Ag coverage was determined with RBS. We conclude that the Ag atoms form two domains of a buckled, quasi-hexagonal structure that is incommensurate with the Al(100) surface unit cell, having a repeat distance of 5 Al(100) interatomic spacings in the [110] direction. The LEED pattern shows a double-domain (5x1) structure with additional intensity in those spots corresponding to a (111) close-packed hexagonal layer. The analysis of the ISS results suggests that the heights of the adsorbed Ag atoms above the Al surface are not all the same, leading to the proposed buckling model. In addition, some Al atoms apparently move from the substrate up into the Ag adlayer.   

MEAM Potentials for Al-Mg Alloy: Application to Defects

The ab-initio calculations based on density functional theory (DFT) are performed for the Al and Mg crystals and their alloy in reference structures, such as NaCl structure. The lattice constant (volume), bulk modulus and shear moduli for each element and the alloy are determined from the total energy calculations. These material parameters are then used to determine the Modified Embedded Atom Method (MEAM) potentials for these elements and their alloys. The transferability of these parameters are tested by obtaining relevant physical quantities on structures different than the reference structures and compare them with the results from ab-initio calculations. MEAM potentials determined for these materials are used to study the structure and morphology of various form of defects of these materials.   

Disorder and Roughening at Surfaces of Silver/Gold Alloys

Attempts to obtain a clean and well-ordered surface in ultrahigh vacuum for several low index faces of a 50 at\% silver/gold alloy gave rise to an unexpected phenomenon. After several cycles of sputtering and annealing, the surfaces appeared clean using Auger spectroscopy but yielded low energy electron diffraction spots of poor quality (in the case of AgAu(110), no diffraction was observed at all). Many further time/temperature annealing protocols were attempted with no improvement. In addition to the diffraction results, which indicate a lack of long-range microscopic order at the surface, continued processing resulted in macroscopic roughening of the surfaces. Electron microscopy revealed the presence of features with sizes on the order of microns at the roughened surfaces. Analysis of the lineshapes of the diffuse LEED beams for the AgAu(111) surface indicates that the sizes of the ordered patches on the surface are less than a nanometer. These results will be presented in more detail and possible explanations for this extraordinary behavior will be discussed.   

Surface Freezing in Liquid Gold-Silicon Eutectic Alloy investigated with Surface X-ray Diffraction

The formation of a 2D AuSi crystalline lattice on top of liquid AuSi eutectic alloy was found in surface X-ray diffraction experiments. Up to 12 degree above bulk melting temperature ($T_ {melt}$ = 361$^{\circ}$C) we observe a powder like, 2D crystalline lattice. Increasing the temperature we find a first order phase transition. GID diffraction data was used to determine the 2D lattice parameters and the domain size was estimated to be larger than 0.9 $\mu m$. Normalized X-ray reflectivity shows an increase of a factor of 20 in comparison to classical systems, indicating that atomic layering normal to the surface is significantly enhanced for AuSi. Synchrotron measurements were performed at ChemMatCARS, Advanced Photon Source, Argonne National Lab and supported by DOE grants DE-FG02-88-ER45379 and DE-AC02-98CH10886.   

Determination of interface compositions by X-ray three-beam resonance diffraction

X-ray three-beam diffraction$(200/\bar {3}\bar {1}1)$under resonant conditions is used to measure the concentrations of the constituent elements of the interface between a (100) CdTe thin film and a (100) InSb substrate. The three-beam diffraction profiles versus the azimuth angle of rotation around [200] reveal a wide variety of change in phase shift due to resonance for photon energies in the vicinity of the Cd $L_{III} $ absorption edge. At different momentum transfers $q_r $ along [200], sensitive to the interfacial structure, the phase shift in the resonant state also provides sufficient information about the distributions of Cd and Te concentrations. With theoretical analysis for the crystallographic phase of the structure-factor triplets and the resonance phase shifts involved in the three-beam diffraction, it allows us to determine the composition of Cd and Te as a function of depth normal to the interface. Via the propagation of the secondary$(\bar {3}\bar {1}1)$reflected beam along the surface, possible interface structures parallel to the surface could also be deduced.   

LEED and Ab-Initio Study of the SmSi(111)-3x2 Reconstruction

The Si(111)3x2-Sm reconstruction that has been observed by STM produces a 3x1 pattern when viewed using LEED [1]. It has been suggested that similar behaviour for Si(111)3x2-Ba is due to the interference of the emergent electron amplitudes between adjacent registry shifted unit cells [2]. We have gathered LEED I(V) curves from this surface and here we present a quantitative comparison of these with a structural model that has been suggested in the literature [3] and with the results of our own ab-initio calculations done using the CASTEP [4] code. \\ \ \\ {[}1{]} {C. Wigren {\it et al},{\it Phys. Rev. B.},{\bf 48} (1993) 11014-11021}\\ {[}2{]} {J. Schafer {\it et al}, {\it Phys. Rev. B.}, {\bf 67} (2003) 85411-85415}\\ {[}3{]} {E. Ehret {\it et al}, {\it Surf. Sci.}, {\bf 569} (2004) 23-32}\\ {[}4{]} {M. D. Segall {\it et al}, {\it J. Phys.: Cond. Matt.}, {\bf 14} (2002) 2717-2743} \\   

Investigation of the interface structure in sputtered WSi$_{2}$/Si multilayers by in-situ synchrotron X-ray scattering.

Multilayer X-ray optics have many applications such as X-ray microscopy, X-ray lithography, and X-ray microanalysis. The interface imperfections are critical to the optical performance of the multilayer structures. We report on the growth of WSi$_{2}$ and Si amorphous thin films by dc magnetron sputtering in a vacuum chamber with 10$^{-9}$ Torr base pressure. In-situ synchrotron X-ray scattering with high temporal resolution has been employed to probe the surface and interface roughness evolution during film deposition. X-ray reflectivity simulations were performed using the IMD software package. It is found that the structure of WSi$_{2}$/Si multilayers is with an alternately smooth and rough interface. While Si layer roughens, WSi$_{2}$ layer is observed to smooth out an initially rough surface. The ion energy and flux assisting the growth may play a role in inducing this asymmetry in the interface roughness.   

Session N13: Focus Session: Ultrafast and Ultrahigh Field Chemistry I: Strong Field Phenomena

Strong-Field Physics with Coherently Prepared Molecular Targets

Intense, short laser pulses can create rotational wavepackets in molecules, resulting in transient preferential molecular alignment in a field free environment. The availability of aligned rather than randomly oriented molecular samples is enabling new strong-field molecular physics experiments which offer additional insight into a variety of complex phenomena. For example, high harmonic generation (HHG) is mediated by electrons that are first tunnel-ionized and then driven back into their parent ions by an intense laser field. Both the initial ionization and recollision events can be strongly dependent on the orientation of the molecular axis with respect to the laser field. Once this dependence is well understood, information regarding the structure of the parent molecule at the instant of the electron/ion recollision might be extracted from the resulting electron and/or photon emission. I will describe methods for manipulating and probing molecular alignment, as well as our recent measurements of the dependence of intense laser ionization rates, HHG yields, and the polarization of high-order harmonics on the alignment of the molecular axis relative to the polarization direction of the intense laser field.   

Non-adiabatic Electronic Excitation of Linear Polyenes in the Strong Field Regime

Using a newly developed unitary transform time-dependent Hartree-Fock (UT-TDHF) algorithm, the electronic response to an ultrashort strong-field laser pulse was studied on a series of molecules -- ethylene, butadiene, and hexatriene -- in which molecular size and conjugation increase systematically. The evolution of electronic subsystem of molecules exposed to 760 nm 8.75$\times $10$^{13}$ W/cm$^{2}$ of 7 fs duration was calculated using the 6-31G(d,p) basis set. Two scenarios are envisioned: in the first, the molecule interacts with the pulse immediately after ionization; in the second, sufficient time elapses for the molecular geometry to relax. The non-adiabatic behavior of the instantaneous dipole moment and the charge distribution in a molecule is more pronounced for the monocations than for the dications or neutrals. For a given charge state and geometry, the non-adiabatic effects increase with the length of the molecule. As Fourier analysis reveals, the residual (after-pulse) oscillations of the dipole moment are mainly due to non-resonant excitations of the lowest excited states with significant oscillator strength. For each molecule, the non-adiabatic coupling is greater for geometry with the lower excitation energies.   

A new type of wavelength dependence in strong-field ionization

It is commonly assumed that in mid-IR region the strong-field ionization approaches quasistatic limit (tunneling, or ADK regime) and ceases to depend on the laser wavelength. Contrary to this notion, ionization yields for the noble gas Xe at intensities from 10$^{13}$-10$^{15}$ W cm$^{-2}$ for wavelengths spanning from 800 to 1500nm reveal strong and counterintuitive wavelength dependence. There is an increasing ionization probability in the strong field regime as the excitation wavelength increases from 800nm to 1500 nm at fixed field intensity. The measured thresholds for the ionization event scale approximately as $\lambda ^{-2}$. We developed a simple quantitative model that extends through-the-barrier tunneling with single photon ionization from a Rydberg intermediate state and captures the observed wavelength dependence. This wavelength dependence will be reduced to some degree if the ionization occurs in a strong DC electric field that is capable to independently ionize the Rydberg states. The wavelength dependence of ionization rate in the quasitstatic regime is of considerable importance for ascertaining the correct physics for various strong field processes.   

Orientation effects in Coulomb explosion of H$_{2}$S in intense laser fields studied by coincidence momentum imaging

The Coulomb explosion of H$_{2}$S in an ultrashort intense laser field (12 fs, 0.33 PW/cm$^{2})$, H$_{2}$S$^{3+} \quad \to $ H$^{+}$ + S$^{+}$ + H$^{+}$, has been studied by the coincidence momentum imaging method to study how the nuclear dynamics depends on the molecular orientation with respect to the laser polarization vector. When the molecular plane, defined as the plane spanned by the fragment momentum vectors, is perpendicular to the laser polarization vector ($\varepsilon )$, the distribution of the total kinetic energy release E shows a peak at E = 21(1) eV. On the other hand, the distribution peak is observed at a substantially smaller value, E = 15(1) eV, when the molecular plane is perpendicular to $\varepsilon $, showing that the Coulomb explosion dynamics of H$_{2}$S depends sensitively on the orientation of the molecular frame to the laser polarization vector. The difference in the peak kinetic energies indicates that the geometrical structure for the perpendicular orientation is less elongated prior to the Coulomb explosion than that for the perpendicular orientation.   

Ultrafast hydogen atom dynamics of small hydrocarbon molecules in intense laser fields - Ejection of H3+ and hydrogen migration

In ultrashort intense laser fields, molecules are decomposed into fragments via a variety of dissociation pathways. Among them, ultrafast migration of hydrogen atoms within molecules as well as efficient ejection of H3+ molecular ions are noteworthy [1]. By referring to our recent studies on small hydrocarbon molecules in intense laser fields [2] by the coincidence momentum imaging method [3], I will show how ultrafast dynamics of hydrogen atoms are induced within duration of ultrashort intense laser pulses. \newline \newline [1] Y. Furukawa, K. Hoshina, K. Yamanouchi, H. Nakano, Chem. Phys. Lett. 414, 117 (2005). \newline [2] T. Okino, Y. Furukawa, P. Liu, T. Ichikawa, R. Itakura, K. Hoshina, K. Yamanouchi, and H. Nakano, Chem. Phys. Lett. 419, 223 (2005). \newline [3] H. Hasegawa, A. Hishikawa, K. Yamanouchi, Chem. Phys. Lett. 349, 57 (2001).   

Femtosecond Laser Ionization of Organic Amines with Very Low Ionization Potential.

The interaction between high intensity femtosecond laser and molecules is one of the most attractive areas in laser chemistry and ionization is the most fundamental subject. Theoretical consideration successfully reproduced the ionization behavior of rare gases. However, the understanding of ionization mechanisms of large molecules is difficult more than those of rare gases due to their complexity. Generally speaking, molecules are harder to ionize than rare gases even if they have the same ionization potential. The suppressed ionization phenomena are one of the important features of molecular ionization. Hankin \textit{et al}. examined 23 organic molecules with ionization potentials between 8.25 and 11.52 eV. We have examined ionization and/ or fragmentation of many organic molecules, including aromatic compounds, halogenated compounds, methane derivatives etc. at various wavelengths below 10$^{16}$ Wcm$^{-2}$. In order to investigate the nature of molecular ionization, it is interesting to examine a variety of molecule in a wide range of ionization potential. In this study, we examined several organic amines because we can explore the uninvestigated ionization potential range down to 5.95 eV. In addition to the significant suppression of the ionization rates, stepwise ionization behavior, which was not observed in rare gases, was observed.   

All-solid-state, ultraviolet, high power laser system using Ce:LiCAF as a gain medium

High peak-power, femtosecond, ultraviolet (UV) lasers have attracted new interest. Chirped pulse amplification (CPA) in the UV region has been demonstrated using Ce:LiCaAlF6 (Ce:LiCAF) crystal as the gain medium. The peak power of the amplified and compressed pulse (115 fs) reached 30 GW at 290 nm. To increase the peak power to the terawatt (TW) level, further pulse compression is desired. Since Ce:LiCAF has a tunability of 281nm to 315nm, it holds promise for 3-fs pulse generation which are required for seeding TW-class Ce:LiCAF lasers. The pulse-width of the frequency-tripled Ti:sapphire regenerative amplifier was measured to be 210 fs. The seed pulses were then focused into a hollow fiber filled with argon to spectrally broaden the pulses due to self-phase-modulation. The pulses were then compressed to 25 fs by dispersion-compensation. The fourth harmonics of a Nd:YAG laser (266 nm) is an ideal pump source as it falls within the absorption band Ce:LiCAF. We have generated 430 mJ fourth harmonics with a total conversion efficiency of 30.5{\%} using Li2B4O7$_{ }$(LB4) crystals. A Ce:LiCAF$_{ }$double-pass power-amplifier was then designed with a peak energy of 98 mJ for a 13 mJ seed pulse and an extraction efficiency of 25{\%}.   

Influence of linear chirp on non-vertical transitions in a dye solution

Chirped laser pulses can manipulate vibrational coherences in dye solutions. We show that the effect of linear chirp also depends strongly on the power spectrum of the ultrafast pulse. We use a programmable phase mask to control the spectral phase of 25-fs visible pulses generated in a noncollinear optical parametric amplifier (NOPA). Following chirped pulse excitation of the oxazine laser dye LD690, coherent oscillations are observed in the time-resolved transient absorption. When a vertical transition is excited, negative chirp leads to strong ground state vibrational coherences as resonant stimulated Raman processes are enhanced by the frequency sweep of the laser pulse. When the chirped pulse excitation is higher frequency than the Franck-Condon transition, the optical response of the dye is significantly affected by excited state absorption. In this case, coherences are established in excited states but no chirp-enhancement of the ground state wavepacket is observed.   

Terahertz absorption spectrum of water vapor at different humidity at room temperature

We measured the absorption spectrum of water vapor in 0.2-2.4THz range at different humidity from 17{\%} to 98{\%} at room temperature using Er: doped fiber laser (IMRA America Inc.) based terahertz time-domain spectroscopy. The experiments were performed in a nitrogen-purged cage at atmosphere environment to obtain the reference and water absorption information. The seventeen absorption lines were observed due to water molecular rotations in the ground vibration state. The first three absorption lines at low frequencies increase with humidity, following the Beer-Lambert Law, while some of high frequency lines were found to decrease with humidity. These effects will be discussed. The observed line broadening is due to collisions occurring among water and nitrogen molecules.   

Session N16: Focus Session: Hydrogen Storage III

Ambient and High Pressure Structural Studies on TiH$_{2}$

Currently metal hydrides attract intense research interest because of their potential application as hydrogen storage materials. We performed in situ high-pressure synchrotron x-ray diffraction as well as high-pressure Raman spectroscopy studies on TiH$_{2}$ at pressures up to 20 GPa. Low temperature ambient pressure x-ray diffraction studies were also carried out. A phase transition from a high symmetry cubic structure to a lower symmetry tetragonal structure was observed as temperature is lowered below room temperature. The unit cell parameters as well as the equation of state were calculated. To the best of our knowledge this is the first report of high pressure synchrotron x-ray diffraction as well as high-pressure Raman spectroscopy studies on TiH$_{2}$.   

Optical Spectroscopy of PdO and Pd thin Films under hydrogen exposure

Palladium oxide (PdO) is a p-type semiconductor with a bandgap appropriate to absorb light in the visible range an thus generating an electrical current. This bandgap, being of the order of 2.5 eV, remains yet not accurately determined. We are currently setting up a new experiment in which we are able to monitor the evolution of reflectivity or transmission, for several wavelengths in the visible spectrum, and the resistivity as a function of the exposure time to hydrogen. We should be able to calculate the evolution of bandgap of the semiconductor as a function of reduction from the reflectivity data. Pd films also change reflectivity properties during hydrogen absorption. The experimental set up consist of a tungsten light, Spectrapro 275 monochromator with a diffraction grid of 1200 lines/mm, and a silicon diode detector. Preleminary results will be shown. Based on these data we expect to depict a model for the changes measured.   

First-principles prediction of a new metallic carbon hydride: bcc-CH$_{2}$

The observation of a new carbon phase in nanoparticles having the body-centered-cubic structure (bcc) has been reported very recently. However, has been suggested that hydrogen is present in the samples forming solid CH$_{2}$ with the anti-cuprite structure. The structural and electronic properties of bcc-C and bcc-CH$_{2}$ are unknown. In the present work we have studied the elastic stability and the electronic structure of these systems by means of first-principles total-energy calculations. The results were obtained with the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have evaluated the structural stability via the elastic properties, we find that bcc-CH$_{2}$ is stable with a lattice parameter very close to the experimental value. In addition, we find that the electronic structure of bcc-CH$_{2}$ exhibits metallic behavior with a relatively high density of states at the Fermi level. The relevance of this new hydride to the problem of hydrogen storage is discussed.   

Ab Initio Thermochemistry and Elastic Properties of Alkaline Earth Hydrides

In addition to comprising a scientifically interesting class of materials, the binary alkaline earth hydrides are important components of hydrogen sorption/desorption reactions. Of critical importance for predicting the thermodynamic stability of hydrides is the enthalpy of hydride formation, $\Delta $H, which links the temperature and pressure of hydrogen sorption via the van't Hoff relation. We compare LDA and GGA predictions of the heats of formation and elastic properties of alkaline earth metals and their binary hydrides BeH$_{2}$, MgH$_{2}$, CaH$_{2}$, SrH$_{2}$, and BaH$_{2}$ using a plane wave density functional method. Phonon calculations using the direct method enabled prediction of the zero point energies of each material and the 0K and 298K heats of formation. We also computed the 0K and 298K cohesive energies for the alkaline earth metals. Born effective charge tensors were computed via the Berry phase method and enabled prediction of the phonon dispersion curves with LO/TO zone center splittings. It was found that the LO/TO splittings have no effect on the computed zero point energies and heats of formation. The elastic constants were computed with a least squares fitting method using a set of sequentially-applied strains to improve the accuracy of each calculation. Comparison of results from the least squares methodology with prior results using the Hartree-Fock method suggest that the former is substantially more accurate for predicting hydride elastic properties.   

Hydrogen uptake and the 18-eletron rule

Hydrogen is considered to be an ideal energy carrier in the foreseeable future; however, the key problem is its storage. Solid state materials capable of storing hydrogen with high gravimetric (9 wt {\%}) and volumetric density (70 g/L) operating under ambient thermodynamic conditions and exhibiting fast hydrogen sorption kinetics are of practical importance. It is clear that the storage material should consist of light elements such as Li, B, and C etc. Hydrides of these elements are too strongly bound to be easily desorbed. Attempts were made to deposit light weight transition metals on carbon surfaces such as fullerenes, nanotubes etc., however, they tend to cluster together reducing hydrogen uptake dramatically. One way to achieve high storage is to functionalize simple organic molecules such as C$_{4}$H$_{4}$, C$_{5}$H$_{5}$ etc. with light weight metals such as Sc and Ti. In this presentation, we will discuss based on DFT computations, the dependence of hydrogen uptake on the nature of substrate, the desportion energies, and the nature of bonding.   

\textbf{\textit{Ab initio}}\textbf{ simulation of hydrogen storage in BN systems }

We model via first principles simulation hydrogen storage in boron nitride systems, such as h-BN sheets, the paradigm BN molecule borazine (B$_{3}$N$_{3}$H$_{6})$ and ammonia-borane (BNH$_{6})$. We found H$_{2}$ preferentially adsorbs on the perfect h-BN surface but strongly bound atomic hydrogen prefers to adsorb on vacancies, with consequences for hydrogen storage. The addition of TM (transition metal) atoms to boron nitride, to act as adsorbents for hydrogen, was investigated using borazine as a prototype system for h-BN. The binding of TM atoms (Sc, Ti, V etc.) to borazine was determined, with the variation in bonding intimately related to the electronic structure. The dopants were found to promote the binding of both hydrogen atoms and molecules to borazine, increasing binding energy by $\sim $300{\%} and 1500{\%}, respectively. Initially TM dihydrides form but as hydrogen concentration increases molecular hydrogen becomes preferred. Bound hydrogen is stable at room temperature and the maximum hydrogen capacity and kinematics of this prototype system will be presented. In addition, the dissociation of BNH$_{6}$ \textit{in vacuo}, on the surface of MgH$_{2}$, and in the presence of TM catalysts is modeled.   

Atomic and Molecular Hydrogen Interaction with Ti-Doped Al (100): Hydrogen Dissociation and Surface Alane Formation

A comprehensive research effort on the atomistic mechanisms underlying hydrogen storage in Ti-doped NaAlH$_{4}$ is aimed at deriving a knowledge base for the rational optimization of this and other related complex hydride materials. Our investigation focuses on the role of the Ti dopants in promoting reversible hydrogenation, a key requirement for any practical hydrogen storage material. The re-hydrogenation reaction proceeds from the crucial initial step of dissociative adsorption of molecular hydrogen on Al or NaH. A specific Al:Ti complex was recently predicted as an active site for H$_{2}$ dissociation on extended Al(100) surfaces [1]. Combining high-resolution surface imaging experiments (scanning tunneling microscopy, low-energy electron microscopy) with density functional theory, we are investigating the dissociative adsorption of H$_{2}$ on Ti-doped Al(100) prepared in ultrahigh vacuum. We will discuss our progress toward identifying catalytically active sites for H2 dissociation on this surface, as well as pathways toward the formation of mobile Al-species. [1] S. Chaudhuri and J.T. Muckerman, J. Phys. Chem. B 109, 6952 (2005).   

Theoretical Study of Hydrogen Dissociation on the TiAl$_{3}$ Surface

In order to better understand the catalytic role played by Ti in enhancing the reaction kinetics of sodium alanate, we present a first-principles investigation of hydrogen dissociation and adsorption on the pure Al surface as well as on the Ti doped surface with a local alloy composition of TiAl$_{3. }$ The most energetically favorable location for Ti near the surface is identified. It is found that the presence of Ti promotes H adsorption on the surface with the H atom sitting on top of an Al atom. The binding between Ti and Al modifies the surface charge distribution near the adsorption site and facilitates the adsorption process. The potential energy surface for H$_{2}$ dissociation over both pure Al and the alloy surfaces are also discussed.   

Phase Stability of Mixed-Alkali Alanates

To date sodium alanate NaAlH$_4$ is the only reversible complex hydride that satisfies the international density targets for hydrogen storage materials of 5 wt.\% and 70 kg/m$^3$. The reversible hydrogenation process takes place at reasonable conditions. Therefore, it is desirable to increase the H wt.% by partially replacing Na with a lighter alkali metal such as Li. To study the stability of these mixed-alkali alantes, we perform first-principles calculations for the alloy systems Na$_{1-x} $Li$_x$Al H$_4$ and Na$_{3(1-x)}$Li$_{3x}$AlH$_6$ within the framework of density functional theory and pseudopotentials. For the compositions we have considered for the tetrahydrides, the mixing energies are all positive, indicating that the sodium and lithium alanates prefer being phase separated. For the hexahydrides, one stable intermediate compound is found. The binding characteristics of these mixed-alkali alanates will be discussed.   

Session N17: Focus Session: Semiconductors for THz and IR I

Terahertz Semiconductor Detectors: Designs to Applications

The work describes terahertz photon detectors based on semiconductor micro- and nano-structures using homojunctions, and heterojunctions. A Homojunction or HEterojunction Interfacial Workfunction Internal Photoemission (HIWIP or HEIWIP) infrared detector, formed by a doped emitter layer, and an intrinsic layer acting as the barrier followed by another highly doped contact layer, can detect Terahertz photons due to intraband transitions. The threshold can be tailored by adjusting the band offset between the emitter and the barrier. This principle can be used with any semiconductor material combination. HIWIPs have the same material (doped and undoped) in the emitters and barriers, while HEIWIPs have different band gap material in the two layers. The detection mechanism involves free carrier absorption in the emitter, followed by the internal photoemission of photoexcited carriers across the junction barrier, and then the collection of carriers by the applied electric field at the contacts. Utilization of nanoplasmonic resonances to enhance the terahertz absorption using engineered and self-assembled metal nanostructures on HEIWIP detectors will also be discussed. The metal nanostructures will act as enhanced frequency couplers, which will allow more efficient absorption of terahertz radiation as it is converted into surface plasmons. The near field of SPs will affect the electron gas in the photodetectors the same way as the far-field does. Thus the local field enhancement known for other phenomena and devices could be achieved. Work supported in part by US NSF and US Airforce.   

III-V Semiconductor Diodes and the Terahertz Technology Gap

The terahertz frequency band, spanning from roughly 100 GHz through 10 THz, is often sited as the most scientifically rich, yet unexplored region of the electromagnetic spectrum. Scientific applications include radio astronomy, chemical spectroscopy, plasma diagnostics, compact range radar, atmospheric remote sensing and electron paramagnetic resonance studies of organic molecules. Recently, many groups have developed rudimentary imaging systems for this frequency band, either for basic scientific investigations or defense and security scanners. However, the inherent difficulty of creating sources of terahertz power that are sufficiently powerful, tunable, reliable and robust is a primary difficulty. Researchers in the field generally speak of the terahertz technology gap, which spans the transition from classical electronics to quantum photonics. This talk will consider the nature of the terahertz technology gap and the technological transition from electronics to photonics. Efforts to develop useful sources and receivers of terahertz energy based on III-V semiconductor diodes will be discussed. Also, important recent results, including the development of all-solid-state sources and receivers for the 0.1 -- 3 THz frequency range will be presented. Finally, the fundamental limitations of this technology will be considered.   

Spontaneous emission from accelerated Bloch electrons -- Bloch oscillation radiation

A theory of spontaneous emission of radiation for a Bloch electron traversing a single band in an external electric field is presented. The radiation field is described by a free space quantized electromagnetic field in the Coulomb gauge. It is shown that the spontaneous emission occurs with frequencies equal to integral multiples of the Bloch frequency without any \textit{ad hoc} assumptions concerning the existence of Wannier-Stark levels. An explicit expression for the transition probability is derived in first-order perturbation theory relative to the radiation field. Although the output frequency of the radiation can be operationally tuned from the gigahertz to terahertz spectral range by varying the constant electric field, it is estimated that a spontaneous emission power output of only about 0.1 of a microwatt is available using GaAs-based superlattices. In this regard, it is noted that the atomic spontaneous emission probability and related transition rates can be enhanced by properly tailoring the surrounding electromagnetic environment. Therefore, considering Bloch oscillations in a resonant microcavity to enhance the spontaneous emission is a noteworthy alternative for exploring tunable gigahertz to terahertz radiation sources.   

Developing a voltage tunable two-color corrugated QWIP focal plane array

Single color quantum well infrared photodetector focal plane array (QWIP FPA) has been fully developed. The trend is toward FPAs with spectral analysis and target discrimination capabilities. The challenges of achieving a two-color QWIP FPA are the identification of an effective coupling scheme for both wavelengths and a voltage tunable QWIP material. The former is needed to ensure high sensitivity in both wavelengths and the latter is needed in high resolution FPAs where only one external connection per pixel may be permissible. In this talk, we will discuss the detector parameters needed for high performance infrared imaging, the corrugated light coupling scheme, and the voltage tunable two-color QWIP materials based on superlattices (SLs). In the coupling design, one approach is to use Fabry-Perot oscillations in the triangular cavities to enhance both incident intensities. In the material design, the focus is on the electron energy relaxation rate in the energy relaxation layers (ERL) lying between the active SL periods. By computing the hot-electron distribution after traversing through the ERL layer, one can determine the doping required to eliminate cross-talk between the two colors.   

Study of a broadband high-gain InGaAs/InGaAsP quantum-well infrared photodetector

Lattice-matched InGaAs/InP quantum well infrared photodetectors (QWIPs) exhibit high photoconductive gain but nonadjustable detection wavelength because of their fixed barrier height. The use of In$_{x}$Ga$_{1-x}$As$_{y}$P$_{1-y}$ (InGaAsP) as the barrier material is superior to that of InP with regard to flexibility of the operating wavelength. In this work we investigate the use of InGaAsP barriers in QWIPs for long-wavelength infrared detection applications. We studied a broadband quantum well InGaAs/InGaAsP detector covering 8-14 $\mu $m and found excellent agreement between observed and calculated responsivity spectra. This result shows the validity of our design model. To determine the usefulness of InGaAsP in long-wavelength detection, we also designed a GaAs/AlGaAs quantum well detector with a similar spectrum and compared its performance with that of the InGaAs/InGaAsP detector. Dark current noise measurement indicates that the gain of InGaAsP is 4.6 times larger than that of AlGaAs, showing that InGaAsP is a good candidate for long-wavelength high-speed infrared detection.   

Interfaces as tools in the design of short period type-II InAs/GaSb superlattices for mid-IR detection

The effect of interface anisotropy on the electronic structure of InAs/GaSb type-II superlattices is exploited in the design of thin-layer superlattices for mid-IR detection threshold. The design is based on a theoretical envelope function model that incorporates the change of anion and cation species across InAs/GaSb interfaces, in particular, across the preferred InSb interface. The model predicts that a given threshold can be reached for a range of superlattice periods with InAs and GaSb layers as thin as a few monolayers. A number of superlattices with periods ranging from 50.6 to 21.2 angstroms for the 4 micron detection threshold were grown by molecular beam epitaxy based on the model design. Low temperature photoluminescence and photoresponse spectra confirmed that the superlattice band gaps remained constant at 330 meV although the period changed by the factor of 2.5.   

Quantum Hall Devices as efficient and fast THz photodetectors

Efficient THz photodetectors on the basis of quantum Hall (QH) system have been developed during the recent years. Engineering of the device shape and selection of the parameters of operation allow to implement QH detectors with response times ranging from 10 ns to milliseconds. The spectral resolution of QH detectors, ranging between 1-2 meV at energies of 8-12 meV of the incoming radiation, is a function of the electron mobility and of the bias voltage. QH photodetectors are tunable by the magnetic field and a gate voltage. The combination of these properties together with the high sensitivity of QH THz detectors serves as a basis for an implementation of reliable spectrometer-on-chip devices for THz spectroscopy and imaging.   

Type-II Strained Layer InAs-GaSb Superlattice Photodiodes For Long Wave IR Detection

We have fabricated and tested p-i-n photodiodes in InAs-GaSb superlattice material with measured cutoff wavelengths from 8.5 -- 10 $\mu $m, and compared their performance with mercury cadmium telluride (MCT) detectors of the same cutoff wavelengths. Impedance area (R0A) products approaching the MCT devices have been demonstrated with quantum efficiencies of over 14 {\%} per micron depth of the intrinsic layer. Progress towards designs with longer cutoff wavelengths, up to 16 $\mu $m, will be discussed along with issues and latest results on fabrication of a focal plane array using this material.   

Electro-optically tunable compact terahertz source

The promise of terahertz technology for surveillance and reconnaissance applications is huge. Despite the technical advantages, the major challenge today in terahertz technology is the development of a portable high-power terahertz source. Of the several available terahertz source technologies those based on the difference frequency technique are very promising, as they can produce a relatively high power terahertz beam over the frequency from 100 GHz to 3.5 THz, which is tunable. However, earlier this technique suffered from a high loss of terahertz signal, and produced a weak terahertz beam, in part due to a large impedance mismatching. Also its frequency tuning was cumbersome and its tuning range was limited since it was typically performed by rotating a nonlinear optical crystal against the pumping beam. In our recent experiments we modified the technique to improve the impedance matching and to replace the mechanical tuning with an electro-optical tuning. With this new technique we demonstrated a terahertz beam output power exceeding 10 mW (occasionally $\sim $ 100 mW) at frequencies around 1 THz. Our new technique the frequency tuning is very convenient and not limited by the geometry of the experimental set up. Detailed experiments and experimental results will be discussed   

Terahertz time domain spectroscopy of hollow polycarbonated metal waveguides

Recently, the terahertz region of the electromagnetic spectrum has gained critical significance in various technical applications and fundamental research problems, involving nondestructive evaluation of material parameters, bio-medical imaging, remote sensing and security screening. However, for applications in which THz radiation needs to be transmitted over a long distance without atmospheric absorption, a flexible waveguide could have potential applications simplifying the propagation of THz radiation in remote locations. Different structures like rigid hollow metallic waveguides, solid wires, or short lengths of solid-core transparent dielectrics such as sapphire and plastic have already been explored for THz guiding purposes. Recently, it has been reported that Cu coated flexible, hollow polycarbonate waveguide has a low loss of less than 4 dB/m in single mode operation, at 1.89 THz. In the present study, using a broadband THz source of photoconductive antennae, we characterize the loss and dispersion profile of Cu coated flexible, hollow polycarbonate waveguide having an inner diameter of 2mm. Insertion loss and the attenuation coefficient were calculated using waveguides of lengths between 40mm and 70mm.   

Single Crystal Si Passive Optical Components for \textit{mm}-Astronomy

Construction of ultrasensitive, cryogenic-focal-planes for \textit{mm}-radiation detection requires simultaneous maximization of detector quantum efficiency and minimization of stray light effects, e.g., optical ``ghosting''. To achieve this task in the focal plane detector arrays of the Atacama Cosmology Telescope, integration of two technologies are envisioned; (1) an antireflective (AR) coating for reducing ghosting from the reflected component and increasing absorption at the focal plane, and (2) a backside absorber for suppressing reflections of the transmitted component. We propose a novel approach, involving single crystal Si components, to fabricate AR coatings and backside absorbers. AR coatings are made from Si dielectric honeycombs, in which their dielectric constant may be tuned via honeycomb dimension and wall thickness. Backside absorbers consist of AR Si honeycomb coated-resistors, and the resistors consist of P-implanted Si wafers. This approach enables us to circumvent the mechanical complexities arising from thermal expansion effects, because the detector array, back-short, and AR coating are fabricated out of the same material. We also extend the functionality of single crystal Si in the field of \textit{mm}-radiation detection by fabricating curved, low-loss, broadband waveguides. These waveguides may enable compact structures for applications requiring variable pathlength, e.g., interferometric spectroscopy.   

Session N18: Focus Session: Carbon Nanotubes: Transport I

Quantum Interference in Multiwall Carbon Nanotubes

Recent low temperature conductance measurements on multiwall carbon nanotubes in perpendicular and parallel magnetic field are reported. An efficient gating technique allows for a considerable tuning of the nanotube doping level. This enables us to study extensively the signature of nanotube bandstructure in electron quantum interference effects like weak localization, universal conductance fluctuations and the Aharonov-Bohm effect. We show that the weak localization is strongly suppressed at peaks at certain gate voltages which can be linked with the bottoms of one-dimensional electronic subbands. This assignment allows a detailed comparison of theoretical calculations with the experimental data. In agreement with the theory, we find clear indications for a pronounced energy dependence of the elastic mean free with a strong enhancement close to the charge neutrality point. In large parallel magnetic field, we observe a superposition of h/2e-periodic Altshuler-Aronov-Spivak oscillations and an additional h/e-periodic contribution. The latter contribution shows a diamond-like pattern in the B/V$_{gate}$-plane, which reflects the magnetic field dependence of the density of states of the outermost shell of the nanotube.   

Three-dimensional images of contact geometry between carbon nanotubes and metal contacts using electron tomography

A significant barrier to the widespread application of carbon nanotube transistors is the variability in contact resistance between metallic leads and nanotubes. Varying by orders of magnitude, the contact resistance has recently been reported to depend on the size of the nanotube. To understand why, we study the three-dimensional contact geometry between metal contacts and nanotubes using electron tomography. Spatially resolved core-level electron energy-loss spectroscopy reveals a change in the local electronic structure of the nanotube in contact with gold-palladium We report successful three-dimensional reconstructions of the metal-nanotube interface for gold, gold-palladium and titanium contacts that explain the change in the electronic structure of the nanotube.   

Electrically Tunable Magnetic Properties of Defective Metallic Carbon Nanotubes

We present a first-principles study of the magnetic properties of metallic carbon nanotubes with various defects under a homogeneous transverse electric field. Single carbon adatoms, hydrogen passivated single carbon adatoms, and the various vacancies in (10,10) nanotubes are shown to play the role of magnetic impurities. The relative energy levels of quazi-localized states of such magnetic impurities with respect to the Fermi level are changeable with the application of a transverse electric field so that the corresponding magnetic ground states are shown to be tunable. Our results suggest that a pure organic nanomagnet could be realizable and their magnetic properties are controllable by electric fields.   

Band structure modulation in carbon nanotube-metal junction

In transport experiments carbon nanotubes (NTs) are embedded in metallic contacts. Distortion of NT shape within the NT-metal junction region may result in the change of NT electronic structure: the deformation can open a band gap in otherwise metallic nanotube, and reduce or completely close the gap in a semiconducting one. Local band structure modulation has non-negligible effect on the electron transmission through NT-metal interface, e.g. the Shottky barrier transparency is changing exponentially with the gap width. To analyze the deformation-induced change of the NT band structure in the contact region, we develop a comprehensive theory of the inhomogeneous deformations in NT, which combines the tight-binding approach with continuous description of the deformation field, and is applicable for NT of any chirality. We calculate the band gap variation in the contact region, and discuss the effect of the deformation on the NT-metal contact resistance.   

Ab initio Calculation of Contact Effects on Transport Properties of Carbon Nanotubes Connected to Metallic Electrodes

Recently, a number of studies have been performed to make carbon nanotube devices. One of the important issues in the developments of the carbon nanotube devices is the control of contact effects of the electrodes. To detect electric signals through nanotubes, electrodes must be connected to the nanotubes. Contact with the electrodes sensitively influences the transport properties. Therefore, it is important to discuss the transport properties on the basis of the detailed electronic state calculation that includes the effect of contact with the electrodes. We have developed a first-principles method of analyzing quantum transport in nanometer-scale systems between electrodes. The electronic states are calculated using a numerical atomic orbital basis set in the framework of the density functional theory, and the conductance is calculated using the Green's function method. We apply the method to calculating transmission spectra of carbon nanotubes connected to metallic electrodes, and discuss the contact effect of the electrodes on the transport properties in the finite size of metallic and semiconducting nanotubes.   

Electronic Structure of Metal-covered Semiconducting Carbon Nanotubes

Carbon nanotube field-effect transistor (CNFET) are regarded as potential building blocks for future nanoelectronics. The interaction between a carbon nanotube and metal contacts and the resulting electronic structure effects are crucial for device properties. In this talk, we present recent results on the properties of semiconducting single wall carbon nanotubes in contact with Pd, in a fully covered geometry that resembles experimental setups. We use first-principles calculations to determine the electronic structure, charge transfer effects, electrostatic potential and Fermi level alignment at the interfaces between the metal contact and various semiconducting single-wall carbon nanotubes.   

Atomic-Scale Theory and Modeling of Electronic and Molecular Transport through Carbon Nanotubes

In this talk, we investigate the application of carbon nanotubes as novel transport channels for electrons and molecules using atomistic simulation. (1) Electronic transport: In this talk, we present a Green's function based self-consistent tight-binding study of electron transport through SWNT junction devices, which takes fully into account the 3D atomistic nature of the electronic processes. We discuss insights obtained from such atomistic study on the contact/diameter dependence of junction conductance and self-consistent study of current transport through metal-SWNT-metal junctions. (2) Molecular transport: Carbon nanotube could also be used to build assemblies for controlled transport of biomolecules for nanofluidic devices. Water confinement inside such nanoscale cylindrical core plays a significant role in determining the insertion and flow of biomolecules through the nanotube channel, which can be strongly affected by surface functionalization. Molecular dynamics simulations have been carried out to study the structure and thermodynamics of water in carbon nanotube and its effect on the spontaneous insertion of DNA molecules inside the nanotube channel. The simulations can provide valuable insights into the transport of molecules through nanoscale pore or channel structures.   

Signatures of Chemical Defects in Carbon Nanotube Electronic Devices

The study of chemical defects in carbon nanotubes has important implications for their operation as electronic devices, and many synthesis and fabrication techniques for such devices result in non-zero defect densities. Scanned probe microscopy techniques are particularly useful for identifying these sites and then examining their electronic properties. We have examined a number of electronic devices in which single defects play important roles in determining the two- and three-terminal device behavior. Using conducting-tip atomic force microscopy to measure local electronic properties, we distinguish one type of defect from another and correlate this with the device characteristics. Furthermore, we can chemically modify and reinvestigate the same defect site. Ultimately, the goal is to use the device characteristics as a kind of signature to reliably infer the presence of particular chemical defects. This work is partly supported by NSF grant DMR-0239842.   

Gated Spin Transport through an Individual Single Wall Carbon Nanotube

We report on the fabrication and characterization of ferromagnetically contacted ``short channel'' SWNT devices that show clear hysteretic switching in the magnetoresistance, and provide strong evidence for SWNT spin transport. The main difference between our work and previous studies is that we have greatly reduced the transport length separating the ferromagnetic contacts to distances on the order of 10 nm. Preliminary measurements demonstrate this reduction to be extremely beneficial. We have observed clear hysteretic switching in the magnetoresistance in 75{\%} of our devices, and are able to modify the magnetoresistance between +15{\%} and -10{\%} as a function of gate voltage. The gate mediated change in magnitude \textit{and sign} of the magnetoresistance switching allows us to discount other non-spin related sources for the observed signal and provides the basis for the first SWNT spin transistor. We note that the short channel contacting scheme is generally applicable to non-ferromagnetic contacts as well, and provides a straightforward technique for fabricating SWNT quantum dot devices.   

Spin transport studies in mesoscopic graphite

We present experimental studies on spin transport in mesoscopic graphite. Two dimensional graphite sheets have been fabricated by means of micromechanical exfoliation. Spin injection has been achieved by employing ferromagnetic Co electrodes. We use the shape anisotropy of the electrodes to uniquely define the magnetic state of the device. Typical two terminal resistances are in the order of 1 k$\Omega$. We will discuss the switching behavior of the device magnetoresistance as a function of temperature, the gate bias voltage and of the source drain bias.   

Ballistic conductance in narrow graphene strips

With structures making them suitable for in-plane device processing, high aspect ratio graphene strips with widths down to tens of nanometers or smaller could ultimately provide important components in carbon-based quantum electronics. However, in comparison to corresponding single-wall carbon nanotubes, such strips will likely have a higher degree of imperfection due to variations in their widths and interactions with the substrate which will degrade their conductance. Also, unlike nanotubes, they can exhibit highly localized edge states which are degenerate with their more extended states at or near the Fermi level. On the other hand, their more extended states near the Fermi level have properties similar to those exhibited by related states in nanotubes, which should suppress the effects of back scattering both due to short and long-range disorder. Stimulated by these observations and recent experiments on graphene sheets, simulations were performed to assess the effects of various types of disorder on the conductance of narrow graphene strips. The results indicate that these strips can exhibit ballistic conductance over large distances in the presence of reasonable disorder making them excellent synthetic targets for carbon-based device applications.   

Probing Biological Processes on Supported Lipid Bilayers with Single-Walled Carbon Nanotube Field-Effect Transistors

We have formed supported lipid bilayers (SLBs) by small unilamellar vesicle fusion on substrates containing single-walled carbon nanotube field-effect transistors (SWNT-FETs). We are able to detect the self-assembly of SLBs electrically with SWNT-FETs since their threshold voltages are shifted by this event. The SLB fully covers the NT surface and lipid molecules can diffuse freely in the bilayer surface across the NT. To study the interactions of important biological entities with receptors imbedded within the membrane, we have also integrated a membrane protein, GT1b ganglioside, in the bilayer. While bare gangliosides can diffuse freely across the NT, interestingly the NT acts as a diffusion barrier for the gangliosides when they are bound with tetanus toxin. This experiment opens the possibility of using SWNT-FETs as biosensors for label-free detection.   

First Principles Properties of Polymeric Photovoltaic Materials

Recent reports suggest that the acceptor-donor junction for bulk heterojunction photovoltaic devices can be achieved using single wall carbon nanotubes (SWNT) and polymers such as poly-3-octothiophenes (P3OT). Optical excitation is believed to occur in the organic polymer which acts as a good hole conductor, with electron transfer to the SWNT which acts as a good electron conductor. An appropriate theoretical understanding of the photovoltaic effect requires knowledge of the electronic states near the Fermi level in these materials. We calculate the electronic structure of infinitely long quasi one-dimensional nanostructures such as carbon nanotubes or electroactive chain polymers, such as polythiophenes using a first principles, all electron, self consistent local density functional (LDF) approach. We present and compare electronic structure calculations for SWNTs and poly-3-alkyl-thiophenes. Further we discuss the variation of effective mass of charge carriers in polymers and SWNTs in the vicinity of Fermi level. This work was supported by the US Office of Naval Research and the DoD HPCMO CHSSI program through the Naval Research Laboratory.   

Session N19: Semiconductor Spin Transport

High field magnetoresistance in $p$-(In,Mn)As/$n$-InAs heterojunctions

The high field magnetoresistive properties of a $p$-In$_{0.96}$Mn$_{0.04}$As/$n$-InAs junction have been measured. The heterojunctions were formed by epitaxially depositing an InMnAs thin film on an InAs substrate using metal-organic vapor phase epitaxy. Under forward bias, a large, nonsaturating magnetoresistance is observed at temperatures from 25 to 295 K in fields up to 9 T. At room temperature, the magnetoresistance increases linearly with magnetic field from 1.5 to 9 T and is greater than 700 {\%} at 9 T. The magnetoresistance can be simulated using a modified diode equation, including a field-dependent series magnetoresistance.   

Room Temperature Tunnel Magnetoresistance and Spin Polarized Tunneling Studies with Organic Semiconductor Barrier

Organic semiconductors, $\pi$-conjugated, with a weak spin-orbit interaction, show promise for spin-conserved transport applications.[1,2] An organic spin-valve utilizing the molecular organic semiconductor tris (8-hydroxyquinolinato)aluminum (Alq$_{3}$), demonstrated giant magnetoresistance at LHe temperatures.[1] The Alq$_{3}$ films in this spin-valve were $>$130nm, and the spin diffusion length was 45nm. Our current study demonstrates spin polarized tunneling through an ultra-thin layer of Alq$_{3} $ in a magnetic tunnel junction. Significant tunnel magnetoresistance was measured in a MTJ structure at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the I-V behavior and temperature and bias dependence of the TMR, show good quality of the organic tunnel barrier. Spin polarization of the tunnel current from Co, Fe and NiFe electrodes through the Alq$_{3}$ layer was directly measured using a superconducting Al electrode as the spin detector. This demonstration of spin-conserved transport through an organic semiconductor at room temperature shows the potential of this material for further study. Supported by KIST- MIT Program and NSF. 1) Z. H. Xiong, \emph{et al}, $Nature$ \textbf{427} 821 (2004).\\ 2) V. Dediu, \emph{et al}, \emph{Solid State Commun.} \textbf{122} 181 (2002).   

Diluted Magnetic Double Barrier Resonant Tunneling Structures for Novel Magnetically-Defined Quantum Dot and Nano Structures

The further development of Spintronics requires the direct control of the spin degree of freedom. A milestone on the path towards this accomplishment has been recently achieved by the demonstration of the successful operation of a magnetic resonant tunneling diode \footnote{A. Slobodskyy \textit{et al.}, Phys. Rev. Lett. \textbf{90}, 246601 (2003)}. We will report our results on manganese doped double barrier tunneling structures (II-IV group) with varying doping, confining potential and well width. These structures, in which both the barrier and well are doped with manganese, show a Zeeman splitting tendency in the vertical transport through the barrier in magnetic field. This tendency can be exploited as spin filter for spintronic applications. Based on this spin voltage transport property of diluted magnetic semiconductor heterostructures and the interaction of superconductors and semiconductors, we propose a novel spin aligned vertical quantum dot device. Progress on the fabrication of this device will also be reported.   

Spin dependent tunnelling in indirect double-barrier structures

Spin-dependent tunelling and polarization in GaAs/AlAs/GaAs based resonant tunelling diode are studied by a tight-binding model. We compare the GaAs/AlAs/GaAs case with similar structures where the barriers are direct and show the advantages of a GaAs/AlAs/GaAs configuration.   

Magnetic Resonances in the circular polarization of light emitted by Fe/InAs QD spin LEDs

The circular polarization $P_{circ}$ of the light emitted from InAs QD LEDs was studied as function of applied magnetic field $B$ in the 5-75 K temperature range. The quantum dots are incorporated at the center of a GaAs quantum well of width $L_{W}$ . At$ T$ = 5 K we observed two distinct resonances in the $P_{circ}$ versus $B$ plot. For $L_{W}$ = 430 {\AA} the resonances occur at $B$ = 4.6 T (strong) and $B$ = 2.3 T (weak). The strength of the resonances depends critically on bias voltage $V$ (very pronounced at low $V $values ) The resonances become weaker with increasing temperature and disappear completely by $T$ = 60 K. No resonances were observed in LEDs in which the QDs were not incorporated in a quantum well. We propose a model that takes into account the confinement conduction subbands of the GaAs quantum well and the dependence of their energies on magnetic field. Acknowledgements: This work is supported by the DARPA SpinS Project, ONR, and NSF   

Tailoring magnetic anisotropy in ferromagnetic metal / semiconductor contacts for spin injection

Robust spin injection across an Fe/AlGaAs interface has recently been demonstrated, producing an electron spin polarization $>$32{\%} in a GaAs QW. In an effort to incorporate a spin injecting metal contact with perpendicular remanence and to explore interface effects on spin injection, we have grown MnGa thin films epitaxially on GaAs(001) LED structures by MBE. Streaky RHEED patterns indicate single crystalline films. Although lattice matched to GaAs, TEM shows that while MnGa crystallizes nicely away from the interface, defects exist at the interface. The insertion of a thin ($\sim $ 5ML) Fe seed layer between MnGa and AlGaAs promotes the initial nucleation of MnGa and provides a means to control the structure of the spin-injecting interface, while the magnetic behavior is determined by the MnGa. Samples are processed to form surface emitting LEDs, and the EL is dominated by QW excitonic emission. A 0.5{\%} remanent circular polarization is observed, which tracks the MnGa magnetization obtained by independent SQUID measurements. Comparison between MnGa spin-LEDs with and without the Fe seed layer (including interface properties), and magneto absorption effects in these heterostructures, will be discussed.   

Quasi-one-dimensional spin polarized states in T-shaped nanostructures

We present results of theoretical and numerical investigations of T-shaped semiconductor nano-wire structures. Such structures have been synthesized in Molecular Beam Epitaxy laboratories by using traditional (GaAs) and magnetic (GaMnAs) semiconductors. The wire is formed in a three-stage MBE process and in its final form, the structure looks as if one quantum well (QW) called Stem well grew perpendicularly into the second QW, the so-called Arm well. The quasi 1D states are formed at the intersection of the two QWs. Typically in the T-shaped structures the thickness of the wire is of order of few nm. For such thickness there is only one conduction channel and the energy states of quasi-particles are indexed by a single quantum number, the 1D linear momentum $k$, and by its spin. There are two distinctive features present in this type of systems: strong spin-orbit coupling, and the lack of square symmetry of the wire cross section. These features have profound implications and make this system very important from theoretical point of view. Results, obtained within the \textbf{\textit{k\textbullet p}} formalism, show that at $k\ne $0 the valence band states are non-degenerate with respect to the spin degree of freedom. Both dispersions of spin up and spin down states are well modeled as parabolic bands but with different effective masses. It opens possible to manipulate the spin degrees of freedom in a T-shaped quantum structure.   

Atomistic spin-orbit effects on the electronic structure of T-shaped quantum wires

The electronic structure and optical properties of GaAs/AlGaAs T-shaped quantum wires are studied by use of an empirical tight-binding method (ETB). This model allows us to study atomistic effects on the electronic structure of wires that have a complicated geometrical cross section. We find that the electronic structure for electrons is similar to that described by effective mass models whereas the electronic structure for holes shows important modifications when spin-orbit coupling is included in the atomistic model. The binding energies of the holes in a T-wire agree with previous effective mass model calculations. However, we find that asymmetries in the spatial distribution of these hole states are induced by atomistic spin-orbit effects. Moreover, the atomistic tight-binding model predicts complex band crossings for hole states that are not predicted by simpler effective mass theories. Consequences for the optical response of T-wires and for excitonic and electron-hole plasma phases in T-wires are discussed.   

Quasi-one-dimensional spin field-effect transistors

We study a spin field effect-transistor(SFET) with multiple transport modes. Energy dispersion relations and spin profiles of eigen-transport modes are examined numerically and analytically for weak and strong Rashba spin-orbit coupling parameters. Electron transport properties of the multiple-mode SFET are investigated including the Fabry-Perot-type interference due to multiple reflections and the peak splitting by external magnetic fields. Impurity scattering effects are also addressed.   

Lateral diffusive spin transport in layered structures

A one dimensional theory of lateral spin-polarized transport is derived from the two dimensional flow in the vertical cross section of a stack of ferromagnetic and paramagnetic layers. This takes into account the influence of the lead on the lateral current underneath, in contrast to the conventional 1D modeling by the collinear configuration of lead/channel/lead. Our theory is convenient and appropriate for the current in plane configuration of an all-metallic spintronics structure as well as for the planar structure of a semiconductor with ferromagnetic contacts. For both systems we predict the optimal contact width for maximal magnetoresistance and propose an electrical measurement of the spin diffusion length for a wide range of materials. This work was supported by NSF DMR-0325599.   

Microscopic calculation of the Gilbert damping in a spin-polarized two-dimensional electron liquid with Rashba spin-orbit interaction

We present a microscopic calculation, based on mode-coupling theory, of (i) the Gilbert damping constants for in-plane and out-of-plane relaxation and (ii)the magnetic anisotropy tensor of the spin-polarized two-dimensional electron liquid in the presence of a spin-orbit interaction of the Rashba form.   

Controlling spin in an electronic interferometer with spin-active interfaces

We consider electronic current transport through a ballistic one-dimensional quantum wire connected to two ferromagnetic leads. We study the effects of the \textit{spin-dependence} of interfacial phase shifts (SDIPS) acquired by electrons upon scattering at the boundaries of the wire. The SDIPS produces a spin splitting of the wire resonant energies which is tunable with the gate voltage and the angle between the ferromagnetic polarizations. This property could be used for manipulating spins. In particular, it leads to a giant magnetoresistance effect with a sign tunable with the gate voltage and the magnetic field applied to the wire.   

Interferometric detection of spin-polarized transport

It is shown that in addition to its sensitivity to spin polarization, the magneto-optic Kerr effect strongly depends on the spatial distribution of spin-polarized charge carriers. Using time-resolved Kerr rotation, the dynamics of spin- polarized electrons can thus be monitored on the nanometer length scale. This is demonstrated experimentally for optically- excited electron spins in the depletion layer of $n$-doped GaAs close to a metallic electrode. The Kerr rotation exhibits fast oscillations that originate from an interference of the light reflected at the electrode with that reflected at the front of the electron distribution moving into the semiconductor. From these oscillations, the dynamics of the electron front is obtained, which is strongly screened by the space-charge field of the excited electron-hole pairs and can be controlled by an electric bias across the Schottky barrier. In addition, the dynamics provides information on the Schottky-barrier height, the depletion-layer thickness and the doping concentration.   

Spin Resolved Current Focusing in InSb Heterostructures

Spin-resolved current focusing has been observed in InSb/AlInSb structures. While InSb has the most significant Rashba and Dresselhaus effects of any of the III-V semiconductors, Dresselhaus effects are expected to dominate in the symmetrically doped structures used here. The double quantum point contact devices were designed with typical dimensions of 0.5 micron which preserve ballistic transport up to 185K as measured in previous experiments. Focusing peaks were observed near the expected values of perpendicular magnetic field; however the first focusing peak was a doublet. With application of a parallel magnetic field the doublet evolved into a singlet as expected for spin resolved focusing.   

Spintronic Ratchet

Carefully designed ratchets are of great interest, practically and conceptually, as means to convert fluctuations into useful work. We argue that a recently proposed ``Spin-Capacitor'' [Appl. Phys. Lett. \textbf{87}, 013115~(2005)] exhibits characteristics that have close resemblance to ratchets. It shows unidirectional current-voltage (I-V) characteristics that depend on the spin excitation spectrum of a neighboring array [http://arxiv.org/abs/cond-mat/0511566]. More interestingly, if the spins in the array are out of equilibrium, useful work can be extracted at the expense of energy/entropy. This is manifested as a \textbf{\textit{non-zero current at zero bias}} and we argue that a recent experiment in an integer quantum hall system [http://link.aps.org/abstract/PRL/v95/e056802] shows evidence for this general principle.   

Session N20: Focus Session: Complex Oxide Thin Films Surfaces and Interfacess III: New Materials, New Techniques, and Effects of Strain

Growth and Properties of a New Correlated Electron Perovskite Thin Film -- PbVO$_{3}$ .

We report the growth of single phase, fully epitaxial thin films of a relatively new perovskite material, lead vanadate (PbVO$_{3})$, using pulsed laser deposition. This growth realizes the first production of PbVO$_{3}$ outside of high-temperature and high-pressure techniques through growth of epitaxial thin films on various substrates. Structural analysis of the PbVO$_{3}$ thin films using transmission electron microscopy, x-ray diffraction, and Rutherford backscattering spectroscopy reveals films that are single phase, highly crystalline, and have a tetragonally distorted perovskite structure, with $a$ = 3.79{\AA} and $c$ = 5.02{\AA} (c/a = 1.32). Electron energy loss spectroscopy and x-ray absorption spectroscopy were used to show the stabilization of vanadium in the V$^{4+}$ state, thereby proving the creation of a new $d^{1}$ system for intensive physical study. Films exhibit semiconducting behavior in plane of the film with thermally activated behavior and distinctly different properties from other $d^{1}$ $A$VO$_{3}$ thin films. Studies of the magnetic and ferroelastic/ferroelectric nature of PbVO$_{3}$ are also underway.   

Phase Transitions of SrFeO$_{3}$ Studied Using a Single-Crystalline Film

To study the electronic nature of SrFeO$_{3}$ (SFO), which is a cubic perovskite containing Fe$^{4+}$ equipped with deep $d$ levels and is, therefore, dominated by $p$-hole character, a single crystalline film was grown and the resistivity (\textit{$\rho $}), Hall effect, magnetoresistance (\textit{MR}) and susceptibility were measured. It is known that this oxide in bulk form becomes antiferromagnetically ordered in a screw spin structure. The $T_{N}$ of the film has been found to be at 120$\sim $125 K from the susceptibility measurement, while the transport properties showed well-defined anomalies at 105 K, rather than at the $T_{N}$. The metallic film (\textit{$\rho $} = 9$\times $10$^{-4} \quad \Omega $cm at 300 K) exhibited a hysteretic, inflectional drop in the \textit{$\rho $} -$T $curve at 105 K after showing a very small anomaly at 125 K; the Hall coefficient was positive and temperature-independent above 110 K but increased quickly below $\sim $100 K; the \textit{MR} changed its sign from negative to positive quite steeply at 105 K. Considering these results together with what is known about bulk samples, we conclude that SFO undergoes its antiferromagnetic transition in two stages, passing an incompletely coherent stage before entering the final coherent state.   

Magnetic Properties of PLD Grown Expitaxial Double Perovskite Thin Films

Transition metal oxides with a double perovskite structure A$_{2}$FeMoO$_{6}$ and (A = Ca, Ba, Sr) has attracted a great deal of attention owing to their high magnetic transition temperatures and spin dependent transport properties. Electronic structure calculations and experimental results show that these materials are half-metallic ferrimagnets with localized up-spin electrons on the Fe ions and itinerant down-spin electrons shared between Fe and Mo. The Fe and Mo atoms are ordered on alternating, corner-shared octahedral sites, however, the ordered array can have imperfections that are dependent upon synthesis conditions. We have grown, using a pulsed laser deposition device, epitaxial double perovskite thin films. These films have been characterized by SQUID, resistivity, and x-ray crystallography measurements. The measurements show that double perovskite thin films can be grown with a high degree of order between the Fe and Mo atoms. Thus these materials can be attractive candidates for spin electronic devices.   

Exotic Single Crystal Thin Films Made From Cobalt Ferrite.

The search for new magnetic materials is driven by technological demands such as increasing the magnetic recording density. Materials possessing a large magnetic anisotropy are suitable media to meet such requirements since a stable magnetization can be promoted in nano-structures. Hard ferrites such as the hexagonal (BaFe$_{12}$O$_{19})$ and the cubic (CoFe$_{2}$O$_{4})$ are attractive for such kind of applications due to their large magnetocrystalline anisotropy and high chemical stability. In this talk we report on exotic properties of films made from CoFe$_{2}$O$_{4}$. Epitaxial CoFe$_{2}$O$_{4}$ thin films have been grown by pulsed laser deposition (PLD) on (100) MgO substrate. Two types of spin-reorientation have been observed in such films upon annealing or increasing the film-thickness. In the as-deposited layers and at low thickness the easy axis of the magnetization is confined to the normal to the film plane whereas at large thickness the film plane becomes the preferential direction of the magnetization. On the other hand annealing induces a reorientation of magnetic anisotropy, which switches from the normal to the film plane in the as-deposited film to be in-plane aligned in the annealed state. The origin of both reorientations is explained in term of competition between stress and magnetocrystalline anisotropies.   

Magnetic Anisotropy of Cr-Substituted Magnetostrictive Cobalt Ferrite

In order to tailor the magnetomechanical response of substituted cobalt ferrite for strain sensing and actuating applications, more needs to be known about the variation of the basic magnetoelastic and magnetic properties with temperature and composition. In this study, the variation of magnetic anisotropy with temperature and composition for a series of Cr-substituted cobalt ferrites, CoCr$_{x}$Fe$_{2-x}$O$_{4}$, (0$\le $x$\le $0.8) was investigated. In order to determine the cubic anisotropy constant $K_{1, }$the ``high field'' regime (from 1 T to 5 T) of the major magnetic hysteresis loops, which were measured at temperatures over the range 10-400 K using a SQUID magnetometer, was fitted using the law of approach approximation $M(T)=M_{S}$(1-8/105$K_{1}^{2}$/\textit{($\mu $}$_{0}$\textit{HM}$_{S})^{2})$ plus a forced magnetization term linear in applied field $H$. It was found that anisotropy increases with decreasing temperature, with the steepest increase coming at progressively lower temperatures for increasing Cr content. For fixed temperatures, anisotropy decreases with increasing Cr content. For the pure cobalt ferrite and x=0.2 Cr samples it appears that for temperatures less than 150 K, 5 Tesla is not enough to saturate the samples, so anisotropy cannot be computed correctly by this method. This research was supported by NSF, Grant No.DMR-0402716, and by NASA, Award No.NAG-1-02098.   

Influence of substrate strain on (La$_{1-x}$Pr$_{x})_{1-y}$Ca$_{y}$MnO$_{3}$ phase transition

The large-scale phase separation between ferromagnetic metallic (FMM) and charge-ordered insulating (COI) domains observed in (La$_{1-x}$Pr$_{x})_{1-y}$Ca$_{y}$MnO$_{3}$ (LPCMO) crystals has attracted a lot of attention. This coexistence of phases is very sensitive to structural and magnetic changes, and is responsible for the enhanced magnetoresistance in LPCMO compared to its parent compounds. The energy balance of the FMM and COI phases is still not well understood. We can change the energy balance by changing the substrate, and therefore the strain on the thin film, and thereby improve our understanding of the phase transition. We have grown and characterized several different thicknesses of LPCMO on LaAlO$_{3}$, SrLaGaO$_{4}$, NdGaO$_{3}$ and SrTiO$_{3}$ substrates. We have observed that the compressive strain from the LaAlO$_{3}$ substrate suppresses the long-range charge ordering in the sample, and enhances magnetoresistance and magnetic hysteresis. The charge ordering is also suppressed in the films on SrLaGaO$_{4}$, even though the strain is negligible. Conversely, the tensile strain from the NdGaO$_{3}$ and SrTiO$_{3}$ substrates enhances the long-range charge ordering and reduces the magnetoresistance and magnetic hysteresis. *Oak Ridge National Laboratory, managed by UT Battelle for the U.S. Dept. of Energy under contract DE-AC05-00OR22725   

The role of strain in the magnetic properties of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ films studied by magnetic force microscopy

In order to elucidate the role of strain in the magnetic properties of manganite films, we studied the behavior of the magnetic domains in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) films grown on SrTiO$_{3}$ (STO) and NdGaO$_{3}$ (NGO) substrates, which are differently strained. Our previous studies on the magnetic properties of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ films grown on STO substrates using magnetic force microscopy showed a distinct magnetic texture within magnetic domains, and spin reorientation and enhancement of T$_{C}$ near grain boundaries. These results were attributed to the strain in the film caused by the lattice mismatch with the substrate and the strain relaxation at the grain boundaries. Our new studies on La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ films grown on NGO substrates, which have very low strain due to a close lattice match between the film and substrate, show no presence of magnetic texture and a very sharp transition from the paramagnetic to ferromagnetic phase.   

Strain Induced Crystal Superstructure in Manganite Films

Using xray diffraction we studied in detail the crystal structure of a 100 nm thick La$_{0.7}$ Sr$_{0.3 }$MnO$_{3}$ film grown on a SrTiO$_{3}$ substrate. Satellite peaks are observed at ($H \quad \pm \quad \delta h$, $K$, $L)$ for nonzero $K$ and ($H$, $K \quad \pm \quad \delta k$, $L)$ for nonzero $H$. No satellite peaks are observed around (0 0 $L)$ reflections. Our measurements show that the modulation wave vectors and polarization vectors representing the atomic displacements are perpendicular to each other and point in the direction parallel to the plane of the film. $L$ scans around the main Bragg peaks and around the satellite peaks exhibit strong Laue oscillations indicating that the superstructure is coherent throughout the whole thickness of the film. We will discuss the results of the xray measurements in connection with the scanning probe microscopy measurements done on the same film.   

Electronic properties of structural twin and antiphase boundaries in materials with strong electron-lattice couplings

Using a symmetry-based atomic scale theory of lattice distortions, we show that in functional materials with strong electron lattice coupling, the electronic properties are distinctly modified near elastic textures such as twin boundaries (TBs) and antiphase boundaries (APBs), which can be directly measured by STM. The results also show that the heterogeneities of electron local DOS are not confined within TBs and APBs, but can propagate into domains in the form of Friedel oscillations for TBs and with the wave vector related to short wave length lattice distortions for APBs. The results are discussed in relation with perovskite manganites and other functional electronic materials. Reference: Ahn, Lookman, Saxena, and Bishop, Phys. Rev. B 71, 212102 (2005).   

Ultra-thin Perovskite Manganite films on SrTiO$_{3}$ and SrTiO$_{3}$/Si heterostructures

We report on the growth and characterization of high-quality ultra-thin La$_{0.7}$Ba$_{0.3}$MnO$_{3}$ and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ epitaxial films on SrTiO$_{3}$ and SrTiO$_{3}$-buffered Si (100) and Si (111) substrates by pulsed-laser deposition. The films demonstrate remarkable magnetic and electrical properties associated with the colossal magnetoresistance behavior at and above room temperature. The enhanced transition temperature of manganite films on buffered Si substrates is discussed in terms of the strain relaxation at the interface between the manganite film and the SrTiO$_{3}$ buffer layer which is caused by the smaller grain size. We also report the doping of Ru (x=0.3 to 0.4) into Mn sites in LSMO films grown both on STO and STO buffered Si. The films display remarkable hardening of $H_{c}$ due to the charge-transfer-enhanced exchange coupling. We have optimized doping for the compensation of hole doping by the valence effect of Ru,. This effect has been explained in terms of the charge transfer between the Mn$^{4+}$ and Ru$^{4+}$ species and ferromagnetic interaction between the resultant Mn$^{3+}$ and Ru$^{5+/4+}$. The electrons in Ru$^{4+}$ partially occupy the degenerated $t_{2g}$ orbitals due to the fact that Ru is a heavy metal, we expect a single-ion anisotropy of Ru spins through a spin-orbit channel. This structure is highly applicable for fabrication of the magnetic tunnel junctions.   

Variation of magnetic domain structure correlated to low-field magnetoresistance hysteresis in La$_{0.66}$Ca$_{0.34}$MnO$_{3}$ film as observed by magnetic force microscopy.

The ferromagnetic domain structure of a 150-nm-thick La$_{0.66}$Ca$_{0.34}$MnO$_{3}$ film was imaged by magnetic force microscopy (MFM) as a function of in-plane applied field at 240K, just below T$_{C}$. The film was grown by sputtering on a (001) SrTiO$_{3}$ substrate. The variation of the domain structure is correlated with the hysteresis in both magnetization and magnetoresistance. The resistance peak in the magnetoresistance coincides with the coercivity of the film and sweeping changes in the MFM images. We show the effect of applying the in-plane magnetic field along both in-plane crystalline axes.   

Anisotropic magnetoresistance in colossal magnetoresistive La$_{1-x}$Sr$_{x}$MnO$_{3}$ thin films

We report on magnetic field and temperature dependent measurements of the anisotropic magnetoresistance (AMR) in epitaxial La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) thin films. While in $3d$ ferromagnetic alloys increasing the magnetization, either by reducing the temperature or increasing the magnetic field, increases the AMR, in LSMO films the AMR dependence on magnetization displays non-monotonic behavior which becomes particularly pronounced in lightly doped compounds. This could imply that these samples may be electronically inhomogeneous, in which increased magnetization yields enhanced uniformity which suppresses spin-dependent scattering and hence reduces the AMR.   

Size effect in percolative phase separation of colossal magnetoresistive (La,Pr,Ca)MnO$_{3}$ films .

La$_{1-x-y}$Pr$_{y}$Ca$_{x}$MnO$_{3}$(LPCMO) (where x=3/8) is electronically phase-separated into a sub-micrometre-scale mixture of ferromagnetic metallic (FMM) and charge-ordered insulating (COI) domains. Transport through the ferromagnetic network depends sensitively on the domain structure, which can be controlled by magnetic field, light, and strain, etc. Enhanced CMR effect can be achieved in the vicinity of the percolative threshold. Using optical lithography, e-beam lithography, and focused ion beam techniques, we can fabricate series micron and sub-micron structures on LPCMO films grown on LaAlO$_{3}$ and SrTiO$_{3}$ using pulsed laser deposition. Size dependant transport properties will be addressed in detail. Research sponsored by the U.S. Department of Energy under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory.   

Combinatorial Hall Effect System for Oxide Films

Combinatorial film growth techniques have made it possible to produce large numbers of high-quality oxide films at one time. Characterizing the samples by traditional methods would be far too slow. Certain measurements, such as the the Hall effect, require careful temperature control and lock-in amplifiers to resolve the small signal. We have built special-purpose, multi-channel resistance bridges to measure the Hall effect simultaneously in 32 samples. The voltage resolution is less than 2 nV for signals on the order of 1 $\mu$V, for a signal-to-noise ratio of 500. We will discuss Hall effect data from samples of La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4}$.   

B-site substitution in LaTiO$_{3}$ thin films: influence on the titanium oxidation state.

LaTiO$_{3}$ (LTO) is a Mott insulator, antiferromagnetic at RT, and exhibits a Metal Insulator Transition (MIT) at the Neel temperature. Despite its complex chemistry, it is an interesting candidate to fabricate field-effect devices. A full device requires the deposition of a dielectric in contact with the LTO thin film such as HfO$_{2}$. This choice will be discussed, and we will present issues related to a possible interdiffusion. Adding Hf in the LTO layer leads to a clear change of the resistivity measured as a function of temperature, and strongly influences the MIT temperature. Starting from a semiconducting LTO, 20{\%} Hf substitution on the perovskite B-site makes the layer become metallic from RT down to 4.2K. The average valence of Ti is increasing from Ti$^{3+}$ towards Ti$^{4+}$ with the substitution of Hf, as shown by XPS. Several explanations can be proposed beyond a real incorporation of Hf into the LTO matrix. Besides a pure electronic effect, structural and catalytic effects have been then investigated in details by means of XRD, XPS and HRTEM. Multilayers as well as single-phase thin films have been fabricated to disentangle these different effects. Our results will be discussed taking also into account a possible material loss in the structure. We will in particular explore the behavior of La-deficient structure to qualitatively explain our data.   

Session N21: Microfluidic Physics II

Microfluidics with Gel Emulsions

Microfluidic processing is usually achieved using single phase liquids. Instead, we use monodisperse emulsions to compartment liquids within microchannel geometries. At low continuous phase volume fractions, droplets self-organize to form well-defined arrangements, analogous to foam. While it is well-known that confined geometries can induce rearrangement of foam compartments at the millimeter-scale, similar dynamics are also expected for gel emulsions. We have studied online generation, organization and manipulation of gel emulsions using a variety of microchannel geometries. ``Passive'' reorganization, based on fixed channel geometries, can be supplemented by ``active'' manipulation by incorporating a ferrofluid phase. A ferromagnetic phase facilitates reorganization of liquid compartments on demand using an electromagnetic trigger. Moreover, coalescence between adjacent compartments within a gel emulsion can be induced using electrical potential. Microfluidics using gel emulsions will be well-suited for combinatorial chemistry, DNA sequencing, drug screening and protein crystallizations.   

In-situ Generation of Focal Conic Defects in Flow of Smectic-A Liquid Crystals in Microchannels

The response of ordered phases of layered liquids such as smectic-A liquid crystals to flow is often dominated not by the direct coupling of flow to molecular degrees of freedom, but by the driven motion of defects in these systems. This is because flow generates instabilities in these systems that lead to the formation of defects. Most attention to date focuses on the global viscoelastic behavior and shear alignment of layered liquids, while little work has been done to understand defect motion and defect/flow interaction in these systems. Here we introduce a new approach, which enables simultaneous generation and observation of focal conic defects in pressure-driven flow of smectic-A liquid crystals. We observe that introducing a sudden change in the cross-sectional area of a microchannel via a small obstacle leads to the formation of a steady stream of defects. Generated defects move along the microchannel and interact with each other and with the flow itself. We measure the pressure drop and defect velocity and relate these to observed defect size and microchannel geometry. These results provide a novel way of probing nonlinear behavior of layered fluids under flow.   

Folding and swirling of viscous threads in microfluidics

We study miscible multiphase flows with large viscosity contrast in microchannels. We investigate the folding instability of a viscous thread surrounded by a less viscous liquid that flow into a diverging microchannel. In this situation, extensional viscous stresses cause the thread to bend and fold rather than dilating in order to minimize dissipation. We show that diffusive mixing at the boundary of the thread can significantly modify the folding flow morphologies. We relate the folding frequency to the characteristic shear rate. We also examine the hydrodynamic coupling between multiple threads and the threads' rupturing into arrays of viscous swirls, reminiscent of the Kelvin-Helmholtz instability.   

Observations of Tipstreaming and Thread Formation in a Microfluidic Flow Focusing Device

We present a novel method of generating sub-micron scale droplets in a microfluidic device. In particular we utilize the interaction of fluid motion and surfactant transport during the tipstreaming mode of droplet formation, which is achieved using a flow focusing microfluidic design. Tipstreaming is a mode of drop breakup in which daughter droplets, usually orders of magnitude smaller than the parent drops, are ejected from the pointed tips of parent droplets or bubbles. An attractive characteristic of tipstreaming is that droplets produced are not limited by the device feature size. In this work we observe that tipstreaming occurs within a specific range of capillary number (Ca $\sim $ 0.1 to 1, consistent with literature values for tipstreaming), flow rate ratio, and surfactant concentration, and that tipstreaming is preceded by the formation of thin threads that follow the pinchoff of larger droplets. We measure the thread diameter and length as a function of dimensionless parameters and discuss our results with respect to surfactant diffusion and kinetic timescales relative to flow timescales.   

Drops and Jets in two Phase Coaxial Flows

We discuss the transition from dripping to jetting from a nozzle in a coaxially flowing fluid. The fact that the outer fluid is viscous and moving affects the transition from dripping to jetting as well jet shape and drop formation mechanism. We relate the physics of drop formation to a recently reported microcapillary device (1,2) that generates monodisperse double emulsions. (1) A. S. Utada, E. Lorenceau, D. R. Link, P. Kaplan, H. A. Stone, D. A. Weitz, Science 308, 537 (2005) (2) E. Lorenceau, A. S. Utada, D. R. Link, G. Cristobal, M. Joanicot, D. A. Weitz, Langmuir in press, (2005).   

Geometrically controlled jet-like instabilities in microfluidic two-phase flows

We demonstrate effects of confinement in microfluidic devices with a two phase co-flowing system. When the flow rate of the inner fluid is small compared to the flow rate of the outer fluid, and the resulting width of the inner fluid is smaller than the height of the channel, the inner fluid breaks into droplets, as expected for a three-dimensional system. On the other hand, when the width of the second phase becomes comparable to the height of the microfluidic device, Rayleigh capillary instabilities are suppressed, and the inner fluid forms a jet that does not break, as might be expected for a purely two-dimensional system. We show that by changing the dimensions of the microfluidic channel we can transition from a stable co-flow to drop break-up. The experimental results are compared with of model of this two-phase flow.   

Playing with Microfluidic Droplets and Actuators

In the lab-on a chips of the future, flows will be handled at the microscale through mazes of microchannels using actuators. Here we concentrate on PDMS based microfluidic systems and we use actuators to introduce localized perturbations on a chip, close to where droplets are formed, i.e. near the intersection of a main and a side channel along which oil and water flows are driven. We observe Arnold tongues and devil staircases leading to the formation of regular or quasiperiodic-like droplets. These behaviors are well accounted for by modelling the system as a non linear oscillator driven by an external forcing. The characteristics of the regimes that are observed depend on the flow-rate conditions. In some range of flow-rates, we show that the droplet sizes can be varied by one order of magnitude by changing the actuation frequency, without modifying the flow-rates.These findings are used to understand the complex behavior of droplet emittors placed in parallel.   

Droplet Traffic Control at a simple T junction

A basic yet essential element of every traffic flow control is the effect of a junction where the flow is separated into several streams. How do pedestrians, vehicles or blood cells divide when they reach a junction? How does the outcome depend on their density? Similar fundamental questions hold for much simpler systems: in this paper, we have studied the behaviour of periodic trains of water droplets flowing in oil through a channel as they reach a simple, locally symmetric, T junction. Depending on their dilution, we observe that the droplets are either alternately partitioned between both outlets or sorted exclusively into the shortest one. We show that this surprising behaviour results from the hydrodynamic feed-back of drops in the two outlets on the selection process occurring at the junction. Our results offer a first guide for the design and modelling of droplet traffic in complex branched networks, a necessary step towards parallelized droplet-based ``lab-on-chip'' devices.   

Manipulation and stretching of bacteria and liposomes by Microfluidics

Microfluidic technology can be useful in lab-on-a-chip applications of biological assays, environmental monitoring, detection of toxic materials, as well as for assembly of nano- and micro-scale objects into more complex systems. In this work we focused on the orientation of rod-shaped bacteria (Bacillus) by employing shear flow and a high rate elongation flow, and stretching of giant liposomes with diameter size of tens of microns, which can be used as a simplified model for cell behavior. This was achieved by flows of dilute rod-like bacteria and liposome suspensions within a micro-channel by means of a capillary-driven motion. Fluidic alignment situations were tested, firstly by Venturi-like flow which produces a sufficiently converging and diverging flow, and secondly by sink-like flow in a converging microchannel. In the first method we found that the converging part of the flow aligns rod-like bacteria, whereas the diverging part disaligns them, while in the second method the rod-like bacteria can perfectly align along the streamlines. In addition we used the same technology to test liposome deformation while they are flowing through a Venturi-like microchannel. The microfluidics devices were fabricated from poly(dimethylsiloxane) (PDMS) by soft lithographic techniques.   

Continuous separation of serum from human whole blood within a microfluidic device

We were able to demonstrate separation of red and white blood cells from their native blood plasma, using a technique known as deterministic lateral displacement. The device takes advantage of asymmetric bifurcation of laminar flow around obstacles. This asymmetry creates a size dependent deterministic path through the device. All components of a given size follow equivalent migration paths, leading to high resolution. A subsequent version of the device will focus on the removal of platelets from whole blood. Samples will be extracted from the microfluidic device and analyzed by conventional flow cytometry.   

Reversible Dialysis in a Microfluidic Formulator.

In order to facilitate the screening of conditions for protein crystallization, we have been using the Microfluidic Formulator chip (Stephen Quake, PNAS Vol. 101, 40 ). This PDMS device allows us to mix up to 40 different reagents and protein solutions. We use this combinatorial approach along with a ``drop-on-demand'' method whereby we employ on-chip positive displacement pumps to form aqueous droplets containing protein and separate them by plugs of oil. Subsequently, the aqueous drops containing protein are guided by surface tension forces into storage chambers. To control the chemical potential of these sub-nanoliter protein samples, we fabricate reservoirs underneath the storage compartments. A thin PDMS membrane that is permeable to water, but not to protein or salt, separates the reservoirs from the storage chambers. Water can permeate into or out of the stored samples until the chemical potentials of the reservoir and the protein solution are equal leading to protein crystallization in some chambers.   

A noble microfluidic device for protein crystallizations.

A high throughput, low volume microfluidic device has been constructed out of poly(dimethylsiloxane) elastomer. We have demonstrated that sub-nanoliter water-in-oil drops of protein solutions of different composition can be rapidly stored in individual wells, which allows screening of 1000 conditions while consuming a total of only 1 microgram protein on a 20 cm$^{2}$ chip. This reduction in protein needed for crystal screens allows high-throughput crystallization of mammalian proteins expressed in tissue culture. A significant advance over current microfluidic devices is that each pot is in contact with a reservoir through a dialysis membrane which only water and other low molecular weight organic solvents can pass, but not salt, polymer or amphiphile. This enables the concentration of all solutes in a solution to be reversibly, rapidly, and precisely varied in contrast to current microfluidic methods, which are irreversible. This microfluidic dialysis technology solves a major problem in protein crystallization, the decoupling of nucleation from growth. The device will also be useful for general studies of the phase behavior of protein solutions.   

Shear induced particle migration in binary colloidal suspensions

We present experimental investigations of the spatial and temporal evolution of particle migration in pressure driven flows of Brownian particle suspensions. Binary suspensions of 1 $\mu $m- and 3 $\mu $m-diameter colloidal particles at a variety of concentration ratios and volume fractions are pumped through a 50 $\mu $m x 500 $\mu $m rectangular-cross-section capillary tube. Shear rate gradients caused by the resulting parabolic velocity profile drive particles away from the walls towards the center of the channel where the shear rate is lowest. Size segregation is observed. We measure the development of the size segregation by tracking the evolution of the particle concentration down the center of the tube of the small and large sized particles. The flows are directly imaged using high-speed confocal microscopy (up to 300 images/second).   

Using permeable microcapsules to deliver nanoparticles on substrates

We present a novel algorithm to simulate nanoparticles in the presence of a substrate, microcapsules and an externally driven flow. Here, the microcapsules consist of an elastic shell that encloses a fluid with either a dissolved chemical component or a suspension of nanoparticles that are small enough to be treated as so-called tracer particles (mutually non-interacting particles without excluded volume). The model couples a lattice-Boltzmann model for the fluid flow, a lattice-spring model for the elastic shell, and a Brownian dynamics model to simulate tracer trajectories. We then apply the model to simulate the release of nanoparticles from a microcapsule as it rolls along a substrate, as well as the subsequent particle adsorption on the wall. We study the effect of flow conditions, reaction kinetics, capsule elasticity, and capsules-substrate interaction on the rate of deposition and the size of the area of deposition at the substrate. The results provide guidelines for designing effective micro-scale delivery systems.   

Session N22: Focus Session: Magnetic Vortices and Exchange Biased Thin Films

Field Dependence of Magnetic Vortex Dynamics

We have used time-resolved Kerr microscopy (TRKM) to investigate the dynamical behavior of micron diameter disks patterned from sputtered Permalloy (Py) films. Different growth conditions yielded grain diameters of $\sim$35~nm and $\sim$80~nm while average roughness (Ra) remained less than 1~nm for 50~nm thick films. The magnetization of the disks relaxes into a vortex ground state, in which broadband spin dynamics include a low frequency vortex translational mode (vortex mode) that is expected to be nearly independent of field, based on simulations and analytical theory. We measured the field dependence of the vortex dynamics of individual disks using 5 Oe field steps from 0 Oe through the vortex annihilation field (H$_{a}$). For a 1~$\mu$m diameter disk the vortex mode has a mean frequency of $\sim$300~MHz, but the frequency fluctuates throughout the entire field range (H$_{a} \sim$ 350~Oe) with a magnitude $\Delta f\sim$200~MHz and a characteristic period $\sim$30~Oe. The fluctuations are not symmetric about zero field and look different in detail for different disks, but are highly repeatable for the same disk. We have also observed non-linear effects including the presence of up to 3 higher harmonics of the vortex mode, with a higher harmonic occasionally dominating the spectrum. A consistent interpretation is that the vortex core samples a distribution of pinning potentials, some of which are anharmonic, as it traverses the disk under the influence of the static applied field. Supported by NSF DMR 04-06029.   

Fractional vortices and composite domain walls in flat nanomagnets

We provide a simple explanation of complex magnetic patterns observed in ferromagnetic nanostructures. To this end we identify elementary topological defects in the field of magnetization: ordinary vortices in the bulk and vortices with {\em fractional} winding numbers ($\pm 1/2$) confined to the edge [1]. Domain walls found in experiments and numerical simulations in strips and rings are composite objects containing two or more of the elementary defects. Allowed compositions of a domain wall in a strip or ring are constrained by simple selection rules of topological origin: (i) An edge contains an odd number of edge defects. (ii) The net winding number of all edge and bulk defects is zero. The walls observed most frequently in experiments and simulations contain a halfvortex and an antihalfvortex (transverse walls in thin and narrow strips) or two antihalfvortices and a vortex (vortex walls in thicker and wider strips). \begin{thebibliography}{1} \bibitem{OT05} O. Tchernyshyov and G.-W. Chern, Phys. Rev. Lett. {\bf 95,} 197204 (2005). \end{thebibliography}   

Magnetic domains in ferromagnetic particles with perpendicular anisotropy

We derive a Derrick-like virial theorem for static states in a disc-shaped ferromagnetic particle with an axially symmetric magnetic configuration. This is applied to elementary magnetic states such as a single domain and a vortex. We calculate the vortex state in a disc-shaped particle with no anisotropy and study the very thin and very thick limits. In the very thin limit the virial relation effectively gives the vortex core radius. We also consider a particle with significant perpendicular anisotropy and show that a vortex is a static state for sufficiently thin particles. For thicker particles the vortex core expands to become comparable to the particle lateral size while the magnetization at the periphery of the particle tilts out of plane opposite to the vortex core region. In sufficiently thick particles, the magnetic state takes the form of a magnetic ``bubble'' (well-known in films) viewed here as a bidomain state. The signature of a bubble is its magnetostatic field which consists of two concentric regions of opposite sign above the particle top surface. Higher order states of multiple concentric domains of opposite magnetization are found in larger particles. We finally study the effect of an external field on magnetic bubble states.   

Composite domain walls in flat nanomagnets: the dipolar limit

Topological defects play an important role in nanoscale ferromagnets. We have previously demonstrated that domain walls in thin strips and rings are composite objects made of bulk vortices (winding numbers $n = \pm 1$) and edge defects (fractional winding numbers $n = \pm 1/2$) and given analytical solutions in the exchange limit [1]. Experimentally accessible systems are in the opposite regime where the dipolar interaction dominate. In this limit the vortex solution remains unchanged, the antivortex and antihalfvortex are deformed but survive, whereas the halfvortex acquires a high magnetostatic energy and becomes unstable. Accordingly, domain walls in this limit consist of two antihalfvortices and a vortex between them. We present a model of the domain wall in the magnetostatic limit in which the location of the vortex core is a variational parameter. As the width and thickness of a strip change, the global and local minima of the total magnetic energy yield the familiar transverse and vortex walls, as well as more exotic configurations such as the ``diagonal wall'' with a vortex hanging close to an edge. [1] O. Tchernyshyov and G.-W. Chern, Phys. Rev. Lett. {\bf 95}, 197204 (2005).   

Single vortex dynamics in patterned ferromagnetic ellipses

Measurements of low frequency dynamics of single magnetic vortices confined in elliptic ferromagnetic dots made of Permalloy with dimensions 2x1 $\mu$m$^2$ and 3x1.5 $\mu$m$^2$, 40-nm thick, have been performed using a microwave reflection method. Resonances were recorded in the sub-GHz range that can be attributed to the vortex translational mode where the vortex core follows an elliptic trajectory around its equilibrium position. The existence of single vortex states in the samples was confirmed by magnetic force microscopy. The frequency of this translational mode varies little under the influence of an in-plane static field H along the easy axis, however, it increases by more than a factor of two when H is applied along the hard axis. Micromagnetic simulations are used to explore the origin of the observed field dependence.   

The relationship between the sign of exchange bias and the magnetization depth profile of Co/FeF$_{2}$

We have used the unique spatial sensitivity of polarized neutron beams in reflection geometry to measure the depth dependence of magnetization across the interface between a ferromagnet (Co) and an antiferromagnet (FeF$_{2})$. Our Co/FeF$_{2}$ bilayer sample is one that exhibits either positive or negative exchange bias depending upon the magnitude of the cooling field. For positive exchange bias, pinned magnetization at the Co/FeF$_{2}$ interface is directed opposite to the cooling field, while in the FeF$_{2}$ bulk, the net pinned magnetization is parallel to the cooling field. For negative exchange bias, the net pinned magnetization near the Co/FeF$_{2}$ interface is parallel to the direction of the cooling field. We propose a model that explains the cooling field dependence of the sign of exchange bias. Work at LANL and UCSD was funded by the U.S. Department of Energy, BES-DMS, and by a University of California Campus-Laboratory Collaboration grant.   

Vortex State in Sub-100 nm Magnetic Nanodots.

Magnetism of nanostructured magnets, which size is comparable to or smaller than ferromagnetic domain size, offers a great potential for new physics. Detailed knowledge of magnetization reversal and possible magnetic configurations in magnetic nanostructures is essential for high-density magnetic memory. Many theoretical and experimental studies are focused on a magnetic vortex which in addition to a circular in-plane configuration of spins has a core, - the region with out-of-plane magnetization. We present a quantitative study of the magnetic vortex state and the vortex core in sub-100 nm magnetic dots. Arrays of single-layer and bilayer nanodots covering over 1 cm$^2$ are fabricated using self-assembled nanopores in anodized alumina. This method allows good control over the dot size and periodicity. Magnetization measurements performed using SQUID, VSM, and MOKE indicate a transition from a vortex to a single domain state for the Fe dots. This transition is studied as a function of the magnetic field and dots size. Micromagnetic and Monte Carlo simulations confirm the experimental observations. Thermal activation and exchange bias strongly affect the vortex nucleation field and have a much weaker effect on the vortex annihilation field. Direct imaging of magnetic moments in sub-100 nm dots is extremely difficult and has not been reported yet. Polarized grazing incidence small angle neutron scattering measurements allow dot imaging in reciprocal space. Quantitative analysis of such measurements performed on 65 nm Fe dots yields the vortex core size of $\sim 15$ nm, in good agreement with the 14 nm obtained from the simulations. \\ \\ This work is done in collaboration with Chang-Peng Li, Zhi-Pan Li, S. Roy, S. K. Sinha, (UCSD), Xavier Batlle (U. Barcelona), R. K. Dumas, Kai Liu, (UC Davis), S. Park, R. Pynn, M. R. Fitzsimmons (LANL), J. Mejia Lopez (Pontificia U. Catolica de Chile), D. Altbir, (U. de Santiago de Chile), A. H. Romero (Cinvestav-Unidad Queretaro), and Ivan K. Schuller (UCSD) and supported by AFOSR, US DOE, NSF, UC-CLE, Spanish MECD, Catalan DURSI, FONDECYT, Millennium Initiative, and Conacyt Mexico.   

Using multiple antiferromagnet/ferromagnet interfaces as a probe of grain size dependent exchange bias in polycrystalline Co/Fe$_{50}$Mn$_{50}$

We have used ferromagnet/antiferromagnet/ferromagnet trilayers and ferromagnet/antiferromagnet multilayers to probe the grain size dependence of exchange bias in polycrystalline Co/FeMn. X-ray diffraction and transmission electron microscopy characterization show that the FeMn grain size increases with increasing FeMn thickness in the Co (30 {\AA}) / FeMn system. Hence, in Co (30 {\AA}) / FeMn / Co (30 {\AA}) trilayers the two Co layers ``sample'' different FeMn grain sizes at the two antiferromagnet/ferromagnet interfaces. For FeMn thicknesses above $\sim $ 100 {\AA}, where simple bilayers have a thickness independent exchange bias, we are therefore able to deduce the influence of grain size on the exchange bias and coercivity (and their temperature dependence) by measuring trilayer and multilayer samples with varying FeMn thickness. Increasing the average grain size results in a large decrease in exchange bias energy. We interpret the results as being due to a decrease in uncompensated spin density with increasing grain size, further evidence for the importance of defect generated uncompensated spins.   

Interplay between reversal asymmetry, training, and anisotropy, in epitaxial NiMn/Ni exchange biased bilayers

We have employed electron and x-ray diffraction, x-ray reflectivity, conventional magnetometry, and polarized neutron reflectivity to probe epitaxial NiMn/Ni bilayers. Binary alloys such as NiMn often require an annealing procedure to induce AF ordering which leads to interdiffusion at the AF/ F interface and a subsequent, and poorly understood, reduction in exchange bias. Our previous work with neutron reflectivity revealed a 35 {\AA} interdiffused region that contains competing AF and F interactions resulting in a very unusual temperature dependant magnetic interface location (M.S. Lund, M.R. Fitzsimmons, S. Park, and C. Leighton, APL 2004). In this work we find that at low temperatures there is a rapid divergence of the exchange bias field, coincident with the onset of strong training, an obvious reversal asymmetry, and the appearance of higher order induced anisotropies. We show in a simple way that the rapid increase in bias field, strong training, and reversal asymmetry are all consequences of the induced anisotropies. In addition, we are able to demonstrate in a single sample that uniaxial anisotropy favors low training, while biaxial anisotropy results in large training, confirming a recent theoretical prediction (A. Hoffmann, PRL 2004). Research was supported by NSF MRSEC.   

Effect of Hard Layer Demagnetization on the Magnetization Reversal of Epitaxial Fe/SmCo Spring Magnets

In epitaxial Fe/SmCo, a classical spring magnet, irreversible magnetization reversal is observed once the SmCo hard layer starts switching [1,2]. To distinguish the soft and hard layer reversibility separately, we studied the effect of partial SmCo layer demagnetization on the reversal behavior of the entire bilayer using the first and second order reversal curve methods (FORC and SORC, respectively). The FORC distribution [2,3] shows two distinct features during the hard layer reversal: a negative/positive pair of features and a single positive peak. The negative/positive pair is from the soft Fe layer reversal and is a manifestation of the interlayer exchange coupling. The single positive peak occurs at larger applied fields and corresponds to the reversal of the hard SmCo layer. A SORC measurements were done at several reversal fields to determine the reversibility along different FORCs. We observe that the Fe layer remains mostly reversible. The partially demagnetized SmCo layer is the main source of irreversibility, particularly when the applied field approaches the hard layer nucleation/saturation field. [1] Fullerton, PRB 58, 12193 (1998). [2] Davies, et al, APL 86, 262503 (2005). [3] Davies, et al, PRB 70, 224434 (2004); PRB 72, 134419 (2005).   

Electrically controlled exchange bias for spintronic applications

Electrically controlled exchange bias (EB) is proposed for novel spintronic applications [1]. Basic effects of electrically controlled EB and its magnetoelectric (ME) switching are studied in a Cr$_{2}$O$_{3}$(111)/(Co/Pt)$_{3}$ heterostructure. Exchange coupling between the ME antiferromagnet Cr$_{2}$O$_{3}$ and a ferromagnetic CoPt multilayer exhibits perpendicular EB. The latter is controlled by applied axial electric fields inducing excess magnetization at the interface. The enhancement of this hitherto weak tuning effect is explored when replacing ME bulk pinning systems by epitaxal thin films. Recently, the sign of the EB field has been tuned via field cooling the system in either parallel or antiparallel axial magnetic and electric fields [2].$_{ }$Here, the crossover from bulk to thin film ME pinning systems is studied and spintronic applications are suggested based on the electrically controlled EB. Pure voltage control of magnetic configurations of tunneling magnetoresistance spin valves is proposed as an alternative to current-induced magnetization switching. In addition we suggest an XOR operation realized in a MEally pinned giant magneto resistance structure. [1] Ch. Binek, B.Doudin, J. Phys. Condens. Matter\textbf{ 17}, L39 (2005). [2] P. Borisov et al., Phys. Rev. Lett. \textbf{94}, 117203 (2005).   

Exchange bias training effect in coupled all ferromagnetic bilayer structures

We study exchange coupled bilayers of soft and hard ferromagnetic (FM) thin films by means of Alternating Gradient Force Magnetometry. A CoCr thin film realizes the magnetically soft layer (SL) which is exchange coupled via a Ru-interlayer with a hard CoPtCrB pinning layer (HL). This new class of all FM bilayers shows remarkable analogies to conventional antiferromagnetic (AF)/FM exchange bias (EB) heterostructures. Not only do these all FM bilayers exhibit a tunable EB effect, they also show a distinct training behavior upon cycling the SL through consecutive hysteresis loops. Training resembles the cycle dependent evolution of the bias field and is to a large extend analogous to the gradual degradation of the EB field observed upon cycling the FM top layer of a AF/FM EB heterostructure through consecutive hysteresis loops. However, in contrast to these conventional EB systems, our all FM bilayer structures allow the observation of training induced changes in the bias-setting HL by means of simple magnetometry. Our experiments show unambiguously that the training effect is driven by deviations from equilibrium in the pinning layer. A comparison of the experimental data with predictions from a theory based upon triggered relaxation phenomena shows excellent agreement.   

Alignment-Sensitive Reversal Mechanisms of Epitaxial-FeF$_{2}$/Polycrystalline-Ni Exchange Biased Thin Films*

Magnetization reversal mechanisms of epitaxial-FeF$_{2}$/polycrystalline-Ni exchange biased thin films were investigated with vector magnetometry and a First Order Reversal Curve (FORC) technique [1]. The FORCs were measured \textbf{\textit{without}} remounting the sample after the vector magnetometry measurements, ensuring consistency between the two methods. Samples were exchange biased by field cooling along the FeF$_{2}$ spin axis. When the applied field is aligned with the spin axis, the transverse hysteresis loop is flat and FORC analysis shows that the magnetization switching is highly irreversible ($\sim $80{\%}), indicating that domain nucleation and motion is the reversal mechanism. With a misalignment of 5\r{ }, the transverse hysteresis loop shows that the reversal is predominantly by rotation [2] and FORC analysis shows that the majority of the magnetic switching is by a reversible mechanism (only $\sim $40{\%} irreversible). These results demonstrate that the magnetization reversal mechanisms are \textbf{\textit{extremely sensitive}} to the alignment of the applied field with the antiferromagnet spin axis and the exchange bias direction [3]. .1. J. E. Davies, et al., Phys. Rev. B \textbf{70}, 224434 (2004); Phys. Rev. B \textbf{72}, 134419 (2005). 2. J. Olamit, et al., Phys. Rev. B \textbf{72}, 012408 (2005). 3. A. Tillmans et al, cond-mat/0509419. *Work supported by ACS-PRF, Alfred P. Sloan Foundation, and DOE.   

Session N23: Focus Session: MAG.THY III: Oxides and Phase Transitions

Antiferromagnetic Heisenberg Spin Layers coupled with Dipolar Interaction -- a Monte Carlo study of Rb$_{2}$MnF$_{4}$

Rb$_{2}$MnF$_{4}$ is a quasi-2D antiferromagnetic (AF) system, in which Mn$^{2+}$ ions carrying spin-5/2 occupy square lattices perpendicular to the c-axis of the tetragonal unit cell. These spins interact via mostly nearest neighbor isotropic AF exchanges, while the dipolar interaction contributes to the effective anisotropy that stabilizes the AF phase at low temperatures. In a magnetic field parallel to the c-axis, the AF phase is terminated along a spin-flop line, and a transverse (XY) phase appears. We perform large scale extensive Monte Carlo simulations to map out the phase diagram and investigate the critical behavior along the phase boundaries. A novel reweighting technique is used to efficiently handle the dipolar interaction. Our results suggest that both the AF phase and the XY phase experience continuous transitions across the spin-flop line, which is consistent with a bicritical point at zero temperature. We also found that the effect of the weak inter-planar coupling is not completely negligible for the spin-flop transition and the properties of the XY phase.   

RVB liquid phase of a quantum dimer model with competing kinetic terms

Starting from a spin-orbital model adapted to the case of LiNiO$_2$, we derived an effective quantum dimer model including 6-dimer loops. We argue that the relevant terms of this model are of kinetic type. Using numerical techniques like exact diagonalizations and Green's function Monte-Carlo we show that a competition between two kinetic terms can lead to a resonating valence bond state for a finite range of the parameters.   

Dynamical correlations of the Quantum Dimer Model on the triangular lattice

Using Green's function Monte Carlo simulations, we have studied the zero-temperature properties of the quantum dimer model (QDM)on the triangular lattice [1] on clusters with up to 588 sites. A detailed comparison of the static properties in different topological sectors as a function of the cluster size and for different cluster shapes has allowed us to identify different phases, and to to show explicitly the presence of topological degeneracy in a phase close to the Rokhsar-Kivelson point, in agreement with an earlier suggestion [2]. We have also extended the Green's function Monte Carlo algorithm to calculate dynamical correlation functions. Preliminary results on the dimer-dimer correlations confirm the extension of the RVB phase and bring new insight on the nature of the transition to the $\sqrt{12} \times \sqrt{12}$ phase and on the type of long-range order realized in that phase.\\ {[1]} A. Ralko, M. Ferrero, F. Becca, D. Ivanov and F. Mila, Phys. Rev. B, {\bf 71}, 224109 (2005).\\ {[2]} R. Moessner and S.L. Sondhi, Phys. Rev. Lett, {\bf 86}, 1881 (2001).   

Towards material-specific simulations of high-temperature superconducting cuprates

Simulations of high-temperature superconducting (HTSC) cuprates have typically fallen into two categories: (1) studies of generic models such as the two-dimensional (2D) Hubbard model, that are believed to capture the essential physics necessary to describe the superconducting state, and, (2) first principles electronic structure calculations that are based on the local density approximation (LDA) to density functional theory (DFT) and lead to materials specific models. With advent of massibely parallel vector supercomputers, such as the Cray X1E at ORNL, and cluster algorithms such as the Dynamical Cluster Approximation (DCA), it is now possible to systematically solve the 2D Hubbard model with Quantum Monte Carol (QMC) simulations and to establish that the model indeed describes $d$-wave superconductivity [1]. Furthermore, studies of a multi-band model with input parameters generated from LDA calculations demonstrate that the existence of a superconducting transition is very sensitive to the underlying band structure [2]. Application of the LDA to transition metal oxides is, however, hampered by spurious self-interactions that particularly affects localized orbitals. Here we apply the self-interaction corrected local spin-density method (SIC-LSD) to describe the electronic structure of the cuprates. It was recently applied with success to generate input parameters for simple models of Mn doped III-V semiconductors [3] and is known to properly describe the antiferromagnetic insulating ground state of the parent compounds of the HTSC cuprates. We will discus the models for HTSC cuprates derived from the SIC-LSD study and how the differences to the well-known LDA results impact the QMC-DCA simulations of the magnetic and superconducting properties. \newline \newline [1] T. A. Maier, M. Jarrell, T. C. Schulthess, P. R. C. Kent, and J. B. White, Phys. Rev. Lett. 95, 237001 (2005). \newline [2] P. Kent, A. Macridin, M. Jarrell, T. Schulthess, O. Andersen, T. Dasgupta, and O. Jepsen, Bulletin of the American Physical Sosciety 50, 1057 (2005). \newline [3] T. C. Schulthess, W. Temmerman, Z. Szotek, W. H. Butler, and G. M. Stocks, Nature Materials 4, 838 (2005).   

Mechanism of Noncollinearity and Magnetoelectric Coupling in Incommensurate Multiferroics

Using extensive Monte-Carlo simulations of a modified orbitally degenerate double-exchange model applied to the multiferroic perovskites $R$MnO$_3$ ($R=$Gd, Tb, Dy), we show that the spiral magnetic phase results from the interplay between the double exchange coupling, superexchange, Jahn-Teller and Dzyaloshinskii-Moriya (DM) interactions. The DM interaction also induces a small ferroelectric moment and provides the mechanism of the strong coupling between the unusual magnetism and ferroelectricity. We also discuss the magnetoelectric effects in the applied magnetic fields.   

Numerical Study of Magnetism in the Periodic Anderson Model

The periodic Anderson model is believed as a candidate of the minimal lattice models for itinerant ferromagnetism. Several numerical methods, including exactly diagonalization, constrained-path Monte Carlo method and mean field method, are employed to investigate the magnetic properties of the model in one dimension and two dimensions. By changing the band-filling, chemical potential of the impurity band and the hybridyzation between conduction band and impurity band, we found that in some parameter regions, different magnetic ordering exist. Some of results confirm the previous works and some are new.   

Bond-diluted Heisenberg spin systems on coupled ladders

We study spin-1/2 Heisenberg spin systems with bond dilution on coupled ladders or striped phases. The diluted bond configurations can be static or dynamic. The dynamic case with motion of the bonds is described by pseudo-spins and modeled by anisotropic Heisenberg spin chains in an external field. The systems are studied using the stochastic series expansion quantum Monte Carlo method. We find the quantum critical point for real spins from the ordered phase to the disordered phase is sensitive to the bond configuration. A study of the ground state energy shows strong differences for different bond configurations, which may be related to phase separation. Under certain conditions, real spin systems with bond-dilution can be described by a coupling-weakened fully coupled spin systems. For the pseudo-spins, an effective field induced by the real spins is observed.   

Thin Ising films with both competing surface fields and a magnetic field gradient: A Monte Carlo study

Extensive Monte Carlo simulations are used to study the interesting effects resulting from a linearly varying magnetic field on a thin Ising film (equivalent to applying gravity to the corresponding lattice-gas model). Besides competing surface fields acting on two LxL free surfaces a distance D apart from each other, we also apply a magnetic field g that varies linearly between the surfaces and which competes with the surface fields. To determine the phase diagram, we look for bulk two-phase conexistence at different values of g and temperature T. In situations with only competing surface fields applied, the interface unbinding transition \footnote{K.Binder, D.P.Landau, A.M.Ferrenberg, Phys.Rev.E {\bf 51}, 2824(1995)} happens at temperature T$_{c}$(D). The addition of the g field produces a phase diagram in which, as g increases, the temperature bounding bulk two-phase coexistence first goes up from T$_{c}$(D), and then decreases. For small g, we find a second order transition, whereas for large g, the transition appears to be first order. We will compare our simulation results with theoretical predictions \footnote{J.Rogiers, J.O.Indekeu, Europhys.Lett. {\bf 24}, 21(1993)}. \\ \\ $^*$Research supported by NSF   

Electronic structure and excitation spectra of transition metal monoxides investigated via orbital-dependent functionals

We are investigating the electronic structure of strongly correlated 3d transition metal monoxides with two orbital dependent functionals, the self-interaction corrected local spin-density method (SIC-LSD) as well as the LDA+U method. Both functionals are known to reproduce the antiferromagnetic insulating ground state of, for example, CoO and NiO. We perform a detailed comparison of magnetic moments, exchange, and electronic structure calculated with the two methods. In addition, we study the interplay between the electronic structure and the electron-hole excitations in both the insulating and the metallic phases. Our results are compared with available experimental data.   

Ab initio calculations on the frustrated magnet ZnCr$_2$O$_4$

The complex oxide ZnCr$_2$O$_4$ is a good realization of the Heisenberg antiferromagnet on a pyrochlore lattice and is a strongly frustrated magnetic system. Recent experiments have shown that ZnCr$_2$O$_4$ undergoes a lattice distortion and a transition from paramagnetic to antiferromagnetic order at $T_c = 12.5 \ K$. Infrared spectroscopy has shown a large splitting of a phonon mode involving magnetic ions. We perform ab initio total energy calculations of the exchange coupling constant and phonon modes using the plane-wave pseudopotential formalism with the LSDA+U method, and we compare the results to experiment. This work was supported by National Science Foundation Grant No. DMR04-39768 and by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Computational resources have been provided by NPACI and NERSC.   

Anisotropic magnetic phases

We study how magnetic phases vary with uniaxial and fourfold anisotropy constants, C and D. We do this for classical magnetic dipoles on cubic lattices with dipolar and nearest neighbor exchange interactions. By mean field and by Monte Carlo calculations, results are obtained for bulk and n-layer film systems under no applied external field. We pay special attention to the spin reorientation (SR) transition. We find (1) a reentrant SR transition for a narrow range of C/D values, and (2) that the ratio of the ordering temperature to the SR temperature varies with C/D but depends rather weakly on the exchange constant.   

Alternative methods to characterize phase transitions in Ising systems

The Binder cumulant (BC) is defined in terms of average values of second and fourth momenta of an order parameter q. Simulations for Ising systems show that the curves for BCs they all cross at the same temperature regardless of the size of the system. We present here two alternative and different methods to obtain the critical temperature after finding the time evolution of any order parameter q(t), after equilibration. First, we consider the time autocorrelation functions for the absolute value of a site order parameter, $\vert $q$\vert $, for different system sizes, showing that they also cross at the same temperature where BCs cross. Second, we show that the ``weight'' in bites of the compressed file containing vector q(t) maximizes at a temperature close to the critical temperature; a scaling analysis takes us back to the temperature of previous crossing. The main advantage of the new methods is its easy physical interpretation.   

Session N24: Structure and Dynamics in Polymer Thin Films

Unique refractive index and thickness values for polymer films via ellipsometry

In this study, elliposometry using multiple ambient media at Brewster's angle is used to determine unique values of refractive index and film thickness for polymer thin films. Measurements were made for polymer thin films on HF etched silicon wafers against air and water. Results obtained for poly(t-butyl acrylate), polystyrene, and trimethylsilyl cellulose confirm that the refractive indices agree well with the literature values and that the film thicknesses agree with values obtained from X-ray reflectivity. This technique provides a rapid unambiguous method for determining a film's thickness and refractive index for polymers.   

Watching How Molecules Orient in a Surface Forces Apparatus, Using Confocal Raman Spectroscopy

Much is known about surface forces, less about where they come from. This laboratory is engaged in direct vibrational spectroscopic measurements of how molecules orient in confined geometries. Regarding force measurements, PDMS (polydimethylsiloxane) was a model system for many years. In this study, we describe direct experiments using a novel version of confocal Raman spectroscopy. This experiment allows direct measurement of how the PDMS molecules orient under confinment as well as under subsequent shear. When the thickness of the fluid film is less than the unperturbed radius of gyration of the polymer, we obtain two novel findings: (a) linewidth analysis of peaks reveals that vibrational relaxation times are perturbed in this confined geometry; (b) orientation of the chain backbone is not everywhere the same within the molecularly-thin film; domains of various orientation are observed instead.   

Polymer dynamics at local scales: origin of ripples formation

A plethora of nanowear patterns in polymers has been obtained by heating the polymers and scanning their surfaces with an atomic force microscope (AFM) tip (1, 2). These morphologies represent the nanoscale realization of aeolian ripples in sandy deserts and are similar to patterns obtained during evolution of surfaces during ion sputtering (3). By means of locally heated AFM probes we studied ripples on various polymer films. While the theory of aeolian ripples formation is very complicated (3), we show that the key morphological features in our results can be explained in terms of the elastic and diffusive properties of the polymer. From measurements of the ripple spacing we study the local dynamics of polymers in the vicinity to the glass transition. (1) B. Gotsmann and U. Durig, Langmuir 20, 1495 (2004) (2) R. H. Schmidt, G. Haugstad and W. L. Gladfelter, Langmuir 15, 317 (1999) (3) T. Aste and U. Valbusa, New Journal of Physics 7, 122 (2005).   

Probing surface relaxation of polystyrene films using gold nano-particles

Polymer thin films are usually used to probe the confinement effects on the dynamics of glass former materials. Many studies show that the glass transition temperature of thin polymer films is decreased below bulk Tg for very thin films. There is evidence that this Tg reduction is due to the existence of a more mobile region near the surface of the film. However, it is very hard to directly measure the existence of this layer, let alone more quantitative aspects such as thickness or viscoelastic properties. In this work we probe the near surface behaviour of thin polystyrene films using a nanorheological technique. To do this, we uniformly distribute gold nanoparticles on the surface. A particle's motion on the surface is then driven by surface capillary waves. The motion of nanoparticles is detected by dynamic light scattering. Since the particles only interact with the surface region (determined independently using AFM), these experiments tell us about the viscoelastic properties of the near surface region.   

Grazing-incidence x-ray scattering studies on surface melting in ultrathin polymer films

The aim of this study is to investigate surface crystal structures formed in ultrathin (thickness below 100 nm) polymer films by using surface-sensitive x-ray scattering techniques. This study was motivated by our current experimental finding that showed a drastic suppression (the decrease of $\sim $50\r{ }C) in the surface melting temperature (T$_{m})$ of ultrathin polymer films, which was determined as the onset of surface softening by using the shear modulation force microscopy (SMFM) method[1]. In order to clarify the relationship between the melting behavior and surface crystal structures, we integrated a variety of grazing-angle x-ray scattering techniques including reflectivity, diffuse scattering, grazing-incidence diffraction, and grazing-incidence small-angle scattering. As a result, we found that diffuse scattering, which is sensitive to surface roughness, drastically changed at T$_{m}$ determined from SMFM, while the surface crystallinity decreased with increasing temperature, but remained up to the bulk melting temperature. A model to explain the mechanism of the surface melting will be discussed. [1]Wang, Y. et al. Macromolecules, \textbf{37}, 3319 (2004).   

The Tg-Nanoconfinement Effect and the Relaxation of Residual Stresses in Spin-Coated Films of Polystyrene and Styrene-Containing Copolymers: Characterization by Intrinsic Fluorescence.

The glass transition temperatures (Tgs) of films of polystyrene (PS) and styrene (S)-methyl methacrylate (MMA) copolymers have been determined using intrinsic fluorescence from styrene units. The Tgs are measured by a break in the temperature dependence of fluorescence intensity measured upon cooling from the equilibrium liquid state. As the film thickness decreases below 50 nm, there is a substantial deviation in Tg from bulk Tg, with PS and high S-content copolymers exhibiting a reduction in Tg and high MMA-content copolymers exhibiting an increase in Tg. This is explained by a competition of free surface effects and the effects of attractive polymer-substrate interactions. As the intrinsic fluorescence is a combination of monomer and excimer fluorescence, it reflects the local conformational population. This is used to determine the conditions at which residual stresses induced by spin coating are relaxed away, leading to a steady-state conformational population and fluorescence signal independent of annealing time. Films must be heated to temperatures well above Tg (Tg + 30 K) for several minutes to achieve constant fluorescence independent of further annealing. Annealing for short times close to Tg is insufficient to achieve an equilibrium conformational population.   

Dynamics of very thin polymer films on supported surface

In this work, we study the effect of solid substrate surface on the viscosity of polymer thin films. We found that the viscosity of polymer thin film increased about two orders of magnitude near the solid substrate. Measurements performed on split layer substrates indicated that this layer was responsible for trapping the subsequent layers, and propagating the effect of the surface interactions to the chains without direct contacts to the surface. If this layer was applied prior to the rest of the film, it in fact screened the surface interactions and even caused auto-dewetting of the other layers in the film. These results will be discussed in terms of the ``two fluid'' hypothesis. This surface interaction was also found to induce the ``melt fracture'' of the polymer thin film during the dewetting process where we see the cracks in the less viscous bottom layer.   

The Distribution of Tgs in Thin and Ultrathin Methacrylate-Based Polymer Films: Percolation of Free Surface and Interface Effects over Tens and Hundreds of Nanometers.

A multilayer/fluorescence method is used to measure the distribution of Tgs in poly(methyl methacrylate) (PMMA) films. The average Tg increases with decreasing total film thickness (h) below 90 nm. In bulk, bilayer films, the free surface layer exhibits a reduced Tg at a thickness of 30 nm or less. The Tg reduction at the free surface is a fraction of that in polystyrene (PS) films; a 14-nm-thick free surface layer exhibits a Tg reduction of $\sim $ 6 K in PMMA and $\sim $ 32 K in PS. The Tg increase observed in the substrate layer of a PMMA bilayer, bulk film exceeds the Tg reduction observed at the free surface and occurs over longer length scales. An amazing effect of confinement of the total multilayer film on the free surface layer Tg of PMMA has been observed. When h $<$ 250 nm, a 12-nm-thick free surface layer exhibits an increase in Tg with decreasing h, and the free surface layer Tg exceeds that of bulk Tg when h $<$ 90 nm. This is the first demonstration that a free surface layer can exhibit Tg $>$ bulk Tg and means that the perturbation of Tg dynamics at the substrate can percolate over hundreds of nanometers in PMMA films. These results will be contrasted with those of poly(isobutyl methacrylate) films which exhibit no average Tg-confinement effect.   

Dynamics of Polymer Thin Film Mixtures

We examined the influence of film thickness and composition on the glass transition temperature ($T_{g})$ and mean square atomic displacements (MSD) of thin film mixtures of deuterated polystyrene (dPS) and tetramethyl bisphenol-A polycarbonate (TMPC) on Si/SiO$_{x}$ substrates using incoherent elastic neutron scattering (ICNS). The onset of dissipative motions, such as those associated with the glass transition and sub-Tg relaxations, are manifested as ``kinks'' in the curve of elastic intensity (or MSD) versus temperature. From the relevant kinks, the $T_{g}$ was determined as a function of composition and of film thickness. The dependence of the $T_{g}$ on film thickness exhibited qualitatively similar trends, at a given composition, as determined by the ICNS and ellipsometry measurements. However, with increasing PS content, the values of $T_{g}$ measured by INS were consistently larger then those measured by ellipsometry. These results are examined in light of existing models on the thin film glass transition and component blend dynamics.   

Dynamics of Block copolymer films

We have investigated the dynamics of thin block copolymer films of ploy(styrene)-b-poly(dimethylsiloxane) using X-ray Photon Correlation Spectroscopy (XPCS). The films were supported on Si substrates and measured at melt. The results are compared with the theory of overdamped thermal capillary waves on thin films. The lateral length scales examined were between 600 and 6000 nm. We selectively measured the dynamics from the surface and from the micelles by changing incident angles and found the different behaviors between them.   

Conformational Anisotropy and the Glass Transition in Polymer Thin Films

A segmental level theory of the ideal kinetic glass transition, or dynamic crossover, temperature $\left( {T_c } \right)$ in confined polymer films has been developed. The theory is based on an anisotropic generalization of a coarse grained polymer mode coupling theory which utilizes conformational and thermodynamic information from anisotropic PRISM theory and computer simulations. Confinement is found to enhance the bulk compressibility and induce anisotropic segmental dynamics. For non-capped films (free standing or supported films on neutral substrates) the theory predicts suppression of $T_c $, with confinement. The underlying mechanism is reduction of the degree of coil interpenetration and intermolecular repulsive contacts due to segmental scale alignment and deformation. The predicted suppression of $T_c $ is nonuniversal and follows an inverse power law dependence on film thickness in reasonable agreement with experiments. For capped films simulations find a weak variation of the dimensionless compressibility with confinement suggesting little or no shift of $T_g $.   

Radial Thickness Profiles of Spincoated Thickness Gradient Films

By dropping a polymer solution onto a spinning substrate at a position displaced from the axis of rotation, one can produce a film in which the thickness increases with increasing radial distance (thickness gradient film). Since each film contains a continuous range of thickness values, one can track subtle changes in the physical properties with film thickness by using a local probe of the film properties. We have used two such local probes, focused ellipsometry and atomic force microscopy, to measure the radial thickness profiles. We have also developed a simple, fluid flow model that describes the dependence of the polymer film thickness on radial distance from the axis of rotation. A detailed comparison between the measured and calculated radial thickness profiles will be discussed.   

Internal Phase Separation Induces Dewetting in Multicomponent Polymer Films

Thin liquid films that dewet from their substrate are ubiquitous as demonstrated by the beading of paint on oily surface. Although most coatings contain more than one component, the dewetting mechanisms in multicomponent films are not understood. Using dPMMA:SAN (50:50) films (550 nm) with or without nanoparticles (NP), we demonstrate, for the first time, that the Laplace pressure induced by internal phase-separated structure is the driving force for roughening and rupture in polymer blend films. Three NP were investigated, namely NP$_{A}$, NP$_{B}$, and NP$_{C}$ which either partition into dPMMA or weakly and strongly segregate to the dPMMA/SAN interface, respectively. NP$_{B}$ are more effective than NP$_{A}$ at stabilizing the film, whereas NP$_{C}$ are able to prevent film rupture. Upon annealing, roughened films display a periodic, lacey structure, resembling patterns from spinodal dewetting. The fluctuation periodicity scales with roughness evolution as \textit{$\lambda $}$_{s} \quad \propto \quad R_{q}^{1/4}$ for neat blends and blends with NP$_{A}$, whereas the scaling breaks down for blends containing NP$_{B}$ and NP$_{C}$. These studies show that phase separation is responsible for film roughening.   

Dewetting Morphology and Dynamics of Ordered Symmetric Block Copolymer Films: Stability of Nanoscopic Liquid Bilayers

Symmetric diblock copolymers, which form lamella upon micro-phase separation, can have unique dewetting properties. In this experimental study we explore the effects of the microphase separation on the dewetting of three different systems. We begin with the dewetting of disordered thin poly(styrene)-b-poly(methyl methacrylate) films on poly(dymethyl siloxane) coated Silicon. In this case, the film is not allowed to relax to its lamellar state before dewetting begins. The complex interplay between dewetting and microphase separation leads to hole growth that appears dendritic and deviates dramatically from the conventional circular hole growth. In a second experiment, the thin films are arrange into their lamellar equilibrium before being transferred onto an unfavourable substrate, which initiates dewetting. On an unfavourable substrate, these films show remarkable stability. Holes that do form are cylindrical but grow at a much-reduced rate when compared to a homopolymer system. Finally, hole growth in free-standing ordered lamellar films is explored. Here we again see significant stability and extremely slow dynamics -- an ordered free-standing film is stable, or nearly stable, even though the liquid film is well above the glass transition temperature and only of order 1 lamella ($\sim $30nm) thick!   

Interfacial segregation and micellization of hydrogen bonding copolymers

An AB diblock copolymer in which A and B have unfavorable interactions will segregate to an interface between A and B homopolymers. The driving force for segregation is increased if the B homopolymer is replaced by a C homopolymer and B and C have favorable interactions. When copolymer accumulates at the interface, the preferred interfacial curvature changes as a function of the copolymer composition. This change in curvature leads to a variety of possible morphologies, including micelles, swollen micelles, or inverted micelles. To examine this effect we use a model system where A is polystyrene (PS), B is poly(4-hydroxystyrene) (PHS), and C is poly(2-vinylpyridine) (PVP), and the PHS and PVP can undergo hydrogen bonding. We measure the interfacial segregation of PS- PHS copolymers at an interface between PS and PVP using dynamic secondary ion mass spectrometry.   

Session N25: Focus Session: Organic Photovoltaics

Organic photovoltaics - towards high performance low bandgap polyfluorene/fullerene bulk heterojunction devices

We use alternating copolymers of polyfluorene (APFOs) in polymer/fullerene blends as used in plastic solar cells. APFO-3/PCBM devices typically give power conversion efficiencies up to 3.4 {\%} (AM1.5, 100 mW/cm$^{2})$. The APFO's are stable, have high mobility and may be fashioned for liquid crystalline phases, as well as for broad optical absorption. By chemical design it is possible to move the optical absorption edge out to 1000 nm, and also to extract this absorption in photocurrent generation out to 1000 nm. As polymer bandgap is reduced, LUMO and HOMO orbitals shift. This requires the use of modified fullerene acceptors, with shifted orbitals, necessary to give the conditions for photoinduced charge transfer with the low bandgap polymers. The APFO polymers therefore give the necessary variability to catch a larger fraction of the solar spectrum. We have developed full optical and electrical model, predicting device performance for multilayer cells and for tandem cells, and using as input empirical determination of optical and electronic transport properties. We find the hole mobility to be a limiting parameter for device function.   

High Performance Organic Light-Emitting Diodes Based on Intramolecular Charge Transfer Emission from Donor-Acceptor Molecules

A clear understanding of the key factors governing the electroluminescence (EL) efficiency of emissive donor-acceptor (D-A) molecules in OLEDs is currently lacking, but is essential to a rational molecular design of future emissive materials. In this study, OLEDs based on intramolecular charge transfer emission from 3,7-[bis(4-phenyl-2-quinolyl)]-10-methylphenothiazine (BPQ-MPT) and 3,6-[bis(4-phenyl-2-quinolyl)]-9-methylcarbazole (BPQ-MCZ) had device performances that differ by orders of magnitude. High performance (44360 cd/m$^{2}$, 21.9 cd/A, 5.78{\%} external quantum efficiency (EQE) at 1140 cd/m$^{2})$ green OLEDs were achieved from BPQ-MPT which has a HOMO level at 5.09 eV and a non-planar geometry. In contrast, diodes with far lower performance (2290 cd/m$^{2}$, 1.4 cd/A, 1.7{\%} EQE) were obtained from BPQ-MCZ which has a HOMO level of 5.75 eV and a planar geometry. These results highlight the pronounced influence of the electron donor strength and molecular geometry on the EL efficiency of D-A molecules.   

Reversible Persistence and Effects of Oxygen on the Photoconductivity of Porphyrin Nanorods

Tetrakis(4-sulfonatophenyl) porphine (TPPS$_{4})$ self assembles$^{1}$ into well-defined nanorods with intriguing photoelectronic properties.$^{2}$ New experiments show that illumination under Ar for several hours induces a change to persistent behavior, i.e. conductivity decays slowly when light is removed, rather than dropping to zero. After resting 24 hours, the sample recovers non-persistent behavior. The dark conductivity of TPPS$_{4}$ aggregates formed by a different technique is sensitive to O$_{2}$.$^{3}$ We find that the conductivity under illumination of nanorod aggregates decreases when 0.2{\%} O$_{2}$ is added, but this change is reversible. By contrast, if the sample is exposed to 21{\%} O$_{2}$ shortly after light is removed, the photoconductivity is permanently lowered. These effects may be due to a combination of O$_{2}$-mediated quenching of excited state porphyrin and oxidation. $^{1}$A.D. Schwab \textit{et al.}, J. Phys. Chem. B \textbf{107}, 11339 (2003). $^{2}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). $^{3}$Y. Otsuka \textit{et al.}, Nanotechnology \textbf{15}, 1639 (2004).   

Photoinduced Charge Transport Spectra for Porphyrin and Naphthalene Derivative-based Dendrimers

Dendrimers are important chemical structures for harvesting charge. We prepared model dendrimers using two porphyrin derivatives and a naphthalene derivative. Films of these porphyrin derivatives have a strong Soret band ($\sim$430nm) and four significant Q-bands; the naphthalene derivative has strong absorption at 365 and 383nm. Two kinds of photovoltaic cell structures [ITO/BaytronP/(thick or thin) dendrimer/Al] are constructed to investigate the optical response spectra of dendrimers under electric potential($V$) on the cell (range from -1V to 2V). To obtain pure optical responses, incident light is modulated with an optical chopper and a lock-in amplifier is used to measure current ($I_{AC}$) and phase ($\theta$). For the excitation of the Soret band, $I_{AC}$ and $\theta$ do not change substantially with change of sign and amplitude of $V$. For Q-bands and naphthalene absorption bands, $\theta$ nearly follows the polarity of V on the cells and $I_ {AC}$ is linear with $V$. Hence, $I_{AC}$ is nearly ohmic for Q- band although there are shifts due to built-in-potential. $I_ {AC}$ for Soret band is almost same for thick and thin active layer cells. In contrast, $I_{AC}$ increases with thickness increase for Q bands. Mechanisms of photogeneration and charge transport will be discussed.   

Carbon nanotube sheets as transparent charge injectors in organic light-emitting diodes

Carbon nanotubes (CNTs) have been recognized for their potential in many applications ranging from high strength materials and fibers to true nanoscale electronics. Recently a method for making strong and transparent CNT sheets has been developed, producing free-standing multiwall nanotube sheets which are easy to process [1]. Their mechanical and electrical properties allow them to meet the needs of a wide range of applications, particularly in optoelectronics. We show here the potential for using these thin, flexible CNT sheets in the development of flexible organic light-emitting diode (OLED) displays. The high transparency of the sheets, the high degree of orientation of tubes and the high work function of the material make them suitable hole injectors for typical hole transport materials used in OLEDs and polymeric LEDs (PLEDs). We show that CNT sheets can be used as anodes for both PLEDs and molecular OLEDs. We also introduce a method for producing inverted OLEDs on existing drive electronics for active matrix displays and a design for a transparent display using CNT sheets as both the electron and hole injector. [1] M. Zhang, S. Fang, A. Zakhidov, S. Lee, A. Aliev, C. Williams, K. Atkinson, R. Baughman, \textit{Science} \textbf{309}, 1215 (2005)   

Dissociation Processes of Singlet and Triplet Excitons in Organic Photovoltaic Cells

The dissociation processes of singlet and triplet excitons were studied based on single-layer photovoltaic cells using: fluorescent Aluminum (III) 8-hydroxyquinoline (Alq$_{3})$ and phosphorescent \textit{fac }tris (2-phenylpyridine) iridium (Ir(ppy)$_{3})$ molecules. We found that triplet exciton dissociation leads to a more efficient photovoltaic response as compared to singlet excitons. The short-circuit photocurrent action spectra suggest that the triplet excitons dissociate mainly at the metallic electrode interface while the singlet excitons exhibit bulk dissociation. This interface dissociation of triplet excitons forms a mechanism for phosphorescent organic materials to show efficient photovoltaic responses. Therefore, control of singlet-to-triplet exciton ratio presents a new pathway to enhance photovoltaic response from organic materials.   

Multilayer polymer devices: light emitting diode and vertical hot carrier transistor

Two new devices based on polymer multilayers are presented:charged-balanced LED and hot carrier transistor. 1. An intermediate liquid buffer layer is introduced to overcome the dissolution problem of solution-processed multilayer polymer light-emitting diodes. This method can be applied to arbitrary combinations of polymers with no restriction on solvents. As an example, a hole-blocking layer is successfully spin-coated on the emissive polymer layer. Three typical p-type polymers, The electron-hole balance is improved by the addition of hole-blockinglayer. The electroluminescence efficiency can be increased up to 5 times, while the luminance up to 7 times. Electron-blocking layer is applied to blue polyfluorene copolymer and the brightness is as high as 30,000 cd/m$^{2}$ while the yield is 4 cd/A. 2. Metal-base hot-carrier transistor with conjugated polymer emitter and collector is demonstrated. The device is fabricated by multiple spin-coating with the metal base sandwiched between two polymers. A thin insulating layer of LiF is inserted between emitter and base to enhance the hot carrier kinetic energy and reduce the mutual dissolution. Using poly(9-vinylcarbazole) as the emitter, Al as base, and poly(3-hexylthiophene) as the collector, common-emitter current gain of 26 is obtained with operation voltage as low as 5V.   

Single-crystal films of a combination of materials (co-crystal) involving DAST and IR-125 for electro-optic applications

Single-crystal film of DAST (4'-dimethylamino-N-methyl-4-stilbazolium tosylate) has been shown [1] to have exceptionally large electro-optic coefficients (r$_{11} \quad \sim $ 770 pm/V at 633 nm). In this report, single crystal film of a combination of materials (co-crystal) involving DAST and a dye molecule IR-125 will be discussed. Modified shear method was used to prepare the co-crystal films. The film has been characterized using polarized optical microscopy, optical absorption spectroscopy and x-ray diffraction. The optical absorption spectrum has two major bands: one at about 350--600 nm corresponding to DAST and the other at about 600-900 nm corresponding to IR-125. The x-ray diffraction results show peaks involving the presence of DAST and IR-125 within the co-crystal film. Since the co-crystal has strong absorption at longer wavelengths it is expected to show higher electro-optic coefficients at longer wavelengths. Preliminary measurements at 1.55 $\mu $m indicate a high electro-optic coefficient of the co-crystal film. [1] Swamy, Kutty, Titus, Khatavkar, Thakur,\textit{ Appl. Phys. Lett.} 2004, $85$, 4025; Kutty, Thakur, \textit{Appl. Phys. Lett.} 2005, 87, 191111.   

Highly efficient third-order optical nonlinearities and their frequency dependence in donor-substituted cyanoethynylethene molecules.

We report on a new class of organic molecules with record efficiency for application in third-order nonlinear optics (NLO). The third-order polarizability, $\gamma$, of several donor-substituted cyanoethynylethene molecules was determined at the off-resonant wavelength of 1.5 microns using four-wave mixing. The nonlinearities were found to be extraordinarily large relative to the small molecular masses and were found to be within 50 times Kuzyk's fundamental limit,\footnote{M. G. Kuzyk, Opt. Lett. 25, 1218 (2000)} with $53\times10^{-48}$ m$^{5}$V$^{-2}$ as the highest $\gamma$ value. Select molecules were further investigated at wavelengths on and surrounding their two-photon (TP) absorption peaks, revealing large TP cross sections and the resonant influence on the real and imaginary parts of $\gamma$. Several members of this molecular family can be vapor-deposited and are likely candidates for third-order NLO devices. When considering their small mass, the \emph{specific} $\gamma$ ($\gamma$ per molecular mass) for this family (off resonance, at 1.5$\mu$m) is up to $6.5\times10^{-23}$ m$^{5}$V$^{-2}$Kg$^{-1}$, approximately one order of magnitude larger than previously known large $\gamma$ molecules.\footnote{J. C. May et al, Opt. Lett. 30, 3057 (2005)}   

A Time-Dependent Density Functional Theory Study of One-and Two-Photon Absorption: Donor-Acceptor Chromophores.

We report time-dependent density functional theory (TDDFT) calculations of one-photon, and two-photon absorption spectra, for a series of compounds, in which electron donating and accepting groups are attached to a core having a delocalized pi-electron structure, such as stilbene or fluorene. We find that the calculated excitation energies are in better agreement with experimental data upon the application of (x-c) functionals that take into account long-range interactions, and also by the inclusion of solvent effects. Furthermore, two-photon absorption cross-section predictions are improved with the application of quadratic response TDDFT, in comparison to experiment.   

Two Photon Absorption in a Novel Nano-optical Material Based on the Nonconjugated Conductive Polymer, Poly(beta-pinene)

As recently reported, the electrical conductivity of the nonconjugated polymer, poly(beta-pinene) increases by more than ten orders of magnitude upon doping with iodine [1]. The FTIR, optical absorption and EPR measurements have shown that radical cations are formed upon doping and charge-transfer involving the isolated double-bond in poly(beta-pinene). In this report, exceptionally large two-photon absorption in iodine-doped poly(beta-pinene) will be discussed. The linear absorption spectrum of medium-doped poly(beta-pinene) have peaks at about 4 eV and 3.1 eV. The first peak is due to the radical cation and the second due to the charge-transfer between the double bond and the dopant. The two-photon absorption of the medium-doped polymer has been measured at 730-860 nm using open-aperture z-scan with 150 femtosecond pulses from a Ti:Sapphire laser. A two-photon peak at about 1.5 eV with a magnitude of more than 1 cm/MW has been observed. The large magnitude of the two-photon absorption coefficient which is proportional to the imaginary part of the third order susceptibility has been attributed to the special structure of the radical cation and the confinement within a sub-nanometer dimension. [1] Vippa, Rajagopalan and Thakur, J. Poly. Sci. Part B: Poly. Phys., 43, 3695 (2005).   

Temperature dependent electrical and optical characterization of polyfluorene based organic light-emitting-diodes

Polyfluorene (PF) conjugated polymers have received widespread attention due to their strong blue emission, high charge mobility and excellent chemical and thermal stability which creates great prospect for optoelectronic device applications. Efficient and well balanced injection of charge carriers and transport capabilities are of paramount importance for high luminescence efficiency of organic light emitting diodes (OLEDs). The maximum current flowing through metal/semiconductor is limited by available space and trapped charges, barrier heights, applied electric field and mobility of the carriers. In this work we present detailed current- voltage (I-V) measurements as a function of temperature from 2- ethylhexyl substituted PF (PF2/6) based OLEDs. PF2/6 is characterized by Tg of ~80 $\rm ^o$C and a nematic liquid crystalline phase above 150 $\rm ^o$C. Barrier heights for current injection were calculated as a function of thermal cycling. The characteristic I-V measurements were fitted with ideal space charge limited conduction (SCLC) with traps to calculate carrier mobilities and trap concentration. Preliminary studies of Raman scattering from these working devices will be discussed.   

Pi-Conjugated Dendrimers for Organic Photovoltaics

Polymer-based organic photovoltaic (OPV) devices are promising candidates for low-cost solar cell fabrication. The operation of such devices is known to be strongly dependent upon the morphology and carrier mobility of the polymer. Here we discuss the use of pi-conjugated dendrimers in OPV devices. Dendrimers have a precisely defined molecular weight, in contrast to pi-conjugated polymers, which leads to a well-defined morphology. This morphology can be highly ordered owing to strong pi-electron interactions between dendrimers. We have synthesized a family of phenyl-cored thiophene dendrimers with a variable number of arms and and variable number of thiophenes in each arm. The optical band gaps of these materials in thin film form range from 2.3 to 2.6 eV. Time-resolved microwave conductivity measurements of the dendrimers showed a power-law dependence of lifetimes extending into the millisecond regime, indicative of a very pure material. Preliminary OPV devices fabricated by blending the dendrimers with a soluble fullerene yielded maximum open-circuit-voltages of 900 mV and external quantum efficiencies of 22\%. A reduced band gap dendrimer was also synthesized by adding strong electron withdrawing groups onto the phenyl core, resulting in an optical band gap of 1.82 eV. This material show good molecular ordering as evidenced by x-ray diffraction.   

Session N26: Focus Session: DNA and Protein Analysis with Micro and Nano Fluidics

Formation of bi-nanopores in silicon chips

Solid-state nanopores are holes with diameter and length on the order of 20 nm or smaller embedded in an insulating solid membrane. These nanopores have been shown to allow linear translocation of DNA molecules in buffer conditions and can be used as an electronic device for detecting and characterizing nucleic acids and proteins. Here we report a novel method of fabricating bi-nanopores in silicon chips using feedback electrochemical etching. The simplicity and low-cost of our approach, taking advantage of the well-known anisotropic etching behavior of silicon in alkaline solutions, bring solid-state nanopores closer to industrial-scale applications.   

Fabrication of nanopores in wax using laser-induced shrinking

We developed a simple laser heating induced shrinking technique for making plastic nanopore bio-sensing devices. Our technique is capable of shrinking thermoplastic pores of diameters up to several hundred micrometers to a few nanometers. We have made nanopore devices by applying this technique to Apiezon W wax (thermoplastics) micropores. Our DNA translocation experiments with$\,48\,\,kilobasepairs\,(kbp)$double-stranded $\lambda \,$DNA ($\lambda \,$dsDNA) have yielded convincing results of the functionality of these devices as biomolecular nanosensors.   

Ion Valence and Solution Temperature Effects on DNA Translocations in Solid-State Nanopores

Solid-state nanopore device provides a sensitive and robust environment for single DNA analysis. When a nanopore is the only partition between two reservoirs filled with an ionic solution and electrical conduction is established any DNA molecule translocation through the nanopore will partially block the open pore current. The current blockage and dwell time depends on both the external parameters, such as applied voltage, ionic strength and on characteristics of the DNA molecule itself, such as the charge and geometry. The properties of the molecule can be modulated by interactions with the ionic solution, and these will produce modifications of the current blockage and translocation time. Here we report on how the DNA translocation signature is modified when: (1) salts of varied valences are used and concentration of the surrounding solution changes; (2) the temperature of ionic solution changes. The mobility and diffusion coefficient of DNA molecule at above conditions are estimated.   

DNA in nanofluidic devices

Nanochannels with a channel cross-section of around 100 nm x 100 nm or less are emerging as a powerful new technique for single-molecule DNA analysis. In these nanochannels, DNA is linearized to a constant fraction of its contour length, and thus spatial locations measured by fluorescence microscopy can be directly related to genomic locations. Because the stretching in nanochannels is caused by lateral confinement, molecules are free to undergo longitudinal fluctuations. Hence, time-averaging over a single molecule is meaningful, and a high resolution can be achieved even using few molecules. We will present how DNA imaging in nanochannels can be applied to common tasks in molecular biology that go beyond simple sizing. In particular, we will discuss the genomic identification of human DNA fragments using fluorescent markers, and how to perform enzymatic reactions, such as restriction mapping using endonucleases, in nanochannels. We will also present our recent progress in the development of ``nanoplumbing'', that is devices that contain junctions of nanochannels. We will show how device dimensions influence the transport of DNA at those nanochannel junctions, and how those properties can be utilized in the design of devices and exotic materials.   

Nanoscale electrical detection of DNA

We try to detect DNA electrically by different nano-devices, including single-walled carbon nanotubes and platinum nano- wires. We will demonstrate the responses of carbon nanotubes conductance to the exposure to DNA, and ac lock-in measurements across metal nano-wires based on the biochemical properties of the DNA bases. The effects of different bases are also studied, which may provide us a real opportunity to sequence DNA electrically.   

DNA Translocation Dependence on Ionic Solution Concentration in a Solid-State Nanopore Device

Our work describes dsDNA translocations through a silicon nitride nanopore subjected to an applied electric field in solutions of different ionic strengths. We demonstrate how the ion concentration affects DNA shielding and, consequently, its effective negative charge. These modifications alter key parameters of the translocation process, such as the dwell time and current drop of the event. In this way, the DNA/salt interaction process can be explored by translocation experiments.   

DNA size and conformations analysis using a synthetic nanopore

Our work reveals the ability of a synthetic nanopore made in a silicon nitride membrane to discriminate between different conformations and lengths of DNA molecules and presents a comparative analysis with the electrophoretic behavior of the same DNA. Double stranded linear, supercoiled and relaxed form of the same DNA, linear restriction fragments, as well as single stranded DNA, are passed through a synthetic nanopore filled with a buffered ionic solution, and a subsequent analysis in terms of current blockage, translocation time and integrated events area shows the analytical ability of such a device. Also, we prove that an intercalating agent increases the temporal resolution by increasing the translocation time up to a factor of two.   

Microfluidic Protein Crystallography

Due to their impressive economies of scale and unique mass transport properties microfluidic devices have become viable technologies for nano-volume protein crystallization screening and growth. In particular, soft microfluidic devices based on multilayer soft lithography (MSL) have been successfully applied to systematic protein solubility studies and efficient nanoliter volume screening by free interface diffusion. While these systems have proven highly effective in identifying crystallization conditions for a large number challenging crystallization targets, realizing the full potential of microscale crystallization requires complementary technologies for crystal optimization and harvesting. In this talk I will briefly review previous studies of protein phase space mapping and crystallization screening, and will present recent work on a microfluidic device which provides a link between chip-based nanoliter volume crystallization screening and structure analysis through ``kinetic optimization'' of crystallization reactions and direct in situ structure determination. Using this device we demonstrate control over crystal quality, reliable scale-up from nanoliter volume reactions, facile harvesting and cryo-protectant screening, and protein structure determination at atomic resolution from data collected in-chip.   

Statics and Dynamics of Single DNA Molecules Confined in Nanoslits

de Gennes provided the scaling predictions for the linear polymer chain trapped in slit with dimension close to the Kuhn length decades ago; however, it has yet to be compared with experiments. We have fabricated nano-slits with vertical dimension similar to the Kuhn length of ds-DNA, $\sim $ 100nm, using standard photolithography. Single DNA molecules with length range from 2 to75 micrometers were successfully inject into the slits and the Brownian motions were imaged by fluorescence microscopy. The distributions of the radius of gyration and the two-dimensional asphericity were measured and it is found that the DNA exhibit highly anisotropic shape distribution. The scaling exponents for the chain extension and the center-of-mass diffusion coefficient will also be discussed.   

Slowing down DNA translocation using magnetic and optical tweezers

Electric-field driven DNA translocation through nanopores can be exploited for DNA sequencing and other applications. However, the DNA translocation under normal patch-clamp-type measurement is too fast to allow detailed measurements of individual or few nucleotides. We propose a concept to slow down the DNA translocation through the nanopore by using magnetic (or optical) tweezers. The 3' end of a single-strand DNA can be attached to a streptavidin-coated magnetic bead through a single biotin molecule. During DNA translocation, the 5' end of DNA will be electrophoretically drawn through the nanopore to the \textit{trans} side while the 3' end of DNA stays in the \textit{cis }side with the magnetic bead. A set of permanent magnets or electric coils can be used to generate a magnetic field gradient large enough to pull the bead, hence the DNA out of the nanopore. The net force on the magnetic bead will determine this back-translocation speed. By carefully tuning the magnetic field gradient and the voltage bias on the nanopore, one can make the back-translocation much slower than the conventional forward-translocation in which case the DNA is driven only by the electric force. We will report our experimental design as well as the preliminary results.   

Patterned Periodic Nanofilter Array for Continuous-Flow Bimolecular Separation

We present an experimental study on sieving process of small biomolecules (i.e., proteins and small DNAs) in one- and two-dimensional periodic arrays of nanofilter. The nanofilters served as artificial sieves with precise pore size characterization and showed exceptional size selectivity and separation efficiency from the periodicity of the environment. A kinetic model is developed to explain the electrophoretic drift of charged molecules across periodically modulated free energy landscapes. Further experimental evidence shows the crossover from Ogston-like sieving to entropic trapping mechanism depending on nanofilter thickness and on electric field strength. We also demonstrate continuous-flow biomolecule separation with a device containing of two-dimensional periodic nanofilter arrays. The interaction between migrating molecules and the two-dimensional physical landscapes cause molecules of different sizes to follow radically different paths leading to separation. Continuous-flow fractionations of small DNA molecules (50bp-766bp) as well as SDS-protein complexes (11kDa-200kDa) were achieved in about 5 minutes with a resolution of 10{\%}. By virtue of its gel-free and continuous-flow operation, this device suggests himself a key component to an integrated biomolecule sample preparation and analysis microsystem.   

Session N27: Electronic Structure I

Tight binding calculations of vibrational and thermal properties of amorphous silicon

By displacing atoms by different amounts and computing atomic forces within the NRL tight binding method we obtain all second order (harmonic) and some third order (anharmonic) coupling constants of a 1000 atom TB-relaxed Wooten CRN model of amorphous silicon. The harmonic force constant results allow us to study various properties including vibrational density of states, dynamic structure factors, specific heat and thermal conductivity within Kubo theory. We shall present results of these applications and compare to experiment and previous work based on the Stillinger Weber potential.   

Manifestation of Negative Compressibility in Low-Density Electron Liquids: Anomaly in the Ion-Pair Distribution Function in Supercritical Fluid Rb

It is a well-known fact that the electronic compressibility $\kappa$ diverges in the 3D electron gas as the density parameter $r_s$ approaches 5.25. A recent investigation clarifies that this divergence is due to the excitonic effect in the electron-hole pair excitation, in particular, to its zero-energy excitation [1]. For $r_s>5.25$, $\kappa$ becomes negative, leading to the negative static dielectric function $\varepsilon(q,0)$ for at least small $q$ owing to the compressibility sum rule. Then we can expect that two positive test charges do not repel but attract to each other in such a system. Keeping this situation in mind, we have calculated the ion-pair distribution function g(R) in the expanded Rb liquid metal by using the Monte Carlo method and found interesting features in g(R) characteristic to the negative $\varepsilon (q,0)$ [2]. Such features have been observed by the recent measurement of g(R) in the supercritical fluid Rb metal with continuously increasing $r_s$ from 5.25 [3]. This confirms the situation of $\kappa<0$ in the low-density 3D electron gas for the first time. [1] YT, J. Superconductivity {\bf 18}, No.3 (2005). [2] H. Maebashi and YT, to be submitted. [3] K. Matsuda and K. Tamura, private communication.   

Finite temperature quasiparticle self-consistent GW approximation

We present a new ab initio method for electronic structure calculations of materials at finite temperature (FT) based on the all-electron quasiparticle self-consistent GW (QPscGW) approximation and Keldysh time-loop Green's function approach. We apply the method to Si, Ge, GaAs, InSb, and diamond and show that the band gaps of these materials universally decrease with temperature in contrast with the local density approximation (LDA) of density functional theory (DFT) where the band gaps universally increase. At temperatures of a few eV the difference between quasiparticle energies obtained in FT-QPscGW and FT-LDA approaches significantly reduces. This result suggests that existing simulations of very high temperature materials based on the FT-LDA are more justified then it might appear from well-known LDA band gap errors at zero-temperature.   

Iterative minimization by Kohn--Sham inversion and potential mixing

Applications of Hohenberg--Kohn--Sham density functional theory to problems in materials physics are critically dependent on algorithms for iterating the Kohn--Sham equations to self-consistency. We present an approach for obtaining the self-consistent solution, which explores a connection between iterative minimization and Kohn--Sham inversion, \textit{i.e.} finding a self-consistent potential for a given density. The central idea is to perform the Kohn--Sham inversion using a position-dependent Lagrange multiplier and to construct a new trial potential from the result. The method is variational, in contrast to commonly-used density mixing approaches, and has excellent convergence. We demonstrate the method using a real-space pseudopotential implementation with applications to small molecules.   

Atomic Forces and Geometry Optimisation with the {\sc onetep} Linear-Scaling DFT Method

{\sc onetep}[1] (Order-$N$ Electronic Total Energy Package), is a density functional method, based on plane-waves, whose computational cost scales only linearly with the number of atoms. \\ {\sc onetep} uses a localised yet orthogonal basis of periodic cardinal sine (psinc) functions[2], also known as Dirichlet or Fourier Lagrange-mesh functions, which are formed from a discrete sum of plane-waves. The localised non-orthogonal generalised Wannier functions (NGWFs) which span the occupied subspace are represented in terms of these psinc functions and are optimised during the calculation. \\ This choice of basis and optimisation of the NGWFs results in smooth potential energy surfaces and enables the use of the Hellmann-Feynman theorem for the calculation of atomic forces. These have been implemented within a quasi-Newton geometry optimisation scheme and preliminary results are presented. \\ \\ {[1]} {\it J.~Chem.~Phys.} {\bf 122}, 084119 (2005). \\ {[2]} {\it J.~Chem.~Phys.} {\bf 119}, 8842 (2003).   

Optimized norm-conserving Hartree-Fock pseudopotentials

We report soft Hartree-Fock based pseudopotentials obtained using the optimized pseudopotential method \footnote{ A.~M.~Rappe, K.~M.~Rabe, E.~Kaxiras, and J.~D.~Joannopoulos, Phys. Rev. B {\bf 41}, 1227 (1990).}. The spurious long range tail due to the non locality of the exchange potential is removed using a self-consistent damping mechanism as employed in exact exchange \footnote{ E.~Engel, A.~H\"ock, R.~N.~Schmid, R.~M.~Dreizler, and N.~Chetty, Phys. Rev. B {\bf 64}, 125111 (2001).} and recent Hartree-Fock pseudopotentials\footnote{J. R. Trail and R. J. Needs, J. Chem. Phys. {\bf 122}, 014112 (2005).}. The binding energies of several dimers computed using these pseudopotentials within a planewave Hartree-Fock code show good agreement with all-electron results.   

Effect of Ti and metal vacancies on the dehydrogenation of Na$_{3}$AlH$_{6}$

Electronic and structural properties of sodium-aluminum hexahydride (Na$_{3}$AlH$_{6})$ formed during the decomposition reaction of sodium alanate (NaAlH$_{4})$ are calculated using density functional theory and generalized gradient approximation for exchange and correlation potential. The roles of Ti substitution at the Na and Al sites as well as that of Na and Al vacancies on the desorption of hydrogen have also been investigated. We show that the improvement in dehydrogenation of Na$_{3}$AlH$_{6 }$is due to the addition of Ti much the same way as that in NaAlH$_{4}$, namely through the weakening of the metal-hydrogen bond. However, as in the case of NaAlH$_{4}$, vacancies are found to be more effective in desorbing hydrogen at lower temperatures than Ti substitution at the Na or Al sites.   

Excitation Energies from Time-Dependent Density Functional Theory within Modified Linear Response: Inclusion of the Electron-Hole Hartree-Fock Interaction

Time-dependent density functional theory (TD-DFT) within linear response (LR) has gained enormous popularity in the calculation of electronic excitations, whereas it is known to give considerably underestimated excitation energies for Rydberg and charge-transfer excitations. Although the incorrect long-range behavior of exchange-correlation (XC) potentials has been blamed for this problem, a different point of view on the LR scheme without any correction of XC potentials is presented here. Analyzing approximate excitation energies from LR within adiabatic local density approximation (ALDA) and the exact exchange (EXX) scheme, we propose a modified LR theory to strictly include the electron-hole Hartree-Fock interaction kernel, and to make excitation energy expression in ALDA explicitly similar to the EXX one. TD-LDA calculations within modified LR on typical diatomic molecules show that excitation energies of both Rydberg and charge-transfer excitations can be greatly improved to the EXX-level accuracy.   

Quasiparticle Corrections to the Electronic Properties of Point Defects

We present a quantitative ab initio method for calculating defect states and charge-transition levels of point defects in semiconductors. It relies on a separation into lattice and electronic energy contributions, which are treated within density-functional theory and many-body perturbation theory, respectively. We use the $GW$ approximation for the self-energy to determine the quasiparticle corrections to defect states in the band gap. As an example, we consider anion vacancies on the (110) surfaces of III-V semiconductors. The calculated charge-transition levels, in particular, show a clear improvement over the local-density approximation and are in close agreement with the available experimental data. As the surface is simulated by a slab within the supercell approximation, we place special emphasis on a convergence analysis of the quasiparticle properties in this approach. The dynamic polarization between the periodic images can be understood within a simple model, which also allows an a posteriori correction.   

Probing the Ab-initio Fermi Surface Efficiently using Wannier Interpolation

Modern {\it ab-initio} techniques are able to provide an accurate description of the electronic structure for a wide range of materials. However, evaluation of the transport properties of metals requires an extremely detailed, and hence computationally expensive, sampling of the Fermi surface. We show that the electron group velocity and effective mass can be obtained directly from the Wannier representation of a system. This leads to an efficient and precise method for the calculation of transport properties using Wannier interpolation. We will present calculations of the ordinary Hall coefficient and thermoelectric power for a variety of materials.   

Nonlinear Optical Response of Polar Semiconductors in the Terahertz Range

Using the Berry-phase finite-field method, we compute from first-principles the recently measured\footnote{ T. Dekorsy, V. A. Yakovlev, W. Seidel, M. Helm, and F. Keilmann, Phys. Rev. Lett. {\bfseries 90}, 055508 (2003).} infrared (IR) dispersion of the nonlinear susceptibility $\chi^{(2)}$ in III-V zincblende semiconductors. At far-IR (terahertz) frequencies, in addition to the purely electronic response $\chi^{(2)}_{\infty}$, the total $\chi^{(2)}$ depends on three other parameters, $C_1$, $C_2$, and $C_3$, describing the contributions from ionic motion. They relate to the TO Raman polarizability and the second-order displacement-induced dielectric polarization and forces, respectively. Contrary to a widely-accepted model,\footnote{C. Flytzanis, Phys. Rev. B {\bf 6}, 1264 (1972).} but in agreement with the recent experiments on GaAs,$^1$ we find that the contribution from mechanical anharmonicity dominates over electrical anharmonicity. By using Richardson extrapolation to evaluate the Berry's phase in $k$-space by finite differences, we are able to improve the convergence of the nonlinear susceptibility from the usual\footnote{P. Umari and A. Pasquarello, Phys. Rev. B {\bf 68}, 085114 (2003).} ${\cal O}[(\Delta k)^2]$ to ${\cal O}[(\Delta k)^4]$, dramatically reducing the computational cost.   

Breathing oscillations accompanying Bloch oscillations of wavepackets in periodic potentials

Using a 1D tight-binding model, we study the evolution of a well-localized wavepacket of Bloch states under an applied electric field. We apply a novel algorithm~\footnote{ along the lines of I. Souza {\it et. al.}, Phys. Rev. B {\bf 69}, 085106 (2004)} for solving numerically the equations of motion which does not rely on the single-band approximation and can thus be used to explore interband Zener tunneling effects. In addition to the well-known Bloch oscillations of the center of the packet, we show that as the waveform moves in k-space, its real-space width varies in response to the change in the local quantum metric,~\footnote{N. Marzari and D. Vanderbilt, Phys. Rev. B {\bf 56}, 12\,847 (1997).} $g(k)$, of the underlying Bloch states. A generalized uncertainty relation is obtained between the spread in position and in {\it crystal} momentum of a wavepacket. It differs from the usual position/momentum uncertainty relation because of the interband matrix elements of the position operator in the crystal-momentum representation, which introduce a correction in terms of $g(k)$.   

Nonadiabatic Transition in the Quantum Hall Effect

We analyze the nonadiabatic transition in a 2D electron system with a periodic potential in the quantum Hall regime. We obtain corrections to the Chern-number term of the Hall conductance and a non-vanishing diagonal conductance. We treat the electric field as a time-dependent vector potential in the Hamiltonian. We calculate the time evolution of the density operator taking account of the first order of the electric field, and thereby study the electric conduction when the system evolves nonadiabatically. We thus obtain analytical expressions of the diagonal and off-diagonal conductances and calculate them numerically.   

Session N28: Polymer Adsorption and Surface Modification

PEG Surface Modification by Thermoreversible Ligand Cleavage in Nanoparticle Composites.

The control of surface chemistry is an increasingly important area of research in the polymer science community; a simple example of a need for controlled surface properties can be found in the need for surfaces that are resistant to bacterial growth for medical applications. In this study, we have successfully modified the surface properties of solvent cast poly(ethylene glycol) (PEG) films, triggered by exposure to an elevated environmental temperature. PEG matrices of varying molecular weights have modified with gold nanoparticles functionalized with thiol terminated, poly(styrene)-PEG block copolymer (P(S-$b$-PEG)) ligands. Gold nanoparticles approximately 15 nm in diameter were first synthesized via reduction of HAuCl$_{4}$ with oleyl amine. Diels-Alder chemistry was then used to create P(S-$b$-EG) ligands that, with increasing temperature, dissociate into simple thiol-terminated PS ligands and PEG oligomers. The ligand-modified gold particles were characterized via small-angle X-ray scattering and TEM. After dissociation occurs, around 90 \r{ }C, the gold particles are suddenly functionalized with only a PS ligand and thus immiscible in the surrounding PEG matrix. The gold nanoparticles are then driven to the surface of the films, measurably denoted by a change in contact angle. After cooling below 60 \r{ }C, the Diels-Alders linkages reform, stabilizing the film surfaces with the new morphology trapped both chemically and kinetically.   

Smart Polymeric Surfaces: Responsiveness and Reconstruction

The ultimate responsive surface is one that instantaneously responds to its environment with a measurable property change. In our research we utilize model poly(vinylmethyl)siloxane elastomer (SE) networks modified with thiol alkanes to provide hydrophobic or hydrophilic surface properties. The cooperative effects of polymer mobility, arising from the high flexibility of the siloxane backbone, and the enthalpic interactions between the outside medium and the SE functionalized surface control the degree of responsiveness as measured by dynamic and static contact angle. The initial parameters screened were alkane chain length, medium temperature, and end-group functionality. Real-time wetting force measurements have been obtained with dynamic contact angle, where the surface reconstruction is measured continuously providing a means to determine the kinetics of reconstruction and reversibility. Our examples show that not only are SE networks excellent stimuli-responsive substrates, but that the magnitude of change and repeated reversibility are unparallel to most polymeric surfaces.   

Conformations of Amphiphilic Comb Copolymer Chains Confined to Two Dimensions Through Self-Organization at the Polymer/Water Interface

Amphiphilic comb copolymers composed of a hydrophobic poly(methyl methacrylate) (PMMA) backbone and short, hydrophilic PEO side chains (PMMA$-g-$PEO) are known to self-organize at the polymer/water interface, resulting in the effective confinement of the backbone to two dimensions for chains at the surface of a PMMA-$g$-PEO film. Conformations of polymers thus confined were studied through selective nanoparticle labeling of PEO side chains of polymer molecules at the film surface. Transmission electron microscopy was used trace the backbone trajectory of nanoparticle labeled chains. The distribution of observed chain lengths is found in good agreement with the distribution determined by gel permeation chromatography. The 2D radius of gyration ($R_{g})$ calculated from the observed conformations was found to scale with number of backbone segments ($N)$ as $R_{g}$\textit{$\sim $N}$^{0.69\pm 0.02}$. This value agrees with Monte Carlo simulations for a system of similar polydispersity, which yield a scaling exponent between that for 2D isolated chains and monodisperse polymer melts ($R_{g}$\textit{$\sim $N}$^{0.64\pm 0.03})$.   

Temperature-responsive polymers and brushes with tunable onset of response

Temperature-responsive polymers are of high interest in the scientific field of stimuli responsive materials, in particular water soluble polymers with a response at $\sim$36.5$^{o}$C. However, difficulties in tailoring this T-response, as illustrated for example from studies of PNIPAM in numerous functionalized and copolymer forms, has hampered their proliferation. Here we present a systematic series of temperature-responsive polymers, which were designed, synthesized, and studied, and we show that we can tailor with high sensitivity their onset of T-response via the design of their monomer. Specifically, we demonstrate lower critical solution temperature (LCST) in water finely tuned between 5 and 70$^{o}$C, by controlling the hydrophilic/hydrophobic balance in the monomer (closely following predictions of phase behavior theories). In addition, we will also show that these polymers maintain their T-responsive characteristics when end-tethered to solid surfaces, over a wide range of grafting densities in combinatorial brushes. This approach allows for controlling contact angle, adhesion and tackiness as a function of temperature.   

Single Molecule Experiments with Adsorbed Polyelectrolytes

We report on the AFM study of single polyelectrolyte (positively charged) molecules (PE) adsorbed on mica surfaces at different conditions (we vary pH and salt concentration). The study was carried out under aqueous solutions in a liquid cell. We observed behavior of PE in real time. The AFM experiments were experiments when possible effects of the AFM tip on PE conformations were minimized. A series of experiments were carried out when PE was adsorbed between two electrodes at applied electrical potential. The AFM images were processed to extract contour length, end-to-end distance, and radii of center of mass. The experiments revealed several interesting facts about adsorption of PE. The chain statistics was found to be consistent with the 2D random walk model. A decrease of charge density resulted in the coil-to-globule transition. The globules appear as a strongly deformed swollen polymer globule. In saline solutions the globules resemble necklace-like globules. PE chains were mobile if an electrical field was applied. The motion of PE chains can be describes as a caterpillar-like motion.   

Quantifying How Polymer Interfacial Diffusion Differs from Bulk

Whereas polymer adsorption-desorption kinetics are reasonably well explored, in-plane diffusion is not. This talk will describe the molecular weight (M) and surface coverage dependence of two polymers, polystyrene (PS) and polydimethylsiloxane (PDMS) adsorbed to quartz from organic solvent. The M scaling of surface diffusion is quantified, and a surprising dependence on surface coverage is described. Time permitting, additional studies will be described in which, for the first time, polymer self-diffusion has been studied within a surface forces apparatus designed for fluorescence spectroscopy. Using FRAP (fluorescence recovery after photobleaching) to study slow diffusion and FCS (fluorescence correlation spectroscopy) to study rapid diffusion, we quantify how the self-diffusion coefficient of PDMS oligomer melts slows with diminishing surface separation, when PDMS is confined between mica surfaces.   

Balancing size exclusion and adsorption of polymers in nanopores

The liquid chromatography at critical condition (LCCC) presents the condition, at which the size exclusion and adsorption of polymer chains are balanced upon interactions with nanoporous substrates. In this study, we investigate how the polymer interactions with nanopores are affected by the solvent quality and nanopore size. Specifically, we measure the retention times of monodisperse polystyrenes in C18-bonded nanoporous silica column as a function of molecular weight, when a mixed solvent of methylene chloride and acetonitrile are used as elutent. C18-bonded silica particles with 70, 100, and 250 A pore size are used as a stationary phase to study how the transition from SEC-like to IC-like retention behavior depends on the condition of temperature and solvent composition. To locate the LCCC at various nanopore sizes, the temperature and solvent composition have been varied from 0 to 60 C and from 51 to 62 v/v{\%} of methylene chloride, respectively.   

Polymer brushes dynamics by evanescent wave dynamic light scattering

Dynamics of swollen brush is experimentally measured by dynamic light scattering in the total internal reflection geometry. Dense thick polymer polystyrene brushes with varying grafting densities are obtained through grafting from synthesis. When highly swollen in good solvent, concentration fluctuations are found to decay through a fast diffusive mechanism, attributed to the expected cooperative diffusion, akin to semi-dilute polymer solutions. Its hydrodynamic size is found to be comparable to the estimated distance between grafting chains. De-swelling of the brush by lowering solvent quality (using cyclohexane at different temperatures) leads to qualitative different dynamics. An extra slower relaxation with a broad distribution of relaxation times is observed, which strongly depends on solvent quality, with an extensive slow down of it characteristic time and an increase of scattered intensity upon cooling. This complex dynamics is discussed in relation to the dynamics in entangled semi-dilute solution in theta solvent. Finally, the brownian dynamics of colloidal particles (radius from 17nm to 100nm) in contact with the brushes are reported. The particles are found to marginally penetrate the brush, but to nonetheless exhibit dynamics reflecting their interactions with the outer part of the brush.   

Electromechanical Recognition of Molecules Adsorbed on Microcantilevers

An alternating current was applied to gold-coated silicon microcantilevers in sodium chloride solution. The cantilever is coated on one side with a thin layer of gold. Since the applied electric field through the cantilever attracts oppositely charged ions onto the gold layer, the variation of surface charges induces the oscillation of the cantilever. The larger the applied voltage is, the more the cantilever oscillates. When the experiment was repeated with self-assembled monolayer coated cantilever, the amplitude of the oscillation is decreased because the monolayer hinders the ions from approaching to the cantilever. In-situ measurements of the adsorption of mercaptohexanol molecules under the electric field clearly showed that the decrease of the oscillation resulted from the formation of the layer. When a square electric field was applied to 1-mercaptoethanol, 1-mercaptopropanol, and 1-mercaptohexanol coated cantilever, the bending profile of the cantilever depended on the kinds of the monolayer. This method can be used to study the diffusion of small ions through thin films as well as the structure of self-assembled monolayer.   

Dependence of surface diffusivity on the molecular conformation of single hydrophobic polyelectrolytes molecules

Hydrophobic polyelectrolytes are found to have their conformation change from an extended chain to globule via neck- lace structures. In this work, surface diffusion of single poly (2-vinylpryridine) (P2VP) molecule was studied under different chemical environment (pH value and ionic strength). Via hydrophobic interaction, P2VP molecules adsorbed to a hydrophobic surface. By fluorescence correlation spectroscopy, fluorescence labeled P2VP molecules were found to raise their surface diffusivity moderately but monotonously when the pH value was tuned from 2.0 to 6.5. The physical mechanism of the diffusivity dependence on molecular conformation is discussed.   

Adsorption of polymers onto selective mixed brushes.

Reversible adsorption of polymers onto selective mixed brushes is studied theoretically. Mixed brushes are recently developed self-adoptive materials that reversibly change their morphology in response to altering external factors (e.g. quality of the solvent). The above changes in the morphology result in the formation of different patterns on the outer surface of the brush. It is shown that thus achieved patterning of the adsorbing surface of the mixed brush drastically enhances the adsorption of polymers, as compared to the adsorption onto the homogeneous brush surface. The density profiles and absorbances of the selected homo- and co-polymers are calculated for the three different morphologies (`ripple', `dimple' and random) of the binary brush. The interplay between conformational entropy and binding energy of the adsorbed polymer leads to the reach adsorption-desorption behavior that is described by the developed theory. The calculated isotherms are compared with the experimental data and Monte-Carlo simulation results. In addition, the developed theory is applied to the study of the polymer adsorption onto the non-uniform binary brush in the presence of the gradient of chemical composition.   

Nano-meter structured three-phase contact line and its surface-guided alignment effect

We report our studies on the air-liquid-solid three-phase contact line on the periodically patterned surface made of polystyrene-b-polymethylmethacrylate (PS-b-PMMA). The difference of the contact angle of the liquid (water and polymer solution) on PS and PMMA generates the contact line with periodic structures of 40 nm length scale. Such a structured contact line was found to produce surface guided alignment effect for single DNA molecules by the molecular combing process and to generate surface-guided morphology of the polymer films through a combination of de-pinning and de- wetting process.   

Surface diffusion of adsorbed polymers studied by molecular dynamics simulation

We study the diffusion of adsorbed polymers near a flat surface by means of molecular dynamics simulations, as a function of chain length~$N$, adsorption energy, and surface coverage~$\phi$. We find that the two-dimensional diffusion coefficient scales as $D\sim N^{-1.017\pm 0.011}$, in agreement with other experimental and simulation results. The relation between lateral diffusion coefficient and surface coverage shows an exponential decay. We also investigate the conformation of the adsorbed chains. The number of ``trains,'' ``loops,'' and ``tails'' per chain, as well as the number of monomers in tails and loops increase as surface density increases and adsorption energy decreases, whereas the number of monomers in trains decreases. The parallel radius of gyration increases as a power law of the chain length, $\langle R_{g\parallel}^2 \rangle \sim N^{2\nu}$, with a power that is in good agreement with the Flory exponent $\nu = 3/4$ for two-dimensional chains. $\langle R_{g\parallel}^2 \rangle$ decreases with increasing surface density and decreasing adsorption energy, whereas $\langle R_{g\perp}^2 \rangle$ remains almost constant with increasing surface density and increases with decreasing surface energy.   

Solvent and salt effects on the adsorption of polymers to charged surfaces

The effect of solvent quality and salt concentration on the adsorption of charged polymers to a planar uniformly charged surface is studied using molecular dynamics simulation. The polyion chains are modeled as chains of charged spheres, the counter ions to the polyions and the surface are modeled as charged spheres, and the solvent molecules are modeled as uncharged spheres. The polyion adsorption is studied as a function of monomer and salt concentration, solvent quality and surface charge density of the surface. The amount of polyion adsorbed on the surface increases with the decrease in solvent quality as the system approaches a bulk phase transition. There are some surprising and counter-intuitive results in this regime. For example, incoprporating a short-ranged attraction between the polymers and the surfaces decreases the number of adsorbed polymers, and the amount of polyion adsorbed decreases as the salt concentration increases.   

Mean-field theory of planar absorption of RNA molecules

Interaction between the viral RNA and the protective protein capsid plays a very important role in the cell infection and self-assembly process of a virus. To better understand this interaction, we study a similar problem of absorption of RNA on an attractive wall. It is known that the secondary structure of a folded RNA molecules without pseudo-knots has the same topology as that of a branched polymer. We use a mean-field theory for branched polymers to analytically calculate the RNA concentration profile. The results are compared to known exact scaling calculations and computer simulations.   

Session N29: Focus Session: Physical Models of Ion Channel Function

The metabolic energy cost of action potential velocity

Voltage changes in neurons and other active cells are caused by the passage of ions across the cell membrane. These ionic currents depend on the transmembrane ion concentration gradients, which in unmyelinated axons are maintained during rest and restored after electrical activity by an ATPase sodium-potassium exchanger in the membrane. The amount of ATP consumed by this exchanger can be taken as the metabolic energy cost of any electrical activity in the axon. We use this measure, along with biophysical models of voltage-gated sodium and potassium ion channels, to quantify the energy cost of action potentials propagating in squid giant axons. We find that the energy of an action potential can be naturally divided into three separate components associated with different aspects of the action potential. We calculate these energy components as functions of the ion channel densities and axon diameters and find that the component associated with the rising phase and velocity of the action potential achieves a minimum near the biological values of these parameters. This result, which is robust with respect to other parameters such as temperature, suggests that evolution has optimized the axon for the energy of the action potential wavefront.   

The Dependence of Ionic Conduction on the Dielectric Properties of Ion Channels

The ion channel OmpF porin is a water filled trimer found in the outer membrane of \textit{Escherichia coli}. Each monomer is a hollow barrel structure with a physical constriction near the center that reduces the width of the pore to approximately 6 {\AA}. Highly charged residues line the inside of the pore constriction, generating an intense electric field that facilitates the dynamics of ions through the channel. The cost of simulating these systems for long times is an oversimplification of key physical features of the ion channel system, most notably, the polarization effects related to the solvent (water) and the protein are poorly represented by a stepwise constant dielectric constant. While the use of this model for the aqueous solution inside the permeation pore is arguably suitable because the ionic hydration shell remains intact (at least away from the central constriction), its validity is questionable when used to describe the polarization response of the protein. In this work, a previously validated P$^{3}$M force-field scheme, self-consistently coupled to a Brownian Dynamics kernel, is used to investigate the influence of the protein dielectric constant on permeation in OmpF porin. The computed channel conductivity is in agreement with experimental measurements. Increased cation selectivity at low ionic concentrations is also observed in the simulations and appears to be dependent on the rings of aspartic acid residues around the mouths of the porin.   

Rocking and Flashing Ratchet Mechanisms of Ion Current Rectification in Asymmetric Nanopores in the Presence of Calcium

We have investigated an engineered system of a single nanopore in a plastic membrane that shows rectification depending on the chemical composition of the surrounding solutions. No lipid bilayer is involved so the system is simple and robust with $>$10 gigohm leak resistance. The single nanopores are tapered cones with openings of diameter $\sim $~600 nm and $\sim $~5 nm. The single nanopores were prepared by the track-etching technique. The walls of the pores have carboxylate groups with surface density $\approx $1.5~(\textbf{\textit{e}}/[nm]$^{2})$. Transport properties of these nanopores were studied by recording current-voltage curves in a variety of solutions. In KCl solutions these single asymmetric nanopores are cation selective and rectify with a ratio of limiting conductances $\approx $~4-10. The K ions flow with lower resistance from the smaller to larger opening. Adding millimolar Ca to both sides reverses the direction of rectification and produces a negative incremental resistance; i.e., larger magnitudes of voltage produce smaller magnitudes of ion current. The rectifying properties of these asymmetric nanopores are described by rocking and flashing ratchet models of directional motion. It will be interesting to compare permeation, selectivity, and gating properties of the polymer nanopores and biological voltage-gated calcium channels.   

Ion selectivity in the ryanodine receptor and other calcium channels.

Biological ion channels passively conduct ions across cell membranes, some with great specificity. Calcium channels are selective channels that range in their Ca$^{2+}$ affinity depending on the channel's physiological role. For example, the L-type calcium channel has micromolar affinity while the ryanodine receptor (RyR) has millimolar affinity. On the other hand, both of these channels have the chemically-similar EEEE and DDDD amino acid motifs in their selectivity filters. An electrodiffusion model of RyR that reproduces and predicts $>$50 data curves will be presented. In this model, ions are charged, hard spheres and the chemical potential is computed using density functional theory of fluids. Ion selectivity arises from a competition between the need for cations to screen the negative charges of the channel and the crowding of ions in the tiny space of the channel. Charge/space competition implies that selectivity increases as the channel volume decreases (thereby increasing the protein charge density), something that has recently been experimentally confirmed in mutant channels. Dielectric properties can also increase selectivity. In Monte Carlo simulations, Ca$^{2+}$ affinity is much higher when the channel protein has a low dielectric constant. This counterintuitive result occurs because calcium channel selectivity filters are lined with negatively-charged (acidic) amino acids (EEEE or DDDD). These permanent negative charges induce negative polarization charge at the protein/lumen interface. The total negative charge of the protein (polarization plus permanent) is increased, resulting in increased ion densities, increased charge/space competition, and there in increased Ca$^{2+}$ affinity. If no negative protein charges were present, cations would induce enough positive polarization charge to prevent flux.   

Modeling Activity: Ions to Hydrophobics in Crowded Biological Solutions

Nonideal solutions play a role in many aspects of chemistry. As concentrations increase, concentration itself becomes a less useful quantity to understand equilibria. Industrial and medicinal chemistry often fail due to the difference between concentration and activity. An understanding of the impact of the crowded conditions in the cytoplasm on its biomolecules is of clear importance to biochemical, medical and pharmaceutical science. Work on the use of small biochemical compounds to crowd protein solutions indicates that a quantitative description of their non-ideal behavior is possible and straightforward. Here, we will show what the structural origin of this non-ideal solution behavior is from expression derived from a semi grand ensemble approach. We discuss the consequences of these findings regarding protein folding stability and solvation in crowded solutions through a structural analysis of the m-value or the change in free energy difference of a macromolecule in solution with respect to the concentration of a third component.   

Entropy driven insulator-metal crossover in ion channels and water filled nanopores

We consider ion transport of an ion channel in a lipid membrane or a water filled nanopore in silicon films [1]. It is known that due to the large ratio of dielectric constants of water (80) and lipid (2), the electric lines of an ion in the channel are squeezed. This should lead to a large electrostatic self-energy barrier for Ohmic resistance [2]. Nevertheless biological channels are well transparent at least for some selected ions. To address this paradox, we study reduction of the electrostatic barrier by a finite concentration of salt in water and/or by immobile charges on the internal channel walls. We show that both types of charges reduce the barrier, leading to insulator-metal crossover resembling metal-insulator transition in excited gas or in doped semiconductors. But here entropy plays the role of quantum mechanics. Evolution of ion channels took into account biological concentration of monovalent salt, and more importantly, made some channels charged from inside to reduce electrostatic barrier for a given sign of ions (cation/anion selectivity). We also show that in the channel with negative wall charges fractionalization of divalent Ca ions into monovalent excitations leads to good Ca-Vs.-Na selectivity of Ca channels. [1] A. Kamenev, J. Zhang, A. I. Larkin, B. I. Shklovskii, Physica A 359, 129 (2006); J. Zhang, A. Kamenev, B. I. Shklovskii, Phys. Rev. Lett. 95, 148101 (2005); cond-mat/0510327. [2] A. Parsegian, Nature 221, 844 (1969).   

Measurement of gating forces of mechanosensitive channels of large conductance in \textit{Escherichia coli}

In order to sense and respond to external mechanical stimuli, cells have evolved schemes to incorporate mechanosensors within their plasma membranes. Mechanosensitive channels of large conductance (MscL) are used by bacterial cells to respond quickly and effectively to hypo-osmotic shock: the opening of this channel permits cells to quickly release large amounts of osmolytes in order to quickly equalize unbalanced osmotic pressure across a membrane. In this study, we are investigating the physical mechanism of the MscL gating within the native environment of the \textit{Escherichia coli }cells. We are using the green fluorescent protein (GFP) and derivative proteins (CFP, BFP) to label the C-termini of MscL subunits in order to observe the channels in live bacteria by fluorescence microscopy. Moreover, we label the opposite termini with a different chromophore system that constitutes an excellent fluorescence resonance energy transfer (FRET) pair with CFP. Channels are activated within the bacterial membrane by osmotic stress and interactions between differently labeled subunits are measured by fluorescence microscopy.   

Calculating Ion Permeation through Biological Channel Proteins

We have developed methodology to simulate the current of ions (Na+, Cl-, etc.) through a general three-dimensional ion channel structure embedded in a lipid bilayer when an electric potential is applied across the membrane. These calculations are done at the level of Brownian dynamics, i.e., ions are treated as particles and their motion is computed using a stochastic algorithm which simulates Brownian motion. Water solvent is treated as a dielectric continuum, which both supplies the thermal agitation underlying the motion of the ions and influences the electrostatic forces on these ions by virtue of its dielectric constant (which differs substantially from that of the protein-membrane complex). Application is made to the Glycine Receptor channel, emphasizing physico-chemical influences on ion current, e.g., charges of critical pore-lining amino acids, channel geometry, etc.   

Morphometric approach to selectivity and gating of ion channels

A physical understanding of selectivity and gating of ion channels requires the free energy of the fluid confined in the channel pore. The free energy depends not only on fluid properties like its density, but also on the interaction between fluid particles and the confining protein, which gives rise to a potentially complicated dependence of the free energy on the conformation of the protein. Here we propose a simple thermodynamic approach that employs the idea that the free energy can be separated into geometrical measures and corresponding thermodynamic coefficients. Our approach enables us to calculate the change in the free energy caused by a change of the pore conformation such as that underlying the gating of an ion channel. We study the connection between the geometrical change of a hydrophobic pore and capillary evaporation, i.e. the effect that water is expelled from the permeation pathway and ion flow is thereby stopped although the pore remains wider than the water or ion diameters. We estimate the energy it takes to remove the water from the pore. Within the same thermodynamic framework, we can also study effects of pore conformation on the equilibrium absorption of ions and thus on ionic selectivity of the channel.   

Voltage Sensor in Voltage-gated ion channels

Voltage-gated ion channels are intrinsic membrane proteins that play a fundamental role in the generation and propagation of the nerve impulse. Their salient characteristic is that the probability of the ion channel of being open depends steeply on the voltage across the membrane where those channels are inserted. Thus, in a membrane containing many channels, the ionic conductance is controlled by the membrane potential. The voltage exerts its control on the channel by reorienting intrinsic charges in the protein, generally arginine or lysine residues located in the 4th transmembrane segment of the channel protein, a region that has been called the voltage sensor. Upon changing the membrane potential, the charged groups reorient in the field generating a transient current (gating current). The properties of the gating current may be studied with a small number of channels to infer the operation of the sensor at the single molecule level by noise analysis or with a large number of channels to infer the details of the energy landscape the sensor traverses in opening the pore. This information is global in nature and cannot pinpoint the exact origin of the charge movement that generates the gating current. The movement of physical charges in the protein has been inferred with site-directed mutagenesis of the charged residues to histidine that allows the study of proton accessibility. The actual movement has been studied with fluorescence spectroscopy, fluorescence resonance energy transfer. The combined information of site-directed mutagenesis, gating currents, fluorescence studies and emerging crystal structures have started to delineate a physical representation of the conformational changes responsible for voltage sensing that lead to the opening of the conduction pore in voltage-gated ion channels.   

Session N30: Block Copolymer Phase Behavior

Equilibrium Phase Diagram of a Model Rod-Coil Block Copolymer

Rod-coil block copolymers can be used to form important self-assembled structures containing functional blocks such as helical polypeptides or conducting polymers. The thermodynamics of these materials is distinct from classical block copolymers due to the conformational asymmetry of the polymer chain and the effect of liquid crystallinity on the microphase structure. We have recently developed a weakly segregated model system, poly(alkoxyphenylene vinylene-b-isoprene) (PPV-b-PI), in which rod-rod and rod-coil interactions are modulated by the presence of short side chains on the rod. We present the phase diagram for rod-coil block copolymers in the weak segregation limit, demonstrating equilibrium lamellar, nematic, and isotropic phases. As molecular weight is increased, subtle order-order transitions in the lamellar phase become obvious. In particular, we will discuss the relative stabilities of smectic phases based on scattering data. Finally, we will discuss the non-lamellar hexagonal phases that are observed as the relative rod-fraction of the block copolymer is decreased.   

Phase behavior of linear ABC triblock copolymer

We report the study of melt phase behavior of poly(isoprene-$b$-styrene-$b$-ethylene oxide) as our model ABC triblock copolymer. Previous investigations on this system have discovered a network phase with O$^{70}$ space group symmetry in an orthorhombic lattice adjacent to network phases with cubic lattice symmetries, namely, alternating gyroid and core-shell gyroid. The present study investigates and expands the phase diagram with varying monomer compositions and temperature. Nearly monodisperse triblock copolymers with controlled molecular weights and block compositions are synthesized by anionic polymerization techniques. Blending of homopolymers with the triblock copolymer is used to refine the phase boundaries. Dynamic mechanical spectroscopy, small angle x-ray scattering, TEM and optical experiments are used to characterize the equilibrium morphologies. Other new phases such as hexagonal cylinders and bcc spheres have been observed. We observe that the phase diagram is not symmetric across the $f_{A}=f_{C}$ isopleth.   

Electron tomography of a novel non-cubic network phase in ABC copolymers

We report the discovery of a novel bicontinuous tetragonal phase in the linear ABC triblock terpolymer system polystyrene (PS), polyisoprene (PI) and polydimethylsilocane (PDMS). The data is consistent with spacegroup Fddd and is distinctly non-cubic. The channel topology is distinct from the better-known cubic bicontinuous mesophases (diamond and gyroid types, with channels). It consists in 2 identical intertwined labyrinths with 3- and 4-connected nodes. Our mesophase differs from an earlier report of a copolymer phase (also in a linear terpolymer system) with the same spacegroup by Epps {\em et al} (Macromolecules {\bf 37}, 8325-41, 2004), who deduced a single channel morphology, based on TEM and SAXS data. Our proposal is based on 3D $e^-$-tomography data. The channel geometry is identified via a medial surface (MS) algorithm. For a labyrinth, the MS is a generalised channel graph consisting in surface patches. In contrast to line graphs, the MS is a complete descriptor of both topology and geometry. It provides robust shape characteristics, and is a useful tool for visualisation of complicated hyperbolic mazes.   

Phase behavior of poly(pentafluorostyrene-b-methyl methacrylate) block copolymers

Fluorine-containing polymers have garnered interest for properties such as chemical inertness, high thermal stability, and low dielectric constants. Previously, the controlled synthesis of fluoropolymers has been difficult due to the electron-withdrawing nature of fluorinated monomers. This issue, however, has been addressed with the development of atom transfer radical polymerization. Using this technique, we have been able to synthesize diblock copolymers containing polypentafluorostyrene (PPfS) and poly(methyl methacrylate), PMMA. The resulting diblock copolymers exhibit narrow molecular weight distributions ($\le $1.1) and undergo microphase separation to form highly-ordered nanostructures at moderate molecular weights. Comparisons of order-disorder transition temperatures with anionically synthesized poly(styrene-b-isoprene), PS/PI, diblocks of comparable molecular weights and compositions suggest that the segregation strength of PPfS/PMMA is within a factor of two of that of PS/PI. This observation is surprising given the chemical uniqueness of PPfS and PMMA but is in fact in agreement with the theoretical segregation strength relative to PS/PI predicted by differences in their solubility parameters.   

Perforated layer structures in liquid crystalline block copolymers

Phase structures of a series of poly(styrene-block-(2,5-bis-(4- methoxyphenyl)oxycarbonyl)styrene) (PS-b-PMPCS) liquid crystalline “rod-coil” block copolymers (LCBCPs) were investigated using thermal analysis, X-ray analysis and transmission electron microscopy. In the low molecular weight asymmetric BCP system, perforated layer structures were observed where the excessive PS molecules punctured the PMPCS domains and these perforations uniquely possess tetragonal in- plane symmetry. In the high molecular weight system, these perforated layer structures were observed in symmetric samples. Randomly initiated perforations became more regular and uniform upon blending with PS homopolymer in symmetric BCPs. These regular perforations also possess tetragonal in-plane symmetry.   

Self-assembly of polydisperse acrylic block copolymers

Self-assembled block copolymers present great interest since they combine at the nanometer scale intrinsic properties of different homopolymers. Over the past decade, remarkable progress in synthetic chemistry has unveiled new opportunities to prepare tailored block copolymers of judiciously chosen monomer type and architecture at reasonable cost. In particular, controlled radical polymerizations (CRP) are suitable to all kinds of vinyl monomers in common mass, suspension or even emulsion processes. Most synthetic efforts in this field have focused on developing a ``living'' character of free radical chain-ends and control polydispersity in length and composition. Here, we discuss a different, though quite common, situation where only one of the copolymer blocks is controlled. Overall composition and molecular weight polydispersities are thus large. Self-assembly and mesoscopic order in these ``asymmetrically polydisperse'' block copolymers and their blends is discussed.   

Electrospun poly(styrene-b-isoprene) fibers that exhibit internal structure

We have used the elelctrospinning process to fabricate fibers from THF / poly(styrene-b-isoprene) (PS-PI) diblock copolymer solutions. We spun fibers with copolymers that had various volume fractions of PI. These fibers had diameters ranging from 200 nm to 5 microns depending on the processing conditions such as solution concentration, needle size, electric field, etc.. The goal of this investigation was to observe the formation of self-assembled microstructures within the fibers. SAXS data indicates that the copolymer microphase separates and that there is some degree of globally ordered domains; however, TEM images indicate that this order is more local which might be due to the short residence time in the electrospinning process. By comparison, SAXS and TEM data of PS-PI films exhibits unambiguous global ordering. In an attempt to improve the long range order within the fibers, we performed various annealing treatments, and found that heating at temperatures below the glass transition temperature only had a small effect.   

Confinement induced novel morphologies of block copolymers

Self-assembly of block copolymers confined in cylindrical nanopores is studied systematically using a simulated annealing method. For diblock copolymers which form two-dimensional hexagonally-packed cylinders with period $L_{0}$ in the bulk, novel structures such as helices and stacked toroids spontaneously form inside the cylindrical pores. These confinement induced morphologies have no counterpart in the bulk system and they depend on the pore diameter ($D$) and the surface-polymer interactions, reflecting the importance of structural frustration and interfacial interactions. On tightening the degree of confinement, transitions from helices to toroids to spheres are observed. Mechanisms of the morphological transitions can be understood based on the degree of structural frustration parametrized by the ratio $D/L_{0}$.   

Self-assembly of three-dimensional morphologies in a diblock copolymer melt confined in a cylindrical nanopore

The microdomain morphologies of an AB diblock copolymer melt confined in a cylindrical nanopore are investigated using three- dimensional real-space self-consistent mean-field theory. We find that many structures self-assemble in the pore, including cylinders, helices, toroids, disks, and spheres. We compute the relative stability of these structures and locate transitions between phases as the diameter of the pore is varied. We focus on narrow pores for simplicity since it appears that the number and complexity of the structures formed increases as the pore size increases. For each of our morphologies, we measure the inter-domain distance, the degree of chain stretching, the area of A/B interface, and the A/B interfacial curvature. We identify which of these factors are driving the structural transitions. Our results will be compared with recent experiments and simulations.   

Effects of confinement on the order-disorder transitionin diblock copolymer melts and crystallization

The effects of confinement, in terms of size and geometry, on the order-disorder transition (ODT) in diblock copolymer melts are studied theoretically. Confinements are applied by restricting diblock copolymers in given geometries of slab, cylinder and sphere, respectively. Within the frame of self- consistent field theory, the second-order fluctuation of free energy functional is studied, and its minimum determines the spinodal point of the homogeneous phase. For the slabs and cylindrical cases the spinodal point $(\chi N)_s$ of the homogeneous phase is independent of the confinement, while in spherical case $(\chi N)_s$ is increased except some suitable radius of the sphere. In addition, using the idea that before nucleation there are fluctuations of the orientation of polymer chains, the puzzling direction of lamellae in the crystallization under confinement can be explained.   

Effect of Cross-linking on the Structure and Thermodynamics of Lamellar Block Copolymers

The effect of cross-linking on the structure and thermodynamics of a lamellar poly(styrene-\textit{block}-isoprene) copolymer was studied using small angle X-ray scattering (SAXS), depolarized light scattering (DPLS) and transmission electron microscopy (TEM). The selective cross-linking of the polyisoprene block took place either in the disordered state, in an isotropic ordered state, or in a shear-aligned ordered state. Using DPLS and TEM, the grain structure as a function of cross-linking density was studied. The order-disorder transition temperature for various block copolymer networks was determined as function of cross-linking density, and comparisons are made to a mean-field theory.   

Influence of Conformational Asymmetry on the Phase Behavior of Ternary Homopolymer/Block Copolymer Blends around the Bicontinuous Microemulsion Channel

We have developed a new ternary polymeric system, poly(ethylene-alt-propylene) (PEP) / poly(butylene oxide) (PBO) / PEP-PBO, to study the complex phase behavior near the bicontinuous microemulsion phase channel. The molecular weights of the PEP and PBO homopolymers are 2600 and 3050 g/mol, respectively, and copolymer is 23.4 kg/mol with volume fraction composition fPBO = 0.49. A combination of small-angle neutron scattering, small-angle X-ray scattering, rheology, optical microscopy and visual oil bath measurements was employed to map out the phase diagrams at five fixed homopolymer PBO/PEP ratios, ranging from 40/60 to 60/40 by volume, with copolymer concentrations ranging from 0 to 100\%. It was found that the bicontinuous microemulsion channel is consistently cut off at low temperature by a hexagonal phase. We attribute this phenomenon to the effect of the conformational asymmetry between the PEP and PBO species, whereby the more flexible PBO component induces a spontaneous curvature toward the PBO domains. These findings complement previous descriptions of the isopleth phase diagrams for the A/B/A-B systems, and identify a new design variable for preparing bicontinuous phases.   

Swelling and Shrinkage of Lamellar Domain of Conformationally Restricted Block Copolymers by Metal Chloride

The lamellar domain spacing (D) of symmetric polystyrene-block- poly(2-vinyl pyridine) copolymer (PS-P2VP) and PS-block-poly(4- vinyl pyridine) copolymer (PS-P4VP) with cadmium chloride (CdCl2) were studied by using rheometry, small angle X-ray scattering and transmission electron microscopy. With increasing amount of CdCl2, D of PS-P2VP increased greatly, but it decreased for PS-P4VP. This is due to different types of the coordination between CdCl2 and nitrogen atoms in the 2-position of pyridine ring (intra-chain coordination), compared with nitrogen atoms in the 4-position (inter-chain coordination).   

Azimuthal Orientational Correlations due to Excluded Volume Epitaxy in Growing Anisotropic Grains

The understanding of the microstructure of anisotropically shaped grains can have a strong influence on a range of material properties, including transport, mechanical and electro-optical properties. A grain-structure related phenomenon, called Excluded Volume Epitaxy (EVE) is reported in this study. EVE is a local, inter-grain orientational correlations effect, which results from a combination of sporadic nucleation of anisotropic grains and impingement of growing grains. Due to EVE, the anisotropically shaped grains have a tendency to be similarly aligned in a local neighborhood, despite the fact that there is no global orientation in the sample. This effect has been verified by transmission electron microscope (TEM) images of lamellar block copolymers and optical micrographs of small molecule crystals. Additionally, to quantify the effect of EVE, a modeling and simulation study involving random nucleation and subsequent growth of anisotropic grains was performed. The simulation study revealed a tendency for azimuthal, inter-grain orientational correlation and re-confirmed the experimental observation of EVE.   

Session N31: Carbon Nanotubes: Theory

Cyanide Nanotubes

The discovery of carbon nanotubes (CNTs) has given birth to an entire field devoted to the study of these one-dimensional (1D) nano-scale structures with extraordinary properties and tremendous promise for applications. To mention but a few, single wall carbon nanotubes are reported to exhibit Luttinger liquid behavior and proximity-induced superconductivity, and can be efficient hydrogen storage systems. The electronic properties of a carbon nanotube are fully determined by its helicity and range from metallic to semiconducting. However, when growing nanotubes, it is not possible to control the helicity; thus, carbon nanotube properties are not a result of design but luck. To overcome this limitation, Cohen and coworkers predicted the existence of insulating boron-nitride nanotubes (BNTs) and Zettl produced such tubes experimentally; these tubes are semiconducting and their properties vary less with helicity. Here we propose another type of structurally simple and energetically stable nanotubes consisting of transition metals and cyanide units. which are semiconductors with large band gaps ($\sim 2-3$ eV). Using first-principles calculations, we study the properties of these nanotubes and find that their helicity does not significantly affect the electronic band gap. The nature of bonding in these systems singles out a particular helicity as energetically more stable, suggesting that only one type of tube will be predominantly formed with well defined electronic properties.   

Electronic Structure of Core-Shell Semiconductor Nanowires

We investigate the electronic structure of silicon/germanium core-shell nanowires with first-principles calculations using the local density approximation (LDA) with pseudopotentials and plane waves. The atomic configurations of the core-shell nanowires are fully relaxed. By examining the wave functions in real space, the electronic states at the band edge are found to be localized in either the core or the shell part of the nanowire. The band offsets are calculated for different core-shell structures. Given the cylindrical band offsets and the associated confined electronic states, a novel doping mechanism in nanowires is proposed for the manufacturing of high-speed nano-devices.   

Dielectric properties of carbon nanotubes from first principles

We characterize the response of single- (SWNT) and multi-wall (MWNT) carbon nanotubes to static electric fields using first-principles calculations and density-functional theory. The longitudinal polarizability of SWNTs scales as the inverse square of the band gap, while in MWNTs it is given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of SWNTs is insensitive to band gaps and chiralities and is proportional to the square of the effective radius; in MWNTs the outer few layers dominate the response. The transverse response is intermediate between metallic and insulating, and a simple electrostatic model based on a scale-invariance relation captures accurately the first-principles results. Dielectric response in both directions remains linear up to very high values of the applied field.   

Atomic-scale and electronic structure of double-walled carbon nanotubes.

Ultra-high vacuum scanning tunneling microscopy (STM) and spectroscopy have been used to elucidate the electronic and atomic-scale structure of double-walled carbon nanotubes$^{1}$ (DWNTs) on the Si(100) 2x1:H surface. Atomically clean DWNT-surface interfaces were facilitated by an \textit{in situ} deposition method$^{2}$ which enables simultaneous resolution of the DWNT chirality and surface atomic structure. A key result includes the observation of periodic 2.7 nm spatial modulation superimposed on the nanotube chirality at both positive and negative scanning biases for a 2 nm diameter semiconducting DWNT. The periodic modulation of the DWNT topography suggests the lattices of the inner and outer nanotubes produce an interference pattern depending on the relative alignment of the constituent carbon atoms. Experimental data (diameter, chiral angle, and local density of state measurements) will be supplemented with simulated STM images which illustrate subtle changes in the outer nanotube topography depending on the inner nanotube chirality. 1. DWNTs synthesized by Nanocyl (www.nanocyl.com) 2. P.M. Albrecht and J.W. Lyding. Appl. Phys. Lett. 83, 5029 (2003).   

All-electron electronic properties of carbon nanotubes through multiresolution analysis

Utilizing our latest developments of density functional calculations using multiresolution wavelet-like basis we compute the electronic properties of carbon nanotubes by including all electrons.   

Molecular Dynamics Simulations of DNA-Functionalized Carbon Nanotube Chemical Sensors

We have conducted all-atom classical molecular dynamics simulations onDNA-functionalized carbon nanotube chemical sensors, including the presence of water. Our simulations verify that single stranded DNA (ssDNA) binds to a single-wall carbon nanotube (swCN) via a pi-pi stacking interaction. Preliminary simulations of a partially hydrated system also suggest that the ssDNA conformation about a swCN exhibits nanoscale pockets that can result in additional binding sites for analytes. Molecular dynamics simulations have also been performed to determine binding orientations of analytes adsorbed to the swCN-ssDNA system. To determine possible chemical gating effects of analytes on the swCN, we numerically calculate changes in the electric potential at the surface of the swCN due to the introduction of ssDNA and analytes. Results of further simulations of a fully hydrated system will also be presented   

Fluxional handles for direct control of conductance in functionalized carbon nanotubes

A class of covalent functionalizations for single-wall carbon nanotubes is identified --- from extensive first-principles calculations --- that preserves the conduction channels of metallic nanotubes. Cycloaddition of carbenes or nitrenes can induce bond cleaving between two adjacent sidewall carbons, restoring their original $sp^2$ hybridization and recovering in the process a transparent $\pi$ manifold, radically at variance with the strong scattering permanently induced by other common covalent functionalizations. The chirality and curvature of the nanotube and the chemistry of the addends can force or inhibit this bond cleavage, that in turn controls very distinctly the transport properties of the functionalized conductor. A well-defined range of diameters can be found for which certain addends - such as dicyanocarbene - exhibit a bistable switchable state, where the opening or closing of the sidewall bonds, and the accompanying on/off switch in the conductance, can be directed with chemical, electrochemical or optical means.   

Silicon-carbon nano-structures to nano-tubes.

There have been continuing efforts to stabilize silicon cage-type nano-structures or nano-tubes which can be used in similar ways as the carbon-based fullerene structures. This is due to the fact that the current semiconductor industries are based on silicon. Silicon carbide is the focus of scientific research due to its potential use even in extreme conditions, such as extreme high-temperature, high-power capabilities and high radiation conditions. In the present study, a set of novel silicon carbon nanosructures (Si$_{2n}$C$_{n})$ in tubular form have been studied which can be extended to form silicon carbide nano-tubes. Generalized gradient approximation to density functional theory has been used with an all electron basis set to study the stability of these structures. A frequency analysis has been performed to ensure that all the frequencies are real. A slight structural shift has been predicted between the hydrogen saturated and --unsaturated nano-tubes.   

\emph{Ab initio} study of semiconducting carbon nanotubes adsorbed on the Si(100) surface: diameter- and registration-dependent atomic configurations and electronic properties

We present a theoretical study within density functional theory in the local density approximation of semiconducting carbon nanotubes adsorbed on the unpassivated Si(100) surface. We find that the interaction between the nanotube and silicon surface results in significant atomic re-arrangment of the surface atoms. Since the spatial configuration of the surface dimers determines to a great extent the electronic properties of the surface, our first-principles calculations indicate a tendency towards metallicity for the semiconducting tube-Si(100) surface system. We confirm this for nanotubes of different diameters and chiral angles, and find the effect to be independent of the orientation of the nanotubes on the surface. Reference: cond-mat/0510477 and references therein.   

A plasmon absorption model for a super-lattice of single-walled carbon nanotubes

Several years ago one of us (P. F. Williams*) developed a self-consistent dielectric response model for one-dimensional metals at high frequency using a tight-binding approximation. At the time this model was found useful in a study of the single-particle excitations and plasmon dispersion curves of tetrathiofulvalene-tetracyano-quinodimethane (TTF-TCNQ). This paper is a report of work in progress to extend this model to arrays of single-walled carbon nanotubes. First the quasi one-dimensional model is extended to represent free electrons confined to the surfaces of cylindrical shells arranged in a 2-D square array. The collective electronic excitations of this system are characterized by a frequency and wavelength dependent complex dielectric constant obtained using the method of self-consistent fields in the random phase approximation. Progress in extending the model to arrays of shells with surface structure will also be discussed. *P. F. Williams and A. N. Bloch, Phys. Rev. B, \textbf{10, }1097 (1974)   

Numerical Results for SU(4) and SU(2) Kondo Effect in Carbon Nanotubes

New numerical results are presented for the Kondo effect in Carbon Nanotube (CNT) quantum dots (QDs). As recently reported by P. Jarillo-Herrero {\it et al.} (Nature {\bf 434}, 484 (2005)), the Kondo effect in CNTs presents an SU(4) symmetry, which arises from the entanglement of orbital and spin degrees of freedom. As the number of co-tunneling processes increases, thanks to the extra (orbital) degree of freedom, the Kondo temperature reaches a high value of $T_K=7.7K$. Interesting considerations can be drawn regarding the change from SU(4) to SU(2) symmetries depending on the hopping matrix elements between the leads and the CNT QD. Our results will analyze the transition between the SU(4) and the so-called two-level SU(2) (2LSU(2)) Kondo regimes induced by the variation of the coupling of the QD to the leads. The effect of an external magnetic field along the tube direction will also be analyzed. Our results will be compared with available Numerical Renormalization Group (NRG) results by M-S Choi {\it et al.} (Phys. Rev. Lett. {\bf 95}, 067204 (2005)). A comparison with the experimental results will be made to gauge the adequacy of the model and approximations made.   

Deep levels in the band gap of the carbon nanotube with vacancy-related defects

We study the modification in the electronic structure of the carbon nanotube induced by vacancy-related defects using the first-principles calculation. Three defect configurations which are likely to occur in semiconducting carbon nanotubes are considered. A vacancy-adatom complex is found to bring about a pair of localized states deep inside the energy gap. A pentagon- octagon-pentagon topological defect produced by the divacancy is structurally stable and gives rise to an unoccupied localized state in the gap. We also discuss the character of partially- occupied localized state produced by a substitutional impurity plus a monovacancy.   

Transport Properties in Carbon Nanotubes films for Hydrogen Sensor Applications

Thin single-walled carbon nanotue (CNT) films doped with palladium (Pd) on surface are shown to be promising in hydrogen(H2) sensor applications. We study electronic transport properties of CNT/Pd/H2 system by combining first-principles band structure calculations with Boltzmann transport theory. The coupling between Pd atoms and carbon nanotues is described by the Kondo model. An effective scattering potential, which creates the major resistance in nanotubes, is calculated by fitting potential parameters to the calculated band structures. In addition, intersection resistance between the nanotubes is also included in our simulations. We will present the band structure of the CNT/Pd system with and without the hydrogen atoms or hydrogen molecules. The conductance change of the whole nanotube films in the presence of hydrogen gas will be shown.   

Theoretical Study of Iron Filled Carbon Nanotubes

We have investigated, using ab-initio methods, the geometry and magnetic structure of free standing and encapsulated iron nanowires, both in perfect and defective single wall carbon nanotubes. The geometries adopted consist of two layers of iron atoms per unit cell, arranged in hcp(0001) and bcc(011) structures, repeated periodically along the wire axis, When the ratio of the nanowire to nanotube diameter is small there is an attractive interaction among them and the density of states at the Fermi energy corresponds to a single spin orientation, as for the free standing nanowires. These systems are therefore potentially interesting for spintronics. When the same ratio is close to one the systems are less stable and a tendency towards antiferromagnetic ordering is observed due to the confinement.   

Session N32: Focus Session: Carbon Nanotubes: Composites and Applications

The Solid-State Fabrication, Structure, and Multifunctional Applications of Strong Carbon Nanotube Yarns and Transparent Sheets

We describe novel methods for producing polymer-free carbon nanotube yarns and transparent sheets (self-assembled textiles), and describe their application as multifunctional materials. These fabrication methods are conducted at room temperature in the solid state for multi-walled carbon nanotubes, which are much cheaper to produce that our previously used single-walled carbon nanotube fibers. The yarns have a maximum failure strength of above 460 MPa (850 MPa after polymer infiltration), they are highly resistant to creep and to knot or abrasion-induced failure, and they provide a giant Poisson's ratio for stretch in the fiber direction. The nanotube textiles have higher gravimetric strength than the strongest steel sheet or the polymers used for ultralight air vehicles and proposed for solar sails. Applications evaluations are described for artificial muscles, thermal and light harvesting, energy storage, field-emission electron sources, electrically conducting appliques, three types of lamps and displays, and sensors.   

Mechanical Reinforcement of Functionalized Carbon Nanotube-Polyethylene Polymer Composites

Carbon nanotube-polymer composites are promising materials for a variety of applications including space exploration and the aerospace industry. In this work, we functionalized and fabricated a series of single walled carbon nanotube (SWNT) composite samples using medium density polyethylene (MDPE). The composites were made by shear mixing melt processing of MDPE with up to 1 wt.{\%} added pristine and functionalized SWNTs including fluoro (F), undecyl and urea SWNT-derivatives. The former two were prepared as described earlier, while the synthesis of the latter is a novel method that has been developed utilizing a solvent free reaction of fluoro-SWNT with molten urea. FTIR, Raman, and AFM data confirm that urea bonds covalently to the SWNT surface and displaces most of the fluorine. Initial tensile strength (TS) of the MDPE composites loaded with urea-F-SWNT reinforced 48{\%} and undecyl-functionalized SWNTs show unprecedented reinforcement up to 185{\%} compared to neat MDPE. These preliminary results show that these functionalized SWNT increased the mechanical strength in of MDPE composites. The FTIR, Raman, AFM, SEM, TEM, XPS, TGA, and TS data of studied materials will be presented.   

Adhesion and Reinforcement in Carbon Nanotube Polymer Composite

The temperature dependent adhesion behavior and reinforcement in carbon nanotube(CNT)-polymer (polyethylene) composite is studied through molecular dynamics (MD) simulations. The interfacial shear stress through van der Waals interactions is found to increase linearly with applied tensile strains along the nanotube axis direction, until the non-covalent bonds between CNTs and molecules break successively. A lower bound value about 46 MPa is found for the shear strength at low temperatures. Direct stress-strain calculations show significant reinforcements in the composite in a wide temperature range, with $\sim $ 200{\%} increase in the Young's modulus when adding 6.5{\%} volume ratio of short CNTs, and comparisons with the Halpin-Tsai formula are discussed.   

Multi-scale Real-Space Characterization of Carbon Nanofiber Composites

Good dispersion of the reinforcement phase in nanocomposites is recognized as critical to achieving material property goals. Hierarchical nanocomposite morphologies can be quantified by a combination of 1) Reciprocal space methods such as scattering, 2) Real space imaging such as AFM and TEM, and/or 3) Inference from established structure-property models. However, none of these techniques alone has proven satisfactory to quantitatively characterize nanocomposite morphologies across multiple length scales and link them to properties. Nor have they been adequate to define quality control metrics for dispersion. The current effort is devoted to characterizing dispersion in carbon nanofiber composites from the nano- to meso-scales (i.e., 10 nm - 10 mm) using the Multi-Scale Analysis of Area Fractions (MSAAF) technique of Spowart et al. This technique uses a fractal analysis of real space images to generate a homogeneous length scale (scale at which the statistical variability in concentration is at some threshold), and a fractal dimension characteristic of the dispersion over a wide range of length scales. This work is part of a larger effort to determine structure/property relations of complex materials systems.   

Radial Elasticity of Nanotubes

The last decade has lead to the discovery of many nanostructures like nanotubes, nanowires or nanobelts. Industrial applications of these nanostructures need practical tools to characterize their optical, electrical or mechanical properties. Here we propose to use state of the art atomic force microscopy to characterize the radial elasticity of nanotubes of different diameters. The nanostructures were elastically strained in the radial direction by applying small indentation amplitudes. In the case of multiwalled carbon nanotubes, this method enables to extract the radial Young modulus from compliance measurements. We find 600 GPa for our smallest tubes with a radius $R$ = 2.2 nm. The values strongly decrease with increasing radii until they reach an asymptotic value of $30\pm 10$ GPa at $R\ge 4$ nm. The normal force vs. indentation curves are in qualitative agreement with molecular dynamics simulations.   

Dissipation in suspended carbon nanotube oscillators

The vibrational properties of doubly clamped suspended single walled carbon nanotubes are studied numerically using continuum, and atomistic methods. Of interest is the dissipation of energy in athermally excited modes. Simple continuum arguments may be used to bound the limits of energy dissipation that arise due to the scale of the nanotubes; however, more detailed atomistic descriptions are required to capture the dissipation due to coupling between phonon modes. This work has implication for the use of carbon nanotubes as high frequency resonators in nanomechanical systems.   

Structure and Mechanical Properties of Model Nanotube Composites

Thermoreversible gels based on solutions of acrylic triblock copolymers have been infused with multi-walled carbon nanotubes at various loadings. The fast transition between liquid and solid states allows for the nanotubes to be frozen into their positions. These composite materials exhibit distinct mechanical and electrical properties from the bulk gel. The storage modulus of the filled gels persists at temperatures well above the gel transition and signifies elasticity that comes solely from the nanotube inclusions. The magnitude of this additional elasticity at high temperatures increases dramatically with increasing nanotube volume fraction above a ``percolation'' threshold that is extremely low. Sensitivity to nanotube interactions is enhanced by the low background levels of gel elasticity above the gel transition temperature. Complementary alternating current impedance spectroscopy measurements were performed to asses the onset of electrical percolation in these systems.   

\textit{In vivo} MRI of single-wall carbon nanohorns through magnetite nanoparticle attachment

Superparamagnetic magnetite (SPM) is used as a contrast agent in magnetic resonance imaging (MRI). Thus, the SPM-attachment to carbon nanotubes (CNTs) will enable to visualize motional behaviors of CNTs in the living body through MRI. We found that the strong attachment of the SPM nanoparticles (ca. 6 nm size) to one type of CNTs, single-wall carbon nanohorns (SWNHs), could be achieved through a deposition of iron acetate clusters on SWNHs in ethanol at room temperature, followed by heat-treatment in Ar. \textit{In vivo} MRI visualized that the SWNHs attached with the SPM nanoparticles accumulated in several organs of mice when injected into mice via tail veins. This simple method for the SPM-attaching on CNTs would facilitate the toxicity assessment of CNTs and the applications of CNTs in bioscience and biotechnology.   

Sensor applications and spin-transport measurements in carbon nanotube nanocomposites

Vertical and horizontal carbon nanotubes have been grown at USF using CVD and PECVD techniques with Ni and Fe nanoparticle catalysts. At NRL we have used CVD to produce carbon nanotube networks on SiO$_{2}$/Si$^{++}$ substrates to build sensors for chemical and bio agents by measuring capacitance and conductance. Various chemical vapors are able to be sensed with a fast response and recovery as well as a high degree of selectivity. A microfluidic flow system has been developed to extend the sensing applications to biological analytes. It is also known that carbon nanotubes are excellent transmission channels for charge and spin transport. In addition to the biosensors, we will also report on our experiments probing charge and spin transport through nanotube networks using point contact Andreev reflection (PCAR) based on superconducting and ferromagnetic junctions. Work at USF supported by DARPA/ARO through grant {\#} W911NF-05-1-0354   

Characterisation of a Hydroxyapatite and Carbon Nanotube Bioceramic Composite

A biocompatible composite for bone replacement applications was investigated. The effects that the microstructure may have on the mechanical properties of the bioceramic have been assessed. Hydroxyapatite was prepared as reported previously[1] with 2, 5 and 10 wt{\%} of carbon nanotubes (CNTs) being incorporated during the production before hot isostatic pressing. Microstructural analysis of the composite has been undertaken by SEM/EDS, TEM/EDS, XRD and ND. The effects of concentration of the CNTs on the mechanical properties of the composite material have been determined. At 2 wt{\%} excellent densification has been achieved, and there is a significant improvement in Vickers Hardness and Young's Modulus. However, as expected fracture toughness is reduced. [1] Lewis, K., Kealley, C., Elcombe, M., van Riessen, A., and Ben-Nissan, B. (2005), J. Aust. Ceram. Soc., 41(2), p52-55.   

The physical properties and possible applications of metal coated carbon nanotubes

We show that Ti atoms can form a continuous coating of carbon nanotubes at various amounts of coverage. The circular cross section of the tubes changes to a square-like form, and the semiconducting tube becomes a ferromagnetic metal with high quantum ballistic conductance. Metallicity is induced not only by the metal-metal coupling, but also by the band gap closing of SWNT at the corners of the square. The magnetic properties of Ti coated tubes depend strongly on the geometry, amount of Ti coverage and also on the elastic deformation of the tube. While the magnetic moment can be pronounced significantly by the positive axial strain, it can decrease dramatically upon the adsorption of additional Ti atoms to the monolayer coting of the nanotube. Besides, electronic structure and spin- polarization near the Fermi level can also be modified by radial strain. On the other hand, it is found that Ti and V decorated carbon nanotubes of various radii and chirality can adsorb large amounts of hydrogen molecules and can be possible candidates for hydrogen storage applications. The other transtions metals like Fe, Co, Cr, and Mn cannot cover nanotube surface uniformly but can only be adsorbed in clustered forms. Depending on the geometry and amount of adsorption these systems can posses high polarization near Fermi level with variable magnetic moments which can be useful in spintronic devices.   

Field Emission from Carbon Nanotubes: From Isolated Nanotubes to Matrix Cathodes.

The high aspect ratio and current carrying ability of carbon nanotubes (CNTs) make them an attractive material for electron sources. Field screening effects are known to occur at high nanotube densities and most large area field emission characteristics (FECs) reflect ensemble averages of the sites with the lowest effective potential barriers. We have studied the FECs and enhancement factor from isolated nanotubes mounted on high resolution manipulators within a scanning electron microscope. We have further developed an in-situ three terminal characterisation facility allowing estimates of the screening factor of the gate electrode and gate transparency. Measurements of the FEC of carbon nanotube -- polymer spin cast composites cathodes have also been made. A range of samples with arc discharge nanotube mass fractions up to 7 {\%} was prepared. Electron emission at low applied electric fields is observed. The transport and emission mechanism of the electrons is discussed in terms of a polymer coating that surrounds the nanotube and acts as a tunnel barrier. This gives rise to fluctuation induced tunnelling between the nanotubes which affects the field emission. The effects of the disordered percolation control network on the field emission along with prospects for applications are discussed.   

Thermal Field Emission from a Single Carbon Nanotube

Carbon nanotubes (CNTs) exhibit excellent characteristics in field-induced electron emission with high brightness, stable emission current, long service time and narrow energy distribution. But it is still not clear how carbon nanotubes behave under high electric field and high temperature. We have characterized the thermal field emission properties of an individual multiwalled carbon nanotube fabricated by a two step process. The characterization was conducted in the transition zone between thermionic emission and field emission. An approximation has been made to the Murphy-Good equation so that the temperature at the CNT apex can be extracted. The boundary of transition zone was determined experimentally by activating thermal field emission at various temperatures. We also show that higher temperature will improve the emission stability and remove disruptions in the emission current.   

Session N33: Focus Session: Instabilities & Turbulence in Complex Fluids

Nonlinear dynamics and flow transitions in viscoelastic shear flows

Dynamical explorations of viscoelastic flows are of fundamental and practical interest. Elastic forces cause flow instability even in the absence of inertia (creeping flow) and greatly modify the onset and ensuing sequence of flow transitions in flows with finite inertia. While past research has yielded much progress, literature on several intriguing nonlinear phenomena has been only slowly emerging. These include: (i) nonlinear transitions in \textit{linearly} \textit{stable}, parallel shear flows, (ii) influence of elastic instability on \textit{pressure-flow rate }relationship under creeping flow conditions, (iii) effect of elasticity on \textit{pattern formation} in \textit{curved} shear flows and (iv) novel instabilities caused by thermal effects induced by \textit{viscous heating}. The recent advances and challenges in the abovementioned areas will be discussed.   

Low-dimensional models for coherent states in viscoelastic turbulent shear flows

We present low-dimensional models for the sustenance of turbulence in shear flows of viscoelastic liquids. We develop these models by systematically investigating the effect of incremental amounts of elasticity on the self-sustaining process maintaining turbulence in shear flows. The recently proposed (Waleffe, 1997) self-sustaining process for shear flows consists of streamwise rolls leading to redistribution of the mean shear into spanwise streaks. A Kelvin-Helmholtz instability of the spanwise streaky flow then results in the regeneration of the streamwise rolls via nonlinear interactions. With the help of our low-dimensional model, we are able to identify which part of the cycle is interrupted or enhanced by the presence of elasticity. Additionally, we explore the effect of fluid rheology on the flow kinematics, particularly the role played by the first and second normal stress differences. For Newtonian liquids, such low-dimensional models have demonstrated their utility by helping to understand the features of full numerical solutions of turbulent flows. We believe that our low dimensional model for viscoelastic turbulent flow will help interpret experiments and direct numerical simulations of turbulent drag reduction by polymers.   

Instabilities in the oscillatory flow of a complex fluid

The dynamics of both a Newtonian and a viscoelastic shear- thinning fluid, subjected to an oscillatory pressure gradient in a vertical tube, is studied experimentally. PIV is used to determine the 2d velocity fields in the vertical plane of the tube axis, for driving amplitudes from 0.8 to 2.5 mm and driving frequencies from 2.0 to 11.5 Hz. The Newtonian fluid exhibits always a laminar flow regime, independent of the axial position. For the complex fluid, instead, the parallel shear flow regime exhibited at low amplitudes [Torralba et al., Phys. Rev. E {\bf 72}, 016308 (2005)] becomes unstable at higher drivings against the formation of symmetric vortices, equally spaced along the tube. At even higher drivings the vortex structure itself becomes unstable, and complex nonsymmetric structures develop. The system studied represents an interesting example of the development of shear-induced instabilities in nonlinear complex fluids in purely parallel shear flow.   

Instability of a Sheared Fluid-Gel Interface

The planar interface between a viscous fluid and an elastic gel is known to be unstable to simple steady shear flow [V. Kumaran and R. Muralikrishnan, Phys. Rev. Lett. \textbf{84}, 3310 (2000)]. By embedding a small number of micron-sized Latex particles at the planar interface between a Newtonian fluid and a soft viscoelastic gel, we use stroboscopic particle tracking to study the onset of this instability in the limit of large gel-to-fluid thickness ratios. The mean-square displacement of the interface and the power spectrum of displacement fluctuations are measured as a function of applied shear rate and gel modulus. Long-wavelength fluctuations with a periodic component are observed in the plane of flow and vorticity, with limited motion normal to the plane of the interface. By relating the power spectrum of fluctuations to the viscoelasticity of the gel, we discuss potential applications in the area of non-Brownian microrheology, where one exploits this instability to optically infer the rheological properties of an otherwise inaccessible soft phase.   

DNS of Viscoelastic Turbulent Channel Flow at High Drag Reduction

A new method has been developed to enable Direct Numerical Simulations (DNS) of viscoelastic turbulent channel flow with high accuracy spectral methods at high values of drag reduction (HDR), when the polymer molecules undergo high extensional deformation. To faithfully represent that we have expressed the conformation tensor, c, as the exponential of another tensor a, c=exp(a) and we solve for a instead of c. Thus, by construction, the positive definite property of c is always preserved. In addition, a stabilizing artificial diffusion has been added to the viscoelastic constitutive model and efficiently implemented numerically using a multigrid method. The Finite-Elasticity Non-Linear Elastic Dumbbell model with the Peterlin approximation (FENE-P) is then used to represent the effect of polymer molecules in solution. To achieve HDR we used high values of the key model parameters: (a) the maximum extensional viscosity, which for the FENE-P constitutive model is proportional to the quantity (1-$\beta )$*L\^{}2, where $\beta $ is the solvent viscosity ratio and L is the maximum extensibility parameter and (b) the friction Weissenberg number, We$\tau $.   

Time Dependent Drag Reduction by Long Chain Polymers in Taylor-Couette Flow

The addition of small amounts of long chain polymers has been shown to dramatically reduce the drag in some aqueous turbulent flows. We examined this effect in flow between concentric rotating cylinders (Taylor-Couette flow). The apparatus is instrumented to measure torque on the inner cylinder and can achieve Reynolds numbers up to $Re=1.4\cdot 10^6$. Reductions in drag of up to 47\% are seen immediately after the addition of the polymer (typical concentrations 10-20 ppm), but this value decays over a time scale of tens of minutes. While the scission of individual polymer molecules may also be important, light scattering measurements, performed on liquid samples, suggest the formation of entangled aggregates of polymer molecules. The polymers used are polyacrylamide with mean molecular weights of 5.5 and 18 MDaltons. Tested concentrations range from 0.5 to 100 parts per million by mass. We examine the dependence on concentration and shear rate (Reynolds number).   

Nonlinear traveling waves as a framework for understanding turbulent drag reduction

Nonlinear traveling waves that are precursors to laminar-turbulent transition and capture the main structures of the turbulent buffer layer have recently been found in all the canonical parallel flow geometries. We study the effect of polymer additives on these ``exact coherent states" (ECS), in the plane Poiseuille geometry. Many key aspects of the turbulent drag reduction phenomenon are found, including: delay in transition to turbulence; drag reduction onset threshold; diameter and concentration effects. The examination of the ECS existence region leads to a distinct prediction, consistent with experiments, regarding the nature of the maximum drag reduction regime. Specifically, viscoelasticity is found to completely suppress the normal (i.e. streamwise-vortex-dominated) dynamics of the near wall region, indicating that the maximum drag reduction regime is dominated by a distinct, and perhaps intrinsically elastic, flow structure.   

Complex dynamics in simple models of shear banding

Complex fluids commonly undergo flow instabilities and flow-induced transitions that result in spatially heterogeneous ``shear banded'' states. Often, these banded states display oscillatory or chaotic dynamics, measured in the bulk rheological signals and in the motion of the interface between the bands. Until recently, however, theory predicted a steady state comprising stationary bands separated by a flat interface. We discuss recent theoretical progress in capturing complex dynamics of the banded state: first in a model in which the interface (or interfaces) remains flat but moves in a chaotic way; second in a model that explicitly allows for undulations along the interface.   

Elastic Instabilities of Polymer Solutions in Extensional Flows

When flexible polymer molecules (in dilute solution) pass near the hyperbolic point of a microchannel cross flow, they are strongly stretched. As the strain rate is varied at low Reynolds number $<$0.01, tracer and particle-tracking experiments show that molecular stretching produces two flow instabilities, one in which the velocity field becomes strongly asymmetric, and a second in which it fluctuates non- periodically in time. The flow is strongly perturbed even far from the region of instability, and this phenomenon can be used to produce mixing. Bulk flow instabilities are not observed in dilute solutions of rigid polymers or Newtonian fluids under similar conditions.   

Delay of Disorder by Diluted Polymers

We study the effect of diluted flexible polymers on a disordered capillary wave state. The waves are generated at an interface of a dyed water sugar solution and a low viscous silicon oil. This allows for a quantitative measurement of the spatio-temporal Fourier spectrum. The primary pattern after the first bifurcation from the flat interface consists of squares. With increasing driving strength we observe a melting of the square pattern. It is replaced by a weak turbulent cascade. The addition of a small amount of polymers to the water layer does not affect the critical acceleration but shifts the disorder transition to higher driving strengths and the short wave length - high frequency fluctuations are suppressed.   

Shear thickening, shear localization and elastic turbulence.

The vast majority of complex fluids is shear thinning. The mechanisms of shear thinning are relatively well understood, and the phenomenon is widely used to tailor the rheology of complex fluids. Shear thickening is the exception to this rule, is incompletely understood and hardly ever used to tailor fluid properties. We study shear thickening in granular pastes (cornstarch), and show that shear localization (banding) is an essential ingredient for shear thickening. For high flow rates, the shear banding is followed by ‘elastic turbulence’. Our measurements provide us with the mechanism of both shear thickening and the flow instabilities that result from it.   

A Transitional Pathway to Turbulence in Elastic Fluids

Multiple scenarios have been discovered by which laminar flow transitions to turbulence, where transitions are caused by inertia or temperature, in Newtonian fluids. Here we show in non-Newtonian fluids a transition sequence that is due to elasticity from polymers, with negligible inertia. Multiple states are found linking the stable base flow to ``elastic turbulence'' in the flow between a rotating and stationary disk, including circular and spiral rolls, and stationary and time-dependent modes. Also, a surprising progression from apparently ``chaotic'' flow to periodic flow and then to ``elastic turbulence'' is found. In these experiments, either shear stress or shear rate is incrementally increased and then held at fixed values. The modes we discover have distinct rheological signatures, and we also image the accompanying secondary-flow field kinematic structures. Finally, we have explored how polymer concentration and gap-to-radius ratio affect (and possibly limit) the transitional pathway. The most concentrated solution tested appears to stabilize an additional, time-periodic mode. In conclusion, we have studied an unexplored route, which we hope, in time, will make it possible to compare experimentally and theoretically the routes to purely elastic turbulence with those for inertial turbulence, leading to a richer understanding of both.   

Session N34: Referee Training Session

Referee Training Session

Representative editors from Physical Review Letters and the Physical Review will provide useful information and tips for referees. The information presented will be relevant to anyone who has recently been asked to referee for a Physical Review journal, or who would like to add to their knowledge and experience of the refereeing process. It will also be of interest to authors who want to know more about the referee reports they receive. Topics we will cover include: (1) how to write a good referee report, (2) the differences between reports for PRL and the PR journals, (3) the role of the referee in the review process, (4) how to submit a referee report, (5) how to use the referee web interface, etc. Following the short presentations from the PRL and PR editors, there will be a moderated discussion where you can ask questions relevant to refereeing.   

Session N35: Focus Session: Organization of Complex Networks

Structural properties of Complex networks

The k-core decomposition was recently applied to a number of real-world networks (the Internet, the WWW, cellular networks, etc and was turned out to be an important tool for visualization of complex networks and interpretation of cooperative processes in them. Rich k-core architectures of real networks were revealed. The k-core is the largest subgraph where vertices have at least k interconnections. We find the structure of k-cores, their sizes, and their birth points --- the bootstrap percolation thresholds. I will show a derivation of exact equations describing the k-core organization of a randomly damaged uncorrelated network with an arbitrary degree distribution. This allows us to obtain the sizes and other structural characteristics of k-cores in a variety of damaged and undamaged random networks and find the nature of the k-core percolation in complex networks. These general results will be applied to the classical random graphs and to scale-free networks, in particular, to empirical router-level Internet maps. We find that not only the giant connected components in infinite networks with slowly decreasing degree distributions are resilient against random damage, as was known, but their entire k-core architectures are robust.   

Skeleton and fractal scaling in complex networks

We find that the fractal scaling in a class of scale-free networks originates from the underlying tree structure called skeleton, a special type of spanning tree based on the edge betweenness centrality. The fractal skeleton has the property of the critical branching tree. The original fractal networks are viewed as a fractal skeleton dressed with local shortcuts. An in- silico model with both the fractal scaling and the scale- invariance properties is also constructed. The framework of fractal networks is useful in understanding the utility and the redundancy in networked systems.   

Analysis of structure of small-world networks

We study the distribution function for minimal paths in small-world networks. We express this distribution in a convex combination form, and use numerical studies to obtain a functional fit for the convex coefficient in the limit of large system sizes and small disorder. Finally, we find analytic expressions for minimal paths distribution based on the functional fit for the convex coefficient. Our analysis can also be considered from the perspective of 1D random walks, our work thus provides a mapping between the structure of small-world networks and the exit problem for a class of 1D random walks.   

Inverted Berezinskii-Kosterlitz-Thouless Behavior on Scale-Free Hierarchical-Lattice Small-World Net

We have obtained exact results for a hierarchical lattice incorporating three key features of real-world networks: a scale-free degree distribution, a high clustering coefficient, and the small-world effect. By varying the probability $p$ of long-distance bonds, the entire spectrum from an unclustered non-small-world network to a highly-clustered small-world system is studied. Expressions for the degree distribution $P(k) $ and clustering coefficient $C$ are obtained for all $p$, as well as for the average path length $\ell$ for $p=0,1$. The Ising model on this network is studied by exact renormalization-group transformation of the quenched bond probability distribution, using up to 562,500 renormalized probability bins for the distribution. For $p < 0.494$, we find power-law critical behavior of the magnetization and susceptibility, with exponents continuously varying with $p$, and exponential decay of correlations away from $T_c$. For $p \geq 0.494$, where the network exhibits a small-world character, the critical behavior radically changes: We find an inverted Berezinskii-Kosterlitz-Thouless singularity, between a low-temperature phase with non-zero magnetization and finite correlation length and a high-temperature phase with zero magnetization and infinite correlation length, with power-law decay of correlations. Approaching $T_c$ from below, the magnetization and the susceptibility respectively exhibit $\exp (-C/\sqrt{T_c-T})$ and $\exp(D/\sqrt{T_c-T})$ singularities.   

Origin of Fractality in the Growth of Complex Networks

The emergence of self-similarity and modularity in complex networks raises the fun- damental question of the growth process according to which these structures evolve. The possibility of a unique growth mechanism for biological networks, WWW and the Internet is of interest to the specialist and the laymen alike, as it promises to uncover the universal origins of collective behavior. Here, we present the concept of renormalization from critical phenomena as a mechanism for the growth of fractal and non-fractal modular networks. We show that the key principle that gives rise to the fractal architecture of networks is a strong effective ``repulsion'' between the most connected nodes (hubs) on all length scales, rendering them very dispersed.   

A rescaling procedure for complex networks

We present here a renormalization scheme for graphs. We introduce a decimation procedure by weighting the different nodes through their centrality. In such a way we obtain rescaled graphs with the same statistical properties of the one at the finest scale. We present the results of such method for some numerical simulations of various models. We also apply this procedure to the real graph composed by Internet Autonomous System. We believe that this procedure can help in detecting the scale free-properties of such structures and can be fruitfully applied whenever the size of a system is that large that it is impossible to be visualized as well as described as a whole.   

Statistical Properties of Sampled Networks

We study the statistical properties of the sampled scale-free networks, deeply related to the proper identification of various real-world networks. We exploit three methods of sampling, and investigate the topological properties such as degree and betweenness centrality distribution, average path length, assortativity, and clustering coefficient of sampled networks compared with those of original networks. It is found that the quantities related to those properties in sampled networks appear to be estimated quite differently for each sampling method. We explain why such a biased estimation of quantities would emerge from the sampling procedure, and give appropriate criteria for each sampling method to prevent the quantities from being overestimated or underestimated.   

Routing and Congestion in Power Law Graphs

We investigate a simple model of packet routing on a power law (scale free) graph where packets arrive at each node at a given rate and are routed to a randomly chosen destination along the shortest path between the source and destination. This mimics the Shortest Path Routing protocol used in the internet. It was previously found that there is a critical rate of packet arrival beyond which there is an onset of congestion and packets start accumulating on the network. This critical rate depends on the maximum betweenness incurred on the network when shortest path routing is used. We analytically find a bound on the maximal betweenness incurred in shortest path routing and compare it to the optimal (least possible) maximal betweenness that can be acheived using an arbitrary routing protocol. This provides an effective quantitative measure of the optimality of Shortest Path Routing.   

Connectivity and Cost Trade-offs in Multihop Wireless Networks

Ad-hoc wireless networks are of increasing importance in communication and are frequently constrained by energy use. Here we propose a distributed, non-hierarchical adaptive method using preferential detachment for adjusting node transmission power to reduce overall power consumption without violating network load limitations. We derive a cost and path length trade-off diagram that establishes the bounds of effectiveness of the adaptive strategy and compare it with uniform node transmission strategy for several node topologies. We achieve cost savings as high as 90{\%} for specific topologies.   

Locating overlapping dense subgraphs in gene (protein) association networks and predicting novel protein functional groups among these subgraphs

Most tasks in a cell are performed not by individual proteins, but by functional groups of proteins (either physically interacting with each other or associated in other ways). In gene (protein) association networks these groups show up as sets of densely connected nodes. In the yeast, Saccharomyces cerevisiae, known physically interacting groups of proteins (called protein complexes) strongly overlap: the total number of proteins contained by these complexes by far underestimates the sum of their sizes (2750 vs. 8932). Thus, most functional groups of proteins, both physically interacting and other, are likely to share many of their members with other groups. However, current algorithms searching for dense groups of nodes in networks usually exclude overlaps. With the aim to discover both novel functions of individual proteins and novel protein functional groups we combine in protein association networks (i) a search for overlapping dense subgraphs based on the Clique Percolation Method (CPM) (Palla, G., et.al. Nature 435, 814-818 (2005), http://angel.elte.hu/clustering), which explicitly allows for overlaps among the groups, and (ii) a verification and characterization of the identified groups of nodes (proteins) with the help of standard annotation databases listing known functions.   

Scaling Properties of Topological Neural Nets

We study the agglomeration of metallic particles in an electric field. Earlier it has been shown that this system is a hardware implementation of a neural net [1]. In this paper we study the growth and topological properties of the emerging networks. In contrast to other networks the conductivity of the connections has a fixed value, but the completeness and number of connections depends on the training patterns. We find that the patterns grow in three stages: growth of shooters, ramification, and expansion [2]. The emerging patterns are hierarchical. For the limiting patterns certain properties are highly reproducible, such as the number of end points and the number of branching points, while other properties are not well reproducible, such as the number of tree structures. Further there are power law relations between the mass and the number of branching points and the number of end points. [1] M. Sperl, A. Chang, N. Weber, and A. Hubler, \textit{Hebbian Learning in the Agglomeration of Conducting Particles}, Phys.Rev.E. \textbf{59}, 3165-3168 (1999). [2] J. K. Jun and A. Hubler, \textit{Formation and structure of ramified charge transportation networks in an electromechanical system}, PNAS 102, 536--540 (2005).   

Session N36: Focus Session: Optical Properties of Nanostructures with S, Se, Te, and Ge

Symmetry considerations for semiconductor nanocrystals

Semiconductor nanocrystals or quantum dots show a wide range of physical properties with respect to their size or shape. In this paper we show that symmetry is also an important characteristic that can lead to different electronic and optical properties, mainly for small nanocrystals. This means that two spherical nanocrystals with similar sizes but different symmetries have different optical and electronic signatures, which should be accessible experimentally. We use pseudopotential density- functional theory, on a real space approach, to address the differences between spherical nanocrystals with similar sizes but different symmetries. We will report differences in the energy gap, the crystal field splitting and the absorption spectra for CdSe nanocrystals. The symmetry of the nanocrystal is also important when studying doping of nanocrystals.   

Optical transitions and the nature of Stokes shift in spherical CdS quantum dots

Resonant Stokes shift observed in CdS quantum dots (QDs) has been previously studied theoretically using {\bf k$\cdot$p} approach. The large values of measured Stokes shift along with the structure of the excitonic levels obtained by the {\bf k$\cdot$p} calculations have suggested an optically forbidden $P$ envelope valence state, thus forming a spatial symmetry induced ``dark exciton'' in CdS QDs, in contrast with the spin-forbidden exchange interaction induced ``dark exciton'' found in CdSe QDs. Since the {\bf k$\cdot$p} method has been known to incorrectly predict the energy levels in other QDs, here we apply {\sl ab initio} accuracy methods to study this problem. Using the LDA-based charge patching method to generate the Hamiltonian, combined with the folded spectrum method to solve the single particle states of thousand-atom nanostructures, we find that the top of the valence band state is $S$-like, thus optically bright, in contrast with all the previous {\bf k$\cdot$p} calculations. Our results also indicate the range of applicability of the {\bf k$\cdot$p} method. The calculated electron-hole exchange splitting suggests that the spin-forbidden valence state may explain the nature of the ``dark exciton'' in CdS quantum dots.   

Bright Exciton Fine Structure Observed in Single CdSe Nanocrystal Quantum Dots

The fine structure splitting of bright excitons in epitaxial quantum dots provides a basis for many quantum computation and entanglement schemes. We demonstrate the existence of a similar splitting in single colloidal CdSe nanocrystals through high- resolution, polarization-resolved, low-temperature photoluminescence (PL) experiments. At 4K, single-dot spectra reveal emission from two distinct, linearly- (and orthogonally- ) polarized bright exciton states. This splitting of the nominally degenerate spin $\pm1$ bright excitons ranges from 1 to 2 meV, depending on nanocrystal size. These values agree well with the splitting recently inferred from spin-polarized resonant PL of nanocrystal ensembles measured in high magnetic fields to 33 Tesla [1]. Similarly to epitaxially-grown quantum dots, the observed fine structure likely results from shape anisotropy of the nanocrystal (i.e. a reduction of axial symmetry), leading to a long-range, anisotropic electron-hole exchange. [1] M. Furis et al., cond-mat/0511567.   

The Peculiar electronic structure of PbSe quantum dots

PbSe quantum dots have recently emerged as promising systems that may realize direct carrier multiplication (DCM) for solar cell applications. We have calculated the underlying electronic/optical structure of PbSe nanocrystals with an atomistic pseudopotential method, finding that the electronic structure is more subtle than k$\cdot$p or tight-binding calculations have previously suggested. The following two effects emerge from our calculations: (i) The bulk-degenerate L states forming the VBM and CBM are split due to (1) valley-valley coupling, (2) valence-conduction interband coupling, and (3) the strong anisotropy of the bulk L valleys. Optical absorption is dictated by transitions among anisotropic dot states characteristic of transverse and longitudinal effective masses. Our calculated optical absorption spectrum is in good agreement with experiment. In particular, our calculation reproduces the measured second obsorption peak that had previously been attributed to forbidden transitions 1S$_{h}\rightarrow$1P$_e$ or 1P$_{h}\rightarrow$1S$_e$ on the basis of k$\cdot$p and tight-binding calculations. (ii) Using our calculated single-particle states, we evaluate DCM mechanism, showing that the rate of X-to-XX (exciton to biexciton) transitions far exceeds the reverse, XX-to-X rate, thus opening the way to efficient DCM.   

Study of colloidal quantum dot surfaces using an innovative thin-film positron 2D-ACAR method

Despite a wealth of information, many fundamental questions regarding the nature of the surface of nanosized inorganic particles and its relationship with the electronic structure remain unsolved. We have investigated the electron momentum density (EMD) of colloidal CdSe quantum-dots via depth-resolved positron 2D angular correlation of annihilation (2D-ACAR) spectroscopy at the Delft intense variable-energy positron beam. This method, in combination with first-principles calculations of the EMD, shows that implanted positrons are trapped at the surface of CdSe nanocrystals. They annihilate mostly with the Se electrons and monitor changes in composition and structure of the surface while hardly sensing the ligand molecules. We thus unambiguously confirm [1] the strong surface relaxation predicted by first-principles calculations [2]. Work supported by the USDOE.\\ \mbox{[1] S.W.H. Eijt {\em et al.}, Nature Materials (in press).}\\ \mbox{[2] A. Puzder, {\em et al.}, Phys. Rev. Lett. 92, 217401 (2004).}   

Fluorescence blinking statistics from single CdSe nanorods

We report that room temperature fluorescence from single colloidally synthesized CdSe nanorods exhibits intermittency (blinking) with truncated power-law off-time and on-time statistics. The nanorods have cross-sectional diameter 5 nm and length 20 nm and are deposited on mica substrates. The aggregated off-time statistics from 67 single nanorods follow a power law: $P(t_{off})\sim t_{off}^{-\alpha }$, with $\alpha \approx $1.1. Power-law behavior extends to off-times of roughly 10 s; longer-time probabilities fall below the best-fit power law. Individual nanorods also show power-law off-time statistics with 1$\le \alpha \le $1.3. On-time probabilities drop below a power law after only $\sim $0.6 s; no on-times longer than $\sim $3 s are observed. These results differ somewhat from those observed with spherical CdSe or CdSe/ZnS core-shell nanocrystals, for which power-law statistics persist to much longer on- and off-times.   

Time-resolved photoluminescence of individual CdS nanowires

We study photoluminescence (PL) dynamics of single VLS-prepared CdS nanowires. AFM imaging reveals that while some nanowires are straight and uniform, the others show significant morphological irregularities. Low temperature PL of uniform nanowires displays a single near band edge (NBE) peak. Spectra of the irregular nanowires exhibit a broad PL band with a high energy shoulder in the same energy range as the NBE peak of the uniform nanowires, as well as an array of narrow peaks at the lower energy. Spatially-resolved PL images indicate that the narrow lines originate at specific locations along the nanowire. Time-resolved PL (TRPL) measurements show that NBE emission in all nanowires is short-lived (lifetime $<$ 50 ps), indicating the presence of non-radiative recombination channels. On the other hand, TRPL of the localized states exhibit are significantly longer (400 ps to 1 ns) and vary from line to line. At room temperature, the PL spectra of all nanowires, regardless of the morphology, consist of the single short-lived NBE emission peak. We acknowledge the support of ACS through the PRF, and NSF through grants 0071797, 0216374, and a graduate fellowship (JLL).   

Morphology and temperature dependence of single CdS nanowire photoluminescence

We study the optical properties of single CdS nanowires (grown by VLS method using 50 nm catalysts) using the technique of micro-PL. We studied ten wires, several that were straight and uniform, and others with morphological irregularities. At room temperature, the PL spectra of all wires are alike and consist of a single line around 2.41 eV. At low temperature (5 K), the PL properties of these two groups of wires differ significantly: the spectra of the uniform wires display a single peak near the band edge, and the spectra of the irregularly shaped wires exhibit a series of sharp lines at lower energies. Detailed PL imaging reveals that the sharp lines are emitted only from particular positions along the wires. Moreover, most of the photons emitted at low temperatures occur at energies below the band edge PL of bulk CdS. This suggests that the sharp lines result from defects or surface states which rapidly trap carriers from the bulk of the wires. As the temperature increases, the sharp lines begin to weaken at about 30 K and completely disappear at 85 K, while a peak which emerges from the high energy shoulder of the low-T emission band becomes dominant and survives up to room temperature. We acknowledge the support of ACS through PRF, and NSF through grants 0071797, 0216374, and a graduate fellowship (JLL).   

Probing the Electronic and Vibronic Structure of Single and Ensemble CdS Nanowires using Resonant Raman Scattering.

Semiconductor nanostructure electronic and vibrational states can be sensitively probed using resonant Raman scattering (RRS) even when such states are not accessible through photoluminescence or transport techniques. We present an investigation of the electronic and vibrational states in both a single CdS nanowire and in an ensemble of CdS nanowires using RRS at room temperature. The CdS nanowire samples were grown using a chemical vapor deposition and gold-catalyzed vapor liquid solid growth technique. We observe strong 1-LO and 2-LO Raman resonances within the broader photoluminescence emission. The energy separation between the peaks of the 1-LO and 2-LO resonance of an ensemble of CdS nanowires was found to be 34 meV. Raman scattering from a single nanowire exhibits similar behavior but with a narrower resonance. These results demonstrate that RRS is a powerful tool for probing the electronic and vibrational properties of semiconductor nanostructures. We acknowledge the support of ACS through PRF, and NSF through grants 0071797, 0216374, and a graduate fellowship (JLL).   

Investigation of the optical gap in Ge nanowires.

We investigate the role of quantum confinement for the optical and electronic properties of Ge nanowires.~ Real space pseudopotentials constructed within density functional theory were used to solve the electronic structure problem.~ We predict the quasi-particle and optical gaps as a function of the diameter up to approximately 3 nm for wires oriented along the (110) and (111) directions.~ We compare our results to previous work on Si wires.~   

Exciton-polariton emission and absorption in inorganic-organic hybrid crystals ZnTe(en)$_{0.5}$

The ultimate accuracy and quality test of nanotechnologies based on either MBE or MOCVD growth is perhaps to make an ultra-short-period superlattice with one monolayer thick alternating components. However, neither artificially nor spontaneously ordered monolayer superlattices (e.g., GaAs/AlAs or GaP/InP) that have been grown by either MBE or MOCVD have been shown to have the desired perfection [1,2]. Recently, we have successfully synthesized a group of II-VI based inorganic-organic crystalline hybrid superlattices with single atomic-layer thick inorganic slabs and single molecular-length organic spacers [3]. We will report experimental and/or theoretical studies on the exciton-polariton emission and absorption, exciton binding energies, and dielectric properties for a prototype hybrid superlattice ZnTe(en)$_{0.5}$ [4,5]. [1] J. Li et al., PRL 91, 106103 (2003). [2] Spontaneous Ordering in Semiconductor Alloys, edited by A. Mascarenhas. [3] X. Huang et al., JACS 122, 8789 (2000); 125, 7049 (2003). [4] B. Fluegel et al., PRB 70, 205308 (2004). [5] Y. Zhang et al., PRL (in press).   

Session N37: Focus Session: Nanoscale Fabrication, Assembly and Semiconductor Nanowires

Single electron transistors without tunnel junctions tailored by local oxidation of metallic ultra thin films

We present the fabrication and low temperature electric properties of nanoscale metallic constrictions made by local oxidation with an Atomic Force Microscope of weakly localized niobium ultra-thin (3nm) strip lines. These constrictions implements nanoscale resistors with resistance of the order of the resistance quantum. Both laterally constrained and variable thickness junctions are made with a lateral gate coupled to the interjunction electrode. Circuits following both geometries exhibits reproducible low contrast gate oscillations at 4K which phase inverts with drain source voltage. The gate modulation of the current is in strong disagreement with the orthodox theory that involves tunnelling. Transport is interpreted as single or multiple islands in series for which Coulomb blockade is induced by the highly resistive sheet resistance.   

Single-Walled Carbon Nanotubes as Shadow Masks for Nanogap Junction Fabrication

We report a technique for fabricating nanometer-scale gaps in Pt wires on insulating substrates, using individual\textit{ single-walled carbon nanotubes} as shadow masks during metal deposition. 83{\%} of the devices display current-voltage dependencies characteristic of direct electron tunneling. Fits to the current-voltage data yield gap widths in the 0.8 - 2.3 nm range for these devices, dimensions that are well suited for single-molecule transport measurements.   

Self-Assembly for Large Scale Fabrication of Integrated Electronic Devices Based on 1-D Nanostructures.

Recently, electronic devices based on 1-dimensional (1-D) nanostructures (e.g. carbon nanotubes (CNTs) and nanowires) have been drawing much attention as next-generation device architecture. However, the shortage of reliable nanomanufacturing methods for such circuits has hindered their practical applications. One promising nanomanufacturing method can be `surface-programmed assembly' process, where functional molecular monolayer on the substrate guides the `selective assembly' and `alignment' of nanowires and nanotubes on the substrate without relying on any external forces. Using this method, we successfully assembled and aligned carbon nanotubes and vanadium oxide nanowires on various substrates including Au, silicon oxide, Si, Al, and polymer. Furthermore, by additional microfabrication process, we demonstrated large-scale fabrication of various device structures such as junctions and top-gate transistors based on CNTs and vanadium oxide nanowires. Significantly, since this process does not require any high-temperature processing steps, it can be applied to virtually general substrates and may remove current difficulty in manufacturing of electronic devices based on 1-D nanostructures.   

Nanoscale devices on thin films by ultra-high-resolution lithography

It is possible to achieve exceptionally high resolution with lithographic techniques that use scanning and transmission electron beams by using a thin film as a substrate. With this approach, numerous structures such as nanowires, nanorings, nanogaps and quantum dots can be made with dimensions under ten nanometers and in some cases even less than one nanometer. The flexibility of this fabrication approach also allows these extremely small structures to be easily contacted by large electrodes and therefore integrated into full electronic devices that exhibit effects due to carrier confinement. Furthermore, because these devices are on thin films, they are compatible with imaging by transmission electron microscopy (TEM). Basic devices made with this approach will be introduced. Extensions to devices with more complicated geometries and those which also include non-lithographically prepared nanostructures will be discussed as well. *This work was supported by ONR (N000140410489), NSF (DMR-0449553), NSF MRSEC (DMR00-79909) and NSF-IGERT (DGE 022166).   

Correlated electrostatic force microscopy and transmission electron microscopy study of nanostructures on silicon nitride membranes

Silicon nitride membrane windows allow correlated electrostatic force microscopy and transmission electron microscopy (EFM/TEM) study of electrical and structural properties of the same nanoscale electronic devices fabricated on top of them. Under EFM, nanoscale charge transport patterns are distinguished and correlated with structural details as imaged by high resolution TEM. Examples of nanostructures studied include lithographically fabricated devices and self-organized nanocrystal arrays. Implications of the results on the transport mechanisms of these nanostructures will also be discussed. This work is supported by ONR Young Investigator Award N000140410489, ACS PRF Grant 41256-G10, NSF Career Grant DMR-0449553, and NSF NSEC Grant DMR-0425780.   

Local Photocurrent Mapping of Nanowire Photodetectors with Ohmic and Schottky Contacts

Near-field scanning photocurrent microscopy (NSPM) was used to determine the mechanisms of carrier transport and collection in CdS nanowire photodetectors. NSPM employs an apertured NSOM probe as a local ($<$100 nm) illumination source to map the local photocurrent as a function of the tip position along the device, i.e., from one metal contact to the other. Striking differences between Schottky and ohmically contacted devices have been observed in maps of the local photocurrent. In the Schottky devices, the photoinduced current is localized to the reverse biased diode, whereas in ohmic devices, the peak photoresponse position shifts continuously with applied bias. Modeling of the photocurrent profiles in ohmically contact devices gives the mobility-lifetime product for electrons and for holes. When independent carrier lifetime measurements are considered, one can extract electron and hole mobilities. As expected for CdS, the electron mobility exceeds the hole mobility, producing the observed shift of the photocurrent peak towards the hole collector. The effects of surface passivation and trap filling on carrier transport have also been explored.   

Negative Differential Resistance in CdSe Nanorod Devices.

Semiconductor quantum rods are expected to exhibit interesting novel behaviors because of their well-defined shape with the long axis preferably grown along the unique c axis. They would also allow for efficient quasi-1D electrical transport. Thus, when organized into arrays of aligned quantum rods separated by insulating barriers, improved and unconventional electronic transport could be achieved compared to that of ``spherical'' nanocrystal arrays. Here, we report on the observation of interesting charging properties in electronic devices consisting of CdSe quantum rod thick films as the active components. The low bias regime of the current-voltage characteristics of such devices displays multiple negative differential resistance behavior and step-like structures at room temperature. This effect may be related to the alignment of localized trap levels in the insulating barriers with the carrier levels in the quantum rods.   

Electron transport of nanoscale P-donor wires in silicon

Three dimensional carrier transport in doped semiconductors has been extensively investigated. However, transport in low-dimensions is much less clear because of the difficulty to confine dopant distribution in a crystal. In the past few years we have created 2D embedded dopant sheets by exposing Si(100) surfaces to phosphine molecules in ultrahigh vacuum followed by growing epitaxial silicon over-layers at room temperature. Electron density in these delta layers can be as high as $\sim $1.5x10$^{14}$ cm$^{-2}$. We find that surface roughness dictates the carrier mobility and activation, even though all surfaces are atomically clean and locally ordered. Furthermore, applying STM e-beam lithography on a single-layer H-resist enables us to define P-donor wires at widths from 200 nm to 5 nm in 2-terminal device templates. The As-implanted electrodes in the device templates provide ohmic contact with P-donor wires. In this presentation we will discuss our electrical and magneto-resistance measurement of various P-donor nanostructures at cryogenic temperatures. The goal of this research is to apply 2D P-donor patterns as building blocks for nanoscale integrated circuits.   

Confined Doping for Control of Transport Properties in Nanowires and Nanofilms

Doping, an essential element for manipulation of electronic transport in traditional semiconductor industry, is widely expected to play important role as well in control of transport properties in nanostructures. However, traditional theory of electronic disorder predicts that doping in one-dimensional and two-dimensional systems leads to carrier localization, limiting practical applications due to poor carrier mobility. Here, a novel concept is proposed that offers the possibility to significantly increase carrier mobility by confining the distribution of dopants within a particular region [1]. Thus, the doped nanostructure becomes a coupled system comprising a doped subsystem and a perfect crystalline subsystem. We showed that carrier mobility in such a dopped nanowire or a nanofilm exhibits counterintuitive behavior in the regime of heavy doping. In particular, the larger the dopant concentration the higher the carrier mobility; we trace this transition to the existence of quasi-mobility-edges in the nanowires and mobility edges in nanofilms. \begin{enumerate} \item J.X. Zhong and G.M. Stocks, Nano Lett., in press, (2005) \end{enumerate}   

Field emission characteristics of GaN nanorods on self-implanted (111) Si

Periodic arrays of GaN nanostructures have been fabricated by MBE growth on self-implanted (111) Si substrates. Nano-capillary condensation was found to be an effective catalytic process fostering the formation of epitaxially aligned GaN nanorods supported by a thin film matrix. Changes of Si substrate surface morphology as a result of ion bombardments prior to the thin-film deposition are responsible for the enhanced nanorod growth. The density of nanorods in relation to implanted ion dosages was studied. Field emission measurement was performed to understand the physical characteristics of functional devices based on such nanostructures. Experimental details and their implications for the future development of nanostructure and nano-device fabrications will be presented.   

Conductance measurement of GaN nanorods

GaN nanorods have been grown by molecular beam epitaxy over a thin-film GaN matrix on Si substrate. We have studied the conductance behaviors of a single nanorod and clusters nanorods. Transport measurement of internal emission of electrons from nanorod-clusters was carried out with metallic contacts over the nanostructure. Vacuum tunneling of externally emitted electrons from individual nanorod was measured using a scanning tunneling microscope-first in constant voltage mode to locate the more conductive nanorods, which was then followed by measurements at various applied voltage. Observations are made to distinguish thin film matrix from the nanorods by their efficiencies of electron emission. The characteristics of I-V curves will be reported and the applications of these nanorods to electron-emission devices will be discussed.   

Structural and Electronic Properties of GaN and InN Nanowires grown using Hot-Wall CVD

We study the electron-mobility dependence on the free carrier concentration \textit{n} exhibited by hot-wall chemical-vapor deposition-grown gallium nitride (GaN) and indium nitride (InN) nanowires. The growth involves flow of ammonia over solid sources of gallium or indium and the substrate, which is covered with metal catalyst (in case of GaN) or is catalyst-free (in case of InN). The nanowires are subsequently deposited on oxidized silicon wafers and fabricated in field-effect transistors using optical lithography. In this way, more than 1000 devices were characterized at room temperature. Both types of nanowires show high carrier concentration ($10^{19}-10^{20}$ cm$^{-3}$ for GaN and $10^{20}-10^{21}$ cm$^{-3}$ for InN), with mobility decreasing with increasing free carrier concentration, consistent with ionized impurity scattering. Mobility levels range between below 1 to 100 cm$^{2}$/Vs. Estimations of the ionized impurity mobility indicate that GaN wires grow heavily compensated, and subsequent anneals in ammonia result in even higher compensation levels. We were also successful in doping GaN nanowires with magnesium, for p-type doping. Similar chemical, structural, and electronic analysis will be presented. This work was partially supported by DARPA through AFOSR, ARO, AFOSR, NASA, by the Department of Homeland Security, and by NSF.   

Session N38: Transport Properties of High-Tc Superconductors

Anomalous dimensional crossover in critical microwave-conductivity fluctuations of superconducting La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films

We demonstrate that there are two dimensional crossover lines separating the phase diagram of La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO) into three regions with different universality classes, by using a dynamic scaling analysis of the microwave complex conductivity. For underdoped LSCO from x=0.07 to x=0.14, we show clear evidence for the 2D-\textit{XY} universality class (the BKT transition in the nearly decoupled CuO$_{2}$ planes), while the 3D-\textit{XY} universality class is observed for nearly optimally doped region (x=0.15, 0.16). Surprisingly, for overdoped LSCO with x$>$0.17, we found that the critical behavior strongly suggested the 2D universality class, in contrast to the reduction of anisotropic properties with hole-doping. We discuss the implication of these results in terms of the effect of the quantum critical fluctuations.   

Finite Size Effects in YBCO Films in Zero and Non-Zero Field

The phase transition in high Tc superconductors in a magnetic field has been intensely studied. However, only a few papers have discussed finite-size effects (Phys. Rev. B \textbf{69}, 214524 (2004)). Neglecting finite size effects can cause misinterpretation of the experimental data leading to incorrect critical exponents. We will report results of DC transport measurements on YBa2Cu3O6.95 films in zero field (less than 50 nT) as well as in fields up to 6 T. The results will be analyzed in terms of field-induced finite size effects and thickness-induced finite size effects.   

Fluctuations at the superconductor/normal phase transition of YBCO thin films

The zero-field phase transition of high $T_c $ superconductors has been studied by a number of techniques, such as penetration depth, magnetic susceptibility, specific heat and thermal expansion, which reveal information about the static, properties of fluctuations. Transport properties (such as the conductivity) which probe the dynamics near $T_c $ are less explored, and a wide range of critical exponents were reported experimentally. We investigated fluctuation effects of $YBa_2 Cu_3 O_{7-d} $~(YBCO) around~$T_c $ by doing frequency-dependent microwave conductivity measurements and dc current-voltage characteristics on the same film. For each experiment the scaling behavior of the data was investigated. The critical exponents $\nu $ and z from the two different experiments will be extracted and compared. We also investigated YBCO frequency-dependent conductivity fluctuation effects for different powers or currents and for different film thickness. The finite size effects in both the microwave and DC voltage current experiments will also be discussed. (This work was supported by NSF grant number DMR-0302596)   

Transport and Noise Properties of High Temperature Superconductor Nanostructures

The study of transport and noise properties of high-$T_c $ superconductor nanostructures provides a sensitive probe of their local electronic structure and may give insight into the proposed mechanisms for the superconductivity and the nature of the anomalous normal phases. We discuss a novel technique for fabricating nanostructures based on the growth of cuprate films on substrates pre-patterned by Focused Ion Beam etching. We report measurements in underdoped YBCO and BSCCO nanostructures fabricated by this technique that are designed to understand anomalous switching noise observed in the pseudogap phase above $T_c$. Our goal is to test if these signal show the existence of dynamical domains characterized by inhomogeneous conductivity or by anisotropic charge stripes.   

Thermoelectric power as evidence for a Quantum Phase Transition in electron-doped cuprates Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ .

We report magnetic field driven normal state thermoelectric power (S) measurement in electron-doped cuprate system Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ as a function of doping (x from 0.11 to 0.19) down to 2K. Consistent with the normal state Hall effect$^{a}$, S in the underdoped region (0.11-0.15) is negative. S changes sign at certain temperatures in overdoped samples (0.16-0.18), which supports the picture of a spin density wave rearrangement of the Fermi surface$^{b}$. More significantly, both S and S/T at 2K (at 9T) increase dramatically from x=0.11 to 0.16, and then saturate in the overdoped region. This kink around x=0.16 is similar to the previous Hall effect result$^{a}$ in Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$. Our results are further evidence for antiferromagnetism to paramagnetism quantum phase transition in electron-doped cuprates. a. Y. Dagan et al, Physical Review Letters, 92 (16) 167001, 2004 b. A. Zimmers et al, Europhysics Letters 70 (2) 225, 2005   

Anomalous Nernst Effect in High-$T_c$ Superconductors by Layer Decoupling

The extended vortex-liquid phase found in high-$T_c$ superconductors at temperatures and magnetic fields that lie above the vortex-lattice melting line can be attributed to weak inter-layer coupling. We compute the diamagnetic contribution to the equilibrium magnetization there, $M$, using the corresponding uniformly frustrated $XY$ model, given in terms of the phase of the superconducting order parameter[1]. A high-temperature expansion that is valid in the vicinity of the mean-field phase transition yields (i) that $-M$ increases monotonically with the external magnetic field, and (ii) that it vanishes as the mean-field phase transition is approached from below. Further, we show (iii) that the assumption of a direct dependence between $M$ and the Peltier transport coefficient results in an anomalous Nernst signal inside of the vortex-liquid phase. These results are compared to recent experimental determinations of the equilibrium magnetization and of the Nernst effect in the vortex-liquid phase of high-$T_c$ superconductors[2]. \newline \newline [1] J.P. Rodriguez, PRB {\bf 66}, 214506 (2002); {\bf 69}, 069901(E) (2004). \newline [2] Y. Wang, et al., PRL, in press (arXiv: cond-mat/0503190).   

Thermoelectric transport near the pair breaking quantum phase transition out of a $d$-wave superconductor

We study electric, thermal, and thermoelectric conductivities in the vicinity of a $z=2$ superconductor-diffusive metal transition in two dimensions, both in the high and low frequency limits. We find violation of the Wiedemann-Franz law, with a Lorentz ratio {\it below} the Sommerfeld value (more charge than heat transport). In addition, the dc thermoelectric conductivity $\alpha$ does not vanish at low temperatures, in contrast to Fermi liquids. We introduce a Langevin equation formalism to study critical dynamics over a broad region surrounding the quantum critical point.   

Competing orders in LSCO probed by heat transport

We elucidate the nature of the thermal metal-to-insulator transition in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) [1] through measurements of the thermal conductivity $\kappa$ performed very close to the transition, down to temperatures as low as 50 mK and in magnetic fields $H$ up to 17 T. For a single crystal with x = 0.15, a monotonic increase in the residual linear term $\kappa_0$/$T$ is observed up to 17 T, as expected for a d-wave superconductor. For a crystal with x = 0.144, however, we observe an initial increase in $\kappa_0$/$T$ at low field, followed by a decrease when $H$ exceeds a critical field $H^*$. This result is consistent with recent neutron scattering measurements on a similar sample [2], which show that static spin-density-wave (SDW) order is not present in zero field, but sets in at a critical magnetic field $H^*$, and then co-exists/competes with superconductivity (SC) for $H > H^*$. Taken together, these two measurements reveal that the SC phase gives way to a phase which is both magnetic and insulating, whether by increasing magnetic field or by decreasing doping. Using low-energy quasiparticle transport, we map out the $T$ = 0 field-doping ($H-x$) phase diagram of LSCO. [1] D.G. Hawthorn et al., Phys. Rev. Lett. 90, 197004 (2003); X.F. Sun et al., Phys. Rev. Lett. 90, 117004 (2003). [2] B. Khaykovich et al., Phys. Rev. B 71, 220508(R) (2005).   

Existence of a boson mode in underdoped YBCO

We have examined the underdoped region of the cuprate phase diagram via a study of heat transport at temperatures down to 50 mK in samples of YBCO with a hole concentration in the vicinity of 5\%. The measured thermal conductivity was found to be well described by a sum of three terms, $\kappa(T) = aT + bT^{\alpha} + cT^3$, which we associate with, respectively, fermionic quasiparticles, phonons, and a new bosonic mode. By comparing data taken at different doping levels on the same sample it was possible to track the evolution of the $T^3$ term. Its coefficient was found to increase with decreased doping, i.e., with proximity to magnetism. We speculate that this term may be associated with the transport of heat by magnons, as observed recently by Li et al. [1] in the undoped cuprate material Nd$_2$CuO$_4$. This would suggest that long range magnetic order in YBCO appears at a doping level very close to that at which superconductivity ends. [1] S.Y. Li et al., Phys. Rev. Lett. {\bf 95}, 156603(2005)   

Low-Temperature Normal State Hall Effect in High-$T_{c}$ La$_{2-x}$Sr$_{x}$CuO$_{4}$

We report Hall effect measurements in the normal state of the high-$T_{c}$ superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$. The Hall resistivity was measured by suppressing superconductivity in 60T magnetic field, thus revealing the normal-state behavior in the low temperature limit. The carrier concentration is varied from overdoped to underdoped regimes by partially substituting La with Sr in a set of thin film samples. We find a discontinuity in the doping dependence of the Hall coefficient suggestive of a phase transition near optimal doping.   

Effect of Disorder Outside the CuO$_{2}$ Planes on $T_{c}$ of Copper Oxide Superconductors.

The effect of disorder on the superconducting transition temperature $T_{c}$ of cuprate superconductors is examined. Disorder is introduced into the cation sites in the plane adjacent to the CuO$_{2}$ planes of single-layer systems, Bi$_{2}$Sr$_{1.6}$Ln$_{0.4}$CuO$_{6+y}$. Disorder is controlled by changing rare earth (Ln) ions with a different ionic radius with the doped carrier density kept constant. We show that this type of disorder works as weak scatterers in contrast to the in-plane disorder produced by Zn, but remarkably reduces $T_{c}$ , suggesting novel effects of disorder on high-$T_{c}$ superconductivity.   

Mixed State $c-$axis Resistivity of Y$_{0.54}$Pr$_{0.46}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ Single Crystals

We report temperature $T$, magnetic field $H$, and angle $\theta$ dependent out-of-plane resistivity $\rho_{c}$ measurements on $Y_{0.54}Pr_{0.46}Ba_{2}Cu_{3}O_{7-\delta}$ single crystals. We performed these measurements in order to investigate the origin of the large $\rho_{c}$ of layered superconductors like cuprates and of its T, H, and $\theta$ dependence. The $\rho_{c}(T, H, \theta)$ data are very well fitted by the Ambegaokar$-$ Halperin expression [V. Ambegaokar and B. I. Halperin, Phys. Rev. Lett. 22, 1364 (1969)] for temperatures up to the critical temperature $T_{c}$ and applied magnetic field up to 14 T. This implies that in the underdoped cuprates the layered structure can be depicted as stacks of Josephson junctions. We calculated the value of the critical current density $J_{c}$ at different temperatures by using the above model and the values of the fitting parameters. Both the magnitude and $T$ dependence of $J_{c}$ are consistent with previous reports. This result supports the applicability of the model and indicates that the mixed state $c-$axis dissipation is mainly due to the Josephson effect.   

C-axis Resistivity and Magnetoresistance of the Electron-doped Cuprate Pr$_{1.85}$Ce$_{0.15}$CuO$_{4}$

C-axis resistivity and magnetoresistance have been studied extensively in the hole-doped high temperature superconductors. Observations, such as a resistivity upturn and associated negative magnetoresistance (n-MR), were attributed to the pseudogap. Recently similar phenomena were reported in the electron-doped superconductor Sm$_{1.85}$Ce$_{0.15}$CuO$_{4}$ (SCCO), and a universal Zeeman splitting of a spin gap (pseudogap) state was proposed $^1$. Here we report transport properties of Pr$_{1.85}$Ce$_{0.15}$CuO$_{4}$ (PCCO) ($T_C\approx 25K$) single crystals for comparison. Our c-axis n- MR can be explained by superconducting fluctuations due to the Aslamazov-Larkin (AL) process and the fluctuating electronic density of states (FDOS) above $H_{C2}$. We find that PCCO does not follow the Zeeman scaling behavior as reported for SCCO. This work is supported by NSF (Grant DMR 0352735). $^1$ T. Kawakami et al., Phys. Rev. Lett. 95, 017001 (2005).   

Resistivity and Hall effect measurements in Pr$_{2-x}$Ce$_{x}CuO$_{4-y }$ up to 60T

We report resistivity and Hall effect measurements in the electron-doped cuprate system Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ in magnetic field up to 60T. We found negative magnetoresistance (MR) in the underdoped region for all magnetic field values, similar to the low field data reported previously$^{a}$. The MR becomes positive at high field in the optimal doped (x=0.15) sample at low temperature. Most surprisingly, we observed a substantial magnetic field dependence of the Hall coefficient at high field (above $\sim $40T) in optimal doped and overdoped samples (from x=0.15 to 0.19) in a certain temperature range. A spin density wave induced Fermi surface reconstruction model can be used to explain this phenomenon. We also report for the first time the parallel upper critical field (H//ab plane) for Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$. (a. Y. Dagan et al., Physical Review Letters 94 (5) 11 2005)   

Role of Disorder and Oxygen Reduction on Transport Properties in Pr$_{1.83}$Ce$_{0.17}$CuO$_{4\pm\delta}$

We present a study on the effects of changing the oxygen content in the electron-doped superconducting cuprate Pr$_{2-x}$Ce$_{x}$CuO$_{4\pm\delta}$ (PCCO). Epitaxial, c-axis oriented, overdoped (x = 0.17) thin films were grown using a pulsed laser deposition technique, and the oxygen content was adjusted during a post-growth annealing process. In addition to the transition temperature (T$_{c}$), measurements of the Hall effect and resistivity were performed at low temperatures (T $<$ T$_{c} $, H $>$ H$_{C2}$) in several films of different oxygen content. We compare the disorder observed in these oxygenated samples with disorder induced by proton irradiation in an optimally annealed (x = 0.17) film. An analysis of the data demonstrates that a change in the oxygen content of PCCO has two separable effects: 1) a disorder effect, and 2) a doping effect similar to that of cerium.   

Session N39: Focus Session: Superconductivity-Thin Films and Junctions Magnesium Diboride and Related Compounds

Clean and Dirty MgB$_2$ Thin Films by Hybrid Physical-Chemical Vapor Deposition

The interband and intraband scattering in MgB$_2$ have significant influences on various properties of this two-band superconductor. In order to explore new physical phenomena and realize the potential for applications in MgB$_2$, it is desirable to control the interband and intraband scattering arbitrarily and independently. Hybrid physical-chemical vapor deposition (HPCVD) is a promising technique towards this goal. It has produced very clean MgB$_2$ films with a residual resistivity ratio of 60. The tensile strain in the films due to crystallite coalescence results in a softening of the $E_{2g}$ phonon and higher-than-bulk $T_c$ values. The long mean free path of the films, which are the cleanest MgB$_2$ materials reported, allow magnetoresistance measurements that reveal rich features of the two-band Fermi surfaces. The HPCVD technique also allows doping of the clean films in a controlled manner, which modifies the interband and intraband scattering and results in record-high upper critical field $H_{c2}$ of over 60 T. The demonstrations of HPCVD for high quality polycrystalline coatings on coated conductor fibers, high deposition speed and thick films, and on various polycrystalline substrates make it a promising technology for high field applications of MgB$_2$.   

Hybrid Physical-Chemical Vapor Deposition of MgB$_{2}$ Film on Flexible Dielectric and Metallic Substrates

The need for flexible dielectric and metallic substrates arises when considering making wires or tapes for high field applications. To accomplish this, Cu wire and foil with a buffer layer, flexible yttrium stabilized zirconium (YSZ), as well as Nb, Ta, and stainless steel foil were used as substrates for polycrystalline MgB$_{2}$ film growth. The foil substrates used range from 1 to 3 mil thickness. The buffer layers deposited on Cu were Ni plating (on 28 BSG wire) as well as TiB$_{2}$ and Nb deposited by sputtering. These served as a barrier to prevent the chemical reaction between Cu and Mg that occurs during deposition of MbB$_{2}$. The resistance vs. temperature (R-T) dependences were recorded for the films successfully grown on these substrates. For the films on YSZ, R-T was recorded initially and then after bending of the film on the substrate over a diameter of 20mm. The T$_{C}$ of MgB$_{2}$ on stainless steel was 38K; on YSZ and Nb it was 38.5K. This is lower than epitaxial films on SiC substrate with T$_{c}$ up to 41.5K. The R-T curve for MgB$_{2}$ on YSZ remained almost completely unchanged after bending. These films hold promise for electromagnetic field generation applications. This work is supported by NSF and ONR.   

Planar MgB$_{2}$ superconductor-normal metal-superconductor Josephson junctions

We have fabricated planar superconductor-normal metal-superconductor (SNS) MgB$_{2}$ Josephson junctions using MgB$_{2}$ films grown by hybrid physical chemical vapor deposition (HPCVD). The junctions exhibit resistively-shunted-junction-like (RSJ-like) current-voltage characteristics up to 31~K. Ac Josephson effect was observed and the behavior of the Shapiro steps are in good agreement with theoretical predictions. The magnetic field modulation of the critical current also agrees with the thin film planar junction behavior. The junction's behavior can be described by Likharev's proximity effect model with rigid boundary at dirty limit. A dc SQUID with modulation depth of 45~\textit{$\mu $}V at 29~K has been demonstrated. This work is supported by ONR, NSF, and AFOSR.   

Fabrication and characterization of MgB$_{2}$/ thermal oxide barrier /Pb Josephson tunnel junctions

Cross-bridge Josephson tunnel junctions were fabricated, using MgB$_{2}$ films grown by hybrid physical-chemical vapor deposition (HPCVD) and barriers made by thermal oxidation at different temperatures. The junctions showed clear Josephson tunneling characteristics with high supercurrents, high $I_{c}R_{N}$ products ($I_{c}R_{N}$ products $\sim $1.8 mV at 4.2 K), and small subgap current leakage. The external DC magnetic field dependence was also measured and showed clear Fraunhofer pattern. The properties of the thermal oxide barrier depend sensitively on the oxidation temperature. The potential height and barrier thickness of 0.7 eV and 1.8 nm, respectively, were inferred from conductance measurements at high bias voltage. Two superconducting gaps of 2.0 meV and 7.5 meV for MgB$_{2}$ were observed from these sandwich-type tunnel junctions. These results suggest the potential of using MgB$_{2}$ thermal oxide layers as a barrier for practical Josephson tunnel junction fabrication. This work is supported by ONR and NSF.   

MgB2 Tunnel Junctions with Native or Thermal Oxide Barriers

MgB$_{2}$ tunnel junctions (MgB$_{2}$/barrier/MgB$_{2})$ were fabricated using an oxide (or a mixture of oxides) grown on the first MgB$_{2}$ film as the tunnel barrier, by exposure to air at 20$^{o}$C (native oxide) or 160$^{o}$C (thermal oxide). Such barriers therefore survived the deposition of the second electrode at 300$^{o}$C, even over junction areas of $\sim $1mm$^{2}$. The sum of the superconducting gaps of the top and bottom electrodes, from conductance-voltage data, was as high as 4.3 mV and this sum gap remained non-zero for temperatures above 30 K. Conductance vs. voltage dependencies of all-MgB$_{2}$ junctions and those of the type MgB$_{2}$/Native or Thermal Oxide/Metal (Pb, Au, or Ag) were used to characterize the height and width of the barriers formed. Such barriers have surprisingly low barrier heights, with typical values for barrier height and width being 0.2 V and 4.5 nm respectively. These values are very different from those reported in the literature. These results show that tunnel barriers grown on MgB$_{2}$ can have different properties (barrier height and width), depending on the film growth, surface composition and oxidation conditions.   

All MgB$_{2}$ tunnel junction with Al$_{2}$O$_{3}$ tunnel barrier

MgB$_{2}$ tunnel junctions are attractive not only from superconducting electronics application part of view but also from the fundamental physics to understand multi-gap superconductors. All MgB$_{2}$ planar junctions with Al$_{2}$O$_{3}$ tunnel barrier were fabricated in situ in an MBE system by coevaporation of Mg and B for MgB$_{2}$ and plasma oxidized Al for tunnel barrier on Si (111) substrate. The junctions exhibit the current-voltage characteristic for quasiparticle and Josephson tunneling including microwave induced Shapiro steps. From conductance spectrum at 1 K, we clearly observe features that correspond to different $\pi $ and $\sigma $ superconducting energy gaps for the two MgB$_{2}$ electrodes. The observed multi-gap structure will be discussed with the difference of crystallographic orientation of MgB$_{2}$ at the interface between tunnel barrier and both superconducting layers.   

Scanning tunneling spectroscopy and high-frequency response of MgB2 films

The critical issues governing thin-film MgB2 applications can be traced to factors both microscopic, arising from the two-gap structure and scattering mechanisms, as well as mesoscopic, determined by connectivity and grain boundary characteristics. These are intertwined to the extent that substitutional doping to control gap characteristics and band scattering can strongly affect connectivity through grain boundary segregation and growth of second phases. These issues can be addressed with a combined approach of gap characterization by low-temperature scanning tunneling spectroscopy, and temperature-dependent microwave conductivity measurements. We discuss preliminary results characterizing gap properties with low-temperature superconducting-tip scanning tunneling microscopy, and 10 GHz cavity-based microwave conductivity measurements.   

Thickness dependence of the properties of MgB$_{2}$ films grown by hybrid physical-chemical vapor deposition

Properties of pure MgB$_{2}$ films of different thicknesses (up to $\sim$1 $\mu$m) grown by hybrid-physical-chemical vapor deposition on sapphire substrates were studied. In accordance with the previous results for the films with thicknesses up to about 400 nm, {\it T}$_{c}$ of the films on Al$_{2}$O$_{3}$ levels off at a value of 40.0 - 40.5 K at thicknesses larger than 200 nm. The residual resistivity, {\it $\rho$}$_{0}$, monotonically decreases with thickness, which is caused by a reduction of the surface and interface scattering (size effect on resistivity). For films with thickness over $\sim$ 800 nm, {\it $\rho$}$_{0}$ is below 0.15 $\mu$$\Omega$.cm and {\it RRR} $>$ 60. X-ray studies of the films did not reveal any other phases besides MgB$_{2}$. In this talk, MgB$_{2}$ films of even larger thickness and the thickness dependence of critical current density will also be reported.   

Far-infrared Studies of the Two-Gap Behavior in Epitaxial MgB$_{2}$ Films

Far-infrared transmission and reflectivity measurements have been carried out for a series of pure and carbon-doped epitaxial MgB$_{2}$ films. While the carbon-doped film exhibits the typical characteristics for a dirty BCS superconductor in the T$_{S}$ /T$_{N}$ and R$_{S}$ /R$_{N}$ ratios, the pure MgB$_{2}$ films can only be understood knowing the multi-gap nature of the superconducting state in MgB$_{2} $. As a function of increasing T$_{c}$, the fraction of the Cooper pairs having the larger gap increases. Both gaps appear to follow the BCS temperature dependence. However, the two gaps exhibit different behavior when a magnetic field is applied along c-axis. While the smaller gap can be suppressed by a relatively small field, the larger gap can persist up to 10 T. These infrared measurements indicate that the two superfluids coexist quite independently in the superconducting state of MgB$_{2}$.   

Penetration Depth Anisotropy in MgB$_{2}$ measured by Small-Angle Neutron Scattering

Traditionally the anisotropy of a type-II superconductor is described either by $\gamma _{\lambda }=\lambda _{c}$/$\lambda _{ab}$ or $\gamma _{H}$~=~$H_{ab}$/$H_{c}$~=~$\xi _{ab}$/$\xi _{c}$. with the two considered to be identical. However, in materials with anisotropic gaps this is generally not the case. MgB$_{2}$ represents an extreme case in which $\gamma _{\lambda }$~$\ne $~$\gamma _{H}$. While there is consensus on the value of $\gamma _{H}(T)$, measurements of $\gamma _{\lambda }$ are still contradictory. Here we demonstrate a novel use of small-angle neutron scattering to determine $\gamma _{\lambda }$ in MgB$_{2}$, by measuring the misalignment between the applied field and the direction of the flux-line lattice as the field is rotated between the $c$ axis and the basal plane. Using a two-band/two-gap model we can fit the angular dependence of the misalignment, yielding $\gamma _{\lambda }$~=~1.1 $\pm $ 0.2 at 4.9 K and 0.4 T.   

Session N41: Insulating and Dielectric Oxides

Oxygen-deficient defects and hydrogen in irradiated Si/SiO2 systems

Performance of Si/SiO2 devices is degraded by ionizing radiation through the production of interface traps and buildup of trapped charge in the oxide. This process is connected to the presence of hydrogen in the oxide. Exposure to ionizing radiation stimulates release of mobile hydrogen, which can migrate through the oxide to the Si/SiO2 interface and depassivate H-terminated Si-dangling bonds. The resulting interface trap states act as charge recombination centers. Our calculations focus on the interaction of hydrogen with oxygen deficient centers in the oxide. In the bulk oxide, these defects can release hydrogen from Si-H groups, or crack H2 molecules. These active sites may also act as border trap recombination centers when near the interface. Our presentation will describe molecular scale mechanisms for radiation-induced generation of free hydrogen using density functional theory applied to fully periodic models. The oxide is represented by both crystalline and amorphous configurations. Sandia is a multiprogram laboratory operated by the Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.   

Photo-catalytic studies of transition metal doped titanium dioxide thin films processed by metalorganic decomposition (MOD) method

We have synthesized pure and transition element (Fe, Co and V) doped Titanium oxide thin films of thickness $\sim $ 350 nm on sapphire, Si, and stainless steel substrates by Metalorganic Decomposition (MOD) method. The films were subsequently annealed at appropriate temperatures ( 500-750C) to obtain either anatase or the rutile phase of TiO$_{2}$. Analysis of the composition of the films were performed by energy dispersive X-ray(EDAX) and Rutherford backscattering spectrometry(RBS). Ion channeling was used to identify possible epitaxial growth of the films on sapphire. Both XRD and Raman spectra of the films exhibit that the films annealed at 550C are of anatase phase, while those annealed at 700C seem to prefer a rutile structure. The water contact angle measurements of the films before and after photoactivation, demonstrate a significant reduction in the contact angle for the anatase phase. However, the variation in contact angle was observed for films exposed to UV ($<$10$^{o}$-30$^{o})$ and dark (25$^{o}$-50$^{o})$. Films doped with Fe show a trend towards lower contact angle than those doped with Co. Results with films doped with V will also be included.   

Proton Conduction and Microstructure of Lanthanum Phosphates

Lanthanum phosphate (LaPO4) has been recently studied as a potential proton conducting ceramic electrolyte for fuel cells. The complexity of the La2O3 - P2O5 system, particularly in the 1000 - 1600K range, leads to the difficulty in forming phase pure LaPO4 by traditional synthesis methods. Complex microstructures involving amorphous and crystalline phases at grain boundaries have been detected by high resolution transmission electron microscopy, and have been shown to affect proton conductivity by several orders of magnitude. A simple model is used to predict conductivity of the material based on grain size and grain-boundary phases, and experimental results match up well with the model. Conductivities as high as 3.6E-5 S/cm at 773 K were found for undoped LaPO4.   

Comparison of the electronic structures of four crystalline phases of FePO$_4$

LiFePO$_4$ in the olivine structure is a promising cathode material for Li-ion batteries. During normal battery operation, an olivine form of FePO$_4$ is formed. Experimental evidence suggests that the olivine phase is meta-stable relative to a quartz-like trigonal phase. In addition, several other meta-stable phases have been reported\footnote{M. S. Whittingham, Chem. Rev. {\bf{104}}, 4271 (2004); Y. Song and co-workers, Inorg. Chem. {\bf{41}}, 5778 (2002); H. N. Ng and C. Calvo, Can. J. Chem. {\bf{53}}, 2064 (1975); J. P. Attfield and co-workers, J. Solid State Chem. {\bf{57}}, 357 (1985); A. S. Andersson and co-workers, Solid State Ionics {\bf{130}}, 41 (2000)} including a monoclinic and a high pressure CrVO$_4$ structure. We have carried first-principles electronic structure calculations on all of these phases in order to investigate their relative stability and other properties using both LAPW and PWPAW methods.\footnote{http://www.wien2k.at/; http://pwpaw.wfu.edu/ } We find that the LSDA approximation systematically underestimates the lattice constants and the calculated stability ordering of the phases is inconsistent with experiment. In contrast the GGA approximation models the phase stability in closer agreement with experiment, although the lattice constants are systematically over-estimated.   

First-principles study of phase stability and phase transition in Ga$_{2}$O$_{3}$

Gallium oxide (Ga$_{2}$O$_{3})$ is a promising opto- and/or electronic wide-band-gap semiconductor. For example, it has been considered as a gate dielectric oxide for MOS device based on GaN or GaAs. In addition to the ground-state monoclinic $\beta $-phase, a rhombohedral $\alpha $-phase metastably exists at ambient conditions. The conditions of stability of either phase are not well understood. In this talk, we will present our recent \textit{ab initio} calculation results of thermodynamic properties of the $\alpha $ and $\beta $ phase. We have computed Gibbs free energies of the two phases based on the total energy density functional theory (DFT) and the statistical quasi-harmonic approximation (QHA), calculated the thermal equations of states, and estimated the equilibrium phase boundary. We have further calculated the pressure dependence of Raman and IR frequencies in the two phases. Our results will be compared with some recent experimental data.   

Phase transition of Ta-modified Pb(Sc$_{0.5}$Nb$_{0.5}$)O$_{3}$ nanoceramics

Ferroelectric relaxors are promising candidates for multilayer ceramic capacitors. We have synthesized nanocrystalline Pb(Sc$_{0.5}$Nb$_{(1-x)/2}$Ta$_{x/2})$O$_{3}$, (0.1$ [Preview Abstract]

Substrate-induced strain effects on the transport properties of pulsed laser deposited Nb doped SrTiO$_{3}$ films.

Thin films of Nb doped SrTiO$_{3 }$(NSTO) are grown via pulsed laser deposition (PLD) on LaAlO$_{3}$ (LAO, 001), MgAl$_{2}$O$_{4}$ (MAO, 001), SrTiO$_{3}$ (STO, 001), and Y-stabilized ZrO$_{2 }$(YSZ, 001) substrates. The film growth is examined under various growth conditions. The dependence of film properties on the film-substrate lattice mismatch, film thickness, and substrate temperature is investigated. The electrical transport in NSTO films is shown to exhibit a strong sensitivity to strain, which is suggested to arise from the dependence of carrier mobility on bond distortions/stretching and related changes in phonon modes.   

Blue-light emission at room temperature from Ar$^{+}$-irradiated SrTiO$_{3}$

SrTiO$_{3}$ is a key material for fabricating oxide-based electronic devices. We found that Ar$^{+}$-irradiated, metallic SrTiO$_{3}$ crystals emit 430-nm blue-light at room temperature. Oxygen-deficient metallic SrTiO$_{3}$ thin films also show the blue-light emission. Reciprocal mapping using synchrotron x-ray radiation at SPring-8 reveals a slight elongation of the lattice parameter along the out-of-plane direction both for these samples. We, therefore, suggest that the Ar$^{+}$-irradiation introduces oxygen deficiency in the crystal surface, and that the deficiencies generate conduction carriers which wait ready for the recombination with photo-exited holes, and play an important role in the emission. It is emphasized that the emitting region could be patterned into any size and shape by combining conventional photolithography and Ar$^{+}$-milling. These new features of SrTiO$_{3}$ will open up new possibilities for the oxide-based electronic devices.   

Measurement of the Electric Field Gradient at $^{181}$\textit{Ta} in \textit{ZrSiO}$_{4 }$and \textit{HfSiO}$_{4}$ using Perturbed Angular Correlation Spectroscopy

Perturbed angular correlation spectroscopy (PAC) is a nuclear technique often used to probe the hyperfine interaction of a nuclear moment with extra-nuclear fields. For example the electric field gradient (EFG) at a $^{181}$Ta probe nucleus in zircon (ZrSiO$_{4})$ depends on the arrangement of the Zr, Si, and O-atoms and is very sensitive to structural rearrangements. Our PAC experiments with zircon show that a very subtle rearrangement of Si-atoms within the unit cell leads to a change in the temperature dependence of the EFG. We are currently performing a series of PAC experiments on the isostructural hafnon (HfSiO$_{4})$. Preliminary results show no evidence of a similar structural rearrangement. In addition to the EFG, we also measure the anisotropy of the $\gamma \gamma $-cascade emitted during the decay of a $^{181}$Ta nucleus. The measured anisotropy depends somewhat on the geometry of the sample and detector arrangement. However, with a given nucleus and a fixed geometry one would not expect a substantial change in the anisotropy during a series of measurements, say as a function of temperature. Yet our PAC spectra of zircon show a consistent decrease of the anisotropy in the temperature range between 650 and 800\r{ }C. Preliminary PAC spectra of hafnon show no change of the anisotropy. Reasons for this apparent loss in anisotropy will be discussed.   

Structure, Energetics, and Clustering Interactions for Cu on TiO$_{2}$ (110) at Various Coverages

TiO$_{2}$ (110) is one of the prototypical metal-oxide surface systems, studied extensively and under a variety of different conditions by experimentalists and theorists alike. Recent experiments have enhanced our understanding of the structure of the stochiometric surface, and our Density Functional Theory calculations show excellent quantitative agreement with these latest results. In addition, strong interactions between metal catalysts and their supporting oxide substrates give rise to enhanced catalytic properties, and we are exploring this phenomenon for the prototypical system of Cu on the (110) surface of rutile TiO$_{2}$. In this talk, we will present our theoretical results for the surface structure and elaborate on the agreement with the latest experimental findings as well as the differences from previous theoretical work on this important system. We will also discuss predictions of the structure and energetics for Cu on TiO$_{2}$ (110) at various coverages and on both stoichiometric and reduced surfaces, where specific focus will be upon clustering interactions and the formation of Cu islands, which has been observed experimentally.   

{\it Ab initio} molecular dynamics of a proton in amorphous SiO$_2$ illustrating the hopping mechanism

The scaling of metal-oxide-semiconductor devices to smaller dimensions is a major issue in current silicon technology. In order to understand the role of hydrogen at silicon-oxide interfaces, we here investigate charged states of hydrogen in amorphous SiO$_2$ ($a$-SiO$_2$) using first-principles calculations (DFT-GGA). We first show that the formation energies of H$^0$, H$^+$ and H$^-$ in $a$-SiO$_2$ are essentially equivalent to those in $\alpha$-quartz. In particular, the H$^+$ and H$^-$ species are always more stable than their neutral counterpart. Then, we focus on the basic diffusion mechanism of the proton in $a$-SiO$_2$. Our molecular dynamics simulations show that the proton hops between O atoms. The hopping does not occur between first O neighbors connected through the network, but takes place across rings when the O--O distance is about 2.3 \AA. The hopping process is favored by the thermal vibrations of the O atoms.   

Unusual compressibility in the negative-thermal-expansion material ZrW$_{2}$O$_{8}$

The negative thermal expansion (NTE) compound ZrW$_{2}$O$_{8}$ has been well-studied because it remains cubic with a nearly constant, isotropic NTE coefficient over a broad temperature range. However, its elastic constants seem just as strange as its volume because NTE makes temperature acts as \textit{positive} pressure, decreasing volume on warming and, unlike most materials, the thermally-compressed solid$_{ }$\textit{softens}. Does ZrW$_{2}$O$_{8}$ also soften when pressure alone is applied? Using pulse-echo ultrasound in a hydrostatic SiC anvil cell, we determine the elastic tensor of monocrystalline ZrW$_{2}$O$_{8 }$near 300 K as a function of pressure. We indeed find an unusual decrease in bulk modulus with pressure. Our results are inconsistent with conventional lattice dynamics, but do show that the thermodynamically-complete constrained-lattice model can relate NTE to elastic softening as increases in either temperature or pressure reduce volume, establishing the predictive power of the model, and making it an important concept in condensed-matter physics.   

Oxide films imaged on a nanometer-scale by single-electron tunneling force microscopy

Recently, we reported a scanning probe technique to manipulate a single electron to and from states in a nonconducting surface by electron tunneling. Each electron is detected by electrostatic force as it tunnels between a scanning probe and the surface. Electrons are manipulated by tuning the probe Fermi level with respect to the states by a dc voltage. This manipulation serves as the mechanism for imaging and for performing electronic spectroscopy of states in dielectric films. The energy distribution of the states is measured by counting the electrons tunneling to the surface at incrementally varied voltages. The spatial distribution of states is imaged on a nanometer-scale by counting each electron tunneling on a 2-D grid. We present spectroscopic and imaging results from silicon dioxide and hafnium oxide. The density and spatial distribution of states is compared for various growth parameters. The measurements reveal evidence for energy relaxation and charge movement in the states. This new nanometer-scale approach provides the means to locate and identify electronic states in nonconducting surfaces, opening for exploration a whole class of materials not accessible to the STM. \newline [1] E. Bussmann, N. Zheng {\&} C. C. Williams, \textit{Appl. Phys. Lett.} \textbf{86}, 163109 (2005).   

A novel approach to the growth of polycrystalline hydroxyapatite thin films

Hydroxyapatite (HA) thin films on metals have been extensively studied in connection with bioimplants. Conventional sputtering techniques have shown some advantages over the commercially utilized plasma spray method; however, the as-sputtered films are usually amorphous which can cause serious adhesion problems when post-deposition heat treatment is necessitated. We present a novel opposing RF magnetron sputtering approach for the room temperature preparation of HA thin films on various substrates at low power levels. The as-sputtered films are found to be polycrystalline and the preferred orientations of the films vary with the substrate material and orientation. The effects of different sputtering parameters on the physical, chemical and structural properties was also studied. Finally, patterned films fabricated both before and after deposition have been prepared for further \textit{in vitro} cell culture experiments.   

Finite temperature properties of (Ba,Sr)TiO$_3$ disordered alloys and BaTiO$_3$/SrTiO$_3$ superlattices from first-principles

We develop and use a first-principles-based scheme to predict properties of (Ba,Sr)TiO$_3$-based systems at finite temperature. This scheme yields a composition-versus-temperature phase diagram of disordered (Ba$_{1-x}$Sr$_{x}$r)TiO$_3$ solid solutions that is in rather good agreement with experimental data. We further use this scheme to reveal and understand the strain-versus-temperature phase diagram of several BaTiO$_3$/SrTiO$_3$ superlattices. A wide variety of dipole patterns, including homogeneous ferroelectric phase and periodic stripe patterns, are predicted to occur depending on the interplay between temperature, strain and superlattice periods.   

Session N42: Focus Session: Simulations of Matter at Extreme Conditions I

Shock-Induced Chemical Reactions of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) have been found in the atmospheres of Jupiter and Titan, and also in meteorites, interplanetary dust, and circumstellar graphite grains. The ubiquity of these complex organic structures and their stability under extreme conditions make them a significant factor in discussions of brebiotic chemistry in the solar system. To study the shock-induced chemistry of PAHs under conditions appropriate for astrophysical impacts, molecular dynamics simulations have been carried out for solid naphthalene and anthracene using a reactive empirical potential. The major reaction channels for these two closely related compounds were found to be substantially different. Product distributions were also found to depend strongly on the orientation of the PAH crystal relative to the shock propagation direction.   

Interatomic bond-order potentials for atomistic simulations of materials at extreme conditions

Molecular dynamics simulations provide an excellent opportunity to address fundamental physics and chemistry of materials at extreme conditions. However, the results of MD modeling can only be as reliable as the ability of the interatomic potentials to properly describe a variety of chemical effects including bond-breaking and bond-making. Our recent MD simulations of shock compression of covalently bonded materials such as diamond and silicon using REBO interatomic potential for diamond and EDIP potential for Si showed that the properties of C and Si systems at large pressures and temperatures are not well described in spite of the fact that the near equilibrium properties of both diamond and silicon are well reproduced. We present new results on development of analytic bond-order potentials (BOPs) for covalently bonded materials at extreme conditions. These BOPs are derived using the powerful concepts of moments of density of states, Green's function and Lanczos recursion, applied within the two-center, orthogonal tight-binding bond representation of electronic structure. Importantly, they include explicit analytic expressions for both the $\sigma $ and $\pi $ bonds. We will describe details of BOP construction including devising a first-principle database of fundamental materials properties, its fitting by the tight-binding (TB) model, and devising the analytic BOPs using the direct link between TB and analytic BOPs via the bond orders. Validation of analytic BOPs by comparison with first-principles high-pressure data will also be discussed.   

First-principles modeling of energetic materials

The prediction of properties of energetic materials using atomic-scale simulation techniques is one of the promising areas of energetic materials (EM) research. One of the challenges is to understand the initial response of EM to shock loading based on fundamental atomic-scale properties of EM crystals. We report the results of first-principles density-functional calculations of static and thermodynamic properties of PETN, HMX and RDX molecular crystals including properties of different crystalline phases and their equations of states (EOS). The EOS are extended beyond simple isotropic constitutive relationships to include materials response upon uniaxial compressions and high pressures up to 100 GPa. The predictions of the theory are compared with recent experimental results.   

Direct Simulation of Detonations: Applications to the H$_{2}$-Cl$_{2}$ System

Earlier simulations in our laboratory showed that ultrafast detonations having steady-state velocities greater than predicted by the Chapman-Jouguet (CJ) and the Zeldovich-von Neumann-D\"{o}ring (ZND) theories could be produced by very fast model reactions. In this paper we will report matching studies incorporating a realistic treatment of the reaction H$_{2}$ + Cl$_{2}\to $ 2 HCl reacting by the Nernst chain reaction mechanism with ignition, propagation and termination steps along with the inclusion of rotational and vibrational degrees of freedom for diatomic species and the realistic treatment of energy exchanges among all species. The H$_{2}$-Cl$_{2}$ system is the prototypical system for studying detonations both experimentally and theoretically, and is an ideal candidate for investigation. Our simulations are made using Bird's direct simulation Monte Carlo method which produces the full details of the coupled gas-dynamic and reaction effects as well as temperature, velocity, density, pressure, and species profiles for the detonation waves. By comparing predictions with available experimental measurements for the system, we will be able to predict the likelihood that ultrafast detonations can be observed for the H$_{2}$-Cl$_{2}$ reaction.   

Molecular Dynamics Simulations of Thermal Induced Chemistry in TATB

Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was developed by van Duin, Goddard and coworkers$^{\dag }$ at CalTech and has already shown promise in predicting the chemistry in small samples of RDX under either shock compression or intense heat. Large-system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. We will show results of 100,000-particle simulations at several temperatures, carried out with the massively parallel GRASP MD software developed at Sandia National Lab. Finally, we will compare the reactions and reaction timescales with those of RDX and HMX. $^{\dag }$ A. C. T. Van Duin, \textit{et al}, \textit{J. Phys. Chem. A}, \textbf{1005}, 9396 (2001).   

Decomposition of Energetic Materials at Extreme Conditions

Detailed description of chemical reaction mechanisms of solid energetic materials at high-density and temperatures is essential for understanding events that occur at the reactive front of these materials, and for the subsequent development of predictive models of materials properties. In this talk, we will report the results of our ongoing ab initio based molecular dynamic simulation of the chemistry of TATB, at density of 2.9 g/cm$^{3}$ and temperature of 1500K, and at density of 2.87 g/cm$^{3}$ and temperature of 2500 K. These conditions are similar to those experienced at the CJ and von Neumann spike Following the dynamics for a time scale of up to forty picoseconds allows the construction of approximate rate laws for typical products such as H$_{2}$O, N$_{2}$, CO, and CO$_{2}$. The approximate reaction rates obtained for these products at the CJ state will be compared to those obtained recently for HMX at similar conditions. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-Eng-48.   

Transitions between strong shock and elastic-plastic shock regimes in tin and Lennard-Jonesium.

We have recently developed the Continuous Hugoniot Method as an efficient numerical approach to study the front dynamics of steady shocks. This method produces a continuous set of shock states as a function of the shock strength driving parameter. We use our method to investigate the shock states at and near the transition between the strong shock and elastic-plastic shock regimes in both single-crystal tin and Lennard-Jonesium along the $\langle 100 \rangle$ directions.   

Multi-scale molecular dynamics simulations of steady shock waves in Lennard-Jones and nitromethane

We compare spatial profiles of steady shock waves using our multi-scale simulation technique (Phys. Rev. Lett. 90, 235503 (2003)) and direct simulation techniques and find good agreement. Multi-scale simulations of shocked amorphous Lennard-Jones are in good agreement with NEMD simulations and multi-scale simulations of shock waves in analytical equations of state of explosives are in good agreement with hydrodynamic simulations. In both cases, agreement improves with distance behind the shock front. We have applied the multi-scale technique to the study of chemically reactive shock waves in condensed nitromethane (CH3NO2) using the density-functional tight-binding (DFTB) method. We study shock waves with speeds ranging from 5.5 km/s to 8 km/s for durations up to 0.5 ns behind the shock front. We believe these are the longest duration tight-binding simulations of shocked matter ever performed.   

First-principles approach to reactivity in the presence of shockwave

Gaining insight into the mechanisms leading to detonation in energetic materials within a planar shock geometry had been thought to be computationally prohibitive. The use of the Multiscale Shock Method (MSSM) of Reed et. al has opened the possibilities to study the chemistry of complex molecular systems undergoing uniaxial shock compression using Kohn-Sham density functional theory (KS-DFT). Here, we present results of nitromethane under various levels of shock loading and reveal the chemical mechanisms underlying the detonation process. We will also discuss an alternative non-Hamiltonian formulation of the MSSM. The aforementioned formulation is present in both CPMD and CP2K software packages. We also will compare and contrast different formulations of KS-DFT (both plane- wave and hybrid schemes) and discuss future of scaling the MSSM method to tera- scale platforms such as Blue Gene/L.   

Finding the structure of phosphorus in phase IV by the first-principles calculation

Phosphorus in phase IV (P-IV) had been unclear since first experimental report. Using the metadynamics combined with the first-principles calculation, we obtained a new structure. The structure is a monoclinic of sc: a=c=4.22{\AA}, b=4.15{\AA}, $\beta$=97.76$^{\circ}$ and a modulation is observed along the b-axis. We noted this modulated pattern and, for modulated structures having other wave-lengths, compared the x-ray diffraction patterns of these structures with experimental one. As a result, limited to the case of commensurate patterns because of the periodic boundary condition for the calculations, the structure whose period is 4 times as long as that of the non-modulated structure is the most compatible for the experimental result. For this pattern, calculating the enthalpy for pressure, it is lower than both sc and sh in the range from 118 GPa to 128 GPa. Recently Akahama \textit{et al.} have determined the structure of P-IV, which is rather close to our structure. Our calculation shows the transition from sc to P-IV was the first order phase transition.   

First Principles Phonon and Elasticity Computations for Iron under extreme conditions

We performed linear-response Linear-Muffin-Tin-Orbital (LMTO) and particle-in-cell (PIC) model calculations to understand and predict the lattice dynamical, thermal equation of state and elastic properties of bcc, fcc, and hcp iron as functions of pressure and temperature. The phonon dispersion and phonon density of states have been calculated at different volumes and show good agreement with experiment. We derived the Helmholtz free energy based on both the linear response LMTO and PIC calculations, and found that the calculated geometric mean phonon frequencies and free energies from these two different methods agree well under pressure, in contradiction to an earlier calculation. We performed detailed investigations on the behavior of elastic constants and various thermal equation of state parameters, including the bulk modulus, the thermal expansion coefficient, the Gr\"{u}neisen ratio, and the heat capacity as functions of temperature and pressure. We made detailed comparison with experiment and earlier theoretical calculations. We do not find the large change in c/a axial change with T. Sound velocities at extreme conditions have also been examined. These first-principles data provide important information to understand shock dynamics and other interesting phenomena under extreme conditions.   

Ab initio studies of magnetism at extreme volume and shape deformation

Magnetic solids constitute a basis of many technologically important materials, however, very little is known how their magnetic behavior changes when a high-strain deformation is applied (as it is, for example, in heavily deformed regions of extended defects, such as grain boundaries, dislocation cores, crack tips etc.). In the present talk, we report on magnetic behavior of iron, nickel, FeCo, Ni$_{3}$Al and Fe$_{3}$Al at the extreme volume as well as tetragonal and trigonal deformation. The total energies are calculated by spin-polarized full-potential LAPW method and are displayed in contour plots as functions of tetragonal or trigonal distortion $c/a$ and volume; borderlines between various magnetic phases are shown. Stability of tetragonal magnetic phases of $\gamma $-Fe is discussed. In case of Fe, Ni and FeCo, the calculated phase boundaries are used to predict the lattice parameters and magnetic states of overlayers from these materials on various (001) substrates. Whereas magnetism does not play an important role in stabilization of the L1$_{2}$ structure in Ni$_{3}$Al, the magnetic effects in Fe and Fe$_{3}$Al are vital.   

Electronic Structure of Actinides under Pressure

The series of heavy radioactive elements known as the actinides all have similar elemental properties. However, when the volume per atom in the condensed phase is illustrated as a function of atomic number, perhaps the most dramatic anomaly in the periodic table becomes apparent. The atomic volume of americium is almost 50{\%} larger than it is for the preceding element plutonium. For the element after americium, curium, the atomic volume is very close to that of americium. The same holds also for the next elements berkelium and californium. Accordingly from americium and onwards the actinides behave very similar to the corresponding rare-earth elements - a second lanthanide series of metallic elements can be identified. This view is strongly supported by the fact that all these elements adopt the dhcp structure, a structure typical for the lanthanides. The reason for this behavior is found in the behavior of the 5f electrons. For the earlier actinides, up to and including plutonium, the 5f electrons form metallic states and contribute most significantly to the bonding. In Np and Pu they even dominate the bonding, while all of a sudden they become localized in Am, very much like the 4f electrons in the lanthanide series, and contribute no longer to the cohesion. This withdrawal of 5f bonding gives rise to the large volume expansion between plutonium and americium. This difference between the light and heavy actinide suggests that it would be most worthwhile to strongly compress the transplutonium elements, thereby forcing the individual 5f electron wave functions into strong contact with each other (overlap). Recently high pressure experiments have been performed for americium and curium and dramatic crystal structure changes have been observed. These results and other high pressure data will be discussed in relation to the basic electronic structure of these elements.   

Session N43: Quantum Optics and Strong Field Physics

Large Double-EIT and Mutual Phase Shifts in Rubidium

We propose a scheme to achieve large double-EIT and mutual phase shifts for two slow, co-propagating pulses of light through a Rubidium gas, with the additional advantages of enabling equal group velocities for the two pulses and avoiding cancellations of nonlinearities at resonance.   

Electromagnetically induced transparency and precision measurement of atomic transitions in a laser-cooled sample of cesium atoms

Electromagnetically induced transparency (EIT) has observed in a cascade system of laser-cooled Cs atoms and the atomic energy levels have been measured to an accuracy of 0.0003 $cm^{-1}$. In our experiment, Cs atoms are loaded into the magneto-optical trap (MOT) from a background vapor that has a pressure of 10$^{- 9}$ torr. The number of Cs atoms is estimated using a CCD camera to be 10$^7$ occupying a roughly spherical volume having a radius of 2 mm and the temperature of the atom cloud is measured using a time of flight technique to be about 100 $\mu$K. A diode laser excites the Cs atoms from $|6\,^2S_{1/2}, F=4\rangle$ state to $|6\,^2P_{3/2}, F=5\rangle$ state, then a dye laser couples the $6\,P_{3/2}$ state to the higher excited, $|9\,^2D_{3/2}\rangle$, $|9\,^2D_{5/2}\rangle$, $|10\,^2D_{3/2} \rangle$, $|10\,^2D_{5/2}\rangle$, and $|11\,^2S_{1/2}\rangle$ atates. The signal is monitored by detecting the trap loss while changing the pump or coupling laser frequency. The effect on the EIT signals by changing the pump laser detuning and the coupling laser power is also discussed. The line positions are measured by comparing the saturation absorption spectrum of molecular iodine (accuracy $<$ 3 MHz).   

Femtosecond Coherent Raman Spectroscopy in Biomolecules

We present a scheme for identification of biomolecules in real time by using Coherent Anti-Stokes Raman Spectroscopy (CARS) with femtosecond pulses. The nonlinear interaction of the pulses with the molecules is calculated analytically as well as numerically and comparison is made with the experimental results. We show that the commonly used rotating wave approximation needs to be waved for our fast dephasing molecules. Propagation effects are simulated, in addition to the nonlinear interaction, and it is shown how one needs to balance the electronic resonance enhancement and the losses due to absorption, in order to maximize the signal at the output of the optically dense medium. In the transient regime, we present and analyze the generation of polarization beats. These beats are generated due to the very broad bandwidth of the femtosecond pulses, which allows for the coupling of more that one vibrational modes at a time.   

Dephasing of excitons in single ZnSe quantum wells using 30 fs pulses

The dephasing of excitons in homogenously broadened 10nm wide ZnMgSe/ZnSe single quantum wells is investigated using ultra short ($<$30fs) light pulses in two beam and three beam four wave mixing (FWM) configuration. The observed FWM traces show marked quantum beats due to excitation of higher exciton transitions within the 80 meV spectrally broad pulses. For pulse delay times shorter than 500 fs, non-Markovian signal decay is observed. For longer delay times the exciton dephasing becomes single exponential indicating the transition from quantum kinetics to classical Boltzmann kinetics. Intensity and polarization dependent FWM measurements give further information on the physical nature and scattering processes that take place in both time regimes. This work is supported by the National Science Foundation (DMR-0305076).   

Dynamics of high amplitude coherent phonons in photoexcited bismuth

We report on studies of high amplitude coherent phonons in photoexcited bismuth. All optical experiments, in which two pump pulses are used to coherently control the amplitude $A_{1g}$ phonon at a fixed carrier density, allow us to separate the effects of carrier dynamics from anharmonicity. The results show that the the time dependent frequency of the phonon is dominated by electronic softening of the interatomic potential. Seperate first-principals theoretical calculations confirm these results for photoexcited carrier densities up to approximately 2\% of the valence electrons. We comment on the possibility of x-ray diffraction and diffuse scattering as a means of measuring the detailed dynamics.   

Optimization and Rate-Equation Model for Second-Harmonic Generation in Mid-Infrared Quantum Cascade Lasers

We present the rate-equation simulation of a mid-infrared quantum cascade laser with optimized second-harmonic generation. The simulation is performed by optimization of the structure design followed by self-consistently solving the rate equations for the carriers in the various levels. The optimized structure was obtained by supersymmetric quantum mechanics with both position-depended mass and band nonparabolicity included. In the rate-equation model, the second harmonic generation process is described by sequentially two single photon absorption and one second-harmonic photon generation. Nonunity pumping efficiency is accounted by all relevant electron-electron and electron-LO phonon scatterings between injector/collector and active region are included. The modal gain, linear power and second-harmonic power can then be calculated based on the steady-state populations in the active region. Results show that the optimized structure has higher modal gain and linear to nonlinear conversion efficiency.   

Observation of a coherent exciton-LO phonon resonance in a ZnSe single quantum well

A new coherent signal has been observed while performing two- beam degenerate four-wave mixing (FWM) experiments on a 3nm ZnMgSSe/ZnSe single quantum well (SQW) using 30fs laser pulses. In this SQW structure the exciton binding energy exceeds the LO-phonon energy (31.6 meV). The observed spectral feature is blue shifted with respect to the heavy-hole bound exciton transition by $\sim $32 meV and indicates the formation of a coherent exciton-LO phonon resonance with a dephasing time of $\sim $500 fs. This tentative assignment is further supported by photoluminescence excitation (PLE) and by reflection measurements. This work is supported by the National Science Foundation (DMR-0305076).   

Vibrational Lifetimes and Frequency-Gap Law of Hydrogen Bending Modes in Semiconductors

Vibrational lifetimes of hydrogen and deuterium related bending modes in semiconductors are measured by transient bleaching spectroscopy and high-resolution infrared absorption spectroscopy. We find that the vibrational lifetimes follow a universal frequency-gap law, i.e., the decay time increases exponentially with increasing decay order, with values ranging from 1 ps for a one-phonon process to 265 ps for a four-phonon process. The temperature dependence of the lifetime shows that the bending mode decays by lowest-order multi-phonon process. Our results provide new insights into vibrational decay and the giant isotope effect of hydrogen in semiconductor systems.   

High field gradient for electron acceleration and ultra-short electron pulse formation

We explore a possibility of strongly inelastic scattering and large energy exchange between the tightly focused laser and electron beams in the ``transverse'' geometry, whereby the beams propagate normally to each other. In the most basic configuration of the laser standing wave we demonstrate that multi-MeV electron acceleration per pass is attainable, if the field has large spatial gradient along the direction of electron motion. The electron motion in this case is relativistic, and the energy gain or loss can be retained by an electron with very high efficiency. The proposed scheme may provide enormous $\sim 0.1$ TeV/cm acceleration gradient. We also show that the transverse electron and laser beam configuration can cause strong temporal electron focusing resulting in formation of ultra-short electron bunches. Such a system has a potential to operate as a full-switch laser gate for electrons, a new base element of a free-electron laser and laser electron accelerators.   

Relativistic effect of ponderomotive force direction reversal in a standing laser wave

In our research, we theoretically discovered a relativistic effect of the direction reversal of the field-gradient (ponderomotive) force (PF) in a standing laser wave. This reversal makes the high-field areas attractive for electrons, in contrast to the regular PF, and it represents the only effect known so far that pins down a distinct borderline between relativistic and nonrelativistic motion. We demonstrated that the collinear configuration, in which the laser wave is linearly polarized with electric field, $\vec{E}$ parallel to the initial electron momentum, $\vec{p}_0$, is the optimal configuration for the relativistic reversal. In that case, the transverse PF reverses its direction when the incident momentum is $p_0 = mc$. The reversal effect vanishes in the cases of circular and linear with $\vec{E} \perp \vec{p} _0$ polarizations. We have discovered, however, that the counter-rotating circularly polarized standing waves develop attraction and repulsion areas along the axis of laser, in the laser field whose intensity is homogeneous in that axis, i.e. has no field gradient.   

Spatial and pulse shape dependence of K$\alpha$ source from high contrast fs laser plasmas in regime of Relativistic Engineering

Interaction of intense Ti: Sapphire laser with Cu foil targets has been studied by measuring hard X-ray generation. Hard x-ray spectroscopy and K$\alpha $ X-ray conversion efficiency (\textit{$\eta $}$_{K})$ from Cu plasma have been studied as a function of laser intensity via pulse duration scan (\textit{60 fs $\sim $ 600 fs}), laser pulse energy scan (\textit{60 mJ $\sim $ 600 mJ}) and target displacement scan from best focus. For intensity \textit{I $>$ 1x10}$^{17}$\textit{ W/cm}$^{2}$, the Cu \textit{$\eta $}$_{K}$ keep on increasing to reach a maximum value of $1x10^{-4}$ at an intensity $I = 1x10^{18}$\textit{ W/cm}$^{2}$. The focusing was varied widely to give a range of intensities from $10^{15}$\textit{ W/cm}$^{2}$\textit{ $\sim $ 10}$^{18}$\textit{ W/cm}$^{2}$. Two individual emission peaks are obtained, one is at best focal spot and the other is at larger target offset corresponding to \textit{$\sim $ 10}$^{15}$\textit{ W/cm}$^{2}$. Each peak is corresponding to different energy absorption mechanism. In addition, when we introduce slightly detuning of compressor gratings at the best focal condition, it shows \textit{$\eta $}$_{K}$ generated by negatively skewed \textit{100 fs} pulse width laser irradiation reach $5x10^{-4}$ and almost $7$ times greater than the case of positively skewed pulse. Vacuum Heating is greatly stimulated in this case and preciously control of pre-plasma is the key factor in tuning control of X-ray emission in relativistic \textit{fs} regime.   

Guiding of 100 TW Relativistic Laser Pulses by 10 mm Plasma Channels

First experiments for laser-gas plasma interaction about electron acceleration have been performed with 30 fs, 100 TW relativistic Ti:Sapphier laser pulse interaction with long slit (\textit{1.2 x 10 mm}$^{2})$ gas plasmas. The world record \textit{10 mm} length plasma channel formed that was longer than 20 times the Rayleigh length. Plasma density was the key factor for this long channel stimulation under \textit{100 TW} laser pulse irradiation that was much higher than critical power for relativistic self-focusing. For the first time, channel characteristics such as laser bending, hosing and cavity formation were demonstrated experimentally. In case of long channel guiding, accelerated electron bunch was tightly collimated with low emmitance \textit{$<$ 0.8 $\pi $ mm mrad} and quasi-monoenergetic electron bunch (\textit{$\sim $70 MeV}) was obtained as well. Accelerated electron charge current with electron energy \textit{$>$ 1 MeV} was \textit{$\sim $ 10 nC/shot} which was highest value in laser accelerator, to our knowledge, and ascribed to the contribution of long plasma channel.   

Session N44: Quantum Criticality and Nematic Ordering

The Effect of the Berry Phase on the Quantum Critical Properties of the Bose-Fermi Kondo model

The theory of the quantum critical point of a $T=0$ transition is traditionally formulated in terms of a quantum-to-classical mapping, leading to a theory of its classical counterpart in elevated dimensions. Recently, it has been shown that this mapping breaks down in an SU(N)$\times$SU(N/2) Bose-Fermi Kondo model (BFKM) [1], a BFKM with Ising anisotropy [2] and the spin-boson model [3]. Here we report the Quantum Monte Carlo results for the scaling properties of the quantum critical point of the BFKM with Ising anisotropy. In addition, using the Lagrangian formulation of the BFKM, we study the critical properties in the presence and absence of the spin Berry phase term. The results of the two cases are compared with the numerical results.\newline [1] L. Zhu, S. Kirchner, Q. Si, and A. Georges, Phys. Rev. Lett. 93,267201 (2004). [2] M. Glossop and K. Ingersent, Phys. Rev. Lett. 95, 067202 (2005). [3] M. Vojta, N-H Tong, and R. Bulla, Phys. Rev. Lett. 94, 070604 (2005).   

Quantum Critical Behaviour Near the Kondo Breakdown Fixed Point

We study the Kondo-Heisenberg model using a fermionic representation for the localized spins. In this model, the mean field Kondo hybridization at $T=0$ can be continuously tuned to zero as a function of the exchange interactions. We calculate the fluctuations of the hybridization and its associated gauge potential at the one loop level, and their contribution to the specific heat and spin susceptibility, near the quantum critical point.   

Kondo Screening and Fermi Surface in the Antiferromagnetic Metal Phase

We address the Kondo effect deep inside the antiferromagnetic metal phase of a Kondo lattice Hamiltonian with SU(2) invariance. The local- moment component is described in terms of a non-linear sigma model. The Fermi surface of the conduction electron component is taken to be sufficiently small, so that it is not spanned by the antiferromagnetic wavevector. The effective low energy form of the Kondo coupling simplifies drastically, corresponding to the uniform component of the magnetization that forward-scatters the conduction electrons on their own Fermi surface. We use a combined bosonic and fermionic (Shankar) renormalization group procedure to analyze this effective theory and study the Kondo screening and Fermi surface in the antiferromagnetic phase. The implications for the global magnetic phase diagram, as well as quantum critical points, of heavy fermion metals are discussed.   

Sign change of the Gr\"uneisen parameter and magnetocaloric effect near quantum critical points

Strong fluctuations near a quantum critical point lead to a singular entropy distribution in the phase diagram. This results in strong signatures of the Gr\"uneisen parameter and the magnetocaloric effect. In particular, a sign change of the Gr\"uneisen parameter coincides with the accumulation point of entropy in the phase diagram. If the quantum critical point is the endpoint of a line of finite temperature phase transitions the sign change generically occurs in the Ginzburg regime of the classical transition as observed in several heavy fermion compounds. In addition, we predict a sharp peak in the Gr\"uneisen parameter at the critical temperature due to the contribution of classical critical fluctuations. For magnetic field tuning these signatures are also reflected in the magnetocaloric effect. Moreover, we discuss the case of metamagnetic quantum criticality where the sign change is located at the critical magnetic field.   

Towards a unification of local moment magnetism and the Kondo lattice

We apply the Schwinger boson scheme to the fully screened Kondo model and generalize the method to include antiferromagnetic interactions between ions. Our approach unifies the Kondo impurity approach of Parcollet and Georges with the Schwinger boson description of antiferromagnetism of Arovas and Auerbach, enabling the formalism to describe magnetically correlated and magnetically ordered heavy electron phases. For the single impurity, our approach captures the Kondo crossover from local moment behavior to a Fermi liquid with a non-trivial Wilson ratio. When applied to the two impurity model, the mean-field theory describes the ``Varma Jones'' quantum phase transition between a valence bond state and a heavy Fermi liquid. We will extend the method to the Kondo lattice, and explore the nature of the phase diagram connecting the heavy electron phase, the magnetic phase and the spin-liquid phase.   

Quasi-particle linewidth close to a quantum critical point: Crossover from non-Fermi liquid to Fermi liquid behavior

Heavy fermion systems frequently display non-Fermi liquid behavior due to a nearby quantum critical point. A nested Fermi surface together with the remaining interaction between the carriers after the heavy particles are formed may give rise to itinerant antiferromagnetism. The order can gradually be suppressed by mismatching the nesting and a quantum critical point is obtained as $T_N \to 0$. The quasi-particle linewidth is calculated in the paramagnetic phase following an approach outlined by Virosztek and Ruvalds (Phys. Rev. B {\bf 42}, 4064 (1990)). The linewidth shows a crossover from non-Fermi liquid ($\sim T$) to Fermi liquid ($\sim T^2$) behavior with increasing nesting mismatch and decreasing temperature. The quasi-particle linewidth is a quantity relevant to the electrical resistivity and the width of the inelastic neutron scattering quasi-elastic peak.   

Curie law, entropy excess, and superconductivity in heavy fermion metals and other strongly interacting Fermi liquids

Low-temperature thermodynamic properties of strongly interacting, itinerant Fermi liquids with fermion condensate are investigated. We demonstrate that the spin susceptibility of these systems exhibits the Curie-Weiss law, and the entropy contains a temperature-independent term. The excessive entropy is released at the superconducting transition, enhancing the specific heat jump $\Delta C$ and rendering it proportional to the effective Curie constant. The theoretical results are favorably compared with the experimental data on the heavy fermion metal CeCoIn$_5$, as well as $^3$He films. \\ Reference: cond-mat/0508275, Phys. Rev. Lett. (December 2005).   

Super-frustration for strongly-correlated fermions in two dimensions

We prove that there exists an exotic ``super-frustrated'' state of strongly-correlated spinless fermions hopping on a two-dimensional lattice. This state is characterized by an extensive ground-state entropy, and very possibly is at a non-Fermi-liquid quantum critical point. We give explicit Hamiltonians which exhibit this behavior. Exploring various lattices and limits, we show how the ground states can be frustrated, quantum critical, or combine frustration with a Wigner crystal.   

Domain formation in electronic nematic phase coupled to lattice deformation

Motivated by the experiments on Sr$_3$Ru$_2$O$_7$, we have investigated the possibility of domain formation in electronic nematic phase coupled to lattice deformations. It has been suggested that the formation of the nematic order may explain the two consecutive metamagnetic transitions observed in Sr$_3 $Ru$_2$O$_7$. Our study may serve as the explanation of the high residual resistivity observed in a range of magnetic fields.   

Non-Fermi Liquid Behavior of Nematic Fermi Fluids

Following the initial study of Ref. 1, we explore the behavior of physically relevant quantities in the vicinity of the quantum critical point between a nematic Fermi fluid and a Fermi liquid. As shown in Ref. 1, this strong coupling fixed point is completely accessible within the method of high dimensional bosonization and we continue the analysis presented therein focusing on quasiparticle properties, such as the fermion residue and the fermion spectral function. We show in particular, that the fermion residue vanishes according to the essential singularity $\exp\big(-1/\sqrt{\delta}\big)$ where $\delta$ is the dimensionless coupling constant measuring the distance to the critical point. Also, at low temperatures, we verify explicitly that the heat capacity obey's the non-Fermi liquid powerlaw of $T^{2/3}$. We conclude with a discussion of the signatures of the nematic phase that would appear in light scattering and angle resolved photo emission spectroscopy experiments. \newline \newline [1] Lawler, Barci, Fernandez, Fradkin and Oxman, unpublished; cond-mat/0508747.   

Phase and amplitude fluctuations for the $l=2$ Pomeranchuk instability in two dimensions.

For a two-dimensional fermionic system, we analyze models that produce shape-distortion instabilities of the Fermi surface in the $l=2$ channel, leading to a non-Fermi liquid with nematic order. The finite-temperature phase diagram contains a transition of the Kosterlitz-Thouless type and a crossover at higher temperatures, corresponding respectively to the disordering of phase and amplitude degrees of freedom.   

Interplay between parallel and diagonal electronic nematic phases in interacting systems

An electronic nematic phase is a spontaneous broken state of a discrete rotational symmetry of a given crystal. There exist two distinct electronic nematic phases in a square lattice. One is the parallel nematic order which breaks the symmetry in $x$- and $y$-direction, and the other is the diagonal nematic order which breaks the diagonal $(x+y)$ and the anti-diagonal $(x-y)$ symmetry. We investigate the different features and the mutual interaction between these two nematic orders. We also discuss the possible implication of our results in the context of neutron scattering and Raman spectroscopy measurements in high $T_C$ superconductors.   

Interacting fermions in two dimensions: singularities in the perturbation theory and the role of collective modes.

We consider a system of interacting fermions in two dimensions. It is shown that even for an infinitesimally weak interaction a straight-forward perturbation theory is ill defined near the mass shell. Starting from the second order, the perturbative expansion for the self-energy is singular at the mass shell. We show that this singularity is a manifestation of a non- perturbative effect: the interaction of fermions with the collective mode. The singularities in the perturbation series for the self-energy is treated by resumming the most divergent diagrams. A threshold for emission of zero-sound waves leads to a non-monotonic variation of the self-energy. Consequently, the spectral function acquires a non-Lorentzian kink-like feature. This feature is reminiscent to spin-charge separation in 1D, as the kink is absent in a spin-polarized system. We examine the possibility of detecting the kink in momentum-conserving tunneling between two parallel layers of a 2D electron gas.   

Ferromagnetic quantum phase transition in an itinerant three-dimensional system

The non-analytic behavior of the spin susceptibility both away and near the quantum critical point signals the breakdown of the Hertz-Millis scenario for a ferromagnetic quantum phase transition in itinerant systems. It is believed that in both 2D and 3D $\chi_{s}$ increases as a function of the magnetic field ($H)$ or momentum ($q),$ which indicates a tendency to either first order transition or ordering at finite $q$. We show that the 3D case is different from the 2D one. Away from the 3D critical point, the non-analytic part of $\chi _{s}$ can be of either sign, depending on microscopic parameters. The non-analyticity in 3D arises from two physically distinct processes: excitations of a single and three particle-hole pairs. Both processes contribute a max\{$H^{2},q^{2}\}\ln \max \{H^{2},q^{2}\}$ term to $\chi _{s}$, but the signs of these contributions are opposite. The single-pair process leads to an increase of $\chi _{s}$ with $H,q$ whereas the three pair one corresponds to a decrease. In the paramagnon model, the three pair contribution always wins sufficiently close to the Stoner instability. We also discuss the behavior of $\chi _{s}$ in the immediate vicinity of the quantum critical point within the spin-fermion model.   

Metallic phase in a two-dimensional disordered Fermi system with singular interactions

We consider a two-dimensional disordered system of gapless fermions interacting with a singular transverse gauge-field. We study quantum corrections to fermion conductivity and show that they are very different from those in a usual Fermi liquid. In particular, the weak-localization effect is suppressed by magnetic field fluctuations. We argue that these fluctuations can be considered static at time scales of fermionic diffusion. By inducing fluxes through diffusive loops that contribute to weak localization, they dephase via the Aharonov-Bohm effect. It is shown that while the flux-flux correlator due to thermal fluctuations of magnetic field is proportional to the area enclosed by the loop, the correlator due to quantum fluctuations is proportional to the perimeter of the loop. The possibility of dephasing due to these quasistatic configurations is discussed. We also study interaction induced effects and show that perturbation theory contains infrared divergent terms originating from unscreened magnetic interactions. We show that due to singular small-angle scattering, the corresponding contributions to the density of states and conductivity are very large and positive indicating that the fermion-gauge system remains metallic at low temperatures.   

Session N45: Exotic Phases in Strongly Correlated Systems

Fractionalization in a strongly correlated exciton system

We show that fractionalized phases arise out of a strongly coupled exciton bose condensate in a multi-band insulator. Based on a world line picture of exciton, we demonstrate that the deconfinement phases can occur in a gauge theory of the exciton model despite an infinite bare gauge coupling. A world sheet of electric flux line in the emergent gauge theory is identified as a web of exciton world lines. It is shown that a deconfined U(1) gauge theory with ``photon'' and either fractionalized boson or fermion can emerge out of a single model depending on the coupling constants. The statistics and spin of the fractionalized particles are shown to be determined uniquely by the dynamics of the model. The exciton model can be numerically simulated without sign problem and some of our results will be shown.   

Criticality in correlated quantum matter

At quantum critical points (QCPs) quantum fluctuations occur on all length scales, from microscopic to macroscopic, which, remarkably, can be observed at finite temperatures, the regime to which all experiments are necessarily confined. But how high in temperature can the effects of quantum criticality persist? That is, can physical observables be described in terms of universal scaling functions originating from the QCPs? We answer these questions by examining exact solutions of models of systems with strong electronic correlations and find that QCPs can influence physical properties at surprisingly high temperatures. As a powerful illustration of quantum criticality, we predict that the zero temperature superfluid density, $\rho_{s}(0)$, and the transition temperature, $T_{c}$, of the copper-oxide superconductors are related by $T_{c}\propto\rho_{s}(0)^y$, where the exponent $y$ is different at the two edges of the superconducting dome, signifying the presence of the respective QCPs. This relationship can be tested in high quality crystals.   

Deconfined quantum-criticality in a 2D $S=1/2$ Heisenberg model

The two-dimensional $S=1/2$ Heisenberg model including a four-spin interaction is studied using a ground state projector quantum Monte Carlo (QMC) method in the valence bond basis. The model is sign-problematic in standard QMC methods formulated in the $S^z$ basis, but not in the valence bond basis. The ground state is studied on lattices with up to $40 \times 40$ spins. The four-spin interaction is shown to suppress the antiferromagnetic order, leading to a phase transition into a valence-bond-solid (VBS) state. The finite-size scaling of the singlet-triplet gap (which can be calculated with the valence bond projector using an improved estimator) scales as $1/L$ at the transition point, indicating a quantum phase transition with dynamic exponent $z=1$. This, and a large spin-spin correlation exponent, $\eta \approx 0.4$, suggests that the transition is a {\it deconfined quantum-critical point}. This would then be the first example of a model Hamiltonian for which this exotic N\'eel--VBS quantum-criticality has been observed.   

Bilayer antiferromagnet with four-spin interaction

We investigate a spin-1/2 Heisenberg antiferromagnet with four-spin interaction on bilayer square and honeycomb lattices. In addition to the standard Neel and quantum disordered phases, these models can be expected to have a valence-bond-solid (VBS) phase [1]. Our aim is to locate the VBS phase and to investigate, in particular, a transition from quantum disorder to VBS. This is potentially a deconfined quantum critical point [1]. We use a recently introduced ground state projection Monte Carlo method which allows us to study these models without negative-sign problems [2].\hfill\break [1] A. Vishwanath et al, Phys. Rev. B {\bf 69}, 224416 (2004). \hfill\break [2] A. W. Sandvik, Phys. Rev. Lett. {\bf 95}, 207203 (2005).   

Simulations of Quantum Spin Models on 2D Frustrated Lattices

Algorithmic advances in quantum Monte Carlo techniques have opened up the possibility of studying models in the general class of the S=1/2 XXZ model (equivalent to hard-core bosons) on frustrated lattices. With an antiferromagnetic diagonal interaction (Jz), these models can be solved exactly with QMC, albeit with some effort required to retain ergodicity in the near-degenerate manifold of states that exists for large Jz. The application of the quantum (ferromagnetic off-diagonal) interaction to this classically degenerate manifold produces a variety of intriguing physics, including an order-by-disorder supersolid phase, novel insulating states, and possible exotic quantum critical phenomena. We discuss numerical results for the triangular and kagome lattices with nearest and next-nearest neighbor exchange interactions, and focus on the relevance of the simulations to related areas of physics, such as experiments of cold trapped atomic gasses and the recent theory of deconfined quantum criticality.   

Quantum phase transition from a valence bond crystal to an antiferromagnet

We use the recently proposed two-step density-matrix renormalization group to study a ground state phase transition from a dimerized phase to a N\'eel phase in a frustrated spatially anisotropic Heisenberg and $t-J$ models. We compute critical exponents for the gap and correlation functions.   

Calculations of Domain Wall and Z$_{4}$ Vortex Energies in the Dimerized Phase of J1-J2 Heisenberg Model

We develop a series expansion method to calculate the Domain Wall Energy per unit length and the Z$_{4}$ vortex energies in the dimerized phase of the J1-J2 Heisenberg Model. The energy difference between the state with and without domain walls is calculated by series expansions around two different dimer configurations. The calculations are used to study the transition away from the dimerized phase. These calculations are compared with other studies of the phase boundaries in this system.   

The nature of quantum phase transition in quantum compass model

In this work, we show that the quantum compass model in two dimension can be mapped to a fermionic model with local density interaction and the quantum phase transition point at the symmetric point Jx=Jz marks a first order phase transition.   

Quantum Nematic Phase in the Emery Model

We investigate one strong coupling regime of the Emery model of a CuO plane in the strong coupling limit first discussed in ref. [1]. In this regime the on-site repulsion energies are much larger than the inter site Coulomb repulsions and the hopping terms. By integrating out the copper sites, we mapped this model into an interacting fermionic model on an effective two-dimensional crossed-chains lattice. We will discuss the simpler case of spinless fermions on this effective lattice in the regime in which the residual interactions are weak. Using a mean-field approach, we discuss the isotropic-nematic phase transition in this system. We show that the nematic phase may exist even for infinitesimally weak interactions. We investigate this phase transition for a range of dopings, temperatures and interactions. For certain choice of parameters, the effective electronic states behave like those of a 2D square lattice model, but for some other choices, its properties are reminiscent of a quasi-one-dimensional system. \newline \newline [1] Steven A. Kivelson, Eduardo Fradkin, and Ted Geballe, PRB 69, 144505 (2004)   

No sliding in time

We analyse the following apparent paradox: As has been recently proved by Hastings, under a general set of conditions, if a \emph{local} Hamiltonian has a spectral gap above its (unique) ground state, all connected equal-time correlation functions of local operators decay exponentially with distance. On the other hand, statistical mechanics provides us with examples of 3D models displaying so-called sliding phases which are characterised by the algebraic decay of correlations within 2D layers and exponential decay in the third direction. Interpreting this third direction as time would imply a gap in the corresponding (2+1)D quantum Hamiltonian which would seemingly contradict Hastings' theorem. The resolution of this paradox lies in the non-locality of such a quantum Hamiltonian.   

Quantum dimer model on a two dimensional pyramid lattice.

We study the Rokhsar-Kivelson (RK) quantum dimer model on a two dimensional corner-sharing pyramid lattice. Contrast to other lattices such as square and triangular lattice, on the RK line (V=t, V' arbitrary), the dimer-dimer correlation is exact zero as long as dimers are a few lattice constants away from each other. More interestingly, a deconfined dimer liquid phase (or RVB phase) is found to the left of the RK line in the phase diagram. There are two kinds of confined valence bond crystal (VBC) states to the very left of the RVB phase. And there is a VBC state to the right of the RK line. Surprisingly, the spinor excitations are deconfined~within the model even though the underlining state breaks the translational symmetry and rotational symmetry.   

A doped interacting quantum dimer model on the square lattice

We introduce a generalized quantum dimer model [1] for interacting dimers on the square lattice [2] which can be mapped to generic 2D classical partition functions. More specifically, we show that the amplitudes of the exact ground state wavefunction are given by the Gibbs weights of a 2D classical doped interacting dimer model. We use this mapping to determine the phase diagram in the interaction - hole density plane. Analytically, we exploit a direct microscopic mapping of the classical dimer model on the square lattice to a special 8-vertex model and generalized Coulomb gases. Numerically, we use a novel rejection-free geometrical cluster algorithm [3] for classical interacting dimers on the square lattice, in the canonical ensemble. We also use simulations to study the system in the grand canonical ensemble. We discuss the structure of the phase diagram and its critical behavior. 1. D.S. Rokhsar and S.A. Kivelson, PRL 61, 2376 (1988), 2. F. Alet et al. PRL 94, 235702 (2005), 3. J. Liu and E. Luijten, PRL 92, 035504 (2004).   

Possible New Physics at Quantum Critical Points: Skyrmions as Elementary Excitations of $2+1$ D Antiferromagnets

It has recently been proposed that there are degrees of freedom intrinsic to quantum critical points that can contribute to quantum critical physics. We point out that intrinsic critical degrees of freedom exist quite generally below the upper critical dimension. We show that in $2+1$ D antiferromagnets skyrmion excitations are stable at criticality and identify them as the critical excitations.   

Possible New Physics at Quantum Critical Points: Skyrmions as critical Spin 1/2 Excitations of 2+1 D Antiferromagnets

We show that despite the absence of a Hopf term and zero Berry phase terms, the N\`eel ordered phase of $2+1$ D quantum antiferromagnets have spin 1/2 excitations, i.e. {\it spinons}. The spinons are skyrmion excitations of a topological nature. Since skyrmion gap is proportional to the spin stiffness, quantum criticality corresponds to skyrmion gap collapse. We speculate that skyrmions are relevant at criticality and are, perhaps, related to recent suggestions of critical fractionalization.   

Quartet condensation of fermions

We investigate quartet condensation in fermion systems with four internal states. Physical examples include spin-3/2 fermionic atoms, transition metal oxides with orbital degeneracy, bi-layered systems with electrons and holes and quadra-layer spin-polarized electron hole systems. We consider a simple SU(4) symmetric model in which the fermions interact among themselves with point attractive interactions. The effective free energy functional of the Cooper Pairs (CP) is found to contain attractive interactions among certain types of CP's. This will allow the CP's to form bound states or quartets. Using a variational calculation based on the Bogoliubov inequality, we find that the system may undergo quartet condensation which will suppress the CP instability. By tuning the interaction away from the SU(4) limit, a phase transition from quartet to CP condensation can occur.   

Session N46: Semiconductor Devices / Semiconductors General

Dielectric and Photovoltaic Physics in Thin-Film Crystalline Sulfides

Solar energy utilization has been the hope and sought-for solution to local energy needs at least since the late 1800's. In today's terms, solar energy is one of the few renewable energy sources with the potential to have a major impact on domestic energy independence. There is a rich, but incomplete scientific literature on the underpinning photovoltaic physics of solar cell development. This literature does however, clearly identify a pervasive, unsolved physics problem -- \textit{deep level electronic states in wide band gap semiconductors quench the electro-optic behavior of solar cells: either p-type or n-type doping is inhibited both of which are required for the basic function of a semiconducting p-n junction solar cell. }We will report on our approach towards solving this problem via layer-sequenced stabilization of thin-film photovoltaics that enable symmetric p or n-type doping. We will bring interface phase physics to the synthesis process for sulfur-based chalcogenides to show that the valence and conduction band energy levels as well as defect formation energies in these systems can be systematically modified in wide bandgap photovolatics.   

Metal-semiconductor-metal junctions with silver sulphide barrier layers

Atomic level electrical switching requires innovative methods of charge transport, wherein the device can be switched between ``on'' and ``off'' states at ambient temperatures by applying reasonably small voltages. Recently, the mixed conducting property of silver sulphide was utilized in making a quantized conductance atomic switch[1] which satisfies these requirements. We present the fabrication of metal-semiconductor-metal junctions where a Ag$_{2}$S layer is sandwiched between two metal electrodes. Current-voltage measurement shows diode characteristics for these junctions at large thickness (100 {\AA}) of Ag$_{2}$S. At lower thicknesses, the nature of transport changes over to a nonlinear tunnel junction like behaviour up to an applied external voltage of 1.5 V. The growth, morphology and transport properties of Ag$_{2}$S layers depend critically on the deposition conditions. Using the tunnel junction, we investigate the effects of parameters such as growth and thickness of semiconducting layers, choice of metal electrodes and the metal-semiconductor interface on the charge transport across the junction. [1] K. Terabe et. al, Nature \textbf{433}, 47 (2005)   

Characterization of hydrogenation processes for c-Si photovoltaics

A commonly used method to introduce H into Si solar cells to passivate bulk defects is by the post-deposition annealing of an H-rich SiN$_{x}$ surface layer that also acts as an antireflection coating.$^{1}$ It previously had been impossible to characterize the small concentration of H that is introduced by this method. Our work on the properties of the transition-metal-H complexes in Si has led us to develop a novel method to characterize the introduction of H into Si.$^{2}$ We have used IR spectroscopy coupled with transition-metal impurities introduced into Si-test samples to act as traps for H. The transition-metal-H complexes can then be detected with high sensitivity to determine the concentration and penetration depth of H in the samples. This model system has been used to obtain insight into what solar-cell processing strategies lead to the best passivation of defects in the Si bulk. We thank V. Yelundur and A. Rohatgi for an enjoyable and fruitful collaboration.. This work is supported by NSF Grant DMR 0403641 and NREL grant AAT-1-31605-04. \newline 1. A. G. Aberle, Sol. Energy Mater. Sol. Cells \textbf{65}, 239 (2001). \newline 2. F. Jiang \textit{et al.}, Appl. Phys. Lett. \textbf{83}, 931 (2003).   

Optical switching and structural properties of Ge2Sb2Te5 and Ge2Sb2Te7 films

Ge2Sb2Te5$_{ }$and Ge2Sb2Te7 are materials important in phase change memory applications, but the structures of both the amorphous and crystalline phases are not well known. Large areas of optically switched material are needed in order to probe the structure. Films of amorphous Ge2Sb2Te5$_{ }$varying in thickness between 20 nm and 100 nm were crystallized by exposure to a focused beam of 532 nm laser light with a power density of approximately 50 kW/cm$^{2}$. Rastering of the crystallized spots produces areas of several square millimeters suitable for experiments to probe the structure of the films. The switching causes little change in surface topography as measured by atomic force microscopy. Ablation of the films occurs if the power density is too high. The structure of optically crystallized films studied by EXAFS will be discussed. Films of Ge2Sb2Te7 will also be discussed.   

Interface Dielectric Function in ZnO/Ag Structures for Applications as Back-Reflectors in Thin Film Solar Cells

Sequential deposition of optically-opaque Ag followed by the transparent conductor ZnO, both by magnetron sputtering on substrates such as stainless steel, is a key process for efficient optical back-reflectors (BRs) of thin film solar cells. The roughness scale investigated in our work is an order of magnitude smaller than that studied previously. We have first analyzed Ag deposition by real time spectroscopic ellipsometry (RTSE) over the energy range from 1.0 eV to 6.5 eV in order to establish the final roughness thickness on the Ag just prior to \textit{in situ} deposition of ZnO. Values from 10 to 50 {\AA} are obtained, for a relatively narrow range of substrate temperature (20-90\r{ }C). We employ the same RTSE probe to analyze the interface and bulk optical properties of ZnO and thus deduce a complete optical model of the BR. Our model for the dielectric function of the interface layer helps explain the losses in the BR structure. It includes contributions from free electrons associated with the Ag component, and bound electrons associated with a metal particle plasmon resonance near 2.7 eV and with interband transitions from Ag and ZnO. The effect of the interface layer on reflectance of BR structures is evaluated.   

High-performance ZnO/ZnMgO FET using a hetero-MIS stricture

We propose a new structure of ZnO/ZnMgO field-effect transistors (FETs) for simplifying the fabrication process as well as for the improvement of the FET characteristics. Recently, we developed a ZnO/ZnMgO heterostructure FET (HFET) by utilizing a two-dimensional electron gas channel layer formed in the selectively-doped single quantum well structure.$^{1)}$ In the HFET fabrication process, the ohmic contact formation is crucial because of the difficulty in removing the top ZnMgO barrier layer with the underlying ZnO channel remained. The use of a very thin (1-2 nm) ZnMgO top barrier layer enables the formations of both good ohmic contacts without the ZnMgO etching and the gate electrode�D We used metal-insulator semiconductor (MIS) gate structure with the use of a 50-nm-thick Al$_{2}$O$_{3}$ gate insulator. The thin ZnMgO barrier acts as a setback layer for the channel electrons from the ZnMgO/Al$_{2}$O$_{3}$ interface. The 1-$\mu $m-gate device showed a complete FET operation with a transconductance of as high as 28 mS/mm and the effective mobility of 62 cm$^{2}$/Vs. 1) K. Koike et al., Appl. Phys. Lett. \textbf{87}, 112106 (2005).   

First-principles calculations of mobilities in novel MOSFETs

Nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs) incorporating novel materials demonstrate unusual electron transport behavior. Straining the silicon lattice results in significant increases in electron and hole mobility. However, mobility calculations using standard approximations have difficulty explaining this increase. MOSFETs using novel gate dielectrics (e.g. hafnium oxide) have mobilities that are much lower than MOSFETs using silicon dioxide as the dielectric. ``Interface quality'' has been invoked as a likely cause of this difference, but few attempts have been made to tie the mobility decrease to scattering mechanisms associated with the novel dielectric structure. In this talk, we report results of mobility calculations in MOSFETs with a strained-Si channel and with alternate gate dielectrics. The calculations employed a recently developed first-principles method based on atomic-scale interface models.[1] Changes in the local environment of atomic-scale interface roughness defects are shown to potentially account for the increase in mobility under strain. Interstitial Hf defects near the silicon-oxide interface can act as traps and are shown to impact the mobility in MOSFETs with hafnium oxide gate dielectrics. [1] M. H. Evans, X.-G. Zhang, J. D. Joannopoulos, and S. T. Pantelides, Phys. Rev. Lett., v. 95, p. 106802 (2005).   

Negative Bias Temperature Instability (NBTI) recovers fully with bake at 325 C or above and the device is equivalent to new.

Negative Bias Temperature Instability (NBTI) is one of the major degradation mechanisms of PMOSFET devices. When the p-channel Field Effect Transistor (PFET) gate is biased negatively with respect to the channel, as in CMOS inverter, at elevated temperature the threshold voltage (Vt) decreases (absolute value increases for application temperatures) and the drive current (Ion) decreases. This degrades the device performance and may lead to circuit failure. NBTI is process dependent and has strong dependence on temperature, gate voltage, time, and gate oxide thickness. It also depends on device area and or geometry. NBTI models used in industry are empirical. I have observed, on different technologies, in the last several years that NBTI recovers with bake. The recovery amount depends on the bake temperature, which can be the stress temperature, and happens very fast at any temperature. Full recovery is achieved at temperatures above 325 degrees C. After full bake recovery the device behaves like new with NBTI equal to the NBTI of the original stress.   

Calculation of the Phonon Lifetime of Photoexcited Bismuth

Phonon lifetimes of the zone-center longitudinal optical phonon in bismuth are calculated with respect to the fraction of valence band electrons excited into the conduction bands. Second order density-functional perturbation theory (DFPT), combined with the frozen phonon technique, is used to calculate the third-order anharmonic couplings between phonons. Calculations on the photoexcited system are performed by constraining the occupations of the valence and conduction bands, giving a certain excited electron-hole plasma density. It is found that the calculated decrease in the phonon lifetime with excitation is due both to the reduction of the phonon frequency and an increase in the coupling to other phonons.   

Defect levels in semiconductors - is the “band gap problem” truly a problem?

Quantitative predictions of defect properties in semiconductors using density functional theory (DFT) have been crippled by standard supercell methods, which have incorrect boundary conditions for an isolated defect, and the “band gap problem,” where DFT drastically underestimates the band gap. I present a generalized supercell method with boundary conditions appropriate to point defects, to fix the electrostatic boundary conditions, remove ambiguity in charge reservoir, include bulk polarization effects, and specifically account for defect level dispersion. I compute formation energies for an extensive set of defects in silicon. The resulting defect level spectrum in silicon exhibits no band gap problem. The results agree remarkably well with experiment for those values that are experimentally known, and predict heretofore unobserved electronic transitions important for the electrical response of irradiated semiconductor devices.   

Structural, optical, and electrochromic properties of V$_{2}$O$_5$ thin films by Metalorganic Decomposition

V$_{2}$O$_{5}$ a n-type semiconductor has been widely used in variety of technological applications such as solid state battery cathodes, solar cell windows, and electrochromic devices as it allows easy intercalation/deintercalation of different ions due its open layered structure. Recently the attention has been focused on the development of thin films of V$_{2}$O$_{5}$ as a cathode material in microbatteries owing to the miniaturization of electronic devices. We report the preparation of V$_{2}$O$_{5}$ thin films by cost effective easy method of metalorganic decomposition technique using vanadium naphthenate oxide precursor. The solution is spin coated on glass and ITO coated glass substrates. The resulting films on annealing at 450\r{ }C are comprised of V$_{2}$O$_{5}$ nanoparticles as evidenced from X-ray diffraction and Raman spectra. UV-VIS studies indicate band gap of $\approx $ 2.4 eV. The dependence of electrochromic properties of these films, heat treated at various temperatures, on microstructure and crystallinity will be presented.   

Structure of III-Sb(001) Surfaces Under Extreme Sb-rich Conditions

We use density functional theory to study the structure of III-Sb(001) (III = Al or Ga) growth surfaces. Various reconstruction models are considered to construct the surface stability diagram under different III-Sb growth conditions. We found that AlSb surface stability diagram identifies experimentally observed surface reconstructions quite well. For GaSb, however, all $(n\times 5)$-like reconstructions proposed to date have too high surface formation energies compared to the ones with wrong periodicities and thus cannot adequately model the structures observed experimentally under extreme Sb-rich growth conditions. Our results indicate that the existing reconstruction models for GaSb(001) surface require revisiting and demonstrate the need for a better reconstruction model.   

A density functional study of the effect of pressure on GeTe

We compare local density approximation (LDA) and generalized gradient approximation (GGA) calculations of GeTe as a function of applied pressure. The LDA yields a poor result for the zero pressure trigonal angle but good to excellent results for the zero pressure lattice constants, energy gap, and relative positions of the two sublattices. More importantly, it yields results within the wide range of experimental values for the critical pressure at which the ferroelectric trigonal to rock-salt transition takes place. We also calculate the zero pressure polarization.   

Pressure-Raman study of optical phonon anharmonicity and metastable phase in $^{68}$Zn$^{76}$Se

The effects of hydrostatic pressure on the one- and two-phonon Raman spectra of isotopic purity $^{68}$Zn$^{76}$Se are studied to 15GPa at 300K. With increasing pressure the TO-TA(X,K) difference-mode shifts rapidly to higher energy, moving above the 2TA overtone band at 5.8GPa with no significant 4 -phonon mixing. Above 10GPa, the one-phonon TO($\Gamma )$ peak broadens rapidly, reaching $\sim $ 60cm$^{-1}$ FWHM and overlapping both TO-TA(X,K) and LO($\Gamma )$. After the sample undergoes the forward and reverse high-pressure transitions, the sphalerite-structure Raman features return (including the strongly broadened TO($\Gamma )$ peak) and a new sharp line attributed to a metastable ZnSe phase appears. The TO($\Gamma )$ broadening in ZnSe is much stronger than that due to pressure-tuning of the anharmonic decay TO($\Gamma )=>$ TA+LA(X,W,K) in GaP and ZnS.[1] Our results suggest that the resonant anharmonic interactions in ZnSe may be strongly enhanced by spatial confinement and disorder in the domains of nucleating phases. [1] J. Serrano et. al., Phys. Rev. B\underline {69}, 014301(2004).   

Raman scattering in Zn$_{1-x}$Fe$_{x}$Te, a van Vleck diluted magnetic semiconductor

Zn$_{1-x}$Fe$_{x}$Te, a zinc blende II-VI diluted magnetic semiconductor(DMSs), exhibits van Vleck paramagnetism, thanks to the electronic level structure of Fe$^{2+}$ with T$_{d}$ site symmetry, Subjected to crystal field and spin-orbit coupling, the lowest level of its ground state multiplet has a $\Gamma _{1}$ non-magnetic level, with a $\Gamma _{4}$ magnetic level just above it. This level ordering leads to its van Vleck paramagnetism. The Raman spectra of this DMS display the $\Gamma _{1}\to \Gamma _{4 }$electronic transition($\Gamma ^{\ast })$ whose Zeeman splittings are interpreted in terms of symmetry considerations and numerical calculations. The magnetic field and the temperature dependence of the spin-flip Raman line of the donor-bound electron in Zn$_{1-x}$Fe$_{x}$Te exhibit characteristics typical of van Vleck paramagnetism and, in combination with magnetization measurements, yield the s-d exchange constant $\alpha $N$_{0}$=236.9$\pm $9 meV. The Raman spectra also show $\Gamma ^{\ast }$ in combination with LO phonons which exhibit an intermediate mode behavior.